Geologic Applications of 

Modern Aeromagnetic Surveys 

---·':'~--· .. 

U.S. Geological Survey Bulleti~ 1924 
Front Cover: Past and present U.S. Geological Survey aircraft 

used for geophysical surveys are depicted flying over terrain 

typical of the foothills west of Denver, Colorado. The aircraft, 

in order of age (from left to right), are Beechcraft Model 17, 

Staggerwing; Douglas DC-3; Convair CV-240; and Fairchild 

Porter PC 6/C-H2. 

Cover design by Art Isom, U.S. Geological Survey. 
Geologic Applications of 

Modern Aeromagnetic Surveys 

Edited by WILLIAM F. HANNA 

Proceedings of the U.S. Geological Survey workshop on 

Geological Applications of Modern Aeromagnetic Surveys held 

January 6-8, 1987, in Lakewood, Colorado 

U.S. GEOLOGICAL SURVEY BULLETIN 1924 
DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Any use of trade, product, or firm names in this publication is for descriptive 

purposes only and does not imply endorsement by the U.S. Government. 

UNITED STATES GOVERNMENT PRINTING OFFICE: 1990 

For sale by the 

Books and Open-File Reports Section 

U.S. Geological Survey 

Federal Center 

Box 25425 

Denver, CO 80225 

Library of Congress Cataloging-in-Publication Data 

U.S. Geological Survey Workshop on Geologic Applications of Modern 

Aeromagnetic Surveys (1987; Lakewood, Colo.) 

Geologic applications of modern aeromagnetic surveys ; proceedings of the 

U.S. Geological Survey Workshop on Geologic Applications of Modern 

Aeromagnetic Surveys, held January 6-8, 1987, in Lakewood, Colorado I 

edited by William F. Hanna. 

p. em. - (U.S. Geological Survey bulletin; 1924) 

Includes bibliographical references. 

Supt. of Docs. no.: I 19.3:1924 

1. Aeromagnetic prospecting-Congresses. 2. GeomagnetismCanada-Congresses. I. Hanna, William F., 1938- . II. Title. Ill. Series. 

QE75.B9 no. 1924 

[TN269] 

557.3 s-dc20 

(662'.153] 90-3272 

CIP 
FOREWORD 

A workshop entitled "Geologic Applications of Modern Aeromagnetic Surveys," 

sponsored by the U.S. Geological Survey, was conducted January (rS, 1987, at the 

Sheraton Hotel Conference Center, Lakewood, Colo. Because of severe constraints 

on time, space, and budget, attendance at the workshop was restricted to invited 

members of academia, private companies, National laboratories, and State and 

Federal governmental agencies. The workshop was originally promoted on short 

notice by the late Frank C. Frischknecht, in concert with Thomas G. Hildenbrand, 

Lindreth E. Cordell, and Gary L. Raines. Hildenbrand, who inherited the 

administrative responsibilities of Frischknecht, subsequently spearheaded a major 

effort to promote airborne electromagnetic and remote sensing techniques as well, in 

later workshops dedicated to these methods. Thus, this proceedings volume on the 

magnetic technique is one of three such workshop volumes documenting the value of 

and the need for a national aerogeophysical program. A list of attendees for the 

present workshop and their respective affiliations is provided in Appendix A. 

The first half of the workshop was devoted to oral presentations made by a 

select number of participants; the second half was devoted to working group sessions 

identified by geographic area or by topic and involving all participants. Titles of 

presentations and the compositions of working groups are summarized in Appendix 

B. Although this proceedings volume does not include narratives for all presentations 

made during the workshop, it does contain a few additional reports relevant to the 

subject of the workshop. 

The motivation for the workshop was based on the strong commitment of the 

U.S. Geological Survey to develop knowledge about efficient techniques for assessing 

mineral, fuel, and ground-water resources as well as geologic hazards. Some of the 

most efficient of these techniques are those which utilize airborne systems because of 

speed of coverage and almost unlimited accessibility. The subject workshop focused 

on a technique that has a proven track record and that has been extensively developed, 

utilized, and promoted by the U.S. Geological Survey over a 45-year period: 

aeromagnetic surveying. 

Many advances have been made since the original implementation of aeromagnetic surveys, some of which relate to development of new instrumentation and 

others of which relate to the formulation of new concepts that bear on data 

acquisition, processing, interpretation, and display. Some of these current and 

emerging technologies were highlighted in presentations made during the workshop; 

others were discussed in working group sessions or were addressed informally in 

conversations among workshop participants. 

William F. Hanna 

Editor 

CONTENTS 

Introduction 1 

Need for aeromagnetic data and a national airborne geophysics program, by 

Thomas G. Hildenbrand and Gary L. Raines 1 

Keynote papers 7 

Aeromagnetic survey program of Canada, mineral applications, and vertical 

gradiometry, by Peter J. Hood 7 

Rock magnetism, the distribution of magnetic minerals in the Earth's crust, 

and aeromagnetic anomalies, by Richard L. Reynolds, Joseph G. 

Rosenbaum, Mark R. Hudson, and Neil S. Fishman 24 

Abstracts and short papers 47 

Worldwide aeromagnetic coverage: The need for data bases and map 

inventory, by William J. Hinze 47 

Explanation for shaded-relief aeromagnetic map of central Minnesota, by Val 

W. Chandler 48 

A State's perspective on a national aeromagnetic program, by Don R. 

Mabey 51 

Some notes on the production of aeromagnetic surveys, by Mike S. 

Reford 52 

Aeromagnetic surveys, by John M. Schmunk 57 

A view from afar: comparison of satellite and aeromagnetic anomalies, by 

Patrick T. Taylor 60 

Dynamic Global Positioning System navigation in an airborne geophysical 

program, by George E. Williams and William F. Hanna 61 

Some historical notes on early magnetic surveying in the U.S. Geological 

Survey, by William F. Hanna 63 

Discussion group reports 75 

Alaska as a frontier for aeromagnetic interpretation, by John W. 

Cady 75 

Notes of the Discussion Group on Western United States: Overview, by 

Robert C. Jachens 8S 

Proposed program based partly on notes of Discussion Group on the Basin 

and Range Province, by Robert C. Jachens, H. Richard Blank, and VJ.S. 

Grauch 86 

Notes of Discussion Group on the Midcontinent, by Lindreth E. Cordell, 

Thomas G. Hildenbrand, and M. Dean Kleinkopf 90 

Notes of Discussion Group on Eastern United States, by Jeffrey D. Phillips 

and John M. De Noyer 92 

Notes of Discussion Group on Technology, by Frank C. Frischknecht, Richard 

H. Godson, and Richard J. Wold 96 

Notes of Discussion Group on Rock Magnetics, by Richard L. Reynolds and 

Charles M. Schlinger 99 

Workshop Participants and Workshop Presentations 103 

Appendix A: Workshop participants 104 

Appendix B: Workshop presentations 106 

Contents V 
FIGURES 

1. Photograph showing biplane flown by the U.S. Geological Survey in 1947 

to collect magnetic data 2 

2. Graph showing yearly number of flight-line miles from geophysical surveys 

flown by the U.S. Geological Survey 2 

3. Aeromagnetic coverage map of the conterminous United States 3 

4-8. Aeromagnetic maps showing: 

4. Interpretive resolution versus flight-line spacing 4 

5. 1981-82 survey of northern Labrador 11 

6. Part of the 1:50,000-scale Lac Lomier map 12 

7. Central portion of the 1:250,000-scale Hebron map 13 

8. Torngat Mountains magnetic map at 1:1,000,000 scale 14 

9. Shaded-relief map of northern Labrador lS 

10. Aeromagnetic and shipborne magnetometer coverage map of Canada to 

December 31, 1986 16 

11. Maps showing various scales utilized in aeromagnetic maps published by 

the Geological Survey of Canada 17 

12. Photograph showing Beechcraft B80 Queenair aircraft of the Geological 

Survey of Canada 19 

13-15. Diagrams showing: 

13. The Fe0-Fe20 3-Ti02 ternary system 26 

14. Reactions that involve exsolution and oxidation and their effects on 

saturation magnetization 27 

15. Summary of the crustal distribution of the aeromagnetically 

important minerals 30 

16. Shaded-relief aeromagnetic map of central Minnesota 49 

17. Generalized geologic map of central Minnesota SO 

18. Aeromagnetic anomaly map showing a linear magnetic low axially 

disturbed along flight paths 15 and 17 53 

19. Contrasting plots of a single flight path showing artificial deviations in 

aircraft X and Y coordinates caused by changes in aircraft altitude 54 

20. Contrasting plots of total-field magnetic anomalies associated with the 

flight path shown in figure 19 54 

21. Aeromagnetic anomaly mosaic of two surveys SS 

22. Contrasting aeromagnetic anomaly maps of a single area derived by gridding in different directions 56 

23. Map showing drainage basins of major rivers of Alaska 76 

24. Color-shaded-relief topographic map of Yukon Flats and vicinity 77 

25. Key to geographic features and faults in figure 24 77 

26. Color-shaded-relief map of the residual total-field magnetic anomaly in 

the area west of Yukon Flats 78 

27. Map showing the flight lines used to obtain aeromagnetic data 79 

28. Index map of study area showing selected geographic features, geophysical 

domains, Cretaceous overlap assemblages, and Cretaceous arc volcanic 

rocks 80 

29. Color-slice contour map of the complete Bouguer gravity anomaly 81 

30. Lithologic map made from interpretation of topographic, gravity, and 

aeromagnetic maps 82 

TABLES 

1. Types of remanent magnetizations that may contribute to aeromagnetic 

anomalies 26 

2. Aeromagnetic survey parameters keyed to map area shown in figure 29 79 

VI Contents 
INTRODUCTION 

Need for Aeromagnetic Data and a 

National Airborne Geophysics Program 

By Thomas G. Hildenbrand and Gary L. Raines 

The U.S. Geological Survey (USGS) pioneered 

airborne geophysical exploration (Hanna, this volume). 

In 1943 James R. Balsley, a USGS geophysicist, recognized the potential importance of a magnetic airborne 

detector (MAD) for geophysical exploration. This device, which was developed by the Navy to detect submarines, utilized a self-orienting fluxgate magnetometer. In 1944, after a few modifications were made to 

the MAD system, the USGS flew the first airborne 

geophysical survey; 10,000 line miles of magnetic data 

were collected over Naval Petroleum Reserve 4 in the 

northernmost part of Alaska. In the following 44 years, 

airborne geophysics evolved into a major component of 

earth science. Today aircraft are capable of acquiring a 

wide variety of critical geophysical data (for example, 

gravity, magnetic, electromagnetic, radiometric, spectral, and thermal), which are critical to solving National 

resource, environmental, and geologic hazards problems. 

Historically the USGS has conducted (fig. 1) or 

contracted with private industry to conduct airborne 

geophysical surveys that involve the yearly collection of 

thousands of line miles of data (fig. 2). A disturbing 

trend in the number of line miles flown has occurred, 

however, over the past 13 years. The USGS has progressively decreased its geophysical data collection effort from 150,000 line miles in 1975 to less than 1,000 

line miles in 1988. This proceedings volume stresses 

the utility of airborne geophysical data and thus the 

need for a national airborne geophysics program to reverse this trend in USGS data collection by systetp.atically acquiring geophysical data to address important 

National needs. 

Manuscript approved for publication, December 13, 1989. 

OTHER NATIONAL AIRBORNE 

GEOPHYSICS PROGRAMS 

Canada, Finland, the U.S.S.R., Zimbabwe, Liberia, 

and Australia have established cost-effective national 

airborne geophysics programs. For example, Canada has 

collected over 6 million line miles of magnetic data, 

published over 9,500 aeromagnetic anomaly ~aps, and 

distributed for many years 30,000 aeromagnetic anomaly 

maps per year (the most requested item of the 

Geological Surey of Canada). The Canadian aeromagnetic program has been a very successful endeavor 

that "has led directly or indirectly to the discovery of 

many ore deposits and its overall cost has been recovere.d 

many times over the years through the general economic 

benefits to the country that result from such discoveries 

and from taxes that are subsequently paid to the 

provincial and federal governments" (Hood, this 

volume). In Finland, magnetic and electromagnetic data 

have been collected at a flight-line spacing of ¥4 mi for the 

entire country. Their national airborne geophysics program includes reflying their country at a flight-line spacing 

of ¥s mi. 

In comparison, the average flight-line spacing in 

the United States (fig. 3) is about 3 mi, which is 

inadequate for most interpretational needs. In figure 4, 

the amount of information that can be derived from 

aeromagnetic data along flight lines spaced ¥4, 1, and 3 mi 

apart is shown for a small area in Minnesota. T~e 

3-mi-spaced magnetic data show only broad features ":tth 

little structural information. The 1-mi-spaced data hmt, 

for example, of structures that trend northwest. Clearly 

the ¥4-mi-spaced data provide a greater wealth of 

information essential to the interpretation of lithologies 

and structures in this area, particularly the northwestand east-trending mafic dikes. 

Introduction 
Figure 1. Biplane flown by the U.S. Geological Survey in 1947 to collect magnetic data. Towed "bird" contained fluxgate 

magnetometer. 

~100 

x 

(/) 

w 

...J 

~ 50 

o~--~----~----~----~----~----~~ 

L!) 

L!) 

CTl 

0 

tO 

CTl 

L!) 

tO 

CTl 

0 

,..... 

CTl 

YEAR 

L!) 

,..... 

CTl 

0 

00 

CTl 

L!) ,..... 

00 00 CTl CTl 

Figure 2. Yearly number of flight-line miles from geophysical surveys flown by the U.S. Geological Survey. Miles flown 

include airborne radiometric, magnetic, and electromagnetic 

surveys. 

NEED FOR A NATIONAL AIRBORNE 

GEOPHYSICS PROGRAM 

An essential dimension to geologic knowledge 

includes data on the subsurface obtained either from 

extrapolation of surface geology or from direct measurement by geophysical methods. Exploitable mineral and 

ground-water resources occur at depths that range from 

1 ft to a few thousand feet. Petroleum resources 

generally occur at greater depths (as deep as a few 

2 Geologic Applications of Modern Aeromagnetic Survey 

miles). Geologic hazards such as landslides and volcanic 

eruptions are associated with processes that may occur 

over an even greater range of depths. 

High-quality maps that are derived from surface 

geologic observations are important and necessary for 

many purposes. Commonly it is possible to extrapolate 

surface geology into the near subsurface· with adequate 

accuracy. Nevertheless, the time has long since passed 

when surface geologic mapping alone meets most needs. 

Vast parts of the United States are covered by surficial 

deposits that prevent observation of the bedrock. Surface 

exposures of flat-lying rocks in the Midcontinent and 

elsewhere provide few clues about underlying structures. 

Most geologic information is obtained from studies 

at scales that range from 1:24,000 to 1:250,000. For 

investigations at these scales, airborne geophysics 

provides a more universal approach to subsurface 

investigations than any other type of method. There are 

several reasons for the broad capabilities of airborne 

methods: 

(1) Aircraft are suitable and commonly optimal 

platforms for acquisition of critical geophysical 

data, including data on potential fields (magnetic 

and gravity) and electromagnetic fields. 

Electromagnetic field data include audio, very low 

and radar frequencies to thermal and near infrared 
:; 

.. 

0 

a. 

c 

a 

0 

:::s 

w 

CQ!lp-11~ •nd p·r·apatcd by: 

P•t lUll. Braac:h of G.ophyai«:.• ..,.u 1966 

-; , . ,A 

:::o·::#>':J:~~.o:;~~: .. 4,.$"' fl'd)fC)tAW'A IU.LI" 

f .. 

FLIGHT LINE SPACING 

. 1 MILE OR LESS • 2 TO 3 MILES 

1 TO 2 MILES 

... 

..,~~ . . ... ~ -·- 1.,. '~< 

} "''!! 

1 .. ] .. 

~~ 

JULY 1916 

Figure 3. Aeromagnetic coverage map of the conterminous United States, showing the best flight-line spacing from geophysical surveys flown prior to April 1986. 

Compiled by U.S. Geological Survey, 1986. 
Figure 4. Interpretive resolution versus flight-line spacing. 

Shaded aeromagnetic anomaly maps (illuminated from the 

northeast) shown at three flight-line spacings from surveys of 

the same area in central Minnesota (magnetic-field intensities 

increase from green to red). The 3-mi-spaced data set (top) 

was compiled from · the National Uranium Resource 

Evaluation (NURE) aeromagnetic survey, which was flown at 

a spacing of 3 mi and at 400 ft above the ground. The 

Minnesota Geological Survey, with funding from the 

Legislative Commission on Minnesota 'Resources, 

conducted an aeromagnetic survey of the same area at a 

flight-line spacing of V.. mi (bottom) and at 450 ft above the 

ground. The aeromagnetic anomaly map from 1-mi-spaced 

data (middle) was derived by using every fourth flight line of 

the V..-mi-spaced data. 

frequencies, and visible-light and gamma-ray 

spectra. Some sensors can be used simultaneously, 

thereby obtaining information on surface and 

subsurface variations of several rock properties in a 

single survey. 

(2) The horizontal resolution of data obtained by 

low-flying aircraft is more than adequate for 

mapping at scales as large as 1:5,000. Images of 

large swaths can be obtained in a single pass by 

high-flying aircraft for small-scale mapping. 

Through use of several sensors, information is 

obtained over a continuum of depths from the 

surface to tens of miles below the surface. 

(3) Aircraft data are now collected in digital form 

and are thus suitable for immediate computer 

processing, analysis, and display by geographic 

information systems. The results form a uniform 

and consistent data: set that can be accessed and 

reinterpreted as often as needed to provide 

information on new problems or as geologic 

concepts change. 

(4) Data can be collected by aircraft over areas that 

are inaccessible on the ground because of natural 

causes, such as rugged topography, or man-made 

causes, such as restrictions imposed by private 

ownership. 

(5) Airborne geophysics is very cost effective when 

applied to large areas. Generally no other set of 

ground-based methods can supply equally useful 

information at such low unit costs. 

The importance of airborne geophysics was 

recognized in past decades as can be seen by the 

relatively large data-acquisition programs of the USGS 

during 1950-1980. Twenty or thirty years ago the rate of 

acquisition of aeromagnetic data far exceeded USGS 

capabilities to interpret and fully utilize the data. Now, 

4 Geologic Applications of Modern Aeromagnetic Survey 

3 MILES 

1 MILE 

1/ 4 MILE 

10 

I 

20 KI LOMETERS 

I 
however, airborne data acquisition by the USGS is at an 

all time low and available data are seriously inadequate. 

Rejuvenation of airborne studies is timely both in 

terms of unsatisfied need for subsurface information and 

new technological developments which greatly enhance 

the capabilities of the various methods. Recent 

improvements in sensors offer order-of-magnitude 

improvement in resolution (both spectral and spatial) 

over the best devices available a few years ago. 

Improvements in electronic navigation for aircraft from 

the Global Positioning System will be operational in the 

near future. 

In coordination with government agencies, 

academic institutions, and private industry, the long-term 

goal of a national airborne geophysics program should be 

to obtain systematic, digital airborne geophysical data for 

the United States. Data acquisition should focus on the 

needs of National programs, including mineral and 

organic fuel assessments, earthquake and volcano 

hazards, and environmental issues. 

Introduction 5 

KEYNOTE PAPERS 

Aeromagnetic Survey Program of Canada, 

Mineral Applications, and Vertical Gradiometry 

By Peter J. Hood1 

Abstract 

A regional aeromagnetic program has been carried 

out in Canada for the past 40 years and now covers about 

60 percent of the land area and a considerable portion of 

the surrounding continental shelves. The basic 

specifications utilized throughout have been to fly at a 

305-m mean terrain clearancy along flight lines spaced 

805 m apart. In the mountainous areas of the country the 

survey technique utilized is to fly a given area at a 

constant barometric elevation. This procedure results in 

contour maps that retain the continuity of the total-field 

contours between the survey areas flown at different 

heights, albeit with some loss of detail. 

The areas surveyed include the economically 

important greenstone belts of the Canadian Shield which 

comprise about 25 percent of the area and most of the 

mineral deposits of the Superior Structural Provin~. The 

aeromagnetic program has been credited with a prime 

role in a number of mineral discoveries in the shield and 

elsewhere in Canada. These include, in addition to the 

direct discovery of iron ore, a pathfinding role in the 

discovery of massive sulphide deposits of the base metals, 

which commonly contain the magnetic mineral pyrrhotite. In recent years, magnetic survey results have also 

been of great assistance in gold exploration programs, 

particularly in delineating the underlying geology in the 

vast drift-covered areas of the Canadian Shield which 

commonly have minimal outcrop. Alkaline syenitecarbonatite complexes, which may contain niobium and 

tantalum mineralization, are easily recognizable because 

they usually generate bulls-eye magnetic anomalies. On 

the continental shelf of eastern Canada, reconnaissance 

aeromagnetic surveys have successfully delineated areas 

underlain by sedimentary rock with hydrocarbon 

potential. 

Vertical gradiometer techniques have been 

developed by the Geological Survey of Canada to provide 

more definitive aeromagnetic survey coverage, which is 

I Geological Survey of Canada, Ottawa, Ontario, KlA OES. 

used in detailed geological mapping programs, many at 

1:20,000 scale. The Geological Survey of Canada initially 

built its own inboard fiXed-wing system on a Beechcraft 

Queenair aircraft using optical absorption magnetometers in the early 1970's. A substantial experimental survey 

program was then carried out to demonstrate the efficacy 

of the gradiometer technique before a technology 

transfer was made to airborne survey contractors. More 

recently the Geological Survey of Canada has fostered 

through research and development contracts the 

development of helicopter-borne towed-boom systems 

for use in mountainous terrain. The results in such 

rugged areas as the Gaspe Peninsula of southeastern 

Quebec have demonstrated the success of this new 

airborne geophysical survey technique. 

PREVIOUS WORK 

In 1947, aeromagnetic surveys were commenced by 

the Canadian Federal government as an aid to both 

geological mapping and mineral exploration programs. 

For a number of years the Geological Survey of Canada 

(GSC) operated its own survey aircraft, and during the 

10-year period 1947-1956, inclusive, an average of 

40,000 line miles were flown each year. During this 

period of time the value of the aeromagnetic survey 

technique as a geologic mapping tool for the Canadian 

Shield was clearly demonstrated; in addition the program 

received a considerable psychological boost in the early 

years as a result of the discovery of a 20-million-ton iron 

deposit at Marmora, Ontario, beneath 120 ft of Ordovician limestone. This aeromagnetic survey was flown by 

Aero Service, Inc., in 1949 and was jointly funded by the 

GSC and the Ontario Department of Mines. Lang (1970) 

carried out a cost-benefit analysis of the Marmora 

discovery and estimated that the 1949 aeromagnetic 

survey cost about $45,000, whereas production and 

reserves at the Marmoraton property to the end of 1969 

amounted to $198 million. Lang stated that this figure of 

$198 million was much more than the total expenditures 

by the GSC from 1842 to 1969! 

Keynote Papers 7 
The prime area of interest in the execution of the 

Canadian aeromagnetic survey program was the wellmineralized part of the Canadian Shield which surrounds 

Hudson Bay. The shield area is in general quite flat 

because the rocks have been eroded for millions of years 

both by the normal process of weathering and to a 

considerable extent by the various glaciers that have 

occurred throughout geologic time. 

In 1960, the need for an accelerated aeromagnetic 

survey program became increasingly apparent to the 

Director of the Exploration Geophysics Division, Dr. 

L.W. Morley, and led to a combined Federal!Provincial 

aeromagnetic survey plan being proposed in which the 

cost was shared equally between the Federal government 

and the various provinces. In retrospect it is clear that the 

timing of the proposal was propitious for the following 

reasons (Hood and Ready, 1976): 

(1) The aeromagnetic method had become an 

established survey technique which could be 

carried out by contract survey companies for a 

reasonable price. In addition, the survey industry 

was actively seeking Canadian government work to 

provide some stability in their operatio~s and thus 

to ensure a continuing Canadian airborne geophysical survey capability. It is of some interest that the 

Canadian airborne geophysical survey industry 

subsequently grew to become the largest in the 

world in terms of overall capabilities and was 

consequently able to obtain a large proportion of 

international business. 

(2) A pilot program had been carried out by the 

GSC itself which demonstrated the value of the 

aeromagnetic survey technique as an aid to 

geologic mapping and to mineral exploration. 

Thus, because the GSC had a cadre of personnel 

who were themselves technically competent in the 

technique, it was possible to draw up an objective 

set of contract specifications and subsequently to 

ensure that these specifications were adhered to 

through field and office inspections. 

(3) It was anticipated that government-funded 

surveys of the Canadian Shield would provide an 

orderly, standardized coverage with common 

specifications in comparison to that which would 

result if the surveys were carried out by various 

private mining companies; in any case, the end 

products of such surveys by private companies 

would remain proprietary. It was also recognized 

that standardized coverage would permit large 

areas to be compiled in order to produce regional 

aeromagnetic survey maps. 

(4) Outside agencies both in the private sector and 

in the provincial governments were enthusiastic 

about the proposed aeromagnetic survey program. 

8 Geologic Applications of Modern Aeromagnetic Survey 

At the 1960 Mines Ministers Conference held in 

Quebec City the resolution to proceed with the present 

Federal-Provincial aeromagnetic survey plan was 

unanimously approved. The aeromagnetic survey of the 

Canadian Shield proceeded as planned, and the rate of 

survey flying jumped 8.5 fold from 40,000 line miles per 

year prior to 1958 to 340,000 line miles per year after the 

inception of the program. However, a number of factors 

combined to reduce the average production rate to a 

lower level of 165,000 line miles per year in subsequent 

years. These factors included the poorer operating and 

weather conditions in northern Canada, where the later 

surveys were conducted; fewer landing strips in northern 

Canada, which increased the percentage of 

nonproductive flying; the fact that the magnetic diurnal 

variation is generally much greater in northern Canada 

due to the auroral zone; and the fact that there was a high 

rate of inflation, which gradually increased the cost of the 

surveys due to a cost-of-living factor that was included in 

the payment schedule. Because the amount of money 

budgeted for aeromagnetic surveys remained essentially 

constant until 1978, the rate of production of aeromagnetic maps decreased inversely as the cost-of-living 

factor increased. In 1978, the budget for aeromagnetic 

surveys was halved, and this caused a further reduction in 

the rate of data acquisition. 

The Federal/Provincial aeromagnetic survey 

program appears to be more generally approved of by the 

mineral exploration industry than any other recent 

Canadian government program. The data from these 

surveys are much used in the planning of mineral 

exploration programs (Roth, 1975). The aeromagnetic 

survey program is credited with a prime role in a number 

of discoveries in Canada and a lesser role in others 

(Pemberton, 1966). Most mineral deposits are located in 

the so-called greenstone belts which are fairly readily 

delineated by aeromagnetic surveys because the belts 

produce regional magnetic lows (Morley and others, 

1967; Grant, 1985). The greenstone belts comprise about 

25 percent of the area of the Superior structural 

province; thus, 75 percent of the area of the Superior 

Province may be eliminated as nonmineralized and nonprospective by carrying out aeromagnetic suiVeys. The 

delineation of iron formation is relatively easy from 

aeromagnetic suiVeys, and there are many iron-ore 

discoveries; for example, in Ontario alone, deposits were 

delineated at Agwa, Briarcliffe, Bruce Lake, Kakabeca 

Falls, Lake St. Joseph, and North Spirit Lake. In 

addition, because there appears to be a close relationship 

between the occurrence of sedimentary iron formation 

and stratiform massive sulphide deposits in a sea-floor 

spreading environment (Stanton, 1976; Grant 1985), a 

follow-up suiVey of distinctive anomalies on aeromagnetic maps in such areas has led to significant 

discoveries, such as the Brunswick No. 12 orebody in the 
Bathurst mining camp (Ward, 1958). Moreover, because 

pyrrhotite has very strong magnetic properties, basemetal sulphides that contain significant amounts of 

pyrrhotite will also produce distinct magnetic anomalies; 

these anomalies appear to be more prevalent on the 

aeromagnetic maps of the Canadian Shield than was 

expected. In British Columbia, aeromagnetic anomalies 

ultimately led to the discovery of the Island copper mine 

in northern Vancouver Island and the Cariboo Bell 

copper mine in the intermontane zone. 

Alkaline syenite-carbonatite complexes, which may 

contain niobium and tantalum mineralization, generate 

circular to elliptical magnetic anomalies (Dawson, 1972). 

The recognition of such features on aeromagnetic maps 

led to the discovery of the Big Beaverhouse, Schryburt 

Lake, and Alpha-B alkaline complexes in Ontario. In 

addition, the Memegos and Lachner Lake ~kaline 

syenite-carbonatite complexes in Ontario and the St. 

Honore complex in Quebec were all identified by aeromagnetic surveys. 

The major geological benefit of the regional aeromagnetic survey of the Canadian Shield has been the 

outlining of the major units and a better understanding of 

the tectonic history of the shield, which has resulted in a 

refinement of the boundaries of the various age provinces. Much of the Canadian Shield is covered with 

glacial overburden, which was deposited during the last 

ice age, and about 30 percent of it is covered by lakes. 

Thus, the percentage of outcrop available to the geologist 

is commonly less than a few percent of the total area. 

With the use of aerial photographs and aeromagnetic 

maps, the geologist can prepare a preliminary geologic 

map that can be used to plan geological traverses and, in 

general, to carry out field work efficiently. The location of 

geologic contacts can usually be more accurately 

estimated using aeromagnetic contour maps, especially 

where outcrops are sparse. Consequently, the aeromagnetic survey data have been of great benefit to the 

geologic mapping program of the GSC. 

In concluding this summary of previous work, it 

should be recognized that the Canadian Federal/Provincial aeromagnetic survey plan has also had a considerable 

influence in promoting the use of the method overseas. 

Consequently the so-called Canadian specifications for 

aeromagnetic surveys have been adopted for many 

overseas surveys, and this has helped the Canadian 

airborne geophysical survey industry to obtain a 

considerable amount of overseas work. In addition, aeromagnetic surveys funded by the Canadian International 

Development Agency (CIDA) were carried out in 

Botswana, Brazil, Cameroon, Guyana, Ivory Coast, 

Lesotho, Mali, Niger, Nigeria, Pakistan, Upper Volta, 

and Zimbabwe by Canadian companies on behalf of the 

national geological surveys of these countries or 

equivalent organizations. 

AEROMAGNETIC SURVEYS IN THE 

MOUNTAINOUS REGIONS OF CANADA 

The mountainous regions of Canada generally 

border the coastal areas, such as the northern coasts of 

Baffin Island and northern Labrador and the Cordillera 

of western Canada. These mountainous areas have posed 

special problems in aeromagnetic surveying because (1) 

it is geperally impossible to conduct drape-flown 

surveying, even using Short Takeoff and Landing 

(STOL) fixed-wing aircraft; (2) it is difficult to maintain 

a constant elevation over the steep terrain; and (3) safety 

considerations are more restrictive here than over flatter 

terrain. 

In the early 1970's, the GSC contracted out 

helicopter-borne aeromagnetic surveys in southern 

British Columbia, but the resultant cost was rather 

excessive, and alternative techniques using fiXed-wing 

aircraft were sought. In 1973, a test was carried out in the 

Kamloops area of British Columbia over an area where 

the mountain peaks reach an elevation of about 1,830 m 

(6,000 ft). An aeromagnetic survey was flown by the GSC 

at a constant barometric elevation (CBE) of 1,980 m 

(6,500 ft) using its twin-engine Beechcraft Queenair 

aircraft equipped with an inboard optical-absorption 

magnetometer. The same area had been drape-flown in 

1967 by a contractor using a fiXed-wing aircraft equipped 

with a fluxgate magnetometer. 

It was apparent that the amplitude of the variations 

of the 1967 drape-flown fluxgate profile was greate_r 

because of the lower survey elevation, but in general 

there was excellent correspondence. Some shortwavelength features were only apparent on the 1973 

CBE profile as an inflection on the side of a larger 

anomaly, which indicates that there was some loss of 

detail. However, in subsequent analysis of the data, it was 

found to be mathematically more valid to downward 

continue constant barometric altitude data, which are 

obtained in a given horizontal plane, than to apply the 

continuation technique to the constant terrain clearance 

data. The exercise also confirmed the widely held view 

that topographic effects are of much less importance than 

magnetization changes in the underlying rocks. However, 

the contoured maps from the Kamloops test clearly 

indicated that useful data could be obtained by 

conducting aeromagnetic surveys of mountainous terrain 

at constant barometric elevation. In addition, there were 

two rather cogent reasons for adopting the CBE 

technique for the Canadian aeromagnetic survey 

program, which is entirely contracted out to airborne 

geophysical survey contractors. Firstly, better quality 

maps will result from the CBE technique in spite of some 

unavoidable loss in detail. This is because: 

Keynote Papers 9 
(1) The control and traverse lines will always cross 

at the same height, which will permit a more 

accurate leveling procedure to be carried out. 

(2) Errors in positioning the aircraft will be 

minimal because the camera will always be 

pointing vertically downward. The accuracy of any 

Doppler system that is utilized will be improved. 

(3) The noise on the magnetic data due to miscompensation effects will be minimized because 

aircraft maneuvers, such as pitch, will }?e minimal 

in straight and level flight. 

Secondly, the cost of the aeromagnetic surveys will 

be lower for CBE surveys than for drape-flown surveys 

because CBE surveys are logistically easier to fly and the 

production is greater for the following reasons: 

(1) A STOL-type aircraft is not required, and 

therefore a less expensive and faster aircraft can be 

utilized. 

(2) The amount of aviation gasoline used will be 

considerably reduced because the aircraft will not 

have to climb up and down over the mountains. 

(3) Typically, in mountainous areas there can be 

fog or mist in the valleys until midmorning which 

might prevent flying drape-flown surveys. 

( 4) The flight lines can be spaced somewhat farther 

apart because the average terrain clearance will be 

greater for most of the survey area. 

(5) There will be fewer reflights because it is easier 

to fly evenly spaced parallel lines. 

(6) Flight-path recovery is simpler at the higher 

elevation. 

(7) The safety factors are much higher. 

(8) There are physically less fatiguing conditions 

and no airsickness problems for the aircrew. 

Subsequently the CBE technique has been applied 

by the GSC to aeromagnetic surveys in northern British 

Columbia, northern Labrador, and Baffin Island, which 

have considerable topographic relief. In planning the 

1981-82 aeromagnetic survey of northern Labrador, the 

first item to be considered was the flight-line direction, 

which should always be oriented at a high angle across 

the main geologic strike. It was clear from the geologic 

maps of northern Labrador published by the GSC at 

1:250,000 scale that the predominant strike direction was 

north-south. Accordingly, the flight-line direction was 

chosen to be east-west. The second item to be considered 

was the division of the area into survey blocks to be flown 

at CBE. This was done using the published topographic 

maps at 1:250,000 scale which indicated that the Labrador Highlands are as high as 1,200-1,800 m (4,00~,000 

ft). The coastal fiords are bounded by cliffs that can be 

more than 600 m (2,000 ft) high. 

The Labrador aeromagnetic survey was divided 

into 14 individual blocks oriented in an east-west 

direction that could be flown at CBE (fig .. 5). It was 

10 Geologic Applications of Modern Aeromagnetic Survey 

decided to survey the inner area, which is remote from 

the coastal mountains, at 300 m (1000 ft) mean terrain 

clearance (MTC) because it was feasible for a normal 

survey aircraft to do so. In carrying out such planning 

exercises, it should be appreciated that the flight lines can 

usually be positioned to straddle isolated peaks so that 

the whole area does not have to be flown at a higher 

elevation. 

Because of the increased terrain clearance of the 

survey aircraft over most of the Labrador survey area, it 

was decided to increase the line spacing from the standard 800 m that is utilized over the Canadian Shield to 1 

km. For the sake of surveying efficiency, the same flightline spacing was designated for the MTC area so there 

could be continuity of the flight lines. The control-line 

spacing was set at 10 km, and the allowable magnetic 

diurnal variation from a linear mean was set at ±10 

gammas between control lines. The typical survey aircraft 

flying at 140 knots takes less than 2 ¥2 minutes to fly 10 

km. The tighter specification (instead of the more usual 

14 km) was chosen because the survey area is located in 

the auroral zone where the diurnal variation is usually 

quite large. The basic sensitivity of the magnetometer 

utilized was set at 0.25 gamma, which was well within the 

capability of good, commercially available protonprecession magnetometers. It is generally accepted that 

standard (or low) sensitivity surveys are those in which 

the sensitivity is 1 gamma or more, medium sensitivity 

surveys are those in which the sensitivity is between 0.1 

and 1 gamma, and high sensitivity surveys are those in 

which the sensitivity is 0.1 gamma or less.· Thus, this 

survey was a medium-sensitivity aeromagnetic survey. 

The noise envelope of the magnetometer data was set at 

0.5 gamma. 

Figure 6 shows one of the 102 resultant 1:50,000-

scale total-field aeromagnetic maps, which is located in 

the central part of the survey area immediately south of 

·tat 58° N. On the published maps the east-west flight lines 

were printed in blue, and the fiducial points were 

numbered along the survey lines (black is the only color 

used on figure 6). The total-field contours were printed 

in red, and the basic contour interval was 5 gammas. The 

thicker contour lines on the map are 100-gamma 

contours. Topographic contours (200-ft intervals) and 

drainage appear on the published maps as a gray base. 

Lakes are stippled with grey dots. The portion of this 

particular sheet that is shown in figure 6 is at the 

intersection of three blocks which were flown at three 

different elevations, namely 300m (1,000 ft) MTC and 

1,070 m (3,500 ft) and 1,220 m (4,000 ft) CBE above sea 

level. It can be seen that, in general, the wavelength of 

anomalies in the area flown at 1,220 m ( 4,000 ft) CBE 

are greater than those in the area flown at 300m (1,000 

ft) MTC, but the continuity of the individual anomalies 

across the elevation boundaries is surprisingly good. 
METERS 

1800 

1200 

900 

600 

300 

150 

0 SEA LEVEL 

100 

200 

400 

1000 

2000 

3000 

100 200 KILOMETE RS 

Figure 5. Map showing 1981-82 aeromagnetic survey of northern Labrador. The survey area has been 

divided into 14 individual blocks, each flown at a given constant barometric elevation, plus an inner area flown 

at 805-m mean terrain clearance. 

Figure 7 shows the central portion of the Hebron 

sheet which includes the area around Saglek Fiord. The 

survey aircraft were mostly based at the airport south of 

Saglek Bay, which has been extensively utilized for 

offshore survey operations by oil companies. A linear 

anomaly strikes in a north-northwest direction across the 

junction of Saglek and U gjuktok Fiords and tends to 

disappear over the water where the anomalies pinch 

considerably. This is due to the fact that the rock has 

been eroded away by glacier movement in the formation 

of the fiord. A similar effect is also seen for the linear 

anomaly that crosses farther to the east along U gjuktok 

Fiord. Thus, there is an inverse topographic effect which 

indicates that the anomalies are mostly produced by the 

magnetic effects of the upper kilometer of crust. 

Nevertheless, it remains clear from the experimental 

evidence that this is another example where 

magnetization changes are much more important than 

topographic effects in the production of magnetic 

anomalies. 

The results of the 1981-82 aeromagnetic survey of 

northern Labrador were also published in the 

1:1,000,000-scale magnetic anomaly map series of the 

GSC (Dods and others, 1985). Figure 8 shows the 

resultant magnetic anomaly map which is a residual 

total-field anomaly map in which the core field of the 

Earth was removed from the measured data using the 

International Geomagnetic Reference Field. The basic 

Keynote Papers 11 
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Figure 6. Part of the 1 :50,000-scale Lac Lomier aeromagnetic map (GSC Map 6322G, published 1983, NTS 14E/13) from the 1981-82 Labrador aeromagnetic 

survey showing the contours for three adjacent areas flown at the constant barometric elevations of 300 m, 1,070 m, and 1,220 m above sea level. 
10 20 KI LOMETE RS 

Figure 7. Central portion of the 1:250,000-scale Hebron aeromagnetic map (GSC Map 7473G, published 1983; NTS 14L, 

K) showing the contours in the area around Saglek Fiord, Labrador. 

color interval for the map is 25 gamma in the central 

portion of the color scale. 

Both aeromagnetic and marine data were utilized 

to compile the map, and no height corrections were 

made. It is clear from this map that the data have not 

been affected by the variety of survey elevations that were 

utilized because the survey boundaries are not marked by 

level discontinuities which would easily show on a 

colored map. The continuity of the north-northweststriking geology is clearly apparent. The blue area is 

mostly underlain by nonmagnetic mylonitized gneiss, 

whereas the adjacent high area (red) to the west consists 

of more magnetic granulite. 

One of the techniques for bringing out the 

distinctive patterns in an aeromagnetic map that provides 

useful structural information is the shaded-relief 

technique (Dods and others, 1985). This technique 

simulates the shadows thrown by a magnetic relief model 

when illuminated by a low-angle light source. Features 

that have low amplitude and that strike at right angles to 

the direction of the illumination will be preferentially 

delineated by the resultant shadows. The technique can 

Keynote Papers 13 
65° 64° 63° 62° 61° 

GAMMAS 

1500 

1000 

750 

500 

400 

350 

300 

275 

250 

225 

200 

175 

150 

125 

100 

75 

50 

25 

0 

-25 

-50 

- 75 

-100 

-125 

-150 

-175 

- 200 

-225 

-250 

-275 

-300 

-325 

- 350 

- 400 

-500 

- 750 

-1000 

-1500 

65° 64° 63° 

50 100 KILOMETERS 

I 

I I I I 

20 40 60 MILES 

Figure 8. Torngat Mountains magnetic anomaly map at 1:1,000,000 scale (GSC Map 20M, published 1985). 

also be used to bring out any misleveling of the data; the 

preferred direction of illumination to bring out any level 

shifts would be parallel to the flight lines because these 

will normally be at right angles to the geologic strike. 

Figure 9 shows the resultant shaded-relief map for 

northern Labrador in which the direction of illumination 

is from the south. No misleveling effects can be detected. 

The structure of the various geologic formations that 

underlie northern Labrador is very apparent. 

14 Geologic Applications of Modern Aeromagnetic Survey 

Thus, the GSC experience indicates that in the first 

phase of a national aeromagnetic survey program, areas 

of rugged topography are most efficiently surveyed at 

CBE using fixed-wing aircraft. 

PRESENT WORK 

The status of the aeromagnetic survey coverage to 

the end of 1986 is summarized in figure 10. To date, a 
0 BO O 

61 ° r-----,------r----~ -----.-----""1-----, 

Cos.\ 

0.95 

0 .79 

0 .63 

0.47 

0.32 

0.16 

0 

Figure 9. Shaded-relief map of northern Labrador with the direction of illumination from the south. 

total of about 8,600,000 line kilometers of regional 

coverage have been flown in Canada, and aeromagnetic 

maps covering all of Newfoundland, Labrador, New 

Brunswick, Quebec, Ontario, and the Canadian shield 

areas of Manitoba, Saskatchewan, and Alberta have been 

issued. The provinces of Prince Edward Island and Nova 

Keynote Papers 15 
EXPlANATION 

Aeromagnetic coverage 

Figure 10. Aeromagnetic and shipborne magnetometer coverage map of Canada to December 31, 1986. 

Scotia were surveyed prior to the start of the Federal/ 

Provincial program. At the present time, more then 

9,500 aeromagnetic maps at various scales have been 

published. 

In 1980, because Canada had adopted metrication, 

a decision was made to change the publication scale from 

the earlier 1 inch to 1 mi (1:63,360) to the more 

compatible scale of 1:50,000. Similarly, aeromagnetic 

maps that were formerly 1:63,360 scale are now being 

issued at 1:250,000 scale. Thus, the scale of the published 

aeromagnetic maps (fig. 11) will increase in three steps 

from 1:50,000 to 1:250,000 to 1:1,000,000 to 1:5,000,000, 

which are jumps of 5:1, 4:1, and 5:1, respectively. This 

appears to be the most logical sequence for a country the 

size of Canada, which has an areal extent of about 10 

million square kilometers. 

For many years, about 30,000 aeromagnetic maps 

were distributed per year, and these maps are the most 

popular single item distributed by the GSC in 

conjunction with the appropriate Provincial agencies. 

16 Geologic Applications of Modern Aeromagnetic Survey 

However, with the downturn in mineral exploration 

activity in the early 1980's, the number of aeromagnetic 

maps distributed has dropped to about 10,000 per year. 

There has been a gradual upgrading in the survey 

specifications since the Federal/Provincial program 

began. For regional surveys, the fluxgate magnetometer 

has been replaced by proton precession magnetometers. 

In addition, digital recording is now mandatory and the 

profile and gridded digital data are now considered the 

prime end product of GSC-funded aeromagnetic surveys. 

For aeromagnetic surveys that utilize proton 

precession or optical absorption magnetometers, an 

aeromagnetic calibration range has been set up in a 

low-gradient area at a crossroads near Bourget, Ontario, 

which is about 45 km east of Ottawa and easily 

recognizable from the air (Hood and Sawatzky, 1983). 

The values at the ground level at the crossroads have 

been tied to the Blackburn Magnetic Observatory using 

calibrated proton precession magnetometers. The values 

at 150-m and 300-m elevation above the crossroads have 
EXAMPLE OF 1 : 50,000-SCALE MAP EXAMPLE OF 1 :250,000-SCALE MAP 

EXAMPLE OF 1 :5,000,000-SCALE MAP EXAMPLE OF 1:1 ,000,000-SCALE MAP 

Figure 11. Various scales utilized in aeromagnetic maps published by the Geological Survey of Canada. 

been measured by flying a survey aircraft at various 

heights across the crossroads to ascertain that the field 

increases by about 12 gammas per 300m rise in height. 

Thus, the total-field values at the two levels above the 

Bourget crossroads have been tied, within a few gammas, 

to the continuously recording magnetometer at the 

Blackburn Magnetic Observatory. Because the diurnal 

variation at the Bourget crossroads can be expected to 

follow closely the diurnal variation at Blackburn, the 

value at Bourget can be calculated ·at any instant of time 

Keynote Papers 17 
by subtracting a constant difference value from the 

Blackburn Magnetic Observatory value. 

Aeromagnetic survey aircraft are normally flown 

along the four cardinal headings across the Bourget 

crossroads with their flight-path cameras operating, and 

the field values for each cardinal heading are ascertained 

at the crossroads. The difference value is subtracted from 

the Blackburn reading at the exact time that the survey 

aircraft crossed the Bourget crossroads to get the true 

reading. The heading errors for the survey aircraft are 

also calculated as part of the same calibration procedure. 

In general, the calibration errors should not exceed 10 

gammas and the heading errors should be within 5 

gammas of the Blackburn values in an acceptable aeromagnetic system. 

Because most of the aeromagnetic survey data in 

Canada were obtained prior to the general adoption of 

digital recording techniques, a great deal of the original 

data were recorded in analog form. As a result, because 

it would have been very expensive to go back to the 

original chart records, the data on the resultant contour 

maps have been digitized to produce the digital data set 

from which 1:1,000,000-scale magnetic anomaly maps 

have been prepared. This procedure also avoids a releveling of the data. Several digitizing techniques have been 

experimented with, and the one that has been finally 

adopted is digitization of the location of the contour 

intercepts along the flight lines. Because the values along 

the flight lines have the greatest accuracy in comparison 

to the interpolated contours between the flight lines, this 

technique retains the accuracy of the original compilation 

prior to contouring. Moreover, there is close 

compatibility between the 0.8128-mm-square size of the 

16-dot matrix pixel utilized on the final, digitally 

produced color map and the flight-line spacing which is 

0.805 mm at the 1:1,000,000-scale. 

An Applicon plotter has been utilized to print the 

final 1:1,000,000-scale magnetic anomaly map. The 

plotter utilizes three color-ink jets, one for each primary 

color (red, yellow, and blue), and these jets are controlled 

by a digital tape. A combination of ink blobs that 

correspond to the correct shade is fired at · a rotating 

drum around which is wound a sheet of paper. The ink jet 

system moves progressively down the rotating paper 

which is consequently painted by the ink raster. It is 

possible by utilizing one ink jet at a time to produce three 

color separations for printing purposes. Unfortunately, 

stable-base plastic material cannot be utilized by the 

Applicon plotter because the ink is not absorbed and 

consequently smears. The three color separations 

therefore have to be photographed the same day they are 

produced to avoid the effects of differential expansion 

along the length and across the width of the paper map. 

18 Geologic Applications of Modern Aeromagnetic Survey 

The resultant end products are utilized together with a 

base map, which shows planimetry, in printing the final, 

colored, 1:1,000,000-scale magnetic anomaly map. 

The color scheme used in the 1:1,000,000-scale 

magnetic anomaly maps is essentially the spectrum of 

white light. Thirty-nine shades, which range from red at 

the positive end to blue at the negative end, are 

employed. The zero value coincides with pure yellow. 

Twenty-six of the shades in the central part of the 

spectrum represent 25-gamma contour intervals with the 

contour interval of the remainder coarsening in discrete 

steps to cover the necessary range. 

It was intended that the 1:1,000,000-scale magnetic 

anomaly maps would be utilized essentially as basic 

building blocks in compiling future editions of the 

1:5,000,000-scale Magnetic Anomaly Map of Canada as 

one of a series of national geologic, tectonic, and gravity 

maps of the country at the same scale. In turn, the 

Magnetic Anomaly Map of Canada has been utilized for 

the Magnetic Anomaly Map of North America, compiled 

as part of the Decade of Notth American Geology to 

celebrate the centenary of the Geological Society of 

America in 1988. However, an additional end product of 

the 1:1,000,000-scale compilation has been a digital 

aeromagnetic data base for Canada which can be utilized 

for many purposes, such as the production of a variety of 

filtered maps that typically emphasize the high- or lowfrequency content of the data. Also of much current 

interest to qualitative interpreters of aeromagnetic data 

are the previously described simulated shaded-relief 

maps which emphasize linear features in the magnetic 

anomaly maps that are not otherwise easily seen. 

Aeromagnetic surveys are conducted overseas as 

part of Canadian aid programs of the Canadian 

International Development Agency, and the preparation 

of regional magnetic anomaly maps has usually been 

made part of the contract at the interpretation stage. It is 

abundantly clear that such magnetic anomaly maps 

should be made a standard end product of regional 

aeromagnetic surveys because of their several uses. The 

regional maps not only serve as indexes to the aeromagnetic survey coverage available on a national basis 

(as well as the gaps in the coverage) but also stimulate 

comparison of the regional magnetic features with those 

appearing on similar-scale geologic and geophysical 

maps and with Landsat imagery. 

DEVELOPMENT OF AN INBOARD 

VERTICAL GRADIOMETER 

In order to keep current with the continually 

advancing technology and to update contract 

specifications, the GSC has maintained its own expertise 

in aeromagnetic surveying over the years. In 1962, the 
Figure 12. Beechcraft 880 Queenair aircraft of the Geological Survey of Canada equipped with twin-boom system for 

aeromagnetic gradiometer surveys. 

GSC purchased a rubidium -vapor magnetometer from 

Varian, Inc., on behalf of the group at the National 

Aeronautical Establishment (NAE) in Ottawa, who 

installed it in their North Star aircraft which is used for 

military aeromagnetic research related to antisubmarine 

warfare. The GSC aeromagnetic survey group has had an 

assocation with NAE that continues to the present day. 

Initially, the optical absorption magnetometer was towed 

in a bird because of the magnetic interference of the 

aircraft. However the NAE group, who are specialists in 

the magnetic compensation of aircraft, developed an 

active compensation system that permitted the magnetometer sensor to be mounted inboard on the· aircraft. 

The GSC instrumentation group, which was headed by 

Peter Sawatzky, worked closely with the NAE group 

during that period and took part, for example, in the 1965 

aeromagnetic survey of the central portion of Hudson 

Bay. The same magnetometer was used to carry out an 

aeromagnetic reconnaissance of the Labrador Sea and 

later of Baffin Bay. The discovery of an extensive 

sedimentary section on the Labrador continental shelf 

(Hood and others, 1967; Hood and Bower, 1973) led to 

the oil companies embarking on an exploration program 

that subsequently resulted in the discovery of significant 

amounts of oil and gas. 

In 1968, the GSC purchased a Queenair B80 

aircraft (fig. 12) and proceeded to equip it with an 

inboard Varian rubidium -vapor magnetometer (using an 

NAE prototype single-cell orienting device). The new 

magnetic compensator system developed by NAE, which 

was being manufactured by Canadian Aviation 

Electronics of Montreal for military antisubmarine 

aircraft, was also utilized. Actually, provision for the 

recording of data from two sensors was made in 

designing the first Queenair data acquisition system. 

Data acquisition was by digital technique, which was 

rather new at that time, and was later incorporated as a 

requirement into the standard GSC aeromagnetic survey 

contracts after the technique was well proven in inhouse 

aeromagnetic surveys. 

A number of high-sensitivity single-sensor surveys 

that were carried out in the period 1970--73 were 

contoured at 2-gamma and even 1-gamma intervals, 

which demonstrated that a light twin-engine aircraft 

could be well compensated magnetically. Then in 1973, 

as a result of theoretical studies in aeromagnetic gradiometry (Hood, 1965), the next, more difficult step was 

taken to build an inboard vertical gradiometer system 

with a 2-m sensor separation. The design was initially 

based on the requirement to delineate the vertical 

gradient at a height of 1,000 ft above a dipping contact 

that had an effective susceptibility contrast equivalent to 

a difference of 0.04 percent magnetite content across the 

contact (Hood, 1981). The contact case is the most 

frequently encountered geometric model for the 

Canadian Shield and is actually the worst case model in 

that the rate of fall-off of the total field with height is 

least. 

The first gradiometer survey was carried out in the 

Spring of 1975 in the White Lake area close to Ottawa 

Keynote Papers 19 
(Hood and others, 1976a, b, 1979), and the results well 

demonstrated the improvement in resolution and other 

desirable properties of the gradiometer which may be 

summarized as follows: 

(1) Resolution of anomalies that are produced by 

closely spaced geologic formations is superior; 

(2) Anomalies produced by near-surface features 

are emphasized with respect to those produced by 

more deeply buried features; 

(3) Direct delineation of vertical contacts by the 

zero-gradient contour value (vertical contact 

mapper) is achieved; and 

( 4) Regional gradient of the Earth's magnetic field 

and diurnal variation are automatically removed. 

The foregoing advantages have been described in 

greater detail in Hood and others (1979) and Hood 

(1981). It follows therefore from the advantages listed 

that vertical-gradient aeromagnetic survey results are in 

fact a better aid to geologic mapping programs than the 

more conventional total-field results. Moreover, because 

many orebodies are located at or near contacts, the 

vertical-gradient technique should prove invaluable in 

tracing such contacts using airborne techniques. 

It has been found that aeromagnetic gradiometer 

surveys are actually best flown at 500ft (152 m) with a 

1,000-ft (305 m) flight-line spacing to obtain the best 

definition of the vertical-gradient anomalies which fall off 

much more rapidly with height. At this height, it is also 

possible to delineate contacts that have only a 0.02 

percent magnetite contrast. Thus the aeromagnetic gradiometer tool is intended for use in detailed geologic 

mapping and for mineral exploration programs, 

particularly those in drift-covered areas. Indeed, when 

the gradiometer results are color contoured using a 

device such as the Applicon color plotter, the result is in 

essence an objective, pseudogeologic map. A number of 

mining companies in central Canada have utilized this 

technique where the area is drift covered and outcrops 

are sparse. 

A series of experimental surveys were then carried 

out to demonstrate the efficacy of the vertical-gradient 

technique in various geologic environments. The 

Radioactive Waste Disposal program was getting 

underway at the same time, and the aeromagnetic gradiometer soon proved its value in unravelling areas of 

complex geology at the various sites. Another particular 

highlight was the survey in the Wollaston Lake area on 

the east side of the Athabasca Sandstone. The gradiometer was able to delineate easily the metasedimentary 

units at the edges of the uranium orebodies. The highsensitivity data could also be high-pass filtered to show 

the dominant glacial direction (because eskers are 

slightly magnetic). Use of glacial direction to trace the 

origin of radioactive boulder trains is a prime exploration 

technique in the area (Kornik, 1982). 

20 Geologic Applications of Modern Aeromagnetic Survey 

With the decline of base-metal exploration in the 

past several years, the only significant activity has been in 

exploration for gold. In September 1979, the Association 

of Prospectors of Quebec asked for a test survey in the 

Abitibi mining camp. Because the Val D'Or 1:50,000-

scale map contains many gold mines, it was chosen for 

the test survey which was carred out in June 1980. The 

resultant colored maps were published using the Applicon color plotter, and compilation techniques were 

developed inhouse. It was readily apparent that many of 

the diorite plugs that are associated with gold are 

magnetic and could readily be detected by the gradiometer (Hood and others, 1982). The release of the maps 

caused a flurry of exploration activity in the area and 

helped in at least one new discovery, the New Pascalis 

prospect. 

By 1980 the time was ripe for a technology transfer 

to take place to the airborne geophysical survey industry, 

although the various airborne contractors were still 

somewhat reluctant to build a system without assurance 

of a market for the product. Fortunately, that year a 

vehicle for the technology transfer manifested itself 

through the Canada-Eastern Ontario Subsidiary 

Agreement, which was administered by the Ontario 

Geological Survey (OGS). R.W. Barlow of OGS and P.J. 

Hood were coleaders of the project, and Dr. N.R. 

Paterson of Paterson, Grant and Watson, Ltd., was 

retained by OGS as a consultant for the project. The 

specifications for the Commercial Aeromagnetic Gradiometer System (CAGS) were prepared by the GSC, and 

proposals were invited from industry. 

Two companies responded, namely Kenting Earth 

Sciences, Ltd., of Ottawa and Questor Surveys, Ltd., of 

Toronto. After considerable deliberation, a contract was 

awarded to Kenting in the Spring of 1982. Under the 

terms of a Memorandum of Understanding between the 

various parties, the GSC undertook to provide Kenting 

with technical support on a day-to-day basis including the 

provision of technical drawings and compilation 

programs. Kenting spent about 15 months fabricating 

their improved system on a Navajo aircraft, and it was 

accepted for survey work in August 1983. The company 

did two test surveys as part of the OGS contract before 

the system was offered commercially. Subsequent work 

included GSC contracts in Nova Scotia, Quebec, Manitoba, and Saskatchewan, which were funded under 

various Federal/Provincial Mineral Development 

Agreements. 

Thus, the history of development of the aeromagnetic gradiometer has extended over two decades. It 

could not have been accomplished without dedicated 

team work, and one essential ingredient was the 

possession of an inhouse survey platform, namely the 

Beechcraft Queenair aircraft. It is also abundantly clear 

that, in the development of an essentially new survey 
technique, it is not sufficient just to build the necessary 

black boxes, test them on a few survey lines, present the 

results at a scientific meeting, and wait for the 

commercial survey contractors to act. The contractors 

have to be convinced that they can make a profit on a 

continuing basis by offering such a service. Otherwise, 

they are unwilling to take the risk. 

One of the important conclusions of the gradiometer program is that national aeromagnetic surveys 

should be carried out in two phases in order to provide 

adequate coverage for multipurpose usage. In the first 

phase, a regional aeromagnetic survey should be carried 

out at a sufficient height to permit a reasonably wide 

flight-line spacing so that costs are not excessive. In the 

second phase, the areas of complex geology and (or) 

interesting mineral possibilities should be reflown at a 

lower elevation and a closer flight-line spacing. Moreover 

vertical gradiometers should be utilized for the second 

survey phase because of their distinct advantages over 

total-field, single-sensor systems. 

HELICOPTER-BORNE AEROMAGNETIC 

GRADIOMETER SURVEYS 

There is a problem in carrying out aeromagnetic 

gradiometer surveys in areas of mountainous terrain. As 

previously described, the GSC has utilized the technique 

of flying at CBE to carry out regional total-field aeromagnetic surveys of mountainous areas, such as the 

Cordillera of British Columbia and the Tomgat 

Mountains of Labrador. However, this procedure does 

not yield useful results when flying vertical-gradient 

surveys of rugged areas, because gradient surveys are 

much more sensitive to the separation between the 

sensors and the causative bodies of anomalies than is the 

case for total-field surveys. Gradiometer surveys must 

always be drape-flown. Accordingly, in 1983 the GSC 

began fostering the development of helicopter-borne 

aeromagnetic gradiometer survey systems through 

research and development contracts to the Canadian 

airborne geophysical industry (Hood and Teskey, 1986). 

The successful companies that initially received funding 

were Geotech, Ltd., of Markham, Ontario, and Geophysical Surveys, Inc., of Ste. Foy, Quebec. Geotech built a 

3-m-separation system using Overhauser magnetometers 

developed under a previous GSC-sponsored contract by 

Gem Systems, Ltd., of Toronto. Two proton oscillators 

(GSM-11A) were installed in a mastlike structure above 

an electromagnetic (EM) bird with a 3-m separation. The 

noise envelope achieved was about 0.1 gamma for two 

readings per second for a single sensor and 0.033 

gammas per meter for the gradient measurements. 

The second helicopter-home vertical gradiometer 

system was developed by Geophysical Surveys, Inc., and 

utilized two V-200 Scintrex split-beam cesium-vapor 

magnetometers with a separation of 2 m mounted on a 

towed bird, which permitted the vertical gradient to be 

measured twice a second with a sensitivity of 0.0005 

gammas per meter. The total-field and vertical-gradient 

fourth-difference noise levels of the system are less than 

0.02 gammas per meter and 0.025 gammas per meter, 

respectively. 

Both systems were first field tested in the relatively 

flat Carleton Place area previously flown by the GSC 

fixed-wing system to ensure that repeatable results could 

be obtained. Then a survey of Mount Megantic, Quebec, 

which is about 700 m high, was carried out in March 1984 

to ascertain whether the technique would produce useful 

results of areas of rugged terrain. The area had been 

previously surveyed in 1951-52 by a frxed-wing aeromagnetic survey flown by the GSC. In general, the 

experimental surveys demonstrated the excellent 

repeatability of total-field survey data, but in the fine 

detail there were many differences that are important to 

the mapping geologist. There is an even more striking 

comparison with the resultant vertical-gradient color 

interval map. This map showed that Mount Megantic 

consists of two concentric rings of magnetic rock that 

extend about 270° in azimuth, which have significant 

magnetization and a central nonmagnetic core. This is 

confirmed by a comparison with the geologic map which 

shows an outer ring of syenite, an inner ring of gabbro, 

and a central core of granite. 

In 1984 a tender for the first contracted helicopterborne vertical gradiometer work was issued for an area in 

the Gaspe Peninsula of southeastern Quebec, and the 

successful contractor was Geophysical Surveys, Inc. The 

published results of the survey demonstrated that the 

helicopter-home gradiometer technique is a viable one 

for geologic mapping programs, and this is confirmed by 

the resultant contour maps. Moreover, the results show a 

significant improvement in resolution over the older, 

drape-flown survey. 

Two other Canadian companies have developed 

helicopter-borne gradiometer systems. Aerodat, Ltd., of 

Mississauga, Ontario, has fabricated a vertical gradiometer that utilizes split-beam cesium-vapor magnetometers with a sensitivity of 0.0005 gamma which are 

manufactured by Scintrex, Ltd. Sander Geophysics, Ltd., 

of Kanata, Ontario, has completed the development of a 

helicopter-borne vertical gradiometer system that utilizes 

Overhauser magnetometers of their own design. The 

sensor separation is 3m, and the 7-m-long bird structure 

is towed by an Aerospatiale AS 3500 Astar helicopter at 

the end of a 30-m cable. They have carried out their first 

contract survey for the GSC in the Gaspe Peninsula of 

southeastern Quebec. 

Helicopter-borne aeromagnetic gradiometer contracts have now been awarded to the forementioned 

Keynote Papers 21 
companies to carry out surveys in four eastern provinces, 

namely Newfoundland, Nova Scotia, New Brunswick, 

and Quebec. It is clear that the helicopter-borne aeromagnetic gradiometer technique combined with very low 

frequency electromagnetic (VLF EM) techniques is 

an excellent geophysical tool to assist geologic mapping 

programs in areas of rugged terrain. 

CONCLUSIONS 

It is generally recognized that the aeromagnetic 

survey program of the GSC has been one of its most 

successful endeavours over the past 40 years. It has led 

directly or indirectly to the discovery of many ore 

deposits, and its overall cost has been recovered many 

times over the years both through the general economic 

benefits to the country that result from such discoveries 

and from the taxes that are subsequently paid to the 

provincial and federal governments. After many years of 

development, the aeromagnetic survey technique is still 

being improved in many new and novel ways and it 

remains the most utilized airborne survey technique in 

terms of the line kilometrage flown each year throughout the world. This continued popularity is due in part to 

a variety of reasons. Of all the airborne geophysical 

survey techniques, the aeromagnetic survey technique 

has by far the greatest depth penetration; this technique 

is able to detect features down to the Curie point 

geotherm, which is about 20 km beneath the Earth's 

surface. Moreover, the aeromagnetic survey technique is 

unaffected by the presence of surficial material such as 

overburden and weathered material or by the presence of 

lakes and swamps. Aeromagnetic surveys also provide a 

continuity of information at low cost that is impossible to 

achieve in ground geophysical or geological surveys. One 

of the outstanding advantages of aeromagnetic surveys 

becomes apparent when large areas are surveyed 

because large regional geologic features are commonly 

discovered. These regional features may not be 

recognizable on the ground because they are very large 

or perhaps obscured by sedimentary formations. Clearly, 

further significant developments in the aeromagnetic 

survey technique can be expected in the next decade, 

especially in the development of aeromagnetic gradiometer techniques and in the methods of presentation and 

interpretation of aeromagnetic data. 

REFERENCES CITED 

Dawson, K.R., 1972, Recent appraisal of niobium (columbium) 

and tantalum deposits in Canada: Canadian Mining 

Journal, v. 93, no. 4, p. 28-30. 

Dods, S.D., Teskey, D.J., and Hood, P.J., 1985, The new series 

of 1:1,000,000-scale magnetic anomaly maps of the 

Geological Survey of Canada-Compilation techniques 

22 Geologic Applications of Modern Aeromagnetic Survey 

and interpretation, in Hinze, W J ., ed., The utility of 

regional gravity and magnetic anomaly maps: Tulsa, 

Okla., Society of Exploration Geophysicists, p. 69--87. 

Grant, F.S., 1985, Aeromagnetics, geology and ore 

environments; II, Magnetite and ore environments: 

Geoexploration, v. 23, p. 335-362. 

Hood, P J., 1965, Gradient measurements in aeromagnetic 

surveying: Geophysics, v. 30, no. 5, p. 891-902. 

__ 1981, Aeromagnetic gradiometry-A superior 

geological mapping tool for mineral exploration 

programs: Tulsa, Okla., Society of Exploration 

Geophysicists, Proceedings of SQUID Applications to 

Geophysics Workshop, Los Alamos, June 2-4, 1980, 

p. 72-77. 

Hood, P J., and Bower, M.E., 1973, Low-level aeromagnetic 

surveys of the continental shelves bordering Baffin Bay 

and the Labrador Sea, in Hood, P J., ed., Earth Science 

Symposium on Offshore Eastern Canada: Geological 

Survey of Canada Paper 71-23, p. 573-598. 

Hood, P J., Holroyd, M.T., and McGrath, P.H., 1979, Magnetic 

methods applied to base metal exploration, in Hood, P.J., 

ed., Geophysics and geochemistry in the search for 

metallic ores: Geological Survey Canada Economic 

Geology Report 31, p. 77-104. 

Hood, P J., Irvine, John, and Hansen, Jens, 1982, The 

application of the aeromagnetic gradiometer survey 

technique to gold exploration in the Val D'Or mining 

camp, Quebec: Canadian Mining Journal, v. 103, no. 9, 

p. 21-39. 

Hood, P J., and Ready, E.E., 1976, Federal-provincial aeromagnetic survey program of Canada-A progress report: 

Geological Survey of Canada Paper 76-1B, p. 267-270. 

Hood, P J., and Sawatzky, Peter, 1983, Bourget aeromagnetic 

calibration range: Geological Survey of Canada Paper 

83-1A, p. 483-485. 

Hood, P J., Sawatzky, Peter, and Bower, M.E., 1967, Progress 

report on low-level aeromagnetic profiles over the 

Labrador Sea, Baffin Bay and across the north Atlantic 

Ocean: Geological Survey of Canada Paper 66-58, 11 p. 

Hood, P.J., Sawatzky, Peter, Kornik, LJ., and McGrath, P.H., 

1976a, Aeromagnetic gradiometer survey, White Lake: 

Geological Survey of Canada Open File 339, 10 p. 

__ 1976b, Aeromagnetic gradiometer survey, White Lake: 

Ontario, Atomic Energy of Canada, Ltd., Technical 

Record TR-11, 13 p. 

Hood, P J. and Teskey, D.J., 1986, Helicopter-borne aeromagnetic gradiometer surveys; a progress report: Tulsa, 

Okla., Society of Exploration Geophysicists, 56th Annual 

Meeting, Houston, p. 152-155. 

Kornik, L.J., 1982, Aeromagnetic gradiometer results in the 

Wollaston Lake area, Saskatchewan, Canada, in 

Uranium exploration methods: Proceedings of the 

Nuclear Energy Agency/International Atomic Energy 

Agency (NEA/IAEA) Meeting, Paris, June 1-4, 1982, 

p. 605-619. 

Lang, A. H., 1970, Discovery and benefits of the Marmora iron 

deposit: Canadian Mining Journal, v. 91, p. 47-49. 

Morley, L.W., McLaren, A.S., and Charbonneau, B.W., 1967, 

Magnetic anomaly map of Canada: Geological Survey of 

Canada Map 1255A, scale 1:5,000,000. 
Pemberton, R.H., 1966, World geophysical discoveries bolster 

future mineral needs: Engineering and Mining Journal, v. 

167,no.4,p. 85-88. 

Roth, Jeremy, 1975, Exploration of the southern extension of 

the Manitoba nickel belt: Canadian Mining and 

Metallurgical Bulletin, v. 68, no. 761, p. 73-80. 

Stanton, R.L., 1976, Petrochemical studies of the ore 

environment at Broken Hill, New South Wales; 1, 

Constitution of banded iron formation; 2, Regional 

metamorphism of banded iron formations and their 

immediate associates; 3, Banded iron formations and 

sulphide orebodies: constitutional and genetic ties; 4, 

Environmental synthesis: Institution of Mining and 

Metallurgy Transactions, Section B, v. 85, p. B33-B46, 

B118-B131, B131-B141, B221-B233. 

Ward, S.H., 1958, The role of geophysics in exploration in New 

Brunswick: Canadian Mining and Metallurgical Bulletin, 

v.51,no.551,p. 162-166. 

Keynote Papers 23 
Rock Magnetism, The Distribution of 

Magnetic Minerals in the Earth's Crust, and 

Aeromagnetic Anomalies 

By Richard L. Reynolds, Joseph G. Rosenbaum, 

Mark R. Hudson, and Neil S. Fishman 

Abstract 

A review of current understanding of relations 

among rock magnetic properties and petrology of 

igneous, metamorphic, and sedimentary rocks, as well as 

of the distribution of magnetic minerals in the Earth's 

crust, shows that rock magnetic and petrologic studies 

can provide important constraints on the interpretation 

of aeromagnetic anomalies. The development of threedimensional geologic maps based on aeromagnetic data 

requires an improved understanding of: (1) petrologic 

and petrochemical controls on magnetic properties of 

rocks in the lower oceanic and continental crust and of 

plutons in the middle and upper continental crust; (2) 

effects of lower and middle crustal temperatures and 

pressures on total magnetizations; (3) relations among 

metamorphic facies, premetamorphic lithostratigraphy, 

and magnetization; and ( 4) relations among types and 

origins of ore deposits, host-rock composition and 

mineralogy, and magnetic signatures that may be 

diagnostic for specific mineral and energy habitats. 

Magnetic contrasts in sedimentary rocks are controlled 

by depositional factors and by geochemical and 

consequent mineralogic alterations that either enhance 

or suppress magnetizations. Examples are drawn from 

the formation of uranium deposits in sandstones and 

from the effects of hydrocarbon seepage. 

INTRODUCTION 

The purpose of this report is to present a broad 

overview of the magnetic properties, petrologic controls, 

and magnetic mineral composition of crustal sources of 

magnetic anomalies. This overview is preceded by a brief 

review of the factors that control magnetic properties of 

minerals and rocks. We also attempt to identify avenues 

of rock magnetic and petrologic investigations that will 

provide important constraints on the interpretation of 

24 Geologic Applications of Modern Aeromagnetic Survey 

aeromagnetic data and thus contribute to the 

development of three-dimensional geologic models. The 

discussion is intended to serve as a general guide for 

interpreters of aeromagnetic data to current knowledge 

and some recognized problems of crustal magnetization. 

Special attention is given to the causes of magnetic 

contrasts in sedimentary rocks. Recently improved 

techniques for the acquisition and analysis of aeromagnetic data, as well as newly developed models and 

guides for the exploration of energy and mineral 

resources, have expanded interest in aeromagnetic 

anomalies that arise from sedimentary rocks. The 

potential of aeromagnetic methods for detecting the 

magnetic-diagenetic effects of vertically directed 

alteration plumes, such as may be caused by hydrocarbon 

seepage (Donovan and others, 1979), is an outstanding 

example. The understanding of such features is currently 

very limited, but where such an understanding exists, it is 

based on detailed rock magnetic, mineralogical, and 

geochemical study. 

ACKNOWLEDGMENTS 

Technical reviews by Bill Hanna and Carol Finn 

greatly improved the manuscript. 

MAGNETIC PROPERTIES OF 

ANOMALY SOURCES 

The physical property that relates magnetic 

anomaly to its source is total magnetization. The 

magnitude and direction of total magnetization (J, ) are 

given by the vector sum of two components: remanent 

magnetization (J, ) and induced magnetization (l; ). J, 

can be computed as: 

J, = J, + 1; = J, + kB, 
where k is magnetic susceptibility (SI, dimensionless), 

and vector B is the magnetic induction of the Earth at the 

location of the body. The measurements of J, and k of 

samples at ambient temperature and pressure are easily 

obtained using laboratory instruments. Another widely 

used quantity is the Koenigsberger ratio of remanent 

magnetization magnitude to induced magnetization 

magnitude. 

The magnetic properties of rocks and minerals 

have been discussed in detail from an aeromagnetics 

perspective by Haggerty (1979), Mcintyre (1980), Grant 

(1984!1985a, b), and Clark (1983). Clark (1983) and 

Carmichael (1982) present summaries of representative 

values of susceptibility, remanent magnetization, and 

Koeingsberger ratio corresponding to many magnetic 

minerals and some rock types. 

The magnitude of l; depends on the quantity, 

composition, and size of the magnetic-mineral grains, as 

well as on the magnitude of the magnetic induction (the 

Earth's field direction). The direction of l; is usually 

parallel to the local magnetic induction, but it may differ 

if anisotropy of magnetic susceptibility is great. In 

general, k, and thus l;, is relatively low for small magnetic 

grains and higher for larger grains. · 

The magnitude of J, is also a function of quantity, 

composition, and sizes of the magnetic grains. In 

addition, naturally occurring J, (natural remanent 

magnetization, NRM) is strongly influenced by the 

physico-chemical environment (which determines the 

type of remanence that is acquired) and the strength of 

the induction field at the time of acquisition. Other types 

of remanence include thermoremanent magnetization-

(TRM), chemical remanent magnetization (CRM), 

isothermal remanent magnetization (IRM), and 

depositional (or detrital) remanent magnetization 

(DRM). The in-situ direction of J, is usually parallel to 

the ambient magnetic induction (the Earth's field 

direction) during the acquisition of remanence, but it 

may be rotated due to later tectonism. In contrast to l;, 

small magnetic grains produce relatively strong and 

stable!,. 

The types of remanent magnetizations that are 

acquired by different rock types are listed in table 1. 

Viscous remanent magnetization (VRM), in the 

direction of the present Earth's field, dominates the 

remanence of rocks in the middle and lower crust as well 

as that of coarse-grained plutonic rocks. The direction of 

NRM for other types of rocks must, in general, be 

determined experimentally. In some cases NRM 

magnitudes can be estimated for known or expected rock 

types (Clark, 1953). However, NRM directions are 

usually difficult to estimate because the NRM may be the 

result of both primary and secondary components, which 

may have been affected by tectonic rotations. An 

excellent review of paleomagnetic methods, including 

problems that involve multiple components of 

magnetizations and tectonic rotations is given by Hillhouse (in press). Not only is knowledge of the direction 

and magnitude of remanent magnetization commonly 

important in constraining the total magnetization vector, 

but knowledge of the type of remanence may be crucial 

to the correct geologic interpretation of magnetic 

anomalies. For example, the implications of anomalies 

over sedimentary rocks are very different if the 

magnetization is dominated by detrital remanent 

magnetization rather than by a chemical remanent 

magnetization related to the presence of hydrocarbons. 

MAGNETIC MINERALS AS 

ANOMAL V SOURCES 

The most important magnetic minerals for aeromagnetic measurements are those of the magnetiteulvospinel (Mt-U SPss) solid-solution series, the titanomagnetites [xFe2Ti04 *(1-x)Fe3 0 4 (Osxs1), fig 13]. A 

number of magnetic properties of titanomagnetites vary 

as a function of composition. For example, Curie 

temperatures decrease systematically from about 580 °C 

for ferrimagnetic magnetite to -153 oc for antiferromagnetic ulvospinel (fig 13). A Curie temperature of 25 oc 

corresponds to titanomagnetite that has about 75 percent 

ulvospinel in solid solution. Saturation magnetization, 

which is the maximum possible magnetization (92 A 

m2/kg for Fe3 0 4 ), similarly decreases with increasing 

content of titanium. Although the Earth's magnetic field 

is too weak to saturate magnetically the common 

magnetic minerals, saturation magnetization is a useful 

indicator of the capacities of different magnetic minerals 

to become magnetized. The susceptibility of Mt-Uspss is 

effectively zero for U SPss contents greater than about 70 

percent. For Mt-Usp88 that is more enriched in Mt, the 

variation of susceptibility with composition depends on 

grain size. Discussions of relations among grain sizes, 

domain structures, and magnetic properties are given by 

Dunlop (1981) and Clark (1983). 

Minerals of the ilmenite-hematite (IIm-Ht88) solidsolution series [xFeTi03 *(1-x)Fe20 3 (0 sxs 1), and 

referred to here as titanohematites], vary complexly in 

magnetic structure and properties as a function of 

composition. In the compositional range 0.5 s x s 0.8, 

between weakly magnetic hematite ( antiferromagnetic) 

and ilmenite (paramagnetic), the titanohematites are 

ferrimagnetic, have Curie temperatures in the range of 

235 to -26 °C, and attain a saturation magnetization of as 

much as a third that of magnetite. 

Keynote Papers 25 
Table 1. Types of remanent magnetizations that may contribute to aeromagnetic anomalies 

[f, temperature; Te, Curie temperature. Based partly on table 4.1 in Tarling (1983)] 

Remanent Abbreviation Rock types magnetization 

Natural NRM All 

Thermal TRM Igneous, 

metamorphic 

Partial thermal pTRM Igneous, 

metamorphic. 

Partial pTCRM Igneous, 

thermochemical. metamorphic. 

Viscous VRM All 

Viscous partial VpTRM All 

thermal. 

Detrital, DRM Sedimentacy 

depositional. 

Postdepositional PDRM Sedimentacy 

Chemical CRM All 

Titanomaghemites (maghemite) form by lowtemperature oxidation of titanomagnetites (magnetite). 

Maghemites are nonstoichiometric spinels with variable 

amounts of cation site vacancies caused by either 

addition of oxygen or removal of metals (shaded pattern 

in fig. 13). The Curie temperature increases with 

progressive degree of oxidation, and the saturation 

magnetization decreases with increased oxidation of titanomagnetites that originally have less than 50 mol 

percent ulvospinel. The saturation magnetization of the 

titanomaghemites is 85 A m2/kg for maghemite (yFe20 3 ). 

The oxidation of magnetite also commonly produces 

hematite (saturation magnetization is 0.5 A m2/kg). The 

factors that dictate the formation of either hematite or 

maghemite are not well understood but may be related to 

presence or absence of water; the presence ·of water 

favors maghemite. 

Other magnetic minerals, which are not illustrated 

on figure 13, that are or may be responsible for magnetic 

anomalies include metallic iron, metal alloys of FeNi-Co-Cu, and the magnetic sulfides, monoclinic pyrrhotite and greigite. In addition, as described later, nonmagnetic iron sulfide minerals, particularly pyrite, can play a 

significant role in changing magnetization in certain 

sedimentary rocks that have been altered at low and high 

temperatures. 

26 Geologic Applications of Modern Aeromagnetic Survey 

Definition or characteristic 

Summation of all components of remanence. 

Acquired during cooling from the maximum Tc to room T 

(Total TRM). 

The TRM acquired during cooling in a T interval having 

maximum T < Tc. The total TRM equals the sum of the 

pTRM's. 

Remanence acquired by products of exsolution-oxidation during 

cooling. 

Secondacy remanence related to thermal agitation of domain 

walls that causes decay of primacy remanence and changes 

domain assemblage to lowest energy state. 

Distinguished from VRM on basis of higher temperature 

conditions below T c . 

Primacy remanence acquired by the physical rotation of detrital 

grains during deposition. 

Acquired during postdepositional rotation of interstitial grains. 

Some investigators include effects of early diagenetic 

chemical-magnetic alterations. 

Secondacy remanence acquired during growth of magnetic 

minerals in presence of magnetic field. Includes growth by 

nucleation or replacement. 

ULVOSPINEL 

FeO 

WUSTITE 

ANATASE 

RUTILE 

TiOz 

FezOJ 

HEMATITE 

MAGHEMITE 

Figure13. TheFe0-Fe2 0 3 -Ti02 ternarysystemshowingthe 

titanomagnetite, titanohematite, and pseudobrookite solidsolution joins, as well as the Curie temperature contours (in 

degrees Celsius) between the titanomagnetite and titanohematite joins. In the compositional range (5Q-80 mol percent 

ilmenite) indicated between the arrows, the titanohematites 

are ferrimagnetic. The shaded area represents the field of 

titanomaghemite. 
PETROLOGIC CONTROLS ON 

MAGNETIC PROPERTIES 

The magnitudes of remanent and induced 

magnetization in rocks depend primarily on the 

abundance, composition, and grain size of magnetic 

minerals in the rocks, as well as on the nature of the 

remanence. In igneous rocks, these factors are controlled 

mainly by bulk chemistry, by initial temperatures of 

formation, and by cooling conditions, including especially 

cooling rate and oxygen fugacity (Haggerty, 1979). In 

metamorphic rocks, such factors are controlled similarly 

by rock and mineral chemistry and by the thermal history 

(Mcintyre, 1980; Grant, 1984/1985a). In clastic 

sedimentary rocks the factors are controlled primarily by 

source area, depositional process, depositional 

environment, and diagenetic alteration. Although most 

chemical and biogenic sedimentary rocks have negligible 

magnetizations, chemical precipitation of abundant 

magnetite from waters, has occurred under appropriate 

conditions of pH, amount and valences of iron and sulfur ' 

oxygen fugacity, and temperature. 

The following sections focus on factors that 

d~termine the types and characteristics of magnetic 

mmerals, as well as the growth or depletion of these 

minerals in rocks, and that thus control rock magnetic 

properties. Although the emphasis is not on links 

between rock magnetization and specific features [such 

as polarity (positive or negative), magnitude, and shape] 

of aeromagnetic anomalies, some general statements on 

these links can be made. In the simplest cases, for 

example, positive magnetic anomalies can result from 

strong induced magnetizations, from a dominant VRM 

and (or) from stable remanence of normal polarity: 

Negative magnetic anomalies can be caused by rocks with 

dominant stable remanence of reversed polarity. 

However, the shape of the magnetic anomaly. depends 

not only on the direction of total magnetization but also 

on the shape of the magnetic body and on the spatial 

relations among different bodies that differ in their 

magnetizations (either magnitude or direction, or both). 

An excellent overview of aeromagnetic methods 

(inclu~ing data acquisition and processing, and 

techmques for interpretation) is given by Blakely and 

Connard (in press); reports by Hinze {1985) and 

Paterson and Reeves (1985) also cover this subject. 

Igneous Rocks 

In general, concentrations of Mt-Usp55 grains tend 

to be higher in basic rocks (about 5-10 percent by 

volume) than in intermediate and acidic rocks (about 1-3 

percent). The difference in magnetization between basic 

and acidic rocks is not as great as one might expect from 

differences in the Fe-Ti oxide contents. In basic rocks the 

titanomagnetites usually have high titanium content, 

weak magnetizations, and low Curie temperatures. 

Conversely, in intermediate and acidic rocks the titanomagnetites have low titanium content, strong 

magnetizations, and high Curie temperatures. In the 

latter, increasing Si02 content is associated with lower 

titanium content and higher oxidation states. 

The aeromagnetic signatures of igneous rocks are 

strongly influenced by their cooling history, primarily 

because of the effects of cooling and oxygen fugacity on 

the solubility of the magnetic oxide solid solutions (Haggerty, 1979). The effects of mineral exsolution and of 

high- and low-temperature oxidation on mineral type, 

and consequently on magnetization, Curie temperature, 

and magnetic stability of the magnetite-ulvospinel solidsolution series minerals are summarized in figure 14. 

Both exsolution and high-temperature oxidation favor 

the isolation of low-temperature titanomagnetite and 

therefore lead to increases in magnetization, Curie 

temperature, and magnetic stability. Advanced hightemperature oxidation of magnetite + ilmenite, 

however, creates weakly magnetic hematite {Curie 

temperature of 680 °C) and nonmagnetic titanium-rich 

phases. Low-temperature oxidation of magnetiteulvospinel to titanomaghemite causes a decrease in 

magnetization, an increase in Curie temperature, and 

little change (perhaps a small increase) in magnetic 

stability. As with high-temperature oxidation, lowtemperature oxidation of low-titanium magnetite to 

hematite greatly diminishes magnetizations. 

Exsolution and oxidation also strongly influence 

rock magnetic properties by decreasing magnetic grain 

sizes. For example, the separation of original grains of 

Mt-Uspss by exsolution or by high-temperature oxidation 

into segmented particles of magnetite + ulvospinel or of 

magnetite + ilmenite, respectively (fig. 14), creates 

REACTIONS SATURATION CURIE MAGNETIC 

MAGNETIZATION TEMPERATURE STABILITY 

MINERAL EXSOLUTION 

Mt-USPss+Usp+Mt Increase Increase Increase 

HIGH-TEMPERATURE 

OXIDATION 

Mt-USPss+Mt+llm Increase Increase Increase 

ADVANCED OXIDATION 

Mt+llm+Ht+Pb+R Decrease Increase Increase 

LOW-TEMPERATURE 

OXIDATION 

Mt-USPss+(Ti) Maghemite 

Mt+Ht Decrease Increase Increase 

Figure 14. Reactions that involve exsolution and oxidation 

and their effects on saturation magnetization, Curie 

temperature, and magnetic stability. Mt-Usp •• , magnetiteulvospinel solid solution; lim, ilmenite; Ht, hematite; Pb, 

pseudobrookite; A, rutile. 

Keynote Papers 27 
smaller intergrowths of ferrimagnetic magnetite with 

antiferromagnetic or paramagnetic phases. In general, 

fine-grained titanomagnetites and magnetite have higher 

remanent and lower induced magnetizations than coarsegrained particles of identical composition. High Koenigsberger ratios ( Q > 1, remanent magnetizations dominant) are thus characteristic of volcanic rocks, which 

typically have undergone substantial high-temperature 

oxidation and contain small magnetic particles, whereas 

low ratios (Q < 1, induced magnetizations dominant) 

are characteristic of many coarse-grained intrusive rocks. 

Metamorphic Rocks 

Magnetic minerals may be produced or destroyed 

during metamorphism. In metasedimentary rocks the 

production of magnetite depends mainly on two factors 

that are largely inherited from the original sediment: (1) 

the total iron content, and (2) the oxidation state of the 

iron (the relative amounts of FeJ + and Fe2 + ). These 

factors control the amounts and types of iron oxide 

minerals that may form by limiting potential production 

of iron oxide and by controlling the partitioning of iron 

between oxides and silicates (Mcintyre, 1980; Grant, 

1984/1985a). Regional metamorphism of sedimentary 

rocks under conditions of lower greenschist to granulite 

facies drive many reactions that are capable of producing 

secondary magnetite (Grant, 1984/1985a). In general, 

formation of magnetite is favored in iron-bearing rocks 

under low-grade conditions or under high-grade 

conditions that promote the dehydration and breakdown 

of hydrous minerals such as biotite and amphiboles. 

Extreme metamorphism, which involves differential 

melting, strongly reduces magnetizations, possibly 

because of recombination of iron and titanium oxides to 

Mt-Usp5 , if titanium is available (Mcintyre, 1980; Grant, 

1984/1985a). 

Other intrinsic properties that influence the 

production of magnetite in metamorphic rocks include 

the contents of silica, carbon, and aluminum. Mcintyre 

(1980) made the following generalizations: (1) Magnetite 

formation is favored in rocks that are undersaturated in 

silica; in the iron-oxygen system, silica saturation expands 

the stability field of fayalite (iron silicate) at the expense 

of magnetite. (2) The presence of carbon may restrict the 

formation of magnetite. Gas phases CO . and C02 

produced from carbon can lower oxygen fugacities below 

the magnetite stability field at high metamorphic 

temperatures (more than 630 °C). (3) In micaceous 

rocks, excess aluminum favors muscovite over biotite, 

and as a result the silicate minerals do not compete 

effectively with the oxides for available iron. Therefore, 

high aluminium content favors the production of 

magnetite. 

28 Geologic Applications of Modern Aeromagnetic Survey 

Strong magnetic contrasts may develop in rocks of 

similar composition that have undergone different metamorphic reactions due to different metamorphic 

conditions. Moreover, metaigneous rocks of similar bulk 

composition may have different magnetizations that 

reflect differences in the original magmatic conditions of 

oxygen fugacity and temperature. 

Magnetizations of igneous rocks are usually 

diminished by metamorphism and metasomatism due to 

destruction of preexisting magnetic minerals (Haggerty, 

1979; Mcintosh, 1983). Magnetite, however, can be 

produced by alteration of olivine and orthopyroxene 

during serpentinization of mafic and ultramafic rocks. 

Evidence for both depletion of primary magnetite and 

creation of secondary magnetite by hydrothermal 

alteration in a plutonic terrane (Criss and Champion, 

1984) will be further discussed. 

Mineralization in Igneous and 

Metamorphic Rocks 

In certain ore-forming environments magnetic 

contrasts are useful as pathfinders to mineral deposits 

(Grant, 1984/1985b; Mcintyre, 1980; Clark, 1983; 

Wright, 1981). Destruction of primary magnetic minerals 

or addition of secondary magnetic minerals, either by 

conversion of preexisting nonmagnetic phases or by 

growth from ore-related fluids, can develop such 

magnetic contrasts. Ferrimagnetic magnetite and pyrrhotite, as well as antiferromagnetic hematite if present in 

sufficiently large quantity, may be sources of magnetic 

anomalies in mineralized rocks. 

Sedimentary Rocks 

The amount, type, and grain sizes of detrital 

magnetic minerals in epiclastic rocks depend on the 

nature of and proximity to the source areas, as well as on 

the depositional environment. Postdepositional diagenetic and authigenic alterations strongly influence the 

magnetic character of sedimentary rocks and can either 

enhance or diminish the original magnetization. In 

general, the degree of oxidation of detrital titanomagnetite, if initially present, to hematite and other ferric 

oxide minerals increases with age (VanHouten, 1968), 

but such a relation has not been quantified in terms of 

magnetization. Rocks that contain detrital and (or) 

secondary hematite from usual depositional and (or) 

diagenetic processes are abundant in the sedimentary 

column, but they have low total magnetizations and are 

of little interest in aeromagnetic studies. However, 

important changes in magnetization of sedimentary rocks 

may be produced by reducing conditions that are related 
to mineralization or to hydrocarbons. Under sulfidic 

reducing conditions the original magnetization may be 

either suppressed by the replacement of the detrital iron 

oxides by iron sulfides or enhanced by the production of 

magnetic sulfide minerals, such as pyrrhotite or greigite. 

The formation of diagenetic magnetite related to 

hydrocarbons has been proposed as another process by 

which the magnetization of sedimentary rocks may be 

increased under reducing conditions (Donovan and 

others, 1979; McCabe, 1986). Geochemically reducing 

fluids, especially those with organic acids, are also known 

to dissolve iron-titanium oxide minerals and thus to 

diminish magnetizations. 

Sedimentary rocks of chemical or biogenic origins, 

such as evaporites and carbonates, usually have low to 

negligible magnetizations. In certain settings, such as salt 

domes, these rocks may be juxtaposed with more 

magnetic rocks and thereby indirectly associated with 

subtle but important negative magnetic anomalies. In 

some strata, magnetic minerals, principally magnetite, 

have precipitated chemically from sea water and may 

comprise or be closely associated with mineral deposits. 

For example, magnetite that formed in this manner 

produces high magnetizations in Lower Proterozoic 

banded iron-formations and in distal sediments related to 

volcanogenic base-metal massive sulfide deposits (Mcintyre, 1980; Large, 1977). 

CRUSTAL DISTRIBUTION OF 

SOME MAGNETIC MINERALS 

A simplified summary of the distribution of 

magnetic minerals that may be important for magnetic 

surveys is presented in figure 15. This figure serves as an 

outline for the remainder of the paper. The shapes of the 

symbols denote either a primary (circle) or secondary 

(square) magnetic mineral. Depletion (destruction) of a 

mineral is indicated by a circle-slash (-) symbol. The 

sizes of the symbols have not been quantitatively derived 

and are only crude indications of relative abundance of 

the different minerals. The symbol sizes have meaning 

only within a single column (crustal setting); a 

comparison of sizes between columns has no significance. 

The summary is not comprehensive and is intended 

simply as a general guide. 

Oceanic Crust 

The dominant magnetic mineral in the uppermost 

layer of newly formed oceanic crust, which is mostly 

composed of pillow lavas erupted from spreading 

centers, is titanomagnetite that has a nearly uniform 

content of ulvospinel (Usp about 60 percent, which 

corresponds to a Curie temperature of about 175 °C) 

(Johnson, 1979). Hydrothermal alteration, however, 

readily converts the primary titanomagnetite to titanomaghemite, so that a CRM largely replaces the original 

TRM. The magnitude and direction of the CRM depend 

on those of the original TRM, the time and degree of 

alteration, and the polarity history during alteration. 

A model of such CRM acquisition (Raymond and 

LaBrecque 1987) helps explain a number of magnetic 

features of the oceanic crust, among them the decrease in 

amplitude of anomalies away from spreading ridges, the 

enhanced magnetization of the Cretaceous Quiet Zones, 

and amplitude and skewness discrepancies in 

intermediate- and short-wavelength magnetic anomalies. 

The alteration also results in increases in Curie 

temperature and in remanent stability. Nevertheless, the 

pattern of positive and negative anomalies, which reflect 

the normal/reversed polarity rock sequence, will be 

preserved if polarity intervals are randomly distributed 

during CRM acquisition (Raymond and LaBrecque, 

1987). 

Magnetite is rarely described as being present in 

samples of oceanic crust, and thus has been considered 

uncommon in these rocks. The paucity of reported 

magnetite occurrences, however, may stem partly from 

the lack of extensive deep sampling below the pillowbasalt layer. Magnetite (or titanomagnetite that has 

higher Curie temperatures and magnetizations than that 

produced near or on the sea floor) may form in the 

oceanic crust in at least two different ways. These 

magnetic minerals may form from Mt-Uspss by oxidation/exsolution during initial cooling of intrusions. This 

mechanism is represented by the circle for magnetite in 

figure 15. Magnetite may also be produced by later 

high-temperature alteration (for example, by hydrothermal reheating) of titanomagnetite under oxidizing 

conditions or by thermal breakdown of titanomaghemite, 

regardless of redox conditions. Kristjansson and Watkins 

(1977), in fact, suggested that the Curie temperature of 

magnetite (580 °C) determines the base of the magnetic 

oceanic crust. Other magnetic minerals, which may be 

responsible for deep-seated magnetic anomalies over 

oceanic crust but for which direct evidence is lacking, are 

alloys of Fe-Ni-Co-Cu and metallic iron that might result 

from the partial serpentinization of ultramafic bodies 

(Haggerty, 1978). Such magnetic phases have Curie 

temperatures in the range 620-1,100 oc and thus, if 

present, would greatly deepen the Curie isotherm. 

The 0.5-km-thick layer of pillow basalts makes a 

substantial contribution to the observed marine magnetic 

anomalies, but it cannot solely account for the magnitude 

of observed anomalies. Recent reviews of the sources of 

the magnetization of the oceanic crust (Johnson, 1979; 

Lowrie, 1979) favor an additional contribution from a 

magnetic layer of intrusive rocks below the pillow basalts. 

Keynote Papers 29 
CRUSTAL SETTING AND ROCK TYPE 

: CONTINENTAL CRUST MAGNETIC OCEANIC I 

MINERAL CRUST I MIDDLE AND I UPPER CRUST AND SURFACE I LOWER CRUST I I I 

IGNEOUS AND METAMORPHIC ROCKS I SEDIMENTARY ROCKS 

Hydrothermal 

alteration/ Diagenetic/ 

Thermal alteration/ Epigenetic 

Fe (-Ti) OXIDES mineralization 

Mag'netite •• • • • " • -• 7 

Titanomaghetite • • • ~ • II' 

Titanomaghemite 

Titanohematite • • " • ~ 

METALLIC Fe ? ? 

Fe·Ni-Co-Cu alloys ? ? 

Fe SULFIDES 

Pyrrhotite 

Fe,Ss • • 

Greigite • Fe)S4 

EXPIANA TION 

• Primary • Secondary - Depleted 7 Diagnetic 

Figure15. Summary of the crustal distribution of the aeromagnetically important minerals. As explained in the 

text, the minerals are divided according to primary (circle) and secondary (square) origin. The primary minerals 

are considered to be (I) minerals crystallized in magma, (2) deuteric alteration products in igneous rocks or 

metamorphic products in metamorphic rocks, or {3) detrital minerals in sedimentary rocks (including chemically 

precipitated magnetite, such as that deposited in banded iron-formations). The circle-slash ~ symbol denotes 

settings or conditions under which minerals may be depleted. Diagenetic magnetite is represented by a bold 

query. Secondary minerals include those formed by replacement of earlier magnetic precursors {such as 

titanomaghemite from titanomagnetite in the oceanic crust) and those formed by nucleation or from a 

nonmagnetic precursor. For example, magnetite in the oceanic crust is considered to be both primary, formed 

during initial cooling by the exsolution of original Mt-Usp •• (titanomagnetite), and secondary, formed by the 

hydrothermal alteration of either titanomagnetite or titanomaghemite. Under conditions of hydrothermal 

alteration, local thermal alteration (as in contact metamorphism), and mineralization in the continental crust 

(shown in the figure as hydrothermal alteration, thermal alteration, and mineralization, respectively), primary 

magnetite may be destroyed or new magnetite may be formed. Secondary magnetite may be destroyed later in 

these settings. The diagenetic and epigenetic alterations are represented in the column on the far right. The sizes 

of the symbols have not been derived quantitatively and are only crude indications of relative abundance of the 

different minerals. The symbol sizes have meaning only within a single column {crustal setting); a comparison of 

sizes between columns has no significance. 

A recent iteration of this two-layer model consists of an 

upper, 0.5-km-thick layer with M=5 A/m (where M is 

the magnetic dipole moment per unit volume, taking into 

account partial oxidation of original titanomagnetite) and 

a lower, 3.5-km-thick, dike and gabbro layer with M = 0.5 

A/m (Banerjee, 1984}. Magnetization in the lower layer 

may be carried by both primary titanomagnetite and 

magnetite of either deuteric or hydrothermal origin 

(Banerjee, 1984). 

The magnetic properties of the first drill-core 

samples from beneath the pillow-basalt layer suggest a 

more complicated picture than that portrayed by twolayer models and provide direct evidence for a role for 

magnetite in oceanic-crust anomalies (Smith and Banerjee, 1986). The drill hole penetrated 1,076 m of crust in 

the Pacific Ocean about 200 km south of the Costa Rica 

Rift and intersected three distinct petrologic and 

magnetic layers. The upper layer of pillow basalts (about 

30 Geologic Applications of Modern Aeromagnetic Survey 

0.5 km thick) has an average NRM moment of 5.5 A/m 

that resides in titanomaghemite. Below this layer is a 

mixed zone (270 m thick) of pillow lavas and dikes which 

have been hydrothermally altered to greenschist facies. 

The alteration produced some magnetite by the oxidation 

of primary titanomagnetite but more importantly 

promoted the replacement of titanomagnetite by silicate 

minerals. Consequently, this layer has a low NRM 

moment (average of 0.74 A/m) and contributes little to 

magnetic anomalies. The bottom 300 of the core consists 

of a sheeted dike complex, in which magnetite production 

has occurred as in the overlying layer but in which 

replacement by silicate minerals is minor. As a result 

NRM moment in the bottom layer is sufficiently large 

(average of 1.4 A/m) to contribute significantly to the 

anomalies. 

Many factors cause vertical and lateral variations in 

the magnetization of the oceanic crust and thereby 
produce irregularities in magnetic anomaly patterns. 

These factors include the geometry, composition, and 

cooling history of intrusions below the upper extrusive 

rocks; the compositions and temperatures of hydrothermal fluids; the geometries of hydrothermal convection 

cells; and the positions of the cells with respect to the 

spreading ridge or to the accreting plate (Johnson, 1979; 

Smith and Banerjee, 1986; Cathles and Fehn, 1985). 

Better understanding of the influences of these factors on 

magnetic contrasts is important in mapping and 

interpreting the geology of the oceanic crust. 

Lower Continental Crust 

Long-wavelength magnetic anomalies sensed 

primarily from satellites are potentially useful for 

mapping large, deep crustal bodies that may reflect the 

tectonic and geochemical evolution of the continents 

(Mayhew and others, 1985; Mayhew and LaBrecque, 

1987; Wasilewski and Mayhew, 1982; Wasilewski and 

others, 1979). These deep-seated anomalies require 

sources with large vertical (about 10-20 km) and lateral 

(about 100 km or more) dimensions and magnetizations 

of about 3-6 A/m, approximately 10-100 times greater 

than that of the average value of upper crustal rocks at 

the surface and even greater than those obtained from 

rare deep-crustal sections now at the surface (Shive and 

Fountain, 1988; Shive, 1989). Induced magnetization is 

an important (probably dominant) contributor to total 

magnetization; VRM may contribute to the 

magnetization of deep crustal sources. 

Nearly pure magnetite is probably the major 

source of high magnetization in the lower crust. This 

conclusion comes from studies of xenoliths and uplifted 

sections of lower crustal rocks and from thermodynamic 

considerations (Wasilewski and Mayhew, 1982; 

Schlinger, 1985; .Williams and others, 1985; Frost and 

Shive, 1986). The rock magnetic and compositional 

studies of lower crustal rocks are consistent with 

thermodynamic predictions of low titanium content in 

the magnetite; typical measured Curie temperatures 

range from about 550 to 580 °C. The few exceptions 

noted by Wasilewski and Mayhew (1982) are xenoliths 

from rift zones with Curie temperatures less than 300 oc 

and were interpreted by them to indicate the presence of 

deep crustal titanomagnetite in anhydrous, perhaps 

relatively reducing, zones in a steep geothermal gradient. 

Thus in most areas the Curie isotherm is controlled by 

nearly pure magnetite. At deeper crustal depths the 

isotherm is elevated from 580 to about 600 oc, because of 

the effect of pressure on Curie temperatures (Schult, 

1970; Frost and Shive, 1986). 

A discontinuity in magnetic mineralogy occurs at 

the mantle-crust boundary (Mohorovicic discontinuity, 

commonly called the "the Moho"); below the Moho 

nonmagnetic spinels dominate (Frost and Shive, 1986; 

Mayhew and others, 1985). Thus the Moho is considered 

to be the bottom of the magnetic crust, except in areas of 

high heat flow where elevated temperatures may raise 

the base of the magnetic crust far above the ·Moho 

(Mayhew and others, 1985). Nevertheless, certain 

satellite anomalies have dimensions and amplitudes that 

indicate very deep sources (Taylor and Frawley, 1987), 

possibly in the upper mantle (P. Taylor, oral commun., 

1987); if this is the case, minerals other than magnetite 

apparently cause the magnetic contrasts. Haggerty (1978, 

1979) and Haggerty and Toft (1985) have argued that 

metallic iron and iron alloys formed in altered serpentinites may account for deep-seated anomalies, but Frost 

and Shive (1986) dismissed a major contribution from 

these phases on thermodynamic, petrologic, and geologic 

grounds. The locations and causes of deep-seated 

magnetic anomalies are topics that deserve more study. 

Other important uncertainties in the distribution of 

magnetization in the lower and middle crust include 

changes in the quantity of magnetite as a function of 

depth and the effects of temperature and pressure on 

magnitudes of susceptibility and VRM. Enhancement of 

susceptibility in magnetite with increasing temperature 

within about 150 oc of the Curie temperature (the 

Hopkinson effect) has been considered an important 

contributor to high magnetizations in the lower crust 

(Wasilewski and others, 1979). Recent experimental 

work by Schlinger (1985), however, suggests that this 

contribution may not be as large as previously thought. 

Another critical factor is the contribution of VRM to 

total remanence. Although the rate of VRM acquisition 

increases with increasing temperature (Dunlop, 1983; 

Shimazu, 1960), data are lacking on the variations in 

VRM magnitude as ·a function of temperature and 

pressure. 

Upper Crust and Surface 

Igneous and Metamorphic Rocks 

At depths shallower than about 5 km in the 

continental crust, minerals of the magnetite-ulvospinel 

series dominate magnetizations of most rocks that are 

the sources of aeromagnetic anomalies. In certain plutonic, volcanic, and uplifted high-grade metamorphic 

settings, however, ferrimagnetic titanohematite may also 

produce anomalies. Many rock types have broad and 

overlapping ranges of magnetic susceptibilities and 

magnitudes of NRM, and thus, correlations among aeromagnetic signatures and lithologies are commonly not 

possible (Clark, 1983). Nevertheless, examples of useful 

relations between the magnetizations and petrology exist. 

An instructive example comes from an aeromagnetic and rock magnetic study of Precambrian 

Keynote Papers 31 
igneous and metamorphic rocks of the Adirondack 

Mountains, New York, in which regional anomalies were 

closely correlated with mineralogy and lithology (Balsley 

and Buddington, 1958). Magnetic highs are caused by 

NRM in titanomagnetites, whereas magnetic lows from 

gneisses are produced by reversed remanent 

magnetizations in titanohematite. These reversed 

directions resulted from a self-reversing process (the 

acquisition of a remanent direction opposite to that of the 

applied field) that is peculiar to a narrow range of 

mineralogic compositions of ferrimagnetic titanohematite. Although potentially important in gneissic terranes, 

ferrimagnetic titanohematites are not the dominant 

magnetic species in most other rock types, based on 

abundant paleomagnetic and rock magnetic data 

acquired over the past 25 years. This conclusion differs 

from those of Grant (1984/1985a), who, from our 

perspective, overemphasized the importance of titanohematite solid-solution series minerals to the NRM of rocks 

and the role of self-reversed magnetizations in producing 

magnetic anomalies. 

An example that illustrates the potential for 

identification of rock types by magnetic contrasts 

concerns granites of different origins. Petrochemical and 

isotopic distinction can be made between granites 

derived primarily from melting of igneous crust (I-type) 

and those from melting of sedimentary crust (S-type ). 

Such a distinction, which reflects regional tectonic and 

magmatic evolution, may, in some settings, be discerned 

aeromagnetically. This is because magnetite tends to be 

sparse in granites derived from carbonaceous metasedimentary rocks (S-type) and abundant in granites 

generated by melting of rocks that have low contents of 

carbonaceous matter (I-type). Clark (1983) stated that 

similar magnetic distinctions exist for amphibolites 

derived from igneous or sedimentary rocks. In a study of 

granitoids of the Piedmont Province in the southern 

Appalachian Mountains, Wenner (1981) and Ellwood 

and Wenner (1981) found that plutons that have low 

initial 180 content correspond to I-type granite and have 

relatively high magnetic susceptibilities (greater than 1.2 

X 1D-2 SI). In contrast, granites that have high initialtSQ 

content correspond to S-type granite and have 

susceptibilities less than about 1.2 x 1o-2 SI. Such 

petrologic-magnetic relations are similar to those 

described by Ishihara (1977, 1981) for "magnetiteseries" and "ilmenite-series" granites. The former 

contain more than 0.1 volume percent iron-titanium 

oxide (dominantly magnetite) and are considered 

important sources for magnetic anomalies, whereas the 

latter contain less than 0.1 percent iron-titanium oxides, 

mostly ilmenite, and hence usually have magnetizations 

(magnetic susceptibility less than about 0.3 x 1o-2 SI) 

too low to produce important aeromagnetic anomalies. 

32 Geologic Applications of Modern Aeromagnetic Survey 

Detailed oxygen-isotopic and magnetic 

susceptibility studies of aeromagnetic anomalies over the 

southern half of the Idaho batholith (Criss and 

Champion, 1984) demonstrate the usefulness of, and 

limits to, the interpretation of the extent and origin of 

magma types from magnetic data. Many unaltered 

samples exhibited relations between o1so values and 

magnetic susceptibility similar to those described for the 

Appalachian Piedmont granitoids (Ellwood and Wenner, 

1981). However, because hydrothermal activity strongly 

modified the original o 1so and susceptibility magnitudes 

in many exposed plutons, as discussed later, simple 

characterization of magma type by magnetic properties is 

not possible for most plutons (Criss and Champion, 

1984). It is worth noting also that some plutons in the 

Idaho batholith defy the standard geochemical 

classification as I or S type, or as magnetite or ilmenite 

series, regardless of hydrothermal alteration. 

Volcanic rocks exhibit many characteristics of their 

intrusive equivalents. However, combinations of exsolution and high-temperature oxidation commonly cause 

enormous variations in the magnetic properties of 

volcanic rocks, even within individual flows (Wilson and 

others, 1968; Haggerty, 1976). Moreover, in some extrusive rocks, notably welded ash-flow tuffs, emplacement 

and cooling phenomena, such as initial thickness and the 

effects of compaction, are capable of locally causing 

large, systematic changes in primary directions and 

magnitudes of NRM (Rosenbaum and Snyder, 1985; 

Rosenbaum, 1986; Rosenbaum and Spengler, 1986). 

Remanence commonly dominates total 

magnetizations in volcanic rocks, mainly because of the 

typically small initial grain sizes and (or) the subdivision 

of magnetic phenocrysts into effectively smaller portions 

by exsolution and oxidation; such grains consequently 

produce relatively high TRM and low susceptibility. For 

example, an aeromagnetic low within the 23.1-Ma Lake 

City caldera, San Juan Mountains, Colorado, is produced 

by the reversely magnetized intracaldera ash-flow tuff 

(Grauch, 1987), the eruption of which caused the caldera 

collapse. Understanding of the magnetic sources was 

aided by modeling that used rock magnetic and paleomagnetic data from the caldera units (Grauch, 1987); the 

conclusions of this study also provide an instructive 

example of the interplay between remanent and induced 

magnetizations. Both types of magnetization exhibit large 

variations vertically and areally in the intracaldera tuff. 

The reversed NRM dominates magnetizations in parts of 

the intracaldera tuff to produce the observed low. 

Elsewhere, reversed remanent components are 

effectively cancelled by induced components to yield 

weak total magnetizations that have little aeromagnetic 

expression. 
Recent contributions have shed light on petrologic 

controls of magnetite production in metamorphic rocks. 

Krutikhovskaya and others (1979) showed that premetamorphic compositions and the degree of metamorphism of rocks in the Ukrainian Shield are the main 

determinates of magnetite content. The quantities of 

magnetite and of iron + magnesium + manganese + 

titanium oxides, as well as the magnitudes of the rock 

magnetization, all increase with a decrease in Si02 

content and with an increase in metamorphic grade. The 

results imply that metamorphic magnetite was derived 

from the breakdown of iron and magnesium silicate 

minerals (Grant, 1984/1985a). 

Even in rocks that have similar original 

compositions, different metamorphic reactions and 

conditions may either produce or consume magnetite. 

For example, Robinson and others (1985) proposed that 

many of the magnetic anomalies associated with 

chemically similar metavolcanic rocks of the Carolina 

slate belt in North Carolina are caused by magnetic 

contrasts produced by different activities of water and 

oxygen during metamorphism. Apparently, iron was 

incorporated into magnetite during greenschist-facies 

metamorphism and into silicate minerals during 

amphibolite-facies metamorphism of equivalent rocks. 

The control on magnetite content by metamorphic 

reactions is also demonstrated by a detailed study of a 

contact metamorphic aureole in pelitic rocks at the 

margins of an Upper Carboniferous granitoid pluton in 

South Carolina (Speer, 1981). Bulk rock compositions in 

the aureole hornfelses do not vary greatly. Controlled 

mainly by temperature but also by changes in fluid 

pressure and composition, many metamorphic reactions 

occurred over a short distance. These reactions produced 

magnetite in the outer and middle parts of the aureole 

but destroyed magnetite near the pluton under 

conditions of relatively high temperature and low oxygen 

fugacity. The metamorphic magnetite produced a ringshaped magnetic high that is concentric with the contact 

between the pluton and the country rock. 

Magnetite is not the only cause of annular 

magnetic anomalies around plutons. Pyrrhotite is also 

reported as the source of magnetic lows from contact 

metamorphic aureoles in Jurassic argillites around Late 

Cretaceous and early Tertiary plutons (mainly granodiorites) in the Alaska Range, south-central Alaska 

(Griscom, 1979; A. Griscom, oral commun., 1987). The 

magnetic lows are caused by reversed remanent 

magnetizations carried by the ferrimagnetic (monoclinic) 

pyrrhotite. 

Metavolcanic rocks of virtually the same lithology 

may have different magnetization because of different 

original conditions in the source magmas. Urquhart and 

Strangway (1985) found distinct magnetization 

differences in a suite of metamorphosed iron-rich 

tholeiites, which had essentially the same major-element 

chemistry. They ascribed the differences to contrasts in 

the original temperature and oxygen fugacity conditions 

of the source magmas. 

We next discuss some effects of hydrothermal 

fluids and of high-temperature, ore-related alteration on 

the original magnetizations of igneous, metamorphic, 

and sedimentary rocks. Although the magnetizations of 

some rocks exposed to such fluids and conditions may 

change little and remain dominated by the original 

magnetic particles, the chemical conditions involved in 

such alteration usually destroy or create magnetic phases 

(fig 15), and (or) may reset earlier remanence. The 

temperatures of such alterations are too low to allow the 

formation of Mt-Usp88 or Ilm-HtSS" Of the magnetic 

oxides, therefore, only magnetite is normally an aeromagnetically · important secondary mineral in these 

settings. Many ores consist of, or are associated with, 

metal sulfide minerals, including magnetic pyrrhotite 

(Fe7S8 ). More common nonmagnetic sulfides, 

particularly pyrite (FeS2), also play important roles in 

decreasing magnetization by replacing magnetic oxides 

or in indirectly enchancing magnetization by providing 

concentrations of iron that are easily converted to 

magnetite by oxidation at elevated temperature. 

Hydrothermal Alteration 

The geochemical and magnetic study of the Idaho 

batholith by Criss and Champion (1984) documented 

both enhancement and depletion of magnetization by 

hydrothermal activity (at temperatures between about 

150 and 400 °C) related to the emplacement of Tertiary 

intrusions into Mesozoic plutons of the Idaho batholith. 

Rocks that underwent strong hydrothermal alteration in 

the interior of the batholith have lower magnetic 

susceptibilities than their unaltered equivalents, 

presumably because of oxidation of primary magnetite. 

Hanna (1969) reached a similar conclusion to explain 

low magnetizations of mineralized plutons of the Boulder 

batholith, Montana. In contrast, similar alteration, but 

probably involving less water and hence lower oxidation 

potential, increased magnetic susceptibility in parts of 

some leucocratic plutons in the Idaho batholith. This 

increase was ascribed to the growth of "hydrothermal" 

magnetite. 

Little is known about the effects of hydrothermally 

driven metasomatism on the magnetizations of silicic 

volcanic rocks. It appears, nevertheless, that metasomatic 

conditions that involve oxygen-rich fluids destroy titanomagnetite. Metasomatic alteration, which resulted in 

enrichment of potassium, decreased magnetic 

susceptibility and remanent intensities of intracaldera 

welded tuffs relative to unaltered material of some 

ash-flow tuffs of the Mogollon-Datil volcanic field, New 

Keynote Papers 33 
Mexico (Mcintosh, 1983). Oxidizing and perhaps slightly 

acidic conditions were thought to be responsible for the 

formation of hematite from magnetite and for the 

dissolution of magnetite. Preliminary study of the 

Carpenter Ridge Tuff (27.61 m.y.), the major host for 

precious- and base-metal vein deposits of the Creede 

mineral district in the Bachelor caldera of the central San 

Juan volcanic field, Colorado, shows that the potassiummetasomatized intracaldera facies (Bachelor Mountain 

Tuff) have much lower magnetization than that of nonmetasomatized outflow facies (D. Sweetkind, written 

commun., 1989). Reduction of magnetization by 

potassium-metasomatism or other pervasive alteration 

may explain, at least in part, why some calderas, such as 

the Bachelor caldera, have indefinite aeromagnetic 

expression, whereas other calderas, which are filled with 

largely unaltered tuffs of nearly identical initial 

compositions, are the sources of large-amplitude 

magnetic anomalies (Williams and others, 1987) 

Magnetization and Mineral Deposits 

The practical value of magnetic exploration for 

mineral deposits has long been recognized (Grant, 

1984/1985b; Clark, 1983; and Wright, 1981 for recent 

summaries) and is based on the experience that high or 

low magnetizations characterize some ore-forming 

environments. Magnetic contrasts may be related to 

formation or destruction of magnetic minerals in direct 

response to mineralization or related to a generally high 

or low magnetic-mineral content of a particular host rock 

or, on a larger scale, of a favorable rock assemblage. For 

example, certain types of mineral deposits were thought 

by Ishihara (1981) to be related to magnetite-series or to 

ilmenite-series magmatism with their respective high and 

low magnetizations. As a specific case, Hattori (1987) 

described an association between magnetite-bearing, 

anomaly-producing felsic intrusions and gold deposits in 

Archean rocks of the Superior Province of the Canadian 

shield. The association of magnetic intrusions and 

mineralization suggests either that ore constituents were 

derived from oxidized magmas or that the intrusions 

were emplaced in zones of deep flow of gold-bearing 

fluids. Other associations among magnetic patterns and 

mineral deposits can be attributed to tectonic setting, in 

particular to the extent of faulting and its controls on the 

chemistry and flow paths of altering fluids (Mcintyre, 

1980). 

Effective use of magnetic patterns in mineral 

exploration depends on understanding relations among 

petrology and structure of the host terrane, 

mineralization, and magnetization. Many such relations 

are discussed in the review by Grant (1984/1985b). Yet 

much remains to be learned about links among ore 

petrology and geochemistry, the genesis of magnetic 

34 Geologic Applications of Modern Aeromagnetic Survey 

minerals, the structure of the host terrain, and magnetic 

anomalies, as well as about the applications of these links 

to effective magnetic exploration in frontier areas 

Although a few mineral-deposit settings have a 

characteristic magnetic signature, most settings do not. 

This lack of uniformity has many causes, including: (1) 

variations in chemistry of mineralizing fluids; (2) 

differences in host-rock composition, structure, and 

crustal level; and (3) variations in the type and degree of 

post-ore alterations. We will not dwell at length on the 

associations among mineral deposits, magnetite, and 

magnetic signatures that are reported by other authors. 

However, we will examine some examples that provide 

typical reasons for different magnetic signatures that 

arise from similar mineral deposits. These examples are 

drawn from skarn and porphyry copper deposits and 

from the magnetite-pyrite association that is common to 

many types of deposits. 

Skarn Deposits 

Skarns are metal-bearing deposits associated with 

the hydrothermal replacement of carbonate rocks by a 

complex assemblage of Ca-Fe-Mg-Al silicate minerals. 

The classification, description, tectonic-magmatic 

settings, and origins of skarn deposits are presented in 

Einaudi and others (1981). Magnetite is common in 

many skarns and is the ore mineral in iron skarns. 

Ma~etite enrichment is associated with many zinc-lead, 

molybdenum, magnesian tin-bearing skarn deposits and 

with some tungsten and calcic tin-bearing skarns. The 

magnetite content in skarns depends on many factors. 

For tungsten skarns, differences in magnetite content 

reflect different depths of emplacement of related 

intrusions and different oxidation states of the altered 

rocks (Einaudi and others, 1981). Tungsten skams that 

were formed at great depth in a carbonaceous-rich, 

low-oxidation, and low-sulfidation environment contain 

abundant magnetite and only a small quantity of sulfide 

minerals, mainly pyrrhotite. In contrast, tungsten skarns 

that were formed at higher levels in carbonaceous-poor, 

intermediate-oxidation and intermediate-sulfidation 

environments may lack magnetite but tend to have 

relatively greater amounts of sulfide, principally pyrite. 

Copper skarn deposits are of special interest because of 

their common association with porphyry copper stocks 

(Einaudi and others, 1981; Einaudi, 1982a, 1982b; Titley, 

1982; Beane, 1982; Titley and Beane, 1981; Beane and 

Tilley, 1981). Magnetite is common in both calcic and 

magnesian copper skarns but is apparently more 

abundant in the magnesian skarns. 

Porphyry Copper Deposits 

The association of magnetite with many porphyry 

copper deposits makes these deposits attractive targets 
for magnetic exploration. Magnetic anomalies may arise 

from both the igneous intrusion and from associated 

peripheral skarns. Jerome (1966) presented a model of a 

copper porphyry that portrays secondary magnetite in 

skarn at the margins of a mineralized intrusion as the 

cause of ground magnetic and aeromagnetic highs that 

might define an annular anomaly around an orebody. 

Grant (1984/1985b) discussed porphyries (based largely 

on the work of Lowell and Guilbert, 1970) and concluded 

that recognition of favorable environments for porphyry 

copper deposits may be based on the magnetic 

identification of felsic-intermediate plutons of 

appropriate size (about 1-10 km in diameter) and on the 

possible existence of a "magnetite halo" within the pluton. The Ely, Nevada, area provides an example of 

positive aeromagnetic anomalies associated with a 

porphyry copper deposit. Magnetic anomalies reflect the 

high magnetizations both of the quartz-monzonite 

intrusion and of the magnetite-bearing skarn in nearby 

carbonate strata (Wright, 1981 ). 

Other important magnetic characteristics of the 

porphyry copper environment have been discerned from 

a regional aeromagnetic map of Arizona which was 

designed to show large-wavelength anomalies from 

deeper and broader sources than those at the ore-deposit 

scale (Sumner, 1985). On this map most of the copper 

districts are associated with regional lows, with arcuate 

magnetic lows at the district level, or with a 400-km-long 

magnetic lineament that is interpreted to reflect a late 

Precambrian transform fault. Sumner (1985) suggested 

that the magnetic lows are caused by the destruction of 

presumable primary magnetite in hydrothermal fluids 

that circulated in deep fractures. Conversely, the regional 

magnetic data also revealed a relation between a few 

deposits and magnetic highs that may be at least partly 

explained by relatively mafic compositions of the 

mineralized intrusions. 

Magnetite-Pyrite Association 

The association between magnetite and nonmagnetic iron sulfide minerals is common to many types of 

mineral deposits. We discuss nonmagnetic iron sulfides 

(referred to here as pyrite, the most common variety) 

because they may play important but not fully recognized 

roles in the magnetic contrasts of some mineral deposits. 

The relationship between magnetite and pyrite in 

some types of mineral deposits can be explained on the 

basis of thermodynamically controlled sequences of 

precipitation (Mcintyre, 1980; Grant 1984/1985b; Beane, 

1982). Nevertheless, other explanations for the 

magnetite-pyrite association should be considered, 

especially in view of the complex thermal, tectonic, and 

geochemical histories of many mineral districts. For 

example, pyrite can be converted to magnetite by the loss 

of sulfur at elevated temperatures. In addition, 

magnetization can be reduced by the replacement of 

primary or secondary magnetic iron oxide minerals by 

nonmagnetic sulfide minerals (Wright, 1981). Such 

distinctions, which can usually be made on petrographic 

criteria, are obviously important for genetic models that 

link mineral assemblages and paragenesis to magnetic 

anomalies. 

The anomaly over the Calico Hills in southwestern 

Nevada is an example of magnetic high that is caused by 

the high-temperature conversion of pyrite to magnetite. 

The anomaly is centered over an area underlain by a 

thick Paleozoic section which is capped by a thin veneer 

of hydrothermally altered Tertiary ash-flow tuffs. Bath 

and Jahren (1984) attributed the anomaly to strongly 

magnetized, thermally altered Paleozoic strata. Measurement of magnetic properties of drill-core samples 

indicates that some of the Paleozoic sedimentary rocks 

are nonmagnetic, whereas others are strongly magnetic 

(Baldwin and Jahren, 1982); the strongly magnetic rocks 

have an average NRM of about 4 Nm. Petrographic 

observations clearly demonstrate that the magnetic rocks 

contain magnetite which formed by the oxidation of 

pyrite and the resultant loss of sulfur. The magnetite 

occurs as replacements of pyrite cubes and pyritohedrons, pyrite veinlets, pyrite that filled cell lumens in 

detrital plant fragments, and framboidal pyrite. In 

contrast, nonmagnetic rocks either contain pyrite in the 

above forms (and little or no magnetite) or show no 

evidence of having ever contained pyrite. 

Sedimentary Rocks 

The magnetizations of epiclastic sedimentary rock 

depend directly on the abundance of detrital magnetic 

minerals and on the geochemical history of the. rocks that 

either preserved or destroyed detrital magnetic minerals 

or that created diagenetic-authigenic magnetic oxide or 

sulfide minerals. Postdepositional effects on rock 

magnetization are especially important because of their 

potential as indicators of mineral and hydrocarbon 

deposits. Although most sedimentary materials contain 

magnetizations that are useable for paleomagnetism, the 

relatively thin sedimentary crust is normally not an 

important contributor to aeromagnetic anomalies. Even 

the most magnetic of the epiclastic sedimentary rocks 

generate low-amplitude, high-wavelength anomalies. 

Because flat-lying or shallowly dipping strata, even if 

strongly and uniformly magnetic, do not generate strong 

magnetic contrasts except locally at their edges, most 

anomalies from sedimentary rocks are those associated 

with steeply dipping or faulted units or with alterations 

controlled by fluid migration paths that either are 

vertically directed or are limited in horizontal extent. 

Ironically, the most magnetic rocks, on the average, 

are sedimentary-the Early Proterozoic quartzKeynote Papers 35 
magnetite banded iron-formations (Clark, 1983). These 

rocks, which contain iron ores, can be easily detected by 

aeromagnetic methods. Magnetite in these rocks was 

chemically precipitated from sea water that was rich in 

dissolved ferrous iron before the buildup of free oxygen 

in the oceans. The magnetics and economic importance 

of banded iron-formations are described by Mcintyre 

(1980) and Grant (1984/1985b), and we will not discuss 

further chemically precipitated, primary magnetite. 

Strongly magnetic detrital minerals in the 

sedimentary rocks are primarily magnetite and titanomagnetite derived from crystalline or other sedimentary 

source areas. Ferrimagnetic titanohematite is generally 

rare or absent, but it may be the dominant magnetic 

oxide in some sedimentary rocks (Reynolds, 1977; 

Reynolds, 1982; Butler, 1982). Concentrated detrital 

magnetic grains can, of course, be responsible for 

anomalies, as shown below; for the most part, however, 

ferrimagnetic oxides are destroyed under most 

conditions of oxidation and reduction in sediments and 

sedimentary rocks. The extent of the destruction depends 

primarily on the duration and nature of altering 

conditions (such as Eh, pH, temperature, bacterial 

activity, concentrations of Fe2 + and Fe3 + , types and 

abundances of sulfur species, and organic matter). Such 

destruction includes dissolution and the oxidative or 

reductive replacement of the detrital oxides by essentially 

nonmagnetic minerals, such as of magnetite by hematite 

or by iron sulfide, respectively. Uncommon, but 

important in certain sulfide reducing environments, is the 

authigenesis of ferrimagnetic sulfide minerals, monoclinic pyrrhotite (FtryS8 ) and cubic greigite (Fe3S4 ). The 

postdepositional growth of magnetite is currently a topic 

of growing interest among aeromagnetists and paleomagnetists. Such growth may be via inorganic geochemical pathways (Murray, 1979) or may be the result of 

bacterial metabolism. Two different types of magnetiteproducing bacteria have been identified. Magnetotactic 

bateria produce small quantities of intracellular 

magnetite (less than 0.2-p.m diameter) and operate in the 

presence of oxygen in marine, brackish and fresh-water 

environments; thus they deposit magnetite in surficial 

sediment layers that contain dissolved oxygen (Blakemore, 1982; Kirschvink and Chang, 1984). In contrast, 

dissimilatory iron reduction by anaerobic bacteria 

appears to be capable of producing large amounts of 

fine-grained (less than 0.2-p.m diameter) magnetite 

outside the microbial cell (Lovley and others, 1987). The 

only known strain of dissimilatory iron-reducing bacteria 

was isolated in 1986 from fresh-water anaerobic 

sediments of the Potomac River (Lovley and others, 

1987; Lovley and Reynolds, 1987). Possible roles of 

anaerobic, magnetite-producing bacteria in generating 

36 Geologic Applications of Modern Aeromagnetic Survey 

magnetic anomalies in sedimentary rocks (especially in 

fault zones above hydrocarbon deposits) are topics 

immediately worthy of intensive research. 

RESEARCH FRONTIERS: PROBLEMS 

INVOLVING SEDIMENTARY ROCKS 

Alteration Associated with 

Sandstone Uranium Deposits 

The following examples from uranium-bearing 

sandstones in the San Juan basin, New Mexico, and in the 

south Texas coastal plain illustrate the destruction of 

detrital magnetic grains. The two areas have either not 

been surveyed or not been assessed in sufficient detail to 

determine whether ore-related alteration has caused 

magnetic anomalies; moreover, rock magnetic studies 

are incomplete or lacking in these examples. However, 

on the basis of the originally high contents of detrital 

magnetic minerals and on the spatial distribution of 

zones that contain or lack the magnetic grains, it seems 

likely that current aeromagnetic techniques are capable 

of detecting the effects of near-surface, uranium-related 

alterations on sandstones that originally contained 

abundant detrital magnetic grains. 

Most uranium deposits that occur in Phanerozoic 

sandstones are classed either as tabular or as roll-type 

deposits (Nash and others, 1981). The largest uranium 

reserves in the United States are found in the tabular ore 

deposits of the Upper Jurassic Morrison Formation on 

the Colorado Plateau. Many of these deposits differ in 

some important genetic aspects, but all share a common 

association with organic matter. The tabular orebodies in 

sandstones of the Westwater Canyon and Brushy Basin 

Members of the Morrison in the Grants uranium region 

in west -central New Mexico consist of uranium 

intermixed with epigenetically introduced organic matter 

that was expelled by compaction from overlying mud-flat 

facies of the Brushy Basin Member (Turner-Peterson 

and Fishman, 1986; Fishman and Turner-Peterson, in 

press). The organic matter, which was derived from 

detrital carbonaceous debris, was apparently transported 

as humic and perhaps as other organic acids in alkaline, 

mildly reducing fluids. Such acids leach iron from the 

Fe-Ti oxide minerals (Lynd, 1960; Schnitzer and Skinner, 

1965; Baker, 1973; Adams and others, 1974; Schnitzer 

and Kodama, 1977). 

Petrographic, geochemical, field and mine 

mapping, and laboratory and borehole magnetic 

susceptibility studies document important relations 

among the genesis of the uranium deposits (including the 

nature and timing of ore formation and paths of 

mineralizing fluids), the alteration of Fe-Ti oxides, and 
the magnetic properties (Adams and others, 1974; 

Adams and Saucier, 1981; Fishman and others, 1985; 

Fishman and Reynolds, 1986; Reynolds, Fishman, Scott, 

and Hudson, 1986; Fishman and Turner-Peterson, in 

press). The Fe-Ti oxide minerals (principally titaniferous 

magnetite and ilmenite) were abundant detrital 

components of the Morrison sandstones that are now 

exposed at the margins of the San Juan basin and are still 

well preserved in places, where they compose nearly 1 

weight percent of the rock. In these sandstones, 

susceptibilities and remanent moments are typically 

greater than 5 x 1Q-4 SI (maximum 8 x 1Q-3 SI) and 10-4 

Nm, respectively. In sandstone beds that encompass the 

uranium deposits (the altered zone), however, the detrital oxides are highly altered by iron dissolution and 

consist typically of relict titanium dioxide~ The 

abundance of such relicts, as well as the spatial and 

paragenetic relations, indicate postdepositional 

alteration of detrital magnetic grains during ore 

formation, which was most likely caused by humic-rich 

solutions that originated in the Brushy Basin Member 

(Adams and others, 1974; Adams and Saucier, 1981; 

Fishman and others, 1985; Turner-Peterson and Fishman, 1986). In the altered zone, magnetic susceptibilities 

and remanent moments are uniformly less than 5 x 10-4 

SI and 1G-4 Nm, respectively. Based on borehole core 

studies, the altered zone is as much as 100 m thick and 

extends from the base of the Brushy Basin Member to 

below the ore lenses, which are typically only 2-10 m 

thick. Outcrop studies reveal that this pattern of 

alteration extends essentially uninterrupted many tens of 

kilometers along the southern and south-western 

margins of the basin. The studies also show that the 

degree of alteration of Fe-Ti oxide minerals decreases 

away from the Brushy Basin Member. Where the mudflat facies of the Brushy Basin Member (the source of the 

humic acids) dies out to the north, the Westwater Canyon 

Member lacks uranium mineralization and contains 

abundant, unaltered Fe-Ti oxides through its entire 

stratigraphic extent (Fishman and Turner-Peterson, in 

press). Thus the presence of uranium deposits and the 

related destruction of detrital magnetic grains in the 

Westwater Canyon Member has been controlled largely 

by distribution of the mud-flat facies of the overlying 

Brushy Basin Member (Turner-Peterson and Fishman, 

1986). 

Roll-type uranium deposits form by invasion of 

geochemically reduced, iron disulfide (FeS2 ) bearing 

sandstone by oxygenated, uranium-bearing ground 

waters, followed by the reduction-precipitation of the 

uranium. Encroachment of such waters into reduced 

strata creates a tongue-shaped zone of oxidized rock (the 

altered or oxidized tongue) and a redox (reductionoxidation) boundary that separates the tongue from 

reduced ground. Uranium is concentrated in a crescentric 

envelope or roll on the reduced side of the redox 

boundary, with the convex side of the roll front pointing 

in the downdip direction into reduced rock. 

Magnetic contrasts may develop in host beds of 

some roll-type deposits as a result of the replacement of 

detrital magnetic grains by iron sulfide minerals. In the 

south Texas coastal plain uranium deposits in the Miocene Catahoula Sandstone (for example, the Benavides 

deposit) and in the overlying Miocene Oakville 

Sandstone (for example, the Felder-Lamprecht deposit, 

Ray Point district), pre-ore sulfide (pyrite and marcasite) 

created a reduced geochemical and mineralogic zone 

that controlled the concentration of uranium. The FeS2 

minerals extensively replaced detrital magnetic grains 

(Reynolds and Goldhaber, 1978; Reynolds, 1982; Goldhaber and others, 1978, 1983). In the Benavides deposit, 

the sulfidization resulted from the invasion of the shallow 

(3~5 m deep) aquifers by sulfidic brines from deep 

sour-gas reservoirs along a normal growth fault that lies 

1.5 km downdip from the roll front. The FeS2 partly to 

completely replaced titaniferous magnetite grains as 

much as 2 km updip from the fault. Ferrimagnetic 

titanohematite, which was initially about half as abundant 

as magnetite, was more resistant to sulfidization, and 

consequently it is the dominant magnetic oxide in the 

reduced zone and in the altered tongue. More than 2 km 

updip from the fault, the sandstone was never sulfidized; 

in these beds the Fe-Ti oxide population has only been 

affected by partial hematite replacement of magnetite 

(Reynolds, 1982). 

Although the magnetic susceptibility of disaggregated core samples was not measured, the abundances 

and distribution of the magnetic oxides were examined 

using separates of magnetic minerals. The magnetic 

fraction, as a percentage of the whole rock, ranges from 

nearly 1 weight percent in the oxidized beds farthest (2.5 

km) updip from the fault to less than 0.1 weight percent 

in reduced rock closer to the fault. The magnetic fraction 

of the samples in between decreases systematically within 

this range with proximity to the fault. This pattern and 

petrographic-geochemical studies strongly suggest that 

the magnetic minerals were nearly uniformly distributed 

in the host beds by depositional processes and that the 

degree of replacement by sulfide minerals decreased 

systematically updip from the fault (Reynolds and Goldhaber, 1978; Goldhaber and others, 1978). 

A similar pattern, but presumably a more subdued 

contrast, appears to be present in the Lamprecht-Felder 

deposit based on laboratory magnetic susceptibility 

measurements of core samples (from about 60-80-m 

depth) from the Lamprecht part of the deposit. Values of 

susceptibility are highest (3-4 x 1G-4 SI) in the host beds 

that lie updip from the roll front and the sulfideproducing Oakville fault (1 and 1.5 km distant, 

respectively) and decrease systematically (to about 

Keynote Papers 37 
2 x 1Q-4 SI) in the roll front and closer to the fault (Scott 

and Daniels, 1976; Scott and others, 1983). Again, the 

increase in magnetization updip from the fault was likely 

caused by a decrease in intensity of sulfidization and the 

corresponding decrease in replacement of detrital 

magnetic grains. 

Observations from the two roll-type uranium 

deposits suggest that the delineation of growth faults by 

their magnetic expressions may be a general guide for 

sulfidically reduced zones with potential for uranium 

mineralization elsewhere along the Texas coastal plain. 

Diminished magnetizations are predicted on the updip 

(northwest) sides of faults. Detailed aeromagnetic 

surveys directly over known faults, many of which tap the 

hydrocarbon-producing deep Edwards reef trend, would 

be desirable, because the faults themselves may have a 

distinctive signature. Such a possibility is based on 

observations that faults and fractures that tap 

hydrocarbon deposits elsewhere can be recognized as 

magnetic features on aeromagnetic maps. An example is 

the association between fractures and magnetic contrasts 

over oil fields in the Williston basin, Montana (R. Wold, 

oral commun., 1985). Geochemical and mineralogic 

causes that change magnetization along faults, especially 

those that may be associated with oil and gas fields, are 

topics of obvious importance to studies of aeromagnetism and rock magnetism. 

In addition to the roll-type uranium deposits in 

sandstones that contain extrinsic (postdepositionally 

introduced) sulfide and that lack detrital organic matter, 

an equally important subclass of roll-type deposits is 

found in sandstones that contain detrital plant debris. 

Examples include the roll-type deposits in the Wyoming 

intermontane basins and in the Eocene Whitsett 

Formation of the south Texas coastal plain. In these 

"biogenic" deposits, the pre-ore sulfide is related to the 

metabolic activity of sulfate-reducing bacteria that feed 

on carbonaceous matter (Rackley, 1972; Granger and 

Warren, 1969; Reynolds and Goldhaber, 1983). In biogenic deposits that we have studied, magnetic oxides have 

been little affected by sulfidization; magnetite may be 

overgrown by pyrite, but replacement by pyrite is rare. 

These associations may be related to the typically low 

concentrations of sulfide in these ore-forming systems. 

Sulfide sulfur averages between 1 and 4 weight percent of 

the host rock in the Benavides, Lamprecht, and Felder 

deposits, but less than 1 percent in a deposit in the nearby 

Eocene Whitsett Formation. These observations would 

appear to contraindicate aeromagnetic exploration for 

roll-type deposits in areas that are remote from faults 

along which sulfidic brines have passed. Nevertheless, the 

fluvial sandstones in which most roll-type deposits occur 

38 Geologic Applications of Modern Aeromagnetic Survey 

may be aeromagnetic targets, based on the likelihood of 

greater concentrations of heavy minerals in these 

relatively coarse-grained, high-energy facies than in 

enclosing mudstone. 

Alteration Associated with 

Hydrocarbon Seepage 

Recently proposed models that link hydrocarbon 

seepage, authigenesis of magnetic minerals in nearsurface strata, and high-frequency magnetic anomalies 

have encouraged magnetic exploration for oil and gas 

deposits (Donovan and others, 1979, 1984; Foote, 1984; 

Saunders and Terry, 1985; McCabe and Sassen, 1986). 

Interest in this approach stemmed mainly from an aeromagnetic survey of the Cement oil field, Oklahoma, in 

which total-field anomalies, which have short 

wavelengths and low amplitudes (less than 40 nT), were 

detected over an area of past oil seepage and current 

production (Donovan and others, 1979). Donovan and 

others (1979) reported as much as 1.2 weight percent 

magnetite in well cuttings from bleached Permian red 

beds over the producing reservoirs. They proposed that 

the magnetite formed by reduction of ferric oxide in the 

presence of upward migrating hydrocarbons and that the 

magnetite was the source of the anomalies. 

Similarly, aeromagnetic suiVeys over vast regions 

of Alaska's North Slope have revealed several areas of 

anomalous magnetizations over known oil fields and over 

structures that appear favorable for hydrocarbon 

deposits based on other geologic and geophysical criteria 

(Donovan and others, 1984). These anomalies were 

likewise attributed to formation of magnetite or other 

magnetic minerals under reducing conditions that were 

related to hydrocarbon seepage. 

Support for an association between magnetite and 

hydrocarbons comes from recent reports of magnetite 

with diagenetic morphologies in bitumen and other solid 

hydrocarbons from surface seeps (McCabe, 1986; 

McCabe and others, 1987; Elmore and others, 1987). In 

addition, magnetite in similar forms has been reported in 

Paleozoic carbonate rocks; in these occurrences diagenetic processes that are related to hydrocarbon 

migration have been the favored interpretation for the 

origin of the magnetite (McCabe and others, 1983; 

Elmore and others, 1986). 

Our investigations of the magnetic anomalyhydrocarbon seepage problem have involved rock 

magnetic, paleomagnetic, petrologic, and geochemical 

studies of samples from the Cement oil field, the Simpson oil field on the North Slope of Alaska, and the 

Jurassic Preuss Sandstone in the Wyoming-Idaho-Utah 

thrust belt. This work has not found a connection among 

magnetite, hydrocarbons, and aeromagnetic anomalies. 
Instead, the work demonstrates an important association 

among magnetic iron sulfide minerals, -leaked 

hydrocarbons, and bacterial activity at Cement, and 

suggests a similar association at the Simpson field 

(Reynolds and others, 1984; Reynolds, Fishman, Hudson, and others, 1986). Studies of the Preuss illustrate the 

case in which magnetic anomalies in an area of 

hydrocarbon potential have been caused by detrital 

magnetite and are unrelated to seepage (Hudson and 

others, 1985; Fishman and others, 1989; Reynolds, Fishman, Hudson, and others, 1986). Diagenetic magnetite is 

represented on figure 3 with a bold query, because, to 

our knowledge, there are as yet no confirmed cases in 

which secondary (authigenic) magnetite has produced 

aeromagnetic anomalies. 

Cement Oil Field 

Studies of well cuttings, including those examined 

by Donovan and others (1979), of shallow (surface to 

about 36 m deep) core and of surface outcrop and quarry 

exposures have not turned up any evidence for diagenetic 

magnetite at Cement. Magnetite is abundant in most well 

cutting samples, but it exhibits synthetic, metallographic 

textures and (or) contains inclusions of industrial steel 

(Reynolds, Fishman, Hudson, and others, 1986; Reynolds 

and others, 1988; Reynolds, Fishman and others, 1990). 

The magnetite is interpreted to be a contaminant related 

to drilling; magnetite is a common product of corrosionoxidation of drill stems (Gray and others, 1980). 

Furthermore, we have found magnetite in the rusty scale 

of a used oil-field drill collar, and this magnetite exhibits 

many textural features similar to those of the Cement 

magnetites. 

Many samples from the cuttings, cores, and 

surface, however, contain authigenic ferrimagnetic 

pyrrhotite, which is commonly intergrown with and (or) 

replaced by pyrite and marcasite. Two lines of evidence-the distribution of the pyrrhotite and the sulfur 

isotopic composition of the sulfide minerals-strongly 

suggest that the pyrrhotite is related to migrated 

hydrocarbons. Pyrrhotite is limited to strata above the 

oil-producing structures, where it appears to be 

concentrated in a shallow (200-500 m deep) zone that 

cuts across lithologic and formational boundaries. Away 

from the main field and the area of hydrocarbon 

alteration at the surface, well cuttings do not contain 

pyrrhotite. Variations in the sulfur isotopic compositions 

of the sulfide minerals and comparison with published 

isotopic values of crude oils at Cement (Lilburn and 

Al-Shaieb, 1984) suggest that the pyrrhotite and other 

sulfide minerals formed from at least two different 

sources of sulfide and by a combination of inorganic and 

organic mechanisms (Reynolds, Fishman, and others, 

1990). Aqueous sulfide in hydrocarbon-related fluids 

that leaked upward from depth along numerous prePermian faults in the area was probably the dominant 

sulfide source for the sulfide minerals at depths greater 

than about 300 m. There the sulfide minerals apparently 

formed by the inorganic reaction of aqueous sulfide with 

available iron. Fault-derived sulfide also probably 

contributed to the iron sulfide minerals at shallower 

depths, but this contribution waned toward the surface. 

With decreasing depth, increasing amounts of sulfide 

produced by sulfate-reducing bacteria were also 

incorporated into the iron sulfide minerals. Because 

these Permian beds contain only small amounts of 

organic carbon (typically less than 0.1 percent) and lack 

detrital debris, the sulfate-reducing bacteria must have 

derived energy from some food source other than plant 

matter. Organic compounds in or derived from 

hydrocarbons were likely sources for bacterial 

consumption in the hydrocarbon seepage plume at 

Cement. 

The contribution of pyrrhotite to the anomaly at 

Cement is difficult to determine with certainty 

(Reynolds, Webring, and others, 1990). The magnetic 

properties of the pyrrhotite-bearing samples (borehole 

cuttings) necessary for magnetic modelling either cannot 

be measured, such as NRM direction and magnitude, or 

cannot be determined with confidence (magnetic 

susceptibility) because of contamination by magnetite. 

Moreover, uncertainty regarding the nature of the 

Cement anomaly and the likely effects of cultural 

interference have clouded the interpretation picture 

(Donovan and others, 1986; Boardman, 1985; Foote, 

1984). We may conclude, nevertheless, that pyrrhotite is 

related to hydrocarbon seepage and that is the only 

possible natural source for anomalous magnetization at 

Cement. 

Simpson Oil Field 

At the Simpson oil field, ferrimagnetic greigite 

(Fe3S4 ) is concentrated locally in Upper Cretaceous 

sandstone, siltstone, and mudstone. Detrital Fe-Ti oxide 

minerals occur with greigite in some samples from borehole cores but rarely compose the majority of magnetic 

grains. Among the detrital oxides, ferrimagnetic titanohematite is more common than titanomagnetite; 

petrographic observations suggest that this is due to the 

preferential dissolution of magnetite after deposition. 

Intensities of NRM of the greigite-bearing samples range 

from 3 x 1Q-3 to 2 x 1Q-1 A/m, whereas those of the 

samples dominated by the magnetic oxides range from 

only 6 x 1Q-4 to 10-2 Nm. Magnetic susceptibilities ( 4 

x 1o-s to 3 x 1Q-3 SI units) are unimportant contributors 

to the total magnetizations. The remanence that resides 

in greigite is therefore probably responsible for the 

anomalies over the Simpson field. 

Keynote Papers 39 
A connection between the greigite and 

hydrocarbon seepage, however, has not been 

unambiguously established. The greigite probably 

formed by processes that involve sulfate reduction by 

bacteria (Berner, 1981) that used either organic 

compounds derived from leaked hydrocarbons or detrital 

organic matter, or both, as food sources. Available 

evidence points to a combination of factors. The greigite 

that is found on the surfaces and within cell lumens of 

abundant detrital plant fragments in many samples from 

the Simpson field may have formed during early diagenesis in the absence of hydrocarbons. However, greigite is present in some beds that are devoid of plant matter 

and absent in others that contain such matter. Geochemical studies that involve detailed sulfur isotopic 

analysis of the organic sulfur and mineral sulfur species 

may help to resolve the problem. 

Preuss Sandstone 

The Preuss Sandstone in the Wyoming-Idaho-Utah 

thrust belt was chosen as one of several targets for testing 

possible connections between hydrocarbon seepage and 

magnetic anomalies for the following reasons. An aeromagnetic survey (U.S. Geological Survey, 1981) showed 

positive magnetic anomalies west of the Absaroka thrust 

fault in the central thrust belt on trend with hydrocarbon 

production to the south. A subsequent ground magnetometer survey (S. Oriel and D. Mabey, written commun., 1982) and a preliminary magnetic study (Fishman 

and others, 1989; Reynolds, Fishman, Hudson, and 

others, 1986) confirmed the Preuss as the source of the 

positive anomalies. The magnetic study also revealed that 

the Preuss possesses a stable remanent magnetization 

carried by magnetite. 

Paleomagnetic analysis indicated that the stable 

magnetization in the Preuss was acquired after 

deposition and at least partly during folding (Hudson and 

others, 1985, 1989). These stable synfolding directions 

are interpreted to be a viscous partial thermoremanent 

magnetization (VpTRM), which was acquired at low 

temperatures (less than 150 °C) over several tens of 

millions of years as thrusting migrated from west to east. 

Similar synfolding behavior in some other sedimentary 

units (principally limestones) elsewhere has been 

attributed to diagenetic magnetite formed from 

hydrocarbon-bearing fluids that were mobilized during 

deformation. In some of these carbonate units, magnetite 

has been identified in magnetic extracts that include 

grains with secondary (authigenic or diagenetic) morphologies (McCabe and others, 1983; Elmore and others, 

1986). In contrast to these rocks, the Preuss lacks diagenetic magnetite. Rather, the magnetization in the 

Preuss resides in large (10-80-p.m diameter), detrital 

titaniferous magnetite (contains as much as 14 mol 

40 Geologic Applications of Modern Aeromagnetic Survey 

percent ulvospinel) grains, which are commonly 

concentrated in heavy-mineral laminations (Hudson and 

others, 1985; Fishman and others, 1989). The abundance 

of the detrital magnetite, which can be estimated from 

magnetic susceptibility, decreases systematically from 

west (nearer the source area for the magnetite) to east 

across the depositional basin. The gradient in the 

susceptibility values has been accentuated by thrusting 

and folding, which shortened the area by about 50 

percent. 

Thus, no evidence was found in the Preuss to link 

the magnetic anomalies with diagenetic magnetite 

formed under the influence of hydrocarbon-bearing 

fluids. Instead, the location of the magnetic anomalies is 

related to the distribution of abundant detrital magnetite. 

The distribution was controlled by the original 

depositional setting, by later thrusting and folding, and by 

the preservation of the titaniferous magnetite by calcite 

and by a favorable ground-water chemistry. 

These results confirm that investigations of sources 

of magnetic anomalies over oil fields should encompass a 

wide variety of stratigraphic, geochemical, and structural 

settings to cover the wide range of diagenetic conditions 

that may affect magnetization in strata above oil deposits. 

Even the results of the limited and largely preliminary 

studies described above indicate that different styles of 

diagenesis can accompany hydrocarbon seepage and that 

aeromagnetic data should be cautiously interpreted in 

the absence of rock magnetic studies. 

CONCLUSIONS 

Magnetite (Curie temperature about 580 °C) is the 

dominant source of magnetic anomalies in continental 

crust. Magnetite is thought by most investigators to be 

the main source of anomalies from metamorphic rocks 

within the lower continental crust and thus to control the 

Curie isotherm in this part of the Earth. The proposed 

existence of metallic iron and iron alloys in the lower 

crust and possibly the upper mantle has fostered 

controversy that will certainly generate more study of this 

topic. Magnetite also carries the induced and remanent 

magnetizations in metamorphic rocks and in many 

acidic plutonic rocks at shallower depths. Titaniferous 

magnetite, which has lower Curie temperatures, is 

abundant primarily in basic igneous rocks at mid-crustal 

and shallower depths. Magnetic titanohematite may 

dominate the magnetizations of some acidic plutonic, 

gneissic, and dacitic volcanic rocks, but it is not an 

important source of anomalies in other settings. 

In contrast, titanomagnetite (Curie temperature 

about 175 °C) and its alteration product, titanomaghemite, dominate magnetizations in upper layers of 

the oceanic crust, and these minerals probably are the 

major source of linear magnetic anomalies from the sea 
floor. At depth, however, magnetite may form by deuteric and hydrothermal alteration of the oceanic crust. 

Thus the depth to the Curie isotherm of the oceanic crust 

is controlled by the intrusive, cooling, hydrothermal 

history of the crust. 

Magnetic contrasts in sedimentary rocks are 

caused by differences in the abundances of detrital 

magnetic minerals or by diagenetic and epigenetic 

destruction or addition of magnetic minerals. The types 

and abundances of detrital magnetic minerals in epiclastic sedimentary rocks reflect the relative and absolute 

abundances of these minerals in the source areas. Thus 

the magnetizations of epiclastic sedimentary rocks reside 

mainly in magnetite and titanomagnetite. Although 

titanohematite is more resistant to oxidation and 

reduction in the sedimentary environment than are the 

Mt-U sp minerals, it is insufficiently common to 

contribute significantly to magnetic signals. Even where 

titanohematite is abundant, the distinction between the 

detrital Mt-Usp and Ilm-Ht magnetic grains is not 

important for aeromagnetic interpretations. 

Destruction of detrital magnetic minerals by postdepositional alteration may produce magnetic contrasts 

that are diagnostic for structural features such as faults 

or for facies changes. Such destruction arises from 

oxidative replacement of magnetite by hematite or from 

dissolution of magnetite or its replacement by sulfide 

minerals. Anomalies related to such magnetic contrasts 

may be useful exploration guides for stratabound mineral 

deposits. 

Magnetic anomalies in areas of oil and gas 

potential are of current interest as possible indicators of 

secondary magnetic minerals that are related to 

hydrocarbon seepage. In such settings rock magnetic, 

mineralogic and geochemical studies can contribute to 

making crucial distinctions between primary and 

secondary magnetic minerals and between primary 

(DRM) and secondary (CRM, or other) remane~ce. In 

addition, the identification of the type of any secondary 

magnetic mineral (for example, iron oxide or iron 

sulfide) is important in developing geochemical models 

for seepage-related magnetizations that can be used for 

predicting its occurrence elsewhere. 

Finally, we emphasize again the importance of 

alteration on the magnetization of rocks and hence on 

their aeromagnetic signatures. This conclusion has been 

stated recently by Mayhew and LaBrecque (1987), and it 

bears repeating. On a global scale, the types and degrees 

of metamorphism and hydrothermal activity in both 

continental and oceanic crust exert fundamental, 

widespread controls on crustal magnetic properties. On 

regional and local scales, rock alterations can cause 

magnetic contrasts that may be diagnostic for certain 

mineral and energy habitats. 

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Schnitzer, M., and Skinner, S.I.M., 1965, Organo-metallic 

interactions in soils; 4, Carboxyl and hydroxyl groups in 

organic matter and metal retention: Soil Science, v. 99, 

p. 278-284. 

Schult, A., 1970, Effects of pressure upon the Curie 

temperature of titanomagnetites: Earth Planetary 

Science Letters, v. 10, p. 81-86. 

Scott, J.H., and Daniels, J.J., 1976, Non-radiometric borehole 

geophysical detection of geochemical haloes surrounding 

sedimentary uranium deposits, in Exploration for 

uranium ore deposits: Vienna, International Atomic 

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Scott, J.H., Daniels, J.J., Reynolds, R.L., and Seeley, R.L., 1983; 

Magnetic-susceptibility logging in sedimentary uranium 

deposits: Log Analyst, v. 24, p. 16-21. 

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Shive, P.N., 1989, Can remanent magnetization in the deep 

crust contribute to long wavelength magnetic 

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p. 89-92. 

Shive, P.N., and Fountain, D.M., 1988, Magnetic mineralogy in 

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v. 93, p. 12,177-12,186. 
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the upper kilometer of the marine crust at the Deep Sea 

Drilling Project Hole 504B, Eastern Pacific 

Ocean: Journal of Geophysical Research, v. 91, 

p. 10,337-10,354. 

Speer, JA., 1981, The nature and magnetic expression of 

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America Bulletin, v. 92, p. 603-609. 

Summer, J.S., 1985, Crustal geology of Arizona as interpreted 

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ed., The utility of regional gravity and magnetic anomaly 

maps: Tulsa, Okla., Society of Exploration Geophysicists, 

p. 164-180. 

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Anniversary Volume, p. 829-839. 

Keynote Papers 45 

ABSTRACTS AND SHORT PAPERS 

Worldwide Aeromagnetic Coverage: 

The Need for Data Bases and Map Inventory 

By William J. Hinze2 

As we enter into a new era of technological development in the acquisition, reduction, processing, and 

interpretation of aeromagnetic data, we look forward to 

increasing utilization of magnetics in contributing to 

the solution of a broad spectrum of earthscience-related scientific and societal problems. However, to realize the full potential of the magnetic 

method we must strive to make all nonproprietary 

magnetic anomaly data, particularly digital data, readily 

available to the geoscience community for individualized processing and interpretation. 

A three-pronged program is recommended to 

achieve this goal: (1) Establish a data management system for all new surveys which includes provisions for 

placing the digital data into a central repository, (2) 

continue to update the national magnetic anomaly data 

base with new digital data observations and digitization 

of existing high-quality magnetic observations, and (3) 

prepare a computer-compatible inventory of available 

magnetic anomaly data bases and maps. In the United 

States an aeromagnetic data management system and a 

national magnetic anomaly data base have been initiated, but not on a formal basis. Problems remain with 

2Purdue University, West Lafayette, IN 47907. 

respect to standardized formats, quality control and assessment, reduction procedures, the role of data centers, and the lack of a central repository. The geophysical community, through the International Lithospheric 

Commission, is currently organizing an effort to 

achieve the third goal, which is to prepare a global inventory of available magnetic anomaly data bases and 

maps. 

The need for an aeromagnetic data inventory is 

widely recognized and has been recommended for the 

United States by several working groups. The effort to 

initiate this inventory on an international level recognizes (1) the increasing importance of obtaining not 

only geologic, but also geophysical data, on a global basis as a necessary step to solving fundamental geologic 

problems; · (2) the significant amount of aeromagnetic 

data that are becoming available in digital data bases 

and maps; and (3) the problems inherent in merging 

available international data bases and maps into a single computer-based, annotated inventory. Such an inventory would be available on tapes or disks and also 

in published catalogs which would be updated periodically. Many issues related to the inventory remain to be 

finalized. Recommendations regarding formats, annotation, types of data to be inventoried, and other details 

are solicited as well as suggestions of sources who 

could prepare an inventory of data from regional areas. 

Abstracts and Short Papers 47 
Explanation for Shaded-Relief 

Aeromagnetic Map of Central Minnesota 

By Val W. Chandler3 

Figure 16 is a shaded-relief presentation of about 

311,000 line kilometers of high-resolution aeromagnetic data from northeastern and central Minnesota. 

These data were collected as part of a state-wide survey that is being conducted by the Minnesota Geological Survey with funding from the Legislative Commission on Minnesota Resources. The data were collected 

using a 400-m flight-line spacing and an average terrain 

clearance of 150 m. Along-line sampling is dopplerscaled to intervals of 40-75 m. The aeromagnetic data 

are crucial to bedrock geologic studies and mineral exploration because of the widespread cover of Pleistocene glacial materials. 

The shaded-relief aeromagnetic image (fig. 16) 

reveals many structural and lithologic details in the 

Precambrian bedrock. A generalized geologic map (fig. 

17) of the surveyed area is provided for comparison. 

The northern part of the image in figure 16 corresponds to a 2,750-Ma greenstone-granite terrane of the 

Superior Province; prominent features include the 

broad fold patterns of migmatitic rocks north of lat 48° 

N., high-amplitude and sinuous magnetic highs over 

iron-formation, irregularly banded anomaly patterns 

over granitic batholiths, and sharp northwest-trending 

8Minnesota Geological Sutvey, St. Paul, MN 55114-1057 

48 Geologic Applications of Modern Aeromagnetic Survey 

linear anomalies from a 2,120-Ma swarm of diabase 

dikes. A sharp lineament that extends northeast from 

near lat 45°30' N., long 96°30' W., corresponds to the 

Great Lakes tectonic zone, which is the boundary between the greenstone-granite terrane to the north and 

a high-grade gneiss terrane to the south. Parts of the 

gneiss terrane are older than 3,500 Ma, and the highly 

complex "birds eye" pattern in the southwestern part of 

the image is typical of rocks that have a high metamorphic grade. The east-central and southeastern part of 

the image corresponds to the Early Proterozoic Penokean orogen. The anomaly features associated with 

this orogen include, from north to south, (1) a broad 

area of subdued magnetic signature, which corresponds 

to a thick sequence of graywacke and slate in the 

Animikie basin and associated outliers; (2) northeastto east-trending anomaly patterns, which delineate the 

Penokean fold and thrust belt; and (3) the somewhat 

irregular anomaly expression of a magnetic terrane. 

The extreme eastern and northeastern part of the image corresponds to 1,100-Ma rocks associated with the 

Midcontinent rift system; major anomaly fea~ures include the extremely complex signature of the mafic 

Duluth Complex (Middle Proterozoic) in the extreme 

northeastern part of the image, the strongly banded 

anomaly pattern over deformed lavas of the St. Croix 

horst in the extreme southeastern part of the image, 

and a series of northeast -striking linear anomalies over 

a dike swarm in the east-central part of the image. 
l> 

0" 

~ 

iil 

; 

I» 

:I 

Q. 

tJ) 

=r 

0 

:::1. 

-: ., 

CD 

.. 

ell 

~ 

co 

This 1:2,500,000 scale, shaded relief map is one of the products of the detailed aeromagnetic survey, funded by the Legislative Commission on Minnesota Resources, that began in 1979 and is continuing . The program is 

supervised by Val W. Chandler. 

The apparent relief is generated by computer and shows magnetic variations in the largely buried Precambrian bedrock. It is based on artificial illumination from the northwest at an inclination of 45°. This image, created at the 

Minnesota Planning Information Center under the supervision of John Hoshal, 

represents data acquired along 400-meter (1 /4 mile) line spacing at a terrain 

clearance of 150 meters (500 feet). 

Products of the survey include digital tapes, 1 :24,000-scale contour maps, 

and 1 :250,000-scale color and contour maps. Inquiries should be addressed to 

the Minnesota Geological Survey, 2642 University Ave., St. Paul, Minnesota 

55114. 

Minnesota Geological Survey and 

The Legislative Commission on Minnesota Resources 

1986 

Figure 16. Shaded-relief aeromagnetic map of central Minnesota. Map was published by Minnesota Geological Survey and the Legislative Commission on 

Minnesota Resources in 1986. 
INDEX MAP 

Figure 17. Generalized geologic map of central Minnesota. 

50 Geologic Applications of Modern Aeromagnetic Survey 
A State's Perspective on a National Aeromagnetic Program 

By Don R. Mabey4 

The combined technical staffs of all the State 

geological surveys is approximately equal to that of the 

Geologic Division of the U.S. Geological Survey 

(USGS). The programs of the State surveys vary considerably, but in the public land states of the western 

United States they are similar to those of the Geologic 

Division. A few States have effective aeromagnetic programs that are largely independent of the USGS, but 

most do not have resources to develop such programs. 

Cooperative programs between the States and the 

USGS to obtain, publish, and interpret aeromagnetic 

data have been an important part of the USGS aeromagnetic program and could be an important p~rt of 

any new National Aeromagnetic Program. In develop4Utah Geological and Mineral Survey, Salt Lake City, UT 

84108. 

ing a cooperative program, the states can contribute: 

(1) political support, (2) funding support, (3) facilities 

for making products available and archiving data, and 

(4) material for studies of rock properties. The USGS 

is generally better able to: (1) manage data acquisition, 

(2) develop interpretation methods, (3) apply some interpretation methods, ( 4) conduct research on rock 

properties, and (5) provide leadership in the effective 

use of magnetic data. Both the State surveys and the 

USGS would have extensive uses for the data. These 

uses include hazards studies and general geologic studies as well as the economic geologic applications that 

are most commonly emphasized. Because of the diverse interests and resources of the State surveys, cooperative programs must be developed individually 

with each State. Nevertheless some method for providing coordinated State input to the development of the 

National program so that it serves both State and Federal needs is desirable. 

Abstracts and Short Papers 51 
Some Notes on the Production of Aeromagnetic Surveys 

By MikeS. Reford5 

Aeromagnetic surveys usually involve two stages, 

each executed by a different group. Typically, the flying 

and compilation are accomplished by a contractor. The 

results then go elsewhere for analysis, which consists of 

a preliminary review of contour maps, followed by, perhaps months later, additional processing and interpretation. These notes concern the interface between the 

two groups. 

Obviously, the client wants the best possible survey for the lowest possible cost. Previous speakers have 

noted that good survey aircraft are not equipped overnight and that good survey pilots are not easy to find. 

A low price will be a false economy if the contractor is 

incapable of meeting the desired specifications. This 

brings up the question of how the client checks the survey results to ensure that the specifications have been 

met. There is no substitute for experience. Here l wish 

to praise the work of the inspectors of the Geological 

Survey of Canada, who check the results of the aeromagnetic contractors. But the prime responsibility for 

survey quality must remain with the contractor. 

The two following examples illustrate this responsibility by detailing the surprises that we have encountered at Geoterrex. Figure 18 shows anomaly data that 

were contoured at a 5-gamma interval. In the middle of 

the figure is a linear low axis. The axis is disturbed by 

lines 15 and 17, which have values that are too high by 

10-15 gammas. These lines were flown consecutively. 

Just before they were flown, the magnetometer measurements suddenly dropped by about 7,000 gammas. 

After the drop, the record appeared normal. The operator concluded that a digital bit had been lost in the recordings, which could therefore be corrected by adding 

a constant. This correction was made, but the resultant 

contours shown in figure 18 still appeared to be incorrect. Also, the levelling adjustments for lines 15 and 17 

were much stronger than usual. Further checking convinced us that a bit had indeed been lost, but in a multiplying rather than an adding sense. The data were 

5Geoterrex Limited, Ottawa, Ontario, Canada. 

52 Geologic Applications of Modern Aeromagnetic Survey 

corrected by multiplying the total-field values by 1.16, 

and this gave a smooth axis. It is interesting to note 

that the uncorrected data gave surprisingly good results 

close to the two control lines on either side of the diagram and that the error would have almost vanished if 

the control line spacing were halved. 

The second example shows an unexpected problem with flight-path recording during our first survey 

with the Global Positioning System (GPS) in 1985. The 

positioning system was supplied and operated by a subcontractor. It allowed LORAN C to supplement or replace the GPS data when satellites went below the horizon. Because this positioning system was new and relatively untried, we decided to have an inertial navigation system (INS) as a supplement. The INS was also 

important as a safety factor to guide the aircraft back 

to base if the GPS/LORAN system failed. The survey extended almost 1,200 kms offshore and was largely 

flown at night due to satellite configurations. Figure 19 

shows a comparison of the simultaneous INS and GPS 

positions. The GPS shows oscillations that sometimes 

exceeded ± 100 m from the mean. These' departures, 

which correlated with small altitude changes of the aircraft, were finally explained as errors that resulted 

from an assumption of constant aircraft altitude in the 

position calculation program. This assumption was necessary due to time constraints .. and only created this 

problem in a very specific satellite configuration. The 

result of using these position errors is shown in figure 

20, where International Geomagnetic Reference Field 

(IGRF) removal transforms the magnetic profile from 

trace 1 to trace 2, thereby introducing false anomalies, 

especially on the left end. The profiles using INS and 

final flight paths, traces 3 and 4, do not show these 

false anomalies. Note that without the INS path recordings, these position errors might never have been 

detected. 

Both these problems-the lost bit and the position program error-illustrate "black box" effects. We 

know what the "black box" is meant to produce. Too 

often we may assume that the expected outcome is 

produced when this may not be the case. The contrac-
KILOMETER 

I I I I I I I I I I 

CONTOUR INTERVAL 5 GAMMAS 

Figure 18. Aeromagnetic anomaly map showing a· linear magnetic low axially disturbed along flight paths 15 and 17. 

Contour interval, 5 gammas (nanoteslas). 

tor, the interpreter, and the client must be judiciously 

skeptical. 

The other points I wish to note concern the final 

products of the survey. Contour maps used to be the final product. Now they serve more for preliminary 

checks and interpretation and also as an ideal index for 

further interpretation. As an index, it is most important 

that they be complete and show flight path and fiducials for each line. The real final products now are the 

digital data files, which are created in at least two data 

sets: Line-by-line and grid values. The line-by-line files 

should contain all the fundamental data from the survey, including both the original magnetic data and the 

data which has been compiled and contoured. The 

original magnetic data should be subject only to editing 

of bad values; the compiled data could be subject to 

various corrections and filters. Both sets might be used 

for additional processing, but it seems more likely that 

the grid values will be used to generate additional map 

products. To meet this purpose, the grid values should 

fulfill several requirements. 

(1) Grid values should fit the line-by-line values. 

Isidore Zietz of the U.S. Geological Survey used 

to complain that the digital contours did not correspond with the line data. This complaint was 

rectified with some gridding-contouring routines. 

(2) Grid values should fit the contours. Any fixing of glitches or other problems in the contours 

should involve revisions of the grid values. If the 

fJXing is done instead by a draftsman, the problem will reappear as a nasty surprise in subsequent processing. 

(3) Grid values should fit a satisfactory set of 

contours that reflect the actual geologic trends in 

the survey area. 

Abstracts and Short Papers 53 
112350 112300 

112450 112400 

112200 112150 

t t l 

112100 L-2050/8 

122060 (Flight path plotted 

fromraw INS x,y's) 

L__ No significant __j 

.------altimeter changer \ I 112150 

112252112200 

112100 L-2050/3 122060 (Flight path plotted 

from post-processed 

GPSILORAN x,y's) 112450 

5 KILOMETERS 

I 

NOTE: Apparent speed change 

EXPlANATION 

f Aircraft high 

J Aircraft low 

Figure 19. Contrasting plots of a single flight path showing artificial deviations in aircraft X andY coordinates caused by 

changes in aircraft altitude. Upper plot shows flight path derived from raw Inertial Navigation System (INS) data. Lower plot 

shows same flight path derived from combined Global Positioning System and long-range radio (LORAN) navigation data; 

deviations along this path correlate with aircraft altitudinal changes. 

T 

5 gammas 

j_ 

Figure 20. Contrasting plots of total-field magnetic 

anomalies associated with the flight path shown in figure 19. 

Trace 1 shows anomalies derived by fourth-difference editing 

and low-pass filtering. Trace 2 shows data of trace 1 with 

International Geomagnetic Reference Field (IGRF) removed 

based on post-processed Global Positioning System X and Y 

coordinates. Trace 3 shows data of trace 1 with IGRF 

removed based on inertial navigation system X and Y 

coordinates. Trace 4 shows data of trace 1 with IGRF 

removed based on final flight path. 

54 Geologic Applications of Modern Aeromagnetic Survey 

Figures 21 and 22 show unsatisfactory contours. 

Figure 21 shows contours from two adjacent surveys. 

North of lat 24° S. the contours were hand-drawn, 

whereas south of lat 24° S. they were machine-drawn, 

using one of the older computer programs. The machine contours do not show the continuity of the several sets of dykes, especially those crossed by the flight 

lines at shallow angles. Further processing of these grid 

values would perpetuate the incorrect blobby pattern. 

Figure 22 shows a more recent case. On the left the 

contours show northwest-southeast-trending features, 

one of which consists of short east-west-trending segments. This actually reflects a pair of dykes, and the 

gridding algorithm has simply joined the wrong peaks 

together. By grid ding in a northwesterly direction, this 

problem has been solved, as shown on the right. The 

new local grid values were then used to interpolate values onto the original grid locations, and these interpreted values were fitted into the complete grid archive. 

These two examples demonstrate the need for 

treating computer products with a suspicious mind. 

Just as with the black boxes, we know what the programs are supposed to do, but they do not always perform as expected. 
CONTOUR INTERVAL 10 GAMMAS 

Figure 21. Aeromagnetic anomaly mosaic of two surveys. In the northern one-third of the area shown, contours are 

hand-drawn; in the southern two-thirds of the area shown, contours are machine-drawn. Machine-drawn contours fail to show 

the continuity of some dykes. 

Abstracts and Short Papers 55 
ORIGINAL CONTOURS TREND-ENHANCED CONTOURS 

Figure 22. Contrasting aeromagnetic anomaly maps of a single area derived by gridding in different directions. On the left, 

the north-south gridding algorithm has joined some peaks together incorrectly, thus obscuring the locations of dykes. On 

the right, this problem has been solved by gridding in a northwesterly direction. 

56 Geologic Applications of Modern Aeromagnetic Survey 
Aeromagnetic Surveys 

By John M. Schmunks 

Airborne magnetic surveying has improved dramatically during the past 15 years. Microelectronics, 

optically pumped sensors, and satellite navigation are 

only part of the reason for that improvement. The technical advancement has been so rapid that awareness of 

this advancement and technical specifications that acknowledge this advancement are far from universal. 

This report recommends methods and specifications 

that are appropriate for taking fuller advantage of 

these latest advances in technology than has been 

achieved in the past. 

For 40 years some members of the geophysical 

profession have devoted their energies to the measurement of the Earth's magnetic field. During that time 

the instrumentation and the procedures have been improved to such a degree that it is now possible to 

measure accurately, from an aircraft, anomalies that 

have amplitudes of 1 gamma (nanotesla) or less. In a 

static configuration such as that of a ground-based diurnal station, this resolution of measurement is increased by a factor of ten. 

We at Airmag Surveys, Inc., originally began our 

aeromagnetic work with an analog system that expressed the magnetic value as an ink line at a sensitivity of 60 gammas per inch. Standard airborne production systems now measure fields to a resolution of 

0.001 gamma and digitally record them to 0.01 gamma. 

Surprisingly, this high-sensitivity capability is seldom 

specified as a technical requirement, except by a few 

private companies. In contrast, most other airborne 

data acquisition programs incorporate rigorous procedures for evaluation and acceptance of equipment or 

results, such as the U.S. Geological Survey requirement 

for periodic calibrations of photogrammetric aerial 

cameras. Our prime objective should be to make the 

most accurate measurement possible within the physical and societal constraints that exist at the time of 

measurement. The specifications, in effect, will define 

the constraints. 

8Airm.ag Surveys, Inc., Philadelphia, PA 19114. 

The resolution and quality of aeromagnetic data 

are determined primarily by the following components: 

(1) Sensor, (2) platform, (3) measurement and recording system, ( 4) navigation and positioning system, ( 5) 

operational procedures, ( 6) diurnal measurements, and 

(7) flight crew. Each of these components can be evaluated and judged with respect to their applicability for 

accomplishing the objectives of a given survey. Standard procedures exist, which if applied, will facilitate this 

evaluation. These procedures are a compendium of 

those that our airborne operators believe to be the 

most effective quality control procedures in use today 

throughout North America. 

SENSOR 

Three prime sensors are in common use today: (1) 

The fluxgate sensor, the voltage output of which is used 

to determine the Earth's magnetic field strength; (2) the 

proton precession sensor, the frequency output of which 

is used to determine the Earth's magnetic field strength; 

and (3) the optically pumped alkali-vapor sensor, the 

electron precession frequency output of which is used to 

determine the magnetic field strength. 

The fluxgate sensor is unique in that it can be 

configured to measure the components of the field as 

well as the total-field vector. The proton sensor has the 

advantages of low cost and ease of operation and 

maintenance. The cesium optically pumped sensor is 

preferred over the others for many reasons, among which 

are its high sensitivity (by a factor of 80 to 1), small size, 

continuity of measurement, low maneuver noise, good 

acceptance angle, high signal-to-noise ratio, low thermal 

noise, high gradient tolerance, and compatibility with 

electromagnetic (EM) systems. 

PLATFORM 

The choice of the platform, if made incorrectly, can 

obviate the benefits derived by using a high-sensitivity 

sensor. Some aircraft are so magnetically contaminated 

Abstracts and Short Papers 57 
that only a low-sensitivity, highly filtered sensor can be 

used. A modern airplane contains electromagnets, 

permanent magnets, induced magnets, and eddy currents 

which produce magnetic fields, and by regulation, an 

airplane must constantly communicate by emitting a 

variety of electromagnetic fields. Airplanes are 

mechanically flexible machines by design, especially 

along their wing axes. This flexing of aircraft components 

adds to the complexity of magnetic fields associated with 

devices attached to moving parts, such as rudder posts, 

control cables, and control surfaces. Despite this 

complexity, however, aircraft symmetry in some cases 

permits the use of certain models to simulate the 

complex fields that are generated. Even so, use of these 

models does not guarantee that all aircraft can be used 

for high-sensitivity magnetic surveys. 

An aircraft should not be used for magnetic 

surveying unless its magnetic signature has been measured and recorded and found to be acceptable. First, 

measurements should be made on the ground while 

pulling the aircraft along cardinal headings. Once it is 

decided that the magnetic signature is of such a character 

that a successful compensation seems probable, then a 

test with a temporary installation can be made by flying 

certain patterns, such as a cloverleaf, followed by a figure 

of merit. A clover-leaf pattern will identify the two 

horizontal vectors of both the permanent and the induced 

fields as measured at the point where the sensor is 

mounted. The cloverleaf roll maneuver will provide a 

close approximation of the vertical vectors of the 

permanent and induced fields at the sensor position. The 

values of these six vectors will indicate whether or not the 

aircraft can be compensated and used as a magnetic 

survey aircraft. 

After this procedure, the aircraft should be 

grounded and the demagnetization process started. Nonmagnetic materials should be substituted for magnetic 

materials where permissible, and landing gear, struts, 

engines, mounts, and other parts that might have been 

magnetized should be removed, checked, and 

demagnetized if necessary. Even a tiny staple near the 

sensor, a speaker in the cabin, or a fuel boost pump near 

a wingtip tank can generate signals that seriously 

interfere with the measurements. All aircraft, as they 

come from the factory, make use of the frame and skin as 

the ground return for the electrical equipment. These 

direct currents generate powerful magnetic fields that 

vary, depending upon what loads are in use, and should 

be eliminated. Certain navigation aids, such as rotary 

beacons, that generate magnetic fields should be 

identified and replaced. The H-type antennae should be 

moved or removed if necessary. 

The optimum sensor position is invariably in a 

stinger attached to the empennage of the plane. This 

position places the sensor along the axis of rotation that 

58 Geologic Applications of Modern Aeromagnetic Survey 

is most symmetrical to the normal fields of the aircraft, 

the roll axis. It also permits the sensor to be placed at the 

greatest distance from the most intense aircraft fields. 

The electromagnetic fields and the component 

motion fields can be measured and corrected on the 

ground. After this has been done, there theoretically 

remain only the permanent and induced fields. These can 

be measured by flying cardinal headings over an area of 

flat magnetic gradient and eliminated. 

MEASUREMENT AND 

RECORDING SYSTEM 

A great variety of options exists with respect to the 

measurement and recording phase of aeromagnetic 

surveying. Once the nature of the signals to be measured 

is defined, the instruments and recording system that are 

most suitable for that measurement can be selected. For 

example, the aircraft operated by our company in the 

Gulf of Mexico has three voltages, two frequencies, and 

three digital outputs available from the sensors that must 

be recorded. We prefer to use a high-quality system 

produced commercially by a company that provides 

strong technical support worldwide. 

NAVIGATION AND 

POSITIONING SYSTEM 

All electronic systems are subject to equipment 

failure. Because it is not always obvious that a failure has 

occurred, an error of position can be generated. The 

most striking example of this is the recent deterioration 

of the time standard in one of the satellites. A more 

mundane example is when the Doppler reading reverts to 

a frequency mode, which causes the sampling to be 

time-based instead of distance-based. 

For all high-sensitivity surveys, the pragmatic 

positioning preference is continuous-strip photography 

or high-resolution video combined with the best 

electronic positioning system available for the survey area. 

This hard-copy concept provides a continuous 

permanent record with no ambiguities of position. It's a 

"show me" philosophy that has evolved as a result of 

equipment malfunctions experienced during the last 40 

years. 

At times we have experienced gross errors in 

position that were computed by every one of the 

electronic navigation systems in use today. Without 

photographic backup it would have been impossible to 

determine when the malfunction occurred. The 

photographic location is more accurate than lower order 

navigation systems such as Doppler, inertial, or very low 

frequency (VLF). The photographic location can be as 
accurate as the Range/Range systems depending upon 

the angle of fix and the altimetric information. 

Global Positioning System (GPS) is the preferred 

electronic system. It is international and passive and has 

the most consistent accuracy. However, it is not yet 

available on a 24-hour basis. LORAN is the next best 

choice over areas where maps do not exist, such as the 

Gulf of Mexico. LORAN has a high degree of 

repeatability but must be calibrated for absolute position 

by correlating it to known positions within the area or to 

GPS. Range/Range positioning systems are accurate, but 

they are the most cumbersome type of system to use. 

Inertial, Doppler, and VLF are secondary systems and 

should be used only if no other options exist. 

PROCEDURES 

The procedures that are followed for the dayto-day operation have evolved over the years and have a 

purpose of eliminating any ambiguity with respect to the 

meaning of the data that are collected. Analog and digital 

annotation of the information must be clearly expressed 

so that it can be easily understood by the user of the data. 

Superimposed on the day-to-day procedures is the 

need to be constantly aware of anything that might 

change the magnetic characteristics of the aircraft. 

Replacement parts or components must be carefully 

checked for magnetic characteristics before being 

installed. 

As the plane moves in latitude, the induced 

signature of the plane will change. A heavy landing can 

change the permanent field. It is standard procedure to 

periodically fly the cloverleaf at a project site to confirm 

that the compensation is still effective. It is also standard 

procedure to monitor carefully the condition of the 

complete system by periodically flying a test profile while 

the project is in process. A system that is operating 

correctly will consistently repeat 1-gamma natural 

anomalies. 

The series of test runs will be an important part of 

the data set. A test line should be included with the 

production for the day the line was flown and also sqould 

be part of the set of test profiles that accompany the final 

delivery products. 

DIURNAL MEASUREMENTS 

For ease of operations, it is wise to use exactly the 

same configuration for the ground station as that 

installed on the aircraft. The site location must be free of 

culturally induced magnetic noise. 

CREW 

Regardless of the extent and correctness of the 

effort and investment in the previously described factors, 

it is absolutely necessary to have an experienced, 

dedicated crew. It is our experience that only a select 

number of pilots prove to be effective survey pilots and 

that even fewer pilots prove to be effective magnetometer 

survey pilots, regardless of whether electronic navigation 

is used. 

SUMMARY 

From the preceding discussion it can be seen that 

our profession has reached a stage where we can 

consistently fly high-sensitivity, high-resolution aeromagnetic surveys. This stage has been reached because of 

desire, equipment, experience, and quality control. The 

hardware is available. A computed parameter known as 

the "fourth difference" monitors the instrument noise. 

The clover leaf and the figure of merit monitor the 

system noise. The repeated flights of the test line during 

the project monitor the quality of the data. 

For the moment, this standardization to high 

sensitivity appears sufficient. Very soon, however, the 

next step· must be taken; that is, three-dimensional 

measurements must be made. The magnetic field is a 

vector, which can be specified by its declination and 

inclination angles. With three-dimensional measurements, the location of the cause of the anomaly may be 

estimated more accurately. Experimental flights that 

measure the declination and inclination have been made 

since the 1970's. On the basis of these experiments, it is 

clear that aeromagnetic surveying is ready to become 

three-dimensional. 

It's a great opportunity. 

(EDITOR'S NOTE: Following the workshop, 

author Schmunk forwarded written and illustrative 

material to workshop participants, regarding some 

measurement prectstons achieved (a numerical 

typographical error was discovered by participant Gerald 

G. Connard and afterward acknowledged by Schmunk) 

on ten successive repeat flights during a project. 

Schmunk also emphasized the importance of measuring 

the Earth's natural magnetic field before cultural 

development, including the anticipated increased usage 

of high-temperature superconductors which generate 

strong magnetic fields, adds further to the abundance of 

artificial magnetic contamination. We thank Schmunk 

for these additional contributions.] 

Abstracts and Short Papers 59 
A View from Afar: Comparison of Satellite and 

Aeromagnetic Anomalies 

By Patrick T. Taylor1 

Aeromagnetic measurements are useful in the 

study of the Earth's magnetic anomaly field from satellite altitude in three respects: (1) To simulate the results ex_Pected at higher altitude by upward continuing 

large data sets, (2) to verify that the satellite field is 

truly recording the crustal anomaly field (this was especially important when Magsat data were relatively 

new), and (3) to aid in the geologic interpretation of 

satellite data. The first two are particularly important. 

In order to use aeromagnetic data to verify that 

Magsat anomalies are recording a true crustal field signature, we need large digital data sets. These data must 

already be in digital form because the cost of digitizing 

large anomaly field maps is prohibitive. If not already 

in a gridded format these data must be put on some 

form of grid. The upward-continued data set is qualitatively compared using standard algorithms with the satellite data for verification. However, we have found 

that long-wavelength magnetic anomalies can represent 

a problem. These long-wavelength anomalies, which 

are apparent in the satellite data, may not be revealed 

in the composite aeromagnetic maps. This problem 

could be due to the difficulty in combining diverse field 

surveys into one map, especially if the composite data 

were acquired over several years. Secular variation is 

believed to be the cause of this problem. Longwavelength anomalies, however, may be erroneously introduced into the composite aeromagnetic anomaly 

map by an incorrect representation of the zero field 

TNational Aeronautics and Space Administration/Goddard 

Space Flight Center, Greenbelt, MD 20771. 

60 Geologic Applications of Modern Aeromagnetic Survey 

level. This bias will be upward continued as a large 

magnetic anomaly. 

A proposed NASA mission, the Geopotential Research Mission (GRM), is being designed to operate at 

an altitude of 160 km, which is an altitude where magnetic field measurements have not previously been 

made. In order to evaluate geologically the results from 

this proposed mission, we must use aeromagnetic data 

to simulate the anomaly field at 160 km. This process of 

upward continuation is the same as anomaly verification except that these data are continued to GRM, 

rather than to Magsat altitude. An alternate method of 

producing a synthetic or theoretical anomaly field at 

any altitude is to produce this field from idealized geometric bodies. This procedure permits a quantitative 

comparison with fields from various satellite altitudes. 

Aeromagnetic and satellite magnetic data may be 

used together to make geological interpretations. Outstanding problems in the interpretation of satellite data 

are: (1) Determination of crustal or subcrustal depth to 

the source(s) of long-wavelength anomalies, (2) the 

lack of an observed anomaly signature over the 

continental-oceanic transition zone, and (3) development of techniques for removing external fields from 

the anomaly data. 

Both Magsat and GRM ~ere designed to record 

the components of the vector magnetic field as well as 

the magnitude of the scalar field. Little, if any, use of 

these components has been made in the study of geological problems. Techniques developed from groundbased or aeromagnetic data could be applied to these 

satellite results. In addition, the magnetic components 

are useful in selecting quiet data, particularly in the 

disturbed polar regions. 
Dynamic Global Positioning System Navigation in an 

Airborne Geophysical Program 

By George E. Williams and William F. Hanna 

Navigation requirements for future aerogeophysical surveys depend upon both the degree to which the 

geophysical sensors are sensitive to position and the degree to which a given airborne measurement must be 

correlated to a point on the Earth's surface (land, ice, 

or water) for the problem at hand. It is well known 

that some of the most difficult problems of current airborne surveying involve flight -path recovery, especially 

over regions that lack readily identifiable cultural features or highly distinctive topographic features. To 

compound the problem, future airborne surveying is 

likely to incorporate an increasingly larger number of 

diverse sensors which have greater sensitivity for measuring quantities that vary with position and which will 

be conducted along lines spaced at increasingly tighter 

intervals. 

Airborne navigation that meets most of the anticipated requirements for dynamic positioning can be 

achieved by use of the Global Positioning System 

(GPS), which is a constellation of Earth-orbiting satellites operated by the U.S. Department of Defense. To 

quote from a report of the Special Interagency Task 

Group on Airborne Geoscience (1989, p. 57), "The advent of navigation by the Global Positioning System has 

reduced positional errors-usually the dominant source 

of errors on aeromagnetic maps-by factors as great as 

10. Thus, new navigational technology has rendered 

most existing aeromagnetic maps obsolete." Although 

the second statement of this quotation should not be 

construed to mean that existing aeromagnetic anomaly 

maps will become useless, it nevertheless serves as a 

reminder that limitations of navigation severely degrade 

the accuracy of many older maps. 

At present, GPS consists of seven Block I satellites of a prototype constellation; they have been joined 

by the first of the Block II satellites of the operational 

constellation, the latter having been launched in February 1989. The operational constellation, which is scheduled to be in place by late 1992, consists of 21 satellites 

(and three in-orbit spares), four of which are in each of 

six orbital planes, which permits at least fo11:r satellites 

to be in view from anywhere on Earth 24 hours per 

day. Each satellite completes its nearly circular orbit in 

12 hours at about 22,000 km above the Earth's surface. 

These satellites transmit signals on two L-band frequencies. The signals are modulated with a binary code 

that contains precise timing information and predicted 

orbital parameters which define the satellite's position 

in an Earth-centered reference frame. More precise orbital parameters are available within about 2 weeks after epoch from the National Geodetic Survey. 

In principle, when at least four satellites are in 

view simultaneously (assuming favorable geometry), a 

four-channel GPS receiver may determine by pseudoranging its three-dimensional point position to an accuracy of about 15 m, using the Precise Positioning 

Source (PPS) signals, or about 50-100 m, using the 

Standard Positioning Source (SPS) signals. By using 

two receivers in a differential mode, the position of one 

receiver relative to the other may be determined to an 

accuracy of less than 5 m, using the SPS. This improved accuracy results from cancellation of commonmode errors tied to satellite position and clock offsets 

by differencing the received transmissions. This use of 

the SPS in a differential mode is considered important 

because the PPS may not be readily available for future 

real-time civilian use, depending upon National security 

policy, which has yet to be established. New Block II 

satellites will transmit the SPS in the clear, but the U.S. 

Department of Defense has reserved the right to encrypt the PPS. A bureaucratic procedure is being implemented to enable government agencies to obtain 

equipment capable of tracking the PPS and thereby utilizing the full accuracy of the GPS. 

Dynamic positioning using GPS incorporates the 

differential mode of operation in one of two highly 

contrasting ways: (1) Differential pseudoranging, in 

which the position of a mobile receiver is continuously 

determined relative to a stationary reference receiver 

to an accuracy of about 5-10 m; and (2) kinematic 

GPS positioning, in which two receivers measure the 

change in phase of the carrier waves between succeedAbstracts and Short Papers 61 
ing measurement epochs to find the corresponding 

change in position of the mobile receiver relative to the 

stationary receiver to an extraordinary accuracy of less 

than 10 em. Although less accurate, differential pseudoranging is more reliable than the kinematic technique, in that momentary loss of signal associated with 

atmospheric disturbances or aircraft maneuvering (aircraft wing briefly passes between receiver and satellite, 

thereby blocking receipt of signal) results only in a very 

small loss of positioning data. In contrast, the much 

more accurate kinematic technique is susceptible to 

"cycle slips" which can render all succeeding data useless for position determination. However, despite this 

shortcoming, the kinematic technique is expected to be 

sufficiently reliable for some applications by virtue of 

the current generation of receivers which track more 

than the minimum of four satellites, improved software 

for processing data, and utilization of careful operational procedures. 

In order to alleviate the "cycle slip" problem of 

kinematic surveying and to provide maximum reliabili~, GPS systems will be integrated with inertial navigation systems (INS). A relatively simple commercialquality INS could assist a phase-tracking mobile GPS 

receiver to patch "cycle slips." 

Although current technologies already provide 

capab~lities for GPS-based navigation, improved technologies are under development or are being tested. 

Examples of these developments include improvement 

of software for speeding up the editing and processing 

o~ navigation data; integration of INS or altimetry data 

~th GPS navigation data; optimization of autopilot design .and control for the GPS navigation application; reduction of systematic errors in phase measurements 

multip~th problems (multiple signals are transmitted b; 

re~ectt~n to the GPS receiver), and imaging problems 

(distortiOn of the antenna's phase response); mitigation 

of the effects of ionospheric disturbances and magnetic 

storms which tend to increase the phase measurement 

n?ise and ~use frequent losses of lock in polar regions; and Improvement of knowledge about various 

sources that contribute to the error budget. Attention 

should also be focused on ensuring that data sets acquired using GPS be as compatible as possible with ex62 Geologic Applications of Modern Aeromagnetic Survey 

isting data sets acquired using older navigation technologies. 

Modern GPS systems may be used in small, as 

well as large, aircraft because they weigh no more than 

7 kg, consume only 8-15 watts, and can function all day 

from a small 12-volt battery. GPS data can be conveniently recorded and stored in flight using conventional 

laptop computers that have, for example, two flexible 

disk drives. Fortunately, aircraft speed does not adversely affect positioning accuracy. In principle, the 

precision of the carrier phase measurement made by a 

receiver degrades when the receiver is in motion because it is necessary to increase the tracking-loop bandwidth to ensure that the receiver maintains lock on the 

satellites. In practice, however, this precision degradation is negligibly small compared to other sources of 

error or noise for speeds of as much as 200 knots. 

The U.S. Geological Survey (USGS) currently 

owns and operates two TI4100 four-channel dualfrequency receivers and three Wild/Magnavox 101 receivers; one of the latter has been converted from 

single-frequency to dual-frequency capability. In addition, the USGS has purchased nine 8- to 12-channel receivers that are capable of utilizing data simultaneously 

acquired from as many as eight satellites. As these systems are implemented, the USGS will have the necessary hardware and software for making kinematic surveys on a production basis. These systems are expected 

to be available for some applications testing. The 

USGS is also supporting the concept of integrating a 

phase-tracking GPS receiver with a commercial-quality, 

off-the-shelf INS to provide a robust, reliable kinematic 

GPS system for map accuracy testing. This system 

would also provide information on sensor orientation 

and would offer added security for aerial surveys of 

large and remote areas, such as Greenland, where 

resurveying lines of lost data would not be economically feasible. 

REFERENCE CITED 

Special Interagency Task Group on Airborne Geoscience, 

1989, Airborne geoscience-The next decade: Greenbelt, Md., National Aeronautics and Space 

Administration, 85 p. 
Some Historical Notes on Early Magnetic Surveying 

In the U.S. Geological Survey 

By William F. Hanna 

INTRODUCTION 

The world's first full-scale airborne geophysical 

survey was conducted in 1945 by the U.S. Geological 

Survey (USGS) and the U.S. Navy over the Petroleum 

Reserve No.4 in Alaska. In 1947 the USGS introduced 

the airborne magnetometer to Canada where it was 

inspected by officials of the Canadian Department of 

Mines and Resources and the Canadian National 

Research Council (Morley, 1969). As a consequence of 

that visit, the National Research Council purchased three 

U.S. Navy airborne magnetometers, which were 

converted for survey application and subsequently used 

for 15 years by the Geological Survey of Canada (GSC). 

As expressed by Peter Hood in his keynote report 

contained in this Workshop Proceedings Volume, the 

GSC has successfully conducted during a period of more 

than 40 years a program of aerogeophysical surveying 

unsurpassed in its combination of line miles flown and 

quality of data by any organization in the world. The GSC 

program should serve in many respects as a model to be 

emulated by the United States and other nations. 

EARLY DISCOVERIES IN MAGNETISM 

The history of magnetic prospecting is Qirectly tied 

to the development of the science of terrestrial 

magnetism. Eight pre-17th-century discoveries in 

magnetism are especially noteworthy: (1) Mutual 

attraction of lodestones (natural magnets composed of 

iron oxides) at various distances from one another, in the 

6th century B.C. by Thales of Miletus, as later reported 

by Aristotle and other Greek philosophers; (2) ability of 

lodestone to magnetize iron over a period of time, as 

reported by the Roman philosopher Lucretius Carus in 

the 1st century B.C.; (3) bipolar nature of lodestones, 

about the 1st century A.D., as reported by the Roman 

savant Pliny the Elder (Gainus Plinius Secundus) in his 

voluminous "Natural History;" (4) directive property of 

lodestone or magnetized iron (and therefore the 

directional property of the Earth's magnetic field), about 

A.D. 83 in China, which marked the birth of geomagnetism; (5) magnetic-needle compass and 

declination of the compass needle, reported by Shen Kua 

in China in 1088; ( 6) spatial variation of magnetic 

declination witnessed by Christoforo Colon (Christopher 

Columbus) in 1492; (7) magnetic inclination, known 

earlier to the Chinese but carefully observed by Georg 

Hartmann of Nuremberg in 1544 and measured by 

Robert Norman of London, prior to his publication of 

1581; and (8) representation of the Earth as a great 

magnet, and the dependency of magnetism on 

temperature, both observed by William Gilbert in his 

monumental "De Magnete" of 1600, the world's first 

treatise on physical science based on the experimental 

method. These historical events, and others, are 

chronicled in Benjamin (1898), Crichton Mitchell (1932, 

1937, 1939), Stoner, (1934), Adams (1938), Johnston 

and others (1939), Heiland (1939b ), Chapman and Bartels (1940), Carlsborg (1963), Needham (1962), Hine, 

(1968), McConnell (1980), and Malin (1987). 

In his "De Re Metallica" of 1556, translated in 

1912 by Herbert and Lovit Hoover prior to Herbert's 

term as U.S. President, Georgi us Agricola was the 

earliest to describe the use of the magnet compass in 

mine surveying (Dibner, 1958; Adams, 1938). However, 

this use of the compass was restricted to geographic 

surveying and did not extend to prospecting for magnetic 

ores. The first document referring to magnetic 

prospecting is found in correspondence from the State 

Chancellor in Sweden, Axel Oxenstjema, dated around 

1630 (Lundberg, 1929). The instrument used was the 

sundial compass, or mariner's declination compass. By 

1640 Daniel Tilas had successfully used it to locate 

Swedish magnetic iron ores by reading anomalies of the 

declination relative to the normal value. As early as the 

18th century this magnetic method of prospecting came 

from Sweden to the United States where a great many 

orebodies were discovered by this technique in New 

Jersey and New York. The sundial compass was 

succeeded by the invention of the miner's compass 

Abstracts and Short Papers 63 
around the end of the 18th century, whereby anomalies in 

inclination of the magnetic field were observed. By the 

middle of the 19th century, the mathematical 

formulation of fields associated with magnetized bodies 

made obvious the need to measure the intensity of 

anomalous fields. Instruments for measuring magnetic 

intensity were developed by Thalen and Tiberg in the 

early 1880's (Lundberg, 1929). At this point we turn our 

attention to early magnetic observations made by 

geologist explorers in government-sponsored surveys, 

neglecting the post-16th-century geomagnetic 

discoveries made by the mathematical genius, Carl 

Friedrich Gauss of Germany, and other European 

scientists, as well as lesser known discoveries made by 

Benjamin Franklin in the United States (Pomerantz, 

1986). 

EARLY GOVERNMENT MAGNETIC 

OBSERVATIONS 

Although Federal government agencies, such as 

the U.S. Army, sponsored several areally extensive 

expeditions during the first half of the 19th century 

(Nolan and Rabbitt, 1982), only the Long Expedition of 

1819 to the Rocky Mountains included carefully 

recorded magnetic measurements using a compass and 

dip circle (Good, 1985; Goetzmann, 1959). The only 

significant State-sponsored surveys to include magnetic 

measurements during this period were conducted by 

John Locke (1844) in Ohio and by Ebenezer Emmons 

and Albert Hopkins in New York (Silliman, 1839). 

Emmons and Hopkins recognized that magnetism 

"might furnish a valuable criterion by which to judge of 

the proximity of iron, of the direction of the bed or vein, 

and perhaps its extent" (Good, 1985, p. 521). 

The first Federal agency responsibility in geomagnetism was assumed by the U.S. Coast Survey, 

founded in 1807 as the "Survey of the Coast" and 

renamed the "U.S. Coast and Geodetic Survey" 

(USCGS) by Congressional correspondence in 1878. 

Geomagnetic work heightened in 1843 when Alexander 

Dallas Bache became Superintendent of the Coast 

Survey (American Philosophical Society, 1941). Bache 

had set up the first American magnetic observatory in the 

garden of his house in Philadelphia in 1830, and as a 

check on the magnetic influence of geographic longitude, 

had set up another observatory at Girard College in 1840 

(Faul and Paul, 1983). He had also aligned his magnetic 

observatory in Philadelphia with a British plan promoted 

by Sir Edward Sabine (Good, 1985; Bloxham and Barraclough, 1985) who, like Alexander von Humboldt of 

Germany (Malin, 1987; Wiederkehr, 1985), had been 

convinced by his fellow scientist Gauss of the scientific 

value of worldwide magnetic coverage (Green, 1984). 

64 Geologic Applications of Modern Aeromagnetic Survey 

According to Paul and Paul (1983, p. 191), Bache 

"directed the Survey until his health failed 21 years later, 

and in that time it became the greatest scientific agency in 

Washington." The geomagnetic program of Bache 

continued at a steady pace until H.S. Prichett became 

superintendent in the late nineties, after having worked 

at several American observatories. Prichette turned the 

USCGS even more toward terrestrial magnetism, having 

set a goal of making one magnetic observation at every 

county seat in the United States (Manning, 1988). The 

continuation of geomagnetic work by the USCGS's 

Division of Terrestrial Magnetism, organized in 1899, 

into the 20th century is summarized in Wraight and 

Roberts (1957). The relationship to the USCGS of 

military agencies, which were also in need of magnetic 

measurements, such as the U.S. Army Corps of 

Engineers and U.S. Navy Naval Hydrographic Office, is 

addressed in Lasby (1966) and in the references of 

Manning (1988). In 1973 virtually all of the geomagnetic 

observatory program of the USCGS was transferred to 

the USGS, although essential geodetic and dataarchiving components remain within the National 

Oceanic and Atmospheric Administration. 

EARLY MAGNETIC OBSERVATIONS BY 

THE USGS AND RELATED AGENCIES 

Returning to consideration of the latter half of the 

19th century, we note that Clarence King, 2 years before 

he was to become the first Director of the USGS, 

published his report of the geological exploration of the 

fortieth parallel in 1877. In this report, S.F. Emmons 

noted that at Anita Peak of the Elkhead Mountains, "The 

rock of the peak has a strong influence upon the 

magnetic needle, which, at the summit, points east 

instead of north" (Hague and Emmons, 1877, p. 179). 

The black basalt to which Emmons referred undoubtedly 

had been remagnetized by lightning, a magnetic phenomenon that had been obserVed 80 years previously by von 

Humboldt (1797) on mountain peaks in Europe. After 

becoming Director of the USGS in 1879, King 

immediately acknowledged the value of making magnetic 

measurements when he stated in his first annual report 

"In this department the work should not stop with an 

exhibition of ores already discovered or outcropping 

upon the surface, but the iron areas should be constantly 

enlarged by the careful working out of the subterranean 

bodies by magnetometrical and stratigraphical methods 

known to geology" (King, 1880, p. 77). This was the only 

reference to what is now considered to be geophysical 

work. 

Geophysical work described in the second and 

fourth USGS annual reports by new Director J.W. Powell (1882, 1884) was limited to the research of Carl Barus 
who invented the nonpolarizing electrode in 1880 and 

successfully used it in measurements of ground potential 

to map a hidden extension of the Comstock lode (Sweet, 

1972, p. 248). Barus and his USGS colleague George F. 

Becker (Rabbitt, 1980) are generally credited with 

establishing that the best method of electrical 

prospecting for ore deposits is to determine the positions 

of the equipotential surface traces on the Earth's surface; 

the superiority of this technique was independently 

confirmed in 1912 and 1913 in studies by Frenchman 

Marcel Schlumberger and the Geological Survey of 

Sweden (Bergsbyran, 1922). With regard to the magnetic 

technique, we note that Becker and Barus later made 

magnetic studies of iron and steel in efforts to distinguish 

the physical properties of these metals. More notably, 

Becker published in 1909 a USGS bulletin on the 

relations between local magnetic disturbances and the 

genesis of petroleum (Becker, 1909). G.F. Becker was 

called "the father of geophysics in the Geological Survey" 

in a 1953 address given by USGS Director W.E. Wrather 

(King, 1953, p. 5). 

Also before the turn of the century, USGS 

geologists who were mapping in Michigan's iron districts 

observed the magnetic attractions of magnetite and other 

iron ores as well as host magnetite-rich gabbro; their 

observations were similar to those made in other States, 

such as New Jersey (Smock, 1875; Farrington, .1852), 

Ohio (Locke, 1840), Wisconsin (Wright, 1880; Irving, 

1880), and California and Arizona (Hanks 1890). 

According to J.R. Henderson (1953, p. 22), Thomas Alva 

Edison made systematic dip-needle surveys in the 1890's 

over New Jersey iron ore deposits. Maps showing some 

results of these measurements are in Bayley (1910). The 

Michigan work of Irving and VanHise (1892), VanHise 

and Bayley (1897), and Smyth (1899), published as 

USGS monographs, indicates that they recognized the 

difficulty of distinguishing some magnetic rocks from 

others, the value of plotting observations before 

returning from the field, and the economy of making 

magnetic measurements before money is spent in 

underground work. Smyth (1899) was the first to derive 

mathematically the anomalous effects of these iron orebodies. 

In the early 20th century, the USGS spawned from 

its organization two important units which later 

conducted much work involving magnetic techniques. 

First, in 1907, the USGS physical laboratory was, in 

effect, reestablished in a recently created Geophysical 

Laboratory of the Carnegie Institution of Washington 

(Nolan and Rabbitt, 1982); the Department of 

Terrestrial Magnetism of the Carnegie Institution had 

been established in 1902 (Good, 1985). Second, in 1910, 

a large staff of USGS mining technologists was split off to 

form the U.S. Bureau of Mines (USBM) (Rabbitt and 

Rabbitt, 1954). From 1910 to 1927 very little magnetic 

exploration work was conducted in either the USGS or 

the USBM; most activity in magnetism was restricted to 

continuing work at the Carnegie Institution and at the 

USCGS. The Carnegie Institution during this period 

used for ocean surveys a specially commissioned nonmagnetic sailing ship, the Carnegie, made of wood with 

bronze fittings (Malin, 1987). This ship was destroyed by 

fire in 1929 while refueling in harbor. Also during this 

period, magnetic investigations of Broderick (1918) on 

the Duluth gabbro incorporated the previously noted 

mathematical work of Smyth (1899) as well as some 

experimental corroborations of Hotchkiss (1915). The 

latter experimentalist, W.O. Hotchkiss, designed the 

famous "Hotchkiss superdip" needle-suspension magnetometer (Hotchkiss, 1923; Stearn, 1932), which together 

with Adolf Schmidt's field balances (Heiland, 1926, 

1929b ), constituted the ground magnetic instruments of 

choice in the USBM and the USGS through the 1950's 

(Dougherty and Fitzhugh, 1946; Hawkes and Balsley, 

1946; James, 1948). Also within the 1910--1927 quiet 

period, F.L. Ransome, Chief of the USGS Metals Section 

and president of the Washington Academy of Sciences 

(Rabbitt, 1986), openly complained, "Research in geophysics was at one time a recognized function of the 

United States Geological Survey, but since the founding 

of the Geophysical Laboratory of the Carnegie 

Institution of Washington this field has been left almost 

entirely to that splendid organization, which is unhampered by some of the unfortunate restrictions of a 

Government bureau. Under these particular and unusual 

conditions this course may have been wise, although it 

does not negat[iv]e this conclusion that, in general, 

investigation[s] in geophysics are logically and properly a 

function of a national geological survey" (Ransome, 

1921, p. 276--277). 

In 1927 exploration geophysics in the USBM was 

stimulated by the arrival of A.S. Eve and D.A. Keys from 

McGill University in Montreal, who almost concurrently 

published as a USBM Technical Paper a 26-page account 

on geophysical methods of prospecting (Eve and Keys, 

1927). The following year a Section of Geophysics was 

established in the USBM. Until then and shortly 

thereafter, the few references on geophysical prospecting 

best known to USBM personnel were journal articles of 

Stearn (1929) and Slichter (1929); a manual of M.C. 

Alexanian (Ayvazoglou, 1931); and textbooks of Eugene 

Haanel (1904), Richard Ambronn (1928), C.A. Heiland 

(1929a), and Alfred Nippoldt (1930), the latter 

translated in-house by Wladimir Ayvazoglou. Ayvazoglou and colleague V.L. Skitsky regularly published 

"Geophysical Abstracts" for the USBM, a function that 

was later coordinated in the USGS by current historian 

Mary C. Rabbitt, who at about that time also served as 

Assistant Chief of the Branch of Geophysics (Kronstedt, 

1957). The 1927 work of Eve and Keys was expanded into 

Abstracts and Short Papers 65 
a 296-page book in 1933 (Eve and Keys, 1933}, which 

much later evolved into the 860-page book of Telford and 

others {1976); the last included Keys as a coauthor. The 

USCGS classical text on geomagnetism (Bauer, 1902} 

and its many successor versions were also considered 

essential background material for USBM personnel. 

Meanwhile, during the period 1925-1928, large oil 

companies began to use magnetometers extensively; use 

of this equipment by these companies significantly 

declined in the 1930's (Sweet, 1972). Costs of magnetic 

surveying, according to a 1930 survey of oil companies by 

the Colorado School of Mines (Wantland, 1930), 

averaged $1 per station in California and Texas, $2-3 per 

station in the wetlands of Louisana, and as much as $11 

per station in inaccessible parts of western Canada. 

According to Sweet {1972, p. 243-244), "Even though the 

earth magnetometer was abandoned by most of the 

major oil companies when the Depression hit, small oil 

companies and individuals continued its use, mostly 

because of the low economic cost of such surveys. In the 

year 1935 there was a single magnetometer working in 

each of the following states, Michigan, Utah, Mississippi, 

Alabama and Florida. Three magnetic parties were 

active in Colorado, four in the Permian Basin and three 

in South Texas. To many, South Texas was judged to be 

the most promising area, because oil fields do occur in 

connection with basaltic plugs." Although data were 

generally proprietary, significant journal articles were 

published, typical among which were those of Barrett 

{1931) on magnetic disturbances caused by buried 

casing; Jenny (1931} on magnetic vector studies of 

geologic structure in oil States; Van Weelden {1933) on 

magnetic anomalies in oilfields; Dick {1936) on the value 

of resurveying areas using newer magnetometer 

technology; and Evjen {1936) on the place of the vertical 

gradient of gravity, which is also applicable to magnetics. 

Applications of magnetic prospecting to oil-bearing terranes were summarized shortly thereafter by Jenny 

{1938) and Heiland (1939a) in the same comprehensive 

way that traditional applications to metal mining were 

presented by Marquardt (1939). Gulf Research and 

Development Company was one large company that 

continued its use of the magnetometer in the 1930's, 

which led later to Gulfs development of an airborne 

magnetometer. Airborne magnetic observations, 

according to Dobrin {1960}, had already been made by 

Hans Lundberg from a captive balloon above Sweden's 

Kiruna orebody in 1921 (Eve, 1932) and by A.A. 

Logachev from an aircraft above Russia's Kursk orebody 

in 1936 (Logachev, 1946). One of the largest and richest 

ore deposits associated with the Kiruna feature had been 

discovered in 1918 by a prospector's wife who noticed 

while picking berries that her miner's compass, which she 

always carried with her, was pointing vertically (Lundberg, 1929). CA. Heiland had also reported on the 

66 Geologic Applications of Modern Aeromagnetic Survey 

advantages of geophysical mapping from the air in 1935 

(Heiland, 1935). At the USBM during this period, 

important in-house publications by Stratton and Joyce 

{1932) and Joyce (1937) focused on magnetic studies of 

iron deposits and on prospecting with the magnetometer. 

In 1936 the Geophysics Section of the USBM was 

transferred to the USGS, in conformance with a 

recommendation made by the Science Advisory Board in 

1934. This recommendation stated that the geophysics 

program, which had been developed in the USBM, "has 

to do with the finding of ore, which is a function of the 

Survey, and not with extraction and technology of ores, 

which are primary functions of the Bureau of Mines" 

(Rabbitt, 1986, p. 343). Near the end of the period 

1936-1942, prior to the retransfer of the geophysics 

program from the USGS to the USBM, the Gulf 

Research and Development Company made significant 

progress toward furthering the development of 

saturation type magnetometers, also called fluxgate or 

fluxvalve magnetometers, which are devoid of movable 

parts subject to gravitational and vehicular accelerations. 

Also about this time a set of scales for evaluating 

components of magnetic attraction was published by the 

Geophysics Section of the USGS (U.S. Geological 

Survey, 1940) and comprehensive textbooks on 

exploration geophysics, including significant sections on 

magnetic techniques, were published by Heiland (1940), 

Jakosky (1940}, and Nettleton (1940). 

BIRTH OF THE AIRBORNE 

MAGNETOMETER 

At Gulf Research and Development Company, 

Victor V. Vacquier is credited with developing, by 1940, 

the first magnetometer element suitable for airborne 

geophysical prospecting (Muffly, 1946). However, 

according to Sweet (1972), two persons were largely 

responsible for development of this airborne magnetometer: J.D.C. Hare, a physicist who held a patent on 

seismic sources and later patents on radiation detectors 

and downhole apparatuses; and Vacquier, an electrical 

engineer trained also in physics, who worked at Gulf 

from 1930 to 1941, when he went to Columbia University 

to work in the Division of War Research. Sweet (1972, p. 

244) stated, "Dr. J.D.C. Hare invented the three element 

magnetometer and Victor Vacquier signed Hare's patent 

application thereto," and referred to this magnetometer, 

initially used in a gyrostabilized mode for submarine 

detection, as "the Hare element and the associated 

Vacquier electrical circuits." An added source of possible 

confusion is that Vacquier, prior to World War II, built 

an absolute magnetometer of new design for the 

determination of declination, vertical intensity, and 

horizontal intensity-not incorporating fluxgate technol-
ogy-at the request of the USCGS (Vacquier, 1945). In 

1939 Vacquier obtained a patent for an "apparatus for 

and method of measuring the terrestrial field" (J akosky, 

1950, p. 245). Supporting the account of Gary Muffly 

(1946), who was development engineer at Gulf, R.D. 

Wyckoff (1948), chief of the Geophysical Research and 

Development Division at Gulf, also credited Vacquier 

only as the one who developed the magnetometer 

element and also as one who did pioneering work on the 

first types of orienting apparatuses for the instrument. 

Wyckoff (1948, p. 183-184) stated, "V. Vacquier 

investigated the properties of iron-cored devices which 

had long been used in certain applications where great 

sensitivity was not required. * * * By 1941, however, he 

had succeeded in developing an element of the flux-gate 

type having a very high sensitivity and a physical form 

amenable to the airborne application." Abstracts of 

Vacquier's 1946 patents for an "apparatus for 

responding to magnetic fields" (Skitsky, 1947, p. 50-51), 

for which 15 claims were allowed, clearly reflect his 

achievements on iron-cored devices and associated 

electronic circuits, fundamental to the Magnetic 

Airborne Detector (MAD) used for submarine detection 

during World War II. Although Sweet (1972, p. 244) 

stated that the need for an airborne magnetometer had 

come originally from the Army because of its interest in 

exploding enemy land mines, Balsley (1946b, p. 4) 

indicated that the original U.S. Navy magnetometer was 

developed from an idea submitted to the Navy in World 

War I by the Department of Terrestrial Magnetism, 

which proposed the magnetic detection of submarines by 

aircraft. 

According to Knoerr (1946), airborne magnetometers were developed jointly by: 

(1) The Gulf Research and Development 

Company, initially as part of their research and 

development program and later under contract to 

the National Defense Research Council (NDRC). 

See, for example, Gulf Research and Development 

Company (1943) for a summary of MAD magnetometers as well as a vertically stabilized 

instrument built to specifications of the Sperry 

Gyroscope Company, and Muffly (1946) and 

Wyckoff (1948) for the history of magnetometer 

development at Gulf. 

(2) The Columbia University Division of War 

Research, through its Airborne Instruments 

Laboratory, also under contract to the NDRC. See, 

for example, Columbia University Airborne 

Instruments Laboratory (1943, 1944) for 

descriptions of mechanical and electronic systems 

used to stabilize MAD's, and Vacquier and Tolles 

(1943) and Tolles and Vacquier (1944) for 

methods of magnetically compensating an aircraft 

for wingtip installation of a MAD. 

(3) The U.S. Naval Ordnance Laboratory and the 

Bell Telephone Laboratories, under the 

sponsorship of the U.S. Navy's Bureau of 

Ordnance and Aeronautics. See, for example, U.S. 

Naval Ordnance Laboratory (1941a, b) for 

descriptions of diverse fluxgate magnetometers 

and the General Electric Company's magnetometer for measuring a ship's magnetic field, Bell 

Telephone Laboratories (1943) for a description of 

a MAD orienting system, and Jensen (1945) for a 

comprehensive analysis of the AN/ASQ-3A 

model. 

The USGS first became involved with the airborne 

magnetometer in late 1942 when Herbert E. Hawkes was 

consulted about interpreting the effects of rock masses 

on the magnetic detector developed by the Naval 

Ordnance Laboratory and Bell Telephone Laboratories. 

Hawkes recognized that the instrument had great 

potential in the Government's exhaustive search for 

strategic minerals (Knoerr, 1946). In late 1943 James R. 

Balsley, Jr., of the USGS initiated discussions with the 

U.S. Naval Ordnance Laboratory on modifying the magnetometer for use in mapping extensive areas. The 

equipment was made available to the USGS through the 

U.S. Navy's Bureau of Aeronautics, and a project was 

established in the U.S. Naval Ordnance Laboratory and 

Bell Telephone Laboratories for instrument modification 

according to requirements of the USGS geophysicists. 

The modified magnetometer was installed in a singleengine biplane furnished by Aero Service Corporation 

who also supplied a pilot experienced in low-altitude 

flying. In April 1944 a test flight was made successfully 

along three traverses at different altitudes along a line 

near Boyertown, Pennsylvania, which had been 

previously surveyed by the USGS using a ground-based 

magnetometer. These flights, which came within a few 

miles of finding a major magnetite deposit (the Grace 

Mine) later discovered by Aero Service (John R. Henderson, written commun., 1975), were followed by other 

single-engine test flights over wood and swamp land near 

Iron River, Michigan, and over the iron-bearing area of 

the Adirondacks. The obvious need to use a twin-engine 

aircraft for more extensive flights came to the attention of 

the Director of the Naval Petroleum Reserves, who 

identified a twin-engine U.S. Navy plane for aeromagnetic use. This more powerful aircraft was used 

cooperatively by the USGS and the U.S. Navy to survey a 

10,000-mi2 area in Naval Petroleum Reserve No. 4 in 

northernmost Alaska. Subsequent surveys over oilproducing structures of the Big Horn basin in Wyoming 

and over the Gulf of Mexico offshore from Texas and 

Louisana were completed by the USGS prior to an 

explosion of publications in 1946 which publicly 

announced the arrival of the aeromagnetic technique, 

when National security classification restrictions were 

Abstracts and Short Papers 67 
lifted (Balsley, 1946a, b, c; Muffiy, 1946; Heiland, 1946; 

Jensen, 1946; Jensen and Balsley, 1946; Mining and 

Metallurgy, 1946; Mining Congress Journal, 1946a, b; 

Skillings' Mining Review, 1946; Oil News, 1946; Knoerr, 

1946). By 1950 the airborne magnetometer was 

recognized by the Department of Interior to. be one of 

the greatest advances in techniques used in geological 

exploration (Horowitz, 1950). 

It is ironic that. the geophysical group that had 

promoted for many years the use of the ground magnetic 

technique in Federal programs was transferred in 1942 

from the USGS to the USBM for wartime purposes at 

the time when Hawkes and Balsley at the USGS 

introduced the aeromagnetic technique in the Federal 

sector. Thus, when the geophysical group at the USBM 

was once again transferred to the USGS in 1946 with the 

strong encouragement of former oil geologist and then 

USGS Director W.E. Wrather, a substantial aeromagnetic program was already in place, spearheaded by 

Balsley. Hawkes remained interested in the 

interpretation of total-field magnetic data, whereas 

Balsley focused more on magnetic properties of rocks 

and t~eir relationship to negative aeromagnetic 

anomalies. Balsley's continued interest in remanent 

magnet~ation led to the USGS program of paleomagnetic research headed by Richard R. Doell and Allan 

Cox . (D.R. Mabey, written commun., 1976), which 

provided th~ fi~st magnetic-reversal time scale widely 

used for esbmatmg rates of seafloor spreading (Frankel, 

1979; Glen, 1982; Opdyke, 1985; Morley 1986). 

Hawkes in 1946 hired Roland G. Henderson and 

Isidore Zietz from the USGS Topographic Branch, who, 

under the supervision of Irwin Roman, initially made 

tabular computations for facilitating the use of complex 

formulas. Roman had previously helped to develop a 

magneti~ gradiometer (Roman and Sermon, 1934) and 

had studied the resolving power of magnetic observations 

(Roman, 1946). As they became familiar with the 

theoretic~ work of Vestine and Davids (1945), Henderson and Zietz (1948, 1949a, b) soon began publishing on 

forward computations of magnetic models, second 

vertical derivatives, and analytical continuation, which 

catalyzed the release of many years' worth of 

mathematical geophysical analysis by scientists of the 

Gulf Research and Development Company [compare, 

for example, earlier articles of Vacquier and Affieck 

(1941) and Nettleton (1942) with later articles of Peters 

(1949), Elkins (1951), and Dean (1958)]. Some of these 

?ulf scientists had benefited at least indirectly from the 

mfluence of geophysicist John Bardeen, later a two-time 

Nobel Prize winner in physics, who worked at Gulf from 

1930--1933, prior to his employment by the U.S. Naval 

Ordnance Laboratory and Bell Telephone Laboratories 

during the period of these laboratories' cooperation with 

the USGS. Theoretical papers also were published by 

68 Geologic Applications of Modern Aeromagnetic Survey 

Scientists of other oil companies, such as the Shell 

Petroleum Corporation and the Carter Oil Company 

(Skeels, 1947; Skeels and Watson, 1949) at about this 

time. Henderson and Zietz later joined colleagues Victor 

Vacquier and Nelson Steenland in producing a landmark 

textbook on the interpretation of aeromagnetic 

anomalies (Vacquier and others, 1951). 

Among the hundreds of aeromagnetic surveys 

conducted since the birth of the technique (Balsley, 

1952), one of the best known was also one of the earliest, 

having been flown in 1947 by the USGS in cooperation 

with the Geological Survey of Missouri (Leney, 1964). 

This survey delineated the Pea Ridge anomaly over a 

major iron orebody, which was located by the St. Joseph 

Lead Company in Missouri. This type of orebody, first 

mined in Missouri about 1815, was last discovered using 

conventional geologic methods in 1845. Thus, more than 

a century elapsed before the fateful 1947 survey which 

revealed the presence of the hidden magnetite and hematite orebody. At that time four types of airborne magnetometer were in general use: The MAD operated by the 

USGS; the MAD modified by the Gulf Research and 

Development Company; a wingtip-installed sensor 

developed by C.A. Heiland for the Heiland Research 

Corporation; and a helicopter-borne magnetometer 

constructed by Hans Lundberg (Lundberg, 1947). By 

1965, more than one hundred major mineral discoveries 

had been made entirely or in part by these and other 

airborne magnetometers (Pemberton, 1966). 

According to the late William J. Dempsey of the 

USGS (written commun., 1979), who served for many 

years as Chief of the Airborne Surveys Section (Kronstedt, 1957), early low-level surveys were exciting. On 

occasion, watered gasoline obtained at small airports 

caused partial engine failure. A few magnetometer 

"birds," which were towed on long cables and which had 

to be manually lowered and raised, were lost in flight; 

however, the only complaint came from an Indiana 

farmer who discovered such a "bomb" dropped in his 

pig-pen. Complaints about the low altitude of the flying 

came from a mink rancher in New York and a turkey 

herder in Minnesota. On one flight, the USGS 

successfully located a bargeload of steel which sank in a 

Chesapeake Bay storm. A survey of Paricutin Volcano 

while still in eruption resulted in a pronounced etching of 

the once clear and shiny plastic nose of the aircraft. Much 

later, in a survey of the Island of Hawaii, a USGS aircraft 

was hit twice by rifle fire that was presumably directed 

from areas of significant marijuana growth. 

EPILOGUE 

During the nearly 45 years that have elapsed since 

the introduction of the airborne magnetometer as a 

geophysical instrument, many millions of line miles have 
been flown by governmental agencies, companies, and 

academic institutions throughout the world using fluxgate, proton-precession, and optically pumped airborne 

magnetometers in scalar, vector, and gradient modes 

aboard a multitude of fiXed- and rotary-wing aircraft. 

During this period, the USGS has actively promoted a 

variety of budget initiatives in support of a national 

aeromagnetic program. Although these initiatives have 

not yet led to the unified program that is desired, they 

have nevertheless facilitated the funding of some surveys 

of limited areas. These regional surveys have been largely 

conducted under contract to private industry, funded 

either as parts of Congressionally mandated programs or 

as cooperative efforts involving various combinations of 

Federal, State, County, and Municipal governmental 

agencies; National laboratories; academic institutions; 

and private industry. Data from all available nonproprietary surveys have been utilized recently for compiling 

magnetic anomaly maps of the United States and North 

America (U.S. Geological Survey and Society of 

Exploration Geophysicists, 1982, 1984; Committee for 

the North American Magnetic Anomaly Map, 1987). 

Index maps showing most nonproprietary aeromagnetic 

coverage of the United States as of 1986 are presented in 

Hill (1986a, b, c, d, e, f, g). Although these index maps 

serve as a reminder of the successful application of the 

aeromagnetic technique over diverse terranes during the 

past 45 years, they also reveal the unfortunate lack of a 

single, coordinated national program for acquiring highquality aeromagnetic data. 

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Abstracts and Short Papers 73 

DISCUSSION GROUP REPORTS 

Alaska as a Frontier for Aeromagnetic Interpretation 

By John W. Cady 

INTRODUCTION 

Alaska is a frontier geological province-a collage 

of many different terranes, kinds of geology, and types of 

potential mineral deposits. It has great potential for 

future mineral development, but at present the economy 

constrains development. The primary goal of geophysical 

investigations should be to understand better the 

regional geology, crustal structure, and tectonic 

framework of Alaska and the surrounding shelf areas. A 

properly designed aeromagnetic map of Alaska would 

provide information on horizontal variations in lithology 

and structure, with resolution that is adequate for 

geologic mapping at scales of 1:63,360 and 1:50,000, as 

well as a synoptic view suitable for a statewide geological 

synthesis. 

The Alaska aeromagnetic map would serve as an 

ideal complement to the Trans-Alaska Lithospheric 

Investigation (TALI), which was designed to reveal 

vertical crustal structure along a transect across the 

State. Together with an improved gravity data set, the 

aeromagnetic data would permit the crustal structure 

determined from seismic and magnetotelluric 

investigations to be extrapolated throughout much of the 

State. The gravity and magnetic data together would be 

of great help in planning more expensive seismic and 

magnetotelluric investigations. 

COVERED AREAS HIDE THE 

KEYS TO ALASKA TECTONICS 

Large areas of Alaska are covered by Quaternary 

and Tertiary deposits that prevent bedrock mapping (fig. 

23). The largest of these areas are, from north to south, 

Yukon Flats (YF), Nowitna lowland (NL), TananaKuskokwim lowland (TKL), Yukon-Kuskokwim delta 

(YKD), Copper River lowland (CAL), Cook Inlet-Susitna 

lowland (CISL), and Nushagak-Bristol Bay lowland 

(NBBL). Other lowland areas where pre-Tertiary rocks 

crop out, but bedrock mapping is made difficult by poor 

exposure, include the Arctic Coastal Plain (ACP) and a 

number of lowlands of the Yukon-Koyukuk province 

(Selawik-Kobuk lowland, SKL; Kanuti Flats, KaF; Koyukuk Flats, KoF). 

Airborne geophysical techniques are especially 

cost effective in Alaska because access to the ground is so 

much more difficult than in the conterminous United 

States. Even where the rocks are well exposed, logistical 

difficulties greatly increase the cost of doing geologic 

field work because helicopters are required virtually 

everywhere. 

Figure 24 is a topographic map in color-shaded 

relief of a 3° swath across nothern Alaska that contains 

abundant lowlands. The map was made from elevations 

digitized at 1-minute intervals on 1 :250,000-scale topographic maps. Highlands, shown in red through yellow, 

and lowlands, shown in shades of blue, are labeled in 

figure 25. The most prominent lowland is the Yukon 

Flats, in which one quadrangle (Yukon Flats, lat 65°-66° 

N., long 144°-147° W.) is nearly devoid of outcrops. The 

area west of the Ruby geanticline and south of the 

Brooks Range (fig. 25) is the Yukon-Koyukuk province, 

which contains several lowlands (Selawik and Kobuk 

lowlands and Koyukuk and Kanuti Flats) separated by 

bedrock hills. 

A detailed study has been made of the part of the 

area of figure 24 west of long 149° W., that is, of the 

northern Yukon-Koyukuk province and its border lands 

(Cady, in press). Excerpts from that study are presented 

below to show the types of results that can be obtained 

from the combined interpretation of digital topographic, 

gravity, and aeromagnetic data. In the remaining area of 

figure 25 east of long 14~ W., aeromagnetic data of 

adequate quality (2 km or narrower flight-line spacing) 

are available east to long 147° W. at all latitudes within 

the swath, as well as for Circle quadrangle (lat 65°-66° 

N., long 144°-147° W.). Four 1 o x 3° quadrangles remain 

to be flown to complete the high-quality aeromagnetic 

data in order to make a study of Yukon Flats and vicinity 

comparable to the study described below for the the 

northern Yukon-Koyukuk province. Such a study would 

be especially valuable in unraveling the tectonics 

of northeastern Alaska, because numerous terrane 

Discussion Group Reports 75 
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Figure 23. Drainage basins of major rivers of Alaska (modified from Wahrhaftig, 1965). ACP, Arctic Coastal Plain; YF, Yukon Flats; SKL, Selawik-Kobuk lowland; KaF, 

Kanuti Flats; KoF, Koyukuk Flats; Nl, Nowitna lowland; TKL, Tanana-Kuskokwim lowland; YKD, Yukon-Kuskokwim delta; CRL, Copper River lowland; CISL, Cook 

lnlet-susitna lowland; NBBL, Nushagak-Bristol Bay lowland. A, northeast corner of outline of figure 24. B, northeast corner of outline of figures 26--30. 
o~~~~~~~~~~~~~~~a~ v 0 v 0 v 0 v 0 v v v v vo v v 

ELEVATION, IN METERS 

Figure 24. Color-shaded-relief topographic map of Yukon Flats and vicinity; illuminated from the south. The color scale is 

nonlinear; it tends to emphasize elevation changes near the bottom of the scale and de-emphasize changes near the top of 

the scale. location of this swatph is shown on figure 24. 

SELAWIK LOWLAND 

+- + + 

NULATO 

HILLS 

+ + + 

KOYUKUK 

FLATS 

+ + 

Figure 25. Key to geographic features and faults in figure 25. 

boundaries disappear beneath Quaternary cover in 

Yukon Flats (Jones and others, 1984), and the Tintina, 

Kaltag, and Kobuk faults (fig. 25) either skirt Yukon 

Flats or disappear beneath it. 

EXCERPTS FROM A STUDY OF THE 

NORTHERN YUKON-KOYUKUK 

PROVINCE 

Figure 26 is an aeromagnetic map, presented in 

color-shaded relief, of the western two-thirds of figure 

24. The aeromagnetic map is a digital composite of many 

EXPlANATION 

[)q:)J Quaternary deposits in Yukon Flats 

different aeromagnetic surveys. The data are variable in 

quality, as shown in figure 27 and table 2. The worst 

quality data (10-km or wider flight-line spacing) are in 

the southwest, where anomalies are blurred and illdefined. These data are not useful for geologic mapping 

at a scale larger than 1:2,500,000. Nevertheless, because 

of its large size, the Koyukuk volcanic arc, which is 9° 

wide and 2° high, stands out boldly in the southwestern 

quarter of the map. 

The remainder of the map was flown with northsouth flight lines that were between 1.2 and 2 km apart 

and were draped nominally 305 m above the ground. 

Such data represent the standard aeromagnetic data set 

Discussion Group Reports 77 
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Figure 26. Color-shaded-relief map of the residual total-field magnetic anomaly in the area west of Yukon Flats, illuminated from the south. The nonlinear color scale 

(color-bar unavailable) shows highs in red and lows in blue and emphasizes magnetic variations in the midrange (yellow and green). Total range about 2,500 nanoteslas. 

To aid in comparing the magnetic map with the gravity map (fig. 27) simplified black contours of the complete Bouguer gravity anomaly are superimposed upon the colored 

magnetic map (contour intervaiiO milligals). 
100 200 KILOMETERS 

I I 

Figure 27. Map showing flight lines that were used to 

obtain aeromagnetic data. See table 2 for details. Flight lines 

are not shown in area A because the data were digitized by 

hand from a contour map and flight lines were not available 

in digital form. 

that is available in those parts of Alaska that have been 

flown by "modern" surveys. In many places, the maps 

were made by analogue methods and were digitized by 

hand. 

Figure 28 is an index map for identification of 

features shown in figures 26, 29, and 30. For a more 

complete discussion of geographic features and geophysical domains shown on the index map, refer to Cady (in 

press). 

Figure 29 is a complete Bouguer gravity anomaly 

map of the same area as figure 26. The station spacing is 

variable; generally it is inferior (wider) to the line spacing 

of the high-quality aeromagnetic data of figure 26 and in 

some places it is equal to or worse than the line spacing 

of the low-quality aeromagnetic data. Major gravity lows 

occur over the low-density and (or) thick crust of the 

Brooks Range. Weaker gravity lows suggest that the 

Ruby geanticline is only a sliver of continental crust. 

The Koyukuk geophysical domain (compare figs. 

26, 28, and 29) is characterized by a distinctive horseshoe 

of magnetic and gravity highs that have been interpreted 

to delineate a Jurassic(?) and Early Cretaceous intraoceanic magmatic arc. Geophysical data suggest a division 

of the Angayucham terrane of Jones and others (1984) 

into the Angayucham domain along the north margin and 

the Kanuti domain along the southeast margin of the 

Yukon-Koyukuk province. Both the Angayucham and 

Kanuti domains contain dense, magnetic oceanic crustal 

rocks that dip inward beneath the Yukon-Koyukuk 

province. The contacts between the Kanuti, Angayucham, and Koyukuk domains are buried by sediments of 

the Kobuk-Koyukuk basin. The gravity and magnetic 

maps show an undulating basement surface, which is of 

varied composition and is cut by faults of the Kobuk fault 

zone. 

Figure 30 is a lithologic map derived from a 

combined interpretation of the magnetic, gravity, and 

topographic maps. It is presented as an example of the 

kind of map that can be made from geophysical data in a 

mostly covered area. In areas of near-total cover, such as 

Yukon Flats, such a map could be made by using 

geophysical data to extrapolate from observable bedrock 

geology on the margins of the basin. Carefully sited 

basement drill holes would be required within the basin 

to test the geophysical hypotheses. 

Table 2. Aeromagnetic survey parameters keyed to map area shown in figure 29 

Map Flight-line Flight Regional removed 

Constant 

Date flown1 Contractor spacing and elevation added area (date of update) direction above ground (In nanoteslas) 

A Al: 1973 Geometries 1.6 km N-S 305m 1965 IGR.FJ (1973-74) 50160 

A2: 1974-75 1.6 km N-S 305m 1965 IGRF (1974) 

A3: 1975 1.2-1.6 km N-S 305m 1965 IGRF (1974-75) 

B 1975 Texas lOkmE-W, 122m 1975 IGRF (1 Jan. 1976)8 50140 

Instruments. 40 km N-S 

c 1976 Texas lOkmE-W, 122m 1975 IGRF (1 Jan. 1976)8 50140 

Instruments. 40km N-S 

D 1978 Geometries 1.2km N-S 305m 1975 IGRF (1978) 50190 

E 1972 Aero Service 2km N-S 305m 1965 IGRF (1972) 0 

lfor complete references to magnetic surveys, see Cady (in press). 

2IGRF, International Geomagnetic Reference Field. 

SDue to apparent errors in IGRF removal in area C by the contractor, areas Band C were difficult to merge. In order to facilitate merger, the 

IGRF of 1 January 1976, about midway between the survey dates, was removed from the raw Earth's magnetic field in areas B and C. 

Discussion Group Reports 79 
co 

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domain; KAN, Kanuti domain), Cretaceous over1ap assemblages (Ks, with names in upper case letters), and Cretaceous arc volcanic rocks (Kv). Geographic features are 

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by black dots. Grid interval 2.032 kilometers. Terrain corrections calculated to a distance of 167 kilometers from the station. 
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The lithologic map (fig. 30), at an approximate 

scale of 1:2,500,000, was derived from aeromagnetic 

maps with a flight-line spacing between 1.2 km and 10 

km. It did not make full use of the resolution available in 

the more closely spaced data. For example, shortwavelength linear magnetic highs are caused by portions 

of the Pzp€s unit in the Brooks Range and by parts of 

the units labeled TOZ and MISC in the southeastern part 

of the map. At a scale of 1:250,000, these magnetic highs 

can be used to map magnetic mafic schist, layered gabbro 

and peridotite, and serpentinized ultramafic rocks, 

respectively. However, existing aeromagnetic maps 

cannot be used as effective geologic mapping tools at a 

scale of 1:63,360. Even for 1:250,000-scale geologic 

mapping, higher quality aeromagnetic maps are 

commonly required to identify the rocks responsible for 

the magnetic anomalies. More detailed aeromagnetic 

maps, accurately draped 300--500 m above the ground 

with a 500-m flight-line spacing, would lend themselves 

to 1:63,360 (or 1:50,000) scale geologic mapping. They 

would also make it much easier to identify the rock type 

of causative sources in 1:250,000-scale mapping. 

RECOMMENDATIONS 

A new aeromagnetic survey of the State of Alaska 

is needed. Although some details of survey design remain 

debatable, several minimum standards are generally 

agreed upon. 

(1) Maximum flight-line spacing should be about 

500 m in areas of exposed or shallow bedrock. Data 

acquired at wider spacings show anomalies that are 

too broad for use in 1:63,360-scale geologic 

mapping. In areas of thick sedimentary cover, 

spacings as wide as 1 km would be useful. 

However, for detection of anomalies that are 

caused by magnetic rocks within the sedimentary 

section, the 500-m spacing may be required. 

(2) Accuracy should be about 0.1-0.01 nT. 

(3) Draped flying should be about 300 m above 

terrain. 

( 4) Digital data file should include x, y, z-location, 

raw magnetic field, and field with DIGRF 

(Definitive International Geomagnetic Reference 

Field) removed, for both profile lines and tie lines. 

The surveys must be designed so that additional 

lines can be interleaved easily by users. Although 

some aerial survey contractors have expressed a 

concern that the existence of a public-domain 

aeromagnetic map with a flight-line spacing of 500 

m would inhibit the market for private mineral 

exploration surveys, I suggest that the opposite is 

the case. The proposed map will enable explorationists to discover many areas where maps with a 

Discussion Group Reports 83 
flight-line spacing of 100 m will be required for 

developing geologic maps at a scale of 1:10,000. 

( 5) Good geographic bases should be provided for 

every aeromagnetic map published. It is especially 

important that 1:63,360- and 1:250,000-scale aeromagnetic maps be published on the same topographic bases that geologists use for field mapping. 

It is commonly convenient to map geology directly 

on a published aeromagnetic map. 

( 6) Smaller scale aeromagnetic maps (1:250,000, 

1:1,000,000, 1:2,500,000, 1:5,000,000) should be 

published in an appropriate combination of colorshaded-relief, color-slice-contour, grey-scale, 

three-dimensional, holographic, and any number of 

other existing and future formats. 

(7) Data should be archived and made available to 

the public. There is an opportunity for private 

vendors to provide services both to the public and 

to government agencies by publishing standard 

84 Geologic Applications of Modern Aeromagnetic Survey 

maps and producing custom products. Compatible 

formats must be developed to facilitate the overlay 

or merging of aeromagnetic, gravity, remotesensing, and digital geologic data sets. 

REFERENCES CITED 

Beikman, H.M., compiler, 1980, Geologic map of Alaska: U.S. 

Geological Survey, scale 1:2,500,000, 2 sheets. 

Cady, J.W., in press, Geologic implications of topographic, 

gravity, and aeromagnetic data in the northern YukonKoyukuk province and its borderlands, Alaska: Journal 

of Geophysical Research. 

Jones, D.L, Silberling, NJ., Coney, P J., and Platker, George, 

1984, Lithotectonic terrane maps of Alaska (west of the 

141st meridian): U.S. Geological Survey Open-File 

Report 84-523-A, 12 p. 

Wahrhaftig, Oyde, 1965, Physiographic divisions of Alaska: 

U.S. Geological Survey Professional Paper 482, 52 p. 
Notes of the Discussion Group on Western United States: 

Overview 

By Robert C. Jachens 

OFFSHORE PACIFIC REGION 

The offshore Pacific region, which includes the 

Pacific Island Territories, is almost entirely covered by 

water and sediments. Therefore, knowledge of 

underlying geology is limited to inferences drawn from 

sparse magnetic coverage, seismic lines, gravity 

observations, dredge hauls, and shallow borings. More 

extensive, detailed magnetic coverage would enable 

preparation of magnetic terrane maps-similar in 

concept to geologic maps-which would delineate 

regions of contrasting magnetization and depths to 

basement; such knowledge is important to the study of 

both structure and lithology. New magnetic coverage is of 

economic interest in hydrocarbon exploration because it 

would help to outline basins, provide improved estimates 

of sediment thickness, and establish limits on the Curie 

isotherm, which is used to predict the geothermal 

gradient that is critical to maturation. From the 

standpoint of geologic hazards, new magnetic coverage 

would sharpen the demarcation of fault locations and 

attitudes near proposed or operating nuclear reactors 

and would provide additional knowledge about the 

lengths of fault segments, which in some .areas directly 

correlate with the magnitudes of earthquakes focused on 

the faults. New magnetic data would contribute to solving 

such geologic problems as delineating the segmentation 

of the Aleutian Arc, tracing lithologic terranes across the 

Bering Shelf, and unraveling the collage of accreted 

terranes that rim the northeast Pacific margin. 

ALASKA 

Although much of Alaskan geology is concealed 

beneath tundra, alluvium, or ice and snow, only about 20 

percent of Alaska is covered by magnetic surveys that 

have flight -line spacings of 1 mi or less. More than any 

other part of the onshore United States, Alaska needs a 

geologic mapping program that includes among its 

highest priorities acquisition, processing, and interpretation of aeromagnetic and other aerogeophysical 

data. Such new coverage would contribute significantly to 

economic geology because characteristic magnetic 

anomalies are commonly associated with gold- or 

chromite-bearing ultramafic rocks and with base- and 

precious-metal deposits contained in volcanic, plutonic, 

and sedimentary rocks (for example, the Brooks Range 

and Seward Peninsula). Such coverage would also 

contribute to tracing the locations and attitudes of strikeslip faults as an aid in assessing possible locations of 

"geometric asperities," where movement along faults may 

be anomalously inhibited. 

CONTERMINOUS UNITED STATES 

Although the geology in the conterminous United 

States is more exposed than in the offshore Pacific region 

and Alaska, large areas of Washington, Oregon, and 

California are covered by Tertiary volcanic rocks and 

alluvium. Improved aeromagnetic coverage would 

contribute especially to knowledge about geothermal 

resources in the Cascade Range and Salton Sea area and 

about petroleum and nonfuel resources in basins that are 

hidden beneath volcanic deposits, such as the Columbia 

River Basalt. Such magn~tic anomaly data would also 

contribute to estimating fault traces, offsets, and 

attitudes in earthquake-prone regions near possible sites 

of nuclear reactors or high-level radioactive waste 

repositories and would help to delineate fissuring 

potential associated with basement geometry. Magnetic 

data would also provide information relevant to groundwater resources, such as estimates of basin depths, 

basement faulting that extends vertically to shallow 

depths, and vertical or subhorizontal barriers to flow. 

New magnetic data would aid analysis of the mechanics 

of accretion and obduction in the Klamath Mountains, 

Blue Mountains, Sierra Nevada foothills, and Oregon 

Coast Ranges, and would permit studies of magnetic 

similarities and contrasts of plutonic rocks of the 

Peninsula, Sierra Nevada, and Idaho batholiths. 

Discussion Group Reports 85 
Proposed Program Based Partly on Notes of 

Discussion Group on the Basin and Range Province 

By Robert C. Jachens, H. Richard Blank, and V.J.S. Grauch 

INTRODUCTION 

Problem 

The United States has a serious minerals deficit, 

and the U.S. mining industry remains in decline. The 

problem is composed of both geologic and economic 

factors: We are running out of identified domestic 

deposits and reserves that contain grades and tonnages 

high enough to be competitive with large foreign reserves 

which generally are developed with cheap labor and 

direct government support. If this trend continues, the 

United States will be ever more dependent on foreign 

sources of metals. This dependence will reduce our 

capacity of produce strategic and critical by-product 

metals and will reduce the ability of the mining industry 

to respond to National emergencies. 

Solution 

Past and present production of minerals from the 

Basin and Range Province of the Western United States 

has contributed significantly to the economic health and 

security of the Nation by providing politically stable 

sources of the raw materials that are needed by U.S. 

industry. Although current production of most minerals 

from the province is dropping and will continue to drop 

as the large, easily located, high-grade deposits are 

exhausted, renewed exploration and development in the 

Basin and Range Province provide an extraordinarily rich 

opportunity to enhance the U.S. position with regard to 

world-wide mineral resources and to reverse the decline 

of the domestic minerals industry. The reason is simple: 

About 50-70 percent of the province is mantled by 

young, unconsolidated materials and volcanic rocks that 

conceal rocks that are potential hosts for mineral 

deposits. In the rich Basin and Range Province, we have 

so far only prospected in the ranges, not in the 

intervening basins. Geologic evidence indicates that the 

86 Geologic Applications of Modern Aeromagnetic Survey 

number and types of deposits concealed beneath the 

basins are probably comparable to those exposed in the 

ranges; nonetheless, the vast basins remain virtually 

unexplored. The potential for new discoveries here is 

extremely high, but a systematic program of the scope 

necessary to realize this potential is beyond the resources 

of the mining industry. We propose a federally funded, 

regional-scale geophysical and geochemical data 

collection program and mineral resource appraisal of the 

covered areas of the Basin and Range Province as a 

means to stimulate and focus domestic exploration for 

new mineral deposits. Inasmuch as this appraisal is 

primarily directed to covered terranes, we envision a 

strong emphasis on geophysical, geochemical, and other 

indirect methods. 

Program Rationale 

Recent advances in diverse fields make it timely to 

undertake a regional-scale program directed toward 

assessing the mineral resource potential of the covered 

areas in the Basin and Range Province. 

( 1) Great strides have been made during the past 

decade in the fundamental understanding of the 

tectonic events and processes of crustal extension 

that have dominated the Basin and Range Province 

during the past 30 m.y. These events and processes 

controlled the formation of basins and generated 

the sediments that now mantle much of the region. 

The increased understanding greatly enhances our 

ability to predict the nature and geologic 

framework of the covered rocks; such information 

is crucial to any assessment of mineral resource 

potential. 

{2) New regional compilations of gravity, aeromagnetic, aeroradiometric, and remote sensing 

data based on state-of-the-art reduction, filtering, 

and merging techniques provide a framework for 

the systematic extrapolation of exposed bedrock 

lithology into covered areas. These computerbased compilations provide the flexibility to 
address specific exploration problems, such as the 

identification of areas favorable for hydrothermal 

mineral deposits based on the distribution of 

concealed plutons. A concealed-pluton map 

derived from geophysical data was recently 

produced for the entire State of Nevada as part of 

a gold resource investigation. 

(3) New regional compilations of radiometric, 

lineament, and limonite maps provide a framework 

for systematic mapping of surficial geology, 

including tectonic features and hydrothermal 

alteration. 

( 4) Recently available Landsat Thematic Mapper 

coverage provides new tools for systematic regional 

mapping of clay types, hydrothermal alteration, and 

unconsolidated basin fill. In addition, SLAR (sidelooking radar) data provide new structural 

information. 

(5) Recent advances in the understanding of the 

magmatic history of the Basin and Range Province 

and of the relationship between specific granitoid 

types and characteristic mineralization 

demonstrate the feasibility of applying magnetic 

and gravity studies at a regional scale to define 

local areas that are favorable for mineral deposits. 

(6) New results from mineral deposit models, 

which are based on analyses of thousands of wellcharacterized deposits world-wide, provide fresh 

insights into the relationships between mineral 

deposits and their tectonic setting, host rock, 

structural control, environment of deposition, and 

many other parameters. Knowledge of these 

relationships is essential to a successful assessment 

of concealed deposits because the geophysical and 

geochemical techniques that can "see through" 

cover only rarely detect the deposits themselves. 

Such techniques are much better able to detect 

related structures and environments. 

(7) Refined geophysical instrumentation, new 

accurate positioning systems, and advanced data 

handling, analysis, interpretation, and display 

techniques now promise to define subsurface 

structure and lithology with unprecedented 

clarity. High-precision magnetometers and 

magnetic gradiometers permit better resolution of 

subtle anomalies from buried sources than has 

been possible in the past. Airborne and groundbased electromagnetic systems that operate over a 

broad frequency range now allow definition of the 

electrical properties of the subsurface materials 

over a wide range of depths, from a few meters to 

well into the lower crust and upper mantle. Gravity 

interpretation techniques now facilitate the 

elimination of anomalies from deep-seated sources 

and permit more precise definition of density 

distributions in the upper few kilometers of the 

crust. Spectral aerial gamma-ray data combined 

with modern display and analysis techniques 

provide a new look at surface lithologies and 

contain information relevant to structural features 

and geochemical environments. Accurate position 

information, which is essential to the effective use 

of new high-resolution data collection systems, can 

be obtained from long-range ground-based 

transponder navigation systems and will be much 

improved with the advent of the Global Positioning 

System that is currently coming on-line. Recent 

developments of computer software for 

manipulating, filtering, interpreting, and displaying 

the enormous data sets that result from regional 

investigations will play a key role in the integrated 

interpretation .of diverse data that is required to 

generate a coherent and well constrained image of 

the subsurface. 

In summary, the needs of the Nation and of the 

domestic minerals industry have converged with the 

increased capabilities of geophysics to explore beneath 

the Earth's surface, with fundamental improvements in 

our understanding of the geologic processes that have 

affected the Basin and Range Province, and with a much 

greater understanding of the relationships between 

mineral deposits and their host environments. This 

convergence sets the stage for a major multidisciplinary 

program designed to provide fundamental geophysical 

and geochemical data to the mining industry and to 

evaluate the mineral resource potential of the covered 

areas of the Basin and Range Province. 

DESCRIPTION OF PROGRAM 

Objectives and Direction 

The primary objectives of the program that we 

envision would be to provide fundamental geophysical 

and geochemical data and to produce a systematic 

mineral resource appraisal of the covered areas in the 

Basin and Range Province. However, a major difference 

between the proposed program and the mineral resource 

appraisal programs of the past is that much of the 

geologic evidence that traditionally has been used in 

appraisals will not be directly available but must be 

inferred from the geophysical and geochemical data 

bases that were compiled by techniques that "see 

through" the cover. This requires that the diverse 

investigations needed to generate the products on which 

the appraisal should be based must have as their main 

goal maximization of the ability of geophysical and geochemical interpretations to characterize accurately the 

subsurface. 

Discussion Group Reports 87 
Investigations under this program will be directed 

toward establishing information in four main areas: (1) 

Tectonic framework of the region; (2) structure and 

composition of the concealed basement in three 

dimensions; (3) structure, thickness, and composition of 

the cover material; and ( 4) types and characteristics of 

mineral deposits that are likely to exist within the 

basement and in the overlying cover material. 

Tectonic Framework 

Background on the tectonic setting of the region is 

an ingredient that is critical to assessing mineral 

resources in general and to focusing exploration activities 

in particular. Reconstruction of the tectonic setting that 

predated Basin and Range spreading coupled with 

studies of deformation and magmatism during the past 

30 m.y. are needed to define the tectonic framework of 

the covered areas. 

An important goal would be to produce a tectonic 

map of the region that identifies ancient rifts, suture 

zones, terrane boundaries, plutonic belts, and major fault 

zones and fracture systems. Particularly significant will 

be definition of the ancient continental margin because 

the margin represents a first -order boundary in the 

distribution of mineral deposits that are related to 

ancient rocks and it may have localized such deposits. 

The map will depend heavily on the analysis of regional 

gravity, magnetic, radiometric, and remote sensing data 

and their derivative products to define tectonic elements 

and boundaries and to provide the third dimension to the 

mapped geology. Detailed studies that include combined 

interpretations of geologic, potential field, electrical, and 

seismic data along critically placed profiles will served as 

nuclei from which crustal structure can be extrapolated 

using the regional data sets. Two such profiles, the PACE 

(Pacific-Arizona Crustal Experiment) profile across 

southern California and western Arizona and the lat 40° 

N. transect recently completed by COCORP 

(Consortium for Continental Reflection Profiling), 

already are underway. Additional profiles that transect 

other important features, such as the Arizona porphyry 

copper province and the western Utah mineral belts, 

should be initiated. 

Structure and Composition of the 

Concealed Basement 

Reliable lithologic, structural, and plutonic maps of 

covered areas (basins) require detailed geologic mapping 

and analysis of the basement that is exposed in adjacent 

ranges, closely coupled with analysis and interpretation 

of geophysical and geochemical data over the same 

88 Geologic Applications of Modern Aeromagnetic Survey 

exposures. Direct modeling of potential field anomalies 

over covered areas; tracing of concealed physical 

property boundaries by potential field, electrical, and 

seismic measurements; and identification of buried 

lithologies by means of their geophysical signatures will 

serve as the basis for a three-dimensional geologic map 

of the basement. Investigations must include extensive 

physical property sampling to define the distributions of 

density, magnetic susceptibility, remanent magnetization, 

electrical conductivity, and seismic-wave velocity, and 

their relationships to rock type and geophysical 

anomalies. Data from basement -penetrating drill holes 

can provide valuable vertical constraints on geophysical 

modeling. Analysis of stress on plant communities with 

deeply penetrating root systems can provide indirect 

information on metal concentrations. 

Thickness, Structure, and 

Composition of the Cover Material 

A systematic mineral resource appraisal should 

take into account the thickness and composition of the 

cover material and should include an appraisal of the 

mineral resource potential within the cover material 

itself. Pediment mapping, ground-water geochemistry, 

and analyses of drill-hole and ground-penetrating radar 

data, coupled with quantitative interpretation of 

magnetic, gravity, electrical, and seismic data, should 

permit effective definition of the thickness and 

composition of the cover material. Remote sensing and 

radiometric data can be utilized for mapping unconsolidated deposits on a regional scale, which is a subject that 

warrants more attention in the Basin and Range 

Province. 

Mineral Deposit Characteristics and 

Relationships 

Detailed information on the mineral deposits that 

are contained within the exposed basement of the Basin 

and Range Province is essential in the assessment of 

covered areas. Studies of these deposits will refine 

mineral deposit models, with special emphasis on 

characterization of the geophysical and geochemical 

signatures of the tectonic setting, host lithology, 

controlling structures, associated alteration, and orebodies themselves. Physical property sampling should be an 

important component of these studies. 

Some of the methods, techniques, background 

information, and data bases required by this program 

already exist. Others will have to be developed early in 

the program, and still other will have to be developed as 

new questions and problems arise. In some areas 
acquisition of new geophysical data will be required. 

From the start, the program will include a component 

that is directly concerned with the application of 

appraisal techniques and a research and development 

component that encompasses topical studies (such as 

tectonic history, magmatic processes, and ore genesis and 

deposition), development of new systems and procedures 

for seeing beneath cover, improved analysis techniques, 

and new methods for integrating data from diverse 

disciplines into a coherent geologic interpretation. 

Anticipated Benefits 

This program will provide a state-of-the-art 

mineral resource appraisal of a vast, mostly unexplored 

region of the United States. It will give the Federal 

Government a much clearer picture of the probable 

mineral wealth of this part of the Nation for use in 

economic and strategic planning. The appraisal and the 

various intermediate data sets and products on which it is 

based may stimulate exploriltion activity and focus the 

limited exploration resources of the mining industry on 

the areas that have the highest potential for success. This 

heightened exploration activity may result in new 

discoveries of ore deposits, thus lessening the 

dependence of the U.S. industry on foreign sources of 

raw materials. 

Other benefits that will result from the program 

are: 

(1) New appraisal and exploration methods, 

systems, and strategies (such as the use of broadband electromagnetic techniques to map the depth 

to and the properties of the basement; the 

fingerprinting of tectonic features, lithology 

structure, and mineral deposits according to their 

geophysical and geochemical signatures; and the 

combined use of gravity, magnetic, and electrical 

data for depth sounding) will be directly 

transferable to the vast covered areas of Alaska, 

the Midcontinent, and the world. 

(2) Refined mineral deposit models with detailed 

characterization of the deposits in terms of geophysical and geochemical parameters will enhance 

exploration strategies in exposed as well as covered 

areas. 

(3) Characterization of basin geometries will 

provide insights into the Cenozoic tectonic 

development of the province, with possible 

implications for geologic engineering and hazard 

mitigation. 

( 4) Determinations of the geometry and nature of 

the cover material in the Basin and Range Province 

will contribute substantially to ground-water 

resource assessments and ground-water modeling, 

which are topics that are of critical importance to 

the increasing permanent population of the 

province. 

Discussion Group Reports 89 
Notes of Discussion Group on the Midcontinent 

By Lindreth E. Cordell, Thomas G. Hildenbrand, and M. Dean Kleinkopf 

INTRODUCTION 

In general, the Midcontinent region, defined here 

to include the continental United States between the 

Cincinnati arch and the Rocky Mountains, is especially 

well suited for aeromagnetic surveys because of the 

extensive cover of alluvium, till, and sedimentary rocks, 

much of which is less than 1 km thick; the absence of 

rugged topography; and the particular nature of the 

geologic problems at hand. Here is where we have the 

best data, the showiest success stories, and the most 

clearly defined and urgent fundamental geologic 

problems that need to be solved. A common view in the 

geological profession is that the Midcontinent is "the next 

(mineral) exploration frontier." State geological surveys 

in the Midcontinent are especially attuned to the use of 

magnetic methods, and some states, such as Kansas and 

Minnesota, have active surveying programs of their own. 

SUCCESS STORIES AND REGIONAL 

TECTONIC PROBLEMS REMAINING 

The discovery of a major Eocambrian graben that 

is entirely hidden beneath younger sedimentary rock 

cover in the Mississippi embayment, and the likely 

relationship of this graben to earthquakes of the region, 

ranks about as highly as recognition of sea-floor magnetic 

stripes as one of the all-time most significant 

achievements of the aeromagnetic method. Similarly, 

Precambrian province boundaries that can be seen in 

outcrop in Canada can now be tracked by aeromagnetic 

data southward into the United States beneath the 

seemingly ubiquitous cover. Specific targets for future 

investigations are the Trans-Hudson belt, the Churchill 

and Wyoming provinces, the Grenville front, the 

southern margin of the Superior Province in Iowa, and 

the northern margin of the Central Plains orogenic belt in 

Nebraska and Kansas. The aeromagnetic data now need 

to be studied more closely to determine the attitude in 

three dimensions and the degree of irregularity of these 

boundaries. A better understanding of such basic 

90 Geologic Applications of Modern Aeromagnetic Survey 

geologic processes as accretion, subduction, and rifting 

will likely be gained. 

A success story of a different sort is provided by the 

high-quality (¥4-mi line spacing) aeromagnetic mapping 

that was conducted by the State of Minnesota. This 

program (like similar efforts in Canada, Finland, and 

elsewhere) points the way and shows how much of the 

geology may be missed in some of the reconnaissance 

aeromagnetic data that are now available. 

A broad northeast-southwest-trending swath of 

anorogenic felsic intrusive rocks (about 1.35 b.y. old) that 

extends from California to Labrador indicates large heat 

flux into the lower crust, without corresponding tectonic 

expression at shallow levels. This belt can only be studied 

by geophysical methods, which incorporate control by 

sparse drill holes. 

Part of the Midcontinent shows characteristic 

northwest-trending linear features, some of which are 

probably Archean faults. There is evidence that these 

faults have been reactivated and have affected 

sedimentation and facies trends in Phanerozoic strata. 

Numerous buried rifts have been mapped geophysically in the Midcontinent. The time of rifting is 

ambiguous in some cases. Rifting probably first occurred 

in the early Proterozoic but was also active about 1.35 

b.y. ago (Anorogenic period), 1.1 b.y. ago (Keweenawan), and 600 m.y. ago (Eocambrian). In-situ paleomagnetic dating of intrusive bodies, by analysis of their 

corresponding magnetic anomalies, offers a chance to fill 

in the uncertain chronology. The rifts also are frequent 

objects of studies by consortiums, such as DOSECC 

(Deep Observation and Sampling of the Earth's 

Continental Crust), which surveyed transects across 

South Dakota, the Keweenawan rift complex, the Grenville front in Ohio, and the Archean in Iowa, as well as 

GLIMPCE (Great Lakes International MultiDisciplinary Program on Crustal Evolution) and 

COCORP. Future studies should progress outward from 

areas where basement geology is known into the 

unknown areas through expansion of state cooperative 

projects and coordination with other programs such as 

GLIMPCE, DOSECC, and Canadian studies. 
A map of the Precambrian basement of the Midcontinent has recently been completed by Paul Sims and 

others (Sims, 1985). The discussion group generally 

agreed that this map represents an end p~oduct: 

Geological and geophysical data resources have been 

exhausted· all of the known data are in the map, and 

there is n~thing further to be done until new geophysical 

data are available. There are perhaps other instances 

where geological mapping has reached a point of 

diminishing returns and geophysical data are needed, but 

the case seems especially well made here. 

TOPICAL RESEARCH PROBLEMS 

The Midcontinent is the birthplace of the concept 

of "Donovan anomalies" over authigenic magnetic 

minerals; such anomalies are possibly associated with 

hydrocarbon exhalative halos. This critical research topic 

for hydrocarbon exploration is receiving attention worldwide inherent problems and alternative hypotheses 

notwithstanding. Other problems of interest include the 

mapping of stream channels by detrital magnetite, as is 

being done in Minnesota; the study of anomaly pattern 

and rock magnetism as a means of mapping metamorphic grade; the direct detection of alteration that 

delineates unsuspected neotectonic faults; and a 

magnetic-based petrology of granites, as a means of 

subdividing and classifying these granites. 

In the core area of the Ouachita Mountains, and 

likely in other favorable areas, crystalline basement rocks 

are sufficiently deep that, with the improved sensitivity of 

today's magnetometers and gradiometers, sedimentary 

strata may have a distinct magnetic expression and thus 

could be the object of magnetic surveys. We consider it 

likely that the aeromagnetic method could, in favorable 

cases, be used to map lithology and, by inference, 

sedimentary rocks, similarly to the way that the aeromagnetic method is now used to map crystalline rocks. 

Many of the goals that were discussed require 

research in potential-field analysis and instrumentation, 

if the goals are to be fully realized. It was strongly 

expressed in the discussion group, as in the full assembly, 

that new data warrant concomitant support in 

methodology and analysis. Specific topics targeted 

include short-baseline gradiometers, stable-platform 

vector magnetometers, optimal position determination, 

interpolation of data from lines to a grid, extension of 

interpretation theory from scalar data to gradient -tensor 

data, and discrimination of cultural noise. 

RECOMMENDED DATA SPECIFICATIONS 

Flight-line spacing should be determined by 

average depth to crystalline basement; the formula that 

was suggested is spacing equal to basement depth, but 

neither less than 400 m nor greater than 2 km. 

Specifically, a 400-m flight-line spacing should b~ us~ if 

basement depth is less than 500 m; 1-km flight-hne 

spacing should be used if basement depth i~ 0.5-2 km; 

and 2-km flight-line spacing should be used If basement 

depth is greater than 2 km. More closely spaced surveys 

could be useful for local, topical research projects and for 

gradiometer surveys. 

In order of preference, and contingent upon future 

development in instrumentation, we favored vector 

measurements, short-baseline gradient-tensor measurements and long-baseline vertical-gradient measurements: We endorsed the view that all surveys will include 

at least some type of difference-gradient (optimally, 

transverse to the flight line) configuration in the very 

near future. 

Flying should be in drape mode and i~ cardi~al 

directions only, preferably along north-south hnes. Tieline spacing should be five times flight-line spacing, not to 

exceed 25 km. Surveys should be in blocks of a minimum 

1 ox 1 o area. Scalar-field observations should be to 

0.25-nT precision and spaced no greater than 25 m apart 

along the track. Global Positioning System (GP~) 

positioning should be accurate to 5-10 m. 

Some existing data are satisfactory within these 

specifications and would not need to be reflown. Other 

existing data sets, such as those which cover much of 

Indiana, even where flight-line spacing is satisfactory, 

would not qualify because of other problems. Loss of 

primary observational data in digital form, positioni~g 

errors, and arbitrary treatment of the geomagnetic 

reference field are especially troubling in many such 

surveys. 

REFERENCE CITED 

Sims, P.K., compiler, 1985, Precambrian basement map of the 

northern Midcontinent, U.S.A.: U.S. Geological Survey 

Open-File Report 85-604, one oversize sheet, scale 

1:1,000,000, 16 p. 

Discussion Group Reports 91 
Notes of Discussion Group on Eastern United States 

By Jeffrey D. Phillips and John M. De Noyer 

INTRODUCTION 

The purposes of this discussion group were to 

define specific geologic problems of the Eastern United 

States that might be amenable to solution using aeromagnetic data and to suggest appropriate aeromagnetic 

survey specifications and data analysis methods. 

GENERAL TOPICS 

Rock Magnetic Studies 

Rock magnetic studies should be a core component 

of a National aeromagnetic mapping program. The 

National Science Foundation (NSF) is sponsoring an 

initiative on the physics and chemistry of geologic 

materials. This subject is considered by NSF to be at the 

"cutting edge" of basic science. A U.S. Geological Survey 

(USGS) airborne mapping initiative would benefit from 

association with this NSF initiative. 

Rock property studies that relate to aeromagnetic 

anomalies are uncommon; the few such studies that have 

been made in Eastern United States, such as in the 

Carolina slate belt and the Adirondack Mountains, have 

shown a strong correlation of aeromagnetic anomalies to 

lithology and metamorphic grade. Unfortunately, oxide 

and sulfide analyses are not made in standard 

petrological studies. USGS petrologists need to be 

encouraged to include such analyses. Good work in this 

area is being done outside the United States. The most 

effective way to encourage rock magnetic measurements 

is to incorporate the data in aeromagnetic 

interpretations. It is possible to interest petrologists and 

paleomagnetists in aeromagnetic anomalies. 

A USGS airborne mapping initiative should 

include a firm statement of the need for rock magnetic 

measurements, including bulk susceptibility and total 

magnetization vector. Problems to be studied should 

include standardization of measurements (calibration of 

instruments, standard samples), relationship of 

properties of rocks at depth to surface measurements, 

92 Geologic Applications of Modern Aeromagnetic Survey 

relationship of properties of a small number of samples 

to the larger body that produces the aeromagnetic 

anomaly, and estimation of rock magnetic properties 

directly from aeromagnetic data in areas of magnetized 

terrain. The USGS has experience in maintaining a 

radiometric rock data base. Generally, maintenance of a 

data base requires either recurrent funding or a person 

dedicated to maintaining the data base. 

Canadian National Aeromagnetic 

Mapper Program 

The Canadian National Aeromagnetic Program 

involves 20 government employees. A Contracts Group, 

which consists of six employees, runs the contracting 

operations, . insures continuity of specifications, and 

monitors the results. An Instrumentation Group 

attempts to push the state-of-the-art in instrumentation 

and feeds its results back into the contract specifications. 

Priorities are set somewhat arbitrarily by committee. 

Current data requirements include ¥4-gamma 

sensitivity, noise levels checked in flight, error diurnal 

variation within 10 gammas, and figure of merit (yaw, 

pitch, and roll) within 4 gammas. Bernoulli disks are used 

rather than tape to store data in the air. Exercising, or 

periodic reading of archived tapes, is not considered cost 

effective; laser disk technology is being explored for 

future archiving. 

The Canadians seldom seek new contractors. 

Ongoing professional relationships are best and yield the 

best data. The companies considered must have all 

necessary aircraft, instruments, personnel, and 

computers, and they must be financially solvent. A point 

rating system is used to evaluate bidders; cost is only one 

factor in the choice. 

The cost of the contracting operation is about 

$500,000 per year in agency funds. The figure does not 

include $600,000-$700,000 in salaries or funds received 

from the Provinces and other Federal agencies. The total 

is close to $2 million per year. 

Geologic problems are decided by the geologists. 

The aeromagnetic data are published in a form that is 
readily usable by the geologists; for example, standard 

colors are used for gradient maps. This encourages the 

use of aeromagnetic data for geologic mapping. No other 

promotional effort is needed. 

Cooperative Programs Between 

Government and Industry 

The USGS Atlantic Continental Shelf (ACS) 

survey was a cooperative effort with industry. The USGS 

bought $600,000 of a $2.5 million effort. The data were 

time-released to the public over a period of 5 years. The 

ACS survey was probably a special case, because 

industry's interest in keeping the data proprietary had a 

time limit related to Federal lease sales. The academic 

community would probably object to a time-release 

contract; this community wants the data while the data 

are still in the forefront of science. However, releasing 

the data eventually is probably better than never 

releasing them. 

GEOLOGIC PROBLEMS OF THE 

EASTERN UNITED STATES 

Introduction 

The Eastern United States contains some of the 

earliest aeromagnetic surveys flown by the USGS. Most 

of these are available only as contour maps at a 20-

gamma intetVal, and are not suitable for analysis using 

modern digital techniques. More recent USGS surveys, 

such as those flown over the Atlantic Coastal Plain, suffer 

from aliasing due to wide flight-line spacing and poor 

quality control. Many parts of the Eastern United States 

have extremely poor coverage or no coverage at all. 

These include large parts of Maine, the Gulf of Maine, 

southern New Hampshire, parts of Virginia, and 

southern Florida. The Exclusive Economic Zone (EEZ) 

is inadequately covered in the vicinity of Maine, Florida, 

and Puerto Rico. It is safe to say that nearly every part of 

the Eastern United States would benefit from resurveying using modern high-resolution technology. 

Environmental problems of the Eastern United 

States that can be addressed using aeromagnetic data 

include earthquake hazards and ground-water 

contamination. Several areas, including the St. Lawrence 

River valley, Cape Cod, New Jersey, Virginia, and South 

Carolina, are sites of active seismicity and occasional 

large earthquakes. The geologic structures related to 

these earthquakes are poorly understood. Modern aeromagnetic data could be used to help identify the causes of 

this seismic activity. 

~. Much of New England is covered by glacial debris 

which obscures the topography and geologic structure of 

the underlying bedrock. Because ground water tends to 

flow within buried bedrock valleys and fault zones, 

accurate mapping of these buried features is important in 

the control of ground-water contamination. Similar 

problems of ground-water flow and contamination are 

present within the sedimentary units of the Atlantic 

Coastal Plain. Combined high-resolution aeromagnetic 

and spectral aeroradiometric surveying are the most 

cost-effective ways of mapping such structures. 

Areas of active mineral exploration in the Eastern 

United States include Maine, the Gulf of Maine, and all 

exposed, buried, and submerged early Mesozoic basins. 

Maine and the Gulf of Maine are known to be underlain 

by geologic structures that are on trend with the sites of 

lead-zinc deposits in New Brunswick; tin and chromite 

also may be present. Accurate assessment of the mineral 

resource potential is hampered by the poor aeromagnetic 

and aeroradiometric coverage of this region. 

Early Mesozoic basins have long been a source of 

iron and copper. Currently they are being explored for 

more exotic strategic minerals, as well as for energy 

resources. Available aeromagnetic and aeroradiometric 

data over exposed Mesozoic basins are frequently 

inadequate for geologic mapping, let alone mineral 

exploration. Large Mesozoic basins are known to lie 

buried beneath the Atlantic Costal Plain, both onshore 

and offshore. Modern aeromagnetic data would help to 

define the boundaries and thicknesses of these buried 

basins and to assess their mineral and energy resource 

potential. 

In addition to the early Mesozoic basins, the 

Appalachian basin and eastern overthrust belt are sites of 

known or potential energy resources. The Appalachian 

basin is the site of the earliest commercial petroleum 

production and is currently producing commercial 

natural gas. Active exploration and drilling for additional 

gas reserves continue. Modern high-resolution aeromagnetic data can be used to help map both the thickness 

of Appalachian basin fill and the basement structures 

that control folding and faulting of the basin; such 

mapping would augment knowledge about structural 

traps that contain oil and gas. Spectral aeroradiometric 

data could be used to improve mapping of the surface 

geology of the basin and thereby identify additional 

geologic units and structures of interest in petroleum 

exploration. 

The eastern overthrust belt is a frontier area of 

energy resource exploration. Recent, deep seismic 

reflection profiles suggest that the crystalline rocks of the 

Blue Ridge Province have been thrust over sedimentary 

rocks of the Appalachian basin. These buried 

Discussion Group Reports 93 
sedimentary units are potential sites for natural gas 

deposits. Modem aeromagnetic data can be used to 

estimate the thicknesses of the crystalline and 

sedimentary units. 

Scientific problems that can be addressed by aeromagnetic surveying in the Eastern United States include 

the general problem of improved geologic mapping and 

specific problems such as the structure of the 

Appalachian Mountains and the origin of Florida. It is a 

common misconception that geologic mapping is 

relatively easy in the Eastern United States because of 

the relatively large areas of exposed crystalline terrane 

and generally low relief. The problems of glacial cover 

and Coastal Plain cover have already been mentioned, 

but even in areas of relatively good exposure of 

crystalline rocks, mapping is made difficult by extensive 

vegetation and thick weathering horizons. The 

availability of high-resolution aeromagnetic and spectral 

aeroradiometric data over the crystalline rocks of the 

Eastern United States would lead to greatly improved 

geologic maps (as would the availability of spectral 

aeroradiometric data over the areas covered by glacial 

deposits and sedimentary rocks). Improved geologic 

maps could in turn result in greater understanding of the 

structural constraints to be placed on the interpretation 

of deep-crustal seismic profiles such as those recently 

collected across the Appalachian Mountains and 

Southern Coastal Plain. 

Maine and the Gulf of Maine 

Of all the states in the Eastern United States, 

Maine contains the largest tracts of land for which 

mineral rights are available. Most of these large tracts of 

land are owned by paper companies. In addition, Maine 

has the largest mineral resource potential of any state in 

New England. Economic lead-zinc deposits in New Brunswick are associated with sedimentary iron deposits lying 

along geologic structures that trend into northern Maine. 

The presence of ultramafic bodies in Maine raises the 

possibility of occurrences of chromium and other 

strategic mineral deposits both onshore and beneath the 

shallow waters of the Gulf of Maine. Shows of tin and the 

presence of potentially tin bearing granites provide 

additional incentive for mineral exploration. 

Aeromagnetic coverage is very poor or absent for 

most of Maine and the Gulf of Maine. Much better 

coverage is needed if meaningful mineral resource 

appraisals are to be made. Spectral aeroradiometric data 

are needed onshore for mapping the large areas of glacial 

cover and would also be useful for identifying potentially 

tin bearing granites. 

Like other areas of New England, the State of 

Maine is concerned with problems of ground-water flow 

and contamination. There is a history of radiometric 

94 Geologic Applications of Modern Aeromagnetic Survey 

contamination of ground water in southwestern Maine, 

and the potential for radiometric or chemical 

contamination exists elsewhere in the State. Aeromagnetic and aeroradiometric mapping would help 

address these problems. 

The Geological Survey of Maine is currently 

involved in the State Geophysical Maps project of the 

USGS and can be expected to support actively additional 

geophysical work within the State. 

Georgia and Florida 

Recent COCORP (Consortium for Continental 

Reflection Profiling) deep-:-crustal seismic data suggest 

that southern Georgia and Florida have been sutured 

onto the North American craton. Existing aeromagnetic 

data in the area of the proposed suture are inadequate 

for testing the COCORP model. Other recent studies 

suggest that oceanic fracture zones can be traced 

completely across the Florida peninsula from the 

Atlantic to the Gulf Coast. Existing aeromagnetic data in 

Florida and the adjacent continental shelf are inadequate 

to test this theory. In Florida, as in most other states, the 

existing aeromagnetic data were collected primarily for 

the purpose of making aeromagnetic anomaly contour 

maps. The specifications of the surveys do not permit the 

data to be used effectively for quantitative analysis with 

today's state-of-the-art computer software. Highresolution aeromagnetic data that are amenable to 

computer processing are needed to address today's 

geologic problems. 

Exclusive Economic Zone 

Although much of the Atlantic EEZ is covered by 

a high-resolution aeromagnetic survey, the relatively high 

altitude and wide flight -line spacing of this survey may be 

inadequate in places for addressing the geologic 

problems that are anticipated as a result of programs 

such as GLORIA (Geological Long-Range Inclined 

Asdic). Some parts of the EEZ, such as the Gulf of 

Maine, the Gulf Coast, and the region around Puerto 

Rico, are not covered at all. Adequate aeromagnetic 

coverage is a necessary first step in assessing the mineral 

and energy resource potential of this huge region. 

Atlantic Coastal Plain 

A decade ago, aeromagnetic data and total-count 

aeroradiometic data were collected over the Atlantic 

Coastal Plain under the auspices of the Coastal Plain 

Regional Commission with the joint purposes of 
completing the aeromagnetic mapping and identifying 

placer deposits of heavy minerals that are associated with 

monazite-bearing radioactive phosphate. The 1.6-km (1 

mi) flight-line spacing and the variable flight-line 

directions serve to limit the usefulness of these data for 

studies of the crystalline basement and the overlying 

early Mesozoic basin deposits which are commonly 

within 1 km of the surface. Adequate modern coverage of 

this large and important region would require 0.4-km (¥4 

mi) spacing, high-resolution aeromagnetic data, and 

spectral aeroradiometric data. 

Radon Hazard 

Spectral aeroradiometric data have been 

mentioned in connection with geologic mapping of glacial 

deposits, Coastal Plain deposits, Appalachian basin units, 

and exposed crystalline rocks. In addition to use as a 

general geologic mapping tool, spectral aeroradiometric 

data can be used to identify lithologies that are associated 

with minerals, such as tin and heavy-mineral placers and 

uranium. Radon, which is a gas associated with the decay 

of uranium, has been identified as an environmental 

health hazard. Accurate mapping of the radon hazard 

should be a primary goal of the USGS airborne mapping 

program. The State of Pennsylvania is very interested in 

a spectral aeroradiometric survey for purposes of radon 

hazard mapping, as well as general geologic mapping and 

resource appraisal. It is expedient to combine spectral 

aeroradiometric surveys with aeromagnetic surveys over 

much of the Eastern United States because in areas of 

magnetized terrane the data collection specifications for 

the two kinds of data are much the same: flight lines 

draped at low altitude over the terrain, and flight-line 

spacing ideally twice the terrain clearance. The National 

Uranium Resource Evaluation (NURE) program of the 

U.S. Department of Energy combined aeromagnetic and 

spectral aerodiometric techniques in a reconnaissance 

survey of the United States which resulted in about 5 

percent coverage of the country. In the Eastern United 

States, coverage should be increased to at least 70 

percent for the reasons outlined above. 

Discussion Group Reports 95 
Notes of Discussion Group on Technology 

By Frank C. Frischknecht, Richard H. Godson, and Richard J. Wold 

MAGNETOMETERS 

Because any new National aeromagnetic program 

will require significant time to be fully implemented, the 

discussion group considered it to be worthwhile to keep 

an eye on probable future developments when 

establishing realistic specifications. Our discussion group 

generally agreed that 0.01 nT is a desirable and 

achievable sensitivity. The magnetometer and data 

acquisition system should be capable of a sample rate of 

10 per second, although this density is not needed unless 

the terrain clearance is small. Some group members 

advocated routine collection and recording at 10 samples 

per second; other members thought this placed an 

unnecessary burden on recording systems and computers 

to store, read, and edit out unneeded data. The sensitivity 

of 0.01 nT can easily be met with optical pumping 

magnetometers that are now generally available, 

although stability of alkali-vapor magnetometers may be 

a problem. 

COMPENSATION 

Various measures of magnetometer performance 

were discussed, and there was general agreement that a 

better scheme for measuring performance is needed. 

Doug Hardwick has a new index, but this is not thought 

to be the most appropriate measure. There was general 

agreement that the following manuevers, if conducted at 

high altitude in a low-gradient area, provide adequate 

data by which to measure magnetometer performance: 

Rolls, ± 10°; pitches, ± so; and yaws, ± S0

• These 

maneuvers should be performed in the magnetic north, 

south, east, and west directions with a period of 3-S sec 

to go from the neutral-to-extreme-to-neutral position for 

each maneuver. At the present time the best figure of 

merit (the sum of all 12 maneuver errors as measured 

above) is 2-4 nT. The corresponding state-of-the-art 

root-mean-square (rms) error is about O.OS nT. Several 

methods for compensation were discussed, including use 

of compensation coils and permalloy strips, real-time 

compensation by onboard computer using manuever 

96 Geologic Applications of Modern Aeromagnetic Survey 

data from a three-component fluxgate magnetometer or 

other motion sensors, and ground compensation by 

computer using recorded attitude data. No strong 

preference for any of these schemes was expressed. The 

minimum peak-to-peak noise envelope that can currently 

be achieved under normal low-level survey conditions is 

about 0.1--0.2 nT. This corresponds to fourth differences 

of about 1.6-3.2 nT. Computation of fourth differences 

in flight, as a means of noise evaluation, is a fairly 

standard procedure. 

VECTOR MEASUREMENTS 

Vector measurements appear to offer important 

information that is not available from scalar measurements. More research on the use of vector measurements 

in interpretation of geologic features is needed. The 

resolution of vector measurements will probably never be 

as good as that of scalar measurements. The performance of existing gyros does not match that of the best 

scalar magnetometers. Stable three-component fluxgate 

magnetometers are used to obtain vector information. 

Alternatively, angles can be read from the mechanism of 

a self-orienting total-field magnetometer such as the 

ASQ-10. If possible, the reference gyro should be 

mounted on the same rigid frame as the magnetometer. 

Flexing of the airframe causes problems if the two 

devices are attached at different locations. Laser gyros 

are likely to become standard for vector measurements. 

A sensitivity of 0.2S0 in direction is useful in studying 

geologic problems. 

GRADIOMETERS 

Technology for measuring vertical and horizontal 

gradients of total field is available. There was general 

agreement that serious consideration be given to 

incorporation of both vertical- and horizontal-gradient 

measurements in any National aeromagnetic program. 

Estimates of the increase in cost over single-sensor 

measurements ranged from "little more" to about 200 
percent. This range of opinion seemed to depend on 

apportionment of a single-time "mobilization" cost (for 

development and implementation of new equipment) 

versus a new "operational" cost (for executing surveys), 

the latter of which may not be much greater than current 

costs for conventional technology. More research on use 

of gradients is needed; for example, the possibility of 

increasing the line spacing when the transverse gradient 

is measured should be thoroughly investigated. The noise 

level that currently can be achieved in total-field gradient 

measurements is about 0.025 nT/m. Although this is 

useful for many purposes, it would be desirable to bring 

the noise level down to about 0.003 nT /m in order to use 

gradient measurements for study of deeply buried 

magnetic basement. 

Superconducting Quantum Interference Device 

(SQUID) magnetometers are capable of very high 

sensitivity and high frequency response, and they offer 

considerable promise for measuring three-orthogonal 

gradients. The U.S. Navy has done considerable work 

with these devices, and contractors are beginning to 

experiment with SQUID gradiometers. The sensitivity of 

SQUID magnetometers is such that high-resolution 

gradient measurements can be made using very short 

baselines. SQUID technology could, in 5 years or less, 

supply the ultimate magnetometer/gradiometer for the 

foreseeable future. Optical-fiber magnetomers were 

mentioned but not discussed. 

NAVIGATION AND 

POSITIONING SYSTEMS 

Full implementation of the Global Positioning 

System (GPS) capability will revolutionize navigation and 

flight-path recovery of geophysical aircraft. When 

operated in the differential mode, the system is accurate 

to within a few meters and should be adequate for almost 

any airborne survey purpose. It was suggested that the 

horizontal position of the aircraft should be recovered to 

within 10 m in the horizontal direction and that the 

altimeter that measures terrain clearance should resolve 

distances to 1 m. This specification for horizontal 

distance can only be met by the GPS system or by 

ground-based transponder systems. Although radar 

altimeters that have the desired resolution are available, 

they tend to average the terrain clearance and respond to 

trees, buildings, and other objects that extend above the 

ground. The best accuracy that is achieved by visual 

means of flight-path recovery (tracking camera or video 

system) is about 20-30 m; at high altitudes the accuracy 

is much poorer. Doppler systems provide a continuous 

track with good relative accuracy, but the absolute 

accuracy of Doppler systems is no better than that of 

visual control, despite the fact that visual control is only 

required at the ends of survey lines. Some members of 

the group expressed the opinion that, in the interim 

period before G PS is fully operational, ground 

transponder systems should be used routinely, despite 

the additional costs. The use of high-resolution recording 

barometric altimeters is recommended. However, to 

obtain good results the barometric altimeter should be 

compared with the radar altimeter at a suitable location 

that has a known elevation at least at the beginning and 

end of each flight. Ideally, the barometric altimeter might 

be checked as often as twice per hour because pressure 

changes can occur very rapidly. A ground-based 

recording barometric altimeter is useful in correcting the 

aircraft altimeter. 

Usual specifications for adherence to flight-line 

spacing are that flight lines should be separated by no 

more than one and one-half of their nominal spacing and 

they should be no closer than one-half of the nominal 

spacing. There was no general agreement on the permissible length of segments that are separated by the 

maximum or minimum amount. 

TEMPORAL OR DIURNAL 

VARIATIONS AND CORRECTIONS 

A recording magnetometer is always needed at the 

aircraft base to determine if temporal variations exceed 

allowable limits for surveying. If ground data are to be 

used for removal of temporal variations from the survey 

data, a ground monitor should be located near the center 

of the survey area. Some contractors recommend use of 

identical magnetometers in the aircraft and on the 

ground; in any case, the sensitivity of the magnetometers 

should be the same. The ground monitor should sample 

the field at least once per second, and the monitor and 

aircraft magnetometers should be synchronized with an 

accuracy of 1 sec or better. There was general consensus 

that many of the specifications used for allowable 

temporal variations while flying are not very good. One 

suggestion was to define temporal variations on the basis 

of 2-min-long chords; deviations from the chords should 

not exceed 2 nT south of the auroral zone. In the auroral 

zone it is necessary to relax the specifications somewhat; 

otherwise the aircraft will be grounded too much of the 

time. Ideally, the specifications for allowable temporal 

variations should be tied to the magnetic latitude. Also, 

the number of tie lines needed depends on magnetic 

latitude. Clearly there is a tradeoff between specifications 

for allowable variations while flying and the number of tie 

lines that are required. Coupling those specifications 

could improve surveying efficiency. It was recommended 

that the ratio of tie lines to flight lines should generally be 

about 1:10; 1:4 was recommended by some persons, and 

1:20 was thought to be too great. Altitude differences 

Discussion Group Reports 97 
between flight and tie lines at intersections should be no 

more than ± 20m, assuming adequate aircraft safety. 

Further investigation of the diurnal correction 

problem is needed before final specifications for a 

National program are developed. In particular, it would 

be very worthwhile to do research on the use of a network 

of ground stations for correction of temporal variations 

when surveying large areas. The Magnetic Observatory 

program also should be strengthened to permit 

development and dissemination of improved models of 

the Definitive International Geomagnetic Field for use in 

computing anomalies. 

DATA PROCESSING 

Most data processing procedures are fairly standard; the largest differences between the practices of 

various contractors are in leveling and gridding 

procedures. In a National program the USGS should 

establish uniform standards for formats. The standard 

coordinate system should be latitude and longitude. 

Some members of the group thought there should be 

uniform standards for contouring and gridding as well. 

The idea was expressed that perhaps the USGS or a 

separate contractor should do all of the data processing. 

In any case, quality control is a large and important task. 

Generally it was agreed that the contractor responsible 

for data processing must be responsible for quality 

control but that the USGS or other organization must 

monitor the processing to ensure that it is done properly. 

Lists should be maintained of all level adjustments made 

in going from the raw data to the final product. Any 

smoothing or other operations must similarly be 

documented; generally no filtering should be allowed. 

Contractor preparation of second-derivative maps would 

be a good means of quality control. It was suggested that 

acceptance of data might take place in two stages. The 

first would be to inspect, evaluate, and accept or reject 

survey data in essentially raw form. Once the raw data 

are accepted, the objective would be to concentrate on 

doing a good job of processing. This would eliminate any 

controversies over the final product due to flaws in the 

raw data. 

The problem of differences in flight heights 

between adjacent flight lines was discussed in detail. It 

was generally agreed that in areas where this is a 

potential problem a race-track pattern for surveying 

98 Geologic Applications of Modern Aeromagnetic Survey 

should be adopted. By race tracking, most adjacent lines 

will be at similar heights although there may be 

substantial differences between blocks of lines. It was 

suggested that in processing, data continuation should be 

used to level the lines to a smooth surface; possibly this 

procedure is already being used by some contractors. 

Measurement of vertical gradients would be useful for 

this purpose in flat terrain but less so in rough terrain. 

Research on this problem is probably warranted. 

Archiving and disseminating aeromagnetic data is 

an enormous task that was not discussed in much detail. 

In a National program resources must be allocated to 

archiving. Available data for visual recovery of the flight 

as well as electronic navigation and magnetometer data 

should be archived. Contractors should keep backup 

copies of all data for a minimum period of, for example, 

18 months. 

OTHER METHODS 

The value and feasibility of using other methods in 

conjunction with aeromagnetics was discussed. Very low 

frequency (VLF) electromagnetic measurements would 

be a valuable addition to aeromagnetics in many areas, 

and the additional cost would be on the order of $1.00 

per line mile provided breaks in the data are acceptable. 

A simple four-channel radiometric system with a crystal 

volume of about 1,000 in.3 would add about 30 percent to 

the cost of aeromagnetic surveys. Radiometric data are 

thought to be very valuable in many regions; in some 

areas radiometric data are more useful than aeromagnetics in constructing surface geologic maps. A 

controlled-source electromagnetic system designed more 

for resistivity mapping than mineral exploration would be 

extremely valuable in many areas but would at least 

double the cost of aeromagnetic surveying. The fivecomponent gravity gradiometer is perhaps 3 years away 

from being ready for routine application; one estimate of 

cost for using such an instrument is $60--100 per line mile 

in 1987 dollars. Specifications for over-water surveys 

were not discussed in much detail, although the group 

agreed that a National aeromagnetic program should 

include surveys of offshore areas. Requirements for 

navigation are obviously very different for over-water 

surveys than for over-land surveys; in addition, faster 

aircraft could probably be used for over-water surveys. 
Notes of Discussion Group on Rock Magnetics 

By Richard L. Reynolds and Charles M. Schlinger 

INTRODUCTION 

An essential component of the geologic 

interpretation of aeromagnetic data is a fundamental 

understanding of how magnetization depends on 

petrology. Traditionally this has often been overlooked 

because of the lack of pertinent data. Aeromagnetic data 

are numerical data; the link to geology is most effective 

:when the magnetization of representative geologic units 

can be measured or reliably estimated. We recommend 

that any National aeromagnetic program should include 

well-defined and adequately funded provisions designed 

to bridge the gaps among aeromagnetic interpretation, 

magnetic properties, and petrology, which will lead to 

meaningful three-dimensional geologic maps and 

interpretations. 

Research in the area of magnetic properties and 

petrology would build on existing results and knowledge 

to address the following problems: (1) The relations of 

aeromagnetic signals to rock magnetizations in 

crystalline rocks and the dependence of these 

magnetizations on bulk rock composition, metamorphism, and hydrothermal alteration, which may be 

closely tied to mineralization; (2) the magnetizations of 

sedimentary rocks and the high-temperature (hydrothermal) and low-temperature (diagenetic) modifications of 

these magnetizations; and (3) the magnetization of the 

crust as a whole and its lateral and vertical magnetic 

variations, as well as the tectonic and petrologic 

significance of these variations. 

Rock magnetic research in support of an aeromagnetic program would include field measurements of 

magnetic susceptibility, as well as field and core sampling 

and laboratory studies. The laboratory work would 

investigate magnetic mineralogy, magnetic properties 

(magnetic susceptibility, remanent direction and 

magnitude, paleomagnetic stability, and temperature 

dependence of magnetic properties), and the petrologic 

controls on magnetizations. In addition, borehole 

magnetic susceptibility studies should be carried out 

whenever possible in areas of interest. 

MAGNETIC TERRANES AND 

METHODS OF STUDY 

Shallow and Exposed Magnetic Sources 

The structural and chronological relations in 

complex igneous and metamorphic regions can be greatly 

clarified by aeromagnetic mapping (Hinze, 1985) with 

support from rock magnetic and petrologic studies of 

samples at and near the surface. Special attention to the 

original petrochemical/mineralogical setting of a given 

unit and its modification by tectonism and mineralization 

may lead to the recognition of accreted terranes or 

environments that are favorable for mineral deposits. For 

example, relations among aeromagnetic signature, metamorphic geology, and rock magnetic properties have 

proved valuable in identifying areas of the Canadian 

Shield that are favorable for mineralization (summarized 

by Grant, 1984/1985a, b). Principles learned in areas of 

exposed basement can be extended to buried crystalline 

terranes with thin to moderate cover. 

Over thick sections of sedimentary rocks, aeromagnetic methods have potential for detecting the effects 

of vertically directed alteration plumes. The 

understanding of such features and their magnetic 

signatures is currently very limited, but where such 

an understanding exists, it is based on detailed rock 

magnetic, mineralogical, and geochemical study 

(Reynolds and others, 1986). Possible targets include 

detection of past and present hydrocarbon seepage and 

of paths of fluid migration in basins leading to mineral 

deposition (for example, lead-zinc deposits in the Midcontinent and sandstone uranium deposits in the San 

Juan basin, New Mexico, and in the south Texas coastal 

plain). 

Intermediate Magnetic Sources 

Rock magnetic data are extremely limited for the 

aeromagnetic interpretation of rocks at intermediate 

crustal levels (2-10 km), below the depths of routine core 

Discussion Group Reports 99 
drilling. Current and future sources of information on 

the magnetic properties of rocks formed at these depths 

include (1) tectonically exposed sections, (2) existing drill 

core and anticipated samples from the deep continental 

drilling program, and (3) assumptions of rock types and 

their magnetic properties deduced from shallower rocks 

of equivalent compositions. 

Among the key geologic problems related to 

structure and events at these depths are the magnetic 

structure associated with thermal plumes, such as the 

Yellowstone hot spot (Eaton and others, 1975); the 

igneous geology and regional tectonics of the Midcontinent region (Sims and Peterman, 1986); and implications 

for mineralization, hydrocarbon traps, and seismic 

activity. Aeromagnetic interpretations in these areas will 

benefit from magnetic property studies of analogous 

lithologies that are exposed at the surface and in drill 

core, with corrections applied for anticipate<:J effects of 

elevated pressure and temperature and for possible 

mineralogic changes related to crustal depth. 

Deep Magnetic Sources 

The long-wavelength aeromagnetic signatures of 

deep crustal (and possibly upper mantle) sources 

generally reflect anomalous magnetizations that persist 

throughout extensive volumes of these regions (Mayhew 

and others, 1985). These anomalies and the causative 

magnetization contrasts can be best understood in terms 

of (1) field and laboratory study of exposed crustal 

sections and xenoliths (Schlinger, 1985; Wasilewski and 

Mayhew, 1982; Wasilewski and Fountain, 1982), (2) 

laboratory experimental study of the pressure and 

temperature effects on magnetization and on magnetic 

properties, and (3) thermodynamic calculations and 

experimental study of the stability of magnetic mineral 

assemblages at depth in the Earth's crust and upper 

mantle (Frost and Shive, 1985; Haggerty, 1978). 

LIMITATIONS 

Methods for sampling each of these source regions 

have limitations. Magnetic properties of rocks ~ow at the 

surface will presumably differ from those properties of 

equivalent rocks at great depths, because of possible 

changes in magnetic minerals and (or) magnetic 

properties produced by changes in pressure, 

temperature, and oxygen fugacity that accompany uplift. 

In some cases surface alteration is important. 

Understanding the magnetic contrasts associated with 

deep crustal sources and the magnetic mineralogy at 

great depth is especially problematic, because only 

indirect samples are available. Deep-continental drill 

100 Geologic Applications of Modern Aeromagnetic Survey 

core will provide valuable but limited samples from 

intermediate depths in the few chosen settings. Obtaining 

representative samples from units at or near the surface 

is even a problem in areas of relatively thin cover or in 

areas in which magnetic contrasts are caused by 

alterations that are limited in horizontal dimensions, 

such as those controlled by faults. 

In all cases, the application of aeromagnetic data is 

most effective when interpretations are constrained by 

one or more other geophysical data sets, including 

gravity, electrical, seismic, and radiometric observations. 

GENERAL RELEVANCE OF ROCK 

MAGNETIC-PETROLOGIC STUDIES 

Rock magnetic-petrologic studies will have broad 

applicability to the geologic interpretation of ground, 

aero-, and satellite magnetic data. In addition, these 

studies will have a substantive overlap with current 

national and international initiatives that address (1) the 

physics and chemistry of rocks and minerals, (2) 

continental lithosphere, and (3) deep continental drilling. 

REFERENCES CITED 

Eaton, G.P., Christiansen R.L., Iyer, H.M., Pitt, A.M., Mabey, 

D.R., Blank, H.R., Jr., Zietz, Isidore, and Gettings, M.E., 

1975, Magma beneath Yellowstone National Park: 

Science, v. 188, p. 787-796. 

Frost, B.R., and Shive, P.N., 1986, Magnetic mineralogy of the 

lower continental crust: Journal of Geophysical 

Research, v. 91, p. 6513-6521. 

Grant, F.S., 1984/1985a, Aeromagnetics, geology, and ore 

environments; I, Magnetite in igneous, sedimentary and 

metamorphic rocks-An ovetview: Geoexploration, v. 

23, p. 303-333. 

__ 1984/1985b. Aeromagnetics, geology, and ore 

environments; II, Magnetite and ore environments: 

Geoexploration, v. 23, p. 335-362. 

Haggerty, S.E., 1978, Mineralogical constraints on Curie 

isotherms in deep crustal magnetic anomalies : Geophysical Research Letters, v. 5, p. 105-108. 

Hinze, W J., ed., 1985, The utility of regional gravity and 

magnetic anomaly maps: Tulsa, Okla., Society of 

Exploration Geophysicists, 454 p. 

Mayhew, M.A., Johnson, B.D., and Wasilewski, P J., 1985, A 

review of problems and progress in studies of satellite 

magnetic anomalies: Journal of Geophysical Research, v. 

90, p. 2511-2522. 

Mayhew, M.A., and LaBrecque, J.L., 1987, Crustal geologic 

studies with Magsat and surface magnetic data: Reviews 

of Geophysics, v. 25, no. 5, p. 971-981. 
Reynolds, R.L., Fishman, N.S., Hudson, M.R., Karachewski, 

J .A., and Goldhaber, M.B., 1986, Magnetic minerals and 

hydrocarbon seepage-Possibilities of magnetic 

detection of oil fields: U.S. Geological Survey Research 

on Energy Resources, U.S. Geological Survey Circular 

974, p. 58-59. 

Schlinger, C.M., 1985, Magnetization of lower crust and 

interpretation of regional crustal anomalies-Example 

from Lofoten and Vesteralen, Norway: Journal of Geophysical Research, v. 90, p. 11,484--11,504. 

Sims,, P.K., and Peterman, Z.E., 1986, Early Proterozoic 

Central Plains orogen-A major buried structure in the 

north-central United States: Geology, v. 14, p. 488--491. 

Wasilewski, P.J., and Fountain, D.M., 1982, The Ivrea zone as 

a model for the distribution of magnetization of the 

continental crust: Geophysical Research Letters, v. 9, 

p. 333-336. 

Wasilewski, P.J., and Mayhew, M.A., 1982, Crustal xenolith 

magnetic properties and long wavelength anomaly source 

requirements: Geophysical Research Letters, v. 9, p. 

329-332. 

Discussion Group Reports 101 

WORKSHOP PARTICIPANTS 

AND 

WORKSHOP PRESENTATIONS 
APPENDIX A 

Workshop Participants 

An ad hoc organizing committee composed of R.J. 

Blakely, H.R. Blank, J.W. Cady, D.L. Campbell, L.E. 

Cordell, F.C. Frischknecht, R.H. Godson, V J.S. Grauch, 

T.G. Hildenbrand, R.C. Jachens, and J.D. Phillips of the 

U.S. Geological Survey developed through an iterative 

process a subject agenda and a tentative list of invitees to 

-the workshop. The original list included more than 100 

names of persons renowned in the field of aeromagnetic 

surveying and analysis. The following shorter list of 

workshop participants was derived within the constraints 

of space, time, and funding resources immediately 

available and in view of the feasibility for attendance on 

short notice during a major holiday period. Several 

invitees, some of whom could not attend the workshop, 

supplied documents of great interest prior to or following 

the workshop. Special acknowledgments are express to F. 

Slade Barker, U.S. Naval Oceanographic Office, on the 

feasibility of using the Project MAGNET aircraft to 

perform selected aeromagnetic vector surveys; Richard 

F. Blakely, U.S. Geological Survey, and Gerald G. Connard, Northwest Geophysical Associates, Inc., on the 

theory and application of aeromagnetic surveying; 

Joseph C. Cain, Florida State University, Tallahassee, 

Fla., on satellite-derived global magnetic models; David 

L. Campbell, U.S. Geological Survey, on profiles showing 

models of magnetic structures in accreted terranes of 

south-central Alaska; John W. ("Jack") Corbett, 

consulting geophysicist, on exploration strategies in the 

minerals industry; Frank C. Frischknecht and M. Dean 

Kleinkopf, U.S. Geological Survey, for providing detailed 

notes taken during the workshop; S. Parker Gay, Applied 

Geophysics, Inc., Salt Lake City, Utah, on results of the 

6th International Conference on Basement Tectonics; 

James R. Heirtzler, National Aeronautics and Space 

Administration, on planned magnetic programs involving 

the National Aeronautics and Space Administration; 

John F. ("Jack") Hermance, Brown University, on 

possibilities for future geomagnetics research in the 

military and civilian sectors; Robert H. Higgs, formerly 

U.S. Naval Oceanographic Office, Bay St. Louis, Miss., 

on the airborne vector magnetic survey capability of the 

104 Geologic Applications of Modern Aeromagnetic Survey 

U.S. Naval Oceanographic Office; William J. Hinze, 

Purdue University, for providing notes and illustrations 

on the need for worldwide magnetic data bases and map 

inventory; Martin F. Kane, U.S. Geological Survey, for 

information on contract aerogeophysical data of Saudi 

Arabia; Elizabeth R. King, U.S. Geological Survey, for 

serving as a technical reviewer for contributions to this 

volume and for providing historical data; Douglas P. 

O'Brien, Comap Exploration Services, Inc., for material 

on the interpretation and display of the relationship of 

aeromagnetic features to geochemical anomalies; notes 

on petroleum exploration data sets and diagenetic 

magnetic mineral indicators; and illustrations showing 

filtered magnetic profiles, seismic data, and models of 

shallowly emplaced concentrations of magnetic mineral 

enrichment; Mike S. Reford, Geoterrex Limited, on 

international institutional support for aeromagnetic 

surveying; John M. Schmunk, Airmag Surveys, Inc., on 

the preciSion of measuring microanomalies as 

determined by the repetition of test flights; Don W. 

Steeples, Kansas Geological Survey, on the combined use 

of aeromagnetic and drill-hole data for interpretation of 

subsurface structure in Kansas; Patrick T. Taylor, 

National Aeronautics and Space Administration, on 

activities of the Interagency Magnetic Field Survey 

Working Group; and Isidore Zietz, consulting 

geophysicist, for providing historical information about 

geophysical work in the U.S. Geological Survey. 

Working Group abbreviations shown for the 

following workshop participants are: [W], Western 

United States; [BR], Basin and Range; [MC], Midcontinent; [E], Eastern United States; [T], Technology; and 

[RM], Rock Magnetism. 

F. Slade Barker, U.S. Naval Oceanographic Office, Bay 

St. Louis, Miss. [BR, T] 

John C. Behrendt, U.S. Geological Survey, Denver, 

Colo. [E] 

Richard J. Blakely, U.S. Geological Survey, Menlo Park, 

Calif. [W] 

H. Richard Blank, U.S. Geological Survey, Denver, 

Colo. [BR] 

Ronald W. Buhmann, National Geophysical Data 

Center, National Oceanic and Atmospheric 

Administration, Boulder, Colo. [T] 
John W. Cady, U.S. Geological Survey, Denver, 

Colo. [W] 

Val W. Chandler, Minnesota Geological Survey, St. Paul, 

Minn. (MC] 

Gerald G. Connard, Northwest Geophysical Associates, 

Inc., Corvallis, Ore. [T] 

John W. Corbett, Consulting Geophysicist, Denver, 

Colo. (BR] 

Lindreth E. Cordell, U.S. Geological Survey, Denver, 

Colo. (MC] 

Richard W. Couch, Oregon State University, Corvallis, 

Ore. (W] 

Richard 0. Crosby, Aero Service, Englewood, Colo. (T] 

W. Minor Davis, National Environmental Satellite, Data, 

and Information Service, National Oceanic and 

Atmospheric Administration, Boulder, Colo. [T] 

John E. Decker, Alaska Division of Mining and Geologic 

Survey, Fairbanks, Alaska. (W] 

John M. De Noyer, U.S. Geological Survey, Reston, 

Va. (E) 

Frank C. Frischknecht, U.S. Geological Survey, Denver, 

Colo. (deceased) [T] 

Richard H. Godson, U.S. Geological Survey, Menlo 

Park, Calif. (T] 

V.J.S. (Tien) Grauch, U.S. Geological Survey, Denver, 

Colo. (BR] 

Andrew Griscom, U.S. Geological Survey, Menlo Park, 

Calif. [W] 

William F. Hanna, U.S. Geological Survey, Reston, Va. 

(W, BR, MC, E, T, RM] 

Richard 0. Hansen, Colorado School of Mines, Golden, 

Colo. (BR] 

-Paul C. Heigold, Illinois State Geological Survey, Champaign, Ill. (M C) 

James R. Heirtzler, Goddard Space Flight Center, 

National Aeronautical and Space Administration, 

Greenbelt, Md. (E) 

Thomas G. Hildenbrand, U.S. Geological Survey, 

Denver, Colo.; currently, Menlo Park, Calif. [MC] 

William J. Hinze, Purdue University, West Lafayette, 

Inc. (MC] 

Peter J. Hood, Geological Survey of Canada, Ottawa, 

Ontario [E] 

Robert C. Jachens, U.S. Geological Survey, Menlo Park, 

Calif. (W, BR] 

Martin F. Kane, U.S. Geological Survey, Denver, 

Colo. [E] 

M. Dean Kleinkopf, U.S. Geological Survey, Denver, 

Colo. [MC] 

Don R. Mabey, formerly Utah Geological and Mineral 

Survey, Salt Lake City, Utah, and currently U.S. 

Geological Survey, Salt Lake City, Utah [BR] 

Michael A. Mayhew, National Science Foundation, 

Washington, D.C. [E] 

Lyle D. McGinnis, Argonne National Laboratory, 

Argonne, Ill. (MC] 

Benjamin A. Morgan, U.S. Geological Survey, Reston, 

Va. [E] 

Douglas P. O'Brien, Comap Exploration Services, Inc., 

Lakewood, Colo. (T] 

Jeffrey D. Phillips, U.S. Geological Survey, Reston, 

Va. (E] 

Gary L. Raines, U.S. Geological Survey, Reno, 

Nev. [BR] 

Mike S. Reford, Geoterrex Limited, Ottawa, Ontario (T] 

Richard L. Reynolds, U.S. Geological Survey, Denver, 

Colo. (RM] 

Charles M. Schlinger, University of Utah, Salt Lake City, 

Utah (RM] 

John M. Schmunk, Airmag Surveys, Inc., Philadelphia, 

Pa. (T] 

Paul K. Sims, U.S. Geological Survey, Denver, 

Colo. (MC] 

Richard L. Summerfelt, EDCON, Inc., Lakewood, 

Colo., [T] 

Don W. Steeples, Kansas Geological Survey, Lawrence, 

Kan. [MC] 

Patrick T. Taylor, Goddard Space Flight Center, 

National Aeronautical and Space Administration, 

Greenbelt, Md. [T] 

Dennis J. Teskey, Geological Survey of Canada, Ottawa, 

Ontario [T] 

Raymond D. Watts, U.S. Geological Survey, Reston, 

Va. [E] 

Richard D. Wold, TerreSense, Inc., Sunnyvale, Calif. [T] 

Discussion Group Reports 105 
APPENDIX B 

Workshop Presentations 

(1) Welcome and introduction, by William F. 

Hanna. 

(2) Worldwide aeromagnetic coverage and how 

we can acquire and make use of it, by 

William J. Hinze. 

(3) Mineral applications of Canadian aeromagnetic coverage and the use of vertical 

gradiometry, by Peter Hood. 

( 4) Perspectives on the usefulness of aeromagnetic data in Kansas, by Don W. 

Steeples. 

(5) Aeromagnetic program of Minnesota, allied 

geophysical studies, and their anticipated 

usefulness, by Val W. Chandler. 

( 6) Utility of aeromagnetic data in exploration 

of mineral district-size or prospect-size 

areas, by John W. Corbett. 

(7) Aeromagnetic anomalies associated with 

sedimentary rocks and with surface geochemical anomalies, by Douglas P. O'Brien. 

(8) Applications of horizontal gradiometry, by 

Richard J. Wold. 

1 06 Geologic Applications of Modern Aeromagnetic Survey 

(9) Low-level vector magnetic data from part of 

the eastern coast of the United States, by F. 

Slade Barker. 

(10) Some examples of high-precision surveys, 

the use of GPS for navigation, and 

comments about the reproducibility of totalfield and vector surveys, by John M. 

Schmunk. 

(11) Brief review of interpretation techniques 

and application to the Curie-temperature 

isotherm problem, by Richard J. Blakely. 

(12) Optimal specifications for acquiring aeromagnetic data, by Frank C. Frischknecht. 

(13) Merging and display techniques for aeromagnetic data, including examples of 

colored shaded relief, by Thomas G. Hildenbrand. 

(14) A view from afar: Comparison of satellite 

and aeromagnetic anomalies, by Patrick T. 

Taylor. 

(15) A view from within: Rock magnetism of the 

sedimentary upper crust and thoughts about 

the magnetic lower crust, by Richard L. 

Reynolds. 

(16) A State's perspective on a national aeromagnetic program, by Don R. Mabey. 

*U.S. GOVERNMENT PRINTING OFFICE: 1990-0-573-049/26005 

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