Airborne Measurements of 

Terrestrial Radioactivity as 

An Aid to Geologic Mapping 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 516-F 

Prepared on behalf of the 

U.S. Atomic Energy Commission 

Airborne Measurements of 

Terrestrial Radioactivity as 

An Aid to Geologic Mapping 

By JAMES A. PITKIN 

GEOPHYSICAL FIELD INVESTIGATIONS 

GEOLOGICAL SURVEY PROFESS ION AL PAPER 516-F 

Prepared on behalf of the 

U.S. Atomic Energy Commission 

Descriptions and evaluation 

of the correlation of airborne 

measurements of terrestrial 

radioactivity with areal geology 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1968 
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary 

GEOLOGICAL SURVEY 

William T. Pecora, Director 

Library of Congress catalog-card No. GS 68-29& 

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 - Price 45 cents (paper cover) 
CONTENTS 

Page Page 

Abstract------------------------------------------- F1 Selected examples-Continued 

Introduction_______________________________________ 1 Valley and ltidge physiographic province, Appalachian Highlands, eastern United States______ F14 

Coastal Plain physiographic province, eastern, 

Alt~Sprogram------------------------------------ 1 

Aeroradioactivity surveying _____________ ------_______ 2 

Aeroradioactivity equipment_________________________ 2 southeastern, and south-central United States____ 18 

Compilation of aeroradioactivity data_________________ 4 Columbia Plateaus physiographic province, northSources of aeroradioactivity __ ___ ____ ___ ____ ___ _ _ ____ _ 6 western United States_________________________ 21 

Cosmic radioactivity____________________________ 6 Great Plains physiographic province, west-central 

ltadioisotopes in the air_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 6 United States____________________________ 21 

ltadioisotopes in the surface layer of the ground____ 6 Denver area________________________________ 21 

Nuclear fallout_____________________________ 6 GNO~E area ________ --- _____ -------------- 22 

Natural radioisotopes_______________________ 6 Pacific Border physiographic province, western 

Variation of radioisotope content with rock type________ 7 United States-------------------------------- 22 

Interpretive technique_______________________________ 7 Glaciated terrain, midwestern and northeastern 

Causative factors in aeroradioactivity interpretation_____ 7 United States-------------------------------- 23 

TopographY------------------------------------ 8 Puerto ltico____ _ _ _ _ _ _ _ _ _ __ _ _ _ _ __ _ _ _ ___ _ _ _ _ _ _ _ _ _ 24 

LithologY-------------------------------------- 8 Use of aeroradioactivity measurements in conjunction 

Soil___________________________________________ 8 with other geophysical methods_____________________ 24 

Selected examples of the correlation of aeroradioactivity Evaluation of the method-correlation of aeroradiowith areal geology_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 8 activity measurements with areal geology____________ 26 

Piedmont physiographic province, Appalachian High- Airborne gamma-ray spectrometry____________________ 27 

lands, eastern United States____________________ 8 lteferences----------------------------------------- 27 

ILLUSTRATIONS 

FIGURE 1. Aeroradioactivity surveys made by the Geological Survey, 1958-6L--------------------------------------

Page 

F3 

3 2. Diagram of aeroradioactivity equipment _____________________________________________________________ _ 

3. Profiles showing aeroradioactivity, topography, and geology in the Valley and ltidge province, eastern Tennessee---------------------------------------------------------------------------------------- 5 

4. ~ap showing aeroradioactivity and geology of an area in Howard County, ~d---------------------------- 9 

5. ~ap showing aeroradioactivity and geology of an area in Montgomery County, Md________________________ 10 

6. Profiles showing characteristic aeroradioactivity and geology in parts of South Carolina and Georgia_________ 11 

7-16. Maps showing aeroradioactivity and geology: 7. Area near Edgefield, S. C _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 12 

8. Part of the Clarks Hill and Warrenville quadrangles, South Carolina and Georgia__________________ 13 

9. Part of Hart County, Ga------------------------------------------------------------------- 15 

10. Area near Clinton, Tenn-------------------------------------------------------------------- 16 

11. Area near Athens, Tenn--------------------------------------------------------------------- 17 

12. Part of the Valley and ltidge province, north-central Georgia----------------------------------· 19 

13. Area in Prince Georges County, Md---------------------------------------------------------- 20 

14. Area in Parker County, Tex----------------------------------------------------------------- 21 

15. Part of the western Mojave Desert, southern California_________________________________________ 23 

16. Part of the Keene and Brattleboro quadrangles, New Hampshire and Vermont______________________ 25 

TABLES 

TABLE 1. Reports and maps resulting from the ARMS-I program--------------------------------------------- F2 

2. General amounts of uranium, thorium, and potassium in common rocks-------------------------------- 7 

m 

GEOPHYSICAL FIELD INVESTIGATIONS 

AIRBORNE MEASUREMENTS OF TERRESTRIAL RADIOACTIVITY AS 

AN AID TO GEOLOGIC MAPPING 

By JAMES A. PITKIN 

ABSTRACT 

Airborne measurements of terrestrial radioactivity (aeroradioactivity) have been interpreted by correlation with areal 

geology. Rock-unit classification provides guidelines to the varying amounts of uranium, thorium, and potassium in the surface 

rock and soil which generate the measured gamma radiation. 

Correlations made between patterns of aeroradioactivity and 

geology are an aid to geologic mapping. 

About 200,000 square miles in the conterminous United States 

and Puerto Rico was surveyed for aeroadioactivity to determine 

background datum levels of environmental radioactivity around 

nuclear facilities. The aeroradioactivity data were obtained with 

continuously recording scintillation detection equipment which 

included a multicrystal array. Corrections were made for cosmic and aircraft background and for variations from the 5oofoot terrain clearance elevation. 

Topography, lithology, and soil are dominant factors controlling aeroradioactivity interpretation. Topographic relief should 

not exceed 1,000 feet because natural radioactivity cannot be 

accurately measured when the airborne detector is more than 

1,000 feet from ground surface. 

Varying bedrock lithology with consequent radioisotope variation results in the varying patterns of aeroradioactivity that 

are required for successful correlation. Homogeneous distributions of radioisotopes as measured by the scintillation detector 

give vague and discontinuous patterns of aeroradioactivity that 

seldom have correlative value. 

The type of soil at ground surface is generally the major 

factor that determines whether correlation is possibl~, because 

for successful correlation the material at ground surface must 

reflect bedrock composition. Residual soils are ideal for correlation, although transported soils that do reflect composition of 

the underlying bedrock are satisfactory. 'V'here bedrock is 

covered by transported soils having a different composition, 

or by alluvium, loess, and under some circumstances glacial 

debris, correlation is usually impossible. 

Examples of correlation are discussed within a general framework of physiographic provinces which include: the Piedmont 

and the Valley and Ridge provinces; the Coastal Plain, Columbia Plateaus, Great Plains, and Pacific Border provinces; glaciated terrain of the midwest and northeast United States; 

and Puerto Rico. 

The Piedmont province is the ideal locale, where relief is 

within desired limits, rocks are lithologically varied, and soils 

are residual. Aeroradioactivity studies can supplement geologic 

mapping, which is hampered by sparse outcrops, structural complexities, and a general lack of fossils. In other areas, where circumstances are less than ideal, evaluation of correlation requires 

analysis within the given framework, noting the effect of topography, lithology, and soil. Aeroradioactivity surveys when combined with aeromagnetic and (or) electromagnetic surveys, in 

areas where the magnetic or conductive rock has a surface 

expression, are extremely effective facets of geophysical 

exploration. 

The Geological Survey is calibrating and flight testing an airborne gamma-ray spectrometer system. Delineation of gammaray energies with this equipment will permit estimations of the 

relative contribution of the uranium, thorium, and potassium 

components of terrestrial radioactivity. 

INTRODUCTION 

Airborne measurements of terrestrial radioactivity 

have been interpreted by correlation with areal geology. 

The radioactivity measured consists of the gamma rays 

emanating from the rock and soil at the surface. This 

radioactivity is the gross effect of the content of uranium, thorium, and potassium in the surface rock and 

soil. The pattern and intensity of the surface radioactivity usually reflect the areal distribution of rock 

types. Thus airborne radioactivity measurements can 

be used as an aid to areal geologic rna pping. This report 

discusses the correlation of aeroradioactivity measurements with areal geology. As used in this report "aeroradioactivity" refers to terrestrial gamma-ray intensity 

measured from an aircraft at 500 feet above ground 

surface. 

ARMS PROGRAM 

The radioactivity data used in the preparation of this 

report were collected during ·airborne surveys made 

by the U.S. Geological Survey on behalf of the Division 

of Biology and Medicine, U.S. Atomic Energy Commission, as part of the Aerial Radiological Measurement 

F1 
F2 GEOPHYSICAL FIE·LD INVESTIGATIONS 

Surveys (ARMS) program. ARMS-I, the subject of 

this report, was conducted by the U.S. Geological Survey, Washington, D.C.; ARMS-II, by Edgerton, Germeshause:t;l, and Grier, Inc., Goleta, Calif. 

The purpose of the ARMS program was to establish 

gamma radiation datum levels (background) for areas 

in and adjacent to nuclear facilities. A datum level was 

determined as the background radioactivity at the time 

of flying. When the datum has been established, resurveying can disclose any significant changes that may 

have occurred as a result of nuclear testing, reactor or 

other Atomic Energy Commission operations, or radioactivity accidents. 

ARMS-I surveying during July 1958-December 1961 

totaled 188,000 line miles over 200,000 square miles in 

23 areas in the conterminous United States and Puerto 

Rico (fig.1). Published maps and reports giving results 

of ARMS-I are listed in table 1. An initial discussion 

of ARMS-I data interpretation was published in 1964 

(Pitkin and others). Guillou (1964) discussed ARMSII surveying. 

TABLE !.-Reports and maps resulting from the ARMS-I program 

Area Bibliography reference Bibliography reference to CEX report 1 to GP map 

Chicago ________________________________________________ _ 

Columbus ____________ Bates (1966a) _____ Bates (1965b). 

Denver ______________ Popenoe (1966a) __ Popenoe (1965). Fort Worth-Dallas _______________________________________ _ 

Georgia Nuclear MacKallor MacKallor 

Laboratory. (1963a). (1963b). · GNOME ____________ MacKallor MacKallor 

(1965a). (1964). Hanford _____________ Schmidt (1962a) ___ Schmidt (1961a). 

Los Angeles __________ Books (1962a) ____ Books (1962b). 

Minneapolis-St. PauL _________________ -------------------- Northeastern United Popenoe (1964b) __ Popenoe (1964a). 

States-north. 

Northeastern United Popenoe (1966b) __ Popenoe (1966c). 

States-south. 

National Reactor Bates (1965a) _____ Bates (1964). 

Testing Station. 

Oak Ridge National Bates (1962b) _____ Bates (1962a). 

Laboratory. Pittsburgh ___________ Bates (1966b) ____ _ 

Puerto Rico__________ MacKallor 

(1966). San Francisco ________ Books (1966) ____ _ 

Savannah River______ Schmidt 

(1962b). Washington, D.C _____ Neuschel (1966) __ _ 

Bates (1966c). 

MacKallor 

(1965b). 

Books (1965). 

Schmidt 

(1961b). 

Neuschel (1965). 

1 Published by the Atomic Energy Commission. Available from the Clearinghouse for Federal Scientific and Technical Information, National Bureau of Standards, 

U.S. Department of Commerce, Springfield, Va. 22151. 

AERORADIOACTIVITY SURVEYING 

A typical ARMS survey covers a square 100 miles 

on a side, centered at a nuclear facility-a total area of 

10,000 square miles. Variations in area are due to either 

(1) more than one nuclear facility within the planned 

survey which increased the area to more than 10,000 

square miles or (2) topographic relief which limited 

the flyable area to less than 10,000 square miles. Typi· 

cally, surveys are made along parallel flight line~ 

spaced 1 or 2 miles apart and oriented normal to the 

strike of the rock units. Traverses along valleys and 

(or) ridges are made where topographic relief precludes regular surveying. 

Radioactivity data are obtained with continuously 

recording scintillation detection equipment installed in 

a twin -engine aircraft. The aircraft maintains a nominal elevation of 500 feet above ground at an average 

air speed of 145 miles per hour. Topographic maps, 

county road maps, and aerial photographs are used for 

pilot guidance. The flight path of the aircraft is recorded by a gyrostabilized continuous strin-film camera, 

and the distance of the aircraft from the ground is 

measured by a continuously recording radar altimeter. 

Fiducial markings which provide a common reference 

for the radioactivity and altimeter data and the camera 

film are made with an electromechanical edgemark 

device operated by a flight observer when the aircraft 

passes over recognizable features on the ground. 

AERORADIOACTIVITY EQUIPMENT 

The gamma radiation detection equipment used for 

the ARMS-I surveys was developed by the Health 

Physics Division of the Oak Ridge National Laboratory 

(Davis and Reinhardt, 1957). It is internally calibrated 

in counts per second. A diagram of the system is shown 

in figure 2. The detecting element consists of six thallium-activated sodium iodide crystals, each 4 inches in 

diameter and 2 inches thick and coupled to a photomultiplier tube. The output of the detecting element is 

amplified and routed through a discriminator and 

pulse shaper that is set to accept only pulses originating 

from gamma radiation with energies greater than 50 

Kev (thousand electron volts) . The signal is then routed 

to two integrating rate meters, each of which exhibits 

a 1-second time constant. One rate meter feeds a circuit that records total or gross radioactivity on a graphic 

milliammeter. The signal from the other r~te meter is 

recorded by a circuit fro~p. which the cosmic and aircraft background has been subtracted. The resulting 

net signal is then compensated for variations from the 

nominal 500-foot surveying altitude by using a correction potentiometer which is driven by the radaraltimeter output shaft. 

At least twice daily during surveying, the aircraft 

aseends to 2,000 feet above ground and measures the 

background radioactivity. At this altitude, the ground 

contribution to the measured count rate is only a slight 
AIRBORNE MEASUREMENTS OF TERRESTRIAL RADIOACTIVITY 

---------..,.,_ __ _ 

HIINFORo / { ------------

. ~ : \ ------r-----------,- ~____ . ! \. i I 

--{ / : ; 

/ <' ... \ NAno ! iuli\IIIIFAF•ous 

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------ I ! I ~------.L I j 

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\ 

\...._ {---......... ....... , \. 

\ .. 0 500 MILES \, 

\ .•. PUERTO RICO 

o ~o<>oo 0 

F3 

FIGURE 1.-Aeroradioactivity suTveys made by the Geological Survey, 1958.-61. Numbers indicate figures in the report covering 

all or part of areas shown 

HIGH 

VOLTAGE 

111 

"""""" 

~ ~ 

~ Na I (TI) 

CRYSTALS 

AND 

PHOTOMULTIPLIER 

TUBES 

MIXING 

PREAMPLIFIER 

RADAR 

ALTIMETER -+ RECORDER +-

SERVOMECHANISM 

~--------, 

~ ~ AND ~ RATE RECORDER 

LINEAR DISCRIMINATOR COMPENSATED tl 

AMPLIFIER PULSE SHAPER METER 

...___ ___ ____. I 500-CPS UNCOMPENSATED 11...___ ___ ____. 

I 

OSCILLATOR ~ RATE ~ RECORDER !+-

METER I 

l__ __ COUNT-RATE-METER CHA~ __j 

FIGURE 2.-Diagram of aeroradioactivity equipment. 

CAMERA 

EDGE MARK 

CIRCUIT 

fraction of that measured at 500 feet, and the 2,000-foot 

value is due mostly to cosmic sources. There are also 

contributions, usually slight, from radioisotopes in the 

air and from gamma-ray sources within the aircraft. 

All these contributions are adjusted or subtracted from 

the altitude-compensated circuit. In this adjustment the 

cosmic component is assumed to be the main contributor 

to background radioactivity. A 10-percent attenuation 

of cosmic-ray intensity is assumed from the 1,500-foot 

interval between 2,000 feet and 500 feet above ground; 
F4 GEOPHYSIOA:L FIELD INVESTIGATIONS 

hence 90 percent of the total background measured at 

2,000 ·feet is subtracted from the altitude-compensated 

circuit. 

The altitude compensator is designed to permit the 

recording of equivalent 500-foot count rate at any flight 

altitude between 100 and 900 feet. The compensation 

is within desired limits, inasmuch as experimental data 

show that a 1,000-foot column of air a;hsorbs alm·ost 

all terrestrial radioactivity except that from the largest 

or most intense natural sources (Davis and Reinhardt, 

1957, p. 719). During surveying it is rarely sa·fe for the 

aircraft to get within 200 feet of the ground, particularly in hilly terrain. 

Figure 3 illustrates altitude compensation for an area 

in the Valley and Ridge province of eastern Tennessee, 

northeast of Knoxville. The aircraft was about 200 feet 

below the survey altitude of 500 feet ( rada.r-altimeter 

profile) when it passed over Clinch Mountain. The top 

of the mountain is composed of Clinch Sandstone, which 

would be expected to have low to moderate aeroradioactivity. The uncompensated profile shows a peak aeroradioactivity of about 1,000 cps (counts per second) 

for the top of the mountain. The compensator nullifies 

the topographic effect, as shown in the compensated 

profile, where the Clinch Sandstone has a more normal 

and correct aeroradioactivity of 300-400 cps. Other less 

striking examples of compensation are also shown. 

Experimental data obtained over land-water interfaces, such as a peninsula, indicate that usually 85 percent of :the radioactivity measured at a flight altitude 

of 500 feet above the ground comes from a surface area 

1,000 feet in diameter, and 95 percent from an area 

2,000 feet in diameter. Thus the effective area of response of the crystal array at the average flight altitude 

is a circle approximately 1,000 feet in diameter. 

Certain calibration procedures that are followed during surveying assure uniformity of equipment response 

and internal consistency of data. A test line, usually part 

of a regular survey line, is flown at 'the beginning and 

end of each surveying day. Comparison of test-line data 

enables estiination of the diurnal variation of radioisotopes (usually radon daughter products) in the air. 

The test-line data are also checked for any artificial 

change (such as would result from fallout) in the background radioactivity. 

Also, a calibrative check is obtained by flying at the 

survey altitude over water, whose measurable aeroradioactivity is negligible. At 500 feet over water, the compensated circuit registe~s zero radioactivity when sufficient adjustment has been made for cosmic and 

extraneous sources. The body of water must be sufficiently large that the crystal array does not measure 

the radioactivity from any exposed land surface. 

The equipment is calibrated several times each surveying day with a low-energy cesium-137 source and 

once :a week with a 10-microgram radium-226 source 

that gives a count rate of 2,700 cps at 90 em ( centimeters) from the crystal array. In other investigations, 

calibrations have been made by flying over varied radioactive sources (Davis and Reinhardt, 1962; Sakakura, 

1957). 

The Geological Survey crystal array loses efficiency 

above 200 Kev (Davis and Reinhardt, 1957, p. 717). The 

detector counts virtually all gamma energy below 200 

Kev, but above that energy level the efficiency falls off, 

owing to greater penetration of the gamma rays, as 

more pass through the crystals without interaction. For 

a monoenergetic source, the count rate varies directly 

with the emission energy of the particular gamma ray. 

The more heteroenergetic the source or field, the more 

important the energy dependence of the efficiency 

function. 

It is not accurate to correlate the count rate at 500 

feet above ground with the gamm·a-ray intensity at or 

near a particular spot at ground surface. The radioactivity at 500 feet above ground is a mixture of energy 

from varied radioisotopes, of which all have been attenuated in the 500-foot column of air. Attenuation 

coefficients vary with the energy of the gamma ray. In 

the aircraft an integrated count rate is measured from 

the approximate 1,000-foot-diameter area of response 

of the detector; hence an individual ground reading 

has no correlative value to the airborne reading made 

directly above. 

COMPILATION OF AERORADIOACTIVITY DATA 

Flight-line locations from the camera film are plotted 

on transparent overlays of base maps,. at .a scale commonly of 1 inch to 1 mile ( 1 : 62,500 or 1 : 63,360). The 

altitude-compensated aeroradioactivity profiles are examined, three to five profiles at one time, and significant 

breaks or changes in level of radioactivity ,are correlated 

from flight line to flight line. These changes in level 

are then plotted on the overlays and connected by solid 

or dashed lines, depending on the degree of correlation. 

The solid lines denote well-defined changes in level, the 

dashed lines minor or subtle changes. The readings between the lines of change are then scanned, and numbers which indicate the individual range or radioactivity 

level are assigned to each area. 

The overlays are then photographically reduced to a 

scale of 1 inch to about 4 miles (1: 250,000). The data 

are plotted on the publication base map, derived from 
~ 

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b 

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(!) 

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2000 1500 

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0 

w I I 1- Altitude-compensated <( I profile I CJ) 

z 500 0 w 

a.. 

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0 I . u I Radar-altimeter I 

I I 1000 

o--+---+-- -J......V-- -+--v..~-r-+- ----\-t -:;;;/-+-- ~-- +--t--~ 5oo 

0 

Clinch Mountain SE 

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2000' 

1000' 1000' 

0 1 2 3 MILES 

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EXPLANATION 

Mississippian: 

Mg, Grainger Formation 

Mississippian and Devonian: 

MDs, shale 

Silurian: 

Sc, Clinch Sandstone 

Ordovician: 

Oj, Juniata Formation 

Omb, Martinsburg Shale 

Om, Moccasin Formation 

Oo, Ottosee Shale 

Och1 , unit 1 of Chickamauga 

Limestone 

Ok, upper part of Knox Group 

Cambrian: 

£k, lower part of Knox Group 

Conasauga Shale: 

£cu, Upper Cambrian part 

£em, Middle Cambrian part 

£r, Rome Formation 

Contact 

Fault 

Arrows indicate directior 

of movernent 

FIGURE 3.-Prorfi.les showing ·aeroradioactivity, topography, and geology in the Valley and Ridge province, eastern Tennessee. 

From Bates ( 1962b, p. 16). 

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F6 GEOPHYSIClliL FIELD INVEiSTIGlATIONS 

Army Map Service sheets, Corps of Engineers 1 : 250.-

000-scale topographic map series. This is substantially 

the form in which the map will be published (table 1), 

with numbe~s and lines of change (called "boundaries") 

in black. Overprint patterns of green indicate generalized or approximate ranges of radioactivity level. These 

r.anges are usually selected very arbitrarily, as their purpose i:s simply to facilitate interpretation and to permit 

broad, general correlations without close perusal of the 

map. The numbers and boundaries permit correlakions 

for specific localities. 

SOURCES OF AERORADIOACTIVITY 

The gamma radiation detected at the 8urveying altitude of 500 feet above the ground results mainly from 

cosmic radiation, radioisotopes in the air, and radioisotopes in the surface layer of the ground, and to a 

lesser extent from extraneous aircraft radiation. With 

the present instrumentation the relative contribution of 

each source cannot be directly measured, but cert'ain 

assumptions and calibration procedures permit reliable 

estimates of the components of the gross gamma 

radiation. 

COSMIC RADIOACT'IVITY 

The cosmic background is measured at least twice each 

surveying day at 2,000 feet above the ground. Ninety 

percent of this value, which includes both cosmic and 

aircraft radiation, is subtracted from the altitudecompensated circuit of the scintillation equipment. A 

further check on this component is to fly at 500 feet over 

a sizable body of water. The measurable natural aeroradioactivity of water is so small that the cosmic-ray 

and aircraft backgrounds are dominant. Proper response would mean measurement of 0 cps over water, 

assuming that the crystal array "sees" only water and 

no exposed land surface and that all other background 

radiations are subtracted. 

RADIOISOTOPES IN THE AIR 

Airborne radioisotopes consist predominantly of 

radon isotopes and their daughter products. Their concentration in surface air is affected by many variables, 

such as moisture content of the soil, barometric pressure, tem·perature, and wind velocity. A tenfold change 

in the atmospheric radon concentration can occur under 

conditions of extreme temperature inversion. Evaluation of these variables during large-scale aeroradioactivity surveying is extremely difficult. A voidance of 

survey activities during or immediately after extreme 

weather conditions eliminates the uncertainty as to 

radon contributions to the aeroradioactivity. 

Presence of artificially radioactive particles in the 

air will increase the airborne component. The largest 

source of such particles is nuelear debris. The adherence 

of airborne debris to the components and skin of the 

aircraft can so elevate the measured. aeroradioactivity 

that further surveying is useless. Such a condition can 

be recognized by an excessively high, nondecreasing 

count rate at all altitudes. Normal operation can be resumed after the aircraft is decontaminated. 

Exhaust stacks can emit contaminants in gaseous 

form, such as argon-41. This isotope, which is emitteil 

at times from some reactor installations, can be detected 

as ·an anomalously high radioactive swath across the 

survey area at altitudes above the limit of terrestrial 

gamma-ray penetrations. 

RADIOISOTOPES IN THE SURFACE LAYER OF 

THE GROUND 

Ninety percent of the ground component of natural 

radioactivity originates in the top 8 to 12 inches of the 

ground (Gregory and Horwood, 1961, p. 13), whereas 

nuclear debris occurs as a surface layer or foliage contaminant. Ground deposits of nuclear debris particulates mask the true intensity of the natural gamma radiation and negate proper data evaluation. 

NUCLEAR FALLOUT 

Most of the ARMS-I data were obtained during the 

first nuclear moratorium, which extended from N ovember 1958 to September 1961. Fallout was assumed to be 

present during all ARMS-I surveys and was definitely 

recognized on some occasions, but it could be considered · 

a uniform contaminant, which would not significantly 

affect geologic interpretation. Most fallout products 

are short lived (Vennart, 1960), ·and experimental data 

indicate that in the spring of 1957 the activity due to 

fallout was less than one-tenth of the total gamma activity of the soil (Gustafson and others, 19·58). 

NATURAL RADIOISOTOPES 

Natural terrestrial radioactivity consists predominantly of gamma rays from potassium-40 and from 

members of the thorium and uranium radioactive decay 

series. Potassium-40 emits a monoenergetic gamma ray; 

this isotope constitutes only 0.0119 percent of total 

potassium. Certain members of the thorium-232 and 

uranium-238 series decay by emission of gamma rays 

and are significant in geologic studies. Concentrations 

of these radioisotopes range from weak to strong. These 

radioisotopes also may occur as single series in various 

proportions, their occurrence depending upon the geochemical environment in which they were formed and 

upon events which may have disrupted decay equilibrium throughout the history of the geologic unit. 
AIRBORNE MEASUREMENTS OF TERRESTRIAL RADIOACTIVITY F7 

VARIATION OF RADIOISOTOPE CONTENT WITH ROCK 

TYPE 

The distribution of uranium, thorium, ·and potassium 

in common rocks depends on several factors and can 

most easily be described by criteria established by general rock classification. The Geological Survey radioactivity instrumentation cannot distinguish between 

contributing radioisotopes, but when the interpreter 

knows the rock types present, he can better appraise the 

aeroradioactivity patterns. Table 2 gives general abundances for geologically significant radioelements in 

common rocks. 

TABLE 2.-General amounts of uranium, thorium, and potassium 

in common rocks 

[Adapted from Turekian and Wedepobl (1961)] 

Common rocks 

Ultramafic ____________________ _ Basaltic ______________________ _ 

Granodiorite __________________ _ Syenite _______________________ _ 

Granite _______________________ _ Shale _________________________ _ 

Sandstone ____________________ _ Carbonate ____________________ _ 

Uranium Thorium 

(parts per million) 

0. 001 

1.0 

3. 0 

3. 0 

3. 0 

3. 7 

.5 

2. 2 

0. 004 

4. 0 

8. 5 

13. 0 

17. 0 

12. 0 

1.7 

1.7 

Potassium 

(percent) 

0. 004 

.8 

2. 5 

4.8 

4. 2 

2. 7 

1.1 

. 3 

Sedimentary rocks.-Shale is usually more radioactive than sandstone or limestone, owing to potassium 

in the clay that composes shale. An increase of shale in 

the sedimentary column is often reflected in an increase 

in radioactivity. Sandstone is generally intermediate in 

radioactivity between shale (high) and limestone (low). 

Most sandstones owe their moderate radioactivity to 

radioactive heavy minerals (thorium in monazite, 

uranium in zircon) and (or) to interbedded shale 

(clay). Quartzose sandstone is an exception; being virtually homogeneous in composition it is nearly devoid of 

radioactivity. Limestone is usually moderately radioactive, because of interbedded (or contained) shale or clay, 

but the purer limestones are virtually nonradioactive. 

Uranium in carbonaceous shale can cause moderate to 

high radioactivity, but such rocks are somewhat rare 

in the sedimentary column. 

Igneous rocks.-Nearly all felsic rocks (granite, 

syenite) are more radioactive than mafic (gabbro, basalt) and ultramafic (peridotite) rocks, grain size seemingly making no difference. The primary reason is the 

greater abund.ance of potassium feldspar in felsic rocks. 

AlSQ, two of the more common accessory mineralsmonazite and zircon-are radioactive and are generally 

confined to felsic rocks. Rocks of intermediate composition (granodiorite, monzonite) often have to be considered individually; however, the comparative potassium 

feldspar abundance of an igneous rock is ·a very good 

index of its radioactivity. Increased mica (biotite, 

muscovite) content can increase the radioactivity of the 

rock; this factor seems to be less important for igneous 

rocks than for metamorphic rocks. 

Metamorphic rocks.-Generalizations about the radioactivity of metamorphic rocks are difficult to make, 

because of the extreme mineralogic variations. Experience shows that the comparative a;bundances of potassi urn feldspar and the micas seem to be the best indices 

for their radioactivity. The composition of a metamorphic rock is determined, basically, by the ·composition of 

the original rock; however, there can be radioisotope 

addition or depletion during metamorphism. Whether 

this has occurred can be determined only by examination of individual rock units. 

INTERPRETIVE TECHNIQUE 

The usual interpretive technique involves the direct 

comparison of radioactivity map pattern with geologic 

map pattern. In the ideal sense, radioactivity boundaries 

will coincide directly with geologic contacts; radioactivity data can in some places enable more precise 

delineation of concealed geologic contacts, which are 

usually drawn on the basis of factors such as topographic expression, soil, and vegetation. 

In aeroradioactivity interpretation, exact or precise 

correlation between boundaries and contacts is seldom 

possible, but association of an areal radioactivity unit 

with an areal geologic unit can be accomplished. In this 

sense, the radioactivity unit reflects the gross radioisotope content of the geologic unit. Internal variation 

within the radioactivity unit can reflect facies changes 

in the geologic unit or other subtle changes at· ground 

surface not known to the interpreter. 

CAUSATIVE FACTORS IN AERORADIOACTIVITY 

INTERPRETATION 

The correlation of aeroradioactivity with areal 

geology is dependent upon evaluation of many geologic 

factors, which include topography, lithology, and soil. 

Evaluation can become difficult, especially if lithologic 

descriptions of detailed features are considered. Examination of gross features is often more rewarding, especially when using the 1- and 2-mile-spaced ARMS-I 

data. The consideration of detailed geologic information may be more rewarding when interpreting data 

obtained on %- or ~-mile spaced flight lines. High 

cost, however, precluded such detailed spacing during 

ARMS-I surveying. 
F8 GEOPHYSICAL FIELD INVIDSTIGATIONS 

TOPOGRAPHY 

The optimum operating limits of the altitude compensator-100-900 feet-have already been discussed. 

Another factor is aircraft safety; attempting to maintain a flight altitude of 500 feet above ground precludes 

surveying over very hilly or mountainous terrain. Ideal 

terrain is mainly flat to moderately rolling, with no sudden changes in relief, and relief of less than 1,000 feet. 

LITHOLOGY 

Variation in lithology, and consequent variation in 

radioisotope content, enables successful correlation, for 

then changes in aeroradioactivity level (boundaries) are 

reflections of changes in radioisotope content (geologic 

contacts). Adjoining rock units that have the same 

radioisotope content cannot be distinguished on the basis 

of aeroradioactivity, even though they can readily be 

individually mapped by the field geologist. Radioisotope 

content and mappability are independent qualities. No 

correlation can be established where there is little or no 

variation in radioisotope content. 

SOIL 

Inasmuch as the radioactivity measured by airborne 

instruments comes almost entirely from the top 8 to 12 

inches of the ground, the presence or absence of soil, 

and the type of soil, may considerably affect the correlation between aeroradioactivity m·ap and geologic (bedrock) map. The aeroradioactivity detected above an area 

will reflect the radioactivity of the bedrock only if the 

bedrock crops out, has a very thin cover of transported 

soil, or is covered by residual soil formed by weathering in place. 

The ~aeroradioactivity of alluviated valleys is difficult to correlate with bedrock units, as the alluvial material is likely to have been derived from several rock 

types with subsequent mixing of radioactive minerals. 

A blanket of loess (wind-laid silt) over bedrock can 

nullify the aeroradioactivity of the bedrock. Glacial 

debris at ground surface has varied effects on bedrock 

aeroradioactivity; under some circumstances correlations can be made. 

SELECTED EXAMPLES OF THE CORRELATION OF 

AERORADIOACTIVITY WITH AREAL GEOLOGY 

ARMS-I surveys (fig. 1) were made in different areas 

having diverse geologic conditions. The survey boundaries usually had little relation to geology. For convenience, the survey areas are discussed according to 

physiographic provinces-areas of similar climate, 

geologic structure, and geomorphic history. The only 

"artificial" category is that on "glaciated terrain," which 

includes several physiographic provinces. Physiographic data are from Fenneman (1931, 1938, 1946). 

PIEDMONT PHYSIOGRAPHIC PROVINCE, 

APPALACHIAN HIGHLANDS, 

EASTERN UNITED STATES 

The P~iedmont is a province of diverse rock types--

igneous, metamorphic, and sedimentary-with consequent radioisotope variation. It extends in an approximately linear direction from eastern Pennsylvania and 

New Jersey southwestward into Georgia. The Piedmont 

~is characterized by ( 1) infrequent outcrops in gently 

rolling terrain covered with thick accumulations of residual soil, (2) structural complexities, (3) variable 

grades of metamorphism, and ( 4) a general absence of 

diagnostic fossils (Fenneman, 1938, ;p. 121-148). The 

Piedmont is thus an excellent locale for aeroradioactivity correlation. 

The Piedmont in Maryland shows "remarkably good 

correlation of radioactivity with geology" (Neuschel, 

1966, p. 26; fig. 4 of this report). Several of the best 

correlations are at the contact of the Baltimore Gneiss 

and the Glenarm Series. The gneiss, of granitic composition, is the more radioactive rock and contains appreciable amounts of microcline, a potassium feldspar 

rarely present in the adjacent Glenarm rocks. The Baltimore Gneiss in the Piedmont is usually a distinct high 

(600-800 cps) which contrasts with the moderate range 

( 400-650 cps) of the adjacent Glenarm rocks. Areas of 

the Baltimore Gneiss shown as having less than 600 cps 

are probably a hornblende gneiss variety of the unit, as 

such ferromagnesian rock is always deficient in radioactive minerals (Neuschel, 1966, p. 26-28). 

The Glenarm Series is internally well defined by 

radioactivity throughout the Piedmont in Maryland 

despite fairly uniform lithology-foliated rocks with 

varying proportions of quartz, chlorite, muscovite, albite, and oligoclase--and a moderate range of· radioactivity. The aeroradioactivity boundary shown in the 

northwest corner of figure 4 is correlative with the 

Sykesville-Wissahickon (Glenarm) contact. The Sykesville is similar to Glenarm rocks in gross composition, 

as it is weakly gneissic rock composed dominantly of 

quartz, plagioclase, and muscovite (Hopson, 1964, p. 

103, 105). 

Interspersed throughout Glenarm rocks are numerous 

mafic and ultramafic bodies of varied (gabbro, diabase, 

serpentine) composition. As expected by their composition, these rocks have low radioactivity and are often 

well defined by this characteristic. In figure 4 a northeast-trending serpentine dike is represented by a radio-
AIRBORNE MEASUREMENTS OF TERRESTRIAL RADIOACTIVITY F9 

0 

Diabase 

Pegmatite 

G + 

+ 

Guilford Granite of 

Knopf and Jon as 

(1929) 

Serpentine 

Ellicott 

~ 

~ 

City Granite of Cloos and 

Broedel ( 1940) 

Gabbro 

Sykesville Formation 

z 

5 

0 

z 

:.:::: 

z 

::J 

w 

(!) 

<( 

2 3 4 MILES 

EXPLANATION 

Peters 

~ Creek Quartzite 

IZl 

~ . .,., <"-· 

~ u rJ3 0 ~ Wissahickon Formation N 

~ Pzwa, alb1:te-chlorite facies 0 1j w ~ Pzwo, oligoclase-mica facies _J 

~ <( 

c:5 0.. 

Cockeysville • Marble and 

Setters Formation z 

<( 

Baltimore 

B 

Gneiss }~ 0::: 

0.. 

Contact 

D 

u 

Fault 

Dashed where approximately 

located. U, upthrown side; 

D, downthrown side 

~ Aeroradioactivity 

boundary 

Numbers indicate general range of 

aeroradioactivity, in counts per 

second 

~ MAPPED AREA 

FIGURE 4.-Aeroradioactivity and geology of an area in Howard County, Md. Flight lines were flown east-west at 1-mile 

spacing. Modified by Neuschel (1966, p. 27) from map by Cloos and Broedel (1940). 
FlO GEOPHYSIO.AlL FIELD INVESTIGIATIONS 

activity low of 200.....:300 cps. The dike, which extends 

some 18 miles along strike in the Piedmont in Maryland, 

is only lj8-'%, mile wide, and yet was detected along 

every flight line that passed over it (Neuschel, 1966, 

p. 32-33). 

Other intrusive bodies shown in figure 4 were evidently too small to be detected by 1-mile-spaced aeroradioactivity data. The pegmatite dikes are not individually identified in figure 4, although they probably 

contribute (because of their high content of potassium 

feldspar) to several areas of 600-800 cps. 

Figure 5 shows several mafic and ultramafic bodies 

outlined by aeroradioactivity in the Piedmont of Maryland. Serpentines and a gabbro are lows of 200-400 cps 

in a higher matrix of 500-600 cps of the Wissahickon 

0 2 3 

Formation. Also delineated is a sill of Triassic diabase 

(Fisher, 1964, p. 14), an obvious low of 200-400 cps. 

An approximate halo effect of radioactivity is associated with several of these bodies. The radioactivity is 

low over the mafic or ultramafic rock and increases in 

several steps to higher radioactivity over the enclosing 

metasedimentary rocks. If these bodies are intrusive 

masses, the halos could represent the effects of alteration 

that occurred during intrusion and persisted during 

several subsequent stages of metamorphism. 

Glenarm rocks in the Piedmont of northern Virginia, 

not mapped in detail, are associated with distinct patterns of radioactivity which include numerous lows 

(Neuschel, 1965). As in the Glenarm of Maryland, 

future mapping probably will show a correlation of 

4 MILES 

Gaithersburg 0 

400-500 

EXPLANATION 

i(~?:m;&;~:wm Alluvium 

New Oxford 

Formation 

>-

}~ :::> 

}i 

J{G S Wissahickon ~ . ~ Formatwn ~ Albite-chlorite 

0 facies 

~}<'-· ~g 

Ijamsville ~ 

Phyllite ~ 

<( 

0.. 

IGNEOUS ROCKS u 

Diabase 

}~ z 

Serpentine 

~}~ Gabbro ~ 

Contact 

CS0 

(9 

<( 

~ 

Aeroradioactivity 

boundary 

Numbers indicate general 

range of aeroradioactivity, in counts per second 

MAPPED AREA 

FIGURE 5.-Aeroradioactivity and geology of an area in Montgomery County, Md. Flight lines were flown east-west at 1-mile 

spacing. Modified by Neuschel (1966, p. 34) from maps by Gloos and Cooke (1953) and Griscom and Peterson (1961). 
AIRBORNE MEASUREMENTS OF TERRESTRIAL RADIOACTIVITY Fll 

lows with mafic rocks. Such correlation is evident in 

the Fairfax quadrangle, Virginia (Bennison and Milton, 1950), where metabasalt correlates with lows of 

200-400 cps and is bounded by metasedimentary rocks 

of 400-500 cps. 

Interpretation is generally easier where there is a 

large range in radioactivity. The numerous correlations 

in the Piedmont of Maryland are accomplished despite 

the handicap of a low total range of radioactivity (200-

800 cps) , and an even more confined range ( 400-800 

cps) where no ferromagnesian rocks are present. This 

accomplishment illustrates that the Piedmont is an 

ideal locale for aeroradioactivity correlation. The Piedmont of northern Virginia contains radioactivity as 

high as 1,600 cps, probably associated with granitic 

zones (migmatite) in metasedimentary rock, which are 

virtually lacking in the Piedmont of Maryland. The 

Piedmont of South Carolina and Georgia has radioactivity as high as 2,100 cps and as low as 150 cps. 

Schmidt ( 1962b, p. 38, 40) , in his study of data from 

South Carolina and Georgia, reported that "Field examination and comparison with geologic maps indicate that 

the radiation level is closely related to the type of soil 

or rock occurring at the surface. The aSSJociation of 

radioactivity levels with rock types is locally so good 

that geologic contacts may be reasonably approximated 

from changes in radiation level, even though the bedrock is mantled by thick residual soil." MacKallor 

(1963a, p. 34) stated that, in north-central Georgia, 

"The pattern of radioactivity reflects much of the geolCl 

~z 

>-0 1500 1-(.) 

-LJJ 2:(/) 

1- CJo::: 1000 

<(LJJ 

00... 

0(1) 

<Ct- 500 

O:::z 

0:::> 

o:::o 0 ~(.) 

z 

ogy, and many of the local changes in the regional trend 

of the rocks are marked by corresponding changes in 

the radioactivity." 

Figure 6 shows profiles of geology and associated 

aeroradioactivity in South Carolina and Georgia. The 

wide range of radioactivity is typical of the southeastern 

Piedmont province. There is often a twofold increase in 

radioactivity from low-level slate-belt rock to higher 

level granite or highly metamorphosed metasedimentary 

rock of the metamorphic complex. 

Erosional windows of crystalline Piedmont rock 

surrounded by Coastal Plain sedimentary rock are 

commonly well defined by aeroradioactivity. These windows occur in headwaters of streams just southeast of 

the Fall Line (Piedmont-Coastal Plain contact). They 

were formed by headward erosion of streams cutting 

through the overlying Coastal Plain sedimentary rock 

and exposing the underlying Piedmont-type crystallines. Nearly all the Piedmont rocks are more radioactive than the Coastal Plain strata, and are dbvious 

radioactive highs on the small-scale map of the area 

(Schmidt, 1961b). 

Figure 7 shows a highly radioactive area in South 

Carolina paralleling the regional strike in what was 

thought to be Carolina slate-belt rock. Field investigation showed the linear area of high radioactivity toresult from gneissic granitic migmatized zones in slate-belt 

rock. There is good but not exact agreement between 

radioactivity and geologic boundaries as the continuity 

-P I E D M 0 N T-ok---------e 0 A S T A L PLAIN--------

NW 

500' 

Erosional SE 

------------------------------------100 MILES------------------------------------

FIGURE 6.-Pro:fi.les showing characteristic aeroradioactivity and geology in parts of South Carolina and Georgia. From 

Schmidt (1962b, p. 22). 
F12 GEOPHYSICkL FIELD INVESTIGATIONS 

C2ix X X X 

X X X 

me 

~ X 400-600 

X X 

X 

0 2 3 4 5 MILES \;> (J' 

MAPPED AREA 

EXPLANATION 

Rocks of the Carolina slate belt 

Metamorphic complex 

Slate, schist, gneiss, granite, and pegmatite 

Granitized gneissic zones in the Carolina slate 

z~ 

c:c:o 

-N 

0::0 

cow 

~....J C:C:c:c: 

~0... 

O::o:: 

o...o 

Contact 

Dashed where inferred/rom widely spaced exposures 

and aerora.dioactivity 

Aeroradioactivity 

~ boundary 

Numbers indicate general range of aeroradioactivity, 

in counts per second. Units over 650 cps are 

vertically lined 

X 

Location where rock was examined in field 

FIGURE 7.-Aeroradioactivity and geology of an area near IDdg·efield, S.C. Flight lines were flown northwest-southeast at 

1-mile spacing. From Schmidt (1962b, p. 33). 

of the altered zones is not known (Schmidt, 1962b, p. 

32-33). 

In the area shown in figure 8, similar circumstances 

enabled reconnaissance mapping of a hitherto unknown 

granite body. The 800- to 1,400-cps high was a striking 

feature in a regional background of 350-850 cps. Field 

examination disclosed biotite granite exposures within 

the radioactivity high along two roads 5 miles apart. On 

the basis of the aeroradioactivity data, radioactivity 

boundaries were assumed to coincide with boundaries of 

the granite. Further fieldwork might disclose that granite or a similar radioactive rock underlies the 850- to 

1,100-cps unit extending northeast to the Coastal Plain 

overlap from the 800- to 1,400-cps unit (Schmidt, 1962b, 

p. 32, 34). 

Another distinct feature shown in figure 8 is the contact of the Piedmont and the Coastal Plain, along which 

nearly every flight line indicates a marked change in 
AIRBORNE MEASUREMENTS OF TERRESTRIAL RADIOACTIVITY F13 

0 2 3 MILES 

~ 

v 

MAPPED AREA 

EXPLANATION 

Barnwell and Tuscaloosa Formations undivided 

Contact adopted/rom Long (1940) 

Granite 

D 

Metamorphic complex 

Mostly schist, gneiss, and granite 

~ ~ 0::; 

wluo::: c:r:W 

1-1-

Wo O:::z Uc:r: 

z~ 

<(0 

-N 

0:::0 

cow 

::?!....I 

<(<( 

~CL 

O:::o::: 

CLo 

Contact 

Dashed where inferred from aeroradioactivity 

Aeroradioacti vi ty boundary 

Dashed where not well defined. Numbers indicate 

general range of aeroradioactivity, in counts per 

second 

X 

Location where granite was examined in the field 

FIGURE 8.-Aeroradioactivity 'and geology of an area in the Clarks Hill-Warrenville quadrangles, South Carolina and 

Georgia. Flight lines were flown northwest-southeast at 1-mile spacing. From Schmidt ( 1962b, p. 34). 

aeroradioactivity from the more radioactive Piedmont 

to the less radioactive Coastal Plain. The 1-mile-spaced 

flight lines have delimited this contact, even though 

numerous erosional outliers of Coastal Plain material 

are not detected. More closely spaced lines would more 

accurately define the contact. 

316-021 Q-68--3 

In north -central Georgia the contact of the Piedmont ( crystallines) and Valley and Ridge (folded 

sedimentary rocks) physiographic provinces is the Cartersville fault, whose surface trace is marked by a major 

radioactivity bounda~y. The sedimenbtry rocks on the 

northwest (Valley and Ridge) range from 450 to 800 
Fl4 GEOPHYSICAL FIELD INVESTIGATIONS 

cps, whereas the crystallines on the southeast (Piedmont) range from 550 to 1,300 cps. Every flight crossing the fault detected a change in aeroradioactivity 

correlative with the fault trace, the general magnitude 

being 100-200 cps (MacKallor, 1963a, p. 23). Even 

though the same contrasting rocks are not present all 

along the fault, the fault has apparently at all locales 

juxtaposed rocks having different levels of radioactivity; in addition, ground-water or hydrothermal solutions may have deposited radioactive minerals along 

the fault plane, although there is no evidence for this. 

The complex geology of the Piedmont is exemplified 

in the map of Hart County, Ga. (fig. 9), where the complicated results of multistage metamorphism are well 

illustrated. In such terrain, the detector will often measure gamma energy from severwl geologic units simultaneously. Nevertheless, some correlations are seen, 

such as the eastern contacts of both outcrops of biotiteplagioclase gneiss; adjacent sillimanite-mica and 

sillimanite-graphite schist is more radioactive, locally 

twice as much or more. Throughout this area are numerous individual boundary correlations of aeroradioactivity and geology. In such complex terrain, lh- or 

even %-mile spacing of flight lines is desirable for 

detailed interpretation. 

VALLEY AND RIDGE PHYSIOGRAPHIC PROVINCE 

APPALACHIAN mGHLANDS, 

EASTERN UNITED STATES 

The Valley and Ridge province is composed of many 

thousands of feet of Paleozoic sedimentary rocks. These 

strata have been folded, faulted, an<;l eroded into long, 

linear, slightly arcuate valleys and ridges. In places 

the ridges are widely spaced or isolated; in others, 

so numerous and closely spaced that the valleys are 

discontinuous or even absent. Owing to differential erosion, the stronger sandstones and conglomerates are the 

ridge formers, whereas the weaker shales and limestones 

underlie the valleys (Fenneman, 1938, p. 195-196). The 

province extends in a long narrow belt from southern 

New York on the north to the Coastal Plain overlap in 

Alabama in the south. It is bordered on the east by the 

Piedmont and Blue Ridge provinces and on the west 

by the Appalachian Plateaus province. Relief in those 

parts of the Valley and Ridge province included in 

several ARMS-I surveys is not usually so extreme as 

to impede low-level aeroradioactivity surveying. 

The linear trend of the geologic units enables many 

correlations with aeroradioactivity. The correlations are 

often made on the basis of gross lithologies, ,because 

the scintillation detector cannot distinguish between 

the thinner stratigraphic units. The larger stratigraphic 

units which have distinctive contents of radioactive 

minerals are separable on the basis of their radioactivity. 

The Valley and Ridge province in eastern Tennessee 

has numerous excellent correlations. 

* * * Some radioactivity units are continuous ·along strike 

* * * [for] a distance of 100 miles. Many can be traced for 

50 miles or more (Bates, 1962b, p. 41). This is particularly 

noteworthy considering the relatively low levels of radioactivity involved and considering that although some adjacent 

radioactivity units differ by only 100 cps, this difference is 

geologically significant. The area is also geologically complex ; 

repeated thrust faulting has resulted in repetition and omission of beds, and units are laterally gradational within the 

area (Bates, 1962b, p. 33). 

Figure 10 shows an area in the Valley and Ridge 

province around Clinton, Tenn., where there is excellent definition by aeroradioactivity of gross lithologies. Southeast to northwest across figure 10, the radioactivity units are labeled A through G. The units may 

constitute as many as three formations. Units A, B, 

D, and G are dominantly shales, and units C, E, and 

F are mostly carbonates. The shales have an overall 

range of 600 to 1,100 cps and the carbonates, 500 to 

800 cps. 

The traces of several thrust faults are clearly indicated by radioactivity boundaries (fig. 10). In several 

places, thrust faults in eastern Tennessee are traceable 

for tens of miles by radioactivity. Where there are different radioactive rocks on opposite sides of the fault 

the resulting radioactivity boundary is as distinctive as 

one resulting from a stratigraphic change. Faulting in 

uniformly radioactive rock is not detectable by aeroradioactivity. 

Figure 11 shows a similar area around Athens, Tenn., 

where many geologic contacts. are accurately defined by 

aeroradioactivity boundaries. However, near Clinton 

the change at the aeroradioactivity bounda.ry was generally about 200 cps. Near Athens (fig. 11), the change 

at the boundary was only 100 cps; nevertheless, numerous boundaries are distinct. linear features for many 

miles. In the southeast corner of figure 11, several geologic units of small areal extent are defined by radioactivity boundaries, in spite of limitations of the area 

of response (1,000-ft diameter) of the scintillation 

detector. Conspicuous within figure 11 is the close definition of several thrust faults, notably the Copper Creek, 

Beaver Creek, and Chestuee. The Knoxville fault, west 

and southwest of Athens, is in uniformly radioactive 

rock, mostly carbonates, and is hence not defined by 

radioactivity. 
34° 

15' 

0 

83°05' 

2 

AIRBORNE MEASUREMENTS OF TERRESTRIAL RADIOACTIVITY 

3 4 5 MILES 

MAPPED AREA 

EXPLANATION 

GRANITIC ROCKS 

Muscovite granodiorite, including 

pegmatitic and porphyritic 

Biotite granodiorite, grading into 

muscovite granodiorite 

Biotite granodiorite gneiss 

Mixed granitic and metamorphic 

rocks; principally biotite-plagioclase gneiss and sillimanite-mica 

schist interlayered with granite 

METAMORPHIC ROCKS 

Staurolite-mica schist 

Sillimanite-mica schist and sillimanite-graphite schist 

Biotite-plagioclase gneis~; minor 

interlayered amphibolitic quartzite 

Feldspathic amphibolite gneiss; 

biotite-plagioclase gneiss 

Contact 

Aeroradioactivity boundary 

Aeroradioactivity levels and points 

of changes along flight line 

Brackets point toward lower level. 

Arrows indicate position of narrow 

high ( t) or low ( U 

FL87 

Flight line showing number and 

direction of flight 

F15 

u 

6 

N 

0 

w 

_J 

<( 

CL 

0:: 

0 

z 

<( 

0:: 

ID 

::2: 

<( 

u 

w 

0:: 

CL 

u 

6 

N 

0 

w 

_J 

<( 

CL 

0 

z 

<( 

z 

<( 

0:: 

ID 

::2: 

<( 

u 

w 

0:: 

CL 

FIGURE 9.-Aeroradioactivity and geology of part of Hart County, Ga. Flight lines were flown n'Orthwest-southeast at 1-mile 

spacing. Modified by MacKallor (1963a, p. 25) from map by Grant (195S). 
F16 GEOPHYSICAL FIE,LD INVESTIGATIONS 

0 2 3 MILES 

EXPLANATION 

~ENNESSEE~ 

MAPPED AREA 

Silurian: 

Ssu, sandstone and shale 

Ordovician: 

Osr, Sequatchie Formation and 

Reedsville Shale 

Chickamauga Limestone: 

Och4 , unit 4 

Olmc, middle and lower parts 

Ole, units 2 and 1 undivided 

Om, Moccasin Formation 

Ordovician and Cambrian: 

O£k, Knox Group 

Ok, Ordovician part 

£k, Cambrian part 

Cambrian: 

£c, Conasauga Shale 

£r, Rome Formation 

----------- Contact 

Dashed where approximately located 

Fault 

¥ ¥ ¥ ¥ ¥ Y TTT 

Thrust fault 

Sawteeth on upper plate. Dashed where 

inferred 

r6do-7oo ") "---.::. - _., Aeroradioactivity boundary 

Dashed where inferred. Numbers indicate general range of aeroradioactivity, 

in counts per second 

FIGURE 10.-Aeroradioactivity and geology of an area near Clinton, Tenn. Flight lines were flown northwest-southeast at 1-mile 

spacing. A-G, radioactivity units referred to in text. Modified by Bates (1962b, p. 36) from map by Rodgers (1953). 
AIRBORNE MEASUREMENTS OF TERRESTRIAL RADIOACTIVITY F17 

84°45' 40' 

35°30' ,-----.n~--~-r---------------,~------OT~r-----~---,----~--------~r---~~-v~~~/7~~----, 

35°25' 

Ordovician: 

01 me, middle and lower parts of 

Chickamauga Limestone 

Oo, Ottosee Shale 

Oh, Holston Formation 

01, Lenoir Limestone 

Oa, Athens Shale 

Ordovician and Cambrian: 

O£k, Knox Group 

Oma, Mascot Dolomite 

Ok, Kingsport Dolomite 

One, Newala, Longview, and 

Chepultepec Formations 

On, Newala Formation 

Olv, Longview Dolomite _ 

Oe, Chepultepec Dolomite 

£er, Copper Ridge Dolomite 

0 2 

EXPLANATION 

Cambrian: 

£e, Conasauga Shale 

£r, Rome Formation 

Contact 

Dashed where aproximately located 

Fault 

• • • • • 

Thrust fault 

Sawteeth on upper plate 

3 MILES 

c 300-500) 

Aeroradioactivi ty boundary 

Numbers indicate general range of aeroradioactivity, in counts per second 

~ENNESSE~ 7 

MAPPED AREA 

},IGURE 11.-Aeroradioactivity and geology of an area near Athens, Tenn. Ji,light lines were flown northwest-southeast ut 1-mile 

spacing. Modified by Bates (1962b, p. 38) from map by Rodgers ( 1953). 
F18 GEOPHYSICA!L FIELD INVESTIGATIONS 

Figure 3 also shows an a.rea in the Valley and Ridge 

province of eastern Tennessee wherein the aeroradioactivity pattern closely follows the pattern of geologic 

units. 

In north-central Georgia (fig. 12), Valley and Ridge 

province, the structural set:ting is more complex. Several stratigraphic units, notably the Rome and Conasauga Formations (shales) and the Knox Group (carbonates), occur both here and in eastern Tennessee. 

':Vhere shale and carbonate adjoin, their contact is 

marked by a distinct radioactivity boundary. Where 

shale units adjoin one another (northeast part, fig. 12), 

they comprise one radioactivity unit owing to their generallithologic homogeneity. 

The 900- to 1,000-cps unit over parts of the usually 

n1oderately radioactive Weisner Formation and Shady 

Dolomite is probably anomalous (eastern part of fig. 

12). Most likely the higher radioactivity results from 

rubble of graphite schist which bounds these sedimentary units on the other side of the nearby Cartersville 

fault. Despite their differing lithologies, the Weisner 

and Sha.dy are not separable by differences in radioactivity, probably because rubble of the more resistant 

vVeisner covers the adjacent Shady and gives the two 

formations a uniform aeroradioac.tivity and because 

they are not extensive (Macl{allor, 1963a, p. 19). 

The Shenandoah Valley of northern Virginia shows 

the same correspondence of geoJogy and aeroradioactivity. Neuschel (1966, p. 36) stated that 

Exee.Uently defined linear radioa<'tivity units parallel the strike 

of the rocks throughout the Appalachian [Shenandoah] Valley. 

Some of the best correlations of aeroradioactivity to geology in 

th~ area survey~d ['Vashington, D.C.] are found in this region. 

In general, low radioactivity is found over the relatively pure 

limestones and dolomites, and higher values over the shales and 

less pure limestones. 

Faeies changes within a strat·igraphic unit are denoted 

by a wide range of radioactivity numbers for the unit, 

whereas the more homogeneous units have a. smaller or 

narrower range of radioactivity numbers. 

The numerous excellent correlations in the Valley and 

Ridge are where sedimentary rocks are considerably 

tilted. Where sedimentary rocks are virtually horizontal 

(such as in the Coastal Plain and the Appalachian 

Pla~eaus), correlations are few and often nebulous, as 

the seintilla.tion detector has only large-scale areal features to measure. Aeroradioactivity patterns in the Appalachian Plateaus are weak and indistinct, a.nd correlations are consequently lacking (Bates, 1962h, p. 28,32; 

1966b, p. 12-13). 

COASTAL PLAIN PHYSIOGRAPHIC PROVINCE, 

EASTERN, SOUTHEASTERN, AND SOUTH-CENTRAL 

UNITED STATES 

In the Coastal Plain province the strata a.re horizontal or ij.early so, virtually undeformed, and often only 

semiconsolidated, and transported soils bearing no resemblance to underlying bedrock are the common surface component. Aeroradioactivity patterns are usually 

discontinuous and seldom correlative to the bedrock 

geology. Also, the low total range of aerora.dioactivity 

hinders interpretation. The low radioactivity is often, 

especially along the coasts, due to abundant marshy or 

swampy terrain with much included water, which absorbs most terrestrial radioactivity present. In certain 

conditions aeroradioactivity correlation is possible. 

One such circumstance is when anomalous radioactivity in the Coastal Plain is due to concentration of 

radioactive minerals derived from nearby crystalline 

rocks. An example is seen along the Rappahannock 

River in Virginia, from Fredericksburg, Va., southeast 

toward Chesapeake Bay. A strip along the river is approximately twice as radioactive as the adjacent Coastal 

Plain. According to N euschel ( 1966, p. 25), "It is probable that the area of higher radioactivity along the 

Rappahannock River is over stream valley alluvium 

which contains radioactive detrital material derived 

from the granitic areas upstream [Piedmont]." The 

flood plains of several rivers in South Carolina and 

Georgia are higher in radioactivity than the adjacent 

Coastal Plain, despite the swampiness and the general 

abundance of water. Schmidt (1962b, p. 38) made the 

similar hypothesis that the anomalies probably indicate 

transport and deposition of radioactive minerals from 

headwaters (in the Piedmont) to flood plains. 

In the Coastal Plain o_f Maryland (fig. 13), correlation is possible owing to presence of the radioactive 

sedimentary mineral glauconite, a complex silicate of 

marine origin, which contains potassium. The Monmouth and Aquia Formations are glauconite sands; the 

other formations are mainly nonglauconitic. The Monmouth probably contains more glauconite than the 

Aquia, as its outcrop is slightly more radioactive than 

that of the Aquia (Neuschel, 1966, p. 23). 

The Hawthorn Formation (Miocene) occurs in the 

Coastal Plain of South Carolina and Georgia. The Hawthorn is known in the subsurface for its content of 

uraniferous phosphatic material; hence, one might conclude that it would he readily detected by aeroradioactivity. However, at the surface, the Hawthorn did not 
20' 

AIRBORNE MEASUREMENTS OF TERRESTRIAL RADIOACTIVITY 

85°00' 

0 5 10 MILES 

EXPLANATION 

Mississippian: 

M ff, Floyd Shale and Fort Payne 

Chert (includes limestone and 

sandstone) 

Silurian: 

Srm, Red Mountain Formation 

(ferruginous shale and sandtone) 

Ordovician and Cambrian: 

O£k, Knox Group (includes limestone and chert) 

Cambrian: 

£c, Conasauga Formation (mostly 

shale; some limestone) 

£r, Rome Formation (shale and 

sandstone) 

£s, Shady Dolomite 

£w, Weisner Formation (shale and 

sandstone) 

Contact 

Aeroradioactivity boundary 

Numbers indicate general range of aeroradioactivity, in counts per second 

MAPPED AREA 

Fl9 

I 

I 

I 

I 

-I 

I 

I 

I 

~ I 

I 

}'IGURE 12.-Aeroradioactivity and geology in part of the Yalley and Ridge provincP. north-N•ntral Georgia. Flight lines werP 

flown northwest-southeast at 1-mile spacing. Modified by :\IaeKallor (196.~a. p. 20) from map by Butts (1946). 
F20 GEOPHYSIC.A!L FIE,LD INVEiSTIGI.ATIONS 

76°45' 

Glen Daleo 

Kpa 

250-300 

0 2 3 4 MILES 

MAPPED AREA 

EXPLANATION 

g 

~{ G Qw }~ z ..., 0::: 

·~ . . . ~ ~ W ICOIDICO FormatiOn ~ 

J{ Chesapeake 

~ Group 

J{ Aquia 

~ Formation 

~ 

~ 

0 

<;>) 

~ 

.s 

<;>) 

(3 

;,.. 

<;>) 

R. 

~ 

Monmouth 

-

Formation 

G 

Patapsco Formation 

and Arundel Clay 

Contact 

Aeroradioactivity 

~ 

boundary 

Numbers indicate general 

range of aeroradioactivity, in counts per second 

0 

>-

0::: 

<( 

i= 

0::: 

w 

1-

(/) 

=> 

0 

w 

u 

<( 

1-

w 

0::: 

u 

FIGURE 13.-Aeroradioactivity and geology of an area in Prince Georges County, Md. Flight lines were fiown east-west at 

1-mile spacing. Modified by Neuschel (1966, p. 24) from map by Cooke and Cloos (1951). 

show appreciable aeroradioactivity. "It can be speculated that the surface exposures of the Hawthorn formation in the Savannah River Plant area are only 

weakly radioactive because the uranium has been 

leached from the surficial layers" (Schmidt, 1962b, p. 

38). 

The previous examples of Coastal Plain aeroradioactivity are concerned with the unusual rather than the 

usual circumstances. The basic difficulty in aeroradioactivity interpretation there is that the individual surface units as seen by the scintillation detector had no 

essential sameness, no integral lithologic homogeneity. 

The result is aeroradioactivity patterns of discontinuous 

nature which are nearly impossible to interpret. 

In the Coastal Plain of :north -central Texas, Upper 

Cretaceous strata in the vicinity of Dallas are not thick, 

yet have a broad areal expression because of their shallow dip. These units-Taylor Marl, Austin Chalk, Eagle 

Ford Shale, and Woodbine Sand-do have an individual 

gross lithologic homogeneity which permits association 

of aeroradioactivity patterns with the particular units. 

Shale is the most radioactive, chalk the least, and sand 

and marl intermediate. Yet some good boundary correlations are possible in this area, especially away from 

the major drainage, where alluvium disrupts the correlative patterns. 

To the west near Forth WoJ;th, Lower Cretaceous 

strata crop out. There is lithologic variation and consequent radioactive mineral variation, but most stratigraphic units have too small an areal extent to permit 

separation by the scintillation detector. Figure 14 shows 

an area west of Forth Worth, where strata are so nearly 

horizontal that geologic contacts approximate topographic contours. For interpretive purposes, varying 

lithologic types were combined into a calcareous unit 

and a quartz-sand unit. This enabled some correlation 

with the aeroradioactivity pattern, but generally in this 

terrain few correlations are possible. 
AIRBORNE MEASUREMENTS OF TERRESTRIAL RADIOACTIVITY F21 

0 2 3 4 5 MILES 

EXPLANATION 

Alluvium 

Calcareous 

~ rocks 

D 

Quartz sands 

Contact 

Aeroradioactivity 

~ 

boundary 

>-

}~ ~ 

0 

CJ) 

~ 

0 

w 

u 

<( 

1-

w 

0::: 

u 

Dashed where transitional. Numbers indicate general range of 

aeroradioactivity, in counts per 

second 

FIGURE 14.-Aeroradioactivity and geology of an area in Parker County, Tex. Flight lines were flown east-west at 1-mile 

spacing. Modified from map by Hendricks ( 1957). 

COLUMBIA PLATEAUS PHYSIOGRAPHIC' PROVINCE, 

NORTHWESTERN UNITED STATES 

The Columbia Plateaus province consists of thick 

almost horizontal lava flows, mainly basalt. The lava 

at ground surface is usually bare and virtually unaltered or covered by only a very thin veneer of soil or 

loess. Any aeroradioactivity measurements should then 

reflect the true radioactivity of the rock, unaffected by 

surface cover. However, in part of the Columbia Plateaus the surface cover does not reflect bedrock composition, and consequently correlation is impossible. 

In southern Idaho, the Snake River Plain section of 

the Columbia Plateaus, where the loess cover is thin 

or ahsent, exhibits excellent correla.tions between 

aeroradioa.etivity and geology. The basalts of the Snake 

River Plain show radioactivity ranging from 300 to 

1,300 cps. The higher radioactivity is probably due to an 

increased content of potassium feldspar. The wide variation in radioactivity permits distinguishing betwee!ll 

several flows. Common sources for some basalt flows 

were suggested hy either their uniform or varying 

ae~oradioactivity. The scattered rhyolites present were 

more radioactive (maximum 2,100 cps) and thus were 

easily recognized (Bates, 1964). 

In southern Washington, correlative aeroradioactivity diminishes, as conditions are quite diff.erent from 

those in southern Idaho. Loess in thick quantities (as 

much as 75 ft) masks the volcanic and sedimentary 

bedrock, and little of its aeroradioaetivity character 

is determinable. Some few exposures of ha.salt are low 

to moderate in radioactivity ( 300-600 cps), whereas 

the loess itself has moderate radioactivity ( 500-750 

cps) (Schmidt, 1962b, p. 18, ·21). 

GREAT PLAINS PHYSIOGRAPHIC PROVINCE, 

WEST-CENTRAL UNITED STATES 

DENVER AREA 

The Denver ARMS-I area lies in the drainage system of the South Platte River, which is within the 

Color.ado Piedmont physiographic section of the Great 

Plains. The Piedmont is the dissected remnant of a 

great fluviatile plain that once extended from the Rocky 

Mountains eastward to the Central Lowlands. The original mantle was unconsolidated silt with lesser a.mounts 

of sand, grave~l, .and loca.lly abundant lime cement. The 

mantle has been destroyed to the extent that the underlying Mesozoic strata are locally deeply eroded. There 

is still, however, a widespread cover of eolian sand, silt, 

and alluvium (Fenneman, 1931, p. 11-13, 30-31). 

Most of the surface material .and most of the underlying Mesozoic sedimentary ~ocks were derived from 

the crystalline rocks of the Front Range of the Rocky 

Mountains, which are a complex assemblage of metamorphic and igneous rocks. They are comparatively 
F22 GEOPHYSIC.A:L FIE>LD INVEiSTIGIATIONS 

rich in potassium feldspar, as well .as accessory minerals 

containing thorium and (or) uranium. The Denver 

area has the highest average aeroradioactivity of all the 

ARMS-I survey ·areas. The average is 800-900 cps; 

tfue total range is 300 to 1,400 cps. These factors have 

resulted in several interesting relationships between 

aeroradioactivity and geology ( Popenoe, 1966a., p. 

14-15). 

The Mesozoic strata., dipping steeply to the east off 

the Front Range, hav·e been differentially e1roded into 

topographic ridges called hogbacks. Where these can be 

surveyed at a 500-foot terrain clearance, these hogbacks 

show linea.r patterns of radioactivity similar to those 

of the V all•ey and Ridge province. Strata underlying 

most of the Denver area. east of the hogbacks are la.rgely 

of arkosic composition (feldspar detritus) , and where 

the eolian and (or) alluvial eover is thin enough, a good 

express~on of their radioactive character has been olbtained ( Popenoe, 1966a, p. 15-25). 

·Most definitive in the Denver area is the outlining 

of the drainage pattern by aeroradioactivity. Granitic 

debris from the Front Range constitutes the bulk of the 

alluvium, and provides such a high radioactive count 

that the Quaternary drainage is accurately outlined by 

aeroradioactivity boundaries. The highest radioactivity 

is along and adjacent to the immediate stream course 

and the radioactivity diminishes toward the upland 

surface or interstrea.m area. The radioactivity of the 

older materials (chiefly gravels) capping interstre•am 

divides is significantly lower than tha.t of the more recent alluviums. "This lower level is proba:hly due to the 

leaching and weathering, and ·accumulation of calcium 

carbonate accompanying soil development" (Popenoe, 

1966a., p. 25). 

Loess and dune sand a.re extensive on interstrea.m 

areas east of the South Pla.tte River, away from the 

Front Range. Where present they preelude determination of bedrock aeroradioactivity. The sand is generally 

moderately radioactive and smne of the loess is highly 

radioactive; t}1ese above-.average levels proibably reflect derivation a.t long range from radioactive Front 

Range rocks. 

GNOME AREA 

In tJhe GNOME ARMS-I area around Carlsbad, N. 

Mex. (Mackallor, 1965a), physiography is similar and 

correlative ae.rora.dioactivity is dissimilar. The1re is no 

source area of radioactive rock within or adjoining the 

GNOME survey are1a., and this lack is reflected by low 

to moder.ate (10(}-600 cps) radioa.etivity. Few bounda.ry 

correlations were possible, although some correlation 

of radioa.ctivity level with type of surface material was 

made. The a.rea is characterized by alluviated plains 

which represent mixtures of minerals from va.rious sedimentary rocks of gene,rally low to· moderate radioactivity. The mixing of radioactive minerals means that the 

radioactivity measured is rarely 11epresentative of a 

single rock type, and thus correlation is seldom 

possible. 

T\vo playas (dry lakes) in the GNOME area were 

distinguishable by aeroradioactivity because :they were 

100-200 cps higher than the adjacent area. The anomalies probably represent concentration through evaporation of radioactive minerals, most likely potassium salts 

(MacKallor, 1965a, p. 27). Detection of two of the 

many playas in the GNOME area was probably fortuitous, as aU are small and the area was surveyed on a 

combination of 1- and 2-mile-spaced flight lines. 

An aeroradioactivity survey at lfs-mile spacing 

around the immediate GNOME test site indicated that 

detailed data would provide correlative information in 

desert terrain despite generally low radioactivity. Distinction was made bet\veen several Quaternary units 

and between the Quaternary and red beds of Permian 

and Triassic age (W. J. Dempsey, written commun., 

1963). 

PACIFIC BORDER PHYSIOGRAPHIC PROVINCE, 

WESTERN UNITED ST.A:TES 

The Pacific Border province consists of a series of 

coastal mountains on the west and an eastern series of 

valleys. Topographic relief in the 1nountains usually 

precludes aeroradioactivity surveying, and thus most 

of the San Francisco and Los Angeles ARMS-I surveys 

were restricted to valley and (or) lowland areas. Where 

data were obtained in the Coast Ranges, mostly north 

and east of San Francisco Bay, correlation is lacking 

because of ( 1) discontinuity of geologic outcrops and 

(2) relief greater than 1,000 feet. There is variation in 

radioactivity due to variation in type of rock, but still 

correlation is lacking (Books, 1962a, 1966). 

The valleys and low lands are mantled by detritus 

(clay, sand, gravel) from varied rock types. Definite 

correlation is possible only where bedrock is exposed. 

at valley margins, or where bedrock controls soil types, 

residual or transported. In marshy areas or near bays, 

the abundant water effectively masks most of the radioactivity. With several exceptions of gross correlations 

with igneous rock-derived soil, correlative data an~ 

nearly totally lacking (Books, 1966, p. 14-22). 

Figure 15 shows the western end of the Mojave Desert. 

of southern California, where radioactivity delineated 

Rosamond and Rogers dry lakes. These are probably 

similar in sedimentologic origin to the playas of New 

Mexico (see Great Plains discussion), although the 

Mojave playas are slightly higher in contrasting radio-
AIRBORNE MEASUREMENTS OF TERRESTRIAL RADIOACTIVITY F23 

0 5 10 MILES 

EXPLANATION 

Quaternary: 

Qal, alluvium 

Ql, lake deposits 

Qs, eolian sand 

Tertiary: 

Tnm, nonmarine rocks 

Tv, volcanic rocks 

Mesozoic: 

Mzgr, granitic rocks (mostly of 

Mesozoic age) 

Precambrian(?): 

p£r, gneiss and schist 

Contact 

Limit of aeroradioactivity survey 

c700-80~ 

Aeroradioactivity boundary 

Numbers indicate general range of aeroradioact,ivity, in counts per second 

MAPPED AREA 

activity. Most likely the waters that were caught and 

then evaporated in the playa carried radioisotopes, 

either in solution or as detritus. The mode and character 

of the radioisotopes being unknown, potassium salts are 

the most likely assumption (Books, 1962a, p. 21) . The 

higher radioactivity in the western Mojave is mostly 

related to granitic rock exposures, but correlations are 

vague and discontinuous. 

GLACIATED TERRAIN, MIDWESTERN AND 

NORTHEASTERN UNITED STATES 

Parts of five ARMS-I surveys lie within the limits of 

Pleistocene glaciation. These are (fig. 1, table 1) Minneapolis-St. Paul (all), Chicago (all) , Columbus 

(part) , Pittsburgh (part) , and northeast United States 

(all). The correlative value of the ARMS-I data varies 

from slight or even nil for the first-named four areas, 

all within the so-called midwestern United States, to 

appreciable for the northeastern United States (New 

England plus southeastern New York). 

Bedrock in glaciated terrain is nearly everywhere 

obscured by debris called glacial drift. Drift has a great 

variation in grain size, from clay to boulders, and in 

lithology, provenance, and thickness. Even though most 

drift has not been carried far from the rock from which 

it was derived, a matter of several tens of miles or less, 

in the Midwest any lithologic or other resemblance of 

drift to underlying bedrock is fortuitous. In west-central Ohio, for example, "* * * the glacial deposits are 

mixtures of material derived almost wholly from local 

bedrock sources which* * * [mostly] have similar radioactivity levels'' (Bates, 1966a, p. 18). The result is 

heterogeneous surface material with a generally homogeneous aeroradioactivity. 

In addition, the upper Midwest is blanketed by varied 

thicknesses of loess, which is interbedded with or overlies drift. Drift and associated loess are so extensive and 

thick as to negate determination of bedrock aeroradioaotivity. The aeroradioactivity measured in the Midwest was that of the drift and loess whose lithology is 

characteristically so heterogeneous that any correlation 

of these geologic units with aeroradioactivity is seldom 

accomplished. 

FIGURE 15.-Aeroradioactivity and geology of part of the western Mojave Desert, southern California. Flight lines were 

flown north-south at 1-mile spacing. Modified by Books (1962a, 

p. 18) from map by Kundert (1955) and information provided 

by T. W. Dibblee, Jr. (U.S. Geol. Survey, written commun., 

1961), and California Division of Mines ( unpub. data, San 

Bernardino 2° sheet). 
F24 GEOPHYSIO.A!L FIELD INVESTIGIATIONS 

Correlation can be accomplished when the glacial 

feature is of homogeneous character, such as a large 

area of sand dunes or sand plains, which resulted from 

winnowing of debris by proglacial streams in an outwash plain. One is the Anoka sand plain, north of 

Minneapolis-St. Paul, Minn. (U.S. Geol. Survey, 1962, 

p. A25). Another is the area of glacial Lakes \Vauponsee and Watseka south and southwest of Chicago, Ill. 

(U.S. Geol. Survey, 1962, p. A25). These areas are principally quartzose sand at the surface, and their extent 

and boundaries are well defined by characteristic low 

radioactivity (100-200 cps). In the general Minneapolis-St. Paul area (exclusive of the sand plain), elongate areas having low radioactivity suggest the location of former drainage courses and the past existence 

of many more lakes (U.S. Geol. Survey, 1965, p. A82). 

As previously mentioned, correlation is effective in 

the northeastern United States, all of which was glaciated during the Pleistocene. Bedrock in the northeast 

is mostly of metamorphic and igneous origin. A thinner 

cover of glacial drift, because of the less readily eroded 

rocks, and a greater variation in radioactivity combine 

to make aeroradioa.ctivity measurement more useful. 

Loess-the major faotor in ~the lack of correlation 

in the Midwest-is virtually absent in the northeast, 

where relief is somewhat greater, sand is more abundant than clay and silt (which constitute loess), and the 

climate is moist--continental rather than steppe (Flint, 

1957, p. 189). "The greater moisture, forest cover, and 

lower wind velocities of the moist-continental climate 

operate to reduce surficial drying of the outwash in 

preparation for deflation" (Flint, 1957, p. 189). 

Also greatly aiding correlation in the northeast is 

the fact that about 90 percent of the gla.cial debris was 

locally derived; the detritus is identical with the underlying bedrock or with the bed~ock a short distance to 

the north. This local origin is reflected in the aeroradioactivity data: the smaller geologic units are not identifiable on the basis of aeroradioactivity, but the larger 

geologic units are distinguished for many miles along 

strike by their characteristic aeroradioa.ctivity. Sharp 

boundary correlatons are rare, because the material 

overlying geologic contacts is intermingled detritus 

from adjacent units, which precludes any abrupt 

changes in aeroradioactivity. The resistant bedrock has 

generally resulted in thinner drift cover on ridgetops 

or highland areas, which permits correlations that are 

not possible over the more thickly filled valleys or lowland area~. 

The northeastern United States ARMS-I survey 

encompassed 28,000 square miles. Some of the numerous 

correlations (Popenoe, 1964b; 1966b) are exemplified 

in figure 16, which shows an area in the eastern New 

England Highlands where distinctive low radioactivity 

(generally 150-400 cps) is associated with rocks of the 

Oliverian Plutonic Series. Higher radioactivity is associated with the IGnsman Quartz Monzonite and the 

surrounding volcanic rocks. Though only a few boundary correlations are shown, the association of areal 

radioactivity units with areal geologic units is well demonstrated. Such association is very noteworthy on the 

small-scale aeroradioactivity maps of the northeastern 

United States (Popenoe, 1964a; 1966c). In addition, the 

radioactivity lows due to swamps and lakes in glaciated 

terrain must be ignored while attempting any correlation with bedrock geology. The published maps show 

all sizable bodies of water as 0 cps (Popenoe, 1964b; 

1966b). 

PUERTO RICO 

Puerto Rico is composed of a central east-trending 

backbone of clastic sedimentary rocks composed chiefly 

of andesi1tic detritus and several granodiorite plutons 1

and bodies of serpentinite. This backbone is flanked by 

north- and south-dipping carbonate strata, chiefly limestones. These dominantly nonradioactive rocks provide 

a moderate range of aeroradioaCJtivity, 50 to 800 cps; 

the range is only 50 to 300 cps for more than 75 percent 

of the island. The andesitic backbone is slightly more 

radioactive than the calcareous flanks (MacKallor, 1966, 

p. 13). 

The extreme relief of the mountainous central backbone required that the flight lines be oriented east-west, 

paralleling the domill!ant topography and the strike of 

most rock units. The relief is mostly more than 1,000 

foot, which negates effective detection of terrestrial 

rrudioactivity. These factors 'are responsible for the 

general lack of boundary correlations. 

USE OF AERORADIOACTIVITY MEASUREMENTS IN 

CONJUNCTION WITH OTHER GEOPHYSICAL METHODS 

Of the common geophysical methods, only those that 

can measure ground -surface phenomena can be used 

in complementary interpretation with aeroradioactivity. Seismology and gravimetry deal almost exclusively 

with thick sections of rock that rarely have surface 

expression. Electromagnetic measurements and total 

field magnetic measurements can, on occasion, be related 

to the ground surface and then be interpreted in conjunction with aerorndioactivity. 

Electromagnetic methods are fundamentally different 

from radioactivity, magnetic, and gravity methods in 

that an artificial source of energy is employed. The 

source of energy is an alternating magnetic field which 

induces eddy currents in conductive earth materials. 

The eddy currents, in turn, set up secondary fields 
AIRBORNE MEASUREMENTS OF TERRESTRIAL RADIOACTIVITY 

0 2 3 

EXPLANATION 

Devonian(?): 

Dk, Kinsman Quartz Monzonite 

Dng, binary granite of New Hampshire 

Plutonic Series 

Oliverian Plutonic Series: 

Doq, quartz diorite 

Dog, granodiorite 

Dom, quartz monzonite 

Devonian: 

D I, Littleton Formation 

Dg, Gile Mountain Formation 

Devonian and Silurian: 

DSw, Waits River Formation 

Silurian: 

Sf, Fitch Formation 

Sc, Clough Formation 

Silurian and Ordovician: 

SOca, Clough Formation and Ammonoosuc 

Volcanics undivided 

4 5 MILES 

Ordovician: 

Op, Partridge Formation 

Oa, Ammonoosuc Volcanics 

Contact 

c 250-400 ) 

Aeroradioactivity boundary 

Numbers indicate general range of aeroradioactivity, in counts per second 

MAPPED AREA 

F25 

FIGURE 16.-Aeroradioactivity and geology of part of the Keene and Brattleboro quadrangles, New Hampshire and Vermont. 

Flight lines were flown east-west at 1-mile spacing. Modified by Popenoe ( 1964b, p. 20) from Billings ( 1956), Doll, 

Cady, Thompson, and Billings ( 1961) , and Moore ( 1949). 
F26 GEOPHYSIOA'L FIELD INVESTIGATIONS 

which can be measured on the ground or from an aircraft. The intensity and direction of these secondary 

fields depend on the conductivity and distribution of 

conductive material within the earth. Airborne electromagnetic measurements are responsive to conductive 

material at depths as great as about 300 feet and are 

most commonly used in exploration for sulfide ore 

bodies. Airborne electromagnetic data can be expected 

to correlate with aeroradioactivity data when the material at the surface is representative of the 1naterials 

within tens or hundreds of feet of the surface. 

The Geological Survey aircraft is equipped with a 

fluxgate magnetometer, and aeromagnetic data are. 

nearly always obtained concurrently with any surveying at 500 feet for aeroradioactivity. The magnetometer 

measures variations in the total intensity of the earth's 

field, which are related to rocks that may or may not 

be at ground surface. Where sedimentary rocks are at 

the surface and the magnetic rocks are buried, magnetic 

data are generally uncorrelatable with surface geology; 

where metamorphic or igneous rocks are at the surface, 

magnetic measurements can be used in conjunction with 

aeroradioactivity data. 

Two combined aeroradioactivity-aeromagnetic interpretations have been made in the Piedmont of the eastern United States, where the magnetic rock is usually 

at ground surface. One area is in Montgomery County, 

Md., within the ARMS-I survey area but flown prior to 

that survey. Airborne data were obtained on %-milespaced lines, and gravity coverage was obtained. Serpentinites are accurately indicated by radioactivity 

lows and magnetic highs, whereas gabbros are radioactivity lows and magnetic lows, bordered by highs. 

The country rock is micaceous quartzofeldspathic 

schists of the Wissahickon Formation, within which 

radioactivity and magnetic trends often correspond and 

prove very useful to the mapping geologist (Griscom 

and Peterson, 1961). 

Another area is the Concord quadrangle of North 

Carolina, where mapping of characteristic Piedmont 

geology is well aided by geophysical methods. The most 

striking feature in the quadrangle is a ring structure 

composed of two arcuate masses of syenite which partly 

surround a large gabbro body. The ring structure has 

distinct radioactivity and magnetic characteristics 

which helped to define field relationships. The geophysical data also permitted more accurate delineation 

of some small granite bodies, by indicating lithologic 

differences and by giving better know ledge of surface 

extent (radioactivity) and subsurface configuration 

(magnetics) in this area of sparse outcrops and thick, 

abundant residual soils. Several granites were discovered by means of geophysical data (Bates and Bell, 

1965). 

Aeromagnetic measurements can also be made at elevations greater than 500 feet above ground. The usual 

technique, particularly in rugged areas, is to fly at some 

given barometric elevation above sea level that permits 

easy and safe clearance of ground obstacles. The greater 

altitude attenuates the intensity of magnetism measured because of the increased distance between the aircraft and the magnetic rock. Aeroradioactivity measurements attempted at more than 1,000 feet above ground 

effectively measure only cosmic radiation; effective 

aeroradioactivity surveying is limited to areas that 

can be surveyed at 100-900 feet above ground. Aeromagnetic surveying is, then, much more versatile as it can 

be accomplished in any topographic region. 

EVALUATION OF THE METHOD-CORRELATION OF 

AERORADIOACTIVITY MEASUREMENTS WITH 

AREAL GEOLOGY 

Numerous examples of the correlations of aeroradioactivity measurements with areal geology have been presented. The ideal conditions are low to moderate topography, material at ground surface that reflects bedrock 

composition, and bedrock lithologies that vary in gross 

radioisotope content. Such ideal conditions are comparatively rare, and the use of aeroradioactivity in less 

than ideal conditions must be considered. 

Topography is a constant, not a variable, factor. The 

aircraft must be able to safely average 500 feet above 

ground surface; if this is not feasible, aeroradioactivity 

surveying cannot be accomplished. 

The material at ground surface as seen by the scintillometer, for any determination of bedrock aeroradioactivity, must in some way reflect bedrock composition 

Also, there must be gross radioisotope variation at 

ground surface that results in aeroradioactivity variation. Distinct patterns of aeroradioactivity, though not 

absolutely necessary, simplify interpretation. A relative 

blandness or uniformity of gross radioisotope content 

greatly complicates effective correlation. Correlation 

also requires some reasonable geologic explanation for 

radioisotope variation. If the interpreter cannot explain 

radioisotope variation, awareness of such variation will 

not aid geologic mapping. 

No attempt has been made to assign actual values in 

counts per second to general rock types, as has been 

accomplished with laboratory-derived parts-per-million 

and percent numbers in table 2, because the counts per 

second for a given rock type differ too much for reasonable usage. This report has emphasized (1) the relative change in count-per-second numbers from unit to 

unit, and (2) the pattern of aeroradioactivity as 

related to the pattern of areal geology. Specfic count-
AIRBORNE MEASUREMENTS OF TERRESTRIAL RADIOACTIVITY F27 

per-second numbers are important in many circumstances, but the magnitude of the change and the 

pattern thereof are more definitive parameters. 

Aeroradioactivity surveying can be both an aid to 

geologic mapping and a combined tool in geophysical 

exploration. The relative usefulness has a wide variance, being dependent on many geologic factors. Once 

the possibility of flying over an area at low elevations 

has been established, the area should be assessed by 

varied geologic criteria to establish the feasibility of 

surveying. Prior assessment. is essential, because airborne geophysical surveying is rather expensive and 

should be carefully planned. To provide greater scientific return for money expended, an aeroradioactivity 

survey can ·be done concurrently with aeromagnetic or 

electromagnetic surveys. 

AIRBORNE GAMMA-RAY SPECTROMETRY 

A recent development in geophysical exploration has 

been the adaptation of gamma-ray spectrometer systems 

for airborne use. Such equipment enables the determination of the relative contributions of uranium and 

thorium daughters and potassium to the total field of 

terrestrial radioactivity. 

Gamma-ray spectrometers have been in use for some 

time in the laboratory (Hurley, 1956; Crouthamel, 

1960; Mero, 1960; Adams, 1964). Spectrometers have 

recently been used in the field, where they were hand 

carried (Adams and Fryer, 1964; Wollenberg and 

Smith, 1964) or mounted in a truck (Moxham and 

others, 1965). The fact that these systems utilize the 

scintillation method in conjunction with pulse-height 

analyzers permits delineation of gamma-ray energies. 

Advances in the manufacture of large crystals and in 

solid-state electronics have permitted their use in airborne geophysical exploration. 

The Geological Survey is at present calibrating and 

flight testing an airborne gamma-ray spectrometer system. The detector for the system will consist of two 

111/2 - hy 4-inch thallium-activalted sodium iodide 

crystals. Gamma energy measured by the detector will 

be processed into both digital and analog form. The 

digital system ·Will include a 400-channel pulse-height 

analyzer and a magnetic tape recorder. The analog system will consist of four single-channel analyzers and 

associated circuitry. Three of the single-channel analyzers will be gated for two high-energy daughter products of uranium and thorium and for potassium-40. 

The fourth analyzer will be gated to record total radioactivity, counting essentially the same spectrum as, and 

being analogous to, the compensated circui·t of the 

ARMS-I instrumentation. It will then be possible to 

relate ARMS-I data to those obtained by the new 

instrumentation. 

The effectiveness of total count aeroradioactivity as a 

viable aid to geologic mapping has been demonstrated; 

the determination of the relative amounts of natural 

radioisotopes will be an even more valuable contribution 

of exploration geophysics. 

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Study, CEX-59.4.16, 25 p. 
F28 GEOPHYSIC.A:L FIE,LD INVESTIGlATIONS 

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M. P., 1961, Centennial geologic map of Vermont: Vermont 

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MacKallor, J. A., 1963a, Aeroradioactivity survey and areal 

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---1963b, Natura,! gamma aeroradioactivity of the Georgia 

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---1964, Natural gamma aeroradioactivity of the Gnome 

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--1965a, Aeroradioactivity survey and geology of the GuomP 

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A.tomic Enei'gy Comm., Civil Effects Study, CEX-59.4.24. 

30 p. 

---1965b, Natural gamma aeroradioactivity map of Puerto 

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West Virginia: U.S. Geol. Survey Geophys. Inv. Map GP475. 

----1966, Aeroradioactivity survey and areal geology of the 

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Geophys. Iuv. Map GP-358. 
AIRBORNE MEASUREMENTS OF TERRESTRIAL RADIOACTIVITY F29 

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Geophysical Field 

Investigations, 1964-67 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 516 

This volume was published 

as separate chapters A-F 
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary 

GEOLOGICAL SURVEY 

William T. Pecora, Director 
CONTENTS 

[Letters designate the separately published chapters] 

(A) Regional geologic implications of the gravity and magnetic fields of a part of eastern Tennessee and southern Kentucky, 

by Joel S. Watkins. 

(B) Geologic interpretation of an aeromagnetic survey of the Iron Springs district, Utah, by H. Richard Blank, Jr., and 

J. Hoover Mackin. 

(C) Regional geophysical investigations of the Moab-Needles area, Utah, by H. R. Joesting, J. E. Case, and Donald Plouff. 

(D) Geologic interpretation of gravity and aeromagnetic maps of Tintic Valley and adjacent areas, Tooele and Juab Counties, 

Utah, by D. R. Mabey and H. T. Morris·. 

(E) A geophysical study in Grand Teton National Park and vicinity, Teton County, Wyoming, by John C. Behrendt, Benton L. 

Tibbetts, William E. Bonini, and Peter M. Lavin, with sections an Stratigraphy and structure by J. D. Dove, 0/IUl, Precambrian rocks, by John C. Reed, Jr. 

(F) Airborne measurements of terrestrial radioactivity as an aid to geologic mapping, by James A. Pitkin.