U.S. Department of the Interior

U.S. Geological Survey

Professional Paper 1674

Mapping Ground Water in Three Dimensions—

An Analysis of Airborne Geophysical

Surveys of the Upper San Pedro River Basin,

Cochise County, Southeastern Arizona
Huachuca Mountains

Fort Huachuca

San Pedro Valley

Tombstone Hills

United States-Mexico border,

looking north from Mexico

Cover. View of the upper San Pedro Valley, looking north from Mexico.
Mapping Ground Water in Three Dimensions— 

An Analysis of Airborne Geophysical

Surveys of the Upper San Pedro River Basin, 

Cochise County, Southeastern Arizona

By Jeff Wynn

Professional Paper 1674

U.S. Department of the Interior

U.S. Geological Survey

Supersedes Open-File Report 00–517
U.S. Department of the Interior

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2006

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Suggested citation:

Wynn, Jeff, 2006, Mapping ground water in three dimensions—An analysis of airborne geophysical surveys of the 

upper San Pedro River basin, Cochise County, southeastern Arizona: U.S. Geological Survey Professional Paper 1674, 

one CD-ROM. (Also available online.)

Library of Congress Cataloging-in-Publication Data

Wynn, Jeffrey C.

Mapping ground water in three dimensions : an analysis of airborne geophysical surveys of the Upper San Pedro River 

Basin, Cochise County, Southeastern Arizona / by Jeff Wynn.

 p. cm. –(Professional paper ; 1674)

“Supersedes Open–File Report 00–517.”

Includes bibliographical references.

1. Groundwater—San Pedro River Watershed (Mexico and Ariz.)—Remote sensing. 2. Groundwater—Arizona—

Remote sensing. 3. Aerial photography in hydrogeology. I. Title. 

GB1025.A6W96 2006

551.4909791’53—dc22

2006041319

ISBN 1-411-30901-4
iii

Contents

Abstract ..........................................................................................................................................................1

Introduction and Background......................................................................................................................1

Previous Hydrological, Geological, and Geophysical Studies ...............................................................4

Data Acquisition for the Airborne Geophysical Surveys ........................................................................5

Quality Control Analysis................................................................................................................................8

Magnetic Data and Depth-to-Basement .................................................................................................10

The Airborne EM Data and Water in the San Pedro River Basin ........................................................14

Conductivity Depth Transforms .................................................................................................................23

Conductivity vs. Depth Conversion Systems—The Inversion “Tuning” Issue ..................................26

Summary of Interpretations .......................................................................................................................28

Interpretations Related to the Hydrology of the Upper San Pedro Drainage 

Derived from the Airborne EM and Magnetic Data.........................................................28

Observations about the Geology of the Upper San Pedro Drainage Derived 

from the EM and Magnetic Data ........................................................................................30

Observations about the Geophysical Methods, Including the CDT 

Conversion Process..............................................................................................................31

Acknowledgments.......................................................................................................................................31

References Cited..........................................................................................................................................31

Figures

1. Index map of Arizona showing the locations of the 1997 San Pedro and 1999 Tombstone 

airborne geophysical surveys............................................................................................................2

2. Map showing boundaries of the four discrete airborne geophysical surveys (one during 

1997 and three during 1999) superimposed on a topographic map of the Fort Huachuca 

area, Arizona.........................................................................................................................................3

3. Map showing survey lines and survey parameters for the three 1999 Tombstone

airborne geophysical survey segments adjacent to the survey lines from 1997 .....................6

4. Combined 1997 and 1999 aircraft survey lines superimposed on a topographic map of

the Fort Huachuca area, Arizona ..................................................................................................... 7

5. Graph showing altimeter calibration for the 1999 airborne geophysical survey .....................9

6. Measured radar altimeter variations superimposed on a shaded-relief topographic

map of the Fort Huachuca area, Arizona, with contour lines of the 1999 EM

channel 10 (z-axis) conductivity ..................................................................................................... 10

7. Radar altimeter variations superimposed on the 1999 EM channel 10 (z-axis)

conductivity, over a shaded-relief topographic map of the Fort Huachuca area, Arizona ...11

Plates

1. Depth to water table superimposed on shallow (1999 EM channel 10) conductivity data 

plotted on a map of the Fort Huachuca area, Arizona (an enlarged version of text figure 17)

2. Conductivity vs. depth fence diagram superimposed on a topographic map of the Fort 

Huachuca area, Arizona (an enlarged version of text figure 22)
iv

Tables

1. Electromagnetic time-domain gates in the 1999 airborne electromagnetic survey.................8

2. Channel equivalencies between the 1997 and 1999 airborne electromagnetic surveys.........8

8. Radar altimeter variations superimposed on a contoured shaded-relief 

topographic map of the Fort Huachuca area, Arizona ................................................................12

9. Aeromagnetic data map showing merged magnetic data from the 1997 San Pedro and

1999 Tombstone airborne geophysical surveys for the Fort Huachuca area, Arizona............13

10. Euler deconvolution solutions superimposed on a topographic map of the Fort

Huachuca area, Arizona...................................................................................................................14

11. Map of the Fort Huachuca area, Arizona, showing magnetic depth-to-source data 

(crystalline basement in most cases) from Euler deconvolution of magnetic data

(Hanning filtered for clarity) for a structural index of 0 ...............................................................15

12. Map of the Fort Huachuca area, Arizona, showing depth-to-basement data

derived from modeling gravity data................................................................................................16

13. Thematic Mapper image of the Sierra Vista subbasin of the upper San Pedro River

drainage...............................................................................................................................................17

14. Map of the Fort Huachuca area, Arizona, showing combined results from the

1997 and 1999 airborne EM surveys, z-axis component, for channel 10 .................................18

15. Map of the Fort Huachuca area, Arizona, showing combined results from the

1997 and 1999 airborne EM surveys, z-axis component, for channel 14 ..................................19

16. Map of the Fort Huachuca area, Arizona, showing combined results from the

1997 and 1999 airborne EM surveys, z-axis component, for channel 18 ..................................20

17. Depth to water table superimposed on shallow (1999 EM channel 10) conductivity data 

plotted on a map of the Fort Huachuca area, Arizona.................................................................22

18. An example CDT profile from survey line 122, with an interpretation superimposed.............23

19. Graph showing comparison of resistivities derived from the airborne EM survey CDT

with resistivities derived from a well log for test well 5 ..............................................................24

20. Graph showing CDT vertical profiles plotted against water-table information and

well-resistivity data (log base 10 of resistivity) for nine test wells............................................25

21. Comparison of two CDT profiles (1997 line 101 and 1999 line 214) that approximately

coincide ...............................................................................................................................................26

22. Conductivity vs. depth fence diagram superimposed on a topographic map of the Fort 

Huachuca area, Arizona...................................................................................................................27

23. A comparison of a true inversion of the 60-channel EM data (using an EM Flow CDI)

with a Geoterrex-Dighem CDT profile for the same survey line ................................................29
v

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Volume

cubic meter (m3) 0.0008107 acre-foot (acre-ft) 

Specific conductance (conductivity) of ground water is given in microsiemens per centimeter 

(μS/cm).

Electromagnetic (EM) conductivity is given in parts per million of the secondary (momentary) 

field strength divided by the primary field strength.

Resistivity is given in ohm-meters. Resistivity measures the extent to which a material offers 

resistance to passage of an electric current. The resistivity of a conductor in ohm-meters is 

defined to be its resistance (in ohms) multiplied by its cross-sectional area (in square meters) 

divided by its length (in meters).

Amplitude of transmitter signal current is given in amperes.

Magnetic dipole moments per unit volume are given in amperes per meter. 

Frequency is given in hertz (equal to one cycle per second).

Electric potential (airborne electromagnetic signal) is measured in picovolts per square meter. 

Pico denotes 10-12 (one-trillionth).

Conversion Factors

notwithstanding, maps of the electrical conductor derived from 

the airborne EM system provide a synoptic view of the presence of water underlying the upper San Pedro Valley, including its three-dimensional distribution. The EM data even show 

faults previously only inferred from geologic mapping.

The magnetic and electromagnetic data together appear to 

show the thickness of the sediments, the water in the saturated 

sediments down to a maximum of about 400 meters depth, and 

even places where the main ground-water body is not in direct 

contact with the San Pedro River. However, the geophysical 

data cannot reveal anything directly about hydraulic conductivity or ground-water flow. Estimating these characteristics 

requires new hydraulic modeling based in part on this report.

One concern to reviewers of this report is the effect that 

clays may have on the electrical conductor mapped with the 

airborne geophysical system. Although the water in the basin 

is unusually conductive, averaging 338 microsiemens per centimeter, reasoning cited below suggests that the contribution 

of clays to the overall conductivity would be relatively small. 

Basic principles of sedimentary geology suggest that silts and 

clays should dominate the center of the basin, while sands and 

gravels would tend to dominate the margins. Although clay 

content may increase the amplitude of the observed electrical conductors somewhat, it will not affect the depths to the 

conductor derived from depth inversions. Further, fine-grained 

sediments generally have higher porosity and tend to lie 

toward a basin center, a fact in general agreement with the 

observed geophysical data.

Introduction and Background

The upper San Pedro Valley in southeastern Arizona is an 

area of concern for ground-water resources, in part due to its 

natural aridity and in part due to multiple competing land uses. 

These land uses include commercial development, ranching, 

military base activities, and the San Pedro National Riparian Conservation Area (SPNRCA). Congress established the 

Abstract 

This report summarizes the results of two airborne 

geophysical surveys conducted in the upper San Pedro Valley 

of southeastern Arizona in 1997 and 1999. The combined surveys cover about 1,000 square kilometers and extend from the 

Huachuca Mountains on the west to the Mule Mountains and 

Tombstone Hills on the east and from north of the Babocomari 

River to near the Mexican border on the south. The surveys 

included the acquisition of high-resolution magnetic data, 

which were used to map depth to the crystalline basement 

rocks underlying the sediments filling the basin. The magnetic 

inversion results show a complex basement morphology, with 

sediment thickness in the center of the valley ranging from 

~237 meters beneath the city of Sierra Vista to ~1,500 meters 

beneath Huachuca City and the Palominas area near the Mexican border. The surveys also included acquisition of 60-channel time-domain electromagnetic (EM) data. Extensive quality 

analyses of these data, including inversion to conductivity vs. 

depth (conductivity-depth-transform or CDT) profiles and 

comparisons with electrical well logs, show that the electrical 

conductor mapped represents the subsurface water-bearing 

sediments throughout most of the basin.

In a few places (notably the mouth of Huachuca Canyon), 

the reported water table lies above where the electrical conductor places it. These exceptions appear to be due to a combination of outdated water-table information, significant horizontal displacement between the wells and the CDT profiles, 

and a subtle calibration issue with the CDT algorithm apparent 

only in areas of highly resistive (very dry) overburden. These 

occasional disparities appear in less than 5 percent of the surveyed area. Observations show, however, that wells drilled in 

the thick unsaturated zone along the Huachuca Mountain front 

eventually intersect water, at which point the water rapidly 

rises high into the unsaturated zone within the wellbore. This 

rising of water in a wellbore implies some sort of confinement below the thick unsaturated zone, a confinement that is 

not identified in the available literature. Occasional disparities 

Mapping Ground Water in Three Dimensions—

An Analysis of Airborne Geophysical

Surveys of the Upper San Pedro River Basin,

Cochise County, Southeastern Arizona

By Jeff Wynn
SPNRCA in 1988 under the Arizona-Idaho Conservation Act 

(Public Law 100–696); among other things, this act set Federal 

reserve water rights for minimum flows in the river. In early 

1997, the U.S. Army requested that an airborne electromagnetic survey (Geoterrex-Dighem, 1997) be flown over the Fort 

Huachuca Military Reservation and immediate surrounding 

areas (Defense Mapping Agency, 1994; Wynn and Gettings, 

1997; Bultman, Gettings, and Wynn, 1999). This survey was 

designed to provide detailed three-dimensional information on 

the regional aquifer of the upper San Pedro River drainage. 

Due to this initial success, in early 1999 a followup 

airborne geophysical survey (Geoterrex-Dighem, 1999) was 

flown over three adjacent land tracts (fig. 1). In addition, an 

experimental line was flown east-west across the San Pedro 

Valley near the Mexican border to help document structure 

and stratigraphy of the sediments and to locate the aquifer in 

the vicinity of the international boundary. Survey outlines are 

shown in figure 2; note that, in this and subsequent figures, 

the coordinate system used is Universal Transverse Mercator (UTM) zone 12. These surveys were designed and carried 

out under U.S. Geological Survey (USGS) supervision (see 

“Acknowledgments”).

These surveys were planned to gather resistivity data 

from the upper 150–400 meters of the subbasin to help map 

the regional aquifer and the geologic structures that might 

control ground-water flow. (Note that the four separate survey 

segments were merged for this report.) In addition, the surveys 

acquired magnetic data, which were used to develop a map of 

the magnetic field strength. This map helps define the location of faults and fractures in the crystalline rocks forming the 

basement beneath the sediments (and, in some cases, volcanic 

rocks within the basin fill). The magnetic data can also be used 

to estimate depth-to-source of the crystalline basement rocks 

and, thus, to provide another set of estimates (in addition to the 

existing, relatively coarsely spaced gravity data) of the basin’s 

sediment fill. The thickness of sediments bears significantly 

on the hydrology of the basin, as shallow crystalline basement 

may interfere with water flow.

Brown and others (1966) identified two different basin fill 

units that constitute the regional aquifer: the relatively more 

porous upper basin fill and the more consolidated, less porous 

underlying lower basin fill. Recharge to the regional aquifer in 

the Sierra Vista subwatershed of the upper San Pedro groundwater basin (defined as the part of the San Pedro drainage 

bounded by the Mexican border on the south and the northern 

extents of the Babocomari River and Walnut Gulch drainages 

on the north and by the Huachuca Mountains on the west and 

the Mule Mountains on the east) is estimated to be about 15.4 

million to 18.5 million cubic meters a year, primarily from 

the Huachuca Mountains and to a lesser extent from the Mule 

Mountains and the Tombstone Hills (fig. 2; see Freethey, 

1982; Pool and Coes, 1999). In addition, Mexico contributes 

up to 4.3 million cubic meters per year of ground-water flow 

(Freethey, 1982).

Ground-water withdrawal from the Sierra Vista subwatershed was probably less than 13.6 million cubic meters per 

year in 1991 (Corell and others, 1996). It peaked at around 

18.5 million cubic meters per year in the early 1980s but has 

declined since then due in part to conservation measures, but 

principally due to the retirement of irrigation pumping. In 

addition, evapotranspiration along the San Pedro River by 

phreatophyte vegetation (for example, cottonwoods, mesquite, and willow trees that use ground water) is estimated 

to be about 7.6 million cubic meters a year (Freethey, 1982; 

Pool and Coes, 1999). Also, ground-water discharge to the 

San Pedro River is about 7.3 million cubic meters annually 

(Freethey, 1982). Total input to the ground-water system is 

about 22 million cubic meters per year. A total outflow from 

the system of about 27.2 million cubic meters per year results 

in depletion of ground-water storage. More importantly, 

ground-water withdrawals intercept the flow of ground water 

to the river and riparian area. Significant interception of the 

flow will affect river flow and will lead to an inevitable reduction in riparian vegetation. The ground-water deficit varies 

from year to year, depending on climate, withdrawals, recent 

conservation measures, and consumption by large mines and 

communal farms (ejidos) in Mexico.

Extensive fine-grained silt and clay layers have been 

identified in the regional aquifer (Pool and Coes, 1999). These 

fine-grained sediments tend to dominate the center of the San 

Pedro Valley, consistent with being more distal from their 

weathering origins in the Huachuca and Mule Mountains, and 

they influence the ground-water flow and interactions between 

the regional aquifer and the San Pedro River. The fine-grained 

Figure 1. Index map of Arizona showing the locations of the 1997 

San Pedro and 1999 Tombstone airborne geophysical surveys.

Base derived from U.S. Geological Survey 

DLG. UTM zone 12.

1997 survey

1999 followup

survey

UTAH

U.S.A.

MEXICO

MOHAVE

COCONINO

NAVAJO

APACHE

YUMA

YAVAPAI

MARICOPA

PINAL

PIMA

GILA

GRAHAM

GREENLEE

COCHISE

SANTA CRUZ

 Mapping Ground Water—An Analysis of Airborne Geophysical Surveys, Cochise County, Arizona
Meters

sediments are overlain and underlain by saturated sands and 

gravels (W. Steinkampf, U.S. Geological Survey, Tucson, 

Ariz., written commun., 2000). Water-level data from wells 

and several springs to the west of the San Pedro River indicate that the water table becomes increasingly shallow toward 

the San Pedro River and that ground water discharges as 

spring flow in places west of the river (Pool and Coes, 1999). 

Conductivity maps and conductivity-depth-transform (CDT) 

profiles discussed below suggest that parts of the San Pedro 

River in the vicinity of the Tombstone volcanic rocks (as 

mapped by Moore, 1993) are not in direct hydraulic contact 

(defined as saturated flow between aquifer and river) with the 

Sierra Vista subwatershed of the upper San Pedro River basin 

ground-water system.

This report describes the airborne geophysical surveys, 

summarizes the operating parameters for the survey system, 

and outlines the steps taken to assess quality of the merged 

datasets. It includes a section describing calculations of depthto-source from the magnetic data (primarily addressing the 

depth to the crystalline basement underlying the San Pedro 

basin sediments). The report then shows several ways in which 

the combined 1997/1999 airborne electromagnetic (EM) data 

can be viewed to understand the three-dimensional aspects of 

the regional aquifer in the Sierra Vista subwatershed. 

Figure 2. Map showing boundaries of the four discrete airborne geophysical surveys (one during 1997 and three during 1999) 

superimposed on a topographic map of the Fort Huachuca area, Arizona. Red lines indicate survey boundaries. An experimental profile 

(10 kilometers long) flown east-west across the San Pedro Valley near the U.S.-Mexico border is denoted by the words in red, “Mexico 

Border Line”; see also figures 3 and 22 and plate 2.

Base from U.S. Geological Survey 1:250,000-scale 

topographic map series. UTM zone 12.

Survey boundaries

Introduction and Background 
 Mapping Ground Water—An Analysis of Airborne Geophysical Surveys, Cochise County, Arizona

Water-table depths (http://www.azwater.gov/dwr/content/Find_by_Program/Wells/default.htm; see also the USGS 

National Water Information System Web site (NWISWeb) at 

http://nwis.waterdata.usgs.gov/nwis) are then compared with 

the shallow electrical conductor derived from the airborne 

electromagnetic survey. The water-table information dates 

from the 1970s through the 1990s; only the most recent value 

for a given well was used, but some depths have not been 

measured for several decades. Finally, a section of the report 

discusses the issue of inversion of the electromagnetic data 

into conductivity-depth-transform profiles (CDTs, or conductivity vs. depth), what CDT profiles tell us about the aquifer, 

and how reliable these inversions might be. A second method 

of doing inversions from the airborne EM data was tested and 

compared with the CDT results. Estimation of depth to water 

from these inversions was generally quite good, especially 

when compared to depths from electrical well logs. With a few 

exceptions discussed below, the CDTs for the entire survey 

area show maximum amplitude of the conductor at the water.

In the following discussion, the electrical properties of 

sediments are described in terms of conductivities, specifically 

in units of microsiemens per centimeter (μS/cm). This metric 

makes viewing the EM maps easier for the reader: red/purple 

hues in images shown later represent higher conductivity and 

show where the water is concentrated, as verified by comparison to electrical well logs. In general, higher clay content 

gives an increased conductor amplitude, represented by the 

darker purple colors. The blue in the images represents lower 

conductivity, generally unsaturated zones and crystalline rock. 

Most ground geophysical measurements are reported as resistivity values (in ohm-meters). The relation between the two is 

straightforward: conductivity = 1/resistivity (for example, 200 

μS/cm = 50 ohm-meters).

Previous Hydrological, Geological, and 

Geophysical Studies

A number of hydrologic studies of the upper San Pedro 

River basin have been produced over the years; one of the 

earliest was by Brown and others (1966). Brown and others 

described an upper basin fill, a lower basin fill, and an underlying, older Pantano Formation lying on crystalline basement. 

The upper basin fill and lower basin fill are lithologically 

similar; the main difference between them is relative porosity. 

Freethey (1982) provided the original ground-water model, 

on which all subsequent models have been based. The most 

recent studies include detailed hydrologic models, such as one 

developed by the Arizona Department of Water Resources 

(Corell and others, 1996). Previous conceptual and numerical 

models assumed a fairly simple basin structure; however, in a 

subsequent section of this report, the reader will encounter a 

more complex basement configuration derived from aeromagnetic data acquired from the 1997 and 1999 airborne geophysical surveys.

Pool and Coes (1999) conducted the most recent hydrologic study of the San Pedro regional aquifer. They identified 

extensive fine-grained units in the regional aquifer and provided detailed hydrogeologic sections of the sediments in the 

Sierra Vista subwatershed of the upper San Pedro River basin 

that were based on well data and vertical electrical soundings. 

Pool and Coes (1999) defined the primary regional aquifer 

as including “upper and lower basin fill, described by Brown 

and others (1966), that accumulated in the structural depression between mountain ranges during the Miocene through 

early Pleistocene ages.” Figure 8 of Pool and Coes (1999) is 

particularly helpful as an overview of the sedimentary package 

hosting the regional aquifer.

Drewes (1980) completed a regional geologic and 

tectonic map, supplemented in part by wilderness maps of 

the Coronado National Forest (Drewes, 1996; Sean Kneale 

and others, U.S. Forest Service, written commun., 1997). 

A 1:50,000-scale geologic map by Moore (1993) outlines a 

newly discovered volcanic complex and shows that parts of 

a large collapse caldera margin (part of what is now generally designated the Tombstone caldera) underlie the eastern 

margins of the Fort Huachuca Military Reservation near the 

San Pedro River, including land covered by the airborne 

geophysical surveys. Together, these reports forced a fundamental reconsideration of the subbasin structure. Reports by 

Halvorson (1984), Gettings and Houser (1995, 2000), and 

Gettings and Gettings (1996) provide an approximate map of 

the depth to bedrock beneath the Tertiary-Quaternary basin 

fill underlying the upper San Pedro River drainage (compared 

below with depth-to-source maps derived from the airborne 

magnetic data). These earlier studies revealed a depression in 

the bedrock as much as a kilometer deep beneath Huachuca 

City and shallow crystalline rock underlying parts of the city 

of Sierra Vista. This latter structural high appears to be an 

uplifted basement block; this interpretation is supported by a 

driller’s log of well D(21-20)35abb, where crystalline basement was encountered 237 meters deep near Fry Boulevard in 

downtown Sierra Vista. Euler deconvolution of aeromagnetic 

data (detailed below) also supports and augments information 

on these structures in the underlying crystalline basement. A 

seismic-reflection study designed to map the water table and 

the aquifer (Environmental Engineering Consultants, 1996) 

gave inconclusive results.

No previous airborne electromagnetic surveys over the 

area are known to exist. Pool and Coes (1999) collected and 

analyzed ground-based electric soundings and profiles at 

selected sites in the area, and the area is included in earlier regional aeromagnetic surveys at 1.6-kilometer spacing 

between flightlines (Andreasen and others, 1965) and proprietary mining company data at 0.5-kilometer spacing. Geophysical logs of nine test wells on the Fort Huachuca Military 

Reservation are available (U.S. Army Corps of Engineers, 

1972, 1974) and were used for evaluation of the 1997 airborne 

geophysical survey data covering the center of the study area 

(Wynn and Gettings, 1997; Bultman, Gettings, and Wynn, 

1999). The Arizona Department of Water Resources pro-
The survey used a CASA C–212 twin turbo-prop aircraft, 

with nominal survey flight speed of about 125 knots (65 

meters per second), flown at a nominal terrain-clearance of 

120 meters. A Rosemount 1241M barometric altimeter was 

used to control survey aircraft elevation; this unit has a sensitivity of about 0.3 meter in a 1-second recording interval. The 

radar altimeter was a TRT AHV–8 model having 2 percent 

accuracy over a 0- to 800-meter range; it also had a 1-second 

recording interval. A Panasonic Super (VHS) camera (model 

WV–CL302) was used to aid navigational recovery and help 

identify cultural interference. Electronic navigation was maintained to a horizontal precision of 10 meters by using a Sercel 

differential Global Positioning System (GPS) receiver (model 

NR103). This GPS system sampled locations once every second to a resolution of 0.00001 degree.

The magnetic data were acquired by using a Scintrex 

CS–2 single-cell cesium-vapor magnetometer, which was 

trailed in a towed sonde called a “bird” behind the aircraft and 

which sampled at 0.1-second intervals. This sampling interval 

gives a magnetic data density of about one sample per 6.5 

meters. Nominal sensor height was 73 meters above ground. 

The contractor estimated that the noise envelope for this survey was about ±0.5 nanoteslas (nT). A base station magnetometer was also operated at the Fort Huachuca airport to provide 

data for the diurnal magnetic field correction. The contractor 

completed corrections to the magnetic field data for diurnal 

drift, for sensor lag (sensor distance behind GPS antenna), 

for leveling (tie-line intersections), and for subtraction of the 

International Geomagnetic Reference Field computed at an 

altitude of 1,430 meters above mean sea level. The contractor 

also supplied the magnetic field data as digital files of data 

along the flightlines on a grid having an interval of 50 meters.

The GEOTEM Airborne ElectroMagnetic (AEM) System 

is a proprietary EM exploration system available only from 

Geoterrex-Dighem. The system uses a transmitter coil of 232 

square meters with six turns, flown at a nominal height above 

ground of 120 meters. The transmitter signal-current was an 

unusual time-domain signal (a 4,080-microsecond half-sine 

followed by a 12,486-microsecond flat off-time) having an 

amplitude of 500 amperes. The dipole moment was 6.96 × 

105 amperes per meter. The multicoil (three-axis: x, y, and 

z) receiver system was towed behind the aircraft in a bird 

maintained about 70 meters above the ground surface and 

approximately 125 meters behind the transmitter coil center. 

For maximum penetration of the underlying sediments, the 

system was set to operate with a base frequency of 30 hertz 

(cycles per second); this provides a transmitter pulse width of 

4,080 microseconds.

The x-axis (in-line) and y-axis (side-looking) receiver 

components were optimally coupled for detecting vertical 

conductors (for instance, vertical-sheet massive sulfide deposits or vertical/subvertical water-filled faults oriented perpendicular and parallel to the flightpath respectively). The z-axis 

(transmitter and receiver both having vertical-axis co-planar) 

component signal is optimally coupled for measuring the conductivity of horizontal layers beneath the ground surface. This 

vided a digital dataset of water-well locations and depths to 

water measured within the last few years for comparison to 

the electromagnetic data interpretation (also available in the 

USGS NWISWeb database). Gettings and Gettings (1996) 

collected and interpreted a detailed ground magnetic profile 

from the Dragoon Mountains southwest across the valley to 

the east gate of Fort Huachuca. Halvorson (1984) conducted 

the first gravity survey to map the depth to basement in the 

San Pedro Valley, followed by Gettings (1996). Gettings and 

Houser (2000) collected, compiled, and interpreted gravity 

anomaly, water-resources, exploration borehole geologic logs, 

and surface geologic data to model the depth and shape of the 

basin in the Sierra Vista to Huachuca City area. Significant 

amounts of proprietary geologic, geophysical, and geochemical data have been collected by mineral exploration companies 

for areas of bedrock outcrop, but these data were not available 

to the author.

Data Acquisition for the Airborne 

Geophysical Surveys

The 1999 airborne geophysical survey (GeoterrexDighem, 1999; referred to as the “Tombstone survey” to 

distinguish it from the Geoterrex-Dighem, 1997, “San Pedro 

survey”) was conducted from February 22 to March 1, 1999, 

and acquired both electromagnetic and magnetic data. Three 

discrete areas were flown to supplement coverage from the 

1997 San Pedro airborne geophysical survey of Fort Huachuca and surrounding areas (reported in Wynn and Gettings, 

1997; Bultman, Gettings, and Wynn, 1999). Areas 1–3 are 

labeled 1999–1, 1999–2, and 1999–3, respectively, in figure 

2 and extend from Lyle Canyon to the city of Tombstone in 

the north, to the Mule Mountains and Greenbush Draw on the 

east, and over to the Huachuca Mountains on the south about 8 

kilometers from the Mexican border (Wynn and others, 2000; 

Wynn, 2002). In addition, a single experimental 10-kilometer-long profile was flown parallel to, but approximately 3 

kilometers north of, the Mexican border to monitor ground 

water between the headwaters of the upper San Pedro drainage 

in Mexico and the regional aquifer in the Arizona part of the 

study area (see p. 26 and pl. 2).

The data were seamlessly merged with the 1997 survey, 

with the exception of the single experimental profile (analyzed 

in detail at the end of this report). A total of 1,366 kilometers 

of data were acquired, including 620 kilometers in Area 

1, 283 kilometers in Area 2, and 454 kilometers in Area 

3. Line-spacing in Areas 1 and 2 was ~400 meters; Area 3 

was flown at 800-meter spacing to fit within survey contract 

specifications but also to maximize the areal coverage on the 

southern margin of the survey. Figure 3 shows the flightline 

spacing (400 meters and 800 meters), the flightline orientation 

for each segment, and the total kilometers covered in each 

section. Figure 4 shows the line-coverage superimposed on the 

local roads and topography for reference.

Data Acquisition for the Airborne Geophysical Surveys 
 Mapping Ground Water—An Analysis of Airborne Geophysical Surveys, Cochise County, Arizona

coil orientation was found in this study to be the most useful 

for mapping shallow horizontal conductors, such as conductive 

ground water. All three components will report anomalies over 

powerlines, pipelines, and grounded fences. The transmitter, 

by virtue of its horizontal layout, is geometrically configured 

to induce (primarily) horizontal-plane eddy currents in the 

ground; the vertical (z-axis co-planar) receiver coil is thus 

optimally coupled to detect a secondary field from these eddy 

currents as deep as 400 meters.

The GEOTEM-transmitted waveform is a unique halfsine-wave-followed-by-an-off-period signal that can be lengthened for maximum penetration depth or shortened for maximum near-surface resolution. For this survey, the waveform 

was lengthened to near its maximum for the deepest possible 

penetration; that is, a 30-hertz repeat rate and a 4-millisecond 

sampling rate. The output channels consisted of sampling windows taken at increasing time gates (which could be translated 

as correlating with increasing depth below the surface) in the 

received secondary waveform for each of three coil orientations. In addition to 60 EM channels (20 time windows each, 

times the three orientations, with an in-phase and a quadrature 

component for each channel), the GEOTEM system acquired 

data from a separate magnetic sensor, plus an additional 

60-hertz monitoring channel to help distinguish manmade, 

grounded metallic structure anomalies (such as pipelines and 

powerlines) from geologic anomalies. These signals were 

location corrected during postflight data processing for the 

distance lag caused by the retarded position of the two detector 

birds trailing behind the aircraft on their respective cables.

The GEOTEM system also acquired Differential Global 

Positioning System (DGPS) location signals, as well as 

continuous videotape of the ground (to resolve any remaining 

manmade-vs.-geologic-source issues and to use as backup to 

the DGPS location information). The DGPS reference beacon 

was located at the Sierra Vista Airport during the survey. The 

EM channels were calibrated periodically by flying over a 

Meters

1997

1999-1

1999-3

1999-2

Area 1

Lines NW/SE@ 0.25 mi (402.3 m)

Tielines NE/SW @ 4.5 mi. (7 km)

Total Lines: 34 Total dist.: 341.73 mi (549.94 km)

Total Ties: 6 Total dist.: 42.97 mi (69.15 km)

 384.70 mi (619.09 km)

Area 2

Lines NE/SW@ 0.25 mi (402.3 m)

Tielines NW/SE @ 4.5 mi. (7 km)

Total Lines: 20 Total dist.: 153.67 mi (247.29 km)

Total Ties: 4 Total dist.: 21.84 mi (35.15 km)

 176.00 mi (282.44 km)

Area 3

Lines NE/SW@ 0.5 mi (800 m)

Tielines NW/SE @ 4.5 mi. (7 km)

Total Lines: 23 Total dist.: 231.99 mi (373.24 km)

Total Ties: 6 Total dist.: 50.23 mi (80.84 km)

 282.00 mi (454.19 km)

Total Miles: 848.70

Total Kilometers: 1365.32

Recon. Line (experimental profile)

1 Line E-W: 6 mi (9.6 km)

Figure 3. Map showing survey lines and survey parameters for the three 1999 Tombstone airborne geophysical survey segments 

adjacent to the survey lines from 1997. Data from Geoterrex-Dighem (1997, 1999). Total distance figures include 10-kilometer-long 

experimental profile (see p. 26 and pl. 2).

1999 Tombstone airborne geophysical survey (Geoterrex-Dighem, 1999)
well-characterized homogeneous target such as seawater and 

were also calibrated daily by flying the aircraft at the beginning and end of each survey at approximately 2,500 meters 

above ground level (AGL), so that any possible conductors 

were well beyond the reach of the rapid-fall-off dipolar transmitted signal, thus making ground contribution negligible. 

The data were also corrected for any residual signal caused 

by interaction of the transmitted pulse with the body of the 

aircraft (this correction is called “compensation”). Barometric 

elevation and radar-altimetry channels were also acquired to 

verify the vertical position of the aircraft.

Because the transmitted signal from an alternating electromagnetic dipole such as the GEOTEM transmitter antenna 

falls off rapidly as 1 over the distance cubed, it is critical to 

maintain the aircraft as close as possible to the ground for 

maximum signal penetration. During the 1997 San Pedro and 

1999 Tombstone surveys, the aircraft was “drape-flown” over 

the terrain at about 125 meters AGL, with the exception of a 

zone flown at 155 meters AGL over populated areas such as 

the city of Sierra Vista and Fort Huachuca. This local exception was necessary to conform with Federal Aviation Administration (FAA) regulations. While flying at 125 meters AGL, 

the magnetometer was nominally about 40 meters below the 

aircraft, and the EM bird was slightly more than 50 meters 

below the aircraft.

The GEOTEM system is a time-domain EM system; 

that is, data were sampled at certain defined time gates, some 

before but most after the transmitter signal was switched off. 

Base from U.S. Geological Survey 1:250,000-scale topographic

map series. UTM zone 12. Data superimposed from 1999 

Tombstone airborne geophysical survey (Geoterrex-Dighem, 

1999).

Figure 4. Combined 1997 and 1999 aircraft survey lines superimposed on a topographic map of the Fort Huachuca area, Arizona.

Data Acquisition for the Airborne Geophysical Surveys 7

Meters
 Mapping Ground Water—An Analysis of Airborne Geophysical Surveys, Cochise County, Arizona

Channel

number

Window starting-point (delay in microseconds from the end of the transmitted pulse)

1 1-3,906

2 1-3,169

3 1-1,975

4 1-782

5 1-44

6 195

7 391

8 630

9 911

10 1,215

11 1,541

12 1,931

13 2,409

14 2,995

15 3,733

16 4,665

17 5,837

18 7,313

19 9,136

20 11,306

These time gates were sampled for each of the three components of the received EM signal detected at the towed bird. 

The base frequency was 30 hertz (the transmitter was switched 

on and then off again 30 times per second). The channels used 

to sample the received signal are listed in table 1. In this table, 

the delays indicate the start of the gated window in microseconds from the end of the transmitted pulse for the given 

channel. Negative delays indicate that the window was started 

before the end of the transmitted pulse; this method was used 

to gather sufficient information to calculate inversions of conductivity vs. depth.

Between the 1997 and 1999 surveys, the GEOTEM 

system was modified slightly; primarily, the channel designations were reordered in a more logical numerical sequence. 

To spline data blocks from both surveys, it was necessary to 

use the same time gates so the system was measuring approximately the same conductivity at the same depth. Because 

the transmitter set up eddy currents in any conductors in the 

ground, the secondary signal received (that is, the signal 

received at the towed bird after the primary transmitter signal 

was turned off) from deeper depths arrived later than a secondary response from shallower depths. Because an averaging 

effect was going on also, there was no direct linear relation 

between gate times and depths. As a rule of thumb, however, 

the longer the time delay in the sample gate, the deeper the 

detected source. For this reason, different time gates can be 

used to effectively sample different depths. Table 2 lists the 

correlation between the sampled channels of the 1997 and 

1999 surveys. These channel equivalencies are presented with 

their relative depth of penetration indicated. Note that depth of 

penetration varies significantly over the target area and is controlled by variables such as aircraft elevation (increased flying 

elevation decreases depth penetration), degree of electrical 

interference (greater electrical interference reduces the signalto-noise ratio and thus reduces depth of effective penetration), 

and ground resistivity (the greater the resistivity, the greater 

the depth of penetration). The CDT inversions discussed below 

compensate for most of the fluctuations in elevation.

Finally, by using the Geosoft Grid Knit software, the 

original 1997 dataset and the three 1999 datasets for the magnetic field, as well as the three EM channel grids, were spliced 

together. The splicing was seamless for the magnetic data and 

generally excellent for the EM channels, except for an imperfect splice made difficult by an area of high topographic relief 

in the Tombstone Hills in Area 2.

Quality Control Analysis

The GEOTEM airborne geophysical system is advanced 

and complex, and as a result there are many variables that 

can affect data quality. These variables include the relative 

locations of the towed birds with respect to the aircraft. The 

dipolar nature of the transmitted signal means that there are 

significant changes in detected signal strength for only small 

Table 2. Channel equivalencies between the 1997 and 1999 airborne electromagnetic (AEM) surveys.

Table 1. Electromagnetic time-domain gates in the 1999 airborne 

electromagnetic survey.

1997 AEM survey channel number 1999 AEM survey channel number Relative depth penetration

2 10 Shallow

6 14 Intermediate

10 18 Deep

1

Transmitter-on data are important to precisely characterize the 

transmitted signal.

[Data from Geoterrex-Dighem 1999]
changes in geometry—even if absolute coil-to-coil separation is unchanged. The relative position of the birds has been 

shown to have a significant effect on the data, both in signal 

strength (fluctuations can thus look like changes in subsurface 

resistivity) and signal-to-noise (the noise threshold can exceed 

the signal, especially for secondary signals from deeper conductors). If the aircraft moves up and down significantly while 

draping rough terrain, then the system will detect apparent 

fluctuations in resistivity (there is a lesser, but still non-zero, 

effect of these fluctuations on the magnetic data). Extensive 

engineering work has gone into compensating for this elevation-fluctuation effect, but procedures to correct for these 

inevitable fluctuations can never be perfect. The GEOTEM 

system was designed, however, to hold these signal-strength 

fluctuation effects to the part-per-million range for most flying 

conditions.

In addition to structural flex in the aircraft-bird geometry, 

there can be short-wavelength errors in the radar altimetry data 

(from electrical interference such as regional lightning or airport radar transmissions) and long-wavelength errors in barometric altimetry data (from weather systems moving through 

the area), both of which affect the signal strength and thus the 

apparent resistivity of the local subsurface sediments. There 

is also the theoretical potential for temperature-controlled or 

vibration-caused changes in the GEOTEM electronic systems. 

Much of the work done by flight engineers and postprocessing 

technicians focuses on compensating for these potential errors.

The survey target areas, especially Area 2, have significant topographic relief that the aircraft must try to drape over. 

In addition, there is always some degree of air turbulence due 

to rising heat from the desert floor, especially in the afternoons. Twice during the 1999 survey, the aircraft was forced to 

return to base early when turbulence began to affect data quality. To check these various effects on the data finally reported 

by the contractor, the author generated several comparison 

figures.

The contractor carried out altimeter calibration by making a series of level overflights over the Sierra Vista Airport 

runway, whose elevation is well known. Results of these 

calibration flights are presented in figure 5, which shows 

barometric elevation fluctuations of less than 15 feet (less than 

5 meters). The GPS elevation noise is also shown in this figure 

for comparison. Because GPS systems cannot take advantage 

of satellite geometry both above and below the aircraft (the 

Earth is in the way), the GPS elevations are always much noisier than the horizontal position. Therefore, the GPS elevations 

were not used in the survey except as a rough quality check.

Figure 6 shows a superposition of measured radar altimeter variations (in color) on shaded-relief topography (digital 

USGS elevation data in gray tones), with contour lines of the 

shallow (1999 EM channel 10) z-axis conductivity. In this figure, we can see the correlation between flightline directions, as 

well as topographic relief and the radar altimetry. Subtle fluctuations in the contours of the conductivity correlate closely 

with the slight “herringbone” effect in the radar altimetry. 

These fluctuations are all within contract specifications (some 

lines were reflown when the fluctuations exceeded contract 

specifications). In the 1997 survey, some of the coverage over 

occupied areas such as Fort Huachuca and Sierra Vista had 

to be flown at 155-meter terrain clearance because of FAA 

flight rules; uninhabited areas in the 1997 and 1999 surveys 

were overflown at an optimal terrain clearance of 125 meters 

to maximize penetration of the EM transmitted signal, while 

minimizing risk to the aircraft.

Figure 7 shows the radar altimeter variations (this time 

as contour lines) superimposed on the 1999 EM channel 10 

z-axis conductivity (in color). Both are laid over shaded-relief 

topography (digital USGS elevation data in gray tones) for 

reference. This figure shows much more clearly that subtle 

wiggles on the edges of the conductivity anomalies in Area 2 

are caused by radar altimeter fluctuations, which in turn are 

caused by the aircraft attempting to drape some of the more 

rugged topography. This figure also shows that the effects of 

uncompensated aircraft motion are relatively small, especially 

considering the nonflat topography of the target area.

Finally, figure 8 shows the radar altimetry fluctuations 

(in color) superimposed on the contoured shaded-relief 

topography. From this view it is clear that the fluctuations 

are almost all due to changes in the topography that the 

aircraft is attempting to negotiate. See, for example, the 

western parts of the Babocomari River (located around UTM 

coordinates 3500000N and 550000E to 555000E), where 

topographic relief caused larger than average variations in 

radar altimetry. Figures 3–8 indicate that the data returned 

by the contractor met (and generally exceeded) the survey 

contract specifications.

These comparisons are self-consistency checks based on 

data released by the contractor. During the 1999 survey, the 

author served onsite as the contracting officer’s representative, 

working with flight engineers and the data-reduction team to 

assure that quality-control procedures were strictly adhered to. 

Figure 5. Graph showing altimeter calibration for the 1999 

airborne geophysical survey. Calibration was based on a series 

of overflights made at the Sierra Vista Airport, Arizona. The GPS 

(Global Positioning System) elevation noise is also shown for 

comparison. Data from Geoterrex-Dighem (1999).

4650

4850

5050

5250

5450

5650

4800 5000 5200 5400

Radar altimeter (feet) + Runway elevation (4612 feet)

Barometric and GPS

elevations (feet)

y = 0.9548x + 458.78

y = 0.9524x + 338.93

Barometric elevation GPS elevation

Quality Control Analysis 9
10 Mapping Ground Water—An Analysis of Airborne Geophysical Surveys, Cochise County, Arizona

The 1997 data have already been released separately (Bultman, Gettings, and Wynn, 1999).

Magnetic Data and Depth-to-Basement

Figure 9 shows merged magnetic data from the 1997 

San Pedro and 1999 Tombstone airborne surveys for the Fort 

Huachuca area; the data are plotted in color and as contour 

lines and imposed on a UTM grid of the area. In the center, 

the data are dominated by a large magnetic high caused by 

the Tombstone volcanic field. In addition, the data show small 

magnetic-noise anomalies caused by Fort Huachuca and Sierra 

Vista. In the northwest, there is a magnetic high thought to be 

caused by Tertiary volcanic rocks. Only small volcanic outcrops are observed on the surface in the northwestern part of 

the survey area (Sean Kneale and others, U.S. Forest Service, 

written commun., 1997), and most of the mapped volcanic 

rocks are beyond the area shown in figure 9, to the north and 

northwest. The anomaly in the northwest implies that a significant amount of Tertiary volcanic material underlies the Tertiary-Quaternary sediments near Huachuca City (Tv outcrops 

in several places north of Fort Huachuca in front of the Mustang Mountains and along the Babocomari River; see Drewes, 

1980). The data in figure 9 are not particularly useful unless 

we have an interest in the geology of the volcanic rocks or the 

basement rocks, subjects beyond the scope of this report. Magnetic contour maps generally show relatively little of the total 

information available in the magnetic data from which they 

are contoured. In this case, the strong magnetic high from the 

Tombstone volcanic field overshadows the remaining dataset.

We can extract additional information hidden in subtle 

variations in the magnetic data to provide depth-to-magneticsource information. Wherever a small lateral change in the 

magnetic field strength occurs (in most cases, changes that 

are smaller than the contour intervals on these maps), we can 

calculate the depth to that source—presumably from a geologic contact between two different lithologies having slightly 

different magnetic-mineral (usually magnetite) content. 

Typically, geologic and fault contacts in crystalline basement rocks beneath a basin such as this cause these changes. 

However, with the presence of nearby Tertiary volcanic rocks 

(Drewes, 1980; Gettings and Houser, 2000), we must consider 

the possibility that there are volcanic flows intercalated in the 

sediments above the crystalline basement.

Figure 6. Measured radar altimeter variations (in color) superimposed on a shaded-relief topographic map of the Fort Huachuca area, 

Arizona (digital USGS elevation data in gray tones), with contour lines of the 1999 EM channel 10 (z-axis) conductivity.

Base derived from U.S. Geological Survey Digital Elevation Model.

Meters
Euler deconvolution was carried out on the merged 

1997/1999 magnetic dataset. The results can be represented in 

several different ways, each of which has advantages and disadvantages. Figure 10 depicts Euler deconvolution solutions 

(shown as circles) superimposed on a topographic map of the 

upper San Pedro drainage. Each of the numerous circles represents a windowed (acceptable quality or low-noise) solution 

for a small change in the magnetic field strength; circles having warmer colors indicate a deeper source. This graphic presentation allows the geophysicist to see both the quality of the 

data (by the coherence of the solution circles) and—equally 

important—areas where data are scarce (by the absence of 

solution circles in low-gradient zones). In addition, structural 

information can be gleaned from this type of presentation. 

Circles that lie along a line represent either a geologic contact 

or a fault-offset in the crystalline basement. They can also 

represent geologic contacts between nonmagnetic sediments 

and thin layers of magnetic volcanic rock interleaved within 

the sediments. Finally, stacked but offset circles imply a 

dip or tilt in the horizontal plane in the geologic or fault 

contact.

One method for calculating depth-to-source from magnetic data is Euler deconvolution (Reid and others, 1990; 

Blakeley, 1995). In an area having many subtle variations in 

magnetic field strength (each caused by a change in magnetite content in the underlying lithology), a model can be 

constructed that breaks the anomaly source into many small 

prism-shaped sources. An analytic model that calculates the 

effect of each simple prism-shaped source can be incorporated 

into an “inversion” system; that is, an algorithm that continually compares the aggregate model solutions for a segment 

of the magnetic map and modifies the model source until it 

obtains a “best fit.” In fact, the inversion effort (that is, going 

backward from the magnetic data to the geologic source) 

is best carried out as a nonlinear process, as experience has 

shown that the comparison and modification part of the process can quickly become unstable. If the algorithm thus developed is passed along a profile, then the algorithm can calculate 

depth-to-source for multiple sources very quickly. Because 

these results vary widely in quality (depending on the noise in 

the original data), they must be “windowed” (that is, selected 

for sensitivity of the result to slight variations in horizontal 

and vertical changes in the small prism model sources).

Figure 7. Radar altimeter variations (as contour lines) superimposed on the 1999 EM channel 10 (z-axis) conductivity (in color), over a 

shaded-relief topographic map of the Fort Huachuca area, Arizona (digital USGS elevation data in gray tones). Altimeter variations were 

small; contours labeled 100 indicate a deviance of 1.00 meter from the target elevation.

Base derived from U.S. Geological Survey Digital Elevation Model.

Magnetic Data and Depth-to-Basement 11

M et e rs

100

100
12  Mapping Ground Water—An Analysis of Airborne Geophysical Surveys, Cochise County, Arizona

We can also present the results of the magnetic depthto-source calculations by using colors, as shown in figure 11. 

Cooler colors in this depth-to-source map represent deeper 

basement, and warmer colors represent shallower basement. 

For reference, figure 12 shows the contours of depth-tobasement derived from gravity data (Gettings and Houser, 

2000). The gravity contours are based on a much sparser 

gravity station dataset. The magnetic data show a much more 

complex basement than the gravity map in figure 12 shows. 

Most of the differences can be ascribed either to the greater 

density of magnetic data or to geologic noise. Geologic noise 

has many causes; one example is different depth solutions 

caused by magnetic rocks lying at two different depths, one 

over the other.

Near Huachuca City, there is an apparent ridge in the 

basement that is almost certainly caused by a narrow channel 

filled with volcanic material from the Tertiary volcanic rocks 

that are now intercalated in the sediment stack in the northwestern part of the survey area. This apparent ridge is located 

at UTM coordinates 3500000N, 564000E in figure 11.

The San Pedro River, with one or two small exceptions 

in the study area, runs over parts of the basin underlain by 

shallow crystalline rock and relatively little sediment. The 

electrically conductive aquifer (discussed on p. 17 and 18) correlates well with the deeper sections of the basement lying to 

the west of the river. In the northeastern quadrant of the study 

area, the San Pedro River, Walnut Gulch, and the Babocomari 

River intersect and are underlain by large sections of shallowly 

buried to surficial exposures of the Tombstone volcanic field. 

The relation between the basin sediments and the bedrock in 

the Tombstone Hills area is complex and may be both stratigraphic (Cenozoic basin sediments unconformably overlie 

the Cretaceous intercalated volcanic flows in the sedimentary 

stack) and fault bounded. This region lies within the mapped 

Tombstone caldera margin (Moore, 1993). Also in figure 11, 

one can see a roughly east-west deep basement feature (the 

blue feature extending from the right to about UTM coordinates 3495000N, 580000E in the middle) south of Tombstone 

that apparently intersects another northwest-southeast deep 

basement zone roughly paralleling the Huachuca Mountain 

front but lying to the east of it (the NNW-SSE blue zone at the 

left-center of figure 11).

The magnetic depth-to-source map in figure 11 agrees 

remarkably well with the gravity depth-to-basement map 

Figure 8. Radar altimeter variations (in color) superimposed on a contoured (for clarity) shaded-relief topographic map of the Fort 

Huachuca area, Arizona (digital USGS elevation data in gray tones).

Base derived from U.S. Geological Survey Digital Elevation Model; 

contours in meters.

Terrain clearance

in meters

Meters
River (Gettings and Houser, 2000). The magnetic depth-tosource map suggests east-west faulting and uplift of basement 

rocks, probably related to 16-million- to 20-million-year-old 

Basin-and-Range faulting, though an east-west orientation is 

unusual in such faulting. There is some geologic evidence that 

these faults have been recently active (Gettings and Houser, 

2000). This map and geologic mapping to the north of the 

study area (Gettings and Houser, 2000) seem to indicate northsouth faulting of an indeterminate character.

The results obtained from the Euler deconvolution of the 

magnetic data should enable hydrologists to better assess the 

thickness of the upper San Pedro drainage sediments that lie 

deeper than the airborne electromagnetic system can penetrate. 

Depending on the degree of human electrical cultural interference present locally, the maximum direct depth of water 

detection by the GEOTEM Airborne ElectroMagnetic System 

Figure 9. Aeromagnetic data map showing merged magnetic data from the 1997 San Pedro and 1999 Tombstone airborne geophysical 

surveys for the Fort Huachuca area, Arizona. Data are depicted in color and as contours and show a large magnetic high caused by the 

Tombstone volcanic field, as well as several smaller magnetic anomalies. Data are plotted on a Universal Transverse Mercator grid of 

the area.

(fig. 12). A structural high (probably a horst or uplifted 

fault-block) underlies the city of Sierra Vista centered around 

UTM coordinates 3490000N, 570000E. A driller’s log of 

well D(21-20)35abb, which encountered crystalline basement 

at 237 meters beneath the city, verifies this feature. A northsouth fault (roughly along UTM longitude line 565000E) also 

extends from the Whetstone Mountains (which lie just beyond 

the northwest corner of the study area) to the Huachuca Mountains on the south (referred to in Gettings and Houser, 2000, 

as the “Range-Front Fault”). The deep (at least 1,000 meters) 

basin beneath Huachuca City (UTM coordinates 3505000N, 

562000E) appears in figure 11 to be more than one basin. The 

apparent ridge in the middle, however, is likely a stringer of 

volcanic materials in the middle levels of the sediment stack 

from the Tertiary volcanic rocks that lie just off the northwestern corner of the map and tiny outcrops in the Babocomari 

Base derived from U.S. Geological Survey Digital Elevation Model.

Magnetic Data and Depth-to-Basement 13

Residual magnetic field

in nanoteslas

Meters

550000E 555000E 560000E 565000E 570000E 575000E 580000E 585000E 590000E 595000E

550000E 555000E 560000E 565000E 570000E 575000E 580000E 585000E 590000E 595000E

3515000N 3510000N 3505000N 3500000N 3495000N 3490000N 3485000N 3480000N 3475000N

3515000N 3510000N 3505000N 3500000N 3495000N 3490000N 3485000N 3480000N 3475000N
14  Mapping Ground Water—An Analysis of Airborne Geophysical Surveys, Cochise County, Arizona

Figure 10. Euler deconvolution solutions superimposed on a topographic map of the Fort Huachuca area, Arizona. The colors of 

the solution circles correlate with depth to magnetic basement; warmer colors indicate deeper sources. The locations of individual 

solutions show magnetic gradients that are generally geologic contacts or faults in the crystalline basement rocks.

Base from U.S. Geological Survey 1:250,000-scale topographic map series.

Meters

(discussed in the following section) could be anywhere from 

100 to 400 meters. However, structures deeper than 300 to 

400 meters are of little hydrologic significance because there 

is generally less ground-water flow with depth and the sediments at great depth have reduced porosity due to consolidation and compaction (Ingebritsen and Sanford, 1998, p. 274). 

The CDTs discussed below appear to support this condition 

by showing closure (termination) of many of the electrical 

conductors at depth.

The Airborne EM Data and Water in the 

San Pedro River Basin

Because they are so similar to the z-axis EM data analyzed in detail below, the x-axis and y-axis airborne EM data 

in the upper San Pedro River basin are not discussed here. 

Instead, the following interpretation focuses almost exclusively on the z-axis imagery because it is an optimally coupled 
signal for mapping horizontal and subhorizontal conductors. 

This approach is just the opposite of that of airborne EM 

investigators searching for mineral resources, where complex 

vertical and subvertical structures are generally the key to 

understanding the mineralization process.

Figure 13 is a Thematic Mapper image of the Sierra Vista 

subbasin of the upper San Pedro River drainage; it was taken 

from a larger Landsat Thematic Mapper image by Dohrenwend and others (2001). The bright colors were deliberately 

enhanced to show subtle differences in surface rocks and soils 

that can be attributed to hydrothermal alteration, a process 

closely related to mineralization. The figure is used here, however, to provide a recent (1997) index of the growing population of Sierra Vista and surrounding areas. Airborne EM data 

are strongly influenced by human cultural interference such as 

powerlines and pipelines.

Figures 14, 15, and 16 show the airborne EM images 

acquired by using the z-axis coil in the GEOTEM system; they 

represent channel 10 (shallow), channel 14 (intermediate), and 

channel 18 (deep) data, respectively. The channels referenced 

here are from the 1999 survey, but the results apply to the 

1997 area, as well. The redder or warmer colors represent 

zones of higher conductivity caused by the occurrence of 

water, with some additional contribution from clays. In these 

figures, lines represent the San Pedro River, the Babocomari

River, and Walnut Gulch. These features were digitized from 

the U.S. Geological Survey 1:250,000-scale topographic map 

of the area.

The San Pedro basin lies in a relatively arid zone, and the 

ground water has a generally higher conductivity from less 

extensive flow-through than that in wetter, more temperate 

regions of the United States. Rainwater normally has very low 

conductivity, but when it rests in an aquifer over an extended 

period, it takes on dissolved ions from the surrounding rocks 

and soils and becomes more conductive, unless flushed by 

recharge and ground-water flow. Ground water in the San 

Pedro regional aquifer is potable and considered good quality in comparison with water in nearby arid basins (Pool and 

Coes, 1999). Conductance of ground water in the regional 

aquifer ranges from about 200 to 1,200 μS/cm and averages 

338 μS/cm, which corresponds to a resistivity of 30 ohmmeters (values from Pool and Coes, 1999, pl. 3). Most geophysicists consider even 200 μS/cm to be conductive ground 

water (Ken Zonge, Zonge Engineering & Research, Tucson, 

Ariz., oral commun., 1999; also personal experience of the 

author). Slightly lower conductivity values in the Sierra Vista 

Figure 11. Map of the Fort Huachuca area, Arizona, showing magnetic depth-to-source data (crystalline basement in most cases) 

from Euler deconvolution of magnetic data (Hanning filtered for clarity) for a structural index of 0. The 0 index was chosen to emphasize 

step-offsets and geologic contacts. Note that in figure 10, warmer colors indicate deeper sources, whereas in this figure, warmer 

colors indicate shallower sources.

Base derived from U.S. Geological Survey Digital Elevation Model.

The Airborne EM Data and Water in the San Pedro River Basin 15

Meters

550000E 555000E 560000E 565000E 570000E 575000E 580000E 585000E 590000E

550000E 555000E 560000E 565000E 570000E 575000E 580000E 585000E 590000E

3510000N 3505000N 3500000N 3495000N 3490000N 3485000N 3480000N 3475000N

3475000N 3480000N 3485000N 3490000N 3495000N 3500000N 3505000N 3510000N
16  Mapping Ground Water—An Analysis of Airborne Geophysical Surveys, Cochise County, Arizona

Figure 12. Map of the Fort Huachuca area, Arizona, showing depth-to-basement data derived from modeling 

gravity data (Gettings and Houser, 2000); contours in meters (m). Compare with figure 11.

Base from Gettings and Houser, 2000.
Mule

Mountains

Fort

Huachuca

Walnut Gulch

San Pedro River

MEXICO

USA

Sierra

Vista

Huachuca Canyon

Whetstone

Mountains

Huachuca

Mountains

Huachuca

City Lyle Canyon

Tombstone

Ramsey Canyon

Charleston

Garden Canyon

Figure 13. Thematic Mapper image of the Sierra Vista subbasin of the upper San Pedro River drainage. Derived from Dohrenwend and 

others (2001), with key features labeled and colors enhanced. The light-purple zones immediately west of the San Pedro River correlate 

with significant vegetation and several springs.

Landsat Thematic Mapper image from 1997.

The Airborne EM Data and Water in the San Pedro River Basin 17

subwatershed in comparison to those of the nearby Tucson 

basin (between 300 and 500 μS/cm) suggest a more recent age 

since time of recharge of the system. The presence of clays in 

the San Pedro basin may further enhance conductivity values, 

but conductivities of ground water in the basin are already 

quite high.

The most electrically conductive parts of the aquifer 

(more than 800 μS/cm conductance or less than 12.5 ohmmeters resistivity) probably correlate with saturated silts and 

clays and with conductive sediments detected in electrical 

well logs, particle size logs (wells TW1, TW3, TW5, TW8, 

TW9, and MW7), and several vertical electrical soundings 

used to define the extent of the fine-grained sediments in the 

Sierra Vista subbasin (Pool and Coes, 1999). Moderately electrically conductive sediments in the aquifer (200 to 800 μS/

cm) generally correlate with saturated sand and gravel. Less 

conductive materials (less than 200 μS/cm) are generally nonaquifer materials such as crystalline rocks. Fine-grained parts 

of the Pantano Formation, a poor aquifer, and the Cretaceous 

Morita Formation, a mudstone deposit, can also be weakly 

to moderately electrically conductive if water saturated and 

can be confused with fine-grained parts of the aquifer. These 
18  Mapping Ground Water—An Analysis of Airborne Geophysical Surveys, Cochise County, Arizona

sediments are likely to occur in the near surface close to the 

Huachuca Mountains (Sean Kneale and others, U.S. Forest 

Service, written commun., 1997).

The clay-contribution-to-conductivity issue bears some 

discussion. On the basis of several hundred laboratory sample 

electrical measurements made while working for an engineering firm in Tucson, plus several hundred line-kilometers of 

resistivity/induced polarization surveys measured and interpreted in southeastern Arizona and other arid environments, 

the author believes that clays have a relatively minor effect 

on the already high conductivities observed in the AEM data 

in the San Pedro Valley. Clays may affect the amplitude of 

airborne EM conductors but not their horizontal or vertical 

location in the inversions. Further, the effect of clays is not 

particularly significant in areas like the San Pedro Valley 

where ground water is already so highly conductive (about 338 

μS/cm or 30 ohm-meters resistivity on average and, in a few 

cases, above 800 μS/cm). Keller and Frischknecht (1966, p. 

449) showed that, above about 5 percent clay content, cation 

exchange capacity becomes saturated. In other words, the 

contribution to conductivity from clays remains constant (and 

even declines) for clay content above 5 percent. In work in a 

similar arid environment in Saudi Arabia (Flanigan and Wynn, 

1979), wadis were found to be conductive only down-drainage 

from mineral deposits, whereas the clay content was uniform 

along the entire length of the wadi. Thus, from the Huachuca 

Mountain front to the center of the basin, clays probably do 

not contribute significantly to the conductivity, no matter 

how much the clay content may increase. Consequently, the 

airborne geophysical data cannot reliably show the variation 

in clay content over the basin but can indicate where water is 

present. By inference, these data can also tell us something 

about the porosity of the sediments.

Figure 14. Map of the Fort Huachuca area, Arizona, showing combined results from the 1997 and 1999 airborne EM surveys, z-axis 

component (which emphasizes flat-lying electrical conductors), for channel 10. Because channel 10 is the shortest time gate, shallow 

electrical conductors are emphasized in the figure. “Speckling” indicates human electrical cultural noise (for example, powerlines 

and pipelines).

550000E 555000E 560000E 565000E 570000E 575000E 580000E 585000E 590000E 595000E

550000E 555000E 560000E 565000E 570000E 575000E 580000E 585000E 590000E 595000E

3520000N 3515000N 3510000N 3505000N 3500000N 3495000N 3490000N 3485000N 3480000N 3475000N

3520000N 3515000N 3510000N 3505000N 3500000N 3495000N 3490000N 3485000N 3480000N 3475000N

2000 0 2000 4000 6000

Meters

Base derived from U.S. Geological Survey Digital Elevation Model.
Dark-blue areas in figures 14–16 correlate closely 

with the Tombstone volcanic field. These blue areas extend 

westward beyond where volcanic rocks appear on the geologic map by Moore (1993), indicating that the volcanic rocks 

extend under the more recent sediments westward of their 

geologic map location. The San Pedro River cuts through the 

Tombstone Hills volcanic rocks in the northern part of the 

survey area, and the AEM data clearly show that no significant 

shallow water is present beneath the river there. An extension 

of the large, central high-conductivity zone (red and purple 

areas in figures 14–16) through the Tombstone Hills correlates 

with a zone of denser vegetation north of Graveyard Gulch, 

roughly northwest of Charleston and east of Huachuca City, 

and extending into the western reaches of Walnut Gulch. Not 

surprisingly, the area where the Babocomari River and Walnut 

Gulch intersect the San Pedro River coincides with a zone 

of higher conductivity (red) that represents the southern end 

of another component of the regional aquifer lying north of 

the study area. A subtle offset in the warm colors in all three 

figures around UTM coordinates 3487000N and 575000E 

coincides with the Sawtooth Canyon fault of Drewes (1980, 

1996). The geophysical data imply a left-lateral throw that 

geologic mapping by itself cannot show.

The speckling effect in figures 14–16 depicts human 

electrical cultural interference (from powerlines and pipelines). Figure 14 clearly shows that the speckled areas west of 

the main high-conductivity/low-resistivity zone correlate with 

the powerlines and pipelines above and beneath Fort Huachuca 

and Sierra Vista. Speckled zones in the south-southwest of 

the survey area correlate well with more recent extensions 

of human settlement to the south toward Garden Canyon and 

Ramsey Canyon.

Figure 16 shows much stronger interference in the survey 

area than figure 14 shows. This finding is counterintuitive, 

Meters

Figure 15. Map of the Fort Huachuca area, Arizona, showing combined results from the 1997 and 1999 airborne EM surveys, z-axis 

component (which emphasizes flat-lying electrical conductors), for channel 14. Because channel 14 is an intermediate time gate, 

intermediate-depth electrical conductors are emphasized in the figure. “Speckling” indicates human electrical cultural noise (for 

example, powerlines and pipelines).

Base derived from U.S. Geological Survey Digital Elevation Model.

The Airborne EM Data and Water in the San Pedro River Basin 19
20 Mapping Ground Water—An Analysis of Airborne Geophysical Surveys, Cochise County, Arizona

as figure 16 represents the output of EM channel 18 (z-axis), 

the most deeply penetrating EM channel. In fact, the deeper 

penetrating (or longer time lag) EM signals are attenuated 

more than the shallower signals by the overlying, conductive 

water-saturated layers. The gains must be adjusted upward to 

bring out the deeper signals, and as a result, the “speckling” 

or human cultural noise from the powerlines and pipelines 

is more strongly represented. Thus, even though the powerlines and pipelines are at the ground surface, they appear 

more strongly in the data representing the deeper conductive 

sources.

The large, high-amplitude high-conductivity zone in the 

extreme west of the survey area (figs. 14–16) suggests a western extension of the regional aquifer around Lyle Canyon and 

Turkey Creek, an area having mesquite bosques (woodlands) 

and springs (Brenda Houser, USGS, Tucson, Ariz., oral commun., March 16, 2000). This conductor is probably caused by 

water present in parts of the Pantano or Morita Formations or 

pre-Tertiary sedimentary rocks that crop out near the Huachuca Mountains and along the Babocomari River (Drewes, 

1980; Sean Kneale and others, U.S. Forest Service, written 

commun., 1997). This conductivity anomaly can occur, however, only if water is present. In the author’s experience with 

hundreds of laboratory resistivity measurements from samples 

throughout Arizona, dessicated clay-bearing samples are not 

conductive. If the clay-bearing samples are hydrated with 

distilled water, they become mildly to moderately conductive, 

but they must be saturated. The relatively high electrical conductivity of the ground water in the San Pedro drainage area 

(Pool and Coes, 1999, pl. 3) makes it clear that high-ion-content water dominates the conductivity anomaly observed here 

in the airborne geophysical surveys. This finding is especially 

true as porosity is usually higher in fine-grained sediments 

near the center of a valley (Driscoll, 1986, p. 67). This conductor should be investigated further to define its character and 

extent beyond the airborne survey boundaries.

Close to the Huachuca Mountain front (the dark-blue 

areas on the southwestern edge of figures 14–16), there appear 

to be some small electrical conductors (narrow fingers of red 

parallel to the mountain front) that may be small, shallow, 

Conductivity

(EM response in parts per million)

Meters

550000E 555000E 560000E 565000E 570000E 575000E 580000E 585000E 590000E 595000E

550000E 555000E 560000E 565000E 570000E 575000E 580000E 585000E 590000E 595000E

3515000N 3510000N 3505000N 3500000N 3495000N 3490000N 3485000N 3480000N 3475000N

3515000N 3510000N 3505000N 3500000N 3495000N 3490000N 3485000N 3480000N 3475000N

3520000N

3520000N

Figure 16. Map of the Fort Huachuca area, Arizona, showing combined results from the 1997 and 1999 airborne EM surveys, z-axis 

component (which emphasizes flat-lying electrical conductors), for channel 18. Because channel 18 is the longest time gate, deep 

electrical conductors are emphasized in the figure. “Speckling” indicates human electrical cultural noise (for example, powerlines and 

pipelines).

Base derived from U.S. Geological Survey Digital Elevation Model.
buried water outliers. Fragments of fine-grained Pantano Formation, in general overlain by sand and gravel basin fill, have 

been mapped in this area, and Pantano Formation also shows 

up in test wells. The Pantano is not a particularly good aquifer, 

however (Brown and others, 1966). These small electrical 

conductors in the Huachuca Mountain front area do not appear 

to correspond to any powerlines or pipelines, and so they must 

be caused by the presence of subsurface water.

Figures 14–16 are in effect depth slices of the regional 

conductor. They appear to suggest a mountain-front-paralleling gap between the small electrical conductors and the large 

conductor (interpreted as the regional aquifer) toward the east. 

This apparent gap is actually a thick, mountain-front-following unsaturated zone, as revealed in inverted conductivity vs. 

depth profiles (discussed in detail below) and water-tabledepth data. The electrical conductor is continuous below this 

thick unsaturated zone between the narrow electrical conductors along the mountain front to the west and the large electrical conductor to the east. Well drawdown has been documented locally to a maximum of about 22 meters in the area 

near the Fort Huachuca East Gate well field, but elsewhere it 

is more typically less than 15 meters. Well drawdown becomes 

nearly insignificant at 3 meters near the San Pedro River (Pool 

and Coes, 1999).

Geologic mapping (Gettings and Houser, 2000) has 

shown that there is a listric fault tilting sedimentary rocks to 

the southwest along the Huachuca Mountain front. Called the 

Nicksville fault, it is probably related to Basin-and-Range 

faulting (Brenda Houser and Mark Gettings, USGS, Tucson, 

Ariz., oral commun., 1999). According to Don Pool (USGS, 

Tucson, Ariz., written commun., 2001), however, this fault 

may be early to mid-Miocene in age because listric faulting 

is not common in mid- to late Miocene Basin-and-Range 

deformation. This faulting has in effect downdropped the 

less hydraulically conductive Pantano Formation more in 

the southwest than in the northeast. However, the younger 

upper-basin-fill sand and gravel sediments are not tilted. This 

southwestern tilting may explain the thick, mountain-front-following sand-and-gravel zone, but not why it is unsaturated, nor 

why the electrical conductor, which apparently represents the 

presence of water, lies deep beneath it. Wells drilled into this 

unsaturated zone encounter water at depth, which then rises in 

the wells, as if it had been confined, to what is then recorded 

as a water table (Brenda Houser, USGS, Tucson, Ariz., oral 

commun., May 16, 2000). The underlying Pantano Formation is an older, pre-Basin-and-Range basin fill unit (Tertiary 

age) with deformed and tilted bedding. It has widely varying 

electrical properties (Pool and Coes, 1999) but is generally 

well consolidated. It is also a poor aquifer (Brown and others, 

1966). There is probably some ground-water discharge from 

this area north to the Babocomari River near Huachuca City 

(Mark Gettings and Don Pool, USGS, Tucson, Ariz., oral 

commun., 1999).

Near the Huachuca Mountain front, there are significant 

variations among nearby wells in the thickness of unsaturated 

resistive sands and gravels (basin fill) overlying the Pantano 

Formation. These variations suggest block-faulting between 

the Huachuca Canyon fault and the Sawmill Canyon fault 

of Drewes (1980) around the mouth of Huachuca Canyon. 

Detailed examination of data in this area (Mark Bultman, 

USGS, Tucson, Ariz., oral commun., March 14, 2001) reveals 

differences between water-table and electrical-conductor 

depth of 10–20 meters in a narrow window near the mouth 

of Huachuca Canyon. These deviations are among the largest between the electrical conductor and the water table in the 

survey area. It is possible that these deviations are an artifact 

in the conductivity-depth-transform calculations (discussed in 

detail below), but they may also be due to seasonal variations 

in water recharge in this canyon-mouth area. The water-table 

data used in this report were acquired as much as several years 

before the airborne EM survey.

Examination of figures 14–16 shows an electrical conductor, interpreted here to be the regional aquifer, underlying 

(and therefore in hydraulic contact with) much of the San 

Pedro River. In the northern part of the study area (north of 

3495000N), however, where the river passes over and through 

the western margins of the Tombstone volcanic field, there 

is little electrical conductivity that we associate with ground 

water in any of the airborne EM channels. Some of this area is 

covered with limestone, but according to the magnetic survey, 

this area has a thin veneer of materials underlain by volcanic rocks. The lack of an electrical conductor here implies 

that the aquifer discharges directly to the San Pedro River in 

at least the southern two-thirds of the survey area (south of 

3495000N) but not in the northern third.

The Landsat image (fig. 13) supports Pool and Coes’ 

(1999) observation that significant water is lost to evapotranspiration through local vegetation. The light-purple zones 

immediately west of the San Pedro River correlate with 

significant vegetation and several springs. These springs 

appear to be caused by relatively insignificant ground-water 

flow reaching the surface as water moves eastward above silt 

and clay layers in the basin fill unit. The water table between 

Fort Huachuca and the San Pedro River becomes increasingly 

shallow as it approaches the river (see fig. 17). The amount 

of discharge from these springs is very small, however, and 

cannot account for most of the ground-water flow from the 

aquifer. The perennial water seen in the eastern reaches of the 

Babocomari River in the northern part of the study area must 

come from the upper San Pedro aquifer.

Figure 17 (an enlarged version of this figure is presented as pl. 1 of this report) shows an overlay of water-table 

depths (numbers in black) onto shallow (1999 EM channel 10) conductivity data. The warm colors (zones of high 

conductivity) are inferred to represent water. The water-table 

data are from Maurice Tatlow (Arizona State Department of 

Water Resources, written commun., 1998; an edited version 

of Tatlow’s data table can be found in appendix 1 of Bultman, Gettings, and Wynn, 1999). Well data can be accessed at 

http://www.azwater.gov/dwr/content/Find_by_Program/Wells/

default.htm and are also available from the USGS NWISWeb 

The Airborne EM Data and Water in the San Pedro River Basin  21
22  Mapping Ground Water—An Analysis of Airborne Geophysical Surveys, Cochise County, Arizona

550000E 555000E 560000E 565000E 570000E 575000E 580000E 585000E 590000E 595000E 600000E

550000E 555000E 560000E 565000E 570000E 575000E 580000E 585000E 590000E 595000E 600000E

3520000N 3515000N 3510000N 3505000N 3500000N 3495000N 3490000N 3485000N 3480000N 3475000N

3520000N 3515000N 3510000N 3505000N 3500000N 3495000N 3490000N 3485000N 3480000N 3475000N

Conductivity

(EM response in parts per million)

2000 0 2000 4000 6000

Meters

Figure 17. Depth to water table (black numbers) superimposed on shallow (1999 EM channel 10) conductivity data (colors) plotted on a 

map of the Fort Huachuca area, Arizona. The redder or warmer colors (zones of high conductivity) are inferred to represent water. The 

water-table data are from Maurice Tatlow (Arizona State Department of Water Resources, written commun., 1998; an edited version of 

Tatlow’s data table can be found in appendix 1 of Bultman, Gettings, and Wynn, 1999). Depth to water table provided in feet, per Tatlow’s 

data table (multiply feet by 0.3048 to obtain meters). Conductivity data from figure 14 are combined results from 1997 and 1999. An 

enlarged version of figure 17 is presented as plate 1 of this report.

Base derived from U.S. Geological Survey Digital Elevation Model; 

contours show elevation in meters.

(http://nwis.waterdata.usgs.gov/nwis). In general, there is good 

agreement between the water-table data and the conductivity 

data. The deeper water table that is 15 to 50 meters just east 

of Fort Huachuca and the Huachuca Mountains coincides 

with the green (weak electrical conductor) zones near Fort 

Huachuca. This area has already been identified as a deep 

unsaturated zone where the conductive water lies much deeper 

than it does farther east in the central part of the valley. Where 

the shallow electrical conductor (the red areas in the figure) 

strongly manifests, the water-table depths are in the 6- to 

15-meter range. In the northeast, where the Tombstone Hills 

outcrop (the dark-blue zone in the upper right of the figure), 

there are very shallow water-table depths, but this is a zone 

where almost no water is present. Shallow (EM channel 10) 

aquifer extensions are also apparent on the east side of the San 

Pedro River, where they correlate closely with drainages that 

channel runoff westward from the Mule Mountains to the main 

regional aquifer.

The deep conductor (the hot-pink, low-resistivity zone 

seen in the EM channel 18 data of fig. 16) is apparently a 

deep, silt-and-clay-rich, high-porosity basin-center deposit. 

This clay-rich zone is also seen in the shallower channels 

(figs. 14 and 15). This is a water-saturated unit that accumulated in the San Pedro basin during late Miocene to Pliocene 

time (about 10 million to 2 million years ago). Weathered-out 

clays tend to concentrate more distally from their source rocks 
than do sands and gravels, and so they tend to predominate in 

basin centers. The colors (bright purple in the CDTs discussed 

below) suggest that the deep conductor is less electrically 

conductive than the shallow aquifer above it. Apparently, this 

phenomenon is a subtle artifact of the airborne CDT inversion 

below about 150 to 200 meters. It is likely that the higher clay 

content in the central parts of the San Pedro basin (but west of 

the San Pedro River) has lower hydraulic conductivity. This 

clay content may cause at least some water flow in the upper 

San Pedro basin aquifer in a more northerly direction before 

discharging into the San Pedro River (for instance, via the 

Babocomari River).

In summary, the airborne EM survey appears to map 

where the water lies in the Sierra Vista subwatershed down 

to depths of 300 to 400 meters. Because overlying-sediment lithostatic pressure tends to reduce porosity with depth, 

most of the water lies in this depth range. Figures 14–16 by 

themselves cannot explain how the water from the Huachuca 

Mountains reaches the main regional aquifer, but CDT profiles 

suggest a deeper hydraulic connection beneath the mountainfront-following thick unsaturated zone. To draw reliable conclusions about ground-water flow, hydraulic data from wells in 

the area are required.

Conductivity Depth Transforms

Conductivity depth transforms (CDTs) are a proprietary 

product provided at extra cost by the airborne survey contractor. CDTs are calculated conductivity vs. depth sections 

derived by a mathematical transform (technically not an 

inversion process) of the 60-channel airborne electromagnetic 

in-phase/quadrature data acquired by the GEOTEM geophysical survey system, using an infinite half-space assumption. In 

earlier reports (Wynn and Gettings, 1997; Bultman, Gettings, 

and Wynn, 1999), CDT profiles were compared first with 

vertical electrical soundings (VES) carried out in the upper 

San Pedro basin and reported in Pool and Coes (1999), with 

available water-table data, and with short and long normal 

electrical logs from test wells on the Fort Huachuca military 

reservation. In general, we found good agreement between the 

CDT information and the VES and particularly good agreement with electrical well logs for the first 150 meters below 

the Earth’s surface. We also found good agreement between 

the CDTs and other sources of San Pedro basin water information (for instance, Corell and others, 1996).

Figure 18 shows an example of a CDT, with an interpretation by Don Pool (USGS, Tucson, Ariz., written commun., 

1999) superimposed; this figure was taken from 1997 survey 

line 122 and was chosen because 122 crossed close to test 

well 5, as discussed in Pool and Coes (1999). This geoelectrical section covers nearly 30 kilometers horizontally from 

southwest to northeast and up to 400 meters vertically (see pl. 

2 for location of line 122 between L97114 and L97126). Blue 

indicates low conductivity, and warm colors represent high 

conductivity. In this area, the warm colors appear to map the 

water of the upper basin fill and lower basin fill (described in 

Brown and others, 1966, and Pool and Coes, 1999), as well as 

some of the Pantano Formation. The figure is topographically 

correct in that the elevations at Fort Huachuca (the southwest) 

are higher than the elevations at the San Pedro River (the 

northeast).

The shallow blue zone beneath and just east of Fort 

Huachuca is the thick unsaturated zone discussed on p. 21 and 

described in Pool and Coes (1999). Note the red beneath the 

zone in this CDT section; that is apparently water, probably 

saturated Pantano Formation sediments. Also note the continuity between the airborne EM conductor near the Huachuca 

Mountains on the left (red area labeled Pantano Fmn?) and 

the electrical conductor interpreted to be the regional aquifer 

(labeled UBF) in the center. This continuity or link was not 

apparent in the depth slices in figures 14–16. The CDTs, however, incorporated data from the longest time gates (deepest 

penetration) of the airborne EM data. The airborne EM system 

mapped a conductive (water-saturated) unit beneath the thick 

Conductivity Depth Transforms  23

Fort Test well 5

Huachuca

San Pedro

River

Southwest Northeast

LBF

Highway

Bedrock

(outcrops offline)

Bedrock Pantano Fmn?

?

? ? ?

Bedrock Bedrock

Thick unsaturated

zone

UBF

LBF ? 400 m

< 30 kilometers

UBF

(silt & clay)

Figure 18. An example CDT profile from survey line 122, with an interpretation by Don Pool (USGS, Tucson, Ariz., written commun., 

1999) superimposed. This figure is technically not an inversion of the airborne EM data, but a proprietary analytical calculation done by 

the contractor to convert the 60 channels of EM data into conductivity vs. depth over the length of survey line 122. LBF, lower basin fill; 

UBF, upper basin fill. Warm colors represent high conductivity.
24  Mapping Ground Water—An Analysis of Airborne Geophysical Surveys, Cochise County, Arizona

Derivative

of CDT

Dashed

line = zero

Log base 10 of resistivity (ohm-meters)

Elevation (meters)

Water

Table

CDT

profile

Well log

0 1 2 3 4

900 1000 1100 1200 1300

Figure 19. Graph showing comparison of resistivities derived 

from the airborne EM survey CDT with resistivities derived from 

a well log for test well 5. Also shown are the water table and a 

derivative of the CDT resistivity. The derivative of the digital CDT 

profile was plotted to indicate changes in slope of the profile. 

Where the derivative is zero, or when it displays slight negative 

slope, it is still possible to have increasing conductivity in the 

deeper portions of the profile. The CDT consistently demonstrates 

a higher resistivity at a given elevation than the well log, but, in 

general, the CDT resistivity profile seems well correlated with 

actual resistivity to depths of 150 meters (for example, elevations 

above 1,100 meters on the left-side scale). At depths greater 

than this, the well log and CDT resistivities begin to diverge. At 

this point, conductivities seem to be conservatively estimated or 

underestimated. From Bultman, Gettings, and Wynn, 1999.

unsaturated zone, showing apparent hydraulic connectivity at 

depth along the profile.

Just to the right of the blue, thick unsaturated zone, there 

is a thin red-purple area with white below it. This area is a 

high electrical conductivity zone that comes very close to the 

surface, perhaps reflecting the presence of effluent-recharge 

ponds found here. The U.S. Army National Environmental 

Protection Act Coordinator (Gretchen Kent, Fort Huachuca 

U.S. Army Garrison, oral commun., 1999) has indicated that 

there is significant water loss (up to 50 percent) in these ponds 

that cannot be explained by evapotranspiration. A possible 

interpretation is that some of this water is recharging the 

regional aquifer. A major highway crosses here, however, and 

so the possibility of cultural influence in the data at this point 

cannot be discounted. The white area underneath is caused by 

the presence of human electrical interference (pipelines and 

powerlines). The CDT algorithm cannot reliably calculate 

the deeper resistivities because the electrical noise from this 

cultural interference overwhelms the data below about 50–100 

meters, and the plot software therefore leaves the area below 

the noise blank.

Farther northeast (to the center and right of fig. 18), there 

is a long, shallowing-to-the-right electrical conductor (deep 

red and purple) and a second deeper, red and orange zone. The 

shallow zone is likely water in the fine-grained upper-basinfill part of the aquifer. The lower purple zone is the lower 

basin fill that is still part of the aquifer. The upper basin fill 

and lower basin fill are not separate but are parts of the same 

aquifer. Farther to the northeast, we see a shallow blue zone 

(labeled “Bedrock”) with a very thin, barely subsurface red 

conductivity zone above it. These zones are the Tombstone 

Hills volcanic rocks, with a veneer of probably damp silt-clay 

making up a thin overlying layer of basin fill.

Figure 19 (from Bultman, Gettings, and Wynn, 1999) 

compares a conductivity depth transform with a resistivity log 

for test well 5 (see also Pool and Coes, 1999). Note that the 

horizontal axes in figures 19 and 20 are in units of resistivity. 

Most ground geophysical measurements are reported in resistivity units, whereas the strength of EM conductors is usually 

reported as conductivities. Resistivity and conductivity are 

related; resistivity is simply the inverse of conductivity (that 

is, resistivity = 1/conductivity). The short and long normal 

lateral electrical log in figure 19 shows resistivities that closely 

parallel the CDT results, but the CDT shows consistently 

higher resistivity (that is, the CDT curve lies to the right of 

the electrical log of the well). Below elevation 1,100 meters, 

the CDT resistivity values continue to increase and begin to 

diverge from the well-log resistivities. Apparently, the CDT 

gives approximately quantitative resistivity values (different 

by a small multiplicative factor) to about 150 meters depth 

and qualitative values (a slow, roughly linear increase) below 

this. It would seem that we should be able to “tune” the CDT 

algorithm to more perfectly match the actual electrical log 

resistivities over the entire survey area. This issue is discussed 

in further detail below.

Note that the first resistivity low of the CDT appears to 

mark the water table in figure 19. In fact, while this is generally the case over most of the San Pedro basin, there are some 

areas where the CDTs don’t exactly mark the water table (for 

instance in a narrow zone near the mouth of Huachuca Canyon). Figure 20 (modified from Bultman, Gettings, and Wynn, 

1999) shows a series of nine CDTs converted to resistivity-vs.-

depth curves, correlated to the reported water table in nearby 

test wells. Six of the CDTs mark the water table closely, 

two mark the water table somewhat lower than where it was 
measured, and one (the worst example we encountered in 

the Huachuca Canyon outflow area) seems to show the water 

table a full 100 meters below where it was measured. There is, 

however, some question about the accuracy of this water-table 

value (see p. 24). Some of the water-table values were older 

measurements taken when the water table was higher, so we 

can ascribe these few apparent discrepancies to either horizontal location imprecision between the CDT vertical section 

and the nearby well it is compared to or to older water-table 

measurements where the water is now further drawn down. 

The water-table values used for this report were always the 

most recent ones available.

There is also some suggestion (Mark Bultman, USGS, 

Tucson, Ariz.) written commun., January 2001) that the CDT 

algorithm may calculate the presence of the conductor above 

where it actually lies in cases of highly resistive (dry) overburden, again raising the question of whether we can tune the 

CDT algorithm better. While efforts are ongoing to resolve 

these occasional discrepancies, it is important to note that such 

discrepancies account for less than 5 percent of the surveyed 

area.

Figure 21 has been included to show another quality-control check of the data, in this case, the CDT inversion process. 

In this figure, a 1997 profile (1997 line 101) is compared with 

a nearly coincident 1999 profile (1999 line 214). The agreement is remarkable, considering that the position of the aircraft 

in three-dimensional space could never be made to coincide 

exactly.

Figure 22 (an enlarged version of this figure is presented 

as pl. 2 of this report) was generated to help readers visualize where the main electrical conductor (which the author 

believes generally correlates with the presence of water) lies 

in three dimensions; this is a CDT fence diagram. For selected 

airborne survey lines, entire CDTs have been scaled and laid 

down along the profile line. These CDTs are topographically 

corrected (that is, the white zone above a CDT on this fence 

diagram compensates for the fact that one end of the line may 

lie at a higher elevation). In these cases, the colored zones 

must be moved up to the surface in the viewer’s mind. Doing 

this corrects a few apparent discrepancies; for instance, where 

the Babocomari River should coincide with a low-resistivity zone on the CDT that crosses or parallels it. This fence 

diagram thus can be considered a broad-brush three-dimensional map of the water underlying the San Pedro Valley in the 

survey area, with the caveats given above.

The CDTs in figure 22 show some interesting structural 

information. One example is a north-dipping conductor beginning at the Babocomari River on line L203 and extending 

down and north from the river. This conductor is almost certainly a water-filled, north-dipping fault system that controls 

the Babocomari. Another example can be seen in the middle 

of line L97–138. The Sawmill Canyon fault that is dashed 

through the basin fill on Drewes’ map (1980) can be seen as 

a left-lateral offset in the conductor (fig. 14) and as an angled 

offset in the conductor in the middle of line L97–138 (fig. 22).

 In Bultman, Gettings, and Wynn (1999), some interesting relations among the digital CDTs, the well resistivity logs, 

and the water tables were observed. These observations (listed 

below) appear to be applicable to the entire 1997/1999 dataset.

The digital CDTs consistently demonstrate a slightly 

higher resistivity at a given depth than the short and long 

normal well logs. Some of this difference may be due to 

an elevation problem that was discovered and reported 

in the 1997 digital CDTs, and some of it may be due to 

parameters selected by the contractor in the CDT algorithm. The elevation problem has been corrected in the 

1997 data and did not appear in the 1999 data.

The shape of the CDTs matches the general shape of the 

well logs in most cases for the first 150 meters depth.

For most of the test wells, the CDT-derived upper conductivity maximum/resistivity minimum seems to correlate 

well (within the vertical resolution of the CDTs) with 

the water table. In a few places, notably near the mouth 

of Huachuca Canyon, the conductivity maximum suggests that the water table is lower than the water table in 

nearby wells. This finding may be due to the significant 

horizontal displacement between the wells and the CDT 

soundings or to outdated water-table information or even 

to a calibration issue with the CDT algorithm in zones of 

highly resistive overburden. The author believes that the 

cause is likely a combination of all three.

1.

2.

3.

Figure 20. Graph showing CDT vertical profiles plotted against 

water-table information and well-resistivity data (log base 10 of 

resistivity) for nine test wells. All test wells were within 200 

meters of a CDT location. Modified from Bultman, Gettings, and 

Wynn, 1999.

Conductivity Depth Transforms  25  

Vertical distance from water table (meters)

Log base 10 of resistivity (ohm-meters)
26  Mapping Ground Water—An Analysis of Airborne Geophysical Surveys, Cochise County, Arizona

Figure 21. Comparison of two CDT profiles (1997 line 101 and 1999 line 214) that approximately coincide. The profiles agree well despite 

the fact that the position of the aircraft in the two overflights could never be made to coincide exactly. Line 214 is also plotted in plate 2 

and figure 22.

1997 Line 101

1999 Line 214 Southwest Northeast

The CDT resistivity profiles, when converted to conductivity vs. depth, correlated well with actual well-log resistivity to depths of about 150 meters. At depths below 150 

meters, resistivities reported in the CDTs increase with 

greater depth over the resistivities measured by well logging. This finding led the author to consider the possibility of “tuning” the CDT algorithm to match the well logs 

down to 400 meters depth, as discussed in detail below.

Conductivity vs. Depth Conversion 

Systems—The Inversion “Tuning” 

Issue

One product of the AEM survey is a conductivity vs. 

depth section derived by a mathematical calculation of the 

60-channel airborne electromagnetic data acquired by the 

geophysical survey system. The airborne geophysical contractor, Geoterrex-Dighem, refers to these calculations as 

conductivity depth transforms or CDTs. Wolfgram and Karlik 

(1995) described the mathematical technique used to obtain 

the CDTs; however, Geoterrex-Dighem considers the actual 

algorithm proprietary. While not technically a true inversion, 

the technique produces results similar to an inversion based 

on the diffusion equation, yet requires much less computation 

time because it is a forward, or analytical, calculation.

Earlier, we observed the generally good correlation 

between well logs and the CDT resistivity values (see p. 24); 

this correlation helps to establish the veracity of the CDT algorithm. The apparent close correlation between CDT resistivities and electrical logs in wells down to depths of 150 meters 

is encouraging; however, the increasing disparity or difference below 150 meters is problematic. To test the accuracy 

of the CDT algorithm, the USGS purchased an EM inversion 

software package called “EM Flow” from ENCOM. ENCOM 

is an Australian Government/private-industry geophysical 

consortium; the inversion method used in the software is 

documented in Macnae and others (1991). Approximately 1 

person-month was spent reformatting the 1999 airborne EM 

4. data and revising the software so that it could invert data from 

parts of the 1999 airborne geophysical survey.

The survey line chosen for the test comparison was 

a separate 10-kilometer-long profile flown parallel to the 

Mexican border and south of the combined Geoterrex-Dighem 

1997/1999 surveys coverage. This comparison survey line 

is called the “Mexico Border Line” here and in figure 2 (see 

L1000 Mex on pl. 2 and fig. 22). The Mexico Border Line is 

important because a large part of the San Pedro basin drainage 

lies south of the Mexican border. This line provides continuity 

between the aquifer surveyed for this report and that part of 

the San Pedro aquifer lying south of the border. It is reasonable to assume that ground water flows across the border, 

down-drainage toward the north. The extent to which an estimated volume of formerly Mexican ground water contributes 

to stream flow on the American side of the San Pedro River, 

however, cannot be quantified at this time. The perennial flow 

in the San Pedro River north of the border is influenced by 

the geometry of the channel alluvium and the volume of bank 

storage present.

Operations at Cananea, the large porphyry copper mine 

that lies in the upper San Pedro drainage in Mexico withdraw a 

considerable amount of water from the upper San Pedro drainage. The mining company Grupo Mexico has acknowledged 

the ongoing withdrawal from the upper San Pedro aquifer of 

more than 15 million cubic meters of water per year for the 

Cananea mine and its local operations (José María GuerraLimón, Reserva Forestal Nacional y Refugio de Fauna Silvestre “Ajos-Bavispe,” Hermosillo, Mexico, oral commun., 2000). 

The withdrawal apparently began in the 1940s and accelerated 

during the 1950s and 1960s to its present level. It is reasonable 

to assume that this withdrawal could have a significant effect 

on the ground-water system on the American side of the border. Note that the Cananea mine is one of many entities withdrawing water from the upper San Pedro aquifer in Mexico, 

including large communal farms (ejidos) and towns.

Figure 23 compares the Geoterrex-Dighem CDT (conductivity-depth-transform) profile and the ENCOM CDI 

(conductivity-depth-inversion) profile for the Mexico Border 

Line. Figure 23A shows the original EM Flow inversion (CDI) 

of the normal off-time gates that the EM Flow software was 

configured to deal with. These time gates are well suited to 
Figure 22. Conductivity vs. dapproximately every 10th CDT survey line) superimposed on the Fort Huachuca area, Arizothe reader to view a three-dimof the conductivities as a funcdimensional sheet. The data pand 1999 airborne EM surveys 1999). An enlarged version of plate 2 of this report.Base from 1:250,000-scale U.S. Armrevised by U.S. Geological Survey, 1in feet. Data superimposed from Sa(1999) airborne geophysical surveys 
28  Mapping Ground Water—An Analysis of Airborne Geophysical Surveys, Cochise County, Arizona

defining details in the upper section of the profile but do not 

sample far out into the late-time windows acquired by Geoterrex-Dighem’s GEOTEM system. Figure 23B shows the original channel-by-channel EM data, for reference. Figure 23C is 

the EM Flow result when the sampling time gates are extended 

back into the on-time of the transmitter (for a closer reference 

to the transmitted signal strength) and as far down the off-time 

window as the software could handle. This method sampled 

as deeply beneath the ground surface as possible for a better 

comparison to the Geoterrex-Dighem CDT profile. Figure 23D

shows the Geoterrex-Dighem CDT profile.

Comparisons between the Geoterrex-Dighem CDTs and 

the EM Flow CDIs show the following:

The EM Flow inversion for a single profile takes up to 10 

minutes to complete on a fast Pentium-III computer. The 

CDI process generally requires several runs, then color 

scaling to reduce color saturation to bring out details. The 

CDT analytical calculation (that is, a forward calculation), 

on the other hand, requires only a few seconds. The heavy 

calculation burden and other inherent limitations make the 

1999 EM Flow software an interesting research tool but 

poorly suited to production processing of large airborne 

EM surveys.

The EM Flow inversion provides fine resistivity detail in 

the upper 50–70 meters below the Earth’s surface—substantially more detail than the Geoterrex-Dighem CDT 

provides.

The EM Flow inversion, when extended to include early 

and late time gates (fig. 23C), agrees well with the CDT 

analytical conversion of the EM data (fig. 23D). The 

minor disparities are caused by the relative strengths of 

the two conversion methods.

The EM Flow results do not reach as deep as the Geoterrex-Dighem analytical CDT solution. In figure 23D, we 

see a second, deeper conductive unit (possibly another 

deeper water body) on the left (west) side of the profile. 

We can see only a hint of this deeper, second water body 

in the EM Flow inversion. This water body is not clearly 

seen on the EM Flow CDI because the software could not 

use the longest time gates in the data.

Silts and clays generally tend to collect distally from 

the source rocks from which they weathered. Along the 

Mexico Border Line, applying this fact suggests that clays 

would tend to collect in the center of the area shown in 

figure 23, near the San Pedro River (between reference 

UTM coordinates 580000E and 586000E). Sands and 

gravels, on the other hand, would likely predominate on 

the edges of a basin. One could then infer that both the 

shallow and deeper electrical conductors seen on the left 

side of figure 23D probably represent accessible bodies of 

water. An observer using the CDT would thus understand 

that any water well located in this area should be extended 

1.

2.

3.

4.

5.

below the first (shallow) electrical conductor, through 

the apparent aquiclude separating it from the underlying 

electrical conductor, and into the apparent second, deeper 

water body. This guidance would not be available if the 

interpreter used only the EM Flow CDI inversion results.

Around location 582000E (the east component of the 

UTM coordinates) on the Mexico Border Line, there is 

an apparent offset in the high-conductivity layers (warm 

colors in fig. 23C,D). This offset is a previously unknown 

feature and probably represents a normal fault that influenced the accumulation of alluvial materials. As such, 

the apparent offset may simply reflect an increase in the 

thickness of the basin fill on the east side.

Summary of Interpretations

Interpretations Related to the Hydrology of the 

Upper San Pedro Drainage Derived from the 

Airborne EM and Magnetic Data

When combined with the hydrologic analysis of Pool and 

Coes (1999), the 1997 and 1999 airborne geophysical surveys 

provide sufficient information to draw the following conclusions:

The EM conductivity maps (figs. 14–16) and the CDT 

profiles (figs. 18, 21–23, pl. 2) show subsurface conductivities in an area of about 1,000 square kilometers. 

This kind of subsurface imaging is possible because the 

local ground water is quite conductive (Pool and Coes, 

1999, pl. 3). While clays are also known to contribute to 

an increased conductivity, they cannot be the dominant 

factor in the conductivity images we see in these figures 

for reasons explained above (see p. 18–19). Nevertheless, 

for conductivities above 300–500 μS/cm, clays definitely 

have contributed to the values measured. Consequently, 

the airborne-EM-derived conductivity maps provide a 

high-resolution, as well as a broad-brush, image of the 

presence of ground water in the Sierra Vista subwatershed of the upper San Pedro regional aquifer, especially 

when combined with existing lithologic and hydrologic 

information. The parameters for the airborne EM surveys 

were optimized for maximum penetration in the San 

Pedro Valley (that is, about 400 meters). In the absence of 

human cultural interference, the CDT profiles generally 

map most of the underlying water (note the closure, or 

bottoms, of the electrical conductors in the CDTs).

The airborne EM data provide a static image of the 

subsurface electrical conductor inferred to be the regional 

aquifer. Figures 14–16 effectively provide three depth 

6.

1.

2.
EM Flow CDI

Geoterrex CDT

1385m1060mRaw Data

1400 m-

(ground)

EM Flow CDI Using Off-Time Taus

Geoterrex-Dighem CDT

μS/m

- 10

- 1

1400 m1100 m1100 mA

B

C

D

SAN PEDRO

RIVER

μS/m

Ch(1)

Ch(2)

Ch(3)

Ch(4)

Ch(5)

Ch(6)

Ch(7)

Ch(8)

Ch(9)

Ch(10)

Ch(11)

Ch(12)

Ch(13)

Ch(14)

Ch(15)

Ch(16)

Ch(17)

Ch(18)

Ch(19)

Ch(20)

1

2

3

4

5

17

18

19

20 (red)

CHANNELS

meters

100

10

1

- 100

Figure 23. A comparison of a true inversion of the 60-channel EM data (using an EM Flow CDI) with a Geoterrex-Dighem CDT profile 

for the same survey line, the so-called Mexico Border Line, acquired approximately 3 kilometers north of the Mexican border (see line 

L1000 Mex in fig. 22). A, A normal (on-time taus only) inversion. B, Original channel-by-channel EM raw data presented for comparison. 

C, The EM Flow inversions using off-time taus (providing a closer reference to the transmitted signal and thereby allowing deeper 

penetration). D, Analytical CDT profile. Warm colors represent high conductivity. Units: m, meter; μS/m, microsiemens per meter; ppm, 

parts per million.

Summary of Interpretations  29
30 Mapping Ground Water—An Analysis of Airborne Geophysical Surveys, Cochise County, Arizona

slices of this electrical conductor beneath the study area. 

The airborne EM images and CDTs do not directly provide information about ground-water flow, although an 

experienced hydrologist can draw some educated inferences from the images.

In many places, the CDTs provide structural information 

about the subsurface, such as the north-dipping, watersaturated fault system controlling the Babocomari River 

(the northward-dipping electrical conductor beginning at 

the Babocomari River on line L203 in fig. 22 and pl. 2).

The airborne EM conductivity images suggest substantially reduced porosity beneath the Tombstone Hills 

volcanic field, over which the San Pedro River flows for 

about 30 percent of its length in the northern part of the 

survey area. In this zone, at least, hydraulic communication is substantially reduced between the San Pedro River 

and the regional aquifer.

The upper-basin-fill sediments in the center of the San 

Pedro Valley appear to be water saturated. The apparent 

offset of the main conductor to the west of the river, combined with basic sedimentary principles that place the silts 

and clays more in the center of a basin, suggest the presence of a large silt-clay body just to the west of the river. 

Both the CDTs and well-log data support this theory. 

This electrically conductive unit, which roughly parallels 

the river but lies to its west, has sands and gravels both 

overlying and underlying it and significantly influences 

the ground-water flow in the regional system (Pool and 

Coes, 1999).

Almost 4 million cubic meters of ground water is estimated to flow from Mexico into the Arizona part of the 

basin each year (Freethey, 1982). The Cananea mine complex in Mexico, which withdraws more than 15 million 

cubic meters of water each year, must have a significant 

effect on water flowing in the San Pedro River in Arizona. 

A further increase in withdrawal in Mexico will almost 

certainly affect the San Pedro National Riparian Conservation Area.

The magnetic depth-to-source maps generated as part of 

this study reveal that there are several deep sections of 

the upper San Pedro basin in the study area (for example, 

beneath Huachuca City and beneath the Palominas area 

near the Mexican border; figs. 11 and 12). These deep 

zones are filled with sediment and, in places, saturated 

with water. Because of overlying lithostatic pressure, 

which generally reduces the porosity, the bulk of the 

regional aquifer probably lies within the 400-meter-depth 

range of the airborne EM system. Nevertheless, the large 

volume of sediment that these huge “potholes” represent 

in the crystalline rock basement must contribute substantially to the bank storage volume in the Sierra Vista 

subwatershed.

3.

4.

5.

6.

7.

The magnetic depth-to-source map (fig. 11) also shows 

that most of the San Pedro River in the southern 70 

percent of the study area (south of the Tombstone Hills) 

has only 100 to 200 meters of sediment underlying it. 

This relatively thin, silt-and-clay-rich section of alluvium 

implies lower transmissivity in the immediate vicinity of 

the river in this segment south of 3493000N. Lines L323, 

L319, and L314 of figure 22 and plate 2 support this 

finding by showing only a thin electrical conductor (that 

is, a thin, shallow water body) on the east side of each 

profile. According to Don Pool, “This feature likely also 

influences the movement of ground water in the system” 

(USGS, Tucson, Ariz., written commun., January 2001).

Observations about the Geology of the Upper 

San Pedro Drainage Derived from the EM and 

Magnetic Data

General geologic observations from the combined 1997 

and 1999 airborne geophysical surveys include the following:

The upper San Pedro basin is bounded on the southwest 

by the Nicksville fault (Drewes, 1980), a high-angle 

basin-margin fault marking the eastern edge of the Huachuca Mountains that is apparent in the magnetic data. 

The airborne EM data (figs. 14–16) show the Sawmill 

Canyon fault (Drewes, 1980, 1996) as a subtle left-lateral 

offset in the shallow conductor in the middle of the basin. 

The Babocomari River appears to coincide with a northdipping fault evidenced as a narrow dipping conductor in 

the CDT data for line L203.

A bedrock high in the Sierra Vista area extends eastward 

about 10 kilometers from the Huachuca Mountains into 

the basin. Figure 11 shows this feature in the inverted 

magnetic data and in the CDT data as a rise in the nearsurface low-resistivity zone. The presence of this high has 

been verified by well D(21-20)35abb, located near Fry 

Boulevard in Sierra Vista (Gettings and Houser, 2000).

South of the Sierra Vista bedrock high, the basin is 

complex but includes a large Basin-and-Range-following 

basement low that appears to extend south into Mexico 

from the Palominas area. The gravity-based model agrees 

closely with the magnetic model but, with fewer data 

points, has a much lower resolution (see also Halvorson, 

1984).

To the east of the basin-bounding fault near the Huachuca 

Mountains and east of the Sierra Vista bedrock high, the 

basin remains deep until it approaches the Tombstone 

Hills. The relation between the basin sediments and the 

bedrock in the Tombstone Hills area is complex and may 

be both stratigraphic (intercalated volcanic flows in the 

sedimentary stack) and fault bounded. This region correlates to the mapped Tombstone caldera margin (Moore, 

1993).

8.

1.

2.

3.

4.
Observations about the Geophysical Methods, 

Including the CDT Conversion Process

The following are observations about the CDT algorithm 

and the conversions released by Geoterrex-Dighem 

(1997, 1999):

The Geoterrex-Dighem CDT conductivities appear to 

map the presence of water to about 150 meters depth over 

most of the survey area. Below about 150 meters depth, 

the CDT conductivities diverge with increasing depth 

(toward lower conductivities) from values measured in 

well logs. In this zone, the CDTs still appear to provide at 

least qualitative data on the presence or absence of water. 

In a few areas, the water-table depths diverge from what 

the CDTs seem to show, notably in a narrow zone near the 

mouth of Huachuca Canyon. This phenomenon may be 

due to (a) out-of-date water-table information in a sharply 

fluctuating recharge channel, (b) horizontal registration 

errors between wells and the CDT profile, or (c) a subtle 

calibration issue with the CDT algorithm apparent only 

in areas of highly resistive overburden. The available 

evidence suggests a combination of all three.

The bedrock under the basin sediments is generally visible in the EM data only where it outcrops or is overlain 

by less than about 300–400 meters of alluvium. The crystalline bedrock, however, is easily mapped by the airborne 

magnetic data converted through the Euler deconvolution 

algorithm. In a few cases, magnetic volcanic flows and 

volcanosedimentary units deposited in the middle of the 

sediment stack above the crystalline basement make the 

basement appear to be locally shallower; for instance, the 

apparent ridge in the middle of the Huachuca City basement low (which according to the gravity data is a single, 

deep subbasin in the San Pedro Valley).

The CDI inversion process (that is, the EM Flow solutions) appears to verify the CDT analytical conversion 

process. Well-log information generally supports the 

veracity of the CDTs and the conductivity maps shown in 

figures 14–16.

Acknowledgments

Gretchen Kent, the National Environmental Protection 

Act Coordinator for the U.S. Army Garrison at Fort Huachuca, Ariz., arranged for funding for the airborne geophysical 

surveys and for the analysis, helped coordinate the surveys 

with the base commanders, provided strong encouragement 

throughout this effort, and provided a review. These surveys 

were designed and carried out under U.S. Geological Survey 

(USGS) supervision at the request of the Environmental and 

Natural Resources Division at the Fort Huachuca U.S. Army 

Garrison, which funded them. Fort Huachuca also provided 

1.

2.

3.

previous data and reports, funding to assist with the analysis 

and interpretation of these data, and logistical support to the 

flight crews. Fort Huachuca’s goal in assisting with these studies was to provide objective, state-of-the-subsurface knowledge to those agencies and organizations trying to plan and 

manage water resources in the region.

Mark Gettings and Floyd Gray of the USGS invited 

the author (Jeff Wynn) to become involved in the Southwest 

Resources project within the USGS and take on this particular 

subtask. During the 1997 and 1999 surveys, the author served 

on site as the contracting officer’s representative, working with 

flight engineers and the data-reduction team to assure that 

quality-control procedures were strictly adhered to.

Mark Bultman (USGS) provided independent calculations to support this work. Jean Lemieux (Geoterrex-Dighem; 

the company is now part of Fugro World Wide) provided 

additional insight and directed the generation of the conductivity depth transforms (CDTs); he offered much helpful advice 

during the interpretation phase of this survey, particularly 

on issues related to gain and system sensitivity. Vic Labson 

(USGS) provided valuable insight in setting up both airborne 

surveys and, in particular, in setting the EM system sampling 

parameters.

This report benefited significantly from reviews by Don 

Pool, Bill Steinkampf, and Mark Bultman (all of the USGS, 

Tucson, Ariz.). In addition, Tom Reilly (USGS, Reston, Va.) 

and Mark Anderson (USGS, Tucson, Ariz.) also provided 

reviews. 

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Wynn—Mapping Ground Water—An Analysis of Airborne Geophysical Surveys, Cochise County, Arizona—Professional Paper 1674