Geology and Geophysics Applied to Groundwater Hydrology at Fort Irwin, 

California

David C. Buesch, Editor

Airborne Electromagnetic Data and Processing within 

Leach Lake Basin, Fort Irwin, California

By Paul. A. Bedrosian, Lyndsay B. Ball, and Benjamin R. Bloss

Open-File Report 2013–1024–G

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2014

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Suggested citation:

Bedrosian, P.A., Ball, L.B., and Bloss, B.R., 2014, Airborne electromagnetic data and processing within Leach Lake 

Basin, Fort Irwin, California, chap. G of Buesch, D.C., ed., Geology and geophysics applied to groundwater 

hydrology at Fort Irwin, California: U.S. Geological Survey Open File Report 2013–1024, 20 p.,

http://dx.doi.org/10.3133/ofr20131024G.

ISSN 2331-1258 (online)
iii

Contents

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

Introduction................................................................................................................................................................. 1

Purpose and Scope .................................................................................................................................................... 2

Geologic Background ................................................................................................................................................. 3

Airborne Electromagnetic Survey ............................................................................................................................... 5

Method and Measurements .................................................................................................................................... 5

Data Preprocessing................................................................................................................................................. 7

Inversion of Electromagnetic Data.............................................................................................................................. 8

Model Assessment and Interpretation ...................................................................................................................... 10

Igneous Framework .............................................................................................................................................. 10

Basin Fill ............................................................................................................................................................... 12

Faulting ................................................................................................................................................................. 15

Geophysical Data Overview...................................................................................................................................... 17

Flightlines.............................................................................................................................................................. 17

Grids ..................................................................................................................................................................... 17

Line Data............................................................................................................................................................... 18

Plots...................................................................................................................................................................... 18

Section Data ......................................................................................................................................................... 18

Acknowledgments..................................................................................................................................................... 18

References Cited...................................................................................................................................................... 19

Figures

 1. Maps showing location of study area within Fort Irwin, California.................................................................. 3

 2. Photographs of the helicopter-borne AeroTEM IV system used during the airborne geophysical surveys in 

Leach Lake Basin, Fort Irwin, California. ....................................................................................................... 6

 3. Maps showing depth slices through the inverted resistivity model at a depth of A, 25 m and B, 125 m below 

land surface at Fort Irwin, California. ........................................................................................................... 11

 4. Example inverted resistivity section along flightline L10400. ....................................................................... 13

 5. Example inverted resistivity section along flightline L10880. ....................................................................... 14

 6. Map showing distribution of interpreted faults from airborne electromagnetic (AEM) models in relation to 

mapped faults in Leach Lake Basin, Fort Irwin, California. .......................................................................... 16

Tables

 1. Generalized sedimentary units within Leach Lake Basin, Fort Irwin, California............................................. 4

 2. System specifications for the airborne electromagnetic survey of Leach Lake Basin, Fort Irwin, California. . 7

 3. Summary of EM1DINV inversion parameters used for final inversion of the airborne electromagnetic (AEM) 

data for Leach Lake Basin, Fort Irwin, California.. ......................................................................................... 9

 4. Digital data organization and description for files and folders for the Leach Lake Basin study area, Fort 

Irwin, California............................................................................................................................................ 17
iv

Appendixes

[Available online only at http://pubs.usgs.gov/of/2013/1024/g/.]

 A. Report on a Helicopter-Borne AeroTEM System Electromagnetic & Magnetic Survey

 B. Airborne Magnetic and Electromagnetic Survey Data 
v

Conversion Factors

SI to Inch/Pound

Multiply By To obtain

meter (m) 3.281 foot (ft)

kilometer (km) 0.6214 mile (mi)

square meter (m2) 0.0002471 acre

square kilometer (km2) 247.1 acre

nanotesla (nT) 1 gamma

Electrical conductivity is given in siemens per meter (S/m) unless otherwise specified.

Electrical resistivity is given in ohm-meters (ohm-m) unless otherwise specified.

Electrical resistivity (ρ, ohm-m) can be converted to conductivity (σ, S/m) as σ = 1 / ρ.

Datum

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD88).

Horizontal coordinate information is referenced to North American Datum of 1983 (NAD83), Universal Transverse Mercator projection, 

Zone 11 North (UTM11N).

Altitude, as used in this report, refers to distance above the vertical datum.
vi

Acronyms

AEM airborne electromagnetic

ECSZ Eastern California Shear Zone

EM electromagnetic

DOI depth of investigation

DTM digital terrain model

FEM frequency-domain electromagnetic

GF Garlock Fault

GPS global positioning system

IGRF International Geomagnetic Reference Field

LCI laterally-constrained inversion

MSSP Mojave strike-slip province

SAF San Andreas Fault

TDS total dissolved solids

TEM time-domain electromagnetic

Tx transmitter

USGS U.S. Geological Survey

Abbreviations

A Ampere

Hz hertz

kHz kilohertz

km kilometer

m meter

nT nanotesla

Ω·m ohm-meter

ppm parts per million

μs microsecond

ms millisecond

s second

V Volt
Airborne Electromagnetic Data and Processing within 

Leach Lake Basin, Fort Irwin, California

By Paul A. Bedrosian, Lyndsay B. Ball, and Benjamin R. Bloss

Abstract

From December 2010 to January 2011, the U.S. Geological Survey conducted airborne 

electromagnetic and magnetic surveys of Leach Lake Basin within the National Training Center, Fort 

Irwin, California. These data were collected to characterize the subsurface and provide information 

needed to understand and manage groundwater resources within Fort Irwin. A resistivity stratigraphy 

was developed using ground-based time-domain electromagnetic soundings together with laboratory 

resistivity measurements on hand samples and borehole geophysical logs from nearby basins. This 

report releases data associated with the airborne surveys, as well as resistivity cross-sections and depth 

slices derived from inversion of the airborne electromagnetic data. The resulting resistivity models

confirm and add to the geologic framework, constrain the hydrostratigraphy and the depth to basement, 

and reveal the distribution of faults and folds within the basin.

Introduction

Groundwater flow in the arid basins of the Mojave Desert of the southwestern United States can 

be strongly influenced by faults. Effective conceptualization and modeling of these groundwater flow 

systems requires knowledge of the distribution and nature of hydrologically significant faults as well as 

the depth to basement rocks and general hydrostratigraphy. In an effort to understand, model, and 

manage groundwater resources in arid environments, the U.S. Geological Survey (USGS) is 

investigating Leach Lake Basin within the National Training Center, Fort Irwin, California (herein 

called Fort Irwin), using airborne and ground-based geophysical techniques. Toward this end, airborne

magnetic and electromagnetic data were collected within Leach Lake Basin, a geologically complex, 

internally drained basin bisected and flanked by numerous Quaternary faults, including the Garlock 

Fault and the Southern Death Valley Fault Zone. Geophysical methods measure the variability in 

subsurface physical properties. In the case of airborne electromagnetic (AEM) methods, the derived 

property is electrical resistivity. This chapter presents the AEM data, while airborne magnetic data 

collected during this survey are discussed elsewhere (Langenheim and Jachens, this volume, chap. I).

Mapping subsurface geology and hydrostratigraphy in Fort Irwin is challenging. Given the 

complexity of the surface geology (Miller and others, this volume, chap. B), mapping subsurface 

structure and basin stratigraphy using direct-sampling techniques, such as drilling, is cost prohibitive

and lends limited insight to the three-dimensional geometry of the subsurface structure. Logistical 

problems also exist: Leach Lake Basin is a year-round live-fire range, thus ground access is severly 

limited. Airborne geophysical methods provide an alternative to direct sampling and provide more 

spatially continuous data than ground-based surveys. Extensive faulting within the region means that 

these physical properties can vary substantially over short distances; thus, the dense spatial coverage 
2

provided by airborne geophysical methods is critical to capturing abrupt changes in hydrostratigraphy 

and basement structure.

The AEM resistivity models provide subsurface constraints in excess of 200 m depth and show 

abrupt changes in earth response across faulted boundaries, reflecting the strong lateral resistivity 

contrast between igneous rocks and basin sediments. Intrabasin faults are identified, and, to a lesser 

extent, faults within the igneous basement can be traced. The distribution of faults throughout the basin 

can thus be directly obtained from the airborne data. The airborne resistivity models have additionally 

been used to trace hydrostratigraphy throughout the basin. Together with hydrologic investigations in 

neighboring basins, these results are being used to estimate groundwater storage within Leach Lake 

Basin.

Purpose and Scope

In December 2010 and January 2011, the U.S. Geological Survey conducted an airborne 

magnetic and airborne electromagnetic survey over Leach Lake Basin within Fort Irwin. Flightlines 

totaling 1,700 km were flown with 400-m nominal line spacing within a 46- by 16-km survey block (fig.

1). The eastern 10 km of the survey block was flown with 800-m line spacing. The majority of 

flightlines are oriented north-south; an additional three tie lines are oriented east-west and run along the 

northern and southern boundaries, as well as through the center, of the survey block. The AEM 

measurements were used to develop inverse models of the electrical resistivity structure along 

flightlines. The AEM resistivity models were combined into spatial grids which reveal the distribution 

of resistivity throughout the survey block. In conjunction with ground-based transient electromagnetic

soundings, laboratory resistivity measurements on hand samples, and borehole geophysical logs from 

nearby basins, these models provide the basis for interpreting subsurface geology and show the 

distribution of faulting within Leach Lake Basin.

This report releases digital data from Leach Lake Basin. These data and the file structure are 

explained in the “Geophysical Data Overview” section. The methodology used to collect, process, and 

invert these data is presented in the “Airborne Electromagnetic Survey” section, and details of the 

inversion process are included in the “Inversion of Electromagnetic Data” section. A limited 

interpretation of the resulting inverse models is discussed in the “Model Assessment and Interpretation”

section. A detailed interpretation is beyond the scope of this report.
3

Figure 1. Maps showing location of study area within Fort Irwin, California. A, Fort Irwin boundary (dashed white 

line) with topographic basins labeled in white. Mapped faults are colored by sense of motion and illustrate eastwest oriented sinistral faults (red), northwest-southeast oriented dextral faults (blue), and oblique faults (green). B,

Leach Lake Basin study area with airborne electromagnetic (AEM) flightlines (black) and surface geology from 

Miller and others (this volume, chap. B). Black plus (+) symbols indicate locations of ground time-domain 

electromagnetic (TEM) soundings discussed in Burgess and Bedrosian (this volume, chap. F). Labeled faults on 

index map include Garlock Fault (GF), Mojave strike-slip province (MSSP), Eastern California Shear Zone (ECSZ), 

and San Andreas Fault (SAF).

Geologic Background

Fort Irwin, within the northeast part of the Mojave strike-slip province (Miller and Yount, 2002), 

accommodates late Cenozoic slip along the boundary of the North American and Pacific Plates east of 

the San Andreas Fault. This region consists of mountains separated by a series of topographic basins 

(fig. 1), all floored by mafic and felsic, volcanic and plutonic rocks. Precambrian basement and 

Paleozoic metasedimentary rocks are minor components, which were subsequently intruded by Jurassic 

and Cretaceous plutonic rocks (Miller and Sutter, 1982; Schermer and others, 1996). Deformation in the 

Middle Jurassic and again in the Early Cretaceous was deep seated and imprinted a mylonitic fabric 
4

upon the older rocks (Schermer and others, 1996). A period of uplift and erosion occurred between Late 

Cretaceous plutonism and the onset of Miocene extension. Miocene and younger volcanism has further 

altered the landscape.

Tertiary and Quaternary sedimentary units (D. Buesch, written commun., 2014) deposited on 

this bedrock framework are described in table 1.

Table 1. Generalized sedimentary units within Leach Lake Basin, Fort Irwin, California.

Unit1 Description

pc Moderate to weakly consolidated Tertiary deposits that may include volcanic or highly altered material. Includes silt and clay 

where weathered.

Qoa+QToa Old alluvial fan deposits consisting of poorly sorted, compact to well-cemented sand and gravel that is less weathered than 

underlying units.

Qya+Qia Young and intermediate alluvial fan deposits consisting of poorly sorted, compact to well-cemented sand and gravel that is less 

weathered than underlying units.

Qyag+Qiag Young and intermediate grus-rich alluvial fan deposits consisting of unconsolidated sand and gravel. Formed mainly from grus 

derived from weathered granite.

Qp Clay-rich playa deposits; typically at the center of many of the internally drained basins.

1

Listed units are a subset of those found within Fort Irwin (shown in fig. 1B).

Excluding clay-rich playa deposits (Qp), a general decrease in hydraulic conductivity with 

increasing depth is expected owing to sediment compaction and sealing of faults and fractures, although 

local variations in depositional environment (for example, provenance and grain size) are also expected 

to be reflected in the hydraulic properties of the sediments (Belcher and others, 2001). Where saturated, 

the older alluvial deposits (QToa) form the primary aquifer within basins of Fort Irwin; however,

interbedded Miocene volcanics and sediments may also be an important source of water, as suggested 

by well production in Nelson Lake and Goldstone Lake Basins (D. Buesch, written commun., 2014).

The region is cut by numerous faults, including the dextral-oblique Southern Death Valley Fault 

Zone and the sinistral Garlock Fault; the latter is an important structure accommodating strain between 

the extensional tectonics of the Basin and Range province and right-lateral motion within the Mojave 

strike-slip province. These faults further reflect the two dominant styles of faulting within the region: (1) 

Cenozoic, sinistral, east-striking and (2) dextral, northwest-striking faults (fig. 1; Schermer and others, 

1996; Miller and Yount, 2002). Additional details of the geology and faulting within the study area are 

described in Miller and others (this volume, chap. B).

Leach Lake Basin, the focus of the AEM survey, is a rhomb-shaped basin at the northern end of 

Fort Irwin that is bordered by the Owlshead and Granite Mountains to the north and south and the Quail 

and Avawatz Mountains to west and east (fig. 1). Over 1,000 m of relief separates the basin from the 

surrounding ranges. The ephemeral Leach Lake lies at the center of the basin and is bisected by the 

Garlock Fault, whose eastern terminus falls within the basin (fig. 1B).
5

Airborne Electromagnetic Survey

Method and Measurements

Within the brittle upper crust, electrical resistivity is sensitive to water content and quality, the 

presence of mineralogical clay, and changes in lithology. In the absence of clay or other conductive 

minerals, low electrical resistivity is typically associated with materials with high porosity, a high 

degree of saturation, or high total dissolved solid (TDS) levels in pore waters. In contrast, high electrical 

resistivity is related to low porosity, a low degree of saturation, and low TDS values. In sediments, 

resistivity also scales with grain size; the greater surface area associated with fine particles promotes the 

transmission of electrical current, in contrast to coarse-grained deposits, such as alluvial sand and 

gravel, which have relatively high bulk resistivity. Electrical resistivity is also highly sensitive to clay, 

small percentages of which can dramatically decrease bulk resistivity.

Helicopter-borne AEM systems transmit an electromagnetic signal from a towed air frame and 

induce current within the earth. The interaction of the earth with the induced current system gives rise to 

signals that are sensed by one or more receiver coils rigidly mounted to the air frame (fig. 2). Two basic 

types of AEM systems, both operating on the same physical principle, are commonly used for 

hydrogeophysical investigations. Frequency-domain electromagnetic (FEM) systems transmit a 

continuous sinusoidal current and measure the earth response in the on-time, while current is passing 

through the transmitter loop. Time-domain electromagnetic (TEM) systems transmit pulses of current 

and measure the earth response in the off-time, when no current is present in the transmitter loop.

Geneally speaking, FEM systems have superior near-surface resolution, whereas TEM systems have a 

greater depth of investigation.
6

Figure 2. Photographs of the helicopter-borne AeroTEM IV system used during the airborne geophysical surveys

in Leach Lake Basin, Fort Irwin, California. Electromagnetic sensors are affixed to a rigid, 12-m-diameter airframe 

that is flown at 50-m nominal height above land surface. Position, altitude, and attitude sensors are also affixed to 

the airframe. Magnetometer is attached to a “bird” that hangs along the tow line between the helicopter and the 

airframe.

For this study, the AeroTEM IV system was flown by Aeroquest, Ltd. Table 2 provides 

specifications of the system flown; a detailed description of data acquisition and contractor-performed 

processing are documented in the contractor’s report (appendix A). Operationally, the AeroTEM IV 

system is analogous to ground-based TEM systems, the details of which are described by Burgess and 

Bedrosian (this volume, chap. F). A current is passed through a transmitter (Tx) loop, which sets up a 

primary magnetic field surrounding the loop. Upon shutting off current to the Tx loop, the collapsing 

magnetic field induces electrical currents in the subsurface. The interaction of these diffusing currents 

with the earth depends on subsurface resistivity structure and results in a secondary magnetic field 

measured at one or more loop receivers located inside of the Tx loop in the plane of the Tx (fig. 2).

When making TEM measurements from an airborne platform, the position, elevation, and

attitude of the air frame must be accurately known. This ancillary information is obtained via additional 

sensors on the air frame, including a global positioning system (GPS) for positioning, a laser altimeter 

for elevation, and a three-component inclinometer for attitude. Principles of the TEM sounding method 
7

can be found in Christiansen and others (2011), Fitterman and Labson (2005), Danielsen and others 

(2003), and Nabighian and Macnae (1991).

Table 2. System specifications for the airborne electromagnetic survey of Leach Lake Basin, Fort Irwin, 

California. [The AeroTEM IV system was flown by Aeroquest, Ltd.]

Parameter Setting

Frame type Rigid frame

Receiver configuration Central-loop

Waveform Bipolar triangular

Base frequency 90 Hz

Measured time range 58 μs–3.2 ms

Transmitter loop turns 5

Peak current 397 A

Transmitter loop area 460 m2

Transmitter moment 237,000 Am2

Receiver altitude (mean) 48 m

Sounding spacing (mean) 3 m

Positioning (helicopter) GPS

Positioning (airframe) GPS

Elevation (helicopter) Radar altimeter

Elevation (airframe) Laser altimeter

Attitude (airframe) Inclinometer

Data Processing

Airborne electromagnetic data, as delivered by airborne survey vendors, have typically 

undergone some degree of processing, most commonly filtering and leveling. The raw data stream (not 

provided by the contractor) consists of on- and off-time data from both X (along the flightline) and Z 

(vertical) receiver coils recorded at a 36-kHz sampling rate. The following data processing steps were 

carried out by Aeroquest Ltd. Data from 36 cycles, or transient decays, were stacked using a 50-percent

overlapping straight stack. Stacked data were subsequently binned into gates of logarithmically 

increasing width using a boxcar window. A compensation procedure was then applied in which the 

effect of the residual primary Tx current was removed from the earliest measured time gates using data 

measured at high altitude (system response data), where the earth response is assumed to be zero. A 

gatewise linear drift correction was then applied between high-altitude system response measurements, 

which were carried out at the start and end of each flight. Filters applied to the data include a 19-point 

Hann window for smoothing and a single-point despiking filter to remove the effect of distant lightning.

Minimally processed data in appendix B incorporate the processing steps described above and 

these data were used in subsequent inversions. Data are additionally provided at three subsequent levels 

of contractor processing. Lag-corrected data are shifted in time relative to ancillary navigational data to 

account for a geometric offset between the electromagnetic sensors located on the air frame and 

navigational sensors on the helicopter and magnetometer bird. This correction was deemed unnecessary 

because additional position and altitude sensors were affixed to the air frame itself and are thus 

colocated with the electromagnetic sensors. Leveled and micro-leveled data have undergone varying 

degrees of tie-line leveling and decorrugation to produce smooth maps of the data at each time gate. 

Leveled data maps are of interest in qualitatively assessing the data and in searching for anomalous data 

responses. Leveling procedures, however, serve to reduce measured variations in signal amplitude 
8

between lines that are associated with system height variations. As system height is modeled within the 

inversion procedure, leveled data, in which these variations are artificially muted, are unsuitable for 

inversion.

Inversion of Electromagnetic Data

The process of inversion is used to recover the distribution of resistivity with depth beneath each 

sounding. Inversion of AEM data is ill posed; that is, many resistivity models can fit the measured data,

and the relationship between data and model is nonlinear. For this reason, regularization is needed to 

stabilize the inverse problem and to enforce specific properties in the inversion model such as proximity 

to a reference model or smoothness. Adjusting or determining the level of regularization is a critical part

of any geophysical inverse problem and is intimately related to the spatial continuity of the problem, 

that is, how rapidly geologic structure and, by inference, resistivity structure vary both laterally and with 

depth. In smoothing regularization, too little regularization results in models that are patchy or 

discontinuous, whereas too much regularization gives rise to overly smooth models that do not capture 

the true degree of resistivity variation in the data. The effects of regularization on the inversion model 

are most pronounced at the greatest depths within the model section, where limited data bandwidth or 

low signal-to-noise levels limit the influence of the data in steering the inversion.

The determination of an appropriate level of regularization is inseparable from the specification 

of data errors. For AEM data, the difficulty lies in determining the data errors from the measured data, 

which, as typically delivered, have been filtered and smoothed, resulting in artificially small data errors.

Additionally, the presence of systematic errors in the data can frustrate one’s ability to accurately define

data errors (Minsley and others, 2012). Most commonly, both data errors and the degree of 

regularization are adjusted during a series of trial inversions, with final parameters chosen that result in 

both a realistic model and an adequate fit to the measured data.

Assuming data errors are accurate and the degree of regularization is set appropriately, an 

inversion model section should reflect the true-earth model from the surface to the calculated depth of

investigation (defined later in this section). We exploit this approach to define errors in the measured 

data. The southern part of the survey area (fig. 1B) consists of exposed Jurassic-Cretaceous granite,

which is known to be resistive and is expected to extend to depths well beyond our depth of 

investigation. Data errors can be specified using a piecewise function that assigns the larger of 5 percent

of the received signal or α·t

-1/2, where α is typically in the range of 10-11 to 10-14 V/(m2

·s

-1/2) and t is the 

time in seconds since the start of current turn-off. The form of this noise function reflects the t

-1/2 noise 

reduction associated with gate widths that increase logarithmically with time. Measured data over the 

granite were inverted from a moderately conductive starting model within a range of α values. Upon 

examining the resulting inverse models, a value of α = 1.3×10-13 V/(m

2

·s

-1/2) was found to produce a 

geologically realistic model: highly resistive from the surface to the maximum depth of investigation 

with no little or no indication of the conductive starting model within this interval.

An effort was made to exclude data from the inversion that show no appreciable earth response

and are predominantly noise. This pertains mostly to late-time data, where signal-to-noise ratios are 

small, but entire soundings may effectively be noise in highly resistive areas. The contrast between 

resistive igneous basement rocks and conductive basin sediments results in highly variable signal levels 

and further demands a rigorous procedure to define the last usable time gate. An automated procedure 

was settled upon in which late-time data were excluded if dBz/dt fell below 10-12 V/m

2 or if the curvature 

of a log10(dBz/dt) versus log10(t) plot exceeded a threshold value of 10. Soundings were entirely omitted 

if early-time data fell below 10-10 V/m

2

.
9

Having established the appropriate subset of data and estimated data errors, data at 16 time gates 

spanning an interval from 100 μs to 2.8 ms were inverted using the inversion code EM1DINV (Auken 

and Christiansen, 2004) coupled with the laterally-constrained inversion (LCI) approach of Auken and 

others (2005). Gate-center times are specified in the contractor’s report (appendix A). The LCI approach 

links one-dimensional (1D) models at neighboring soundings together using regularization that enforces 

lateral continuity along a flightline. Data were decimated by a factor of 10 prior to inversion resulting in 

a 30-m nominal spacing between soundings. Blocks of 20 soundings (roughly 600-m line segments) 

were jointly inverted for linked 20-layer 1D models starting from a 100 Ω·m halfspace with no prior 

model enforced. The thickness of model layers varied from 8 m in the near surface to 42 m at depth with 

a total model depth of 380 m.

Vertical regularization, which effects how smoothly model resistivity varies with depth, was 

relaxed by a factor of 20 relative to lateral regularization. This ratio of vertical to lateral regularization

reflects the expected high degree of lateral continuity within the basin sediments relative to abrupt 

changes in resistivity with depth, such as the sediment-basement contact. While this assumption appears 

to generally hold, the highly-faulted geologic setting is observed to produce abrupt lateral changes in 

received signal strength. To address such areas within the context of a laterally constrained inversion, 

the lateral regularization was locally relaxed in proximity to known or suspected structures using 

maxima in the along-profile gradient in dBz/dt as a proxy for suspected faults or contacts. Table 3 

provides a summary of the final inversion parameters applied to the Leach Lake Basin AEM data.

Table 3. Summary of EM1DINV inversion parameters used for final inversion of the airborne electromagnetic 

(AEM) data for Leach Lake Basin, Fort Irwin, California.

Parameter Setting

Minimum/maximum gate centers 106 μs / 2.772 ms

Resistivity constraints 20

Thickness constraints 0.001 (fixed)

Vertical resistivity regularization 20

Vertical thickness regularization 0.001 (fixed)

Horizontal resistivity regularization 1

Horizontal thickness regularization 0.001 (fixed)

System altitude constraints ±3 m

Starting model 20 layers, 100 Ω·m, 380 m to top of half-space

Sounding exclusion threshold dBz/dt of first time gate less than 10-10 V/m2

Data exclusion threshold dBz/dt at all time gates less than 10-12 V/m2

Curvature cut-off criteria All time gates beyond which curvature of log10(dBz/dt) 

versus log10(t) exceeds 10 V/(m·s)

2

Data errors Greater of 5 percent dBz/dt or (1.3×10-13)t

-1/2 V/m2

System bandwidth 70 kHz single-pole low-pass 

Modeled current waveform Bipolar triangular pulse
10

A TEM system must be properly characterized in order to recover realistic subsurface resistivity 

models (Christiansen and others, 2011). For both ground and airborne TEM systems, the frequency 

response of the system, the system geometry, and the current waveform must all be modeled accurately.

For the AeroTEM IV system, the current waveform is known to vary during and between flights (both 

in timing and amplitude) and so is monitored by the contractor. We model a current waveform that 

varies dynamically throughout the survey area according to waveform parameters specified in appendix 

B. We further model two repetitions of the current waveform to account for the late-time effect of 

induction from earlier current pulses (Christiansen and others, 2011). 

For the purpose of inversion, an accurate estimate of system height relative to land surface is 

also needed. Measurements of system height were obtained from the laser altimeter mounted on the air 

frame but were incorporated into the inversion with a standard error of 3 m. The inversion treats system 

height as a free parameter subject to the specified standard error. This approach, in which system height 

is allowed to vary relative to a measured starting value, accounts for inaccuracy in the measured system 

height associated with tilt of the air frame or canopy reflections. Topography is not explicitly 

incorporated within the inversion algorithm, however all inverse models have been hung on topography 

extracted from a 30-m resolution National Elevation Dataset grid.

Depth of investigation (DOI) is a valuable tool for evaluating the approximate depth in an 

inversion model to which data are sensitive. In the context of regularized inversion, the DOI helps 

discriminate between parts of the inverse model that are determined by the data and those that simply 

reflect the regularization applied to the inversion. We calculated a linear sensitivity-based DOI for each 

sounding, based upon the work of Christiansen and Auken (2010). In this approach, a columnwise sum 

of the Jacobian, or sensitivity matrix, is calculated for each layer resistivity within the model. The 

cumulative sum of the values for each layer resistivity, starting at the bottom of the model, is then used 

to estimate the DOI, assuming a threshold value of 0.8. This choice of threshold is subjective, and the 

DOI is best thought of as a relative, rather than absolute, indicator of the depth below which model 

resistivities are poorly constrained by the measured data.

The DOI is a very useful tool when displaying the final inversion model images because it 

provides a means for displaying areas of confidence in the model. A simple approach is to blank out 

regions of the model which fall below the DOI, preserving only those portions of the model that are well

constrained by the measured data. This procedure has been applied to the inverted resistivity data 

provided in the SECTIONDATA folder under the ResINVDOI channel (appendix B).

Model Assessment and Interpretation

Igneous Framework

Examination of the resistivity model in map view (fig. 3) reveals strong resistivity contrasts 

within and surrounding Leach Lake Basin. The most resistive model features (>200 Ω·m) correspond to 

volcanic and plutonic rocks which floor and flank the basin. Mafic plutonic rocks, including diorite in 

the Avawatz Mountains (fig. 3, location 1a) and eastern Granite Mountains (fig. 3, location 1b), are 

extremely resistive (>1,000 Ω·m) and can be identified even in the measured data because received 

signal amplitudes plummet to the level of background noise. These regions can be identified on the

model depth slices (fig. 3) as gaps in the resistivity grid.
11

Figure 3. Maps showing depth slices through the inverted resistivity model at a depth of A, 25 m and B, 125 m 

below land surface at Fort Irwin, California. Labeled features are described in the text. 

Granitic rock of the western Granite Mountains (fig. 3, location 2a) are generally resistive but in 

the near surface exhibit significant variability in resistivity over short distances. Upon close inspection,

the spatial variability is closely correlated with exposed granitic ridges (resistive) and weathered 

depressions with localized sedimentary input (less resistive). This mottled appearance is a common 

geophysical expression of exposed intrusives and is evident in the magnetic field strength as well 

(Langenheim and Jachens, this volume, chap. I). Between Leach Lake and the Granite Mountains, 

shallow granite is imaged within the subsurface (fig. 3, location 2b) and is centered about small outcrops 

of granite within the basin fill (fig. 1B).

Felsic volcanic rocks in the Quail Mountains between the Garlock Fault and Owl Lake Fault 

(fig. 3, location 3a) exhibit similar resistivity values to the granite. Where mapped further east (fig. 3, 

location 3b), this rock unit has a more discontinuous resistive signature, possibly the result of dissection 

and translation along the Garlock Fault. Within the survey area, mafic volcanic rocks are mapped almost 

exclusively in the northwest corner of the survey area, are on average quite conductive (fig. 3, location 

4a), and are juxtaposed against felsic volcanic rocks across Owl Lake Fault. The high conductivity of 

the mafic volcanic rocks in contrast to resistive felsic volcanic rocks may reflect a difference in the age 
12

of emplacement or the degree of chemical or physical weathering. Two isolated resistive regions (fig. 3, 

location 4b) within the area of mapped mafic volcanic rock may be petrologically distinct from the 

surrounding material. This interpretation is further supported by color differences in Landsat imagery

(fig. 4).

Basin Fill

The extent and shape of Leach Lake Basin, as well as the edges of surrounding basins, is clearly 

demarcated in map view by slices through the resistivity models (fig. 3) and by cross-sections along 

individual profiles (figs. 4, 5). Fault control on these tectonic basins is apparent by the sharp linear 

resistivity contrasts which bound the basin and in many cases correlate with mapped or inferred faults.

A resistivity-based basin statigraphy can be derived from the AEM models. The extent and distribution 

of near-surface conductive deposits correlates with localized playa deposits (Qp) of Leach Lake

(compare figs. 1 and 3). Basinwide, a change is typically observed between a thick package of 

moderately resistive (80–150 Ω·m) older alluvium (Qoa+QToa) and a thin surface veneer of young and 

intermediate age alluvium (Qya+Qia) (30–50 Ω·m) (fig. 4B). A subtle contrast in the resistivity of this 

surface veneer is sometimes observed between young alluvial fan deposits (Qya) in the northern half of 

the basin and grus-rich alluvial fan material (Qyag) in the southern half of the basin. This contrast is 

quite visible in Landsat imagery (fig. 4A). The most pronounced contrast, however, is between 

Qoa+QToa and underlying, highly conductive (<10 Ω·m) material (fig. 4B). This boundary can be 

traced throughout much of the basin. Significant topography on this interface (for example, fig. 5B)

leads us to interpret this boundary as a lithologic change rather than a saturation boundary. This 

conclusion is supported by ground-based TEM models in Nelson Lake Basin, which show little 

correlation with water table elevations in nearby boreholes (Burgess and Bedrosian, this volume, chap.

F) and further suggest the water table is not a significant resistivity boundary in this environment.
13

Figure 4. Example inverted resistivity section along flightline L10400. A, Index map with the location of flightline 

L10400 highlighted. B, Inverted resistivity model cross section along flightline L10400; vertical exaggeration 2:1.
14

Figure 5. Example inverted resistivity section along flightline L10880. A, Index map with the location of flightline 

L10880 highlighted. B, Inverted resistivity model cross section along flightline L10880; vertical exaggeration 2:1.

The average depth to the top of the conductor is consistent with the average combined thickness 

of Qya+Qia (as much as 10 m) and Qoa+QToa (as much as 100 m) (Miller and others, this volume, 

chap. B). The base of this conductive unit is imaged within southcentral Leach Lake Basin, where it is

inferred to rest directly on granite basement. Beyond Leach Lake Basin, this conductive stratum is 

imaged along the northern edge of Nelson Lake and Drinkwater Lake Basins and the western edge of 

southern Death Valley. The horizon is folded and faulted throughout the study area, suggesting 

significant tectonic movement syn- or postdeposition. As such, it may serve as an important tectonic 

marker in understanding the evolution of the region.

The highly conductive sediments are believed to reflect clay-rich, partially lithified sediments, 

probably Mio-Pliocene in age (fig. 1, map unit pc). These older sediments have limited surface exposure 

and are poorly understood; they have been mapped by several authors along the eastern boundary of 

Fort Irwin but may not be correlative even throughout this limited region (Brady and Troxel, 1999; 

Beratan and others, 1999; Sobieraj, 1994; Spencer, 1990). In other basins of Fort Irwin, ground-based 

TEM resistivity models image a similar conductive layer (<10 Ω·m) (Burgess and Bedrosian, this 
15

volume, chap. B). In most cases, the conductor is at least 50–100 m below land surface, consistent with 

the limited exposure of map unit pc within Fort Irwin. 

As part of an effort to identify the origin of the high conductivity sediments, laboratory 

resistivity measurements (Bloss and Bedrosian, this volume, chap. E) were carried out on hand samples 

of exposed sediments within Nelson Lake Basin and the Central Corridor that are thought to be older 

than map unit Qoa and possibly QToa. Laboratory measurements were also carried out on drill core 

samples within Nelson Lake Basin. Both the deepest drill core samples and the surface hand samples 

were found to be highly conductive (<10 Ω·m) when saturated to simulate conditions below the water 

table (Burgess and Bedrosian, this volume, chap. F). X-ray diffraction and X-ray fluorescence analyses

were subsequently carried out on a subset of the drill core samples. The electrically conductive samples 

were found to have a high percentage of smectite clay as well as a large percentage of amorphous glass. 

The reaction of glass with water of moderate pH and low ionic strength is known to produce clay, 

predominantly smectite, as an alteration product (Vaniman, 2006). Miocene volcanism in the region is 

believed to be the source of the glass in the pre-Quaternary sediments. Some of this glass is interpreted 

to have been chemically weathered to smectite in the presence of water, giving rise to the high electrical 

conductivity. The Quaternary sediments, in contrast, contain less volcanic source material, have a low

clay fraction, and are observed to be more resistive in laboratory measurements.

The AEM resistivity models suggest Leach Lake Basin is a relatively shallow basin. This finding 

is in agreement with gravity-derived depth-to-basement estimates (Jachens and Langenheim, this 

volume, chap. H). Both gravity and AEM methods suggest that the basin fill within Leach Lake Basin is 

no more than 200–300 m deep. Within the AEM survey area, the deepest sedimentary pile occurs 

outside of Leach Lake Basin along the west edge of the Southern Death Valley fault, where Qoa alone 

can exceed 300 m in thickess. This is again consistent with the gravity-derived depth-to-basement 

model; however, gravity coverage is very limited in this area. In Leach Lake Basin, three localized 

depocenters are identified on AEM models and confirmed by gravity models—in the west near the 

intersection of the Garlock and Desert King Spring Faults, near the dry Leach Lake bed, and in the east 

between the Garlock Fault and the eastern Granite Mountains. Quaternary sediments are interpreted to 

be thin or absent along an east-southeast striking corridor at the south edge of the Owlshead Mountains.

Here, the conductive pc unit is imaged in the near surface and has been tightly folded and possibly 

faulted against the Owlshead Mountains.

Faulting

Understanding the geometry, extent, and activity of faults provides constraints on slip 

partitioning at the intersection between the Garlock and Southern Death Valley Fault Zones. The 

distribution of faults is also important when estimating groundwater storage because faults can reduce 

horizontal permeability and effectively compartmentalize groundwater into subbasins (see Burgess and 

Bedrosian, this volume, chap. F for a discussion of compartmentalization of groundwater in Nelson 

Lake Basin). The AEM resistivity models, both in map view and cross section, permit the direct 

identification of faults as abrupt, subvertical contrasts in resistivity. Basin-bounding faults, such as the 

Desert King Spring Fault, are most prominently imaged by the contrast between igneous basement and 

conductive sediments (fig. 4B). The top of conductive pc strata is often locally folded in the vicinity of 

such faults. Intrabasin faults, such as the Garlock Fault Zone, have a more subtle resistivity signature 

but can nevertheless be identified as disruptions in the otherwise flat-lying sedimentary strata (fig. 5B). 

In some cases, inactive fault strands can be inferred from the resistivity cross sections as continuous, 

flat-lying, near-surface sediments over discontinuous, older strata (for example, fig. 5B). 
16

In map view, the broad pattern of faulting along the eastern terminus of the Garlock Fault Zone 

is evident (fig. 3). A series of east-southeast trending linear resistivity highs marks a series of positive 

flower structures in the basin sediments that diverge westward from the through-going Garlock Fault 

and may continue as far west as the Owl Lake Fault. Within the Avawatz Mountains, these push-ups 

incorporate slivers of basement rock, including felsic plutonic (fp), felsic volcanic (fv), and mafic 

plutonic (mp) rocks (fig. 1). The Southern Death Valley Fault Zone is imaged in cross section (not 

shown) as a set of tight folds that have been faulted and exhibit a clear southwest vergence, consistent 

with an oblique sense of slip. A similar structure is imaged in the Celestite Hills, southwest of the 

Southern Death Valley Fault Zone, albeit with no clear sense of vergence direction (fig. 5B).

In order to attain the distribution of faulting within the survey area, the resistivity cross sections 

were sequentially examined. Faults were inferred where abrupt lateral changes in resistivity or vertical 

offsets in subhorizontal layering were observed. The locations of faults inferred from the resitivity 

models are marked with circles on fig. 6. A remarkable correspondence is observed between faults 

identified from aerial photography and light detection and ranging (lidar) (C. Menges, unpub. data, 

2012) and those independently identified from the AEM model (fig. 6). Major strands of the Garlock 

and Southern Death Valley Fault Zones are clearly identified from the AEM models, as are the Owl 

Lake and Desert King Spring Faults. In some cases, additional fault strands or previously unidentified 

faults are suggested by the AEM models (fig. 6, black dashed lines). These include northwest-striking 

faults between the Quail and Owlshead Mountains, a sub-parallel, older strand of the Garlock Fault at

the northern edge of the Granite Mountains, and faulting in the Celestite Hills between the Southern 

Death Valley Fault Zone and the Garlock Fault Zone. Additionally, a series of northwest-striking linear 

features are suggested within the central and eastern Granite Mountains, in particular along their contact 

with basin sediments. These structures have an unusual orientation with respect to regional faulting (fig. 

1) and may reflect local deformation associated with the termination of the Garlock Fault Zone within 

the Avawatz Mountains.

Figure 6. Map showing distribution of interpreted faults from airborne electromagnetic (AEM) models (white 

circles) in relation to mapped faults (solid black lines) in Leach Lake Basin, Fort Irwin, California. Dashed black 

lines indicate inferred fault traces or contacts. Background is a depth slice through the resistivity model at 125 m 

below land surface.
17

Geophysical Data Overview

Digital data are provided for the Leach Lake Basin survey area (see appendix B). Several 

products and data formats have been provided and are summarized in table 4 and described in the 

following sections in more detail.

Table 4. Digital data organization and description for files and folders for the Leach Lake Basin study area, Fort 

Irwin, California.

Folder Description

FLIGHTLINE

Geospatial information consisting of survey flightlines (FLIGHT_LINES). Subfolders exist for AutoCAD files 

(LINE_DXF folder, *.dxf), shape files (LINE_SHP folder, *.shp), and keyhole markup language1 (LINE_KML, 

*.kml) formats.

GRIDS

Gridded geophysical models created by the USGS. All grids are in Geosoft Oasis Montaj2 format (GRID_GRD 

folder, *.grd), a standard of the geophysical industry used in many map display computer algorithms. Grids are 

also provided as georeferenced images in geoTIFF format (GRID_GEOTIFF folder, *.tif).

LINEDATA AEM data in ASCII standard (*.xyz) and Geosoft Oasis montaj2 (*.gdb) database formats. The README file in 

this directory contains channel heading descriptions.

PLOTS Images of inverted resistivity sections (INVERTED_SECTIONS folder, *.pdf) and inverted model depth slices 

(INVERTED_SLICES folder, *.pdf).

SECTIONDATA Databases of the inverted resistivity depth sections in ASCII standard (*.xyz) and Geosoft Oasis montaj2 (*.gdb)

database formats.

1

Information on the keyhole markup language format and free GoogleEarth software can be found at 

http://www.google.com/earth/index.html.

2

Information on the Oasis montaj program and a free data viewer can be found at http://www.geosoft.com/.

Flightlines

The FLIGHTLINE folder contains geospatial datasets of the flightline paths. The flightline 

location files are formatted in AutoCAD format (LINE_DXF folder, *.dxf), Esri Shapefile format 

(LINE_SHP folder, *.shp and associated files), and in GoogleEarth keyhole markup language format 

(LINE_KML folder, *.kml).

Grids

The GRIDS folder contains interpolated grids of various channels of the model section data from 

the Leach Lake Basin used to produce map plots. One of the challenges of gridding airborne 

geophysical data is that the spacing between flightlines (hundreds of meters or more) is much greater 

than the sampling down-line (a few meters). Gridding has been carried out using the minimum curvature 

method implemented by Webring (1981) for airborne geophysical data. This gridding method is 

implemented in Geosoft’s Oasis montaj program (http://www.geosoft.com/resources/technologypapers/solution-earth-modeling). We have used this algorithm to produce 30-m-resolution grids of the

inverted resistivity data from each model layer. 
18

Grids are provided in two formats: Geosoft grid format (GRID_GRD folder, *.grd) and 

georeferenced geoTIFF images (GRID_TIFF folder, *.tif). The nomenclature for the grid names is given 

in the README file. The Geosoft grids can be viewed and analyzed in free software distributed by 

Geosoft (http://www.geosoft.com/support/downloads/viewers/oasis-montaj-viewer) or in various other 

mapping software packages using free plug-ins provided by Geosoft 

(http://www.geosoft.com/downloads). For example, the Geosoft-formatted grids can be viewed directly 

in the ESRI ArcMap application with the Geosoft ArcGIS plug-in 

(http://www.geosoft.com/support/downloads/plug-ins/plug-arcgis). Once the plug-in is installed and 

loaded in ArcMap, Geosoft grids can be handled within ArcMap in a similar manner to other types of 

raster data. GeoTIFF images can be viewed geospatially in most standard geographic information 

system software. The geoTIFF files can also be opened in several standard image-viewing software 

packages, including Windows Picture and Fax Viewer.

Line Data

The LINEDATA folder contains AEM survey data. The data are presented in Geosoft database 

format (*.gdb) and in ASCII format (*.XYZ contained within a *.zip file) with column headings as 

described in the README file. The contractor’s report in appendix A also describes the digital 

flightline data.

Plots

The PLOTS folder contains plots of the inverted resistivity sections (INVERTED_SECTIONS 

folder, *.jpg) and depth slices (INVERTED_SLICES folder, *.pdf) for the Leach Lake Basin study area.

Resistivity section plots have been generated for each flightline; the index map panel shows the location 

of the selected flightline being displayed in a given plot. Depth slices have been generated for the first 

16 inversion model layers; the deepest three model layers fall largely below the estimated depth-ofinvestigation and are not provided in plots. All plots have been produced with a common color scale for 

all sections to allow comparison between flightlines. 

Section Data

The SECTIONDATA folder containts the resistivity structure as a function of depth along the 

flightlines, as determined from the previously described inversion process. The DOI cutoff has been 

applied to the inverted resistivity values provided in the ResINVDOI channel. The depth intervals for 

each inversion model layer (DepTop and DepBot channels) are relative to land surface. Land-surface

elevation along flightlines (NED30 channels) have been sampled from the 30-m resolution USGS 

National Elevation Dataset (http://ned.usgs.gov) and is specified in meters relative to the North 

American Vertical Datum of 1988 (NAVD88).

Acknowledgments

Airborne data collection was funded by the Fort Irwin National Training Center. The authors

thank Aeroquest, Ltd. for their efforts in collecting this data set. In particular, we acknowledge the field 

crew (Lee Harper, Thomas Wade, and John Douglas) and the data processing team (Chris Kahue and 

Marion Bishop). We thank Jonathan Rudd for extended discussion and clarification of many aspects of 

the AeroTEM IV system and data processing. Matt Burgess, Sarah Falk, and Jill Densmore are thanked 
19

for their help in acquiring ground-truth data for this survey. Bill Benzel is thanked for running X-ray 

diffraction and X-ray fluorescence analyses on drill core samples, the results of which informed our 

interpretation. Discussions with Chris Menges, David Miller, and David Buesch provided the geologic 

and tectonic context for these studies. John Dohrenwend processed the Landsat scenes to enhance 

features of geologic interest. Thanks to Ted Asch, Marya Deszcz-Pan, and Jeff Lucius for constructive 

reviews of this manuscript.

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