U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2011–1289

Prepared in cooperation with the Upper Loup Natural Resources District ,the University of Prepared in cooperation with the Upper Loup Natural Resources District, the University of

Nebraska Conservation and Survey Division, and the Nebraska Environmental T Nebraska Conservation and Survey Division, and the Nebraska Environmental Trust rust

Hydrostratigraphic Interpretation of Test-Hole and

Geophysical Data, Upper Loup River Basin, Nebraska,

2008–10
Front and back cover. Test-hole drilling south of Whitman, Nebr., Grant County, 2009.

Back cover. Grass covered eolian dunes, Thomas County, 2008 (upper photo). Strategem transmitter used in Audio Magnetotelluric survey, 

Thomas County, 2008 (lower photo).
Hydrostratigraphic Interpretation of TestHole and Geophysical Data, Upper Loup 

River Basin, Nebraska, 2008–10

By Christopher M. Hobza, Theodore H. Asch, and Paul A. Bedrosian

Prepared in cooperation with the Upper Loup Natural Resources District, the 

University of Nebraska Conservation and Survey Division, and the Nebraska 

Environmental Trust

Open-File Report 2011–1289

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2011

For more information on the USGS—the Federal source for science about the Earth, its natural and living 

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U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Hobza, C.M., Asch, T.H., and Bedrosian, P. A., 2011, Hydrostratigraphic interpretation of test-hole and geophysical 

data, Upper Loup River Basin, Nebraska, 2008–10: U.S. Geological Survey Open-File Report 2011-1289, 60 p. 
iii

Contents

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

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

Purpose and Scope ..............................................................................................................................3

Study Area Description........................................................................................................................3

Geologic Setting....................................................................................................................................3

Methods...........................................................................................................................................................6

Site and Method Selection..................................................................................................................6

Test-Hole Drilling and Borehole Geophysics ...................................................................................7

Test-Hole Drilling Method and Procedure...............................................................................7

Borehole Geophysical Data Collection ....................................................................................7

Surface-Geophysical Data Collection...............................................................................................8

Time-Domain Electromagnetic ..................................................................................................8

Time-Domain Electromagnetic (TDEM) Method............................................................8

Audio Magnetotelluric (AMT)..................................................................................................10

The Magnetotelluric Method..........................................................................................10

Audio-Magnetotelluric (AMT) Data Collection............................................................11

Audio-Magnetotelluric (AMT) Data Analysis...............................................................15

Hydrostratigraphic Interpretation of Test-Hole and Surface-Geophysical Data..............................16

Test-Hole Lithologic and Borehole Geophysical Logs..................................................................16

TDEM Survey Results.........................................................................................................................18

TDEM Models.............................................................................................................................18

Time-Domain Electromagnetic (TDEM) Model Uncertainty ...............................................19

Time-Domain Electromagnetic (TDEM) Model Interpretation ...........................................19

Audio-Magnetotelluric (AMT) Survey Results...............................................................................23

2008 and 2009 Two-Dimensional Inversion Modeling Results............................................23

2010 Two-Dimensional (2D) Inversion Modeling Results....................................................23

Conclusions and Implications for Future Study.............................................................................34

Summary........................................................................................................................................................34

References Cited..........................................................................................................................................35

Supplemental Data ......................................................................................................................................39
iv

Figures

1. Map showing location of the upper Loup study area within the Elkhorn-Loup 

groundwater model area.............................................................................................................2

2. Map showing location of drilled test holes and time-domain electromagnetic 

(TDEM) and audio-magnetotelluric (AMT) soundings, upper Loup study area, 

2008 to 2010....................................................................................................................................4

3. Stratigraphic, lithologic, and hydrostratigraphic column of the High Plains 

aquifer and confining units of the upper Loup study area.....................................................6

4. Schematic illustration showing system layout for audio-magnetotelluric survey 

using STRATAGEM EH-4 system ..............................................................................................11

5. Composite of generalized lithologic description and geophysical logs for test

hole 2-UL-08, McPherson County, Nebr..................................................................................17

6. Generalized lithologic description of test hole1-UL-08, Logan County, Nebr...................40

7. Composite of generalized lithologic description and geophysical logs for test 

hole 3-UL-08, Thomas County, Nebr ........................................................................................41

8. Composite of generalized lithologic description and geophysical logs for test 

hole 1-UL-09, Grant County, Nebr.............................................................................................42

9. Composite of generalized lithologic description and geophysical logs for test 

hole 2-UL-09, Hooker County, Nebr..........................................................................................43

10. Composite of generalized lithologic description and geophysical logs for test 

hole 3-UL-09, Cherry County, Nebr...........................................................................................44

11. Composite of generalized lithologic description and geophysical logs for test 

hole 1-UL-10, Thomas County, Nebr ........................................................................................45

12. Composite of generalized lithologic description and geophysical logs for test 

hole 2-UL-10, Thomas County, Nebr ........................................................................................46

13. Composite of generalized lithologic description and geophysical logs for test hole 

3-UL-10, Blaine County, Nebr....................................................................................................47

14. Graphs showing relations between A, central-loop resistivity, out-of-loop 

vertical magnetic field (time-derivative, right axis), and time; B, depth below 

land surface and subsurface resistivity modeled from TDEM sounding for 

site CN5 ........................................................................................................................................18

15. Graphs showing relations between A, central-loop resistivity, out-of-loop 

vertical magnetic field (time-derivative, right axis), and time; B, depth below 

land surface and subsurface resistivity modeled from TDEM sounding for sites

CN1 and CN2................................................................................................................................48

16. Graphs showing relations between A, central-loop resistivity, out-of-loop

vertical magnetic field (time-derivative, right axis), and time; B, depth below 

land surface and subsurface resistivity modeled from TDEM sounding for sites

 CN3 and CN4................................................................................................................................49

17. Graphs showing relations between A, central-loop resistivity and time; B, depth 

below land surface and apparent resistivity modeled from TDEM sounding for 

sites SH7 and SH8.......................................................................................................................50

18. Graphs showing relations between A, central-loop resistivity, out-of-loop

vertical magnetic field (time-derivative, right axis), and time; B, depth below 

land surface and subsurface resistivity modeled from TDEM sounding for sites 

UL1 and UL2 .................................................................................................................................51
v

19. Graphs showing relations between A, central-loop resistivity, out-of-loop

vertical magnetic field (time-derivative, right axis), and time; B, depth below 

land surface and subsurface resistivity modeled from TDEM sounding for sites 

UL3 and UL4 .................................................................................................................................52

20. Graphs showing relations between A, central-loop resistivity, out-of-loop 

vertical magnetic field (time-derivative, right axis), and time; B, depth below 

land surface and subsurface resistivity modeled from TDEM sounding for sites 

UL5 and UL6 .................................................................................................................................53

21. Graphs showing relations between A, central-loop resistivity, out-of-loop 

vertical magnetic field (time-derivative, right axis), and time; B, depth below 

land surface and subsurface resistivity modeled from TDEM sounding for sites 

UL7 and UL8 .................................................................................................................................54

22. Graphs showing relations between A, central-loop resistivity, out-of-loop 

vertical magnetic field (time-derivative, right axis), and time; B, depth below

land surface and subsurface resistivity modeled from TDEM sounding for sites 

UL9 and UL10 ...............................................................................................................................55

23. Graphs showing relations between A, central-loop resistivity, out-of-loop 

vertical magnetic field (time-derivative, right axis), and time; B, depth below 

land surface and subsurface resistivity modeled from TDEM sounding for sites 

UL11 and UL12 .............................................................................................................................56

24. Graphs showing relations between A, central-loop resistivity, out-of-loop 

vertical magnetic field (time-derivative, right axis), and time; B, depth below 

land surface and subsurface resistivity modeled from TDEM sounding for sites 

UL13 and UL14 .............................................................................................................................57

25. Graphs showing relations between A, central-loop resistivity, out-of-loop 

vertical magnetic field (time-derivative, right axis), and time; B, depth below 

land surface and subsurface resistivity modeled from TDEM sounding for sites 

UL15 and UL16 .............................................................................................................................58

26. Graphs showing relations between A, central-loop resistivity, out-of-loop 

vertical magnetic field (time-derivative, right axis), and time; B, depth below 

land surface and subsurface resistivity modeled from TDEM sounding for sites 

UL17 and UL18 .............................................................................................................................59

27. Graphs showing relations between A, central-loop resistivity, out-of-loop 

vertical magnetic field (time-derivative, right axis), and time; B, depth below 

land surface and subsurface resistivity modeled from TDEM sounding for site 

UL19...............................................................................................................................................60

28. Map showing resistivity at the inferred base of High Plains aquifer.................................20

29. Map showing inferred base-of-aquifer elevation from minimum-layer 

inverse time-domain electromagnetic (TDEM) models and contours

derived primarily from test-hole data......................................................................................21

30. Inferred depth from land surface to base of High Plains aquifer from 

minimum-layer time-domain electromagnetic (TDEM) inverse models............................22

31. Mixed-mode two-dimensional (2D) inversion results for stations near test 

hole 5-SH-87 ................................................................................................................................24

32. Map showing location of audio-magnetotelluric (AMT) stations along 

Whitman Road, Cherry and Grant Counties, Nebr., where 2010 stations 

occurred.......................................................................................................................................25
vi

Tables

1. Field name, site number, location, and elevation of test holes .............................................5

2. Upper Loup time-domain electromagnetic (TDEM) site coordinates, elevation,

county, and acquisition year.......................................................................................................9

3. Summary by site of audio-magnetotelluric (AMT) stations, geometry, location, 

and elevation for AMT data collected in 2008 and 2009.......................................................12

4. Location and elevation for all audio-magnetotelluric (AMT) stations where 

AMT data were acquired in 2010 along Whitman Road profile, Cherry and Grant

Counties, Nebr.............................................................................................................................13

5. Location and inferred elevation of base of High Plains aquifer, north segment 

of Whitman Road profile, Grant and Cherry Counties, Nebr., 2010.....................................30

6. Location and inferred elevation of base of High Plains aquifer, south segment 

of Whitman Road profile, Grant and Cherry Counties, Nebr., 2010.....................................32

33. Mixed-mode two-dimensional (2D) inversion results from 2010 audiomagnetotelluric (AMT) stations for the north segment (A to A’) along 

Whitman Road, Cherry and Grant Counties, Nebr ................................................................26

34. Mixed-mode two-dimensional (2D) inversion results from 2010 audiomagnetotelluric (AMT) stations for the north segment (A to A’) along 

Whitman Road, Cherry and Grant Counties, Nebr ................................................................27

35. Mixed-mode two-dimensional (2D) inversion results from 2010 audiomagnetotelluric (AMT) stations for the south segment (B to B’) along 

Whitman Road, Cherry and Grant Counties, Nebr ................................................................28

36. Mixed-mode two-dimensional (2D) inversion results from 2010 audiomagnetotelluric (AMT) soundings for the south segment (B to B’) along 

Whitman Road, Cherry and Grant Counties, Nebr ................................................................29
vii

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 4,047 square meter (m2)

acre 0.4047 hectare (ha)

acre 0.4047 square hectometer (hm2) 

acre 0.004047 square kilometer (km2)

square foot (ft2) 0.09290 square meter (m2)

Frequency

Cycles per second 1 hertz

Magnetic moment

Ampere-square foot (A-ft2) 0.09290 Ampere-square meters (A-m2)

Conversion Factors

Inch/pound to SI

Abstract

Nebraska’s Upper Loup Natural Resources District is 

currently (2011) participating in the Elkhorn-Loup Model to 

understand the effect of various groundwater-management 

scenarios on surface-water resources. During Phase 1 of the 

Elkhorn-Loup Model, a lack of subsurface geological information in the Upper Loup Natural Resources District, hereafter 

referred to as the upper Loup study area, was identified as 

a gap in current knowledge that needed to be addressed. To 

improve the understanding of the hydrogeology of the upper 

Loup study area, the U.S. Geological Survey, in cooperation with the Upper Loup Natural Resources District and the 

University of Nebraska Conservation and Survey Division, 

collected and described the lithology of drill cuttings from 

nine test holes, and concurrently collected borehole geophysical data to identify the base of the High Plains aquifer. Surface 

geophysical data also were collected using time-domain 

electromagnetic (TDEM) and audio-magnetotelluric (AMT) 

methods at test-hole locations and between test holes, as a 

quick, non-invasive means of identifying the base of the High 

Plains aquifer.

Test-hole drilling has indicated greater variation in the 

base-of-aquifer elevation in the western part of the upper 

Loup study area than in the eastern part reflecting a number 

of deep paleovalleys incised into the Brule Formation of the 

White River Group. TDEM measurements within the upper 

Loup study area were shown to be effective as virtual boreholes in mapping out the base of the aquifer. TDEM estimates 

of the base of aquifer were in good accordance with existing 

test-hole data and were able to improve the interpreted elevation and topology of the base of the aquifer. In 2010, AMT 

data were collected along a profile, approximately 12 miles 

(19 kilometers) in length, along Whitman Road, in Grant and 

Cherry Counties. The AMT results along Whitman Road indicated substantial variability in the elevation of the base of the 

High Plains aquifer and in the distribution of highly permeable 

zones within the aquifer. 

Introduction

Newly adopted legislation in Nebraska requires a sustainable balance between long-term water supplies and uses 

of surface- and groundwater (Ostdiek, 2009) and requires 

Natural Resources Districts (NRDs) to understand the effect of 

groundwater use on surface-water systems when developing a 

groundwater management plan. The recent (2000–06) drought 

has increased concerns about the long-term sustainability of 

surface- and groundwater resources as well as concerns about 

the effect of groundwater use for irrigation on streamflow in 

Nebraska (Peterson and others, 2008). 

The Upper Loup Natural Resources District (ULNRD) is 

located in the west-central part of the Sand Hills and overlies 

the nationally important High Plains aquifer (fig. 1). The Loup 

River and its tributaries, with their headwaters in the ULNRD, 

contribute substantially to supplying base flow to more 

intensively irrigated areas downstream. Although groundwater 

use for irrigation is fairly limited in the ULNRD compared 

to other areas of Nebraska, the streams within the ULNRD 

are sensitive to consumptive groundwater use because of 

interaction between groundwater and surface water (Peterson 

and others, 2008). The complexity of this hydrologic system 

and the interaction between the available surface-water and 

groundwater resources is not yet fully understood. 

The ULNRD currently is participating in studies developing the Elkhorn-Loup Model (ELM) (Peterson and others, 

2008; Stanton and others, 2010) to understand the impact of 

various groundwater management scenarios on surface-water 

resources. The ELM development is a coordinated effort 

involving 10 NRDs, the Nebraska Department of Natural 

Resources, the University of Nebraska, Conservation and Survey Division (CSD), and the U.S. Geological Survey (USGS). 

The ELM studies began in 2006 and are being completed in 

three separate phases. During Phase 1 of the ELM studies, 

a lack of subsurface geological information in the ULNRD 

was identified as a gap in present knowledge that needed to 

be addressed. Test holes drilled to the base of aquifer in the 

ULNRD were as much as 25 miles apart. 

Given the variable character of the hydrostratigraphic 

units that compose the High Plains aquifer, substantial 

Hydrostratigraphic Interpretation of Test-Hole and 

Geophysical Data, Upper Loup River Basin, Nebraska, 

2008–10

Christopher M. Hobza, Theodore H. Asch, and Paul A. Bedrosian
2 Hydrostratigraphic Interpretation of Test-Hole and Geophysical Data, Upper Loup River Basin, Nebraska, 2008–10

97° 98° 99° 100° 101° 102° 103° 104°

43°

42°41°40°

0 50 100 MILES

100 KILOMETERS 0 5 0

High Plains

aquifer

Sand Hills boundary

Elkhorn-Loup model areaSand Hills boundary

Upper Loup study area

and

Upper Loup 

National Resources 

District

Base from U.S. Geological Survey digital data, 2002, 1:100,000Lambert Conformal Conic projectionStandard parallels 40oN and 43oN, central meridian 100oWHorizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83)

Upper Loupstudy area

NEBRASKA

NEBRASKA

EXPLANATION

SOUTH DAKOTA

KANSAS

COLORADO

Plum

Cr

Cr

LongPine

Niobrara

River

Eagle Cr

Cr

Cr

Cr

Cr

Cr

bdeR ri d

eer C k

Creek

Verdigre

Creek

Bazile

North Fork 

Elkhorn River

Elkhorn River

Elkhorn River

Holt South ForkCache

Cr

Clearwater

CedarCr

Beaver

Cedar

Cedar River

River

River

River

River

River

River

Creek

Calamus

North

Loup

Loup

Shell

Wood River

Platte River

Oak Cr

Clear Mud Cr

South

Loup

Dismal

Middle

Loup

River

Goose Cr

Mud Cr

North Br

Middle

Loup

Snake

River

North

Platte

W Birdwood

Creek

R

Figure 1. Location of the upper Loup study area within the Elkhorn-Loup groundwater model area.
Introduction 3

uncertainty in aquifer thickness and characteristics can exist 

between test holes. In 2008, the USGS, in cooperation with 

the ULNRD and University of Nebraska Conservation and 

Survey Division, began a hydrogeologic study of the ULNRD 

to describe the lithology and thickness of the High Plains 

aquifer and the topology of the bedrock surface at the base of 

the aquifer. Knowledge of these characteristics is important for 

assessing water supplies and understanding groundwater flow 

systems.

Purpose and Scope

The purpose of this report is to provide a hydrostratigraphic interpretation of test-hole and surface geophysical data 

collected in the ULNRD. This report documents the methods 

of data collection and analysis, presents test-hole and surface 

geophysical survey results and interpretations, and examines 

the utility of these techniques in improving the hydrostratigraphy. The geologic and geophysical data collected from the 

additional test holes and surface geophysical data are intended 

to refine interpretations in the base-of-aquifer elevation map 

being used for the Phase 3 ELM groundwater model. 

Of nine test holes drilled to the base of the High Plains 

aquifer (fig. 2; table 1), six were drilled since McGuire and 

Peterson (2008) previously interpreted the base of the aquifer 

beneath the ELM area. Surface geophysical data also were 

collected using time-domain electromagnetic (TDEM) and 

audio-magnetotelluric (AMT) methods at selected locations 

to supplement test holes for identifying the base of the High 

Plains aquifer. Included in this report are the generalized lithologic and borehole geophysical logs from all nine test holes 

and surface geophysical data collected.

Study Area Description

The ULNRD is the study area for this report, hereafter 

referred to as the upper Loup study area, which is located in 

the west-central Sand Hills at the headwaters of the North, 

Middle, and South Loup Rivers and almost the entire Dismal River drainage basin (fig. 1). The Sand Hills region of 

Nebraska is the largest dune field in the Western Hemisphere 

currently stabilized by vegetation (Bleed, 1989). Of the 

4.13 million acres composing the upper Loup study area, 

approximately 91.5 percent is rangeland, 4.7 percent is open 

water and wetlands, 1.4 percent is barren, 1.1 percent is irrigated cropland, with riparian forest and woodland, dry-land 

crops, and unclassified land uses all constituting less than 

1 percent each (Center for Advanced Land Management Information Technology, 2007). 

The climate in the upper Loup study area is characterized by cold winters and warm summers typical of continental 

mid-latitude locations (Mast and Turk, 1999). Mean monthly 

temperatures ranged from 22.3 degrees Fahrenheit (oF) in 

January to 74.7oF in July during 1931–89 (National Oceanic 

and Atmospheric Administration, 2010). The mean annual 

precipitation was 20.9 inches (in.) at the Halsey 2W weather 

station, near Halsey, Nebr. (fig. 2) from 1931–90 (National 

Oceanic and Atmospheric Administration, 2010) with the 

greatest precipitation occurring during the spring and summer 

months. Potential evaporation, similar to that of precipitation, 

increases during the growing season, peaking in July, and 

often exceeds precipitation (Chen and others, 2003). 

The surface of the upper Loup study area is mantled 

with deposits of Quaternary age: eolian dunes and alluvium in 

modern stream valleys (Swinehart and Diffendal, 1989). The 

eolian dunes presently are stabilized with grasses, but during 

the past 15,000 years severe droughts triggered their remobilization because of vegetative cover loss (Loope and Swinehart, 

2000). Numerous lakes and wetlands formed when migrating sand dunes blocked stream systems (Loope and others, 

1995; Mason and others, 1997). Such lakes and wetlands are 

hydrologically connected to the High Plains aquifer (Winter, 

1986) and can persist even during times of prolonged drought 

(Loope and Swinehart, 2000). 

Unique physical characteristics of the Sand Hills have 

allowed a net gain to the groundwater system with time and 

resulted in storage of a substantial amount of water beneath 

the upper Loup study area. Chen and others (2003) noted that 

the permeability of the soils in the Dismal River and Middle 

Loup River areas (fig. 2) combined with thick unsaturated 

zones beneath dunes increases the amount of precipitation 

reaching the water table and reduces groundwater loss to 

evapotranspiration. Additionally, snowmelt can contribute substantial amounts of recharge because evapotranspiration rates 

typically are very small at that time of year. 

One aspect unique to streams in the study area compared 

to other streams in Nebraska is the steadiness of streamflow, which is the result of highly permeable soils and large 

amounts of groundwater in storage. Runoff-induced high-flow 

events are rare within the upper Loup study area because of 

the ability of permeable soils to capture precipitation. This 

results in streamflow that is dominated by groundwater discharge or base flow. Bentall (1989) estimated that the Middle 

Loup River and the Dismal River above Dunning, Nebr. 

(fig. 2) have groundwater contributions exceeding 95 percent 

of total streamflow. Szilagyi and others (2003) have reported 

similar findings for areas in the west-central Sand Hills. 

Streamflow of the Middle Loup and Dismal Rivers remains 

steady even during periods of drought and responds more 

slowly to changes in climatic conditions than do other streams 

in the Sand Hills east of the upper Loup study area. This was 

attributed to greater aquifer thicknesses, increased groundwater storage, and steeper hydraulic gradients near the Middle 

Loup and Dismal Rivers (Chen and others, 2003).

Geologic Setting

The upper Loup study area overlies the thickest parts of 

the High Plains aquifer system, where saturated thicknesses 

can exceed 1,000 feet (300 m) (McGuire and Fischer, 1999). 
4 Hydrostratigraphic Interpretation of Test-Hole and Geophysical Data, Upper Loup River Basin, Nebraska, 2008–10

100° 101° 102°

42°30'

42°

41°30'

Base from U.S. Geological Survey digital data, 2002, 1:100,000Lambert Conformal Conic projectionStandard parallels 40oN and 43oN, central meridian 100oWHorizontal coordinate information is referenced to the North AmericanDatum of 1983 (NAD 83)

SHERIDAN CHERRY BROWN ROCK

GRANT

HOOKER

THOMAS BLAINE

ARTHUR MCPHERSON LOGAN CUSTER

LOUP

GARDEN

KEITH LINCOLN

Hyannis

Whitman

Mullen

Seneca

Thedford

Stapleton

Gandy Arnold

Anselmo

Merna

Broken Bow

Brewster

Halsey

Johnstown

Ainsworth Long Pine

Dunning

Lewellen

River

Snake

Mud Creek

Loup

River

North Branch Middle Loup River

M di dle Branch Middle

Dismal

River

South

Fork

Di ms al River

West BirdwoodCreek

North Fork Birdwood Cr Birdwood Cr

North

Middle

Loup

Loup

River

River

Goose

Creek

Calamus

River

North

Platte

River

Plum

Cr

Cr

Long Pine

Upper Loup

study area

Elkhorn-Loup 

model area

NEBRASKA

0

0

10

20 KILOMETERS 10

20 MILES

!(

")

E

!(

*#

2010 AMT profile2008 AMT2008 AMT and TDEM2008 TDEM2009 TDEM and AMT2010 TDEMTest holes

EXPLANATION

[AMT, Audio magnetotellurics; 

TDEM, Time-domain electromagnetic]

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Figure 2. Location of drilled test holes and time-domain electromagnetic (TDEM) and audio-magnetotelluric (AMT) soundings, upper Loup study area, 2008 to 2010. 
Introduction 

5

Here the High Plains aquifer is considered to include all 

hydrologically connected units of Tertiary and Quaternary age. 

According to Gutentag and others (1984), these rocks are the 

upper units of the Oligocene-age Brule Formation of the White 

River Group (herein referred to as the Brule Formation), the 

Miocene-age Arikaree Group, the Miocene-age Ogallala For-

mation, and Quaternary-age alluvial and eolian deposits. The 

Brule Formation forms the base of the High Plains aquifer in 

the upper Loup study area. All previously mentioned units are 

shown in the hydrostratigraphic column presented in figure 3. 

The geologic history relevant to this present study begins 

roughly 70 million years ago. During Cretaceous time, the 

Sand Hills region was covered by a shallow inland sea, and 

marine sediments, including the thick Pierre Shale, were 

deposited. Following regression of the Cretaceous sea, uplift 

resulted in formation of the Chadron and Cambridge Arches 

striking northwest to southeast across the upper Loup study 

area (Swinehart and others, 1985). Subsequent fluvial erosion 

removed as much as 1,800 ft (550 m) of the Cretaceous sec-

tion and created a structural low over the previously uplifted 

region. 

Overlying the Pierre Shale is the Brule Formation (fig. 

3), which underlies the entire study area. The Brule Formation 

is a massive siltstone composed of primarily eolian silt, with 

some alluvial deposits. Deposits of volcanic ash derived from 

eruptions in the western United States compose two-thirds of 

its volume. The Brule Formation is relatively impermeable 

when unfractured (Cannia and others, 2006).

Overlying the Brule Formation is the Arikaree Group, 

which is largely limited to the western part of the study area 

or may exist in paleovalleys in the eastern part of the study 

area (Swinehart and Diffendal, 1989). The Arikaree Group is 

a massive, very-fine to fine-grained sandstone with localized 

beds of volcanic ash, silty sand, and sandy clay (Darton, 1903; 

Condra and Reed, 1943). The Arikaree Group is considered 

part of the High Plains aquifer system; however, it does not 

yield large quantities of water to wells (Gutentag and others, 

1984). Within the upper Loup study area, the Arikaree Group 

typically is not used as a water source.

The Ogallala Formation (referred to herein as the Ogal-

lala) is the principal geologic unit in the High Plains aquifer 

system and can reach a thickness of 800 ft (240 m) beneath the 

upper Loup study area (Diffendal, 1991). The Ogallala is com-

posed of a poorly sorted mixture of sand, silt, clay, and gravel 

(Condra and Reed, 1943). The Ogallala generally is unconsoli-

dated or weakly consolidated, but contains layers of sandstone 

cemented by calcium carbonate. When covered by younger 

deposits, the Ogallala has not been subdivided into formations 

recognized in other areas because of the difficulty correlat-

ing these units in the subsurface. The Ogallala was deposited 

by aggrading streams that filled paleovalleys eroded into 

older rocks (Swinehart and others, 1985). Beneath the upper 

Loup study area, the Ogallala typically is composed of fine to 

medium-grained sand and sandstone (Swinehart and Diffen-

dal, 1989). Although the lithology of this unit can be fairly 

uniform, the base of the Ogallala is a complex surface formed 

Table 1. Field name, site number, location, and elevation of test holes.[Legal descriptions: T., township; R., range; S., section; ABCD, codes for the quarter section, quarter-quarter section, quarter-quarter-quarter section, and quarter-quarter-quarter-quarter sections A, B, C, and D, respectively, where A is northeast, B is northwest, C is southwest, and D is southeast quarter of the next larger unit; Predicted minus drilled, indicates the predicted base-of-aquifer elevation from Scientific Investigations Map 2008–3042 minus the drilled base-of-aquifer elevation at test hole; -- indicates test hole was included in Scientific Investigations Map 2008–3042 and a difference not calculated]

Field name County Site number1 Legal description Latitude2 Longitude2 Elevation3 (feet) Date drilled Depth to base of aquifer (feet) Predicted minus drilled (feet)

1-UL-08 Logan 412611100384403 T.17N R.29W S.22ABBB 41.43639 -100.64556 3,038 8/22/2008 800 --2-UL-08 McPherson 413735100540503 T.19N R.31W S.9CCCC 41.62639 -100.90139 3,260 8/25/2008 795 --3-UL-08 Thomas 415840100340803 T.23N R.28W S.9DBBB 41.97778 -100.5685 2,855 8/28/2008 690 --1-UL-09 Grant 415909101314803 T.23N R.36W S.7ABAC 41.98583 -101.53000 3,600 8/4/2009 1,040 3002-UL-09 Hooker 415554101145403 T.23N R.34W S.27CCCD 41.93167 -101.24833 3,370 8/7/2009 920 1353-UL-09 Cherry 421016100233303 T.25N R.26W S.6BADD 42.17111 -100.39250 2,749 8/12/2009 600 51-UL-10 Thomas 415301100421703 T.22N R.29W S.17BBAA 41.88333 -100.70444 3,060 7/27/2010 800 -152-UL-10 Thomas 415848100200803 T.23N R.26W S.9ACCB 41.98000 -100.33556 2,850 7/30/2010 695 -1573-UL-10 Blaine 415043099433103 T.22N R.21W S.26CCBB 41.84528 -99.72528 2,540 8/4/2010 605 -40

1Site identification number assigned by U.S. Geological Survey.2Decimal degrees; North American Datum of 1983 (NAD 83).3Land-surface elevation above National Geodetic Vertical Datum of 1929 (NGVD 29); elevations surveyed to nearest foot.
6 Hydrostratigraphic Interpretation of Test-Hole and Geophysical Data, Upper Loup River Basin, Nebraska, 2008–10

by multiple episodes of erosion. The location of Ogallalafilled paleovalleys has been proposed by previous researchers 

(Swinehart and Diffendal, 1989; Swinehart and others, 1985), 

but may represent only a fraction of the drainage systems that 

existed during Miocene time. Much of the deposition was 

restricted to valleys along drainage systems originating from 

mountains in Wyoming and Colorado (Swinehart and others, 

1985), but deposition may have occurred on broad, low-relief 

plains as well (Swinehart and Diffendal, 1989).

An unconformity of at least 1.5 million years separates 

the Ogallala from the Pliocene Broadwater and the Pleistocene 

Long Pine formations (Swinehart and Diffendal, 1989). These 

sediments, from sources in central Wyoming and northern 

Colorado (fig. 1) (Stanley and Wayne, 1972) are unevenly 

deposited and preserved but contain coarse sand and gravel 

separated by finer-grained deposits covering the Ogallala in 

much of the upper Loup study area. Pliocene and Pleistocene 

fluvial deposits average 50 ft (15 m) in thickness, but can be 

as thick as 300 ft (91 m) (Swinehart and Diffendal, 1989), and 

are in hydrologic connection with the underlying Ogallala. 

The Broadwater and Long Pine Formations, parts of the High 

Plains aquifer, are used as a water source where sufficient 

saturated thicknesses are present.

Much of the Quaternary deposits of gravel, sand, silt, 

and clay resulted from stream and eolian erosion and deposition that heavily reworked the Tertiary units (Swinehart and 

Diffendal, 1989; Weeks and Gutentag, 1988). Quaternary-age 

eolian deposits mantle the entire upper Loup study area except 

for the Middle and North Loup River valleys and southern 

Logan County (Swinehart, 1989). Most eolian deposits are 

mainly above the regional water table and typically are not 

used as a water source (Peterson and others, 2008). Quaternary-age alluvial sand and gravel are found in modern stream 

valleys of the Loup River system and are often used as a water 

source. 

Methods 

Site and Method Selection

This section of the report details the rationale for the 

selection of test-hole and geophysical data-collection locations. Site selection for test-hole drilling and surface geophysical data collection was determined by several factors, but sites 

primarily were chosen to fill existing spatial gaps left in the 

test-hole database (University of Nebraska-Lincoln, Conservation and Survey Division, 2011). Test-hole drilling and 

subsequent monitoring-well installation locations were chosen 

in coordination with the ULNRD to locate nested monitoring 

wells for long-term water-level and water-quality information 

as part of the long-term monitoring plans (Anna Baum, oral 

commun., 2008). Surface geophysical data were collected to 

provide a quick, non-invasive method to fill spatial gaps in 

the existing test-hole coverage. For data collected in 2008 and 

2009, surface geophysical sounding locations were selected 

PERIOD EPOCH STRATIGRAPHIC UNIT LITHOLOGY HYDROSTRATIGRAPHY

Quaternary

Holocene Alluvium and eolian

deposits

Gravel, sand, silt,

and clay

High Plains aquifer

Pleistocene

Long Pine Formation 

and undifferentiated 

alluvial and eolian 

deposits

Tertiary

Pliocene Broadwater Formation Gravel and sand

Miocene

Ogallala Formation Gravel, sand, silt,

and clay

Arikaree Group

Very fine to finegrained sandstone

Oligocene Brule 

Formation

Upper

Siltstone and 

sandstone

Confining unit(s)

Lower

Siltstone and 

claystone

Cretaceous Late Pierre Shale Shale

Modified from Swinehart and Diffendal, 1989, and 

Gutentag and others, 1984 

Figure 3. Stratigraphic, lithologic, and hydrostratigraphic column of the High Plains aquifer and confining units of the upper 

Loup study area.
Methods 7

close to previously drilled or planned test holes to verify that 

these geophysical techniques could identify the base of the 

aquifer reliably. Test-hole drilling and geophysical sounding 

locations also depended on site accessibility and landowner 

permission.

Geophysical methods were selected based on physical 

characteristics of the hydrostratigraphic units of the upper 

Loup study area and the anticipated depth to the base of the 

aquifer. Examination of previously collected CSD borehole 

geophysical logs indicated a sharp contrast in electrical resistivity between the Brule Formation and the overlying units 

of the High Plains aquifer system. After consideration of the 

anticipated depth to the base of the aquifer (top of the Brule 

Formation), two surface geophysical methods were chosen: 

time-domain electromagnetic (TDEM) and audio-magnetotelluric (AMT) methods. These surface geophysical methods 

detect variations in the electrical properties of earth materials 

— in particular, electrical resistivity, or its inverse, electrical conductivity. A lower electrical resistivity corresponds to 

higher porosity or smaller grain size, or both, because of the 

higher surface area associated with fine particles that promote 

the transmission of electrical current (Biella and others, 1983; 

Kwader, 1985). Higher electrical resistivity is associated with 

coarse-grained deposits such as alluvial sand and gravel or 

sandstone. As such, electrical resistivity can be correlated with 

geologic units on the surface and at depth using lithologic logs 

to provide a picture of subsurface geology. The principles of 

operation and field application of each technique are explained 

in detail in the Surface-Geophysical Data Collection section of 

this report. More information regarding the electrical properties of rocks can be found in Keller (1987, 1989), Palacky 

(1987), Hearst and Nelson (1985), Hallenburg (1998), and 

Hearst and others (2000).

The depth of investigation was a consideration in the 

selection of the surface geophysical method. The depth of 

investigation for the TDEM method is variable, but is roughly 

2 to 3 times the size of loop length. The AMT method is able 

to resolve subsurface features from tens of feet to nearly 2,500 

ft (tens of meters to 760 m) in depth. For this reason, use of 

the AMT technique was particularly important in areas where 

the base of aquifer was deepest. Another advantage of the 

AMT technique is the ability to examine the lateral variability 

of the subsurface with depth. In contrast, the TDEM method 

provides a one-dimensional view of the subsurface, which 

can be viewed as a virtual borehole. Further discussion of the 

TDEM and AMT methods and their application is presented in 

the Surface-Geophysical Data Collection section of this report.

Test-Hole Drilling and Borehole Geophysics

Presented in the Test-Hole Lithologic and Borehole Geophysical Logs section and Supplemental Data section of this 

report are the generalized lithologic descriptions and borehole 

geophysical logs for nine test holes drilled to the base of the 

High Plains aquifer (top of Brule Formation). Original copies 

of the lithologic logs of each test hole are filed at the U.S. 

Geological Survey in Lincoln, Nebr. All borehole geophysical logs are archived in accordance with USGS protocol (U.S. 

Geological Survey, 2009). At the present time (2011), final 

stratigraphic interpretations have not been completed. Consequently, the stratigraphic units composing the High Plains 

aquifer are undifferentiated, and only the base of the aquifer at 

each test hole has been reported. 

Test-Hole Drilling Method and Procedure

Test-hole drilling with mud-rotary drilling equipment has 

been an integral part of groundwater and geologic studies in 

Nebraska for many years (Goeke, 2000). Mud-rotary test-hole 

drilling and sampling required the use of drilling fluid suitable for geologic conditions. As the drill stem was advanced, 

typically in 5-ft (1.5-m) increments, the time required to 

advance each increment was recorded along with the drilling 

action. Drill cuttings were circulated to the surface, collected, examined immediately, and lithologically described. 

Described samples were bagged, labeled, and provided to the 

CSD for further examination under a petrographic microscope 

before the final assignment of stratigraphic intervals and 

their publication in county test-hole log books. Although the 

precise contacts of certain hydrostratigraphic units have not 

been determined, logged lithologic characteristics can provide 

information on the hydrostratigraphic units sampled. In many 

test-hole logs published by CSD (Goeke, 2000; Wigley, 2000; 

University of Nebraska-Lincoln, Conservation and Survey 

Division, 2011), the Ogallala is indicated by presence of 

siliceous plant roots (rootlets). In most areas of western and 

north-central Nebraska, late-Tertiary rocks contain numerous 

fossil seeds of grasses and forage herbs. These provide the 

best means to identify Tertiary units from the upper part of the 

Arikaree Group to the top of the Ogallala (Condra and Reed, 

1943). 

Data from the three test holes drilled in 2008 were 

included in the Scientific Investigations Map (SIM) 2008-3042 

for the ELM area (McGuire and Peterson, 2008). Additional 

test holes drilled in 2009 and 2010 have allowed further 

improvements in the interpretations of the base of aquifer and 

will be used to refine the base-of-aquifer configuration used 

for the Phase 3 ELM. Locations of all test holes are shown in 

figure 2, and test-hole locations and dates drilled are listed in 

table 1. 

Borehole Geophysical Data Collection

Borehole geophysical data were collected at each testhole location using a Century 8144 Multi-Parameter electricallog tool (Century Geophysical Corp., 2011a). The Century 

8144 tool measures long-normal [64 in. (1.6 m)] and shortnormal [16 in. (0.41 m)] electrical resistivity, natural gamma 

radiation, spontaneous potential, and fluid resistivity. The 

types of geophysical logs collected are described briefly in the 
8 Hydrostratigraphic Interpretation of Test-Hole and Geophysical Data, Upper Loup River Basin, Nebraska, 2008–10

following paragraphs. Further information regarding borehole 

geophysics can be found in Keys (1990).

Normal-resistivity logs measure the electrical resistivity of sediment, rocks, and water surrounding the borehole. 

Electrical resistivity measurements consisted of long-normal 

[64 in. (1.6 m)] and short-normal [16 in. (0.41 m)], each having different volumes of investigation. Short-normal logs have 

a smaller volume of investigation and, therefore, are affected 

more by the resistivity of the drilling fluid and the invaded 

zone that develops on the borehole wall. Correspondingly, 

long-normal logs have a larger volume of investigation and are 

less affected by drilling fluid. Intervals where these two logs 

diverge indicate areas of permeability where drilling fluid has 

invaded the formation (Anderson and others, 2009).

Natural gamma logs measure natural gamma radiation 

being emitted by the formation surrounding the borehole 

(Keys, 1990). Clay tends to accumulate radioisotopes through 

adsorption and ion-exchange processes. Most clays, particularly illites, have high gamma activity because of the presence 

of potassium in their crystal structure. As such, zones of high 

gamma activity typically are interpreted as being clay rich. 

Volcanic ash, organic shale and feldspathic or arkosic sandstones also can give higher gamma responses (Anderson and 

others, 2009).

Spontaneous-potential logs (often referred to as selfpotential or SP) measure differences in the electrical potentials 

that develop in a borehole at lithologic or water-quality interfaces (Anderson and others, 2009). Differences in electrical 

potential often are caused by differences between the salinity 

of the borehole fluid and that of the formation fluid (Keys, 

1990). When the borehole fluid is fresher than the formation fluid, electric current flow is such that SP will deflect in 

a negative direction; conversely, when the formation fluid is 

fresher than the borehole fluid then electric current flow is 

such that SP will deflect in a positive direction (Stanton and 

others, 2007).

Electrical conductivity (inverse of resistivity) also was 

recorded with an electromagnetic-induction tool, which measures the conductivity of formations and water surrounding 

the borehole. The electromagnetic induction log was collected using a Century 9512 slim-hole induction tool (Century 

Geophysical Corp., 2011b). The tool measured formation 

conductivity within an area 10 to 50 in. (0.25 m to 1.3 m) from 

an open borehole. The tool is not affected by nearby borehole 

fluid or where drilling fluid has invaded the formation. The 

induction tool was calibrated according to specified guidelines (Century Geophysical Corp., 2011b). The induction tool 

was lowered into the borehole and the temperature of the tool 

was allowed to equilibrate with the surrounding formation. 

Before data collection, the induction tool was quickly raised 

to land surface and calibrated “in air” for a zero conductivity 

measurement and using a 690 millimhos per meter (mmho/m) 

conductivity ring.

All test holes were logged with the Century 8143 MultiParameter E-Log tool with the exception of test hole 1-UL-08. 

A wiring problem in the draw works prevented data collection, 

and only the generalized lithologic log is provided. Intermittent problems were encountered with the Century 8144 

multi-parameter tool: negative long-normal resistivity values 

were recorded at some test holes. As a result, the long-normal 

resistivity was not included for the 1-UL-09, 2-UL-10, and 

3-UL-10 borehole logs. All logged test holes were logged to 

a depth below the base of the aquifer with the Century 8143 

multi-parameter tool, with the exception of test hole 3-UL10. An irregularity in the test hole at 3-UL-10 prevented the 

multi-parameter tool from being lowered below the base of the 

aquifer. As a result, resistivity and natural gamma data were 

collected using a Century 9512 electromagnetic induction tool 

below the base of aquifer at that site. 

Surface-Geophysical Data Collection

Time-Domain Electromagnetic

Time-Domain Electromagnetic (TDEM) Method

TDEM is an inductive electromagnetic technique that 

provides a measure of near-surface resistivity by passing a 

current through a wire loop which, as explained by Ampere’s 

law, generates a primary magnetic field. The primary current 

is rapidly turned off, thereby causing a time-varying change 

in magnetic flux, which induces voltages, and hence eddy 

currents, in conductive bodies, according to Faraday’s law. In 

TDEM, a secondary magnetic field is produced by the decay 

of these subsurface eddy currents, and typically is measured 

as a voltage with time after primary current turnoff at one or 

more surface receivers. An apparent resistivity is calculated 

from the measured voltage at the receiver coil and the time 

elapsed after turnoff.

The apparent resistivity is solely a mathematical transform; however, it serves two important purposes. First, 

subtle differences in the measured receiver voltage caused 

by changes in subsurface resistivity are accentuated by the 

power-law dependence of apparent resistivity on elapsed time 

and voltage. Second, the TDEM response of a homogenous, 

isotropic earth is such that the apparent resistivity is time 

dependent, but asymptotic to the true half-space resistivity at 

late times after turnoff. For more complicated, heterogeneous 

earth structure, the variation of apparent resistivity with time 

can be viewed as a proxy for the variation of true-earth resistivity with depth. The actual electrical resistivity distribution 

in the earth is computed from the measured apparent resistivity through the process of inversion. Additional details on the 

TDEM sounding method can be found in Christiansen and 

others (2011), Fitterman and Labson (2005), Danielsen and 

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

Time-Domain Electromagnetic (TDEM) Data Collection

A total of 26 TDEM soundings were collected within 

the upper Loup study area between 2008 and 2010 (fig. 2). In
Methods 9

2008, two soundings were acquired in close proximity to previously drilled CSD test holes. An additional five soundings 

were collected in 2009, three of which were followed up by 

new test holes. The 2010 data-collection plan was expanded to 

19 soundings specifically targeting locations where little information existed. The location and site name for each sounding 

collected are presented in table 2. 

TDEM data were collected using two systems: a Zonge 

NanoTEM/ZeroTEM system was used in 2008, and a Geonics 

ProTEM system was used in 2009–10. All data were collected 

using a 328-ft by 328-ft (100-m by 100-m) square transmitter 

loop. The depth of investigation scales with the transmitter 

moment (number of wire turns times transmitter current times 

transmitter loop area), with a rule of thumb placing the depth 

of investigation at 2–3 times the length of a side of the square 

transmitter loop; therefore, depth investigated for this study 

corresponds to 656 to 984 ft (200 to 300 m). At all locations, 

data were collected in a central-loop configuration where the 

receiver loop was placed at the center of the transmitter loop. 

From 2009 onward, additional data were collected outside of 

the transmitter loop at a distance of 328 ft (100 m) from the 

transmitter loop edge. Out-of-loop data were used as a qualitative check on the assumption of one-dimensionality (layercake earth structure) required for subsequent modeling and 

inversion. Out-of-loop data is recorded as the time derivative 

of the secondary magnetic field or dB/dt, where B is the magnetic induction. The out-of-loop data also were modeled and 

inverted jointly with the central-loop data to better constrain 

the final resistivity models.

Calibration of a TDEM system is essential to an accurate 

recovery of near-surface resistivity structure (Christiansen and 

others, 2011). Calibration, in this context, refers to a particular combination of data logger, transmitter, and receiver, and 

involves the characterization of system filters, the knowledge 

Table 2. Upper Loup time-domain electromagnetic (TDEM) site coordinates, elevation, county, and acquisition year. 

[Northing and easting in Nebraska State Plane coordinates]

Site Latitude1 Longitude1 Northing

(feet)

Easting 

(feet) County Elevation2(feet) Year

SH7 42.28773 -100.81356 895092 1420332 Cherry 3,039 2008

SH8 41.47070 -100.87394 597630 1400990 McPherson 3,192 2008

CN1 41.98464 -101.53369 787331 1223579 Grant 3,587 2009

CN2 41.92688 -101.25098 765054 1300107 Hooker 3,399 2009

CN3 41.43396 -100.65705 583723 1460306 Logan 3,044 2009

CN4 42.16822 -100.39113 850754 1534411 Cherry 2,760 2009

CN5 42.29998 -100.39816 898768 1532725 Cherry 2,823 2009

UL1 42.39735 -101.23127 936364 1307910 Cherry 3,297 2010

UL2 42.38449 -101.36321 932215 1272207 Cherry 3,379 2010

UL3 42.31846 -101.21419 907558 1312117 Cherry 3,385 2010

UL4 42.24699 -101.26374 881714 1298341 Cherry 3,416 2010

UL5 42.29881 -101.20844 900378 1313573 Cherry 3,372 2010

UL6 41.96938 -100.82020 779132 1417436 Thomas 3,103 2010

UL7 42.03190 -100.78048 801807 1428438 Thomas 3,078 2010

UL8 42.02801 -100.82353 800498 1416734 Thomas 3,081 2010

UL9 41.87274 -100.38286 743101 1536176 Thomas 2,807 2010

UL10 41.93864 -100.48038 767239 1509756 Thomas 2,874 2010

UL11 41.79325 -100.32051 714074 1553043 Thomas 2,763 2010

UL12 42.06173 -100.47670 812077 1511005 Thomas 2,937 2010

UL13 42.34067 -100.67524 914059 1457901 Cherry 2,929 2010

UL14 42.25532 -100.69699 883009 1451772 Cherry 2,966 2010

UL15 41.76738 -100.73760 705348 1439262 Thomas 3,105 2010

UL16 41.74462 -100.61729 696799 1472014 Thomas 2,994 2010

UL17 41.92342 -101.36381 764260 1269396 Hooker 3,475 2010

UL18 41.89260 -101.04908 751838 1354877 Hooker 3,258 2010

UL19 41.88077 -100.69763 746561 1450496 Thomas 3,039 2010

1

Decimal degrees; North American Datum of 1983 (NAD 83).

2

Land-surface elevation above National Geodetic Vertical Datum of 1929 (NGVD 29); elevations surveyed to nearest foot.
10 Hydrostratigraphic Interpretation of Test-Hole and Geophysical Data, Upper Loup River Basin, Nebraska, 2008–10

of system geometry, measurement of the transmitted waveform, an assessment of system bias, and correction for timing 

and normalization errors. The ProTEM and Zonge systems 

used in this study were calibrated in 2009 at an established 

test site at Lyngby, Denmark (Geological Institute, 2002a and 

2002b). 

In 2009–10, data were collected at a range of base frequencies (285, 75, 30, 7.5, and 3 hertz [Hz]) using a combination of high- and low-frequency receivers together with 

low- and high-current transmitters. Average current was 2.5 

and 8.0 amperes (A) for the low- and high-current transmitters, giving rise to an average transmitter moment of 270,000 

ampere-square feet (A-ft2) (25,000 A-m2) and 915,000 A-ft2

(85,000 A-m2), respectively. All data were collected with air 

coil receivers with moments of 340 A-ft2 (31.4 A-m2) and 

2,150 A-ft2 (200 A-m2) for high- and low-frequency coils, 

respectively. At each base frequency, individual voltage decay 

curves, corresponding to a single current pulse, were averaged 

over a time interval between 4 and 15 seconds. A minimum of 

20 such readings were made to permit robust error calculation. 

Background noise measurements also were made with each 

receiver at each station by acquiring data with the transmitter 

turned off. Such noise measurements are used during processing to determine the time at which actual data fall below the 

measured noise envelope.

Data acquired with the Zonge system utilized low- and 

high-power transmitters, coupled with a single-turn 16.4-ft 

by 16.4-ft (5-m by 5-m) loop receiver and a TEM/3 induction coil receiver, respectively. Data were recorded at base 

frequencies of 32 and 2 Hz, with average currents of 7 and 

26 A, respectively, giving rise to transmitter moments ranging 

from 322,000 A-ft2 (30,000 A-m2) to 2,800,000 A-ft2 (260,000 

A-m2). At each base frequency, as many as 1,024 transients 

were averaged to form a single data record. A minimum of 20 

data records were collected to permit robust error calculation. 

Noise measurements also were made with each receiver at 

each station by acquiring data with the transmitter turned off.

Time-Domain Electromagnetic (TDEM) Data Analysis

Data analysis for all soundings consisted of data format conversion, statistical analysis and averaging, forward 

modeling, data inversion, and model assessment. Data were 

processed and inverted using the SiTEM data processing 

and Single-Site Electromagnetics Data Inversion (SEMDI) 

software packages (Auken and Nebel, 2001). The SEMDI 

inversion permitted full waveform specification, the modeling 

of system filters, and the incorporation of data errors. Furthermore, SEMDI reported error bounds on inverted parameters 

(layer thicknesses and resistivities). Data were inverted for 

two end-member model classes – minimum-layer models and 

20-layer, smooth or Occam-style, inverse models (Constable 

and others, 1987). The former class of models seeks to fit the 

measured data with as few distinct layers as possible, whereas 

the latter seeks to fit the data with a large number of thin 

layers of fixed thickness under the constraint that the resistivity varies slowly between adjacent layers. Minimum-layer 

models are more appropriate in settings where abrupt changes 

in resistivity are expected, whereas smooth inverse models are 

more realistic when gradual changes are expected. Independent knowledge from geologic mapping, borehole lithology, 

and geophysical borehole logs typically are used to assess 

which of the above model classes may be more appropriate in 

interpreting subsurface structure. 

The modeling and inversion of TDEM data are commonly one-dimensional (1D) analyses. This assumption is 

reasonable given the nature of the regional geology (laterally expansive sedimentary deposits), the compact footprint 

of the TDEM method, and the limited depth of investigation 

(as much as 1,000 ft [305 m]). Furthermore, lithologic and 

stratigraphic logs from surrounding test holes indicate lateral 

correlation across the study area. Finally, with the exception of 

the 2008 soundings, where out-of-loop data were not recorded, 

joint inversion of the in- and out-of-loop data are consistent 

with a 1D subsurface resistivity structure. 

Some geophysical models, including TDEM models, 

exhibit a degree of non-uniqueness. As such, available groundtruth from test-hole logs was needed to model TDEM data 

and constrain interpretations. The non-uniqueness stems from 

inherent resolution limitations, and incomplete and inexact 

data. Thus, for any measured data set, typically a range of 

models can be determined that adequately fit the data. For 

most electrical methods, equivalence exists between models 

with equal conductance (depth-integrated conductivity). Thus, 

for example, a model with a 10 ohm-m, 164-ft (50-m) thick 

layer produces nearly identical measured data as a 33-ft (10-m) 

thick layer with a resistivity of 2 ohm-m. Additionally, the 

TDEM method has difficulty resolving resistive layers because 

of the low current densities induced within them as compared 

to conductive layers. 

Audio Magnetotelluric (AMT)

The Magnetotelluric Method

The magnetotelluric (MT) method is a passive surface 

geophysical technique that uses the Earth’s natural electromagnetic fields to investigate the electrical resistivity structure 

of the subsurface. It does so by measuring time variations in 

the Earth’s natural electric and magnetic fields. Worldwide 

lightning activity at frequencies of about 10,000 to 1 Hz and 

geomagnetic micropulsations at frequencies of 1 to 0.001 Hz 

provide the main source of signals used by the MT method. 

The natural electromagnetic waves propagate vertically in the 

Earth because of the large contrast in the resistivity between 

the air and the Earth, which causes a vertical refraction of the 

electromagnetic wave at the Earth’s surface (Vozoff, 1972). 

The MT method is well-suited for studying complicated geological environments because the electric and magnetic fields 

are sensitive to vertical and horizontal variations in electrical 

resistivity. 
Methods 11

“Skin depth” is considered to be the depth of investigation at a particular frequency, given an apparent resistivity of 

the medium. Skin depth calculations were used during survey 

design to determine what frequencies would be sampled and, 

thus, how long the magnetotelluric fields would be sampled. 

At least 20–30 cycles of the data were measured at the lowest frequencies. The definition of skin depth, δ, in meters, is 

described in equation 1 as,

δ

ωμσ

ρ = ≈ 2 503

f

(1)

where ω is the angular frequency, which is equal to 

2πf, in hertz

μ is the magnetic permeability of the material 

through which the electromagnetic wave is 

passing through, in henries per meter

σ is the electrical conductivity, in siemens per 

meter

ρ is the apparent resistivity, in ohm-meters

f is the frequency, in hertz.

Skin depth is defined as the depth at which the primary magnetic field has fallen off (decreased) by approximately 1/ e, 

where e is a constant equal to 2.718281828.

The natural variations of the Earth’s magnetic and electric field were recorded in two orthogonal, horizontal directions at each MT station as a function of time. The recorded 

time-series signals were used to derive apparent resistivity 

and phase after first converting them to complex cross spectra 

by using fast Fourier transform techniques and least-squares, 

cross-spectral analysis (Bendat and Piersol, 1971). For a twodimensional (2D) Earth, the MT fields can be decoupled into 

transverse electric (TE) and transverse magnetic (TM) modes 

with “transverse” indicating that the field direction is either 

parallel or perpendicular to the geologic strike (fig. 4). For 

this study 2D geology was assumed with the survey transect 

oriented perpendicular to the geologic strike of interest, the 

MT data for the TE mode are for the electric field parallel to 

geologic strike (or perpendicular to the survey transect, Ex on 

figure 4) and the data for the TM mode are for the electric field 

across strike (parallel to the survey transect, Ey on figure 4). 

2D modeling was done to fit both modes simultaneously.

The magnetotelluric response of a resistive body, such as 

an alluvial paleochannel incised into siltstone bedrock, is such 

that the TE mode is relatively insensitive to narrow resistors 

along the strike of the resistive paleochannel, and the TM 

mode is sensitive to narrow resistors oriented along the strike 

of the resistive paleochannel. On the other hand, the magnetotelluric response to the geologic strike of a narrow, electrically 

conductive prism, such as mineralized dikes and veins or 

water flow along faults and joints, is such that the TE mode is 

more sensitive to narrow conductors oriented along strike, and 

the TM mode is relatively insensitive to narrow conductors 

along strike. These factors illustrate the importance of considering that the estimated geologic strike must be accounted for 

when designing magnetotelluric surveys. An introduction to 

the MT method and references for a more advanced understanding are in Dobrin and Savit (1988), and Vozoff (1991). 

The high-frequency range of magnetotellurics (ranging 

from approximately 10 Hz to as much as 100 kilohertz [kHz]) 

is usually called the audio-magnetotelluric (AMT) frequency 

range. With AMT methods, variations in ground electrical 

resistivity can be investigated from tens of feet to nearly 2,500 

ft (tens of meters to 760 m) in depth. Traditionally, the use of 

the AMT technique was limited by a natural paucity of MT 

signals from around 1 to 5 kHz. However, the natural MT 

fields in this band were augmented by providing additional 

signal power with a small battery-powered transmitter (a 

controlled source) that reduces the potential loss of resolution 

at greater depths of investigation. The AMT technique is used 

to image shallow stratigraphy, subsurface channels, alluviumbedrock contacts, and many kinds of bedrock structures 

including those associated with fracture zones.

Audio-Magnetotelluric (AMT) Data Collection

In the summer of 2008, 2009, and 2010, a total of 117 

AMT soundings were acquired in the upper Loup study area 

(fig. 2). To take advantage of the 2D modeling capabilities 

Ex

Ey

STRATEGEM 

transmitter

Hx

Hy

STRATEGEM 

receiver

Ground

stake

Not to scale

Estimated geologic strike 

Figure 4. System layout for audio-magnetotelluric survey using 

STRATAGEM EH-4 system. Hx and Hy represent magnetic field 

receivers, and Ex and Ey represent the orthogonal ends of the 

dipoles used to measure the Earth’s electric field. STRATAGEM 

transmitter usually was placed from 425 to 740 feet away from 

receiver station, depending on local near-surface geology. 

Azimuths and dipole lengths varied by site and are provided in 

tables 3 and 4.
12 Hydrostratigraphic Interpretation of Test-Hole and Geophysical Data, Upper Loup River Basin, Nebraska, 2008–10

Table 3. Summary by site of audio-magnetotelluric (AMT) stations, geometry, location, and elevation for AMT data collected in 2008 

and 2009.

[AMT, audio magnetotellurics; dipole length is 82 feet; azimuth given in degrees from North; northing and easting in Nebraska State Plane coordinates]

Year County AMT 

Station

Azimuth 

of 

Ex Dipole

(fig. 4)

Azimuth 

of 

Ey Dipole

(fig. 4)

Latitude1 Longitude1 Easting

(feet)

Northing

(feet)

Elevation2

(feet)

Site 5-SH-87-AMT

2008 Cherry 18 0 90 42.28383344 100.8143334 1420109.5 893674.5 3,060

17 42.28474989 100.8142223 1420142.7 894008.1 3,056

16 42.28572233 100.8141112 1420176.1 894362.1 3,054

19 42.28666664 100.8138611 1420247.0 894705.5 3,050

20 42.2876111 100.8139444 1420227.7 895049.8 3,041

Site 8-SH-89-AMT

2008 Grant 25 0 90 41.88355554 101.7596113 1161435.2 751684.8 3,802

26 41.8842779 101.7596666 1161425.5 751948.2 3,788

24 41.88511122 101.758889 1161643.3 752247.4 3,780

23 41.88588888 101.7583054 1161807.9 752527.4 3,764

22 41.88669435 101.7578056 1161949.9 752818.0 3,738

Site 28-SH-87-AMT

2008 Hooker 31 0 90 41.84341663 101.0312222 1359521.9 733863.6 3,259

30 41.84455556 101.0316946 1359398.2 734280.0 3,255

29 41.84544448 101.0319724 1359326.4 734604.7 3,246

28 41.84633329 101.0321389 1359284.9 734929.0 3,241

27 41.84722224 101.0321667 1359281.2 735252.9 3,219

Site Grant-AMT

2009 Grant 1 0 90 41.9844302 101.5328736 1223799.7 787251.9 3,587

2 41.98483688 101.5328053 1223820.9 787399.7 3,590

4 41.98528899 101.5327266 1223845.2 787564.0 3,595

Site Hooker-AMT

2009 Hooker 5 0 90 41.92631125 101.2500157 1300365.1 764844.3 3,399

6 41.92676198 101.2499953 1300373.0 765008.4 3,398

7 41.92676198 101.2499953 1300373.0 765008.4 3,398

Site Logan-AMT

2009 Logan 8 0 90 41.43348388 100.6572313 1460255.7 583548.1 3,046

9 41.43396096 100.6572434 1460253.7 583721.9 3,047

10 41.43438465 100.6572306 1460258.4 583876.2 3,044

Site Cherry South-AMT

2009 Cherry 11 270 0 42.16733104 100.3906835 1534529.4 850430.7 2,768

13 42.16738668 100.3912779 1534368.4 850451.7 2,772

14 42.16744259 100.3918722 1534207.4 850472.8 2,776

Site Cherry North-AMT

2009 Cherry 15 0 90 42.29886574 100.397966 1532776.2 898361.6 2,768

16 42.29931619 100.3979518 1532780.8 898525.7 2,772

17 42.29977499 100.3979861 1532772.3 898692.9 2,776

1

Decimal degrees; North American Datum of 1983 (NAD 83).

2

Surveyed land-surface elevation above National Geodetic Vertical Datum of 1929 (NGVD 29).
Methods 13

Table 4. Location and elevation for all audio-magnetotelluric (AMT) stations where 

AMT data were acquired in 2010 along Whitman Road profile, Cherry and Grant 

Counties, Nebr.

[AMT, audio magnetotellurics; dipole length is 50 feet; azimuth is given in degrees from North; 

azimuth of Ey dipole is 90 degrees; northing and easting in Nebraska State Plane coordinates]

AMT 

Station Latitude1 Longitude1 Easting(feet)

Northing

(feet)

Elevation2

(feet)

North segment

61 42.04364 101.5237959 1226646.8 808773.7 3,657

62 42.04588 101.5243195 1226519.0 809592.1 3,660

63 42.04804 101.5251784 1226299.7 810383.2 3,605

64 42.05 101.5266461 1225913.9 811106.7 3,621

65 42.05187 101.5279643 1225568.0 811791.3 3,618

66 42.05393 101.5288684 1225335.9 812549.4 3,597

67 42.05629 101.5289858 1225319.2 813408.5 3,597

68 42.05859 101.5289924 1225332.2 814244.9 3,644

69 42.06091 101.5291208 1225312.3 815090.9 3,681

70 42.06311 101.5296919 1225171.5 815896.0 3,663

71 42.06542 101.5299648 1225112.3 816738.1 3,615

72 42.06776 101.5301544 1225075.9 817590.4 3,641

73 42.07004 101.5305474 1224984.0 818425.6 3,643

74 42.07218 101.5311279 1224840.2 819204.5 3,617

75 42.07428 101.5310785 1224867.2 819972.2 3,599

76 42.07643 101.5303421 1225080.9 820752.0 3,635

77 42.07874 101.5297324 1225261.2 821587.2 3,665

78 42.08081 101.530794 1224986.5 822348.1 3,632

79 42.0817 101.5335133 1224254.4 822687.0 3,621

80 42.08336 101.5360455 1223578.0 823302.1 3,619

81 42.08533 101.5376231 1223162.7 824028.5 3,621

82 42.0875 101.5396442 1222628.4 824829.0 3,618

83 42.0898 101.5400743 1222526.6 825667.3 3,617

84 42.09189 101.5390941 1222806.1 826425.0 3,644

85 42.0937 101.5372917 1223306.8 827073.8 3,636

86 42.09535 101.535375 1223837.5 827667.1 3,598

87 42.09739 101.5339205 1224245.3 828403.4 3,598

South segment

1 41.98585 101.5298516 1224630.1 787753.7 3,597

2 41.98494 101.5273654 1225299.9 787410.7 3,595

3 41.98335 101.5249368 1225949.7 786821.5 3,587

4 41.98173 101.5226156 1226570.1 786218.9 3,575

5 41.97991 101.5208419 1227040.5 785548.8 3,578

6 41.97891 101.5179179 1227828.8 785171.2 3,588

7 41.97686 101.5159541 1228349.4 784413.2 3,636

8 41.97524 101.5137424 1228940.2 783813.5 3,644

9 41.97371 101.51262 1229235.5 783249.6 3,639

10 41.9715 101.5116766 1229477.9 782442.4 3,594

11 41.96932 101.5113008 1229566.1 781643.2 3,574
14 Hydrostratigraphic Interpretation of Test-Hole and Geophysical Data, Upper Loup River Basin, Nebraska, 2008–10

Table 4. Location and elevation for all audio-magnetotelluric (AMT) stations where 

AMT data were acquired in 2010 along Whitman Road profile, Cherry and Grant 

Counties, Nebr.—Continued

[AMT, audio magnetotellurics; dipole length is 50 feet; azimuth is given in degrees from North; azimuth of Ey dipole is 90 degrees; northing and easting in Nebraska State Plane coordinates]

AMT 

Station Latitude1 Longitude1 Easting(feet)

Northing

(feet)

Elevation2

(feet)

12 41.9674 101.5097291 1229981.1 780936.5 3,579

13 41.96554 101.5081837 1230389.4 780252.6 3,580

14 41.9636 101.5064909 1230837.3 779539.6 3,580

15 41.96082 101.5047651 1231288.8 778518.5 3,579

16 41.95883 101.503541 1231609.0 777787.6 3,576

17 41.95668 101.5031186 1231710.2 777001.7 3,579

18 41.95444 101.5028381 1231772.3 776185.8 3,580

19 41.95225 101.5020287 1231978.5 775384.7 3,580

20 41.94993 101.5016359 1232070.6 774536.4 3,579

21 41.94811 101.4996466 1232600.0 773863.8 3,599

22 41.94617 101.4982199 1232975.7 773150.0 3,629

23 41.94397 101.4973738 1233191.9 772345.8 3,639

24 41.9425 101.4950969 1233801.8 771798.7 3,633

25 41.9402 101.4941388 1234047.9 770958.8 3,621

26 41.93817 101.4929981 1234345.3 770211.3 3,605

27 41.93645 101.4916405 1234703.7 769579.2 3,600

28 41.9345 101.4911303 1234830.2 768865.8 3,614

29 41.93234 101.4911791 1234803.4 768081.6 3,616

30 41.93025 101.492147 1234527.0 767324.1 3,586

31 41.92814 101.4912687 1234752.6 766549.5 3,579

32 41.92628 101.4895321 1235213.3 765866.2 3,576

33 41.92405 101.4886854 1235429.6 765048.8 3,577

34 41.92155 101.4898063 1235109.0 764143.1 3,595

35 41.91959 101.4918042 1234553.2 763438.0 3,624

36 41.91741 101.4923704 1234385.5 762648.9 3,656

37 41.91528 101.4931074 1234171.6 761875.7 3,612

38 41.91321 101.4946908 1233727.8 761129.3 3,578

39 41.98457 101.5276408 1225222.7 787278.6 3,589

40 41.98598 101.5302556 1224521.2 787804.9 3,599

41 41.9885 101.5305121 1224467.7 788722.5 3,650

42 41.991 101.5300194 1224617.7 789629.5 3,674

43 41.99318 101.5296355 1224736.1 790424.5 3,614

44 41.99517 101.5282896 1225114.6 791140.7 3,596

45 41.99724 101.5274903 1225345.1 791891.2 3,601

46 42.00016 101.5275212 1225355.5 792953.9 3,683

47 42.00199 101.5278838 1225268.8 793625.2 3,695

48 42.004 101.5280719 1225230.6 794356.1 3,619

49 42.00711 101.5279292 1225289.4 795488.2 3,592

50 42.00931 101.5284151 1225171.6 796293.6 3,626

51 42.0113 101.5303537 1224657.8 797028.7 3,687
Methods 15

afforded by the magnetotelluric technique, short AMT profiles 

using three to five stations were recorded at or near each location (tables 3 and 4; fig. 2). AMT data in 2008 were nominally 

acquired every 328 ft (100 m), approximately 164 ft (50 m) 

in 2009, and 656 ft (200 m) in 2010 using the arrangement 

shown in figure 4. The exact survey locations and short-profile 

orientation were selected based on either (a) being in the vicinity of the borehole sites that were being studied in 2008 or, 

(b) in 2009, were directed away from cultural electromagnetic 

interference features, such as electrically powered center-pivot 

irrigation systems, fences, pipelines, communication lines, 

railways, and other manmade conductors. Buried power lines 

along roads resulted in some stations having to be relocated. 

A summary of site location and AMT system geometry for 

each AMT station where data were collected in 2008 and 2009 

is provided in table 3. The geologic strike was unknown at 

specific survey locations, but was postulated from paleodrainage patterns from Swinehart and Diffendal (1989) and other 

sources. In 2010, the Whitman Road profile in Cherry and 

Grant Counties (fig. 2) was designed to be perpendicular to the 

assumed general strike of the subsurface paleochannel features 

under investigation. The AMT stations, system geometry, 

location, and surface elevation for each AMT station along the 

Whitman Road profiles are listed in table 4.

All AMT surveys in the upper Loup study area used a 

Geometrics EH-4 STRATAGEM system (Geometrics, 2007). 

A schematic of the EH-4 STRATAGEM system as used in 

this study is shown in figure 4. The STRATAGEM transmitter 

complements the natural signal in the range of 1 to 72 kHz. 

The STRATAGEM does not measure the vertical component 

of the magnetic field. Electric field components, Ex and Ey, 

were measured using long multi-strand, single-wire dipoles 

having lengths of either 50 ft (15 m) or 82 ft (25 m), and magnetic fields were measured using EMI BF-6 high-frequency 

magnetic induction coils (Geometrics, 2007). 

Audio-Magnetotelluric (AMT) Data Analysis

Electric and magnetic field time-series data were acquired 

and field processed by fast Fourier transforms into spectral 

crosspowers (Jiracek, 2011) in real time by the STRATAGEM 

acquisition software. This allowed for immediate quality control of the acquired data in the field. During post processing 

the recorded AMT time-series data were transformed to the 

frequency domain and processed to determine a 2D apparent 

resistivity and phase tensor (Jiracek, 2011) at each station. 

Time-series data sets were selected based on optimal signalto-noise characteristics before the crosspower calculations. 

Noisy data in the time series, spectral, and resistivity data were 

culled at this stage. Crosspower and MT impedance data files 

were created with the STRATAGEM data-acquisition program 

(IMAGEM) and used as input to the 2D inversion-modeling 

program. Because the STRATAGEM is a single-station 

system, data acquisition cannot be acquired simultaneously at 

more than one location. 

During the 2D analysis and interpretation process, each 

station was rotated to a fixed angle defined by the perpendicular direction to the nominal profile orientation. Rotation of the 

impedance tensor (Jiracek, 2011) into this coordinate system 

thus allows for decoupling into the quasi-TE and quasi-TM 

modes. The true TM and TE modes are uniquely determined 

by the geologic strike, which in most cases is unknown at the 

survey location.

After data collection and field processing, 2D forward 

and inverse modeling of the AMT data was performed. 

Table 4. Location and elevation for all audio-magnetotelluric (AMT) stations where 

AMT data were acquired in 2010 along Whitman Road profile, Cherry and Grant Counties, 

Nebr.—Continued

[AMT, audio magnetotellurics; dipole length is 50 feet; azimuth is given in degrees from North; azimuth 

of Ey dipole is 90 degrees; northing and easting in Nebraska State Plane coordinates]

AMT 

Station Latitude1 Longitude1 Easting(feet)

Northing

(feet)

Elevation2

(feet)

52 42.01294 101.5324714 1224093.1 797635.3 3,693

53 42.01503 101.5341726 1223644.5 798404.0 3,663

54 42.01719 101.5338841 1223736.8 799188.9 3,611

55 42.0194 101.5328128 1224042.1 799989.5 3,613

56 42.02179 101.5324965 1224143.4 800856.5 3,658

57 42.02396 101.532595 1224130.7 801650.0 3,638

58 42.02608 101.5337783 1223823.0 802426.9 3,599

59 42.02813 101.5344033 1223666.5 803176.1 3,603

60 42.02988 101.5362004 1223189.8 803823.1 3,600

1

Decimal degrees; North American Datum of 1983 (NAD 83).

2

Surveyed land-surface elevation above National Geodetic Vertical Datum of 1929 (NGVD 29).
16 Hydrostratigraphic Interpretation of Test-Hole and Geophysical Data, Upper Loup River Basin, Nebraska, 2008–10

Wannamaker (1983) determined that although some MT 

responses are fundamentally three-dimensional (3D) in 

nature, for elongated structures 2D modeling could be used to 

construct reasonable estimates of the resistivity cross sections 

along each profile. Wannamaker and others (1984) demonstrated that approximating a 3D structure beneath a centrally 

located profile with 2D modeling is best achieved when fitting 

the TM curve, even at the expense of a poor fit of the TE 

curve. However, because TM data are relatively insensitive to 

the vertical extent of a subsurface conductive body (EberhartPhillips and others, 1995), the depths to the base of the bodies 

in the model were not well constrained. Hence, mixed-mode 

analysis (modeling the TM and TE mode together) was used to 

help clarify the modeling results. Mixed-mode analysis commonly is referred to as TMTE mode and is an average of all 

data in orthogonal directions.

Two-dimensional inverse-resistivity models were constructed for each of the short profiles acquired at each AMT 

station. Not all soundings were included in each inversion if 

an unusual amount of cultural noise (for example, center-pivot 

irrigation systems) was observed. 

First, 2D inversions of the AMT data were conducted 

using the computer program, RLM2DI (Mackie and others, 

1997; Rodi and Mackie, 2001) within GEOTOOLS (Geotools, 

1998), a shell program specifically designed to process and 

facilitate interpretation of MT and AMT data. RLM2DI uses 

a finite-difference network analog to the Maxwell’s equations 

governing magnetotellurics (Jiracek, 2011) to calculate the 

forward solution. A nonlinear conjugate gradient optimization approach (Rodi and Mackie, 2001) is applied directly 

to the minimization of the objective function for the inverse 

problem. 

Inversion was followed by the application of the 2D 

forward-modeling program, PW2D, developed by Wannamaker and others (1987). The results of the RLM2DI twodimensional inversion were used as the initial input model for 

the forward modeling (PW2D) where a sensitivity analysis 

was performed on the conductive structures derived from the 

inversion results. PW2D is a finite-element algorithm that 

simulates transverse electric and magnetic fields across each 

finite element (Wannamaker and others, 1987). The number 

of iterations of forward modeling (PW2D) depended on how 

complex the profile inversion results were from RLM2DI. 

Hydrostratigraphic Interpretation Of 

Test-Hole And Surface-Geophysical 

Data 

Test-Hole Lithologic and Borehole Geophysical 

Logs

Generalized lithologic descriptions and selected borehole 

geophysical logs for all test holes are presented in figures 5 

through 13. An example test hole with generalized lithologic 

description and borehole geophysical logs is shown in figure 5. 

Logs from all other test holes (figs. 6–13) can be found in the 

Supplemental Data section of this report. Borehole geophysical logs include data for long- and short-normal resistivity, 

natural gamma activity, and spontaneous potential. Examination of the resistivity data collected from the borehole logs in 

comparison with the lithologic test-hole data has enabled the 

following interpretations of geophysical data. 

The base of aquifer is indicated by the top of the siltstone 

unit of the Brule Formation in the generalized lithologic log. 

The long- and short-normal resistivity logs indicate the base of 

the principal aquifer by sharp contrasts in resistivity between 

the Brule Formation and the overlying units. Examining the 

long- and short-normal resistivity, the sediments overlying 

the Brule Formation range from less than 50 to greater than 

250 ohm-m. Resistivity for the Ogallala (units containing 

rootlets) ranged between 50 to 200 ohm-m (and more typically between 75 to 150 ohm-m), with higher resistivity values 

indicating the presence of coarser deposits or a greater degree 

of cementation of the sandstone units. Coarse sand and gravel 

deposits typically had resistivity ranges for 150 ohm-m to 

more than 250 ohm-m. In contrast, the resistivity of the Brule 

Formation typically was less than 25 ohm-m and, in most 

cases, less than 15 ohm-m. 

Other logs provide further support for the base-of-aquifer 

interpretation. Natural gamma logs indicate an increase in 

activity near the base of the aquifer because of increased 

volcanic ash content of the Brule Formation. Spontaneouspotential logs can indicate the degree to which the drilling 

fluid invaded the formation, and the relative impermeability of 

the Brule Formation typically results in a positive deflection 

indicating that minimal drilling fluid invaded this interval. 

Differences in the elevation of the base of the principal 

aquifer for test holes drilled after the completion of SIM 2008-

3042 (McGuire and Peterson, 2008) are presented in table 

1. The range of differences between the elevations of the 

drilled base of aquifer and the predicted base of aquifer was 

from -157 to 300 ft (-48 to 91 m). In test holes 1-UL-09 and 

2-UL-09, within the western part of the upper Loup study area, 

the test-hole base-of-aquifer elevation was more than 100 ft 

lower than that predicted by McGuire and Peterson (2008). 

This may indicate that the test holes were drilled into paleovalleys eroded into the Brule Formation. Furthermore, both of 
Hydrostratigraphic Interpretation Of Test-Hole And Surface-Geophysical Data 17

Short-normal resistivity (16 in.) Generalized lithology

0 Ohm-m 300

Long-normal resistivity (64 in.)

0 Ohm-m 300

Spontaneous-potential

0 Millivolts 100

Gamma

0 250

Depth,

in feet

100

200

300

400

500

600

700

800

0

API units

Sand, very fine to medium; 

some silt

Gravel, some sand

Very fine to medium sand

and sandstone, some silt,

rootlets

Silt, siltstone, trace clay

Silty very fine to fine sand

and sandstone, some

interbedded medium sand

Brule Formation silt/siltstone

Figure 5. Composite of generalized lithologic description and geophysical logs for test hole 2-UL-08, McPherson County, Nebr. Shortnormal resistivity, in ohm-meters; long-normal resistivity, in ohm-meters; natural gamma activity, in American Petroleum Institute units; 

spontaneous potential, in millivolts.
18 Hydrostratigraphic Interpretation of Test-Hole and Geophysical Data, Upper Loup River Basin, Nebraska, 2008–10

these test holes lie in close proximity to the proposed location 

of paleovalleys in Grant and Hooker Counties (table 1; fig. 2) 

(Swinehart and Diffendal, 1989).

Researchers have indicated that Pliocene-age or younger 

gravel covers much of the upper Loup study area (Swinehart 

and Diffendal, 1989). Gravel overlying intervals of sandstone 

containing rootlets was encountered in every test hole with the 

exception of 1-UL-09 (fig. 8) and 3-UL-09 (fig. 10). Although 

the age was not determined for this study, the authors infer by 

superposition that these sand and gravel units are of Miocene 

age or younger. At the location of test hole 1-UL-09, Miocene 

or younger sand and gravel were absent and only finer-grained 

deposits overlie sand and sandstone containing siliceous 

rootlets. At test hole 3-UL-09, rootlets were encountered 

10 ft below land surface, which is indicative of the Ogallala 

Formation of Miocene age. It can be inferred that if Miocene 

or younger gravel had existed at in this location, it would have 

been eroded and reworked by the modern North Loup River 

(fig. 2; table 1). Where present, thicknesses of sand and gravel 

deposits were less than 50 ft with the exception of test hole 

3-UL-10 (fig. 13). Here, sand and gravel deposits were nearly 

75 ft (23 m) thick. 

TDEM Survey Results

TDEM Models 

Figures 14 through 27 present the measured data from the 

26 TDEM stations together with best-fit inverse models. An 

example sounding is shown in figure 14. All other soundings 

(figs. 15–27) are located in the Supplemental Data section of 

this report. At each station, the left panel shows the forward 

responses whereas the right panel shows the best-fit minimumlayer (red) and smooth (blue) inverse model. The right panel, 

additionally, shows the forward responses of the two inverse 

models. For all soundings, the central-loop data are plotted as 

apparent resistivity. Out-of-loop data for the secondary magnetic field, which were collected for all soundings collected 

1,300

1,200

1,100

1,000

900

800

700

600

500

400

300

200

100

0

Depth, in feet

Depth, in meters

Apparent resistivity, in ohm-meters

Time, in seconds

dB/dt (time derivative of the out-of-loop secondary magnetic field), 

in volts per square meter

100

200

300

400

150

250

350

50

10 0 -5

10-6

10-7

10-8

10-9

10-10

10-11

103

102

101

100

CN5, r.m.s. =

 0.85 (m), 0.72 (s)

285

75

30

30, 7.5, and 3—Collected from high-current transmitter

Repetition frequency data, in hertz

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Root mean square error—

 Minimum-layer model (m) 

 and smooth model (s), in 

 ohm-meters

r.m.s. 

Best-fit smooth inverse model

 Vertical error bar indicates variance of data point 

 with mean at center

Resistivity, in ohm-meters

10 1 10 100 1,000 -5 10-4 10-3 10-2 10-1

A B

Figure 14. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (time-derivative, right axis), and time; B, 

depth below land surface and subsurface resistivity modeled from TDEM sounding for site CN5 (table 2).
Hydrostratigraphic Interpretation Of Test-Hole And Surface-Geophysical Data 19

in 2009 and 2010, are plotted as the time derivative (dB/dt) in 

volts per square meter. In most cases the model response falls 

within the measured data errors; exceptions are for the outof-loop vertical magnetic field data, which over conductive 

ground suffers a sign change at early times. This sign change 

is associated with passage of time for the peak current density 

outward from the transmitter loop, and is extremely sensitive 

to the position of the receiver relative to the transmitter loop.

In general, data quality was high, reflecting the low 

density of human population, and subsequent low level of 

cultural noise (power lines, pipelines, well pumps, electric 

fences) within the survey area. A small number of soundings 

suffered from cultural noise, identified as oscillatory behavior 

in the vertical component of the secondary magnetic field (dB/

dt) decay curves. At these locations data quality was compromised at late times; however, removal of the late-time data 

resulted in interpretable sounding data, albeit with a reduced 

depth of investigation.

Time-Domain Electromagnetic (TDEM) Model 

Uncertainty

The TDEM data were processed and inverted using 

the SiTEM data processing and SEMDI software packages 

(Auken and Nebel, 2001). For a detailed discussion on TDEM 

noise and inversion calculation of the root-mean-square 

(r.m.s.) errors in the SiTEM program refer to Effersø and 

others (1999). The TDEM models in almost all cases reproduce the measured data to within estimated measurement 

(r.m.s. error is less than 1 ohm-meter). The r.m.s. error for all 

soundings collected for this report are indicated in figures 14 

through 27. In general, the r.m.s. error is slightly larger for the 

minimum-layer models, than for the smooth models, reflecting 

the greater difficulty in fitting the data with a small number of 

model parameters. Average errors were, however, similar for 

the smooth and minimum-layer models, suggesting that the 

minimum-layer parameterization can adequately represent the 

subsurface resistivity structure, and by inference, the hydrostratigraphy. This is consistent with the previously known 

geologic history, with a number of (sharp) unconformities 

associated with erosion and changes from marine to alluvial to 

eolian deposition.

Time-Domain Electromagnetic (TDEM) Model 

Interpretation 

The data generally supported a 3–5 layer model for resistivity stratigraphy, and beyond 300–400 ft (91–120 m) depth 

exhibited decreasing resistivity with increasing depth. Within 

the upper Loup study area, some spatial trends were observed 

in the deep resistivity structure, with greater-than-average 

resistivities observed along a northwest to southeast trend axis 

through parts of southwest Cherry, Grant, Hooker, Thomas, 

and McPherson Counties (fig. 28). Although the Brule Formation is expected beneath this entire area, the TDEM data, at 

later times, reflect a volume average of the deeper stratigraphic 

section. Thus, although the depth to the conductive interface 

reflects the top of the Brule Formation, the apparent resistivity 

itself reflects an average of the Brule Formation and underlying units (for example, Pierre Shale of Cretaceous age). The 

greater resistivities observed along this northwest-southeast 

corridor are consistent with thinning or removal of the Pierre 

Shale near the Chadron Arch, and suggest that the TDEM 

data are sensing (but not imaging) structural changes within 

the Cretaceous section. Some of the greatest resistivity values 

occurred along the eastern flank of the Chadron Arch and were 

more than double the average base resistivity of 11 ohm-m.

The models in figures 14 to 27 show conductors extending to 1,300 ft (400 m) deep; however, the depth of investigation is, in most cases, less than this. The TDEM models do 

accurately recover the top of this “basement” conductor and 

its conductivity; however, the greater conductance of the Brule 

Formation or underlying Pierre Shale precludes defining stratigraphic contacts much below the Ogallala-Brule interface. The 

models should, thus, be viewed as a proxy for the geologic 

structure for the section extending to the depth at which the 

conductive horizon is encountered.

Assuming that this conductive horizon represents the 

base of the High Plains aquifer, the base-of-aquifer elevation was examined, as inferred from the TDEM models, and 

compared to the base of aquifer as determined from test-hole 

derived mapping (McGuire and Peterson, 2008) (fig. 29). An 

eastward decrease in the base-of-aquifer elevation is evident in 

both data sets, reflecting the southeast dip of the stratigraphic 

column. With few exceptions, the base of aquifer inferred 

from the TDEM models indicates close correspondence with 

the test-hole-derived base of aquifer. Outliers occur in western 

Hooker (site UL17; inferred base-of-aquifer elevation 3,181 ft) 

and southern Cherry Counties (site UL4; inferred base-ofaquifer elevation 2,977 ft), where modeled soundings suggest 

a shallower (higher-elevation) conductor than either the testhole data or nearby TDEM models indicate.

In general, greater variation in the base-of-aquifer elevations is observed in the west. This reflects a number of deep 

paleovalleys incised into the Brule Formation, some of which 

have been identified from test-hole data. This is evident at 

site CN1 in Grant County, where the TDEM model indicated 

a base-of-aquifer elevation nearly 400 ft (120 m) deeper than 

McGuire and Peterson (2008) (fig. 29; table 2). The drilled 

base-of-aquifer elevation from nearby test-hole 1-UL-09 was 

300 ft (91 m) deeper than predicted, and thus, was in general 

agreement with the TDEM estimates. 

The topology of the base-of-aquifer surface, in particular 

the configuration of paleochannels, is of primary importance 

in understanding groundwater flow. The depth to the base of 

the aquifer (fig. 30), however, provides an estimate of total 

aquifer thickness and is important in assessing groundwater 

supply. The depth to the base of the High Plains aquifer varies 

by more than a factor of two within the surveyed area. The 

thickest section falls within Grant, Hooker, and Thomas Counties, with 900 ft (270 m) or more between the land surface and 

the base of the aquifer. 
20 Hydrostratigraphic Interpretation of Test-Hole and Geophysical Data, Upper Loup River Basin, Nebraska, 2008–10

100° 101° 102°

42°30'

42°

3 41° 0'

SHERIDAN

CHERRY BROWN ROCK

GRANT

HOOKER

THOMAS BLAINE

ARTHUR MCPHERSON LOGAN CUSTER

LOUP

GARDEN

LINCOLN

River

Snake

Mud Creek

Loup

River

North Branch Middle Loup River

M di dle Branch Middle

Dismal

River

South

Fork

Di ms al River

West Birdwood

Creek

North Fork Birdwood Cr Birdwood

Cr

South Loup

River

North

Middle

Loup

Loup

River

River

Goose

Creek

Calamus

River

Plum

Cr

Cr

Long Pine

North

Platte

River

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

6

7.5

5.6

7.2

7.6

9.8

8.7

6.4

3.3

5.5

9.9

7.3

12

12

17

10

12

12

12

27

11

14

29

20

11

13

Chadron Arch

Kn

Kb

Kp

Kd

Kn

Kd

Kn

Base from U.S. Geological Survey digital data, 2002, 1:2,000,000Lambert Conformal Conic projectionStandard parallels 40oN and 43oN, central meridian 100oWHorizontal coordinate information is referenced to the North AmericanDatum of 1983 (NAD 83)

0

0

10

20 KILOMETERS 10

20 MILES

EXPLANATION

Time-domain electromagnetic (TDEM)  sounding—Number indicates the  resistivity at the inferred base of  aquifer, in ohm-meters

! 11

Cretaceous-age uplift structures and underlying Cretaceous-unit contacts from Swinehart and Diffendal, 1989

Stratigraphic unit 

Upper Loup

study area

Elkhorn-Loup 

model area

NEBRASKA

Chadron

Arch

Niobrara FormationBenton GroupDakota GroupPierre Shale

KnKbKdKp

Stratigraphic unit boundary

Figure 28. Resistivity at the inferred base of High Plains aquifer.
Hydrostratigraphic Interpretation Of Test-Hole And Surface-Geophysical Data 21

100° 101° 102°

42°30'

42°

3 41° 0'

SHERIDAN

CHERRY BROWN ROCK

GRANT

HOOKER

THOMAS BLAINE

ARTHUR MCPHERSON LOGAN CUSTER

LOUP

GARDEN

LINCOLN

River

Snake

Mud Creek

Loup

River

North Branch Middle Loup River

M di dle Branch Middle

Dismal

River

South

Fork

Di ms al River

West Birdwood

Creek

North Fork Birdwood Cr Birdwood

Cr

South Loup

River

North

Middle

Loup

Loup

River

River

Goose

Creek

Calamus

River

Plum

Cr

Cr

Long Pine

North

Platte

River

3,100

3,000

2,900

2,800

2,400

2,500

2,000

2,000

2,800

2,900

3,000

2,700

2,700

2,600

2,400

2,300

2,200

2,100

1,800

1,900

3,100

3,000

2,900

2,800

2,400

2,500

2,000

2,000

2,800

2,900

3,000

2,700

2,700

2,600

2,400

2,300

2,200

2,100

1,800

1,900

3,100

3,100

3,200

2,000

3,000

2,400

2,400

2,200

2,300

2,100

1,900

1,900

1,800

1,900

1,800

1,800

1,800

1,800

1,900

1,700

2,500

2,600

3,100

3,100

3,200

2,000

3,000

2,400

2,400

2,200

2,300

2,100

1,900

1,900

1,800

1,900

1,800

1,800

1,800

1,800

1,900

1,700

2,500

2,600

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

2,534

2,607 

2,488

2,513 2,977 2,387

2,166

2,299

2,232

1,932

2,000

1,998

1,807

2,035

2,166 2,138

2529

2140

2,206 2,317

2,403

3,181

2,442

2,210

2,148

2,297

Upper Loup

study area

Elkhorn-Loup 

model area

NEBRASKA

Base from U.S. Geological Survey digital data, 2002, 1:2,000,000Lambert Conformal Conic projectionStandard parallels 40oN and 43oN, central meridian 100oWHorizontal coordinate information is referenced to the North AmericanDatum of 1983 (NAD 83)

0

0

10

20 KILOMETERS 10

20 MILES

EXPLANATION

Time-domain electromagnetic (TDEM) 

sounding—Number indicates inferred base-of-aquifer elevation for minimumlayer inverse TDEM model; datum is NGVD 29, in feet

Base of aquifer contour—Shows elevation

of base of aquifer. Contour interval 100 feet. Datum is NGVD 29, in feet

3,200

! 2,534

TDEM models and countours from McGuire and Peterson, 2008

Figure 29. Inferred base-of-aquifer elevation from minimum-layer inverse time-domain electromagnetic (TDEM) models and contours derived primarily from test-hole data.
22 Hydrostratigraphic Interpretation of Test-Hole and Geophysical Data, Upper Loup River Basin, Nebraska, 2008–10

100° 101° 102°

42°30'

42°

3 41° 0'

SHERIDAN

CHERRY BROWN ROCK

GRANT

HOOKER

THOMAS BLAINE

ARTHUR MCPHERSON LOGAN CUSTER

LOUP

GARDEN

LINCOLN

River

Snake

Mud Creek

Loup

River

North Branch Middle Loup River

M di dle Branch Middle

Dismal

River

South

Fork

Di ms al River

West Birdwood

Creek

North Fork Birdwood Cr Birdwood

Cr

South Loup

River

North

Middle

Loup

Loup

River

River

Goose

Creek

Calamus

River

Plum

Cr

Cr

Long Pine

North

Platte

River

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

845.1 689.6

897

858.9

439

651.9

629.9

799.9

591.2

828.1

936.7

727.7

999.7

876 833

955.1

967.2

828.1

784.4

868.4

294.3 996.4

940.9

1145

662.7

904.5

Base from U.S. Geological Survey digital data, 2002, 1:2,000,000Lambert Conformal Conic projectionStandard parallels 40oN and 43oN, central meridian 100oWHorizontal coordinate information is referenced to the North AmericanDatum of 1983 (NAD 83)

0

0

10

20 KILOMETERS 10

20 MILES

EXPLANATION

! Time-domain Electromagnetic (TDEM) 

sounding—Number indicates estimated depth from land surface to inferred base of aquifer from minimum-layer inverse TDEM model; datum is NGVD 29, in feet

845.1

Upper Loup

study area

Elkhorn-Loup 

model area

NEBRASKA

Figure 30. Inferred depth from land surface to base of High Plains aquifer from minimum-layer time-domain electromagnetic (TDEM) inverse models. The thickest part of the aquifer within the study area is located in Grant, Hooker, and Thomas Counties.
Hydrostratigraphic Interpretation Of Test-Hole And Surface-Geophysical Data 23

Audio-Magnetotelluric (AMT) Survey Results

2008 and 2009 Two-Dimensional Inversion 

Modeling Results

In 2008, a series of short reconnaissance profiles were 

surveyed near previously drilled CSD test holes shown in 

figure 2 and listed in table 3. An example profile is presented 

in figure 31. These inversion results illustrate the variability in 

subsurface resistivity over short lateral distances of a few hundred feet where zones of resistive material (the warm colors in 

fig. 31) alternate with zones of more conductive material (cool 

colors) in the profile. This profile indicates that there is variability not only at the base of aquifer, but also within the High 

Plains aquifer itself. 

To model AMT data and accurately image subsurface 

features with short reconnaissance profiles, the geologic strike 

of the contact at the base of aquifer must be known. At the 

time of data collection the true strike direction of subsurface 

structures, including the base-of-aquifer contact was unknown 

at the survey locations. A regional approximation of strike was 

used but is not adequate when collecting short reconnaissance 

profiles. As a result, visual depictions of the mixed-mode 2D 

inversion results potentially are misleading and, therefore, are 

not presented in this report. For the locations surveyed in 2008 

and 2009, a pilot survey needs to be completed first to assess 

the actual strike direction of subsurface features before collecting a profile, so that it can be inverted and used for interpretation. It also was determined from the 2008 and 2009 data 

that short profiles are not the best way to apply this technique 

when only a regional estimate of subsurface features, including the base-of-aquifer strike direction, is applied. 

2010 Two-Dimensional (2D) Inversion Modeling 

Results

In 2010, AMT data were not collected as short reconnaissance profiles; rather data were collected along an 

approximately 12-mi-(19-km-)long profile along Whitman 

Road, which runs approximately north-south in Grant and 

Cherry Counties (figs. 2 and 32). This location was selected 

for resurvey because test hole 1-UL-09 indicated the base-ofaquifer topography and the surface was poorly constrained in 

this area. AMT stations, listed in table 4, were spaced approximately 656 ft (200 m) apart along the road. Data were not 

acquired within the town limits of Whitman because of the 

presence of substantial electromagnetic interference. The data 

were divided into northern (A to A’) and southern (B to B’) 

segments. The northern segment consists of data acquired 

north of Whitman, whereas the southern segment consists of 

data acquired south of Whitman. The results of the 2D inversion modeling are presented in figures 33 and 35 as vertically 

exaggerated 2D cross sections and in figures 34 and 36 as 3D 

views shown from two different oblique aerial viewpoints to 

depict the sinuosity of the actual profile line.

The inverted 2D model for the northern segment (figs. 33 

and 34) depicts, in simplest terms, a four-layer model: a resistive zone (layer 1) about 60 to 70 ft (18 to 21 m) thick over a 

relatively continuous conductive zone (layer 2), about 60 to 75 

ft (18 to 23 m) thick, over a moderately to very resistive zone 

(layer 3), greater than 150 ft (45 m) in thickness, over a very 

thick basal conductive zone (layer 4). The upper conductive 

zone (layer 2) may consist of fluvial deposits of silts and clays, 

possibly from riverine overbank flows. Some resistive material 

occurs below the base-of-aquifer surface (interface between 

zones 3 and 4). These more resistive zones are not interpreted 

as coarser sediments within the Brule Formation; rather, they 

are the result of edge effects and inversion artifacts.

In the inverted 2D model for the southern segment (figs. 

35 and 36), the upper conductive layer does not appear to be 

as continuous as in the northern segment. In fact, it appears 

that the upper conductive zone pinches out around AMT stations 4 to 5 and from there northward is discontinuous. Data 

north of stations 51 extending to station 59 suffer from cultural 

noise caused by a registered USTA (National Earthquake 

Information Center, 2011) seismic station (L28) collecting 

data nearby. As such, the deeper data near the base-of-aquifer 

surface below these stations are suspect and were not used for 

interpretation.

The generalized lithology log from test hole 1-UL-09 

(fig. 8) was compared to the resistivity section at about AMT 

station 1 (fig. 35), which was collected 30 ft (10 m) west of 

the 1-UL-09 test-hole location. A visual interpretation of the 

resistivity model results for the data collected in 2010 (fig. 35) 

indicate the depth to the base of aquifer is around 1,100 feet, 

which agrees with the test hole 1-UL-09 (fig. 8) and baseof-aquifer elevation TDEM site CN1 (fig. 15). Examination 

of the borehole geophysical log also indicates the absence of 

layer 2, which is consistent with the AMT profile (fig. 35). At 

this location the modeled profile indicates moderately resistive material that extends to the base of the aquifer. Similarly 

modeled AMT data collected in 2009, collected approximately 

1,500 ft (460 m) west of 1-UL-09 (fig. 32) displays a different subsurface resistivity distribution including some of the 

upper conductor described in the 4-layer interpretation. This 

is another example of considerable variation in subsurface 

hydrostratigraphy over a short distance. 

The inferred elevation of the base of High Plains aquifer 

along the northern and southern segments using the inverted 

models of the 2010 AMT data is listed in table 5 and table 6, 

respectively. These points indicate only where base-of-aquifer 

elevations were picked and do not indicate individual AMT 

stations. The inferred elevation of the base of the aquifer from 

the AMT models indicates abrupt changes in the topology over 

short distances. This interpretation is consistent with AMT 

data collected in 2008–09 elsewhere in the upper Loup study 

area where AMT-based models indicated abrupt changes in the 

elevation of the base of the High Plains aquifer as compared 

with test-hole data. 
24 Hydrostratigraphic Interpretation of Test-Hole and Geophysical Data, Upper Loup River Basin, Nebraska, 2008–10

3,500

3,000

2,500

2,000

1,500

1,000

500

250

100

80

60

40

15

-500

0

Apparent resistivity, in ohm-meters

Site 5-SH-87

Land surface

893800 894000 894200 894400 894600 894800 895000

Elevation, in feet above vertical datum (NGVD 29)

No vertical exaggeration

Figure 31. Mixed-mode two-dimensional (2D) inversion results for stations near test hole 5-SH-87. No vertical exaggeration.
Hydrostratigraphic Interpretation Of Test-Hole And Surface-Geophysical Data 25

101°20' 101°25' 101°30' 101°35' 101°40'

42°05'

42°

41°55'

0 2

2

3 4 MILES 1

1 0 3 4 KILOMETERS

A

A ́

B

Whitman

H

gi h

w

a

y

2

Test hole 1-UL-09

Base from U.S. Department of Agriculture, Aerial Photography Field OfficeDigital orthophotography, 2007, 1:100,000Universal Transverse Mercator projection, Zone 14Horizontal coordinate information is referenced to the North AmericanDatum of 1983 (NAD 83)

HOOKER

GRANT

CHERRY

#*#*#*#*#*#*#*#*#*#*#*#*#*#*#*#*#*#*#*#*#*#*#*#*#*#*#*#*#*#*#*#*#*#*#*#*#*#*

#*#*#*#*#*#*#*#*#*#*#*#*#*

#*#*#*#*#*#*#*#*

#*

#*#*#*#*

#*#*#*#*#*#*#*#*#*#*#*#*#*#*

#*#*

#*#*#*#*#*#*

#*

!.!(

B ́

Grant 2009 AMT station

Index map

Upper Loupstudy area

Whitman RoadAMT profiles

Elkhorn-LoupModel area

9

!.#*

!(

Test hole 1-UL-09AMT stationGrant 2009 AMT profile

EXPLANATION

[AMT; audio magnetotellurics]

9

8

7 6 5

4

3 1

2

87

86 85

84 83

78 77

74 73

70 69

66 65

62 61

60

59 58

55 54

51

47 44

38 37

33 32

27 2

21

6

20

16 13

Figure 32. Location of audio-magnetotelluric (AMT) stations along Whitman Road, Cherry and Grant Counties, Nebr., where 2010 stations occurred. Stations north of Highway 2 compose the north segment (

A to A’), and stations south of Highway 2 compose the south segment (

B to B’).
26 Hydrostratigraphic Interpretation of Test-Hole and Geophysical Data, Upper Loup River Basin, Nebraska, 2008–10

Elevation above vertical datum NGVD 29, in feet

Apparent resistivity, in ohm-meters

3,7503,5003,2503,0002,7502,5002,2502,0001,750

25010080604015

812000 814000 816000 818000 820000 822000 824000 826000 828000

Northing, in feet

Vertical exaggeration is 10:1

63

63

64 65 63 63 63

69 70

71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 Land surface

Audio-magnetotelluric (AMT) station and location numberInferred elevation of the base of High Plains aquifer 

SOUTH

A

NORTH

A'

Layer 1

Layer 2

Layer 3

Layer 4

EXPLANATION

[Map of profile trace in figure 32]

Figure 33. Mixed-mode two-dimensional (2D) inversion results from 2010 audio-magnetotelluric (AMT) stations for the north segment (

A to A’) along Whitman Road, Cherry and 

Grant Counties, Nebr. 
Hydrostratigraphic Interpretation Of Test-Hole And Surface-Geophysical Data 27 A A’ A

A’

SOUTHSOUTH

NORTH

NORTH

Resistivity, in ohm-meters

15 40 50 80 100 250

[Sections are shown from two different oblique, aerial viewpoints;

no vertical exaggeration; map of profile trace in figure 32]

EXPLANATION

Figure 34. Mixed-mode two-dimensional (2D) inversion results from 2010 audio-magnetotelluric (AMT) stations for the north segment (A to A’) along Whitman Road, Cherry and Grant Counties, Nebr. 
28 Hydrostratigraphic Interpretation of Test-Hole and Geophysical Data, Upper Loup River Basin, Nebraska, 2008–10 Layer 1 Layer 2 Layer 4 Layer 3

Elevation above vertical datum NGVD 29, in feet

3,7503,5003,2503,0002,7502,5002,2502,0001,750

Apparent resistivity, in ohm-meters

25010080604015

8

37

36 3534 33 32 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 19 11 10 9 8 7 6 5 4 3 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58

800000

SOUTH

B

NORTH

B'

795000 790000 785000 780000 775000 770000 765000

1-UL-09

8 Audio-magnetotelluric (AMT) station and location number

Inferred elevation of the base of High Plains aquifer 

EXPLANATION

[Map of profile trace in figure 32]

Northing, in feet

Vertical exaggeration is 10:1

59

Figure 35. Mixed-mode two-dimensional (2D) inversion results from 2010 audio-magnetotelluric (AMT) stations for the south segment (

B to B’) along Whitman 

Road, Grant County. 
Hydrostratigraphic Interpretation Of Test-Hole And Surface-Geophysical Data 29 Resistivity, in ohm-meters 15 40 50 80 100 250 B

B’

SOUTH

B

SOUTH

NORTH

B’

NORTH

[Sections are shown from two different oblique, aerial viewpoints;no vertical exaggeration; map of profile trace in figure 32]

EXPLANATION

Figure 36. Mixed-mode two-dimensional (2D) inversion results from 2010 audio-magnetotelluric (AMT) soundings for the south segment (B to B’) along Whitman Road, Grant County, Nebr.
30 Hydrostratigraphic Interpretation of Test-Hole and Geophysical Data, Upper Loup River Basin, Nebraska, 2008–10

Table 5. Location and inferred elevation of base of High Plains aquifer, north segment of Whitman Road profile, Grant and Cherry 

Counties, Nebr., 2010.

[Northing and easting in Nebraska State Plane coordinates]

Latitude1 Longitude1 Northing

(feet)

Easting

(feet)

Elevation2

(feet)

42.04536 101.5242 809404 1226552 2,279

42.04439 101.524 809048 1226608 2,884

42.04563 101.5243 809504 1226534 2,877

42.04664 101.5246 809871 1226458 2,851

42.04783 101.5251 810308 1226329 2,805

42.0483 101.5254 810479 1226253 2,751

42.04887 101.5258 810691 1226149 2,706

42.04922 101.526 810817 1226079 2,653

42.04944 101.5262 810900 1226033 2,590

42.05001 101.5266 811109 1225913 2,542

42.05058 101.5271 811319 1225799 2,493

42.05168 101.5278 811722 1225600 2,481

42.05255 101.5283 812044 1225469 2,502

42.05345 101.5287 812372 1225371 2,533

42.05437 101.5289 812708 1225319 2,551

42.05563 101.529 813167 1225312 2,581

42.05703 101.529 813676 1225325 2,595

42.05829 101.529 814137 1225331 2,588

42.0588 101.529 814322 1225334 2,563

42.06 101.529 814759 1225332 2,545

42.06099 101.5291 815122 1225305 2,565

42.06167 101.5293 815369 1225269 2,622

42.06216 101.5294 815547 1225234 2,669

42.06289 101.5296 815815 1225187 2,740

42.06326 101.5297 815949 1225165 2,780

42.06375 101.5298 816130 1225147 2,814

42.06444 101.5299 816380 1225127 2,846

42.06488 101.5299 816540 1225120 2,855

42.06638 101.53 817088 1225100 2,851

42.06732 101.5301 817431 1225084 2,823

42.06782 101.5302 817613 1225074 2,769

42.0685 101.5302 817863 1225055 2,728

42.06906 101.5303 818067 1225034 2,681

42.07023 101.5306 818493 1224969 2,663

42.07102 101.5308 818782 1224912 2,672

42.07169 101.531 819027 1224868 2,715

42.07194 101.5311 819117 1224855 2,774

42.07312 101.5312 819549 1224827 2,828

42.07521 101.5308 820307 1224945 2,855

42.07775 101.5299 821229 1225216 2,833

42.07827 101.5298 821419 1225248 2,789

42.07847 101.5297 821490 1225257 2,735
Hydrostratigraphic Interpretation Of Test-Hole And Surface-Geophysical Data 31

Latitude1 Longitude1 Northing

(feet)

Easting

(feet)

Elevation2

(feet)

42.07928 101.5298 821787 1225243 2,692

42.08018 101.5302 822115 1225147 2,653

42.08084 101.5309 822358 1224963 2,629

42.08135 101.5322 822550 1224613 2,667

42.08159 101.5331 822643 1224359 2,735

42.0821 101.5344 822835 1224015 2,833

42.08241 101.5349 822950 1223883 2,873

42.08311 101.5358 823210 1223650 2,943

42.08469 101.5371 823791 1223301 2,964

42.08592 101.5382 824246 1223008 2,953

42.08697 101.5392 824634 1222734 2,955

42.08821 101.5399 825086 1222555 2,953

42.08863 101.5401 825242 1222525 2,987

42.08863 101.5401 825242 1222525 3,009

42.0905 101.5399 825920 1222584 2,993

42.0907 101.5398 825993 1222609 2,950

42.09118 101.5395 826167 1222678 2,898

42.09152 101.5393 826292 1222736 2,837

42.09209 101.5389 826497 1222853 2,774

42.09395 101.537 827166 1223395 2,771

42.09472 101.5361 827442 1223650 2,880

42.0951 101.5357 827576 1223756 2,993

42.09484 101.5359 827485 1223689 3,074

42.09586 101.5351 827851 1223919 3,175

42.09738 101.5339 828398 1224242 3,175

1Decimal degrees; North American Datum of 1983 (NAD 83).

2Land-surface elevation above National Geodetic Vertical Datum of 1929 (NGVD 29); elevations surveyed to nearest foot.

Table 5. Location and inferred elevation of base of High Plains aquifer, north segment of Whitman Road profile, Grant and Cherry 

Counties, Nebr., 2010.—Continued

[Northing and easting in Nebraska State Plane coordinates]
32 Hydrostratigraphic Interpretation of Test-Hole and Geophysical Data, Upper Loup River Basin, Nebraska, 2008–10

Table 6. Location and inferred elevation of base of High Plains aquifer, south segment of Whitman Road profile, Grant County, Nebr., 

2010.

[Northing and easting in Nebraska State Plane coordinates]

Latitude1 Longitude1 Northing

(feet)

Easting

(feet)

Elevation2

(feet)

41.9131 101.4948 761088 1233700 2,374

41.91508 101.4932 761802 1234135 2,492

41.9165 101.4926 762316 1234314 2,546

41.91833 101.4922 762981 1234435 2,621

41.92056 101.4909 763789 1234812 2,659

41.92205 101.4894 764324 1235222 2,605

41.92411 101.4887 765070 1235417 2,535

41.92515 101.4889 765451 1235368 2,465

41.92828 101.4913 766601 1234736 2,460

41.93027 101.4921 767329 1234541 2,444

41.93221 101.4913 768034 1234781 2,422

41.93429 101.4911 768790 1234834 2,492

41.93621 101.4916 769490 1234722 2,562

41.93741 101.4924 769932 1234516 2,654

41.93792 101.4928 770121 1234404 2,756

41.93844 101.4932 770310 1234291 2,831

41.9398 101.494 770811 1234093 2,880

41.9427 101.4954 771874 1233712 2,896

41.94293 101.4958 771958 1233624 2,847

41.94326 101.4963 772082 1233485 2,777

41.94369 101.497 772243 1233289 2,713

41.94561 101.4981 772945 1233016 2,729

41.94661 101.4984 773311 1232921 2,783

41.9479 101.4994 773788 1232653 2,837

41.94897 101.5007 774184 1232312 2,820

41.9496 101.5013 774414 1232151 2,745

41.95193 101.502 775266 1231987 2,724

41.95423 101.5028 776109 1231791 2,681

41.95588 101.5031 776709 1231721 2,627

41.95771 101.5032 777377 1231683 2,611

41.95901 101.5037 777853 1231579 2,670

41.9597 101.504 778105 1231484 2,777

41.96063 101.5046 778448 1231324 2,869

41.96211 101.5055 778993 1231087 2,907

41.96358 101.5065 779533 1230836 2,864

41.96371 101.5066 779580 1230810 2,794

41.96486 101.5076 780003 1230549 2,783

41.96575 101.5084 780332 1230344 2,864

41.96716 101.5095 780851 1230032 2,901

41.96882 101.511 781460 1229651 2,896

41.9704 101.5115 782038 1229510 2,869

41.97145 101.5117 782423 1229475 2,820
Hydrostratigraphic Interpretation Of Test-Hole And Surface-Geophysical Data 33

Latitude1 Longitude1 Northing

(feet)

Easting

(feet)

Elevation2

(feet)

41.97246 101.512 782793 1229396 2,767

41.9733 101.5124 783099 1229292 2,681

41.97552 101.5141 783915 1228849 2,664

41.97766 101.5166 784708 1228169 2,632

41.97907 101.5184 785231 1227702 2,589

41.98056 101.5217 785787 1226824 2,546

41.98311 101.5244 786731 1226087 2,508

41.98433 101.5275 787188 1225252 2,514

41.98621 101.5305 787887 1224463 2,541

41.98772 101.5307 788440 1224418 2,632

41.98989 101.5302 789227 1224564 2,675

41.99216 101.5299 790053 1224660 2,659

41.99449 101.5288 790898 1224977 2,621

41.99563 101.528 791307 1225183 2,632

41.99563 101.528 791307 1225183 2,654

41.99651 101.5277 791627 1225288 2,713

41.99863 101.5274 792396 1225382 2,702

41.99909 101.5274 792565 1225379 2,670

41.9997 101.5275 792788 1225369 2,740

42.00045 101.5276 793063 1225342 2,826

42.00075 101.5276 793172 1225328 2,917

42.0018 101.5278 793553 1225279 2,998

42.00224 101.5279 793717 1225261 2,966

42.00255 101.528 793827 1225250 2,869

42.00472 101.5281 794618 1225241 2,777

42.00504 101.528 794734 1225249 2,697

42.00647 101.5279 795256 1225281 2,659

42.00846 101.5281 795983 1225248 2,713

42.00945 101.5285 796343 1225138 2,799

42.00945 101.5285 796343 1225138 2,966

42.0101 101.529 796583 1225009 3,106

42.0106 101.5295 796767 1224880 3,197

42.01143 101.5305 797074 1224614 3,256

42.01248 101.5319 797463 1224251 3,176

42.01283 101.5323 797593 1224132 3,047

42.01356 101.5331 797862 1223918 2,933

42.01392 101.5335 797998 1223818 2,794

42.01562 101.5342 798618 1223637 2,691

42.01892 101.533 799814 1223986 2,702

42.0255 101.5334 802213 1223912 2,681

42.03291 101.5356 804924 1223385 2,681

1Decimal degrees; North American Datum of 1983 (NAD 83).

2Land-surface elevation above National Geodetic Vertical Datum of 1929 (NGVD 29); elevations surveyed to nearest foot.

Table 6. Location and inferred elevation of base of High Plains aquifer, south segment of Whitman Road profile, Grant County, Nebr., 

2010.—Continued

[Northing and easting in Nebraska State Plane coordinates]
34 Hydrostratigraphic Interpretation of Test-Hole and Geophysical Data, Upper Loup River Basin, Nebraska, 2008–10

Conclusions and Implications for Future Study

The differences between the predicted base-of-aquifer 

elevation and the elevation from McGuire and Peterson (2008) 

suggests that in areas of the upper Loup study area the base of 

aquifer is not well-constrained. The range of differences was 

from -157 to 300 feet (-48 to 91 m). Test holes 1-UL-09 and 

2-UL-09, drilled in the western part of the upper Loup study 

area, had base-of-aquifer elevations more than 100 feet (30 m) 

deeper than predicted by SIM 2008-3042. This indicates that 

the test holes likely were drilled in paleovalleys eroded into 

the Brule Formation. 

TDEM-based estimates of the base-of-aquifer elevation 

were in good accordance with those from existing test-hole 

data. The TDEM method was able to constrain the elevation and topology of the base of the High Plains aquifer. The 

TDEM method also provided information regarding the thickness of the Quaternary eolian sands, Pliocene or younger sand 

and gravel, and the Miocene Ogallala Group. TDEM is, thus, a 

valuable and cost-effective tool for mapping hydrostratigraphy 

throughout the High Plains aquifer and can provide additional 

subsurface characterization needed in regional groundwater 

models. 

Short reconnaissance profiles were collected with the 

AMT method to determine the base of aquifer. To model 

subsurface features using this approach, the geologic strike 

of subsurface features, including the base-of-aquifer surface, 

must be known in advance. When collecting short profiles 

(roughly 650 to 1,300 ft; 200 to 400 m), the assumption of 

west-to-east alignment of drainages and paleovalleys may be 

invalid in the upper Loup study area; therefore, a pilot survey 

would need to be completed before collecting a short profile. 

In 2010, AMT data were collected along an approximately 12-mile-(19-km-) long profile along Whitman Road, 

in Grant and Cherry Counties. The AMT results from the 

Whitman Road profile are in general accordance with TDEM 

sounding CN1 and test hole 1-UL-09. The AMT profile 

indicated substantial variability in the topography of the base 

of the High Plains aquifer and in the distribution of high 

permeability zones within the aquifer. This indicates that 

the subsurface geology in this area is poorly understood and 

local-scale variability of the High Plains aquifer cannot be 

captured with test-hole drilling alone. Parallel profiles, such as 

those provided by airborne TDEM methods, would be needed 

to adequately characterize the hydrostratigraphy in the upper 

Loup study area.

A combination of test-hole drilling and surface-geophysical data collection was used to improve the understanding of 

the hydrogeology in the upper Loup study area in support of 

the ELM groundwater model. The data collected for this study 

can be incorporated into existing data sets to improve the 

estimated thickness of the aquifer and the configuration of the 

base-of-aquifer surface. 

Summary 

The Elkhorn-Loup Model (ELM) and related studies were collaboratively begun to understand the impact 

of various groundwater management scenarios on surfacewater resources. During Phase 1 of the ELM study, a lack of 

subsurface geological information in the Upper Loup Natural 

Resources District (ULNRD) area was identified as a gap in 

current (2011) understanding that needed to be addressed. To 

improve the understanding of the hydrogeology of the upper 

Loup study area, the U.S. Geological Survey, in cooperation 

with the ULNRD and the University of Nebraska Conservation and Survey Division, included lithologic description 

of circulated drill cuttings collected from nine test holes 

and borehole geophysical data to improve aquifer thickness 

estimates and enhance the understanding of the groundwater 

system. Surface geophysical data also were collected using 

time-domain electromagnetic (TDEM) and audio-magnetotelluric (AMT) methods at test-hole locations and between test 

holes as a quick, non-invasive means of identifying the base of 

the High Plains aquifer. 

Sites primarily were chosen to fill existing spatial gaps in 

the University of Nebraska Conservation and Survey Division 

test-hole database. Geophysical methods were selected based 

on the physical characteristics of the hydrostratigraphic units 

of the study area and the anticipated depth to the base of the 

aquifer (top of the Brule Formation). Two surface geophysical 

methods were chosen: TDEM and AMT methods. 

Nine test holes were drilled to below the base of the aquifer and logged with a borehole geophysical logging system. 

A total of 26 TDEM and 117 AMT soundings were collected 

within the study area between 2008 and 2010. Soundings collected in 2008 and 2009 were in close proximity to previously 

drilled or planned test holes. The data collection in 2010 was 

expanded to specifically include locations where little information previously existed. 

Previous test-hole drilling has indicated greater variation in the base-of-aquifer elevation in the west, reflecting a 

number of deep paleovalleys incised into the Brule Formation. 

Two of the new test holes were located in close proximity to 

the previously interpreted paleovalleys in Grant and Hooker 

Counties.

TDEM soundings within the study area were documented 

to be effective as virtual boreholes in mapping the base-ofaquifer elevation beneath the study area. TDEM estimates 

of the base of aquifer were in good accordance with existing 

test-hole data and are useful in improving topology of the base 

of aquifer, and details of the overlying stratigraphic section. 

Uplift structures, such as the Chadron Arch, were identified 

in the modeled TDEM results as subtle changes in the deep 

resistivity structure. 

In 2010, AMT data were collected along an approximately 12-mile-(19-km-) long profile along Whitman Road, 

in Grant and Cherry Counties. The AMT results indicated 

substantial variability in the elevation of the base of the High 
References Cited 35

Plains aquifer and in the distribution of high permeability 

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A combination of test-hole drilling and surface-geophysical data collection was used to improve the understanding of 

the hydrogeology in the upper Loup study area in support of 

the ELM groundwater model. The data collected for this study 

can be incorporated into existing data sets to improve the 

estimated thickness of the aquifer and the configuration of the 

base-of-aquifer surface. TDEM is a valuable and cost-effective 

tool for mapping hydrostratigraphy throughout the High Plains 

aquifer. The AMT technique is a useful tool when assessing 

the variability within the High Plains aquifer and distribution 

of high permeability zones. Both techniques, when properly 

applied, are useful in improving the regional hydrostratigraphic framework needed in groundwater models. 

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Wannamaker, P.E., 1983, Resistivity structure of the northern 

Basin and Range: Davis, Calif., Geothermal Resources 

Council, Special Report no. 13, p. 345–361.

Wannamaker, P.E., Hohmann, G.W., and Ward, S.H., 1984, 

Magnetotelluric responses of three-dimensional bodies in 

layered earths: Geophysics, v. 49, no. 9, p. 1,517–1,533.

Wannamaker, P.E., Stodt, J.A., and Rijo, L., 1987, PW2D— 

Finite element program for solution of magnetotelluric 

responses of two-dimensional earth resistivity structure 

(User documentation): Salt Lake City, Utah, Earth Science 

Laboratory, University of Utah Research Institute, 40 p.

Weeks, J.B., and Gutentag, E.D., 1988, Region 17, High 

Plains, Chap. 20, in, Back, W., Rosenshein, J.S., and Seaber, 

P.R., eds., Hydrogeology, Geology of North America, v. 

O-2: Boulder, Colo., Geological Society of Amercia, p. 

157–164.

Wigley, P.B., 2000, Logan County test-hole logs: Lincoln, 

Nebr., University of Nebraska-Lincoln, Conservation and 

Survey Division, Nebraska Water Survey Test-Hole Report 

no. 57, 17 p.

Winter, T.C., 1986, Effect of ground-water recharge on 

configuration of the water table beneath sand dunes and on 

seepage in lakes in the Sandhills of Nebraska, U.S.A: Journal of Hydrology, v. 86, p. 221–237

Supplemental Data 39

Supplemental Data
40 Hydrostratigraphic Interpretation of Test-Hole and Geophysical Data, Upper Loup River Basin, Nebraska, 2008–10

Depth,

in feet

Generalized lithology

Sand, very fine to fine, trace medium

sand, some silt, trace clay

Coarse sand to fine gravel, trace silt

Sandstone, some coarse sand, rootlets

Sandstone with coarse sand, some

medium gravel, rootlets

Fine sand/sandstone, some interbedded

silt, trace clay

Silt, siltstone, moderately clayey

Very fine to coarse sand, trace fine

gravel, some clayey lenses, some

siltstone clasts

Brule Formation silt/silstone, trace clay

100

200

300

400

500

600

700

800

0

Figure 6. Generalized lithologic description of test hole1-UL-08, Logan County, Nebr.
Supplemental Data 41

Generalized lithology

Depth,

in feet

Sand and gravel fine to

medium

Sand, fine, silty sand to sandy

silt, sandstone, rootlets

Silt to clayey silt

Sand, fine, silty sand to sandy

silt, sandstone, rootlets

Sandy silt, interbedded

siltstone

Sand, fine, silty sand to sandy

silt, sandstone, some clayey

silt, rootlets

Sand, very fine to fine, some

medium, some silt

Brule Formation silt/siltstone

100

200

300

400

500

600

700

0

Short-normal resistivity

0 Ohm-m 300

Long-normal resistivity

0 Ohm-m 300

Spontaneous-potential

0 Millivolts 100

Gamma

0 API units 250

Figure 7. Composite of generalized lithologic description and geophysical logs for test hole 3-UL-08, Thomas County, Nebr. Shortnormal resistivity, in ohm-meters; long-normal resistivity, in ohm-meters; natural gamma activity, in American Petroleum Institute units; 

spontaneous potential, in millivolts.
42 Hydrostratigraphic Interpretation of Test-Hole and Geophysical Data, Upper Loup River Basin, Nebraska, 2008–10

Short-normal resistivity (16 in.) Generalized lithology

0 Ohm-m 300

Spontaneous-potential

-100 Millivolts 100

Gamma

0 250

100

200

300

400

500

600

700

800

900

1,000

1,100

0

Sand, very fine to fine, silty

sand, some clay

Sand, very fine to fine, silty

sand, sandstone, rootlets

Sand, very fine to fine,

sandstone, some interbedded

gravel, rootlets

Siltstone/sandstone

Sand, fine to medium, some

sandy silt, trace of gravel,

intebedded

sandstone/siltstone, rootlets

Silty sand, interbedded

Sand, fine to medium, some

sandy silt, trace of gravel,

interbedded

sandstone/siltstone, rootlets

siltstone/sandstone

Brule Formation silt/siltstone

Depth,

in feet

API units

Figure 8. Composite of generalized lithologic description and geophysical logs for test hole 1-UL-09, Grant County, Nebr. Shortnormal resistivity, in ohm-meters; long-normal resistivity, in ohm-meters; natural gamma activity, in American Petroleum Institute units; 

spontaneous potential, in millivolts.
Supplemental Data 43

Short-normal resistivity (16 in.) Generalized lithology

0 Ohm-m 300

Long-normal resistivity (64 in.)

0 Ohm-m 300

Spontaneous-potential

-100 Millivolts 100

Gamma

0 250

100

200

300

400

500

600

700

800

900

0

Depth,

in feet

API units

Sand, very fine to fine, silty

sand to sandy silt, interbedded

sandstone

Sand and gravel, fine to

medium, some coarse

Sand, very fine to fine, some

silt, sandstone, rootlets

Sand, fine to medium,

sandstone, rootlets

Sand, very fine to fine, some

medium, silty sand, sandstone,

rootlets

Sand, fine some medium, silty

sand, interbedded sandstone

to sandy siltstone, rootlets

Brule Formation silt/siltstone

Figure 9. Composite of generalized lithologic description and geophysical logs for test hole 2-UL-09, Hooker County, Nebr. Shortnormal resistivity, in ohm-meters; long-normal resistivity, in ohm-meters; natural gamma activity, in American Petroleum Institute units; 

spontaneous potential, in millivolts.
44 Hydrostratigraphic Interpretation of Test-Hole and Geophysical Data, Upper Loup River Basin, Nebraska, 2008–10

Short-normal resistivity (16 in.) Generalized lithology

0 Ohm-m 300

Long-normal resistivity (64 in.)

0 Ohm-m 300

Spontaneous-potential

-50 Millivolts 100

Gamma

0 250

100

200

300

400

500

600

0

Depth,

in feet

API units

Sand, very fine to medium,

interbedded sandstone, some

silt and siltstone, rootlets

Sand/sandstone, very fine to 

medium, rootlets

Sand/sandstone, fine to 

medium, some silty sand,

rootlets

Sand/sandstone, very fine to 

fine, some medium, silty sand

to siltstone, interbedded olive

green siltstone, rootlets

Sand/sandstone, very fine to 

fine, interbedded silt and 

siltstone, rootlets

Sand/sandstone, fine to 

medium, interbedded olive

green siltstone, some silt,

rootlets

Brule Formation silt/siltstone,

trace of gravel

Figure 10. Composite of generalized lithologic description and geophysical logs for test hole 3-UL-09, Cherry County, Nebr. Shortnormal resistivity, in ohm-meters; long-normal resistivity, in ohm-meters; natural gamma activity, in American Petroleum Institute units; 

spontaneous potential, in millivolts.
Supplemental Data 45

Short-normal resistivity Generalized lithology

0 Ohm-m 300

Long-normal resistivity

0 Ohm-m 300

Spontaneous-potential

-100 Millivolts 100

Gamma

0 250

100

200

300

400

500

600

700

800

0

100

200

300

400

500

600

700

800

0

Depth,

in feet

API units

Sand to silty sand, very fine to

fine

Silt to sandy silt, very fine to

fine, some medium, trace of

coarse, some clay

Gravel, fine to medium

Silty sand to sandy silt, very

fine to fine, sandstone, traces

of gravel

Silt, slight clay, some sand,

very fine to fine

Sand and gravel, with

sandstone, some silt, rootlets

Silt, slight clay, trace of

sandstone

Sand/sandstone, very fine to

fine, some medium, silty sand

to sandy silt, few rootlets

Silty sand to sandy silt, fine,

some medium, occasional

interbedded sandstone and

claystone

Brule Formation silt/siltstone

Figure 11. Composite of generalized lithologic description and geophysical logs for test hole 1-UL-10, Thomas County, Nebr. Shortnormal resistivity, in ohm-meters; long-normal resistivity, in ohm-meters; natural gamma activity, in American Petroleum Institute units; 

spontaneous potential, in millivolts.
46 Hydrostratigraphic Interpretation of Test-Hole and Geophysical Data, Upper Loup River Basin, Nebraska, 2008–10

Short-normal resistivity Generalized lithology

0 Ohm-m 300

Spontaneous-potential

-100 Millivolts 100

Gamma

0 250

100

200

300

400

500

600

700

0

Silty sand to sand, very fine to

fine

Sand, fine, some medium,

trace of coarse

Sand and gravel, fine to

coarse

Sand to silty sand, fine,

interbedded medium to coarse

sand, rootlets

Sand to silty sand, fine, trace

medium, interbedded

sandstone, rootlets

Silty sand to sandy silt, very

fine to fine, trace medium

sand, sandstone

Brule Formation silt/siltstone

Depth,

in feet

API units

Figure 12. Composite of generalized lithologic description and geophysical logs for test hole 2-UL-10, Thomas County, Nebr. Shortnormal resistivity, in ohm-meters; long-normal resistivity, in ohm-meters; natural gamma activity, in American Petroleum Institute units; 

spontaneous potential, in millivolts.
Supplemental Data 47

Short-normal resistivity (16 in.) Generalized lithology

0 Ohm-m 300

Spontaneous-potential

-100 Millivolts 100

Gamma

0 250

100

200

300

400

500

600

0

EM Induction resistivity

0 Ohm-m 300

Depth,

in feet

API units

Sand to silty sand, fine

Sandy clay to clayey sand,

with silty sand to sand silt, fine

Gravel, fine to medium, some

coarse

Silty sand, fine, interbedded

sandstone, rootlets

Silty sand to sand, fine, traces

of interbedded medium to

coarse sand with sandstone,

rootlets

Silty sand to sand, very fine to

fine, interbedded sandstone,

marl, rootlets

Brule Formation silt/siltstone

Figure 13. Composite of generalized lithologic description and geophysical logs for test hole 3-UL-10, Blaine County, Nebr. Shortnormal resistivity, in ohm-meters; long-normal resistivity, in ohm-meters; electromagnetic induction resistivity, in ohm-meters; natural 

gamma activity, in American Petroleum Institute units; spontaneous potential, in millivolts.
48 Hydrostratigraphic Interpretation of Test-Hole and Geophysical Data, Upper Loup River Basin, Nebraska, 2008–10

CN1, r.m.s. =

1,300

1,200

1,100

1,000

900

800

700

600

500

400

300

200

100

0

Depth, in feet

Depth, in meters

Apparent resistivity, in ohm-meters

1 10 100 1,000

1,300

1,200

1,100

1,000

900

800

700

600

500

400

300

200

100

0

Time, in seconds Resistivity, in ohm-meters

 1.98 (m), 2.01 (s)

CN2, r.m.s. =

 0.88 (m), 0.91 (s)

100

200

300

400

150

250

350

50

0

100

200

300

400

150

250

350

50

0

10-5

10-5 10-4 10-3 10-2 10-1

10-6

10-7

10-8

10-9

10-10

10-11

10-5

10-6

10-7

10-8

10-9

10-10

10-11

103

102

101

100

103

102

101

100

285

75

30

30, 7.5, and 3—Collected from high-current transmitter

Repetition frequency data, in hertz

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Root mean square error—

 Minimum-layer model (m) 

 and smooth model (s), in 

 ohm-meters

r.m.s. 

Best-fit smooth inverse model

 Vertical error bar indicates variance of data point 

 with mean at center

dB/dt (time derivative of the out-of-loop secondary magnetic field), in volts per square meter

A B

A B

Figure 15. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (time-derivative, right axis), and time; B,

depth below land surface and subsurface resistivity modeled from TDEM sounding for sites CN1 and CN2.
Supplemental Data 49

CN3, r.m.s. =

1,300

1,200

1,100

1,000

900

800

700

600

500

400

300

200

100

0

Depth, in feet

Depth, in meters

Apparent resistivity, in ohm-meters

1 10 100 1,000

1,300

1,200

1,100

1,000

900

800

700

600

500

400

300

200

100

0

Time, in seconds Resistivity, in ohm-meters

 0.64 (m), 0.63 (s)

CN4, r.m.s. =

 0.75 (m), 0.58 (s)

100

200

300

400

150

250

350

50

0

100

200

300

400

150

250

350

50

0

10-5

10-5 10-4 10-3 10-2 10-1

10-6

10-7

10-8

10-9

10-10

10-11

10-5

10-6

10-7

10-8

10-9

10-10

10-11

103

102

101

100

103

102

101

100

dB/dt (time derivative of the out-of-loop secondary magnetic field), in volts per square meter

285

75

30

30, 7.5, and 3—Collected from high-current transmitter

Repetition frequency data, in hertz

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Root mean square error—

 Minimum-layer model (m) 

 and smooth model (s), in 

 ohm-meters

r.m.s. 

Best-fit smooth inverse model

 Vertical error bar indicates variance of data point 

 with mean at center

A B

A B

Figure 16. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (time-derivative, right axis), and time; B,

depth below land surface and subsurface resistivity modeled from TDEM sounding for sites CN3 and CN4.
50 Hydrostratigraphic Interpretation of Test-Hole and Geophysical Data, Upper Loup River Basin, Nebraska, 2008–10

SH7, r.m.s. =

1,300

1,200

1,100

1,000

900

800

700

600

500

400

300

200

100

0

Depth, in feet

Depth, in meters

Apparent resistivity, in ohm-meters

1 10 100 1,000

1,300

1,200

1,100

1,000

900

800

700

600

500

400

300

200

100

0

Time, in seconds Resistivity, in ohm-meters

 0.27 (m), 0.20 (s)

SH8, r.m.s. =

 0.21 (m), 0.20 (s)

100

200

300

400

150

250

350

50

0

100

200

300

400

150

250

350

50

0

10-5 10-4 10-3 10-2 10-1

103

102

101

100

103

102

101

100

32

2

Repetition frequency data, in hertz

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Root mean square error—

 Minimum-layer model (m) 

 and smooth model (s), in 

 ohm-meters

r.m.s. 

Best-fit smooth inverse model

Vertical error bar indicates variance of data point 

 with mean at center

A B

A B

Figure 17. Relations between A, central-loop resistivity and time; B, depth below land surface and apparent resistivity modeled from 

TDEM sounding for sites SH7 and SH8.
Supplemental Data 51

UL1, r.m.s. =

1,300

1,200

1,100

1,000

900

800

700

600

500

400

300

200

100

0

Depth, in feet

Depth, in meters

Apparent resistivity, in ohm-meters

1 10 100 1,000

1,300

1,200

1,100

1,000

900

800

700

600

500

400

300

200

100

0

Time, in seconds Resistivity, in ohm-meters

 1.06 (m), 0.93 (s)

UL2, r.m.s. =

 0.88 (m), 0.86 (s)

100

200

300

400

150

250

350

50

0

100

200

300

400

150

250

350

50

0

10-5

10-5 10-4 10-3 10-2 10-1

10-6

10-7

10-8

10-9

10-10

10-11

10-5

10-6

10-7

10-8

10-9

10-10

10-11

103

102

101

100

103

102

101

100

285

75

30

30, 7.5, and 3—Collected from high-current transmitter

Repetition frequency data, in hertz

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Root mean square error—

 Minimum-layer model (m) 

 and smooth model (s), in 

 ohm-meters

r.m.s. 

Best-fit smooth inverse model

 Vertical error bar indicates variance of data point 

 with mean at center

dB/dt (time derivative of the out-of-loop secondary magnetic field), in volts per square meter

A B

A B

Figure 18. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (time-derivative, right axis), and time; B,

depth below land surface and subsurface resistivity modeled from TDEM sounding for sites UL1 and UL2.
52 Hydrostratigraphic Interpretation of Test-Hole and Geophysical Data, Upper Loup River Basin, Nebraska, 2008–10

UL3, r.m.s. =

1,300

1,200

1,100

1,000

900

800

700

600

500

400

300

200

100

0

Depth, in feet

Depth, in meters

Apparent resistivity, in ohm-meters

1 10 100 1,000

1,300

1,200

1,100

1,000

900

800

700

600

500

400

300

200

100

0

Time, in seconds Resistivity, in ohm-meters

 1.44 (m), 1.40 (s)

UL4, r.m.s. =

 0.96 (m), 0.86 (s)

100

200

300

400

150

250

350

50

0

100

200

300

400

150

250

350

50

0

10-5

10-5 10-4 10-3 10-2 10-1

10-6

10-7

10-8

10-9

10-10

10-11

10-5

10-6

10-7

10-8

10-9

10-10

10-11

103

102

101

100

103

102

101

100

dB/dt (time derivative of the out-of-loop secondary magnetic field), in volts per square meter

285

75

30

30, 7.5, and 3—Collected from high-current transmitter

Repetition frequency data, in hertz

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Root mean square error—

 Minimum-layer model (m) 

 and smooth model (s), in 

 ohm-meters

r.m.s. 

Best-fit smooth inverse model

 Vertical error bar indicates variance of data point 

 with mean at center

A B

A B

Figure 19. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (time-derivative, right axis), and time; B,

depth below land surface and subsurface resistivity modeled from TDEM sounding for sites UL3 and UL4.
Supplemental Data 53

UL5, r.m.s. =

1,300

1,200

1,100

1,000

900

800

700

600

500

400

300

200

100

0

Depth, in feet

Depth, in meters

Apparent resistivity, in ohm-meters

1 10 100 1,000

1,300

1,200

1,100

1,000

900

800

700

600

500

400

300

200

100

0

Time, in seconds Resistivity, in ohm-meters

 0.89 (m), 0.81 (s)

UL6, r.m.s. =

 1.14 (m), 1.14 (s)

100

200

300

400

150

250

350

50

0

100

200

300

400

150

250

350

50

0

10-5

10-5 10-4 10-3 10-2 10-1

10-6

10-7

10-8

10-9

10-10

10-11

10-5

10-6

10-7

10-8

10-9

10-10

10-11

103

102

101

100

103

102

101

100

dB/dt (time derivative of the out-of-loop secondary magnetic field), in volts per square meter

285

75

30

30, 7.5, and 3—Collected from high-current transmitter

Repetition frequency data, in hertz

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Root mean square error—

 Minimum-layer model (m) 

 and smooth model (s), in 

 ohm-meters

r.m.s. 

Best-fit smooth inverse model

 Vertical error bar indicates variance of data point 

 with mean at center

A B

A B

Figure 20. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (time-derivative, right axis), and time; B,

depth below land surface and subsurface resistivity modeled from TDEM sounding for sites UL5 and UL6.
54 Hydrostratigraphic Interpretation of Test-Hole and Geophysical Data, Upper Loup River Basin, Nebraska, 2008–10

UL7, r.m.s. =

1,300

1,200

1,100

1,000

900

800

700

600

500

400

300

200

100

0

Depth, in feet

Depth, in meters

Apparent resistivity, in ohm-meters

1 10 100 1,000

1,300

1,200

1,100

1,000

900

800

700

600

500

400

300

200

100

0

Time, in seconds Resistivity, in ohm-meters

 0.88 (m), 0.67 (s)

UL8, r.m.s. =

 0.59 (m), 0.56 (s)

100

200

300

400

150

250

350

50

0

100

200

300

400

150

250

350

50

0

10-5

10-5 10-4 10-3 10-2 10-1

10-6

10-7

10-8

10-9

10-10

10-11

10-5

10-6

10-7

10-8

10-9

10-10

10-11

103

102

101

100

103

102

101

100

dB/dt (time derivative of the out-of-loop secondary magnetic field), in volts per square meter

285

75

30

30, 7.5, and 3—Collected from high-current transmitter

Repetition frequency data, in hertz

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Root mean square error—

 Minimum-layer model (m) 

 and smooth model (s), in 

 ohm-meters

r.m.s. 

Best-fit smooth inverse model

 Vertical error bar indicates variance of data point 

 with mean at center

A B

A B

Figure 21. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (time-derivative, right axis), and time; B,

depth below land surface and subsurface resistivity modeled from TDEM sounding for sites UL7 and UL8.
Supplemental Data 55

UL9, r.m.s. =

1,300

1,200

1,100

1,000

900

800

700

600

500

400

300

200

100

0

Depth, in feet

Depth, in meters

Apparent resistivity, in ohm-meters

1 10 100 1,000

1,300

1,200

1,100

1,000

900

800

700

600

500

400

300

200

100

0

Time, in seconds Resistivity, in ohm-meters

 0.72 (m), 0.51 (s)

UL10, r.m.s. =

 0.95 (m), 0.85 (s)

100

200

300

400

150

250

350

50

0

100

200

300

400

150

250

350

50

0

10-5

10-5 10-4 10-3 10-2 10-1

10-6

10-7

10-8

10-9

10-10

10-11

10-5

10-6

10-7

10-8

10-9

10-10

10-11

103

102

101

100

103

102

101

100

dB/dt (time derivative of the out-of-loop secondary magnetic field), in volts per square meter

285

75

30

30, 7.5, and 3—Collected from high-current transmitter

Repetition frequency data, in hertz

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Root mean square error—

 Minimum-layer model (m) 

 and smooth model (s), in 

 ohm-meters

r.m.s. 

Best-fit smooth inverse model

 Vertical error bar indicates variance of data point 

 with mean at center

A B

A B

Figure 22. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (time-derivative, right axis), and time; B,

depth below land surface and subsurface resistivity modeled from TDEM sounding for sites UL9 and UL10.
56 Hydrostratigraphic Interpretation of Test-Hole and Geophysical Data, Upper Loup River Basin, Nebraska, 2008–10

UL11, r.m.s. =

1,300

1,200

1,100

1,000

900

800

700

600

500

400

300

200

100

0

Depth, in feet

Depth, in meters

Apparent resistivity, in ohm-meters

1 10 100 1,000

1,300

1,200

1,100

1,000

900

800

700

600

500

400

300

200

100

0

Time, in seconds Resistivity, in ohm-meters

 1.36 (m), 0.89 (s)

UL12, r.m.s. =

 0.41 (m), 0.38 (s)

100

200

300

400

150

250

350

50

0

100

200

300

400

150

250

350

50

0

10-5

10-5 10-4 10-3 10-2 10-1

10-6

10-7

10-8

10-9

10-10

10-11

10-5

10-6

10-7

10-8

10-9

10-10

10-11

103

102

101

100

103

102

101

100

dB/dt (time derivative of the out-of-loop secondary magnetic field), in volts per square meter

285

75

30

30, 7.5, and 3—Collected from high-current transmitter

Repetition frequency data, in hertz

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Root mean square error—

 Minimum-layer model (m) 

 and smooth model (s), in 

 ohm-meters

r.m.s. 

Best-fit smooth inverse model

 Vertical error bar indicates variance of data point 

 with mean at center

A B

A B

Figure 23. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (time-derivative, right axis), and time; B,

depth below land surface and subsurface resistivity modeled from TDEM sounding for sites UL11 and UL12.
Supplemental Data 57

UL13, r.m.s. =

1,300

1,200

1,100

1,000

900

800

700

600

500

400

300

200

100

0

Depth, in feet

Depth, in meters

Apparent resistivity, in ohm-meters

1 10 100 1,000

1,300

1,200

1,100

1,000

900

800

700

600

500

400

300

200

100

0

Time, in seconds Resistivity, in ohm-meters

 0.63 (m), 0.48 (s)

UL14, r.m.s. =

 0.87 (m), 0.64 (s)

100

200

300

400

150

250

350

50

0

100

200

300

400

150

250

350

50

0

10-5

10-5 10-4 10-3 10-2 10-1

10-6

10-7

10-8

10-9

10-10

10-11

10-5

10-6

10-7

10-8

10-9

10-10

10-11

103

102

101

100

103

102

101

100

dB/dt (time derivative of the out-of-loop secondary magnetic field), in volts per square meter

285

75

30

30, 7.5, and 3—Collected from high-current transmitter

Repetition frequency data, in hertz

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Root mean square error—

 Minimum-layer model (m) 

 and smooth model (s), in 

 ohm-meters

r.m.s. 

Best-fit smooth inverse model

 Vertical error bar indicates variance of data point 

 with mean at center

A B

A B

Figure 24. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (time-derivative, right axis), and time; B,

depth below land surface and subsurface resistivity modeled from TDEM sounding for sites UL13 and UL14.
58 Hydrostratigraphic Interpretation of Test-Hole and Geophysical Data, Upper Loup River Basin, Nebraska, 2008–10

UL15, r.m.s. =

1,300

1,200

1,100

1,000

900

800

700

600

500

400

300

200

100

0

Depth, in feet

Depth, in meters

Apparent resistivity, in ohm-meters

1 10 100 1,000

1,300

1,200

1,100

1,000

900

800

700

600

500

400

300

200

100

0

Time, in seconds Resistivity, in ohm-meters

 0.88 (m), 0.76 (s)

UL16, r.m.s. =

 1.10 (m), 0.78 (s)

100

200

300

400

150

250

350

50

0

100

200

300

400

150

250

350

50

0

10-5

10-5 10-4 10-3 10-2 10-1

10-6

10-7

10-8

10-9

10-10

10-11

10-5

10-6

10-7

10-8

10-9

10-10

10-11

103

102

101

100

103

102

101

100

dB/dt (time derivative of the out-of-loop secondary magnetic field), in volts per square meter

285

75

30

30, 7.5, and 3—Collected from high-current transmitter

Repetition frequency data, in hertz

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Root mean square error—

 Minimum-layer model (m) 

 and smooth model (s), in 

 ohm-meters

r.m.s. 

Best-fit smooth inverse model

 Vertical error bar indicates variance of data point 

 with mean at center

A B

A B

Figure 25. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (time-derivative, right axis), and time; B,

depth below land surface and subsurface resistivity modeled from TDEM sounding for sites UL15 and UL16.
Supplemental Data 59

UL17, r.m.s. =

1,300

1,200

1,100

1,000

900

800

700

600

500

400

300

200

100

0

Depth, in feet

Depth, in meters

Apparent resistivity, in ohm-meters

1 10 100 1,000

1,300

1,200

1,100

1,000

900

800

700

600

500

400

300

200

100

0

Time, in seconds Resistivity, in ohm-meters

 0.79 (m), 0.42 (s)

UL18, r.m.s. =

 0.83 (m), 0.69 (s)

100

200

300

400

150

250

350

50

0

100

200

300

400

150

250

350

50

0

10-5

10-5 10-4 10-3 10-2 10-1

10-6

10-7

10-8

10-9

10-10

10-11

10-5

10-6

10-7

10-8

10-9

10-10

10-11

103

102

101

100

103

102

101

100

dB/dt (time derivative of the out-of-loop secondary magnetic field), in volts per square meter

285

75

30

30, 7.5, and 3—Collected from high-current transmitter

Repetition frequency data, in hertz

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Root mean square error—

 Minimum-layer model (m) 

 and smooth model (s), in 

 ohm-meters

r.m.s. 

Best-fit smooth inverse model

 Vertical error bar indicates variance of data point 

 with mean at center

A B

A B

Figure 26. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (time-derivative, right axis), and time; B,

depth below land surface and subsurface resistivity modeled from TDEM sounding for sites UL17 and UL18.
60 Hydrostratigraphic Interpretation of Test-Hole and Geophysical Data, Upper Loup River Basin, Nebraska, 2008–10

UL19, r.m.s. =

1,300

1,200

1,100

1,000

900

800

700

600

500

400

300

200

100

0

Depth, in feet

Depth, in meters

Apparent resistivity, in ohm-meters

 0.63 (m), 0.64 (s)

100

200

300

400

150

250

350

50

10 0 -5

10-6

10-7

10-8

10-9

10-10

10-11

103

102

101

100

dB/dt (time derivative of the out-of-loop secondary magnetic field), 

in volts per square meter

1 10 100 1,000

Time, in seconds Resistivity, in ohm-meters

10-5 10-4 10-3 10-2 10-1

285

75

30

30, 7.5, and 3—Collected from high-current transmitter

Repetition frequency data, in hertz

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Root mean square error—

 Minimum-layer model (m) 

 and smooth model (s), in 

 ohm-meters

r.m.s. 

Best-fit smooth inverse model

 Vertical error bar indicates variance of data point 

 with mean at center

A B

Figure 27. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (time-derivative, right axis), and time; B,

depth below land surface and subsurface resistivity modeled from TDEM sounding for site UL19.
Publishing support provided by:

Rolla Publishing Service Center

For more information concerning this publication, contact:

Director, USGS Nebraska Water Science Center

5231 South 19 Street

Lincoln, Nebraska 68512

(402) 328–4100

Or visit the Nebraska Water Science Center Web site at:

http://ne.water.usgs.gov/
Hobza and others—Hydrostratigraphic Interpretation of Test-Hole and Geophysical Data, Upper Loup River Basin, Nebraska, 2008–10—Open-File Report 2011-1289