U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2012–1227

Prepared in cooperation with the Nebraska Department of Natural Resources; and the Upper 

Elkhorn, Lower Elkhorn, Upper Loup, Lower Loup, Middle Niobrara, Lower Niobrara, Lewis and 

Clark, and Lower Platte North Natural Resources Districts; and the University of NebraskaLincoln Conservation and Survey Division

Hydrostratigraphic Interpretation of Test-Hole and Surface 

Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 

2008 to 2011
Front cover. Time-domain electromagnetic data collection in the Lower Elkhorn Natural Resources District, 2011.
Hydrostratigraphic Interpretation of TestHole and Surface Geophysical Data, 

Elkhorn and Loup River Basins, 

Nebraska, 2008 to 2011

By Christopher M. Hobza, Paul A. Bedrosian, and Benjamin R. Bloss

Prepared in cooperation with the Nebraska Department of Natural Resources; 

and the Upper Elkhorn, Lower Elkhorn, Upper Loup, Lower Loup, Middle 

Niobrara, Lower Niobrara, Lewis and Clark, and Lower Platte North Natural 

Resources Districts; and the University of Nebraska-Lincoln Conservation and 

Survey Division

Open-File Report 2012–1227

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: 2012

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Suggested citation:

Hobza, C.M., Bedrosian, P.A., and Bloss, B.R., 2012, Hydrostratigraphic interpretation of test-hole and surface geophysical 

data, Elkhorn and Loup River Basins, Nebraska, 2008 to 2011: U.S. Geological Survey Open-File Report 2012–1227, 

95 p.
iii

Contents

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

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

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

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

Geologic Setting....................................................................................................................................4

Methods...........................................................................................................................................................5

Site and Method Selection..................................................................................................................5

Test-Hole Drilling and Borehole Geophysics ...................................................................................6

Test-Hole Drilling Methods.........................................................................................................6

Borehole Geophysical Data Collection ....................................................................................8

Surface Geophysical Data Collection and Analysis .......................................................................8

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

TDEM Data Collection........................................................................................................9

Time-Domain Electromagnetic (TDEM) Data Analysis........................................................13

Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data...............................13

Test-Hole Lithologic and Borehole Geophysical Logs..................................................................13

TDEM Resistivity Models...................................................................................................................15

Stratigraphic Interpretation of TDEM-Based Resistivity Models......................................16

Evaluation of Stratigraphic Interpretation of TDEM-Based Resistivity Models..............17

Summary........................................................................................................................................................24

References Cited..........................................................................................................................................25

Supplemental Data ......................................................................................................................................29

Figures

1. Map showing location of the Elkhorn-Loup Model study area and Natural

Resources Districts, Nebraska...................................................................................................2

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

soundings, Elkhorn-Loup Model study area, 2008–2011 ........................................................7

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

hole 1-LL-10, Custer County, Nebraska...................................................................................14

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

hole 2-LL-10, Custer County, Nebraska...................................................................................30

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

hole 3-LL-10, Custer County, Nebraska...................................................................................31

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

hole 4-LL-10, Custer County, Nebraska...................................................................................32

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

hole 6-LL-10, Custer County, Nebraska...................................................................................33

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

hole 7-LL-10, Custer County, Nebraska...................................................................................34
iv

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

vertical magnetic field, and elapsed time after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for site UE11...................................................................16

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

vertical magnetic field and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites CGG1 and CGG2.............................................35

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

vertical magnetic field and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites CGG3 and CGG4.............................................36

12. Graphs showing relations between A, central-loop resistivity and time elapsed

after primary current turnoff; and B, depth below land surface and subsurface

resistivity modeled from time-domain electromagnetic sounding for sites CGG5

and CGG6......................................................................................................................................37

13. Graphs showing relations between A, central-loop resistivity and time elapsed

after primary current turnoff; and B, depth below land surface and subsurface

resistivity modeled from time-domain electromagnetic sounding for sites CGG7

and CGG8......................................................................................................................................38

14. Graphs showing relations between A, central-loop resistivity and time elapsed

after primary current turnoff; and B, depth below land surface and subsurface

resistivity modeled from time-domain electromagnetic sounding for sites CGG9

and CGG10....................................................................................................................................39

15. Graphs showing relations between A, central-loop resistivity and time elapsed

after primary current turnoff; and B, depth below land surface and subsurface

resistivity modeled from time-domain electromagnetic sounding for sites CGG11

and CGG12....................................................................................................................................40

16. Graphs showing relations between A, central-loop resistivity and time elapsed

after primary current turnoff; and B, depth below land surface and subsurface

resistivity modeled from time-domain electromagnetic sounding for sites CGG13

and CGG14....................................................................................................................................41

17. Graphs showing relations between A, central-loop resistivity and time elapsed

after primary current turnoff; and B, depth below land surface and subsurface

resistivity modeled from time-domain electromagnetic sounding for sites CGG15

and LL27........................................................................................................................................42

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

vertical magnetic field, and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites CN1 and CN2..................................................43

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

vertical magnetic field, and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites CN3 and CN4..................................................44

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

vertical magnetic field, and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites CN5 and CN6..................................................45
v

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

vertical magnetic field, and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites CN8 and CN9..................................................46

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

vertical magnetic field, and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites LL1 and LL2.....................................................47

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

vertical magnetic field, and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites LL3 and LL4.....................................................48

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

vertical magnetic field, and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites LL5 and LL6.....................................................49

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

vertical magnetic field, and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites LL7 and LL8.....................................................50

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

vertical magnetic field, and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites LL9 and LL10...................................................51

27. Graphs showing relations between A, central-loop resistivity and time elapsed

after primary current turnoff; and B, depth below land surface and subsurface

resistivity modeled from time-domain electromagnetic sounding for sites LL11

and LL12........................................................................................................................................52

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

vertical magnetic field, and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled from time-domain 

electromagnetic sounding for sites LL13 and LL14 ...............................................................53

29. Graphs showing relations between A, central-loop resistivity and time elapsed

after primary current turnoff; and B, depth below land surface and subsurface

resistivity modeled from time-domain electromagnetic sounding for sites LL15

and LL16........................................................................................................................................54

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

vertical magnetic field, and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites LL17 and LL18.................................................55

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

vertical magnetic field, and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites LL19 and LL20.................................................56

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

vertical magnetic field, and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites LL21 and LL22.................................................57
vi

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

vertical magnetic field, and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites LL23 and LL24.................................................58

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

vertical magnetic field, and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites LL25 and LL26.................................................59

35. Graphs showing relations between A, central-loop resistivity and time elapsed

after primary current turnoff; and B, depth below land surface and subsurface

resistivity modeled from time-domain electromagnetic sounding for sites SH1

and SH2. .......................................................................................................................................60

36. Graphs showing relations between A, central-loop resistivity and time elapsed

after primary current turnoff; and B, depth below land surface and subsurface

resistivity modeled from time-domain electromagnetic sounding for sites SH3

and SH4. .......................................................................................................................................61

37. Graphs showing relations between A, central-loop resistivity and time elapsed

after primary current turnoff; and B, depth below land surface and subsurface

resistivity modeled from time-domain electromagnetic sounding for sites SH5

and SH6. .......................................................................................................................................62

38. Graphs showing relations between A, central-loop resistivity and time elapsed

after primary current turnoff; and B, depth below land surface and subsurface

resistivity modeled from time-domain electromagnetic sounding for sites SH7

and SH8. .......................................................................................................................................63

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

vertical magnetic field, and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites TDEM8 and TDEM9.......................................64

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

vertical magnetic field, and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites TDEM11 and TDEM16...................................65

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

vertical magnetic field, and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites TDEM20 and TDEM41...................................66

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

vertical magnetic field, and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites TDEM42 and TDEM43...................................67

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

vertical magnetic field, and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites TDEM44 and TDEM48...................................68

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

vertical magnetic field, and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites TDEM49 and TDEM68...................................69
vii

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

vertical magnetic field, and time elapsed after primary current turnoff; and 

B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites TDEM69 and TDEM70...................................70

46. Graphs showing relations between A, central-loop resistivity and time elapsed

after primary current turnoff; and B, depth below land surface and subsurface

resistivity modeled from time-domain electromagnetic sounding for sites

TDEM71 and TDEM79.................................................................................................................71

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

vertical magnetic field, and time elapsed after primary current turnoff; and 

B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites TDEM84 and TDEM86...................................72

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

vertical magnetic field, and time elapsed after primary current turnoff; and 

B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites TDEM89 and TDEM90...................................73

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

vertical magnetic field, and time elapsed after primary current turnoff; and 

B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites TDEM91 and TDEM99...................................74

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

vertical magnetic field, and time elapsed after primary current turnoff; and 

B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites TDEM140 and TDEM156...............................75

51. Graphs showing relations between A, central-loop resistivity and time elapsed

after primary current turnoff; and B, depth below land surface and subsurface

resistivity modeled from time-domain electromagnetic sounding for sites

TDEM158 and TDEM177.............................................................................................................76

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

vertical magnetic field, and time elapsed after primary current turnoff; and 

B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for site TDEM178...........................................................77

53. Graphs showing relations between A, central-loop resistivity and time elapsed

after primary current turnoff; and B, depth below land surface and subsurface

resistivity modeled from time-domain electromagnetic sounding for sites

TDEM210 and TDEM211.............................................................................................................78

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

vertical magnetic field, and time elapsed after primary current turnoff; and 

B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites TDEM212 and TDEM218...............................79

55. Graphs showing relations between A, central-loop resistivity and time elapsed

after primary current turnoff; and B, depth below land surface and subsurface

resistivity modeled from time-domain electromagnetic sounding for sites

TDEM223 and TDEM224.............................................................................................................80

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

vertical magnetic field, and time elapsed after primary current turnoff; and 

B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites TDEM228 and TDEM232...............................81
viii

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

vertical magnetic field, and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites UE1 and UE2...................................................82

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

vertical magnetic field, and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites UE3 and UE4...................................................83

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

vertical magnetic field, and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites UE5 and UE6...................................................84

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

vertical magnetic field, and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites UE7 and UE8...................................................85

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

vertical magnetic field, and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites UE9 and UE10.................................................86

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

vertical magnetic field, and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites UL1 and UL2 ...................................................87

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

vertical magnetic field, and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites UL3 and UL4 ...................................................88

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

vertical magnetic field, and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites UL5 and UL6 ...................................................89

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

vertical magnetic field, and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites UL7 and UL8 ...................................................90

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

vertical magnetic field, and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites UL9 and UL10 .................................................91

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

vertical magnetic field, and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites UL11 and UL12 ...............................................92

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

vertical magnetic field, and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites UL13 and UL14 ...............................................93

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

vertical magnetic field, and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites UL15 and UL16 ...............................................94
ix

Tables

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

aquifer system and confining units, Elkhorn-Loup Model study area, Nebraska..............5

2. Field name, site number, legal description, location, and selected drilling data of

test holes, Elkhorn-Loup Model study area, 2010....................................................................6

3. Summary by site of Elkhorn-Loup Model time-domain electromagnetic sounding

locations and acquired resistivity data...................................................................................10

4. Stratigraphic contacts resulting from minimum-layer time-domain

electromagnetic models of sounding sites, Elkhorn-Loup Model study area,

2010 ...............................................................................................................................................19

5. Comparison of elevations of selected stratigraphic contacts based on

minimum-layer time-domain electromagnetic model results and test-hole

stratigraphy at Quality Control sites, Elkhorn-Loup Model study area, Nebraska ..........23

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

vertical magnetic field, and time elapsed after primary current turnoff; and 

B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for sites UL18 and UL19 ...............................................95

71. Graphs showing comparison of results from time-domain electromagnetic

minimum-layer model, University of Nebraska-Lincoln Conservation and Survey

Division test holes, and Nebraska Oil and Gas Conservation Commission test

holes..............................................................................................................................................18

72. Map showing five regions within the Elkhorn-Loup Model study area containing

Plio-Pleistocene gravel deposits identified by modeled time-domain

electromagnetic soundings ......................................................................................................22
x

Conversion Factors

Inch/Pound to SI

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)

Rate

inches per year (in./yr) 25.4 millimeters per year (mm/yr)

Hydraulic gradient

foot per mile (ft/mi) 0.1894 meter per kilometer (m/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)

square mile (mi2

) 259 hectare (ha)

square mile (mi2

) 2.590 square kilometer (km2) 

Flow rate

gallon per minute (gal/min) 0.06309 liter per second (L/s)

Frequency

Cycles per second 1 hertz (Hz)

Magnetic moment

Ampere-square foot (A-ft2

) 0.09290 Ampere-square meters (A-m2)

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

 °C=(°F-32)/1.8

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 

(NGVD 29)

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83)
Hydrostratigraphic Interpretation of Test-Hole and 

Surface Geophysical Data, Elkhorn and Loup River Basins, 

Nebraska, 2008 to 2011

By Christopher M. Hobza, Paul A. Bedrosian, and Benjamin R. Bloss

Abstract

The Elkhorn-Loup Model (ELM) was begun in 2006 to 

understand the effect of various groundwater-management 

scenarios on surface-water resources. During phase one of the 

ELM study, a lack of subsurface geological information was 

identified as a data gap. Test holes drilled to the base of the 

aquifer in the ELM study area are spaced as much as 25 miles 

apart, especially in areas of the western Sand Hills. Given the 

variable character of the hydrostratigraphic units that compose 

the High Plains aquifer system, substantial variation in aquifer 

thickness and characteristics can exist between test holes. To 

improve the hydrogeologic understanding of the ELM study 

area, the U.S. Geological Survey, in cooperation with the 

Nebraska Department of Natural Resources, multiple Natural 

Resources Districts participating in the ELM study, and the 

University of Nebraska-Lincoln Conservation and Survey 

Division, described the subsurface lithology at six test holes 

drilled in 2010 and concurrently collected borehole geophysical data to identify the base of the High Plains aquifer system. 

A total of 124 time-domain electromagnetic (TDEM) soundings of resistivity were collected at and between selected testhole locations during 2008–11 as a quick, non-invasive means 

of identifying the base of the High Plains aquifer system.

Test-hole drilling and geophysical logging indicated the 

base-of-aquifer elevation was less variable in the central ELM 

area than in previously reported results from the western part 

of the ELM study area, where deeper paleochannels were 

eroded into the Brule Formation. In total, more than 435 test 

holes were examined and compared with the modeled-TDEM 

soundings. Even where present, individual stratigraphic units 

could not always be identified in modeled-TDEM sounding results if sufficient resistivity contrast was not evident; 

however, in general, the base of aquifer [top of the aquifer 

confining unit (ACU)] is one of the best-resolved results from 

the TDEM-based models, and estimates of the base-of-aquifer 

elevation are in good accordance with those from existing 

test-hole data. Differences between ACU elevations based on 

modeled-TDEM and test-hole data ranged from 2 to 113 feet 

(0.6 to 34 meters). The modeled resistivity results reflect the 

eastward thinning of Miocene-age and older stratigraphic 

units, and generally allowed confident identification of the 

accompanying change in the stratigraphic unit forming the 

ACU. The differences in elevation of the top of the Ogallala, estimated on the basis of the modeled-TDEM resistivity, and the test-hole data ranged from 11 to 251 feet (3.4 to 

77 meters), with two-thirds of model results being within 

60 feet of the test-hole contact elevation. The modeled-TDEM 

soundings also provided information regarding the distribution of Plio-Pleistocene gravel deposits, which had an average 

thickness of 100 feet (30 meters) in the study area; however, in 

many cases the contact between the Plio-Pleistocene deposits and the overlying Quaternary deposits cannot be reliably 

distinguished using TDEM soundings alone because of insufficient thickness or resistivity contrast.

Introduction

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

surface water and groundwater (Ostdiek, 2009), and requires

Natural Resources Districts (NRDs) to include the effect of 

groundwater use on surface-water systems as part of their 

groundwater-management plan. Recent drought (2000–2006) 

has increased concerns about the long-term sustainability of 

surface-water and groundwater resources, and the effect of 

groundwater irrigation on streamflow in the State of Nebraska 

(Peterson and others, 2008). The Elkhorn-Loup Model (ELM) 

was begun in 2006 (Peterson and others, 2008; Stanton and 

others, 2010) to understand the effect of various groundwatermanagement scenarios on surface-water resources. The ELM 

study area covers about 30,800 square miles (mi2

) and includes 

the entire drainage area of the Loup River and its tributaries and the upper part of the Elkhorn River drainage area 

(fig. 1). Upper parts of both stream systems provide substantial 

amounts of water as base flow to more intensely irrigated areas 

downstream. Streamflow within the ELM area can be sensitive to consumptive groundwater use because of interaction 

between groundwater and surface water. The complexity of 
Figure 1. Location of the Elkhorn-Loup Model study area and Natural Resources Districts, Nebraska. Niobrara River North Loup River South Loup River Wood River Beaver Cr. Snake River Loup River Cedar River Dismal River Oak Cr. Plum Cr Goose Cr 

Bazile Cr W

Eagle Cr Mud Cr Long Pine Cr Cedar Creek Holt Cr 

Clear Cr West Birdwood Creek Cr Cedar S Fork Elkhorn River Cache Cr Turkey Cr 

Mud Cr S Fork Elkhorn River Redbird CrVerdigre Creek Clearwater Cr Bone Cr North Branch Middle Loup R Middle Loup River North Fork Dismal River Big CedarCr North Loup River Niobrara River Middle Loup River Elkhorn River Beaver Cr. Loup R

Calamus River Niobrara River Mud Cr Lower Loup Upper Loup Twin Platte Upper Elkhorn Central Platte Middle Niobrara Lower Niobrara 

Lower Pl

Platte Rive

North Platte River 102° 101° 100° 99° 98° 43° 42° 41° INDEX MAP NEBRASKA Sand Hills boundary High Plains aquifer Elkhorn-Loup Model study area 0 30 

0 30 

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

E
Introduction 3

this hydrologic system and the interaction between the available surface-water and groundwater resources are not fully 

understood at present.

The ELM study is a coordinated effort involving the 

Upper Elkhorn, Lower Elkhorn, Upper Loup, Lower Loup, 

Middle Niobrara, Lower Niobrara, Lewis and Clark, and 

Lower Platte North NRDs (collectively referred to hereinafter as the ELM NRDs), the Nebraska Department of Natural 

Resources, the University of Nebraska-Lincoln, Conservation 

and Survey Division (CSD), and the U.S. Geological Survey 

(USGS). The ELM study approach comprises three separate 

phases. Phase one included the initial model construction 

and calibration of simulated groundwater flow (Peterson and 

others, 2008). Phase two included updates to the groundwaterflow simulation using newly collected data and supporting 

analyses completed in 2007 and 2008, improved simulation 

calibration methods, and additional approaches for analyzing 

the effects of agricultural irrigation (Stanton and others, 2010). 

The newly collected data include revisions to the base-of-aquifer map (McGuire and Peterson, 2008) using test-hole drilling 

and borehole geophysics. Phase three will incorporate additional geologic and geophysical data collected in the Upper 

Loup NRD (Hobza and others, 2011), including the datasets 

reported herein.

During phase one of the ELM study, a lack of subsurface geological information was identified as a data gap. Test 

holes drilled to the base of aquifer in the ELM study area are 

spaced up to 25 miles (mi) apart, especially in areas of the 

western Sand Hills, which requires interpolation of subsurface 

data over long distances. Given the variable character of the 

hydrostratigraphic units that compose the High Plains aquifer 

system, substantial variation in aquifer thickness and characteristics can exist between test holes. To improve the hydrogeologic understanding of the ELM study area, the USGS, 

in cooperation with the Nebraska Department of Natural 

Resources, the ELM NRDs, and the University of NebraskaLincoln Conservation and Survey Division, described the 

lithology of drill cuttings collected from six test holes, and 

concurrently collected borehole geophysical data to identify 

selected hydrostratigraphic contacts at these locations. Surface 

geophysical data also were collected from 2008 to 2011 at 

124 locations using time-domain electromagnetic (TDEM) 

methods at and between selected test-hole locations across the 

ELM study area as a quick, non-invasive means of identifying 

the base of the High Plains aquifer system. The data collected 

for this study allow improved knowledge of the aquifer thickness and the topography of the base-of-aquifer surface, which 

are important for assessing water supply and understanding 

groundwater flow.

Purpose and Scope

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

data collected from 2008 to 2011 in the ELM study area. 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 for 

improved understanding of the hydrostratigraphic framework. 

Included in this report are the generalized lithologic and borehole geophysical logs from all six test holes and additional 

TDEM surface geophysical data that were collected. The geologic and geophysical data collected from additional test holes 

and surface geophysical soundings across the ELM study area 

are intended to refine interpretations of the base-of-aquifer 

surface being used for phase three of the ELM study.

A previous, related report was aimed at improving understanding of the hydrostratigraphic framework of the Upper 

Loup NRD (fig. 1) using surface geophysical and test-hole 

drilling information (Hobza and others, 2011). This report 

expands the previous study area to encompass the entire ELM 

study area and includes refinements to the initial interpretations provided by Hobza and others (2011).

Study Area Description

The ELM study area extends from the Niobrara River 

on the northern boundary to the Platte River on the southern 

boundary (fig. 1). The western boundary coincides roughly 

with the western boundary of the Upper Loup and Middle Niobrara NRDs, and the eastern boundary coincides roughly with 

the approximate location of the westernmost extent of glacial 

till in eastern Nebraska (Peterson and others, 2008). The ELM 

study area covers approximately 60 percent of the Sand Hills 

and overlies the nationally important High Plains aquifer 

system (fig. 1). The Sand Hills region of Nebraska is characterized as the largest dune field in the Western Hemisphere and 

currently is stabilized by vegetation (Bleed, 1989).

Land use varies across the study area. Most of the Sand 

Hills, such as in the headwaters of the Loup and Elkhorn Rivers, primarily is undeveloped rangeland. Areas to the east are 

dominated by row-crop agriculture, with corn and soybeans 

being the principal crop types (Strauch and Linard, 2009).

The climate in the ELM study area is characterized 

by cold winters and warm summers typical of continental 

midlatitude locations. Strauch and Linard (2009) examined 

records from precipitation gages in and around the study area 

from October 1939 to September 2005 and reported that mean 

annual precipitation increases from west to east from 17 to 

27 inches (in.), with the highest precipitation occurring during 

the summer months. Potential evaporation, similar to precipitation, is greatest during the crop-growing season of April 

through September, peaks in July, and often exceeds precipitation (Chen and others, 2003). Mean annual evapotranspiration 

(ET) in the study area ranges from 19 inches per year (in/yr) in 

the east to 16 in/yr in the west (Dugan and Zelt, 2000).

More than one-half of the surface of the ELM study area 

is covered by the Sand Hills, where Quaternary eolian dunes 

and alluvial deposits in modern stream valleys cover the land 

surface (Swinehart and Diffendal, 1989). Unique physical 
4 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

characteristics of the Sand Hills have allowed a net gain to 

the groundwater system from recharge of meteoric water 

with time and resulted in a substantial amount of water being 

stored. Runoff-induced high-flow events are rare within the 

Sand Hills because of the ability of permeable soils to infiltrate 

even heavy precipitation. Strauch and Linard (2009) reported 

the simulated maximum deep percolation rate in the Sand Hills 

can exceed 12 in/yr. This results in streamflow that is dominated by groundwater discharge (base flow). Bentall (1989) 

estimated that the Middle Loup and Dismal Rivers (fig. 1) 

above Dunning, Nebr. (Dunning not shown in fig. 1; located 

near the confluence of the Middle Loup and Dismal Rivers), 

have base flows exceeding 95 percent of total streamflow; 

therefore, streamflow in the Middle Loup and Dismal Rivers 

remains steady even during periods of drought and responds 

more slowly to changes in climatic conditions compared to 

streams in the eastern Sand Hills where base-flow contribution 

to total streamflow is less (Chen and others, 2003).

Geologic Setting

The ELM study area overlies the High Plains aquifer 

system, where saturated thicknesses can exceed 1,000 feet 

(ft) [305 meters (m)] (McGuire and Fischer, 1999). The 

High Plains aquifer system is considered to be inclusive of 

all hydrologically connected Tertiary and Quaternary units. 

According to Gutentag and others (1984), these units include 

the fractured part of the Tertiary rocks, the Oligocene Brule 

Formation of the White River Group (herein referred to as the 

Brule), the Miocene Arikaree Group (herein referred to as the 

Arikaree), the Miocene Ogallala Formation (herein referred 

to as the Ogallala), and Quaternary-age alluvial and eolian 

deposits. The unfractured Brule or, where the Brule is not 

present, the Cretaceous Pierre Shale generally form the base 

of aquifer. The base of aquifer surface generally dips 8 feet per 

mile (ft/mi) east and contains paleovalleys that also drain eastward (Peterson and others, 2008). All previously mentioned 

units are shown in the hydrostratigraphic column presented in 

table 1.

The geologic history relevant to this study begins roughly 

70 million years ago with the deposition of Cretaceous-age 

sediments. During the Cretaceous period, much of the study 

area was covered by a shallow inland sea, where marine sediments were deposited to form the Dakota Sandstone, Carlile 

Shale of the Colorado Group (herein referred to as the Carlile 

Shale), Niobrara Formation of the Colorado Group (herein 

referred to as the Niobrara), and Pierre Shale. Following 

regression of the Cretaceous sea, uplift resulted in formation 

of the Chadron and Cambridge arches trending from northwest to southeast across the ELM area (Swinehart and others, 

1985). Subsequent fluvial erosion removed up to 1,800 ft 

(550 m) of the Cretaceous section and created a structural low 

over the previously uplifted region.

In the western part of the study area, the Brule and 

Chadron Formation of the White River Group (herein referred 

to as the Chadron) unconformably overlie the Pierre Shale 

(table 1). The Chadron consists of tuffaceous claystone, 

siltstone, and mudstone (Swinehart and others, 1985). Condon 

(2005) and Swinehart and others (1985) indicated that the 

Chadron underlies the extreme western part of the study area 

and isolated remnants underlie the eastern part (Diffendal and 

others, 2008). The unfractured Brule forms the base of aquifer 

in much of the western part of the ELM area. The Brule is a 

massive siltstone composed of primarily eolian silt but also 

may contain embedded fluvial units. Deposits of volcaniclastic sediments and some ash derived from volcanic complexes 

from the western United States make up most of its volume 

(Souders, 2000). When unfractured, the Brule is impermeable 

relative to the overlying units.

Overlying the Brule is the Arikaree, which is largely 

limited to the western part of the study area but may exist in 

paleovalleys to the east (Swinehart and Diffendal, 1989). The 

Arikaree 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 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 ELM area, the Arikaree typically is 

not used as a water source.

The Ogallala is the principal geologic unit in the High 

Plains aquifer system and reaches a thickness of 800 ft 

(240 m) beneath the western ELM area (Diffendal, 1991). 

The Ogallala is composed of a poorly sorted mixture of sand, 

silt, clay, and gravel (Condra and Reed, 1943). The Ogallala 

is generally unconsolidated or weakly consolidated but can 

contain layers of sandstone cemented by calcium carbonate or 

limestone. When covered by younger deposits, the Ogallala 

has not been subdivided into stratigraphic units recognized 

in other areas because of the difficulty correlating these units 

in the subsurface. The Ogallala was deposited by aggrading 

streams that filled paleovalleys that were eroded into preOgallala rocks (Swinehart and others, 1985). The base of the 

Ogallala is a complex surface formed from multiple episodes 

of erosion. The location of some Ogallala paleovalleys has 

been proposed by previous researchers (Swinehart and Diffendal, 1989; Swinehart and others, 1985), but these may 

represent only a fraction of the drainage network 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 Formation and 

the Pleistocene Long Pine Formations (Swinehart and Diffendal, 1989). The Pleistocene Long Pine Formation is used 

by the CSD and is not formally recognized by the USGS. 

These sediments, eroded from central Wyoming and northern 

Colorado (Stanley and Wayne, 1972), are unevenly deposited 

and preserved, contain coarse sand and gravel separated by 

finer-grained deposits, and cover the Ogallala through much 

of the study area. In the ELM area, Pliocene and Pleistocene 
Methods 5

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 hydrologically connected with the underlying Ogallala. 

The Broadwater and Long Pine Formations, as parts of the 

High Plains aquifer system, are used as a water source where 

sufficient saturated thicknesses are present. Distinguishing 

the age of some of these deposits is difficult, if not impossible 

(Condon, 2005); as such, these deposits are referred to as PlioPleistocene in age.

Quaternary deposits, which consist of wind-deposited 

silt, clay, and fine-grained sand (together called loess), and 

alluvial deposits of sand and gravel cover much of the ELM 

area. Eolian sand and loess deposits typically are not used as 

a water source because they are mostly above the water table. 

Quaternary-age alluvial sand and gravel are present in modern 

stream valleys and are often used as a water source. Where 

sufficient thicknesses are present, these coarse deposits can 

sustain well withdrawals of more than 1,000 gallons per minute (63 liters per second) (Peterson and others, 2008). Quaternary deposits can be as thick as 700 ft (210 m) but typically 

are 150 to 300 feet (46 to 91 m) thick and are absent from 

uplands near the Niobrara River (University of NebraskaLincoln Conservation and Survey Division, 2011).

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 guided by several factors, but primarily 

was intended to fill existing gaps in the test-hole database 

Table 1. Stratigraphic, lithologic, and hydrostratigraphic column of the High Plains aquifer system and confining 

units, Elkhorn-Loup Model study area, Nebraska.

Period Epoch Stratigraphy Lithology Hydrostratigraphy

Quaternary

Holocene Alluvium and eolian

deposits 

Gravel, sand, silt, 

and clay 

High Plains aquifer

system

Pleistocene

Long Pine Formation

and undifferentiated

alluvial and eolian

deposits 

Tertiary

Pliocene Broadwater Formation Gravel and sand

Miocene

Ogallala Group Gravel, sand, silt, 

and clay 

Arikaree Group Very fine to finegrained sandstone

Oligocene

Brule 

Upper Siltstone and

sandstone 

Great Plains

confining system 

Lower Siltstone and

claystone 

Eocene Chadron Formation Siltstone and 

claystone 

Cretaceous Upper

Pierre Shale Shale

Niobrara Formation Chalk, calcareous

shale 

Carlile Shale Shale, limestone,

and sandstone 

Lower Dakota Sandstone Sandstone and

shale 

Great Plains aquifer

system 

Modified from Souders, 2000; Swinehart and Diffendal, 1989; 

Burchett and others, 1975; Gutentag and others, 1984; and 

Helgesen and others, 1993.

White River Group Colorado Group
6 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

(University of Nebraska-Lincoln Conservation and Survey 

Division, 2011). A subset of surface geophysical sounding 

sites was located near previously drilled or planned test-hole 

locations for ground-truth purposes. Test-hole drilling and 

geophysical data-collection locations also depended on site 

accessibility and landowner permission.

Geophysical methods were selected on the basis of physical characteristics of the hydrostratigraphic units of the ELM 

area and the anticipated depth to the base of aquifer. Examination of previously collected CSD borehole geophysical logs 

indicated a sharp contrast in electrical resistivity between the 

Brule or Pierre Shale and the overlying Tertiary units of the 

High Plains aquifer system. After considering the anticipated 

depth to the base of aquifer (top of the Brule or Pierre Shale), 

the TDEM method was chosen. This surface geophysical 

method detects variations in the electrical properties of earth 

materials—in particular, electrical resistivity in units of ohmmeters (ohm-m), or its reciprocal, electrical conductivity in 

units of siemens/meter (S/m). Electrical resistivity measurements are sensitive to water content and quality, and changes 

in lithology. In the absence of clay or other conductive material 

(such as dissolved solids in pore water), lower electrical resistivity corresponds to higher porosity, smaller grain size, or both 

because of the greater surface area associated with fine particles 

that promotes the transmission of electrical current (Biella and 

others, 1983; Kwader, 1985); higher 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 and 

Analysis” section of this report. More information regarding the 

electrical properties of differing types of rocks can be found in 

Keller (1987, 1989), Palacky (1987), Hearst and Nelson (1985), 

Hallenburg (1998), and Hearst and others (2000).

Test-Hole Drilling and Borehole Geophysics

The generalized lithologic descriptions and borehole 

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

High Plains aquifer (top of the silt and siltstone of Brule or 

Pierre Shale) are presented in the “Test-Hole Lithologic and 

Borehole Geophysical Logs” section and “Supplemental 

Data” section of this report. A map of all test holes is shown 

in figure 2, with location coordinates and other information 

presented in table 2. Original copies of the lithologic logs of 

each test hole are kept on file at the USGS Nebraska Water 

Science Center in Lincoln, Nebr. All borehole geophysical logs are archived in accordance with USGS protocol 

(U.S. Geological Survey, written commun., 2009). At the time 

of this report (2012), final stratigraphic interpretations of the 

test-hole data have not been completed. Consequently, the 

stratigraphic units composing the High Plains aquifer system 

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

hole has been interpreted.

Test-Hole Drilling Methods

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 drilling 

and sampling requires the use of mud suitable for geologic 

conditions encountered during drilling. As the drill stem was 

advanced, the time required to advance each increment was 

recorded as well as 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 publication in the CSD test-hole 

database.

Table 2. Field name, site number, legal description, location, and selected drilling data of test holes, Elkhorn-Loup Model study area, 

2010.

[Site number, site identification number assigned by U.S. Geological Survey; legal descriptions, T., township; N, north; R., range; W, west; 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; northing and easting in Nebraska State Plane feet; landsurface elevation in feet above National Geodetic Vertical Datum of 1929 (NGVD 29), elevations to the nearest foot; estimated minus drilled, indicates the 

estimated base-of-aquifer elevation, from McGuire and Peterson (2008), minus the drilled base-of-aquifer elevation at test hole, in feet]

Field 

name

Site number Legal 

description

Northing, 

in feet

Easting, 

in feet

Land-surface 

elevation, 

in feet

Date 

drilled

Drilled depth 

to base of 

aquifer, 

in feet

Estimated 

minus 

drilled, 

in feet

1-LL-10 414426099325301 T.20N R.19W S.6 ABBB 695,061 1,763,705 2,629 8/9/2010 755 26

2-LL-10 413433099281101 T.19N R.19W S.35 ACDC 635,160 1,785,459 2,563 8/11/2010 805 -8

3-LL-10 412407099265301 T.17N R.19W S.36 DABB 571,818 1,791,775 2,461 8/12/2010 695 34

4-LL-10 413143099430701 T.18N R.21W S.15 DBDC 617,629 1,717,462 2,646 8/16/2010 715 -131

6-LL-10 411613099425101 T.15N R.21W S.15 CABB 523,546 1,718,974 2,539 8/19/2010 645 -44

7-LL-10 411308099261401 T.14N R.19W S.1 AABB 505,171 1,795,195 2,463 8/24/2010 565 -48
Figure 2. Location of test holes and time-domain electromagnetic (TDEM) soundings, Elkhorn-Loup Model study area, 2008–2011.

!

!

6-LL-10

6-LL-10

3-LL-10

7-LL-10

2-LL-10

1-LL-10

!

!

!

!

UL19 UL18 UL17

UL16 UL15

UL14

UL13

UL12

UL11

UL10

UL9

UL8 UL7

UL6

UL5

UL4

UL3

UL2

UL2

UL1

UE11

UE10

UE9 UE8

UE7

UE6

UE5

UE4

UE3 UE2

UE1

SH8

SH7

SH6

SH5

SH4

SH3

SH2

SH1

LL27

LL26

LL25

LL24

LL23 LL22

LL21

LL20

LL19

LL18

LL17

LL16

LL15

LL14

LL13

LL12LL11

LL10

LL9

LL8

LL7

LL6

LL5

LL4

LL3

LL1

CN9

CN8

CN6

CN5

CN4

CN3

CN2

CN1

CGG14 CGG15

CGG13

CGG12

CGG11

CGG10

CGG9

CGG8

CGG7

CGG6

CGG5

CGG4

CGG3

CGG2

CGG1

TDEM228

TDEM224

TDEM223

TDEM218

TDEM212

TD

TDEM178

TDEM177

TDE

TDEM156

TDEM99

TDEM91

TDE

TDEM71

TDEM69

TDEM68

TDEM49

TDEM48

TDEM20

TDEM11

LL2

TDEM232

TDEM140

4-LL-10

CHERRY

HOLT

CUSTER

ROCK BROWN

LINCOLN

KNOX

BUFFALO

GRANT

HALL

ARTHUR

BLAINE

LOUP

KEITH

BOONE

DAWSON

HOOKER

ANTELOPE

VALLEY

LOGAN

THOMAS

MCPHERSON

HOWARD

GARFIELD

NAN

WHEELER

MERRICK

SHERMAN

GREELEY

BOYD

SHERIDAN GARDEN

KEYA PAHA

PHELPS

HA

KEARNEY

GOSPER

ADAMS

102° 101° 100° 99° 98°

43°42°41°

[TDEM

! 

Elkhorn-Loup Model area

NEBRASKA

0 25 50 MILES0 25 50 KILOMETERS

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)

Natural Resources District boundaries
8 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

For this report, even though precise contacts of certain 

hydrostratigraphic units have not been determined, lithologic characteristics were noted that can provide information on the hydrostratigraphic units sampled. The top of the 

Ogallala was determined by the presence of siliceous plant 

roots (rootlets) that are easily recognized in drill cuttings. 

This approach has been used by other investigators (Cast, 

2000; Goeke, 2000; Wigley, 2000; University of NebraskaLincoln Conservation and Survey, 2011). Furthermore, 

Condra and Reed (1943) stated that in most areas of western 

and north-central Nebraska, rocks of late Tertiary age contain numerous fossil seeds of grasses and forage herbs that 

provide the best means to identify Tertiary units from the 

upper part of the Arikaree to the top of the Ogallala (Condra 

and Reed, 1943).

Borehole Geophysical Data Collection

Borehole geophysical data were collected at each testhole location using a Century 8144 Multi-Parameter E-Log 

tool (Century Geophysical Corp., 2012). The Century 8144 

tool measures long-normal [64 in. (1.6 m) tool electrode spacing] and short-normal [16 in. (0.41 m) tool electrode spacing] 

resistivity, natural gamma, spontaneous potential, temperature, 

and fluid resistivity. The types of geophysical logs collected 

are described briefly in the following paragraphs. Further 

information regarding borehole geophysics can be found in 

reports such as Keys (1990).

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

resistivity measurements consisted of short-normal [16 in. 

(0.41 m)], and long-normal [64 in. (1.6 m)], each having 

different volumes of investigation. Short-normal logs have 

a smaller volume of investigation and, therefore, are more 

affected by the resistivity of the drilling fluid and mud cake 

that develops on the borehole wall. Correspondingly, longnormal 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).

A natural gamma log measures natural gamma radiation being emitted by the formation surrounding the borehole (Keys, 1990). Clays tend to accumulate radioisotopes 

through adsorption and ion-exchange processes. Most clays, 

particularly illites, have high gamma emissions because of 

the presence of potassium in their crystal structure. Zones of 

high gamma activity are typically interpreted as being clayrich. Volcanic ash, organic shales, 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 in salinity between the borehole fluid and the formation fluid (Keys, 

1990). When the borehole fluid is fresher (less saline) than 

the formation fluid, electric current flow is such that SP will 

deflect in a negative direction; conversely, when the formation fluid is fresher (less saline) than the borehole fluid, then 

electric current flow is such that SP will deflect in a positive 

direction (Stanton and others, 2007).

Test holes were logged to a depth below the base of the 

High Plains aquifer, with the exception of test holes 3-LL-10 

and 7-LL-10. These two test holes were drilled through the 

geologic units composing the High Plains aquifer system and 

into the Pierre Shale. When the drill stem was removed before 

logging these test holes, drill cuttings from the Pierre Shale 

stuck on the bit, and the drill stem fell down hole and bridged. 

This prevented the multi-parameter E-log tool from being lowered down to the bottom of the test holes, resulting in incomplete borehole geophysical logs for both test holes. As a result, 

the base of the High Plains aquifer system at these locations 

was interpreted solely on the basis of lithologic description of 

recovered drill cuttings.

One test-hole drilling location, site 5-LL-10 (not shown 

on map), was abandoned after several attempts of drilling 

because of drilling-fluid loss. Drilling-fluid loss occurred 

several times before the water table was reached, which likely 

was caused by cracks and voids in the thick loess deposits 

above the water table in the area. This test-hole location subsequently was moved to site 7-LL-10.

Surface Geophysical Data Collection and 

Analysis

Time-Domain Electromagnetic (TDEM) Sounding 

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 of 

induction. In applying the TDEM method, secondary magnetic 

fields are produced by the decay of these subsurface eddy 

currents, and typically are measured with one or more receiver 

wire loops at land surface. An apparent resistivity is calculated 

from the measured voltage at the receiver coil and the time 

elapsed after primary current turnoff.

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

subtle differences in the receiver-measured 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 resistivity at late times 

after turnoff. For more complicated, heterogeneous earth 
Methods 9

structures, 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 measured apparent resistivity data 

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).

TDEM Data Collection

A total of 124 TDEM soundings were collected within 

the ELM study area between 2008 and 2011 (fig. 2; table 3). 

In 2008, eight soundings were acquired in close proximity to 

previously drilled CSD test holes. An additional eight soundings were collected in 2009, three of which were followed 

up by new test holes. The 2010 data collection expanded 

to 57 soundings, and in 2011 an additional 51 soundings 

were collected; in both years, locations with sparse test-hole 

coverage were specifically targeted. A subset of the TDEM 

soundings within the Upper Loup Natural Resources District 

was described previously in Hobza and others (2011), but for 

completeness is repeated in this report.

TDEM data were collected using two systems: a Zonge 

NanoTEM/ZeroTEM system (Zonge International, Tuscon 

Ariz., http://www.zonge.com/Zonge.html) was used in 2008, 

and a Geonics ProTEM system (Geonics Limited, Mississauga, Ontario, http://www.geonics.com/) was used during 

2009 through 2011. All data were collected using a 328- by 

328-ft (100- by 100-m) square transmitter loop. The depth 

of investigation (DOI) scales with the transmitter moment 

(number of wire turns times transmitter current times transmitter loop area), with a rule of thumb placing the DOI at 2 to 3 

times the length of a side of the square transmitter loop. 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 central receiver location. Out-of-loop data were used 

as a qualitative check on the assumption of one-dimensionality 

(layer-cake earth structure) required for subsequent modeling and inversion. Out-of-loop data are recorded as the time 

derivative of the secondary magnetic field or dB/dt, where B

is the magnetic induction and t is time. The out-of-loop data 

also were modeled and inverted together with the central-loop 

data to better constrain the final resistivity models; however, 

at many sites the early time out-of-loop data were inconsistent with the early time central-loop data. This inconsistency 

can indicate lateral variability in near-surface resistivity; 

however, early time data also are extremely sensitive to the 

exact positioning of the out-of-loop receiver, and as such, the 

inconsistency may reflect inaccuracy in the receiver position. 

Only the in-loop data were modeled at sites that demonstrated 

this inconsistency.

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 

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). The ProTEM system was re-calibrated in 2011 following system repairs.

During 2009 through 2011, 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 2.0 amperes (A) for the low-current transmitter and 

10.0 A for the high-current transmitter; however, when signal 

levels were low (for example, at the 3-Hz base frequency) 

the current was raised to 20 A. Average transmitter moments 

of 215,280 ampere-square feet (A-ft2) (20,000 A-m2); 

1,076,390 A-ft2 (100,000 A-m2); and 2,152,780 A-ft2

(200,000 A-m2) were produced at 2-A, 10-A, and 20-A currents, respectively.

Data acquired with the Zonge system utilized low- and 

high-power transmitters, coupled with two receivers, a 

single-turn 16.4- by 16.4-ft (5- 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 753,470 A-ft2

 (70,000 A-m2) to 

2,798,620 A-ft2 (260,000 A-m2). At each base frequency, 

as many as 1,024 transients (data curves) were averaged 

to form a single, stored data record. A minimum of 20 data 

records were collected to permit robust error calculation. 

Background noise measurements also were made with each 

receiver at each station by acquiring data with the transmitters turned off. Such noise measurements are used during 

processing to determine the time beyond which measured 

data are contaminated by background noise. As a general 

rule, late-time data were culled when signal-to-noise ratios 

fell below 10.

All data collected with the ProTEM system used air 

coil receivers with moments of 338 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, transients were averaged 

for elapsed time between 4 and 15 seconds to form a single, 

stored data record. A minimum of 20 such records were collected 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. In 

general, late-time data were culled when signal-to-noise ratios 

fell below 10.
10 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

Site 

identifier

Northing, 

in feet

Easting, 

in feet

Land-surface 

elevation, 

in feet

County DOI, 

in feet

MLM r.m.s., 

in ohm-meters

Smooth r.m.s., 

in ohm-meters

Acquisition 

year

CGG1 619,658 1,565,060 2,819 Logan 1,578 1.403 1.194 2011

CGG2 710,781 1,680,430 2,521 Blaine 1,578 2.369 1.704 2011

CGG3 755,845 1,615,650 2,757 Blaine 1,578 0.325 0.735 2011

CGG4 795,948 1,715,440 2,512 Blaine 1,335 0.353 0.303 2011

CGG5 833,235 1,605,900 2,660 Brown 1,453 0.36 0.334 2011

CGG6 895,370 1,612,010 2,722 Brown 1,453 0.345 0.292 2011

CGG7 942,590 1,720,300 2,545 Brown 1,335 0.364 0.37 2011

CGG8 819,890 1,804,420 2,423 Loup 1,227 0.283 0.276 2011

CGG9 730,256 1,741,840 2,458 Loup 1,227 0.463 0.4 2011

CGG10 562,946 1,716,160 2,646 Custer 1,125 0.535 0.437 2011

CGG11 642,151 1,732,320 2,644 Custer 1,227 0.22 0.245 2011

CGG12 587,543 1,641,010 3,026 Custer 1,578 0.716 0.475 2011

CGG13 882,821 1,257,210 3,508 Cherry 1,335 0.424 0.343 2011

CGG14 701,211 1,237,790 3,575 Grant 1,578 0.513 0.401 2011

CGG15 706,582 1,296,100 3,421 Hooker 1,732 0.214 0.204 2009

CN1 787,332 1,223,580 3,587 Grant 2,041 1.98 0.406 2009

CN2 765,055 1,300,110 3,399 Hooker 1,886 0.766 0.796 2009

CN3 583,721 1,460,310 3,044 Logan 1,732 0.638 0.546 2009

CN4 850,755 1,534,410 2,760 Cherry 1,578 0.742 0.509 2009

CN5 898,768 1,532,730 2,823 Cherry 1,732 0.836 0.627 2009

CN6 981,823 1,606,710 2,627 Brown 863 0.703 0.489 2009

CN8 1,049,770 1,545,540 2,720 Cherry 1,453 0.595 0.496 2009

CN9 985,040 1,612,190 2,621 Brown 1,578 0.481 0.407 2009

LL1 674,679 1,953,370 2,163 Valley 945 0.736 0.648 2010

LL2 713,463 1,964,950 2,138 Garfield 1,227 0.472 0.432 2010

LL3 742,801 1,931,490 2,201 Garfield 1,335 0.7 0.639 2010

LL4 680,045 2,001,320 2,188 Greeley 1,578 0.75 0.718 2010

LL5 659,843 1,986,900 2,283 Greeley 1,578 0.878 0.718 2010

LL6 707,648 2,009,740 2,037 Wheeler 1,453 0.397 0.419 2010

LL7 591,210 1,977,870 2,122 Valley 1,227 1.36 1.165 2010

LL8 624,244 2,015,620 2,098 Greeley 863 1.126 0.953 2010

LL9 666,355 2,041,590 2,131 Greeley 1,335 0.662 0.571 2010

LL10 558,710 2,013,980 1,926 Howard 863 0.423 0.384 2010

LL11 502,722 2,016,080 1,985 Howard 479 0.41 0.342 2010

LL12 472,753 2,015,650 1,989 Howard 650 0.404 0.406 2010

LL13 502,630 1,953,930 2,186 Sherman 1,227 0.9 0.794 2010

LL14 433,666 1,892,430 2,192 Buffalo 863 0.542 0.475 2010

LL15 560,276 1,924,580 2,212 Sherman 1,030 0.464 0.399 2010

LL16 581,490 1,949,800 2,191 Valley 787 0.311 0.36 2010

LL17 791,687 1,831,070 2,360 Loup 1,453 1.094 0.873 2010

LL18 766,258 1,827,410 2,292 Loup 1,125 0.523 0.548 2010

Table 3. Summary by site of Elkhorn-Loup Model time-domain electromagnetic sounding locations and acquired resistivity 

data.

[Northing and easting in Nebraska State Plane coordinates; land-surface elevation in feet above National Geodetic Vertical Datum of 1929 

(NGVD 29); DOI, estimated depth of investigation; MLM r.m.s., minimum-layer model root-mean-square resistivity error; smooth r.m.s., smooth 

model root-mean-square resistivity error]
Methods 11

Site 

identifier

Northing, 

in feet

Easting, 

in feet

Land-surface 

elevation, 

in feet

County DOI, 

in feet

MLM r.m.s., 

in ohm-meters

Smooth r.m.s., 

in ohm-meters

Acquisition 

year

LL19 720,956 1,836,210 2,416 Loup 1,030 0.439 0.433 2010

LL20 689,741 1,810,020 24,40 Custer 1,227 1.162 0.961 2010

LL21 619,666 1,838,190 2,319 Custer 1,030 0.308 0.332 2010

LL22 575,615 1,835,500 2,333 Custer 945 1.771 1.283 2010

LL23 571,551 1,791,820 2,467 Custer 1,453 0.715 0.604 2010

LL24 543,306 1,834,790 2,413 Custer 1,335 0.748 0.654 2010

LL25 490,508 1,835,850 2,295 Custer 1,227 0.9 0.729 2010

LL26 683,565 1,877,120 2,370 Valley 1,453 1.509 1.188 2010

LL27 648,907 1,901,310 2,284 Valley 1,125 0.337 0.276 2010

SH1 703,139 1,949,260 2,239 Garfield 1,125 0.474 0.387 2008

SH2 900,203 1,870,970 2,326 Holt 1,030 0.478 0.401 2008

SH3 758,285 1,886,520 2,356 Garfield 945 1.836 1.166 2008

SH4 648,822 1,856,070 2,365 Valley 1,125 0.238 0.224 2008

SH5 765,610 1,795,980 2,353 Loup 1,227 1.299 0.824 2008

SH6 647,381 2,044,570 2,130 Greeley 1,030 0.34 0.325 2008

SH7 895,092 1,420,330 3,039 Cherry 1,030 0.198 0.176 2008

SH8 597,630 1,400,990 3,193 McPherson 945 0.201 0.136 2008

TDEM8 767,368 2,193,390 2,000 Antelope 1,030 0.817 1.146 2011

TDEM9 810,352 2,196,470 1,824 Antelope 1,125 0.641 0.525 2011

TDEM11 825,238 2,037,680 2,035 Holt 1,335 0.416 0.488 2011

TDEM16 597,872 2,203,610 1,698 Nance 1,030 0.645 1.093 2011

TDEM20 794,794 2,135,630 1,950 Antelope 1,453 0.957 0.956 2011

TDEM41 628,152 2,260,090 1,681 Platte 945 0.746 0.582 2011

TDEM42 829,643 2,251,240 1,821 Madison 1,227 0.914 0.742 2011

TDEM43 622,377 2,202,250 1,854 Boone 945 0.822 0.651 2011

TDEM44 711,724 2,200,300 1,943 Boone 1,030 0.442 0.997 2011

TDEM48 585,432 2,074,960 1,995 Greeley 1,453 1.017 1.607 2011

TDEM49 614,318 2,076,010 2,001 Greeley 715 0.67 1.036 2011

TDEM68 597,213 2,135,340 1,882 Nance 1,125 0.539 0.443 2011

TDEM69 621,494 2,138,750 1,933 Boone 1,125 0.487 1.247 2011

TDEM70 850,756 2,248,890 1,762 Pierce 1,125 0.373 0.364 2011

TDEM71 699,973 2,079,710 2,108 Wheeler 1,227 0.557 0.458 2011

TDEM79 656,089 2,333,860 1,651 Platte 715 0.51 0.418 2011

TDEM84 871,215 2,198,140 1,847 Antelope 1,125 0.432 0.369 2011

TDEM86 941,520 2,240,480 1,723 Pierce 787 0.24 0.606 2011

TDEM89 811,358 2,312,160 1,560 Madison 787 0.448 0.38 2011

TDEM90 777,910 2,313,560 1,704 Madison 863 0.46 0.899 2011

TDEM91 487,751 2,080,900 1,880 Howard 945 0.357 0.822 2011

TDEM99 668,366 2,074,890 2,014 Greeley 945 0.462 0.876 2011

TDEM140 888,185 1,968,590 2,099 Holt 1,125 0.638 0.753 2011

TDEM156 702,988 2,109,730 2,126 Boone 863 0.63 1.534 2011

Table 3. Summary by site of Elkhorn-Loup Model time-domain electromagnetic sounding locations and acquired resistivity 

data.—Continued

[Northing and easting in Nebraska State Plane coordinates; land-surface elevation in feet above National Geodetic Vertical Datum of 1929 

(NGVD 29); DOI, estimated depth of investigation; MLM r.m.s., minimum-layer model root-mean-square resistivity error; smooth r.m.s., smooth 

model root-mean-square resistivity error]
12 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

Site 

identifier

Northing, 

in feet

Easting, 

in feet

Land-surface 

elevation, 

in feet

County DOI, 

in feet

MLM r.m.s., 

in ohm-meters

Smooth r.m.s., 

in ohm-meters

Acquisition 

year

TDEM158 933,403 2,198,670 1,747 Antelope 1,030 0.623 0.517 2011

TDEM177 916,371 2,067,880 1,909 Holt 787 0.511 0.428 2011

TDEM178 900,104 2,061,040 1,933 Holt 945 0.618 1.124 2011

TDEM209 835,359 2,196,680 1,873 Antelope 1,125 1.144 0.82 2011

TDEM210 875,958 2,242,030 1,709 Pierce 1,030 0.457 0.377 2011

TDEM211 906,612 2,279,380 1,723 Pierce 945 0.774 0.625 2011

TDEM212 923,617 1,951,490 2,130 Holt 1,030 1.073 0.872 2011

TDEM218 598,191 2,013,350 2,045 Greeley 1,030 0.533 0.448 2011

TDEM223 799,172 1,198,470 3,728 Grant 1,732 0.659 0.521 2011

TDEM224 824,062 1,193,740 3,659 Grant 1,732 0.555 0.457 2011

TDEM228 777,070 1,187,490 3,678 Grant 1,886 0.499 0.751 2011

TDEM232 782,646 1,286,340 3,436 Hooker 1,732 0.592 0.462 2011

UE1 947,419 1,956,570 2,071 Holt 1,335 0.815 0.696 2010

UE2 915,620 1,950,450 2,131 Holt 1,335 0.566 0.47 2010

UE3 899,780 1,846,380 2,379 Rock 1,453 0.505 0.45 2010

UE4 925,047 1,877,700 2,250 Holt 1,227 0.475 0.484 2010

UE5 957,904 1,875,210 2,209 Holt 945 0.79 0.673 2010

UE6 944,357 1,956,270 2,083 Holt 1,227 0.928 0.673 2010

UE7 881,159 1,958,390 2,118 Holt 1,227 1.272 1.04 2010

UE8 854,451 1,954,880 2,172 Holt 1,335 0.58 0.49 2010

UE9 840,477 2,001,090 2,075 Holt 1,227 0.665 0.568 2010

UE10 871,712 2,006,140 2,050 Holt 863 0.935 2.263 2010

UE11 1,003,050 1,863,850 2,160 Holt 1,125 0.368 0.278 2010

UL1 936,364 1,307,910 3,297 Cherry 1,732 0.941 0.769 2010

UL2 932,214 1,272,210 3,379 Cherry 1,886 0.857 0.706 2010

UL3 907,558 1,312,120 3,385 Cherry 2,041 1.319 1.074 2010

UL4 881,714 1,298,340 3,416 Cherry 1,732 0.857 0.716 2010

UL5 900,378 1,313,570 3,372 Cherry 2,041 0.791 0.706 2010

UL6 779,134 1,417,440 3,103 Thomas 2,195 1.157 0.976 2010

UL7 801,809 1,428,440 3,078 Thomas 1,732 0.635 0.58 2010

UL8 800,500 1,416,730 3,081 Thomas 1,732 0.544 0.488 2010

UL9 743,101 1,536,170 2,807 Thomas 1,732 0.716 0.459 2010

UL10 767,240 1,509,760 2,874 Thomas 1,578 0.772 0.722 2010

UL11 714,076 1,553,040 2,763 Thomas 1,578 0.815 0.783 2010

UL12 812,076 1,511,000 2,937 Thomas 1,453 0.379 0.328 2010

UL13 91,4058 1,457,900 2,929 Cherry 1,578 0.461 0.408 2010

UL14 88,3009 1,451,770 2,966 Cherry 1,886 0.694 0.555 2010

UL15 70,5348 1,439,260 3,105 Thomas 1,578 0.778 0.654 2010

UL16 69,6800 1,472,010 2,994 Thomas 1,578 0.733 0.678 2010

UL18 75,1838 1,354,880 3,258 Hooker 1,578 0.754 0.579 2010

UL19 74,6562 1,450,500 3,039 Thomas 1,886 0.646 0.562 2010

Table 3. Summary by site of Elkhorn-Loup Model time-domain electromagnetic sounding locations and acquired resistivity 

data.—Continued

[Northing and easting in Nebraska State Plane coordinates; land-surface elevation in feet above National Geodetic Vertical Datum of 1929 

(NGVD 29); DOI, estimated depth of investigation; MLM r.m.s., minimum-layer model root-mean-square resistivity error; smooth r.m.s., smooth 

model root-mean-square resistivity error]
Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data 13

Time-Domain Electromagnetic (TDEM) Data 

Analysis

Data analysis for all soundings consisted of data format 

conversion, statistical analysis and averaging, data inversion, model assessment, and forward modeling. 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 was chosen because it permits full waveform 

specification, the modeling of system filters, and the incorporation of data errors. Furthermore, it provides error bounds on 

inverted parameters (layer thicknesses and resistivities). Data 

were inverted for two end-member model classes: minimumlayer inverse 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 

assumption that resistivity varies gradually between adjacent 

layers. The fit of the model to the data is evaluated by comparing the model’s forward response, or synthetic “measured” 

data, to the data. Minimum-layer models are more appropriate 

in settings where abrupt changes in resistivity are expected, 

whereas smooth models are more realistic when gradational 

changes are expected. Independent knowledge from geologic 

mapping, borehole lithology, and geophysical borehole logs 

typically are used to evaluate which model class is more applicable for interpreting subsurface structure at a specific study 

area.

The DOI estimates at each site were calculated using the 

approach of Christiansen and Auken (2010). Based on a linear 

sensitivity analysis, this approach provided an estimate of the 

maximum model depth that was constrained by the measured 

data. These DOI estimates are based on a specified sensitivity threshold, and are viewed as relative, rather than absolute. Within the ELM study area, the average DOI of TDEM 

soundings was 1,288 ft (393 m), and values ranged from 479 

to 2,195 ft (146 to 669 m). The DOI estimate for each TDEM 

sounding is presented in table 3.

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

given the nature of the regional geology (laterally expansive 

sedimentary deposits), the compact footprint of soundings by 

the TDEM method, and the limited depth of investigation. 

Furthermore, lithologic and stratigraphic logs from surrounding test holes show lateral correlation across the study area. 

Finally, with the exception of the 2008 soundings, where outof-loop data were not recorded, joint inversion of the in-loop 

and out-of-loop data were consistent with a 1-D subsurface 

resistivity structure.

Geophysical methods, including TDEM, exhibit a degree 

of model non-uniqueness. As such, available ground truth 

from test-hole logs was incorporated during and following 

modeling and inversion of TDEM data, and provided important guidance during interpretations. The non-uniqueness 

stems from inherent resolution limitations, and incomplete 

and inexact data. Thus, for any set of measured data, a range 

of models can typically be constructed 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 resistivity of 10 ohm-m 

and a 164-ft- (50-m-) thick layer produces a nearly identical 

forward response as a model with a resistivity of 2 ohm-m and 

a 33-ft- (10-m-) thick layer. Additionally, the TDEM method 

has difficulty resolving resistive layers because of the lowcurrent densities induced within them as compared to conductive layers.

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 the six test holes are presented in 

figures 3 through 8. An example test hole is shown in figure 3 

with generalized lithologic description and borehole geophysical logs. Logs from all other test holes are located 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 from the borehole logs in comparison with the 

test-hole lithologic data led to the following hydrostratigraphic 

interpretations.

The base of aquifer is indicated in the generalized lithologic log by the top of the siltstone unit of the Brule or the 

top of the Pierre Shale. When a complete geophysical log was 

collected, the long- and short-normal resistivity logs indicated 

the base of the High Plains aquifer system by sharp contrasts 

in resistivity between the Brule or Pierre Shale and the overlying units. Examining the long- and short-normal resistivity, the 

resistivity of sediments overlying the Brule ranged from less 

than 50 to greater than 250 ohm-m. Within these sediments, 

resistivity for the Ogallala (units containing rootlets) ranged 

between 50 and 200 ohm-m (and more typically between 

75 and 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 from 150 ohm-m to more than 

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

was less than 25 ohm-m and, in most cases, less than 15 ohmm, and the underlying Pierre Shale had resistivity typically 

less that 15 ohm-m.

Other logs provided further support for the base-of-aquifer interpretation. Natural gamma logs indicated an increase 

in activity near the base of the aquifer because of increased 
14 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

Depth, 

in feet Gamma

0 Counts per second 003

Spontaneous potential

001- Millivolts 001

Short-normal resistivity (16 inches)

0 Ohm-meters 005

Long-normal resistivity (64 inches)

0 Ohm-meters 005

Generalized lithology

Topsoil, sandy silt, trace

clay

Silty very fine sand, some

sandstone

Very fine to fine sand

Silt, some clay

Fine to coarse gravel, silty

in parts

Fine to coarse sand, some

gravel, silty in parts

Silty sand, sandstone,

very fine to fine, rootlets 

Brule Formation

silt/siltstone

100

200

300

400

500

600

700

800

0

Figure 3. Composite of generalized lithologic description and geophysical logs for test hole 1-LL-10, Custer County, Nebraska. 

Short-normal resistivity, in ohm-meters; long-normal resistivity, in ohm-meters; natural gamma activity, in counts per second; 

spontanenous potential, in millivolts.
Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data 15

volcanic ash content of the Brule or clay content in the Pierre 

Shale. Spontaneous-potential logs can indicate the degree to 

which the drilling fluid invaded the formation, and the relative 

impermeability of the Brule and Pierre Shale typically resulted 

in a positive deflection indicating that minimal drilling fluid 

invaded this interval.

While drilling test hole 2-LL-10, a hard, sandy claystone 

was encountered at 805 to 807 ft (245 to 246 m) below land 

surface, which plugged the drill bit and caused a loss in circulation of drilling fluid. Consequently, the drill stem had to be 

promptly removed and a geophysical log was collected immediately. Because the hole was only drilled 2 ft (0.6 m) into the 

claystone unit, the geophysical log did not measure any of the 

physical properties in the claystone unit, making an interpretation of the base of aquifer difficult. Cast (2000) reported that 

in test hole 12-A-66, approximately 6 miles (mi) [10 kilometers (km)] northwest (not shown on map), a thin layer of Brule 

was drilled before the Chadron was encountered. Although 

no Brule cuttings were noted in the cuttings from 2-LL-10, it 

is possible that the sandy claystone encountered was another 

eastern outlier of the Chadron. For this report, the authors have 

picked the base of aquifer at the top of this claystone unit at 

805 feet (245 m) below land surface.

Differences in the elevation of the base of the principal 

aquifer for test holes drilled after the completion of Scientific Investigations Map 3042 (McGuire and Peterson, 2008) 

were computed by subtracting the estimated base-of-aquifer 

elevation from the drilled (table 2). The range of differences 

between the elevations of the drilled base of aquifer and the 

previously estimated base of aquifer (McGuire and Peterson, 

2008) was from -131 to 34 ft (-40 to 10 m). The upper end of 

this range is less than those previously reported for the western 

part of the ELM area (Hobza and others, 2011), where deeper 

paleochannels were eroded into the Brule.

Researchers have indicated that Pliocene-age or younger 

gravel covers much of the ELM area (Diffendal and others, 

2008; Souders, 2000; Swinehart and Diffendal, 1989). Sand 

and gravel deposits overlaid intervals of sandstone containing 

rootlets in all test holes. Other researchers, including Souders 

(2000), have noted a Plio-Pleistocene gravel unit that ranged 

from 45 to 165 ft (13 to 50 m) in thickness and that frequently 

was interbedded with silt or clay. The sand and gravel layers 

encountered in test holes in Custer County were thicker than 

those encountered by previous drilling in the Upper Loup 

NRD (Hobza and others, 2011).

TDEM Resistivity Models

Figures 9 through 70 present the measured data from the 

123 utilized (1 TDEM site not used, discussed later in this 

section) TDEM stations together with best-fit inverse models. 

An example sounding is shown in figures 9A and 9B. All other 

soundings data are located in the “Supplemental Data” section 

of this report. At each station, the left panel (A) shows the 

measured data (error bars) as well as the forward responses 

of the two inverse models (curves), whereas the right panel 

(B) shows the best-fit minimum-layer (red) and smooth 

(blue) inverse model. For all soundings, the central-loop data 

are plotted as apparent resistivity. Out-of-loop data for the 

secondary vertical magnetic field, which were collected for 

all 2009–2011 soundings, are plotted (when used for interpretation) as the time derivative of the induced magnetic 

field (dB/dt) in volts per square meter (upper set of curves, 

see figure captions for details on plots). In most cases, the 

model response falls within the variance of the measured data; 

exceptions are noted for the out-of-loop data collected above 

conductive ground, which demonstrate a sign change at early 

times. This sign change is associated with the passage of time 

for peak current density to travel outward from the transmitter 

loop, and is extremely sensitive to the position of the receiver 

relative to the transmitter loop.

In general, the low root-mean-square (r.m.s.) errors for 

the minimum layer and smooth models (table 3), and small 

error bounds on the data (figs. 9–70), even into later times, 

yielded high-quality data. This reflects the low density of 

human population within the survey area, and consequent low 

level of cultural noise (power lines, pipelines, well pumps, 

electric fences). A small number of soundings (eight) 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 

DOI. One sounding (UL17) proved to be completely contaminated by cultural noise; the data were found to be inconsistent with nearby TDEM soundings as well as lithologic/

stratigraphic logs from nearby test holes. Consequently, this 

sounding was removed from the dataset.

The resistivity models, in most cases, reproduced the 

measured data to within the estimated uncertainty of measurement r.m.s. error of less than 1 ohm-m. The r.m.s. errors for 

all soundings for the two models are indicated in table 3. In 

general, the r.m.s. error is slightly larger for the minimumlayer models than for the smooth models, reflecting the greater 

difficulty in fitting the data with a small number of model 

parameters. R.m.s. errors (table 3) 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, which includes a number of sharp unconformities associated with erosion and changes from marine to 

alluvial to eolian deposition.

Each minimum-layer model is the result of an unregularized, parameterized inversion and, as such, error bounds on the 

inverted parameters (layer thicknesses, depth-to-layer interfaces, and resistivities) can be calculated from a model covariance matrix output by the inversion (Aarhus Hydrogeophysics 

Group, 2008). The resulting standard errors define an asymmetrical error window, where the upper and lower bounds of 

parameter p are calculated by equations 1 and 2:
16 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

 (1)

 (2)

where

p is the parameter (resitivity, thickness, or 

depth)

pu is the upper bound of parameter 

pl is the lower bound of parameter 

σp is the standard error of parameter, and is 

computed using equation 3.

 (3)

where

e is an exponential

cov(p,p) is the model covariance matrix of parameter p.

The resulting parameter bounds, in particular the bounds on 

layer-interface depths, are important in assessing the relative 

uncertainty of geophysical and test-hole constraints on aquifer 

and aquifer-confining-unit (ACU) stratigraphy. Model-derived 

constraints on stratigraphic boundaries, together with their 

associated upper and lower bounds, discussed in the next section, are provided in spreadsheet format that can be accessed 

on the Internet at http://pubs.usgs.gov/of/2012/1227/.

Stratigraphic Interpretation of TDEM-Based 

Resistivity Models

To aid in assigning stratigraphy to the minimum-layer 

TDEM resistivity models, a comparison was made of each 

model to lithologic and stratigraphic logs from all test holes 

within roughly 6 mi (10 km) of the TDEM sounding. In total, 

more than 435 test holes were examined in relation to the 

TDEM resistivity models. This comparison proved useful in 

Figure 9. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, right 

axis), and elapsed time after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled from timedomain electromagnetic sounding for site UE11.

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

Apparent resistivity, in ohm-meters

Time derivative of out-of-loop magnetic field, in volts per square meter

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

Depth, in feet

Depth, in meters

285

75

30

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

Repetition frequency data, in hertz

EXPLANATION

Vertical error bar indicates variance of data point

with mean at center

Best-fit minimum-layer model

EXPLANATION

Best-fit smooth inverse model

A B

SITE UE11

Time, in seconds Resistivity, in ohm-meters

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

pu p = ∗ p ( ) 1+ σ

pl

p

p =

1+ σ

σp

p p = − e cov , ( ) 1
Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data 17

identifying characteristic resistivity ranges and thickness patterns for each stratigraphic unit, in addition to spatial trends in 

the elevation and depth to stratigraphic interfaces. An example 

of the type of comparison made is shown in figure 71. A 

limitation of CSD test-hole data, however, is that few boreholes extend beyond the base of the High Plains aquifer 

system, defined generally by the Brule in the west, the Pierre 

Shale in the east, and the Niobrara along the eastern edge of 

the ELM study area. To examine the correlation with deeper 

stratigraphic layers, more than 93 deep boreholes within the 

Nebraska Oil and Gas Conservation Commission (2012) database were examined. These wells complement the test-hole 

database because they typically contain stratigraphic information only for the Pierre Shale and below. These comparison 

studies led to further modeling of subsets of the TDEM data; 

the final stratigraphic picks in table 4 and the online spreadsheet in the Supplemental Data are the result of several additional rounds of modeling and test-hole comparison.

The resulting stratigraphic picks (table 4) include the 

base of Quaternary deposits (BQT), the top and base of PlioPleistocene gravel (TPP/BPP), the top and base of the Ogallala (TOG/BOG), and the top of the ACU (Brule, Pierre Shale, 

Niobrara,or Carlile Shale). In addition, stratigraphy below the 

ACU can often be identified (Carlile Shale or Dakota Sandstone) and, for sites where such units were distinguished, is 

provided in the spreadsheet available at http://pubs.usgs.gov/

of/2012/1227/. Not all stratigraphic units are present at all 

sounding locations, and the ACU varies throughout the ELM 

study area from the Brule in the west to Pierre Shale, Niobrara, or Carlile Shale in the east. Even where present, individual stratigraphic units could not always be picked if sufficient 

resistivity contrast was not evident. This is particularly true 

for the Miocene and younger deposits, the thickness and 

resistivity of which are highly variable throughout the ELM 

study area. This reflects the complex fluvial deposition pattern 

of Ogallala and younger sediments relative to older marine 

and eolian deposits. In general, the base of aquifer (top of the 

ACU) is one of the best-resolved interfaces from the resistivity 

models. Estimates of the base-of-aquifer elevation are in good 

accordance with those from existing test-hole data.

The TDEM data generally support four- to nine-layer 

models for resistivity stratigraphy. The modeled resistivity 

results reflect the eastward thinning of Miocene-age and older 

stratigraphic units, and generally allow confident interpretation 

of the accompanying change in the stratigraphic unit forming 

the ACU. TDEM resistivity models from the western ELM 

study area (Cherry, Grant, Hooker, Thomas, Arthur, McPherson, and Logan Counties) exhibit decreasing resistivity with 

increasing depth below about 300–400 ft (91–120 m) below 

land surface. In many cases, these models indicated a thick 

Ogallala sequence atop the low-resistivity (12.6±4.9 ohm-m) Brule 

or Pierre Shale. The underlying Pierre Shale (4.7±1.5 ohm-m) 

was identified except in parts of Cherry, Grant, Hooker, and 

McPherson Counties, where a thick Tertiary section limited 

the utility of TDEM resistivity models to resolve stratigraphy beneath the Brule. The central ELM study area (Blaine, 

Brown, Custer, Loup, and Rock Counties) is characterized by 

a thin or absent Brule and eastward thinning Pierre Shale; at 

most locations, the underlying Niobrara (20.7±17.3 ohm-m) 

also was distinguished. Within the eastern ELM study area, the 

Brule is absent, except in parts of Holt and Antelope Counties, 

where test-hole data define an east-west trending strip of Brule 

in the subsurface. Along the easternmost edge of the ELM 

study area, no rocks of Miocene or Oligocene age (Ogallala 

and Brule) are preserved, and the Pierre Shale and Niobrara 

also are absent in several locations. This part of the ELM 

study area contains the oldest geologic structure, and a number 

of TDEM resistivity models allowed the tops of the Carlile 

Shale (6.8±2.5 ohm-m) and Dakota Sandstone (21.4±9.1 ohmm) to be distinguished.

The TDEM resistivity models also provided information regarding the distribution of Plio-Pleistocene (PP) gravel 

deposits within the study area. These deposits are typically 

more resistive (about 100 ohm-m) than the overlying Quaternary sediments or the underlying Ogallala. The average 

thickness of the PP deposits in the ELM study area is 100 ft 

(30 m), although in many cases the PP cannot be uniquely 

distinguished from the overlying Quaternary deposits and/or 

the underlying Ogallala Formation using only TDEM soundings. In such cases, the estimated PP thickness is likely an 

overestimate. PP deposits are unevenly distributed throughout the ELM study area; however, five regions (fig. 72) with 

PP deposits can be broadly identified from the TDEM-based 

resistivity models. In the southwest, two small regions are 

observed: one in Grant and Hooker Counties, and the other in 

Thomas, Logan, and potentially McPherson Counties. These 

regions are separated by several TDEM soundings without an 

identifiable PP layer. Furthermore, this separation is in agreement with Swinehart and Diffendal (1989), who identified 

areas of thin or absent gravels. Further east, a largely contiguous region includes eastern Loup County; much of Garfield, 

Valley, and Sherman Counties; and smaller parts of Wheeler 

and Greeley Counties. A smaller region of PP deposits was 

evident in south-central Holt County. Finally, at the northeast 

edge of the ELM study area, PP deposits are inferred near the 

intersection of Antelope, Pierce, and Madison Counties.

Evaluation of Stratigraphic Interpretation of 

TDEM-Based Resistivity Models

For this study, a subset of all the TDEM sites was 

collected near existing or planned test holes. These sites, 

referred to hereinafter as quality-control (QC) sites, were 

located across the ELM area. Efforts were made to collect 

TDEM data as close as possible to the test hole, but often the 

TDEM sounding was up to 0.31 mi (0.5 km) away because of 

the proximity of cultural noise. For other soundings, contacts between various stratigraphic intervals (table 4) were 

identified, where distinguishable, based on the minimumlayer TDEM resistivity-model results presented in figures 9 

through 70. The depths of the stratigraphic picks were 
18 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

 26115050020000

 26071050040000 26115050050000

 LL17

-30,000 -25,000 -20,000 -15,000 -10,000 -5,000 0 5,000 10,000 15,000 20,000 25,000 30,000 35,000

Elevation, in meters above National Geodetic Vertical Datum of 1929 

-35,000 -30,000 -25,000 -20,000 -15,000 -10000 -5,000 0 5,000 10,000 15,000 20,000 25,000 30,000

500

750

1,000

1,250

1,500

1,750

2,000

2,250

2,500

Elevation, in feet above National Geodetic Vertical Datum of 1929 

 2-S-84

 4-CH-82

 5-B-83

 57-HP-79

 LL17

-30,000 -25,000 -20,000 -15,000 -10,000 -5,000 0 5,000 10,000 15,000 20,000 25,000 30,000

Distance, in feet

 2-S-84 4-CH-82

 5-B-83

 57-HP-79

 LL17

1 50 100 150 200 250

A B C

Niobrara 

Carlile Shale

Dakota Sandstone

No data

 

Clay

Gravel

Sand

Shale

Silt

Siltstone

Sandstone

200

250

300

350

400

450

500

550

600

650

700

750

Quaternary, undifferentiated

Plio-Pleistocene gravel

Ogallala 

Niobrara 

Carlile Shale

Dakota Sandstone

Pierre Shale No data

University of Nebraska-Lincoln Conservation and

Survey Division (CSD) stratigraphy

University of Nebraska-Lincoln Conservation and 

Survey Division (CSD) lithology

Nebraska Oil and Gas Conservation Commission 

(NOGCC) stratigraphy

Quaternary, undifferentiated

Plio-Pleistocene gravel

Ogallala 

Pierre Shale

EXPLANATION

Minimum-layer model resistivity, in ohm-meters for time-domain electromagnetic (TDEM) sounding LL17

Figure 71. Comparison of results from time-domain electromagnetic minimum-layer model, University of Nebraska-Lincoln Conservation and Survey Division (CSD) test holes, and Nebraska Oil and Gas Conservation Commission (NOGCC) test holes. The modeled resistivity data 

collected in 2010 from time-domain electromagnetic (TDEM) sounding site LL17 are compared to A, CSD test-hole stratigraphy, B, CSD lithologic descriptions, and C, NOGCC test-hole stratigraphy.
Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data 19

Table 4. Stratigraphic contacts resulting from minimum-layer time-domain electromagnetic models of sounding sites, 

Elkhorn-Loup Model study area, 2010.

[Elevation, land-surface elevation above National Geodetic Vertical Datum of 1929 (NGVD 29); all depths given in feet below land surface; 

TQT, top of Quaternary; BQT, depth to bottom of Quaternary; TPP, depth to top of Plio-Pleistocene gravel; BPP, depth to bottom of Plio-Pleistocene 

gravel, italicized where top of unit is not distinguishable with overlying Quaternary units; TOG, depth of top of Ogallala; BOG, depth of bottom of 

Ogallala; TACU, depth to top of aquifer confining unit; ACU, aquifer confining unit; ND, depth of contact not determined; BR, Brule; --, contact 

not present; PI, Pierre Shale; NI, Niobrara Formation; CA, Carlile Shale]

Site 

identifier Elevation TQT BQT TPP BPP TOG BOG TACU ACU

CGG1 2,819 0 ND ND 152 152 754 754 BR

CGG2 2,521 0 166 -- -- 166 474 474 BR

CGG3 2,757 0 118 -- -- 118 855 855 BR

CGG4 2,512 0 54 -- -- 54 710 710 BR

CGG5 2,660 0 117 -- -- 117 693 693 BR

CGG6 2,722 0 52 -- -- 52 668 668 BR

CGG7 2,545 0 200 -- -- 200 543 543 PI

CGG8 2,423 0 136 136 247 247 616 616 BR

CGG9 2,458 0 67 -- -- 67 578 578 BR

CGG10 2,646 0 322 -- -- 322 912 912 PI

CGG11 2,644 0 187 -- -- 187 868 868 PI

CGG12 3,026 0 190 -- -- 190 1,163 1,163 BR

CGG13 3,508 0 98 -- -- 98 758 758 BR

CGG14 3,575 0 ND ND 347 347 921 921 BR

CGG15 3,421 0 97 -- -- 97 860 860 BR

CN1 3,587 0 ND ND 166 166 1,145 1,145 BR

CN2 3,399 0 158 158 312 312 1,042 1,042 BR

CN3 3,044 0 161 -- -- 161 760 760 BR

CN4 2,760 0 68 -- -- 68 553 553 BR

CN5 2,823 0 104 -- -- 104 557 557 BR

CN6 2,627 0 ND ND 82 82 568 568 BR

CN8 2,720 0 41 -- -- 41 355 355 BR

CN9 2,621 0 ND ND 46 46 513 513 BR

LL1 2,163 0 74 -- -- 74 643 643 PI

LL2 2,138 0 ND ND 237 237 554 554 PI

LL3 2,201 0 45 45 172 172 534 534 PI

LL4 2,188 0 ND ND 51 51 656 656 PI

LL5 2,283 0 ND ND 114 114 764 764 PI

LL6 2,037 0 ND ND 80 80 495 495 PI

LL7 2,122 0 77 -- -- 77 472 472 PI

LL8 2,098 0 182 -- -- 182 467 467 PI

LL9 2,131 0 212 -- -- 212 566 566 PI

LL10 1,926 0 55 -- -- 55 324 324 PI

LL11 1,985 0 107 -- -- 107 334 334 PI

LL12 1,989 0 43 -- -- 43 269 269 PI

LL13 2,186 0 71 71 144 144 532 532 PI

LL14 2,192 0 155 155 259 259 375 375 PI

LL15 2,212 0 0 -- 260 260 701 701 PI

LL16 2,191 0 73 -- -- 73 482 482 PI

LL17 2,360 0 80 80 241 241 717 717 PI

LL18 2,292 0 23 23 125 125 558 558 PI

LL19 2,416 0 138 138 249 249 572 572 PI
20 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

Site 

identifier Elevation TQT BQT TPP BPP TOG BOG TACU ACU

LL20 2,440 0 67 67 246 246 538 538 PI

LL21 2,319 0 98 -- -- 98 589 589 PI

LL22 2,333 0 167 -- -- 167 743 743 PI

LL23 2,467 0 317 -- -- 317 679 679 PI

LL24 2,413 0 222 -- -- 222 606 606 PI

LL25 2,295 0 124 -- -- 124 453 453 PI

LL26 2,370 0 ND ND 392 392 742 742 PI

LL27 2,284 0 55 55 96 96 896 896 PI

SH1 2,239 0 ND ND 327 327 604 604 PI

SH2 2,326 0 92 -- -- 92 520 520 PI

SH3 2,356 0 260 -- -- 260 580 580 PI

SH4 2,365 0 193 -- -- 193 670 670 PI

SH5 2,353 0 215 -- -- 215 590 590 PI

SH6 2,130 0 195 -- -- 195 515 515 PI

SH7 3,039 0 64 -- -- 64 630 630 BR

SH8 3,193 0 ND ND 592 592 754 754 BR

TDEM8 2,000 0 216 -- -- 216 635 635 NI

TDEM9 1,824 0 ND ND 192 192 759 759 NI

TDEM11 2,035 0 130 -- -- 130 622 622 PI

TDEM16 1,698 0 107 -- -- 107 228 228 NI

TDEM20 1,950 0 ND ND 299 299 414 414 PI

TDEM41 1,681 0 ND ND 43 43 145 145 NI

TDEM42 1,821 0 ND ND 284 284 387 387 NI

TDEM43 1,854 0 179 -- -- 179 409 409 NI

TDEM44 1,943 0 119 -- -- 119 345 345 NI

TDEM48 1,995 0 120 -- -- 120 354 354 PI

TDEM49 2,001 0 100 -- -- 100 413 413 PI

TDEM68 1,882 0 97 -- -- 97 444 444 NI

TDEM69 1,933 0 73 -- -- 73 366 366 PI

TDEM70 1,762 0 ND ND 122 122 307 307 PI

TDEM71 2,108 0 ND ND 245 245 580 580 PI

TDEM79 1,651 0 278 -- -- -- -- 278 CA

TDEM84 1,847 0 56 56 124 124 325 325 BR

TDEM86 1,723 0 ND ND 54 54 310 310 PI

TDEM89 1,560 0 ND ND 158 158 -- 158 NI

TDEM90 1,704 0 130 -- -- -- -- 130 NI

TDEM91 1,880 0 225 -- -- -- -- 225 PI

TDEM99 2,014 0 83 -- -- 83 504 504 PI

TDEM140 2,099 0 43 -- -- 43 403 403 PI

TDEM156 2,126 0 183 183 344 344 536 536 PI

TDEM158 1,747 0 16 -- -- 16 339 339 PI

Table 4. Stratigraphic contacts resulting from minimum-layer time-domain electromagnetic models of sounding sites, 

Elkhorn-Loup Model study area, 2010.—Continued

[Elevation, land-surface elevation above National Geodetic Vertical Datum of 1929 (NGVD 29); all depths given in feet below land surface; 

TQT, top of Quaternary; BQT, depth to bottom of Quaternary; TPP, depth to top of Plio-Pleistocene gravel; BPP, depth to bottom of Plio-Pleistocene 

gravel, italicized where top of unit is not distinguishable with overlying Quaternary units; TOG, depth of top of Ogallala; BOG, depth of bottom of 

Ogallala; TACU, depth to top of aquifer confining unit; ACU, aquifer confining unit; ND, depth of contact not determined; BR, Brule; --, contact 

not present; PI, Pierre Shale; NI, Niobrara Formation; CA, Carlile Shale]
Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data 21

Table 4. Stratigraphic contacts resulting from minimum-layer time-domain electromagnetic models of sounding sites, 

Elkhorn-Loup Model study area, 2010.—Continued

[Elevation, land-surface elevation above National Geodetic Vertical Datum of 1929 (NGVD 29); all depths given in feet below land surface; 

TQT, top of Quaternary; BQT, depth to bottom of Quaternary; TPP, depth to top of Plio-Pleistocene gravel; BPP, depth to bottom of Plio-Pleistocene 

gravel, italicized where top of unit is not distinguishable with overlying Quaternary units; TOG, depth of top of Ogallala; BOG, depth of bottom of 

Ogallala; TACU, depth to top of aquifer confining unit; ACU, aquifer confining unit; ND, depth of contact not determined; BR, Brule; --, contact 

not present; PI, Pierre Shale; NI, Niobrara Formation; CA, Carlile Shale]

Site 

identifier Elevation TQT BQT TPP BPP TOG BOG TACU ACU

TDEM177 1,909 0 133 -- -- 133 248 248 PI

TDEM178 1,933 0 46 -- -- 46 380 380 PI

TDEM209 1,873 0 188 -- -- 188 414 414 PI

TDEM210 1,709 0 67 -- -- 67 284 284 PI

TDEM211 1,723 0 138 -- -- 138 231 231 PI

TDEM212 2,130 0 ND ND 69 69 432 432 PI

TDEM218 2,045 0 66 -- -- 66 392 392 PI

TDEM223 3,728 0 233 -- -- 233 1,168 1,168 BR

TDEM224 3,659 0 168 -- -- 168 1,162 1,162 BR

TDEM228 3,678 0 171 -- -- 171 1,395 1,395 BR

TDEM232 3,436 0 163 163 293 293 1,173 1,173 BR

UE1 2,071 0 38 -- -- 38 462 462 BR

UE2 2,131 0 ND ND 71 71 383 383 PI

UE3 2,379 0 135 -- -- 135 574 574 PI

UE4 2,250 0 ND ND 204 204 545 545 PI

UE5 2,209 0 36 -- -- 36 472 472 PI

UE6 2,083 0 46 -- -- 46 509 509 BR

UE7 2,118 0 61 -- -- 61 447 447 PI

UE8 2,172 0 97 -- -- 97 526 526 BR

UE9 2,075 0 ND ND 212 212 424 424 BR

UE10 2,050 0 -- -- 176 176 498 498 PI

UE11 2,160 0 100 -- 100 325 325 PI

UL1 3,297 0 ND ND 136 136 701 701 BR

UL2 3,379 0 78 -- -- 78 833 833 BR

UL3 3,385 0 89 -- -- 89 849 849 BR

UL4 3,416 0 44 -- -- 44 690 690 BR

UL5 3,372 0 24 -- -- 24 854 854 BR

UL6 3,103 0 217 -- -- 217 936 936 BR

UL7 3,078 0 ND ND 262 262 695 695 BR

UL8 3,081 0 242 -- -- 242 737 737 BR

UL9 2,807 0 73 -- -- 73 588 588 BR

UL10 2,874 0 ND ND 111 111 812 812 BR

UL11 2,763 0 ND ND 302 302 693 693 BR

UL12 2,937 0 276 -- -- 276 735 735 BR

UL13 2,929 0 149 -- -- 149 528 528 BR

UL14 2,966 0 131 -- -- 131 714 714 BR

UL15 3,105 0 218 -- -- 218 898 898 BR

UL16 2,994 0 ND ND 255 255 753 753 BR

UL18 3,258 0 143 -- -- 143 990 990 BR

UL19 3,039 0 160 -- -- 160 839 839 BR
Figure 72. Five regions within the Elkhorn-Loup Model study area containing Plio-Pleistocene gravel deposits identified by modeled time-dom

soundings. CHERRY HOLT CUSTER ROCK BROWN LINCOLN KNO

BUFFALO GRANT HALL ARTHUR BLAINE LOUP KEITH BOON

DAWSON HOOKER ANTELOPE VALLEY 

LOGAN THOMAS MCPHERSON HOWARD GARFIELD NA

WHEELER MERRIC

SHERMAN 

GREELEY BOYD

SHERIDAN GARDEN

KEYA PAHA

PHELPS

H

KEARNEY

GOSPER

ADAMS

102° 101° 100° 99° 98°

43°42°41° EXPLANATION

[TDEM, Time-domain electromagnetic]

TDEM sounding and identifier with no

Plio-Pleistocene gravel

TDEM sounding and identifier with 

Plio-Pleistocene gravel

Elkhorn-Loup Model area

Area underlain with Plio-Pleistocene gravel

0 25 50 MILES0 25 50 KILOMETERS

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)

!

!

!

!

!

!!

!

!

!

!

!

!

!

!

!

!

!

! !

!

!

!

!

!

!

!

!

! !

!

!

!

!

!

!

!

! SH8

CN3

UL19 UL18 UL17

UL16 UL15

UL14

UL13

UL12

UL11

UL10

UL9

UL8 UL7

UL6

UL5

UL4

UL3

UL2 UL1

UE11

UE10

UE9 UE8

UE7

UE6

UE5

UE4

UE3 UE2

UE1

SH8

SH7

SH6

SH5

SH4

SH3

SH2

SH1

LL27

LL26

LL25

LL24

LL23 LL22

LL21

LL20

LL19

LL18

LL17

LL16

LL15

LL14

LL13

LL12LL11

LL10

LL9

LL8

LL7

LL6

LL5

LL4

LL3

LL1

CN9

CN8

CN6

CN5

CN4

CN3

CN2

CN1

CGG14 CGG15

CGG13

CGG12

CGG11

CGG10

CGG9

CGG8

CGG7

CGG6

CGG5

CGG4

CGG3

CGG2

CGG1

TDEM228

TDEM224

TDEM223

TDEM218

TDEM212

T

TDEM178

TDEM177

T

TDEM156

TDEM99

TDEM91

TD

TDEM71

TDEM6

TDEM6

TDEM49

TDEM48

TDEM20

TDEM11

LL2

TDEM232

TDEM140
Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data 23

determined and compared to the corresponding values for 

nearby test holes. The PP layers may not have a depth-to-top 

layer defined because of inadequate contrast in resistivity or 

in thickness (table 4). In cases where the stratigraphy was not 

interpreted for CSD test holes, the stratigraphic picks used in 

this report were based on the described lithology and available 

geophysical logs (University of Nebraska-Lincoln Conservation and Survey Division, 2011). Differences between the 

modeled TDEM results and test-hole stratigraphy constituted 

the evaluation dataset for the QC sites. Results for the top of 

Ogallala (TOG) and top of ACU (TACU) are presented in table 5.

Determining total aquifer thickness, or the depth of the 

ACU below land surface, was one of the primary objectives of 

this study. The TDEM technique was chosen to determine the 

depth to the ACU because those units underlying the aquifer 

have a much lower resistivity compared to the overlying units 

of Tertiary age. Differences between ACU elevations based on 

the modeled-TDEM resistivity and test-hole data ranged from 

2 to 113 ft (0.6 to 35 m) (table 5). Some of these differences 

may be attributed to abrupt changes in subsurface geology. 

Of the 12 QC sites given in table 5, one-half are located in 

the Upper Loup NRD (fig. 1), where Hobza and others (2011) 

reported large changes in the elevation of the top of the ACU 

across short distances [less than 1,640 ft (500 m)].

Subdividing the hydrostratigraphy within the aquifer 

on the basis of resistivity-model results is a more challenging task because often the ranges in resistivity corresponding 

to individual stratigraphic units are very similar or overlap. 

Resistive targets, such as Plio-Pleistocene gravel sequences, 

are difficult to resolve because of the low-current densities 

Table 5. Comparison of elevations of selected stratigraphic contacts based on minimum-layer time-domain electromagnetic 

model results and test-hole stratigraphy at Quality Control sites, Elkhorn-Loup Model study area, Nebraska.

[Elevation datum is National Geodetic Vertical Datum of 1929 (NGVD 29); TOG, depth below land surface of top of Ogallala; TOG elevation, elevation 

of top of Ogallala; ABS difference, absolute value of the difference in elevation between the time-domain electromagnetic (TDEM)-based elevation and 

the test-hole elevation; TACU, depth below land surface of top of aquifer confining unit; TACU elevation, elevation of top of aquifer confining unit; --, not 

applicable]

Site

identifier

Elevation,

in feet

TOG,

in feet

TOG elevation, 

in feet

TOG ABS 

difference, 

in feet

TACU, 

in feet

TACU elevation, 

in feet

TACU ABS 

difference, 

in feet

Data 

source1

CGG2 2,521 166 2,355 -- 474 2,047 -- 1

28-SH-85 2,520 154 2,365 11 532 1,988 60 3

CN1 3,587 166 3,421 -- 1,145 2,442 -- 1

1-UL9 3,600 147 3,453 32 1,100 2,500 58 2

CN2 3,399 312 3,087 -- 1,042 2,357 -- 1

2-UL9 3,390 290 3,100 13 920 2,470 113 2

CN4 2,760 68 2,693 -- 553 2,207 -- 1

3-UL9 2,749 10 2,739 47 600 2,149 58 2

LL1 2,163 74 2,090 -- 643 1,520 -- 1

1-B-83 2,153 35 2,118 28 635 1,518 2 2

LL17 2,360 241 2,119 -- 717 1,643 -- 1

57-HP-79 2,360 200 2,160 40 690 1,670 27 2

LL23 2,467 317 2,150 -- 679 1,788 -- 1

3-LL-102 2,461 245 2,216 66 695 1,766 22 2

SH4 2,365 193 2,173 -- 670 1,696 -- 1

15-A-59 2,352 127 2,225 53 722 1,631 65 2

SH5 2,353 215 2,139 -- 590 1,763 -- 1

56-HP-79 2,379 200 2,179 40 610 1,769 6 2

SH6 2,130 195 1,935 -- 515 1,615 -- 1

48-A-57 2,151 305 1,846 89 561 1,590 25 3

SH8 3,193 592 2,601 -- 754 2,439 -- 1

6-S-82 3,172 320 2,852 251 735 2,437 2 3

UL19 3,039 160 2,879 -- 839 2,200 -- 1

1-UL-10 3,060 275 2,785 94 800 2,265 60 2

1

Source of hydrostratigraphic pick, where 1 is TDEM-based model, 2 is interpreted by the authors, and 3 is interpreted by University of Nebraska 

Conservation and Survey Division.
24 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

induced within resistive layers as compared to conductive layers. As such, differentiating Plio-Pleistocene deposits from the 

base of the Quaternary deposits or top of the Ogallala may not 

be possible using the data collected for this study if there is 

insufficient thickness and contrast in resistivity of the deposits. The differences in elevation of the top of the Ogallala, 

estimated on the basis of the TDEM resistivity models, and the 

test-hole data ranged from 11 to 251 ft (3.4 to 77 m) (table 5), 

with two-thirds of model results being within 60 ft (20 m) of 

the test-hole contact elevation. The largest difference was from 

site SH8 (test hole 6-S-82), where the minimum-layer and 

smooth-layer models indicated very uniform resistivity across 

the Quaternary-Ogallala contact, and no abrupt change in 

resistivity occurred until the depth of the ACU was reached. In 

such cases, the modeled resistivity values of the Ogallala and 

overlying units are very similar and potentially overlap. Thus, 

any hydrostratigraphic interpretation of the top of the Ogallala 

based solely on modeled, TDEM-based resistivity data should 

be carefully considered.

Summary

The Elkhorn-Loup Model (ELM) was begun in 2006 to 

understand the effect of various groundwater-management 

scenarios on surface-water resources. During phase one of 

the ELM study, a lack of subsurface geological information 

was identified as a data gap. Test holes drilled to the base of 

the aquifer in the ELM study area are spaced up to 25 miles 

apart, especially in areas of the western Sand Hills. Given the 

variable character of the hydrostratigraphic units that compose 

the High Plains aquifer system, substantial variation in aquifer 

thickness and characteristics can exist between test holes. To 

improve the hydrogeologic understanding of the ELM study 

area, the U.S. Geological Survey, in cooperation with the 

Nebraska Department of Natural Resources, the ELM NRDs, 

and the University of Nebraska-Lincoln Conservation and 

Survey Division, described the subsurface lithology at six test 

holes and concurrently collected borehole geophysical data to 

identify the base of the High Plains aquifer system. Surface 

geophysical data also were collected using time-domain electromagnetic (TDEM) methods at and between selected testhole locations as a quick, non-invasive means of identifying 

the base of the High Plains aquifer system. The data collected 

for this study allow improved knowledge of the aquifer thickness and the topography of the base-of-aquifer surface, which 

are important for assessing water supply and understanding 

groundwater flow.

Site selection for test-hole drilling and surface geophysical data collection was guided by several factors, but 

primarily was intended to fill existing gaps in the test-hole 

database. Geophysical methods were selected on the basis 

of physical characteristics of the hydrostratigraphic units 

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

of aquifer. In 2010, six test holes were drilled to below the 

base of aquifer, and borehole geophysical data were logged. 

A total of 124 TDEM soundings (of which 123 were used)

were collected within the ELM study area between 2008 

and 2011. TDEM data were inverted for two end-member 

model classes—a minimum-layer inverse model and 20-layer 

smooth, or Occam-style, inverse model—both as one-dimensional models.

Results from test-hole drilling and geophysical logging 

indicated differences between elevations of the drilled base 

of aquifer and the previously estimated base of aquifer that 

ranged from -131 to 34 ft (-40 to 10 m). This range in differences is less than those previously reported for the western 

part of the ELM study area, where deeper paleochannels were 

eroded into the Brule.

In total, more than 435 test holes were examined and 

compared with the modeled-TDEM soundings. Even where 

present, individual stratigraphic units could not always be 

identified in modeled-TDEM sounding results if sufficient 

resistivity contrast was not evident; however, in general, the 

base of aquifer (top of the aquifer confining unit) is one of the 

best-resolved results from the resistivity models. Estimates 

of the base-of-aquifer elevation are in good accordance with 

those from existing test-hole data.

The TDEM data generally support four- to nine-layer 

models for resistivity stratigraphy. The modeled resistivity results reflect the eastward thinning of Miocene-age and 

older stratigraphic units, and generally allowed confident 

identification of the accompanying change in the stratigraphic unit forming the aquifer confining unit (ACU). 

Models from the western ELM study area (Cherry, Grant, 

Hooker, Thomas, Arthur, McPherson, and Logan Counties) 

exhibit decreasing resistivity, with increasing depth below 

about 300–400 ft (91–120 m) below land surface. In many 

cases, these models indicated a thick Ogallala sequence 

atop the low-resistivity Brule or Pierre Shale. The modeledTDEM soundings also provided information regarding the 

distribution of Plio-Pleistocene (PP) gravel deposits within 

the study area. These deposits are typically more resistive 

(about 100 ohm-m) than the overlying Quaternary sediments 

or the underlying Ogallala. The average thickness of the PP 

deposits in the study area is 100 ft (30 m), although in many 

cases the Quaternary-PP contact cannot be reliably distinguished using only TDEM soundings because of insufficient 

thickness or resistivity contrast.

A subset of all the TDEM sites were collected near existing or planned test holes. Determining total aquifer thickness, 

or the depth of the ACU, was one of the primary objectives 

of this study. Differences between ACU elevations based on 

modeled-TDEM and test-hole data ranged from 2 to 113 ft 

(0.6 to 34 m). Generally, larger differences were associated with deeper ACUs and occurred in the western study 

area. Subdividing the hydrostratigraphy within the aquifer 

on the basis of TDEM-based resistivity-model results is a 

more challenging task because often the ranges in resistivity corresponding to individual stratigraphic units are very 

similar or overlap. The differences in elevation of the top of 
References Cited 25

the Ogallala estimated on the basis of the modeled-TDEM 

resistivity and the test-hole data ranged from 11 to 251 ft 

(3.4 to 77 m), with two-thirds of model results being within 

60 ft (18 m) of the test-hole contact elevation. As such, any 

hydrostratigraphic interpretation of the top of the Ogallala 

based solely on modeled, TDEM-based resistivity data should 

be carefully considered.

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28 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11
Supplemental Data 29

Supplemental Data
30 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

Depth,

in feet Gamma

0 Counts per second 052

Spontaneous potential

0 Millivolts 001

Short-normal resistivity (16 inches)

0 Ohm-meters 051

Long-normal resistivity (64 inches)

0 Ohm-meters 051

Generalized lithology

Topsoil, silt, some clay

Silty sand, very fine to fine

Gravel fine to medium,

interbedded silt at 235 feet

Silty sand, sandstone,

trace medium to coarse

sand, rootlets

Sandy silt, sandstone

Silty sand, sandstone,

very fine, trace medium,

calcareous in parts

Sandy silt, calcareous,

some clay

Clayey silt, clayey sand, hard

claystone at 805–807 feet

100

200

300

400

500

600

700

800

0

Figure 4. Composite of generalized lithologic description and geophysical logs for test hole 2-LL-10, Custer County, Nebraska. 

Short-normal resistivity, in ohm-meters; long-normal resistivity, in ohm-meters; natural gamma activity, in counts per second; 

spontanenous potential, in millivolts.
Supplemental Data 31

Figure 5. Composite of generalized lithologic description and geophysical logs for test hole 3-LL-10, Custer County, Nebraska. 

Short-normal resistivity, in ohm-meters; long-normal resistivity, in ohm-meters; natural gamma activity, in counts per second; 

spontanenous potential, in millivolts.

Depth,

in feet

Gamma

0 Counts per second 002

Spontaneous potential

52- Millivolts 57

Short-normal resistivity (16 inches)

0 Ohm-meters 001

Long-normal resistivity (64 inches)

0 Ohm-m 001

Generalized lithology

Clayey silt, sandy 

in parts, calcareous in

parts

Silty sand interbedded

with medium to coarse

sand

Silty sand, sandstone,

calcareous

Silty sand, sandstone,

rootlets, calcareous

Sand, fine to medium,

trace silt, sand fine at

bottom

Silty sand, sandstone,

calcareous in parts

Pierre Shale, yellowish to

100

200

300

400

500

600

700

0

gray
32 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

Depth,

in feet

Gamma

0 Counts per second 052

Spontaneous potential

001- Millivolts 001

Short-normal resistivity (16 inches)

0 Ohm-meters 002

Long-normal resistivity (64 inches)

0 Ohm-meters 002

Generalized lithology

Topsoil, silt, firm and clayey 

in parts, some very

fine sand

Gravel, fine to coarse,

some interbedded silt and

clay

Sandy silt to silty sand,

calcareous sandstone,

rootlets begin at 280 feet

Sand, sandstone, fine,

some medium sand, trace

silt

Silty sand, fine,

calcareous in parts, more

silt at bottom

Brule Formation,

silt/siltstone, brown and

pinkish, trace very fine

sand

100

200

300

400

500

600

700

0

Figure 6. Composite of generalized lithologic description and geophysical logs for test hole 4-LL-10, Custer County, Nebraska. 

Short-normal resistivity, in ohm-meters; long-normal resistivity, in ohm-meters; natural gamma activity, in counts per second; 

spontanenous potential, in millivolts.
Supplemental Data 33

Depth,

in feet

Gamma

0 Counts per second 052

Spontaneous potential

001- Millivolts 001

Short-normal resistivity (16 inches)

0 Ohm-meters 052

Long-normal resistivity (64 inches)

0 Ohm-meters 052

Generalized lithology

Topsoil, silt, some clay,

more very fine sand at

bottom

Coarse sand to coarse

gravel

Silty sand to sandy silt,

trace fine to medium

gravel

Gravel, fine to coarse

interbedded with fine

sand

Silty sand, sandstone,

interbedded fine to

medium gravel

Silty sand, sandstone,

very fine to fine, rootlets,

trace medium sand to fine

gravel

Brule Formation,

silt/siltstone, some very

fine sand 

Brule Formation and

weathered Pierre Shale

100

200

300

400

500

600

700

0

Figure 7. Composite of generalized lithologic description and geophysical logs for test hole 6-LL-10, Custer County, Nebraska. 

Short-normal resistivity, in ohm-meters; long-normal resistivity, in ohm-meters; natural gamma activity, in counts per second; 

spontanenous potential, in millivolts.
34 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

Depth,

in feet Gamma

0 Counts per second 003

Spontaneous potential

0 Millivolts 001

Short-normal resistivity (16 inches)

0 Ohm-meters 052

Long-normal resistivity (64 inches)

0 Ohm-meters 052

Generalized lithology

Topsoil, silt, some silty

sand at bottom, 

calcareous in parts

Very fine to coarse sand,

some fine gravel, silty at

bottom

Silty sand, sandstone,

rootlets, calcareous in

parts

Sand, very fine to fine

Silty sand, very fine to

fine, sandstone,

calcareous in parts,

rootlets, trace fine gravel

Pierre Shale, weathered,

trace sand at top, firm at

bottom

100

200

300

400

500

0

600

Figure 8. Composite of generalized lithologic description and geophysical logs for test hole 7-LL-10, Custer County, Nebraska. 

Short-normal resistivity, in ohm-meters; long-normal resistivity, in ohm-meters; natural gamma activity, in counts per second; 

spontanenous potential, in millivolts.
Supplemental Data 35

Apparent resistivity, in ohm-meters

Time, in seconds

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

Time derivative of out-of-loop magnetic field, in volts per square meter

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

Depth, in feet

Depth, in meters

1 10 100 1,000

Resistivity, in ohm-meters

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

Best-fit smooth inverse model

A

A

B

B

SITE CGG1

SITE CGG2

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

Vertical error bar indicates variance 

 of data point with mean at center

Figure 10. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled 

from time-domain electromagnetic sounding for sites CGG1 and CGG2.
36 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11 Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

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

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE CGG3

SITE CGG4

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

Figure 11. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis for site CGG3), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface 

resistivity modeled from time-domain electromagnetic sounding for sites CGG3 and CGG4.
Supplemental Data 37

Apparent resistivity, in ohm-meters

Time, in seconds

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

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

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE CGG5

SITE CGG6

100

104

103

102

101

100

104

103

102

101

Figure 12. Relations between A, central-loop resistivity and time elapsed after primary current turnoff; and B, depth below 

land surface and subsurface resistivity modeled from time-domain electromagnetic sounding for sites CGG5 and CGG6
38 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11 Apparent resistivity, in ohm-meters

Time, in seconds

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

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

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE CGG7

SITE CGG8

100

104

103

102

101

100

104

103

102

101

Figure 13. Relations between A, central-loop resistivity and time elapsed after primary current turnoff; and B, depth below land 

surface and subsurface resistivity modeled from time-domain electromagnetic sounding for sites CGG7 and CGG8.
Supplemental Data 39

Apparent resistivity, in ohm-meters

Time, in seconds

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

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

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE CGG9

SITE CGG10

100

104

103

102

101

100

104

103

102

101

Figure 14. Relations between A, central-loop resistivity and time elapsed after primary current turnoff; and B, depth below land 

surface and subsurface resistivity modeled from time-domain electromagnetic sounding for sites CGG9 and CGG10.
40 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

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

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE CGG11

SITE CGG12

100

104

103

102

101

100

104

103

102

101

Apparent resistivity, in ohm-meters

Time, in seconds

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

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

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

Figure 15. Relations between A, central-loop resistivity and time elapsed after primary current turnoff; and B, depth below land 

surface and subsurface resistivity modeled from time-domain electromagnetic sounding for sites CGG11 and CGG12.
Supplemental Data 41

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

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE CGG13

SITE CGG14

100

104

103

102

101

100

104

103

102

101

Apparent resistivity, in ohm-meters

Time, in seconds

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

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

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

Figure 16. Relations between A, central-loop resistivity and time elapsed after primary current turnoff; and B, depth below land 

surface and subsurface resistivity modeled from time-domain electromagnetic sounding for sites CGG13 and CGG14.
42 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

Figure 17. Relations between A, central-loop resistivity and time elapsed after primary current turnoff; and B, depth below land 

surface and subsurface resistivity modeled from time-domain electromagnetic sounding for sites CGG15 and LL27.

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE CGG15

SITE LL27

100

104

103

102

101

100

104

103

102

101

Apparent resistivity, in ohm-meters

Time, in seconds

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

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

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

10-5 10-4 10-3 10-2 10-1
Supplemental Data 43

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE CN1

SITE CN2

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

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

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Figure 18. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled 

from time-domain electromagnetic sounding for sites CN1 and CN2.
44 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A B

B

SITE CN3

SITE CN4

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

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

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Figure 19. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled 

from time-domain electromagnetic sounding for sites CN3 and CN4.
Supplemental Data 45

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A B

SITE CN6

100

104

103

102

101

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

A B

SITE CN5

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

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

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Figure 20. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis for site CN5), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity 

modeled from time-domain electromagnetic sounding for sites CN5 and CN6.
46 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0 0

1,600

1 10 100 1,000

A

A

B

B

SITE CN8

SITE CN9

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

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

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Figure 21. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis for site CN9), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface 

resistivity modeled from time-domain electromagnetic sounding for sites CN8 and CN9.
Supplemental Data 47

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE LL1

SITE LL2

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

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

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Figure 22. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled 

from time-domain electromagnetic sounding for sites LL1 and LL2.
48 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

3

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE LL3

SITE LL4

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

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

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Figure 23. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled 

from time-domain electromagnetic sounding for sites LL3 and LL4.
Supplemental Data 49

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE LL5

SITE LL6

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

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

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Figure 24. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled 

from time-domain electromagnetic sounding for sites LL5 and LL6.
50 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE LL7

SITE LL8

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

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

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Figure 25. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled 

from time-domain electromagnetic sounding for sites LL7 and LL8.
Supplemental Data 51

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE LL9

SITE LL10

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

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

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Figure 26. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled 

from time-domain electromagnetic sounding for sites LL9 and LL10.
52 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE LL11

SITE LL12

100

104

103

102

101

100

104

103

102

101

Apparent resistivity, in ohm-meters

Time, in seconds

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

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

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Figure 27. Relations between A, central-loop resistivity and time elapsed after primary current turnoff; and B, depth below land 

surface and subsurface resistivity modeled from time-domain electromagnetic sounding for sites LL11 and LL12.
Supplemental Data 53

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE LL13

SITE LL14

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

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

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Figure 28. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled 

from time-domain electromagnetic sounding for sites LL13 and LL14.
54 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE LL15

SITE LL16

100

104

103

102

101

100

104

103

102

101

Apparent resistivity, in ohm-meters

Time, in seconds

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

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

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Figure 29. Relations between A, central-loop resistivity and time elapsed after primary current turnoff; and B, depth below land 

surface and subsurface resistivity modeled from time-domain electromagnetic sounding for sites LL15 and LL16.
Supplemental Data 55

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE LL17

SITE LL18

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

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

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Figure 30. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis for site LL17), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface 

resistivity modeled from time-domain electromagnetic sounding for sites LL17 and LL18.
56 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE LL19

SITE LL20

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

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

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Figure 31. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled 

from time-domain electromagnetic sounding for sites LL19 and LL20.
Supplemental Data 57

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE LL21

SITE LL22

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

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

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Figure 32. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis for site LL22), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface 

resistivity modeled from time-domain electromagnetic sounding for sites LL21 and LL22.
58 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

600

500

400

300

200

100

0

150

250

350

50

450

550

1,000

800

600

400

200

0

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE LL23

SITE LL24

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

1,400

1,200

1,800

1,600

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

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

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Figure 33. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled 

from time-domain electromagnetic sounding for sites LL23 and LL24.
Supplemental Data 59

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

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

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE LL25

SITE LL26

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

Figure 34. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled 

from time-domain electromagnetic sounding for sites LL25 and LL26.
60 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

100

104

103

102

101

100

104

103

102

101

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE SH1

SITE SH2

Apparent resistivity, in ohm-meters

Time, in seconds

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

285

75

Repetition frequency data, in hertz

Figure 35. Relations between A, central-loop resistivity and time elapsed after primary current turnoff; and B, depth below land 

surface and subsurface resistivity modeled from time-domain electromagnetic sounding for sites SH1 and SH2.
Supplemental Data 61

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE SH3

SITE SH4

100

104

103

102

101

100

104

103

102

101

Apparent resistivity, in ohm-meters

Time, in seconds

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

285

75

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Repetition frequency data, in hertz

Apparent resistivity, in ohm-meters

Time, in seconds

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

285

75

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Repetition frequency data, in hertz

Figure 36. Relations between A, central-loop resistivity and time elapsed after primary current turnoff; and B, depth below land 

surface and subsurface resistivity modeled from time-domain electromagnetic sounding for sites SH3 and SH4.
62 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

A

A

B

B

SITE SH5

SITE SH6

100

104

103

102

101

100

104

103

102

101

Apparent resistivity, in ohm-meters

Time, in seconds

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

285

75

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Repetition frequency data, in hertz

Figure 37. Relations between A, central-loop resistivity and time elapsed after primary current turnoff; and B, depth below land 

surface and subsurface resistivity modeled from time-domain electromagnetic sounding for sites SH5 and SH6.
Supplemental Data 63

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE SH7

SITE SH8

100

104

103

102

101

100

104

103

102

101

Apparent resistivity, in ohm-meters

Time, in seconds

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

285

75

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Repetition frequency data, in hertz

Figure 38. Relations between A, central-loop resistivity and time elapsed after primary current turnoff; and B, depth below land 

surface and subsurface resistivity modeled from time-domain electromagnetic sounding for sites SH7 and SH8.
64 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE TDEM8

SITE TDEM9

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

285

75

30

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

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Repetition frequency data, in hertz

Figure 39. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis for site TDEM8), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface 

resistivity modeled from time-domain electromagnetic sounding for sites TDEM8 and TDEM9.
Supplemental Data 65

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE TDEM11

SITE TDEM16

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

285

75

30

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

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Repetition frequency data, in hertz

Figure 40. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis for site TDEM16), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface 

resistivity modeled from time-domain electromagnetic sounding for sites TDEM11 and TDEM16.
66 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

A

A

B

B

SITE TDEM20

SITE TDEM41

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

285

75

30

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

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Repetition frequency data, in hertz

Figure 41. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis for site TDEM20), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface 

resistivity modeled from time-domain electromagnetic sounding for sites TDEM20 and TDEM41.
Supplemental Data 67

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE TDEM42

SITE TDEM43

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

285

75

30

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

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Repetition frequency data, in hertz

Figure 42. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis for site TDEM42), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface 

resistivity modeled from time-domain electromagnetic sounding for sites TDEM42 and TDEM43.
68 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

600

500

400

300

200

100

0

150

250

350

50

450

550

1,200

1,000

800

600

400

200

0

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE TDEM44

SITE TDEM48

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

1,400

1,800

1,600

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

285

75

30

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

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Repetition frequency data, in hertz

Figure 43. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled 

from time-domain electromagnetic sounding for sites TDEM44 and TDEM48.
Supplemental Data 69

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,000

800

600

400

200

0

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE TDEM49

SITE TDEM68

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

1,800

1,600

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

285

75

30

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

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Repetition frequency data, in hertz

Figure 44. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis for site TDEM49), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface 

resistivity modeled from time-domain electromagnetic sounding for sites TDEM49 and TDEM68.
70 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE TDEM69

SITE TDEM70

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

285

75

30

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

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Repetition frequency data, in hertz

Figure 45. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis for site TDEM69), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface 

resistivity modeled from time-domain electromagnetic sounding for sites TDEM69 and TDEM70.
Supplemental Data 71

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE TDEM71

SITE TDEM79

100

104

103

102

101

100

104

103

102

101

Apparent resistivity, in ohm-meters

Time, in seconds

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

285

75

30

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

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Repetition frequency data, in hertz

Figure 46. Relations between A, central-loop resistivity and time elapsed after primary current turnoff; and B, depth below land 

surface and subsurface resistivity modeled from time-domain electromagnetic sounding for sites TDEM71 and TDEM79.
72 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

Figure 47. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis for site TDEM86), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface 

resistivity modeled from time-domain electromagnetic sounding for sites TDEM84 and TDEM86.

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,000

800

600

400

200

0

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE TDEM84

SITE TDEM86

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

285

75

30

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

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Repetition frequency data, in hertz

1,800

1,600
Supplemental Data 73

Figure 48. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis for site TDEM90), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface 

resistivity modeled from time-domain electromagnetic sounding for sites TDEM89 and TDEM90.

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE TDEM89

SITE TDEM90

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

285

75

30

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

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Repetition frequency data, in hertz
74 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE TDEM91

SITE TDEM99

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

285

75

30

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

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Repetition frequency data, in hertz

Figure 49. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis for site TDEM99), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface 

resistivity modeled from time-domain electromagnetic sounding for sites TDEM91 and TDEM99.
Supplemental Data 75

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE TDEM140

SITE TDEM156

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

285

75

30

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

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Repetition frequency data, in hertz

Figure 50. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled 

from time-domain electromagnetic sounding for sites TDEM140 and TDEM156.
76 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE TDEM158

SITE TDEM177

100

104

103

102

101

100

104

103

102

101

Apparent resistivity, in ohm-meters

Time, in seconds

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

285

75

30

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

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Repetition frequency data, in hertz

Figure 51. Relations between A, central-loop resistivity and time elapsed after primary current turnoff; and B, depth below land 

surface and subsurface resistivity modeled from time-domain electromagnetic sounding for sites TDEM158 and TDEM177.
Supplemental Data 77

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE TDEM178

SITE TDEM209

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

Apparent resistivity, in ohm-meters

Time, in seconds

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

Best-fit minimum-layer model

EXPLANATION

Best-fit smooth inverse model

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

Time derivative of out-of-loop magnetic field, in volts per square meter

Time derivative of out-of-loop magnetic field, in volts per square meter

Time, in seconds

285

75

30

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

EXPLANATION

Vertical error bar indicates variance 

 of data point with mean at center

Repetition frequency data, in hertz

Figure 52. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled 

from time-domain electromagnetic sounding for site TDEM178. (Site TDEM209 only includes out-of-loop vertical magnetic field 

data in left panel.)
78 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE TDEM210

SITE TDEM211

100

104

103

102

101

100

104

103

102

101

Apparent resistivity, in ohm-meters

Time, in seconds

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

285

75

30

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

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Repetition frequency data, in hertz

Figure 53. Relations between A, central-loop resistivity and time elapsed after primary current turnoff; and B, depth below land 

surface and subsurface resistivity modeled from time-domain electromagnetic sounding for sites TDEM210 and TDEM211.
Supplemental Data 79

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE TDEM212

SITE TDEM218

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

285

75

30

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

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Repetition frequency data, in hertz

Figure 54. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis for site TDEM212), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface 

resistivity modeled from time-domain electromagnetic sounding for sites TDEM212 and TDEM218.
80 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE TDEM223

SITE TDEM224

100

104

103

102

101

100

104

103

102

101

Apparent resistivity, in ohm-meters

Time, in seconds

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

285

75

30

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

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Repetition frequency data, in hertz

Figure 55. Relations between A, central-loop resistivity and time elapsed after primary current turnoff; and B, depth below land 

surface and subsurface resistivity modeled from time-domain electromagnetic sounding for sites TDEM223 and TDEM224.
Supplemental Data 81

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE TDEM228

SITE TDEM232

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

285

75

30

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

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Repetition frequency data, in hertz

Figure 56. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis for site TDEM228), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface 

resistivity modeled from time-domain electromagnetic sounding for sites TDEM228 and TDEM232.
82 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE UE1

SITE UE2

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

285

75

30

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

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Repetition frequency data, in hertz

Figure 57. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled 

from time-domain electromagnetic sounding for sites UE1 and UE2.
Supplemental Data 83

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE UE3

SITE UE4

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

285

75

30

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

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Repetition frequency data, in hertz

Figure 58. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled 

from time-domain electromagnetic sounding for sites UE3 and UE4.
84 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE UE5

SITE UE6

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

285

75

30

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

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Repetition frequency data, in hertz

Figure 59. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled 

from time-domain electromagnetic sounding for sites UE5 and UE6.
Supplemental Data 85

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE UE7

SITE UE8

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

285

75

30

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

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Repetition frequency data, in hertz

Figure 60. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled 

from time-domain electromagnetic sounding for sites UE7 and UE8.
86 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE UE9

SITE UE10

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

285

75

30

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

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Repetition frequency data, in hertz

Figure 61. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled 

from time-domain electromagnetic sounding for sites UE9 and UE10.
Supplemental Data 87

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE UL1

SITE UL2

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

285

75

30

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

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Repetition frequency data, in hertz

Figure 62. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled 

from time-domain electromagnetic sounding for sites UL1 and UL2.
88 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE UL3

SITE UL4

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

285

75

30

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

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Repetition frequency data, in hertz

Figure 63. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled 

from time-domain electromagnetic sounding for sites UL3 and UL4.
Supplemental Data 89

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE UL5

SITE UL6

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

285

75

30

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

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Repetition frequency data, in hertz

Figure 64. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled 

from time-domain electromagnetic sounding for sites UL5 and UL6.
90 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE UL7

SITE UL8

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

285

75

30

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

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Repetition frequency data, in hertz

Figure 65. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled 

from time-domain electromagnetic sounding for sites UL7 and UL8.
Supplemental Data 91

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE UL9

SITE UL10

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

285

75

30

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

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Repetition frequency data, in hertz

Figure 66. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled 

from time-domain electromagnetic sounding for sites UL9 and UL10.
92 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

600

500

400

300

200

100

0

150

250

350

50

450

550

1,000

800

600

400

200

0

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE UL11

SITE UL12

1,400

1,200

1,800

1,600

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

285

75

30

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

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Repetition frequency data, in hertz

Figure 67. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled 

from time-domain electromagnetic sounding for sites UL11 and UL12.
Supplemental Data 93

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE UL13

SITE UL14

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

285

75

30

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

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Repetition frequency data, in hertz

Figure 68. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled 

from time-domain electromagnetic sounding for sites UL13 and UL14.
94 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE UL15

SITE UL16

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

285

75

30

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

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Repetition frequency data, in hertz

Figure 69. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled 

from time-domain electromagnetic sounding for sites UL15 and UL16.
Supplemental Data 95

Figure 70. Relations between A, central-loop resistivity, out-of-loop vertical magnetic field (upper set of curves time-derivative, 

right axis), and time elapsed after primary current turnoff; and B, depth below land surface and subsurface resistivity modeled 

from time-domain electromagnetic sounding for sites UL18 and UL19.

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

600

500

400

300

200

100

0

150

250

350

50

450

550

1,400

1,200

1,800

1,000

800

600

400

200

0

1,600

1 10 100 1,000

A

A

B

B

SITE UL18

SITE UL19

100

104

103

102

101

100

104

103

102

101

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-3

Apparent resistivity, in ohm-meters

Time, in seconds

Time derivative of out-of-loop magnetic field, in volts per square meter

Depth, in feet

Depth, in meters

Resistivity, in ohm-meters

285

75

30

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

EXPLANATION

Best-fit minimum-layer model

EXPLANATION

Best-fit smooth inverse model

Vertical error bar indicates variance 

 of data point with mean at center

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

Repetition frequency data, in hertz
96 Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11

Publishing support provided by: 

Rolla Publishing Service Center

For more information concerning this publication, contact:

Director, USGS Nebraska Water Science Center

5231 South 19th Street

Lincoln, Nebraska 68512

(402) 328–4100

Or visit the Nebraska Water Science Center Web site at:

http://ne.water.usgs.gov/
Back cover. Photos of test-hole drilling in Custer County, Lower Loup Natural Resources District, 2010.
Hobza and others— Hydrostratigraphic Interpretation of Test-Hole and Surface Geophysical Data, Elkhorn and Loup River Basins, Nebraska, 2008–11—OFR 2012–1227