Prepared in cooperation with the U.S. Army Corps of Engineers Sacramento District

Characterization of Major Lithologic Units Underlying 

the Lower American River Using Water-Borne 

Continuous Resistivity Profiling, Sacramento, 

California, June 2008

By Lyndsay B. Ball and Andrew P. Teeple

Open-File Report 2013–1050

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2013

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Suggested citation:

Ball, L.B., and Teeple, A.P., 2013, Characterization of major lithologic units underlying the lower American 

River using water-borne continuous resistivity profiling, Sacramento, California, June 2008: U.S. Geological 

Survey Open-File Report 2013–1050, 13 p., http://pubs.usgs.gov/of/2013/1050/.

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iii

Contents

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

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

Acknowledgments............................................................................................................................................. 4

Methods............................................................................................................................................................ 4

Continuous Resistivity Profiling..................................................................................................................... 4

Two-Dimensional Inverse Modeling .............................................................................................................. 8

Geophysical Characterization of Major Lithologic Units.................................................................................... 9

Section 1..................................................................................................................................................... 11

Section 2..................................................................................................................................................... 11

Section 3..................................................................................................................................................... 11

Section 4..................................................................................................................................................... 11

Section 5..................................................................................................................................................... 11

Conclusions .................................................................................................................................................... 12

References Cited............................................................................................................................................ 13

Tables

1. Water-borne resistivity cable schematic...................................................................................................... 7

2. Summary of inversion parameters used in processing water-borne resistivity data .................................... 9

Figures

1. Maps showing A, the study area with respect to the city of Sacramento and B, direct-current 

resistivity lines on the lower American River .......................................................................................... 3

2. Photographs of water-borne resistivity and auxiliary equipment set up on the boat and the 

water-borne resistivity cable................................................................................................................... 5

3. Schematic diagram showing electrode geometry of the water-borne resistivity cable. Brackets 

indicate the potential-electrode pair used in each measurement for the adapted inverse 

Schlumberger array, numbered in order of increasing depth penetration............................................... 7

4. Three-dimensional map, looking down on the study area to the Northwest, showing inverted 

resistivity profiles below the partially transparent aerial photography. Delineated sections indicate 

areas of similar resistivity structure ...................................................................................................... 10
iv

Plates

1. Inverted resistivity profiles for section 1.....................................................................................................link

2. Inverted resistivity profiles for section 2.....................................................................................................link

3. Inverted resistivity profiles for section 3.....................................................................................................link

4. Inverted resistivity profiles for section 4.....................................................................................................link

5. Inverted resistivity profiles for section 5.....................................................................................................link

Conversion Factors

SI to Inch/Pound 

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

kilometer (km) 0.5400 mile, nautical (nmi) 

meter (m) 1.094 yard (yd) 

Area

square kilometer (km2) 247.1 acre

square meter (m2) 10.76 square foot (ft2) 

hectare (ha) 0.003861 square mile (mi2) 

square kilometer (km2) 0.3861 square mile (mi2)

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

°F=(1.8×°C)+32

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 depth below water surface.

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83)
Characterization of Major Lithologic Units Underlying 

the Lower American River Using Water-Borne 

Continuous Resistivity Profiling, Sacramento, 

California, June 2008

By Lyndsay B. Ball and Andrew P. Teeple

Abstract

The levee system of the lower American River in Sacramento, California, is situated above 

a mixed lithology of alluvial deposits that range from clay to gravel. In addition, sand deposits 

related to hydraulic mining activities underlie the floodplain and are preferentially prone to scour 

during high-flow events. In contrast, sections of the American River channel have been observed to 

be scour resistant. In this study, the U.S. Geological Survey, in cooperation with the U.S. Army 

Corps of Engineers, explores the resistivity structure of the American River channel to characterize 

the extent and thickness of lithologic units that may impact the scour potential of the area. Likely 

lithologic structures are interpreted, but these interpretations are non-unique and cannot be directly 

related to scour potential. Additional geotechnical data would provide insightful data on the scour 

potential of certain lithologic units. Additional interpretation of the resistivity data with respect to 

these results may improve interpretations of lithology and scour potential throughout the American 

River channel and floodplain. 

Resistivity data were collected in three profiles along the American River using a waterborne continuous resistivity profiling technique. After processing and modeling these data, inverted 

resistivity profiles were used to make interpretations about the extent and thickness of possible 

lithologic units. In general, an intermittent high-resistivity layer likely indicative of sand or gravel 

deposits extends to a depth of around 30 feet (9 meters) and is underlain by a consistent lowresistivity layer that likely indicates a high-clay content unit that extends below the depth of 

investigation (60 feet or 18 meters). Immediately upstream of the Watt Avenue Bridge, the highresistivity layer is absent, and the low-resistivity layer extends to the surface where a scourresistant layer has been previously observed in the river bed. 

Introduction

The lower American River levee system is situated on Quaternary deposits of clay, silt, 

sand, and gravel as well as sand deposits derived from hydraulic mining activities (Asch and 

others, 2008). If located near the ground surface, these sand deposits have the potential to

preferentially scour during flood events and may threaten the integrity of the levee system. To 

identify areas of high scour potential, it is important to understand not only the extent of these sand 
2

deposits but also the extent and thickness of scour-resistant layers that may limit the river’s access 

to these deposits. To assist the U.S. Army Corps of Engineers (USACE) with the management of 

the lower American River levee system, the U.S. Geological Survey (USGS) has used a series of 

geophysical techniques to improve our understanding of the geology of the American River 

floodplain and channel. In 2007, the USGS collected frequency-domain electromagnetic, 

capacitively coupled resistivity, and direct-current (DC) resistivity data on the right-bank 

floodplain (with respect to the downstream direction of flow) to identify sand deposits 

preferentially prone to scour (Asch and others, 2008). In this study, water-borne DC resistivity data

were collected to characterize potentially scour-resistant units within and below the American 

River channel.

The surface geology of the lower American River channel and floodplain is dominated by 

Holocene stream channel and alluvial deposits that are absent in some locations and may reach up 

to 80 feet (ft) (24 meters (m)) in thickness (Helley and Harwood, 1985). The upper member of the 

Pleistocene Modesto Formation, mapped in the southern floodplain of the study area, consists of 

unconsolidated, unweathered gravel, silt, sand, and clay and is generally considered to be 

topographically higher than the Holocene deposits in the area (Helley and Harwood, 1985). The 

Modesto Formation has been identified by the U.S. Army Corps of Engineers as a potentially 

scour-resistant unit that they suspect underlies parts of the American River. Several electrically 

conductive layers (approximately 50 ohm-meters (ohm-m)) identified and interpreted by Asch and 

others (2008) likely indicate a high-clay content unit that may in some places represent this

suspected scour-resistant layer. In this study, the collection of geophysical data was extended into 

the American River channel using a water-borne continuous DC resistivity profiling (CRP) 

technique. A 5-mile (mi) (8 kilometer (km)) long stretch of the American River, from the Campus 

Commons Golf Course (about 3,000 ft or 0.9 km north of the H Street Bridge) to the Rio 

Americano High School (about 1.5 mi or 2.4 km east of the Watt Avenue Bridge) was targeted for 

characterization (fig. 1). The study area extent is similar to that of work completed by USGS along 

the north bank of the river in the summer of 2007 (Asch and others, 2008).

This report presents the methods and techniques used in this study. Results are presented, 

including a characterization of the extent and thickness of possible lithologic units underlying the 

investigated stretch of the lower American River that may potentially be scour resistant. 

 
3

Figure 1. A, the study area with respect to the city of Sacramento and B, direct-current resistivity lines on the lower American River.
4

Acknowledgments

This work was funded through cooperation with the USACE Sacramento District. We 

gratefully acknowledge Ayres Associates for their support throughout this study, and we specially 

thank Bill Spitz, Tom Smith, and Adrian Aguilar for providing and operating the boat. Many 

thanks to Ginny McGuire and Jeff Lucius (USGS) and Nate Bradley (University of Colorado at 

Boulder) for lending their computer-programming talents to the data-processing effort; their work 

has made substantial contributions to the consistency and timeliness of these results. 

Methods 

Continuous Resistivity Profiling

Direct-current resistivity measurements are made by transmitting a known current through 

two current electrodes and measuring the electrical potential across two other electrodes; the 

resistance is calculated by dividing the measured voltage by the applied current using Ohm’s Law. 

By multiplying the resistance by a geometric factor derived from the relative position of the current 

and potential electrodes, the apparent resistivity of the subsurface is calculated. Apparent resistivity 

is the electrical resistivity over an equivalent electrically homogeneous and isotropic subsurface 

and is used to represent the average resistivity of a more realistic, heterogeneous subsurface (Loke, 

2000). Deeper apparent resistivity values may be calculated by increasing the distance between 

electrodes. Through inverse modeling of multiple apparent resistivity measurements, likely 

heterogeneous electrical structures can be identified and used for lithologic interpretation. The 

inverse-modeling procedure is explained in more detail in the following subsection.

The water-borne CRP technique, a specialized application of the DC resistivity method, was 

used to identify major lithologic units underlying the lower American River. Advanced resistivity 

meters with multiple data channels allow multiple potential measurements to be taken 

simultaneously with a single current transmission. By using an array of electrodes with various 

spacings, a vertical resistivity profile can be simultaneously developed. When the electrodes are 

placed in water, electrical contact is maintained without driving electrodes into the ground. Data 

are collected while the array is slowly pulled behind a boat through the water. CRP allows rapid 

collection of resistivity data with high horizontal resolution and in areas that could otherwise be 

logistically difficult to survey, such as below rivers. A more detailed explanation of the resistivity 

method can be found in Zohdy and others (1974). Further discussion of the water-borne CRP 

technique can be found in Ball and others (2006). 
5

Figure 2. A, water-borne resistivity and auxiliary equipment set up on the boat (T. Smith, Ayres Associates) 

and B, the water-borne resistivity cable (L. Ball, U.S. Geological Survey).

Resistivity data were collected between June 24 and June 26, 2008, in three transects (right 

bank, center channel, and left bank) along the lower American River (fig. 1) with the Syscal Pro 

DC resistivity meter (IRIS Instruments, Orleans, France). A floating, 13-electrode cable was towed 

behind a boat (fig. 2). Geographic positioning data were collected using a DSM-232 decimeter

horizontal accuracy global positioning system (GPS) (Trimble, Sunnyvale, California) with dualfrequency antenna and OmniStar HP real-time correction service (OmniStar, Inc, Houston, Texas). 

Bathymetric data were collected using a 455XPe single-frequency depth sounder (Innerspace 

Technologies, Carlstadt, New Jersey) with decimeter resolution. Changes in water-surface 

elevation were monitored through regular checks of GPS elevation. However, inconsistencies in 

elevation (greater than 3 ft or 1 m) were found to be unacceptable and therefore were not used to 

correct the elevation of the final data. 

Bathymetric control provided by the depth sounder vertically references every data point to 

the river bed. The water-surface elevation is referenced to NGVD29 through bathymetric 
6

correlation between this survey and the 2006 Ayres Associates thlaweg profiles at River Mile 6 and 

11, both locations showing less than 3 ft (1 m) of vertical movement between 1987 and 2006 

(Ayres Associates, 2008). A linear trend along the line distance is used between those two mile 

markers to provide an approximated water-surface elevation. Relative water-level changes were

monitored at the Howe Avenue boat ramp and found to be negligible throughout the day, with the 

largest change, a water-level drop of 0.2 ft (6 cm), occurring between June 25th and June 26th. 

Water-temperature and conductivity data were collected three times throughout the survey using an 

Orion Model 122 field conductivity meter (Orion Research Inc., Boston, Massachusetts) and were 

consistently found to be about 54 microsiemens per centimeter (μS/cm) at 17°C, or 164 ohm-m. 

The resistivity cable used in this study was designed to define the thickness and depth of 

electrically contrasting layers up to a total depth threshold of 60 ft (18 m). To maximize the depth 

of investigation while maintaining the resolution of shallow layers that may represent thin (6 ft or 

about 2 m thick), scour-resistant lithologic units, an inverse Schlumberger array was used with 

shorter distances between electrodes towards the center of the cable (table 1, fig. 3). Forward 

models were developed using RES1DINV (Geotomo Software, Panang, Malaysia) with a series of 

electrical structures representing various water depths and conductive-layer thicknesses to optimize 

the cable design. The final electrode array positions are listed in table 1 and the cable schematic is 

illustrated in figure 3. 

Several steps that are not common to more traditional land-based DC resistivity surveys are 

necessary to prepare CRP data for inversion and interpretation. Because the geometric factor used 

to calculate apparent resistivity from the measured resistance is dependent on a consistent distance 

between electrodes, it is necessary for the resistivity cable to remain relatively straight throughout 

data collection. However, changes in water-flow direction and surface velocity can drag and bend 

the cable in ways that violate the assumption of consistent electrode spacing. Careful attention was 

paid to the position of the cable throughout the survey, and locations where electrode spacing could 

not be maintained were removed during data processing. 
7

Table 1. Water-borne resistivity cable schematic 

[C, current electrode; P, potential electrode]

Distance (feet)1

49

Electrode

P11

102 P9

154 P7

180 P5

207 P3

220 P1

231 C1

241 C2

253 P2

266 P4

292 P6

318 P8

371 P10

1 Represents the distance from the beginning of the cable, not the distance from the boat.

Figure 3. Schematic diagram showing electrode geometry of the water-borne resistivity cable. Brackets 

indicate the potential-electrode pair used in each measurement for the adapted inverse Schlumberger 

array, numbered in order of increasing depth penetration.
8

To be useful for mapping lithologic layers, resistivity data must be matched with accurate 

spatial-positioning data. Because the GPS and depth sounder are located in the boat while the 

resistivity data are approximately located at the center of the electrode array, a spatial offset was 

applied to each measurement that is equivalent to the distance to the center of the array from the 

GPS antenna mount; on average this offset is 67.5 m. Because the GPS is located in the boat, the 

first several tens of meters of every collection event, or line, lack spatial and bathymetric data. For 

some lines, the end of a previous line may provide an overlapping boat track that can be used to 

manually replace these data; however, river conditions (shallow water, navigation hazards, 

recreational use, and so forth) did not always allow lines to overlap and resulted in small gaps in 

survey coverage between individual collection lines. In addition, bathymetric data were smoothed 

using a combination of non-linear and low-pass filters to minimize the influence of small changes 

(less than 1-m wide) on the resistivity inversion results. 

Two-Dimensional Inverse Modeling

To help determine the probable heterogeneous distribution of electrical resistivity from 

apparent resistivity measurements, a numerical inverse-modeling process is used. Data were 

inverted using a two-dimensional (2-D) finite-element least-squares approximation through 

EarthImager 2-D version 2.2.9 (Advanced Geosciences, Inc., Austin, Tex.). A starting model 

consisting of rectangular blocks of individual resistivity values was developed using a combination 

of the average measured apparent resistivity values and a priori knowledge of the electrical 

structure, including water depth and conductivity data. The inversion program determined the 

calculated system response over that model, referred to as the “calculated apparent resistivity,” on 

the basis of the field data-collection parameters. The root-mean-square (RMS) error between the 

measured and calculated apparent resistivity sections was used to determine the appropriateness of 

the model. The inversion program then iteratively reduced the RMS error by altering the model 

resistivity values and recalculating the apparent resistivity. When the RMS error between the 

calculated and measured apparent resistivity no longer improved between iterations by more than 

1 percent of the total RMS error or a total of 8 iterations were completed, a solution was reached. 

The final inverse model represents a non-unique estimate of the probable distribution of electrical 

resistivity within the subsurface. This inversion process is described in detail by Loke (2004); 

inversion parameters used in this study are provided in table 2. 

Because CRP data are collected while moving, individual lines of data can be several 

kilometers in length. A full 2-D inversion of such long lines is computationally expensive, time 

consuming, and unnecessary given the relatively small electrode array over which measurements 

are made. Long lines of field data were divided into 300-m (984-ft) segments with 60 percent

overlap between segments to minimize inversion time. These segments were rejoined into a single 

line at the end of the inversion process. This approach occasionally results in a segmented 

appearance in the final displayed profiles. While in some lines this is visually noticeable, it is 

typically cosmetic and does not affect the overall geologic interpretation. Following inversion, data 

were reunited with their original geographic coordinates and water-depth data for visualization. 
9

Table 2. Summary of inversion parameters used in processing water-borne resistivity data. 

[Min., minimum; Max., maximum; mV, millivolt; ohm-m, ohm-meter; %, percent] 

Parameter Setting used

Inversion method

Robust data conditioner

Robust model conditioner

Mesh thickness incremental factor

Depth factor

Min. voltage accepted (mV)

Min. resistance accepted (ohm)

Min. apparent resistivity accepted (ohm-m)

Max. apparent resistivity accepted (ohm-m)

Rough conditioner

Max. number of iterations

Max. RMS model error (%) 

Starting model (below riverbed) (ohm-m)

Water-column resistivity (ohm-m)

Damping factor

Horizontal/vertical roughness ratio

Lagrange multiplier

Robust

1

1

1.1

1.1

0.2

0.0001

1

1000

0.2

8

3

Average section apparent resistivity

164

100

0.1

100

Geophysical Characterization of Major Lithologic Units

The inverted resistivity profiles reveal a variety of resistivity features ranging from 25 to 

1,000 ohm-m, shown on a linear color scale ranging between 50 to 500 ohm-m in figure 4. There is 

consistently a low-resistivity layer (less than 100 ohm-m) in the bottom of half of the profile, from 

about 30 to 60 ft (9 to 18 m) below the water surface. This low-resistivity layer is likely indicative 

of a high-clay-content layer. Also consistent throughout the study area is a moderate-resistivity 

layer (about 150 ohm-m) that represents the water column and correlates well to the bathymetric 

data. There are more variations in the resistivity structure of the upper 30 ft (9 m) below the river 

bed, which we discuss in sections from downstream to upstream, as labeled in figure 4. A 

consistent color scale is used throughout this study and has been chosen to represent interpreted

low-resistivity features as blue, moderate-resistivity features as cyan, green, and yellow, and highresistivity features as red.

While distinct contrasts in resistivity were resolved by these data, the lithologic source of 

these contrasts cannot be known with certainty without more geotechnical data, such as coring in 

the river. In the following discussion, we interpret likely lithologic structures, but these 

interpretations are non-unique and cannot be directly related to scour potential. Results from 

erodibility studies being performed by Ayres Associates (T. Smith, Ayres Associates, oral 

commun., June 2008) on recently cored boreholes may provide insightful data on the scour 

potential of certain lithologic units. Additional interpretation of the resistivity data presented here 

and in Asch and others (2008) with respect to these results may improve interpretations of lithology 

and scour potential throughout the American River channel and floodplain. 
10

Figure 4. Three-dimensional map, looking down on the study area to the Northwest, showing inverted resistivity profiles below the partially 

transparent aerial photography. Delineated sections indicate areas of similar resistivity structure.
11

Section 1

Section 1 is the farthest downstream section, beginning at the Campus Commons Golf 

Course and extending to about 1,000 ft (300 m) south of the H Street Bridge. This section is 

characterized by a high-resistivity layer (greater than 500 ohm-m) to an average depth of about 

30 ft (9 m), underlain by a low-resistivity layer (plate 1). This structure may represent a low claycontent unit, such as a sand and gravel layer, overlying a unit of higher clay content that extends 

below the depth of investigation. This structure is generally consistent and seen in the left-bank, 

center, and right-bank profiles. 

Section 2

Section 2 extends from about 1,000 ft (300 m) south of the H Street Bridge to immediately

west of the Howe Avenue Bridge. This section is characterized by an extensive low-resistivity 

layer that extends from about 10 to 20 ft (3 to 6 m) depth to below the depth of investigation 

(plate 2). Immediately above this layer, we see inconsistent moderate- to high-resistivity layers that 

generally extend to the riverbed. These layers are generally thinner and lower in resistivity value 

than the upper layer in Section 1. This structure may represent a sand and gravel unit immediately 

above a higher clay content unit. Borehole logs that exist for this floodplain near this area (Tom 

Smith, Ayres Associates, written commun., August 2008) indicate substantial clay units at depth, 

intermixed with some sand and silt units—these log data generally support this interpretation. 

Section 3

Section 3 extends from just west of the Howe Avenue Bridge to the Watt Avenue Bridge.

This section is characterized by an intermittent high-resistivity layer that, when present, is between 

15 and 35 ft (4.5 to 10.5 m) in depth (plate 3). The remainder of the section is dominated by lowresistivity features similar to those seen in other sections. This structure may represent intermittent 

sand and gravel deposits overlying a higher clay content unit. 

Section 4 

Section 4 begins at the Watt Avenue Bridge and extends east for about 3,500 ft (1.6 km). In 

this section, the low-resistivity layer occurs immediately below the river bed (plate 4). The river 

here is quite shallow and the river bed comprises a unit that has shown increased stability in 

repeated bathymetric surveys between 1987 and 2006 (Ayres Associates, 2008). This layer may be 

continuous within the depth of investigation. However, there are some small increases in resistivity 

in the subsurface that may indicate a slight reduction in clay content or coarsening of material at 10 

to 25 ft (3 to 7.5 m) depth, noted by the transition from deep blue to cyan on the resistivity color 

scale. Borehole logs indicate that there are substantial sandy silt layers that exist below clay units in 

the floodplain. This small increase in resistivity may indicate that these sandy silt layers also exist

below the river. 

Section 5

Section 5 extends from 3,500 ft (1.6 km) east of the Watt Avenue Bridge to the upstream 

portion of the study area near the Rio Americano High School (plate 5). This section is similar in 

resistivity structure to section 2, with intermittent moderate to high resistivity features at the 

surface extending to inconsistent depths between 10 and 30 ft (3 to 6 m), and infrequently these 
12

moderate resistivity features extend to nearly 60 ft (18 m). Below this, a low-resistivity layer 

similar to that seen throughout the study area extends below the depth of investigation. This 

resistivity structure may represent intermittent sand and gravel deposits above a higher clay 

content unit. 

Conclusions

The American River in Sacramento, California, is situated above a mixed geology of 

alluvial deposits that range from clay to gravel. Sand deposits related to historic hydraulic mining 

activities underlie the floodplain and are preferentially prone to scour during high-water events. In 

contrast, sections of the American River channel have been observed to be scour resistant. In order 

to understand how floods may impact the integrity of the levee system, an improved understanding 

of the geology is required. In this investigation, the USGS explored the resistivity structure of the 

American River channel to characterize the extent and thickness of lithologic units that may impact 

the scour potential of the area. 

Resistivity data were collected in three profiles along the American River using a waterborne continuous resistivity profiling technique. After processing and modeling these data, inverted 

resistivity profiles were used to make interpretations about the extent and thickness of possible 

lithologic layers. In general, an intermittent high-resistivity layer extends to a depth of up to 30 ft 

(9 m). This layer is underlain by a consistent low-resistivity layer that likely indicates a high-clay

content unit that extends below the depth of investigation of 60 ft (18 m). The high-resistivity layer 

is completely absent immediately upstream of the Watt Avenue Bridge, and the low-resistivity 

layer extends to the surface where a scour-resistant unit has been observed in the river bed. 

Additional boring data may improve the lithologic interpretations presented in this report, and 

results of ongoing erodibility analyses would allow for more robust interpretation of scour potential 

to be made.
13

References Cited

Asch, T.A., Deszcz-Pan, M., Burton, B.L., and Ball, L.B., 2008, Geophysical characterization of 

the American River levees, Sacramento, California, using electromagnetics, capacitively coupled 

resistivity, and direct-current resistivity: U.S. Geological Survey Open-File Report 2008-1109, 

23 p., 11 plates. (Also available at http://pubs.usgs.gov/of/2008/1109/.) 

Ayres Associates, 2008, Lower American River aerial atlas with historic thalweg profiles: Prepared 

for U.S. Army Corps of Engineers Sacramento District, June 17, 2008, Project # 32-1006.06, 

8 plates.

Ball, L.B., Kress, W.H., Steele, G.V., Cannia, J.C., and Andersen, M.J., 2006, Determination of 

canal leakage potential using continuous resistivity profiling techniques, Interstate and Tri-State 

Canals, Western Nebraska and Eastern Wyoming, 2004: U.S. Geological Survey Scientific 

Investigations Report 2006–5032, 53 p. (Also available at http://pubs.usgs.gov/sir/2006/5032/.) 

Helley, E.J., and Harwood, D.S., 1985, Geologic map of the late Cenezoic deposits of the 

Sacramento Valley and northern Sierran Foothills, California: U.S. Geological Survey Map MF 

1790, 5 plates. (Also available at http://pubs.usgs.gov/mf/1985/1790/.) 

Loke, M.H., 2000, Electrical imaging surveys for environmental and engineering studies—A 

practical guide to 2-D and 3-D surveys: Penang, Malaysia, Geotomo Software, 62 p., accessed 

April 2009 at http://www.geoelectrical.com/downloads.php

Loke, M.H., 2004, RES2DINV, version 3.54—Rapid 2D resistivity and IP inversion using the 

least-squares method—Geoelectrical imaging 2-D and 3-D: Penang, Malaysia, Geotomo 

Software, 130 p., accessed April 2009 at http://www.geoelectrical.com/downloads.php

Zohdy, A.A.R, Eaton, G.P., and Mabey, D.R., 1974, Application of surface geophysics to groundwater investigations: U.S. Geological Survey Techniques of Water-Resources Investigations 

book 2, chap. D1, 116 p.