Electrical Resistivity Surveys in Prospect 

Gulch, San Juan County, Colorado 

By Robert R. McDougal 

Open File Report 2006-1171

U.S. Department of the Interior 

U.S. Geological Survey 
ii

U.S. Department of the Interior

Dirk A. Kempthorne, Secretary 

U.S. Geological Survey 

P. Patrick Leahy, Acting Director 

U.S. Geological Survey, Reston, Virginia 2006 

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Suggested citation: 

McDougal, R.R., 2006, Electrical Resistivity Surveys in Prospect Gulch,

San Juan County, Colorado: Denver, CO, U.S. Geological Survey

Open File Report 2006-1171, 35 p. 

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Although this report is in the public domain, permission must be secured from the 

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iii

Contents 

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

Introduction ...................................................................................................................................2

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

Physical Setting .........................................................................................................................3

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

Methods of Investigation ...............................................................................................................5

Electrical Properties of Earth Materials ......................................................................................5

Collection of Resistivity Data......................................................................................................6

Results..........................................................................................................................................8

Line 1 .........................................................................................................................................9

Line 2 .......................................................................................................................................10

Line 3 .......................................................................................................................................11

Line 4 .......................................................................................................................................11

Line 5 .......................................................................................................................................12

Conclusions ................................................................................................................................13

References Cited ........................................................................................................................15

Appendix 1..................................................................................................................................24

Appendix 2..................................................................................................................................30

Figures 

Figure 1. Location map of the Prospect Gulch study area in the upper Animas River 

watershed. ........................................................................................................................17

Figure 2. Location of resistivity surveys (digital ortho photo quad base image). ..............18

Figure 3. Conductivity line 1.............................................................................................19

Figure 4. Conductivity line 2.............................................................................................20

Figure 5. Conductivity line 3.............................................................................................21

Figure 6. Conductivity line 4.............................................................................................22

Figure 7. Conductivity line 5.............................................................................................23
1

Electrical Resistivity Surveys in Prospect 

Gulch, San Juan County, Colorado 

By Robert R. McDougal1

Abstract 

Prospect Gulch is a major source of naturally occurring and mining related metals to 

Cement Creek, a tributary of the upper Animas River in southwestern Colorado. Efforts to improve 

water quality in the watershed have focused on Prospect Gulch because many of its abandoned 

mines and are located on federal lands. Information on sources and pathways of metals, and related 

ground-water flow, will be useful to help prioritize and develop remediation strategies. It has been 

shown that the occurrence of sulfate, aluminum, iron, zinc and other metals associated with 

historical mining and the natural weathering of pyritic rock is substantial. In this study, direct 

current resistivity surveys were conducted to determine the subsurface resistivity distribution and to 

identify faults and fractures that may act as ground-water conduits or barriers to flow. Five lines of 

resistivity data were collected in the vicinity of Prospect Gulch, and cross-section profiles were 

constructed from the field data using a two-dimensional inversion algorithm. The conductive 

anomalies in the profiles are most likely caused by wet or saturated rocks and sediments, clay rich 

deposits, or high TDS ground water. Resistive anomalies are likely bedrock, dry surficial and subsurface deposits, or deposits of ferricrete. 

 

1

 U.S. Geological Survey, Denver, Colorado 80225 
2

Introduction 

Prospect Gulch is a major source of naturally occurring and mining related metals to 

Cement Creek, a tributary of the upper Animas River in southwestern Colorado. In this highly 

mineralized sub-basin it has been shown that the occurrence of sulfate, aluminum, iron, zinc and 

other metals associated with historical mining is substantial, but relatively insignificant compared 

to contributions from areas of natural weathering of highly altered, pyritic rock (Wirt and others, 

2001). While many of the streams in the area have low pH and elevated metal loads caused by 

acid-rock weathering, acid-mine drainage from historical mining has contributed to the degredation 

of ground and surface water (Church and others, in press). Efforts to improve water quality in the 

watershed have focused on Prospect Gulch because many of its abandoned mines and waste-rock 

piles are located on land managed by the U.S. Bureau of Land Management (BLM). Information on 

sources and pathways of metals, and related ground-water flow, will be useful to the BLM to help 

prioritize and develop remediation strategies. 

The direct current (dc) resistivity method used in this study is a geophysical technique that 

uses variations in electrical resistivity of the earth to help characterize subsurface features related to 

geologic structures and ground-water geochemistry. The surveys were conducted to determine the 

subsurface resistivity distribution and to identify faults and fractures that may act as ground-water 

conduits or barriers to flow. 

This study was conducted as part of the Process Studies of Contaminants Associated with 

Mineral Deposits Project by members of the U.S. Geological Survey’s Central Region Crustal 

Imaging and Characterization Team. The results of the electrical geophysical surveys presented 

here are part of an integrated effort, including seismic surveys, drilled core and monitoring wells, 

stream tracer injection studies, and geochemical analyses, to characterize and model ground-water 
3

flow in Prospect Gulch. These data sets will be used to help constrain the conceptual models of 

current ground-water conditions. 

Purpose and Scope 

This report describes the process and results of a geophysical investigation of subsurface 

geologic and hydrologic conditions as they relate to ground-water flow in a mountainous sub-basin. 

Four lines of resistivity data were collected at the base of Prospect Gulch using an automated multichannel system. An additional line of data was collected near the Henrietta and Lark mines to 

record pre-remediation subsurface conditions. Cross-section profiles were constructed from the 

field data using a two-dimensional inversion algorithm. 

Physical Setting 

Prospect Gulch, located approximately 10 km north of Silverton, Colorado, is a steepgradient alpine to sub-alpine mountain sub-basin that drains the southern flank of Red Mountain 

No. 3 (fig. 1). The stream in Prospect Gulch has an elevation change of 800 m and is approximately 

2.4 km in length. The majority of precipitation in the area occurs as snowfall (94 cm/year), with an 

average annual precipitation of about 114 cm (Wirt and others, 2001). During the summer months 

thunderstorm events can produce substantial amounts of precipitation. 

Much of Prospect Gulch is above tree line, which is approximately 3,536 m at this latitude. 

Non-forested areas primarily consist of alpine vegetation, poorly developed soils, talus, and 

exposures of bedrock. 

Several mine adits and large waste dumps are located on BLM property in the drainage. 

These are associated with the Lark, Henrietta, and Joe and John mines. Other notable historical 

mines include the Hercules and Galena Queen mines (fig. 1). 
4

Geologic Setting 

Prospect Gulch is located on the margin of the historic Red Mountain mining district, near 

the northwest structural margins of the San Juan and Silverton calderas. Rocks in Prospect Gulch 

formed following formation of the 28.2 Ma San Juan Caldera, and are coincident with and or postdate formation of the 27.8 Ma, Silverton caldera (Yager and Bove, 2002). These intermediate- to 

felsic- composition igneous rocks and minor volcaniclastic sedimentary rocks were deposited on 

the flanks of central vent volcanoes. This igneous sequence, referred to as the Silverton Volcanics, 

formed from lavas and volcaniclastic sediment that infilled the San Juan caldera over an area of 14 

km2

 to a thickness of nearly a kilometer (Lipman, and others 1973). In order of their relative 

abundance, the primary minerals of the intermediate composition porphyritic lavas of the Silverton 

Volcanics include plagioclase, quartz, hornblende, pyroxene, ± biotite, and opaque oxide minerals. 

Following caldera formation and simultaneous with Silverton Volcanic deposition, regional 

propylitic alteration occurred throughout much of the study area. As the San Juan caldera cooled 

and degassed, the primary mineral assemblages were altered to secondary assemblages containing 

chlorite, ± epidote, ± calcite, ± pyrite, fine grained muscovite, and iron oxide minerals (Yager and 

Bove, in press). 

Several small-volume plugs and dikes of dacite to rhyolite composition intruded along the 

structural margins of the calderas long after caldera activity ceased. Based on the isotopic ages of 

the intrusions near Prospect Gulch, indications are that they formed several million years after the 

eruption of the intermediate composition Silverton Volcanics lavas during the Miocene Epoch. The 

intrusions are thought to have provided the heat source for the hydrothermal alteration and 

mineralization of the Red Mountain mining district (Lipman and others, 1976; Bove and others, 

2001). 
5

A number of alteration types are found in Prospect Gulch and vary substantially from north 

to south across the sub-basin (Bove and others, in press). Near Red Mountain #3, acid sulfate 

mineralization is exposed and is associated with pervasive silicification that formed resistant 

ridges. The acid sulfate mineralization is characterized by high sulfate content and includes the 

mineral assemblage of quartz, alunite, pyrophyllite (QAP) and pyrite. More easily weathered 

argillic alteration occurs on the margins of the QAP assemblage. The most abundant alteration 

assemblage in Prospect Gulch is the quartz-sericite-pyrite (QSP), locally containing ten to twenty 

volume percent pyrite. In the lower part of the sub-basin, regional propylitic alteration is dominant. 

Structural interpretation of mapped veins in the sub-basin indicates that general trends of 

northwest, north, and northeast are predominant. There is evidence that these veins may have 

formed coincident with mineralization along structural zones of weakness. East-west trending 

veins, although uncommon, do occur in Prospect Gulch (Bove and others, in press). 

Methods of Investigation 

Electrical Properties of Earth Materials 

Electrical resistivity, which can be expressed inversely as electrical conductivity, is a 

fundamental property of all earth materials. The degree of resistance or conductance varies with 

rock or sediment type, porosity, clay mineral type and content, and the quantity and quality of 

moisture. Poorer quality ground water (that is, water with higher concentrations of dissolved solids) 

or sediments with higher clay content are usually more conductive (Zohdy and others, 1974). The 

presence of electrically conductive minerals, such as metallic sulfides can also result in higher 

conductivity values. 

Resistivity is expressed in ohm-meters, and is an estimate of the earth’s resistivity 

calculated using the relationship between resistivity, an electric field, and current density (Ohm’s 

Law), and the geometry and spacing of the current and potential electrodes. When the earth is not 
6

homogeneous and isotropic, which is usually the case, this estimate is called the “apparent 

resistivity”, which is an average of the true resistivity in the measured section of the earth (Dobrin 

and Savit, 1988). Often, in geophysical studies associated with ground-water characterization it is 

useful to convert values of resistivity to conductivity to identify or emphasize conductive 

anomalies or zones. The range of resistivity values from 1 to 1,000 ohm-meters collected in the 

field can be expressed in conductivity units of 1,000 to 1 millisiemens per meter using the 

conversion factor: 

1000/ohm-meters = millisiemens/meter (McNeill, 1980) 

Because of the many factors that affect terrain conductivity, interpretation of geophysical 

surveys can often result in a non-unique explanation for a conductive or resistive anomaly. 

Therefore, in this study, the availability of information from drilled core samples (Johnson and 

Yager, 2006) and seismic profiles aided in the interpretation of the resistivity data. 

Collection of Resistivity Data 

In July 2004, four lines of resistivity data were collected on and down gradient from the 

alluvial fan at the base of Prospect Gulch (fig.2). Line 1, paralleling County Road 140, starts at the 

north end of the Prospect Gulch alluvial fan, crosses Prospect Creek, and ends at the southern 

extent of the fan. Lines 2 and 3 coincide with the location of two seismic lines collected 

contemporaneous with the resistivity. Line 4 extends from the southern extent of the alluvial fan 

past an iron bog adjacent to Cement Creek. Line 5 was located below the Lark Mine on an old road 

grade composed of waste rock from the Henrietta Mine. Subsequent remediation at this site was 

supervised by the BLM. Post-remediation seismic surveys have been completed in this area, and 
7

future resistivity surveys are planned to help evaluate the effectiveness of removal of the waste 

rock. 

The resistivity system used in this study is an Advanced Geosciences, Inc. (AGI) 

SuperSting R8/IP eight channel multi-electrode instrument. The system was used to record lateral 

and vertical resistivity variations in the subsurface. Multi-electrode systems have the capability of 

recording many channels of data simultaneously and allow for the collection of very dense dc 

resistivity data in a relatively short amount of time. The electrode array was connected to the R8 

transmitter/receiver, and consisted of 76 dual mode switches in contact with the ground using 

stainless steel stakes. To minimize contact resistance with the ground, the stainless steel stakes 

were watered with salt water as needed. The dual mode electrodes are automatically switched to 

allow each to operate as either current or potential electrodes in the array. Equidistant spacing 

between each electrode varied depending upon the desired line length, depth of investigation, and 

data resolution. 

In this study, the surveys were conducted using an inverse Schlumberger array (Telford and 

others, 1990). A resistivity measurement in this configuration uses from four to ten of the 

electrodes simultaneously, half of the electrodes on either side of a central point. The inner two 

electrodes transmit current, while the outer electrodes measure the electrical potential. After the 

instrument finishes collecting data from the electrode array, a group of electrodes can be moved 

from the beginning of the line to the end of the line and data collection is then re-initiated over the 

section of the line that had not been previously measured (referred to as a “roll-along”). This 

method allows for the collection of lines of data of effectively any length, without sacrificing 

resolution. An electronic distance meter (EDM) survey system was used to accurately record the 

position and elevation of the resistivity lines. 
8

The data collected in the field were converted from apparent resistivity to inverted 

resistivity with depth using a numerical inversion of the data. The inversion routine used for this 

study was EarthImager 2D, a DC resistivity inversion program from Advanced Geosciences, Inc. 

The result of the inversion process is a highly detailed cross-section of resistivity corrected for 

changes in topography along the profile using the EDM elevation and position data. The inversion 

routine minimizes the root mean square (RMS) error between the measured apparent resistivity and 

the resistivity calculated from the inverted model. To further minimize the RMS error, multiple 

iterations of the inversion process were run and noisy or poorly fit data points were identified and 

removed after each inversion. The RMS error for all of the resistivity lines was reduced to less than 

five percent. Profiles of the measured apparent resistivity pseudosections, calculated apparent 

resistivity pseudosections, and inverted resistivity section are shown in appendix 1. Crossplots of 

measured versus predicted apparent resistivity are shown in appendix 2. 

Results 

Inverted data from the dc resistivity surveys were used to construct profiles of sub-surface 

electrical properties of the Prospect Gulch alluvial fan, the area along the Cement Creek road from 

Prospect Gulch to Georgia Gulch, and the waste rock below the Lark Mine (fig. 2). Resistivity 

values in ohm-meters obtained from the inversion program were converted to conductivity values 

in millisiemens per meter to emphasize conductive anomalies as previously described. Data grids 

were calculated using the minimum curvature gridding method in Surfer v.8 from Golden 

Software, Inc., and plotted as contoured profile maps ranging in value from 0 to 37 

millisiemens/meter. 

The conductive anomalies in these profiles are most likely caused by wet or saturated rocks, 

clay-rich sediments, or high TDS ground water. Resistive anomalies are likely bedrock, dry 

surficial and sub-surface deposits, or deposits of ferricrete. Identification of these conductive and 
9

resistive zones is important to help characterize ground-water flow in the sub-basin and help 

constrain ground-water modeling efforts. The conductivity profiles presented in this study were 

plotted to the same horizontal and vertical scale. The end points for each line are given in Universal 

Transverse Mercator (UTM) coordinates, Zone 13N, North American Datum (NAD) 1927. 

It is important to note that two-dimensional lines of resistivity data are collected in threedimensional space, and must be interpreted bearing in mind the possibility of off-survey-line 

anomalies. The magnitude of this “3D effect” depends upon the lateral distance of the anomaly 

from the survey line and can result in misinterpretation of its size and location. 

 Line 1 

Line 1 is located along the western side of the Cement Creek road and extends from the 

northern end of the alluvial fan at the base of Prospect Gulch to the southern end, just past the creek 

draining the sub-basin (fig. 2). This survey line was positioned to identify conductive anomalies 

that are possibly related to ground water discharging to Cement Creek from Prospect Gulch. The 

line was not positioned on the east side of the road closer to the creek, as originally planned, 

because of the presence of a buried phone line, metal pipes, and the remnants of an old railroad 

line. 

This line consisted of a total of 124 electrode positions (76 initial electrodes with three rollalongs of 16 electrodes each), using 2 m spacing, for a total line length of 248 m (fig. 3). In this 

configuration the maximum depth of investigation was approximately 30 to 35 m. However, the 

data resolution and signal-to-noise generally decrease below 20 to 25 m. 

The conductive zone on the northeastern end of the line between approximately 170 and 

230 m occurs in the thickest part of the Prospect Gulch alluvial fan. The highest conductivity 

values are interpreted as wet or saturated alluvium where they occur near the surface, and wet or 

saturated bedrock where they occur at depth although the alluvium-bedrock interface is not well 
10

defined. Conductive anomalies occurring in bedrock may be associated with fracture controlled 

ground-water flow and conductive anomalies in the alluvium may be associated with increased clay 

content. A well-core log from an exploration well drilled near Line 1 shows that bedrock (Silverton 

Volcanics) in this area is highly fractured and hydrothermally altered (Johnson and Yager, 2006). 

The small near surface conductive zone at 130 m along the line is likely caused by a buried metal 

cultural object, given its size and electrical contrast with the adjacent area. 

The resistive near surface anomaly between 75 and 125 m is located in an area where 

ferricrete and dry surficial deposits were observed near the intersection with Line 3. The resistive 

zone at the surface from 0 to 60 m along the survey line in the vicinity of Prospect Creek occurs 

where bedrock outcrops and dry surficial deposits are found along the banks of the creek. 

Line 2 

Line 2 extends across the alluvial fan at the base of Prospect Gulch (fig. 2). The location of 

this line was selected to correspond with the position of a contemporaneous seismic survey line. 

Line 2 consisted of a total of 92 electrode positions (76 electrodes in the initial array with one rollalong of 16 electrodes), using 2 m spacing, for a total line length of 182 m (fig. 4). 

As with Line 1, Line 2 is characterized by high resistivity at the surface, which is the result 

of dry surficial deposits and ferricrete. In some areas near the center of the line, ferricrete was 

exposed at the surface. The very dense nature of the ferricrete made placement of the electrodes 

difficult and resulted in very high contact resistance and low signal-to-noise response from the 

instrument. 

The large conductive zone extending laterally from 20 to 85 m is interpreted to be the result 

of a saturated clay-rich interval encountered in the well-core between a depth of approximately 10 

to 25 m. Below this interval, a highly fractured and altered bedrock interval to a depth of 

approximately 40 m was recorded in the well log (Johnson and Yager, 2006). 
11

Line 3 

Line 3 extends from the break in slope at the top of the Prospect Gulch alluvial fan to the 

Cement Creek road, and intersects Line 2 near the test well. The location of the line was coincident 

with a seismic survey line. The array consisted of 76 electrodes using 2 m spacing, for a total line 

length of 150 m (fig. 5). 

The conductive zone extending laterally from 25 to 55 m correlates with the position and 

depth of the conductive anomaly interpreted as the clay-rich interval in Line 2. This conductive 

anomaly extends semi-continuously to the northwest with another conductivity high occurring 

between 115 and 140 m. The relative depth of these conductive anomalies indicates that the 

saturated alluvial zone is deeper near the bottom of the alluvial fan and may also indicate the 

changing depth of clay-rich sediments. The conductive zone at the surface at approximately 120 m 

is near an area of saturated ground observed to be associated with an iron bog. The thin resistive 

layer beneath this conductive zone is interpreted as a layer of ferricrete separating the conductive 

shallow ground water from deeper conductive water. 

The resistive zone at the surface, which extends from 0 to 100 m, is interpreted as ferricrete 

and dry surficial deposits. The well-core from here shows that ferricrete extends from near the 

surface to a depth of approximately 6 m (Johnson and Yager, 2006). 

Line 4 

Line 4 extends along the Cement Creek road from near the southwestern end of Line 1 to 

the alluvial fan at the base of Georgia Gulch. The line consisted of 92 electrode positions (76 

electrodes in the initial array with one roll-along of 16 electrodes) using 5 m spacing for a total line 

length of 455 m (fig. 6). The line was positioned to identify conductive zones down gradient from 

Prospect Gulch that might be associated with an iron bog along Cement Creek. 
12

The line is generally resistive at the surface along its length, resulting from ferricrete and 

extremely dry soil and surficial deposits. The highly resistive surface layer at the northeast end of 

the line coincides with the resistivity high at the southwest end of Line 1, and is the result of 

observed ferricrete and dry surficial deposits. The “cat’s eye” shaped resistive zone that extends 

from 120 to 140 m at a depth of 5-35 m occurs in an area of mapped mineralized veins (Yager and 

Bove, 2002). Therefore, given the shape and depth of the anomaly, this resistive feature is possibly 

a concealed fault or vein that is likely silicified. 

The conductive near surface layer extending from 260 to 430 m is interpreted as wet clay 

and alluvium eroded off of the ridge between Prospect Gulch and Georgia Gulch. The resistive 

zone below this layer is likely bedrock that becomes more conductive down gradient to the 

southwest. 

The prominent conductive zone on the southwest end of the line between 30 and 180 m is 

interpreted as deep circulating ground water draining from Prospect Gulch. Well and piezometer 

samples in this area indicate that ground-water temperatures are relatively warm and that there is a 

strong upward gradient in flow (Johnson and Yager, 2006). Therefore, we conclude that deep 

circulating ground-water flow is likely influenced by the resistive structure near the upper bog and 

that the near surface layer of ferricrete acts as a confining layer. The highly conductive anomaly 

between 40 and 110 m is interpreted as the result of concentrated high TDS ground water that is 

mantled and confined by the ferricrete. However, this zone could also contain clay-rich sediments, 

but this alternate interpretation cannot be confirmed without direct core data. 

Line 5 

Line 5 is located midway up Prospect Gulch on an old road grade below the Lark mine. The 

line was positioned to intersect ground water draining from the tailings and waste rock associated 
13

with the mine in an effort to record pre-remediation subsurface conditions. The electrode array 

consisted of 76 electrodes using 3 m spacing for a total line length of 225 meters (fig. 7). 

Several seeps and springs were observed below the embankment to the north of the line, some 

having orange colored water with obvious iron precipitation. The monitoring well drilled near the 

line shows a perched water-table system in colluvial material above fractured bedrock. The 

conductive zone between 30 and 110 m is likely caused by the interaction of the perched nearsurface water table with mine waste rock. The well data, core, and water levels show that the 

colluvial material and fractured bedrock below a depth of approximately 15 m is commonly 

unsaturated, with a deeper water table occurring at a depth of approximately 50 m, near the limit of 

the depth of investigation for the resistivity data. The near-surface conductive anomaly between 

155 and 200 m along the line is coincident with seeps and saturated surface soils. This zone is 

interpreted as a plume of conductive shallow ground water draining from the Lark mine waste rock. 

The sharp interface between conductive and resistive zones suggests that ground water here is 

perched on a resistive surface of ferricrete or bedrock. 

Conclusions 

The resistivity profiles presented in this study generally identify zones of varying near 

surface geology (for example, clay-rich sediments and ferricrete) as well as zones of conductive 

ground water. Conductive zones at depth indicate the presence of a deeply circulating ground-water 

system that occurs either in saturated bedrock or faults and fractures. Where ground water is 

associated with mine waste the conductivity is generally high, suggesting poor water quality. 

This study illustrates the utility of combining geophysical investigations with available 

well-core logs to expand the interpretation and understanding of the ground-water and geologic 
14

systems in complex mountainous terrains. By correlating well and piezometer data with the 

resistivity profiles, the often non-unique interpretation of the geophysical data can be improved. 
15

References Cited 

Bove, Dana J.; Hon, Ken; Budding, Karin E.; Slack, John F.; Snee, Lawrence W.; Yeoman, Ross 

A., 2001, Geochronology and geology of late Oligocene through Miocene volcanism and 

mineralization in the western San Juan Mountains, Colorado, U.S. Geological Survey 

Professional Paper 1642, 30p. 

Bove, D.J., Mast, M.A., Dalton, J.B., Wright, W.G., and Yager, D.B, in press, Major styles of 

mineralization and hydrothermal alteration and related solid- and aqueous-phase geochemical 

signatures, in Church, S.E., von Guerard, Paul, and Finger, S.E., eds., Integrated investigations of 

environmental effects of historical mining in the Animas River Watershed, San Juan County, 

Colorado: U.S. Geological Survey Professional Paper 1651. 

Church, S.E., Mast, M.A., Martin, E.P., and Rich, C.L., in press, Mine inventory and compilation 

of mine-adit chemistry data, Chapter E5 in Church, S.E., von Guerard, Paul, and Finger, S.E., 

eds., Integrated investigations of environmental effects of historical mining in the Animas River 

Watershed, San Juan County, Colorado: U.S. Geological Survey Professional Paper 1651. 

Dobrin, M.B. and Savit, C.H., 1988, Introduction to Geophysical Prospecting, fourth edition, 

McGraw-Hill, 630 p. 

Johnson, R.H., Yager, D.B., 2006, Completion reports, core logs, and hydrogeologic data from 

wells and piezometers in Prospect Gulch, San Juan County, Colorado: U.S. Geological Survey 

Open-File Report 2006-1030. 

Lipman, P.W., Steven, T.A., Luedke, R.G., and Burbank, W.S. 1973, Revised volcanic history of 

the San Juan, Uncompahgre, Silverton, and Lake City calderas in the western San Juan 

Mountains, Colorado: U.S. Geological Survey Journal of Research, v. 1, 627-642. 

Lipman, P.W., Fisher, F.S., Mehnert, H.H., Naeser, C.W., Luedke, R.G., and Steven, T.A. 1976, 

Multiple ages of mid-Tertiary mineralization and alteration in the western San Juan Mountains, 

Colorado: Economic Geology, v. 71, 571-588. 

McNeill, J.D., 1980, Electrical Conductivity of Rocks and Soils, Technical Note TN-5, Geonics 

Limited, Ontario, Canada, 22 p. 

Stanton, M.R., Yager, D.B., Fey D.L., and Wright, W.G., in press, Formation and geochemical 

significance of iron bog deposits, in Church, S.E., von Guerard, Paul, and Finger, S.E., eds., 

Integrated investigations of environmental effects of historical mining in the Animas River 

Watershed, San Juan County, Colorado: U.S. Geological Survey Professional Paper 1651. 

Telford, W.M., Geldart, L.P., and Sheriff, R.E., 1990, Applied Geophysics, 2nd ed., Cambridge 

University Press, 770 p. 

Wirt, Laurie, Leib, J.L., Melick, Roger, Bove, D.J., 2001, Metal loading assessment of a small 

mountainous sub-basin characterized by acid drainage – Prospect Gulch, Upper Animas River 

Watershed, Colorado, U.S. Geological Survey Open-File Report 01-0258, 36p. 
16

Yager, D.B. and Bove, D.J., 2002, Generalized Geologic Map of Part of the Upper Animas River 

Watershed and Vicinity, Silverton, Colorado: U.S. Geological Survey Miscellaneous Field 

Studies Map MF-2377, 1:48.000. 

Yager, D.B., and Bove, D.J., in press, Geologic framework, Chapter E1 in Church, S.E., von 

Guerard, Paul, and Finger, S.E., eds., Integrated investigations of environmental effects of 

historical mining in the Animas River Watershed, San Juan County, Colorado: U.S. Geological 

Survey Professional Paper 1651. 

Zohdy, A.A.R., Eaton, G.P., and Mabey, D.R., 1974, Application of Surface Geophysics to 

Ground-Water Investigations, Techniques of Water-Resources Investigations of the United States 

Geological Survey, Book 2, Chapter D1: United States Geological Survey, Denver, CO, 116 p. 
17

Figure 1. Location map of the Prospect Gulch study area in the upper Animas River watershed. 
18

Figure 2. Location of resistivity surveys (digital ortho photo quad base image).





24

Appendix 1 

Results of two-dimensional inversion of resistivity field data showing the measured apparent 

resistivity pseudosection, calculated apparent resistivity pseudosection, and the inverted resistivity 

section.
25
26
27
28
29
30

Appendix 2 

Crossplots of Measured versus predicted apparent resistivity.
31
32
33
34
35