Open-File Report 2011–1035 

U.S. Department of the Interior 

U.S. Geological Survey 

Prepared in cooperation with the Bureau of Land Management 

Geophysical Investigation of Red Devil Mine Using 

Direct-Current Resistivity and Electromagnetic 

Induction, Red Devil, Alaska, August 2010

By Bethany L. Burton and Lyndsay B. Ball 
Cover: Looking northeast along DC resistivity line 1 at Red Devil mine. Photo by Bethany L. 

Burton.
U.S. Department of the Interior 

KEN SALAZAR, Secretary 

U.S. Geological Survey 

Marcia K. McNutt, Director 

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

For product and ordering information: 

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its natural and living resources, natural hazards, and the environment: 

World Wide Web: http://www.usgs.gov 

Telephone: 1-888-ASK-USGS 

Suggested citation: 

Burton, B.L., and Ball, L.B., 2011, Geophysical investigation of Red Devil mine using directcurrent resistivity and electromagnetic induction, Red Devil, Alaska, August 2010: U.S. 

Geological Survey Open-File Report 2011−1035 , 53 p. 

Any use of trade, product, or firm names is for descriptive purposes only and does not imply 

endorsement by the U.S. Government. 

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

copyright owners to reproduce any copyrighted material contained within this report.

Contents 

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

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

Site Background ............................................................................................................................................................ 2

Site History................................................................................................................................................................. 3

Site Geology and Hydrogeology ................................................................................................................................ 5

Purpose and Scope ....................................................................................................................................................... 6

Methods of Investigation................................................................................................................................................ 6

Direct-Current Resistivity Method .............................................................................................................................. 6

Data Acquisition ..................................................................................................................................................... 7

Data Processing and Inversion..............................................................................................................................10

Electromagnetic Induction Method............................................................................................................................10 

Data Acquisition ....................................................................................................................................................11

Data Processing and Inversion..............................................................................................................................10

Results of Investigation.................................................................................................................................................10

DC Resistivity............................................................................................................................................................10

Line 1 ....................................................................................................................................................................11

Lines 2a and 2b.....................................................................................................................................................13

Line 3 ....................................................................................................................................................................14

Line 4 ....................................................................................................................................................................22

Three-Dimensional Grid ........................................................................................................................................24

Line 5 ....................................................................................................................................................................25

Line 6 ....................................................................................................................................................................20

Line 7 ....................................................................................................................................................................21
Electromagnetic Induction.........................................................................................................................................22

Summary and Discussion.............................................................................................................................................36

Acknowledgments ........................................................................................................................................................40

References Cited..........................................................................................................................................................41

Appendix A. DC Resistivity Pseudosections.................................................................................................................43 

Appendix B. Digital Data...............................................................................................................................................52 
Figures 

 1. Index map showing location of Red Devil mine ........................................................................................ 3 

 2. Aerial photograph of Red Devil mine showing location of site features .................................................... 4 

 3. Aerial photograph of Red Devil mine showing location of geophysical surveys and 

relevant monitoring wells and soil borings ..................................................................................................................... 8 

 4. Photograph of shoulder-slung electromagnetic induction system........................................................... 11 

 5. Photograph of DC resistivity equipment deployed on line 1 ................................................................... 14 

 6. Line 1 inverted resistivity profiles............................................................................................................ 15 

 7. Photograph of DC resistivity equipment deployed on line 2a ................................................................. 16

 8. Line 2a inverted resistivity profiles.......................................................................................................... 18 

 9. Line 2b inverted resistivity profiles.......................................................................................................... 19 

 10. Photograph of DC resistivity equipment deployed on line 3 ................................................................... 20 

 11. Line 3 inverted resistivity profiles............................................................................................................ 21 

 12. Photograph of DC resistivity equipment deployed on line 4 ................................................................... 22

 13. Line 4 inverted resistivity profiles............................................................................................................ 23 

 14. Photographs of DC resistivity equipment deployed on the three-dimensional grid and 

three electrodes along the southwest edge of the grid ................................................................................................ 24

 15. Diagram of three-dimensional resistivity inverted volume....................................................................... 26 

 16. Photographs of DC resistivity equipment deployed on line 5.................................................................. 27 

 17. Line 5 inverted resistivity profiles............................................................................................................ 28 

 18. Photograph of DC resistivity equipment deployed on line 6 ................................................................... 29 

 19. Line 6 inverted resistivity profiles............................................................................................................ 30 

 20. Photographs of DC resistivity equipment deployed on line 7.................................................................. 32

 21. Line 7 inverted resistivity profiles............................................................................................................ 33 
22. Map showing apparent conductivity data for 93,030 Hz ......................................................................... 34 

23. Map showing apparent conductivity data for 19,110 Hz ......................................................................... 35 

24. Map showing apparent magnetic susceptibility data for 93,030 Hz ........................................................ 37 

25. Map showing locations of annotated anomalies from the DC resistivity profiles and 

approximate locations of settling ponds and monofills................................................................................................. 38 

Tables 

1. Summary of two-dimensional DC resistivity line acquisition parameters.................................................. 9

2. Summary of three-dimensional DC resistivity grid acquisition parameters............................................... 9
Conversion Factors 

[SI to Inch/Pound] 

Multiply By To obtain 

Length 

centimeter (cm) 0.3937 inch (in.) 

meter (m) 3.281 foot (ft) 

meter (m) 1.094 yard (yd) 

kilometer (km) 0.6214 mile (mi) 

Area 

square meter (m2

) 10.76 square foot (ft2) 

Volume 

cubic centimeter (cm3

) 0.06102 cubic inch (in3) 

cubic meter (m3

) 1.307951 cubic yard (yd3) 

Mass 

megagram (Mg) 1.102 ton, short (2,000 lb) 

megagram (Mg) 0.9842 ton, long (2,240 lb) 

metric ton (t) 1.102 ton, short (2,000 lb) 

Megagram (Mg) 1.102 ton, short (2,000 lb) 

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

°F=(1.8×°C)+32. 

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88). 

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83). 

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

Geophysical Investigation of Red Devil Mine Using DirectCurrent Resistivity and Electromagnetic Induction 

Methods, Red Devil, Alaska, August 2010

By Bethany L. Burton and Lyndsay B. Ball 

Abstract 

Red Devil Mine, located in southwestern Alaska near the Village of Red Devil, was the state’s 

largest producer of mercury and operated from 1933 to 1971. Throughout the lifespan of the mine, 

various generations of mills and retort buildings existed on both sides of Red Devil Creek, and the 

tailings and waste rock were deposited across the site. The mine was located on public Bureau of Land 

Management property, and the Bureau has begun site remediation by addressing mercury, arsenic, and 

antimony contamination caused by the minerals associated with the ore deposit (cinnabar, stibnite, 

realgar, and orpiment). In August 2010, the U.S. Geological Survey completed a geophysical survey at 

the site using direct-current resistivity and electromagnetic induction surface methods. Eight twodimensional profiles and one three-dimensional grid of direct-current resistivity data as well as about 

5.7 km of electromagnetic induction profile data were acquired across the site. On the basis of the 

geophysical data and few available soil borings, there is not sufficient electrical or electromagnetic 

contrast to confidently distinguish between tailings, waste rock, and weathered bedrock. A water table is 

interpreted along the two-dimensional direct-current resistivity profiles based on correlation with 

monitoring well water levels and a relatively consistent decrease in resistivity typically at 2–6 meters depth. 

Three settling ponds used in the last few years of mine operation to capture silt and sand from a flotation 

ore processing technique possessed conductive values above the interpreted water level but more 

resistive values below the water level. The cause of the increased resistivity below the water table is 

unknown, but the increased resistivity may indicate that a secondary mechanism is affecting the 

resistivity structure under these ponds if the depth of the ponds is expected to extend below the water 

level. The electromagnetic induction data clearly identified the three monofills and indicate, in 

conjunction with the three-dimensional resistivity data, additional possible landfill features on the north 

side of Red Devil Creek. No obvious shallow feature was identified as a possible source for a spring that 

is feeding into Red Devil Creek from the north bank. However, a discrete, nearly vertical conductive 

feature observed on the direct-current resistivity line that passes within 5 meters of the spring may be 

worth investigating. Additional deep soil borings that better differentiate between tailings, waste rock, 

and weathered bedrock may be very useful in more confidently identifying these rock types in the 

direct-current resistivity data. 

1
Introduction 

Mercury ore deposits and mines are located throughout southwestern Alaska (Webber and 

others, 1947). Several of the minerals associated with these mercury-rich deposits, including the target 

mineral cinnabar (HgS), are sources of potential contamination of the surrounding soil, plants, and water 

and pose subsequent health risks to humans and wildlife. Mercury (Hg) is a heavy metal that is toxic to 

various degrees to any organism in all forms in which it exists. The inorganic forms of mercury that 

include cinnabar, liquid mercury (Hg0), and mercuric ion (Hg2+) can be converted to the more highly 

toxic organic form of methylmercury (meHg) by aerobic and anaerobic bacteria. These bacteria often 

occur in mercury-contaminated soils and stream environments (Bailey and Gray, 1997; Rytuba, 2000). 

Elevated levels of the heavy metals arsenic (As) and antimony (Sb) from ore-related minerals and lead 

(Pb) from the mercury production process also pose potential health and environmental risks. The 

tailings and production byproducts present at these mine sites can be a source of contamination owing to 

the increased rock surface area that is exposed to weathering and erosional processes. 

Red Devil mine, located near the Kuskokwim River (figs. 1 and 2), was Alaska’s largest 

mercury producer and operated intermittently between 1933 and 1971, during which time 36,000 flasks 

(1,224 metric tons) of mercury were produced (Bailey and Gray, 1997). The mine was officially 

abandoned in 1981, at which time groundwater pumping ceased, and the mine workings below the 

natural water table flooded (Weston, 1989). Throughout the mine’s development and operation, land 

ownership remained with the Bureau of Land Management (BLM), but the land has been selected for 

conveyance to Alaska Native corporations under authority of the Alaska Native Claims Settlement Act 

of 1971. In 1986, site inspections began under the Comprehensive Environmental Response, 

Compensation, and Liability Act of 1980 (CERCLA), commonly referred to as the Superfund program. 

Since this time, the BLM, under its CERCLA authority, has begun remediation by beginning to address 

Hg, As, Sb, Pb, and petroleum contamination at the site. To reassess potential risks to human health and 

environment that still exist after the remediation activity since 1999, a Remedial 

Investigation/Feasibility Study was initiated in accordance with CERCLA in 2009 (Bureau of Land 

Management, 2009). 

In August 2010, in cooperation with the BLM and in conjunction with the Remedial 

Investigation/Feasibility Study, the U.S. Geological Survey (USGS) completed a surface-based 

geophysical survey at Red Devil mine to aid in the remediation efforts. Direct-current (DC) resistivity 

and frequency-domain electromagnetic induction (EMI) methods were employed to determine tailings 

and waste rock thicknesses across the site and to identify additional unknown sources of potential 

contamination. 

Site Background 

The Red Devil mine is located approximately 2 kilometers (km) southeast of the village of Red 

Devil, Alaska, along the south bank of the Kuskokwim River. The village of Red Devil is located about 

400 km west of Anchorage, Alaska, and 260 km northeast of Bethel, Alaska (fig. 1). Located within the 

Kuskokwim Mountain Range, the region is heavily forested. The climate is continental with a total 

average annual precipitation of 51 centimeters (cm) including 216 cm of snowfall. Annual temperatures 

range from −50 to 32C (Alaska Division of Community and Regional Affairs, 2010). 

2
Figure 1. Index map showing the location of Red Devil mine. 

Red Devil Creek bisects the site with the mining operations and mine camp on the north side and 

the second generation processing and retort operations to the south (fig. 2). The creek flows northeast 

into the Kuskokwim River and is bordered on both banks through sections of the site by tailings and 

waste rock. Mine tailings are the rock left over after the ore has been processed, and in the case of retort 

processing, are commonly referred to as calcines and have a characteristic red color due to the oxidation 

of iron sulfides caused by the roasting process (Rytuba, 2000). Waste rock has ore concentrations that 

are uneconomical to extract or is not mineralized and so has not been processed. Both tailings and waste 

rock are distributed throughout the site and used as fill to recontour the land to support the mine-site 

buildings (Weston, 1989). 

Site History 

The mined mineralized zone, located on the north side of the creek, covered an area of 

approximately 40,500 square meters (m2) and extended about 190 meters (m) below the ground surface. 

By 1958, about 2,900 m of mine workings had been created including shafts, adits, drifts, crosscuts, 

winzes, and stopes (MacKevett and Berg, 1963). 

The mine was primarily operated using underground mining methods but did include 

exploratory surface trenching and hydraulic sluicing of overburden in the earlier years of development 

(MacKevett and Berg, 1963). Except for the last few years of production when the concentrated ore was 

shipped to Japan for further processing, the ore was fully processed on site using retorts and furnaces 

(Tryck, Nyman, and Hayes, Inc., 1987). The processing method involved heating the ore to 600–700C 

to vaporize the mercury and collect elemental liquid mercury through a condensing process (Rytuba, 

2000). The original mill equipment, located on the north side of Red Devil Creek, was destroyed in 

October 1954 and rebuilt in 1955 on the south side of the creek (Tryck, Nyman, and Hayes, Inc., 1987). 

Owing to several episodes of mining and milling operations throughout the lifespan of the mine, the 

3
6,848, 450 6,848, 500 6,848, 550 6,848, 600 6,848, 650 6,848, 700 6,848, 750 6,848, 800

61°45'30"

588,900 588,950 589,000 589,050 589,100 589,150

61°45'35" 61°45'40"

-157°19'00" -157°18'50" -157°18'40"

B02

B05

MW01

MW03

MW04

MW06

Line 7

Line 6

Line 5

Line 3

Line 4

Line 1 Line 2b

Line 2a

Red Devil Creek

Kuskokwim

River

Monofill #2

Monofill

#3

Monofill #1

Settling Ponds

#1

#2

#3

25 0 25

(meters)

NAD83 / UTM zone 4N

Easting, in meters

Northing, in meters

EXPLANATION

DC resistivity line Spring

GPS base station

EMI calibration station

Monitoring well

Main shaft

Concrete pad

DC resistivity 3-D grid

Soil boring

Stopes

Underground mine

workings

Figure 2. Aerial photograph of Red Devil mine showing the locations of site features including the

three settling ponds, three monofills, concrete pads, monitoring wells and soil borings, two springs, and

underground mine workings (M. Longtine, Ecology and Environment, Inc., June 2010) relative to the DC

resistivity profiles and three-dimensional grid. The base photograph was taken in 2001 prior to the

construction of the monofills, and no above-ground structures remained at the site at the time of the

geophysical survey. The monofill and settling pond outlines are approximate and based on historical

photographs. 

4
resulting tailings were deposited in various locations around the mine site and differ in composition and 

texture (Weston, 1989). 

Furnacing and retorting ceased in 1963, but a flotation unit was operated from 1970 to 1971 to 

concentrate the cinnabar before shipment to Japan. Lead acetate was used as a foaming agent in the 

concentration process to increase cinnabar recovery rates. Sand and silt were deposited through a chute 

from the flotation system into a series of three cascading settling ponds (fig. 2) with minimal effluent 

released with the particulates, because it was valuable and therefore continually recycled for additional 

processing (Tryck, Nyman, and Hayes, Inc., 1987). 

Site remediation from 2000 to 2005 included demolition of existing structures, creation of 

several monofills, or landfills, the removal of waste or contaminant sources, demolition of five aboveground storage tanks, and excavation of 1,070 cubic meters of petroleum-contaminated soil from the 

former above-ground storage tank area. In 2002, monofill #1 was created near the main shaft on the 

north side of the creek and filled with onsite nonhazardous debris and building materials. Two building 

foundation concrete pads still exist in this area as well. Monofill #2 is located on top of the site of the 

demolished retort and flotation mill and contains treated contaminated material including retort building 

debris, tailings, and other items of contaminated waste from the immediate area. This monofill has an 

impermeable liner and cap. An additional, smaller monofill #3 was established in 2003 and located west 

of monofill #2. It contains the steel remains of the mill hopper and above-ground storage tanks (Bureau 

of Land Management, 2010). 

Site Geology and Hydrogeology 

The Red Devil mine is located in Cretaceous Kuskokwim Group rocks, a thick sequence of 

postaccretionary, marine turbidite facies of interbedded greywacke (hard, poorly sorted, angular, impure 

sandstone) and shale with a predominantly 30 to 45 degree northwest strike and variable dip between 45 

and 55 degrees southwest (Webber and others, 1947; MacKevett and Berg, 1963). These rocks are 

locally overlain by loess and alluvium consisting of Kuskokwim River floodplain and Red Devil Creek 

fluvial deposits and minor amounts of slope wash. The loess deposits range in thickness from a few 

centimeters to 9 m, and fluvial deposits up to 6 m thick have been observed. The fluvial deposits include 

gravel, sand, and silt, and they generally postdate the loess (MacKevett and Berg, 1963). 

The dominant faults strike northwest, possess predominantly right-lateral motion, and generally 

parallel bedding planes. Although individual faults are difficult to trace for long distances because of the 

highly fractured nature of the rocks and the lack of outcrops, it is believed that these are an en echelon 

system of faults (MacKevett and Berg, 1963). 

The ore is present in veins, vein breccias, and massive replacements localized along and near 

intersections between northeast-trending late Cretaceous and early Tertiary hydrothermally altered 

mafic dikes and northwest-trending faults. The ore deposits in this region are thought to be late Miocene 

or early Pliocene (11.6 to 3.6 Ma) in age and hydrothermally developed as a result of the northwesttrending faulting. Mineralized veins are mostly small (typically less than 1-cm wide) and discontinuous, 

but veins as much as 1-m wide and 10-m long are present. Principal metallic minerals are cinnabar and 

stibnite (Sb2S3); minor amounts of realgar (As4S4), orpiment (As2S3), and pyrite (FeS2) are also found. 

High-grade ore contained as much as 30 percent Hg, but most ore averaged about 2 to 5 percent Hg 

(Webber and others, 1947; MacKevett and Berg, 1963). 

Groundwater in the vicinity of Red Devil mine is likely to occur in an unconfined aquifer 

composed of mine-related overburden, alluvial and loess deposits, and the fractured sedimentary rocks 

of the Kuskokwim Group. General hydraulic head contours were interpreted from water levels taken in 

5
five monitoring wells during September 2010 (Mark Longtine, written commun., Nov. 2, 2010), the 

elevation of two springs, and the general land-surface topography. Hydraulic head gradients were 

assumed to be uniform between wells, and the creek and river were assumed to be hydrologically 

connected with the aquifer. These contours show that the groundwater-flow direction near the mine is 

generally northeast towards the Kuskokwim River. The presence of faults, veins, dikes, and subsurface 

mine workings may create permeability heterogeneities that would affect the distribution of hydraulic 

head, groundwater-flow directions, and travel times. These heterogeneities are difficult to identify and 

are not considered in the general hydraulic head contours used to help interpret the DC resistivity data. 

Purpose and Scope 

In August 2010, the USGS, in cooperation with the BLM, completed a surface-based 

geophysical survey at the Red Devil mine site using DC resistivity and frequency-domain EMI methods. 

The goals of the survey were to determine if (1) there is sufficient contrast in electrical properties 

between the surface tailings and bedrock to quantify the volume of tailings present, (2) a substantial 

deposit of tailings exists across the Red Devil Creek delta at its intersection with the Kuskokwim River, 

(3) mine workings in the vicinity of monofill #1 on the north side of Red Devil Creek could be resolved, 

(4) the source of the spring depositing iron precipitates (“yellow boy”) into Red Devil Creek below 

monofill #1 could be determined, and (5) contamination exists from the three settling ponds on the south 

side of Red Devil Creek below the mill. To this end, eight DC resistivity profiles were acquired across 

the main processing area of the mine site and one three-dimensional DC resistivity grid was acquired 

across the vicinity of monofill #1. In addition, frequency-domain EMI data were collected in two grids 

on the north and south sides of Red Devil Creek. 

Methods of Investigation 

Direct-Current Resistivity Method 

Resistivity is an intrinsic material property that is defined by resistance to the flow of electric 

current in that material. Resistivity measurements are made by injecting a known current into the 

subsurface using two current electrodes and measuring the resulting voltage difference between two 

potential electrodes. For each profile, a linear array of stainless steel electrodes spaced at regular 

intervals is placed in the ground, and many different combinations of current and potential electrodes 

are used. Information about lateral variability in the subsurface is gained as the measuring electrodes are 

translated across the array, while information about greater depths is obtained by increasing the 

separation between the electrodes. For the three-dimensional grid, the electrodes were arranged in a 

rectangular grid pattern with consistent in-line and cross-line distances. As with the profiles, different 

electrode combinations are used for the measurements, but the active electrodes for a given 

measurement are not confined to a single row or column within the grid. On the basis of Ohm’s law, the 

resistance is computed by taking the ratio of the measured voltage and the transmitted current. The 

apparent resistivity of the material, expressed in ohm-m, can then be determined by multiplying each 

resistance value by the corresponding geometric factor, which is based on the electrode geometry and 

spacing (Butler, 2005). 

The main factors that affect the resistivity of a material are the amount of interconnected pore 

water present, the water quality (level of total dissolved solids, TDS), and the amount of mineralogical 

6
clay present. In the unsaturated zone, if no mineralogical clay is present, a fine-grained material (for 

example, silt or clay) will generally retain more interconnected water due to capillary forces than a 

coarse-grained material (for example, sand or gravel). The fine-grained material will therefore have a 

lower resistivity compared to coarser grained materials. For cemented sedimentary rock such as shale or 

greywacke, the primary (intragranular) effective porosity and secondary (fracture) effective porosity can 

differ widely on the basis of the interconnectedness of pores and open fractures and joints. Water quality is 

an important factor because the concentration of ions in the water affects its ability to conduct 

electricity. Materials containing water with high TDS will have a lower resistivity than materials 

containing water with low TDS. Even a small amount of mineralogical clay minerals can sharply 

decrease the overall bulk resistivity of a material, because current is conducted both through ion 

mobilization in the pore fluid (electrolytically) as well as through cation exchange in the clay 

(electronically). Because of the relation between grain size, water quality (TDS), and resistivity, the 

resistivity method may be a useful tool in differentiating surficial mine tailings and bedrock as well as 

delineating contaminant plumes emanating from the abandoned settling ponds. Reynolds (1997), 

Sharma (1997), and Butler (2005) provide more detailed descriptions of the resistivity method and 

resistivity values for common geologic materials. 

Data Acquisition 

DC resistivity data were acquired using an Advanced Geosciences, Inc. SuperSting R8 

resistivity/induced polarization meter with a maximum of 112 electrodes. Each solid stainless steel 

electrode is about 45 cm long and 1 cm in diameter. The 8-channel resistivity meter uses a command 

file to acquire measurements from predetermined electrode and potential electrode configurations. 

Measurements were made for a period of 0.8 seconds, equivalent to about 2 hertz (Hz), during which the 

polarity of the current electrodes is reversed in order to minimize electrode polarization effects to 

further improve contact resistance. The resistivity meter is powered by two 12-volt batteries and is 

capable of injecting up to 2,000 milliamperes (mA) of current into the ground. At this site, transmitted 

currents were generally between 10 and 800 mA. 

All electrodes were hammered into the ground as far as possible to minimize electrical contact 

resistance between the electrode and ground. It is important to note that the site was extremely wet during 

the majority of this survey. Ponded surface water was present throughout much of the survey, so the addition 

of the saltwater solution to the electrodes to further reduce contact resistances was not deemed necessary 

unless particularly high contact resistances were measured. High contact resistances can cause lower-quality 

data because of greater difficulties in both injecting current into the ground and measuring accurate voltages. 

Overall, the observed contact resistances were low to moderate at this site, typically ranging from 0.5 to 5 k 

ohm. For the three-dimensional grid acquired over monofill #1, three electrodes were located on the 

rectangular concrete pad on the southwestern edge of the grid, which required that these electrodes be placed 

on a small pile (about 1,000 cm3

) of bentonite mud instead of being driven into the ground to decrease the 

contact resistance. 

An inverse Schlumberger array geometry was used for all eight profiles (fig. 3), which allows 

for rapid data acquisition and provides a good balance between lateral and vertical sensitivity to 

resistivity changes. With this geometry, the four electrodes used for each measurement are symmetric 

about a central location, with the current electrode pair located nearer to the central location. Line 

lengths were varied on the basis of available accessible space, and electrode spacing varied between 1 

and 2 m on the basis of the project objectives while maintaining high levels of lateral and vertical 

sensitivity. A summary of the acquisition parameters for the eight profiles are provided in table 1. 

7
6,848,450 6,848,500 6,848,550 6,848,600 6,848,650 6,848,700 6,848,750 6,848,800

588,900 588,950 589,000 589,050 589,100 589,150

61°45'30" 61°45'35" 61°45'40"

-157°19'00" -157°18'50" -157°18'40"

B02

B05

MW01

MW03

MW04

MW06

Line 7

Line 6

Line 5

Line 3

Line 4

Line 1 Line 2b

Line 2a

Red Devil Creek

Kuskokwim

River

25 0 25

(meters)

NAD83 / UTM zone 4N

Easting, in meters

Northing, in meters

EXPLANATION

DC resistivity line Spring

GPS base station

EMI calibration station

EMI path

Monitoring well

Main shaft

Concrete pad

DC resistivity 3-D grid

Soil boring

Figure 3. Aerial photograph of Red Devil mine showing the locations of the geophysical surveys

and relevant monitoring wells and soil borings. 

8
Table 1. Summary of two-dimensional DC resistivity line acquisition parameters. 

Line Total line 

length Geometry Heading Total number of electrodes 

Electrode 

spacing Date acquired 

1 278 m 

2 overlapping 112-

electrode spreads at 

electrode 29 

SW - NE 140 2 m 8-Aug-10 & 

9-Aug-10 

2a 144 m 1 73-electrode spread SE - NW 73 2 m 12-Aug-10 

2b 210 m 1 106-electrode spread SE - NW 106 2 m 12-Aug-10 

3 132 m 1 112-electrode spread w/ 1 

21-electrode roll SE - NW 133 1 m 10-Aug-10 

4 97 m 1 98-electrode spread SW - NE 98 1 m 7-Aug-10 

5 111 m 1 112-electrode spread SE - NW 112 1 m 10-Aug-10 

6 111 m 1 112-electrode spread SE - NW 112 1 m 11-Aug-10 

7 104 m 1 105-electrode spread SE - NW 105 1 m 14-Aug-10 

For the three-dimensional grid, a radial dipole-dipole array geometry with 110 electrodes was 

employed (fig. 3). The electrodes were deployed as 10 parallel strings along the longest axis of the 

rectangular grid (southwest-northeast trending), each with 11 electrodes. The in-line electrode spacing 

was 6 m and the cross-line spacing was 3 m. This geometry was chosen to maximize the surveyed area 

and therefore the depth of investigation while minimizing interference from the two large concrete pads. 

The acquisition geometry parameters are summarized in table 2. 

Table 2. Summary of three-dimensional DC resistivity grid acquisition parameters. 

Geometry Number of 

electrodes 

In-line electrode 

spacing 

Cross-line 

electrode spacing 

Total grid size 

[electrodes] Array type 

Rectangular grid 110 6 m 3 m 60 x 27 m 

[11 NE x 10 NW] 

Radial 

dipole-dipole 

The three-dimensional grid was acquired in response to the resistivity character observed on the 

north side of Red Devil Creek on two-dimensional lines 2a, 2b, and 3. There appeared to be imaging 

artifacts from improper resolution of three-dimensional objects in the two-dimensional profiles. In a 

two-dimensional inverted model, the resistivity values are allowed to vary in one direction (parallel to 

the line) but are assumed to be constant perpendicular to the line. This assumption is therefore valid 

when the profiles are perpendicular to the strike of elongated structures or geologic layers. When a 

profile is acquired across a discrete three-dimensional object, however, the object is typically not well 

resolved and the resulting modeled shape is dependent on the sensitivity pattern of the electrode array 

(Loke, 2010). In the case of two-dimensional DC resistivity lines 2a, 2b, and 3, these imaging artifacts 

are often manifested as arcuate anomalies or as alternating highly resistive or conductive values. 

9
All electrode locations were positioned using a Leica GPS1200 differential global positioning 

system (GPS) in the cleared areas and supplemented with a Sokkisha SET2 electronic total station in the 

forested areas. The GPS base station was set up over a single unknown position throughout the survey (figs. 

2 and 3), and the raw observational data were converted to receiver independent exchange (RINEX) format 

and submitted to the National Geodetic Survey’s Online Positioning User Service (OPUS) to determine the 

true location, including elevation. The rover GPS data were corrected by first postprocessing the base and 

rover raw observational data using Leica Geo Office ver. 7.0 and second by applying horizontal and vertical 

translations on the basis of the OPUS base station location solution. 

Data Processing and Inversion 

The raw DC resistivity data consist of measured voltage and injected current values and the 

down-line locations of the electrodes used for each measurement. For each of these measurements, an

apparent resistivity is calculated using the raw data, Ohm’s law, and the assumption of a homogeneous and 

isotropic half-space. In most cases, the subsurface is heterogeneous and sometimes anisotropic, and it 

requires that apparent resistivity data be inverted to determine a best-fit earth resistivity model of the 

subsurface. 

The DC resistivity profile data were inverted using the robust inversion method in EarthImager 

2D version 2.3.3 build 586. The robust method is based on the assumption of an exponential distribution 

of data errors and minimizes an L1-norm parameter that is a combination of the model data misfit and 

stabilizing function. The method typically performs well on datasets containing low-quality data and 

resolves resistivity boundaries well (Advanced Geosciences, Inc., 2008a). Topographic information was 

incorporated into the inversion in order to account for the influence of the irregular earth surface on the 

distribution of subsurface electrical currents, which is an important step towards determining a more 

accurate subsurface resistivity model. The inversions were allowed to run a maximum of 10 iterations 

with a stop criterion of 3 percent or less root-mean-square error between the measured and calculated 

forward model values or an L2-norm value of 1.0 or less. If neither of the stopping criteria was met, the 

lowest-quality data were removed using a percent-data misfit threshold. This threshold was chosen on 

the basis of a good balance between the noise levels of the data and on the overall percentage of data 

that would be removed at that data misfit level. The inversion was then run another time with the edited 

data. The raw data plots (pseudosections) for all eight profiles are placed in appendix A, and the raw and 

inverted ASCII data are placed within this report and described in appendix B. 

The three-dimensional DC resistivity dataset acquired in the vicinity of the main shaft and 

monofill #1 was inverted using the finite difference smooth model in EarthImager 3D version 1.5.4.361 

(Advanced Geosciences, Inc., 2008b). Topographic information was not incorporated during the 

inversion process, but the survey area was relatively flat with a consistent, gentle slope that should not 

affect the inversion results. The inversion procedure for the three-dimensional dataset was similar to that 

described for the two-dimensional datasets. The raw and inverted ASCII datasets are included with this 

report, and the data formats are included in appendix B. 

Electromagnetic Induction Method 

The EMI method uses time-varying, induced electrical and magnetic fields to indirectly measure 

subsurface conductivity, which is the reciprocal of resistivity. EMI typically requires two coils of wire, 

one transmitter coil and one receiver coil, separated by some distance. Alternating current is supplied to 

the transmitter coil at some frequency, which creates an alternating magnetic field, of which the lines 

are perpendicular to the plane of the coil. This primary magnetic field induces electric, or eddy, currents 

10
in subsurface conductors that in turn induce a secondary, alternating magnetic field that is superimposed 

on the primary magnetic field measured by the receiver coil of the EMI instrument. The measured 

output values are typically the in-phase (real) and quadrature (90 degrees out-of-phase; imaginary) 

components of the secondary field normalized to the primary field, with units of parts per million (ppm). 

The amplitude of the secondary magnetic field for a given coil configuration is dependent on the 

electromagnetic properties and geometry of the subsurface conductor. The quadrature component is 

linearly proportional to ground conductivity values if the instrument is considered to be within the low 

induction number environment, which is dependent on ground conductivity, measured frequency, and 

transmitter-receiver offset (Spies and Frischknecht, 1991). The in-phase component is dependent on the 

magnetic susceptibility of the subsurface and therefore particularly sensitive to buried metallic objects. 

The depth of investigation can be increased by increasing the separation distance between the 

coils or by decreasing the frequency of the primary current. Additional information on the EMI method 

is presented in Frischknecht and others (1991), West and Macnae (1991), and Lowrie (1997). 

Data Acquisition 

The EMI survey was acquired with a Geophex GEM-2 multi-frequency, dual-coil induction 

instrument. This is a single component, 1.66 m fixed-coil spacing instrument that uses multiple 

frequencies to vary the depth of investigation. Data were acquired in the co-planar, horizontal coil mode 

(fig. 4). The GEM-2 instrument can be used to measure relative variations in ground conductivity versus 

lateral position and depth for the top 10 to 15 m of subsurface material. 

Figure 4. Photograph of the shoulder-slung electromagnetic induction system in horizontal co-planar mode 

integrated with a backpack-mounted global positioning system. The operator is standing at the calibration station 

facing north. 

11
Seven frequencies of EMI data were acquired: 3,930, 6,630, 11,310, 19,110, 32,430, 54,930, and 

93,030 Hz. Data were acquired with the EMI instrument carried 1 m above the ground surface and 

integrated with the GPS. The EMI and GPS data acquisition rates were 10 Hz and 1 Hz, respectively. 

Approximately 5.7 km of data were acquired in the accessible areas in the main working areas of 

the site. The data were acquired in a snaking grid style with lines spaced roughly 5 m apart. Data were 

acquired throughout three data acquisition events over the course of two days. 

In order to properly process and potentially invert the data, an instrument calibration procedure 

was followed according to Abraham and others (2006). A calibration station location was selected (figs. 

2 and 3) on the basis of its proximity to the survey area, relative distance from known infrastructure (for 

example, monofills or other observed surface debris) to minimize interference, and relatively level 

ground surface within several meters of the station in all directions. The calibration station was occupied 

repeatedly during each acquisition event, including pre and post-survey, with the operator remaining 

stationary at the station for 2–3 minutes. 

Data Processing and Inversion 

EMI data editing included (1) removal of instrument drift during the surveys using the 

calibration station data and (2) the application of a 20-point low-pass filter to remove high-frequency 

noise on the quadrature and in-phase data (Abraham and others, 2006). Apparent conductivity and 

magnetic susceptibility values were computed from the drift-leveled and filtered quadrature and inphase data for each frequency using a conversion utility named Invertor by Geophex. Magnetic 

susceptibility is a measure of the degree to which the subsurface can be magnetized with the application 

of a magnetic field and is useful for mapping subsurface metallic objects or magnetic minerals. Lowerfrequency electromagnetic energy penetrates deeper into the subsurface (maximum 10–15 m in resistive 

ground), and higher-frequency electromagnetic energy provides higher resolution near the surface 

(upper 2–3 m). Depth of investigation decreases with increasing ground conductivity. The apparent 

conductivity data can be inverted to produce conductivity, or resistivity, depth images similar to those 

produced by the DC resistivity methods, but the EMI data from this instrument typically require 

calibration from additional data at the site, such as the DC resistivity data. This calibration procedure 

was not done with these datasets, however, owing to time constraints. The relative variations observed 

in the EMI are useful for mapping anomalies, but the absolute apparent conductivity and magnetic 

susceptibility values calculated without calibration are not realistic. The EMI data are therefore 

presented in this report as frequency-dependent, relative apparent conductivity and magnetic 

susceptibility data. The raw ASCII EMI dataset is included with this report, and the data format is 

included in appendix B. 

Results of Investigation 

DC Resistivity 

The DC resistivity profiles and three-dimensional dataset were interpreted with the aid of 

digitized soil borings from 2000 (Harding Lawson Associates/Wilder Construction, 2001). The majority 

of the 86 soil borings was relatively shallow (less than 2 m) and was concentrated on the future site of 

monofill #2. The borings associated with monitoring wells MW01, MW03, MW04, and MW06 and soil 

borings B02 and B05 were deeper and provided the most useful information with regard to interpreting 

the DC resistivity data (figs. 2 and 3). 

12
Although the soil-boring logs provided engineering soil texture and grain size classifications as 

well as additional descriptions, the information provided did not definitively differentiate between 

tailings or waste rock and alluvium, loess, or weathered bedrock that may have been present before the 

mining activity. There also does not appear to be a strong, consistent contrast in the electrical properties 

in the shallow subsurface that is indicative of tailings or waste rock. 

However, a mappable contrast between unsaturated and saturated sediments and rock is possible 

by using the estimated hydraulic head contours determined from the September 2010 measured water 

levels (Mark Longtine, written commun., Nov. 2, 2010) as a guideline. The unsaturated zone is 

generally more resistive than the underlying saturated zone, but there are several instances in this dataset 

where the trend is reversed, and possible interpretations are discussed where this is observed. 

A distinct bedrock contact is not consistently identifiable but has been interpreted on several of 

the profiles. Where indicated, the bedrock was interpreted on the basis of its increased resistivity 

(greater than 200 ohm-m) and is assumed to indicate a more competent surface. Although this is still 

relatively conductive for a competent rock, there appear to be relatively thick sections of either intensely 

fractured or weathered bedrock that possess lower resistivities and are electrically similar to the 

overlying overburden. 

There are also many anomalies, both resistive and conductive, that are present throughout the 

dataset that are not interpretable without additional information. Many of these discrete anomalies are 

present in the upper 5 m and may be due to unidentified buried items. The underground mine workings 

that are present in the western half of the survey area (fig. 2) are deeper than the maximum depth of 

investigation of both methods and are not expected to directly affect the data. The estimated depth of 

investigation, also referred to as the median depth of investigation, is based on the array geometry and 

maximum electrode spacing and is approximately equal to one-fifth of the maximum electrode spacing 

for an inverse Schlumberger array. The median depth of investigation refers to the depth at which the 

overlying section has the same influence on the measured potential as the underlying section (Loke, 

2010). Although the sensitivity of the inverted model to variations in layer thicknesses and resistivity 

values decreases with depth, it does not become zero at the median depth of investigation. Resolution of 

features or interfaces below this depth, however, is reduced. 

For this survey, the maximum electrode spacing used for a single measurement with a full 112-

electrode layout with an electrode spacing of 1 m is 100 m, which is equivalent to about a 20 m median 

depth of investigation. When less than a full layout of electrodes is used, the investigation depth is 

shallower, and when the nominal electrode spacing of the profile is increased to 2 m, the investigation 

depth is likewise doubled. 

The DC resistivity inversion profiles are presented and discussed individually. All profiles (or 

resistivity sections) are presented at the same scale with no vertical exaggeration and with the same 

color scale. The lower horizontal axis is downline distance along the line azimuth and the upper 

horizontal axis is the projected northing or easting coordinates along the line. For each profile, two 

identical inverted models are shown: one that is not interpreted and one that shows the interpretations 

and annotations. Interpreted water levels are shown for all profiles as well as bedrock contacts for 

selected profiles. 

Line 1 

Line 1 is a southwest-northeast trending profile that begins at monofill #3, crosses the post-1955 

retort area and three settling ponds, and ends just south of Red Devil Creek (fig. 5). The resistivity 

section generally exhibits three layers: a resistive (greater than 200 ohm-m) 3–5 m thick surface layer 

13
overlying a less resistive layer (40–100 ohm-m) that varies in thickness from 1–8 m, all overlying what 

is interpreted to be more competent bedrock that exhibits resistivity values greater than 200 ohm-m (fig. 

6). The variations observed in the bedrock may be due to varying fracture intensities. The surface layer 

is interpreted to be unsaturated unconsolidated overburden and the middle layer is interpreted to be 

saturated overburden or weathered bedrock. The water level appears to follow a smoothed topographic 

contour. The fact that the interpreted water level on line 1 is about 2.5 m higher than the well MW01 

water level is consistent with an average hydraulic gradient of about 0.1 and a separation distance of 

about 25 m. 

Figure 5. Photograph of the DC resistivity equipment deployed on line 1, looking northeast toward settling 

pond #1. 

Monofill #3 is located from 0–20 m downline distance and appears to be about 6 m deep with a 

resistive layer overlying a more conductive zone. The exact depth and electrical signature may not be 

properly resolved because of the metallic material present in the monofill. 

The three settling ponds exhibit discrete conductive anomalies (less than 80 ohm-m) across each 

pond in the unsaturated zone that is truncated at each berm on the downgradient side. These anomalies 

are interesting, however, because they become more resistive below the water table. If the anomalies 

were due to the presence of concentrated deposits of silt and sand from the flotation process, then these 

14
 MW03

 MW01

Monofill #3 Line 2b Line 2a Line 3 Line 5

Anomaly A Settling ponds

Sandy gravel

Silty sandy gravel

Gravel

Sandy silt

Silt

Wood

EXPLANATION

Water level

Bedrock

MW water level

A

B

Figure 6. Line 1 inverted resistivity profiles that are (A) uninterpreted and (B) have interpretive annotations and soil boring logs displayed for wells MW01 and MW03. The locations of the cross-line intersections are shown.

15
sediments would be expected to become more conductive when saturated if the base of these ponds 

extended below the water table. One possibility is that the conductive anomalies indicate the presence of 

a contaminant that undergoes a change in electrical properties under different saturation conditions. 

These anomalies may also indicate that a contaminant is mobile under saturated conditions, and that it 

has been carried away or substantially diluted by groundwater flow. 

Anomaly A is a moderately conductive anomaly 92–112 m downline distance that becomes 

more prominent below the water table. This is the area over which the petroleum-contaminated soil was 

stored on an impermeable liner for several years before removal just prior to the geophysical surveys. 

This anomaly is interesting because it is also observed in the apparent conductivity EMI data, which 

will be presented in the electromagnetic induction results section. 

Lines 2a and 2b 

Lines 2a and 2b are obliquely intersecting profiles that generally trend southeast-northwest and 

together span the north-south extent of the survey site from monofill #2, across Red Devil Creek, and 

over monofill #1 (figs. 2 and 3). Two lines were necessary to span this area because of the location of a 

narrow clearing across the creek through which equipment could be deployed. A bridge used to stand at 

this clearing, and there are now large piles of loose waste rock on the south side of the creek (fig. 7). 

Figure 7. Photograph of the DC resistivity equipment deployed on line 2a taken from monofill #2, looking 

northwest. 

16
On the south side of Red Devil Creek, the interpreted water level on both lines 2a and 2b 

generally follows topography and is 2–4 m deep (figs. 8 and 9). The presence of a surface conductor in 

the top 2 m on the slope leading up to monofill #2 from 22–28 m downline distance on line 2a and from 

20–40 m downline distance on line 2b may indicate that the water level becomes much shallower, there 

is a change in rock type or texture (becoming finer grained material), or there is change in soil or water 

quality causing the resistivity to decrease (anomaly B). The presence of the liner surrounding monofill 

#2 causes the monofill to be imaged as a resistor, and the conductive anomaly adjacent to the monofill 

(figs. 8 and 9), may be a result of imaging artifacts due to the liner. 

The interpretation of water levels in the DC resistivity data on the north side of Red Devil Creek 

is difficult. The water level measured in well MW04 is below the level of the creek and indicates that 

this stretch of the creek is a losing stream. The well MW04 water level, however, is difficult to correlate 

with the resistivity data. The blue water level interpretation on Figures 8 and 9 attempts to uphold the 

measured water level in MW04, but an alternate, shallower water level is also presented that more 

closely follows topography and is more consistent with the resistivity transitions observed on the south 

side of the creek. Unfortunately, there are no other water level measurements on this side of the creek to 

aid in the interpretation, and the three-dimensional effects observed on the profile, including monofill 

#1, obscure the water level on the northern end of Line 2b (fig. 9). 

There are several discrete conductive anomalies below the water table on the south side of the 

creek (anomaly C) that are generally consistent between both resistivity lines. The largest anomaly from 

88–100 m downline distance on line 2a is not observed on line 2b. The causes of these anomalies are not 

known but are interpreted to be either within the alluvium or weathered bedrock. 

Anomaly D is a very strong discrete conductor (less than 30 ohm-m) that is centered below the 

creek and present in both lines 2a and 2b. Its cause is unknown, but it is not believed to be an artifact of 

the presence of the creek or of the steep slopes on either side of the creek. 

Line 3 

The southeastern end of line 3 starts at the base of the outcrop exposed by the road cut, continues 

down a steep, loose dirt slope, crosses settling pond #1 and Red Devil Creek, and passes about 7 m 

northeast of the main shaft on the north side of the creek (fig. 10). This line is closest to the spring that 

is feeding into Red Devil Creek and is depositing iron precipitate, also referred to as yellow boy, on the 

creek bed below (fig. 3). The spring is surfacing about 1 m above the creek water level, and its source is 

unknown. 

On the basis of the resistivity section and the measured water level in well MW03, the 

interpreted water level diverges from the ground surface below the settling pond to the level of Red 

Devil Creek and then rises again on the north side of the creek (fig. 11). The distinctive bisected 

layering centered on the settling pond berm (48 m downline distance) is most likely a result of the 

construction process, and artifacts may be produced in the resistivity images if debris, such as the noted 

wood layer in the lithology log, is buried. As discussed earlier, discrete, three-dimensional objects in the 

subsurface are not well resolved using two-dimensional resistivity profiles. Well MW03 is located on 

the berm northeast of line 3 and its measured water level does correlate with the elevated conductive 

zone centered below the berm on the line 3 resistivity section. The interpreted water level as shown on 

figure 11, however, transitions more smoothly across this peak and follows the top of a conductive layer 

(40–100 ohm-m) that is interpreted to be the saturated alluvium or weathered bedrock. 

As presented for lines 2a and 2b, two water levels are shown on the north side of the creek. The 

blue water level is consistent with the well MW04 measurements, and the alternate water level follows a 

contact between a resistive layer overlying a highly conductive layer that is about 3 m above the well 

MW04 water level. The lithology logs for both well MW04 and soil boring B05 indicate a silty sandy 

17
 Line 1 Line 2b Line 4

 MW04

 B02

Sandy gravelly silt

Silty sandy gravel

EXPLANATION

Water level

Bedrock

Alternate water level

Silty gravelly sand

Gravelly silt

Shale

MW water level

Monoll #2

Anomaly C

Anomaly D

Anomaly B

A

B

Figure 8. Line 2a inverted resistivity profiles that are (A) uninterpreted and (B) have interpretive annotations and soil boring logs displayed for wells B02 and MW04.

The locations of the cross-line intersections are shown. The blue water level interpretation abides by the MW04 measured water level, and the pink dashed alternate water

level is based on similar resistivity transitions observed on the south side of the creek. 

18
 Line 1 Line 2a Line 4

 MW04

Sandy gravelly silt

Alternate water level

Sandy gravel

Silty sandy gravel

Gravel

Sandy silt

Silt

EXPLANATION

Water level

Bedrock

MW water level

 MW01 Anomaly C Anomaly D

Anomaly B

Monoll #2

Monoll #1

A

B

Figure 9. Line 2b inverted resistivity profiles that are (A) uninterpreted and (B) have interpretive annotations and soil boring logs displayed for wells MW01 and MW04. The locations of the cross-line intersections

are shown. The blue water level interpretation abides by the MW04 measured water level, and the pink dashed alternate water level is based on similar resistivity transitions observed on the south side of the creek. 

19
Figure 10. Photograph of the DC resistivity equipment deployed on line 3 taken from monofill #2, looking 

northwest. 

gravel layer throughout the sections with sandstone encountered at the base of B05. No water level was 

indicated for B05 (Harding Lawson Associates/Wilder Construction, 2001). The relatively flat 

alternating layers observed on the north side of the creek on line 3 may be indicating changes in water 

content due to subtle variations in grain sizes that were not observed during drilling. 

The highly conductive layer (less than 30 ohm-m) from 84–110 m downline distance that is 

interpreted as the saturated layer below the alternate water level (fig. 11) is interesting because of its 

proximity to the spring. The character and shape of this feature, however, do not resemble a point source 

feeding the spring but rather a feature that would promote more widespread seepage across the slope 

leading down to the creek. Although the survey area was wet during most of the survey from the 

frequent rain, seepage was not observed on this slope and it has not been previously reported. There is a 

discrete, nearly vertical, conductive anomaly that extends to within 2 m of the surface (anomaly E) that 

is closer to the creek level and has a character more indicative of a point source. Anomaly D that was 

observed on lines 2a and 2b is also observed on line 3, and it is therefore a feature below the creek that 

extends for a minimum length of 60 m. 

A competent bedrock contact is not interpreted along this profile, because the resistivity values 

at depth are generally lower as compared to lines 1, 2a, and 2b, and there is no additional evidence as to 

the depth of bedrock in this area. The interpreted bedrock contact at the line 1 intersection is at 63 m 

elevation, which is just below the depth of investigation of line 3 at this location and is therefore 

20
 MW03

 B05

 Line 1 Line 4

Silty sandy gravel

Sandy silt

Wood

EXPLANATION

Water level

Sandstone

Alternate water level

MW water level

Anomaly D

Anomaly E

Settling pond #1

A

B

Figure 11. Line 3 inverted resistivity profiles that are (A) uninterpreted and (B) have interpretive annotations and soil boring logs displayed for well

MW03 and soil boring B05. The locations of the cross-line intersections are shown. The blue water level interpretation abides by the MW04 measured water

level, and the pink dashed alternate water level is based on similar resistivity transitions observed on the south side of the creek.

21
consistent with the line 1 interpretation. The high resistivity feature observed from 0–20 m downline 

distance is interpreted to be a result of the road construction and loose material present on the slope 

down to the settling pond. 

Line 3 passes over the western side of settling pond #1, where the surface layer is more resistive 

than the highly conductive surface layer observed on the eastern half of the pond crossed by line 1. For 

all three ponds, the conductive surface anomaly observed on line 1 (fig. 6) is strongest on the 

downgradient side of the pond against the berm. 

Line 4 

Line 4 was acquired across the pre-1955 retort area as close as possible to the spring feeding into 

Red Devil Creek (fig. 12). The objective was to image a possible spring source, but no source was 

observed on this profile (fig. 13). Access for placement of a resistivity line was difficult owing to the 

steep slope and dense trees immediately above the spring. The line was located on the cleared pre-1955 

retort area as close to the top of the slope that parallels the creek and as far northeast as possible, but 

access to the northeast was limited by trees. 

Figure 12. Photograph of the DC resistivity equipment deployed on line 4 across the pre-1955 retort area, 

looking southwest.

22
 Line 2a Line 3

 MW04

Sandy gravelly silt

Alternate water level

Silty sandy gravel

EXPLANATION

Water level

MW water level

 Line 2b

spring

Anomaly F Anomaly G

Anomaly H

A

B

Figure 13. Line 4 inverted resistivity profiles that are (A) uninterpreted and (B) have interpretive annotations and

soil boring log displayed for well MW04. The locations of the cross-line intersections are shown. The blue water level

interpretation abides by the well MW04 measured water level, and the pink dashed alternate water level is based on

similar resistivity transitions observed on the south side of the creek. 

23
The water level that corresponds to well MW04 is presented but does not correlate with an 

increasingly conductive transition in the resistivity section as would be expected below the water table. 

An alternate water level is presented that correlates better with the resistivity section and is consistent 

with the alternate water levels interpreted on lines 2a, 2b, and 3. 

There are several features observed in the top 5 m of this profile, but the ground surface across 

the profile did not indicate any obvious changes. There is a 2-m thick high resistivity feature (greater 

than 400 ohm-m) on the surface (anomaly F) that may be a result of being reworked or regraded at some 

time. The two conductive anomalies (less than 30 ohm-m; anomaly G) with a resistor centered between 

would be interpreted as possible shallow buried metallic objects that are not well resolved, but the threedimensional dataset presented next better resolves this feature. Anomaly H exhibits the character of an 

8-m wide trench or pit about 4 m deep, but it may only be lateral discontinuity due to grain size 

variations. 

Three-Dimensional Grid 

The three-dimensional resistivity dataset was acquired (fig. 14) after observing the complicated 

resistivity structure exhibited by the two-dimensional resistivity lines 2a, 2b, 3, and 4 on the north side 

of Red Devil Creek. The objectives were to better resolve the resistivity structure of what appeared to be 

three-dimensional features in the two-dimensional profile data as well as an additional attempt to image 

the source of the spring. 

Figure 14. Photographs of the (A) DC resistivity equipment deployed on the three-dimensional grid across the 

pre-1955 retort area, from the southeast corner of the grid looking northeast and (B) the three electrodes along the 

southwest edge of the grid placed on the concrete using bentonite mud, looking northwest. 

The maximum modeled depth of investigation is 13.5 m. The depth of investigation is shallower 

than for the profiles because of the reduced maximum possible separation between the current and 

potential electrode pairs. The origin is on the southeast corner with negative x along the short axis in the 

northwest direction and positive y along the x-axis in the northeast direction. 

24
The surface resistivity of the block exhibits similar trends to the 93,030 Hz apparent 

conductivity data with lower resistivity areas in the southwest corner and north of the main shaft (fig. 

15a). There are several discrete resistive features throughout the volume, and only one of these features 

extends to the bottom of the model. This anomaly is below the concrete pad (centered at about 20 m on 

the x axis) along the southwest side and may be an artifact of the presence of the pad. The bottom of the 

anomaly becomes shallower and ends at about 10 m along the y axis, indicating that it may be due to the 

three electrodes placed on the concrete pad not properly resolving the subsurface below the concrete. 

The resistive anomaly centered at 18 m on the y axis is below anomaly G on line 4 (fig. 13). There is an 

indication of two conductive features above the resistor that may correspond with the anomaly G 

conductive bodies, and the size of about 8 m depth by 8 m wide roughly corresponds with the total size 

of the two anomaly G conductors. Although the character of the anomaly in the three-dimensional 

volume is much different than the character of anomaly G in the line 4 profile, it is expected that the 

three-dimensional dataset more correctly imaged the feature, and it is therefore interpreted as a potential 

trench or landfill that extends north to about 10 m on the x axis. This interpretation is also consistent 

with a discrete low apparent magnetic susceptibility anomaly in this location presented in the 

electromagnetic induction results section. There are additional resistive anomalies in the vicinity of the 

concrete pad that are also consistent with lower measured apparent magnetic susceptibility data (fig. 

15c). 

It also appears that the main shaft in the upper 5–6 m is imaged as a resistive anomaly that is 

about 6 m square (fig. 15c). At about 7 m depth, the resistive feature transitions to a consistent 

conductive layer that is observed throughout most of the northern half of the volume. There are a couple 

of shallow resistive features from 30–60 m along the y axis that also terminate at this conductive layer 

and may be due to increased water content. This would be consistent with the water level measured in 

well MW04. However, the conductive layer at depth is not as consistent from 0–30 m in the y direction 

but that may be due to the high density of resistive anomalies observed in this region. 

Line 5 

The south end of line 5 begins at the base of the steep slope north of the access road on the south 

side of Red Devil Creek. The line continues immediately east of settling pond #3, crosses line 1 and Red 

Devil Creek, and continues across the access road north of the creek and ends about 20 m short of the 

clearing of where the petroleum-contaminated soil was seeded earlier in the summer of 2010. The 

vegetation was very dense in this area on both sides of the creek (fig. 16), so the orientation of the line 

was determined by a relatively clear path through the woods that required the minimum amount of tree 

clearing and that still intersected line 1. 

The average resistivity for the profile is moderate (100–200 ohm-m) to depth (fig. 17). Although 

there are high resistivity layers (greater than 400 ohm-m) observed below the interpreted water level, 

these areas are not consistent enough to make a confident competent bedrock interpretation across the 

entire profile. Where line 1 intersects line 5, the bedrock appears to drop sharply, and the resistivity 

structure in line 5 at 38 m downline distance may possess the same trend. This is also consistent with 

the absence of highly resistive bedrock on line 3 on the south side of the creek. Bedrock is interpreted to 

exist, however, from about 80–111 m downline distance on line 5, which corresponds with the north 

side of the access road to the end of the profile. This section exhibits a very high resistivity (greater than 

1,000 ohm-m) with the top 2–3 m consistently higher than the underlying layer. The interpreted 

25
A

B

C

Anomaly G

Main shaft

Main shaft

Anomaly G

Main shaft

Anomaly below

concrete pad

Anomaly below

concrete pad

N

N

Figure 15. Oblique views of the three-dimensional resistivity inverted volume: (A) looking north, (B) looking south, and

(C) a threshold limit of 450 ohm-m and greater displayed, looking south. 

26
Figure 16. Photographs of the DC resistivity equipment deployed on line 5 (A) across Red Devil Creek facing the 

low end of the line toward the southeast and (B) from the same position looking northwest up the steep slope to the 

access road. 

resistivity section in figure 17b has a stretched color scale to better display this contrast. The 

interpretation is a water level that generally follows topography through the bedrock, but another 

possibility is a relatively consistent soil horizon overlying bedrock with a water level remaining deeper, 

near 62 m elevation, across the last quarter of the resistivity profile. The well MW06 lithology log 

indicates gravel throughout, which would generally show as a resistor, but no additional data are 

available to aid in the interpretation. 

The surface topography along the high resistivity section from 80 m to the end of the profile 

appears terraced but is interpreted as artificial terracing from mining activity. The terraced topography is 

observed on line 6 as well, but the absence of similar terracing along the banks of McCally Creek, 

located about 1.1 km downstream (north) on the Kuskokwim River from Red Devil Creek, is not 

observed. 

There is a discrete, vertical conductive feature at 90 m downline distance that is coincident with 

the interpreted water level (anomaly I), but its cause is not known. There is also a conductive feature 

immediately north of the creek that is similar to anomaly D noted in lines 2a, 2b, and 3 but much 

shallower and has slightly higher resistivity (30–50 ohm-m). 

27
Sandy gravel

Silty sandy gravel

EXPLANATION

Water level

MW water level

 Line 1

 MW06

Alternate water level

A

B

Anomaly D?

Anomaly I

Figure 17. Line 5 inverted resistivity profiles that are (A) uninterpreted and (B) have interpretive annotations and soil boring log displayed for well

MW06 with a stretched color scale to better display the variations in the high resistivity section on the north side of Red Devil Creek. The location of the

line 1 intersection is shown. 

28
Line 6 

Line 6 was located further downstream than originally proposed, because the original location 

was densely forested with no obvious path through the trees to deploy the equipment. The construction 

of the bridge across Red Devil Creek earlier in the summer provided a clear route across the creek in an 

area that was still upstream of the creek delta. Line 6, from southeast to northwest, started in the 

moderately forested section, continued over a pile of rock, fine sediments, and small pieces of wood 

debris, continued across the support slope of the bridge that was mostly cobble-sized rip-rap or ballast 

material with minor amounts of finer grained sediments, crossed the creek and access road, and 

proceeded up a near-vertical slope to the more gently sloped and thinly forested area for the remainder 

of the profile (fig. 18). 

Figure 18. Photograph of the DC resistivity equipment deployed on line 6, looking northwest. 

The line 6 resistivity section has an overall higher resistivity than any other profile acquired at 

the site. Figure 19b presents the interpreted section of line 6 with the same stretched color scale used for 

line 5 to better show detail, but there is no resistivity cross-line to compare this profile with and no 

monitoring wells or soil borings. As observed on line 5, the area north of the access road is highly 

resistive (greater than 1,000 ohm-m) and is interpreted as bedrock. The very conductive anomaly (less 

than 20 ohm-m) centered below the creek (anomaly J) is much wider than the anomaly D feature 

discussed on the previous lines that crossed the creek, and it may be an unrelated feature. The shape of 

the conductor is most likely not well resolved, because its upper surface tends to mimic the topographic 

features. 

29
EXPLANATION Water level

Anomaly J

A

B

Figure 19. Line 6 inverted resistivity profiles that are (A) uninterpreted and (B) have interpretive annotations with

a stretched color scale to better display the variations throughout the profile. There are no nearby soil borings and no

line intersections. 

30
Although this line contained noisier data than the other profiles, it is thought to be due to the 

nearly vertical slope on the north side of the access road at 44 m downline distance. It was necessary to 

smooth the topography across the slope in order to complete the inversion. The resolution of the features 

at depth below this slope is questionable, because data were removed to achieve a better inverted model 

fit (see the pseudosection in appendix A). 

A water level was interpreted across the profile with depths to the saturated zone between 0 and 

3 m. The water level is interpreted in a consistent manner with line 5 and follows topography from 0–44 

m downline distance through what is interpreted to be bedrock. The water level is interpreted to 

intersect with the creek bed for consistency with the other DC resistivity profiles that cross the creek, 

but the presence of highly resistive areas (greater than 450 ohm-m) between the ground surface and 

anomaly J make this interpretation less certain. If the entire section is composed of bedrock with 

varying degrees of saturation and weathering, then the water level may intersect the creek bed because 

competent, saturated bedrock can have such high resistivity values. 

An interpreted bedrock surface is not presented for this line, however, because the low resistivity 

values of anomaly J may be masking the real resistivity structure at depth below the anomaly. Current 

will preferentially flow through conductive zones and may not properly resolve the subsurface below 

strong conductors. The resistivity structure from 80–111 m downline distance is also highly resistive but 

with a moderately resistive (80–150 ohm-m) layer that is interpreted to be saturated alluvium or 

weathered bedrock. Without the aid of lithologic logs, it is impossible to make confident interpretations 

of the features observed in this profile. 

Line 7 

Line 7 was acquired across the upper part of the Red Devil Creek delta below the narrow creek 

valley. The line was located along the access road that parallels the Kuskokwim River and extended 

across the creek to a densely wooded area (fig. 20). 

The resistivity section generally contains a two-layer resistivity structure with a more resistive 

(300–450 ohm-m) layer ranging in thickness from 2 to 5 m overlying a moderately resistive layer (70–

150 ohm-m) that extends to depth (fig. 21). This section is interpreted to be unsaturated and saturated 

alluvial deposits. Variations observed in the saturated section may be due to differences in grain size 

distributions within the alluvium. 

There is an indication at the base of the resistivity section (about 35 m elevation) of a more 

resistive layer at depth. The lower layer is interpreted as weathered bedrock but could also be more 

consolidated alluvium. Because resolution of resistivity profiles decreases with increasing depth of 

investigation, a specific interpreted contact for this lower layer is not possible. The creek bed elevation 

corresponds precisely with the measured river level within two days of acquiring line 7 (51.5 m 

elevation). There are no nearby soil borings or monitoring wells with which to correlate this resistivity 

profile. 

31
Figure 20. Photographs of the DC resistivity equipment deployed on line 7, (A) looking northwest along the 

access road and (B) from the southeast end of the line looking northwest toward Red Devil Creek in the trees.

Electromagnetic Induction 

The apparent conductivity maps of the site are presented for two frequencies: the highest 

frequency, 93,030 Hz (fig. 22), and a middle frequency, 19,110 Hz (fig. 23). A 93,030 Hz apparent 

magnetic susceptibility map is also presented. The EMI data have been drift-leveled and filtered, but 

without calibration, the absolute calculated apparent conductivity and magnetic susceptibility values are 

not correct. The relative variations in these values, however, are useful in mapping anomalies and 

correlating with the DC resistivity data. Figures 22 and 23 are plotted on the same color scale for direct 

comparison between the two datasets. The most prominent conductive anomalies observed on the 

93,030 Hz dataset, however, are very similar between the two datasets. 

The highest-frequency apparent conductivity data contain the highest conductivity values over 

monofill #2 and in the vicinity of monofill #1 and the two concrete pads (fig. 22). Much of the pre-1955 

retort area has higher conductivity values than the majority of the site and is not confined to monofill #1 

and the concrete pads, which is consistent with the consistent conductive surface layer observed along 

DC resistivity lines 2a and 3 in this area (figs. 8 and 11). The very low conductivity area (dark blue) 

between the northeast side of the L-shaped concrete pad and soil boring B05 are measured values, but 

the extension of the low conductivity data along the northwest side of the concrete pad along the edge of 

the grid is a gridding extrapolation artifact. Because both B05 and well MW04 are described as mostly 

sandy silty gravel and because the same low values are not observed in the apparent conductivity map 

32
Weathered bedrock

EXPLANATION

Water level

A

B

Figure 21. Line 7 inverted resistivity profiles that are (A) uninterpreted and (B) have interpretive annotations. The

interpreted weathered bedrock is an approximate contact. There are no nearby soil borings and no line intersections.

33
1.1

1.3

1.5

1.7

2.0

2.3

2.7

3.1

3.5

4.0

4.6

5.3

6.1

7.0

8.1

9.3

10.7

12.3

14.1

16.3

18.7

21.5

24.7

28.4

32.6

mS/m

6,848,450 6,848,500 6,848,550 6,848,600 6,848,650 6,848,700 6,848,750 6,848,800

61°45'30"

588,900 588,950 589,000 589,050 589,100 589,150

61°45'35" 61°45'40"

-157°19'00" -157°18'50" -157°18'40"

B02

B05

MW01

MW03

MW04

MW06

Line 7

Line 6

Line 5

Line 3

Line 4

Line 1 Line 2b

Line 2a

Red Devil Creek

Kuskokwim

River

25 0 25

(meters)

NAD83 / UTM zone 4N

Easting, in meters

Northing, in meters

EXPLANATION

DC resistivity line Spring

GPS base station

EMI calibration station

Monitoring well

Main shaft

Concrete pad

DC resistivity 3-D grid Soil boring

Settling Ponds

#1

#2

#3

Monofill

#3

Monofill #1

Monofill #2

Figure 22. Map showing apparent conductivity data for 93,030 Hz. The absolute apparent conductivity values

are not correct because they have not been calibrated with the DC resistivity data, but the relative variations are

useful for identifying anomalies. The monofill and settling pond outlines are approximate and based on historical

photographs. 

34
1.1

1.3

1.5

1.7

2.0

2.3

2.7

3.1

3.5

4.0

4.6

5.3

6.1

7.0

8.1

9.3

10.7

12.3

14.1

16.3

18.7

21.5

24.7

28.4

32.6

mS/m

6,848,450 6,848,500 6,848,550 6,848,600 6,848,650 6,848,700 6,848,750 6,848,800

61°45'30"

588,900 588,950 589,000 589,050 589,100 589,150

61°45'35" 61°45'40"

-157°19'00" -157°18'50" -157°18'40"

B02

B05

MW01

MW03

MW04

MW06

Line 7

Line 6

Line 5

Line 3

Line 4

Line 1 Line 2b

Line 2a

Red Devil Creek

Kuskokwim

River

25 0 25

(meters)

NAD83 / UTM zone 4N

Easting, in meters

Northing, in meters

EXPLANATION

DC resistivity line Spring

GPS base station

EMI calibration station

Monitoring well

Main shaft

Concrete pad

DC resistivity 3-D grid Soil boring

Settling Ponds

#1

#2

#3

Monofill

#3

Monofill #1

Monofill #2

Figure 23. Map showing apparent conductivity data for 19,110 Hz. The absolute apparent conductivity values

are not correct because they have not been calibrated with the DC resistivity data, but the relative variations are

useful for identifying anomalies. The monofill and settling pond outlines are approximate and based on historical

photographs. 

35
near MW04, the observed low conductivity anomaly near B05 is not related to grain size and is most 

likely not due to changes in water content because of its discrete nature. 

On the south side of Red Devil Creek, the boundary of settling pond #1 is well defined and has 

moderately more conductive values compared to the surrounding area. Similar conductivity values exist 

in the recently regraded area to the north and east of the EMI calibration station where the petroleumcontaminated soil was stored for several years before disposal (anomaly A), along the road leading 

down to the creek, and along the creek past the spring. On the basis of line 1, anomaly A increases in 

conductivity below the interpreted water level, which may indicate that finer-grained sediments are 

present in this area. 

The low conductivity (blue) areas along the road leading to the above-ground storage tank area 

and along the southern and western edges of monofill #3 are interpreted as shallow bedrock. Although 

there is only a weak response from monofill #3 in the 93,030 Hz dataset, it becomes better defined in 

the lower frequency 19,110 Hz dataset (fig. 23). In general, the 19,110 Hz dataset is more conductive 

than the highest 93,030 Hz dataset and is interpreted to be a result of increased water content at depth. 

The 93,030 Hz apparent magnetic susceptibility data highlight all three monofills and the two 

concrete pads (fig. 24). The concrete pads most likely possess elevated magnetic susceptibilities owing 

to rebar. The monofills were expected to have elevated values as well, and in particular monofill #3, 

because it contains the steel remains of the hopper and above-ground storage tanks. Monofills #1 and #2 

were described as containing building debris, but a photo taken during construction of monofill #1 

(Bureau of Land Management, 2010) shows mostly wood debris with several tires as well as several 

metal drums. The drums do not appear to produce an elevated signature as expected. One possible 

explanation for the observed low values is that the tailings, waste rock, alluvium, or bedrock that existed 

in the areas of these monofills prior to construction was removed and replaced with lower-magneticsusceptibility material. 

There is no indication in either the apparent conductivity or magnetic susceptibility datasets of a 

potential source for the spring that feeds into Red Devil Creek. No linear features or discrete conductive 

zones in the vicinity of the spring are observed. Delineation between waste rock and tailings is not 

feasible without additional borehole data to possibly help identify distinguishing electromagnetic 

characteristics between the rock types. Assuming that these data would produce similar results to the 

DC resistivity models when inverted and without the aid of additional soil borings with more specific 

descriptions identifying waste rock, tailings, and alluvium or weathered bedrock, a volume of tailings 

and waste rock cannot be estimated at this time. 

Summary and Discussion 

The DC resistivity and EMI data acquired in August 2010 at Red Devil mine have provided 

subsurface information about the site, but available soil borings that sample deeper than 2 m are sparse. 

Figure 25 displays the annotated anomalies from the DC resistivity data on a map to better illustrate the 

spatial relationships between those anomalies and other site features. A georeferenced .DXF file 

containing these anomalies is included in appendix B. 

The EMI data were presented as frequency-dependent apparent conductivity and apparent 

magnetic susceptibility maps and have not been calibrated and inverted. Inverted EMI data provide 

resistivity depth images similar to the DC resistivity sections but at a generally lower resolution. The 

success of the inversion is dependent on the quality of the original EMI data acquired and the robustness 

of the DC resistivity inversions used to calibrate the EMI data. If successful, inverted EMI data can 

provide additional useful information to help distinguish between rock types in the shallow subsurface 

36
0.06

0.09

0.12

0.18

0.25

0.36

0.52

0.75

1.08

1.55

2.22

3.18

4.57

6.56

9.41

13.50

19.37

27.80

39.90

57.25

82.16

117.90

169.19

242.80

348.42

SI x 10-3

6,848,450 6,848,500 6,848,550 6,848,600 6,848,650 6,848,700 6,848,750 6,848,800

61°45'30"

588,900 588,950 589,000 589,050 589,100 589,150

61°45'35" 61°45'40"

-157°19'00" -157°18'50" -157°18'40"

B02

B05

MW01

MW03

MW04

MW06

Line 7

Line 6

Line 5

Line 3

Line 4

Line 1 Line 2b

Line 2a

Red Devil Creek

Kuskokwim

River

Settling Ponds

#1

#2

#3

Monofill

#3

Monofill #1

Monofill #2

25 0 25

(meters)

NAD83 / UTM zone 4N

Easting, in meters

Northing, in meters

EXPLANATION

DC resistivity line Spring

GPS base station

EMI calibration station

Monitoring well

Main shaft

Concrete pad

DC resistivity 3-D grid Soil boring

Figure 24. Map showing apparent magnetic susceptibility data for 93,030 Hz. The absolute apparent

magnetic susceptibility values are not correct because they have not been calibrated with the DC resistivity

data, but the relative variations are useful for identifying anomalies. The monofill and settling pond outlines

are approximate and based on historical photographs. 

37
6,848,450 6,848,500 6,848,550 6,848,600 6,848,650 6,848,700 6,848,750 6,848,800

61°45'30"

588,900 588,950 589,000 589,050 589,100 589,150

61°45'35" 61°45'40"

-157°19'00" -157°18'50" -157°18'40"

B02

B05

MW01

MW03

MW04

MW06

Line 7

Line 6

Line 5

Line 3

Line 4

Line 1 Line 2b

Line 2a

EXPLANATION

DC resistivity line Spring

GPS base station

EMI calibration station

Anomaly Monitoring well

Main shaft

Concrete pad

DC resistivity 3-D grid

Red Devil Creek

Kusko

Riv

kwim

er

25 0 25

(meters)

NAD83 / UTM zone 4N

Underground mine Stopes

workings

Monofill

#3

Monofill #1

Settling Ponds

#1

#2

#3

Monofill

#2

Monofill

#2

B

C

D

D

D?

I

J

E

F

G

H

Easting, in meters

Northing, in meters

Figure 25. Map showing the locations of the annotated anomalies from the DC resistivity profiles

with the outlines of the approximate locations of the settling ponds (orange polygons) and monofills

(purple polygons) included.

38
or to help constrain the tailings volume estimate. This dataset would provide better constraints on 

interpretations of contacts over the entire site and fill in the gaps between the DC resistivity profiles 

down to a maximum depth of 10–15 m. Additional soil borings from the site would also aid in the 

interpretation of this dataset. 

On the basis of the measured water levels in four monitoring wells scattered throughout the site 

(MW01, MW03, MW04, and MW06), a water level was interpreted on the resistivity profiles. On the 

south side of Red Devil Creek, the water levels in monitoring wells MW01 and MW03 and an observed 

decrease in resistivity at depths typically ranging from 2–5 m depth were relatively consistent and 

tended to follow topography. The correlation of the water level in MW04 with the observed resistivity 

transitions in DC resistivity lines 2a, 2b, and 3 and the three-dimensional grid on the north side of Red 

Devil Creek, however, was difficult. The profiles in particular were affected by what appear to be threedimensional discrete features that are imaged much better with the three-dimensional dataset, but the 

apparent frequent reworking of the shallow subsurface in this area has complicated the interpretations of 

the observed features and even the water level. 

The measured water level in well MW04 is below the elevation of Red Devil Creek and 

indicates that this stretch of Red Devil Creek is a losing stream. The measured water levels in the 

remaining wells are above the elevation of the creek bed, indicating that gaining conditions exist in 

other locations. If water levels in MW04 are representative of the aquifer, then the hydraulic interaction 

between the creek and the aquifer may be variable throughout the study area. Additional hydrologic 

data, such as those acquired through high-resolution stream gaging or tracer tests, may help quantify this 

interaction. Identifying the losing stream stretches may aid in determining which areas of the creek bed 

have experienced greater aggradation due to waste rock and tailings deposits. 

There was no obvious shallow source of the spring that feeds into Red Devil Creek along its 

north bank in the DC resistivity or EMI data that was acquired over the spring in the pre-1955 retort 

area. DC resistivity line 3 passes within 5 m to the north of the spring and exhibits an interesting 

shallow, near-vertical feature (anomaly E) that is roughly consistent with the elevation at which the 

spring surfaces. This particular feature is not observed on lines 2a, 2b, or 5. There is a similar but 

generally larger conductive anomaly imaged below the creek bed (anomaly D) that is also observed on 

lines 2a, 2b, and possibly 5. This is another interesting anomaly whose source is not known but is not 

interpreted to be related to the spring because of its location below the level of the creek bed. 

The three settling ponds that are located downgradient from the post-1955 retort and flotation 

facility produce a conductive anomaly in the unsaturated zone in DC resistivity line 1. Because silt and 

sand deposits are expected in these ponds, a decrease in resistivity is logical since a relatively large 

deposit of fine grained material would have an increased water content in the vadose zone. The 

transition to higher resistivity below the water level may indicate that the base of the ponds did not 

extend below this depth. If the base of the ponds is expected to have extended below the water level, 

however, then the increase in resistivity may indicate that a secondary mechanism besides grain size is 

contributing to the resistivity structure observed. 

A competent bedrock contact was interpreted on the south side of Red Devil Creek along DC 

resistivity lines 1, 2a, and 2b. This contact is characterized by resistivity generally greater than 200 

ohm-m and is consistent with the depth at which shale was encountered at the bottom of soil boring soil 

boring B02. This is the only soil boring that indicates shale and also describes highly weathered shale 

existing about 3–4 m above the bottom of the hole. On DC resistivity line 2a, there is not a strong 

resistivity contrast between what is described for B02 as sandy gravelly silt and weathered bedrock, so 

the variations observed in the resistivity sections above and near the competent bedrock interpretation 

are assumed to be those of grain size and degrees of weathering and fracturing in the bedrock. 

39
Competent bedrock is also interpreted to exist on the north side of Red Devil Creek from DC 

resistivity lines 5 and 6, which possess very high resistivity values (greater than 1,000 ohm-m) that 

approach the surface. Line 6 may indicate that competent bedrock dominates much of the area through 

which the line crosses, but is difficult to confidently interpret because the resistivity section is so 

dissimilar to the other sections and there are no borings that far downstream along Red Devil Creek. 

There is a large range in observed resistivity values in what is interpreted as bedrock between the 

upstream and downstream portions of the site. The higher resistivity values (greater than 1,000 ohm-m) 

observed on lines 5 and 6 may be due to unsaturated conditions if the water level does not follow 

topography closely on the north side of the creek, to increased competency, or to a change in lithology 

(for example, dominated by a more sand-rich greywacke instead of shale). 

An indication of weathered bedrock is interpreted at depth on DC resistivity line 7 but the 

resistivity section is dominated by moderately resistive values (70–150 ohm-m) that are interpreted to 

indicate alluvial material deposited from the creek and river. Distinctions between alluvial material and 

possible tailings or waste rock that have been transported downstream the creek cannot be made with 

the available data. 

There is not an obvious electrical signature contrast between the tailings, waste rock, and 

bedrock, and the soil borings do not provide enough detail as described to help differentiate these rock 

types in the DC resistivity data. The tailings and waste rock may be too mixed to be able to differentiate 

these rock types, but additional deep soil borings that have more detailed descriptions of the rock type 

encountered may help to identify characteristic electrical or electromagnetic signatures that would help 

differentiate these rock types from the bedrock in the geophysical data. A useful additional analysis 

would be to analyze surface and downhole soil and rock samples from throughout the site with 

petrophysical laboratory equipment to determine how water content affects the bulk resistivity values. 

Well-described lithologic logs accompanied by borehole geophysical logs (particularly resistivity or 

induction logs) would also be beneficial to constraining interpretations of the data presented in this 

report. By defining a range of expected resistivity values for each rock type, further information may be 

gained from the surface geophysical data. 

Acknowledgments 

This work was completed in cooperation with the Bureau of Land Management under 

intergovernmental order L10PG00707. We are grateful for the overall logistical support provided by 

Mike McCrum and Larry Beck from the Bureau of Land Management and by Bryan Lund from Marsh 

Creek, LLC. We thank Starling Rogers from Marsh Creek, LLC, for providing necessary field support 

during the data acquisition phase of the project. Mark Longtine of Ecology and Environment, Inc. 

provided water sample and GIS data to aid in the interpretation of the data. Jeff Lucius and Jared 

Abraham, U.S. Geological Survey, provided helpful comments and suggestions through their review of 

the report. 

40
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42
Appendix A. DC Resistivity Pseudosections 

DC resistivity measured data apparent resistivity pseudosection (top image), calculated forward 

model apparent resistivity pseudosection (middle image), and inverted resistivity section (bottom image) 

are shown for each two-dimensional line. For each figure, the vertical axis on the two pseudosections is 

estimated depth, in meters, of the apparent resistivity datum and on the inverted section is the NAVD88 

elevation, in meters. The horizontal axis for all three images is the downline distance, in meters, with 

electrode locations represented by the black squares near the top of each figure. The black dots in the 

two pseudosections are individual apparent resistivity datums, and missing datums indicate areas where 

noisy data were removed using the percent data misfit threshold removal tool to improve the inverted 

model fit. 

43
44
45
46
47
48
49
50
51
Appendix B. Digital Data 

Direct-Current Resistivity Data 

Digital direct-current resistivity data are provided in two compressed files: 

1. Raw data: RDM_DCres_RawData.zip and 

2. Inverted data: RDM_DCres_InvData.zip. 

The raw data compressed file contains 4 files, 1 for the profiles and 3 for the three-dimensional grid. 

The inverted data compressed file contains 2 files, 1 for the profiles and 1 for the three-dimensional grid 

data. 

The raw profile data are provided in an ASCII, comma-separated, format 

(RDM_2D_DCres_RawData.csv). The data headings are as follows: 

 Line: line number, 

 Dist: local downline distance, in meters, 

 Depth: negative depth below the ground surface, in meters, 

 X and Y: easting and northing coordinates of resistivity data in NAD83, UTM zone 4N, 

in meters, and 

 Z: surface elevation of resistivity data in NAVD88, in meters. 

The raw three-dimensional grid data are provided in three ASCII Advanced Geosciences, Inc., 

data formats: 

 RDM3D.stg: raw data file containing the acquisition parameters and measurements, 

 RDM3D.crs: contact resistance readings file, and 

 RDM3D.cmd: command file that contains the electrode configurations used for each 

measurement and the electrode locations in local coordinates, with electrode #1 located 

at coordinate (0,0) in the southeast corner of the grid and the x-coordinates increasing to 

the northwest and y-coordinates increasing to the northeast. 

The inverted profile data are provided in an ASCII array, comma-separated, format 

RDM_2D_DCres_InvData.csv. Unavailable or null data are denoted by a 9990.00 value. The data 

headings are as follows: 

 Line: line number, 

 X and Y: easting and northing coordinates of resistivity data in NAD83, UTM zone 4N, 

in meters, 

 Z: surface elevation of resistivity data in NAVD88, in meters, 

 Dist: local downline distance, in meters, 

 elev[#]: elevation horizontal array containing 25 elements, in meters, and 

 res[#]: modeled resistivity value horizontal array containing 25 elements, in ohm-m. 

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The inverted three-dimensional grid data are provided in an ASCII comma-separated format 

with the following data headings (RDM_3D_DCres_InvData.csv): 

 X and Y: easting and northing coordinates of resistivity data in NAD83, UTM zone 4N, 

in meters, and 

 NAVD88: elevation of resistivity data in NAVD 88, in meters, 

 Res: modeled resistivity values, in ohm-m, 

 Xlocal and Ylocal: local grid coordinates of resistivity data as originally acquired, in 

meters, and 

 Z: negative depth below the ground surface of resistivity data, in meters. 

Electromagnetic Induction Data 

The digital electromagnetic induction data are provided in a compressed ASCII, commaseparated, file RDM_EMI_Data.zip. The data headings are as follows: 

 Line: line type and number, 

 Sample: sample number, 

 X and Y: easting and northing coordinates of EMI data in NAD83, UTM zone 4N, in 

meters, 

 Z: surface elevation in NAVD88, in meters, 

 Date: acquisition date in yyyy/mm/dd format, 

 Time_ms: GPS time of the measurement, in milliseconds of the day, 

 Time_hhmmss_sss: GPS time of the measurement in hours, minutes, seconds, 

 I_#Hz: raw in-phase measurement for a single frequency, in parts per million (ppm), and 

 Q_#Hz: raw quadrature measurement for a single frequency, in ppm. 

Individual I_#Hz and Q_#Hz headings exist for each of the seven acquired frequencies: 3,930 Hz, 6,630 

Hz, 11,310 Hz, 19,110 Hz, 32,430 Hz, 54,930 Hz, and 93,030 Hz. The line name prefixes indicate either 

line data (L) or calibration data acquired at the calibration station (P). A line number followed by .1

indicates data acquired during the first acquisition event on Aug. 7, 2010, and a .2 indicates data 

acquired during the second data acquisition event on that day. Line numbers without these indications 

were acquired on Aug. 6, 2010. 

Digitized Anomalies 

 The annotated anomalies from the DC resistivity profiles have been digitized and are provided in 

a .DXF AutoCAD® drawing interchange file format, RDM_DCres_anomalies.DXF. The ASCII .DXF 

format retains the horizontal and vertical extents in NAD83 UTM zone 4N, in meters and NAVD 88, in 

meters, respectively. 

 

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