U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2006-1215

Magnetotelluric Data, Rainier Mesa/Shoshone 

Mountain, Nevada Test Site, Nevada
Magnetotelluric Data, Rainier Mesa/Shoshone

Mountain, Nevada Test Site, Nevada 

By Jackie M. Williams, Jay A. Sampson, Brian D. Rodriguez, and Theodore H. Asch 

Open-File Report 2006-1215 

U.S. DEPARTMENT OF THE INTERIOR 

U.S. GEOLOGICAL SURVEY 
U.S. Department of the Interior 

DIRK KEMPTHORNE, Secretary 

U.S. Geological Survey

Mark D. Myers, Director 

U.S. Geological Survey, Reston, Virginia 2006 

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Contents

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

Electrical Rock Properties .............................................................................................................1 

Magnetotelluric Method.................................................................................................................3 

Magnetotelluric Survey..................................................................................................................3 

Magnetotelluric Data .....................................................................................................................5 

References Cited...........................................................................................................................7 

Figure 

1. Index map showing magnetotelluric (MT) stations in the areas of Rainier Mesa and 

Shoshone Mountain..................................................................................................2 

Table 

1. Station Locations.................................................................................................4

Appendix 

Magnetotelluric Data Plots ............................................................................................................9 

iii
Introduction 

The United States Department of Energy (DOE) and the National Nuclear Security 

Administration (NNSA) at their Nevada Site Office (NSO) are addressing ground-water 

contamination resulting from historical underground nuclear testing through the Environmental 

Management (EM) program and, in particular, the Underground Test Area (UGTA) project. 

From 1951 to 1992, 828 underground nuclear tests were conducted at the Nevada Test 

Site northwest of Las Vegas. Most of these tests were conducted hundreds of feet above the 

ground-water table; however, more than 200 of the tests were near or within the water table. 

This underground testing was limited to specific areas of the Nevada Test Site, including Pahute 

Mesa, Rainier Mesa/Shoshone Mountain, Frenchman Flat, and Yucca Flat. 

One issue of concern is the nature of the somewhat poorly constrained pre-Tertiary 

geology, and its effects on ground-water flow. Ground-water modelers would like to know more 

about the hydrostratigraphy and geologic structure to support a hydrostratigraphic framework 

model that is under development for the Rainier Mesa/Shoshone Mountain Corrective Action 

Unit (Bechtel Nevada, 2006). 

During 2005, the U.S. Geological Survey (USGS), in cooperation with the DOE and 

NNSA-NSO, collected and processed data from twenty-six magnetotelluric (MT) and audiomagnetotelluric (AMT) sites at the Nevada Test Site. The 2005 data stations were located on 

and near Rainier Mesa and Shoshone Mountain to assist in characterizing the pre-Tertiary 

geology in those areas. These new stations extend the area of the hydrogeologic study 

previously conducted in Yucca Flat. This work will help refine what is known about the 

character, thickness, and lateral extent of pre-Tertiary confining units. In particular, a major goal 

has been to define the upper clastic confining unit (UCCU – late Devonian to Mississippian-age 

siliciclastic rocks assigned to the Eleana Formation and Chainman Shale) from the Yucca Flat 

area and west towards Shoshone Mountain, to Buckboard Mesa in the south, and onto Rainier 

Mesa in the north. Subsequent interpretation will include a three-dimensional (3-D) character 

analysis and a two-dimensional (2-D) resistivity model. The purpose of this report is to release 

the MT sounding data for the twenty-six stations shown in figure 1. No interpretation of the 

data is included here. 

Electrical Rock Properties 

Electromagnetic geophysical methods detect variations in the electrical properties of 

rocks-in particular, electrical resistivity, or its inverse, electrical conductivity. Electrical 

resistivity can be correlated with geologic units on the surface and at depth using lithologic logs 

to provide a 3-D picture of subsurface geology. In the upper crust the resistivity of geologic 

units is largely dependent upon their fluid content, pore-volume porosity, interconnected fracture 

porosity, and conductive mineral content (Keller, 1989). Although there is not a one-to-one 

relation between lithology and resistivity, there are general correlations that can be made using 

typical resistivity values, even though values can be found at other localities that may fall 

outside of the ranges presented below (Palacky, 1987). Fluids within the pore spaces and 

fracture openings, especially if saline, can reduce resitivities in what would otherwise be a 

resistive rock matrix. Resistivity also can be lowered by the presence of electrically conductive 

clay minerals, graphitic carbon, and metallic mineralization. It is common, for example, for 

altered volcanic 

1
Figure 1. Index map showing magnetotelluric (MT) stations in the areas of Rainier 

Mesa and Shoshone Mountain. The 26 stations were collected in 2005. Dashed lines 

are Nevada Test Site areas.

2
rocks to contain replacement minerals that have resistivities ten times lower than those of the 

surrounding rocks (Nelson and Anderson, 1992). Fine-grained sediments, such as clay-rich 

alluvium, marine shales, and other mudstones, are normally conductive from a few ohm-m to a 

few tens of ohm-m (Keller, 1987; Palacky, 1987). Metamorphic rocks (non-graphitic) and 

unaltered, unfractured igneous rocks are normally moderately to highly resistive (a few hundreds 

to thousands of ohm-m). Carbonate rocks can have similarly high resistivities depending on 

their fluid content, porosity, and impurities (Keller, 1987; Palacky, 1987). Fault zones may be 

moderately conductive (tens of ohm-m) when comprised of rocks fractured enough to have 

hosted fluid transport and consequent mineralogical alteration (Eberhart-Phillips and others, 

1995). Higher subsurface temperatures cause higher ionic mobility that reduces rock resistivities 

(Keller, 1987; Palacky, 1987). Tables of electrical resistivities for a variety of rocks, minerals, 

and geological environments may be found in Keller (1987) and Palacky (1987). 

Magnetotelluric Method 

The MT method is a passive-surface electromagnetic geophysical technique that 

measures variations in the Earth's natural electromagnetic fields to investigate the electrical 

resistivity structure of the subsurface from depths of tens of meters to tens of kilometers (Vozoff, 

1991). Worldwide lightning activity at frequencies of 10,000 to 1 Hertz (Hz) and geomagnetic 

micro-pulsations at frequencies of 1 to 0.001 Hz provide the majority of natural signal used by 

the MT method. The natural electromagnetic wave propagates vertically into the Earth due to 

the large resistivity contrast between the air and the Earth, causing a vertical refraction of the 

electromagnetic wave transmitted into the Earth (Vozoff, 1972). 

The natural fields are recorded in the xyz direction for the magnetic field and the xy 

direction for the electric field at the Earth’s surface. The resulting time-series signals are used to 

derive tensor apparent-resistivities and phases by first converting them to complex cross-spectra 

using Fourier-transform techniques. Least squares, cross-spectral analysis (Bendat and Piersol, 

1971) are used to solve for a tensor transfer function. Prior to conversion to apparent resistivity 

and phase, the tensor is normally rotated into principal directions that usually correspond to the 

direction of maximum and minimum apparent resistivity. For a two-dimensional (2-D) Earth, in 

which Earth’s resistivity structure varies with depth and in one lateral direction, the MT fields 

can be decoupled into transverse-electric (TE) and transverse-magnetic (TM) modes; 2-D 

resistivity modeling is generally computed to fit both modes. When the geology satisfies the 2-D 

assumption, the MT data for the TE mode represents electric fields that are oriented parallel to 

geologic strike, and the data for the TM mode represents electric fields oriented perpendicular to 

strike. The MT method is well suited for studying complicated geological environments because 

the electric and magnetic fields are sensitive to vertical and horizontal variations in resistivity. 

The method is capable of establishing whether the electromagnetic fields are responding to 

subsurface rock bodies of effectively 1, 2, or 3 dimensions. An introduction to the MT method 

and references for a more advanced understanding are contained in Dobrin and Savit (1988) and 

Vozoff (1991). 

Magnetotelluric Survey

In May of 2005, data were collected at 26 stations on and near Rainier Mesa and 

Shoshone Mountain. The station locations were chosen to constrain the 

geologic/hydrostratigraphic interpretation, for proximity to roads, and to avoid, where possible, 

3
electrical noise from power lines and vehicles. The low-frequency data (0.0002 to 200 Hz) were 

collected with an Electromagnetic Instruments, Inc., (EMI) MT24/LF 24-bit system (EMI, 

2002), and the high-frequency data (4 Hz to 23,000 Hz) were collected with a portable EMI MT1 system (EMI, 1996). For the low-frequency data, horizontal electric fields were measured 

using three copper/copper sulfate porous-pot electrodes placed in an L-shaped array with dipole 

lengths of 30 meters (m). Titanium electrodes were used in a similar array for the highfrequency data acquisition. The orthogonal, horizontal magnetic fields in the direction of the 

electric-field measurement array were sensed using EMI’s high-magnetic-permeability, mumetal-cored induction coils. For the low-frequency data, two single-station recordings of the 

orthogonal, horizontal components of the electric and magnetic fields and the vertical magnetic 

field were acquired at Global Positioning System (GPS) referenced times and were used as 

remote references for each other as shown in Table 1. The high-frequency data were recorded as 

non-remote referenced single stations. 

The following table lists the 26 MT and AMT station locations as recorded using a GPS 

during field acquisition. Coordinates are referenced to the 1866 Clarke spheroid and North 

American 1983 Western United States datum. Longitude and latitude format is degrees, 

minutes, seconds. Universal Transverse Mercator (UTM) Zone 11 North units are in meters. 

Station elevation is given in meters (NAVD29) above sea level. The accuracy of the x, y, 

component is ±5 m. The accuracy of the z component is ±10 m.

 Table 1. Station Locations [d,m,s degrees, minutes,seconds; m, meters]

Station Latitude 

 (d,m,s) 

Longitude 

(d,m,s) 

Elevation 

above sea 

level 

(m) 

Zone 11 

Northings 

(m) 

Zone 11 

Eastings 

(m) 

Remote 

Reference

d to 

Station 

1 36,55'28.2 116,16’20.1 1,603 4086744 564819 5

2 36,56'03.7 116,15'14.9 1,729 4087851 566420 6

3 36,56'48.6 116,14'52.9 1,929 4089239 566954 7

4 36,57'15.4 116,13'30.8 1,648 4090080 568979 8

5 37,04'45.8 116,19'17.8 1,454 4100906 562963 1

6 37,02'45.2 116,15'42.8 1,537 4100216 565636 2

7 37,02'31.6 116,14'01.8 1,665 4099819 568133 3

8 37,02'49.1 116,12'36.8 1,602 4100375 570229 4

9 37,05'53.5 116,19'15.2 1,521 4105982 560348 12

10 37,06'05.6 116,17'42.4 1,581 4106370 562632 21

11 37,06'06.1 116,15'48.0 1,632 4106408 565460 17

12 37,05'02.6 116,13'42.5 1,640 4104474 568573 9

13 37,07'04.2 116,20'38.3 1,566 4108146 558283 22

14 37,07'21.3 116,18'28.3 1,578 4108693 561479 20

15 37,07'42.9 116,16'33.1 1,686 4109380 564321 26

16 37,08'53.4 116,15'41.2 1,733 4111563 565505 25

17 37,10'33.3 116,14'48.5 1,828 4114650 566860 11

18 37,09'57.1 116,13'35.8 2,096 4113551 568664 24

19 37,10'31.7 116,11'00.7 1,772 4114652 572478 23

20 37,10'36.5 116,10'12.5 1,867 4114810 573666 14

21 37,14'00.6 116,18'23.2 2,127 4121001 561521 10

4
22 37,13'30.6 116,16'30.0 2,063 4120102 564318 13

23 37,13'05.9 116,15'02.1 2,097 4119357 566489 19

24 37,13'30.7 116,13'19.1 2,107 4120130 569030 18

25 37,13'36.7 116,12'20.6 2,059 4120333 570460 16

26 37,13'03.8 116,10'37.8 2,062 4119342 573002 15

Magnetotelluric Data 

The recorded time-series data were transformed to the frequency domain and processed 

to determine a 2-D apparent resistivity and phase tensor at each site. Rotation of the impedance 

tensor allows for decoupling into the TE and TM modes. The data provided here have not been 

rotated from the original north-south, east-west acquisition orientation. During the analysis and 

interpretation process, each station will be rotated to a fixed angle determined by the given 

nominal profile orientation. Low-frequency time-series data were edited, and cross-power files 

were created with Egbert’s (1997) multiple-station magnetotelluric data-processing algorithms

using remote references. Cross-power files were sorted to select optimal signal-to-noise timeseries data sets (see appendix 1). 

The effects of near-surface resistivity anomalies can cause what are known as “static 

shifts” in the data (Sternberg and others, 1988). Cultural features also can affect the measured 

magnetotelluric responses. These include fences, pipelines, communication lines, railways, and 

other manmade conductors. 

The figures in appendix 1 represent the field-processed MT data for each station, after the 

time-series data were converted to the frequency domain and the tensor-transfer function was 

developed. 

For each station, nine separate plots are given: 

1. Apparent Resistivity (x and y symbols are xy and yx components) 

2. Impedance Phase (x and y symbols are xy and yx components) 

3. Rotation Angle 

4. Impedance Skew 

5. Multiple Coherency (x and y symbols are xy and yx components) 

6. Impedance Polar Plots 

7. Tipper Magnitude 

8. Tipper Strike 

9. HzHx (x symbol) and HzHy (o symbol) Coherency 

Error bars (],[) on the Apparent Resistivity, Impedance Phase, Skew, Tipper Magnitude, 

and Tipper Strike plots represent probable errors within one standard deviation of the sample 

variance (Gamble and others, 1979). 

Apparent resistivity is the approximate ratio of the electric-field strength to the 

magnetic-field strength at a given frequency. The impedance phase is proportional to the slope 

of the apparent resistivity curve on a log-log plot, but from baselines at ±45 degrees (Vozoff, 

1991). A measure of the dimensionality for MT data is provided by the impedance skew of the 

impedance tensor (Vozoff, 1972). If the effective measured resistivity response to the geology 

beneath a MT station truly is one or two dimensional, then the skew will be zero. Instrumental 

5
and environmental sources of electrical noise can cause non–zero skew values. Skew values 

typically are small (about 0.1) for relatively low-noise recordings. Higher skews (above 0.2) are 

an indication of either the resistivity response to 3-D geology or higher levels of noise. 

Manmade electrical noise, such as power lines, power generators, and moving vehicles and 

trains, can have a negative effect on MT data quality. All of these local disturbances can 

produce incoherent noise that mainly affects frequencies above 1 Hz. Other manmade electrical 

noise, such as direct-current electric trains and active cathodic protection of pipelines, produces 

coherent electromagnetic signals that mainly affect frequencies below 1 Hz. 

In the survey area, noise from a number of small power lines and small moving vehicles 

was negligible at distances greater than 0.4 km from the noise source. Power-line signal levels 

were measured at each site and typically were less than 20 percent of the maximum recordable 

signals. Noise from larger power lines, power generators, pipelines, and trains was negligible at 

distances greater than 5 km. Local lightning, wind, and rainstorms also can degrade data 

quality. Burying the magnetic induction coils and the electric dipole wires minimized wind 

noise. 

Predicted values of the electric field can be computed from the measured values of the 

magnetic field (Vozoff, 1991). The coherence of the predicted electric field with the measured 

electric field is a measure of the signal-to-noise ratio provided in the multiple coherency plots. 

Values are normalized between 0 and 1; values at 0.5 signify signal levels equal to noise levels. 

For this data set, coherencies generally were at an acceptable level, except at times in the 

frequency ranges of 0.01 to 5 Hz (often referred to as the “dead band”). 

The field-processed MT data include some scatter and poor signal-to-noise ratios. 

Spectral results were inspected visually for noisy data, and the best signal-to-noise field data 

were combined into the final plots. 

The magnetotelluric impedance polar plots provide a measure of MT data dimensionality 

(Reddy and others, 1977). For 1-D resistivity structures, the principal impedance polar diagram

(dashed line) is a circle. For 2-D or 3-D resistivity structures, the principal impedance polar 

diagram (dashed line) elongates either parallel or perpendicular to strike direction. Over 

resistors, the principal impedance polar diagram elongates perpendicular to strike direction, and 

over conductors, it elongates parallel to strike direction. For 2-D resistivity structures, the 

additional impedance polar diagram (solid line) attains the shape of a symmetric clover leaf. For 

3-D resistivity structures, the additional impedance polar diagram (solid line) elongates in one 

direction, and its amplitude is comparable to that of the principal impedance polar diagram

(dashed line). 

The magnetotelluric “tipper” is calculated from the vertical component of the magnetic 

field. The tipper magnitude is a measure of the “tipping” of the magnetic field out of the 

horizontal plane (Vozoff, 1991). It will equal zero for the 1-D case. It typically increases to 

values between 0.1 to 0.5 and seldom approaches 1, as it responds primarily to vertical and 

subvertical structures. The tipper magnitude of the stations discussed in this report ranged from

0.1 to 0.6 over the lower frequencies, indicating some vertical structure at depth. The tipper 

strike is used to help resolve the 90-degree ambiguity in the impedance rotation angle. The 

HzHx and HzHy coherency is a measure of the signal-to-noise ratio of the vertical magnetic field 

with respect to each of the orthogonal, horizontal magnetic-field directions. Values are 

normalized between 0 and 1; values at 0.5 signify signal levels equal to noise levels. These 

three-component magnetic-field coherencies provide a check on the signal-to-noise ratio of the 

measured values in the tipper magnitude and tipper strike plots. 

6
References Cited 

Bechtel Navada, 2006, A hydrostratigraphic model and alternatives for the ground-water 

flow and contaiminant transport model of Corrective Action Unit 97: Yucca 

Flat-Climax Mine, Lincoln and Nye counties, Nevada: Report DOE/NV/11718-1119, in 

press. 

Bendat, J.S., and Piersol, A.G., 1971, Random data-analysis and measurement procedures: 

 New York, Wiley Interscience, 407p. 

 

Dobrin, M.D., and Savit, C.H., 1988, Introduction to geophysical prospecting (4th ed.): New 

York: McGraw-Hill, 867 p. 

Eberhart-Phillips, Donna, Stanley, W.D., Rodriguez, B.D., and Lutter, W.J., 1995, Surface 

seismic and electrical methods to detect fluids related to faulting: Journal of 

Geophysical Research, v. 100, no. B7, p. 12,919-12,936. 

Egbert, G.D., 1997, Robust multiple station magnetotelluric data processing: Geophysics Journal 

International, 130, 

 p. 475-496. 

EMI, Inc., 1996, MT-1 magnetotelluric system operation manual, version 3.2: Richmond, 

Calif., ElectroMagnetic Instruments, Inc., 220 p. 

EMI,Inc., 2002, MT24/LF system operation and maintenance manual, version 1.0: Richmond, 

Calif., ElectroMagnetic Instruments, Inc., 72 p. 

Gamble, T.D., Goubau, W.M., and Clarke, J., 1979, Error analysis for remote reference 

magnetotellurics: Geophysics, v. 44, no. 5, p. 959-968. 

Keller, G.V., 1987, Rock and mineral properties, in Nabighian, M.N., ed., Electromagnetic 

 methods in applied geophysics theory: Tulsa, Okla., Society of Exploration 

 Geophysicists, v. 1, p. 13-51. 

Keller, G.V., 1989, Electrical properties, in Carmichael, R.S., ed., Practical handbook of 

 physical properties of rocks and minerals: Boca Raton, Fla., CRC Press, p. 359-427. 

Nelson, P.H., and Anderson, L.A., 1992, Physical properties of ash flow tuff from Yucca 

Mountain, Nevada: Journal of Geophysical Research, v. 97, no. B5, p. 6,823-6,841. 

Palacky, G.J., 1987, Resistivity characteristics of geologic targets, in Nabighian, M.N., ed., 

 Electromagnetic methods in applied geophysics theory: Tulsa, Okla., Society of 

Exploration Geophysicists, v. 1, p. 53-129. 

7
Reddy, I.K., Rankin, David, and Phillips, R.J., 1977, Three-dimensional modelling in 

magnetotelluric and magnetic variational sounding: Geophysics Journal of the 

Royal Astronomical Society, v. 51, p. 313-325. 

Sternberg, B.K., Washburne, J.C., and Pellerin, Louise, 1988, Correction for the static shift 

 in magnetotellurics using transient electromagnetic soundings: Geophysics, v. 53, 

 p. 1, 459-1,468. 

Vozoff, Keeva, 1972, The magnetotelluric method in the exploration of sedimentary basins: 

Geophysics, v. 37, p. 980-1041. 

Vozoff, Keeva, 1991, The magnetotelluric method, in Nabighian, M.N., Electromagnetic 

methods in applied geophysics: Tulsa, Okla., Society of Exploration Geophysicists, v. 2, 

pt. B, p. 641-711. 

 

8
Appendix 

Magnetotelluric Data Plots 

There are nine separate plots for each station: 

1. Apparent Resistivity for the rotated maximum (x symbol) and minimum (o symbol) modes 

2. Impedance Phase for the rotated maximum (x symbol) and minimum (o symbol) modes 

3. Rotation Angle for the impedance tensor (corresponds to the direction of maximum apparent 

resistivity) 

4. Impedance Skew for the impedance tensor 

5. Multiple Coherency for the rotated maximum (x symbol) and minimum (o symbol) modes of

the electric field 

6. Impedance Polar Plots (at 12 selected frequencies) 

7. Tipper Magnitude for the vertical magnetic field 

8. Tipper Strike for the vertical magnetic field 

9. HzHx (x symbol) and HzHy (o symbol) Coherency 

Refer to the “Magnetotelluric Data” section in this report for an explanation of these 

plots. The priorities listed on the plots were determined prior to data acquisition. During postprocessing the priority ranking was changed.

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