Audiomagnetotelluric Data to Characterize the 

Revett-Type Copper-Silver Deposits at Rock Creek 

in the Cabinet Mountains Wilderness, Montana

By Jay A. Sampson and Brian D. Rodriguez

Open-File Report 2011–1174

U.S. Department of the Interior

U.S. Geological Survey
ii

U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

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

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Suggested citation:

Sampson, J.A., and Rodriguez, B.D., 2011, Audiomagnetotelluric data to characterize the Revett-type 

copper-silver deposits at Rock Creek in the Cabinet Mountains Wilderness, Montana: United States 

Geological Survey Open-File Report 2011-1174, 73 p.

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

endorsement by the U.S. Government. This report has not been reviewed for stratigraphic nomenclature.

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.
iii

Contents

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

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

Electrical Properties of Rock.......................................................................................................................................... 3

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

Audiomagnetotelluric Survey Across the Revett-Type Copper-Silver Deposits at Rock Creek...................................... 4

Audiomagnetotelluric Data Collected in the Rock Creek Survey ................................................................................... 5

References Cited........................................................................................................................................................... 8

Audiomagnetotelluric Data Diagrams ............................................................................................................................ 9

Appendix

Audiomagnetotelluric Data Diagrams ...........................................................................................................................16

Figure

1. Audiomagnetotelluric profile across the Revett-type copper-silver deposits at Rock Creek in the 

Cabinet Mountains Wilderness, Montana ...................................................................................................................... 2

Table

1. Audiomagnetotelluric station coordinates. ........................................................................................................... 5

Abbreviations Used in This Report

Ω-m ohm-meter

AMT udiomagnetotellurics

FFT fast Fourier transform

km kilometer

m meter

MT magnetotelluric

S/m Siemens/meter

TE transverse electric

TM transverse magnetic

UTM Universal Transverse Mercator

Vertical coordinate information is referenced to the 1866 Clarke Spheroid.

Horizontal coordinate information is referenced to the North American Datum of 1927 (NAD 27).

Altitude, as used in this report, refers to distance above the vertical datum.
1

Audiomagnetotelluric Data to Characterize the 

Revett-Type Copper-Silver Deposits at Rock Creek 

in the Cabinet Mountains Wilderness, Montana

By Jay A. Sampson and Brian D. Rodriguez

Abstract

The Revett-type deposits at Rock Creek are part of the concealed stratabound coppersilver deposits located in the Cabinet Mountains Wilderness of Montana. The U.S. Geological 

Survey is conducting a series of multidisciplinary studies as part of the Assessment Techniques 

for Concealed Mineral Resources project.

Geologic, geochemical, geophysical, and mineral resources data are being evaluated with 

existing and new mineral deposit models to predict the possibility and probability of 

undiscovered deposits in covered terranes. To help characterize the size, resistivity, and depth of 

the mineral deposit concealed beneath thick overburden, a regional southwest-northeast

audiomagnetotelluric sounding profile was acquired. Further studies will attempt to determine if 

induced polarization parameters can be extracted from the magnetotelluric data to determine the 

size of the mineralized area. The purpose of this report is to release the audiomagnetotelluric 

sounding data collected along that southwest-northeast profile. No interpretation of the data is 

included.

Introduction 

The Western United States holds great potential for concealed mineral deposits. The U.S. 

Geological Survey is conducting a series of multidisciplinary studies as part of the Assessment 

Techniques for Concealed Mineral Resources project. 

Geologic, geochemical, geophysical, and mineral resources data are being evaluated with 

existing and new mineral deposit models to predict the possibility and probability of 

undiscovered deposits in covered terranes. This study uses the magnetotelluric (MT) method to 

determine if a polarizable mineral deposit (indicative of metallic content) can be detected when 

buried beneath thick overburden. Conventional geophysical exploration methods employing 

induced polarization surveys produce signals of limited penetration depth and require large, 

cumbersome transmitters. The MT method measures and records the Earth’s natural timevarying electromagnetic signals as they pass through the subsurface. A relatively new technique, 

known as natural field induced polarization (Gasperikova and Morrison, 2001), extracts induced 

polarization parameters from MT data. To do so, however, the size and electrical resistivity of 

the target must be known in order to distinguish the electromagnetic response of the mineral 

deposit from the response of the surrounding host medium.
2

The Revett-type deposits at Rock Creek are part of the concealed stratabound coppersilver deposits located in the Cabinet Mountains Wilderness of Montana. To help characterize 

the size, resistivity, and depth of the mineral deposit concealed beneath thick overburden, a 

regional southwest-northeast audiomagnetotelluric (AMT) sounding profile was acquired the last 

week of August, 2009 (fig. 1). Further studies will attempt to determine if induced polarization 

parameters extracted from the AMT data can also be used to determine the size of the 

mineralized area. The purpose of this report is to release the AMT sounding data collected along 

that southwest-northeast profile. No interpretation of the data is included.

Figure 1. Audiomagnetotelluric profile across the Revett-type copper-silver deposits at Rock Creek 

in the Cabinet Mountains Wilderness, Montana. Audiomagnetotelluric stations acquired in late August 

2009 are numbered purple triangles. Base map from Elephant Peak, Montana, 1:24,000 topographic 

quadrangle.
3

Electrical Properties of Rock

Electromagnetic geophysical methods detect variations in the electrical properties of rock 

units—in particular, electrical resistivity, in units of ohm-meters [Ω−m], or its inverse, electrical 

conductivity in units of Siemens/meter (S/m). Electrical resistivity can be correlated with

geologic units on the surface and at depth by using lithologic logs to provide a three-dimensional 

picture of subsurface geology. In the upper crust, the resistivities of geologic units largely 

depend upon their fluid content, pore-volume porosity, interconnected fracture porosity, and the 

presence of conductive minerals (such as clay, graphite, and metallic minerals). Fluids in the 

pore spaces and fracture openings, especially saline fluids, can increase electrical conductance in 

an otherwise electrically resistive rock matrix (Keller and Frischknecht, 1966; Hearst and 

Nelson, 1985; Keller, 1987; Palacky, 1987; Hallenburg, 1998; Hearst and others, 2000). Although 

no one-to-one relation exists between lithology and resistivity, some general correlations can be 

made by using typical values, even though different values can be found at other localities 

(Palacky, 1987) that may fall outside of the ranges presented below. Fine-grained sediments—

such as clay-rich alluvium, marine shales, and other mudstones—are normally conductive and 

have resistivities of a few ohm-meters to tens of ohm-meters (Keller, 1987; Palacky, 1987). 

Metamorphic rocks (not containing graphite) and unaltered, unfractured igneous rocks are 

normally moderately to highly resistive (a few hundred to thousands of ohm-meters). It is 

common for altered volcanic rocks to contain replacement minerals that have resistivities only 

one tenth as high as those of the surrounding rocks (Nelson and Anderson, 1992). Porous 

carbonate rocks having low fluid content and few impurities can have similarly high resistivities 

(Keller, 1987; Palacky, 1987). Fault zones may be moderately conductive (tens of ohm-meters) 

when composed of rocks fractured enough to have hosted fluid transport and consequent 

mineralogical alteration (Eberhart-Phillips and others, 1995). At greater depths, higher 

subsurface temperatures increase the mobility of ions in the fluids and reduce rock resistivity 

(Hallenburg, 1998). Tables of electrical resistivity for a variety of rocks, minerals, and geological 

environments may be found in Keller (1989) and Palacky (1987).

Magnetotelluric Method

The MT method is a passive ground-based electromagnetic geophysical technique that 

investigates the distribution of electrical resistivity below the surface at depths of tens of meters 

to tens of kilometers (Vozoff, 1991). It does so by measuring time variations in the Earth’s 

natural electric and magnetic fields. Worldwide lightning activity at frequencies of about 1 Hertz 

to 10,000 Hertz and geomagnetic micropulsations at frequencies of 0.001 Hertz to 1 Hertz 

provide the main source of signals used by the MT method. The natural electromagnetic waves 

propagate vertically in the Earth because the very large resistivity contrast between the air and 

the Earth causes a vertical refraction of the electromagnetic wave at the Earth’s surface (Vozoff, 

1972).

The horizontal electric and magnetic fields are recorded in two orthogonal directions and 

the vertical magnetic field is also recorded. The resulting time-series signals are used to derive 

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

using fast Fourier transform (FFT) techniques and least-squares, cross-spectral analysis (Bendat 

and Piersol, 1971) to solve for a tensor transfer function. If one assumes that the Earth consists of 

a two-input, two-output linear system in which the orthogonal magnetic fields are input and the 
4

orthogonal electric fields are output, then a transfer function can be calculated that relates the 

observed electric fields to the magnetic fields. Before the tensor is converted to apparent 

resistivity and phase, it is normally rotated parallel to geologic strike. Subsurface geologic strike 

can be estimated by determining the horizontal direction (Hxy or Hyx) in which the vertical 

magnetic field “tips” (the tipper strike direction).

For a two-dimensional Earth, the MT fields can be decoupled into transverse electric and 

transverse magnetic modes (commonly referred to as the TE and TM modes). Two-dimensional 

resistivity modeling is generally computed to fit both modes. When the geology satisfies a

two-dimensional assumption, the MT data for the transverse electric mode are assumed to 

represent the electric field oriented along geologic strike, and the data for the transverse 

magnetic mode are assumed to represent the electric field oriented across strike. The MT method 

is well suited for studying complicated geological environments because the electric and 

magnetic field transfer functions are sensitive to vertical and horizontal variations in resistivity. 

High-resolution shallow-subsurface characterization is possible for closely spaced MT stations, 

but the resolution decreases for deeper measurements and for widely spaced stations. The 

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

subsurface rock bodies of effectively one, two, or three dimensions. An introduction to the MT 

method and references for a more advanced understanding are contained in Kaufman and Keller 

(1981), Dobrin and Savit (1988), and Vozoff (1991).

Audiomagnetotelluric Survey Across the Revett-Type Copper-Silver 

Deposits at Rock Creek

Six AMT soundings were collected in August 2009 along a 5-km-long profile in 

northwestern Montana (fig. 1). The profile starts about 9 kilometers (km) northeast of Noxon, 

Montana, in the Kaniksu National Forest of the Cabinet Mountains, then continues northeasterly, 

passing just south of Chicago Peak and Saint Paul Peak, and ends about 1 km northwest of Saint 

Paul Lake. The profile location dissects patented claims of Revett-type copper-silver deposits at 

Rock Creek in the Cabinet Mountains Wilderness, Montana.

To avoid vehicle and electrical noise, station locations were chosen in remote areas and 

next to roads that had no power lines in the vicinity. Data at stations 0, 1, and 2 (Table 1) were 

collected with an Electromagnetic Instruments, Inc., MT-1 system (Electromagnetic Instruments, 

Inc., 1996). Horizontal electric fields were recorded using titanium electrodes placed in an Lshaped, three-electrode array with dipole lengths of 15 (station 2), 20 (station 0), and 30 (station 

1) meters. The orthogonal horizontal magnetic fields were measured in the direction of the 

electric-field array and were sensed using high magnetic permeability mu-metal-cored induction 

coils. Frequencies were sampled from about 4 Hertz to 23,000 Hertz at each station. 

Data at stations 1, 3, 4, and 5 (Table 1) were collected within the Cabinet Mountains 

Wilderness with a portable Electromagnetic Instruments, Inc., MT24HF system (Electromagnetic 

Instruments, Inc., 2003), since there were no roads for access with the MT-1 system. Horizontal 

electric fields were recorded using stainless steel electrodes placed in a cross-shaped, fourelectrode array with dipole lengths of 24 meters (m). The orthogonal horizontal magnetic fields 

were measured in the direction of the electric-field array and were sensed using high magnetic 

permeability mu-metal-cored induction coils. Frequencies were sampled from about 10 Hertz to 

54,000 Hertz at each station. Station 1 has dual recordings from both the MT-1 system and the 
5

MT24HF system for calibrating the MT24HF data correction (see “Audiomagnetotelluric Data

Collected in the Rock Creek Survey” section below).

Table 1. Audiomagnetotelluric station coordinates. [Unannotated station numbers are MT-1 system stations. Annotated (asterisk) station numbers are MT24HF system 

stations. Coordinates are referenced to the 1866 Clarke spheroid and North American 1927 Western United States 

datum. Longitude and latitude format is degrees:minutes:seconds. Universal Transverse Mercator (UTM) zone 11N 

units and station elevations are in meters. The accuracy of the north and east component is ±5 m and is ±10 m for 

the elevation. X direction (X Dir) is in degrees clockwise from true north]

Station X Dir Latitude Longitude North (m) East (m) Elevation (m)

0 246 48:03:25 -115:42:21 5,323,223 596,446 1,490

1 010 48:03:42 -115:41:51 5,323,740 597,064 1,720

1* 010 48:03:42 -115:41:51 5,323,740 597,064 1,720

2 160 48:03:59 -115:41:40 5,324,300 597,267 1,920

3* 000 48:04:13 -115:41:10 5,324,723 597,884 2,040

4* 000 48:04:24 -115:40:52 5,325,087 598,246 2,050

5* 000 48:05:56 -115:39:59 5,327,922 599,290 1,250

Audiomagnetotelluric Data Collected in the Rock Creek Survey

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

determine the impedance tensor, which is used to derive apparent resistivities and phases at each 

site. Rotation of the impedance tensor allows for decoupling into the transverse electric and 

transverse magnetic modes. The data provided here have not been rotated from the original 

acquisition orientation (X direction) listed in table 1. During the analysis and interpretation 

process, each station should be rotated to a fixed angle determined by the given nominal profile 

orientation. Cross-power files were sorted to select optimal signal-to-noise time-series data sets 

(see Appendix). Cultural features such as fences, pipelines, communication lines, moving 

vehicles and trains, and other manmade sources of electromagnetic noise, can contaminate the 

responses of the MT system. Care was taken to avoid these sources of noise when we acquired

these data. 

The figures in the Appendix represent the field-processed AMT data for each station, 

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

function was developed. 

Data for each station is presented in eight diagrams: 

1. Apparent resistivity (x and o symbols are xy and yx components) 

2. Impedance phase (x and o symbols are xy and yx components) 

3. Impedance skew 

4. Multiple coherency (x and o symbols are xy and yx components) 

5. Impedance polar diagrams 

6. Tipper magnitude 

7. Tipper strike 

8. HzHx (x symbol) and HzHy (o symbol) coherency
6

Units of measure for the apparent resistivity, phase, and tipper strike are given on the 

diagrams. The coherency diagrams, the skew, the tipper magnitude, and the polar diagrams have 

unitless vertical axes. The data for all diagrams are shown over a range of frequencies. Error 

bars (],[) on the diagrams of apparent resistivity, impedance phase, skew, tipper magnitude, and 

tipper strike (tipper direction) diagrams represent probable errors within one standard deviation 

of the sample variance (Gamble and others, 1979).

Apparent resistivity is calculated from the ratio of the electric field strength magnitude 

over the magnetic field strength magnitude for a given frequency. The impedance phase is 

proportional to the slope of the apparent-resistivity curve on a log-log diagram, relative to a 

baseline 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 an MT station is truly one- or two-dimensional, then 

the skew will be zero. Both instrument and environmental sources of noise contribute to nonzero 

skew values but are typically small (about 0.1) for relatively low-noise-level recordings. Higher 

skews (more than 0.2) indicate either the resistivity response to three-dimensional geology or 

higher levels of noise. Skews with large error bars are due to higher levels of noise.

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

was negligible beyond 0.25 km from the noise source. Power-line signal amplitudes were 

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

Local lightning, wind, and rainstorms also can degrade data quality, but these noise sources were 

avoided by not recording during active thunderstorms. Burying the magnetic induction coils and 

keeping the electric dipole wires flat on the ground helped to minimize wind noise.

The figures in the Appendix represent the field-processed AMT data at each station, and 

they include some data scatter and poor signal-to-noise ratios. The only effort aimed at removing 

noisy data points was to visually inspect and select the best signal-to-noise field data to combine 

into the final data diagrams.

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 

diagrams. Values are normalized between 0 and 1, where values at 0.5 signify signal levels equal 

to noise levels. For this data set, coherencies were generally above 0.8, except in the higher 

frequencies (greater than about 1,000 Hertz).

The impedance polar diagrams provide a measure of the MT data dimensionality (Reddy 

and others, 1977). For one-dimensional resistivity structures, the principal impedance 

(off-diagonal elements) polar diagram (dashed line) is a circle. For two-dimensional or threedimensional resistivity structures, the principal impedance polar diagram (dashed line) elongates. 

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

while over conductors the principal impedance polar diagram elongates parallel to strike 

direction. For two-dimensional resistivity structures, the additional impedance (on-diagonal 

elements) polar diagram (solid line) attains the shape of a symmetric clover leaf. For threedimensional 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), although high noise levels can produce the same effect on the polar 

diagram. A three-dimensional analysis of polar diagrams at each frequency should also take into 

account the corresponding coherence and skew values along with their associated error levels.
7

The polar diagrams computed for our data show the electromagnetic response for all stations was 

three-dimensional over all frequencies measured at acceptable noise levels, except for station 2 

that showed a mix of two- and three-dimensional electromagnetic response at the lower 

frequencies.

The tipper can be 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). The magnitude is zero for the one-dimensional case, typically increases to values 

between 0.1 and 0.5, and rarely is as great as 1 as it responds to vertical and subvertical 

structures. The “tipper strike” (tipper direction) typically is used to help resolve the 90-degree 

ambiguity in the impedance rotation angle. The tipper magnitudes of these stations were all 

above 0.1 indicating vertical and subvertical structure at depth.

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, where values of 0.5 signify signal levels equal to noise 

levels. These three components of magnetic-field coherence provide a check on the quality of the 

measured values in the tipper magnitude and tipper strike diagrams.

Data collected with the MT24HF system requires a data correction to match the data 

recorded with the MT-1 system, which is a proven system for collecting reliable AMT data. We 

collected AMT data with both the MT-1 system and the MT24HF system on the same day at 

station 1 to determine the data correction required to match the MT24HF system apparent 

resistivities with the MT-1 system apparent resistivities. Data diagrams for station 1 collected 

with the MT-1 system appear in the Appendix labeled “Station 1”. Data diagrams for 

uncorrected station 1 collected with the MT24HF system appear labeled “Station 1?”. Data 

diagrams for the corrected station 1 collected with the MT24HF system are labeled “Station 1*”. 

Through trial and error using the MTR95 data processing program provided by Electromagnetic 

Instruments, Inc., we found that applying a “Fix Spectra” factor of 6 to both the x and y channels 

of the electric field parameter resulted in the best overall match to the apparent resistivities 

collected with the MT-1 system. A factor of 5 resulted in better matching apparent resistivities 

for the upper apparent resistivity curve, while a factor of 7 resulted in better matching apparent 

resistivities for the lower curve. We applied a “Fix Spectra” factor of 6 to both x and y channels 

of the electric field parameters to all of the data collected with the MT24HF system (see 

Appendix stations labeled 1*, 3*, 4*, and 5*).

Outstanding differences that remain between the data collected with the MT-1 system and 

the corrected MT24HF system include an order of magnitude reduction of tipper magnitude 

between the two systems at the same frequencies. Another difference included tipper strikes 

(tipper direction) with the MT24HF system that consistently fluctuate (with small oscillations) at 

either +90 degrees (measured clockwise from North) or +270 degrees (-90 degrees). We found 

that applying a “Fix Spectra” factor of 10 to the MT24HF system vertical magnetic field channel 

(Hz) corrected the tipper magnitudes to better match the MT-1 tipper magnitudes, but without 

any effect on the tipper strike data. We elected to not make any corrections to the vertical 

magnetic field channel (Hz), since applying a factor of 10 correction did not resolve the tipper 

strike mismatch between the MT-1 and MT24HF systems. Outstanding differences in impedance 

skew and coherencies are likely caused by differences in data quality rather than system 

calibration issues.
8

References Cited

Bendat, J.S., and Piersol, A.G., 1971, Random data—Analysis and measurement procedures: 

New York, Wiley Interscience, 407 p.

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. 12919–12936.

Electromagnetic Instruments, Inc., 1996, MT-1 magnetotelluric system operation manual, 

version 3.2: Richmond, Calif., Electromagnetic Instruments, Inc., 220 p.

Electromagnetic Instruments, Inc., 2003, MT24/HF system operation and maintenance manual, 

revision AA: Richmond, Calif., Electromagnetic Instruments, Inc., 60 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.

Gasperikova, Erika, and Morrison, H. F., 2001, Mapping of induced polarization using natural 

fields: Geophysics, v. 66, no.1, p.137–147.

Hallenburg, J.K., 1998, Non-hydrocarbon methods of geophysical formation evaluation: Boca 

Raton, Fla., Lewis Publishers, 265 p.

Hearst, J.R., and Nelson, P.H., 1985, Well logging for physical properties: New York, McGrawHill, 571 p.

Hearst, J.R., Nelson, P.H., and Paillet, F.L., 2000, Well logging for physical properties (2d ed.): 

New York, John Wiley, 483 p.

Kaufman, A.A., and Keller, G.V., 1981, The magnetotelluric sounding method, in Methods in 

geochemistry and geophysics, 15: Amsterdam, Elsevier Scientific Publishing Company, 595 p.

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.

Keller, G.V., and Frischknecht, F.C. 1966, Electrical methods in geophysical prospecting: 

Oxford, Pergamon Press Inc., 519 p.

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. 823–841.

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

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

Geophysicists, v. 1, p. 53–129.

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

magnetotelluric and magnetic variational sounding: Geophysical Journal of the Royal 

Astronomical Society, v. 51, p. 313–325.

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

Geophysics, v. 37, p. 980–141.

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.
9

Appendix

Audiomagnetotelluric Data Diagrams

Units of measure for the apparent resistivity, phase, and tipper strike are given on the diagrams. 

The coherency diagrams, the skew, the tipper magnitude, and the polar diagrams have unitless 

vertical axes. The data for all diagrams are shown over a range of frequencies. Data for each 

station is presented in eight diagrams: 

1. Apparent resistivity for the unrotated xy (x symbol) and yx (o symbol) modes 

2. Impedance phase for the unrotated xy (x symbol) and yx (o symbol) modes 

3. Impedance skew for the impedance tensor 

4. Multiple coherency for the xy (x symbol) and minimum (o symbol) modes of the electric field

5. Impedance polar diagrams (at 12 selected frequencies)

6. Tipper magnitude for the vertical magnetic field

7. Tipper strike for the vertical magnetic field

8. HzHx (x symbol) and HzHy (o symbol) coherency

Refer to the “Audiomagnetotelluric Data Collected in the Rock Creek Survey” section in this 

report for an explanation of these diagrams.
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