Audio-Magnetotelluric Survey to Characterize the 

Sunnyside Porphyry Copper System in the 

Patagonia Mountains, Arizona

By Jay A. Sampson and Brian D. Rodriguez

Open–File Report 2010–1311

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

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Suggested citation:

Sampson, J.A., and Rodriguez, B.D., 2010, Audio-magnetotelluric survey to characterize the Sunnyside 

porphyry copper system in the Patagonia Mountains, Arizona: U.S. Geological Survey Open-File Report

2010–1311, 57 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.

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

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

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

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

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

Audio-Magnetotelluric Survey Across the Sunnyside Porphyry Copper System............................................................ 4

Audio-Magnetotelluric Data Collected in the Sunnyside Survey .................................................................................... 5

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

Audio-Magnetotelluric Data Diagrams ........................................................................................................................... 9

Appendix

Audio-Magnetotelluric Data Diagrams ..........................................................................................................................10

Figure

1. Audio-magnetotelluric profile across the Sunnyside porphyry copper system in the Patagonia 

Mountains, Arizona........................................................................................................................................................ 2 

Table

1. Audio-magnetotelluric station coordinates.............................................................................................................. 5 

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.

Abbreviations Used in This Report

Ω−m ohm-meter

km kilometer 

m meter

AMT audio-magnetotelluric

FFT fast Fourier transform

Hz hertz

MT magnetotelluric

S/m Siemens/meter

TE transverse electric

TM transverse magnetic 

UTM Universal Transverse Mercator

iii
Audio-Magnetotelluric Survey to Characterize the 

Sunnyside Porphyry Copper System in the 

Patagonia Mountains, Arizona

By Jay A. Sampson and Brian D. Rodriguez

Abstract

The Sunnyside porphyry copper system is part of the concealed San Rafael Valley 

porphyry system located in the Patagonia Mountains of Arizona. To help characterize the size, 

resistivity, and skin depth of the polarizable mineral deposit concealed beneath thick overburden, 

a regional east-west audio-magnetotelluric sounding profile was acquired. Further studies will 

attempt to determine if induced polarization parameters extracted from the magnetotelluric data 

can also be used to determine the size and resistivity of the mineralized area. The purpose of this 

report is to release the audio-magnetotelluric sounding data collected along that east-west profile. 

No interpretation of the data is included. 

Introduction 

The Great Basin province of the western United States holds great potential for concealed 

mineral deposits. This study uses the magnetotelluric (MT) method to determine if a polarizable 

mineral deposit (indicative of metallic content) can be detected 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 time-varying 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 skin depth 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. One must know the general size, depth, and electrical resistivity of the target to 

calculate the skin depth.

The Sunnyside porphyry copper system is part of the concealed San Rafael Valley 

porphyry system located in the Patagonia Mountains of Arizona. To help characterize the size 

and resistivity of the mineralized area beneath cover, a regional east-west audio-magnetotelluric

(AMT) sounding profile was acquired the first week of May, 2008 (fig. 1). Resistivity modeling 

of the audio-magnetotelluric (high frequency MT) data can be used to help resolve the size and 

resistivity of the target. The purpose of this report is to release the AMT sounding data collected 

along that east-west profile. No interpretation of the data is included. 

1
Figure 1. Audio-magnetotelluric profile across the Sunnyside porphyry system in the Patagonia 

Mountains, Arizona. Audio-magnetotelluric stations acquired in May 2008 are numbered yellow 

squares (station 2 not acquired). Base map from Cumero Canyon, Harshaw, Patagonia, and Mount 

Hughes, Arizona, 1:24,000 topographic quadrangles.

2
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, graphitic carbon, 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. 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). 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). 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 decrease fluid viscosity that causes greater mobility of ions in the fluids 

and reduces 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 (or its inverse, electrical conductivity) 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 10,000 hertz (Hz) to 1 Hz and geomagnetic 

micropulsations at frequencies of 1 Hz to 0.001 Hz provide the main source of signals used by 

the MT method. The natural electromagnetic waves propagate vertically in the Earth because the 

very large contrast in the resistivity of 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 

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

3
observed electric fields to the magnetic fields. Before it is converted to apparent resistivity and 

phase, the tensor is normally rotated parallel to geologic strike. Subsurface geologic strike can be 

estimated by determining the horizontal direction (Hxy or Hyx) that 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 the 

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

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

magnetic mode is 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 of the subsurface 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 Dobrin and Savit 

(1988) and Vozoff (1991). 

Audio-Magnetotelluric Survey Across the Sunnyside Porphyry Copper 

System

Six AMT soundings were collected in May 2008 along a 10-km-long profile in southern 

Arizona (fig. 1). The profile starts along the western foothills of the Patagonia Mountains about 6 

kilometers (km) northeast of Nogales International Airport, then continues northeasterly, passing 

roughly halfway between the Sunnyside mine and Thunder mine, and ends almost 2 km 

northeast of the town of Harshaw. The profile location dissects the northwest-trending Sunnyside 

porphyry copper system, whose location is coincident with the phyllic alteration zone mapped by 

Graybeal (1996). 

Station locations were chosen for proximity to roads and in order to avoid electrical noise 

from power lines. All data at the stations were collected with a portable Electromagnetic 

Instruments, Inc., MT-1 system (Electromagnetic Instruments, Inc., 1996). Horizontal electric 

fields were recorded using titanium electrodes placed in an L-shaped, three-electrode array with 

dipole lengths of 30 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-metalcored induction coils. Frequencies were sampled from about 4 Hz to 23,000 Hz at each station.

Table 1 lists the six AMT station locations (data from station 2 was not acquired because 

of site access problems). Remote reference stations were not used because man-made noise 

sources were assumed to be minimal in this inactive mining area. 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) units 

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

elevation it is ±10 m. 

4
Table 1. Audio-magnetotelluric station coordinates.

[m, meter; X direction, electrode-array direction in degrees clockwise from true north]

Station X 

direction Longitude Latitude North(m)

East

(m)

Elevation 

(m)

01 19 –110°47′21′′ 31°26′32′′ 3,478,446 520,029 1,377

03 241 –110°44′48′′ 31°27′15′′ 3,479,792 524,068 1,737

04 0 –110°44′34′′ 31°27′33′′ 3,480,321 524,436 1,718

05 320 –110°44′08′′ 31°27′41′′ 3,480,590 525,131 1,622

06 45 –110°43′40′′ 31°27′28′′ 3,480,174 525,872 1,561

07 208 –110°41′19′′ 31°28′17′′ 3,481,710 529,577 1,551

Audio-Magnetotelluric Data Collected in the Sunnyside 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 are 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 plots 

6. Tipper magnitude 

7. Tipper strike 

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

Error bars (],[) on the diagrams of 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). 

5
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 plot, 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. 

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.

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

more than 5 km. 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 plots.

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, where values at 0.5 signify signal levels equal to noise 

levels. For this data set, coherencies were generally above 0.9, except in the higher frequencies 

(greater than about 1 kHz) for stations 1, 3, 4, and 7. 

The impedance polar plots 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 three-dimensional 

resistivity structures, the principal impedance polar diagram (dashed line) elongates either 

parallel to or perpendicular to strike direction. 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 polar diagram (solid line) attains the shape of a symmetric 

clover leaf. For three-dimensional 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 plots at each frequency should also 

take into account the corresponding coherence and skew values along with their associated error 

levels. The polar plots computed for our data show the electromagnetic response for all stations 

was three-dimensional throughout all frequencies measured at acceptable noise levels.

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 

6
between 0.1 and 0.5, and rarely is as great as 1 as it responds to vertical and subvertical 

structures. The tipper strike 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 plots. 

7
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. 

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. 

Graybeal, F.T., 1996, Sunnyside, a vertically preserved porphyry copper system, Patagonia 

Mountains, Arizona: Society of Economic Geologists Newsletter, no. 26, p. 1, 10–14. 

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. 

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: Geophysics 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. 

8
Appendix

Audio-Magnetotelluric Data Diagrams

Data for each station are 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 plots (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 “Audio-Magnetotelluric Data Collected in the Sunnyside Survey” section in this 

report for an explanation of these diagrams. 

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