Magnetotelluric Data, Taos Plateau Volcanic Field, New 

Mexico 

By Chad E. Ailes and Brian D. Rodriguez 

Open-File Report 2010–1245 

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:
 

Ailes, C.E., and Rodriguez, B.D., 2010, Magnetotelluric data, Taos Plateau Volcanic Field, New Mexico: United States 

Geological Survey Open-File Report 2010–1245, 65 p.
 

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Contents 

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

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

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

Magnetotelluric Survey...................................................................................................................................................4
 

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

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

Appendix 

Appendix 1. Magnetotelluric Data Plots .........................................................................................................................9
 

Figure
 Figure 1. Magnetotelluric profile across the Taos Plateau Volcanic Field, New Mexico ..........................................2
 

Table
 Table 1. Magnetotelluric station coordinates. .........................................................................................................5
 

iii 
Conversion Factors 

Inch/Pound to SI 

Multiply By To obtain 

Length 

foot (ft) 0.3048 meter (m) 

mile (mi) 1.609 kilometer (km) 

mile, nautical (nmi) 1.852 kilometer (km) 

yard (yd) 0.9144 meter (m) 

SI to Inch/Pound 

Multiply By To obtain 

Length 

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi) 

kilometer (km) 0.5400 mile, nautical (nmi) 

meter (m) 1.094 yard (yd) 

Vertical coordinate information is referenced to the 1866 Clarke Spheroid.
 

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

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

iv 
Magnetotelluric Data, Taos Plateau Volcanic Field, New 

Mexico 

By Chad E. Ailes and Brian D. Rodriguez 

Introduction 

The population of the San Luis Basin region of northern New Mexico is growing. Water 

shortfalls could have serious consequences. Future growth and land management in the region depend 

on accurate assessment and protection of the region’s groundwater resources. An important issue in 

managing the groundwater resources is a better understanding of the hydrogeology of the Santa Fe 

Group and the nature of the sedimentary deposits that fill the Rio Grande rift, which contain the 

principal groundwater aquifers. The shallow unconfined aquifer and the deeper confined Santa Fe 

Group aquifer in the San Luis Basin are the main sources of municipal water for the region. 

The U.S. Geological Survey (USGS) is conducting a series of multidisciplinary studies of the 

San Luis Basin. Detailed geologic mapping, high-resolution airborne magnetic surveys, gravity surveys, 

an electromagnetic survey called magnetotellurics (MT), and hydrologic and lithologic data are being 

used to better understand the aquifers. This report describes a regional east-west MT sounding profile 

acquired in late July 2009 across the Taos Plateau Volcanic Field where drillhole data are sparse (fig. 1). 

Resistivity modeling of the MT data can be used to help map changes in electrical resistivity with 

depths that are related to differences in rock types. These various rock types help control the properties 

of aquifers. The purpose of this report is to release the MT sounding data collected along the east-west 

profile. No interpretation of the data is included. 

1
Figure 1. Magnetotelluric profile across the Taos Plateau Volcanic Field, New Mexico. Magnetotelluric stations 

acquired in late July 2009 are numbered yellow squares (stations 6 and 8 not acquired). Blue circle is No Agua 

well. Base map from La Segita Peaks, Cerro de la Olla, Sunshine, Cerro, Tres Piedras, Tres Piedras NE, 

Guadalupe Mountain, and Questa, New Mexico, 1:24,000 topographic quadrangles. 

2
Electrical Rock Properties 

Electromagnetic geophysical investigation methods detect variations in the electrical properties 

of rock units, in particular electrical resistivity, which is measured in units of ohm-meters (Wm), 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 using lithologic logs to provide a three dimensional 

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

upon their fluid content, pore volume porosity, interconnected fracture porosity, and conductive mineral 

content (Keller, 1987). 

Although there is not a one-to-one relationship between lithology and resistivity, there are 

general correlations that can be made using typical values, even though values can be found at other 

geographic locations (Palacky, 1987) that may fall outside of the ranges presented below. Fluids within 

the pore spaces and fracture openings, especially if saline, can reduce resistivities in what would 

otherwise be a resistive rock matrix (Hallenburg, 1998; Hearst and Nelson, 1985; Hearst and others, 

2000; Keller, 1987; Keller and Frischknecht, 1966; Palacky, 1987). Resistivity can also be lowered by 

the presence of electrically conductive clay minerals, graphitic carbon, and metallic mineralization. It is 

common, for example, for altered volcanic 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, 

with resistivities ranging from a few Wm to tens of Wm (Keller, 1987; Palacky, 1987). Metamorphic 

rocks (non graphitic) and unaltered, unfractured igneous rocks are normally moderately to highly 

resistive (a few hundred to thousands of Wm). 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 Wm) 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 cause higher ionic mobility that reduces rock resistivities 

(Keller, 1987; Palacky, 1987). 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 magnetotelluric method is a passive surface geophysical technique that uses 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). Natural variations of the Earth's magnetic 

and electric fields are measured and recorded at each magnetotelluric station. Worldwide lightning 

activity at frequencies of about 1 to 20,000 Hertz and geomagnetic micro-pulsations at frequencies of 

about 0.0001 to 1 Hertz provide the majority of the signal sensed by the magnetotelluric 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 surface 

(Vozoff, 1972). 

The orthogonal horizontal electric and magnetic field components (Ex, Ey, Hx, and Hy) and the 

vertical magnetic field component (Hz) are recorded. The resulting time-series signals are used to 

derive tensor apparent resistivities and phases after first converting them to complex cross-spectra 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, twooutput 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 observed electric fields to the 

magnetic fields. For resistivity modeling, magnetotelluric data are normally rotated into directions that 

3
are parallel and perpendicular to the subsurface geologic strike. Subsurface geologic strike can be 

estimated by determining the horizontal direction (Hxy or Hyx) that the vertical magnetic field (Hz) 

“tips” (the Tipper strike direction). 

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

transverse magnetic (TM) modes. Two-dimensional resistivity modeling generally is computed to fit 

both modes. When the geology satisfies the two-dimensional assumption, the MT data for the TE mode 

are assumed to represent the electric field oriented along geologic strike, and the data for the TM 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 relations 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 1, 2, or 3 dimensions. An 

introduction to the magnetotelluric method and references for a more advanced understanding are in 

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

Magnetotelluric Survey 

Seven MT soundings were collected in July 2009 along a 35-km-long east-west profile in 

northern New Mexico (fig. 1). The profile starts north of the No Agua Peaks about 2.5 kilometer east of 

State Highway 285, then continues easterly, until a bend south of Cerro de la Olla mountain, then 

continues east across the Rio Grande Gorge, and ends in the Sangre de Cristo Mountains about 3 km 

east of State Highway 522, east of the town of Cerro and northeast of the town of Questa, New Mexico. 

The profile location was selected to map electrical resistivity with depth related to the properties of 

aquifers beneath the Taos Plateau Volcanic Field. 

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. 

(EMI), MT-1 system (Electromagnetic Instruments, Inc., 1996). Horizontal electric fields were recorded 

using copper sulfate 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 electricfield array and were sensed using high magnetic permeability mu-metal-cored induction coils. 

Frequencies were sampled from about .01 Hertz to about 100 Hertz at each station. Station 9 was 

located close to the No Agua well to help calibrate future resistivity modeling of the MT data. The No 

Agua well is a 354 meters test well with lithologic descriptions that encountered granite at 350 meters 

feet. Table 1 lists the seven MT station locations. Stations 6 and 8 were not acquired because of site 

access problems. Remote reference stations were not used because manmade noise sources were 

assumed to be minimal in this remote area. 

4
 

 

 

 

 

 

 

 

Table 1. Magnetotelluric station coordinates. [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 and is ±10 m for the elevation. X direction (X Dir.) is in 

degrees clockwise from true north] 

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

01 

02 

03 

04 

05 

07 

09 

295 

004 

171 

041 

005 

279 

141 

-105:33:40 

-105:37:38 

-105:39:08 

-105:42:05 

-105:44:30 

-105:50:48 

-105:57:08 

36:45:11 

36:43:09 

36:44:15 

36:43:32 

36:42:25 

36:47:10 

36:46:52 

4,067,415 

4,063,693 

4,065,746 

4,064,453 

4,062,421 

4,071,274 

4,070,799 

449,925 

443,996 

441,778 

437,367 

433,753 

424,446 

415,019 

2,608 

2,325 

2,353 

2,300 

2,317 

2,363 

2,537 

Magnetotelluric Data 

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 TE and TM modes. The data provided 

here have not been rotated from the original acquisition orientation (X Dir.) listed in table 1 above. 

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 1). 

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 is taken to avoid these sources of noise when acquiring the data. 

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

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

developed. 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. 

For each station, eight separate plots are given: 

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 

5
Error bars (],[) on the Apparent Resistivity, Impedance Phase, Impedance 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 ratio at a given frequency of the electric field strength magnitude to 

the magnetic field strength magnitude. The impedance phase is proportional to the slope of the apparent 

resistivity curve on a log-log plot, relative to a baseline at –45o

 (Vozoff, 1991). A measure of the 

dimensionality for magnetotelluric data is provided by the impedance skew of the impedance tensor 

(Vozoff, 1972). If the effective, measured resistivity response to the geology beneath an 

magnetotelluric station is truly one- or two-dimensional, then the skew will be zero. Both instrumental 

and environmental sources of noise contribute to non zero 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 3-D geology or higher levels of noise. 

In the study area, noise from a number of small powerlines and small moving vehicles was 

negligible at distances of 0.25 km and farther from the noise source. Powerline amplitude levels 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 greater than 

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

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

keeping the electric dipole wires flat on the ground helped to minimize 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, where values at 0.5 signify signal levels equal to noise levels. For this data set, 

coherencies were generally above 0.75, except in the middle frequencies (from about 0.1 Hertz to 5 

Hertz) at most stations, and also in the lower frequencies for station 7, where a nearby thunderstorm 

made it difficult to collect low-frequency data with high amplifier gains. 

The figures in appendix 1 represent the field-processed magnetotelluric data at each station and 

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

points was to visually inspect and digitally select the best signal to noise field data to combine into the 

final data plots. 

The impedance polar plots provide a measure of the magnetotelluric data dimensionality (Reddy 

and others, 1977). For 1-D resistivity structures, the principal impedance (off diagonal elements) 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 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 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), although high noise levels can produce the same effect on the polar diagram. A 3-D 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 the data show the 

electromagnetic response for all stations was 3-D over the middle to low frequencies measured at 

acceptable noise levels. The electromagnetic response for stations 2, 3, and 4 was 3-D over all 

frequencies. 

6
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 0 for the 1-D case, typically increases between 0.1 to 0.5, and rarely is as large as 1 as 

it responds to vertical and subvertical structures. The tipper strike typically is used to help resolve the 

90o ambiguity in the impedance rotation angle. The tipper magnitude of these stations were all above 

0.1 indicating vertical and sub-vertical 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, 

N.Y., 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. 

EMI, 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. 

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

Fla., Lewis Publications, 265 p. 

Hearst, J.R., and Nelson, P.H., 1985, Well logging for physical properties: McGraw-Hill Book Co., 

New York, NY, 571 p. 

Hearst, J.R., Nelson, P.H., and Paillet, F.L., 2000, Well logging for physical properties, 2nd edition: 

New York, John Wiley & Sons, 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. 

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., ed., Electromagnetic methods 

in applied geophysics: Tulsa, Okla., Society of Exploration Geophysicists, v. 2, pt. B, p. 641-711. 

8
 

Appendix 1. Magnetotelluric Data Plots 

There are eight separate plots for each station: 

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 

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

plots. 

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