Magnetotelluric Data Collected Near 

Geophysically Logged Boreholes in the 

Española and Middle Rio Grande Basins, 

New Mexico 

By Jackie M. Williams and Brian D. Rodriguez 

Open-File Report 2006-1120 

U.S. DEPARTMENT OF THE INTERIOR 

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

Gale A. Norton, Secretary 

U.S. Geological Survey

P. Patrick Leahy, Acting Director 

U.S. Geological Survey, Reston, Virginia 2006 

Revised and reprinted: 2006 

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ii
Contents

Introductioṇ̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣1 

Electrical Rock Propertieṣ̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣1

 

Figure 1̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣2

Magnetotelluric Methoḍ̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣3

Magnetotelluric Surveỵ̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣4

 

Magnetotelluric Datạ̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣4

Referenceṣ̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣6

Appendix 1 Magnetotelluric Data Plotṣ̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣̣9

iii
Introduction

The Santa Fe region is growing rapidly. The Santa Fe Group aquifer in the 

Española Basin is the main source of municipal water for the region (fig. 1), and water 

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

region depend on accurate assessment and protection of the region’s ground-water 

resources. An important issue in managing the ground-water resources is a better 

understanding of the hydrogeology of the Tertiary Santa Fe Group. The Santa Fe Group 

includes the sedimentary deposits that fill the Rio Grande rift and contain the principal 

ground-water aquifers. 

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

studies of the Española Basin in northern New Mexico. Detailed geologic mapping, 

high-resolution airborne magnetic surveys, electromagnetic surveys, and hydrologic, 

lithologic, and hydro-geochemical data are being used to better understand the aquifer 

systems. Magnetotelluric (MT) surveys were completed as part of these studies (Williams

and Rodriguez, 2001; 2003; 2005a; 2005b). The primary purpose of the MT surveys was 

to map changes in electrical resistivity with depth that are related to differences in various 

rock types that help control the properties of aquifers in the region. 

Resistivity modeling of the MT data can be used to investigate buried structures 

related to the basic geologic framework of the study area. The purpose of this report is to 

release MT sounding data collected near geophysically logged boreholes in the study 

area, including the nearby Middle Rio Grande Basin. This MT data can be used in 

subsequent resistivity modeling. No interpretation of the data is included in this report. 

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 also at depth 

by using lithologic logs to provide a three-dimensional (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). While 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 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 resistivities in what would otherwise be a 

resistive rock matrix. 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). 

Saturated, fine-grained sediments, such as clay-rich alluvium, marine shales, and other 

mudstones, are normally conductive from a few ohm-meters (ohm-m) to a few tens of 

ohm-m (Keller, 1987; Palacky, 1987). In-situ measurement from local data in Deszcz-Pan 

and others (2000) show that coarser-grained sediments, such as gravels and clean 

1
Figure 1. Magnetotelluric (MT) profile index map of northern New Mexico. MT stations 

acquired in September 1999, July 2002, and July 2004 are numbered solid black boxes. 

Open circles are nearby geophysically logged boreholes (CWKF1-Castle West Kelly 

Federal 1, SSF3 - Shell Santa Fe 3, RR9- Rio Rancho 9, SFP1 - Santa Fe Pacific 1, 

RR18- Rio Rancho 18. Numbered black lines are major highways. Dashed lines are 

Española and Middle Rio Grande basin boundaries. Map datum is Clark 1866 Spheroid, 

NAD 1927 CONUS with geographic projection.

2
sands, are commonly moderately conductive (tens of ohm-m). Others have reported 

similar in-situ resistivity measurements for sands and gravels elsewhere in North 

America (Gilson and others, 2000: Alger, 1971). 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 they are 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 resistivity for a variety of rocks, minerals, and geological environments may 

be found in Keller (1987) and Olhoeft (1989), although these resistivity measurements 

were laboratory measurements, and using them for in-situ measurements is problematic. 

Palacky (1987) presents a figure of typical ranges of resistivities of various earth 

materials, but whether the figure was derived from entirely in-situ measurements is not 

clear. 

Magnetotelluric Method 

The MT method is a passive surface electromagnetic geophysical technique that 

measures variations in the Earth's natural electromagnetic field 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 in 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 electric and magnetic fields are recorded in two orthogonal, horizontal 

directions at the Earth’s surface (the vertical magnetic field is also recorded). 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 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. The 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 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 

3
for a more advanced understanding are contained in Dobrin and Savit (1988) and Vozoff 

(1991). 

Magnetotelluric Survey

Nine MT soundings were collected in September 1999, July 2002, and July 2004 

near geophysically logged boreholes in the study area, including the nearby Middle Rio 

Grande Basin. Station locations were chosen for proximity to roads and to avoid 

electrical noise such as power lines. All data at the stations were collected with a 

portable Electromagnetic Instruments, Inc. (EMI), MT-1 system (EMI, Inc., 1996). 

Horizontal electric fields were recorded using copper sulfate porous pots placed in an 

 L-shaped, three-electrode array with dipole lengths of 30 meters (m). The orthogonal, 

horizontal magnetic fields in the direction of the electric-field measurement array were

sensed using high magnetic permeability, mu-metal-cored induction coils. Frequencies 

were sampled from 0.009 to 70 Hz, using single-station recordings of the orthogonal, 

horizontal components of the electric and magnetic fields and the vertical magnetic field. 

The following table lists the 9 MT station locations. 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 x, y 

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

Table 1 

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

EB01 -106:02:02 35:56:50 3,978,396 13,406,745 1795 

EB02 -105:48:29 35:59:23 3,982,900 13,427,161 2509 

EB03 -105:55:03 35:56:09 3,977,013 13,417,246 2098 

RR01 -106:54:48 35:17:32 3,906,919 13,326,002 1775 

RR02 -106:46:40 35:20:50 3,912,809 13,338,440 1829 

RR20a -106:37:05 35:20:51 3,912,588 13,352,962 1682 

RR21a -106:40:11 35:21:20 3,913,545 13,348,281 1754 

RR22a -106:43:35 35:20:52 3,912,787 13,343,119 1780 

RR23a -106:50:09 35:18:19 3,908,252 13,333,083 1886

Magnetotelluric Data 

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

processed to determine a resistivity and phase tensor at each site. Rotation of the 

impedance tensor to maximum and minimum directions allowed for decoupling into the 

TE and TM modes. 

 Although remote reference techniques were not used in the study, cross-power 

files were sorted to select optimal signal-to-noise time-series data sets (see Appendix 1). 

The effects of near-surface resistivity anomalies can give rise to “static shifts” in 

the data (Sternberg and others, 1988). Static shifts were significant at stations EB02, 

EB03, RR2, RR21a, RR23, and RR23a. Cultural features can furthermore affect the 

response of the MT system. Fences, pipelines, communication lines, power lines, power 

generators, railways, moving vehicles and trains, and other man-made sources of 

electromagnetic noise can contaminate the responses. 

4
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 after the 

tensor-transfer function was rotated into principal directions as described in the 

“Magnetotelluric Method” section of this report. 

For each station, nine separate plots are given:

1. Apparent Resistivity (x and o symbols are xy and yx components, respectively) 

2. Impedance Phase (x and o symbols are xy and yx components, respectively) 

3. Rotation Angle 

4. Impedance Skew 

5. Multiple Coherency (x and o symbols are xy and yx components, respectively) 

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 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 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 power lines and small moving 

vehicles was negligible at distances of 0.4 km and greater from the noise source. 

Power-line amplitude was measured at each site and was typically less than 20 percent of 

the maximum recordable signals. Noise from larger power lines, power generators, 

pipelines, and trains was negligible at distances of more than 5 km from the site. Local 

lightning, wind, and rainstorms also can degrade data quality, but these were avoided by 

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

and keeping the electric dipole wires flat on the ground surface 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 generally were at 

an acceptable level, except at times in the frequency “dead band” (0.01 to 5 Hz) when 

they dropped below 0.75 (Dobrin and Savit, 1988). 

The figures in Appendix 1 represent the field-processed MT data at each station, 

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

5
removing noisy data points was to visually inspect and select the best signal-to-noise 

field data to combine into the final data plots. 

The impedance polar plots provide a measure of the MT 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 take into account the corresponding coherency and skew values along 

with their associated error levels. The polar plots computed for our data show station 

EB01 was 3-D below 0.06 Hz. Station EB02 was 3-D below 0.6 Hz. Station EB03 was 

3-D over all 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 1-D case, typically 

increases to between 0.1 to 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 magnitude of these stations 

typically ranged between 0.1 and 0.6 over the lower frequencies, indicating some 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. 

References 

Alger, R.P., 1971, Interpretation of electric logs in fresh water wells in unconsolidated 

formations: Transactions of the Society of Professional Well Log Analysts 7th

Annual Logging Symposium, May 9-11, 1966, Houston, TX, 24 p. 

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

procedures: New York, NY., Wiley Interscience, 407 p. 

Deszcz-Pan, M., Rodriguez, B.D., Doucette, J.P., Godbout, M., Williams, J.M., 

Sawyer, D.A., Stone, B.D., and Grauch, V.J.S., 2000, Digital airborne 

 time-domain electromagnetic data from surveys over Cochiti Pueblo, Rio Puerco, 

and Rio Rancho, New Mexico: U.S. Geological Survey Open-File Report 00-502 

(CDROM). 

6
Dobrin, M.D., and Savit, C.H., 1988, Introduction to Geophysical Prospecting (4th ed.): 

New York, NY. 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. 

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. 

Gilson, E.W., Nimeck, G., Bauman, P.D., and Kellett, R., 2000, Groundwater exploration 

in prairie environments, in Powers, M.H., Ibrahim, A., and Cramer, L., 

Proceedings of the Symposium on the Application of Geophysics to 

Environmental and Engineering Problems, p. 955-959.

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. 

Olhoeft, G.R., 1989, Electrical properties of rocks, in Touloukian, Y.S., Judd, W.R., and 

 Roy, R.F., eds., Physical Properties of Rocks and Minerals, CINDAS Data Series 

 on Material Properties. Vol.II-2, Hemisphere Publishing, New York, NY, 

p. 257-329.

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. 

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. 

7
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. 98-141. 

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

Methods in Applied Geophysics: Tulsa, Oklahoma, Society of Exploration 

Geophysicists, v. 2, part B, p. 641-711. 

Williams J.M., and Rodriguez, B.D., 2001, Magnetotelluric data in the Middle Rio 

Grande Basin at Cochiti Pueblo, New Mexico: U.S. Geological Survey Open-File 

Report 01-442, 66 p. 

(http://greenwood.cr.usgs.gov/pub/open-file-reports/ofr-01-0442/) 

Williams J.M., and Rodriguez, B.D., 2003, Magnetotelluric data in the southwest 

Española Basin, northern New Mexico: U.S. Geological Survey Open-File Report 

03-199, 128 p. (http://greenwood.cr.usgs.gov/pub/open-file-reports/ofr-03-0199/) 

Williams, J.M., and Rodriguez, B.D., 2005a, Magnetotelluric data in the Española Basin 

west of Santa Fe, New Mexico: U.S. Geological Survey Open-File 

Report 2005-1037, 263 p. (http://pubs.usgs.gov/of/2005/1037/) 

Williams, J.M., and Rodriguez, B.D., 2005b, Magnetotelluric data along the Pajarito 

Fault in the Española Basin west of Santa Fe, New Mexico: U.S. Geological 

Survey Open-File Report 2005-1054, 173 p. (http://pubs.usgs.gov/of/2005/1052/) 

8
Appendix 1

 Magnetotelluric Data Plots 

There are nine 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. Rotation Angle for the impedance tensor (corresponds to the direction of xy 

component) 

4. Impedance Skew for the impedance tensor 

5. Multiple Coherency for the xy (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. 

9