Magnetotelluric Data In the Española

Basin, West of Santa Fe, New Mexico

By Jackie M. Williams and Brian D. Rodriguez

Open-File Report 2005-1037

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY
Contents

Introduction3

Magnetotelluric Method3

Magnetotelluric Survey5

Magnetotelluric Data6

Reference Cited8

Appendix Magnetotelluric Data Plots10

2
Introduction

The Santa Fe region is rapidly growing. 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 Santa Fe Group, the sedimentary deposits that

fill the Rio Grande rift and contain the principal ground-water

aquifers.

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

studies of the Española Basin in northern New Mexico. One

objective of these studies is to understand the hydrogeologic

framework of the Española Basin to help land managers plan and

develop water supplies. These studies involve a

multidisciplinary approach to better understand the critical

aquifers in the intracontinental rift environment. Detailed

geologic mapping, high-resolution airborne magnetic surveys,

electromagnetic surveys, and hydrologic, lithologic, and

hydrogeochemical data are being used to better understand the

aquifer systems in the Española Basin. A magnetotelluric (MT)

survey was done as part of these studies. The primary purpose of

the MT survey was to map changes in electrical resistivity with

depth that are related to lithologic variations important to the

critical aquifers. The purpose of this report is to release the

MT sounding data; no interpretation of the data is included.

Magnetotelluric Method

The MT method is a passive surface geophysical technique

that uses the Earth's natural electromagnetic fields to

investigate the electrical resistivity structure of the

subsurface. The resistivity of geologic units is largely

dependent upon their fluid content, porosity, degree of

fracturing, temperature, and conductive mineral content (Keller,

1989). Saline fluids within the pore spaces and fracture

openings can reduce resistivities in a resistive rock matrix.

Resistivity also can be lowered by the presence of conductive

clay minerals, carbon, and metallic mineralization. It is common

for altered volcanic rocks to contain authigenic minerals that

have resistivities 10 times less than those of the surrounding

rocks (Nelson and Anderson, 1992). Increased temperatures cause

higher ionic mobility and mineral activation energy, reducing

rock resistivities significantly. Unaltered, unfractured igneous

rocks are moderately to highly resistive (hundreds to thousands

of ohm-m), whereas fault zones will show low resistivity (less

than 100 ohm-m) when they are composed of rocks that are

fractured enough to have hosted fluid transport and consequent

mineralogical alteration (Eberhart-Phillips and others, 1995).

Carbonate rocks are moderately to highly resistive (hundreds to

3
thousands of ohm-m) depending upon their fluid content, porosity,

fracturing, and impurities. Marine shales, mudstones, and clayrich alluvium are normally conductive (a few ohm-m to tens of

ohm-m). Unaltered metamorphic rocks (non-graphitic) are

moderately to highly resistive (hundreds to thousands of ohmmeters[ohm-m]). Tables of electrical resistivity for a variety

of rocks, minerals, and geological environments are included in

Keller (1987) and Palacky (1987).

The MT method can be used to probe the crust from depths of

tens of meters to depths of tens of kilometers (Vozoff, 1991).

Natural variations of the Earth's magnetic and electric field are

measured and recorded at each MT station. The primary frequency

bands used by the MT method are 10,000 to 1 hertz (Hz) from

worldwide lightning activity and 1 to 0.0001 Hz from geomagnetic

micro-pulsations. The natural electric and magnetic fields

propagate vertically in the Earth because the large resistivity

contrast between the air and the Earth causes a vertical

refraction of both fields transmitted into the Earth (Vozoff,

1972).

The natural electric and magnetic fields are recorded in two

orthogonal, horizontal directions. The vertical magnetic field

(“tipper”) also is recorded. The resulting time-series signals

are used to derive the tensor apparent resistivities and phases.

First, the signals are converted to complex cross-spectra using

Fast-Fourier-transform (FFT) techniques. Then, least-squares,

cross-spectral analysis (Bendat and Piersol, 1971) is used to

solve for a transfer function that relates the observed electric

fields to the magnetic fields with the assumption that the Earth

consists of a two-input, two-output linear system, with the

magnetic fields as input and the electric fields as output.

Prior to conversion to apparent resistivity and phase, the tensor

is normally rotated into principal directions that correspond to

the direction of maximum and minimum apparent resistivity. For a

two-dimensional (2-D) Earth, the MT fields can be de-coupled into

transverse electric (TE) and transverse magnetic (TM) modes; 2-D

modeling generally is done to fit both modes. When the geology

satisfies the 2-D assumption, the MT data for the TE mode are for

the electric field parallel to geologic strike, and the data for

the TM mode are for the electric field 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.

The MT method is capable of establishing whether the

electromagnetic fields are responding to subsurface terranes of

effectively one, two, or three dimensions. An introduction to

the MT method and references for a more advanced understanding

are in Dobrin and Savit (1988) and Vozoff (1991).

4
5

Magnetotelluric Survey

Data were collected at 18 stations in 2002 and 2003 to

represent the area of this study. The station locations were

chosen for constraining gravity modeling across the fault, and

for proximity to roads, and for avoidance of electrical noise

such as power lines. All data at the stations were collected

with a portable EMI MT-1 system (EMI, 1996). Horizontal electric

fields were sensed 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

permalloy-cored induction coils. Frequencies sampled ranged from

4 to 23,000 Hz (4.3945, 7.3242, 12.2070, 19.0430, 28.3203,

41.5039, 60.0586, 79.0000, 85.9375, 100.0000, 122.0703, 150.0000,

172.3633, 210.0000, 270.0000, 340.0000, 460.0000, 580.0000,

720.0000, 885.0000, 1170.0000, 1500.0000, 1870.0000, 2200.0000,

2730.0000, 3550.0000, 4900.0000, 6500.0000, 9000.0000,

11590.0000, 15290.0000, 19500.0000, 23370.0000) and 0.009 to 70Hz

(0.0088, 0.0146, 0.0244, 0.0381, 0.0566, 0.0830, . 0879, 0.1201,

0.1465, 0.1719, 0.2441, 0.3447, 0.3809, 0.5664, 0.8301, 1.2012,

1.7578, 2.4414, 2.9297, 3.4473, 4.8828, 7.6172, 11.3281, 16.6016,

24.0234, 34.3750, 48.8281, 68.9453)usingsingle-station

recordings of the orthogonal, horizontal components of the

electric and magnetic fields and the vertical magnetic field.

The following table lists the 18 MT station locations as

recorded using a global positioning system during field

acquisition. 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 are in meters. Station

elevation is given in meters. The accuracy of the x, y, z

component is ±5 m.

UTM UTM

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

8 -106:12:08 35:37:25 3,942,655 13,391,127 2017

9 -106:09:31 35:44:24 3,955,517 13,395,221 2030

10 -106:10:05 35:41:53 3,950,901 13,394,319 2170

11 -106:11:19 35:38:47 3,945,184 13,392,391 2025

12 -106:23:20 35:46:03 3,958,861 13,374,449 2381

13 -106:07:58 35:39:16 3,946,026 13,397,459 1976

16 -106:06:50 35:42:27 3,951,868 13,399,230 2002

17 -106:21:42 35:41:09 3,949,770 13,376,785 1760

18m -106:16:37 35:23:32 3,917,087 13,384,020 1845

19m -106:10:09 35:34:20 3,936,915 13,394,057 1886

20m -106:11:36 35:36:16 3,940,539 13,391,897 1986

21m -106:11:52 35:38:16 3,944,249 13,391,560 2010

22m -106:09:31 35:43:35 3,954,039 13,395,225 2042

23m -106:09:58 35:42:48 3,952,588 13,394,527 2100

24m -105:56:01 35:24:25 3,918,370 13,415,194 1860

25m -106:10:09 35:40:42 3,948,914 13,394,185 2100

26m -106:07:07 35:40:08 3,948,043 13,398,749 1973

27m -106:09:22 35:47:06 3,960,730 13,395,460 2010
6

Magnetotelluric Data

The recorded time-series data were transformed to the

frequency domain and processed to determine a 2-D apparent

resistivity and phase tensor at each site. Rotation of the

impedance tensor to maximum and minimum directions allows for

decoupling into the TE and TM modes.

Although true remote reference techniques were not used

in the survey, cross-power files were sorted to select optimal

signal-to-noise time-series data sets (see Appendix at the back

of the report).

The effects of near-surface resistivity anomalies caused

“static shifts” in the data (Sternberg and others, 1988). Static

shifts were significant at stations 9, 13, 16, 20, 22, 24, and

27. Cultural features can affect the response of the MT system.

Fences, pipelines, communication lines, railways, and other

manmade conductors can contaminate the responses.

The figures in the Appendix represent the field-processed MT

data for each station after the time-series data were converted

to the frequency domain, and the tensor-transfer function was

rotated into principal directions as described in the

“Magnetotelluric Method” section.

For each station, nine separate plots are given:

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

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

3. Rotation Angle

4. Impedance Skew

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

6. Impedance Polar Plots

7. Tipper Magnitude

8. Tipper Strike

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

Error bars (],[) on the Apparent Resistivity, Impedance

Phase, Skew, Tipper Magnitude, and Tipper Strike plots represent

probable errors within one standard deviation of the sample

variance (Gamble and others, 1979).

Apparent resistivity is the 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, but

from 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 a 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 (above 0.2)

are an indication of either the resistivity response to 3-D

geology or higher levels of noise. Manmade electrical noise,

such as power lines, power generators,and moving vehicles and

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

these local disturbances produce an incoherent noise mainly

affecting frequencies above 1 Hz. Other manmade electrical

noise, such as direct current electric trains and active cathodic

protection of pipelines, produce coherent electromagnetic signals

mainly affecting frequencies below 1 Hz.

In the survey 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 signal 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 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 minimized wind noise.

Predicted values of the electric field can be computed from

the measured values of the magnetic field (Vozoff, 1991). The

coherence of the predicted electric field with the measured

electric field is a measure of the signal-to-noise ratio provided

in the multiple coherency plots. Values are normalized between 0

and 1, where values at 0.5 signify signal levels equal to noise

levels. For this data set, coherencies were generally at an

acceptable level, except at times in the frequency range “dead

band” (0.01 to 5 HZ).

The figures in the Appendix represent the field-processed MT

data at each station, which includes some data scatter and poor

signal-to-noise ratios. The only effort at removing noisy data

points was to visually inspect and select the best signal-tonoise 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 polar diagram (dashed line)

is a circle. For 2-D or 3-D resistivity structures, the

principal impedance polar diagram (dashed line) elongates either

parallel or perpendicular to strike direction. Over resistors,

the principal impedance polar diagram elongates perpendicular to

strike direction, and over conductors, 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

7
diagram (dashed line). Station 27 indicates a 3-D response over

all frequencies measured.

The tipper can be calculated when the vertical component of

the magnetic field is measured. 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 and typically increases between 0.1 to 0.5, and rarely

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 was 0.1 to 0.6 over

the lower frequencies, indicating some vertical structure at

depth. The HzHx and HzHy coherency is a measure of the signalto-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 at 0.5

signify signal levels equal to noise levels. These threecomponent magnetic-field coherencies provide a check on the

signal-to-noise ratio of the measured values in the tipper

magnitude and tipper strike plots.

References Cited

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

and measurement procedures: New York, Wiley Interscience,

407p.

Dobrin, M.D., and Savit, C.H., 1988, Introduction to

Geophysical prospecting (4th ed.): New York, McGraw-Hill,

867p.

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, 1996, MT-1 magnetotelluric system operation manual, version

3.2: Richmond, California, 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.

Keller, G.V., 1987, Rock and mineral properties, in Nabighian,

M.N., ed., Electromagnetic methods in applied geophysics

theory: Tulsa, Oklahoma, Society of Exploration

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

8
Keller, G.V., 1989, Electrical properties, in Carmichael, R.S.,

ed., Practical handbook of physical properties of rocks and

minerals: Boca Raton, Florida, 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. 6823-6841.

Palacky, G.J., 1987, Resistivity characteristics of geologic

targets, in Nabighian, M.N., ed., Electromagnetic methods in

applied geophysics theory: Tulsa, Oklahoma, Society of

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

Reddy, I.K., Rankin, David, and Phillips, R.J., 1977, Threedimensional modelling in magnetotelluric and magnetic

variational sounding: Geophysics Journal of the Royal

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

Sternberg, B.K., Washburne, J.C., and Pellerin, Louise, 1988,

Correction for the static shift in magnetotellurics using

transient electromagnetic soundings: Geophysics, v. 53, p.

1459-1468.

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,

Oklahoma, Society of Exploration Geophysicists, v. 2, pt. B,

p. 641-711.

9
Appendix

Magnetotelluiric Data Plots

There are nine separate plots for each station:

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

minimum (o symbol) modes

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

(o symbol) modes

3. Rotation Angle for the impedance tensor (corresponds to the

direction of maximum apparent resistivity)

4. Impedance Skew for the impedance tensor

5. Multiple Coherency for the rotated maximum (x symbol) and

minimum (o symbol) modes of the electric field

6. Impedance Polar Plots (at 12 selected frequencies)

7. Tipper Magnitude for the vertical magnetic field

8. Tipper Strike for the vertical magnetic field

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

Refer to the “Magnetotelluric Data” section in this report

for an explanation of these plots.

10