Magnetotelluric Data, Southern San Luis 

Valley, Colorado 

By Jackie M. Williams and Brian D. Rodriguez 

Open-File Report 2007–1291 

U.S. Department of the Interior 

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

DIRK KEMPTHORNE, Secretary 

U.S. Geological Survey 

Mark D. Myers, Director 

U.S. Geological Survey, Reston, Virginia 2007 

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under Interagency Agreement DE-AI52-07NV28100. 

Suggested citation: 

Williams, J.M. and Rodriguez, B.D., 2007, Magnetotelluric data, southern San Luis 

Valley, Colorado: U.S. Geological Survey Open-File Report 2007-1291, 208 p. 

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this report. 

2
 
Contents
 

Introduction .................................................................................................................................4
 

Electrical Rock Properties .............................................................................................................4
 

Magnetotelluric Method.................................................................................................................6
 

Magnetotelluric Survey..................................................................................................................6
 

Magnetotelluric Data .....................................................................................................................7
 

References....................................................................................................................................9
 

Figure 

1. Index image showing magnetotelluric stations in the San Luis Valley Area ............................. 5
 

Table 

1. Magnetotelluric Station Locations..............................................................................................7
 

Appendix 

Magnetotelluric Data Plots ..........................................................................................................10
 

3
 
Introduction 

The population of the San Luis Valley region is growing rapidly. The shallow 

unconfined and the deeper confined Santa Fe Group aquifer in the San Luis Basin is the main 

sources of municipal water for the region. 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 and the nature of 

the sedimentary deposits that fill the Rio Grande rift, which contain the principal ground-water 

aquifers. 

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

the San Luis Basin located in southern Colorado. 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 aquifer systems. 

The primary goal of the MT survey is to map changes in electrical resistivity with depth that are 

related to differences in rock type. These various rock types help control the properties of 

aquifers in the region. This report does not include any interpretation of the data. Its purpose is 

to release the MT data acquired at the 22 stations shown in figure 1. 

Electrical Rock Properties 

Electromagnetic geophysical methods detect variations in the electrical properties of 

rocks. Electrical resistivity, or its inverse, electrical conductivity, are of particular interest. 

Electrical resistivity can be correlated with geologic units on the surface and at depth 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). 

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

general correlations that can be made using typical resistivity values even though values vary 

from one location to another (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. Resistivities 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 from a few ohm-meters 

(ohm-m) to a few tens of ohm-m (Keller, 1987; Palacky, 1987). Metamorphic rocks 

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

resistive (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 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 resistivities for a variety of rocks, 

minerals, and geological environments may be found in Keller (1987) and Palacky (1987). 

4
 
Figure 1. Location of magnetotelluric stations in the San Luis Valley Area. 

5
 
Magnetotelluric Method 

The MT method is a passive-surface electromagnetic geophysical technique that 

measures variations in 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). 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. A small amount of electromagnetic energy reflects and propagates vertically 

into the earth due to the resistivitiy contrast at the air-earth interface (Vozoff, 1972). 

The natural fields are recorded in the xyz direction for the magnetic field and the xy 

direction for the electric field at the Earth’s surface. 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) is 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’s 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; 2-D 

resistivity modeling is generally computed to fit both modes. When the geologic structures are 

assumed to be primarily 2-D, the MT data for the TE mode represents electric fields that are 

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 for a more advanced understanding are contained in Dobrin and Savit 

(1988) and Vozoff (1991). 

Magnetotelluric Survey 

In July and August of 2006 we collected 22 MT stations in the southern San Luis Valley 

(fig. 1) of south central Colorado. The station locations were chosen to constrain the 

geologic/hydrostratigraphic interpretation, for proximity to roads, and to avoid, where possible, 

electrical noise from power lines and vehicles. The MT data were collected with an 

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

were measured using three copper/copper sulfate porous-pot electrodes placed in an L-shaped 

array with dipole lengths of 30 meters (m). The orthogonal magnetic fields in the direction of the 

electric-field measurement array were sensed using EMI’s high-magnetic-permeability, mumetal-cored induction coils. The MT data were recorded as non-remote referenced single 

stations. 

Table 1 lists the 22 MT station locations as recorded using a GPS 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. Station 

elevation is given in meters. Universal Transverse Mercator (UTM) Zone 13 Northing and 

Easting units are in meters. The accuracy of the x, y component is ±5 m. The accuracy of the z 

6
 
component is ±10 m. The X direction is given in degrees and defines the direction of the 

measured magnetic (Hx) and electric (Ex) fields. 

Table 1. Station Locations 

Latitude Longitude Elevation Northing Easting X Station (d:m:s) (d:m:s) (m) (m) (m) dir 

1 37:05:06.9 105:14:46.6 3357 4104156 478113 239 

2 37:06:24.7 105:52:55.5 2369 4106886 421623 153 

3 37:05:54.3 105:50:52.5 2382 4105927 424527 133 

4 37:01:16.5 105:29:24.2 2612 4097140 456409 269 

5 37:01:16.1 105:27:14.6 2596 4097117 459608 333 

6 37:02:24.0 105:24:17.9 2559 4099182 463985 182 

7 37:03:01.1 105:33:54.3 2342 4100400 449752 183 

8 37:00:35.0 105:45:03.9 2302 4095989 436144 95 

9 37:00:36.9 105:39:48.6 2298 4096014 440973 185 

10 37:04:39.6 105:47:00.6 2322 4103572 430355 5 

11 37:05:17.2 105:48:44.0 2366 4104752 427800 19 

12 37:21:59.6 105:29:30.8 2390 4135445 456444 4 

13 37:20:42.3 105:31:58.8 2378 4133085 452788 186 

14 37:19:39.3 105:36:03.1 2349 4131175 446771 2 

15 37:16:14.1 105:40:01.3 2347 4124895 440858 283 

16 37:17:39.5 105:28:41.8 2410 4127423 457608 5 

17 37:14:32.4 105:37:14.7 2382 4121733 444942 201 

18 37:23:47.7 105:40:52.0 2317 4138885 439714 137 

19 37:05:05.1 105:28:38.0 2659 4104178 457586 81 

20 37:08:30.4 105:28:09.0 2587 4110500 458332 306 

21 37:04:59.8 105:34:06.0 2352 4104061 449483 151 

22 37:09:20.7 105:37:39.3 2344 4112132 444274 272 

 ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄

Magnetotelluric Data 

The recorded time-series data were transformed 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 (X dir) listed in

Table 1. During the analysis and interpretation process, each station will 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). 

The effects of near-surface resistivity anomalies can cause what are known as “static 

shifts” in the data (Sternberg and others, 1988). Cultural features also can affect the measured 

magnetotelluric responses. These features include fences, pipelines, communication lines, 

railways, and other manmade conductors. 

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 the tensor-transfer function was 

developed. 

7 
For each station, nine 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. Rotation Angle 

4. Impedance Skew 

5. Multiple Coherency (x and o 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 approximate ratio of the electric-field strength to the magneticfield strength at a given frequency. The impedance phase is proportional to the slope of the 

apparent resistivity curve on a log-log plot, but from baselines 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 truly is one or two dimensional, then the skew will be zero. Instrumental and 

environmental sources of electrical noise can cause non–zero skew values. Skew values typically 

are small (about 0.1) for relatively low-noise 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 can produce incoherent 

noise that mainly affects frequencies above 1 Hz. Other manmade electrical noise, such as 

direct-current electric trains and active cathodic protection of pipelines, produces coherent 

electromagnetic signals that mainly affect frequencies below 1 Hz. 

In the survey area, noise from small power lines and small moving vehicles was 

negligible at distances greater than 0.4 km 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 may also degrade data quality. 

Burying the magnetic induction coils and the electric dipole wires 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; 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 ranges of about 0.1 to 5 Hz (traditionally referred to as the “dead band”). 

The field-processed MT data include some scatter and poor signal-to-noise ratios. 

Spectral results were inspected visually for noisy data, and the best signal-to-noise field data 

were combined into the final plots. 

The magnetotelluric impedance polar plots provide a measure of 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 

8
 
 

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, it 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). 

The magnetotelluric “tipper” is 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). It will equal zero for the 1-D case. Typically, tipper value 

increases from 0.1 to 0.5 and seldom approaches 1, as it responds primarily to vertical and 

subvertical structures. The tipper strike is used to help resolve the 90-degree ambiguity in the 

impedance rotation angle. 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 magneticfield directions. Values are normalized between 0 and 1; values at 0.5 signify signal levels equal 

to noise levels. These three-component 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 

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. 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. 

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. 

9 
 

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. 

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. 980–1041. 

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. 

Appendix 1 Magnetotelluric Data Plots 

There are nine separate plots for each station: 

1. Apparent Resistivity for the x-y direction (x symbol) and y-x direction (o symbol) modes 

2. Impedance Phase for the x-y direction (x symbol) and y-x direction (o symbol) modes 

3. Rotation Angle for the impedance tensor (corresponds to the x-y direction) 

4. Impedance Skew for the impedance tensor 

5. Multiple Coherency for the x-y direction (x symbol) and y-x direction (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