Magnetotelluric Data, San Luis Valley, 

Colorado 

By Brian D. Rodriguez and Jackie M. Williams 

Open-File Report 2007–1405 

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 2008

For product and ordering information: 

World Wide Web: http://www.usgs.gov/pubprod 

Telephone: 1-888-ASK-USGS 

For more information on the USGS—the Federal source for science about the Earth, 

its natural and living resources, natural hazards, and the environment: 

World Wide Web: http://www.usgs.gov 

Telephone: 1-888-ASK-USGS 

Suggested citation: 

Rodriguez, B.D. and Williams, J.M., 2008, Magnetotelluric data, San Luis Valley, 

Colorado: U.S. Geological Survey Open-File Report 2007-1405, 227 p. 

Any use of trade, product, or firm names is for descriptive purposes only and does not 

imply endorsement by the U.S. Government. 

Although this report is in the public domain, permission must be secured from the 

individual copyright owners to reproduce any copyrighted material contained within 

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 ..........................................................................................................11

3
Introduction 

The San Luis Valley region population 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 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 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 located in southern Colorado. Detailed geologic mapping, high-resolution 

airborne magnetic surveys, gravity surveys, an electromagnetic survey (called magnetotellurics , 

or MT), and hydrologic and lithologic data are being used to better understand the aquifers. The 

MT survey primary goal is to map changes in electrical resistivity with depth that are related to 

differences in rock types. These various rock types help control the properties of aquifers. This 

report does not include any data interpretation. Its purpose is to release the MT data acquired at 

24 stations shown in figure 1. Two of the stations were collected near Santa Fe, New Mexico, 

near deep wildcat wells. Well logs from those wells will help tie future interpretations of this 

data with geologic units from the Santa Fe Group sediments to Precambrian basement. 

Electrical Rock Properties 

Electromagnetic geophysical methods detect variations in the electrical properties of 

rocks. Electrical resistivity, or its inverse, electrical conductivity, is 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 electrically conductive clay minerals, graphitic 

carbon, and metallic mineralization. For example, it is common 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 (Palacky, 1987). Metamorphic rocks and unaltered, unfractured igneous rocks are 

normally moderately to highly resistive (hundreds to thousands of ohm-m). Carbonate rocks can 

also have high resistivities depending on their fluid content, porosity, and impurities (Keller, 

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). Tables of electrical resistivities for a variety of 

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

4
B 

A 

Figure 1. A. Magnetotelluric (MT) stations in the San Luis Valley Area. Open squares 

(23-44) are MT stations collected in July 2007. Open circles (1-22) are MT stations from 

Williams and Rodriguez (2007). B. MT stations (01-02) collected near Santa Fe, New 

Mexico. Solid black circles are deep wildcat wells. 

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 resistivity 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 of 2006 we collected 22 MT stations in the San Luis Valley of south central 

Colorado (numbered 23-44, fig. 1). Two additional stations (01 and 02, fig. 1) were collected 

west of Santa Fe, New Mexico, near deep wildcat wells. Well logs from those wells will help tie 

future interpretations of this data with geologic units from the Santa Fe Group sediments to 

Precambrian basement. The station locations were chosen to constrain the geologic and 

hydrostratigraphic interpretation, for proximity to roads, and to avoid, where possible, electrical 

noise from power lines and vehicles. The MT data were collected with 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, mu-metal-cored 

induction coils. The MT data were recorded as non-remote referenced single stations. 

Table 1 lists the 24 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 

6
Easting units are in meters. The accuracy of the x, y component is about ±5 m. The accuracy of 

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

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

Table 1. Magnetotelluric Station Locations 

Station Latitude 

( d° m’ s” ) 

Longitude 

( d° m’ s” ) 

Elevation

(m) 

Northing 

(m) 

Easting 

(m) 

X-Dir 

(deg) 

01 35° 22' 37.3" 106° 06' 16.3" 1905 3915216 399671 143 

02 35° 38' 29.1" 107° 08' 05.3" 1866 3946078 306708 64 

23 37° 53' 02.7" 105° 41' 55.1" 2336 4192978 438564 90 

24 37° 51' 29.2" 105° 37' 37.3" 2402 4190052 444841 165 

25 37° 50' 12.9" 105° 42' 13.3" 2331 4187750 438080 166 

26 37° 53' 25.7” 105° 46' 52.1" 2304 4193745 431314 149 

27 37° 47' 58.3" 105° 48' 34.1" 2297 4183676 428735 91 

28 37° 40' 52.7" 105° 35' 27.1" 2360 4170417 447898 181 

29 37° 45' 37.0" 105° 38' 12.8" 2331 4179204 443899 198 

30 37° 02' 04.4" 105° 47' 30.4" 2323 4098796 429580 249 

31 37° 01' 43.8" 105° 51' 01.3" 2326 4098205 424364 188 

32 37° 01' 07.8" 105° 55' 03.3" 2369 4097151 418374 127 

33 37° 13' 56.7" 105° 29' 03.2" 2403 4120563 457047 6 

34 37° 04' 00.7" 105° 19' 19.1" 2873 4102135 471376 265 

35 37° 22' 14.7" 105° 21' 52.5" 2516 4135861 467719 291 

36 37° 28' 33.3" 105° 20' 11.2" 2475 4147518 470252 206 

37 37° 13' 15.9" 105° 50' 57.8" 2369 4119532 424641 215 

38 37° 14' 06.8" 105° 45' 23.3" 2298 4121031 432896 247 

39 37° 01' 07.6" 105° 45' 23.3" 2297 4097019 432705 101 

40 36° 59' 46.9" 105° 59' 25.2" 2448 4094723 411876 296 

41 37° 21' 53.5" 105° 44' 39.5" 2300 4135404 434089 126 

42 37° 25' 43.9" 105° 44' 31.5" 2301 4142505 434342 244 

43 37° 29' 21.7" 105° 38' 19.4" 2318 4149150 443533 273 

44 37° 36' 07.2" 105° 41' 28.6" 2294 4161677 438980 162 

 ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄ ̄

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 orientation (X-dir) 

listed in Table 1 above. 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. 

7
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. 

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. 

8
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 

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 signalto-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. 

9
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. 

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. 

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. 

10
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. 

11