In cooperation with the New Mexico Bureau of Geology and Mineral Resources

Time-Domain Electromagnetic Soundings to Characterize 

Water Quality Within a Freshwater/Saline-Water

Transition Zone, Estancia Valley, New Mexico,

July 2005—A Reconnaissance Study

During July 2005, the U.S. Geological Survey, in cooperation with the New Mexico Bureau of Geology and Mineral 

Resources, conducted a reconnaissance study in the Estancia 

Valley in central New Mexico (fig. 1) to characterize water 

quality using time-domain electromagnetic 

(TDEM) surface-geophysical soundings. 

TDEM sounding (Fitterman and Labson, 

2005) is one of a number of surfacegeophysical methods that provide a 

relatively quick and inexpensive means 

to characterize subsurface geologic and 

hydrogeologic properties. TDEM surfacegeophysical methods can be used to detect 

variations in the electrical resistivity of the 

subsurface, which in turn can be related 

to variations in the physical and chemical 

properties of soil, rock, and pore fluids.

Seven TDEM soundings were collected along an east-southeasterly profile 

about 24 kilometers long (fig. 2) in which 

apparent electrical resistivity was measured 

to depths of about 80 meters. This report briefly describes the 

hydrogeologic setting, how TDEM soundings work, the methods of data acquisition and processing, inversion modeling, 

and modeling interpretation and findings. The interpretation 

involves characterizing water quality on the basis of geologic 

characteristics and resistivities.

Hydrogeologic Setting

The study area encompasses about 525 square kilometers 

in the Estancia Valley in central New Mexico. Altitudes in the 

study area range from about 2,300 meters along the western 

edge of the area to about 1,900 meters along the north-south 

central axis of the study area, where a series of playas are 

incised into the valley floor (Bachhuber, 1982).

The study area is composed of deformed Paleozoic and 

Mesozoic rocks overlain by Cenozoic (Quaternary) sediment 

(Smith, 1957; Hawley and Hernandez, 2003). Figure 3 shows

a conceptual geologic section along the TDEM profile. 

Paleozoic- and Mesozoic-age strata generally dip eastward 

(except for a trough centered at about 9,000 meters from the 

NEW MEXICO

Estancia Valley

Torrance County

LOCATION MAP

Figure 1.  Study area of the Estancia Valley,

New Mexico.

750

2,000

1,400

4,000

2,000

1,400

750

4,000

2,000

!

!

!

!

! !

!

UV41

EVT05

EVT10

EVT20

EVT25

EVT30

EVT40

EVT50

Base from U.S. Geological Survey digital elevation model

North American Datum 1983

Universal Transverse Mercator Zone 13S

Altitude,

in meters

above

NAVD 88

2,300

EXPLANATION

! EVT05

1,900

Line of equal

specific

conductance, in

microsiemens

per centimeter at

25 degrees

Celsius—Intervals

variable (Smith,

1957)

TDEM

sounding

location

and number

750

106°01'30" 105°56'

34°51'

34°56'30"

105°50'30"

0 1 2 3 4 5 KILOMETERS

0 1 2 3 4 5 MILES

Figure 2.  Time-domain electromagnetic (TDEM) sounding sites, July 2005, and lines of

equal specific conductance, fall 1957, Estancia Valley study area, New Mexico.

U.S. Department of the Interior

U.S. Geological Survey Printed on recycled paper

Fact Sheet 2007–3011

April 2007
west end of the section), and Quaternary sediment overlies

progressively younger strata from west to east. The early

Pennsylvanian-age Sandia Formation is overlain by the middleto-late Pennsylvanian-age Madera Limestone. The Madera 

Limestone is capable of producing large quantities of freshwater (specific conductance less than about 1,700 microsiemens 

per centimeter at 25 degrees Celsius [μS/cm]) (Hawley and 

Hernandez, 2003). Overlying the Madera Limestone are the 

Permian-age Abo and Yeso Formations and Glorieta Sandstone. The Yeso Formation and Glorieta Sandstone can contain 

slightly to moderately saline water (specific conductance 

about 1,700 to 5,000 μS/cm) (Smith, 1957). The Permian-age 

San Andres Limestone and Triassic-age Chinle Formation 

unconformably overlie the Glorieta Sandstone. The erosional 

surface of the Paleozoic and Mesozoic strata forms an angular 

unconformity that is overlain by Quaternary valley fill deposits, 

which thin from west to east, and lake deposits along the eastern margin of the study area. The valley fill and lake deposits 

contain freshwater to slightly saline water. Faults and fractures 

characterize the subsurface, particularly the Madera Limestone, 

Yeso Formation, and Glorieta Sandstone (Smith, 1957).

Average rainfall in the area is about 32 centimeters per 

year. Because the Estancia Valley is a topographically closed 

basin, the principal means of discharge for precipitation that 

falls within the basin is evapotranspiration.

The dissolved mineral content of ground water in the valley fill deposits increases from west to east (Smith, 1957). In 

the western part of the study area, the water quality generally is 

satisfactory for irrigation, stock, and domestic and community 

water supply. To the east, water quality rapidly deteriorates 

and generally is unsatisfactory for irrigation or human consumption, although it is still used for watering livestock. The 

historical lateral transition between freshwater and slightly 

saline water along the profile in the valley fill deposits (fig. 2) 

is relatively sharp, occurring over a distance of less than about 

2 kilometers.

How Time-Domain Electromagnetic Soundings Work

Electromagnetic measurements are made by transmitting an alternating current into a square loop of insulated 

wire deployed on the land surface. The current consists of 

equal periods of time-on and time-off base frequencies (North 

Carolina Division of Water Resources, 2004) that produce a 

primary electromagnetic field in the immediate vicinity of the 

wire loop. Termination of the current flow is not instantaneous, 

but occurs over a period of a few microseconds, known as the 

ramp time, during which the magnetic field is time-variant. 

The time-variant nature of the primary electromagnetic field 

creates a secondary electromagnetic field in the ground beneath 

the loop. This secondary field immediately begins to decay, 

in the process generating additional eddy currents (fig. 4) that 

propagate downward and outward into the subsurface like a 

series of smoke rings. The secondary currents are measured 

by a receiver located in the center of the transmitter loop only 

during the time-off period. The depth of investigation depends 

EVT50

EVT30

EVT40

EVT20

EVT25

EVT05

EVT10

Geology modified from Hawley and Hernandez (2003, plate 3e)

10 x vertical exaggeration

2,000

1,700

1,400

1,100

800

500

0

ALTITUDE,

IN METERS 

ABOVE

NAVD 88

WEST EAST EXPLANATION

Valley fill gravel

deposits

Yeso Formation

Abo Formation

Madera Limestone

Sandia Formation

Lake deposits

Glorieta Sandstone

Chinle Formation

Upper clastic and limestone

member of the San Andres

Limestone

TDEM sounding

Fault—arrows indicate

direction of movement.

Dashed where inferred

Shear zone

Geology

EVT05

MESOZOIC PALEOZOIC

PERMIAN PENNSYLVANIAN

TERTIARY AND

QUATERNARY TRIASSIC

CENOZOIC

0 1 2 3 4 5 KILOMETERS

0 1 2 3 4 5 MILES

Figure 3.  Conceptual model of subsurface geology and time-domain electromagnetic (TDEM) sounding locations, Estancia Valley study area,

New Mexico.
on the time interval after shutoff 

of the current, because at later 

times, the receiver is sensing 

eddy currents at progressively greater depths. The 

intensity of the eddy 

currents at specific 

times and depths is 

determined by the 

combined conductivity of the subsurface 

rock and fluid (Stewart and Gay, 1986).

Data Acquisition

Seven sounding 

sites were selected to provide a uniform distribution 

of data to determine variations 

in electrical properties across the 

study area. A Geonics Protem-47 

system (fig. 5A) using a 60- by 

60-meter square transmitter loop 

(Tx) was used to collect the TDEM 

soundings (Geonics Ltd., 2005). 

Transmitter loop

Protem-47 transmitter

Eddy currents

Receiver loop

Receiver

Figure 4.  Diagram showing time-domain electromagnetic sounding

layout and operation (modified from North Carolina Division of Water

Resources, 2004).

(C)

(B)

(A)

Figure 5.  Time-domain electromagnetic sounding equipment

used in the Estancia Valley study area, New Mexico: (A) Protem-47

system, (B) transmitter loop, and (C) Protem-47 transmitter.

A 1-meter-diameter multi-turn loop receiver 

(Rx) with a 31.4-square-meter area measured magnetic fields at the center 

of the transmitter loop (fig. 5B). 

The Protem-47 Tx transmitter 

(fig. 5C) used an injection current ranging from 

2.3 to 3.5 amps for the 

various soundings. Three 

frequencies were used to 

collect voltage data, with 

preset base frequencies of 

315 hertz [ultra-high], 75 

hertz [very-high], and 30 

hertz [high] configured in 

the Protem TDEM unit. 

The Rx unit samples signal 

amplitudes in 20 subintervals (gates) during each 

measuring step, averaging 

the signal amplitudes in 

each gate over a specified 

number of duty cycles 

(eight in this application). 

The 20 gate widths are distributed exponentially over 

a measuring interval in each step of 0.8 millisecond (ultra-high 

range), 3.0 millisecond (very-high range), and 8.0 millisecond 

(high range). The Rx was set to collect three measurements of 

voltage for each base frequency.

Data Processing

For each sounding, the voltage data were averaged and 

evaluated statistically using NTEMAVG v. 3.04, a computer 

program written by David V. Fitterman (U.S. Geological Survey, written commun., 2006). Using this program, the raw field 

data (voltage data) were first averaged to reduce error; then the 

averaged data were used to estimate uncertainty by calculating 

the standard deviation of the voltage; and finally, the apparent 

resistivity of each gate for each frequency was calculated. Voltages that yielded a standard deviation greater than about 3 percent were not used, which eliminated data from later-time gates 

that yielded the highest noise. Figure 6 illustrates the percentage 

standard deviation of the average voltages from the ultra-high-, 

very-high-, and high-frequency data. The ultra-high-frequency 

data yielded the lowest standard deviation through all 20 gates. 

In the very-high-frequency data, standard deviations increased 

sharply after the 14th gate, whereas in the high-frequency data, 

standard deviations increased sharply after the sixth gate. The 

amplitude of the recorded voltage decayed below electromagnetic noise level at lower (very-high and high) frequencies 

because the power of the Tx did not induce a large enough 

magnetic field to be measured accurately during later times.

Inverse Modeling

IX1D, a program developed by Interpex Ltd. (2006), was 

used to visually analyze the apparent resistivity data calculated 

by NTEMAVG. The apparent resistivity data were plotted 

as a function of time on a log scale. Data points that deviated 
severely (a judgment decision) from the curve were deleted 

before inverse modeling. Inverse modeling, using the smooth 

modeling technique based on Occam’s inversion principle 

(Constable and others, 1987), was used to estimate the geologic or hydrogeologic structure for each sounding along the 

profile. The IX1D defaults were used for all smooth-modeling 

parameters except the number of modeling units, which was set 

to 30 with the first layer of units starting at 1 meter. A multiple-iteration, smooth-model inversion was computed until the 

root-mean-square error reached an acceptable limit of about 5 

percent or less. The smooth-model inversion technique minimizes model roughness subject to the constraint that the model 

fits the data to a desired tolerance (Interpex Ltd., 2006).

Modeling Interpretation and Findings

The smooth-model inversion results of each sounding 

collected in the study area produced models at each sounding 

site with similar resistor, conductor, and resistor resistivity 

structure. To depict the spatial changes in resistivity based on 

the smooth-model inversion results, a two-dimensional profile 

of the TDEM soundings with superimposed faults (Hawley and 

Hernandez, 2003) was created (fig. 7).

The principal finding of this study is the potential relation 

between faulting and the occurrence of freshwater and slightly 

saline water, based on the configuration of resistivity along 

the TDEM profile. A low-resistivity zone along the profile 

GATES

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

STANDARD DEVIATION, IN PERCENT

100

90

80

70

60

50

40

30

20

10

0

100

90

80

70

60

50

40

30

20

10

0

100

90

80

70

60

50

40

30

20

10

0

EXPLANATION

Soundings for

each frequency

EVT05

EVT10

EVT20

EVT25

EVT30

EVT40

EVT50

HIGH FREQUENCY

ULTRA-HIGH FREQUENCY

VERY-HIGH FREQUENCY

Figure 6.  Percentage standard deviation of the average voltages of the

ultra-high-, very-high-, and high-frequency data collected from time-domain

electromagnetic soundings, Estancia Valley study area, New Mexico.
(resistivity ranging from greater than 0 to 10 ohm-meters) 

occurs between sites EVT10 and EVT20. The western part of 

this low-resistivity zone extends to the base of the profile. The 

occurrence of this zone cannot be interpreted on the basis of 

known geologic characteristics. Another low-resistivity zone is 

indicated near EVT40 below an altitude of about 1,865 meters 

above NAVD 88. Faults near EVT40 could provide preferential flow paths for upward movement of slightly saline water 

from the Yeso Formation and Glorieta Sandstone, which could 

account for the relatively low subsurface resistivities. A sharp 

DISTANCE, IN METERS

0 5,000 10,000 15,000 20,000 25,000

ALTITUDE, IN METERS ABOVE NAVD 88 

1,920

1,900

1,880

1,860

1,840

1,820

1,800

1,780

1,760

050100150200250300350

050100

050100

050100150

050100150200250300350400

050100150200250

050100150200

EVT05

EVT10

EVT20 EVT25 EVT30 EVT40

EVT50

Resistivity, in ohm-meters

Fault Resistivity, in ohm-meters

5 25 50 75 100

Figure 7.  Two-dimensional section showing resistivity distribution based on the smooth-model inversion results with superimposed faults,

Estancia Valley study area, New Mexico.

conductive/resistive boundary (interface) from greater than 0 to 

50 ohm-meters appears at EVT25 and marks the beginning of a 

high-resistivity, near-surface zone that extends laterally through 

EVT40. Faults near EVT30 could provide preferential flow 

paths for upward movement of freshwater from the Madera 

Limestone, which could account for the high resistivities in that 

zone.

To adequately define the relations between subsurface 

resistivity measured by TDEM methods and spatial changes in 

water quality, additional water-quality, geology, and borehole-geophysical data are needed. The low-resistivity zones at 

the base of the profile beneath EVT10 and EVT40 might be 

defined less accurately than they could have been had a more 

powerful transmitter been used, probably in conjunction with a 

loop size larger than 60 by 60 meters.

—Sachin D. Shah1, Wade H. Kress1, and Lewis A. Land 2

1

 U.S. Geological Survey.

2

 New Mexico Bureau of Geology and Mineral Resources.
References Cited

Bachhuber, F.W., 1982, Quaternary history of the Estancia Valley, central New Mexico: New Mexico Geological Society, 

Guidebook 33, p. 343–346.

Constable, S.C., Parker, R.L., and Constable, C.G., 1987, 

Occam’s inversion—A practical algorithm for generating 

smooth models from EM sounding data: Geophysics, v. 52, p. 

289–300.

Fitterman, D.V., and Labson, V.F., 2005, Electromagnetic induction methods for environmental problems, in Butler, D.K., ed., 

Near-surface geophysics—Part 1. Concepts and fundamentals: 

Tulsa, Society of Exploration Geophysics,

p. 295–349.

Geonics Ltd., 2005, TEM-57 transmitter, accessed August 1, 

2005, at http://www.geonics.com/html/tem57-mk2.html

Hawley, J.W., and Hernandez, J.W., 2003, A new perspective on

the hydrogeologic framework and brackish-groundwater 

resources of the eastern Estancia Basin, central New Mexico, 

in Second New Mexico Symposium on Hydrologic Modeling, New Mexico Tech, August 12, 2003: Symposium 

Program, p. E-25.

Interpex Ltd., 2006, IX1D v. 3 inversion software, accessed 

June 2006 at http://www.interpex.com/SoftwareIndex.htm

North Carolina Division of Water Resources, 2004, Time 

domain electromagnetic geophysics, accessed July 31, 2006, 

at http://www.ncwater.org/Education_and_Technical_

Assistance/Ground_Water/TDEM/.

Smith, R.E., 1957, Geology and ground-water resources of 

Torrance County, New Mexico: State Bureau of Mines and 

Mineral Resources Ground-Water Report 5, 186 p.

Stewart, M., and Gay, M.C., 1986, Evaluation of transient 

electromagnetic soundings for deep detection of conductive 

fluids: Ground Water, v. 24, p. 351–356.

Vertical coordinate information is referenced to the North 

American Vertical Datum of 1988 (NAVD 88).

Horizontal coordinate information is referenced to the North 

American Datum of 1983 (NAD 83).

This fact sheet is based on the journal article:

Shah, S.D., and Kress, W.H., in press, Time-domain electromagnetic soundings to characterize water quality within a 

freshwater/saline-water transition zone, Estancia Valley, 

New Mexico, USA—A reconnaissance study: Association of 

Exploration Geophysicists, India Journal of Geophysics,

v. XXVIII.

Any use of trade, product, or firm names is for descriptive 

purposes only and does not imply endorsement by the U.S. 

Government.

For additional information contact:

Director E-mail: dc_tx@usgs.gov 

USGS Texas Water Science Center Phone: (512) 927-3500

8027 Exchange Dr. FAX: (512)927-3590

Austin, TX 78754-4733 World Wide Web:

http://tx.usgs.gov/