by Jeffrey E. Lucius ¹ Robert J. Bisdorf ¹, and Jared Abraham ¹

Open-File Report 01-331

2001

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

CONTENTS

SUMMARY

In June 2001, the U.S. Geological Survey conducted electrical geophysical surveys to determine the thickness of colluvium in the Red River and Straight Creek valleys in northern New Mexico. The electrical surveys consisted of 23 direct current (DC) resistivity soundings, 8 time-domain electromagnetic (TDEM) soundings, and 4 multiple-electrode DC (Swift/Sting) sounding profiles. For the two DC methods, electric current was introduced into the ground using two metal electrodes and potential difference was concurrently measured at two other metal electrodes. For the TDEM soundings, a direct current was passed through a loop of wire on the ground. The current was abruptly turned off, and another wire loop measured the rate of change in the magnetic flux produced by the decaying eddy current system in the ground. Both methods provide information concerning the electrical resistivity structure of the earth. For detailed discussions of these geophysical methods, look in the following sources: Dobrin (1976), Fitterman and Stewart (1986), Kaufman and Keller (1983), Koefoed (1979), Spies and Eggers (1986), and Zohdy and others (1974).

Resistivity in the earth is strongly affected by water saturation and water quality. Resistivity generally decreases with increasing water content and with decreasing water quality (that is, an increase in dissolved solids). Before visiting the site, we hypothesized that the valley fill (the colluvium) would have a higher water content than the bedrock, and consequently the valley fill would have a lower resistivity than the bedrock. Electrical geophysical methods should detect this contrast in resistivity. Unfortunately, interpretation of the data from all three methods used at the Red River site showed that there is not a strong electrical contrast between the colluvium and the bedrock. In fact, many of the DC soundings indicate that shallow layers are more resistive than deeper layers. All three methods had low noise levels in the Straight Creek valley. In the Red River valley, the DC measurement contained low noise levels. However, the TDEM and Swift/Sting measurements along the Red River were very noisy, making these two methods unsuitable there. In all cases, the boundary between colluvium and bedrock cannot be determined with confidence from the electrical soundings alone. Additional information from wells and seismic surveys may help us to improve the interpretation.

GEOPHYSICAL SURVEYS

The DC resistivity sounding surveys employed a Schlumberger arrangement of the electrodes. For this method, all the electrodes are placed in a straight line, current electrodes are positioned at the ends, and potential electrodes are located symmetrically about the center at various distances. The distance between the potential electrodes, MN, is no more than one fifth that between the current electrodes, AB. A sounding is obtained by making measurements at increasing current electrode separations. Twelve DC resistivity soundings were measured in the Straight Creek Valley and eleven soundings were measured along the Red River (figure 1). The field measurements and best-fit models are presented in the Appendix.

The area designated here as the Well Line, about 300 meters north of the Sewage Plant, was of special interest because the USGS plans to drill a series of wells there across the valley. DC soundings S11, S16, S17, and S18 are in this area. Figure 2 shows a cross section of interpreted resistivity from the DC soundings near the Well Line. The cross section does not show features that can be easily interpreted as the colluvium-bedrock interface.

The Swift/Sting Resistivity system functions similarly to the DC sounding system, except that 48 electrodes are used (instead of four), permitting easy collection of multiple Schlumberger soundings along a line. Swift/Sting resistivity data were collected along four profiles (figure 3). The field measurements and best-fit models are presented in the Appendix.

The Swift/Sting data were interpreted using the computer program RESIX IP2DI v4 (http://www.interpex.com/). Figure 4 shows the data collected along the Well Line. The figure includes a pseudosection of observed apparent resistivities (data), a pseudosection of apparent resistivities calculated from an interpreted model(synthetic) and a model of interpreted resistivities. As with the DC soundings, the thickness of the colluvium is not easily determined from the Swift/Sting profiles.

The time-domain electromagnetic sounding method uses the transient induction properties of electromagnetic fields to determine electrical resistivity as a function of depth. A direct current is passed through a square loop, which was 38-m on a side for seven of our eight surveys, and the current is abruptly shut off. The quick termination of current in the loop causes eddy currents under the loop that diffuse downward and outward at a rate controlled by the resistivity of the ground. The receiver coil, in the center of the transmitter loop, senses the magnetic fields produced by the decaying currents in the ground. The receiver coil measures the time rate of change of the magnetic flux over various small time periods called gates. The duration of the gate depends upon the frequency of the transmitter. The measurements reported here were made with the Protem 47 transmitter set at the ultrahigh (UH) frequency. Measurements at the lower frequencies (very high, VH, and high, H) were very noisy and so not used. Six sets of measurements were recorded at each station and averaged to calculate the apparent resistivity values (avg-r) reported in the Appendix. The locations of the TDEM sounding stations are shown in figure 5.

Because of various problems with the equipment and noise levels, only the three TDEM stations near the Well Line are presented here and in the Appendix. For these soundings, the transmitter was set to the UH frequency, for which the corresponding voltage measurements are affected primarily by shallow sediments and bedrock. Even at these frequencies, the TDEM method at this site is not sensitive to layers shallower than about 20 meters. The Interpex computer program TEMIXGL was used to produce the resistivity models shown in figure 6.

REFERENCES

Dobrin, M.B., 1976, Introduction to Geophysical Prospecting, 3rd ed.: New York, NY, McGraw-Hill, 630 p.

Fitterman, D.V. and Stewart, M.T., 1986, Transient electromagnetic sounding for groundwater: Geophysics, v. 51, p. 995-1005.

Kaufman, A.A. and Keller, G.A., 1983, Frequency and Transient Sounding: Amsterdam, Elsevier, 685 p.

Koefoed, Otto, 1979, Geosounding Principles, 1. Resistivity Sounding Measurements: New York, NY, Elsevier Scientific Pub. Co., 276 p.

Spies, B.R. and Eggers, D.E., 1986, The use and misuse of apparent resistivity in electromagnetic methods: Geophysics, v. 51, p. 1462-1471.

Zohdy, A.A.R., Eaton, G.P., and Mabey, D.R., 1974, Application of Surface Geophysics to Ground-Water Investigations: Techniques of Water-Resources Investigations of the U.S. Geological Survey, Book 2, Chapter D1, 116 p.

For each of the four profiles, the top image is the contoured apparent resistivity calculated from the Swift/Sting DC soundings presented in pseudosection format. The middle image is the contoured synthetic apparent resistivity data from the model in pseudosection format. The bottom image is the resistivity model. Click on an image below so see the full-size image in a new window.