INTRODUCTION Controlled source audio-frequency magnetotellurics (CSAMT) is a frequency-domain electromagnetic sounding technique which uses a fixed grounded dipole or horizontal loop as an artificial signal source.CSAMT is similar to the natural-source magnetotellurics (MT) and audio-frequency magnetotellurics (AMT) techniques; the chief differences center around the use of the artificial CSAMT signal source at a finite distance. The source provides a stable, dependable signal, resulting in higher-precision and more economical measurements than are usually obtainable with natural-source measurements in the same spectral bands. However, the controlled source can also complicate interpretation by adding source effects, and by placing certain logistical restrictions on the survey. In most practical field situations these drawbacks are not serious, and the method has proven particularly effective in mapping'the top 2 to 3 km of the earth's crust.The CSAMT source usually consists of a grounded electric dipole about ! to 2 km in length, ideally located at least four skin depths (4g = 2012 X/O/o/f; p = resistivity, f = signal frequency) from the area where soundings are to be made. Measurements are made within the 0.1 Hz to 10 kHz frequency band. Magnitude and phase are measured for two to five electric and magnetic field components (Ex, Ey, Hx, Hy, Hz), using either one or two sources. Grounded dipoles detect the electric field and magnetic antennas sense the magnetic field. The ratio of orthogonal, horizontal electric and magnetic field magnitudes yields the apparent resistivity. The difference between the phase of the electric and magnetic fields yields the phase of the impedance. In tensor measurements, these quantities may be treated by standard MT processing techniques. CSAMT depth of investigation is roughly equal to g/X/• m, when the separation between the transmitter dipole and the receiver station is greater than 4g, although we have found that maximum depth of investigation is limited in many cases to about 3 km due to the practical constraints of the measurements. Lateral resolution is controlled by the electric field dipole length, which normally is between 10 and 200 m. Vertical resolution is .5 percent to 20 percent of the depth of penetration, depending upon resistivity contrasts.Since its introduction in the mid-1970s, CSAMT has been used in exploration for petroleum, geothermal resources, massive sulfides, base and precious metals, structure, lithology, and sources of groundwater contamination. Much of this work has been quite successful due to the inherent capabilities of the technique.However, much remains to be learned about CSAMT interpretation, particularly in regard to static and source effects. By exploring the present understanding of the theory and practice of CSAMT we hope to provide a basis for further development. ObjectivesIn recent years the controlled source audio-frequency magnetotellurics (CSAMT) sounding technique has gained acceptance as a viable geophysical exploration tool. Its hig...

A simple equivalent circuit model and field measurements show that dipolar electric field measurements can be changed by up to 50% due to the effects of electrode contact resistance (R C). The equivalent circuit model shows that a high R C enhances the effective wire-to-ground capacitive coupling, leading to a complex dependence of received voltage on frequency, electrode contact resistance, wire length, and wire capacitance. The model shows that measured electric field voltages will fall between a perfectly grounded asymptote (R C  0) and an ungrounded asymptote (R C  ). Field tests were made of this model using the controlled source audio-frequency magnetotelluric (CSAMT) technique. By varying the effective R C and the signal frequency, the behavior predicted by the model was confirmed. The tests indicate that electrode contact resistance or ECR effects cannot be ignored in CSAMT data, and that they may influence complex resistivity measurements in certain conditions. A simple, workable solution to the ECR problem was devised by inserting a high-impedance amplifier in series with the electrodes and by shielding the lead wires, grounding the shield to a common-mode reference pot. Measurements using this configuration show that ECR effects virtually can be eliminated even at high R C values.

Ammonium perchlorate, a risk to human health, was used formerly to manufacture rocket fuel at the Naval Weapons Industrial Reserve Plant (NWIRP) McGregor, McLennon County, Texas. Perchlorate exists in several groundwater contaminant plumes, whose geometries were suspected to be influenced by transmissive bedrock structures. To identify these possible contaminant-transport pathways, a towed-array ground-penetrating radar (GPR) system was used to acquire 118 line-km of data across [Formula: see text] of the property. The shallow geology consists of bedrock limestone overlain by [Formula: see text] of clay soil. For the [Formula: see text] antennas used, the conductive clay limits depth penetration to less than [Formula: see text] and yields a [Formula: see text] wavelength, reducing lateral and vertical resolution. Nevertheless, GPR data resolved the top of bedrock in many areas. Linear discontinuities in bedrock were interpreted as weathered fracture zones, and linear areas of signal loss were attributed to deeper clay weathering along fracture zones. GPR-interpreted fractures have orientations corresponding to known lineament and fault trends, appear to control plume geometries, and tend to have higher hydraulic transmissivities. GPR results led to a more complete contaminant-transport model and were used to optimize the positions of monitoring wells needed to define the extent of contamination. This reduced the cost and time required for an environmental investigation at the site. GPR was helpful also in positioning remedial trenches across contaminated structures, resulting in plume containment at the property boundaries.

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