In recent years, the use of induced polarization (IP) data for delineating buried landfills has increased significantly, due to technological advances that have made this method faster and less expensive, and therefore more applicable to the environmental industry. As the database of IP information grows to include information over waste of differing composition and in differing environments, interpretation has improved significantly. Problems and limitations still exist, of course, but the IP method has become an efficient and economic tool in evaluating waste sites. We discuss here progress in interpretation, including methods to process large amounts of data rapidly in order to decrease costs, and we discuss problems that still exist, such as electrode stability, which still limit the field survey speed.

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The seismic reflection method has been a highly successful tool in oil and gas exploration for half a century, and it presently accounts for about 98% of all geophysical expenditures worldwide. However, the relatively high cost of seismic exploration and its limitations in certain geologic environments are continuing problems. Some help has been provided by the magnetotelluric (MT) sounding technique, but the cost of MT is also quite high due to the low natural signal strengths being measured. The controlled-source audio-frequency magnetotellurics (CSAMT) technique is a shallower-penetrating variation of MT which uses an artificial signal source. This permits faster and more economical data acquisition. CSAMT has a penetration of about 2 km in typical petroliferous environments. CSAMT does not replace seismic but functions in three specific roles: (1) as a reconnaissance tool to help focus seismic coverage, or to help avoid 'no-record' zones; (2) to assist in static corrections and in interactive seismic interpretation; (3) as a primary tool in certain environments (volcanics, complex thrust areas) where seismic data acquisition is limited. An example of the application of CSAMT to structure mapping comes from data taken over Trap Spring Field, located in the frontier Great Basin of the western United States. The field produces oil from fractured volcanics at the edge of a major graben fault. The CSAMT data delineate the major subsurface faulting and stratigraphic relationships in the area. The resolution of the CSAMT survey is significantly better than previously obtained induced polarization (IP) data. Detailed comparisons with electric log, drill hole, and air-photo data show an excellent correlation between the CSAMT features and known geology. The work suggests that CSAMT could be used in this area for reconnaissance mapping to develop seismic prospects, at approximately one sixth the cost of seismic.

Laboratory tests and field examples of the horizontal field component measurements in shallow transient electromagnetic (TEM) surveys show the utility of these data in target characterization in reconnaissance, random-walk surveys for unexploded ordnance (UXO) and underground utilities. For example, prior work has shown that the Hx component (which we define as the horizontal field component in the direction of travel of the measurement system) is often useful in distinguishing small 3-D targets from linear features (such as buried pipelines or power lines). The current work expands on this capability, which is particularly important in UXO projects in which random transects are evaluated to determine a statistical estimate of density and distribution of potential ordnance and explosives. Anomalies from pipelines or power lines can skew the statistical evaluation toward overestimating the number of potential UXO in a given area. The current research provides examples of the additional information that is available in the horizontal components of TEM surveys, particularly in the early time after transmitter turnoff. For example, the Hy component data are useful in estimating the orientation of linear features such as pipes and power lines with respect to the survey lines.

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