When direct current introduced into the ground at two points is interrupted, a small voltage, which may take several minutes to decay, appears between another pair of electrodes. This effect, called by C. Schlumberger “provoked polarization,” is found useful in prospecting for ground water. Laboratory experiments indicate that induced polarization (I.P.) depends on cation exchange in the clay minerals contaminating the aquifers. It is suggested that the effect is due to electrodialysis of the clay within the aquifer, which acts as a distributed electronegative membrane. The magnitude of the I.P. depends on the kind of clay and the kind of positive ions in the water. It is inversely proportional to the conductivity of the water and independent of the kind of negative ion dissolved therein. The rapidity with which the I.P. decays appears to depend only on the grain size of the aquifer. Field trials and experiments on laboratory models give promise of useful application of the method to depths of about 350 feet.

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The induced polarization (IP) technique is based on the observation that variations of earth conductivity with frequency may be indicative of buried metallic mineralization. Conventional IP involves the use of grounded electrodes, the installation of which can be quite time consuming. However, the electromagnetic fields about an oscillating magnetic dipole also depend on ground conductivity. Thus, it is conceivable that we might be able to detect IP anomalies with an inductive system, thereby eliminating the need for grounded electrodes. An airborne induced polarization method is a theoretical possibility. Theoretical calculations based on a conductivity model determined experimentally at frequencies less than 30 hz suggest that the effect of polarizable material on electromagnetic response is quite small. In order to check the theory and to determine experimentally whether inductive IP is feasible, field tests were conducted in two areas in Nevada which exhibit strong conventional IP anomalies. The field tests consisted of measurements of the amplitudes of the electric and magnetic fields about a horizontal loop of wire carrying current at frequencies ranging from 15 hz to 1500 hz. The presence of the polarizable material is not evident in the inductive data; in fact, the observations can be fitted to theoretical curves for nonpolarizable models. Hence, on the basis of both theory and field tests, it is concluded that inductive IP based on amplitude measurements is not a practical exploration tool for environments such as that of the southwestern United States.

In response to the Geothermal Energy Research, Development, and Demonstration Act of 1974, a federal geothermal program has been established with the objective of stimulating the commercial development of geothermal resources. The program goal is to increase the annual rate of energy utilization from the present 0.04 quads (500 MWe) to 0.3–0.5 quads in the near term (about 1985), 4.0–9.0 quads in the mid‐term (1985–2000), and 16.0–28.0 quads in the long term (by about 2020). The realization of these goals depends upon the discovery and exploitation of many new geothermal resource areas. The Department of Energy program for geothermal exploration and assessment has been structured to address technological barriers presently hindering the economical discovery and delineation of geothermal resources. We describe the program elements—exploration technology, reservoir assessment, reservoir confirmation, and reservoir engineering—in light of the need to evaluate some 1500 new prospects in order to meet the federal midterm electric power goal of 20,000 MWe on‐line by the year 2000. We illustrate the program elements with suggested sequences for exploration, assessment, and confirmation of a 200-MWe resource in the eastern Basin and Range physiographic province. The estimated costs for these sequences are $385,000, $565,000, and $3,190,000, respectively. Deep drilling constitutes the major element in the confirmation costs. An economical exploration strategy requires use of cost‐effective techniques; thus, we have initiated a number of technology assessment studies. Pursuant to these studies, we present a summary of our initial findings and discuss the status and needs for topics such as system modeling, thermal methods, rock and fluid properties, seismic, and electrical methods. Accurate reservoir assessment requires new developments in logging instrumentation for high‐temperature, hostile environment boreholes and improved means for interpreting acquired data. The logging instrumentation program is aimed at upgrading logging systems for operation at 275°C and 48.3 MPa in the near term (1982) and 350°C and 138 MPa by 1986. Existing hardware is being upgraded, components and materials are being developed, and critically needed prototype tools for temperature, flow, and pressure measurements will be evaluated. A program addressing log interpretation problems uses industrial expertise to analyze specific shortcomings in our ability to infer critical reservoir parameters from acquired data. The program will also establish and maintain test and calibration wells and support research logging and petrophysical studies.

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