Telluric Mapping Over the Mesa Geothermal Anomaly,Imperial Valley,California,

摘要

The telluric prospecting method employs naturally occurring AC electric fields in the earth to measure the variations in lateral electrical conductance within a sedimentary basin. It is a relatively inexpensive,rapid and deeply penetrating method of estimating electrical conductivity. Telluric measurements were made at thirty-nine stations in the vicinity of the Mesa Geothermal Anomaly,Imperial Valley,California in order to evaluate the usefulness of the method as a geothermal prospecting tool. The area of investigation is characterized by thick (about 3km) and highly conductive (resistivity on the order of 5ohm-m) sediments. The large sediment depth and conductivity necessitated a more sensitive (p-p noise level = uV) apparatus operating at lower frequencies (.0067hz) than is normally employed for telluric investigations. Relative ellipse areas were determined for the stations and plotted as total field and residual contour maps. These maps clearly reveal the presence of the thermal anomaly. They coincide well in geometry with the anomaly shown by shallow thermal gradient data and are consistent with temperature data from five deep wells in the area.