DEEP RESISTIVITY PROBES IN THE SOUTHWESTERN UNITED STATES

Eight deep resistivity probes have been made along a traverse extending from southwest Colorado to southern California to determine the electrical properties of the deep crust. The dipole method for measuring resistivity was used, and dipole spacings as great as 50 to 100 km were obtained at each survey location. In the Colorado Plateaus province, the electrical basement usually does not coincide with the top of the Precambrian crystalline basement, but rather, corresponds with the top of the carbonate rocks of Paleozoic age. Because of the large electrical conductance in the sedimentary column, resistant basement rock was detected at only three of the five sounding locations in the Colorado Plateaus province. At these three locations, the minimum possible basement resistivity was 2000 to 6000 ohm‐m. Electrical basement was detected at the survey locations in the Basin and Range province. The minimum possible basement resistivity was 600 to 1000 ohm‐m. A survey near Barstow, California, indicates that the resistant portion of the crust may be 9 to 10 km thick and may have a resistivity of 600 to 700 ohm‐m. A conductive zone underlies the resistant portion of the crust.