GEOPHYSICAL INVESTIGATIONS IN THE UPPER SAN PEDRO RIVER BASIN, BENSON, ARIZONA

Abstract

Four geophysical surveys were conducted at the Nature Conservancy about 20 miles north of Benson, AZ, in the Upper San Pedro River Basin, in order to better understand the nature of the sub-surface features of the basin. The geophysical methods included TEM (Transient Electromagnetic), seismic, EM34 and magnetic surveys. The TEM, seismic and magnetic surveys were conducted perpendicular to the river basin while the EM34 lines followed the riverbed. The perpendicular surveys were divided into two regions, referred to as the South and North Lines. The TEM, seismic, and magnetic surveys revealed a consolidated bedrock structure at shallow depths (30-40 m) along the South Line. The feature has an east-west extension of approximately 500 meters, and is located just east of the San Pedro River. None of the perpendicular surveys were able to detect bedrock features along the North Line, implying that the depth to bedrock exceeds the maximum depth of this investigation (360 m). Both lines showed regions of high porosity, and, potentially, of saturated materials. These regions were more prevalent along the North Line, where numerous highly porous areas were detected at various depths (including one region beginning at a depth of 50 meters and extending at least to 360 m). The EM34 failed to detect any appreciable long-wavelength changes in conductivity along the riverbed, though localized point anomalies were found.