Investigation of Structural Controls on Groundwater Flow Using Ground-based Magnetics, VLF, and DC Resistivity Near the San Andreas Oasis, CA

The San Andreas Oasis is a rare riparian and wetland habitat within the arid Colorado Desert where the convergence of several faults plays a significant role in the flow and distribution of groundwater. In historical times the oases in this region served as a collection of vital watering holes for those travelling through the desert. However, water levels at these springs and oases have been on the decline in recent years, putting wildlife at risk. Recharge ponds were constructed upgradient of the oases, near the Coachella Canal, to replenish groundwater in the area, but unfortunately water levels at the San Andreas Oasis, located about 4 km southwest of these recharge basins, continue to decline. Limited literature is available on the geology of this area and its subsurface structure is poorly understood. To better understand the local controls on groundwater flow, we use several geophysical methods, including ground-based magnetic, very low frequency (VLF), and direct current (DC) resistivity surveying.

A GEM proton precession magnetometer is used with a VLF attachment to collect total magnetic intensity data over an area of about 2 km² of the San Andreas Oasis. VLF profiles varied from 200 to 400 m in length and traversed both the Hidden Springs Fault (HSF), which bounds the oasis to the west, and the eastern boundary of the San Andreas Oasis. Anomalies seen in the resulting magnetic data appear to correlate with the known trace of the HSF as well as a second hypothesized fault, which trends roughly parallel to the eastern boundary of the oasis. VLF profiles also show anomalies that correspond to the trace of the HSF and provide additional evidence for the existence of the hypothesized fault. DC resistivity data was collected using an IRIS Syscal Kid with 24 electrodes at 5 m spacing and gave insight into the subsurface structure near the HSF and unmapped fault. Near the HSF, resistivity profiles show a vertically oriented, low resistivity feature which may represent saturated or clay-rich fault gouge. A resistivity profile near the unmapped fault reveals a low resistivity zone at about 4 m depth which may represent the approximate depth to groundwater. Further surveys will be done to examine the extent and geometry of the unmapped faults and their role in promoting or inhibiting groundwater flow.