THE EFFECTS OF FAULTING ON GROUNDWATER FLOW IN THE SAN ANDREAS OASIS FROM MEASUREMENTS OF GROUND-BASED MAGNETICS, VERY LOW FREQUENCY AND DIRECT CURRENT RESISTIVITY

The San Andreas Oasis is an oasis within the Dos Palmas Preserve. The Oasis is located to the east of the Hidden Springs Fault (HSF) in the Colorado Desert, several kilometers from the northeastern shore of the Salton Sea. The Oasis is a riparian habitat, home to endangered species such as the desert pupfish. In the last few years, water levels near the Oasis have declined despite the use of recharge ponds in the area. The subsurface is thought to have significant faulting that directly affects the flow of groundwater. Without a clear image of the subsurface, it is difficult to determine the cause of decreased water levels. We survey the area using ground-based magnetic, very low frequency (VLF), and direct current (DC) resistivity techniques in order to get a better picture of the subsurface groundwater flow with respect to local faulting.

We used a GEM Systems proton precession magnetometer with a VLF attachment for the magnetic and VLF surveys, and created a map of total magnetic field intensity and Fraser filtered VLF measurements. Anomalies are seen in both the VLF and magnetic surveys across the mapped trace of the HSF, and additional anomalies suggest the presence of at least one additional unmapped fault. Several DC resistivity profiles were collected using the IRIS Syscal Kid and Pro Switch units to confirm these structures and generate models of the subsurface resistivity. Profiles were created with 24 electrodes, with spacings of 2.5 m, 5 m, and 10 m across the HSF. Resistivity models near the Oasis show vertically oriented features, suggesting saturated or clay rich fault gouge. South of the Oasis, we see decreased resistivity near the surface, suggesting the water table is near the surface as the HSF steps over to the east. The mapped faults are likely to affect the flow of groundwater from the north.