GEOPHYSICAL AND REMOTE SENSING APPLICATIONS FOR BETTER UNDERSTANDING OF THE STRUCTURAL (FAULTS/BASEMENT UPLIFTS) CONTROLS ON GROUNDWATER FLOW IN THE LUCERNE VALLEY, CALIFORNIA

The Mojave Desert is characterized by a series of staggered, NW trending dextral strike-slip fault systems (e.g., Helendale, Lenwood) and proximal and subparallel disconnected basement uplift. Large differences in groundwater levels were reported across stretches of these faults. To investigate whether these faults or basement uplifts act as barriers or conduits (or both), an integrated (geophysics, remote sensing, field, subsurface modeling) study was conducted in the Mojave Basin. Fault traces were identified using LIDAR data, basement outcrops were mapped from VNIR ASTER images, and geophysical transects (Very Low Frequency [VLF], magnetometer, and resistivity) were conducted along the Helendale fault over 3 summers (2009, 2010, 2011). The VLF repeatability was reasonably good, but different amplitudes for the Fraser-filtered dip were recorded for each of the three years, probably due to different signal strengths. The greater response in 2011 is due to greater moisture content in the fault zone because of the earlier survey date and greater winter/spring precipitation. VLF measurement across the fault traces showed significant radio field dip angles (up to 60%) consistent with the presence of sub-vertical, sheet-like conductors in the shallow subsurface. Many of the acquired VLF tilt peaks coincide with changes in the magnetic profiles to be expected across faults which juxtapose rocks with varying magnetic properties. The Vertical Electrical Soundings (VES) character is different on opposite sides of the fault. On the east side, the VES’s show descending final branches that are due to a deeper saturated zone. On the west side, two and possibly three of the VES’s indicate shallow basement. In other words, the ridge that is visible to the SE continues in the subsurface almost to the town of Lucerne Valley. Thus, this bedrock ridge to the W of the Helendale fault is what probably has a large influence on the difference in water table elevations across the fault. It is a barrier to eastward flow. There is only a narrow “gate” on the order of a mile wide where groundwater is more “free” to flow across the fault, at the bottom of the valley. Results indicate the faults act as conduits for groundwater flow, whereas the basement uplift are probably connected at depth forming long ridges that restrict lateral flow across the faults.