Seismic Shear Wave Reflection Imaging at the Former Fort Ord, Monterey, California

At the former Fort Ord in Monterey County, California, contamination threatens an aquifer that provides drinking water for local communities. Assessment and remediation require accurate hydrological modeling, which in turn require a thorough understanding of aquifer stratigraphy. In order to help guide remediation efforts at the site, the U.S. Geological Survey, in cooperation with the U.S. Army Corps of Engineers, has undertaken seismic reflection surveys, testing compressional (P) and horizontally polarized shear (SH) waves. Sledgehammer-source SH data show reflections from interfaces up to approximately 60 m deep, which correspond with the major boundaries between aquifers and aquitards. In contrast, P-wave data show only the reflection from the water table at approximately 30 m depth. We collected SH data along two transects and processed these data to produce reflection images. The interpreted SH-wave images agree with available well information, constrain the geology for ground-water models, and provide guidance for future geophysical studies. These favorable results demonstrate the effectiveness of SH reflection methods for imaging unconsolidated aquifer layers at the former Fort Ord and at other sites with similar geologic conditions.