MULTI-CHANNEL ANALYSIS OF SEISMIC SURFACE WAVES IN KARST TERRAINS: IMPLICATIONS FOR DETECTING SUBSIDENCE FEATURES AND BEDROCK LINEAMENTS

Multi-Channel Analysis of Surface Waves (MASW) has been employed in covered, karst terrain near Albany, GA to investigate potential subsidence features and lineament orientations within bedrock. The 2-D shear-wave velocity profiles indicate that observed sinkhole development at the ground surface can be correlated with zones of low velocity at depth. Since sinkhole formation is typically related to preferential weathering along lineaments within bedrock, the feasibility of detecting fracture orientations was also assessed.

The study was conducted on a well field in southwest Georgia operated by the Albany Water, Gas, and Light Commission. The site is characterized by approximately nine meters of unconsolidated sand and clay overlying the karstified Ocala limestone. Shear-wave profiles were obtained with a 24-channel recording system along seven parallel lines spaced 20 meters apart. The uppermost 15 meters was imaged using one-meter station spacing and a hammer source. Restricting the analysis to the nearest 12 traces enhanced lateral resolution within the uppermost 5 meters. Depth penetration was increased to 25 meters using two-meter spacing and a moving passenger van as the seismic source. Shear-wave velocities within the uppermost 10 meters range from 150-350 m/s. A sharp increase in velocity at 9-10 meters is interpreted as the soil-bedrock interface, in agreement with coincident borehole data. Shear-wave velocities within fractured bedrock range between 400-750 m/s. Backfilled sinkholes and deeper voids in bedrock are marked by well-defined velocity anomalies.

Identification of subtleties on the seismic records improved the lateral resolution of potential collapse features. Noticeable changes in apparent velocity of the surface waves indicate significant velocity variations over portions of the seismic lines. To resolve those variations, subsets of traces for anomalous gathers were extracted and analyzed separately. The velocity anomalies in the seven profiles are being analyzed for trends to compare with existing fracture data.