Multi-channel Analysis of Surface Waves (MASW) in Karst Terrain, Southwest Georgia: Implications for Detecting Anomalous Features and Fracture Zones

摘要

Multi-channel analysis of surface waves (MASW) was used to map bedrock topography, image anomalies in the soil overburden, and detect variations in the degree of limestone weathering in covered, karst terrain near Albany, Georgia. Surface-wave data were acquired along seven parallel lines and two perpendicular cross-lines. Two surveys using a hammer and moving van-source were conducted to address the trade-off between lateral resolution and depth penetration. Resolution matrices show that the depth ranges most effectively imaged are 0.5-4 m for half-spread hammer data, 3-12 m for full-spread hammer data, and 8-20 m for van-source data. A steep S-velocity gradient marked by the 350 m/s contour at depths of 9-12 m is interpreted as the bedrock surface, in agreement with coincident borehole data. The models indicate variations within limestone bedrock (S-velocity: 350-700 m/s) and anomalous zones within the sandy-clay overburden. Standard penetration tests at two locations 45-m apart confirm variations in limestone rigidity suggested by the velocity models. At one location, a dissolution feature (N-value: 0) within highly weathered, saturated limestone (N-values: 12 and 19) correlates with low S-velocities (<350 m/s). Borehole data from the second location shows more rigid limestone (N-values: 25 and 34) that correlates with S-velocities between 350-400 m/s. A zone of weathered rock at depths of 12-20 m imaged along a suspected fracture zone is consistent with borehole data and sinkhole formation at the ground surface. An anomaly associated with a burn pit at the intersection of two perpendicular profiles was imaged at two different scales. S-velocity models and distinct changes in apparent velocity of direct P-waves and surface waves across shot gathers constrain the lateral and vertical extent of the anomaly, and borehole data verify changes in soil composition and depth (5 m) to the base of the anomaly.