Conventional Processing Techniques and Nonlinear Refraction Traveltime Tomography for Imaging Bedrock at an Eastern Massachusetts Coastal Site

A seismic refraction survey was conducted at a coastal site in eastern Massachusetts to determine bedrock depth and locate areas where overburden thickness is sufficient for the construction of a replacement stormdrainage system. Since the site is in a tidal zone, the saturated thickness of the overburden is constantly changing. Additionally, the depth to bedrock varies from a few feet to 50 or 60 feet. We use a nonlinear refraction traveltime tomography technique (Zhang et. al., 1996) to evaluate this complex tidal environment and compare the modeled results with those acquired using more conventional processing techniques such as the crossover-distance and iterative ray tracing methods. We find good correlation of the methods along those lines closest to shore and least influenced by tidal changes and organic deposits. However, conventional methods fall short where organics and changing saturation conditions cause abrupt changes in lateral and vertical velocities.