Misleading interpretation of shallow seismic methods: Three case studies from MASW, P-wave refelection and shwave reflection surveys

Despite the reliable performance of high resolution seismic methods for imaging the shallow subsurface, in some cases interpreting seismic results can be misleading. We present three case studies where results from, P-wave reflection, SH-wave reflection, and MASW surveys were incorrectly interpreted because of inadequate constrains on either the surface or subsurface conditions of surveyed sites. A feature from a P-wave reflection survey was first interpreted as a shallow fault zone, but it was later] determined to result from a high level of background noise as the acquisition passed through a road intersection. A feature from an SH-wave reflection survey was interpreted to be a reverse dip-slip fault, but targeted drilling showed it was deep local erosion into the bedrock surface. Finally, in an MASW survey, a steeply dipping feature was first interpreted as a bedrock valley. However, later exploratory drilling showed the feature to be a shallow layer of very soft lake sediment that severely damped most of the applied surface wave frequency band. Although initial interpretations were incorrect, they stimulated discussions among geophysicists and geologists and underscored the need for meaningful cooperation and discourse between the scientists before, during, and after geophysical data acquisition.