Seismic imaging of the shallow subsurface: shear-wave case histories

Obtaining high-resolution images of the geology and hydrogeology of the sub surface in the depth range from ground level to 50 m is one of the major ch allenges of modern geophysics. The methods which are commonly used (such as compressional-wave surveys and ground-penetrating radar) often suffer from adverse effects caused by the near-surface conditions, changes in water sa turation and various sources of noise. This paper demonstrates some of the advantages offered by the use of shear-wave seismology and by the combinati on of shear- and compressional-wave seismic methods in shallow subsurface i nvestigations. Multicomponent shallow seismic tests were carried out at four different sit es to examine the effectiveness of different acquisition geometries under a variety of near-surface geological conditions. Near-surface conditions enc ountered at the sites included thick clays, clay/sand sequences overlying C halk, mudstone overlying granodiorite bedrock and landfill material. Under all conditions, shear-wave data acquisition was found to have advanta ges over compressional-wave acquisition for the investigation of the shallo w subsurface. Shear head waves, being unaffected by water saturation, achie ved penetration to greater depths at a site in Crewkerne, Dorset where comp ressional head-wave penetration was limited to the near-surface layers. Bet ter vertical resolution was achieved at shallow depths using shear-wave ref lection energy at a landfill site. Shear-wave reflections from shallow inte rfaces were in some cases less affected by noise compared with the equivale nt compressional-wave reflections. Combinations of shear- and compressional -nave data recording allowed the measurement of a Poisson's ratio log and g ave indications of seismic anisotropy at two sites where dipping clay layer s were present.