The geotechnical engineer has always been faced with the problem of characterizing near-surface materials. The near-surface region is often within 10 to 100 m of the ground surface. Traditionally, field exploration programs involved boring, sampling, and penetration testing. In the 1960s, in-situ geophysical measurements began to be employed in geotechnical engineering. This work primarily involved seismic (stress wave) measurements at small strains which were adapted from exploration geophysics. These measurements were used to characterize geotechnical sites (e.g.