Characterizing the upper few tens of meters of Earth materials by seismic methods other than classical refraction was virtually nonexistent in the scientific literature before 1980. This was due partly to lack of interest, but also to the limited dynamic range of analog seismographs prior to 1960 and to the limited instantaneous dynamic range of digital seismographs between 1960 and the 1980s. Furthermore, access to adequate digital processing facilities was not easily available to those outside of oil companies or major geophysical contractors until the late 1980s.

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