EFFECTS OF SOIL-MOISTURE CONTENT ON SHALLOW SEISMIC DATA

Repeated shallow-seismic experiments were conducted at a site on days with different near-surface moisture conditions in unconsolidated mate rial. Experimental field parameters remained constant to ensure compar ability of results. Variations in the seismic data are attributed to t he changes in soil-moisture content of the unconsolidated material. Hi gher amplitudes of reflections and refractions were obtained under wet ter near-surface conditions. An increase in amplitude of 21 dB in the 100-300 Hz frequency range was observed when the moisture content incr eased from 18% to 36% in the upper 0.15 m (0.5 ft) of the subsurface. In the time-domain records, highly saturated soil conditions caused la rge-amplitude ringy wavelets that interfered with and degraded the app earance of some of the reflection information in the raw field data. T his may indicate that an intermediate near-surface moisture content is most conducive to the recording of high-quality shallow-seismic refle ction data at this site. This study illustrates the drastic changes th at can occur in shallow-seismic data due to variations in near-surface moisture conditions. These conditions may need to be considered to op timize the acquisition timing and parameters prior to collection of da ta.