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 material. Experimental field parameters remained constant to ensure comparability of results. Variations in the seismic data are attributed to the changes in soil-moisture content of the unconsolidated material. Higher amplitudes of reflections and refractions were obtained under wetter near-surface conditions. An increase in amplitude of 21 dB in the 100-300 Hz frequency range was observed when the moisture content increased 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 large-amplitude ringy wavelets that interfered with and degraded the appearance of some of the reflection information in the raw field data. This may indicate that an intermediate near-surface moisture content is most conducive to the recording of high-quality shallow-seismic reflection data at this site. This study illustrates the drastic changes that can occur in shallow-seismic data due to variations in near-surface moisture conditions. These conditions may need to be considered to optimize the acquisition timing and parameters prior to collection of data.