EXPERIMENTAL RESULTS OF ENERGY AND SOIL-MOISTURE EFFECTS ON NONLINEAR DEFORMATION ASSOCIATED WITH NEAR-SURFACE SEISMIC REFLECTION SOURCES

The ability to collect seismic-reflection data in the upper 3m of the subsurface (ultrashallow seismic reflection; USR) is important because it offers an alternative to ground-penetrating radar in regions exhibiting high attenuation of electromagnetic signal. Previous work has demonstrated that the critical controllable factors in obtaining USR data are dense spatial sampling and high (>400 Hz) frequency content. The frequency content is strongly controlled by the characteristics of the sources used to generate the seismic energy. Qualitative experiments have indicated that the higher-energy near-surface seismic reflection projectile sources lack some of the high-frequency energy exhibited by smaller-energy sources. In addition, soil moisture content has been shown to be an important variable in the frequency content of data even if the same source is used. In both cases, a reduction in the frequency content of the data presumably correlates with greater amounts of near-source nonlinear deformation.

In order to test this hypothesis, experiments were conducted in a controlled laboratory setting to quantitatively determine the relationship between source energy, soil-moisture conditions, and the amount of nonlinear deformation. Projectile sources with different energy (ammunition) were fired into a sand pit having varied moisture content, and the resulting voids were filled with plaster. The volume of the casts (representing the most substantial region of nonlinear deformation) was then measured. Results indicate that source energy and soil moisture have a direct correlation on the amount of near-source nonlinear deformation.