Seismic parameter evaluation of gravel and sand samples subjected to stress using general purpose piezocrystals

Abstract

High resolution geophysical techniques based on the transmission of elastic waves in dry samples of gravels and sands were developed and implemented. Laboratory specimens of alluvial sand and gravel were prepared at different compaction levels. Low cost piezocrystals transducers were used as sources and receivers; and TL084 microprocessors as analog amplifiers. A TDS 210 Tektronix digital oscilloscope was used for A-D conversion. The specimens were subjected to stress simulating sediments at different subsurface depths. Propagation velocity, VP and frequency spectra were found to correlate with the applied effective stress and reflected nonlinear inter-particle contact response. During the discharge cycle, VP values exceeded those of the initial load cycle, as grains were relocated and compacted in the loading process. Thus, compaction in a given sample may be obtained from seismic waves propagation, before and after loading.