Cross-borehole compressional wave imaging of unconsolidated sediments

Cross-borehole elastic wave tomography offers great potential for nondestructive imaging of near-surface sediments, fluids, and soils. This method was used to create a high-resolution tomographic image of velocity structure from simulated sedimentary layers obtained using compressional wave travel times. The image shows three distinct layers and a water-filled container located in the center of the middle layer. An image obtained from computer-simulated travel times shows good agreement with the image from laboratory data. The high-quality data and the resulting image were due in part to a novel transmitter and receiver design. These were composed of cylindrical piezoelectric transducers individually sealed in oil-filled Teflon containers and stacked to form stringers. The transmitters were extremely efficient at coupling energy into the medium. The transmitter frequency was 135 kHz, velocities ranged from 1.4 to 1.8 km/s, and thus wavelengths were 1.0-1.4 cm. The distance between boreholes was 30 cm, with about 60 cm of vertical coverage