Repair, Evaluation, Maintenance, and Rehabilitation Research Program. High-Resolution Seismic Reflection Investigations at Beaver Dam, Arkansas

摘要

Seismic refraction surveys require line lengths four to five time the desired depth of investigation. For many geotechnical applications, these are physical and geometrical constraints which inhibit seismic refraction, such as narrow river valleys, changes in direction of the center line of a dam, or legal inaccessibility to land surrounding a site. In the past, the required data processing, limitations of engineering seismographs, and inapplicability of oil exploration seismic recording systems very shallow targets (<30 m) precluded effective utilization of the seismic reflection method for geotechnical applications. Rapid advances in microcomputer technology, development of digital engineering seismographs, development of high frequency seismic sources, and adaption of oil exploration field procedures to account for inherent problems of shallow depths of investigation now make shallow,high-resolution seismic reflection survey a viable tool for geotechnical applications. Field investigations of shallow, high-resolution seismic reflection methodology at an existing structure site, dike 1, Beaver Dam, AR are discussed. Dike 1 is built across a graben with vertical offset in excess of 70m. The down dropped block consist of solutioned cherty limestone. The top of rock is highly irregular, and solution cavities and solution-widened joints exist in the rock. The seismic reflection results effectively mapped the irregular top of rock (3 to 10 m depth), detected previously unknown faults within the graben, and mapped sandstone and shale formations at the base of the graben (70 to 75 m depth). (SDW)