Seismic reflection investigations of a bedrock surface buried under alluvium

Abstract

Seismic reflection techniques were used to characterize a bedrock surface buried under alluvium near a construction site on the campus of Baylor University in Waco, Texas. One of the objectives of the study was to determine if either compressional or shear seismic profiling could be used to reduce the number of engineering boreholes required to determine the bedrock depth and relief prior to building construction. The upper few meters of the alluvium is dry but the lower portion is below the water table, making the bedrock surface a difficult target for compressional waves. The compressional reflection coefficient at the water table is an order of magnitude greater than that at the bedrock surface, and the dry alluvium reduces the signal bandwidth such that the two reflections cannot be distinguished. Also, the multimode Rayleigh ground roll, traveling along the surface at about half the speed of the compressional wave, swamps the reflections. By using shear waves to profile the alluvium/bedrock interface, it was possible to avoid the water table and ground roll problems associated with compressional profiling. Walkaway survey results and analytical models presented demonstrate that shear waves do not “see” the water table, and masking of the bedrock target by the reflection at the dry/wet alluvium interface does not occur. Nor was ground roll a problem because the Love “ground roll,” traveling at a velocity almost as fast as the shear body wave, moves across the geophone spread before the return of the shallow reflections. Common depth point (CDP) and optimum offset shear profiles are presented. Uncertainty in determining the depth to bedrock from the seismic data was estimated to be 3 ft (0.9 m), which is sufficiently accurate to be useful in reducing the number of preconstruction boreholes required in the Brazos floodplain.

Seismic Shear Wave Reflection Imaging at the Former Fort Ord, Monterey, California

“…Also, the strong impedence contrast of the water table traps much of the seismic energy above this reflector, leaving very little energy to image deeper layers. This phenomenon has been observed at other sites (for example, Goforth and Hayward, 1992): the reflection coefficient of the water table is significantly greater than that of the target, making the target nearly "invisible". A previous Pwave reflection study conducted elsewhere at the former Fort Ord was able to image the top of the FO-SVA using a higher-frequency source (a seis-gun fired 0.6 m below the surface), but the energy could not penetrate to deeper layers (Merey and others, 1992).…”

Shallow seismic reflection section—Introduction

“…Surveying with shallow S-wave reflections has been done (e.g., Hasbrouck and Padget, 1982;Hasbrouck, 1991, Goforth andHayward, 1992), but it has not been widespread. Separating S-wave reflections from the Love waves that usually arrive at the same time on seismograms has been and continues to be the major obstacle to progress in this area.…”

Effects of soil‐moisture content on shallow‐seismic data

“…Changes in the moisture content of the near-surface material may also affect some of these factors. The effect of changing surface-moisture content has been noted in passing (Goforth and Hayward, 1992) for shallow-seismic work, but there appears to be a great deal of uncertainty as to the significance of such variation.…”