Determination of depths to an irregular interface in shallow seismic refraction surveys using a pocket calculator

Abstract

Engineering geologic problems commonly involve the determination of depth to bedrock where it is overlain by residual soil or dry and saturated alluvium. The buried bedrock surface is commonly irregular and the nature of such irregularities must be known, for example, in (1) foundation studies where irregularities on the bedrock surface could indicate faults, or (2) groundwater studies where low areas in the bedrock may indicate channels for groundwater flow which may be targets for drilling. Seismic refraction surveys have proven to be a simple and accurate means of providing adequate data for describing irregularities on the bedrock surface, especially when the data are analyzed by the “plus‐minus” method of the wavefront approach. The method can be programmed readily for a Hewlett‐Packard HP-45 hand calculator, enabling rapid reduction of the data in the field. On‐site data reduction and interpretation result in greater efficiency by allowing immediate recognition of those areas where more detailed information may be needed, thereby precluding repeat surveys and reducing costs. For this purpose, suitable computer programs for the two‐ and three‐layer cases are developed, as well as programs for elevation and weathered zone corrections commonly needed in such shallow seismic surveys.