Seismic imaging: Receiver-based refraction processing
For nearly all the records in this study, the initial energy recorded at the receivers is either the direct energy wave traveling through the ground to the receiver, or energy which has traveled down to the top of the bedrock and then refracted back to the ground surface (Figure 3). For this reason, the first breaks, which were carefully measured for reflection static correction, can also be used for refraction analysis of the near surface. We developed a simple refraction analysis method to take advantage of this large amount of near-surface data. This analysis provides an estimate of the thickness of the near-surface materials (soil and weathered bedrock) every 10 ft (3 m) along the survey line. It is designed for the specific conditions along IL 29 and has the following restrictions:
- Near-surface materials (soil and weathered bedrock) are treated as a single shallow layer over bedrock;
- The shallow layer is not thicker than approximately 30 ft
In most of the surveyed area, these assumptions are valid. One exception has been found on the most southern section, where two shallow layers are present above bedrock. Since mine voids are not observed or suspected in this short section, depth to bedrock has not been computed.