Seismic imaging: Methodology
Theoretically, underground voids, such as mine workings or collapsed mine workings, exhibit very strong impedance contrasts with surrounding rock and have good potential for being observed by the reflection method. The impedance contrast of Quaternary sediments or weathered bedrock (both having low velocity and density) with unweathered bedrock (having high velocity and density) is also very strong. In practice, the complex geometry of the voids often makes them difficult to distinguish from background noise in the data. Also, irregularities in the shallow sediments (thickness and material type) introduce strong distortions in the reflection images that may lead to erroneous interpretations of voids where none exist. Special attention has been given to the correction of this problem.
During the processing and interpretation phases we added a special refraction interpretation routine similar to refraction statics corrections. This process resulted not only in the static correction, but in a simple, 2-layer model of the shallow subsurface. We used this model to explain several irregularities in the reflection data, and it can also be used as an engineering tool for road design. In particular, where the refraction model indicated that the bedrock was very shallow (less than 10 ft beneath the ground surface) the reflection data frequently contained strong, spurious coherent noise, known as guided waves (Robertson et al., 1995). We were only partly successful in removing this noise from the data set, so having the refraction data as an independent check helped to confirm where signal degradation was caused by actual subsurface conditions or was an artifact of the surface conditions.
Preliminary tests across known underground coal and clay mines revealed that standard refraction methods are not compatible with the subsurface conditions at the site. Signal variations attributable to the possible presence of collapsing structures cannot be easily separated from signal variations caused by changes in bedrock depth and other subsurface variations. However, the same tests showed that seismic reflection methods would give good results.