Seismic imaging: Results
As an example of the results we achieved in this study, we present a 7000 ft (2.33 km) section of one of our seismic lines south of the village of Sparland (Figure 6). Mine maps indicate that the old clay mine underlies at least part of this section. In contrast with the section displayed in Figure 3, this section shows strong changes in amplitude and oblique reflections which can possibly be interpreted as diffractions. Some of these apparent diffractions collapse during the migration process. However, because many of the 3-dimensional subsurface structures causing these diffractions are off-line relative to the seismic line, the 2-dimensional migration algorithm is not successful in removing all the oblique noise. Slope stability has been an ongoing problem in this stretch of IL-29. A tied-back retaining wall was installed along a half-mile section in order to stabilize the base of the bluff. In this illustration, static corrections and filtering have been applied. Approximate depth scales are shown. Geophysical logs from Borehole 3 are shown for correlation. Borehole 3 was terminated at a depth of 251 ft when drilling operation encountered a void, presumably in the old clay mine. Incoherent noise that disturbs the reflections is present in several areas at that depth. We have circled areas where the noise is not associated with either surface features such as culverts or processing artifacts from guided waves. These areas are within the boundaries of the mapped clay mine and we believe they indicate voids in the bedrock. Several smaller diffractive features are also present in this section, but are not individually noted. Both the northern and southern edges of the mine appear to be independently imaged in this section.