Seismic imaging: Borehole analysis, Seismic calibration and Horizon picking
Three boreholes were drilled to provide geologic control for this project. Data from one of these boreholes, borehole 3, is presented in Figure 5, location in Figure 1. This borehole was drilled in an area suspected of being undermined. It was located near the foundations of abandoned surface structures associated with the old clay mine. Drilling operations were halted when a void was encountered at a depth of 251 ft. According to mine notes on file at the ISGS, this is the depth of the mine.
Lithologic and stratigraphic analyses were based on core extracted from the borehole. A suite of geophysical logs provided correlation with the seismic reflection data. Several attempts were made to acquire a Vertical Seismic Profile (VSP) and a velocity log based on first breaks using a down hole geophone and a down hole multi-channel hydrophone array (provided by the Geological Survey of Canada, Ottawa). However, signal quality, particularly at this site, was so poor that first breaks could not be picked. We suspect that this is related to fracturing associated with the mine operation. Signal quality was somewhat better in borehole 2 (location in Figure 1) which was located in an unmined area. We were able to determine an average P-wave velocity of 8500 ft/s (2590 m/s) for the bedrock. This is the same value determined by the refraction analysis. The seismic reflection profiles were then converted to depth sections using the constant average velocity of 8500 ft/s and exported to the Kingdom Suite seismic reflection interpretation package.
Using the lithologic log and the electric logs, we were able to correlate strong reflections with the Hanover limestone (Figure 6, yellow horizon), the base of the Pleasantview sandstone, (Figure 6, light-blue horizon) and the base of the Isabel sandstone (Figure 6 green horizon). A fourth reflector is probably correlative to the Babylon sandstone or the Pennsylvanian-Devonian unconformity (Figure 6, red reflector).