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Jere ?0, 2000 Puge 6 <br />t used. The resolution and accuracy would be improved by conducting seismic surveys several times <br />over the life of the cavern. Tlris takes advantage of seeing changes in seismic response with cavern <br />growth. A full 3D seismic monitoring approach over one square mile would cost about $180,000 <br />' (BxplorTech, LLC-J. Edward Blott). <br />4.3 Vertical Seismic Profiling <br />Vertical seismicprofilingisbasicallyahybridof3Dseismicandcross-hole seismic. Sources <br />' are sent from the surface and received by a downhole geophone (Figure 2). Resolution by this <br />method is improved over 3D seismic, as both direct and reflected waves are used in interpretation. <br />A system of sources can be tailored to provide ample data fora 3D interpretation of the cavern <br />geometry. Some problems may exist with the instrumentaticnatelevatedtemperaturesabove350°F <br />' (Schlumberger-Web site). Tools capable of 450°F exist, but these tools are not three-component <br />tools, leading to lower resolution. Configurations using a dedicated drilled hole for the downhole <br />' geophone are possible. This hole could be located in cooler rock; however, extra cost is associated <br />with the dedicated hole. Certain well hole completion setups, for example in large-diameter <br />' production wells, can also present problems and reduce resolution. A basic vertical seismic profiling <br />survey ofone well could cost as much as $120,000 (Schlumberger-Brian Pluemer). Alternatively, <br />' one setup could be used to determine the shape ofseveral surrounding caverns, thereby reducing the <br />_ cost per cavern. <br />4.4 Cross-hole Seismic <br />Cross-hole seismic technology is similar to that of standard seismic methods, except that in <br />cross-hole methods signals are transmitted and received from within boreholes (Figure 3). This <br />method has been shown successful in detecting and characterizing subsurface fractures (Niitsuma <br />et al. 1997, Hayles et al. 1999, and Tweeton et al. 1988). For the best results, several source and <br />receiver locations would be used to obtain many seismic ray paths. This would require a series of <br />monitoring wells to be developed around the well. Several sources claim cross-hole seismic methods <br />to be expensive (Cook 1974 and Schlumberger-Brian Pluemer). It is possible that sun'ounding wet I <br />holes could be utilized to provide enough ray paths for acceptable resolution of cavern geometry. <br />Currently, it is repoiKed that problems witli this technology exist at temperatures greater than 175°F <br />Agapito Associates, Inc. <br />