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June 2Q 2000 Page 4 <br />' Daring Comatercial Operrrtimrs: <br />' As caverns reach diameters of 100 to 200 R, 3D seismic methods become an alternative or <br />supplemental method to volumetric analysis as the preferred method. Comparison of 3D seismio, <br />cross-hole seismic, and vertical seismic profiling methods reveals advantages and disadvantages of <br />each. All these methods are based on interpretation of direct andlor reflected waves. Resolution by <br />' downhole seismic techniques is slightly better than by surface 3D seismic methods. In general, <br />vertical seismic profiling is preferred over cross-hole techniques because the later requires a <br />sufficient number of available holes for good resolution of a cavity boundary. Por characterization <br />of a single cavity, vertical seismic profiling may yield the best resolution per dollar spent, relative <br />' to 3D seismic or cross-hole seismic. Por a larger scale seismic survey of an entire well field, 3D <br />seismic techniques would be preferred over the downhole seismic methods. <br />3.0 OBJECTIVES <br />There are two main objectives of characterizing the nahcolite cavern geometry. One <br />' objective is to verify the integrity of the crown pillar between the top of the cavern and the <br />dissolution surface. The second objective is to demonstrate that pillars surrounding the caverns arc <br />intact, with no hydraulic communication between wells or caverns. <br />' 4.0 CANDIDATE TECHNOLOGIES <br />4.1 Volumetric Analysis <br />' By means of mass balance, a record of pressure and flow rate can be used to estimate the <br />volume of gas in the cavern at various stages by replacing the cavern fluid with gas (Berest 1986). <br />' The volume can also be estimated by monitoring the water level during the process of replacing the <br />' cavern fluid with gas. Assuming lateral isotropy with respect to growth preference, the shape of the <br />cavern can be estimated by stacking disc-shaped volumes calculated from the described mass balauce <br />information. A weakness of this methodology is an inability to identify changes in porosity. If a <br />great deal of caving in the leached zone were to occur, this method would under-predict the actual <br />cavern extents in rubblized zones and over-predict the boundaries in the corresponding void areas. <br />t <br />' Agapito Associates, hrc. <br />