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June 2Q 2000 Pogo 8 ' A basic survey of one well would cost about $10,000 to $15,000 (Chinook Geoconsulting-Karen Christopherson and Digital Magnetotellurics "technologies Daniel Woods). 1 ' 4.6 Iixploration Drilling Drilling exploration holes around and/or into the cavern is a relatively accurate method to ' probe the solution cavern boundary. To determine a 3D rendition of the cavern geometry would require several holes (Figure 5). Drilling exploration holes for this information is impractical when ' considerations such as cost, permitting, and interference with production are accounted for. Tests conducted by Shell Oil in the early 1970s provide experience with using core holes to ' determine nahcolite cavern geometries. These tests are of particular interest because they were conducted in the same nahcolitic oil shale resource that American Soda will be mining. Afrer some ' test solution mining, three core holes were developed through and near the cavern volume. One hole was drilled through the body of the cavern and the other two were drilled just outside the cavern. ' From measuring sonic transit times between the core holes and interpreting the cores, the geometry of the cavern was estimated to be as shown in Figure 6 (Prats et al. 1977). Also used in this ' interpretation of the cavern geometry were temperature surveys, injection tests, and volume measurements. These tests provide useful information about the material inside the cavern and the possible scenarios of the processes taking place during mining. However, little definitive information about the leached zone boundary has been learned from the rock core. The boundaries shown in Figure 6 were primarily estimated from cross-hole sonic measurements and are based heavily on interpretation. As previously discussed, invasive techniques are less favorable to non-invasive techniques with respect to production, expense, and permitting. 4.7 Magnetics Magnetics is the measure of variations in the earth's magnetic field. Magnetic anomalies or distortions in the magnetic field can be detected with magnometers. Sedimentary rocks, such as those found in the Piceance Basin, are typically low in magnetic susceptibility (Dorbin 1976). If maguetic material were injected into the wells, a magnetic anomaly at the leached zone interface would be created, making possible the detection of the cavern extents. The shape of the cavern Agapito Associates, Inc.