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.hrne 20, 2000 Page 5 ' Inaccuracies due to changes in porosity throughout the cavern might be reduced by application of geophysical logging. Also, core assays could be utilized to minimize error in porosity estimation. ' With respect to well field stability, cavity shape characterization byvolumetric analysis from ' production records may be sufficient. Considering the mine plan, the well field has been laid out into panels of wells. The panels are separated by several hundred feet and wells are not currently ' planned between these panels. Within the panels the wells are spaced such that, ultimately, a 40 percent extraction ratio, in the areal sense, will be achieved if cavities reach 200 ft in diameter. ' Production monitoring of the total nahcolile removed from the panel will give constant updates into the percentage of the resource extracted. As long as the design is not exceeded (i.e., 40 percent ' extraction for the panel), it may be of I ittle consequence whether some pillars are larger than planned and others are smaller than planned. The arching mechanism of the overburden strata will result in ' load transfer to larger pillars within the panel and to the panel margins where no cavities are mined. Arching and the panel-based design of the well fields will result in the long-term stability of a panel ' that contains variations in pillar sizes that are equivalent to the long-term stability of a panel containing uniform pillar dimensions. 4.2 3D Seismic ' In seismic techniques, waves are propagated into the subsurface where variations in rock type cause reflections which can be received at the surface and interpreted (Figure I ). Utilizing multiple ' source and receiver locations, tomography can produce 2D or 3D representations of the cavern. ' Resolution obtainable by this method is generally YQ of the signal wave length (Blackhawk Geometries-Mark Blohm). High frequency waves give greater resolution, but propagation losses ' are also greater. Accuracies on the order of 20 ft should be feasible at depths between l 500 and 2000 ft (Geotrace Technologies, Inc.-Brad Feild). Ian Howrie of Scotia Group suggested that ' conventional3Dseismicmethodsarethemostreliabletechnologycapableofcharacterizingsolotion caverns at reasonable resolutions. A 3D seismic survey would consist of a grid of geophones set out on a predetermined spacing, probably between 20 to 60 ft. Sourcing could be generated by mechanical impact, ' explosive, or vibratory sources and would likely be set up on the same grid spacing. Truck rigs are typically used for sourcing, but where access is difficult ATV units or hell-portable units can be Agapito Associates, Inc.