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Pilot Test for Mapping of Solution-Mined Cavern Using NSA's <br />Seismic Reflection Technology, TRTTM <br />Section 1.0 Introduction <br />American Soda, L.L.P. conducts solution mining to recover nahcolite from Federal lands <br />located in northwestern Colorado. American Soda contracted NSA Geotechnical <br />Services, Inc., to (1) evaluate the effectiveness of using NSA's seismic reflection <br />technology, TRTTM, to estimate of the size and shape of the caverns resulting from this <br />solution mining, and (2) recommend a system and procedure for the evaluation of other <br />caverns based on the outcome of the test. <br />At the time of the survey, the cavern of We1128-21 was filled with brine heated to a <br />temperature of approximately 200 to 275 degrees Fahrenheit. The crown of the cavern <br />was estimated to be at 1,680 feet below the surface, and the floor of the cavern was <br />estimated to be at 2,150 feet below the surface. Specifically, the objective of this project <br />was to produce two- and three-dimensional tomographic images of the average cavern <br />perimeter as defined by seismic energy reflected from the cavern boundaries. <br />This report includes a description of the test procedures, data analysis, and a brief <br />interpretation of results. <br />TRTTM Background <br />TRTTM uses seismic waves to identify structures within the rock mass that reflect seismic <br />waves. The technique is based on acoustic impedance contrasts (the product of density <br />and seismic velocity) that occur at boundaries between geological layers or <br />discontinuities. These discontinuities act as imperfect mirrors, returning part of seismic <br />energy to a detector; this energy is then analyzed to determine the location and nature of <br />the reflecting boundary. A transition from a material with lower acoustic impedance to <br />one with a higher value results in a positive reflection coefficient, and vice versa. <br />Features such as fractured zones within a more solid rock mass will also give rise to <br />reflections. The larger the acoustic impedance contrast, the larger the reflection <br />coefficient, and the easier it is to detect the echo. <br />A typical TRTTM survey uses a fixed array of about 10 sensors (receivers) and seismic <br />sources (blasts or hammer blows) initiated at different locations. For each source, a <br />seismograph records a data file that contains seismic signals received at each sensor. The <br />seismic signals are inspected to obtain the arrival times of P- and/or S-waves that <br />determine the seismic velocity of each wave type in the rock mass. These velocity values <br />are used to define the velocity model within athree-dimensional orthogonal block <br />selected for seismic data processing. The volume of the block is subdivided by a <br />regularly spaced cubic grid, the size of which determines the resolution of the ground <br />image to be reconstructed by the TRTTM from reflected seismic waves. <br />