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expected. Some of the cables have been compromised by the infiltration of water. Due to <br />operational difficulty, unreliability, and the propensity for failure, TDR methodology will not <br />be used by NS to address subsurface monitoring issues in future mining areas. <br />3.1.1 Borehole Geophysics <br />Borehole geophysical logs will be run periodically in subsidence monitor wells which are in <br />service and associated with active solution mining. These geophysical logs will be compared <br />to an original baseline geophysical log. A natural gamma ray (GR) and casing collar locator <br />(CCL) log will be used to detect subsurface movement. Due to elemental differences in the <br />chemical compositions of nahcolite and oil shale, distinct gamma ray signatures for each <br />material can be readily identified. Changes in the nature and depth of these signatures can <br />be used to determine the extent of subsidence. Likewise, changes in the position of a <br />casing collar as identified on a casing collar locator log can be used to monitor the extent of <br />any subsidence, as the grouted casing string may subside or fail along with the rock strata. <br />A gamma ray log will be run: a) during or immediately following completion of the well to <br />establish a credible baseline; b) during mining of the cavity associated with the well; and <br />c) following cessation of cavity mining. <br />3.1.2 Subsurface Monitoring Frequency and Duration <br />Borehole geophysical logging will typically be conducted four times for each well in service, <br />during the life of an injection well: (1) a baseline log following initial drilling of the well, (2) <br />after a production of 50% of the permitted tons of nahcolite from a cavity, (3) after a <br />production of 75% of the permitted tons of nahcolite from a cavity, and (4) after retirement <br />of the cavity. A sample comparison of subsurface subsidence geophysical logging is <br />presented in Figure 5. <br />Future wells will have a subsidence monitor well associated with every two cavities <br />pursuant to the EPA UIC Permit requirements. For a cavity pair, geophysical logging will be <br />conducted four times during the life of the cavity pair: (1) a baseline log following initial <br />drilling of the first well, (2) after the first well of the pair reaches a production goal of 50% <br />of the permitted tons of nahcolite, (3) after the first well of the pair reaches a production <br />goal of 75% of the permitted tons of nahcolite, and (4) after retirement of the cavity pair. <br />NS may submit a request for a reduction in the number geophysical logs required providing <br />justification with data obtained that indicates a lesser number of geophysical logs are necessary <br />for adequate subsurface monitoring.' <br />Daub & Associates, Inc. <br />Revised: 9/23/2015 <br />Page 13 Natural Soda LLC 2015 EMP <br />