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Apri! IS, /999 Page 2 to the subsidence monitoring plan as mining progresses and these markers fall within the potential azea of influence of mining. 23 Surface Monitoring Frequency and Duration Surface monument measurements will be conducted annually. Monitoring of surface monuments will continue for the duration of the Yankee Gulch Project. Upon :reclamation of a well Geld panel (or area), monitoring will continue for five years. If movement is detected, subsidence monitoring will continue for a period of five years following the last detection movement. The surface subsidence monitoring frequency beyond the cessation of commercial operations at the Yankee Gulch Lease will be detem~ined prior to lease relinquishment. 3.0 SUBSURFACE MONITORING The purpose of the subsurface monitoring is to ascertain movement of strata overlying the solution cavities. A combination of time domain reflectometry ('TDR) and borehole geophysical techniques will be employed for determination of subsurface movement. 3.1 Time Domain Reflectometry TDR will be used for monitoring subsurface strata movement. Coaxial cable for TDR S measurements will be installed in the fast 23 of the first 26 production wells during the fast phase (~5 years) of commercial operations. TDR is an electrical pulse-testing technique originally developed bythe telecommunications industry for locating faults in coaxial power transmission cables (Dowding et al. 1988). A cable tester is used to create a voltage pulse m the cable. When faults, crimp:,, or elongations are encountered in the cable, a reflected signal returns to the cable tester as a result of the change in electrical wire properties. Knowledge of the properties of the wire permit location of cable damage by measurement of travel times. A coaxial cable will be attached to the outside of the steel casing installed as part of a solution cavity well completion (Figure 2). The cable will be grouted in place in the drillhole by tremie placement of cement during well completion. Once installed, TDR waveforns will be collected according to the monitoring schedule in Section 3.3. Cable testing hardware will be connected to the coaxial cable installed in each well and • Agapito Associates, Inc.