During 1997 a suite of borehole geophysical surveys were completed at the Dover National Test<br>Site (DNTS), Dover Air Force Base (DAFB) to demonstrate the efficiency and accuracy of<br>geophysical methods in determining the areal extent of cement bentonite subsurface barriers. Two<br>barriers were emplaced as vertical walls keyed into a clay aquitard with a modified jetting<br>technique. These barriers are denoted as the ‘shallow’ active’ and ‘deep passive’ barriers extending<br>to 5 and 13 m, respectively. The active and passive descriptors refer to the hydraulic and gaseous<br>tracer work performed at the sites. Demonstrated methodologies included electromagnetic<br>induction logging (EM), ground penetrating radar (GPR), electrical resistivity (ER), and borehole<br>seismic. The geophysical instruments were deployed from boreholes or permanently emplaced<br>vertical electrode arrays (VEAs).<br>The purpose of the work was to develop a robust methodology for the verification and monitoring<br>of a subsurface barrier. Site characterization data were used to estimate the physical properties of<br>the background host and laboratory measurements were performed to estimate the properties of the<br>grout. Based on these data numerical models were computed for survey design and interpretation.<br>Representative data and discussion for each of the methods are presented. Additionally, a brief<br>discussion is presented on the site preparation activities. Preliminary results indicate that the GPR<br>and ER methods are successful at imaging the area1 extent of the barrier and assist with leak<br>detection through ER monitoring of a salt water flood test. Future work will combine the<br>information obtained from the geophysical measurements with the hydraulic and gaseous tracer<br>results to produce a comprehensive understanding of the quality and performance of the barrier.


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