Tomography Between Wells, A Transient Dielectric Logging Tool, And The Very Early Time Electromagnetic (Vetem) System

This paper reviews progress on three site characterization technologies we are developing<br>at the U.S. Geological Survey (USGS).<br>We produced hole-to-hole electromagnetic and seismic tomograms of fractured crystalline<br>rock at the Mirror Lake, New Hampshire, hydrology research site. Both electromagnetic<br>and seismic tomograms between well pairs correlate with zones of high hydraulic<br>conductivity. We processed the electromagnetic full wave recordings in a variety of ways<br>to produce velocity tomograms, attenuation tomograms, and dispersion tomograms. Since<br>tomograms image not only fracture zones, but lithology as well, it is important to examine<br>all additional available geological, hydrological, and geophysical information in order to<br>achieve the most accurate interpretation of the tomograms.<br>Dielectric single-hole tools should be useful for direct detection of non-aqueous phase<br>liquids. Borehole dielectric logging tools were developed by the petroleum industry for<br>measuring oil saturation in cases where electrical resistivity is too high for resistivity tools<br>to be effective. However, for many shallow environmental applications existing commercial<br>borehole tools are physically too large to be practical. Previous experience with single-hole<br>borehole radar suggested to us that a time-domain dielectric logging tool might be feasible.<br>An experiment in a laboratory sandpack confirmed that supposition. A further experiment<br>is in progress to assess resolution of such a tool and to measure the effects on the calculated<br>values of dielectric permittivity of borehole diameter and tool location in a borehole.<br>The very early time electromagnetic (VETEM) system is a time-domain, surface-deployed<br>system that has potential applications to direct detection of non-aqueous phase liquids as<br>well as to the detection of buried objects. It is designed to operate in environments that are<br>too conductive for ground penetrating radar to be effective, and too.shallow for standard<br>electromagnetic systems. The VETEM system provides a faster profiling alternative to the<br>frequency-domain high frequency sounder (HFS) we been built and successfully applied in<br>a number of field experiments. The VETEM system construction is essentially complete.<br>Laboratory system evaluation tests are in progress, and field tests will soon follow.