The two-loop variable-spaced induction logging tool for containment applications: Design and field results

Clay zones near to but not intersecting an emplacement hole near the work point are a serious containment concern but are not detectable by any NTS logging tool. To develop such a detection capability a variable spaced two loop induction logging tool was designed, built and tested. The two loop induction tool consists of a locally powered 10 kHz resonance oscillated transmitter coil located below a receiver coil that can be spaced from 6 ft. to 150 ft. from the transmitter. All rigging is nonconducting and the transmitter current reference is sent via fiber optic cable to the logging cable head. A lock in amplifier compares the receiver signal to the transmitter reference and separates in-phase and quadrature components. These parameters are stored, along with the values of the tool depth, in an LSI-11 computer as the tool is raised. Logs can be plotted in the field or transferred to other computer systems for subsequent processing. The induction logging tool was fielded in holes U8n and U20aw. The results show the induction tool to be very sensitive to low formation resistivity and that it responded strongly to a known clay lens in U8n. The tool is weakly responsive to highmore » formation resistivities. The induction tool logs were very repeatable, of low noise, and agreed with known hole geologies and BHDH resistivity logs. 7 refs., 13 figs.« less