A New Approach to Measuring Formation Resistivity with Capactive Electrodes

摘要

A new Russian designed probe for measuring formation resistivities has been built under a collaborative research project funded jointly by the Geological Survey of Canada (GSC) and IFG Corporation, a Canadian manufacturer of borehole geophysical instrumentation. The resistivity measurements are based on the principle of electrical capacitance. The GSC carried outfield tests with a prototype probe, in several boreholes around Ottawa and at several massive sulfide deposits in Ontario and Quebec, Canada. The geological environments at these field evaluation sites are varied and include sedimentary, metamorphic and igneous rocks with a large resistivity range (<1.0 Ωm to>30,000.00 Ω-m). The capacitive resistivity field data were compared to normal array galvanic resistivity and EM39 inductive conductivity data. Results indicate that the capacitive technique is sensitive in the resistivity range from100 to10000 Ω-m and compares well with galvanic resistivities (100 ― 10000 Ω-m) and inductively measured resistivities (100 ― 1000 Ω-m). Field tests were also carried out using several transmitter-receiver spacings and in plastic-cased fluid-filled boreholes. Capacitive resistivity logs acquired in a plastic-cased borehole are almost identical to those acquired in the same but uncased borehole and, therefore, promise to be valuable logs for use in environmental applications where boreholes drilled in unconsolidated sediments often have plastic casing. Accurate resistivity measurements can also be acquired in air-filled boreholes and, therefore, provide formation resistivities in vadose zones.