SIGNALS OF A RESISTIVITY LOGGING TOOL WITH TOROIDAL COILS: PHYSICAL AND MATHEMATICAL MODELING

The study compares in detail the signals of an electromagnetic tool with toroidal coils which were measured in an electrolytic tank with a borehole and numerically calculated in its three-dimensional geoelectric model. For each electrical resistivity of the electrolyte, we performed the profiling of the air–tank and tank–borehole boundaries during round-trips of the tool. The values of the coupling coefficient of the measured and modeled signals have been determined for the entire set of frequencies and positions of the measuring coils in the summary and differential operating regimes. We have identified a pair of signals with a virtually constant coupling coefficient at varying electrolyte mineralization. Drawing on this pair, transformations of the tool signals into the apparent electrical resistivities of the geoenvironment have been constructed. The resulting transformation graphs allow a reliable recalculation of the measured signals of the toroidal tool into the apparent electrical resistivity distribution in the near-wellbore space, which is necessary for the petrophysical interpretation of the field log data.