A Perspective On The Problems Of Borehole Magnetic Susceptibility And Conductivity Measurements In Highly Magnetic And Conductive Base Metal Massive Sulphide Environments

摘要

The basic system for single-coil induction probes for measuring magnetic susceptibility andrnconductivity consists of an induction coil in an electrical bridge circuit with an alternating fieldrndriving the circuit. When the probe passes through magnetically susceptible material, the coilrninductance changes, causing the bridge to become unbalanced. There is a change in the signalrnamplitude across the coil arising from magnetic changes in the rock formation due to therninducing field and this is converted to magnetic susceptibility units. There is also a phase shiftrn(imaginary component) arising from eddy currents and hysteresis losses. The eddy currents arernnormally assumed to be negligible in common rocks, because of extremely low conductivity.rnMagnetic hysteresis effects in the magnetic susceptibility anomalies can cause apparentrnconductivity signals that are not separable from true conductivity signals generated by eddyrncurrents in the formation. Negative magnetic susceptibilities are observed in highly conductivernenvironments and the magnitude of this effect appears to be a function of instrumentation.rnIn this poster we present data that illustrate the cross-talk between the magnetic susceptibilityrnand conductivity signals. These data were acquired at a several sites including nickel sulphiderndeposits that are highly conductive and also contain pyrrhotite and magnetite with a highrnmagnetic susceptibility. Data are also presented from two VMS deposits; the Kidd Creek minern(Cu-Zn-Ag-Pb) and the Heath Steele mine (Zn-Cu-Pb) where magnetic pyrrhotite and/orrnmagnetite are common occurrences within the massive sulphides. We also compare the responserncharacteristics of two commercially available single-coil conductivity/magnetic susceptibilityrnprobes (IFG and OMS-Logg).