Physical Property Laboratory Measurements

Complex Resistivity

The complex resistivity (CR) or spectral IP determinations performed in the physical property laboratory are measured in the frequency or phase domain. The CR responses of these samples are determined utilizing a 4-electrode sample holder with isolation amplifiers, measured by a GDP series Geophysical Data Processor. The resistance of each sample is measured and an equivalent resistor is inserted in the measuring setup to balance out any stray capacitance which may influence the high frequency phase values.

All rocks are cut to provide a sample between 2 and 4 cm in length with parallel planar ends, vacuum impregnated with distilled water, and additionally soaked in distilled water for 3 days. The exception to this procedure is when the samples are cored for magnetic measurements. Then the samples are made into 1.0 inch bicylindrical cores as described in the magnetic susceptibilities procedure.

The results reported are the stacked and averaged values for 0.125, 1.0, and 8.0 Hz, including the 3rd, 5th, 7th, and 9th harmonics for each fundamental frequency. Complex-plane plots and magnitude/phase plots are presented along with tabulations of the spectral results, and apparent resistivity and phase at 0.125 Hz.

The spectral-type parameters are based on the slopes of the CR curves for the 0.125 to 1 Hz, 1 to 8 Hz, and 8 to 72 Hz data, using alphabetic notations of A, a, B, b, c, and C. The range of slopes for each spectral type are:

          Type        Slope
            A       greater than 20%
            a          10%  to   20%
            B           0%  to   10%
            b           0%  to  -10%
            c         -10%  to  -20%
            C       less than   -20%

Rock resistivities measured by laboratory techniques are often much higher than observed field resistivities. Laboratory measurements on small core samples do not adequately sample the large volume resistivities, which are grossly influenced by fractures and faults containing mineralized solutions. Fresh drill core samples which have not been exposed to the air or to elevated temperatures will sometimes yield representative rock resistivities in the laboratory.

The CR induced polarization values determined in the laboratory for disseminated sulfide samples are not similarly affected. Laboratory measurements on small core samples are often more representative of the macro-scopic induced polarization responses, and usually correlate very well with surface measurements. Exceptions to this are cores which contain massive sulfide mineralization or which contain through-going, mineralized fractures or veinlets.