| Description |
Complex resistivity and dielectric data may be interpreted quite simply in terms of relaxations. Ridge regression inversion has been used to fit various simple relaxation models to complex resistivity measurements of mineralized and clay-bearing terrestrial rocks, and to dielectric measurements of returned lunar samples. The model-fitting procedure allows 1) limited identification of the physical processes responsible for conductive and dielectric behavior, 2) parameterization of data so that slight variations in spectra due to changes in temperature, composition, concentration, or qrain size may be characterized accuretely , and 3) easy transformation between the frequency, time and distribution function domains via the analytic forms for the relaxations in each domain, and the relaxation parameters determined by inverston of the observed data. Of all the relaxation models which have been considered, the Cole -Cole model has been found to provide the simplest, yet most accurate, representation of the complex resistivity behavior of mineralized rocks. This model has been used to fit in-situ measurements made over the frequency range 10_2 to 10 +5 hz on 26 North American magnetite, pyrrhotite, graphite, massive sulfide and porphyry copper deposits. Two of the model parameters, the DC resistivity and the frequency dependence, show relatively minor variation for different types of mineralization. As a result of the typically low value of frequency dependence (around 0.25) for natural mineralization, it is relatively easy to identify and remove, through inversion, the effects of inductive electromagnetic coupling (which tends to have a frequency dependence near 1.0). The main variation in model parameters w ith different types of mineralization occurs in the chargeability, and particularly in the time constant, of the relaxation. Graphite, pyrrhotite, and "wet" porphyry deposits are associated with large time constants whereas volcanogenic massive sulfides, magnetite and "dry" porphyry deposits are associated with small time constants. Since studies of the impedance of the mineral-electrolyte interface reveal relatively small variations with mineral type, the differences in time constant and chargeability appear mainly due to characteristic differences in minerel texture rather than differences in composition. Measurements over mineral deposits of limited size result in apparent complex resistivity spectra which are different from the true spectra. However, the relaxation parameters describing apparent spectra may be predicted from the true relaxation parameters and from the same dilution factor used to obtain apparent polarizability from true polarzability. Calculation of the dilution factor is straightforward only if the geometry and resistivity of the source is known. A simple program, developed to invert apparent resistivity and poIarizability pseudosections, provides this information. |
