b'Minerals geophysics Minerals geophysicsThe range of resistivities of geologicIP-resistivity pseudosection (see materials is extremely large (100 000Figure1). These results show that for ohm-metres and more for some igneousresistive disseminated sulphide targets, rocks to 0.00001 ohm-metres and less fordetection of which is considered a some massive sulphides). Furthermore,strong point for the IP method, using the distribution of resistivities appearsconventional Metal Factors could to be logarithmic in nature, rather thandiminish, or even obliterate, associated linear. That is, resistivities can only beIP anomalism. So, when dealing with positive, and the scaling increases byhistoric IP-resistivity data that comprise multiplication rather than by addition.only Metal Factors and resistivities, Thus, the contrast between 10 andit would be prudent to consider 11 ohm-metres is more equivalent toback-calculating the IP effects before Terry Harveythe contrast between 1000 and 1,100proceeding with further processing and Associate Editor ohm-metres, than to that betweenassessment.for Minerals geophysics 1000 and 1001 ohm-metres as in a linear distribution. Because of the hugeImplications from the logarithmic nature terry.v.harvey@glencore.com.au dynamic range and the logarithmicof resistivity distribution can be taken nature of the distribution, we tend to usefurther. What about inversions? Most log resistivities in presentations; base 10conventional resistivity inversion routines logs are more familiar, but natural logsuse resistivity as the input parameter. Log Rho, MF, LIP and HIP (base e) work just as well. Could it be more appropriate to use log resistivity? Ive run both numerous The petro-physical property electricalThe logarithmic nature of resistivitytimes, and the end results range from resistivity (rho) may be defined asdistribution does have somequite similar to very different. Taking the a measure of the resisting power ofimplications. Consider the historicalantilog at the end of the log resistivity a specified material to the flow ofMetal Factor (defined as inducedinversion to recover resistivities can an electric current. In the SI system,polarisation (IP) effect divided bydramatically amplify minor variations in the resistivity unit of measurementresistivity and multiplied by a constant).the log resistivity inversion. Arguably, is the ohm-metre (-m). That is, ifMetal factors were introduced in thein this case, the logarithmic process is a one metre solid cube of materialearly days of IP-resistivity to addressworking against a reasonable result.has sheet contacts on two oppositeconcerns about the diminution of IP faces, and the resistance betweeneffects with decreasing resistivity, andHowever, thinking about log resistivity these contacts is one ohm, then thewere even presented in some surveyin inversions did have an unexpected resistivity of the material is one ohm- results to the exclusion of IP effects.benefit. As I wrote in a previous column metre. Electrical conductivity (),However, in my experience, Metal Factor(see Mineral geophysics in Preview the measure of a specified materialspatterns were typically dominated215), the development of MIMDAS ability to conduct electricity, is definedby resistivity variations because of(MIM Distributed Acquisition System) as the reciprocal of electrical resistivity;resistivitys much larger dynamicin the late 1990s led to a resurgence the SI unit of electrical conductivityrange. Perhaps the denominatorin the use of IP-resistivity within our is Siemens per metre (S/m). In mineralshould have been log resistivity? Togroup. The MIMDAS pole-dipole geophysics, we tend to use resistivity forinvestigate further, I calculated bothdipole-pole array IP-resistivity results galvanic methods and conductivity forconventional and log resistivity basedwere processed in-house using the electromagnetics, but either is valid. Metal Factors for a dipole-dipole2D inversion software packages then Figure 1.Dipole-dipole IP-resistivity Pseudosections a) Resistivity, b) Induced Polarisation, c) Conventional Metal Factor, d) Metal Factor using log Resistivity.JUNE 2022 PREVIEW 36'