Dear Colleagues,

The goal of mineral exploration is the discovery of new deposits of economic value with the purpose of extracting them to feed the needs of industry. Geophysics describes a set of non-intrusive techniques useable to image the subsurface and extract their physical properties. It is used in mineral exploration to reduce risk associated with geochemical and geological data and drilling. In the last several decades, a great deal of progress has been made in the development and use of geoelectrical and electromagnetic geophysical methods in mineral exploration—especially regarding the exploration of targets located the first kilometer below the ground surface. In addition to electrical conductivity, induced polarization is a very sensitive technique that can be used to localize and decipher minerals in the subsurface of the Earth. This Special Issue targets contributions from laboratory to field measurements, numerical forward and inverse modeling, as well as petrophysical models able to connect the properties of minerals to the geophysical properties of interest. Our goal is to provide an updated view of the state-of-the-art in terms of geoelectrical and electromagnetic methods applied to mineral exploration. This Special Issue is also open to a variety of methods, including magneto-resistivity, magneto-induced polarization as well as seismoelectric and electroseismic effects, just to cite a few.

Dr. André Revil
Dr. Damien Jougnot
Dr. Jacques Deparis
Guest Editors

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• induced polarization
• mineral exploration
• electrical conductivity
• petrophysics
• inverse modeling

Tentative title: Estimation of electrical spectra of irregular embedded samples
Authors: Malte Wichmann and Andreas Hördt, TU Braunschweig
Abstract:
In order to interpret spectral induced polarization( SIP) data measured in the field for the purpose of mineral exploration, laboratory investigations are necessary that establish relationships between electrical parameters and mineral properties. For massive ores, and in particular for seafloor massive sulfides (SMS), samples may be difficult to obtain, and it is often not desired to cut cylindrical plugs, out of the available hand pieces. We suggest a method to obtain the electrical spectra of hand pieces from measurements on the samples embedded in a non-polarizing medium, in our case quartz sand. This way, destroying potentially precious samples is not necessary. The frequency-dependent phase spectrum of the sample is calculated by dividing the bulk spectrum with a so-called dilution factor, which is obtained from numerical simulation and has a real and constant value. We evaluate the method with a set of SMS samples where conventional cylindrical plugs are available. We can estimate the phase shift maximum of 73% of the samples with a deviation less than 50% from the reference. The estimation quality slightly decreases if the dilution factor is approximated by volumetric share of the sample. We consider the performance acceptable if the general difficulty to obtain reproducible and representative laboratory measurements for massive sulfides is taken into account.

Tentative title: Measurement of dried seafloor massive sulphides
Authors: Malte Wichmann and Andreas Hördt, TU Braunschweig
Abstract:
We carried out spectral induced polarization (SIP) measurements on a set of dried seafloor massive sulphide samples and compared the results with those obtained with the same samples fully saturated with NaCl solution. We find that the conductivity and polarizability are generally high for both dried and saturated samples, i.e. exhibiting phase shifts in the order of 100s of mrad and imaginary conductivities up to 1 S/m. Depending on the particular sample, the polarizabilities of the dried samples remain as high as for the saturated samples, or are slightly reduced. The high polarizability of dried samples and the fact that polarizability cannot be destroyed by drying are significant observations, because most of the existing theories to explain the polarization of mineralized rock assume a pore space filled with an electrolyte. We also found that the often used agar gel is unsuitable to couple the dried samples to the electrodes, because it releases water into the sample. Coupling with plasticine is a feasible alternative, because significantly less fluid is absorbed by the sample when it is incorporated into the sample holder.