Earthing Project - PLTMG Jayapura (2017)Electrical resistivity studies in geophysics may be understood in the context of current flow Grassberg prospecting (2013) Gorontalo Prospecting (2010) Resistivity through a subsurface medium consisting of layers of materials with different individual resistivities. For simplicity, all layers are assumed to be horizontal. The resistivity of a material is a measure of how well the material retards the flow of electrical current. Resistivities vary tremendously from one material to another. For example, the resistivity of a good conductor such as copper is on the order of 10^-8 Ohm.m, the resistivity of an intermediate conductor such as wet topsoil is 10 Ohm.m, and the resistivity of poor conductors such as sandstone is 10^8 Ohm.m. Due to this great variation, measuring the resistivity of an unknown material has the potential for being very useful in identifying that material, given little further information. In field studies, the resistivity of a material may be combined with reasoning along geologic lines to identify the materials that constitute the various underground layers. Resistivity is often first encountered in physics when discussing the resistance of an ideal cylinder of length L and cross-sectional area Aof uniform composition. The resistivity appears as the material-specific constant of proportionality in the expression for the total resistance of the cylinder. - R = rho (L/A) (1)
- The total resistance R may be obtained experimentally through Ohm’s law,
R =V/I, where V is the potential difference between the ends of the cylinder and I is the total current flowing through the cylinder. Edge effects are not considered. The resistivity of the material, an intrinsic property of the material, is then related to experimentally measured extrinsic parameters by - rho = (V/I )*(A/L) = Rhoapp*K (2)
Induced PolarizationInduced polarization (IP) is a geophysical imaging technique used to identify the electrical chargeability of subsurface materials, such as ore. The method is similar to electrical resistivity tomography, in that an electric current is transmitted into the subsurface through two electrodes, and voltage is monitored through two other electrodes. Time domain measurementsTypical transmitted current waveform and potential response for time domain resistivity and induced polarization measurements. Time domain IP methods measure considers the resulting voltage following a change in the injected current. The time domain IP potential response can be evaluated by considering the mean value on the resulting voltage, known as integral chargeability or by evaluating the spectral information and considering the shape of the potential response, for example describing it with a Cole-Cole model. Frequency domain measurementsFrequency domain IP methods use alternating currents (AC) to induce electric charges in the subsurface, and the apparent resistivity is measured at different AC frequencies. |

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