Abstract:Spectral induced polarization (SIP) is an effective means to explore an induced polarization target and deep minerals in geophysical prospecting. Induced polarization (IP) is a geophysical imaging technique used to identify subsurface materials, such as ore. The method of IP is similar to electrical resistivity tomography, in that an electric current is induced into the subsurface through two electrodes and voltage is monitored through two other electrodes. IP is a measure of a delayed voltage response in earth materials. The IP effect is caused by a current-induced electron transfer reaction between electrolyte ions and metallic luster minerals and is a measurement of the electrical energy storage capacity of the earth. The IP effect can be determined by passing an induced current into the ground and measuring the change in voltage with respect to time (TDIP), or by measuring changes in phase at a given frequency with respect to a reference phase (FDIP). To produce an IP effect, fluid-filled pores must be present, since the rock is essentially an insulator. The IP effect becomes evident when these pore spaces are in contact with metallic luster minerals, graphite, clays, or other alteration products. IP effects make the apparent resistivity of the host rock change with frequency. Generally, the rock resistivity decreases as the measurement frequency increases. Understanding the rock spectrum parameters, which are obtained through resistivity and phase inversion, is the most important step. There are many methods applied in rock spectrum inversion. Generally, when using the least squares algorithm, the inversion result is primarily determined by the initial value of each parameter. In this paper, the authors will show that the initial value based on the least squares algorithm can be selected randomly from the scope of various parameters. In order to demonstrate the practicality of this method, the theoretical data determined by Luo Yan-zhong is compared to shale measurement data (complex resistivity and phase) measured by an SI-1260A Impedance Analyzer. The shale was saturated before starting the experiment and the measuring conditions were room temperature and standard pressure. During the measurement, the frequency was applied at a total of 61 points, varying from 0.01 Hz to 10 kHz and the measuring mode was a symmetric quadrupole. In order to prevent the partial saturation fluid to evaporate and alter the electrical characteristics of the rock samples, both of the shale core's ends were soaked in the saturation fluid. In order to reduce the electrode polarization effects, the annular high-purity platinum network was used as unpolarized electrodes. In order to reduce noise disturbance, each core was measured a minimum of three times until the error of two adjacent measurements was within the permissible range. The experiment was automatically controlled by a computer during the entire process. Impedance and phase parameters were obtained from the experiment. Complex resistivity could be calculated through the impedance according to the geometric parameters of the shale cores. The single and double Cole-Cole models are selected to study the complex resistivity properties of shale. The inversion results show that the complex resistivity and phase curves can be described well and the spectrum parameters also can be obtained precisely. Simultaneously, relative to blindly selecting the initial value, this method has the advantages of avoiding local minima and performing at a high speed, and it overcomes the previous detrimental impact on the results.
