Electromagnetic coupling effect on the spectral induced polarization (SIP) data

Summary :Spectral Induced Polarization (SIP) is widely used for environmental and hydrogeophysics, but one major limitation concerns the electromagnetic (EM) coupling effect. In this study, an overview of the mutual impedance of a polarizable multilayered homogeneous half-space is done (it consist both of electromagnetic coupling and the Spectral Induced Polarization effects). Then, the effect of electrode array parameters and the subsurface electrical resistivity on mutual impedance response is investigated. The results show the EM coupling effect can be seen at frequencies less than 10 Hz. It will increase with increasing frequency of excitation current and the conductivity. So by choosing the optimal arrangement for current cable, SIP can be surveyed using electrode arrays such as Schlumberger electrode array (with higher signal to noise ratio) and the EM coupling effect is also reduced as much as possible.
Induced polarization (IP) is the main geophysical method in mineral deposits prospecting. Spectral induced polarization (SIP), an extension of the IP method, in the past few decades has been used extensively in mineral prospecting and increasingly in environmental investigations, hydro-geophysics, archaeo-geophysics, bio-geophysics. SIP measurements are very sensitive to the low frequency capacitive properties of rocks and soils. One of the major limitations of SIP method is the EM coupling effect. In SIP method, the amplitude and phase components of the earth’s resistivity are measured in a frequency range (typically 0.001 Hz to 10 kHz). At low frequencies, the inductive coupling effect may affect the spectrum ohmic responses and normal polarization effect of the subsurface material. In SIP, there are three types of the EM coupling effect: the first is the removal of the EM coupling effect from SIP field data. In the second type, the mutual impedance of the earth is calculated using Cole-Cole equation as IP dispersion of the earth. SIP data and mutual impedance are compared using an inversion algorithm in order to recover the earth IP parameters. In this method, since the SIP method employs alternative fields using grounded wires, it should be characterized as an EM method. The third uses a current cable arrangement in order to reduce the EM coupling effects from SIP data.
Due to the increasing application of SIP method during the recent years, investigating and finding solutions to its limits is necessary. Because in this method, time-varying current signal is used, electromagnetic coupling occurs between the Earth, current cable and potential cable and electrical response affected by this phenomenon. So the calculation of earth's mutual impedance response and the separation of earth's electrical and electromagnetic responses are necessary.
Methodology and Approaches :In this study, an overview of the electromagnetic coupling effect on the Spectral Induced Polarization data and calculating this effect on the total response using linear dipole-dipole electrode array with dipole arbitrary length on a polarizable multilayered homogeneous half-space is done. Then, the effect of electrode array parameters and the subsurface electrical resistivity on mutual impedance response is investigated. We used CR1D mod code in order to calculate the mutual impedance of the earth (it consists both of SIP and electromagnetic response of the earth). At the end, the effect of current cable arrangement on electromagnetic coupling and mutual impedance response is investigated.
Results and The results show that the electromagnetic coupling effect can be seen at frequencies less than 10 Hertz. The induced impedance response will increase by increasing the survey's dipoles length as well as increasing the distance between the two consecutive dipoles. So we can conclude, when electrode array is pole-dipole, pole-dipole and linear Schlumbeger, the electromagnetic coupling effect will be much larger when electrode array is dipole-dipole; if the rectangular arrangement is chosen for current cable, the Schlumberger electrode array with low signal to noise ratio is used for SIP survey. On the other hand, the EM coupling effect will increase with increasing frequency of excitation current and the conductivity. So by choosing the optimal arrangement for current cable, SIP can be surveyed using electrode arrays such as Schlumberger electrode array and the EM coupling effect is also reduced as much as possible
Due to the increasing application of SIP method during the recent years, investigating and finding solutions to its limits is necessary. Because in this method, time-varying current signal is used, EM coupling occurs between the Earth, current cable and potential cable and electrical response affected by this phenomenon. So the calculation of earth's mutual impedance response and the separation of earth's electrical and EM responses is necessary. In this study, an overview of the EM coupling on the SIP data and calculating this effect on the total response using linear dipole-dipole electrode array with dipole arbitrary length on a polarizable multilayered homogeneous half-space is done. Then, the effect of electrode array parameters and the subsurface electrical resistivity on mutual impedance response is investigated. At the end, the effect of current cable arrangement on EM coupling and mutual impedance response is investigated.
The results show that EM coupling effect can be seen at frequencies less than 10 Hz. The induced impedance response will increase by increasing the survey's dipoles length as well as increasing the distance between the two consecutive dipoles. So we can conclude, when electrode array is pole-dipole, pole-dipole and linear Schlumbeger, the EM coupling effect will be much larger when electrode array is dipole-dipole; if the rectangular arrangement is chosen for current cable, the Schlumberger electrode array with low signal to noise ratio is used for SIP survey. On the other hand, the EM coupling effect will increase with increasing frequency of excitation current and the conductivity. So by choosing the optimal arrangement for current cable, SIP can be surveyed using electrode arrays such as Schlumberger electrode array and the EM coupling effect is also reduced as much as possible.