3D forward modeling and parametric inversion of induced polarization with electromagnetic coupling

Abstract

The induced polarization (IP) method is commonly used in mineral exploration. In particular, it has been successfully applied to explore for disseminated sulfide and/or porphyry deposits, which often have strong induced polarization responses. IP methods can provide information about the chargeability and resistivity distribution of the subsurface. However, these methods may underperform under certain geological conditions, such as in the presence of highly conductive or resistive cover. In the presence of resistive cover, the direct current (DC) signal cannot penetrate beneath this zone, and in the presence of conductive cover, a short circuit results. In such cases, the inductive response of the Earth can potentially contain information about the subsurface geological units due to complimentary physics to the DC resistivity method. This is because the current flow generated by electromagnetic (EM) induction can penetrate resistive features. However, EM coupling is generally considered to be noise and unwanted content in the DC/IP signal. In the traditional processing and interpretation of IP data, inductive effects are usually eliminated. Although removing the electromagnetic effect to increase the sensitivity to IP signal is the main focus of the mineral exploration industry, there is a possibility to extract more information about the subsurface by using EM induction. However, in order to handle this multi-parameter physical problem, traditional DC/IP modeling is deficient. Moreover, the conductivity model needs to be defined by a frequency or time-dependent complex conductivity. Throughout this thesis, we investigate the relationship between IP and EM by simulating grounded source time-domain DC/EM/IP data with a complex conductivity model called the stretched exponential with SimPEG and SimPEG EMIP. This method allows us to model IP relaxation in the time-domain instead of using the Cole-Cole model (CC) in the frequency-domain. Cole-Cole in the frequency-domain is a standard approach in the context of complex conductivity modeling. However, it is computationally expensive to apply in time-domain since Fourier transform is needed. With the stretched exponential, one can simulate a grounded source time-domain DC/EM/IP survey with complex conductivity more efficiently. We used different geoelectrical models such as a chargeable conductive block in a resistive subsurface, a chargeable resistive block in a conductive subsurface, a chargeable conductive block buried under a relatively conductive cover, a chargeable conductive block buried under a relatively resistive cover, and a 3D porphyry-mineral deposit model. By using the full DC/EM/IP step response, we calculated apparent resistivities by using the peak arrival time of the signal obtained from normalized pseudoimpulse response. Our results show that this method contains more information about the subsurface than the traditional DC/IP method. Furthermore, we apply a parametric inversion to a simple block model and show that grounded source DC/EM/IP data with complex conductivity can be inverted. However, a full 3d inversion is necessary to characterize this highly non-unique and difficult EM problem.