2-D Magnetotelluric Multiparameter Joint Inversion Considering the Induced Polarization Effect

Abstract

Magnetotelluric (MT) is an important geophysical exploration method that uses natural sources to study the electrical structure of the earth. This method is advantageous owing to its low cost, large exploration depth range, and high resolution for low-resistivity bodies. However, traditional MT modelling approaches can only invert the resistivity parameters of geological bodies, while natural geological bodies also exhibit the induced polarization (IP) effect. Applying the IP effect of geological bodies could be effective for exploring mineral resources, such as polymetallic ores, oil (gas) fields, and coal fields. To incorporate the IP information into MT inversion, we proposed a multi-parameter MT joint inversion algorithm that considers the IP effect. We used the Cole-Cole model to integrate four IP parameters into the forward algorithm: the zero-frequency resistivity ρ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> , chargeability η, frequency exponent <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</i> , and time constant <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</i> . The influence of each IP parameter on the forward response was analyzed by forward simulations, and we concluded that the parameters ρ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">and</i> η should be considered in the inversion. A cross-gradient function was introduced into the objective function of the Occam inversion method to constrain the structural consistency of ρ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> and η. Model testing and practical application results illustrated that the algorithm can not only obtain the subsurface resistivity structure that can be achieved by the traditional MT method but also reveal the distribution of the chargeability parameter. The additional information obtained using this algorithm is conducive to interpreting specific geological structures that cannot be distinguished by the traditional MT method.