Feasibility of joint inversion using gravity, magnetic and DC resistivity data around SMS

Due to the growth in the demand of metal resources, submarine massive sulphides (SMS) have become of great interest as one of new types of natural mineral resources. Several geophysical and geological surveys have been conducted for SMS, and there are a few examples that report the approximately estimated amount of deposits. However, naturally speaking, much sophisticated and accurate interpretation of the deposit amount is necessary for the development of SMS. Potential field measurements using gravity and magnetic susceptibility, and electric surveys have been widely used over the years as major technology in the exploration of metallic mineral deposits. The inversion of the potential field data, however, has been known as a non-uniqueness problem expressed in the Green's equivalent layer theory. We first developed an inversion code for gravity data in which the subsurface structure is discretized to a set of rectangular blocks using a least square criterion. We estimate the density, i.e., model parameter, of each block through the minimization of the quadratic norm of the residuals of the synthetic potential field calculated for the model from the observed one. We also minimize the roughness of the model parameter. The regularization parameters that control the degree of roughness are determined in the L-curve criterion scheme. Our results showed that the inversion result has lower resolution in depth than in horizontal directions only with gravity data. We then developed a similar inversion code for magnetic susceptibility data. Our magnetic inversion results also showed low resolution in depth. We then developed an inversion algorithm of DC resistivity data. Our electrical resistivity inversion results showed high resolution in depth and low resolution in horizontal directions. As is clearly implied in the results of inversions, every method has its own good and bad points. We finally developed an inversion algorithm to use all the above different physical parameters, i.e., gravity, magnetic and DC resistivity survey data assuming a linear relationship in each pair of density, magnetization and resistivity. As a preliminary attempt, we conducted a joint inversion using gravity and magnetic data sets and our results showed higher accuracy and resolution than those by the individual inversions. In this study, we present a 3D joint inversion method of gravity, magnetic and electrical surveys to improve the resolution of subsurface structure and to estimate physical parameters of buried materials around a SMS region.