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Recently, controlled-source electromagnetic methods are widely used for shallow sub-seafloor explorations. In this paper, we propose a new controlled-source electromagnetic method using an autonomous underwater vehicle (AUV) for the exploration of submarine massive sulphides (SMS). A numerical simulation code for 2.5-D or 3-D electromagnetic field in the frequency domain is employed in order to estimate electromagnetic responses from possible conductivity structures. As a result, we confirmed that we could detect the area of SMS effectively using Ty-Hx and Tz-Ez for transmitter and receiver. This is because the electric field is attenuated and the electric flux is concentrated to the interior of conductive zones. We also confirmed that the effects of topography were relaxed using normalized electromagnetic fields in the simulation. Based on our results, we think that the new controlled-source electromagnetic method is realizable.

We present a 3D joint inversion method to estimate two physical parameters, density and magnetization of subsurface materials using field intensity measurements. In the method, we introduce the fuzzy c-means (FCM) clustering technique to relate gravity with magnetic data. In the approach, the subsurface structure is discretized to a set of rectangular prisms. For estimating the density and magnetization of each prism, we minimize the quadratic norm of the residuals between the observed data and the calculated responses from inversion model. Two regularization terms, i.e. the roughness and the similarity of the two physical parameters, are introduced in our joint inversion to control the degree of model roughness and similarity. We determine their regularization parameters using the L-curve criterion. We apply our method to a numerical model which represents submarine massive sulphides (SMS). The joint inversion results, which have the advantages of both gravity and magnetic inversion, show better accuracy and resolution than the individual ones.

Seismic full-waveform inversion (FWI) method has been used to estimate subsurface velocity structure. FWI directly utilizes observed waveforms that could include information on the properties of subsurface materials. In seismic time-lapse surveys, we observe the difference between waveforms as a function of time for the change such as fluid alteration. Residual waveforms between the observed before and after a certain time interval are used to estimate the changes in the fluid distribution in terms of seismic velocities in FWI method. In contrast to the previous FWI applications, our research focuses directly on the properties in the hydrocarbon reservoir in order to estimate the fluid distribution and alteration. We simulate the wave propagation based on the Biot theory that includes the effects of fluid in porous media. The simulation model is composed of a block of sandstone saturated with water and gas. We assume a transition zone around the fluid contact, whose vertical profile of the saturation rate varies gradually in time in this zone. The result inspires that the combination of elastic parameters is necessary for estimating the seismic velocity contact in fluid transition zone relating to the fluid-contact movement by FWI.