ALBERT

Description: We propose a three-dimensional (3-D) inversion scheme for deep ocean magnetotelluric (MT) data that enables us to incorporate both the local small-scale topography effect and regional large-scale topography effect, whose length scale is comparable with that of the mantle structure to be resolved by the inversion. We assume that the MT impedance tensor Z is approximately equal to the MT response of the total structure Z ts , expressed by the product of the local topographic distortion term D lt and the response to the regional structure Z rs that consists of the regional topography over the mantle electrical conductivity structure Z Z ts = D lt Z rs . We also assume that D lt may be treated as a site correction term, which is not dependent on the conductivity structure to be solved in the inversion. D lt is calculated before the inversion iteration is carried out through the forward modelling of Z ts and Z rs using a good initial guess for the mantle structure. However, the initial Z ts is separately modelled by another forward modelling program, which is more efficient in terms of computation cost. The model space of the inversion includes only the regional structure so that the modelled response is Z rs . Thus, in the inversion, Z ts is calculated from Z rs and D lt ; Z rs is modelled at every iteration but D lt is fixed throughout the inversion. The sensitivity of Z ts to the conductivity is calculated considering D lt as a constant. This scheme requires modelling only of the regional scale structure in the inversion process, saving computational resources and time, which is critical in the 3-D case. Tests using a synthetic model and data demonstrate that incorporating the local topography effect in this scheme was successful and produced an accurate reconstruction of the given mantle structure.