Practical incorporation of local and regional topography in three-dimensional inversion of deep ocean magnetotelluric data

Abstract

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 Zts, expressed by the product of the local topographic distortion term Dltand the response to the regional structure Zrsthat consists of the regional topography over the mantle electrical conductivity structure Z ≈ Zts= DltZrs. We also assume that Dltmay be treated as a site correction term, which is not dependent on the conductivity structure to be solved in the inversion. Dltis calculated before the inversion iteration is carried out through the forward modelling of Ztsand Zrsusing a good initial guess for the mantle structure. However, the initial Ztsis 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 Zrs. Thus, in the inversion, Ztsis calculated from ZrsandDlt; Zrsis modelled at every iteration butDltis fixed throughout the inversion. The sensitivity of Ztsto the conductivity is calculated considering Dltas 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. © The Authors 2013. Published by Oxford University Press on behalf of The Royal Astronomical Society.