Frequency-domain magnetotelluric footprint analysis for 3D earths

Abstract

Abstract For magnetotelluric (MT) data with a large coverage, inversion interpretation for the whole area is usually impossible, due to the physical limitation of a computer. However, for each MT station, it is only sensitive to a region (called MT footprint) localized immediately underneath the station, much smaller than the whole survey area. Thus the whole large survey area can be subdivided into many sub-areas and inverted using the moving footprint technique efficiently. Therefore, to understand the MT footprint is of great importance for developing such an inversion algorithm. In this paper, we study the influence of frequency, resistivity and existence of anomalous structure in the MT footprint. We first use the finite difference method to evaluate the total electromagnetic (EM) fields, then apply the numerical volume integration of tensor green's function to calculate the total induced fields at the receiver (summation of the contribution for each cell). The footprint is obtained based on the contribution of each cell to the total induced fields. Different models are designed to understand the footprint for MT problems. An increase in frequency or conductivity results in a decrease in the size of footprint. Existence of anomalous bodies within the footprint affects the MT footprint size to a degree, dependent on the overall resistivity. Outside the footprint, the existence of anomalous bodies have little effect on the footprint size and shape.