High resolution electrical imaging of fault zones

Two-dimensional numerical modelling studies of a generic conductive fault zone indicate that very closely spaced TM mode magnetotelluric (MT) data are capable of providing high-resolution images of the fault structure. The degree of resolution that is possible is largely controlled by the conductance of the overburden and is also dependent on the spacing of measurement sites. In the absence of any overburden, or in the case of very thin overburden, the resolution is limited by the minimum electrode dipole length which is practicable. As an example, the results of two-dimensional inversion of closely spaced data measured across the Alpine Fault are compared with an earlier model derived from more widely spaced measurement sites. Much higher resolution of the fault zone is obtained from the closely spaced data and some features of the earlier model appear to arise due to the sparseness of the data. In the case of a very narrow fault zone, it is shown that the MT data exhibit very little lateral variation in phase and therefore independent control of static-shift is necessary.