One‐dimensional magnetotelluric inversion using an adaptation of Zohdy's resistivity method

Abstract

An iterative refinement method for determining a layered resistivity model from a Schlumberger or Wenner sounding curve is adapted to determine a layered resistivity model by using apparent resistivity and phase derived from the magnetotelluric impedance. Magnetotelluric observations presented as a function of period are first converted to an approximate resistivity–depth profile using Schmucker's transformation and this is used to construct an initial guess (starting) model. A two‐stage procedure is then invoked. Keeping resistivities constant, layer boundaries are first adjusted to give a minimum misfit between measured data and responses and this is followed by resistivity adjustments with fixed layer boundaries to reduce the misfit further. The method is illustrated by application to some synthetic data both exact and with added noise, to a real field data set and to some magnetotelluric profile data obtained in a survey over the Carnmenellis granites in south Cornwall. The method is validated by recovering conductivity models from the exact and noisy 1D synthetic data. For complicated three‐dimensional data at a single site and along a profile of stations, the method is shown to produce acceptable solutions which may be used as starting models in further two‐ or three‐dimensional studies.

Reply to comment on: ‘One‐dimensional magnetotelluric inversion using an adaptation of Zohdy's resistivity method’ by Chien‐chih Chen and Shi‐zhe Xu

“…enhancing the role of the phase at the four highest frequencies, which they justify by discussion. Hobbs and Dumitrescu (1997) imposed no a priori conditions and allowed the iterations to proceed unhindered.…”

“…For comparison, the method described by Hobbs and Dumitrescu (1997) has been applied to the COPROD data investigated by Chen and Xu (1999) and the resulting model and responses, together with the field data, are shown in Fig. 1.…”