ONE-DIMENSIONAL MAGNETOTELLURIC INVERSION USING AN ADAPTATION OF ZOHDYS RESISTIVITY METHOD

An iterative refinement method for determining a layered resistivity m odel from a Schlumberger or Wenner sounding curve is adapted to determ ine a layered resistivity model by using apparent resistivity and phas e derived from the magnetotelluric impedance. Magnetotelluric observat ions presented as a function of period are first converted to an appro ximate resistivity-depth profile using Schmucker's transformation and this is used to construct an initial guess (starting) model. A two-sta ge procedure is then invoked. Keeping resistivities constant, layer bo undaries are first adjusted to give a minimum misfit between measured data and responses and this is followed by resistivity adjustments wit h fixed layer boundaries to reduce the misfit further. The method is i llustrated by application to some synthetic data both exact and with a dded noise, to a real field data set and to some magnetotelluric profi le data obtained in a survey over the Carnmenellis granites in south C ornwall. The method is validated by recovering conductivity models fro m the exact and noisy 1D synthetic data. For complicated three-dimensi onal data at a single site and along a profile of stations, the method is shown to produce acceptable solutions which may be used as startin g models in further two-or three-dimensional studies.