DIRECT INTERPRETATION OF MAGNETOTELLURIC SOUNDING DATA-BASED ON THE FREQUENCY-NORMALIZED IMPEDANCE FUNCTION

An important aspect of any non-linear inversion method is the generati on of a suitable or good initial model as this controls the rate of co nvergence and accuracy of the result. To overcome the problem, a numer ical method is presented for direct interpretation of magnetotelluric sounding data based on the frequency-normalized impedance (FNI) functi on. The expressions used to calculate the parameters are developed, fi rst for a two-layer case under the assumption that deeper layers do no t contribute to the early part of the FNI curve, and they are then gen eralized for an n-layer situation. The parameters of the first layer a re computed by using successive sample values and the final estimate i s obtained from the arithmetic mean of selected values by excluding un acceptable results in the logarithmic space. The top layer is then rem oved using a reduction equation. The repetition of the procedure on su ccessive branches of the FNI function gives successive layer parameter s, the resistivity of the substratum being obtained at the final step, when the reduction equation becomes equal to the square root of that resistivity. The proposed method can be used as a complementary method for iterative inversion as it creates an initial guess which is close to the optimal solution. The solution produced by the direct interpre tation may also be modified by the interpreter to incorporate prior ge ological information before being input to iterative interpretation sc hemes.