Three-dimensional magnetotelluric modelling and inversion

An outline is presented of the nature of the problem of making quantitative use of magnetotelluric data for geophysical exploration. The authors discuss their view and experiences in dealing with the modeling and interpretation problems. The key step is considered to be the modeling step, which is that of predicting the response of a given distribution of electrical properties within the Earth to the electromagnetic excitation. Decisions about modeling methods also consider the interpretation process that will be used. The electromagnetic problem is three-dimensional which makes it necessary to use numerical methods. Difference equations are used and the authors investigate spacing requirements and two approaches to solving the system of equations: a relaxation method and a direct solution method. The relaxation method is found to be much faster, but it is very difficult to eliminate all the errors in the resulting solution. The interpretation problem is approached using a scheme that minimizes the joint probability of fitting the observed data and adhering to an a priori conductivity model. A relaxation procedure is used to solve this problem and the behavior of the procedure is examined associated with the application of magnetotellurics to exploration problems. Some of the problems in signal resolution that arise when interpreting the data are noted