Cubic spline regularization applied to 1D magnetotelluric inverse modeling in geothermal areas

Electrical resistivity interpretations in geothermal areas typically show a three domains geoelectric structure consisting in an upper resistive horizon, an intermediate highly conductive layer (the 'cap-rock') and a fairly resistive zone, potentially hosting the geothermal reservoir. The clay cap usually give rise to low electrical resistivity anomalies that represent attractive targets for magnetotelluric exploration. Different algorithms are available to get 1D realistic pictures of the geoelectric structure from magnetotelluric soundings, such as blocky/sharp boundaries or smoothed ('Occam') approaches. Here we introduce a new method to model 1D resistivity profiles. In our approach, a cubic spline is assumed to be a realistic approximation of the true resistivitydepth distribution. The advantages of the method are twofold. First, cubic splines need a much smaller number of model parameters to be defined than the layered models used in smoothed inversion algorithms; this can be particularly useful in pseudo 2D/3D inversions or for Monte Carlo approaches, where a large number of 1D models must be solved. Second, being the spline intrinsically a smooth 1D resistivity depth profile, there is no need of solution regularization. An example of application of this approach to MT single site data from the geothermal field of Menengai is presented, and a comparison with blocky and smoothed models and the data from a deep exploratory well is made.