2-DIMENSIONAL NONLINEAR INVERSION OF VLF-R DATA USING SIMULATED ANNEALING

The VLF-R (very low frequency-resistivity) data, i.e. the apparent res istivity (rho(a)) and phase (phi) data, were inverted individually and jointly using the VFSA (very fast simulated annealing) global inversi on approach. Global inversion results for synthetic data without and w ith various amounts of random and normally distributed Gaussian noise reveal that the inversion of neither the rho(a) nor phi data alone yie lds the true parameters of the structures. However, the joint inversio n of the rho(a) and phi data yields very good estimates of the model p arameters. Five models, representing typical subsurface structures in the shield areas, are studied here. Various models achieved after 10 V FSA runs were used to compute the mean model and the corresponding cov ariance and correlation matrices, which were used to estimate the unce rtainties in the mean model parameters and correlations between the mo del parameters. We observe that these correlations follow the physics associated with the problem. VLF-R held data due to a nearly vertical contact Structure and a very thick dyke-like structure were also inver ted to demonstrate the efficacy of the approach in the delineation of the parameters of 2-D structures.