Automatic 1D interpretation of DC resistivity sounding data

In the last few years, much of the work carried out on DC resistivity has m ainly concentrated on 2D and 3D techniques for data acquisition and interpr etation. However, when the resistivity changes are smooth, 1D techniques ca n be used to interpret the data. In the present paper, we present an automa tic 1D inversion scheme for DC resistivity sounding. The inversion scheme i s based on an iterative least squares procedure (ILSQP) with Singular Value Decomposition (SVD). In order to reinforce the convergence of the inversio n scheme towards a global minimum, the ILSQP is combined with a logarithmic parameterization of the unknown model parameters, and splitting of the dat a set into parts. The data are then inverted stepwise, i.e., starting from the first data part which corresponds to the smallest electrode spacings. A s the inversion continues more data parts, corresponding to increasingly la rger electrode spacings, are included in the inversion until the whole data set is inverted. In this way, the shallower part of the model becomes well estimated first, and as the inversion advances and more data parts are inc luded, the deeper structure is better resolved. The solution of the resisti vity inverse problem by standard least squares procedure with SVD allows us to distinguish between well and poorly resolved linear combinations of mod er parameters. For a given inversion step, the construction of models that give a better data fit can be done with truncation of eigenvectors belongin g to the least resolved combinations. Thus, as the iteration process advanc es those singular values can be activated one by one. This approach has bee n tested on synthetic data representing some layering that is assumed commo n in shallow studies. The model studies suggest that there is a strong coup ling between the resolving power of DC resistivity data and their random er rors. For high to moderate data quality, the resolution power of the scheme is generally good, depending upon the degree of complexity of the model. F or larger error levels, however, there is a loss of resolution. The inversi on scheme is suitable for fast interpretation of data that are collected in shallow studies in connection with environmental, hydrogeological and geot echnical investigations if the lateral changes in the study area are smooth . The method may also be used as a first interpretation prior to a 2D or 3D survey. (C) 1999 Elsevier Science B.V. All rights reserved.