Improvement in 1D TDEM Interpretation by Simultaneous Inversion of Data from Two Loop Configurations

Abstract

This paper presents simultaneous inversion performed on data from central loop and offset receiver TDEM soundings. It is demonstrated that central loop and offset soundings complement each other and that the simultaneous 1D inversion of both data sets (1) yields rapid convergence and better resolution of the model parameters, (2) reduces the importance of distortion caused by induced polarization on interpretation, and (3) provides a good indication of the subsurface geometry in the measurement zone. Simultaneous inversion of central loop and different offset data sets are initially performed over a layered earth. The simultaneous inversion drastically improves interpretation obtained from the long offset data alone, but yields no noticeable improvement for the short offset sounding interpretation. Individual and simultaneous interpretation of central loop and offset soundings are then compared over a layered earth with a polarizable surface layer. The simultaneous interpretation reduces the importance of IP distortion affecting the central loop data and yields a more reliable model. Finally, central loop and offset data are calculated over a 3D conductive body embedded in layered host and individual and simultaneous inversions are performed. Lack of convergence and mediocre fit in simultaneous inversion indicate the presence of lateral conductivity variations. The simultaneous inversion is applied to two field examples from data collected in the Chalco Sub-Basin (Mexico). The first example consists in simultaneous inversion of two neighboring central loop soundings over an earth with no significant lateral conductivity changes. A slight enhancement in the resolution of model parameters is observed, but no substantial improvement in terms of misfit error or number of iterations is achieved compared to the single-site inversion. The second example shows simultaneous inversion of central loop and offset TDEM soundings when lateral conductivity changes occur. The combination of the two soundings never yields a final model that satisfies both data sets, showing the presence of a 2D or 3D heterogeneity in the vicinity of the two sounding sites.