Optimization of self-potential interpretation of 2-D inclined sheet-type structures based on very fast simulated annealing and analysis of ambiguity

• Interpretation technique is developed for self-potential anomaly.
• Model parameter of isolated and multiple sheet-type conductors is optimized.
• Ambiguities in the interpretation for different forward approaches are analyzed.
• Very fast simulated annealing global optimization is utilized in the study.

Interpretation of self-potential anomaly over multiple 2-D inclined sheet-type structure employing very fast simulated annealing (VFSA) global optimization is performed and ambiguity in the interpretation is also investigated simultaneously. Various scientists have used different formulations to compute forward response over 2-D thin sheet-type structures. Each forward formulation is represented by a set of 5 unknown variables. In principle, the forward response computed by various formulations for a given target is the same; however, optimization of different variables in the inversion leads to different kinds of ambiguity in the interpretation. Therefore, in the present study, ambiguities in the interpretation of self-potential anomaly measured over 2-D thin sheet-type structures have been analyzed for various forward approaches and the most suitable approach has been worked out that yields the least ambiguous result. It has been observed that uncertainty in the interpretation is the smallest when model parameters are described with respect to the center of the causative body (Approach 1). Moreover, this approach also yields an equivalent solution (geologically irrelevant model where the sheet extends into the air). To avoid geologically irrelevant models from Approach 1, the model is described with respect to one edge of the sheet (Approach 2); however, inversion using Approach 2 yields a large uncertainty in the interpretation. Another approach is where x and z coordinates of the negative and positive poles (Approach 3) are optimized to overcome the limitation of Approach 1. This yields smaller uncertainty in comparison to the results obtained by Approach 2. Further, Approach 3 can deal with multiple sheet-type structures without any complexity in the optimization. The study concludes that Approach 1 can be used for interpretation of isolated anomalies by discarding geologically irrelevant models, while Approach 3 is the best model to interpret the data over multiple targets. Finally, a field data has been interpreted considering Approach 3 over a multiple sheet-type structure and excellent fit between the observed and model data is demonstrated. The present optimization approach developed using VFSA global optimization for self-potential interpretation is general and can handle a large number of variables associated with a large number of sheet-type structure together.