Inversion of ERT-3D data using PSO and weighting functions

Abstract

In the last decade, a new data acquisition technique has emerged in 3D electrical resistivity tomography studies, where the target to study is beneath a structure such as a building and it is not possible to use the traditional technique. This is a very common scenario when investigating urban areas or archaeological sites. The technique, called Perimeter Data Acquisition (PDA) in this paper, allows the acquisition of irregularly distributed data beneath structures. Although electrical anomalies can be detected, the solution does not correctly determine their vertical location because of the irregular distribution of the data and the lack of data towards the central portion of the survey region limited by the electrodes.The methodology presented here aims to improve the calculation of the vertical location of the anomalous bodies in the subsoil using weighting functions in conjunction with a stochastic inversion method called Particle Swarm Optimization (PSO). Three weighting functions defined from the theoretical depth of the arrays and the resistivity standard deviation were tested by using synthetic models and comparing the solutions between them and with non-weighted solutions. Finally, the algorithm and weighting functions were applied on a case study to compute the vertical location of a cavern beneath a Mayan pyramid.The results indicate that when using the PDA technique, it is necessary to employ a weighting function to obtain an acceptable and reasonable solution. It is found that the weighting function defined from the resistivity standard deviation allowed to determine with the least error the vertical characteristics of the anomalous bodies.