Efficient ERT measuring and inversion strategies for 3D imaging of buried antiquities

The optimum processing technique (21) vs. 3D inversion) to interpret and visualize parallel and/or orthogonal two-dimensional surface Electrical Resistivity Tomography data collected from archaeological sites is examined in this work. A simple modification of a standard resistance-meter geophysical instrument was implemented in order to collect parallel two-dimensional sections along the X-, Y- or XY-direction in a relatively short time, employing a pole-pole array. The sensitivity analysis showed that the distance between the parallel 2D lines must be smaller or, at the most, equal to the basic inter-electrode spacing in order to produce reliable 3D resistivity images of the subsurface. This was confirmed by modelling and inversion of both synthetic and real data. Direct comparisons of the quasi-3D images, resulting from a posteriori combination of the inverted 2D sections, with the full 3D inverted resistivity models indicated the superiority of the 3D inversion algorithm in the reconstruction of buried archaeological structures, even in complex archaeological sites. Due to the inherent three-dimensionality of many archaeological targets, quasi-3D images suffer from artefacts. The combination of a single survey-direction with a full 3D processing and interpretation scheme is adequate to image the 3D subsurface resistivity variation in detail. Furthermore, the implementation of a quasi-Newton Jacobian matrix update technique reduced the processing time by one-half without any significant loss of accuracy and resolution.