L‐ and CORNER‐arrays for 3D electric resistivity tomography: an alternative for geophysical surveys in urban zones

Abstract

ABSTRACTThree‐dimensional electric resistivity tomography surveys carried out on heavily urbanized areas represent a cumbersome task since buildings, houses, or other types of obstacles do not allow parallel electric resistivity tomography lines to be deployed. This paper proposes applying any four‐electrode configuration to provide subsurface information in complex urban areas. Such a procedure allows acquiring information beneath a construction by simply surrounding the structure of interest by a series of electric resistivity tomography profiles. Apparent resistivity is obtained from ‘L’‐ and ‘Corner’‐shaped profiles, where alternations between current and potential electrodes are carried out in an automatic way. Four ‘L’‐arrays and four ‘Corner’‐arrays are employed in a square geometry that allows surrounding the studied target to cover the subsurface. The first mentioned array will provide deep information. The second array will cover more of the shallow subsurface information. For the ‘L‐’ and ‘Corner’‐arrays, a mixture of traditional arrays are employed, like the Wenner–Schlumberger, axial, equatorial, azimuthal, and perpendicular dipole arrays.Two synthetic examples are presented to demonstrate the possibilities of the proposed electric arrays. A resistive cube set at the centre of a working cube is modelled. The ‘L‐’ and ‘Corner‐’ arrays are capable to detect such a model; however, dimensions are exaggerated. Later on, an extended wall model is dealt with. Similar results as in the first synthetic example are obtained in terms of geometry and resistivity. However, depth to the top of the wall model is not adequately recovered in comparison with the traditional methodology.Finally, the ‘L’‐ and ‘Corner’‐arrays are applied in an archaeological site named El Pahñu, located in Central Mexico. The new methodology described here is compared with the traditional 3D procedure employing a grid of electric resistivity tomography transects. As expected, the approach discussed in this investigation produced a reasonable solution towards the central portion of the working cube. However, shallow resistive anomalies (size about the electrode interval) were not fully detected, in comparison to a traditional 3D survey, where parallel lines forming a grid could be deployed. The reason is that no electrodes were set towards the central portions of the structure under study. However, the L‐ and Corner‐arrays are more sensitive to anomalies produced by deeper objects, which cannot be observed in the traditional method, especially when objects are located in between the electric resistivity tomography transects.