Special 3D electric resistivity tomography (ERT) array applied to detect buried fractures on urban areas: San Antonio Tecómitl, Milpa Alta, México

Abstract

Geophysical techniques can be employed to understand the physical characteristics of the subsurface in highly populated areas, where urban settlements present structural problems. An interesting example is presented in this investigation, where three-dimensional Electric Resistivity Tomography (ERT-3D) is applied employing alternative arrays that allow investigating the subsoil beneath the affected constructions. A small neighborhood comprised by a block of houses within the town of San Antonio Tecómitl is studied. The area is found towards the outskirts of the Chichinautzin range, in the southern limit of the Mexican Basin. This settlement suffers of strong damage in their structures due to fractures and subsidence within the subsoil.
ERT-3D was carried out to characterize the subsoil beneath this urban complex. A special resistivity array (‘horse-shoe’ geometry) employing a combination of ‘L’, equatorial-dipole and minimum-coupling methods was carried out to fully ‘illuminate’ the subsoil beneath the block of houses. Computed resistivity models depicted a high resistivity pattern that coincides with the affected houses. Such pattern seems to extend beyond the limits of the surveyed areas, and is probably part of a more complex fracture system. It is very likely that fractures have been produced due to the poorly consolidated soil, which is part of a transition zone; the intense water extraction, that form ‘voids’ in the subsoil causing subsidence effects and finally the existence of regional faults to greater extent that might control these shallow features.