Andrés Tejero-Andrade

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

Three-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 fourelectrode 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 Pahnu, 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.

Karst Detection Beneath the Pyramid of El Castillo, Chichen Itza, Mexico, by Non-Invasive ERT-3D Methods

Currently, archaeologists perform excavations determined by previous geophysical studies to accurately establish the prospective targets and minimize site disturbance. Among others, one of the methods most widely employed is the Electrical Resistivity Tomography (ERT-2D, -3D). However, investigation of the subsoil of archaeological buildings is not possible to carry out with traditional geophysical methods, because the structure itself prevents it. Therefore, it is necessary to design non-invasive special arrays capable of characterizing the subsoil of such buildings, while preserving their historical context. Here we show how this procedure combined with sequences of resistivity observations at depth allowed us to detect a low resistivity body beneath the pyramid of El Castillo in Chichen Itza (Mexico). This feature may be associated with a cavity (karst) partially filled with sweet water. On the other hand, a natural cavity was discovered under El Osario pyramid (south of El Castillo), at the end of the 19th century. Therefore, this pyramid was also studied to validate the effectiveness of this methodology, obtaining outstanding results. This method provides an interesting procedure to investigate the subsoil of archaeological structures for unveiling evidences that allow specialists to understand the religious meaning of these temples.

A two‐dimensional gravity inversion from the Guadalcazar intrusive, San Luis Potosí, Mexico

A gravity interpretation is presented from an outcropping intrusive structure, located in the San Luis Potosi State, Mexico. Such a body is of granitic zomposition, with a density of 2.47 g/cm . The surrounding rocks are of carbonated3type, with a mean density of 2.65 g/cm . A residual gravity low associated with the intrusive body suggested that the bulk of the structure lies at depth to the southeast of the exposed portion. Therefore, the outcrop could be an apophysis of the main body. Surface geological surveys and several exploratory wells drilled on that area seemed to confirm the gravity evidence. An automatic non-linear inversion algorithm was designed and applied to interpret seven gravity profiles taken perpendicular to the major axis of the anomaly. The Marquardt modified approach combined with the singular value decomposition technique was used, introducing a scheme of normalized residuals to speed up convergence of the solution. A parametric conditioning allowed to insert a priori information. Three to four iterations were needed to reach a solution that reasonably well satisfied the observed anomaly. A pseudo three-dimensional model was then computed from the interpreted profiles, in order to visualize the structure as a whole. Such a model is in agreement with the available information of the area.