Andrés Tejero-Andrade

When collecting data using ERT2D (2D electrical resistivity tomography) and TDIPT2D (2D time domain-induced polarization), different phenomena can occur, which can cause natural or anthropogenic noise, contaminating the data and making its processing, analysis, and interpretation difficult. Different techniques have been developed to eliminate or reduce these effects on the data, such as noise filtering or the development of new techniques to improve data collection in the field. In the present work, an iterative, weighted, least-squares filter was employed after voltage normalization using current and geometrical factor correction on data collected in rough topographic terrains. The selected filter basis function should be able to represent the natural behavior of the function to be filtered. Stationary or variable voltages in electrical prospecting decay with the inverse of the distance, which can be represented by an expansion in Legendre polynomials. On the other hand, uneven spacing of the electrodes leads to using the incorrect geometric factor, resulting in an error in the calculation of the electrical anomaly. The efficiency of the proposed technique was analyzed and tested with field examples using different filters and by comparing applying and not applying the proposed correction factor. The results indicated low RMS and L2-Norm errors, and better definition of the inverted resistivity image was obtained. For the TDIP data, a better correspondence between the inverted images of resistivity and chargeability was obtained.

When collecting data from ERT2D (electrical resistivity tomography) and TDIPT2D (time domain IP tomography), different phenomena occur, whether natural or anthropogenic noise, which contaminate the data, and it can make their processing, analysis and interpretation difficult. Different techniques have been developed to eliminate or reduce these effects on the data, such as noise filtering or the development of new techniques for better data collection in the field. In the present work an iterative weighted least squares filter is employed after voltage normalization by current and a geometrical factor correction applied on strong topographic terrains. The selection of the filter basis function is recommended to be able to represent the natural behavior of the function to be filtered. Stationary or variable voltages in electrical prospecting decay with the inverse of the distance, which can be represented by an expansion in Legendre polynomials. On the other hand, uneven spacing of the electrodes leads to not using the correct geometric factor which results in an error in the calculation of the electrical anomaly. The efficiency of the proposed technique is analyzed and tested with field examples using different filters and applying the proposed correction factor or without using it. The results indicate a low RMS and L2-Norm error, whereby a better definition of the inverted resistivity image is obtained. For the TDIP case a better correspondence between the inverted images of resistivity and chargeability can be reached.

This work analyzes the progress that the degree of geophysical engineer has made in the last fifty years, specifically in the School of Engineering of the Universidad Nacional Autónoma de México (unam), Mexico. This raises the vision, a little history, the teaching, and learning process that gives identity to the geophysical engineer, the forms of learning and the competences that applicants must have and acquire to obtain the professional title; In addition, the experience in the Introduction to Geophysics course, at the beginning of the COVID-19 pandemic, is detailed; finally, some challenges and areas of opportunity are proposed that geophysical engineers will face in the near future. Se hace un análisis del avance que ha tenido la carrera de Ingeniería geofísica en los últimos 50 años, específicamente en la Facultad de Ingeniería de la Universidad Nacional Autónoma de México (unam), México. Se plantea la visión, un poco de historia, forma de enseñanza y proceso de aprendizaje que da identidad al ingeniero geofísico, formas de aprendizaje y competencias que deben tener y adquirir los aspirantes para obtener el título profesional; también, se detalla lo experimentado en la asignatura Introducción a la Geofísica, ante la COVID-19, en el primer confinamiento. Finalmente, se proponen algunos retos y áreas de oportunidad que enfrentarán los ingenieros geofísicos en el futuro cercano.

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 ofEl Castilloin 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 underEl Osariopyramid (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.

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 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.