René E. Chávez

Cavity Detection in the Southwestern Hilly Portion of Mexico City by Resistivity Imaging

Resistivity imaging (electric tomography) is a methodology for defining lateral variations of resistivity associated with structural anomalies such as caves, water contamination, and fractured zones among others. Important urban areas in Mexico City are currently at high risk of collapse. Cavities and shallow fractures have been created as a result of mining in hilly regions beneath several of today’s urbanized neighborhoods located in the southwestern portion of Mexico City. Selected areas have been investigated. Caves and tunnels were found for depths ranging between 5m up to 15m. Diameters of such features were between 5m up to 30m. Some of these structures are across paved roads and beneath buildings and houses, running for several tens of meters in length. Topographic effects were also studied in one profile. It showed important inaccuracies in resolving the geometry of the cave, mainly in terms of depth to the top, where differences of 2mto3m were encountered. GPR (Ground Penetrating Radar) was also...

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.

Imaging Fractures beneath a Residential Complex Using Novel 3-D Electrical Resistivity Arrays

ABSTRACT Typical 3-D electrical resistivity tomography sampling schemes, which require a grid of electrode lines to be deployed, are limited by physical conditions of the area under study. New array techniques are needed to characterize the subsoil beneath anthropogenic or natural structures to define hazardous zones. Use of multiple L-shaped arrays overcome the need for a grid of electrodes by surrounding an area in a square of electrode lines; however, in some instances, the physical environment does not allow closure of a square of electrodes. An alternative array introduced in this investigation is termed the horseshoe array. The horseshoe array combines the L-shaped arrays with equatorial and minimum coupling arrays to overcome array closure problems. Three synthetic examples were investigated to establish the limitations of the horseshoe array, and to describe the geological conditions of the subsoil, e.g., building foundations and fractures. The first two examples represent two resistive cubes init...

A 3D Gravimetric Crustal Model for the northeastern region of the Iberian Peninsula

We combine different methodologies normally used separately in exploration geophysics to obtain the topographies of the Moho discontinuity and the boundary between the upper and lower crusts by separating their effects in the Bouguer anomaly map. The SFM method was used to define the average depth of the layers causing the anomalies. The resulting three-dimensional model which reproduces the observed gravity anomaly was estimated by inverting the corresponding domains of the power spectrum. A good correlation exists between the calculated depths and the upper and lower boundaries of the layered lower crust observed along the reflection seismic lines in the area.

Application of the fast Fourier transform to interpret geoelectric anomalies : preliminary results

Archaeological structures are commonly buried at shallow depths, nevertheless, their geophysical signatures measured at the surface may be weak and contaminated with noise, thus, difficult to interpret. However, regional-residual separation techniques commonly used in gravity and magnetics to enhance the signal of interest, also can be applied to the interpretation of one-dimensional resistivity profile data. Such a process is done based on a frequency analysis. The resistivity response of synthetic models has been computed analytically by the images method for a Wenner array. Different curves were analysed varying the observation step ( p ), the width ( t ) of two vertical layers, the resistivity (p) of the media, and the electrodes interval ( a ). The Fourier transform is applied to the theoretical resistivity profiles to compute and analyse its amplitude spectrum. Such a function presented three regions of interest. A low-frequency zone was selected to design a low-pass filter to calculate the “regional” of the resistivity curve. A “residual” resistivity anomaly was calculated by taking the difference between that and the observed profile. Such an anomaly depicted highs that corresponded to the location of the resistive interfaces. After assessing the technique with theoretical data, an application to a real case was studied. The real data are from the city of Denia (Alicante), located in the eastern coast of Spain, in the Mediterranean sea. The area of interest was covered by eight resistivity profiles. Two of them lay on a road under construction, where remains of a Roman structure were discovered during the construction works. Residual resistivity anomalies were computed by applying the method of regional-residual separation in the wave-number domain. A correlation process between the computed residual and the theoretical response of the “wall” model was carried out to locate the position of the boundaries of the village foundations. The geophysical interpretation enabled the archaeological work to discover the remains of a warehouse complex related to the ancient Roman port of Dianium, buried close to the surface below the road. So far no excavation has been undertaken in the reminder electrical profiles.

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.

Geothermal Model of the Chicxulub Impact Region.

We established the three-dimensional sub-surface structural model of Chicxulub impact crater. The complete gravity data set of northern Yucatan was inverted to obtain the buried topography of the central basin, which included the central structural high of the crater. Borehole information was used to constraint the gravity interpretation. The computed gravity model was subsequently employed to establish the thermal state of the impact basin and its relation to the ground water flow. We determined the heat flow from a geothermal study based in the UNAM boreholes. The mean temperature gradient in the study area is 0.03°C/m. The thermal conductivity varies from 2.11 to 2.67 W/m°C for the different rock types present. We obtained a mean heat flow of 64 ± 8 mW/m 2 . A model of the conduction-convection heat and mass transfer was computed, based on the 3-D sediment-basin interface. The results confirmed that water flow is controlled by the crater structure, modifying the thermal state in the area (as inferred from temperature profiles). Ground water circulation through the fractured limestone is the cause of a generally low heat flow in the crater’s rim.

Structure, And Geothermal Potential of the Laguna Salada Basin, BCN, Mexico.

Based on gravity and vertical magnetic field data we established the structure of the sediment-basement interface of the Laguna Salada basin. A three-dimensional inversion for the gravity was carried out in the wavenumber domain using an iterative scheme. The maximum density contrast of -400 kg/m and the mean depth of 3 km constrained the inversion. The resulting model indicated that the sedimentary infill is up to 4.2 km thick at its deepest point. Vertical magnetic field measurements available were interpreted on four selected two-dimensional profiles. The magnetic anomalies were interpreted successfully using the gravity derived basement-sedimentary infill interface as top of the magnetic bodies. An elongated N-S to NW-SE trending highly magnetized body running from south to north along the basin is observed to the west of the basin. This magnetic anomaly has no gravity signature, and can be interpreted as an intrusive body emplaced along a fault running through Laguna Salada Basin. Two-dimensional thermal modeling was carried out in three profiles across the Laguna Salada basin. We used a finite difference scheme to solve the coupled Darcy and Fourier differential equations. Computed models showed fluid flow in the sedimentary layers and a redistribution of heat flow from the basin axis toward its rims (Sierra de Juarez and Sierra Cucapah). The numerical and chemical analyses support the hypothesis of fluid circulation between the clay-lutite layer and the fractured granitic basement. Thermal modeling shows low heat flow values along the Laguna Salada basin. Deep fluid circulation patterns were observed that redistribute such flow at depth. Two patterns were distinguished; one displays the heat flow increasing from the basin axis towards its borders (temperature increases at 20o). The second pattern shows an increasing heat flow from south to north of the basin. Such behavior is confirmed by the temperature measurements in the thermometric boreholes.

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.

Inversion of gravity data from the Laguna Salada Basin, B.C., Mexico

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