We expressed electromagnetic measurements at low induction numbers as spatial averages of the subsurface electrical conductivity distribution and developed an algorithm for the recovery of the latter in terms of the former. The basis of our approach is an integral equation whose averaging kernel is independent of the conductivity distribution. That is, the recovery of conductivity from the measurements leads to a linear inverse problem. Previous work in one and two dimensions demonstrated that using a kernel independent of conductivity leads to reasonably good results in quantitative interpretations. This study extended the approach to 3D models and to data taken along several profiles over a given area. The algorithm handles vertical and horizontal magnetic dipoles with multiple separations for appropriate depth discrimination. The approximation also handles issues like negative conductivity measurements, which commonly appear when crossing near-surface conductors. This happens particularly when using vertical magnetic dipoles; whose averaging kernel has significant negative weights in the space between the dipoles, something that does not happen for the horizontal dipoles. In general, the more complex the kernel, the more complicated the signature of any given anomaly. This makes qualitative interpretations of pseudosections somewhat difficult when dealing with more than one conductive or resistive body. The algorithm was validated using synthetic data for imaging data from horizontal or vertical coils or from a combination of them. Imaging of field data from a mine tailings site recovered a shallow 3D conductive anomaly associated with the tailings.
Abstract. In the XIX century when Mexico City was much smaller than at present, there was non-regulated mining of building materials in a region of tuffs northwest of the city in an inhabited countryside. With the growth of the city during the XX century, this region was increasingly populated and in the 1970's many two-level bricks houses were built, without regard for underground caves created by the earlier extractions. Some ground sinkings in adjacent areas alarmed the residents who now are worried about this permanent hazard. An association of residents contracted a private company for a geophysical study in order to know the distribution of the caves.Resistivity measurements were taken in the area to detect the caves in order to alert city authorities. Resistivity data along most of the streets were collected with the array pole-dipole that consisted of three grounded electrodes. We performed 2-D dimensional inversions to the data in order to get a 2-D resistivity image of every street. This is similar to a resistivity cross-section of the ground but obtained from the inversion of pole-dipole and Schlumberger resistivity data simultaneously. Using the information of previous drills we modified our programming code in order to perform constrained inversion and to get more accurate resistivity models in agreement with the drills. From the resistivity models obtained for every street it was possible to produce a map which shows the horizontal distribution of the resistive bodies at a depth of 12 m. These resistive bodies show coherent alignments that seem to correspond with a distributions of interconnected caves or tunnels used for extracting the sandy-tuffs. From these kind of interpretation method it was intended to get a more accurate horizontal distribution of the excavated areas in order to better know the urbanized area affected and lead the authorities to remedy the area with refill material.
La conductividad eléctrica de la corteza en la región sísmica de Ojos Negros, en el norte de Baja California, México, fue investigada mediante un perfil de 45 km de longitud con 17 sitios de observación magnetotelúrica (MT). En esta región la actividad sísmica consiste principalmente de micro-sismos, que ocurren a profundidades entre 13 y 15 km de profundidad, según lo muestran los registros de una red sismológica local colocada específicamente para estudiar la zona. La distribución espacial de epicentros muestra que esta actividad no ocurre a lo largo de las fallas conocidas, sino que se distribuye difusamente dentro del valle de Ojos Negros. Los datos MT fueron interpretados aplicando algoritmos de inversión en 2 ‘D, tanto a las respuestas invariantes serie y paralelo como a las respuestas tradicionales TE y TM de Groom-Bailey. Los modelos de resistividad encontrados con ambos procedimientos son similares. Los modelos revelan que la distribución de hipocentros está fuertemente correlacionada con una anomalía de baja resistividad. La mayor parte de los eventos sísmicos se aglutinan en una zona con un fuerte gradiente de resistividad, localizada en el borde de la anomalía conductora. La forma de la anomalía conductora y la ubicación de los eventos sísmicos sugieren que la falla Ojos Negros es una importante estructura lístrica que acomoda movimiento normal y lateral derecho. El gran contraste de resistividad en ambos lados del plano de falla está relacionado probablemente con un contraste litológico, entre rocas metamórficas en el bloque de piso y rocas plutónicas en el bloque de techo de la falla. La extensión a profundidad del plano de falla parece separar rocas metamórficas dúctiles, altamente conductoras, de rocas graníticas más quebradizas de resistividad intermedia. El modelo de resistividad obtenido a partir de los datos MT junto con la información sísmica nos ha permitido construir explicaciones que no podrían obtenerse con cada una de estas técnicas separadamente, y que proporcionan información valiosa sobre las condiciones físicas de las rocas de la corteza media en el área de estudio.
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