Abstract Saturated thickness is a vital parameter in the assessment of hydrogeological reserves. By considering thickness variations associated with faults, the present study aimed to evaluate the water reserves of an unconfined aquifer. The study area was the Barreiras Aquifer in the lower course of the Maxaranguape River in Brazil. The methodology involved morphotectonic and lithostratigraphic analysis, well profile correlation, and hydrogeological and geoelectrical analysis (vertical electrical sounding, VES). The morphotectonic analysis employed topographical (Shuttle Radar Topography Mission) and hydrographical data to support the structural characterization, considering the lack of outcrops with specific features in the area. The results of these assessments enabled the creation of a morphotectonic lineament map, highlighting the prevailing NE and NW directions. The hydrogeophysical cross sections obtained from geoelectrical inverse models and well data reveal saturated thickness values ranging from 19 to 66 m, featuring some lineaments as faults. The hydrogeological reserve was calculated based on 18 subareas associated with blocks resulting from structural compartmentalization. Based on an area of approximately 250 km2, the average saturated thickness for each subarea, and an average effective porosity of 7.6%, the saturated reserve is estimated to be 8.7 × 108 m3.

Abstract This study used Electrical Resistivity Tomography (ERT) to identify the subsurface cavities in the carboniferous region on the State of Coahuila caused by underground coal mining and the hazards they induce. Measurements were taken next to roads and bridges support in three sectors with mining activity. Authors used roads to run the ERT profiles in order to avoid private properties, with some of these profiles including areas with subsidence. It is considered that subsidence is an anthropogenic related to both shallow and relatively deep underground mining activity. The electrical resistivity cross-sections obtained from the geophysical inversion process identified several cavities with a variety of dimensions ranging upwards from a depth of 7 m. Most of these structures cause deformations on surface in the form of subsidence or fractures. In general, the cavities identified in two of the sectors analyzed are filled with water or sediments. In a third sector analyzed in this study, all cavities were either empty or partially filled with collapsed material from their walls and ceiling. Geoelectrical sections reveal other characteristics of the geological environment, such as zones in which aquifer levels decrease, strata dipping (indicating prior subsidence), and faults and fractures that affect the stratigraphic sequence. The decrease in aquifer levels indicates changes in the hydrodynamic groundwater regime that have been caused by the voids generated by mining. These changes cause the ground to sink due to lithostatic pressure and the weight of the vehicles passing above. As a final result, the areas at the greatest potential risk of hazards related to ground subsidence are located within the three analyzed sectors.

Resumen En este estudio se utilizó Tomografía de Resistividad Eléctrica para identificar cavidades en el subsuelo de la región carbonífera del Estado de Coahuila, originadas por la minería del carbón, y con ello se valora el riesgo geológico inducido. Las mediciones se tomaron a lo largo de perfiles ubicados en las proximidades de carreteras y puentes, dentro de sectores de actividad minera. Estos perfiles se ubican evitando las propiedades privadas y abarcando algunas zonas de hundimientos de las carreteras. Se considera que la subsidencia es antropogénica y está relacionada con la actividad minera somera y relativamente profunda. En las secciones transversales de resistividad eléctrica obtenidas del proceso de inversión se identifican varias cavidades con diferentes formas desde los 7 m de profundidad. La mayoría de estas estructuras causan deformaciones en la superficie, generando hundimientos y fracturas en las carreteras. De forma general, las cavidades identificadas en dos de los sectores analizados se encuentran rellena de agua o sedimentos. En un tercer sector todas las cavidades están vacías o parcialmente rellenas con material colapsado de sus paredes y techos. Las secciones eléctricas revelan otras características del ambiente geológico, tales como zonas de disminución de los niveles acuíferos, inclinación de estratos (que indican paleo-subsidencia del terreno), fallas y fracturas que afectan la secuencia estratigráfica. La disminución de los niveles de los acuíferos indica cambios en el régimen hidrodinámico de las aguas subterráneas que han sido ocasionados por los espacios vacíos generados por la minería. Estos cambios provocan que el subsuelo se hunda debido a la presión litostática y el peso de los vehículos que pasan por encima de las carreteras. Como resultado final, se ubican en los tres sectores analizados, las áreas con mayor riesgo potencial de peligros relacionados con el hundimiento del subsuelo.

Una investigación geofísica de superficie a partir de métodos resistivos y magnéticos fue realizada en las inmediaciones de fuentes termales de Ikogosi en el suroeste de Nigeria, para delimitar la secuencia geológica y evaluar la estructura por debajo de fuentes termales. Las mediciones de campo magnético total y los Sondeos Eléctricos Verticales (SEV)se tomaron a lo largo de cinco líneas transversas en dirección N-S. Se incluyó una modelación inversa en la interpretación de los datos magnéticos y resistivos (SEV). Los modelos magnéticos inversos identificaron áreas de cuarcita fracturada y fallada dentro de cuarcitas masivas frescas a diferentes profundidades por debajo de todas las transversas. Las secciones geoeléctricas se desarrollaron a partir de la interpretación de SEVs, que también identificaron una secuencia de suelo/capa meteorizada, cuarcita fresca, cuarcita fracturada yunos estratos de cuarcita poco alterada. Se concluyo que la cuarcita fracturada pudo actuar como conducto para el flujo de las aguas subterráneas cálidas desde grandes profundidades a la superficie, mientras que la fuente de salida fue localizada en una interfaz geológica (lineamiento).

An integrated surface geophysical investigation involving resistivity and magnetic methods was carried out in the immediate vicinityof the Ikogosi warm spring situated in south-western Nigeria with a viewto delineating its subsurface geological sequence and evaluating the structural setting beneath the warm spring. Total field magnetic measurements and vertical electrical sounding (VES) data were acquired along five N-S traverses. Magnetic and VES data interpretation involved inverse modelling. The inverse magnetic models delineated fractured quartzite/faulted areas within fresh massive quartzite at varying depths and beneath all traverses. The geoelectrical sections developed from VES interpretation results also delineated a subsurface sequence consisting ofa topsoil/weathered layer, fresh quartzite, fractured/faulted quartzite and fresh quartzite bedrock. It was deduced that the fractured/faulted quartzite may have acted as conduit for the movement ofwarm groundwater from profound depths to the surface while the spring outlet was located on a geological interface (lineament).

The determination of hydraulic conductivity is essential for the assessment of fluid migration rates in the subsurface. Geoelectric methods are often used in hydrogeological studies as quick and inexpensive tools. The relation between saturated soil hydraulic conductivity (K), clay content and soil resistivity allows an estimation of clay content and K from electrical measurements made in the field or laboratory. In this work, a methodology for estimating fines (silt+clay) content and K from electrical measurements in silt-loam soils is presented. A textural analysis was performed in 73 soil samples collected from 21 boreholes located in the municipality of Guasave, Sinaloa, Mexico, to determine the sections with a greater infiltration of water from the Valle del Fuerte irrigation canal to the local aquifer. The calculated values of texture and K were used to develop a new empirical equation, K = 0.101176 * FINES-1.62, which achieved a new relation to properly estimate the K values from the percentage of fines content. Electrical measurements were performed in the laboratory for each soil sample to determine the fines content. The K values, which were determined by a new empirical equation, showed an acceptable correlation with the values obtained by traditional techniques for silt and silt loam soils with a clay content < 35%. Water resistivity measurements were performed for samples collected from wells and flumes, showing that the water salinity of the canal is significantly lower than the groundwater. Due to the location of the Valle del Fuerte canal and soil K values, the water infiltrated from the flume into the subsoil creates a barrier preventing or slowing down the advance of the saltwater intrusion from the Sea of Cortez. Short Electrical Resistivity Tomography (ERT) profiles were carried out to determine the surface stratigraphy. The results achieved by the application of ERT and the groundwater salinity values allowed the recalculation of the geoelectrical sections in the fines content and K sections, resulting in a new, faster and less expensive procedure for the determination of hydraulic and petrophysical parameters.

Vertical Electrical Sounding (VES) method is used extensively in environmental impact studies including hydrocarbon contamination. In this work, the results of the geoelectrical characterization of a contaminated site caused by pipeline leakage are presented. Geoelectrical study was performed with multi-electrode technology and 2D profile data interpretation. VES results from six parallel profiles were presented in resistivity sections and maps. Layered model of the site was found including aquifer and aquitard layers. Although the contamination grade of the site is low, we found two contaminated zones into sandy aquifer. Aquifer and aquitard were characterized by its resistivity, clay content, porosity and cation exchange capacity values. Recalculation of resistivity data into petrophysical sections and maps was performed by an inversion algorithm taking into account pore water salinity. Petrophysical parameters for uncontaminated areas estimated from resistivity are close to real values; meanwhile, in contaminated zones petrophysical parameters have anomalous values. Similar effects of contamination influence on petrophysical parameters were found in laboratory by resistivity measurements made at clean and contaminated sand samples.

El método Sondeo Eléctrico Vertical (SEV) es ampliamente utilizado en estudios de impacto ambiental incluyendo el caso de contaminación por hidrocarburos. En este trabajo se presentan los resultados de la caracterización geoeléctrica de un sitio contaminado por hidrocarburos relacionado con una fuga en línea de ducto. El estudio geoeléctrico fue realizado utilizando el método SEV en la variante de tomografía, realizándose una interpretación 2D de los datos observados. Seis perfiles paralelos de SEV fueron medidos y presentados sus resultados en secciones y mapas. Se determinó un modelo estratificado que incluye acuitardo y acuífero. Aunque el grado de contaminación en este sitio es bajo fue posible localizar dos zonas contaminadas dentro del acuífero. El acuífero y el acuitardo fueron caracterizados con base en su resistividad, contenido de arcilla, porosidad y capacidad de intercambio catiónico. Los valores de resistividad fueron recalculados a valores de parámetros petrofísicos utilizando un algoritmo de inversión que toma en cuenta la salinidad del agua de poro. En zonas no contaminadas los parámetros petrofísicos estimados a partir de datos de resistividad presentan valores cercanos a los reales, mientras que en zonas contaminadas se obtienen valores anómalos. Efectos similares de la influencia contaminación sobre los parámetros petrofísicos fueron encontrados en laboratorio realizando mediciones de resistividad en muestras de arena limpia y contaminada.

ABSTRACT A geoelectrical characterization of an oil-contaminated site, where thirty years ago an oil well was drilled, was carried out using vertical electrical sounding (VES) method on 2D Resistivity Imaging technology and 2D interpretation. VES data were added by water resistivity measurements and soil resistivity analysis. The contaminated zones feature with low resistivity anomalies situated above groundwater level. The geoelectrical interpretation was compared with geochemical sampling. An inverse correlation between electrical resistivity and concentration of contaminants confirms that the oil-contamination is mature and for this reason it creates low resistivity anomalies. Petrophysical modelling allowed estimating boundary resistivity value separating clean and contaminated zones. Resistivity data were visualized as cross-sections and maps and also were recalculated into sections and maps of petrophysical parameters, useful for more precise and detailed characterization of contaminated zones.

RESUMEN La caracterización geoeléctrica de un sitio contaminado por hidrocarburos, donde hace treinta años fue perforado un pozo petrolero, fue realizada utilizando el método de Sondeo Eléctrico Vertical (SEV) con la tecnología de Imagen de Resistividad e Interpretación 2D de los datos adquiridos. La interpretación de los datos de SEV estuvo apoyada por mediciones de resistividad en muestras de agua y suelo. Las zonas contaminadas corresponden con anomalías de baja resistividad ubicadas encima del nivel freático. Los resultados de la interpretación geoeléctrica fueron comparados con los resultados del muestreo geoquímico. La correlación inversa existente entre la resistividad eléctrica y la concentración de contaminantes confirma la existencia de contaminación madura, la misma que crea anomalías de baja resistividad. Mediante la modelación petrofísica es posible estimar el valor de resistividad que define la frontera entre zona limpia y contaminada. Los datos de resistividad son presentados a manera de secciones y mapas, además de haber sido recalculados a secciones y mapas de parámetros petrofísicos, como herramientas útiles en la caracterización más precisa y detallada de las zonas contaminadas.

O presente trabalho compreendeu estudos geofísicos conduzidos com o método da eletrorresistividade na região de Porto Seguro (BA), que tiveram o objetivo de localizar o aqüífero mais promissor para a exploração de água subterrânea naquela região. Os trabalhos recobriram uma área de aproximadamente 60km² situada imediatamente ao norte de Porto Seguro. A área pesquisada insere-se num contexto geológico de ambiente sedimentar Terciário (Formação Barreiras) e Quaternário (cordões litorâneos). Foram realizadas 60 sondagens elétricas verticais (SEVs) utilizando o arranjo Schlumberger. Destas, 14 foram realizadas para reconhecimento regional; 36 localizaram-se dentro da área de interesse e 10 distribuiram-se ao longo de um perfil regional ligando as cidades de Eunápolis e Porto Seguro. Estas últimas foram realizadas com o objetivo de delinear o topo do embasamento cristalino entre aquelas cidades. Para a interpretação quantitativa das SEVs foi desenvolvido um programa Fortran baseado no método de inversão que utiliza a técnica da inversão linearizada com reponderação múltipla dos desvios (ILRMD). Este método foi testado na inversão de SEVs sintéticas e em seguida aplicado para inversão das SEVs obtidas nos trabalhos de campo. Os resultados demonstraram que o método de ILRMD é mais robusto e eficaz comparado ao método de inversão linearizada governado pela norma L² ou L¹. Para auxiliar a interpretação foram utilizados dados disponíveis de perfis litológicos de poços perfurados na região. Utilizando-se as estimativas de espessura e resistividades de subsuperfície, obtidas através das inversões das SEVs, foram construídas seções geoelétricas. A interpretaçcão destas seções permitiu determinar o aqüífero de interesse para a exploração de água subterrânea naquela região. Este é formado predominantemente por arenito saturado com água doce e ocupa a porção centro-norte da área com o topo na profundidade a cerca de 80m e espessura não inferior a 70m. Tal aqüífero representa a melhor opção para a captação de água subterrânea no município de Porto Seguro.

This paper comprises geophysical studies carried out with the resistivity method in the Porto Seguro region, in the state of Bahia (Brazil). The objective was to find the main aquifer of that region. The work was performed on a area of approximately 60 km² located north of the Porto Seguro city. The studied area has sedimentary units with Tertiary (Formação Barreiras) and Quaternary (coastal shore) ages. Sixty vertical electrical soundings (VES) were performed using the Schlumberger array: 14 for regional reconnaissance; 36 for detailed study and 10 in a profile between the cities of Eunápolis and Porto Seguro. The VES in the regional profile were carried out to define the basement topography between these cities. A Fortran program was written for the quantitative interpretation of the VES, based on the method that uses the linearized inversion with multiple weight for the deviations (LIMWD). This proposition was tested on synthetic VES data and then it was used with field data. The results showed that this method is more stable and powerful than the traditional linearized inversion method which uses L¹ and L² fixed norms. The available information about well drilling data of the region was used to aid the interpretation. Geoelectrical sections were drawn from the inverted results of the thickness and resistivity of the layers. The interpretation of these sections provided the definition of the main aquifer for ground water exploitation in this region. This aquifer is confined to saturated sandstone layer in the mid-north portion of the area at about 80 m deep and an average thickness of 70 m. This aquifer represents the best option for ground water exploitation to supply the city of Porto Seguro.