Vertical Electrical Sounding Interpretation Research Articles

Aplicação de sondagem elétrica vertical na estimativa do topo e da espessura de unidades geológicas da bacia do Paraná na cidade de Rondonópolis-MT

Este estudo foi realizado na cidade de Rondonópolis, estado de Mato Grosso, e teve como objetivo estimar a profundidade e a espessura das unidades geológicas da área, usando Sondagem Elétrica Vertical (SEV). Os resultados das SEVs mostram que a espessura máxima da Formação Ponta Grossa é 63 m; a espessura da transição Furnas/Ponta Grossa e da Formação Furnas é superior a 700 m. A espessura da mencionada transição não foi estimada por falta de evidências nas curvas das SEVs. Foi observada uma variação litológica considerável na Formação Furnas. Os dados de perfis geológicos de poços, usados na interpretação das SEVs, permitiram maior confiabilidade nos resultados obtidos.

Geoelectric/electromagnetic VLF survey for groundwater development in a basement terrain – a case study

A geophysical investigation for groundwater development involving the electrical resistivity and electromagnetic VLF methods was carried out in the premises of the Conference Centre, Obafemi Awolowo University, Ile-Ife, southwestern Nigeria. The investigation involved three reconnaissance VLF-EM profilings and six vertical electrical soundings (VES). The VLF normal and filtered real component anomalies identify five major geological interfaces suspected to be faults/fractured zones. One of the interfaces coincided with a river channel which is suspected to be structurally controlled. The geoelectric section prepared from VES interpretation results delineate four subsurface layers which include the topsoil, weathered layer, partly weathered/ fractured basement and the fresh basement. The weathered layer and the partly weathered/fractured basement constitute the aquifer units. The weathered layer is relatively thin (1.8m - 4.7m) while the partly weathered/fractured basement is significantly thick (15.7m - 67.3m) and extensive with tendency for significant groundwater discharge capacity. The lithological log from a test borehole at one of the VES stations corroborated geophysically predicted subsurface sequence and its structural disposition. Fractured basement columns were identified at depth ranges of 2.5 - 7m and 12 - 45m within a column of fresh basement rock. The borehole whose aquifer is primarily fractured basement discharges about 2.0 L/s. Key Words: electromagnetic, electrical resistivity, lithological log, groundwater development. Ife Journal of Science Vol.6(1) 2004: 74-78

Geoelectric investigations in Bakreswar geothermal area, West Bengal, India

A cluster of hot springs in Bakreswar geothermal area, which belongs to the Chotanagpur Gneissic Complex in the eastern part of Peninsular India, is characterised by varying temperature and similar chemical composition. Vertical electrical sounding (VES) investigations in and around Bakreswar reveal the presence of two to four prominent lithologic layers under prevailing hydrodynamic conditions. The intermediate weathered zone and the fractured rocks constitute a single aquifer system of varied hydraulic conductivity under water table condition. Lithology and groundwater conditions, as inferred from the VES, as well as hydrological studies, are in agreement with the nearby bore hole lithologs. Water table contours accompanied by VES findings of the region indicate that the occurrence and movement of groundwater take place mostly within the weathered and fractured rocks under unconfined condition. 1D interpretation of VES results reveals few promising groundwater potential zones in the eastern part of the region. Wenner resistivity profiling, coupled with VES and geological studies, indicates the presence of a nearly N–S striking buried fault providing passage for hot water to emerge in the form of springs.

A collocation method to fit geophysical survey data

A self adaptive collocation method has been applied to the quantitative interpretation of geophysical survey data. Examples of interpretation of vertical electrical soundings (VES) and magneto-telluric soundings (MTS) have been chosen for this work due to the high nonlinearity of the response functions and to the high degree of correlation of the parameters involved. In both cases horizontally isotropic multi-layered models have been adopted. For comparison two different computing algorithms have been considered. In the first, using a generalized inverse formulation a new different decomposition into characteristic values has been used. In the second the Marquardt procedure has been modified in an original way to give a faster convergence using a strategy based on the contemporary adjustment of the step length and the direction of the regression. The confidence limits of the results have been evaluated. Results are given for interpretation of both MTS and VES curves using the two different algorithms. Model data and field data have been considered for an effective comparison of speed of convergence and accuracy.