Reducing ambiguities in vertical electrical sounding interpretations: A geostatistical application

The Vertical Electrical Sounding (VES) is the most widely used geophysical technique for groundwater prospecting. However its interpretation has been subjected to several indistinctness and efforts are on to tackle them. Vertical Electrical Soundings were carried out at 86 sites in a small watershed of about 60 km2 in a granitic terrain with the objective of delineating the aquifer layer parameters viz. weathered zone, fissured zone and depth to the bedrock. The thickness and resistivity of the weathered, fissured zone and depth to bedrock were determined from VES data. Due to wide variation in resistivity and in the absence of good resistivity contrast, the resolution has been less precise and thus the layer parameters interpreted could be ambiguous.Based on the VES results and hydrogeological considerations, 25 wells were initially drilled and later on 8 more wells were drilled intercepting through the bedrock. In addition, lithologs from 6 additional wells are available in the same watershed. Thus with the help of lithologs from 39 wells, thicknesses of various layers and bedrock depths were determined. This set of data from the lithologs were analysed geostatistically and an estimation of these parameters was made at all the 86 locations using a final variogram obtained from the variographic analysis. This has provided a range for the estimated values of the above three parameters using the standard deviation of the estimation error. The interpreted parameters from VES were compared with the range thus obtained in the above procedure. The interpreted VES results that could not be found within the stipulated range provided by the geostatistical estimation, were categorized separately and a suitable reinterpretation was made for them by fitting some parameters obtained from the nearby well data. After a few iterations, a large number of VES results were found falling in the estimated range and thus reduced the ambiguities in the VES results. The study has provided a new and additional method of reducing the ambiguities in VES interpretation as well as providing a quality indicator to each interpretation.