Schlumberger Vertical Electrical Soundings Research Articles

A combination of self-potential and Schlumberger VES measurements for subsurface layering investigations

Self-potential (SP) anomalies can be useful in locating discontinuities in underground structure where water-flow occurs, but their interpretation is sometimes ambiguous. This paper was initiated by an effort to form a relatively accurate picture of the subsurface structure by combining SP measurements with Schlumberger soundings. The data employed come from a 1996 series of measurements carried out for a hydrogeological study near the village of Pigi of Rethimnon province in Crete Island. Data from 8 vertical electrical soundings (VES) were first used to determine the structure and possible lateral variations in the subsurface. For this, two parallel geoelectrical cross-sections were drawn, by considering simplified models. One of these cross-sections was employed in reducing the smoothed SP data to a homogeneous medium. This was necessary in order to make use of an interpretation algorithm (SPIN). The program is based on Fitterman's model of a dipping polarized plane for calculating SP anomalies, when the anomalies are generated by water flow along a boundary between rocks with different cross-coupling properties, that is through thermoelectrical or electrokinetical mechanism. Two major SP anomalies were observed, which suggest water presence. The interpretation algorithm led to the model parameters of the two sources, that reproduced accurately the anomalies. Their positions are in good agreement with those of the lateral discontinuities revealed by VES results. On examining the geological map of the area, we concluded that one of the sources might well be the prolongation of a fault disappearing about 2km from the SP line, while a second fault could explain slight lateral differences between the two geoelectrical crosssections.

Electric-hydraulic conductivity correlation in fractured crystalline bedrock: Central Landfill, Rhode Island, USA

Remote sensing and geoelectrical methods were used to find water-bearing fractures in the Scituate granite under the Central Landfill of Rhode Island. These studies were necessary to evaluate the integrity of the sanitary landfill and for planning safe landfill extensions. The most useful results were obtained with fracture trace analysis using Landsat and SLAR imagery in combination with ground-based resistivity measurements using Schlumberger vertical electrical soundings based on the assumption of horizontally layered strata. Test borings and packer tests confirmed, in the presence of a lineament and low bedrock resistivity, the probable existence of high bedrock fracture density and high average hydraulic conductivity. However, not every lineament was found to be associated with high fracture density and high hydraulic conductivity. Lineaments alone are not a reliable basis for characterising a landfill site as being affected by fractured bedrock. Horizontal fractures were found in borings located away from lineaments. High values of hydraulic conductivity were correlated with low bedrock resistivities. Bedrock resistivities between 60 and 700 Ω m were associated with average hydraulic conductivities between 4 and 60 cm/day. In some cases very low resistivities were confined to the upper part of the bedrock where the hydraulic conductivity was very large. These types of fractures apparently become narrower in aperture with depth. Bedrock zones having resistivities greater than 1000 Ω m showed, without exception, no flow to the test wells. Plots of bedrock resistivity versus the average hydraulic conductivity indicate that the resistivity decreases with increasing hydraulic conductivity. This relationship is inverse to that found in most unconsolidated sediments and is useful for estimating the hydraulic conductivity in groundwater surveys in fractured bedrock. In appropriate settings such as the Central Landfill site in New England, this electric-hydraulic correlation relationship, supplemented by lineament trace analysis, can be used effectively to estimate the hydraulic conductivity in bedrock from only a limited number of resistivity depth soundings and test wells.

Geoelectric sounding for the determination of aquifer characteristics in parts of the Umuahia area of Nigeria

Seventeen Schlumberger vertical electrical soundings (VES) were carried out in parts of the Umuahia area of Nigeria, using a maximum current electrode separation of 1 km. The data were interpreted using a conventional partial curve‐matching method to obtain initial model parameters, which were used in a computer program to obtain final parameters. Three soundings were made at existing boreholes for comparison. Aquifer parameters of hydraulic conductivity and transmissivity were obtained by analyzing pumping test data from existing boreholes. The results of the interpretation revealed three distinct geoelectric layers overlying a conductive geoelectric basement. Based on the model obtained, aquifer hydraulic conductivity and transmissivity were calculated. The hydraulic conductivity calculated agreed closely with that determined from pumping test data. The results of the present study also indicate that the entire area is divided into two zones, each of which is homogeneous in hydraulic properties and water quality, but distinct from the other zone. A local groundwater divide between the Imo and Kwa‐Ibo drainage systems was also defined.

Application of Resistivity Sounding in the Exploration for Nigerian Tarsand

The extensive tarsand deposits of Ondo and Ogun States of Nigeria have previously been investigated only by the use of geological methods. In a recently completed exploration programme covering a 17 km2 area north of Agbabu, in the Ifesowapo Local Government area of Ondo State, the electrical resistivity technique was used extensively along with other exploration methods. The electrical resistivity survey was made up of 32 Schlumberger vertical electrical soundings (VES). The resistivity curves were interpreted by curve matching and computer based automatic interpretation techniques. Apparent resistivity values obtained varied between less than 10 ohm-m to greater than 1000 ohm-m. The results permitted fair to excellent correlation between the geoelectrical and Stratigraphic data, particularly the delineation of lithological boundaries and zone of bitumen saturation. The lithological boundaries and zone of bitumen saturation. The geophysical results also confirmed the regional block faulting and the consistent, increase in overburden thickness southwards.

Geoelectric investigation of saline contamination of a chalk aquifer by mine drainage water at Tilmanstone, England

Arising from the soakage of mine water pumped from a coal mine in the Kent coalfield which underlies the Chalk of East Kent in England, an area of 27 km 2 of a Chalk aquifer has become contaminated with saline water having concentrations of chloride as high as 5000 mg/l. Twenty-six Schlumberger vertical electrical soundings were made as part of a hydrogeological study of the area. Interpretation of the VES data showed a spatial variation in contamination of the aquifer which correlated closely with data obtained from boreholes. Laboratory measurements of effective porosity and resistivity of Chalk core samples were made and the form of Archie's formula was determined. A synthesis of the surface geophysical, hydrochemical and laboratory data led to quantitative mapping of formation water salinity in the fully saturated aquifer.

INTERPRETATION OF APPARENT RESISTIVITY DATA FROM APODI VALLEY, RIO GRANDE DO NORTE, BRAZIL

One hundred and twelve Schlumberger vertical electrical soundings were made as part of a hydrogeological study in the Apodi Valley, Brazil. Most of the data have been interpreted using an automatic ridge regression inversion algorithm in conjunction with a fast digital filter forward algorithm. As a result, the inversion costs are very low. The increase in speed and accuracy in the evaluation of the forward problem has also allowed calculation of the Schlumberger apparent resistivity from potential differences, instead of the electric field. Consequently, there is no difficulty in the interpretation of data where very large receiver electrode (MN) spacings have been used or where discontinuities have been introduced by changing the MN spacing on a layered earth containing large resistivity contrasts. The soundings were conducted primarily to map the thicknesses of a known alluvial aquifer and a potential sandstone aquifer. These thicknesses have been determined to within an error of 20 percent as estimated from analysis of the parameter standard deviations and comparison with available drill hole information.