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Geo-electrical Resistivity Survey in the High BARInd for the Delineation of Aquifer Geometry

A.H.M.Selim Reza
Departmetnt of Geology and Mining, University of Rajshahi, Rajshahi-6205, Bangladesh

Quamrul Hasan Mazumder
Departmetnt of Geology and Mining, University of Rajshahi, Rajshahi-6205, Bangladesh

Mushfique Ahmed
Departmetnt of Geology and Mining, University of Rajshahi, Rajshahi-6205, Bangladesh

Abstract/ Executive Summary:

Geo-electrical resistivity techniques have been employed in the High Barind as a reconnaissance investigation for the subsurface lithology. Out of 13 upazillas, comprising the Barind tract, Sapahar and Porsha Upazillas, situated in the High Barind are badly affected by water shortage. Vertical electrical soundings (VES) were executed using Schlumberger configuration in different areas taking the current electrode separation up to 170 meters. Direct method technique was employed to evaluate the geo-electric layer such as resistivity and thickness of the discrete layers. The results of the resistivity survey give the subsurface information consisting of three to four layers. The results of geo-electric sounding have been compared with the geological section wherever available.

Keywords: Geo-electric resistivity; Aquifer geometry

Location:

Start Date:

End Date:

Program Area: Crop-Soil-Water Management

Commodity: Groundwater

Objectives:

Electrical resistivity survey is very effective method for characterizing aquifer in terms of more resistive from the less resistive. In view of the geological conditions and available geophysical information on the area an electrical resistivity survey has been carried out in the High Barind.

Methodology:

Location and Extent The study area lies between 24°54′ N and 25°13′ N latitudes and 88°23′ E and 88°39′ E longitudes. It comprises a total area of 497.3 sq km. Geology The study area lies within the High Barind. This is the uplifted part of the Barind tract. It was formed by the deposition of sediments carried by the river Ganges and the Brahmaputra and their tributaries in the Pleistocene age. This alluvium is composed of mostly clay, silt and fine sand. The Pleistocene sediments are well- oxidized and typically reddish, brown or tan. They commonly contain ferruginous or calcareous nodules, which were deposited by the Ganges- Brahmaputra fluvial system. Water content is low due to firm and compact materials. Hydrogeology Hydrogeological cross section has been constructed along NS directions of the study area. According to the lithological constituents and on the basis of hydrogeological types, the subsurface layers are divided into three zones. Zone 1: The zone extends vertically up to an average depth of 6 to 36 m below ground surface and is composed mainly of clay with occasional fine sand and silt. Zone 2: The thickness of this thin zone varies from 15 to 51 m and is mainly composed of fine, medium and coarse sand with gravel. Zone 3: in some part of the study area, there is another silty shale sequence below zone 2 ranging in thickness of about 2 to 17m. Data Acquisition and Processing The area of the investigation was covered by sounding surveys. Vertical electrical soundings (VES) have been executed using Schlumberger electrode configuration taking the current electrode separation up to 170m. Twelve soundings were made (Fig.1). Each location is assigned an individual VES number such as VES 1, VES 2, VES 3, VES 4, VES 5, VES 6, VES 7, VES 8, VES 9, VES 10, VES 11 and VES 12. Resistivity values with respect to distance were recorded for each sounding point. The resistivity data are plotted from sounding data. Interpretation of these sounding curves is done by using USGS software programme. The DD resistivity meter was used in the field. Basic Principles of Electrical Resistivity Method The basic principles of electrical resistivity method is to send current into the ground y means of two electrodes and the potential difference between two other electrodes is measured in time. From the magnitude of potential difference and current and knowledge of the electrode separation a quantity known as the apparent resistivity can be calculated. For a quantitative treatment, let us consider a homogeneous isotropic earth layer of resistivity ρ, length L, resistance R and cross sectional area A, through which a Current I is following: V = ρI/2πr This is the basic equation, which enables the calculation of the potential distribution in a homogeneous conducting medium. In the above equation r is the geometric factor, which is different electrode configuration. Fig. 3 shows the distribution of the potential and the lines of current flow in the vertical section of a homogeneous conducting underground medium due to a pair of current electrodes. Data Acquisition and Processing The area of the investigation was covered by sounding surveys. Vertical electrical soundings (VES) have been executed using Schlumberger electrode configuration taking the current electrode separation up to 170m. Twelve soundings were made . Each location is assigned an individual VES number such as VES 1, VES 2, VES 3, VES 4, VES 5, VES 6, VES 7, VES 8, VES 9, VES 10, VES 11 and VES 12. Resistivity values with respect to distance were recorded for each sounding point. The resistivity data are plotted from sounding data. Interpretation of these sounding curves is done by using USGS software programme. The DD resistivity meter was used in the field.

Source of Information: Rajshahi University J. of Sci. 39, 27-37 (2011) ISSN 1681-0708

Article Link (Online Journal):

Funding Source:

Total Budget (Tk.) :

Achievement/Findings:

For the present study two isopach maps have been prepared for the aquitard and aquifer. The thickness of the top sandy and silty clay layer varies from 4 to 41m. The highest thickness is observed at Shironti of Sapahar Upazilla and the lowest thickness is observed in the southern portion of the area. Overall the thickness of the top sandy and silty clay layer is higher in northwestern part of Sapahar Upazilla and lower in other parts of the study area. The thickness of the aquifer varies from 25 to 51.5m. The highest thickness (51.5m) is observed at Khatirpur station of Porsha Upazilla and the lowest thickness (25m) is observed in the Karmudanga of Sapahar Upazilla. Overall the thickness of aquifer is high in Porsha Upazilla. Considering the thickness of aquifer it is ascertained that the aquifers of Porsha Upazilla show better potentiality as compared to Sapahar Upazilla.

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