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

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 geoelectric 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. Introduction Geophysical methods provide useful information on subsurface geological structure and lithology. With the advancement of technology different methods have been developed for exploration work. Among the others the electrical method [1] is much more effective for shallow exploration giving information on subsurface features. Electrical method studies the electrical properties of the earth and distinguishes different types of material. In resistivity method vertical electric sounding (VES) [2] is one kind of measurement in which a succession of apparent resitivity is recorded with an increasing electrode separation, the center of the configuration and its orientation remaining fixed. 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. Twelve vertical soundings were conducted in the area. Institute of Water Modeling (IWM) carried out a detailed geophysical survey for groundwater resources study and decision support system development of Thakurgaon, Panchagarh, Dinajpur, Joypurhat districts and also remaining districts of Rajshahi Division through mathematical model study under Barind Multipurpose Development Authority (BMDA). A.H.M.Selim Reza et al. : Rajshahi University J. of Sci. 39, 27-37 (2011) 28 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. The resistivity survey has been carried out at twelve different stations as shown in fig.1. 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 [3]. This alluvium is composed of mostly clay, silt and fine sand. The Pleistocene sediments are welloxidized and typically reddish, brown or tan. They commonly contain ferruginous or calcareous nodules, which were deposited by the GangesBrahmaputra 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 (Fig.2). 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 to51 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 [5]. The DD resistivity meter was used in the field. Geo-electrical Resistivity Survey in the High Barind 29 Fig.1. Location map of the study area showing the location of VES points and boreholes A.H.M.Selim Reza et al. : Rajshahi University J. of Sci. 39, 27-37 (2011) 30 Fig.2. Cross section along NS direction Geo-electrical Resistivity Survey in the High Barind 31 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, legth L, resistance R and cross sectional area A, through which a Current I is following: