Determination of Aquifer Porosity Using Resistivity Imaging Technique
Determination of Aquifer porosity using Resistivity Imaging Technique
By:
Abdelatif Mokhtar Ahmed1 and Wan Norazmin Sulaiman2
1-College of Water Science and Technology, Sudan University of Science and Technology, PO Box 352, Khartoum North, Sudan, 2-Department of Environmental Sciences, College of Science and Environmental Studies, Universiti Putra Malaysia, Malaysia.
KEYWORDS: Resisyivity, Porosity and Groundwater
ABSTRACT
This paper is an attempt to investigate the groundwater aquifer porosity in the area south of Kuala Lumpur City, Malaysia. Other groundwater aquifer characteristics such as the aquifer thickness, depth to aquifer, and aquifer type were determined. Resistivity imaging survey was used as the main technique to investigate the aquifer characteristics, and mainly the aquifer porosity. Electrical resistivity imaging survey line was conducted in the area south of Kuala Lumpur City, and around the Malaysian Agricultural Research Institute. A relationship between the resistivity and formation factor was established, and used for plotting of porosity distribution along the same line. Porosity values along this line were found to range between 6 to 33%. Three porosity zones were determined; arenitic schist with porosity values between 6 and 10%, schist with porosity values between 10 and 17%, and fissured phyllite with porosity values between 17 and 33%. Thickness of the top layer (Arenitic Schist of kenny Hill Formation), was estimated to range between 1.2 and 10 ms, while the depth to aquifer was estimated to range between 7 and 10 ms. The aquifer was determined to be unconfined and of good quality and can yield good quantities of water.
الملخص:
هذه الورقة محاولة للتعرف على مسامية صخور الخزان الجوفي في المنطقة جنوب كوالالمبور (ماليزيا). وكذلك للتعرف على بعض خصائص الخزان الجوفي مثل عرض الخزان ونوعيته وبعده عن سطح الأرض، وذلك بطريقة المقاومة الأرضية.
لقد تم التعرف على عدة قيم للمسامية تتراوح ما بين 6 – 33%. وتم التعرف على بعد الخزان من سطح الأرض ووجد أنه يتراوح ما بين 7 - 10 متر. ولقد تعرفت الدراسة على أن الخزان الجوفي نوعيته شبه محصور وذو نوعية جيدة وبه كميات وافرة من المياه.
INTRODUCTION
Groundwater is important as a source of drinking water as well as for irrigation and industrial use (Canter and Knox, 1995). The amount of water required to society has increased as population and urbanization increased. As a result there is a growing demand for groundwater (Kobus, 1996). Large percentage of population worldwide still depends on ground water as the main supply for a variety of purposes such as drinking, irrigation, recreation, and industry. The amount of groundwater in a certain aquifer is a function of its porosity. The availability of groundwater in Malaysian aquifers is abundant and the storage is estimated about 64 billion cubic meters per year, and is still far untapped for public and industrial consumption except in Kota Bharu and several places in Sarawak (Nawang, 1993). This study has been conducted around Kuala Lumpur City (Malaysia), within a tropical humid climate characterized by high rainfall, to determine the extend of porosity of Kenny Hill Formation aquifer. The main technique used was the resistivity imaging survey. The electrical imaging survey is a technique recently developed to investigate areas of complex geology where the use of resistivity sounding and profiling is inadequate (Griffiths and Barker, 1993). The 2-D resistivity model for subsur-face is obtained using an automatic inversion computer program (RES2DINV), which uses the techniques developed by (Loke and Barker 1996).
LOCATION AND GEOLOGY
The study area extending from the landfill of Sri Petaling in the north to Serdang, UPM (Universiti Putra Malaysia), and the Malaysian Agricultural Research and Development Institute (Mardi) areas in the south as can be seen from (Fig. 1). The study area covering an area of about 57.4 km and underlain by the Kenny Hill formation. Other rock units surrounding the area comprise the granitic rocks and Kuala Lumpur Limestone (Fig. 2). The name of the ‘Kenny Hill Formation’ has been used for a number of years for a sequence of clastic sedimentary rocks occurring in Kuala Lumpur area. It was shown by that the clastic rocks fall into two groups, one underlying and the other group overlying the limestone, which is now known to be at least in part Silurian (Gobbett and Hutchson, 1973). The younger clastic sequence has been called the ‘Kenny Hill Formation’ after exposures in Kenny Hill residential area of Kuala Lumpur town. Kenny Hill Formation exists as a broad synclinal belt, generally 7 to 10 km wide, running from Kuala Lumpur city center southward through the shrubs of Petaling and Petaling Jaya and further to the south for at least 30 km (Fig. 2).
Fig. (2): Geological Setting of the Study Area
MATERIALS AND METHODS
An OYO McOhm Resistivity Meter was used to obtain a 2 Dimensional electrical resistivity image along Line Mardi 1 (located at the southern part of the study area), which was taken to investigate the aquifer porosity. The electrodes were connected to the central switching system. The current and potential poles of the McOhm OYO Resistivity Meter were connected to the central switching system. Four electrodes were chosen at any one time for resistance measurement. Currents were injected into the ground via two current electrodes located to the exterior of the potential electrodes. The potential difference between the potential electrodes was measured and the resistance of the ground was calculated automatically by the meter. The measured resistances were recorded into a pre-prepared data entry sheet. The electrode configuration used in the present survey is that of Wenner Array. Resistance values were converted into apparent resistivity values ra using an equation: ra = 2p aR
Schist Arenitic Schist of Kenny Hill Formation
Fissured Phyllite
Fig. (3): Resistivity Image alongLine Mardi 1
Where (a) is the spacing used in the measurement and (R) is the resistance of the ground recorded by McOhm OYO Resistivity Meter. The x position of measurement along the resistivity traverse, the electrode spacing, and the calculated apparent resistivity values were entered into the data files, which were subsequently used by the RES2DINV 2 Dimensional Resistivity Imaging Interpretation software. The interpretation programmer essentially calculates the true resistivity and true depth of the ground from the inputted data file using Jacobian Matrix Calculation and Forward Modelling procedures. The results of the interpretation are displayed as the 2-D electrical resistivity image of the subsurface along the line of traverse. In this study the line used and the generated resistivity image is that of Mardi 1 (Fig. 3). Laboratory measure-ments on the electrical resistivity and effective porosity of rock and soil samples were conducted so as to be able to appreciate the significance of the resistivity values measured in the field.
In a fully saturated, clay-free granular formation the rock resistivity Rt is related to the resistivity of the formation water by the following expression: Rt = F * Rw
The constant of proportionality, F, being known as the formation factor. This linear relationship is often referred to as Archie’s Law (Archie, 1942). The formation factor, F, is found to be related to the effective porosity, f, through the following equation: F = af-m
Where and m are imperical constants within a formation or a section of a formation. Laboratory investigations have been carried out to determine these constants. The laboratory established relationship between formation factor and the effective porosity was applied to the field data (In this study data was collected from Line Mardi 1), to calculate the porosity values of the formation. These values were contoured and plotted using DIPIX-PLUS software (Stoyer, 1987). The contouring is in a form of a pseudosection (as shown by (Fig. 4), of Line Mardi 1).
RESULTS AND DISCUSSION
Resistivity Image along Line Mardi 1: This line was conducted in Universiti Putra Malaysia (UPM) farms, west of the main building of the university. The length of this line is 250 m with 392 total number of datum points. (Fig. 3) shows the resistivity image of this line. The most prominent features which can be seen from the resistivity image along this line are the high resistive layer on the top of the image with resistivity values ranging from 194 to 2156 Wm and a thickness of about 7 to 15 m. This layer reflects an irregular topography of the conductive zone lying below it. This layer is interpreted as arenitic schist of Kenny Hill Formation and it is a dry top layer and not indicative of potential water bearing zone (Venkateswara and Briz-Kishore 1991). The low resistive zone (7.8 to 17.4 Wm) situated in the center of the image is interpreted as fissured phyllite. The large zone occupying most of the image with resistivity values of 17.4 to 194 Wm represents the porous aquifer was interpreted as schist. The interpretation of this image was based on the lithological logs of bore holes drilled in this area.
Porosity Distribution along Line Mardi 1: Porosity values along this line range between 6 to 33%. (Fig. 4) shows the porosity distribution along this line. By comparing this section with the resistivity distribution along the same line (Fig. 3), three porosity zones are determined; arenitic schist with porosity values between 6 and 10%, schist with porosity values between 10 and 17%, and fissured phyllite with porosity values between 17 and 33%. Both the schist and fissured phyllite represent the aquifer materials. Thickness of the top layer (Arenitic Schist of Kenny Hill Formation), was estimated to range between 1.2 and 10 ms. The aquifer was determined to be unconfined. The depth to aquifer was estimated to range between 7 and 10ms. Aquifer porosity ranging between 20% and 33% was regarded as excellent (Cosse, 1993), while the study area (Line Mardi 1), ranging between 10% and 33% can be estimated as good porosity.
Fig. (4): Subsurface porosity distribution along line MARDI 1
CONCLUSION
From the discussion of the resistivity section and the porosity distribution along Line Mardi 1, it can be concluded that, the resistivity imaging survey technique is useful for the determination of groundwater aquifer porosity, and hence is useful for determination of water quantity in the groundwater aquifers of different formation.
The wide difference in resistivity values between the different lithological beds could be attributed to the significant difference in porosity, mineralogy, water saturation and type of fluid content i.e., water quality either fresh or saline which greatly affect the resistivity values. From the above discussion it can be concluded that the groundwater aquifer of Mardi area is of good quality and can yield good quantities of water.
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