25-01-2013, 03:53 PM
GROUND WATER PROSPECTING IN HARD ROCK TERRAIN USING ELECTRICAL RESISTIVITY TOMOGRAPHY TECHNIQUE
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INTRODUCTION TO GROUND WATER
Groundwater is water stored under the surface of the ground in the tiny pore spaces between rock, sand, soil, and gravel. It occurs in two “zones” an upper unsaturated zone where most of the pore spaces are filled with air and a deeper saturated zone in which all the pore spaces are filled with water.
HYDROLOGICAL CYCLE
In most of the groundwater investigations which will be carried out by using the electrical resistivity method. In this method we have to identified the subsurface features and various geological formations and most of the geophysical methods, especially the electrical resistivity is used to identify the groundwater zones in different geological terrains. We are going to discuss below the electrical resistivity method the commonly used technique in groundwater prospecting and exploration studies.
Groundwater applications of near-surface geophysics include mapping the depth and thickness of aquifers, mapping aquitards or confining units, locating preferential fluid migration paths such as fractures and fault zones and mapping contamination to the groundwater such as that from saltwater intrusion.
The Movement of Ground Water
most ground water moves relatively slowly through rock underground
because it moves in response to differences in water pressure and elevation, water within the upper part of the saturated zone tends to move downward following the slope of the water table
Movement of ground water beneath a sloping water table in uniformly permeablerock. Near the surface the ground water tends to flow parallel to the sloping water table.
INTRODUCTION TO ELECTRICAL RESISTIVITY METHOD
Electrical and electromagnetic methods have proved particularly applicable to groundwater studies as many of the geological formation properties that are critical to hydrogeology such as the porosity and permeability of rocks can be correlated with electrical conductivity signatures.
The electrical resistivity surveying is based on the principle that the distribution of electrical potential in the ground around a current-carry in electrode depends on the electrical resistivity and distribution of the surrounding soils and rocks.
2D ELECTRICAL RESISTIVITY METHOD
The purpose of electrical surveys is to determine the subsurface resistivity distribution by making measurements on the ground surface. The 2D Electrical Resistivity Tomography (ERT) is widely used for mapping the subsurface geological formations and different rock structures like namely fault, fractures, joints, etc. with much more precision compared to the conventional resistivity method. The true resistivity of the subsurface geological formations estimated based on the type of the geological terrain. The ground resistivity is related to various geological parameters such as the porosity and degree of water saturation of the rock matrix.