Ground water hydraulics presentation vtu

Ground water exploration and development
Electrical resistivity and seismic method

1. KLE Institute Of Technology, Hubballi - 27 Department Of Civil Engineering K.L.E Society’s 2023-2024 GROUNDWATER EXPLORATION AND DEVELOPMENT - ELECTRIC RESISTIVITY AND SEISMIC METHOD Presentation By: Sahana A Sarwade SpoortHi mathapati

2. GROUNDWATER EXPLORATION Groundwater is an invisible natural resource. It is available in different proportions, in various rock types and at various depths, on the surface layer of the earth. Groundwater is not uniformly distributed everywhere. The occurrence of groundwater varies from formation to formation. In a typical crystalline hard rock terrain, the quantitative occurrence of groundwater depends on the weathered and fractured zones. The occurrence of groundwater in a sedimentary terrain will be more promising.

3. TYPES OF GROUNDWATER EXPLORATION METHODS 1. SURFACE METHOD - GEOLOGICAL AND GEOPHYSICAL METHOD 2. SUBSURFACE METHOD GEOPHYSICAL METHODS - geophysics is an applied branch of geophysics and economic geology, which uses physical methods at the surface of the Earth, such as seismic, gravitational, magnetic, electrical and electromagnetic, to measure the physical properties of the subsurface, along with the anomalies in those properties. ELECTRIC RESISTIVITY METHOD SEISMIC METHOD GRAVITY MRTHOD MAGNETIC METHOD REMOTE SENSSING

4. ELECTRIC RESISTIVITY METHOD The electrical resistivity of a rock formation limits the amount of current passing through the formation when an electrical potential is applied. It may be defined as the resistence in ohms between opposite faces of a unit cube of the material. If a material of resistance R has a cross- sectional area A and a length L, then its resistivity can be expressed as P = (RA)/L

5. The Wenner arrangement, shown in Figure a, has the potential electrodes located at the one-third points between the current electrodes. The apparent resistivity is given by the ratio of voltage to current times a spacing factor. For the Wenner arrangement, the apparent resistivity where a is the distance between adjacent electrodes, V is the voltage difference between the potential electrodes, and I is the applied current. Ρα = 2 παV/I The Schlumberger arrangement, shown in Figure b, has the potential electrodes close together. The apparent resistivity is given by Ρα = π (1/2)² - (b/2)² V/I b where L and b are the current and potential electrode spacings, respectively. Theoretically, L >> b, but for practical application good results can be obtained if L≥ 5b

6. Table : Resistivity values for some types of water

7. SEISMIC METHOD The seismic method is developed from the science of earthquakes called Seismology. This method is used in two different kinds, seismic refraction and seismic reflection . The seismic refraction method creates a small shock on the earth’s surface and measures the time of returning sound from the surface. The seismic reflection is also based on the same principle. However, it provides information for thousands of meters below the surface. In contrast, the refraction method is used to conduct the study only 100 meters deep below the surface.

8. Analyzing Seismic velocities A basic understanding of the characteristic seismic velocities for a variety of geologic materials is necessary. These velocities help to identify the nature of alluvium or bedrock. In coarse alluvial terrain, seismic velocity increases markedly from unsaturated to saturated zones. In seismic method, the depth to water table car be mapped, with an accuracy of 10 percent, where the geologic conditions are relatively uniform. The changes in seismic velocities are governed by changes in the elastic properties of the formations. The greater the contrast of these properties, the more clearly the formations and their boundaries can be identified

9. Seismic refraction method applied to determine depth to water table. (a) Wave front advance (b) Time-distance graph

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