Subsurface investigation of groundwater
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The document discusses various methods for subsurface investigations of groundwater, including geologic logging, resistivity logging, spontaneous potential logging, radiation logging, temperature logging, television logging, and acoustic logging. Each method provides critical data regarding the geological and hydrological character of the subsurface, aiding in aquifer delineation and understanding groundwater conditions. Key technologies and applications are described, revealing their relevance in assessing lithology, porosity, and fluid dynamics within boreholes.
Subsurface investigation of groundwater
- 1. Subsurface Investigations of Groundwater D. Naresh Kumar Assistant Professor Civil Engineering Department
- 2. Geologic log ▸ A Geologic log is constructed from sampling and examination of well cuttings collected at frequent intervals during the drilling of a well or test hole. Such log description of the geologic character and thickness of each stratum encountered as a function of depth, thereby enabling aquifers to be delineated. ▸ It is good practice to store samples of well cuttings systematically. These not only permit detailed geologic logs to be prepared but also enable grain size analyses and correlations with other nearby wells to be made after drilling finished
- 4. Geologic log
- 5. Geophysical logging Resistivity logging ▸ Current and potential electrodes can be lowered to measure electric resistivities of the surrounding media and to obtain a trace of their variation with depth. The result is a resistivity log. Such a log is affected by fluid within a well, by well diameter by the character of surrounding strata, and by groundwater.
- 6. ▸ Four electrodes, two for potential measurement, constitute the system. Recorded curves are termed normal or lateral, depending on the electrode arrangement. ▸ In the normal arrangement the effective spacing is considered to be the distance AM and the recorded curve is designed AM. ▸ Sometimes a long normal curve AM’ is recorded based on the same electrode arrangement as the normal but with a larger AM distance. ▸ The spacing for lateral AO curves is taken as the distance AO, measured between A and a point midway between the electrodes M and N
- 7. Typical electrode arrangement and standardized distances for resistivity logs. (a) Short normal (b) Long normal (c) Later
- 8. Spontaneous potential logging ▸ This method measures natural electrical potentials found within the earth. Measurements, usually in millivolts, are obtained from a recording potentiometer connected to two like electrode is lowered in an uncased well and the other is connected to the ground surface. The potentials are primarily produced by electrochemical cells formed by the electrical conductivity differences of drilling mud and groundwater where boundaries of permeable zones intersect a borehole. ▸ Potential values range from zero to several hundred millivolts. Spontaneous potentials resulting from electrochemical potentials can be expressed by
- 10. Radiation Logging Radiation logging, also known as nuclear or radioactive logging, involves the measurement of fundamental particles emitted from unstable radioactive isotopes. Logs having application to groundwater are natural gamma, gamma- gamma, and neutron.
- 11. Natural Gamma logging ▸ Rocks are emit natural-gamma radiation, a record of this constitutes a natural- gamma log. The radiation originates from unstable isotopes of potassium, uranium and thorium. In general, the natural-gamma activity of clayey formations is significantly higher than that of quartz sand and carbonate rocks. The most important application to groundwater hydrology is identification of lithology, particularly clayey or shale- bearing sediments, which possess the highest gamma intensity.
- 12. ▸ Utilises a sodium iodide crystal connected to photomultiplier to count photons produced through capture of gammas as they pass through the scintillation system.
- 13. Gamma-Gamma Logging Gamma radiation originating from a source probe and recorded after it is backscattered and attenuated within the borehole and surrounding formation constitutes a gamma-gamma log. The source probe generally contains cobalt-60 or cesium-137. which is shielded from a sodium iodide built into the probe. Primary applications of gamma-gamma logs are for identification of lithology and measurement of bulk density and porosity of rocks
- 15. Temperature logging A Vertical traverse measurement of groundwater temperature in a well can be readily obtained with a recording resistance thermometer. Such data can be of value in analyzing subsurface conditions. Ordinarily, temperatures will increase with depth in accordance with the geothermal gradient, amounting to roughly 30C for each 100m in depth. Abnormally cold temperatures may indicate the presence of gas or in deep wells, may suggest recharge from ground surface.
- 16. Miscellaneous logging techniques ▸ Television logging ▸ A convenient tool with increasing use is a television camera lowered in a well. Specially designed wide angle cameras typically less than 7 cm in diameter, are equipped with lights and provide continuous visual inspection of a borehole; with videotape a record of the interior can be preserved.
- 18. Acoustic logging ▸ Acoustic or sonic logging measures the velocity of sound through the surrounding an uncased, fluid-filled hole. Sound velocity in rock is governed by the velocity of the rock matrix and the fluid filling the pore space. Therefore, the greater the porosity, the closer the measured sound velocity approaches that of the fluid. Chief applications of the acoustic log include determining the depth and thickness of porous zones, estimating porosity, identifying fractures zones, and determining the bonding of cement between the casing and the formation