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Basic Geologic and Hydrogeologic Investigations

Basic Geologic and Hydrogeologic Investigations. 7.1 Key drilling and Push technologies 7.2 Piezometers and water-table observations wells 7.3 Installing piezometers and water-table wells 7.4 Making water-level measurements 7.5 Geophysics applied to site investigations

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Basic Geologic and Hydrogeologic Investigations

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  1. Basic Geologic and Hydrogeologic Investigations 7.1 Key drilling and Push technologies 7.2 Piezometers and water-table observations wells 7.3 Installing piezometers and water-table wells 7.4 Making water-level measurements 7.5 Geophysics applied to site investigations 7.6 Groundwater investigations

  2. KEY DRILLING AND PUSH TECHNOLOGIESdownload this file from textbook home page

  3. Piezometers and Water-Table Observation Wells • PIEZOMETER: A borehole or standpipe installed to some depth below the water table piezometer Water-table observation well

  4. Piezometers and Water-Table Observation Wells • PIEZOMETER: A borehole or standpipe installed to some depth below the water table piezometer Water-table observation well

  5. Basic design for piezometers and water-table observation wells • Screen for water to enter the standpipe • Sand pack around the screen to increase the effective size of the screen and support material placed above • Seal above the screen to prevent water from leaking along the casing • Screen and casing materials that do not react with groundwater or contaminants • Casing protector to finish the top of piezometer and prevent unauthorized access

  6. Piezometers and Water-Table Observation Wells • PIEZOMETER: A borehole or standpipe installed to some depth below the water table piezometer Water-table observation well

  7. Caving Materials with Seal • Hollow-stem auger used to drill the hole to required depth • Center rod is removed to provide access to formation • Auger itself holds the hole open like a temporary casing • Now, we are ready to emplace the piezometer

  8. Making water level measurements • Electric tape Measures depth of water level from fixed point at top of well (usually top of casing) When the electrode hits water, electric circuit is completed and light goes on. Actual elevation of water = elevation of fixed point – measurement on tape • Pressure transducer (logger) Continuous measurement Data is collected in digital format Measurement taken at intervals of few seconds

  9. Making water level measurements • Electric tape Measures depth of water level from fixed point at top of well (usually top of casing) When the electrode hits water, electric circuit is completed and light goes on. Actual elevation of water = elevation of fixed point – measurement on tape • Pressure transducer (logger) Continuous measurement Data is collected in digital format Measurement taken at intervals of few seconds

  10. Making water level measurements • Electric tape Measures depth of water level from fixed point at top of well (usually top of casing) When the electrode hits water, electric circuit is completed and light goes on. Actual elevation of water = elevation of fixed point – measurement on tape • Pressure transducer (logger) Continuous measurement Data is collected in digital format Measurement taken at intervals of few seconds

  11. Making water level measurements • Electric tape Measures depth of water level from fixed point at top of well (usually top of casing) When the electrode hits water, electric circuit is completed and light goes on. Actual elevation of water = elevation of fixed point – measurement on tape • Pressure transducer (logger) Continuous measurement Data is collected in digital format Measurement taken at intervals of few seconds

  12. In-Situ Minitroll

  13. Making water level measurements • Electric tape Measures depth of water level from fixed point at top of well (usually top of casing) When the electrode hits water, electric circuit is completed and light goes on. Actual elevation of water = elevation of fixed point – measurement on tape • Pressure transducer (logger) Continuous measurement Data is collected in digital format Measurement taken at intervals of few seconds

  14. In-Situ Minitroll

  15. Geophysics applied to site investigation • Surface geophysical techniques Used to map features of geological setting and location of abandoned hazardous waste disposal sites • Electrical resistivity • Electromagnetic methods • Ground penetrating radar (GPR) • Seismic reflection • Seismic refraction • Magnetic • Borehole Geophysics Provides stratigraphic and hydrogeologic information

  16. Making water level measurements • Electric tape Measures depth of water level from fixed point at top of well (usually top of casing) When the electrode hits water, electric circuit is completed and light goes on. Actual elevation of water = elevation of fixed point – measurement on tape • Pressure transducer (logger) Continuous measurement Data is collected in digital format Measurement taken at intervals of few seconds

  17. In-Situ Minitroll

  18. Geophysics applied to site investigation • Surface geophysical techniques Used to map features of geological setting and location of abandoned hazardous waste disposal sites • Electrical resistivity • Electromagnetic methods • Ground penetrating radar (GPR) • Seismic reflection • Seismic refraction • Magnetic • Borehole Geophysics Provides stratigraphic and hydrogeologic information

  19. Electric Resistivity method • A measure of Electrical conductivity (or resistivity) • Conductance is controlled by: • content of dissolved mass (TDS) • relative abundance of clay minerals

  20. Making water level measurements • Electric tape Measures depth of water level from fixed point at top of well (usually top of casing) When the electrode hits water, electric circuit is completed and light goes on. Actual elevation of water = elevation of fixed point – measurement on tape • Pressure transducer (logger) Continuous measurement Data is collected in digital format Measurement taken at intervals of few seconds

  21. In-Situ Minitroll

  22. Geophysics applied to site investigation • Surface geophysical techniques Used to map features of geological setting and location of abandoned hazardous waste disposal sites • Electrical resistivity • Electromagnetic methods • Ground penetrating radar (GPR) • Seismic reflection • Seismic refraction • Magnetic • Borehole Geophysics Provides stratigraphic and hydrogeologic information

  23. Electric Resistivity method • A measure of Electrical conductivity (or resistivity) • Conductance is controlled by: • content of dissolved mass (TDS) • relative abundance of clay minerals

  24. Electric Resistivity method • Measuring electric potential difference between two electrodes in an electrical field as induced by two current electrodes. • Apparent resistivity • Two modes: • 1. profiling method (electrode spacing constant) • 2. sounding method ( increasing electrode spacing)

  25. Schlumberger Array

  26. Electromagnetic Methods • a current is induced in the ground with an alternating current transmitting coil • Magnetic field around coil induces electric field • depth of electric field controlled by: • Background properties of medium • Moisture content • Relative difference in conducting properties of medium and target • Most important application: • Detect buried objects, waste disposal sites

  27. Ground Penetrating RadarGPR • Method used to: • delineate features of the geologic setting • Map distribution of buried objects • Define configuration of water table and stratigraphic boundaries • Establish the distribution of liquids GPR is well suited for surveying abandoned waste disposal sites

  28. GPR Principle • Reflection of Radio waves from subsurface discontinuities • A transmitting antenna at surface radiates short pulses of radio waves into the ground. • An antenna that is moved along the surface recovers the reflected energy from subsurface • Radar energy is reflected due to changes in dielectric constants and electrical conductivity (reflecting variation in properties, degree of saturation, material density) • GPR works like reflection seismic method (electromagnetic reflections instead of acoustic energy)

  29. Investigative depth of method is determined by electrical conductivity of earth material: •  = electric cond. in mS/m • Depth range: few m to 100 m (< 30 m in most cases)

  30. GPR

  31. GPR

  32. Reflection Seismic Method • Reflected • Refracted • Reflection seismic is most useful in environmental applications • Useful in: • Determining top of bedrock surface • Structural features • Pattern of stratigraphic layering

  33. Seismic Methods • Method is based on measuring velocity and paths of seismic waves in subsurface • Energy to produce seismic waves • Explosion • Vibroseis • Rifle bullet • Weight drop (Hammer)

  34. Seismic waves • Surface waves: Waves transmitted from source to receiver along ground surface • Refraction wave: Wave moving along the boundary before being reflected • Reflection Wave: • Wave reaching lower boundary and reflected back to ground surface

  35. Seismic waves Surface wave reflected wave refracted wave

  36. Reflection from multiple layers

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