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Field Methods and Groundwater Models

Field Methods and Groundwater Models. Field Methods. Hydrogeologists can apply field methods in exploration for groundwater supplies and various aspects of environmental hydrogeology.

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Field Methods and Groundwater Models

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  1. Field Methods and Groundwater Models

  2. Field Methods • Hydrogeologists can apply field methods in exploration for groundwater supplies and various aspects of environmental hydrogeology. • Field methods (techniques) are: fracture-trace methods, aerial photographs, remote sensing, site evaluation methods, surface and borehole geophysical methods.

  3. Fracture-Trace Analysis • Groundwater is in general concentrated in fracture zones found in many different rocks. • The fractured zones are less resistant to erosion, hence valley and stream segments tend to run along them. • Fractured zones are related to regional tectonic activity. • Water productivity and yield from wells located on fractured zones is much higher than those not on fractured zones. • There are many types of fractures: Linear features (300m to 1500m), Lineaments (1500 m to 150 km), Fracture traces and joints. • Mapping fractures and lineaments is very important for selecting places for monitoring wells and water exploration wells using remote sensing and surface mapping techniques.

  4. Geophysical Methods • Most common geophysical methods applied in water exploration are: (1) surface geophysics: electrical methods, electromagnetic methods, seismic refraction, gravity, and magnetic methods. Ground Penetrating Radar (GPR) can be used in detecting buried waste for safety and avoid groundwater pollution. (2) Borehole geophysics: Well Logging. • The geophysical methods can help in understanding: depth to the water table, location of subsurface faults, depth of the basement rocks, thickness of unconsolidated materials. • The combination of geophysical data and well logs data is more reliable.

  5. Electrical Methods Electrical methods help to identify aquifers. Groundwater aquifers have low electrical resistivity and high electrical conductivity.

  6. Electromagnetic Methods Interpretations of central-loop time-domain EM soundings in terms of seawater intrusion (red-colored areas) into the coastal Motril-Salobrena aquifer of southern Spain (Duque et al., 2008) Electromagnetic survey in Al-Ain (H. SAIBI et al., 2018)

  7. GPR Method

  8. Gravity Method Measured Gravity = f (time) + x _ High Low Surface Unsaturated area Porous media Mass gain Mass gain Mass loss y x Saturated area Porous media z

  9. Spontaneous Potential (Electrical method) Figure: SP anomaly produced by pumping from a well.

  10. Seismic Reflection

  11. Radiometric Method

  12. Well Logging Techniques

  13. Well Logging Methods • Caliper Logs: used to measure the diameter of borehole. • Temperature Logs: used to measure temperature in the well at different depths (continuous record). Examples: Temperature log can revealed zones of different temperature in the well. Temperature logs can be used also to trace the movement of injected water into an aquifer. • Single-Point Resistance: used to measure the electrical resistance in borehole. Examples: Sand and gravel have high resistance. Clay and shale have low resistance. If salinity increase, resistance decrease. If borehole diameter increase, resistance decrease. • Nuclear Logging methods

  14. Hydrogeologic Site Evaluations • Many types of construction and similar projects require evaluation of the hydrogeology of the site. • Environmental impact studies often include sections on geology and hydrogeology. • Hydrogeology can help ensure safety of structures in areas of high water-table, spring discharge. • Hydrogeology can also locate the sources of groundwater contamination and make plan for corrective action. • Costs of hydrogeological studies are mainly for collecting data: geophysical surveys, aquifer tests, permeability laboratory tests. • Results of hydrogeology are generally presented on maps (overly on topography and geology maps).

  15. Responsibilities of the field hydrogeologist • Collected data by field hydrogeologist is very important. • The collected data should be recorded directly to a field notebook. • Notebook characteristics: weatherproof pages, use waterproof ink. • Data to be included in the notebook during the field is: date, time of arrival, name and affiliation of all people, weather conditions, specific data collected, time of absence of the hydrogeologist, any other observations, well number, well conditions, depth to water in the well, equipment conditions, …

  16. Groundwater Models & Software • MODFLOW (https://water.usgs.gov/ogw/modflow/) • Video: https://www.youtube.com/watch?v=8mjcuu5TJgU Figure: Aquifer with geology. Figure: Contaminant Transport Figure: Finite-Difference Grid.

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