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 TECHNOLOGIES download this file from textbook home page

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

4. Piezometers and Water-Table Observation Wells
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
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

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

14. 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. 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. 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

26. In-Situ Minitroll

27. 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

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

29. 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

32. In-Situ Minitroll

33. 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

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

35. 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)

36. Schlumberger Array

37. 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

39. Ground Penetrating Radar GPR
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

40. 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)

41. 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)

42. GPR

44. GPR

46. 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

47. 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)

48. Seismic waves Surface waves: Refraction wave: Reflection Wave:
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

49. Seismic waves
Surface wave
reflected wave
refracted wave

50. Reflection from multiple layers

52. Procedures of seismic survey
Data acquisition
Processing
Interpretation
Seismograph:
records and enhances sound waves to detect geologic features
Geophones:
receivers for detecting reflected acoustic signals

53. Common Depth Point (CDP)

54. Example: VSP + 1441 shotpoints, 24-fold CDP, 3 lines

55. Borehole Geophysics Methods
Not all techniques are utilized in groundwater industry because of economics
Caliper logs
Resistivity log
spontaneous potential (SP) log
Natural gamma log

56. Caliper logs Record variation of borehole diameter with depth Used to:
Interpret other logging methods affected by hole size
Provide info on fracture distribution and lithologies
Estimate quantities of cement or gravel to complete a water well

57. Resistivity logs
Measures change in resistance between a lead electrode in borehole and a fixed electrode at surface.
Log provides info on resistivity (ohms) changes vs depth
Useful in distinguishing different types of lithologies (sand vs clay or shale)
Sand: high resistivity (log reflection to the right)
Clay: less resistivity (log reflection to the left)

58. Spontaneous potential (Sp)
Measures natural electrical potentials (voltages) that develop at contact between clay beds and sands (as a result of differences in lithology and in chemistry between drilling water and formation water)
Sand: less potential (left on the log)
Used in water quality investigations

59. Gamma log Measures total intensity of natural gamma radiation
Radionuclides: K-40, thorium, Uranium
Run in cased wells
Higher count rate in finer-grained units
Useful for determining clay content of units, boundaries between units, regional correlation between boreholes

60. Groundwater investigations
Regional investigation:
Large area, overall evaluation of groundwater conditions
Local investigations:
More detailed study of an area; geology, hydrogeology, water quality
Site investigations:
Specific site: e.g., well field, leaking refinery, abandoned industrial site. Done with other investigations as risk assessment, air-quality monitoring…etc

61. Steps of Groundwater investigations
Objectives
Workplan
Collecting data
Interpreting data
Developing conclusions
Presenting results

62. Sources of information
Topo maps
Soil maps
Geol maps
Aerial photos
Sat. images
Quality data
Climate data
Previous reports
Government reports
Personal interviews
outcrop mapping
Borehole logging
Core samples
Geop. Logs
Textural analyses
Hydraulic head
Hydraulic conductivity
Pumping tests
Tracer tests

64. Chapter Highlights
A variety of different drilling methods can be used to sample the subsurface and to provide boreholes for the installation of wells or piezometers. Hollow-stem and solid-stem augers are well suited for drilling in unlithified sediments.
Push technologies involve pushing or hammering a casing string into the ground, carrying sensors, sampling tools, or permanent monitoring devices.
A piezometer is a standpipe that is installed to some depth below the water table. Water entering the piezometer rises up the casing and reaches a stable elevation, which is a measure of the hydraulic head at the open part of the casing at the bottom. A water-table observation well is a standpipe with a large screened section spanning the water table. The water-level elevation provides the elevation of the water table at that location.

65. In practice, piezometers are designed to have a screen, attached to the bottom of the casing, a sand-pack to support the screen, and a seal to prevent leakage of water down the borehole. The seal is usually constructed with granular bentonite, a low-permeability material that expands when it becomes wet.
Water-level elevations in wells or piezometers are determined using an electric tape or a transducer system that provides a continuous measurement of pressure with time. An electric tape is a tape measure that buzzes when the electrode on the end touches the water surface.
A variety of geophysical techniques are used in hydrogeological investigations. Most useful surface surveying methods include electrical resistivity, electromagnetics, ground penetrating radar, and seismic.
Geophysical logs are also run in boreholes. Key logs used in ground-water applications include single-point resistance, spontaneous potential (SP), and gamma.
Ground-water investigations are carried out at regional, local, and site scales. The type of investigative approach changes as a function of scales.