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
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KEY DRILLING AND
PUSH TECHNOLOGIES
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Piezometers and Water-Table
Observation Wells
• PIEZOMETER:
A borehole or standpipe installed to some
depth below the water table
piezometer
Water-table
observation well
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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
Basic design for piezometers and
water-table observation wells
1.
2.
3.
4.
5.
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
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Piezometers and Water-Table
Observation Wells
• PIEZOMETER:
A borehole or standpipe installed to some
depth below the water table
piezometer
Water-table
observation well
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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
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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
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
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
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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
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16
In-Situ Minitroll
17
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
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19
20
In-Situ Minitroll
21
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
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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
23
24
25
In-Situ Minitroll
26
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
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Electric Resistivity method
• A measure of Electrical conductivity
(or resistivity)
• Conductance is controlled by:
– content of dissolved mass (TDS)
– relative abundance of clay minerals
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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
29
30
31
In-Situ Minitroll
32
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
33
Electric Resistivity method
• A measure of Electrical conductivity
(or resistivity)
• Conductance is controlled by:
– content of dissolved mass (TDS)
– relative abundance of clay minerals
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Electric Resistivity method
• Measuring electric potential difference
between two electrodes in an electrical field as
induced by two current electrodes.
V
• Apparent resistivity
a  K I
• Two modes:
– 1. profiling method (electrode spacing constant)
– 2. sounding method ( increasing electrode
spacing)
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Schlumberger Array
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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
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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
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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)
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• Investigative depth of method
is determined by electrical
conductivity of earth material:
Z
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
(meter )
•  = electric cond. in mS/m
• Depth range: few m to 100 m
(< 30 m in most cases)
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GPR
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GPR
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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
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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)
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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
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Seismic waves
Surface wave
reflected wave
refracted wave
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Reflection from multiple layers
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Procedures of seismic survey
1.
2.
3.
Data acquisition
Processing
Interpretation
Seismograph:
records and enhances sound waves to detect geologic
features
Geophones:
receivers for detecting reflected acoustic signals
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Common Depth Point (CDP)
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Example: VSP + 1441 shotpoints, 24-fold CDP, 3 lines
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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
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Caliper logs
Record variation of borehole
diameter with depth
Used to:
1. Interpret other logging
methods affected by hole
size
2. Provide info on fracture
distribution and lithologies
3. Estimate quantities of
cement or gravel to
complete a water well
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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)
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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
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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
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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
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Steps of Groundwater investigations
1.
2.
3.
4.
5.
6.
Objectives
Workplan
Collecting data
Interpreting data
Developing conclusions
Presenting results
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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
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Chapter Highlights
1.
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.
2.
Push technologies involve pushing or hammering a casing string
into the ground, carrying sensors, sampling tools, or permanent
monitoring devices.
3.
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.
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4. 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.
5. 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.
6. A variety of geophysical techniques are used in hydrogeological
investigations. Most useful surface surveying methods include electrical
resistivity, electromagnetics, ground penetrating radar, and seismic.
7. Geophysical logs are also run in boreholes. Key logs used in ground-water
applications include single-point resistance, spontaneous potential (SP),
and gamma.
8. Ground-water investigations are carried out at regional, local, and site
scales. The type of investigative approach changes as a function of scales.
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