Basic Geologic and Hydrogeologic Investigations

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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 TECHNOLOGIESdownload
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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
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Basic design for piezometers and water-table
observation wells

1. Screen for water to enter the standpipe
2. Sand pack around the screen to increase the
   effective size of the screen and support material
   placed above
3. Seal above the screen to prevent water from
   leaking along the casing
4. Screen and casing materials that do not react
   with groundwater or contaminants
5. 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

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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

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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

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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

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In-Situ Minitroll
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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

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In-Situ Minitroll
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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

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In-Situ Minitroll
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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

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In-Situ Minitroll
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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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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)

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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 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

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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
• ? electric cond. in mS/m
• Depth range few m to 100 m (lt 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

• 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
• 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

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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 monitoringetc

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Steps of Groundwater investigations

1. Objectives
2. Workplan
3. Collecting data
4. Interpreting data
5. Developing conclusions
6. 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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1. 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.
2. 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.
3. A variety of geophysical techniques are used in
   hydrogeological investigations. Most useful
   surface surveying methods include electrical
   resistivity, electromagnetics, ground penetrating
   radar, and seismic.
4. Geophysical logs are also run in boreholes. Key
   logs used in ground-water applications include
   single-point resistance, spontaneous potential
   (SP), and gamma.
5. Ground-water investigations are carried out at
   regional, local, and site scales. The type of
   investigative approach changes as a function of
   scales.