Basic Geologic and Hydrogeologic Investigations

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Basic Geologic and Hydrogeologic Basic Geologic and Hydrogeologic InvestigationsInvestigations

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 KEY DRILLING AND PUSH TECHNOLOGIESPUSH TECHNOLOGIES

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Piezometers and Water-Table Piezometers and Water-Table Observation WellsObservation Wells

• PIEZOMETER:

A borehole or standpipe installed to some depth below the water table

Water-table observation well

piezometer

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• PIEZOMETER:



piezometer

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Basic design for piezometers and Basic design for piezometers and water-table observation wellswater-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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• PIEZOMETER:



piezometer

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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 measurementsMaking water level measurements

• Electric tapeMeasures 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 Geophysics applied to site investigationinvestigation

• Surface geophysical techniquesUsed 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 GeophysicsProvides stratigraphic and hydrogeologic information

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In-Situ Minitroll

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Electric Resistivity methodElectric 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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• 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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• 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 ArraySchlumberger Array

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Electromagnetic MethodsElectromagnetic 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 RadarGround Penetrating RadarGPRGPR

• 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 PrincipleGPR 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 (< 30 m in most cases)

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GPRGPR

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GPRGPR

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Reflection Seismic MethodReflection 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 MethodsSeismic 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 wavesSeismic 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 wavesSeismic waves

Surface wave

reflected wave

refracted wave

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Reflection from multiple layersReflection from multiple layers

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Procedures of seismic surveyProcedures of seismic survey

1. Data acquisition

2. Processing

3. 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)Common Depth Point (CDP)

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ExampleExample: : VSP + 1441 shotpoints, 24-fold CDP, 3 linesVSP + 1441 shotpoints, 24-fold CDP, 3 lines

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Borehole Geophysics MethodsBorehole 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 logsCaliper 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 logsResistivity 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)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 logGamma 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 investigationsGroundwater 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 investigationsSteps 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 informationSources 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

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•Hydraulic head

•Hydraulic conductivity

•Pumping tests

•Tracer tests

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Chapter HighlightsChapter 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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Basic Geologic and Hydrogeologic Investigations