Geophysical Techniques
Electrical Resistivity Surveying
ATS
International, Inc. is proficient in the application of surface
electrical resistivity imaging. Electrical resistivity is a versatile
tool that offers the ability to obtain high-density subsurface data
quickly and cost-effectively.
Electrical resistivity is the
characteristic of earth materials to inhibit the flow of an electrical
current. Electrical resistivity is a medium property that is effected
primarily by:
Geologic information obtainable by electrical resistivity include:
Electrical
resistivity is measured by inducing a current between two electrodes
and measuring the resulting potential at other electrodes. The more
widely spaced the electrodes, the deeper the sampling depth.
The raw data collected from a resistivity profile are the apparent
resistivities in a cross-section of the earth. Apparent resistivities
are different than the actual resistivities of the profile because of
changes in the electric current that result from its pathway through
various earth materials. Therefore, the apparent resistivities often
require inversion modeling to convert the raw data to actual
resistivities.
After the resistivity profiles are interpreted in cross-section,
multiple resistivity profile data can be laterally interpolated to
provide three-dimensional geologic models.
Seismic Surveying
Seismic
exploration is a powerful geophysical technique. The same principles
which have achieved great success in the petroleum industry can also
enhance ground water exploration, geotechnical engineering,
environmental site investigations, and mining exploration. Seismic
reflection surveys are conducted by inducing a sound wave into the
ground with a hammer blow on the ground or an explosion in a shallow
hole. The sound waves travel through the subsurface and are reflected
off geologic features before returning to the ground surface. The
returning waves are recorded with geophones. By measuring the arrival
time at successive surface locations we can produce a profile or
cross-section of seismic travel times. The seismic profile gives us
information on the subsurface such as:
Spectral Analysis of Surface Waves • SASW
A
relatively new in-situ seismic method for determining shear wave
velocity profiles, the SASW method measures the dispersive
characteristic of Rayleigh waves when traveling through a layered
medium. The SASW testing is applied from the surface which makes the
method nondestructive and non-intrusive, therefore boreholes are not
required. An impact or vibration is applied at the ground surface where
two or more vertical transducers record the propagation of surface
waves. By analyzing the phase information for each frequency contained
in the wave train, the Rayleigh and shear wave velocity can be
determined.
This data can be used for:
The
SASW method offers a much more accurate means of measuring stiffness
than traditional methods including oedometer testing, triaxial testing,
and penetration testing. Numerous studies indicate these methods
significantly under-predict stiffness, sometimes by as much as a factor
of 10. Compared to borehole measurements, which are point estimates,
SASW testing is a global measurement, where a much a larger volume of
subsurface is measured. SASW is more cost effective because it is
non-invasive and non-destructive to undeveloped land or building
structures already present. Greater productivity is achieved over
traditional invasive methods because SASW is not inhibited by
subsurface obstacles such as cobbles or boulders. SASW method can
provide stiffness data below the soil-bedrock interface where
traditional methods are limited.
Crosshole Sonic Logging • CSL
Crosshole
Sonic Logging was developed to provide a comprehensive in-situ
evaluation of newly placed concrete, drilled shafts, seal footings, and
slurry walls. The test can be accomplished as long as two or more
access tubes or coreholes are present that are capable of holding
water. Moreover, CSL can be used to evaluate the integrity of submerged
concrete piers and foundations by strapping access tubes to the side of
the structures.
The CSL
ultrasonic transmitter/ receiver probes are lowered to the bottom of a
pair of water-filled access tubes. The two probes are then pulled up
simultaneously to maintain near horizontal ray paths between the
transmitter and receiver. The transmitter probe emits an ultrasonic
signal of known strength, and the receiver measures both the velocity
of sonic waves and the strength of the signal. Weak spots in the
concrete will display a loss of signal or slow wave velocities.
Crosshole Sonic Logging reveals:
Ground Penetrating Radar • GPR
Ground
Penetrating Radar is a non-invasive and non-destructing geophysical
technique that provides 2-dimensional or 3-dimensional images of
subsurface conditions.
GPR can reveal:
The
data can also be represented as horizontal slices at various depths.
The example to the left illustrates 19th-century graves in map view. The grave on the right was unmarked.
GPR not only detects buried objects, it also detects changes or
disturbed soils indicating a small or deeply buried object. Virtually
any manmade disturbance of the soil will result in disruption of
natural stratigraphy and cause redistribution of soil moisture. GPR
measurements over areas can readily detect these changes, which may
have occurred hundreds or even thousands of
years previously. GPR can be used to detect buried bodies, hidden
weapons, or other contraband. In addition, the technique responds to
localized metal or rock objects buried in soils making it a powerful
tool for direct detection of buried artifacts or foundations.
- Electrical Resistivity Surveying
- Seismic Surveying
- Spectral Analysis of Surface Waves • SASW
- Crosshole Sonic Logging • CSL
- Ground Penetrating Radar • GPR
- Electromagnetic Induction • EM
- Bedrock Mapping
- Borehole Geophysics
Electrical Resistivity Surveying
ATS
International, Inc. is proficient in the application of surface
electrical resistivity imaging. Electrical resistivity is a versatile
tool that offers the ability to obtain high-density subsurface data
quickly and cost-effectively.
Electrical resistivity is the
characteristic of earth materials to inhibit the flow of an electrical
current. Electrical resistivity is a medium property that is effected
primarily by:
- Water Saturation
- Ionic Strength of Pore Water
- Grain Size
- Porosity
- Clay Content
Geologic information obtainable by electrical resistivity include:
- Water Table Surface
- Preferential Flow Paths
- Lithology Changes
- Geologic Structures
- Contaminant Plumes
Electrical
resistivity is measured by inducing a current between two electrodes
and measuring the resulting potential at other electrodes. The more
widely spaced the electrodes, the deeper the sampling depth.
The raw data collected from a resistivity profile are the apparent
resistivities in a cross-section of the earth. Apparent resistivities
are different than the actual resistivities of the profile because of
changes in the electric current that result from its pathway through
various earth materials. Therefore, the apparent resistivities often
require inversion modeling to convert the raw data to actual
resistivities.
After the resistivity profiles are interpreted in cross-section,
multiple resistivity profile data can be laterally interpolated to
provide three-dimensional geologic models.
Seismic Surveying
Seismic
exploration is a powerful geophysical technique. The same principles
which have achieved great success in the petroleum industry can also
enhance ground water exploration, geotechnical engineering,
environmental site investigations, and mining exploration. Seismic
reflection surveys are conducted by inducing a sound wave into the
ground with a hammer blow on the ground or an explosion in a shallow
hole. The sound waves travel through the subsurface and are reflected
off geologic features before returning to the ground surface. The
returning waves are recorded with geophones. By measuring the arrival
time at successive surface locations we can produce a profile or
cross-section of seismic travel times. The seismic profile gives us
information on the subsurface such as:
- Depth and Character of the Bedrock Surface
- Buried Channel Definition
- Depth of Water Table
- Depth and Continuity of Stratigraphic Interfaces
- Lithologic Competency Determination
- Mapping of Faults and Other Structural Features
Spectral Analysis of Surface Waves • SASW
A
relatively new in-situ seismic method for determining shear wave
velocity profiles, the SASW method measures the dispersive
characteristic of Rayleigh waves when traveling through a layered
medium. The SASW testing is applied from the surface which makes the
method nondestructive and non-intrusive, therefore boreholes are not
required. An impact or vibration is applied at the ground surface where
two or more vertical transducers record the propagation of surface
waves. By analyzing the phase information for each frequency contained
in the wave train, the Rayleigh and shear wave velocity can be
determined.
This data can be used for:
- Profile Shear Stiffness vs. Depth
- Predict Ground Deformation Under Loading
- Assess Integrity of Concrete Structures
- Assess Liquefaction Potential
- Determine Earthquake Site Response
The
SASW method offers a much more accurate means of measuring stiffness
than traditional methods including oedometer testing, triaxial testing,
and penetration testing. Numerous studies indicate these methods
significantly under-predict stiffness, sometimes by as much as a factor
of 10. Compared to borehole measurements, which are point estimates,
SASW testing is a global measurement, where a much a larger volume of
subsurface is measured. SASW is more cost effective because it is
non-invasive and non-destructive to undeveloped land or building
structures already present. Greater productivity is achieved over
traditional invasive methods because SASW is not inhibited by
subsurface obstacles such as cobbles or boulders. SASW method can
provide stiffness data below the soil-bedrock interface where
traditional methods are limited.
Crosshole Sonic Logging • CSL
Crosshole
Sonic Logging was developed to provide a comprehensive in-situ
evaluation of newly placed concrete, drilled shafts, seal footings, and
slurry walls. The test can be accomplished as long as two or more
access tubes or coreholes are present that are capable of holding
water. Moreover, CSL can be used to evaluate the integrity of submerged
concrete piers and foundations by strapping access tubes to the side of
the structures.
The CSL
ultrasonic transmitter/ receiver probes are lowered to the bottom of a
pair of water-filled access tubes. The two probes are then pulled up
simultaneously to maintain near horizontal ray paths between the
transmitter and receiver. The transmitter probe emits an ultrasonic
signal of known strength, and the receiver measures both the velocity
of sonic waves and the strength of the signal. Weak spots in the
concrete will display a loss of signal or slow wave velocities.
Crosshole Sonic Logging reveals:
- Honeycombing
- Segregation of Concrete
- Washout of Cement from Groundwater Flow
- Cracks in Pile Shafts due to Shrinkage
- Foreign Material Contamination of Concrete
- Necking and Arching After Collapse of Side Walls During Withdrawl of Temporary Liners
Ground Penetrating Radar • GPR
Ground
Penetrating Radar is a non-invasive and non-destructing geophysical
technique that provides 2-dimensional or 3-dimensional images of
subsurface conditions.
- GPR transmits radio waves into the ground through a transducer or antenna.
- The
radar waves travel through the ground and encounter buried objects or
subsurface strata with different electrical properties. - GPR waves reflect off the object or interface; while the rest of the waves pass through to the next interface.
- The GPR stores the data for immediate viewing or future reporting.
GPR can reveal:
- Thickness of Asphalt or Concrete
- Voids in Concrete
- Rebar in Concrete
- Buried Utilities, USTs, or other objects
The
data can also be represented as horizontal slices at various depths.
The example to the left illustrates 19th-century graves in map view. The grave on the right was unmarked.
GPR not only detects buried objects, it also detects changes or
disturbed soils indicating a small or deeply buried object. Virtually
any manmade disturbance of the soil will result in disruption of
natural stratigraphy and cause redistribution of soil moisture. GPR
measurements over areas can readily detect these changes, which may
have occurred hundreds or even thousands of
years previously. GPR can be used to detect buried bodies, hidden
weapons, or other contraband. In addition, the technique responds to
localized metal or rock objects buried in soils making it a powerful
tool for direct detection of buried artifacts or foundations.
عدل سابقا من قبل في 2007-09-21, 5:34 pm عدل 6 مرات
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