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:
- 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.
Electromagnetic Induction • EM
Electromagnetic Induction (EM), also called terrain conductivity, is a measure of how well the subsurface materials conduct electric current.
EM surveys are usually conducted along traverses through the area of interest, with measurements taken at fixed distances along the traverse. By conducting parallel traverses, a grid of measurements is acquired. The data can then be contoured to look for anomalies in the spatial distribution. Measurements taken at different depths over the same area can reveal vertical changes in subsurface characteristics.
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EM surveys detect:
- Buried drums or tanks
- Landfills and trenches
- Archeological interests
- Fracture zones
- Contaminant plumes
- Voids or mines
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Time-Domain EM
Time-domain EM is used specifically for detecting metallic objects. Like terrain conductivity, a transmitter coil is used to produce eddy currents in the subsurface. Unlike terrain conductivity, the transmitter is turned off before the receiver measures the secondary field. The eddy currents in metals decay slower than in earth materials, thereby discriminating between metallic and non-metallic objects. Time-domain EM is used for:
- Unexploded ordnance
- Buried drums
- USTs
- Metallic utilities
Bedrock Mapping
On any excavation project, it's not hard to get from the existing grade to the proposed grade to calculate the total volume.
EXISTING GRADE |
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PROPOSED GRADE |
What costs money is the uncertainty in the volume of soil versus rock!
With resistivity imaging, we can fill in the gaps between scattered borings to map the bedrock surface and distinguish rock "float" from in-place bedrock. The information can also be used to interpret rock quality.
Application to Excavation and Quarrying
- Resistivity Imaging is a reliable method of mapping bedrock, soil, and rock "float" over large cross-sectional areas.
- Resistivity Imaging detects variations in mineralogic properties of the rock.
- The information can be used to make more accurate volume calculations on earth-moving and quarrying prospects.
Other Applications
- Water Table Mapping
- Void and Cavern Detection
- Sand and Gravel Mapping
- Landfill Delineation
- Waste Pit and Trench Mapping
- Bedrock Fault and Fracture Identification
- Contaminant Plume Identification
- Sinkhole and Other Karst Featuer Identification
Borehole Geophysics
Utilizing borehole tools can provide valuable geologic information that is often not obtainable by traditional surface logging methods. Geophysical logging is the measurement of various physical properties by way of sensors lowered into a well or borehole. Borehole video and geophysical logging tools can provide critical answers to a number of questions encountered in water resources and geotechnical work including:
- Fracture Zone Identification
- Inspection and Verification of Well Construction
- Detailed Stratigraphic Evaluation
- Hydrostratigraphic Delineation
- Identification of Seperate Phase Contaminants
Tools applied in borehole geophysical logs include:
- Borehole Caliper
- Spontaneous Potential
- Fluid Resistivity
- Natural Gamma Ray
- Heat Pulse Flow Meter
- Downhold Video Camera
Caliper Logging
A caliper log measures changes in hole diameter with depth. The caliper tool has three mechanical arms that open at the bottom of the well, where they expand to the diameter of the borehole. As the tool is drawn up the well, the arms expand and contract as the hole diameter changes. The caliper provides an accurate log of borehole diameter and the depth of significant fractures.
Spontaneous Potential and Fluid Resistivity
Stratigraphic units and water quality can be identified from logs of electrical resistivity and natural electric potentials. Fluid resistivity is a measurement of the bulk resistance between a surface electrode and the downhole probe. Spontaneous potential (SP) measures natural voltages produced by electrochemical differences between various lithologies and the borehole fluid.
Natural Gamma Ray Logging
All rocks and soils emit naturally occurring gamma radiation in varying amounts. The primary gamma emitting materials are potassium 40, uranium, and thorium. These elements tend to be more abundant in fine-grained sediments and certain igneous rocks. When interpreted with other geophysical logs, natural gamma can assist in the hydrostratigraphic characterization of the subsurface.
Heat Pulse Flow Meter
The heat pulse flow meter is used to detect the direction and magnitude of vertical flow within a borehole or well. A wire mesh on the probe instantaneously heats the water surrounding it, and sensors above and below the mesh detect minute changes in fluid temperature at the sensors. By measuring the fluid temperature at the sensors and the travel time between the heat source and the sensors, calculations of vertical water velocities are obtained.
Downhole Video Camera
While not strictly a geophysical method, downhole video photography is extremely useful in providing detailed screen inspections, identifying cracks and holes in casings, delineating fracture zones and ground water flow, and lithologic identification. A video camera built into a waterproof casing is lowered into the well via coaxial cable. The image is transmitted back through the cable to a VHS recorder and monitor at the surface. This arrangement provides real-time viewing and a permanent videotape recording, which facilitates detailed inspections of features of interest.
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