Geophysics
Geophysical methods respond to the physical properties of the sub-surface media (rocks, sediments, water, voids, metals, etc.) in which artificially generated signals are transmitted into the ground, which then modifies those signals in ways that are characteristic of the materials through which they travel. The altered signals are measured by appropriate detectors whose output can be displayed and ultimately interpreted. It is our goal to interpret the data and communicate the results clearly to the benefit of the surveying team and client. The various geophysical methods rely on different physical properties and are important that the appropriate technique be used for a given type of application. Below are the different geophysical methods and their main applications
Ground Penetrating Radar (GPR)
Ground penetrating radar (GPR) offers the means to detect embedded or hidden objects within the near subsurface. This non-destructive method uses electromagnetic energy to detect reflected signals from subsurface structures. GPR can be used in a variety of media including concrete, brick, asphalt, and soil. GPR can detect objects, changes in material, as well as voids and fractures.
GPR uses transmitting and receiving antennas. The transmitting antenna radiates short pulses of the high-frequency (usually polarized) radio waves into the surface. When the wavelet hits an embedded object or a boundary with different dielectric constants the receiving antenna records variations in the reflected return signal.
GPR uses transmitting and receiving antennas. The transmitting antenna radiates short pulses of the high-frequency (usually polarized) radio waves into the surface. When the wavelet hits an embedded object or a boundary with different dielectric constants the receiving antenna records variations in the reflected return signal.
Applications for GPR
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Electromagnetic Surveying
The electromagnetic (EM) method uses induction to measure the electrical conductivity of the sub-surface. A primary alternating electric current of known frequency and magnitude is passed through a coil creating a primary magnetic field in the space surrounding the coil. The eddy currents generated in the ground induce a secondary current in underground conductors which results in an alternating secondary magnetic field, which is sensed by the receiving coil. The secondary field is distinguished from the primary field by a phase lag. The ratio of the magnitudes of the primary and secondary currents is proportional to the terrain conductivity. The depth of penetration is influenced by the coil separation and orientation.
No ground contact is required in electromagnetic (EM) surveys. This eliminates direct electrical coupling problems and allows much more rapid data acquisition. This application can give way to extremely sensitive and accurate data acquisition, capable of detecting variations in conductivity of as little as 3%.
No ground contact is required in electromagnetic (EM) surveys. This eliminates direct electrical coupling problems and allows much more rapid data acquisition. This application can give way to extremely sensitive and accurate data acquisition, capable of detecting variations in conductivity of as little as 3%.
Applications for Electromagnetic (EM) Surveys
- Regional geologic studies
- Engineering site investigations
- Hydrogeological investigations
- Detection of sub-surface cavities
- Mapping of Leachate and contaminant plumes
- Location and orientation of buried metallic objects
- Archaeogeophysical investigations
Resistivity Surveying
Using the geophysical application of resistivity involves inserting electrodes into the surface, using a known spacing, and injecting a current through the ground to be recorded. Variations in depth profile can be measured when the electrode spacing is altered. The current flowing through the earth (a resistive material) develops a voltage/potential difference. This voltage drops resulting from the flow of current and is measured by the electrodes providing a data set from which an electrical profile model of the subsurface can be calculated.
Applications for Resistivity
- Regional geologic studies
- Engineering site investigations
- Hydrogeological investigations
- Detection of sub-surface cavities
- Mapping of Leachate and contaminant plumes
- Archaeogeophysical investigations
Down-hole Well Logging
Down-hole Well Logging involves gamma ray emissions of high frequency electromagnetic radiation derived from the radioactive decay of various elements within rocks and soil. Gamma ray logs provide a clear indication of variations in lithology and also accurately define bed thickness. Down-hole well logging can be done during any phase of a well's history: drilling, completing, producing, or abandoning. Well logging is performed in boreholes drilled for oil and gas, groundwater, mineral and geothermal exploration, as well as part of environmental and geotechnical studies.
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Detroit, MI 48226 (734) 780-6849 |
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Ann Arbor, MI 48103 (734) 474-4591 |