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The term radar is derived from "radio detection
and ranging," and this name was used by the U.S. and its allies during
World War II for a variety of devices concerned with radio detection and position finding.
Although originally developed as an instrument of war, radar is now used extensively to
map geologic structures, archeological sites, and a wide range of buried objects.
A transmitting antenna broadcasts a beam of
electromagnetic waves into the subsurface. When these waves strike an object in the path
of the beam, or a geologic unit with contrasting dielectric properties, some waves are
reflected from the object, forming an echo signal. The receiving antenna collects the
reflected energy at the ground surface. Through an amplification process and computer
processing, the radar receiver produces a visual signal on the screen of the indicator,
essentially a computer display monitor.
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GPR data over 3 underground tanks,
associated piping, and reinforcement mesh.  Return to Applications Page
Seismic reflection detects boundaries between different
kinds of rocks, assisting in the mapping of geologic structures. It is a
powerful and proven geophysical method that has been employed primarily by the petroleum
industry. The method was first applied commercially in 1927 by the Geophysical Research
Corporation, but met more widespread use in the 1940s. In more modern time, the reflection
method has been increasingly used to shallower applications, such as groundwater
exploration and civil engineering applications.
S eismic
prospecting methods use explosives and other means to initiate seismic waves at a given
point. Geophones are deployed at other points to determine the time of arrival of the
energy that is reflected by discontinuities in rock formations. These time measurements
constitute the fundamental data for seismic data analysis and interpretation.
Seismic reflection is typically applied to identify geologic contacts at depths
greater than 200 feet (70 meters). At greater depths, reflection provides the highest
level of accuracy and resolution of stratigraphic and structural information than any
other geophysical method. Since 1980, advances have been made in applying the technique to
shallower investigations. Return
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The seismic refraction technique is an established, proven method for mapping
buried bedrock surfaces. In 1922, the Seismos Company furnished two crews to conduct
refraction surveys in the Gulf Coast area of the United States and in Mexico. Today, the
method is particularly well suited for measuring the depths to bedrock in connection with
the construction of large buildings, dams, tunnels, and highways.
Although seismic refraction generally has lower resolution than seismic
reflection, it is generally the favored seismic method for shallow surveys because of the
ability to obtain superior resolution in areas of thick alluvial fill (i.e. buried bedrock
valleys), and because acquisition and processing costs are generally lower. Return to top of page
Forward Looking Seismic is a new method of mapping bedrock conditions ahead of
an active tunneling boring machine (TBM). First applied to a project in Columbus, Ohio,
where the method detected anomalies ahead of the TBM. These anomalies were confirmed
to correlate with nearly vertical fracture zones, and areas of decreased
bedrock integrity. This area was independently known to require grouting before being
tunneled. By anticipating geological features ahead of the TBM, Forward Looking Seismic
can provide early warning of changing conditions, reduce TBM down time, and anticipate the
need for different support systems. Return to top of page
Forward Looking Seismic Test
Gravity surveys have been
extensively used to map buried valleys and cave features with demonstrated success. A highly sensitive instrument, called a gravimeter, can be used
to measure changes in the gradient of the gravitational potential—that is, the force
of gravity- within a survey area. Changes in the gravity measurements can reflect bedrock
structures, buried bedrock valleys, caves, tunnels, or other features that exhibit
contrasting material densities. Differences in relative gravity due to variations in the
earth's density below the measurement site are referred to as Bouguer anomalies.
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page
Electromagnetic surveys map variations in the
conductivity or capacitance of rocks. Buried objects and geologic discontinuities can be
detected by artificially applying known electric or magnetic fields to the ground surface,
and recording the presence of disruptions to the known field. These disruptions, termed
electromagnetic anomalies, can result from geological changes or the presence of metallic
objects, such as pipes, drums, cables, tanks, etc. Electromagnetic methods have also been
applied to prospecting for mineral and energy resources. Return to top of page
Measurements
of the magnetic field of the earth, and local disruptions to the field, can be used to map
geologic contacts and structures, the presence of naturally occurring ore bodies, and
manmade objects such as buried drums, tanks, or ordinance. In ground magnetic
surveys, the earth's magnetic field is measured at closely spaced stations. The data can
be filtered and plotted to define localized magnetic anomalies attributable to the
presence of buried structures or objects. Return to top of
page
Electrical resistivity is a proven method for mapping contrasting geological
materials - no other surface geophysical method has been more widely used in groundwater
studies. The method can effectively determine both vertical and horizontal extent of
geologic units. Resistivity data may be collected relatively rapidly and inexpensively. In
comparison to seismic surveys, this method has no noise or impact on surrounding community
or environment. Return
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Natural Gamma Logging is a method of measuring the relative amount of clay in
rocks and sediments. A gamma log can be run in open holes, steel or plastic casing, and in
open augers in wet or dry conditions. This method can be valuable in geologic
correlations between borings and wells, or in determining well screen placement.
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