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Instrument: GPR : Ground Penetrating Radar
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Ground Penetrating Radar (GPR)is a noninvasive electromagnetic
geophysical technique for subsurface exploration,
characterization and monitoring (history). It is widely used in
locating lost utilities, environmental site characterization and
monitoring, agriculture, archaeological and forensic
investigation, unexploded ordnance and land mine detection,
groundwater, pavement and infrastructure characterization,
mining, ice sounding, permafrost, void, cave and tunnel
detection, sinkholes, subsidence, karst, and a host of other
applications. It may be deployed from the surface by hand or
vehicle, in boreholes, between boreholes, from aircraft and from
satellites. It has the highest resolution of any geophysical
method for imaging the subsurface, with centimeter scale
resolution sometimes possible.

Resolution is controlled by wavelength of the propagating
electromagnetic wave in the ground. Resolution increases with
increasing frequency (shorter wavelength). Depth of
investigation varies from less than one meter in mineralogical
clay soils like montmorillonite to more than 5,400 meters in
polar ice. Depth of investigation increases with decreasing
frequency but with decreasing resolution. Typical depths of
investigation in fresh-water saturated, clay-free sands are
about 30 meters. Depths of investigation (and resolution) are
controlled by electrical properties through conduction losses,
dielectric relaxation in water, electrochemical reactions at
the mineralogical clay-water interface, scattering losses, and
(rarely) magnetic relaxation losses in iron bearing minerals.
Scattering losses are the result of spatial scales of
heterogeneity approaching the size of the wavelength in the
ground (like the difference between an ice cube and a snowball
in scattering visib le light). Detectability of objects in the
ground depends upon their size, shape, and orientation relative
to the antenna, contrast with the host medium, as well as
radiofrequency noise and interferences.

Additional information available at

[Summary provided by Gary R. Olhoeft]