Geology Reference
In-Depth Information
10
GROUND PENETRATING RADAR
Radio-wave echo-sounding was first patented by Christian Hulsmeyer in
1904 as a device (the
telemobiloscope
) to prevent ships colliding. The term
RAdio Detection And Ranging (RADAR) was not coined until 1934, by
Robert Watson-Watt. The radar frequencies, from one to several thousand
megahertz, were originally thought to be too high for useful ground pen-
etration, but in the 1950s USAF pilots were crash landing on ice fields in
Greenland because radar altimeter reflections from the base of the ice were
misread as reflections from the landing surfaces.
Ground penetrating radar
(
GPR
) was born, and it was only a short step from its use in determining ice
thickness to studies of permafrost. It was then realised that some penetration
was being achieved into the deeper, unfrozen ground and that the depth
of investigation, although unlikely to ever amount to more than a few tens
of metres, could be increased by processing techniques almost identical to
those applied to seismic reflection data.
Today GPR is one of the most extensively used of all geophysical methods.
Numerous applications can be found in geological mapping, engineering,
structural and archaeological investigations, as well as forensic and environ-
mental surveys.
10.1 Radar Fundamentals
Ground penetrating radar uses electromagnetic waves with frequencies be-
tween 10 MHz and 4 GHz to detect changes in electrical properties. There
are many resemblances to seismic (particularly seismic reflection) methods,
although some terminologies seem to have been developed with the aim
of disguising this fact (Figure 10.1). Transmission velocities are crucially
important in interpretation, and are almost independent of frequency at radar
frequencies for a wide range of materials. This contrasts with the situation
below about 1 MHz, where conduction currents dominate and velocities and
frequencies decrease together.
10.1.1 Radar parameters
Radar waves obey Maxwell's equations (see Section 5.2.2) but in GPR
work it is usually assumed that the relative magnetic permeability,
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