Geology Reference
In-Depth Information
1000
Medium
100
Granite
Limestone
10
Schist
Best coal
Coal-clay
1
Shales
Gouge
Fig. 9.23 The relationship between
probing distance and frequency for
different materials. (After Cook 1975.)
1
10
100
1000
Frequency, f (MHz)
2. Velocity sounding (Fig. 9.24(b)), in which transmit-
ter and antenna are moved apart about a fixed central
point (the common depth point (CDP) method), or one
kept stationary while the other is progressively moved
away (the wide-angle reflection and refraction (WARR)
method). The methods are designed to show how the
radar velocity changes with depth. Without this infor-
mation, velocities might be determined by correlating
the radargram with a borehole section or with signals
reflected from a body at known depth. In many cases,
however, the velocities are guessed.
3. Transillumination (Fig. 9.24(c)), in which the trans-
mitter and antenna are mounted on either side of the
object of interest (e.g. a pillar in a mine). If it is arranged
that there are many different configurations of transmit-
ter and antenna, radar tomography can be carried out in
a similar fashion to seismic (see Section 5.10) and resis-
tivity (see Section 8.2.7) tomography.
Filtering of the radar signal can be applied during data
acquisition, but is more conveniently performed on the
digital output provided by modern instruments. The
radar reflections can subsequently be enhanced by
digital data-processing techniques very similar to those
used in reflection seismology (see Section 4.8), of which
migration is particularly important.
Interpretation of a radargram is commonly per-
formed by interface mapping, similar to that used in the
interpretation of seismograms. If amplitude fidelity has
been retained in the radargram, zones of high attenua-
tion can be recognized which represent high-conduc-
tivity areas such as are produced by clay accumulations.
However, the identification of each band on a radargram
as a distinct geological horizon would be incorrect
because of the effects of multiples, interference with a
previous reflection wavetrain, sideswipe (see Section
4.8), noise, etc. Processing of the radargram is simplified
by deconvolution (see Section 4.8.2), which restores the
shape of the downgoing wavetrain so that primary events
can be recognized more easily. Migration is also particu-
larly useful in that diffraction hyperbolae are removed
and correct dips restored.
A GPR profile and its interpretation are shown in
Fig. 9.25, which illustrates the detailed information
provided by the technique.
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