Geology Reference
In-Depth Information
Figure 10.18 SIR System-3 variable area ground penetrating radar (GPR)
record (B-scan) showing sigmoidal oblique reflectors in the Lower Green-
sand. Reproduced by permission of Dr C. Bristow.
of target, standard GPR systems produce field data that can be interpreted
without further ado.
If processing is done, it is vital to preserve an audit trail so that any
outcome can be traced back to source. With the world-wide proliferation
in the use of GPR in civil engineering, it is only a matter of time before
someone, somewhere is called on to defend an interpretation in a court of law.
10.3.1 Display of GPR data
A GPR 'trace' or A-scan (Figure 10.1) is recorded as a series of digital
values equally spaced in time. An ideal A-scan would consist of a flat
line punctuated by occasional 'events' produced by energy arriving at the
surface after reflection from points vertically below the mid-point between
the receiver and transmitter antenna (Figure 10.17). Display can be either as
a grouping of A-scans ( wiggle trace ), or as variable area B-scans in which
excursions on one side of the zero line are shaded. Colour is sometimes used
to emphasise polarity.
On a normal B-scan, the horizontal axis represents distance and the ver-
tical scale is in two-way reflection time (TWT). With most instruments, a
B-scan formed from lightly processed traces is displayed on a screen in 'real
time' as the antennas are moved along the traverse. This makes GPR one of
the most interesting geophysical methods to actually use. The resemblance
between B-scans and seismic reflection sections is extremely close (com-
pare Figure 10.18 and Figure 12.8), and it is sometimes only the fact that the
two-way time scale is in nanoseconds rather than milliseconds that shows
that the section was produced by electromagnetic rather than seismic waves.
As in seismic reflection, radar work now includes 3D surveys, in which
traverses are very closely spaced and the results are considered in terms
of data volumes rather than individual sections. Presentations of the type
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