Geology Reference
In-Depth Information
12.1.2, they are routine in seismic reflection, where cheap
geophone
detectors can be laid out in large numbers).
The advent during the last few years of multi-channel (multi-antenna
or array) GPR systems promises to revolutionise many engineering, en-
vironmental and archaeological applications. Apart from the obvious pro-
ductivity gains, the probability of detecting localised or linear small tar-
gets increases with the number of antennas deployed. The multi-channel
systems now available, from several manufacturers, include single central-
frequency UWB systems giving improved illumination and detectability of
targets within a selected range, multiple central-frequency UWB systems
providing improved illumination across a range of depths in a single pass,
and stepped-frequency systems for providing improved illumination and
detectability across a range of depths in a single pass.
10.2.3 Selecting survey parameters
Antenna separation, station separation, record length, transmission fre-
quency and sampling frequency can all be varied to some extent in most
GPR work. Transmission frequency is the most important single variable,
since it constrains the values of many other parameters. Resolution criteria
may, and usually do, conflict with penetration requirements, and field oper-
ators should at least know the extent to which, for the instruments they are
using, the trade-off between penetration and resolution can be compensated
for in processing.
Resolution is also affected by station spacing. Targets that would oth-
erwise be resolved will not be properly defined if the distance between
adjacent stations in a common offset profile is more than one quarter of the
wavelength in the ground, i.e. approximately 75
/
f
√
ε
, where f is measured
in MHz. Separations of about one-fifth of this value usually give good results
(Figure 10.16) but smaller ones may be used for operational convenience.
The various criteria are summarised in Table 10.3. Correct choices are par-
ticularly important if buried features must be mapped in real-time, since this
implies interpretation in the field.
Radar signals are recorded digitally and must be sampled often enough
to ensure that waveforms are fully defined. If there are fewer than two
samples in each full period,
aliasing
(see Figure 1.7) will occur. Because
the maximum frequency present in a GPR signal is approximately twice
the nominal central frequency, the sampling rate should be at least four
times the central frequency. A safety factor of two is usually added, giving a
sampling frequency of 800 MHz for a 100-MHz signal (i.e. a sample interval
of 1.25 ns). Most GPR systems are set up to collect many more samples
than necessary. This can become important when survey speed/resolution
tradeoffs are being considered.
