Geology Reference
In-Depth Information
waveform over a selected time interval (measured in nanoseconds). Although
feasible in acoustics and seismics, this has only recently become possible
at radar frequencies, because Analogue to Digital Converters (ADCs) with
enough dynamic range and sufficiently high sampling rates did not exist.
To get around this, sequential sampling of repeated transmissions is widely
used, with only one sample collected for each transmission. Thus, if 256
samples are being collected for every transmission, then 256 transmissions
are required to collect a single sample at each sampling time (stack fold
of 1). GPR systems generally have pulse-repetition frequencies (PRFs) of
100 kHz for central frequencies below 400 MHz, and the 1000-fold stack
needed to increase the signal-to-noise ratio by 30 dB would require 256 000
transmissions and 2.6 s for a single scan. This is not a problem if antennas
are being moved by hand, but it is not practical to collect more than 32-64
stacks per scan using continuously towed systems. This is a fundamental
limit on the SNR. Practical and regulatory restrictions limit the scope for
improving things by increasing transmitter power.
High-speed ADCs have recently become available that allow entire wave-
forms to be collected in a single pass. These offer the possibility of increasing
the stack fold (up to a practical limit of about 10 000), and can provide an
additional 30-40 dB for the same antenna. These systems are likely to be-
come routine during the lifetime of this edition.
10.2 GPR Surveys
The key factors controlling the performance of a GPR system are the central
frequency,
f
c
, and bandwidth,
B
(see Section 10.1.6). High frequencies are
needed to resolve small objects but have limited penetration. Lower fre-
quencies are required to detect deeper objects, which consequently must be
larger. Table 10.2 provides typical centre frequency selections for a range
Table 10.2
Indicative depths of investigation for
different centre frequencies
Centre frequency (MHz)
Approximate depth (m)
10
50
25
30
50
10
100
5
200
3
500
2
1000
1
