Geology Reference
In-Depth Information
Figure 10.6
Radiation footprint from a ground-coupled dipole antenna.
A
is the long axis of the ellipsoid-shaped footprint.
B
is the short axis.
D
is
depth.
10.1.7 Continuous-wave radar
Continuous-wave (stepped-frequency) radar systems are less common than
impulse systems, and require more complicated antennas. They transmit
series of sine waves with stepwise increasing frequency. Figure 10.7 shows
an example of just one of these 'packets', or 'chirps'. Fourier transformation
is then required to convert the data (collapse the phases) to produce the
familiar narrow-pulse response.
Stepped-frequency data can be useful for characterising material proper-
ties, and a single system sweeping from say 100 MHz to 2 GHz in steps of
2 MHz (Figure 10.8) can provide detailed images of both shallow and deep
features in a single pass. The trade-off comes in the relatively longer time
required to collect the data. The
dwell-times
between each transmission can,
and must, be defined by the user.
