Geology Reference
In-Depth Information
Figure 10.4
Ultra wideband (UWB) signal in the form of a short-duration
'Ricker wavelet' with broad frequency content centred around the central
frequency
f
c
.
W
is the pulse-width measured between the two side-lobe
minima, and
B
is the bandwidth, measured between the 50% amplitude
points. For an ideal Ricker wavelet,
B
=
0.78/W
.
10.1.6 Impulse radar
Impulse or ultra wideband (UWB) radar systems are the most common,
employing short bursts of electromagnetic energy spanning a range of
frequencies from about 50% below to 50% above some specified central
frequency
f
c
. The bandwidth is thus numerically equal to the central fre-
quency, and a typical 100-MHz signal has significant energy at frequencies
as low as 50 MHz and as high as 150 MHz. The pulse-width, in seconds,
is approximately equal to the reciprocal of the bandwidth, measured in Hz
(Figure 10.4), and narrow pulses therefore imply large bandwidths.
The range resolution,
R
, defines the minimum distance between inter-
faces that can be resolved by a radar signal and is related to the pulse width,
W
, and the transmission velocity,
V
, by the equation:
R
≥
W
·
V
/
4
Thus the range resolution of a survey can be improved by decreasing the
pulse width, i.e. by increasing the signal bandwidth (Figure 10.5).
Lateral resolution is defined as the minimum horizontal distance that must
exist between two objects if they are to be seen as separate. It is related to
distance from the target,
d
, the pulse width,
W
, and the transmission velocity,
V
, by the equation:
L
=
√
(
V
·
d
·
W
/
2)
