Geology Reference
In-Depth Information
ε
1 (air)
4
9
16
4
V (m ns
-1
)
V
0.30
0.15
0.10
0.075
0.15
200 MHz
800 MHz
0
12.5
25.0
37.5
Time (ns)
Figure 10.17
'One-dimensional' A-scan responses at 200 and 900 MHz for
a horizontally layered model in which a thin (0.1-m) layer is sandwiched
between two layers with relative permittivities as shown (top). Note the
greater resolution at the higher frequency, and the opposite polarity of the
reflection from the top of the low-permittivity layer. Polarity can be a useful
diagnostic for determining material type.
very strong if the objects are metallic. Features actually at the surface can
produce strong sideswipe because there is substantial radiation of energy
along the ground/air interface if ground conductivity is high.
A common source of interference or low SNR is the presence of what
is often termed
clutter
, i.e. inhomogeneities in the materials surrounding a
target. The signal then loses significant energy by reflection from irrelevant
features before reaching the target depth. This could, for example, occur in
a survey over a pocket of coarse uncompacted gravels or industrial rubble.
Lower frequencies than would otherwise be desirable may have to be used.
10.3 Data Processing
GPR data may need extensive processing. The reduction in size and increase
in capacity of small computers has made it possible to process in the field,
sometimes using the field instruments themselves. However, much time can
be wasted for very little gain, and it is a useful principle to only process the
raw data if there is a clearly identifiable benefit in doing so. For many types
