Civil Engineering Reference
In-Depth Information
In order to ensure an application of geophysical methods that is satisfactory for all con-
tract parties, the performance capabilities and technical limits of the method used must
be known in advance and an appropriate specification decided. One essential factor is the
resolution capability and thus the size of features, which can be detected geophysically
depending on distance. Fig. 4-47 shows the size of the anomalies that can be detected by
a geoelectrical process [104]. This limit line has been determined empirically and takes
into account the current geology. As long as the geology is homogeneous, the limit line
is lower so that smaller anomalies can be detected, and vice-versa. The reliability of the
described process is negatively influenced by metallic support elements, which have no
contact to the reinforcing steel in the outer support layer, so the limit line rises.
Figure 4-47
Limit line of
detectable caves and voids
(determined from experi-
ence).
With seismic methods, the lateral and vertical resolution and the resolution of individual
features depend on the signal frequency and the propagation velocity of the waves, the
bandwidth of the signals and the depth of exploration. The lateral resolution is determined
by the size of the Fresnel zone and the distance of the underground scanning. Guideline
values for the lateral and vertical resolution capability at various depths, assuming a vari-
able velocity distribution, are compared in Table 4-4 independent of frequency.
Table 4-4
Seismic - resolution capability.
Depth interval
[m]
Wave velocity
[ms]
Signal frequency
[Hz]
50
100
150
50
100
150
Resolution capacity
[m]
vertical
lateral
< 25
1500
3.8
1.9
1.3
6.9
4.8
3.8
25 to approx. 40
2000
5
2.5
1.7
10
6.9
5.5
40 to approx. 100
3000
7.5
3.8
2.5
18.9
13.1
10.7
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