Geology Reference
In-Depth Information
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(b)
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Fig. 5.15
The undetected layer problem in refraction
seismology. (a) A hidden layer: a thin layer that does not give
rise to first arrivals. (b) A blind layer: a layer of low velocity
that does not generate head waves.
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result from the thinness of the layer, or from the closeness
of its velocity to that of the overlying layer. In such a case,
a method of survey involving recognition of only first ar-
rivals will fail to detect the layer. It is good practice to ex-
amine the seismic traces for possible arrivals occurring
behind the first arrivals.These should then be examined
to ensure they are compatible with the structural model
derived from the first arrivals.
A
blind layer
presents a more insidious problem,
resulting from a low-velocity layer, as illustrated in
Fig. 5.15(b). Rays cannot be critically refracted at the
top of such a layer and the layer will therefore not give
rise to head waves. Hence, a low-velocity layer cannot
be detected by refraction surveying, although the top
of the low-velocity layer gives rise to wide-angle
reflections that may be detected as later arrivals during a
refraction survey.
In the presence of a low-velocity layer, the interpreta-
tion of travel-time curves leads to an overestimation of
the depth to underlying interfaces. Low-velocity layers
are a hazard in all types of refraction seismology. On a
small scale, a peat layer in muds and sands above bedrock
may escape detection, leading to a false estimation of
foundation conditions and rockhead depths beneath a
construction site; on a much larger scale, low-velocity
zones of regional extent are known to exist within the
continental crust and may escape detection in crustal
seismic experiments.
