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be useful (although never convenient) but the need to do so will only be
recognised if preliminary interpretations are being made on a daily basis.
13.3.4 Blind zones
If velocity decreases at an interface, critical refraction cannot occur and no
refracted energy returns to the surface. Little can be done about these blind
interfaces unless vertical velocities can be measured directly. Surface wave
methods (see Chapter 14) may have to be used.
Thin high-velocity layers such as perched water tables and buried terraces
often rest on blind interfaces. The refracted waves within thin layers lose
energy rapidly with increasing distance from the source and ultimately
become undetectable. Much later events may then be picked as first-arrivals,
producing discontinuities in the time-distance plot. A similar effect is seen
if a layer ends abruptly.
13.3.5 Limitations of drilling
Despite the limitations of refraction surveys, interpretations are not always
wrong when they disagree with drill-hole data. Only a very small subsurface
volume is sampled by the drill, and many drill tests of drift thickness have
been terminated in isolated boulders some distance above the true top-of-
bedrock. It is always important to find explanations for differences between
drilling and seismic results.
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