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velocities, the refraction method is fundamentally flawed in that the depth equations require vertical velocities but what are actually measured are horizontal velocities. If there is significant anisotropy, errors will be introduced. Figure 3.7 Hard copy of a single stored data set played back at two different amplifications. The first arrivals clearly visible on (a) would probably be overlooked or dismissed as noise on (b). A direct wave velocity based on (b) would be roughly correct provided that the best-fit line was not forced through the origin. The cross-over distance would also be wrong but the intercept time would not be affected, provided that the refracted arrivals were amplified sufficiently.This is a problem for interpreters rather than field observers but the latter should at least be aware of the importance of using any boreholes or recent excavations for calibration or to measure vertical velocities directly. 3.3 Hidden layers A refractor that does not give rise to any first arrivals is said to be hidden. A layer is likely to be hidden if it is much thinner than the layer above and has a much lower seismic velocity than the layer below. Weathered layers immediately above basement are often hidden. The presence of a hidden layer can sometimes be recognized from second arrivals but this is only occasionally possible, in part because refracted waves are strongly attenuated in thin layers.
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