{[ promptMessage ]}

Bookmark it

{[ promptMessage ]}

Delays in the timing circuits may also play a part

Info iconThis preview shows pages 60–63. Sign up to view the full content.

View Full Document Right Arrow Icon
Delays in the timing circuits may also play a part but can be determined by direct experiment, with a detonator or a light hammer blow close to a geophone. A more important reason may be that the amplifier gains at geophones close to the shot point have been set so low that the true first arrivals have been overlooked (Figure 3.7). Full digital storage of the incoming signals should allow the traces to be examined individually over a range of amplifications, but if this is not possible, then the most reliable velocity estimates will be those that do not treat the origin as a point on the line.
Background image of page 60

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
61 3.2 Vertical velocities However much care is taken to obtain valid direct-wave or refracted-wave 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.
Background image of page 61
62 A layer may also be hidden even if the head wave that it produces does arrive first over some part of the ground surface, if there are no appropriately located geophones. Concentrating geophones in the critical region can sometimes be useful (although never convenient) but the need to do so will only be recognized if preliminary interpretations are being made on a daily basis. 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. Thin high-velocity layers such as perched water tables and buried terraces often create blind zones. The refracted waves within them lose energy rapidly with increasing distance
Background image of page 62

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
Image of page 63
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

Page60 / 66

Delays in the timing circuits may also play a part but can...

This preview shows document pages 60 - 63. Sign up to view the full document.

View Full Document Right Arrow Icon bookmark
Ask a homework question - tutors are online