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  Limitation of Refraction Methods, Physics tutorial

Introduction:

First-arrival refraction work utilizes only small proportion of information enclosed in seismic traces, and it is not surprising that interpretation is subject to strict limitations. These are particularly significant in engineering work; in low-velocity-layer studies only time delay estimate is sought and short shots alone are frequently adequate.

Direct Waves:

Ground roll comprises of complex of P-and S-body waves, and Love and Rayleigh surface waves. Such waves travel with different but usually slow velocities. There is frequently some doubt as to which component really produces first break, as conventional geophones react only poorly to horizontal ground motions of direct P-waves. Close to source, sufficient energy is related with P-waves for response to be assessable, but at greater distances, first breaks may record arrival of S-waves, surface waves or even air wave.

Complex character of direct wave may be among reasons for usually observed failure of best-fit arrival line to pass through origin. Delays in timing circuits may also play the part but can be determined by direct experiment, with detonator or light hammer blow close to geophone. More significant reason may be that amplifier gains at geophones close to shot point have been set so low that true first arrivals have been overlooked. Full digital storage of incoming signals must permit traces to be studied individually over range of amplifications, but if this is not possible, then most reliable velocity evaluates will be those that don't treat origin as point on line.

Vertical Velocities:

However, much care is taken to get valid direct-wave or refracted wave velocities. Refraction method is essentially flawed in that depth equations need vertical velocities but what are actually estimated are horizontal velocities. If there is important anisotropy, errors will be introduced.

First arrivals clearly visible would possibly be overlooked or dismissed as noise. The direct wave velocity would be approximately correct given that best-fit line was not forced through origin. Cross-over distance would also be wrong but intercept time wouldn't be affected, if the refracted arrivals were amplified adequately.

This is a problem for interpreters rather than field observers but latter must at least be aware of significance of using any boreholes or recent excavations for calibration or to compute vertical velocities directly.

Hidden Layers:

Refractor which doesn't give rise to any first arrivals is said to be hidden. Layer is likely to be hidden if it is much thinner than layer above and has a much lower seismic velocity than layer below.

Weathered layers immediately above basement are frequently hidden. Presence of hidden layer can at times be recognized from second arrivals but this is only rarely possible, in part as refracted waves are strongly attenuated in thin layers. The layer may also be hidden even if head wave which it produces does arrive first over some part of ground surface, if there are no suitably located geophones. Concentrating geophones in critical region can at times is helpful (though never convenient) but need to do so will only be identified if preliminary interpretations are being made on daily basis.

Blind Zones:

If velocity decreases at interface, critical refraction can't take place and no refracted energy returns to surface. Little can be done about the blind interfaces unless vertical velocities can be estimated directly. Thin high-velocity layers like perched water tables and buried terraces frequently create blind zones. Refracted waves within them lose energy rapidly with increasing distance from source and eventually become undetectable. Much later events may then be chosen as first arrivals, producing discontinuities in time-distance plot. Similar effect is seen if layer itself ends suddenly.

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