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Abstract

Substantial progress has occurred during the past 15 years in development of shallow<br>CDP seismic-reflection techniques, but there are occasional interpretation problems with the<br>resulting data. We discuss examples of the pitfalls of the method, along with some procedures<br>to help avoid them. Problems that often occur include spatial aliasing of ground roll, interpreting<br>processed ground-coupled air waves as true seismic waves, misinterpreting refractions as<br>reflections on stacked CDP sections, and not recognizing processing artifacts. Aliasing occurs<br>when data are not sampled often enough in time and/or space. Decreasing the geophone interval<br>by a substantial amount (such as a factor of two) will improve coherency of a true reflector, but<br>will destroy coherency of spatially aliased ground roll. It is often difficult to separate shallow<br>reflections from shallow refractions during processing. Reflected energy from shallow depths<br>tends to have frequency content close to that of the direct wave and/or early refracted arrivals on<br>field seismograms. Refractions on a stacked section tend to be a bit lower in frequency because<br>the NM0 correction in a CDP stack assumes hyperbolic moveout, while refractions arrive as a<br>linear time-distance function. Hence, they don’t stack as coherently as reflections, which<br>decreases their frequency. Processing artifacts from inadequate velocity analysis and inaccurate<br>static corrections are at least as troublesome on shallow reflection sections as they are on classical<br>reflection surveys from petroleum exploration. It has been our experience that occasional field<br>records will display unusually good reflections. These field seismograms can be used to<br>correlate to the processed seismic sections. Unequivocally separating shallow reflections from<br>shallow refractions is clearly one of the major limitations of the shallow-seismic reflection<br>method at present.

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/content/papers/10.3997/2214-4609-pdb.208.1994_002
1994-03-27
2024-03-29
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http://instance.metastore.ingenta.com/content/papers/10.3997/2214-4609-pdb.208.1994_002
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