Avoiding pitfalls in shallow seismic reflection surveys

Abstract

Acquiring shallow reflection data requires the use of high frequencies, preferably accompanied by broad bandwidths. Problems that sometimes arise with this type of seismic information include spatial aliasing of ground roll, erroneous interpretation of processed air- waves and air-coupled waves as reflected seismic waves, misinterpretation of refractions as reflections on stacked common-midpoint (CMP) sections, and emergence of processing artifacts. Processing and interpreting near- surface reflection data correctly often requires more than a simple scaling-down of the methods used in oil and gas exploration or crustal studies. For example, even un- der favorable conditions, separating shallow reflections from shallow refractions during processing may prove difficult, if not impossible. Artifacts emanating from in- adequate velocity analysis and inaccurate static correc- tions during processing are at least as troublesome when they emerge on shallow reflection sections as they are on sections typical of petroleum exploration. Conse- quently, when using shallow seismic reflection, an inter- preter must be exceptionally careful not to misinterpret as reflections those many coherent waves that may ap- pear to be reflections but are not. Evaluating the validity of a processed, shallow seismic reflection section there- fore requires that the interpreter have access to at least one field record and, ideally, to copies of one or more of the intermediate processing steps to corroborate the interpretation and to monitor for artifacts introduced by digital processing.