AVOIDING PITFALLS IN SHALLOW SEISMIC-REFLECTION SURVEYS

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 o f ground roll, erroneous interpretation of processed airwaves and air- coupled waves as reflected seismic waves, misinterpretation of refract ions as reflections on stacked common-midpoint (CMP) sections, and eme rgence of processing artifacts. Processing and interpreting nearsurfac e 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 under favorable conditions, separating shallow ref lections from shallow refractions during processing may prove difficul t, if not impossible. Artifacts emanating from inadequate velocity ana lysis and inaccurate static corrections during processing are at least as troublesome when they emerge on shallow reflection sections as the y are on sections typical of petroleum exploration. Consequently, when using shallow seismic reflection, an interpreter must be exceptionall y careful not to misinterpret as reflections those many coherent waves that may appear to be reflections but are not. Evaluating the validit y of a processed, shallow seismic reflection section therefore require s that the interpreter have access to at least one field record and, i deally, to copies of one or more of the intermediate processing steps to corroborate the interpretation and to monitor for artifacts introdu ced by digital processing.