Pitfalls in Shallow Seismic Reflection

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