Imaging deep reflectors in the presence of signal-generated noise

Abstract

Summary. In contrast to the continuous character of reflectors seen in seismic data from the sedimentary column, a pervasive feature of deep crustal reflections is their laterally discontinuous nature. Deep reflections often appear on the seismic section as groups of flat-lying or dipping segments. The length of the individual segments may vary by an order of magnitude. Moreover, the segments are often shorter than the Fresnel zone for a specular reflection. The apppearance of deep flat-lying reflectors of this type has motivated geologic hypotheses of a layered and heterogeneous lower crust. Dipping bands of discontinuous reflectors are interpreted variously as decollement surfaces, sutures, or faults penetrating to midcrustal or deeper levels. A significant geophysical concern is whether truly discontinuous reflectors in the deep crust can be distinguished from an inaccurate or incoherent image. Incoherence can result from the presence of signal-generated noise created at the earth's surface or along the transmission path. Given the types of signal-generated noise likely to be present in deep crustal data, we wish to know how well we can resolve both laterally continuous and discontinuous reflectors using standard reflection data acquisition and processing. To test the effects of such noise on the CMP stack we have conducted finite-difference synthetic seismogram experiments for two similar layered models each having two specular reflectors. One model contained an irregular surface layer, the other contained a zone of random velocity fluctuations between the two reflectors. We processed the data normally, from shot gather to CMP stack and migrated section. The two numerical experiments produced CMP sections with substantially different character. Processing the surface model data produced a clear image of the two deep reflectors as the CMP stack suppressed noise generated in the irregular surface layer. The surface generated noise limited the vertical resolution as it degraded the action of predictive deconvolution applied before stack. Signal-generated noise in the second model did not affect the deconvolution process because the scatterers were located below the zone of surface layer reverberations. The laterally inhomogeneous zone degraded the image of the deepest reflector. The signal-generated noise in the second model may be viewed as signal reflected from the random zone. The dip-filtering action of the CMP stack resulted in reflection events from the random zone having greater horizontal continuity than is present in the velocity model, causing further smearing by a standard time-migration technique.