Abstract
Abstract. Interpretation of deep seismic data is challenging due to the lack of direct geological constraints from drilling and the more limited amount of data available from 2-D profiles in comparison to hydrocarbon exploration surveys. Thus other constraints that can be derived from the seismic data themselves can be of great value. Though the origin of most deep seismic reflections remains ambiguous, an association between seismic reflections and crustal strain, e.g. shear zones, underlies many interpretations. Estimates of the 3-D orientation of reflectors may help associate specific reflections, or regions of the crust, with geological structures mapped at the surface whose orientation and tectonic history are known. In the case of crooked 2-D onshore seismic lines, the orientation of reflections can be estimated when the range of azimuths in a common midpoint gather is greater than approximately 20∘, but integration of these local orientation attributes into an interpretation of migrated seismic data requires that they also be migrated. Here we present a simple approach to the 2-D migration of these orientation attributes that utilizes the apparent dip in reflections on the unmigrated stack and maps reflector strike, for example, to a short linear segment depending on its original position and a migration velocity. This interpretation approach has been applied to a seismic line shot across the Younami Terrane of the Australian Yilgarn Craton and indicates that the lower crust behaved differently from the overlying middle crust as the newly assembled crust collapsed during the Late Archean. Some structures related to approximately east-directed shortening are preserved in the middle crust, but the lower crust is characterized by reflectors that suggest N-NNE-oriented ductile flow. Deployment of off-line receivers during seismic acquisition allows the recording of a larger range of source-receiver azimuths and should produce more reliable future estimates of these reflector attributes.
Highlights
Deep seismic reflection surveys that image the entire continental crust are typically acquired as 2-D profiles due to cost and are able to provide subsurface images with a resolution of the order of 100 m or better
The preprocessing of the prestack seismic data for orientation analysis included resampling to 8 ms, refraction statics, residual statics, amplitude recovery with a T 1.2 gain, time-variant spectral whitening, automatic gain control (AGC) with a 0.5 s window, zero-phase Ormsby filtering to 5-10-30-40 Hz, trace muting, and the combination of 64 adjacent common depth point (CDP) into supergathers every 2 CDP; an additional mute of data stretched more than 30 % is included in the orientation estimation analysis
The error depends on the distribution of sources and receivers in the supergather used for the estimate, and their relation to the CDP bin centre; in practice, those parts of the seismic line where it is difficult to obtain orientation angles are reasonably well predicted by the range of useful source-receiver azimuths, which is defined to be the number of 1◦ azimuth bins for which there are seismic data available (Fig. 2)
Summary
Deep seismic reflection surveys that image the entire continental crust are typically acquired as 2-D profiles due to cost and are able to provide subsurface images with a resolution of the order of 100 m or better. The interpretation of these deep seismic profiles, is often limited by the presence of reflections that can originate from locations out of the plane of the seismic profile, resulting in cross-cutting reflections in the migrated seismic section. Doublier: Migration of reflector orientation attributes stacking velocity These results, for example the angles and error estimates, are displayed as a function of time at each CDP on unmigrated seismic sections. The importance of obtaining more accurate orientation estimates for positioning reflectors in 3-D, by for example deploying additional crossline receivers, will be discussed
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