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Acquisition footprint is a pattern on seismic data which is mainly caused by the sparseness of the acquisition geometry. It hinders seismic interpreters from accurate structure analysis by masking geological significances such as fault lineaments, channels and karsts. It is triggered by coherent changes in case of variable acquisition parameters and/or methods in the field, or alignments caused by the direction of data acquisition, and it is predominant on sparse acquisition geometries. Furthermore, it might be accentuated by multi-channels seismic processing such as stacking, DMO and pre-stack time migration if the aliased noise produced by the sparse geometry does creep into the seismic data. In general, the acquisition footprint signature is rather strong in the shallow events due to relatively lower fold. Hence, it is strongly dependent on trace mute designs and sensitive to the amount of moveout and NMO stretch, where the influence is progressively healed with depth due to higher fold, wider mute parameters and smaller NMO shift. It is known that remnant acquisition footprints often exist on the data after the application of the 3D F-Kx-Ky wave number notch filter. Although it may not always appear on the vertical cross-sections and time slices prominently after the attenuation, it might be pronounced on edge detection attributes, such as coherency cube, curvature and/or seismic impedance. There is some ongoing debate in Abu Dhabi between processing geophysicists and interpretation geophysicists as to whether apply harsh filter to clean up the residual footprint or to be more conservative while preserving some desirable subtle geological features as possible. This paper advocates the necessity of time-variant 3D notch filtering approach regardless of the acquisition geometry and/or the area by showing some case histories from 3D seismic surveys offshore and onshore Abu Dhabi with three different acquisition geometries; orthogonal OBC patch layout, orthogonal land layout, and parallel OBC patch layout. Change of footprint characteristics in time, in association with adaptiveness of various notch filters in temporal/spatial domain, will b e discussed and demonstrated for each geometry type.
Merging 3D seismic surveys into a seamless single 3D volume, either post-stack stage or pre-stack stage, is a challenging task on seismic data processing. This study describes some tips from coastline Abu Dhabi where we successfully managed merging two partially overlapping surveys during post-stack stage, one from transition zone (land and shallow marine) while the other one from offshore 3D OBC seismic survey, in order to understand subtle geological structure relationship among two areas. Since spatial sampling between two surveys are greatly diverse due to different orientations and grid sizes, a conjugate grid which is identical to a main cube had been applied over the subordinate one that enable the whole dataset to interpolate and process as a same grid. Then we deployed pre-conditioning steps over the subordinate dataset to minimize their quality differences where we particularly focused on residual noise, multiples, frequency contents and event timings. Lastly, a matching filter was designed and applied to the subordinate side to compensate residual amplitude, frequency and phase and produce a final dataset for structure interpretation. A single consolidated seamlessly merged volume was produced throughout the steps as described above along with well-to-seismic calibrations. Seismic interpretation was conducted over the main reservoir between two datasets/fields with a good degree of confidence. The present day structure separation and structure growth history were analyzed as a part of the structure interpretation. Moreover, this case study illustrates the add values of the seismic 3D merge from the aspect of regional structure restoration, and revealing structure relationship between the overlapping surveys. The final merging result outcome of this case study has an amenable structure continuity and seamless horizon mapping of the common reservoir target level between the two surveys. Additionally, the merged seismic cube did showcase a lateral zero phase wavelet stability of the existing wells that verified the reliability of the conducted post-stack seismic merging processing workflow. This case study has successfully demonstrated and summarized key technical tips that are recommended for merging datasets on post-stack domain in the future. However, pre-stack merge is also and still strongly recommended since static corrections, velocity pickings and imaging processing can be applied throughout the two surveys in one go while these cannot be fixed on post-stack merge. From data acquisition perspectives; enough overlap to reaching the full-fold rim of the overlapping surveys is highly recommended.
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