Variable-depth streamer acquisition: Broadband data for imaging, inversion and interpretation

Variable-depth streamer acquisition is a solution for broadband marine seismic where the depth profile of the streamer is optimized in order to create receiver ghost diversity, which in turn enables deconvolution of the residual ghost at theimaging stage, either pre-stack or post-stack. Increasing the seismic bandwidth improves the interpretability of the seismic image, but it is necessary to increase both the high frequency and the low frequency spectrum. The high frequencies improve resolution but the low frequencies are necessary to suppress wavelet sidelobes which interfere with interpretation. For this reason, the addition of one octave of low frequencies (e.g. 2.5-5 Hz) is as beneficial as adding one octave of high frequencies (e.g. 75-150 HZ). Variable-depth streamer acquisition has a maximum depth of 50m and an average depth as deep as 40m resulting in improved receiver sensitivity at low frequencies which yields a better low frequencysignal-to-noise ratio. This technique also benefits from the use of solid streamers which further improves the signal-to-noise ratio at low frequencies. This broadband marine towed streamer solution has now been used in production in a variety of locations around the world, in different water depths and over different geologies. The receiver de-ghosting after migration/mirror-migration is fully 3D and true amplitude and completely removes the effect of the receiver ghost. In addition to the early 2D results, 3D, AVO and inversion data have now been obtained, all ofwhich show significant improvements over conventional data, achieving bandwidths of up to 6 octaves (2.5Hz -160 Hz). Interpretation of this broadband data is aided by the reduced noise and additional low-frequency energy, which helps to delineate deep structures, improves sedimentary package differentiation anddelineation and shows clear local impedance contrasts and heterogeneities. This is due to the lack of side lobes to the wavelet and the excellent phase control of the low frequencies, which also provide texture and continuity to the data. The broad bandwidth provides stratigraphic detail, allowing direct discrimination of rock or fluid properties. This broad bandwidth translates into improved results for seismic inversion. Pre-stack elastic inversion has been performed, providing both impedance and Vp/Vs sections, so proving the feasibility of pre-stack deghosting of variable-depth streamer data. All of these factors enhance interpretation.