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High Resolution Processing of crustal data to study gas and gas hydrates across Rock Garden

Most seismic datasets in data archives worldwide have been acquired and processed for deeper targets than gas hydrates, mostly for oil and gas exploration or crustal studies. Resulting seismic images are often lacking the resolution necessary to identify gas hydrate related features, such as BSRs. We therefore re-processed crustal seismic reflection data acquired on the Hikurangi margin in 2001 (North Island GeopHysical Transect, NIGHT) for high resolution, focussing on Rock Garden, a ridge offshore of Hawke's Bay. We discovered BSRs and bright, shallow reflections that we interpret as gas conduits (Pecher et all.,2004).

The U.S. R/VIB N.B. Palmer acquired generator/injector (GI) airgun data across that area in 2003 giving us the opportunity to compare our images from high-resolution processing of crustal data to those from real high-resolution data (Zhu et all.,2004). Key processing steps for the crustal data included shot interpolation from 100 to 25 m, CDP sorting at 12.5 m, and NMO-corrected migration of the CDP-neartrace. Except for layer pinchouts at the seafloor, all the key features related to gas hydrates (BSRs, gas conduits) are resolved in the crustal data. We conclude that high-resolution re-processing of existing low-resolution data is a viable first step towards identifying possible gas hydrate deposits even if standard processing of these data did not reveal any evidence for gas hydrates.

References

  1. Pecher, I.A., S.A. Henrys, and H. Zhu (2004), Seismic images of gas conduits beneath vents and gas hydrates on Ritchie Ridge, Hikurangi margin, New Zealand, N. Z. J. Geol. Geophys. , 47 , 275-279.
  2. Zhu, H., I.A. Pecher , S.A. Henrys, and G. Maslen, Improving the Resolution of Deep-Crustal Seismic Data to Study Shallow Gas Hydrates on the Hikurangi Margin, New Zealand , in ASEG/PESA , pp. 4 pp., Sydney , 2004.

BSR and normal faults at the eastern edge of Rock Garden.
A: NIGHT crustal data after high-resolution processing,
B: NIGHT pre-stack time migration, which would be considered high resolution on crustal scales,
C: N.B. Palmer data, highest-possible resolution, similar to that for NIGHT data in (A),
D: N.B. Palmer data, pre-stack time migration [Zhu et al. , 2004] .
Except for features right beneath the seafloor, (A) has a similar resolution as (C) and (D). Note: The BSR as a gas hydrate proxy would probably not be recognised in (B).

Faults at Rock Garden