Abstract
Horizon velocity analysis and pre-stack depth migration of seismic profiles collected by R/V Maurice Ewing in 1995 across the accretionary prism off SW Taiwan and along the continental slope of the northernmost South China Sea were implemented for identifying gas hydrates. Similarly, a survey of 32 ocean-bottom seismometers (OBS), with a spacing of about 500 m, was conducted for exploring gas hydrates on the accretionary prism off SW Taiwan in April 2006. Travel times of head wave, refraction, reflection and converted shear wave identified from the hydrophone, vertical and horizontal components of these OBS data were applied for imaging P-wave velocity and Poisson’s ratio of hydrate-bearing sediments. In the accretionary prism off SW Taiwan, we found hydrate-bearing sediment, with a thickness of about 100–200 m, a relatively high P-wave velocity of 1.87–2.04 km/s and a relatively low Poisson’s ratio of 0.445–0.455, below anticlinal ridges near imbricate emergent thrusts in the drainage system of the Penghu and Kaoping Canyons. Free-gas layer, with a thickness of about 30–120 m, a relatively low P-wave velocity of 1.4–1.8 km/s and a relatively high Poisson’s ratio (0.47–0.48), was also observed below most of the bottom-simulating reflectors (BSR). Subsequently, based on rock physics of the three-phase effective medium, we evaluated the hydrate saturation of about 12–30% and the free-gas saturation of about 1–4%. The highest saturation (30% and 4%) of gas hydrates is found below anticlines due to N–S trending thrust-bounded folds and NE-SW thrusting and strike-slip ramps in the lower slope of the accretionary prism. We suggest that fluid may have migrated through the relay-fault array due to decollement folding and gas hydrates have been trapped in anticlines formed by the basement rises along the thrust faults. In contrast, in the rifted continental margin of the northernmost South China Sea, P-wave velocities of 1.9–2.2 km/s and 1.3–1.6 km/s, and thicknesses of about 50–200 m and 100–200 m, respectively, for a hydrate layer and a free-gas layer were imaged below the remnant and erosional ridges in the upper continental slope. High P-wave velocity of hydrate-bearing sediment below erosional ridges may also indicate high saturation of hydrates there. Normal faults due to rifting in the South China continental crust may have provided conduits for gas migration below the erosional ridges where P-wave velocity of hydrate-bearing sediment in the passive continental margin of the northernmost South China Sea is greater than that in the active accretionary prism off SW Taiwan.
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References
Andreassen K, Hart PE, Grantz A (1995) Seismic studies of a bottom simulating reflection related to gas hydrate beneath the continental margin of the Beaufort Sea. J Geo Res 100:12659–12673
Ashi J, Tokuyama H, Taira A (2002) Distribution of methane hydrate, BSRs and its implication for the prism growth in the Nankai Trough. Mar Geol 187:177–191
Bünz S, Mienert J, Vanneste M, Andreassen K (2005) Gas hydrates at the Storegga slide: constraints from an analysis of multi-component, wide-angle seismic data. Geophysics 70:B19–B34
Carcione JM, Gei D, Rossi G, Madrussani G (2005) Estimation of gas-hydrate concentration and free-gas saturation at the Norwegian-Svalbard continental margin. Geophys Prospect 53:803–810
Chi WC, Reed DL, Liu CS, Lundberg N (1998) Distribution of the bottom-simulating reflector in the offshore Taiwan collision zone. Terr Atmos Oceanic Sci 9:779–794
Chiu JK, Tseng WH, Liu CS (2006) Distribution of gassy sediment and mud volcanoes offshore southwestern Taiwan. Terr Atmos Oceanic Sci 17:703–722
Guerin G, Goldberg D, Meltser A (1999) Characterization of in situ elastic properties of gas hydrate-bearing sediments on the Blake Ridge. J Geophys Res 104:17781–17795
Hamilton EL (1979) Vp/Vs, and Poisson’s ratios in marine sediments and rocks. J Acoust Soc Am 66:1093–1101
Hyndman RD, Spence GD (1992) A seismic study of methane hydrate marine bottom simulating reflectors. J Geophys Res 97:6683–6698
Korenaga J, Holbrook WS, Singh SC, Minshull TA (1997) Natural gas hydrate on the southeast US margin: constraints from full waveform and traveltime inversions of wide-angle seismic data. J Geophys Res 102:15345–15365
Lin S, Hsieh WC, Lim YC, Yang TF, Liu CS, Wang Y (2006) Methane migration and its influence on sulfate reduction in the good weather ridge region, South China Sea continental margin sediments. Terr Atmos Oceanic Sci 17:883–902
Lin CC, Lin AT, Liu CS, Schnürle P, Chen GY, Liao WZ (2009) Geological controls on BSR occurrences in the incipient arc-continent collision zone offshore southwest Taiwan. Mar Pet Geology 26:1118–1131
Liu CS, Schnürle P, Wang Y, Chung SH, Chen SC, Hsiuan TH (2006) Distribution and characters of gas hydrate offshore of southwestern Taiwan. Terr Atmos Oceanic Sci 17:615–644
Paull CK, Matsumoto R, Wallace PJ, Dillon WP (2000) Proceedings of the ocean drilling program, scienific results vol. 164
Schnürle P, Liu CS, Hsiuan TH, Wang TK (2004) Characteristics of gas hydrate and free gas offshore southwestern Taiwan from a combined MCS/OBS analysis. Mar Geophys Res 25:157–180
Schnürle P, Liu CS, Lee CS (2006) Acoustic and shear-wave velocities in hydrate-bearing sediments offshore southwestern Taiwan: tomography, converted waves analysis and reverse-time migration of OBS records. Terr Atmos Oceanic Sci 17:757–779
Shipley TH, Houston MH, Bufer RT, Shaub FJ, McMillen KJ, Ladd JW, Worzel JL (1979) Seismic evidence for widespread possible gas hydrate horizons on continental slopes and rises. Am Assoc Pet Geol Bull 63:2204–2213
Sibuet JC, Hsu SK, Le Pichon X, Le Formal JP, Reed D, Moore G, Liu CS (2002) East Asia plate tectonics since 15 Ma: constraints form the Taiwan region. Tectonophysics 344:103–104
Singh SC, Minshull T, Spence G (1993) Velocity structure of a gas hydrate reflector. Science 260:204–207
Tinivella U, Accaino F (2000) Compressional velocity structure and Poisson’s ratio in marine sediments with gas hydrate and free gas by inversion of reflected and refracted seismic data (South Shetland Islands, Antarctica). Mar Geol 164:13–27
Wang TK, Chen MK, Lee CS, Xia KY (2006) Seismic imaging of the transitional crust across the northeastern margin of the South China Sea. Tectonophysics 412:237–254
Wang TK, Lee CS, Chen CW, Chen CH, Wang Y, Chen SC (2007) OBS imaging of gas hydrates off SW Taiwan—the first phase of geological exploration. Eur Assoc Geophys Eng P166 (extended abstracts)
Wang TK, Lin SS, Cheng WB, Lee CS, Schnürle P (2009) OBS imaging of gas hydrates in the northernmost South China Sea. Am Assoc Pet Geol Annual meeting, Poster 21F
Yang BJ, Wang TK (2009) Pre-stack depth imaging and horizontal velocity analysis of gas hydrates off SW Taiwan. Am Assoc Pet Geol Annual Meeting, Poster 22E
Yilmaz O (2001) Seismic data analysis: processing, inversion, and interpretation of seismic data, series: investigations in Geophysics, no. 10, Doherty SF (ed.), SEG, Tulsa, pp 2027
Yu HS (2004) Nature and distribution of the deformation front in the Luzon Arc-Chinese continental margin collision zone at Taiwan. Mar Geophys Res 25:109–122
Zelt CA, Smith RB (1992) Seismic traveltime inversion for 2-D crustal velocity structure. Geophys J Int 108:16–34
Zillmer M (2006) A method for determining gas-hydrate or free-gas saturation of porous media from seismic measurements. Geophysics 71:N21–N32
Acknowledgments
We appreciate the team work for conducting surveys through the NTOU’s OBS group. This research was continuously supported by the Central Geological Survey, Ministry of Economic Affairs, Taiwan since 2004.
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Wang, T.K., Yang, B.J., Deng, JM. et al. Seismic imaging of gas hydrates in the northernmost South China sea. Mar Geophys Res 31, 59–76 (2010). https://doi.org/10.1007/s11001-010-9096-7
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DOI: https://doi.org/10.1007/s11001-010-9096-7