Earth:Bottom simulating reflector

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Short description: Anomalous reflections in marine seismic data


Bottom simulating reflectors (BSRs) are, on seismic reflection profiles, shallow seismic reflection events, characterized by their reflection geometry similar to seafloor bathymetry.[1] . They have, however, the opposite reflection polarity to the seabed reflection,[1] and frequently intersect the primary reflections.[2]

Cause of Reflection

Seismic reflection is a sound wave bounced back from subsurface at the interface between media with different acoustic properties (density and wave velocity).[3] In geology, the reflections normally occur at the contacts between different rocks, for example, between layers of sedimentary rocks (stratification).[4][5] The acoustic properties of sedimentary rocks are influenced by their rock materials, pore space and fluid content.[6] Reflections are generally parallel to sedimentary layering or bedding surfaces.[6] Fluid content in pore space, however, sometimes becomes the dominant influence factor for the acoustic properties,[6] therefore, reflections in such case, may not be parallel to bedding surfaces.[6][7] BSRs are such a case of crossing bedding surfaces.[8]

Drilling results show BSRs approximately marking the base of gas hydrated sediments below the seafloor and the reflection is primarily caused by the free gas contained in sediments below the gas hydrated section.[9] Gas presence in sediments is well known for its drastically lowering the sediment acoustic impedance and hence, generates high amplitude reflection at the interface of gas bearing formation.[6] Formation of gas hydrate in deep sea sediments depends on its ambient pressure and temperature, both which are largely influenced by the depth below seafloor. This is the primary reason for BSRs grossly parallel to the seafloor reflection on seismic profiles.[10]

Formation and Occurrence

Gas hydrates are made of molecules of natural gas, mostly biogenic or thermogenic methane, contained in solid water molecule lattice. They are formed by combining methane with water under elevated pressures and at relatively low temperatures. Hence BSRs are widespread in arctic permafrost regions and in shallow sedimentary columns below seabed in deepwater continental margins[11]

Application

Geologcial hazard studies

Identification of natural gas hydrate in deep sea sediments is crucial for offshore petroleum exploration. Without adequate equipment installed prior to drilling, blowout may occur if penetrating the gas hydrate sediments. Furthermore, presence of gas hydrates in marine sediments may alter sea floor stability, and induce submarine slumping.[12][13]

Alternative energy resource

Although current production technology has not been proven to be commercially viable, gas hydrates’ global occurrence in deep sea sediments, have still been considered as a potential alternative energy resource. It should be pointed out that areal distribution of BSRs alone is not adequate to properly estimate the potential reserve, since other techniques are needed to address the thickness of sedimentary columns which contain the hydrates.[14] In addition, seismic acquisition parameters and acoustic properties of sediments with free gas in pores may all influence acoustic impedance contrast, which inevitably affects the reflection amplitude. This would cause the uncertainty of the relationship between BSRs and the presence of gas hydrate.[15]

Climatic impact

Because gas hydrates are only stable in a range of low temperatures and moderate pressures, atmospheric and ocean warming may trigger the hydrates instability and release significant amounts of methane from both permafrost and marine sediments. This could aggravate the greenhouse effect on the earth climate.[16]

References

  1. ↑ 1.0 1.1 MacKay, Mary E.; Jarrard, Richard D.; Westbrook, Graham K.; Hyndman, Roy D. (1 May 1994). "Origin of bottom-simulating reflectors: Geophysical evidence from the Cascadia accretionary prism". Geology 22 (5): 459–462. doi:10.1130/0091-7613(1994)022<0459:OOBSRG>2.3.CO;2. Bibcode: 1994Geo....22..459M. 
  2. ↑ Shedd, William; Boswell, Ray; Frye, Matthew; Godfriaux, Paul; Kramer, Kody (June 2012). "Occurrence and nature of "bottom simulating reflectors" in the northern Gulf of Mexico". Marine and Petroleum Geology 34 (1): 31–40. doi:10.1016/j.marpetgeo.2011.08.005. Bibcode: 2012MarPG..34...31S. 
  3. ↑ William Ashcroft (2011) Chapter 1.2： The principle of seismic reflection survey in A Petroleum Geologist’s Guide to Seismic Reflection. Wiley-Blackwell ISBN:9781444397857
  4. ↑ William Ashcroft (2011) Chapter 7.2： Linking well Geology to the Seismic Section in A Petroleum Geologist’s Guide to Seismic Reflection. Wiley-Blackwell ISBN:9781444397857
  5. ↑ William Ashcroft (2011) Chapter 7.3“Choosing Reflections to Pick. in A Petroleum Geologist’s Guide to Seismic Reflection. Wiley-Blackwell ISBN:9781444397857
  6. ↑ 6.0 6.1 6.2 6.3 6.4 "Characteristics of seismic events". Exploration Seismology. 1995. pp. 145–190. doi:10.1017/CBO9781139168359.007. ISBN 9780521462822. 
  7. ↑ Jack Dvorkin, Mario A. Gutierrez, Dario Grana (2014) Seismic Reflections of Rock Properties. Cambridge University Press,Science ISBN:1107782732, 9781107782730
  8. ↑ Dvorkin, J., Gutierrez, M., & Grana, D. (2014). Gas hydrates*. In Seismic Reflections of Rock Properties (pp. 262-274). Cambridge: Cambridge University Press. doi:10.1017/CBO9780511843655.022
  9. ↑ Ruppel, C.; Boswell, R.; Jones, E. (1 November 2008). "Scientific results from Gulf of Mexico Gas Hydrates Joint Industry Project Leg 1 drilling: Introduction and overview". Marine and Petroleum Geology 25 (9): 819–829. doi:10.1016/j.marpetgeo.2008.02.007. Bibcode: 2008MarPG..25..819R. 
  10. ↑ Haacke, R. Ross; Westbrook, Graham K.; Hyndman, Roy D. (September 2007). "Gas hydrate, fluid flow and free gas: Formation of the bottom-simulating reflector". Earth and Planetary Science Letters 261 (3–4): 407–420. doi:10.1016/j.epsl.2007.07.008. Bibcode: 2007E&PSL.261..407H. 
  11. ↑ Sloan, E. D., and C. Koh (2007), Clathrate Hydrates of Natural Gases, CRC press, Boca Raton, Fla.
  12. ↑ Maslin, Mark; Owen, Matthew; Betts, Richard; Day, Simon; Dunkley Jones, Tom; Ridgwell, Andrew (28 May 2010). "Gas hydrates: past and future geohazard?". Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 368 (1919): 2369–2393. doi:10.1098/rsta.2010.0065. PMID 20403833. Bibcode: 2010RSPTA.368.2369M. 
  13. ↑ Phrampus, Benjamin J.; Hornbach, Matthew J. (25 October 2012). "Recent changes to the Gulf Stream causing widespread gas hydrate destabilization". Nature 490 (7421): 527–530. doi:10.1038/nature11528. PMID 23099408. Bibcode: 2012Natur.490..527P. 
  14. ↑ Boswell, Ray; Collett, Timothy S. (2011). "Current perspectives on gas hydrate resources". Energy Environ. Sci. 4 (4): 1206–1215. doi:10.1039/c0ee00203h. 
  15. ↑ Majumdar, Urmi; Cook, Ann E.; Shedd, William; Frye, Matthew (16 July 2016). "The connection between natural gas hydrate and bottom‐simulating reflectors". Geophysical Research Letters 43 (13): 7044–7051. doi:10.1002/2016GL069443. Bibcode: 2016GeoRL..43.7044M. 
  16. ↑ Archer, David; Buffett, Bruce; Brovkin, Victor (8 December 2009). "Ocean methane hydrates as a slow tipping point in the global carbon cycle". Proceedings of the National Academy of Sciences 106 (49): 20596–20601. doi:10.1073/pnas.0800885105. PMID 19017807. Bibcode: 2009PNAS..10620596A. 



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