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== Method ==
== Method ==


=== Acquisition of data ===
=== Data acquisition ===
Seismic oceanography is based on marine [[Reflection_seismology|seismic reflection profiling]], in which a ship tows specialised equipment for generating underwater sound. This equipment is known as the acoustic source, and produces sound by releasing either compressed air or [[Electric_charge|electrical charge]] into the sea. The ship also tows one or more cables along which are arranged hundreds of [[Hydrophone|hydrophones]], which are instruments for recording underwater sound. These cables are referred to as streamers, and are between 1 km and 10 km long. Both the acoustic source and the streamers lie a few metres beneath the sea surface.
Seismic oceanography is based on marine [[Reflection_seismology|seismic reflection profiling]], in which a ship tows specialised equipment for generating underwater sound. This equipment is known as the acoustic source, and produces sound by releasing either compressed air or [[Electric_charge|electrical charge]] into the sea. The ship also tows one or more cables along which are arranged hundreds of [[Hydrophone|hydrophones]], which are instruments for recording underwater sound. These cables are referred to as streamers, and are between 1 km and 10 km long. Both the acoustic source and the streamers lie a few metres beneath the sea surface.
The acoustic source generates sound waves once every few seconds. Most of these sound waves travel downwards towards the seabed, and a small fraction of the sound is reflected from boundaries at which the temperature or salinity of seawater changes<ref name="SallarèsBiescas2009">{{cite journal|last1=Sallarès|first1=V.|last2=Biescas|first2=B.|last3=Buffett|first3=G.|last4=Carbonell|first4=R.|last5=Dañobeitia|first5=J. J.|last6=Pelegrí|first6=J. L.|title=Relative contribution of temperature and salinity to ocean acoustic reflectivity|journal=Geophysical Research Letters|volume=36|year=2009|issn=0094-8276|doi=10.1029/2009GL040187|hdl=10261/18510|hdl-access=free}}</ref>. Reflected sound is recorded by the hydrophones. As the ship moves forwards, the positions of the acoustic source and hydrophones change with respect to the reflecting boundaries. Over a period of 30 minutes or less<ref name="FalderWhite2016">{{cite journal|last1=Falder|first1=Matthew|last2=White|first2=N. J.|last3=Caulfield|first3=C. P.|title=Seismic Imaging of Rapid Onset of Stratified Turbulence in the South Atlantic Ocean|journal=Journal of Physical Oceanography|volume=46|issue=4|year=2016|pages=1023–1044|issn=0022-3670|doi=10.1175/JPO-D-15-0140.1}}</ref><ref name="DickinsonWhite2017">{{cite journal|last1=Dickinson|first1=Alex|last2=White|first2=N. J.|last3=Caulfield|first3=C. P.|title=Spatial Variation of Diapycnal Diffusivity Estimated From Seismic Imaging of Internal Wave Field, Gulf of Mexico|journal=Journal of Geophysical Research: Oceans|volume=122|issue=12|year=2017|pages=9827–9854|issn=2169-9275|doi=10.1002/2017JC013352}}</ref>, multiple different configurations of acoustic source and hydrophones sample the same point on a boundary.
The acoustic source generates sound waves once every few seconds. Most of these sound waves travel downwards towards the seabed, and a small fraction of the sound is reflected from boundaries at which the temperature or salinity of seawater changes (these boundaries are known as thermohaline boundaries)<ref name="SallarèsBiescas2009">{{cite journal|last1=Sallarès|first1=V.|last2=Biescas|first2=B.|last3=Buffett|first3=G.|last4=Carbonell|first4=R.|last5=Dañobeitia|first5=J. J.|last6=Pelegrí|first6=J. L.|title=Relative contribution of temperature and salinity to ocean acoustic reflectivity|journal=Geophysical Research Letters|volume=36|year=2009|issn=0094-8276|doi=10.1029/2009GL040187|hdl=10261/18510|hdl-access=free}}</ref>. Reflected sound is recorded by the hydrophones. As the ship moves forwards, the positions of the acoustic source and hydrophones change with respect to the reflecting boundaries. Over a period of 30 minutes or less<ref name="FalderWhite2016">{{cite journal|last1=Falder|first1=Matthew|last2=White|first2=N. J.|last3=Caulfield|first3=C. P.|title=Seismic Imaging of Rapid Onset of Stratified Turbulence in the South Atlantic Ocean|journal=Journal of Physical Oceanography|volume=46|issue=4|year=2016|pages=1023–1044|issn=0022-3670|doi=10.1175/JPO-D-15-0140.1}}</ref><ref name="DickinsonWhite2017">{{cite journal|last1=Dickinson|first1=Alex|last2=White|first2=N. J.|last3=Caulfield|first3=C. P.|title=Spatial Variation of Diapycnal Diffusivity Estimated From Seismic Imaging of Internal Wave Field, Gulf of Mexico|journal=Journal of Geophysical Research: Oceans|volume=122|issue=12|year=2017|pages=9827–9854|issn=2169-9275|doi=10.1002/2017JC013352}}</ref>, multiple different configurations of acoustic source and hydrophones sample the same point on a boundary.


=== Sensitivity and resolution ===
=== Image creation ===
Comparison with direct measurements of temperature has shown that seismic oceanographic data can be sensitive to temperature changes at least as small as 0.03[[Celsius|°C]]<ref name="NandiHolbrook2004">{{cite journal|last1=Nandi|first1=Papia|last2=Holbrook|first2=W. Steven|last3=Pearse|first3=Scott|last4=Páramo|first4=Pedro|last5=Schmitt|first5=Raymond W.|year=2004|title=Seismic reflection imaging of water mass boundaries in the Norwegian Sea|journal=Geophysical Research Letters|volume=31|issue=23|doi=10.1029/2004GL021325|issn=00948276|hdl=1912/3317|hdl-access=free}}</ref>.


=== Signal processing ===
==== Idealised case ====
Seismic data record how the [[Intensity (physics)|intensity]] of sound at each hydrophone changes with time. The time at which sound reflected from a thermohaline boundary is recorded at a particular hydrophone depends on the location of the hydrophone relative to the acoustic source, on the depth of the boundary, and on the [[speed of sound]] in seawater. The depth of the boundary and the local speed of sound, which can vary between approximately 1450 m/s and 1540 m/s<ref>{{Cite book|last=Brekhovskikh|first=L. M.|url=https://www.worldcat.org/oclc/56066920|title=Fundamentals of ocean acoustics|date=2003|publisher=Springer|others=I︠U︡. P. Lysanov|isbn=0-387-21655-3|edition=3rd ed|location=New York|oclc=56066920}}</ref>, are both initially unknown. By analysing records from multiple different configurations of acoustic source and hydrophones, the boundary depth and speed of sound can be determined using a set of algorithms collectively known as [[seismic migration]]. After migration, different records that sample the same point on a boundary are added together to increase the [[signal-to-noise ratio]] (this process is known as stacking). Migration and stacking are carried out at every depth and at every horizontal position to make a spatially accurate seismic image.
Processing seismic records in the water column broadly follows the same steps as processing geological data. The techniques have been adapted from methods commonly used in seismic imaging of the solid Earth, and can be divided into two key stages. First, noise is removed in order to reveal clear reflections from oceanic structure. Secondly, seismic records are geometrically corrected to yield a stacked image.


The intensity of sound recorded by hydrophones can change due to causes other than reflection of sound from thermohaline boundaries. For instance, the acoustic source produces some sound waves that travel horizontally along the streamer, rather than downwards towards the seabed. Aside from sound produced by the acoustic source, the hydrophones record background noise caused by natural processes such as breaking of [[Wind wave|wind waves]] at the ocean surface. These other, unwanted sounds are often much louder than sound reflected from thermohaline boundaries. Use of [[Filter (signal processing)|signal-processing filters]] quietens unwanted sounds and makes reflections from thermohaline boundaries more obvious.
Noise removal incorporates a number of processes. Typically, the first step is band-pass filtering using a filter band of approximately 10-100 Hz. This step removes low frequency (2 Hz) swell noise (i.e. waves breaking near the sea-surface close to the streamer) as well as high frequency ambient noise (>100 Hz). At this point it is appropriate to remove reflections from the solid Earth, since they are typically of order 100 times greater in amplitude than oceanic reflections. Due to their large amplitudes, these reflection swamp the lower amplitude oceanic reflectivity. In order to remove the solid Earth noise, reflections from the seabed and below are muted (i.e. set to zero). Now, the direct wave is removed. This noise appears as a linear band of high amplitudes at the shallowest portion of the dataset. It is the energy that has travelled along the streamer (rather than down into the ocean and reflected upwards). Since most of the oceanic realm has abrupt temperature gradients in the upper 500~m, known as the thermocline, it is important to effectively remove the direct wave to observe the entire vertical extent of water column reflectivity. A number of methods have been developed to remove the direct wave, including eigenvector filtering<ref>{{cite journal |last1=Tang |first1=Q |last2=Gulick |first2=S |last3=Sun |first3=J |last4=Sun |first4=L |last5=Jing |first5=Z |title=Submesoscale Features and Turbulent Mixing of an Oblique Anticyclonic Eddy in the Gulf of Alaska Investigated by Marine Seismic Survey Data |journal=Journal of Geophysical Research: Oceans |date=2020 |volume=125 |issue=1 |pages=1-16}}</ref> and linear filtering.<ref>{{cite journal |last1=Gunn |first1=K. L. |last2=White |first2=N |last3=Caulfield |first3=C. P. C |title=Time-Lapse Seismic Imaging of Oceanic Fronts and Transient Lenses Within South Atlantic Ocean |journal=Journal of Geophysical Research: Oceans |date=2020 |volume=125 |issue=7}}</ref> There are a number of other noise-removal techniques that can be applied. The three described above are the minimal required steps.

Secondly, the seismic data are mapped from the source and receiver spatial domains to coordinates of x, y, and z (i.e. longitude, latitude, and depth). The first step is to correct for the source-received separation in the experiment, which causes many traces to be recorded at the same location. This correction is called the normal-moveout correction, and is described in detail here. After correcting for normal-movement, data which records the same spatial locations are stacked to yield a single time series with an increased signal-to-noise ratio. Stacked traces belonging to adjacent locations are placed next to each other, resulting in the final seismic image.


=== Interpretation ===
=== Interpretation ===

Revision as of 16:55, 23 August 2021

Main articles: Oceanography and Underwater acoustics

Seismic oceanography is a form of acoustic oceanography, in which sound waves are used to study the physical properties and dynamics of the ocean. It provides images of changes in the temperature and salinity of seawater. Unlike most oceanographic acoustic imaging methods, which use sound waves with frequencies greater than 10,000 Hz, seismic oceanography uses sound waves with frequencies lower than 500 Hz. Use of low-frequency sound means that seismic oceanography is unique in its ability to provide highly detailed images of oceanographic structure that span horizontal distances of hundreds of kilometres and which extend from the sea surface to the seafloor. Since its inception in 2003[1] , seismic oceanography has been used to image a wide variety of oceanographic phenomena, including fronts[2], eddies[3], thermohaline staircases[4], turbid layers[5] and cold methane seeps[6]. In addition to providing spectacular images, seismic oceanographic data have given quantitative insight into processes such as movement of internal waves[7] and turbulent mixing of seawater[8].

Method

Data acquisition

Seismic oceanography is based on marine seismic reflection profiling, in which a ship tows specialised equipment for generating underwater sound. This equipment is known as the acoustic source, and produces sound by releasing either compressed air or electrical charge into the sea. The ship also tows one or more cables along which are arranged hundreds of hydrophones, which are instruments for recording underwater sound. These cables are referred to as streamers, and are between 1 km and 10 km long. Both the acoustic source and the streamers lie a few metres beneath the sea surface.

The acoustic source generates sound waves once every few seconds. Most of these sound waves travel downwards towards the seabed, and a small fraction of the sound is reflected from boundaries at which the temperature or salinity of seawater changes (these boundaries are known as thermohaline boundaries)[9]. Reflected sound is recorded by the hydrophones. As the ship moves forwards, the positions of the acoustic source and hydrophones change with respect to the reflecting boundaries. Over a period of 30 minutes or less[10][11], multiple different configurations of acoustic source and hydrophones sample the same point on a boundary.

Image creation

Idealised case

Seismic data record how the intensity of sound at each hydrophone changes with time. The time at which sound reflected from a thermohaline boundary is recorded at a particular hydrophone depends on the location of the hydrophone relative to the acoustic source, on the depth of the boundary, and on the speed of sound in seawater. The depth of the boundary and the local speed of sound, which can vary between approximately 1450 m/s and 1540 m/s[12], are both initially unknown. By analysing records from multiple different configurations of acoustic source and hydrophones, the boundary depth and speed of sound can be determined using a set of algorithms collectively known as seismic migration. After migration, different records that sample the same point on a boundary are added together to increase the signal-to-noise ratio (this process is known as stacking). Migration and stacking are carried out at every depth and at every horizontal position to make a spatially accurate seismic image.

The intensity of sound recorded by hydrophones can change due to causes other than reflection of sound from thermohaline boundaries. For instance, the acoustic source produces some sound waves that travel horizontally along the streamer, rather than downwards towards the seabed. Aside from sound produced by the acoustic source, the hydrophones record background noise caused by natural processes such as breaking of wind waves at the ocean surface. These other, unwanted sounds are often much louder than sound reflected from thermohaline boundaries. Use of signal-processing filters quietens unwanted sounds and makes reflections from thermohaline boundaries more obvious.

Interpretation

The key advantage of seismic oceanography is that it provides high-resolution (up to 10 m) images of oceanic structure, that can be combined with quantitative information about the ocean. The imagery can be used to identify the length, width, and height of oceanic structures across a range of scales. If the seismic data is also 3D, then the evolution of the structures over time can be analyzed too.[13][14]

Inverting for temperature and salinity

Combined with its imagery, processed seismic data can be used to extract other quantitative information about the ocean. So far, seismic oceanography has been used to extract distributions of temperature, and salinity, and therefore density and other important properties. There is a range of approaches that can be used to extract this information. For example, Paramo and Holbrook (2005)[15] extracted temperature gradients in the Norwegian Sea using the Amplitude Versus Offset methods. The distributions of physical properties were limited to one-dimension however. More recently, there has been a move toward two-dimensional technique. Cord Papenberg et al. (2010)[16] presented high-resolution two-dimensional temperature and salinity distributions. These fields were derived using an iterative inversion that combines seismic and physical oceanographic data. Since then, more complex inversions have been presented that are based on Monte Carlo inversion techniques,[17] amongst others.

Spectral analysis for vertical mixing rates

Aside from temperature and salinity distributions, seismic data of the ocean can also be used to extract mixing rates through spectral analysis. This process is based on the assumption that reflections, which show undulations at a number of scales, track the internal wave field. Therefore, the vertical displacement of these undulations can give a measure of the vertical mixing rates of the ocean. This technique was fist developed using data from the Norwegian Sea and showed the enhancement of internal wave energy close to the continental slope.[18] Since 2005, the techniques have been further developed, adapted, and automated so that any seismic section may be converted into a two-dimensional distribution of mixing rates [19] [20][21]

References

  1. ^ Holbrook, W. S. (2003). "Thermohaline Fine Structure in an Oceanographic Front from Seismic Reflection Profiling". Science. 301 (5634): 821–824. doi:10.1126/science.1085116. ISSN 0036-8075.
  2. ^ Nakamura, Y.; Noguchi, T.; Tsuji, T.; Itoh, S.; Niino, H.; Matsuoka, T. (2006). "Simultaneous seismic reflection and physical oceanographic observations of oceanic fine structure in the Kuroshio extension front". Geophysical Research Letters. 33 (23). doi:10.1029/2006GL027437. ISSN 0094-8276.
  3. ^ Pinheiro, Luis Menezes; Song, Haibin; Ruddick, Barry; Dubert, Jesus; Ambar, Isabel; Mustafa, Kamran; Bezerra, Ronaldo (2010). "Detailed 2-D imaging of the Mediterranean outflow and meddies off W Iberia from multichannel seismic data". Journal of Marine Systems. 79 (1–2): 89–100. doi:10.1016/j.jmarsys.2009.07.004. ISSN 0924-7963.
  4. ^ Fer, I.; Nandi, P.; Holbrook, W. S.; Schmitt, R. W.; Páramo, P. (2010). "Seismic imaging of a thermohaline staircase in the western tropical North Atlantic". Ocean Science. 6 (3): 621–631. doi:10.5194/os-6-621-2010. hdl:1912/3915. ISSN 1812-0792.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  5. ^ Vsemirnova, E. A.; Hobbs, R. W.; Hosegood, P. (2012). "Mapping turbidity layers using seismic oceanography methods". Ocean Science. 8 (1): 11–18. doi:10.5194/os-8-11-2012. ISSN 1812-0792.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  6. ^ Jiang-Xin, CHEN; Hai-Bin, SONG; Yong-Xian, GUAN; Sheng-Xiong, YANG; Yang, BAI; Ming-Hui, GENG (2017). "A PRELIMINARY STUDY OF SUBMARINE COLD SEEPS BY SEISMIC OCEANOGRAPHY TECHNIQUES". Chinese Journal of Geophysics. 60 (1): 117–129. doi:10.1002/cjg2.30032. ISSN 0898-9591.
  7. ^ Tang, Qunshu; Wang, Caixia; Wang, Dongxiao; Pawlowicz, Rich (2014). "Seismic, satellite and site observations of internal solitary waves in the NE South China Sea". Scientific Reports. 4 (1). doi:10.1038/srep05374. ISSN 2045-2322. PMC 4064323.
  8. ^ Kubichek, Robert; Helfrich, L. Cody; Klymak, Jody M.; Lizarralde, Daniel; Schmitt, Raymond W.; Fer, Ilker; Holbrook, W. Steven (2013). "Estimating Oceanic Turbulence Dissipation from Seismic Images". Journal of Atmospheric and Oceanic Technology. 30 (8): 1767–1788. doi:10.1175/JTECH-D-12-00140.1. hdl:1912/6229. ISSN 0739-0572.
  9. ^ Sallarès, V.; Biescas, B.; Buffett, G.; Carbonell, R.; Dañobeitia, J. J.; Pelegrí, J. L. (2009). "Relative contribution of temperature and salinity to ocean acoustic reflectivity". Geophysical Research Letters. 36. doi:10.1029/2009GL040187. hdl:10261/18510. ISSN 0094-8276.
  10. ^ Falder, Matthew; White, N. J.; Caulfield, C. P. (2016). "Seismic Imaging of Rapid Onset of Stratified Turbulence in the South Atlantic Ocean". Journal of Physical Oceanography. 46 (4): 1023–1044. doi:10.1175/JPO-D-15-0140.1. ISSN 0022-3670.
  11. ^ Dickinson, Alex; White, N. J.; Caulfield, C. P. (2017). "Spatial Variation of Diapycnal Diffusivity Estimated From Seismic Imaging of Internal Wave Field, Gulf of Mexico". Journal of Geophysical Research: Oceans. 122 (12): 9827–9854. doi:10.1002/2017JC013352. ISSN 2169-9275.
  12. ^ Brekhovskikh, L. M. (2003). Fundamentals of ocean acoustics. I︠U︡. P. Lysanov (3rd ed ed.). New York: Springer. ISBN 0-387-21655-3. OCLC 56066920. {{cite book}}: |edition= has extra text (help)
  13. ^ Dickinson, A; White, N; Caulfield, C. P. C (2020). "Time-Lapse Acoustic Imaging of Mesoscale and Fine-Scale Variability within the Faroe-Shetland Channel". Journal of Geophysical Research: Oceans.
  14. ^ Gunn, K. L.; White, N; Caulfield, C. P. C (2020). "Time-Lapse Seismic Imaging of Oceanic Fronts and Transient Lenses Within South Atlantic Ocean". Journal of Geophysical Research: Oceans. 125 (7).
  15. ^ Páramo, P; Holbrook, S. W. (2005). "Temperature contrasts in the water column inferred from amplitude- versus-offset analysis of acoustic reflections". Geophysical Research Letters. 32: 1–4.
  16. ^ Papenberg, C; Klaeschen, D; Krahmann, G; Hobbs, R. W. (2010). "Ocean temperature and salinity inverted from combined hydrographic and seismic data". Geophysical Research Letters. 37 (4): 6–11.
  17. ^ Tang, Q; Hobbs, R; Zheng, C; Biescas, B; Caiado, C (2016). "Markov Chain Monte Carlo inversion of temperature and salinity structure of an internal solitary wave packet from marine seismic data". 121 (6): 3692–3709. {{cite journal}}: Cite journal requires |journal= (help); Missing |author2= (help)
  18. ^ Holbrook, W. S.; Fer, I (2005). "Ocean internal wave spectra inferred from seismic reflection transects". Geophysical Research Letters. 32.
  19. ^ Sheen, K. L.; White, N; Hobbs, R (2009). "Estimating mixing rates from seismic images of oceanic structure". 36 (24): 1–5. {{cite journal}}: Cite journal requires |journal= (help)
  20. ^ Holbrook, S; Fer, I; Schmitt, R W; Lizarralde, D; Klymak, J. M.; Helfrich, L. C.; Kubichek, R (2013). "Estimating oceanic turbulence dissipation from seismic images". Journal of Atmospheric and Oceanic Technology. 30 (8): 1767–1788.
  21. ^ Dickinson, A; White, N; Caulfield, C. P. C. (2017). "Spatial Variation of Diapycnal Diffusivity Estimated From Seismic Imaging of Internal Wave Field, Gulf of Mexico". Journal of Geophysical Research: Oceans. 122: 1–28.
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