Coincident reflection images of the Gulf Stream from seismic and hydrographic data

Mirshak, Ramzi; Nedimović, M. R.; Greenan, B. J. W.; Ruddick, Barry; Louden, K. E.

doi:10.1029/2009gl042359

Cited by 16 publications

(18 citation statements)

References 11 publications

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“…One possible explanation for the feature is the strongly depressed isotherms/isopycnals (distinct water boundaries; temperature profiles with only negative gradients, not shown here), rather than the water boundaries of a cold tongue wrapping around the warm eddy as reported by Mirshak et al . [].…”

Section: Discussionmentioning

confidence: 99%

“…While subsurface eddies (mostly Meddies) have been frequently studied using the seismic method [e.g., Biescas et al, 2008;Menesguen et al, 2012;Pinheiro et al, 2010], only two surface anticyclonic eddies have been briefly reported. One is a joint oceanographic and seismic survey by Mirshak et al [2010]. They presented a robust view but simple description of a surface eddy in the Gulf Stream region [Mirshak et al, 2010].…”

Section: Citationmentioning

confidence: 99%

“…One is a joint oceanographic and seismic survey by Mirshak et al . []. They presented a robust view but simple description of a surface eddy in the Gulf Stream region [ Mirshak et al ., ].…”

Section: Introductionmentioning

confidence: 99%

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Seismic observations from a Yakutat eddy in the northern Gulf of Alaska

2014

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Recent works show that the seismic oceanography technique allows us to relate water column seismic reflections to oceanic finescale structures. In this study, finescale structures of a surface anticyclonic eddy have been unveiled by reprocessing two seismic transects acquired in the northern Gulf of Alaska using an 8 km hydrophone streamer and 6600 cu in linear airgun array in September 2008. The eddy was a typical bowl-like structure with around 55 km width and 700 m depth. It has two fringes around the eddy base and a spiral arm at the NE edge. The in situ sea surface temperature and salinity data from a shipboard thermosalinograph help to confirm our interpretations of a spiral arm shed from the warm eddy and the entrained cold water from elsewhere. Nearby the eddy and offshore the shelf-break, there is a strong frontal feature, probably the Alaska Current. The eddy likely formed offshore Yakutat shelf and transported along the offshore shelf-break by tracking the sea level anomalies. Its equivalent diameter of 65 km was measured using the along-track altimeter and the seismic constraints. It was comparable with results from the representative conventional algorithms of eddy detection. Geostrophic velocities of the eddy were estimated from the dipping seismic reflections under the assumptions of approximate isopycnals and geostrophic balance. Measured water properties including sea surface temperature, salinity, and chlorophyll revealed that eddy translation transports coastal water to the pelagic regions. Structures synthesized from CTD profiles that sampled an earlier eddy suggest that thin striae around the base might be a common feature in Gulf of Alaska eddies.

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Section: Discussionmentioning

confidence: 99%

Section: Citationmentioning

confidence: 99%

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Seismic observations from a Yakutat eddy in the northern Gulf of Alaska

2014

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“…Reflection coefficients were computed for the seismic frequency range 50–500 Hz, using equation and following the method detailed in section 3. Results plotted in Figure b show that the thermocline reflection coefficients vary between −87 and −59 dB, and span the range of the typical reflectivity of oceanographic structures [e.g., Holbrook et al ., ; Mirshak et al ., ]. These reflection coefficients are low compared to those typically found in the solid Earth (−40 dB) and at the seafloor (−20 to −8 dB) [ Telford et al ., ].…”

Section: Seismic Imaging Of Shallow Oceanographic Structuresmentioning

confidence: 99%

Seismic reflection imaging of shallow oceanographic structures

et al. 2013

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[1] Multichannel seismic (MCS) reflection profiling can provide high lateral resolution images of deep ocean thermohaline fine structure. However, the shallowest layers of the water column (z < 150 m) have remained unexplored by this technique until recently. In order to explore the feasibility of shallow seismic oceanography (SO), we reprocessed and analyzed four multichannel seismic reflection sections featuring reflectors at depths between 10 and 150 m. The influence of the acquisition parameters was quantified. Seismic data processing dedicated to SO was also investigated. Conventional seismic acquisition systems were found to be ill-suited to the imaging of shallow oceanographic structures, because of a high antenna filter effect induced by large offsets and seismic trace lengths, and sources that typically cannot provide both a high level of emission and fine vertical resolution. We considered a test case, the imagery of the seasonal thermocline on the western Brittany continental shelf. New oceanographic data acquired in this area allowed simulation of the seismic acquisition. Sea trials of a specifically designed system were performed during the ASPEX survey, conducted in early summer 2012. The seismic device featured: (i) four seismic streamers, each consisting of six traces of 1.80 m; (ii) a 1000 J SIG sparker source, providing a 400 Hz signal with a level of emission of 205 dB re 1 mPa @ 1 m. This survey captured the 15 m thick, 30 m deep seasonal thermocline in unprecedented detail, showing images of vertical displacements most probably induced by internal waves.

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“…The physical basis for seismic oceanography has been well established by several studies that combined seismic surveys with simultaneous in situ measurements of temperature, salinity, and density (Nakamura et al 2006;Nandi et al 2004;Sallar es et al 2009): weak, but clear, reflections from within the water column come from vertical changes in (primarily) sound speed and (secondarily) density. Over the past few years, numerous studies have demonstrated the ability of seismic imaging to detect and map major features in the ocean, including fronts (Holbrook et al 2003;Mirshak et al 2010;Sheen et al 2009), internal waves (Holbrook and Fer 2005;Krahmann et al 2008), eddies and warm-core rings (Biescas et al 2008;Ruddick et al 2009;Yamashita et al 2011), thermohaline staircases (Biescas et al 2010;Fer et al 2010), lee waves (Eakin et al 2011), and internal tide beams ). SO has several unique capabilities, including the ability to image fine structure over large sections of the ocean and to full ocean depth, provided that sufficient fine structure is present to produce reflections.…”

Section: Introductionmentioning

confidence: 99%

Estimating Oceanic Turbulence Dissipation from Seismic Images

Holbrook

Fer

Schmitt

et al. 2013

Journal of Atmospheric and Oceanic Technology

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Seismic images of oceanic thermohaline finestructure record vertical displacements from internal waves and turbulence over large sections at unprecedented horizontal resolution. Where reflections follow isopycnals, their displacements can be used to estimate levels of turbulence dissipation, by applying the Klymak-Moum slope spectrum method. However, many issues must be considered when using seismic images for estimating turbulence dissipation, especially sources of random and harmonic noise. This study examines the utility of seismic images for estimating turbulence dissipation in the ocean, using synthetic modeling and data from two field surveys, from the South China Sea and the eastern Pacific Ocean, including the first comparison of turbulence estimates from seismic images and from vertical shear. Realistic synthetic models that mimic the spectral characteristics of internal waves and turbulence show that reflector slope spectra accurately reproduce isopycnal slope spectra out to horizontal wavenumbers of ;0.04 cpm, corresponding to horizontal wavelengths of 25 m. Using seismic reflector slope spectra requires recognition and suppression of shot-generated harmonic noise and restriction of data to frequency bands with signal-to-noise ratios greater than about 4. Calculation of slope spectra directly from Fourier transforms of the seismic data is necessary to determine the suitability of a particular dataset to turbulence estimation from reflector slope spectra. Turbulence dissipation estimated from seismic reflector displacements compares well to those from 10-m shear determined by coincident expendable current profiler (XCP) data, demonstrating that seismic images can produce reliable estimates of turbulence dissipation in the ocean, provided that random noise is minimal and harmonic noise is removed.

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Coincident reflection images of the Gulf Stream from seismic and hydrographic data

Cited by 16 publications

References 11 publications

Seismic observations from a Yakutat eddy in the northern Gulf of Alaska

Seismic observations from a Yakutat eddy in the northern Gulf of Alaska

Seismic reflection imaging of shallow oceanographic structures

Estimating Oceanic Turbulence Dissipation from Seismic Images

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