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Seismic Oceanography: Imaging Oceanic Finestructure with Reflection Seismology

W. Steven Holbrook*

INTRODUCTION: OLD DOG, NEW TRICK

Seismic oceanography (SO) is a new approach to studying interior ocean structure by applying an old tool—marine seismic reflection profiling. Reflection seismology is a standard technique used in industry and academia for imaging the solid earth using reflected sound waves. We have recently discovered that finestructure—temperature variations at vertical scales of meters to tens of meters caused by internal waves, intrusions, and mixing processes—can be imaged quite well with seismic reflections at 10-150 Hz—the frequency range commonly used in seismic reflection profiling. Our results (e.g., Holbrook et al. 2003) show spectacular images of thermohaline finestructure in the ocean (Figure 1); features such as intrusions, internal waves, and mesoscale eddies are clearly visible. These images show the ocean in a way it has never been seen before.

The past several years have seen rapid progress in defining this new tool. We have achieved a basic physical understanding of the origin of the acoustic reflections (predominantly temperature finestructure at the 10 m vertical scale). The physical basis for SO is the presence of “boundaries” in the ocean caused by strong vertical gradients in either density or sound speed. The strength of reflections from a “sharp” discontinuity can be described by the “reflection coefficient,” R= (ρ2c2 – ρ1c1)/(ρ2c2 + ρ1c1),

*

University of Wyoming



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Seismic Oceanography: Imaging Oceanic Finestructure with Reflection Seismology W. Steen Holbrook* INTRODUCTION: OLD DOG, NEW TRICK Seismic oceanography (SO) is a new approach to studying interior ocean structure by applying an old tool—marine seismic reflection pro- filing. Reflection seismology is a standard technique used in industry and academia for imaging the solid earth using reflected sound waves. We have recently discovered that finestructure—temperature variations at vertical scales of meters to tens of meters caused by internal waves, intrusions, and mixing processes—can be imaged quite well with seis- mic reflections at 10-150 Hz—the frequency range commonly used in seismic reflection profiling. Our results (e.g., Holbrook et al. 2003) show spectacular images of thermohaline finestructure in the ocean (Figure 1); features such as intrusions, internal waves, and mesoscale eddies are clearly visible. These images show the ocean in a way it has never been seen before. The past several years have seen rapid progress in defining this new tool. We have achieved a basic physical understanding of the origin of the acoustic reflections (predominantly temperature finestructure at the 10 m vertical scale). The physical basis for SO is the presence of “boundar- ies” in the ocean caused by strong vertical gradients in either density or sound speed. The strength of reflections from a “sharp” discontinuity can be described by the “reflection coefficient,” R= (ρ2c2 – ρ1c1)/(ρ2c2 + ρ1c1), * University of Wyoming 15

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15 OCEANOGRAPHY IN 2025 FIGURE 1 Image of a mesoscale eddy beneath the North Atlantic Front. Holbrook_Fig1.eps bitmap image where ρ and c represent density and sound speed, respectively, and the subscripts represent layers (layer 1 overlies layer 2). Because sound speed (i.e., temperature) dominates, to first order these images can be thought of as maps of dc/dz at vertical scales of ~10 m. Our group and others have produced fascinating images of finestruc- ture in numerous ocean settings, including fronts, Meddies (Figure 2), FIGURE 2 Image of a Meddy in the Gulf of Cadiz. The Meddy is visible as the prominent oval shape on the left side of the figure. Note the strong contrast in finestructure characteristics on either side of the Gorringe Bank (white protru- sion centered at 130 km). Image courtesy of Berta Biescas, Marine Technology Unit—CSIC (Spanish National Research Council).

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159 W. STEVEN HOLBROOK intrathermocline lenses, warm core rings, watermass boundaries, and thermohaline staircases, some of which raise unexpected questions about the processes controlling the distribution of oceanic finestructure. We have also shown that quantitative information on, for example, internal- wave spectra (Figure 3) and temperature can be gleaned from these data. These observations raise the intriguing possibility that seismic reflection profiling may become a tool of great usefulness to physical oceanogra- phers in observing and characterizing ocean structure and dynamics. Note that SO differs from traditional acoustic oceanography in several ways, including the dominant sound frequencies (and thus the resolu- tion), the targets, and the acquisition methods. Reflection seismology uses much lower frequencies (10-200 Hz) than traditional ocean acoustics. The resolution is thus lower (vertical resolution O(5m)), which means that our targets are fundamentally different: rather than scattering from mil- FIGURE 3 Horizontal wavenumber (Kx) spectra produced from seismic reflec- Holbrook_Fig3.eps tion images in the Norwegian Sea. Gray field is the GM76 tow spectra. Two sets bitmap image of reflectors were tracked: open-ocean reflectors, which show good agreement with GM76, and near-slope reflectors, which show enhanced internal wave energy levels. From Holbrook and Fer 2005.

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10 OCEANOGRAPHY IN 2025 limeter- or centimeter-scale objects (e.g., plankton), we record specular reflections from temperature and density finestructure. Specialized equip- ment is necessary at sea; sound sources are usually pneumatic sources that release high-pressure air into the ocean, and the reflected wavefield is recorded on a kilometers-long hydrophone streamer towed behind the vessel. ADvANTAGES AND DISADvANTAGES OF THE TECHNIqUE The principal advance that SO offers is the ability to track oceanic finestructure laterally at relatively high spatial resolution: the typical lateral sampling of seismic images is 6.25 m. These images provide, first and foremost, a means of “flow visualization” (to borrow Larry Armi’s description) akin to schlieren images, which show structural detail that can provide intuition into dynamical processes. Other advantages offered by reflection profiling as a complement to standard oceanographic mea- surements include the ability to simultaneously image large volumes of ocean, over full ocean depth, and the ability to do 3D and timelapse imag- ing. Especially when combined with in situ physical oceanography (PO) observations (either from expendables or from more detailed measure- ments), these images have the potential to add great value to traditional PO investigations of ocean mixing processes. The principal disadvantage of SO is that it cannot provide informa- tion where finestructure is weak or absent. This means, for example, that the method is ill-suited to imaging the abyss, where the low stratification prevents gradients in c and ρ of sufficient magnitude to produce acoustic reflections. WHAT MIGHT SO PROvIDE? We are now poised to make several significant advances in seismic oceanography. Two developments are particularly promising. First, we now have the means to create the first 3D and timelapse 3D (“4D”) images of oceanic finestructure, which enable 3D maps of, for example, internal wave energy. Second, recent work shows that reflection images have the potential to produce quantitative estimates of turbulence dissipation by applying the Klymak & Moum theory of horizontal wavenumber spectra. Because of the spatial density of such data, we have the possibility of pro- ducing maps of dissipation over large regions of the ocean. This approach is in its infancy and requires testing and truthing, but the potential appli- cations are obvious. SO is applicable to studying any process that creates, destroys,

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11 W. STEVEN HOLBROOK disrupts, or deforms finestructure in the ocean. Problems that can be addressed by SO thus include (but are not limited to): • Where does mixing occur—in particular, where are mixing hot- spots in the ocean? • How is boundary mixing influenced by critical slopes and sea- floor roughness? • How, and where, do fronts and eddies shed energy into the inter- nal wave field? • What are the lateral length scales of oceanic finestructure, and what controls these length scales? • How does isopycnal stirring create temperature variance in the ocean? • What are the 3D shapes of internal wave packets in the ocean? • What controls the generation of strong internal waves in places such as the South China Sea? SEISMIC OCEANOGRAPHY IN 2025 Predicting the role of SO in oceanography in 2025 is quite difficult. By that time, SO could either be a historical footnote or (one hopes) a widely used technique in oceanography. The technique has much promise in imaging (and thus mapping) processes that have an expression in tem- perature/density finestructure. Fulfilling that promise will require: • improved and continued collaboration and communication between the PO and seismology communities via workshops; • successful development and testing of techniques to invert low- frequency acoustic returns for oceanic properties of interest (tem- perature, density, internal wave energy, and turbulence); and • a willingness on the part of funding agencies to invest in this methodology by supporting focused field and laboratory work. A large extant database of seismic reflection profiles contains useful information that should be mined, but substantial progress needs joint PO/seismic field programs that collect state of the art data at the same time and place. RESOURCES Holbrook’s web page: http://www.steveholbrook.com/research/ seismic_oceanography/

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12 OCEANOGRAPHY IN 2025 Recent ESF-sponsored workshop on SO: http://www.cmima.csic. es/sow/ EU-GO project: http://www.dur.ac.uk/eu.go/ REFERENCE Holbrook, W.S., P. Páramo, S. Pearse, and R.W. Schmitt. 2003. Thermohaline Fine Structure in an Oceanographic Front from Seismic Reflection Profiling. Science. 301 (5634): 821- 824. Holbrook, W.S. and I. Fer. 2005. Ocean Internal Wave Spectra Inferred from Seismic Re- flection Transects. Geophysical Research Letters, 32: L15604, doi:10.1029/2005GL023733, 2005.