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Seismic Oceanography
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2012
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Melville
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Project Title: |
Seismic Oceanography of Agulhas Mixing |
Project Status: |
Submitted |
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Principal Investigator: |
Warren T. Wood, NRL |
Project Institution: |
NRL_SSC |
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Project ID: |
102743 |
Version #: |
1 |
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Date Submitted: |
12/10/2010 5:20:00 PM |
Created By: |
Warren T. Wood |
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Date Last Modified: |
12/10/2010 5:20:00 PM |
URI Serial #: |
None |
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Funding Agencies: |
NAVY/NRL - NRL 6.1 Base Funds - Funded |
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Summary of Field Work: |
Objective/Goals: The project goals are to quantify physical processes of diapycnal mixing across a major ocean front with a focus on mesoscale eddy stirring and cross-frontal water mass exchange by making use of new seismic oceanography methodology that provides high lateral resolution (order of 10 meters), full water column sections of isothermal fine-structure.
Motivation/Background: Diapycnal mixing is a significant mechanism for exchange of sub-tropical and sub-polar waters across major ocean fronts and is an important term in global thermohaline balances and for global circulation models. It is perhaps most important in the intersection regions of the sub-polar and sub-tropical frontal systems, as in, for example, the North Atlantic Current and Labrador Current, the Kuroshio and Oyashio, and the Agulhas and the Circumpolar Current System. Past work has shed some light on mixing mechanisms in these energetic regions, particularly on isopycnal exchange of water parcels, but, diapycnal mixing and eddy stirring quantification has been fundamentally limited by the point nature of turbulence microstructure measurements made from research ships.
The technique of seismic oceanography (SO) is capable of tracking individual “fingers” or “slabs” of water at lateral resolutions of meters to tens of meters over arbitrary distances and over the full water column; impractical tasks with XBT or CTD profiling. Data are acquired by recording controlled sound blasts, typically used for reflection seismic imaging of the sedimentary structures below the seafloor. SO has recently imaged intriguing fine-scale and mesoscale structures at major ocean fronts (Holbrook et al., 2003; Nakamura et al. 2006), and also shown to be useful in direct quantification (Páramo and Holbrook, 2005; Wood et al., 2008) of thermohaline fine structure. Past SO studies have been somewhat limited by the relative lack of coincidental physical oceanography measurements needed for more complete interpretation of the seismic results.
Approach: The Agulhas Return Current is a particularly good study region for this topic because it is characterized by very large horizontal temperature gradients (Lutjeharms and Valentine, 1984) and has considerable eddy activity (Lutjeharms, 1981). Thus the signal to noise will be maximized for seismic methods and there is considerable energy for intrusion and instability processes to mix sub-tropical waters with surrounding waters. In particular, the region identified in this current that would maximize these factors extends from just east of the Retroflection towards the east (roughly from 38-40°S and 20-28°E) and includes the first meander over the Agulhas Plateau. The exact positioning of the study area should be relatively compact to allow for adequate coverage with seismic tows taken at ship speeds of 4 knots and therefore will likely be limited to spatial coverage of an area on the order of 300 km by 300 km (approximately the eddy and meander scale of the system).
The approach will be to first use high-resolution tow-yo CTD (Scanfish) measurements to initially locate the Agulhas Front and any associated eddies. Then 6-8 long-line moorings will be deployed across and in the Agulhas Return Current with 500-1000 m range Acoustic Doppler Current Profilers (ADCPs) and temperature/salinity sensors to measure the barotropic and baroclinic streamfunction during the study period. High-resolution seismic measurements will then be taken simultaneously with additional Scanfish measurements if possible or with some switching between methods if required. The seismic measurements will provide full water-column images of temperature gradients at order of 10 m horizontal resolution and the Scanfish will provide 0-400 m casts of temperature, salinity, and other data at 500 m resolution and thus the two can be combined to invert and estimate isopycnal structure at very high resolution. XBTs will additionally be used during seismic tows for ground-truth temperature profiles. Finally at identified points of interest, dedicated full water column CTDs and micro-structure turbulence casts will be made to further quantify mixing parameters. Additionally, ship board ADCP and meteorological measurements would be useful for interpreting results.
We will also be collaborating with other projects in the Oceanography Division and thus gain access to the extensive remote sensing and numerical modeling products that are produced daily. For the cruise period we should be able to obtain, at a minimum, MODIS sea-surface-temperature and chlorophyll-a images, and high-resolution COAMPS meteorological and HYCOM ocean nowcast/forecast products.
Logistics:
Ship Days: We estimate that the minimum amount of ship-days needed to conduct the science work is 17 days at sea including the transit to and from the study site. Additional days would be of great benefit to the science of the project. 1 complete day of loading and 1complete day of offloading would be required in port.
Personnel: We will need space to embark 5 NRL scientists and 3 NRL technicians for the cruise work. A minimum of 3 additional collaborating scientists, technicians, and students will likely be needed to maintain 24 hour watches. We would certainly welcome many more collaborating participants than these, if additional berthing is available.
Equipment: NRL will provide the Scanfish tow-yo and dedicated winch, the microstructure profiler and dedicated winch, the long-line mooring equipment, and numerous XBTs. NRL does not own its own seismic system so one would need to be rented by NRL or provided by a collaborator. We would need to make use of the ship’s CTD system. Additionally, a shipboard meteorological measurement suite, a shipboard ADCP, and a flow-through water sampling system would be useful but not essential equipment.
Operations and Deck space requirements: The seismic reflection technique requires the towing of two pieces of gear, the streamer and the source. The streamer is an array of hydrophones and is essentially a 5-10 cm diameter cable deployed off the ship’s fantail. The streamer we use will likely be between 500 and 1500 m long. We prefer to use GI air guns as seismic sources, deployed off the fantail immediately adjacent to the streamer. Air guns are steel cylinder-shaped devices about 1 m long and 0.2 m in diameter, and can be deployed singly or in an array. They are powered by compressed air that is generated by a compressor system (approx. 1.5 x 1.5 x 1.5 m) sitting on the fantail and drawing power from the ship’s electrical system. Both the guns and streamer are towed about 2 m below the surface. A low fantail (less than 2 m) is ideal for seismic acquisition but larger freeboard is acceptable. A ship no smaller than 50 m long is likely needed.
The Scanfish is a towed body of dimensions 0.9 m x 0.26 m x 1.80 m. It is cabled to a dedicated hydraulic smart winch (54 kW, 1.3 m x 1.3 m x 2 m) that can automatically pay cable in and out to control depth. It is common for seismic systems to be towed with the streamer in the center and two lines of sources to either side (3 lines total). We would like to replace one line of guns with the Scanfish if this can be safely done on the particular ship used for the cruise. If this is not possible, than the Scanfish tows should be alternated with seismic tows. The microstructure profiler can deployed off the ship using a very small A-frame or crane. It is a free-falling package and has a very small winch which can assist with retrieval. The profiler is deployed when the ship is drifting or keeping station. At the beginning and end of the cruise, deck space will be needed for the deployment of 6-8 full ocean depth long-line moorings with applicable expendable anchors. XBTs and CTDs will be taken following typical practice. |
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Summary of Facility Requirements: |
We will require a ship capable of conducting operations in the rough seas expected over the Agulhas Plateau for 14 days on site (17 days total). We will need a small multichannel seismic system, and enough back deck space to launch and recover 2 deep-water moorings. |
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Summary of other requirements and comments: |
Multichannel Seismic equipment (2x205 cu. in. GI guns plus >600m streamer) with technicians. Also require cranes, A frame etc for launing and recovering deep sea moorings. |
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Type of Request: |
Primary Ship Use |
Request Status: |
Submitted |
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Request ID: |
1004222 |
Created By: |
Warren T. Wood |
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Date Last Modified: |
12/10/2010 5:20:00 PM |
Date Submitted: |
12/10/2010 5:20:00 PM |
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Year: |
2012
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Ship/Facility: |
Melville
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Optimum Start Date:
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1/7/2012 |
Dates to Avoid: |
Our objectives take us below about 40 degrees south where weather is most likely best in Dec-Mar. All other times are more likley to have bad weather |
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Earliest Start Date: |
1/7/2012 |
Multi-Ship Op: |
No |
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Latest Start Date: |
3/1/2012 |
Other Ship(s): |
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Operating Days Needed: |
Science Days |
Mob Days |
De-Mob Days |
Estimated Transit Days |
Total Days |
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14 |
1 |
1 |
3 |
19 |
Repeating Cruise?
(within same year) |
No |
Interval: |
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# of Cruises: |
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Description of Repeating cruise requirements: |
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Justification/Explanation for ship choice, dates,
conflicts, number of days & multi-ship operations: |
A Global Class ship is required for work in expected heavy seas. |
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Short Description of Op Area
for use in schedules: |
Agulhas Plateau |
| Description of Op Area: |
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| Op Area Size/Dia.: |
250 nm |
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Lat/Long |
Marsden Grid |
Navy Op Area |
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Beginning
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Ending
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Show Degrees Minutes |
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Foreign Clearance Required? |
Yes |
Coastal States:
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 Important Info on Foreign Research Clearances
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Are you or any member in your science party bringing in any science equipment items which are regulated for export by the International Traffic in Arms Regulations (ITAR) and/or the Export Administration Regulations (EAR)?
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No |
If yes, have you applied for the necessary permits through your export control office?
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No |
| Questions about ITAR/EAR regulations?
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Comments about foreign clearance requirements or
description of any other special permitting requirements
(e.g., MMPA, ESA, IHA, Marine Sanctuaries, etc.) |
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Requested Start Port |
Intermediate Port(s) |
Requested End Port |
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Cape Town, South Africa |
None |
Cape Town, South Africa |
Explanation/justification for requested
ports and dates of intermediate stops
or to list additional port stops |
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Important Info on Working in Foreign Ports
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Chief Scientist: |
Warren T. Wood, NRL |
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# in Science Party |
18 |
# of different science teams |
2 |
# Marine Technicians to be
provided by ship operator:
(include in science party total)
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4 |
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Explanation of Science Party Requirements and Technician Requirements |
We are expecting 2 seismic technicians and 2 marine mammal observers |
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 Dynamic Positioning | ADCP | Multibeam |  Seismic |
| Dredging/Coring/Large Dia. Trawl Wire | Stern A-frame | Fiber Optic (.681) | 0.680 Coax Wire |
| SCUBA Diving | Radioisotope use - briefly describe | NO Radioisotope use/Natural level work | Other Operator Provided Inst. - Describe |
| 0 PI-Provided Vans - briefly describe | MOCNESS | | |
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Explain Instrumentation or Capability
requirements that could affect choice
of ship in scheduling. |
This objective requires seismic gear 2 Gi guns and at least a 600m hydrophone streamer
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Explain Major Ancillary Facilities
Requirements and list description
and provider for "other" systems. |
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