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Acoustic inversion of the cold water filaments off the Southwest coast of Portugal

P. Felisberto and S.M.Jesus ,
ISR-Universidade do Algarve, 8005-139 Faro, Portugal
P. Relvas
CIMA - Universidade do Algarve, Portugal

Comments: download pdf file .
Ref.: Proc. OCEANS MTS/IEEE ' 2007, (ISBN:), p., Vancouver, Canada,  2007. (to appear)

Abstract : Cold water filaments have important implications in the biological and chemical exchanges between the coastal and offshore ocean. The Cape Sao Vicente area in the Southwest coast of Portugal is a well know region where such phenomena are observed.
Recently, a multidisciplinary project called "Acoustic Tomography Monitoring System" (ATOMS), involving oceanographers and acousticians was conducted in order to characterize the filament structure in the water column. The ATOMS sea trial took place from 22th to 25th October 2004, a season when these phenomena are unlikely. However, satellite images of sea surface temperature (SST) showed the presence of a relatively well formed cold water filament. The SST images where used to limit the area of interest, which is approximately a square box with a side of 100km. A set of 42 CTD casts covering the upper 400m of the water column allowed to find an image of the vertical structure of the cold water filament. Due to weather and technical conditions during the project sea trial, only oceanographic measurements were performed (including currents and chemical water analysis).

Although, real acoustic data lacks, the filament was fairly well characterized by the oceanographic measurements. These at sea collected data together with archival data from the NODC database, and bathymetric data, allowed to find a realistic scenario of the 3D temperature distribution in the area, including layers bellow 400m, until the deepest point in the area around 2000m. Archival data of temperature profiles suggest the occurrence of other important oceanic phenomena in deeper water layers, such as the subduction of warm Mediterrenean water, that should also influence the acoustic propagation. With the help of forward acoustic modelling the significance and signature of the individual oceanographic phenomena on the acoustic propagation, regarding different sampling strategies of the area by acoustic means is investigated.

These preliminary studies on acoustic propagation allowed to develop strategies to settle the main problem addressed by this work: invert the cold water filament structure by acoustic means in a complex environment where acoustic propagation is affected also by other important oceanic and bathymetric features. Matched-field and ray tracing based tomography techniques are used in the inversion procedures.

Since, the objective of this work, is to evaluate the ability to perform a 3D characterization of vertical structure of the ocean, a minimal transmit-receive acquisition composed of a suspended source from a ship and a drifting vertical array, is assumed. The ship moves around the borders of the interest area emitting acoustic signals at predetermined locations. Thus, the 3D structure of the filament is obtained from a set of 2D inversions. To keep realistic assumptions it is considered that an SST image of the area is available to define the area of interest. Also it is assumed that temperature profiles acquired by some conventional mean are performed at the acoustic array location and at the locations of acoustic signal emissions. The planned sampling strategies and necessary acoustic equipment to resolve such oceanic features is discussed having in mind future sea trials.

ACKNOWLEDGMENT: this work was partially supported by FCT project ATOMS - POCTI/MAR/15296/1999.

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