INternal TIde Measurements with Acoustic Tomography Experiments


[PRAXIS XXI contract 2/2.1/MAR/1698/95]

Sérgio M.
UCEH - Universidade do Algarve,
Campus de Gambelas,
PT-8000 Faro, Portugal
CMO - Service Hidrographique Oceanographique de la Marine,
Brest, France
Emanuel F. Coelho
Instituto Hidrográfico,
Rua das Trinas 49,
PT-1296 Lisboa, Portugal
Michael B. Porter
Centre of Applied Mathematics,
New Jersey Institute of Technology,
NJ Newark, USA


Today acoustics is one of the main tools for ocean exploration. Inspired by a technique from biomedical engineering, Ocean Acoustical Tomography (OAT) consists of the use of one or more acoustic sensors/sources strategically distributed in a volume of water and simultaneously emitting and receiving coded signals. The ``comparison'' of the emitted and received signals allows for the acoustical characterization of the environment in between the source(s) and the receiver(s). The experiments that have been performed to date, mainly in deep water, showed that it is possible to estimate: (1) the temperature over the water column, (2) the physical characteristics of the seabottom and (3) the velocity and direction of currents.

Due to the large water mass involved and its periodic nature, internal tides signatures have been frequently observed during ocean tomography experiments in the Bay of Biscay. In the continental shelf, internal tides are extremely important to the study of the coastal dynamics with a very special relevance close to the shelf break and to topographic irregularities like submarine canyons. This project aims at proving the concept of using vertical arrays of sensors and acoustic sources in the band 50 - 400 Hz for tomographic estimation of the oceanographic characteristics of internal tides on the continental shelf up to 200 m depth. During the project, areas of interest near the coast will be selected for at sea testing including the validation of the tomographic results with ground truth measurements. The conclusions of this study are hoped to bring a better knowledge of the internal tide phenomena and its impact on the coast dynamics. This should lead to improved prediction accuracy for coastal dynamical models and to a better management of existing resources. The results will also be important in providing more refined predictions of the impact of potential ecological accidents and climate changes.


Plan of activities

During INTIMATE the following important milestones are forseen:

INTIMATE'96 sea trial

Short description

The first small scale sea trial, INTIMATE'96, took place near the Nazaré site from 14 to 19 of June 1996.

Two research vessels were involved: the NRP ANDROMEDA and the BO D'ENTRECASTEAUX. BO D'ENTRECASTEAUX was towing a sound source and emmiting coded signals along pre-determined geometries. NRP ANDROMEDA was lying dead in the water nearby a 4 hydrophone vertical array receiving radio signals and recording the data. The receiving vertical array was loaned by courtesy of SACLANT Undersea Research Centre, La Spezia, Italy.

Ground truth measurements

A number of environmental measurements were performed simultaneously with the acoustic data transmissions:

Data analysis (see Ocean Acoustic Library for more results on line)

Superficial analysis of the CTD data at the vertical array location shows evidence of a approximately 12 hour period movement of the thermocline with an amplitude of about 10 m/s extending from 10 to 30 m depth.

Cross-correlograms of the received and emitted signals shows a very stable channel impulse response both for the flat bottom and range dependent transects.

Source localization using single hydrophone data and the full source frequency band has been possible for both the stationnary and the moving source events.

INTIMATE'98 sea trial

INTIMATE'98 toke place during July 1998 in the shallow water zone at the border of the continental platform SW from Brest, France. Two sources and two receiving arrays were employed along a number of transects for tomographic reconstruction. Three research vessels were involved: the ALCYON, the LAPEROUSE and the HMS ROEBUCK.

INTIMATE'99 sea trial

INTIMATE'99 toke place from 19 to 23 July 1999 in the Troia Peninsula, near Setubal, Portugal (see INTIFANTE'99 Sea Trial )

INTIFANTE'00 sea trial

INTIFANTE'00 is taking place from 9 to 29 October 2000 in the Tróia Peninsula, near Setubal, Portugal (see INTIFANTE'00 Sea Trial )

Publications and reports

last update on Jun 21, 2002