Geophysical seafloor exploration with a towed array in shallow water

Contract: MAS2-CT920022

Sérgio M. Jesus sjesus@ualg.pt
UCEH - Universidade do Algarve,
Campus de Gambelas,
PT-8000 Faro, Portugal
A. Caiti andy@dist.unige.it
DIST - Universitá di Génova
Via Opera Pia, 13
IT-16145, Génova, Itália
H. Zambujo
Instituto Hidrográfico,
Rua das Trinas 49,
PT-1296 Lisboa, Portugal
A.Kristensen (Associated Partner)
SACLANT Undersea Research Centre
Viale San Bartolomeo, 400
IT-19135 La Spezia, Italy

Motivation

The geophysical properties of marine sediments just beneath the seafloor are of great importance for modelling of coastal areas. Accurate knowledge of these properties allows for correct initialization and testing of environmental models and increase their capability for predicting changes in the morphology of the coastal line. Moreover, a geophysical description of shallow marine sediments is a requirement in a large number of applications, such as environmental monitoring, ecological studies, underwater acoustics, geotechnical exploration and soil stability testing.

Objectives

The project aimed at the study and development of a towed remote-sensing acoustic instrumentation able to quantitatively measure in a survey fashion the geophysical/geoacoustic properties of the seafloor in shallow waters (coastal areas and continental platforms). The instrumentation consists in a low frequency acoustic source and an horizontal array of acoustic receivers. Both source and receivers are towed from the the same platform. The source is transmitting pure tones at selected frequencies in continuous wave (cw) mode. Both source and receivers are submerged at a certain depth below the sea surface. The measured acoustic field at the receiver positions is exploited to estimate the geophysical properties of the seafloor sediments by using appropriate inversion algorithms.

The specific objective of the project were defined as follows:

• to determine the optimal configuration and mode of operation of the above described instrumentation, with the objective of being able to resolve geophysical structures within the sediment to scales of 0.5 - 1 meter, down to at least 30 meters from the seafloor surface; the optimization had to keep into account the constraints imposed by the operation in shallow water of the instrumentation proposed;

• to design computationally efficient data inversion techniques that rely on the optimal geometry;

• to assemble and test an experimental towed array localization system;

• to conduct a feasibility test at sea of the proposed methodology/instrumentation;

• to assess the merits and drawbacks of the proposed system, and come with recomendation as for design parameters and mode of operation of a commercial survey system

Results

The project involved the interaction of expertise coming from system engineering, signal processing, functional optimization, acoustic modelling and geophysics. It comprised a theoretical study with simulated data, the design and assembly of a non-acoustic positioning system on an existing acoustic instrumentation, a major sea trial for data collection, and a validation study on field data. The main result of the project is the conclusion that the proposed methodology is indeed feasible to obtain a detailed and spatially localized estimate of the seabottom structure with a considerable saving in ship time and cost of the operations required. The more detailed findings of the project can be resumed as follows:

• a towed array of 156m length, with 40 receivers spaced at 4 meters from each other is able to collect data sufficient for the estimation of sea bottom geophysical properties; the length of the array is such to allow its operation in shallow water with minimum navigation risk; source and receivers should be towed at about mid water depth, and with about 10 meters difference in depth between them; in any case, the towed instrumentation should stay below the thermocline; in the sea trial the experimental configuration has been of 40m depth for the source, 50-60m depth for the receiver array, a total distance of 685m between the towing ship and the tail of the array, all this in 120m deep waters;

• cw signals in the frequency range from 100 to 200 Hz were experimentally shown able to resolve structures with thickness of 3 meters, and with penetration up to 20 meters; use of lower frequencies can increase the penetration ability of the method, while higher frequencies will enhance resolution; incoherent broadband summation of different signals did not enhance the performance;

• position of the receiving sensors has to be known with an accuracy of lambda/2 in range and lambda/5 in depth (lambda being the acoustic wavelength); a system measuring the deformation of the array shape under tow has to be included in the instrumentation, and it is critical for the success of the estimate; the system developed and tested in the project, composed by non-acoustic sensors as tiltmeters, compasses and pressure gauges can measure in real time the array deformation with the required accuracy and without interfering with the acoustic measurement;

• data inversion and bottom parameter estimation require the use of global search methods; genetic algorithms and neural networks were tested and proved able to come with correct estimates in a variety of environments with simulated data; with field data, both methods produced estimates in agreement with an independent bottom model of the same area obtained from standard instrumentation (coring, seismic surveys, geophone stations); neural network inversion allows for a great reduction in the number of computation required; genetic algorithms allow for a more robust inversion, and can take into account in a much easier way the changes in the environment (like sloping bottoms, etc.).

Conclusion

The towed array system for geophysical exploration has been proved successful in operation at sea and in the analysis of the experimental data. A great wealth of recomendation and results are now available for the application of such a system in shallow surveys. These results can be considered as a fesibility test of the approach, and lead towards the development of easier-to-use and less expensive systems for seafloor exploration.

Publications

Journal papers

• [J1] S.M. Jesus and A. Caiti, ``Estimating geoacoustic bottom properties from towed array data'', (abstract), Journal of Computational Acoustics, (gziped ps file), Vol.4, No. 3, p.273-290, 1996.
• [J2] P. Felisberto and S.M. Jesus, ``Towed array beamforming during ships' maneuvering'', (abstract), IEE Proc. Radar, Sonar and Navigation, (gziped ps file), vol. 143, no. 3, p. 210-215, 1996.
• [J3] A. Caiti and S.M. Jesus, ``Acoustic estimation of seafloor parameters: a Radial Basis Functions approach'', (abstract), Journal of the Acoustical Society of America, (gziped ps file), Vol. 100(3), p. 1473-1481, 1996.
• [J4] A. Caiti, S.M. Jesus and A. Kristensen, ``Geoacoustic seafloor exploration with a towed array in a shallow water area of the Strait of Sicily'', (abstract), IEEE Journal of Oceanic Engineering, (gziped ps file), Vol. 21, No. 4, pp. 355-366, 1996.

Conferences with proceedings

• [C1] S.M. Jesus ``A sensitivity study for full-field inversion of geoacoustic data with a towed array in shallow water'' Full field Inversion Methods in Ocean and Seismo-Acoustics, O. Diachock, A. Caiti, P. Gerstoft and H. Schmidt (eds.), Kluwer, Dordrecht, 103-108 (1995).
• [C2] A.Caiti, T. Parisini and R. Zoppoli, ``Seafloor parameters estimation: approximating the inverse map through RBF networks'', Full field Inversion Methods in Ocean and Seismo-Acoustics , O. Diachock, A. Caiti, P. Gerstoft and H. Schmidt (eds.), Kluwer, Dordrecht, 177-182 (1995).
• [C3] S.M. Jesus ``A sensitivity study for full-field inversion of geoacoustic data with a towed array in shallow water'', 2nd European Conf. on Underwater Acoustics, Copenhagen, L. Bjorno (ed.), 899-904 (1994).
• [C4] P. Gerstoft and A.Caiti, ``Acoustic estimation of bottom parameters: error bounds by local and global methods'', 2nd European Conf. on Underwater Acoustics, Copenhagen, L. Bjorno (ed.), 887-892 (1994).
• [C5] S.M. Jesus and M.C. Jesus, ``On bottom properties estimation with a towed array in shallow water'', 2nd Conf. on Theoretical and Comput. Acoustics, Hawaii, USA, (1995).
• [C6] S.M. Jesus and A. Caiti, ``Geophysical seafloor exploration with a towed array in shallow water'', 2nd MAST Days and EUROMAR Market Conference, Sorrento, Italy, (1995).
• [C7] O. Lotsberg and S.M. Jesus, ``Matched-field inversion of geoacoustic properties from towed array data in shallow water'', 3rd European Conf. Underwater Acoustics, Heraklion, Greece, (1996).
• [C8] P. Felisberto and S.M. Jesus, ``Towed array plane and non-planewave beamforming under ship's maneuvering'', 3rd European Conf. Underwater Acoustics, Heraklion, Greece, (1996).
• [C9] A. Caiti, S.M. Jesus and A. Kristensen, ``Geoacoustic seafloor exploration with a towed array in a shallow water area of the Strait of Sicily'', to be presented at OCEANS'96 Conference, Ft. Lauderdale, USA, 23-26 September, (1996).

Other documents (reports, technical reports, internal publications,...)

• [R1] O. Rodriguez, ``Modelos de Propagação Acústica Submarina: comparação de resultados com a solução analítica do problema de 3 camadas'', Provas de Aptidão Científica (MsC Thesis), UCEH - Universidade do Algarve, (1996).
• [R2] P.S. Felisberto, ``Impacto da deformação de uma antena horizontal na direccionalidade do campo acústico submarino'', Provas Públicas (MsC Thesis), EST - Universidade do Algarve, (1995).
• [R3] E.S. Dias, ``Calculating the shape of a towed acoustic array'', Anais do Instituto Hidrográfico, 13, 83-90, (1995).

last update on August 20, 1997