E. Coelho

An experimental demonstration of blind ocean acoustic tomographya)

Despite the advantages clearly demonstrated by ocean acoustic tomography (OAT) when compared to other ocean monitoring techniques, it suffers from several technical-related drawbacks. One is the requirement for rather expensive equipment to be maintained and operated at several locations in order to obtain sufficient source–receiver propagation paths to cover a given ocean volume. This paper presents the preliminary feasibility tests of a concept that uses ships of opportunity as sound sources for OAT. The approach adopted in this paper views the tomographic problem as a global inversion that includes determining both the emitted signal and the environmental parameters, which is a similar problem to that seen in blind channel identification and was therefore termed blind ocean acoustic tomography (BOAT). BOAT was tested on a data set acquired in October 2000 in a shallow-water area off the west coast of Portugal, including both active and passive (ship noise) data. Successful results show that BOAT is abl...

Filtering Drifter Trajectories Sampled at Submesoscale Resolution

In this paper, a variational method for removing positioning errors (PEs) from drifter trajectories is proposed. The technique is based on the assumption of statistical independence of the PEs and drifter accelerations. The method provides a realistic approximation to the probability density function of the accelerations while keeping the difference between the filtered and observed trajectories within the error bars of the positioning noise. Performance of the method is demonstrated in application to real data acquired during the Grand Lagrangian Deployment (GLAD) experiment in the Northern Gulf of Mexico in 2012.

Environmental inversion using high-resolution matched-field processing

This paper considers the inversion of experimental field data collected with light receiving systems designed to meet operational requirements. Such operational requirements include system deployment in free drifting configurations and a limited number of acoustic receivers. A well-known consequence of a reduced spatial coverage is a poor sampling of the vertical structure of the acoustic field, leading to a severe ill-conditioning of the inverse problem and data to model cost function with a massive sidelobe structure having many local extrema. This causes difficulties to meta-heuristic global search methods, such as genetic algorithms, to converge to the true model parameters. In order to cope with this difficulty, broadband high-resolution processors are proposed for their ability to significantly attenuate sidelobes, as a contribution for improving convergence. A comparative study on simulated data shows that high-resolution methods did not outperform the conventional Bartlett processor for pinpointing the true environmental parameter when using exhaustive search. However, when a meta-heuristic technique is applied for exploring a large multidimensional search space, high-resolution methods clearly improved convergence, therefore reducing the inherent uncertainty on the final estimate. These findings are supported by the results obtained on experimental field data obtained during the Maritime Rapid Environmental Assessment 2003 sea trial.

Tidal Effects on Source Inversion

In the summer of 1996, an experiment was conducted off the coast of Portugal to study the effects of internal tides on sound propagation. This experiment—called INTIMATE’96 (Internal Tide Investigation by Means of Acoustic Tomography Experiment)—has provided a great deal of insight about the variability of pulse transmission over space and time. In contrast to a common view of shallow-water propagation as complicated and unpredictable, we find a steady pattern of echoes. The echo-pattern stretches and shrinks in a systematic way with the tides and allows us to infer the components of the first few oceanographic modes. We also used the echo-pattern to track the source over a period of several days. During this period the isotherms in the ocean wavered by 20 m as a result of the tides, providing a challenge for model-based tracking. We will discuss these acoustic results with emphasis on the source tracking.

NONLINEAR SOLITON INTERACTION WITH ACOUSTIC SIGNALS: FOCUSING EFFECTS

The problem of nonlinear interaction of solitary wave packets with acoustic signals has been intensively studied in recent years. A key goal is to explain the observed transmission loss of shallowwater propagating signals, which has been found to be strongly time-dependent, anisotropic, and sometimes exhibited unexpected attenuation vs. frequency. Much of the existing literature considers the problem of signal attenuation in a static environment, without considering additional effects arising from groups of solitons evolving both in range and time. Hydrographic and acoustic data from the INTIMATE’96 experiment clearly exhibit the effects of soliton packets. However, in contrast with reported observations of signal attenuation, the observed transmission loss shows a pronounced signal enhancement that behaves like a focusing effect. This focusing is correlated with peaks in current, temperature, and surface tide. That correlation suggests that the nonlinear interaction of solitary wave packets with acoustic signals can lead to a focusing of the signal. To clarify this issue, hydrographic data was used to generate physically consistent distributions of “soliton-like” fields of temperature and sound velocity. These distributions were then used as input for a range-dependent normal-mode model. The results strongly support the hypothesis that the soliton field causes the observed signal enhancement.

Experimental testing of the blind ocean acoustic tomography concept

Acoustic focalization is a well known concept that aims at estimating source location through the adjustment of multiple environmental parameters. This paper uses the same concept for inverting water column sound speed in a blind fashion, where both source location and source emitted waveform are not known at the receiver — that is Blind Ocean Acoustic Tomography (BOAT). The results obtained with BOAT, using ship noise data received on a vertical line array in a shallow water area off the coast of Portugal, show that it is indeed possible to obtain reliable joint estimates of source location and water column sound speed. During that process, it was shown that source range and depth, and Bartlett power, where good indicators of the degree of focus of the model being used.

Estimating uncertainties in bio-optical products derived from satellite ocean color imagery using an ensemble approach

We propose a methodology to quantify errors and produce uncertainty maps for satellite-derived ocean color bio-optical products using ensemble simulations. Ensemble techniques have been used by the environmental numerical modeling community to propagate initialization, forcing, and algorithm error sources through-out the full simulation process, but similar approaches have not yet been applied to satellite optical properties. Uncertainties in retrievals of bio-optical properties from satellite ocean color imagery are related to a variety of factors, including sensor calibration, atmospheric correction, and the bio-optical inversion algorithms. Errors propagate, amplify, and intertwine along the processing path, so it is important to understand how the errors cascade through each step of the analysis, to assess their impact and identify the main factors contributing to the uncertainties in the final products. Also, we are interested in producing short-term forecasts of the bio-optical property distributions, by coupling the satellite imagery with physical circulation models. So, in addition to the uncertainties in the satellite-derived bio-optical properties due to the above-mentioned factors, the uncertainties in the model currents used to advect the bio-optical properties add another layer of complexity to the problem. We outline these processes and present preliminary results for this approach.

Intimate ’96: Shallow water tomography in the Sea of the Condemned

As is well‐known, the tidal force of the moon and the sun can cause notable changes in the sea level. Besides this so‐called barotropic effect, the tidal force also drives internal waves in a daily rhythm. Thus, the internal wave spectrum is often dominated by a single component with perhaps 10 km from crest to crest. This ‘‘internal tide’’ tends to propagate toward shore and has its greatest height near the shelfbreak. As this tide propagates it modulates the surface duct and its acoustic signature is often seen in data. The Intimate ’96 experiment (conducted off the coast of Portugal) was specifically designed to acoustically image the internal tide with an eye toward a more precise understanding of its structure and acoustic impact. A towed source emitted chirps every 8 s for several days and the chirps were received on the SACLANTCEN portable array. The data show a textbook multipath structure with early refracted paths followed by some 30 distinct bottom and surface echoes which shift with the intern...

Internal tide impact measured by acoustic tomography experiment

The INTIMATE (internal tide impact measured by acoustic tomography experiment) project is devoted to the study of internal tides by use of acoustic tomography schemes. The first exploratory experiment was carried out in June 1996 on the continental shelf off the coast of Portugal using a towed broadband acoustic source (500–800 Hz) and a four‐hydrophone vertical array. Acoustic data were collected for 5 days, including legs where the source ship was moving and legs with the ship on station. Intensive environmental surveys (XBT, CDT, bottom and hull‐mounted ADCP, thermistor chain, bathymetry, geoacoustic characteristics of the sediments) were also conducted. The purpose of the presentation is to give the preliminary results of acoustic data processing and to evaluate the impact of internal tides on the acoustic propagation in a shallow water environment. Future works will deal with inversions in terms of sound speed and geoacoustic parameter estimation. [Work supported by SHOM‐IH‐UAL. The hydrophones array...

Overview of U.S. Navy Operational Oceanographic Models in Support of Acoustic Applications

The Naval Oceanographic Office operational global 1/8° Navy Coastal Ocean Model assimilates satellite and in‐situ data to produce daily 72‐hr forecasts. Output includes 3D fields of temperature, salinity, u‐ and v‐components of ocean currents at standard depth levels, and these support derived fields including sound speed and sonic layer depth. The global model provides initial/boundary conditions for nested regional models, primarily relocatable NCOM. The relocatable NCOM modeling system can be set up quickly for areas of interest, includes river and tidal forcing, and is forced with a high‐resolution atmospheric mesoscale model. Local and remote observations are incorporated into the models through the Navy Coupled Ocean Data Assimilation system, which assimilates sea surface temperature data from satellite, ships and buoys, profile data from floats and gliders, xbts, CTDs, fixed and drifting buoys as well as altimeter‐derived sea surface heights and ice concentration. In this presentation we will dis...