X. Demoulin

Neural Direct Approaches for Geoacoustic Inversion

This paper presents different neural network approaches for geoacoustic inversion. The basic idea of neural inversion is to approximate the inverse function from a set of behaviors, i.e. relations between acoustic fields and geoacoustic parameters. In this work, such methods have been applied in two different forms: a global approach which aims to estimate all parameters from all data, and a hierarchical approach in which the most sensitive parameters are estimated before the least sensitive. The methods are tested using synthetic data. Statistical results, as well as benchmark results show that such approaches are efficient and have similar performances.

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.

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...

Dynamics of Acoustic Propagation Through a Soliton Wave Packet: Observations from the Intimate’96 Experiment

Experimental observations of acoustic propagation through a Soliton Wave Packet (SWP) show an abnormally large attenuation over some frequencies, that was found to be significantly time dependent and anisotropic. Nevertheless, by considering the problem of signal attenuation, the approach used in most of the studies can be considered as “static” since no additional effects were taken into account as a SWP evolves in range and time. Hydrographic and acoustic data from the INTIMATE’96 experiment clearly exhibit traces of the presence of soliton packets, but in contrast with known observations of attenuation, its frequency response also reveals a sudden increase of signal amplitude, which may be due to a focusing effect. This signal increase coincides with a significant peak found in current and temperature records. However, the correlation of both acoustic and hydrographic features is difficult to support due to the different time scales between the rate of hydrographic data sampling and the rate of signal transmissions. To study the possibility that a SWP could be responsible for the observed signal increase, the INTIMATE’96 hydrographic data was used to generate physically consistent distributions of “soliton-like” fields of temperature and sound velocity, which were used as input for a range-dependent normal-mode model; it was found that for a particular soliton field, the set of “dynamic” (i.e., range-dependent and time-dependent) acoustic simulations reveals an acoustic signature similar to that observed in the data. These results contribute to a better understanding of underwater propagation in shallow-water coastal environments and therefore provide a potential basis for range-dependent temperature and sound-speed inversions.