Laurent Guillon

Sound speed profile characterization by the image source method

This paper presents the first results of an imaging technique that measures the geoacoustic structure of a seafloor in shallow water areas. The devices used were a broadband (100 Hz-6 kHz) acoustic source towed by a ship and a vertical array. Among all the acoustic paths existing in the water column, two are used: the direct one and the seabed-reflected one, the latter being composed of the reflections from the seafloors surface as well as that from each buried layer. Due to the good time resolution of the signal and to the short range configuration, the reflected signal can be modeled as a sum of contributions coming from image sources relative to the seabed layers. The seabed geometry and the sound speed profile can then be recovered with the detection and localization of these image sources. The map of the image sources is obtained by a function that combines back-propagation of signals and knowledge of the emitted pulse. The thickness and sound-speed of each layer is finally obtained by a position analysis of the image sources. The results obtained by this data-driven algorithm on both at-sea and synthetic data are satisfactory.

Range dependent sediment sound speed profile measurements using the image source method

This paper presents a range dependent sediment sound speed profile measurement obtained using the image source method. This technique is based on the analysis of the seafloor reflected acoustic wave as a collection of image sources which positions are linked with the thick-nesses and the sound speed of the sediment stack. The data used were acquired by the NURC in 2009 during the Clutter09 experiment. The equipment used was an autonomous undersea vehicle towing a 1600-3500 Hz frequency band source and a 32 m horizontal line array of 32 hydrophones at 12 m above the seabed. Under the assumption of locally range independent seabed properties, the moving horizontal array provides successive range independent sediment sound speed profiles along a track to obtain the range and depth dependent structure of the seafloor. Two key steps include recovery of the time-varying unknown array shape from the data and spatial filtering of the successive sound speed profiles. A comparison of the image source method result and seismic data along nearly the same 14 km track indicates that the seabed stratigraphy is correctly mapped by this method.

T-wave generation and propagation: a comparison between data and spectral element modeling.

T-waves are underwater acoustic waves generated by earthquakes. Modeling of their generation and propagation is a challenging problem. Using a spectral element code-SPECFEM2D, this paper presents the first realistic simulations of T-waves taking into account major aspects of this phenomenon: The radiation pattern of the source, the propagation of seismic waves in the crust, the seismic to acoustic conversion on a non-planar seafloor, and the propagation of acoustic waves in the water column. The simulated signals are compared with data from the mid-Atlantic Ridge recorded by an array of hydrophones. The crust/water interface is defined by the seafloor bathymetry. Different combinations of water sound-speed profiles and sub-seafloor seismic velocities, and frequency content of the source are tested. The relative amplitudes, main arrival-times, and durations of simulated T-phases are in good agreement with the observed data; differences in the spectrograms and early arrivals are likely due to too simplistic source signals and environmental model. These examples demonstrate the abilities of the SPECFEM2D code for modeling earthquake generated T-waves.

Image source detection for geoacoustic inversion by the Teager-Kaiser energy operatora)

This letter presents an improvement of the image source method for geoacoustic inversion. The algorithm is based on the Teager-Kaiser energy operator which amplifies the discontinuities in signals while the soft transitions are reduced. This property is exploited for accurate detection of time arrivals and thus for location of the image sources. The effectiveness of the method is shown on both synthetic and real data and the inversion results are, overall, in good agreement with ground truth and other inversion results with a significant reduction of computation time.

Spherical wave reflection in layered media with rough interfaces: Three-dimensional modeling.

In the context of sediment characterization, layer interface roughnesses may be responsible for sound-speed profile measurement uncertainties. To study the roughness influence, a three-dimensional (3D) modeling of a layered seafloor with rough interfaces is necessary. Although roughness scattering has an abundant literature, 3D modeling of spherical wave reflection on rough interfaces is generally limited to a single interface (using Kirchhoff-Helmholtz integral) or computationally expensive techniques (finite difference or finite element method). In this work, it is demonstrated that the wave reflection over a layered medium with irregular interfaces can be modeled as a sum of integrals over each interface. The main approximations of the method are the tangent-plane approximation, the Born approximation (multiple reflection between interfaces are neglected) and flat-interface approximation for the transmitted waves into the sediment. The integration over layer interfaces results in a method with reasonable computation cost.

Bayesian Geoacoustic Inversion With the Image Source Method

This paper develops a Bayesian approach to the image source method (ISM) for efficient inversion of seabed reflection data to estimate geoacoustic parameters and uncertainties. Based on the representation of layered seafloor-reflected signals by image sources, ISM is a very efficient method which provides the local sound-speed profile (SSP) of the sediment structure. It is a two-step method: first, the image sources are detected and localized from the recorded signals, and second, from these locations, the thickness and sound speed of each sediment layer are estimated from the Snell-Descartes law of refraction. This work focuses on the definition and construction of the image sources with a distinction between real and virtual image sources which has consequences on the uncertainties of ISM. The localization of the image sources is performed within a Bayesian formulation based on sampling the posterior probability density to estimate the median SSP and uncertainties. The algorithm is tested first on synthetic data, with results in excellent agreement with Bayesian travel-time inversion but a much lower computational cost. Finally, the Bayesian ISM is applied to at-sea data measured in the Scattering And ReverberAtion from the sea Bottom (SCARAB) experiment, which took place near Elba Island off the west coast of Italy in 1998, and the resultant sediment SSP agrees well with previous results of other geoacoustic inversion methods.

High-frequency sediment sound speed and attenuation measurements during TREX13 (Target and Reverberation Experiment 2013) with a new portable velocimeter

During the Target and Reverberation Experiment 2013 (TREX13), high-frequency measurements of sediment sound speed and attenuation were collected throughout the experiment site. These measurements were performed using the INSEA, a diver-portable array of sources and receivers conceived and developed by French companies in collaboration with research institutions. During each deployment of the instrument, the INSEA was inserted 10–15 cm into the sediment and narrow-band pulses covering the 70 to 350 kHz range were transmitted through the sediment. The sound speed is determined from the time-of-flight and attenuation is determined from the amplitude ratio of the transmissions through the sediment and through the water. The variability of the TREX13 site made it possible to collect data in several different sediment types including mud, silty-sand, and sand sediments each with low to high concentrations of shells. In addition to the acoustic measurements, diver cores and shell samples were also collected. The...

Cross-Spectral Analysis of Midfrequency Acoustic Waves Reflected by the Seafloor

Direct path measurements of a single-bottom interacting path on a vertical array are used to probe the seabed structure. The phase of the cross-spectrum, commonly used in engineering acoustics, permits examination of the importance of subbottom paths. When the cross-spectral phase is linear with frequency it implies that source to receiver propagation is dominated by a single path. A linear cross-spectral phase would also satisfy the linear seabed reflection coefficient phase approximation sometimes employed in forward modeling and geoacoustic inversion approaches. Shallow water measurements of the cross-spectrum, however, evidence a strongly nonlinear phase, below about 1500 Hz at one site, and 600 Hz at another site, implying that: 1) the subbottom structure plays an important role (i.e., a seabed half-space approximation would be inappropriate); and 2) the linear reflection phase approximation would be violated at those frequencies.

On the inversion of sediment density profile by the image source method

Image Source Method is a recently developed method for geoacoustic inversion. Under the Born approximation, the reflection of a spherical wave above a stratified seafloor is modeled as the simultaneous emission of image sources, symmetric of the real one relatively to the layers. These image sources are detected and located by the use of Teager-Kaiser Energy Operator (TKEO), which amplifies sudden changes in signal amplitudes, and by a triangulation scheme. Time and arrival angle of the recorded signals coming from the image sources are the input values of the inversion algorithm which gives the thickness and sound speed of each detected sediment layer by the use of Snell-Descartes laws. The objective of the present work is to extend this method to the inversion of the sediment density profile. To this end, one supplementary data is required. Experimental amplitudes corresponding to arrival times of the reflected signals, detected by TKEO, are compared to theoretical ones computed by a numerical evaluation of the Sommerfeld integral. Having first inverted sound speed profile and neglecting absorption coefficient, density is the only remaining parameter and thus can be obtained. The effectiveness of this method is tested on both synthetic and real data.

Very low frequency wave propagation numerical model

This paper shows preliminary results of modeling T waves propagation (acoustic waves generated by underwater earthquakes). These waves can be used to detect and localize low magnitude earthquakes. The computer code SPECFEM2D, which uses the spectral finite element approach, is selected among the large number of methods and codes available to perform the simulations. This choice was based on its capability to model the fluid/solid media coupling, and to model precisely a seismic source by considering its moment tensor. In the first set of simulations we have tested the influence of environmental parameters such as the bathymetry and the vertical sound speed profile on the propagation of T-waves. According to these simulations, it seems that the effect of bathymetry is much more significant than that of the sound speed profile. The next set of simulations concerned an actual seismic event, whose location and moment tensor are listed in seismic catalog. We have found good agreement between the simulated and ...