Academic literature on the topic 'Passive underwater tomography'

Abstract. This paper is a review which briefly describes a selection of acoustic observation techniques and certain aspects of underwater video technology suitable for observations in an underwater environment. The review is divided into two sections, one for each subject, where each section concludes with a discussion of the current challenges within the respective fields. The acoustic section of the review covers bathymetric and geometrical measurements, imaging sonars, subsurface penetrating profilers, positioning methods, acoustic underwater communication and sensor networks, and water speed measurements. The section ends by considering temperature measurements by ocean acoustic tomography and passive acoustic monitoring. The underwater video section initially deals with questions of acquisition including underwater visibility, the type of platform, and video formats, image sensors and specialized cameras. This is followed by notes on processing techniques including mosaicking, stereo video, structured light, recording and transmission, image enhancement techniques and ends with a short discussion of underwater holographic cameras.

Abstract Due to the efficient propagation of sound in water, sound in the deep ocean propagates such great distances that soundscapes are influenced not only by local conditions but also by distant sound sources. Ocean Sound is now an Essential Ocean Variable within the Global Ocean Observing System making passive acoustic monitoring routine. Active acoustic probing of the environment informs us about ocean topography, currents and temperature, and abundance and type of marine life vital to fisheries and biodiversity related interests.Efficient sound propagation is the foundation of a proposed multipurpose acoustic network. Judiciously placed low-frequency acoustic sources transmitting to globally distributed passive acoustic systems provide: (1) high temporal resolution measurements of large-scale ocean temperature/heat content variability using tomography; and (2) underwater geo-positioning (UW-GPS) and communication services enabling basin-scale underwater operation of floats, gliders, and AUVs. Every platform (fixed or moving) equipped with a hydrophone becomes a “GPS” receiver, while listening to the ocean soundscape. The combined active and passive acoustic technology will lead to multi-disciplinary discovery and improved understanding of ocean ecosystem health and biodiversity, climate variability and change, marine hazards, and maritime safety. The same system will improve the operation of gliders, floats and AUVs.

A high-resolution passive seismic investigation was performed in a [Formula: see text] area around the Rio-Antirio Strait in central Greece using natural microearthquakes recorded during three months by a dense, temporary seismic network consisting of 70 three-component surface stations. This work was part of the investigation for a planned underwater rail tunnel, and it gives us the opportunity to investigate the potential of this methodology. First, 150 well-located earthquake events were selected to compute a minimum (1D) velocity model for the region. Next, the 1D model served as the initial model for nonlinear inversion for a 3D P- and S- velocity crustal structure by iteratively solving the coupled hypocenter-velocity problem using a least-squares method. The retrieved [Formula: see text] and [Formula: see text] images were used as an input to Kohonen self-organizing maps (SOMs) to identify, systematically and objectively, the prominent lithologies in the region. SOMs are unsupervised artificial neural networks that map the input space into clusters in a topological form whose organization is related to trends in the input data. This analysis revealed the existence of five major clusters, one of which may be related to the existence of an evaporite body not shown in the conventional seismic tomography velocity volumes. The survey results provide, for the first time, a 3D model of the subsurface in and around the Rio-Antirio Strait. It is the first time that passive seismic tomography is used together with SOM methodologies at this scale, thus revealing the method’s potential.

Abstract. Tunnelling below water passages is a challenging task in terms of planning, pre-investigation and construction. Fracture zones in the underlying bedrock lead to low rock quality and thus reduced stability. For natural reasons, they tend to be more frequent at water passages. Ground investigations that provide information on the subsurface are necessary prior to the construction phase, but these can be logistically difficult. Geophysics can help close the gaps between local point information by producing subsurface images. An approach that combines seismic refraction tomography and electrical resistivity tomography has been tested at the Äspö Hard Rock Laboratory (HRL). The aim was to detect fracture zones in a well-known but logistically challenging area from a measuring perspective. The presented surveys cover a water passage along part of a tunnel that connects surface facilities with an underground test laboratory. The tunnel is approximately 100 m below and 20 m east of the survey line and gives evidence for one major and several minor fracture zones. The geological and general test site conditions, e.g. with strong power line noise from the nearby nuclear power plant, are challenging for geophysical measurements. Co-located positions for seismic and ERT sensors and source positions are used on the 450 m underwater section of the 700 m profile. Because of a large transition zone that appeared in the ERT result and the missing coverage of the seismic data, fracture zones at the southern and northern parts of the underwater passage cannot be detected by separated inversion. Synthetic studies show that significant three-dimensional (3-D) artefacts occur in the ERT model that even exceed the positioning errors of underwater electrodes. The model coverage is closely connected to the resolution and can be used to display the model uncertainty by introducing thresholds to fade-out regions of medium and low resolution. A structural coupling cooperative inversion approach is able to image the northern fracture zone successfully. In addition, previously unknown sedimentary deposits with a significantly large thickness are detected in the otherwise unusually well-documented geological environment. The results significantly improve the imaging of some geologic features, which would have been undetected or misinterpreted otherwise, and combines the images by means of cluster analysis into a conceptual subsurface model.

The functional anatomy of the split compound eyes of whirligig beetles Dineutus mellyi (Coleoptera: Gyrinidae) was examined by advanced microscopy and microcomputed tomography. We report the first 3D visualization and analysis of the split compound eyes. On average, the dorsal and ventral eyes contain 1913 ± 44.5 facets and 3099 ± 86.2 facets, respectively. The larger area of ventral eyes ensures a higher field of vision underwater. The ommatidium of the split compound eyes is made up of laminated cornea lenses that offer protection against mechanical injuries, bullet-shaped crystalline cones that guide light to the photoreceptive regions, and screening pigments that ensure directional light passage. The photoreceptive elements, made up of eight retinular cells, exhibit a tri-tiered rhabdom structure, including the upper distal rhabdom, a clear zone that ensures maximum light passage, and an enlarged lower distal rhabdom that ensures optimal photon capture.

Abstract Ophthalmosaurids were highly derived and the youngest clade of ichthyosaurs. Their evolutionary history comprises ~76 Myr (from the early Middle Jurassic to the final extinction of the group at the Cenomanian–Turonian boundary). Fossil records indicate that soon after they emerged they achieved a widespread geographical distribution. Analyses of disparity based on craniodental morphologies, and the co-occurrence of different bone histology and microstructure among the members of the group, indicate that ophthalmosaurids are not only taxonomically but also ecologically diverse. A region of the skull that is particularly complex among ophthalmosaurid taxa is the external nares and surrounding areas, but there have been few attempts to explore this topic. We describe a new ophthalmosaurid from the Late Jurassic of Patagonia (Argentina) in the genus Arthropterygius and analyse its phylogenetic relationships. A detailed exploration of the skull, using computed tomography, reveals the division of the external nares by means of a stout bony pillar. This condition was previously known only in some Cretaceous forms and, within a phylogenetic framework, its distribution among ophthalmosaurids indicates that this morphology was the result of convergent evolution. Based on available data on extant (and some extinct) forms, we explore putative soft structures involved in the narial region. We propose that the division of the external nares (complete or partly osseous) was related functionally to the separation of the air passage from the outlet of nasal salt glands in such a way that the salt glands could be evacuated underwater, while the air passage could be closed by a valvular system.

Recent theoretical and experimental studies have demonstrated that an estimate of the Green's function between two hydrophones can be extracted passively from the cross‐correlation of ambient noise recorded at these two points. Hence monitoring the temporal evolution of these estimated Green's functions can provide a means for noise‐based acoustic tomography using a distributed sensor network. However, obtaining unbiased Green's function estimate requires a sufficiently spatially and temporally diffuse ambient noise field. Broadband ambient noise ([200 Hz-20 kHz]) was recorded continuously for 2 days during the SWAMSI09 experiment (next to Panama City, FL) using two moored vertical line arrays (VLAs) spanning 7.5m of the 20‐m water column and separated by 150 m. The feasibility of noise‐based acoustic tomography ([300-1000 Hz]) was assessed in this dynamic coastal environment over the whole recording period. Furthermore, coherent array processing of the computed ocean noise cross‐correlations between all pairwise combinations of hydrophones was used to separate acoustic variations between the VLAs caused by genuine environmental fluctuations-such as internal waves-from the apparent variations in the same coherent arrivals caused when the ambient noise field becomes strongly directional, e.g., due to an isolated ship passing in the vicinity of the VLAs.

Les contraintes de rapidité et de discrétion imposées à un système moderne de caractérisation du milieu océanique ont conduit au développement de la tomographie passive, définie comme un moyen discret et rapide d'estimation des paramètres d'un canal acoustique. Ce concept fait appel aux signaux existants dans le milieu et transmis par des sources d'opportunité. Les signaux d'opportunité sont inconnus à la réception mais contiennent des informations relatives aux paramètres physiques du canal défini entre la source et le récepteur. Le travail de recherche présenté dans ce mémoire est d´edié à la caractérisation des milieux sous-marins en utilisant des signaux bioacoustiques d'opportunité (sifflements à modulation fréquentielle). La méconnaissance du signal transmis, de la position et de la vitesse de la source acoustique d'opportunité rendent la tomographie passive difficile à mettre en oeuvre. La propagation dans l'environnement océanique et le mouvement inconnu de la source transforment conjointement les signaux d'opportunité enregistrés. Dans un premier temps, nous présentons de nouvelles méthodes d'estimation simultanée des paramètres environnementaux et des déformations engendrées par le mouvement dans le plan d'ambiguïté large-bande, dans un contexte d'émissions actives (le signal transmis est supposé connu). Ces méthodes, permettant de compenser les effets du mouvement dans les scénarios d'´emissions actives, sont appliquées et validées sur différents jeux de données simulées et réelles enregistrées en mer. Puis nous nous intéressons à la tomographie océanique acoustique passive sur un unique hydrophone. Dans ce contexte, le signal transmis, la position et la vitesse de la source sont entièrement inconnus. A partir des estimateurs développés pour les scénarios d'émissions actives, nous présentons une nouvelle méthodologie permettant d'estimer les paramètres environnementaux en utilisant des vocalises de mammifères marins enregistrées sur un unique hydrophone. Les informations extraites sur les signaux naturels d'opportunité sont ensuite utilisées pour estimer la position puis le vecteur vitesse de la source d'opportunité. Ces méthodes sont appliquées et validées sur différents jeux de données simulées et réelles enregistrées en mer<br>Quickness, secrecy and loudness constraints imposed by modern oceanic characterization led to passive tomography which is defined as a quick, secretive and quiet mean of estimating underwater propagation canals. This concept uses signals naturally existing in the medium and transmitted by opportunity sources. Opportunity signals are unknown at the receiver but they also carry information about canal physical properties. This research work is dedicated to underwater environments characterization using opportunity bioacoustic signals (dolphin whistles). Opportunity signals are simultaneously transformed by underwater propagation and the unknown motion effects. Firstly, we propose new methods for estimating simultaneously environmental parameters and transformations created by motion effects. These parameters are estimated in the broadband ambiguity plane for active tomography (the emitted signal is known) with unknown motion in the system. This work, allowing to compensate for motion effect in active scenarios, is validated on different simulated and real data. Then, we apply our signal processing methods to passive underwater tomography, using a single hydrophone. In this context, both the transmitted signal, source position and source speed are completely unknown. From the theory we developed for active tomography, we derive new methods allowing the estimation of impulse response using underwater mammals vocalization recorded on a single hydrophone. Information extracted on opportunity signals is then used for source position and speed estimation. These methods are applied and validated on different simulated and real data from at sea experiments