Description: A study to investigate the use of the acoustic vector field, separately or in combination with the scalar field, to invert for geoacoustic properties of the seafloor was conducted. The analysis was performed in the context of the 2004 Sediment Acoustics Experiment (SAX04) conducted in the Northern Gulf of Mexico (GOM) where a small number of acoustic vector sensors were deployed in close proximity to the seafloor. The acoustic vector sensors were located both above and beneath the seafloor interface where they measured the acoustic pressure and the acoustic particle acceleration. A variety of acoustic waveforms were transmitted into the seafloor at normal incidence. Motion data provided by the buried vector sensors were affected by a suspension response that was sensitive to the mass properties of the sensor, the sediment density, and shear wave speed. The suspension response for the buried vector sensors included a resonance within the analysis band of 0.4–2.0 kHz. The response was sufficiently sensitive to the local geoacoustic properties, that it was integrated into the inverse methods developed for this study. Inversions of real and synthetic data sets showed that information about sediment shear wave speed was carried by the suspension response of the buried sensors, as opposed to being contained inherently within the vector acoustic field.

Description: The cubic approximation of random wave drag forces is studied in this paper to simplify the applications of nonlinear wave drag forces in spectral analysis. Nonlinear spectral analysis and numerical simulations are used in the study. The drag force expression is approximated by a cubic function whose coefficients are determined by equating the corresponding autocorrelation function with that associated with the original bilinear form of Morison's equation. Numerical verification indicates that the cubic approximation approaches the original expression satisfactory for engineering applications. Application examples indicate that the spectral densities of wave forces per unit pile length and total wave forces on vertical cylinders, as well as the root mean squares (RMSs) of wave forces, can be easily obtained using cubic approximations.

Description: The fusion of several images of the same scene into a single and larger composite is known as photomosaicing. Unfortunately, the seams along image boundaries are often noticeable, due to photometrical and geometrical registration inaccuracies. Image blending is the merging step in which those artifacts are minimized. Processing bottlenecks and the lack of medium-specific processing tools have restricted underwater photomosaics to small areas despite the hundreds of thousands of square meters that modern surveys can cover. Large underwater photomosaics are increasingly in demand for the characterization of the seafloor for scientific purposes. Producing these mosaics is difficult due to the challenging nature of the underwater environment and the image acquisition conditions, including extreme depth, scattering and light attenuation phenomena, and difficulties in vehicle navigation and positioning. This paper proposes strategies and solutions to tackle the problems of very large underwater optical surveys (gigamosaics), presenting contributions in the image preprocessing, enhancing, and blending steps, resulting in an improved visual quality in the final photomosaic. A comprehensive review of the existing methods is also presented and discussed. Our approach is validated by a large optical survey of a deep-sea hydrothermal field, leading to a high-quality composite in excess of 5 Gpixel.

Description: Hyperbolic-frequency-modulated (HFM) waveforms offer detection performance that is “Doppler insensitive”—relatively unaffected by target range rate in matched filtering. They have been applied in radar; but more they are particularly useful in sonar where the wideband Doppler insensitivity is especially prized. However, Doppler insensitivity does come hand in hand with a range bias, which would significantly degrade the tracking performance. In this paper, we model the range bias as a function of range rate and system parameters, and then utilize that to calibrate the measurement equation in very precise target tracking. By doing so, the tracking performance can be improved, particularly for fast maneuvering targets.

Description: One of the main challenges in mine detection using sidescan sonar images is the high density of mine-like objects (MLOs) in a clutter environment. This paper proposes an image change detection technique for bitemporal images which suppresses false alarms efficiently without involving large training data sets. The proposed approach uses the spatial dependence of a stationary object between bitemporal images to eliminate the differences caused by a position error. Bayes theory is then employed to classify the changed and unchanged objects. In particular, the a priori probabilities are formulated by the Markov random field (MRF). The likelihood functions are modeled using the coarseness difference of objects as the test statistics, and the parameters are estimated using the expectation–maximization (EM) method. Real sidescan sonar data are used to validate the proposed method. Results show that the proposed MRF change detection method is robust to the poor quality of object boundaries due to speckle noise, and outperforms the conventional pixel-level change detection methods.

Description: Autonomous underwater vehicles (AUVs) are frequently used for deep-water ocean applications such as surveying and cable laying, where accurate control of vehicle depth and attitude is needed. The water level in the onboard ballast tanks is typically manually set for neutral buoyancy before each mission, while the vehicle is on the surface. The ballast tank contents are not normally adjusted to control vehicle depth and orientation while the AUV is in operation. As a result, vehicle trajectory and orientation is exclusively controlled using the vehicle's control surfaces during a mission. The challenges with controlling the depth and trim of an underwater vehicle include nonlinear hydrodynamic forces, as well as inherent time delays (latencies) associated with water tank level changes and valve adjustments. Furthermore, small changes in the location of the vehicle's center of gravity (i.e., due to the deployment of the AUV's payload equipment) can reduce the control authority of the AUV's control surfaces. To meet these challenges, this paper proposes two unique variable ballast system (VBS) control approaches. The first proposed VBS controller changes the weight of the AUV to help control vehicle depth and vertical (inertial) velocity. The second proposed VBS controller attempts to shift the center of gravity $x_{G}$ along the body-fixed $x$ -(longitudinal)-axis to reduce depth and pitch angle error while restoring control authority to the bowplane and sternplane deflection fins. The ballasting system consists of two water tanks positioned aft and forward of amidships. The ballast tanks are then automatically filled or emptied of ocean water as desired. Numerical simulations have been carried out on a 2-D underwater vehicle simulator to test and compare the performance of the proposed ballast and fin deflection control systems. Th- simulation results show that, for the assumptions and conditions tested, the proposed controllers are capable of achieving a setpoint depth and pitch angle with minimal error by effectively utilizing the ballast tanks and deflection fins. As a result, the work presented in this paper helps increase the autonomy of large AUVs on long-duration missions.

Description: This index covers all technical items - papers, correspondence, reviews, etc. - that appeared in this periodical during the year, and items from previous years that were commented upon or corrected in this year. Departments and other items may also be covered if they have been judged to have archival value. The Author Index contains the primary entry for each item, listed under the first author's name. The primary entry includes the co-authors' names, the title of the paper or other item, and its location, specified by the publication abbreviation, year, month, and inclusive pagination. The Subject Index contains entries describing the item under all appropriate subject headings, plus the first author's name, the publication abbreviation, month, and year, and inclusive pages. Note that the item title is found only under the primary entry in the Author Index.

Description: Provides a listing of current staff, committee members and society officers.

Description: Provides a listing of current staff, committee members and society officers.

Description: High activity of seismic surveying in Norwegian waters has caused concerns about the impact the acoustic noise from the seismic airguns may have on marine life. There is evidence that this noise can cause reactions on the behavior of the fish resulting in reduced catches. To mitigate the problem and the conflict of interest between the fishing industry and the seismic exploration interest, the Norwegian Petroleum Directorate (NPD) commissioned SINTEF Information and Communication Technology (ICT, Trondheim, Norway) and the Department of Biology, University of Oslo (Oslo, Norway) to develop an acoustic–biological model to predict the impact of seismic noise on the fish population. The ultimate goal is to develop an acoustic–biological model to use in the design and planning of seismic surveys such that the disturbance to fishing interest is minimized. This acoustic module of the model is based on ray theory and can deal with range-dependent bathymetry and depth-dependent sound-speed profiles. The bottom is modeled as a sedimentary fluid layer over a solid elastic rock and the model requires the thickness and seismoacoustic properties of the sediments layer and the rock with compressional speed, shear speed, and absorption. The model simulates the total sound field, both in the time domain and in the frequency domain, out to very large distances. Calculated sound exposure levels are compared with startle response levels for cod. Preliminary conclusions indicate a required distance in the range of 5–10 km, but dependent on the depth and the season. In additions, under certain conditions, there will appear regions with hot spots where the sound level is significantly higher due to caustics and focusing of sound. Modeled results are compared with results obtained from a joint seismoacoustic survey conducted in summer 2009 at Vesterålen-Lofoten area (Nordland VII). In this experiment, signals were recorded at fixed hydrophone po- itions as the seismic vessel approached from a maximum distance of 30 km toward the receiving positions. The same situation was modeled using available geological and oceanographic information as input to the acoustic model. The agreement between the real and recorded signals and the model results is good. This indicates that in the future acoustic–biological models may be used in the design and planning of seismic surveys such that the disturbance to fishing is minimized.