Observing seafloor crustal deformation is often composed of acoustic ranging and GPS positioning techniques, which involves positioning of a single seabed transponder. Generally, the positional uncertainty of the seabed transponder is evaluated in terms of the slant range residuals. In order to further verify the relative positioning accuracy between seabed transponders, this study designed and fabricated a transponder lander. Three transponders were mounted on the lander to form a triangle with sides 1.505, 1.505 and 1.160 m. In addition, a data acquisition and logging system is developed to collect the lander¡¦s attitude, including pitch, roll, heading, and accelerations in three orthogonal axes. A field experiment to verify the relative positioning accuracy between seabed transponders was carried out off the coast of Kaohsiung Harbor, Taiwan. The transponder lander was deployed on the seabed at a water depth of about 300 m. Based on the attitude data collected by the logging system, the heading, pitch and roll of the transponder lander on the seabed are 123.5 degrees, 0.63 degrees and 0.62 degrees, respectively. A vessel was sailed along predetermined paths to collect observations of GPS and acoustic slant range. Then, an optimization technique combined with ray tracing method was used to estimate the positions of three transponders. The position estimates of the three transponders form a triangle with sides 1.533, 1.518 and 1.184 m, which shows that, comparing with the true values, the relative positioning error between transponders is less than 3 cm. Furthermore, based on the estimates of the transponder positions, the heading, pitch and roll of the transponder lander are calculated as 120.45 degrees, 6.43 degrees and 2.51 degrees, respectively, which are in good agreement with that measured by the attitude logging system. The experimental and evaluation results indicate that the optimization technique along with the ray tracing method is practical for precisely estimating the transponder position.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第22703号 / 工博第4750号 / 新制||工||1743(附属図書館) / 京都大学大学院工学研究科都市社会工学専攻 / (主査)教授 小池 克明, 教授 林 為人, 准教授 柏谷 公希 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

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Seafloor hydrothermal systems have been intensively studied for the past few decades; however, the location of recharge zones and details of fluid circulation patterns are still largely uncertain. To better understand the effects of anhydrite precipitation on hydrothermal flow paths, we conduct 2-D numerical simulations of hydrothermal circulation at a mid-ocean ridge using a NaCl-H2O numerical code. The simulations focus on East Pacific Rise hydrothermal system at 950N due to availability of key observational data to constrain the models. Seismicity data that is available suggests that fluid flow is primarily along axis and that recharge is focused into a small zone near a 4th order discontinuity in the ridge axis. Simulations are carried out in an open-top square box 1500 m on a side maintained at a surface pressure of 25 MPa, and nominal seawater temperature of 10 C. The sides of the box are assumed to be impermeable and insulated. A constant temperature distribution is maintained along the bottom of the box consisting of a 1000 m long central-heated region maintained at 450 C to represent the axial magma chamber and ensure P-T conditions for phase separation; a linearly decreasing temperature profile from 450 to 300 C is maintained along the 250 m long segments adjacent to the heated region to delineate the recharge zone. We constructed a homogeneous model with a uniform cell size of 25 m with a permeability of 10-13 m2 and a similar model with a 200 m thick layer 2A region with a permeability of 10-12 m2. For the homogeneous model the simulations were run for 100 years to approximate steady state conditions and the model with layer 2A was run for 50 years. Assuming that anhydrite precipitation resulted from the decrease in solubility with increasing temperature as downwelling fluid gets heated, the rate of porosity decrease and sealing time was calculated at 50 and 100 years. The results showed that sealing occurred most rapidly at the bottom of the recharge areas near the base of the high-temperature plumes, where complete sealing occurred after ~55-625 years for an initial porosity of 0.1. The simulations also suggested that sealing would occur more slowly at the margins of the ascending plumes, with times ranging between ~ 80 and 5000 years. The sealing times in the deep recharge zone determined in these simulations are considerably greater than estimated from 1D analytical calculations, suggesting that with a 2D model, focused recharge at the EPR 950N site may occur, at least on a decadal time scale. More detailed analyses are needed to determine whether such focused recharge can be maintained for longer times. / Master of Science

A poroelastic finite element formulation originally derived for modeling porous absorbing material in air is adapted to the problem of acoustic scattering from a poroelastic seafloor with a one-dimensional randomly rough interface. The developed formulation is verified through calculation of the plane wave reflection coefficient for the case of a flat surface and comparison with the well known analytical solution. The scattering strengths are then obtained for two different sets of material properties and roughness parameters using a Monte Carlo approach. These numerical results are compared with those given by three analytic scattering models---perturbation theory, the Kirchhoff approximation, and the small-slope approximation---and from those calculated using two finite element formulations where the sediment is modeled as an acoustic fluid. / text

Title from PDF of title page. / Document formatted into pages; contains 105 pages. / Thesis (M.S.)--University of South Florida, 2003. / Includes bibliographical references. / Text (Electronic thesis) in PDF format. / ABSTRACT: Relative plate motion history since 30 Ma between the Pacific and the southern portion of the Nazca (Farallon) plates is examined. The history is constrained by available seafloor magnetic anomaly data and a two-minute grid of predicted bathymetry derived from satellite altimetry and shipboard sensors. These data are used to create a new plate motion reconstruction based on new magnetic anomaly identifications and finite poles of motion. The new identified magnetic isochrons and tectonic reconstruction provides greater resolution to the tectonic history between chrons 7y (24.73 Ma) and 3 (4.18 Ma) than previous interpretations. Shipboard magnetics and aeromagnetic data from over 250 expeditions were plotted and used to extrapolate magnetic anomalies picked from 2D magnetic modeling from selected cruises. Magnetic anomalies were further constrained by tectonic features evident in the predicted bathymetry. / ABSTRACT: Previously published magnetic anomaly locations consistent with this work were used where interpretation could not be constrained by 2D modeling and map extrapolation. Point locations for anomalies were used as input for calculation of finite poles of motion for chrons 10y, 7y, 6c, 5d, 5b, 5aa, 5o, 4a and 3a. An iterative process of anomaly mapping, pole calculation and anomaly point rotations was used to refine the finite poles of motion. Eleven stage poles were calculated from the nine finite poles from this study and two published instantaneous Euler vectors. Tectonic reconstructions indicate a history dominated by two major southward ridge propagation events, the first starting by 28 Ma and completed by 18 Ma. The second event initiated in association with breakup of the Farallon plate around 24 Ma and ceased by about 11 Ma. Lithosphere was transferred from Nazca to Pacific during the first event and in the opposite sense during the second. / ABSTRACT: Development of the Mendoza microplate east of the later propagator occurred at about 20 Ma and this dual spreading process appears to have lasted until about 15 Ma. / System requirements: World Wide Web browser and PDF reader. / Mode of access: World Wide Web.

Occurrences of carbonate-cemented nodules and concretions exposed on the seafloor that contain cements with light carbon isotopes, indicating a contribution of methane-derived carbon, are commonly interpreted to be indicators of seafloor fluid venting. Thus, their presence is commonly used as an indicator of the possible occurrence of methane gas hydrate within the near subsurface. While some of these carbonates exhibit facies that require formation on the seafloor, the dominant fine-grained lithology associated with these carbonates indicates they were formed as sedimenthosted nodules within the subsurface and are similar to nodules that are obtained from the subsurface in Deep Sea Drilling Project, Ocean Drilling Project, and Integrated Ocean Drilling Project boreholes. Here we present the hypothesis that the occurrence of these carbonates on the seafloor may instead indicate areas of persistent seafloor erosion.

A one-kilometer-diameter carbonate/hydrate mound in Mississippi Canyon Block 118 has been chosen to be the site of a multi-sensor, multi-discipline sea-floor observatory. Several surveys have been carried out in preparation for installing the observatory. The resulting data set permits discussing the mound’s structure in some detail. Samples from the water column and intact hydrate outcrops show gas associated with the mound to be thermogenic. Lithologic and bio-geochemical studies have been done on sediment samples from gravity and box cores. Pore-fluid analyses carried out on these cores reveal that microbial sulfate reduction, anaerobic methane oxidation, and methanogenesis are important processes in the upper sediment. These microbial processes control the diffusive flux of methane into the overlying water column. The activity of microbes is also focused within patches near active vents. This is primarily dependent upon an active flux of hydrocarbon-rich fluids. The geochemical evidence suggests that the fluid flux waxes and wanes over time and that the microbial activity is sensitive to such change. Swath bathymetry by AUV combined with sea-floor video provides sub-meter resolution of features on the surface of the mound. Seismic reflection profiling with source-signature processing resolves layer thicknesses within the upper 200-300m of sediment to about a meter. Exploration-scale 3-D seismic imaging shows that a network of faults connects the mound to a salt diapir a few hundred meters below. Analyses of gases from fluid vents and hydrate outcrops imply that the faults act as migration conduits for hydrocarbons from a deep, hot reservoir. Source-signature-processed seismic traces provide normal-incidence reflection coefficients at 30,000 locations over the mound. Picking reflection horizons at each location allows a 3-D model of the mound’s interior to be constructed. This model provides a basis for understanding the movement of fluids within the mound.

It was proposed that erosion of subsea ridges on the Hikurangi margin may be linked to a fluctuating level of the top of gas hydrate stability in the ocean. Since publication of this hypothesis, three field campaigns were conducted in the study area. Here we summarize relevant results from these cruises. We found that water temperature fluctuations occur at lower frequencies and higher amplitudes than previously thought, making it more likely that temperature changes reach sub-seafloor gas hydrates. Dredge samples encountered numerous consolidated mudstones. We speculate that gas hydrate “freeze-thaw” cycles may lead to dilation of fractures in mudstones due to capillary forces, weakening the seafloor. Ubiquitous gas pockets beneath the ridge may lead to overpressure that may also contribute to seafloor fracturing.

This study focuses on the potential of using a single beam echosounder as a tool for recognition and assessment of seafloor vegetation. Seafloor vegetation is plant benthos and occupies a large portion of the shallow coastal bottoms. It plays a key role in maintaining the ecological balance by influencing the marine and terrestrial worlds through interactions with its surrounding environment. Understanding of its existence on the seafloor is essential for environmental managers. / Due to the important role of seafloor vegetation to the environment, a detailed investigation of acoustic methods that can provide effective recognition and assessment of the seafloor vegetation by using available sonar systems is necessary. One of the frequently adopted approaches to the understanding of ocean environment is through the mapping of the seafloor. Available acoustic techniques vary in kinds and are used for different purposes. Because of the wide scope of available techniques and methods which can be employed in the field, this study has limited itself to sonar techniques of normal incidence configuration relative to seafloors in selected regions and for particular marine habitats. For this study, a single beam echosounder operating at two frequencies was employed. Integrated with the echosounder was a synchronized optical system. The synchronization mechanism between the acoustic and optical systems provided capabilities to have very accurate groundtruth recordings for the acoustic data, which were then utilized as a supervised training data set for the recognition of seafloor vegetation. / In this study, results acquired and conclusions made were all based on the comparison against the photographic recordings. The conclusion drawn from this investigation is only as accurate as within the selected habitat types and within very shallow water regions. / In order to complete this study, detailed studies of literature and deliberately designed field experiments were carried out. Acoustic data classified with the help of the synchronized optical system were investigated by several methods. Conventional methods such as statistics and multivariate analyses were examined. Conventional methods for the recognition of the collected data gave some useful results but were found to have limited capabilities. When seeking for more robust methods, an alternative approach, Genetic Programming (GP), was tested on the same data set for comparison. Ultimately, the investigation aims to understand potential methods which can be effective in differentiating the acoustic backscatter signals of the habitats observed and subsequently distinguishing between the habitats involved in this study.