Abstract The object of this study is to investigate the flow induced vibration of cable structures. Two various methods, Eigenvalue method and Cable dynamics method, are used to evaluate, the natural frequencies of structures. Vortex shedding frequencies are introduced in order to compare with the natural frequencies mentioned above. The results determine whether the phenomenon of resonance or Lock-in occur. This study has four parts. The first part is the dynamic analysis of an underwater cable Depending on the various boundary conditions, the study discusses, the system of towing cable and mooring cable in which the maximum horizontal displacement and the maximum dynamic response amplitude are found in different ocean environment. The second part is a linear Eigenvalue analysis for natural frequencies of cable structures. The third part compare the results based on the two methods , Cable dynamics and Eigenvalue method. The former obtain tension results that are important to determine the natural frequencies of structures by theoretical formula. The results of natural frequencies from the letter are compared to those in the former. The fourth part is mainly to calculate vortex shedding frequencies resulting in the relative motion between structures and fluid. The final results found in the fourth part are necessary to compare with part three, so as to determine whether the resonance or Lock-in occur.

Oceanographers are currently investigating small-scale ocean turbulence to understand how to better model the ocean. To measure ocean turbulence, one must measure fluid velocity with great precision. The three components of velocity can be used to compute the turbulent kinetic energy dissipation rate. Fluid velocity can be measured using a five-beam acoustic Doppler current profiler (VADCP). The VADCP needs to maintain a tilt-free attitude so the turbulent kinetic energy dissipation rate can be accurately computed to observe small-scale ocean turbulence in a vertical column. To provide attitude stability, the sensor may be towed behind a research vessel, with a depressor fixed somewhere along the length of the towing cable. This type of setup is known as a two-part towing arrangement. This thesis examines the dynamics, stability and control of the two-part tow. A Simulink simulation that models the towfish dynamics was implemented. Through this Simulink simulation a parametric study was conducted to see the effects of sea state, towing speed, center of gravity position, and a PID controller on the towfish dynamics. A detailed static analysis of the towing cable's effects on the towfish enhanced this dynamic model. The thesis also describes vehicle design and fabrication, including procedures for trimming and ballasting the towfish. / Master of Science

No description available.

Geophysical methods are non-invasive and allow an effective way of understanding subsurface structures and their physical properties. One of the main challenges is the often non-uniqueness of the geophysical models and that several different models can explain a dataset to an agreeable fit. Moreover, noise and limitations in resolution, which are inherent to field data, are additional obstacles for obtaining a true physical property model of the subsurface. Facing all these challenges, geophysicists have dedicated their efforts for decades to recover models that represent, as close as possible, the true subsurface. Joint inversion and integration of multiple geophysical data are two main approaches that I studied to better resolve subsurface structures. I further used these approaches, together with new software and hardware implementations for data acquisition and inversion, for near-surface applications.  In this thesis, radio-magnetotelluric (RMT), boat-towed RMT, boat-towed controlled source MT (CSMT), electrical resistivity tomography (ERT), and first-arrival traveltime tomography are jointly used for quick clay investigations and fracture zone delineation under shallow water-bodies. The joint approach, as compared with any individual method, shows a better ability to both resolve the geological targets and to assist in understanding the subsurface geology that hosts these targets. For examples: by performing the joint inversion of lake-floor ERT and boat-towed RMT data, a fracture zone is better delineated with greater details compared with single inversion; by employing boat-towed CSMT measurements and jointly inverting with boat-towed RMT data, the subsurface structures, especially at greater depth, are better resolved than by inverting each dataset alone. During my PhD studies, two types of new implementations were employed. (1) Boat-towed data acquisition system was implemented to expand the RMT and CSMT method from land to shallow-water applications. This is significant since many large-scale underground infrastructures are likely to cross these water zones (for example multi-lane train or bypass tunnels, such as the Stockholm bypass). (2) The modification of a well-structured code EMILIA allows joint inversion of boat-towed RMT and lake-floor ERT datasets, and the modification of another well-structured code MARE2DEM can accurately model high frequency CSMT data and handle joint inversion of boat-towed RMT and boat-towed CSMT datasets. Thus, the code modification as another type of new implementation guarantees the success of near-surface applications using the boat-towed RMT and CSMT data acquisition systems. Studies conducted during my PhD work, included under the SEG-GWB (the Society of Exploration Geophysicists - Geoscientists Without Borders) program and the TRUST (TRansparent Underground STructure) umbrella project, are useful for near-surface applications including, for examples, engineering purposes such as planning of underground infrastructures, site characterization in connection with energy or waste storage, and geohazard investigations.

Sampling small scale ocean turbulence is one of the most important problems in oceanography. The turbulence can be near the noise level of current microscale profiling techniques and these techniques do not provide spatially and temporally dense measurements and can be labor intensive. A 5 beam acoustic Doppler current profiler (VADCP) can more accurately measure three components of fluid velocity in a column. By towing such a device in a sensor platform, called a towfish, one may measure turbulent mixing in a vertical swath of the ocean. If the towfish attitude is not precisely regulated, however, the turbulence measurements can be irreversibly corrupted. A two-part tow that includes a depressor weight between the towing vessel and the towfish can provide some degree of disturbance rejection. Passive devices alone, however, can not meet the performance requirements for measuring ocean turbulence. This thesis presents a design for a two-stage towing system which will be used to measure ocean turbulence. The focus is on the towfish, which includes independently actuated stern planes for pitch and roll disturbance rejection. The thesis also describes design and analysis of an active control system to precisely regulate the pitch and roll attitude of a streamlined towfish. A three dimensional numerical model is presented and a PID controller is developed to provide active attitude stabilization. The effect of random depressor motions on the towfish dynamics is assessed for both the uncontrolled and the feedback-controlled case. The numerical investigation also considers variations in parameters such as tether length and CG location. / Master of Science

Unmanned ground vehicles (UGVs) will increasingly be used for tasks such as retrieving injured soldiers from a battlefield, transporting supplies, and towing other small vehicles and payloads. To date, the unmanned test community has not standardized on an apparatus or test operating procedure (TOP) specifically for evaluating the towing capacity of small unmanned ground vehicles. Draw-bar testing has been adapted by several groups to quantify small unmanned ground vehicle (SUGV) tow capacity; however, these devices are inherently limited to measuring peak static towing force. This paper describes an alternative method using a variable-resistance tow sled for quantifying the dynamic towing capacity of SUGVs. The tow sled contains a frontal skid plate and a rear axle and wheel arrangement. A weighted carriage is transferred from the rear of the sled to the front of the sled by a cable geared to the rear axle. As the sled is pulled along the ground, towing resistance increases in a controlled linear fashion. An encoder on the rear axle and a load cell in the tow chain provide motion and force data. Testing of the tow sled has been conducted on a TALON SUGV at the Southwest Research Institute (SwRI) Small Robot Test Facility and a MATILDA SUGV at the Joint Unmanned Systems Test, Experimentation, and Research (JOUSTER) site. / Master of Science

The towed streamer controlled source electromagnetic (CSEM) system collects data faster than the conventional static node-based CSEM system. However, the towed streamer CSEM is typically much noisier than the conventional static node-based CSEM. Identifying and quantifying various sources of noise is important for the development of future robust electromagnetic streamer system. This is the problem I address in this thesis. I achieve this in three parts. First, I examine the idea that the towed streamer suffers from noise induced by its motion through the Earth’s magnetic field according to Faraday’s law of induction. I derive expressions for the motionally-induced noise for the cases of a horizontal streamer parallel to the acquisition vessel’s path and a curved streamer caused by a constant cross-current. These expressions demonstrate that the motionally-induced noise is sensitive to the magnitude of the feather angle at the head and at the tail of the streamer, and to the vertical and lateral motion of the streamer. The key finding is that no motionally-induced noise is generated when the streamer is horizontal and moving in a constant magnetic field. By contrast, when the streamer shape is curved because of cross-currents, motionally-induced noise is generated if the velocity of the streamer varies over time. Second, I analyse and compare the noise recorded using the first generation of towed streamer with the noise recorded using a static ocean bottom cable (OBC) CSEM. I find out that within the frequency range of interest, 0.01–1 Hz the towed streamer noise is 20 dB greater (factor of 10) than the noise recorded with the OBC CSEM. I show also that the motion of the telluric cable between the pair of electrodes in the towed streamer is responsible for this difference in amplitude between the two systems. In the frequency ranges, 0.03–0.1 Hz and 0.03–0.2 Hz, the motionally-induced noise is shown to be uncorrelated across all channels. However, within the frequency band 0.1–0.3 Hz, the motionally-induced noise correlation gradually increases and becomes well correlated at about 0.2 Hz. This correlated noise could be caused by ocean swell from surface waves, water flowing around the streamer or cross-currents. Finally, to identify and quantify the contribution of several distinct sources of noise, and to describe the mechanisms generating each source of noise, I co-designed a prototype towed streamer CSEM. I carried out an experiment with the prototype streamer suspended 1 m below the water surface in the controlled environment of the Edinburgh wave tank located in King’s building campus (the University of Edinburgh). I then subjected the streamer to flow running at velocities of 0–1ms−1 along its length and to waves propagating in the same direction, at 45°, and perpendicular relative to the streamer direction.

The three key steps of modern seismic imaging are (1) multiple attenuation, (2) velocity estimation, and (3) migration. The multiple-attenuation step is essentially designed to remove the energy that has bounces at the free surface (also known as "multiples"), since velocity estimation and migration assume that data contain only primaries (i.e., seismic events that have reflected or diffracted only once in the subsurface and have no free-surface reflection). The second step consists of estimating the velocity model such that the migration step can be solved as a linear inverse problem. This thesis concerns the multiple attenuation of towed-streamer data. We have proposed a new method for attenuating multiples and discussed how this method affects velocity estimation and migration. The multiple-attenuation approach used today in the E&P industry is based on the scattering theory. It is carried out in two steps: (1) the prediction of multiples using data only, and (2) the subtraction of multiples contained in the data using predicted multiples. One of the interesting features of these multiple-attenuation methods is that they do not require any knowledge of the subsurface. However there are still two drawbacks that limit the usage of these methods. They are (1) the requirement of acquiring very large 3D datasets which are beyond the capability of current seismic acquisition technology, and (2) the requirement of acquiring near-offset (including zero-offset) data. The method developed in this thesis can potentially overcome these two problems. The novelty of our approach here is to image receiver ghosts of primaries--events which have one bounce in the subsurface and one bounce at the free-surface that is also the last bounce--instead of primaries themselves. We propose to predict two wavefields instead of a single wavefield, as is presently done. One wavefield contains all free-surface reflections, including receiver ghosts of primaries, ghosts of multiples, and multiples. The other wavefield does not contain receiver ghosts of primaries. We pose the problem of reconstructing receiver ghosts of primaries as solving a system of two equations with three unknowns. The two wavefields are used to construct the two equations. The three unknowns are (1) the receiver ghosts of primaries, (2) the multiples contained in the wavefield containing the receiver ghosts of primaries, and (3) the multiples contained in the other wavefield. We solve this underdetermined system by taking advantage of the fact that seismic data are sparse. We have validated our approach using data generated by finite-difference modeling (FDM), which is by far the most accurate modeling tool for seismic data. Starting with a simple 1D model, we verified the effectiveness of predicting data containing multiples and receiver ghosts of primaries. Then we used the sparsity of seismic data to turn the system of two equations with three unknowns into a system of two equations with two unknowns on a datapoint basis. We have also validated our method for complex geological models. The results show that this method is effective, irrespective of the geology. These examples also confirm that our method is not affected by missing near-offset data and does not require special seismic 3D acquisition.

Abstract Kaohsiung Harbor is one of the most important international sea ports in the world. Due to the long-standing lack of in-situ current data, the complex variations of the flow field in this basin still remain unclear. As a consequence, the related environmental problems such as oil spills, water quality management and ship maneuvering safety , have long been a great concern in this harbor. The present study is conducted to better understand the flow field in the Kaohsiung Harbor. A series of synoptic flow observations of the Kaohsiung Harbor were conducted by using towed-ADCP or EM current meters. From these observations it can be shown that the flow field of the Kaohsiung Harbor is¡GWater entering the harbor through the second entrance and exiting the harbor through the first entrance during ebbs. During floods the flows are reversed. A 3-D numerical model (from POM) is developed for the Kaohsiung Harbor. The flows are more complicated by the M2 tide driven than by the mixed tide driven. From the results by the M2 tide driven show the ocean current is variable, especially the south ocean current. Therefore, the flows are more complicated owing to the mixed tidal characteristics and shoreline geometry. The maximum current speeds amount to 30 - 40 cm/s in the narrow first entrance and 10 - 20 cm/s in the second entrance. It is clearly demonstrated from the model results that drainage from the Chien-Chen River affects greatly the salinity and circulation patterns of the Kaohsiung Harbor, causing the salinity of the first entrance to be lower than that of the second entrance, and the surface layers flowing outward toward the ocean while the lower layers displaying tidal oscillations. From the modeling results, the influence of the wind on the harbor flows is insignificant and the tide is main force in the harbor. Under the simultaneous forcing of river and wind, flood and ebb tidal streams leaving the two entrances are found to diverge in a flow stagnation area inside the harbor near Pier 45 and 61, respectively. Based on the modeling results, it can be concluded that the main factors affecting the flow patterns of the Kaohsiung Harbor are (1) mixed tidal nature, (2) shoreline geometry and (3) river runoff

Multisource Least-Squares Migration (LSM) of phase-encoded supergathers has shown great promise in reducing the computational cost of conventional migration. But for the marine acquisition geometry this approach faces the challenge of erroneous misfit due to the mismatch between the limited number of live traces/shot recorded in the field and the pervasive number of traces generated by the finite-difference modeling method. To tackle this mismatch problem, I present a frequency selection strategy with LSM of supergathers. The key idea is, at each LSM iteration, to assign a unique frequency band to each shot gather, so that the spectral overlap among those shots—and therefore their crosstallk—is zero. Consequently, each receiver can unambiguously identify and then discount the superfluous sources—those that are not associated with the receiver in marine acquisition. To compare with standard migration, I apply the proposed method to 2D SEG/EAGE salt model and obtain better resolved images computed at about 1/8 the cost; results for 3D SEG/EAGE salt model, with Ocean Bottom Seismometer (OBS) survey, show a speedup of 40×. This strategy is next extended to multisource Full Waveform Inversion (FWI) of supergathers for marine streamer data, with the same advantages of computational efficiency and storage savings. In the Finite-Difference Time-Domain (FDTD) method, to mitigate spectral leakage due to delayed onsets of sine waves detected at receivers, I double the simulation time and retain only the second half of the simulated records. To compare with standard FWI, I apply the proposed method to 2D velocity model of SEG/EAGE salt and to Gulf Of Mexico (GOM) field data, and obtain a speedup of about 4× and 8×. Formulas are then derived for the resolution limits of various constituent wavepaths pertaining to FWI: diving waves, primary reflections, diffractions, and multiple reflections. They suggest that inverting multiples can provide some low and intermediate-wavenumber components of the velocity model not available in the primaries. In addition, diffractions can provide twice or better the resolution as specular reflections for comparable depths of the reflector and diffractor. The width of the diffraction-transmission wavepath is on the order of λ at the diffractor location for the diffraction-transmission wavepath.