Of all the forms of radiation known, sound travels through the sea the best, therefore it is widely used in underwater detection and signal transmission. However, the ambient noise in the ocean will affect the quality of these applications, so that understanding of ambient noise is important for improving acoustic capability. Previous literatures show that ambient noise at deep water should decrease with depth, but would it be the same in the shallow? Most studies of ambient noise were made at deep water in the past, the reason is that great depth variation of ambient noise is easily observed. The ASIAEX 2001 South China Sea experiment is on the continental shelf-break, so the depth effect of ambient noise in shallow water can be explored. A typhoon shall affect experimental zone during May 10-14, 2001. From the results of this study, afternoon effect shall affect distribution of temperature and ambient noise under water depth. The other, a typhoon can not only affect seriously ambient noise, but also intercept the shine and variation of sea temperature, and reduce afternoon effect. Another, ambient noise has insignificant change with depth, however it¡¦s is much affected by the variation of temperature, frequency, and occurrence of internal wave.

Hindcasts from the Directional Ambient Noise Estimation System (DANES) model were compared to in situ ambient noise measurements to determine the accuracy of the U.S. Navy's ambient noise model. One hundred fifty eight (158) sonobuoy ambient noise measurements were acquired at eight locations in the Northeast Pacific (NEPAC) Ocean during November and December 1980. For each sonobuoy observation a DANES hindcast was made using archived fields from Fleet Numerical Oceanography Center for the simultaneous time and location. The difference between the predictions and measurements was calculated. A maximum mean error of 4.9 dB occurred at 200 Hz which appeared to be due to errors in the DANES Historical Temporal Shipping (HITS) data base. The model was insensitive to synoptic shipping, sound speed profiles and wind field inputs when the HITS data base was utilized.

It can be a real challenge to construct the ambient noise modeling among the harbors of very frequent shipping. Therefore, in these harbors, a great amount of resource has to be utilized for precisely control the ambient noise occurring parameters. In this study, the underwater measuring skill is adapted to collect the ambient noise raw data among the first harbor area of Kaohsiung Harbor and the sound intensity of various ships as well. Besides, the signal processing skill is also conducted for ship tracing experiment. The harbor ambient noise mainly comes from ships. Thus, the understanding of ship activity and their different sound intensity can be a great help to construct the ambient noise modeling. The results of this study indicate¡GShips outside the harbor create more low-frequency noise than those inside the harbor.Waves inside the harbor create more middle-frequency noise than those outside the harbor.In the harbor, those ships with onboard electricity create more noise and there is less noise around the wharfs without any ships. In the part of ship noise study, the intensity is usually correlated with ship types, displacement, ship speed, primary engine and auxiliary engine, etc. However, only two ship types, small and large ships, are categorized in this study. Small ships can be operated easily with the most power outcome, which can create the sound intensity of 155 to 165 dB in average. On the contrary, large ships are usually limited within their operating range and can not move with the most power outcome under the security consideration. Therefore, the sound intensity of large ships is measured between 169 to 177 dB. In the ship tracing experiment, operations are the basis for constructing the harbor ambient noise model.

Ambient noise generated by surface random processes is the primary contribution to the noise-field energy in the intermediate frequency band, and thus is important in many applications of underwater sound. In this study, the noise field is analyzed with respect to the effects of random source spectrum, waveguide structure of the water column, and seabed stratification upon the noise-field intensity as well as spatial correlation. Based upon a noise-generation model due to continuous random sources, incorporating several analytical models for seabed stratification, a formulation may then be derived to facilitate the numerical implementation. Many results shall be generated and analyzed. In this study considers the noise field generated by surface random processes in an oceanic environment with a sediment layer possessing a continuously varying density and sound-speed profile. This model closely resembles the oceanic waveguide environment and therefore enables the simulation of surface noise generation. Many results of the noise field were generated, including the noise intensity distribution, vertical and horizontal correlations. It is demonstrated that the noise intensity may be affected by the stratification mainly through the continuous spectrum, in that the continuous spectrum is equally important as the normal modes in the present analysis. Moreover, the results for the correlations show that the noise field in the horizontal direction becomes more coherent when the noise sources are more correlated, while in the vertical direction, the results tend to reverse. The horizontal correlations of the noise field due to surface random sources with non-isotropic power spectrum, such as nonisotropic Gaussian and Pierson-Moskowitz, were generated and analyzed.

High population density and the presence of many more motorcycles than cars make the noise environment in Taiwan different from that in other countries. There is growing concern about the electromagnetic effects within this environment. The electromagnetic environment is unique and the information about radio noise is not sufficient at this time. The interference of wireless communication system may be caused by the noise environment. Thus, we need to consider the influence that the noise causes. With the measured radio noise, the minimum suggested receive power in an urban environment ranges from 354 MHz to 426 MHz. It is analyzed by the means of Cumulative Distribution Function (CDF), Amplitude Probability Distribution (APD), Noise Amplitude Distribution (NAD), Pulse Duration Distribution (PDD), Pulse Spacing Distribution (PSD) and Average Crossing Rate (ACR). We measured the properties of noise at an urban center, a nearby port, and a freeway exit, which are located in the same city, and on a hill lying adjacent to the city. We chose an urban center and a nearby hill as the noise environment for the following reasons: (a) The noise margin at urban areas is smaller than that at suburban and rural areas. (b) The coverage of the measurement on a hill is larger than that in a city. (c) The relation of the noise environment between a hill and an urban center can be obtained. The statistical distributions of the four particular noise environments are shown and design constraints for a broadcasting system are revealed. Secondly, we also provide a technology for designing miniature Rogowski coils on glass substrates to obtain current sensors for high operating frequencies in this thesis. The coils are useful for measurement of a small current on a microstrip line at high frequencies. In our experiments, a 50 Ohm microstrip line is driven by an input voltage of 100 mV. A frequency as high as 6 GHz has been used. The highest frequency is limited by the oscilloscope available to us. Geometric effects of the device were investigated to obtain the sufficient output voltage at high frequencies. The induced output voltage can approach approximately 7 mV by modifying the structure of Rogowski coils. At the same time, On-chip solenoid inductors for high frequency magnetic integrated circuits are proposed. The eddy current loss was reduced by dividing the inductor into three consecutive inductors connected in series. The inductor has an inductance of 1.1 nH and the maximum quality factor (Qmax) of 50.5. The self-resonant frequency and the operating frequency at Qmax are greater than 17.5 GHz and 16.7 GHz, respectively.

Sound waves are highly conductive in the ocean; therefore, they are used in underwater detection and signal transmission. During these applications, we may receive some signals, such as radiated noise, self-noise, reverberation noise, target noise, and ambient noise. These signals are influential in the application of underwater acoustics, so many people study underwater noise and create numerous mathematical and physical models in order to improve the applications. According to the past researches, ambient noise is the most complicated one among all the other underwater noise. Until now, there are still some unknown factors in the ambient noise study, many of which are too intricate to be modulated. Accordingly, it is very important to know more about the ambient noise for the enhancing of the quality of underwater communication and detection, which is the reason why people keep researching on ambient noise. Among all kinds of ambient noise, wind-generated noise is not only loud in volume, but also wide in frequency scale, which makes it the most detectable noise during sound detection. In order to understand wind-generated noise, we have compiled past papers first to look for the correlation between physical mechanisms and models of mathematics. In addition, we have constructed a measuring system for underwater sound in the ocean and another measuring system for wind on land. Data from the two measuring system were analyzed by statistics. One of the correlations between wind and ambient noise is that the stronger wind appears, the louder ambient noise is generated. But wind does not always generate ambient noise; it has to be strong enough to generate ambient noise. However, even when the wind is strong enough, ambient noise is not generated immediately, but with a delay. By analyzing the data, we have not only proved the credibility of the data from the old researches, but also presented a method of statistics for analyzing the wind delay effect, and the minimum velocities of wind which generates ambient noise.

Ambient noise generated by surface random processes is the primary contribution to the noise-field energy in the intermediate frequency band, and thus is important in many applications of underwater sound. In this study, the noise field is analyzed with respect to the effects of random source spectrum, waveguide structure of the water column, and seabed stratifica¬tion upon the noise-field intensity as well as spatial correlation. Based upon a noise-generation model due to continuous random sources, incorporating several analytical models for seabed stratification, a formulation may then be derived to facilitate the numerical implementation. Many results shall be generated and analyzed. In this study considers the noise field generated by wave in an oceanic environment with a sediment layer possessing a constant density and sound-speed profile. This model closely resembles the oceanic waveguide environment and therefore enables the simulation of surface noise generation. Many results of the noise field were generated, in¬cluding the noise intensity distribution, vertical and horizontal correlations. It is demonstrated that the noise intensity may be affected by the strat¬ification mainly through the continuous spectrum, in that the continuous spectrum is equally important as the normal modes in the present analysis. Moreover, the results for the correlations show that the noise field in the horizontal direction becomes more coherent when the noise sources are more correlated, while in the vertical direction, the results tend to reverse. The horizontal correlations of the noise field due to surface random sources with non-isotropic power spectrum, such as non-isotropic Gaussian and, were generated and analyzed.

No description available.

This thesis is directed at exploiting information in the coda of seismic phases and the ambient noise field to provide new constraints on the structure of the Australian Continent. ¶ The exploitation of the immediate coda following the onset of P waves from a distant earthquake using radial receiver functions is now a well established method. The 40 sec interval following P contains reverberations and conversions, by deconvolving the radial component trace with the vertical components, the conversions are emphasized by canceling the part of the response that are common to both components. A member of different styles of such deconvolution, are investigated and a variant of the multitaper method is adopted for subsequent applications. The TASMAL experiment 2003-2005 spans the expected location of the transition between Precambrian and Phanerozoic Australia. The 20 portable broadband stations were exploited in receiver function studies to extract S wave crustal structure through the inversion of stacked receiver functions using the Neighbourhood Algorithm. There is no clear crustal transition associated with the presence of Tasman Line. The Precambrian Cratons tend to exhibit crustal thicknesses close to 40 km but such values are also found in some Phanerozoic sites. ¶ The second part of the thesis is directed at the exploitation of ambient noise or seismic coda to gain information on the Green's function between seismic stations. The TASMAL experiment covered a significant fraction of the Australian continent with a simultaneous deployment of portable broadband stations. From these continuous records, it has proved possible to extract very clear Rayleigh wave signals for station separations up to 2000 km, and to demonstrate the frequency dependent variations in group velocity behaviour. The combination of the paths between the 20 stations localize such behaviour, but detailed images needed more data. The entire archive of portable broadband data recorded by RSES was mined, and combined with data from permanent stations to provide more than 1100 estimates of interstation Green's functions within Australia. Group velocity analysis as function of frequency was followed by nonlinear tomography with the Fast Marching Method. The resulting images of group velocity patterns as a function frequency show pronounced regions of lowered group velocities, most of which match regions of thick sediment. The frequency dependence is not consistent with just sedimentary structure and low midcrustal velocities, most likely due to elevated temperatures, are also needed. ¶ The surface wave portion of the interstation Green's function is the most energetic, and is normally all that seen in ambient noise studies. However, in the coda of events record at the broadband Warramunga seismic array in the Northern Territory, the P and S body wave components also emerge. The characteristics of these arrivals match those observed from nearby small earthquakes. The stacked cross-correlation is the normal approach to enhance Green's function information from ambient noise, but a broader spectral band width with the same phase response can be found by spectral division. It appears advantageous to compare both approaches and select the best result, since very little modifications to procedures are needed. ¶ The properties of the ambient noise at a single station have been investigated in the logarithmic spectral domain and a station dependent signal can be extracted by stacking. The signal appears to be related to the local structure beneath the station, and when fully characterized may provide a new means of investigating structure.

Virtual source gathers were generated using the principles of seismic interferometry from 135 hours of ambient noise recorded during a controlled-source survey across the Salton Sea Geothermal Field in southern California. The non-uniform nature of the noise sources violated a primary assumption of the method and generated artifacts in the data. The artifacts generated by the high-energy impulsive sources (e.g. earthquakes, shots) were removable using traditional methods of amplitude normalization prior to cross-correlation. The continuous source artifacts generated by the geothermal wells and highways required an unconventional approach of utilizing only normalized impulsive sources to successfully reduce the artifacts. Virtual source gathers were produced successfully that contained strong surface waves at 0.4-2.5 Hz, an order of magnitude below the corner frequency of the geophones, and modest body waves at 22-30 Hz, which are generally more difficult to obtain due to the need for many large, well-distributed subsurface sources. The virtual source gathers compare well to nearby explosive shots and are more densely spaced, but have a much lower signal-to-noise ratio. Analysis of the surface waves was complicated by strong higher-order modes. Spectral analysis of virtual source gathers required utilization of the geothermal plant energy, which produced usable signal at offsets required for mode separation. The virtual source dispersion curve compared well to a dispersion curve from a nearby explosive shot. P-waves were observed on the virtual source gathers. Creation of a low-quality multichannel reflection stack revealed two weak reflectors in the upper 2 km. / Master of Science