1,720,952 research outputs found
Classifying sediments on Dutch riverbeds using multi-beam echo-sounder systems
No full textThe economic importance of the Dutch rivers is very high as they are heavily used for inland waterway transport between the Netherlands and their neighbouring countries. A minimum depth must be guaranteed to keep the rivers navigable but also to ensure that the ships can carry maximum cargo. An attractive system for obtaining information about the riverbed bathymetry is the multi-beam echo-sounder (MBES). Furthermore, the MBES received echoes due to acoustic backscatter from the sediments in theory also allow for discriminating between different sediments. The aim of the research presented in this thesis was to develop methods for discriminating between different river sediments using MBES measurements. In order to fulfil this aim, MBES surveys were performed in the Rhine river and the Meuse river between 2007 and 2010. The research shows that indeed the MBES system can be used for discriminating between the different sediments present in the river areas. In addition an important finding is that areas differing in sediment type require different classification approaches.Air Transport and OperationsAerospace Engineerin
Aircraft noise calculation and synthesis in a non-standard atmosphere
No full textThe atmosphere modifies the emitted sound waves of an aircraft during propagation and is therefore important in the calculation of noise contours or synthesis. Noise contours present the resulting noise levels on the ground and are, as such, often applied for regulatory purposes. Aircraft noise synthesis is a technique that allows to transform a calculated prediction into audible sound that can be experienced in a virtual reality environment. Noise synthesis techniques allow people to be subjected to aircraft, routes or procedures that are still being designed. This dissertation describes recent research to improve the modeling of atmospheric propagation effects in aircraft noise contours as well as aircraft noise synthesis. Multi-event noise contours are usually calculated with standardized models that take non-standard propagation into account in an empirical fashion. A propagation algorithm was developed to augment such a model. Signal processing steps can be applied to transform a source noise prediction into an audible result. Furthermore, such steps can be utilized to apply propagation effects to a source noise signal. For a non-standard atmosphere this is not trivial. The role of a non-standard atmosphere is described by a dedicated simulation framework developed in this dissertation. The framework is applied to a flyover to demonstrate the effects associated with multiple ray paths and shadow zones. Besides demonstrating non-standard atmospheric effects, the framework was used to create synthesized results of actual flyovers near an airport. Subsequently, a comparison between measured results and synthesized results was executed. Furthermore, a method was designed to include the effect of turbulence-induced coherence loss of the direct and ground reflected ray in noise synthesis.Aircraft Noise & Climate EffectsAerospace Engineerin
The inaudible symphony: On the detection and source identification of atmospheric infrasound
No full textSound becomes inaudible if it consists of frequencies lower than 20 Hz, i.e. the human hearing threshold, and is called infrasound. Low frequency acoustic signals were first discovered after the eruption of the Krakatoa (Indonesia) in 1883. Due to its low frequency content, this infrasound traveled up to seven times around the globe while reaching altitudes over 100 kilometer. The ability to detect explosions with infrasound resulted in substantial scientific and societal interest during World War I and the era of atmospheric nuclear testing. This interest diminished as nuclear tests were confined to the underground under the Limited Test Ban Treaty in 1963. Recently, with the signature of the Comprehensive Nuclear-Test-Ban Treaty, infrasound gained renewed attention as it is being used as a verification technique. This thesis describes the complete sequence of measuring, processing and interpreting infrasound data. A microbarometer was developed and deployed in arrays to measure infrasound. Array processing and statistical detection techniques are applied to extract signals of interest from the continuous recordings. Knowledge on the atmospheric propagation appeared essential in identifying infrasonic sources, like meteors, volcanoes, sea waves and accidental chemical explosions. A large amount of coherent infrasound is continuously being detected from both natural and man-made sources, i.e. the inaudible symphony. Applications are foreseen in acoustic remote sensing where infrasound can be used as passive probe for the upper atmosphere. Non-acoustic phenomena, like gravity waves, can also be detected with the methods described in this thesis.Aerospace Engineerin
Infrasound source location: An automated method to estimate the location of infrasound sources as detected by microbarometer array recordings in the Netherlands over 2007
No full textBy recording infrasound signals one is able to listen to the sound of highly energetic events such as volcanoes, meteorites and the supersonic flight of aircraft, even if these occur hundreds of kilometres away. Recordings of infrasound signals are made using infrasound arrays, which can determine the incoming direction of the signals. In this research a method is developed to use the recordings of multiple infrasound arrays to estimate the location, in latitude and longitude, of infrasound sources. The method is applied to recordings of three infrasound arrays in the Netherlands which are operated by the Koninklijk Nederlands Meteorologisch Instituut (KNMI) and three additional arrays located in Germany and France.GeomaticsRemote SensingAerospace Engineerin
Aircraft Noise Shielding Assessment: The creation of a software tool to predict aircraft noise shielding
No full textThe amount of air traffic is increasing and cities become more and more populated. Since airports tend to be located close to or inside cities, an increasing amount of people suffer from aircraft noise pollution. More silent aircraft are needed to be able to reduce this noise pollution. One way to do this is through blocking the engine noise propagation with elements of the aircraft itself such as the wings. This method is called aircraft engine noise shielding To be able to shield the aircraft engine noise efficiently, a noise shielding assessment tool is required. In this thesis project, such a tool is developed for conventional tube with wing aircraft. This tool needs to have low computational cost while the accuracy should be sufficient for the preliminary design. To create a fast and accurate noise prediction tool, the aircraft's fuselage is modelled as a cylinder and the wings as planar surfaces. The engine is represented by a monopole sound source. The most suitable noise shielding calculation methods are found to be the barrier shielding method for the wings and the analytic solution of a cylinder for the fuselage. The noise shielding assessment tool is validated against results obtained with the Kirchhoff theory of diffraction. The noise shielding prediction tool developed in this thesis project has shown to be able to determine the noise attenuation by a point source of different aircraft configurations with low computational cost. The addition of the analytic results of the cylinder to the barrier shielding method has increased the accuracy by including the creeping rays. Keywords: Noise, Shielding, Barrier, Attenuation, Scattering, DiffractionANCEAerospace Engineerin
Converted waves in shallow marine environments: Modelling and field experiments
No full textThe shallow marine subsurface is explored for various engineering purposes e.g. constructing installations and platforms, laying pipelines and dredging for sand. Knowledge of the soil properties is essential to minimize the risks involved with these offshore activities. Energy resources in the form of gas hydrates and hydrocarbon reservoirs are often found a few hundred meters to a few kilometers below the seafloor. Accurate estimates of the sediments properties at shallow depths may considerably improve the image at the target depth. Seismic waves converted from compressional (P-) to shear (S-) mode can be useful to retrieve these properties. The acquisition and processing of converted-wave data in shallow marine environments remains challenging. This is due to the high velocity ratio between the P- and the S-wave in the sediments. In this thesis, the acquisition and processing of converted waves is addressed through field experiments and synthetic modelling.GeotechnologyCivil Engineering and Geoscience
Underwater detection, classification and localisation: Improving the capabilities of towed sonar arrays
No full textThe end of the Cold War and the collapse of the Warsaw pact have resulted in a change of operational theatre for the naval forces of the North Atlantic Treaty Organisation (NATO). In particular, the focus of Anti Submarine Warfare forces has shifted from tracking Soviet nuclear ballistic missile submarine in the deep waters of the Atlantic ocean to hunting smaller and quieter Diesel electric submarines in coastal water. In most scenarios, towed array sonars are the best sensor to detect, classify and localise submarines. The long passive towed array sonars used during the Cold war are more difficult to use in coastal waters and are being replaced by most Navies by Low Frequency Active Sonars (LFAS) using a towed source and shorter towed receiving array. These shorter towed arrays can be used in both active and passive modes. In passive mode, their reduced size offer limited performance compared to their longer equivalent. In active mode, they can detect submarines at long ranges in shallow waters but are plagued by false alarms caused by echoes from features of the seafloor. This thesis deals with algorithms improving Detection, Classification and Localisation for towed sonar arrays, with a specific focus on LFAS sonars. In Chapter 2, we derive, analyse and apply to measured data a method for improving detection performance with short passive towed arrays. An important issue in detection of quiet acoustic source with short towed arrays is the improvement in signal-to-noise ratio (SNR) and bearing resolution for targets emitting low frequency signals. One of the techniques believed to improve these characteristics is Synthetic Aperture Sonar (SAS). The method is based on the artificial enlargement of a sonar array by coherently integrating acoustic snapshots at different antenna positions. We first derive theoretical measures of performance of passive SAS and report on its application in combination with other signal-processing algorithms. Its theoretical performance is compared with that of the frequently used incoherent integration. The used passive SAS algorithm is the method known as Extended Towed Array Measurement (ETAM) or the overlap correlator. It is based on the correlation of data snapshots on overlapping hydrophones. Correlation is a key issue in this method and since it is affected by noisy targets, some gain can be expected from noise cancellation. The influence on the performance of ETAM of a method of tow ship noise cancelling at hydrophone level (Inverse Beam Forming, IBF) is analysed. This approach increases ETAM performance by removing a loud and highly correlated noise source, the tow ship, and thus enhancing the other targets in the beam pattern. The results of the algorithms applied to two experimental datasets show that they bring an improvement close to theoretical expectations. Port starboard discrimination and the successful combination of IBF with ETAM make this approach innovative. In Chapter 3, methods for improving the localisation of a source with a short towed array are analysed and applied to data, both simulated and measured at sea. Localisation performance with sonar towed array is related to the array length. The knowledge of the position of a given acoustic source gives a critical tactical advantage to a ship. There are a limited number of ways to estimate the range of a source with a towed passive sonar, most requiring the towing platform to execute a manoeuvre. These manoeuvres are undesirable as they take a lot of time, cause bending of the towed array and can even put the towing platform in harm’s way. We present a number of source position estimation methods for both broadband and narrowband sources suitable for short towed arrays. Recursive methods based on the extended Kalman filter are first examined. A new method based on the integration of time delay of arrival measurements along the sonar path is described. We derive theoretical performance indicators and show that this method gives the possibility to estimate the position and speed of a source without a manoeuvre. In Chapter 4, the Classification performance of a broadband waveform is analysed and measured on data collected at sea. Like any long-range active sonar system, LFAS produces a large amount of unwanted sea bottom echoes or clutter. These echoes give rise to false alarms that increase the computational load of target trackers and jeopardise the correct classification of each echo. The number of false alarms due to clutter can be reduced either through echo classification techniques or through Doppler filtering provided the targets of interest are in motion. Much research has been carried out on waveform investigation for the efficient use of bandwidth capabilities of modern sonar transmitters. Among the quantity of waveforms, Binary Phase Shift Keyed (BPSK) pulses have emerged as exhibiting cross-correlation properties relevant to Doppler filtering while maintaining a range resolution comparable to Frequency Modulated (FM) pulses. We have successfully applied a false alarm reduction technique using contacts obtained with an FM pulse subsequently processed by Doppler filtering with a BPSK pulse. The Doppler classification performance for this pulse is evaluated on an experimental dataset and a few limitations of BPSK are identified.Aircraft Transport and OperationAerospace Engineerin
Adaptive motion compensation in sonar array processing
No full textIn recent years, sonar performance has mainly improved via a significant increase in array ap-erture, signal bandwidth and computational power. This thesis aims at improving sonar array processing techniques based on these three steps forward. In applications such as anti-submarine warfare and mine hunting motion of the sonar needs to be accounted for. For towed anti-submarine warfare sonar, beamforming methods are developed for the port/starboard (PS) discrimination problem and for Doppler compensation. In mine hunting, synthetic aperture sonar (SAS) is a promising technique to improve sonar performance by combining of multiple pings. Efficient imaging techniques with adequate motion compensa-tion are examined to control the data flow. Each processing method is implemented, tested and assessed. For the PS discrimination prob-lem three beamformers are investigated. These beamformers and the Doppler compensation methods are investigated theoretically, with simulations and with datasets recorded at sea. For mine hunting a complete SAS processing chain with motion estimation, motion compensation and imaging is developed. It is tested on simulations and on five independent experimental datasets. The processing techniques proposed substantially improve sonar performance. Triplet tech-nology is a successful solution for the PS discrimination problem. The ambiguous direction is sufficiently suppressed with the beamformers proposed. The requirements for Doppler com-pensation are derived analytically and are met without complicated adaptations. The SAS re-search showed that for the cases considered, wavenumber frequency imaging is preferred when using adequate motion compensation. A key-finding while comparing the imaging algo-rithms was enhancement of an important sonar classification clue, the acoustic shadow.Applied Science
Sediment characterization by geo-acoustic inversion of marine seismic data received on a towed hydrophone array
No full textAcoustic Remote SensingAerospace Engineerin
Developing a data based method to quantify the effects of flight track, aircraft weight and engine setting on the received aircraft noise levels
No full textAirports in The Netherlands are subjected to tangent environmental laws to restrain pollution and noise nuisance. Amsterdam Airport Schiphol (AAS) is one airport dealing with this regulatory framework but nevertheless they are resolute to continue growth with respect to aircraft movements. To cope with the law related to aircraft noise, the department Stakeholder Strategy and Development (SSD) of AAS is responsible for the implementation of Noise Abatement Measures (NAMs). NAMs are used to minimize aircraft noise as to be able to maximize the number of aircraft movements within the environmental law as set by the Dutch government. SSD demands to be able to visualize the effect of a NAM by measuring aircraft noise with its Noise Monitoring System (NOMOS). However, in practice it appears that the effect of a NAM to the exposed noise level cannot easily be determined since the total set of measurements show a high degree of scattering. This is caused by the fact that many other parameters are contributing to the exposed noise level as, for example, engine setting and aircraft configuration. Therefore, AAS encounters difficulties evaluating the effectiveness of implemented noise reducing measures using the noise levels as measured by NOMOS. Hence, the research question becomes: How can the distinctiveness between noise measurements effectively be improved as to evaluate the direct effect of a Noise Abatement Measure to the measured noise level? As a first step towards answering this question, aircraft mass m and aircraft engine setting N1 were identified which were expected to mask the effect of a NAM to the measured noise level. Then, the Peak Find Method (PFM) is developed to determine N1 from the associated acoustic time series as retrieved from NOMOS. Thirdly, aircraft mass m was found to be very difficult to determine from aircraft performance theories. Therefore, the lift-off speed at take-off Vlof2 is taken as an aircraft mass representative. With the two predictors N1 and Vlof2 available and the measured maximum loudness levels Lmax retrieved from NOMOS, a Multivariate Linear Regression Analysis (MLRA) is carried out to assess the effect of the two predictors to variations in Lmax. Last, the identified MLRA model is used to subtract the contribution of N1 and Vlof2 from the received noise levels, hence leaving the direct effect of a NAM to the measured noise level. Initial correlation analysis showed no correlation between N1 and Lmax and neither between Vlof2 and Lmax. While the MLRA model is based upon the identified values of the predictors, it was therefore not expected that the high variations in Lmax would decrease when using these predictors, bearing in mind the results of both correlation analysis. And ultimately, by using the MLRA model only 7% of the total variation in Lmax could be explained, which turned out to be too less to evaluate the direct effect of a NAM to the measured noise level.Air Traffic Performance and the EnvironmentControl & OperationsAerospace Engineerin
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