Sonar systems for object recognition

The deep sea exploration and exploitation is one of the biggest challenges of the next century. Military, oil & gas, o shore wind farming, underwater mining, oceanography are some of the actors interested in this eld. The engineering and technical challenges to perform any tasks underwater are great but the most crucial element in any underwater systems has to be the sensors. In air numerous sensor systems have been developed: optic cameras, laser scanner or radar systems. Unfortunately electro magnetic waves propagate poorly in water, therefore acoustic sensors are a much preferred tool then optical ones. This thesis is dedicated to the study of the present and the future of acoustic sensors for detection, identi cation or survey. We will explore several sonar con gurations and designs and their corresponding models for target scattering. We will show that object echoes can contain essential information concerning its structure and/or composition

Object characterisation using wideband sonar pulses

Characterisation of objects in an underwater environment is challenging. Success in the task can be beneficial in a variety of scenarios, which include oil and gas pipe maintenance, archaeology, and assistance to general underwater object identification. This work focuses on object characterisation, providing a solution for material identification. To do this, one must sense the underwater environment for which there are several different ways. Some of the most popular rely on sonar images. These provide limited information about the objects,mostly the shape, size and distance to the object. The study of acoustic wave scattering over a wide frequency range provides more information about the targets characteristics. This work builds on the principles of sound scattering. An acoustic echo reflected from an object has a different pulse shape and frequency composition than its initial pulse. These changes in the pulse are due to the interaction of the sound wave with an object during the reflection process and the pulses interaction with the transmission medium. Study of the reflected pulse can provide information about physical properties such as size, material and shell thickness. The objects used in this work are limited to spherical shells made of a variety of materials, and filled with different liquids or air. The task of material identification is approached in two different ways. The first one is a machine learning based approach. The classification is not based on the object’s shape, but on its physical properties including the composition material. Two approaches will be presented: one, where the spherical shell is described by the echo’s representation in time frequency domain and one, where it is described by the form function. The objects are classified using a number of machine learning techniques including support vector machine, gradient boosting and neural networks. The machine learning approaches give good results for a number of tasks, but are not sufficient to distinguish between materials with similar properties, like water and salt water. An alternative solution is presented in this thesis, which identifies the filler and the shell materials separately. This material identification approach is based on the timing of the sound scattering components. The echo reflected from an object is formed by a number of processes. The information about these processes can be extracted from the echoes and used to identify the material. This approach does not require any training data and shows good results, which are demonstrated on both the simulated and experimental data. This work focuses on object characterisation, providing a solution for material identification using underwater acoustics and signal processing techniques

Amplitude and phase sonar calibration and the use of target phase for enhanced acoustic target characterisation

This thesis investigates the incorporation of target phase into sonar signal processing, for enhanced information in the context of acoustical oceanography. A sonar system phase calibration method, which includes both the amplitude and phase response is proposed. The technique is an extension of the widespread standard-target sonar calibration method, based on the use of metallic spheres as standard targets. Frequency domain data processing is used, with target phase measured as a phase angle difference between two frequency components. This approach minimizes the impact of range uncertainties in the calibration process. Calibration accuracy is examined by comparison to theoretical full-wave modal solutions. The system complex response is obtained for an operating frequency of 50 to 150 kHz, and sources of ambiguity are examined. The calibrated broadband sonar system is then used to study the complex scattering of objects important for the modelling of marine organism echoes, such as elastic spheres, fluid-filled shells, cylinders and prolate spheroids. Underlying echo formation mechanisms and their interaction are explored. Phase-sensitive sonar systems could be important for the acquisition of increased levels of information, crucial for the development of automated species identification. Studies of sonar system phase calibration and complex scattering from fundamental shapes are necessary in order to incorporate this type of fully-coherent processing into scientific acoustic instruments

Interferometric synthetic aperture sonar system supported by satellite

Tese de doutoramento. Engenharia Electrotécnica e de Computadores. Faculdade de Engenharia. Universidade do Porto. 200

Analysis of the potential for coded excitation to improve the detection of tissue and blood motion in medical ultrasound.

Doppler ultrasound imaging modalities arguably represent one of the most complex task performed (usually in real time) by ultrasound scanners. At the heart of these techniques lies the ability to detect and estimate soft tissues or blood motion within the human body. As they have become an invaluable tool in a wide range of clinical applications, these techniques have fostered an intensive effort of research in the field of signal processing for more than thirty years, with a push towards more accurate velocity or displacement estimation. Coded excitation has recently received a growing interest in the medical ultrasound community. The use of these techniques, originally developed in the radar field, makes it possible to increase the depth of penetration in B-mode imaging, while complying with safety standards. These standards impose strict limits on the peak acoustic intensity which can be transmitted into the body. Similar solutions were proposed in the early developments of Doppler flow-meters to improve the resolution / sensitivity trade-off from which typical pulsed Doppler systems suffer. This work discusses the potential improvements in resolution, sensitivity and accuracy achievable in the context of modern Doppler ultrasound imaging modalities (taken in its broadest sense, that is, all the techniques involving the estimation of displacements, or velocities). A theoretical framework is provided for discussing this potential improvements, along with simulations for a more quantitative assessment. Colour Flow Imaging (CFI) modalities are taken as the main reference technique for discussion, due to their historical importance, and their relevance in many clinical applications. The potential achievable improvement in accuracy is studied in the context of modern velocity estimation strategies, which can be broadly classified into narrowband estimators (such as the “Kasai” estimator still widely used in CFI) and time shift based wideband strategies (normalised crosscorrelation estimator used, for instance, in applications like strain or strain rate estimation, elastography, etc.). Finally, simulations and theoretical results are compared to experimental data obtained with a simple custom-designed experimental set-up, using a single-element transducer

Broadband Echosounder Calibration and Processing for Frequency Dependent Target Strength and Phase Measurements

An analysis technique is developed for the calibration and processing for the target strength and phase spectra using a broadband echosounder. A new variable ‘residual phase’ is introduced, which could be used as a target classifier. Implementation of the method to characterise marine organism from the open ocean, demonstrated consistent target strength and residual phase and the matching of both the variables to the output of the numerical scattering model verified the method