Maria Palmese

An Efficient Digital CZT Beamforming Design for Near-Field 3-D Sonar Imaging

A planar array of sensors is required to collect the signals coming from a 3-D scene to generate volumetric underwater acoustic images. The method most frequently used to process the acquired signals is a digital beamforming algorithm. In general, owing to the high number of sensors and beam signals, the computational load is prohibitive for real-time image generation. In the literature, a frequency-domain beamforming technique based on the chirp zeta transform (CZT), which is efficient and computationally advantageous, has been introduced for linear and planar arrays working in the far field. This paper proposes an extension of the CZT beamforming that has been specifically devised to cope with the requirements of volumetric sonar imaging. In particular, the processing of wideband signals collected by a planar array and generated by a scene placed in both near-field and far-field conditions is enabled with a computational load that is one or two orders of magnitude lower than that of the traditional frequency-domain and time-domain beamforming implementations. To attain this result, the Fresnel delay approximation, a useful definition of steering angles, and the setting up of multiple focal regions are adopted. In addition, a computationally convenient technique to generate cubic resolution cells is introduced.

Devising an Affordable Sonar System for Underwater 3-D Vision

In this paper, the preliminary design and assessment of a high-resolution 3-D acoustic imaging system based on a sparse planar array of sensors, which is particularly intended for underwater applications, are presented. Critical issues in the development of high-resolution 3-D sonar systems are 1) the cost of hardware, which is associated with the huge number of sensors that compose the planar array, and 2) the computational burden of processing the signals that were gathered. Here, such problems are overcome by the optimized synthesis of an aperiodic sparse array that allows the device to operate at different frequencies, yielding an acceptable sidelobe level and a good tradeoff between the field of view and the resolution. The array optimization is performed using an efficient stochastic method, in which the number of sensors is minimized, whereas their positions and weights are simultaneously optimized. To test the validity of the designed system, the signals that the sparse array received in response to the insonification of a scene with a wideband pulse are simulated, and voxel-based beamforming is applied to generate the 3-D image. The obtained images show high fidelity to the geometrical characteristics of the scene, in accordance with the expected performance of the 3-D sonar system.

Three-Dimensional Acoustic Imaging by Chirp Zeta Transform Digital Beamforming

To generate volumetric underwater acoustic images, a planar array of sensors is required to collect the signals coming from a 3-D scene. The method most frequently used to process the acquired signals is a beamforming algorithm. In general, owing to the high number of sensors and beam signals, the computational load is prohibitive for real-time image generation. An efficient frequency beamforming implementation based on the use of the chirp zeta transform (CZT) has been proposed in the literature for the case involving linear arrays. In this paper, we propose an algorithm to perform CZT beamforming on the wideband signals collected by a planar array generated by a scene placed in the far field. This extension becomes simpler and more intuitive thanks to a nonconventional method of defining the azimuth and elevation angles. In terms of computational complexity, the analysis compares the solution developed with traditional beamforming techniques carried out in both time and frequency domains, showing several advantages for the proposed method.

Acoustic imaging of underwater embedded objects: signal simulation for three-dimensional sonar instrumentation

This paper presents a method able to emulate the signals received by a sonar system exploring a submerged environment. The simulation of the response of an underwater scene insonified by a wideband pulse has been based on the reproduction of the interactions of the acoustic field with a buried object, with the seabed surface, and with the sediment volume. The signals gathered by the sonar array are emulated by modeling the surfaces of both the seabed and objects as a dense random grid of small discrete scatterers, by following Rayleighs law, and by integrating the responses of the scatterers. The roughness effects are simulated by allowing random distances from the actual scatterer positions to their nominal positions on the surface. Sediment-volume inhomogeneities are modeled by a random distribution of the small asymmetrical scattering volumes, characterized by their three-dimensional (3-D) dimensions, densities, and sound velocities. In general, the simulator is very flexible in defining the object, the related scenario, and the major physical parameters involved. A voxel-based beamforming to generate the 3-D images starting with the simulated signals is also presented and briefly discussed. The 3-D images obtained appear very realistic and contain all the expected elements in the right relationships, including the typical speckle noise

A comparative analysis of multi-pulse techniques in contrast-enhanced ultrasound medical imaging

One of the important issues in the field of ultrasound medical imaging using contrast agents is the development of techniques able to separate the response of the contrast media from that of the biological tissues. In the literature, one can find various solutions involving the use of multiple transmitted signals and the combination of related echoes. However, the quality of these techniques may be reduced due to some undesired effects that are seldom considered, despite the fact that they are always present in real systems. These effects are the signal distortions introduced by the hardware equipment, the thermal noise in the electronic circuitry, and body motion between successive pulses. In this paper we propose a simulation tool that will allow the calculation of the backscattered echo from a population of contrast agents immersed in a biological tissue, considering all the mentioned effects. With this tool, an assessment of the comparative robustness of three well-known multi-pulse techniques has been carried out under realistic working conditions and the performance of the three techniques has been evaluated in terms of contrast-to-tissue ratio and signal-to-noise ratio. The results show that the undesired effects have a strong impact on these techniques and that there are notable differences in their robustness. Finally, some suggestions on the choice of the particular technique to be applied are provided on the basis of the specific work conditions.

On the Optimization of the Transmitted Beam in Contrast-Enhanced Ultrasound Medical Imaging

The optimization of the ultrasound pulses transmitted by an echographic scanner is considered here in order to attain an insonification beam having a quasi-constant pressure profile over a large depth interval. An equalized pressure intensity allows a correct exploitation of the ultrasound contrast agent injected into the human body and, in turn, permits one to fully exploit its nonlinear response. A stochastic method of synthesis (based on the simulated annealing scheme) that is able to produce the desired transmission beam profile, acting simultaneously on the carrier frequencies of the acoustic pulses emitted by the arrays transducers and on the apodizing weights, has been developed and assessed. The results obtained by the joint optimization, in terms of acoustic pressure profile, sidelobe level, and main lobe shape, are reported, discussed, and compared with those obtained by the traditional emission strategy and by the exclusive optimization of the apodizing weights or the carrier frequencies. A significant improvement of the ultrasound beam generated by the joint optimization over those generated by the exclusive optimization is pointed out.

Design and Assessment of a Low-Cost 3-D Sonar Imaging System Based on a Sparse Array

In this paper, the design and the assessment of a high-resolution three-dimensional acoustic imaging system based on a sparse planar array of sensors are presented. The aim is to generate useful acoustic 3D images in underwater context. Towards this end, a planar array is mandatory, as a linear aperture does not allow one to discriminate signals coming from a 3D space. One critical issue in the development of high-resolution 3D sonar systems is the hardware cost associated to the necessary huge number of sensors. In this paper, an innovative 3D imaging system able to operate at different resolution levels is proposed that is based on a single sparse planar array consisting of only 584 elements. Such a limited number of sensors represents an important stage in designing 3D acoustic imaging systems, making feasible the achieving of a drastic reduction in both costs and successive processing associated to the system. The array optimization is performed by an efficient stochastic method based on the simulated annealing algorithm, in which the positions and the weights of the array elements are optimized simultaneously. To test the validity of the proposed system, the signals received by the sparse array as the response of a given scene insonifled by a pulse are simulated. To move from the simulated signals to the 3D image of the scene, a voxel-based beamforming in the time domain is designed. To assess the proposed acoustic imaging system, several complex scenes are taken into account and images are obtained that exhibit a high fidelity to the geometrical and physical characteristics of the assumed underwater environments

From 3-D Sonar Images to Augmented Reality Models for Objects Buried on the Seafloor

The investigation of man-made objects lying on or embedded in the sea floor can be carried out with acoustic imaging techniques and subsequent data processing. In this paper, we describe a processing chain that starts with a 3-D acoustic image of the object to be examined and ends with an augmented reality model, which requires minimal user involvement. Essentially, the chain includes blocks devoted to statistical 3-D segmentation, semi-automatic surface fitting, extraction of measurements, and augmented reality modeling. In particular, the 3-D segmentation method presented here is based on a volume-growing approach, which is essentially a 3-D extension of the traditional 2-D region growing. The volume-growing operation is guided by a statistical approach based on the optimal decision theory. The surface-fitting block is based on predefined geometric models, i.e., one of them is tentatively selected by the user after a preliminary study of the segmented object and is automatically or partially manually adapted to the segmented data by exploiting an inertial tensor. The proposed chain was successfully applied to the analysis of some 3-D acoustic images obtained from both simulated and real signals acquired by different sonar systems and containing objects that were completely or partially buried. The segmentation results provided an effective help in the identification of the objects shape, i.e., facilitating the subsequent surface-fitting step and the extraction of related measurements.

Spread Spectrum Modulation for Acoustic Communication in Shallow Water Channel

A wireless underwater acoustic communication system based on the combination of chirp modulation and direct-sequence spread-spectrum signaling is presented. The use of chirp signals takes advantage of the low Doppler sensitivity in the matched filter operation whereas the choice of pseudo noise (PN) sequences allows one to reduce narrowband interference arising from other users and self-interference due to multipath propagation. The system here described is made of a transmitter encoding the bits with linear chirps multiplied by PN sequences, and a rake receiver that allows one to positively exploit the energy present in the most significant propagation paths. Moreover a tracking procedure that allows an adaptation to the instantaneous Doppler shift has been devised and tested. Results, obtained on real data over a long-range shallow-water channel with a moving transmitting platform, are presented and discussed.

Digital Near Field Beamforming for Efficient 3-D Underwater Acoustic Image Generation

To generate volumetric underwater acoustic images a planar array of sensors is required to collect the signals coming from a 3-D scene. The method most frequently used to perform the spatial elaboration of the acquired signals is the beamforming algorithm. In general, owing to the huge number of sensors and the massive number of beam signals, computational load becomes prohibitive for real-time image generation. An efficient frequency beamforming implementation based on the use of the Chirp Zeta Transform (CZT) has been proposed in the literature for the case of a scene placed in the far-field and involving linear arrays. In this paper, we propose an algorithm to perform CZT beamforming on the wideband signals collected by a planar array and generated by a scene placed in the near-field. This extension becomes simpler and feasible thanks to (i) a non-conventional method of defining the azimuth and elevation angles; (ii) the Fresnel approximation in the delay computation; (iii) an adequate organization of the different operations. In terms of computational complexity, the analysis compares the solution developed with traditional beamforming techniques, showing several advantages of the proposed method.