Communications – electrical: acoustic wave systems and devices – Echo systems – Side scanning or contour mapping sonar systems
Patent
1996-11-12
1998-08-11
Pihulic, Daniel T.
Communications, electrical: acoustic wave systems and devices
Echo systems
Side scanning or contour mapping sonar systems
367 7, G01S 1589
Patent
active
057937036
DESCRIPTION:
BRIEF SUMMARY
1. INTRODUCTION
This invention relates to improvements in or relating to digital time-delay acoustic imaging. The invention particularly, though not exclusively, relates to an acoustic method and apparatus intended for marine survey applications. The invention is also applicable to other applications of acoustic imaging.
2. PRIOR ART
2.1 GENERAL
The general approach to digital acoustic imaging given in the literature is to store the set of samples received from a 2D or 3D array of transducers in a "space-time memory". Processing of this data set should enable an
Time-delay beamforming is the most widely used analog technique, being robust and either narrowband or wideband. It can also be extended from beamforming to imaging in polar coordinates by using a large number of beams simultaneously, separated in angle by a few degrees. A digital version preserves these advantages, while adding some important new capabilities, particularly the ability to image in a Cartesian or other coordinate system, and the selection of the region of the scene to be imaged under computer control ("pan and zoom").
Conventional analog time-delay beamforming uses a fraction of the information received from an array of transducers to generate a line-scan of the scene in one particular direction. This direction is defined by a differential time delay, inserted between each pair of transducer echoes, before summation to form a composite signal with the required directivity. Time-delay imaging relies on fine compensation for phase-shifts between samples received at adjacent transducers. Hence a digital implementation requires either high sampling rates which is expensive both in digitizing hardware and computational load, or some method of interpolation between digital samples. techniques for interpolating or "upsampling" a sequence of digital samples, and proposed specific beamforming procedures using interpolated sampled signals from a transducer array. These interpolation techniques are based on local spectral estimation of FFT components by quadrature including Pridham and Muccis' formulation. Ries gave an analytic solution for optimising the filter kernels, but did not discuss the possibility of avoiding explicit interpolation in the time domain.
2.2 THE INVENTOR AND OTHERS
The acoustic imaging method claimed there uses essentially the same phase-shifting technique as Pridham and Mucci, employing a set of adjacent digital filters spanning the signal bandwidth (quadrature comb filter) for local spectral estimation. However explicit interpolation in the time domain is avoided with consequent simplification in the computation.
This earlier Patent proposed a wideband version of the technique in which local spectral components of the composite echo are cross-multiplied by local spectral components of a frequency modulated source pulse to achieve pulse compression at little extra cost. However it gives no idea of how long the time window used in the comb filter for local spectral estimation should be, or the shape of the window function.
It turns out that the shape of the window function is extremely important for the success of match-filtered imaging. This window function determines the bandwidth of each spectral component used in the comb filter. Unless the bandwidth of the source signal is spanned uniformly by the spectral components of the comb filter, time-delay correction by phase-shifting the spectral components becomes inaccurate. The shape of the window function also determines the shape of the match-filtered output, for example the width of the peak, and discrimination against background noise. It is not at all obvious that the same window function can satisfy these different requirements simultaneously. In the present Patent Application a method of designing the window function is given for any kernel length. It turns out that the required shape is not standard, so the availability of at least one design technique is very important. Furthermore, using this design technique, good performance can be obtained with few
REFERENCES:
patent: 5090245 (1992-02-01), Anderson
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Bofors Underwater Systems AB
Pihulic Daniel T.
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