Digital time-delay acoustic imaging

• US 5,793,703 A
• Filed: 11/12/1996
• Issued: 08/11/1998
• Est. Priority Date: 03/07/1994
• Status: Expired due to Term

• Current Assignee: Saab AB
• Original Assignee: Bofors Underwater Systems AB (Saab AB)
• Inventors: Shippey, Geoffrey Arthur
• Primary Examiner(s): Pihulic, Daniel T.
• Application Number:
  US08/714,054
• Time in Patent Office: 637 Days
• Field of Search: 367/88, 367/7, 342/25
• US Class Current: 367/88
• CPC Class Codes: G01S 15/104 wherein the transmitted pulses use a frequency- or phase-modulated carrier wave
• CPC Class Codes: G01S 15/8904 using synthetic aperture techniques

Abstract

This invention relates to improvements in or relating to digital time-delay acoustic imaging. According to the invention there is provided an acoustic imaging method for marine survey and other purposes comprising the steps of: insonifying a target area by periodic acoustic pulses; detecting reflections of said pulses by an array of at least two receiver transducers, which transducers generate respective output signals in response thereto, wherein further the signals are: sampled in time at a predetermined rate during an interval between said pulses; converted from analog to digital form; processed so as to generate a composite acoustic signal for each pixel in the insonified target area required to be imaged; and match-filtered with optimised filters in the frequency domain with an estimated source pulse signature.

Claims

1. An acoustic imaging method comprising the steps of periodically transmitting wideband acoustic pulses to insonify a target area, detecting reflections of said pulses at a first set of at least two receiving positions using at least one receiver transducer to generate transducer output signals, and processing the transducer output signals corresponding to the set of receiving positions so as to produce an image of the target area, said processing step comprising:

(a) sampling said transducer output signals at a predetermined rate during an interval between the transmitted pulses;

(b) digitizing the sampled signals by an analog-to-digital conversion;

(c) quadrature match filtering the digitized sampled signal for each receiving position using a complex kernel derived from an estimated signature of each of the transmitted pulses, sampled at a same rate as the transducer output signals, so as to generate a set of complex filtered samples for each receiving position;

(d) taking, for each pixel in a required image, a weighted sum of selected phase-shifted complex filtered samples for the first set of receiving positions to form a complex composite signal by;

(i) selecting from said sets of complex filtered samples, samples whose echo times are closest to an estimated time of flight of the transmitted pulse to the point in the target area corresponding to the pixel and back to the transducer, and

(ii) interpolating the selected complex samples by phase-shifting to compensate for a difference between required and selected time delays, wherein all phase-shifts are determined from the time differences using a single frequency.

2. The acoustic imaging method of claim 1, wherein the complex kernel used in step (c) is derived from the estimated signature, optimized for at least one of a shorter kernel length and a different compressed pulse shape.

3. The acoustic imaging method of claim 1, wherein the complex kernel used in step (c) is derived from the estimated signature, modified for a Doppler shift caused by relative movements of the transducer and a source of the reflections.

4. The acoustic imaging method of claim 1, wherein the single frequency used in step (d)(ii) for calculating phase shifts corresponds to a center frequency of the source pulse.

5. The acoustic imaging method of claim 1, wherein the wideband source pulse is of a balanced spectrum.

6. The acoustic imaging method of claim 1, wherein the source pulse comprises a linear swept-frequency chirp.

7. The acoustic imaging method of claim 1, wherein a plurality of receiver transducers are provided in an array, each transducer defining a respective receiving position.

8. The acoustic imaging method of claim 7, wherein a second set of receiving positions is defined by moving the transducer array between successive pulses, and samples received from said second receiving positions combined with those of the first set receiving positions to obtain a synthetic aperture image.

9. The acoustic imaging method of claim 1, wherein repeated imaging of a set of pixels derived from successive identical insonification pulses after lateral movement of the receiver transducer(s) is used to generate a phase-coherent synthetic aperture image by combination of the complex composite signals given for each pixel by step (d).

10. The acoustic imaging method of claim 1, further comprising the step of:

(e) generating a magnitude for each pixel by taking modulus of the complex composite signal given by step (d).

11. The acoustic imaging method of claim 10 wherein the repeated imaging of a set of pixels derived from successive, not necessarily identical, insonification pulses after lateral movement of the receiver transducer(s) is used to generate a non-coherent synthetic aperture image by a combination of the pixel magnitudes given by step (e).

12. The acoustic imaging method of claim 1, wherein the pulses are selected at frequencies and magnitudes transmittable in liquid.

25. The acoustic imaging method of claim 1, wherein the acoustic imaging method is performed for underwater search and survey.

13. An acoustic imaging apparatus comprising a transmitter for periodically transmitting wideband acoustic pulses to insonify a target area, at least one receiver transducer for detecting reflections of said pulses at a first set of at least two receiving positions to generate transducer output signals, and a processor for processing the transducer output signals corresponding to the first set of receiving positions to produce an image of the target region, said processor comprising:

a digitizer for sampling said transducer output signals at a predetermined rate during an interval between the transmitted pulses and digitizing the sampled signals by an analog-to-digital conversion;

a filter for quadrature match filtering the digitized sampled signal for each receiving position using a complex kernel derived from an estimated signature of one of the transmitted pulses, sampled at a same rate as the transducer output signals, so as to generate a set of complex filtered samples for each receiving position;

a signal compositor for taking a weighted sum of selected phase-shifted complex filtered samples to form a complex composite signal of each pixel in the required image, the signal compositor comprising

(i) a selector for selecting from said sets of complex filtered samples, samples whose echo times are closest to an estimated time of flight of the transmitted pulse to the point in the insonified area corresponding to the pixel and back to the transducer, and

(ii) an interpolator for interpolating the selected complex filtered samples by phase-shifting to compensate for a difference between required and selected time delays, all phase-shifts being determined from the time differences using a single frequency.

14. The acoustic imaging apparatus of claim 13, wherein said complex kernel is derived from the estimated signature, optimized for at least one of a shorter kernel length and a different compressed pulse shape.

15. The acoustic imaging apparatus of claim 13, wherein said complex kernel is derived from the estimated signature, modified for Doppler shift caused by a relative movement of the transducer(s) and a source of the reflections.

16. The acoustic imaging apparatus of claim 13, wherein the single frequency used by the signal compositor for calculating phase shifts corresponds to a center frequency of the source pulse.

17. The acoustic imaging apparatus of claim 13, wherein the wideband source pulse is of a balanced spectrum.

18. The acoustic imaging apparatus of claim 13, wherein the source pulse comprises a linear swept-frequency chirp.

19. The acoustic imaging apparatus of claim 13, wherein a plurality of receiver transducers are provided in an array, each transducer defining a respective receiving position.

20. The acoustic imaging apparatus of claim 19, wherein a second set of receiving positions is defined by moving the transducer array between successive pulses, and samples from said second set of receiving positions combined with those of the first set of receiving positions to obtain a synthetic aperture image.

21. The acoustic imaging apparatus of claim 13, wherein the repeated imaging of a set of pixels derived from successive identical insonification pulses after lateral movement of the receiver transducer(s) is used to generate a phase-coherent synthetic aperture image by a combination of the complex composite signals given for each pixel by step (d).

22. The acoustic imaging of claim 13, further comprises means for generating a magnitude for each pixel by taking a modulus of the complex composite signal given by the signal compositor.

23. The acoustic imaging apparatus of claim 22, wherein the repeated imaging of a set of pixels derived from successive insonification pulses after lateral movement of the receiver transducer(s) is used to generate a non-coherent synthetic aperture image by a combination of the pixel magnitudes given by said magnitude generating means.

24. The acoustic imaging apparatus of claim 13, wherein the signals are selected in frequencies and magnitudes transmittable in liquid.

26. The acoustic imaging method of claim 13, wherein the acoustic imaging method is performed for underwater search and survey.