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1. (WO2007000580) CONTRAST ENHANCEMENT BETWEEN LINEAR AND NONLINEAR SCATTERERS
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CLAIMS

1. A method for creating an acoustic observation of a target volume, the method comprising the steps of transmitting a group of at least two acoustic pulses towards the target volume, receiving at at least one detector an echo of the group scattered from the target volume, the echo having linear and nonlinear components, processing the scattered signal in such a way as to enhance at least part of the nonlinear component (and suppress the linear component) of the scattered signal in a signal P+ , processing the scattered signal in such a way as to suppress at least part of the nonlinear component (and enhance the linear component) of the scattered signal in a signal P., and producing a detection signal from a mathematical combination of the signals P+ and P..

2. The method of claim 1 in which the mathematical combination comprises a ratio of said signals P+ and P., or a function thereof, the combination being so chosen as to enhance or suppress at least part of the non-linear component, and suppress or enhance the linear component, according to requirements.

3. The method of claim 2 in which the mathematical combination comprises the ratio PJP+ whereby some of the nonlinear components are further suppressed and the linear components are further enhanced.

4. The method of claim 2 in which the mathematical combination comprises the ratio P+IP. whereby some of the non-linear components are further enhanced and the linear components are further suppressed.

5. The method of claim 2 in which the mathematical combination comprises the product of the ratio P+ IP. with the numerator P+ of the ratio.

6. The method of claim 2 in which the mathematical combination comprises the product of the ratio PJP+ with the numerator P. of the ratio .

7. The method of claim 1 comprising switching between two different mathematical combinations in order to provide a contrast in the detection signal between two different levels of enhancement/suppression of the non-linear components of the scattered signal.

8. The method of any one of claims 1 to 7 in which the second pulse of the group of acoustic pulses is substantially identical to the first pulse but of opposite polarity, the acoustic signal being of the form P^≡^Tφ-Tft—t^ , where F is a pressure function, t is time, and t, corresponds to the time delay between the two pulses.

9. The method of claim 8 in which the time between the centre of a first pulse of the group and the centre of a second pulse of the group is greater than half of the characteristic decay time of the signal between the pulses.

10. The method according to any one of the preceding claims applied to a target volume containing bubbles of a range of sizes, in which the driving frequency of the acoustic pulses is chosen to be lower than the resonance frequency of the majority of the bubbles.

11. The method of claim 10 applied to a target volume of water containing oceanic bubbles, the driving frequency of the acoustic pulses being less than 100 kHz.

12. The method of claim 11 in which the driving frequency of the acoustic pulses is less than 50 kHz.

13. The method of claim 12 in which the driving frequency of the acoustic pulses is less than 20 kHz.

14. The method of claim 11 in which the time delay t, between the pulses is greater than 10 μs.

15. Apparatus for creating an acoustic observation of a target volume in accordance with the method of any one of the preceding claims, the apparatus comprising an acoustic pulse transmitter and an acoustic pulse receiver, a signal processing unit responsive to the output of the receiver, the signal processing unit being so configured as in use to enhance at least part of the nonlinear component (and suppress the linear component) of the scattered signal to produce a signal P+ , and also to suppress at least part of the nonlinear component (and enhance the linear component) of the scattered signal to produce a signal P., and a combiner unit arranged to produce in use a detection signal by mathematically combining the signals P+ and P. in a manner such as to further enhance the contrast between said part of the nonlinear component and the linear component.

16. Apparatus for creating an acoustic observation of a target volume in a human or animal body, the apparatus comprising an acoustic pulse transmitter and an acoustic pulse receiver adapted to be positioned adjacent to a human or animal body, a signal processing unit responsive to the output of the receiver, the signal processing unit being so configured as in use to enhance at least part of the nonlinear component (and suppress the linear component) of the scattered signal from the target volume to produce a signal P+, and also to suppress at least part of the nonlinear component (and enhance the linear component) of the scattered signal from the target volume to produce a signal P., and a combiner unit arranged to produce in use a detection signal by mathematically combining the signals P+ and P. in a manner such as to further enhance the contrast between said part of the nonlinear component and the linear component.

17. A transmitting/receiving apparatus for observing a target by transmitting a pulsed electromagnetic signal towards the target and monitoring the receipt of signals scattered by the target, the transmitter being arranged to transmit a group of at least two pulses towards the target volume, the group of pulses being so configured that the scattered signal comprises linear and nonlinear components, the detector being arranged to process the scattered pulses resulting from said group in such a way as to modify the appearance of at least part of the nonlinear component of the scattered pulses in the receiver output signal.

18. Apparatus as claimed in claim 17 in which the electromagnetic signals are RADAR signals.

19. Apparatus as claimed in claim 17 in which the electromagnetic signals are LIDAR signals.

20. Apparatus as claimed in any one of claims 17 to 19 in which said part of the nonlinear component of the scattered electromagnetic pulses is suppressed (and the linear component enhanced) in the receiver output signal.

21. Apparatus as claimed in any one of claims 17 to 19 in which said part of the nonlinear component of the scattered electromagnetic pulses is enhanced (and the linear component suppressed) in the receiver output signal.

22. Apparatus as claimed in any one of claims 17, 18 or 19 in which a first receiver signal P+ is produced by the receiver by processing the received scattered signal so as to enhance part of the nonlinear component (and suppress the linear component) of the scattered electro-magnetic pulses, and a second receiver signal P. is produced by processing the received scattered signal in such a way as to suppress at least part of the nonlinear component (and enhance the linear component) , and a receiver output signal is produced from a mathematical combination of the signals P+ and P. in a manner such as to further enhance the contrast between said part of the nonlinear component and the linear component.