Patent Description:
SONAR systems often have difficulty in bubbly conditions, which can be brought about by interaction with physical features of the coastline or seabed as well as by ships and boats. The measurement and modelling of surface ship bubbly wakes is of great importance in military applications. The wake may interfere with acoustic operations through scattering and absorption while also providing a method for detection, tracking and identifying a particular ship.

When SONAR is used to identify a target (which may be a target in the military sense or, more generally, an object of interest), problems may be encountered with the acoustic backscatter from bubbles. When insonified (i.e. subjected to acoustic energy from a SONAR transmitter), the backscatter from bubbles can hinder the detection of 'real' targets in the water. Furthermore, when excited by a high amplitude acoustic pressure, the bubbles demonstrate their inherently nonlinear behaviour if they are of the correct size, which usually means they are close to their pulsation resonance or, if the amplitude is sufficiently high (i.e. sufficient to enable the water to briefly go into tension during the peak rarefaction of the acoustic cycle), smaller than their pulsation resonance.

There are known prior art techniques which use a number of two-pulse sonar techniques which can be employed to separate linear and nonlinear scatterers. TWin Inverted Pulse Sonar (TWIPS) and Biased Pulse Summation Sonar (BiaPSS) are known processes that exploit nonlinear bubble dynamics to perform such a classification, with clutter reduction. Consequently, these techniques can be used to enhance target detection in bubbly waters. TWIPS and BiaPSS rely upon bubbles being driven to large nonlinear pulsations, but this is dependent upon availability of a high-amplitude SONAR source.

<CIT> discloses a system using chirp reversal ultrasound contrast imaging.

<CIT> discloses a synthetic aperture SONAR and related processing method.

"<NPL> discloses a marine sonar system with a highly repeatable source signature to facilitate the acquisition of correlated data with high accuracy.

"<NPL>, discloses a processing approach motivated by the neural processing of echolocation waveforms by certain species of bats.

<CIT> discloses a target extraction system and associated method.

<CIT> discloses a memory system using filterable signals.

It is an aim of embodiments of the present invention to address issues with the prior art techniques, whether mentioned explicitly herein or not.

For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings in which:.

As discussed with reference to the prior art, the use of a pair of time-reversed chirp signals has been proposed previously for biomedical applications. However, according to an embodiment of the present invention, it is possible to enhance the scatter from both linear targets and bubbles, with a greater effect on the former, owing to the difference between bubble responses, to an increasing or decreasing frequency sweep of the driving chirp signal.

When a bubble is insonified at a frequency that is much greater than its resonance, it barely responds. It does not pulsate to large amplitudes because it is being driven off-resonance in the inertia-controlled regime. However, when a bubble is driven off-resonance but at a frequency that is less than its resonance, it can still pulsate. This is because a small bubble responds with a fast response time.

Considering a pair of down-chirp pulses (i.e. the chirp signal is reduced from a higher to a lower frequency), prior to being driven at resonance each bubble has been insonified by frequencies higher than its resonance, to which it barely responds, as stated before. Therefore, the initial condition is similar to a stationary bubble wall. Subsequently, the lower frequencies in the down-chirp can delay the damped decay of its resonant oscillations. Therefore, two down-chirps give reproducible echoes from the bubbles, with greater amplitudes in nonlinear response.

In contrast, a pair of up-chirp pulses (i. e the chirp signal is increased from a lower to a higher frequency) will both drive the bubble into oscillation before its resonance and, as such, the initial conditions are not so reproducible (and therefore the cancellation and enhancement provided by the prior art TWIPS technique is not so effective). In addition, the damped decay is more rapid and reduces the energy of the nonlinear signal components.

This is illustrated in <FIG>, which shows the response of a <NUM> bubble when insonified by both a linearly decreasing - as shown at (a) - and increasing - as shown in (b) - sine sweep. It is immediately obvious that the responses are different, confirming this by reversing the response to the up-chirp and overlaying it on top of the response to the down-chirp - as shown at (c). The circled region at the right of the plot clearly shows the different bubble responses to the up and down chirp signals.

Analysis of the responses during the time for which the driving signal is below the bubble's natural frequency indicates that the down-chirp yields a greater nonlinear response.

For a linear scatterer, the responses will be the same. Therefore a subtraction of the two will cancel each other, whereas P+ (the sum of the echoes from the two pulses) will be non-zero. However, using time-reversed pulses that are also in anti-phase, the subtraction of the two reflections should enhance the linear component while suppressing the even-numbered harmonics too. The addition of the two will only contain the odd-numbered harmonics thus providing a similar effect to the prior art TWIPS technique with identical, anti-phase pulses.

In the prior art TWIPS technique, there is a necessity for both pulses to have a sufficient delay between them to allow echoes from the first pulse to end before the second pulse is emitted. However, the delay also needs to be short enough to ensure that the same bubble cloud is insonified by both pulses.

Embodiments of the present invention address this issue and allow for a greater range of detection (ignoring transmission loss and attenuation owing to bubbles and other particulates) by emitting two pulses in close proximity or at once. Simultaneously emitting identical pulses in anti-phase (as used in TWIPS) would result in emitting nothing as the two pulses would cancel each other out. Therefore, embodiments of the invention use two different pulses that give similar (match-filtered) responses by a bubble cloud.

In order to ensure that the transmitted pulses do not simply cancel each other, the profiles of the up and down chirp signals are selected such that this does not happen. Furthermore, the degree of simultaneity required can be selected to ensure the desired result. Prior art systems, such as TWIPS, require a distinct separation between the transmitted pulses. According to the invention, a degree of overlap is required. The degree of overlap can range from partial overlap up to complete overlap where the transmit time of one pulse is the same as or comprised within the transmit time of the other pulse.

By using a time-reversed pulse pair, it becomes possible to emit the up and down-chirp simultaneously and subsequently suppress reflections from either chirp by using appropriate matched filters. By employing this method, the effect of inter-pulse delay on detection range no longer becomes relevant. Furthermore, simultaneously emitting both pulses allows for the same bubble population to be insonified by both the up and down chirp simultaneously.

This differs from the technique used in the prior art TWIPS technique, which uses a pair of pulses which are in anti-phase and which, if transmitted simultaneously would simply cancel each other out.

In a first embodiment of the invention, the frequency change in the chirp (both up and down variants, since they are time-reversed versions of each other) is linear i.e. the chirp contains a sweep from frequency A to frequency B, in a linear fashion, over a fixed time period. The rate of change of frequency is ([frequency B - frequency A]/time period).

In a second embodiment of the invention, the frequency change in the chirp (both up and down variants, since they are time-reversed versions of each other) is logarithmic i.e. the chirp contains a sweep from frequency A to frequency B, in a logarithmic fashion, over a fixed time period. An advantage of this arrangement, compared to the linear embodiment, is an immunity to Doppler effects, which is advantageous in practical situations.

It is the filtering process, on receipt of echoes from the pair of pulses, that separates the echoes into the responses to one particular component of the pair which have been emitted simultaneously. Consequently, any harmonics that might have existed in the echo will be removed from the filter. The filters are designed to match the contents of individual chirps, one for the up-chirp and one for down-chirp. Each filter's transfer function is the time-reversed replica of the transmitted (up or down) chirp.

However, it is important to recall how differently a bubble responds to an up-chirp, compared to a down-chirp, as shown in <FIG>. <FIG> show the match filtered response of a cloud of bubbles, and a target arranged behind the cloud of bubbles, to an up-chirp superimposed with a down-chirp.

<FIG> shows the match-filtered response of a bubble cloud and linearly-scattering target when insonified by an up-chirp. <FIG> shows the match-filtered response of a bubble cloud and linearly-scattering target when insonified by a down-chirp. <FIG> shows the responses of <FIG> together to highlight the incoherence in the bubble cloud response (illustrated by the darker portions in connection with the bubble scatter compared to the target scatter, which is substantially identical in the case of the up and down chirp).

The bubble cloud responses are different in <FIG>, after separating the echo into the scattered energy from the up-chirp and down-chirp only. In contrast, the target responses are almost the same. A sum of the two responses would result in the level at the target location being enhanced more than the level within the bubble cloud region owing to the incoherence between responses of the latter.

<FIG> show plots from an echolocation trial using a down-chirp and an up-chirp transmitted simultaneously. <FIG> shows standard SONAR processing; <FIG> shows the prior art TWIPS processing - using the ratio: P/P+ (i. e the ratio of the difference between the two pulses and the sum of the two pulses); and <FIG> shows an embodiment of the present invention: P/P2+ (i.e. the ratio of the difference between the two pulses and a processed form of P+, filtered using a filter with a centre frequency that is double the centre frequency of the driving signal).

In each case, in this trial, a small cloud of bubbles was released from the bottom of a water tank approximately <NUM> from the sound source and receiver. Over time, the bubbles travelled upwards towards the water surface. However, the target remained stationary at approximately <NUM> from the sound source. The approximate location of the bubbles and the target are shown in <FIG>.

Linear enhancement and bubble scatter suppression is seen when emitting the time-reversed pulses simultaneously, however, it is P-/P2+ (i.e. <FIG>) that provides the best results.

This is illustrated more clearly still in <FIG> which compares the two ratios of <FIG>, with a focus on amplitudes between -30dB and <NUM> dB (as shown on the right hand axis). Although there is still some scatter from the bubbles, the P-/P2+ plot (on the right hand side) shows much less noise elsewhere while maintaining the enhanced presence of the real target, shown by the clear white vertical line.

Embodiments of the invention may be implemented using known hardware modules, with adapted processing modules to form and synchronise the transmit pulses and receive pulses and process the received echoes. <FIG> shows a hardware implementation suitable for use with embodiments of the invention. The system of <FIG> comprises a processor <NUM> which performs the digital processing necessary to prepare and code waveforms for transmission. This is connected to an analog processing section <NUM> which includes a transmitter and a receiver, each connected to a SONAR transmitter <NUM> and a SONAR receiver <NUM> respectively. The processor <NUM> is programmed to create the transmit pulses and process the received echoes, as set out previously in connection with embodiments of the invention.

Claim 1:
A SONAR system comprising:
a SONAR transmitter (<NUM>) operable to transmit a pair of pulses including an up-chirp signal and a down-chirp signal wherein the down-chirp signal is a time-reversed version of the up-chirp signal, characterised in that the up-chirp signal and the down-chirp signal at least partially overlap in time;
a SONAR receiver (<NUM>) operable to receive echoes of the pair of pulses as echo pulses; and
a processor (<NUM>) operable to determine a ratio of a difference between the echo pulses and a sum of the echo pulses, wherein the sum of the echo pulses is filtered using a filter having a centre frequency that is double a centre frequency of the transmitted pair of pulses.