WO2005057234A1

WO2005057234A1 - Sonar apparatus with a curved array and method therefor

Info

Publication number: WO2005057234A1
Application number: PCT/GB2004/005175
Authority: WO
Inventors: Peter Ian Hogarth
Original assignee: Geoacoustics Limited
Priority date: 2003-12-12
Filing date: 2004-12-09
Publication date: 2005-06-23
Also published as: GB0328877D0

Abstract

A sonar apparatus comprises a line-array transducer for transmitting or receiving an acoustic pulse, the line-array transducer being curved along its length. The apparatus may advantageously be used to insonify a substantially planar surface, such as a bed of a body of water positioned beneath the transducer. Acoustic signals transmitted from different points along the length of the transducer may then arrive substantially simultaneously at a sensing line on the planar surface or, equivalently, signals reflected from the sensing line may arrive substantially simultaneously at different points along the length of the transducer.

Description

SONAR APPARATUS WITH A CURVED ARRAY AND METHOD THEREFOR

Field of the Invention

The invention relates to a sonar apparatus, particularly a side-scan sonar apparatus, and to a method for forming a sonar image.

Background

Side-scan and sector sonars are types of sonar that produce short acoustic pulses (pings), which are transmitted sideways in narrow, fan-shaped beams. The acoustic pulses are produced by one or more transducers mounted on a side face of the sonar and used underwater, such as in the sea, travel at the speed of sound in water. As each pulse travels away from the transducer or transducers, it will insonify a narrow strip of sea-bed and some of the signal will be reflected. Different shapes and topographies on the sea-bed will cause different levels of reflection of the signal, thus, the intensity or amplitude of the reflected signal will vary. The variation in amplitude of the returning reflected signal from the insonified strip of sea-bed is plotted against time, and thus against range. The amplitude can be recorded as a time-related variation in gray scale on a raster (or sector) display, each raster line corresponding to a single ping or transmit/receive cycle. A two-dimensional image is built up line- by-line as the transducer physically moves forward over the sea-bed in a direction perpendicular to the insonified strip, and this image is called a side- scan record. Side-scan images typically show fine, almost photographic, textural details of the sea-bed to ranges of several hundreds of metres each side of the sonar, which is typically carried by a towed vehicle. Typically, a sub-decimetre across-track resolution can be achieved, but along-track resolution (i.e. resolution in the direction of travel) is often poorer.

Conventional transducer arrays used in side-scan or sector scanning sonar are typically straight line-arrays much longer in the horizontal, or length, dimension than in the vertical, or height, dimension (references to horizontal and vertical directions here refer to a line-array used for scanning a horizontal plane, such as the sea bed, using a towfish as illustrated, for example, in Figure 2). Transducer elements are typically mounted on a side face of a line array transducer. The beam patterns generated by such arrays are correspondingly wide in the vertical dimension and narrow in the horizontal dimension.

Reducing the width of a beam generated by a line-array transducer increases along-track resolution.

Beamwidth is inversely proportional to line-array length. Thus, for a given sonar frequency, increasing the line-array length will produce a narrower beam. However, this will also disadvantageously result in a greater near- field/far-field transition distance. The near-field/far-field transition is essentially the point on the normal to the centre of the transducer face where the differential distance to the centre of the transducer and to the edge of the transducer (the end of the transducer array) is equal to one half wavelength at the frequency used. Inside this distance (within the near-field) a complex pattern of constructive and destructive interference exists, and only beyond this distance is the desired narrow beam fully formed. If attempts are made to significantly reduce beamwidth then the near-field can even extend beyond the useful, i.e. signal-to-noise limited, range of the transducer, significantly degrading performance.

An alternative approach to decrease beamwidth uses the fact that beamwidth is proportional to wavelength. Thus, decreasing the wavelength will narrow the beam. For a constant wave speed, a decrease in wavelength will produce an increase in frequency, but higher frequency waves are attenuated more quickly than lower frequency waves and thus have a disadvantageously shorter range. In addition, decreasing the wavelength will increase the near-field/far-field transition distance.

In practice, a typical industry-standard dual-frequency side-scan sonar array is approximately 0.5m long and operates at 100kHz and 400kHz. The resultant horizontal beamwidths are 1.5° and 0.4° respectively. The near-field/far-field transition distances will be a few metres and perhaps a few tens of metres respectively. Typical line-array transducers have around eight or more small narrow piezoelectric ceramic elements in a line, or comprise a continuous line of composite material. These are each wired up to an electrical connector giving a simple robust two terminal transducer. For such a side-scan, if the array length were, for example, doubled to reduce the beamwidths, then the near-field effect would be a problem over an increased range.

It is apparent that a problem exists in increasing the along-track resolution of a side-scan sonar.

Summary of the Invention

The invention provides in its various aspects a sonar apparatus and a method of imaging as defined in the appended independent claims, to which reference should now be made. Preferred or advantageous features of the invention are set out in dependent sub-claims.

In a first aspect, the invention provides a sonar apparatus comprising a line- array transducer for transmitting or receiving an acoustic pulse, the line-array transducer being curved along its length. Preferably, the curved line-array transducer has a length dimension which is greater than its height dimension.

In a preferred embodiment of the invention, the line-array transducer comprises a plurality of discrete transducer elements. Preferably these elements are piezoelectric ceramic elements and are mounted in a curved line. Particularly preferably the elements are mounted on a side-face of the line- array transducer.

In a further preferred embodiment, the line-array transducer comprises a continuous, curved, line of composite material. Particularly preferably the composite material is mounted on a side-face of the line-array transducer.

In a further aspect, the invention provides an apparatus for sonar imaging of a first plane lying beneath the surface of a body of water. The body of water will usually be a sea, lake or river. The first plane will usually be a sea-bed, lake- bed or river-bed.

In use, the line-array transducer will, preferably, be curved in a second plane, substantially perpendicular to the first plane being imaged. The line-array transducer will, advantageously, be curved so that the ends of the transducer are closer to the first plane than the middle of the transducer.

The transducer element or elements will, preferably, be mounted on a face of the sonar apparatus that is in a plane substantially perpendicular to the first plane.

Preferably, the curve of the line-array transducer is or approximates to an arc with a radius equal to or less than twice a predetermined operational distance between the transducer and the first plane.

The operational distance can be defined along a line extending from a point (O) in the first plane, the line extending perpendicularly from the first plane to the apex of the curve in the line-array transducer.

Particularly preferably, the curve of the line-array transducer is or approximates to an arc with a radius substantially equal to the predetermined operational distance.

Preferably, the transducer elements mounted along the length of the curved line-array transducer are, in use, substantially equidistant from the point (O) lying in the first plane.

Advantageously, acoustic pulse signals transmitted from the curved array will be substantially in phase and focussed at point (O) in the first plane.

Particularly advantageously, the returning reflected signals from point (O) will be received by each of the transducer elements substantially simultaneously.

Particularly advantageously, the acoustic pulse signals are focussed, in both transmit and receive, on a line lying in the first plane. This line of focus extends from point (O), in a direction perpendicular to the length dimension of the line-array transducer. The fact that the sonar apparatus may automatically focus the transmitted signal as well as receiving a focussed reflected return signal, results in a further benefit. The focussed transmitted signal may provide a received signal having a significantly increased signal-to-noise ratio for a given transmitted source level.

Advantageously, the sonar apparatus of the invention may improve the rejection of nuisance signals from the surface of the body of water, e.g. the sea surface. An acoustic pulse can be reflected from the sea-surface just as it is from the sea-bed, and the downward curve of the line-array transducer may result in signals reflected from the surface being de-focussed. This means that these signals may be easier to identify and remove or ignore.

The apparatus for sonar imaging may be mounted on the hull of a boat or submarine.

The apparatus for sonar imaging is preferably mounted on a towed vehicle, or tow-fish, typically towed behind a boat, or on a self-propelled autonomous underwater vehicle (AUV). Advantageously the depth of the towed vehicle or the AUV, and thus the operational distance f the apparatus, can be controlled.

In a further embodiment, the radius of the curve of the line-array transducer can be changed so as to allow different operational distances. This embodiment would be particularly beneficial if used in conjunction with a hull- mounted sonar apparatus, mounted for example on the hull of a boat.

The focussing effect produced by curving the transducer array may alleviate problems associated with near-field effects. This may allow longer arrays to be utilised and, consequently, an acoustic pulse with a narrower beamwidth to be formed. Advantageously, the narrower beamwidth and increase in directivity may increase along-track resolution at all ranges and increase maximum useful range for a given transmit power. This may be achievable without penalties at shorter ranges; for example, the near-field far-field transition distance may not be increased as would be the case if a conventional straight- line array were to be increased in length. Although the preferred curvature of the line-array transducer is an arc of a circle, other curves may be utilised and still provide advantages over the prior art, e.g. ellipses, parabola etc.

In a further aspect, the invention also provides a method for imaging a line lying in a plane beneath the surface of a body of water and a method for imaging the plane. The sonar apparatus may be arranged in the water, preferably at a predetermined operational distance from the plane to be imaged that is substantially equal to the radius of the curve in the transducer array. At this operational distance the transducer elements along the length of the array are, advantageously, equally spaced from any point in a line of focus lying in the plane. The sonar apparatus then transmits an acoustic pulse. Acoustic waves from the pulse are focussed on the line of focus and some of the signal is reflected. The reflected signal from the line of focus will arrive at each transducer element at substantially the same time and can thus also be considered to be in focus.

Advantageously, the reflected signals received by the sonar apparatus may be processed so as to form an image of the line.

Advantageously, if the sonar apparatus is moved in a direction perpendicular to the line of focus, and further acoustic pulses are transmitted at predetermined intervals, an image of the plane can be built up. Particularly advantageously, acoustic pulses may be transmitted at intervals corresponding to the beamwidth of the signal.

The image produced using the apparatus according to any embodiment of the invention may have an advantageously high resolution in the along-track direction as well as the across-track direction.

Specific Embodiment of the Invention

Specific embodiments of the invention will now be described by way of example, with reference to the accompanying drawings, in which; Figure 1 illustrates an autonomous underwater vehicle (AUV) supporting a first prior art linear transducer array.

Figure 2 illustrates a tow-fish supporting a second prior art linear transducer array.

Figure 3 shows a cross-section through a typical prior art dual-frequency side- scan transducer.

Figure 4 is a perspective view of an embodiment of the invention; and

Figure 5 is a further perspective view of the embodiment of the invention.

Figure 1 illustrates a prior art Synthetic Aperture Sonar (SAS) side-scan sonar apparatus. A 1.8m long straight, flat, receive array 100 is suspended below an AUV 110. The array is made of advanced composite materials and has up to 80 individually addressable elements, pre-amplifiers and ADC (analogue-to- digital converter) channels per side to accomplish focussing and beam-forming functions. The result is a high resolution side-scan sonar, but the cost of the transducer is very high.

Figure 2 illustrates a simple prior art side-scan sonar apparatus. This example has a 0.5m long transducer 120 fitted laterally along the body of a tow-fish 130. The transducers are simple units with all elements wired in parallel. No focussing or beam-forming is employed and a single pre-amp and ADC channel is required per transducer. The cost is low, as is the along-track resolution.

A typical simple low-cost prior art transducer element is shown in cross-section in Figure 3. Here, the semi-circular cross-section of a body of acoustically- matching potting material 200 is shown housing an aluminium channel 210 (this channel is for mechanical support only), the channel containing a high density backing material 220 (a closed-cell foam) and two piezoceramic blocks 230, 240. In this example, the larger block 230 is for transmission and reception at 114kHz and the smaller block 240 is for transmission and reception at 410kHz. The blocks are continuous along the length of the array and have a plurality of silvered electrodes front and back, wired in parallel, placed end-to-end along the length of the array. The electrodes thus effectively define a plurality of transducer elements along the length of each piezoelectric block. The array is usually mounted with the flat side mounted against the body of the tow vehicle. The outer surface of the final potted shape should be smooth but is otherwise not critical. The array is tilted slightly to angle the main lobe of the transmit/receive beam downwards towards the sea floor. It is normal on most conventional side-scan arrays to transmit and receive acoustic pulses on the same elements using a transmit/receive switch.

In the following description of embodiments of the invention, the term sea-bed 10 is used to refer to a plane lying beneath the surface of a body of water.

In a first embodiment a sonar apparatus is housed within, or mounted on, a tow-fish or an AUV in substantially the same arrangement as in the prior art, for example as illustrated in Figures 1 and 2. As illustrated schematically in Figures 4 and 5, the sonar apparatus comprises a curved line-array transducer 20 for transmitting and receiving an acoustic pulse. The transducer comprises a number of discrete, piezoelectric ceramic transducer elements, mounted in a curved line 30, so that the end elements of the array (E & F) are closer to the sea-bed than the central elements. The curve is an arc of a circle. The transducer elements are mounted on a side-face of the line-array transducer and face in a side-scan direction normal to the side-face.

In an alternative embodiment, the transducer elements may be defined by a plurality of electrodes on surfaces of a continuous block of piezoelectric material, mounted in an acoustically-matching potting material, as illustrated in cross-section in Figure 3.

In either embodiment, single frequency or multiple frequency transducer elements may be used.

In an embodiment in which the sonar apparatus is housed in or carried by a tow-fish, the tow-fish is towed at a predetermined operational distance above the sea-bed. The operational distance is measured along a line (r) extending from the apex of the curved array (G), to a point directly beneath on the seabed (O). The operational distance is ideally substantially equal to the radius of the curve. Thus, the lengths of lines GO (r), EO (r-,) and OF (r₂) are equal and each of the transducer elements is equidistant from point O on the sea-bed.

When an acoustic pulse is emitted from the transducer array, the pulses from E, F and G will all reach point O at the same time and be in the same phase, and will therefore be focussed at point O. Any reflected signals will be received by each transducer element at the same time, as the distances from point O to any point on the transducer array are substantially equal.

What is less obvious is that this focus of both the transmitted and received signals is maintained at all points on a line (the insonified line) along the seabed extending from point O in a direction perpendicular to the length of the line-array transducer (line OP in Figure 5).

This can be visualised by imagining a cone on its side, where the apex of the cone is a point (P) on the line on the sea-bed and the centre of the circular base of the cone is the point O on the sea-bed, directly below the centre of the line-array. This construction is illustrated in Figure 5.

The curved line-array transducer is then an arc of the circumference of the base of the cone. Keeping the circular base of the cone unchanged, it can be seen that, for any cone length, the distance from any point on the arc to the cone's apex (on the line OP) will be equal, and therefore acoustic signals at that point will be in phase and focussed. Therefore, just as lines r, η and r₂ in Figure 4 are equal in length, for the cone apex at point P lines I, and l₂ in Figure 5 are equal in length.

Advantageously, for any distance of the curved array above the sea-bed between zero and twice the radius of the cone-base, the focussing should be significantly better than for a straight line-array of similar length. At distances of greater than twice the radius of the cone-base the focussing advantage is likely to be reduced. However, at any distance nuisance reflections from the sea surface may, advantageously, be relatively suppressed, as these will always be defocused.

For any length of array, there may be advantage in using a curved array. The performance advantage over a conventional straight array increases substantially, however, the longer the lengths of the arrays being compared. For example, for a 1.5m long array embodying the invention, transmitting at 400kHz and with an operational distance of 25m, the end elements would be vertically displaced by 7mm and the array would generate an extremely narrow beamwidth of 0.13° focussed precisely at any point on a line of focus extending along the sea-bed in the side-scan direction outwards from the transducer. This performance is advantageously significantly better than a straight array of the same length and operating frequency.

Claims

1. A sonar apparatus comprising a line-array transducer for transmitting or receiving an acoustic pulse, the line-array transducer being curved along its length.

2. An apparatus according to claim 1 in which the curve is substantially an arc of a circle.

3. An apparatus according to claim 1 or 2, in which the apparatus is used for sonar imaging of a first plane lying beneath the surface of a body of water, and the line-array transducer is curved, in use, in a second plane perpendicular to the first plane.

4. An apparatus according to claim 3, in which the ends of the line-array transducer are, in use, closer to the first plane than the middle of the line-array transducer.

5. An apparatus according to claim 3 or 4, in which the apparatus, in use, has a predetermined operational distance defined along a line extending perpendicularly from the first plane to an apex of the curve of the transducer, and a radius of the curve is greater than or equal to half the operational distance.

6. An apparatus according to claim 3 or 4, in which the apparatus, in use, has a predetermined operational distance defined along a line extending perpendicularly from the first plane to an apex of the curve of the transducer, and the radius of the curve is substantially equal to the operational distance.

7. An apparatus according to any of claims 3 to 6, in which the first plane is a sea-bed, river-bed, or lake-bed.

8. An apparatus according to any preceding claim, in which the line-array transducer comprises a plurality of discrete transducer elements.

9. An apparatus according to any of claims 1 to 7, in which the line-array transducer comprises a continuous-line transducer.

10. An apparatus according to any preceding claim, in which the curvature of the line-array transducer is variable for operation at different predetermined operational distances.

11. A method of imaging a line beneath a body of water comprising the steps of; transmitting an acoustic pulse from a curved source or a curved array of sources: and receiving acoustic signals reflected from points along the line.

12. A method according to claim 11 , in which the source or sources is or are substantially equally spaced from the line.

13. A method of imaging a line beneath a body of water comprising the steps of; transmitting an acoustic pulse; and receiving acoustic signals reflected from points along the line at a curved receiver or a curved array of receivers.

14. A method according to claim 13, in which the receiver or receivers is or are substantially equally spaced from the line.

15. A method of imaging a plane lying beneath the surface of a body of water comprising the steps of; using a sonar apparatus to form an image of a line according to the method of claim 11 or 13; and moving the sonar apparatus in a direction substantially perpendicular to the line and repeating the method of claim 11 or 13 so as to produce images of a plurality of parallel lines lying in the plane.

16. A method according to claim 15, in which the movement of the sonar apparatus is continuous.

17. A sonar apparatus substantially as described herein, with reference to the drawings.

18. A method of imaging substantially as described herein, with reference to the drawings.

19. An image produced using an apparatus or method as defined in any preceding claim.