WO2011021018A1

WO2011021018A1 - Acoustic reflector

Info

Publication number: WO2011021018A1
Application number: PCT/GB2010/051163
Authority: WO
Inventors: Carl Tiltman; Andrew Malcolm Tulloch; Carl Duncan Holmes
Original assignee: Subsea Asset Location Technologies Ltd
Current assignee: Subsea Asset Location Technologies Ltd
Priority date: 2009-08-19
Filing date: 2010-07-16
Publication date: 2011-02-24
Anticipated expiration: 2012-02-19
Also published as: GB2485698A; GB201202036D0; AU2010284850A1; EP2467848A1

Abstract

An acoustic reflector (10) comprises at least one elongate member (12, 14) having a plurality of reflector means (16) disposed around its axis. Various embodiments are discussed. In one two such members (12, 14) are mounted substantially orthogonally to each other. In another the reflectors are arranged in opposed pairs about an axis, adjacent pairs being rotated with respect to the next pair to enable each incoming acoustic waves from various directions to be reflected.

Description

Acoustic Reflector

[0001] This invention relates to acoustic reflectors. There is a demand for a simple to make, very cheap reflector for use in less demanding situations. Previous attempts. For example,

Patent Citation 0001: US 3195677 B (HILLARY ET AL). 1965-07-20.

and

Patent Citation 0002: US 4303144 B (WIRT). 1981-12-01.

have limited directional characteristics. Omnidirectional devices have been proposed in

Patent Citation 0003: WO WO 2006/075167 A (THE SECRETARY OF STATE FOR DEFENCE). 2006-07-20.

and

Patent Citation 0004: WO WO 2009/122184 A (THE SECRETARY OF STATE FOR DEFENCE). 2009-10-08.

, while these have excellent characteristics they may be rather expensive for mass deployment in some circumstances.

[0002] According to the present invention an acoustic reflector comprises at least one

elongate member having one or a plurality of acoustic reflecting elements disposed around its axis.

[0003] In this context "an elongate member" means a member whose length along a central axis is greater than a cross section measured orthogonally to the axis.

[0004] In one embodiment of the invention an acoustic reflector as aforesaid comprises at least two such members mounted substantially orthogonally to each other.

[0005] The acoustic reflecting elements may comprise elongate corner reflectors substantially parallel to the axis of the member(s).

[0006] In an alternative the acoustic reflecting elements may comprise elongate parabolic reflectors substantially parallel to the axis of the member.

[0007] In a further alternative, the acoustic reflecting elements comprise elongate partially elliptical reflectors substantially parallel to the axis of the member.

[0008] Where the reflector comprises two or more elongate members the acoustic reflecting elements are different on at least two different members.

[0009] In any of the embodiments described in paragraphs [0002] to [0008] one or more of the members may have one or more annular pieces disposed around them, to present surfaces orthogonal to the surface of the members.

[0010] In a further embodiment of the invention the elongate members are substantially cylindrical with acoustic reflecting elements formed on the outer surface of the cylinder parallel to the axis.

[0011] In a further embodiment of the invention the elongate members are arranged in a T configuration.

[0012] In a further embodiment of the invention the reflector comprises parallel elongate members joined by a cross member.

[0013] In a further embodiment of the invention individual acoustic reflecting elements are formed in the surface of the elongate members. These individual reflecting elements may be corner reflectors, parabolic reflectors or partial elliptical reflectors.

[0014] In a still further embodiment of the invention an acoustic reflector according to the invention has an elongate member comprising a plurality of pairs of individual reflecting elements the pairs being disposed in back to back pairs around the axis of the elongate member. The pairs may be arranged to be rotated with respect to one another around the axis such that the angle of rotation of one pair with respect to an adjoining pair is π/xn where n is the number of pairs along the length of the member and x is a whole number, normally x is 1. In a preferred arrangement of this embodiment n=3 and x=l and thus the angle of rotation is 60°.

[0015] In a development of the embodiment described in the previous embodiment a corner reflector is provided at each end of the stack wherein the corner of the reflector is perpendicular to the longitudinal axis of the stack.

[0016] Acoustic reflector of paragraphs [0002 to 0015] may be made of any suitable

material. However, conveniently the members are made from polyurethane which may be extruded or moulded in the elongate shape required. If the reflector comprises a plurality of members to be made from extruded polyurethane or the like, they can be cut to the required lengths and lengths fixed together in the required arrangement by friction or hot air welding. To improve acoustic reflecting characteristics the reflecting elements can be lined with a good acoustic reflecting material such as aluminium.

[0017] Where the acoustic reflector comprises pairs of individual reflecting elements as described in paragraphs [0014 and 0015], the reflectors can be fabricated separately or in pairs and mounted on a central rod.

[0018] By following the process described in the previous paragraph it can be seen that an acoustic reflector can be made extremely economically.

[0019] As an alternative to polymers and other plastic materials, glass reinforced plastics may be used for the members as can metals such as aluminium. However it will be seen readily that these latter materials are more difficult and expensive to make it in the required monolithic shape, although metals such as aluminium may be particularly useful for cheaply fabricating the acoustic reflector described in paragraph [0015] and subsequent injection moulded elements may be cheaply formed from glass filled polymers and welded together to form a monolithic reflector. [0020] The applicants have found that use of reflectors of the acoustic reflectors of the kind described in this invention can provide the acoustic underwater equivalent of air borne chaff which is widely used to divert or confuse heat seeking missiles and the like which locate hot spots, such as engines on aircraft, by using infra red guidance.

[0021] Underwater the normal target seeking mechanism for submersibles is sonar.

[0022] Thus a system to disguise or protect an underwater object comprises a plurality of acoustic reflectors according to this invention.

[0023] Systems of this kind can be used for protecting vessels against threats, say from a target seeking torpedo, which identifies potential targets by sonar sounding, or for rendering the vessel invisible to the sonar of submarines or ships or to so called dunking sonar systems. The system can also be used to mask the emitted acoustic signal of a vessel by scattering the emitted sound and allowing a vessel to leave port, for example, without being detected by acoustic listening devices.

[0024] Systems can be very simple. For a surface ship a number of acoustic reflectors can be stored in a net, released when needed, and recovered when the threat which led to deployment had been removed. In submarines, the reflectors could be deployed through tubes.

[0025] In some situations buoyancy may be an issue, with the reflectors tending to sink or rise to the surface of the water. If this is the case the reflectors could be weighted to provide neutral buoyancy at normal deployment depths. The cheapness of reflectors described in this invention enables large numbers to be deployed, and if not recovered the economic loss would be low.

[0026] The invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0027] figure 1 shows an acoustic reflector according to the invention in a T-configuration;

[0028] figure 2 shows the acoustic reflector figure 1 sectioned on the line A-A' of figure 1;

[0029] figure 3 shows an acoustic reflector according to the invention in an I shape configuration;

[0030] figure 4 shows the reflector of figure 3 sectioned on the line B-B' of figure 3;

[0031] figure 5 shows an acoustic reflector according to the invention in which individual reflectors are formed in the surface of the members;

[0032] figure 6 shows a section on the line C-C of figure 5; and

[0033] figure 7 shows a perspective view of a simple reflector according to the invention;

[0034] figure 8 shows a further embodiment of the invention sectioned on the line A-A' figure 9;

[0035] Figure 9 is a side view of the further embodiment;

[0036] Figure 10 shows an alternative arrangement of the further embodiment shown in figures 8 and 9; [0037] Figure 11 shows a pipe section with elongate acoustic reflectors having a cross section as shown in figure 2; and

[0038] Figure 12 shows the use of elongate acoustic reflectors according to the invention to identify scouring around an object, such as a bridge support, in water.

[0039] In figures 1 and 2, an acoustic reflector 10 according to the invention comprises a pair of elongate orthogonal members 12 and 14 mounted in a T configuration. The members 12 and 14 are substantially cylindrical with elongate corner reflectors 16 formed in the surfaces 18 of the cylinders, the reflectors are parallel to the axes 20 of the members 12 and 14. The reflectors 16 can be seen in cross-section in figure 2. The members 12 and 14 have been cut to length from an extruded length of polyurethane and friction welded together at their join 22. Annular members 24 are disposed around the orthogonal members 12 and 14, tightly fitting into groves in the surface 18 of the members 14 and 16 formed by the elongate corner reflectors 16. The annular members 24 are mouldings with their inner holes shaped to correspond with the grooves formed by the corner reflectors 16. The annular members are glued in place. The shape of the annular members 24, is such as to provide two surfaces that are orthogonal to the axis of members 12 and 14, to form a pair of right angles with respect to the surfaces of reflectors 16.

[0040] In figures 3 and 4, a reflector 30 according to the invention comprises parallel

members 32 and 33 bridged at their mid-points by member 34, which is orthogonal to each of members 32 and 33,. The members 32, 33 and 34 are substantially cylindrical with elongate parabolic reflectors 36 formed in the surfaces 38 of the cylinders parallel to their axes 40. The parabolic reflectors 36 can be seen in cross-section in figure 4. The members 32, 33 and 34 had been cut to length from an excluded length of polyurethane and friction welded together at their joints 42 and 43. Annular members 44 are disposed around the orthogonal members 32, 33 and 34 tightly fitting into groves in the surface 38 of the members 32, 33 and 34 formed by the elongate parabolic reflectors 36. The annular members 44 are mouldings with their inner holes shaped to correspond with the grooves formed by the parabolic reflectors 36. The annular members are glued in place. The shape of the annular members 44 is such as to provide two surfaces that are orthogonal to the axis of members 32, 33 and 34, to form a pair of right angles with respect to the surfaces of reflectors 36.

[0041] In the figures 1 to 4, corner and parabolic reflectors are illustrated. Partial elliptical reflectors can be used instead. Equally reflector designs can be interchanged so that one member has reflectors of one type and another member another type.

[0042] In the embodiments shown in figures 1 to 4, the orthogonal members 12, 14, 32, 33 and 34 will typically be around 200mm in diameter. These members can be as long as required, ranging up to several metres. The annular members 24 and 44 would be disposed along the length of members 12, 14, 32, 33, and 34 at least one every metre, but their separation can be much less than this, say, every 200mm.

[0043] Instead of the elongate reflectors, individual corner, parabolic or partial elliptical reflectors shapes can be formed in the surface of members. To form these, the material of the reflector would have to be punched, moulded or cast, as straightforward extrusion through a die would no longer be a practical method of manufacture. This arrangement is shown in figure 5. In figure 5 a T-shaped acoustic reflector 50 similar to that of figure 1 is shown, with orthogonal cylindrical members 52 and 54 but instead of the elongate reflectors, individual reflectors 56 are formed into the surface of the cylinder. An individual reflector is shown by the cross section shown in figure 6, where it is seen as a corner reflector having a right angle 58 between two adjacent faces 59 and 60 of the reflector. Under water an incoming acoustic wave 64 striking one face 59 will be reflected to the other face 60, which in turn reflects the wave back. The final reflected wave 66 travels in the opposite direction to the incoming wave 64 and can be detected conventionally. Parabolic and partial elliptic reflectors work in the same way by initially reflecting an incoming acoustic wave internally from one surface to another, the second surface then reflects the wave out of the reflector in the opposite direction to the incoming wave. In the embodiments shown in figures 1 to 4 the combination of reflectors 16 or 36, with annular members 24 or 44 respectively behaves in the same way as the individual corner reflectors of figures 5 and 6, but is easier to make in large sizes.

[0044] Although T and I acoustic reflectors have been described, in practice the orthogonal members can be assembled in many ways, as a X, E, FI, H etc..

[0045] Figure 7 shows a very simple device according to the invention. Here, an acoustic reflector 70 comprises a pair of rectangular crossed planks 72 and 74 (the planks may be continuous, and joined by conventional carpentry joints, or the planks may be in parts and simply glued or welded together at the crossing points. However assembled, the planks intersect orthogonally so that four corner acoustic reflectors 76 are created in each of the four right angles formed by the crossing point. The planks may have annular members similar to those shown as 24 and 44, but with a rectangular central hole, disposed as a tight fit around their surface, the annular member is shaped in such a way that it has two surfaces that form rights angles with the surface of the plank.

[0046] For some circumstances, where incident acoustic waves of interest will not be

expected from one side of the X formed by the intersecting planks, a backing sheet can be attached to the X formed by the planks such that the right angles are also formed between the planks and the backing sheet. Further enhancements are possible, for example the planks may have a partially parabolic surface, if this provides the reflecting characteristics needed. [0047] In figures 1 to 7, for cheapness the main components can be extruded from poly- urethane, to improve acoustic reflecting characteristics the reflecting elements can be lined with a good acoustic reflecting material such as aluminium.

[0048] In figure 8 an acoustic reflector element 110, part of an elongate member comprising an acoustic reflector according to the invention, comprises a pair of corner reflectors 11OA and HOB arranged back to back. Each reflector 11OA and HOB comprises a pair of reflective surfaces 112, 114, arranged perpendicularly to each other, with reflective top and bottom surfaces 116 of the reflector element 110 orthogonally mounted with respect to both surfaces 112 and 114. Between them the surfaces 112, 114 and 116 define an acoustically reflecting box section and reflector 11OA is thus a box-corner acoustic reflector to reflect an incoming acoustic wave from, for example, sonar, from that side of the acoustic reflector and reflector HOB reflecting acoustic signals from the opposite direction. The right angled corners of each reflector 11OA and HOB are positioned back to back along a common axis 118.

[0049] The areas 119 between the acoustic reflecting surfaces 112 and 114 is non

acoustically reflecting material formed as described in paragraph [0048] below.

[0050] To form an acoustic reflector device 101 in accordance with the present invention, element 110 is located above two substantially identical elements 120 and 130 as shown in figure 2. Each element 20, 30 is rotated by π/3 (60°) from element 110 and from each other so that the entire device gives six responses equally spaced at 60° intervals viewed azimuthally. Lugs 105 are inserted into location holes 103 to secure the elements together in their desired relative positions. The outside of the acoustic reflector device 101 is generally of cylindrical shape. The device 101 is supported by a rod 107 which may be arranged to extend through a hole along the central axis of the device.

[0051] Conveniently the acoustic reflector 101 is manufactured by preparing three cylinders of a foamed plastics material such as polyurethane each corresponding to the shape of a reflector element 110, 120, 130. The material is one that will not itself reflect acoustic waves at the frequencies of interest. The material is then cut along the surfaces defining the reflector surfaces 112, 114 and the top and bottom plates 16. Each of the surfaces is lined with a material that will reflect acoustic waves at the frequency of interest. This may be aluminium, for example. The three corner reflecting pairs 110, 120, 130 prepared in this way are to each other by means of holes 103 and lugs 105 in the top and bottom plates 116, rod 107 passes through the longitudinal axis of the device 101 in combination with lugs and holes 103 and 105 maintains the individual elements 110, 120, 130 in place element si 10, 120, 130 may be glued together.

[0052] To provide omnidirectional performance the ends of the stack may be finished with further reflectors, reflecting acoustic signals in the parallel to the longitudinal axis of the stack.

[0053] An alternative to the embodiment of figures 8 and 9 is illustrated in Figure 10. In this case the reflector 170 is collapsible. It comprises a stack of three pairs 182, 184, 186 of corner reflectors. Each corner reflector comprises aluminium reflector plates 172 and 174, and hinged along axis 176, orthogonal to the longitudinal axis 90 - 90' of the element. The assembly may be spring-loaded into its erected disposition and each pair of aluminium plates 172 and 174, held in its position by a spacer 178, such that an internal angle of 90° is maintained between the reflector plates 172 and 174. Each member of each pair of corner reflectors is mounted back to back with their hinges 176 common. The pairs 182, 184, 186 of reflectors are mounted in a stack with circular aluminium plates between them and the d the longitudinal axis 90-90' at their centres and perpendicular to their planes. Optionally the inactive spaces 192 formed between the various plates when in position can be filled with acoustic absorbing foam to help maintain the plates 172 and 174 in position when the device is deployed. In plan, the axis 176 of the of reflectors 184 is at π/3 (60°) to the axis 176 of the reflector pair 182; likewise the axis 176 of reflector pair 186 turned by a further π/3 (60°) to the axis 176 in respect of the reflector pair 182. The edges of each plate 172 and 174 are cut such that in plan, looking in the direction 90 to 90' the whole device is circular.

[0054] Although in figures 8 to 10 a three element stack device is preferred for reasons of economy, the stack may comprise four reflector elements with adjacent elements being relatively rotated by substantially π/4 instead of π/3 as would be the case for the three element stack. Again reflectors can be provided at each end additionally to reflect acoustic waves along the longitudinal axis of the device.

[0055] In the embodiments of figures 8 to 10 all the angles mentioned above are preferable values; substantial variation may occur without an excessive fall in performance. For example it is possible for the elements to be arranged at up to 15° to the horizontal instead of precisely horizontal.

[0056] It will be seen that when assembled together to form an acoustic reflector device, the pairs of reflectors shown in figures 8 to 10 form an elongate member.

[0057] Figure 11 shows a schematic diagram of a pipe section 530 fitted with a number of acoustic reflectors 532 each according to the invention and comprising an elongate aluminium member having a cross section as shown in figure 2. Each reflector comprises an aluminium rod 536, with elongate reflectors 534 formed in its surface (the detail of these can be seen in figure X). Each of elongate cylindrical ends 538 of the reflectors 532 is capped and closed off by a conventional electrically insulating material, which, in particular isolates the aluminium rods from the pipe section 530 and prevents any electrical conduction there between. The reflectors 532 are attached to the pipe section 530 by upstanding lugs 542 fitted to the pipes. The acoustic reflectors 532 are electrically isolated from the pipe sections 530 with insulating bushes (not shown) fitted to the end of the lugs in a conventional manner. Aluminium members 540 fitted around the rods 536 may be provided in a similar manner to the annular members 24 mentioned in paragraph [0039]. The pipe section 530 has conventional end flanges 531 with holes therein allowing it to be bolted to another pipe section. The pipe section with the acoustic reflectors can be prefabricated on land and joined by means of the bolt holes in the flanges 531 to another like fitted pipe. In this way, a pipe line fitted with acoustic reflectors can be assembled as part of the normal process for laying an underwater pipeline. The invention thus provides a neat and economical way of ensuring that underwater pipelines can be identified.

[0058] As an alternative, rather than using aluminium, plastic extrusions may be used for the reflectors with annular members fitted in the same way. If such materials are used insulation of the lugs and rod ends is avoided. Performance will not be as good as the aluminium extrusion, but if the reflectors 536 are lined with aluminium and aluminium annular members 540 provided the performance will be similar to the wholly aluminium reflector. Although aluminium has been described because of its lightness and ease of extrusion, other metals and acoustic reflecting materials can be used instead.

[0059] Although the elongate reflector in figure 11 has been described in relation to a pipe section, the reflectors can be applied to other underwater objects, such as oil rig platforms, accommodation platforms for workers at sea, and other objects to be placed under water.

[0060] An example of one of these alternative applications is shown in figure 12. In the figure the lower portion of a bridge pier 660 is shown extending below surface of the sea bed 666. The progressive effects of scouring of the sea bed are shown by lines 667 and 668. The pier 660, (although it could be a drilling platform, stanchion or the like) extends below the sea surface 665, is mounted in the sea bed 666. A series of acoustic reflectors 661, 662, 663 and 664 is mounted on the pier. The reflectors are elongate aluminium rods having a cross section as shown in figure 2 with orthogonal aluminium annuli mounted thereon as described in paragraph [0039]. The series of rods may be formed around the pier, or a single rod bend all the way round in effect as shown in figure 12. Reflectors 661 and 662 are permanently above the sea bed and can be used to mark the pier 660. Currents will scour the sea bed preferentially around the pier 660, eventually lowering the sea bed level to 667 exposing reflector 663 which was initially below the sea bed. Identification of this reflector 663 by interrogation by a sonar signal will provide an early warning of scouring, and the need for possible attention. As scouring continues and the sea bed drops further as indicated by line 668, a second reflector 664 is exposed, which may indicate that a potentially dangerous situation has developed and the underwater mounting of support item 660 may need urgent attention.

Although four acoustic reflectors have been illustrated in figure 12, more or less may be used to suit the relevant design criteria. Although aluminium reflectors have been described the rods can be made from any suitable material. As an alternative to rods reflectors of the configurations shown in figures 1 to 4 could be used.

Claims

[0001] An acoustic reflector characterised in that it comprises at least one elongate member having one or a plurality of acoustic reflecting elements disposed around its axis.

[0002] An acoustic reflector according to claim 1 characterised in that it comprises at least two members intersecting substantially orthogonally to each other.

[0003] An acoustic reflector according to claim 1 or 2 characterised in that said elongate member has one or a plurality of acoustic reflecting elements that are disposed along the surface of said members.

[0004] An acoustic reflector according to any one of claims 1 to 3 characterised in that said elongate member is substantially cylindrical having elongate reflecting elements formed in its surface, said elongate reflecting elements being parallel to the axis of the elongate member.

[0005] An acoustic reflector according to any one of claims 1 to 4 characterised in that two said elongate members are arranged in a T configuration.

[0006] An acoustic reflector according to claim 1 to 4 in which two said elongate are parallel to each other and joined by a further cross member.

[0007] An acoustic reflector according to any one of claims 1 to 4 characterised in that the reflector comprises three said elongate members two of which are parallel to each other and the third joining the other two is orthogonal to them.

[0008] An acoustic reflector according to any one of claims 1 to 7 characterised in that one of said elongate members reflecting elements comprise elongate corner reflectors substantially parallel to the axis of the member.

[0009] An acoustic reflector according to any of claims 1 to 7 characterised in that one of said elongate members reflecting elements comprise elongate parabolic reflectors substantially parallel to the axis of the member.

[0010] An acoustic reflector according to any of claims 1 to 7 characterised in that one of said elongate members reflecting elements comprise elongate partially elliptical reflectors substantially parallel to the axis of the member.

[0011] An acoustic reflector according to any one of claims 1 to 10 having a plurality of said elongate members characterised in that said elongate members include different reflecting elements.

[0012] An acoustic reflector according to any one of claims 1 to 7 characterised in that said elongate member has individual acoustic reflectors.

[0013] An acoustic reflector according to claim 12 characterised in that the individual reflectors are selected from the group comprising corner reflectors, parabolic reflectors and partial elliptical reflectors. [0014] An acoustic reflector according to any preceding claim characterised in that at least one annular member is disposed around said elongate member orthogonally to the axis of the elongate member.

[0015] An acoustic reflector according to claim 14 characterised in that a plurality of annular members are disposed around the elongate member at intervals of no more than a metre along the length of the elongate member.

[0016] An acoustic reflector according to claim 12 characterised in that the reflector comprises n pairs of corner reflectors, each pair comprising two acoustic reflecting surfaces arranged substantially perpendicularly to each other wherein the n pairs of corner reflectors are mounted in pairs about longitudinal axis to form a stack, two pairs of corner reflectors being arranged back-to-back in each layer, and wherein the corners of each corner reflector are substantially parallel to the longitudinal axis and wherein the sides of each corner reflector are turned by an angle of π/xn with respect to the sides of the corner reflectors the next adjacent pair in the stack where x is a whole number.

[0017] An acoustic reflector according to claim 16 characterised in that n=3.

[0018] An acoustic reflector as claimed in claim 16 or 17 characterised in that a corner reflector is provided at each end of the stack wherein the corner of the reflector is perpendicular to the longitudinal axis of the stack.

[0019] An acoustic reflector as claimed in any one of claims 16 to 18 characterised in that wherein the two pairs of corner reflectors in each level constitute a reflector element, said reflector element being of substantially circular shape in plan view.

[0020] An acoustic reflector according to claim 19 characterised in that acoustic

absorbing material is between each pair of corner reflectors.

[0021] An acoustic reflector as claimed in any preceding claim characterised in that the stack has end corner reflectors mounted such that they reflect substantially along the axis of the stack.

[0022] An acoustic reflector according to any preceding claim wherein the acoustic reflecting surfaces are constituted by an acoustic reflective lining on faces of a solid body.

[0023] An acoustic reflector according to any one of claims 16 to 21 claim in which the acoustic reflecting material is aluminium.

[0024] An acoustic reflector comprising n pairs of corner reflectors, each pair

comprising two acoustic reflecting surfaces arranged substantially perpendicularly to each other wherein the n pairs of corner reflectors are mounted in pairs about longitudinal axis to form a stack, two pairs of corner reflectors being arranged back-to-back in each layer, and wherein the corners of each corner reflector are substantially parallel to the longitudinal axis and wherein the sides of each corner reflector are turned by an angle of π/xn with respect to the sides of the corner reflectors the next adjacent pair in the stack wherein x is a whole number.

[0025] An acoustic reflector according to claim 24 characterised in that n=3.

[0026] An acoustic reflector as claimed in claim 24 or 25 characterised in that a corner reflector is provided at each end of the stack wherein the corner of the reflector is perpendicular to the longitudinal axis of the stack.

[0027] An acoustic reflector as claimed in any one of claims 24 to 26 characterised in that wherein the two pairs of corner reflectors in each level constitute a reflector element, said reflector element being of substantially circular shape in plan view.

[0028] An acoustic reflector according to claim 28 characterised in that acoustic

absorbing material is between each pair of corner reflectors.

[0029] An acoustic reflector device according to any one of claims 24 to 28 wherein the acoustic reflecting surfaces are constituted by an acoustic reflective lining on faces of a solid body.

[0030] An acoustic reflector according to any claim 29 characterised in that the acoustic reflecting material is aluminium.

[0031] An acoustic reflector as claimed in any one of claims 24 to 30 characterised in that the stack has end corner reflectors mounted such that they reflect substantially along the axis of the stack.

[0032] An acoustic reflector according to any one of claims 1 to 31 characterised in that it is attached to an object to be deployed under water.

[0033] An acoustic reflector according to claim 32 characterised in that it is attached to a portion of pipe.

[0034] An acoustic reflector according to any one of claims 1 to 31 characterised in being fixed to an underwater object.

[0035] An acoustic reflector according to claim 34 being one of a plurality of acoustic reflectors as a part of a scouring monitoring system.

[0036] An acoustic reflector according to any one of claims 32 to 35 characterised in that the acoustic reflector comprises aluminium.

[0037] An acoustic reflector substantially as hereinbefore described with reference to the accompanying drawings.