Sub-seabed scanning

Abstract

A sub-seabed scanning assembly for scanning a seabed is disclosed. The sub-seabed scanning assembly, comprises: an elongate structure extending along a major elongate axis; an acoustic array positioned on the elongate structure and arranged to produce an acoustic swath aligned along the elongate axis; and a plurality of magnetometers positioned on the elongate structure and arranged to have a plurality of overlapping magnetic detection zones aligned along the elongate axis and overlapping the acoustic swath.

Description

[0001] SUB-SEABED SCANNING

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a sub-seabed scanning assembly for scanning a subseabed and a method.

[0004] BACKGROUND

[0005] Apparatus for sub-seabed scanning are known. Although such apparatus exist, each have their own shortcomings. Accordingly, it is desired to provide an improved technique for sub-seabed scanning.

[0006] SUMMARY

[0007] According to a first aspect, there is provided a sub-seabed scanning assembly for scanning a sub-seabed, comprising: an elongate structure extending along a major elongate axis; an acoustic array positioned on the elongate structure and arranged to produce an acoustic swath aligned along the elongate axis; and a plurality of magnetometers positioned on the elongate structure and arranged to have a plurality of overlapping magnetic detection zones aligned along the elongate axis and overlapping the acoustic swath.

[0008] The first aspect recognizes that a problem with existing arrangements is that the coherence of any acoustic and magnetic detection data can be poor, since such data is typically obtained using separate sweeps of the seabed or produced by an apparatus having their acoustic array and magnetometers positioned on separate structures located at different positions along an along-track direction.

[0009] Accordingly, a sub-seabed scanning assembly is provided. The sub-seabed scanning assembly may be for scanning a sub-seabed or underground or under the seabed. The scanning may be to localise objects typically in 3 dimensions. The scanning may be with multiple sensors. The sub-seabed scanning assembly may comprise towed vehicle providing an elongate or a longitudinally-extending structure. The elongate structure may extend or be dimensioned along a major or principle elongate axis. The sub-seabed scanning assembly may comprise an acoustic array. The acoustic array may be a 3 dimensional underground acoustic beamformed synthetic aperture sonar (SAS) array. The towed vehicle may house the acoustic array. The acoustic array may be positioned on or coupled to the elongate structure. The acoustic array may be arranged to produce an acoustic swath aligned with or along the elongate axis. Thesub-seabed scanning assembly may comprise a plurality of magnetometers. The towed vehicle may house the plurality of magnetometers. The plurality of magnetometers may be positioned on or coupled to the elongate structure. The acoustic array and the plurality of magnetometers may be positioned in a fixed spatial relationship. The plurality of magnetometers may be arranged to have a plurality of magnetic detection zones. The magnetic detection zones may be overlapping or intersecting. The magnetic detection zones may be aligned along or with the elongate axis. The magnetic detection zones may overlap or intersect the acoustic swath. In this way, the acoustic swath and magnetic detection zones are preferably aligned and overlapping due to the spatial coupling or co-location of the acoustic array and magnetometers on the elongate structure, which helps to improve the coherence of the acoustic and magnetic detection data and which helps to improve the detection of targets of interest. This provides an arrangement where the assembly can be towed at a variable or selectable scanning height to enable a three-dimensional localization of objects below the seabed (underground).

[0010] The underground SAS acoustic array and the plurality of magnetometers may be spatially aligned such that the acoustic swath and magnetic detection zones are coregistered or aligned in a three-dimensional coordinate system based on a predetermined scanning height above the seabed.

[0011] The plurality of magnetometers may be spaced to provide a total or overall magnetic capture width that is at least equal to an across-track footprint of the acoustic swath. Such spacing may facilitate or enable three-dimensional localization of any underground object.

[0012] The elongated structure may be at least around 5m across to allow for the sensors to be aligned for varying depth direction regardless of seabed altitude as sensors penetrate the seabed for detections. Such an arrangement may allow for an operational efficient apparatus for single-pass detections.

[0013] The elongate structure may be configured for movement in an along-track direction of travel over the seabed and the elongate axis is orientated in a cross-track direction transverse to the direction of travel. Hence, the elongate structure may be arranged to extend along the elongate axis which is orthogonal to the direction of travel, in order to provide for an acoustic swath and overlapping magnetic detection zones scanning orthogonally to the direction of travel.The elongate structure may be configured to retain the acoustic array and the plurality of magnetometers in a fixed spatial relationship. Accordingly, rather than the acoustic array and magnetometer being moveable with respect to each other, the elongate structure may be used to secure the acoustic array and magnetometer to be substantially immoveable with respect to each other.

[0014] The elongate structure may be rigid to reduce relative movement of the acoustic array and / or the plurality of magnetometers. Providing an elongate structure which is rigid, stiff, unbending or inflexible helps to reduce or decrease relative movement of the magnetometers and / or of the acoustic array with respect to the magnetometers, which helps to improve the coherence of the acoustic and magnetic data.

[0015] The beamforming 3D underground synthetic aperture sonar may operate below around a 30kHz acoustic frequency and may create a swath of at least 120° in the across-track direction (60° either side of Nadir).

[0016] The plurality of magnetometers may comprise passive magnetometers with a sensitivity of at least a 1pTA / Hz and may have a sample rate of at least 30Hz.

[0017] The assembly may have an inertial navigation system with a coupled acoustic doppler current profiler (ADCP) for calculating assembly motion and position.

[0018] The assembly may have a measuring probe for determining real-time sound velocity of water within which the assembly is immersible.

[0019] The elongate structure may comprise a wing having at least two ailerons operable to adjust a scanning height above the seabed when moving in the direction of travel over the seabed. Hence, a scanning height, altitude or distance above the seabed can be adjusted or changed by actuating the ailerons. The ailerons also provide for stabilisation.

[0020] The elongate structure may be non-magnetic. Hence, at least a portion of the elongate structure may be formed of non-magnetic material to reduce interaction with the magnetometers.

[0021] The elongate structure may comprise carbon fibre tubes.The assembly may comprise a controller. The controller may be configured to adjust the at least one aileron to maintain or achieve the scanning height when moving in the direction of travel over the seabed.

[0022] The elongate structure may be configured for movement at the scanning height above the seabed.

[0023] The acoustic array and the plurality of magnetometers may be arranged so that the acoustic swath and plurality of magnetic detection zones overlap at the scanning height to provide an overlapping, and preferably continuous, detection area aligned along the elongate axis. Hence, the magnetic detection zones may overlap each other and may together overlap the acoustic swath when at the scanning height.

[0024] The acoustic array and the plurality of magnetometers may be arranged so that, at the scanning height, the overlapping detection area extends to a sub-seabed or underground depth. Accordingly, the detection area may extend or penetrate into the seabed to a sub-seabed depth in order to scan below the seabed.

[0025] The acoustic array and the plurality of magnetometers may be arranged so that, when at the scanning height, the plurality of overlapping magnetic detection zones and the acoustic swath are configured to have a matching effective detection range for a buried or underground target having desired characteristics. Accordingly, the detection range of the magnetic detection zones and the detection range of the acoustic swath may match or correspond in order to identify a target having desired or predetermined characteristics. The target may be ferrous or non-ferrous

[0026] The acoustic array and the plurality of magnetometers may be arranged so that, when at the scanning height, the acoustic swath extends to the sub-seabed depth of at least around 2m and the magnetic detection zones extends to the sub-seabed depth of around at least 2m for a target having desired characteristics.

[0027] The desired characteristics may comprise a ferrous mass target of between around 25kg and 50kg. The desired characteristics are typical for legacy munition.

[0028] The plurality of magnetometers may be distributed along the major elongate axis with a lateral spacing d defined by:

[0029]

[0030] where H is the scanning height, 0 is the acoustic swath angle of at least 120° and n is the number of magnetometers. The provides magnetic detection zones with continuous coverage across the acoustic swath.

[0031] The lateral spacing d may be constrained by a magnetic detection range Rmagof the magnetometers, such that for any point (x, y, z) within the acoustic swath, there exists at least one magnetometer at a slant range distance Rssatisfying the condition:

[0032]

[0033] where (Xj.yi.zi) represents the coordinates of the i-th magnetometer. This provides for co- registration of targets throughout the entire acoustic swath.

[0034] The lateral spacing d may be configured to maintain a slant range from an outermost magnetometer to an edge of the acoustic swath that is less than a maximum magnetic detection range Rmagfor a target. Such a target may have a ferrous mass of at least around 10 kg.

[0035] The underground SAS acoustic array may be configured to produce the swath having an across-track component aligned with the major elongated axis at an angle of at least 120° and the along-track component may be determined by synthetic aperture processing that uses at least a lm synthetic array to achieve a resolution less than 0.04m.

[0036] The acoustic array may be configured to produce an acoustic swath extending towards the seabed having an across-track component aligned with the elongate axis and an along-track component aligned transverse to the elongate axis.

[0037] The across-track component may extend from the acoustic array at an angle of around 120 degrees.

[0038] The along-track component may extend from the acoustic array at an angle of around between 0.3 and 1.7 degrees.

[0039] The acoustic array may be configured to produce the acoustic swath having, when at the scanning height, an across-track elongate footprint extending between footprintlimits and at least one of the plurality of magnetometers is positioned on the elongate structure to provide magnetic detection zones extending to at least one of the footprint limits. Accordingly, the acoustic swath may be configured to provide an across-track elongate footprint on the seabed extending between footprint ends or limits at either end of the footprint. At least one of the magnetometers may be positioned so that its detection zone extends to or encompasses that footprint limit. This ensures that the overlapping magnetic detection zones extend at least to the ends of the acoustic swath.

[0040] The acoustic array may be configured to produce the acoustic swath having, when at the scanning height, an across-track elongate footprint extending between footprint limits and associated ones of the plurality of magnetometers are positioned on the elongate structure to provide magnetic detection zones extending to an associated one of the footprint limits. Accordingly, magnetometers may be positioned so that their magnetic detection zones overlap or encompass both ends of the acoustic swath.

[0041] The acoustic array may be configured, when at the scanning height of around 5m and with the across-track component extending from the acoustic array at an angle of around 120°, to produce the acoustic swath having the across-track elongate footprint extending for around 18.6m and having an along-track footprint extending for between around 0.03m and 0.15m.

[0042] The plurality of magnetometers may be configured to have the magnetic detection zone of around 8.7m for a target having the desired characteristics.

[0043] The assembly may be configured, when at the scanning height of around 5m, with the lateral spacing d and with a length of the elongate structure length to provide overlapping and preferably continuous detection areas extending at least around 8.7 m laterally from a central axis of the assembly.

[0044] The plurality of magnetometers may be configured to detect at around a 1.5 nT amplitude, a ferrous mass of around 50kg with around an 8.7m detection range for any magnetometer in the plurality of magnetometers.

[0045] The elongate structure may extend for around 6 metres. Accordingly, the elongate structure may support scanning a distance on the seabed multiple times its own length.The acoustic array may comprise a plurality of acoustic sensors spaced apart along or on the elongate axis.

[0046] The plurality of acoustic sensors may be grouped together along or on the elongate axis. The plurality of acoustic sensors may be grouped together towards a central, mid or inner portion or region of the elongate structure.

[0047] The acoustic array may comprise a synthetic aperture sonar.

[0048] The acoustic sensor may be operable at acoustic frequencies of less than around 30 kHz. Such frequencies typically to enable sub-seabed penetration of at least 2m

[0049] The acoustic sensor may comprise an around 5 kHz to 25 kHz synthetic aperture sonar.

[0050] The acoustic sensor may be operable to provide at least around a 120° swath made up of around < 2° beamformed detection footprints.

[0051] The underground SAS acoustic array may comprise an array of receiving acoustic transducers and at least 2 transmitting transducers.

[0052] The plurality of magnetometers may be spaced apart along, on or aligned with the elongate axis.

[0053] The plurality of magnetometers may be spaced evenly or at constant intervals along, on or aligned with the elongate axis.

[0054] The plurality of magnetometers may be located at distal ends of said elongate structure.

[0055] The plurality of magnetometers may be spaced evenly apart along said elongate axis.

[0056] The plurality of magnetometers may be grouped together towards at least one outer or end portion of the elongate structure.

[0057] The plurality of magnetometers may be positioned as two groups of magnetometers located or positioned towards outer or end portions of the elongate structure.Accordingly, two groups of magnetometers may be provided, each located towards ends of the elongate structure.

[0058] The plurality of magnetometers may comprise small atomic magnetometers.

[0059] The plurality of magnetometers may comprise micro-fabricated atomic caesium pumped magnetometers.

[0060] The plurality of magnetometers may operate at a sampling rate of around 1000 kHz.

[0061] The plurality of acoustic sensors may interpose or be located between the two groups of magnetometers.

[0062] The plurality of acoustic sensors and the plurality of magnetometers may be positioned along, on or aligned with the elongate axis.

[0063] The plurality of acoustic sensors and the plurality of magnetometers may be positioned along the elongate axis with a fixed along-track positional offset. Accordingly, the acoustic sensors and magnetometers need not be exactly positionally aligned on the elongate axis and positional offset is possible. Typically, in these circumstances, the positional offset is provided by the assembly to assist in subsequent data analysis.

[0064] The assembly may comprise a positioning device configured to determine a position of the elongate structure.

[0065] The assembly may comprise a pressure sensor configured to provide depth below sealevel measurements.

[0066] The assembly may comprise a data processor configured to receive and preferably store acoustic data from the acoustic array and magnetic data from the plurality of magnetometers. The data process may be configured to receive and preferably store data from any ancillary or additional sensors.

[0067] The assembly may comprise a communications device configured to convey the acoustic data and the magnetic data. The communications device may be configured to convey data from any ancillary or additional sensors.The assembly may comprise a fixing configured for towing by a vessel.

[0068] The assembly may comprise a fixing configured for fixing to a surface vessel hull.

[0069] The assembly may comprise a fixing configured for fixing to a sub-surface vessel hull.

[0070] The assembly may comprise a data processor configured to receive the acoustic data and the magnetic data and perform data processing to identify at least one of seabed and target characteristics.

[0071] According to a second aspect, there is provided a method of scanning a sub-seabed, comprising: providing an elongate structure extending along a major elongate axis; positioning an acoustic array on the elongate structure; positioning a plurality of magnetometers on the elongate structure; producing, with the acoustic array, an acoustic swath aligned along the elongate axis; and producing, with the plurality of magnetometers, a plurality of overlapping magnetic detection zones aligned along the elongate axis and overlapping the acoustic swath.

[0072] The method may comprise spatially aligning the underground SAS acoustic array and the plurality of magnetometers such that the acoustic swath and magnetic detection zones are co-registered or aligned in a three-dimensional coordinate system based on a predetermined scanning height above the seabed.

[0073] The method may comprise spacing the plurality of magnetometers to provide a total or overall magnetic capture width that is at least equal to an across-track footprint of the acoustic swath to facilitate or enable three-dimensional localization of any underground object.

[0074] The method may comprise dimensioning the elongated structure to be at least around 5m across to allow for the sensors to be aligned for varying depth direction regardless of seabed altitude as sensors penetrate the seabed for detections.

[0075] The method may comprise configuring the elongate structure for movement in an along-track direction of travel over the seabed orientating the elongate axis in a crosstrack direction transverse to the direction of travel.The method may comprise retaining the acoustic array and the plurality of magnetometers in a fixed spatial relationship on the elongate structure.

[0076] The method may comprise configuring the elongate structure to be rigid to reduce relative movement of the acoustic array and / or the plurality of magnetometers.

[0077] The method may comprise operating the beamforming 3D underground synthetic aperture sonar below around a 30kHz acoustic frequency and creating a swath of at least 120° in the across-track direction (60° either side of Nadir).

[0078] The plurality of magnetometers may comprise passive magnetometers with a sensitivity of at least a 1pT / > / Hz and may have a sample rate of at least 30Hz.

[0079] The method may comprise providing an inertial navigation system with a coupled acoustic doppler current profiler (ADCP) for calculating assembly motion and position.

[0080] The method may comprise providing a measuring probe for determining real-time sound velocity of water within which the assembly is immersible.

[0081] The method may comprise providing the elongate structure with a wing having at least two ailerons and operating the at least one aileron to adjust a scanning height above the seabed when moving in the direction of travel over the seabed.

[0082] The method may comprise configuring the elongate structure to be non-magnetic.

[0083] The method may comprise forming the elongate structure from carbon fibre tubes.

[0084] The method may comprise adjusting the at least one aileron with a controller to maintain the scanning height when moving in the direction of travel over the seabed.

[0085] The method may comprise configuring the elongate structure for movement at the scanning height above the seabed.

[0086] The method may comprise arranging the acoustic array and the plurality of magnetometers so that the acoustic swath and plurality of magnetic detection zones overlap at the scanning height to provide an overlapping, and preferably continuous, detection area aligned along the elongate axis.The method may comprise arranging the acoustic array and the plurality of magnetometers so that, when at the scanning height, the overlapping detection area extends to a sub-seabed or underground depth.

[0087] The method may comprise arranging the acoustic array and the plurality of magnetometers so that, when at the scanning height, the plurality of overlapping magnetic detection zones and the acoustic swath have a matching effective detection range for a buried or underground target having desired characteristics.

[0088] The method may comprise arranging the acoustic array and the plurality of magnetometers so that, when at the scanning height, the acoustic swath extends to the sub-seabed depth of at least around 2m and the magnetic detection zones extends to the sub-seabed depth of around at least 2m for a target having desired characteristics.

[0089] The desired characteristics may comprise an around 50kg ferrous mass target.

[0090] The method may comprise distributing the plurality of magnetometers along the major elongate axis with a lateral spacing d defined by:

[0091]

[0092] where H is the scanning height, 0 is the acoustic swath angle of at least 120° and n is the number of magnetometers.

[0093] The method may comprise constraining the lateral spacing d by a magnetic detection range Rmagof the magnetometers, such that for any point (x, y, z) within the acoustic swath, there exists at least one magnetometer at a slant range distance Rssatisfying the condition:

[0094]

[0095] where (Xj.yi.zi) represents the coordinates of the i-th magnetometer.

[0096] The method may comprise configuring the lateral spacing d to maintain a slant range from an outermost magnetometer to an edge of the acoustic swath that is less than a maximum magnetic detection range Rmagfor a target.The method may comprise configuring the underground SAS acoustic array to produce the swath having an across-track component aligned with the major elongated axis at an angle of at least 120° and to produce the along-track component determined by synthetic aperture processing that uses at least a lm synthetic array to achieve a resolution less than 0.04m.

[0097] The method may comprise configuring the acoustic array to produce the acoustic swath extending towards the seabed having an across-track component aligned with the elongate axis and an along-track component aligned transverse to the elongate axis.

[0098] The across-track component may extend from the acoustic array at an angle of around 120°.

[0099] The along-track component may extend from the acoustic array at an angle of between around 0.3° and 1.7°.

[0100] The method may comprise arranging the acoustic array to produce the acoustic swath having, when at the scanning height, an across-track elongate footprint extending between footprint limits and positioning at least one of the plurality of magnetometers on the elongate structure to provide magnetic detection zones extending to at least one of the footprint limits.

[0101] The method may comprise configuring the acoustic array to produce the acoustic swath having, when at the scanning height, an across-track elongate footprint extending between footprint limits and positioning associated ones of the plurality of magnetometers on the elongate structure to provide magnetic detection zones extending to an associated one of the footprint limits.

[0102] The method may comprise configuring the acoustic array to produce the acoustic swath, when at the scanning height of around 5m and with the across-track component extending from the acoustic array at an angle of around 120°, having the across-track elongate footprint extending for around 18.6m and having an along-track footprint extending for between around 0.03m and 0.15m.

[0103] The method may comprise configuring the plurality of magnetometers to have the magnetic detection zone of around 8.7m for a target having the desired characteristics.The method may comprise configuring the acoustic array and the plurality of magnetometers, when at the scanning height of around 5m, with the lateral spacing d and with a length of the elongate structure length to provide overlapping and preferably continuous detection areas extending at least around 8.7 m laterally from a central axis of the assembly.

[0104] The method may comprise configuring to detect at around a 1.5 nT amplitude, a ferrous mass of around 50kg with around a 8.7m detection range for any magnetometer in the plurality of magnetometers.

[0105] The method may comprise configuring the elongate structure to extend for around 6m.

[0106] The acoustic array may comprise a plurality of acoustic sensors and the method may comprise spacing the plurality of acoustic sensors apart along the elongate axis.

[0107] The method may comprise grouping the plurality of acoustic sensors together along the elongate axis.

[0108] The method may comprise grouping the plurality of acoustic sensors together towards a central portion of the elongate structure.

[0109] The acoustic array may comprise a synthetic aperture sonar.

[0110] The method may comprise operating the acoustic sensor at acoustic frequencies of less than around 30 kHz.

[0111] The acoustic array may comprise an around 5 kHz to 25 kHz synthetic aperture sonar.

[0112] The method may comprise operating the acoustic sensor to provide at least around a 120° swath made up of around < 2° beamformed detection footprints.

[0113] The underground SAS acoustic array may comprise an array of receiving acoustic transducers and at least 2 transmitting transducers.

[0114] The method may comprise spacing the plurality of magnetometers apart along the elongate axis.The method may comprise spacing the plurality of magnetometers evenly or at constant intervals along the elongate axis.

[0115] The method may comprise locating the plurality of magnetometers at distal ends of said elongate structure.

[0116] The method may comprise spacing the plurality of magnetometers evenly apart along said elongate axis.

[0117] The method may comprise grouping the plurality of magnetometers together towards at least one outer portion of the elongate structure.

[0118] The method may comprise positioning the plurality of magnetometers as two groups of magnetometers located towards outer portions of the elongate structure.

[0119] The plurality of magnetometers may comprise small atomic magnetometers.

[0120] The plurality of magnetometers may comprise micro-fabricated atomic caesium pumped magnetometers.

[0121] The plurality of magnetometers may operate at a sampling rate of around 1000kHz.

[0122] The method may comprise interposing the plurality of acoustic sensors between the two groups of magnetometers.

[0123] The method may comprise positioning the plurality of acoustic sensors and the plurality of magnetometers along the elongate axis.

[0124] The method may comprise positioning the plurality of acoustic sensors and the plurality of magnetometers along the elongate axis with a fixed along-track positional offset.

[0125] The method may comprise providing a positioning device and determining a position of the elongate structure with the positioning device.

[0126] The method may comprise providing a pressure sensor and providing depth below sealevel measurements with the pressure sensor.The method may comprise providing a data processor and receiving and preferably storing acoustic data from the acoustic array and magnetic data from the plurality of magnetometers with the data processor.

[0127] The method may comprise receiving and preferably storing data from any ancillary or additional sensors.

[0128] The method may comprise providing a communication device and conveying the acoustic data and the magnetic data with the communication device.

[0129] The method may comprise conveying data from any ancillary or additional sensors.

[0130] The method may comprise providing a fixing and towing the elongate structure with a vessel using the fixing.

[0131] The method may comprise providing a fixing and fixing the elongate structure to a hull of a surface vessel using the fixing.

[0132] The method may comprise providing a fixing and fixing the elongate structure to a hull of a sub-surface vessel using the fixing.

[0133] The method may comprise providing a data processor, receiving the acoustic data and the magnetic data with the data processor and performing data processing to identify at least one of seabed and target characteristics.

[0134] According to a third aspect, there is provided a sub-seabed scanning assembly for scanning underground to localize objects in 3 dimensions with multi-physics sensors, comprising: an elongate structure extending along a major elongate axis; a 3D underground acoustic beamformed synthetic aperture sonar (SAS) array coupled to the elongate structure and arranged to produce an acoustic swath aligned along the elongate axis; a plurality of magnetometers coupled to the elongate structure and arranged to have a plurality of magnetic detection zones overlapping the acoustic swath; and a towed vehicle for housing the sensors and elongated structure; wherein the underground SAS acoustic array and the plurality of magnetometers are spatially aligned such that the acoustic swath and magnetic detection zones are co-registered in a three-dimensional coordinate system based on a predetermined scanning height H;and wherein the plurality of magnetometers are spaced to provide a total magnetic capture width that is equal to the across-track footprint of the acoustic swath, to enable three-dimensional multi-physics localization of any underground object. The third aspect may have the optional features of the first aspect set out above.

[0135] According to a fourth aspect, there is provided a method of scanning a sub-seabed, comprising: providing an elongate structure extending along a major elongate axis; positioning a 3D underground SAS acoustic array and a plurality of magnetometers on said elongate structure in a fixed special relationship; producing an acoustic swath aligned along said elongate axis; and producing a plurality of magnetic detection zones that overlap with the acoustic swath at a variable scanning height to enable a three-dimensional localization of objects below the seabed (underground). The fourth aspect may have the optional features of the second aspect set out above.

[0136] Further particular and preferred aspects are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims as appropriate, and in combinations other than those explicitly set out in the claims.

[0137] Where an apparatus feature is described as being operable to provide a function, it will be appreciated that this includes an apparatus feature which provides that function or which is adapted or configured to provide that function.

[0138] BRIEF DESCRIPTION OF THE DRAWINGS

[0139] Embodiments of the present invention will now be described further, with reference to the accompanying drawings, in which:

[0140] FIG. 1 illustrates schematically underside perspective views of components of a subseabed scanning assembly according to one embodiment;

[0141] FIG. 2 is a schematic plan view of the sub-seabed scanning assembly;

[0142] FIG. 3 illustrates schematically a cross-track view showing the scanning pattern of the sonar array of the sub-seabed scanning assembly;

[0143] FIG. 4 is a schematic plan view showing the arrangement of the acoustic swath in relation to the magnetic detection zones of the sub-seabed scanning assembly;

[0144] FIG. 5 is a schematic cross-track view showing the arrangement of the acoustic swath and magnetic detection zones of the sub-seabed scanning assembly in more detail; and

[0145] FIG. 6 illustrates schematically example towing of the sub-seabed scanning assembly.DESCRIPTION OF THE EMBODIMENTS

[0146] Before discussing embodiments in any more detail, first an overview will be provided. A sub-seabed scanning assembly is provided. The assembly has a structure configured for sub-surface towing by a vessel at a configurable height above the seabed. The assembly has a structure which is elongate such that its length is longer than its width. The structure holds both acoustic sensors and magnetometers which are positioned along the structure length. In other words, the assembly is configured as a towed vehicle housing multiple sensors (acoustic sensors and magnetometers) in a fixed spatial relationship. These can then be used for scanning a sub-seabed (in other words scanning underground, under the seabed) to determine the location of objects under the seabed. The scanning localises objects typically in 3 dimensions, typically with both acoustic sensors and magnetometers. Typically, the acoustic sensors are provided as a 3 dimensional underground acoustic beamformed synthetic aperture sonar (SAS) array. Typically, the acoustic sensors are located towards a central region of the structure and the magnetometers are located on the outer portions of the structure. The acoustic sensors and magnetometers are configured such that their detection zones extend along and are aligned with the elongate axis and overlap or intersect. This provides for spatial alignment of the detection zones and spatial alignment of data provided by the acoustic sensors and magnetometers. In other words, the acoustic swath and magnetic detection zones are co-registered or aligned in a three-dimensional coordinate system based on a predetermined scanning height above the seabed. This spatial alignment improves the coherence of that data and improves accuracy of targets detected on and below the seabed, correlating features identified by the acoustic sensors and the magnetometers. Also, by co-locating the acoustic sensors and magnetometers on the elongate structure, the coherence of the data is improved and the need to perform different scanning tracks at different times with different sensors is obviated. Forming at least a portion of the structure into a wing helps to improve the stability of the assembly when being towed through the water and enables a desired height above seabed to be achieved and maintained.

[0147] Seabed Scanning Assembly

[0148] FIG. 1 illustrates schematically underside perspective views of components of a subseabed scanning assembly 10 according to one embodiment. The sub-seabed scanning assembly 10 has a central portion 20 and a pair of wing portions 30A, 30B extending port and starboard from the central portion to form an elongate structure. Each wing portion 301 , 30B ends or terminates with a winglet 40A, 40B. The elongatestructure extends along an elongate axis AA. The central portion 20 houses an Ultra Short Base Line (USBL) propagated Global Navigation Satellite System (GNSS) positioning beacon 50, an Inertial Navigation System (INS) device and Doppler Velocity Logger (DVL) device 60, as well as a speed sensor 70 and a pressure sensor (not shown). A sonar array is formed from a pair of omnidirectional acoustic transmitters 80 and a set of acoustic receiver panels 90. Typically, the sonar array is a multichannel hydrophone array with dual omni-directional FM transmitters 80 operating between -5-25 kHz which creates a 3D swath [around 120 degree] with real aperture and synthetic processing (done in real-time).

[0149] As can be seen in more detail in FIG. 2 which is a schematic plan view of the upper side of the sub-seabed scanning assembly 10, the wings 30A, 30B are formed from a plurality of individual wing panels 35, the central portion 20 has ailerons 100 aft and a tow fixing 110 forward which allows the sub-seabed scanning assembly 10 to be attached to a fibre tow cable 190. The tow fixing 110 allows the sub-seabed scanning assembly 10 to be towed in an along-track direction B, with the elongate axis AA then being aligned with a cross-track axis orthogonal to the along-track direction B. Located within the wings 30A, 30B are a number of magnetometers 120A - 120D which are typically evenly spaced along the length of the elongate axis AA. Hence, it can be seen that the omnidirectional acoustic transmitters 80 and acoustic receiver panels 90 forming the sonar array are located on the central portion 20, with a first group of magnetometers 120A, 120B located on the wing 30A and a second group magnetometers 120C, 120D located on the wing 30B. As can be seen, the magnetometers 120A - 120D, the acoustic transmitters 80 and the acoustic receiver panels 90 are located along the elongate axis AA. Although coherence in the data produced is best when the magnetometers 120A - 120D, the acoustic transmitters 80 and the acoustic receiver panels 90 are completely positionally aligned on the elongate axis AA, a slight along-track positional offset (forward / aft) is possible as shown in FIG.

[0150] 2 since they still remain in a fixed spatial relationship due to the fixed structure provided by the central portion 20 and wings 30A, 30B. Although four magnetometers 120A-D are provided in this arrangement, it will be appreciated that more or fewer may be provided. Having four magnetometers 120A-D helps to provide for improved triangulation of detected targets. The sonar array is typically a 3 dimensional underground acoustic beamformed synthetic aperture sonar (SAS) array.

[0151] Scanning and Detection ZonesFIG. 3 illustrates schematically a cross-track (front) view showing the scanning pattern of the sonar array forming the acoustic swath 230 projecting onto the seabed 260. In this example, the sonar array scans with an across-track beam width of around 124 degrees made up of individual pings generating footprints 240 on the seabed 260. Each ping has an across-track (port / starboard) beam angle of less than 2 degrees and typically around 1.4 degrees and an along-track (forward / aft) beam angle of less than 2 degrees and typically between around 0.34 and 1.7 degrees. At a scanning height of around 5 metres above the seabed, this forms individual footprints 240 extending typically less than around 0.05 metres across-track and between around 0.03 and 0.15 metres along-track. The total along-track distance of the acoustic swath 230 is therefore around 20 metres (varying with scanning height), centred on the central portion 20 of the elongate structure.

[0152] FIG. 4 is a schematic plan (top) view showing the arrangement of the acoustic swath 230 in relation to the magnetic detection zones 150A - 150D for each of an associated magnetometer 120A - 120D projecting onto the seabed 260. As can be seen, the acoustic swath 230 is elongate, extending port / starboard in the cross-track direction along the elongate axis AA and has an along-track dimension extending forward / aft in the direction B. The magnetic detection zones 150A - 150D overlap each other and overlap the acoustic swath 230 to provide an overlapping and continuous detection zone where. Typically, the beamforming 3D underground synthetic aperture sonar operates below around a 30kHz acoustic frequency (and typically between 5kHz and 25kHz) and creates a swath 230 of at least 120° in the across-track direction (60° either side of Nadir). The underground SAS acoustic array is typically configured to produce the swath 230 having an across-track component aligned with the major elongated axis AA at an angle of at least 120° and the along-track component may be determined by synthetic aperture processing that uses at least a lm synthetic array to achieve a resolution less than 0.04m.

[0153] To cover the acoustic swath 230 with passive sensing magnetometers 120A-D, a purpose-built structure is needed. Micro-Fabricated Atomic caesium pumped magnetometers haven been found to be suited to be mounted close to electromagnetic and ferrous objects with less interference than was previously possible with conventional caesium pumped magnetometers due to new build methods and higher sampling rates (1000kHz). Magnetometers are passive sensors and detection ranges vary with object sizes and magnetic characteristics. A Minimum Detectable Target (MDT) is determined, which is given by the maximum distance to target (slant range of run line spacing and altitude off bottom or height above seabed) and the backgroundnoise of the instrument. Typical unexploded ordinance (UXO) surveys are looking for 50 kg ferrous mass targets as an MDT size. This MDT was used to inform the base design of the structure, with an MDT calculated at 8.7 metres when at a scanning height of 5m. To propagate the 8.7 metres slant range coverage to correlate with the 20 metres coverage of the sonar, a rigid structure that extends 6 metres perpendicular to the direction of travel are needed. Carbon fiber tubes were used to extend a housing structure from the primary remote operated towed vehicle (ROTV) forming the central portion 20. The ROTV can house the high-grade Inertial Navigation System (INS), Doppler Velocity Logger (DVL), and 3D SAS Sub-bottom Sonar. The carbon fiber extensions in the form of wings 30A, 30B provide a strong non-magnetic structure that can position the magnetometers 120A-D to align the detection range of both systems for a 50 kg ferrous mass target. A solid structure allows for tight coupling of both data sets positionally and temporally. Typically, magnetometers 120A-D comprise passive magnetometers with a sensitivity of at least a 1pTA / Hz and have a sample rate of at least 30Hz. The magnetometers 120A-D are distributed along the major elongate axis AA with a lateral spacing d defined by:

[0154]

[0155] where H is the scanning height, 0 is the acoustic swath angle of at least 120° and n is the number of magnetometers. The provides magnetic detection zones 150A-150D with continuous coverage across the acoustic swath 230. The lateral spacing d is typically constrained by a magnetic detection range Rmagof the magnetometers, such that for any point (x, y, z) within the acoustic swath 230, there exists at least one magnetometer at a slant range distance Rssatisfying the condition:

[0156]

[0157] where (Xj.yi.zi) represents the coordinates of the i-th magnetometer. This provides for co- registration of targets throughout the entire acoustic swath 230. The lateral spacing d is typically configured to maintain a slant range from an outermost magnetometer to an edge of the acoustic swath 230 that is less than a maximum magnetic detection range Rmagfor a target. Such a target may typically have a ferrous mass of at least around 10 kg. Typically, the plurality of magnetometers are configured to detect at around a 1.5 nT amplitude, a ferrous mass of around 50kg with around an 8.7m detection range for any magnetometer in the plurality of magnetometers.

[0158] The relationship between sonar acoustic swath 230 and magnetometer detection ranges 150A-D when in this structure scales with MDT size. If an MDT of 25 kg ferrousmass is used then the magnetometers 120A-D require a lower flying altitude (height above seabed) as the slant range to detect the target is decreased. This would decrease the angular coverage of the sonar such that the detection ranges of both systems still correlate. Thus, the configuration and dimensions of the structure maintain a correlated detection range for most / all use cases. The ROTV provides altitude control to ensure the system stays within the detection range of the MDT by using alerions to control the systems altitude above seabed. The INS and DVL provide tightly coupled positioning that can be further constrained by Ultra Short Baseline (USBL) propagated GNSS positioning. Ensuring an International Hydrographic Organization (I HO) Exclusive Order degree of positioning. Pressure sensors in the ROTV also provide depth below sea level that allows for accurate tidal computation. When all combined, this arrangement provides a low uncertainty, tightly integrated acoustic and magnetic data set, in real world positions. An acoustic doppler current profiler (ADCP - not shown) may be provided for calculating assembly motion and position, along with a measuring probe (not shown) for determining real-time sound velocity of water within which the assembly is immersible.

[0159] FIG. 5 is a schematic cross-track (front) view showing the arrangement of the acoustic swath 230 and magnetic detection zones 150A - 150D projecting onto the seabed 260 in more detail. As can be seen, the acoustic swath 230 extends (port / starboard) along the elongate axis AA in the cross-track direction and the magnetic detection zones 150A - 150D extend from their respective magnetometers 120A - 120D. As can be seen, when the sub-seabed scanning assembly 10 is moved at a height H above the seabed 260, the acoustic swath 230 and one or more of the magnetic detection zones 150A-D are configured to detect the presence of a target 170 buried at a depth D below the seabed 260.

[0160] FIG. 6 illustrates schematically a side view of example towing of the seabed scanning assembly 10. In this example, the seabed scanning assembly is towed by a surface vessel 180 using the fibre tow cable 190. The sub-seabed scanning assembly 10 is towed in the along-track direction B. As can be seen, when towed at above seabed height of 5 metres, the acoustic swath 230 has a beam width which provides for an along-track (forward / aft) scanning distance of between around 0.03 and 0.15 metres and the acoustic swath 230 can extend below the seabed 260 to detect the target 170 at a depth of around 2 metres. Furthermore, the at least one of the magnetic detection zones 150A-D related to an associated magnetometer 120A-D, which typically extends at a radius of 8.7 metres, is also able to detect the presence of the target 170.Although this arrangement shows the sub-seabed scanning assembly 10 being towed by the surface vessel 180, it will be appreciated that this need not be the case and the sub-seabed scanning assembly 10 may instead be attached to the hull of the surface vessel 180 or to a hull of a sub-surface vessel.

[0161] In operation, the sub-seabed scanning assembly 10 moves through the water. The ailerons 100 operate to adjust the scanning height to a desired heigh H above the seabed 260. The LISBL positioning beacon 50, INS device and DVL device 60, as well as the speed sensor 70 and a pressure sensor take measurements related to the position, speed and depth of the sub-seabed scanning assembly 10 and provides that data to a data processor. The acoustic array performs scans using the acoustic swath 230 and provide that data to the data processor (not shown) typically housed in the central portion 120. The magnetometers 120A-D perform detection withing their detection zones 150A-D and provide that data to the data processor. The data processor performs and pre-processing and provides that data to a communications device (not shown) typically housing in the central portion 120. The data is conveyed to a remote processor typically on the surface vessel 180. In this arrangement, the data is conveyed using a fibre link provided by the fibre tow cable 190. The data is then processed in a conventional manner to identify features of and below the seabed and any targets. The data provided by the sub-seabed scanning assembly 10 enables more accurate identification of features and targets due to the temporal and spatial coherence of the data provided by the acoustic array and the magnetometers 120A-D.

[0162] Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents.

Claims

CLAIMS1. A sub-seabed scanning assembly for scanning a sub-seabed, comprising: an elongate structure extending along a major elongate axis;an acoustic array positioned on said elongate structure and arranged to produce an acoustic swath aligned along said elongate axis; anda plurality of magnetometers positioned on said elongate structure and arranged to have a plurality of overlapping magnetic detection zones aligned along said elongate axis and overlapping said acoustic swath.

2. The sub-seabed scanning assembly of claim 1 , wherein said elongate structure is at least one of:configured for movement in an along-track direction of travel over said seabed and said elongate axis is orientated in a cross-track direction transverse to said direction of travel;configured to retain said acoustic array and said plurality of magnetometers in a fixed spatial relationship;rigid to reduce relative movement of said acoustic array and said plurality of magnetometers;non-magnetic; andcomprised of carbon fibre tubes3. The sub-seabed scanning assembly of claim 1 or 2, wherein said elongate structure comprises a wing having at least one aileron operable to adjust a scanning height above said seabed when moving in said direction of travel over said seabed.

4. The sub-seabed scanning assembly of claim 3, further comprising a controller configured to adjust said at least one aileron to maintain said scanning height when moving in said direction of travel over said seabed.

5. The sub-seabed scanning assembly of claim 3 or 4, wherein said elongate structure is configured for movement at said scanning height above said seabed and said acoustic array and said plurality of magnetometers are arranged so that said acoustic swath and plurality of magnetic detection zones overlap at said scanning height to provide an overlapping detection area aligned along said elongate axis.6 The sub-seabed scanning assembly of any one of claims 3 to 5, wherein said acoustic array and said plurality of magnetometers are at least one of:arranged so that, when at said scanning height, said overlapping detection area extends to a sub-seabed depth;arranged so that, when at said scanning height, said plurality of overlapping magnetic detection zones and said acoustic swath are configured to have a matching effective detection range for a target having desired characteristics; andarranged so that, when at said scanning height, said acoustic swath extends to said sub-seabed depth of at least around 2m and said magnetic detection zones extends to said sub-seabed depth of around at least 2m for a target having desired characteristics.

7. The sub-seabed scanning assembly of claim 5, wherein said desired characteristics comprise a ferrous mass target of between around 25kg and 50kg.

8. The sub-seabed scanning assembly of any preceding claim, wherein said acoustic array is configured to produce said acoustic swath extending towards said seabed having a cross-track component aligned with said elongate axis and an along-track component aligned transverse to said elongate axis.9 The sub-seabed scanning assembly of claim 8, wherein said cross-track component extends from the acoustic array at an angle of around 120° and preferably wherein said along-track component extends from the acoustic array at an angle of between around 0.3° and 1.7°.

10. The sub-seabed scanning assembly of any one of claims 3 to 9, wherein said acoustic array is at least one of:configured to produce said acoustic swath having, when at said scanning height, a cross-track elongate footprint extending between footprint limits and at least one of said plurality of magnetometers is positioned on said elongate structure to provide magnetic detection zones extending to at least one of said footprint limits; configured to produce said acoustic swath having, when at said scanning height, a cross-track elongate footprint extending between footprint limits and associated ones of said plurality of magnetometers are positioned on said elongate structure to provide magnetic detection zones extending to an associated one of said footprint limits; andconfigured, when at said scanning height of around 5m and with said crosstrack component extending from the acoustic array at an angle of around 120°, to produce said acoustic swath having said cross-track elongate footprint extending for around 18.6m and having an along-track footprint extending for between around 0.03m and 0.15m.

11. The sub-seabed scanning assembly of any one of claims 6 to 10, wherein said plurality of magnetometers are configured to have said magnetic detection zone of around 8.7m for a target having said desired characteristics.

12. The sub-seabed scanning assembly of any preceding claim, wherein said elongate structure extends for around 6m.

13. The sub-seabed scanning assembly of any preceding claim, wherein said acoustic array comprises a plurality of acoustic sensors spaced apart along said elongate axis.

14. The sub-seabed scanning assembly of any preceding claim, wherein said plurality of acoustic sensors are at least one of:grouped together along said elongate axis; andgrouped together towards a central portion of said elongate structure.

15. The sub-seabed scanning assembly of any preceding claim, wherein said acoustic array comprises at least one of:a synthetic aperture sonar; andan around 5 kHz to 25 kHz synthetic aperture sonar.

16. The sub-seabed scanning assembly of any preceding claim, wherein said plurality of magnetometers are at least one of:spaced apart along said elongate axis;grouped together towards at least one outer portion of said elongate structure; andpositioned as two groups of magnetometers located towards outer portions of said elongate structure.

17. The sub-seabed scanning assembly of any preceding claim, wherein said plurality of magnetometers comprise at least one of:small atomic magnetometers; andmicro-fabricated atomic caesium pumped magnetometers.

18. The sub-seabed scanning assembly of any preceding claim, wherein said plurality of magnetometers operate at a sampling rate of around 1000kHz.

19. The sub-seabed scanning assembly of any one of claims 16 to 18, wherein said plurality of acoustic sensors interpose said two groups of magnetometers.

20. The sub-seabed scanning assembly of any preceding claim, wherein said plurality of acoustic sensors and said plurality of magnetometers are at least one of:positioned along said elongate axis; andpositioned along said elongate axis with a fixed along-track positional offset.

21. The sub-seabed scanning assembly of any preceding claim, further comprising at least one of:a positioning device configured to determine a position of said elongate structure;a pressure sensor configured to provide depth below sea-level measurements; a data processor configured to receive acoustic data from said acoustic array and magnetic data from said plurality of magnetometers;a communication device configured to convey said acoustic data and said magnetic data;a fixing configured for one of towing by a vessel, fixing to a surface vessel hull and fixing to a sub-surface vessel hull; anda data processor configured to receive said acoustic data and said magnetic data and perform data processing to identify at least one of seabed and target characteristics.

22. A method of scanning a sub-seabed, comprising:providing an elongate structure extending along a major elongate axis; positioning an acoustic array on said elongate structure;positioning a plurality of magnetometers on said elongate structure; producing, with said acoustic array, an acoustic swath aligned along said elongate axis; andproducing, with said plurality of magnetometers, a plurality of overlapping magnetic detection zones aligned along said elongate axis and overlapping said acoustic swath.

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