Method for imaging objects in a body of water using an array of remotely operated underwater vehicles

Abstract

A method for acoustic imaging of objects in a body of water incudes disposing a plurality of remotely operated underwater vehicles, ROVs (12, 14), in a body of water. One of the ROVs has an acoustic receiver (12A, 14A) and others have an acoustic transmitter (12A, 14A). The acoustic transmitters are actuated. Acoustic signals are detected by the receiver. A response of the detected signals is beam steered by applying different time delays among actuations of the acoustic transmitters, and / or, if the ROVs comprises a plurality of acoustic receivers, applying different time delays to signal recordings of detected acoustic signals. If an object is identified in the detected acoustic signals, the plurality of ROVs is moved to predetermined positions in the body of water based on a type of the object. The actuating, detecting and beam steering are repeated to a plurality of points on the object based on the type thereof.

Description

METHOD FOR IMAGING OBJECTS IN A BODY OF WATER USING AN ARRAY OF REMOTELY OPERATED UNDERWATER VEHICLES BACKGROUND

[0001] This disclosure relates to the field of imaging objects submerged in a body of water, or located below the bottom of a body of water, using acoustic measurements. More particularly, the disclosure relates to methods for imaging using acoustic transponders disposed on a plurality of remotely operated underwater vehicles (ROVs).

[0002] ROVs are known in the art and are used for, among other things, optical imaging of objects submerged in a body of water. Optical imaging requires that the water have sufficient clarity to optically detect presence of the object(s). It is sometimes the case that optical imaging is not possible due to opaqueness of the body of water. It is also necessary for the objects being imaged to be located within the illumination area of one or more lamps disposed on the ROV. In cases where very large lamps are needed to optically image at great distance, the power requirements for the ROV may make impractical the use of batteries to provide power to components on the ROV. Further, some objects may be disposed below the water bottom and are thus not susceptible to optical imaging.

[0003] There exists a need for techniques using ROVs to image objects other than optically to enable imaging at greater distances, below the water bottom and without the need for optical lamps to illuminate the objects with visible light.SUMMARY

[0004] A method for acoustic imaging of objects in a body of water according to one aspect of the present disclosure incudes disposing a plurality of remotely operated underwater vehicles (ROVs) in a body of water. At least one of the plurality of ROVs has an acoustic receiver and others of plurality of ROVs have an acoustic transmitter. The acoustic transmitters are actuated. Acoustic signals are detected by the receiver in response to the actuations of the acoustic transmitters. A response of the detected signals is beam steered by applying different time delays among the times of actuations of the acoustictransmitters, and / or, if the plurality of ROVs comprises a plurality of acoustic receivers, applying different time delays to signal recordings of the detected acoustic signals. If an object is identified in the detected acoustic signals, the plurality of ROVs is moved to predetermined positions in the body of water based on a type of the object. The actuating, detecting and beam steering are repeated to a plurality of points on the object based on the type of object.

[0005] In some implementations, the type of object is determined based on prior determined characteristics of the beam steered response.

[0006] In some implementations, the prior determined characteristics are determined by training an artificial neural network (ANN) using detected acoustic signals returned from at least one known type of object.

[0007] In some implementations, the at least one known type of object comprises one or more of an explosive mine, a buried cable or an anchor.

[0008] Some implementations further comprise determining a geodetic position of each of the plurality of ROVs at any time using signals from an inertial navigation device on each of the plurality of ROVs.

[0009] In some implementations, each of the inertial navigation devices is initialized to a position of a vessel on a surface of the body of water from which the plurality of ROVs is deployed.

[0010] In some implementations, the detected acoustic signals are communicated to a vessel on a surface of the body of water using a tether comprising at least one of an electrical conductor and an optical fiber.

[0011] In some implementations, electrical power to operate thrusters on each of the ROVs is provided by batteries disposed on each ROV.

[0012] In some implementations, if an object is not identified in the detected acoustic signals after initially beam steering the response, then the plurality or ROVs is moved and the actuating, detecting and beam steering are repeated until an object is identified.

[0013] In some implementations, the plurality of RO Vs is deployed in a predetermined pattern, and moving the plurality of RO Vs comprises maintaining the predetermined pattern at different spatial positions.

[0014] Other aspects and possible advantages will be apparent from the description and claims that follow.BRIEF DESCRIPTION OF THE DRAWINGS

[0015] FIG. 1 shows an example implementation of ROVs submerged in a body of water to perform an imaging method according to the present disclosure, including equipment that is located aboard a vessel on the surface of the body of water

[0016] FIG. 2 shows another example implementation of ROVs to perform a method according to the present disclosure.

[0017] FIG. 3 shows an example arrangement of ROVs for scanning the water bottom for targets or objects.

[0018] FIG. 4 shows an example process for generating images from returned acoustic energy by weighted beam steering.

[0019] FIG. 5 shows a flow chart of an example implementation of a method according to the present disclosure.DETAILED DESCRIPTION

[0020] FIG. 1 shows an example implementation of remotely operated underwater vehicles (ROVs) 12, 14 submerged in a body of water 15 to perform an imaging method according to the present disclosure. The ROVs 12, 14 may be part of a system deployed from a vessel (not shown) on the surface of the body of water 15. The system may, in addition to the ROVs 12, 14 include equipment such as a docking and control station 10 and a control and data processing system 10A located on the vessel (not shown). Each ROV 12, 14 may comprise one or more acoustic transducers 12A, 14 A. The acoustic transducers 12 A, 14A may perform the function of emitting and / or detecting acoustic energy. In the present example implementation, it is contemplated that to perform a method according to thepresent disclosure there are at least two ROVS (e.g., at 14) each comprising a transducer 14A capable of emitting acoustic energy, and at least one ROV (e.g., at 12) having a transducer 12A capable of detecting acoustic energy. The particular type and configuration of the transducers are not limitations on the scope of the present disclosure; transceivers capable of both emission and detection of acoustic energy may be used in any or all of the ROVs 12, 14. Further, it is contemplated that each ROV 12, 14 comprises a tether (or umbilical cable) that is capable at least of communicating signals between the respective ROV 12, 14 and the docking and the control station 10 and the control and data processing system 10A. The tether (not shown for convenience) may comprise at least one insulated electrical conductor or an optical fiber capable of communicating such signals. In some implementations, electrical power to operate the ROVs 12, 14 may be supplied by batteries (not shown) on board the ROVs 12, 14, which may provide the benefit of minimizing the required size of the umbilicals or tethers, and thus may facilitate making the umbilicals or tethers neutrally buoyant in the body of water 15.

[0021] The present example implementation may comprise one ROV 12 having an arm and gripping feature 12B capable of performing manipulating functions on an object (not shown) in the body of water 15. Others of the ROVs 14 may not have such feature. For purposes of performing a method according to the present disclosure, whether any particular ROV comprises an arm and gripping feature is not a limitation on the scope of the present disclosure.

[0022] A non-limiting possible implementation as shown in FIG. 1 may comprise one of the ROVs, e.g., at 12, performing the function of a “master” ROV wherein electrical and / or optical signal connections from each of the other ROVs 14 may be made through suitable umbilical cables or tethers (not shown) between the other ROVs 14 and the “master” ROV 12. Signal communication to the control system 10A from the other ROVs 14 may be made through the master ROV 12.

[0023] FIG. 2 shows another example implementation of ROVs 14 to perform a method according to the present disclosure. In any implementation, it is desirable to have the onesof the RO Vs acting as transmitters to be located spaced apart from each other so as to illuminate an object or target (not shown) being imaged from more than one direction.

[0024] In some implementations, each of the plurality of RO Vs 14 may comprise an atomic clock or other absolute time reference (not shown separately). Each of such ROVs 14 may further comprise an inertial navigation device (not shown separately) that may be initialized at the time each ROV 14 is deployed from the surface vessel (not shown), e.g., by the control system (10A in FIG. 1). Geodetic position and depth departure from the deployment position, such as the geodetic position of the surface vessel, of each ROV 14 may thus be determined at any point in time subsequent to deployment. An internal controller (not shown) may be provided on each ROV 14 to enable semi-autonomous movement of each ROV 14 to a predetermined position in an array upon detection of a particular object or target in the body of water; such movement will be further explained below.

[0025] In performing a method according to the present disclosure, a command signal may be sent to each of the ROVs 14 from the control system (10A in FIG. 1) to perform a measurement. The ones of the ROVs 14 acting as transmitters may actuate their respective transducers 14A to emit an acoustic signal, to be explained further below, and record the time of actuation. The one(s) of the ROVs 14 acting as receivers may begin locally recording signals detected by the respective receiving transducer 14A, while time stamping such recording using, e.g., the respective atomic clock. A corresponding time stamped record of the geodetic departure from the inertial navigation device (not shown) may be made in each ROV 14. When a measurement sequence is completed, the ROV(s) 14 acting as receivers may transmit to the control system (10A in FIG. 1) or to the “master” ROV (12 in FIG. 1) as the case may be, the signal recordings made in response to the actuations of the transmitter ROVs. The transmitter ROVs may communicate to the control system (10A in FIG. 1) or the master ROV the absolute time reference corresponding to each actuation of the respective transmitters (the transducers in the transmitter ROVs). Signals thus communicated to the control system (10A in FIG. 1) may then be processed as will be further explained below.

[0026] For purposes of the following description, the term “transmitter” will be used to describe those of the ROVs 14 acting as an acoustic transmitter. “Receiver” is used to describe the one of more of the ROVs 14 acting to detect and record signals received in response to acoustic energy generated by the transmitters. The recorded signals are processed in order to generate an image corresponding to at least one point on an object of interest in the body of water 15. Some implementations may use a specific and dedicated type of transducer to act as a respective transmitter or receiver; other implementations may use a single transducer to perform both the transmit and receive functions, wherein each ROV 14 may perform the function of transmitter or receiver as applicable. It will be appreciated that when coded signals are used (see below), it may be advantageous to use separate ROVs 14 for transmitting and receiving functions.

[0027] It is contemplated that each transmitter ROV will be actuated a plurality of times for each imaging point on an object or target. The received signals thus may comprise acoustic energy emitted by more than one transmitter. In order to identify the transmitter from which detected acoustic energy originated, each transmitter may be programmed to emit acoustic energy in binary or other coded sequences rather than short duration impulses. Non-limiting examples of coded sequences include, without limitation, Kasami code, Gold code, Walsh-Hadamard code and m-sequencing to generate pn-code.

[0028] Processing the detected acoustic signals may include stacking the recordings from the one or more receiver ROVs (receivers) for part or all of the plurality of actuations of the transmitter(s), and then beam steering response of the receivers. Beam steering the response may comprise adding a selected time delay to each of the respective detected signals in order to orient the response in a particular direction from the position(s) of the receiver(s). The beam steering may be in response to the requirements of focusing algorithms that rely on accurate known location and positioning of the receiver ROV(s) such that at least one image point is equivalent to a focal point of a response of the receiver to the plurality of transmitters associated with the object. It is also possible to beam steer the acoustic energy emitted by thetransmitters by applying suitable time delay to the actuations of the respective transmitters.

[0029] Because of the multiple projection traverses and positions of the ROVs as a programmed array in the body of water 15, movement of the ROVs (thus acting as antenna elements) may be registered precisely from the inertial navigation system coreferenced to a navigational grid, with a specific atomic clock time stamp. When received signals are recorded, having an adaptively changing array of ROVs in underwater space with exactly located ROVs enables the transmitted and received acoustic energy to be beamformed. This is enabled in some implementations through specific ROV positions in underwater space to create a predetermined antenna pattern. This enables parallel target or object modelling and learning (e.g., using artificial intelligence) to take place, which can guide the focusing routines through deep learning algorithms to capture a target or object, recognize the type and expected shape of the target or object and take action to flag it for further signal acquisition and processing or to remove it from further analysis.

[0030] Due to the nature of forming a substantial diameter synthetic aperture underwater, it is expected that the along-track resolution can be customized. Quarterwavelength spacing between adjacent transmitters / receivers may be expected to provide sufficient spatial resolution without aliasing.

[0031] A particular feature of a method according to the present disclosure is to use the geodetic position of each ROV at any moment in time with aided guidance of onboard navigation devices (e.g., three-axis vectorable thrusters and the previously described inertial navigation devices) to direct and correct the trajectory and position of each of the various ROVs (acting as transmitters and receivers individually) as may be required to fully image each identified object. Such a “synthetic aperture” can then be used to illuminate targets or objects buried below or disposed on the water bottom, or in the body of water.

[0032] The transmitted acoustic energy can be caused, in the near-field, to accentuate each scattering point on any particular target or object. With the displacement obtainedwithin the synthetic aperture, coherent reorganization of the received signals from all the actuations of all the transmitters may provide highly sharpened detection and characterization of the targets’ three spatial coordinates, and amplitudes and phases of inherent spectral components returning from the targets or objects illuminated by the transmitted acoustic energy. As a practical matter, the transmitted acoustic energy is redistributed in the body of water by attenuation, backscatter, and reflection, depending on what object(s) the propagating acoustic waves encounter. An object in this case is any thing or device having a different acoustic impedance than the water. When the acoustic waves encounter a distinct target or object, a non-specular response occurs, and the diffused displacement of the non-specular response is captured within the synthetic aperture as coherent diffractor returns. Further actuation of the transmitter arrays, received acoustic signals, and the subsequent beamforming and steering of the detected acoustic energy sharpen the imaging process: initially, coherent signals are stacked to diminish effects of the incoherent background noise, resulting in a ‘good focus’ where the target’s amplitudes and phases of inherent spectral components become stronger, indicating the detection of the target or object that is being illuminated.

[0033] A method according to the present disclosure utilizes the RO Vs’ accurately determined locations at required moments in time to provide an advantage for spatially fine-tuning and sharpening the transmission outputs and for range-independent, along-water bottom trajectory resolution. Synchronizing the individual ROV receivers that form the sensing array with accurate individual (inertial navigation determined) locations and accurate base station (the control system 10 in FIG. 1) geodetic position (e.g., from GPS or GNSS satellite location signals) allows for constructive phasing and shading to deliver narrow beam steering and sharpened beam steered responses. The beam steered responses relate to the final determination of the detection and subsequent tracking of the target if it is in motion.

[0034] The overall “antenna” provided by the array of RO Vs can be adaptively tuned to focus on a particular object’s or target’s non-specular returned acoustic energy. Through this approach, it is possible to capture a target's or object’s geometric profilesand monitor the target's or object’s character of interest as it evolves during imaging through multi-aspect views and through stacking to improve signal to noise ratio. Imaging can also evolve from a two-dimensional image of a target’s scattering surface to a pseudo-three-dimensional rendition that includes in its formation of the third- dimension the locations of individual scattering points and their distribution in space, which includes the aspect sensitivity of the scattering, and the derived apparent-motion vectors if the target is a moving target in the body of water (15 in FIG. 2). From the disclosed method’s imaging, information may be obtained about possible moving targets in the water body, particularly about the target's approximate size (uniquely through the method’s beamformed-shaded cross-section). The beamformed crosssection provides for the detection resolution cell used to delineate a target.

[0035] High-resolution images may be obtained by capturing target focused changes by sweeping the ROV array’s receiver beams as steered through the signal processing, fully exploiting the synthetic aperture and dynamically shaped antenna array with its associated proximate tunable broad frequency-based transmitters. The acoustic transmissions may be individually beam-steered with temporal stacking to obtain sufficient signal-to-noise for the beamforming and beam steering to be more easily obtained as stronger signals. A Bartlett (delay and sum) beamformer may be used to maximize signal in the intended direction of observation. The beamformer relies on multiple positions created to form a large aperture. Deterministic weights (constant, equal) may be applied to the beamformed received signals. Data processing uses a delay-and-sum beamformer, implemented in either or both time and frequency domains; frequency domain processing enables the determination of scale-dependent scattering and interpretation, while range and offset-dependent amplitude correction functions are applied. Geometric spreading and the expected scattering phase function form the total amplitude correction function. The method relies on constructive and destructive interference, similar to all migration routines. The imaged volume can have various shapes, but the default is a cone with the apex at the center of the receiver array, opening away from the transducer array at angles not exceeding 45°.

[0036] Shaped transmitted fields may be obtained by a combination of geometric disposition of the several transmitters that form part of the ROV “constellation” together with the controlled phasing, e.g., as explained above by suitably time sequencing transmission of acoustic energy from the various transmitter ROVs.

[0037] A feature of the disclosed method is to form as many beams as required, sweeping, for example, particular buried or water bottom disposed targeted fields of complex forms of differing sedimentary, biological fauna / flora and man-made debris or fragments or actual high concern objects. The resulting images and acoustic attributes thus formed at a given spatial location, at a beam focusing point or area of interest in the subject target that is being specifically illuminated, may have as many independent points as there are independent beams formed. It is desirable to use iterative focusing strategies such as using edge detection algorithms to enhance and improve the focusing and thereby increase the available information about the debris and its underwater environment, hence the desirability to illuminate specific points or areas on the target surface from different angles with very high resolving strengths.

[0038] The disclosed method relies and focuses on the underwater target and acoustic propagated fields, highlighting the spatial resolution and the dominance of the wavefield. To ensure the target of interest is identified, the range (distance from the ROV array to the object / target), a desirable resolution should be at least on the order of the size of object / target being imaged. To obtain such resolution, the disclosed method uses the relative positions of each of its receivers to the same order as the spatial resolution desired to detect and characterize. To obtain such resolution, the disclosed method uses the relative positions of each of its receivers to the same order as the spatial resolution desired to detect and characterize. This spatial position resolution can be obtained by GPS / GNSS location stamping (i.e., using the GPS / GNSS geodetic location as a reference) the surface control system (10 in FIG. 1), using the spatial position information from the onboard inertial navigation device on each ROV, together with atomic clock timing to generate for each ROV a time-based trajectory with reference to the position of the surface control system (10 in FIG. 1). In the case of a transmitter ROV that is surrounded by receiver ROVs, it is also possible todetermine transmitter ROV position estimates from Time Difference Of Arrival (TDOA) measurements of the signals at each of a plurality of receiver ROVs from the transmitted energy scattered back from a target or object.

[0039] An example implementation of a method according to the present disclosure will now be explained with reference to FIGS. 3 through 5. In FIG. 3, a plurality of ROVs 14 may be deployed, for example in a line array on or proximate to the water bottom in order to detect and image objects or targets on or below the water bottom. The ROVs 14 on the left hand side of FIG. 3 may act as transmitters. The ROVs 14 may remain in fixed position on or near the water bottom during signal acquisition. In some implementations, the transmitter ROVs are actuated a plurality of times, each time with a different time delay among the transmitter ROVs 14 to cause the acoustic energy to be beam steered along a plurality of points on and below the water bottom, generally along a line defined by the deployment of the transmitter ROVs. The plurality of transmitters in this way acts as a synthetic aperture array. A corresponding line array of receiver ROVs, shown on the right hand side of FIG. 3, may be deployed in front of, behind, or interspersed between the transmitter ROVs. Signals detected by the receiver ROVs and recorded (in the ROV or, e.g., in the master ROV 12 in FIG. 1 or the surface control system 10A in FIG. 1) may be processed, e.g., using phased array processing 22, initially by applying an individual time delay among each of the various receiver signals to beam steer the response of the receiver ROVs to one or more specific points in the body of water, or on and / or below the water bottom. Thus, emission and response of the acoustic energy may be beam steered to a plurality of points in the space relative to the positions of the transmitter and receiver ROVs.

[0040] If no object or target is identified when the ROVs are at first, fixed positions in a predetermined pattern, the ROVs may be moved to second fixed positions, e.g., in the same pattern but spaced apart from the first fixed positions. The second fixed positions may be, for example, along a linear path relative to the first fixed positions. Surveying an area of the water bottom, for example, may comprise terminating the linear path at a particular path length, moving the ROVs a predetermined distance transverse to the linear path, and moving the ROVs along a transversely displacedlinear path in the direction opposed to the previous, terminated linear path. The foregoing movement of the RO Vs may continue until a predetermined area of the water bottom has been surveyed or, as explained below, until presence of an object is detected in the received acoustic signals.

[0041] If the received acoustic signals indicate presence of an object in the water, or a target below or on the water bottom, automatic repositioning of the RO Vs may then begin. Indications of a target or object having a specific shape may comprise diffractor returns in the receiver signals. The shape may be viewed by examining the returns from different offsets and angles of illumination. When such returns are combined, the form of the object becomes observable.

[0042] If the received acoustic signals indicate presence of an object in the water, or target below or on the water bottom, automatic repositioning of the RO Vs may then begin. Indications of a target or object having a specific shape may comprise diffractor returns in the receiver signals. Depending on a rough spatial distribution of the diffractor returns, a specific type of target or object (e.g., an explosive mine, abandoned anchor, buried communication cable, etc.) may be indicated. In some implementations, an artificial neural network (ANN) may be trained on data sets representing spatial distribution of diffractor returns for particular objects on the water bottom and / or at specific depths below the water bottom. The diffractor returns from the ROV arrays deployed as shown in FIG. 3 may be used by the ANN (whether in the master ROV or surface control system) to make an initial estimate of the object or target type and expected shape. Based on such initial estimate, the ROV arrays may be automatically redeployed into a selected pattern most suitable for imaging the particular estimated target or object. Examples of redeployment patterns may comprise, for example and without limitation, semicircular or hemispherical arrays of transmitter RO Vs and receiver RO Vs.

[0043] Signal acquisition and processing may be repeated using the redeployed ROVs.In the present example implementation, and referring to FIG. 4, for each receiver ROV’s detected acoustic signal corresponding to each transmitter ROV’s actuation(s),a first pass beam steering process may comprise applying individual time delays among the receiver signals to beam steer the received response to a plurality of selected points relative the receiver ROV array, e.g. , below the array. Initially, the time delayed receiver signals, shown at 30, may be individually weighted by unity. Then, each individual time delayed received signal 32 may be separately weighted at 36, and the weighted results summed at 35 to revise the diffractor output at each point in the scanned area or volume below the ROV array. Results of the weighed, summed receiver signals may be examined. If the sharpness of the image thus generated improves, the weighting factors or coefficients at 36 may be adjusted, the weighted, time delayed receiver signals summed and the results once again examined for sharpness. Sharpness improvement may be observed as an increasing signal amplitude relative to background noise and reverberation. The foregoing may be repeated until improvement in sharpness falls below a selected threshold. Further, the foregoing may be repeated for each beam steered transmitter point in the volume below the ROV array, wherein such transmitter points, as explained above, may be determined by suitable time delays between individual transmitter ROV actuations in the ROV array.

[0044] A flow chart of an example implementation of a method according to the present disclosure is shown in FIG. 5. At 50, the area / volume below the ROV array is discretized into a model space. In the model space, four RO Vs per wavelength at the highest acoustic frequency are deployed, e.g., as shown in FIG. 3. Acoustic energy is emitted and detected as explained with reference to FIG. 3. Time delays are applied to the received signals, and in some implementations the transmitter signals are time delayed also as explained with reference to FIG. 3 to effect beam steering. Upon detection of an object or target, at 51, the ROVs may be repositioned as explained with reference to FIG. 3, for example, using a preprogrammed deployment geometry related to the type of target identified.

[0045] At 52, Green’s function may be determined at each point in the image space (below the ROV array) using, e.g., a finite-difference approximation.

[0046] At 53, a differential target debris (or object / target) scattering cross-section may be determined.

[0047] At 54, optimal phase adjustments, through correlation or eigenvalue methods, may be determined for maximum focus to update debris (or object / target) detection and such debris / target / object characteristic model.

[0048] At 55, the object detection model may be updated, through ray-based travel time inversion or wave equation-based travel time inversion, and using focusing phase corrections for voxels with sufficient scattering strength.

[0049] At 56, recompute the Green’s function at each point in the area / volume and apply a beamformer to obtain the final image.

[0050] For enhanced accuracy, the atomic clocks onboard the ROVs may directly measure the ROV’s detected received signal travel times. From these data, it is possible to obtain the target's coordinates in three dimensions. By measuring at a similar interval to a target’s movement in the water or by its presence at a particular location on or below the water bottom, the resultant range and sizing resolution may be expected to be within the target size, providing indications of the target's size and shape.

[0051] To obtain such resolution, high bandwidth is needed. Cross-correlating received signals with the transmitted signals can detect target resolution. The time duration and bandwidth will set the signal to noise ratio.

[0052] The spacing of the transmitters / receivers should be such that the time difference between received signals from an object at range R is measurable and thus on the order of several wavelengths of the acoustic energy. The disclosed method exploits this in the near field, where the resulting acoustic waves are typically spherical, with the distance from the transmitters being comparable to the source dimensions and wavelength. The near field region is where direct sound energy predominates over reflected boundaries such as the water bottom; hence, the receiver is designed to be surrounded by a pseudo-circular or randomized array of at least three transmitters overa varying radius to exploit these curved propagation characteristics fully. The receiver may be surrounded by a pseudo-circular or randomized array of at least three transmitters over a varying radius. More transmitters are preferable to better characterize the aperture, thereby improving the dynamic range.

[0053] On reception, the cross-correlation between the received signals and the transmitted signals may have peaks separated in time, corresponding to different objects. Individual cross-correlation peaks may have detectable structure, e.g., amplitude with respect to time structure, to target dependent characteristics. The time overlaps of transmitted and received sequences will govern the duration of the transmitted sequence.

[0054] It will be appreciated by those skilled in the art that by the principle of reciprocity, any and all references to “transmitter” in the present disclosure may be substituted by “receiver”, and vice versa. Therefore, in defining the scope of the present disclosure, and as recited in the appended claims, “transmitter” may be substituted by “receiver” and vice versa in equal scope to the claims as literally recited herein.

[0055] In light of the principles and example implementations described and illustrated herein, it will be recognized that the example implementations can be modified in arrangement and detail without departing from such principles. The foregoing discussion has focused on specific implementations, but other configurations are also contemplated. In particular, even though expressions such as in “an implementation," or the like are used herein, these phrases are meant to generally reference implementation possibilities, and are not intended to limit the disclosure to particular implementation configurations. As used herein, these terms may reference the same or different implementations that are combinable into other implementations. As a rule, any implementation referenced herein is freely combinable with any one or more of the other implementations referenced herein, and any number of features of different implementations are combinable with one another, unless indicated otherwise. Although only a few examples have been described in detail above, those skilled in the art will readily appreciate that many modifications are possiblewithin the scope of the described examples. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims.

Claims

AMENDED CLAIMSreceived by the International Bureau on May 15, 2026 (15.05.2026) Claims:

1. A method for acoustic imaging of objects in a body of water, comprising:disposing a plurality of remotely operated underwater vehicles (ROVs) in a body of water, at least one of the plurality of ROVs comprising an acoustic receiver and others of the plurality of ROVs comprising an acoustic transmitter;adaptively moving the plurality of ROVs into antenna patterns to enable beamforming a response of the at least one acoustic receiver and the acoustic transmitters based on expected objects to be detected; actuating the acoustic transmitters;detecting acoustic signals by the acoustic receiver in response to actuations of the acoustic transmitters; beam steering a response of the detected acoustic signals by applying different time delays among the times of actuations of the acoustic transmitters, and / or, if the plurality of ROVs comprises a plurality of acoustic receivers, applying different time delays to signal recordings of the detected acoustic signals;if an object is identified in the detected acoustic signals, moving the plurality of ROVs to predetermined positions in the body of water based on a type of the object; andrepeating the actuating, detecting and beam steering to a plurality of points on the object based on the type thereof.2 The method of claim 1 wherein the type of object is determined based on prior determined characteristics of the beam steered response.3 The method of claim 2 wherein the prior determined characteristics are determined by training an artificial neural network (ANN) using detected acoustic signals returned from at least one known type of object.4 The method of claim 3 wherein the at least one known type of object comprises one or more of an explosive mine, a buried cable or an anchor.5 The method of claim 1 further comprising determining a geodetic position of each of the plurality of ROVs at any time using signals from an inertial navigation device on each of the plurality of ROVs.6 The method of claim 5 wherein each of the inertial navigation devices is initialized to a position of a vessel on a surface of the body of water from which the plurality of ROVs is deployed.

7. The method of claim 1 wherein the detected acoustic signals are communicated to a vessel on a surface of the body of water using a tether comprising at least one of an electrical conductor and an optical fiber.

8. The method of claim 7 wherein electrical power to operate thrusters on each of the ROVs is provided by batteries disposed on each ROV.

9. The method of claim 1 wherein if an object is not identified in the detected acoustic signals after initially beam steering the response, moving the plurality or ROVs and repeating the actuating, detecting and beam steering until an object is identified.

10. The method of claim 9 wherein the plurality of ROVs is deployed in a predetermined pattern, and moving the plurality of ROVs comprises maintaining the predetermined pattern at different spatial positions.

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