Internal wave ambient noise tomography for antisubmarine warfare

A system and method thereof identifies and locates a submarine or other similar object located in the ocean. The thermocline in the water column is utilized to identify objects based on signals carried by the internal wave (IW) in a mixed layer (ML) of water established between a less-dense and/or warmer upper layer of water and a more-dense and/or cooler lower layer of water. The system can detect objects moving in the ML. Additionally, the system detects non-moving objects in the ML based on shadowing and/or scattering effects established by naturally occurring waves contacting the object.

BACKGROUND

Technical Field

The present disclosure relates generally to systems and methods for locating a remote object. More particularly, the present disclosure relates to locating a submerged object spaced apart at a remote distance from a sensor. Specifically, the present disclosure relates to analyzing and processing an internal wave (IW) in a fluid column to detect signatures of an object, such as a submarine, located near an interface between two fluid layers having differing parameters, such as different temperatures, different densities, or both.

Background Information

Antisubmarine warfare (ASW) refers generally to the process of locating submarines operating in the ocean. Typically, submarines can be located through visual means or acoustic means. Each manner for locating submarines have advantages in their own respect. Visual inspection of submarines can usually be accomplished when the submarine is operating at or near the surface, especially when the ocean water is clear. The acoustic manners of locating submarines can usually be performed when the submarines are operating at deep cruising depths.

Sometimes submarines operate near a transition layer below the ocean's surface. The transition layer of the ocean occurs naturally when less dense warmer water establishes a first layer located above more dense cooler water establishing a second layer. The transition layer is defined by an interface between the first and second layers.

The interface, established by the transition layer, acts as a medium along which waves travel. More particularly, oceanographic IWs propagate through the fluid/water column along the interface. Typically, there is a significant amount of omnibus ambient IWs that are random and are generated through natural currents and weather conditions. The naturally occurring IWs are scattered and have no clear directions or uniformity.

Due to the naturally occurring IWs at the interface, acoustic and visual detection of submarines operating in or near the transition layer can be difficult. More particularly, the natural IWs in the transition layer make acoustic detection difficult because of the scattering of the natural wave creates significant feedback and noise, rendering acoustic detection extremely difficult. Visual detection is further difficult in the transition layer due to the depth and movement of waves which may cause visual detections means to operate in an ineffective manner.

Passive and active acoustic sensors describe the current state of the art for long range ASW detection problem. For localization, acoustic sensors can be supplemented by passive and active optical sensors. Acoustic methods are becoming increasingly insufficient in light of the proliferation of quiet, inexpensive submarines. Further, the efficacy of acoustic and optical detection methods is diminished in acoustically noisy and optically turbid environments such as the littorals.

SUMMARY

Issues continue to exist with systems and methods for locating objects, such as a submerged vehicle, in a fluid column. Particularly, existing ASW capabilities are no longer keeping pace with the evolving submarine threat. Responding to this threat will require not only the development of new technologies, but also the identification of new ASW detection modalities. The present disclosure addresses these and other issues by providing a system and method to locate an object operating near a transition layer defined at an interface between a first fluid layer and a second fluid layer having different parameters, such as differing temperatures or different densities. The present disclosure implements a modality for ASW operations, wherein submarines are detected by persistent surveillance of the internal wave field, exploiting both IW wakes and the scattering of naturally occurring IWs by moving or stationary vehicles, thereby enabling low-cost, long-duration, wide-area surveillance. Stated otherwise, the present disclosure exploits information carried in the ocean IW field to provide a new ASW detection modality.

The present disclosure may provide a system for detecting the IW analogue to a seismic array. Such an array can support both active and passive ASW detection modes. In the active mode, time of arrival differences for a submarine's induced IWs at different array nodes allows target detection by triangulation and back-projection techniques. In the passive mode, submarines can be detected by their scattering effects on ambient-noise IWs by use of ambient noise tomography (ANT) techniques. This new modality would be insensitive to both the acoustic stealth of the target (the limiting factor in passive acoustic ASW) and to the optical turbidity of the water (the limiting factor in optical ASW).

In one aspect, an embodiment of the present disclosure may provide a method comprising: deploying at least one sensor into a fluid column to sense an IW in the fluid column near an interface between two fluid layers of differing parameters; sensing an IW in the fluid column with the sensor; locating a remote object from the IW sensed at the sensor; and reporting the location of the remote object. This exemplary embodiment or another exemplary embodiment may further provide mapping a dispersion relationship between a wavelength and frequency of the IW; and back projecting the IW to a ping, wherein the ping is generated by the remote object located near the interface between the two fluid layers. This exemplary embodiment or another exemplary embodiment may further provide sensing naturally generated ambient IWs; and filtering out the naturally generated ambient IWs. This exemplary embodiment or another exemplary embodiment may further provide identifying scatter in the IW generated by the remote object submerged in the fluid column; and processing the scatter to locate the remote object when the remote object is stationary in the fluid column. This exemplary embodiment or another exemplary embodiment may further provide processing signatures in the Ms moving in a first direction generated by the remote object; and processing deficiencies in the IW moving in an opposite second direction generated by the remote object. This exemplary embodiment or another exemplary embodiment may further provide sensing the remote object when the remote object transitions through the interface, wherein a ping is generated when the remote object transitions through the interface between the two layers in the fluid column. This exemplary embodiment or another exemplary embodiment may further provide wherein the remote object is a submersible vehicle, further comprising: identifying whether the submersible vehicle is within an area of interest based on the ping generated by the transition between the two layers in the fluid column; prosecuting the ping to determine the location of the submersible vehicle; and sensing the IW at the interface with at least two strings of thermistors. This exemplary embodiment or another exemplary embodiment may further provide sensing the IW at the interface with an expendable thermograph. This exemplary embodiment or another exemplary embodiment may further provide sensing the IW at the interface with an unmanned submersible vehicle carrying the sensor and oscillating within the fluid column. This exemplary embodiment or another exemplary embodiment may further provide wherein the differing parameters between the two layers are temperature dependent, sensing a temperature profile of the fluid column with LIDAR on an aircraft flown above the fluid column. This exemplary embodiment or another exemplary embodiment may further provide wherein the different parameters refer to differing densities and differing temperatures between the two layers. This exemplary embodiment or another exemplary embodiment may further provide locating the sensor when the signal is received; and accounting for the sensor drifting in the fluid column. This exemplary embodiment or another exemplary embodiment may further provide determining the depth of the at least one sensor in the fluid column when the IW is sensed at the at least one sensor. This exemplary embodiment or another exemplary embodiment may further provide surveying, persistently, an area of interest for an extended period of time. This exemplary embodiment or another exemplary embodiment may further provide determining frequency characteristics of the IW over a period of time, wherein the period of time exceeds at least one day so as to effectuate persistent surveillance of an area of interest in an ocean. This exemplary embodiment or another exemplary embodiment may further provide processing local oceanographic data, wherein the ambient IW is filtered out based, at least in part, on the local oceanographic data. This exemplary embodiment or another exemplary embodiment may further provide recording, at least temporarily, the location of the remote object in computer implemented medium.

In another aspect, an exemplary embodiment of the present disclosure may provide a system for detecting a remote object submerged in a fluid column comprising: at least one sensor for sensing at least one of a temperature profile of the fluid column or a density profile in the fluid column, wherein a fluid interface is defined between a less dense warmer first fluid layer and a more dense cooler second fluid layer, wherein the second layer is below the first layer; at least one non-transitory computer readable storage medium having instructions encoded thereon that, when executed by at least one processor, performs operations to locate a submerged vehicle in the fluid column located in an area of interest remotely from the at least one sensor, and the instructions including: logic to sense a signal carried by an IW moving along the interface; logic to back propagate the sensed signal to determine a signal source location; and logic to record the signal source location. This exemplary embodiment or another exemplary embodiment may further provide a string of thermistors submerged in the fluid column, wherein the at least one sensor is a thermistor on the string. This exemplary embodiment or another exemplary embodiment may further provide a pressure transducer on the string adapted to determine depth of the thermistor. This exemplary embodiment or another exemplary embodiment may further provide a shadow in the IW created by the remote object remaining stationary near the interface, or a scattering of the IW. This exemplary embodiment or another exemplary embodiment may further provide a signature in the IW created by the remote object moving near the interface. This exemplary embodiment or another exemplary embodiment may further provide a ping generated by the remote object transition from the first fluid layer to the second fluid layer, or vice versa. This exemplary embodiment or another exemplary embodiment may further provide local oceanographic data including an ambient IW spectrum, wherein the ambient IW spectrum is filtered out to locate the remote object based, at least in part, on the sense signal. This exemplary embodiment or another exemplary embodiment may further provide wherein the at least one sensor is submerged in the fluid column at a depth in a range from about 1 m to about 100 m.

In accordance with one aspect of the present disclosure, an exemplary embodiment may provide a system comprising: a temperature sensor submerged in a fluid; at least one non-transitory computer readable storage medium operatively connected with the temperature sensor, the storage medium having instructions encoded thereon that, when executed by at least one processor, performs operations to detect an object in the fluid based on Ms of the fluid, the operations including: detect a first temperature of the fluid at a first time; detect a second temperature of the fluid at a second time later than the first time; and determine a location of the object in the fluid based on the first temperature at the first time and the second temperature at the second time.

In accordance with another aspect of the present disclosure, an exemplary embodiment may provide a method of detecting the presence of an object submerged in fluid comprising: submerging at least one temperature sensor in a fluid; recording a first temperature of the fluid at a first time; recording a second temperature of the fluid at a second time; wherein an IW passes the at least one temperature sensor between the first time and the second time; and determining a location of an object in the fluid based on the first temperature at the first time and the second temperature at the second time.

DETAILED DESCRIPTION

FIG. 1depicts a system in accordance with the present disclosure for detecting signals in an internal wave (IW) generally at10. System10includes at least one sensor12for sensing at least one of a temperature profile of the fluid column14or a density profile in the fluid column14, wherein a fluid interface16is defined between a less dense warmer first fluid layer18and a more dense cooler second fluid layer20, wherein the second layer20is below the first layer18, and at least one non-transitory computer readable storage medium22having instructions encoded thereon that, when executed by at least one processor, performs operations to locate a submerged vehicle24in the fluid column14located in an area of interest remotely from the at least one sensor12. The instructions include instructions to sense a signal carried by an IW moving along the interface16, and instructions to back project the sensed signal to determine a signal source location (i.e., the remote object to be detected, such as a submarine or submersible vehicle), and instructions to record the signal source location.

With continued reference toFIG. 1, the sensor12is generally depicted as a thermistor. In one example, the thermistor may be a first thermistor12A positioned along a first string26. The first string26may include other thermistors12B-12N where N is any integer. The thermistors on the string may vary by embodiment; however, it is envisioned that each thermistor12A-12N will be spaced apart along the string at least semiregular intervals of about one meter. In accordance with one aspect of the present disclosure, the string26is suspended in the fluid or water column14at a depth that transects interface16so as to provide some thermistors located in the first layer18and some thermistors being located in the second layer20. Stated otherwise, the interface16is positioned below an upper end of string26and the interface16is positioned above a lower end of string26.

At least one of the temperature sensors12on the string26is in communication with the storage medium22through a network28. Network28may be effectuated by a wireless or wired connection so as to enable data from the sensor12to be sent or transferred to the storage medium22as one having ordinary skill in the art would understand. In one particular embodiment, the storage medium22may be carried by a remote computer30. The remote computer30may be located onboard a ship that processes the data sent from the sensors12or may be located remotely onshore at a central command center.

System10may further include a second string of temperature sensors. The second string32is spaced apart a distance34from the first string26. In one embodiment, the separation distance between the first and second strings26,32may be on the order of about one kilometer. The reason the distance34is significantly separated between the first string26and the second string32is that the amplitude and wave lengths of signals transmitted by the IW near the interface16may be very large; thus, the spacing distance34between the first string26and the second string32should also be large. Second string32may also include similar sensors12A-12N having similar configurations spaced along the second string32in operable communication with medium22. When the sensors12are carried by a string, such as first string26or second string32, the sensors12may have pressure transducers thereon, such as pressure transducer36, so as to recognize the depth of each respective sensor12in the fluid column14. The depth of the sensor12is used when calculating the location of the submersible vehicle24.

In accordance with another aspect of the present disclosure, it is not required that the at least one temperature sensor12be carried by strings26,32. For example, an expendable thermograph could be utilized to record temperatures of the fluid column14as it is dropped and sinks within the fluid column14measuring temperature during the sinking of the thermograph. Alternatively, another sensor12could provide a single thermistor oscillating up and down on an unmanned submersible vehicle so as to obtain various temperature measurements in the fluid column14between the upper first layer18and the lower second layer20. Alternatively, additional manners of observing the temperature in the fluid column14may be accomplished by providing LIDAR15from an airborne vehicle13. The present disclosure may be effectuated by any temperature sensor that will establish and provide a temperature profile through the fluid column14. LIDAR15, which stands for Light Detection and Ranging, is a remote sensing method that uses light in the form of a pulsed laser to measure ranges (variable distances) to the Earth. LIDAR is a detection system that works on the principle of radar, but uses light from a laser.

The interface16is established between the less-dense and/or warmer first layer18being positioned above a more-dense and/or cooler second layer20. Ms operate at or near the interface16between the first layer18and the second layer20. The IW spectrum is a known range that is frequently studied by oceanographers. The system10exploits the ambient IW spectrum so as to filter it out in order to locate the submersible vehicle24when a ping38indicates that the submersible vehicle24is operating at or near the interface16between the first layer18and the second layer20.

In accordance with an aspect of the present disclosure, the system10maps the dispersion relationship so as to enable the system10to back project the ping38to the point of origin so as to identify the potential location of the submersible vehicle24. The self-mapping of the dispersion relationship through the water and the IW spectrum is accomplished based on the wave length (i.e., the speed) and the frequency of the IW occurring near the interface16. The ambient IW spectrum is identified so that it may be filtered out during the processing so as identify the location of the submersible vehicle24based on the signatures coming from the submersible vehicle24.

In operation, one of the initial steps for ASW is to determine whether there is a submarine anywhere within an area of interest. Once it determines that there is a submarine somewhere within the area of interest, a variety of known detection manners may be implemented to more precisely locate the submarine. These known manners include acoustical and visual detection mechanisms. The system10helps identify whether or not there is a ping38(i.e., yes, there is a submarine operating in the area of interest). Once a ping38is determined, then system10can alert other identification systems to execute their methods for locating the submersible vehicle24with other signatures (i.e., visual or acoustic).

The system10and its method of implementation will first obtain a temperature and/or density profile of the fluid column14through the first layer18(i.e., the mixed layer (ML)). In addition to obtaining the temperature of the first layer18, the method may further include obtaining a density profile of the fluid column14. When a temperature profile has been identified, the interface16can be accurately located so as to know where to place one or more sensors12in the fluid column14. Depth of the interface16is known to position some thermistors or other sensors above the interface16inside the ML or first layer18and position some of the sensors12N below the interface16in the more-dense or cooler second layer20.

During the operation of system10, the sensors12measure temperatures and fluctuations of the interface16as the waves move past the string of sensors12. The sensors12are then able to back project using ambient noise techniques applied to the transverse wave moving along interface16so as to identify the source of the ping38coming from submersible vehicle24.

The sensors12are correlated together after obtaining a large time series of events from at least two strings26,32. Instructions in the storage medium22, when executed by one or more processors, perform a frequency dependent cross correlation analysis to determine how long the signal moving along interface16took to get from the first string26to the second string32. This enables the system10to determine a dispersion relationship along that path. The data is timestamped and the location of the strings26,32is known. From this information, the back project technique can be utilized so as to identify the location of the ping38. It is to be understood that the sensors12carried by strings26,32may be drifting in the ocean; therefore, additional calculations may be utilized to filter out the drifting effect of the sensors12when the location of the sensors12is to be determined prior to back propagating the source of the signal. The operation of the method further filters out natural background perturbations in the ambient IW.

Because the ML (i.e., near interface16) of the ocean varies at different locations, the system10takes into account different oceanographic information which may be obtained from oceanography sources. This enables the system10to optimize the temperature sensors12in a manner that most accurately transect the ML across the interface16. Thus, in one embodiment, the network28may persistently feed oceanographic data to the sensors12so as to enable the sensors12to persistently monitor the interface16along which the IW moves to appropriately ensure that the string or other sensors carried by another device appropriately transect the interface16.

In accordance with another aspect of the present disclosure, system10provides an “IW seismic array analogue” solution to exploit information in the ambient and active components of the IW field via persistent observation. Persistent measurements of the IWs can be obtained, for example, by a string of thermistors that transects the thermocline. The passage of an IW manifests as a temperature differential that oscillates along the string. A measurement of the IW field can be performed by an array of such temperature strings.

In passive-mode processing, cross correlation terms can be computed between array nodes (strings); and these cross correlation functions can be used to estimate the Green's functions between nodes (i.e., sensors12). The Green's function emerges after sufficient stacking of the cross-correlograms, and is obtained as the time derivative of the correlation. These Green's function estimates can be used to estimate the dispersion relation within the array and to image reflective sources within the field. An object (i.e., submersible vehicle24) within the IW field can be detected by its effect on the ambient noise background, even if it is not generating any IW energy itself. In active mode processing, the array can be used to locate sources of induced IWs (e.g., submarines) by back projection to the source location. Back projection accuracy can be enhanced through application of IW dispersion measurements extracted from passive mode processing.

System10takes advantage of the phenomenology of ocean IWs, both from active and passive sources, and the measurement of those waves with techniques derived from ANT. Wind and waves can mix the shallowest waters to provide near-uniform temperature and salinity in the ML (i.e., first layer18). The thickness of the ML can range from a few meters to about 200 meters [consistent]. At the base of the ML, near interface16, the thermocline can be very steep, with temperature decreasing quickly as depth increases. The vertical density gradient between shallow warm (less dense) water and deeper cool (more dense) water enables IWs to propagate. The internal-wave-generation potential is characterized by the peak Brunt-Väisälä (BV) frequency N, given by: N2≅−(g/ρ)∂ρ/∂z (radians/sec)2, where r is the density and g=9.8 m/sec2. IW propagation speeds are typically of order 10-100 cm/s and have periods measured in cycles per hour.

The ambient IW energy is limited at the high frequencies by the BV frequency N and at low frequencies by the Coriolis frequency f, f=2Ω sin(φ) where Ω is the Earth's rotational angular frequency and φ is the latitude. The spectral energy distribution of the ambient noise field is typified by the Garrett-Munk spectrum, which falls off approximately as (frequency)−2with a cutoff near N.

A vehicle24moving through or near the thermocline or interface16will excite IWs either directly, or via its turbulent wake. These waves are excited as the vehicle pushes a volume of higher density water across the thermocline into the lower density region above, or vice versa. This action sets up buoyancy oscillations in the water column; such oscillations have their primary mode of oscillations near the BV frequency. Because the density differences across the base of the ML are on the order of parts per thousand, the amplitude of the wave can be very large, on the order of many meters. The exact frequency spectrum of these excited IWs will depend on the size, speed, and configuration of the vehicle.

Ambient noise measurements have been exploited in fields such as geophysics, seismology, and acoustics to obtain information about the medium through which noise propagates. The technique, broadly known as ANT, is based on the observation that cross correlations of noise measurements within a system can be used to determine the Green's function of that system. If CAB(τ) is the expectation value of the cross correlation between two points xA, xB, in an isotropic ambient noise field, then: ∂CAB(τ)/∂τ∝[G(xA,xB, τ)−G(xA,xB, −τ)], where G(xA,xB, τ) is the Green's function. The Greens function characterizes the impulse-response behavior of the system, and, with an estimate of the Green's function, the medium's dispersion relation can be estimated and mapped. This technique can be applied to any ambient wave field in which non-zero point-to-point cross correlation function can be measured.

FIG. 2depicts a method in accordance with the present disclosure shown generally at200. Method200includes deploying at least one sensor12in a fluid column14to sense an IW in the fluid column near the interface16between two fluid layers18,20of differing parameters, shown generally at202. Method200further includes sensing an IW in the fluid column with the sensor12, shown generally at204. The method200further includes a remote object, such as submersible vehicle24, from the IW sensed at the sensor12, shown generally at206. Method200may further include reporting the location of the remote object, shown generally at208. Method200may include additional steps or additional processes. By way of non-limiting example, some additional processes of method200may include mapping a dispersion relationship between a wave length and frequency of the IW and back projecting the IW to a ping, wherein the ping is generated by the remote object located near the interface between the fluid layers. Method200may further include sensing naturally generated ambient IWs, and filtering out the naturally generated ambient waves during the step of locating a remote object. Method200may further include identifying a scatter in the IW generated by the remote object submerged in the fluid column and processing the scatter to locate the remote object when the remote object is stationary in the fluid column near the interface16. Method200may further include processing signatures in the IWs moving in a first direction generated by the remote object and processing deficiencies in the IW moving in an opposite direction generated by the remote object. The method200may further include sensing the remote object when the remote object transitions through the interface wherein the ping is generated when the remote object transitions through the interface between the two layers in the fluid column. This may be accomplished when the submersible vehicle24is surfacing from a deep depth and must transition through the interface or when the submersible vehicle24is diving to a cruising depth and transitions through the interface16. Method200may further include identifying whether the submersible vehicle is within an area of interest based on the ping generated by the transition between the two layers in the fluid column, then prosecuting the ping to determine the location of the submersible vehicle, and sensing the IW at the interface with at least two strings of thermistors.

The above-described embodiments can be implemented in any of numerous ways. For example, embodiments of technology disclosed herein may be implemented using hardware, software, or a combination thereof. When implemented in software, the software code can be executed on any suitable processor or collection of processors, whether provided in a single computer or distributed among multiple computers.

Furthermore, the logic(s) presented herein for accomplishing various methods of this system may be directed towards improvements in existing computer-centric or internet-centric technology that may not have previous analog versions. The logic(s) may provide specific functionality directly related to structure that addresses and resolves some problems identified herein. The logic(s) may also provide significantly more advantages to solve these problems by providing an exemplary inventive concept as specific logic structure and concordant functionality of the method and system. Furthermore, the logic(s) may also provide specific computer implemented rules that improve on existing technological processes. The logic(s) provided herein extends beyond merely gathering data, analyzing the information, and displaying the results.

Moreover, the description and illustration of the preferred embodiment of the disclosure are an example and the disclosure is not limited to the exact details shown or described.