Specification

[001. j This invention was made with .Government support under Contract WC-133 -15- C -0079 awarded by NQAA, The Government has certain rights in the invention.

Baek ground

[002.] This invention provides a system a para us and method for a low-drag ship-towed deployment of anon- linear voiumetrie array of hydrophones, allowing line-intersect or iine-transeet sampling of marine mammal populations, where "ship-towed" is defined as any type of vehicle capable of movin the appa.rat.ns through the water.

[003.] Passi e acoustic monitoring (P AM) is the preferred technique for detecting marine mammals, because the marine mammals use low-frequency sounds for their own echo-location and communication, and the passive monitoring- does not interrupt nor distort those sounds. The low-frequency sounds are very efficiently transmitted in seawater and travel great distances Ijecause ^' the..hydroac6ustic- hpedaaee- properties of seawater favor lower frequencies and disfavor higher Frequencies.

[004.] Fixed-location hydrophones have been used to monitor marine mammals, but cannot monitor the vast amounts of deep-ocean habitat as required. Presently, line arrays of hydrophones, towed one behind the other, are used, in order to obtain -useful location information, the line array needs to detect more than a single isolated click in order to obtain enough information to make a triangulation. A massive amoun of data from line arrays must be processed and analyzed after the fact in order to derive the location information.

[005.] The community organization PAMGUARD ha been established to address the fundamental limitations of existing cetacean passive acoustic monitoring (PAM) software capabilities, and continues to develop open-source PAM software for acquiring and analyzia g h droaeoustic data related to marine mammals. Any . quipment or methods developed for this field should be operable or interoperable with this evolving

PAMGOA D software.

[006.] Sound will reflec at the interface boundary of materials with differing acoustic impedance, Sound will also attenuate in materials. Hydrophones, when towed in the ocean,, need a boundary layer to separate the hydrophone from the water flow or significan flow noise will result. In order to optimize hydrophone measurements in the ocean environment, one should surround the hydrophones with materials having low attenuation and close acoustic impedance matches to ^' seawatex.

[007.] he: towing of a hydrophone arra across great distances at a reasonable speed to cover those distances, which is about 10 knots, put a huge amount of stress on any hydrophone -array, whether a line array or otherwise _* and on any tow cable used, in order to tow the hydrophone array far enough behind the towing ship to avoid the

hydroaeoustic noise of the ship, the tow cable need to be at least 100 meter long, and preferably 300 meters long. Such tow cables, which are integrated with data-transfer cables, are known, and any new equipment or methods should make use of such existing tow cables, if possible. The existing cables can withstand 1000 pounds of tension, and that breaking point could be reached if the towed hydrophone array generated even a few hundred pounds of hydrodynamic drag force,

[008,] The monitored sounds are very faint, and can be overwhelmed by turbulence in the vicinity of the hydrophones. Existing practice of the PAM technique suffers from turbulence caused by cable drag and by hydrodynamic drag or turbulent, flow around whatever housing is provided for a hydrophone array being lowed at 10 knots at the end of a 300 meter to cable.

[009.3 Because the PAM technique requires detection of the small differences in the time of a given sound reaching each hydrophone in. an array, the latitude, longitude, and depth position of the whole array, pins the position of the individual hydrophone with respect to one another, must be known in order to analyze the data, and preferably should be stable in order to avoid additional complexity in the analysis, and because syc!i instability is likely correlated with hydrodyyjamic drag _* ; turbulence, seif-nofee, and strain on the hydrophone array structure arid the t w cable.

[010,] The advantages of an array of hydrophones arranged in a tetrahedron are known. Each hydrophone i equidistant from each other hydrophone, simplifying the

calculations, and the relative position of each hydrophone to the others is fixed.

However, a tetrahedron is difficult to tow underwater at the end of along cable, and any roll, pitch, or yaw in the travel of the hydrophone arra will: alter the .relative position of each ^' hydrophone with respect t the underwater sound source. Any such change of orientation o attitude of the hydrophone array must be captured and accounted for in the analysis of the data. Such changes of orientation or attitude are also likely to correlate with increased hydrodynamic drag, self-noise, and strain on the hydrophone array structure and the to cable,

[01 1.] There is a. need for a small, low cost volumetric array _* integrated with

PAMGtlARD, for the use of the government, military, and universities for marine mammal population studies, mitigation for military and commercial activities in the ocean, and detection and localization of submerged assets such: as downed _. -planes,, moorings, A t ⁾ Vs - or RQ Vs.

.Summary of tlx: Invention

IOI .3 This invention provides a system apparatus and method for ship-towed deployment of a nonlinear volumetxic: array of hydrophones, allowing line-iatersect or line-transect sampling of marine mammal populations through: passive acoustic monitoring, enabling unambiguous real-time three-dimensional iocalifcatiors of single sounds received through a low-cos.,. modular, robust, stable, small, light, .neutrally to slightly negatively buoyant volumetric arra havi.Bg low self-noise and low flow noise, that avoids puttin high tension on the tow cable and th t is compatible with: standard hydrophor!es, imtriimentalion, cabling, and analytical software.

Brief Description of Drawings

[01.3,] Reference will now be made to the drawings, wherein like parts are designated by like num rals:, and wherein;

[014.] FIG. 1 is a schematic view of the ship-towed hydrophone volumetric array system of th in vention in use;

[0.15. j iG. 2 is an orthographic -..axonometric view of the ship-towed hydrophone volumetric array system of the invention;.

[016..] FIG. 3 is a section, view of the fin profile of the ship-towed hydrophone volumetric array system of the invention;

[0.17.] FiG . 4 i s an orthographic axononie e view of the ship-towed hydrophone volumetric array system of the invention;

[018.] FIG, 5 is a fron -view of the ship-towed hydrophone voltnnetric arra system of the invention; _.

[01 .] FIG. 6 is a back- view of ihe ship-towed hydrophone volumetric array system of the invention;

[020. J FIG. 7 is bottom view of the ship-lowed hydrophone volumetric array system of ihe invention;

[021.] FIG. 8 is a. top view of the ship-towed hydrophone volumetric array system of the invention;

{022.;] FIG. 9 is a side view of the ship-towed hydrophone volumetric array system of th invention;

[023.] FIG, 30 is a section view of the ship-towed hydrophone; volumetric array system of the invention;

[024.] PIG. 11 is a schematic: view of the interna! components of the ship-lowed hydrophone olumetric array system of the in vention;

[025.] FIG. 12 is an exploded view of the ship-towed hydrophone volumetric array system of the invention;

[026.] FIG. 13 is a schematic view of the ship-vowed hydrophone volumetric array system of the invention, in trim;

[027. ] FIG _* 14 is a schematic view of the ship-towed hydrophone volumetric array system of the invention roiling;

[028.] FIG. 15 is a schematic view of the ship-towed hydrophone volumetri arra system of the invention yawing;

[029.] FIG. 16 I a schematic view of the ship-towed hydrophone volumetric array system of the invention pi ching; and [030.] FIG. 17 is a schematic view of the shipboard -or topside operation of the ship- lowed hydrophone volumetric array system of the in ention in use.

Detailed ¹ Description o the Iri Vention

[031.. ) Referring : to FIG. 1 , the ship-towed hy rophoiie volumetric array system invention method 100 and apparatus 10 are illustrated schematically. The activity is line- fnterseet or line-transect sampling of marine mammal populations through passi ve acoustic monitoring. The wate is deep and the area covered is wide, so the underwater sound of the ship and of any equipment used must be minimized in order to be able to pick up the desired signals. The hydrophone volumetric- array system is towed behind a ship on a cable about 300 meters in length, in order to get sufficiently far away from the ship's- noise. The tow speed is about 10 knots, because so much area must be covered in a single run in order to collect sufficient data.

[032,] An underwater sound wave from a single click frorn a whale propagates through the water and hits each individual hydrophone 1 , .2, 3, 4 in the ship-tewed hydrophone volumetric an¾y system. 10 apparatus at slightly difrerent times correlated with- ihe slight differences in straight-line distances to the sound source. Aboard the towing shi are the topside electronics 90, recei ing, signals from a GPS antenn 91 and communicating with the hydrophone volumetric array through a multi-channel power and data cable 92 incorporated into o onto the to cable 93. The topside electronics are able to analyze and display in real time the signals being received from, the hydrophone arra and the location ^' of the sound source in relation to the hydrophone array, the location of which is knows from applying the proper corrections to the GPS data. The result is real-time, tl«¾e-dimepsional location information. [033.] Referring to FIG. 2, She four hydrophones 1, 2, 3, are. fixed in a tetraoedral relationshi one to the other in the ship-towed hydrophone volumetric army 10 system apparatus. With reference to the direction of travel there is a fore hydrophone 1, aft hydrophone 4, port, hydrophone 2, and starboard hydrophone 3. The four hydrophones generate: data that is: consolidated and amplified in an electronics housing 5, which also consolidates and amplifies dat from: a p essore-depih sensor 6 aad an orientation sensor 7, and all of the data is sent: through the tapered cable clamp 65 through the tow cable and to the topside electronics.

|034.j The ship-towed hydrophone volumetric array 10 system apparatus is streamlined, to eliminate iiydrodynaraic drag and to enhance stability as much as possible. Structural components are t ei¾re-straet«re 20, aft- structure 30, and .roid-stmcture 40. The mid- structure 40 has a main tube housing: 41 enclosing the ^■' electronics housing 5, pressure - depth seusof 6 and orientation sensor 7. The central axis of the main tube housing 41 passes through th projected center point of the tetrahedron &i hydrophones. This central axis Is also the axis of the direction of travel of the whole unit. There are three hydrophone tubes 42, 43, 44 arrayed around the main tube housing 41 with 120 de rees separation of one tube to ffie others _* and located at 60 degrees, 180 degrees, and 300 degrees around: the central, axis of the main tube housing 41. The .three- hydrophone tubes are the fore-and-aft hydrophone tube 42 located at 1.8G degrees or directly below the main tube housing in use. The port hydrophone tube 43 and the starboard hydrophone tube 44 are located at. 60 degrees and.300 degrees:.

[035.] The central axis of the m in tube housin 41 passes through the projected center point of the tetrahedron of hydrophones,: with the consequence that the lore and aft hydrophones 1, and the center ροίόί are in a plane along the ^■ central axis, and the port and starboard hydrophones 2, 3 and the center point are is plane perpendicular to the central axis _* The distance between the central axis of the- mmn tube housing: 41 and the central axis of the fore-and-aft hydrophone tube 42 is therefore one-fifth shorter, or 80*%· of the oon'espondhig distance for the port and starboard hydrophone tubes 43, 4.

[036.] The foi¾-sti-uciisre 20 functions: ¾$ fairing and stabilization of the m -stmeiore 40 and is attached in smooth continuations of the cylinder and tubes of the mid-structure 40, both as to continuation of the relevan central axes and to■continuation of outer surfaces, in order to avoid turbulence and hydrodynamtc drag. The fore-structure 20 has a streamlined main nose cone 21 on the main body and three subsidiary nose cones 22, 23, 24 affixed to fore-fins 25, 26, 27 that connect the streamlined main nose cone 21 to the three subsidiary nose cones 22, 23, 24. The nose cones 21 , 22, 23, 24 and fore-fuis.25, 26, 2? are hollow or partially hollow to provide paths lor Wiring from the hydrophones- to the electronics- housing 5, and fo lighter weight. The three subsidiary nose cones are the same size and are designated as fore-and-aft hose cone 22, port nose cone 23, and starboard nose cone 24. The three fore-fins are, not all of equal length from the main nose cone to the subsidiary nose: cone. The fore-and-aft fore- fin 25 is shorter than the port and Starboard fore-fins 26» 27, which are of length equal one to the other.

[037,] The aft-sttiieture 30 supports and stabilizes the hydrophone tubes 42, 3, 44, and, with the fote-strnetiir 20, stabilizes the ship-towed hydrophone volumetric array system 10 apparatus in use. As with the attachmen t of the fore-structure 20 and mid-structure 40, the attachment of the: rnid-structhre 40 and aft-stniet re 30 smoothly continues the relevant central axes and outer surfaces. The aft-strueiure 30 has a main open aft collar 31 and three subsidiary open aft collars, a fore-and-aft open aft collar 32, a port open aft collar 33, and a starboard open aft collar 34, with each subsidi ry open aft colla connected to the main open aft collar 31 by the fore-and-aft aft fin 35. port aft fra 36, and starboard aft fin 37, corresponding to the κ?!& ¾αί portions of the fore-structure 20 and miri-strvseture 40.

[038.] The main open aft collar 31 and the three subsidiary open aft collars 32, 33, 34 are ail open cylinders or tube ^' s, which allow water to fill all of the mid-structure 40 and the hollow portions of the fore-structure when the Invention: is in use under water, and to drain out of those structures when the invention is retrieved or handled topside out of the water. It is highly desirable to fill the ship-towed hydrophone volumetrie array system 10 with water when in use, because the water continues the trarismission of sound to the hydrophones, where air would not and where any h droaeoustieally different substance would caus reflection of the sound waves "at the boundaries. Also, the smooth, stable operation of the hydrophone array under water e ui es that the hydrophone array, when in use. has neutral buoyanc or slightly negative buoyancy of five pounds negative plus or minus five pounds. Otherwise, any tendenc to float, combined with the natural tendency o ihe tow cable itself to rise when pulled at significant speeds, is Hfcely to prevent the hydrophone array from- .staying, consistently at the desired depth, which is on the order of ten meters deep. With neutral or slightly negative buoyancy of the hydrophone array, depressor weight only in the -amount needed to couriteract the effects of cable drag, or to dampen cable strum, will he required, with none to very little depressor weight needed for the hydrophone array. The addition of solid weight to the simeture of the hydrophone array, or filling portions with a heavier-than-seawater liquid, would increase the topside weight and would detrimentall m en the bydroaeowsiie transparency of the hydrophone r

[039.] The topside or dry weight of a preferred embodiment, of the ship-towed hydrophone volumetric: array 10, built with the preferred material arid equipped with the preferred equipment, is approximatel 30 pounds, and the size is 11 Q centimeters ^' by 55 centimeters, just over on e meter by one-half meter. The size and weight make the ship- towed hydrophone volumetric array 10 easy for one e son to handle, deploy, and .retrieve..

[040,] With the water entering through the open aft collars, abaft the hydrophones, and with no significant water entering through the fare-structure, there is no water actively flowing past the hydrophones inside the hydrophone tubes, but instead there is a envelope of relatively still wate in direct contact with the hydrophones. This provides an advantage because the seawater inside the hydrophone tubes has the same hydroaeousiic properties as the seawater outside the tubes, but the noisy effects of having the seawater constantly flowing directly over the hydrophones is avoided. The open, collars on. the aft end do not present any significant hydrodynamic pertitrhances,

[04 ! ..] A tapered cable clamp 65 is provided at the center of the ti of the mai nose cone 21 for the purposes of towing, low- oltage power, and data eommunications through data cables incorporated into or onto the tow cable,

[042.] Referring t FIG..3, illustrating the streamlined NACA 0012 win or fin profile, the cross sectional profile of the fore-fins 25, 26, 27 and aft-fins 35, 36, 37 must be streamlined to promote laminar flow and suppress turbulent flow, in order to avoid the generation of noise at or ahead -of the hydrophones, which the hydrophones will subsequently be pulled through. Suppression of turbulence behind the hydrophones is somewhat less critical,, but still necessar since any resulting noise travels at muc greater speed than 10 knots, and will reach the hydrophones, hi a preferred embodiment of the invention, the streamlined fore-fins and ait-fins have the ACA 001:2 profile.- [043,] Referring to FIG. 4, the ship-towed hydrophone volumetric array 10 system optionally ' provides a cab !e-eoiineetor fairing boot 66 for the purpose of streamlining o fairing the tow-cable connection and integrating its fairing effect with the fairing effect of the main nose cone 21. The main nose, cone 21 has a spherically blunted tangent, ogive profile, which minimizes hydrodynauMC drag in that area upon which, a greater amount of pressure is concentrated. Fairing is provided at the joining of nose cones and fins in the fore-structure 20 and of eollai's- and fins in the aft- structure 30.

[044.] Referring additionally to FIG. 5 & FIG. 6, the four hydrophones are supported in a tetrahedra! relationship, by defintiion with equal straight-line distances between any given two hydrophones. In a preferred embodiment of the invention, that distance is 50 centimeters, the equivalent of one-half meter. That distance will figure into the

volumetric or three-dimensional analysis of data from the hydrophone array. It has been found to be a sufficient distance for obtaining meaningful and quantifiable differences in the arrival of sound waves. Based on such a 50-eentimete.r separation, the overall longitudinal length of the central or main ^■ portion of the hydrophone array embodiment is 1 10 centimeters, the diameter of the main tube housing 1 is 6 centimeters, and the diameter of the three subsidiary hydrophone tubes 42, 3, 44 is 5 centimeters: each.

Measuring from the central axes of each tube _* which are parallel each to the others, the distance from the center of the main tube housing 41 to the center of the port tube 43 is 25 centimeters, with the is anc to the starboard tube 24 being the same. The distance to the center of the fore-and-aft tube 42 is 20 centiaieiers, which is 80% of the other distances; A functioning full-scale prototype of the preferred embodiment was built using schedufc-80 pipe of 4-meh and 1.5 -inch ID size for the main central and Use subsidiary tubes.

[045.] Referring to FIG. 7 & FIG. 8. the bilateral symmetry along the central axis In the bottom and top views, and the two-dimensional spatial relationships among the

hydrophones, can he seen, -Referring; to FIG. 9, the asymmetry along the central axis caused by the accommodation of the tetrahedral arra in the three-dimensional stricture of the ship-towed hydrophone volumetric array system ! 0 apparatus, and the two- dirnensional spaeial relationships among the hydrophones can he seen.

[046.] Referring- to FIG, 10 <& FIG. 1 L the tapered cable clamp 65 has a taper corresponding to a taper built into the main nose cone 21. The tapered cable clamp 65 grips the tow cable 93 and Inco porated power and data, cable 92. The force of the towing tightens the connection becatsse of the taper. The tow cable 93 and -incorporated power and data cable 92 is then connected to the electronics housing 5 leaving a small amount of slack in the cable, providing strain relief and a backu cable attachment that can be used for a slow retrieval of the shtp-iowed. hydrophone volumetric array in the event of a failure of the tapered cable clamp. Optionally, a: cable-connector fairing boot 66 can be provided to further streamline the assembly,

[047, j Appropriate hydrophones are tire BENTHOWAVE ΒΠ-7094, with a sensitivity of "204 dB Y/uPas a capacitance of 13 nF, and the HTI ίΤϊ-9&-ΜΙΝ, with a sensitivity of -201 dBV/Was and a- capacitance of 10 nF, Neither of these hydrophones have built-in preanips. The prgaoips are incorporated in the electronics housing: 5, where they can be more easily accessed for adjustment and replacement if necessary.

[048.] in a preferred embodiment the three subsidiary hydrophone tubes 42, 4 4 are made from polyvinylidene difiuoride (PVDF) tubing. PVDF plastics are heavier than water but ydroacoasticail transparent

[049.] An appropriate material i r eonsiritctsng the fore -structure 20 and aftsirueture 30 is FIBRE GLASI ^" 2060 epm resin used wiih a mix o f carbon composiie arid fiberglass cloths for the fairing sections. Many epoxies have similar characteristics to plastics that have good acoustic properties and these materials exhibit high strength-to-weight characteristics,

[050.] In a preferred embodiment, the electronics bousing 5 is a pressure balanced. Castor-oil filled nylon plastic housing. Castor oil is a close acoustic impedance match to seawaier, and is an environmentally safe oil that is adily available. The electronics housing 5 can be milled rom black 6/6 nylon rod, and an acrylic cover can be machined fr m 0.5" acrylic sheet.

1051.] Four two-slage preamplifiers, one for each hydrophone, are provided in the electronics housing :5, The two-stage preainps are all differential input and differential output. The fi st reamplifier stag has a gain of 40 dB,. and the second stage has a gain of 21 dB, for a total gain of 61 dB, The 3 dB bandwidth of die preamps is 3.0 kHz to 200 kHz.

1052,] Also provided are two linear regulators that accept the 2VDC from, the topside batteries and regulates down to the +/- 3,0VDG used in the low noise preamps, 12 VDC power is provided to the pressure-depth senso 6 and the .orientation sensor 7; [053.1 A ess»re-d.epth sensor 6 is provided w measure, ta real time, the water pressur , front which the depth can be derived. The depth must be known in order to locate the ship-towed volumetric array in: three-dimensional space using topside, GES data and knowledge of the length of the tow cable and the direction of movement. Sound travels in water through pressure waves, which the hydrophones can detect. The pressure -depth sensor 6 also provides real-time measurements of the baseline or background pressure in hich the hydrophones, are operating. An appropriate pressure sensor is the

HONEYWELL PX2ASIXX250PAGHX, which has a 4-2¾tA loop out ut and a full scale range of 250 psi. It is possible that the ship-towed volumetric hydrophone array 10 could reach this depth and pressure if the vessel ..ever stops in deep water with the array deployed with a 300 meter cable. This sensor will not be destroyed should this happen.

[054,] An orientation sensor 7 measures, i n real time, any rolL-yaw, or pitch in the ship- towed hydrophone volumetric array's attitude along three axes of acceleration. Precise knowledge of the orientation is necessary in orde to know the positioning of the hydrophones each with the others so that meaningful directional locations of sound sources can be calculated. The ship- to wed Irydrophone volumetric array is designed to be stable and stay in trim while being towed, but some variation in orientation will likely occur and must be measured in real time and factored into the analysis of hydrophone data. The orientation sensor 7 is ideally placed at the projected center point of the tetrahedron formed by the four hydrophones,; A appropriate orientation senso is an JAC-I-03 that provides three axes of acceleration with 4-20mA loop signaling,

[055.] Referring to FIG. 1 , an exploded view further illustrates the internal and external components of the ship-towed volumetric hydrophone array 10 system and their spatial relationships each to the other,

[056.] Referring to FIG. 13, with the ship-to ved hydrophone volumetric array 10 traveling in trim, with its centrai axis or roll axis in lin with: the direction of travel, the orientation sensor 7 reports zero degrees deviance about all three axes,

[057.] Referring to PIG. 14, with the ship-towed hydrophone volumetric array 10 rolling, the .orientation- sensor 7 reports the roll, and the consequential shifting of the hydrophones 1 , 2, 3, 4 is available to be accounted for in the analysis of data.

[058.] Referring fa FIG. ..1 S, with, the ship-towe hydrophone volumetric array 10 yawing, the orientation sensor 7 reports the yaw, and the consequential shifting of the hydrophones 1, 2, % 4 is available to be accounted for in the analysis of data,

[059.] Referring to FIG. 16, with the shtp-toxved hydrophone voluTOetrie array 10 pitching, the orientation sensor 7 reports the pitch, and the consequential shifting of the hydrophones 1, 2, 3, 4 is: available to be accounted for in the analysis of data.

[060.] Referring to FIG, 17, a schematic view of the topside electronics 90 aboard the towing ship, the system is receiving signals . rom a GPS ^' antenna- 91 and communicating with the hydrophone volumetric array through a multi-channel power and data cable 92. The signals being received from the hydrophone array and the calculated, derived three- dimensional location of the sound source are displayed in real time and are recorded for further analysis and comparisons.

[061..] Many changes and modifiea ioM can be made m the present invention without .departing from the spirit thereof. I therefore pray thai rights to the present invention be limited only by the scope of the appended claims.