US3732533A

US3732533A - Underwater explosive-acoustic transducer and systems

Info

Publication number: US3732533A
Application number: US00541918A
Authority: US
Inventors: D Epstein; S Epstein
Original assignee: VADYS ASS Ltd
Priority date: 1966-04-04
Filing date: 1966-04-04
Publication date: 1973-05-08
Anticipated expiration: 1990-05-08

Abstract

An underwater sonic detection system wherein underwater pulses are generated in uniformly spaced selected time sequence to produce a water traversable multi-line spectrum made up of a plurality of discrete reflectable narrow band consecutively sequenced harmonic sonic frequencies of substantially uniform energy content and receiver means are employed to (a) discriminatorially sense reflections thereof from a target engaged thereby or (b) sense the relative radial velocity component of an engaged target in a transient type Doppler system or (c) sense emanated-vibratory frequency modes induced in a target engaged thereby.

Description

w States Patent 1 1 111 3,732,533 Epstein et a1. May 8, 1973 [54] UNDERWATER EXPLOSIVE- 3,022,852 2/1962 Pavey, Jr. ..1s1 .5 ACOUSTIC TRANSDUCER AND 3,052,185 9/1962 Apstein 1 ..102 70.2 SYSTEMS 33353313 22322 2132112111: 1 51513 [75] Inventors: David Epstein; Sidney Epstein, both 3,262,388 7/1966 McCarty ..340/5 E of Brmklyn FOREIGN PATENTS OR APPLICATIONS [73] Assigme: $5 Associates Bmoklyn 447,597 7 1927 Germany ..340 5 E [22] Filed: Apr. 4, 1966 Primary Examinen-Samuel Feinberg pp NOJ 541,918 Assistant Examiner-R. Kinberg Related US. Application Data Continuation-impart of Ser. No. 367,607, May 12,

Att0rneyRobert E. Isner [57] ABSTRACT An-underwater sonic detection system wherein un 1964, abandoned.

derwater pulses are generated in uniformly spaced 52 U.S. c1. ..340 3, 340/5 E, 340/6, Selected time seque'lce w Pmduce a Water "aversflble 181/5 XC multi-line spectrum made up of a plurality of discrete [51] Int. Cl. ..G0ls 9/66 reflectable narrow band consecutively sgquenced 58 Field of Search ..1s1/.5; 340/6, 5 E, frequeniies substamiany 340/12 gy content and rece1ver means are employed to (a) discriminatorially sense reflections thereof from a tar- [56] References Cited get engaged thereby or (b) sense the relative radial velocity, component of an engaged target in a transient UNITED STATES PATENTS type Doppler system or (c) sense emanated-vibratory frequency modes induced in a target engaged thereby. 1,504,247 8/1924 Jacques ..340/6 2,320,248 5/1943 Shimek ..181/.5 C 7 Claims, 9 Drawing Figures 0 SlN6LE-CHAR6E SPECTRUM ENVELOPE A, T db.

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UNDERWATER EXPLOSIVE-ACOUSTKC TRANSDUCER AND SYSTEMS This invention relates to underwater detection phenomena and more particularly to improved underwatei: sonic detection systems and apparatus utilizable therein. This invention is a continuation in part of our copending application Ser. No. 367,607 filed May 12, I964 now abandoned.

Recent years have witnessed increased attention and development effort relative to seismic techniques for geophysical exploration. Such attention, however, has been largely confined to earth exploration and comparatively little progress has been effected in the underwater field. Such comparative lack of progress has been reflected in the efforts of those skilled in the art being primarily directed to the improvement of known electro-sonic apparatus and techniques for underwater detection systems. Some of the reasons for the comparative lack of progress in other types of underwater detection systems have been the inherent problems posed thereby and the nature of the underwater mileu which have effectively precluded the practical reduction of theoretical concepts to realistically operable and utilizable realities.

One such system employing seismic phenomena for geophysical earth exploration having possibletheoretical advantage was that proposed in Shimek US. Pat. No. 2,320,248. This patent teaches and discloses the generic application of a specific mathematical analysis of a rectangular wave form as described by Shea in the text Transmission Networks and Wave Filters (th Ed. 1929, D. Van Nostrand & Co.) at pages 417 426 thereof to seismic prospecting. Shimek, however, indiscriminately adopted Sheas rectangular wave form and spectral envelope on the assumption that such was readily attainable from explosive charge detonation and apparently also assumed that the contemplated results would flow therefrom. Unfortunately, however, the practical results that are theoretically feasible from the use of such system have never been attained in actual practice.

We have discovered that one of the reasons for the inability of the art to realize practical advantage from the Shimek disclosure has been a continued failure to recognize the criticality of precise pulse timing and the deleterious effect of detonation time jitter relative to the formation and character of the discrete spectral lines and the inherent inability of conventional pyrotechnic fusing and firing techniques to provide the necessary discrete frequency spectra required. Still another reason has been the inability of the art to accommodate the necessary spatial separation of solid explosive charges to avoid sympathetic detonation thereof and yet satisfy the antithetical requirement of I effectively common point source detonation necessary to achieve effective and utilizable spectral lines.

Other reasons include lack of fundamental frequency flexibility due to an inherent difficulty or inability to control the duration andshape of the explosive seismic impulse and a progressive and rapid diminution in the magnitude of the discrete spectral lines as well as the complete loss of the even order harmonic lines.

This invention may be briefly described as a novel underwater sonic Doppler type detection system and apparatus utilizable therein embodying the general theoretical approach outlined in the saidShimek patent but modified in a number of critical particulars to effectively transform such theory into practical, attainable reality. In more particularity, the subject invention includes the generation of a series of a predetermined number of explosively shaped pulses through explosive detonation in an underwater milieu in association with means for selecting and closely controlling the variance with respect to the mean repetition rate of said pulses within sufiiciently narrow limits as to effectively insure the creation of selective and discrete narrow band, high energy multi-line spectra of harmonically related water traversable frequencies of finite and discrete band widths and which may be readily received and discriminated by receivers selectively tuned thereto and/or incrementally displaced therefrom in the frequency domain.

Among the advantages of the subject invention is a widespread adaptability of the system to specific underwater detection systems such as would be utilizable in geophysical prospecting, underwater target detection and related underwater systems utilizing seismic phenomena. Further advantages of the subject invention include the ready selection and modification of the fundamental frequency of the high energy spectral system by control of the time interval intermediate successive explosively shaped pulses which provides a flexibility of operation heretofore unattainable by conventional explosive methods and the provision of an essentially omnidirectional energy transmission system that avoids the limitation and deficiencies inherent in directional type transmission systems.

The primary object of this invention is the provision of an improved underwater detection system.

A further object of this invention is the provision of improved explosive acoustic transducer apparatus for effecting broad band underwater energy conversion and transmission.

Another object of this invention is to provide an improved underwater sonic detection system employing an explosive acoustic transducer apparatus adapted to effect electronic detonation of a sequence of a plurality of explosive charges at a selected predetermined repetition rate within such narrow limits as to give a temporally precise underwater pressure pulse train a with the time jitter of detonations about the desired pulse period maintained within such predetermined stringent bounds as to insure the creation of a discrete, high-energy narrow band spectrum of harmonically-related, water traversable acoustic frequencies of predetermined band width and substantially uniform energy content and whose fundamental frequency is related to the explosive pressure pulse detonation rate.

Still another object of the invention is the provision of a self-contained, expendable broad band explosiveacoustic transducer apparatus capable of creating a discrete, high-energy, narrow band spectrum of harmonically-related, water traversable acoustic frequencies of predetermined band width and fundamental frequency.

Still another object of this invention is to provide a reverberation suppression underwater detection system utilizing a multiple frequency Doppler signal processing technique to indicate relative radial components of target velocity in relation to transmitted energy source.

A further object of this invention is the provision of a novel underwater sonic energy transmission system capable of use as a boomer type of research tool for ascertaining fine structure frequency behavior of submerged bodies and for conjointly inducing sympathetic vibrations within a submerged target object and detecting ensuing vibratory radiation emanating therefrom.

Other objects and advantages of the subject invention will be alluded to in the following disclosure and claims and will be apparent to those skilled in this art therefrom and from the accompanying drawings which delineate by way of illustrative example, the incorporation of the principles of this invention in presently preferred embodiments of underwater target detecting sonic signal systems and the like.

Referring to the drawings:

FIG. 1 is a schematically idealized plot of pressure versus time for uniform underwater explosive pulse train of N charges spaced T,. seconds apart;

FIG. 2 is a plot of the narrow band energy spectrum resulting from the uniform pulse train of FIG. I in the practice of this invention and illustrating three representative harmonics, with attendant side bands, within the flat portion of the single charge frequency spectrum envelope;

FIG. 3 schematically illustrates, in section, the essentials of an explosive acoustic transducer incorporating the principles of this invention and arranged in a general spherical configuration to effect omnidirectional energy transfer therefrom;

FIG. 4 schematically illustrates, in section, the essentials of acoustic explosive transducer incorporating the principles of this invention and arranged in a generally linear configuration to effect an essentially directional energy transfer;

FIG. 5 is an idealized semi-log plot of comparative magnitude and phase relationship of the narrow band comb spectrum resulting from the practice of this invention;

FIG. 6 is a schematic block diagram illustrating the incorporation of the principles of this invention in apparatus for ascertaining fine frequency structure of submerged objects and for inducing and detecting the emanation of sympathetic vibrations therefrom;

FIG. 7 is a schematic representation of the utilization of this invention for the airborne detection of moving submerged objects;

FIG. 8 is a schematic block diagram of the components of a self-contained expendable explosive acoustic transducer incorporating the principles of this invention; and

FIG. 9 is a schematic block diagram of the multiple frequency Doppler effect processing and presentation components includable in the signal receiver components of a system incorporating the principles of this invention.

By way of general introduction, the subject invention includes the generation of a discrete, narrow band, high energy spectrum of harmonically related water traversable frequencies of finite and discrete band width by detonation of a series of explosive charges in an underwater milieu. We have found that the effective realization of the above is critically dependent upon a number of interrelated parameters, which include the spacing and location of explosive charges and the sequence and timing of charge detonation. To the above ends we have developed a novel explosive acoustic transducer apparatus which is of a self contained and expendable character and is adapted for individual use in an underwater milieu as exemplified by its inclusion in the hereafter described systems.

TRANSDUCER APPARATUS The underwater explosive acoustic transducer apparatus embodying the principles of this invention comprises an assemblage of a plurality of explosive charges or elements disposed in a selective spatial configuration adapted to accomodate the particular application at hand. In all such arrangements however, the minimum inter-charge spacing must be sufficiently large to insure against sympathetic detonation of other charges and possible mechanical rupture of the transducer body or frame. In accord with this invention, the transducer apparatus is so constituted as to structure and mode of operation as to provide an energy output in the form of discrete narrow bands, rather than in the continuous distribution that is extant throughout the single explosive charge spectrum. By detonating a number of charges, N, in rapid succession, with period T seconds, (see FIG. 1) the transducer and herein described system transforms the continuous spectrum of a single explosive charge into a series of narrow band spectral lines, of bandwidth Af=f,/N, spaced at regular intervals, f 1 IT, cycles/second apart, throughout the spectrum (FIG. 2). A feature of the transducer apparatus and described system is that by proper adjustment of the pulse repetition rate and the number and positioning of the explosive charges, the spectral lines can be made quite sharp (practically monochromatic" or CW); moreover, the spectral lines can be positioned, in regular harmonic sequence, almost at will within the single charge spectrum. However, in order to generate a practical, operationally useful, discrete spectrum, an extremely high degree of pulse timing accuracy must be satisfied.

Present day multiple charge explosive systems utilize convention pyrotechnic fuzing and firing techniques. Although a discrete frequency spectrum may be desired, the inherent random variations of the pulse period, resulting from conventional techniques, causes rapid deteriorationof the periodic spectrum, starting with the higher harmonics. The subject invention overcomes these difficulties, at least in part, by maintaining the pulse detonation time interval between explosive charges, which may be of practically arbitrary duration, within extreme accuracy limits by means of the hereinafter described electronic firing techniques.

Referring now to FIG. 3, which illustrates a sectional view of a presently preferred embodiment of an underwater omnidirectional transducer, the explosive elements or charges 2 are disposed in essentially a spherical array as by being embedded in a plastic sphere or by being located at the extremities ofa multi-armed star frame 4. The radius arms of said frame 4 are of suitable lengths so that the outer layer of charges effectively lie on the surface of a sphere in such a manner that the necessary minimum distance is maintained between any given charge and its neighbors to prevent sympathetic detonation of adjacent charges and/or mechanical rupture of the frame. As will be now apparent, the actual distances required will be determined by the strength of the charges. It should be noted however, that since the total charge is equally distributed amongst a large number of elements, the strength of each individual charge may be small, permitting close juxtaposition thereof. The packing density should desirably be as great as possible so that this embodiment of the transducer tends to act as or approach a point source in its characteristics.

More specifically, the charge elements 2 are adapted to be electrically detonated as by inclusion of electrically actuatable detonating means of uniform response characteristics with each charge. Preferably the time for effecting detonation should be in the order of microseconds and fast acting detonators 6 such as are firing detonation caps as the X-98 manufactured and sold by El. DuPont de Nemours & Co. Inc. are preferred. By way of example, the firing time for these caps is specified as 2.6 microseconds i 0.2 microseconds (the tolerance being a measure of the time jitter to be expected) when energized by a 3 microfarad capacitor previously charged to a potential of 5000 volts.

The essentials of a timing system, for controlling both the firing order and the explosion pulse repetition rate, are illustrated in FlG. 3 in conjunction with such spherical array (shown on in equatorial plan thereof). As there shown, electrical energy is supplied from a self contained power supply 18 to a pulse generator 16 and a presettable electronic timer unit 14. The timer is so arranged as to permit setting by a user of the desired pulse repetition rate for the repetitive pulse outputs of the pulse generator 16. The output of the pulse generator 16 is fed into stepping switch 12 via line 10. The contacts on stepping switch 12 are selectively and individually connected to individual charges 2 by means of cable 9, junction box 7 and leads 8 and such individual connections permit preselection of the desired firing order. By means of the above, the uniform train of sharp electrical firing energy pulses emanating on line 10 and spaced in time as per FIG. 1 are electrically steered so that each fast acting detonator 6 receives its firing pulse at the proper time. As previously pointed out, the particular firing order is one of the critical parameters and the considerations relevent thereto will be pointed out in a later portion of this specification.

FIG; 4 illustrates the essentials of an alternate embodiment in the nature of a directional array version of an underwater explosive acoustic transducer. Here, the explosive charges 2 are placed along a linear frame 5 at uniformly spaced intervals. The caps 6 are again energized in uniform time sequence. However, in lieu of the stepping switch 12 pulse generator 14 combination heretofor described, this embodiment employs a plurality of N single-pulse generators 17 in conjunction with the timing unit 14. This results in each of the charges 2 receiving its firing pulse in the desired sequence. A more detailed disclosure of requisite elec tronic circuitry suitable for use in either transducer array will be presented in a later portion of this specification.

SYSTEM THEORY The theoretical background for the present invention is based on report entitled Theoretical Feasibility The energy spectrum of the finite pulse train is u) x l l 2 where to is the angular frequency, and

fr) N2 (3) where M is one of the desired harmonics of discrete spectrum.

The explosion repetition rate, T determines the spectral line separation, i.e., the frequency interval between successive principal maxima; the spectral line width, Af, is fixed by T and the number of explosive charges, N:

Af=1/NT,=f,/N 4 FIG. 2 shows the compression of energy into spectral bands, with attendant side bands, centered about harmonies of f,, where fr= r (5) In light of the above described system theory, the hereinbefore described transducer apparatus can be used as a signal source in underwater detection systems and as a tool to uncover unusual frequency dependent effects in media or structures. I-leretofore, broad band signals were frequently used in the search for frequency fine structure behavior; although radiation from a single charge provides a continuous spectrum, from about zero frequency on up, relatively little energy appears in any given narrow band. Thus, to obtain an appreciable signal/noise ratio may require averaging over a fairly broad band which may serve to obscure just those fine structure effects one wishes to observe. However, by compressing the energy in the broad band (M l)f Mf, into the narrow band f,./N, the new explosive acoustic source yields a narrow band signal at the spectral line of interest, 10 log N decibels higher than its continuous counterpart. In particular, the herein described transducer and its described mode of operation provides easy access to the very low frequency range.

Referring to FIGS. 1 and 2, the discrete spectral lines appear at regular intervals, f, l/T, cycles/second throughout the single charge spectrum. By way of illustration, suppose it is desired to study submarine hull resonance phenomena; suppose further that this calls for a thorough exploration, in one cycle/second increments, of the low frequency range from 20 cycles/second with monochromatic" probing signals of approximately 1 cycle/second band width. It is clear that a single choice of T, will not, in general, yield a sequence of signals sufficient to blanket the entire frequency interval of interest. However, many different combinations of T, and N will give spectral line sequences with components in this band. For example, with T,= 50 milliseconds and N 20, one cycle/second bands appear at 20, 40, 60, 80, cycles per second; this set of narrow band frequencies is labeled 19 on FIG. 5. To fill in the gaps, a readjustment of T, is required; e.g., set T,= 52.6 milliseconds, then since f,= 19 cycles/second, spectral lines appear at 19, 38, 57, "76, cycles/second. It is seen that by continued readjustments of the pulse period, the target can be isonified by all frequencies in the band and any appreciable resonance effects detected.

Directional effects are basically a function of the inter-element spacing, charge array configuration, and any relative phase delays produced by charge element positioning and/or by controlled variation of the pulse detonation interval. Referring now to FIGS. 4 and 6, if the charges of the linear array 20 are detonated at time intervals of T, (in sequence from left to right), the effect on an object 24 placed along the centerline of the charges (endfire, 0) will be that of a sequence of charges detonated at time intervals of T, T, 8; where 8 is the small water delay due to the finite separation of charges. Now, if said object 24 to be irradiated is moved broadside (0 1r/2) to the linear array transducer 20, the spectral lines will shift because 8 is a function of 6. Therefore, the spatial separation may be said to introduce a frequency shift as function of the departure from omnidirectionality; in this sense, the linear array is directional. On the other hand, if charges are placed on and within the surface of a sphere, shown in section on FIG. 3, the phase differences introduced by 8 will tend to cancel out because of the particular radiation interference pattern set up by virtue of the three-dimensional spherical symmetry. The degree of a cancellation is a function of the number of charges and their relative spacing, the explosive pulse repetition rate and the frequency harmonic number. Thus, an essentially omnidirectional, i.e., omnidirectional in both the signal strength sense and the comb frequency spectrum independence of viewing angle sense) transducer may be physically realized.

With reference to the pulse repetition period, 7",, and in sharp contrast with conventional explosive arrays where the firing time delay between neighboring charges is set either by slow burning time fuzes, which the inherently incapable of providing the timing accuracy called for herein, or by prima-cord where the firing time delay is set by the speed of deflagration z 50 microseconds/foot), or by standard electric blasting cap method, the use of the herein described firing techniques affords time delays over many orders of magnitude of time and with greater practicality, accuracy and repeatability.

As pointed out earlier, timing accuracy and repeatability of the explosion pressure pulses is a critical parameter because random time jitter of the underwater explosions, about their assigned detonation times, causes a deterioration of the desired spectrum comb or line).

The available energy shifts from the line spectrum into the continuous spectrum, starting with the higher harmonics. Thus, jitter must be effectively controlled if the desired process is not to be degraded to the point of uselessness.

To illustrate this problem, in the context of the aforementioned example, assume that the system designer has set the highest desired harmonic frequency of the comb spectrum, f, N/T,, and that in this worst case he is willing to allow a given amount of the n' harmonics energy to be transferred to the noise continuum. Thus, he would set the ratio of the energy strength of the n" harmonic component of the line spectrum with jitter to the energy strength of the n" harmonic component of the line spectrum without jitter to be -0.5 db. for example,; then (from curve b of FIG. 3, Vadys, op. cit.) the harmonic-jitter parameter,

m ssengea=21rnp (6) and p=p/ r where n harmonic,p R MS time jitter, and

p normalized time jitter parameter.

For T, 50 milliseconds and N 20, the highest harmonic of interest, n* 4 (corresponding to cycles per second). Combining Equ. 6 and 7 and substituting numbers or an explosion timing precision of approximately 1 percent is required. For higher harmonics, timing precision required is correspondingly higher. If, instead of the 4th, it was necessary to utilize the 40th harmonic timing precision would have to be on the order of 0.1 percent of 50 milliseconds or 50 microseconds. Therefore, the useable discrete spectrum has a fundamental frequency of l/ T, cycles/second and harmonics thereof up to an upper frequency bound which is determined by the lowest tolerable ratio of narrow band energy which is contained in said upper boundary frequency band as compared to that of the neighboring continuum energy.

Another upper frequency bound should be satisfied if the discrete spectrum is to constitute a set of harmonically related, narrow band frequencies of constant energy magnitude. This upper boundary is obtained from the single charge waveform. Although the time dependence of an underwater explosion waveform may be quite complicated, (See, for example, FIG. 1.5, page 11, Underwater Explosiions, RH. Cole, Princeton U. Press (1948) reprinted by Dover Publications, New York, which shows a waveform characteristic of an underwater explosion.) for the purposes of this analysis the underwater pressure pulse can be adequately represented mathematically as a single exponential function with time constant 1'. The time constant, 1', of this idealized underwater pressure pulse, of the pulse train shown on FIG. 1, is in the order of microseconds. The Fourier integral transform of the single charge spectrum is given by the formula The relative amplitude, |(m)| 29 and phase 31 are shown as idealized semi-log plot's on FIG. 5, whereupon the seromechanisms engineers well known straight line approximations have been used. Beyond the break point 33, the envelope 29 falls off with a slope 35 of 20 decibels per decade. In lieu of the rather stringent 0.5 db upper frequency bound constraint used in the preceeding example, a looser constraint based upon the half-power 6 db) criterion conventionally associated with said break point might be utilized.

As will be apparent from what hereinafter follows, the subject system is of versatile character and capable of many uses in the underwater milieu. Some of these are as follows:

As a Boomer/Resonance research tool An application of the explosive acoustic transducer apparatus embodying the principles of this invention is as a boomer, capable of irradiating an underwater object with high energy, narrow band, harmonically related spectral lines for NT, seconds. Prior art boomers, exemplified by the relatively low energy, broad band system, used for underwater sea bottom profiling, is described by Edgerton, Gereshausen and Grier Inc. in their data sheet C 64-3, Boomer Profiling Systems. Their representative electromechanical transducer system has an energy output in the order of a thousand joules per boom (as compared with roughly l l0 joules of useful acoustic energy in 1 pound of TNT), has continuous and repetitive boom intervals in the order of seconds, and generates a spherical pressure pulse which has significant output components in the range from 30 to well over 1000 cycles/second. Because of the broad band nature of this transducer signal, the associated receiver uses relatively broad band filters, approximately 2% octaves wide, to obtain some sort of frequency discrimination.

As a Hull Resonance research tool FIG. 6 schematically illustrates the application of a system of this invention to investigate hull resonances of a submerged vessel. The linear array version of the explosive acoustic transducer 20 is adapted to be lowered into the water from a surface vessel 22 so as to be in position to irradiate a submerged structure of vessel 24 with, for example, endfire (0 0) radiation 25. Subsequent sympathetic vibrations, induced by said radiation in hull 24 may be detected and measured in conventional ways; as for example, either directly by means of applied sensors, such as strain gages 26 or accelerometers 28, strategically positioned and attached to the hull of 24, or acoustically and at a distance via induced radiation 27 to hydrophone 30. Although only one direct sensor of each type is shown with its attendant circuitry, it will be apparent to those skilled in the art that a multiplicity of said direct sensors would each be connected to a parallel bank of strain gage amplifiers 32 and accelerometer amplifiers 34. The hydrophone 30 is connected to sonar receiver 36. The [a(t)] outputs of band of accelerometer amplifiers 34 is integrated once by bank of integrators 37 to give velocity [v(t)] of the induced hull vibrations and is integrated a second time by a second bank of integrators 38 to give displacement [x(t)]. Each of these signals, plus the aforementioned outputs of the bank of strain gage amplifiers 32, may be stored, as for example, on assigned tracks of a multi-track tape recorder 40. The stored records are selected and read-out through a multi-point switch 42 and repeatedly played back 43, in real time or fast time, so that a spectrum analysis 44 of the signals may be obtained. The macroscopic (overall) hull resonance phenomena of hull 24 may be detected via hydrophone 30 and receiver 36. Storage and anlysis of received induced radiation signal 27 may proceed, for example, in fast (compressed) time utilizing a fast time storage device 39 such as the Delay Line Time Compressor apparatus disclosed in the Anderson U.S. Pat. No. 2,958,039 to feed an electronic read-out unit 41 which in turn feeds the spectrum analyzer 44. As a general rule, the linear transducer 20 will be preferred when the target position with respect to the transducer is known a priori and the spherical transducer 46 when it is not.

As a multiple frequency, transient type Doppler, target detection system Another use for the subject transducer is as a component in the hereinafter and preferred Doppler type system for the detection of moving underwater targets. Referring to FIG. 7, an aircraft 21 drops a radio telemetery sonobuoy 23 in the vicinity of a suspected submarine 24. Concurrently, an expendable and self-contained explosive acoustic transducer 46, of the type hereinbefore described and preferably of the spherically shaped array, is dropped also. Transducer 46 is set to initiate detonation at the desired water depth by means of a timing device or hydrostatic switch. The detailed circuitry for the presently preferred embodiment of a self-contained, watertight and shock resistant transducer is shown on FIG. 8.

Referring now to FIG. 8, the set of individual electrical energy storage networks or capacitors 48 are electrically precharged, preferably just prior to the dropping of transducer 46 into the water, via the set of high reverse voltage diodes 50 and the charging resistor 52 through coupling 54. For safety, an arming switch 56 and/or a water activated battery 58 (responsive to immersion in the milieu) is used to energize the timing and pulse steering electronics. Qnce in the water, electrical power from battery 5% energizes the precise, stable, high frequency oscillator 60 whose sine wave output is fed to the high gain isolation amplifier 62. The output of amplifier 62 is clipped to form a rectangular wave by the action of the double ended Zener diode 64 and differentiated by capacitor 66, resistor 68 and diode70, in combination, to give a continuous train of sharp, positive, electric pulses with a period of T,/v seconds, where 11 may be any integer greater than 1, preferably a power of 2. When transducer 46 has reached the desired water depth, hydrostatic switch 72 closes thereby applying a positive step voltage firstly to delay line 74 (delay sufficient to allow reset pulse on line 8 0 to act) an secondly to differentiating network, capacitor 76 and resistor 78, which outputs a positive pulse on said reset line 80 to insure that the pulse counter and divider unit 82 and the ring counter,84 are initially set to zero. Immediately thereafter, and-gate 86 is energized by positive step voltage through delay 74 thereby gating the train of positive timing pulses to the input of the pulse counter and divider 82 which emits a positive pulse every T, seconds. By judiciously choosing the oscillator 60 frequency and the integer v of pulse count divider 82, T, may be changed to accomodate any one of a predetermined set of pulse periods. Each positive pulse of the new pulse train, of period T,, steers pulse output of ring counter 84 to an adjacent output line thereby activating, in desired sequence, trigger inputs of the set of detonator switches 87 via set of pulse transformers 88 and high resistance resistors 90. Detonator switches with firing delay below 1 microsecond and firing delay jitter below 0.3 microseconds such as the miniature Sprytron or Krytron tubes as manufactured and sold by Edgerton, Gereshausen and Grier Inc. of Boston Mass. are preferred. The high reverse holdoff voltage, high pulse current, fast acting switches 87 gate the energy stored in the storage elements 48 to their individual detonator caps 6 which in turn detonate individual charges 2. The result is the train of underwater pressure pulses, as per FIG. l, with time jitter about the design detonation time, mT,, where l s m s (N I), kept within required bounds. The value of the statistical, normalized jitter parameter, p, is a function of the precision and repeatability of all the components in the firing chain, i.e., timing, switching, and the amount of electrical energy and power that is used to actuate the detonator cap.

Referring again to FIG. 7, the transmitted signal 25 and the return echo 27 from the transducer 46 and the moving submerged target 24, respectively, are picked up by the hydrophonc of the sonobuoy 23 and transmitted to the aircraft 21 wherein the signals are processed for moving target information. The main factors limiting system performance are underwater pressure pulse time jitter and spurious Doppler. Spurious Doppler is caused by the departure from omnidirectionality, of the underwater transducer radiation pattern, caused by the spatial distribution of charges. The problem arises, in a search situation, because location of target with respect to transducer is only known a posteriori, if at all. Furthermore, as previously discussed, spectral lines generated by the linear array transducer 20 will experience a frequency shift as a function of the angle but target radial velocity, with respect to transducer velocity, also manifests itself as a frequency shift of the spectral lines, hence spurious Doppler. If the spherical array transducer 46 configuration is used, phase differences introduced by the finite separation of said charges tend to cancel out thereby reducing spurious Doppler effects to negligible proportions. Because of the essentially point source character of the spherical array explosive charge system, said system is of particular utility in a target search situation.

Although explosive array systems have been used for detecting and ranging on submarines, they are and have been of limited utility because the target echo is often, if not usually, exessively corrupted by reverberation noise. In the subject invention, however, because the reverberation is due to an essentially stationary background, reverberation effects are minimized. If the echo 27 spectrum from submerged and moving target 24 is compared with the spectrum from the transducer 46, which is essentially stationary, and/or the reverberation, the spectral lines are found to be displaced by the Doppler frequency shift of f 7X l 0' cycles/second/knot/kilocycle 9 The minimum detectable radial component of target 24 velocity, V relative to transducer 46, is V,,,,,,. V,,,,, is related to the minimum Doppler frequency that can be resolved a=1 p=0.5% V,,,,,,= 3 knots After some manipulation of Equ. 9 and 10,

n= l.42Xl0 /NV,,,,,, l.42X1() /(2O)(3) 24 Therefore, the lower frequency bound on the sub-set of narrow band spectral lines is nf,= l.2 l 0 cycles/second and as previously illustrated in connection with the resonance research tool example,

where n* is the highest useable harmonic consistent with allowable deterioration of discrete spectra as given by harmonic-jitter parameter, a, and degree of control over normalized jitter parameter, p. Referring back to FIG. 5, the nine vertical arrows 45 represent the sub-set of narrow band harmonics, M f,, 24 s M s 32, of I (m)| for T, 20 milliseconds, located within the single charge spectrum ](w)| 29. It should be noted that in the interest of symbol economy, M stands for both the M narrow-band frequency component and the total number of said sub-set of frequencies in the desired line spectrum.

Now system V,,,,,,, is limited by velocity ambiguity to maz mIn (11) But, whereas V(n),,,,,, 3 knots is the minimum detectable velocity, V(n*),,,,,, 2.22 knots must be used to avoid velocity ambiguity when the upper bound frequency of the sub-set 45 is Doppler shifted 50 cycles/second. From Equ. l 1,

ista 1 =42 knots There are a total of twenty-nine radial velocity, V,,, resolution, AV in 3 knot increments obtainable, or

V =:3kknots,0 s k s 14 12 The aircraft 21 receives the transmitted signal spectrurn from sonobuoy 23 and after demodulating same aquires the radiated signal from transducer 46, reverberation, and the echo or Doppler shifted spectrum 27 from the moving target 24. Referring now to FIG. 9, the demodulated output of the airborne receiver 92 may be processed in the time or frequency domains for Doppler information. In the presently preferred embodiment of the subject invention, a frequency domain processor will be described. The acoustic signals from receiver 92 are fed to a 9X29 matrix of bandpass filters, 261 in all. The center frequency, f,, of each one of the bandpass filters 94 is given by f f(M,k) Mf,iO.7(50M)(3k) cycles/second (l3) and the bandwidth of the corresponding bandpass filters 94 in the matrix are each adjusted in accordance with Af(M,k) =0.7(50M)(3k) cycles/second where,24 s M s 32 andO k s 14.

The output of each bandpass filter 94 is fed to a detector such as the square-law diodes 96 which in turn feed resistors 98. The nine resistors 98 of each vertical column in the filter matrix are connected to a summing buss 100, there being 29 busses in all. Each of the busses 100 are inturn connected to the input of an operational amplifier 102 which is shunted by a capacitor 104; the combination of the nine resistors 98, the capacitor 104 and the operational amplifier 102 being a summing integrator. The ganged switch 106, when closed, keeps the integrator output lines 108 at ground potential; the switch 106 is opened just prior to the expected detonation time of transducer 46.

Because the signal output on any one of the 29 summing integrator output lines 108 is the sum of nine narrow band signals, an improvement of approximately 10 decibels, over that of a single channel narrow band filter Doppler system, in signal to noise ratio obtains. Each line of the set of integrator output lines 108 is connected to a sampling gate, exemplified by the set of dual-input and-gates 1 l0. Said sampling gates are actuated in a uniform time sequence, at a rate consonant with well known sampling theorems, by the sequential scanner 122; the sampling cycle may start with the zero speed (k filter column which is interrogated by a pulse on line 112. The output of the zero speed sampling gate, and the 28 other sampling gates 110, are fed to resistors 124 which in turn feed operational amplifier 126 and feedback resistor 128. The combination of set of resistors 124, operational amplifier 126 and feedback resistor 128 constitute a memoryless summing amplifier.

- To complete the description of the multiple frequency Doppler processor, an operational cycle will be given. Sonobuoy 23 and explosive transducer 46 are dropped into the water. At a time just prior to the expected onset of detonations of transducer 46, the ganged switch 106 is put into the position as shown on FIG. 9. The positive step voltage from the battery 120 is differentiated by capacitor 130 and resistor 132 to provide a reset pulse to zero horizontal deflection staircase generator 134 and sequential scanner 122. The horizontal deflection voltage from 134 now being at ground potential, the spot on the face of the oscilloscope 136 is centered in the zero speed slot. Suitable visual readout is provided by a Hewlett-Packard variable persistence oscilloscope described in Electronics 3.8

24 66-70 (1965). Simultaneously, the sequential scanner 122 pulses the zero speed sampling gate of set via line 122 so that the signal content of zero speed filter column is fed to the vertical input of scope 136 along line 138. After a delay 140 (equal to &7}, the scan pulse period of the scan pulse generator 1 18), the positive voltage from battery energizes and-gate 142 which thereafter passes the uniform scan pulse train to index horizontal deflection staircase generator 134 and sequential scanner 122. The scan pulse train appears on line 123 and the horizontal staircase output on line 135. The first scan pulse from generator 118 causes staircase generator 134 to output a step voltage of sufficient magnitude to move spot to the +3 knot slot (target moving away from transducer 46) on face of scope 136; simultaneously, the sequential scanner 122 indexs thereby interrogating the +3 knot filter column integrated signal which is fed to the vertical input of scope 136 via line 138. Succeeding scan pulses on line 123 increment staircase generator output voltage on line and index scanner 122 so that all velocity bins intermediate +3 and +42 knots are interrogated and presented in sequence to scope 136. The +42 knot bin is interrogated by a pulse on line 114; upon receipt of the next scan pulse on line 123, staircase generator output 135 drops abruptly so as to present integrated signal from 42 knot bin, simultaneously being sampled by sample pulse on line 116. Thus, all velocity bins are sampled in sequence and in 3 knot increments over and over until such time as maximum range of the transducer 46 system is realized and the search is terminated. At this time, ganged switch 106 is operated so as to zero bank of memory capacitors 104 and disable and-gate 142. Doppler data stored on face of scope 136 is cleared manually and processor awaits the dropping of another transducer 46 to commence another search.

Use of the above described system by ships, either surface or submerged, instead of aircraft, can be readily effected. Referring back to FIG. 7, the sonobuoy 23 may be replaced by an omnidirectional sonar receiving transducer. For receiving transducers moving through the water at the search ships velocity, the system operation will be the same except that the radial component of the search vessels velocity, with respect to the spherical explosive acoustic transducer 46, will cause the transducer 46 transmitted spectrum 25 and ensuing reverberation to be Doppler shifted also and such must be accomodated in the interpretation and presentation of results.

Similiarly, the heretofor described use of the system as a hull resonance tool can be modified, in the manner described now in detail, to effect submarine detection.

Referring back to FiG. 6, in lieu of the linear transducer 20 lowered from vessel 22, a self-contained spherical array transducer 46 is dropped from a surface vessel or aircraft as shown on FIG. 7. Just prior to expected detonations of transducer 46, a record of signals received via receiving

transducers

30 or 23 are stored, preferably in compressed time storage unit 39, read out 41, and subjectedto spectral analysis 44 as previously described in connection with the resonance research tool example.

In operation, the set of harmonically related frequencies are adjusted so that at least one of the narrow band frequencies of the set corresponds to a hull vibrational mode. If frequencies of said modes are known a priori, so much the better. If not, they may be determined in the field by irradiating the suspected waters with a multiplicity of transducers 46, in sequence; each one adjusted so as to isonify the target with all frequencies of possible interest, thereby detecting any appreciable resonances of target 24.

Having thus described out invention, we claim: 1. An underwater sonic detection system comprising means for detonating a predetermined number of explosive charges beneath the water surface to generate a series of a predetermined number of pulses of a shape generally characteristic of underwater explosions, means for electronically initiating the detonation of said explosive charges to produce said pulses in selected time sequence and with a maintained uniformity of pulse repetition period therebetween within predetermined limits set by a selected timejitter parameter 0 so as to produce a water traversable multi-line spectrum made up of a plurality of discrete reflectable, narrow band consecutively sequenced odd and even harmonic sonic frequencies of substantially constant energy content and wherein the magnitude of the energy concentrated within said narrow band relative to the energy contained in the noise continuum attendant thereto is determined by a signal-to-noise parameter a that will be desirably extant at the upper boundary frequency of said multi-line spectrum, each of said discrete harmonically related water traversable sonic frequencies having a band width of l/(N T,), where N is a whole integer equal to said predetermined number of explosive charges and T, is the pulse repetition period intermediate said pulses, and said multi-line spectrum having a fundamental frequency of 1/ T, and component frequencies of f,

of n/T, Hertz where n is an integer that is equal to or greater than zero and equal to or less than the harmonic number of the upper boundary frequency of said multi-line spectrum having a magnitude that is a function of said parameters 0, a, N, T,.

2. The system as set forth in claim 1 including receiver means selectively tunable to said water traversable sonic frequencies for discriminatorially sensing reflections thereof from a target engaged thereby.

3. The system as set forth in claim 2 including means responsive to a frequency shift of the reflected discrete frequencies comprising the multi-line spectrum to sense the relative radial component of velocity of said target engaged thereby.

4. The system as set forth in claim 2 including means to sense the relative radial component of velocity of said target.

5. The system as set forth in claim 1 including receiver means selectively tunable to emanated vibratory frequency modes induced in a target by exposure to said energy spectrum created by said series of explosively shaped pulses.

6. A system as set forth in claim 1 including an expendable underwater transducer a paratus comprising means for positioning a plurality 0 said detonata le explosive charges in predetermined spatial array. v V

7TT lieunderwater transducer apparatus as set forth in claim 6 wherein said charges are disposed in a three-dimensional generally spherical configuration to generate an interference pattern effective to nullify phase shifts introduced because of unavoidable finite spatial displacement of individual charges to thereby create an omnidirectional radiation pattern whose field strength and comb frequency spectrum are independent of the relative angle vis-a-vis a target.

Claims

1. An underwater sonic detection system comprising means for detonating a predetermined number of explosive charges beneath the water surface to generate a series of a predetermined number of pulses of a shape generally characteristic of underwater explosions, means for electronically initiating the detonation of said explosive charges to produce said pulses in selected time sequence and with a maintained uniformity of pulse repetition period therebetween within predetermined limits set by a selected time-jitter parameter 0 so as to produce a water traversable multi-line spectrum made up of a plurality of discrete reflectable, narrow band consecutively sequenced odd and even harmonic sonic frequencies of substantially constant energy content and wherein the magnitude of the energy concentrated within said narrow band relative to the energy contained in the noise continuum attendant thereto is determined by a signal-to-noise parameter Alpha that will be desirably extant at the upper boundary frequency of said multiline spectrum, each of said discrete harmonically related water traversable sonic frequencies having a band width of 1/(N Tr), where N is a whole integer equal to said predetermined number of explosive charges and Tr is the pulse repetition period intermediate said pulses, and said multi-line spectrum having a fundamental frequency of 1/Tr and component frequencies of fn of n/Tr Hertz where n is an integer that is equal to or greater than zero and equal to or less than the harmonic number of the upper boundary frequency of said multi-line spectrum having a magnitude that is a function of said parameters ''''o'''', '''' Alpha '''', N, Tr.

6. A system as set forth in claim 1 including an expendable underwater transducer apparatus comprising means for positioning a plurality of said detonatable explosive charges in predetermined spatial array.

87. The underwater transducer apparatus as set forth in claim 6 wherein said charges are disposed in a three-dimensional generally spherical configuration to generate an interference pattern effective to nullify phase shifts introduced because of unavoidable finite spatial displacement of individual charges to thereby create an omni-directional radiation pattern whose field strength and comb frequency spectrum are independent of the relative angle vis-a-vis a target.