GB2085591A

GB2085591A - Method of Classifying Underwater Objects

Info

Publication number: GB2085591A
Application number: GB8131043A
Authority: GB
Original assignee: Fried Krupp AG
Current assignee: Fried Krupp AG
Priority date: 1980-10-15
Filing date: 1981-10-14
Publication date: 1982-04-28
Also published as: DE3038900A1; FR2492112A1

Abstract

Underwater objects are classified by directing upon them from a surface vessel 10 a beam of sound which excites a located object 19 to resonance. Apparatus in the vessel receives the resulting echo signal and derives from it a spectrum of the received frequencies. Classification is effected by comparison of this spectrum with the known frequency spectra of underwater objects likely to be of interest. <IMAGE>

Description

SPECIFICATION Method of Classifying Underwater Objects The invention relates to a method of classifying underwater objects of the kind set out in the introductory portion of Claim 1.

In a known classifying procedure of this kind, a located underwater object is irradiated by a series of wave trains of constant duration and frequency (e.g. so called CW pulses), which are reflected from the object and received by a receiver. The so-called echo signals are utilized to determine the nature of the object by determining from the entire structure of the echo signals the external form of the object which makes its classification possible.

To achieve by this method an echo structure capable of giving reasonably reliable conclusions in regard to the located object it is necessary to use transmitters and/or receivers of high resolution, which is often not possible or only possible at high cost. Also, owing to the limited resolution capability of the transmitter and/or receiver this method is only applicable to classification of large objects, such as ships' hulls, and not to small barrels, pipe ends or the like.

The object of the invention is to provide a method of the above kind which makes possible with limited technical expenditure an effective classification in particular of small technical underwater objects, such as barrels, pipe ends and the like, and therefore yields traces of such objects when located.

This object is achieved in accordance with the invention in a method of the kind set out in the introduction of claim 1 by the characteristics set out in the latter part of claim 1.

In the method according to the invention, use is made of the fact that the so-called "technical" objects, such as barrels, pipe ends and the like, are distinguished from natural objects of similar dimensions, such as rocks, pebbles and the like, in that they have simple forms, smooth surfaces and generally a constant wall thickness and so have special patterns of natural frequency. In accordance with the invention these "technical" objects are excited by acoustic irradiation from a distance to natural resonance, whereby the natural or resonance frequencies of the objects are selectable from the sound energy reflected from the objects and received by the receiver in accordance with their individual frequency pattern.For example, it has been determined that a thick-walled steel cylinder having a length of 40 cm, a diameter of 40 cm and a wall thickness of 1.7 cm exhibits 26 natural frequencies in the range of 0.8 to 8.3 kHz (Acoustics 1979, volume 42, page 93). If this natural frequency pattern of the underwater object is known, it is possible to derive from the signals received by the receiver a frequency pattern of selected resonance frequencies and from these to determine the nature of the located object. The individual frequency patterns of the normally limited number of objects to be classified can be determined, preferably empirically or also by calculation, and tabulated to afford easy comparison of the natural frequency pattern in the received signals with known and recorded natural frequency patterns of the located underwater objects.

The method according to the invention permits in particular classification of small underwater objects located at the bottom of a river or the sea and embedded between stones or in a coral reef.

The resolution of the transmitter and/or receiver need be no greater than in conventional sonar equipment. The method according to the invention achieves not only classification of a located underwater object but also the tracing of a known underwater object between other objects, e.g. rocks.

An advantageous embodiment of the invention is given in claim 2. The wide band width of the sound beam ensures that the range of natural frequencies of the underwater objects to be classified is fully embraced so that reliable results are achieved. If one is interested only in tracing quite definite underwater objects, e.g. pipe ends, of certain dimensions, it is sufficient to suit the band width to the range of resonant frequencies of the object sought.

An advantageous embodiment of the invention is given in claim 6. This measure affords reliable selection of objects lying relatively close together.

A further advantageous embodiment of the method according to the invention is given in claim 7. By use of a sonar system operating on the known parametric principle broad band width of the sound beam and sharp focussing can be obtained. The sound beam can consist of a pulse sequence (known as interrupted carrier wave CW pulses) in which the individual pulses are modulated at different frequencies, preferably increasing or decreasing monotonically in the pulse sequence. The echo signals from the located underwater objects are received in the intervals between the pulses. It is, however, also possible to transmit the sound beam continuously (often called continuous wave signal), this being composed either of a frequency modulated sine wave signal with monotonic frequency variation or of background noise or pseudo background noise.

The invention will now be further described with reference to an embodiment of apparatus for carrying out the method shown in the drawings, in which: Fig. 1 shows schematically a surface vessel carrying apparatus for classifying underwater objects, and Fig. 2 is a schematic block circuit diagram of an apparatus according to claim 1.

As shown schematically in Fig. 1, a transmitting transducer 11 and a receiving transducer 1 2 are fitted to the keel of a surface vessel 10. The transducer 11 and the transducer 12 are connected by respective electrical connections 15, 1 6 to a transmitter 13 and a receiver 1 4 disposed inside the ship. Fig. 1 shows at 1 7 the bottom of the sea or of a river on which a "technical" object 19, e.g. a barrel, a pipe end or the like, is embedded between pebbles or rocks 1 8. The transducers 11, 12, the transmitter 13 and the receiver 14 constitute the apparatus for carrying out the method of classifying underwater objects, referred to below as the classifying device.

In the method according to the invention of classifying an underwater object, e.g. the "technical" object 1 9 in the form of a barrel or pipe end, the object 1 9 is acoustically irradiated by the classifying device. A sound beam of very wide band width is used. This is produced by two directional sound beams of different frequency from which, owing to the non-linear transmission properties of water, a frequency is derived corresponding to the difference frequency of the two beams. The transmitter 1 3 therefore includes a signal generator 20 supplying electrical energy at frequency f, and a signal generator 21 supplying electrical energy at frequency f2, the frequency f2 being continuously variable.The frequencies f1 and f2 are added in an adding circuit 22 and fed to the transmitting transducer 11 through a gate circuit 23 and an amplifier 24. The gate circuit 23 is controlled by a timing circuit 25, which switches it on for a determined time, so that the transducer 11 transmits a series of wave trains of determined length, also known as a pulse train, and so transmits sound energy of the frequencies f, and f2 in the non-linear medium, water.

Owing to the non-linear transmission properties of the water, there results in known manner a directional sound radiation having a frequency f1-f2, which corresponds to the difference frequency of the two sound beams. By appropriate focussing of the two sound beams the resulting radiation can be so focussed that the surface impinged by sound in the locality of the object 19 does not substantially exceed that of the object 19.

Irradiation by sound in this way excites the object 1 9 to natural resonance. Since the frequency f2 is varied monotonically for each pulse of the train, the beam of sound striking the object 1 9 can be varied over so wide a frequency range that it will certainly cover the range in which the object 1 9 manifests resonance.

The object 19 excited to resonance manifests a characteristic natural frequency pattern, which is determined by the material, the dimensions and the wall thickness of the object 19. If the natural frequency pattern of the object 1 9 is known, classification can take place on the basis of the characteristic resonance frequencies.

The receiver 14 accordingly includes a spectrum analyzer. Such a spectrum analyzer, which operates pn the basis of a fast Fourier transformation algorithm, is described in IEEE Transactions on Instrumentation, No. 4, November 1 971, pages 198-201.

The sound signals emitted by the object 19 are received by the transducer 12, converted into electrical signals and fed by the conductor 1 6 to the receiver 1 4. After passage through an amplifier 27 the electrical signals are applied to the spectrum analyzer 26. This produces a spectrum of the received frequencies in which the resonance frequencies of the object 1 9 can be clearly recognized. This spectrum is rendered visible on a display 28. Assuming the natural frequency pattern of a large number of possible underwater objects is known, comparisons of the frequency pattern shown on the display with the available known frequency patterns enables the object 1 9 to be classified as a barrel or pipe end of determined dimensions.As in practice a limited number of interesting objects is required to be classified, it is possible to determine the frequency patterns of the interesting objects empirically or by calculation and to tabulate them.

By successive comparison of the frequency pattern of the located object 1 9 shown on the display with all tabulated frequency patterns the nature of the object 1 9 can be specified.

Instead of direct comparison by an operator of the received frequency pattern with known patterns, the receiver 14 may include a computer 29 and a store 30, in which the frequency patterns of all known interesting objects are stored. The spectrum of the received signal obtained in the spectrum analyzer 26 is fed to the computer 29, to which the frequency patterns in the store 30 are applied in succession and compared with that from the spectrum analyzer 26. Upon coincidence a signal is given.

The invention is not limited to the abovedescribed embodiment of a method of classifying underwater objects. Thus the sound beam can also be frequency modulated, the frequency being varied monotonically. The sound beam can also be formed of background noise or pseudo background noise.

It is also not essential, in the apparatus for carrying out the method, to use separate transmitting and receiving transducers. These may be combined in a single transducer, the conductors 1 5 and 1 6 being connected to the transducer through an electrical switch.

Claims

1. A method of classifying an underwater object, in which the object is subjected to acoustic irradiation and determined by the reception of the sound signals generated by the object, characterised in that the object is so irradiated as to excite it to its natural resonance and determination of the object is undertaken on the basis of its characteristic resonance frequencies.

2. A method according to claim 1, characterised in that the object is irradiated by acoustic irradiation of wide band width and preferably by beamed irradiation.

3. A method according to claim 2, characterised in that the acoustic irradiation consists of pulse trains, successive pulses of which are modulated by different frequencies, preferably by monotonically varying frequencies.

4. A method according to claim 2, characterised in that the acoustic irradiation is frequency modulated and in that the frequency is preferably varied monotonically and continuously.

5. A method according to claim 2, characterised in that the acoustic irradiation consists of background noise or pseudo background noise.

6. A method according to one of claims 1 to 5, characterised in that the acoustic irradiation is so focussed that its dimensions in the region of the impinged surface of the object do not exceed those of the object itself.

7. A method according to one of claims 1 to 6, characterised in that the acoustic irradiation is produced by two directional sound beams of different frequency so that, owing to the nonlinear transmission properties of water, the acoustic irradiation proceeds at a frequency corresponding to the difference frequency of the two sound beams and there is frequency wobbling in at least one sound beam.