WO1992011546A1

WO1992011546A1 - Transmission-reception device of acoustic signals for marine environment exploration

Info

Publication number: WO1992011546A1
Application number: PCT/IT1991/000112
Authority: WO
Inventors: Giovanni Bosco Cannelli; Enrico D'ottavi
Original assignee: Consiglio Nazionale Delle Ricerche
Priority date: 1990-12-21
Filing date: 1991-12-18
Publication date: 1992-07-09
Also published as: IT9048608A1; AU9157691A; IT9048608A0; IT1241558B

Abstract

A device is described for the transmission and reception of acoustic signals, particularly for submarine exploration, comprising a group of electroacoustic transducers (17) arranged around a reception electroacoustic transducer (18). All transducers are of a paraboloidal reflector type and are arranged with their axes parallel to each other and having their openings on the same plane orthogonal to these axes. Each of the transmission transducers has a pair of electrodes (30), disposed near the focus, connected to an electrostatic energy supplier (3), in order to simultaneously receive from it a predetermined energy amount, which is different for each transducer, so as to generate an electric discharge and, consequently, an acoustic pulse. The reception transducer has a piezoelectric detector near its focus which is connected to circuits and devices (5) for processing the received signals.

Description

Transmission-reception device of acoustic signals for marine environment exploration

Technical field

The present invention is concerned with acoustic techniques for submarine exploration, for submarine

communications at great distances, and for geophysical prospecting of the soil at low depths, and more particularly, it deals with a transmitting-receiving high resolution device useful in applications regarding the aforementioned

techniques.

The conventional acoustic SONAR-type devices are able to deliver "acoustic images" of the sea bottom surface with sufficient resolution properties, but they really are not suited for what concerns their capability of penetrating below the sea-bottom or transmitting underwater acoustic signals at great distances.

Background Art

In the last years some progress has been made in the field of high-power and high resolution acoustic transducers for the exploration of the subsoil and sea-bottom. For example, in the Italian patent No. 1.178.206 granted to the applicant on the day 9 Sept. 1987 (inventors: G.B. CANNELLI, E.D'OTTAVI e S. SANTOBONI), corresponding to US patent no.

4.734.894, an electroacoustical transducer is disclosed, which comprises a paraboloidal metallic reflector having a pair of electrodes near its focus, and an electrostatic energy generator, comprising capacitors connected to the electrodes in order to cause therebetween an electric discharge, so as to generate, in such a way, an acoustic wave in the medium which contacts the transducer.

With such a device one obtains high-power impulsive acoustic signals having good directivity, whose frequency and power distinctive features may be varied appreciably by varying the capacitance of the capacitors of the generator and the location of the electrodes inside the reflector.

In a subsequent Italian patent No. 1.211.963 granted to the applicant on 8 Nov. 1989 (inventor: G.B.CANNELLI and E.D'OTTAVI), corresponding to US patent no. 4.899.845, a device is disclosed for the transmission and reception of acoustic signals, comprising a transmitting portion,

constituted by an electro-acoustic transducer of the type disclosed in the aforementioned patent No. 1.178.206, and a receiving portion, constituted by an apparatus having a paraboloidal reflector similar to the transmission one and a piezoelectric detector near the reflector's focus, said last mentioned receiving portion further comprising means for adjusting the position of the detector in the point of

optimum focusing of the received acoustic wave.

Such a device is characterized by a good directivity and by a good capability of adapting itself to different structures of the soil to be explored, it has however a restricted vertical resolution, due to the circumstance that the signal produced by the transmission paraboloid is affected by the cavitation phenomenon, which, as known, considerably increases the duration of the acoustic

prospecting signal, and by a horizontal resolution which, in many applications, is considered to be insufficient.

To minimize the cavitation pulse effect, in the Italian patent application No. 48370-A/89 filed by the applicant on 15 Sept. 1989 (inventors: G.B.CANNELLI and E.D'OTTAVI), corresponding to PCT/IT90/00075, it is proposed to employ a plurality of transducers, each of them being supplied with a different amount of electrostatic energy and excited

simultaneously. With this method one obtains an acoustic signal having a primary pulse, given by the sum of the simultaneous primary pulses of each transducer, and a series of cavitation pulses, each due to one of the transducers, which are time-offset due to the dependence from the

emission power of the delay between primary pulse and

cavitation pulse, so that the ratio between the amplitude of the total primary pulse and the amplitude of the cavitation pulse is great, and this allows a better vertical resolution.

It has also been proposed, as results from the paper presented in the "18th International Symposium on Acoustical Imaging" on September 1989 by G.B.Cannelli and E.D'Ottavi, entitled "Tuned Array of Paraboloidal Transducers for high-resolution marine prospecting", to dispose the transducers around a circle, that is with the axes of the paraboloidal reflectors parallel to each other and passing through a circumference on a plane perpendicular to the axes, for experimentally demonstrating the effectiveness of a group of tuned transducers according to the method described in the aforementioned patent application No. 48370-A/89. In that paper, however, no practical application is described for the disclosed transmission structure.

Disclosure of the invention

It is an object of the present invention, to provide a device for transmitting and receiving acoustic signals, which, by using the teachings of the known aforementioned techniques, concerning the structure of the transmission transducers and their different excitation, will have better performance than the ecograph disclosed in the aforementioned patent No. 1.211.963, not only with respect to vertical resolution, but also to horizontal resolution.

Another object is to realize a device for transmitting and receiving acoustic signals, capable of generating a high- intensity acoustic wave, and having a high signal to noise ratio.

Still another object is to realize a device for transmitting and receiving acoustic signals, which is capable of obtaining "acoustic images" of the structures present in the propagation medium of the transmitted acoustic signals.

Brief description of the drawings

These objects are attained by providing a device as defined and characterized in the claims at the end of the present description, such device being described in detail in the following, in relation to an illustrative embodiment thereof, which is in no way restrictive, reference being made to the annexed drawings, in which:

Fig. 1 is a schematic view of one embodiment of the device according to the invention,

Fig. 2 is an useful plot for illustrating a

mathematical representation of the radiation of an acoustic radiator,

Fig. 3a is an useful plot for illustrating a mathematical representation of the impulsive response of an ecographic system of the type disclosed in aforementioned patent No.

1.211.963.

Fig. 3b is a graph which represents the relative

amplitude, as a function of the relative horizontal distance, of a received signal, according to the plot of fig. 3a,

Fig. 4a is a plot, similar to that of fig. 3a of a device according to the present invention,

Fig. 4b is a graph representing the relative amplitude, as a function of the relative horizontal distance, of a signal received according to the plot of Fig. 4a,

Figs. 5a and 5b are graphs representing the relative amplitude as a function of time, of a received signal, by an echographic system of the type described in aforementioned patent No. 1.211.963 and by a device according to the

invention, respectively, and

Fig. 6 is a longitudinal sectional view of an elctrode supporting arrangement for an electroacustical transmission transducer utilized in a preferred embodiment of the

invention. Best mode for carrying out the invention

In the preferred embodiment of the invention,

represented schematically in Fig. 1, there are shown

essentially four different parts of the device: an

electroacoustic and mechanical system for transmission and reception, indicated generally by reference 1, an

electrostatic energy supplier 3 with an electronically controlled transmission, electronic reception circuits 5, and a signal processing system 7. The transmitting-receiving system is dipped in water, for example for prospecting the sea-bottom, and it is hold by appropriate mechanical means (not shown), at an average predetermined distance from the sea surface 9.

In Fig. 1, line 11 represents the sea-bottom, and lines 13 and 15 the limiting surfaces between layers of different acoustic impedance below the sea-bottom.

The transmitting-receiving system 1 is constituted by an array of transmission transducers 17 and by a reception transducer 18, mounted on a supporting frame, which is not represented in order to simplify the drawing. The transducer array 17 comprises nine electroacoustic transducers having a paraboloidal reflector, arranged so as to have their axes parallel to each other and the openings of the reflectors on the same plane orthogonal to these axes, the centres of said openings being located at the vertices of a regular polygon whose center C is on the axis of symmetry of the system.

Each transducer 17 has a pair of electrodes 20, arranged near the focus of the relevant transducer, and it is substantially built as described in the aforementioned patents, but it comprises, in a preferred embodiment,

modified electrodes so as to obtain a greater useful power of the transmitted signal, as will be described in detail in the following, reference being made to Fig. 6.

The electrode pairs of the nine transducers 17 are connected in parallel by appropriate cables 19, to the supplier 3, which comprises, as described in detail in patent application No. 48370-A/89, groups of capacitors connected to the electrode pairs and dimensioned so as to supply every transducer with a different electrostatic energy amount, the supplier 3 further comprises electronic control circuits to simultaneously induce an electric discharge between the electrodes of each electrode pair.

The electric discharges of different intensity generate, on the other hand, acoustic pulses which last for a very short period of time and which preferentially propagate in water in a direction parallel to the reflector axis. In Fig. 1, the directions of propagation are shown as to be

convergent to the central axis of the device, but this is done only for a more intuitive representation of the axial optimization of the acoustic emission.

The reception transducer 18 is mounted on the same supporting frame to which are fixed the transmission

transducers, by means of fixing and acoustic coupling means, not shown, so as to avoid crosstalk phenomena.

It is built in a similar manner to the reception

transducer disclosed in patent No. 1.211.963, that is, it comprises a parabolic reflector with a piezoelectric detector 21 arranged near the focus. According to the invention, this transudcer is mounted in the region bounded by the transmission transducers, and, more precisely, with its axis along the symmetry axis of the array of transmission

transducers 17, its opening being located on the same plane as that of the transmission transducers 17.

Detector 21 is connected by cable 23 to the electronic reception circuits 5 which comprise filters and an analog-to -digital converter, indicated generally by 25, which are not described in detail, since they are known to those skilled in electronic design.

The output of the filter and analog-to-digital converter cirucits 25, which carries numeric signals, is applied to the input of the signal processing system 7. The signal is processed preliminarly by a "dedicated" processor DSP and then sent to a PC-programmed computer for general purposes, to realize graphical representations in two and three dimensions and in order to represent on a screen DY, in the form of "acoustic images", the received signals

corresponding to layers or objects of different density which are on the sea-bottom or below it.

In a practical realization of the device, the nine reflectors of each transmission transducer had an internal height of 21 cm., an internal diameter of the opening of 20 cm., and a focal length of 1.2 cm., and the reflector of the reception transducer had an internal height of 52.1 cm., an internal diameter of the opening of 50 cm., and a focal length of 3 cm .

The circumference containing the centres of the openings of the transmission reflectors had a radius of 45 cm.

Obviously, these dimensions may vary in a remarkable way according to the different kinds of use of the device. For example, the number of transmission transducers is chosen in such a way that the ratio between the total primary signal and the cavitation signal will be great enough to allow a safe identification of the useful signal in every practical case. In the embodiment of the invention described above, good results have been obtained with a ratio equal to 6:1. However, in some application, also lower ratios may be acceptable, for example as low as 3:1, and in other

applications also ratios higher than 6:1 may be advantageous.

The transducer dimensions are chosen on the basis of the frequency transmission and reception bandwiths of the

relevant acoustic waves in every particular application.

In the above described example, the opening of the transmission reflectors was chosen so as to be comparable with the wavelength approximately corresponding to the central frequency (f=7500 Hz) of the transmission band (0,1 - 15 kHz), that is λ = 0.20 m, and the opening of the

reception reflector was chosen greater than that of the transmission reflector, so as to be able to receive also signals characterized by relatively lower frequencies than the central transmission one, in this case the opening corresponds to a frequency of 3000 Hz. Generally it is useful that the opening of the reception transducer is as large as possible, taking advantage of all space delimited by the transmission transducers. Furthermore, the distance between the adjacent transmission transducers and their number are chosen in such a way as to minimize the effect of false signals which are emitted laterally, known as "grating lobes".

In the following a technique will be briefly described developed by the authors of the present invention, to demonstrate why and to what extent the device of the invention allows a lateral resolution greater than that of known device of the type disclosed in patent No. 1.211.963.

This technique is based on the concept of "impulsive response" of an ecographic system. This concept may be illustrated starting from the simple plot of Fig. 2

representing a circular transducer resembling to an ideal piston of radius d, vibrating in an infinitely rigid screen.

The acoustic pressure produced by the system in a point

Q (x₁, y₁, z₁) in a distant region may be obtained as the time-derivative of the convolution of the displacement velocity V(t) of the acoustic wave with a function h (Q.t) known as "impulsive response of the system":

The function h (Q,t) is directly connected to the

"directivity function" of the system and therefore its determination allows to give an estimation of the lateral resolution of the same. In the more involved case of a system constituted by two distinct transducers, a transmission and a reception

transducer, it is possible to make the simplifying assumption of an excitation by a monochromatic continuous wave, without loss of generality for the results of the comparison between the invention and the prior art.

Consider first the case, represented in Fig. 3a, of two paraboloidal transducers, one for the transmission (T) and the other for the reception (R). both resembling to a first approximation, to two ideal pistons of the same radius d. located to a distance x_o from the origin of a cartesian coordinate system (x, y, z). This case corresponds to the ecographic system described in aforementioned patent No.

1.211.963.

The impulsive response of this system may be obtained by takinq account of the fact that it is proportional to the distribution function of the transmission transducer times the directivity function of the reception transducer (see the paper by G. B. CANNELLI, E.D'OTTAVI and L.PITOLLI "Role of the impulsive response of ecographic acoustical wave systems", Rivista It. di Acustica, 2, pages 33-41, 1987):

where C is a constant, λ is the wavelength, J₁ is the first kind and first order Bessel function, r₁, r₂ and u₁, u ₂ have the meaning shown in Fig. 3a.

The computation of the function h in equation (2) has been done in the standard conditions given by y=0, z=4m and with the following values of the other parameters: d=0.25m,

2x_o=3m, λ=0.4m obtaining the curve given in Fig. 3b which shows along the ordinate the relative amplitude AR of the acoustic signal and along the abscissa the relative

horizontal distance DOR. This curve has the following physical meaning: it represents the behaviour of the peak value of the signal, which is received at the output of the reception transducer, which would be obtained by placing a point scatterer at the point Q_o (0,0,4) and by making a scanning along the x-axis, the distance 2x_o between the centres of both transducers being held constant. From the same curve the relative value of the lateral resolution R₁ of the system may be determined, as the distance along the abscissa between the two points having their ordinate equal to one half times the maximum value of the curve. One thus obtains R₁ = 0.65.

Consider now the case of an array of paraboloidal transmission transducers (T_A) and one paraboloidal reception transducer (R_A), placed at the centre of the array T_A as shown in Fig. 4a, which illustrates in a schematic way the device of the invention shown in Fig. 1.

Making the simplifying assumption that the transducers resemble to two ideal pistons and by using a relation similar to (2) in which the parameters relative to the different openings of the transducers d₁ = 0.25m and d₂ = 0.45m and to their mutual distance (x_o = 0) have been introduced, the curve of Fig. 4b is obtained. Also in this case, the curve has been obtained for y=0, z=4m and λ=4m. As in the

foregoing case, the lateral resolution can be determined, which in this case is equal to R₁ = 0.39. It is therefore evident, that the device of the present invention has a better lateral resolution, i.e., it is capable of

distinguishing objects placed on a horizontal plane at a closer distance, in a better manner than the known ecographic system disclosed in patent No. 1.211.962.

It has also been found that the device of the invention has a better vertical resolution than that which would have been expected by the application of the method described in the patent application No. 48370-A/89. This effect seems to be mainly due to the capability of the system to discriminate the received signals by promoting and enhancing those

arriving from "axial" directions, that is those characterized by a shorter duration, being less influenced by the

phenomenon of acoustic cavitation.

For what concerns the signal/noise ratio, the device according to the invention, due to the directivity of the transmission transducers and the reception transducers is substantially insensitive to noise sources which act in a transversal direction to its axis.

Industrial applicability

In particular, in the case of its application for submarine exploration, it is less sensitive than the known devices to noise produced by acoustic waves reflected first by the sea-bottom and then by the interface water-air, and to noise produced by scattering of acoustic waves caused by slight unevenesses of the sea bottom, which contribute in limiting the capability of distinguishing acoustic signals arriving from points at the sea-bottom placed at near horizontal distance to each other.

Finally, the result of a test is shown (Figg. 5a, 5b) which makes a comparison between the response of the

ecographic system already known (Fig. 5a) with that of the system of the present invention (Fig. 5b).

The experiment has been carried out in a sea basin having a depth of about 5m, with the same experimental conditions and an electrostatic shooting energy of about 1000 joule. Both diagrams representing the relative amplitude AR as a function of time, show a first signal corresponding to the transmission pulse and a second signal corresponding to the reflected pulse which carries the information relative the sea-bottom, located approximately at time t=6.5 ms, detected on the respective receivers.

By comparison one clearly notices that the known

ecographic system is not able to distinguish the signal reflected by the sea-bottom from the other signals partly due to the phenomenon of acoustic cavitation and to different underwater environment noises on the other hand.

Instead, the system of the present invention clearly shows a reflected signal from the sea bottom after a time of about 6.5 ms which clearly corresponds to the time it takes for an acoustic wave to propagate at a speed of about 1500 m/s in both directions along the known path which had a length of 10 meters. One notices also, that the prospecting signal has properties of duration, amplitude and content in high-frequency harmonic components, such as to insure a remarkable improvement of the horizontal and vertical resolution of the system according to the invention with respect to the known one. In order to obtain the maximum efficiency of the transmission transducers, that is, a greater intensity of the acoustic wave emitted from a paraboloidal reflector, according to a preferred embodiment of the invention, the electrodes, instead of being located transversally with respect to the axis of the paraboloid, as described in the above cited patents, are directed along its axis, so as to minimize the shielding effect due to their size and their supporting structure.

More particularly, as shown in Fig. 6, the electrodes, indicated by 30, are located one above the other along the axis of the electrode supporting structure, which

coincides with the axis of the parabolic reflector (not shown). The lower electrode is held in position by a

relatively thin U-shaped electric conductor 32, so that, between the point of occurrence of the electric discharge and the inner halls of the reflector there are practically no obstacles. Even if only one particular embodiment of the present invention has been described, it is obvious that many modifications or variants may be realized without departing from the scope of the inventive concept, as defined in its essential features, in the first claim. For example, the reflectors of the transmission transducers, instead of being all equal and supplied with different electrostatic energy amounts, could be different from each other and supplied with the same energy, that is dimensioned so as to emit acoustic pulses of different intensity, although they are supplied with the same electrostatic energy supply. In this case it is convenient that the opening of the reception transducer is at least equal to the greatest opening of the greatest

transmission transducers.

Claims

1. Device for the transmission and reception of acoustic signals in a propagation medium, comprising:

- a supporting frame,

- a plurality of electroacoustic transmission transducers, fixed to the supporting frame, each comprising a

paraboloidal reflector having, near its focus, a pair of electrodes,

- an electroacoustical reception transducer,

- electrostatic energy supply means, electrically

connected to the pairs of electrodes of the paraboloidal reflectors of the transmission transducers, capable of supplying simultaneously to each pair of electrodes a predetermined amount of electrostatic energy, so as to cause between said electrodes an electric discharge, and generating in this way an acoustic pulse in the

propagation medium, the intensity of the acoustic pulse being different for every transmission transducer, and

- electric reception circuits electrically connected to the electroacoustic reception transducer, characterized in that

- the transmission transducers are disposed with the

axes of the respective parabolic reflectors

substantially parallel to each other, with the openings of said reflectors on the same plane substantially orthogonal to the axis of the reflectors, the centres of the openings being at the vertices of a polygon, and

- that the reception transducer comprises a paraboloidal reflector, having a piezoelectric detector near its focus, disposed in a fixed position with respect to the reflectors of the transmission transducers, with the axis of its reflector substantially parallel to the axes of the transmission reflectors, its opening being located substantially on the same plane of said

transmission reflector openings and the centre of its opening being inside said polygon.

2. Device according to claim 1, wherein the transmission transducers are substantially equal to each other, the polygon is a regular polygon, and the axis of the reception reflector passes through the centre of the polygon, the electroacoustic energy amount supplied from the supply means to the transmission transducers is different for every transducer.

3. Device according to claim 1 or 2, wherein the number of transmission transducers is such that the ratio between the total primary signal, given by the constructive contribution of the primary signals of the single transducers, and the cavitation signal, is at least 2:1.

4. Device according to claim 3, wherein the number of

transmission transducers is such that the ratio between the total signal and the cavitation signal is 6:1.

5. Device according to any of the preceding claims, wherein the opening of the parabolic reception reflector is at least equal to that of the greatest among the transmission

parabolic reflectors.

6. Device according to any of the preceding claims, wherein the distance among adjacent transmission reflectors is lower than the distance at which the effects of false signals emitted laterally, known as "grating lobes", will noticeably interfere with the principal acoustic signal emitted by adjacent transducers.

7. Device according to any of the preceding claims, wherein the reception circuits comprise electronic circuits for the analog-to-digital conversion of the received signals, and devices for the processing of the digital signals, capable of representing the informations relating to the received signals in the form of "acoustic images".

8. Device according to any of the preceding claims, wherein the paraboloidal reception transducer is fixed to the

supporting frame by means of acoustic coupling means.

9. Device according to any of the preceding claims, wherein the electrodes of each paraboloidal transducer are located one above the other along the axis of the paraboloidal transducer and are held in position by an electrode

supporting structure whose lower portion is a thin U-shaped electric conductor, thereby maximizing the space between the point of occurrence of the electric discharge and the inner walls of said paraboloidal transducer.