The present invention relates to a device for emitting acoustic pulses, particularly suitable for use in echometric apparatus, for example in echometric apparatus used for marine and inland- water exploration.

As is known, an echometric apparatus for marine and inland-water exploration generally comprises a device adapted to generate acoustic pulses that propagate through water to strike the sea-floor, penetrating the underlying layers to variable depths depending on their permeability and on the characteristics of the acoustic signal.

In this way, it is possible to obtain an “acoustic mapping” of the sea-floor, enabling study of its composition and structure, for example in order to locate possible buried deposits of hydrocarbons.

Devices for emitting acoustic pulses, commonly referred to as “sparkers”, have been known for some time. These devices comprise one or more pairs of electrodes adapted to be immersed in water and electrically connected to an electric power source.

When using these devices, the electrical power source provides electrical energy to the aforementioned electrodes so as to induce an electrical discharge between them and the consequent transformation into a plasma state (a so-called “plasma bubble”) of water in a volume in the proximity of the aforementioned electrodes.

The formation and subsequent sudden collapse or implosion of this “plasma bubble” generate a pulse acoustic wave that propagates through water to the sea-floor and is reflected or refracted by the layers therein.

“Sparker” type devices currently available do not allow controlling in a satisfactory manner the wave form (pulse duration and amplitude) and the direction of propagation of the acoustic pulses emitted. This is reflected in the relatively poor quality (for example with vertical resolution values much higher than 1 m) of the “acoustic images” of the sea-floor that can be obtained from the reflection and reception of the acoustic pulses transmitted.

The main task of the present invention is to provide a device for emitting acoustic pulses for echometric apparatus that makes it possible to overcome the drawbacks of known art as described above.

In the context of this task, an object of the present invention is to provide a device for emitting acoustic pulses capable of emitting acoustic pulses of high quality in terms of duration and amplitude. A further object of the invention is to provide a device for emitting acoustic pulses capable of emitting acoustic pulses with an easily controllable direction of propagation.

Another object of the invention is to provide a device for emitting acoustic pulses with high power, for example in the order of tens of KJ.

Yet another object of the invention is to provide a device for emitting acoustic pulses with relatively simple installation in the field and easy practical use.

A further object of the present invention is to provide a device for emitting acoustic pulses that is easy and economical to produce on an industrial scale.

This task and these objects, together with other objects that will become evident from the following description and accompanying drawings, are achieved, according to the invention, by a device for emitting acoustic pulses, according to claim 1 and the related dependent claims set out below.

In a general definition, the device for emitting acoustic pulses, according to the invention, comprises an enclosing structure having at least one reversibly deformable wall (preferably elastically deformable) and adapted to be put in contact with a liquid of propagation of the acoustic pulses (sea water, for example), which is external to said enclosing structure.

The enclosing structure is arranged so as to define, at least partially, an internal volume of the device confined with respect to said liquid of propagation of the acoustic pulses.

Said internal volume is filled, at least partially, with a conductive liquid having an electrical conductivity lower than 5 mS/cm, at a temperature of 20 °C.

The device, according to the invention, also comprises an assembly for generating the acoustic pulses comprising at least a pair of electrodes, including a first electrode (or anode electrode) and a second electrode (or cathode electrode) spaced one from another.

Said first and second electrodes are arranged so as to be in contact with said conductive liquid and spaced from one another. The separation space between said first and second electrodes is thus occupied by said conductive liquid.

According to the invention, said first and second electrodes are electrically connectable to a power source and are adapted to receive electrical energy (a pulse of current, for example) from said power source so that an electric discharge is generated between said first and second electrodes and, consequently, an acoustic pulse is generated. This acoustic pulse is transmitted from said conductive liquid to said liquid of propagation of the acoustic pulses through the deformable wall of the enclosing structure.

Preferably, said conductive liquid has an electrical conductivity equal to 3 mS/cm, at a temperature of 20 °C. Preferably, said conductive liquid is a solution of water and sodium-chloride, with a concentration of sodium-chloride lower than 10 g/litre.

Preferably, said conductive liquid is a solution of water and sodium-chloride, with a concentration of sodium-chloride lower than 5 g/litre.

Preferably, in said assembly for generating the acoustic pulses, at least one of said first and second electrodes is made of copper.

Preferably, in said assembly for generating the acoustic pulses, said second electrode (or cathode electrode) has an enlarged shape with respect to said first electrode (or anode electrode).

According to a preferred embodiment, said second electrode (or cathode electrode) has the shape of a paraboloid of revolution, advantageously with the concavity turned towards the deformable wall of said enclosing structure. Said first electrode (or anode electrode) has an elongated shape and is arranged along the main axis of symmetry of said paraboloid. Preferably, the second electrode shaped as a paraboloid of revolution has its focal point positioned along the first electrode having a rectilinear shape, between the ends of said first electrode.

According to this embodiment, the enclosing structure comprises a deformable sheet made of plastic material coupled to the second electrode at a free edge of said paraboloid, so as to define, in cooperation with said second electrode, an internal volume for containing said conductive liquid.

In another aspect, the present invention relates to an echometric apparatus comprising the device for emitting acoustic pulses, according to the invention.

Preferably, said echometric apparatus is an echometric apparatus for marine and inland-water exploration.

Further characteristics and advantages of the device for emitting acoustic pulses, according to the invention, will be better understood by referring to the following description and the accompanying drawings, provided by way of non-limiting example, wherein: figure 1 shows a schematic view of the device for emitting acoustic pulses, according to the invention; and figures 2-3 show a schematic view of an embodiment of the device for emitting acoustic pulses, according to the invention; and figure 4 shows a schematic view of some experimental measurements relating to operation of the device for emitting acoustic pulses, according to the invention, in the embodiment shown in figures 2-3. With reference to the aforementioned figures, the present invention relates to a device 1 for emitting acoustic pulses.

The device 1 is particularly suitable for use in echometric apparatus designed for prospecting on the seabed or underwater in general, and will be described with particular reference to this type of application for obvious reasons of brevity and, in any case, without limiting the scope of application.

The device 1 can, in fact, be used in any type of echometric apparatus, for example in echometric apparatus for industrial, military, and medical applications, and so on.

With reference to the aforementioned figures, the device 1 comprises preferably a support structure 4 adapted to support and hold in position several components of the device. According to certain embodiments of the invention, for example when the device 1 is intended to be used in an echometric apparatus for marine and inland-water exploration, the support structure 4 may advantageously comprise or be connected to one or more floating elements 40 or similar.

According to the invention, the device 1 comprises an acoustic pulse generating assembly 3 with at least one pair of electrodes, including a first electrode 31 and a second electrode 32 spaced one from another.

When the device 1 is in operation, the first electrode 31 (or anode electrode) is advantageously set at a positive electric voltage while the second electrode 32 (or cathode electrode) is earthed or set at an analogous reference voltage (e.g. a negative reference voltage).

Preferably, the acoustic pulse generating assembly 3 is operatively connected to the support structure 4, for example by being equipped with appropriate elements (not illustrated) for mechanical connection and electrical insulation (not illustrated).

In principle, the first and second electrodes 31, 32 can be of any shape, depending on needs. Preferably, the second electrode 32 has an enlarged shape with respect to the first electrode 31, for example as in the preferred configuration shown in figures 2-3.

Preferably, the first electrode 31 has an elongated shape with a substantially punctiform transversal size (i.e. greatly reduced or negligible with respect to the length).

According to the invention, the device 1 comprises an enclosing structure 2 which can be operatively connected to the support structure 4 and/or to the pulse generating assembly 3.

The enclosing structure 2 comprises at least one reversibly mechanically deformable wall 21 and adapted to be put in contact with a liquid of propagation (not illustrated) of the acoustic pulses, which is external to said enclosing structure. When used in an echometric apparatus for marine and inland-water exploration, the enclosing structure 2 may be at least partially immersed, so that the deformable wall 21 is in contact with the sea water or fresh water (which in this case constitutes the liquid of propagation of the acoustic pulses).

Preferably, the deformable wall 21 of the enclosing structure 2 is constructed so as to be reversibly mechanically deformable (elastically deformable, for example) with respect to a pressure wave coming from inside the device 1.

Preferably, at least the deformable wall 21 of the enclosing structure 2 is made of an electrically insulating material.

Preferably, in particular when the device 1 is intended for use in an echometric apparatus for marine and inland-water exploration, the enclosing structure 2 is advantageously made of an impermeable material, at least around the parts intended to come into contact with the water.

In general, the enclosing structure 2 is adapted to define, at least partially, an internal volume 20 of the device confined with respect to the liquid of propagation of the acoustic pulses.

In principle, the enclosing structure 2 can have any shape, depending on needs.

As illustrated better below, according to some embodiments of the invention (figure 2), the enclosing structure 2 defines the internal volume 20 in cooperation with parts of the pulse generating assembly 3, in particular with the second electrode 32 (cathode electrode) thereof. According to other embodiments of the invention (not illustrated), the enclosing structure 2 consists of a self-standing container or defines the internal volume 20 in cooperation with (electrically insulating) parts of the support structure 4.

According to the invention, the internal volume 20 of the device (defined, at least in part, by the enclosing structure 20) is filled, at least partially, with a conductive liquid 200 having controlled electrical conductivity.

According to the invention, the first and second electrode 31, 32 are arranged so as to be in contact with the conductive liquid 200 and so that the separation space between said electrodes is occupied by said conductive liquid. In this way, the conductive liquid 20 is subjected to an electric field between the electrodes 31, 32, when an electrical voltage differential is applied to them.

According to certain embodiments of the invention (figure 1), the electrodes 31, 32 are arranged inside the internal volume 20 so as to be immersed in the conductive liquid 200. According to certain embodiments of the invention (figure 2), the first electrode 31 (anode electrode) is advantageously arranged inside the internal volume 20 so as to be immersed in the conductive liquid 200, while the second electrode 32 (cathode electrode) delimits the internal volume 20 in cooperation with the enclosing structure 2, and is therefore put in contact with the conductive liquid.

Preferably, at least one of said first and second electrodes 31, 32 is made of copper.

According to other preferred embodiments, both electrodes 31, 32 are made of copper. According to the invention, the conductive liquid 200 has an electrical conductivity lower than 5 mS/cm, at a reference temperature of 20 °C.

Preferably, the conductive liquid 200 has an electrical conductivity equal to 3 mS/cm, at a temperature of 20 °C.

Preferably, the conductive liquid 200 is a solution of water and sodium-chloride, with a concentration of sodium-chloride lower than 10 g/litre.

Even more preferably, the conductive liquid 200 is a solution of water and sodium-chloride, with a concentration of sodium-chloride lower than 5 g/litre.

According to the invention, the electrodes 31, 32 of the acoustic pulse generating assembly 3 are electrically connectable to an electrical power source 6.

The electric power source 6 is adapted to provide a pulsed quantity of electrical energy (current pulse) to the electrodes 31, 32 of the device 1.

When said electrodes receive voltage from the power source 6, an electrical discharge is generated between the electrodes and, consequently, an acoustic pulse is generated, which is transmitted from the conductive liquid 20 to said liquid of propagation of the acoustic pulses through the deformable wall 21 of the enclosing structure 2.

Said electrical discharge causes the formation of a region with a plasma state in the conductive liquid 200 around the electrodes 31, 32.

This region of the conductive liquid with a plasma state expands during the electrical discharge and suddenly collapses when the electrical discharge ceases at the end of the pulse of electrical energy provided by the power source 6.

The process of formation, expansion and subsequent collapse of this region of the conductive liquid with a plasma state generates an acoustic pulse which is transmitted to the liquid of propagation of the acoustic pulses through the deformable wall 21.

Preferably, the electric power source 6 comprises a positive terminal T1 electrically connectable to the first electrode 31 (anode electrode) and a negative terminal T2 electrically connectable to the second electrode 32 (cathode electrode).

In principle, the power source 6 is not included in the device 1. It can be advantageously located on the site where the device 1 is used to ensure its operation.

According to certain embodiments of the invention, however, the power source 6 could be part of the device 1 and transportable therewith.

In general, the power source 6 can be of a capacitive type. For example, it may comprise a bank of power capacitors.

Figures 2-3 show a preferred embodiment of the device 1 for emitting acoustic pulses, according to the invention.

According to this embodiment, shown in figure 2, the first electrode 31 (anode electrode) consists of an elongated electrically conductive element (preferably made of copper or tungsten or another metal with high electrical conductivity and high resistance to corrosion), for example a cylindrical conductive element with substantially punctiform bases (i.e. with dimensions that are very small or negligible with respect to the length of the cylindrical element).

The first electrode 31 comprises a first free end 311 and a second end 312, opposite said first end 311 and electrically connectable to the positive terminal T1 of the power source 6. Advantageously, the acoustic pulse generating assembly 3 comprises a hollow insulating element 33, also with a substantially cylindrical shape and joined coaxially to the first electrode 31 so as to cover it along at least part of its length and insulate it electrically from the second electrode 32.

The second electrode 32 (cathode electrode) consists of a conductive element (preferably made of copper) with the shape of a paraboloid of revolution, with a vertex P, a focal point F and an axis of symmetry A passing through the vertex P and focal point F.

The second electrode 32 comprises a coupling edge 320 which delimits its concavity in a position distal to the vertex P. This coupling edge is mechanically connected to the support structure 4 by means of suitable connecting means (electrically insulating mechanical connectors, for example).

The second electrode 32 is electrically connected directly to the terminal T2 (negative or earth, for example) of the power source 6 by means of a known type of electrical connecting element (an electrical clamp, for example), advantageously passing, at least partially, through the insulating element 33.

According to the embodiment shown in figure 2, the first electrode 31 is placed along the axis of symmetry A.

Advantageously, the first electrode 31 and the insulating element 33 are operatively associated with the second electrode 32 at the vertex P thereof. In particular, they are arranged so as to pass through a hole 321 made at the vertex P of the electrode 32.

Advantageously, the first electrode 31 and the insulating element 33 are mechanically connected to the second electrode 32 by means of known mechanical connection means (one or more mechanical flanges, for example - figure 3).

According to the aforementioned embodiment (figure 2), the enclosing structure 2 comprises a flexible sheet of plastic material, joined (by means of known mechanical coupling means) to the second electrode 32 at the joining edge 320 thereof.

The enclosing structure 2 thus defines, in cooperation with the second electrode 32 (cathode electrode), the internal volume 20 for containing the conductive liquid 200.

Operation of the device 1 for emitting acoustic pulses, in the embodiment shown in figures 2- 3, will now be briefly illustrated.

At the start of an operating cycle, the power source 6 is connected electrically to the electrodes 31, 32 in response to a command from the user.

The power source 6 provides a current pulse having a pre-determined duration (a few ms, for example) which causes the formation of an electrical discharge in the conductive liquid 200 between said electrodes.

The electrical discharge in the conductive liquid 200 causes the formation of a region wherein the conductive liquid 200 is transformed into a plasma state.

For the entire duration of the current pulse provided by the power source 6, the region of the conductive liquid 200 in a plasma state expands in the volume occupied by said conductive liquid with expansion lobes even along the inner surface of the paraboloid- shaped electrode 32.

At the end of the current pulse provided by the power source 6, since the electrical discharge generated ceases as it is no longer powered, the region of the conductive liquid 200 in a plasma state collapses suddenly.

The process of expansion and collapse of the region of the conductive liquid 200 in a plasma state generates an acoustic pulse that propagates towards the deformable wall 21 and is transmitted to the liquid of propagation of the acoustic pulses (e.g. sea water) through the deformable wall 21, which deforms elastically.

Experimental tests have demonstrated how the acoustic pulse generated propagates substantially along the axis of symmetry A of the paraboloid- shaped electrode 32 (cathode electrode).

It should be noted that the process of expansion and collapse of the region of the conductive liquid 200 in a plasma state does not lead to any substantial variation in the overall volume of the conductive liquid 200.

Figure 4 shows several measurements conducted during an experimental test on the operation of the device 1 for emitting acoustic pulses, in the embodiment shown in figure 2.

The experimental test was conducted in a tank structure filled with fresh water, in which the device for emitting acoustic pulses was partially immersed.

The internal volume 20 of the device 1 was filled with a solution of water and sodium- chloride, with a concentration of sodium-chloride lower than 5 g/litre and electrical conductivity equal to 3 mS/cm, at a temperature of 20 °C.

The power source 6 (capacitor bank) was arranged so as to provide a voltage of 4.5 kV between the terminals Tl, T2 (and thus to the electrodes 31, 32).

The curve Cl shown in figure 4 illustrates the current provided by the power source 6 (measured using an ammeter) during an operating cycle of the device (t=0, moment of activation of the power source 6 and start of operating cycle).

The presence of an IC pulse is evident, with a duration of around 2 ms centred at the moment t=2ms.

The curve C2 shown in figure 4 illustrates an audiometric measurement conducted by means of a hydrophone placed at around 1.5 m from the device 1 and immersed in the liquid of propagation of the acoustic pulses (fresh water) in which the device 1 was in turn immersed. The presence of a first acoustic pulse AC1 is evident, with a shorter duration (a few qs) and large amplitude (in the order of 80-100 dB) at the moment t=12 ms. This acoustic pulse constitutes the acoustic pulse generated by the device 1 and transmitted to the liquid of propagation of the acoustic pulses.

Note the presence of a second pulse AC2 with a smaller amplitude than the first pulse AC1, mentioned above. This acoustic pulse is the echo of the first acoustic pulse AC1 generated by the reflection of said acoustic pulse from the walls of the tank structure.

The device for emitting acoustic pulses, according to the invention, has notable advantages with respect to known art.

A very important aspect of the present invention is the fact that the electrical discharge between the electrodes 31, 32 is generated in a conductive liquid 200 with reduced electrical conductivity, according to the values given above.

It has been verified that this solution makes it possible to obtain an improved quality of the acoustic pulse generated. Indeed, the pulse, given the same amount of power used, has a shorter duration and higher amplitude than an acoustic pulse generated by a traditional “sparker” device with the electrodes immersed in water (typically with a duration of a few ms).

It should be noted that, in a conductive liquid 200 with relatively reduced conductivity (in particular lower than 5 mS/cm), the region of the conductive liquid in a plasma state expands more slowly than with a traditional “sparker” device. This makes it possible to considerably reduce the presence of spurious acoustic components generated by turbulence in the plasma region during the expansion phase.

Another important aspect of the device for generating acoustic pulses, according to the invention, consists in the use, in the acoustic pulse generating assembly 3, of electrodes 31, 32 with different shapes, with the second electrode 32 (cathode electrode) having an enlarged shape with respect to the first electrode 31 (anode electrode).

In the first place, this solution makes it possible to significantly improve control of the direction of the acoustic impulse generated. For example, in the embodiment shown in figures 2-3, the concavity of the cathode electrode 32 results in the concentration of the acoustic energy in a specific direction, similar to the phenomenon of the concentration of explosive energy typical of a hollow charge (the Munroe effect). This makes it possible to obtain an acoustic pulse that is highly directional, which propagates along the axis of symmetry A of the paraboloid.

Furthermore, the solution described above makes it possible to obtain acoustic pulses with even greater amplitude, using the same amount of electrical power. For example, in the embodiment shown in figures 2-3, the concavity of the cathode electrode 32 results in the expansion of the region of the conductive liquid 200 in a plasma state towards the outer edge 320 of the electrode (according to the aformentioned preferential lobes of expansion), increasing the volume occupied by the region of the conductive liquid 200 in a plasma state and, consequently, the power of the acoustic pulse generated, using the same amount of electrical power.

The device for generating acoustic pulses, according to the invention, is therefore able to deliver very high performance in terms of the power and quality (duration and amplitude) of the acoustic pulses generated and control of the direction of emission of said pulses.

If used in an echometric apparatus for marine and inland-water exploration, the device for generating acoustic pulses, according to the invention, makes it possible to obtain very high quality “acoustic images” of the sea-floor, typically with resolutions below 0.2 m where vertical resolution is concerned. Note that these vertical resolution values cannot in any way be obtained using echometric apparatus provided with traditional “sparker” devices for generating acoustic pulses.

The device for generating acoustic pulses, according to the invention, is easy to use and does not require complex calibration procedures or fine-tuning during it operational life. The device for generating acoustic pulses, according to the invention, has a very compact structure that can be easily produced at an industrial level using known components, at competitive costs with respect to solutions available in known art.