WO1990000135A1

WO1990000135A1 - Control equipment for a towed submarine acoustic-wave emitter, in particular for mine dredging

Info

Publication number: WO1990000135A1
Application number: PCT/FR1989/000331
Authority: WO
Inventors: Jean-Louis Vernet
Original assignee: Thomson-Csf
Priority date: 1988-07-04
Filing date: 1989-06-27
Publication date: 1990-01-11
Also published as: EP0423203A1; FR2633584A1; FR2633584B1

Abstract

Equipment for controlling output parameters of a submarine acoustic wave emitter (10) towed by a vessel (20), comprising at least one receptor (40) towed by said vessel along with the transmitter, the acoustic signal received by the receptor being subsequently compared with the signal produced by the transmitter. The receptor according to the present invention is towed behind the transmitter and at a distance therefrom generally equal to that separating the transmitter from a point (41) situated in the perpendicularity of the vessel, or from another reference point for controlling said transmission parameters. The signal received by the receptor can in particular be used to regulate the transmission level of the transmitter so that the acoustical level produced by the transmitter in the perpendicularity of the vessel or in said reference point is maintained below a predetermined limit.

Description

Control equipment for a towed submarine acoustic wave transmitter, in particular for dredging mines

The present invention relates to equipment for controlling the emission parameters of an underwater acoustic wave transmitter towed by a ship.

The invention applies mainly to the dredging of mines with acoustic influence, and will be described mainly in the case of this application; however, this application is in no way limiting, and the invention can be applied equally to other applications - in particular all underwater prospecting applications - in which the acoustic signal produced by one or more points is picked up a source of acoustic waves at a distance therefrom and, by bringing together (in particular by correlation) the signals transmitted and the signals received, certain information is derived, for example concerning the nature of the seabed.

In the case of mine dredging, towing is carried out several hundred meters behind the towing vessel (the mine hunter), a dredge consisting of a broadband acoustic wave emitter that simulates the mechanical and hydrodynamic noise of 'a naval vessel. The aim is thus to cause the explosion of mines laid at the bottom of the sea or submerged at a certain level above the bottom of the sea; for this, the transmitter is towed with a constant β-immersion relative to the bottom, to maintain a constant emission level relative to the sea bottom. The emission is generally a directive emission, extending essentially in a plane perpendicular to the towing direction, obtained by transducers located on the side of the dredge so as to obtain a lateral and oblique sweep of the seabed.

A zone of influence or "insonification zone" is thus determined for which the threshold of acoustic sensitivity of the mines is reached, this zone being very extended in the direction perpendicular to the towing direction, and relatively little extended in the direction of towing.

However, in this latter direction, the area still has a certain width, and it is obviously essential that the boat is outside this area so as not to trigger the explosion of mines located near or near it.

One of the difficulties in this process is that the acoustic levels depend on many parameters (variation in the immersion of the transmitter, acoustic propagation characteristics, nature of the seabed, etc.), so that the area of influence can present, in the direction of towing, a very variable width which is difficult to estimate in advance.

To be sure that the sound level below the ship will always remain below a given threshold, it is known, as shown in FIG. 1, to tow an acoustic receiver 30 near the bottom, where it is assumed be the mines, and at a distance d called "safety" corresponding to the distance separating the towing vessel from the supposed limit of the zone of influence, extended by a certain safety margin. The transmission level of the transmitter 10 is then controlled by the level measured on the receiver.

This system is nevertheless not very effective due to the significant variations in the acoustic levels due to the various phenomena indicated above. It is indeed observed that, very often, the acoustic level received near the bottom plumb with the ship 20 - and even in front of it - is sometimes higher than that which exists at the safety distance, where is towed receiver 30.

One could certainly increase the safety distance, but this would lead to prohibitive towing lengths.

Ideally, the sound level should be measured directly above the ship, since this is where the ship is most vulnerable. But we then come up against a practical difficulty which is to tow a receiver located just under the boat, near the bottom.

The invention proposes to overcome these difficulties with equipment having an original configuration and allowing a substantial improvement in the protection of the ship, without excessive increase in safety distances.

To this end, according to the invention, the receiver is towed behind the transmitter and at a distance from the latter substantially equal to the distance separating the transmitter from a point located directly above the ship or other point. of reference for the control of said emission parameters. Generally (as in the aforementioned application of minesweeping), the signal received by the receiver is used to control the emission level of the transmitter, so that the acoustic level produced by the transmitter at the base of the ship or said reference point remains below a given threshold.

According to a first configuration, the receiver is towed at the end of a cable connected to the ship.

According to a second configuration, the receiver is towed at the end of a cable connected to the transmitter, itself towed at the end of a cable connected to the ship. Preferably, the receiver comprises a flute formed of a plurality elementary receivers arranged in a linear network extending substantially in the direction of towing.

In an advantageous embodiment, the aforementioned equipment comprises a second receiver towed aft of the ship but in front of the transmitter, at a distance from the latter such that the transmitter is located substantially halfway of the two receivers.

Other characteristics and advantages of the invention will appear on reading the detailed description below, made with reference to the attached _Q drawings in which:

- Figure 1, above, shows the configuration of the equipment that was used until now,

FIG. 2 is homologous with FIG. 1, for the configuration according to the present invention,

FIG. 3 is an alternative embodiment of FIG. 2, and

FIG. 4 is an improved variant of FIG. 3.

Essentially, the invention consists in towing a receiver 40 _Q behind the transmitter 10 (and no longer in front, as in the prior art), and at a horizontal distance D from the transmitter 10 substantially equal to that separating the transmitter 10 of the ship 20 (here and in the following, unless otherwise indicated, we will always consider distances projected on the horizontal plane). 5 The emission characteristics of the transmitters being generally symmetrical in front and behind the zone of influence, it can be seen that, by simple geometric symmetry, the acoustic level produced by the transmitter at the location of the receiver 40 will be substantially the same as that present at the point referenced 41, that is to say at a point located directly above the ship 20 0 and at the same level with respect to the sea bottom as the receiver 40 (if it is assumed that the bottom is substantially flat over the extent covered by the equipment, which is generally the case).

This configuration has the advantage that the measurement can be made at a reference point as close as desired to the plumb line of the boat, or even possibly at a reference point located in front of it, without being encounter obvious practical difficulties in towing a receiver to locate it at this point. Another advantage is that, as the receiver is moved away from the towing vessel, the noise radiated by the latter is received with less intensity by the receiver, since advantage is taken of the propagation losses between the ship and the receiver. It is thus possible, for the control of the transmitter, to better separate the signal produced by the transmitter from the noise of the ship. Thus, if the receiver 40 is towed to a depth of approximately 100 m and to a vessel-receiver distance of approximately 500 m, the noise of the boat will be attenuated by approximately 15 dB compared to the signal from the transmitter (although understood, this theoretical figure is in reality very variable taking into account the conditions of propagation, in particular at the lowest frequencies, where the propagation is done by modes).

This gain will allow the signal delivered by the receiver to be processed in better conditions, in particular by allowing better control of the transmitter's emission level.

It is also possible to envisage other processing operations, in particular by comparing the signal emitted and the signal received, which may provide information on the propagation conditions, the nature of the seabed, etc.

The receiver 40 is preferably made up of a plurality of hydrophones placed in a chain so as to form a linear network or flute; the detection thus covers an area having, in the direction of towing, a certain width. The emission level of the transmitter is then slaved to the maximum level detected, which avoids the effects of certain propagation singularities or interference phenomena which can locally produce attenuations in the level of the received signal.

In the case of FIG. 2, the receiver 40 is towed indirectly by the ship, by means of a connecting cable fixed to the transmitter 10, itself connected to the towing ship 20 by another cable.

In this case, it is generally necessary to provide a depressor 42 making it possible to maintain the receiver 40 at the desired constant immersion depth, corresponding to the depth- at which one might think that the mines are located (generally, the level of the bottom from the sea, or a small height above it).

In the variant of FIG. 3, the receiver 40 is towed directly by a cable connected to the ship, the rest of the configuration being identical elsewhere.

In this case, the weight of the cable is preferably calculated and adjusted so that, in a manner known per se, one is at the critical angle for the dredging speed, which avoids the use of a depressant.

In FIG. 4, a variant is shown in which a safety distance d is also provided, that is to say that the point 41 where the acoustic level is measured is no longer located directly above the ship as in Figures 2 and 3, but aft of it.

In this case, if the transmitter 10 is towed at a distance D from the ship 20, it will be ensured that the receiver 40 is towed at a distance D - d from the transmitter 10.

Depending on the needs, it would also be possible to carry out a detection corresponding to a point 41 which is no longer plumb or behind the ship, but in front of it: it would suffice for this purpose to tow the receiver 40 to a distance D + d from the transmitter 10, d being the distance in front of the boat at which we want to perform the detection.

In this FIG. 4, a second receiver 30 has also been shown, which is of the same type as that of the prior art (FIG. 1), and which is towed aft of the ship at distance d; it is thus seen that the two systems, namely the system of the prior art and that of the present invention, can coexist so as to provide the most complete measurements possible.

Claims

1. Equipment for controlling the emission parameters of an underwater transmitter (10) of acoustic waves towed by a ship (20), comprising at least one receiver (40) towed by the ship at the same time as the transmitter, the acoustic signal picked up by the receiver then being used for comparison with the signal produced by the transmitter, characterized in that the receiver is towed behind the transmitter and at a distance from the latter substantially equal to the distance separating the transmitter from a point (41) located below the ship or other reference point for checking said emission parameters.

2. The equipment of claim 1, wherein the signal received by the receiver is used to control the emission level of the transmitter, so that the acoustic level produced by the transmitter above the ship or audit reference point remains below a given threshold.

3. The equipment of one of claims 1 and 2, wherein the receiver is towed at the end of a cable connected to the ship.

4. The equipment of one of claims 1 and 2, wherein the receiver is towed at the end of a cable connected to the transmitter, itself towed at the end of a cable connected to the ship.

5. The equipment of one of claims 1 to 4, wherein the receiver comprises a flute formed of a plurality of elementary receiversdisposed in a linear network extending substantially in the direction of towing.

6. The equipment of one of claims 1 to 5, comprising a second receiver towed aft of the ship but in front of the transmitter, at a distance from the latter such that the transmitter is located substantially halfway distance from the two receivers.