The present invention relates to a method for sweeping marine mines having a magnetic sensor, according to which spaced electrodes are towed by a vessel and said electrodes are supplied with electric current from the vessel to set up a magnetic field in the water surrounding the electrodes a magnetic field. Sweeping of marine mines having a magnetic sensor means that a magnetic field has to be set up in the water said field being strong enough to be sensed by the mine as a vessel target, so that the mine is brought to detonation. In order to protect the vessel carrying out the mine sweeping, it is desired to limit the magnetic field of said strength to an area of a safe distance from the mine sweeping vessel so that a mine brought to detonation by the magnetic field, cannot damage the mine sweeping vessel. In practice the mine sweep arrangement is towed behind the mine sweeping vessel at a distance of approximately 200 to 600 .

A sweeping operation must fulfil two primary demands. The first demand is to make mines having a low sensitivity detonate even if they are displaced a large distance in the transverse direction of the track of the vessel. This is the so-called sweeping width preferably chosen to be of a size of the order of 100 to 500 m. The second demand is that mines having a high sensitivity shall not be initiated within a certain security zone surrounding the sweeping vessel. These demands are partially conflicting because a strong magnetic field required to satisfy said first demand makes difficult to satisfy said second demand.

The procedure of sweeping marine mines having a

magnetic sensor by means of an electrode sweeping arrangement is as follows. Two or more electrodes are placed in the water and towed by one or more vessels. The electrodes are supplied with electric current through cables from the towing vessel, the current in the cables and through the water generating the desired magnetic field. In the so-called two electrode sweeping arrangement two rod-shaped electrodes made of some conducting material and associated feeding cables are utilized. This type of mine sweeping arrangement, the most simple one, has been improved in many ways according to prior art technique.

US-A-2 937 611 discloses a system in sweeping marine mines by means of a plurality of vessels, each vessel towing two electrodes. The system provides a pulsating magnetic field between the several electrodes. US-A-2 397 209 relates to a system in mine sweeping according to which a pulsating magnetic field is provided between two of the electrodes towed by the vessel. A more complicated system in mine sweeping is disclosed in US-A-3 946 696. The system comprises two electrodes, a controlled current generator, and a magnetic field sensor. There is also included a control system controlling the current through the electrodes in dependence on the magnetic field in the vicinity of the mine sweeping vessel. By measuring the magnetic field adjacent to the mine sweeping vessel the desired safety of the mine sweeping vessel can be achieved. Another simple contructive step to improve the protection of the mine sweeping vessel without imparing the desired mine sweeping properties is to extend the mine sweeping arrangement behind the vessel. However, practical problems in handling long cables limit the length of the mine sweeping

arrangements.

A device in sweeping mines actuatued both acoustically and magnetically is described in EP Al 0 205 887.

An object of the present invention is to provide a method for sweeping marine mines initiated magnetically, which meets the demand of a safe detonation of mines, even if the mines are displaced a distance in the transverse direction of the track of the vessel, as well as the demand of a satisfactory safety of the mine sweeping vessel. This is accomplished by imparting to the generated magnetic field a desired propagation characteristic with a sufficiently weak magnetic field adjacent to the mine Sweeping vessel by the steps stated in claim 1. The invention will be explained in more detail by means of embodiments, reference being made to the accompanying drawings, in which

FIG. 1 is a diagrammatic view of a prior art two-electrode sweeping arrangement,

FIG. 2 shows a model to be applied in calculating the field propagation from a two-electrode sweeping arrangement according to FIG. 1,

FIG. 3 is a graph showing the field propagation of a two-electrode sweeping arrangement according to FIG. 1,

FIG. 4 is a diagrammatic view of a prior art three-electrode sweeping arrangement,

FIG. 5 is a graph showing the field propagation of the three-electrode sweeping arrangement according to FIG 4.,

FIG. 6 is a graph showing the field propagation from the three-electrode sweeping arrangement according to FIG. 4, the ambient conditions being changed, and

FIG. 7 is a diagrammatic view of a three-electrode sweeping arrangement according to the present invention.

The two-electrode sweeping arrangement according to FIG. 1 comprises a first electrode 10 which is towed next to the vessel during the sweeping operation, and a second farther electrode 11. Current is supplied to the electrodes from a generator, and if direct current is used, from a rectifier aboard the ship. By approximating the rod shaped electrodes with point shaped electrodes a model is provided by means of which the magnetic field set up by the electric current between the electrodes can be calculated with high accuracy, at least at a distance from the sweeping arrangment. FIG. 2 shows this model.

The propagation characteristic of the magnetic field set up by the electrode configuration according to FIG. 1 is shown in the graph of FIG. 3. The magnetic field shown in the graph is set up on one hand by the current through the conductor leading to electrode 10 and 11, respectively, and on the other hand by the current through the water between the electrodes. The graph of FIG. 3 shows the magnetic field from a fictitious electrode sweeping arrangement having two electrodes arranged at a spacing of 20 m and fed by 200 A. The magnetic field is expressed by the absolute value of the magnetic flux density in nT.

A development of the two-electrode sweeping arrangement is shown in FIG. 4. A third electrode 13 is inserted between the forward electrode 10 and the vessel. The graph of FIG. 5 shows the propagation of the magnetic field set up by the three electrodes when current is supplied to said three electrodes according to FIG. 4. The front electrode 13 suppresses the propagation of the field in the forward direction

towards the mine sweeping vessel and thus maintains a high level of protection of the vessel. In the example II = 13 = 200 A, the distance LI between the two front electrodes is 100 m, and the distance L2 between the rear electrode 11 and the centre electrode 10 is 250 τ ^~ . The total length of the sweeping arrangement of FIG. 5 is approximately 600 m, which is equal to the total length of the sweeping arrangement of FIG. 3.

As mentioned initially two partly conflicting demands must be satisfied in mine sweeping. The sweeping width should be at maximum, resulting in the magnetic field being strong enough to activate mines in an area as large as possible. In the examples of FIG. 3 and FIG. 5, respectively, the area covered by a magnetic field of the strength 100 nT, has a width of a little bit over 400 m. 100 nT will be sensed by most mines as a vessel target, and thus the first demand can be said to be satisfied in an adequate way. The second demand is the safety zone of the mine sweeping vessel. The flux density allowed in the vicinity of the mine sweeping vessel varies depending on different factors, but if 5 nT is the maximum tolerated strength below and ahead of the vessel it is clear from FIGS. 3 and 5 that it is only the three-electrode sweeping arrangement according to FIG. 5 that fulfils this second demand.

A crucial factor of the field propagation characteristic of a three-electrode sweeping arrangement is the relationship between the current II in the front electrode 13 and the current 13 in the rear electrode 11 and the spacing between the electrodes 10, 11 and 13. In FIG. 5, LI is 100 and L2 is 350 (see also FIG. 4). The relationship between II and 13 is 1, i.e. the currents II and 13 are of the same size and have the same direction. FIG.

6 shows the changed propagation characteristic of the magnetic field when the relationship between the currents II and 13 is instead 0.5, the electrode spacing being unchaged. It is apparent from FIG. 6 that the demand of a safety zone of the mine sweeping vessel is not fulfiled. The changed relationship between the currents II and 13 may be the effect of changes of the conductivity of the water. Since the conductivity is varying within broad limits, no adequate safety will be obtained by this type of three-elctrode sweeping arrangement as far as the magnetic field propagation in the vicinity of the mine sweeping vessel is concerned.

According to the present invention the desired safety of the mine sweeping vessel is indeed obtained, while at the same time the propagation of the magnetic field in the transverse direction can be controlled as desired. This is accomplished by means of a three-electrode sweeping arrangement according to FIG. 7, all three electrodes being towed in line by a mine sweeping vessel, by supplying the current to each electrode of the electrode sweeping arrangement separately and by controlling individually the current for each electrode. To provide a magnetic sweeping arrangement according to the present invention the electrodes first of all are arranged in a suitable manner as to the types of electrodes, types of cables, and the spacing between the electrodes. Starting with these fundamentals the desired relationship between the currents II to the front electrode 13 and the current 13 to the rear electrode 11 is determined. The currents II, 12 and 13 are then adjusted to suitable values so as to achieve the desired current relationship. Then, the mine sweeping can start and continue over areas having a highly varying water

conductivity, the safety of the mine sweeping vessel being maintained. Thus, the relationship between the current II to the front electrode 13 and the current 13 to the rear electrode 11 is maintained at the preset value by the current to each electrode being positively controlled.

The method according to the invention also allows an adjustment of other propagation characteristics selected in accordance with the actual situation. Thus, mine sweeping of extremely non-sensitive mines and sweeping arrangement having a considerably larger sweeping width are easily provided. It is also Possible to make the sweeping arrangement function as a two-electrode sweeping arrangement by completely cutting off the current for one of the electrodes. To achieve currents which can be individually controlled to all of the electrodes a device according to FIG. 7 can be utilized. The device comprises a current generator, not shown, and a control and regulator device 14 for controlling separately the currents II and 13. In another embodiment, not shown, the device comprises an AC-generator and a controlled thyristor rectifier for each of the outer electrodes 11, 13.

* The electrodes and the cable of conventional construction.