Method of towing sources of seismic energy behind a vessel, and an arrangement for use in the method
BACKGROUND OF THE INVENTION 
     The invention relates to a method of towing sources of seismic energy behind a vessel, especially in seismic surveys in waters that are covered with ice, with a source of energy or arrays of sources of energy being suspended from buoyancy means which are towed behind the vessel. The invention also relates to an arrangement for use in the method. 
     When seismic surveys are carried out in waters that are covered with ice, other kinds of problems arise than those commonly encountered in seismic surveys at sea. Such problems are mainly caused by the ice and the noise generated by ice being broken up when the towing vessel is propulsed in order to permit seismic surveys to be made. Icebreaking causes noise of such a level that the seismic signals are no longer detectable in the streamer cable. These problems are avoided by carrying out measurements as disclosed in NO Patent Application Norwegian 90 1616 by the same applicant. Another problem prevailing in seismic surveys in waters which are covered with ice is connected with towing the sources of seismic energy. The sources of seismic energy which are towed behind the vessel are suspended on some kind of buoyancy means which maintain a plurality of sources of energy at a predetermined mutual distance in order to permit the sources together to emit a seismic impulse which is defined in the desired manner and is detected by the hydrophones of the streamer cable. In waters which are covered with ice, however, the buoyancy means for the sources of energy will be influenced by broken ice, which may partly damage them, but at least will cause interference of their advance. Ice may pile up in front of the buoyancy means and may hamper advance. Under the surface of the water such a pile-up may also disturb the suspension lines of the sources of energy, so that the latter are displaced from their correct position. Consequently, the seismic signals may be distorted and, thus, render recordings dubious. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method and arrangements to prevent such an influence on the sources of seismic energy and to ensure a maximum controlled and safe advance of the sources of energy behind a towing vessel. 
     The objects are achieved by a method and arrangements characterized by the novel features of this invention. 
     In the method according to the invention two towing cables are used, which will stabilize the system. The special arrangement of the towing cables, one of which, towing the buoyancy means, follows the waterline and is secured to the vessel in the area of the waterline, and the other second towing cable, being connected directly with the array of sources of energy, preferably air guns, is arranged as far as possible under water, provides for the safest possible towing conditions without influence by ice formations. 
     It is, in fact, previously known from U.S. Pat. No. 3,718,207 to tow a seismic source of energy by a cable from a location under water and directly to the source of energy, but this will not provide sufficient security for steady advance in waters which are covered with ice, so that a double system is, as mentioned, used in the present case. By designing the buoyancy means as described hereinafter ice floes are in the best possible manner cut or pushed aside, etc. At the same time the knife-like arrangement at the suspending location beneath the buoyancy means will prevent ice from piling up beneath the buoyancy means, which might otherwise lift the buoyancy means and the air guns so that the latter are displaced. In order to ensure that the souces of energy are kept in place and that their towing line has a direction which is as horizontal as possible, the fastening points of the towing line are provided as far as possible under the surface of water, and means to lower the fastening point on the vessel after launching the tow may suitably be used. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is disclosed in more detail below with reference to an embodiment shown in the accompanying drawing, wherein: 
     FIG. 1 is a schematic elevational view which shows diagrammatical sections of a towing vessel towing sources of seismic energy according to the invention; 
     FIG. 2 shows three is a side elevational view illustrating a buoyancy means designed according to the invention; 
     FIG. 3 is a top plan view of FIG. 2; and 
     FIG. 4 is a left end view of FIG. 2. 
    
    
     DETAILED DESCRIPTION 
     A towing vessel 1 is diagrammatically shown in FIG. 1. The towing vessel tows a seismic streamer 2 which is not disclosed in detail here, since it does not constitute part of the present invention. The towing vessel, furthermore, tows an array of sources of seismic energy which is generally designated by numeral 3. Said array of sources of seismic energy comprises an array of buoyancy means 4 and the sources of energy 5, which may be air guns, are suspended beneath said buoyancy means 4 by the aid of lines 6. The tow of sources of energy 3 is towed behind vessel 1 by the aid of two lines 7 and 8. One line 7 is secured to buoyancy means 4 on one side and to the vessel in the area of the waterline on the other end and the fastening point is generally indicated at 10 in the figure. Line 7 which may be a wirerope, preferably a chain, will thus extend in the area of the waterline and will cut or push aside any ice collected behind the vessel. The sources of seismic energy 5 are also connected with the vessel, via line 8, which is secured to the vessel at a location below the waterline and which is generally designated by numeral 11 in the figure. From the sources of seismic energy there are also connecting cables for operation of the sources which are designated 9. Cables 9 are of a conventional kind. 
     Line 8 may be introduced into the water through special ducts on the ship&#39;s side or through the ship&#39;s hull, which open below the waterline. Such duct members may be extendable or displaceable to permit regulation of the depth of towing point 11. The duct line 8 is connected with a winch means 12 for hauling in and paying out the towing line 8. 
     In order to minimize the influence of ice on buoyancy means 4 the latter may have a shape as illustrated in FIG. 2. The buoyancy means has a rounded shape with pointed front and rear ends. In the front and rear portions fastening lugs 13, and 14, respectively are integrated with the buoyancy means and are, preferably, vertical to be able to cut through any ice floes or the like. The towing line 7, preferably a chain to break up any drifting ice, is secured to front lug 13. To lug 14 a coupling means for the following buoyancy means 4 is secured, so that a series of buoyancy means which are close behind each other is provided. On the top of the buoyancy means a fastening lug is provided for lifting and lowering the buoyancy means. Below the buoyancy means a rigid and pointed fin-shaped fastening means 17, extending in the longitudinal plane of the buoyancy means is at its lower edge provided with an integrated fastening lug 16 for the suspension line leading to source of energy 5. When the buoyancy means are towed among drifting ice floes and crushed ice, part of such ice will collect below the buoyancy means, but will be crushed and pushed aside by said fin-like fastening means 17, so that ice will not affect the suspending lines 6 to any degree worth mentioning and interference with operations are substantially avoided. Said fin-like member in combination with use of two towing lines 7 and 8, is highly important to a steady advance of the sources of energy. 
     In the drawing only one embodiment of the invention is shown, but many modifications will be possible, based on local conditions and on the kind of towing vessel used. Such modifications are intended to be within the scope of the present invention. It should also be mentioned that there is nothing preventing the invention from being used in other waters than waters which are covered with ice, e.g. in places where drifting objects may be expected on the surface of the water, or the like.