This invention relates to a device for adjusting the buoyancy of a seismic cable.

In seismic studies at sea cables are used being towed after a vessel. The cables comprises different kinds of sensors for the measuring of acoustic waves being reflected from geological formations beneith the bottom of the sea, after having been generated from one or more sources by the vessel or vessels. Typically the sensors will be hydro- phones, or possibly geophones, located along the longitudinal direction of the seismic cables in order to receive different parts of the reflected waves. To provide accurate results from the measurments it is important that the positions of the sensors are well defined, with a given depth and position in relation to the seismic source. It is therefore important that the cables have a linear and smooth progress through the water, and that the whole cable is kept at the same depth. It is also usual to fill the cable with a liquid, e.g. oil or kerosene, which will behave like a buoyant medium, so that the cable is provided with a net weight comparable to that of the surrounding water, and is kept with a stable shape at different depths.

In order to maintain the cable at a chosen depth it must be balanced in relation to its weigth and buoyancy so that is as neutral as possible in relation to the surrounding water. This may, however, be problematic, since the buoyancy of the cable will vary with the salt content of the water, and the buoyancy therefore must be regulated in accordance to the enviroment. As the buoyancy of the cable in relation to the surrounding water can vary during an operation it must be possible to change the buoyancy in a simple manner. This has previously been done by fastening different types of weights to the cable. The weights have been attached to the outside of the cable using tape. This solution has, however, major disadvantages, as the tape often dissolves in sea water, and the weights fall off. Also the fastening of the weights with tape is time consuming, and it is awkward to take off and move the weights to another position along the cable, especially since these operations often must be performed at sea. Also

the necessary quality of tape is expensive, and can not be used more than once. Another disadvantage related to this solution is that the tape and the weights generate noise and vibrations when the cable is towed through the water, which may disturb the sensors in the cable.

In Norwegian patent application 90.5497 (US patent 5,278,804) a solution is described in which the weights are fastened around the cable using simple locking mechanisms so that they are clamped to the outside of the cable. This provides a simpler way to fasten the weights, but has the disadvantage that they extend outside the diameter of the cable. Thus, this solution will also generate noise when the cables are towed, and in addition increase the towing resistance of the cable. The present invention is aimed to solve these problems.

The present invention is based upon the known technique relating to the positioning of blocks in the longitudinal direction of the cable. These are used to suppress vibrations propagating along the cable in the liquid it contains, dampen the movements in this liquid, keep electrical conductors and wires protected and separated, as well as to keep the shape of the cable stable. The blocks are preferrably located at even distances along the whole of the cables length. The present invention is also related to devices which may function as such blocks. The new and inventive is related to a buoyancy adjustment device for the adjustment of buoyancy in a seismic cable, the device comprising a main part adapted for firm, integrated mounting at a chosen point in the longitudinal direction of the cable, wherein the main part comprises fastening means for at least one weight or buoyancy element, and at least one exchangeable weight or buoyancy element for removable mounting in the main part.

This way a possibility is obtained to regulate the buoyancy of a seismic cable in a simple manner using units which will be installed in the cable anyway.

The invention will be described below by way of example and with reference to the accompagnying drawings. Figure 1 shows a main part of a device according to the invention as seen from two sides.

Figure 2 show sections of different alternative embodiments of the invention. Figure 3 shows a device according to the invention mounted in a cable. Figure 1A and IB shows the main part of the device with a room 3 for a weight or buoyancy element, in which figure 1A shows the main part as seen from the side and figure IB shows it as seen from one end. The main part 1 is preferrably made from plastic or a similar material with a relatively neutral buoyancy in water, and will also comprise means 4 for the fastening of weight elements to the main part, passages 9 for wires extending along the seismic cable and similar passages 10 for electric conductors. The main part may in addition comprise valves 5,6 with relating channels 7,8 provided for the emptying and filling of the cable. In some cases it may be preferrable to let the liquid move through a suitable channel (not shown) , possibly controlled by valves located in the channel, so that all the liquid may be filled in or poured out in the same position in the cable, preferrably at the ends. The valves may be of any known type, depending on the use of the cable and other characteristics, as well as the characteristics of the liquid.

The embodiment shown in figures 1A and IB is adapted to a cable comprising three wires running in the logitudinal direction of the cable through the channels 9. This number will normally vary from one to 5, but more may also be possible. Correspondingly the number of channels for electric conductors may vary depending on the cable at hand. Fastening of the main part to the cable may be performed in many different, known ways and will not be discussed any further here. Preferrably the fastening mechanism and the main part have the same cross sectional dimensions, so that noise generated from unevenness along the cable is avoided.

Figures 2A-2E show sections of different other embodiments of the invention. Figure 2A shows a cross section of another embodiment comprising one weight or buoyancy element 2. In this case the main part is adapted to a cable with two wires and one bundle of conductors.

Figure 2B shows a variant with two weight elements. This will give an increased flexibility to vary the buoyancy of the seismic cable, since weight elements 2 of different kinds may be located at approximately the same position in the cable and together provide a buoyancy between what may be obtained with one of the materials. This version also shows a combination adapted to three wires and one bundle of conductors.

In figure 2C a cross section of another version is shown comprising two weight elements 2 adapted to two wires and one bundle of conductors. In this case there will also be a possibility of using one sentrally positioned wire and two conductor bundles. The embodiment of figure 2C comprises weight or buoyancy elements occupying a substancial part of the cross section of the device. This provides a great freedom in the ability to vary the buoyancy of the cable, as it provides a large range of variation of the weight of each element 2. If there is a need for a positive buoyancy in the elements 2 this is a advantageous solution as the volumes of the elements are relatively large. Figure 2D shows a partial section in the longitudinal direction of the same embodiment in which the positions of the valves 5,6 are indicated.

Essentially the same embodiment is shown in figure 2E as in figure 2D, but the main part is provided with taps 12 engaging into corresponding recesses in the weight or buoyancy elements 2, so that they are able to take up some of the strain in the device. This solution may of course also be employed in other embodiments of the invention. The weight or buoyancy elements 2 may be made from different types of materials depending on the situation, from heavy materials such as lead or tungsten to light materials such as plastic or gas containers. Variants with pieces of metal cast into plastic are also possible, and may provide a possibility to obtain a certain weight.

In all of the embodiments shown here the weight or buoyancy elements 2 are fastened to the main part using screws. Other solutions may of course be conceived, e.g. spring devices. Figure 3 shows another embodiment of the invention

mounted in a cable. The surface of the device is essentially in line with the surface 14 of the cable. The figure shows a cable with two wires 13 and one bundle of conductors 15. The device according to the invention will normally be positioned at intervals along the cable, with an intermediate distance of e.g. 12.5 meters in a cable section of 75 meters. The total seismic cable will comprise a number of such sections coupled together. Other embodiments than those illustrated in the drawings may of course be conceived, for example an embodiment in which the main part of the device according to the invention is split into two or more parts and may be mounted in an existing seismic cable by joining the parts together around the longitudinally extending wires and electric conducters in the cable. The weight and buoyancy elements may be covered by an adapted lid for waterproof enclosure of the elements, and in such cases the room 3 for the elements may be empty, or possibly the resulting room may in itself function as a weight or buoyancy element, depending on it containing water or air.

The main part of the device may be made from plastic, or possibly plastic with an outer protecting layer of another material. Metal may be used in some situations, depending on the desired weight.