Arrangement for towing

Apparatus for towing a surface buoy or float, in particular a reference position buoy (1), from a marine seismic streamer (4) being adapted during operation to be immersed in the sea. The towing takes place by a means of a length of cable or the like (5A,5B) being attached to a section of the streamer and to the surface buoy respectively. At an intermediate portion of the length of cable (5A,5B) there is provided a weight or deflector device (2) being adapted during towing to impose an angular deviation of the length of cable so that the direction thereof downwards (5A) and upwards (5b) from the weight or deflector device (2), forms a mutual angle (Vb) differing significantly from 180.degree.. The point of attachment (3) at the streamer is located at an active section (4) thereof.

BACKGROUND OF THE INVENTION 
In marine seismic exploration it is important to be able to determine the 
position or shape of one or more streamer cables being towed behind an 
exploration vessel. It is common practice to let the position 
determination be based upon a navigation system of higher level, as, for 
example, GPS satellites, which gives a number of reference positions in 
the total arrangement being towed. The number of reference positions is 
mostly relatively low, and as a rule the reference positions are 
associated with the forward part of the whole assembly being towed as well 
as to one or more end buoys which terminate the assembly. The shape or 
path of the streamers over the relatively long distance between the 
forward and the rearward end of the towed assembly, is usually determined 
by means of a number of magnetic compasses being provided on the streamer, 
in particular at the active sections thereof. Moreover there are known 
proposals based upon trilateration (Norwegian published patent application 
No. 173.206) for the purpose of avoiding the dependancy on magnetic 
compasses. 
It is also known per se to tow surface buoys or floats being anchored to 
sections of the streamer, but in such cases the buoys or floats are not 
anchored to the active sections of the streamer. The problem in this 
connection consists therein that this kind of buoy or float will introduce 
noise which interferes with the detection and the recording of reflected 
seismic signals. 
Thus, on the above background this invention takes as a starting point an 
apparatus for towing a surface buoy or float, in particular a reference 
position buoy from a marine seismic streamer adapted to be immersed in the 
sea during operation, whereby the towing takes place by means of a length 
of cable or the like being attached to a section of the streamer and to 
the surface buoy respectively. 
SUMMARY OF THE INVENTION 
What is novel and specific to the apparatus according to the invention in 
the first place consists therein, that at an intermediate portion of the 
length of cable there is provided a weight or deflector device being 
adapted during towing to impose an angular deviation of the length of 
cable so that the direction thereof downwards and upwards from the weight 
or deflector device forms a mutual angle differing significantly from 
180.degree., and that the attachment point at the streamer is located at 
an active section thereof. 
With this solution the geometrical relationships associated with the towing 
cable length for the surface buoy will result in a substantially reduced 
transfer of vertical forces between the buoy and the active streamer 
section, so that the movements of the buoy because of waves or other 
irregularities, will not be transferred as vertical forces, and thereby 
noise, to the streamer.

DETAILED DESCRIPTION OF THE INVENTION 
The drawing shows a buoy or float 1 at the sea surface and preferrably 
provided with a GPS receiver 1A with an associated antenna 1B so as to 
serve as a reference position in a seismic assembly being towed. Through a 
length of cable consisting of two parts 5A and 5B the buoy 1 is connected 
to a point of attachment or an adapter 3 at an active section 4 of a long 
streamer. The cable length 5A, 5B primarily comprises a flexible tension 
element adapted to sustain the tensional forces occuring, for example a 
wire element, but besides usually comprises leads or cables for electrical 
connections, i.e. in particular signal or data transmission between the 
buoy 1 and the streamer 4 or units provided in or on the streamer. 
In contrast to known arrangements for towing a surface buoy or float, 
whereby the towing wire extends directly from a point of attachment at the 
underwater streamer concerned, to the underside of the buoy or float, the 
length of cable 5A-5B in the drawing has a significant angular deviation 
provided for by a weight device 2 being preferrably streamlined and being 
located at an intermediate portion of the whole cable length 5A-5B. With a 
sufficiently heavy weight of the device 2 this will pull down the cable 
length 5A-5B so that the angle Va between the cable length part 5A and the 
active streamer section 4 itself, will be substantially reduced in 
relation to a direct towing wire as mentioned above. In a corresponding 
manner the angle Vb between the two parts 5A and 5B of the towing cable 
length, will be significantly reduced in relation to such an angle of 
180.degree., which would be found in the case of direct towing. 
When seismic streamers are towed, the depth D of the streamer as indicated 
in the drawing, will be maintained at a certain magnitude by means 
provided specifically for this purpose. Therefore this depth D can be 
regarded as of known magnitude in the geometric relationships being of 
interest with respect to the relation between the reference position 
represented by the buoy 1 and points of interest along the length of the 
streamer 4. Moreover the length of the cable 5A-5B is of known magnitude, 
whereby the part 5A is the more significant part, whereas the part 5B in 
the first place is usually much shorter than the part 5A and in the second 
place will run almost vertically upwards from the weight device 2 to the 
buoy 1. Accordingly the relative position of the buoy 1 as seen in the 
longitudinal direction of the streamer 4, is quite well defined. 
For a still more accurate definition of the reference position represented 
by the buoy 1 in relation to the longitudinal extension of the streamer 4, 
the embodiment of the drawing shows acoustic positioning units, i.e. 
transceivers 6 and 7 being located at the streamer 4 and at the upper 
cable part 5B respectively. The positioning unit 6 is preferrably located 
close to the attachment point 3 and the positioning unit 7 is suspended 
closely underneath the buoy 1. Then the position of the latter unit will 
vary only to a very small extent in relation to the buoy. In a manner 
known per se the acoustic positioning units 6 and 7 can then cooperate, as 
illustrated by the signal path 8, through the seawater between these 
units, in order to determine the mutual distance between the buoy 1 and 
the point at the streamer 4 being represented by the positioning unit 6. 
For further reliability in this relative position determination, the 
acoustic unit 7 can also cooperate with another acoustic positioning unit 
6A on the streamer 4 at a distance rearward of the positioning unit 6. The 
signal path between the units 7 and 6A is illustrated at 8A. 
The geometric relationships discussed above are valid for the relative 
position determination, provided that the buoy 1, and besides the weight 
device 2, are located substantially in a vertical plane through the 
streamer 4. In practical operations this will generally be satisfied. 
Considering the so-called weight device 2 more in detail, it is a great 
advantage to design this device as a streamlined body having a significant 
weight in water. So as to contribute to the above discussed angular 
deviation represented by the angle Vb, the device or body 2 can be 
provided with deflector surfaces or wings 2A, the angle of which can be 
adjustable if desired. 
The length of cable 5A-5B, and in particular the cable part 5B can be 
provided in a manner known per se with friction or drag reducing elements, 
such as ribbons or threads which result in a decrease of water resistance. 
This can contribute, inter alia, to obtaining a more vertical direction of 
the cable part 5B. 
It is obvious that a streamer may have a number of reference position buoys 
located along its total length, so that the employment of magnetic 
compasses or the like will be superfluous. In many cases it will be 
sufficient in practice to provide a reference position buoy adjacent to 
the forward end of the active streamer section or sections, and besides 
such a reference position buoy approximately midway between the end points 
of the active section or sections. As previously known per se it will also 
be suitable to provide for a reference position at the rear end, for 
example at an end buoy for each streamer. 
Moreover when two or more streamers are towed side by side, the acoustic 
transceiver units 6, 6A and 7 belonging to each such streamer will be able 
to cooperate laterally for establishing a network for further securing an 
accurate position determination of the respective streamers. 
As mentioned above, the towing length of cable 5A-5B preferrably comprises 
leads or a cable for communication between equipment on the buoy 1 and the 
acoustic positioning unit 7 at one side, and equipment or units at the 
streamer 4 at the other side. Signal and data communication between these 
components in the total arrangement being towed and the towing vessel, 
therefore can take place by electrical cable so that there is no need for 
a radio link for this purpose. This gives a secure and reliable 
communication, since a radio connection can involve problems such as 
damage to the antennae and also radio interference as well as lack of 
available channel capacity.