GB2142432A - Float assembly for seismic sources - Google Patents

Abstract

An elongate continuous seismic float system (10) has a flexible float chamber (24) which may be filled with a buoyant material such as air or foamed plastic. The float system is towed behind a seismic boat and is adapted to supports number of seismic sources (12), for instance air guns or sleeve exploders, from a stress element (28). Cable (32) carries electrical signals to fire the guns (12) and air to supply the guns. The float chamber may be divided into smaller, separable elements. <IMAGE>

Description

SPECIFICATION Float assembly for seismic sources This invention relates to an improved seismic source float system. More particularly it relates to a simple flexible continuous float apparatus which supports a number of seismic sources, such as air guns, and is hydrodynamically more suitable for towing than those of the prior art.

In prospecting in subsea and other areas underlying a body of water, it is desirable to provide a source of energy for propagating sonic pulses or shock waves into the water. Since water is a good conductor of sound, it normally is not necessary to generate pulses near the floor of the waterbody; the pulses can be, and desirably are, produced near the water's surface. These pulses propagate down through the water, across the waterfloor interface, into the subfloor geologic formations and are, to some extent, reflected back across the same path to an array of hydrophones waiting near the surface of the water. Analysis of the signals produced by hydrophones can provide information concerning the structure of the subfloor geological formations and attendant petroleum accumulation in those formations.

The term "water" as used herein is meant to include swampwater, mud, marshwater and any other liquid containing sufficient water to enable operation of the invention.

There are many ways of generating a sonic pulse in a liquid. For instance, explosives introduce strong pulses into the water and accordingly achieve substantial penetration into subfloorformations. Certain obvious drawbacks exist in their use: they are dangerous to store, handle, and fire. When used in open water, they kill marine life. In crowded areas such as harbors, explosives cannot be used at all.

Explosives are orders of magnitude more expensive to use, on a per-shot basis, than are gas guns.

Modification of the explosive source's sonic signature to achieve an acceptable spectrum distribution is difficult.

Another method of generating a sonic pulse is by discharge of a bank of capacitors through a subsurface electrode to produce a quickly collapsing gaseous bubble. However, the efficiency of this method is quite low in that only a few percent of the energy charged to the capacitors is found in the shock wave produced on discharge.

Apparatus using explosive gas mixtures, e.g., propane and air, to produce a sonic pulse have gained wide acceptance. The two major types of explosive gas guns are those which operate by exploding a gas mixture behind a flexible membrane which in turn is in contact with the water and those which operate by allowing the abrupt bubble from the gas explosion to pass directly into the water. An example of the former apparatus can be found in U.S. Patent No.3,658,149; and example of the latter can be found in U.S. Patent No.4,193,472.

Open guns using high pressure compressed gases, instead of an explosive mixture, have achieved a wide acceptance in the industry. Typical designs for open-ported compressed gas guns are found in U.S. Patent No. 3,653,460 to Chelminski and U.S. Patent No.4,141,431 to Baird. These guns employ two pressurized chambers, i.e., a control chamber and a gaswholding chamber, which are sealed by a spool-shaped valve or shuttle. The gun is fired by abruptly releasing gas from the control chamber. The gas in the gas-holding chamber forces the shuttle into the control chamber and thereby simultaneously exposes the exhaust ports. These ports allow the gas stored in the gas-holding chamber to exit explosively into the water. The control chamber is then repressurized, thereby moving the shuttle back into a position sealing the gas-holding chamber.The gun is again ready to "fire".

A compressed gas gun eliminating several problems associated with prior guns is disclosed in U.S.

Patent No. 4,180,139 to Walker, issued in December, 1979. This patent discloses a gun having a single cylindrical gas chamber with central exhaust ports about its periphery. Inside the gas chamber resides a moveable shuttle also having ports about its center.

When the shuttle is moved from one end of the gas chamber to the other end, via the action of an integrated actuator, the ports in the shuttle momentarily align with those in the gas chamber wall and allow an amount of compressed gas to escape. Once the shuttle reaches the other end, the gun is in position to "fire" again awaiting only the build-up of pressure in the gas chamber and actuating mechanism.

Another desirable gas powered seismic source is disclosed in U.S. Patent No. 4,381,044 to Kirby, issued April 26, 1983. That device utilizes a single shuttle which moves through a stroke. At the opposite ends of the stroke the shuttle seals simul taneouslytwo separate gas storage cylinders. Movement of the shuttle through its stroke will allow gas from both chambers to exit through ports provided for that purpose into the surrounding water.

Whatever source is used, however, it must be supported in some manner near the water's surface.

Typically the sources are supported by discrete floats which are variously round, pear-shaped, or cylindrical in form.

Round floats are described in U.S. Patent No.

3,460,064 to Giles et al, issued August 5, 1969; U.S.

Patent No. 3,602,878 to Sullivan, issued August 31, 1971; U.S. Patent No.3,893,539 to Mott-Smith, issued July 8, 1975; U.S. Patent No.4,064,479 to Ruehle, issued December 1977; U.S. Patent No.

4,146,871 to Ruehle, issued March 27, 1979; and U.S.

Patent No. 4,313,392 to Guenther et al, issued February 2, 1982. The seismic sources in each of these patents are supported via lengths of chain or similar supports to the buoys.

U.S Patent No. 2,607,842 to Reid, issued August 19, 1952, discloses a method for varying the depth of a series of marine seismometers by utilizing a series of discrete, flexible, and variably buoyant floats. The patent does not suggest the use of such floats on marine seismic sources.

U.S. Patent No. 3,718,207 to Babb, issued February 27, 1973, discloses double cylinder rigid floats supporting a water powered seismic source. The tow line in Babb is connected to the source rather than to the float assembly. Babb shows-but a single seismic source in operation.

U.S. Patent No. 3,953,826 to Brundrit et al, issued April 27,1976, discloses an elongated seismic source for use in marine seismic exploration. A number of single cylinderical floats are towed behind a vessel.

Each float supports a number of seismic sources.

Again, the tow cable is attached to the sources rather than to the floats.

Another buoy widely used to support seismic sources is an air filled, pear-shaped float typically constructed of plastic and known as a Norwegian buoy. Each seismic source is supported at a predetermined depth via a chain connected to an individual float.

None of the prior art discloses a continuous float suitable for supporting a number of air guns or other subsea seismic sources.

According to the invention there is provided a float assembly adapted to support several seismic sources below and near the surface of a water body, comprising: an elongate flexible floatation chamber wall formed in a generally cylindrical shape and adapted to float and support several seismic sources, a stress member situated within and along said floatation chamber wall, means for towing said float assembly, and means for attaching a number of seismic sources to said stress member.

A preferred embodiment comprises a continuous, air filled seismic source float system which is capable of supporting a large number of air guns or other seismic sources. Because of its generally cylindrical shape it is hydrodynamically more suitable for towing behind a seismic vessel than the shapes found in the prior art.

The cylindrical shape floatation member may be of a single chamber or may be of a number of separate airfilled chambers to preventthe loss of an air gun array should the floatation member be punctured.

For the purposes of this application, the term "cylindrical" is meant to include floatation members having circular and oval cross sections and those having cross sections such as those depicted below having a major circular portion and minor V-shaped section. The floatation member may be filled with a buoyant material.

Typically the floatation member will have a flexible lead-in made up of a stress member, high pressure air supply hoses for the air guns (if such are used), and electrical control and monitoring leads.

The lead-in may also have slight positive buoyancy through the inclusion of a buoyant filler such as foamed plastic or oil. The lead-in desirably is connected to the floatation member through a streamlined transition piece. Fittings and connectors on the transition piece allow the floatation member to be quickly disconnected from the hoses, wires and stress member in the lead-in.

The seismic sources may be attached to the stress member through chains and eyes. Provision for an explosive gas, e.g., propane or butane, line and an oxygen or air line would also need to be made if exploding type seismic source such as a sleeve exploderwereto be used.

The invention will be better understood from the foilowing description, given by way of example and with reference to the accompanying drawings, in which Figure 1 shows a schematic representation of one form of continuous floatation device in accordance with the invention being towed behind a seismic vessel, and Figure 2 shows, in perspective partial cutaway, details of the continuous float assembly.

Description of the preferred embodiments Figure 1 shows a schematic of the continuous seisic source float system in place as it would be towed behind a seismic boat. The floatation member 10 supports a number of air guns 12 via support chains 14. Although air guns are shown, other known sources such as the sleeve exploders discussed above are suitable. Two or more floatation members may be towed in parallel to produce an areal array as with any other floatation arrangement The float system is towed via a seismic boat 16 using lead-in 18. The lead-in, as mentioned above, is made up of a stress member of a high tensile strength wire rope, a high pressure air supply hoses for providing the air guns with a motive fluid, and electric control and monitoring circuits to fire the guns simultaneously and monitor their performance.The leadin enters the floatation member through a transition piece 20. Transition piece 20 is streamlined to allow smooth flow of water from the lead-in to the body of floatation member 10. The transition piece may also contain connectors for parting the stress member, high pressure air hose connections, and electrical connectors for the control and monitoring circuits.

Lead-in 18 may be stored on winch 22 found on the aft end of seismic boat 16.

Figure 2 shows the details of construction for floatation member 10 in a perspective partial cutaway drawing. There it can be seen that floatation member 10 is made up of a floatation chamber 24 and a wall 26. As noted above, this wall may be cylindrical or oval in cross section as well as having the V-shaped wet section depicted in Figure 2.

Floatation chamber 24 may be a single chamber all the way from the aft end of the transition piece 20, as shown in Figure 1,to the tail end of the float system.

Alternately, it may be divided into a series of smaller chambers, each sealed with respect to the other.

Those same chambers may be filled with a loose buoyant material such as foamed polyurethane or may have a similar material cast therein upon initial construction. Similarly, walls 26 may be constructed of a sea-resistant material such as a synthetic rubber which may cause the floatation member 10 to act as a bladder. The walls may be reinforced with textile fiber or fabric, or may be relatively stiff in configuration. It is alternatively possible to place a number of similar sections of floatation member end to end and join them in some suitable manner.

The seismic source float system also contains, within floatation member 10, stress member 28 located generally within the area sitting lowest in the water. Air supply hoses and electrical control and monitoring lines are shown schematically at 30 and may be contained within their own conduit 32. Air guns 12 having ports 34 are attached to the stress member 28 through chains 14 and pad eyes 36.

Individual air supply hoses and electrical control lines 38 are taken out of floatation member 10 through streamline pods 40. Pods 40 may contain synchronization apparatus to insure the proper and precise firing of each of the air guns in a particular array. Desirably pod 40 would also contain connectors for the individual air control lines and firing and control circuits 38 so that an individual gun 12 could be removed, installed, or deactivated by connecting or disconnecting the air gun 12 at pod 40.

As an example of the size a constructor of the float system might find suitable: a 12 inch diameter float containing air will support a weight of about 50 pounds per linear foot in sea water. Since the weight of air guns often range approximately from 75 pounds to 200 pounds in the air, depending on the type and chamber size, a floatation member of 12 inches diameter is practical.

The use and handling of the seismic source float system should be apparent to those skilled in this art. In any event, it is handled much in the same way as a portion of streamer cable containing hydrophones would be on the same boat. The accompanying hydrophone streamer cable used in virtually every offshore seismic prospecting setup has been omitted from the drawings and discussion of the disclosed embodiments for the sake of clarity.

Claims (11)

1. A float assembly adapted to support several seismic sources below and near the surface of a water body, comprising: an elongate flexible floatation chamber wall formed in a generally cylindrical shape and adapted to float and support several seismic sources, a stress member situated within and along said floatation chamber wall, means for towing said float assembly, and means for attaching a number of seismic sources to said stress member.

2. A float assembly according to claim 1, also comprising a transition between said towing means and said floatation chamber adapted to provide a smooth transition therebetween and containing at least means for disconnecting said stress member from said towing means.

3. A float assembly according to claim 1 or 2, additionally comprising means for supplying air to said seismic sources.

4. A float assembly according to claim 1, wherein said elongate flexible floatation chamberwall comprises discrete chambers.

5. A float assembly according to claim 4, wherein said chambers are separable from each other.

6. A float assembly according to any preceding claim, wherein said elongate flexible floatation chamber wall is filled with loose buoyant material.

7. A float assembly according to any one of claims 1 to 5, wherein said elongate flexible floatation chamber is substantially filled with buoyant material cast therein.

8. A seismic source array comprising a float assembly according to any preceding claim and a number of seismic sources attached to said stress member by said attaching means.

9. A seismic source array of claim 8 wherein said seismic sources are air guns.

10. A seismic source array according to claim 8, wherein said seismic sources comprise sleeve exploders.

11. A float assembly adapted to support several seismic sources below and near the surface of a water body, substantially as hereinbefore described with reference to the accompanying drawings.