Patent Application
20080144434
Not applicable.
Not applicable.
1. Field of the Invention
The invention relates generally to the field of marine seismic survey apparatus and methods. More specifically, the invention relates to structures for marine seismic streamers that have reduced noise induced by effects of towing such streamers in the water.
2. Background Art
In marine seismic surveying, a seismic vessel travels on the surface of a body of water such as a lake or the ocean. The seismic vessel typically contains seismic data acquisition equipment, which includes devices such as navigation control, seismic source control, seismic sensor control, and signal recording devices. The seismic acquisition equipment causes a seismic source towed in the body of water, by the seismic vessel or another vessel, to actuate at selected times. The seismic source may be any type well known in the art of seismic acquisition, including air guns or water guns, or most commonly, arrays of air guns. Seismic streamers, also called seismic cables, are elongate cable-like structures that are towed in the body of water by the seismic survey vessel or by another vessel. Typically, a plurality of seismic streamers is towed behind the seismic vessel laterally spaced apart from each other. The seismic streamers contain sensors to detect the seismic wavefields initiated by the seismic source and reflected from acoustic impedance boundaries in the subsurface Earth formations below the water bottom.
Conventionally, seismic streamers contain pressure-responsive sensors such as hydrophones, but seismic streamers have also been proposed that contain particle motion sensors, such as geophones, in addition to hydrophones. The sensors are typically located at selected intervals along the length of seismic streamers.
Seismic streamers also include electronic components, electrical wiring and may include other types of sensors. Seismic streamers are typically assembled from sections, each section being approximately 75 meters in length. A number of such sections are joined end to end, and can extend the assembled streamer to a total length of many thousands of meters. Position control devices, such as depth controllers, paravanes, and tail buoys are affixed to the streamer at selected positions and are used to regulate and monitor the movement of the streamer in the water. During operation, the seismic sources and streamers are typically submerged at a selected depth in the water. The seismic sources are typically operated at a depth of 5-15 meters below the water surface and the seismic streamers are typically operated at a depth of 5-40 meters.
A typical streamer section consists of an external jacket, connectors, spacers, and strength members. The external jacket is formed from a flexible, acoustically transparent material such as polyurethane and protects the interior of the streamer section from water intrusion. The connectors are disposed at the ends of each streamer section and link the section mechanically, electrically and/or optically to adjacent streamer sections and, therefore, ultimately link it to the seismic towing vessel. There is at least one, and are usually two or more such strength members in each streamer section that extend the length of each streamer section from one end connector to the other. The strength members provide the streamer section with the capability to carry axial mechanical load. A wire bundle or cable also extends the length of each streamer section, and can contain electrical power conductors and electrical data communication wires. In some instances, optical fibers for signal communication are included in the wire bundle.
Typically, hydrophones or groups of hydrophones are located within the streamer section. The hydrophones are frequently mounted within corresponding spacers for protection. The distance between hydrophone containing spacers is ordinarily about 0.7 meters. A hydrophone group, typically comprising 16 hydrophones, thus extends for a length of about 12.5 meters. The hydrophones in a group are typically connected in series to cancel effects of certain types of noise to which the streamer may be exposed. The interior of the seismic streamers is typically filled with a void filling material to provide buoyancy and desired acoustic properties. Many seismic streamers have been filled with a liquid, such as oil or kerosene.
Ideally, in a streamer moving at constant speed, all the streamer components including the jacket, the connectors, the spacers, the strength members, wire bundle, sensors and liquid void filling material all move at the same constant speed and do not move relative to each other. Under actual movement conditions, however, transient motion of the streamers takes place, such transient motion being caused by events such as pitching and heaving of the seismic vessel, movement of the paravanes and tail buoys attached to the streamers, strumming of the towing cables attached to the streamers caused by vortex shedding on the cables, and operation of depth-control devices located on the streamers. Any of the foregoing types of transient motion can cause transient motion (stretching) of the strength members. Transient motion of the strength members displaces the spacers or connectors, causing pressure fluctuations in the liquid void filling material that are detected by the hydrophones. Pressure fluctuations radiating away from the spacers or connectors also cause the flexible outer jacket to compress in and bulge out in the form of a traveling wave, giving the phenomenon “bulge waves” its name.
In addition, there are other types of noise, often called “flow noise”, which can affect the quality of the seismic signal detected by the hydrophones. For example, vibrations of the seismic streamer can cause extensional waves in the outer jacket and resonance transients traveling down the strength members. A turbulent boundary layer created around the outer jacket of the streamer by the act of towing the streamer can also cause pressure fluctuations in the liquid core-filling material. In liquid filled streamer sections, the extensional waves, resonance transients, and turbulence-induced noise are typically much smaller in amplitude than the bulge waves, however they do exist and affect the quality of the seismic signals detected by the hydrophones. Bulge waves are usually the largest source of vibration noise because these waves travel in the liquid core material filling the streamer sections and thus act directly on the hydrophones.
It is known in the art to replace the liquid core material in a streamer section with a soft, flexible solid core material, such as gel. The introduction of a softer, flexible solid material may block the development of bulge waves compared to a liquid core material. Using a soft, flexible material will eliminate a substantial portion of the problem with “bulge waves”, but the so-called Poisson effect from the strength members can increase. Because of the relatively high tensile stiffness of the strength members, transients generally travel along the strength members at velocities near to or greater than that of the sound velocity in water, such velocities typically in the range of 1000 to 1500 meters per second. The actual velocity of transients along the strength members depends mainly on the elastic modulus of the strength member material and the tension applied to the streamer as it is towed in the water. The lower the elastic modulus the more compliant the streamer will be, and thus the more transient energy it will dissipate as heat and the less will pass through the strength member. Special elastic sections are normally placed at either end of a streamer cable to reduce the effects of transients.
There is still a need to further improve the attenuation of longitudinal waves transmitted through the strength members of marine seismic streamers.
One aspect of the invention is a seismic streamer. A seismic streamer according to this aspect of the invention includes at least one array of sensors each disposed in a holder at longitudinally spaced apart locations. Each sensor in the at least one array may be more sensitive to pressure waves traveling one longitudinal direction than in an opposed direction. At least one sensor or at least one sensor holder is oriented differently than the others in the array.
A seismic streamer according to another aspect of the invention includes a jacket covering an exterior of the streamer and at least one strength member extending along the length of the jacket and disposed inside the jacket. At least one array of sensors is disposed inside the jacket along the strength member. Each sensor in the at least one array is disposed in a respective sensor holder. Each sensor holder is configured to expose the sensor therein to the interior of the jacket in only one longitudinal direction. The holders are arranged such that at least one holder is longitudinally oriented differently than the other holders. In some embodiments, the holders are oriented in a pattern that will provide optimum symmetry after array forming. An acoustically transparent material fills the void spaces in the interior of the jacket.
Other aspects and advantages of the invention will be apparent from the description and claims which follow.
During operation, certain equipment (not shown separately) in the recording system 16 causes the source 18 to actuate at selected times. When actuated, the source 18 produces seismic energy 19 that emanates generally outwardly from the source 18. The energy 19 travels downwardly, through the water 12, and passes, at least in part, through the water bottom 20 into the formations 21, 23 below. Seismic energy 19 is at least partially reflected from one or more acoustic impedance boundaries 22 below the water bottom 20, and travels upwardly whereupon it may be detected by the sensors in each sensor array 24. Structure of the formations 21, 23, among other properties of the Earth's subsurface, can be inferred by travel time of the energy 19 and by characteristics of the detected energy such as its amplitude and phase.
Having explained the general method of operation of a marine seismic streamer, an example embodiment of a streamer section according to the invention will be explained with reference to
The streamer segment 10A in the present embodiment may be about 75 meters overall length. A streamer such as shown at 10 in
The segment 10A can include a number of buoyancy spacers 34 disposed in the jacket 30 and coupled to the strength members 42 at spaced apart locations along their length. The buoyancy spacers 34 may be made from foamed polyurethane or other suitable, selected density material. The buoyancy spacers 34 have a density selected to provide the segment 10A preferably with approximately the same overall density as the water (12 in
The segment 10A includes a generally centrally located conductor cable 40 which can include a plurality of insulated electrical conductors (not shown separately), and may include one or more optical fibers (not shown). The cable 40 conducts electrical and/or optical signals from the sensors (which will be further explained below with reference to
Sensors, which in the present embodiment may be hydrophones, can be disposed inside sensor holders, shown in
In the present embodiment, and as will be explained below with reference to
In a particular implementation of the invention, there are thus six such arrays, spaced apart from each other at about 12.5 meters. The spacing between individual hydrophones in each array should be selected so that the axial span of the array is at most equal to about one half the wavelength of the highest frequency seismic energy intended to be detected by the streamer (10 in
At selected positions along the streamer (10 in
In the present embodiment, the interior space of the jacket 30 may be filled with a material 46 such as buoyancy void filler (“BVF”), which may be a curable, synthetic urethane-based polymer. The BVF 46 serves, to exclude fluid (water) from the interior of the jacket 30, to electrically insulate the various components inside the jacket 30, to add buoyancy to a streamer section and to transmit seismic energy freely through the jacket 30 to the sensors (in sensor holders 32). The BVF 46 in its uncured state is essentially in liquid form. Upon cure, the BVF 46 no longer flows as a liquid, but instead becomes substantially solid. However, the BVF 46 upon cure retains some flexibility to bending stress, substantial elasticity, and freely transmits seismic energy to the sensors (in sensor holders 32). It should be understood that the BVF used in the present embodiment only is one example of a gel-like substance that can be used to fill the interior of the streamer. Other materials could be also used. For example, heating a selected substance, such as a thermoplastic, above its melting point, and introducing the melted plastic into the interior of the jacket 30, and subsequent cooling, may also be used in a streamer according to the invention. Oil or similar material may also be used to fill the interior of the streamer.
The sensor spacers 32, as explained in the Background section herein, are typically molded from a rigid, dense plastic to better protect the seismic sensors therein from damage during handling and use. An exterior configuration of the holder 32 is preferably such that the holder 32 fits snugly within the jacket (30 in
The sensor 56 may be mounted inside the passage by interference fit, or preferably by highly compressible elastomer rings (not shown) to provide acoustic isolation between the holder 32 and the sensor.
Another embodiment of sensor holder 32, shown in
The effect of the structure of the sensor spacers 32 shown in
One possible arrangement of an array having alternatingly disposed front 32F and rear 32R closed spacers each having a sensor (56 in
It has been determined by testing in the laboratory that hydrophones may have some longitudinal directionality to its response to pressure waves. Such longitudinal directionality appears to manifest itself by higher response amplitude to longitudinally traveling acoustic energy with the sensor oriented in one longitudinal direction along the streamer than along the other longitudinal direction. In some embodiments, the longitudinal orientation of at least one sensor in the array is different than the longitudinal orientation of the other sensors in the array. In some embodiments, the seismic sensor longitudinal orientation may alternate along the array. In one example, such alternating longitudinal orientation may be symmetric about a longitudinal center of the array. Other types of seismic sensors may be used in other implementations according to the present embodiment to ensure optimum symmetry after array forming, in the case such other sensors have longitudinal directionality. Referring to
The aft 132R and forward 132F sensor holders (with respectively oriented sensors in each holder) are shown in
A streamer made using sensor spacers and sensor arrays as described herein may provide substantially reduced effect of noise related to axial vibrations than streamers made according to structures known in the art prior to the present invention.
While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.
