Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for towing widely separated seismic sources in water.
Marine seismic data acquisition and processing generate a profile (image) of the geophysical structure (subsurface) under the seafloor. While this profile does not provide an accurate location for the oil and gas, it suggests, to those trained in the field, the presence or absence of oil and/or gas. Thus, providing a high-resolution image of the subsurface is an ongoing process for the exploration of natural resources, including, among others, oil and/or gas.
The seismic sources generate seismic waves such as 120a and 120b that propagate through the water layer 30 toward the seafloor 32. At interfaces (e.g., 32 and 36) between layers (e.g., water layer 30, first layer 34, and second layer 38) inside which the seismic waves propagate with different wave propagation velocities, the waves' propagation directions change as the waves are reflected and/or transmitted/refracted/diffracted. Seismic waves 120a and 120b are partially reflected as 122a and 122b and partially transmitted as 124a and 124b at seafloor 32. Transmitted waves 124a and 124b travel through first layer 34, are then reflected as waves 126a and 126b, and transmitted as 128a and 128b at interface 36. At the surface of reservoir 40, waves 128a and 128b are then partially transmitted as waves 130a and 130b and partially reflected as waves 132a and 132b. The waves traveling upward may be detected by receivers 116. Maxima and minima in the amplitude versus time data recorded by receivers carry information about the interfaces and traveling time through layers.
The marine seismic data acquisition system 100 does not illustrate the connections present between the front-end rigging 117 and the umbilicals 119. In this regard,
Deflectors 224 are provided on the sides of this arrangement to maintain a transverse distance (relative to the path of the vessel) between streamers 115. The distance between sources 112a and 112b is about 50 m. The distance between the sources is maintained with the links 230. Thus, essentially, the deflectors 224 also maintain the source separated via spread ropes 226 and links 230. Note that the terms “rope,” “cable,” “link,” and “wire” are used sometimes interchangeably in this document. Thus, these terms should not be construed in a narrow sense, but rather as those skilled in the art would expect.
Each of source 112a and 112b may have one or more subarrays, each sub-array including plural source elements 112a-i and 112b-i, respectively.
A single sub-array 300 is shown in
Source bases 318 are connected to a bell housing 380 via a connection 382. In one application, bell housing 380 and connection 382 may form an enclosure in which the various cables 364 and 366 are entering. Bell housing 380 may be made of a resistant material, for example, stainless steel. A bend restrictor device 390 may be connected to the bell housing 380 and also to vessel 110 via an umbilical 392. Bend restrictor device 390 is configured to prevent an over-bending of the front part of the source array due to the towing force applied via umbilical 392.
As the front-end rigging for the streamers and the umbilicals for the sources are mechanically connected to each other, the current separation between the sources is about 50 m, with a maximum of about 80 m. Also, the front-end rigging of the streamers and the umbilicals have been designed for such limited separation.
However, the modern seismic survey systems require today a much larger source separation, for example, between 200 and 300 m. Such wide separation between the sources is not possible with the existing rigging. Thus, there is a need for a rigging system that can handle a large source separation.
According to an embodiment, there is a source front-end gear for towing sources. The gear includes first and second umbilicals for connecting first and second seismic sources to a towing vessel; first and second deflectors; first and second connecting ropes that connect the first and second deflectors to the vessel; and first and second spur lines that connect the first and second deflectors to the first and second seismic sources. The source front-end gear is free of any mechanical connection to a front-end rigging that connects streamers to the vessel.
According to another embodiment, there is a seismic acquisition system for acquiring seismic data. The system includes first and second seismic sources; a source front-end gear that tows the first and second sources; plural streamers having seismic sensors for recording seismic waves; and a front-end rigging that tows the plural streamers. The source front-end gear is free of any mechanical connection to the front-end rigging.
According to still another embodiment, there is a method for acquiring seismic data. The method includes deploying plural streamers with a front-end rigging, deploying at least two sources with a front-end gear, towing the plural streamers while actuating the at least two sources, and recording the seismic data with the plural streamers. The source front-end gear is independent from the front-end rigging.
The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate one or more embodiments and, together with the description, explain these embodiments. In the drawings:
The following description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to the terminology and structure of a single seismic vessel that tows streamers and three sources. However, the embodiments to be discussed next are not limited to this configuration, but may be applied to a vessel that tows more or less than three sources.
Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner with other features or structures in one or more embodiments.
According to an embodiment, there is a seismic data acquisition system 400 that includes a source front-end gear 410 that connects exclusively to sources 412A to 412C. This means that the source front-end gear 410 is connected between vessel 402 and the three sources (more or less sources may be used), but there is no link, wire, cable or rope mechanically connecting the source front-end gear 410 to the front-end rigging of the streamers. To reflect this independence of the source front-end gear 410 from the streamers and the front-end rigging of the streamers, the source front-end gear 410 is considered in the following to be an independent source front-end gear.
The source front-end gear 410 includes umbilicals 419A to 419C, one for each source. The umbilicals are configured to exchange compressed air (if air guns are used as the source elements), power and data between the vessel and the sources. The source front-end gear 410 also includes two source deflectors 420 and 422 connected to the outer sources 412A and 412C, respectively, with corresponding spur lines 420A and 422A. The deflectors are also connected with corresponding connecting ropes 424 and 426 to the vessel. Each deflector may have a corresponding transceiver (e.g., radio transceiver) 420B and 422B for communicating with a vessel transceiver 404. A management system 406 located on the vessel controls the communications between the vessel transceiver 404 and the deflector transceivers 420B and 422B. The radio communication between the transceivers is used, as discussed later, for adjusting a position of the deflector in water, for providing an adjustable force to the sources, so that a gap between the sources is maintained constant.
Note that
One will notice that the source front-end gear 410 and the front-end rigging 440 for the streamers are now decoupled, i.e., each mechanism is independent of the other. In other words, placing the sources 412A to 412C at a desired inline position (inline axis X) and also at a desired cross-line position (cross-line axis Y is perpendicular to the inline axis X in
Connecting ropes 424 and 426 of the source front-end gear 410 may be connected to corresponding winches 424A and 426A located on vessel 402, so that a length of connecting ropes 424 and 426 may be adjusted. By adjusting the length of connecting ropes 424 and 426 and the lengths of umbilicals 419A to 419C, the inline position of the sources 412A to 412C can be adjusted.
However, the addition of the source front-end gear 410 adds new elements to an already complex and extensive front-end rigging 440 and a risk exists that some ropes or other elements of the source front-end gear 410 may interfere with some ropes or other elements of the front-end rigging 440. This might become especially a serious problem given the possible configurations of the source deflectors. In this respect,
To prevent rope entanglement between the source front-end gear 410 and the front-end rigging 440, the two mechanisms are distributed/positioned in water as now discussed with regard to
With the configuration illustrated in
Having the source front-end gear deployed independent of the front-end rigging for the streamers, it is now possible to locate the sources close to the streamers' heads or even in line with the streamers' heads. In this regard,
In this way, the source is located very close to the first receiver 116 of the streamer 450, which permits to record very low-offset seismic data, which is not possible with traditional sources because they are linked to the lead-ins 446. Being able to record very low-offset seismic data is advantageous because this data is responsible for high spatial and vertical resolution of the final image of the surveyed subsurface.
In addition, the presence of the source front-end gear allows to move the sources before operational transitions operations, as, for example, vessel turn at the end of a line, speed change, source recovery, etc. In one embodiment, it is possible to deploy the sources behind even the heads of the streamers. The source elements of the sources discussed above may be airguns, marine vibrators, or a combination of the two.
A method for acquiring seismic data with the streamer front-end rigging and the source front-end gear discussed above is now presented with regard to
The disclosed embodiments provide a marine seismic system and a method for towing two or more sources with a desired separation gap, independent of a front-end rigging of the streamers. It should be understood that this description is not intended to limit the invention. On the contrary, the embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details.
Although the features and elements of the present embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein.
This written description uses examples of the subject matter disclosed to enable any person skilled in the art to practice the same, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims.