Patent Publication Number: US-2019176936-A1

Title: Method and system for towing widely separated sources

Description:
BACKGROUND 
     Technical Field 
     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. 
     Discussion of the Background 
     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. 
       FIG. 1  illustrates a marine seismic data acquisition system  100 . In this side view, a vessel  110  tows seismic sources  112   a  and  112   b  and a streamer spread  114  having plural streamers (only one streamer  115  is shown in the figure) at predetermined depths under the water surface  111 . Although only one streamer  115  is visible in this vertical view, plural streamers are typically spread in a three-dimensional volume. Streamer  115 , which has a tail buoy  118  and likely other positioning devices attached, houses seismic receivers/sensors  116 . The streamer spread  114  is connected to the vessel through a front-end rigging  117  while sources  112   a  and  112   b  are connected to the vessel through umbilicals  119 . 
     The seismic sources generate seismic waves such as  120   a  and  120   b  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&#39; propagation directions change as the waves are reflected and/or transmitted/refracted/diffracted. Seismic waves  120   a  and  120   b  are partially reflected as  122   a  and  122   b  and partially transmitted as  124   a  and  124   b  at seafloor  32 . Transmitted waves  124   a  and  124   b  travel through first layer  34 , are then reflected as waves  126   a  and  126   b , and transmitted as  128   a  and  128   b  at interface  36 . At the surface of reservoir  40 , waves  128   a  and  128   b  are then partially transmitted as waves  130   a  and  130   b  and partially reflected as waves  132   a  and  132   b . 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,  FIG. 2  shows the front-end rigging  117  including wide tow lines  220  and plural lead-ins  222 . The two wide tow lines  220  connect to deflectors  224  and spread ropes  226  while the lead-ins connect to streamers  115  and spread ropes  226 .  FIG. 2  also shows the umbilicals  119  that connect the vessel  110  to the sources  112   a  and  112   b . Each source  112   a  and  112   b  may include plural source elements, e.g.,  112   b - i , where “i” is between 2 and 20. The umbilicals  119  are connected to lead-ins  222  through various links  230  and  230 ′ as illustrated in  FIG. 2 . 
     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  112   a  and  112   b  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  112   a  and  112   b  may have one or more subarrays, each sub-array including plural source elements  112   a - i  and  112   b - i , respectively.  FIG. 2  shows that each source  112   a  and  112   b  includes two sub-arrays. 
     A single sub-array  300  is shown in  FIG. 3 . Sub-array  300  includes one or more floats  360  from which individual source elements  316  are suspended with cables, chains or ropes  362 . In one application, clusters of individual source elements are provided at location  316 . Various cables and hoses connect individual source elements  316  to the vessel for providing electric power, compressed air, data transmission, etc. For example, a hose  364  provides compressed air and a cable  366  provides electric power and/or data transmission. 
     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. 
     SUMMARY 
     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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a schematic diagram of a conventional marine seismic acquisition configuration; 
         FIG. 2  is a top view of a marine seismic acquisition configuration that uses mechanical links between the sources and the lead-ins; 
         FIG. 3  is a schematic diagram of a source sub-array; 
         FIG. 4  is a top view of a marine seismic acquisition system that uses a dedicated source front-end gear for towing the sources; 
         FIG. 5  is a top view of a marine seismic acquisition system that uses a dedicated source front-end gear for towing the sources and a dedicated front-end rigging for towing the streamers and there are no mechanical links between these two mechanisms; 
         FIG. 6A  illustrates a deflector with a float and  FIG. 6B  illustrates a deflector with no float; 
         FIG. 7  is a side view of a marine seismic acquisition system that uses a dedicated source front-end gear for towing the sources and a dedicated front-end rigging for towing the streamers; 
         FIG. 8  is a side view of another marine seismic acquisition system that uses a dedicated front-end gear for towing the sources and a dedicated front-end rigging for towing the streamers; 
         FIG. 9  is a top view of a marine seismic acquisition system that uses a dedicated source front-end gear for towing the sources and a dedicated front-end rigging for towing the streamers and the sources are located near the heads of the streamers; 
         FIG. 10  is a flow chart of a method for collecting seismic data with a marine seismic acquisition system that uses a dedicated source front-end gear for towing the sources and a dedicated front-end rigging for towing the streamers. 
     
    
    
     DETAILED DESCRIPTION 
     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  412 A to  412 C. 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  419 A to  419 C, 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  412 A and  412 C, respectively, with corresponding spur lines  420 A and  422 A. 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)  420 B and  422 B 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  420 B and  422 B. 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. 
       FIG. 4  shows the sources being separated by a distance G, where G can have a value between 50 and 400 m. One skilled in the art would know how to size up the deflectors for achieving even a larger gap between the sources. A deflector is an element having a surface that makes a certain angle with the direction of the vessel so that a force is generated by the movement of the deflector in water. This force is used to move the deflectors  420  and  422  away from each other, so that the separation gap G between the sources is maintained. Various shapes and sizes of a deflector are disclosed in U.S. Pat. No. 9,676,454, assigned to the assignee of this application. Note that the streamer deflectors  452  and  454  (shown in  FIG. 5 ), which are different from the source deflectors  420  and  442 , are used to maintain a separation gap between the streamers. Also, the streamer deflectors  452  and  454  maintain the separation ropes  456  between the streamers stretched. 
     Note that  FIG. 4  shows the vessel  402  towing only the source front-end gear  410  and the sources  412 A to  412 C for simplicity. In fact, vessel  402  also tows the front-end rigging  440  of the streamers and the streamers  450 , as shown in  FIG. 5 .  FIG. 5  also shows the wide tow cables  442  and  444 , the lead-ins  446 , and associated streamer deflectors  452  and  454 . The streamer deflectors are connected to the wide tow cables and spread ropes  458 . The streamers  450  are separated by spread ropes  456 . Each streamer  450  may have a corresponding head buoy  450 A, attached to the streamer head  450 B and to spread ropes  456 . 
     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  412 A to  412 C 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  FIG. 5 ), and implementing a desired gap G between adjacent sources is now possible with only the source front-end gear  410  and absolutely no involvement from the front-end rigging  440  of the streamers. The source front-end gear  410  includes at least connecting ropes  424  and  426 , source deflectors  420  and  422 , spur lines  420 A and  422 A, and umbilicals  419 A to  419 C while the front-end rigging  440  includes the wide tow lines  442  and  444 , the plural lead-ins  446 , deflectors  452  and  454 , and separation ropes  456  and  458 . 
     Connecting ropes  424  and  426  of the source front-end gear  410  may be connected to corresponding winches  424 A and  426 A 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  419 A to  419 C, the inline position of the sources  412 A to  412 C 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,  FIG. 6A  shows a first possible deflector  600  that has its body  602  underwater and the body is connected to a float  606 . While  FIG. 6A  shows the body  602  directly connected to the float, it is possible that the body is connected to the float with one or more ropes.  FIG. 6A  also shows the connecting rope  424  and the spur line  420 A connected to the deflector. In one embodiment, it is possible that the connecting rope  424  and spur line  420 A connect to a control mechanism  608 , which is capable of changing the attach angle of the diverter to the inline direction. Such a control mechanism  608  is known in the art and thus, not described herein. The control mechanism  608  is connected to a transceiver  610 , that is located on the float  606 . In this way, instructions from the vessel may be received at the transceiver  610  during the seismic survey, to adjust the attach angle of the diverter, if necessary to apply a different separating force to the sources. 
       FIG. 6B  shows another possible implementation of the deflector  420 , in which its body  602  is not connected to a float. The shape of the body  602  is selected in such a way that the forces generated by the water while the deflector moves in the water maintain the deflector at a constant depth under water, as discussed, for example, in U.S. Pat. No. 9,676,454. 
     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  FIGS. 7 and 8 .  FIG. 7  shows an embodiment in which the source deflectors are connected to corresponding floats while  FIG. 8  shows an embodiment in which the source deflectors have no floats. In both embodiments, the umbilicals  419 A to  419 C (only one is shown for simplicity) are located, in terms of depth (the depth is shown on the Y axis, with zero indicating the water level), below the wide tow line  442  of the streamers  450  and above the lead-ins  446 . Also note that the source  412 A is located below the wide tow line  442  and above the lead-ins  446 , along a depth direction.  FIG. 7  shows the float  606  of the source deflector  420  floating at the water surface, together with the float  360  of the source  412 A. Plural links  362  connect the float  360  to the source  412 A (see  FIG. 3  for more details) and a spur line  420 A connects the source deflector  420  to the source  412 A. Note that in  FIG. 8 , the source deflector  420  has no float, and the source deflector  420  is connected to source  412 A through a spur line  420 A and the source deflector  420  is fully underwater. 
     With the configuration illustrated in  FIGS. 7 and 8 , the interaction between the elements of the source front-end gear  410  and the front-end rigging  440  is minimized, and thus, the entanglement between the ropes of the two mechanisms can be maintained to a minimum. 
     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&#39; heads or even in line with the streamers&#39; heads. In this regard,  FIG. 9  shows the source  412 A being aligned along the inline direction (X axis) with the head-buoy  450 A of the streamer  450 . Note that the head-buoy  450 A of the streamer  450  is connected at the head portion  450 B of the streamer as shown in  FIGS. 7 and 8 . Thus, as illustrated in  FIG. 9 , the source  412 A (and of course the other sources) may be located to have the same inline position as the head streamer because of the independence and flexibility offered by the source front-end gear. 
     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  FIG. 10 . The method includes a step  1000  of deploying plural streamers with a streamer front-end rigging, a step  1002  of deploying at least two sources with a source front-end gear, a step  1004  of towing the plural streamers while actuating the at least two sources, and a step  1006  of recording the seismic data with the plural streamers. The source front-end gear is independent from the streamer front-end rigging. 
     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.