Abstract:
A method for improving a 4-dimensional (4D) repeatability by modifying a given path to be followed by a source during a seismic survey. The method includes receiving the given path at a control device associated with a vehicle that caries the source; following the given path during a first seismic survey that is a baseline survey for the 4D seismic survey; deviating from the given path to follow a new path when encountering an obstacle on the given path; and updating the given path, based on the new path, to obtain an updated given path when a deviation condition is met.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority and benefit from Provisional Patent Application No. 61/722,439, filed Nov. 5, 2012, for “Seismic Vibrator Guidance System based on a Field Acquired Trajectory to Improve 4D repeatability of the Following Surveys,” the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for determining a field trajectory for a land-based seismic survey to improve 4-dimensional (4D) repeatability. 
         [0004]    2. Discussion of the Background 
         [0005]    During the past years, interest in monitoring oil and/or gas reserves has increased. Time-lapse (or 4D) seismic monitoring of producing oil fields is an accepted optimization method for field development and product recovery, providing significant recovery rate improvements and drilling cost savings. 
         [0006]    Time-lapse seismic reservoir monitoring is the comparison of 3D seismic surveys at two or more points in time. Time-lapse seismic reservoir monitoring also has potential for increasing ability to image fluid movement between wells. A traditional configuration for achieving a 4D land seismic monitoring is illustrated in  FIG. 1 , which shows a system  10  that includes plural receivers  12  positioned over an area  12   a  of a subsurface to be explored and in contact with the surface  14  of the ground. A number of vibroseismic sources  16  (e.g., located on corresponding trucks) are also placed on the surface  14  in area  16   a , in the vicinity of area  12   a  of the receivers  12 . A recording device  18  is connected to the plurality of receivers  12  and placed, for example, in a station truck  20 . Each source  16  may be composed of a variable number of vibrators, typically between one and five, and may include a local controller  22 . A central controller  24  may be present to coordinate the sources&#39;  16  shooting times. A global positioning system (GPS)  26  may be used to time-correlate the sources  16  and receivers  12 . 
         [0007]    With this configuration, sources  16  are controlled to generate seismic waves, and the plurality of receivers  12  record waves reflected by the oil and/or gas reservoirs and other structures. The seismic survey may be repeated at various time intervals, e.g., months or years apart, to determine changes in the reservoir. For reservoir monitoring, it is traditional to maintain the receivers at their locations in the field over the entire time of the 4D surveys (i.e., not to remove the receivers at the end of a first survey and to deploy them again at the beginning of a second survey). It is also customary to have mobile sources that move from location to location and shoot seismic waves. For this case, when the first survey ends, the sources are removed, and when the second survey starts, the same sources or other sources are brought back in. 
         [0008]    For this situation, it is desirable to position and shoot the sources at the same geographic positions during each survey, i.e., the first survey, the second survey, etc., of the 4D survey. However, the following problems are noted in practical situations.  FIG. 2  shows a system  200  that includes plural sources  202  and plural receivers  204 . A single source  202  is shown for simplicity. The receivers are fixed, i.e., their locations do not change during the 4D survey. However, the sources are truck-mounted and they carried from one shooting position to another by truck. This means that the truck driver  206  is instructed to follow a given path  210  each time an area  211  is surveyed. Path  210  is traditionally pre-calculated by the operator of the seismic survey, usually at its facilities, which can be hundreds, if not thousands, of miles from the surveyed area  211 . 
         [0009]    Because the operator relies on maps for determining path  210 , which may not be accurate or omit certain details that influence the path actually followed by the driver, path  210  can intersect with an obstacle or avoidance  214  (e.g., a hill, a pond, a man-made structure, a boulder, etc.). In one case, the given path does not match the truck specifications (e.g., steering angle too wide, steep slope which the truck cannot climb, etc.). Consequently, the driver cannot follow given path  210  and cannot shoot source  202  at the required locations  210 A,  210 B, etc. In these situations, the driver takes the liberty of deviating from given path  210  and following another path  216 , which the driver believes to be close enough to given path  210 . This means that source  202  is shot at locations  216 A,  216 B, etc. However, the driver&#39;s choice may change from survey to survey, thus, introducing undesirable inaccuracies in the collected seismic data (geographic discrepancies  220  between the intended shooting positions and the positions actually shot, which vary along the path). 
         [0010]    Given the fact that operators try to understand the behavior of the reservoir over time by qualitatively and quantitatively quantifying these effects, 4D reservoir monitoring is very sensitive to shooting sources at the same locations when the survey is repeated. 
         [0011]    Thus, there is a need for developing a device and a method for minimizing geographic discrepancies between shoots intended to be performed at the same position over time. 
       SUMMARY 
       [0012]    According to one exemplary embodiment, there is a method for modifying a given path to be followed by a source during a 4-dimensional (4D) seismic survey. The method includes receiving the given path at a control device associated with a vehicle that caries the source; following the given path during a first seismic survey that is a baseline survey for the 4D seismic survey; deviating from the given path to follow a new path when encountering an obstacle along the given path; and updating the given path, based on the new path, to obtain an updated given path when a deviation condition is met. 
         [0013]    According to another embodiment, there is a control device configured to modify a given path to be followed by a source during a 4-dimensional (4D) seismic survey. The control device includes an interface configured to receive the given path; and a processor connected to the interface. The processor is configured to monitor the given path during a first seismic survey that is a baseline survey for the 4D seismic survey, track a deviation from the given path when the source follows a new path when encountering an obstacle along the given path, and update the given path, based on the new path, to obtain an updated given path when a deviation condition is met 
         [0014]    According to still another exemplary embodiment, there is a computer readable medium including computer executable instructions, wherein the instructions, when executed by a processor, implement the above discussed method. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    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: 
           [0016]      FIG. 1  is a schematic diagram of a conventional onshore seismic data acquisition system; 
           [0017]      FIG. 2  is a schematic diagram of a 4D land seismic data acquisition system; 
           [0018]      FIG. 3  is a schematic diagram of a novel 4D land seismic data acquisition system according to an exemplary embodiment; 
           [0019]      FIG. 4  is a flowchart illustrating a method for determining a new given path according to an exemplary embodiment; and 
           [0020]      FIG. 5  is a schematic diagram of a computing device. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    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 seismic system having a single source that is being shot at desired geographical positions over a period of time. However, the embodiments to be discussed next are not limited to a single source, but are applicable to systems with many sources. Also, the following embodiments are discussed with regard to a land seismic survey. However, the following embodiments are equally applicable to a marine seismic survey. 
         [0022]    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 in one or more embodiments. 
         [0023]    According to an exemplary embodiment, there is a method for modifying a given path that needs to be followed by a source during a first seismic survey to account for various obstacles and avoidances the source encounters. Once the modified given path has been followed during a first seismic survey that is part of a 4D seismic survey, the system uses this modified given path as the new given path to be followed by sources in subsequent surveys. In this way, discrepancies between geographical locations of the sources during repeat shootings over time are minimized. 
         [0024]    According to an exemplary embodiment illustrated in  FIG. 3 , a seismic survey system  300  includes one or more trucks  306  (only one is shown) carrying corresponding sources  302  and plural receivers  304  distributed over an area  305  to be surveyed. The truck  306  is given a path  310  to follow, and this path is calculated prior to the survey, e.g., at the operator&#39;s facilities. However, an obstacle  314  or an avoidance induced by truck specifications, unknown to the operator, disrupts given path  310 . Thus, when the truck is in the field and follows given path  310  and has to shoot the source  302  at given locations  310 A and  3108 , the driver is suddenly faced with obstacle  314 . The driver then takes the liberty, as is typically done, of altering the truck&#39;s path and following a new path  316  that (partially) deviates from given path  310 . However, new path  316  avoids obstacle or avoidance  314 . 
         [0025]    The truck  306  has on board a control device  340  (that may include a processor, storage device, input/output interface, screen, Internet connection, etc.) and a location device  342  (e.g., GPS) that tracks the location of the truck relative to Earth and/or given path  310 . The control device  340 , or its operator in the truck, when faced with following a new path  316  instead of given path  310 , may decide to accept new path  316  as the given path, i.e., to alter/modify given path  310  to coincide with new path  316 . This process may alter/modify one or more portions of given path  310 . In this way, new given path  316  takes into account obstacle or avoidance  314  and allows, the next time the survey is performed, the truck to follow this path. 
         [0026]    According to an exemplary embodiment, this alteration/modification of the new path  310  is allowed only the first time the truck performs the survey, i.e., the first seismic survey in a series of seismic surveys that constitute the 4D seismic survey. In one application, this alteration/modification is only allowed when deviation of new path  316  relative to given path  310  is larger than a given threshold. This last condition (deviation condition) may be imposed for the following reason. The location device  342  acquires a location of the truck with a given error. Considering a simple example, suppose that this error is 1 m. If the truck is exactly on given path  310 , and the location device  342  determines that the truck is 1 m away from the given path, it is undesirable to consider this deviation as a new path and to modify the given path. For this reason, a given threshold is entered and only if the deviation of the truck from the given path, including the error of the location device, is larger than the threshold, the control device will modify the given path. For the example considered in this paragraph, the threshold may be set to 2 m. However, these numbers are exemplary and not intended to limit the applicability of the novel method. Other values for the threshold may be used, depending on the accuracy of the location device and other characteristics of the seismic survey. 
         [0027]    Regarding the shooting positions  310 A,  310 B, etc., note that these positions are also affected by the new given path  316 . The shooting positions are input into the control device and they specify a geographical location where the source should be and a time at which that source is shot. By changing given path  310  to new given path  316 , the shooting positions are also changed to lie on new given path  316 . Note that the sources may be shot in various modes, i.e., sequentially, simultaneously, flip-flop, flip-flop with a certain delay, etc. In one exemplary embodiment, geographical locations  310 A,  310 B, etc., are translated into geographical locations  316 A,  316 B, etc., by simply drawing perpendicular segments  360  from geographical locations  310 A,  310 B, etc. to the new given path  316 . In one application, the segments are not perpendicular on the given path or the new path, but rather are defined to have minimum values. 
         [0028]    Returning to  FIG. 3 , suppose that the truck tried to follow given path  310  for the first time, but was unable to do so because of obstacle or avoidance  314 . Thus, the truck followed path  316 . In this situation, the control device (or the operator) accepts new path  316  as the given path, i.e., original given path  310  is updated with the geographical locations of new path  316  to obtain the new given path ( 310 ,  316 ). Also, assume that the truck follows new path  316  at time t 0 . 
         [0029]    When the survey is repeated again at time t 1 &gt;t 0 , for example, after a few months or more, the new given path the truck needs to follow is given path  310  combined with new path  316 , and not original given path  310 . Note that truck  306  might physically be another device that the one that was used during the previous survey. However, in the field the truck  306  might follow an actual path  320  that is neither given path  310  nor new path  316 . This may happen because of location device  342  inaccuracy or other reasons. However, the deviation  330  between the actual path  320  and the new given path  310  and  316  is usually below the threshold. In one application, deviation  330  is smaller than deviation  220 . Note that parts  350  of original given path  310  may be identical to corresponding parts of new given path  310  and  316 , while only some parts  352  of original given path  310  are modified. 
         [0030]    A method that illustrates modification of the given path for a 4D survey is discussed now with respect to  FIG. 4 . In step  400 , a given path  310  is entered into the control device of the truck that carries a source for seismic shooting. The given path may include not only the geographical path for the truck to follow, but also shooting positions and shooting times associated with the given path. In step  402 , the truck follows for the first time given path  310  and starts shooting the sources at the given locations and given times. Note that the truck may stop at the shooting positions for shooting the source. However, at a certain position along given path  310 , the driver faces obstacle  314  and decides in step  404  to abandon the given path and follow a new path that avoids the obstacle. The new path  316  is recorded in step  406  by the truck&#39;s control device until the truck arrives back at given path  310 . The coordinates of new path  316  may be used for future surveys instead of the corresponding coordinates of given path  310 . If that is the case, given path  310  is modified in step  408  to incorporate new path  316 . This step may be performed automatically, by the control device, or manually by the operator of the seismic survey. If performed automatically, the control device may have a given threshold that is compared with the deviation (a difference) between the two paths. If the deviation is higher than the threshold, the control device automatically modifies given path  310 . Other algorithms may be used to make this decision. The deviation may be defined in various ways by using various metrics. For example, deviation  360  (see  FIG. 3 ) may be a segment between given path  310  and new path  316 , and deviation segment  360  may be perpendicular on at least one of the two paths. In another application, a length of segment  360  is minimized. Any deviation metrics between two paths can be used for this purpose. 
         [0031]    In step  410 , the control device verifies whether the end of the path has been reached. Note that “end the path” can be the end of a subsection of the survey that will be repeated before the whole survey has been shot. If the answer is no, the algorithm returns to step  402 . If the answer is yes, the algorithm advances to step  412 , in which the modified path becomes the new given path for further surveys. As noted above, step  408  applies only when the truck follows the given path for the first time. Once the truck has followed the given path and it was modified as discussed above, the control device stores the modified given path and does not allow the system to change it, even if the truck deviates from the modified/new given path the next time a survey is performed. This ensures that all seismic surveys that are part of the 4D survey follow the same given path. 
         [0032]    Step  408  may also include a sub-step of recalculating the shooting positions for the sources. In this sub-step, the new shooting positions are recalculated based, for example, on perpendicular lines as previously discussed. However, it is possible to calculate the new shooting positions based on other criteria. Various distance or time metrics can be here used or specifically developed. This step may be performed, in real time, by the control device associated with the source. In another device, the step is performed remotely, e.g., at the operator facility and transmitted in real time to the source. 
         [0033]    For purposes of illustration and not of limitation, an example of a representative computing device capable of carrying out calculations in accordance with the exemplary embodiments is illustrated in  FIG. 5 . Hardware, firmware, software or a combination thereof may be used to perform the various steps and operations described herein. 
         [0034]    The exemplary computing device  500  suitable for performing the activities described in the exemplary embodiments may include a server  501 . Such a server  501  may include a central processor (CPU)  502  coupled to a random access memory (RAM)  504  and to a read-only memory (ROM)  506 . The ROM  506  may also be other types of storage media to store programs, such as programmable ROM (PROM), erasable PROM (EPROM), etc. The processor  502  may communicate with other internal and external components through input/output (I/O) circuitry  508  and bussing  510  to provide control signals and the like. The processor  502  carries out a variety of functions as are known in the art, as dictated by software and/or firmware instructions. 
         [0035]    The server  501  may also include one or more data storage devices, including hard disk drives  512 , CD-ROM drives  514 , and other hardware capable of reading and/or storing information such as DVD, etc. In one embodiment, software for carrying out the above-discussed steps may be stored and distributed on a CD-ROM  516 , portable media  518  or other form of media capable of portably storing information. These storage media may be inserted into, and read by, devices such as the CD-ROM drive  514 , the disk drive  512 , etc. The server  501  may be coupled to a display  520 , which may be any type of known display or presentation screen, such as LCDs, LED displays, plasma displays, cathode ray tubes (CRT), etc. A user input interface  522  is provided, including one or more user interface mechanisms such as a mouse, keyboard, microphone, touch pad, touch screen, voice-recognition system, etc. 
         [0036]    The server  501  may be coupled to other computing devices, such as a landline and/or wireless terminals, via a network. The server may be part of a larger network configuration as in a global area network (GAN) such as the Internet  528 , which allows ultimate connection to the various landline and/or mobile client devices. 
         [0037]    The disclosed exemplary embodiments provide a system and a method for modifying a given path to be followed by a seismic source during a 4D seismic survey. It should be understood that this description is not intended to limit the invention. On the contrary, the exemplary 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 exemplary 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. 
         [0038]    Although the features and elements of the present exemplary 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. 
         [0039]    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.