Patent Application
20130188448
Embodiments of the subject matter disclosed herein generally relate to methods and systems for marine seismic data acquisition and, more particularly, to mechanisms and techniques for improved azimuth and/or offset distribution of marine seismic data acquisition.
Marine seismic data acquisition and processing techniques are used to generate a profile (image) of a geophysical structure (subsurface) under the seafloor. This profile does not necessarily provide an accurate location for oil and gas reservoirs, but it may suggest, to those trained in the field, the presence or absence of oil and/or gas reservoirs. Thus, providing better image of the subsurface is an ongoing process.
For a seismic gathering process, as shown in
One of the shortcomings of existing technology relates to the poor azimuth/offset distribution of the data collection points, i.e., detectors 122, positioned along streamers of equal length, and the number of streamers 104 attached to the survey vessel 102. Generally, a single survey vessel 102 tows approximately ten to sixteen streamers 104, of uniform length, with detectors 122 equally spaced along the length of each streamer. In this configuration, the azimuth of the collection points is narrow. Narrow azimuth distribution leads to problems associated with multiple (reflective) removal at locations on the streamers in close proximity to the source arrays 108. It should be noted that a survey vessel is limited in the number of streamers 104 it can tow, regardless of their length, i.e., adjusting the length of a portion of the streamers 104 to vary detector 122 density does not result in the ability to tow a greater number of streamers 104.
Another shortcoming associated with existing acquisition methods relates to the collected data in relation to its intended use, i.e., different streamer collection configurations lend themselves to different uses of the data, such as multiple removal, imaging and model building. Narrow azimuth distribution streamer configurations are not focused on a specific use of the collected data, resulting in less than optimal seismic image results.
Given a certain area 200 that needs to be surveyed, as illustrated in
Other methods for traversing the survey area 204 have been proposed. For example, U.S. Pat. No. 4,486,863 (French) discloses a method in which a streamer towing vessel follows a circular path so that the streamers follow this circular path. Each of the circle paths is offset from the next one along an advancing line. The towing vessel completes a full circle and then leaves the completed circle to move on to the next circle path. However, because a streamer will only accept a finite amount of curvature, a large track distance ratio (i.e., a large ratio between the actual distance traversed by the vessel compared to the nominal sail-line distance) will be produced, which is an inefficient way to collect seismic data. In addition, this method increases the time taken to acquire the data, which results in an increase in the acquisition's cost.
Another method is disclosed in U.S. Pat. No. 4,965,773. This method gathers and maps seismic data of a marine region by defining a spiral path using a point on the region as the origin of the spiral, and towing a transmitter/receiver streamer along the spiral path to gather seismic data. One embodiment discloses that the radial distance between the spiral turns is constant as given by an Archimedean spiral. This is also an inefficient way to collect seismic data, and the additional time taken to acquire the data equates to an increase in acquisition cost.
Still another acquisition method is disclosed in U.S. Patent Publication Number 2008/0285381, which describes towing a seismic spread including a single source and a plurality of streamers, with all the streamers being actively steered to maintain each streamer on a generally curved advancing path. The radius of the generally curved advancing path is described as being around 5,500-7,000 m, resulting in a curved path with a circumference of around 34,000-44,000 m. Given an average streamer length of around 6,000 m, it can be seen that the length of each streamer covers only a small arc-length of the circular path being traversed. This acquisition method inherently has only a small amount of deviation from traditional linear 3D acquisition systems, with the added expense of having to actively steer a plurality of streamers to keep them from becoming entangled during acquisition.
Accordingly, it would be desirable to provide systems and methods that avoid the afore-described problems and drawbacks, and improve the azimuth/offset distribution of the collected seismic data.
According to an exemplary embodiment, there is a method for determining a seismic survey configuration of a multi-vessel acquisition system. The method includes a step of receiving a number that corresponds to vessels to be used in the multi-vessel acquisition system; a step of receiving a cross-line distance between first and last straight line paths corresponding to first and last vessels, respectively, of the multi-vessel acquisition system; a step of receiving an inline distance between the first and last vessels; a step of selecting shapes of undulating paths for the vessels of the multi-vessel acquisition system; a step of receiving a desired azimuth and/or offset distribution of receivers towed by one or more streamer vessels of the multi-vessel acquisition system relative to source vessels of the multi-vessel acquisition system; and a step of calculating amplitudes (Ai), periods (Ti) and phases of the undulating paths.
According to another exemplary embodiment, there is a computing device for determining a seismic survey configuration of a multi-vessel acquisition system. The computing device includes an interface for receiving a number that corresponds to vessels to be used in the multi-vessel acquisition system, for receiving a cross-line distance between first and last straight line paths corresponding to first and last vessels, respectively, of the multi-vessel acquisition system; and for receiving an inline distance between the first and last vessels; and a processor connected to the interface. The processor is configured to select shapes of undulating paths for the vessels of the multi-vessel acquisition system, receive a desired azimuth and/or offset distribution of receivers towed by one or more streamer vessels of the multi-vessel acquisition system relative to source vessels of the multi-vessel acquisition system, and calculate amplitudes (Ai), periods (Ti) and phases of the undulating paths.
According to yet another exemplary embodiment, there is a method for determining a seismic survey configuration of a multi-vessel acquisition system. The method includes a step of receiving a bin size to be used in the multi-vessel acquisition system; a step of receiving a nominal fold for the bin size; a step of receiving a desired azimuth and/or offset distribution of receivers towed by one or more streamer vessels of the multi-vessel acquisition system relative to source vessels of the multi-vessel acquisition system; and a step of calculating lateral displacements between undulating paths to be followed by the one or more streamer vessels and the source vessels of the multi-vessel acquisition system.
According to another exemplary embodiment, there is a computing device for determining a seismic survey configuration of a multi-vessel acquisition system. The computing device includes an interface for receiving a bin size to be used in the multi-vessel acquisition system, for receiving a nominal fold for the bin size, and for receiving a desired azimuth and/or offset distribution of receivers towed by one or more streamer vessels of the multi-vessel acquisition system relative to source vessels of the multi-vessel acquisition system; and a processor connected to the interface and configured to calculate lateral displacements between undulating paths to be followed by the one or more streamer vessels and the source vessels of the multi-vessel acquisition system.
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. Some of the following embodiments are discussed, for simplicity, with regard to the terminology and structure of two streamer vessels and two source vessels. However, the embodiments to be discussed next are not limited to this configuration, but may be extended to other arrangements that include more or fewer streamer vessels and/or source vessels. Also, the figures show a particular order of the streamer and source vessels along a cross-line direction. This order is exemplary and not intended to limit the novel embodiments.
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.
According to an exemplary embodiment, both the streamer vessel and the source vessel follow curved paths. For example, the streamer vessel and the source vessel may follow periodic curved paths having amplitudes, periods and phases that may or may not be the same. The amplitudes, periods and phases may be optimized to obtain better azimuth and offset diversity for the recorded data. However, it is possible that only the streamer vessel follows the curved path and the source vessel follows a straight line path, or the other way around, as long as the azimuth and/or offset distribution of the collected seismic data is improved relative to the existing acquisition systems.
Such an example of a novel acquisition system is illustrated in
Regarding the streamers, it is conventional to tow them at the same depth below the water surface or slanted to the water surface. However, a more modern arrangement is described with reference to
Regarding the sources, it is conventional to tow a source array that includes three sub-arrays. Each sub-array includes a float to which individual source elements are attached. Thus, all the individual source elements are located at a same depth. However, a more novel source array is presented in
The depths z1 to z4 of the source points of the first sub-array 502 may obey various relationships. In one application, the depths of the source points increase from the head toward the tail of the float, i.e., z1<z2<z3<z4. In another application, the depths of the source points decrease from the head to the tail of the float. In another application, the source points are slanted, i.e., provided on an imaginary line 514. In still another application, the line 514 is a straight line. In yet another application, the line 514 is a curved line, e.g., part of a parabola, circle, hyperbola, etc. In one application, the depth of the first source point for the sub-array 502 is about 5 m, and the greatest depth of the last source point is about 8 m. In a variation of this embodiment, the depth range is between 8.5 and 10.5 m or between 11 and 14 m. In another variation of this embodiment, when the line 514 is straight, the depths of the source points increase by 0.5 m from one source point to an adjacent source point. Those skilled in the art would recognize that these ranges are exemplary and these numbers may vary from survey to survey. A common feature of all these embodiments is that the source points have variable depths so that a single sub-array exhibits multiple-level source points.
Returning to
Further, each path may have its own amplitude A. However, in one embodiment, all the amplitudes are equal. Still in another exemplary embodiment, the amplitudes are divided into subsets, and each subset has a same value. A subset may include any number of paths, from one to the maximum number of paths. The amplitude A may be defined as the maximum deviation of the vessel from a straight line path. For example, for vessel 302, the maximum deviation from the straight line path 322 is shown as A1. The amplitudes A2 to A4 of the remaining vessels are also illustrated in
A third parameter that may be used to characterize the undulating paths 312, 314, 316 and 318 is the phase. The phase may be measured from a given cross-line reference 340 (extending along a Y axis) that is substantially perpendicular on the straight line paths 322, 324, 326 and 328. The phase represents the distance of a vessel to the reference 340. The phase may be represented as an angle if the undulating path is a sinusoid. A position of the vessel or the source measured along the straight line paths determine the phase φi for each vessel. As with the amplitudes and periods, the phases may be different from vessel to vessel. In one embodiment, the phases must be different from vessel to vessel to achieve a better azimuth and/or offset distribution. In another application, the sources are shot in a staggered way, i.e., if the sources are shot simultaneously, a distance stag 345 (inline distance along inline axis X), between the first source 304B and the last source 302B along the straight line paths is maintained during the seismic survey.
A further parameter of the seismic survey system 300 is the cross-line offset 350, i.e., the distance between the first straight line path 322 and the last straight line path 324. For a given cross-line offset 350, the cross-line distance between adjacent straight line paths may vary from vessel to vessel. In one application, there is a minimum cross-line distance D1 between two adjacent straight line paths and a maximum cross-line distance D2 between another two adjacent straight line paths. Thus, all cross-line distances fall between D1 and D2. However, the cross-line distances may be the same in one application.
According to an exemplary embodiment, the novel survey system may use undulating paths that are sinusoids having the same amplitudes, the same periods, and different phases. However, it is possible to optimize these quantities (amplitude, period and phase or other quantities) based on the number of streamer vessels and source vessels to further improve the offset/azimuth distribution as discussed next. Note that various optimization methods may be implemented as discussed next.
According to an exemplary embodiment illustrated in
Having these parameters fixed, an undulating shape for the trajectories 312, 314, 316 and 318 is selected in step 806. The shape may be the same for all undulating paths or different. For simplicity, a sinusoidal shape is selected in step 806. Thus, the variables to be determined for the sinusoidal shape are the amplitude, period and the phase (e.g., distance between the vessels). In step 808, the operator may enter the desired azimuth and/or offset distribution. An example of azimuth and/or offset distribution was shown in
An optimization algorithm may be applied in step 810 for determining the amplitude and period and phase for the undulating paths of the streamer and source vessels. The optimization algorithm may be any known algorithm, e.g., a weighted least square method. An objective function may be defined based on the variables of the problem (amplitude and period), and the objective function is minimized or maximized taking into account the parameters introduced in steps 800 to 804, the shape of the undulating path, and the desired azimuth and/or offset distribution. The result of the optimization algorithm is the amplitudes, periods and phases of the vessels' trajectories. For simplicity, the flowchart in
According to another exemplary embodiment, the phases for the undulating paths are calculated as illustrated in
Then, in step 904, the desired azimuth and/or offset distributions are entered. Based on the information received in steps 900, 902 and 904, an optimization algorithm is performed in step 906. This optimization algorithm may be similar to that discussed in
According to an exemplary embodiment, a more general optimization scheme is disclosed. In general, the problem of finding an optimal path with respect to several optimal criteria can be formulated as a multi-objective (or multi-criteria), non-linear and constrained optimization problem; it consists of finding the vector of admissible variables x for a set of given parameters that represent the minimum of the objective function F(x). The optimization problem can be formulated as follows:
where ∥·∥ denotes any Lp norm with common choices including L1, L2 and L∞, x=[x1,x2, . . . , xn] is a vector of the n variables, S is the search space, the sets of functions hi(x) and gi(x) describe possible inequality and equality constraints, respectively, that need to be satisfied. For the case of a multi-objective optimization problem, the function F(x) can be written as:
F(x)={f1(x),f2(x), . . . , fp(x)},
where fi are individual objective functions.
In other words, this is a procedure looking for a compromise balance between the different objectives resulting in a configuration (called a Pareto optimal solution, which is defined as follows: given an initial allocation of goods among a set of individuals, a change to a different allocation that makes at least one individual better off without making any other individual worse off is called a Pareto improvement) out of an infinite number of alternatives. Optimization for a multi-objective problem is solved by using known mathematical methods. Depending on the topology/complexity (e.g. linear, uni-modal or multi-modal) of the cost function F(x), various mathematical methods can be used, e.g., conjugate gradient, genetic algorithm, simulate annealing, etc.
For a multi-vessel seismic survey as in the present case, the optimization problem could be formulated as follows:
Vessels paths can be described by smooth curves. For instance, the generalized sine curve,
f(x)=A sin((2π/T)x+φ),
for which the parameters A, T, and φ are the amplitude, period, and phase (in radians), respectively, may describe one curve. More generally, periodic undulating vessel paths can be approximated by twice differentiable piecewise polynomial curves (e.g., B-splines). Arbitrary undulating paths, not only defined by amplitude, period and phase may also be used. Note also, as all non-holonomic vehicles, marine vessels have a minimum turning radius which has to be considered in the procedure (e.g., inserted in the optimization as an external constraint). Further, note that the algorithm described above may be modified for each seismic survey, and fewer or more parameters, variables and constraints may be imposed to optimize the paths of the vessels participating in the multi-vessel seismic survey. In one application, one or more variables may be fixed and moved to the parameters category, or one or more constraints (i.e., objectives) may be made variables or parameters, as the circumstances require.
A plurality of parameters may be used as constraints for the optimization algorithms used with regard to
Note that the algorithms illustrated in
Based on the above algorithms (illustrated in
The sources illustrated in
It should further be noted in the exemplary embodiment that a firing sequence includes the sequential one-time firing of each source. In another exemplary embodiment firing sequence, the sources are fired either simultaneously or almost simultaneously, with random time delays between firings. The seismic system 1100 of the exemplary embodiment of
The methods and algorithms discussed above may be implemented in a computing device 1200 as illustrated in
The above-disclosed exemplary embodiments provide a system and a method for improving an azimuth and/or offset distribution for a seismic survey by using undulating paths for the source vessels and/or the streamer vessels. 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.
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.
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.
| Number | Date | Country | |
|---|---|---|---|
| 61589990 | Jan 2012 | US |
