Abstract:
To performing positioning of survey equipment, measurements of acoustic signals reflected from at least one boundary of a marine environment are received. The reflected acoustic signals are reflected from the at least one boundary in response to acoustic signaling originated by an acoustic source. A positioning model is updated based on the measurements of the acoustic signals, wherein the positioning model contains information relating to positions of components of a positioning system that includes the acoustic source and acoustic receiver

Description:
TECHNICAL FIELD 
       [0001]    The invention relates generally to determining positioning of survey equipment using a model. 
       BACKGROUND 
       [0002]    Seismic surveying is used for identifying subterranean elements, such as hydrocarbon reservoirs, fresh water aquifers, gas injection reservoirs, and so forth. In performing seismic surveying, seismic sources are placed at various locations above an earth surface or sea floor, with the seismic sources activated to generate seismic waves directed into the subterranean structure. Examples of seismic sources include explosives, air guns, or other sources that generate seismic waves. In a marine seismic surveying operation, the seismic sources can be towed through water. 
         [0003]    The seismic waves generated by a seismic source travel into the subterranean structure, with a portion of the seismic waves reflected back to the surface for receipt by seismic receivers (e.g., geophones, hydrophones, etc.). These seismic receivers produce signals that represent detected seismic waves. Signals from seismic receivers are processed to yield information about the content and characteristic of the subterranean structure. 
         [0004]    It is often useful to know the positions of seismic receivers (and other components of survey equipment). Typically, in marine acquisition, the positions of the seismic receivers are determined using a high-frequency acoustic network. High-frequency sources in this network can emit signals that are recorded by the acoustic receivers. Note that such high-frequency sources used for positioning differ from seismic sources in that seismic sources usually operate in a lower frequency range. Note also that the acoustic receivers of the acoustic network can be the same as (or can be a subset of) the seismic receivers. 
         [0005]    However, many conventional acoustic positioning techniques suffer from inaccuracies. 
       SUMMARY 
       [0006]    In general, according to an embodiment, to determine positioning of survey equipment, acoustic signals originated by acoustic source(s) and reflected from at least one boundary (e.g., water surface and/or bottom surface in a marine environment) are considered. The acoustic signals reflected from the at least one boundary are used to update a model that can be used to determine positioning of components of a survey system. 
         [0007]    Other or alternative features will become apparent from the following description, from the drawings, and from the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  illustrates an example marine survey arrangement that includes an acoustic positioning system, according to some embodiments. 
           [0009]      FIGS. 2 and 3  illustrate ray paths that are considered for producing a model that is to be used for performing positioning of survey equipment. 
           [0010]      FIG. 4  is a flow diagram of a process of producing a model to be used for determining positioning of survey equipment, according to an embodiment. 
           [0011]      FIG. 5  is a block diagram of a computer that includes processing software to perform tasks according to an embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    In the following description, numerous details are set forth to provide an understanding of the present invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these details and that numerous variations or modifications from the described embodiments are possible. 
         [0013]      FIG. 1  illustrates an example marine survey arrangement that includes a marine vessel  100  for towing a survey spread  104  in a body of water  106  between a water surface  102  and a bottom surface  108  (e.g., sea floor). The survey spread  104  includes at least one streamer  105 , as depicted in  FIG. 1 . Note, that typically a survey spread can include multiple streamers. 
         [0014]    The survey spread  104  can have survey sources and survey receivers, where the survey sources are able to emit signals propagated through the body of water  106  and into a subterranean structure  110 . Signals are reflected from one or more bodies of interest  114  in the subterranean structure  110 , with the reflected signals received by survey receivers in the survey spread  104 . 
         [0015]    In some embodiments, the survey spread  104  includes seismic acoustic sources (not shown) and seismic acoustic receivers. Examples of seismic sources include air guns, vibrators, explosives, and so forth. Examples of seismic receivers include geophones, hydrophones, and so forth. 
         [0016]    In addition to the seismic sources and receivers, each streamer  105  can also includes acoustic positioning equipment, including, as examples, positioning acoustic sources  116  and positioning acoustic receivers  118 . The positioning acoustic receivers  118  can be the same as the seismic receivers (or some subset of the seismic receivers). The positioning acoustic sources  116  can be higher frequency acoustic sources (which are distinguished from lower frequency seismic sources used for performing a seismic survey operation). 
         [0017]    Note also that the acoustic positioning system can be separate from the streamer on which seismic equipment is deployed. Moreover, note that although reference is made to seismic surveying, similar positioning techniques can be applied for positioning other types of survey equipment, such as electromagnetic (EM) survey equipment that includes EM sources and EM receivers. 
         [0018]    The marine vessel  100  also includes a controller  120 , which can be implemented with a computer, for example. The controller  120  is able to control activation of the acoustic sources  116  of the acoustic positioning system. In addition, the controller  120  is able to receive observed data collected by acoustic receivers  118  responsive to acoustic signals generated by the acoustic sources. Travel times of acoustic signals can be derived from the observed data. The controller  120  can then process the observed travel times and other information to produce (or update) a positioning model to enable estimation of positioning of the survey equipment (e.g., depth of a streamer, distances between seismic receivers, and so forth). 
         [0019]    In accordance with some embodiments, in producing (or updating) the positioning model, the controller  120  takes into account acoustic signals that are emitted by the acoustic sources  116  and reflected from the water surface  102  (an upper boundary) and sea floor  108  (a lower boundary). Thus, rather than just take into account direct (non-reflected) acoustic signals traveling from an acoustic source to an acoustic receiver, techniques according to some embodiments also take into account acoustic signals reflected from an upper boundary (e.g., water surface  102 ) and a lower boundary (e.g., sea floor  108 ). Considering the reflected signals in updating the positioning model allows for provision of a more accurate model. Note that reflected signals from just one boundary (either upper or lower boundary) could be considered, in conjunction with direct acoustic signals. 
         [0020]    Moreover, techniques according to some embodiments do not have to use an assumption that velocity in the body of water  106  is constant throughout the body of water. In fact, the velocity of acoustic signals in the body of water can spatially vary (such as in different layers of the body of water) due to variations in salinity and/or temperature. Techniques according to some embodiments can take into account differences of velocities of acoustic signals in the body of water. 
         [0021]    The waves emitted by an acoustic source approximately satisfy the Helmholtz equation: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
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         [0000]    where w(t) is the wavelet emitted by the acoustic source, s is the source position, u is the pressure recorded at x (acoustic receiver position), ω is circular frequency, and c is acoustic velocity in water. An efficient way to solve Eq. 1 is by using asymptotic ray theory, which assumes that the acoustic energy propagates along ray paths. For this purpose, the following equation can be inserted into Eq. 1: 
         [0000]        u ( x ,ω)= A ( x ) e   iωT(x) ,  (Eq. 2) 
         [0000]    where A(x) is the acoustic signal amplitude received at receiver x, and T represents travel time from the source to receiver. Up to highest order in frequency ω, it is determined that: 
         [0000]      |∇ T ( x )| 2   =c   2 ( x ).  (Eq. 3) 
         [0022]    Eq. 3, the eikonal equation, is an equation for the wave fronts T of the acoustic waves. The equation for the amplitudes of the acoustic waves, the transport equation, follows by putting together terms of the second highest order in ω. The equations for the ray paths themselves follow by applying the method of characteristics to the eikonal equations. It is therefore determined that: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
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         [0000]    where x(t) is the ray path and p(t) the tangent vector to the ray path. The travel time T at x is given by the value of the independent parameter t. The initial conditions, x(0)=s and p(0) (the take-off direction), are given. 
         [0023]    The ray equations can be solved using a variety of methods, such as Runge-Kutta or any other numerical solver for ordinary differential equations. In the case of acoustic positioning, the velocity is smoothly varying. Rays from the source to the receiver that are not reflected from the water surface or sea floor can be found easily using two point ray tracing. However, rays reflected from these boundaries are harder to find numerically. It is therefore useful to obtain good initial conditions for the two point ray tracing. These may be obtained from analytical expressions for the ray paths if the water velocity is constant. 
         [0024]      FIG. 2  shows an example arrangement including a source position s, receiver position r, a sea floor V 1 , and the water surface V 2 . The general situation of rays propagating in a medium between two flat boundaries (V 1  and V 2  in  FIG. 2 ) with arbitrary orientation is considered. The equation for these boundaries are given by (x,N i )=a i . 
         [0025]    Given a source position s and receiver position r on the same side of a boundary (e.g., sea floor V 1 ), the reflection point t of the ray from s to r (as depicted in  FIG. 2 ) is defined as: 
         [0000]    
       
         
           
             
               
                 
                   
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         [0000]    where r′ is the mirror point of r with respect to v 1  (see  FIG. 2 ). The mirror point r′ is the point across the boundary V 1  that is a mirror of r. The mirror point r′ is defined by: 
         [0000]        r′=r+ 2( a −( r,N )) N.   (Eq. 5) 
         [0026]    For ray paths reflected from the water surface V 2 , similar reflection points t can be defined. 
         [0027]    The travel time T and amplitude A now follow from the length of s-r′. Note that it is easier to compute the length of s-r′ since s-r′ is a straight line. These formulas can be extended to include any number of reflections. For example, in the case of three reflections (as depicted in  FIG. 3 ), the following mirror points can be defined: 
         [0000]        r   1   =r+ 2( a   1 −( r   1   ,N   1 )) N   1    
         [0000]        r   2   =r   1 +2( a   2 −( r   1   ,N   2 )) N   2 , 
         [0000]        r   3   =r   2 +2( a   1 −( r   2   ,N   1 )) N   1    
         [0000]    where r 1  is the mirror point of r to V 1 , and similar definitions hold for mirror points r 2  and r 3 . Note that r 2  is a mirror point across the water surface V 2 . Multiple reflections are depicted in  FIG. 3 , where the multiple reflections are between the sea floor V 1  and water surface V 2 . Reflection points (or rays at boundary V 1  or V 2 ) are defined as t 1 , where i=1, 2, . . . , n (n being the number of reflections). 
         [0028]    From geometric considerations, t 1  is the intersection of the line sr 3  with V 1 , t 2  is the intersection of the line t 1 r 2  with V 2 , and t 3  is the intersection of the line t 2 r 1  with V 1 . From these equations, one can determine the travel time and amplitude of the multiply reflected acoustic wave. The general case of n reflections can be solved in similar fashion. 
         [0029]    Thus, in accordance with some embodiments, travel times of acoustic signals reflected once or multiple times from an upper boundary and from a lower boundary can be considered in producing (or updating) a positioning model for use in determining a position of survey equipment. 
         [0030]    The above discussion has assumed a constant velocity in the body of water  106 . However, that is usually not the case, as acoustic velocity in water can vary in different layers of the water, such as due to salinity or temperature variation. 
         [0031]    If the velocity in the body of water is heterogeneous, e.g., c=c 0 +c 1 , then the ray paths and the corresponding travel times and amplitudes can change. The above assumes two different velocities c 1  and c 2 ; more generally, there can be additional different velocities. The above ray equations can be used in this case, but one of various types of perturbation theories for the travel times and ray paths can also be used, which are faster to process but may be less accurate. To first order, the correction in travel time δT due to velocity variation in a body of water is found by integrating the heterogeneous velocity along the unperturbed ray: 
         [0000]    
       
         
           
             
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         [0000]    The second correction to apply would be to compute the perturbed ray paths. These can be found to first order by integration using the unperturbed ray paths. 
         [0032]    Basically, if constant velocity is assumed, then a ray path from a source to a boundary or from a boundary to a receiver is straight. However, if velocity varies, then the ray path is curved (made up of various straight height segments in corresponding layers of constant velocity). 
         [0033]    As noted above, the information computed above can be used to update a positioning model that includes positions of acoustic sources and receivers and a velocity profile. The velocity profile refers to velocity of acoustic signals in various layers of a body of water. 
         [0034]    Given an initial positioning model, including source and receiver positions and a velocity profile, and observed data, the positioning model can be updated using an inversion technique. With the inversion technique, the following objective function 
         [0000]    
       
         
           
             
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         [0000]    is minimized, where T o,i (r,s) is the observed travel time of ray type i from source s to receiver r, and T c,i (r,s,c) is the corresponding calculated travel time. Ray type i refers to one of several possible ray types: (1) direct ray from source to receiver; (2) ray reflected from lower boundary to receiver, and (3) ray reflected from upper boundary to receiver. This problem can be linearized as follows: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
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         [0000]    where the various partial derivatives can be computed. This reduces the nonlinear minimization problem to a linear problem: 
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         [0000]    where i is an index that goes over all data (ray types and source-receiver combinations) and A is a matrix that contains the partial derivatives. This linear problem can be solved using any least squares solver. Particularly appropriate in this case is an algorithm such as LSQR, which is a sparse matrix solver (note that matrix A is large and sparse). Often the least squares problem is augmented using damping and smoothing factors to stabilize the inversion procedure. The damping factor ensures that solutions close to the starting model are favored, and the smoothing factor (which is especially appropriate for the part of the inversion that determines the velocity model) ensures that the inverted model is smooth. In this case the matrix equation to be inverted becomes: 
         [0000]    
       
         
           
             
               
                 
                   
                     
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         [0000]    where I is an identity matrix, λ 1  and λ 1  are smoothing parameters, D is a difference operator, and δm is the change in the model that is to be computed. The vector δm contains δs (change in source position), δr (change in receiver position), and δc (change in velocity). Note that δT is a vector containing δT i  over all ray types i and source-receiver combinations. 
         [0035]    If the covariance matrix for the data is known, then a covariance matrix for the positioning model can also be determined. This is particularly important if the absolute locations are determined using for example GPS receivers located at various places on the streamers. Finally, the geometrical amplitude and even the whole waveform can be used if necessary, to refine the inversion result, using a similar method as in H. Keers et al., “Acoustic Crosswell Imaging Using Asymptotic Waveforms,” Geophysics, 65, pp. 1569-1582 (2000). Also the attenuative properties of the water can be determined using such method as shown by H. Keers et al., “Viscoacoustic Crosswell Imaging Using Asymptotic Waveforms,” Geophysics, 66, pp. 861-870 (2003), in a different context. 
         [0036]      FIG. 4  is a flow diagram of a process of producing a model that can be used for determining positions of components of survey equipment, according to some embodiments. An initial model is provided (at  402 ), where the initial model includes positions of acoustic sources and receivers in the acoustic positioning system. The initial model can also include a velocity profile, which specifies different velocities in different layers of a body of water in which the acoustic positioning system is placed. 
         [0037]    Next, after deployment of the acoustic positioning system in a body of water, observed data including travel times, is received (at  404 ). The travel times refer to travel times associated with acoustic signals being received by acoustic receivers of the acoustic positioning system. The received acoustic signals include: (1) direct acoustic signals (from acoustic source to acoustic receiver), (2) acoustic signals reflected from an upper boundary (water surface), and (3) acoustic signals reflected from a lower boundary (bottom surface such as a sea floor). 
         [0038]    Moreover, using the initial model, travel times of various ray types (direct ray, reflected ray from water surface, and reflected ray from bottom surface) for different source-receiver combinations are computed (at  406 ). An objective function is then minimized to solve for the vector δm. As noted above, the vector δm contains δs (change in source position), δr (change in receiver position), and δc (change in velocity). 
         [0039]    The vector δm is then used to update (at  410 ) the model. The updated model can then be used to predict positions of components of survey equipment, such as positions of acoustic receivers. The predicted positions can be relative positions (e.g., positions or receivers relative to each other or other components), or absolute positions (which can be calculated from known reference positions of one or more components of the survey equipment that have GPS receivers). 
         [0040]    Using the techniques according to some embodiments, locations of an acoustic receiver network of a seismic spread, for example, can be determined, using direct arrivals of acoustic signals as well as reflected arrivals of acoustic signals from an upper boundary and a lower boundary. The techniques according to some embodiments can also correct for velocity variations in the body of water. In addition to using travel times, amplitudes and whole waveforms can also be used. Moreover, error estimates of the locations can be determined, using error estimates from the acoustic receiver network, and from GPS receivers located at various locations in the seismic spread. 
         [0041]      FIG. 5  shows components of the controller  120 , which includes processing software  502  to perform the various tasks discussed above. The processing software  502  is executable on one or more central processing units (CPUs)  504 , which is (are) connected to a storage  506 . The storage  506  can store observed data  508  (e.g., observed travel times of acoustic signals), computed data  510  (e.g., computed travel times of acoustic signals), and models  512  (e.g., initial model and/or one or more updated models). 
         [0042]    The processing software  502  is able to provide an output  514  based on the processing discussed above, where the output  514  can be presented for display in a display device, or can be communicated over a data network to a remote terminal. 
         [0043]    Instructions of the processing software  502  can be loaded for execution on a processor (such as one or more CPUs  504 ). The processor includes microprocessors, microcontrollers, processor modules or subsystems (including one or more microprocessors or microcontrollers), or other control or computing devices. A “processor” can refer to a single component or to plural components. 
         [0044]    Data and instructions (of the software) are stored in respective storage devices, which are implemented as one or more computer-readable or computer-usable storage media. The storage media include different forms of memory including semiconductor memory devices such as dynamic or static random access memories (DRAMs or SRAMs), erasable and programmable read-only memories (EPROMs), electrically erasable and programmable read-only memories (EEPROMs) and flash memories; magnetic disks such as fixed, floppy and removable disks; other magnetic media including tape; and optical media such as compact disks (CDs) or digital video disks (DVDs). 
         [0045]    While the invention has been disclosed with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover such modifications and variations as fall within the true spirit and scope of the invention.