Abstract:
Techniques or mechanisms are provided to improve accuracy in determining headings and/or shapes of carrier structures based on measurements made by one or more compasses that are attached to or provided with the carrier structures. The carrier structures are used to carry survey receivers that detect survey signals affected by a subterranean structure.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates generally to improving accuracy of a compass provided on a carrier structure used in subterranean surveying. 
       BACKGROUND 
       [0002]    Marine survey (seismic survey or electromagnetic (EM) survey) exploration investigates and maps the structure and character of subterranean geological formations underlying a body of water. For large survey areas, a survey spread may have vessels towing multiple streamers through the water, and one or more survey sources (seismic or EM sources) by the same or different vessels. Survey sources are propagated or emitted downwardly into the geological formations. The signals affected by the geological formations are detected by survey receivers attached to the survey streamers, and data representing detected signals is recorded and processed to provide information about the underlying geological features. 
         [0003]    Often, one or more compasses are provided on a streamer to aid in determining the heading of the streamer. However, compasses can be adversely affected by magnetic fields that are generated by components of the streamer. As a result, conventionally, compasses are typically mounted externally of the streamer to reduce the amount of magnetic disturbance that each compass experiences from streamer components and electric power fields. However, locating a compass externally of a streamer has various disadvantages, including having to attach the compass to the streamer during deployment of the streamer into the water and having to remove the compass during retrieval of the streamer from the water. Another disadvantage is that batteries have to be used to power the compasses, which leads to having to change, store, and dispose of such batteries. Also, the locations on a streamer where external compasses can be mounted are relatively limited, since compasses have to be located where magnetic coil lines are located (for the purpose of communicating data through the coil lines). 
         [0004]    Another issue associated with compasses is that compasses are assumed to be substantially parallel to the streamer that the compasses are mounted in, and it is assumed that the shape of the streamer is substantially straight. “Substantially straight” used in this context means that the spatial frequency of the compasses on the streamer provides enough heading samples to determine changes in streamer shape. Models can be used for fitting measurements to the model unknowns such that changing shapes of the streamer can be determined based on compass readings. However, the assumption that the shape of the streamer is substantially straight is often not correct, such that conventional models that are used do not provide accurate results. A streamer typically includes steering devices to cause steering of the streamer, which deforms the streamer in a deterministic way. The steering devices apply lateral forces on the streamer, such that the streamer shape becomes non-straight. In the presence of such lateral forces applied by streamer steering devices, the models that are conventionally used are not accurate, since the streamer does not have a shape that matches model shapes, and because the streamer shape changes with lateral forces exerted by the steering devices. As a result, in view of the forces applied by steering devices of a streamer, the determination of streamer shapes and streamer headings based on compass readings may be inaccurate. 
       SUMMARY 
       [0005]    In general, according to an embodiment, techniques or mechanisms are provided to improve accuracy in determining headings and/or shapes of carrier structures based on measurements made by one or more compasses that are attached to or provided with the carrier structures. The carrier structures are used to carry survey receivers that detect survey signals affected by a subterranean structure. 
         [0006]    Other or alternative features will become apparent from the following description, from the drawings, and from the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a schematic plan view of a towed streamer spread that incorporates some embodiments of the invention. 
           [0008]      FIG. 2  is a schematic side view of a streamer insert that includes a steering device and compass, in accordance with an embodiment. 
           [0009]      FIG. 3  illustrates a compass that is positioned proximate an electrical cable, where the electrical cable has electrical wires having a predefined arrangement for reducing magnetic interference with the compass, in accordance with an embodiment. 
           [0010]      FIG. 4  illustrates another cable arrangement positioned proximate to a compass with reduced magnetic interference characteristics in accordance with another embodiment. 
           [0011]      FIG. 5  is a schematic diagram that illustrates various forces applied on a streamer section, according to an embodiment. 
           [0012]      FIG. 6  is another schematic diagram illustrating various parameters associated with a streamer section. 
           [0013]      FIG. 7  shows curved streamer sections between steering devices. 
           [0014]      FIG. 8  is a flow diagram of determining bias associated with a compass that is mounted on a streamer that is subject to forces applied by a steering device, according to an embodiment. 
           [0015]      FIG. 9  is a block diagram of an exemplary computer that includes processing software to perform tasks according to some embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    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. 
         [0017]    Generally, according to some embodiments, a compass can be provided as part of a streamer that carries survey receivers. More specifically, the compass can be provided in-line inside the streamer, rather than mounted externally to the streamer. To reduce magnetic interference with the compass, electrical wires in the streamer are arranged such that magnetic fields from the individual electrical wires substantially cancel each other. In addition, the housing surrounding the compass is formed of a non-magnetic material, and the compass is also positioned sufficiently far away from magnetic components in the streamer to reduce magnetic interference. In this manner, both soft and hard magnetic fields are eliminated or reduced, such that the compass provided inside a streamer section (or streamer insert that is provided in-line with the rest of the streamer) can provide accurate compass readings. 
         [0018]    Additionally, according to other embodiments, a technique is provided to determine a bias of a compass that results from forces exerted by one or more steering devices in the streamer. A “bias” refers to the difference between the compass heading resulting from forces (including lateral forces) exerted by a steering device and the heading of the compass without the forces exerted by the steering device. By determining the bias of the compass due to forces applied by steering devices on the streamer, more accurate determinations of streamer headings and/or streamer shapes can be determined based on the compass readings. 
         [0019]      FIG. 1  illustrates an exemplary marine survey arrangement for a marine environment, in which one or more marine streamers  102 ,  104  are towed by tow cables  106 ,  108 , respectively, attached to a marine vessel  110 . Each streamer  102 ,  104  includes survey receivers  112  (represented as small circles) arranged along the length of each streamer. As further depicted in  FIG. 1 , a survey source  114  is towed behind the marine vessel  110 , where the survey source  114  is activated to generate survey signals that are propagated into a subterranean structure (underneath the water bottom surface). The survey signals affected by the subterranean structure are detected by the survey receivers  112 . The survey source  114  can be a seismic source or an electromagnetic (EM) source, and the survey receivers  112  can be seismic or EM receivers. 
         [0020]    As depicted in  FIG. 1 , a water current represented by arrow C tries to force streamers off the path intended by the survey operator. To address this, steering devices  116  are provided along the length of each of the streamers  102  and  104 . The steering devices  116  are used to maintain the streamers  102 ,  104  close to the intended path. However, due to the interaction of the steering devices  116  and the current C, the streamers  102  and  104  may assume a non-straight shape, with some portions between the steering devices  116  bowed. Thus, any determination of the streamer headings and/or streamer shapes that is based on the assumption that each streamer is straight would produce errors. 
         [0021]    During periods when other methods of positioning are not available, such as periods when the streamer is without power for acoustics, battery powered compasses can be used to determine the streamer position using the straight streamer assumption as long as there is no steering occurring. Periods when power may not be available include streamer deployment and retrieval, and when power is lost on the streamer due to earth leakage. Since steering is almost always an advantage, some embodiments of this invention introduce a method for using compasses alone or with any other combination of positioning instrumentation, such as GNSS (global navigation satellite system) control points anywhere along the streamer, for streamer positioning even when steering. 
         [0022]    During such periods, the accuracy of the positioning required is not as high as during production. Yet positions have to be determined well enough to avoid streamers colliding. By determining the average heading of the streamer and the streamer take off angles between steering devices, the streamers can be positioned well enough with compasses to allow steering. This is achieved by using a force model to estimate the difference between the steering device heading when misalignment forces are present and the heading (β below) when the misalignment forces are not present. In addition, the shaping of the streamer between steering devices is facilitated by knowing the angle the streamer has going into and out of the curved shape between steering devices (α and ψ in  FIG. 6 ). 
         [0023]    Positioning with compasses is improved even further by calibrating the force model during periods when additional information is available, such as acoustically determined coordinates along the streamer and at the steering devices. Parameters of streamer shape can be estimated when acoustically determined points are available to give measured points along the shape. Thus the amount of curvature actually resulting from the steering can be estimated independent of the force or mathematical shape models. In addition, the non misaligned steering device heading can be estimated acoustically and compared to the compass heading to estimate errors in the compass instrument. After these calibration factors have been recorded in software, they can be applied during period when positions are determined only with compasses and the force or mathematical shape, (e.g., hyperbola parameters) models are used. 
         [0024]    As further depicted in  FIG. 1 , buoys (or floats)  118 ,  120 ,  122 , and  124  are provided at respective leading and trailing ends of each streamer  102 ,  104 . Global positioning system (GPS) receivers can be provided at the buoys  118 ,  120 ,  122 , and  124  to provide GPS positions of the streamers  102 ,  104 . Other components (not shown) can also be part of the streamer spread depicted in  FIG. 1 . 
         [0025]    In accordance with some embodiments, for improved convenience and efficiency, compasses  126  can be provided in respective steering devices  116 . For example, each steering device  116  can have a housing in which a compass  126  can be provided. Alternatively, each compass  126  can be part of a streamer insert that is placed in-line with the streamer  102  or  104 , or alternatively, each compass  126  can be part of another streamer section. The compass can be part of an active streamer section containing seismic recording devices, or part of a towing section. In some embodiments, the compass may be mounted external to the streamer, in which the compass is attached to an external part of the streamer by some attachment mechanism. 
         [0026]    As noted above, placing a compass  126  in a streamer section or streamer insert can subject the compass to magnetic field interference caused by components and electric power fields in the streamer. 
         [0027]    In one example,  FIG. 2  shows a compass  126  mounted inside a steering device  116 . More specifically, the steering device  116  has a housing  204  that defines an inner chamber in which the compass  126  is provided. The steering device  116  also has steering wings  202  mounted to the housing  204 , where the wings  202  are used to provide steering. 
         [0028]    Moreover, the housing  204  also contains one or more electrical cables that run from one end of the housing  204  to another end of the housing  204 . The electrical cable(s) also run(s) through sections  206  and  208 . The section  206  is connected to a front connection assembly  210 , and the section  208  is connected to a communications module  212 , which in turn is connected to a tail connection assembly  214 . 
         [0029]    The overall assembly depicted in  FIG. 2  is a streamer insert  200 , where the front connection and tail connection assemblies  210  and  214  are used to connect to other streamer sections such that the streamer insert  200  is provided in-line with the remainder of the streamer. The communications module  212  is used to perform communications, including communications of commands to control the steering device  116 , communications of compass readings, and so forth. 
         [0030]    As depicted in  FIG. 2 , the electrical cable(s) is (are) located proximate the compass  126 , and can potentially cause magnetic interference with the compass  126  such that the compass  126  may not provide accurate measurement readings. 
         [0031]    In accordance with some embodiments, to address this issue, each electrical cable that is provided relatively close to the compass  126  has electrical wires that are arranged to provide for reduction or cancellation of magnetic fields.  FIG. 3  depicts one example arrangement of electrical wires  302 ,  304 ,  306 ,  308 ,  310 ,  312 , and  314  in a power cable  300  (which delivers power to components of the streamer). The electrical wire  314  is a ground wire. The “+” symbol and the “−” symbol indicates direction of electrical current flow. The “+” symbol indicates current flow in a first direction through the electrical cable  300 , while the “−” symbol indicates current flow in an opposite direction. As depicted in  FIG. 3 , the “+” and “−” electrical wires are provided in an alternating arrangement such that any “+” electrical wire is between two adjacent “−” electrical wires, and similarly, any “−” electrical wire is between two adjacent “+” electrical wires. 
         [0032]    Electrical current flowing through an electrical wire produces a magnetic field surrounding the electrical wire. By positioning two electrical wires of opposite current flows right next to each other, the magnetic fields generated by such electrical wires will substantially cancel each other out. It is noted that there would be portions of the magnetic fields that are not completely cancelled out since there are just a limited number of electrical wires provided in the cable  300 . 
         [0033]    Improved magnetic field cancellation can be provided by using a cable having an even larger number of electrical wires with the alternating arrangement of “+” and “−” electrical wires. However, the cable  300  having the six alternately arranged “+” and “−” electrical wires provides substantial magnetic field cancellation such that the compass  126  that is positioned a distance D 2  from the cable  300  experiences no or very little magnetic field interference from the magnetic fields produced by the electrical wires in the cable  300 . In  FIG. 3 , the electrical cable  300  has a diameter D 1 . 
         [0034]    In one example, the diameter D 1  can be 10 millimeters (mm), while D 2  is 20 mm. In other examples, other values of D 1  and D 2  can be used, with D 2  set such that the compass  126  is positioned sufficiently far away from the cable  300  such that any remaining or residual magnetic field that has not been cancelled by the alternating arrangement of electrical wires in the cable  300  does not cause magnetic field interference with the compass  126 . 
         [0035]      FIG. 4  shows another cable  404  that has electrical wires  406 ,  408 ,  410 , and  412 . Note that the cable  300  in  FIG. 3  can be the main power cable that is provided in the streamer. On the other hand, the cable  404  can be a network cable that includes two twisted pairs, with a first twisted pair for receive (Rx) data, and a second twisted pair for transmit (Tx) data. For example, Rx data can be provided on a first twisted pair of wires  406 ,  408 , while the Tx data can be provided on a second twisted pair of wires  410 ,  412 . 
         [0036]    In addition to communicating Tx and Rx data, power can also be injected into the cable  404  for powering devices connected to the network cable  404  (that do not receive power from the main cable  300  in  FIG. 3 ). Power can be injected, for example, by injecting “−” current in a first pair  400  of electrical wires ( 408 ,  412 ), and by injecting “+” current in a second pair  402  of electrical wires ( 406 ,  410 ). The arrangement of electrical wires  406 ,  408 ,  410 , and  412  depicted in  FIG. 4  is referred to as a quad arrangement. 
         [0037]    Magnetic field cancellation provided by the quad arrangement depicted in  FIG. 4  is less than the magnetic field cancellation provided by the 6-wire arrangement depicted in  FIG. 3 . However, since the magnitudes of power current flow in the cable  404  is likely less than the magnitudes of power current flow in the cable  300 , the magnetic field cancellation features of the quad arrangement of cable  404  is likely to be acceptable. As further depicted in  FIG. 4 , the compass  126  is positioned some distance away from the cable  404 , such that any residual magnetic field produced by the cable  404  does not cause interference at the compass  126 . 
         [0038]    In an alternative implementation, instead of providing the compass  126  inside the housing  204  of the steering device  116 , the compass  126  can be part of another module that is connected to either the front connection assembly  210  or the tail connection assembly  214 . As yet another alternative, compasses can be provided in all three locations (one inside the steering device  116 , and one each connected to the front and tail connection assemblies  210  and  214 ). 
         [0039]    To further reduce magnetic interference at the compass  126 , the housing  204  that contains the compass  126  is formed of a non-magnetic material. Also, to reduce magnetic interference, the motor of the steering device  116  that drives the wings  202  can be positioned a sufficiently large distance away from the compass  126 . A motor contains some amount of magnetic material. When the motor is running, changes to the magnetic field produced by the motor is mainly contained inside the motor. 
         [0040]    Also, the wings  202  are also formed mainly of non-magnetic material. Batteries inside the steering device  116  are also positioned a sufficiently large distance away from the compass  126 . 
         [0041]    Another issue associated with the use of the compass  126  in a streamer is that the streamer can be subjected to forces (including lateral forces) of the steering device  116  that can cause bias in the compass. The “bias” of a compass is the difference between the compass heading resulting from forces applied by the steering device  116 , and the compass heading without the forces applied by the steering device. Actual compass headings can be compared with computed compass headings that are computed based on a streamer force model. 
         [0042]    The force model receives the following input parameters: tension in the streamer section that contains the compass; side force applied by the steering device  116  on the streamer section; wing angle (which is the angle of the wings  202  of the steering device  116 ); lift experienced by the wings  202  of the steering device  116 ; and velocity of the water current (C in  FIG. 1 ). Based on these input parameters, the force model outputs a computed heading. This computed heading can then be compared to the actual compass heading, and the difference between the computed and actual headings constitutes compass bias that can be used to either calibrate the force model or to calibrate the compass. If the force model is assumed to be accurate, then the difference between the computed heading and the actual heading represents a bias of the compass due to forces applied by the steering device  116 . This bias can then be used to correct actual readings received from the compass during streamer operation. 
         [0043]    However, if the compass heading is known to be accurate (such as due to the compass having been calibrated using another technique), then the difference between the computed heading and the actual heading can be used to calibrate the force model. The calibrated force model can then be used to compute the heading of the streamer section that contains the compass when no steering side forces are applied by a steering device. Further, with a calibrated force model, the streamer shape can be computed, allowing improved positioning of the seismic instruments contained in the streamer section. 
         [0044]    In addition, calibration of a compass can be accomplished by using an acoustic mechanism. For example, acoustic devices can be provided ahead and behind the location of the compass, and the acoustic devices are then used to accurately determine the heading of the corresponding streamer section. The acoustic devices that are mounted ahead of and behind the compass location may be acoustic transponders that are part of an acoustic ranging, such as an IRMA (intrinsic range modulated acoustics) system. The acoustic transmitter emits acoustic waves that are received by the streamer seismic hydrophones. The line between each acoustic hydrophone positioned gives a direction that is equal to a tangent point along the streamer. If this tangent point is also the location of a compass, this compass heading determined acoustically can be used to calibrate the compass such that the compass reading from the compass matches the heading determined acoustically. 
         [0045]      FIG. 5  illustrates forces that are experienced by the steering device housing  204 . Tensions T are applied by the streamer on the steering device housing  204 . Also, the steering device housing  204  experiences a moment M due to the wings  202  of the steering device  116 . In addition, R represents the fin lift due to lift experienced by the wings  202  of the steering device  116 . In  FIG. 5 , the angle φ is the angle caused by misalignment due to fin moment M, and γ is the angle caused by misalignment due to moment resulting from fin lift (R) and drag due to water friction. The angle α represents the angle between a first streamer section and horizontal, and the angle χ represents the angle between a second streamer section and horizontal. β is the angle the steering device housing would have if there were no misaligning forces, i.e., no bias. Misaligning forces are moments and lateral forces due to the steering device wing angles, or lift. The steering device body heading is the sum of β, which is ideally the compass heading in the non-misaligned steering device body, the misalignment angle due to the moment (φ) and the misalignment angle due to the fin lift (γ): β+φ+γ=compass heading. 
         [0046]    β is also the direction of the straight streamer. Any distortion of the streamer such as curvature due to side forces will result in tangent points along the curve that are not parallel with β. But the line between the steering devices is parallel with β despite the curved streamer ( FIG. 7 ) between the steering devices. This allows the computation of β acoustically with an error that is due to the cross line (direction perpendicular to β) acoustic determination error. It is assumed that the error associated with the acoustical determination of β is normally distributed and so will average to zero over many independent determinations: β=arctan(dy/dx). 
         [0047]    What follows is the method of estimating the misalignment due to fin lift. In this development, γ is the misalignment due to fin lift. Fin lift (L) is a function of angle of attack which includes current and vessel speed, but will not be further discussed here, The lift (L) has the following relationship to various parameters shown in  FIG. 6 : 
         [0000]        L=K 1 +K 2 =T  sin(α)+ T  sin(ψ)
 
         [0000]        K 1 ·X 1 =K 2·( X 1 −X 2)
 
         [0000]      γ=α−ψ
 
         [0048]    The Q-fin body has wing shaft X 1  distance from rear and X 2  distance from front. To solve for K 1  and K 2 : 
         [0000]        K 1 =K 2*( X 2 −X 1)/ X 1; 
         [0000]        XX =( X 2 −X 1)/ X 1; 
         [0000]    Substitute for K 1  in terms of L 2  into L=K 1 +K 2 ; 
         [0000]        L=K 2 *XX+K 2 =K 2*( XX+ 1). 
         [0049]    So, 
         [0000]        K 2 =L /( XX+ 1) 
         [0000]        K 1 =L−L /( XX+ 1). 
         [0050]    Therefore, to solve for α, ψ and γ using K 1  and K 2 : 
         [0000]        K 1 =T  sin(α) and  K 2 =T  sin(ψ);
 
         [0000]      γ=α−ψ
 
         [0000]    where γ is the bias due to fin lift. 
         [0051]    Next, the formula for getting the component of misalignment due to moment is calculated: 
         [0000]    
       
         
           
             φ 
             = 
             
               arc 
                
               
                   
               
                
               
                 
                   sin 
                    
                   
                     ( 
                     
                       M 
                       
                         
                           T 
                           · 
                           X 
                         
                          
                         
                             
                         
                          
                         2 
                       
                     
                     ) 
                   
                 
                 . 
               
             
           
         
       
     
         [0000]    Combining this information for various values of tension (T), lift (L) and moment (M) gives a table of biases for these conditions. If β is the corrected steering device heading (non-biased, with no misalignment due to fin lift or moment), then 
         [0000]      β=Compass Heading−φ−γ+ r,  
 
         [0000]    where r is residual compass error due to instrumentation and any other errors. 
         [0052]    At different lifts (R) and tensions (T), the biases (difference between computed headings and actual compass headings) can be determined and compiled. The bias values can be stored in a table. The biases stored in this table can be used to either calibrate the compass or calibrate the force model, depending on which is assumed to be less accurate. 
         [0053]    Also, a mathematical function fit can be applied to the biases contained in the table for extrapolation at zero lift (in other words, no steering is being applied by the steering device). The zero lift values correspond to values when the streamer is substantially straight. These zero lift values can then be used in performing positioning of the streamer sections based on compass measurements. Effectively, the zero lift values relate to values of a compass that is not subjected to forces applied by steering devices. 
         [0054]      FIG. 8  shows an exemplary procedure for determining bias associated with a compass (whether the compass is provided in-line with the streamer or provided externally of the streamer). The tension at the front of the streamer can be measured (at  602 ), using tension measurement devices mounted at the front of the streamer. Alternatively, note that tension measurement devices can be mounted elsewhere in the streamer. 
         [0055]    Next, a tension model is retrieved regarding how tension is reduced along the length of the streamer from the front of the streamer. Using this tension model, the tension at the location of the compass is obtained (at  604 ). 
         [0056]    The angles of the wings  202  of the steering device  116  are also measured (at  606 ) using angle measurement devices of the steering device  116 . From the wing angles, the lift and side forces can be computed (at  608 ). Next, the heading of the streamer section is computed (at  610 ) based on the force model by applying the tension, lift force, side force, and water current velocity (C in  FIG. 1 ) to the force model. 
         [0057]    The actual compass reading is also received (at  612 ). Based on the received compass reading, the bias associated with the compass can be computed (at  614 ) by determining the difference between the actual compass heading and the computed heading. This bias can be used to correct either the compass or the force model, as noted above. Using the corrected compass headings or outputs of corrected force model, correct headings of sections of a streamer or shapes of the streamer can be determined. 
         [0058]    Referring again to  FIG. 6 , comparing the heading measured by the compass to the angles a and x gives the tangent direction of the streamers at the Q-fin body in global north reference frame. Combining this information with the coordinates of the compass (determined acoustically), boundary conditions for fitting a streamer shape are given between the Q-fin bodies. The shape can be based on fitting a mathematical curve to other acoustically determined points along the streamer or fitting a streamer force model based shape on the acoustically determined points. 
         [0059]    In some implementations, quality control can also be performed (at  616 ) using an acoustic measurement mechanism to check whether the computed bias is accurate. For example, the acoustic measurement mechanism is able to determine the heading of the streamer section in which the compass is located. This heading can be compared with the received compass heading, and the two values can be compared to determine whether it is the compass that requires correction or the force model that requires correction. 
         [0060]    The computations in  FIG. 8  can be performed using processing software, such as processing software  702  executable in a computer  700 , as shown in  FIG. 9 . The processing software  702  is executable on one or more central processing units (CPUs)  704 , which are connected to storage  706 . The storage  706  can be used to store compass measurements  708 , a force model  710 , and a bias table  712  (that contains biases as a function of tension and/or lift). 
         [0061]    Instructions of software described above (including processing software  702  of  FIG. 9 ) are loaded for execution on a processor (such as one or more CPUs  704  in  FIG. 9 ). 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. 
         [0062]    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). 
         [0063]    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.