The invention relates generally to improving accuracy of a compass provided on a carrier structure used in subterranean surveying.
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.
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).
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.
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.
Other or alternative features will become apparent from the following description, from the drawings, and from the claims.
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.
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.
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.
As depicted in
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.
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
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.
As further depicted in
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.
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.
In one example,
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.
The overall assembly depicted in
As depicted in
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.
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.
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 D2 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
In one example, the diameter D1 can be 10 millimeters (mm), while D2 is 20 mm. In other examples, other values of D1 and D2 can be used, with D2 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.
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
Magnetic field cancellation provided by the quad arrangement depicted in
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).
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.
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.
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.
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
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.
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.
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 (
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
L=K1+K2=T sin(α)+T sin(ψ)
K1·X1=K2·(X1−X2)
γ=α−ψ
The Q-fin body has wing shaft X1 distance from rear and X2 distance from front. To solve for K1 and K2:
K1=K2*(X2−X1)/X1;
XX=(X2−X1)/X1;
Substitute for K1 in terms of L2 into L=K1+K2;
L=K2*XX+K2=K2*(XX+1).
So,
K2=L/(XX+1)
K1=L−L/(XX+1).
Therefore, to solve for α, ψ and γ using K1 and K2:
K1=T sin(α) and K2=T sin(ψ);
γ=α−ψ
where γ is the bias due to fin lift.
Next, the formula for getting the component of misalignment due to moment is calculated:
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
β=Compass Heading−φ−γ+r,
where r is residual compass error due to instrumentation and any other errors.
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.
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.
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).
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
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.
Referring again to
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.
The computations in
Instructions of software described above (including processing software 702 of
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).
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.