Survey, Navigation and Positioning Guidelines for 3D Marine Seismic Survey Specifications

Abstract

Guidelines for readily achievable survey, navigation and positioning specifications for 3D marine seismic surveys are provided in a check list type format detailing: the survey and configuration parameters; the type, quantity and criteria for the equipment; checks, verifications and calibrations; rejection and termination criteria; and deliverables. Each system or component required is described together within these topics. These guidelines could be considered the minimum standards of performance, required proofs, and quality control criteria for the acceptance of the survey, navigation and positioning for 3D marine seismic surveys.

Introduction

During the author's career as a Client Navigation Representative, he has had to implement a variety of contracts and specifications for 3D marine seismic surveys, with respect to survey, navigation and positioning. Some of these have omitted details, which were critical to or would have significantly improved the quality of the survey.

As part of the author's procedures a list of survey, navigation and positioning criteria was complied which went through a number of style and content revisions. The end result are the tables below which could be used as a check list. The various equipment required and the expected performance of that equipment are described. Most of the values and other criteria suggested have been compared under actual survey conditions and found to be acceptable.

Not every table is complete. Some information will be supplied by the company, and other information will be supplied and negotiated with the contractor. However, the majority of the information provided below should not have to be altered as the specifications detailed should suit most 3D marine seismic surveys. A tabular format is used to highlight the values and criteria suggested so as not to become buried in extensive explanations. Consequently, only brief comments follow some of the tables where applicable.

The reader is encouraged to use these tables for reference and guidance. The values adopted by a company for any item must meet the survey objectives of the company, and be negotiated with the contractor. Both should ensure that the vessel to be used is capable of meeting the criteria set out in the subsequent contract. The company may be required to accept a lower standard due to time or budget constraints.

3D marine seismic survey operations have been described in various articles some of which are listed in the references. These and other articles should be referred to if the reader is uncertain of the relevance of some of the topics listed.

1. General Information

2. Configuration

The contractor should supply a cable and source positioning system integrating at least the vessel(s) positioning; actively positioned source sub array(s), front (gun) float(s), and tailbuoy(s); laser positioning; front and tail (and middle if used) acoustic network(s); streamer compasses; and gyro compass(es). The contractor should use an integrated navigation system and a post-processing system that output compatible results (for example, the software should be based on the same processing models and algorithms).

These guidelines cover the use of all of the above although it is possible that a system may not used, such as laser positioning, or a front (gun) float(s) is not deployed. If that is the case then reference to the affected item(s) which follows can be disregarded.

3. Expected a posterior Positioning Accuracy Relative to the Vessel

The above accuracies should be achievable for each source; for the front (gun) float(s); for the first, middle and last trace on each streamer; and for each tailbuoy. Sufficient redundancy should be incorporated in the network(s) design, to be able to achieve these results throughout the survey.

The effect of any permanent failure(s) on the accuracy of the network(s) should be demonstrated by the contractor on a MOVE' (or similar) package, and should ensure that the positioning of any node should not be greater than ± 1.5 metres when compared to the previous network analysis. Realistic a priori standard deviations (1σ) should be used which should be approved by the company.

The corresponding horizontal mid point (HMP) accuracy would be 2.8 metres for the first and last common mid point (CMP), and 3.1 metres for the middle CMP for each streamer.

The above calculations are based on the bivariate probability of a point lying with in an error ellipse drawn with semi-major and semi-minor axes of (σ_max) and (σ_min). The correlation between the in-line and cross-line errors has been estimated.

Using the error estimates for the in-line (σ_x) and cross line (σ_y) directions then 0m,,, and 0mmcan be computed assuming a realistic value for the correlation (σ_xy) . The correlation values are included to ensure all possible errors were considered instead of being assumed not to apply. The formula are:

σ_max² = 1/2 { σ_x² + σ_x² + [ (σ_x² - σ_x²)² + 4σ_xy² ] ^1/2 }

σ_min² = 1/2 { σ_x² + σ_x² - [ (σ_x² - σ_x²)² + 4σ_xy² ] ^1/2 }

The derived σ_max can then be used to calculate 95% (2.447σ_max).

4. Streamer Parameters

*The maximum feather angle will depend on the geophysical objectives. All available tidal and current information should be utilized to predict the expected feather angles.

Streamer compasses are specified below under Navigation and Positioning Systems.

5. Source and Streamer Geometry

*Larger adjacent tail streamer separations caused by currents and sea conditions may be acceptable if approved by the company.

^△The minimum offset from the centre source to the inner streamers will be at least the minimum safe distance.

^▽Instantaneous variations would occur due to sea conditions and should be for not more than 20 shot points.

6. 3D Coverage Parameters

*The streamer(s) can be subdivided into various subsections, which can be equal in length, or whatever length is required.

^△Maximum possible without creating seismic noise or strumming on any streamer(s) .

^▽The fun-in for any line or part of a line should allow the streamer compasses to stabilize, ensure the streamer(s) is straight for best coverage, and to ensure that the acoustic positioning system is operating satisfactorily, and the gyro compass(es) have stabilized to minimize any Schuler effects.

7. Binning Displays Required

8. Line and Shot Point Numbering

*Other conventions may be more appropriate to the particular needs of the company. The line naming convention should ensure that the seismic recording system does not truncate the line suffix number.

^△The shot point renumbering suggested for reshoots and infill may not be required.

9. Local or National Datum

10. Alternate Datum

11. Datum Shift Parameters from WGS-84 to Local Datum

The multiple regression shift parameters may be specified instead of the seven parameter transformation above, if the information is available for the area in which the 3D survey will be carried out.

12. Geoidal Height

13. Projection Parameters on Local Datum

There are other grid projections, such as Transverse Mercator, Rectified Skew Orthomorphic, or Gauss-Kruger, which will have different parameters than UTM.

14. Example of Co-ordinate Conversion

These are very useful to confirm the contractor's software agrees with the company's.

15. Prospect Boundary Points

Insert as many points as required to describe the prospect full fold area.

16. Block Boundary Points

Insert as many points as required to describe the block area. The block boundary should be shown on all maps, plans and aerial plots produced by the contractor.

17. Navigation and Positioning Systems – Type, Manufacturer, Software Version and Date

*The single frequency GPS receivers should support P-code resolution on the CA-code measurement (Ll) type technology, provide parallel tracking, and be ail-in-view type GPS receivers. The components of the primary DGPS positioning system should utilize identical GPS and telemetry equipment, pseudo range correction (PRC) software and firmware versions. The components of the secondary DGPS positioning system should have the same characteristics but should be of a different type. (See next table.)

The number of rGPS units, acoustic sensors, and streamer compasses should meet at least the above minimum criteria and ensure that the resulting uncertainty in the front and tail network positioning of any node should not be greater than (1.5 metres when compared to the previous network analysis.

^△The number of acoustic units should meet at least the above and the following minimum criteria:

1. The cable separations at the front and tail should be directly measured, by duplicate range measurements if possible.
2. There should be at least 2 acoustic sensor units mounted approximately 100 metres apart, one on either side of the first live trace. The first acoustic sensor should be mounted within 15 metres of the first live trace.
3. There should be at least 2 acoustic sensor units mounted approximately 100 metres apart, one on either side of the last live trace. The last acoustic sensor should be mounted within 15 metres of the last live trace.
4. The network should have at least 30% redundant acoustic range observations to enable biases to be determined, and to ensure there are no uncontrolled observations.

^▽The number of streamer compasses should meet at least the above and the following minimum criteria:

1. There should be 2 compasses mounted approximately 100 metres apart, one on either side of the first live trace. The first compass should be mounted within 25 metres of the first live trace.
2. There should be 2 compasses mounted approximately 100 metres apart, one on either side of the last live trace. The last compass should be mounted within 25 metres of the last live trace.
3. The maximum distance from the last active streamer compass to the tailbuoy on each streamer should be less than 100 metres.

18. Minimum DGPS Equipment Requirements for Primary and Secondary Positioning

*Twelve channel CPS receivers are preferred as these provide enhanced tracking capabilities, faster acquisition and re-acquisition times, better performance during high dynamic maneuvers, with an update rate of less than 1 second per cycle.

^△The individual DGPS reference stations should be chosen so that they are equally spaced around the prospect to minimize distortions caused by ionospheric and tropospheric effects. As much as possible the following guidelines should be adhered to with respect to the maximum distance from each DGPS reference station to the prospect centre. The optimum locations should be chosen for the GPS antenna ashore and onboard to minimize electromagnetic interference, multipath effects, and any obstructions, GPS health (site) audits of the DGPS reference stations and the DGPS monitor station (if used) to check for multipath effects, electromagnetic interference, and obstructions should have been updated as specified later in these guidelines.

^▽Distances from the prospect to the individual DGPS reference stations should be kept to a minimum.

19. Required Accuracy for DGPS Reference and Monitor Station Co-Ordinates

The GPS land survey observations to establish the geodetic control for the DGPS reference and monitor (if used ) stations should meet the latest version of an approved national standard, such as the following:

1. "Guidelines and Specifications for GPS Surveys", Natural Resources Canada, Geodetic Survey Division, Ottawa, Ontario, Canada, Edition 2.1, December 1992.
2. "Geometric Geodetic Accuracy Standards and Specifications for Using GPS Relative Positioning Techniques", Federal Geodetic Control Committee, National Geodetic Survey, NOAA, United States, Version 5.0, May 1988.

All DGPS reference and monitor stations should be tied (referenced) to the primary control network of the country in which the stations are located. The national control points to which the GPS land survey should be tied should be at least second (2nd) order accuracy (1 in 50,000) or better.

20. Required Offset Measurement Accuracies

All static offsets should be accurately measured by land survey methods. All offsets and laybacks should be shown on an appropriate set of diagrams and the layout approved by the company. The angle of deflection for towed acoustic sensors would vary depending on their weight.

Contractor should endeavor to ensure that the offsets and laybacks are unchanged during the survey unless operational reasons justify change. Any changes should be re-measured and the network definition(s) and effected diagrams should be revised and approved by the company.

Any or all offsets and laybacks should be checked by contractor at each streamer deployment when required by the company.

All offsets of the in-sea units should be measured to the centre of the measurement sensor on each acoustic unit and streamer compass, and the centre of each individual gun or gun cluster. Allowance should be made for the stretch in the front and tail stretch sections.

When the shortest possible source and streamer geometry have been established allowing for operational and safety concerns, all towing wires and chains should be marked to ensure the geometry remains the same following each deployment.

21. DGPS and rGPS Positioning Criteria

*Whenever possible, the maximum available number of healthy satellites that meet the minimum elevation criteria should be used in the positioning computation(s). Satellite prediction software with the latest updated almanac should be available throughout the survey. Such software should be used to identify any poor coverage windows and if possible plan operations accordingly.

^△Provided the DGPS and rGPS software is capable of handling such low elevation satellites by the use of appropriate weighting. The weighting should be documented and approved by the company.

A continuous integrity check could be carried out during mobilization and the survey for each line or part of a line. If an integrity check is not possible then all DGPS positioning equipment should be verified in situ with the vessel(s) fast alongside prior to the commencement of seismic acquisition.

A re-radiation check of all rGPS positioning units could be carried out during mobilization, prior to deployment, and, as required, during the survey. If a re-radiation check is not possible then all rGPS positioning units should be verified in situ with the vessel(s) fast alongside prior to the commencement of seismic acquisition.

22. Active Tailbuoys - rGPS and Acoustic Positioning Acceptable

An active tailbuoy is defined as any tailbuoy with operational rGPS and acoustic positioning. With 3 or more streamers, acceptable active tailbuoys should not be adjacent to each other. The maximum number of active tailbuoys would be one for each tailbuoy.

The contractor should ensure that each tailbuoy unit has sufficient power to satisfy the requirements of the positioning system and data telemetry systems. Non-gassing batteries should be used.

23. Active Gun Sub Array(s) or Front (Gun) Float(s) - rGPS and Acoustic Positioning Acceptable

Active front (gun) fIoats(s) may not be required if adequate reps units are deployed on the source sub array(s). The maximum number of active rGPS and acoustic units would be one of each on each gun sub array.

24. Laser Positioning Criteria

*Orientation from the navigation gyro compass, should be combined with the laser directions to derive the laser bearing in the integrated navigation system and the post-processing system.

^△To allow direct comparisons to be made with the primary navigation gyro compass.

Laser positioning may be redundant with sufficient rGPS units deployed on the source arrays and front (gun) float(s). The prisms should be waterproofed to prevent water ingress. To prevent damage during deployment and recovery of a source sub-array, the prisms should be protected by a bracket or other strong feature, if operationally feasible. Repeated failed attempts to establish acceptable laser positioning to the source sub-arrays should require alternative arrangement(s) to be devised with company approval.

25. Acoustic Positioning Criteria

The contractor should provide a medium or high frequency acoustic positioning system capable of providing acceptable data in the conditions expected during the survey. These conditions would include but not be limited to, the expected water depths, tides and currents, and the temperature and salinity changes. The acoustic positioning system should be designed such that the co-ordinate accuracy of the source(s) and streamer traces should be within the tolerance specified by the company.

A unique serial number should be clearly engraved or marked in some other durable manner on an easily visible portion of the acoustic sensor housing.

The redundancy and geometry of the observed acoustic ranges should meet good survey practices and standards, and should allow an effective, statistically adjusted network(s) to be computed. The survey network should contain sufficient redundant observations to enable biases to be detected. The redundancy should be evenly spread such that the network contains no uncontrolled observations. A well controlled network will have the redundant observations in the design network. If the processing method is based on a phased adjustment of sub-networks, this redundancy requirement should apply to each sub-network. The company preferred testing method for all observables is by applying Baarda statistical testing for outliner detection using the testing values specified by the company.

Part II – Continued in June 2007 RECORDER.