PRAKLA-SEISMOS INFORMATION No. 43

I Streamer Positioning for 3-D Surveys

Introduction

Advanced 3-D marine surveys nowadays require a much higher degree of accuracy in streamer positioning than was possible until recently. Dense grids in conjunction with multi-line and strike line shooting make the tolerances ailowable for the determination of streamer positions even more stringent. In order to obtain ail angles and distances with the required accuracy PRAKLA-SEISMOS added new features to the existing measuring systems and the associated computer software. These efforts aim to pinpoint each common reflection point to within ± 10 m. The new techniques to resolve the difficulties involved were adopted after careful research into the problems of sensing angles and tensions, pertaining to the seismic streamer, as primary input para­meters in deriving the streamer position .

Streamer Tracking System

General The foilowing hardware/software components are util ized in deriving streamer locations und er the new scheme:

• Gyro Compass System: to reduce the error against true North to within ± 0.12 degree. .

• Magnetic Heading Compasses: arrayed along the streamer to record the subsurface direction of the streamer sections.

• Precision Direction Finding System: to determine the tail end of the streamer to within ± 0.2 degree.

Fig. 1: Presentation of Hardware Components

Gyro Compass System

Lateral Deflection Angle

The Precision Direction Finding System, developed by PRAKLA-SEISMOS, controls and adjusts the streamer compasses. Reference North is provided by a speciaily assembled multiple gyro system. An auto­mated control system monitors all on-board events in realtime.

In the post-processing stage, a highly reliable motion model is established by cross-correlating multiple se­quences of angle measurements, thereby taking fu" advantage of the redundancy in the data. This procedure improves the realtime data significantly.

• Lateral Deflection Angle Measuring Unit: to measure the horizontal angle between the vessel 's axis and the towing-cable of the streamer to within ± 0.2 degree.

• Stretch Section Distance Measuring Unit: to record the actual distance between the towing-cable and the first hydrophone group to within ± 1.0 metre.

• Realtime Processing and QuaHty Control: to provide a quick reference of headings, positions, and coverage on screen and table plotter.

/ Magnetic heading compasses:---

MeasUri~~U~ni:t~~~~~t;;;~t:~~~~~~~"-'~~~~ Transmitting tailbuoy

Precision Direction Finding System

2

Streamer Tracking System

Description of Components:

Gyro Compass System

The gyro system is the master insrrument which provides the reference North for all measurements of bearing conducted from the survey vessel . The optimization of the reference North is achieved by integrating a multiple set of gyro compasses manufactured by C. PLATH and specially designed for PRAKLA-SEISMOS. A microprocessor checks the plausibility of the individual gyro readings and compensates these readings to true North to within ± 0.12 degree.

Fig. 2: C. PLATH Standard Gyro System Fig. 3: Improved Reference North

Gyro discs

Ring shaped damping container filled with oil

C. PLATH NAVIGAT 11 Gyro systems

MICROPROCESSOR

REFERENCE NORTH = ± 0.12°

3

Streamer Tracking System

Magnetic Heading Compasses

The relative positions of the streamer are derived from the angle indications of miniature magnetic compasses arrayed along the streamer at set intervals. The compasses used are manufactured either by DigiCOURSE or by SYNTRON.

Fig. 4: Streamer with Heading Sensors

True North of Gyro Compass System

Magnetic North wlth specific errors

Send ---,.

Dlrectlon of streamer asreported from magnetic

Current Proposed line

-~- -~._ ,_._~ - - -~--' -' Ship's headlng

SaJled trac

Lateral Deflection Angle Measuring Unit

The deflection angle between the ship 's axis and the towing-cable is continuously recorded -10 determine the ship/streamer lateraloffset. The device consists of universal joints. flexible shafts and synchro units. Angles are recorded 10 wilhin ± 0.2 degree.

Fig. 5: Lateral Deflection Angle Measuring Unit

4

Simplified mechanism--

InductJon Synchro Unit

Deflection angle

Streamer Tracking System

Precision Direction Finding System

Precision Direction Finder The PRAKLA-SEISMOS Precision Direction Finder which operates in the VHF band compares the phase angle of a continuous wave (CW) transmitted from the tailbuoy. The Precision Direction Finder provides the angle between true North and the tai! end of the streamer continuously. The obtained accuracy is better than ± 0.2 degree.

Manual Direction Rnders

Three Manual Direction Finders serve as the function control for the Precision Direction Finder. They are installed in such a way that uninterrupted sighting of the tailbuoy may be maintained at all times. A push-button release mecha­nism permits instantaneous freezing of the angle of the tailbuoy with respect to the main axis of the ship. The navigation computer derives the north orientation automati­cally by adding the ship 's heading to this measurement. The statistical average of a set of observations, computed in realtime, provides the adjusted bearings for calibration and control of the Precision Direction Finder.

Fig. 6: Precision Direction Finding System

Manual direction finders with

telescope sight

PreClsion direction finding "'\Jc"<>,m

antennae with gear box

Continuous wave tram the tailbuoy

,/ /'

Stretch Section Distance Measuring Unit

1 INDAS V Mamframe 2 Multiplexer

3 Direction Rnding Computa 'on

4 True North Computation

5 C. PLATH Gyro System

The variation of the length of the streamer stretch sections is sensed by asounding system developed by HONEYWELL ELAC. It records the required data to within ± 1.0m.

Fig. 7: Stretch Section Distance Measuring Unit

~..o;==..,.-"t~-..-_ Varyingdlstance Lr-"""T'- -------

S ~d ~. Uvesections oun er R~ver

5

Streamer Tracking System

Bin Coverage Display Offsets

.. i .. 100 % 0 %

50 %

cross-line binning (each bin) ~

Time Lag Heading Control Display r---:"IiI~~:ar.::::~

I Other I SUbsy~efT

'-'- '-t;_. I r-._._ I _.

i Radio av Rec • Gravltymetl I Magnelome'

eIe .L....._._._.

--Live Srrear

6

Streamer Tracking System

Realtime Processing and 3-D Quality Control

Streamer tracking data are controlled and processed in realtime to attain optimized and reliable coverage. These tasks are performed with the Streamer- and 3-D Sub­systems of the Integrated Navigation and Data Acquisition System INDAS V (see block diagram).

Essential control components of these subsystems are:

• Plausibility control and error statistics

• Dynamic offset navigation

• Heading control display (see presentation lower lett)

- displays all relevant heading sensors dynamically

- enables correlation of heading sensors by time lag display

Fig. 8: Realtime Processing and 3-D Quality Control Block Diagram

3-D Quality Control

+--.-.oj Subsystem

._.-.J

INDAS V MAINFRAME

Magnetic Compasses Stretch Section Sections--------~~ Monitoring

• Subsurface Scattergram (see plotter sketch immediately below)

- shows feathering variations

- updates knowledge of sea currents

- reports survey progress

• Bin Coverage Display (see presentation upper lett)

- updates bin coverage dynamically

- reports coverage for three selected streamer offsets (usually near / centre/ far)

- shows past and future situations to support coverage tracking .

'""'I

Operator I I DIsplay

Bridge Display

cn c: -w­I/)QI QI> uQl 0-' ... a.

... o I/) I/) Q) u-oQl ... > a.QI ... ::s Q. c:

...

-'

0-I/)Q) c: > Q)QI (/)...J

7

Post-Processing

General

Post-mission processing is to determine the true streamer shape and locations of all streamer segments in the geo­detic grid and thus to derive the most accurate subsurface positions. They are supplied on a positioning data file for further seismic processing. Various graphie representa­tions are plotted at suitable scales for control purposes and to support the seismic processing.

The raw information from all sensors ist enhanced using an integrated, advanced processing technique in order to achieve a positioning accuracy of better than ± 10 metres for shot and receiver locations.

Built-in streamer compasses are strongly influenced by varying torsion und tension forces. Experiences show that their calibration state changes with variations in the local environment such as, in particular, the revolving of the streamer. And therefore, one major task in post-processing is to redefine the pre-mission calibration values on a conti­nuous basis, and not to consider them invariant over the whole survey. Special correlation techniques in conjunction with the in­dependent PRAKLA-SEISMOS Precision Direction Finding System (controlled by precise gyro compass reference) result in linewise calibration functions for each individual streamer compass unit.

Processing

The 3-D positioning sequence consists of 4 main phases (see flow diagram) ;

Phase 1 Phase 3

• Sensor data preparation • Determination of the streamer polygon

Phase 2 • Polygon closure to tailbuoy

• Positioning of ship based locations Phase 4

• Processing of sensor data • Positioning of common reflection points

• Graphics Representation

Streamer Positioning Sequence

Input Output

I Non-sefsmic Field Data

-~ '~

I Formal Review --. Control Display -• I Interactive Terminal .. --. Ediling ProfilePlols

• • Dala Organisalion I I Ship Positioning ..1 . Sholpoinl Localion Map -.-

I Calibralion I and Fillering - Final Sensor Display

• 'I Streamer Polygon I I Parameters for Areal Graphics

I Tailbuoy Adjustmenl -"" Streamer Location Map -I~

-~ ,.-.

I COP Positioning -"" Subsurface Scattergram -• -I Bin Orienlalion I J I Bin ParameIers Bin Graphics I and Coverage I I

I

• I 3-0 Data Collection . 3-0 Data Tape

~ I Seismlc Processing

8

I

I

I

I

I

J

I I

-

I

Editing of gaps, spikes and sensor malfunctions along with a formal review of all input data are the main tasks during Phase 1. Plots and displays as shown in fig . 10a assist in performing the edits .

Phase 2 splits into ship based positioning and 3-D sensor data processing. Ship based positioning results in highly accurate geodetic positions for antenna and source (see fig. 9a, location map for antenna) .

3-D sensor data are processed in an "instantaneous position domain", by removing the time-lag of the events affecting the various sensors. Sensor data are correlated, filtered, and calibrated (see fig . 10b). Heading sensor data are calibrated relative to each other after application of corrections such as magnetic deviations. At this stage, the residual systematic error for all compasses becomes approximately the same.

Additional reference angles for the streamer polygon are derived from the original compass data in the instantaneous position do~ain (see fig . 11 a) in order to maintain continuity over the entlre polygon length on a piecewise basis. Final sens.or displays as shown in fig.10c are produced by trans­formmg back to the time domain, after the interpolation process is completed .

Fig. 9a: Location Map Based on Antenna Positions

Fig. 9b: Location Map Based on

Reference Reflection Point

P F3 9 Skm

PE39 Skin

Post-Processing

Cable deflection angles, streamer angles, and stretch section variations are input into the streamer polygon process in Phase 3. Displacements of precalculated streamer polygons against the tailbuoy-track result in a linewise calibration function for the streamer, which is further examined for reliability. The reviewed function represents the residual systematic error (see phase 2). The residual error is removed by a polygon closure to the tailbuoy (see fig . 11 b) .

Phase 4 consists of final steps such as the positioning of the reference data points, which are kept on the proposed line during the survey (see fig . 9b) . Positioning of common reflection points from source and receiver locations terminates with the generation of a 3-D positioning data file. In addition , various graphie representations are produced. Special streamer control maps with rotated streamer poly­gons for enhanced control are plotted, as weil as the sub­surface scattergram, which is shown as an example in fig . 12. Apart of a bin coverage map is shown as a perspec­tive view (2-D and 3-D) on the backcover as an example of bin graphics.

9

Post-Processing

OPTICAL TAILBUOY BEARING 6 I

El.ECTRICAL TAILBUOY BEARING I 5"

FAR SENSOR

I

~ CALI8RATION FUNCTION (1-21 for example ' 0.2 ~

-0.4' OFFSET TRACK

Fig. 10a: Raw Sensor Plot Fig. lOb: Edited Sensor Plot

Fig. 11 b: Polygon Closure

400 375 350

~_~~,~,_, __ ~.~ ____ !~, ____ ~x ____ ~~~, __ ~I --~:~,O~~--T_R-~~~~~~,---Jl __ ~' ______ x __ ,,~, __ ~~~~3_2LI5~X(~--~-~-,---x

SCALE

0.5 KM

'--_-:y-----.:..' y----- .... ' .;:.y-TArLBUOY TRACK ---y---'-y~--

\ TAILBUOY

10

/890

OPTICAl ~l.8UOY BEARING I, ____ ------------~

El.ECTRICAl TAILBUOY BEARING I

NEAR SENSOR I

SHIP'S HEADING

I-~ 200 ~

~------~, ------------- ~ -----

Fig. 10c: Final Sensor Plot

' . . , ' ..

........ , '"""7"" * ,

Post-Processing

SENSORS METHOO

HEADING CORRELATION

CABLE DEFLECTION

-NEAR ~ lot n~ar trac~1

- 2 ND a»4PASS

~ - FAR COMPASS

I lot lar traceI

~ -TAILBUOY

PlECEWlSE

..... 'MeI •• CGIINSS COtI_

CABlE DRUM

TRAlUNG

N

INTERPOLATION

CORRELATION

TAILBUOV

Fig. 11a: Streamer Polygon Method

Fig. 12: Subsurface Scattergram

= n iL!Aa; - '

--r=-

5 _ _ _ +4 -,

11

f\ PRAKlA· SUSMOS

\J V

BIN COVERAGE

aboTe SO. 0 55. 0 - 60. 0 50. 0 - 55. 0 45. 0 - SO. 0 40. 0- 45 . 0 35. 0- 40. 0 30. 0- 35. 0 25. 0- 30. 0 20. 0 - 25. 0 15. 0- 20. 0 10. 0- 15 . 0 5. 0 - 10. 0 1.0- 5. 0

bei,", I. 0

PRAKLA-SEISMOS GMBH· BUCHHOLZER STR. 100 . P.O.B.51 0530 . 0-3000 HANNOVER 51 PHONE: (511) 6460-0 . TELEX: 922847 + 922419 + 923250 . CABLE: PRAKLA GERMANY

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5000883