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`Ex. PGS 1055
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`EX. PGS 1055
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`Investigation of Interactions between Cable Leveling and Heading Measurement
`
`SA 2.2
`
`Ted L. Amrine, Jt, Oryx Energy Co.; Sam A. Loree, Halhburton Geophysical Services Inc.; and Dave Ridyard, Q.C Tools
`
`SUMMARY
`
`CALIBRATION TECHNIQUES
`
`a
`to play
`positioning
`pre- and
`
`continue
`sensors
`heading
`Magnetic
`streamer
`role
`in marine
`3D seismic
`crucial
`sophisticated
`a variety
`of apparently
`Despite
`post-mission
`calibration
`techniques,
`navigation
`processors
`are still
`routinely
`required
`to make
`subjective
`assessments
`of
`individual
`compass biases,
`or use modelling
`techniques
`to produce
`reasonable
`looking
`final
`location
`data.
`tc
`Although
`several mechanisms may contribute
`and spatially
`variant
`nature
`of
`these biases,
`time
`the
`the authors
`believe
`that
`a critical,
`and previously
`un-noted
`source of
`these biases
`has been
`isolated.
`Controlled
`experimental
`data
`is described,
`a mechanisrr
`is discussed,
`and some potential
`remedies
`are
`suggested.
`
`INTRODUCTION
`
`heading
`than 10 years, magnetic
`For more
`the primary method
`for positioning
`sensors have been
`for 3D surveys.
`Ini tiallx,
`compasses
`marine
`streamers
`the streamer,
`in special
`sections.
`were built
`into
`as
`it became required
`that
`regualar
`in-line
`However,
`group spacing be maintained,
`compasses migrated
`to
`pods externally
`mounted on the streamer.
`These
`of
`externally
`mounted pods
`tended
`to act as sources
`streamer
`noise,
`and over
`the
`last
`few years,
`there has
`been a very positive
`move to
`integrate
`heading
`sensors
`into
`the depth controllers
`(birds).
`The estimation
`of compass biases
`taken up
`(misalignment,
`non-linearity,
`etc.)
`has
`geophysical
`considerable
`effort
`from manufacturers,
`different
`contractors
`and oil
`companies.
`However,
`techniques
`of bias
`estimation
`often
`give different
`The
`final
`judgement
`on what bias
`corrections
`results.
`to apply on a
`line
`by
`line,
`even shot by shot,
`basis
`is
`left
`to
`the navigation
`processors.
`Their
`judgement
`sometimes
`aided by a computer generated model,
`relies
`heavily
`on a subjective
`perception
`of what streamer
`shapes are “realistic”.
`is
`made
`is generally
`that
`The assumption
`is a non-rigid
`body,
`fixed
`since
`the streamer
`that
`This often
`“kinks”
`in
`the cable
`are not acceptable.
`to
`leads
`the conclusion
`that
`some compasses exhibit
`different
`biases
`from
`line
`to
`line.
`In some cases,
`such compasses are simply
`excluded
`from
`the streamer.
`the ambiguitv
`in “bias”
`is
`In most cases,
`but 1.0
`to 1.5 degrees
`are not
`than 0.5 degrees,
`less
`exceptional.
`Often biases
`are
`random down the
`and
`tend
`to self
`cancel,
`thus minimizing
`streamer,
`Vessel
`crcvs are
`trained
`total
`positioning
`error.
`place better
`compasses at
`the
`front
`of
`the streamer,
`where biases would have
`the maximum detrimental
`the
`(The authors will
`suggest
`later
`that
`effect.
`putting
`good compasses at
`the
`front
`nay well be a
`As a result
`self
`fulfilling
`prophesy).
`of all
`these
`phenomena, major positioning
`errors
`due
`to compass
`rare . . .but
`biases
`have been
`far
`from unique.
`
`to
`
`640
`
`the cornpasse:
`of
`the packagin,:
`of
`Regardless
`remained
`as
`the cornerstones
`of
`3 techniques
`have
`bias
`estimation
`technology.
`In varying
`degrees
`of
`sophistication,
`these
`techniques
`are:
`1 : Bench calibration
`: The compasses are
`at accurately
`known magnetic
`headings,
`and
`placed
`C-O values
`comuted at each point
`on the compass rose,
`This
`is highly
`accurate,
`and generally
`repeatable
`technique,
`but
`it
`fails
`to comprehend
`the magnetic
`environment
`of
`the streamer,
`the magnetic
`inclinatior
`of
`the survey
`area,
`and any dynamic effects.
`2 : Short Tow calibration
`: All
`compasses
`causing
`are placed
`as close
`as possible,
`without
`to avoid
`interaction,
`far enough behind
`the vessel
`Given a
`any perturbation
`of
`the Earth’s
`field.
`sufficient
`drag,
`the assumption
`is made that
`all
`Various
`compasses point
`in
`the same direction.
`statistical
`methods are used
`to extract
`bias
`estimates.
`
`the
`
`: Production
`3 : Production
`calibration
`compass data
`is statistically
`evaluated
`based on
`the assumption
`of a “water-pulley”
`effect,
`causing
`compass headings
`to he the same as
`they pass over
`This
`technique
`has
`little
`the same point
`in space.
`statistical
`merit
`over a few
`lines,
`but given
`sufficient
`statistics,
`good results
`can be achieved.
`
`INITIAL OBSERVATIONS
`
`1988,
`late
`in
`to a vessel
`a visit
`During
`that during
`crew noted
`an observer
`on the vessel
`two compasses 4 meters
`apart
`at
`the
`production,
`One
`tail
`of
`the streamer
`read 5-7 degrees
`different.
`of
`these birds was at a fixed wing angle
`(10 degrees)
`was
`in depth keeping mode,
`while
`the other
`resulting
`in a wing angle of minus 8-10 degrees.
`When both birds were put
`to zero,
`their
`headings
`The experiment
`was repeated with
`the
`agreed.
`were
`compasses on the other
`streamer,
`and results
`two
`The
`tests were
`repeated with
`the
`similar.
`the
`pairs
`of adjacent
`compasses at
`the head of
`and no significant
`heading
`differences
`streamer,
`could be observed.
`
`FIRST EXPERIMENT
`
`II.
`
`was performe
`tow compass calibration
`A short
`(30)
`of a 30 survey.
`Thirty
`the start
`to
`prior
`calibration
`compass-birds
`were placed
`on a 150 meter
`section,
`300 meters behind
`the M/V Cecil H. Green
`1200 meters of streamer was towed behind
`the
`calibration
`section,
`to act as a drag anchor.
`and
`the streamer
`floated
`weather was perfect,
`10 meters +/-
`1.5 m with
`all
`the wing angles
`zero.
`
`The
`at
`set
`
`to
`
`A 45 minute
`line
`in each
`line was run
`in 300 samples of each compass.
`resulting
`direction,
`TWO compasses showed major
`failures,
`and one
`
`Downloaded 03/31/14 to 173.226.64.254. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/
`
`Ex. PGS 1055
`
`

`

`2
`
`exhibited
`deviations
`excellent.
`
`Investigation of compass-birds
`-.. _
`
`(Fig. 1) 5 t andnrd
`a bias of over 1 degree.
`on all
`‘28 operational
`compasses wert’
`
`, steady state
`in which adjacent
`to be reached
`‘,irds are
`forced
`to fight
`each other on the
`calibration section.
`for Cal Line 8,
`Figure 5 shows the results
`in which discontinuities
`in heading can be seen at
`each compass-bird with non-zero wing angles,
`The
`data collected
`on Lines 5 through 9 showed a near
`linear
`increase
`in
`the spread of deviations
`apparent at
`the compass birds.
`
`MECHANISMS TO PRODUCE FIN ANGLE BIAS
`
`Several people have noted that acceleration
`in a magnetic
`field
`can induce systematic bias
`in
`the observed heading.
`Since compass birds undergo
`more dynamic movement than pure compasses,
`it has
`been suggested
`that
`this could cause the effect.
`the magnitudes of this error are not
`However,
`Nor would this
`consistent with our observed biases.
`mechanism explain
`the streamer
`tension dependency
`which has been circumstantially
`observed.
`Another common theory concerns the
`movement of the motor within
`the compass-bird.
`theory breaks down since
`tests conducted
`in
`flow
`tanks by various other groups, with compass-birds
`attached
`to rigid beams indicate
`no such
`significant
`bias.
`the compasses
`that
`The authors believe
`They are measuring
`fact working correctly.
`in
`are
`above them.
`the heading of the cable directly
`the birds pull not
`Unfortunately,
`in
`the real world,
`directly
`down, but have some component of pull
`to
`The cable
`is
`thus systematically,
`left
`or right.
`locally
`“kinked” by the bird.
`This
`theory
`is by no
`means proven, but fits
`the data well.
`The process
`of birds pulling
`to
`left
`or right
`is generally
`that
`referred
`to as “kiting”.
`It should be noted
`the compasses are sampling
`if
`this
`is
`the case,
`the cable heading at
`the most anomalous possible
`point.
`
`This
`
`CONCLUSIONS AND RECOMMENDATIONS
`
`1 : one component of perceived heading
`sensor bias
`is
`the
`local deformation of cable shape
`fin angle,
`in
`integrated
`cable
`leveller/
`caused by
`The magnitude of this deformation
`heading sensors.
`is approximately
`0.6 degrees
`for 1200 meters of
`This
`streamer, with 15 degrees of
`fin angle.
`magnitude may not be accurately
`estimated
`from this
`data set, due to the presence of the noise
`induced
`by the hypercyclic
`current observed
`in
`the survey
`area.
`
`2 :
`hypothesis on the
`the authors’
`If
`mechanism is correct
`the effect
`should be: -
`(at
`only a weak function of vessel speed.
`(a)
`is
`the streamer
`tension
`higher vessel speeds,
`and the water
`flow over the bird wings
`increased,
`This would result
`in some
`is also
`increased.
`cancellation
`between the two effects.)
`(b)
`inversely
`proportional
`to the amount of
`streamer behind
`the compass.
`approximately
`directly
`proportional
`angle
`in
`the bird.
`
`(c)
`fin-
`
`to
`
`the
`
`3 : The effect
`-
`can be minimized by:
`careful
`streamer ballast
`at zero wing angle,
`
`(a)
`
`641
`
`The second line was extended by 15 minutes,
`and +15 degrees of wing angle was forced
`into
`the
`front 10 compasses, and -15 degrees
`in
`the mid 10
`compasses.
`The standard deviations
`remained
`low,
`but significant
`biases of up to 6 degrees now
`(Fig. 2)
`appeared.
`These wing angles are quite excessive
`normal operation,
`and although
`it was realized
`an important phenomena had been observed,
`the
`-
`results
`asked more questions
`than they answered:
`1 :
`If
`the effect was due to some dynamic,
`mechanical effect,
`was it cumulative?
`2 : Was the effect
`linear with speed/fin
`angle, or was there some threshold
`level?
`With
`these questions
`in mind it was agreed
`to perform a more controlled
`experiment at
`the
`next opportunity.
`
`for
`that
`
`CONTROLLED TESTING
`
`The objective
`
`of the second experiment
`
`were:
`
`-
`
`1 : Eliminate
`any possible cumulative
`by changing
`fin angles only on a small
`effect,
`number of birds within
`the data set.
`2 : Determine differences
`different manufacturers.
`3 : Determine
`linearity
`the
`mechanism, by using more reasonable
`fin angles.
`4 : Determine
`
`between
`
`of the
`incremental
`
`the effect
`
`of streamer
`
`tension.
`
`To this end, 10 Syntron RCL-4 integrated
`compass-birds,
`22 Digicourse model 396 compass-birds
`and 6 Digicourse model 321 pods were assembled
`aboard
`the M/V Northern Surveyor.
`The calibration
`cable was configured as in the
`first
`experiment
`described above.
`line was run at 5.7 kts,
`A continuous
`taking 60-120 samples at each of 9 configurations.
`Wing angles were only adjusted on 6-30 per cent
`Three degree
`increments were used,
`of the birds.
`from 0 to 15 degrees.
`
`RESULTS
`
`The sea state was 2-3, but a strong cross
`wind caused the vessel
`to crab up to 6 degrees, with
`a feather of 3-6 degrees
`in
`the opposite direction.
`The crossed winds and seas caused a strong
`hypercyclic
`near surface current.
`Unfortunately,
`with 38 birds on the calibration
`section,
`a strong
`This caused a
`slant was observed on the cable.
`“bowing” of the cable, somewhat
`systematic
`obscuring any curvature
`induced by compass-birds.
`Figure 3 shows the variation
`in depth and heading
`deviation
`on Cal Line 1 - with all
`zero wing angles.
`Of the 38 units deployed, only compass
`to work throughout,
`and has been
`from all calculations.
`on heading
`Figure 4 shows the effect
`caused by putting
`the birds
`in depth
`deviation
`keeping mode on Cal Line 2a. Note that
`the
`algorithm used to minimize bird battery
`usage causes
`
`30 failed
`excluded
`
`Downloaded 03/31/14 to 173.226.64.254. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/
`
`Ex. PGS 1055
`
`

`

`investigation of compass-birds
`
`3
`
`in pods at
`
`the tail
`
`(b) Use of heading sensors
`the streamer.
`tow
`(c) A new fin angle control mode for short
`the
`but
`calibrations,
`in which depth
`is maintained,
`birds constantly
`seek to return
`to zero
`fin angle.
`(This would result
`in excessive battery
`usage in
`production.)
`
`of
`
`1.2 T
`‘t
`
`0.0
`0 0.6
`
`E 1
`
`G 0.4..
`
`4 : The observed strength of the depth
`dependency implies
`that
`in some area, considerably
`more sampling may be required,
`especially
`if
`flared
`streamer work is
`to be considered.
`
`-0.4
`-0.6 i
`
`FIG. 1 Heading deviations observed at zero-wing angle.
`
`-6-
`
`0
`
`FIG. 2. Heading deviations observed with excessive wing
`angles.
`
`FIG. 3. Heading deviations due to
`hypercyclic currents.
`
`FIG. 4. Heading deviations due to
`depth-keeping activity.
`
`FIG. 5. Heading deviations due to
`controlled wing angles.
`
`642
`
`Downloaded 03/31/14 to 173.226.64.254. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/
`
`Ex. PGS 1055
`
`