ASPE49061Societyd PetroleumEnginearsAmiication
ofIntemolationIn-FieldReferencingtoRemote
OffshoreLocationsH.S~Williamson,SPE. BPExdoration, P.A.Gurden, SPE,
Baker~uqhes INTEQ. D.J.Kerridae, BritishGeolo~ical Survev-.and
G.Shiells, SPE, Baker HughesINTEQCopydght iWS, SWkty of
PefmleumEngineers, inc.Thispaper waspreparedforpresenlafional the
iWS SPE Annual Technical Conference andExhibition held in
NewOrfeans, Louisiana, 27-SOSeptember 1998,Thispaper
wasselectedforpresentationbyanSPE ProgramCommitea
IollowingreviewofInformationcontained inanabrdract aubmited bythe
author(a). Contents ot the paper, aspresentwl, havenot
keenreviewedby theSocietyofPetroleumEngineemandaresubject
tooorrecfion bythe author(s). The material, as presented, dces not
necessarily reflect anyposition of theSociety01 PetroleumEnginews,
ihnc.fficem, ormembers. PapempresentedatSPE meetings amsubject to
publication reviewby Edtorial Committees G+the Sociely
01PetroleumEngineers. Electronicreproduction, dktribution, or
storageof anypadof thispaperfor commemial purposeswithoul
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Permission toreproduceinprint isrestrictedtoanabslracf of not more
then300wordq illustrations may not be copied. The ebatracf must
contain conspicuousacknowledgment of whereandby whomthepaper
waspresented Wtie Librarian, SPE, P.O.Sox S3S8Ss, Richardson,
TX750SS-282S, U. S.A., fax 01-972.952434S5.AbstractA method for
modelling the crustrd magnetic field vector fkomtotal intensity
datahasbeenused to determinethemagneticfield snapshot re@red for
Interpolation In-Field Referencing(IIIR), Themethod has been
validated in anumber of ways,including comparison of magnetic
andgyroscopic survey datain three UK fields,IntroductionThe
objective of the wellbore surveyor is simply stateddeliveryof
wellbore position to the required accuracyatminimum cost to the
operation. The technique of InterpolationIn-HeldReferencing(IIFR)
described by Russell et a1102hasenabled survey accuracy previously
only obtainable horngyroscopic systems to be achieved with standard
MeasurementWhile Drilling (MWD) tools.The diftkultyof making
accurate magnetic measurementsat seahas until
reeentlylimitedtheoffshoreapplicationofIIFR to near-shore locations
where the gradient of the erustalfield was knownto beslight.
Thecurrentauthorshave beenable to surmount this diftlculty through
innovative use of totalfielddat&typicallyacquired with an
airborne survey. Bymaking certain simplifying assumptions, this
data may be usedto deduee mathematically the direction of the
crustal field,Operators are increasingly inviting directional
chillingcontractors to share in project goats andto provide
assurancethat their design activities and operations are
technicallysound. In developing aneffective IIFR survey service
withinthistype of relationship, the
directionaldrillingcontractoristasked with the co-ordination of
data flowbetween as many asfour organisations. A casual approach to
this task willinevitably lead to confusion and disillusionment of
thecustomer. For the examples described in this
paper,responsibilityfor the implementationand operationof
theservice was taken by an autonomous
surveymanagementgroupwithinthedirectional drillingcompany. Inthis
way,timelydelivery of theIIFRdata totheoperationwaskeptcentral to
the targer positioning objectives of the wetl.TheGeomagnetic
FieldThe geomagneticfieldvector, B, maybe specifiedbyitsdeclination
D (the angle between true north and the horizontalprojection of the
field vector, measured positive eastwards), itsdipangleI
(theanglebetweenthehorizontal andthefieldvector, measured positive
downwards), and its intensity F. Thefield can also be described in
terms of its orthogonalcomponents X (north), Y(east) and Z
(vertically downwards).X, Y,Z and Fare usually expressed in units
of nanoteslas, nT.At any Wation near the Earths surface, B may
beexpressed as the vector sum of the main field generated in
theEarths core (&), the crust.alfield from beat reeks (E) and
acombined disturbance field due to electrical currents flowing
inthe upper atmosphere and magnetosphere, and currentsinduced in
the sea and in the ground (BJ:B. accounts for approximately 98% of
the field strength at theEarthssurface, anditsstrengthanddirection
vary relativelyslowlywithtime. In the North Seathe rate of change
istypically some tens of nT per year in F, and a few arc-minutesper
year in direction. B. may be regarded as a static fiekl onlyvarying
over geological timescates. Incontrast, B~ fluctuatesontimescales
of minutes tohours. Duringseveremagneticstorms the intensity of B~
may vary by afew thousand nTatNorthSea latitudes, andit cantake any
direction, leadingtovariations in the direction of B of several
degrees.The normal practice of drilling surveyors in anatysing
datacollected by magnetic survey tools has been to obtain
estimatesofthegeomagnetic field strengthrmd directionat
adrillinglocation from a global model of the geomagnetic field sueb
asthe British Geological SurveyGlobal Geomagnetic Model(BGGM).
However,global models are designedtoprovide3872 H.S.WIIJJAMSON, P.
A. GURDEN, D, J. KERRIDGE, GSHIELLS SPE 49(X1estimatesof B. only.
The contributions of BcandBdareeffectivelyerrors when B. alone is
taken as an estimate of thelocal field, B.Ideally, magnetometers
would be situated at a drillingsiteto measure the local strength
and direction of Bcontinuouslyand with high accuracy. This has
rarely proved to be a rerdisticproposition. The techniqueof IIFRhas
beendevelopedtoprovide a practical approximation to the ideal. IIFR
combinesa local one-off absolute measurement of the geomagnetic
fieldwithcontinuousmeasurements madeat
oneormoreremotemagneticobservatories toestimatethelocal vrdues ofB,
Ineffect a virtualmagnetic observatory is run at the site,
takingadvantage of the stringent quatity control procedures applied
atthe remoteobservatories,Russell ef d and Shiells ef alzdescrhxl
an application of IIFR where, txxause of theproximity of the
drilling site to the shore, heat absoluteobservations of
declination, dip angle and total intensity couldbe made using
standard land survey methods.Magnetic FieldModelling
withAeromagnetic DataAeromagneticsurveyshave been carried out in
oil explorationareas for manyyears toprovide data to assist in
defining sub-surface geology. Theaeromagnetic datasets typicatly
consistofclosely-spaced spotmeasurementsof total intensitymadeusing
an absolute instrument. Conventionally, the datacollected
arecorrected for time-varying fields by reference tomeasnrementa
madebya magnetometeroperatedat a basestation. An estimate of the
mainfield isthenremoved, oftenusing a global geomagnetic field
model, andthe residuals arepresented in the form of crustal anomaly
maps,A number ofanthors45,have considered the problem of using
total intensitydata sets to estimate the vector components of the
crustal field,but this work has received little attention to date
because of thelimited value perceived for geological
interpretations, There is,however, clearly considerable value in
using these methods toextend the application of IIFRoffshore.The
Method: Assumptions and Limitations. Themathematical formulation of
the method is given in theAppendix, A number of assumptions are
made,1.2.3.The crustal field vector, intheregionwhere the
dataarecollected,maybeexpressedas the gradient of a
scalarpotential, Asaeromagneticobservations aremadeintheair, where
there areno electrical currentsflowing andnomagnetised materiat
this assumption is vatid.Thescalar potentialis harmonic, i.e. it
satisfies Laplacesequation. This follows ftomthefurther
assumptionthatmagnetic monopoles do not exist.The anomaly in field
intensity is, to a god approximation,the component of the crustat
field vector in the direction ofthe local main field vector. This w
be shown to be a goodassumption provided
thestrengthoftheanomalyfieldismuch less than that of the main
field. (If the field intensityis about 50000 nT and the anomaly is
500 nT the errorin4.5.making this approximation is about 2,5
nT.)Theanomalyinfieldintensityisharmonic. This istrueprovided the
direction of the main field is constant over thearea of the
aeromagnetic survey.The data points are collectedon a horizontal
surface.Aeromagnetic surveys offshore are flown at a
well-definedaltitude, approximating to a horizontal surface.The
final two assumptions limit the total areafor which datamay be
analysed, but in practical terms analysis of data over anareaof
about 50 km by 50 kmis generally adequate andthedeviations from the
exact requirements of these assumptions isthen small. An implicit
assumption is that an accuraterepresentationof thegeomagnetic
mainfield isavailableforthe survey area. In estimating the crustat
field vectorcomponents no assumptions are made about the
geometriczdorgeophysical propertiesofthesources ofthecrustai
anomalyfieldsuchastheir dimensions, orientations,
ordirectionsofmagnetisation. However,in practice the magnetic
anomalydata should be examined carefully and local
geologicalinformationused toas.wss thelikelihood
ofthepresenceofsignificant shallow magneticsources, Insome
circumstances,where such sources exisf, extrapolating the crustal
componentvaluescomputedat the aircraft altitude to the
subsurfaceenvironment may lead to significant errors.Validation of
the Technique. The successof the aeromagneticdata transformation
technique has been tested using bothsyntheticandreal data. Fig.
1illustrates the results of asynthetic test. The left-hand diagrams
show calculatedrmomrdyfields in F, X, 1and Z due to a dipole source
assumedto be magnetised in a direction representative of the North
Searegion. ThecentrediagramsshowtheX, Y andZanomalyfields computed
from the F anomrdy data using thetransformationmethod. The
right-hand diagrams showtheerrorsinthetransformed values.
Theanomalyvalues areofthe order of 100 nT, the errors are only a
few nT.The tests with reddata were made for two areas on land inthe
UKover whichaeromagneticsurveys hadbeenflown67.The transformation
technique was applied to the aeromagneticdataand theresults
comprwed withvectorcomponent datameasured atanetwork of sites
ontheground. Inoneoftheareas (approximately 40 km by 50 km) the
anomaly values intotat intensity were relatively small, with a
range of about150nT. Ground observations were made at19 sites in
the area andthe root mean square differences between the
transformed andobserved valueswere0,05 indeclinationand0.02
indipangle. The second area provided a more challenging test.
Thearea(approximately 50 km by 50 km)contained ananomalywith a
range of about 750 nT - greater thanobserved in mostNorthSeaareas.
Inaddition, therequirement for dataonahorizontal surfacewasnot well
satisfiedbecausethe flightlines followed the undulating terrain,
The transformationtechnique produced estimates of theanomalies
indeclinationand dip angle with ranges over the area of about 1.4
and 0.5388SPE 49001 APPMCAT)ON OF lNTERPOLATtON IN-FIELD
REFERENCING TOREMOTE OFFSHORE LOCATIONS 3respectively. Thercmt
meansquaredifferencestwtweenthetransformed andobserved values at36
sites intheareawere0.16 in declination and 0,08 in dip angle. Fig.
2ashows thetotal intensityanomaliesover the area and
theobservationpoints on the ground. Figs. 2b and 2Cboth
showthedeclinationanomaliesoverthearea. Fig. 2bwascalculatedusing
the transformation method, Fig. 2Cwas calculated fromthe
measurements at the observation poin[s. The closesimilarity between
these two figures is a striking confirmationof thevalidity of
thetransformationmethod. Itisalso
worthnotingthecharacteristicpatternofthedeclinationanomaly.The
field is Ixmt in towards the (positive) total field
anomaly,generatingapositive declinationanomaly to thewest,
andanegative anomaly to the east.Data requirements for offshore
mapping. In medemaeromagnetic surveyscarried out for exploration
purposesoffshore, dataaretypically collected at anattitudeofabout80
m with aflight-line separation of 500 m. Caesium
vapourmagnetometers with an accuracyof atwut 0.1 nT in
fieldintensity are routinely used, typically sampled at about 10
Hz.Similarmagnetometersareoperatedat abasestation.
Goodquatitypositional andtiminginformationisavailableusingGPS.
Compensation methods areused toeliminatefromthedatathe magnetic
interference caused by the aircraft,
Forthepurposesofgatheringdataforuseinestimatingthecrustalcontribution
to the local vector magnetic field for use in
IIFRaflight-lineseparation of2kmwill besuftlcientunlessthearea is
unusuatlymagneticallycomplex, If the main
flightlinesarenorth-souththenaset of east-west linesshouldbeflown
ata separation of about 6 kmto provide data atcross-over points.
The accuracies in magnetic field, navigationat andtiming data
whichare routinelyachievedin aeromagneticsurveys
aremorethanadequate forIIFR. Toallowaccuratedata processing it is
recommended that the base stationmagnetometer is operated for
atleast 24 hoursatthetime ofthe survey, not just while the survey
is being flown.Survey DataProcessingThere areatleast five options
of applyingbeat field modelsandobservatory datato magnetic wellbore
surveys which cank deseribed as IIFR. The simpler methods give the
most rapidturn-around time and require the least complex
techniealassurance. The more advanced methods offer the
potentiatofgreater accuracy, but atthe expense of greater
computationaland logistical complexity. Typieatly, an operation
will need toselect two correction methods: one for
instantaneousapplication at the rig site to enable drilling ahead,
and one forproduction of definitive wellboreposition data. Only
where themagnetic observatory data is available atthe rigsite,
togetherWithappropriate software and expertise, ean definitive data
begeneratedintruereal time. The mainoptions for azimuthcorrection
are as follows.Option l-Correction for crustrd field declination.
Themagneticdeclination at thedrill site asdetermined
fromtheaeromagnetic data is used in preference to the value
predictedby amainfield modeltocorrectmagneticsurvey
azimuths.Sincethisvalue representsasnap-shotintime, itsaccuracywill
quickly degrade unless it is corrected for secular (ie. long-term)
variation inthefield, Thismay be done by
calculatingthedeclinationanomaly(ie. thedifference
betweenthetruevalue and the main field value) at the time of the
observations,and adding this valuetoall subsequent main
fieldmodelpredictions.Option 2-Correction for instantaneous
fielddeclination. Abetter estimate for declinationmay be obtained
by addinginthe short-termvariations detectedat one or more
nearbyobservatories. This will largely eliminate the effects
ofmagnetic disturbances, Where observatory data are notavailable in
real-time, this correction will typicallybe made byshore-based
staff, delayingdeliveryof the final data tothedrilling operation by
a day or two.Option 3-Correction for crustal field declination
anddrillstring magnetic interference. The limitations ofestablished
magnetic interference correction algorithms toerrors in the assumed
values of magnetic field strength and dipangle are well
known89.Hitherto, this has restrictedtheirapplication offshore.
Where values for magnetic field
strengthanddipanglemoreaccuratethanthose provided by amainfield
model areavailable, these limitations aremitigated, butnot
eliminated. Thepractical effect isthat thesecorrectionsmay be
applied with confidence to surveys over a greater rangeof hole
orientations, with only those withinafew degrees ofhorizontal
east-westbeingexcluded. Estimates of magneticfield strength and dip
angle at the drill site should be made asin Option 1 and updated
for each well drilled.Option 4-Correction for instantaneous field
declinationand drillstring magnetic interference. Acombination
ofOptions2and3, Estimatesof
instantaneousmagneticfieldstrengthanddipangleareusedasinputsinanestablishedinterference
correction rdgorithm, The resulting magneticazimuths are corrected
for instantaneous declination.OptionS-Correction fortoolsensor
errors, fieldvariationandinterference using near real-time data.
Corrections formagnetic survey azimuthswhich use multiple survey
stationsto track tool sensor perfcmnanee as well as
externalinterference are now entering widespread uselOl 1.
Theireffectiveness isgreatlyenhancedbytheavailabilityofreal-time
estimates of the magnetic field, Indeed, the increased useof IIFR
has encouraged their development. Generallyspeaking, these methods
have yet to be successfullyautomated,and their application remains
the preserve of oftke-basedexperts within the directional drilling
companies.For the operational examples included in this
paper,correction Options 3 and 5 were used for real-time rig site
dataand definitive positional data respectively.3894 H. S.
WILLIAMSON, P. A. GURDEN, D. J. KERRIDGE, G. SHIELLS SPE
49061Operational Sequence ofanOffshore IIFRServiceThe
implementation of the technique can be divided intotwoparts :-G
Acquisition and validation of the crustal anomaly data,. Useof
IIFRin dailydrilling operations. This includesapplying the
improveddeclinationdata, quality controlmonitoring,
decidingwhich,if any,correction techniquesare to be applied, and
delivering the data.Acquisition of Aeromagnetic Data. Prior
tocommissioningthe survey, it is important to obtain as much
existing magneticdata as possible surrounding the field, This will
help determinethe character of the magnetic gradients andcrustal
anomaliesin the area. The scaleand locationof these features
willdetermine the extent, sample frequencyand potential benefit
ofany new aeromagnetic data. For an isolated field in the NorthSea
covering a10 km by 10 km area, an aeromagnetic surveyof an area 50
kmby 50 km would typically be required.The potential for errors of
hundreds of metres in thegeographical locationof magneticanomalies
shownonoldairborne surveysmust be recognised. Use of such data for
IIFRwouldrequire significant relaxation of surveyperformancemodels
to allow for these errors.Wh.h atypical flight height of about 80
metres above sealevel for aeromagnetic data acquisition, it is
important that allonshore and offshore personnel in the area are
informed whenand where the survey is to be undertaken.Computation
of magnetic field values. Once the dataacquisition is complete, a
new digital magnetic field map maybe calculated (see Appendix).
This will showanomaliesrelative to a particular main field model.
The exact co-ordinatesofeachstructureor
wellheadwithintheareaareinterpolated on the new magnetic anomaly
mapto determinethecrustal offset relativetothismodel.
Shouldanupdatedmain fieldmodel be availableat the time of the
drillingoperations, newoffsets for each drilling facilitymust
becalculated. Computingthelocal magneticfieldas anoffsetfroma main
field model allows long-termsecular fieldvariations to be properly
allowed for in the IfFR correction.For simplicity, thesamecomputed
magneticfieldvalues(those for the wellhead) are used throughout the
length of eachwell. The aeromagnetic map must k reviewed to ensure
thatthe variation of the crustal field along each wellbore is not
inexcess of the values which have been incorporated into the
toolperformance model.Validation of magnetic field values.
Sincethey arenot theresult of direct observation andrely on
thevalidity of severalassumptions, the magnetic field values
obtained fromtheanomalymapmust bevalidatedbyindependent
techniquesprior to use. This is typically done by comparing
IIFRcorrectedMWDsurveyswithhigh accuracyinertial
gradedownholegyrosurveys.There are three variations on thismethod,
all of which have been used to good effeccGGGRun gyros in the fiist
one or two wells drilled after themagnetic mapping exerciseIf gyro
surveys have been taken in previous wells, and ifsuitable MWD data
are available, re-process the data andcompare results
retrospectively.CompareIIFRMWDresultswith results
in-holereferencedtoa gy&copictoo112. This validationmethod was
used in BPsWytchFarmfield.Application of IIFRdata. At a basic
level, the application ofthe IIFR technique is simple, The rig
drills ahead correcting itssurveys for declination only. These are
replaced with a batch ofIIFR corrected surveys as and when required
(Fig. 3).Knowledge of the crustal anomaly atthe drillsite
benefitsthe real-time survey dataintwo ways. First, amore
accuratedeclination value may be used for correcting data at the
rig. Toavoidconfusion,a singlevalueisnormallyadoptedfor thewhole
well, but for some extended reach or deep wells use ofmore than one
value may bejustified, Second, improvedestimates of total field
strength and dip angle allow better real-time magnetic data quality
control to be applied on the rig.All MWD surveys acquired on the
rig are compiled into afile and sent to the processing centre,
typicallyon a daily basis,Each survey must be time stamped using a
time basesynchronous with the magnetic observatory data.
Exceptduringmagneticdisturbances,particularlyat
highlatitudes,synchronisation accurate to +/- 1 minute is
suftlcient, and mayke achieved by the MWD/ Survey engineers using a
broadcastradiosignal. For quality control purposes itis important
thatthe six-axis accelerometer and magnetometer data be suppliedfor
full analysis using triaxial bias and scale factor errorevaluation
techniques.Frequency of Data Delivery. Priorto commencing
theIIFRservice, it is important to determine the appropriate
frequencyof data delivery to the rig, Thishelps establish the
techniquewithin theoperational sequenmand
enablesfit-for-purposecommunicationinfrastructureto
beinstalledandtested.Thedecision depends on operational
requirements and is inevitablya compromisebetween:GGGkeepingthe
rigappraised of the best estimate of thewellposition at all
timesavoiding the proliferation of datasets with
differentprocessing appliedminimizing operational cost390SPE 4=>
APPL}CAT\ON OF INTERPOLATION IN-FIELD REFERENCING TOREMOTE OFFSHORE
LOCATIONS5During times of relative magnetic calm, the field
variationscmbe adequately monitored on a daily basis, and delivery
oftheupdated wellbore position may be left to theendof eachhole
section. In periwis of high magnetic disturbance, or whenknowledge
of wellboreposition is criticrd, near real-timeupdates will be
required to maintain
surveyperformancewithinthepre-determinedbounds.IftheprimaryreasonforapplyingIIFRis
for futurewell collisionavoidance, or forimproved reservoir
mapping, prmessing need not beundertaken until drillingis complete.
This of course removestheadvantageof improvedconfidencein the
while-drillingwell position.Experience has shown that monitoring
magnetic variationsdaily andreactingto problems asthey
cccurisasatisfactorycompromise. This allows update to the wellbore
position dailyor at casing point dependent on circumstances,Service
Management andCommunications. Acquisition anddelivery of IIFR
datainvolves uptofour organisations. Theoperator,
thedirectionaldrillingcompany, theMWDsurveycompany andthe providers
of themagneticobservatory dataall play an important role. Eftlcient
management of the serviceis a non-triviat exercise in
communication. In particular, rapidand robust verbal and digital
data links are critical to success.Transmissionof dataeither
byhand-entryor faxhas kenshowninpractice tobe inadequateexcept as a
temporarymeasure.The applications of IIFRdiscussed in this paper
areassigned to a survey management service operatingindependently
within a directional drilling company. Thesurvey management service
is given full responsibility for theset up, monitoring and control
of the survey service. They alsoprovide an operational position to
whom all questionsregarding theapplicationandimplicationsof
IIFRmaybeaddressed.Results fromUsingIIFROffshoreBPis
currentlyapplyingIIFRinthreeoffshorefieldsoverwhich the magnetic
field has been mapped using aeromagneticdata.. The Foinaven and
Schiehallion fields are neighbors intheWest of Shetlandprovince.
Magneticvariationdata forthese fields is supplied by the BGS
observatory at Lerwick intheShetlandislands, 165 kmtotheeast.
AndrewisintheCentral North Sea. Its IIFR service uses magnetic
datainterpolated between Lerwick, 275 km to the northwest, andthe
BGSobservatory at Eskdalemuir, 415kmtothe southwest. Fig. 4 shows
locations of the fields and observatories.Magnetic Anomaly Maps.
The aeromagneticdata for theWest of Shetlandfields was partof
alarger datasetacquiredfor exploration purposes in 1993and 1994,
The cost
ofreprocessingwasthereforetheonlyexpensetothedrillingoperation.
Processing the data(see Appendix) revealed a250nT total field
anomaly centred some 5 km to the south of thefields. Angular
anomalies are small over both fields. Thecomputed values for
declination and dip anomaly at theFoinaven B and Schiehallion
Central drillingsites are atllessthan O.1O.The anomalies
surrounding the Andrew field are illustratedin Figs. 5a, 5band5c,
The data from which these maps werecomputedwereacquiredin 1997.
Theaeromagneticsurveycoverage was extendedtothewest (not shown) to
covertheAlba and Britanniafields, operated by Chevron. This
reducedthe capital cost of data acquisition to both Operators,Fig.
5a shows the totat intensity anomalies over an area 40km by 52 km
containing the Andrewplatform. A large feature,of intensityjust
over 400nT can be seen centred some 23 km tothe south east of the
platform. The declination anomaly (Fig.5b) shows the characteristic
positive-negativepattern describedabove. The dip angle anomaty(Fig,
5c) is less obviouslyrelated to the total field anomaly, but it is
worth noting that thecentre of the main feature, where the dip
angle anomalyexceeds0.2, does not coincide with the centre of the
totat fieldanomaty. It happens that the direction (but not the
strength) ofthe magnetic field at the Andrew platform is very close
to thefield predicted by the BGGMmain field model. The
magneticeffect of the platform itself, although clearly visible in
the rawaeromagnetic data, was removed numerically at an early
stagein the processing.Comparison of IIFR vs. Gyro and Inertial
Surveys. Allthree fieldsinthis studyhave twowellsin whicha
highaccuracy gyroscopicor inertial tool and IIFR-processed MWDdata
are simultaneously available. Fig. 6shows the results
ofthedatacomparisons for all sixwells. Thewhite circleandbarsshow
themeanandstandard deviation of thedifferencebetween the
gyro/inertiat survey azimuth and the MWDazimuthcorrected
onlyformainfield declinationThemainfield model used was the current
BGGM. The black square andbars compare the gyrdinertial survey
azimuthwith the MWDazimuths after full IIFRcorrection (Gption
5),For all sixwells, applicationof IIFRproducesa meandifference
ktween MWD andthe gydinertial survey oflessthan 0.3. For five of
the six wells, this represents a significantimprovement, There is
little effect on the standard deviation ofthe difference. Thisis
probably dueto the(random)residualerrors leftafter applicationof
themagneticdataprmessingalgorithm. At these accuracy levels the
gyrdinertial survey canno longer he regarded as the truth. They
must be treated, likethe IIFR data, as an approximation to it.There
is no evidence that the IIFR technique is correcting asystematic
azimuthdifference common to all wells inanyofthe three tields. This
is unsurprising given the size of the localdeclination anomalies in
comparison to other sources of error.SomeTechnical Issues Affecting
IIFRDaily andMagnetic StormVariations. There aretwo mainsources of
magnetic field which vary on timescales of minutesto hours; the
regular daily variation and magnetic stormvariations. The daily
variation has a fundamental peria.i of 24hours and has a very
similar structure over the whole region of3916 H. S. WILLIAMSON, P.
A. GURDEN, D. J, KERRIDGE, Q. SHIELLS SPE 49061the British Isles.
Its typical range,which varies with latitude,theseasons andwith
the1l-year solar cycle, is afew tens ofnTintotal intensity,
approximately 0.2 indeclinationandabaut 0.03 indipangle. Thephaseof
thedailyvariationdepends on local time.Magnetic storm variations,
on the other hand, areessentially simultaneous over large regions
and their amplitudedepends on geomagnetic latitude. In the area of
the North Sea,the amplitude of variations maybeof the order of a
fewthousandnanoteslasinfieldintensityandafew degreesindirection
during severe magnetic storms.Tnrbitt and Chtrk*3 compareddata for
1991fromthepermanent magnetic observatories at Lerwick,
Dombas(IWrway)and Brorfelde (Denmark). They useddata fromtick and
Brorfeldeto studylongitudinal effects. Theyfoundthat onquiet
daysthecross-comelations betweenthedata sets peaked at lags
consistent with the difference in locattime between thetwo sites
(about 51 minutes).Ondisturbeddays the cross-correlationspeaked at
zero lag, showing that thedisturbance fields affected both sites
simultaneously andcaused similar field perturbations.Change ofMain
FieldModel. Thetransformation techniquedescribed
inthispaperestimates thelocal magneticfield bythe combination of a
main field model and aeromagnetic data.The crustal
fieldvalueswhichare derivedare apparentvalues consistent with the
main field model used in theanalysis. Whena newmain fieldmodel is
adopted, thesecrustat field values are adjusted to be consistent
with thenewmodel, so there is no jump in the local field estimates,
Strictly,thewhole crustal field analysis should be
repeatedusingthenew mainfield model, but thiswill have very
littleeffect onthe results, Any discrepancy arisingfollowing model
revisionwill be a result of differences in the secular variation
models.This will be a small effect over a few years.IIFRat High
Latitude. Duringperiodsofsevere magneticdisturbance there are
intense electrical currents flowing in theauroral regions, at a
height of about 100 km, and the strengthand the pattern of the
currents can change rapidly. As a result,the ability to use
magnetic observatorydata to estimatevariationsat remotelocations,
possible overa few hundredkilometres in the UKregion, will decrease
to tens ofkilornetres at high-latitude locations close to the
auroralelectrojet currents. Becauseof the increased rate of
fieldvariation, efforts should be made to improve
thesynchronisation between theMWD system andthemagneticobservatory
to better than 1 minute.Induced Currents. Short
termgeomagneticfieldvariationsmay be viewedas electromagnetic waves
which propagate
intotheEarthgeneratinginducedcurrentsastheyareabsorbed.The
characteristic length scale for absorption, the skin depth,depends
on the electrical conductivity of thesolid earth(andthesea)
andthefrequency of thevariations. Inshallowseawater only short
period variations will induce significant392currents
(theskindepthinseawater for variations withaperiod of 10 seconds is
about 1 km). This means that there willhe effects duringsevere
magnetic disturbances, but in generalthe uncertainties due to
fluctuations in the source currents willbe more significant than
the induced currents, This alsodemonstrates that there is, on most
days, very littlescreeningoffieldvariationsbythesea. Themost
significant inducedcurrent effects are expected to k in coastal
areas where thereisacontrast inconductivity
betweenthelandandthesea.Experience at Liverpool Bay12hasshown that
theabilitytointerpolate accurately falls slightly during
disturbances.Impact of IIFRonSurveying UncertaintyFuII
apprwiationof the impact of HFRon survey performancedepends on
understandingthedifference between uncertaintyreduction and error
reduction. Just as highintensity magneticstorms are relatively
infrequent events, so high intensityCrustal anomalies are
relatively sparsely scatteredgeographically. As a consequence, IIFR
corrected survey datawill frequentlyshowonlyslight
differencesfromthe samedataset processedconventional y. Anatural
reaction is toquestionthevalue ofthetechnique, butconsideration
oftheassociated survey uncertainties shows this reaction to
bemistaken. Since both high-intensity storms and
crustalanomaliesareessentially
randomandunpredictableintheiroccurrence, ampleallowancemust bemade
for themina-priori estimates of survey uncertainty for services
which do notattempt to eliminate t.km. This justifies the
apparentlypessimisticvaluesfor magnetic
fielduncertaintywhicharecurrently in use. A technique which can be
shown to eliminatemost of the effects of magneticstorms
andcrustalanomaliescan be assigned a much tighter uncertainty
model, withconsequent benefits for overatl wellbore surveying
costs.Magnetic field predictions based onmain field models.
ABritish Geological Surveyreport14 commissionedby BakerHughes INTEQ
estimated the magnitude of the various effectscausing the
instantaneous magnetic field at a randomly chosentime andplaceto
differfromthepredictionofamainfieldmodel. Although not a
comprehensive study, the results can beusedtoderive robustestimates
ofthetotal uncertaintyinatypical main field model prediction in the
North Sea regionDeclination 0.5 perjieldDip angle 0,2 perjleldTotal
Field Strength 130 nTperjieldThe characterization of these
uncertainties, quotedat onestandard deviation, as per j7eld
indicates that whenimplemented as error terms in a tool performance
model, theyshould betreated asactingsystematically throughout
afield.This is becausethe bulk of the uncertainty is attributable
to thecrustal field, thespatial variations of whicharerather
largerthanmost oil andgas fields. Whileopentochallenge, theauthors
consider these values to be the best currently available.SPE 49061
APPLICATION OF INTERPOLATION IN-FIELD REFERENCING TOREMOTE OFFSHORE
LOCATIONS 7Magnetic field predictions based onIIFR. Likely
errorsintheestimates of theinstantaneousmagneticfield at
thedrillsite may be split into errors in the combined main and
crustatfields, B., and errors in the short-term variations, B4.Main
Field plus Crustul Fiehi. The transformationtechniqueprovides an
estimateof the combinedmain andcmstal tields, and any errors apply
to the sumof thesecomponents. The UKland studies discussed above
usedaeromagnetic data acquired in1962 and1956 respectively. Itis
likely thatsimilarstudies with aeromagnetic dataacquiredwith modem
equipment would show better agreement
betweenthemeasuredandcalculatedfields. Theresultsof the
landstudiesindicatethati particulmlyinDandI, errors in
thetransformation method increase with the size of the anomaly ina
roughlylinear fhshion. Usingthis assumption,
individualuncertainties may be calculated for particular locations
basedon the size of local anomalies. Given our knowledge of
typicalanomalyintensityin the NorthSearegion, the
1standarddeviationuncertaintiesinthecomponents of themainfieldplus
cmstal field vector at a typical drilling
locationareestimatedas0.08inD, 0.025in1 and40uTinF. Thesevrdues do
not include the possible effects of short wavelengthdown-hole
anomalies which would not h detected byaeromagnetic
surveys.Short-Term Variations. Anumber of studieshavebeencarried
out using data Born the three UK magneticobservatoriesand
observatories in Norwayand Denmark to testthe accuracy of the
interpolation technique in bothmagnetically quiet and disturbed
conditions. Barraclough
andMacmiIlan*5examinedthedegreetowhichfieldvariationsobserved at
Eskdalemuir Observatory (55,3N) could bereproduced using data from
Lerwick Observatory (60.1N) andHartland Observatory (51.0N) over a
selection of 83magnetically disturbed days during the three-year
period1987-89. Setting thresholds of 5 arc-minutes (0.083) in
declinationand dip angle and 50 nT in field intensity they
foundagreements between theobserved andinterpolatedvatues for96.6%,
94.3% and 93.9% of the time, respectively. Given thatEskdatemuir
ismorethan450kmfrombothLerwickandHartland, this result gave
confidence in the ability tointerpolate between observatories even
on disturlxxl days, Theintermittent, spikynatureof
short-termmagnetic variationsmakes these values conservative as 2
standard deviationestimates of the errors in B~,These results are
borne out by the success of theinterpolation technique for
Liverpool Bayi2.Over a period of 4months the standarddeviationof
differencesbetweendatahorn au on-site monitor and data interpolated
fromEskdalemuirandHartlandObservatories was less thanO.O1in
Eothdeclination and dip angle, and less than10 nT in
fieldintensity. These resultsshow errorsless thanthose found
byBarraclough andMacmillan15.Thisis beeause Liverpool Bayis at a
comparatively low latitude and the distance over whichthe data were
interpolated was less than in their study.Error Model Parameters.
From the above data, bestestimates for the 1 standard deviation
magnetic fielduncertainty when using IIFR in the North Sea region
are:Declination 0.08 perjield 0.04 per surveyDip angle 0.025
perjield 0.04 per surveyField Strength 40 nT perjeld 25nTper
surveyThe characterizationof the uncertaintiesdue toshort
termvariations in the field as per surveyindicates that error
modelterms arising horn themare to be treated as
systematictx?tweenstations in the same survey, but not between
differentsurveysin the samewell. This reflects the
distinctlynon-random variations characteristic of some disturbance
events, InthecaseofMWD holesection surveys,
whicharenormallyacquiredover a periodof severat days, this is a
distinctlyconservative assumption. Withelectronicmukishot
surveys,which are acquired over a few hours, the assumption will
stillbe conservative, but far less so.The above vatues are thought
to be readily attainable for anHFR serviceset upandmanagedina
waysimilar tothatdescribedinthis paper. Significant change
ingeographicallocation, magnetic environment, or data acquisition
andprocessing would necessitatea review.Impact onTotal MWDError
Budget. In most applications,total azimuth uncertainty of an MWD
measurement isdominated by two sources of error: declination
uncertainty anddrillstring magnetic interference. The
figuresabovesuggestthat use of HFR will typically reduce the impact
of declinationuncertainty by a factor of about five. If no attempt
is made tocorrect for drillstring magnetic interference, IIFR will
have noeffect on this error source, However, the improved
knowledgeof the total fieldstrengthand dip angle
affordedbyIIFRsignificantlyreduces the errors inherent in the
correctionalgorithms currently in use. Used with care,these
correctionsor their successors will replacetheerror
duetouncorrectedinterference with a residual error several times
smaller.Both declinationuncertainty andtheimpactof
cirilIstringinterference arestrongly dependentongeographical
locationaud/or hole direction. This precludes generalization about
theoverall reduction in azimuth uncertainty available through useof
IIFR. Nevertheless, two statements can be made withreasonable
confidence. Careful use of IIFR:G enables substantial reductions in
azimuth uncertaintycompared with conventional MWD surveying.
Theevidence presented in this papersuggests that the averageazimuth
error over a well can generally be reduced to lessthan 0.3.G
greatly reduces the dependency of azimuthuncertainty onexternal
factors, such as the presence of magneticanomalies.
Thisbuildsconfidenceintheuseof thetoolperformance model for
quantitative risk-based decisions,Impact of IIFRonWell
PositioningThe development of HFRfromits fwst
ground-br~lngapplication into anestablished survey service
hasenabled the3938 H. S. WILLIAMSON, P. A. GURDEN, D. J. KERRIDGE,
G. SHIELLS SPE 49081benefits forecast in a previous paperl to lx
substantiallyreatiied. These can be summarised under four
headings.Survey Program Optimisation. Gyroscopic surveys
aregenemlly no longer required in the deeper hole sections. Theyare
retained at shallowdepths where external magneticinterference may
be expected. Electronic multishot surveys areno longer required for
end-of-section quality control, They addlittle extra assurance if
the MWD tool performance hasbeenanalysed using one of the latest
magnetic processingalgorithms.Near Real-Tbne Quality Control. The
new magneticprocessing algorithms highlight the deterioration in
MWD toolperformance characteristic of imminent failure.
Changingoutsuch a todat the next convenient opportunity may save a
trip.Near ReaI-Thne Definitive Survey. Theeffect of IIFR on
thedirectional
drillingoperationhasbeenlikenedtogettingagyrosurveyeveryday(an
unusuatly reliableone). Missedtargets andcorrection runscaused by
misleadingsurveysareelimimted.Interpretation of Well Position.
Thereductioninpositionuncertaintyafforded byIIFRreducesthenumber of
seismiclines against which a match with the well logs may need to
besought. The increased confidence in absolute well position
alsoreducesthetemptationto runextrasurveys whenthewellsposition
relative to the existing reservoir model is in doubt.Future
DevelopmentsAs nse of IIFR becomes widespread, itwill beincumbent
onthe directional drilling industry to develop
operationsprocedures, dataformats andperformance models which
are,asfar aspossible,standardisedacrosscompanies. This willimprove
theOperatorsconfidence inthetechnique, andthevalue they derive from
it.Integral to the type of IIFR service described here is the useof
the new generation of magnetic survey processingalgorithms. These
methods are not claimed to be 100%accurate, nor will they produce
good surveys fromfimdamentally poor quality data, but they are
highly indicativeoferroneoussurveys. The sizeof the residurderrors
tobeexpected after prcxessing requires deeper investigation
beforethe somewhat conservative performance estimates currently
inuse can be tightened. This will be the subject of
furtherpublication.Conclusions1. Mathematical analysis of
aeromagnetic total field dataprovides anaccurate andcost-effective
means of mappingthecrnstrd fieldvector
whichisparticularlysuitableforoffshore locations.2. IIFRcombined
with recently develo@ magnetic dataprocessing methcds can deliver
MWDsurveys to anaccuracy normally associated only with
high-accuracysurvey systems,3. Provision of an effective HFR
service requires a significanteffort of co-ordination and surveying
expertise.Communication systems, data processing methods and
datadelivery schedules must be tailored to the
individualoperation.NomenclatureA(u,v) Fourier transform of the
magnetic field potentialb unit vector in direction of magnetic
fieldB magnetic field vectorBOOMBritish Geological Survey Globat
Geomagnetic ModelC(u,v) Fourier transformof the total intensity
anomalyvaluesD magnetic declinationF total magnetic field
strengthAF total intensity anomalyI magnetic dip angleIIFR
interpolation in-field referencingMWD measurement while drillingu
wave number in x-directionv wave number in y-directionv magnetic
field potentialfunction relating A(u,v) and C(n,v):,x north
directionY,y east directionz, z down directionSubscriptso
excludingexternal field sourcesdue to crustat anomaly: due to
external field sourcesm due to main fieldAcknowledgmentsTheauthors
wishtothank BPExploration, Baker HughesINTEQand the Director of the
British Geological Survey(NERC) for permission to publish this
paper.A number of the studies that established the basis of
IIFRweresupportedbySperry-SunDrilling Services, whoalsomanage the
IIFR service for BP Exploration on the Foinavenfield.References1,
Russell, J,P., Shiells, G. and Kerridge, D.J.: Reduction
ofWell-Bore Positional Uncertainty Through Application of aNew
Geomagnetic In-Field Referencing Technique, paperSPE30452 presented
at the1995 SPEAnnual TechnicalConference and Exhibition, Dallas,
TX, Oct. 22-25.2. Shiells, G., D. J. Kerridge and J. P. Russell,
1996.Reduction of Wellbore Positionat Lhcertaintywith a
newGeomagnetic Referencing Technique.IPT (March1996)~43-2@,3.
Rixse, M.R. and Thorogood, J.L.: Case Study - Building aSystemin a
Service Company to Assure Technical Integrityand Institutionalize
Organizational Learning, paper394SPE 49061 APPLICATION OF
INTERPOLATION IN-FIELD REFERENCING TOREMOTE OFFSHORE LOCATIONS
94.5.6.7.8.9.IADC/SPE 35067 presented at the 1996 IADC/SPEDrilling
Conference, NewOrleans, LA, Mar. 12-16.LeMout21,J.-L., Lelev~
a&omagn&iquedelaFrance.Calcul des composantes du champ
?Ipartirdes mesures delintensit&Annales deGkophysique,
26,2,229-258.,1970Lourenco, J.S., and Morrison, H,F., Vector
magneticanomalies derived from measurements of a singlecomponent of
the field Geophysics, 38, 359-369.,1973S., and Barraclough, D.R.,
Areviewofmethods for deriving allelements of
thecrustatmagneticfield from total intensity observations, British
GeologicalSurvey TechnicaJReport WM/90/18C, 1990.Macmillan, S., and
Barraclough, D.R., Transformations oftotal intensity aeromagnetic
data in Norfolk, BritishGeological SurveyTechnical Report
WM/90/25C, 1990.Nljden Twilhaar, G.D,, Accurate Magnetic Surveying
in asingle NMDC: Analysis of magnetic interferencecorrection, paper
SPE 16530 presented at Offshore Europe87, Aberdeen, UK, Sep.
8-11.Brooks, A.G,,andWilson, H., AnImproved Met.hl forComputing
Wellbore Position Unc&aint y and itsApplication to Collision
and Target Intersection ProbabilityAnalysisfl paper
SPE36863presentedat the 1996 SPEEuropean PetroleumConference, Mihm,
Itaty, Oct. 22-24.10. McElhinney, G,, Sognnes R., and Smith, R,,
CaseHistories Demonstrate a New Method for Well AvoidanceandRelief
Well Drilling, paper 37667presentedat the1997SPE/L4DCDrilling
Conference, Amsterdam, TheNetherlands, Mar. 4-6.11.Brooks, A.G.,
Gurden, P.A., and Noy, K,, PracticatApplication of a
Multiple-SurveyMagnetic CorrectionAlgorithm, paperSPE 49060
presented at the1998 SPEAnnual Technical Conference and Exhibition,
NewOrleans, LA, Sep. 27-30.12.Thorogood, J.L., andKnott,
D,R.,Surveying TechniquesWithaSolid-StateMagneticMultishot
Device,SPEDESept. 1990, p. 209-214.13.Turbitt, C, W. andT, D. G.
Clark. The use of Lerwickvariometer measurements to estimate
magneticdisturbances over the North Sea. British GeologicalSurvey
Technical Report WM/94/21C, 1994.14.Macmillan, S., Firth, M.D.,
Clarke, E., Clark, T.D,G. andBarraclough, D.R., Error estimates for
geomagnetic fieldvalues computedfromthe BGGM, British
GeologicalSurvey Technical report WM193t28C, 1993.15.Barraclough,
D,R. and S. Macmillan. Correctionofmagneticsurvey
observationsfortheeffects ofmagneticdisturbance. British Geological
Survey Technical ReportWM/90/22C, 1990.AppendixTransformation
Methodof CalculatingCrustal FieldVector fromTotal Intensity
DataSupposethat B. is the geomagneticfieldvectorat a pointwhere an
aeromagnetic spot measurement of field intensity ismade, andassume,
tosimplify thealgebra, that thereisno395contribution from external
field sources. If B~is the main fieldand B. is the crustal field
vector at the sample point, thenBO=BW+BCand the measurement made by
the magnetometer is[Bo[,The total intensity anomaly AF is defined
asAF=lBO[-lB~lwhere IB~ I maybeestimatedusingaglobal
geomagneticfield model. The total intensity anomaly is not the
magnitudeof the crustal field becauseAF=lB~+BC[-lB,nl #lBelHowever,
ifthecrustal fieldissmall comparedtothemainfield, i.e. [BmI>>
\ BC\, it follows that) 1AF=(Bm.Be / Bm= BC.b~= XCX~ I Fm+lCY~I F~
+ZcZ,~ I Fm. . ..(A-l)where BC=(Xc, k,, ZJand b. = (X#F., YJF.,
Z#F.) is theunit vectorinthedirectionof B~. Thus the total
intensityanomaty is the magnitude of the component of the crustal
fieldvector in the direction of the main field
vector.Astheaeromagneticdata are collectedin a
source-fleeregionthecrustal field maybewrittenasthegradient
ofascalar potential satisfying Laplaces equation, soBC= -VVC and
V%c= oFromthis it follows that thecomponentsofB.
alsosatisfyLaplaces equation,If b. is constant for the area of the
survey thenV2AF = V2(b~.BC) = b~. V2BC = OHence the total intensity
anomaly values satisfy Laplacesequation, A general solution to
Laplaces equation for thecrustat field potentiat V, in cartesian
coordinates (x - north, y -east, z- vertically down) maybewritten
as{V=(x, y,z) = ] J exp 2ti(m +,7) +2nz(u2 +1,2)2}A(u. r)
d~4dv--.-whereuandvare wavenumtwrs
inthexandydirectionsrespectively, and A(u,v) is the Fourier
transform of VC(X,Y,O),If the altitude at which the aeromagnetic
data werecollected istaken as z =O, thenasimilar expression may
be10 HS.WILLIAMSON, P. A. GURDEN, D. J. KERRIDGE, G. SHIELLS SPE
49061used to describe the total intensity anomaly
values.-AF(x,y,O)=~~exp{2?ri(ux +vy)}C(u,v) dudv . .
..(A-2)----whereC(Wv) is theFourier transformof the total
intensityanomaly values. The components of the crustal field in the
x, yand z directions on the surface z = Oare then given
byX.k,o=+z=o=-d 7i uexp{2@P+vy) }A( uv) ~dv.-.0
-80Z(X,Y,O)=~=o=-2zj ~ivexp{2ti((x+*y)}A(u,v)&dvz
-co--Inserting these expressions into Eq. A-1 and then using Eq.
A-2 gives a relationship between A(u,v) and C(u,v):A(u,v) = C(u,
v)/2zwwherew= i(uXjJFm + vYJFjjJ + (U2+ v2)1nUFm.Hence,
ffomthetotal intensityanomalyvaluesthefunctionC(u,v) maybe
computed, then the function A(u,v) and, finalIy,the values of Xc,
Ycand z.SynfheticDeta Transformed Defa Differencesmntour interval.
10nT contour interval =10nT contwr mtewal =1nlmmmGlrEIFig.
1Synthettctest of the transformation methodof
magneticfletdmodelling. Thedirectionof the mainffeld vector is
typical of:$+Y3ij&f!* ,--. GG270 230 290 3(X) 310 320Easiings
(km)Fig. 2aUK land teat of the transformation methodof
magnatlcflafd modafllng. Aeromagnetfctotal intensityanomalies(nT)
andgroundobservation locations.396SPE 49081 APPLICATION OF
INTERPOLATION IN-FIELD REFERENCING TOREMOTE OFFSHORE LOCATIONS
11270 280 280 300 310 320Eaetings(km)FfrJ. 2b UK land teat of the
tranaformatfonmethodof meanettcfl~d modalling.
Declinationanomalies(arcminutes) calculatedfrom~!eeo i:- 12to880
:,) s.. . . 0 ,P1 ,--- ,fn~,,, , !670-\,,, , : ,,!. .=,l:,, ,.am-l
\i- t- 12\040,,270 280 280 350 310 320Eaetings(km)Take MWD Process
QC, reporlsuweys + -Real timeusing and drillat rig Option 1
ahead(rig site)4Compilesurveysinto dailytile1Get
dailyHProcessmagnetic usingvariation Optiondata 3,4or 5dRepxt
resultsto rig (frequency
Regulardependsonturn-aroundIoperationalI(office)requirements)Fig.
3Typical flow of MWDsurveydatein anIIFR operation6W 4w. . . . .. .
. . ., ----- -- .,...-;,!1~ Foinaven ~ : ~00Schiehsllion.. . . .
..!. ..-. .,,,! ;)):,!,w,W;, b. . . . . . . . . . . ..1..,,;.!d1,
T. . . . . . . . . . . . . . .a ;...*.0e, . ,!,,.,.Fig. 4 Location
mapfor Foinaven, Schiehallion and Andrewffeldsand
BGSmagneticobservatories at Lemvick and Eskdalemulr4,>! . .,,,
,;,$ ,,:.. , ... . . ...,,56N,(:!!!,,,:,,, ;,;,, ,,,,>,!,,, ,.
.,. ,: *NFig. 2cUKland test of the traneformatfonmethodof
magneticflefd mcdalling. Declinationanomaliea(arcminutes) from
groundobservations.39712H. S. WILLIAMSON, P. A. GURDEN, D. J.
KERRIDGE, G. SHIELLSSPE 49061J1 1 I 1 # I 1\1Ko0/----------*
----..~ ----.-~--------8*-*1,.~#-./,---,390 400 410 420UTMZorP331
Eestings (km)Fig. 5aTotal fieldanomalies(nT) for the Andrewfield
andsurrounding area.390 400 410 420UTMZone31Eeetinge(km)1 I 1 i I I
i 16460-------- e-----L,. 2..; 7------/ ------ -------0 .6450~.t.~-
,(,g-------- .,--1 #--.2-); 6440- ./~.------..;czlg!!3,------Andrew
.o5p!atioml2~> 6430 0 4+3e68J12 106420ItI-- -2--------- , .-
-I_041U ~ . t1 I I 1 1 I I Ir360 400 410
420UTMZone31Eastings(km)Fig. 5CMagneticdip angleanomaiiaa (nT) for
the Andrewfieldan-dsurroundingarea.~ +1 ,5EFJ] T: +1 ,0g5= +05;: 1,
, ;f -T- ;, :, :G o.gEI 1g .0,5 -$[- 1- C02AIIZS05-1.oO-(39w
weys)(64aufveya)(39Wiwy$)A07g(3! surveys)S06-1.5- (21Wtveys)C03(50
Wlww)Foinaven Schiehallion AndrewFig. 6 Comparfeonof MWDva.
gyroscopic reference azimufhsforsix wells. Uncorrected
azimuths(white circleand bars) have onlytheBGGMdeclinationapplied.
llFR+orrected azimuths(blackequareand bars)arecorrectedusing
Option5 (aaetext).Fig. 5b Declinationanomalies(arcminutes) for the
Andrewfieldandsurroundingarea.398