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CAS
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Y Downhole Magnetometer Surveyon the Isle of Dogs
Instrumentation
• Mag-03MSSthree-axisfluxgatemagneticfieldsensor
• Spectramag-6dataacquisitionunit
Objectives
The principal aim of this survey was todeterminethedepthbelowgroundlevelofthetoeofasteelsheetpilethatseparatesthesitefromtheRiverThames.
Application
This site is being redeveloped. The clientwished to establish the pile length so as todetermine whether the existing piles wouldmeet the requirements of the proposedredevelopment.
Background
In April 2013, Arrow Geophysics Ltd carriedout a downhole magnetometer survey oftwoboreholes locatedatasite in the IsleofDogs,LondonE14.Thesurveywasconductedwith a Mag-03MSS sensor from Bartington
Instruments, using a Spectramag-6 DataAcquisitionUnitforcollectionandanalysisofthedata.
The survey aimed to establish the points atwhich themagnetic field of the pile exertedzeroinfluenceandmaximuminfluenceonthesensor.AsshowninthepaperbyJoetal,1thetoeof thepilecanbe judgedasbeingat thelocationmidwaybetweenthesetwobaselines.
Method
Dynamic downhole measurements werecollected in two boreholes from 0m to 20mbelowgroundlevel,usingaloggingspeedofapproximately1/6metrespersecond.
First borehole
This borehole is a horizontal distance ofapproximately 1.5 metres from the steelpile.ReadingsforthisboreholeareshowninFigure1.
1 Jo,C.H.,Cha,Y.H.andChoi,J.H.,Aboreholemagnetic
logging tool for estimating unknown foundation
depths,Presentedatthe3rdInternationalConference
onAppliedGeophysics,Dec8-12,2003.
CS001716/08
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Thetotalfieldmagneticintensityrecordedinthisboreholevariessubstantiallybetween0s(groundlevel)and33s,presumablyduetothepresence of various items of ferrous debrislocatedwithintheshallowsubsurface.
Between33sand56s,theintensitystabilisesat approximately 30,000nT, which we regardas the unadulterated influence of the sheetpile. Below 56s, the intensity begins toincrease to an ambient total magnetic fieldintensity2ofapproximately51,000nT,whichisfirstrecordedat71s.Thetwobaselinesofthissurveyaretherefore56s(fullinfluenceofpile)and71s(zeroinfluenceofpile).
Assuming the toe of the steel sheet pile tobe located midway along the linear slopeconnectingthetwobaselines,thislocationliesatarecordingoffsetof63.5s,whichequatestoanapproximatedepthbelowgroundlevelof10.6metres.2 The ambient total magnetic field intensity is the
intensity which would be recorded at depth within
a borehole in the absence of any nearby ferrous
material–inotherwords,theapproximatemagnetic
fieldoftheEarthitself.
Second borehole
Readingsforthesecondboreholeareshowninfigure2.
Thisboreholeislocatedahorizontaldistanceof approximately 2.7 metres from the sheetpile, compared with a horizontal distanceof approximately 1.5 metres for the firstborehole.Thereforeinthesecondboreholewewouldexpectatransitionofgreaterwidthandlower amplitude between the full influenceandzeroinfluencebaselines.
The rotary borehole log for the secondborehole revealed the presence of a 0.3mthick interval of strong dark grey concreteat a depth of 5.0m to 5.3m below surface.This further complicated the interpretationof the totalmagnetic field intensity log. Thenarrow peak between 30s and 33s and thebroaderpeakbetween33sand41sinthetotalmagneticfieldintensitylogareinterpretedtobeduetosteelassociatedwiththisconcrete,and make it very difficult to determine thestartpointofthelinearslopeconnectingthetwobaselines.
Figure 1: readings for first borehole Figure 2: readings for second borehole
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Intheabsenceofmorediagnosticinformation,we have interpreted this start point to belocated at 46s. The end point of the linearslopehasbeeninterpretedat70s.Thelocationof thetoeof thesteelsheetpile is thereforeinterpretedtolieatarecordingoffsetof58s,whichequatestoanapproximatedepthbelowgroundlevelof9.7metres.
Conclusions
A downhole magnetometer survey of twoboreholes at a site in the Isle of Dogs hasproduced interpreted depths below groundlevel of 10.6 metres and 9.7 metres for thetoeofasteelsheetpilelocatedtotheeastofthesetwoboreholes,separatingthissitefromtheRiverThames.
Theseresultswillhelptheusersofthesitetoassessthesuitabilityofexistingstructuresforfuturedevelopment.
Notethattheinfluenceofthelayerofconcreteinthesecondboreholeshowsthatthedepthofthepileisbestmeasuredfromaboreholeclosertothepileandasfarawayaspracticablefrominfluencesorthogonaltothedirectionofthe pile. Such influences can be uncoveredduringtheboringprocess.
