uk 2007-2008 repeat station report
DESCRIPTION
UK 2007-2008 Repeat Station Report. T J G Shanahan and S Macmillan. June 2009 MagNetE Workshop Helsinki, Finland. 2009 MagNetE Workshop – Helsinki, Finland. UK Measurement Programme. Daily occupation for ~ 7 hrs Site survey for anomalies 8 absolute observations True North found using gyro - PowerPoint PPT PresentationTRANSCRIPT
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UK 2007-2008 Repeat Station Report
T J G Shanahan and S Macmillan
June 2009 MagNetE WorkshopHelsinki, Finland
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UK Measurement Programme• Daily occupation for ~ 7 hrs
• Site survey for anomalies
• 8 absolute observations
• True North found using gyro
• 60 second F samples
• Accurate location using GPS and distance measurements
• Repeat period of ~ 5 yrs
• About 10 stations per year
• Total of 51 stations
• 1st order stations (observatory reduction)
• Average spacing < 70 km
2009 MagNetE Workshop – Helsinki, Finland
Measurements at Alnwick Repeat Station
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Primary Instrumentation• Carl-Zeiss Theodolite with Bartington Mag-01
Probe attachment
• GEM GSM-19 Proton Precession Magnetometers
• Wild-GAK-1 North Seeking Gyroscope
2009 MagNetE Workshop – Helsinki, Finland
Repeat Station InstrumentationInstrument Function AccuracyWild GAK-1 north seeking gyro-attachment for theodolite
To acquire a horizontal circle reading for true north
10 arc-seconds
Bartington Mag-01 fluxgate magnetometer sensor mounted on Zeiss non-magnetic theodolite
Detection of magnetic meridian in the horizontal plane for measurement of D and detection of magnetic field vector in the magnetic meridian for measurement of I
~ 6 arc-seconds
GEM GSM-19 Overhauser effect proton precession magnetometer
Measurement of F 0.2 nT
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Production of UK Model
• Reduced using UK observatory data (latitude weighting) corrections
• Reduced to epoch using piece-wise linear SV models
• Main model uses least squares polynomial fit for data for latitude and longitude (up to degree 4)
• IGRF used as a priori model to constrain data over areas of ocean
• Declination model chosen for 2008.5 used degree 2 polynomial fit
2009 MagNetE Workshop – Helsinki, Finland
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Locations Visited in 2007-2008
Stations Occupied:
• 9 stations in 2007
• 8 stations in 2008
Observatories Used:
• Lerwick
• Eskdalemuir
• Hartland
2009 MagNetE Workshop – Helsinki, Finland
2007
2008
Observatory
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2008 Model Results
2009 MagNetE Workshop – Helsinki, Finland
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Regional Model (2008.5)
Declination (degrees) Inclination (degrees) Total Field (nT)
2009 MagNetE Workshop – Helsinki, Finland
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Secular Variation (2007-2008)2009 MagNetE Workshop – Helsinki, Finland
Declination (minutes east/year) Inclination (minutes/year) Total Field (nT/year)
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Change in Secular Variation
Rate of change of SV for LER, ESK, HAD, GRW and ABN
2009 MagNetE Workshop – Helsinki, Finland
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Grid Magnetic Angle (2008.5)
Convergence Angle Grid Magnetic Angle Map Requests from OS
Provides magnetic Grid North data for UK Ordnance Survey for 200 topographic and 30 geological maps
2009 MagNetE Workshop – Helsinki, Finland
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Declination Residuals (2008)
• Model includes crustal sources
• Accuracy (95 % confidence) of 2008 model: 0.5 degrees
• Source of NW residuals due to strong crustal fields – known igneous rock geology
2009 MagNetE Workshop – Helsinki, Finland
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Changes to NetworkReducing Network SizeWhy:
• ‘Cultural’ Noise
• Core field, secular variation sources are dominant > 400 km wavelengths - current spacing is < 75 km
• We are interested in the changes with time (SV) – faster sampling would improve understanding of ‘jerks’
• Better to improve accuracy using few ‘clean’ sites than many poor sites and trade spatial sampling for improved temporal sampling
How:
• 25 % reductions would enable repeat period of ~ 3 years
• Sites of lower quality to be repeated every 10 years
• Average spatial distribution to increase to ~ 100 km
2009 MagNetE Workshop – Helsinki, Finland
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Questions?
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Improving Crustal RepresentationPossible use of high-resolution
aeromagnetic survey data to determine near-surface crustal field
contribution