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UK 2007-2008 Repeat Station Report

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

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UK 2007-2008 Repeat Station Report

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  1. UK 2007-2008 Repeat Station Report T J G Shanahan and S Macmillan June 2009 MagNetE Workshop Helsinki, Finland

  2. 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 • 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 Measurements at Alnwick Repeat Station

  3. 2009 MagNetE Workshop – Helsinki, Finland Primary Instrumentation • Carl-Zeiss Theodolite with Bartington Mag-01 Probe attachment • GEM GSM-19 Proton Precession Magnetometers • Wild-GAK-1 North Seeking Gyroscope

  4. 2009 MagNetE Workshop – Helsinki, Finland 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

  5. 2009 MagNetE Workshop – Helsinki, Finland Locations Visited in 2007-2008 Stations Occupied: • 9 stations in 2007 • 8 stations in 2008 Observatories Used: • Lerwick • Eskdalemuir • Hartland 2007 2008 Observatory

  6. 2009 MagNetE Workshop – Helsinki, Finland 2008 Model Results

  7. 2009 MagNetE Workshop – Helsinki, Finland Regional Model (2008.5) Declination (degrees) Inclination (degrees) Total Field (nT)

  8. 2009 MagNetE Workshop – Helsinki, Finland Secular Variation (2007-2008) Declination (minutes east/year) Inclination (minutes/year) Total Field (nT/year)

  9. 2009 MagNetE Workshop – Helsinki, Finland Change in Secular Variation Rate of change of SV for LER, ESK, HAD, GRW and ABN

  10. 2009 MagNetE Workshop – Helsinki, Finland 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

  11. 2009 MagNetE Workshop – Helsinki, Finland 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

  12. 2009 MagNetE Workshop – Helsinki, Finland Changes to Network Reducing Network Size Why: • ‘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

  13. Questions?

  14. Improving Crustal Representation Possible use of high-resolution aeromagnetic survey data to determine near-surface crustal field contribution

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