© NERC All rights reserved UK Repeat Station Report T J G Shanahan and S Macmillan June 2009 MagNetE Workshop Helsinki, Finland

© NERC All rights reserved 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 1 st order stations (observatory reduction) Average spacing < 70 km 2009 MagNetE Workshop – Helsinki, Finland Measurements at Alnwick Repeat Station

© NERC All rights reserved 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 Instrumentation InstrumentFunctionAccuracy Wild 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 F0.2 nT

© NERC All rights reserved 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 used degree 2 polynomial fit 2009 MagNetE Workshop – Helsinki, Finland

© NERC All rights reserved Locations Visited in Stations Occupied: 9 stations in stations in 2008 Observatories Used: Lerwick Eskdalemuir Hartland 2009 MagNetE Workshop – Helsinki, Finland Observatory

© NERC All rights reserved 2008 Model Results 2009 MagNetE Workshop – Helsinki, Finland

© NERC All rights reserved Regional Model (2008.5) Declination (degrees) Inclination (degrees) Total Field (nT) 2009 MagNetE Workshop – Helsinki, Finland

© NERC All rights reserved Secular Variation ( ) 2009 MagNetE Workshop – Helsinki, Finland Declination (minutes east/year) Inclination (minutes/year) Total Field (nT/year)

© NERC All rights reserved Change in Secular Variation Rate of change of SV for LER, ESK, HAD, GRW and ABN 2009 MagNetE Workshop – Helsinki, Finland

© NERC All rights reserved 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

© NERC All rights reserved 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

© NERC All rights reserved 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 2009 MagNetE Workshop – Helsinki, Finland

© NERC All rights reserved Questions?

© NERC All rights reserved Improving Crustal Representation Possible use of high-resolution aeromagnetic survey data to determine near-surface crustal field contribution