UK 2007-2008 Repeat Station Report

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UK 2007-2008 Repeat Station Report

T J G Shanahan and S Macmillan

June 2009 MagNetE WorkshopHelsinki, 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

2009 MagNetE Workshop – Helsinki, Finland

Measurements at Alnwick Repeat Station


Primary Instrumentation• Carl-Zeiss Theodolite with Bartington Mag-01

Probe attachment

• GEM GSM-19 Proton Precession Magnetometers

• Wild-GAK-1 North Seeking Gyroscope


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


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



Locations Visited in 2007-2008

Stations Occupied:

• 9 stations in 2007

• 8 stations in 2008

Observatories Used:

• Lerwick

• Eskdalemuir

• Hartland


2007

2008

Observatory


2008 Model Results



Regional Model (2008.5)

Declination (degrees) Inclination (degrees) Total Field (nT)



Secular Variation (2007-2008)2009 MagNetE Workshop – Helsinki, Finland

Declination (minutes east/year) Inclination (minutes/year) Total Field (nT/year)


Change in Secular Variation

Rate of change of SV for LER, ESK, HAD, GRW and ABN



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



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



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



Questions?


Improving Crustal RepresentationPossible use of high-resolution

aeromagnetic survey data to determine near-surface crustal field

contribution

UK 2007-2008 Repeat Station Report

Documents

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