© NERC All rights reserved Measuring the Earth’s Magnetic Field Chris Turbitt

© NERC All rights reserved Measurement of Total Field (F) Proton Precession Magnetometer (PPM) Pole mounted sensor

© NERC All rights reserved Measurement of Field Direction (D & I) Fluxgate (D/I) theodolite: Non-magnetic theodolite Elsec 810 fluxgate magnetometer Mounted on non-magnetic tripod

© NERC All rights reserved Magnetic Observation Site Reference Object (RO) ~ 20m Field Site (F) Observation Site (D/I)

© NERC All rights reserved 27th and 29th September Preliminary results: D = -26.4° I = -67.4° F = nT Y = nT X = nT Z = nT Tristan da Cunha (Munich University)

© NERC All rights reserved General Procedure 1.Concurrently measure the Total Field at the F site and the D/I site to establish the F difference 2.Set-up the tripod and theodolite over the D/I site 3.Take the circle reading of the reference object 4.Take a set of four declination readings 5.Verify the circle reading to the reference object 6.Take a set of four inclination readings 7.Repeat steps 3-6 With only one Proton Magnetometer, measure at the D/I site only

© NERC All rights reserved Total Field (F) Measurements Proton Magnetometer Procedure: Switch on both consoles and check the battery voltage Establish an F site and erect poles (three sections) at this and the D/I site Mount sensor bottles on poles, align east-west Connect sensors to consoles Configure the console including tuning (Falklands 29, Antarctic Peninsula 40, Hartland 48) Check field value and signal strength for 4 measurements in quick succession Take a reading every 15s for 5 minutes at both sites and record Switch off D/I console Remove D/I PPM and poles

© NERC All rights reserved Total Field (F) Measurements

© NERC All rights reserved Points to note when taking readings: Occupy the correct measurement sites Be non-magnetic! Remove any magnetic items or sources of RF interference (mobile phones, GPS receivers) Use non-magnetic tripod The axis on the sensor bottle should point magnetic east-west Consistent readings - consecutive reading should not change by more than 2nT On GSM-19T the signal strength value should be above 95 All readings should be taken with the bottle on 3 sections of pole Fully document site details and measurements Total Field (F) Measurements

© NERC All rights reserved D/I Measurements True North Azimuth Note observer, date, site details and reference object details on the observation form Set-up the tripod & theodolite over the D/I mark and level Take two readings (face right / face left) on the reference object and record

© NERC All rights reserved D/I Measurements Declination Measurements – Fluxgate magnetometer principle N Magnetometer aligned along field vector, the output will be the magnitude of this vector This instrument measures the Earth’s magnetic field strength along its axis N Magnetometer aligned at right angles to the field vector, the magnetometer output will be zero When the magnetic field is at right angles to the magnetometer axis the magnetometer output is zero In practice – rotate the magnetometer to search for a zero reading

© NERC All rights reserved D/I Measurements Declination Measurements Connect electronics and check battery level Check that the offset switch is in the middle (OFF) position Align telescope approximately horizontal and to magnetic east with magnetic sensor down (“ED” reading) Set vertical circle to level (270° 00’ 00”) Set magnetometer to “uT” and manually rotate horizontal circle to find zero magnetic reading Clamp horizontal circle and set magnetometer to “nT” scale Search for zero magnetic reading using horizontal fine adjust Note time and ED circle reading Check for magnetic contamination and external field activity

© NERC All rights reserved D/I Measurements Declination Measurements EUWUWD ED N S EW

© NERC All rights reserved True North 090° 180° 270° CZ 000° Theodolite Horizontal Circle H WU, WD EU, ED D Mean D/I Measurements

© NERC All rights reserved D/I Measurements Inclination Measurements Take second set of two readings (face right / face left) on the reference object and record Calculate the circle reading of the magnetic meridian i.e. the mean of the four declination readings Record this mean and its reciprocal Align telescope approximately to magnetic south with magnetic sensor up (“SU” reading) Set horizontal circle to meridian or its reciprocal Set magnetometer to “uT” and manually rotate vertical circle to find zero magnetic reading Clamp vertical circle and set magnetometer to “nT” scale Search for zero magnetic reading using vertical fine adjust Take an F sample using the PPM at the F site Note time, SU circle reading and F value

© NERC All rights reserved D/I Measurements Inclination Measurements NUSUND SD N S EW

© NERC All rights reserved F F Theodolite Vertical Circle Magnetic North 000° 180° 270° 090° SD NU ND SU D/I Measurements

© NERC All rights reserved Points to note when taking readings: Occupy the correct measurement sites Be non-magnetic & use non-magnetic tripod Be gentle! These instruments are fragile. Never handle the instrument by the magnetometer and don’t pull the cable Be focused! The most common errors are mis-readings and transcription mistakes The instruments are not waterproof – protect them from precipitation Fully document site details and measurements Always calculate and compare the results – the field does not change very much year to year D/I Measurements

© NERC All rights reserved Equipment List D/I theodolite and electronics Non-magnetic tripod 2x proton magnetometer sensors, electronics & poles 12x D-cell batteries for D/I electronics Scientific calculator (DMS calculations) Timepiece set to Zulu Hand-held compass Tape Measure D/I and F forms Site details Previous results