Magnetometers

Mon, 10/07/2023

Updates..

The Seismological Association of Australia have a very nice 'vault' at Kangaroo Valley in South Australia, and have been loaned a high accuracy Magson MFG-1S triaxial magnetometer which we installed almost 3 years ago. The data from the Magson MFG-1S instrument is publicly accessible here. TPSO Magnetometer

'Speake and Co' from whom we purchased our magnetometer flux gate sensors from are no longer operating, however, you can still purchase suitable sensors and DIY kits from FGM Magnetic Field Sensors 

  • We now have 13 years of magnetometer data recorded from this instrument located at Nairne in South Australia at 1 sample per second. Over 410 million samples!

An update to the original magnetometer was done in 2022, when I replaced the 12 bit digitizer for the LM35 analogue temperature sensor with a 16 bit digitizer module (Z6221) purchased from my local Altronics store.

The module uses an Texas Instruments ADS1115 chip, well, it was supposed to, it actually came with something else, that was more like an ADS1015, a 12 bit device and I eventually realized that it was a clone / fake device.

Unfortunately, cloned and fake devices are a rather common issue with electronics purchased from non certified suppliers (eBay for example). I ordered a couple of replacement ADS1115 chips from Element 14, and finally got 16 bit resolution.. which was the original plan.. This hardware update allows far better temperature corrections to be made to the data

  • I discussed the issue with the staff at Altronics, they weren't aware of the fake / cloned components (they are now!)

Proton Precession Magnetometers

I had the opportunity to purchase 2 PPM's of unknown condition a few years ago very cheaply (what could possibly go wrong!) and now have 2 (1980's vintage) GEOMagnetics proton precession magnetometers (PPM's). One arrived in a working condition and the 2nd one had almost every IC blown in it. I eventually got a circuit from the manufacturer and spent considerable hours understanding how it worked and repairing it.

Now I have 2 working and portable PPM's., so absolute magnetic field strength can now be measured.

PPM's are really 'cool' instruments operating at the quantum level!

My first exposure to PPM's was when I was in Tennant Creek in the mid 1970's by the company LA Richardson and Associates , who had one mounted on a boom on an aluminium Land Rover and later on a Toyota Land Cruiser and one that hung out the back of a twin engine Cessna plane being used for mineral exploration by GeoPeko (I worked for GeoPeko as a tech contracted to run the Warramunga Seismic Station)

Ian Ogilvie from LA Richardson (LAR) spent many hours explaining to me (a very young techy in those days) how PPM's worked and how LAR went about their business finding underground ore bodies with these instruments. Thank you Ian! You and a lot of others had faith in me..

One of the PPM's I now own has been modified with the addition of a GPS and an Arduino mega data logging board to allow logging of latitude, longitude, time and flux to a uSD card. This now gives us the ability to carry out magnetic 'site' surveys and generate 'heat maps' of flux variations across an area. I used this logging PPM at the TPSO seismic vault when we installed the Magson Triaxial device and discovered 2 spots of higher flux some distance from the vault, discussing these 'hot spot' findings with the previous property owner, revealed that there had been some 'junk metal' buried at both these locations on the property. 

Wed, 20/01/2010

A Magnetometer is a scientific instrument used to measure the strength and/or direction of the magnetic field in the vicinity of the instrument. Magnetism varies from place to place and differences in Earth's magnetic field (the magnetosphere) can be caused by the differing nature of rocks and the interaction between charged particles from the Sun and the magnetosphere of a planet.

Magnetometers are a frequent component instrument on spacecraft that explore planets.

John purchased two fluxgate magnetometer sensors (model FGM-3H from Speake & Co, Llanfapley, England). Fluxgate magnetometers typically are very temperature sensitive (~ 100nT/degC) so, for temperature stability, one unit is now installed underground at the Wurillba Observatory site.

 

 

I measured the sensitivity of the magnetometers as better than 1 nano-Tesla (nT), and completed calibration using a precision current calibrator. Testing of the custom interface electronics (based on a PIC microcontroller) is completed and is producing data comparable to professional monitoring stations anywhere in the world.


Sample of data captured on the 16th March 2010 (green) compared to the previous day 15th March 2010 (yellow)

The interface computes the period of the magnetometer (frequency) output using the onboard 4MHz clock crystal as a timing reference. Using a 24-bit counter the unit will be capable of measuring magnetic field strengths ranging from 0 to 100,000 nT with an output resolution of 0.1nT.

References: 1. AN AURORA DETECTOR: HOW TO MONITOR THE EARTH'S MAGNETIC FIELD by David Olean (1.4MB PDF)

  • Our FGM-3H magnetometers

  • Status. Feb 2018
  • 2 units were manufactured in 2010, one unit was installed at our Wurillba site in early 2010, the other unit being used for software development, testing and public demonstrations.
  • The magnetometer sensor was buried at around 1 meter at the rear of the property. We now have  8 years of excellent data at a sample rate of 1 per second.
  • For the Technical minded a short discussion on the interface electronics. Flux gate magnetometers in general suffer from temperature changes affecting the sensitivity of the material used to make the flux gate core., figures quoted are from 40 to 100nT per degree C. Both our units have a temp coefficient of around -50nT/degree C. An LM35 analogue temperature sensor is attached to the FGM-3H magnetometer sensor and temperature variations are able to be compensated for in the software. The units are usually buried under the ground for temperature stability and we did this at our remote site.
  • Double power supply regulation was implemented for the sensor as suggested by the manufacturer.
  • In simple terms, The FGM-3H sensors are of the fluxgate design and output a 'square' wave with a frequency in the range of 40KHz to 140KHz and a sensitivity of approx 1 Hz per nT (nanoTesla) at a frequency around 62KHz. The FGM-3H sensors are more linear in their response by measuring the period of the output waveform than by measuring the frequency of the output waveform so the electronics has been designed to measure the period with high accuracy.
  • The output waveform is divided by 2 to the 14th (16384) to give an output frequency with a range of approx 2.4Hz to 8.5Hz. This is used to gate a 4MHz clock producing groups of 4MHz clock pulses. Each group of clock pulses (approx 800,000) are counted by a 24 bit counter. At the end of each gate period, the number of clock pulses captured by the counter are read by a 16F877 PIC micro. The counter is then reset ready for the next gate period. With this arrangement, we end up with approx 13 counts per nT, and a very large full scale range (approx +_ 30,000nT).
  • The count value is in 3 bytes and at the same time, the signal from the LM35 temperature sensor is measured as a single byte. At an appropiate time interval (user adjustable from 1 to 240 seconds), these 4 bytes are passed onto a computer (via RS232) and are displayed in graphical form. The data is logged to a file for further analysis as required. At this stage, we are logging data every second.
  • The effects of CME's on Earth's magnetic field have been regularly recorded.
  • Future plans. A tri-axial version and a proton precession magnetometer. There is some interest in sampling at 10 samples per second at the same sensitivity, however this will require some changes to hardware.