Just what can we accomplish with this device? Here I want to compare the resolution of the sensor I have chosen to the magnitude of various magnetic anomalies to see if we can actually measure anything geophysical.
Before I began this exercise I looked around for examples of magnetic field anomalies seen in the field, especially associated with ore bodies. I found some excellent examples from a variety of sources, including the classic book Applied Geophysics by Telford, Geldart, and Sheriff on my office bookshelf. It seems that over ore bodies, anomalies of between one hundred and several thousand nano Tesla are common (see the example from the paper by Gunn and Denith: Magnetic responses associated with mineral deposits; although not surprisingly reports by Integrated Geophysics Corporation show considerably smaller anomalies of about less than to a few nano Tesla for shallow sedimentary structures.
So, given that we want to resolve >100 nT anomalies here is what I did.
I began by reading the following report: Fluxgate Magnetometer by Carl Moreland where he mentions the Speake & Co. FGM-3 sensor, which according to Moreland has resolution of about 10 nT.
There is also the description of home-brew magnetomteters: Practical Guidelines for building a Magnetometer by Hobbyists Part 2 by W. Bayot.
These sensor types required more electronics development than I felt was necessary in this day and age, so after further investigation I purchased the best quality and easy to interface, but less than $100 magnetic field sensor I could find: the PNI RM3100 evaluation module.
This has a resolution of 13 nano Tesla, which should be sufficient to detect many mineral deposit signals. This uses magneto-inductive technology: see Magneto-Inductive Technology Overview; Andrew Leuzinger & Andrew Taylor, February, 2010) or Slawomir Tumanski’s article “Induction coil sensors – a review”.
By comparison, for example, looking at Sparkfun Electronics and Digikey Corporation we see it is costly to obtain better than a 10 nano Tesla device. There may also be several options using magneto-resistance, but I did not pursue these.
For an overall comparison of resolution and cost for the easier to interface options as of January, 2015:
| Device | resolution | cost | source |
|---|---|---|---|
| Honeywell HMC5883L Breakout board | 500 nT | $15 | Sparkfun |
| Freescale MAG3110 Breakout board | 100 nT | $15 | Sparkfun |
| PNI RM3100 Evaluation module | 13 nT | $61 | PNI Corp |
| Honeywell HMC2003 module | 4 nT | $328 | Digikey |
| Honeywell HMR2300-rs232 module | 6.7 nT | $871 | Digikey |
Now that the tiny magnetometer is up and running here in Ecuador with the PNI RM3100, we'll see if it lives up to my dreams.