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.

PNI Sensor Ad
PNI RM3100 Sensor

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:

Honeywell HMC5883L
Breakout board
500 nT$15Sparkfun
Freescale MAG3110
Breakout board
100 nT$15Sparkfun
PNI RM3100
Evaluation module
13 nT$61PNI Corp
Honeywell HMC2003
4 nT$328Digikey
Honeywell HMR2300-rs232
6.7 nT$871Digikey

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.

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