Prospecting for Placer Gold with Geomagnetism (Conclusion)

(Geometrics Model 256 magnetometer kit.)

In this post I'll be outlining a few tips and suggestions for operating a portable magnetometer (mag) in your gold prospecting endeavors. So without further adieu, let's get going.

No "One Size Fits All"

First off, please understand that not all portable mags are the same and that the "one size fits all" idea doesn't apply. There are a number of portable mag manufacturers out there, as well as a range of models, so each unit will have its own operating steps and unique characteristics. That said, however, it should go without saying that the basic principles of geomagnetism apply to all mag makes and models. For the sake of simplicity, I'll use the Geometrics Model 856 Proton Precession Magnetometer as my operating example.

Like most of its portable counterparts, the Model 856 is lightweight (about 10 pounds) and mounted via a harness rig strapped to the mag sensor. It is also computerized with a thousand line memory bank for crunching and analyzing raw field data. Major data functions for the Model 256 include base time corrections, regional and local gradients, digital "filtering," and the ability to plot magnetite (black sand) profiles. You can manually adjust the Model 856 for use in a small localized area or, conversely, set it up for detecting and profiling large areas.

Grid Parameters

The size of your "target" is very important when using a portable mag in terms of establishing spacing and line intervals. In general, richer gold-bearing gravels tend to occur in pockets or narrow paystreaks that could be totally missed in a grid search where your line spacing is greater than 100 feet. So you want to use search grid lines a little less than 100 feet apart and take readings or measurements with your mag about every 20 feet or so. Again, you can manually adjust these grid parameters downward if you plan to survey a much smaller area.

One person can operate a unit like the Geometrics Model 856, but a two-person mag survey team is your best bet. With two people, one can carry the instrument and note specific data while the other person or "helper" can carry any extraneous gear as well as mark locations with high magnetite readings. Even a relatively inexperienced experienced two-person mag team like this can survey about 6,000-8,000 linear feet per day, depending on the terrain. Still, you have the option of going it alone using most portable mags if that's your desire. The only "negative" will be less ground covered over the long haul.

Good Data is Critical

If you're wondering about accuracy with portable mags, you can breathe easy for the most part. The most significant mag error encountered out in the field is caused by variations in surrounding magnetic fields (what's known as background "noise"). Solar flares and sunspot activity tend to be the biggest culprits in this regard, but the odds of you running into a solar storm out there are pretty slim. Just be aware of the potential impacts that any type of magnetic "interference" can have on your surveying activities.

In the science and technology realms accurate data is critical. One way to ensure your mag survey data is viable is, to put it simply, to get lots of data points for evaluation. The greater the amount of data you acquire, the more likely that potentially erroneous anomalies or fluctuations get weeded out. Needless to say, this means a resultant increase in the overall reliability of your data. What you want data-wise is good stuff in and good stuff out. On the flip side, what you want to avoid is the old "garbage in, garbage out" routine.

(Remember this adage when conducting your mag surveys.)

Portable mags offer a unique means of identifying heavy black sand deposits and their main constituent, magnetite. As you already know, magnetite concentrations can mean gold-bearing gravels. Gold-bearing gravels mean gold in your pocket if you have the right knowledge, skills, and gear. Anything that can help increase your gold take is worth consideration so a portable mag could be just the right ticket in certain placer contexts, wet or dry.

In other words, whatever works, "works!"

(c) Jim Rocha (J.R.) 2014

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