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 Meteorite Detecting Articles


posted January 5, 2002

Meteorite Madness

by Ed Gerken

Air Tests with Metal Detectors


Pictured are some of the detectors we have used in our meteorite tests:
Left front, the pinpointers: Falcon MD-10 and a home-brew BFO.
Center, The Oldies: First Texas Search Master Tracker DX-8500, Radio Shack Micronta 3001 (Alert re-brand), Compass Relic Magnum 7.
Rear, Prospecting and Coinshooting: Fisher Gold Bug 1, Bounty Hunter Time Ranger.
Book: Rocks From Space by O. Richard Norton.
Space Rocks: Courtesy of Black Hills Institute
.

Introduction

Metal detectors have been used for many years to prospect areas for gold and other minerals. They can also be effective instruments in the search for extraterrestrial objects. No, not LGM in their UFO's, we are leaving that hunt to others with more experience in such matters! Our quest is a little more down to earth, though what we seek is indeed from outer space. We're experimenting with the best ways to hunt meteorites with our metal detectors.

There were two basic questions I wanted to answer. The first question, is it necessary to have a state-of-the-art detector, or would most any machine be able to detect a variety of meteorite types? The simple answer is that probably any detector will work, from the least expensive to most feature-laden. Every detector I have tried so far was able to respond to nearly all of the test meteorites.

The second question was how might the signal from a meteorite compare to other more familiar targets. Again, we were able to categorize most of the meteorites as having basic and predictable responses that were similar to common objects.

There is a catch, though. As far as my testing was concerned, we saw only two basic types of meteorites; iron and stony. The irons would detect in one of two ways; either the same as a rusty can, or similarly to a new nail or bolt. The stony meteorites would respond as either a rusty can or they would exhibit a "hot rock" signal. The problem, as I'm sure you can guess, is that rusty cans, nails and hot rocks are all things that detectorists studiously avoid digging and detector manufacturers try to prevent their machines from detecting!

Even with these drawbacks, metal detectors can still be very useful tools in the search for fallen rocks from space.

To put my recommendations in a nutshell, try the machine you already own before investing in a new machine in hopes of a superior response. An older, used detector would probably work just fine, even the lowly "obsolete" BFO! Manual ground balance is generally better than auto. A detector capable of both a "positive" metal and a mineral "negative hot rock/null" response is ideal. Discrimination is not desired, unless it has a notch feature useful for knocking out zinc or aluminum while retaining iron and hot rock signals. Discrimination can be used to "assay" the relative iron content of stonys.

Hunt with your eyes and hands as well as with your detector's coil. Does the rock look out of place, oddly shaped or colored? Is it too light or too heavy? An air check of a suspect rock found with your eye is perhaps easier and more telling than finding it with coil alone on the ground. While it's my experience that an actual sample in hand is your best guide to what a meteorite looks, feels and detects like, the information presented here will give you a good idea of what to expect in the field.

I hope you are able to use the information in these articles to learn what settings to use on your detector, what responses to expect, and perhaps most importantly, to gain a better understanding of what a meteorite might look like to the untrained eye.


The Tests

Bob Farrar from Black Hills Institute of Geological Research, Inc. and I tested eleven meteorites, using a Bounty Hunter Time Ranger with an 8" standard coil. We also tested a home-brew BFO pinpointer. For a brief comparison, we checked one of the stonys with a White's XLT, a programmable ID detector. The White's essentially responded similarly to the Time Ranger, but it is more sensitive to electrical disturbances, in this case all the fluorescent lights and large metal specimen cabinets in the meteorite storage area. Since the test was indoors with lots of metal around, we were unable to properly ground balance the White's, I'm sure this affected our ability to test with it for this article. It was apparent it was able to respond in a manner close enough to that of the Time Ranger, so we left it out of further testing.

We did all the tests in about 30 minutes, so I hope I got all the info logged properly! I know I'm leaving a few observations out, although there may have been differences from one sample to the next, the detectors responded in a more or less predictable fashion to the types of meteorites we tested.

Some of the meteorites were cut, some were fragments, others were intact more or less as discovered, their listed sizes are approximate. We read various angles and edges as well as flat sides at various distances in both all metal and disc. modes. These were all air tests.

Iron Meteorite- Large 4" cut slab. Read 299, full scale on the numeric ID, a higher reading than silver, but gave a $1 target id, same reading as a rusty can might give. Occasional lower nickle reading of around 34 showed some of that metal was present in the sample.

Low Iron Stony- small 1.5" chunk. Read as a "hot rock". That is, in all metal mode, it gave a strong negative signal on the ground monitor and nulled the threshold tone. In disc. mode, except if we hit an iron-rich spot, it would sound upon entering and exiting from beneath the coil, but not when under the center of ithe coil. Depth readings were unreliable.

In general, the BFO detector would also read the "stonys" as a null. However, the BFO could be adjusted to read them as positive if the unit was tuned to react to the mineral signal. It had no problem responding to any of the iron samples.

H5 "Stony", small 1" round, intact surface- Hot rock, similar response as above. No sound in all-metal, but ground monitor moved. Most of the stonys had to be quite close to the 8" coil to read.

Rusty Iron - 3.5" oblong, intact surface. Numeric 299, but readings varied with coil distance. Perhaps reading other metals in the sample.

Pallasite - 5" cut slab. True iron reading, that is, the Time Ranger's numeric indicator field is blank, but the iron id target locks. Similar reading to a large nail or small bolt. Read at great depth reliably, but behaved just like any other fairly rust-free iron target would.

Pallasite - weathered chunk with surface intact. Read as a hot rock on rusty side, 299 rusty iron signal on shiny cut side, go figure!

Polished Iron - 1.5" chunk, etched and polished surface, showing iron crystalline structure in 3D. Read as true iron, similar to the 5" cut Palasite sample above.

LL - stony chondrite, 2" intact surface. Time Ranger - no signal at all. It had a speck of iron, which was visible, but not large enough to read. The BFO with its half-inch coil got a very slight hot rock null.

H4 Stony, Gold Basin - Small 1.5" cut in half. Random, varying ID metal signal on Time Ranger, showing a mixed metallic makeup. Good signal as metal on BFO.

Carbonaceous Chondrite - about 1" intact. Weak Hot Rock. Nothing on Time Ranger, null on BFO.

High Iron Carbonaceous - 1.5" chunk. Varied on the Time Ranger, depending on the angle it was scanned at, apparently showing metals other than iron as well. BFO - positive metal signal.

Well, it was a fun half-hour, and showed that some sort of response was possible on most of our samples, even if it was to a null or hot rock signal. If any signal as I noted above is detected in the strewn fields, it should be investigated by passing individual stones beneath the coil in air till you note the one that caused the reading. Set it aside for further study or comparison to "native" hot rocks. I'd say any rock that shows a metallic response is worth checking, especially if it is unusually heavy.

The lowest cost detector around should respond to an iron-rich sample, depending on the size of the sample, of course. Better models would probably search deeper or pick up smaller meteorites.

Hot rock responses may be much trickier, depending upon the natural occurance of terrestrial hot rocks in the vicinity. Lacking a better field test than my detector alone, I'd treat random "oddball" hot rocks as something worth hanging onto, at least on a guess till a sample could be checked by an expert. Hunting a known field, you have the advantage in that you expect to find anomalous rocks, that's why you're there in the first place!

Most of the stony's appeared to have a crusty coating or shell, and were close to a "normal" rock in weight. Most of the native hot rocks in my area are smoother and more polished, usually hematite, but I have found a piece of native graphite that read like iron but looked like gray slate. Iron meteorites can be a rusty-looking slag ball or have a definite metallic look, with possible pockmarks and holes. When cut, or if the makeup can be viewed, they all have a jumbled appearance, crystalline or just a bunch of inclusions.

Older-style BFO detectors that can be tuned to react positively to the mineral content might be a useful tool to hunt for the stonys. Compared to the Time Ranger, it read some of the stonys more reliably. We did not test any of the older tr/vlf models I have on hand, but if they can be set to react to mineralized ground or hot rocks instead of tuning that out, they may be somewhat effective searchers.

I hope this article was at least informative, and that it might help someone with a field identification sometime!

Happy Hunting -Ed

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