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Testing Meteorites

with Metal Detectors

posted January 5, 2002

Shooting Star Shootout

PART ONE

by Ed Gerken

As I promised all my detecting friends, I have conducted more meteorite testing,, a four hour session this time! Part One of the article is geared more towards generalizations based on our findings. Part Two has photographs of the specimens we used in these tests and a detailed listing of the measurements. In Part Three we test a single specimen in highly mineralized ground.

We once again borrowed several samples from Black Hills Institute's museum; a Henbury Iron, and four stonys; an Allende (CV 3.4), a Mahbes L6, a Zag H3-6 and a Gold Basin H4. For fun, I included two "Leaverites" in my testing, one an apparent magnetite sample, and an odd-looking nodule I found in Wyoming.

The contenders this time consisted of seven different metal detectors, including a Fisher Gold Bug 1, Bounty Hunter Time Ranger, Radio Shack Micronta 3001 (Alert re-brand), First Texas Search Master Tracker DX-8500, Compass Relic Magnum 7, and for pinpointers, the Falcon MD-10 and a home-brew BFO. Ron from Texas has already conducted some excellent tests using a White's XLT, so I did not borrow that model from the museum to bring along to this "shootout."

Results were comparable to what we reported the last time we tested some "space rock" samples. Excepting the Henbury Iron, of course, all the stony meteorites detected similarly to terrestrial "hot rocks."

As before when testing stonys, I noted the BFO machine and also the VLF portion of the Compass could be tuned to give a positive response. The rest of the machines and the TR/Disc mode of the Compass all responded with a null signal of some kind.

Of all the tests we conducted, the Fisher Gold Bug had the best depth at 14", achieved while reading the Henbury Iron. The Radio Shack was weakest on the Henbury at 9". Not counting the pinpointers, of course!

When it came to the stonys, I was surprised to find the First Texas DX-8500 had the best depth of all the machines, detecting some stonys over 6 inches away from the coil. The Compass and Radio Shack Units were slightly less responsive, but similar in depth, with the Fisher close behind.

Of the tested detectors, the Time Ranger gave the poorest performance on the stonys, reading some of the stony meteorites slightly over 3.5 inches away. It was weakest on the Allende at 1" versus the DX-8500's 4" on the same specimen. I admit to being a little disappointed with the depths the Time Ranger was achieving in these air tests.

In its defense, the Time Ranger displayed a lot of other info about the target's ID, which I feel might give it an edge in the field where one would encounter trash and other targets, plus it is by design less sensitive to hot rock signals.

The pinpointers both did a good job of reading all the samples, with the BFO having a very slight edge on depth reading the stonys, to a maximum of about 1.25". Respectable for a pinpointer, the Falcon put in a reading of 3.5" on the Henbury.

The best indicator of a null signal that I found was that the sample would not respond when brought straight at the face of the coil, but did sound off when pulled away. If scanned in a side to side motion across the face of the coil, the stonys would read upon leaving the coil's field rather than when centered under it. On some detectors, the audible difference between a 25-cent piece and a stony meteorite was startling. On the machines that had meters or digital equivalents, the meter would deflect to the negative or left side. The audio "generally" dropped in volume or frequency when nulled. On the few machines that could be adjusted to read the mineral content, the samples then responded normally in the center of the coil, which was a big advantage in some instances.

The "magnetite" sample read so identically to the stonys that I began to suspect it might be a meteorite itself! However, it was the only sample, including the large Henbury, that could actually pull a compass needle off magnetic north and hold it there. Our final test specimen, the "nodule," is probably just an agate of some sort, but I did note that the BFO machine gave a very mild null when reading it. The BFO was the only machine that responded to the nodule at all.

Of course the Iron sample was strongly magnetic. All the rest, except for the Allende, had a small attraction to a magnet. Three of the stonys could also sway a compass needle a small amount when moved about above the stationary compass.

All of the detectors responded in some fashion to the majority of the samples. I tested all the various operating modes and also tried the discrimination and ground balance controls when available. For depth comparison's sake, I also read a quarter on each machine.

The most important bit of info I noted is that even the least expensive and most-outdated of all the machines still did a credible job of detecting all of the samples I had here for testing.

My thanks once again go to Bob Farrar at Black Hills Institute of Geological Research, www.bhigr.com, for graciously lending me the specimens and for his help with identifications and info on the specimens themselves. BHI sells meteorites and books on the subject and has an excellent public display, besides the collection to which I was granted access.

Some people may find a closer examination of my results to be interesting. If so, read on for all the gruesome details. It's a lot of info to compile, and even drier than this little essay, if you can imagine! This series of articles came about due to a few questions posted on a metal detecting forum. Since I had access to several detectors and the museum's collection, I decided to undetake these few simple tests. It's my conclusion that anyone can take most any detector and conduct their own tests or searches.

Thanks to everyone for all your interest, I've learned a few new tricks while I was at it and have enjoyed the experimenting. Now, let's all go find some meteorites!

-Ed


First posted January 5, 2002

Shooting Star Shootout

PART TWO

The Meteorites

Click the photos for enlargements

Can you tell from this photo which of these "rocks" are true Meteorites,
and which two are Meteorwrongs? I suppose not, small as it is!
Note the compass needle deflected from magnetic North by the sample placed next to it.


The Gear

The contenders consisted of seven different metal detectors, including a Fisher Gold Bug I, Bounty Hunter Time Ranger, Radio Shack Micronta 3001 (Alert re-brand), First Texas Search Master Tracker DX-8500, Compass Relic Magnum 7, and for pinpointers, the Falcon MD-10 and a home-brew BFO. Paul Janke, owner of Pan Terra, provided the Compass detector which allowed me to include it in the testing. Thanks, Paul!

I tested each sample for maximum depth, and reported the measurement that gave the first clear response. A mild waver in tone was not counted. By tuning or switching modes, each machine was set to peak on the test specimen. Any problems I encountered or differing responses were noted. For complicated detectors with several modes, the results are more detailed. For detectors that saw no improvements with additional tweaking, just the depth of detection and basic info is reported. I also tried to include a subjective comment on the apparent strength of the detector's overall response to each meteorite. The response each detector gave for a particular specimen is listed below the photos for that meteorite.

Using a small speaker magnet, I checked each specimen for iron content by noting its attraction to the magnet. Some specimens were also magnetized as well as or instead of being magnetic, that is, they attracted a compass needle when moved above the stationary compass.

For a signal strength comparison, I tested a clad quarter for maximum clear signal depth. To avoid cluttering up the meteorite test results below, here are the 25c readings and a general description of each machine:

Time Ranger: Read a quarter at 6" in discrimination mode, and at 8" in all-metal. At the time of our tests in 2002, the Time Ranger was Bounty Hunter's top model, a position I believe it still holds (now there are the Teknetics T2 and Fisher F-75, all made by the same company). The Time Ranger has two basic modes of operation, Motion Discrimination and No-Motion All-Metal. We restricted our testing to these two modes, with discrimination set to zero when in the motion mode. We did not test the disc notching, blanking or sniff modes. It read the iron meteorite very well, but its designed-in rejection of hot rock signals made it less responsive to the stonys than the other manually ground-balanced detectors appearing in this test. I got the best responses in no-motion mode by increasing the sensitivity until a threshold tone was heard. The Time Ranger has automatic ground balance in either mode. Though it can be set manually, it self-adjusts during use. The Time Ranger could be set to null out a stony meteorite by pressing the ground track button in no-motion mode, but it was still able to recover to a degree and detect the meteorite after balancing it out. By this I mean that even if it is badly ground balanced such that it will not detect stony meteorites, it will tend to self-tune out of that condition. To a degree. It will take some field testing to determine the effectiveness of the automatic ground balance circuit on the stony meteorites, particularly in hot ground.


Falcon MD-10 Pinpointer: Depth: 3/4". The Falcon MD-10 is a prospecting pinpointer, so it is sensitive to both minerals and metals. It has only two dials, sensitivity and ground balance, these were set for maximum response to the test targets. It read clearly and easily on all of the meteorites. This is a fine pinpointing detector for any purpose and it has proven to read meteorites of nearly all types well. Out of production for a time, I believe it is being offered once again, though it's a bit spendy for a pinpointer. One could travel very light using only this machine, relying on eye for discovery followed by air tests of a suspect rock.


Home-Built BFO: Depth on a quarter: 1". The slightly greater depth of this pinpionter compared to the Falcon is probably due to a slightly larger coil winding. Based on Steve Hegeman's design, it is a simple one-IC, two-transistor Beat Frequency Occillator detector. Like any BFO, it can be set to respond with a positive signal to either metal or mineral. It is the only detector that gave a signal on all seven of the test targets. It's my opinion "obsolete" BFO machines can find new uses as meteorite detectors.


Fisher Gold Bug I: Depth on a quarter: 7". Primarily a gold prospecting machine, the Gold Bug is well-known and was a popular detector choice in the 1980's and 90's. It has no discrimination, but has an audio threshold and 10-turn manual ground balance control. It read the best of all on the iron meteorite, but was middle-of-the-pack on the stonys.


First Texas DX-8500: Depth: 7". A late 1970's design low-cost VLF detector, with simple discrimination and ground balance controls. I bought this unit from Fingerhut in the early 1980's. It made a weak coinshooter and was hard to use in bad soils. Surprisingly, it topped all the rest in the detection of the stony meteorites in air. Another machine that will need field testing to see if these phenomenal depth readings will still hold true in mineralized soil. Somehow, I doubt it. Probably best kept to air testing samples if the soil is bad.


Radio Shack Micronta 3001: Depth: 4.5". An all-transistor design that is probably almost as old as I am. Radio Shack dropped the Micronta name years ago, this particular machine dates from the 1970's, while the circuit's design is probably much older. Ancient as it is, I believe this same detector, or one with a very similar circuit, is still available today from various sources, while used units abound. It is adjusted depending on whether you are searching for ferrous or non-ferrous targets and has a center-reading meter which is good to detect nulls from stony meteorites. A cheap and reasonably capable meteorite machine. Depth on Irons may be less, but it makes up for that with good sensitivity to the stonys.


Compass Relic Magnum 7:  Depth on a quarter: 7". Another older design, but a good solid one. This model is a combined TR Disc/VLF All-Metal machine. The VLF mode was the best mode to use to detect the stonys. It has a discrimination control in the TR mode, which could be used to indicate the relative amount of iron in the sample, by noting how quickly it discriminated out the signal.

Please note the following were all indoor air tests.

Henbury Iron

Num 132a Henbury, Iron Octahedrite fragment
Northern Territory, Australia
795  grams

Strongly attracted to a magnet, but did not deflect the compass needle. Highly rusted and weathered surface. Some sculpting and ablation of the surface. Felt very heavy to hold for its size, compared to a common rock.

Time Ranger: All-Metal Motion (Disc mode): 10" max depth, most any number might appear on the numeric target ID, showing a mixed metallic makeup. An ID number of 299, the maximum of the machine, was predominate. This is a "rusty iron" signal, identical to that made by a long-buried tin can. All-Metal No-Motion Mode: 11", iron reading (no numeric display) a typical indication made by a nail or other non-rusted iron, some nickle readings around 35, proving the iron/nickle mix of this type of meteorite.

Falcon: Depth, 3.5". I got differing responses with this machine between the iron and the stonys. As I have often commented, stony meteorites read like a hot rock signal, producing a null response from a detector that is tuned to detect metal instead of mineral. These signals generally appear when the target is at the edge of or leaving the coil's detection field. A more definitive test  of this nulling effect is noted when you either draw the sample towards the coil or pull it away from it..Metal responds when going towards the coil, hot rocks and stony meteorites respond on the pull away stroke. On the Falcon, I also noticed the Henbury did not need motion to continue to be detected, while the stonys needed motion or the detector would become silent.

BFO: Depth, 3" to 4". Strong iron (metal) signal.

Gold Bug 1: Depth, 14". Very strong, postive response. Detection went out past 14" but not reliably. Detector was set for all tests with a definite audible threshold tone.

First Texas: 12". Iron response, the signal could be fully discriminated out. Set as the Gold Bug, with an audible threshold tone. A good response, but a little more "wavery" than the Gold Bug at extreme distances.

Radio Shack: 9". Iron reading, detector was tuned to a peak on the ferrous side of the control.

Compass: 12". In TR Disc, the iron signal could be completely discriminated against. A good solid signal all the way out.

General observations: Bring a big shovel, because you will find decent-sized iron meteorites as deeply as your detector can send a signal into the soil. Hopefully the area is not too trashy with nails and tin cans or you will be digging a lot of junk as well.

Allende

CV 3.4

Carbonaceous Chondrite

Mexico

Apparently not attracted to my small Alnico speaker magnet. Did deflect the compass needle, however. Relatively light weight, black oxide crust on portions of the specimen, but much of that had flaked away from exposure to the elements. Many visible small chondrules, the lighter-colored flecks seen in the photos. The most difficult to detect of the 5 meteorite samples.

Time Ranger: No reading in motion mode. 1" in All-Metal with a null signal, reads when pulled away from coil.

Falcon: 1/8". Null reading, same as for Time Ranger. Motion needed or detector becomes silent.

BFO: Weak reading, positive response.

Gold Bug 1: 3" depth, with a weak null reading.

First Texas: 4" depth, the best for this sample. Weak null, the Allende's signal did not discriminate out.

Radio Shack: 2" depth, non-ferrous setting, weak null.

Compass: 2.5" positive reading, in VLF mode. Signal did not disc out.

General observations: Hard to read, it would be best to hunt this type by eye with primary use of the detector to confirm a hot rock signal in a separate air test. The oxide crust, relatively light weight and highly visible chondrules are the keys to identifying this meteorite type. A VLF detector might be used with some success, but ground mineralization may make this a difficult task. Resort to an air test for better checking of a null response.

Mahbes

num: 151 name: BHI-NWA-013
L6 Chondrite, end cut with crust
Mahbes, Kem Kem, Morocco
181.1 grams
Very mildly magnetic, with a mild compass needle deflection. Pretty ordinary-looking externally, although crust is somewhat obvious, and rust is evident where the crust is missing,. Relatively heavy due to high iron content. Cut side shows many flecks of iron seen by reflected light. High degree of internal rust banding caused by internal oxidation along faults and cracks.

Time Ranger: 2" in motion mode, rusty iron 299 ID, null signal read on edge of coil or when pulled away from coil. 2.5" in no-motion, with weak audio when placed right against coil, otherwise all-metal mode was mostly silent until the threshold tone was increased, then a null was evident. Ground monitor indicator showed strong deflection to negative.

Falcon: 1.25", iron-type signal, read when run towards coil, no motion needed to sustain a signal.

BFO: 1.5", Positive (metal) response.

Gold Bug 1: Depth 6", null "hot rock" reading.

First Texas: Amazing 8" depth, some discrimination change as control was advanced, strong null-type reading.

Radio Shack: 6.5", non-ferrous setting. Strong null observed.

Compass: 6.5" in VLF mode, strong disc. effect in TR mode.

General observations: The iron content aided detection for most of the machines, but a few still read it as a hot rock nuling signal. Certainly this form of meteorite can be deteced at depth, with enough strength to the signal to clue the finder to have it checked further. If your detector has iron discrimination, it can possibly be used to "assay" the iron content of the sample.

Gold Basin

Num: 112a  Name: Gold Basin
H4 Chondrite, weathered individual
Mohave County, Arizona
27.8 grams
Very mildly magnetic, did not deflect the compass needle. Probably one of the most common types recovered in the US, due to the extensive hunting of the Mohave Desert area where they are found. Quite similar in weight and appearance to many earthly rocks, but the rusty, crusty surface and weathered missing portions are clues to its more fragile and easily-eroded nature.

Time Ranger: 1" with 299 ID in motion mode, more random variation in ID readings. 1/2" in no-motion, with both clean iron and rusty iron readings, weak audio when against coil, similar to Mahbes readings.

Falcon: 3/4",  positive acting iron signal, no motion needed for detector response.

BFO: 1", positive metal-type response noted.

Gold Bug 1: 4.5" null-type reading.

First Texas: 6",  very respectable depth with mid-strength null response,  discrimination control had no effect on the reading.

Radio Shack: 3.5", non-ferrous tuning with a good null-type reading as a response.

Compass: 4", VLF mode., no discrimination effects in TR Disc.

General observations: Readily detected by all the test machines.

Zag

H3-6 Chondrite "Zag"
Western Sahara
weight not noted
Mildly magnetic and also magnetized, as it deflected the compass needle. A split piece, with the metallic content easily visible in the break. The specimen showed many hairline fractures in the relatively thick, pebbly crust. Some rusting visible on surface. Heavier by feel than a common rock. Very comparable in size and reading to the Mahbes.

Time Ranger: 2" with 299 rusty iron ID and typical null response in motion mode. 3.5" null reading in no-motion.

Falcon: 1" metallic reading, with no motion required to sustain a reading.

BFO: 1" Positive response. Depth on the stonys with the BFO were similar to the Falcon's.

Gold Bug 1: 6" strong null reading.

First Texas: 7" depth, with strong discrimination effects and a heavy null.

Radio Shack: 6" non-ferrous with strong null reading.

Compass: 6" in VLF mode, strong discrimination response and strong null.

General observations: There were some variations in the readings between the stonys, which may be attributable to the varying iron content and percentage of other metals and minerals in each specimen. The size of the specimen played a part in the strength of the reading, but the discrimination control would disclose the relative amount of iron present.

Meteor Wrongs

Septarian Nodule

Magnetite

The nodule was recovered from the Oligocene fossil deposits in northeastern Wyoming. I included it in the tests because it had a some visible white specks on the surface and a thin, rusty-looking layer beneath the crust. It was simply an unusual-looking stone that caught my eye and I wondered if there might be an electrical response. The surface is somewhat translucent and the interior is somewhat homogenous, despite the many inclusions. Only the BFO machine gave a very mild response, all the other detectors completely ignored this specimen.

The Magnetite was discovered while gold panning and sluicing as a heavy object that stuck in the gold pan. It has a high degree of surface luster, some small rusty spots and appears to be heat melted, but that is probably due to stream action. It was strongly attracted to a magnet and gave the highest deflection of the compass needle of any of the test specimens. It could pull and hold the needle in any position showing the specimen to be magnetized to fairly high degree. It did not have enough attraction to lift a small paper clip or a pin, however. After Bob Farrar identified it as either hematite or magnetite, I did a streak test, which left a black mark. This indicated magnetite, as hematite leaves a reddish streak (I know, the pics still say hematite). I then ground and polished one end of the specimen to expose the interior. The residue from polishing and the appearace of the surface was more akin to an electric motor commutator brush than a metal object. The interior was very homogenous with some minor cracks.

Since the BFO was the only detector with any response to the Septarian nodule, the following results are from the Magnetite sample.

Time Ranger: 2", 299 rusty iron ID in motion mode. In no-motion (all metal) mode, I got different readings depending on how fast I scanned the object. A slow scan gave either a blank numeric readout, indicating "clean" iron, or the rusty iron 299 reading. A faster scan gave a 299 reading only. An audio response and deep null at a 3" depth was observed in no motion mode..

Falcon: 1/2" depth,  mineral response, that is it signalled when pulled away from the coil, motion was required to give a response.

BFO: Once again a positive signal was noted. Incidentally, I tested this BFO detector more thoroughly in a previous set of experiments, see the Meteorite Madness page.

Gold Bug 1: Depth 5", null reading.

First Texas: 6", no discrimination effects, good null.

Radio Shack: 4" non-ferrous tuning.

Compass: 4.5" in VLF, some discrimination noted in TR Disc.

General observations: The readings taken from the Magnetite were so very similar to that of the stonys, I was nearly convinced that this was a small iron meteorite. The strong compass needle deflection, and the absence of crust and lack of interior differentiation gave it away as terrestrial in origin. It also gave null readings, while a true high-iron meteorite will always be positive.

Posted January 13, 2002

Shooting Star Shootout

PART THREE

Metal Detector Tests of a Meteorite on Mineralized Ground

The objects at our test bed

From palm to finger we have a ferrite AM antenna rod, a Moroccan stony-iron meteorite and a quarter. The rocks are rusty slates, with embedded garnets and occasional quartz outcrops, the soil is mostly pine needle mulch and decomposed slate, with some clays. The vertically shifted slate bedrock is mere inches beneath the soil. A compass showed some localized magnetic anomalies near the test site, which was about 75 feet from my front door, and a check with the Gold Bug showed highly varied mineralization. I searched for and found no trash targets, which typically would be mostly spent 22 shells. These give a very characteristic response, so I assumed the area to be trash-free. There were no electric wires nearby, but the phone cable is buried some 20 feet away.

This test was done to determine if there is a difference in how a metal detector reacts to a meteorite in air versus soil, particularly in highly mineralized soil. We had conducted many tests of meteorites in air and wanted to know if we could expect a change in detector performance in a field test. The short answer is yes, all three machines I tested suffered a loss of depth in mineralized soil compared to free-air.

When conducting the ground tests, the detector's response to the ground alone was checked first and ground balance was adjusted for minimum response, just as you would do in a normal hunting situation. The meteorite was placed on the surface in that same spot and checked for a reading. After testing for a bit, the specimen was removed, the ground rechecked, and then the specimen was replaced and read again to verify the response was coming from the meteorite and not the soil. For comparison, a clad quarter was also read in place of the meteorite.

Depth of detection was hard to estimate, a ruler was held next to the coil and a measurement was taken from the top of the specimen to the bottom of the coil as near as I could figure it. The coil was swung back and forth and slowly raised until it did not respond,, this was done several times to get a more accurate estimate of depth.

Each detector was then ground balanced to the ferrite rod instead of the soil, which setting always gave a response to the meteorite, but the detector then also tended to be out of balance with the surrounding soil. In another test, ground balancing normally, the ferrite rod was checked on the ground in the same way as the meteorite was in our first test. In this final test, the rod always gave a stronger response than the meteorite, so I concluded from this that the ferrite rod was too large to be a comparable target to this particular meteorite. I'm not sure if breaking the rod in two and using just half of it would help set the detector better than just ground balancing normally. I stole the ferrite rod from a radio and wanted to return it, so I did not break it in half to see if that made a difference, but I suspect it would have.

Setting the ground balance on either the Gold Bug or the First Texas to give a positive response to the meteorite on the ground did not appreciably increase detection depths, and tended to upset the ground balance, causing false signals to appear. It was not possible to set the Time Ranger to give anything other than a null response, since it does not have a manual ground balance.

More or less randomly choosing spots within my mineralized test bed, I moved the meteorite about and rechecked it. In the midst of the iron-rich slates, it became much harder to pick up. Compared to the air tests, each machine lost at least a little depth in my mineralized test bed on both the quarter and the stony meteorite. Basically, the surrounding soil matrix was giving a response that slightly masked the meteorite or coin. Interestingly, the Time Ranger in motion/disc mode did not lose much depth on either the quarter or the meteorite, but it also had the least depth on the meteorite to begin with. Despite the loss in depth, each detector responded quite similarly in either air or ground tests. A null response was a null response, no matter the location of the test. Had I picked a more neutral area to test in, I would expect the greater depths seen in the air tests to be more evident.

Our magnetite sample was also briefly tested, it too reacted about the same on the ground as it had in our indoor tests. As in the other ground tests, we again noted a similar slight loss in depth.

The Moroccan sample I was using is a good example of a stony-iron meteorite. It would tend to read a bit lower for its size than a Gold Basin, for example, but a good response is possible and the readings obtained were similar to other stonys I have tested. An iron meteorite would be much easier to detect in any soil condition than a stony, so I did not bother to obtain one for this series of ground tests.

While local ground effects will vary in any search locale, we did note the basic responses the detector gave at our test bed did not change from what we observed in our previous air tests. What this means to me is that air testing is a valid method for determining the "typical" detector response to a given meteorite type. That's great news to me, because it is snowing outside once again, and my outdoor tests are probably curtailed until spring. To be fair, a detector that has a hard time generally in difficult ground will not fare any better at meteorite hunting in such ground. I have only one auto-ground balancing detector to test with, but it did not fare well in depth, although it did provide a lot of additional information via its digital readouts.

Depending on how well your detector works in some highly mineralized areas, it might be necessary to rely on air tests to check samples spotted visually, rather than trying to detect them directly with the coil on the ground. Still, even in these areas, the detector may be used to check for anomalous null signals, then these may be inspected more closely by eye.

A "null" signal is one that causes a reduction in audio threshold rather than an increase. You will notice that if the specimen is held in the hand and moved straight towards the face of the coil, the detector will not respond or the audio will weaken, but it will respond more strongly when you pull it away from the coil. Scanned side to side across the coil, it will respond as the specimen is passing the edge of the coil, rather than as it approaches the center. It is the direct opposite of what you get when a metal object is detected and is similar to a "hot rock" type of signal. Still, it is a response and can be used as such to search for meteorites or help identify a suspect rock.

Because of the nulls caused by normal mineralization of the soil, some machines may not prove to be effective at on the ground searches for stony meteorites. However, that same machine may prove to be quite good in an air test of a suspected space rock. In this case, hunt mainly by eye and use the detector for air testing rocks that appear odd-looking.

While a piece of ferrite might be useful to "preset' the detector for finding meteorites as some people have suggested, in my opinion a natural magnetite specimen might be a touch better. Granted, my ferrite rod was a little on the large side. After an hour or so of testing, however, the best sample target I could find to set the detectors for stony meteorite hunting, was an actual stony meteorite.

(Click the photos for enlarged views)

Our test specimen -

NW Moroccan Meteorite

Stony Iron

Uncataloged specimen

43 grams weight

A very non-descript rock on first glance, most of my friends called it a "leaverite," aka "sex rock," when I gave it to them for inspection. It has some minor ablation depressions and the fusion crust nearly completely covers the specimen, such that it appears to be merely a rusty brown rock. On closer inspection it has some tell-tale signs that show it to be a meteorite. Magnetic, it was swayed and pulled a bit by my small loudspeaker magnet. Magnetized as well, it caused a compass needle to wiggle a bit when the meteorite was passed above. It was not till it was ground on one face and the iron inclusions were exposed, that it became readily apparent visually.

This side of the meteortie does not show much except a rusty, mottled and weathered surface with some small ablation depressions.

This view shows the fractured face, which is seen facing the bottom in the above photo. The fractured side is heavily rusted and pitted, and seems to show some secondary crust, meaning it may have broken off while the main mass was still burning from reentry heat. A few small flecks of iron seem to be visible on the upper right surface.

This close-up shows the texture of the rusted  fusion crust, and the iron flecks are a bit more visible here. The camera intensified them slightly, they are barely visible to the naked eye.

I ground down the right side of the flat face to expose a bit of the interior. In the image above, it is shown with normal lighting.

Here is the same face, with the angle of the lighting changed to show the iron, seen as white specks on the right side.

This is a close-up of the roughly ground surface as seen above right, this view showing the iron a little better.

In this photo, I tried to accentuate the iron as much as possible with the lighting. When I ground it down, some of the iron made a smear or trace, as displaced iron particles became embedded into the stone. In this picture, the effect gives the appearance of more iron than there actually is in the specimen.

How the detectors responded-

Bounty Hunter Time Ranger:
On the Ground Test- All-metal mode, 1/2" to 1" depth, null responses, read when coil was moving away from the target, of audio quieting, negative-reading ground monitor. A "typical stony," with a 299 "rusty iron" numeric ID and $1 silver target ID. Disc. mode, depth 1-1.5", read when coil was moving away from the target. Read a quarter at 6" in the same test spot in either mode. I moved the meteorite about to different areas and retested. In some places, the specimen matched the surrounding ground so well, there was almost no response, but careful ground balancing could adjust for a reading. Air Test, same specimen- All-Metal,  2" depth, Disc. mode 1", same basic null readings observed in both modes as in the ground portion of the test. Air-tested a quarter at 6" in discrimination mode, and at 8" in all-metal.
Were it not for the rusty iron ID signature, this particular space rock would be easy to pass by with this machine.

Fisher Gold Bug 1:
On the Ground Test- 3.5" depth. The Gold Bug's manual ground balance control could be adjusted to give a positive response, but then mineralization in the area gave problems with false readings. We set it for maximum response from the meteorite and minimum interference due to mineralization. A quarter in the same spot read at over 7". Air Test- 5" maximum depth, the ground balance control could again be adjusted to give either a negative or positive response, but depth was unaffected by the setting. Air test of a quarter read 7", showing that mineralization masked the meteorite, but not the quarter.

First Texas DX-8500:
On the Ground Test- 3.5" depth. The ground cancel control was adjusted for best balance for the test area, but it was obvious this older VLF detector would not perform well in this type of soil. If the ground balance was turned clockwise, a greater depth could be obtained, but with many resulting false readings from soil mineralization. Like the Gold Bug, we reported the greatest depth using a setting that did not give false readings. The First Texas read a quarter at 5.25" in the test area. Air Test- 5" to 5.5" depth, the ground cancel knob had no effect. The specimen could be nearly discriminated out by advancing the disc. control, but some reading was always obtained. Air test on a quarter gave 7", showing this detector's sensitivity was indeed affected by the bad ground.

This small face shows the fusion crust quite well.


Other detectorists have conducted extensive field testing and I have placed a few of their articles and posts on the subject on the Articles page. Hope you can check them out! -Ed


More test reports on the Meteorite Articles page!

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