[meteorite-list] Cold hunting

From: David Freeman <dfreeman_at_meteoritecentral.com>
Date: Thu Apr 22 10:29:53 2004
Message-ID: <3F5FE606.2070800_at_fascination.com>

Dear List;
One more point, I recognized my 53.7 gram meteorite at about 40 feet
away, and could confirm it to be meteoritic at about 30 feet by the
excellent crust. I would have not seen it at all probably if I were in
a vehicle, or riding a four wheeler.
My method is walk ten steps, scan left to the comfortable sight range to
see a golf ball sized black item (about 100 feet in good light and clear
skies), look across in front of me in a long sweep of the eyes, and off
to the right the same distance. Ten more steps (about 30 feet for long
legged me) and do it again. Keep in mind, no rocks in my playa lake bed
but I am sorting through antelope droppings, sheep droppings, and horse
piles. The main plan is to discern the meteorites from the animal
droppings. Riding a four wheeler, or bicycle wouldn't work, as there had
been previous 4 wheeler tracks in a methodic pattern running back and
forth across the playa and about 100 foot swaths and they failed to find
the one, and I am pretty sure it was one of my student meteorite hunters
that missed the golden opportunity by traveling too fast. Attention to
detail was what helped me locate that fusion crust, and handling and
looking at maybe a hundred pounds of individual NWA's really imprinted
that fusion crust in my mind. That was important to me...that and
putting down that remote control, the mouse, and getting out to where
the meteorites should be found.Time spent in the field where the
meteorites live is the most important factor for my find. They don't
usually come through the roof.
Best,
DAve F.

Matson, Robert wrote:

>Hi Norm and List,
>
>>As an exploration geologist, I spend at least 15-20 long days every
>>month wandering the alluvial fans and dry lakes of Nevada searching
>>for mineralized float (and, unofficially, meteorites!). Some of
>>the remote dry lakes are almost certainly unsearched, and can cover
>>many square miles.
>>
>
>You might be surprised. Aside from playas on military installations (which
>in Nevada is a pretty big fraction!) I'm fairly confident that most
>of the major dry lakes in Nevada (and California) have been searched at
>least once by one or more competent meteorite hunters. Of course, they
>haven't been searched "completely"; indeed, no location can ever be
>completely searched due to the dynamic nature of playas.
>
>>Putting along on my ATV, I can give a reasonably large area a pretty
>>decent search. After several years of this, still NO cold finds.
>>
>
>This is actually a bit surprising, though I can think of a couple
>factors which could bring it about:
>
>1. your size threshold (how small a stone you will stop for)
>2. too broad a search image
>
>By broad search image I mean that your primary target (mineralized
>float) is so dissimilar to your secondary target (meteorites) that
>the subconscious pattern-matching that your brain is trying to
>accomplish will suffer. (It's hard enough picking out meteorites
>among terrestrial brown and black rocks).
>
>>I think the point may be that there's a pretty good chance that
>>stones have at some point fallen on most any square mile of earth's
>>surface.
>>
>
>Absolutely. Even if each fall produced only a single meteorite, in
>any square mile you could expect a fall about once every 5000
>years. However, the average number of meteorites per fall is
>certainly more than one (perhaps in the neighborhood of a half
>dozen?), so this improves things. Maybe a meteorite every 2500
>years per square mile (a complex problem that requires some
>Monte Carlo modeling to come up with a good figure). This leads
>to Norm's next point:
>
>>However, in most areas, survival times are short. In many areas,
>>erosion has erased the record. In other areas, deposition has
>>buried every trace.
>>
>
>Exactly. I liked this wording:
>
>>From the cosmic perspective, every square mile is created equal, but
>>for us grunts on the ground, that's far from the case. Some square
>>miles are just right. Most are not.
>>
>
>If you want to reduce search time per (cold) meteorite find (i.e.
>non-strewnfield), the #1 factor is "survival time" -- you must
>maximize it any way you can:
>
>1. Low deposition rate -- ideally a ~negative~ deposition rate: you'd
>prefer a surface that is "deflating"
>
>2. Low annual precipitation
>
>3. Minimal human presence
>
>Fortunately point #3 tends to go naturally with #2. It's only in the
>modern era of weekend warriors (and meteorite hunters!) that #3 has
>become an issue. Still, the historical importance of point #3 can
>not be ignored for iron meteorites. Chondrites wouldn't have been
>particularly useful to native Americans 500 years ago; irons certainly
>were.
>
>But detection isn't just about "signal" (signal in this case being
>the presence of meteorites); it's about signal-to-noise ratio (SNR)
>and scan rate. What else is sharing space with your meteorites,
>and how does this impact the rate you can search an area? It's
>not much fun looking for meteorites in a sea of basalt.
>
>Scan rate, incidentally, is the reason one should never hunt cold for
>meteorites with a metal detector. It is more than 50 times less
>efficient that visual searching. But you also don't want your scan
>rate to be too high or you'll miss the small stuff. Remember that
>there are far greater numbers of small meteorites than large ones.
>Searching in a car or truck is fine for finding the big ones, but
>most places that ~can~ be searched by car/truck in Nevada and
>California already have been. You'll have better luck looking for
>small meteorites, and this means getting out of your car and its
>limited viewing angles. Most of my searching is done on foot.
>
>An ATV is even better than walking: you're still close to the ground
>with an unobstructed view, but you have the comfort and convenience
>of motorized transport. ATVs can also get into distant areas that
>cars can't (and by extension, walking searchers).
>
>So to summarize: search old, simple surfaces on foot or by ATV,
>and you will eventually find meteorites.
>
>Cheers,
>Rob
>
>
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Received on Wed 10 Sep 2003 11:03:34 PM PDT