[meteorite-list] Cold hunting

From: Matson, Robert <ROBERT.D.MATSON_at_meteoritecentral.com>
Date: Thu Apr 22 10:29:53 2004
Message-ID: <AF564D2B9D91D411B9FE00508BF1C86901B4ED39_at_US-Torrance.mail.saic.com>

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
Received on Wed 10 Sep 2003 02:57:41 PM PDT