[meteorite-list] Strewn fields vs Distribution Ellipses

From: meteoriteshow <meteoriteshow_at_meteoritecentral.com>
Date: Thu Apr 22 10:17:49 2004
Message-ID: <001801c3c498$3f4eae60$c48b4052_at_monordinateur>

Thanks for those accurate information.

Frederic
----- Original Message -----
From: "E. L. Jones" <jonee_at_epix.net>
To: <Meteorite-list_at_meteoritecentral.com>
Sent: Wednesday, December 17, 2003 10:55 AM
Subject: Re: [meteorite-list] Strewn fields vs Distribution Ellipses


> Hello List,
> Let me throw this out to help clarify the concepts. I have a
> statistical major but after my accident some procedures are fuzzy to me
> and today most is done by computers anyway, so bear with me. I'll try
> too post a field technique for estimating an ellipse another time.
>
> The strewn field is the physical distribution of the meteorites on the
> ground. It's true density, content and numbers are never known because
> you never know if you have recovered the very last meteorite. It tends
> to fit within an elliptical form, but when actually plotted can look
> like a Rorschach figure. It dosen't have a line drawn around it--not
> yet anyway.
>
> The Distribution Ellipse is the geometric/mathmatical representation of
> the "predicted" boundary or"limit" or "edge" of the strewn field. The
> first reason for computing an ellipse is to focus search and recovery
> efforts. Its secondary use is to estimate angle of fall and altitude of
> fragmentation. Its tertiary use is to confuse novice meteorite hunters.
> When all the math and geometry is done you have an ellipse to overlay on
> your map board. The statistical significance of this a prediction is
> that 95%(2Standard Deveations-SD) or 99 %(3SD) of all the meteorites in
> this fall lie within the ellipse.
>
> For actual use You duplicate the ellipse overlay on tracing paper and
> give it to each hunter team and send them off to cover each sector with
> an agreed upon search pattern( grid walk, etc.), adjusting assigned
> sectors based on the terrain and best judgement of the search director
> who may be playing every position if they are first at the scene. It
> maybe wise to search the centerline first.
>
> As previously mentioned you need a minimum 4 points to start. Actually
> there can be a little cheating as to the number of points if one of the
> points includes what is clearly a large mass it is treated as two close
> points--i.e. The large mass is assumed to have fallen ON axis and/or
> you have additional clues as to the direction of fall. Remember this is
> a recovery tool which is refined as more finds trun up.
>
> That said-- for an ellipse to be of search value you must have good
> sampling from the whole field which you are trying to estimate and that
> can be a catch-22 loop. For practical use, a laptop with a spread sheet
> template or statistical package is the only efficent way to get to the
> 95% confidence level and add additional locations. The inital plot will
> likely become skewed as more finds are plotted and the ellipse has to be
> adjusted and the new information shared with the searchers. It also
> assumes that you have a simple fall and you do not have multiple
> concentrations due to multiple fragmentations at altitude.
>
> The eccentricity of an ellipse and the length of the two axies give a
> clue as to the angle of the fall and altitude of fragmentation. A narrow
> ellipse suggests a lower level fragmentation because the fragments have
> less time to disperse on the lateral axis before reaching the ground. An
> elongated ellipse suggests a low angle of fall (shallow) with fragments
> dropped along a path where the bulk of the mass carried forward momentum
> longer before reaching the ground. Conversely, a wider ellipse suggests
> a high altitude fragmentation. A short ellipse suggests a high overhead
> (steep) angle of fall.
>
> Conventional wisdom says that larger masses are found at the far end of
> the direction of fall. And perhaps this holds true for a simple
> fall--one that has a single fragmentation episode. However Portales
> Valley had the larger fragments fall to the near end of the field.
> <http://www.lpi.usra.edu/meetings/LPSC99/pdf/1964.pdf>
>
> An example of a complex distribution can be seen at
> <http://www.saharamet.com/meteorite/data/map/ellipse.html> . Note that
> there are several ellipses on the direction of fall suggesting several
> fragmentation episodes. This page shows the initial ellipse from the
> first expedition<http://www.saharamet.com/expedition/CO3/part1.html> See
> how the ellipse plot was expanded as new material was found? The actual
> strewn field recoveries are plotted both inside and outside the ellipses
> where they were recovered over two or more expeditions.
>
> Elton
> (Thank you, Dr. Jean Dyer,USARI, where ever you are)
>
> mike miller wrote:
>
> > Hello, my question is about "strewnfields" in general, not how they
> > relate to a dry lake bed. I started as a meteorite hunter and have
> > evolved into a hunter/collector.
> > I think I understand the end of a strewnfield, the elliptical shape
> > is created by most of the lighter material falling away to earth and
> > the larger pieces continuing on a path down the middle of the
> > strewnfield. They create a point that more or less follows the middle
> > of the strewnfield. My first question is what is the reason for the
> > point at the beginning of a strewnfield?
> > The second part of my question is are there other types of
> > "strewnfields"? More to the point, explosions that occurred closer to
> > the surface and that might be described as a shotgun pattern? I would
> > be interested in any other types as the only types I have heard about
> > are the classic elliptical and a few related pieces.
> > Thanks in advance for your thoughts and opinions.
> >
> > Mike Miller
>
>
>
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Received on Wed 17 Dec 2003 07:21:06 AM PST