John

About fifteen years ago I went to a non-classified briefing about a land
version of  a gravity gradiometer.  The Defense Mapping Agency had it.

It was in a Winabago of some sort and had three disks that rotated at a
fairly low rpm, a few Hz at most.   The disks were less than a meter in
diameter.   The disks had two accelerometers on them.  The acceleration as a
function of position of the two accelerometers was reccorded and some sort
of auto-correlation or FFT was done to get the direction and magnitude of
the gradient.

It was cute.


Tom Schmitt

tschmitt@..............

----- Original Message -----
From: John Hernlund 
To: 
Sent: Saturday, April 29, 2000 10:58 PM
Subject: Re: Article on gravimeters
> > George Bush
>
> George,
>    There was a horribly written article about an incredibly interesting
> subject a few years back in Scientific American.  Even though the article
did
> not belong in the magazine, you might be interested in reading it.  It was
> about a gravity gradiometer that had been designed to help submarines
"see"
> the topography on the ocean floor and avoid collision with sea mounts.
They
> needed a passive method to see the ocean floor so that they would not be
> detected.  Supposedly this thing worked great (too bad all the data they
> collected is classified).  Anyways, the company that made it was allowed
to
> contract its use out to some geophysical concerns, such as finding oil and
> natural gas resevoirs.  The company takes a huge amount of money, throws
this
> thing in a boat and drives around for a while.  They take the data, reduce
it,
> and then sell portions of it for commercial use.  Nobody is allowed to buy
> one of these instruments, or use one for themselves.  This is the reason
it
> did not belong in the magazine: it was really an ad for the company.  The
> exact technology is still classified, so it is like a magical black box.
>
>    Anyways, this device produces higher resolution images for finding
density
> anomalies than seismic methods, and does not require a whole lot of fancy
> deployment except for position tracking and recording instruments, and so
is a
> lot easier to use than seismic arrays.  I found this hard to believe when
I
> first heard it, but then again I was only used to absolute vertical
gravity
> measurements.  So I had to find out about it a little further.
>
>    The gravity gradient is the change in the gravity for each direction
(down,
> east, north) within a given distance and with respect to each direction.
So
> the downward component of gravity changes differently in the down, east
and
> west directions, and likewise for the east and north components of
gravity.
> This makes nine different combinations of gradients that can be examined.
One
> of those nine components can tell you an awful lot more than an absolute
> gravity measurement, so just imagine having nine of them.  Well, it turns
out
> that some of the components are equal, and the total number of unique
> components is reduced to six.  For example, the change in the down
component
> of gravity with respect to north is equal to the change in the north
component
> of gravity with respect to the down direction.  In addition, the three
terms
> where the component changes in each of its directions is regulated by the
fact
> that a gravitational field is divergenceless, which means these three
> gradients must sum to 0.  So that makes five unique components, from which
the
> other four can be determined.  All nine components are referred to as the
> "gravity gradient tensor."
>
>    This machine is supposed to have rotatiing parts and sensors inside
some
> kind of black spherical shell.  It is very mysterious, and I would love to
see
> inside one some day.  I am sure everyone else would be interested too...
I
> guess I would have to hijack one of their ships or something if I wanted
to
> get it, but that won't happen any time soon.  Perhaps the PSN could mount
an
> expedition, and the media would try and figure out if PSN stood for some
kind
> of terrorist organization or something.
>
>    Anyhow, enjoy!
>
> John Hernlund
> E-mail: hernlund@.......
> WWW: http://www.public.asu.edu/~hernlund/
>
>
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