PSN-L Email List Message

Subject: Re: Article on gravimeters
From: John Hernlund hernlund@.......
Date: Sat, 29 Apr 2000 19:58:10 -0700 (MST)

On Sat, 29 Apr 2000, George Bush wrote:
> I just discovered an interesting article on the development of an absolute
> differential gravimeter that appeared in an unusual place. As a ametuer
> seismometer builder, I found the article fascinating to see what the big
> boys do. The article appeared in the March 2000 issue of "Laser Focus
> World," on pp20-24. For you non-subscribers the article can be seen on the
> web at:
> PUBLICATION_ID=12&Section=CurrentIssue
> Now I have had trouble emailing long URL's before, so if this isn't
> clickable, try to copy and paste it into the address box of your browser.
> And if that doesn't work you can go the the magazine's main web address
> and then select "current Issue" then World News" then the article
> "Interferometry, Fiber Coupling improves Gravimeters"
> George Bush

   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@.......



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Larry Cochrane <cochrane@..............>