To whom it may interest

I have been reading the comments on gravitation with great interest and it
started me wondering about plumb lines and if at certain locations
(assuming no interference/vibration) they hang truly vertical all the time
- i.e. pointing directly through the centre of the earth, and even change
due the core rotating at a different speed to the crust. Likewise if a
skyscraper is truly vertical at one moment, could it not be vertical at
another time as compared with a plumb line... I know the amount by which it
would be out of plumb would be so insignificant as not to matter in the
real world.

I enjoy reading the discussions that the group put up and one day, GV, I
hope to take a more active role in the earth sciences


------------

Ted Rogers.

      

----------
> From: Tom Schmitt 
> To: psn-l@..............
> Subject: Re: Article on gravimeters
> Date: Monday, May 01, 2000 4:40 AM
> 
> 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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