Ted

The stories about plumb bobs not hanging vertical are wonderful.  The
British Royal Surveyers in India
found the plumb bobs leaning away from the mountains.  They figured this out
because the distance between
two points was different when measured by astronomical and standard survey
techniques.  There is a little more
to the story and this effect was  known before that.  However  that is the
example used in text books because the
story is so good.  (So were the surveyers, thousands of miles and they were
worried about less than few meters errror
if someone has a text book fill us in on the ammount of error they found).

By the way, plumb bobs (lacking wind electostatic and or magnetic
influences) always point in such a way
to minimize their potential energy.  Because of density variations in the
earth, that may not be "down".
The ammount of variation is slight and insignificant for most purposes.
It is important for scientific purposes, it is important for  very critical
surveys is a few areas in the world,  and it is very important for launching
nuclear weapons  at people.  The later is why the Defense Maping Agency is
concerned with it.  The missles follow potential energy paths, which may be
curved relative to other coordinates.  If you want to hit dead on you need
the shape of gravity field between where you are and where you want it to
hit.

Tom Schmitt

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




----- Original Message -----
From: Ted Rogers 
To: 
Cc: Ted Rogers 
Sent: Monday, May 01, 2000 12:54 AM
Subject: Re: Articles on gravimeters


> 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/
> > >
> > >
> >
>
****************************************************************************
>
> > **
> > >
> > > __________________________________________________________
> > >
> > > Public Seismic Network Mailing List (PSN-L)
> > >
> > > To leave this list email PSN-L-REQUEST@.............. with
> > > the body of the message (first line only): unsubscribe
> > > See http://www.seismicnet.com/maillist.html for more information.
> >
> > __________________________________________________________
> >
> > Public Seismic Network Mailing List (PSN-L)
> >
> > To leave this list email PSN-L-REQUEST@.............. with
> > the body of the message (first line only): unsubscribe
> > See http://www.seismicnet.com/maillist.html for more information.
> __________________________________________________________
>
> Public Seismic Network Mailing List (PSN-L)
>
> To leave this list email PSN-L-REQUEST@.............. with
> the body of the message (first line only): unsubscribe
> See http://www.seismicnet.com/maillist.html for more information.

__________________________________________________________

Public Seismic Network Mailing List (PSN-L)