PSN-L Email List Message

Subject: Re: Article on gravimeters
From: "Tom Schmitt" tschmitt@..............
Date: Tue, 9 May 2000 10:58:54 -0400

A couple of comments.

I am begining to remember more about the device that was talked about at the

Some one said tens of millions of dollars.  I suspect that would be a
minimum.  The device
relied alot on technology that the Navy had been working on for decades.
They had very precise
accerometers in the 1950's.  Nuclear subs have used inertial navigation
since the begining and that
was in the 1950's.   The gradiometer had  GPS though, that was before the
commercial units
( this was in 1984 or 1985).

The rotating accelerometers were on an inertial table.  That is a table that
had three gyros and at least
two accelerometers on it.  This is importantant because basically what the
device does is
measure the direction of the vector we call "DOWN".  The magnitude measured
is the difference in g
over twice the rotational vector.

For the direction of down to have any special meaning ( we can measure that
with a rock :)  ) we have
to compare it to the direction that we think down should be.  We know what
direction it should be
from our assumtion of the shape of the earth ( the elipsoid ).  Now the
second tricky part is having an
independent frame of reference at the site which tells us where down is
based on the elipsoid.  That is
where the inertial table comes in.   The gyros etc keep track of the presice
orientation of the table
with respect to an inital calibration points ( more likely points along the
way previously established
by classical geodetic methods).

Bumps by the way are canceled out more or less.  The sudden acceleration due
to a bump is more or less the
same at both accelrometers.  Since it is the difference  between
acceleration at the two units that is calculated, the bumps tend to camcel.
Tend to because they would have to have the same responce at the range of
acceleration tey are
subjected to and the disk would have to be perfectly stiff (  relative to
the integration time of the measurement).

I was interested in teh device because at the time I had considerable
interest in the 1886 Charleston earthquake.
I wanted to get it down to Charleston to see if there was any basement
expression of the faults Paradeep Talwani
an Don Calhoun had propsed in the Charleston Area.

Tom Schmitt



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