PSN-L Email List Message

Subject: Re: Article on gravimeters
From: George Harris gjharris@.............
Date: Mon, 08 May 2000 21:12:20 -0400

John Hernlund wrote:

> On Mon, 8 May 2000, Charles R. Patton wrote:
> > Good work, John.
> Thanks, just a half hour out of my day.  I'll post the whole derivation some
> day soon when I can recall what "time" means.  The point of my analysis was to
> get only at the gradient, not the absolute field.  The absolute field is noise
> for gradient determination.  I think the idea though is that the gradient is
> far more useful in "resolving" sub-surface structures.  I am still skeptical
> about the claim that these out perform seismic reflection techniques in terms
> of time and quality though.  Who knows, maybe they are right.  I can think of
> a million academic applications for this thing once it reaches the relatively
> poor and destitute realm of science (as compared to defense and oil of
> course).PSN-L-REQUEST@.............. with
> the body of the message (first line only): unsubscribe
> See for more information.

George Harris suggests:

Based on experience with inertial navigation and air bearings, I believe that I
can sow some light on what was probably used in the rotating gradiometer.

First, as some may know, inertial navigation depends on the use of very precise
gyros to stabilize a platform about all three axes.   Mounted on the platform are
two very precise calibrated accelerometers which measure the two compnents of the
horizontal acceleration.  The outputs of these acceleromaters is doubly
integrated, and used to correct the vertical orientation of the platform by
precessing the gyros.  Back in the 50's, such a platform could be stabilized to
something like a minute per hour or less.  By now they should be much better.

On this platform, supported in a precision air bearing, would be a second platform
which could be rotated at a desired speed.  This would have two of the same
accelerometers (or perhaps four) with pairs on opposite sides of the platform.
When rotated, they would show identical accelerations as long as the vertical did
not move.  A motion of the vertical would affect the opposite acclerometers in
opposite direction,  one acceleration would be slightly increased the other
decreased.  By subtracting the two,  the vertical shift  would be known.   Any
lateral acceleration would also affect the diference, but this would be at a
frequency other than the rotation frequency.   Proper data analysis should be able
to separate out the differences which are at the rotation frequency, and these
would be the gradient changes.

With GPS, the whole thing should be much easier.  In the original system,  long
term corrections were applied by looking at the stars.  Yes, they can be seen in
the daytime if you know about where to look with.
with proper sensors!  A precise GPS system should be able to cancel out most of
the lateral acceleration by doing a double differentiation on the GPS position and
subtracting from the measured acceleration.

The whole thing sounds like a fascinatimg, expensive program.


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