PSN-L Email List Message

Subject: RE: strainmeters/creepmeters
From: "George Harris" gjharris@.............
Date: Wed, 24 May 2000 20:38:27 -0700

> [Original Message]
> From: S-T Morrissey 
> To: 
> Date: 5/20/00 7:43:15 PM
> Subject: strainmeters/creepmeters
> Jim,
> The first task when considering strainmeters or tiltmeters is just
> what range of strain or displacement is expected over what period of
> time. 

> Among the best strainmeters are the 750 meter baseline laser
> systems at Pinon Flat that operate in evacuated tubes and have massive
> end monuments with "optical anchors" thru the decomposed granite of the
> surface to virgin rock 20 meters down. They have achieved stability of
> better than 10^-6 / year but at great expense and effort.
The above started me thinking about what type of an optical system might be
The follwing seems like it should work, and is not too difficult to try.

It would use two laser pointers of the penlight size and a pair of
receivers consisting 
of two solar cells each.  The lasers woud be positioned on one side of the
fault on a 
pier with their beams at 90 degrees to each other,and at 45 degrees with
respect to the
fault.  On the other side of the fault would be the two receivers, one on
laser beam.  The solar cells should be placed horizontally adjacent to each 
other so that the laser is half on each cell.  The cells are connected so
that the 
plus of one is connected to the minus of its adjacent cell.  These two
connections go 
directly to the plus and minus input of an opamp with suitable feedback
resistor.  When
the beam is half on each cell, the output will be zero, and the sensitivity
can be
adjusted to microinch sensitivity by proper resistor choice and cell

It must be assumed that there might be some rotary motion of the laser
pier, so use of 
two beams would detect this and show similar signals on both beams.  It
would probably 
be necessary to put some heavy filtering in the system so that atmospheric
effects due to 
wind or heat would be smoothed out. The system should measure both
components of the motion. 

I'd be happy to work with someone who would wish to try this.  I have one
of the pointers,
and all of the other components immediately available.  In fact, a single
laser with a 
beam splitter is probably even easier.  I have that too.  Anybody

--- George Harris (San Francisco area)
--- gjhar


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