> [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 interferometer > 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 easier. 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 each 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 positioning. 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 interested? --- George Harris (San Francisco area) --- gjhar __________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>