A friend of mine has co-developed a laser seismograph. Really, it is a strain meter. The way it works is to lay out a long line of fiber optic cable and bounce light signals. One end is reflecting, while the other end has a transmitter/receiver. The "time of flight" variations give you the change in length. Dividing the change in length by the over all length gives the strain as a function of time. His large prototype is buried in his back yard in Kansas... He caught on to this while gabbing with an engineer who uses this same technique to measure strain in structures (eg. bridges) and apparently it works quite well. They are working on a proposal to rent fiber optics communications lines and hook this stuff up...it could be a lot of fun. The longer term arm-waving goal is to eventually measure precursory strain transients near an active fault (eg. San Andreas). They could also work on other fun problems with sufficient sensitivity, such as the Chandler wobble... Most people I have known prefer an interferometric method to time of flight methods, but I am not sure how each technique stacks up against the other with the current technology. The interferometric method uses a steady transmitted signal and measures the relative interference of the waves to back out the changes in length. For a laser with a wavelength around 500 nm, the resolution is a small fraction of this. Averaged out over a very long cable, this would allow very small strains to be measured. The nice thing about these types of instruments is that they are truly broadband...limited only by the number and frequency of measurements. The astronomers working on binocularizing the Keck scope have run into difficulties in connecting the two sources via an underground tunnel...this also uses interferometric methods. I wonder how they are doing now; whether they have fixed this problem or not. It may be interesting to find out how they fix it all, since the Keck could easily become a strain meter as well as a bad ass telescope when they do figure it out. Cheers! John Hernlund Department of Earth and Space Sciences University of California, Los Angeles http://geodyn.ess.ucla.edu/~hernlund/ hernlund@............ __________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>