Chris, I'm sorry that I never posted the AGU abstract. I still think that I can get much more done than reality allows. I used to be able to run a dozen projects at the ends of the earth, but my disabilities are now an unfortunate limitation. And then someone shared the two-week flu that is going around. My major effort in early October was to complete the preliminary technical report on the beam-balance tiltmeter: a very difficult effort to define the project from all the theoretical, mechanical, and electronic aspects, as well as to present some very initial data. The report runs 60 pages of text and 40 of figures, and none of it is in a condition to mark up to HTML. I have sent it to those who I thought could provide a really critical review, but so far I have only got back "nicely written ... as far as I got ... about page 5". So I guess that if it was complicated for me to write, maybe it IS unreadable. I had hoped for (and they were anxious to provide) good criticism from ASL (the national Albuquerque Seismological Lab.) but they are in the process of relocating because their lease on the reservation went up by over 10X. They moved their servers, etc, on Oct. 18. I don't know the fate of the ANMO test site (a very quiet deep tunnel) where we were planning on testing the new tiltmeter. (So I am setting up a side-by-side test with an STS-2 with an 8-channel, 24-bit digitizer in my remote vault.) Another problem came up last week when the leaf-spring vertical seis in the basement here bonked; details next. Here is the AGU abstract. I have submitted it with Dr. Mitchell as a second author and presenter, since I don't think I will be able to attend; but I am looking to attend the SSA meeting in SFO in April with new data. Regards, Sean-Thomas ________________________________________________ An abstract for the fall 2000 AGU meeting. A Beam-Balance Broadband Tiltmeter That is Insensitive to Horizontal Acceleration. Sean-Thomas Morrissey (sean@............ Brian J. Mitchell (mitchell@............ (Dept. of Earth and Atmospheric Sciences, St.Louis University, 3507 Laclede; St. Louis MO, 63103; 314 977 3129; A new beam-balance tiltmeter has been developed that does not respond to horizontal ground movement. Since all horizontal seismometers are also tiltmeters they are sensitive to tilt, especially at longer periods, and seismic data can be compromised in the period range of 20 to 3000 seconds. The most important noise source in the horizontal data is tilting of the pier, mostly due to barometric loading. But all tiltmeters, up to now, are also seismometers, (ie. they are sensitive to horizontal translation), so can not be used to separate tilt noise from seismic signals. Our new beam balance tiltmeter does not respond to horizontal acceleration because the masses at each end of the horizontal beam are suspended through the exact center of mass. This system is inherently unstable, so broadband feedback is used to control it. With appropriate feedback, the beam remains relatively horizontal when the base is tilted, and the output is the relative motion at the displacement detector. There is no rotation or output when the base is translated horizontally along the axis of the beam. The beam-balance tiltmeter is designed with separated lead masses mounted in an aluminum bar that is suspended exactly at the center of mass of the horizontal beam with a new low torque hinge flexure. The center of mass is trimmed by a unique vertical mass centering adjustment above the flexures. Displacement transducers and compact force feedback coils with rare-earth magnets are placed at both ends of the beam. Three sensors have been assembled as "proof of concept" prototypes. One instrument is operating at station CCMO, near Saint Louis University, and data from it are being digitized. Another is on a table and can be used to demonstrate that tilt can be separated from horizontal acceleration by simply sliding it horizontally. The static or DC tilt sensitivity of the prototype is about 120 millivolts per microradian, and the resolution is better than 0.1 nanoradian. Initial comparisons using data generated by large quakes on the horizontal components of nearby broadband seismic stations (18 km distant) show that the response to horizontal acceleration is reduced by a factor greater than 1000 while maintaining the equivalent tilt sensitivity of the seismometer. Ideally, the noise recorded in the tiltmeter output will exactly emulate the tilt noise from the seismometer, at least in the flat portions of their broadband velocity response. The success of this new instrument has a tremendous implications for broadband stations in all regions of the world where tilting from barometric, thermal, hydrologic, etc., effects may limit the usefulness of the horizontal data. ____________________ __________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>