As soon as I posted this note, I saw a serioius error: here is the corrected version. Delete the first version. Regarding locating a long period horizontal seis in a basement. Orientation of the sensitive axis: The LP seis is sensitive to floor/pier tilt in the direction that the mass moves, which is at right angles to the boom. So to minimize local tilt where a single instrument is involved, one has to orient the sensor so that the boom motion is at a right angle to the suspected tilt direction, ie. the boom axis is is in the direction of the tilt. Most LP "garden gate" seismometers are rectangular in design, so it is convenient to place the long axis of the box against the basement wall. This then has the mass moving towards or away from wall, which is the undesirable direction of greater floor tilting. If you can imagine the floor tilting as a slab with one edge fixed at a hinge, the foundation wall is the hinge. The LP seis sensitive axis should be at a right angle to this hinge. This means that the boom axis, or longer axis of the box, will extend out from the wall. A further reduction of slab deformity noise is realized if the transverse leveling or mass centering screws are at the mass end of the boom and closest to the wall. I am operating a prototype of the VBB-BBT (very broadband beam balance tiltmeter) in the basement of the farmhouse here. The east basement wall has been reinforced with cinderblock because of added floor beams since termites devoured the ends of the joists in the '40s. The room used to be the coal bin, so the floor is competent. The BBT is installed close to this wall, immediately in front of the pier for the VBB vertical shown in a photo on the web site, on a 1" high "pier" made with grouted ceramic tiles. The sensitive axis, the axis of the beam, is parallel to the wall, so going down and sitting beside it results in a tilt of only about 0.6 microradian, whereas today the barometric loading noise is running about 1 microradian P-P at a period of around 240 seconds. The BBT still senses large trucks on the street about 30 m away and parallel to the wall, with of the tilt impulse indicating the direction of travel at the 0.1 to 0.3 microradian level. Hopefully this will help you get quieter data from your LP horizontal seismometers. As always, I encourage the construction of vertical sensors, since they are relatively immune to pier tilt. Regards, Sean-Thomas __________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>