PSN-L Email List Message

Subject: long period seis orientation
From: sean@...........
Date: Sun, 3 Dec 2000 14:58:50 -0600 (CST)

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 is at a right angle
to the suspected tilt direction.

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.


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