Bob,

Thank you for your comments.

Your first comment is very interesting.  The boom is a pendulum.  In order
for it to be in balance as a "gate" it must first be in balance as a
vertical pendulum.  I first found the vertical balance point of the entire
boom assembly.  I then suspended the assembly in the Lehman "gate" position.
I marked the suspension point and cut a shallow, vertical groove in the face
of the support.  I adjusted the position of the seismometer until the boom
centred.

I find very little drift in the two horizontal instruments.  I may have to
adjust the physical instruments once every four to six months.  The
magnitude correction in Winquake needs to be adjusted a little more often,
but not by much.

As to your second point, I cannot say very much more, except that a vertical
knife edge support of a balanced pendulum eliminates any source of noise
from vertical displacement.

Martin Brewer


_____________________________________________________________________

Public Seismic Network Mailing List (PSN-L)