Hi Steinar,
Outside of having the horizontal seismometer very mechanically rigid put together; almost every horizontal like yours I've set up seems to need
a metallic stress adjustment "settling in period of time" for the machine to adjust to its temperature environment; which can even be 2-3 months of time.
This can be true for a seismometer like yours and I've even also seen such on a smaller S-G; simple hanging mass gravity zeroing pendulum.
Chris Chapmans set screws advise is quite valid. I usually have taken it perhaps a bit further....and by that I literally mean, NO possible slippage;
or, the setscrews tightly prevented from ANY wobble movement with using other nuts/locking washers thereon to prevent ANY movement. If, you need to
literally tightly anchor such with using a one or two fixed or adjustable wrenchs...so be it.
Whats your natural seismometer period? Running too long a period on them makes them ultra sensitive to even very slight tilts. Try reducing the
natural period if the problem keeps recurring too often. Its quite possible you will have to run them lower like at 15 seconds; even though your
machine is likely capable of like up to 20-30 seconds; all the frequent leveling screw adjustments can just be too much to content with over time.
Big distant quakes will still temporarily tilt your machine anyway to whatever period they eventually radiate from their epicenter.
It also sounds like a "seasonal tilt"; which in escense is a outside environmental temperature change thats worked its temperature change way down
into the soil/rock surrounding your area....which for there is now summer. After the (~ 1 month) beginning onset of real winter cold, you'll see more sudden tilts.
Regardless of cause, all you can do is keep on recentering the pendulum; the tilt/s will be a constant very minor procedure you'll always have
to content with.....its just the actual nature of your seismometer/tiltmeter.
Hmmm....over time....you might even mentally "consider" switching the pivot to one like Chris Chapman has recently done; and that is putting a flat
on the end of the boom, and putting a fixed ball on the mast for better stability and operational results. Its "sounds" perhaps drastic for your new
seismometer; but I doubt if it is mechanically tough to consider/do/try.
Take care, Meredith Lamb
On Tue, Jul 29, 2008 at 12:42 PM, Steinar Midtskogen
<steinar@.............> wrote:
I've had a SEP seismometer running for a month or so and everything
has been fine. However, a couple of days ago while we were away on
vacation it began reporting no activity. When I got back home I found
that the mass had moved all the way to the side, so I had to change
the tilt a little so it would move back to the centre again.
Is this normal behaviour for a Lehmann seismometer? The mass moved
away from the center at a time the house was empty, so I can't really
think of any external disturbances which could have caused this.
--
Steinar Midtskogen
http://voksenlia.net/met/seismometer/
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