The Importance of a Proper
Seismometer Pier
You need to have a solid pier for your seismometer
to rest on, simply because if their isn't the seismometer will be tilting
in all manner of directions without it, which more or less nullifys all
the effort you may have put into the construction of your seismometer.
If you have a place on your home area or outside it
which has solid rock formations, and it can be sheltered from wind effects,
you may not have to do more than build up a level layer of concrete where
you can put your seismometer. If it can also be somewhat away from
trees which induce seismic noise, this would be even better. Additionally
if it can be away from roads, highways, train tracks or any source of noise
it would be better still.
If you're like most of us, meeting the above criteria
is not very likely; they would be rather choice, but impossible sites for
where we actually live. More likely you live in a home which either
has a basement or crawlspace, possibly with a attached garage or detached
garage. With a crawlspace you can build up your pier from the ground
up. Basement floors are usually concrete and you can build a pier
upon that. Garages, have the problem of their being used to park
cars therein (Tilting Effect). Even utilizing a piece of yard area
which is furtherest away from noise, might be a fair solution, but with
the string attachment of having to build a shelter over the pier/seismometer
to shield from weather effects.
Years ago, I used to live on the ground floor (concrete)
of an apartment house. I simply set the seismometer on the floor
furtherest away from doors and traffic. It worked pretty good, which
surprised me. Nowadays I have a home, and I do use my crawlspace
for a homebrew seismometer vault. I picked an area furtherest from
active household noise; i.e., below the bedrooms. I do see tilt noise,
the house transfers this via people walking on the floor near the bedrooms.
My pier/vault
Although my crawlspace vault is not considered ideal
in any sense, it does have to serve its purpose as is. I ended up
picking a corner of the house, where I built a shelter around the area
I wished to use to keep out air motion effects. I used common materials,
but the most important aspect was installing insulation in the wood walls
and building an extension away from the house foundation concrete to enable
air space insulation installation. Changing temperature effects on
a seismometer simply lead to its material expansion and contraction effects
on the output of the instrument. Over this I put on a wall, both
to keep the insulation therein, and to further add more insulation to help
slow down the invariable temperture changes which will occur.
My seismometers were rather tall, so to begin I had
to dig down about 6 inches in clay, and braced this pit, with wood laced
with a preservative. Next I leveled out the pit as best I could.
The clay here is rather moist year around, and is rather soft and rather
dirty to crawl around on (of course-ha). I next laid a thin layer
of dry premixed concrete over the bottom of the pit and leveled it as best
I could. After that came a sprinkling of water over the concrete,
to help accelerate the solidification from what moisture came from the
ground. Although not as strong as properly laid wet concrete, my
vault area is too far from the entrance to risk hauling buckets, spreading,
leveling, for the time would probably have solidified it before it was
done. It ended up more like a loose surface conglomerate, but strong
enough to support myself and the piers....kind of a foundation. After
drying out afew days, I next covered the entire floor area with a thick
plastic tarp, and ran it up the concrete and homebrew walls a foot or so.....this
is a groundwater moisture barrier, which prevents the piers from absorbing
water. Next came the piers building. Ideally a flat thick piece
of rock would serve the purpose, however that was not available and would
probably be impossible to put in the vault regardless.
I used common bagged premix concrete, stepping stones,
masonry concrete, to build up the piers on top of the plastic tarp.
First a layer of concrete perhaps 3/4" thick, then four wet stepping stones
measuring 20"X20" set in a square pattern adjacent to each other.
I leveled with a carpenters level with its bubble. I also filled
inbetween the edges to insure the pieces locking together. I let
this dry, acouple days.
I next laid masonry cement on the top of the pier
about 1/2" thick, and laid 4 more wet stepping stones with the cement between
the stones as before. As above, I let it dry a day or so. I
next added another final layer of stepping stones, but atop this I laid
out
porcelain tiles, also cemented to the underlying concrete
stepping stones. After drying out once again for afew days,
and painting with a latex/acrylic combination paint, the pier was basically
ready for the seismometer. An additional couple of weeks later, the
pier basically stabilizes from the concrete cure, and the seismometer doesn't
seem to need but only infrequent adjustments due to my clay water content
and/or seasonal ground changes.
In addition to the insulation mentioned, I also covered
the seismometers shield frame with insulation, both inside and out.
In effect, this makes the outside air, have to work its way through the
house/foundation, into the crawlspace. Then it has to work its way
through the vault wall, and finally the seismometer cover which in itself
is kind of a temperatue trap. Thus any rapid weather changes can
take quite some time to transverse the barriers, and by then the temperature
could oscillate around its natural cycles, with a lesser effect on the
seismometer output.
This was my vault/pier makeup. For additional
ideas I would suggest your reviewing the email files on the subject particulary
the 1998 first quarter email file. See, click the PSN main site of
Redwood City referenced on the main or home page. Therein are are
other individuals ideas and suggestions for your particular site or option.
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