PSN-L Email List Message
Subject: Re: Digging in...
From: Dennis Wieck dwieck@............
Date: Fri, 12 Aug 2005 21:29:23 -0500
> Hi Dennis,
> Is it a timber house or one with brick/stone walls?
> How many stories?
> Are they the 4.5 Hz geophones?
> So long as you stay away from the foundations, you should not get
> too much house noise. I suggest that you try putting the geophone
> under the house with the three legs pushed firmly into the soil and
> see what results you get. Look for significant changes in the
> background noise between no wind and strong wind conditions and keep
> some recordings for reference. Also look out for spikes and
> interference signals, traffic or other noise over 24 hrs.
The good news is I am 200 yards from the nearest road ( except for what
comes in my driveway)
> I connected my geophones up to the amplifier in a portable radio and
> listened with headphones. OK, these signals aren't the seismic ones,
> but you can often recognise noise sources by ear easier than trying to
> figure them out from the traces. I start to hear fast cars on the main
> road at about 1 km. There is a slightly depressed water drain on the
> main road about 100 yards away which gives thump signals occasionally
> with passing lorries..... Check for slamming doors and windows, fridge
> start/stop, cooking loads, central heating timing, wind noise.....
> If the under house installation is OK, you can scoop out several
> inches of the topsoil and lay a 3 ft square concrete base in a wood
> frame. You need to cover it with polythene to keep it 'wet' for maybe
> a month to 'cure' fully. You use a 50:50 cement and sand mixture, no
> gravel. Professional installations use a vibrator to remove air
> bubbles from the wet mix after it is poured. You can get a lot out
> with a stick. Cement can be quite corrosive in contact with metal, but
> you can use special paints. You will have to leave adequate space
> for the thermal / draft screening box if you are definitely going to
> use a SG horizontal; maybe a longer slab?
> The second option would to build a small vault probably dug in
> the ground 3 or
> 4 feet deep. ( Most likely I would not be able to get to bedrock).
> I have seen
> several of this type on the web. I live pretty much in the country
> on about 5
> acres. Most of the land is pretty hilly with a lot of trees.
> You would have to make it water proof / adequately drained / dry
> inside. See various websites for ways of doing this.
> http://www.guralp.net/services/stations/eskdalemuir/ is a professional
> example in soil which saturates with water. See also
> I have seen references saying that you need to be relatively far
> from trees etc or the wind moving them will affect the sensors. If
> I do this I would have further to run the cables back to the
> house. Is it better to have the longer run from the sensors to
> the boards or to have the A/D output have the longer runs?
> Trees may give broad band noise, but this may be more serious for
> the longer period instruments. Aim to keep your sensors at a distance
> equal to the height of the trees, minimum. This is very roughly the
> expected extent of the roots.
This is about impossible on my lot. Even the under the house spot would
not meet this but would probably be the closest.
> Exposed houses will also experience wind noise.
> You have the problem of long cable runs and probably
> some lightning protection required for installations away from the house.
I am used to this. I am a ham and have dealt with tower and coax to my
> Some soils have such poor electrical conductivity / are so dry that
> the effective 'electrical ground surface' is several feet below ground
> level. All your house electrical wiring is effectively sticking out
> above the electrical ground! If lightning is a severe local hazard,
> you can use intermittently charged batteries and a length of fibre
> optic cable to isolate the digital signals. There are several websites
> with advice on this for various states. You can bury cables in plastic
> water pipe. Pipe end fittings designed for water tanks are readily
Is there a problem with moderate ( probably less than 100 ft) runs
between the geophones and the amp/filter/A/D or do I need to remote them
( that adds to the waterproof question)?
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