PSN-L Email List Message

Subject: Re: Digging in...
From: Dennis Wieck dwieck@............
Date: Fri, 12 Aug 2005 21:29:23 -0500

ChrisAtUpw@....... wrote:

> Hi Dennis,
>     Is it a timber house or one with brick/stone walls?

Its brick

> 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. 
> 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 
radios grounding.

> 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 
> available.

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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