PSN-L Email List Message

Subject: Re: My seismometer vault solution
From: ChrisAtUpw@.......
Date: Sat, 28 Oct 2006 19:14:32 EDT


In a message dated 2006/10/28, geodynamics@....... writes:

> The meltwater and rain percolation are just the things I'm concerned about. 
> It is much more efficient to seal the small housing of the geophones than it 
> is to seal a plastic bucket lid and the access hole. 

Hi Tim,

       I tend to use a belt + braces policy - two barriers are better than 
one. Also, you may need something for the geophone housing to sit on and dry 
sand is just fine.
       It is not too difficult to make a good seal to a plastic bucket with a 
clip on lid. You can easily buy tank connectors and plastic water pipe. 
Alternatively, waterproof cable gland seals are readily available. 

As long as Jon gets rid of the external splices (1-meter pigtail coming out 
from the 
> geophone case) the geophone case should be sufficiently sealed to easily 
> withstand the local climate and long-term burial. If he chooses to leave the 
> splices then all bets are off because he's probably left a channel for moisture 
> migration into the geophone case.

       The way that I deal with splices is to fit adhesive heatshrink to both 
the wires and to the main cable. You slide heatshrink over one end, solder 
the two wires, slide the heatshrink over the joint and shrink it onto both wire 
insulations. Then you join the other wires etc, slide the outer heatshrink 
over the bundle of joins and shrink it. This gives a waterproof seal. Adhesive 
heatshrink is a bit more expensive, but you don't need much of it - and it works 
fine!  If you want added rigidity, you can use two overlapping heatshrink 
layers on the main cable. 
       I prefer the alternative of replacing a short length of cable when 
possible. I strongly suggest using multicore cable with a woven copper screen. I 
don't like the CAT5 cable very much for this job - it has a single copper wire 
cores and usually no screen. It doesn't take many repeat bends for a wire to 
break - the copper is hard drawn.  

> Frankly, if it was mine, I wouldn't even bother with the plastic bucket. 
> However, for Jon's installation the bucket does afford some additional 
> protection, particularly when it comes time to dig it up again. I've had countless 
> installations, both surface and subsurface for monitoring of both ground and 
> structural vibrations. Many were in-place over a period of years using geophone 
> cases very similar to the one Jon is using. We've never had a problem with 
> moisture migration but we've always been careful to seal the cases with 
> non-corrosive silicon sealant. One installation had triaxial sets about 20-feet 
> below the groundwater level for over two years and they are still being used on 
> other jobs, some 10-years later. Here in Michigan, we don't have the extremes 
> that Iceland does but I 'd suspect that we have more freeze-thaw cycles -- 
> and that's what typically causes the greatest problems.

       For sealing the case, I use an adhesive mastic called BluTack or 
WhiteTack. I don't know if you can get an equivalent product. It is like a non 
drying slightly tacky putty and is sold in stationery shops etc for mounting 
posters to notice boards, to wallpaper.... any temporary job. It sticks well 
enough, but you can peel it off and it does not leave a mark. A rolled ring of this 
makes an excellent waterproof seal for a geophone case. You can demount the 
join, re roll the ring and use it again - unlike silicone rubber. If you use 
silicone rubber, it must be the variety which sets with no smell. The type which 
smells strongly of acetic acid / vinegar should NOT be used - it corrodes 
copper wire like fun.
       You can buy porous paper sachets containing silica gel in a range of 
sizes. Some may have a color indicator to tell you when they need to be baked 
to remove absorbed moisture. Placing a drying sachet inside the geophone case 
should prevent condensation problems.  DON'T use lime or calcium chloride. An 
alternative method if you have a chest type of Deep Freezer, is to separately 
lower the lid and the case of the geophones slowly over a few minutes to the 
bottom of the freezer and assemble them there. So long as you don't 'stir it 
up', the air inside a chest freezer is cold and very dry - in spite of any ice on 
the sides. You have to be careful not to brush off any ice crystals into the 
geophone case during assembly.
       The temperature 20 feet below ground will be above freezing and nearly 
constant, summer and winter. Jon lives quite close to the shore in northern 
Iceland. I would expect the ground to freeze, to maybe over two feet down, but 
the surface temperature changes to be quite variable around zero - due to the 
ocean and the wind direction. The results of likely surface freeze / thaw 
cycles was concerning me. A sealed plastic drum part filled with dry sand should 
be OK.
       See climate links in http://www.vedur.is/english/

       Regards,

       Chris Chapman
In a me=
ssage dated 2006/10/28, geodynamics@....... writes:

The meltwater and rain percolat= ion are just the things I'm concerned about. It is much more efficient to se= al the small housing of the geophones than it is to seal a plastic bucket li= d and the access hole.


Hi Tim,

       I tend to use a belt + braces policy -=20= two barriers are better than one. Also, you may need something for the geoph= one housing to sit on and dry sand is just fine.
       It is not too difficult to make a good=20= seal to a plastic bucket with a clip on lid. You can easily buy tank connect= ors and plastic water pipe. Alternatively, waterproof cable gland seals are=20= readily available.

As long as Jon gets rid of the external splices (1-meter pigtail coming out=20= from the

geophone case) the geophone ca= se should be sufficiently sealed to easily withstand the local climate and l= ong-term burial. If he chooses to leave the splices then all bets are off be= cause he's probably left a channel for moisture migration into the geophone=20= case.


       The way that I deal with splices is to= fit adhesive heatshrink to both the wires and to the main cable. You slide=20= heatshrink over one end, solder the two wires, slide the heatshrink over the= joint and shrink it onto both wire insulations. Then you join the other wir= es etc, slide the outer heatshrink over the bundle of joins and shrink it. T= his gives a waterproof seal. Adhesive heatshrink is a bit more expensive, bu= t you don't need much of it - and it works fine!  If you want added rig= idity, you can use two overlapping heatshrink layers on the main cable.
       I prefer the alternative of replacing a= short length of cable when possible. I strongly suggest using multicore cab= le with a woven copper screen. I don't like the CAT5 cable very much for thi= s job - it has a single copper wire cores and usually no screen. It doesn't=20= take many repeat bends for a wire to break - the copper is hard drawn. =

Frankly, if it was mine, I wou= ldn't even bother with the plastic bucket. However, for Jon's installation t= he bucket does afford some additional protection, particularly when it comes= time to dig it up again. I've had countless installations, both surface and= subsurface for monitoring of both ground and structural vibrations. Many we= re in-place over a period of years using geophone cases very similar to the=20= one Jon is using. We've never had a problem with moisture migration but we'v= e always been careful to seal the cases with non-corrosive silicon sealant.=20= One installation had triaxial sets about 20-feet below the groundwater level= for over two years and they are still being used on other jobs, some 10-yea= rs later. Here in Michigan, we don't have the extremes that Iceland does but= I 'd suspect that we have more freeze-thaw cycles -- and that's what typica= lly causes the greatest problems.


       For sealing the case, I use an adhesive= mastic called BluTack or WhiteTack. I don't know if you can get an equivale= nt product. It is like a non drying slightly tacky putty and is sold in stat= ionery shops etc for mounting posters to notice boards, to wallpaper.... any= temporary job. It sticks well enough, but you can peel it off and it does n= ot leave a mark. A rolled ring of this makes an excellent waterproof seal fo= r a geophone case. You can demount the join, re roll the ring and use it aga= in - unlike silicone rubber. If you use silicone rubber, it must be the vari= ety which sets with no smell. The type which smells strongly of acetic acid=20= / vinegar should NOT be used - it corrodes copper wire like fun.
       You can buy porous paper sachets contai= ning silica gel in a range of sizes. Some may have a color indicator to tell= you when they need to be baked to remove absorbed moisture. Placing a dryin= g sachet inside the geophone case should prevent condensation problems. = ; DON'T use lime or calcium chloride. An alternative method if you have a ch= est type of Deep Freezer, is to separately lower the lid and the case of the= geophones slowly over a few minutes to the bottom of the freezer and assemb= le them there. So long as you don't 'stir it up', the air inside a chest fre= ezer is cold and very dry - in spite of any ice on the sides. You have to be= careful not to brush off any ice crystals into the geophone case during ass= embly.
       The temperature 20 feet below ground wi= ll be above freezing and nearly constant, summer and winter. Jon lives quite= close to the shore in northern Iceland. I would expect the ground to freeze= , to maybe over two feet down, but the surface temperature changes to be qui= te variable around zero - due to the ocean and the wind direction. The resul= ts of likely surface freeze / thaw cycles was concerning me. A sealed plasti= c drum part filled with dry sand should be OK.
       See climate links in http://www.vedur.i= s/english/

       Regards,

       Chris Chapman

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