Dave,

The main reasons for burying geophones are
 - keeping them away from the wind
 - keeping them away from raindrops (which are very noisy)
 - reducing air-borne environmental noise
 - providing a firm planting base
 - avoiding the slow weathering layer

Temperature is not much of a factor for geophones -- neither is 
barometric pressure.

Before planting the geophone be sure to fully understand which of the 
three axes are doing what.  Also, test the polarity in advance.  Don't 
plant the geophone until you are 100% sure.  Take lots of pictures and 
label all the electrical outputs ahead of time.  In oscilloscope is very 
helpful in figuring out the polarities.

The important thing is making sure the geophone doesn't leak.  Apply 
exterior grade (typically black) silicone over all the cracks, openings, 
and cable-entry.  Allow it to dry fully -- this usually takes a couple 
of days.   I would recommend digging down at least a foot or two, 
preferably stopping when you hit something more solid than sand or 
topsoil. Plant the geophone making sure to note the orientation (align 
the big arrow to true north) and level.  Then, back-fill the hole, 
ensuring that the geophone's orientation and level is unchanged.  I 
would personally stay away from PVC pipe as it has the chance of acting 
as a conduit for surface noise. PVC is good for retrieving deeply-buried 
geophones (10m), but for something that is a 3-4 feet deep, I wouldn't 
worry about it.  

The geophone cable is usually very durable so long as it doesn't get 
tripped over.  After years of getting sunlight, it will start to weaken 
and crack.  For industrial installations, I usually run geophone cable 
through rubber-coated liquid-tight conduit.  If it were my back yard, I 
wouldn't bother, but I would probably dig a shallow trench to avoid 
having the kids trip on it.

Regards,
Henry Bland


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