PSN-L Email List Message

Subject: Re: Geophone layout
From: John Hernlund hernlund@.......
Date: Thu, 20 Jan 2000 10:25:29 -0700 (MST)

On Thu, 20 Jan 2000, Thomas wrote:
> I am 4th year student working a project using geophones to measure ice
> thicknes. Is there a special method to layout the geophones to get an
> accurate profile an induced vibration? The purposal is to generate an
> acoustic pulse through possibly several kilometers of ice and have the
> geophones detect this pulse and derive the ice thickness. Would anyone have
> suggestions on a detonation mechanism? I've heard people use 1 kg of
> dynamite for 1 km of ice.

Yes, there is a general rule of thumb, but dynamite in ice will probably be
messy, not to mention all the permits and other difficulties involved.  In
addition, if you are recording on a triggered system you may destroy the
trigger each time...  The hard thing in explosives is that for the energy to
be transmitted into the solid they must be buried to a certain depth, so that
you aren't just blowing up the ground.  However, a deeper source will give you
a slightly more complicated waveform and at greater depths reflections become
broader as reflections from the surface are super-imposed onto the signal.
Also, far better results are obtained from various different shot locations.

> As you can see, I am not too familiar with geophones but I do know they work
> like an accelerator. If anyone knows of a good book, please suggest it
> because none of our group members are geology people.

Honestly, if you are able to switch methods I would strongly suggest using
electromagnetic methods, which will be able to discern changes in conductivity
between ice and rock quite well.  You won't have to deploy fidgety geophones
and worry about the complicated routine of getting a nice source.  The
drawback here is the cost of the equipment, unless you can borrow it some how. 
We use a $60,000 instrument from Zonge Engineering in Tucson, which has a very
good square wave generator (quick turnoff).  Several methods exist that simply
require you to make a loop on the surface and place an antenna in the middle
(another coil or special device).  A square wave is sent through the large
loop and the response recorded on the antenna can be fourier transformed to
give a conductivity vs. depth profile.  The deeper you want to probe, the
larger the outer loop must be.  Depending on the resistance of your loop, at
some point a large loop will require a gas powered generator to drive the
current, because batteries will only do so much.  But all of this can be put
into a single vehicle and carted around.  The moving of the coil is the
hardest part.

There are many academic books on the method of reflection seismology, but from
experience many people can tell you that there are a lot of practical
difficulties that are mostly learned from doing it.  Some of these problems
will be specific to the equipment you are using. 

John Hernlund
E-mail: hernlund@.......



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Larry Cochrane <cochrane@..............>