Hi Greg,

I doubt your a/d board will be capable to record low frequencies.
Usually audio board has a high-pass filter at about 10Hz.
This cut out all low frequency needed for earthquake recording.
If you put a large capacitor inseries to the input this can only
decrease the low frequency response.
The only way you may have in recovering low frequencies
is to use a digital inverse filter and use Lehman sensors instead
of geophones.
This because horizontal long period sensors (like lehman) are
very sensitive to the low frequencies and give a lot of signal
that sometime need to be cut-out with HP filters.
Geophones has poor performances at low frequencies
so they are absolutely not adequate for a standard audio board.

Some expensive audio board has a hardware register
to setup the frequency of the internal HP filter and could be used.
You should refer to the manufacturer datasheet and see what you
can do with your board.

Regards
mauro



At 20:30 18/07/2005, you wrote:
>I installed some larger-capacitance-value coupling
>capacitors on my 24-bit sound card so I could resolve
>lower frequencies.  What is a good datalogging app to
>use with this?
>
>Also, I put out a request for geophones and got one
>response and never heard anything back.  I am looking
>for geophones - tell me about what you have -
>dimensions - frequency, etc., and how much you want
>for them.
>
>Thanks,
>
>Greg
>
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