>>   I have a Mark Products L10B geophone. It has a metal case and is fairly well sealed. The lowest temperatures we normal get here is -10 C and inside the enclosure it will be slightly warmer.

Hi Ian,

     Those temperatures should not be a any problem at all.

>>   The SEQUA10 looks interesting, though there isn't much detail on the webside. Does it work by just boosting the lower frequencies?  

     You set up your geophone to be critically damped. The additional amplifier has a frequency selective gain which tailors into the damping curve. The gain down to 1/10 the natural resonance is increased to a max of x100 to compensate for the f^2 natural rolloff. The overall response stays flat with velocity from 0.5 Hz to your low pass filter rolloff of maybe 10 Hz. Below 0.5 Hz the amplification falls off sharply again.

>>   If so, I can adjust my programmable filter to do the same thing, never thought about that...  Presumably this lowers the overall sensitivity of the device(?). Would you move the upper frequency down too or keep it just above its natural frequency - 4.5 Hz?

     You will quickly run into digitisation step / noise problems if you try to do it that way, unless you have the overall amplifier gain set WAY UP. Can you even set up your filter to have a reciprocal f^2 response reducing to unity at about 9 Hz and falling again below 0.5 Hz? The overall output voltage sensitivity stays the same, just the bandwidth is increased. There is a small increase in the overall noise. The major ++ is being able to detect the P and S waves directly with a 4.5 Hz geophone. 

     The original Roberts' circuit used a high pass filter which was still sensitive to 1/f VLF noise. Using a modified filter circuit has removed most of that noise.

     Regards,

     Chris Chapman
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