PSN-L Email List Message

Subject: Questions from a novice
Date: Fri, 14 Sep 2001 11:38:00 +1100


My name is Dale Hughes and I have been receiving mail from this list for
some time
and have found it very interesting. I have almost completed a basic system
consists of the following:

- A home made vertical geophone with a natural frequency of about 4Hz,
- An amplifier system which can give me a voltage gain of up to 2e7 or 146
- 4 pole low pass filter with selectable cutoff frequencies of 20Hz, 10Hz,
5Hz, 2Hz & 1Hz,
- Software written in LabVIEW 5 running on a 486PC to acquire data at up to
20HZ using
  a 12 bit ADC card.

The geophone uses 2 rare earth magnets attached to a 375gm aluminium
cylinder supported
by a spring inside a 50mm tube. The pickup coil is 12000 turns of fine wire
on the outside
of the tube. Damping has yet to be optimised and is currently acheived by
felt strips
stuck to the side of the Al cylinder so that friction brings the mass to
rest in a few 
seconds - otheriwse it oscillates for many minutes.

I live in a rural location about 30KM South East of Canberra in south
eastern Australia
and plan to install the geophone several hundred meters from the house. This
location will be
about 1KM from the nearest road.

I have made test recordings over the last few weeks while getting the system
and have recorded what, to my untrained eye, look like seismic events -
the recent event near Indonesia. Even with the geophone siting on the desk
in the study
the system appears to be very sensitive and is driven into saturation by
footsteps within
a few meters.

So my questions are:
- Is the above system likely to be sensitive enough for useful work?
- I have run the system with the LPF at 5Hz and logging at 10Hz, is this a
- Any other suggestions?

Many thanks

Dale Hughes  VK2DSH

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