In a message dated 06/08/2000, gmvoeth@........... writes:

I  am interested in knowing the cheapest way possible to build your own  
Geophone that consistently receives teleseismic signals over a long  period of time



Hi there,
 
    The cheapest seismic sensor that I know of  is a piezo sounder disk with 
a weight attached to the centre. Look for a 41  - 50 mm disk with a natural 
capacity from 40 to 100 nF. I get a sensitivity out  to over 4 sec this way and 
it gives rather more output than a traditional  geophone. The disk I use costs 
about $1. I stick a screw head to the centre  of the disk with modified 
acrylic twinpack glue (not epoxy), mount it in a tube,  screw on a 1.5" cylindrical 
weight ~50 gm and mount a V suspension at the free  end of the weight. You 
can also use 1/2" shim strip.
 
    This gives a predominantly single axis sensitivity,  you can use it 
either vertically or horizontally and it acts as a sensitive  accelerometer. My 
loaded disk has a natural resonance at ~170 Hz, so damping is  not a real 
problem, so long as I use a low pass filter. However, you can damp it  with 
sorbothane, polyurethane foam, or magnetically.
 
    I use a low noise CMOS opamp with a synthetic  impedance of ~100 M Ohms 
immediately followed by a high pass filter set  to 7 sec. You do have to 
provide fairly good thermal isolation, as the  disk behaves like a very tiny battery 
as the temperature of the PZT  drifts. You need to mount it in a dry 
container, preferably with some  silica gell and have / provide electrostatic 
screening. 
    You need two signal diodes connected opposite  ways across the disk to 
protect the input of the opamp. The disks on their own  can generate many 10s of 
volts if knocked ~ instant death to an opamp. Remember  that the ordinary 
glass silicon signal diodes are usually quite photo sensitive.  It took me about 
1 hr to find out where all the power hum was coming from -  optically from the 
striplight in the kitchen!
 
    If you just want really low cost sensors for P  & S waves, ~4 sec to 10 
Hz, this is about the bottom line.....
 
    Regards,
 
    Chris Chapman






In a message dated 06/08/2000, gmvoeth@........... writes:
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>  I=20
  am interested in knowing the cheapest way possible to build your own=20
  Geophone that consistently receives teleseismic signals over a long=20
  period of time


Hi there,
 
    The cheapest seismic sensor that I know of=
=20
is a piezo sounder disk with a weight attached to the centre. Look for=20=
a 41=20
- 50 mm disk with a natural capacity from 40 to 100 nF. I get a sensitivity=20=
out=20
to over 4 sec this way and it gives rather more output than a traditional=20
geophone. The disk I use costs about $1. I stick a screw head to the ce=
ntre=20
of the disk with modified acrylic twinpack glue (not epoxy), mount it in a t=
ube,=20
screw on a 1.5" cylindrical weight ~50 gm and mount a V suspension at the fr=
ee=20
end of the weight. You can also use 1/2" shim strip.
 
    This gives a predominantly single axis sensitiv=
ity,=20
you can use it either vertically or horizontally and it acts as a sensitive=20
accelerometer. My loaded disk has a natural resonance at ~170 Hz, so damping=
 is=20
not a real problem, so long as I use a low pass filter. However, you can dam=
p it=20
with sorbothane, polyurethane foam, or magnetically.
 
    I use a low noise CMOS opamp with a synthetic=20
impedance of ~100 M Ohms immediately followed by a high pass filter set=20
to 7 sec. You do have to provide fairly good thermal isolation, as=
 the=20
disk behaves like a very tiny battery as the temperature of the PZT=20
drifts. You need to mount it in a dry container, preferably with some=20
silica gell and have / provide electrostatic screening. 
    You need two signal diodes connected oppos=
ite=20
ways across the disk to protect the input of the opamp. The disks on their o=
wn=20
can generate many 10s of volts if knocked ~ instant death to an opamp. Remem=
ber=20
that the ordinary glass silicon signal diodes are usually quite photo sensit=
ive.=20
It took me about 1 hr to find out where all the power hum was coming from -=20
optically from the striplight in the kitchen!
 
    If you just want really low cost sensors for P=20
& S waves, ~4 sec to 10 Hz, this is about the bottom line.....
 
    Regards,
 
    Chris Chapman