In a message dated 30/05/2009, DSaum@............ writes:

Hi  Chris
Sounds interesting in theory, but how well does it actually  work?
Do you have any data / spectra from your piezo device that shows  it
picks up teleseismic events as well as a my horizontal 4.5 Hz  geophone?
Dave



Hi Dave,
 
    There are more sensor construction details and  circuits at 
_http://www.jclahr.com/science/psn/chapman/piezo/index.html_ 
(http://www.jclahr.com/science/psn/chapman/piezo/index.html)  Please  Note that the stated circuit 
components and their values are critical to  success.
 
    The device acts as an accelerometer, but the  internal noise of 
capacitor is very low and the voltage sensitivity to bending  stresses is high.
 
    I initially extended the period of my 4.5 Hz  geophones by x10 to 0.5 
Hz using a modified Roberts circuit and it worked fine.  See 
_http://www.jclahr.com/science/psn/roberts/index.html_ 
(http://www.jclahr.com/science/psn/roberts/index.html)  The  increased response to teleseismic P and S waves is 
very evident. The  modifications were to effectively add a high pass function 
at about 0.3 Hz  to remove most of the 1/f  VLF noise, that the original 
circuit displays  only too well. You can use CAZ opamps for the input stages 
($$), but a filter  also reduces 6 second microseism noise, which is also 
quite obvious. I do not  find microseisms particularly interesting.
 
    Lennartz put a negative input impedance amplifier  onto 4.5 or 2 Hz 
geophones and amplify the current required to keep the mass  stationary. They 
can increase the response period to 20 seconds this way,  but you need 
careful temperature compensation and a very low noise  amplifier. See 
_http://www.lennartz-electronic.de/Pages/Seismology/Seismometers/Seismometers.html_ 
(http://www.lennartz-electronic.de/Pages/Seismology/Seismometers/Seismometers.htm
l)  The  NoeMax triaxial sensor used in French schools appears to be 
similar  but they are not exactly cheap either. See 
_http://www.agecodagis.com/WebData/Documentation/Commercial/NoeMax/NoeMax.pdf_ 
(http://www.agecodagis.com/WebData/Documentation/Commercial/NoeMax/NoeMax.pdf)    The apparent output 
response below 0.05 Hz concerns me and it crosses the NHNM  high ground noise 
plot. Extending 4.5 Hz geophones to 20 seconds is a factor of  x90. This 
would be below the noise level of the 'standard' seismic amplifier  that I 
used. 
 
    I first made a very simple piezo detector by  sticking a disk onto two 
ridges of adhesive silicone rubber spaced 1.25" apart  on a backing plate. 
This silicone adhesive does NOT smell of acetic acid /  vinegar. I then stuck 
a brass rod parallel to the ridges on the top centre of  the disk. I used a 
unity gain FET amplifier to provide a signal for the same  amplifier used 
for observing the geophone signal. The piezo signal for the  background noise 
was about 5x that of the geophone, measured on an  oscilloscpe. I then 
tried to optimise the design and extend the period as  far as practicable. My 
sensors do work fine to well below 1 Hz. I do  not know about yours, Dave.
 
    Regards,
 
    Chris Chapman
 
 






In a message dated 30/05/2009, DSaum@............ writes:
Hi 
  Chris
Sounds interesting in theory, but how well does it actually 
  work?
Do you have any data / spectra from your piezo device that show=
s 
  it
picks up teleseismic events as well as a my horizontal 4.5 Hz 
  geophone?
Dave


Hi Dave,
 
    There are more sensor construction details an=
d 
circuits at http:/=
/www.jclahr.com/science/psn/chapman/piezo/index.html Please 
Note that the stated circuit components and their values are critical=
 to 
success.
 
    The device acts as an accelerometer, but the=
 
internal noise of capacitor is very low and the voltage sensitivity to ben=
ding 
stresses is high.
 
    I initially extended the period of my 4.5 Hz=
 
geophones by x10 to 0.5 Hz using a modified Roberts circuit and it worked=
 fine. 
See http://www.j=
clahr.com/science/psn/roberts/index.html The 
increased response to teleseismic P and S waves is very evident. The 
modifications were to effectively add a high pass function at about=
 0.3 Hz 
to remove most of the 1/f  VLF noise, that the original circuit displ=
ays 
only too well. You can use CAZ opamps for the input stages ($$), but a fil=
ter 
also reduces 6 second microseism noise, which is also quite obvious. I do=
 not 
find microseisms particularly interesting.
 
    Lennartz put a negative input impedance ampli=
fier 
onto 4.5 or 2 Hz geophones and amplify the current required to keep the ma=
ss 
stationary. They can increase the response period to 20 seconds this=
 way, 
but you need careful temperature compensation and a very low noise 
amplifier. See http://www.lennartz-electronic.de/Pages/Seismology/Seismo=
meters/Seismometers.html The 
NoeMax triaxial sensor used in French schools appears to be simi=
lar 
but they are not exactly cheap either. See http://www.agecodagis.com/WebData/Documentation/Commercial/Noe=
Max/NoeMax.pdf  
The apparent output response below 0.05 Hz concerns me and it crosses the=
 NHNM 
high ground noise plot. Extending 4.5 Hz geophones to 20 seconds is a fact=
or of 
x90. This would be below the noise level of the 'standard' seismic amplifi=
er 
that I used. 
 
    I first made a very simple piezo detector by=
 
sticking a disk onto two ridges of adhesive silicone rubber spaced 1.25"=
 apart 
on a backing plate. This silicone adhesive does NOT smell of acetic acid=
 / 
vinegar. I then stuck a brass rod parallel to the ridges on the top centre=
 of 
the disk. I used a unity gain FET amplifier to provide a signal for the sa=
me 
amplifier used for observing the geophone signal. The piezo signal for the=
 
background noise was about 5x that of the geophone, measured on an 
oscilloscpe. I then tried to optimise the design and extend the perio=
d as 
far as practicable. My sensors do work fine to well below 1 Hz. =
I do 
not know about yours, Dave.
 
    Regards,
 
    Chris Chapman