In a message dated 21/12/02, lehmancj@........... writes:

Hi Jim Lehman,

       Geophones have a high sensitivity and can reach the noise floor. While 
the higher frequency models are commonly used for seismic prospectingy, the 
1, 2 and 4.5 Hz models are useful for seismic monitoring, particularly in 
volcanic regions. See http://www.geospacelp.com/ and 
http://www.sercel.com/Products/frproduct2.htm 

       The response of a geophone can be extended down to about 1/10 of it's 
natural resonant frequency by two methods. The simplest type uses a circuit 
similar to a bass boost circuit on an audio amplifier to amplify the falling 
response below the resonant frequency. You are eventually limited by 
amplifier, 1/f and thermo-emf noise. Another system uses a negative impedance 
converter to severely damp the geophone. This flattens the normal 'dog leg' 
in the response but reduces the signal a lot. A very low noise amplifier / 
filter system is used to boost the lower frequency signals. The extended 
range covers P and S seismic wave frequencies. 

       The more elaborate system reported by Barzilai of Standford involved 
opening up the geophone, fitting a precision distance transducer to the 
armature and using feedback through the built in sense coil to control the 
response. If you choose a geophone between 1 and 4.5 Hz, they have an 
armature mass of 1 oz or more, which is about the minimum from Brownian 
motion noise considerations. The 2 and 4.5 Hz ones are relatively 
'affordable'. The electronics controls the effective period and a broad band 
response from 30 sec to 50 Hz has been achieved. See 
http://micromachine.stanford.edu/smssl/projects/Geophones/ for several 
papers. 

       Regards,

       Chris Chapman
In a message dated 21/12/02, lehmancj@........... writes:



Hi Jim Lehman,



       Geophones have a high sensitivity and can reach the noise floor. While the higher frequency models are commonly used for seismic prospectingy, the 1, 2 and 4.5 Hz models are useful for seismic monitoring, particularly in volcanic regions. See http://www.geospacelp.com/ and http://www.sercel.com/Products/frproduct2.htm 



       The response of a geophone can be extended down to about 1/10 of it's natural resonant frequency by two methods. The simplest type uses a circuit similar to a bass boost circuit on an audio amplifier to amplify the falling response below the resonant frequency. You are eventually limited by amplifier, 1/f and thermo-emf noise. Another system uses a negative impedance converter to severely damp the geophone. This flattens the normal 'dog leg' in the response but reduces the signal a lot. A very low noise amplifier / filter system is used to boost the lower frequency signals. The extended range covers P and S seismic wave frequencies. 



       The more elaborate system reported by Barzilai of Standford involved opening up the geophone, fitting a precision distance transducer to the armature and using feedback through the built in sense coil to control the response. If you choose a geophone between 1 and 4.5 Hz, they have an armature mass of 1 oz or more, which is about the minimum from Brownian motion noise considerations. The 2 and 4.5 Hz ones are relatively 'affordable'. The electronics controls the effective period and a broad band response from 30 sec to 50 Hz has been achieved. See http://micromachine.stanford.edu/smssl/projects/Geophones/ for several papers. 



       Regards,



       Chris Chapman