From: mike brown 
To: psnlist 
Sent: Sun, 6 Nov 2011 19:31
Subject: 2Hz geophone question



Hello,

Hopefully this will be my first successful post to the list!   I was wonder=
ing what would be some of the benefits of using a 1 or 2Hz geophone vice a =
4.5 geophone in terms of detecting seismic events.   I understand that the =
lower resonance frequency of the 2Hz should help but I have no idea how to =
quantify the differences.   Could anyone take a few moments and expound on =
what they thought could be gained by going from a 4.5Hz geophone to a 2Hz g=
eophone? Of course the one negative thing is the cost!! ;) Thanks for your =
time!

Hi Mike Brown,=20

    The output of a geophone is flat with velocity above the resonant frequ=
ency - if it is correctly damped.=20
    The output falls as the square of the frequency ratio below the resonan=
t frequency.
    The P wave frequency is roughly twice the S wave frequency.=20
    The P wave may be about 1 Hz for very distant teleseismic quakes, but u=
p to about 10 Hz for very local quakes.=20
    So the amplitude of a 1 Hz P wave will be a factor of 4.5 x 4.5 =3D 20.=
25 down and the accompanying S wave a factor of 81 down in sensitivity on a=
 4.5 Hz geophone.=20
    Roberts has described how to build an electronic compensation circuit t=
o make a 4.5 Hz geophone behave like a 1 Hz geophone. It needs TWO amplifie=
r stages.=20
    Larry usually has some 4.5 Hz geophones for sale.
    Alternatively, you can make a 0.25 Hz sensor from a 44 mm OD Piezo Soun=
der disk and a 1 oz mass for a few $s, but you have to use a very high inpu=
t impedance amplifier.=20
    It rather depends on your constructional and electronics skills!!=20

    See PSN and John Lahr's websites. I hope that this helps!

    Regards,

    Chris Chapman
=20



=
From: mike brown <hawkchalk01@...........>

To: psnlist <psnlist@..............>

Sent: Sun, 6 Nov 2011 19:31

Subject: 2Hz geophone question













Hello,



Hopefully this will be my first successful post to the list!   I was w=
ondering what would be some of the benefits of using a 1 or 2Hz geophone vi=
ce a 4.5 geophone in terms of detecting seismic events.   I understand=
 that the lower resonance frequency of the 2Hz should help but I have no id=
ea how to quantify the differences.   Could anyone take a few moments =
and expound on what they thought could be gained by going from a 4.5Hz geop=
hone to a 2Hz geophone? Of course the one negative thing is the cost!! ;) T=
hanks for your time!



Hi Mike Brown, 


 


    The output of a geophone i=
s flat with velocity above the resonant frequency - if it is correctly=
 damped. 


    The output falls as t=
he square of the frequency ratio below the resonant frequency.


    The P wave frequency =
is roughly twice the S wave frequency. 


    The P wave may be about 1 =
Hz for very distant teleseismic quakes, but up to about 10 Hz for very loca=
l quakes. 


    So the amplitude of a 1 Hz=
 P wave will be a factor of 4.5 x 4.5 =3D 20.25 down and the accompanying S=
 wave a factor of 81 down in sensitivity on a 4.5 Hz geophone. 


    Roberts has described how =
to build an electronic compensation circuit to make a 4.5 Hz geophone =
behave like a 1 Hz geophone. It needs TWO amplifier stages. 


    Larry usually has some 4.5=
 Hz geophones for sale.


    Alternatively, you can mak=
e a 0.25 Hz sensor from a 44 mm OD Piezo Sounder disk and a 1 oz mass =
for a few $s, but you have to use a very high input impedance amplifier. 



    It rather depends on your =
constructional and electronics skills!! 


 


    See PSN and John Lahr=
's websites. I hope that this helps!


 


    Regards,


 


    Chris Chapman