PSN-L Email List Message

Subject: Re: Geophone Velocity Question
From: chrisatupw@.......
Date: Fri, 9 Nov 2012 15:44:13 -0500 (EST)


From: GMVoeth GM 


To: psnlist 
Sent: Fri, 9 Nov 2012 14:10
Subject: Geophone Velocity Question


Hello PSN,
I know with a velocity sensor the voltage increases with frequency after F0=
 if given=20
the same vertical motion amplitude. This seems to be because as the frequen=
cy=20
increases so does the velocity and thus, the voltage output.

Hi Geoff,=20

    The voltage output of a new geophone is flat with velocity above it's r=
esonant frequency if it is 0.7 x=20
critically damped. Otherwise, it has a peaked response at the resonant freq=
uency tailing off to a flat with=20
velocity response at higher frequencies. If the amplitude of the vibration =
is A.sin(wt), the output voltage will=20
be A.w.k.cos(wt), where k is the sensitivity of the geophone and w =3D Omeg=
a. =20
    A lot of traffic noise may be in the 10 to >20 Hz frequency range and y=
ou need to filter this out, probably with=20
a 5th to 6th order Butterworth Filter. You DON'T get a 6th order filter by =
stringing three 2nd order filters in=20
sequence! There is a good FREE filter design program called FilterPro suppl=
ied by Texas Instruments. The=20
response is also available graphically.
    I suggest that you set the turnover frequency to between 5 and 3 Hz to =
minimise the road noise. The MAX280 is=20
a five pole Butterworth filter in an 8 pin chip. You change one capacitor t=
o vary the switching frequency which=20
controls the LP turn over frequency.
    If you are connecting the geophone into a -ve input impedance circuit, =
you get an output response rising=20
with f.

    Regards,

    Chris Chapman
    =20

=20
=20
From: GMVoeth GM <gmvoet=
h@.........>
To: psnlist <psnlist@..............>
Sent: Fri, 9 Nov 2012 14:10
Subject: Geophone Velocity Question

Hello PSN,
I know with a velocity sensor the voltage increases with frequency after F0=
 if given 
the same vertical motio= n amplitude. This seems to be because as the frequency
increases so does the v= elocity and thus, the voltage output. Hi Geoff,
The voltage output of a new g= eophone is flat with velocity above it's resonant frequency if it is 0.7 x =
critically damped. Otherwise, it has a peaked response at the resonant freq= uency tailing off to a flat with
velocity response at higher frequ= encies.
If the amplitude of the vibration is A.s= in(wt), the output voltage will
be A.w.k.cos(wt), where k is the sensitivity of the geopho= ne and w =3D Omega.
A lot of traffic noise= may be in the 10 to >20 Hz frequency range and = you need to filter this out, probably with
a 5th to 6th order Butterworth Filter. You DON'T get a 6th= order filter by stringing three 2nd order filters in
sequence!
There is a good FREE filter design program called F= ilterPro supplied by Texas Instruments. The
response is also available graphically.
I suggest that you set= the turnover frequency to between 5 and 3 Hz to minimise the road noise. T= he MAX280 is
a five pole Butterworth filter in an 8 pin chip. You chang= e one capacitor t= o vary the switching frequency which
controls the LP turn over frequency.
<= /font>
If you are connecting = the geophone into a -ve input impedance circuit, you get an output response rising
with f.

Regards,

Chris Chapman
 

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