Geoff:

What I think you were saying is that a geophone makes a poor detector 
for distant quakes.  Quite true.

If you turn up the gain high enough to see distant quakes, the local 
ground noise plus the instrument noise, is overwhelming.  When you 
reduce the sensitivity enough to make the noise look reasonable you 
will only see nearby quakes.  Dave's geophone setup which goes from 
0.5Hz to perhaps 50 Hz, gives beautiful traces for the small quakes 
that often occur within 100 km of LA.  Their traces closely match 
what the broadbands see.

Rarely, there will be a teleseism that is large enough for its p-wave 
to show up on the geophone above the noise, but they have to be 
pretty big and don't happen often.

Even the commercial geophone-based sensors are quite noisy when you 
look at their specs.

Geophone sensors are a great tool for recording nearby quakes.

Regards,
Brett



At 10:31 AM 11/9/2012, you wrote:
>On 11/9/2012 6:30 AM, Bob Smither wrote:
>>
>>On Fri, 2012-11-09 at 06:10 -0800, GMVoeth GM wrote:
>>>Hello PSN,
>>>
>>>I know with a velocity sensor the voltage increases with frequency after
>>>F0 if given the same
>>>vertical motion amplitude. This seems to be because as the frequency
>>>increases
>>>so does the velocity and thus, the voltage output.
>>>
>>>This means to me the energy also increases as the square of the velocity.
>>>So like, double the velocity, quadruple the energy.
>>>
>>>What i would like to do is: filter at f0 (1.0Hz) so that given
>>>a steady shake amplitude (sine wave) but incresing frequency
>>>that the voltage will remain constant.
>>>
>>>Now I understand this may not be possible.
>>
>>I think it is.  A simple RC low pass filter (R from signal to C, C to
>>ground, output at junction of R and C) gives:
>>
>>    T(S) = 1 / (R*C*S + 1)
>>
>>Above w = 1/(R*C) this is
>>
>>    T(S) ~ 1 / (R*C*S)
>>
>>exactly what you need to reduce gain proportional to frequency.
>>
>>For your example, w = 2*pi*1Hz = 1/RC so C = 1ufd, R = 160K should work.
>
>I find this interesting.
>
>I have eliminated filters except for a single HPF n=1 at like 100seconds
>and am looking at pure noise.
>
>At 8AM MST today I get occasional over driving (jamming) of my
>preamplifier with no filtering.
>
>I need this level of amplification to see the smaller seismic signals.
>
>If I apply n=2 LPF at 1.0Hz using winquake I get the best flattening
>of the noise baseline above 1.0Hz. Noise is human vehicular related.
>
>It is an absolute must for me to apply n=2 at 1.0 Hz to prevent
>jamming by vehicular and other noise.
>
>Does this mean there is a problem with my geophone
>or is vehicular noise increasing with frequency like n=1
>to add to the velocity profile of the geophone ?
>
>None of this is speaking of Alizing problems.
>
>this is an extremely noisy town for vehicular activity.
>(Apache Junction AZ USA)
>
>Would it be best to stop recording during the day
>or best to heavily filter to keep the system from being
>jammed. Heavy filtering terribly affects the resulting
>waveforms ?
>
>Regards,
>geoff
>
>
>
>
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