Hello All,

For a good discussion of signal to noise ratio, we can begin by 
separating seismic noise and instrument noise. 

Seismic noise--Any motion of the ground or environment that generates a 
signal on the motion detector that is not the desired quake:
1.  Local movement causing ground vibrations
2.  Wind or air density change, causing a change in the "float" depth of 
a vertical sensing seismometer
3.  Air movement around the seismometer

Instrument noise--Any condition of the sensor or electronics that 
registers as data in the visual display:
1.  Seismometer hinge--non-linearity or friction causes vibration
2.  Electronic noise-- visible even when the seismometer is locked
3.  Damper characteristics--Oil dampers will have internal fluid waves
4.  Temperature changes

Some conditions affect the amount of signal captured but do not affect 
the seismic signal or ground motion:
1.  The size of the seismometer rest mass.
2.  The period of the seismometer.  Longer periods allow increased 
ground motion before the seismic mass will move. 
3.  The damper.   The damper absorbs seismic energy, and can also be 
frequency sensitive or non-linear in response

After the signal is captured, the electronics can reduce or absorb the 
signal:
1.  Improperly set filters
2.  Resistance 

Each of these generally named sources of noise or signal loss can be 
discussed in much greater detail.

The effects of the signal to noise ratio can be seen by looking at the 
postings on the PSN website.   The omnipresent microseims caused by 
ocean wave action are detected by many instruments and create a waveform 
which precludes recording a straight line trace.   Small seismic 
signals, whether generated locally or distant, should appear as higher 
frequency deviations "riding" on  the microseim trace.   Only rarely 
will the seismic signal frequency exactly match the microseim frequency 
and therefore be masked as being a "greater microseism". 

BW,  Roger



psn-l-digest-request@.............. wrote:
> .------ ------ ------ ------ ------ ------ ------ ------ ------ ------.
> | Message 1                                                           |
> '------ ------ ------ ------ ------ ------ ------ ------ ------ ------'
> Subject: Signal To Noise Ratio of a Seismic Sensor
> From:    "Geoffrey" 
> Date:    Sat, 25 Mar 2006 02:25:33 -0700
>
> Hello PSN:
>
> Can someone out there run down a list of
> best ways to increase the signal to noise
> ratio of a seismic sensor ?
>
> Things like:
> 1. in a mass spring system use a larger mass
> 2. use a phased array
> 3. use low resistance's in the circuitry
> 4. use a narrow bandwidth
> 5. use a high Q
> 6. shield everything against anything not a seismic signal.
> 7. a high signal to noise ratio is not possible for the average amateur due to cost
>    it might take a bunch of amatures to pool their monies and talents to
>    create a decent seismic sensor, possibly such a thing is only possible
>    at a university where both money and talent exists.
>
> Does any know of a system out there with the
> highest signal to noise ratio...if so can we see it ?
>
> More then likely it is in military hands
> and is secret ?
>
> In all the discussions I have seen in this group
> none seem to concentrate on signal to noise ratio.
>
> Sincerely;
> gmvoeth
>
>
>   
>   
__________________________________________________________

Public Seismic Network Mailing List (PSN-L)