Larry, The primary reason for poor signal to noise ratio at long periods wit= h a coil/magnet system involves the physics employed. Such a sensor functi= ons on the basis of Faraday's law, which says that a time changing magnetic= flux gives rise to a voltage. The key word in this process is 'changing';= the output depends on the flux rate. If the boom of the seismometer is sw= inging sinusoidally at a frequency f, then the amplitude of the output volt= age of the sensor is proportional to f-which is the very nature of the time= derivative of the sine function, involving the chain rule. Because the si= gnal is proportional to f, the signal level decreases by the same amount as= f decreases. Regardless of the nature of the noise, this means there is a= significant reduction in the signal to noise ratio for accelerations havin= g a frequency below the natural frequency of the instrument. When operatin= g without the high-pass filter with your S-G instrument, your output is not= rate sensitive; therefore it isn't afflicted with this low frequency loss.= The 'wandering' you experience is actually representative of the various = changes occurring. Although some of those changes are oftentimes the undes= irables of instrument thermal coefficient or whatever, there are also signi= ficant variations associated with the Earth itself. When you high pass the= output, any earth motions having frequencies lower than the cutoff are su= ppressed. I operate the VolksMeter with a high pass and also low pass filt= er of recursive type in WinSDR. The primary reason is so that the helicord= record is well behaved for purpose of seeing earthquakes. But for viewing= tides, or magnetoelastic phenomena, or earth hum, or .... It is necessary = to look at the unfiltered output, which is what I save. The obsession with 'velocity' sensing is almost universal. Even forc= e balance instruments of the highest dollar type use a network that causes = the seismograph to behave just like a magnet/coil system with a low corner = frequency, usually about 30 s. Thus the greatest advantage of the capaciti= ve sensor is actually lost, for signals having frequencies lower than the d= esign corner. For signal variations having a frequency below the corner, t= he output from such an instrument is not 'velocity'; it is instead the deri= vative of acceleration, called the 'jerk'. Only for drive frequencies abov= e the corner can one use the term 'velocity' appropriately. RandallLarry,
The primary reason for = poor signal to noise ratio at long periods with a coil/magnet system involves th= e physics employed. Such a sensor functions on the basis of FaradayR= 17;s law, which says that a time changing magnetic flux gives rise to a voltage.= The key word in this process is ‘changing’; the output depends = on the flux rate. If the boom of the seismometer is swinging sinusoidall= y at a frequency f, then the amplitude of the output voltage of the sensor is proportional to f—which is the very nature of the time derivative of = the sine function, involving the chain rule. Because the signal is propor= tional to f, the signal level decreases by the same amount as f decreases. Regardless of the nature of the noise, this means there is a significant reduction in the signal to noise ratio for accelerations having a frequency below the natural frequency of the instrument. When operating without= the high-pass filter with your S-G instrument, your output is not rate sensitiv= e; therefore it isn’t afflicted with this low frequency loss. The = ‘wandering’ you experience is actually representative of the various changes occurring. Although some of those changes are oftentimes the undesira= bles of instrument thermal coefficient or whatever, there are also significant v= ariations associated with the Earth itself. When you high pass the output, any earth motions having frequencies lower than the cutoff are suppressed= .. I operate the VolksMeter with a high pass and also low pass filter of recur= sive type in WinSDR. The primary reason is so that the helicord record is = well behaved for purpose of seeing earthquakes. But for viewing tides, or magnetoelastic phenomena, or earth hum, or …. It is necessary to look= at the unfiltered output, which is what I save.
The obsession with R= 16;velocity’ sensing is almost universal. Even force balance instruments of the highest dollar type use a network that causes the seismograph to behave jus= t like a magnet/coil system with a low corner frequency, usually about 30 s. Thus the greatest advantage of the capacitive sensor is actually l= ost, for signals having frequencies lower than the design corner. For sign= al variations having a frequency below the corner, the output from such an instrument is = not ‘velocity’; it is instead the derivative of acceleration, called the ‘jerk’= .. Only for drive frequencies above the corner can one use the term ‘vel= ocity’ appropriately.
Randall