PSN-L Email List Message

Subject: Re: Measuring Time Delay
From: ChrisAtUpw@.......
Date: Sun, 9 Jan 2000 21:45:48 EST

In a message dated 10/01/00 00:52:44 GMT Standard Time, GeE777@....... writes:

>> I have invented a device to do just that on a vertical geophone. I use a 
DC voltage and apply it to the geophone output. This lifts the coil (or 
depresses it depending on the polarity), then this voltage is switched off 
and the output is connected to an oscilloscope or some other device that can 
immediately measure the coil voltage output as it returns to the normal 
resting place.

    Well, no actually. What you will get is a transient as you turn the bias 
current off which depends on the L, C and R of the geophone and it's circuit. 
This will be followed by the induced voltage transient as the system 
physically returns to it's rest position, which will again depend on the 
damping etc. of any connected circuit. I would not like to guess what you 
will actually see at the output end of a six pole Butterworth filter. 
    I'm puzzled at the general use of Butterworth and not Bessel filters in 
the circuits on PSN. There won't be much difference on second order filters, 
but sixth order filters are another matter. 

>>One can use this to measure the natural frequency and relative amplitude of 
the geophone. 
>>You can also measure the damping factor. 
    Yes if the damping is sub critical. Over damped systems just show a 
broadening of the response, which is difficult to measure accurately.

    Have you tried testing a geophone by sitting it on a flat plate on small 
li-lo cushion filled with water? Then use a syringe to inject a ml of water.
    An impulse could be given to a horizontal seismometer by discharging a 
capacitor through a nearby coil of wire. The induced currents give a 
repulsive force.
    I'm puzzled why such accurate timing would be needed anyway for our use. 
With V = 6000 ft / sec, even 100 milliseconds will give an error of  ~ 0.1 
mile. Using geophones and explosives to do depth surveys is another ball game 
    A piezo sounder disk makes quite a good accelerometer if you want to 
thump the floor. Attach the case with plasticene, bluetack, chewing gum etc 
and feed the output directly into the 'scope. I compared a 38mm sounder disk 
with a $200+ accelerometer and up to 300 Hz, the only difference was that the 
sounder disk gave the bigger signal. Otherwise you could overlay the traces 
on a dual beam 'scope.

    Regards Chris Chapman


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Larry Cochrane <cochrane@..............>