Extend Frequency Range of 4.5hz Geophone

Thread Starter

daveos

Joined Mar 19, 2023
39
Hi Everyone.

Please go easy. I'm an electronics noob with little experience. I'm good with programming, mostly in Python.

I've been messing with a 4.5hz natural frequency geophone and looking at ways to extend the bandwidth to 0.1hz at decent gain. I found a paper and got some help trying to design the circuit, built it, and it's somewhat working but I'm still not getting the response at very low frequencies.

Paper on extending natural frequency of 4.5hz geophone.
https://www.academia.edu/40257314/B..._5_Hz_geophone_for_seismic_monitoring_purpose

I don't fully understand the circuit. The first part is just a preamp? I built it using 2 10k resistors and an LM358 opamp.

For the second part, I set the gain to 30 (1 + R4/R3), 10K for R3 and 290K for R4. I used a 0.1uF cap to set the bandwidth. Also with an LM358.

I'm feeding +5 and -5v into the system for the opamps, the signal goes to a 16bit ADC with ability to increase gain there.

I've attached the specs on the geophone.

All in all, this is really above my head. I know enough to mess around and build a test circuit, but a lot of the theory and approach is beyond me.

If some expert on opamps and bandwidth extension could help me figure out a circuit to extend the bandwidth on this geophone, I'd be eternally grateful.
 

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ericgibbs

Joined Jan 29, 2010
19,084
hi daveos,
I have quickly read through the PDF.
Do you have your circuit to post, I can use LTSpice simulator to check out the performance of your circuit?
I assume your Sensor coil is, 375R.
E
 

Thread Starter

daveos

Joined Mar 19, 2023
39
I don't know how to use any software to build it there. The coil specs are above, so it is 375R yep. For R1 and R2 I used 10K resistors. I used 10K for R3 and 290K for R4. The cap was 0.1uF. Both of the opamps were LM358 with +5 and -5 v going into them. Is that enough info?
 

ericgibbs

Joined Jan 29, 2010
19,084
Hi daveos,
I would recommend you build and test this prototype circuit and post your results, we can then refine the design.

Regarding the 10Hz > 50Hz, the Sensor has a possible natural resonance at 4.5Hz also local power mains supplies are 50Hz.

E
 

Alec_t

Joined Sep 17, 2013
14,405
The post #6/7 circuit provides a flat gain below a few Hz, but won't a lot of boost be required instead, to compensate for the geophone's output drop-off with frequency? Something along the lines of the post #4 link circuit?
 

Janis59

Joined Aug 21, 2017
1,863
The idea is rather nice, however I believe the signals are weak thus the name of operationals have a large meaning. You was right, the all magnification factor comes from Op 1 and Op2 works as integrator cutting off the faster responses. Be watchy about fig 1 where R2 makes the flip-flop regime like Schmitt trigger like to do. In fig 2 it is correctly eliminated.

Can understand the Italien interest in topic because of Syrien and Turkian earthquake but need to add that exists an Ukrainian team invented microgravimeter capable to detect even so small as nanoG changes, and they claim their apparatus is capable to foresee the earthquakes very reliable. Sad the war in many ways cut their efforts, but at least few weeks ago they was alive and tried to write an European Grant application with us. Be welcome in consortium (I guess) if may help with something to build it or test it or advertize it. If yes, then may write me more personally at Janis.Blahins at LU.LV. Obey I shall contact Your lab boss with their lab boss, and mine.
 

ericgibbs

Joined Jan 29, 2010
19,084
hi,
If you are attempting to model a geophone in the real world on your project you could add them, but if you have the actual geophone, the answer is you do not need to add the components;
They have been created by Alec to mimic a real geophone. OK.?
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Jon Chandler

Joined Jun 12, 2008
1,095
I think you're missing the key feature here. If I understand correctly, you want to see data down to 0.1Hz. You're out of gas. There's no signal to amplify at 0.1Hz. The slope of the response below 5.4Hz is 40dB/decade. At 0.1Hz, the response is about 50dB down (assuming the slope continues off the graph) - about 1/300 less response than at 5.4Hz.

acc-geophone freq response.jpg
 
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