Convert the output of a small geophone

[Kleinar]

Need some help, I need to convert the output of a small geophone (30mv to 1 v) to an audible signal so it can be recorded. The impedance is 375 Ω. Most available preamp circuits have a lower max input. Any suggestions? Thanks.

 

[davenn]

Hi Kleinar
welcome to PF what is the make and model of your geophone ?
if this is a geophone used for seismology, seismic reflection studies, its likely to have an output signal in the 4.5 to 20 Hz region, 4.5, 8 and 15Hz are the 3 most common ones.

I have geophones in my seismic station here in Sydney, Australia
http://www.sydneystormcity.com/g_phones.htm

in that link you will see the preamp board and the Analog to Digital board. Then there's the software for doing the recording.
To hear sound from your geophone the frequency output range would have to be converted to a higher freq within the human hearing range.
Do you really need to do that ? or are you just wanting to record earthquakes like many others and I do ?

Tell me a bit more about your goals and I'm sure I can lead you in the right direction

Dave

 

[Kleinar]

Hi Dave,
I believe the device is a SM-24, it is 10Hz with a spurious frequency of 240Hz, I have read that the sensitivity of these devices is incredable. My experimentation is less noble than your field of expertice, but when you need help you go to the experts. My goal is to build a circuit that will allow me to record the output of the geophone onto audio software on my computer for further analysis. Will the circuit on your site allow me to do that? Thanks for responding.

 

[f95toli]

The audio input on your compute is unlikely to work well below 20 Hz, this is the usual cut-off since everything below that is outside the audiable part of the spectrum.
You would be better off using a dedicated data-aquisition card, there are quite a few USB based ones that are reasonable priced.

 

[davenn]

Hi Kleinar

as I said earlier and f95toli restated, there will ne no audible signal from the geophone, the freq is way, way below human hearing range

so again I ask, what are you trying to record ? that is, what is the source of the vibrations ?
if it is earthquakes, then the frequencies you are interested in are ~ 15Hz and lower
In fact when I do analysis on the quake files from the geophones, I'm doing a 5Hz low pass filter on the data. Most signals 10Hz and up are mainly manmade noise.

This A to D unit would be a good place for you to start.
This is their data acquisition software page

this company has been around for a long while, their gear is good ( I have no personal attachment to them!)

Depending on the input sensitivity of the AtoD unit, you may well need a preamplifier between the geophone and the AtoD input that preamp circuit near the bottom of that www page of mine would be suitable

cheers
Dave

 

[Baluncore]

The problem here is that the audio input to a PC is usually AC coupled. You need a DC coupled recorder to reliably record the important low frequencies without phase shift or attenuation.

There are humanly audible components in seismic signals. Seismic surveys that use explosives produce a distinct audible click nearby which you can hear if you put your head to the ground.

I have also heard the multiple clicks from an earthquake swarm by lying on the ground. I was trying to find the epicentre by time of arrival measurements with a portable seismic array. I reached a predicted closer site on the afternoon of a 40°C day, so I slid in under my car for some shade. It took me a short while to realize that the clicks I was hearing came from below and not from the car above. Measurements at the time showed no difference in TOA over my array, so I had found the epicentre.
As one might expect, the clicks sounded the same with my ear to the ground as they did when the digital recordings from the geophones were later replayed through the loud speakers of an audio system. In both cases the important seismic low frequencies were missing from the human senses.