PSN-L Email List Message

Subject: Re: Modified sound card and datalogging and geophones
From: Gordon Couger gcouger@..........
Date: Wed, 20 Jul 2005 16:43:51 -0500


Out of sequence and possible not on subject but it has some 
thoughts on timing that are important. I must have come in the 
middle and don't understand your set up. I thought you were 
recording data to disk with a computer sound card.

I am working on a sismomenter that records several sismometers 
at one time in the 0-50 HZ rang on the two channels of a sound 
card for imaging shallow aquifers looking for larger gravel to 
find better water well in a shallow aquifer along Red River 
between Oklahoma and Texas so I can afford to put in irrigation.

I have probably imposed my project on yours without realizing it.

A GPS will give you the best time stamp you could ask for some 
even have a 100 kHz clock as well a 1 second pulse that are 
linked to GPS clocks. They can be used to phase lock frequency 
sources to for super accurate osilators.

If you are not interested in 1 to 20 Hz signals sound cards are 
pretty good tools. If you are really serious about low 
frequencies and accurate timing Windows poses a lot of problems.

First Windows is an event driven operating system and there is 
no assurance when a program will run. The sound card gets around 
that but you have to put a chirp on the signal to synchronize it 
with time you can depend on the computer clock or the RS232 port.

Having worked with embedded systems for the last 20 years I 
would choose a real time computer. There are real time versions 
of Linux but I think that's over kill for dataloging geophones.

A GPS for a time base, a good a/d converter and fast 
microcontroler would be the way I would do it. I am working with 
a very fast board 60 MHz with 4 10 bit A/D channels


ChrisAtUpw@....... wrote:
> In a message dated 18/07/2005 19:31:08 GMT Daylight Time, 
> mike8s2@......... writes:
>     installed some larger-capacitance-value coupling capacitors on my
>     24-bit sound card so I could resolve lower frequencies.  What is a
>     good datalogging app to
>     use with this?
> Hi Greg,
>     I suggest that you look at Larry's website at 
> and read some of the articles. Look at other 
> websites on
>     A soundcard by itself has a frequency range of about 10 Hz to 24 KHz 
> and it is basically an AC only device. The 10 Hz lower frequency is 
> limited by the input C + R circuit. The standard PC drivers set the 
> sample rate from 8,000 to 48,000 sps. Using this, you end up with simply 
> massive files on a daily basis and no quick way of monitoring them. I 
> haven't seen an application which enables you to use the soundcard ADC 
> at really low sample rates, say 20 / sec and return them to a file.
>     Assuming that you are in the States, you can buy a $25 10 bit ADC 
> from Dataq, but it is usual to use 12 to 16 bit ADCs. Following on a 
> geophone, you will need a low noise amplifier and a filter. See 
> or 
>     It is usual to limit amateur seismic sensors to frequencies of less 
> than 10 Hz. This cuts out most of the urban traffic and environmental 
> noise, which is of no interest to most of us and may swamp everything 
> else above 20 Hz.  
>     You also need a timing system which is good to better than 1 sec. 
> Unfortunately, most computers are fitted with something called a 
> 'software clock', which can vary by minutes per day. The first seismic P 
> waves travel at maybe 10 km / sec, so a 1 minute error would give a 
> location error of ~380 miles. This really is useless for seismic work 
> and why you need a dedicated data recording program which takes this 
> into account.
>     The P and S waves are of most interest to us, since they enable you 
> to determine the distance of the quake from your station. Long distance 
> P waves are at about 1 Hz and S waves are at about 0.5 Hz, but nearby 
> quakes have higher frequency components.
>     The 'cheap' 4.5 Hz geophones can be extended down to about 0.5 Hz 
> with a special amplifier, but they will also pick up local and near 
> regional quakes on their own.
>     It is also possible to make a really cheap vibration detector / 
> seismometer using piezo disks and added weights. I use one from about 
> 0.25 Hz to 10 Hz, but I need a FET input opamp and good screened cable 
> to do this.    
>     *Do 'read up' about earthquakes and seismometers before you start. 
> There is a lot to learn!*
>     To go beyond this very basic advice, I would need to know where you 
> are located, your knowledge of electronics, what tools / constructional 
> skills you have and how much you are prepared to spend. I am not being 
> inquisitive, merely practical. Commercial seismometer systems may cost 
> several 10's of thousands of $, but amateur systems may be made from a 
> few $100 upwards. You can exchange construction time + skills for ready 
> made equipment, but the effort may be considerable. The real art lies in 
> not making too many mistakes.....
>     I hope that this hslps...
>     Regards,
>     Chris Chapman


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