PSN-L Email List Message

Subject: Re: Modified sound card and datalogging and geophones
From: Gordon Couger gcouger@..........
Date: Thu, 21 Jul 2005 01:03:18 -0500

ChrisAtUpw@....... wrote:
 > In a message dated 20/07/2005, gcouger@.......... writes:
 >     Chris,
 >     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 seismometer that records several 
seismometers at
 >     one time in the 0-50 HZ range on the two channels of a 
sound card
 >     for imaging shallow aquifers looking for larger gravel to 
 >     better water well in a shallow aquifer along Red River
 >     between Oklahoma and Texas so I can afford to put in 
 >     I have probably imposed my project on yours without 
realizing it.
 >  Hi Gordon,
 >     We were discussing sound cards and I was rejecting this as
 > impractical for seismic purposes. The data files are massive 
and I
 > haven't found a way of using the 24 bit soundcard ADC for 20 
sps sampling.
 >     Chuck a bit of cash at it and get rid of the your 
problems? Larry
 > sells a fairly good 8 channel serial board, which you can use 
with a GPS
 > receiver.
 >     If you want to play, fine. If you want to work, get rid 
of problems
 > that you do not need to have.
 >     Just what rate of water rise do you get along the Red 
river?? Even
 > for marine waves, 20 sps is adequate and most geological 
systems will go
 > to 100sps, some even more.
 >  >>    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 osillators.
 >     I am NOT asking for the best system, just one that 
 > inexpensive, low power and ADEQUATE for my purposes.
 >     I use VLF timing signals. You can get relatively inexpensive
 > receiver boards with a ferrite aerial: or gut a radio clock. 
You get
 > about 10 mS + the transit time. The only folks who need it 
more accurate
 > monitor volcanoes of do reflection seismometry. I have been 
all along
 > this road. I would be great if Larry would fit the software 
to decode
 > WWVB directly, but he seems concentrate on GPS.  GPS is power 
hungry and
 > needs a clear line of sight to satellites; 60 KHz VLF radio 
is low power
 > and can be used most anywere. I can use it where I can't get 
MW radio,
 > even underground. Just keep it away from striplights, radio
 > transmitters, TVs and computer monitors. I have used it 
underground near
 > radio and TV transmitters - find a nice hole and bury it to 
screen it!
 > It is no accident that submarine communications use the VLF 
range of
 > frequencies.
 >     If you are not interested in 1 to 20 Hz signals sound 
cards are
 >     pretty good tools. If you are really serious about low 
 >     and accurate timing Windows poses a lot of problems.
 >     0 to 10 Hz is my range of interest. I want about +/-0.1 sec
 > accuracy, 24 hrs / day. This is why I use an ADC which has 
it's own on
 > board timing and correction. The data goes to the hard disk 
with a time
 > stamp regardless of the rubbish timing on the operating system.
 >     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.
 >     Can you say that again please? I don't quite understand.
 >     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.
 >     You have the real and the software clocks on computer 
boards. You
 > used to be able to get 4.194 MHz AT crystal clocks on 
computers, which
 > could be made moderately reliable. All the boards that I have 
 > recently use 32 KHz crystals which appear to be rejects from 
 > manufacturers. A random error of 20 sec to 2 min a day is 
what I call
 > utter ****. If you had a quartz watch which was that bad, you 
would take
 > it back to the shop and demand your money back.
 >     A GPS for a time base, a good a/d converter and fast 
 >     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
 >     Does 10 bits give you enough resolution, taking into 
account sensor
 > noise? The Dataq $25 starter ADCs give you 4 Channels at 240 
sps total -
 > 60 sps / channel.
 >     I can't figure out at the moment why you would need a high
 > monitoring rate for looking at water levels?
 >     Regards,
 >     Chris Chapman

I am not looking for water levels I putting out sensors and 
making a shock moving over and making another shock ...... and 
another shock and resetting the sensors and doing it over again 
to do a seismic study of the aquifer of the water bearing sand 
and 30 feet down. I need a post process GPS signal to know where 
I am I might as well use the time spruce on it.

Almost everything I work with has a GPS. I did the first vehicle 
units for and all the software for spryer 

I agree they are power hogs and take a lot of resources for time 
for seismic senors that VLF will do just as well as GPS. I have 
seen jitter on VLF from WWV in a really bad magnetic storm but 
not as often as I have lost lock with the GPS.

If you want to time sync a signal on a sound card you can depend 
on windows for time. It is too uncertain when the windows 
routine will time stamp it. Windows can't be made work in real time.

If you want to record 0 to 20 Hz signals on a sound card a mixer 
and local oscillator at 500 Hz will move the 0 to 20 Hz to 500 
to 520 and the inverse of it to 480 to 500. You can use DSP to 
get the signal back from either one.

Ten bits is enough for most situations in vehicles any more is 
lost in the noise of the vehicle. In the case of seismometer in 
an open field a more sensitive A/D could be used.



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