From: andy@pangea.Stanford.EDU (Andy Michael USGS Guest)
Subject: Re: Seismology Tests in N. CA
Organization: Stanford Univ. Earth Sciences
Date: Wed, 2 Sep 1992 08:58:29 GMT

In article <1992Sep2.030404.15143@qualcomm.com> karn@servo.qualcomm.com (Phil Karn) writes:
>In article <1992Sep2.015855.16773@gordian.com> johnk@gordian.com (John Kalucki) writes:
>>How will time be calibrated and syncronized for the event? WWV? Or will
>>all of the seismographs be assembled in a big room, syncronized, and
>>then moved out to the field?
>
>I don't know if they're actually using it, but GPS essentially makes clock
>synchronization a solved problem. It's easy to synchronize clocks to within 1
>microsecond with GPS, which should be more than adequate for this task.
These particular instruments don't use GPS or other radio receiver
clocks.  They are a bit too old for that and its not really worth
upgrading them for it.  We do also have some instruments that have
WWV clocks but they won't be used for this experiment.

The clock timing goes as follows:
Back at Menlo Park there are a series of master clocks that are kept
synched to UTC with an accuracy somewhat better than a millisecond.
It might be a lot better, but for our work it wouldn't really matter.

A field master clock is synched to the master clocks before the
instruments leave the lab and after they return.  These field master
clocks drift at less than a millisecond per day.  To maintain this
they are run with a heater that keeps the electronics stable.

Each instrument has its own internal clock that may drift at up to
a few milliseconds a day.  The instrument clocks will be synched to
the field master clock before being deployed and after being
retrieved.  This will cover a time span of about 3 days.  There
shouldn't be any drifts of more than 0.01 seconds which is the
sampling rate of the network, although the portables will probably
be run at a higher rate.  But still it isn't a big deal.

However, to reduce the errors all the clock drifts are assummed
to be linear and the drifts are removed from the data by applying
clock corrections.  The linear assumptions have been tested and
are quite adequate for these purposes.

It may all sound complicated but its been done enough times that
the process does go smoothly.

Some of the newer instruments may have internal GOES satellite
clocks but I haven't worked with them yet so I am not sure.  One
problem with GPS over the past several years while these instruments
were developed was that there weren't enough satellites to ensure
coverage at all times so other methods were still needed.

BTW, the press release wasn't actually referring to the seismograph
clocks.  Those are always kept carefully not just for shots.  It
was referring to the time of the blast which will be controlled by
a blasting box with the same type of clock as the instruments.

On another note about explosive sources: a local fisherman accidently
dredged up an old MK-13 mine this weekend and the Navy nicely detonated
it just off Daly City, almost directly over the San Andreas fault.
While they didn't know the exact time the detonated it they were nice
enough to tell us where they did it from their GPS readings.  It'll
make a nice calibration shot for the events offshore in that area.  And
you can bet that without the driving force of a public safety threat we
would have had great trouble getting permission to shoot 760 lbs of
explosive there.

Andy
Xref: sparky ca.earthquakes:1046 sci.geo.geology:1885