Hal Murray wrote:
Maybe they were actually interested in the humidity as a source of phase
distortion or attenuation?
Maybe they wanted to prove that the temperature did not cause a problem?
Or maybe just a government paid-for helicopter ride.
Geologists are interested in plate motions over extended periods of time
(years) so there should be lots of opportunities for rides. Are they
creeping or jumping? How far?
You can measure motion along a fault if you can measure the distance between
two places on opposite sides of the fault. In this area, the San Andreas
splits into several faults. That pair of mountains straddles most of them.
Distance is speed-of-light times time. They can measure the round trip time.
very accurately.. that's the whole Electronic Distance Measuring thing
which lasers revolutionized a few decades ago.
The speed of light in air depends upon the density which depends upon
temperature. I think I could work out the details with enough time/work, but
a quick search didn't find it.
Yes, humidity probably is important too.
yes it is.. and gas constituents and particulates as well.
I don't think they are interested in attenuation as long as they have a
strong enough signal to get a clean reading on the time.
Yes..
These days, geodetic GPS receivers are dispersed everywhere, BUT, they
need to calibrate and check, so they still do optical measurements over
some baselines. If you can measure India, measuring California is
straightforward (no worries about tigers eating the survey crew,
malaria, etc.)
Check out the Southern California Integrated Geodetic Network
http://www.scign.org/
250 receivers..
now part of the Plate Boundary Observatory
http://pboweb.unavco.org/
These days, lasers are used in strain meters...
"Strainmeters can resolve strain changes of less than one part per
billion (1 mm in 1000 km) at short periods, which makes them ideal for
capturing short-term transient deformation over time intervals ranging
from seconds to months; at longer periods, GPS techniques are more
stable, and by using GPS and strainmeters together, EarthScope will
capture a fuller range of Earth deformation processes. Borehole
strainmeters have been used for more than two decades to measure crustal
deformation near active faults and volcanoes in the United States,
Taiwan, China, Iceland and Japan. "
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