In a message dated 2007/04/05, tchannel@.............. writes:

> Hi Chris,  Thanks for the explanation.  Have they also use lasers, in such 
> devises?   Ted

Hi Ted,

       Lasers have been used, but I don't know how successfully. In seismic 
applications you tend to be limited by 1/2 the wavelength of light for fringe 
counting - a step size of say ~0.3 micron = large. Fringe counting may sound 
simple, but it is not a 'cheap and easy technique'. Your average microseism may 
be ~2 microns. You want to measure seismic movements down to nano metres. 
Measuring to 1/100 of a wavelength tends to be very costly. Especially when a 
small filament bulb and a couple of Si photocells can give you an optical sensor 
with a noise level <15 nano metres! The intensity stability of semiconductor 
lasers in my experience is lousy.
       The huge open trough tiltmeters in Washington State use floats with 
'free armature' LVDT magnetic sensors. See http://www.geodesy.cwu.edu/tilt/   
       The smaller ones at Walferdange use capacitative sensors of at least 
two types. There are also small bubble tiltmeters, which use AC fluid 
resistance measurement, but they do not have such a high resolution. They tend to be 
used for monitoring the tilts of buildings, bridges, roads, etc. See several des
criptions at http://www.geomechanics.com/bibliography.cfm
       The Earth Tide tilts may be ~0.25 micro radian = 1/4 mm at 1 km = not 
very large. 

       Regards,

       Chris Chapman   
In a me=
ssage dated 2007/04/05, tchannel@.............. writes:



Hi Chris,  Thanks for the=20=
explanation.  Have they also use lasers, in such devises?   T=
ed




Hi Ted,



       Lasers have been used, but I don't know=
 how successfully. In seismic applications you tend to be limited by 1/2 the=
 wavelength of light for fringe counting - a step size of say ~0.3 micron=20=
=3D large. Fringe counting may sound simple, but it is not a 'cheap and easy=
 technique'. Your average microseism may be ~2 microns. You want to measure=20=
seismic movements down to nano metres. Measuring to 1/100 of a wavelength te=
nds to be very costly. Especially when a small filament bulb and a couple of=
 Si photocells can give you an optical sensor with a noise level <15 nano=
 metres! The intensity stability of semiconductor lasers in my experience is=
 lousy.

       The huge open trough tiltmeters in Wash=
ington State use floats with 'free armature' LVDT magnetic sensors. See http=
://www.geodesy.cwu.edu/tilt/   

       The smaller ones at Walferdange use cap=
acitative sensors of at least two types. There are also small bubble tiltmet=
ers, which use AC fluid resistance measurement, but they do not have such a=20=
high resolution. They tend to be used for monitoring the tilts of buildings,=
 bridges, roads, etc. See several descriptions at http://www.geomechanics.co=
m/bibliography.cfm

       The Earth Tide tilts may be ~0.25 micro=
 radian =3D 1/4 mm at 1 km =3D not very large. 



       Regards,



       Chris Chapman