EPOS-N - European Plate Observing System
Seismometers / vault construction
Stations: Upgrade/install notes
Ideas for improvements
Installation / Inventory
Last update: February 17, 2020, at 01:26 PM
There is no universally recognized body in the world of seismology that has the authority to establish standards related to seismometer instrumentation. Instead, each organization issues its own specification. Let's look at some.
It seems we can specify that seismometer orientation should be: ≤ 3°
Excerpts from Measurements of Seismometer Orientation at USArray Transportable Array and Backbone Stations, by Göran Ekström and Robert W. Busby (also local copy):
A plot of heading accuracy vs. latitude for Teledyne Marine FOG model CDL-TOGS-S (specified 0.5 deg secant latitude heading accuracy), between 70 and 81 deg N, with some geographic locations added:
Plot of heading accuracy vs. latitude for Teledyne Marine FOG model CDL-TOGS-S (specified 0.5 deg secant latitude heading accuracy), between 70 and 81 deg N, with some geographic locations added.
US Transportable Array sensor North alignment is now based Octans manufactured by iXBlue - ref. information in Measurements of Seismometer Orientation at USArray Transportable Array and Backbone Stations, by Göran Ekström (Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY) and Robert W. Busby (Incorporated Research Institutions for Seismology, Washington). Photo below is found in that article.
STS-2 sensor North alignment, using Octans devise. Image source.
CTBTO/IMS uses this optical gyro. However, it has a specified operating latitude of ±70° (ref datasheet). All new EPOS-N/SVALBARD stations will be above 76 deg N - how will the gyro operate at such latitudes?
Perhaps this limitation is a consequence of the physics behind optical gyros, which then has to be studied in more detail.
Model CDL-TOGS is intended for subsea use, with 3000 meter depth rating in basic version. Presumably such features drives price upwards. Aren't there any optical gyros for surface use, that could be suitable - and much less expensive? We have to search ...
When heading accuracy of gyros is specified to be "0.5° secant latitude" - which is the case for "CDL TOGS-S" - ref datasheet - it means it will be accurate to 0.5 degrees at the equator, and show reduced accuracy according to the secant of latitude, which is inverse of the cosine of latitude.
A plot of heading accuracy vs. latitude for this model, with some geographic locations added:
Model "MR-2" made by Topcon
Model AsteRx-U made by Septentrio/Altus.
Yes, we agree, this method is old-fashioned.
VADS seismometer North alignment, September 2016. Edge of compass is aligned to metal "ruler" placed at vault bottom; rope attached to one end of this ruler makes it easy to alter its azimuth angle. Afterwards, North alignment lines are drawn on concrete floor - see image below. Magnetic declination at that location was given as 13.6 degrees, so compass rotating ring is adjusted accordingly. All magnetic material removed when measuring incl keys, watch ...
VADS seismometer North alignment, September 2016. Magnetic declination at that location was given as 13.6 degrees.
Example from Jan Mayen. Declination at that location (70.9225N, 8.7138W) is according to NOOA on-line "Magnetic Declination Estimated Value" 8.4 grader West.