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    SEISMOMETER NORTH ORIENTATION    

     Specifications

UNIVERSITY OF BERGEN
Department of Earth Science
 Allé gt. 41, N-5007 Bergen, Norway 
             
0.1   DRAFT - request for comments - 19 October, 2017 OM OM -
VER. STATUS CHANGE DATE BY CKD APPD

1 REQUIREMENTS


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.

Seismological Research Requirements for a Comprehensive Test-Ban Monitoring System

Quoting from page 31:

w. Seismometer Orientation (known within 1 degree). This is an extraordinary high accuracy, not routinely achieved with any boreholde instrument (KS3600I and 54000I orientation is +/- 3 degrees), and vault-type instruments can be oriented this accurately only if a suitable accurate survey mark is provided in the vault. More recent GSE documents suggest that 3 degree is an acceptable specification. The panel recommends relaxation of the orientation tolerance.

CTBTO/IMS (Comprehensive Test Ban Treaty Organization / International Monitoring System) CTBTO, Preparatory Commission, doc # CTBT/WGB/TL-11,17/15/Rev.5: OPERATIONAL MANUAL FOR SEISMOLOGICAL MONITORING AND THE INTERNATIONAL EXCHANGE OF SEISMOLOGICAL DATA - DRAFT (alos local copy), Appendix I, page 33: Seismometer orientation: ≤ 3°

It seems we can specify that seismometer orientation should be: ≤ 3°

2 PROBLEMS RELATED TO SEISMOMETER NORTH ORIENTATION


Excerpts from Measurements of Seismometer Orientation at USArray Transportable Array and Backbone Stations, by Göran Ekström and Robert W. Busby (also local copy):

  • "In the SEED convention, as well as in other data distribution formats, the precision of this paramater [azimuth of horizontal components] is given to at least 0.1 degree."
  • "Despite the precision with which seismometer orientations are given, the accuracy of the reported azimuths is neither well-known nor easily verified."
  • "In particular, when a magnetic compass is used for the alignment, there is frequently a significant site-specific declination correction to be made. Errors are introduced when this correction is not made, when a correction is made in the opposite sense to that required, or when the wrong correction for the location is applied."
  • " Obtaining direct measurements of the orientation of the seismometer after installation is associated with the same difficulties and compounded by the presence of strong magnets within the sensor itself."

3 METHODS OF SENSOR NORTH ALIGNMENT


3.1 FOG = Fiber optic gyro


3.1.1 What is the difference between FOG and RLG (Ring Laser Gyro)


3.1.2 How geographical latitude affects FOG heading accuracy


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.

3.1.3 Model "Octans", made by iXBlue


  • Model: Octans
  • Data sheet: Link (PDF)
  • Manufacturer: iXBlue
  • Measures orientation to < 0.2 deg.
Click to enlarge.

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.

3.1.4 Model "Quadrans", made by iXBlue


  • Model: Quadrans
  • Data sheet: Link (PDF)
  • Manufacturer: iXBlue
  • Heading accuracy:
  • Secant of latitude means inverse of the cosine of latitude.
Click to enlarge.

Model "Quadrans", made by iXBlue.

3.1.5 Model "CDL TOGS-S", made by Teledyne Marine


  • Model: CDL TOGS-S
  • Data sheet: Link (PDF)
  • Manufacturer: Teledyne Marine
  • Heading accuracy: 0.5° secant latitude RMS
  • Secant of latitude means inverse of the cosine of latitude.
Click to visit product web page.

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:

3.2 APS = Azimuth Pointing System, as used by EarthScope Transportable Array / Alaska


Source: IRIS-PASSCAL: Installing a "Cold and Dark" Seismic Station in the Yukon

Source: IRIS-PASSCAL: Installing a "Cold and Dark" Seismic Station in the Yukon

3.3 Dual-antenna GNSS receiver


3.3.1 Model "MR-2" made by Topcon


Click to visit product web page.

Model "MR-2" made by Topcon

3.3.2 Septentrio/Altus products


Model AsteRx-U made by Septentrio/Altus.

AsteRx-U

  • Model AsteRx-U product web page.
  • Heading accuracy:
    • 0.15° with 1 m antenna separation
    • 0.03° with 5 m antenna separation

3.4 Compass alignment


Yes, we agree, this method is old-fashioned.

Click to enlarge.

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.

3.4.1 Compensating for magnetic declination


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.

4 MISC


NORTH orientation:

  • Online-kalkulator som gir asimut til sola for vilkårlige klokkeslett: http://www.susdesign.com/sunangle/
  • Det finnes også en app for mobilen (suncalc.org) som beregner solas posisjon ut fra sted og klokkeslett.
  • Sola flytter seg 1/4 grad i minuttet så det burde bli ganske nøyaktig.
  • Se også http://suncalc.net/
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Page last modified on January 10, 2019, at 12:52 PM
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