Installation of 100m laser strainmeters in a 1km deep tunnel for the purpose of geodynamic study
Abstract
We are now installing two orthogonal laser strainmeters, each of which has a length of 100m, in a deep tunnel at the Kamioka Mine, Gifu prefecture, Japan. The geographic coordinates of the observation site are 36.42 N, 137.30 E and 370 m above mean sea level. The laser strainmeter system is installed about 1km below the ground surface. The geological constituent around the tunnel is identified as the metamorphic rock of Hida gneiss. The velocities of P and S waves at the observation site were determined from in-situ blast data to be 5.45 km/s and 3.10 km/s, respectively. A frequency-doubled YAG laser with a wavelength of 532 nm is used as the laser source. The laser frequency is locked on an iodine absorption band of saturation spectroscopy and the stability of 2 × 10-13 is attained. The light paths of the laser strainmeter system are enclosed in SUS304 stainless steel pipes having a diameter of 190 mm and a thickness of 3 mm. In order to reduce outgas, the inside of the pipe is taken a process of electrochemical buffing. Inside pressure is kept to be 10-5 Pa. Consequently, quantitative measurement of crustal strains of the order of 10-13 becomes possible by the laser strainmeter system at Kamioka. This resolving power corresponds to that of the superconducting gravimeter. Therefore, we can expect to determine parameters of the fluid core resonance, core modes and core undertone as well as other geodynamic signals such as slow strain changes caused by silent earthquakes or slow earthquakes by employing data of the new laser strainmeter system.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....4911T