Description of the installation:
(from Widmer, Zürn and Masters, Geophys. J. Int. 1992, 111, 226-236)
The strainmeter array is installed at the very end of the tunnel system under about 170 m of overburden and a horizontal distance of 650 m from the mine entrance.
An air - lock reduces air pressure variations near the instruments and temperature variations are on the order of 1 mK. Three tunnels at angles of 120 degrees and 60 m long are the sites of three strainmeters with azimuths of N2E, N120E and N60W.
The strainmeters are slightly modified versions of the invar wire devices described by King and Bilham (1976) and Agnew (1986) and have a length of 10 m. One modification is the inclusion of an in-situ calibration device in the form of an expandable bearing plate. The output signals from the inductive displacement transducers (LVDTs) are fed into free mode bandpass filters with amplifications of about 600 and cutoff periods of about 80 s and 3000 s, respectively.
The output signals of these filters are then anti-alias filtered with a cutoff period of 25 s and digitized in intervals of 2 seconds and a resolution of 16 bits. The least significant bit in the passband corresponds to approximately 0.3*10**(-12) of strain, and because of the design it corresponds to a displacement of 0.03*10**(-9) m at the LVDTs (length of 10 m, mechanical amplification of 10 by lever arms).
Presently both mode and tide channels are recorded from all three strain meters at a sample rate of 1 sample every 5 seconds with a 21 bit ADC.