November 03, 2015. Japan , 09.51 PM (CET): A violent earthquake had occurred in the south of Japan that night. According to the Japan Meteorological Agency (JMA), it reached a magnitude of 6.7. The epicenter was located off the coast of the southern island of Kyushu. Almost at the exact same time several instruments at the Moxa Geodynamic Observatory (Germany, Thuringia) registered large signal deviations in measurements of the usually sinusoidal tide. Some of these instruments use laser interferometers from SIOS Meßtechnik GmbH.
Highly sensitive instruments enable the detection and recording of deformations and tremors of the earth's crust during violent events such as earthquakes, even in regions thousands of kilometers away. However, seismology is not only used to investigate purely geoscientific questions such as the origin of earthquakes or the structure of the Earth's interior, but have an important political dimension as well. This is because seismic measuring stations can register tremors caused by man-made events, such as nuclear weapons tests.
The Moxa geodynamic observatory is part of a network of several seismological facilities in Germany. Earth movements are registered using various different methods, studied and compared at the observatory, which is part of the Institute of Geosciences at the Friedrich Schiller University in Jena. The latest development in the field of recording geodynamic processes is the 1m laserstrainmeter from SIOS Meßtechnik GmbH.
The 1 m laser strainmeter is installed in an underground gallery of the observatory. There are three strainmeters in the north-south tunnel shown in Figure 2:
The larger measuring distance grants the 26 m Strain system a better signal-to-noise ratio (blue curve).
The main advantages of the Meterstrain system are undoubtedly its compact size, robust build and ease of use. Thus, it is predestined as a portable universal measuring instrument for measuring the smallest mechanical deformations on buildings or for investigative purposes in geodesy. High temperature stability and an internal environmental correction allow measurements even under extreme environmental conditions, such as 100% humidity.
Strainmeters determine deformations between fixed points by measuring the relative changes in length, i.e. the change in length relative to the base length.
At the heart of the instrument is the differential interferometer SP-DI series from SIOS. With its strictly symmetrical optical design the use of high-quality thermally stable materials and the associated extremely high long-term stability of the length measurement it offers the best prerequisites for measurements under difficult climatic conditions such as those prevailing underground. Since the amplitudes to be measured are in tiny orders of magnitude of a few 100 nm differential measurement is especially important.
Two parallel beams record the relative motion between a reference point and the measuring point with the highest resolution and stability. The factory-calibrated beam spacing enables high-precision angular measurement.
The standard differential interferometers of the SP-DI series have a measuring range of 2 m with a resolution of 20 pm. The beam spacing is 21 mm. Angle measurement is possible in a range of up to ±1.5 arcmin with a resolution of 0.0002 arcsec. The reference and measurement beams are fed out of the interferometer head, which is made from stainless steel. Both beams travel identical paths and are equally affected by environmental changes, such as temperature variations. Thus, a temperature stability of < 20 nm/K is achieved.
The setup of the meter strain is shown schematically in Figure 5.
The reference reflector is firmly connected to the sensor head at fixed the point A. The measuring reflector is attached to a quartz glass tube and suspended freely.
The other end of the 1 m long glass tube is attached to the fixed point B. The beamline was shielded and the fixing points were decoupled by joints. The reflectors are spherical reflectors
Based on the differential interferometers of the SP-DI series, laser interferometric measurement systems have been developed that are capable of monitoring mechanical deformations in the nanometer range, which can occur over a few seconds up to several months or years. High frequency bandwidth and resolution as well as compact dimensions make these measurement systems stand out. Their robust build with respect to environmental influences allows their use even in extreme climates.
Possible application scenarios include monitoring areas prone to landslides, tectonic movements along fault zones or monitoring volcanically active areas. In addition to this use cases arise in areas with man-made deformations, such as near dams, around mines, near geothermal power plants as well as near other underground storage facilities of any kind. Furthermore they can be used for monitoring compliance with the Nuclear Test Ban Treaty.