Tidal earth crust deformation measurements

Long-term measurements of earth crust deformation

Deformation of the earth crust mainly results from the tidal forces of sun and moon acting on the Earth, but also comes from seismic wave propagation or regional and local sources. Strainmeters allow the observation of crustal deformation with a resolution better than 1 nm. At the Geodynamic Observatory Moxa in Thuringia/Germany an assembly of strainmeters of different types records the deformation. The analysis of the strainmeter data shows the comparability of the data from the different instruments as well as the good data quality connected to the very low noise level at the Geodynamic Observatory Moxa. The strainmeter systems of SIOS Meßtechnik GmbH described in this paper are long range laser interferometers of Michelson type. These interferometers with corner cube reflectors are precision length measurement instruments and coupled by optical fibers. They are specially designed for long range operation under difficult environmental conditions. The interferometers are installed in the Geodynamic Observatory in a gallery dug horizontally to the adjacent slope. The measuring ranges are 26 m and 38 m. The resolution of the systems is about 1 nm. All interferometers are designed for long-term measurements over several weeks.
To minimize the influence of temperature and air pressure changes on the interference fringes due to the dependence of the refractivity index of air on these values, the horizontal borehole for the diagonal 38m strain is sealed at both ends with a special glass and the 26m strains use the hermetic metal tubes for the laser beam protection.
All data are sampled every 10 s. In addition, several environmental sensors measure variations of temperature, air pressure and humidity inside the gallery. At the laser strainmeter, another air pressure sensor is installed and five temperature sensors are placed at different points along the laser beam. Outside the observatory building, a meteorological station records environmental  parameters.

Fiber-coupled homodyne interferometer

Displacement measuring homodyne interferometers are interferometers which use only one frequency light source and compare the measuring displacement with a light wavelength. The devices allow ultra precise measurements with nanometer-scale resolutions. A metrological analysis of this measuring method shows the opportunities they afford and the metrological limits of laser-interferometric systems. For that purpose, consider a Michelson interferometer, the basic type of interferometer for displacement measurements, and assume that the laser light source employed emits plane waves that are split into a pair of coherent, partial waves and interfere through superposition.
The homodyne interferometers presented here are Michelson interferometers. The figure below shows schematically such interferometer, configured in the form interferometer with cube corner reflector.

Fully fiber-coupled homodyne interferometer sensor
1. Fully fiber-coupled homodyne interferometer sensor

Both the laser light source and power-supply/signalprocessing unit are separated from the measurement head. Light from the frequency-stabilized He-Ne laser is transmitted to themeasurement head on a single-mode fiberoptic lightguide, which allows keeping heat sources well away from the location where measurements are conducted. The advantage of the metrological method presented here is based on transmitting just a single beam that is reflected by the moving retroreflector per measurement axis, which allows configuring the metrological setup such that there will be a well-defined point of contact with the object being measured and that the laser beam will remain accurately aligned on the measurement axis, which, in turn, means that the configuration of an Abbé comparator will be maintained. Abbé errors, a typical error source, will thus be minimized or totally eliminated. Such interferometers have nanometer precisions and excellent linearity over displacements in meter range. The presented interferometer type is used in the application for the earth crust deformation measurement, because
of the low heat generation and simple design.

Interferometer application for tidal crust deformation measurements

The Geodynamic Observatory Moxa consists of a three-part building located at the foot of a hill, and a gallery dug horizontally into the adjacent slope. The gallery is separated from the building by several doors to keep environmental conditions, especially temperature, as stable as possible. Its instrumentation consists of different types of measurement systems, including besides strainmetersalso tiltmeters, seismometers, a superconducting gravimeter as well as a spring gravimeter. Additionally, outside the observatory and in the surrounding area a large number of sensors record environmental parameters like air pressure, temperature, wind, or soil moisture. All strainmeters are installed inside the gallery. The 26 m long quartz tube strainmeters and the 38 m long laserstrainmeter are connected to the ground by steel pillars whereas the borehole instrument is installed at about 10 m depth and the borehole filled up with concrete. The quartz tube strainmeters are oriented in east-west and north-south direction, respectively, along the two arms of the gallery. Distance changes along the two quartz tube strainmeters are measured by inductive sensors. The laser diagonal instrument connects the far ends of the quartz tube strainmeters through a horizontal borehole, and the borehole strainmeter is installed at the gallery elbow. The new laser based strainmeters have been installed paralle to quartz strainmeters, so they build now the completely triangular system for crust deformation monitoring.

2. Assembly of laser strainmeters at the Geodynamic Observatory Moxa.

The Figure 2 shows an assembly of the laser-interferometric strainmeters in the observatory.

The interferometers used as laser strainmeters are modified fiber-coupled homodyne interferometers with cube corner reflector (Figure 1). The high humidity at the measuring location of about 100% and the long stand-off distance of the reflector cause main problems for the
interferometer design. To minimize the influence of temperature and air pressure changes on the interference fringes due to the dependence of the refractive index of air on these values, the horizontal bore 38m hole is sealed
at both ends with a special glass and the 26m strains use the hermetic metal tubes for the laser beam protection.

 

Figure 3 presents the measuring principle of the laserinterferometric strainmeters.

3. Measuring principle of the laser-interferometric strainmeter

The light source for the interferometer is a modified fiber coupled SL-03 He-Ne laser of SIOS Meßtechnik GmbH, which is placed outside the device and outside the gallery behind the styropor door. The light comes to the interferometer via singlemode PM-fiber. The measuring beam of the interferometer goes through the tube with two glass windows. The tube is intended to be hermetic sealed. The air pressure sensor is installed on the tube and four temperature sensors are placed at different points along the laser beam. In addition, the one temperature sensors measure the temperature inside the gallery. This interferometer setup is used for both N-S and E-W strains. Also diagonal laserinterferometric strainmeter has similar setup.

Figure 4 shows the interferometers of the strains in gallery.

4. Thermally isolated north-south strain(left) and interferometer base for N-S and E-W strains (right)

Measurement results

The usual correlations of the interferometer measuring results in air to the temperature and to the air pressure are very high. The natural temperature stability in the gallery is only some tens of Kelvin over the year. But the air pressure changes are directly influences by the external natural effects. Therefore the variations of the air pressure along the measuring beam plays the main role for the stability of the measuring results.

Stability of the temperature of N-S strain (left) and air pressure of N-S and E-W strains
5. Stability of the temperature of N-S strain (left) and air pressure of N-S and E-W strains

The Figure5 presents the stability of temperature of N-S strain over 8 h and the stability of the air pressure over 12 d. The natural standard deviation of the air temperature in the gallery over 8 h is about 5mK. This temperature influences only short part of the interferometer measuring beam before and after the hermetic tube. The hermetic tube is additionally isolated from the environment by styropor material. Four other temperature sensors are connected to the tube and measure the temperature variations along the measuring beam. These sensors are used for monitoringpurposes only. As is presented in Figure 5 the standard deviation of the temperature along the beam is in the range of 1.5 mK.

The variation of the air pressure for N-S and E-W strains is presented in Figure 5 (right). It is measured inside the hermetic tube along the measuring beam of interferometers. For the air pressure measurements the high-precision resonance pressure sensor RPT201 is used. The sensor is temperature compensated and provides the air pressure dependent frequency output. In order to minimize the power consumption of the electronics these sensor digital outputs are converted in the data pre-processing box into the analogue harmonic signals and transferred over more than 66 m to the main data processing station. As is presented in the Figure 5, the high stability of the air pressure in the tube was achieved. Over the 12 d both sensors show the standard deviations of variations of the air pressure in the range of 1.6 Pa. Therefore the influence of these parameters to the measuring result of the interferometer can be neglected.

The high stable temperature and air pressure conditions of the measurements allow the direct monitoring of the earth crust deformation. Figure 6 presents the representative measuring curve of the N-S laser-interferometric strainmeter.

Typical earth deformation curve over 24 h
6. Typical earth deformation curve over 24 h

The amplitude of the curve in Figure 6 is about 1.2 μm. For the stand-off distance of 26m it is a relative value of 4.6 × 10−8. Such values are usually not achievable for the interferometric displacement measurements in air. The measuring curve is not noisy and is not drift affected. Even the earthquake effects, which are monitored in the observatory by different instruments can be detected in the measuring results. The data is saved every 10 s continuously over 24 h.

The data processing of the measured earth crust deformation and comparison to the other measuring instruments is a complex mathematical task.

Summary

The article presented an application of the fiber coupled homodyne interferometers for the tide caused earth crust deformation measurements. Three laser-interferometric strainmeters measure the crust deformation continuously over 24 h. The interferometers for this application were designed for long-range application under the high humidity conditions. The achieved high stable condition of the measurements specially in respect to the air pressure and advanced instrumentation for measurement of the environmental parameters provided the background for successful installation of the system.

This joint research project was realized between the FSU Jena, Institute of Geosciences in Jena and SIOS Meßtechnik GmbH.