First field tests with the fiber optic high-temperature borehole seismometer

Recent geophysical investigations require highly sensitive monitoring of seismic events under extreme pressure and temperature conditions. Typical areas of interest are very deep boreholes and volcanic areas either for disaster prediction, fundamental research, or examining geothermal anomalies for exploration of natural energy resources. These non-trivial environmental conditions include temperatures at the measuring sites of more than 300°C, signal transmission lines for analogue signals over several km and extremely corrosive environments. There are several reports [1,2,3] on fiber optic readout of seismic mass, but until now several of the necessary components were unsuitable for long-term high temperature application. We report the first construction and practical application of a complete triaxial seismometer with fiberoptic interfmometric measurement of seismic mass vibrations (with <0.1nm. 100µm amplitude) in a 6cm diameter borehole geometry, and with a long-term working range of 0..260°C (300° for the fiber optic part). Construction and performance parameters of the high-temperature seismometer are now presented. This uses temperature-stable fiber optic components previously reported [4]. The first results of seismic measurements at high temperatures are also shown.