High-frequency accelerometer
Similar to geophones, accelerometers are pendulum-based seismometers that are capable to record seismic waves, but accelerometers are able to record higher frequencies and outperform geophones in the kHz range. The recording of kHz waves is necessary to record magnitudes smaller than -2, which correspond to fractures on meter or centimeter scale.
In the Bedretto Reservoir Project we install two kinds of accelerometers: First, we decided for six Wilcoxon 736 sensors, as this type of sensor covers the broadest frequency band up to 25 kHz. Second, we installed three Endevco 45A19 sensors from Megitt that are capable to record up to 6 kHz and come with a higher sensitivity. IMS South Africa built both sensors into a housing that allows cementing the sensors in the boreholes and that allow operations at pressures up to 100 bar. We relied on the experiences from IMS in sensor construction, because building a housing without introducing unwanted resonances is very challenging in this frequency range. One to three accelerometers are installed per borehole.