In-situ Acoustic Emission sensor

Piezoelectric in-situ acoustic emission (AE) sensors are ten times more sensitive than geophones or accelerometers, recording frequencies up to several hundred kHz. GMuG GmbH has designed highly sensitive AE sensors, which reduced resonances. They are optimized to monitor frequencies between 1 and 80 kHz, ideal to monitor small cracks of centimeter size within the reservoir. The sensor is made for installation via cementation and able to withstand high pressures > 100 bar. Eight to sixteen AE sensors are placed per borehole. Data is digitized with a 200 kHz sampling frequency.

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Multi-Disciplinary Monitoring Networks for Mesoscale Underground Experiments

Geothermal Testbed Monitoring Sensors

Geophones for Background Monitoring

Geophones are pendulum-based seismometers that are capable to measure seismic waves emitted by small seismic events.

Three 100-Hz geophones have been manufactured by IMS South Africa and installed in the deep boreholes. Geophones were retrofitted specially to record high frequencies up to the kHz range. The sensors must be spurious free when recording seismic events from short distances and able to withstand up to 100 bar background pressure. During an experiment, these geophones record larger seismic events (magnitudes up to M 0.5) without having clipped signals.

The geophone network is extended during the experiment with some additional 100Hz geophones installed temporarily in short boreholes in the tunnel and a geophone chain installed in the injection/extraction borehole.

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Multi-Disciplinary Monitoring Networks for Mesoscale Underground Experiments

Pore-Pressure Sensors and Multi-packer System

Pore pressure refers to the pressure of groundwater held within a soil or rock, in gaps between particles (pores). To measure pore pressure in different intervals of the experiment volume, we installed a multi-packer that separates the monitoring borehole "MB2" into several segments, each coming with one pressure sensor. Our multi-packer comes with 7 intervals separated by packers of 1m length. The intervals were chosen based on the observed fracture/fault clusters from the core and logging observations.

Multi-Packer system

Installed in borehole ST1 in the geothermal testbed. It is composed of 14 packers delimitating 15 intervals. The borehole ST1 has a total length of 400 m while the Multi Packer system has a total length of 388.32 m. It is equipped with a heatable fiber optic cable over the total length of the Multi packer system, a geophone packer installed at the bottom of the Multi packer system and pressure and temperature sensors at each interval. The pressure sensors of the intervals 1 and 4 suffered damage and thus, no pressure information is available at these intervals.

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Hydromechanical characterization of a fractured crystalline rock volume during multi-stage hydraulic stimulations at the BedrettoLab Multi-Disciplinary Monitoring Networks for Mesoscale Underground Experiments

6 Wilcoxon 736 sensors - this type of sensor covers the broadest frequency band up to 25 kHz. 3 Endevco 45A19 sensors (Megitt) - capable to record up to 6 kHz with a higher sensitivity.

IMS South Africa built both sensors into a housing that allows cementing the sensors in the boreholes allowing for operations at pressures up to 100 bar.

Ultrasonic Transmitters

We have installed sources in the monitoring boreholes suitable for cementation, which emit a repeatable signal in the long term. A ultrasonic transmitter has been installed to monitor the evolution of fracture geometries, of centimeter to meter scale. Manufactured by GMuG and retrofitted to withstand the pressures in the reservoir.

The transmitters are based on piezoelectric ceramics that emit after receiving a 1200 V delta impulse, a stress wave similar to seismic waves. Two transmitters are placed per borehole and distributed in such a way that the reservoir center is fully scanned from all directions.

Four seismic stations are permanently installed within the Bedretto tunnel.

Small device developed at GMF that sits inside a well-bore deep underground, which can create a fracture and measure its aperture in real-time, using the same instrument.

The sensors were arranged along chains with ten sensors each. As it is important that the specific wavelengths of each FBG sensor, that are arranged in series along the same fiber chain, must not overlap, the number of sensors per chain are limited to 10. In each borehole we use two sensor chains with alternating sensors for redundancy purposes. The sensors come with cable (multimode) loops to allow access from both sides, again for redundancy purposes. Because FBG sensors are sensitive to both strain and temperature, a single mode cable for temperature measurements is incorporated in the backloop cable.

Single-mode cables are suitable both for temperature measurements (Brillouin- or Raman based) and strain measurements (DSS=Brillouin or DAS=Rayleigh). The kind of measurement performed is defined by the interrogator used.

The Silixa Ultima XT DTS offers the established high standard of resolution, accuracy and reliability of the ULTIMA DTS family, but over a class-leading operating temperature range. The unit can operate in temperatures from -40◦C to +65◦C; all without compromising the superior accuracy and reliability. Complete with a self-calibrating utility and on-board solid state storage, the XT-DTS has been designed for long-term unattended measurements.

These geophones complement the microseismic monitoring of the largest events expected, by extending the network horizontally. The triaxial geophones contain 100Hz sensors (RTC 100Hz) and come with 100m cable. The tool is suitable for 3'' boreholes. The coupling in the boreholes is realized by two pneumatic cylinders per geophone.

Three tiltmeters complement the geomechanical monitoring in BedrettoLab.