ALBERT

Description: In this paper, an underground experiment at the Äspö Hard Rock Laboratory (HRL) is described. Main goal is optimizing geothermal heat exchange in crystalline rock mass at depth by multistage hydraulic fracturing with minimal impact on the environment, that is, seismic events. For this, three arrays with acoustic emission, microseismicity and electromagnetic sensors are installed mapping hydraulic fracture initiation and growth. Fractures are driven by three different water injection schemes (continuous, progressive and pulse pressurization). After a brief review of hydraulic fracture operations in crystalline rock mass at mine scale, the site geology and the stress conditions at Äspö HRL are described. Then, the continuous, single-flow rate and alternative, multiple-flow rate fracture breakdown tests in a horizontal borehole at depth level 410 m are described together with the monitoring networks and sensitivity. Monitoring results include the primary catalogue of acoustic emission hypocentres obtained from four hydraulic fractures with the in situ trigger and localizing network. The continuous versus alternative water injection schemes are discussed in terms of the fracture breakdown pressure, the fracture pattern from impression packer result and the monitoring at the arrays. An example of multistage hydraulic fracturing with several phases of opening and closing of fracture walls is evaluated using data from acoustic emissions, seismic broad-band recordings and electromagnetic signal response. Based on our limited amount of in situ tests (six) and evaluation of three tests in Ävrö granodiorite, in the multiple-flow rate test with progressively increasing target pressure, the acoustic emission activity starts at a later stage in the fracturing process compared to the conventional fracturing case with continuous water injection. In tendency, also the total number and magnitude of acoustic events are found to be smaller in the progressive treatment with frequent phases of depressurization.

Description: 〈span〉〈div〉Summary〈/div〉Knowledge of the position of lithological boundaries is key information for a realistic interpretation of geological settings. Especially in the mining environment, the exact knowledge of geometrical boundaries and characteristics of rock structures has a great impact for both economic decisions and safety awareness. For this purpose, we investigate the P-coda of high frequency acoustic emission events (picoseismicity) and test the application of array seismology techniques, usually used to study the Earth's deep interior, on a much smaller scale in a mining environment. In total 52 events were used, all of them recorded in the Asse II salt mine in Lower Saxony (Germany) using a network of 16 piezoelectric sensors. Many of these events show a pulse-like arrival in the late P-coda, suggesting the presence of a well-defined structure which scatters seismic energy. To explore the directional information of the signals in the seismograms we use the sliding-window slowness-backazimuth analysis, performed on the waveform envelope of the entire recording. Strong direct P-wave arrivals are clearly visible with observed slowness and backazimuth as expected for a homogenous medium. This implies straight ray paths from event to sensors indicating that the medium between the events and the sensors is homogeneous for wavelengths larger than about 60 cm. In the late P-coda we observe out-of-plane arrivals from South-East and, assuming single P-to-P scattering, we find that the scatterers responsible for these observations are clustered in space defining a sharp reflector corresponding to a known lithological boundary located at the southern flank of the salt dome. In agreement with the established geological model we observe no other dominant reflections in the analyzed waveforms that would indicate previously unknown lithological boundaries. This study shows that array seismology can be applied to acoustic emissions in mines to gain more information on structures and heterogeneities located in the vicinity of the monitored rock volume. In micro-acoustically monitored mines, this technique could be a valuable addition to increase hazard awareness and mining efficiency at little or no extra costs.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉Knowledge of the position of lithological boundaries is key information for a realistic interpretation of geological settings. Especially in the mining environment, the exact knowledge of geometrical boundaries and characteristics of rock structures has a great impact for both economic decisions and safety awareness. For this purpose, we investigate the P-coda of high frequency acoustic emission (AE) events (picoseismicity) and test the application of array seismology techniques, usually used to study the Earth's deep interior, on a much smaller scale in a mining environment. In total 52 events were used, all of them recorded in the Asse II salt mine in Lower Saxony (Germany) using a network of 16 piezoelectric sensors. Many of these events show a pulse-like arrival in the late P-coda, suggesting the presence of a well-defined structure which scatters seismic energy. To explore the directional information of the signals in the seismograms we use the sliding-window slowness-backazimuth analysis, performed on the waveform envelope of the entire recording. Strong direct 〈span〉P〈/span〉-wave arrivals are clearly visible with observed slowness and backazimuth as expected for a homogenous medium. This implies straight ray paths from event to sensors indicating that the medium between the events and the sensors is homogeneous for wavelengths larger than about 60 cm. In the late P-coda we observe out-of-plane arrivals from southeast and, assuming single P-to-P scattering, we find that the scatterers responsible for these observations are clustered in space defining a sharp reflector corresponding to a known lithological boundary located at the southern flank of the salt dome. In agreement with the established geological model we observe no other dominant reflections in the analysed waveforms that would indicate previously unknown lithological boundaries. This study shows that array seismology can be applied to AEs in mines to gain more information on structures and heterogeneities located in the vicinity of the monitored rock volume. In micro-acoustically monitored mines, this technique could be a valuable addition to increase hazard awareness and mining efficiency at little or no extra costs.〈/span〉

Description: We investigate the source characteristics of picoseismicity (Mw 〈 −2) recorded during a hydraulic fracturing in situ experiment performed in the underground Äspö Hard Rock Laboratory, Sweden. The experiment consisted of six stimulations driven by three different water injection schemes and was performed inside a 28-m-long, horizontal borehole located at 410-m depth. The fracturing processes were monitored with a variety of seismic networks including broadband seismometers, geophones, high-frequency accelerometers, and acoustic emission sensors thereby covering a wide frequency band between 0.01 and 100,000 Hz. Here we study the high-frequency signals with dominant frequencies exceeding 1000 Hz. The combined seismic network allowed for detection and detailed analysis of 196 small-scale seismic events with moment magnitudes MW 〈 −3.5 (source sizes of decimeter scale) that occurred solely during the stimulations and shortly after. The double-difference relocated hypocenter catalog as well as source parameters were used to study the physical characteristics of the induced seismicity and then compared to the stimulation parameters. We observe a spatiotemporal migration of the picoseismic events away and toward the injection intervals in direct correlation with changes in the hydraulic energy (product of fluid injection pressure and injection rate). We find that the total radiated seismic energy is extremely low with respect to the product of injected fluid volume and pressure (hydraulic energy). The radiated seismic energy correlates well with the hydraulic energy rate. The obtained fault plane solutions for particularly well-characterized events signify the reactivation of preexisting rock defects under influence of increased pore fluid pressure on fault plane orientations in good correspondence with the local stress field orientation. ©2018. American Geophysical Union. All Rights Reserved.

Description: In-situ acoustic emission (AE) monitoring is carried out in mines, tunnels and underground laboratories in the context of structural health monitoring, in decameter-scale research projects investigating the physics of earthquake nucleation and propagation and in research projects looking into the seismo-hydro-mechanical response of the rock mass in the context of hydraulic stimulations or nuclear waste storage. In addition surface applications e.g. monitoring rock faces of large construction sites, rock fall areas and rock slopes are documented in the literature. In geomechanical investigations in-situ AE monitoring provides information regarding the stability of underground cavities, the state of stress and the integrity of the rock mass. The analysis of AE events recorded in-situ allows to bridge the observational gap between the studies of faulting processes in laboratory and studies of larger natural and induced earthquakes. This chapter provides an overview of various projects involving in-situ AE monitoring underground with a focus on recent achievements in the field. In-situ AE monitoring networks are able to record AE activity from distances up to 200 m, but the monitoring limits depend strongly on the extension of the network, geological and tectonic conditions. Very small seismic events with source sizes on approximately decimeter to millimeter scale are detected. In conclusion in-situ AE monitoring is a useful tool to observe instabilities in rock long before any damage becomes directly visible and is indispensable in high-resolution observations of rock volume deformation in decameter in-situ rock experiments.