ALBERT

Description: Estimation of the recording completeness of seismic catalogs recorded with small networks in a heterogeneous observation volume, for example, in mines, is difficult. Local heterogeneities have a strong influence on the wave path and attenuation and must be taken into account. In order to analyze the spatially varying completeness of such catalogs in three dimensions, we present a new approach based on the probability-based magnitude of completeness (PMC) method of Schorlemmer and Woessner (2008). We demonstrate that the traditional approach of Schorlemmer and Woessner (2008) is insufficient in very complex and heterogeneous settings. To account for this problem, we extend the PMC method, taking into account the direction of incoming seismic waves. This allows us to analyze the influence of small heterogeneities on the recording completeness in high resolution. We compare the results with results obtained by a traditional Gutenberg–Richter frequency-magnitude analysis for the JAGUARS catalog: The Japanese German Underground Acoustic Emission Research in South Africa (JAGUARS) project recorded approximately 500,000 seismic events with magnitudes -5

Description: We investigate the source parameters of picoseismic and nanoseismic events (Mw〉-4.1) recorded with a high-sensitivity seismic network at the Mponeng gold mine in South Africa to gain new insights into the scaling of small seismic events. The Japanese–German Underground Acoustic Emission Research in South Africa (JAGUARS) network, composed of one three-component (3C) accelerometer (sensitivity 50 Hz to 25 kHz) and 8 acoustic emission (AE) sensors, (sensitivity 1 kHz to 180 kHz) is located at a depth of 3268 m and covers the limited volume of approximately 300×300×300 m. The AE sensors are calibrated with respect to the 3C accelerometer in the frequency band 400 Hz–17 kHz. The waveform data of two datasets are analyzed; (1) the aftershock sequence of an Mw 1.9 event that occurred approximately 30 m from our network, and (2) the postblasting activity recorded during working days, located at a distance 〉90 m from the network at the exploitation level. For the analysis we applied spectral fitting and spectral ratio methods. The calculated values of Mw range from -0.8 down to -4.1 with corner frequencies 0.8 kHz–13.6 kHz (source sizes from 8 cm to 1.3 m). We observe static stress drops ranging from 1 MPa to 10.0 MPa with apparent stresses of 0.01 MPa–1.00 MPa. Stress drops are independent of the moment, suggesting self-similar behavior in the analyzed magnitude range -4.1

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: We have determined the locations of more than 20,000 aftershocks (as small as moment magnitude M (sub w) -4.4 or even smaller) following an M 2 event in a South African gold mine, using manually picked arrival times. Spatial clustering into five groups was clearly discerned. A majority of the aftershocks formed a planar cluster ( approximately 4 m in apparent thickness, approximately 100 X 80 m in areal extent). This cluster is thought to delineate the rupture area of the mainshock because its orientation and spatial extent were consistent with the nodal plane of the centroid moment tensor (CMT) solution and with the corner frequency of the mainshock, respectively. The cluster's attitude suggests that the mainshock was a Mohr-Coulomb failure (or formation of a shear rupture surface in intact rock at an angle that obeys the Coulomb failure criterion) that took place in a vertical compression stress field that is indicated by borehole breakout patterns. The aftershock distribution also shows that the mainshock rupture was largely confined to the interior of a 25-m-thick vertical dike, although there are indications of interactions taking place between the rupture and the dike's material boundary with the host rock.