Passive Velocity Tomography Research Articles

Experimental Study on Damage Characteristics of Coal Samples under True Triaxial Loading and Dynamic Unloading

Abstract Coal bursts are one of the formidable hazards in underground coal mines, yet it is still not fully explored due to the complex stress environment that exists during mining. To better understand the bursting mechanism of coal under real-time mining conditions, it is necessary to develop an experimental method capable of reproducing in situ stress and loading-unloading paths of coal in in situ conditions. In this study, a self-developed true triaxial testing system was used to investigate the damage and failure characteristics of coal samples under true triaxial loading and dynamic unloading conditions. Acoustic Emission (AE) monitoring was used to capture the fracturing of the loaded coal. Passive Velocity Tomography (PVT) and Cumulative AE Energy Density (CAEED) were used to analyse damage evolution characteristics of the coal samples under true triaxial loading conditions. A high-speed camera was used to record the failure of the coal samples when the minimum principal stress σ3 was suddenly unloaded. It was found that continuous coal damage occurred primarily during the true triaxial loading period. The peak number of AE events in the coal samples increased and then dropped as σ1 levels increased. High and low wave velocity zones in the coal samples represent regenerations of the high-density zone and fracture emergence, respectively. Significant energy release zones transferred and expanded as the triaxial loading level increased. Under significant triaxial loads, the coal samples failed and were severely damaged, and the dynamic unloading of σ3 caused ejections of coal fragments at low velocities. The outcome of this study provides in-depth understanding of the failure mechanism of coal under in situ conditions.

Regional local integrated rockburst monitoring and early warning for multi-seam mining

Abstract Rockburst has become a main problem affecting the safety and efficiency of coal mine production. Early warning of it is essentially important for safety management. The dynamic and static load sources of rockburst, induced by multi-coal seam mining, are more complex, making rockburst prediction more difficult. The problem of rockburst early warnings in multi-seam mining was addressed by establishing a regional local integrated rockburst monitoring and early warning method based on the following criteria. (i) In regional space, the microseismic (MS) frequency ratio index and dispersion index were used to predict the regional dynamic load. (ii) In the local excavation and mining space, the risk degree of local static load concentration was characterised by the overrun amplitude indexes of electromagnetic radiation (EMR) intensity and pulse. (iii) According to the predicted spatial location of dynamic load and the attenuation law of regional dynamic load, the influence weight of the regional dynamic load on the excavation and mining space was determined. Based on this, the danger degrees of regional dynamic load and local static load were superposed to realise the regional local integrated warning of rockburst. This prediction method was applied for predicting rockburst in the Xinzhouyao mine, and the result indicated that the method can effectively predict the occurrence of dynamic load, the risk degree of local static load and rockburst induced by multiple types of dynamic and static loading combined together. The reliability of this prediction method was verified by records of roadway deformation and rockburst, and detection results from passive velocity tomography.

A New Method to Assess Coal Burst Risks Using Dynamic and Static Loading Analysis

Mining-induced seismicity is one of the dynamic energy sources that can trigger coal burst. This paper presents a new methodology to assess coal burst risks under different loading conditions by examining seismic energy attenuation and fracture size. Two new indices are proposed: (1) Dynamic Load Index (DLI) quantifies the magnitude of dynamic loading induced by seismic events, based on the relationships between the seismic energy, peak particle velocity and dynamic stress; (2) dynamic–static loading assessment index, $$I_{\text{PD}}$$, links the DLI with passive velocity tomography (PVT) to assess the coal burst risks in a longwall panel under dynamic and static loading. A total of 3080 seismic records were examined to validate $$I_{\text{PD}}$$ in a typical burst-prone longwall panel in China. Based on the rate of occurrence of high-magnitude seismic events, $$I_{\text{PD}}$$ thresholds were determined to identify low-, medium- and high-risk coal burst zones. Using this new zoning approach, coal burst risks were assessed in the same longwall panel while mining through a fault structure. The proposed risk classes correlated well with the recorded high-magnitude seismic events with energies over 10 kJ. The analysis indicated that 69% of the high-magnitude events occurred in the high-risk zones, where $$I_{\text{PD}}$$ was between 0.35 and 1; and 31% of the events occurred in the medium-risk zones, where $$I_{\text{PD}}$$ was between 0.2 and 0.35.

Experimental study of rock burst in coal samples under overstress and true-triaxial unloading through passive velocity tomography

The influence of overstress state on rockbursts in deep underground coal mining was investigated. Laboratory experiments were conducted using a true-triaxial rockburst system. Passive velocity tomography, using acoustic emission (AE), was used to study the internal microfractures and energy evolution of coal samples during true-triaxial compression. AE activities were monitored using eight sensors and every 130 consecutive AE events were used for tomographic calculations. For each coal sample, three phases and between four and six typical tomography results were obtained which reflected significant variations in velocity redistribution. Experimental results show that there was a significant time-delay effect under different loading and unloading stress paths. Compared to triaxial loading, coal samples are more easy to deform and bring to failure under triaxial unloading. Under a constant confining pressure, bursting failure occurs when the axial stress is more than three times the uniaxial compressive strength of the coal sample. The wave velocity inversion results show that the macrofracture positions in coal samples after failure have a positive correlation with the region of wave velocity anomalies, and the low wave velocity zone corresponds to areas of microfracture evolution.

Passive velocity tomography for mudstone under uniaxial compression using acoustic emission

A passive velocity tomography method using acoustic emission (AE) was used to study characteristics of AE responses and velocity redistributions in mudstone during uniaxial deformation. Two standard cylindrical samples were uniaxially deformed until failure with axial loading rates of 1.00 × 10–3 mm/s and 2.50 × 10–3 mm/s, respectively. AE activities were monitored using eight sensors and every 100 consecutive AE events were used for tomography calculations. For each sample, three typical tomography results were obtained which reflected significant variation of velocity redistributions. From the experimental data, it can be concluded that the stress drop point observed in the stress-strain curves with high energies and AE events indicated coalescence of micro-cracks and formation of the main shear plane. In the initial tomography phase, the velocity difference was low and few AE events were detected. As loading increased, AE events clustered and velocity differences became obvious with high velocities being mainly located near the sample boundary, whereas low velocities begun to propagate from the bottom corner to the core. When approaching failure, velocity anomaly regions further expanded and low velocity regions interconnected with the position being consistent with macro-fractures in the post-failure samples. The positions of the AE events with large energies over 50 μV·s were found to correlate well with high velocity regions in the tomography results whose calculation phase was conducted prior to the occurrence of large energy AE events. This method can be used for the prediction of large energy AE events in rocks under unconfined pressures.

Case study of seismic hazard assessment in underground coal mining using passive tomography

The relation between applied stress and wave velocity is examined firstly at the laboratory-scale by using ultrasonic technique. Subsequently, an island longwall face under hard-thick strata, suffering from the threat of seismic hazards, e.g. rock burst, strong tremor, was chosen for a case study of passive velocity tomography. Based on continuous seismic monitoring during mining operation, passive tomographic imaging has been used to locate high seismic activity zones and assess seismic hazard around the face. The results show that the high velocity and velocity gradient regions correlated well with the high seismic activities occurred in future mining period. In addition, the high velocity or velocity gradient regions redistributed with the retreat of coalface and large-scale movement of key strata. Passive velocity tomography can be a promising tool to continuously monitor the relatively high stress zone or evaluate seismic hazard in underground coal mining.

Seismic tomography of the crust and lithospheric mantle in the Betic Cordillera and Alboran Sea

The Alboran Domain lies between the South of Spain and the North of Morocco and contains a Miocene extensional basin which formed during continuous compression between the African and Eurasian plates. We have performed a 1-D and 3-D passive velocity tomography study of the structure of the crust and lithospheric mantle of the Betic Cordillera and Alboran Sea region. An important drawback encountered in most passive seismic tomography studies is that the quality of the data set used is inhomogeneous and it is often impossible to measure the error. This occurs as the data set needs to be very large which implies using several years’ worth of data from different seismic networks with different station noise levels and phase travel time picking accuracy. We present a simple method to quantitatively determine the errors in the travel time data typically used in passive seismic tomography studies. We use this method to determine the standard error of our data set and present the tomography results with quantitatively determined error bars. An additional advantage of performing the error analysis is that the data can be weighted in terms of its standard error, and hence the contribution of noisy data can be reduced in the inversion, resulting in substantially enhanced tomography images. The main structural results obtained in the 1-D inversion of the data are: (1) the average velocity of the crust in the Betic Cordillera is V p=6.0±0.05 km/s and V s=3.5±0.05 km/s, indicating an average V p/ V s ratio of 1.71 for the crust. (2) The average Moho depth in the Betic Cordillera is less than 36 km. (3) The average crustal velocity in the Alboran Sea is V p=6.4±0.1 km/s and V s=3.4±0.2 km/s, indicating an average V p/ V s ratio of 1.88 for the crust. (4) The average depth of the Moho in the Alboran Sea is greater than 20 km. The 3-D inversion provides new structural information of the Alboran basin over the depth range 24–60 km. For the depth range 24–31 km we image the centre and north western margin of the basin and observe velocities of the order of 7.2–7.5±0.45 km/s, which could correspond to lower crust material or else anomalous low velocity mantle material. For the depth range 31–40 km we image the centre and the southern margin of the basin and observe P phase velocities of 8.0–8.2±0.55 km/s which we interpret as normal mantle material. Below this layer, in the depth range 40–60 km we image the centre and northern margin of the basin and observe low P velocities ( V p=7.8±0.55 km/s). At present the preferred model used to explain the formation of the Alboran Sea Basin makes use of the process of Lithospheric Delamination. The low velocity layer observed in the mantle starting at approximately 40 km depth could be interpreted to be hot mantle material or the asthenosphere, however, the normal mantle velocities observed in the 31–40 km layer above it do not support replacement of the lithosphere by hot asthenospheric material in the lower crust or immediately beneath the crust. If this is so, then delamination must have occurred within the lithospheric mantle at an approximate depth of 40 km, or else not occurred at all.