Microseismic Method Research Articles

Theoretical and Experimental Studies of Localization Methodology for AE and Microseismic Sources Without Pre-Measured Wave Velocity in Mines

The acoustic emission (AE) and microseismic (MS) monitoring are efficient methods to detect faults/breaking signals for both healthy evaluation and disaster control in mining engineering. This paper presents an MS or AE source location method without the need for a pre-measured wave velocity. It can eliminate the location errors for MS/AE monitoring systems caused by the deviations of the wave velocity. To verify the applicability of the proposed method, first, tests of both the pencil lead break and the thermal fracture in granite were carried out, and location errors were compared and analyzed. Results show that the location accuracy of the proposed method is significantly improved, which is superior to the results of the traditional location method (TM) using pre-measured wave velocity. Second, blasting experiments were carried out in Dongguashan copper mine in China. The blasts were used as simulated seismic sources. Average values of absolute distance errors of the MS/AE source locations resulting from the proposed method without wave velocity and the traditional method using average measured wave velocity are 10.16 and 17.55 m, respectively. It shows that the calculated locations by the proposed method are in better agreement with the real blast coordinates. Third, the proposed method is also applied to previously published data. It gives superior results compared with the considered existing methods. Results of the pencil lead break tests, the thermal fracture experiment in granite, and the blasting experiments (including published data) have demonstrated that the proposed method can not only decrease the location errors induced by measurement deviation of velocity, but also locate the MS/AE source in real time, which is a beneficial complement to the method TM in mines.

A Method for Dynamic Risk Assessment and Management of Rockbursts in Drill and Blast Tunnels

Focusing on the problems caused by rockburst hazards in deep tunnels, such as casualties, damage to construction equipment and facilities, construction schedule delays, and project cost increase, this research attempts to present a methodology for dynamic risk assessment and management of rockbursts in D&B tunnels. The basic idea of dynamic risk assessment and management of rockbursts is determined, and methods associated with each step in the rockburst risk assessment and management process are given, respectively. Among them, the main parts include a microseismic method for early warning the occurrence probability of rockburst risk, an estimation method that aims to assess potential consequences of rockburst risk, an evaluation method that utilizes a new quantitative index considering both occurrence probability and consequences for determining the level of rockburst risk, and the dynamic updating. Specifically, this research briefly describes the referenced microseismic method of warning rockburst, but focuses on the analysis of consequences and associated risk assessment and management of rockburst. Using the proposed method of risk assessment and management of rockburst, the occurrence probability, potential consequences, and the level of rockburst risk can be obtained in real-time during tunnel excavation, which contributes to the dynamic optimisation of risk mitigation measures and their application. The applicability of the proposed method has been verified by those cases from the Jinping II deep headrace and water drainage tunnels at depths of 1900–2525 m (with a length of 11.6 km in total for D&B tunnels).

Microseismic Real-time Monitoring System Based on Virtual Instruments

Geological disasters such as rock burst coal and gas outburst in coal mines have inevitable link with micro-seismic phenomenon intrinsically. Monitoring the underground microseismic signals could determine the scope and extent of the underground rock and provide strong evidence for underground mine safety monitoring. A microseismic real-time monitoring system design based on virtual instrument was presented, using thirty two channels front microseismic signal processing unit, data acquisition card DAQ-6343, realizing multi-channel analog microseismic signals acquisition, achieved a real-time monitoring and analysis of microseismic signals combined with the microseismic method and improved adaptive wavelet transform processing algorithms in the LabVIEW platform. System monitoring results agree well with the actual mine earthquake, the error of obtained positioning results is small, realizing a real-time microseismic monitoring and analysis.

Integrated Analysis of the Formation Mechanism of Cracks in a Concrete Dam Using Microseismic Monitoring and Numerical Simulation

The dam of Guanyinyan hydropower station is composed of a concrete gravity dam in the left bank and a rockfill dam in the right bank. During the operation of the hydropower station, several surface cracks occurred in the concrete gravity dam, which threatened the stability of the dam. To evaluate the evolution trend of the cracks and forecast the potential risk of the dam, the microseismic (MS) monitoring technique and finite-element method were used. First, the concrete three-point bending field test was performed to prove the reliability of the MS technique in monitoring the concrete cracks. The MS monitoring results were consistent with the simulation results. Then, the MS monitoring system was installed in the dam body. By analysing the MS activities before and after the impoundment, the evolution trend of the cracks and potential risk of the dam were evaluated and forecasted. The simulation results were also consistent with the monitoring results. These results can provide significant references for the operation safety of the dam and also present a new thought for the risk evaluation of similar dam engineering.

Microseismic location: using both P and S waves with new methods

In this paper we develop a new method on the basis of the two exist two methods, using both P and S waves recorded with surface array and borehole data. The new microseismic location method take the full advantage of Generalized Pattern Search Method and Simulated Annealing Method, improving the location accuracy. We use this method to do the cast study. Consequently, using both P and S waves information in the location technique reduces the position uncertainty as compared to single P or S wave relative location. And it will be more applicable to low signal-to-noise ratios data.

Multi-Index Monitoring and Evaluation on Rock Burst in Yangcheng Mine

Based on the foreboding information monitoring of the energy released in the developing process of rock burst, prediction system for rock burst can be established. By using microseismic method, electromagnetic radiation method, and drilling bits method, rock burst in Yangcheng Mine was monitored, and a system of multi-index monitoring and evaluation on rock burst was established. Microseismic monitoring and electromagnetic radiation monitoring were early warning method, and drilling bits monitoring was burst region identification method. There were three identifying indexes: silence period in microseismic monitoring, rising period of the intensity, and rising period of pulse count in electromagnetic radiation monitoring. If there is identified burst risk in the workface, drilling bits method was used to ascertain the burst region, and then pressure releasing methods were carried out to eliminate the disaster.

Monitoring of hydraulic fracturing in Moomba, South Australia, using magnetotellurics

The microseismic method is the dominant geophysical tool for monitoring hydraulic fracturing. High-pressure injection of fluids creates fractures in the sub-surface, recorded as small seismic events. Inverting such events yields the temporal and spatial extent of deformation; however, there is no straightforward relationship between the location of micro-deformations and the migration of injected fluid. In this peer-reviewed paper, an alternative approach to deep-fluid monitoring using the magnetotelluric (MT) method is presented. As MT is primarily sensitive to the presence of fluids (through changes in the bulk electrical resistivity) and the orientation of fluid connection, this technique may yield important constraints on how fluids propagate from the well with time. This technique has been successfully used to monitor hydraulically induced fractures in two geothermal settings at depths of more than 3 km. A 10-stage horizontal-well hydraulic-fracture in the Cooper Basin for a shale-gas reservoir was monitored using MT. From the central injecting well, four lines of instruments were placed radially in a cross-pattern. These new electric and magnetic loggers were deployed for a continuous monitoring period of 65 days. As changes in the magnetic field are spatially uniform across the extent of the survey area, only two magnetic-field data recorders were placed in the array. A third logger was placed 10 km from the well, for redundancy and as a remote reference. Measurements were taken from mid-May to mid-July 2014, while the injection and drawback occurred for approximately one month. Processing regimes were created to convert field data into MT responses and resistivity models of the Earth. Theory, survey practice and outputs from this experiment are presented in this peer-reviewed paper.

A Microseismic Method for Dynamic Warning of Rockburst Development Processes in Tunnels

Early warning of rockbursts remains a worldwide challenge in rock engineering. In this work, a microseismicity-based method of rockburst warning in tunnels is proposed to warn of and reduce the risk of rockburst. The method uses real-time microseismic data and an established rockburst warning formula to provide dynamic warning of rockburst risk during excavation of a tunnel. The establishment of the rockburst warning formula involves several key parts. These include a rockburst database, selection of typical rockburst cases, functional relationships between microseismicity and rockbursts, optimal weighting coefficients, and dynamic updating. By using the proposed method, the probability of strain and strain-structure slip rockbursts of different intensity (extremely intense, intense, moderate, slight, and none) can be warned of in real time. The method has been successfully applied to rockburst warning in deeply buried tunnels at the Jinping II hydropower project (about 11.6 km in total for D & B tunnels). This success illustrates the applicability of the proposed method. In addition, it is found that during the rockburst development process, the microseismic eigenvalues for strainbursts are bigger than in strain-structure slip rockbursts of the same intensity.

Characterisation of induced fracture networks within an enhanced geothermal system using anisotropic electromagnetic modelling

As opinions regarding the future of energy production shift towards renewable sources, enhanced geothermal systems (EGS) are becoming an attractive prospect. The characterisation of fracture permeability at depth is central to the success of EGS. Recent magnetotelluric (MT) studies of the Paralana geothermal system (PGS), an EGS in South Australia, have measured changes in MT responses which were attributed to fracture networks generated during fluid injection experiments. However, extracting permeabilities from these measurements remains problematic as conventional isotropic MT modelling is unable to accommodate for the complexities present within an EGS. To circumvent this problem, we introduce an electrical anisotropy representation to allow better characterisation of volumes at depth. Forward modelling shows that MT measurements are sensitive to subtle variations in anisotropy. Subsequent two-dimensional anisotropic forward modelling shows that electrical anisotropy is able to reproduce the directional response associated with fractures generated by fluid injection experiments at the PGS. As such, we conclude that MT monitoring combined with anisotropic modelling is a promising alternative to the micro-seismic method when characterising fluid reservoirs within geothermal and coal seam gas reservoirs.

A new microseismic location method accounting for the influence of the hydraulic fracturing process

In the hydraulic fracturing process, the velocity model of traditional inversion is usually constructed by well logs, seismic data or calibration shots. The variation of pore pressure and fractures in this process has a major influence on the velocity. However, this influence is usually ignored in the velocity estimation model. In this paper, we propose a new microseismic location method that accounts for the influence of hydraulic fracturing on velocity. Firstly, we simulate the 3D hydraulic fracturing process based on mass conservation, the seepage equation and fracture mechanics. Then the equivalent velocity model is constructed using the Coates–Schoenberg method and fracture compliances. The 3D ray-tracing method is applied to forward-model the microseismic data and traditional inversion methods are applied to locate events and analyse the inversion error. Finally, we introduce a new method, called the equivalent velocity inversion method (EVIM), taking into account the influence of the fracturing process. The simulation results show that the hydraulic fracturing process affects the velocity and thus makes the inversion accuracy of the same receiver array vary considerably with the sources in the traditional methods. The new EVIM can effectively reduce the inversion error.

Active monitoring of hydraulic fracture propagation: an experimental and numerical study

Hydraulic fracturing is known as one of the most common stimulation techniques performed in oil and gas wells for maximising hydrocarbon production. It is a complex procedure due to numerous influencing factors associated with it. As a result, hydraulic fracturing monitoring techniques are used to determine the real-time extent of the induced fracture and to prevent unwanted events. Although the well-known method of monitoring is the microseismic method, active monitoring of a hydraulic fracture has shown capable of providing useful information about the fracture properties in both laboratory conditions and field operations. In this study, the focus is on laboratory experiment of hydraulic fracturing using a true-triaxial cell capable of simulating field conditions required for hydraulic fracturing. By injecting high-pressure fluid, a hydraulic fracture was induced inside a 20 cm cube of cement. Using a pair of ultrasonic transducers, transmission data were recorded before and during the test. Both cases of an open and closed hydraulic fracture were investigated. Then, using a discrete particle scheme, seismic monitoring of the hydraulic fracture was numerically modelled for a hexagonally packed sample and compared with the lab results. The results show good agreements with data in the literature. As the hydraulic fracture crosses the transducers line, signal dispersion was observed in the compressional wave data. A decrease was observed in both the amplitude and velocity of the waves. This can be used as an indicator of the hydraulic fracture width. As the fracture closes by reducing fluid pressure, a sensible increase occurred in the amplitude of the transmitted waves while the travel time showed no detectable variations. The numerical model produced similar results. As the modelled hydraulic fracture reached the source-receiver line, both amplitude and velocity of the transmitted waves decreased. This provides hope for the future real-time ability to monitor the growth of induced fractures during the fraccing operation. At present, however, it still needs improvements to be calibrated with experimental results.

Assessment of excavation damaged zone using a micromechanics model

Abstract It is well known that acoustic emission (AE) and microseismic (MS) events are indicators of rock fracturing or damage as the rock is brought to failure at high stress. By capturing the microseismic events, underground excavation induced rock mass degradation or damage can be located. The use of microseismic method has been shown as a valuable tool in a number of nuclear waste repository research programs to monitor the extent of the excavation damaged zone (EDZ), but most of the works are limited to a qualitative assessment. This paper presents a study on the quantification of the degree of damage, in terms of crack density calculated from the crack length, and the extent, in terms of crack density distribution, from microseismic event monitoring data. The approach builds on the finding that a realistic crack size corresponding to a microseismic event can be established by applying a tensile cracking model instead of the traditional shear model, commonly used in earthquake data analysis. It can be shown that brittle rock failure is the result of tensile crack initiation, propagation, accumulation, and interaction. Tensile stress can be generated in a confined rock with heterogeneous material properties. When a crack is formed by tensile cracking in this fashion, its orientation tends to become parallel to the direction of maximum compressive stress. A method is developed to take microseismic event monitoring data as input to determine the damage state and the extent of the EDZ by crack distribution. Based on the crack orientation and crack density information, the rock is modeled by a micro-mechanics based constitutive model which considers the anisotropic material properties. Numerical examples are presented using field monitoring data from a tunnel in granite to demonstrate how microseismicity can be quantitatively linked to dynamic rock mass properties.

Microseismic Monitoring of Highwall Mining Stability at Moura Mine, Australia

We present results obtained from a microseismic monitoring at Pit 20DU at Moura Mine in central Queensland. The objective of this study was to investigate the feasibility of using the microseismic method to map roof fracturing associated with highwall mining for highwall mining stability assessment. 14 triaxial geophones were installed in 7 boreholes across the highwall bench, covering an area of 300 m by 400 m. The experiment was earned out from Jun 12 to Jul 25, 1999. More than 7,000 events were recorded.Generally, the recorded events were weak in seismic energy. Many of the events only triggered the nearest one or two geophones. The events were classified into three types on the basis of then’ frequency content and seismic duration. Each type appears to be associated with different fracturing mechanisms.The first type consisted of two groupings of events. One was located in the main sandstone roof and constrained by existing faults. They may be caused by the release of a localised stress concentration. The second group was dominated by events located in the floor of the DU Seam near the highwall bench. They appeal to be controlled by the fault and floor stress concentration near the highwall face.The second type of events occurred across the mining area and followed the general sequence of the mining. These are inferred to represent a general ground response to the mining. Events of the third group were located mainly in the immediate roof of the DU Seam. They were found to be associated with minor fracturing near the mine entries.During the monitoring period, about 30 entries were mined and no significant entry or pit stability problems were encountered. Therefore, the fracturing giving rise to the microseismic activity observed at 20DU did not have an adverse impact on the highwall mining stability.

Microseismic techniques for monitoring the behaviour of rock structures : Report. 27F. US BUR. MINES, BULLETIN B665, 1974, 65P

From Introduction: This report presents the results of a 7-year Bureau of Mines program to review, revise, and make an effective engineering tool of the microseismic method of determining the stability of rock structures.

Relation of microseisms to meteorology

Microseismic data obtained by the Naval Aerological Service from its ten seismograph stations show that severe wind storms over water cause large sections of the Earth's crust to vibrate. These oscillations are called microseisms and have been recorded on sensitive seismographs up to 1600 mi from the source. The direction of propagation of certain dominant microseisms, especially those produced by hurricanes and typhoons, can be determined with great accuracy when they pass over a tripartite seismograph station. The microseismic method of storm detection as outlined in this paper is being developed by the Naval Aerological Service and is now used to supplement other methods of detecting and forecasting severe storms.