Abnormal characteristics and effectiveness evaluation of the micro-seismic signal before the Debao MS4.8 earthquake

The Debao MS4.8 earthquake occurred in western Guangxi on August 5,2021,near where the Jingxi MS5.2 earthquake occurred in 2019.To study the increasing seismicity in western Guangxi,it is necessary to determine whether there was an anomaly related to the earthquake source near the Pingxiang gravity station,which is located approximately 100 km from the epicenter of the Debao MS4.8 earthquake.In this study,the R-value scoring method was used to analyze the anomaly and evaluate the prediction efficiency of the double frequency(DF)micro-seismic signal vertical displacement(referred to as vertical displacement,VD)and the absolute value of monthly extreme rate(referred to as the monthly rate).Results show that earthquakes larger than MS4.0 in the 350 km range from the Pingxiang station tend to coincide with yearly typhoons,and the VD of micro-seismic signals correspondingly changes from low to high.The Debao MS4.8 earthquake occurred during a gradual VD increase from 0.05×10^(-6)to 0.10×10^(-6)m.When discussing the relationships among R,the rate threshold,and the effective duration of prediction,the rate threshold of the micro-seismic signal converges from 0.00039×10^(-6)to 0.00031×10^(-6)m/month,the effective duration of prediction is approximately 6-10 months,and R also converges from 0.29 to 0.31.By comparing the results of three gPhone gravity stations in Guangxi,we found that the increase of short-term VD before the Debao earthquake was related to the enhancement of the DF micro-seismic signal excited by the typhoon.When the typhoon track was perpendicular to the coastline of China,the possibility of an earthquake occurring was increased.This study provides evidence and reference for the future occurrence period of earthquakes above MS4.0 in western Guangxi.

Seismogenic Structure around the Epicenter of the May 12,2008 Wenchuan Earthquake from Micro-seismic Tomography

A three-dimensional local-scale P-velocity model down to 25 km depth around the main shock epicenter region was constructed using 83821 event-to-receiver seismic rays from 5856 aftershocks recorded by a newly deployed temporary seismic network. Checkerboard tests show that our tomographic model has lateral and vertical resolution of -2 km. The high-resolution P-velocity model revealed interesting structures in the seismogenic layer： （1） The Guanxian-Anxian fault, Yingxiu-Beichuan fault and Wenchuan-Maoxian fault of the Longmen Shan fault zone are well delineated by sharp upper crustal velocity changes; （2） The Pengguan massif has generally higher velocity than its surrounding areas, and may extend down to at least -10 km from the surface; （3） A sharp lateral velocity variation beneath the Wenchuan-Maoxian fault may indicate that the Pengguan massif＇s western boundary and/or the Wenchuan-Maoxian fault is vertical, and the hypocenter of the Wenchuan earthquake possibly located at the conjunction point of the NW dipping Yingxiu-Beichuan and Guanxian-Anxian faults, and vertical Wenchuan-Maoxian fault; （4） Vicinity along the Yingxiu- Beichuan fault is characterized by very low velocity and low seismicity at shallow depths, possibly due to high content of porosity and fractures; （5） Two blocks of low-velocity anomaly are respectively imaged in the hanging wall and foot wall of the Guanxian-Anxian fault with a -7 km offset with -5 km vertical component.

Determining areas in an inclined coal seam floor prone to water-inrush by micro-seismic monitoring

The failure depth of the coal seam floor is one important consideration that must be kept in mind when mining is carried out above a confined aquifer.Determining the floor failure depth is the essential precondition for predicting the water-resisting ability of the floor.We have used a high-precision microseismic monitoring technique to overcome the limited amount of data available from field measurements. The failure depth of a coal seam floor,especially an inclined coal seam floor,may be more accurately estimated by monitoring the continuous,dynamic failure of the floor.The monitoring results indicate the failure depth of the coal seam floor near the workface conveyance roadway（the lower crossheading） is deeper and that the failure range is wider here compared to the coal seam floor near the return airway（the upper crossheading）.The results of micro-seismic monitoring show that the dangerous area for water-inrush from the coal seam floor may be identified.This provides an important field measurement that helps ensure safe and highly efficient mining of the inclined coal seam above the confined aquifer at the Taoyuan Coal Mine.

Frequency spectrum analysis on micro-seismic signal of rock bursts induced by dynamic disturbance

Blasting and breaking of hard roof are main inducing causes of rock bursts in coal mines with danger of rock burst,and it is important to find out the frequency spectrum distribution laws of these dynamic stress waves and rock burst waves for researching the mechanism of rock burst.In this paper,Fourier transform as a micro-seismic signal conversion method of amplitude-time character to amplitude-frequency character is used to analyze the frequency spectrum characters of micro-seismic signal of blasting,hard roof breaking and rock bursts induced by the dynamic disturbance in order to find out the difference and relativity of different signals.The results indicate that blasting and breaking of hard roof are high frequency signals,and the peak values of dominant frequency of the signals are single.However,the results indicate that the rock bursts induced by the dynamic disturbance are low frequency signals,and there are two obvious peak values in the amplitude-frequency curve witch shows that the signals of rock bursts are superposition of low frequency signals and high frequency signals.The research conclusions prove that dynamic disturbance is necessary condition for rock bursts,and the conclusions provide a new way to research the mechanism of rock bursts.

Application of micro-seismic monitoring technology in mining engineering

Micro-seismic phenomena, occurring when rock masses are subjected to forces and failures, allow the determination of their unstable states and failure zones by analyzing micro-seismic signals. We first present the principles of micro-seismic monitoring and location, as well as an underground explosion-proof micro-seismic monitoring system. Given a practical engineering application, we describe the application of micro-seismic monitoring technology in determining the height of a "two-zone" overburden, i.e., a caving zone and a fracture zone, the width of a coal-pillar section and the depth of failure of a floor. The workfaces monitored accomplished safe and highly efficient mining based on our micro-seismic monitoring results and provide direct proof of the reliability and validity of micro-seismic monitoring technology.

An improved multidirectional velocity model for micro-seismic monitoring in rock engineering

An improved multidirectional velocity model was proposed for more accurately locating micro-seismic events in rock engineering. It was assumed that the stress wave propagation velocities from a micro-seismic source to three nearest monitoring sensors in a sensor's array arrangement were the same. Since the defined objective function does not require pre-measurement of the stress wave propagation velocity in the field, errors from the velocity measurement can be avoided in comparison to three traditional velocity models. By analyzing 24 different cases, the proposed multidirectional velocity model iterated by the Simplex method is found to be the best option no matter the source is within the region of the sensor's array or not. The proposed model and the adopted iterative algorithm are verified by field data and it is concluded that it can significantly reduce the error of the estimated source location.

Application of micro-seismic facies to coal bed methane exploration

A neural network is applied to high-quality 3-D seismic data during micro-seismic facies analysis to perform the waveform analysis and training on single reflection events. Modeled seismic channels are established and the real seismic channels are classified. Thus, a distribution of micro-seismic facies having a high precision over a fiat surface was acquired. This method applied to existing geological data allows the distribution of areas rich in coal bed methane to be clearly defined. A distribution map of the micro-seismic facies in the research area is shown. The data accord well with measured methane con- tents, indicating that the analysis using micro-seismic facies is reliable and effective. This method could be applied to coal bed methane exploration and is of great importance to future exploration work and to an increase in the drilling success rate.

Signal characteristics of coal and rock dynamics with micro-seismic monitoring technique

In this study, differences of signal characteristics between mine shocks and coal and gas outbursts in coal mines were examined with the micro-seismic monitoring technique and time-frequency analysis. The duration of the mine shock is short while the coal and gas outburst lasts longer. The outburst consists of three stages： the pre-shock, secondary shock and main shock stage, respectively. The velocity amplitude of the mine shock is between 10 s and 10-3 m/s, which is higher than that of the outburst with the same energy level. In addition, in both cases, the correlation between the velocity amplitude and energy is positive while the correlation between the signal frequency band distribution and energy is negative. The signal frequency band of the high energy mine shock is distributed between 0 and 50 Hz, and the low energy mine shock is between 50 and 100 Hz. The fractal characteristics of mine shocks were studied based on a fractal theory. The box dimensions of high energy mine shocks are lower than the low energy ones, however, the box dimensions of outbursts are higher than that of mine shocks with the same energy level. The higher box dimensions indicate more dangerous dynamic events.

Micro-seismic Event Detection of Hot Dry Rock based on the Gated Recurrent Unit Model and a Support Vector Machine

Micro-seismic monitoring is one of the most critical technologies that guide hydraulic fracturing in hot dry rock resource development. Micro-seismic monitoring requires high precision detection of micro-seismic events with a low signal-to-noise ratio. Because of this requirement, we propose a recurrent neural network model named gated recurrent unit and support vector machine(GRU;VM). The proposed model ensures high accuracy while reducing the parameter number and hardware requirement in the training process. Since micro-seismic events in hot dry rock produce large wave amplitudes and strong vibrations, it is difficult to reverse the onset of each individual event. In this study, we utilize a support vector machine(SVM) as a classifier to improve the micro-seismic event detection accuracy. To validate the methodology, we compare the simulation results of the short-term-average to the long-term-average(STA/LTA) method with GRU;VM method by using hot dry rock micro-seismic event data in Qinghai Province, China. Our proposed method has an accuracy of about 95% for identifying micro-seismic events with low signal-to-noise ratios. By ignoring smaller micro-seismic events, the detection procedure can be processed more efficiently, which is able to provide a real-time observation on the types of hydraulic fracturing in the reservoirs.

Relocation method of microseismic source in deep mines

A new method, named relocation, was proposed to reduce the impact of sensor errors systematically, especially whenavailable data of sensors are abundant. The procedure includes evaluating the reliability of every sensors datum, processing the initiallocation by the credible data, and selecting a set of equations with optimal noise tolerance according to the relative relationshipbetween the initial location and sensors location, then calculating the final location by k-mean voting. The results obtained in thisresearch include comparing traditional location method with the presented method in both simulation and field experiment. In thefield experiment, the location error of relocation method reduced 41.8% compared with traditional location method. The resultssuggested that relocation method can improve the fault-tolerant performance significantly.

Numerical simulation of rock progressive failure on samples with a pre-existed weak zone during brittle failure

Considering the heterogeneity of geomechanical materials, seismicity during brittle rock failure under compressive loading on the sample with an original weak zone is simulated by using rock failure process analysis code (RFPA2D). The run-through process of weak zone, the forming of new fault and associated micro-seismicities are studied. The modeling demonstrates the total process of source development of earthquake from deformation, micro-failure to collapse and the behavior of temporal-spatial distribution of micro-seismicities. The stress, strain and the temporal-spatial distribution of micro-seismicities life-likely portrayed the phenomena of localization and temporal-spatial transitions, which is similar to those observed in our real crust. Also, the results obtained in simulations are in agreement with or similar to the reported experimental observations.

Gradient principle of horizontal stress inducing rock burst in coal mine

Based on the stress distribution characteristics of rock burst multiple sites, the criterion of horizontal stress inducing layer dislocation rock burst was established. Accordingly, the influencing factors were analyzed. The analysis results indicate that the stress condition, edge of etastic zone depth, supporting strength, and the friction angle and cohesion among coal stratum, roof and floor are sensitive factors. By introducing double-couple model, the layer dislocation rock burst was explained and the energy radiation characteristics were analyzed. The SOS micro-seismic monitoring system was applied to observe the rock burst hazards about a mining face. The results show that P- and S-wave energy radiations produced by rock burst have directional characteristics. The energy radiation characteristics of the 22 rock bursts occurring on 79Z6 long-wall face are basically the same as theoretical results, that is, the ratio of S-wave energy of sensor 4 to 6 is about 1.5 and that of P-wave is smaller than 0.5. The consistency of the monitored characteristics of the energy radiation theoretically increases with the total energy increasing.

3D visualization of hydraulic fractures using micro-seismic monitoring:Methodology and application

In this paper,a new 3D visualization technical method was developed for hydraulic fractures using micro-seismic monitoring.This technical method consists of four steps:i.interpret the geologic hydraulic fracture model based on seismic source location data from micro-seismic monitoring;ii.develop a hydraulic fracture indication model,relying on the 3D spatial freeze-frame of micro-seismic monitoring sources from hydraulic fracturing;iii.construct a hydraulic fracture density model using the intensity from the micro-seismic monitoring;and iv.implement a 3D visualization of the hydraulic fractures,relying on the spatial constraints of the density model,the hydraulic fracture indication model,and the properties of the hydraulic fractures.This proposed technical method was used to produce 3D visualizations of the hydraulic fractures in well X in the Jiao reservoir,China,and the 3D visualizations of the distribution,development,extent and cutting relationships of hydraulic fractures were successfully realized.The results show that this technical method can be used as a practical and reliable approach to characterize hydraulic fractures.

Research on the Surface Subsidence Monitoring Technology Based on Fiber Bragg Grating Sensing

In order to monitor the process of surface subsidence caused by mining in real time, we reported two types of fiber Bragg grating （FBG） based sensors. The principles of the FBG-based displacement sensor and the FBG-based micro-seismic sensor were described. The surface subsidence monitoring system based on the FBG sensing technology was designed. Some factual application of using these FBG-based sensors for subsidence monitoring in iron mines was presented.

Fiber Bragg Grating （FBG） Sensors Used in Coal Mines

The fiber Bragg grating （FBG） strain sensors were used for on-line monitoring of the stress variation of the lined wall in the gateway retained along the goaf of No. 3203 coal mining face in Dongtan Mine. The results showed that the FBG strain sensor with the wide measuring range could measure the stress variation accurately during the support process of the gateway retained along the goaf and could provide the basis to further optimize the support structure and to determine the support plan of the gateway retained along the goaf. The FBG micro-seismic sensors were used in Xinglong Mine to detect the micro-seismic signal. The signals were well received and analyzed to determine the location and energy level of the source of the micro-seismic event warning. The FBG sensors and detecting system show a significant potential for micro-seismic detection and geological disasters detection.