WO2015002558A2

WO2015002558A2 - Method and system for analysis of geological structure and relative changes in stress in the layers located above the workings of an underground mine

Info

Publication number: WO2015002558A2
Application number: PCT/PL2014/000128
Authority: WO
Inventors: Zbigniew ISAKOW; Przemysław SIERADZKI; Zenon PILECKI; Kazimierz SICIŃSKI; Rafał CZARNY
Original assignee: Instytut Technik Innowacyjnych Emag
Priority date: 2014-10-30
Filing date: 2014-11-07
Publication date: 2015-01-08
Also published as: RU2587521C1; UA118543C2; WO2015002558A3; PL409989A1; CN105765408A; PL230219B1; CN105765408B

Abstract

In the proposed method of analysis of geological structure and relative changes in stress in the layers located above the mining workings of an underground mine the data from the mobile measurement data recorder (3) and from the central station of mine seismic system (10) obtained as a result of closely correlated in time recording of low-frequency seismic noise (D_n.cz.) from the surface system and of seismic bursts generated by mining (D_w.cz.) are transmitted to the stationary processing center (1). Then the recorded measurement data in the time windows of preferably 30-seconds, in the form of the 3-axis recordings of low-frequency seismic noise (D_n.cz.) and the seismic bursts generated by mining (D_w.cz.) are processed using the method of seismic interferometry for the noise recordings, and passive velocity and / or attenuation tomography for the recordings of the mine bursts. On this basis the isolines of the transverse wave velocity and the isolines of the longitudinal wave velocity and / or attenuation in the method of passive velocity and / or attenuation tomography are determined for the studied area of the rock mass (7). These will ultimately represent the average state of relative changes in stress (ΔΝΡ) in the layers above the mining workings (B). At the moment of mining burst (W), the parameters of location coordinates (X, Y, and Z) and the calculated time (To) of its occurrence in the outbreak, are correlated with the times (Tp) of the first enters of the longitudinal wave generated by this rock burst in the recordings of the 3-axis low-frequency measuring stations (5) of the wave recorded on the mine surface, and the corresponding rise times of the signal from the first input of the longitudinal wave until the recorded signal of the rock burst (W) reaches the first maximum in each measurement station (5). In the proposed measuring system the stationary processing center (1) is connected, preferably via the GSM modem (2) to the mobile measurement data recorder (3), and to the central station of mine seismic system (10) which is connected to the clock (GPS) and the visualization-signaling module (11) and via the intrinsically safe digital transmission system (12) and the mine tele-transmission network (13) to at least four underground seismometric stations (14) and / or to at least four underground seismic geophone stations (15).

Description

Method and system for analysis of geological structure and relative changes in stress in the layers located above the workings of an underground mine

The present invention relates to a method and system for analyzing geological structure and relative changes in stress in the layers located above the workings of an underground mine, intended to determine hazard for an infrastructure on surface resulting from a possibility of rock burst caused by mining in this area.

The state of the art

Currently, determination of geological structure of the rock layers above the mining workings is carried out with invasive methods involving excitation of seismic waves propagating in the rock mass, generated by high-power actuators or explosives localized on the mine surface. Usually seismic tomography tools are used.

Applications of the method of low frequency passive seismics (LFS) and phenomena relating to the noise of a regional nature, which are generally caused by natural processes, e.g. earthquakes, volcanic activity, waves of the seas and oceans, the impact of ocean currents on the continents or the impact of rapid flow of air masses are known from the publications of Dangel S. "Phenomenology of tremor-like signals observed over hydrocarbon reservoirs" (Journal of Volcanology and Geothermal Research, 2003, 128 (1-3), pp. 135-158), or Gorbatikov A.V., Kalinina A.V., Volkov V.A. et al. "Results of Analysis of Data of Microseismic Survey at Lanzarote Island, Canary, Spain" (Pure Appl. Geophys., 2004, 161 , pp. 1561 -1578), or Boullenger B. "Finite Difference Feasibility Modelling of Time-lapse Seismic Noise Interferometry for CO2 monitoring" (TU Delft, Master Thesis, 2012). Local noises are induced by vibrations from passing vehicles, working of equipment, plants or humans, or bumps induced by mining or other activity. Frequency of analyzed seismic signals is generally from 0.1 Hz to 3 Hz in case of regional noise, and up to 30 Hz in case of local noise. Maximum deep range associated with frequency of analyzed surface seismic wave can range from tens of meters to several thousand meters.

Development of methods for low frequency passive seismics, LFS, is now possible through the use of modern technologies in production of broadband sensors and increase of computing capacity through the use of parallel computing. The LFS methods use the records of multiple-hours data, which forces creation of new applications for recording, processing and interpretation of these data. Among the LFS methods one can distinguish two fundamental ones: the method of microseismic sounding (MS) and the method of seismic interferometry (IS).

When using the method of seismic sounding the recordings are carried out on a number of displaced measurement stations and a fixed reference station, and the interpretation is based mainly on vertical component of seismic noise. The recording is performed at each point by at least 1 hour to get its stationarity.

In turn, using the seismic interferometry method recording is performed continuously with sampling selected for a particular task without moving the sensors. The shallower depth of investigated layers, the smaller is sampling step. Recording can be carried out using the vertical component of surface wave of Rayleigh type and / or horizontal Love wave. Record length is dependent on: the characteristics of the wave field of tested area, including dominant frequency and direction of propagation, and the type of task carried out, for example: monitoring of changes, localization of weakened areas, determination of the medium structure. International patent application WO2012044480 (A2) discloses the data processing method of seismic interferometry to study the geological structure of the seabed by means of low-frequency seismic sensors towed along the sea bottom by the research vessel, which allows for three-dimensional imaging of the geological structure under the seabed. The method uses the Green's function for calculations. American patent application US201 1069580 (A1 ) describes the means for modifying the directivity of seismic interferometry in determining subterranean geological structure of the Earth.

Another American patent application US2010315902 (A1 ) discloses a method of imaging the subterranean structures with passive tomography using seismic interferometry recording seismic signals generated by seismic phenomena that occur in the rock mass. This method further includes adaptation of filtering speed of the detected signals.

Czarny R. in "An overview of the method of seismic interferometry" (Mining Review, 2014, No. 7) describes seismic interferometry method that consists in mapping the impulse response of the medium (Green's function) between a pair of sensors with an operation of mutual correlation or deconvolution of the seismic signals recorded by these sensors. This method offers a very wide range of applications, from imaging subsurface structures of the Earth's crust and monitoring the changes, to geo- engineering applications. Marcak H., Pilecki Z., Isakow Z., and Czarny R. in "Possibilities of using seismic interferometry method in mining" (Mining Review, 2014, No. 7) describe the trends in use of this method in geophysics of mining, and present the results of the analysis of noise generated by mining, which can be used in research with the methods of seismic interferometry. The mathematical and physical basis of this method is presented. As it has been mentioned above, in the method of seismic interferometry recording is performed continuously with sampling adjusted individually to specific geological and mining conditions. The shallower the geological structure of investigated layers, the smaller is the sampling step. Recording is carried out using vertical component of Rayleigh surface wave and / or horizontal Love wave. The length of the recording depends on the characteristics of the wave field of tested area, i.e. dominant frequency and direction of propagation, and the type of task, i.e. monitoring changes of localization of weakened zones or of determination of the medium structure. Isakow Z., Pilecki Z., and Sierodzki. P. in "The modern LOFRES system of low- frequency passive seismics" (Mining Review, 2014, No. 7) show a system LOFRES designed for testing with a low-frequency passive seismics LFS method of subsurface geological layers using seismic noise. This system is due to its measurement functions designed to carry out the method of microseismic sounding and the method of seismic interferometry. The system consists of a stationary central processing unit connected wirelessly to a mobile data acquisition module, and subsequently via WI-FI autonomous access points to measuring stations and the reference station.

The 3-axis low-frequency measuring stations are equipped with 3-axis low-frequency seismic sensors connected via an analog-digital converter to a microprocessor and equipped with an internal memory of large capacity, a GPS receiver, a wireless transmission system and a battery. The measuring stations record velocity of vibrations in three components using 3-axis low-frequency seismic sensors. The data are stored in the internal non-volatile memory of the sensors with a capacity of several gigabytes. Time synchronization is provided by a GPS receiver attached to each measuring station. The autonomous access points WI-FI working in WDS regime increase WI-FI transmission range in wide area.

Discussion of the prior art

The main drawback of previously used methods and systems for analysis of geological structure and relative changes in stress in the layers located above the mining workings using the active seismic tomography is their onerousness. This is associated to usage of invasive methods especially in the area where the surface of the mine in the observed area is used for residential or industrial buildings and the related technical infrastructure. Disclosed by Isakow Z., Pilecki Z., and Sierodzki. P. in the above mentioned article "The modern LOFRES system of low-frequency passive seismics", a method of low- frequency passive seismics is used to identify geological structures and relative changes in stress in the layers located above selected mining workings in a version of seismic sounding MS. Although non-invasive and with good horizontal resolution (approximately 3.5% of the wavelength) it is ineffective and difficult in practical use because it requires a reference station arranged in an area free of an influence of mining and is sensitive to non-stationarity of low-frequency noise used for imaging. MS method has also low resolution of vertical imaging (about 30% of the wavelength). The method of seismic interferometry IS although insensitive to the noise non-stationarity and comfortable in long-term field use is also characterized by a low resolution of vertical imaging and in this known form is not useful for analysis of geological structure and relative changes in stress in the layers located above the workings of an underground mine.

Aim of the invention The aim of the invention is to provide new and more reliable method and system used for non-invasive determination of a geological structure and relative changes in stress in the layers above selected mine workings, by low-frequency passive seismics. This is necessary to enable alarming on a periodic or ongoing basis about the states of geological anomalies and increased concentrations of relative changes in stress preceding local tectonic shocks.

Substance of the invention

The method according to the invention is characterized in that the data from a mobile measurement data recorder and from central station for mine seismic system obtained by strictly correlated in time recording of low-frequency seismic noise from the surface system, and seismic bumps generated by mining are transmitted to a stationary processing center. Then the recorded measurement data in the time windows of preferably 30 seconds, in the form of 3-component recordings of low- frequency seismic noise and seismic bursts generated by mining are processed using the method of seismic interferometry for the noise recordings, and passive velocity and / or attenuation tomography for the shock recordings.

On this basis isolines of transverse wave velocity and isolines of longitudinal wave velocity and / or attenuation in the method of passive velocity and / or attenuation tomography are determined for an investigated part of the rock mass. Ultimately, they reflect average state of relative changes in stress in the layers located above the mining workings. At the moment of a mining shock, the parameters localizing coordinates of its hypocenter and time of occurence of this event calculated in the rock burst center are correlated with the moment of first arrival of a wave generated by the shock in the recordings of 3-axis low-frequency longitudinal wave measurement stations, and the corresponding rise times of the signal calculated from first arrival of the longitudinal wave to reaching first maximum of the shock recording. The above mentioned longitudinal wave is recorded on the surface of the mine.

State of relative changes in stress in the layers of studied area of the rock mass above the mining workings is subjected to ongoing comparative analysis in the stationary processing center with adopted acceptable threshold relative changes in stress for the investigated area of the rock mass. In case of increase of the currently measured relative changes in stress over a threshold the places where such a condition has occurred are signaled. Then a spatial result of the seismic interferometry tomography and averaged tomography are transmitted from the stationary processing center to the central station of mine seismic system, where relative changes in stress are visualized in the visualization-signaling module.

In the system according to the invention stationary processing center is connected, preferably via a GSM modem, to a mobile measuring data recorder, and to the central station of mine seismic system. This station is connected to the clock and visualization-signaling module and via the intrinsically safe digital transmission system to the mine tele-transmission network with at least four seismic underground seismometric stations and / or with at least four seismic geophone stations. Effects of the invention

Allowing for current effective analysis of relative changes in stress in the layers above the mining workings by correlated use of method of passive low-frequency seismic interferometry and method of passive seismic tomography using seismic shocks generated by mining increases functionality, resolution and accuracy of measurements carried out on a specified mining area of underground mine. The invention allows for earlier determination of places with increased concentration of stress that precede regional tectonic bursts. This is extremely important in application of appropriate preventive means, in areas where on the mine surface residential and / or industrial buildings and technical infrastructure are located.

Analysis of relative changes in stress allows for alarming that anticipates hazardous conditions. The invention allows for deep, extending to several hundred meters noninvasive penetration of geological layers, without necessity to apply methods of artificial seismic wave excitation such as with the explosives. Measuring only requires installation of sensors and creation of a system for measuring low-frequency passive seismics associated with mining seismic system. It is important to shorten the intervals between successive analyzes, by using seismic interferometry method, in which due to correlating the noise records stationarity of recorded noise is not required.

The invention is demonstrated in the exemplary embodiment in the drawing, wherein Fig. 1 - shows schematically the measuring system together with the mutual location of its components on the surface and the underground of the mine, and Fig. 2 - shows a block diagram of the measurement system. The method according to the invention is carried out using the method of seismic interferometry using preferably forty 3-axis low-frequency measurement stations 5-5, located from the beginning of the measurement in five profile lines "k". Seismic interferometry method involves mapping impulse response of the rock mass, the so- called Green's function by using cross-correlation or deconvolution of seismic signals recorded by a pair of 3-axis low-frequency measurement stations 5.

For continuous monitoring with seismic interferometry method it is necessary to ensure continuity of power supply with safe voltage from a DC power supply 8 and wireless receiving of data using wireless data transmission network WI-FI with local autonomous WI-FI access points 4 and a mobile measuring data recorder 3 which records data and sends them advantageously in block transmission GPRS through a GSM modem 2 to a stationary processing center 1. The recorded measurement data in time windows of preferably 30 seconds in the form of 3-axis low-frequency seismic noise D_n.cz. from the 3-axis low-frequency measuring stations 5 are processed using the seismic interferometry method.

The seismic bursts generated by mining D_{w cz.} recorded in the 3-axis low-frequency measurement stations 5 are processed with use of passive seismic tomography method, using for this purpose automatically determined transit times of seismic wave along seismic rays from the place of occurrence of seismic bursts W to the place of their recording on the surface in the 3-axis low-frequency measurement stations 5. On this basis isolines of transverse and longitudinal wave velocity for studied area of the rock mass 7 are determined. They represent relative changes in stress ΔΝΡ in the layers above the mine workings B located above the investigated area of the rock mass 7. At the moment of burst W the localization parameters of coordinates X, Y and Z, and the calculated time To in the hypocenter of mining burst are correlated with time Tp of the first arrival of longitudinal wave recorded in the recordings of 3-axis low- frequency measuring stations 5 on the surface of the mine A. This is done in order to automatically determine the transition times of seismic wave along the paths of seismic rays. These times are necessary to determine the speed of the wave propagation in these directions. Besides, in order to allow additionally usage of the method of spatial passive attenuation tomography, which also identifies relative changes in stress after identification in the 3-axis low-frequency measurement stations 5 of the burst signal recording, signal rise times from the first arrival Tp of longitudinal wave induced by the burst to the first maximum value of recorded signal are determined automatically.

Surface waves that carry information about the structure and properties of geological medium, as described in the form of dispersion curves, are used in the method of seismic interferometry. Processing in a stationary processing center 1 includes: data quality control by eliminating the trend and recording errors, filtering of surface waves in the range of 0.2 Hz to 15 Hz, a one-bit normalization, data cross-correlation for each pair of measuring stations, selection of correlation plots with the best signal to noise ratio, identification of the phase velocity dispersion curves of Rayleigh waves, inversion of the dispersion curves for the 1 D models of the transverse wave velocity, development of a 2D model of the transverse wave velocity field, and using data from multiple measurement line profiles "k" of 3-axis low-frequency measurement stations 5 development a spatial (3D) model of the transverse wave velocity field.

Software with dedicated algorithms allows to perform measurements and to process them in real time with use of computerized methods of seismic interferometry. As a result of the processing, spatial visualization of controlled area is obtained in the form of isoline of transverse wave velocity. Reduction in amplitude is observed in less dense places, with less stress of the transverse wave velocity, while in more dense places with larger stress increase in amplitude is observed. In the method according to the invention localized mine bursts of known coordinates X, Y and Z, and the calculated time To in the burst hypocenter are applied, as well as the identified times Tp of the first arrival of generated wave, in the recordings of the measuring stations 5 arranged at the nodes of a measuring grid P on the surface, for spatial passive velocity or attenuation tomography. The results of this tomography are complementary to low-frequency tomography and after correlating the obtained tomographic maps they increase the reliability of the analysis.

The spatial distribution of propagation velocity of transverse waves is created from the averaged maps of low-frequency tomography along particular profile lines "k", obtained by seismic interferometry method, based on low-frequency noise D_{n cz} recorded on the surface. In turn, the spatial distribution of propagation velocity of longitudinal waves is created from passive velocity and / or attenuation tomography based on mining bursts W, which represent the state of relative changes in stress ΔΝ in the layers of the studied area of the rock mass 7 above the mine workings B.

Then the relative changes in stress ΔΝΡ are treated with ongoing comparative analysis in the stationary processing center 1 with the adopted acceptable threshold relative changes in stress ANPgr for the studied area of the rock mass 7. In case of increase of measured ongoing relative changes in stress ΔΝΡ over the thresholds, ΔΝΡ≥ ANPgr, alarm is switched-on and spatial result of both tomographies and of averaged tomography is sent from stationary processing center 1 to the central station of mine seismic system 10. There, in the visual-signaling module 11 connected to the station the examined relative changes in stress ΔΝΡ are visualized.

In the arrangement according to the invention shown in Fig. 1 and Fig. 2, stationary processing center 1 is wirelessly connected in a WI-FI network via the GSM modems 2 to the mobile measurement data recorder 3, and successively through two autonomous WI-FI access points 4 in the mesh configuration to digital outputs of forty stationary 3-axis low-frequency measuring stations 5 which were grouped in five measuring panels 6 located along the profile lines "k".

Measuring nodes P of the measurement grid with the 3-axis low-frequency measurement stations 5 (SP) and the measuring panels 6-6k are arranged above investigated mining area 7. Their number and positions are determined in each case individually, depending on the size of this area. The 3-axis low-frequency measurement stations 5 are placed in the ground and are supplied with the constant voltage DC power supply 8 via the underground telecommunication cable lines secured with arresters. Each of the 3-axis low-frequency measuring stations 5 is equipped with the 3-axis low-frequency seismic sensors 5a connected via an analog- to-digital converter 5b to the microprocessor 5c with internal non-volatile memory 5d of large capacity. The microprocessor 5c is connected to the battery 5e and through the automatic charging circuit 5f to the DC power supply 8 and the GPS clock. The stationary processing center 1 is also connected in the Ethernet network 9 to the central station of mine seismic system 10 equipped with the GPS clock and visualization-signaling module 11. In turn, the central station of mine seismic system 10 via the intrinsically safe digital transmission system 12 is connected through the mine digital telecommunication network 13 to digital outputs of underground seismometric stations 14 and seismic geophone stations 15. The 3-axis low- frequency measuring stations 5 record continuously and synchronously with the GPS clock time the measurement data from the 3-axis low-frequency seismic sensors 5a in the form of low-frequency seismic noise D_{n C}z . After processing these data by the microprocessor 5c with analog-digital converter 5b into digital form the data are stored in the internal non-volatile memory 5d of large capacity.

Then the 3-axis low-frequency measurement stations 5 send the above-mentioned measurement data over a distance of several hundred meters through autonomous access points WI-FI 4 which increase the transmission range, to the mobile measurement data recorder 3. They are also stored there and periodically or continuously transmitted, preferably by the GSM communication modem 2 or alternatively via wired Internet network, to the stationary processing center 1.

Claims

A method of analysis of geological structure and relative changes in stress in the layers located above the mining workings of an underground mine comprising the measurement of properties of structure of these layers by seismic interferometry with 3-axis recording of seismic noise with use of autonomous low-frequency measurement stations measuring vibrations of the rock mass installed on the surface above investigated area of the rock mass, and transmission of measurement data to a mobile measurement data recorder, characterized in that the data from the mobile measurement data recorder (3) and from the central station of mine seismic system (10) obtained as a result of closely correlated in time recording of low-frequency seismic noise (D_n._cz.) from the surface system and of seismic bursts generated by mining (D_{w cz} ) are transmitted to the stationary processing center (1), and then the recorded measurement data in the time windows of preferably 30-seconds, in the form of the 3-axis recordings of low-frequency seismic noise (D_{n C}z.) and the seismic bursts generated by mining (D_{w cz.}) are processed using the method of seismic interferometry for the noise recordings, and passive velocity and / or attenuation tomography for the recordings of the mine bursts, and on this basis the isolines of the transverse wave velocity and the isolines of the longitudinal wave velocity and / or attenuation in the method of passive velocity and / or attenuation tomography are determined for the studied area of the rock mass (7), which will ultimately represent the average state of relative changes in stress (ΔΝΡ) in the layers above the mining workings (B), wherein at the moment of mining burst (W), the parameters of location coordinates (X, Y, and Z) and the calculated time (To) of its occurrence in the outbreak, are correlated with the times (Tp) of the first arrivals of the longitudinal wave generated by this rock burst in the recordings of the 3-axis low-frequency measuring stations (5) of the wave recorded on the mine surface, and the corresponding rise times of the signal from the first arrival of the longitudinal wave until the recorded signal of the rock burst (W) reaches the first maximum in each measurement station (5).

The method according to claim 1 , characterized in that the state of relative changes in stress (ΔΝΡ) in the layers of the studied area of the rock mass (7) above the mine workings (B) is subjected to an ongoing comparative analysis in the stationary processing center (1) with accepted threshold relative changes in stress (ANPgr ) for the studied area of the rock mass (7) and in the case of increase of currently measured changes in stress (ΔΝΡ) above the threshold values (ΔΝΡ≥ ANPgr), the signaling of the places where this state has occurred is realized and then the spatial result of tomography performed by seismic interferometry and averaged tomography is transmitted from the stationary processing center (1) to the central station of mine seismic system (10), where in the visualization-signaling module (11) the measured relative changes in stress (ΔΝΡ) are visualized.

A system for analysis of geological structure and relative changes in stress in the layers located above the mining workings of an underground mine comprising a stationary processing center connected wirelessly to a mobile measurement data recording unit and subsequently via autonomous access points WI-FI to the 3-axis low-frequency measuring stations, which are equipped with the 3-axis low-frequency seismic sensors connected via an analog-digital converter to a microprocessor with an internal non-volatile memory of large capacity, a GPS receiver, a wireless transmission system and a battery, for use of the method as defined in claims 1 to 2, characterized in that the stationary processing center (1) is connected, preferably via the GSM modem (2) to the mobile measurement data recorder (3), and to the central station of mine seismic system (10) which is connected to the clock (GPS) and the visualization-signaling module (11) and via the intrinsically safe digital transmission system (12) and the mine tele-transmission network (13) to at least four underground seismometric stations (14) and / or to at least four underground seismic geophone stations (15).