International Journal of Mining Science and Technology 24 (2014) 75–80

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International Journal of Mining Science and Technology

journal homepage: www.elsevier .com/locate / i jmst

Applying real time seismic monitoring technology for slope stabilityassessment—An Indian opencast coal mine perspective

2095-2686/$ - see front matter � 2014 Published by Elsevier B.V. on behalf of China University of Mining & Technology.http://dx.doi.org/10.1016/j.ijmst.2013.12.013

⇑ Corresponding author. Tel.: +91 9994212416.E-mail address: vino.ceg@gmail.com (S. Vinoth).

Vinoth S. ⇑, Ajay Kumar L.Department of Mining Engineering, College of Engineering Guindy, Anna University, Chennai 600025, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 1 May 2013Received in revised form 15 June 2013Accepted 20 July 2013Available online 2 January 2014

Keywords:Seismic monitoringInduced seismicitySeismic clustersSlope stabilityOpencast mining

This paper outlines the results obtained from real time microseismic monitoring of an opencast coal minein South India. The objective of the study is to investigate the stress changes within the rockmass alongthe slope due to underground mine development operation and their impact on the stability of thehighwall slope. The installed microseismic systems recorded the seismic triggerings down to �2 momentmagnitude. In general, most of the events recorded during the monitoring period are weak in seismicenergy. The study adopts a simple and more reliable tool to characterize the seismically active zonefor assessing the stability of the highwall in real time. The impact of underground working on the slopeis studied on the basis of the seismic event impact contours and seismic clusters. During the monitoringperiod, it is observed that the intensity of the overall microseismic activity along the slope due to themine development operations did not cause any adverse impact on the highwall stability.

� 2014 Published by Elsevier B.V. on behalf of China University of Mining & Technology.

1. Introduction

Mining coal and subsequent removal of overlying rock masswill lead to increase in stress along the highwall, resulting in defor-mation or even complete collapse of the slope. Appreciableresearch on the aspects of slope stability in open pit mining hasbeen carried out during 1960s and 1970s [1]. Later, very littledevelopment is achieved in the evaluation of slope stabilitytechniques during the next 30 years [2]. Till then, slope stabilityremains a concern even in the most conservative slopes, due tothe unknown properties and conditions of rock mass [3]. The appli-cation of mine induced seismicity for understanding the stresschanges within the rockmass has been investigated in many coun-tries with mining tradition [4]. In general, rock fracturing and seis-mic activity are unavoidable consequences of mining activities.This can be directly related to interaction of mine excavationsand geological structures [5]. Many valid attempts have been madeto monitor such microseismicity for delineating and predictingrock falls, rock burst, and outburst [6–11]. While monitoring ofmicroseismic radiations is common in underground miningoperations, the method is implemented in opencast mines since2002 in Australia, Canada and South Africa [12]. This is necessi-tated due to the increase in large number of deep opencast mines,often beyond the working experience and knowledge base of theindustry. With the development of sophisticated seismic instru-mentation in the recent years, the application of microseismic

monitoring techniques in surface mines has provided a useful toolin understanding the rock mass response to mining operations[13–15]. Routine seismic monitoring in mines enables thequantification of seismicity induced in the rock mass in threedimensional volume surrounding the slope in real time. This willprovide a logical and effective tool to prevent any impendingfailure caused by evaluation of fractures in rockmass, activationof known or unknown geological structures, stress changes andvibration threshold for the safe and productive mining operations[3,16,17]. They will also yield useful information, in understandingthe relation between the slope stability, mine seismicity and min-ing operations [18]. Hence, it is important to monitor the level ofextraction versus the seismicity induced along the slope due tosuch operations.

In Indian opencast mines, generally geotechnical methods arepreferred for design of slope and investigating stability issues[19]. The earlier applications of seismic monitoring method in Indiaare restricted mainly to various underground mines [20–23]. Chou-han has reported induced seismicity from the Raniganj coal seam inthe form of bumps from the Chinakuri colliery [20]. Later the seis-mic method has been adopted for rock burst prediction and forunderstanding the geomechanical characters of the Kolar GoldMines in India [21–23]. Summarising the earlier effects and to over-come the new geotechnical problems associated with slope stabil-ity, it is proposed to apply the microseismic monitoring techniquefor an opencast coal mines in India for predicting slope failures.

Since most of the research in seismic monitoring is restricted tounderground mining applications in India, this paper summarisesin brief the preliminary results of real time microseismic

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monitoring carried out on the slope of a highwall in an opencastmine in Godavari coal basin of India.

The details of the seismic system configuration, array design,network settings, data processing and the results obtained fromthe five triaxial geophones are discussed. The study is initiated tomonitor the impact of construction of underground mine openingson the slope stability of the highwall in terms of induced seismicityin real time.

The existence of seismic event clusters suggests that the rockmass exhibits distinct planes of weakness [24]. The study aimedto estimate the impact of the advancing underground openingson such weak planes along the slope for monitoring the stabilityof the rockmass. The work also underlines the adoptability ofseismic technology as a monitoring tool, for efficient mitigationof seismic hazard in an open pit mine.

2. Geological setting

The study area is a part of Manuguru–Cherla belt under the coalbearing Pranhita–Godavari Basin, a principle Gondwana Basin coaldeposit of South India (Fig. 1). The Pranhita–Godavari valley is amajor NNW-SSE trending belt resting on the Precambrian SullavaiGroup rocks and extending over 470 km in strike length. The basinpreserves entire Gondwana periods records of peninsular Indiacovering the entire sedimentation from Permian to upperCretaceous times. It is well known that geological discontinuitiessuch as joints, fractures, faults, etc., play a significant role inseismic activity associated with active mining [25]. The area hasbeen interrupted by an oblique fault trending ENE-WSW. Threemine entries of 4 m height and 6 m width, dipping easterly areplanned for the development of new underground coal mine fromthe existing open pit highwall. The conversion of the open pit slopeto underground entry uses an aggressive and novel combination ofblasting and machinery excavation using continuous miners.Hence, a sophisticated microseismic monitoring system is em-ployed to understand the response of rock mass to such distur-bances and for monitoring slope instabilities in real time. The

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study is initiated as a preliminary tool to identify the impact ofthese three entries on the highwall slope of the erstwhile opencastmine.

3. Methodology

For achieving the best possible results and industrial benefits,few important parameters such as low cost, reliability of sourceparameters, geological disturbances, nature of rock are consideredprior to the design of the sensors array. The primary aim of theinvestigation is also to determine the event frequency, momentmagnitude (Mo), peak particle velocity (PPV) more precisely in acontinuous mode and to superimpose the data on the mine mapwithout human interaction in real time [26]. Hence, it is decidedto go for triaxial geophones array along the monitoring slope.The P-wave arrivals from the seismograms are utilized for fullyautomatic seismic data processing.

4. Hardware settings and experimental setup

In order to monitor the mining induced microseismicity, a ded-icated whole waveform stand alone recording system is installedon the highwall. The number of sensors and channels are basedon the volume of rockmass, the desired location accuracy, the sig-nal level expected, rock formations and the local site conditions.The system is based on the ESG’s paladin recorders, which offeredsix channels, 24 bit resolution continuous data acquisition at asampling frequency of up to 10 kHz and Ethernet TCP/IP telemetry,GPS time receiver for time synchronisation of the events, omni-anduni-directional antennas and radio set up for transmitting the seis-mic signals.

4.1. Sensors

For reliable measurement of seismicity, the sensors shouldsurround the volume of rock being monitored [13]. An array of fivehigh sensitivity (43.3 V/m/s), 15 Hz omni directional triaxial

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geophones with the frequency range of 6–1000 Hz are installed infive vertical boreholes at various depths (Fig. 2). The array is de-signed such that the inter-sensor distance is less than 200 m toachieve event location accuracy within 5 m. The use of tri-axialsensors in the array allows for waveform polarization analysisand to reduce uncertainties in P-and S-wave identification [15].

4.2. Data acquisition networks

The seismic data collection and analysis at the central pro-cessing site have multiple stages of data transformation (Fig. 3).In the first stage, the analogue seismic signals from the geo-phones are transmitted to the seismic data loggers. Each datalogger is configured with a separate Internet Protocol (IP) ad-dress. The data logger has the ability to store and transmit theinformation to a centralised repeater. In the second stage, thedata can be stored at the data logger or transferred to a centralrepeater point through a combination of wireless uni-andonmi-directional radio antennas. In the third stage, the repeaterstation transmits the data to the data acquisition PC or a centra-lised computer. The data is available in real time mode at theweb enabled centralised processing station and accessible toany internet network. Later, the triggers are examined for dis-crete seismic ‘‘events’’. In earlier systems, the use of mining com-pany’s general purpose LAN’s for seismic data transmission hasmet with the mixed success [26]. However, in the present casethe data processing and acquisition is successful with wirelesssetup on the real time mode. High standard of maintenance ofthe connectivity for uninterrupted flow of data across thenetwork is ensured, which became the major requirement in ex-treme opencast mine weather conditions.

5. Observation

The main task during the preliminary phase is to determine thelevel of seismic activity based on the number of seismic events inthe monitoring region. Magnitude and locations are the first orderoutput for recorded microseismic events [27]. The P-wave arrivalsare preferred over the S-wave arrivals, due to relative simplicity,and accuracy, which are most essential for locating the seismicevent hypocenter within the given volume of rock [28,29]. Thesource location accuracy depends on the efficiency in reductionof the noise from the recorded seismic signal [25]. A trial blast is

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conducted with known source co-ordinates for determining thevelocity of P waves in the given rock mass stratum, and for the sen-sor orientation. In order to monitor the seismic response of therock mass, a simple 2-D model consisting of the geology log infor-mation is developed. This is used to create a layered velocity modelwith different layers representing the litho stratigraphy of the site.The simplex ray trace routine is used as an advanced event locationalgorithm. The routine allowed the layered velocity model to be asinput. It also makes use of azimuth and dip information of the stra-ta from the P wave polarization analysis in the location solution.The P-wave polarization of each event is determined with the firstarrival P wave hodogram analysis.

6. Results and discussion

A total of 11,458 triggers are recorded in the geophones withinthe study period. The study is based on the 228 events which exhi-bit clear seismogram recorded in the geophones, developed at mi-cro levels in various places around the monitoring volume ofrockmass. 219 events had their hypocenters around the monitoredpit slope. Generally, the seismic events will tend to align as a groupeither in the existing geological structures or around the mineoperation taking place underground or along the direction of planeof weakness. This principle is applied to monitor the mine highwallfor understanding the behaviour of seismicity induced due tounderground mining operations against the highwall failure. It isexpected that the stress developed due to the underground open-ing might also get prolonged with the excavation direction thatwould affect the stability of the highwall. But the results provedthat the events aligned along the direction perpendicular to thedirection of extraction. This suggested that the plane of weaknessmay exists in the direction perpendicular to the direction of under-ground opening, which are confirmed from the borehole section(Fig. 4). Also, the results suggested that the oblique fault trendingeast–west which is expected to play a critical part in the stabilityof the overlying highwall stratum, is not disturbed due to theunderground extraction.

6.1. Seismic clusters as a tool

Seismic clusters are used to classify the points within thegiven seismic volume into zones of similar seismic response. Inthe present study, the events are classified into clusters on the

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Trending direction of the recorded seismic events

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Fig. 4. Seismic events recorded in the geophones along the highwall slope.

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basis of seismic attributes such as the location of hypocenters ofthe seismic events in space, Mo calculated in a scale of �2 to +2and PPV from 5 to 250 mm/s. Few seismic events clusters areidentified along the slope at depths of 10, 35 and 75 m respec-tively (Fig. 5).

The map with the seismic clusters clearly depicted the progres-sive path of underground mining entries. It is observed that thesource parameter estimates for events with moment magnitudesless than �1 are insignificant, with the seismic cluster surroundinggeophones [14]. The seismic energy released by these events willbe very low. The cluster surrounding the most top layer of the stra-ta consisted predominantly of events with the moderate magni-tude values. A gradual development in the underground mineoperation is marked by the increase in higher magnitude eventswith increased PPV values. At early stage of the monitoring, theevents are characterised with lower magnitude values, aligningin the direction perpendicular to the direction of undergroundexcavation. The cluster at the greater depth (>70 m) is character-ized by more number of high magnitude events and the PPVvalues.

6.2. Seismic event impact contour

Apart from the seismic event clusters discussed above, largenumbers of events are found highly scattered along the highwallwhich might have significant impact on the highwall stability. Asignificant number of events in this category possess higher Movalues (0–1) and PPV values greater than 75 mm/s surroundingthe underground excavation.

It is observed that these events have a critical part if the slopefails since the size of rock failure events can be directly relatedto the moment magnitude [29]. The highest moment magnituderecorded is 0.83 with PPV value of 100 mm/s beneath the workingpit. The seismic impacts with high magnitude at the bottom of thepit indicated the effect of blasting, the initial method practised forremoving the overburden rock mass to make entry for under-ground mine. A sharp decrease in the Mo and PPV values of theevents is marked with shift in drilling mechanism from blastingto continuous miner.

The seismic event impact contour map is based on the averagelevel of event density, peak particle velocity and moment

Moment magnitude2-2

Direction of UG entry High magnitude events triggered due to blasting at the earlier stages

Fig. 5. Location of apparent seismic clusters along the slope with recorded events.

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Fig. 6. Event impact contour map of the monitoring regions based on the seismic event density.

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magnitude of the events. Fig. 6 represents the typical event impactcontour map for analysing the impact of induced seismicity. Stagewise reports of induced seismicity versus excavation progress aregenerated and analysed for the estimation of the seismic impactalong the region. The significant results are obtained from the seis-mic potentiality within the sensor range for a period of time. Thisprovided a visualization tool for analyzing the status of slope andthe level of seismic activity on the region.

The event impact contour map suggests that the seismic impactaround the slope due to these events decreases as the excavationadvances. Few strong events at the early stages are triggered atthe slope face due to blasting. The results suggest that there areno serious stability issues noticed along the highwall slope bythe seismic events induced by the mining operations generateddue to the development of underground mine opening. The rockmass is found stable during the entire monitoring period and nosign of failure has been observed during the time.

7. Conclusions

The preliminary results obtained from the seismic monitoringtechnique employed to assess the stability of highwall slope inan Indian opencast coal mine has been discussed. Numerousseismic signals are recorded in three seismic stations during the

observation period. The seismic source parameters such as peakparticle velocity (PPV), moment magnitude (Mo) and the eventhypocenter localization are used for characterizing the behaviourof seismic events inside the rock mass along the highwall slopeof the mine. The simplex ray trace routine is adopted as backendtool for the first arrival P wave hodogram analysis. The P wave ar-rival timings are used for defining the locations of the recordedseismic events and hypocenter. An event accuracy of 5 m on allthe planes is targeted with considerable success.

The observations discussed in the paper are based on the 229seismic recordings from the geophones which exhibit clear seismo-gram. The seismic event clusters and event impact contour mapsprovided an excellent tool to assess the in situ stability conditionsof the slope in real time. They also provided resourceful informa-tion regarding the behaviour of seismic events within the givenrockmass. From results, it is understood that there are few highmagnitude events of range greater than +0.5 are generated dueto the blasting at the slope and the gradual decrease in momentmagnitude and PPV is marked with a change in excavation methodat later stages. The mining operations carried out undergroundbelow the monitored slope are not intensive to cause neither anyeffect on the pre existing geological structures along highwallnor slope failure and the rock mass is stable. With the understand-ing of induced seismicity around the slope and by improved day to

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day reporting, it will also be possible to forecast the mechanismsassociated with slope movements in future.

Acknowledgments

The research work is a part of the S&T project ‘‘High resolutionmicroseismic monitoring for early detection and analysis of slopefailure in opencast mines’’ funded by Ministry of Coal, Governmentof India and The Singareni Collieries Co Ltd (SCCL), Andhra Pradesh.The Officials of Central Mining Planning and Design Institute, Indiaand Manuguru Opencast Mines, SCCL, Andhra Pradesh are thankedfor the extended support offered by them during the researchwork.

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