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Qualification of the microseismic monitoring technique applied to the risk of collapse in iron ore mines Gloria Senfaute, Mohamad Khir Abdul-Wahed, Jack-Pierre Piguet, Jean-Pierre Josien To cite this version: Gloria Senfaute, Mohamad Khir Abdul-Wahed, Jack-Pierre Piguet, Jean-Pierre Josien. Qual- ification of the microseismic monitoring technique applied to the risk of collapse in iron ore mines. International symposium of the international society for rock mechanics (EUROCK 2000), Mar 2000, Aachen, Germany. <ineris-00972194> HAL Id: ineris-00972194 https://hal-ineris.ccsd.cnrs.fr/ineris-00972194 Submitted on 3 Apr 2014 HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destin´ ee au d´ epˆ ot et ` a la diffusion de documents scientifiques de niveau recherche, publi´ es ou non, ´ emanant des ´ etablissements d’enseignement et de recherche fran¸cais ou ´ etrangers, des laboratoires publics ou priv´ es.

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Page 1: Quali cation of the microseismic monitoring technique ... · 2000-11 Qualification of the microseismic monitoring technique applied to the risk of colfapse in iron ore mines G. Senfaute1,

Qualification of the microseismic monitoring technique

applied to the risk of collapse in iron ore mines

Gloria Senfaute, Mohamad Khir Abdul-Wahed, Jack-Pierre Piguet,

Jean-Pierre Josien

To cite this version:

Gloria Senfaute, Mohamad Khir Abdul-Wahed, Jack-Pierre Piguet, Jean-Pierre Josien. Qual-ification of the microseismic monitoring technique applied to the risk of collapse in iron oremines. International symposium of the international society for rock mechanics (EUROCK2000), Mar 2000, Aachen, Germany. <ineris-00972194>

HAL Id: ineris-00972194

https://hal-ineris.ccsd.cnrs.fr/ineris-00972194

Submitted on 3 Apr 2014

HAL is a multi-disciplinary open accessarchive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come fromteaching and research institutions in France orabroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, estdestinee au depot et a la diffusion de documentsscientifiques de niveau recherche, publies ou non,emanant des etablissements d’enseignement et derecherche francais ou etrangers, des laboratoirespublics ou prives.

Page 2: Quali cation of the microseismic monitoring technique ... · 2000-11 Qualification of the microseismic monitoring technique applied to the risk of colfapse in iron ore mines G. Senfaute1,

2000-11

Qualification of the microseismic monitoring technique applied to the risk of colfapse iniron ore mines

G. Senfaute1, M. Abdul Wahed2, J.P. Piguet2, J.P. Josien1

Institut National de l'Environnement Industriel et des Risques (INERIS), 54042 Nancy, France2Ecole des Mines de Nancy, Laboratoire Environnement, Geomecanique et Ouvrages (LAEGO), 54042 Nancy, France

ABSTRACT: Experiments carried out in a working iron mine validated the microseismicmonitoring technique äs a means of detecting fracture noise emissions regarded äs Signalsindicating an incipient collapse. In the experiment surface recordings were made of themicroseismic Signals corresponding to fractures and local collapse phenomena generated at themine bottom by deliberately destroying pillars. The pillar removal operations and the collapse ofthe roof were systematically correlated with a series of microseismic events. The experimentserved to validate the microseismic monitoring technique äs a means of detecting surfaceprecursors of a collapse, to demonstrate the ejfectiveness of the technique, and to calibrate theprincipal parameters of a microseismic monitoring System adapted to detection and monitoring inareas where there is a risk of collapse.

Key words: Microseismic monitoring, collapse, torpedoing, robbing of pillars.

ZUSAMMENFASSUNG : Dieser Bericht beschreibt die Validation, durch einen in einem aktievenEisenerzbergwerk durchgeführten Feldtest, der Mikroseismik zum Abhöhren von Brüchen imGebirge welche anzeichenfür einen bevorstehenden Tagesbruch im Kammer-Pfeiler-Bergbau seinkönnen. Der Feldtest bestand darin, an der Tagesoberfläche, mikroseismische Signaleaufzunehmen, welche durch das nach der gewollten Sprengung bestimmter Pfeiler nachbrechendeHangende provoziert wurden. Die über Tage aufgenommenen Signale konnten mit den unter Tagegesprengten Pfeilern und dem nachbrechenden Hangenden sowie den darauf folgenden Brüchenim Deckgebirge koreliert werden. Dieser Feldtest hat somit bewiesen dass sich die Mikroseismikzur Überwachung von alten, nicht mehr befahrbaren, Kammer-Pfeiler Bauten eignet und dass siedort in der Lage ist Tagesbrüche vorherzusagen.

1. Introductio n

The iron ore basin in Lorraine extendsbetween the eitles of Luxembourg in the northand Nancy (France) in the south. The iron oredeposit is sedimentary Aalenian and aninsertion in the geological series between theLias and the Jurassic (Tincelin, 1958). Thisdeposit is virtually horizontal with a reguläraverage slope of 3% and a mean thickness of30 metres (figure l ) . The sterile rocks lyingabove the iron-bearing formation known äs"dead ground" have an average thickness of

150 metres. In the deepest mines in the basiniron ore has been recovered some 250 metresbelow ground level.

Over the greater part of the deposit, recoveryinvolved removing the roof Supports andcollapsing the residual pillars. Howeverbeneath sensitive zones (where there werehouses and surface infrastructure) partialrecovery methods were used : abandonedchambers and pillars, with Islands separated bylongwalls. In these areas of partial recovery,collapse phenomena have occurred severaltimes, usually owing to the failure of

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abandoned pillars (Piguet et al, 1999, Vinkleret al, 1998).

The microseismic monitoring technique isused today in a number of different situations:to study seismic phenomena induced byactivities in working mines (Senfaute et al,1994, 1997 and 1999), the injection of fluids,geothermal phenomena (Moriya H., NiitsumaH., 1996), the recovery of gas and oil, and thesurveillance of Underground storage facüities orsensitive installations. As regards abandonediron mines exposed to the risk of sudden andunexpected collapse affecting the surface, therehas hitherto been no means of predicting ordetecting these phenomena.

For this reason we undertook a researchprogram with the objective of validating themicroseismic monitoring technique to detectthe noise emitted during the fracturing thatconstituted the first signs of incipient collapse(failure of pillars in the mine, then of thesurrounding roof, and finally of theoverburden). The technique was validated bymeans of an experiment carried out in the lastworking iron mine with the support ofARBED's mine. The experiment involvedsurface recordings of the Signals correspondingto the fracturing and local collapses generateddeliberately at mine bottom by destroying(torpedoing) pillars.

2. Description of the mine and themicroseismic monitoring System

The usual method of mining iron in Lorrainewas to separate the deposit into panels which in

tum were divided into pillars, i.e., intoparallelepiped blocks of the order of 10 metresin size. In cases of total recovery, thedimensions of these pillars were graduallyreduced and they were then removed withexplosives. This Operation known äs "robbingpillars" led to the collapse of the roof of thestrata known äs "caving".

Whenever it was considered necessary toprotect the infrastructure on the surface (towns,roads, railways, etc.) the methods was theneither to leave the pillars in place (the so-called"abandoned pillars" method) or to limi t thedimensions of the panels (the method known äs"Islands"). In the latter case, the collapseheight of the roof is less the smaller the size ofthe panel. In the Arbed mines, experienceshows that the collapse cone reaches a height ofabout 6 metres above an island 25 metres inlength.

The geometry of the mining unit monitoredby the microseismic technique is shown onfigure 2. In this unit, the width of the Islands is40 metres, the strips between the Islands are 30metres wide and the mining height is 4 metres.The microseismic monitoring Station wasinstalled at the surface, in a borehole 30 metresin depth and about 300 metres from the miningoperations. The microseismic Station is athree-directional unit consisting of sensors ofthe accelerometer type with a pass band of 2 to2300 Hz and a sensitivity of 500 mV/g. TheSignals are fed to a local Computer with asampling frequency of 10 kHz (figure 3).

MEUSE

la Wœvre plateau

M E U S E M O S E L LB MOSELLB

EgsBS Oxfordien-CalloTieni l l Upper Bathonien (blue clay marl)

p g î) Lower Bajoden (chaUcy mal! and graveJ)O H Micaceous marl

Oolithic iron ore

Toarcien (sandy marl wilh chalk insertions)

Ë 2H Upper Bajocien(oolithic Aalte "oolithe de Jaumont")

Figure l: Simplified geological section of the Lorraine iron ore basin

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50m

P S as to be toipedoed

Mined areas

Surface microseismic Station

Fault

Figure 2: Plan view of the mine workings andlocation of the microseismic monitoring Stationat the surface

Data acquisition Signal processing

S 4

II1

I—~i Jonction b

I—Figure 3: Microseismic measuring system

3. lUUcroseismi c events recorde d

The microseismic Station on the surface wasleft switched on for 34 days during theoperations of removing roof supports and thetorpedoing of pillars at mine bottom. Duringthis period 260 microseismic events wererecorded. These were classified into threetypes:

• Class l : Events associated with blasting.• Class 2: Events associated with fractures in

the caving of the immediate roof.• Class 3: Events associated with fractures

away from the roof caving.

The events in class l were identified by thetime at which blasting took place. The eventsin class 2 were identified by the time at whichthe localised slip or collapse occurred,following the torpedoing of the pillars.Experience showed that collapse might beimmediate or take place a few hours or a fewdays arter the shot was fired. The third class ofevents were relatively isolated incidents. Theyoccurred at times other than those at whichblasting took place to remove roof supports orfor plotting purposes äs well äs roof collapse.Table l below shows all the recordings madethroughout the period of microseismicmonitoring.

Class lEvents

associated withblasting

26

Class 2Seismic events:

fractures incaving

66

Class 3Seismic eventsfractures away

from caving168

Table l: Total microseismic events recordedduring the monitoring period

3.1 Microseismic events associated withblasting

Al l the blasting done during miningoperations located 300 metres from the seismicStation was systematically recorded by theseismic monitoring system. The Signalsrecorded were of very high amplitude, leadingto Signal Saturation. The signal frequency isabout 200 Hz with peaks of up to 500 Hz.

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Other blasts located about 600 metres from themonitoring Station were also recorded. Themaximum frequency of these events is lower(under 100 Hz) than the frequency of theevents recorded during blasts located at300 metres from the monitoring Station.Finally, blasts some 900 metres from themonitoring Station were not detected by theSystem. Figure 4 shows examples of seismicSignals and of the frequency spectrum of theevents recorded.

lftl. a-l.Val:Z™-K:>a-z™-T;»l ka,xla.;*m-toa.aa.:tm

MZh-tMat.P.E:&mm:1H.aKm.Y:xl

^

up

Figure 4: Examples of seismic Signals andfrequency spectra recorded during blastingoperations some 300 metres from the seismicmonitoring Station on the surface.

3.2 Microseismic events associated withfractures in the caving

The collapse of the roof following thetorpedoing of pillars A and B (see figure 3)generated 66 microseismic events. Theseevents were characterised by their arrival inbursts, i.e., a number of events recorded in ashort period of time. The signature of theSignals and the spectral content of these eventsare appreciably different from those associatedwith blasting. They are very short events withmaximum frequency peaks between 400 and500 Hz (figure 5).

^M^ t M^Ww ^

Figure 5: Examples of seismic Signalsrecorded during the collapse of the roof. Thedijference with the signature of the seismicevents associated with blasting can be clearlyseen (see figure 4)

Collapse following the torpedoing of pillar A

The area left unsupported by the torpedoingof pillar A is about 500 m2. E the height of thecollapse cones is assumed to be of the order of6 metres, the volume of collapsed material isestimated at about 2800 m3. A series ofmicroseismic events was recorded in theminute following the final blast, correspondingto the removal of the pillar. Observations madeat mine bottom confirmed the immediatecollapse following the torpedo shot making itpossible to correlate the microseismic eventsrecorded with the rockbursts generated by theadvent of collapse.

Collapse following the torpedoing of pillar B

The area left unsupported by the torpedoingof pillar B is of the order of 625 m2, theconfiguration of this pillar being different fromthat of pillar A. Pillar B is about 6 metres froma pillar already torpedoed. This configurationmeans that the dimensions of the collapse coneof this pillar are greater than those associated

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with pillar A. This new collapse cone added tothe collapse cone of the neighbouring pillar.The volume of the collapse cone duringtorpedoing of pillar B was estimated directiy onthe spot at about 3000 m3.

At the moment when pillar B was torpedoed,no microseismic event (apart from the blasts)was recorded. Observations at mine bottomconfirmed that there was no collapse after theblasts torpedoing the pillar. Further shots werefired 24 hours later, without immediatecollapse. A burst of events was recorded 7hours after these torpedo shots (20microseismic events in 40 minutes).Verification at mine bottom showed that theseevents were concomitant with part of the roofcollapse. However observations showed thatthe collapse was not complete. A further burstof events, greater man the previous one (40events in an hour) was recorded 16 hours afterthis first collapse. Checks at mine bottomshowed that these events were associated withthe occurrence of a second collapse. Followingthis second burst of events, no other eventswere recorded and the collapse ceasedspreading.

3.3 Microseismic events associated withfractures away from the roofcollapse

These events were characterised in that theywere all recorded at times other man those ofthe blasting and roof collapses. These eventsdid not occur in bursts, but were isolated intime and could arrive at any time, includingduring the night. The signature and spectralcontent are similar to the events associated withcollapses. These events were interpreted äsbeing subsequent adjustments in the rock in thevicinity of the mined zone (failures of the roofor pillars).

3.4 Sensitivity of the monitoring Systemto the seismic events recorded

In view of the frequency ränge of the eventsoccurring at mine bottom and that of thesensors installed to record them, it appearedthat no blasting event taking place at more than900 metres away was detected by the

experimental monitoring Station. Howeverevery blast taking place at a distance within600 metres was detected by the seismic Station.

Also, fractures occurring in the mined strataat a distance of about 300 metres from theseismic monitoring Station tripped the Systemand recorded interpretable seismic signals. Forthis reason the sensitivity radius of the Systemwas fixed at 400 metres äs a firstapproximation.

4. Conclusion s

The operations to remove roof supports atmine bottom and in particular the fracturing ästhe roof collapsed were unambiguouslycorrelated with a characteristic series ofmicroseismic events recorded by a 3-directionalseismic Station on the surface. Themicroseismic events associated with fracturesin the roof collapse are very short events withmaximum frequency peaks between 400 and500 Hz. Other events, producing signalssimilar to those of the collapse of the roof butmore isolated in time were interpreted äs lateradjustments within the rock.

The experiment made it possible to validatethe microseismic monitoring technique äs away of detecting precursor signs of the collapseprocess before the phenomenon producessurface effects, to demonstrate the effectivenessof the technique and to calibrate the principalParameters of a microseismic monitoringSystem adapted to the detection andsurveillance of zones exposed to the risk ofcollapse in the Lorraine iron ore basin.

The System was applied for the first time atthe towns of Joeuf and Homecourt. Thesubsoil of these towns is entirely underminedby mine workings that are now abandoned. Acontinuous microseismic monitoring systemwas installed on this site and on the basis of theresults from these preliminary qualificationtests, a warning procedure has been set up.

Acknowledgements

The authors wish to thank the LORMINESand ARBED companies and the Ministry for

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Industry for their generous funding of thisexperiment. We offer our particular thanks tothe management and staff of ARBED's TerresRouges mine for their support during ourresearch on the site.

5. Bibliographie

Moriya H., Niitsuma H.; (1996), Doubletanalyses for characterizing regional structuresin three-component microseismic measurement.The 6th conference on AE/MA in GeologieStructures and Materials.Piguet J.P., Josien J.P, Kouniali S., Bigarre P.,Vouill e G.; (1999), The contribution of rockmechanics for long term risk assessment andmanagement in abandoned mines - The case ofiron mines in Lorraine. International Societyfor Rock Mechanics. Paris / France / 1999 (pp.317-323).Senfaute G., Bigarre P., Josien J.P., MathieuE.; (1994) Real-time microseismic monitoring :Automatic wave processing and multilayeredvelocity model for accurate event location. InRock Mechanics in Petroleum Engineering,EUROCK'94 (Balkema, Delf/Netherlands1994) pp. 631-638.Senfaute G., Chambon C. Bigarre P., Guise Y.and Josien J.P. (1997), Spatial Distribution ofMining Tremors and the Relationship toRockburst Hazard, Pure and AppliedGeophysics. Birkhäuser Verlag, BaselSenfaute G., Al-Heib M., Josien J.P., NoirelJ.P.; (1999), Detection and monitoring of highstress concentration zones by numerical andmicroseismic methods. International Society forRock Mechanics. Paris / France / 1999 (pp.1065-1070)

E. Tincelin (1958), Pressions et Deformationsde Terrains dans les Mines de Fer de Lorraine.These ä la faculte des sciences de l'universitede Nancy, 283 pages.Vinkler F., Piguet J.P. (1998). Numericalanalysis of long term stability of the abandonedmines. Impact of the groundwater level rise.FLAC Symposium on Numerical Modeling inGeomechanics.