Seismic Monitoring

DIN EN ISO 9001Reg.-Nr. 73 100 1022

K U EC SONDERSHAUSEN

seismic monitoring / GY / 12 / 2003

Seismic Monitoring

Kali – Umwelttechnik GmbH (K – UTEC) Am Petersenschacht 7 99706 Sondershausen Germany

Phone: ++49 – 3632 - 610 - 0 Fax: ++49 – 3632 - 610 - 105 e-Mail: kutec@kutec.de

Managing Director: Dr Heiner Marx Dr Holger Thoma

Sector: Geophysics

Person in charge: Dr. M. Jordan

Phone: ++49 – 3632 - 610 - 190 Fax: ++49 – 3632 - 610 - 105 e-Mail: geophysik@kutec.de

Specialist for the field of seismic monitoringof Potash mines:

Cert. Geophys. Olaf Klippel Cert. Geophys. Astrid Gessert

K-UTEC GmbHAm Petersenschacht 799706 SondershausenGermanySeismic monitoring system

Tel.: +49 36 32 - 61 01 00 Fax: +49 36 32 - 61 01 05 Email: geophysik@kutec.de http://www.kutec.de

Introduction

A seismic monitoring system consists ofa number of seismometer stations dis-tributed within the observation area andthe data recording and processing cen-tre.The arrangement of the seismometersdepends mainly on the observation taskbut also on the geology and local pos-sibilities regarding accessibility and sig-nal transmission. In general, the targetarea has to be spatially encircled byseveral seismometer stations, whichalso includes the positioning of thegeophones in different depth levels.Possible locations for seismometer sta-tions are for example the basement of abuilding, within a borehole or within anunderground mine. The number or thedensity of the seismometer stations,respectively, determine the accuracy ofthe location of a seismic event.

To just locate a seismic event, 1D-geophones (only vertical component)are usually sufficient. Most important isa high sensitivity and a good couplingto the ground. The frequency charac-teristic of the used seismometersshould be in the range of 2..200 Hz fora larger scale monitoring system andrespectively higher (in the kHz-range)for micro systems or seismo-acousticapplications.However, to further qualify the recordedseismic event regarding magnitude andsource parameters (e.g. stress-drop,seismic moment, dislocation, size of thefault area, etc.) we strongly recommendto include at least one or two 3D-geophones in the seismometer net-work.

Principle of locatingseismic events

K-UTEC GmbHAm Petersenschacht 799706 SondershausenGermanySeismic monitoring system

Tel.: +49 36 32 - 61 01 00 Fax: +49 36 32 - 61 01 05 Email: geophysik@kutec.de http://www.kutec.de

Signal transmission:

A lot of possibilities exist to transmit thesignal from the seismometer stations tothe recording centre (e.g. signal cable,telephone cable, radio; analogue ordigital). Especially for the application inmines and for very long distances oncable the signal transmission via pulsefrequency modulation (PFM) has beenproven to be the most reliable one.Modulators of this kind are also li-censed for the use within gas-/explosion endangered mine workings(certificate I M2 EEx ia I).

data recording:

The signal from each seismometer istransmitted to the recording centre,where it is demodulated and processedfor data storage and further interpreta-tion. The heart of the data centre is thetransient recorder, which makes the

simultaneous and continuos monitoringof numerous input channels possible.A very accurate absolute time base isneeded to identify and catalogue ob-served seismic events. This is achievedby using a DCF-77 or a GPS time mod-ule to synchronise the timer in the tran-sient recorder.To limit the amount of data, the seismicmonitoring system does not record thedata continuously as for example seis-mological observatories but event ori-entated. This means that only the rele-vant seismic events with an appropriatetime window before and after the eventis stored on the disk.The decision whether or not there is anevent which has to be stored on thedisk is made by the definition of triggerfunctions.

Demodulation

Transient-recorder

Time base: DCF-77 / GPS

Seismometer

PC for data storage andprocessing

PFM-modulator

K-UTEC GmbHAm Petersenschacht 799706 SondershausenGermanySeismic monitoring system

Tel.: +49 36 32 - 61 01 00 Fax: +49 36 32 - 61 01 05 Email: geophysik@kutec.de http://www.kutec.de

Transient recorder:

The transient recorder is a measuringsystem enabling the simultaneousdigitally recording of data from numer-ous input channels. The system usedby K-UTEC has connections for up to32 analogue or digital input channelsand the integrated signal processorpermits the definition of complex triggerfunctions and the online data process-ing.The modularised design of this meas-uring system as well as the almostendless possibilities to configure it’scontrolling software makes it also pos-sible to record other types of data (e.g.gas detectors, air conditions an manymore) in addition to the primary seismicdata. Moreover, the integrated signalprocessor together with the variousdigital and analogue output channelsallow to communicate with or evencontrol other peripheral systems.

Trigger functions:

The trigger functions are defined withinthe controlling software of the transientrecorder. Possible trigger functionscould be for example a simple thresholdor the ratio of the short time and longtime average. The different seismome-ters are often grouped together and thedata recording is triggered when theconditions for one or more groups aremet.All input channels are monitored con-tinuously and the data is stored in amemory loop. Once the trigger condi-tions are met, the data is copied fromthe memory loop to a file system on the

disk. This procedure allows to alsostore an appropriate time window be-fore and after the event. Especially thetime window before the actual triggertime is necessary to ensure that thebeginning of the seismic event as wellas smaller leading events are re-corded. The size of the pre-trigger timewindow can be freely chosen and isonly limited by the number of used inputchannels and the size of the memoryloop.

Data storage:

The data is stored on the hard disk of anormal PC, which can be connected tothe transient recorder via a standardethernet network. It is also possible tointegrate a PCMCIA-type hard disk orflash card in the transient recorder,which then enables the stand aloneoperation of the system without thecontrolling PC.

Data processing and interpretation:

The identification and visualisation ofthe seismic events can be done directlyon the PC where the data is stored.However, to ensure undisturbed dataacquisition, we strongly recommend todo further data processing such as theestimation of the magnitude of theevent and the calculation of the coordi-nates of the epicentre etc. on a differ-ent System.

K-UTEC GmbHAm Petersenschacht 799706 SondershausenGermanySeismic monitoring system

Tel.: +49 36 32 - 61 01 00 Fax: +49 36 32 - 61 01 05 Email: geophysik@kutec.de http://www.kutec.de

Numerous types of seismic shocks arerecorded with the monitoring systemand most of the events are not of anatural origin or do not come fromwithin the observation area. Thismeans that besides the main task to

observe naturally occurring seismicevents, all kinds of blasting in andaround the target area as well as largerearthquakes from afar are recorded.Since it is impossible to automaticallydistinguish between these types ofevents, it is very important to correctlyidentify the event manually to avoidmisinterpretation when assessing thegeneral seismicity in the target area.

Locating the events:

K-UTEC has developed a special toolto calculate the origin of the seismicevent. The tool is easy to use and runson any Windows 9x/NT based system.It uses the intercept times from thevarious seismometers and the informa-tion about the propagation conditionsand especially the velocities of seismic

waves from a given geological model tocalculate the coordinates of the focalorigin.To be able to locate a seismic event, ithas to be recorded by at least three,better by four or more seismometers.

Assessing the seismicity:

Seismic events, whether they arecaused by local tectonics or induced bylocal mining, are only sporadically oc-curring events, thus the assessment ofthe general seismicity within an areacan only be realised over a longer pe-riod of time.

K-UTEC has a long year experiencewith this subject and offers extensivesupport both in planning and installingseismic monitoring systems but we alsooffer extensive assistance for the op-eration of the monitoring system, theinterpretation of the single events aswell as the general assessment of theseismicity in the target area.

Seismogram, Estimation of intercept times at thevarious seismometer stations

Tool SeismoLocal for locating local seismicevents

K-UTEC GmbHAm Petersenschacht 799706 SondershausenGermanySeismic monitoring system

Tel.: +49 36 32 - 61 01 00 Fax: +49 36 32 - 61 01 05 Email: geophysik@kutec.de http://www.kutec.de

Technical properties of the transient recorder (basic system):

• 32 analogue input channels single ended or 16 differential• 3 special channels, AC-coupled for current fed accelerometers• synchronous sampling of all input channels (2 time bases: intern + DCF-

77 / GPS)• 80 kHz max. sampling rate, 2 MB memory• 15 bit resolution, input range: ±100 mV, ± 500 mV, ± 2 V, ± 10 V• Thermistors and PT100-resistors directly connectable• 32 digital I/O channel, incl. 4 channels for incremental encoders• 8 analogue output channels, 12 bit• integrated signal processor:• connection for PC via Ethernet RJ-45 (NetBios)• optional: PCMCIA-type HD / flashcard

Data recording and processing centre:

consisting of

• demodulation unit• transient recorder• PC for data storage and processing

transient recorder

K-UTEC GmbHAm Petersenschacht 799706 SondershausenGermanySeismic monitoring system

Tel.: +49 36 32 - 61 01 00 Fax: +49 36 32 - 61 01 05 Email: geophysik@kutec.de http://www.kutec.de

Signal transmission

Demodulation unit with integratedtransient recorder

PFM-modulator and various types of seismometers

technical properties PFM-modulator:

• power supply: 12 V, 24 V, 42 Vor 230 V

• nearly all types ofseismometers connectable

• up to 72 dB dynamic range

optional:• intrinsic protection against

explosive gases according toEN 50014 and EN 50020,EEx-ia-I (only 12 V powersupply)

Introduction

The tasks for seismic monitoring systems can bedefined in the following way:

Ever since the beginning of the salt mining activities incentral Germany at the end of the last century it repeat-edly came to fracturing and deformation processes ofthe rock formations resulting in rock bursts, contourbreaks and sink holes leading to considerable damageto the mines and the earth surface. From 1900 on 15larger sink holes and rock bursts occurred in theGerman potash mining areas reaching magnitudes ofup to 5.5 and intensities of 5 to 8.5 on the Medwedew-Sponheuer-Karnik (MSK) scale.A large number of the now closed mines is situatedvery close to or even directly under dense populated

areas, thus special care has to be taken for the long-time stabilization and plugging of the mine workings toavoid larger damage on infrastructure and buildings.Figure 1 shows a sink hole from the Staßfurt VI/VIImine. The sink hole with a diameter of approximately140 m was formed shortly after the mine was flooded in1975 and has been growing ever since to the todayssize of almost 300m.

1) from the intercept times of various wave types:- spatial coordinates of the origin of the seismicevent

2) from signal amplitude:- particle displacement, velocity and acceleration-magnitude of the event (localmagnitude, after Iida)- released seismic energy- intensity- fault plane solutions

3) from frequency characteristics:- seismic moment- dislocation at fault plane- fault plane radius or area- stress drop

Each monitoring network consists of a number of verti-cal component seismometers (currently 8-17) distrib-uted within the mine and on the earth surface above(Figure 3). Additionally, a number of independentmobile 3C-sensors are placed in public buildings or onthe surface. These 3C-seismometers are primarilyused for the damage assessment of infrastructure andbuildings for larger events but provide also data for thecalculation of the magnitude of the event and faultplane parameters (seismic moment, stress drop, etc.).Right now, they can not be used for the event localisa-tion because of differences in the time base of the seis-mometer network and the independent stations.The focal origin is calculated from the intercept times ofP- and S-wave using a given velocitymodel.Themodelis extracted from borehole data as well as from seismicand sonar surveys.

Figure 4 shows the development of the seismicity overthe last years of a carnallitite mining field. This field wasmined between 1982 and 1991. The deformationcharacteristics of carnallitite-salt and the rather difficultgeomechanical situation of the field resulted in adangerous weakening and almost disintegration of thepillars. As a consequence, high deformation velocitiesand a rapidly increasing seismicity (Magnitudes largerthan 1.5) could be observed. To avoid the collapse(rock burst) of the entire field, it had to be stabilized byrefilling the chambers with rock salt. The backfilling andstabilization of the field was carried out between 1991and 1996. The seismicity started to decline alreadyduring the refilling and plugging process. Moreover,the seismic monitoring together with the deformationmeasurements made it possible to identify especiallyweakened zones where special safety measures hadto be considered.

The increased mining of Carnallitite and the extensiveexploitation of the potash salt deposits lead to anincreasing seismic activity, asking for the installation oflarge scale seismic monitoring networks. Figure 2shows the location of the monitoring systems which arecurrently operated by K-UTEC.

Beginning in the early 80ies, a more complexmethology was introduced for the assessment of themining field conditions, including:

- geophysicalmethods such as- seismicmonitoring- sonar and radar measurements

- geomechanicalmethods- hydraulic fracturing (in-situ stress measurement)- conventional deformation measurements- borehole video and photographic documentation

- remote sensing techniques:- arial and satellite images- laser scanning and others

From all the applied methods, the seismic monitoring isabout the only way to obtain any instant informationabout ongoing deformation processes in those parts ofthemine which are no longer accessible.

Monitoring of the mining process and assessment ofinduced and naturally occurring seismicity

Monitoring of underground disposal sites

Overall seismic monitoring and assessment of theseismicity of sylvinite and mixed salt fields forsupervision and steering of stowing and stabilizationmeasures

Production phase:

Aftertreatment phase:

Closing and plugging of themine:

The following parameters are provided by seismicmonitoring systems:

1. Stabilization of a carnallititemining field

Locating the seismic event

Case examples

Abstract

The intensified exploitation of potash-deposits in central Germany in the last decades lead to an increasing seismicity inthe region and large subsidences on the surface. The potentials of seismic monitoring systems in surveying and steeringof stabilization measures for the mines will be discussed by selected case studies as the redevelopment of a Carnallitemining field and the assessment of a mixed salt field with multiple level mining.

Table 1: Heavy rock bursts in German rock salt and potash mining

date mine type of salt area[km2]

magnitude

17.08.1879 Leopoldshall I/II,Staßfurt

Carnallitite 0.02

23.12.1880 Leopoldshall I/II,Staßfurt

Carnallitite 0.12

11.11.1901 Ludwig II, Staßfurt Carnallitite 0.06 17 people dead22.01.1916 Hallesche Salzwerke,

TeutschenthalCarnallitite 0.03 3.0

24.05.1940 Krügershall,Teutschenthal

Carnallitite 0.6 5.0 42 people dead,damages on buildings

05.03.1943 Kleinschierstedt,Ostfeld

Sylvinite 0.3 4.0

22.02.1953 Wintershall, Heringen Carnallitite 0.7 5.2 damages on buildings,fault on earth surface

28.08.1955 Kleinschierstedt,Westfeld

Sylvinite 0.6

07.08.1958 Merkers Sylvinite /Carnallitite

3.0 5.2

30.12.1959 Neustaßfurt VI/VII Carnallitite 0.129.06.1961 Merkers Carnallitite 0.2 4.104.04.1971 Aschersleben V,

NordfeldCarnallitite 0.32 4.6

23.06.1975 Unterbreizbach Carnallitite 3.0 5.3 damages on buildings02.07.1983 Bleicherode Carnallitite 0.1 3.313.03.1989 Merkers Carnallitite 7.0 5.7 damages on buildings11.09.1996 Teutschenthal Carnallitite 2.5 4.8 damages on buildings

Figure 2:Installed seismic monitoring systems incentral Germany

Demodulation

Transient-recorder

Timebase:DCF-77/GPS

Seismometer

PC fordatastorageandprocessing

PFM-modulator

Figure 3: Scheme of a seismic monitoring network

Figure 4:Development of the seismicity of a Carnallititemining field during (1991-1996) and afterstabilization measures

Kali- Umwelttechnik GmbHAm Petersenschacht 799706 SondershausenGermany

Continous seismic monitoring as one way to assesssources of danger in potash and rock salt mining

Holger Thoma, Bernd Bode and Olaf Klippel*Contact: Olaf.Klippel@kutec.de http://www.kutec.de

Figure 1:Sinkhole Neustaßfurt VI/VIIDiameter: 300 m

Part I

2. Stabilization of a mixed salt field

3. Controlled flooding of a mixed salt field

References

Acknowledgements

Conclusions

Asimilar situation was found in a mixed salt field, whichhas been mined out on several levels. Here, thestowing and stabilization measures are not yetcompleted and a rather high seismicity withmagnitudes of up to 2.0 can be observed. Thedistribution of the seismic events in the mining field isshown in figure 5. As can be seen in Figure 5, o

he vast majority of theseismic events is concentrated on lines which can berelated to local tectonics. This indicates that theongoing deformation processes focus on thereactivation of old, already existing fault systems,which inturn implements the risk of the opening ofwater-ways into the salt complex.

Figure 6 shows a cross section through the mining fieldand gives an overview of the depth distribution of thelocated seismic events. As can be appreciated fromFigure 6, the majority of the seismic events with highermagnitudes have their origin above the mining horizonin the more competent layers such as the lower NewRed sandstone (SU) or the main anhydrite (T3/A3).Events within or close to the mining horizons arerelated to the fracturing or even collapse of singlepillars.

An other way of characterising the development of theseismicity over time is shown in figure 7. There, thecumulative released seismic energy or respectivelythe square root of the energy is plotted over time(Benioff, 1951). The slope of the curve is related to theamount of stress released by the seismic events. Theshape of the curve gives information of how the stressis released: a large number of small events (smoothcurve) or a small number of stronger events (blockycurve). A change in the mean slope of the curveindicates a change of the characteristic of thedeformation processes. In case of this mixed saltmining field (figure 7, field 2b), an increasing activitycan be observed, indicating that the ongoingstabilisation measures have not yet taken any effect.Moreover, the deformation and disintegration of thefield seems to accelerate.

The mining field as shown in Figure 8 was abandonedand in parts already plugged . In orderto prevent an uncontrolled flooding of the mining fielddue to brine inflows to the mine (partly saturated withNaCl), it was decided to geomechanically stabilize andplug the field by a controlled flooding with highlyconcentrated MgCl brine.

In preparation of the flooding, experiments were madein isolated blocks to investigate how the NaCl-brine isreacting on the salt formation (consisting of sylviniteoverlaying Carnallitite at a depth of 800 to 850 m) butalso to test if the MgCl brine can be produced withintheminewithoutrisking the collapse of themining field.

The development of the solution processes, thedeformation dynamics as well as the development ofthe induced seismicity had to be monitored. Theinflowing brine (partly saturated with NaCl) is ledthrough the parts of the mining field which has alreadybeen plugged (Figure 8, area A) in order to mineralizewith MgCl . In that way, the brine is almost saturatedand has no relevant solution power left when it enters

the more sensitive areas. Thus,the geomechanical effect of theflooding is mainly limited to themobilisation of deformationprocesses due to reduced frictionby the migrating brine and therelated stress reallocation.

In areas B and C (Figure 8), anotably seismicity was developednot until 2 years after thebeginning of the flooding, whentwo third of the blocks were filled.The seismicity was significantlyreduced after one more yearw h e n t h e s e a r e a s w e r ecompletely flooded. The seismice v e n t s w e r e g e n e r a l l ycharacterised by rather smalle n e r g i e s w i t h m a x i m u mmagnitudes of M =0.6. The originof the events were located withinthe mining horizon and the layersdirectly above or below.

A rather quick and also more intense reaction on theflooding could be observed in area D, which ischaracterised by a more complex tectonic situation.The period of high energy release was limited to a fewmonth, where seismic events with magnitudes largerthan M =1.0 were registered at very short intervals .The strongest event reachedmagnitudes of M =1.9

As can be seen in Figure 8, most of the events arelocated along a NW-SW striking structure indicating aclose relation to the local tectonics. Moreover, theorigins of the seismic event were partly located in depthregions far below the mining horizon, which also pointstowards a mobilisation of local tectonics.

The development of the seismicity of the field over time(BENIOFF-curve) is shown in figure 9. The period ofhigh seismic activity in area D can be clearly identified.

Seismic monitoring systems play an essential part inthe surveying of mining fields and rock formations inGerman potash and rock salt mining.

They are capable of detecting fracturing processes inthe rock formations and provide appropriateparameters for the assessment and interpretation ofthe seismic activity.

By observing the development of the seismicity overt ime and the complex interpretat ion withrockmechanical and other surveying data, seismicmonitoring systems allow for:

steer reconstruction or plugging measures andcontrol its effects

Benioff, H.;1951; Colloquium on plastic flow anddeformation within the earth; Trans. Am. Geophys.Union;pp508-514

Benioff, H.; 1951; Global strain accumulation andrelease as revealed by great earthquakes; Bull.Geol. Soc. Am.;pp331-338

We like to thank the people from GVV mbH and GSESfor the good cooperation in the project.

nly aminor part of the events is spread more or lessrandomly over the field. T

after production

early recognition and assessment of critical orpossible hazardous conditions

2

2

2

L

L

L

Development of the seismicity:

cumulative stress release 1994 - 2001

0,00E+00

5,00E+04

1,00E+05

1,50E+05

2,00E+05

2,50E+05

year

entire mine. SW-field

start of seismicmonitoring

migration to areaB and C

Area B and C2/3 f looded

area B and C completelyf looded

migration of thebrine to area D

first observationof brine inf low s

flooding of area A

Periods of brine migration in the SW-Field

Figure 5:Distribution of seismic events in a mixed salt field.The majority of the events are located at structuresrelated to local tectonics

Figure 6:Cross section through the mining field and depth distribution of the seismic events

cumulative stress release 04/86 - 04/02

0,0E+00

5,0E+04

1,0E+05

1,5E+05

2,0E+05

2,5E+05

3,0E+05

3,5E+05

86 87 88 89 90 91 92 93 94 95 96 97 98 99 00 01 02

year

0,0E+00

2,0E+04

4,0E+04

6,0E+04

8,0E+04

1,0E+05

1,2E+05

1,4E+05

mine field1 field2

stabilization measures field Istabilization measures field2

acceleration of deformationprocesses in field2

sudden initiation of seismic activity inmining field2

Figure 7:Cumulative stress release (BENIOFF-curve) of miningfield1 (carnallitite field), field2 (mixed salt field) and theentire mine

Figure 9:Cumulative stress release (BENIOFF-curve) of theflooded mining field and the entire mine

Figure 8:Distribution of seismic events a mixed salt fieldduring the flooding of

Kali- Umwelttechnik GmbHAm Petersenschacht 799706 SondershausenGermany

Continous seismic monitoring as one way to assesssources of danger in potash and rock salt mining

Part II