APPENDICES

APPENDIX ADetail of original 017 Proposal

APPENDIX BLiterature search

APPENDIX CList of Documents received

APPENDIX A

Detail of original 017 Proposal

APPENDIX B

Literature search

APPENDIX B

Literature Search

An attempt was made to assess how wide and comprehensive the literature review of the GAP017 endeavour had been. In the report 333 papers are listed. A cursory search through this givesevery indication that it includes very few items that are not recent or relevant.

Papers dealing with the problem of rockbursts from a simple descriptive perspective or treatmentat a level that could be considered superficial or non-technical (eg. by local South Africanauthors) are conspicuously absent. This is considered to be an indication of a well-founded,appropriate selectivity or discernment. Treatments at a higher, more abstract, level of statistical,mathematical and physics insight appear to be common. This too is considered to be appropriatefor a problem as complex as prediction of mechanical instability of inhomogeneous material inindeterminable stress situations must be. The majority of papers listed were published in thejournals of the more elevated academic and technical institutions of geophysics, specificallyearthquake science, material science, physics and mechanics.

Briefly stated, it is considered that the review of the literature by the project team has been verysound, comprehensive yet selective, discerning and deep. It indicates clearly that assembly ofwell-known relevant references and search for new material was guided by a very soundunderstanding of the fundamental needs of the subject matter and of the project.

This conviction was reinforced when the reviewer instituted a search through the literature fieldto see if there had been any obvious ommissions by the ISS team. Using the SRK computer-accessed search facilities an enquiry was made into several appropriate data bases. The twokeywords of earthquake and prediction were coupled together to constrain the selection. Theresult was unsatisfactory and frustrating.

A large number of titles (just over 1000) were turned up by the search, very few of whichcoincided with the ISS list. The majority were highly specific, relating to particular districts andprovinces in earthquake-prone countries or to particular structures such as buildings, bridges andpipe-lines. From this exercise it soon became clear that a useful and relevant enquiry can onlybe launched from an already well-informed knowledge base such as ISS have jointly andindividually amongst their team.

It is concluded that a high quality literature review has been reported in the GAP 017 project.

APPENDIX C

List of Documents received

APPENDIX C

List of Documents received

Summary of Final Report to the Safety in Mines Research Advisory Committee, Department ofMineral & Energy. RSA, for research Project GAP 017 (1993-1995) 34 pages.

(In ringbound volume this is entitled Summary of Final Report. The letter is identical to thissummary except that it was printed on 6 & 7 May while the separate summary was printed on theMay {pp21-34} and 23rd May pp1-21)

• Ringbound ‘Final Project Report’ (approved)- consists of Summary and ten chapters

• 1993 Annual Project Report. (about 70mm thick)- consists of 9 chapters

• Project Report Year-end report 1994 (about 45mm thick)- consists of summary and 10 chapters

Chapter 10 details 9 case studies

Report on the Audit/ Review of Project GAP 017

Seismology for Rockburst Prevention, Control and Prediction

By WD Ortlepp

1 INTRODUCTION

Following a request conveyed in a fax dated 25 November 1996 from Mr DP Allen, thesecretary to SIMGAP, a proposal was submitted to the Advisory Group early inDecember.

The project title given in the proposal was:

Audit/ review of project GAP 017.

The use of the words audit and review in the title description followed from the fact thatboth appeared in the original request from SIMGAP.

In order to clarify any possible ambiguity arising from this duality, the following is givenas the reviewer’s interpretation of the meanings of the two terms:

Audit: a verification (signed and attested by the reviewer) that the organization(ISS Int. Ltd.) has satisfactorily executed its undertaking to SIMGAP to do what itundertook to do. This would be judged against what was set out explicitly in the originalISS proposal to SIMGAP.

(A copy of the relevant parts of that proposal is included as an appendix to this report)

Review: is interpreted as being more of an appraisal which is rather less rigorousand less explicit than a strict audit would be. It would essentially reflect the reviewer’sinformed opinion of how valuable a contribution the project has made to providingworkable methodologies and technologies for use in combatting the problem ofrockbursts.

The understanding in the primary output of the formal proposal, to “... compile a criticalreview of the output of GAP 017...” was thus carried out in the spirit of the aboveunderstanding.

In places in the appraisal where it might appear that a somewhat critical (evenhypercritical) comment has been made, it was invariably the result of some frustration feltby the reviewer and was certainly not intended as a disparagement of the quality of thework. This frustration arises out of the reviewer’s inability to understand much of thehigh-level mathematical treatment demanded by the subject matter at that level. However, there is, at the same time, the underlying belief by the reviewer that thecompilers of the report have an obligation to try to make the description of the good workthat was done, more comprehensible to the uninitiated readers. If this is not done thereis a danger that similar frustration or cynicism may lead to a wrong perception regarding

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the real value of the work or, at best, a failure to appreciate its true quality.

2 APPRAISAL

The necessary prelude to determining the successfulness of the SIMGAP project, is toexamine its aims and goals. The following paragraphs give the reviewer’s understandingof the thrust of the ISS undertaking in this regard. This is essentially an interpretation ofthe detailed proposal given in the 1995 project proposal to SIMRAC dated 96/04/28- seeAppendix A.

2.1 Overall objectives of GAP 017

The primary objectives are the development of strategies, methodologies andtechnologies for proper seismological monitoring, with appropriate analyses andinterpretation, in rockburst-prone mines. (In this context, proper means producing datawhich is quantitative in terms of accepted physical parameters). The ultimate purposeof this endeavour would be to provide the means for guiding preventative and controlmeasures employed on the mine and predicting rockbursts. Prediction can be taken tomean where and when damaging and potentially life-threatening seismic events canoccur.

The detailed procedures and methodologies that it was anticipated would be followed inthe striving to achieve these objectives, were grouped into 3 sub-goals:

2.1.1 Rockburst Alert

The endeavour would be to detect zones in the rock mass WHERE potential forsignificant seismic activity is developing . The hope being that preventative action(layout, sequence, support) could be taken.

2.1.2 Rockburst Alarm

The endeavour would be to determine WHEN the build-up in activity could be close to‘instability’. The intention would then be to ‘defuse’ the threatened danger by triggeringthrough a special blasting procedure or to reduce the risk by improving control measureseg. increasing the quality of support or reducing exposure of personnel (eg. preventingentry of the night shift).

2.1.3 Rockburst Scram

The hope expressed implicitly here is that the beginning of self-nucleation of rockbursts(onset of instability) might be detected so that the evacuation of people from specificareas could be initiated.

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3 OPINION

3.1 General

It is necessary to state at the outset that it is in the nature of research or the developmentof new technology that ultimate success is usually not achieved without requiringrepeated efforts to overcome early failures and disappointments.

Because fundamental research (by definition) is exploring into the unknown, no priorpath of established methodology exists. Sometimes the best that can be done is to adopta systematic trial-and-error approach and probe successively into different avenues asthey present themselves. The researcher has to be prepared to run into ‘dead-ends’ andretrace his steps, again and again.

It is of course fundamentally important for a research agency to make every endeavourto expedite the search for the truth by comprehensively exploring the literature in the fieldof study and all other closely related fields.

In this respect there can be no doubt that ISS has satisfied this requirement more thanadequately.

It is probably true that there is not an extensive body of literature in the field of appliedmine seismology and certainly very little specifically available in the library of minerockburst prediction.

A total of 333 papers is given in the list of references which probably includes all of themost authoritative voices in the English language literature on the subject. An appraisalof the relevance and value of this compilation is given in Appendix B.

However, for the obvious reason that earthquakes have historically presented thegreatest threat to mankind in the way of natural catastrophes, very considerable effortshave been devoted to the search for means of predicting crustal earthquakes.

It is a salutory realization, but one that must be faced, that there is, as yet, no success inthe field of earthquake prediction. Because it has been clearly established that inter-plateshallow earthquakes and large rockbursts are essentially similar phenomena, there is avast literature documenting the efforts that have been made, particularly in the last 3decades or so, in the search for means to predict earthquakes.

Probably the main value to the ISS research effort of their thorough search of thisliterature has been that it has kept them from entering many blind alleys in the search forprediction. It has obviously also been of great value in developing understanding ofsource parameters, rock transmission characteristics and refinement of locationprocedures.

The fact that earthquake prediction has been unsuccessful does not mean that it is aforlorn waste of time to continue with efforts to find useful pre-cursory indicators to large

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rockbursts.

Because large rockbursts occur frequently, are often surrounded in 3 dimensional rockspace by sophisticated detection and analytical equipment and can have their sourceregions actually ‘dissected’ and exposed for direct examination, they are much moreamenable to being completely understood.

Therefore, there is greater hope that useful predictive techniques may be developed.

The reservations expressed above must not be taken to mean that no success has beenachieved to date. The reviewer is quite prepared to believe that there have been somesuccessful predictions made. It is nevertheless true that useful routine prediction ofrockbursts is still a long way off. However, the success that has been achieved inGAP 017 is sufficient to justify the continuation of effort, indeed at an increased level ofintensity, towards developing a useful system of prediction.

3.2 Evaluation of the extent to which specific objectives have been met

The specific evaluation that follows addresses each objective in the order which has beenlaid out in 2.1 to 2.1.3 above.

a) Primary objectives:

It is the decided opinion of this reviewer that strategies, methodologies andtechnologies for proper seismological monitoring have been developed as partof GAP 017.

Within the ambit of a broad knowledge of the western-world literature andpersonal experience (which covers Europe, North America, South America,Africa and Australia) the ISS technology is the best in the mining world.

b) Preventative and control measures:

Before the advent of seismic monitoring, the success or otherwise of anypreventative or control measures could only be judged empirically or by referenceto recorded data such as accident or production statistics. In the undergroundsituation, particularly, human memory of its own experience is very fallible andaccidents are so often determined by fortuitous circumstances that are not readilyamenable to proper statistical controls.

The availability of quantified, physical measures in the form of properseismological parameters does now provide the means for objectively measuringand comparing the success or otherwise of different preventative and controlmeasures that may be introduced.

This objective has therefore also been satisfied.

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c) Prediction

The delineation of WHERE the potential for seismic activity is developing, doesnow seem to be possible with a considerable degree of reliability. It seems thatsuch monitoring procedures are being pursued routinely on mines which haveinstalled substantive ISS systems and which employ properly-trainedseismologists.

The determination of WHEN the build-up in seismic activity is about to becomeunstable is much more problematic. Some notable successes have beenachieved: the most dramatic single instance of which was that of the Trough faultat Western Deep Levels. The Postma dyke example at Western Holdings was acase where most of the 11 or so repeated events that occurred were predictable. In some of these, warnings were actually given before the event whilst the otherswere ‘predicted’ retrospectively. It is a pity, in general, that a greater effort hasnot been made to give a quantitative estimate of the percentage success rateachieved under various geotechnical circumstances or in different geometricalsituations.

4 COMMENT ON SPECIFIC SECTIONS OF REPORT

The following comments are given chapter by chapter. The amount of detail givenreveals, more-or-less, the depth of perceptive criticism possible or appropriate on the partof the reviewer.

CHAPTER 1:CHARACTERISTICS OF SEISMIC MONITORING SYSTEMS FORMINES

The key elements of the system are:

1.1 Transducers

To correctly measure moment we need frequencies down to at least one octave (5 spectralpoints) below the corner frequency (fo) of the largest event likely. To correctly measureseismic energy we need frequencies at least 5 times above (fo) of the smallest event ofinterest. Moment magnitude range from -3 to +5 (≡105 - 1016 Nm of seismic moment).Geophone for events < M=0. Accelerometers sensitive for very small high frequencyevents but sufficiently insensitive at low frequencies to record close, strong groundmotions.

1.2 Data Transmission

1.3 Data Acquisition : computer at sensor triggers and stores event data.Dynamic range by gain ranging.

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1.4 Central control siteAssociates, processes, analyses, archives and communicates.

All of these systems have been developed over several years now. They have been continuouslyupgraded, proved and improved, and tested and tried in real-life operational conditions (roughenvironment).

This section gives every indication of having been properly researched, based on a thoroughliterature survey, and the development of software appears to have been carried out withparticular care to achieve efficiency in processing without compromising accuracy and flexibility.

The total system, represents state-of-the-art technology in mine-based seismological monitoring. It is the reviewer’s belief that it is the best system operating at present in the mining world.

CHAPTER 2:CONFIGURATION AND SENSITIVITY

There is convincing proof that network design is based on a complete use of available statistical tools and a through understanding of the factors that influence wave propagation in elasticmedia.

This permits i) optimum location of stations in a network

ii) performance expectation (contours of expected location errors and ofsensitivity i.e. detectability of small events)

iii) quality index of located events ie. the performance characteristics of theset of geophones that were actually used for the location.

CHAPTER 3:LOCATION OF SEISMIC EVENTS AND VELOCITY INVERSION

3.1 Location of seismic events

‘Arrival time difference’ technique is used, or ‘minimization of misfit function’.

These appear to be well established statistically based procedures with a well-researched(and referenced) foundation in seismological literature.

3.2 Li, Adaptive Lp-Norm and simplex minimization procedure

The evaluations and comparisons of these various techniques appear to have been donevery searchingly. The explanations are also carefully detailed but, to the uninitiated (likethe reviewer is), totally incomprehensible!

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3.3 Velocity Inversion

3.3.1 Travel time equations and solutions:

Joint Hypocentres and Velocity Determination (JHVD) is velocity inversion of clustersof events.

A great deal of research has gone into determining the best location models and indesigning efficient software for their solution.

CHAPTER 4:SEISMIC RAYTRACING

The treatment of this aspect of the problem of hypocentral location gives the impression, as withall the other sections of the report, of great thoroughness and of careful examinations ofalternative methods. However, the approach is totally esoteric with no attempt made tointerpolate simple physical explanations for the uninitiated reader.

It is not clear which comparative exercises were carried out by the analytical work done in theSIMRAC project and which are examples drawn from the papers explored during the literaturesearch. This search appears, as always, to have been an ongoing process carried outcomprehensively and with deeply founded insight and understanding. But it is not at all obviouswhich methods were finally chosen for use in the analyses of the GAP 017 Project.

CHAPTER 5:SEISMIC WAVE ATTENUATION AND SITE EFFECT

Qo , the quality factor is a measure of the attenuation - which is due to inelastic processes and toscattering. It is an important parameter for classification of the rock eg. homogenous rock hasa large value of Qo while fractured rock has a small value.

Treatment of this chapter is somewhat textbook-like rather than a straightforward account of howand why Qo values were determined. No attempt is made to explain things to the uninitiated.

Site effects (at geophone) can corrupt data and therefore have to be removed at the source beforesource parameters can be estimated. A site amplification of 5 times at a surface station inWelkom has been detected (at16 Hz).

It is not indicated how much of the material in the chapter was developed during the SIMRACproject and how much was culled from the literature and which was used in pursuance of thegoals of the project GAP 017.

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CHAPTER 6:SOURCE PARAMETERS

6.1 Spectral Analysis6.2 Moment Tensor

Automatic picking of the arrival time defines the beginning of the seismogram. Calculation of the source spectral parameters requires proper calculation of the Fourierspectrum through a time window of 10 cycles for the P wave and 15 cycles for the SHand SV waves.

6.3 Source Parameters

Strength of a seismic event is described by the moment magnitude scale, M where:

M = 2/3 log Mo -6,0 where Mo is in Nm

Dynamic stress drop (effective stress drop) is the difference between the initial stress and thekinetic friction (stress) level on the level below and can be calculated from the seismic data.

_ σ = 7/16 Mo/ ro

For most mine tremors _σ varies from 0,01 MPa to 10 MPa.

Seismic moment Mo and radiated seismic energy E allows the calculation of apparent stress:

σa = M

E

o

µ

where µ = shear modulusE is radiated seismic energy

Seismic moment and stress drops allow the calculation of source volume A (interpreted as theregion with the largest inelastic shear strain drop).

The above parameters are crucial to proper quantification of seismic events. Thus it is surprising that the description of these important quantities was not given a better explanation - somethingthat the interested lay reader could understand.

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CHAPTER 7:STATISTICAL ANALYSIS OF SEISMICITY

7.1 Gutenberg - Richter relation

log n = a - b m

Various procedures for estimating parameter b (seismic activity rate) are described. Three methods for assessment of Mmax are given. These have been used with data fromFar West Rand for period 1972 - end 1991.

There is no statement as to how the determination of these parameters affects the overallthrust of the total SIMRAC undertaking. Which procedures are to be used is notexplicitly stated.

7.2 Integrated volume of ground motion

The values of magnitude and PPV considered to be important are 1,0 and 100 mm/srespectively . Again, it is not stated how this will be used in the main thrust of theproject.

7.3 Space-time clustering of seismicity

The clustering analysis which is to be used (presumably in pursuance of the objectivesof the project) are described. Events encountered by a moving time window are linkedto all remaining events in the window by a space/ time/ distance criterion. Length of timewindow is a crucial choice - 25 events seems to be a suitable window length. Seismicitywithin each cluster is characterized by monitoring all the seismic parameterscontinuously.

7.4 Probability model based on temporal variations of stress-strain release

Although claims have been made regarding temporal variations of seismic activity(especially Gutenberg-Richter a and b) as useful indicators of probability of large eventsoccurring, direct monitoring of stress-strain release (is proposed in the GAP 017 projectreport).

The definition of instability is reassuringly similar to the instability precepts of excessenergy supplied by a ‘soft’ loading system to a strain-softening fractured rock volumeproposed by NGW Cook in 1963. The notion that .... (4th paragraph, p.9 of Chapter 7)...“...potential instability... can be identified through .... the changes of rates of stress andelastic strain....” thus appears to be soundly based. This is referred to as logisticdistribution.

However, the examples shown, namely Figs. 7.3 and 7.4 and Figs. 7.5 and 7.6 are notconvincing. Perhaps this is because the explanations are not clear- the reader is left to

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draw his own inferences as to which changes in decreasing stress and increasing straindirectly precede which events. (The events are presumed to be the 15 little ‘beads’ alongthe top horizontal axis of the figures but it is nowhere stated that this is the case).

7.5 Probability Model for Seismic Event Occurrence based on Fractal Formalism

The choice of a statistical model for seismicity appears to derive from a careful searchof the literature. Again there is no attempt to explain the development of the model tothe uninitiated reader. For example, the central concept of “... conditional intensityfunction...” is defined, its earlier development attributed and discussed, but its meaningin the context of mining and mine-induced seismicity is neither described norexemplified.

Later the probability assessment approach is tested against actual data, in Figure 7.6, ofevents of energy greater than 5,5 x 106 J. Nine out of ten of the events occurred only afterperiods of sharply-increased ‘calculated’ probability. This was taken to be encouragingproof of their expectation.

7.6 Correlation between seismicity in adjacent areas

The cross-correlation between clusters of seismic events is nicely demonstrated and, asmight be expected, the interaction becomes weaker with increasing distance betweenclusters.

CHAPTER 8:LIMITS OF PREDICTABILITY

This chapter introduces certain notions of non-linearity, chaos and phase space. It is clear thata great deal of research into the literature has been necessary in order to choose the mostappropriate conceptual model.

However, it requires an act of faith on the part of the uninitiated reader to accept the centralnotion that states that demonstrating the chaotic nature of seismicity is proof that rockbursts areintrinsically predictable. (From the parallel with long-range weather forecasting, where, forexample, the reliability of el Ni_o predictions has improved dramatically in the last few yearsthrough the advent of super-computers and the adoption of chaos theory, the reviewer has nodifficulty in committing himself to that act of faith!)

However, it is much easier to accept the idea that different sets of seismic data (from differentareas of the mine) pose different degrees of predictability.

Regrettably (and perhaps inevitably), the discourse on the entire topic of chaos, how one canquantify the amount of chaos, the existence of phase space with many more than fourdimensions etc. is incomprehensible to the ordinary reader.

There appears to be little effort made to attempt to describe these esoteric concepts in lay terms. Even the important concept of seismic flow in a rock mass is not very easy for mining men to

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conceptualize.

As an illustration of this apparent lofty disregard of the need to try to make these matterscomprehensible to the ordinary reader, I find the treatment of Figures 8.8, 8.9 and 8.10informative. There is no attempt to anticipate the likely response of the reader to these clear buttotally enigmatic diagrams viz. What do these plots mean in physical terms? Perhaps there isno answer possible because there is no meaning to the question. However, one feels that thereis an obligation on the writers to explain the real physical meaning of the subtle differenceswhich these three sets of data display (which is presumably the purpose behind comparing threesets of similar data).

CHAPTER 9: QUANTITATVE SEISMOLOGY AND ROCK MASS STABILITY

9.1 Seismic Moment Source Size and Stress Drops

Idealized Planar Source:

M = GûA

where A = source areaû = average slipG = modulus of rigidity (note that µ is used in Chapter 6)M = seismic moment

This is one of the important relationships which form the basis of quantitativeseismology. It would have been informative and reassuring to see some actually observedexamples from the field with elaboration on the extent to which these conform to thetheoretical expectations, how frequently are events observed which are simple, and howoften are sources complicated and the surrounding rock mass in homogeneous.

It is not clear how much of this section describes work that was done in pursuance of theobjectives of the SIMRAC project and how much is discourse outlining textbookprocedures that have been used in previously developing the system.

9.2 Seismic Energy

As in 9.1, the value of this section would have been greatly improved by inclusion of field examples. An indication of how small the fractures would be that would cancel the“acoustic term” would be helpful.

An isometric drawing illustrating the single fracture source and how the propagation ofthe fracture and the slip are correlated, would have been informative.

9.3 Apparent Stress and Energy Index

Apparent stress σa is an important, model-independent measure of the dynamic stress

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release in the source region.

The statement that, in a complex event, the rapid deformation process at the initial sourcecan push the stresses in the adjacent volume to a much higher level than can normally besustained thus producing higher apparent-stress subevents, is an interesting one. It wouldhave been valuable to have seen practical examples or a diagrammatic sketch.

9.4 Source Volume, Clustering of Sources and Seismic Damage

Apparent volume:

VA

... is defined as VA = M2/2GE. As this is such an important parameter it is sad that noattempt has been made to describe the concept in more physical terms or to phrase thedefinition more simply and clearly.

Clustering of Seismic Sources in Space:

Although it seems that clustering is going to be an important concept, the term is notdefined. Example is given of the quantitification of the term but the meaning of thequantity CSS2 is not explained.

Seismic Damage

This brief section is a good example of the way in which the seismological terminology(or mind state) distances itself in a frustrating manner from down-to-earth engineeringunderstanding.

The second sentence: "... the source of the seismic event .... could be considered asdamage within the rock mass ..." promises a possible common understanding. But thefollowing thought viz "... cumulative damage ... could be defined as the ratio of totalvolume of ..., ... to the volume of interest..." is completely meaningless to the averageengineer! It takes damage out of the realm of simple understanding back into theesoteric! What happens to the simple case where one single large event produces a burstfracture (shear rupture) which causes major damage to a tunnel, for example, and thereis no “... volume of interest...”? Does a CSD of zero mean that there is no seismicdamage because "... there is no overlapping volume between seismic events ..."?

Why is there a need for a concept to embrace, (at a value of one), the totally improbablecase of "... an infinite number of seismic events of identical volume ... occurred in thesame place"?!

9.5 Seismic Strain and Seismic Stress

This passage is one of many in the report which appear more like extracts from anadvanced textbook than accounts of what was done (and why) in the pursuance of the

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project objectives. One can accept, as a matter of faith, that the procedures developed inthese sections are necessary for the building up of the total methodology. However oneis left with a vague regret that it is not possible (or appears to be not possible) to give theabstract notions a small measure of tangible or visualizable form.

9.6 Seismic Softening

This section appears to be centrally important for the conceptual understanding of theonset of instability. It is easy to accept that there is a need to define rigorously theconditions which permit strain localization, nucleation and the subsequent initiation ofunstable slip. But is it not possible also to illustrate it visually or descriptively in termsthat would make these notions more meaningful to the uninitiated reader?

9.7 Seismic Viscosity and Deborah Number

Seismic viscosity is the resistance to flow of the (coseismic inelastic) deformation. Deborah number characterises the nature of the flow process, it can be interpreted as theratio of elastic to viscous forces. When De < 1 elastic effects dominate while De > 10describes a system that behaves as an elastic solid. The concepts date, respectively, to1988 (Kastrov and Das) and 1969 (Reiner).

9.8 Seismic Diffusion

In 1976 McGarr proposed that the sum of seismic moments released during a period oftime would be proportional to the volume of closure. This is a concept that is simpleenough to be understood by the informed lay person.

9.9 Seismic Schmidt Number

Seismic viscosity and seismic kinematic diffusion can be combined into one parametercalled seismic Schmidt number which encompasses 4 independent parameters describingseismicity: average time between events; average distance between consecutive events;cumulative seismic energies and cumulative seismic moments. This is stated very matter-of-factly with no explanation. Since these subsequently become very importantparameters in prediction, it would have been worth considerable effort to make the wholeconcept (involving the combining of the four parameters) more meaningful.

9.10 Unstable Deformation and Unstable System

This is a long section (p30 - 39) dealing with the onset of instability initially in anapparently very abstract way eg. "... inelastic potential surface is always normal to theinelastic strain rate vector ... in 6 - dimensional stress space".

The comments made by the reviewer under seismic damage apply also to this initialportion of this section.

Further on there is frequent reference made to theoretical and experimental work of other

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experimental rock physicists and seismologists into nucleation zones, shear-banding andbifurcation phenomena. These references offer brief, intriguing glimpses of what, attimes, seems to be real-life behaviour as distinct from conceptualized behaviour. However with the references being unavoidably brief the reviewer is usually not surewhen the researchers are dealing with real behaviour observed in rock specimens onlaboratory scale and when they are extrapolating their concepts into the inferred worldof crustal rock masses. The author of this section, as is usually the case, does not appearto try to relate this work directly to the real physical world of the earth’s crust that ourrockburst problems occur in!

9.11 Nucleation of Instability

Critical size of the fractured rock

Softening

Accelerated deformation

These phenomena, which are conceived to be happening in the nucleation zone, appearto crucially identify and circumscribe the items of knowledge that need to be knownbefore identifying or developing a prediction methodology. Being so important to thisvital methodology, it means to the reviewer that it would have been invaluable to thefostering of understanding in the mining community if there had been a serious effort tohave described where these phenomena could be expected to be found in the actual rockspace around mining excavations.

9.12 Phenomenological Model and Time to Failure

This section importantly describes the prediction rationale by identifying the suspectedor inferred micro phenomena which precede or are believed to precede the onset ofinstability.

The model is ingenious, logical and seems to be entirely feasible. Since it is not possibleto physically probe into the rockmass to search for actual tangible evidence of theexistence of such micro phenomena, the test of validity can only be established byexperience gained from continued use of the system and conscientious reporting ofsuccesses and failures.

9.13 Phenomenonological Model and Time to Failure

Growth of deformation processes up to the point of instability (ie. the nucleationbehaviour) is accompanied by several observable processes which are part of a precedingphase of accelerating deformation.

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• acceleration in apparent volume increase• lower seismic stress or Energy Index within the nucleation zone• an increased distance between consecutive events

It appears that these are going to form the basis of the ‘prediction’ procedure. Theywould have been made more meaningful and more memorable with further descriptionor illustration in a visual way.

CHAPTER 10: APPLICATIONS OF QUANTITATIVE SEISMOLOGY IN MINES

10.2 Criteria for the Recognition of Potential Instabilities

The potential for instability increases with :

High stress, high strain rate, high gradient in cumulative strain.

10.3 Analysis and Interpretation of Seismic Events

Examples are given which, by and large, are clearly explained and illustrated and whichmake it easy to appreciate the practical value of applied quantititative seismology.

10.4 Analysis and Interpretation of Seismicity

This section also gives interesting examples which underline the potential usefulness ofapplied seismology eg. in delineating unknown geological structures and suggestingstoping sequences eg. mining the strip of reef adjacent to a fault loss first and thenstoping away from the fault (as in the Tanton fault on President Steyn Mine).

10.4.7 In this section a clear description of the 7 seismic parameters which seem to be the mostfrequently-used in time-history plots for the prediction of unstable rockmass behaviour,is given.

This section of the report is described quite clearly in a non-mathematical way, which isobviously a very useful guideline to the owner of an ISS system. However, how it is related tothe specific objectives and milestones of the SIMRAC project is still not sufficiently carefullyidentified.

10.5 Velocity of ground motion estimates

This is a somewhat diffident discussion of how estimates of ground velocity may be madeon which to base design of support.

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These afford some food for thought but also pose the question as to whether or not theyfall within the scope of the SIMRAC proposal.

10.6 Benchmark Case Studies

This is perhaps the most vital portion of the project (at least the prediction part) wherethe criteria for ALARM are tested retrospectively.

The figures 10.12(a) to 10.12(b) on pp. 32-33 are not sufficiently elaborated in thecaptions to allow the critical reader to see just how unique the drop in Energy Index (EI)(or the other dependent parameters) was before the alert was given. Reference to theoriginal in annual report of 1994 was not any more reassuring.

Perhaps the graphs need to be supplemented by a ‘zoom-in’ supplemental graph wherethe time axis scale is stretched by a factor of 5 or 10.

10.6.2 For example event WSN-9406270610 is said to have been “.... predicted quite well(Fig 10.13 a - 10.13 c)...”, but it is not explained in the text or caption how large a dropwas experienced in EI or in seismic viscosity and over what time period. In fact it seemsas if the EI remained constant for 3 days prior to the event. Also the rate of increase incumulative VA over the previous six days was not as great as it had been between 7th and10th of June when there was also a significant drop in the other two parameters but nolarge event occurred.

The summary given in table 10.4 is a good way of assessing overall performance of theprecursor (or predictors?) but more explanation of this table would have been warranted. It is not quite clear as to whether or not any warnings were given for the 11 cases whichdisplayed the appropriate precursory behaviour or whether they were seen asretrospectively successful.

10.6.3 WH No 6 shaft

Similarly Fig 10.16(f) warrants a more searching description of just how it was decidedthat the three warnings between 26 July and 10th August should be given. There was nowarning given before the second event (01.08.93) although there seems to have been asteady high rate of VA increase for 3 days before, together with the start of a fairly markeddrop in energy index. On the other hand the third warning was given after a day or so ofno VA change and a steady EI.

10.6.4 The Trough Event

The ‘seismic gap’ which depicts the suddenly changed behaviour very visibly, is adifferent representation of precursory behaviour than has been used previously -presumably the indicators of increasing VA and decreasing viscosity would have also beenpresent? The time history plot of seismic viscosity and seismic diffusion was sufficiently

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dramatic. More information on the nature of the damage caused by this magnitude 4,0would have been informative.

10.6.5 81-122 longwall

It is not clear just what purpose is served (in respect of establishing predictability) inpresenting this data in such detail.

It appears to be a somewhat non-typical area in so far as there is such a strong biastowards events occurring with the blast and so few (<2,4%) happening outside blastingtime. Perhaps this is because there were very few large events (which normally are morerandomly distributed in time). Are the fewer large bursts an indication of the effectivestabilizing pillars in the area of interest?

General Comment

Not enough attention has been given to synthesizing the results of the prediction aspect of theproject eg. How many alerts or alarms have been given? What has the overall success rate been?

W D ORTLEPP Pr Eng., BSc, M. EngAssociate ConsultantSteffen, Robertson and Kirsten (CE) (Pty) Ltd

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