Cave front tracking experiment

Green, Mark (2016-12)

Thesis (MSc)--Stellenbosch University, 2016

Thesis

ENGLISH ABSTRACT : Seismic tomography, a procedure to trace seismic wave velocity variation, has been used for more than 100 years to “see through rock”. Initially the recording of seismic waves from natural sources (earthquakes) using seismographs, provided the first evidence of the crust - mantle boundary. Further development of recording techniques and use of explosive seismic sources were employed for the delineation of the geological structures of mineral deposits (primarily oil and gas) as well as detailing the earth - mantle location and geometry. Ultra sonic techniques have been applied in the laboratory experiments to characterise rock sample velocities and its relationship to changes in temperature and stress. Active seismic tomography for hydrocarbon resources determination as well as for the earth structure utilized both reflection and refraction techniques. During the early 60’s seismic techniques were employed in active mines to investigate rock properties for geotechnical purposes. The objective of the Cave Front Tracking Experiment was to investigate whether active seismic tomography could potentially be used to track the cave back progression of a block cave mine. A small scale experiment was conducted to study the variations in seismic travel times associated with ray path refraction caused by stress changes and changes in the rock fabric induced by sub-level mining. A piezoelectric transducer was employed as an active seismic source and a seismic recording system was installed to monitor a volume of rock through which sub-level mining was propergated. Implementation involved the deployment of 11 seismic detectors and the recording of pre-stacked data. The experimental details and hardware characteristics are discussed, which includes technical problems encountered, the process and techniques of significant data stacking in order to recover weak signals generated by a piezoelectric seismic source and travel time variation calculation using signal cross-correlation. Signal travel time variation measurements were made over the course of a year and a half (2013-14), however only five months are investigated in detail and related to mining activities. Finite difference numerical modelling was also employed in order to create a better understanding of the sub-level mining process and its effect on seismic signal delay change. Technical problems associated with the signal processing and signal behavior interacting with an advancing mining front were identified and a number of techniques were employed to overcome problems faced by routine cave front estimation in a working mine environment. The field testing of a prototype impact source that is pneumatically driven and an eccentric rotating mass device is discussed.

AFRIKAANSE OPSOMMING : Seismiese tomografie, ’n prosedure om die variasies in seismiese golfsnelhede na te spoor, word reeds meer as 100 jaar gebruik om “deur rots te sien”. Deur die meting van seismiese golwe van aardbewings het die eerste seismograwe inligting verskaf oor die bestaan – en posisie van die kors-mantel oorgang. Wanneer golwe van natuurlike seismiese bronne ge-analiseer word verwys ons na ’passiewe’ tomografie terwyl ’aktiewe’ tomografie verways na die gebruik van kunsmatige seismiese bronne, soos ontploffings of vibreerders. Verbeterde opnametegnieke en die gebruik van ontploffings as seismiese bronne het gehelp om aardstruktuur en ook die strukturele aard van ertsafsettings en die van olie en gasvelde te detaileer. Ultrasoniese tegnieke word in laboratoriumeksperimente aangewend om die seismiese golfsnelhede van rotsmonsters te karakteriseer, asook om die verband tussen die veranderinge in golfsnelhede en veranderinge in temperatuur en ook veranderinge in spanning te bepaal. Beide reflektiewe en refraktiewe tegnieke word gebruik in aktiewe seismiese tomografie vir die studie van koolwaterstof-bronne en die aardstruktuur. Gedurende die vroeë 60’s is seismiese tegnieke in myne gebruik om die geotegniese aard van die rots te bestudeer vir ’n beter begrip van die onstaan van rotsbarste en die algemene meganiese gedrag van rots. Die doel van die mynboufront-volgingseksperiment is om aktiewe seismiese tomografie te gebruik om variasies in seismiese reistye (Engels: travel times) te bestudeer. Hierdie variasies hou verband met golfroete-refraksie wat veroorsaak word deur verandering in spanning en in die geotegniese aard van die rotsmedium onder die invloed van mynbou. ’n Piësoëlektriese transduktor is as aktiewe seismiese bron gebruik en ’n seismiese waarnemingstelsel is geïnstalleer om die rotsvolume te moniteer waardeur ’n groot myn-opening sou vorder. In hierdie verhandeling word verslag gegee oor die tegniese probleme en die vordering van die projek. Die projek behels die ontplooiing van 11 seismiese sensors en die opname van vooraf-gestapelde data – deur groot hoeveelhede digitale seismogramme van dieselfde bron op mekaar te stapel kan klein veranderinge in golfsnelhede bespeur word. Die eksperimentele detail en eienskappe van die hardeware word bespreek, wat die proses en tegnieke van beduidende data-stapeling vir die uitsifting van swak seine vanaf ’n piësoëlektriese seismiese bron insluit. Metings van die variasies in sein-reistyd wat met mynbouaktiwiteite verband hou, is oor die verloop van ’n jaar (2013-14) gedoen. Die numeriese modeleringstegniek van eindige verskille is gebruik om ’n beter begrip te kry van die migreerende mynboufront en die effek daarvan op seismiese seine se fasevertraging. Die probleme wat geassosieer word met die ondersoek van die seismiese data van ‘n migreerende mynboufront is geïdentifiseer en ’n aantal tegnieke is aangewend om probleme te oorkom wat verband hou met roetine skatting van die posisie and mynboufront’n myn. Die veldtoetsing van ’n prototipe impakbron wat deur lugdruk aangedryf word, en ’n bron gebaseer op ’n eksentries-roterende massa, word beskryf.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/100103
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