We present statistical analyses of pore pressure and other signals, aimed at characterizing the properties of a geologic porous medium and, particularly, of the Excavation Damaged Zone ("EDZ") around underground galleries. The purpose is not only to identify the material properties, but also to quantify their evolution during excavation (creation of an EDZ). The methods are applied to two Underground Research Laboratories ("URL’s") in claystone sites: Mont Terri in Switzerland (gallery Ga98) and Tournemire in France (gallery Ga03). This work is part of the LP 14 experiment, an international research program of the Mont Terri Consortium, aimed at assessing the isolation properties of claystone as a potential geologic repository for radioactive waste (LP 14 experiment: Long term Pressure experiment, phase 14)

Ababou, Rachid

Fatmi, Hassane

Matray, Jean-Michel

Nussbaum, Christophe

Open Archive Toulouse Archive Ouverte

   Open Archive TOULOUSE Archive Ouverte (OATAO)  OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible.  This is an author-deposited version published in : http://oatao.univ-toulouse.fr/ Eprints ID : 10648 To cite this version : Fatmi , Hassane and Ababou , Rachid and Matray, Jean-Michel and Nussbaum, Christophe Statistical analyses of pressure signals, hydrogeologic characterization and evolution of Excavation Damaged Zone (claystone sites of Mont Terri and Tournemire). (2011) In: MAMERN11: 4th International Conference on Approximation Methods and Numerical Modelling in Environment and Natural Resources, 23 May 2011 - 26 May 2011 (Saidia, Morocco) Any correspondance concerning this service should be sent to the repository administrator: staff-oatao@listes-diff.inp-toulouse.fr Statistical analyses of pressure signals, hydrogeologiccharacterization and evolution of Excavation DamagedZone (claystone sites of Mont Terri and Tournemire)Hassane FATMI1, Rachid ABABOU1, Jean-Michel MATRAY2,Christophe NUSSBAUM31 IMFT - Institut de Me´canique des Fluides de Toulouse,1 Alle´e du Professeur Camille Soula, 31400 Toulouse, Franceababou@imft.fr; hassane.fatmi@hotmail.fr2 IRSN - Institut de Radioprotection et de Suˆrete´ Nucle´aire,Av. du Ge´ne´ral Leclerc BP 17, 92262 Fontenay-aux-Roses, Francejean-michel.matray@irsn.fr3 SwissTopo & Mont Terri Consortium,Fabrique de Chaux 65, 2882 St-Ursanne, Switzerlandchristophe.nussbaum@swisstopo.chKeywords: Signal processing and analyzis; wavelets; correlation functions;Fourier spectrum; random process; statistical envelope; hydrogeology; clay-stone; specific storativity; porosity; compressibility; excavation; EDZAbstract. We present statistical analyses of pore pressure and other sig-nals, aimed at characterizing the properties of a geologic porous medium and,particularly, of the Excavation Damaged Zone (”EDZ”) around undergroundgalleries. The purpose is not only to identify the material properties, butalso to quantify their evolution during excavation (creation of an EDZ). Themethods are applied to two Underground Research Laboratories (”URL’s”) inclaystone sites: Mont Terri in Switzerland (gallery Ga98) and Tournemire inFrance (gallery Ga03). This work is part of the LP 14 experiment, an interna-tional research program of the Mont Terri Consortium, aimed at assessing theisolation properties of claystone as a potential geologic repository for radioac-tive waste (LP 14 experiment: Long term Pressure experiment, phase 14).1 Introduction and objectivesWe present methods for statistical analyzis and hydrogeological interpreta-tion of pressure signals in claystone formations. The purpose is to characterizethe hydromechanical behaviour of the formation, and to study the evolution ofits EDZ (Excavation Damaged Zone) during the excavation of galleries. Wefocus on galleries Ga98 and Ga03 in the sites of Mont Terri (Jura, Switzer-land) and Tournemire (France, Aveyron), through data collected in the BPP-1and PH2 boreholes, respectively. This work is part of ongoing studies on theisolation properties of deep claystone repositories for Mid-Level/High-Leveland Long-Lived radioactive wastes.The Mont Terri site, crossing the Aalenian Opalinus claystone, is an under-ground laboratory managed by an international consortium, namely the MontTerri project (Switzerland). The Tournemire site, crossing the Toarcian clay-stone, is an Underground Research facility managed by IRSN (France).We have analysed porewater and atmospheric pressure signals at these sites,sometimes in correlation with other data. The methods of analysis are basedon the theory of stationary random signals (correlation functions, Fourier spec-tra, transfer functions, envelopes), and on multiresolution wavelet analysis(adapted to nonstationary and evolutionary signals). These methods are alsocombined with filtering techniques, and they can be used for single signals aswell as pairs of signals (cross-analyses).The objective of this work is to exploit pressure measurements in selectedboreholes from the two compacted clay sites, in order to (i) evaluate phenom-ena affecting the measurements (earth tides, barometric pressures, etc.); (ii) toestimate some properties of the geologic formation (specific storativity, dy-namic porosity) prior to excavation, and to compare them with those estimatedby pulse or slug tests on shorter time scales; and finally (iii) to analyze the ef-fects of drift excavation on pore pressures before, during, and after excavation.2 Methods (overview)To achieve the above objectives, we have analyzed atmospheric and porepressure signals using various types of statistical methods, listed below1:1. Auto-correlation function versus lag time, cross-correlation between two signals, andtheir input/output transfer functions (deconvolution).2. Estimation of Fourier spectrum of each signal, and cross-spectral analyses (barometriceffect in frequency space, via spectral gain of atmospheric/relative pore pressures).3. Alternative cross-correlation analyses of porewater versus atmospheric pressure (Atmo-spheric Correction Factor, residual pore pressure, re-interpretation of barometric effect).1All methods were developed and implemented by the authors as custom-made MATLAB TOOLBOXES4. Multi-resolution wavelet analysis (wavelet approximation and residual, selection of com-ponents at specified scales, cross-component analyses).5. Statistical envelope of signals computed with Hilbert transform (Cramer-Leadbetter en-velope); application: evolution of EDZ during excavation.Before these methods can be implemented, it is necessary to first pre-process the raw signals obtained from data acquisition devices. In this work,the raw signals were pre-processed using a number of techniques, aimed athomogenizing irregular time steps, detecting outliers, and/or reconstructingmissing data. 2 Furthermore, some of the methods enumerated just above alsoneeded to be pre-calibrated or tested (e.g., cut-off parameters for spectral es-timation filters). For all these reasons, a number of tests were developed tovalidate and illustrate the various processing tools (e.g. testing statistical en-velopes for synthetic signals, or comparing Fourier/Wavelet decompositions).3 Hydrogeologic characterization via barometric and earth tideeffects (effective dynamic porosity and specific storativity)The pressure signals were interpreted hydromechanically, in terms of com-pressibility effects, in order to assess the hydrogeologic characteristics of theclaystone. In particular, the statistical analyses of atmospheric and pore pres-sures allowed us to detect and to characterize:1. at diurnal time scales, the relationship between piezometric and atmospheric pressure(barometric efficiency in frequency space); and2. at semi-diurnal time scales, the statistical influence of earth tides, identified with mul-tiresolution wavelets (whence specific storativity).For the sake of brevity, we only summarize some of the results obtainedfrom long term pressure signal analyses at Mont Terri’s gallery Ga98 (PP1and PP2). The effective dynamic porosity Φ (in response to compressibilityeffects) was found to be quite low, on the order of 1% or even less on aver-age (however, it peaks at 4% during the post-excavation phase). The effectivespecific storativity (SS) was found to be approximately 3E − 07m−1 on av-erage (except during excavation). These average values correspond to longterm pressure signal analyses at the scale of several years (April 1997 – April2005). The next section analyses in more detail, and on shorter time scales, theresponse to excavation (pre-, syn-, and post-excavation phases).4 Excavation effects: evolution of pore pressure and storativityWe now focus on the time evolution of pressure signals during the passageof the excavation front, and on the infered evolution of claystone properties2Fatmi H., R. Ababou, J.-M. Matray (2008): ”Statistical pre-processing and analyses of hydrogeo-meteorological time series in a geologicclay site (methodology and first results for Mont Terri’s PP experiment)”. Physics and Chemistry of the Earth: Special Issue on Clays in Natural& Engineered Barriers for Radioactive Waste Confinement (Internat. Conf. CLAY’2007, Lille, France) 33(2008) S14-S23.(such as SS) in the EDZ of gallery Ga98 at Mont Terri. 3Figure 1(a) presents a comparison plot of two time evolutions, namely: theabsolute pore pressure P (t) measured in chamber PP1, and the distance D(t)between pressure chamber PP1 and the excavation front of gallery Ga98. Notethat the smallest distance is 4m, attained on the 4th of March 1998 (excavationworks stopped for a few weeks after that). The pressure signal clearly indicatesthe emergence of a damaged zone (EDZ) as the excavation front approachesnear the pressure sensor. The sharp rise of pore pressure coincides with thesyn-excavation phase, which lasts about 11 days.Figure 1(b) shows the resulting evolution of the specific storativity SS(t)as a function of time, superimposed on the excavation distance D(t). Specificstorativity SS(t) was obtained from multiresolution wavelet analyzis (identi-fication of the semi-diurnal ”earth tide component” of the pressure signal); inaddition, in order to better capture the fine evolution of SS(t) during exca-vation, statistical envelope analyzis was applied using the Hilbert Transform.There is clearly a sharp decrease of SS(t) by 1 to 2 orders of magnitudes, for afew days, as the excavation front gets close to the pressure sensor (about 6m).// //(a) (b)Figure 1: Effects of gallery excavation on pore pressure, and on specific storativity, atMont Terri gallery Ga98. (a) Evolution of absolute pore pressure P (t) measured in chamberPP1 of borehole B-PP1, and distance D(t) between chamber PP1 and the moving excavationfront. (b) Evolution of D(t) and evolution of specific storativity SS(t) infered from pressuresignal analyses during excavation (SS(t) is in m−1 units, and it is shown here on a log-scale).Acknowledgments. This work was developed within Mont Terri’s LP 14 experiment(Long term Pressure, phase 14), as part of the international research programs of the Mont TerriConsortium on the properties of claystone as a potential geologic repository for radioactivewaste. The authors gratefully acknowledge the help provided by the following persons andinstitutions in support of LP 14: Paul Bossart (SwissTopo); Martin Cruchaudet (ANDRA);David Jaeggi (SwissTopo); Herwig Mueller (NAGRA); Eric Sykes (NWMO).3For details, the reader may consult the following Ph.D. thesis (freely available on line): Fatmi H. (2009), ”Me´thodologie d’analysedes signaux et caracte´risation hydroge´ologique: Application aux chroniques de donne´es obtenues aux laboratoires souterrains du Mont Terri,Tournemire et Meuse/Haute-Marne”. Doctoral thesis, Institut National Polytechnique de Toulouse, France, June 2009, 248 pp.

Statistical analyses of pressure signals, hydrogeologic characterization and evolution of Excavation Damaged Zone (claystone sites of Mont Terri and Tournemire)

International audienceWe present statistical analyses of pore pressure and other signals, aimed at characterizing the properties of a geologic porous medium and, particularly, of the Excavation Damaged Zone ("EDZ") around underground galleries. The purpose is not only to identify the material properties, but also to quantify their evolution during excavation (creation of an EDZ). The methods are applied to two Underground Research Laboratories ("URL's") in claystone sites: Mont Terri in Switzerland (gallery Ga98) and Tournemire in France (gallery Ga03). This work is part of the LP 14 experiment, an international research program of the Mont Terri Consortium, aimed at assessing the isolation properties of claystone as a potential geologic repository for radioactive waste (LP 14 experiment: Long term Pressure experiment, phase 14)

HAL Descartes

Institute for Radiation Protection and Nuclear Safety (IRSN)

Hal-Diderot

http://oatao.univ-toulouse.fr/10648/1/fatmi_10648.pdf

Statistical analyses of pressure signals, hydrogeologic characterization and evolution of Excavation Damaged Zone (claystone sites of Mont Terri and Tournemire)

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Hal-Diderot