Near Surface 2008 - keynotes
Keynote: 9.50 Monday 15 September 2008
Statistical properties of the mining-induced seismic process
by Stanislaw Lasocki
AGH University of Science and Technology
Results of some recent investigations into the statistical properties of the parameter series of seismic events from mines are addressed. The seismic process in mines is controlled by complex and changeable anthropogenic factors. It is not surprising, therefore, that the differences in statistical properties between mining-induced seismicity and natural seismicity are substantial. Unlike tectonic seismicity, which is permanent in both space and time, mining seismicity is transient. The event occurrence process in mines is not Poissonian. It is time-dependent and, at best, can be regarded as quasi-stationary. The event magnitude processes in mining-induced seismicity is also time-dependent. Consequently, the hazard parameters: the exceedance probability, the return period , the maximum credible magnitude etc., vary in time. Furthermore, for the majority of mining event parameterizations their stochastic features cannot be ignored. The interevent times, magnitudes and the interevent distances of the seismic process in mines are internally interrelated. One of the possible ways for these interactions among mining-induced seismic events is the static Coulomb stress transfer. Finally, the magnitude distribution of mining seismic events is complex, often multimodal and cannot be accurately approximated with the Gutenberg-Richter model. These unique features of the seismicity process in mines show that it is complex. This complexity complicates the practice of the statistical analysis of mining seismic data. On the other hand, however, the non-stationarity and interrelations mean that the mining seismic event generation process is intrinsically predictable.
Stanisław Lasocki is a professor of geophysics in the Department of Geophysics, Faculty of Geology, Geophysics and Environmental Protection, AGH University of Science and Technology. Graduated initially as a physicist, since 1975 he is involved in geophysics with a particular interest in mining seismology. His main research interest is the application of statistical methods to seismic hazard assessment and rockburst prediction. He has authored or co-authored over 100 research papers and has taken part as an advisor and instructor in various projects for mining industry in Poland and also in South Africa. As a consultant of UNESCO he ran a strong motion monitoring trainings in Libya. Dr. Lasocki is chairing the Triggered and Induced Seismicity (TAIS) working group of IASPEI. He is also the Editor-in-Chief of Acta Geophysica, an international journal dealing with all aspects of general and applied geophysics, co-published by Springer-Verlag GmbH. The title of his keynote address is "Statistical properties of the mining-induced seismic process ".
Keynote: 8.30 Tuesday 16 September 2008
Seismic methods to detect gas-hydrate bearing sediments
by José M. Carcione
Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS)
Gas hydrate is composed of water and natural gas, mainly methane, which forms under conditions of low temperature, high pressure, and proper gas concentration (1 m3 hydrate is equal 164 m3 free gas plus 0.8 m3 water). Joseph Priestley is believed to have produced oxygen hydrate in 1778, and Humphrey Davy in 1810 and Michael Faraday in 1823 synthesised chlorine hydrate. Methane hydrate occurs in two types of geologic settings: on land in permafrost regions, and beneath the ocean floor at water depths greater than about 500 m where conditions of low temperature, high pressure, and proper gas concentration dominate. Gas hydrate is a potential energy resource, and can be the cause of slope failure offshore (a submarine geohazard) and an important issue in global warming (greenhouse effect).
We first present a theory to obtain the wave velocities and quality factors of gas-hydrate bearing sediments as a function of pore pressure, temperature, frequency and partial saturation. The model is based on a Biot-type three-phase theory that considers the existence of two solids (grains and gas hydrate) and a fluid mixture. Attenuation is described with the constant-Q model and viscodynamic functions to model the high-frequency behavior. We apply a uniform gas/water mixing law that satisfies Wood's and Voigt's averages at low and high frequencies, respectively. The acoustic model is calibrated to agree with the patchy-saturation theory at high frequencies (White's model). Pressure effects are accounted by using an effective stress law for the dry-rock moduli and permeabilities. The dry-rock moduli of the sediment are calibrated with data from the Cascadia margin. Moreover, we calculate the depth of the BSR below the sea floor as a function of sea-floor depth, geothermal gradient below the sea floor, and temperature at the sea floor.
Then, we propose a modeling algorithm for wave simulation in a three-phase porous medium composed of sand grains, ice (or hydrate) and water. We first obtain the time-domain stress-strain relations for non-uniform porosity and the corresponding differential equations based on the Lagrangian.The displacements of the rock and ice frames and the variation of fluid content are the generalized coordinates, and the stress components and fluid pressure are the generalized forces. An example shows the simulation wave propagation in a frozen porous medium with fractal variations of porosity and, therefore, realistic freezing conditions. The low-frequency limit yields the generalized Gassmann modulus, used to estimate the hydrate concentration.
We present two field examples. First, an area located at the western Svalbard margin. The method is based on P-wave velocities computed by reflection tomography applied to multi-component ocean-bottom seismometer (OBS) data. The tomographic velocity field is fitted to theoretical velocities obtained from the poroelastic model based on the Biot-type approach. Next, we estimate the concentration of gas hydrate at the Mallik 2L-38 research site using P- and S-wave velocities obtained from well logging and vertical seismic profiles (VSP). The dry-rock moduli are estimated from the log profiles, in sections where the rock is assumed to be fully saturated with water.
In the Svalbard case, the quality factor is also estimated from the data by using attenuation tomography based on the frequency-shift method.
José M. Carcione was born in Buenos Aires, Argentina, in 1953. He received the degree "Licenciado in Ciencias Físicas" from Buenos Aires University in 1978, the degree "Dottore in Fisica" from Milan University in 1984, and the Ph.D. in Geophysics from Tel-Aviv University in 1987. In 1987 he was awarded the Alexander von Humboldt scholarship for a position at the Geophysical Institute of Hamburg University, where he stayed from 1987 to 1989. From 1978 to 1980 he worked at the "Comisión Nacional de Energía Atómica" at Buenos Aires. From 1981 to 1987 he was a research geophysicist at "Yacimientos Petrolíferos Fiscales", the national oil company of Argentina. Presently, he is a senior geophysicist at the "Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS)" (former "Osservatorio Geofisico Sperimentale") in Trieste, where he was Head of the Department of Geophysics from 1996 to 2000. He is editor of GEOPHYSICS since 1999. His current research deals with numerical modeling, the theory of wave propagation in acoustic and electromagnetic media, and their application to geophysical problems.
Keynote: 8.30 Wednesday 17 September 2008
Rocks under stress: Geomechanical monitoring using time-lapse seismic data
by Jorge Herwanger
WesternGeco
Time-lapse seismic data are traditionally interpreted in terms of fluid saturation and pressure changes. Recent field examples have shown that seismic time-lapse changes also occur above and below the reservoir. These are time-lapse effects that cannot be explained using pressure or saturation changes in the reservoir. However, time-lapse signal in the overburden can be explained by studying the geomechanical response of the reservoir and surrounding rock to pressure decrease in the reservoir. Due to decrease in pore-pressure, reservoir rocks can compact. A seismic wave "sees" an increased velocity and a thinner reservoir interval. The rock surrounding the reservoir (overburden and "underburden") reacts to reservoir compaction by vertical expansion and an associated decrease in seismic velocity, causing observable seismic traveltime changes.
The lecture gives an in-depth introduction into the causes of geomechanically induced time-lapse seismic changes. This is achieved by coupled reservoir and geomechanical modelling using full-field finite-element modelling, converting geomechanical changes to velocity changes using stress-sensitive rock-physics models and the prediction of time-lapse seismic attributes. Using this workflow, I show how to explain 4D time-shift observations in the overburden and shear-wave splitting observation associated with the formation of a subsidence bowl above producing oil-fields. Both of these effects are
observed in field data.
In my lecture, I argue and demonstrate that time-lapse seismic experiments are a useful geomechanical monitoring tool.
Jorg Herwanger is a reservoir geophysicist with WesternGeco. The focus of his current research is on monitoring of stress-changes induced by reservoir depletion using time-lapse seismic data. His current work combines coupled fluid-flow and geomechanical modelling, stress-sensitive rock-physics modelling and time-lapse seismic data analysis. Previous to joining WesternGeco, Jorg worked at Imperial College. His main interest was in geophysical characterization of fractured rock. This led him to pioneer 3D anisotropic resistivity tomography and to study the relationship between acoustic and electric anisotropy and rock fractures. Jorg Herwanger holds a PhD degree from Imperial College, London, UK and a Diplom (equivalent to a Masters Degree) from Technical University Clausthal, Germany, both in Geophysics. He is a member of EAGE and SEG.
