http://hdl.handle.net/1842/1631 Wed, 22 May 2013 10:18:06 GMT 2013-05-22T10:18:06Z http://hdl.handle.net/1842/6537 Title: Seismic body-wave anisotropy beneath continents Authors: Singh, Jasbinder Abstract: A search for the effects of anisotropy on seismic body-waves predicted by theory is described. Preliminary studies were based on long-period data from the WWSSN, HGLP and SRO networks. These showed that data from the WWSSN network are unsuitable for anisotropy studies because of features in the geometry of the recording system which lead to misalignment of the digitizer relative to the galvanometer-swing (which it is not always possible to correct) and the fact that the horizontal components are not always well matched. Digital data from the HGLP (recorded after 1976) and SRO networks are more suitable for anisotropy studies but eventually it was found that the anisotropic differences are too small to be resolved by long-period instruments. Analysis of short-period teleseismic shear-waves observed at LRSM stations located in United States and southern Canada has revealed shear-wave splitting diagnostic of anisotropy somewhere along the path. The shear-wave splitting is often seen as two separate shear-wave arrivals on the rotated horizontal components. All cases of shear-wave splitting are indicated by an abrupt change in the direction of particle-motion in the horizontal plane. A selection of seismograms and associated particlemotion diagrams is presented in order to illustrate shear-wave splitting. The polarizations of the first arrival shear-waves and the delays between the shear-wave arrivals were measured and are presented in the form of stereograms. The maximum shear-wave delay observed is 2.75 seconds and on the basis of this, we calculate the thickness of the anisotropic layer to be 248 kms for a model with 4.5% differential shearwave velocity anisotropy. For a model with much higher differential shear-wave velocity anisotropy (8.4%), the thickness of the layer is only 136 kms. Our results do not allow us to constrain the depth to the top of the anisotropic layer, although on the basis of other studies we believe the anisotropic layer to be situated immediately below the Mohorovicic discontinuity. The polarizations are broadly similar to those obtained theoretically for the y- and z-cuts of olivine, transversely isotropic olivine and mixture of transversely isotropic olivine/isotropic material. On the basis of this, we tentatively identify N50°E as a direction of symmetry and note that it is approximately parallel to the absolute motion of the North-American plate. We therefore suspect a causal relationship between plate motion and the generation of anisotropy. The most likely hypothesis is that as the continental lithosphere moves across the asthenosphere, the drag on the lithosphere sets up a horizontal compression in the direction of motion of the lithosphere relative to the asthenosphere and olivine crystals align by {Okl} [100] pencil glide so that the a-axis points into the direction of plate motion while the b and c axes form girdles perpendicular to the a-axis. This would result in transverse isotropy with the axis of symmetry horizontal, an orientation which is consistent with our results. The existence of anisotropy in the upper mantle has implications for other seismological studies. In particular, focal mechanism studies which rely solely on S-wave polarizations will be erroneous and studies of travel-time residuals will need to take account of the anisotropy. Sat, 01 Jan 1983 00:00:00 GMT http://hdl.handle.net/1842/6537 1983-01-01T00:00:00Z http://hdl.handle.net/1842/6224 Title: Complexity, aftershock sequences, and uncertainty in earthquake statistics Authors: Touati, Sarah Abstract: Earthquake statistics is a growing field of research with direct application to probabilistic seismic hazard evaluation. The earthquake process is a complex spatio-temporal phenomenon, and has been thought to be an example of the self-organised criticality (SOC) paradigm, in which events occur as cascades on a wide range of sizes, each determined by fine details of the rupture process. As a consequence, deterministic prediction of specific event sizes, locations, and times may well continue to remain elusive. However, probabilistic forecasting, based on statistical patterns of occurrence, is a much more realistic goal at present, and is being actively explored and tested in global initiatives. This thesis focuses on the temporal statistics of earthquake populations, exploring the uncertainties in various commonly-used procedures for characterising seismicity and explaining the origins of these uncertainties. Unlike many other SOC systems, earthquakes cluster in time and space through aftershock triggering. A key point in the thesis is to show that the earthquake inter-event time distribution is fundamentally bimodal: it is a superposition of a gamma component from correlated (co-triggered) events and an exponential component from independent events. Volcano-tectonic earthquakes at Italian and Hawaiian volcanoes exhibit a similar bimodality, which in this case, may arise as the sum of contributions from accelerating and decelerating rates of events preceding and succeeding volcanic activity. Many authors, motivated by universality in the scaling laws of critical point systems, have sought to demonstrate a universal data collapse in the form of a gamma distribution, but I show how this gamma form is instead an emergent property of the crossover between the two components. The relative size of these two components depends on how the data is selected, so there is no universal form. The mean earthquake rate—or, equivalently, inter-event time—for a given region takes time to converge to an accurate value, and it is important to characterise this sampling uncertainty. As a result of temporal clustering and non-independence of events, the convergence is found to be much slower than the Gaussian rate of the central limit theorem. The rate of this convergence varies systematically with the spatial extent of the region under consideration: the larger the region, the closer to Gaussian convergence. This can be understood in terms of the increasing independence of the inter-event times with increasing region size as aftershock sequences overlap in time to a greater extent. On the other hand, within this high-overlap regime, a maximum likelihood inversion of parameters for an epidemic-type statistical model suffers from lower accuracy and a systematic bias; specifically, the background rate is overestimated. This is because the effect of temporal overlapping is to mask the correlations and make the time series look more like a Poisson process of independent events. This is an important result with practical relevance to studies using inversions, for example, to infer temporal variations in background rate for time-dependent hazard estimation. Mon, 25 Jun 2012 00:00:00 GMT http://hdl.handle.net/1842/6224 2012-06-25T00:00:00Z http://hdl.handle.net/1842/6185 Title: Analysis of P-wave seismic response for fracture detection: modelling and case studies Authors: Xu, Yungui Abstract: This thesis addresses a few specific issues in the use of wide azimuth P-wave seismic data for fracture detection based on numerical modelling and real data. These issues include the seismic response of discrete fractures, the effects of anticline and uncertainties in real data analysis. For this, I implemented the finite difference scheme for modelling the seismic response in 3D fractured media; appropriate approaches are then selected to study discrete fracture models and the effect of the anticline with 3D seismic modelling, followed by an integrate real case study. Finite difference (FD) is widely used in seismic modelling. There are three FD schemes described in this thesis, the standard staggered grid (SSG), the rotated staggered grid (RSG), and the diamond staggered grid (DSG). Both qualitative and quantitative comparison has been made to reveal their capability in modelling 3D fractured media. The SSG has shown best performance for anisotropic media with orthorhombic symmetry or higher symmetry system. For lower anisotropy symmetry, the DSG is preferred than the RSG in terms of computation efficiency. A new solution to the diamond grid issue is developed which can simplify the DSG implementation, and an optimized workflow is proposed to simulate large 3D fractured models. The SSG scheme is implemented in three dimensions and it provides a useful tool for various practical modelling studies. With the above tool, two modelling studies have been carried out, on the effects of the discrete fractures and of the presence of anticline: the Discrete Fracture Model (DFM) study provides many insights into seismic response of discrete fracture and the link between the discrete fractures and aligned micro cracks, as well as the features in scattering waves. The modelling results demonstrate that, P-wave seismic anisotropy increases with the decrease of discrete fracture spacing, and different spacing leads to different patterns in scattering waves. The study also reveals the azimuthal AVO variation on the top of discrete fracture layer, which is similar to that we find in homogenous anisotropic media. The study of the anticline structure with vertical fractures, which is built with the parameters from a real case, is to assess the anticline structure effect on fracture parameter inversion based on the Singular Value Decomposition (SVD) method. The fracture density can be resolved accurately at the top of the anticline, whilst that on the flanks tends to be over-estimated. The results also indicate that the SVD method is a reliable approach for directly estimating the fracture density. P-wave azimuthal attributes are commonly employed to invert fracture density and orientation. Many factors may affect the accuracy of the inversion results. The integrated study in this thesis shows that azimuthal coverage, offset-depth ratio, data quality and geological structures all affect the final prediction, and different attributes shows different sensitivities to these factors. Furthermore, the combined analysis of both geological observation and pre- and post-stack seismic attributes can reduce the uncertainties for fracture detection. Mon, 25 Jun 2012 00:00:00 GMT http://hdl.handle.net/1842/6185 2012-06-25T00:00:00Z http://hdl.handle.net/1842/6184 Title: Non-physical energy in seismic interferometry Authors: King, Simon James Abstract: Non-physical arrivals produced by seismic interferometry, the process whereby Green’s functions are synthesized between two points by cross-correlation, crossconvolution or deconvolution, are often considered to provide little information about the Earth’s subsurface. Their contributions are usually suppressed in interferometric Green’s function estimates to suit existing methods of seismic velocity estimation which favour the more familiar physical arrivals. In this thesis we show that the non-physical arrivals retrieved in exploration-type settings are useful for determining the long-wavelength seismic velocity structure and can be used to obtain improved Green’s function estimates. First, we estimate the seismic velocity and layer thickness by measuring the signal coherency along traveltime curves between two receivers in a collection of traces consisting of cross-correlated wavefields, known as the correlation gather. The traveltime curves represent the traveltime differences between wavefields recorded at the two receivers. When the procedure is used to find the velocity and thickness of the uppermost layer, the traveltime curves implicitly incorporate the physical and non-physical wavefields in the Green’s function estimates. When the procedure is applied to a model with more than one layer, the traveltime curves correspond to non-physical wavefields only in the Green’s function estimates. Instead of suppressing multiple reflections as in conventional methods, the procedure incorporates the traveltimes of multiple reflections to constrain velocity and thickness estimates. The procedure above is most suitable for recovering the first-layer seismic velocity. We propose a simpler method to estimate the seismic velocities corresponding to deeper layers. We find that the Green’s functions contain very weak reflections, but are dominated by non-physical refractions if retrieved using a limited source aperture. The seismic velocities are easily identifiable as repeating bright spots after transforming the refraction-dominated Green’s functions to the − p domain. We show that non-physical reflections can be used constructively to provide physical reflections, and therefore improved Green’s function estimates, by using a cross-convolution operation in a new variant of seismic interferometry, called source-receiver interferometry. We also show that non-physical reflections associated with the cross-correlation of reflections from different interfaces allow for the direct estimation of interval velocities and layer thicknesses. This method removes the necessity to first find the root-mean-square velocities and two-way traveltimes required to compute the interval velocities by Dix inversion. Overall, this thesis significantly improves our understanding of how nonphysical energy in seismic interferometry both provides useful information about the Earth’s subsurface and contributes to physical energy in particular interferometric methods. Mon, 25 Jun 2012 00:00:00 GMT http://hdl.handle.net/1842/6184 2012-06-25T00:00:00Z