No description available.

No description available.

Although there is no obvious reason why seismic 6-value and the spatial fractal dimension of earthquakes, D, should be related, there are several reports of observed empirical correlations between these two quantities. In order to investigate this phenomenon, and attempt to relate it to different types of earthquakes, industrially induced seismicity in The Geysers geothermal area, California and earthquake swarms in Long Valley caldera, California were analysed. Raw seismograms from the Unocal-NEC-Thermal network in The Geysers were processed automatically, calculating magnitudes from coda lengths and locating them using a three-dimensional velocity model. Seismicity correlated with the locations of commercial wells and surface fault locations. The entire Geysers dataset was too complex for clear correlations between b, D, seismicity and injection to be observed. In several cases, short pulses of injection induced bursts of seismicity of either small-magnitude, clustered events or large-magnitude diffuse seismicity, resulting always in a transient anomaly of negative b/D. However, sometimes pulses of injection were not accompanied by b/D transients and sometimes b/D transients were not accompanied by known injection. The latter cases may or may not indicate undisclosed injection activity. A seismic crisis in Long Valley caldera was associated with major b/D anomalies that accompanied migration of the activity from a hydrothermal zone on the south edge of the resurgent dome to the right-lateral, blind, near-vertical South Moat fault to the immediate south. The results indicated that the hydrothermal zone is an inhomogeneous structure whereas the South Moat has a mature, coherent fault plane, capable of generating magnitude M = 6 earthquakes and posing a threat to the town of Mammoth Lakes.

Iceland has long been thought to be underlain by a thermal upwelling, or plume, rising from deep within the mantle. This study tests this hypothesis, by a) seeking evidence for a plume in the lower mantle in azimuth anomalies at the NORSAR array and b) mapping the three-dimensional structure of the mantle beneath Iceland using teleseismic tomography and data from an Iceland-wide broadband seismometer network. A temporary network of 30 digital broadband, three-component seismographs was deployed 1996-1998 to complement the existing, permanent seismic network on Iceland. This created a dense, well-distributed network. 3159 P-wave and 1338 S-wave arrival times were measured and inverted for velocity structure using the ACH method of teleseismic tomography. The preferred models are well-resolved down to -400 km, and reveal a low-velocity body with anomaly up to -2.9% in V(_p)) and -4.9% in V(_s) beneath central Iceland. This persists throughout the entire model depth range. The amplitudes of the anomalies imply an excess temperature of 200-300 K relative to the surrounding mantle. The morphology of the anomaly changes from cylindrical to tabular at 250-300 km depth, a feature that resolution tests suggest is real. This is consistent with the predictions of some convection models and suggests that the plume is restricted to the upper mantle. Anomalies in v(_p) and v(_s) provide evidence for lateral flow of material beneath the Reykjanes Ridge to the southeast in the depth range 50-200 km. Similar anomalies are present beneath the Kolbeinsey Ridge to the north only beneath 160 km. This shows that flow outwards beneath the Kolbeinsey Ridge is blocked by the Tjörnes Fracture Zone above 160 km. Azimuthal anomalies detected on the NORSAR array for rays travelling beneath Iceland at 1,500 km depth are consistent with a plume beneath Iceland at this lower-mantle depth with a Gaussian radius of 125 km and a strength of 1.5%. The observations do not serve as proof for such an anomaly because the solution is not unique. V(_p)/V(_s) ratios are 1% high throughout most of the plume, and up to 3.2% high at depths of 100-300 km beneath central and east-central Iceland. This suggests that up to a few percent of melt pervades the entire plume.

The purpose of the thesis is the determination of potentially seismic active sources and of the dynamic response evaluation of surface and subsurface structure at sites where the geometric and dynamic properties of the ground can strongly amplify seismic motions. A combination of methods involving the study of geology, geophysics and seismology disciplines permitting cross-comparison of techniques in a robust approach is applied to address these issues. The study area is focused in Kastelli-Kissamou and Paleohora half graben basins in northwestern and southwestern Crete that is located in one of the most seismically active parts of the Africa-Eurasia collision zone. Ground truthed geological field survey, 2D Electrical Resistivity Tomography (ERT), Horizontal to Vertical Spectra Ratio (HVSR) technique using microtremors and microseismicity study are conducted. Microseismicity study involves two different earthquake dataset acquired from a regional permanent network installed on Crete and local temporal network installed on Paleohora. 2D Electrical resistivity tomography (ERT) reveals seven faults in the territory of Kastelli-Kissamou and three faults large scale faults in the territory of Paleohora basin. HVSR technique using microtremors is applied only in the populated area of Kastelli and Paleohora basins and reveals five fault zones in Kastelli and four major fault zones in Paleohora crosscutting the densely populated areas. The effects of the surface and subsurface structure are well patterned in the horizontal to vertical spectra ratios. One amplified clear frequency, two high amplified clear frequencies, broad and flat or low amplitude HVSR peaks attributes the effects of surface and subsurface structure on seismic ground motion. The effects of soft rocks, stiff soils, thick and thin alluvial deposits, fault zones, lateral heterogeneities and discontinuities on seismic ground motion are determined. The higher ground amplification level is observed in Paleohora (A=5.7) compared to Kastelli (A=3.4). Three case studies of building vulnerability evaluation in Paleohora half-graben basin using HVSR technique and microtremors are presented. Temporal seismological network is installed in the territory of Paleohora to study the seismotectonic setting of southwestern Crete. Microseismicity using data from the permanent seismological regional network of Crete is used to compare the seismicity of the study areas.

Seismology is the study of earthquakes and the Earth’s internal structure using seismic waves. Traditional seismology is constrained by the timing and location of seismic sources, and by the location of seismometers with which energy from the sources are recorded. Improvements in the global seismometer networks have reduced the latter constraint. Furthermore, recent advances into Seismic Interferometry (SI) have enabled detailed information about the Earth’s interior to be obtained using ambient seismic noise, hence even in areas with low natural seismicity. The most common approach to SI is to use the cross-correlation of ambient noise recordings to construct an estimate of the Green’s function between two seismometer locations. The Green’s function estimate is then analysed or inverted for seismic properties of the Earth. This method of noise interferometry is now a popular approach in earthquake seismology as in some situations it renders active seismic sources (earthquakes or synthesised explosions) obsolete, as subsurface information can be obtained even in times of seismic quiescence. This thesis investigates a different method: Source-Receiver Interferometry (SRI). SRI can be used to construct earthquake seismograms on seismometers that were not necessarily deployed when the earthquakes occurred - a form of ‘retrospective seismology’. This might be useful if, for example, we wish to analyse old earthquakes with newly installed seismometers. The application of SRI involves evaluating two interferometric integrals. The first integral is evaluated using ambient noise interferometry: at least 6 months of noise data is cross-correlated to estimate the Green’s functions between pairs of seismometers. These inter-receiver Green’s functions are then used as the “propagators” for SRI. Their role is to project earthquake signals recorded on a backbone array of seismometers to the location of a target sensor at which a new, novel earthquake seismogram is to be constructed - a form of spatial redatuming. To spatially redatum the earthquake data, the second interferometric integral is evaluated using either processes of correlation (resulting in correlation-correlation SRI) or convolution (correlation-convolution SRI). The method used depends on the relative location of the target sensors with respect to both the backbone seismometer array and the earthquake epicentre. The SRI process is completed by integrating (summing) over all projected earthquake signals. To regularise the spatial distribution of the projected earthquake data and to invoke this second interferometric integral more precisely, the backbone seismometers are embedded within 2D spatial Voronoi cells. New seismograms for 87 earthquakes were reconstructed on up to eight target sensors, seven of which were deployed when the earthquakes occurred and are used to test the success of the method by comparing with the SRI results with the directly-recorded seismograms. The seismogram reconstructions on the eighth target sensor are truly novel. The SRI method was developed to operate over two length scales. The first focusses on relatively small length scales in which the inter-station distance between the eight target sensors and the backbone array seismometers is between ~ 210 km and 540 km. Both correlation-correlation SRI and correlation-convolution SRI are used to reconstruct the earthquake seismograms on four of the same target sensors. Applying correlation-convolution SRI is shown to remove spurious signals associated with correlation-correlation SRI. Second, a significantly larger length scale is considered where a second set of target sensors are located up to 2420 km from a second backbone seismometer array. The correlation-correlation and correlation-convolution SRI methods are used in parallel to increase the spatial extent of the study. The quality of the SRI seismograms constructed is shown to depend on the quality of three components: 1) the SRI propagators constructed using ambient noise interferometry, 2) the earthquake signals recorded on the backbone seismometer array, and 3) the correlation (or convolution) functions that are summed in the second interferometric integral to construct the final SRI seismogram. The quality of each component is quantified by its signal-to-noise ratio and root-mean-square value, and criteria are proposed to obtain optimal earthquake seismogram reconstructions using SRI. SRI is most successful when the target sensors are located less than 540 km from the backbone array seismometers. Such SRI seismograms are being used to create a catalogue of new, ‘virtual’ earthquake seismograms that are available to complement real earthquake data for use in future earthquake seismology studies. An alternative approach to noise interferometry is also considered: the recordings from just 15 earthquakes are used to perform multidimensional deconvolution (MDD) to estimate the Green’s functions between pairs of seismometers. This is the first time such data has been used to perform MDD, which is valid in attenuating media and is thus theoretically more valid in earthquake seismology settings than correlational interferometry. The Green’s functions estimated using MDD are compared with those same Green’s functions estimated using ambient noise interferometry and the results are comparable on several occasions, despite using far fewer data for MDD. However, the quality of the results of MDD is significantly affected by the illumination of the receiver array from the earthquake sources. A greater density of earthquakes that sufficiently illuminates all backbone array seismometers is required to obtain accurate Green’s functions by MDD.

No description available.

Anelasticity and inhomogeneity in the Earth decreases the energy and modifies the frequency of seismic waves as they travel through the Earth. This phenomenon is known as seismic attenuation. The associated physical process leads to amplitude diminution, waveform distortion and phase delay. The level of attenuation a wave experiences depends on the degree of anelasticity and the scale of inhomgeneity in the rocks it passes through. Therefore, attenuation is sensitive to the presence of fluids, degree of saturation, porosity, fault, pressure, and the mineral content of the rocks. The work presented in this thesis covers attenuation measurements in seismic data; estimation of P- and S-wave attenuation in recorded well logs; attenuation analysis for pore fluid determination; and attenuation compensation in seismic data. Where applicable, a set of 3D seismic data or well logs recorded in the Gullfaks field, North Sea, Norway, is used to test the methods developed in the thesis. A new method for determining attenuation in reflection seismic data is presented. The inversion process comprises two key stages: computation of centroid frequency for the seismic signal corresponding to the top and base of the layer being investigated, using variable window length and fast Fourier transform; and estimation of the difference in centroid frequency and traveltime for the paired seismic signals. The use of a shape factor in the mathematical model allows several wavelet shapes to be used to represent a real seismic signal. When applied to synthetic data, results show that the method can provide reliable estimates of attenuation using any of the wavelet shapes commonly assumed for a real seismic signal. Tested against two published methods of quality factor (Q) measurement, the new method shows less sensitivity to interference from noise and change of frequency bandwidth. The method is also applied to seismic data recorded in the Gullfaks field. The trace length is divided into four intervals: AB, BC, CD, and DE. The mean attenuation (1/Q_m) calculated in intervals AB, BC, CD, and DE are 0.0196, 0.0573, 0.0389, and 0.0220, respectively. Results of attenuation measurements using the new method and the classical spectral ratio method (Bath 1974, Spencer et al, 1982) are in close agreement, and they show that interval BC and AB have the highest and lowest value of attenuation, respectively. One of the applications of Q measured in seismic records is its usage for attenuation compensation. To compensate for the effects of attenuation in recorded seismograms, I propose a Q-compensation algorithm using a recursive inverse Q-filtering scheme. The time varying inverse Q-filter has a Fourier integral representation in which the directions of the up-going and down-going waves are reversed. To overcome the instability problem of conventional inverse Q-filters, wave numbers are replaced with slownesses, and the compensation scheme is applied in a layer-by-layer recursive manner. When tested with synthetic and field seismograms, results show that the algorithm is appropriate for correcting energy dissipation and waveform distortion caused by attenuation. In comparison with the original seismograms, the Q-compensated seismograms show higher frequencies and amplitudes, and better resolved images of subsurface reflectors. Compressional and shear wave inverse quality factors (Q_P^(-1) and Q_S^(-1)) are estimated in the rocks penetrated by well A-10 of the Gullfaks field. The results indicate that the P-wave inverse quality factor is generally higher in hydrocarbon-saturated rocks than in brine-saturated rocks, but the S-wave inverse quality factor does not show a dependence on fluid content. The range of the ratio of Q_P^(-1) to Q_S^(-1) measured in gas, water and oil-saturated sands are 0.56 – 0.78, 0.39 – 0.55, and 0.35 – 0.41, respectively. A cross analysis of the ratio of P-wave to S-wave inverse quality factors, (Q_P^(-1))/(Q_S^(-1) ), with the ratio of P-wave to S-wave velocities, V_P/V_S , clearly distinguishes gas sand from water sand, and water sand from oil sand. Gas sand is characterised by the highest (Q_P^(-1))/(Q_S^(-1) ) and the lowest V_P/V_S ; oil sand is characterised by the lowest (Q_P^(-1))/(Q_S^(-1) ) and the highest V_P/V_S ; and water sand is characterized by the V_P/V_S and (Q_P^(-1))/(Q_S^(-1) ) values between those of the gas and oil sands. The signatures of the bulk modulus, Lame’s first parameter, and the compressional modulus (a hybrid of bulk and shear modulus) show sensitivities to both the pore fluid and rock mineral matrix. These moduli provided a preliminary identification for rock intervals saturated with different fluids. Finally, the possibility of using attenuation measured in seismic data to monitor saturation in hydrocarbon reservoirs is studied using synthetic time-lapse seismograms, and a theoretical rock physics forward modelling approach. The theory of modulus-frequency-dispersion is applied to compute a theoretical curve that describes the dynamic effects of saturation on attenuation. The attenuation measured in synthetic time-lapse seismograms is input to the theoretical curve to invert the saturation that gave rise to the attenuation. Findings from the study show that attenuation measured in recorded seismograms can be used to monitor reservoir saturation, if a relationship between seismogram-derived attenuation and saturation is known. The study also shows that attenuation depends on other material properties of rocks. For the case studied, at a saturation of 0.7, a 10% reduction in porosity caused a 5.9% rise in attenuation, while a 10% reduction in the bulk modulus of the saturating fluids caused an 11% reduction in attenuation.

Over recent years geological, geochemical and geophysical surveys of mid-ocean ridges have revealed a significant degree of along-axis variability not only in seabed morphology, but also in crustal structure, particularly Numerous geophysical surveys of the Valu Fa Ridge, southwest Pacific, have mapped the extent of an axial mid-crustal reflector. This reflector has been interpreted as representing the top of a sill-like melt lens, comprising a high percentage of partial melt, lying at the top of a crustal magma chamber. In 1995, a controlled-source, wide-angle seismic dataset was acquired at the Valu Fa Ridge during RN Maurice Ewing cruise EW9512, to investigate the mid-deep crustal structure at this ridge, and particularly the crustal magma chamber associated with the melt lens beneath the ridge axis. The EW9512 acquisition geometry was primarily two-dimensional in design, and modelling of these 2-D profiles revealed the presence of an axial low velocity zone beneath the melt lens. This low velocity zone is thought to represent a region of crystal mush comprising a much lower percentage of partial melt than is present in the overlying melt lens. Similar structures have been modelled beneath a number of other mid-ocean ridges. The primary aim of this study was to build on this 2-D interpretation by taking advantage of three-dimensional ray coverage in the axial region in order to assess the along-axis continuity of the magmatic system, correlate this to any ridge segmentation apparent in the seabed morphology, and determine if ridge segmentation is related to the magma supply. The 3-D data were analysed using a tomographic inversion technique. The inversion results suggest that the axial low velocity zone may be segmented on a scale of 5-10 km, which correlates with the morphological segmentation of the ridge crest and is believed to reflect episodic magma supply with different ridge segments at different stages of a cycle of magmatic and amagmatic extension. However, three- dimensional ray coverage is not ideal owing to the dominantly 2-D acquisition geometry. Therefore a detailed assessment of data uncertainty and resolution was undertaken to enable a meaningful interpretation of the inversion results in terms of which features have a geological origin and which are artefacts of the inversion process. P-S mode converted arrivals arising from mid-crustal interfaces were also modelled in order to obtain improved geological constraints on the crustal structure than is possible from P-wave studies alone. This modelling indicates that the uppermost crust is pervaded by thin cracks. In addition, techniques were developed for modeling the polarisation of 5-wave arrivals with low signal strength. Application of these methods suggests that the thin cracks have a preferred orientation parallel to the ridge crest on-axis, and oblique to the ridge crest off-axis which is thought to reflect the pattern of southward propagation of the ridge system inferred from regional tectonic and bathymetric studies. Modelling of P-S mode converted arrivals arising from conversion at the top of the melt lens provided additional constraints on the properties of the melt lens. In conjunction with the 3-D tomographic results, this work suggests that the southernmost ridge segment in the study area has recently become magmatically active following a period of amagmatic extension suggested by its morphology, thus providing evidence for episodic melt supply at this ridge. As part of the suggestions for further work, a theoretical investigation of survey resolution was undertaken to test commonly adopted acquisition geometries with a view to optimising the design and cost-effectiveness of future 3-D controlled-source tomographic experiments.

In exploration seismology, seismic waves are emitted into the structurally complex Earth. Its response, consisting of a mixture of arrivals including primary reflections, conversions, multiples, and transmissions, is used to infer the internal structure and properties. Waves that interact multiple times with the inhomogeneities in the medium probe areas of the subsurface that are sometimes inaccessible to singly scattered waves. However, these contributions are notoriously difficult to use for imaging because multiple scattering turns out to be a highly nonlinear process. Conventionally, imaging algorithms assume singly scattered energy dominates data. Hence these require that energy that scatters more than once is attenuated. The principal focus of this thesis is to incorporate the effect of complex nonlinear scattering in the construction of subsurface elastic images. Reciprocity theory is used to establish an exact relation between the full recorded data and the local (zero-offset, zero-time) scattering response in the subsurface which constitutes our image. Fully nonlinear, elastic imaging conditions are shown to lead to better illumination, higher resolution and improved amplitudes in pure-mode imaging. Strikingly it is also observed that when multiple scattering is correctly handled, no converted-wave energy is mapped to any image point. I explain this result by noting that conversions require finite time and space to manifest. The construction of wavefield propagators (Green’s functions) that are used to extrapolate recorded data from the surface to points in the Earth’s interior is a crucial component of any imaging technique. Classical approaches are based on strong assumptions about the propagation direction of recorded data, and their polarization; preliminary steps of wavefield decomposition (directional and modal) are required to extract upward propagating waves at the recording surface and separate different wave modes. These algorithms also generally fail to explain the trajectories of multiply scattered and converted waves, representing a major problem when constructing nonlinear images as we do not know where such energy interacted with the scatterers to be imaged. A secondary aim of this thesis is to improve on the practice of wavefield extrapolation or redatuming by taking advantage of the different nature of multi-component data compared with single-mode acoustic data. Two-way representation theorems are used to define novel formulations in elastic media which allow both up- and downward propagating fields to be back-propagated correctly without ambiguity in the direction, and such that no cross-talk between wave modes is generated. As an application of directional extrapolation, the acoustic counterpart of the new approach is tested on an ocean-bottom cable field dataset acquired over the Volve field, North Sea. Interestingly, the process of redatuming sources to locations beneath a complex overburden by means of multi-dimensional deconvolution also requires preliminary wavefield separation to be successful: I propose to use the two-way convolution-type representation as a way to combine full pressure and particle velocity recordings. Accurate redatumed wavefields can then be obtained directly from multi-component data without separation. Another major challenge in seismic imaging is to construct detailed velocity models through which recorded data will be extrapolated. Nowadays the information contained in the extension of subsurface images along either the time or space axis is commonly exploited by velocity model building techniques acting in the image domain. Recent research has shown that when both extensions are taken into account, it is possible to estimate the data that would have been recorded if a small, local seismic survey was conducted around any image point in the subsurface. I elaborate on the use of nonlinear elastic imaging conditions to construct such so-called extended image gathers: missing events, incorrect amplitudes, and spurious energy generated from the use of only primary arrivals are shown to be mitigated when multiple scattering is included in the migration process. Finally, having access to virtual recordings in the subsurface is also very useful for target-oriented imaging applications. In the context of one-way representation, I apply the novel methodology of Marchenko redatuming to the Volve field dataset as a way to unravel propagation effects in the overburden structure. Constructed wavefields are then used to synthesize local, subsurface reflection responses that are only sensitive to local heterogeneities, and detailed images of target areas of the subsurface are ultimately produced. Overall the findings of this thesis demonstrate that, while incorporating multiply scattered waves as well as multi-component data in imaging may be not a trivial task, such information is vital for achieving high-resolution and true-amplitude seismic imaging.