In this thesis, I have introduced and presented a new time-frequency distribution, termed the Signal-Dependent Distribution (SDD), which can by-pass the Gabor Uncertainty Principle, with a trade-off instead between joint time-frequency transform and suppression of transform artefacts. In two different synthetic seismic data sets, the SDD provided estimates of attenuation in closer agreement to the input attenuation than those from a fixed- (short-time-Fourier-transform) and a variable-window (Stockwell transform) time-frequency transform. The SDD also provided spectral ratio surfaces from a pre-stack gathers, using the pre-stack Q inversion (PSQI) method, more consistent with a frequency-independent attenuation model than the fixed- and variable-window transforms. Frequency-dependent Q can be estimated in the PSQI method if the seismic attenuation quality factor, Q, is assumed to follow a powerlaw frequency-dependence, Q=af^b. Utilising the SDD to form spectra, the modified PSQI method found a frequency exponent, b, of only 0.074±0.001 (median 0.06) for the Kinnoull field in the North Sea, implying that an approximation of frequency-independent Q is valid for the Kinnoull dataset. Higher b values were found to coincide with sudden, localised, drops in centroid frequency beneath amplitude anomalies: they were inferred not to be due to a genuine frequencydependence of Q, but to interference on the spectral ratio surface. The SDD was then used to estimate spectra in the PSQI method applied to: the Kinnoull pre-stack surface seismic dataset; a spectral ratio method applied to a stacked surface seismic survey; and VSP data for well 16/23-7. The three sets of attenuation estimates were compared to each other, and also to average energy and average centroid frequency maps derived from the stacked seismic dataset. Only the attenuation values estimated from the stacked seismic data and the VSP data correlated well with each other (median 1000/Q of 9.2±0.1 and 10.4±2.0 respectively). 1/Q attenuation maps from stacked and pre-stack seismic data did not correlate coherently with the centroid frequency or energy maps, nor did the pre-stack attenuation values correlate with the VSP data. This inconsistency remains unexplained. This method of estimating frequency-dependent seismic attenuation quality factor was then applied to 6 VSP datasets located in the Barents Sea. Although the formation-averaged frequency exponent b varied between -0.1 and 0.2, the median value was 0.02, again supporting an assumption of frequency-independent attenuation within the seismic bandwidth. Using coincident well logs, statistically significant correlations were found between intrinsic attenuation and bulk, shear, and Young’s moduli, and Poisson’s ratio. In contrast, no robust relationship was found between petrophysical parameters and attenuation in the seismic bandwidth. However, a squirt flow model fitted to the estimated power-law curves of frequency-dependent 1/Q implied crack aspect of between 1 and 6 x 10^−3, similar to the crack aspect ratio of 0.1-1.0 x 10^−3 expected to be responsible for the majority of squirt flow induced attenuation.

Accurate seismic exploration demands sophisticated seismic techniques that can be applied to any complex geological structures. The key of most recent seismic processing techniques is wave propagation modelling. For accurate simulation of the seismic wavefield, the characteristics of the real Earth media should be appropriately considered, such as attenuation effects and anisotropy. This dissertation aims to address the attenuation problems in seismic wavefield modelling in attenuative media and its related applications. My work presented in this dissertation includes 1. A viscoelastic wave equation for isotropic media is proposed. The attenuation operator applied in the wave equation consists of separate terms, which are related to velocity dispersion and energy absorption effects respectively. It is derived that the two effects are completely decoupled. 2. A numerical scheme based on low-rank approximation is presented for wave simulation in heterogeneous attenuative media. This method can achieve very high accuracy while the computational cost is greatly reduced compared with conventional method. 3. In order to take account of the anisotropic attenuation, a generalized viscoelastic wave equation is presented. The velocity dispersion and energy absorption effects are also decoupled in the encapsulated attenuation operators. Based on this, the seismic wave propagation in arbitrary anisotropic media with anisotropic attenuation can be accurately simulated. 4. Q-compensated reverse-time migration (RTM) is investigated. It is shown that the velocity dispersion and energy absorption effects should be taken care of separately. Attributed to the completed decoupled attenuation effects in the derived viscoelastic wave equation, the \Q-compensation operator is derived. As a result, the dispersion correction and energy compensation is successfully separated and can be implemented simultaneously. Based on this, different \Q-compensation schemes are analysed and compared.

The mechanism of support for the present topography of Scandinavia is not well explained by current crustal models. High topography in Norway and Sweden (Scandinavian Alps) is directly in contact with a relatively flat region (the Baltic shield). A crustal root beneath the Scandinavian Alps, expected from Airy isostatic calculations, is not present and the gravity anomaly map suggests lateral variations in density support present day topography. To bring new constrains on the variation of crustal properties we use P-receiver functions and ambient seismic noise. Using these methods we quantify Moho depth and depth-velocity variations across the crust. P-receiver functions indicate the presence of a high velocity layer at the base of the crust, interpreted as magmatic underplating or eclogitization. Ambient seismic noise provides data from Rayleigh wave seismic tomography and identifies the presence of a low velocity layer associated with granite intrusion in the upper-middle crust. Our new crustal thickness map shows that the topography at the surface is not reflected by the Moho topography in this region. We observe a relative thickening of the crust from the Atlantic coast (40 km) to the Gulf of Bothnia (44 km). Secondly, Moho sharpness analysis and 1D depth-velocity profiles show a difference in the transition from crust and upper mantle. A high velocity layer (Vp > 7.1 km.s−1) beneath the Baltic shield is missing beneath the Scandinavian mountains. This observation explains the crustal thickness variations beneath the mountain belt and the shield, with magmatic underplating beneath the shield and a delamination of the lower crust due to eclogitization process beneath the mountain range. In addition the presence of a local low density layer (granitic body) in the upper crust beneath the northern mountains seems to be a possible mechanism to explain the presence day topography in this region.

A proof of concept for using microseismics as a site investigation tool has been developed and presented as a feasibility study utilising changes in the seismic wave Peak Particle Velocity (PPV) (m/s) and dominant frequency (Hz). The key significance of this thesis is the enhancement of near surface seismic imaging applications using a novel concept. Researchers have observed low frequencies when detecting geological features at depths greater than 100m. Mitchell, Derzhi et al. (1997) and Dilay and Eastwood (1995) have shown previously that the dominant frequency observed moved towards the low frequency range (<100Hz). Marfurt (1984), investigated how the dominant frequency varies with geological feature thickness and Marfurt and Kirlin (2001) used this concept to resolve geological features with a thickness of <20m. This thesis identified the following gaps in knowledge and identified that there is:- No study to demonstrate if the relationship between dominant frequency and geological feature thickness is observed in the near surface (i.e. depths less than 100m) at metre scale accuracy (i.e. <10m); - No study has used micro seismometers to apply this technique for near surface applications; and - No study which has considered if the dominant frequency and PPV characteristics can be used to develop a concept for a near surface site investigation tool deployed in the near surface. Considering the effect of medium properties there was significant effect on seismic wave characteristics such as PPV and frequency when utilising low frequency seismic sources in the range of 1-100Hz. The changes in the seismic wave characteristics during wave propagation through geological features characterised by different central feature widths and low Pressure (P) wave velocity zones were investigated. COMSOL Multiphysics Finite Element software modelled seismic excitations using the linear elastic equations that govern mechanical wave propagation. Dominant frequency was more responsive than PPV to material property changes. Considering the presence of a material property boundary, there was a significant effect on the PPV and dominant frequency characteristics, allowing a novel prediction methodology to be developed. The presence and width of a geological feature was detected at sub metre scale accuracy. Considering the presence of a geological feature surrounded by a low P wave velocity zone, the differentiation between the material zones can be detected numerically at sub metre scale accuracy, and this was validated in “blind” tests and pilot field trials with a systematic error of +0.4m and a random uncertainty of ±0.39m. Plotting PPV as a horizontal profile across the monitoring cross section allowed the visualisation of geological feature width, which inferred that geological feature location can be visualised with good accuracy. This research has confirmed that we can use the seismic wave characteristics i.e. PPV and frequency, to effectively map and locate near surface geological and manmade structures using a novel concept which can be deployed in the near surface. The range of validity of this novel concept is the detection and location of geological structures of a known type (i.e. a vertical dyke formation) for a range of different geological parameters such as, width and material properties in a low ambient noise environment. The effect of noise is important in terms of resolution and applicability of the method, and was investigated by adding noise to the sensitivity analysis. This research intentionally selected a field site that was characterised by a low ambient noise environment removing the requirement to utilise signal processing filtration methods as the impact from ambient noise was deemed insignificant. Consideration was given to sites that may be characterised by high ambient noise. When noise was increased to 2 x source PPV the “worst case” systematic error was - 2m and the random uncertainty was ± 1.6m. Both of which are greater than systematic error of +0.4m observed in the field trial. In high noise environment it would be advantageous prior to the experiment to establish if the PPV of the seismic source is powerful enough to overcome the effect of ambient noise. Future work could consider the application of filtration via various signal processing methods to minimise the effect of ambient noise. Preliminary simulations were conducted to consider the feasibility of future applications. There is potential to utilise the changes in the dominant frequency and PPV of the seismic signal as it propagates to locate voids and other subsurface features at depth. Future work will have to be conducted to determine subsurface feature location capabilities. Numerical simulations and pilot field trials demonstrate that this novel concept can be applied effectively achieving sub metre scale accuracy for a site with specific material properties and metre scale accuracy for site characterised by high ambient noise. These results are significant in forming the theoretical basis for the development of a novel microseismic site investigation tool.

High-resolution velocity models, at near surface and deeper reservoir depths are produced with three-dimensional, acoustic, anisotropic, full-waveform inversion. Industry experts show eager interest in the development of this technology with a drive to push its application to reflection-dominated streamer datasets as well as ocean-bottom-node datasets in geologically complex environments. Here, a robust methodology employing the use of key strategies to address the inversion of such datasets, attaining increased resolution and depth penetration, is explored. Synthetic tests were undertaken to exploit the use of reflected energy. Key strategies: muting of direct arrivals, time windowing, and layer-stripping, all produced highly resolved, full waveform inversion models. These strategies have been incorporated into inversion schemes focusing solely on reflection targets. Strategies to further improve model resolution for field datasets were then investigated. Close examination of a full-waveform inversion model for a shallow-water ocean-bottom-node dataset, revealed a systematic mismatch between the observed and predicted data. After conducting a series of tests, it was illustrated that systematic errors in the starting model, source wavelet, incomplete convergence, or an inadequate finite-difference mesh did not cause the mismatch. Instead, inadequacies in the physics used during inversion are believed to be the cause. The introduction of an offsetvariable density scheme during inversion, compensated efficiently and heuristically for these inaccuracies, removing the mismatch and increasing the model resolution. The sensitivity of full-waveform inversion to local minima, where the computed model is stuck away from the real global-minimum solution and further iterations of the optimisation bring no reward, was kept in mind during the inversion of two deep-water ocean-bottom node datasets. Thus, full-waveform inversion was undertaken using conditioned data obtained through adaptive matching, incorporating higher frequencies and a greater weight on reflected energy, valuably pushing the limits of resolution and depth penetration of the update. The use of all these robust methodologies improved the travel-time match; better flattened common-image gathers giving a closer fit to well logs and an improvement in the pre-stack depth-migrated image. Effectively, the reflectivity was non-linearly migrated into the velocity model via the inversion acting on raw unprocessed waveforms. Thus, full waveform inversion can eventually replace conventional processing and migration - all that is needed, is a full-bandwidth velocity model.

A surface wave dispersion study has been conducted on high frequency (0.5-5 Hz) crustal Rayleigh waves propagating across the 65 Ma. Chicxulub impact structure in Mexico. These were recorded on a 20 station seismic array deployed along 4 radial arms across the region and originated from nearby quarries within the array. Events originating from the same quarry were stacked prior to the application of a multiple filter technique to produce group velocity dispersion curves. Using a genetic algorithm several one-dimensional shear wave velocity-depth models have then been obtained through the optimisation of the fundamental and higher mode dispersion curves. The models provide information on the velocity structure of the upper few kilometres of the crust and suggest an infilling of the crater from the crater rim inwards. An inverted velocity gradient is modelled over the upper few hundred metres across most of the region with the exception of a central radial area. This inverted velocity zone may be connected to dolomitization during a late Miocene regression. The base of the Tertiary sequence is modelled at c. 1-1.5 km depth and shows increased velocities compared to the overlying sediments. This velocity increase may imply some form of hydrothermal alteration of the sediments caused by a thermal blanket effect created by the underlying crater breccia and melt. Immediately below the Tertiary sediments a c. 200 m thick low velocity zone is interpreted as a layer of suevitic impact breccia. Models obtained at c. 35-45 km radius from the crater centre are consistent with the existence of a peak ring as a topographic high above the crater floor. The results from the velocity models provide fresh information on the sedimentation of the region and some constraints on the crater morphology.

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Since 1900, earthquakes worldwide have been responsible for over 2 million fatalities and caused nearly $2 trillion of economic damage. Accurate assessment of earthquake hazard is therefore critical for nations in seismically active regions. For a complete understanding of seismic hazard, the temporal pattern of strain accumulation, which will eventually be released in earthquakes, needs to be understood. But earthquakes typically occur every few hundred to few thousand years on any individual fault, and our observations of deformation usually only cover time periods of a decade or less. For this reason, our knowledge of the temporal variation in strain accumulation rate is limited to insights gleaned from kinematic models of the earthquake cycle that use measurements of present-day strain to infer the behaviour on long time scales. Previous studies have attempted to address this issue by combining data from multiple faults with geological estimates of long-term strain rates. In this thesis I propose a different approach, which is to observe deformation at multiple stages of the earthquake cycle for a single fault with segments that that have failed at different times. In the last century the North Anatolian Fault (NAF) in Turkey has accommodated 12 large earthquakes (Mw >6.5) with a dominant westward progression in seismicity. If we assume that each of these fault segments are at a different stage of the earthquake cycle then this provides a unique opportunity to study the variation in along-strike surface deformation, which can be equated to variation of deformation in time. In this thesis I use Interferometric Synthetic Aperture Radar (InSAR) and Global Navigation Satellite System (GNSS) observations to examine the spatial distribution of strain along the NAF. InSAR is an attractive technique to study surface displacements at a much higher spatial resolution (providing a measurement every 30 m) compared to established GNSS measurements, with station separations between 10 km to 100 km in Turkey. I specifically address a key technical challenge that limits the wide uptake of InSAR: phase unwrapping, the process of recovering continuous phase values from phase data that are measured modulo 2π radians. I develop a new unwrapping procedure for small baseline InSAR measurements that iteratively unwraps InSAR phase. For each iteration, this method identifies pixels unwrapped correctly in the previous iteration and applies a high cost to changing the phase difference between these pixels in the next iteration. In this way, the iterative unwrapping method uses the error-free pixels as a guide to unwrap the regions that contained unwrapping errors in previous iterations. I combine measurements of InSAR line-of-sight displacements with published GNSS velocities to show that an ∼80 km section of the NAF that ruptured in the 1999 Izmit earthquake (Mw 7.4) is creeping at a steady rate of ∼5 mm/yr with a maximum rate of 11 ± 2 mm/yr near the city of Izmit within the observation period 2002-2010. I show that in terms of the moment budget and seismic hazard the effect of the shallow, aseismic slip in the past decade is small compared to that from plate loading. Projecting the shallow creep displacement rates late into the earthquake cycle does not produce enough slip to account for the 2-3 m shallow coseismic slip deficit observed in the Izmit earthquake. Therefore, distributed inelastic deformation in the uppermost few kilometers of the crust or slip transients during the interseismic period are likely to be important mechanisms for generating the shallow slip deficit. I used similar techniques to confirm that a ∼130 km section of the central NAF near the town of Ismetpasa, is also undergoing aseismic creep at a steady rate of 8±2 mm/yr. Using simple elastic dislocation models to fit fault perpendicular velocities I show that there is an eastward decreasing fault slip rate in this region from ∼32 mm/yr to ∼21 mm/yr over a distance of about 200 km. The cause of this decrease remains unclear, but it could be due to postseismic effects from the 1999 Izmit and Duzce earthquakes and/or long-term influence from the 1943 (Mw 7.4) and 1944 (Mw 7.5) earthquakes. Finally, I combine line-of-sight displacements from 23 InSAR tracks to produce the first high resolution horizontal velocity field for the entire continental expression of the NAF (∼1000 km). I show that the strain rate does not vary significantly along the fault, and since each segment of the NAF is at a different stage of the earthquake cycle, the strain rate is invariant with respect to the time since the last earthquake. This observation is inconsistent with viscoelastic coupling models of the earthquake cycle, which predict a decreasing strain rate with time after an earthquake. My observations imply that strain accumulation reaches a steady-state fairly rapidly after an earthquake (<7-10 years) after which strain is localised on a narrow shear zone centred on the fault and does not vary with time. A time-invariant strain rate is consistent with a strong lower crust in the region away from the fault with a viscosity ≥1020 Pas. My results imply that short term snapshots of the present-day strain accumulation (as long as it is after the postseismic period) are representative of the entire earthquake cycle, and therefore geodetic estimates of the strain rate can be used to estimate the total strain accumulation since the last earthquake on a fault, and be used as a proxy for future seismic hazard assessment. The techniques I developed to explore the spatial and temporal pattern of aseismic fault creep and long-term strain accumulation along the NAF are general and can be ap- plied to all strike-slip faults globally. The archived ERS-1/2 and Envisat satellite data are an extremely valuable resource that can and should be used to extend InSAR time series measurements back to the early 1990s. Together with the new Sentinel-1 data sets, this provides an unprecedented opportunity to explore tectonic deformation over several decades and on continental scales. Despite the availability of numerous correction techniques (in this thesis I use global weather models to calculate the atmospheric contribution), atmospheric delays remain the major challenge to exploiting Sentinel-1 data for global strain mapping, the mitigation of these delays are an important goal for the InSAR community.

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