In this paper we present the results from an anisotropic converted-wave (C-wave) prestack time migration (PSTM) of a 2D multicomponent (4-C) line over Lomond Field. This is a gas-condensate field 233 km east of Aberdeen, in the Central North Sea, Block 23/21, close to the border between the UK and the Norwegian sectors, on the east flank of the Central Graben. The geologic setting is excellent for testing C-wave imaging tools; the quite complex structure is dominated by a salt-induced anticline (fractured on top) and a large gas cloud (leaked from the fractured reservoir). In addition, a thick overburden of laminated shales is likely to produce polar anisotropic (VTI) effects. The large gas cloud was the main motivation for C-wave acquisition. In fact, the imaging results obtained using P-waves (Figure 1) include a large zone of amplitude dimming, circled in black, and the push-down effects on the above events. This cloud partially obscures the area of the reservoir, which is composed of Paleocene sandstones of the Forties Formation, 200 m in thickness, and sealed by mudstones from the Rogaland Group. The presence of polar (VTI) anisotropy was confirmed by observations of angular-dependent velocity in deviated sonic logs.

There is general agreement between different theories giving expressions for the overall properties of materials with dry, aligned cracks if the number density of cracks is small. There is also very fair agreement for fluid‐filled isolated cracks. However, there are considerable differences between two separate theories for fluid‐filled cracks with equant porosity. Comparison with recently published experimental data on synthetic sandstones gives a good fit with theory for dry samples. However, although the crack number density in the laboratory sample is such that first‐order theory is unlikely to apply, expressions correct to second order (in the number density) provide a worse fit. It also appears that the ratio of wavelength to crack size is not sufficiently great for any detailed comparison with effective‐medium theories, which are valid only when this ratio is large. The data show dispersion effects for dry cracks and scattering, neither of which will occur at sufficiently long wavelengths. Data from the water‐saturated samples indicate that the effect of equant porosity is significant, although the two theories differ strongly as to just how significant. Once again, and in spite of the reservations mentioned above, a reasonable fit between theory and observation can be shown.

The movement of interstitial fluids within a cracked solid can have a significant effect on the properties of seismic waves of long wavelength propagating through the solid. We consider three distinct mechanisms of wave-induced fluid flow: flow through connections between cracks in an otherwise non-porous material, fluid movement within partially saturated cracks, and diffusion from the cracks into a porous matrix material. In each case the cracks may be aligned or randomly oriented, leading, respectively, to anisotropic or isotropic wave speeds and attenuation factors. In general, seismic velocities exhibit behaviour that is intermediate between that of empty cracks and that of isolated liquid-filled cracks if fluid flow is significant. In the range of frequencies for which considerable fluid flow occurs there is high attenuation and dispersion of seismic waves. Fluid flow may be on either a wavelength scale or a local scale depending on the model and whether the cracks are aligned or randomly oriented, resulting in completely different effects on seismic wave propagation. A numerical analysis shows that all models can have an effect over the exploration seismic frequency range.

A similarity may be found between various approaches for determining the effects of parallel fractures or aligned cracks on seismic wave propagation at wavelengths that are long compared with the scale length of the cracks. Fractures can be modeled using an empirical linear slip condition; however, natural fracture surfaces can also be simulated directly as planar distributions of small isolated areas of slip (cracks) (model 1) or, conversely, as planar distributions of imperfect interfacial contacts (model 2). An alternative is plane surfaces separated by thin continuous layers of viscous fluid or a soft material (model 3). We present analytic expressions for the fracture compliances for these three models and, using these analytic results, compute the effective compliances and stiffnesses of the fractured material. As a result, it is possible to relate the measured compliances or stiffnesses directly to the statistics of the microstructural details of a fracture, given appropriate a priori information on the fracture surfaces. The results for model 1 are equivalent to a volume distribution of cracks as studied by Hudson [1980, 1981] for small crack density; the results for model 2 are basically the same as those given by White [1983] for a packing of spheres; and finally, the results for model 3 are in agreement with those given by Backus [1962] for combinations of two constituent layers. These results can be extended to the case of nonaligned fractures and to allow for fluid flow between cracks and into a porous matrix rock. Finally, it is shown that the ratio of the normal to shear fracture compliance is a good indicator of the properties of the fracture infill.

Volcano seismology often deals with rather shallow seismic sources and seismic stations deployed in their near field. The complex stratigraphy on volcanoes and near-field source effects have a strong impact on the seismic wavefield, complicating the interpretation techniques that are usually employed in earthquake seismology. In addition, as most volcanoes have a pronounced topography, the interference of the seismic wavefield with the stress-free surface results in severe waveform perturbations that affect seismic interpretation methods. In this study we deal predominantly with the surface effects, but take into account the impact of a typical volcano stratigraphy as well as near-field source effects. We derive a correction term for plane seismic waves and a plane-free surface such that for smooth topographies the effect of the free surface can be totally removed. Seismo-volcanic sources radiate energy in a broad frequency range with a correspondingly wide range of different Fresnel zones. A 2-D boundary element method is employed to study how the size of the Fresnel zone is dependent on source depth, dominant wavelength and topography in order to estimate the limits of the plane wave approximation. This approximation remains valid if the dominant wavelength does not exceed twice the source depth. Further aspects of this study concern particle motion analysis to locate point sources and the influence of the stratigraphy on particle motions. Furthermore, the deployment strategy of seismic instruments on volcanoes, as well as the direct interpretation of the broad-band waveforms in terms of pressure fluctuations in the volcanic plumbing system, are discussed.

The motivation for analysing time-dependent seismic data is to understand how temporal changes in the elastic wavefield can be generated by external influences such as those which occur during improved oil recovery (IOR).

A similarity between various approaches for determining the effects of fractures or cracks on the seismic wave propagation is found. Fractures may be modeled using infinite planes with a linear slip condition giving rise to the definition of macroscopic fracture compliances. However, natural fracture surfaces can also be simulated by planar distributions of small isolated areas of slip (cracks), or conversely as planar distributions of imperfect interfacial contacts (rough surfaces). An alternative is plane surfaces separated by thin continuous layers of fluid or a soft material. Using these boundary conditions, the fracture compliances are computed for these various models. Consequently, it is possible to relate the measured compliances directly to the microstructural details of the fracture given appropriate a priori information on the fracture surfaces.

The effects of movement of interstitial fluids within a cracked solid are examined. We consider two distinct mechanisms: flow through connections between otherwise isolated cracks, and diffusion into a porous matrix material (Hudson, Liu & Crampin 1996). These models are used to obtain the overall effective elastic constants for a medium whose parameters are considered geologically realistic. We determine the phase velocities and attenuations for seismic waves passing through such a rock and analyse their dependence on the permeability and the background porosity. We find that the properties of and waves are very sensitive to the transport of fluids through interconnected pathways. The limit of zero flow is equivalent to isolated fluidfilled cracks, the opposite extreme of free flow is that of dry or gas-filled cracks. The transitional behaviour which is observed occurs over a range of permeabilities that encompasses those of most oil and gas bearing lithologies. The introduction of fluid flow causes Pwaves to travel slower in the direction across the cracks than parallel to them, as expected intuitively. There is also a marked increase in the attenuation from negligible values to the order of 1. Similar effects arise from the introduction of an equant porosity.

We have developed a method to retrieve overcritical PdP reflections from the top of D'' from the data. The differential travel-time, amplitude ratio and phase advance of the reflected PdP waveform relative to the direct P waveform are accurately obtained. Kirchhoff synthetics have been used to model the reflected waveforms for a set of different incidence angles i and velocity contrasts Delta alpha at the reflector which were then used as reference wavelets for the deconvolution.The results from a Grafenberg data set differ markedly from the values predicted by the one-dimensional PWDK model derived from the same data. However, there are large uncertainties in i and Delta alpha. This could be caused by the presence of significant lateral heterogeneity in D'' or the need to model other phases which may exist such as the diving waves PDP and PDDP. The method is also applied to two other data examples. The trade-offs between different model parameters are examined.

The observation of seismic reflections from the top of D″ is restricted to the epicentral distance range near the critical angle where the amplitude is large and a phase distortion occurs. This phase advance in the reflected wavelet is examined and implications for commonly used seismic processing techniques such as phase picking, stacking and deconvolution are made. Kirchhoff synthetics are generated to study several models of D″ reflectors, such as steeply inclined interfaces and laterally varying velocity contrasts. Observations of strong amplitude variations in seismic reflections, the ‘bounce points’ of which lie well within a Fresnel zone, are successfully explained. In a search for reflections from D″ investigators have exclusively used models of the D″ layer parallel to the core-mantle boundary. We speculate about a steep slope as it may be produced by deposited slab material and model the influence of such features on travel-time and waveform.