This paper presents a method for elastic and viscoelastic imaging of multicomponent seismic data. The method is based on Claerbout's survey-sinking concept and the (visco)elastic Kirchhoff integral for the displacement field. Assuming a multishot and multireceiver experiment, the migration process is formulated as a wavefield reconstruction problem, using the (visco)elastic Kirchhoff integral twice. First, the receiver coordinates are downward continued. Second, the source coordinates are downward continued. The multicomponent seismic data are treated as a vector wavefield in which the data measurements may be displacement velocity or traction (pressure). The theoretical formulation is based on the viscoelastic Hooke's law and Newton's equation of motion as the physical model for seismic wave propagation. It is valid for linear viscoelastic media with any anisotropic symmetry. When the lowest-order ray approximation is introduced, the migration equation takes a form similar to conventional Kirchhoff migration. To obtain the imaging equations, the downward-continued wavefield is related to the ray-Born approximation. Numerical results are shown from elastic imaging of synthetic and real marine walkaway vertical seismic profiling (VSP) data.

In prestack depth migration using explicit extrapolators, the attenuation and dispersion of the seismic wave has been neglected so far. We present a method for accommodating absorption and dispersion effects in depth migration schemes. Extrapolation operators that compensate for absorption and dispersion are designed using an optimization algorithm. The design criterion is that the wavenumber response of the operator should equal the true extrapolator. Both phase velocity and absorption macro models are used in the wavefield extrapolation. In a model with medium to high absorption, the images obtained are superior to those obtained using extrapolators without compensation for absorption.

The aim of the Iceland Deep Drilling Project is to drill into supercritical geothermal systems and examine their economic potential. The exploratory well IDDP-2 was drilled in the Reykjanes geothermal field in SW Iceland, on the landward extension of the Mid-Atlantic Ridge. The Reykjanes geothermal field produces from a b300°C reservoir at 1 to 2.5 km depth and is unusual because it is recharged by seawater. The well was cased to 3000 m depth, and then angled towards the main up-flow zone of the system, to a total slant depth of 4659 m (~4500 m vertical depth). Based on alteration mineral assemblages, joint inversion of wireline logging, and rate of heating measurements, the bottom hole temperature is estimated to be about 535°C. The major problem encountered during drilling was the total loss of circulation below 3 km depth and continuing to the final depth. Drilling continued without recovering drill cuttings, consequently spot coring provided the only deep rock samples from the well. These cores are characteristic of a basaltic sheeted dike complex, with hydrothermal alteration mineral assemblages that range from greenschist to amphibolite facies, hornblende hornfels, and pyroxene hornfels, allowing the opportunity to investigate water-rock interaction in the active roots of an analog of a submarine hydrothermal system. As they have not yet been sampled, the composition of the deep fluids at Reykjanes is unknown at present. Cold water is currently being injected with the aim of enhancing permeability at depth, before allowing the well to heat up prior to flow tests planned for early 2019. The well has at least two fluid feed zones, a dominant one at 3.4 km depth and a second smaller one at 4.5 km. Extensive geophysical surveys of the Reykjanes Peninsula completed recently allow correlation of geophysical signals with rocks properties and in-situ conditions in the subsurface. Earthquake activity monitored with a local seismic network during drilling the IDDP-2 drilling detected abundant small earthquakes (M L ≤ 2) within the depth range of 3-5 km. A zone at 3-5 km depth below the producing geothermal field that was generally aseismic prior to drilling, but became seismically active during the drilling. The drilling of the IDDP-2 has achieved number of scientific and engineering firsts. It is the deepest and hottest drill hole so far sited on an active mid-ocean spreading center. It penetrated an active supercritical hydrothermal environment at depths analogous to those postulated as the high temperature reaction zones feeding black smoker systems.

Compensation for geometrical spreading is important in prestack Kirchhoff migration and in amplitude versus offset/amplitude versus angle (AVO/AVA) analysis of seismic data. We present equations for the relative geometrical spreading of reflected and transmitted P-and S-wave in horizontally layered transversely isotropic media with vertical symmetry axis (VTI). We show that relatively simple expressions are obtained when the geometrical spreading is expressed in terms of group velocities. In weakly anisotropic media, we obtain simple expressions also in terms of phase velocities. Also, we derive analytical equations for geometrical spreading based on the nonhyperbolic traveltime formula of Tsvankin and Thomsen, such that the geometrical spreading can be expressed in terms of the parameters used in time processing of seismic data.Comparison with numerical ray tracing demonstrates that the weak anisotropy approximation to geometrical spreading is accurate for P-waves. It is less accurate for SV-waves, but has qualitatively the correct form. For P waves, the nonhyperbolic equation for geometrical spreading compares favorably with ray-tracing results for offset-depth ratios less than five. For SV-waves, the analytical approximation is accurate only at small offsets, and breaks down at offset-depth ratios less than unity. The numerical results are in agreement with the range of validity for the nonhyperbolic traveltime equations.

The seismic imaging of salt diapirs in the Nordkapp Basin gave rise to considerable problems in defining their shape and volume. Independent information was added by integrating the interpretation with high resolution gravity and magnetic data. We developed a novel, iterative workflow, separated into sub‐categories: sediments, salt structures, basement and Moho. Distinctions between the sources of the anomalies from different depths was achieved by utilizing the different decay characteristics of gravity, gravity gradiometry and high resolution magnetic anomalies. The workflow was applied to the southern part of the Nordkapp Basin. It started with the sedimentary model derived from seismics, populated with measured densities and magnetic susceptibilities and a starting model for the base salt. The residual after the removal of this model was interpreted in terms of a crustal model, including flexural isostatic calculations for the Moho with the sedimentary load. The residual after the removal of crustal and early sedimentary model was used to tune the salt model. As these major and minor modelling steps depend on each other, an iterative process was applied to stepwise improve the density and magnetic susceptibility model. The first vertical gradient of gravity and the magnetic field were found to give most information about the cap rock of the diapirs. The improvement in salt imaging, integrated with results from controlled‐source electromagnetic and magneto‐telluric modelling is shown for the salt diapir Uranus, where a well, terminated in the salt, constrains the minimum of the depth to base salt.