Moritz M. Fliedner

Integrated wide‐angle and near‐vertical subbasalt study using large‐aperture seismic data from the Faeroe—Shetland region

Acquiring large-aperture seismic data (38 km maximum offset) along a profile crossing the Faeroe–Shetland basin in the North Atlantic enables us to use wide-angle reflections and refractions, in addition to conventional streamer data (0–6 km), for subbasalt imaging. The wideangle results are complemented and confirmed by images obtained from the conventional near-vertical-offset range. Traveltime tomography applied to the wide-angle data shows a low-velocity layer (3.5–4.5 km/s) underneath southeastward-thinning lava flows, suggesting a 2.5–3.0-km-thick sedimentary layer. The velocity model obtained from traveltime tomography is used to migrate wide-angle reflections from large offsets that arrive ahead of the water-wave cone. The migrated image shows base-basalt and sub–basalt reflections that are locally coincident with the tomographic boundaries. Application of a new multiple suppression technique and controlled stacking of the conventional streamer data produces seismic sections consistent with the wide-angle results. Prestack depth migration of the near-vertical offsets shows a continuous base-basalt reflection and a clearly defined termination of the basalt flows.

Seismic structure of basalt flows from surface seismic data, borehole measurements, and synthetic seismogram modeling

We use the wide‐angle wavefield to constrain estimates of the seismic velocity and thickness of basalt flows overlying sediments. Wide angle means the seismic wavefield recorded at offsets beyond the emergence of the direct wave. This wide‐angle wavefield contains arrivals that are returned from within and below the basalt flows, including the diving wave through the basalts as the first arrival and P‐wave reflections from the base of the basalts and from subbasalt structures. The velocity structure of basalt flows can be determined to first order from traveltime information by ray tracing the basalt turning rays and the wide‐angle base‐basalt reflection. This can be refined by using the amplitude variation with offset (AVO) of the basalt diving wave. Synthetic seismogram models with varying flow thicknesses and velocity gradients demonstrate the sensitivity to the velocity structure of the basalt diving wave and of reflections from the base of the basalt layer and below. The diving‐wave amplitudes of the...

Automated velocity model building with wavepath tomography

Wavefield-continuation migration generally is recognized as superior to Kirchhoff methods in complex velocity models, such as below rugose salt bodies. It accounts for multipathing, sharp velocity contrasts, and the limited bandwidth of seismic wave propagation. Wavepath tomography builds the velocity model in a way that is consistent with the wavefield-migration operator. Traveltime residuals are back-projected along a wavepath instead of rays. The actual wavefield-continuation operator is used to represent the wave propagation between surface source/receiver pairs and subsurface reflection points. A wavepath is obtained by multiplying impulse responses from a surface location and a reflection point. The inversion matrix is kept to a manageable size by restricting the wavepath to the first Fresnel zone. The considerable expense of computing a single wavepath kernel in comparison to ray tomography is partially offset by the smaller number of back projections necessary to sample the velocity model adequately. We have used wavepath tomography to build subsalt velocity models using 2D synthetic data. Wavepath tomography was implemented in 3D.

Velocity model building by wavefield-continuation imaging in the deepwater Gulf of Mexico

Wavefield-continuation-based migration algorithms that downward extrapolate the 3D prestack wavefield (commonly known as “wave-equation migration”) have been recently shown to produce better imaging results than Kirchhoff migration in many synthetic and real data cases (Popovici, 2000). Wavefield-continuation methods are potentially more accurate and robust because they are based on the full wave equation and not on an asymptotic solution based on ray theory. In addition, wavefield-continuation methods handle multipathing naturally in contrast to Kirchhoff methods, focusing and defocusing effects of velocity variations are correctly modeled, antialiasing is handled implicitly, and amplitudes are consistent with the wave equation.

Using Wide-angle Seismic Data For Basalt And Sub-basalt Imaging

The imaging of basalt flows and underlying structures can be improved by using data recorded at long offsets. Travel-times and amplitudes of wide-angle refractions and reflections allow the construction of better velocity models. The high near-critical amplitudes of wide-angle seismic data and the absence of low-velocity multiples can be exploited in imaging basalt and sub-basalt arrivals by pre-stack migration of selected portions of the wide-angle wavefield.

Seismic velocity structure of basalt flows

SUMMARY Basalt flows form a significant impediment to seismic imaging of structures beneath them. This arises both from the internal layering of the basalts, which may generate complex inter-bed multiples and interference patterns, and from the large impedance contrasts where high-velocity basalts overly low-velocity sediments. By making use of the wide-angle arrivals, both velocity and structural information can be derived from the refracted and reflected waves that pass through the basalts. We show examples of wide-angle seismic data recorded from the Atlantic volcanic margin, using two-ship acquisition to attain large offsets, and of normal incidence data from stacked lava flows in Iceland. Both are modeled using synthetic seismogram methods. We summarize the velocity structure and gradients of volcanic sequences by a compilation taken from sub-surface measurements around the world.

Wave Equation Imaging Comparisons: Survey Sinking Vs. Shot Profile Methods

We examine the differences and similarities between the two common categories of wave-equation migration by looking at their mathematical formulations and examining their imaging results. We compare different implementations of shot profile and double square root downward continuation imaging in the context of the overall imaging challenge, including image accuracy, velocity model building, and the ability to generate prestack gathers. We examine the strengths and advantages of the methods by considering the approximations that go into them, the resulting images, and the relative costs of the methods.

Fast Stochastic Inversion of Marine CSEM And Seismic Data With the Neighbourhood Algorithm

Stochastic (“Monte Carlo”) inversion methods, which do not involve the calculation of misfit function gradients, are now gaining interest. One such approach is via the Neighbourhood Algorithm (Sambridge, 1999a) allows to replace forward modeling for some of the examined models by the construction of an approximate error surface. This is accomplished with the use of Voronoi cells, whose dimension equals the number of parameters describing a given forward model. A brief description of the theory and its underlying assumptions is then illustrated with two examples. The first is a 2-layer gas reservoir overlain by water, while the second is based on a single depth profile from the SEG SEAM model, which intersects a reservoir partially filled with oil.

Fast velocity model building by plane‐wave migration in tilted coordinates, automated volume‐based picking, and tomography

In tomography, reducing the turnaround time for each velocity iteration remains a major challenge. Ideally, we would like to have a fast and accurate migration technique combined with a fast and accurate tomography. Then, the interpreter can spend more time QCing the results and make geophysical decisions. With this in mind, we propose a workflow combining plane-wave migration (PWM) in tilted-coordinates and a tomographic scheme with automatic volume-based picking.

Building high‐resolution velocity models in the Gulf of Mexico with wave‐equation depth migration

Obtaining an accurate velocity model is fundamental to successfully imaging complex salt bodies in the deep Gulf of Mexico. With the introduction of faster, full volume wavefield solutions that output finely-sampled angle gathers, velocity models can be constructed allowing the full potential of the wavefield method to be applied to our most complex subsurface problems. We present a case history of building a high-resolution velocity model in the Gulf of Mexico, using multiple iterations of wave-equation migration and angle common image gathers (ACIG). We show how in certain instances picking in depth slices can build more detail in the velocity model, and how we integrated well log information and other stratigraphic geological information into the velocity model in collaboration with the interpreter team, to produce an optimal depth migrated image.