Ian F. Jones

Seismic imaging in and around salt bodies

Seismic imaging of evaporite bodies is notoriously difficult due to the complex shapes of steeply dipping flanks, adjacent overburden strata, and the usually strong acoustic impedance and velocity contrasts at the sediment-evaporite interface. We consider the geology of salt bodies and the problems and pitfalls associated with their imaging such as complex raypaths, seismic velocity anisotropy, P- and S-wave mode conversions, and reflected refractions. We also review recent developments in seismic acquisition and processing, which have led to significant improvements in image quality and in particular, reverse time migration. We tried to call attention to the form, nature, and consequences of these issues for meaningful interpretation of the resulting images.

Application of Anisotropic 3D Reverse Time Migration to Complex North Sea Imaging

Following completion of model building, amplitude preserving 3D depth migration is usually performed using a Kirchhoff scheme for modest structural problems with steep dips. For structures where multi-valued ray-paths exist (e.g. complex salt bodies), we generally use a oneway Wavefield Extrapolation (WE) algorithm instead. However, more recently, full two-way solutions of the wave equation, such as Reverse Time Migration (RTM) have become commercially available: - these are suited for highly complex environments, where both steep dips and multipathing are an issue. Standard shot-based one-way WE preSDM techniques image the subsurface by extrapolating the source and receiver wavefields for each shot. The imaging condition is invoked by cross correlating these two wavefields at each depth level, and then summing the contributions from all shots in the aperture to form the image. One of the assumptions made in using this technique is that the wavefields travel along the direction of extrapolation only in one direction: downwards for the source wavefield, and upwards for the receiver or scattered wavefield.

Anisotropic ambiguities in TI media

The consequences of ignoring polar anisotropy vary depending on the degree and type of anisotropy. Typically, using isotropic imaging in an anisotropic medium results in mis-ties between the preSDM and the well depths. Such mis-ties, in some extreme cases, can exceed 10% of the true depth (for example, in the Franklin-Elgin field operated by TFE in the North Sea there is a 600m mis-tie at a depth of 5km). In addition to the vertical depth error, there is also a lateral shift, most pronounced for the steepest dips, and noticeable in fault surface reflections. Experience has shown that one cannot image simultaneously flat and steep dips with an isotropic velocity field.

A modeling study of preprocessing considerations for reverse-time migration

Much of the thinking behind conventional geophysical processing assumes that we wanted to image energy that propagates down from the surface of the earth, scatters from a reflector or diffractor, and then propagates back up to the recording surface without being reflected by any other feature. Such travel paths conform to the assumptions of one-way wave propagation, and most contemporary migration schemes are designed to image suchdata.Inaddition,themoveoutbehavioroftheseprimaryreflection events in the various prestack domains is well understood, and many of our standard data-preprocessing techniques relied on the assumption that this behavior adequately describes the events we wanted to preserve for imaging. As a corollary, events that do not conform to this prescribed behavior are classifiedasnoise,andmanyofourstandardpreprocessingtechniques were designed to remove them. We assessed the kinematics of moveoutbehaviorofeventsthatarisefromtwo-waywavepropagationandtheeffectofcertainpreprocessingtechniquesonthose events. This was of interest to us because the recent rapid increase in available cost-effective computing power has enabled industrialimplementationofmigrationalgorithms—particularly reverse-time migration—that in principle can image events that reflect more than once on their way from source to receiver. We used2Dsyntheticdatatoshowthatsomeconventionaldata-processingsteps—particularlythoseusedinsuppressionofcomplex reverberations “multiples”—remove nonreverberatory primary events from seismic reflection data. Specifically, they remove events that have repeated or turning reflections in the subsurface such as double-bounce arrivals but that otherwise are imageableusingreverse-timemigration.

Resolving near-seabed velocity anomalies: Deep water offshore eastern India

Imagingindeep-waterenvironmentsposesaspecificsetof challenges, both in data preconditioning and velocity model building. These challenges include scattered, complex 3D multiples,aliasednoise,andlow-velocityshallowanomalies associatedwithchannelfillsandgashydrates.Wedescribean approach to tackling such problems for data from deep water offtheeastcoastofIndia,concentratingourattentiononiterativevelocitymodelbuilding,andmorespecificallytheresolutionofnear-surfaceandothervelocityanomalies.Intheregion under investigation, the velocity field is complicated by narrowburiedcanyons500 mwidefilledwithlow-velocity sediments,whichgiverisetoseverepull-downeffects;possible free-gas accumulation below an extensive gas-hydrate cap,causingdimmingoftheimagebelowperhapsasaresult of absorption; and thin-channel bodies with low-velocity fill. Hybrid gridded tomography using a conjugate gradient solver with 20-m vertical cell size was applied to resolve small-scale velocity anomalies with thicknesses of about 50 m. Manual picking of narrow-channel features was used to define bodies too small for the tomography to resolve. Prestack depth migration, using a velocity model built with a combination of these techniques, could resolve pull-down and other image distortion effects in the final image. The resulting velocity field shows high-resolution detail useful in identifying anomalous geobodies of potential exploration interest.

Prestack Depth Migration and Velocity Model Building

During the past decade, a radical change has taken place in the way seismic processing delivers a subsurface image. Previously, we employed a purely linear processing sequence, moving from data preconditioning to velocity estimation and culminating in a single time-migrated 3D image. The work of the contractor’s data processor was separate and distinct from that of the client’s interpreter.

Imaging deepwater salt bodies in West Africa

The deep offshore of the Gulf of Guinea is a major challenge to seismic processing and imaging techniques, due to the complexity of the salt body structures (Figure 1). Even though Kirchhoff prestack time migration (preSTM) can image the top of salt and the wider, simple sedimentary basin reflections, it fails elsewhere.

Continuous 3-D preSDM velocity analysis

Over the past few years, many techniques have been presented for updating the velocity depth model required for full-volume 3-D prestack depth migration (preSDM).

Tutorial: Velocity estimation via ray-based tomography

Tomographic inversion forms the basis of all contemporary methods of updating velocity models for depth imaging. Over the past 10 years, ray-based tomography has evolved to support production of reasonably reliable images for data from complex environments for models with modest lateral velocity variation scale lengths. In this tutorial, the basics of ray-based tomography are described along with the concepts behind full waveform tomography.

Tutorial: migration imaging conditions

Migration of seismic data is the process that attempts to build an image of the Earth’s interior from recorded field data, by repositioning these data into their ‘true’ geological position in the subsurface, using various numerical approximations of either a wave-theoretical or ray-theoretical description of the propagation of sound waves in the subsurface. This migration can be described as being performed in a number of stages, both for ray and wave-extrapolation based methods. The final stage of the migration process is that which forms the image, via what is known as an imaging condition. In this tutorial, I will outline the various methods involved in forming imaging conditions, primarily for the case of wave-extrapolation methods, and describe some of the techniques used to build gathers of pre-stack-migrated data for use in post-migration velocity analysis.