Anders Sollid

Imaging through gas‐filled sediments using marine shear‐wave data

Marine multicomponent sea-floor data of excellent quality have been acquired over the Tommeliten Alpha field. The most dominating wave modes are interpreted to be conventional compressional PP-waves and converted PS-waves. The most important geophysical problem associated with the Tommeliten Alpha field is the presence of a gas chimney obscuring the conventional 3-D seismic image of the reservoir zone. The converted PS-waves effectively undershoot the gas chimney, leading to substantially improved images of the reservoir. Subsequent interpretation indicates the Tommeliten Alpha structure is a faulted dome.

Scattering‐angle migration of ocean‐bottom seismic data in weakly anisotropic media

Scattering‐angle migration maps seismic prestack data directly into angle‐dependent reflectivity at the image point. The method automatically accounts for triplicated rayfields and is easily extended to handle anisotropy. We specify scattering‐angle migration integrals for PP and PS ocean‐bottom seismic (OBS) data in 3D and 2.5D elastic media exhibiting weak contrasts and weak anisotropy. The derivation is based on the anisotropic elastic Born‐Kirchhoff‐Helmholtz surface scattering integral. The true‐amplitude weights are chosen such that the amplitude versus angle (AVA) response of the angle gather is equal to the Born scattering coefficient or, alternatively, the linearized reflection coefficient.We implement scattering‐angle migration by shooting a fan of rays from the subsurface point to the acquisition surface, followed by integrating the phase‐ and amplitude‐corrected seismic data over the migration dip at the image point while keeping the scattering‐angle fixed. A dense summation over migration dip...

Seismic angle migration

Common image gathers in the offset domain are used extensively in velocity analysis, and AVO/AVA studies using a technique described by Bleistein (1987). Imaged with the correct background velocity model, the events will appear horizontal in the offset gathers. Any curvature or moveout in these gathers can be used as a criterion for updating migration velocities. If the geology is complex and the ray field becomes multipathed, the assumptions made for imaging data in the offset domain are violated. This will especially influence the quality of common image gathers and, hence, make it difficult to perform any form of AVA or velocity analysis. Such complicated problems arise typically in seismic imaging beneath gas clouds, salt, and basalt.

Multicomponent Seabed Seismic Data: A Tool for Improved Imaging and Lithology Fluid Prediction

This paper was selected for presentation by the OTC Program Committee following review of information contained in an abstract submitted by the author(s).

Imaging through gas-filled sediments with marine S-wave data

This paper shows how multicomponent marine data acquired using three component geophones planted into the seafloor could be used to image through a gas filled overburden. Data from the Tommeliten chalk field contains strong S-wave arrivals which are interpreted as P to S-converted waves, where the conversion takes place at the target reflector. Subsequent processing using the Common Conversion Point (CCP) concept produced a section were the major target reflectors could be identified and mapped inside the gas filled area. The conventional 3D seismic data provides unreliable structural images and the interpretation is dubious in an area with an horizontal extent of approximately 2000 m. Using the S-wave data, the horizontal extent of the uncertain area has been narrowed down to approximately 800m.

A Practical Approach to Automated PP Angle Tomography

The use of the differential semblance misfit function on common image point gathers in the angle domain, lends itself to an automated tomographic approach through a gradient based search of the model space. The common image point gathers are created by the AVA-corrected or incomplete generalized Radon transform. We present a real data example of PP scattering events and an approach to regularize the optimization by respecting geological features.

Prestack time migration applied to marine P‐SV seismic waves

Reflectors on a stacked P-SV section often deviate from P-P events in character and amplitude, sometimes making correlation between the two sections difficult. To reduce this problem, the migration scheme can be designed to output P-SV migrated constant-of&et sections in P-P travel time. The data is smoothly compressed in a time variant manner according to the depthand laterally variant ratios. The P-wave RMS velocities utilized in the migration may be obtained from stacking velocity analysis of conventional P-wave data or the vertical particle velocity component aquired at the sea bottom. The S-wave RMS velocities may be estimated via the ratios and Dix’ equation. More accurate S-velocities are achieved by migrating the data to S-S travel time, backing out the S-S velocity function in the migrated data and picking new migration velocities on S-S time scaled gathers.

Reflection‐angle migration in weak TI media

The purpose of true-amplitude migration is to invert for structure and the angle-dependent subsurface reflectivity. Kirchhoff migration is usually implemented as a diffraction-stacking along traveltime curves in the prestack data using some source-receiver coordinates as integration variables. For a given source-receiver offset, it is then a problem how to map migrated data from offset to incidence angle. If the mapping is unique we may use ray-tracing, or as Bleistein (1987) suggested, perform an additional prestack migration using a modified weight function to determine the specular reflection angle of a reflection event.

Offset dependent geometrical spreading for converted waves using constant velocity gradient models

Summary A method for accurately removing the effect of geometrical spreading in time processing schemes is presented. It bases on the approximation of the given rms velocity function by constant velocity gradient media for which the spreading can be calculated analytically. The method is applicable to single-mode and converted wave data. For the latter, high accuracy is only achieved if the converted wave mode is processed together with the PP mode. This allows to properly split the total spreading effect into the contributions along the individual legs.

Parallelization of 2-D And 3-D Depth Migration

We have ported serial codes for 2-D and 3-D seismic depth migration to parallel platforms. The porting was done using portable parallel primitives, such that the same code can be run on a . number of parallel machines, workstation clusters, as well as serial machines. Only small modifications of the serial codes were necessary. Efficient parallel codes were obtained with moderate modification of the serial codes.