EAGE Subsalt Imaging Workshop Cairo 2009

Though ray tracing in its standard forms is not able to produce complete synthetic seismograms, as may be required for processing tests, it may be used in completely different manners and thus providing other type of information useful for the industry. A now classic example is the use of ray-based illumination studies to test surveys for subsalt imaging, where the output are various maps of attributes along given horizons, e.g., so-called hit maps (reflection points), etc. This is especially interesting when testing various survey azimuth geometries. Another technique allows to find azimuth and offset ranges of a survey to illuminate given points on a horizon, e.g., below a salt dome. Going further, an efficient method can also generate migration amplitudes for the same horizons without explicitly generating synthetic seismograms. In a more local approach, a PSDM simulator uses ray-generated Green’s functions to simulate 3D PSDM cubes, including 3D illumination and resolution effects. Such simulated PSDM results allow quickly testing subsalt imaging effects on detailed reservoir targets, without needing to compute synthetic data and process them.

Variable illumination cause significant problems when imaging subsalt or other geologic regions with velocity variations. Much effort is taken to identify and improve the artifacts from these illumination variations, with limited success. A key challenge is that the illumination variations are a strong function of dip angle. Methods based on a single illumination value at each depth location or based on predefined reflector dip will be incomplete. We propose using “density bubbles” to identify the illumination variations as a function of dip. These density bubbles are spheres of moderate density increase inserted in a model used to produce synthetic data. After the synthetic data is processed and migrated, a human can easily see illumination variations as a function of dip. This can be an essential QC mechanism to evaluate different acquisition approaches and imaging approaches. This an effective method to test RTM imaging conditions. And after imaging, these density bubbles can be an effective interpretation tool for the interpreter to guide him on the imperfections of the seismic image produced with the real data that corresponds to the synthetic data.

The challenges associated with sub-salt imaging are well known and well documented within the industry, and the recent successes of imaging beneath salt in the Gulf of Mexico might encourage the optimist to believe that the industry has solutions to all of these problems. These imaging successes have established azimuth-rich towed-streamer acquisition as the method of choice for exploration in the Gulf of Mexico. The azimuth-rich data acquired to date has delivered better illumination, imaging, a higher signal-to-noise ratio, and improved seismic resolution. However, the azimuth-rich towed-streamer acquisition configurations used in the Gulf of Mexico are all multi-vessel, and as such have a limited azimuth range, and very often a compromised near trace offset. The sub-salt imaging challenges can be broken into three major categories, illumination, multiples and imaging, the subject of this paper is to illustrate how a single-vessel azimuth-rich acquisition technique can help all three categories of sub-salt imaging challenges, and has positive advantages over the multi-vessel techniques used in the Gulf of Mexico.

We present a new method for broadband marine acquisition and processing capable of imaging deep targets while maintaining full high frequency bandwidth data. The method uses a deep interpolated streamer coverage approach (DISCover), which comprises a 3D shallow towed-streamer spread, designed to optimize the mid- and high-frequency parts of the bandwidth, together with data simultaneously acquired from a small number of deeper towed streamers. The depth of these deeper streamers is optimized for the low frequencies such that the combined overall bandwidth is enhanced. Because the deep streamers are only going to provide the low-frequency part of the bandwidth, we can more sparsely sample these data thus enabling efficient acquisition scenarios as fewer streamers are required. A 3D case study using this new acquisition method was acquired off the NW Shelf of Australia. The streamer spread consisted of six shallow streamers towed at a depth of 6m and two deeper streamers (below shallow streamers 2 and 5) towed at a depth of 20m. The resulting data exhibit both high resolution and deep penetration for subsalt and sub-basalt imaging, for example. Inversion for acoustic impedance, imaging, and velocity model building, also benefit from the broadband result.

As the exploration for hydrocarbons faces more and more challenging structures, reverse-time migration, which is capable of imaging complex geological settings, becomes the preferred imaging algorithm. Especially for the costly acquired wide-azimuth data sets, application of this high-end imaging method is even more appealing. Recently, reverse-time migration has been extended to transversely isotropic (TI) medium. The rapid advance in computing technology has made its routine application feasible. In this presentation, we will summarize the procedures undertaken at Hess to implement our proprietary reverse time migration software, which focuses on the discussion of numerical dispersion error in isotropic media, the stability and artifact issues in TI media and the noise problem introduced by the imaging condition.

In recent years exploration targets have become more challenging due to their deeper position and higher complexity. In order to map these targets correctly, accurate 3D PSDM is usually required. Interval velocity analysis in such areas becomes a critical process. The major concept of most velocity analysis procedures is that after PSDM, common image gathers (CIGs) should be flat if the migration was carried out with the correct velocity function. When the CIGs contain non-flat events the problem is how to relate this non-flatness to errors in the given velocity function.

The method used to build an anisotropic model for TTI migration is described. The method includes calibrating seismic depth to well depths, estimation of the tilted axis of symmetry of the anisotropy, iterative depth migration and tomography, and salt horizon interpretation. Cycle time is reduced by using a fast 3D, wide azimuth, beam, prestack depth migration. Results are shown on a wide azimuth marine survey in the deep water Gulf of Mexico.

A case study for 3D seismic land data from Mexico uses high-resolution CRS attribute volumes for improving the imaging and model buildung in salt geology. Initial CRS time processing provides both, an initial outline of the salt body, and general information for constructing the depth model. On one hand, sediment and salt areas are more clearly separated by a conspicuous drop of reflection continuity in the CRS time images. On the other hand, the CRS attributes contain abundant information for constructing a reliable velocity depth model by CRS tomography. The smooth CRS depth model is well suited for initial poststack and prestack depth migration (PostSDM and PreSDM). PostSDM transfers the good structural resolution and salt body delineation of the CRS time domain images to depth, thus supporting the initial salt body definition in depth. Furthermore, a CRS-based noise suppression and regularisation of the prestack data improves the iterative sequence of PreSDM imaging and model refinement. The CRS depth processing approach finally leads to a 3D PreSDM volume with a strongly increased resolution and signal-to-noise ratio.

Hammam Faroun area has a particular importance due to its geothermal activity which constitutes the main geothermal resource in Egypt. The area is a part of Sinai Peninsula which considered as sub plate bounded by two seismic active zones (Gulf of Suez and Gulf of Aqaba). Sinai Peninsula, including our study area, is covered by high resolution ground gravity and magnetic survey (by the Egyptian Geological Survey and Mining Authority) in order to be used for mineral, geothermal, and hydrocarbon exploration. In the present study, the gravity and magnetic data were analyzed using new and recent techniques; e.g. Source Edge Detection (SED), to image the subsurface structure which in general controlled the mineral, geothermal, and hydrocarbon exploration. The results of the analysis indicated that the area is dissected by a set of faults trending in the NW direction, parallel to the Gulf of Suez. Additionally, several trends perpendicular to the NW direction can be recognized which could be utilized as a proof for the Gulf of Suez rift. The analysis of the potential field data highly contributed for imaging the subsurface structures of Hammam Faroun area and led to the fact that the area is structurally complex.

Subsalt imaging is seen as one of the most difficult imaging challenges in seismic processing. Although that is a discussion in itself, no one will question the fact that processing seismic for subsalt exploration is very complicated due to the structural complexities associated with salt, the complex multiples, as well as the associated velocity contrasts. When people discuss subsalt imaging challenges, they almost immediately associated these with the deepwater Gulf of Mexico issues. However, each salt basin has its own characteristic set of problems. In this paper we will discuss and illustrate different salt ‘types’ in different parts of the world, each of which has a specific set of issues and potential seismic solutions. We will discuss differences and simularities.