№ 6 (June 2008)
Application of Reverse Time Migration to Complex Imaging Problems in the North Sea
Seismic time and depth migration is commonly carried out using either integral or conventional (one-way) wavefield extrapolation techniques.
By Paul Farmer, Ian F. Jones
Application of Reverse Time Migration to Complex Imaging Problems in the North Sea
Paul Farmer, Ian F. Jones
Seismic time and depth migration is commonly carried out using either integral or conventional (one-way) wavefield extrapolation techniques. Although adequate in many cases, both methods have significant limitations that reduce imaging quality in some instances, especially in geophysically complex domains. In 2005 GXT introduced Reverse Time Migration, RTM, a depth migration technique that overcomes the limitations of these existing techniques. Since then it has been proven on dozens of projects spread throughout the world.
We start by discussing the issues affecting existing methods, and then go on to show how RTM addresses these limitations, illustrated using an example from the North Sea.
Standard shot-based one-way wavefield extrapolation (WE) pre-Stack Depth Migration techniques image the subsurface by continuing the source and receiver wavefields for each shot downward in depth. An image is formed by cross correlating these two wavefields at each depth and each position, and the partial images formed for all shots are summed to form the final image. A key, limiting, assumption made is that the source and receiver wave fields only travel in one direction along the direction of extrapolation: forwards for the source wave-field, and backwards for the receiver or scattered wavefield. In practice, each of these wave-fields will generally travel both up and down if the velocity model is complex or exhibits strong velocity gradients. This produces turning (or diving) rays and multiples. In addition, approximations in the one-way WE techniques generally limit the allowed dips in the final image to less than 70 degrees. Steep dips and turning rays are usually imaged using Kirchhoff techniques, but these fail when either the source or receiver wavefields becomes sufficiently complex that multipathing occurs. The RTM migration technique handles these problems by using the two-way acoustic wave equation without approximations or assumptions. The source wavefield is propagated forward in time and the recorded receiver wavefield is propagated back in time, hence Reverse Time Migration, though note that it is a depth migration technique. Because RTM uses the exact acoustic wave equation it can image data through velocity structures of arbitrary complexity without error or dip limitations, and even has the potential to image multiples when the multiple generators are present in the model.
The RTM technique is computationally very demanding, and as such was considered economically impracticable. However, the recent step-change advances in computing power have made it commercially viable and GXT have been to the fore in applying it on a large number of projects in the Gulf of Mexico, West Africa, and the North Sea. This has initially been with towed streamer data, but it is equally applicable to multi or wide azimuth streamer data, as well as OBC scenarios. GXT also have considerable experience in using RTM for VSP imaging. RTM has the potential to migrate all multiples, however practice has shown that multiples are perhaps more effectively handled before RTM application, and GXT have found that employing their proprietary 3D SRME anti-multiple tools has had a significant effect on imaging quality.
3D Field Data Example
We now illustrate the effectiveness of anisotropic 3D RTM on a classic “mushroom-shaped” salt dome typical of the North Sea. A “final” 3D model was available from a prior WE migration, as was the migration result, Fig. 1 and 2 respectively show the velocity-depth model and Kirchhoff preSDM result for a selected crestal line, where the deeper (sub-chalk) targets are not adequately imaged, due in part to multipathing issues.
Forward modelling was applied to predict expected outcomes, specifically 2D ray tracing through a model of a crestal line from this salt body. The aim was to ascertain what classes of energy illuminate the steep salt flank events. Fig. 3 shows the ray trace results for PP events passing through the top salt, and illuminating the salt flanks. Fig. 4 shows the ray paths for the PS-SP arrivals, while Fig. 5 shows ray paths for double bounces (or prism waves) that reflect off both the salt flanks and the flat lying adjacent events. The energy illuminating the salt flank is dominated by double bounce arrivals. Using our conventionally-derived final 3D velocity-depth model (Fig. 1) we ran an RTM, limiting the migration frequency to 17Hz. The results of the RTM clearly indicated the inadequacies of the conventional model, and we proceeded to refine the model based on iterative RTM model update. Fig. 6 and 7 show respectively a one-way wave extrapolation preSDM and corresponding RTM for a crestal line (using the “final” model from the prior project). It should be noted that there are some differences in the precise processing sequence of these results, however, these differences have no impact on the conclusions drawn here.
We can see from the RTM that we have very steep salt flank arrivals, possibly from an upturned chalk interface. Ray trace studies concluded that these steep events probably result from double-bounce illumination. The original salt interpretation in this model, based on WE results, looks to be incorrect near the edges of the salt, especially on the right hand side. The RTM result indicates that the salt is probably a bit less wide, with near-vertical edges below a slight overhang. Furthermore, the RTM result indicates that the surrounding flat horizons probably turn-up sharply to abut the salt, rather than “rolling-through” the model with a gentle anticlinal slope, as previously modelled.
Complex bodies such as salt domes are illuminated by many wave paths that cannot be imaged by conventional one-way wave equation techniques. Significant improvement can be achieved both in the model building and final migration by employing the two-way Reverse Time Migration technique. The combination of model building and migration is the key to successful imaging. Iterative application of RTM can be used to delineate salt geometries in areas where both Kirchhoff and one-way wave equation methods fail.
ION and LARGEO Announce Geophysical Alliance
ION and LARGEO, a Moscow-based seismic data processing company, have formed an alliance to provide advanced imaging services for seismic data acquired in the Russian market.
Launch and “Buzz” in St. Petersburg
The official signing took place at ION’s booth during the EAGE’s Bi-annual Trade Show and Technical Conference in St. Petersburg on April 8, 2008. The alliance ceremony was represented by key officials from both companies and the agreement was signed onsite by LARGEO General Director, Andrei Elistratov and ION CEO, Bob Peebler.
After the proceedings, customers and guests were treated to a champagne reception and details of the partnership were explained by ION host, Jean Januard, Vice President, Russia and CIS countries. Immediately following the briefing by Januard, there was plenty of opportunity for informal discussions between ION and LARGEO personnel, as well as many other conference guests who attended the event, whilst sipping champagne and enjoying great hospitality.
Local Capabilities, International Expertise
The alliance, known as LARGEO-ION, combines the technological strengths of ION’s GX Technology (GXT) Imaging Solutions group with the local market knowledge and extensive regional processing experience of LARGEO to bring best-in-class imaging services to E&P firms operating in Russia.
LARGEO was established in 2004 and since its inception, has rapidly gained a presence in the highly competitive Russian market. The company’s client base includes Rosneft, LUKOIL, Sibneft, RITEK, YUKOS, ONGC, Shell, Gaither Petroleum and other Russian and foreign oil and gas companies. The company’s key strategies focus on technical excellence and response service, which contribute to LARGEO’s success in tackling the most complex imaging projects in Russia.
The LARGEO-ION Imaging Center will be staffed mainly by LARGEO geophysicists who have been trained by GX Technology at other geophysical service centers around the world.
“We believe our collective processing experience, advanced technology, market knowledge and personnel will allow LARGEO-ION to deliver the highest quality seismic imaging with reduced turnaround time. The opening of this Russian processing center marks a key milestone in the global expansion of GXT’s data processing services,” stated Nick Bernitsas, Senior Vice President of ION GXT Imaging Solutions.
A Logical Union
Russia is the world’s second largest oil producer and the largest holder of proven natural gas reserves. The Russian market attract vast numbers of E&P participants with keen interest from major national and international oil and gas companies.
Strategically located in Moscow, the center will deliver a wide range of advanced seismic data processing services with an initial focus on data acquired in the marine environment.
Imaging services will use the latest Linux-based server hardware and will include:
• Pre-stack depth and time migration;
• Reverse time migration (RTM);
• Azimuthal velocity analysis;
• Full-wave imaging;
• AVO and inversion;
• Velocity modelling;
• Data conditioning.
“The alliance between our two companies will create one of the most powerful seismic data processing centers in Russia. We believe LARGEO-ION will be capable of delivering the highest quality seismic images and will allow our E&P customers to rapidly take advantage of the latest data processing innovations such as reverse time migration and full-wave imaging,” stated Alexander Yakovlev, LARGEO General Manager.