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Seismic imaging is a key processing in O&G exploration and production. This is the ultimate stage of a long, heavy and complex data processing sequence, which leads to images of the earth’s subsurface used thereafter by geologists for interpretation. The huge amount of data acquired and to process is however a strong constrain on the type of processing which can be reasonably applied. Consequently, the industry has always tried to develop methods to first reduce the amount of data in a summation approach, leading hopefully also to better signal-to-noise ratio, then perform an imaging of the reduced data. But the reduction process (summation) was based on rather simple models of the subsurface, which implied post corrections to hopefully compensate for such approximations.
Nowadays, it is well accepted that the only proper way to treat the data is an iterative model-based processing, where the imaging process itself focus all available information down to a depth model. Such imaging is called Prestack Depth Migration (PSDM) and is the ultimate stage of the seismic processing. But the challenge is still huge, despite incredible developments in computer technology. The actual PSDM codes are designed for the oil industry and lack flexibility with respect to acquisition geometry, interactivity, etc. The competency of NORSAR in ray tracing approaches, known to be flexible in use and efficient, has already led to commercial software which can be used for PSDM applications. But we believe at NORSAR that it is also possible to develop new PSDM tools for other types of applications, especially in a target-oriented ray-based mode, to give more flexible imaging solutions, including interactive analysis tools.
NORSAR was granted in 2007 a new research project funded by the Research Council of Norway and dedicated to seismic imaging (PSDM). Though PSDM imaging has been studied at NORSAR for many years, yielding indeed new modeling approaches such as the SimPLI (PSDM simulator) patented method used in the new SeisRoX software, it is the first time that a project is entirely dedicated to this activity. This gives us the opportunity to demonstrate through prototypes our ideas within PSDM technology. In a tight integration with our other R&D activities (ray tracing, tomography, rock physics), new ideas and concept are developed in order to come up with complementary tools.
As an example, the figure below shows PSDM performed in a so-called anisotropic media, here a tilted-transverse isotropic (TTI) case, and using ray-tracing technology. The latter has indeed been re-designed to better fit the needs in PSDM using a powerful coordinate-transform approach (Iversen and Pšenčík, 2006). TTI data simulated by the latest version of the NORSAR-3D Ray Tracing software were imaged in depth using 3 different models, i.e., an isotropic one, a so-called vertical Transverse isotropic one (VTI) and the true TTI model. The final PSDM images show the reflectors at their proper depth only in the TTI case, as expected. This also illustrates how important it is to have the proper background velocity field in PSDM in order to properly image and locate the geological structures. In that respect, a combined tomography-imaging approach would be a must, and time has also come at NORSAR to work towards such goals.
Prestack depth imaging in anisotropic models. a) model with 6 anisotropic layers of TTI-type. b) Local ray-tracing coordinate system used at NORSAR for efficient calculation of the necessary imaging parameters in anisotropic media with rotation along the raypaths. c) Imaging results using different velocity models, from isotropic on left towards the correct TTI model on right, via a vertical-TI (VTI) model in the middle. The actual reflectors are superimposed in green. From Kaschwich et al. (2006).
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