SeismicCity has upgraded its depth imaging to offer prestack depth migration (PSDM) services for full elastic wave equation.
SeismicCity has upgraded its depth imaging to offer prestack depth migration (PSDM) services for full elastic wave equation.
‘Due to computational and algorithmic limitations, until recently most seismic data processing has been carried out under the acoustic assumption. However, a geological rock property model which better matches the response of the earth is an anisotropic elastic solid’ said Dan Kosloff, chief geophysicist of SeismicCIty.
The model correctly predicts the generation of shear waves in addition to the pressure waves which are recorded in any seismic survey, both onshore and offshore. ‘There are numerous practical issues in implementing elastic solution schemes,’ added Allon Bartana, director of R&D for SeismicCity. ‘A large number of variables are required, and for realistic cases these variables do not necessarily fit within a compute nodes’ memory. By use of domain decomposition, and by incorporating optimized numerical schemes these algorithmic and optimization challenges are solved, enabling the move to full elastic imaging in a production environment.’
Elastic migration improves imaging and enables more accurate determination of the subsurface material parameters, said SeismicCity. Using elastic migration, two types of waves are present, P-waves and S-waves which usually travel in different paths. As the subsurface is illuminated by both waves, in areas of poor illumination by one type of wave, there is a chance of good illumination by the other type. ‘Using both P-waves and S-waves in PSDM enable us to finally image low illumination zones beneath salt, helping our clients to achieve more successful drilling results in these challenging areas,’ said Jeff Codd, VP technology for SeismicCity. In addition, shear waves are travelling with a velocity which is different from the pressure waves and therefore its determination will help in resolving subsurface material properties.
David Kessler, president of SeismicCity, said. ‘In the early 1990s ray-based Kirchhoff summation algorithms were used. These were based on the simpler ray trace solution of the wave equation. In the early 2000s we used the wave equation, but due to computer limitations had to use downward propagation algorithms that were limited in the ability to image steep dip structures.
‘Around 2010 we could finally use a more correct solution of the wave equation by industrial implementation of Reverse Time Migration (RTM). However, this was done still using acoustic assumptions (ARTM). As we move to the next decade, we can finally switch to using a correct representation for wave propagation in solid material via a direct and complete solution of the elastic wave equation. This will result in an industry move from the use of Acoustic RTM (ARTM) to elastic RTM (ERTM) as the leading algorithm for PSDM.
‘With depth imaging results that we have seen so far by using the full wave field in PSDM (i.e. both P-waves and S-waves) we are pleased to provide operators with a step-change improvement in the ability to image seismic data in low illumination zones as well as increased seismic resolution by incorporating converted waves in the seismic image.
