MIGRATION AND ATTENUATION OF SURFACE RELATED AND INTERBED MULTIPLES
Zhiyong JIANG, University of Utah
The theories for imaging free-surface multiples, interbed multiples, and filtering high-order multiples are presented and applied to synthetic and field data. Unlike the conventional imaging methods, which only migrate primary reflections and treat multiples as noise, the multiple imaging strategy presented in this dissertation treats multiples as valuable signals and uses them to map the subsurface reflectivity distribution.
There are three main chapters in this dissertation.
Chapter 2. Three methods are presented for imaging free-surface multiples. The first method is model-based and the other two methods use natural information from the observed data. These methods are tested on three synthetic data sets and three field data sets. Numerical results with VSP synthetic and field data show that multiples provide a wider subsurface coverage and more balanced illumination than primary reflections. Results also show that multiples are able to illuminate subsurface structures above the receivers, which is invisible to primaries. An application of the multiple imaging strategy for VSP field data shows that VSP multiples provide a subsurface image comparable to that obtained with surface seismic (CDP) data. The test with CDP synthetic data demonstrates that a weighted summation of the primary and multiple migration images produces an image with fewer artifacts than the primary migration image.
Chapter 3. Two new imaging methods are proposed to migrate interbed multiples: the first method applies Fermat's principle to find the specular reflection point along the interbed multiple generating interface, and the second method sums the diffraction energy from all the diffraction points along the multiple generating interface. This Fermat method is applied to two synthetic data sets and one field data set. The two synthetic tests show that interbed multiples from the upper salt boundary can be used to image the lower salt boundary and subsalt structures, and interbed multiples from the lower salt boundary can be used to image subsalt structures. The results also show that interbed multiples from the lower salt boundary provide a better illumination of subsalt structures than the free-surface multiples. Numerical tests with the field data demonstrate that interbed multiples from the sea bed are able to image salt boundaries. Moreover, interbed multiples are able to image the structures above receivers, which can not be illuminated in conventional primary imaging. The conclusion is that interbed multiple migration should be employed as a tool for salt boundary imaging.
Chapter 4. A major problem with low-order multiple imaging is the interference from high-order multiples. I propose attenuating high-order multiples in multiple imaging. I use two steps to attenuate high-order multiples: first a model-based approach is used to predict the high-order multiples, and then the predicted multiples are subtracted from the data. This strategy is applied to both synthetic and field data sets. Numerical results show that the high-order multiples are effectively attenuated, and the multiple migration images are much cleaner. The multiple filtering process makes the multiple migration method a more practical tool in seismic imaging.