סמינר בחוג לגיאופיזיקה: Estimating and mapping anisotropy in the East Mediterranean Subsurface
Anastasya Yankelson, a PhD defense lecture
Recent advances in drilling technology together with gas discoveries offshore Israel in the beginning of 2000, led to an extensive exploration effort under the deep water of the Levant basin. As it was the first drilling phase in this area, there had been no available information on the physical properties of the subsurface. The targets for the drilling were based totally on isotropic models. As a result, different miss-ties between seismic reflections and well tops were reported all over the basin. In order to understand this phenomena, regional analysis of anisotropy was performed.
Eight 2D Lines together with well data were processed using conventional methods to estimate anisotropic Epsilon and Delta. The processing resulted in the detection of three regions that demonstrated anisotropic behavior. The upper layer is in the Plio-Pleistocene section which is overlying Messinian salt layer. The additional layer is the mass transport complex that is related to Middle Miocene. Deep anisotropy in this unit, is demonstrated in this study for the first time. An additional layer is detected in the shallow water environment where anisotropy is observed due to carbonates buildups.
The main challenge of this research was the insufficient amount of available well information that is conventionally required for depth calibration. The flatness of the seismic events in the prestack domain is usually a sign of the correctness of the used model. In areas such as the Mediterranean Sea, where no prior knowledge on the subsurface properties exist and well information is very limited, two different models can produce results with flat events: a model with lateral velocity variation and an additional one with anisotropic parameters.
To overcome this ambiguity, we investigate a new approach for anisotropy analysis. This study introduces a non-conventional method to discriminate between the effects of seismic anisotropic and lateral velocity variation. The suggested technique utilizes constant depth slices from multi-parameter CIGs. It supplies an additional constrain to the analysis, the scattering dip angle, making the investigation of the subsurface model more detailed. Implementation of this technique succeeded in providing a solution to the ambiguity we were facing in the EMED. It supported a verification technology for our regional anisotropic maps.
The principal conclusion of this work is that applying isotropic imaging is no longer justified in the EMED deep water environment. This study has demonstrated that neglecting seismic anisotropy could consequently produce seismic depth errors hundreds of meters out of the correct location.
מארגן הסמינר: פרופ' איל חפץ