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Module 2: Wave Generator and Seismic Modelling


MODULE 2: WORKFLOW


MODULE 2: RESULTS & DISCUSSION

In acoustic modelling, the model reflects the acoustic impedance properties of layered rock. The acoustic impedance of rock can be obtained by multiplying the compressional wave velocity of the rock and its density. The nature of the compressional wave, P-wave is controlled by a parameter which known as bulk modulus and this modulus deals with the volume change of the medium as the wave travel through it. The impedance contrast at the geological interface determines how much the energy is reflected and transmitted through the layers. The greater the impedance contrast, the greater the wave energy that will be reflected. As shown in Figure 6, the bright colour of amplitude at the top of part of model are due to high acoustic impedance contrast between the layers. The diffraction tail and apex of the diffractions was also observed which might represent the boundary between the salt body and the strata. Furthermore, the wave also loss its energy as going deeper, resulting in losing amplitude as shown in the Figure 6. To maintain the simplicity of the modelling process as well as to identify the nature of propagation more effectively, no multiple is allowed to generate (invisible reflector) in the modelling.


Figure 5: Salt dome model.



Figure 6: Salt dome 2D acoustic modelling.

In the elastic modelling, the process considers two types of wave; shear wave (S-wave) and compressional wave (P-wave) which represent the reality of the seismic acquisition. When the source is fired, the compressional wave hits the geological interface, generating reflected and transmitted P-wave as well as reflected and transmitted S-wave. This phenomenon is known as wave transmission. S-wave is controlled by the shear modulus which deals with the change of shape of the medium as the wave travel through it. S-waves is generally important for fracture analysis and gas cloud image improvements as it is not affected by gassy media. Based on the result of the elastic modelling as shown in the Figure 7, many seismic events are recorded such as direct arrival, first break and reflection and the image produced is also clearer compared to acoustic modelling as the shape of the top part of salt dome is more obvious.




Figure 7: Salt dome 2D elastic modelling.


In ray tracing modelling, each ray is calculated as a string of points progressing with time and normal to the wavefront or in other word, it is a form of estimated wave propagation with high frequency approximation. This kind of the modelling however restricted to the geometrical optical theory which cannot explain most wave phenomenon such as diffraction and other effect. The accuracy and computational process is affected by complexity of model and wide incidence. The higher the complexity of the model, the lower the resolution of the ray tracing modelling. This problem is supported by the results obtained after performing the modelling process as shown in Figure 8. As we can see in the figure, the shape of the top salt dome is not properly created as in elastic modelling due to the complexity of the salt dome model. Besides that, the diffraction apex is also not clearly recorded in the image.




Figure 8: Salt dome 2D ray tracing modelling.






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