Impact of intra-layer velocity and density variations on seismic characterization of a thin layer

Abstract

Seismic exploration has progressed greatly in the past 75 years due to advances in seismic acquisition and processing. One interpretation technique used by geoscientists is measuring amplitude-variations-with-offset (AVO). AVO analysis helps in understanding elastic property changes, lithology identification, rock properties and pore fluid properties to assist in gas and oil exploration. Interpretations of AVO responses could be complicated by the presence of thin layers which cause tuning. Tuning occurs when the thickness between two layers is very small to the extent that it would be hard to distinguish the boundaries between two interfaces. The main focus of my thesis is to investigate the effect of varying velocity and density within a thin layer on seismic characterization using tuning curves and AVO analysis. For my study, I chose the Primary Pulse Method. This method considers gradual changes in velocity and density between two welded elastic half spaces but ignores the effect of multiples. I coded the equations of the Primary Pulse Method then compared my results with the published results and got good similarities. I extended the work of the published method to investigate the impact of varying velocity and density on waveform shapes. The results showed that the intra-layer velocity and density changes do have an impact on waveform shapes for the top and bottom of a transition zone model. My investigation of tuning curves indicated that the Primary Pulse Method results diverge as thickness tends to zero. I hypothesized that this effect could be a result of ignoring multiples in the Primary Pulse Method and confirmed this by comparing it with the Reflectivity Method. To asses the practical implications of these results, I analyzed seismic responses from a well log data set. The Primary Pulse Method still gave unrealistic high amplitudes which confirms the importance of ignoring multiples. On the other hand, one of the advantages of the Primary Pulse Method is considering the effect of ray bending which my results showed it could have a potential impact on AVO analysis. I concluded that varying velocities and densities can have implications on thin layer characterization but I recommend that the Primary Pulse Method should not be used to study thin layers.