Impact of intra-layer velocity and density variations on seismic characterization of a thin layer
View/ Open
Date
30/11/2020Author
AlZawad, Amjad
Metadata
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.