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I am Nur Khalisa Binti Abdul Hamid. Matric number: 20381. Welcome to my blog Mr. Halim! Hope you like it!
For our lab assignment, as a student who is taking Exploration Geophysics as a major on the final semester, we are expected to go through every lab module and understand the concept of model building, wave propagation, velocity and migration which is part of seismic wave imaging analysis. The software that we use is called the Tesseral software which is used for all kinds of geological-geophysical workflows ranging from academic research up to exploration geophysics. Not only that, it can also be used as a learning tool as available here in UTP.
There are a total three (3) modules for this assignment in which we have been provided with an instruction and procedure to guide us getting familiar to Tesseral software.
MODULE 1: BUILDING A SUBSURFACE MODEL
The objective of this lab session is to get familiarize with Tesseral environment by building a structure model and designing seismic surface acquisition parameter.
1) Velocity Model
The model is first being defined by its region boundary as shown on Figure below. This is done by creating a gradual linear velocity gradient as the base of the model framework. The main important panels are the database pane, modelling qizard, and graphic pane.
Next is to run the acquisition WIZARD in which the receiver position is fixed as shown in the figure below.
2) Polygon
Polygon was define in order to represent the structure required. The shape of the polygon is created as shown in the figure below. Top of the polygon is < 1000m depth.
3) Subsurface Model
The structure velocity model in designed as figure below in which different velocities represents different sediments.
Seismic acquisition was evaluated as follow:
a. 1 source (center) and receivers of 50m interval.
b. 1 source (center) and receivers of 20m interval.
c. 2 source (50m interval) and receiver of 50m interval.
d. Create a VSP shot with 3 source (200m interval) and receiver of 30 interval.
Defining the correct acquisition parameter is very important as it gives a great impact on processing techniques especially on S-wave velocity profile. The shear wave velocity of soils is a vital parameter in geotechnical design. It is, for example, widely used for seismic design criteria. Uncertainties in the choice of Vs can have large consequences on project economy and design methods in practice.
The goal of forward modelling is want to determine the seismic trace as calculating the amplitude impedance by multiplying reflection coefficient and wavelet. In contrast, inverse modelling is when we are calculating the wavelet from known seismic trace which is more complicated. This scenario is similar with velocity forward and inverse modelling in which forward modelling is when we want to get the output by defining the acquisition parameter whereas inverse modelling is when the output is known, what we are finding is the input/acquisition parameter.
MODULE 2: GENERATING AND MODELLING WAVE
The objective of this lab session is to generate an acoustic model.
1) 2D Acoustic Modelling
Wave propagation is generated with parameters (scalar, acoustic, elastic) of different physical properties distribution. The acoustic equation modelling is generated by simply clicking on "Run Modelling" with specific settings being set. The modelling properties is changed from 2D Acoustic to 2D Elastic, 2D ray tracing.
2) 2D Elastic Modelling
This is done, again by simple clicking the Run Modelling, then 2D Elastic setup is being created as shown below. After that, modelling is run.
3) 2D Ray Tracing (Eikonal)
This is done, once again by simple clicking the Run Modelling, then 2D Ray tracing (Eikonal) setup is being created as shown below.
4) 2D Velocity Model
This is done from the generated 2D acoustic, elastic and eikonal ray tracing modelling. 2D velocity model for a salt dome is built within 3000m length and 3000m depth. Acquisition geometry is used with a single source with 50m receivers' interval.
Acoustic medium model effectively approximates 2-D 2.5-D and 3-D wave effects of seismic energy propagation in a real geological situation. Elastic medium model permits to precisely and consistently model 2-D 2.5-D and 3-D seismic energy propagation in the solid medium, including all wave effects appropriate to geological media, such as wave P-S and S-P conversions. In case of marine observations user can model true effects of water-bottom discontinuity. On the other hand, ray tracing is generated when each ray is calculated as a string of points along the timefield gradient. Ray-tracing algorithms emulate the wave propagation nominally, as the infinite-frequency approximation of the wave propagation in relatively smooth heterogeneous media. Rays may be treated separately, for example, for tomography applications. Different ray clusters can be separated for analysis
MODULE 3: PRE-STACK AND POST STACK MIGRATION
The objective of this lab session is to design and understand simple surface data acquisition to by performing velocity analysis. This is done by designing a simple structure as parameter follow:
- Frequency = 100Hz
- Ricker wavelet
- 1st Polygon (Vp = 1500 m/s), 2nd Polygon (Vp = 2000 m/s)
Acoustic Modelling was generated from 1st shot point until 101st shot point. Figure below are showing a dipping model of acquisition geometry: moving receiver array, same number of sources and 48 geophones with spacing of 12.5m.
Dipping model
After that, velocity and migration model was run. Then, both Post-stack migration and Pre-stack migration is run for salt dome model (as in Module 2) as shown in figure below.
Before and after pre-stack migration.
Before and after post-stack migration
A better seismic image in terms of its positioning is seen after the true subsurface model is being migrated in both pre and post-stack. This is due to the redistribution of energy which are able to produce better/clear position of subsurface image. Besides that, pre-stack migration usually require to image significant structure complexity while post-stack migration is used for imaging simple to moderate structure complexity. Therefore, that explains the minimum difference of image produced before and after migration as the dipping angle showed earlier represents a simple structure.
Velocity Model
Seismic image of salt dome before any migration
Velocity model of salt dome
(a) Before pre-stack migration (b) after pre-stack migration is applied
(a) Before pre-stack migration (b) after pre-stack migration is applied
It can be seen that the dipping events were not properly positioned besides absence in collapsing of diffraction events before the seismic image was migrated. This caused an event which we call it as an overlapping event (black box). However, after migration was applied, all of the stated problems were solved.
A major difference in migration algorithms arises from the way the velocity field is utilised. These assumptions led to time-migration - a process which collapses diffractions and moves dipping events toward the true position but leaves the migrated image with a time axis which must be depth converted at a later stage.
Time migration assumes that the diffraction shape is hyperbolic and ignores ray bending at velocity boundaries which depth migration assumes that the arbitrary velocity structure of the earth is known and will compute the correct diffraction shape for the velocity model.
If the velocity model for the depth migration is incorrect then the migration will be incorrect and the error may be difficult to detect if the migration is performed post-stack. Prestack depth migration will provide an error estimate of the migrated result.
Post-stack depth migration is often performed for reasons of economy but pre-stack depth migration is almost always required since it is almost impossible to define an accurate velocity model using purely post-stack processing.
The time-migration is always incorrect, but in practice the results turn out to be resilient to a large variation of different velocity models and geological structures and can be very useful for an initial interpretation such as for building models for depth migration. The interpreter will often treat a time-migrated section as a geological image. Time migration is very fast and is robust to errors in the velocity model. Further, errors in the shallow velocity model do not affect imaging of deeper structures.
Polygon was define in order to represent the structure required. The shape of the polygon is created as shown in the figure below. Top of the polygon is < 1000m depth.
3) Subsurface Model
The structure velocity model in designed as figure below in which different velocities represents different sediments.
Seismic acquisition was evaluated as follow:
a. 1 source (center) and receivers of 50m interval.
b. 1 source (center) and receivers of 20m interval.
c. 2 source (50m interval) and receiver of 50m interval.
d. Create a VSP shot with 3 source (200m interval) and receiver of 30 interval.
Defining the correct acquisition parameter is very important as it gives a great impact on processing techniques especially on S-wave velocity profile. The shear wave velocity of soils is a vital parameter in geotechnical design. It is, for example, widely used for seismic design criteria. Uncertainties in the choice of Vs can have large consequences on project economy and design methods in practice.
The goal of forward modelling is want to determine the seismic trace as calculating the amplitude impedance by multiplying reflection coefficient and wavelet. In contrast, inverse modelling is when we are calculating the wavelet from known seismic trace which is more complicated. This scenario is similar with velocity forward and inverse modelling in which forward modelling is when we want to get the output by defining the acquisition parameter whereas inverse modelling is when the output is known, what we are finding is the input/acquisition parameter.
MODULE 2: GENERATING AND MODELLING WAVE
The objective of this lab session is to generate an acoustic model.
1) 2D Acoustic Modelling
Wave propagation is generated with parameters (scalar, acoustic, elastic) of different physical properties distribution. The acoustic equation modelling is generated by simply clicking on "Run Modelling" with specific settings being set. The modelling properties is changed from 2D Acoustic to 2D Elastic, 2D ray tracing.
2) 2D Elastic Modelling
This is done, again by simple clicking the Run Modelling, then 2D Elastic setup is being created as shown below. After that, modelling is run.
3) 2D Ray Tracing (Eikonal)
This is done, once again by simple clicking the Run Modelling, then 2D Ray tracing (Eikonal) setup is being created as shown below.
4) 2D Velocity Model
This is done from the generated 2D acoustic, elastic and eikonal ray tracing modelling. 2D velocity model for a salt dome is built within 3000m length and 3000m depth. Acquisition geometry is used with a single source with 50m receivers' interval.
Acoustic medium model effectively approximates 2-D 2.5-D and 3-D wave effects of seismic energy propagation in a real geological situation. Elastic medium model permits to precisely and consistently model 2-D 2.5-D and 3-D seismic energy propagation in the solid medium, including all wave effects appropriate to geological media, such as wave P-S and S-P conversions. In case of marine observations user can model true effects of water-bottom discontinuity. On the other hand, ray tracing is generated when each ray is calculated as a string of points along the timefield gradient. Ray-tracing algorithms emulate the wave propagation nominally, as the infinite-frequency approximation of the wave propagation in relatively smooth heterogeneous media. Rays may be treated separately, for example, for tomography applications. Different ray clusters can be separated for analysis
MODULE 3: PRE-STACK AND POST STACK MIGRATION
The objective of this lab session is to design and understand simple surface data acquisition to by performing velocity analysis. This is done by designing a simple structure as parameter follow:
- Frequency = 100Hz
- Ricker wavelet
- 1st Polygon (Vp = 1500 m/s), 2nd Polygon (Vp = 2000 m/s)
Acoustic Modelling was generated from 1st shot point until 101st shot point. Figure below are showing a dipping model of acquisition geometry: moving receiver array, same number of sources and 48 geophones with spacing of 12.5m.
Dipping model
After that, velocity and migration model was run. Then, both Post-stack migration and Pre-stack migration is run for salt dome model (as in Module 2) as shown in figure below.
Before and after pre-stack migration.
Before and after post-stack migration
Velocity Model
Seismic image of salt dome before any migration
Velocity model of salt dome
(a) Before pre-stack migration (b) after pre-stack migration is applied
(a) Before pre-stack migration (b) after pre-stack migration is applied
It can be seen that the dipping events were not properly positioned besides absence in collapsing of diffraction events before the seismic image was migrated. This caused an event which we call it as an overlapping event (black box). However, after migration was applied, all of the stated problems were solved.
Time migration assumes that the diffraction shape is hyperbolic and ignores ray bending at velocity boundaries which depth migration assumes that the arbitrary velocity structure of the earth is known and will compute the correct diffraction shape for the velocity model.
If the velocity model for the depth migration is incorrect then the migration will be incorrect and the error may be difficult to detect if the migration is performed post-stack. Prestack depth migration will provide an error estimate of the migrated result.
Post-stack depth migration is often performed for reasons of economy but pre-stack depth migration is almost always required since it is almost impossible to define an accurate velocity model using purely post-stack processing.
If the velocity model for the depth migration is incorrect then the migration will be incorrect and the error may be difficult to detect if the migration is performed post-stack. Prestack depth migration will provide an error estimate of the migrated result.
Post-stack depth migration is often performed for reasons of economy but pre-stack depth migration is almost always required since it is almost impossible to define an accurate velocity model using purely post-stack processing.
The time-migration is always incorrect, but in practice the results turn out to be resilient to a large variation of different velocity models and geological structures and can be very useful for an initial interpretation such as for building models for depth migration. The interpreter will often treat a time-migrated section as a geological image. Time migration is very fast and is robust to errors in the velocity model. Further, errors in the shallow velocity model do not affect imaging of deeper structures.
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