OBJECTIVE: Invert gravity and seismic data for salt model. Find best model that accounts for both types of data.

BACKGROUND: Straight ray tomography does not take into account ray bending so we use a curved ray method now.  You will recognize that the problem is non-linear so you need to update the seismic velocity model for more than one iteration (as opposed to straight rays which is a linear problem and only one iteration is needed).  The minimum resolution of the tomogram should meet two requirements.  There should be three times more traveltimes than cells (aka smoothing schedules), and each cell size should be no less than 1/2 the first-arrival wavelength in meters.
 

Procedure for Seismic Phase I:

Note: The working directory is c:\New Folder.  All required files may be found here.

1. Open Vista and retrieve the seismic project started last week.  Open up a shot gather and determine the wavelength in meters of several first-arrivals (both near and far offset) and determine the average.

2. Now prepare an input file (coord.temp) for the inversion.  This is done using the Matlab program mkcoord.m.  Start Matlab, open the file mkcoord.m, and review it.  What input files does it require?  Check c:\New Folder to make sure these files are present.

3. Using Matlab, run mkcoord.  Discuss what the plots mean.

4. An identical copy of coord.temp has already been moved to the laptop.  Normally, it would be placed in c:\pc_gui\m.  Working together as a group, open up Matlab on the laptop and type c:\pc_gui/startup.  When a new window appears, select Invert Data > Units in meters.

5. The inversion main window should appear next.  Select three smoothing sizes by clicking and holding the Dynamic Schedule button.  For each schedule, the number of iterations should be 10, 5, and 5.  The grid size should be 1 for all schedules.  Discuss the appropriate selections for the starting model velocities and grid points.

6. While the data is inverting, work on the gravity portion of the lab.

Procedure for Gravity:

1. Return to the desktop computers and open the gravity program GM-SYS.

2. Open the existing file c:\New Folder\Class Exercise.sur.  Rebuild a salt diaper starting model and experiment with various inverting parameters until the observed gravity matches the calculated and a realistic model is achieved.  Parameters to adjust are the movement direction of the block nodes.  Remember, they can move in the X direction, Z direction, or both.  In addition, deselect some nodes to keep them from moving, and see what happens.

Procedure for Seismic Phase II:

1. When the inversion on the laptop is finished, inspect the tomogram by selecting Show Results > Velocity Field > Dynamic Result.  Interpret where the salt is.  Inspect the plot of the residual by selecting Show Results®Total traveltime residual.  Discuss its meaning.

Submit a Report:

What is the velocity of the salt?  What is the velocity of the surrounding material?

Use seismic to estimate how far the top of the diapir is below the surface, and report your value.

Use gravity to estimate the distance from the surface to the bottom of the salt, and report your value.  Remember, your gravity based salt diaper must be geologically reasonable!  Include a printout of your gravity model with your report.

Comment on the sensitivity of the final result with regards to traveltime noise and starting models.

Comment on usefulness of getting salt model from both gravity and seismic data compared to just using one data set. Is it worth it?  Why or why not?

Hand in the report in the usual format.  Key objective is to find salt model that fits both gravity and traveltime data.  State assumptions.