2D Crosscorrelation Interferometric Migration of VSP Multiples





Figure 1. (a). The velocity model used to generate the synthetic VSP data. In this model, there are 100 shots evenly deployed on the surface, and 16 geophones evenly placed in the leftside well (offset 0 m) at the depth range from 490 m to 790 m. (b). A common receiver gather of the synthetic VSP data. (c). The interferometric migration image of the VSP multiples.

Objective: Learn the 2D interferometric migration methods by using crosscorrelation, and study the advantages of VSP multiple migration.

Introduction: Interferometric migration methods use the multiples to delineate the subsurface structures. VSP interferometric migration has several advantages. It is insensitive to receiver-related static errors, and it has wider subsurface illumination than the migration of primary reflections. The implementation of VSP interferometric migration consists of the following steps

  1. Separate the VSP data into upgoing and downgoing waves (The data provided in this lab are already separated. We do not need this part in this lab.).
  2. Get the direct waves and multiples by proper muting process.
  3. Crosscorrelate the direct waves with multiples to generate the crosscorrelated virtual SSP data.
  4. Migrate the virtual SSP data.

Procedure:

  1. Make a directory, and load in the file: The main program: main.m, diffraction migration codes:mig1.m, direct wave muting codes:mutedata.m, raytracing code:Mray.m.
  2. Load the velocity file vel.mat and the traveltime table traveltimesrc.mat. Load the synthetic VSP data set data.zip, and use command 'unzip data.zip' to extract the data into the same directory.
  3. Run program main.m in Matlab to redatum the VSP data to virtual SSP data and migrate the virtual SSP data (It costs about half an hour).
  4. Try to subsample the shot gathers and show the effects of sparsity on the migration image.
  5. 16 VSP common receiver gathers are used in this lab to generate the virtual SSP data set. Try to use only one VSP common receiver gather and repeat the test. Use stationary phase theorem to explain why we can not get a decent virtual SSP data but can get a fairly good migration image in this case.

Questions:

  1. What are the advantages and disadvantages of migrating VSP multiples compared to migrating VSP primaries?
  2. In practice, we can crosscorrelate the multiples with the whole traces instead of the direct waves to generate the crosscorrelogram. Why does this strategy work? Modify the code to generate crosscorrelogram by whole traces. Compare the migration images obtained by different strategies. Which one is better? Show a ray diagram that redatums VSP to SWP events to show why the latter strategy is more powerful.