Seismic Tomography Field Test at KAUST


Objective:

Pick first arrival and invert the picked data to get a subsurface tomogram

Skill learned:

  1. Picking seismic data.
  2. Prepare input files for PC_GUI.
  3. Invert seismic data.
Introduction:

Data for a 2D tomography line is collected near building # 1, KAUST. Bison-9000 is used to collect the data with 48 receivers at receiver interval of 5 m and 47 shots at shot interval of 5 m. The shot at receiver point # 29 is missing due to concreat ground.

Procedure:

  1. Download the picking software (DataPick) DataPick.CAB, setup.exe, and SETUP.LST. Download the data files kaust_unfiltered.dat and kaust_filtered.dat.
  2. Run the setup.exe file on you windown machine and follow the on-screen instructions. This will install the DataPick software.
  3. Run the DataPick software from your "All Programs" in the "Start" menu.
  4. Load either the unfiltered or the filtered version of the data using "File -> Open Profile -> ASCII file", then enter 2256, 2000, and 400 for No. of Traces, No. of Samples, and Total Time.
  5. Pick the first arrivals. Suggested display parameters: Traces (1 : 48), Normalization (Trace), Gain Value (2000), Clip Value (100), and Traces/CSG (48). Then press replot button.
  6. Save the picked data using "File -> Save Picking".
  7. Modify the first-arrival-time file, the topography file, and the mkcoord.m as required (see Curved ray synthetic lab for more details).
  8. Download MKCoord file.
  9. Download and unzip the Inversion Code, copy it to a directory from your choice.
  10. Download the topography file Topography
  11. Download Plot Results.
  12. Start MatLab and open mkcoord_2017.m file.
  13. make all required changes to fit your data set, which includes
    • ds, s1_x, and ns: the source interval, x-position of the first source, and number of source points, respectively
    • dg, g1_x, and ng: the receiver interval, x-position of the first receiver, and number of receiver points, respectively
    • zmax: the expected maximum depth
    • cutoff: Accepted reciprocity error (in Sec)
    • tshift: Const. value to subtrace from all travel times (in Sec)
    • Rm_Rec: =1 to remove reciprocity error, = 0 to keep all data
  14. Generate the inversion parameter file (coord.temp) using the mkcoord_2017.m.
  15. Copy the coord.temp file to the pc_gui/m directory, then in the STARTUP.m change the workdir to your actual working directory of the PC_GUI. Run STARTUP.m
  16. Go to "Invert Data" menu and chose "Units in meters"
  17. Select "Show Input Data" to test the input data
  18. From the "Dynamic Schedule" select the number of schedule you need
  19. In each schedule enter:
    • Iteration: The number of iterations of this schedule
    • Grid Spacing (m): The grid spacing for the forward modeling
    • Vtop (m/s) and Vbottom (m/s): The top and bottom velocities for the initial velocity model, it is gradient (1D) model
    • Vmin (m/s) and Vmax(m/s): The min and max velocity of the tomogram
    • lrsx(grid points) and lrsz(grid points): The size of the pixel in the velocity model
    Change the Dynamic Schedule to fit your data set, and start the inversion.