2D Refraction Tomography
Part-1: Picking Seismic Data
OBJECTIVE:
Pick and invert first arrivals seismic refraction data to generate subsurface velocity tomograms.
INTRODUCTION:
In this lab we will pick the first arrival travel times and invert them to get the subsurface velocity tomogram. The first part of the lab is covring picking travel times and the second part covering the inversion process.
This data set is collected over Qademah Fault, 25 km north of KAUST. A total of 109 CSG with 109 traces/CSG are fired at 3 m shot/receiver interval.
Figure 1. CSG # 1, raw data.
Figure 2. CSG # 1, filtered data.
Figure 3. CSG # 1, super virtual data.
Download:
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Data Pick software ver. 4-1-1 DataPick4_1_1.zip
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Down load Qademah Data (raw, filtered, and super virtual data sets) from FTP site.
PROCEDURE:
Unzip the Pick Data software and install it on your windows OS machine.
Pick the first arrival traveltimes for one of the three data sets.
Save the picked traveltimes in a PIK file.
2D Refraction Tomography
Part-2: Inverting Traveltimes
PROCEDURE:
Download MKCoord file.
Download and unzip the Inversion Code, copy it to a directory from your choice.
Download the topography file Topography
Download Plot Results.
Start MatLab and open mkcoord_2017.m file.
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
Generate the inversion parameter file (coord.temp) using the mkcoord_2017.m.
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
Go to "Invert Data" menu and chose "Units in meters"
Select "Show Input Data" to test the input data
From the "Dynamic Schedule" select the number of schedule you need
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.
Requirements:
Write a report that includes the following:
1. The traveltime image as displayed by mkcoord_2010.m, comment on the missing pickings (if any).
2. What is the effect of using bigger cuttoff value in the mkcoord_2010.m file.
3. The used Dynamic Schedule(s).
4. Final tomogram with comment.
5. Final RayPath image with comment.
6. Time residuals curve with comment.
7. What would be the final tomogram if you used different initial model?