2D Refraction Tomography

Part-1: Picking Seismic Data, Running Inversion Code, Selecting Inversion Parameters

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 covering picking travel times and the second part covering the inversion process. This data set is collected over Qademah Fault, 35 km north of KAUST. A total of 288 CSG with 288 traces/CSG are collected.

To download the data and get the shot/receiver layout click on the following link

Download the 2D Wadi Qadied data set from here

Get a copy of the 3D active-source data set from here

Get a copy of the 2D Aqaba data set from here

Data Picker Software

This traveltime picking software is written using Visual Basic language, and it runs only under Windows operating system. We tested it with Windows 97, XP, 7, 8, and 8.1, no problems recorded.

The software has few bugs, if you found any, please contact me to fix it.

Download the software from here, install it in your windows machine.

Samples of the collected CSGs

Figure 1: A sample CSG. This is CSG no. 25 shows only inline receivers (receivers 25 to 48).

Figure 2: A sample CSG. This is CSG no. 1 shows all 288 receivers

PROCEDURE:

1. Unzip the Pick Data software and install it on your windows OS machine.

2. Pick the first arrival traveltimes for the 2D data set (each student is assigned 2 2D profiles).

3. Save the picked traveltimes in a *.pik file.

2D Refraction Tomography

Part-2: Inverting Traveltimes, Running Inversion Code and Selecting Inversion Parameters

PROCEDURE:

1. Download mkcoord.mat file. This file will (1) do reciprocity test and (2) create input file (coord.temp) for the inversion code.

2. Download and unzip the Inversion Code, copy it to a directory from your choice. Copy the (coord.temp) file, from previous step, to the directory ./PC_GUI/m

3. Create the topography file, it is an ASCII file with two columns, the first column is x-position in meters and the second column is the z-value in meters.

4. Download Plot Results file. This file will plot the final tomogram.

5. Start MatLab and open mkcoord_2017.m file.

6. Make all required changes to fit your data set.

7. Generate the inversion parameter file (coord.temp) using the mkcoord_2017.m.

8. Make sure that the data rejected due to reciprocity is minimum and cannot be fixed, re-pick these traces if required.

9. 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, 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_2017.m, comment on the missing pickings (if any).

2. What is the effect of using bigger cuttoff value in the mkcoord_2017.m file.