Simple 2-Layer Dipping Refractin Model


Figure 1. (Top) Traveltimes-offset (t-x) plot and (bottom) its corresponding offset-depth (x-z) plot.

Figure 2. Equations used in 2-layer dipping case.

OBJECTIVE:

    Pick refraction first arrival traveltimes and calculate its corresponding geological model.

INTRODUCTION:

    Two short profiles are collected very close to building 7 at KAUST. Each profile contains 3 shots; a forward, split, and reverse shot with 24 receiver/shot gather. The geophone interval is 2 m while the shot-geophone interval is 1 m, i.e. forward shot is at x=0 m, split shot is at x= 24 m , and revers shot is at x=48 m, while geophones are at x=1, 3, 5, 7, ..., 47 m. The total recorded time is 0.125 s with time interval of 0.5 ms. The direction of the two profiles is almost perpendicular (forming an X shape).

Download:

  1. Profile 1 (all three shots) as matlab file profile_1.mat

  2. Profile 2 (all three shots) as matlab file profile_2.mat.

    3. If you have a good experience with matlab, use these two files to plot and pick the data. If your matlab experience is limited, then download the following jpg files and use them to pick the first arrivals.

  3. Profile 1, forward shot (jpg format) Profile_1_Forward.jpg

  4. Profile 1, forward shot (jpg format) Profile_1_Split.jpg

  5. Profile 1, forward shot (jpg format) Profile_1_Reverse.jpg

  6. Profile 2, forward shot (jpg format) Profile_2_Forward.jpg

  7. Profile 2, forward shot (jpg format) Profile_2_Split.jpg

  8. Profile 2, forward shot (jpg format) Profile_2_Reverse.jpg

PROCEDURE:

  • Pick the first arrivals of the collected shot gathers

  • Plot the x-t curves of each profile

  • How many layers in the subsurface?

  • Are they horizontal or dipping layers?

  • What is the reciprocal time?

  • Find the apparent and true velocities of the subsurface layers

  • Find the thickness and depth of each refractor under all shot points

  • Calculate: Critical angle(s), Refractor dipping angle(s), Cross-over distance, and Critical distance

  • Plot the x-z sketch under the corresponding t-x curves