In this exercise, you will: derive a two-layer model by using the refraction records you acquired in the vault area. VISTA dataset containing the shot records is in this zipped directory. Download and unpack it, and use VISTA for viewing and processing.
The line was shot along the VLF line, at 2-m geophone spacings starting from the edge of t road. In your data analysis, consider the line to be straight and horizontal, with equally spaced geophones. After you derive your refraction model, you can apply the surface elevations measured for the VLF line.
The analysis of the data will consist of three steps:
Picking travel times;
Travel-time inversion using the Plus-Minus method;
Plotting the model, applying surface elevations, and interpretation.
Open the VISTA project and look through the shots. Select scaling (AGC and trace gain) so that the records look reasonable and you can see the first P-wave arrivals. Select filtering cutting the low-frequency noise, to improve the quality and sharpness of first arrivals.
Look through the different shots. Select 4-6 shots you would use for the inversion. The shots should provide both forward and reverse-direction coverage of the line by head waves.
Pick the travel-time curves from these shots. Use manual picking. Try ensuring consistency in the phases picked in different shots.
Export the travel-time picks in text format for loading into Matlab. This can be done by:
Putting the first breaks in trace headers, and then
Listing the trace headers in a text file.
In the text fie, output the coordinates of the receivers and the first-break travel-time headers, so that these two columns can be directly copied into Matlab scripts.
Wubing is going to provide a Matlab function that should allow you to:1) interactively identify the direct and head wave, 2) measure the direct-wave velocity, and 3) extract the head-wave segment from a picked travel-time curve. Use this tool with each of the picked shots. This will provide estimates for the velocity V1 in the low-velocity layer and the headwave segments for each shot.
Plot all forward and reverse head waves. Select a pair of forward and reverse shots for which the reciprocal times can be observed. Verify that these reciprocal times are equal.
Using the selected forward shot, extend its forward head-wave travel-time curve in both directions by using the forward-direction records from other shots. The extension is done by shifting segments of the headwave curves along the time axis so that they merge in a continuous line across the entire length of the survey. This procedure is called "phantoming".
Using the "phantomed" head-wave curves:
Add them together, subtract the reciprocal time, and divide by 2.This gives the delay times Dt at receiver locations along the line;
Subtract them and divide by 2. This gives a travel-time curve with slope 1/V2, where V2 is the velocity beneath the refractor. Measure this V2. Watch for possible variations of this quantity along the line.
Transform the delays Dt(x) into depth variations h(x) by using the formula for delay times given in class notes.
Plot the resulting h(x) relative to the VLF-line topography. Describe the shape of the refractor (does it correlate with the topography of the hill?). See whether there may be some correlations with the water table or the vault.