Hydrogeophysics - Seismic Refraction Lab

Due Friday, September 19

This lab is available to WWW viewers at the URL: http://www.seismo.unr.edu/ftp/pub/louie/class/hydro/refrlab.html

This lab exercise employs software that runs only on Macintosh computers. We will meet at the MSM Mapping Modeling and Visualization (MMV) Lab in LMR 286 from 2:00 to 5:00 PM (3:30 to 5:00 if there is an Advanced Seismology Seminar) on Thursday, September 11. Four Macintoshes will be reserved for our use then. Bring a 3 1/2-inch diskette with you to copy programs and data for later use.

When you enter the MMV Lab, first get access to our programs and data on the file server. (Click on the links within this paragraph to see a screen shot of the operation.) Insert your floppy into the Mac, and wait for virus checks and formatting as necessary. Then select Chooser from the Apple menu (top left corner of the screen). In the Chooser select AppleShare from the upper left window, then MSM LMR from the lower left window (if it is not already selected), then double-click on ``mmv'' from the right window. You will then get a login dialog window. Check the Guest button, and hit return. Then make sure in the next dialog that the ``d2'' disk is selected, and hit return again. When this is successful you can quit the Chooser (by clicking on the little box at the top left corner of the Chooser window) and you should see a new network disk named ``d2'' on the desktop. Double-click to open it, and inside double-click on the users folder, the louie folder, and the hgph folder. Also open the Burger folder. Click WL0EW.pick to select it. Drag the file to your floppy's icon on the desktop, to copy it to your floppy. Also drag the Chapter 3 folder to your floppy. After opening the floppy to make sure you do have the files, you can log out of the mmv server by trashing the disk that came up.

If you have Burger's software already, click here to download a Mac binhex archive of the WL0EW.pick file.

Now double-click on your copy of WL0EW.pick to start the RefractSolve program for Washoe Lake refraction data the class recorded four years ago, with Dave Burger's help. If the double-click on WL0EW.pick does not start the RefractSolve program immediately, open the Chapter 3 folder in your copy of the Burger folder and start RefractSolve directly by double-clicking it. Then use the File->Open menu to load the WL0EW.pick data. Copies of the field sketch map and the data records are attached below. First examine the travel time picks Louie made from the data records. Use Edit->Revise Data (select the Revise Data item from RefractSolve's Edit menu) to see the survey line parameters. Our shot offset and geophone spacing was actually 100 feet = 30.48 meters. This program will not allow a geophone spacing of more than 10 m. So ALL TIMES AND DISTANCES WERE DIVIDED BY 10 BEFORE THEY WERE ENTERED IN THE PROGRAM. With this trick, velocities will come out correctly in meters/second, but you must multiply all distances, depths, and times the program gives by 10 before you put them in your lab report.

Continue looking at the input data by clicking on Done in the survey setup dialog. The next dialog will give you the forward and reverse first-arrival times Louie picked (of course divided by 10). The >>> button goes to the next geophone. Compare the original data plots to the picked times, and consider:

You may print any results on the MMVLaserJet5, found also in the Chooser. Please make sure you use the Chooser to select the printer before you try printing from a program. The Macintosh you are at may be set to print to some unknown printer, and you should change it to MMVLaserJet5.

Exercises

1 - Make the simple 2-layer interpretation. Looking at the time-pick data, notice the pointer turns to a cross when it is in the time plot. You can hold down the mouse button and drag the mouse to make a line. Start with the forward shot, which should be the picks in solid black. First draw the V1 line from (0,0) through the nearest-offset pick. Then draw another line at the higher velocity V2 through the rest of the points. (Always draw the shallowest layer's line first, then the next layer down, etc.) Now draw lines for the reversed shot's times by selecting Direction->Reverse. You can select Line->Edit to change the lines by clicking on them and dragging the handles that appear at their ends, and improve your fits.

When you have satisfactory line fits for two layers, select File->Draw Structure to see a cross section. Then select Window->Table to see numerical values. Report the velocities and thicknesses (remember to multiply by 10). The dip given in the Table may be wrong; to get dip use:


Report the dip. It it a north or south dip?
2 - Examine from the table you got above, for the Forward picks only, the surface-layer velocity V1, the apparent velocity Va2 of the second layer, and its intercept time ti (multiply times and distances by 10 again). Also estimate a crossover distance xcross from the time plot. Use the equations from the notes for a simple layer-over-half-space with no dip to compute the depth to the refractor using ti. Compute it again using the equation for xcross. Are they different? Might one be more accurate than the other?
3 - Now Edit your line fits to make the highest-velocity interpretation you can, especially considering the error inherent in each pick (particularly the far-offset picks). Use Line->Edit to move the lines around on both the Forward and Reverse directions. Select File->Draw Structure when you are done to recompute the Section and Table. Report depths, velocities, and dip.
4 - Now make the lowest-velocity interpretation you can. Report depths, velocities, and dip. How do these compare with the high-velocity model?
5 - Make a three-layer, three-velocity interpretation of our data. What evidence do the pick times show for an intermediate velocity? Erase your previous picks with Line->Erase All, and remember to start drawing V1, then V2, then V3; and the same on the reversed times. Report velocities, thicknesses, and dips from your best line fits.
6 - Supposing the intermediate layer is really a hidden thin layer, use Line->Move to move your V2 lines back to just intersect the V1 to V3 crossover. Report velocities, thicknesses, and dips for this minimum amount of the intermediate thin layer. How do the depths to the deepest V3 layer compare with those from your best-fit 3-layer interpretation above? How do the depths compare with those from your 2-layer interpretation? What constraints do you have on the depth of the deepest refractor?
7 - Starting with your 3-layer interpretation, compute the minimum depth to basement having an assumed velocity of 4 km/s. Assume that the last geophone (forward and reverse) just missed the refraction from this deepest layer. Guess on the slopes and compute a structure repeatedly to get close to 4 km/s. Do the times show any hints of such a refractor? How does the minimum depth change if you assume the basement is 6 km/s?
8 - Evaluate the effect of the time picks not landing exactly on your fit lines. Do the data show any delays that seem consistent on both forward and reverse shots? Measure the maximum delay by fitting a line to the slope of the main refraction arrivals, and then moving it to a) just graze the earliest times from that layer, and b) just graze the latest times. In each case compute a structure and examine the Table to see the refractor's intercept time. Subtracting the two intercept times should give you an estimate of the maximum .
9 - Assume all your delay results from structural deflections h in the refractor. Using the equations in the notes, compute this maximum deflection. Ignore any dip, or intermediate layers. What proportion of the average refractor depth is this deflection?
Later we can look into assuming the refractor is also a density difference of, say, 0.1 g/cc, and use the simple infinite-plate formula from gravity to estimate what is the maximum number of milligals of gravity anomaly these structural deflections might produce; and whether we could observe such structure with our gravity instruments.
10 - Next assume all your delay results from lateral velocity changes in the shallowest layer (again ignoring dip and intermediate layers). Compute the new surface velocity V0. What proportion of V1 is the velocity change V1 - V0?

Assuming that resistivity in this layer changes in proportion to changes in velocity due to changes in porosity, it is possible to estimate the resistivity changes if you have some locations where resistivity and seismic data overlap in the same environment. We may try this later.


Sketch map of a reversed seismic refraction survey carried out on Oct. 9, 1993 by a Hydrogeophysics class at Washoe Lake, with Dave Burger's assistance. The survey consisted of 24 geophones nominally spaced at 100 ft intervals (labeled G1 to G24 from south to north) Shot 1 (S1) was set off 50 ft south of G1, and the reverse Shot 2 was set 100 ft north of G24, for a total survey length of 2400 ft. Note that the channel numbers that appear on the seismic records below increase from Channel 1 (C1) at G13 to Channel 12 (C12) at G24 northward, and then southward from Channel 13 (C13) at G12 to Channel 24 (C24) at G1.


Seismic record of Shot 1. Note that Channel 24 as labeled on the plot (C24 on map above) is closest to the shot, and Channel 12 is farthest. Time increases down from the time of the blast, with each minor division representing 10 ms.


Seismic record of Shot 2. Note that Channel 12 as labeled on the plot (C12 on map above) is closest to the shot, and Channel 24 is farthest. Time increases down from the time of the blast, with each minor division representing 10 ms.