1. Geology 5660/6660 Applied Geophysics 22 Mar 2016 Lab 6 © A.R. Lowry 2016 Gravity Start by discussing lab 3 assignment & your assignment for two weeks from now Due noon Apr 5 For Wed 23 Mar: Burger 349-378 (§6.1–6.4)

2. Lab 3: Seismic Reflection 02000 40006000800010000 0 0.5 1.0 1.5 2.0 2.5 3.0 (m) (sec)

3. (1)These are synthetic (idealized) data generated using a numerical (finite element) approach. There are (at least) four primary reflections present in the data. Pick ten travel-times (i.e. at ten different distances) from each of at least three of those reflections. Using Excel (or some similar software that allows you to regress a line fit– you can do this by hand calculator and in GMT, or on graph paper if you prefer) plot x 2 versus t 2 for each of your reflections, and calculate the velocities and thicknesses these would imply assuming each reflection is from a single-layer interface over a halfspace. (2) Now do the same for one of the multiple reflections.

4. 02000 40006000800010000 0 0.5 1.0 1.5 2.0 2.5 3.0 (m) (sec)

6. V 2 = 2660 m/s; h 2 = 1280 m V 3 = 2620 m/s; h 3 = 1670 m V 4 = 2470 m/s; h 4 = 2810 m V 1 = 1540 m/s; h 1 = 400 m (2) Multiple: V m = 1490 m/s; h m = 790 m (1) Layer / Halfspace: V 1 = 1500; h 1 = 400 V 2 = 3048; h 2 = 600 V 3 = 2439; h 3 = 400 V 4 = 3048?; h 4 = 1160? “True” model:

7. (3) Again, without using Reflect, use the intercepts and slopes of lines on your x 2 – t 2 plots with Dix equations to calculate velocities and thicknesses. (4) Do similar calculations for velocities and thicknesses using Dix equations, but now instead of using taken from the inverse slope of x 2 – t 2, use the first- order T NMO relation (i.e., use the inverse slope of a line fit to a plot of T NMO versus x 2 /2t 0 ). How does this change the model? Which is likely to be more accurate and why? (5) Now plug your travel-time picks into Reflect and calculate a model of velocities and thicknesses. How does this compare to your “hand-calculated” models?

8. V 2 = 3580 m/s; h 2 = 770 m V 3 = 2500 m/s; h 3 = 390 m V 4 = 2280 m/s; h 4 = 1130 m V 1 = 1540 m/s; h 1 = 400 m (3) Dix equations V 1 = 1500; h 1 = 400 V 2 = 3048; h 2 = 600 V 3 = 2439; h 3 = 400 V 4 = 3048?; h 4 = 1160?

9. V 2 = 940 m/s; h 2 = 197 m V 3 = 4908 m/s; h 3 = 712 m V 4 = 2003 m/s; h 4 = 1032 m V 1 = 2929 m/s; h 1 = 2817 m (4) Using T NMO 1st order binomial approximation V 1 = 1500; h 1 = 400 V 2 = 3048; h 2 = 600 V 3 = 2439; h 3 = 400 V 4 = 3048?; h 4 = 1160? Dix equations: “True” model:

10. (5) Modeling in Reflect: V 1 = 1500; h 1 = 400 V 2 = 3048; h 2 = 600 V 3 = 2439; h 3 = 400 V 4 = 3048?; h 4 = 1160? V 1 = 1540; h 1 = 400 V 2 = 3580; h 2 = 770 V 3 = 2500; h 3 = 390 V 4 = 2280; h 4 = 1130

11. (6) Create a record-section plot for all of the arrivals using Reflect and compare/contrast with the synthetic record section from which you derived your travel-times. Why do you suppose they are different?

12. Lab 6: Gravity Data Modeling Introducing the field locale: East slope of Little Mountain, West of Franklin, ID Two reversed seismic lines One DC resistivity profile Twelve relative gravity measurements Collocated magnetic field measurements (vertical component) Grav collected on east slope… Note missing GPS location at one grav/mag site!!! With thanks to Dan Munger for arc-image! (Estimated Location) (GPS/RTK Base Site) (Far-Field Site)

13. The Raw Gravity Observations… GPS Base 11 1 12 Note: Estimated position at this site!

14. (1) Acquire & Reduce the Gravity Data Gravity spreadsheet is posted on the website Treat site 1 as the reference site, with value zero. (& add all corrections at site 1 back to all sites to keep it at zero). I have already corrected dial reading to relative gravity using constants for the LR-meter that we used, and also corrected for tidal effects You will need to do the rest… a. Calculate drift (in mGal/hour) from repeated measurements and do a drift correction b. Calculate GRS67 reference gravity (see text) and do latitude correction (Watch your units!!!) c. Calc free air correction  free air anomaly d. Calc Bouguer slab corr  simple Bouguer anomaly e. I give approximate 2D terrain correction using GravMag and elevation data  you need to use to get complete Bouguer anomaly.

15. Strong Hint: There is a function in Excel Called RADIANS(). You will need to use it!

16. (2) Evaluate possible errors. There was some uncertainty regarding whether the dial readings from site 1 were correct! This calls into question both the measurements at site 1 and the estimate of drift correction. Plot your data at the following steps with and without the drift correction : Free air anomaly, simple Bouguer anomaly, complete Bouguer anomaly. Are the values at site 1 consistent with the other measurements? Does the profile of data seem cleaner with or without the drift correction? Does this help us determine whether our dial reading edits were reasonable? Note also that the position at site 10 was not measured but approximated (by interpolating between adjacent sites). Do you see any evidence this estimate of position might be off? If so, by about how much?

17. (3) Model the Complete Bouguer Gravity Anomaly Using GravMag We examined three possible end-member models to explain why Little Mountain is there: a. A horst associated with active normal faulting b. An allochthonous remnant from Laramide thrusting c. Paleotopographic surface that was not buried by Cache valley normal faulting & sedimentation Create a GravMag prismatic model for each end member, and vary the density contrasts/prism endpoints as needed to optimize fit for that model type Remember that the RMS residual is key to evaluating the validity of your model! Discuss your results. Can any of the end member models be ruled out based solely on the gravity data? Why, or why not?