Geology 5660/6660Applied Geophysics

25 Apr 2014Final Project

© A.R. Lowry 2014

Final Project Assignment

Due 5:00 pm May 1

West Cache Fault Project AssignmentPart I: Seismic Investigation

Record sections are up on the website; Xiaofei has originals.

W to E (Forward)

E to W (Reverse)

West Cache Fault Project AssignmentPart I: Seismic InvestigationDownload the record section images from the website (the originals are with Xiaofei if you need to look at them!)The East-West (cross-fault) line is given bottom of p1 & top of p3 (E-to-W forward) and top of p2 (W-to-E reverse). The North-South (along-fault) line is shown bottom of p2 & top of p3 (S-to-N forward) and top of p1 (N-to-S reverse). Note the spacing in each case was 3m with 3m source offset.(1) Pick first arrivals for each record section and plot times vs distance. It’s not clear that the earliest of the obvious arrivals are indeed the first arrivals (the “direct” on the first several phones has air velocity) but treat them as if they are. I recommend looking at both forward and reverse (remembering reciprocity!) on both sections before finalizing & modeling your picks.

West Cache Fault Project AssignmentPart I: Seismic Investigation Cont’d Remember that you don’t need to populate every pick in Refract… Modeling only the picks you are confident in at first may help you decide on picks that are less obvious!(2) First model the N-S data in Refract; then model the E-W. • Are the best-fitting models for the two lines consistent with one another (I.e., do they have the same velocities and ~ same depth where they cross, ~15 m from the E shotpoint)? • What effect do you expect topography to have on arrival times and/or your model?

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Profile Distance E-to-W (m)

Elevation (m)

Elevation vs distance on E-W profile

West Cache Fault Project AssignmentPart I: Seismic Investigation Cont’d

• Is there unequivocal evidence for dip on the layer interfaces generating your arrivals? If the E-W model has a “dipping” layer, is it consistent with what you’d expect?

(3) Model the arrival times expected for a reflection off of the interface you got on the north-south profile. Can you find any coherent arrivals at about those times that are consistent with a reflection? If so, pick those arrivals and model in Reflect. Are the resulting velocities and thicknesses reasonably consistent with those from your refraction modeling?

West Cache Fault Project AssignmentPart I: Seismic Investigation Cont’d(4) Our target should have travel-times somewhere along the spectrum between a layer over a vertical contact and an offset layer boundary, on the fault-perpendicular line:

There are no Xcel spreadsheet models of these in the Burgercodes, but I’ve created one (on the website; E-W_Seismic.xls).

V2 = 2000 m/s V3 = 1000 m/s

V1 = 500 m/s

m = 1/V2

m = 1/V2

m = 1/V3

m = 1/V3

m = 1/V1m = 1/V1

V1 = 500 m/s

V2 = 2000 m/s

zAB

m = 1/V2

m = 1/V2

m = 1/V2

m = 1/V2

m = 1/V1m = 1/V1

West Cache Fault Project AssignmentPart I: Seismic Investigation Cont’d(4) Use the spreadsheet to model both “fault-like” cases, using the E-side thickness, layer 1 velocity, & E-side layer 2 velocity you got from Refract modeling of the N-S seismic profile.

(That leaves only two free parameters in each case.) Is onemodel more successful? Do residuals suggest anything else?

V2 = 2000 m/s V3 = 1000 m/s

V1 = 500 m/s

m = 1/V2

m = 1/V2

m = 1/V3

m = 1/V3

m = 1/V1m = 1/V1

V1 = 500 m/s

V2 = 2000 m/s

zAB

m = 1/V2

m = 1/V2

m = 1/V2

m = 1/V2

m = 1/V1m = 1/V1

West Cache Fault Project AssignmentPart II: DC Resistivity Investigation• Download the data file from the website, along with the Excel spreadsheet for two-layer modeling.

• Use the two-layer modeling routine 2LayerRes.xls (up on the course website to model 1D structure at profile distances near 18m, 28m, and 37m. Note that you will need to linearly interpolate between measurements horizontally to maximize constraint. Interpret the resulting structure in terms of what you know about the area.

West Cache Fault Project AssignmentPart II: DC Resistivity Investigation

Notes on Linear Interpolation: Because the center points of measurements are not going to occur at exactly x = 18, 28, & 37 m, you will have to interpolate apparent resistivities found in the spreadsheet to the desired profile distance x0 using

In the modeling script, substitute your app’s into the column marked “Observed” and choose an upper layer , lower layer to fit the data (left). Vary the model parameters to minimize the RMS misfit of the model and data. Your write-up should include plots of model fits, the best-fit model parameters and RMS misfit values for each depth sounding.

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0 = ρ i− +ρ i+ − ρ i−( ) x0 − xi−( )

xi+ − xi−i- i+

xi- xi+

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x0

West Cache Fault Project AssignmentPart II: DC Resistivity Investigation

• Using your 1D model results, draw (by hand or using a graphics application of your choice) a cross-section interpreting the true resistivity of the pseudosection.

• How does your interpretation based on the 1D modeling compare to inversion results from the AGI EarthImager software, given above?

West Cache Fault Project AssignmentPart II: DC Resistivity Investigation• Next, extract the traverse of measurements with smallest a-spacing (n = 1, a = 2 m, corresponding to pseudo-depths of 1.0333 m) from the Xcel file and plot them versus profile distance.

• Use the Xcel modeling script for Table 5.5 (in the course CD software) to model the data as a traverse across a vertical contact. (Minimize the rms misfit to your data, similar to what was done in 2LayerRes.xls). How are the results similar to/ different from expectations for this model and the 1D results?

Finally, consider &factor into yourinterpretation theIP chargeability image(at bottom).

NOTE that theseimages go fromeast to west; thebreak in slope wasnear the center.

West Cache Fault Project AssignmentPart III: Gravity Investigation

• Download the Excel spreadsheet from the website, and reduce the data as you did previously for Lab 6 (if you did it correctly, you can just cut-paste formulae & update column expressions!)• The two measurements at Station 9 were very different; we resolved the discrepancy by throwing out the second measurement. Does the complete Bouguer anomaly look most reasonable using only the first measurement, only the second measurement, or the average of the two? Why?

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~Profile Distance (m)

Observed Gravity (mGal)

West Cache Fault Project AssignmentPart III: Gravity Investigation• The rms of the differences in RTK heights were about 5 cm, and two of the sites (Stations 2 and 5) had particularly large height discrepancies (12 and 11 cm, respectively). How much error does a 5 cm height error introduce in the combined free-air plus Bouguer slab corrections? How much would a 12 cm error introduce?• Plot the free air and complete Bouguer anomalies. Do sites with possible errors show evidence of being anomalous?• You can estimate uncertainties as the square-root of the sum of squares of the height uncertainty and measurement uncertainty. Assume height errors are 12 cm at Station 2, 11 cm at 5, and 3 cm at all other sites; use measurement errors from repeatability of measurements (in column N of the Xcel spreadsheet). Then, enter the uncertainties along with the complete Bouguer anomaly for modeling in GravMag!

West Cache Fault Project AssignmentPart III: Gravity Investigation• Model using GravMag as a normal fault using as simple a polygon as possible. (!!!!!! There should be only four things you can vary: x0-position of the fault at the surface; dip of the fault; depth h to the base of the sedimentary basin; and density contrast .)• Is there significant misfit? If so, what is the simplest additional polygon needed to fit the data? Can we model the data even better using a single simple polygon with a different shape?• What, if anything, does this tell us?

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h

West Cache Fault Project AssignmentPart IV: Magnetic Investigation• Download the Excel spreadsheet from the website. Note the file contains all raw measurements and various massages; but you will only need the results (rows 22-38; columns A-H) for modeling. Reduce the data by first correcting for the WMM. Plot the data and calculate data variance before and after. Does the reduction improve the scatter, or make it worse?

(Note:Profiledistance ispositive-W;zero is thegravity basestationlocation)

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Profile Distance (m)

Vertical Magnetic Anomaly (gammas)

West Cache Fault Project AssignmentPart IV: Magnetic Investigation (Cont’d)• What is the range of variation of the WMM for these data?• Like you did previously for the gravity data, model using GravMag as a normal fault using as simple a polygon as possible. Start by using the exact same polygon that gave you the best fit for the gravity data, and systematically vary the magnetic susceptibility (and remanent magnetization, if you wish) to minimize the misfit. • Note this is a measurement of the vertical field anomaly, so you need to set to “Vertical Field” for the RMS misfit options in the data window! Also need to estimate azimuth of the profile (which you can do in Xcel). In preferences, check “Use uncertainties” & “Enter magnetic field”. All of these (azm, main field, etc) should be reported in your write-up.• What, if anything, does your best-fit model tell us about the WCF &/or its environs?