Geology 5660/6660 Applied Geophysics 19 Apr 2016 Final Project © A.R. Lowry 2016 Final Project Assignment Maple Grove Hot Springs Data Due 5:00 pm, Thurs May 5

Maple Grove Hot Springs Project Part I: West Cache Fault Seismic Investigation Record sections are up on the website; Xiaofei has originals. W to E (Forward) E to W (Reverse)

WCF Maple Grove Hot Springs Project Part I: West Cache Fault Seismic Investigation Background Information:

special_studies/SS-98.pdf Two events on a 7 m (!) scarp: One 4.4–4.8 ka; the other 15–25 ka. Long- term slip rate ~ 0.13 mm/year… Would imply a ~27 ka recurrence for a 3.5 m event (but with caveats!)

Maple Grove Hot Springs Project Part I: West Cache Fault Seismic Investigation Background Information: Data were collected crossing the West Cache Fault near Mendon, UT, roughly 100 m from the site of the UGS trench Forward and reverse seismograms were collected for two profiles: One ~E-W crossing the fault scarp; the other ~N-S, both with 3m spacing and 3m shot offsets. The N-S profile intersected the E-W profile at its center, ~15 m from the east shotpoint. Positions (elevation shown here) had to be approximated several weeks later

Maple Grove HS Project Assignment Part I: Seismic Investigation (from WCF 2014) Download 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.

Maple Grove HS Project Assignment Part I: WCF 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? Elevation vs distance on E-W profile

Maple Grove HS Project Assignment Part I: WCF 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?

Maple Grove HS Project Assignment Part I: WCF Seismic Investigation Cont’d (4) The target should have travel-times somewhere on 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 Burger codes, but I’ve created one (on the website; E-W_Seismic.xls). V 2 = 2000 m/s V 3 = 1000 m/s V 1 = 500 m/s m = 1/V 2 m = 1/V 3 m = 1/V 1 V 1 = 500 m/s V 2 = 2000 m/s z A B m = 1/V 2 m = 1/V 1

Maple Grove HS Project Assignment Part I: WCF 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 one model more successful? Do residuals suggest anything else? V 2 = 2000 m/s V 3 = 1000 m/s V 1 = 500 m/s m = 1/V 2 m = 1/V 3 m = 1/V 1 V 1 = 500 m/s V 2 = 2000 m/s z A B m = 1/V 2 m = 1/V 1

Maple Grove HS Project Assignment Part 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 22 m, 40.5 m, and 59 m. Use a = 3 m and linearly interpolate between measurements horizontally to maximize constraint. Interpret the resulting structure in terms of what you know about the area. Uncorrected Elevation corrected

Maple Grove HS Project Assignment Part II: DC Resistivity Investigation Notes on Linear Interpolation: Because the center points of measurements are not going to occur at exactly x = 22, 40.5, & 59 m, you will have to interpolate apparent resistivities found in the spreadsheet to the desired profile distance x 0 using In the modeling script, substitute your interpolated  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.  i-  i+ x i- x i+ 00 x0x0

Maple Grove HS Project Assignment Part II: DC Resistivity/IP 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? (With kudos to Matt for doing the work!)

Maple Grove HS Project Assignment Part II: DC Resistivity/IP Investigation Next, extract the traverse of measurements with smallest a -spacing ( n = 1, a = 3 m, corresponding to pseudo-depths ~1.5 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? Uncorrected

Maple Grove HS Project Assignment Part II: DC Resistivity/IP Investigation The apparent chargeabilities and their signal-to-noise ratios (expressed as meas/ 1  ) are shown below: I’ve mentioned several times in class that chargeability negatively correlates with resistivity. Download the file containing the IP measurements and plot apparent chargeability vs apparent resistivity. Do they negatively correlate in this case? EWEW

Maple Grove Hot Springs Position Data

Maple Grove HS Project Assignment Part II: DC Resistivity/IP Investigation The apparent chargeabilities and their signal-to-noise ratios (expressed as meas/ 1  ) are shown below: Examine the SNR values. What do they appear to really mean in this case? Negative apparent chargeability has a physical meaning. What is that meaning and what does it imply here? (Hint: google scholar is a good resource for finding literature…) EWEW

Finally, consider & factor into your interpretation the IP chargeability inversion image (at bottom). (Be sure to factor in location relative to surface features on the previous page!)

Maple Grove HS Project Assignment Part III: Magnetic Investigation Download the Excel spreadsheet from the website. Note the file contains all raw measurements and various massages; you will need to decide which results to use for modeling (& explain why). 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: Profile distance is positive-~E; zero is first magnetic station location)

Maple Grove HS Project Assignment Part III: Magnetic Investigation (Cont’d) What is the range of variation of the WMM for these data? Model using GravMag as a simple square polygon near where we observed hematite staining in outcrop (roughly at profile distance 120–170 m). 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 you 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. Does the shape of a simple induced magnetization polygon look similar to the variation of the measurements? Systematically vary the magnetic susceptibility, polygon shape (and remanent magnetization, if you can make a case for why this might matter) to minimize the misfit.

Maple Grove HS Project Assignment Part III: Magnetic Investigation (Cont’d) What, if anything, does your best-fit model tell us about the Maple Grove Hot Springs &/or its environs? How does the magnetic modeling aid in the interpretation of the resistivity/IP data, and/or vice-versa?