A Seismic Survey Across a Suspected Fault in Mason Valley, Nevada Camille Amdall , Ian Pierce, Kate Curtin, Eric, Conner, Amanda, Lauren, somebody, John N. Louie GPH 492/692 Applied Geophysics class 2017, University of Nevada Reno Refraction Analysis Reflection Analysis Example refraction record with 1st-arrival picks:

Background Conducted March 23, 2017 in Mason Valley, NV Objective: identify and characterize the “Little Mason Fault” discovered by Ian Pierce of UNR Two seismic reflection-refraction lines with 48 channels each Survey lines were east-west-trending 3 m spacing between each channel with groups of six 100 Hz geophones A plate and a 16 lb sledge hammer provided the seismic source, 10 hits per flag A Bison recorder saved all of the data generated by the geophones Seismic processing with Louie’s free JRG/Viewmat software from crack.seismo.unr.edu/jrg

Reflection Filter tests showed prominent reflections between 80-250 Hz Constant-velocity stacks testing velocities from 500 to 2500 m/s CMP stacking with velocities picked for maximum continuity on the CV stacks Fault breaks identified on the CMP stack at the apex of diffraction hyperbolas Post-stack Stolt migration at 1700 m/s collapses diffractions to show stratigraphy better

Refraction The velocity of a wave is dependent on the stiffness of the rocks through which it travels Some channels were missing due to problems with the instrumentation (bad cables, geophones not plugged in) In an area where the geology is composed of discrete homogenous layers modeling can be quite easy. Mason Valley offered a much more complicated geological environment Refraction analysis required optimization of first-arrival travel times with SeisOpt® @2d™ from Optim, a sophisticated velocity tomography First-arrival picks are being reworked for LMF2, to verify the large discontinuities in the P-velocity section The LMF1 P-velocity section suggests velocities are lower uphill of the fault- Why?

Data Processing 3 steps application of array geometry to all of the raw data we picked the first p-wave arrivals using Dr. John Louie’s viewmat application exporting all of the data to SeisOpt@2D Problems 1. Due to the geology of the site and the relatively small amount of energy released by the source, this presented a difficult task requiring the use of automatic gain control to “normalize” the records and increase the visibility of faint first arrivals. 2. Some channels were missing due to physical problems with the instrumentation (not plugged in)