ABSTRACT NEAR-SURFACE GEOPHYSICAL IMAGING OF THE INTERNAL STRUCTURE OF EL CAPITAN MEADOW ROCK AVALANCHE IN YOSEMITE NATIONAL PARK, CALIFORNIA Rock avalanches are a large form of mass movement which, while uncommon, are particularly destructive due to their very high volumes and long flow distances. Due to their rarity, they are typically studied well after they occur. Mass movement events can subject humans to risks and casualties as well as cause massive infrastructural damage. The El Capitan Meadow rock avalanche lies at the foot of El Capitan in Western Yosemite Valley. I investigated this avalanche using Electrical Resistivity Tomography (ERT) and Ground Penetrating Radar (GPR) to image the internal structures, depth, and topography of the underlying paleo- surface of the valley. GPR and ERT surveys were conducted along three profile lines over the avalanche deposit. ERT results showed a strong but gradual resistivity contrast between the low resistive soil of the prior valley surface and the highly resistive rock avalanche deposits, however, detecting the interface with precision was difficult. GPR results revealed several sharp interfaces within the subsurface making it impossible to identify which one was the interface to the valley floor. Selected interfaces from the GPR model were incorporated into the ERT inversion process, where the GPR interfaces, which indicated the true location of the valley floor, was the one which produced the sharpest resistivity contrast in its ERT model. At the intersection points of the profiles, the estimated depths to the paleo-valley floor had similar elevation revealing a flat extension of the exposed valley floor beneath the rock avalanche deposits. Christine Horngjen Liu August 2018 NEAR-SURFACE GEOPHYSICAL IMAGING OF THE INTERNAL STRUCTURE OF EL CAPITAN MEADOW ROCK AVALANCHE IN YOSEMITE NATIONAL PARK, CALIFORNIA by Christine Horngjen Liu A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geology in the College of Science and Mathematics California State University, Fresno August 2018 APPROVED For the Department of Earth and Environmental Sciences: We, the undersigned, certify that the thesis of the following student meets the required standards of scholarship, format, and style of the university and the student's graduate degree program for the awarding of the master's degree. Christine Horngjen Liu Thesis Author Alain Plattner (Chair) Earth & Environmental Sciences John Wakabayashi Earth & Environmental Sciences Greg Stock National Park Services (Yosemite NP) For the University Graduate Committee: Dean, Division of Graduate Studies AUTHORIZATION FOR REPRODUCTION OF MASTER’S THESIS I grant permission for the reproduction of this thesis in part or in its entirety without further authorization from me, on the condition that the person or agency requesting reproduction absorbs the cost and provides proper acknowledgment of authorship. X Permission to reproduce this thesis in part or in its entirety must be obtained from me. Signature of thesis author: ACKNOWLEDGMENTS I want to express my gratitude to Dr. Alain Plattner for his advising in my project. Alain’s guidance and enthusiasm has provided me the knowledge and ability to accomplish this research and go beyond my limits. I also want to thank Sandi Smart, Austin Robbins, Alex Briand, and Mikhael Bratanata for their services in helping me in the field. This work would not have been done without you and your support. I am grateful for Michelle Johnson's guidance in grammar writing skills and counseling me throughout my time at Fresno State. I want to thank Deborah Camper and Iesha Rosenboro for their assistance in proofreading. Thank you to Greg Stock for the open communications about this research project and providing more insight about my project area. Many thanks to the department members, Chris Pluhar, Alex Pytlak, Sue Delcroix, Amanda Ausman, Jerome De Graff, John Wakabayashi and many more for their friendship, assistance in completing paperwork, aiding me in presentations, and encouraging me to push forward. I also want to thank Dr. Laurent Montesi, Dr. Vedran Lekic, and Dr. Daniel Lathrop for their support for my educational advancement. Lastly I also want to thank Dr. Freja Nordsiek for her support, friendship, and mentorship throughout the early transition of my graduate career. This paper would not have been possible without the support of my mother, Susan Liu and my boyfriend, Paul Young, who both encouraged me to pursue my passion in geophysics. Thank you, Paul, for your steadfast patience when I needed to vent and emotional assistant throughout my thesis process. TABLE OF CONTENTS Page LIST OF TABLES ................................................................................................. vii LIST OF FIGURES ............................................................................................... viii INTRODUCTION .................................................................................................... 1 Geology of Yosemite Valley ............................................................................. 3 Near-Surface Geophysical Methods ................................................................. 7 LITERATURE REVIEW ....................................................................................... 10 METHODOLOGY ................................................................................................. 14 Gravity Mapping ............................................................................................. 14 Seismic Refraction .......................................................................................... 15 Seismic Reflection .......................................................................................... 16 ERT .............................................................................................................. 17 GPR .............................................................................................................. 21 Survey ............................................................................................................. 24 Field Deployment ............................................................................................ 26 Data Processing ............................................................................................... 36 Selecting Interfaces in GPR ............................................................................ 63 Combining ERT and GPR ............................................................................... 68 RESULTS ............................................................................................................... 71 ERT Inversion ................................................................................................. 71 Ground Penetrating Radar ............................................................................... 73 Combining GPR and ERT ............................................................................... 79 DISCUSSION ......................................................................................................... 85 CONCLUSION ...................................................................................................... 88 vi vi Page BIBLIOGRAPHY .................................................................................................. 89 APPENDIX: SUPPLEMENTAL FIGURES ......................................................... 95 LIST OF TABLES Page Table 1. Near Surface Geophysical Methods Versus Units ................................... 15 LIST OF FIGURES Page Figure 1. Location of El Capitan situated in Yosemite Valley generated from a LiDAR-based digital elevation model (Brody et al., 2015, Stock, 2016). ............................................................................................. 5 Figure 2. Representation of a pseudosection displaying raw data. ........................ 18 Figure 3. ERT inversion flowchart ......................................................................... 19 Figure 4. 2-D discretization of parameter mesh (Gunther et al., 2006) ................. 19 Figure 5. Applied regularization “smoothing” on three resistivity models (a, b, c) from the observed data (d, e, f). (Obtained from Oldenburg et al., 1999) ............................................................................................. 20 Figure 6. Diagram of how common-offset GPR works to obtain a profile radargram. ............................................................................................... 21 Figure 7. Raw radargram with white-black color pixels using the gray colormap. 22 Figure 8. Walkaway WARR procedure (modified from Morozov lecture, 2018) . 23 Figure 9. Representation of a raw WARR radargram. ........................................... 24 Figure 10. The southeast face of El Capitan, showing the location of the El Capitan Meadow rock avalanche. ........................................................... 24 Figure 11. LiDAR image obtained from Google Earth of the El Capitan Meadow Rock Avalanche depicting three profile lines chosen for the data collection for both ERT and GPR (LiDAR image provided by Stock (2017)). .................................................................................................... 25 Figure 12. Arrangement of current (A and B) and potential (C and D) electrodes for Dipole-Dipole, Wenner, and Schlumberger configuration arrays (Lowrie, 2007). ......................................................................................