UNIVERSITY OF CALIFORNIA, SAN DIEGO Application of Broadband Marine Magnetotelluric Exploration to a 3D Salt Structure and a Fast-Spreading Ridge A dissertation submitted in partial satisfaction of the requirements for the degree Doctor of Philosophy in Earth Sciences by Kerry W. Key Committee in charge: Steven Constable, Chairperson Charles Cox Yuri Fialko Graham Kent Larry Milstein 2003 Copyright Kerry W. Key, 2003 All rights reserved. The dissertation of Kerry W. Key is approved, and it is acceptable in quality and form for publication on micro- film: Chair University of California, San Diego 2003 iii For my parents Clare and Bill, and my brother Kevin iv Table of Contents Signature Page . iii Dedication . iv Table of Contents . v List of Figures . viii List of Tables . xi Acknowledgements . xii Vita, Publications and Abstracts . xiv Abstract . xvi 1 Introduction ................................ 1 References . 3 2 Magnetotelluric Exploration on the Seafloor ............ 4 2.1. The Magnetotelluric Method: Basic Physics . 4 2.1.1. Behavior of Electric and Magnetic Fields . 5 2.1.2. Relationship Between Electric and Magnetic Fields . 9 2.1.3. Apparent Resistivity . 9 2.1.4. Electrical Impedance Tensor . 12 2.2. Working in the Conductive Ocean . 13 2.2.1. Filtering Effect of Seawater . 13 2.2.2. Amplitudes of Seafloor Electric and Magnetic Fields . 16 2.2.3. Vertical Current at the Seafloor . 19 2.2.4. Distortions from Bathymetry . 20 2.3. Broadband Instrumentation . 28 v 2.4. Data Processing . 33 2.4.1. Basics . 33 2.4.2. Least Squares Analysis . 34 2.4.3. Remote Reference Analysis . 35 2.4.4. Robust Remote Reference Analysis . 36 2.4.5. Robust Multiple-Station Processing . 37 2.5. Seafloor Orientations of MT Instruments . 40 References . 44 3 Mapping 3D Salt with 2D Marine MT: Case Study from Gem- ini Prospect, Gulf of Mexico ...................... 47 3.1. Introduction . 48 3.2. Gemini Prospect Salt Structure . 50 3.3. MT Surveys . 54 3.4. Data Processing . 54 3.5. Modeling . 58 3.5.1. Two-Dimensional Inversion . 58 3.5.2. Synthetic 2D Inversions of 3D Forward Data . 64 3.6. Discussion of Modeling Results . 65 3.7. Joint Analysis with Seismic Reflection Data . 75 3.8. Conclusions . 77 References . 80 Appendix A: MT Responses . 82 4 Broadband Marine MT Exploration of the East Pacific Rise at 9◦500N .................................... 94 4.1. Introduction . 95 4.2. Structure of the EPR Near 9◦500N .................. 97 4.3. Sensitivity to Ridge Structures . 99 vi 4.4. Data and Modeling . 100 4.5. Discussion and Conclusions . 102 References . 107 vii List of Figures 2.1 Skin depth versus period for different half-space resistivities. 8 2.2 1D Earth resistivity models and MT responses for the deep oceans and a sedimented margin. 11 2.3 Attenuation of electric and magnetic fields by a seawater layer. 15 2.4 Electric and magnetic fields in the ocean as a function of depth. 17 2.5 Expected seafloor magnetic and electric field amplitudes. 18 2.6 MT responses from a sinusoidal bathymetry step. 21 2.7 Electric and magnetic fields for a sinusoidal bathymetry step of 1 Ωm at 1 and 100 s period. 26 2.8 Electric and magnetic fields for a sinusoidal bathymetry step of 100 Ωm at 1 and 100 s period. 27 2.9 SIO Broadband marine MT instrument. 28 2.10 Time series of electric and magnetic fields collected at Gemini Prospect, Gulf of Mexico. 31 2.11 Example of an eigenvalue spectrum. 40 2.12 Example of rotating magnetic transfer tensor to determine the rel- ative instrument orientation angle. 43 3.1 Location of Gemini Prospect in the northern Gulf of Mexico. 50 3.2 Regional isopach map. 52 3.3 Bathymetry and MT site locations at Gemini Prospect. 53 3.4 Top-of-salt depth map from 3D seismic data and MT site locations. 55 3.5 Base-of-salt depth map from 3D seismic data and MT site locations. 55 3.6 Salt isopach map from 3D seismic data and MT site locations. 56 3.7 Example of high quality marine MT data. 57 3.8 Map showing the profiles used in the 2D inversions. 59 3.9 Inversion results for Line A. 66 3.10 Inversion results for Line B. 67 viii 3.11 Inversion results for Line C. 67 3.12 Inversion results for Line D. 68 3.13 Inversion results for Line E. 68 3.14 Inversion results for Line G. 69 3.15 Inversion results for Line H. 69 3.16 Inversion results for Line I. 70 3.17 Inversion results for Line J. 70 3.18 Inversion results for Line K. 71 3.19 Inversion results for Line L. 71 3.20 Inversion results for Line 1. 72 3.21 Inversion results for Line 2. 72 3.22 Inversion results for Line 3. 73 3.23 Line A: combined MT and reflection models. 77 3.24 Line I: combined MT and reflection models. 78 3.25 Gemini Prospect MT responses for sites 05a, 06a, 08d, and 10c. 83 3.26 Gemini Prospect MT responses for sites 11e, 12c, 14b, and 18a. 84 3.27 Gemini Prospect MT responses for sites 19a, g05, g12, and g16. 85 3.28 Gemini Prospect MT responses for sites g17, g18, g19, and g20. 86 3.29 Gemini Prospect MT responses for sites g21, g22, g23, and g24. 87 3.30 Gemini Prospect MT responses for sites g25, g26, g31 and s02. 88 3.31 Gemini Prospect MT responses for sites s03, s05, s06, and s08. 89 3.32 Gemini Prospect MT responses for sites s09, s11, s13, and s14. 90 3.33 Gemini Prospect MT responses for sites s15, t03, t19, and t36. 91 3.34 Gemini Prospect MT responses for sites t38, t45, t51, and t60. 92 3.35 Gemini Prospect MT responses for sites u01 and u04. 93 4.1 Regional map of showing location of the experiment on the East Pacific Rise near 9◦500 N......................... 96 4.2 Bathymetry map of the experiment location. 97 ix 4.3 Simplistic 2D forward model study of the response generated by possible ridge resistivity structures. 99 4.4 MT data and model responses. 102 4.5 Inversion model fitting the data to an RMS misfit of 1.0. 103 x List of Tables 3.1 Table of number of sites and misfit for TE and TM mode two- dimensional inversions. 63 xi Acknowledgements I wish to thank my advisor, Steven Constable, who invited me to partici- pate (i.e. provide free labor) in the 1997 Gemini Prospect MT cruise when I was a mere undergraduate student. At that time I was still bobbing around freely in an ocean of undecided possibilities for a future career; Steve’s subsequent encourage- ment to join the SIO graduate program and his gusto for geophysical research have since guided me along a rewarding path. Steve and Cathy Constable are thanked for the many parties at the “Constabulary,” which promoted familiarity between faculty, old students, and new students and made IGPP a friendly place to be. I wish to also thank the other members of my committee: Chip Cox, Yuri Fialko, Graham Kent, and Larry Milstein. Their comments and interest in my work have added greatly to this dissertation. My wife Carolyn and stepson Alexander have made the past three years a joy. Their love fuels my every day. My parents are thanked for their dedication to my education. Their unending support, encouragement, and wisdom have been invaluable. I thank my older brother Kevin for showing me early on that its okay to take your toys apart, and that computers and electronics are wonders to be explored. I thank my friends and colleagues at Scripps Institution of Oceanography, in particular James Behrens and Adrian Borsa for many fun adventures. Chester Weiss of Sandia National Laboratories spent the fall of 2002 at IGPP and is thanked for his friendship and collaboration. Arnold Orange and AOA Geophysics are thanked for their ongoing collab- oration with the SIO Marine EM Laboratory, in particular for providing personnel for the seagoing experiments presented in this dissertation. Lisl Lewis of AOA Geophysics is a friend and colleague; her dedication and attention to detail has been pivotal to the success of many of our research cruises. Funding for the research presented in this dissertation was provided in xii part or in full from the Seafloor Electromagnetic Methods Consortium at Scripps Institution of Oceanography. Current and past members include: Anadarko, Shell, Texaco, Chevron, AGIP, BP, BHP, British Gas, ExxonMobil, GERD, AOA Geo- physics, Norsk-Hydro, Statoil, OHM, and ISL. The text of Chapter 3, in part or in full, is a reprint of the material as has been submitted for publication. The dissertation author was the primary re- searcher and author of the published work. The co-author listed in the publication directed and supervised the research which forms the basis for Chapter 3. The text of Chapter 4, in part or in full, is a reprint of the material as it appears in Geophysical Research Letters. The dissertation author was the primary researcher and author of the published work. The co-authors listed in the publication directed and supervised the research which forms the basis for Chapter 4. xiii Vita November 24, 1974 Born, Arlington Heights, Illinois 1998 B.Sc. University of California, San Diego 1998–2003 Research Assistant, University of California, San Diego 1998-1999 Teaching Assistant, Department of Earth Sciences University of California, San Diego 2003 Doctor of Philosophy University of California, San Diego Publications Heinson, G., A.