Geology of the Faroe Islands

PhD thesis Propagation and scattering of reflection seismic waves in a basalt succession Uni K. Petersen NVDRit 2014:01 Propagation and scattering of reflection seismic waves Heiti / Title in a basalt succession Høvundur / Author Uni K. Petersen Jarðfeingi, Tórshavn Faroese Earth and Energy Directorate, Tórshavn [email protected] Vegleiðari / Supervisor R. James Brown University of Stavanger and University of Faroe Islands [email protected] Hjálparvegleiðari/Co-supervisor Morten S. Andersen Geological Survey of Denmark and Greenland, Copenhagen and University of Faroe Islands [email protected] Ritslag / Report Type PhD ritgerð/PhD thesis Latið inn / Submitted 27.01.2014 NVDRit 2014:01 © Náttúruvísindadeildin og høvundurin 2014 ISSN 1601-9741 Útgevari / Publisher Náttúruvísindadeildin, Fróðskaparsetur Føroya Bústaður / Address Nóatún 3, FO 100 Tórshavn, Føroyar (Faroe Islands) Postrúm / P.O. box 2109, FO 165 Argir, Føroyar (Faroe Islands) @ +298 352550 +298 352551 [email protected] What if reality is just a dream? What if the dreams are the reality? Figure on cover: Photo of Tórshavn taken from the hillside above Tórshavn towards south. Glyvursnes can be seen in the upper right corner. The photo has been filtered with cartoon filter using the Gimp (photo and editing: Uni K. Petersen). Foreword Foreword This PhD thesis is the completion of a PhD study in the years 2002 – 2006 at the department of Science and Technology, University of the Faroe Islands, initially submitted in 2008. Although acknowledging the work to be sufficient for a PhD, the opponents had some critical suggestions for improvement, mostly regarding the well log tie to seismic data and regarding the thesis needing additional proofreading. I therefore decided to do additional work, addressing the suggestions of the opponents. During this time, an opportunity to perform refraction seismic modelling, on the very same data as used for the thesis, showed up. The modelling greatly improves the constraint on the velocity model and verifies the consistency of the results, and was therefore included in the thesis. Apart from the added refraction modelling, the thesis basically has the same content, the same structure, and the same results as the 2008 submission. In 2008 I changed supervisor from Morten S. Andersen, previously professor in geophysics at the department of Science and Technology and now at GEUS, to R. James Brown. This was in association with Jim having taken over the position as professor in geophysics after Morten. All work done before 2008, the structure and content of the thesis, is done under the supervision of Morten S. Andersen and all work after 2008, the refractions seismic modelling and amending of the thesis, is done under supervision of R. James Brown. i Abstract Abstract In 2003, surface seismic data were acquired at Glyvursnes in the Faroe Islands as part of the SeiFaBa project. The objective was to investigate seismic wave propagation in an area, where the elastic properties of the basalt flows were relatively well known. This thesis presents a study of the seismic response, observed in both surface seismic data and downhole data from an over 4000 m thick basalt succession below Glyvursnes. The aim was to compare different acquisition methods, to better understand the seismic facies of basalt formations and describe scattering and attenuation, and to do geological mapping. The surface seismic reflection data were densely sampled and acquired with a combined dynamite (land source, shot-point interval 10 m), airgun (shot-point interval ~20 m), geophone (station interval 5 m), and streamer (group interval 6.25 m) setup centred at a 700 m deep well at the shoreline. Wireline log data from the three deep wells in the Faroes were used for modelling the seismic response and for tying to VSP data and seismic data. The data processing comprised reflection seismic processing, refraction seismic modelling, prestack migration and VSP processing. Comparisons of the acquisition methods showed that low frequencies are best suited for penetrating thick basalt successions and that the airgun was superior to the dynamite in generating seismic energy for penetrating successive basalt flows, while the dynamite source provides the most details in the shallow section. The stacked sections imaged at least the upper 1400 m of the basalt succession. Interbed multiples were not identified as events but energy from multiples was present as wave-train tails deteriorating the seismic image within and below the basalts. The frequency content of the wave-train is an imprint of the properties of the basalt formations through which the signal has travelled. Significant differences in seismic facies for different basalt formations were found. These are related to the frequency of interbedded sediments and basalt flow thickness. Of the 3 major formations [Upper (UBF), Middle (MBF), and Lower ( LBF)] comprising the basalt succession below Glyvursnes, the MBF is relatively transparent to seismic signals and appears with lower amplitudes than the UBF and LBF above and below, expressing lack of sedimentary beds. Further, an attenuation study showed that the MBF is significantly less attenuative than the UBF and LBF. The MBF-LBF boundary, the A-horizon, was imaged to be at ~1400 m depth overlain by the MBF of ~1050 m thickness. This is less than the stratigraphic thickness of 1350 – 1400 m of the MBF in the northern part of the Faroes, and thus in accordance with a thinning towards the south. Further, the A-horizon was identified and mapped over a large area on conventional seismic data to the east of Glyvursnes. Refraction seismic modelling and prestack migration strongly indicate that in the uppermost 1000 m dip orientation of interfaces have the same trend as that from surface mapping, towards the south (relative to the north-south orientation of the profile), while at 1400 m depth the dip of the A- horizon is towards the north. A significant lesson to learn from the thesis study is that basalts are not just basalts. Although general properties such as velocities for core, crust, hyaloclastics, and volcaniclastic sediments as well as the VP/VS ratio are consistently similar, the combination of these can form basalt formations with quite different properties when considering seismic facies and attenuation. An implication of this is that determining base of basalts directly from seismic data is difficult. ii Acknowledgements Acknowledgements First of all I want to thank my wife Maria and our daughter Ruth for their patience during the last many years of writing. I would like to offer my special thanks to my supervisors Jim Brown and Morten S. Andersen. I would also like to thank Jarðfeingi and Heri Ziska for providing facilities for refraction seismic modelling and data processing, without which the completion of this thesis would not have been possible. I am also grateful for having worked together with many members of the SeiFaBa project: Michael Worthington and Felicia Shaw, University of Oxford; Robert White, Nick Mohamed and Giovanni Bais, University of Cambridge; Peter Japsen at GEUS; Lars O. Boldreel, Copenhagen University. I would also like to thank the Sindri group for funding the SeiFaBa data acquisition making this thesis possible. Thanks to Aarhus University for providing seismic equipment and for providing me access to their ProMAX system; thanks also to Holger L. Andersen and Egon Nørremark for their assistance during my stays in Aarhus. Special thanks to Per Trinhammer for leading the technical part of the acquisition with great success. Also I want to thank Regin Waagstein, GEUS, and Simon Passey, Jarðfeingi, for widening my knowledge to the geology of the Faroes; Magnus Danielsen and Knud Simonsen, NVD, Jana Olavsdóttir, Jarðfeingi, and Birgit Remmel for proofreading. Finally, I want to thank Thomas Meyer Hansen, Copenhagen University, for discussions and help with finite-difference full- waveform modelling. A special thanks to the farmer Sjurður Patursson for his goodwill during the data acquisition on his land. And finally it is with great love that I dedicate this work to the women in my life: my wife Maria Smith, our daughter Ruth Smith, my mother-in-law Jórun Smith and our now 7-year-old daughter Jórun Mai Smith, who still puts our prejudice to shame on a daily basis while ice skating and riding on her bicycle. And also to my late parents, who, I know, with great pride, from above, are observing my achievements by the completion of this thesis. iii Contents Contents PROPAGATION AND SCATTERING OF REFLECTION SEISMIC WAVES IN A BASALT SUCCESSION ................................................................................................................................................... I FOREWORD .................................................................................................................................................... I ABSTRACT .................................................................................................................................................... II ACKNOWLEDGEMENTS ........................................................................................................................... III CONTENTS ................................................................................................................................................... IV 1. THESIS OBJECTIVES ........................................................................................................................... 1 2. GEOPHYSICAL

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