Seismic Oceanography: A New Tool to Characterize Physical Oceanographic Structures and Processes

Abstract

[eng] Large scale global oceanic circulation redistributes heat and freshwater and therefore affects global climate. One of its main forcing mechanisms is, in addition to surface heat and freshwater fluxes, the diapycnal (across lines of equal density) mixing in the ocean interior. The energy needed to drive the mixing processes is mainly provided by tides and wind [Wunsch, 2002]. It is transformed into internal wave energy, cascading through a range of smaller scales leading finally into turbulence and molecular dissipation. Water masses in the ocean are stratified and often separated by relatively thin layers with strong gradients in temperature and/or salinity across which heat and mass transfer occur in order to maintain global circulation and stratification. However, these processes are difficult to observe in practice. Below a few meters, the ocean is opaque to light, and thus to direct optical observations of deep processes [Thorpe, 2005]. Therefore, the development of scientific methodologies and instruments to directly or indirectly measure processes in the ocean interior are of high importance to understanding those processes and their implications. The motivation behind this research is two-tier: 1) broadly, and academically, it is the scientific curiosity of understanding the ocean in order to better comprehend its role in the context of Earth systems; 2) expressly, the motivation is to develop the methodological toolset necessary to observe the ocean on a spatial and temporal scale not possible with traditional oceanographic techniques, thus allowing the foundation of more accurate models of ocean circulation and thereby, ocean-climate interactions. The toolset is emerging as a robust technique of physical oceanography known as 'seismic oceanography'. By definition, seismic oceanography is the application of multichannel seismic (MCS) reflection profiling to physical oceanography. This definition, however, could be subject to future revision and refinement because the development of seismic oceanography observational tools will inevitably lead to newer perspectives. The Mediterranean Outflow Water (henceforth, MOW) is a natural laboratory for seismic oceanography. The MOW was chosen to test seismic reflection in oceanography for three main reasons: 1) The strong oceanographic signature of the MOW. Due to the penetration of the MOW into the North Atlantic through the Strait of Gibraltar, strong characteristic contrasts in temperature (1.5 °C) and salinity (0.3 psu) and thus, density (0.4 kg/m3) are observed between the MOW and the surrounding Atlantic waters [Baringer and Price, 1997]. These contrasts in density (along with sound speed) are the contributing factors to reflection coefficient, making the identification of structures and processes possible. 2) The large variety of oceanographic and topographic features, such as a continental slope, undulating seafloor (including seamounts and basins) and mesoscale Mediterranean salt lenses (meddies). These structures and processes are believed to play an important role in maintaining the temperature and salinity distribution in the north Atlantic [Bower et. al., 1997]. 3) Finally, extensive archived data sets of bothoceanographic and seismic data place interpretive constraints on the data collected. Part I of this thesis consists of two peer-reviewed papers published by the author and coauthors (Chapters 1 and 2), one manuscript submitted for publication (Chapter 3) and two published peer-reviewed research letters that the author played a lesser role developing (Chapter 4). Part II of the thesis addresses the seismological (Chapter 5) and oceanographic backgrounds (Chapter 6) in the context of some of the structures and processes that are amenable to seismic ensonification. Part III consists of general discussions and conclusions (Chapter 7) and potential future research and development (Chapter 8).

[cat] La motivació que hi ha al darrera d'aquest treball es pot separar en dos nivells: 1) En termes generals, i acadèmicament, és la curiositat purament científica d'estudiar l'oceà per tal de comprendre millor el seu paper en el context de les Ciències de la Terra; 2) Concretament, la motivació és desenvolupar les eines necessàries per a observar l'oceà a una escala espaial i temporal que no és possible amb les tècniques tradicionals oceanogràfiques, permetent així generar models més precisos de circulació oceànica i, per tant, de les interaccions oceà-clima. L'objectiu a curt i mitjà termini d'aquesta tesi és el desenvolupament de noves eines d'oceanografia física que proporcionin noves perspectives sobre la dinàmica oceànica. Aquest conjunt d'eines està emergint com una metodologia sòlida dins de l'oceanografia física coneguda com 'oceanografia sísmica'. Per definició, l'oceanografia sísmica és l'aplicació de la sísmica de reflexió multicanal (MCS) a l'oceanografia física. Aquesta definició, però, podria estar subjecte a una futura revisió i perfeccionament, ja que el desenvolupament d'eines d'oceanografia sísmica inevitablement donarà lloc a noves perspectives. La part principal d'aquesta tesi la constitueixen quatre articles, publicats per l'autor i co-autors al llarg del seu període d'investigació (capítols 1, 2 i 4); així com un article sotmès (Capítol 3). En la segona part, com a suport per als lectors no familiaritzats, s'aborden els antecedents sismològics (Capítol 5) i oceanogràfics (Capítol 6) en el context d'algunes de les estructures i processos que són susceptibles de ser identificats mitjançant la sísmica. Les conclusions generals es presenten en el Capítol 7 i en el Capítol 8 es donen algunes recomanacions per a futures investigacions i desenvolupaments (Part III). La tesi es complementa amb quatre apèndixs, on a banda d'un ampli resum en català (Apèndix I), hi ha els diagrames de flux utilitzats per l'autor en el processament de les dades sísmiques (Apèndix II), un conjunt de set desplegables de les seccions sísmiques en gran format (Apèndix III) i finalment un glossari de termes útils per ajudar als lectors no iniciats en qualsevol de les branques de la sismologia o l'oceanografia (Apèndix IV).