Oceanografia sísmica. Una nova eina per entendre els oceans

L'oceanografia sísmica s'està convertint en una eina pràctica per estudiar la circulació oceànica a gran escala, els processos de mescla a mesoescala i la seva dinàmica. A més, s'ha demostrat la seva utilitat per quantificar paràmetres com ara la temperatura i la salinitat. Des de 2003, s'ha emprès la recerca en la millora i l'adaptació de la sísmica de reflexió, una eina robusta ben acceptada en el món acadèmic i la indústria dels hidrocarburs per visualitzar l'escorça profunda i els marges de les plaques tectòniques, i per localitzar possibles reservoris de petroli, respectivament. La necessitat urgent d'identificar amb precisió els mecanismes responsables del canvi climàtic fa que l'oceanografia sísmica sigui encara de més interès per als oceanògrafs físics. Atesa la gran contribució dels oceans al transport de calor (més o menys equivalent a l'atmosfera encara que amb molt menys gruix), és necessari entendre els processos oceànics i les imatges detallades de les estructures oceàniques, com ara els remolins, fronts i escales termohalines. L'oceanografia sísmica proporciona aquesta imatge detallada, així com la quantificació de les propietats intrínseques oceàniques. La sobreabundància d'arxius amb dades sísmiques marines, molts d'ells amb registres de reflexions relativament febles de l'oceà, ofereix un conjunt de dades a escala mundial pràcticament il·limitat amb el qual es pot estudiar la circulació oceànica. L'oceanografia sísmica no només ofereix l'oportunitat de representar espacialment l'estructura termohalina, sinó que l'accés a bases de dades històriques pot donar informació sobre el comportament temporal de la circulació, i això és especialment important de cara a la comprensió del canvi climàtic global

Oceanografia sísmica. Una nova eina per entendre els oceans

L'oceanografia sísmica s'està convertint en una eina pràctica per estudiar la circulació oceànica a gran escala, els processos de mescla a mesoescala i la seva dinàmica. A més, s'ha demostrat la seva utilitat per quantificar paràmetres com ara la temperatura i la salinitat. Des de 2003, s'ha emprès la recerca en la millora i l'adaptació de la sísmica de reflexió, una eina robusta ben acceptada en el món acadèmic i la indústria dels hidrocarburs per visualitzar l'escorça profunda i els marges de les plaques tectòniques, i per localitzar possibles reservoris de petroli, respectivament. La necessitat urgent d'identificar amb precisió els mecanismes responsables del canvi climàtic fa que l'oceanografia sísmica sigui encara de més interès per als oceanògrafs físics. Atesa la gran contribució dels oceans al transport de calor (més o menys equivalent a l'atmosfera encara que amb molt menys gruix), és necessari entendre els processos oceànics i les imatges detallades de les estructures oceàniques, com ara els remolins, fronts i escales termohalines. L'oceanografia sísmica proporciona aquesta imatge detallada, així com la quantificació de les propietats intrínseques oceàniques. La sobreabundància d'arxius amb dades sísmiques marines, molts d'ells amb registres de reflexions relativament febles de l'oceà, ofereix un conjunt de dades a escala mundial pràcticament il·limitat amb el qual es pot estudiar la circulació oceànica. L'oceanografia sísmica no només ofereix l'oportunitat de representar espacialment l'estructura termohalina, sinó que l'accés a bases de dades històriques pot donar informació sobre el comportament temporal de la circulació, i això és especialment important de cara a la comprensió del canvi climàtic global

Stochastic Heterogeneity Mapping as a tool to quantify turbulence in reflectivity layers of thermohaline staircases in the Tyrrhenian Sea

European Geosciences Union General Assembly 2013, 7-12 April 2013, Vienna, AustriaProcessed multi-channel seismic data acquired in the Tyrrhenian Sea in April-May 2010 with the B/O Sarmiento de Gamboa provide images of oceanic thermohaline staircases. Thermohaline staircases are regular, well-defined step-like variations in vertical profiles of temperature and salinity. In the ocean they are thought to be the result of double diffusion driven by the two order of magnitude difference in the diffusivities of heat and salt. Staircases are believed to have an anomalously weak internal wave-induced turbulence, making them suitable for the estimation of a lower limit of turbulent disturbances detectable by multi-channel seismics. We apply stochastic heterogeneity mapping based on the band-limited von Kármán function to post-stack time-migrated seismic data to extract stochastic parameters such as the Hurst number (a measure of reflection interface roughness) and correlation length (scale length). For scale sizes smaller than the correlation length, the von Kármán model describes a power law (fractal) process where the Hurst number is its exponent. We present the results of our analysis corroborated by benchmark tests performed on synthetic seismic data generated from random fractal surfaces. The synthetic tests are found to verify the robustness of the technique. Lower Hurst numbers represent a richer range of high wavenumbers and therefore correspond to a broader range of heterogeneity in reflection events. We interpret a broader range of heterogeneity as indicative of a greater degree of turbulence. Some areas of the seismic data show a spatial variation in Hurst number across several frequency bands that may indicate some preferential coupling of energy at different depthsPeer Reviewe

Seismic Oceanography in the Tyrrhenian Sea – Thermohaline Staircases, Eddies and Internal Waves

We use seismic oceanography to document and analyze oceanic thermohaline finestructure across the Tyrrhenian Sea. Multichannel seismic (MCS) reflection data were acquired during the MEDiterranean OCcidental survey in April-May 2010. We deployed along-track expendable bathythermograph probes simultaneous with MCS acquisition. At nearby locations we gathered conductivity-temperature-depth data. An autonomous glider survey added in-situ measurements of oceanic properties. The seismic reflectivity clearly delineates thermohaline finestructure in the upper 2,000 m of the water column, indicating the interfaces between Atlantic Water/Winter Intermediate Water, Levantine Intermediate Water, and Tyrrhenian Deep Water. We observe the Northern Tyrrhenian Anticyclone, a near-surface meso-scale eddy, plus laterally and vertically extensive thermohaline staircases. Using MCS we are able to fully image the anticyclone to a depth of 800 m and to confirm the horizontal continuity of the thermohaline staircases of more than 200 km. The staircases show the clearest step-like gradients in the center of the basin while they become more diffuse towards the periphery and bottom, where impedance gradients become too small to be detected by MCS. We quantify the internal wave field and find it to be weak in the region of the eddy and in the center of the staircases, while it is stronger near the coastlines. Our results indicate this is because of the influence of the boundary currents, which disrupt the formation of staircases by preventing diffusive convection. In the interior of the basin the staircases are clearer and the internal wave field weaker, suggesting that other mixing processes such as double-diffusion prevail. Synopsis We studied the internal temperature and salinity structure of the Tyrrhenian Sea (Mediterranean) using the multichannel seismic reflection method (the same used in the hydrocarbon industry). Low frequency sound (seismic) waves are produced at the surface with an explosive air source and recorded by a towed cable containing hydrophones (underwater microphones). The data are processed to reveal 'stratigraphy' that result from contrasts in density that are themselves caused by changes in temperature and salinity. In this way we can map ocean circulation in two-dimensions. We also deployed in situ oceanographic probes to measure temperature and salinity in order to corroborate and optimize the processing of the seismic data. We then quantified the internal gravity wave field by tracking the peaks of seismic trace wavelets. Our results show that the interior of the Tyrrhenian Sea is largely isolated from internal waves that are generated by a large cyclonic boundary current that contains waters from the Atlantic ocean and other parts of the Mediterranean. This isolation allows the thermohaline finestructure to form, where small scale vertical mixing processes are at play. Understanding these mixing processes will aid researchers study global ocean circulation and to add constraints that can help improve climate models

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

[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)

Seismic and geological constraints on the structure and hydrocarbon potential of the northern Whitehorse trough, Yukon, Canada

The Whitehorse trough is an Early to Middle Jurassic underexplored and undrilled sedimentary basin in the northern Canadian Cordillera that is prospective for oil and gas. It records deposition in a collapsing forearc that evolved to become a synorogenic piggyback basin within the nascent Cordilleran orogen. The basin developed atop the Triassic and older arc terranes of Stikinia and Quesnellia, and locally overlaps the Cache Creek accretionary complex. A regional seismic survey and bedrock mapping across the northern Whitehorse trough in Yukon indicate that Jurassic strata of the Laberge Group are up to 3000 m thick and were deformed by southwest-verging folds and thrust faults. These structures have an overall en échelon, sigmoidal pattern that is consistent with dextral transpression associated with strike-slip displacement along the Teslin and Braeburn faults. The structures likely began developing in Middle to Late Jurassic and were further enhanced during the mid-Cretaceous. Fold-and-thrust structures of the northern Whitehorse trough present suitable structural plays in areas where Jurassic strata with good source rock potential and adequate thermal maturity are most prospective. © 2012 by the Canadian Society of Petroleum Geologists. All Rights Reserved.Peer Reviewe

A decision-theoretic algorithm for bundle purchasing in multiple open ascending-price auctions

This paper presents an algorithm for decision-making where the buyer needs to procure one of possibly many bundles of complementary products, and items are sold individually in multiple open ascending-price (English) auctions. Auctions have fixed start and end times, and some may run concurrently. Expected utility of a bidding choice is computed by considering expected utility of choices at future decisions. The problem is modeled as a Markov decision process, and dynamic programming is used to determine the value of bidding/not bidding in each state. Three techniques for reducing the state space are given. Results show that a bidding agent using this algorithm achieves significantly higher utility on average when compared with one that does not consider the value of future choices.Peer reviewed: YesNRC publication: Ye