Final Report Summary - OCEANSEIS (Oceanic exploration with seismic reflection data)
Oceans constitute invaluable sources of energy and material for human life and strongly influence Earth’s climate. Scientific community has been always conscientious of the importance of the oceans in the Earth System and at the same time have always tried to overcome the challenges related to their exploration.
In 1948, F.P. Shepard, a famous American marine geologist wrote: "Man’s perpetual curiosity regarding the unknown has opened many frontiers. Until recent years much more was known about the surface of the moon than about the vast areas that lie beneath three-fourths of the surface of our own planet.” Because of the high pressure, the lack of light and the lack of oxygen, the exploration of the deep ocean is still a challenge for oceanographers.
OCEANSEIS (www.utm.csic.es/so/projects/oceanseis) aims to give a step forward towards the improvement of ocean exploration from a point of view of its physical processes and properties. This project explores the potential of the acoustic discipline called Seismic Oceanography (SO), which consists on the use of multi-channel seismic reflection (MCS) technique, traditionally used for geological prospection, to detect the physical properties of the oceanic water. As in several scientific fields such as medicine, astronomy, geology, etc., inaccessible media are explored using waves. For example, the X rays are used to observe the bones, the ultrasound scan are used to image the babies or the sound can be used to examine the subsurface of the Earth. Similarly, the acoustic data used in Seismic Oceanography allow to observe the temperature and salinity of the ocean from the surface to the seafloor and along lateral scales from 100 m to 100000 m.
Therefore, the OCEANSEIS’ main objectives and the work carried out to achieve them are the following ones:
Objective 1. We aim to recover temperature, salinity and potential density data from acoustic reflectivity in the ocean, because physical oceanographers need to explore the water in terms of these three variables to understand how is its motion and how the heat and salt are transported around the oceans. With this objective in mind we developed three different methods: a processing-oriented one, the full-waveform inversion (FWI), which is deterministic and the very fast simulated annealing (VFSA), which is statistical. These three methodologies have very different principles behind and each one has their own advantages and issues. The FWI and VFSA methods have still important issues to be solved in order to be successfully applied to real data. Therefore, up to the present time and based on our comparison between the main advantages and issues of these three methodologies , the processing-oriented method is the one we recommend to apply for real oceanic data inversion. The most relevant achieved results have been: i) the development and application of the processing-oriented method to recover temperature, salinity and potential density in the Mediterranean Undercurrent that flows from the Strait of Gibraltar into the Atlantic Ocean (Figure 1a and 1b); and ii) the development of the 1D FWI method applied to invert temperature and salinity directly from acoustic impedance. The detailed methodologies, as well as the results and accuracy of the methods are described in Biescas et al 2014- JGR-Oceans and Bornstein et al 2013- GRL, which summarize the most important results of OCEANSEIS in terms of inversion methods.
These obtained results provide 2D sections of temperature, salinity and potential density of the sea water with high resolution (100 m in the lateral and 10 m in the vertical) over hundreds of kilometers. These variables had never been explored before with such a high lateral resolution. We can now observe in these results how the masses of different temperature mix , which are the fine structures that are created by the mixing processes, and finally understand which are the mechanisms governing mixing. This new knowledge will help us to better understand the circulation in the ocean, including the formation and development of currents, eddies, or fronts. This type of information is needed to understand how the ocean interacts with the atmosphere and how does it influence the Earth's climate; where are the warm/cold currents where we can get fishes; how a pollutant will spread in the middle of the ocean if an accident occurs, etc. This research is relevant to oceanic modelers, fishery policy makers, environmental policy makers and physical oceanographers.
Objective 2. Detailed study of the acoustic reflectors and their physical causes. We aim to improve our ability to relate reflections to the ocean dynamics that creates them. The main result achieved so far regarding the understanding of the acoustic reflectivity in the ocean and its relation with physical oceanographic processes, is the relation between oceanic reflectivity and potential density in the water, which is the variable that determines the dynamics. Previous works had assumed that acoustic reflectors follow isopycnal (constant potential density) surfaces, mainly concerning two different topics, the study of the energy cascading from internal waves to turbulence, and the interpretation of front dynamics. Using the inverted temperature, salinity an potential density, we were able to compare the isopycnal surfaces with the acoustic reflectivity data. These results highlight that the hypothesis of considering acoustic reflectivity as a proxy of isopycnal surfaces is not valid for all dynamic contexts, but it is case-by-case dependent, and it is specially weak in the case of lateral interleaving or fronts, while the studies of internal wave energy from undulating reflectors within a vertical thermal gradient should be valid.
In addition, we carried out new surveys and processed new data, which image the finestructure of shallow eddies in the Northeastern coast of the Pacific Ocean, shallow thermohaline finestructure in the Mediterranean Sea and the outflow of Mediterranean water through the Strait of Gibraltar. All these new data have been included in the seismic oceanography catalog of our web site www.utm.csic.es/so/map with the aim of sharing the results with the international scientific community.
These results will help physical oceanographers to interpret seismic reflection images in terms of the mixing processes involved.
Objective 3. Optimization of MCS experiment and systems for oceanographic purposes. All the SO surveys carried out so far have used standard instrumentation and standard parameters that were originally designed for geological purposes. We aim to adapt this technology to marine exploration and find the most suitable acoustic sources and design of the experiment to be applied to the water layer. We had two main goals in mind: i) designing a portable system that could be installed in mid-size oceanographic vessels; and ii) find affordable acoustic sources, smaller and cheaper than the ones that are used in seismic surveys, which need a big and very expensive seismic vessel.
Alternative acoustic sources were tested using synthetic modeling. The most promising alternatives to conventional airgun sources are based on the theory of RADAR signals, which allows to decrease the power of the signal by making the signal longer in time. We are contacting potentially interested companies in Europe and Canada, which could be interested in the developing of new marine technology applied to the observation of the ocean's temperature field at high resolution.
OCEANSEIS aims to ocean exploration research. Rapid population growth and global warming are creating unprecedented challenges in which the oceans will play an important role. The significant role of the oceans in climate patterns, carbon cycle and life on Earth is one of the statements of the ”Galway Declaration”. OCEANSEIS aims at contributing to the progress in oceanic observation and will provide new knowledge about thermohaline structures, interaction between oceanic scales and mixing processes. This advance in the knowledge of oceans and fluids dynamics will have potential impacts in: material/nutrients transport; heat/energy transport; environmental studies including pollutants dispersion and detection of deep contaminants such as oil spills; oceanic global, mesoscale and submesoscale modeling; climate change; developing of sustainable fishing industry and renewable energies.
Humans are driven to explore the unknown, discover new worlds, push the boundaries of our scientific and technical limits, and then push further.
Figure Captions
Figure1a: Thermal anomalies recovered from acoustic reflectivity in the Gulf of Cadiz, NE Atlantic Ocean.
Figure1b: Thermal structure of a Mediterranean water eddy recovered from acoustic reflectivity in the Gulf of Cadiz, NE Atlantic Ocean.
In 1948, F.P. Shepard, a famous American marine geologist wrote: "Man’s perpetual curiosity regarding the unknown has opened many frontiers. Until recent years much more was known about the surface of the moon than about the vast areas that lie beneath three-fourths of the surface of our own planet.” Because of the high pressure, the lack of light and the lack of oxygen, the exploration of the deep ocean is still a challenge for oceanographers.
OCEANSEIS (www.utm.csic.es/so/projects/oceanseis) aims to give a step forward towards the improvement of ocean exploration from a point of view of its physical processes and properties. This project explores the potential of the acoustic discipline called Seismic Oceanography (SO), which consists on the use of multi-channel seismic reflection (MCS) technique, traditionally used for geological prospection, to detect the physical properties of the oceanic water. As in several scientific fields such as medicine, astronomy, geology, etc., inaccessible media are explored using waves. For example, the X rays are used to observe the bones, the ultrasound scan are used to image the babies or the sound can be used to examine the subsurface of the Earth. Similarly, the acoustic data used in Seismic Oceanography allow to observe the temperature and salinity of the ocean from the surface to the seafloor and along lateral scales from 100 m to 100000 m.
Therefore, the OCEANSEIS’ main objectives and the work carried out to achieve them are the following ones:
Objective 1. We aim to recover temperature, salinity and potential density data from acoustic reflectivity in the ocean, because physical oceanographers need to explore the water in terms of these three variables to understand how is its motion and how the heat and salt are transported around the oceans. With this objective in mind we developed three different methods: a processing-oriented one, the full-waveform inversion (FWI), which is deterministic and the very fast simulated annealing (VFSA), which is statistical. These three methodologies have very different principles behind and each one has their own advantages and issues. The FWI and VFSA methods have still important issues to be solved in order to be successfully applied to real data. Therefore, up to the present time and based on our comparison between the main advantages and issues of these three methodologies , the processing-oriented method is the one we recommend to apply for real oceanic data inversion. The most relevant achieved results have been: i) the development and application of the processing-oriented method to recover temperature, salinity and potential density in the Mediterranean Undercurrent that flows from the Strait of Gibraltar into the Atlantic Ocean (Figure 1a and 1b); and ii) the development of the 1D FWI method applied to invert temperature and salinity directly from acoustic impedance. The detailed methodologies, as well as the results and accuracy of the methods are described in Biescas et al 2014- JGR-Oceans and Bornstein et al 2013- GRL, which summarize the most important results of OCEANSEIS in terms of inversion methods.
These obtained results provide 2D sections of temperature, salinity and potential density of the sea water with high resolution (100 m in the lateral and 10 m in the vertical) over hundreds of kilometers. These variables had never been explored before with such a high lateral resolution. We can now observe in these results how the masses of different temperature mix , which are the fine structures that are created by the mixing processes, and finally understand which are the mechanisms governing mixing. This new knowledge will help us to better understand the circulation in the ocean, including the formation and development of currents, eddies, or fronts. This type of information is needed to understand how the ocean interacts with the atmosphere and how does it influence the Earth's climate; where are the warm/cold currents where we can get fishes; how a pollutant will spread in the middle of the ocean if an accident occurs, etc. This research is relevant to oceanic modelers, fishery policy makers, environmental policy makers and physical oceanographers.
Objective 2. Detailed study of the acoustic reflectors and their physical causes. We aim to improve our ability to relate reflections to the ocean dynamics that creates them. The main result achieved so far regarding the understanding of the acoustic reflectivity in the ocean and its relation with physical oceanographic processes, is the relation between oceanic reflectivity and potential density in the water, which is the variable that determines the dynamics. Previous works had assumed that acoustic reflectors follow isopycnal (constant potential density) surfaces, mainly concerning two different topics, the study of the energy cascading from internal waves to turbulence, and the interpretation of front dynamics. Using the inverted temperature, salinity an potential density, we were able to compare the isopycnal surfaces with the acoustic reflectivity data. These results highlight that the hypothesis of considering acoustic reflectivity as a proxy of isopycnal surfaces is not valid for all dynamic contexts, but it is case-by-case dependent, and it is specially weak in the case of lateral interleaving or fronts, while the studies of internal wave energy from undulating reflectors within a vertical thermal gradient should be valid.
In addition, we carried out new surveys and processed new data, which image the finestructure of shallow eddies in the Northeastern coast of the Pacific Ocean, shallow thermohaline finestructure in the Mediterranean Sea and the outflow of Mediterranean water through the Strait of Gibraltar. All these new data have been included in the seismic oceanography catalog of our web site www.utm.csic.es/so/map with the aim of sharing the results with the international scientific community.
These results will help physical oceanographers to interpret seismic reflection images in terms of the mixing processes involved.
Objective 3. Optimization of MCS experiment and systems for oceanographic purposes. All the SO surveys carried out so far have used standard instrumentation and standard parameters that were originally designed for geological purposes. We aim to adapt this technology to marine exploration and find the most suitable acoustic sources and design of the experiment to be applied to the water layer. We had two main goals in mind: i) designing a portable system that could be installed in mid-size oceanographic vessels; and ii) find affordable acoustic sources, smaller and cheaper than the ones that are used in seismic surveys, which need a big and very expensive seismic vessel.
Alternative acoustic sources were tested using synthetic modeling. The most promising alternatives to conventional airgun sources are based on the theory of RADAR signals, which allows to decrease the power of the signal by making the signal longer in time. We are contacting potentially interested companies in Europe and Canada, which could be interested in the developing of new marine technology applied to the observation of the ocean's temperature field at high resolution.
OCEANSEIS aims to ocean exploration research. Rapid population growth and global warming are creating unprecedented challenges in which the oceans will play an important role. The significant role of the oceans in climate patterns, carbon cycle and life on Earth is one of the statements of the ”Galway Declaration”. OCEANSEIS aims at contributing to the progress in oceanic observation and will provide new knowledge about thermohaline structures, interaction between oceanic scales and mixing processes. This advance in the knowledge of oceans and fluids dynamics will have potential impacts in: material/nutrients transport; heat/energy transport; environmental studies including pollutants dispersion and detection of deep contaminants such as oil spills; oceanic global, mesoscale and submesoscale modeling; climate change; developing of sustainable fishing industry and renewable energies.
Humans are driven to explore the unknown, discover new worlds, push the boundaries of our scientific and technical limits, and then push further.
Figure Captions
Figure1a: Thermal anomalies recovered from acoustic reflectivity in the Gulf of Cadiz, NE Atlantic Ocean.
Figure1b: Thermal structure of a Mediterranean water eddy recovered from acoustic reflectivity in the Gulf of Cadiz, NE Atlantic Ocean.