IEA QIWIFrench-New Zealand International Emerging Action in Geosciences
France: Dr. Y. Le Gonidec (CNRS)
New Zealand: Dr G. Lamarche and Dr. Y. Ladroit (NIWA)
December 2019: QIWI meeting at NIWA (New Zealand, Wellington)
Paper published in EOS – Gas Bubble Forensics Team Surveils the New Zealand Ocean (2019).
June 2019: QIWI meeting at IFREMER (France, Brest)
The IEA QIWI (International Emerging Action “Quantitative Imaging of Water-column Inhomogeneities using backscatter acoustic signal“) is managed by Dr. Yves Le Gonidec (Géosciences Rennes, CNRS – Université de Rennes 1) in collaboration with the National Institute of Water and Atmospheric research (New Zealand, Wellington).
Missions and research themes
Detecting liquid or gaseous features in the ocean is generating considerable interest in the geoscience community because of their potentially high economic values (oil & gas, mining, freshwater), their significance for environmental management (oil/gas leakage, biodiversity mapping, greenhouse gas monitoring) and, in New Zealand, cultural and traditional values. Modern marine multibeam echosounders provide the most reliable, accessible and technologically advanced means to develop systematic, measurable and repeatable means of analysis of such features by using the acoustic energy backscattered by gas, oil bubbles, freshwater plumes, particulate matter, etc. Identifying and characterising flares and plumes from the marine acoustic backscatter signal is a difficult task due to the often very weak contrast of acoustic impedance between scatterers and sea-water, the transient and dynamic behaviour of the scatterers, and the complexity of the physics involved in marine acoustic signal analysis in this dynamic environment. In 2018, the QUOI (Quantitative Ocean-Column Imaging using hydroacoustic sources) oceanographic voyage, leads by the NIWA, was performed in the hydrothermal vent field in the shallow waters of Bay of Plenty (New Zealand) to tackle some of the issues pertinent to this topic.
Main OBJECTIVES OF THE PROJECT
The aim of the IEA QIWI is to enhance understanding of the origin, behaviour and quantity of physical features in the water column recorded in marine acoustic systems: the main projects of research deal methodological development and specific processing of multi-sensor and multi-target acoustic datasets acquired during the QUOI voyage. Complementary measurements are also available, including an optical towed-camera (IMAS) over active gas seeps and ground-truth sampling data for direct observation and characterization (natural targets), and a Synthetic Seep Generator (SSG, UNH) used to generate gas bubbles automatically released in the water column (artificial targets). Passive acoustic experiments performed to record ambient acoustic noises are analysed in order to identify acoustic sounds associated to natural gas bubbles released at the seafloor, but this remains challenging because of the noisy environnement. Acoustic experiments deal with the use of different echosounders used during the QUOI voyage. A single-beam transducer, mounted to a Pan and Tilt system (IFREMER) to acquire acoustic profiles with different incident angles, and two multibeam systems with a large acoustic fan aperture were recorded simultaneously, allowing a cross-calibration experiment performed on the SSG deployed in a seafloor area free of natural seeps. A multifrequency approach has been performed with a set of calibrated singlebeam echosounders in the frequency range 18-200 kHz and is used to identify the bubble size and gas viscosity: the quantification of these two frequency dependent parameters can contribute to discriminate between CO2 and CH4 gases, and between small and large bubbles associated to different rising speeds in the water column: the approach may inform to better understand the origin of the seep and the flare morphology from the seafloor to the sea surface.
institutions and laboratories involved
- Géosciences Rennes, CNRS – Université de Rennes 1: Yves Le Gonidec
- IFREMER, Brest: Jean-Marie Augustin, Arnaud Gaillot, Cyrille Poncelet
- NIWA, Wellington: Yoann Ladroit, Geoffroy Lamarche (leader of the QUOI voyage), Arne Pallentin, Sally Watson
- IMAS/CSIRO, Hobart: Vanessa Lucieer, Amy Nau, Erika Spain
- UNH, New Hampshire: Tom Weber, Elizabeth Weidner
Perspective view of the Calypso Hydrothermal Vent Field with acoustic flares generated by gas bubbles backscattered acoustic echos (QUOI voyage).
Active gas seep observed by the towed-camera (IMAS): video sample (2 s) of natural gas bubbles released at the seafloor in the hydrothermal vent of the Bay of Plenty, New Zealand (QUOI voyage).
The Synthetic Seep Generator (SSG) developed by the University of New Hampshire (T. Weber) generates artificial bubbles in the water column (QUOI voyage). Credit photo: G. Lamarche