Acoustic monitoring of gas emissions from the seafloor. Part I: quantifying the volumetric flow of bubbles

Three decades of continuous ocean exploration have led us to identify subsurface fluid related processes as a key phenomenon in marine earth science research. The number of seep areas located on the seafloor has been constantly increasing with the use of multi-scale imagery techniques. Due to recent advances in transducer technology and computer processing, multibeam echosounders are now commonly used to detect submarine gas seeps escaping from the seafloor into the water column. A growing number of en-route surveys shows that sites of gas emissions escaping from the seafloor are much more numerous than previously thought. Estimating the temporal variability of the gas flow rate and volumes escaping from the seafloor has thus become a challenge of relevant interest which could be addressed by sea-floor continuous acoustic monitoring. Here, we investigate the feasibility of estimating the volumetric flow rates of gas emissions from horizontal backscattered acoustic signals. Different models based on the acoustic backscattering theory of bubbles are presented. The forward volume backscattering strength and the inversion volumetric flow rate solutions were validated with acoustic measurements from artificial gas flow rates generated in controlled sea-water tank experiments. A sensitivity analysis was carried out to investigate the behavior of the 120-kHz forward solution with respect to model input parameters (horizontal distance between transducer and bubble stream, bubble size distribution and ascent rate). The most sensitive parameter was found to be the distance of the bubble stream which can affect the volume backscattering strength by 20 dB within the horizontal range of 0–200 m. Results were used to derive the detection probability of a bubble stream for a given volume backscattering strength threshold according to different bubble flow rates and horizontal distance.     

Aerosol optical properties and its radiative forcing over Yulin,China in 2001 and 2002

The aerosol optical properties and direct radiative forcing over the Mu Us desert of northern China, acquired through a CE318 sunphotometer of the ground-based Aerosol Robotic Network (AERONET), are analyzed. The seasonal variations in the aerosol optical properties are examined. The effect of meteorological elements (pressure, temperature, water vapor pressure, relative humidity and wind speed) on the aerosol optical properties is also studied. Then, the sources and optical properties under two different cases, a dust event and a pollution event, are compared. The results show that the high aerosol optical depth (AOD) found in Yulin was mostly attributed to the occurrence of dust events in spring from the Mu Us desert and deserts of West China and Mongolia, as well as the impacts of anthropogenic pollutant particles from the middle part of China in the other seasons. The seasonal variation and the probability distribution of the radiative forcing and the radiative forcing efficiency at the surface and the top of the atmosphere are analyzed and regressed using the linear and Gaussian regression methods.     

Paleo-deviatoric stress magnitudes from calcite twins and related structural permeability evolution in minor faults: Example from the toarcian shale of the French Causses Basin, Aveyron, France

The relationship between deformation and so-called fluid paleotransfers in minor faults has been analysed in an argillaceous formation located in the Causses Basin in France. The fluid paleotransfers are related to the fault activity to a large extent. We attempt to estimate the intensity of paleo-deviatoric stress magnitudes under which the fault activity may have occurred and consequently, the change in the structural fault permeability. The paleo-deviatoric stress magnitudes were calculated with the inverse method of Etchecopar applied to calcite twinning. The measured crystals are contained within the core zone of minor faults and this study is based on a previous complete microtectonic and microstructural analysis of the faults. In this paper, analysis of calcite twinning has been applied for the first time to vein fillings associated small faults in a context of relatively weak deformation, a condition ensured by the tectonic and structural analysis. Calculation and discussion of the paleo-deviatoric stress tensors in relation to the evolution of the structural fault permeability and to the hydraulic behaviour of the faults are the aim of this paper. The analysed faults, created and active during the same tectonic event, were permeable under a (σ₁–σ₃) mean value of 40–50 MPa. On the other hand, the reactivation of faults during a second tectonic event implies mean (σ₁–σ₃) value higher than 40–50 MPa, especially for the faults that are poorly oriented with respect to the principal tectonic stress directions. The core zone of these faults remained sealed and impermeable or became permeable by development of microcracks inside the pre-existing fillings.