•Passive acoustic techniques can be used to identify and quantify underwater gas release.•We propose an adaptive single bubble identification technique, incorporating bubble acoustic characteristics ...of time, frequency and strength.•In a controlled gas release experiment, the bubble radius is acoustically determined as 0.15–0.3 cm, and optically determined as 0.2–0.5 cm.•The gas flux is acoustically estimated as 32–88 kg/day in response to a known gas injection flow rate 143 kg/day.
Passive acoustic techniques can be used to identify and quantify underwater gas release at natural sites, or at locations related to anthropogenic activities. There are still significant issues in extracting bubble signals from background noise, particularly for bubble counting and sizing techniques relying on inversion of the time-averaged acoustic spectrum. In this work we propose an adaptive single bubble identification technique, which incorporates bubble acoustic characteristics including pulsation time interval, frequency bandwidth and radiation strength. The method applies a cross-spectrogram, enabling an increase in signal-to-noise ratio resulting in a reduction of the false alarm rate on bubble identification. We demonstrate this technique using an array of hydrophones to determine the bubble size distribution and gas flux at a controlled CO2 release site, 4 m beneath the seabed, at 120 m water depth in the central North Sea. The results show that the bubble radius, as estimated using acoustics has a distribution with a peak in the 0.15–0.3 cm range, while an estimate based on optical method suggests a range of 0.2–0.5 cm. The gas flux is acoustically estimated as 32–88 kg/day in response to a known gas injection flow rate 143 kg/day, indicating 22–62% of the injected CO2 is emitted from the seabed in gaseous form, with the remainder being trapped, or dissolved.
•CO2-induced changes in chimney’s porosity/permeability are minor at decadal scales.•Seabed CO2 leakage via chimneys depends on the pre-existing hydrogeological state.•We only predict leakage with ...chimney permeabilities >10−14 m2 and CO2 content >30%.•Chimney permeabilities of 10−14 m2 result from fracture apertures >0.05 mm.
Sub-vertical geological structures that cut through the overburden, usually called chimneys or pipes, are common in sedimentary basins. Chimneys behave as conduits that hydraulically connect deep strata with the overburden and seabed. Hence, if stored CO2 migrates to a sufficiently high permeability chimney the risk of CO2 leakage at the seabed increases. Despite the possible negative effects these structures may have on the integrity of CO2 storage sites, little is known about (i) their effective permeability distribution, controlled by the combined role of fractures and matrix, and (ii) feedback mechanisms between porosity-permeability, CO2 reactivity and mineralogy within them. Reactive transport modelling is used to perform 2D axisymmetric radial simulations of geological systems containing chimneys. CO2 saturations of 10%, 30% and 50% are imposed on a cell located next to the symmetry axis at the base of the model. Under hydrostatic conditions, CO2 reaches the seabed, at 500 m above the injection point, in less than 100 yr using injected CO2 saturations at or above 30% and with overburden isotropic permeabilities and chimney vertical permeabilities above 10−14 m2. Vertical fractures with apertures larger than 0.05 mm for volume fractions below 1% are sufficient to sustain such high vertical permeabilities in the chimney with a relatively high cap rock matrix permeability of 10−16 m2. Over 100 yr of CO2 injection, changes in porosity and permeability due to mineral precipitation/dissolution are negligible. For this time scale, in systems containing chimneys sufficiently far away from the injection well, the risk of CO2 leakage at the seabed is primarily controlled by the pre-existing hydrogeological state of the system.
Anthropogenic noise, e.g., shipping noise, are usually unavoidable and radiate over a certain area, e.g., 10 km depending on the noise source level and acoustic propagation channel, which might ...affect seabed acoustic engineering as well as species’ communication, behaviour, fitness and consequently their survival. Therefore, better understanding of these noise sources allows better prediction of performance of seafloor acoustic research, engineering, and biological environment. Measurements from a hydrophone system mounted on the seafloor in the central North Sea permit comparisons between temporal and spectral seafloor noise as a vessel transits nearby. The measured data indicates that the peak energy in the power spectral density (PSD) of seafloor noise is dominated by that of multiple vessel tonal noise. The tidal level has a strong negative correlation with the seafloor noise variance, and the vessel to hydrophone angle has a modest negative correlation with both the noise variance and kurtosis, particularly within a certain distance (6.6 km at depth 150 m here). As the vessel approaches, the seafloor noise is sensitive to the noise radiated from the vessel manoeuvre dominated at frequencies 400–900 Hz. Further, as the vessel speed increases from 2 knots to 15 knots, the vessel tonal increases the PSD by 10–20 dB at multiple narrow frequency bands (770 and 850 Hz). Results reported here advance the knowledge of seafloor acoustic sensitivity to nearby transit vessels.
•Seafloor noise has been measured as a vessel transits nearby in the central North Sea.•Peak energy of seafloor noise is dominated by vessel tonal noise.•Strong negative correlation between tidal level and seafloor noise variance.•Seafloor noise is sensitive to vessel manoeuvre at frequencies 400–900 Hz.
Plasmonic nanoparticles have been demonstrated to enhance photocatalysis due to their strong photon absorption and efficient hot-carrier generation. However, plasmonic photocatalysts suffer from a ...short lifetime of plasmon-generated hot carriers that decay through internal relaxation pathways before being harnessed for chemical reactions. Here, we demonstrate the enhanced photocatalytic reduction of gold ions on gold nanorods functionalized with polyvinylpyrrolidone. The catalytic activities of the reaction are quantified by
in situ
monitoring of the spectral evolution of single nanorods using a dark-field scattering microscope. We observe a 13-fold increase in the reduction rate with the excitation of d-sp interband transition compared to dark conditions, and a negligible increase in the reduction rate when excited with intraband transition. The hole scavenger only plays a minor role in the photocatalytic reduction reaction. We attribute the enhanced photocatalysis to an efficient charge separation at the gold-polyvinylpyrrolidone interface, where photogenerated d-band holes at gold transfer to the HOMO of polyvinylpyrrolidone, leading to the prolonged lifetime of the electrons that subsequently reduce gold ions to gold atoms. These results provide new insight into the design of plasmonic photocatalysts with capping ligands.
Polyvinylpyrrolidone, a capping ligand adsorbed on gold nanorods, induces electron-hole separation to prolong the hot carrier lifetime and increase the efficiency of plasmonic photocatalysis under interband excitation.
•This paper has demonstrated the utility of beamforming as a tool for analysis of acoustic data from gas releases.•It has shown that the MVDR beamformer offers worthwhile performance gains over CBF ...in terms of greater enhancement of SNR and in terms of localisation accuracy.•Using beamforming it was possible to enhance the SNR and in so doing extend the range over which the passive acoustic system can detect and monitor a gas leak.
Passive acoustics has been identified as an important strategy to determine underwater gas flux at natural sites, or at locations related to anthropogenic activities. The ability of an acoustic system to detect, quantify and locate a gas leak is fundamentally controlled by the Signal to Noise Ratio (SNR) of the bubble sounds relative to the ambient noise. This work considers the use of beamforming methods to enhance the SNR and so improve the performance of passive acoustic systems. In this work we propose a focused beamforming technique to localise the gas seeps. To achieve high levels of noise reduction an adaptive beamformer is employed, specifically the minimum variance distortionless response (MVDR) beamformer. The technique is demonstrated using an array of five hydrophones collecting data at the controlled CO2 gas release experiment conducted as part of STEMM-CCS (Strategies for Environmental Monitoring of Marine Carbon Capture and Storage) project. The experimental results show that the adaptive beamformer outperforms the conventional (delay and sum) beamformer in undersea bubble localisation. Furthermore, the results with a pair of hydrophone arrays show an improvement of the localisation compared to the use of one hydrophone array.
•Time-lapse pseudo-3D seismic reflection imaging of CO2 migration in marine sediment.•CO2 is mapped via enhanced reflectivity, acoustic shadowing, and time-shifts of seismic horizons.•CO2 migration ...initially occurs via stable fracture propagation, then via dynamic fracture propagation.•CO2 began pooling when the gas injection rate exceeded the rate at which gas could escape the sediment.•Dynamic fracture propagation increases sediment permeability and removed gas pooling.
The ability to detect and monitor any escape of carbon dioxide (CO2) from sub-seafloor CO2 storage reservoirs is essential for public acceptance of carbon capture and storage (CCS) as a climate change mitigation strategy. Here, we use repeated high-resolution seismic reflection surveys acquired using a chirp profiler mounted on an autonomous underwater vehicle (AUV), to image CO2 gas released into shallow sub-surface sediments above a potential CCS storage site at 120 m water depth in the North Sea. Observations of temporal changes in seismic reflectivity, attenuation, unit thickness and the bulk permeability of sediment were used to develop a four-stage model of the evolution of gas migration in shallow marine sediments: Proto-migration, Immature Migration, Mature Migration, and Pathway Closure. Bubble flow was initially enabled through the propagation of stable fractures but, over time, transitioned to dynamic fractures with an associated step change in permeability. Once the gas injection rate exceeded the rate at which gas could escape the coarser sediments overlying the injection point, gas began to pool along a grain size boundary. This enhanced understanding of the migration of free gas in near-surface sediments will help improve methods of detection and quantification of gas in subsurface marine sediments.
•Passive acoustics detected and quantified offshore natural CO2 emissions.•Method was developed to quantify gas flux and derive bubble sizes.•CO2 bubble plume emitting gas at 2.3 L/min was detected ...up to distances of 8 m.•Quantification of CO2 was possible using a hydrophone at distances of up to 4 m.•The bubble signal needs to be 10 dB above ambient noise levels.
Estimating the range at which an acoustic receiver can detect greenhouse gas (e.g., CO2) leakage from the sub-seabed is essential for determining whether passive acoustic techniques can be an effective environmental monitoring tool above marine carbon storage sites. Here we report results from a shallow water experiment completed offshore the island of Panarea, Sicily, at a natural CO2 vent site, where the ability of passive acoustics to detect and quantify gas flux was determined at different distances. Cross-correlation methods determined the time of arrival for different travel paths which were confirmed by acoustic modelling. We develop an approach to quantify vent bubble size and gas flux. Inversion of the acoustic data was completed using the modelled impulse response to provide equivalent propagation ranges rather than physical ranges. The results show that our approach is capable of detecting a CO2 bubble plume with a gas flux rate of 2.3 L/min at ranges of up to 8 m, and determining gas flux and bubble size accurately at ranges of up to 4 m in shallow water, where the bubble sound pressure is 10 dB above that of the ambient noise.
The passage of greenhouse gases, from both natural and anthropogenic sources, through the upper sedimentary succession and into overlying aquatic systems is a poorly understood process. Our ...understanding of it however conditions our ability to detect potential leaks from Carbon Capture and Storage sites (CCS) as well as our overall knowledge of the global carbon cycle. In this thesis repeated high-resolution seismic reflection surveys are used to image carbon dioxide (CO2) gas released into shallow sub-surface sediments above a potential CCS storage site in the North Sea. Observations of temporal changes in seismic reflectivity, attenuation, unit thickness and the bulk permeability of sediment were used to develop a four-stage model of the evolution of gas migration in shallow marine sediments: Proto-migration, Immature Migration, Mature Migration, and Pathway Closure. Variations in ebullition rates from natural methane (CH4) seeps in Lake Constance (central Europe) are observed over a 9-month period using physical and acoustic measurement techniques, demonstrating a significant negative correlation between gas flux and in-situ pressure. The water level (hydrostatic pressure) dictates flux rates on monthly timescales, while atmospheric pressure causes minor fluctuations on daily to weekly periods, the effect of land-lake breeze cycles are observed for the first time. Exploiting this relationship, we find that long-term ebullition rates are best estimated by quantifying the relationship between in-situ pressure and gas flux, and then using this relationship to predict gas flux from more easily measurable in-situ pressure data. Finally, the use of passive acoustic flux inversion techniques is refined by measuring the initial amplitude of a bubble's excitation when released from sediment, a previously poorly constrained parameter, demonstrating a strong correlation with the bubble equilibrium radius. We demonstrate the use of this refined acoustic inversion technique by measuring the flux from a volcanic CO2 seep in offshore Panarea (Italy), seeing a significant increase in precision with estimates consistent with optical and physical flux measurements. These findings have enhanced our understanding of gas migration in the near surface and improved our ability to measure gas emissions.
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