Depth-dependent Intensity Research Articles

Listening to the acidity of the ocean: Inversion of passive deep sea acoustic data for pH

Ocean acidification is an ongoing concern due to its impact on the marine ecosystem. The volume integrated pH of sea water can be determined from the depth-dependence of ambient sound, which depends on the acoustic absorption properties of seawater. For a wind-driven noise in the ocean over the band 1–10 kHz, two main contributions to sound attenuation are associated with the ionic relaxation of boric acid (<3 kHz), related to pH, and magnesium sulfate (>3 kHz), unrelated to pH. When local winds are strong (>10 m/s), the ambient noise is dominated by locally generated surface noise and has a depth-independent directionality and a weakly frequency and depth-dependent intensity, due to sound absorption. By measuring the attenuation of sound in a wide frequency band, it is possible to estimate pH by comparing the experimentally measured attenuation with an analytical theory of passive acoustic absorption spectroscopy. Measurements of the depth-dependent ambient sound field were carried out in the Philippine Sea, Mariana Trench, and Tonga Trench throughout 2009—2021. The wideband (5 Hz—30 kHz) acoustic data were recorded with untethered free-falling autonomous recording systems carrying two or four hydrophones.

A scheme for a priori generation of the optical patterns for magnetic nanoemulsion based defect detection in carbon steel specimens

A numerical scheme is proposed for a priori generation of the optical patterns for magnetic nanoemulsion (MNE) based defect detection in ferromagnetic steel specimens. External static magnetic field (HDC) induced tunable optical contrast in MNE has been exploited for the development of the wide-area non-contact optical sensor, which utilizes the magnetic flux leakage (MFL) from the defect regions to generate visually discernible optical patterns, where the image contrast varies with the severity of the defects. For theoretically generating the optical patterns, a three-step numerical scheme is proposed, where the first step involves the physical characterization of the MNEs. In the presence of a HDC, the MNE droplets exhibit an oriented linear arrangement, where the inter-droplet separation decreases with increasing HDC, resulting in a blue shift of the wavelength corresponding to the maxima of the Bragg's reflection peaks (λmax). Unique λmax-HDC calibration curves are obtained for various types of MNEs depending on the nature of the stabilizing moieties. In the second step, the surface distributions of the MFL are obtained using finite element modelling. Subsequently, in the third stage, using the λmax-HDC calibration curves, the surface distributions of MFL are converted to surface distributions of λmax values, which are then pseudo colour-coded to theoretically generate the optical patterns. The presence of the rectangular slots, double rectangular, cylindrical, and buried defects are clearly discernible from the simulated optical patterns, which are found to be in good agreement with the experimentally recorded patterns. Defect depth-dependent intensity variations and estimated defect widths from the simulated images are in good agreement with the data obtained from the experimental images, thereby validating the efficacy of the proposed scheme for a priori prediction of the optical patterns for MNE-based non-contact wide area defect detection. Further, the proposed scheme will be beneficial for the selection of appropriate MNE-based sensors, reducing inspection time, and developing automated inspection routines with enhanced detection capabilities.

Towards developing a generalized semi-coherent three-dimensional energy flux model

The energy flux method can be interpreted as an integration over propagation angle of a semi-classical (WKB) modal continuum, which yields an averaged incoherent intensity field with depth-dependent intensity bands that decay in range. Range dependence is typically incorporated via the adiabatic modes approximation and use of the ray invariant to map propagation angles. Though the energy flux method averages out nearly all of the field structure, it has the primary advantages of avoiding finding eigenvalues and eigenrays and its computational effort does not directly scale with frequency or range. The method has also been extended in the past decade to include a convergence factor derived from the interference of neighboring modes, producing a semi-coherent solution that captures some convergence structure analogous to high-frequency caustics. Since the development of the semi-coherent energy flux method, it has so far only been applied to Nx2D environmental models, but should theoretically be applicable in a 3D environment without the assumption of azimuthal symmetry or the exclusion of horizontal refraction. This paper will discuss the theoretical derivation and numerical implementation of a generalized semi-coherent three-dimensional energy flux model. [This study is funded by the NDSEG Fellowship program.]

Fluorescence detection of deep intramucosal cancer excited by green light for photodynamic diagnosis using protoporphyrin IX induced by 5-aminolevulinic acid: an ex vivo study.

.Significance: The diagnostic depth of photodynamic diagnosis (PDD) for gastric cancer with protoporphyrin IX (PpIX) is limited, which leads to missing intramucosal cancers in screening and surgery.Aim: The reason is that the excitation light, whose wavelength is determined by the highest absorption peak of PpIX (), is strongly attenuated by mucosal tissues. We investigated an excitation wavelength that can extend the diagnostic depth of PpIX fluorescence at the mucosal subsurface.Approach: By calculating the depth-dependent intensity of the excitation light in porcine gastric mucosa for each wavelength, relationships among the wavelength, fluorophore depth, and fluorescence intensity were assessed and fluorescence images of PpIX pellets located at different fluorophore depths were compared experimentally by changing the excitation wavelength.Results: The numerical calculation showed that a 505-nm excitation light provided the highest fluorescence intensities at a fluorophore depth deeper than 1.1 mm. In the fluorescence observation, the fluorescence intensities at fluorophore depths of 0 and 1.0 mm at 405 nm were and units, whereas those at 505 nm were and units, respectively.Conclusion: The experimental results suggest that the diagnosis depth of PDD with PpIX for intramucosal cancer can be extended by 505-nm excitation light.

The measurement of ocean acidity using the depth-dependence of ambient noise

The absorption of sound in seawater is due to the viscous and chemical relaxation of different compounds. Over the wind noise band of 1–10 kHz, the frequency dependence of the absorption is due to the mechanisms of chemical relaxation for magnesium sulfate (f > 3 kHz) and for boric acid (f 10 m/s), the ambient noise field is dominated by locally generated surface noise and has a depth-independent directionality and a weakly frequency and depth-dependent intensity, due to sound absorption. By comparing measurements with theory, estimates of ocean acidity can be made from the depth profiles of ambient noise. [Work supported by ONR.]The absorption of sound in seawater is due to the viscous and chemical relaxation of different compounds. Over the wind noise band of 1–10 kHz, the frequency dependence of the absorption is due to the mechanisms of chemical relaxation for magnesium sulfate (f > 3 kHz) and for boric acid (f 10 m/s), the ambient noise field is dominated by locally generated surface noise and has a depth-independent directionality and a w...

The spatial properties of breaking wave generated and bedload transport generated noise in the sediment layer of a shallow water wave guide

In May of 2012, three weeks of ambient noise measurements from a hydrophone buried 30 cm deep in the sediment were recorded at Advocate Beach, a 1:10 sloped beach at the head of the Bay of Fundy, Nova Scotia. While tides varied the mean water depth between 0 and 4 m, 0.8 m surface waves passed overhead, driving sediment bedload transport and creating an ambient noise field in the sediment consisting of two primary components: noise generated by bubbles formed in breaking waves at the surface and noise generated by the collisions of sand, gravel, and cobble in the bedload transport along the seabed. Both of these noise sources are stochastic and can be described by their second order statistics: power spectral density, spatial coherence, and directional density. In an effort to distinguish these two noise sources, the spatial properties of three full wave models of the noise field in the sediment are compared, using an infinite sheet of sources placed near the surface of a Pekeris waveguide to model breaking wave noise, near the fluid–fluid interface of a Pekeris waveguide to model bedload transport noise, or near the fluid–fluid interface of two infinite half-spaces to model bedload transport noise. Using integral transforms to solve the wave equation, each noise model is shown to be spatially inhomogeneous with a unique depth dependent intensity and coherence.

Simulating Magnetic Susceptibility Profiles in Loess as an Aid in Quantifying Rates of Dust Deposition and Pedogenic Development

Magnetic susceptibility profiles χ(z) in loess sequences reflect a combination of two climatically modulated processes: dust deposition and pedogenic development. Prominent soils with high χ values, for example, likely reflect periods of slow dust deposition, and warm wet conditions favorable for rapid chemical weathering. To refine our understanding of the climate records contained in loess, we develop a numerical model as a tool for exploring quantitatively the integrated record of the temporal variation in rates of loess aggradation and soil development contained in χ profiles. In our model, the aggrading loess is pedogenically altered in a reactive zone near the ground surface. The strength of the χ signal is dictated by both the depth-dependent intensity of pedogenic processes and the rate of dust accumulation, which dictates the total time a loess parcel spends in the near-surface reactive zone. The model can accommodate both pedogenic production of magnetically susceptible minerals and arrival of magnetically susceptible grains as eolian dust. To explore the model performance and develop a sense of time and length scales implicit in χ profiles, we first examine simple synthetic cases with idealized steady and cyclic climatic forcing. Reported χ profiles in three Chinese loess sequences at varying distances from the western China dust source are then modeled in two illustrative ways: (i) by imposing a specific dust deposition rate history that is proportional to the dust accumulation rate history reported from western Pacific deep-sea cores, allowing the time variation of pedogenic rates to be calculated directly from χ profiles; and (ii) by imposing both dust accumulation and pedogenic rate histories that are independently scaled by the deep-sea δ18O history, which reflects global climate cycles to which regional climate forcing is linked. We find that the zone of pedogenic activity is roughly 0.5–1.0 m thick, both deposition rate and pedogenic intensity have varied dramatically over the last 140,000 yr, the age structure of the Luochuan loess sequence is best fit by driving the model with the δ18O record, and environmental conditions must have been anomalously favorable for pedogenesis during isotope stage 3 at all three sites. Finally, we advocate the assembly of a variety of data types at a suite of sites within any loess field that taken together will better constrain the temporal and spatial patterns of climatically modulated deposition and pedogenic processes.