Underwater Measurements Research Articles

Object Depth Profile and Reflectivity Restoration From Sparse Single-Photon Data Acquired in Underwater Environments

This paper presents two new algorithms for the joint restoration of depth and reflectivity (DR) images constructed from time-correlated single-photon counting measurements. Two extreme cases are considered: 1) a reduced acquisition time that leads to very low photon counts; and 2) imaging in a highly attenuating environment (such as a turbid medium), which makes the reflectivity estimation more difficult at increasing range. Adopting a Bayesian approach, the Poisson distributed observations are combined with prior distributions about the parameters of interest, to build the joint posterior distribution. More precisely, two Markov random field (MRF) priors enforcing spatial correlations are assigned to the DR images. Under some justified assumptions, the restoration problem (regularized likelihood) reduces to a convex formulation with respect to each of the parameters of interest. This problem is first solved using an adaptive Markov chain Monte Carlo (MCMC) algorithm that approximates the minimum mean square parameter estimators. This algorithm is fully automatic since it adjusts the parameters of the MRFs by maximum marginal likelihood estimation. However, the MCMC-based algorithm exhibits a relatively long computational time. The second algorithm deals with this issue and is based on a coordinate descent algorithm. Results on single-photon depth data from laboratory-based underwater measurements demonstrate the benefit of the proposed strategy that improves the quality of the estimated DR images.

Towards automating underwater measurement of fish length: a comparison of semi-automatic and manual stereo–video measurements

Abstract Underwater stereo–video systems are widely used for counting and measuring fish in aquaculture, fisheries, and conservation management. Length measurements are generated from stereo–video recordings by a software operator using a mouse to locate the head and tail of a fish in synchronized pairs of images. This data can be used to compare spatial and temporal changes in the mean length and biomass or frequency distributions of populations of fishes. Since the early 1990s stereo–video has also been used for measuring the lengths of fish in aquaculture for quota and farm management. However, the costs of the equipment, software, the time, and salary costs involved in post processing imagery manually and the subsequent delays in the availability of length information inhibit the adoption of this technology. We present a semi-automatic method for capturing stereo–video measurements to estimate the lengths of fish. We compare the time taken to make measurements of the same fish measured manually from stereo–video imagery to that measured semi-automatically. Using imagery recorded during transfers of Southern Bluefin Tuna (SBT) from tow cages to grow out cages, we demonstrate that the semi-automatic algorithm developed can obtain fork length measurements with an error of less than 1% of the true length and with at least a sixfold reduction in operator time in comparison to manual measurements. Of the 22 138 SBT recorded we were able to measure 52.6% (11 647) manually and 11.8% (2614) semi-automatically. For seven of the eight cage transfers recorde,d there were no statistical differences in the mean length, weight, or length frequency between manual and semi-automatic measurements. When the data were pooled across the eight cage transfers, there was no statistical difference in mean length or weight between the stereo–video-based manual and semi-automated measurements. Hence, the presented semi-automatic system can be deployed to significantly reduce the cost involved in adoption of stereo–video technology.

In-air versus underwater comparison of 3D reconstruction accuracy using action sport cameras

Action sport cameras (ASC) have achieved a large consensus for recreational purposes due to ongoing cost decrease, image resolution and frame rate increase, along with plug-and-play usability. Consequently, they have been recently considered for sport gesture studies and quantitative athletic performance evaluation. In this paper, we evaluated the potential of two ASCs (GoPro Hero3+) for in-air (laboratory) and underwater (swimming pool) three-dimensional (3D) motion analysis as a function of different camera setups involving the acquisition frequency, image resolution and field of view. This is motivated by the fact that in swimming, movement cycles are characterized by underwater and in-air phases what imposes the technical challenge of having a split volume configuration: an underwater measurement volume observed by underwater cameras and an in-air measurement volume observed by in-air cameras. The reconstruction of whole swimming cycles requires thus merging of simultaneous measurements acquired in both volumes. Characterizing and optimizing the instrumental errors of such a configuration makes mandatory the assessment of the instrumental errors of both volumes.In order to calibrate the camera stereo pair, black spherical markers placed on two calibration tools, used both in-air and underwater, and a two-step nonlinear optimization were exploited. The 3D reconstruction accuracy of testing markers and the repeatability of the estimated camera parameters accounted for system performance. For both environments, statistical tests were focused on the comparison of the different camera configurations. Then, each camera configuration was compared across the two environments. In all assessed resolutions, and in both environments, the reconstruction error (true distance between the two testing markers) was less than 3mm and the error related to the working volume diagonal was in the range of 1:2000 (3×1.3×1.5m3) to 1:7000 (4.5×2.2×1.5m3) in agreement with the literature. Statistically, the 3D accuracy obtained in the in-air environment was poorer (p<10−5) than the one in the underwater environment, across all the tested camera configurations. Related to the repeatability of the camera parameters, we found a very low variability in both environments (1.7% and 2.9%, in-air and underwater). This result encourage the use of ASC technology to perform quantitative reconstruction both in-air and underwater environments.

Study on improvement of CRI using RGB LED lights for underwater environments

This study presents a novel LED (light-emitting diode) light source. The device was simulated in both air and underwater environments by using TracePro. This device can adjust the current to change the power of different colors (M-E-L-R, M-E-L-G and M-E-L-B) to improve its color rendering index (CRI) in different scattering environments. The simulated results were analyzed and used to build a design module for use in underwater LED devices. In underwater experiments, the ordered sequence of light efficiency values was M-E-L-G>M-E-L-B>M-E-L-R. Underwater measurements of the performance of the device confirmed the simulated results.

Effects of inelastic radiative processes on the determination of water-leaving spectral radiance from extrapolation of underwater near-surface measurements.

Extrapolation of near-surface underwater measurements is the most common method to estimate the water-leaving spectral radiance, Lw(λ) (where λ is the light wavelength in vacuum), and remote-sensing reflectance, Rrs(λ), for validation and vicarious calibration of satellite sensors, as well as for ocean color algorithm development. However, uncertainties in Lw(λ) arising from the extrapolation process have not been investigated in detail with regards to the potential influence of inelastic radiative processes, such as Raman scattering by water molecules and fluorescence by colored dissolved organic matter and chlorophyll-a. Using radiative transfer simulations, we examine high-depth resolution vertical profiles of the upwelling radiance, Lu(λ), and its diffuse attenuation coefficient, KLu (λ), within the top 10 m of the ocean surface layer and assess the uncertainties in extrapolated values of Lw(λ). The inelastic processes generally increase Lu and decrease KLu in the red and near-infrared (NIR) portion of the spectrum. Unlike KLu in the blue and green spectral bands, KLu in the red and NIR is strongly variable within the near-surface layer even in a perfectly homogeneous water column. The assumption of a constant KLu with depth that is typically employed in the extrapolation method can lead to significant errors in the estimate of Lw. These errors approach ∼100% at 900 nm, and the desired threshold of 5% accuracy or less cannot be achieved at wavelengths greater than 650 nm for underwater radiometric systems that typically take measurements at depths below 1 m. These errors can be reduced by measuring Lu within a much shallower surface layer of tens of centimeters thick or even less at near-infrared wavelengths longer than 800 nm, which suggests a requirement for developing appropriate radiometric instrumentation and deployment strategies.

Research on a micromachined flexible hot-wire sensor array for underwater wall shear stress measurement

This paper reports an innovative flexible hot-wire senor microarray and its experimental studies for underwater wall shear stress measurement. 20 parallel channels of the hot-wire sensor are embedded between two polyimide layers for curved surface applications, which is fabricated based on micromachining processes, including platinum sputtering, nickel electroplating, etc. An accurate method to calculate the conversion factor of the compensation loop is proposed and verified for the temperature-compensated circuit. An average temperature coefficient of resistance (TCR) is measured 2136.7 ppm/°C with linearity better than 0.05 % for platinum thermal resistors. Output instabilities of 0.22 and 0.63 mV are obtained in static flow and moving flow, respectively. The feasibility of the hot-wire sensors for detecting underwater wall shear stress is verified with an average sensitivity of 0.0123 $${\text{V}}^{2} /{\text{Pa}}^{1/3}.$$ According to the experiment results, the flexible sensor microarray is very promising for characterizing underwater wall shear stress distributions in the boundary layer.

The effect of photosynthetically active radiation and temperature on the photosynthesis of two Vietnamese species of Sargassum , S. mcclurei and S. oligocystum , based on the field and laboratory measurements

SUMMARY The effects of photosynthetically active radiation (PAR) and temperature on the photosynthesis of two Vietnamese brown algae, Sargassum mcclurei and S. oligocystum (Fucales), were determined by field and laboratory measurements. Dissolved oxygen sensors and pulse-amplitude modulated (PAM) fluorometry were used for the measurements of photosynthetic efficiency. A Diving-PAM revealed that underwater measurements of the effective quantum yield (Φ PSII ) of both species declined with increasing incident PAR, with minimum Φ PSII occurring during noon to early afternoon. Φ PSII recovered in the evening, indicating photo-adaptation to excessive PAR. In laboratory experiments, Φ PSII also decreased under continuous exposure to 1000 μmol photons m−2 s−1; and full recovery occurred after 12 h of dark acclimatization. The net photosynthesis – PAR experiments of S. mcclurei and S. oligocystum conducted at 28°C revealed that the net photosynthetic rate quickly increased at PAR below the saturation irradiance of 361 and 301 μmol photons m−2 s−1 and nearly saturated to maximum net photosynthetic rates of 385 and 292 μg O2 gww − 1 min−1 without photoinhibition, respectively. Gross photosynthesis and dark respiration experiments determined over a range of temperatures (12–40°C), revealed that the maximum gross photosynthetic rates of 201 and 147 μg O2 gww − 1 min−1 occurred at 32.9 and 30.7°C for S. mcclurei and S. oligocystum, respectively. The dark respiration rates increased exponentially over the temperature ranges examined. The estimated maximum value of the maximum quantum yield occurred at 19.3 and 20.0°C and was 0.76 and 0.74, respectively. Similar to the natural habitat of the study site, these two species tolerated the relatively high temperatures and broad range of PAR. The ability of these species to recover from exposure to high PAR is one of the mechanisms that allow them to flourish in the shallow water environment.

Magnetic-Sphere-Based Phononic Crystals

Periodic arrays in one, two, and three dimensions, made of magnetic spheres embedded in a fluid matrix, are considered in this study and utilized as phononic structures. The propagation of acoustic waves through these structures is analyzed experimentally, in low- and high-frequency region, via laser vibrometry, as well as standard underwater acoustic measurements. A first comparison to theoretical calculations obtained through multiple-scattering techniques and multipole models reveals a distinct behavior depending on the immersion fluid and/or frequency regime. Our results show that the elastodynamic response of these systems can be, under conditions, simply described by classical elastic theory without taking directly (ab initio) into account the magnetic character of the spherical particles. The structures considered above could offer several possibilities including facility of construction and use in filtering applications, but they are also of interest from a theoretical point of view, as a means to investigate the validity of several approximate theoretical descriptions.

Experimental findings on the underwater measurements uncertainty of speed of sound and the alignment system

Speed of sound is an important quantity to characterize reference materials for ultrasonic applications, for instance. The alignment between the transducer and the test body is an key activity in order to perform reliable and consistent measurement. The aim of this work is to evaluate the influence of the alignment system to the expanded uncertainty of such measurement. A stainless steel cylinder was previously calibrated on an out of water system typically used for calibration of non-destructive blocks. Afterwards, the cylinder was calibrated underwater with two distinct alignment system: fixed and mobile. The values were statistically compared to the out-of-water measurement, considered the golden standard for such application. For both alignment systems, the normalized error was less than 0.8, leading to conclude that the both measurement system (under and out-of-water) do not diverge significantly. The gold standard uncertainty was 2.7 m-s-1, whilst the fixed underwater system resulted in 13 m-s-1, and the mobile alignment system achieved 6.6 m-s-1. After the validation of the underwater system for speed of sound measurement, it will be applied to certify Encapsulated Tissue Mimicking Material as a reference material for biotechnology application.

A high transmission broadband gradient index lens using elastic shell acoustic metamaterial elements.

The use of cylindrical elastic shells as elements in acoustic metamaterial devices is demonstrated through simulations and underwater measurements of a cylindrical-to-plane wave lens. Transformation acoustics of a circular region to a square dictate that the effective density in the lens remain constant and equal to that of water. Piecewise approximation to the desired effective compressibility is achieved using a square array with elements based on the elastic shell metamaterial concept developed by Titovich and Norris [J. Acoust. Soc. Am. 136(4), 1601-1609 (2014)]. The sizes of the elements are chosen based on availability of shells, minimizing fabrication difficulties. The tested device is neutrally buoyant comprising 48 elements of nine different types of commercial shells made from aluminum, brass, copper, and polymers. Simulations indicate a broadband range in which the device acts as a cylindrical to plane wave lens. The experimental findings confirm the broadband quadropolar response from approximately 20 to 40 kHz, with positive gain of the radiation pattern in the four plane wave directions.

Underwater measurements using laser induced breakdown spectroscopy

New data is presented related to high-pressure underwater laser-induced breakdown spectroscopy measurements.

レーザー分光分析による海底岩石のリアルタイム化学分析

Long-pulse Laser-Induced Breakdown Spectroscopy (LIBS) is a promising method for an in-situ analysis of chemical compositions of rocks and sediments on the seafloor. We have developed a real-time LIBS device, “Chemicam” and have successfully performed in-situ analysis at the bottom of the deep sea using the device. In order to develop a real-time quantitative analytical method, principal component regression analysis is applied to underwater LIBS measurements. The non-linear effects of plasma temperature fluctuation on the signals are treated as systematic errors in the analysis. The effect of these errors on the analytical performance is evaluated by comparing the conventional PCR, PCR with a temperature segmented database, and calibration-free LIBS. Improvement of accuracy in the proposed PCR model indicates that non-linear systematic effects contribute to fluctuation of underwater plasmas, where temperature-based segmentation can improve the performance of PCR. The results suggest that the proposed method can be applicable to real-time automated quantitative analysis of rocks in the deep sea.

水中計測用干渉計型光ファイバAEセンサの無指向性化

Directivity of the Mach-Zehnder interferometer type optical fiber acoustic emission (AE) sensor having the U-shaped sensing region for the use of underwater measurement was discussed. It was demonstrated that detected waveforms by the sensor depend on the incidence angles of the AE waves although the amplitude of the first peak in the waveform caused by the arrival of the AE wave indicates no change. It was also found that directivity of the sensor tends to disappear by setting masking tubes on the sensing fiber that avoid the influence of undesirable elongation of the sensing region. Nearly omnidirectional sensor was successfully made with the effective ratio for receiving waves given by masking tubes was less than 50%. It was concluded that the J-shaped sensing region is preferable for waveform analyses by taking sensitivity of the sensor into accounts.

Underwater 3D Surface Measurement Using Fringe Projection Based Scanning Devices.

In this work we show the principle of optical 3D surface measurements based on the fringe projection technique for underwater applications. The challenges of underwater use of this technique are shown and discussed in comparison with the classical application. We describe an extended camera model which takes refraction effects into account as well as a proposal of an effective, low-effort calibration procedure for underwater optical stereo scanners. This calibration technique combines a classical air calibration based on the pinhole model with ray-based modeling and requires only a few underwater recordings of an object of known length and a planar surface. We demonstrate a new underwater 3D scanning device based on the fringe projection technique. It has a weight of about 10 kg and the maximal water depth for application of the scanner is 40 m. It covers an underwater measurement volume of 250 mm × 200 mm × 120 mm. The surface of the measurement objects is captured with a lateral resolution of 150 μm in a third of a second. Calibration evaluation results are presented and examples of first underwater measurements are given.

Towards an Interoperable Field Spectroscopy Metadata Standard with Extended Support for Marine Specific Applications

This paper presents an approach to developing robust metadata standards for specific applications that serves to ensure a high level of reliability and interoperability for a spectroscopy dataset. The challenges of designing a metadata standard that meets the unique requirements of specific user communities are examined, including in situ measurement of reflectance underwater, using coral as a case in point. Metadata schema mappings from seven existing metadata standards demonstrate that they consistently fail to meet the needs of field spectroscopy scientists for general and specific applications (μ = 22%, σ = 32% conformance with the core metadata requirements and μ = 19%, σ = 18% for the special case of a benthic (e.g., coral) reflectance metadataset). Issues such as field measurement methods, instrument calibration, and data representativeness for marine field spectroscopy campaigns are investigated within the context of submerged benthic measurements. The implication of semantics and syntax for a robust and flexible metadata standard are also considered. A hybrid standard that serves as a “best of breed” incorporating useful modules and parameters within the standards is proposed. This paper is Part 3 in a series of papers in this journal, examining the issues central to a metadata standard for field spectroscopy datasets. The results presented in this paper are an important step towards field spectroscopy metadata standards that address the specific needs of field spectroscopy data stakeholders while facilitating dataset documentation, quality assurance, discoverability and data exchange within large-scale information sharing platforms.

Localization of the Chelyabinsk Meteorite From Magnetic Field Survey and GPS Data

The Chelyabinsk meteorite fragment that landed in the Chebarkul lake in Russia on February 15, 2013 weighed over half a ton. We provide magnetic field maps that were obtained during underwater measurements above the fragment. The data acquisition process was multiple global position system referenced magnetic surveys 0.5–1 m above the top of the lake sediment layer at 10 m water depth. Gradiometric configuration of the survey using two triaxial fluxgate magnetometers helped to suppress local geological anomalies. The location of the ice crater and the underwater magnetic anomaly provided final meteorite landing coordinates, which were made available during meteorite recovery.

Calibration-free analysis of immersed brass alloys using long-ns-duration pulse laser-induced breakdown spectroscopy with and without correction for nonstoichiometric ablation

Long-ns-duration, single pulse laser-induced breakdown spectroscopy (LIBS) is known to be an effective method to observe well resolved spectra from samples immersed in water at high hydrostatic pressures. The aim of this study is to investigate whether the signals obtained using this method are suitable for quantitative analysis of chemical composition. Six certified brass alloys consisting of copper (Cu), zinc (Zn) and lead (Pb) were measured underwater using a laser pulse of duration 250ns, and their compositions were determined using calibration-free LIBS (CF-LIBS) and corrected CF-LIBS (CCF-LIBS) methods. The mass fractions of Cu and Zn calculated using CF-LIBS showed better agreement with the certified values than those determined using CCF-LIBS, with relative errors of Cu 4.2±3.3 % and Zn 7.2±6.4 %. From the results, it can be said that the difference of preferential evaporation and ablation among elements does not need to be considered for underwater measurements with the long-pulse LIBS setup used in this work. While the results indicate that the CF-LIBS method can be applied for in situ quantitative analysis of major elements with concentrations>~10 %, the mass fractions determined for Pb, with concentrations<5 % had large relative errors, suggesting that an alternative method is required to quantify minor elements.

A novel underwater measurement method for mooring system using self-contained technique

This article introduces a prototype measurement method for investigating deep-water mooring systems. Measurement information and specifications are determined according to mooring system analysis. Mooring line information is difficult to measure underwater, particularly for aging platforms. To solve this problem, a novel mooring line tension measurement strategy based on lumped mass method is proposed. A type of self-contained sensor is designed and manufactured to realize dynamic response measurement of mooring systems underwater. The sensor is described in detail, including its design requirement, hardware design, and installation mode. A comprehensive field test system was constructed on a semi-submersible platform in South China Sea. Prototype data, including the data measured during typhoons, are introduced and discussed to improve the accuracy, feasibility, and stability of the proposed measurement method.

Underwater acoustic measurements: Sound-source-verification techniques — Drift-buoy and towed-array solutions

Underwater sound is recognized as an important environmental issue because of the effects of sound on marine life. Government agencies and regulators worldwide are showing an increasing interest in sound-source-verification (SSV) techniques, analysis, and results. Analysis provides the necessary information to assess the impact of sound on marine mammals, which can range from behavioral changes to auditory damage, as well as to assess the impact on the environment. Marine mammals generally rely heavily on their sense of hearing for communication, prey location, predator detection, navigation, and echolocation, so prolonged exposure to continuous sound is potentially harmful.

Low-cost small action cameras in stereo generates accurate underwater measurements of fish

Small action cameras have received interest for use in underwater videography because of their low-cost, standardised housing, widespread availability and small size. Here, we assess the capacity of GoPro action cameras to provide accurate stereo-measurements of fish in comparison to the Sony handheld cameras that have traditionally been used for this purpose. Standardised stereo-GoPro and Sony systems were employed to capture measurements of known-length targets in a pool to explore the influence of the type of camera, distance to camera rig, angle to the optical axis and target speed on measurement accuracy. The capacity to estimate fish length in situ was also compared by measuring the same fish on a coral reef with two baited remote underwater video systems, each fitted with both a GoPro and a Sony camera system. Pool trials indicated that the GoPros were generally less accurate than the Sonys. Accuracy decreased with increased angles and distance for both systems but remained reasonably low (<7.5%) at 5m distance and 25° angle for GoPros. Speed of target movement did not result in any consistent decrease in accuracy. In situ measurements revealed a strong correlation (R2=0.94) between Sony and GoPro length measurements of the same individual fish, with a slope not different from 1 and an intercept not different from 0, suggesting that GoPro measurement errors do not result in a consistent bias at the level of individual fish. Moreover, the investigation of kernel density functions of the length distribution of the entire fish assemblage indicated that difference in measurement accuracy becomes negligible for purposes of comparing population size structure. We suggest a measurement protocol for the use of GoPro stereo-camera systems that improves accuracy, where distance to target is limited to 5m and angle to optical axis is restricted to 25°. For distances up to 7m, angles should be restricted to 15°. This protocol supports the use of small action cameras such as the GoPro system, providing reductions in cost and increases in effective sampling efforts, compared with traditional rigs based on relatively expensive handheld cameras.