Underwater Measurements Research Articles

High-accuracy calibration method for an underwater one-mirror galvanometric laser scanner.

Three-dimensional (3D) perception of deep-sea targets is the key to autonomous operation of underwater equipment (e.g., underwater robots). Underwater one-mirror galvanometric line-laser scanner has advantages for short-range measurement, but it is difficult to achieve high calibration accuracy due to installation errors and refraction effects. For this reason, in this paper, a high-accuracy refraction-considered and installation-error-independent calibration method is proposed for the vision system. Firstly, to address the difficulty of aligning the incident light plane with the galvanometer shaft, a high-accuracy land-based installation-error-independent model is proposed, which avoids the influence of the installation errors and allows the real shaft axis and the light-plane cluster poses to be calculated using only three light planes. Subsequently, considering the underwater refraction, a 3D model is established for simulating refractive behaviors of the light-plane cluster, and then a partition-based method is proposed for calibrating the underwater light-plane cluster, which further improves the calibration accuracy of the scanner in underwater measurement scenarios. Finally, a one-mirror galvanometric laser scanner is developed in the laboratory to verify the calibration accuracy and to perform the 3D measurement experiments of underwater targets. The results show that the calibration accuracy of the proposed land-based installation-error-independent model is improved 2 times more compared with the traditional installation-error-dependent model. Additionally, the measurement accuracy of the scanner for the standard sphere is 11.98 µm and 12.75 µm in the air and underwater measurement scenarios, and the two measurements are in good agreement. The above results comprehensively verify the high accuracy of the calibration method proposed in this paper.

Conversion of underwater concrete images to air in detection of hydraulic structures

Taking underwater concrete images with an optical camera is an important measure for underwater defect detection. However, the underwater low-light environment and light refraction on the surface of different media result in poor image quality. In order to make the collected underwater images effectively reflect the real situation of underwater concrete defects, we propose an image conversion algorithm combining underwater image color enhancement and refraction distortion correction, which can convert underwater images into aerial equivalent images. In this paper, two conversion models of underwater image to air conversion are proposed, one of which models the refractive distortion of the underwater multilayered media through the ray projection method to correct the refractive distortion of underwater images and image field of view (FOV) conversion. The other is that by analyzing the problem of low image-imaging quality and loss of edge information due to the uneven illumination environment underwater, we process the dark channel priority adaptive enhancement algorithm to improve underwater image quality. The experimental results from real scenes show that the imaging quality of underwater images is improved by converting underwater images to air. The pixel error of images converted into the air is ⩽1 pixel, and the FOV error of images is ⩽8.5%. The high-precision underwater image conversion provides strong support for subsequent underwater measurement.

A Bio-Inspired Simultaneous Surface and Underwater Risk Assessment Method Based on Stereo Vision for USVs in Nearshore Clean Waters

Ensuring safety of unmanned surface vehicles (USVs) during nearshore field operations is challenging as they face unknown risks of collision with surface and underwater obstacles. To address this problem, we propose a bio-inspired collision risk-assessment method for ensuring safe USVs operation in nearshore clean waters. Collision risks are typically unknown due to the imprecision of underwater measurements for the water-air boundary light refraction, and disturbances caused by wave-induced USV motion. To tackle these two causes of imprecision, our collision risk-assessment method employs one single stereo camera and an IMU. We use visual information from the stereo camera to model the light rays at the water-air interface, rectify the underwater measurements, and monitor for surface obstacles, and IMU data to model wave disturbances via time-frequency analysis. We verify the performance of this new method in both the indoor and outdoor experiments, which demonstrates that it achieves a 65.44% improvement in the depth accuracy at the water-air interface compared with raw depth measurements, and provides a range of safety levels for guiding USV operations. This new collision risk-assessment method significantly expands the USVs working areas while ensuring safety during field exploration. Additionally, the simplicity of the sensor setup and thus minimal cost of this method underscore its utility for supporting USV-based various explorations in clear waters.

Underwater determination of calcium and strontium ions in oilfield produced water by laser-induced breakdown spectroscopy (LIBS).

Laser-induced breakdown spectroscopy (LIBS) was applied to the determination of scaling ions in oilfield-produced water employing underwater measurements. Initially, the stability of plasma was verified using four different optical setups and expansion of the laser beam, and a combination of an achromatic lens with a meniscus lens were necessary to stabilize the plasma. Preliminary experiments demonstrated that only the determinations of Ca(II) and Sr(II) ions were feasible while the signal for the Mg(II) ion was absent and the sensitivity for Ba(II) was very low. The laser pulse repetition rate was evaluated and rates of 10 and 20 Hz provided a more stable breakdown in water compared to repetition rates of 2 to 7 Hz, besides imparting higher intense signals. The increase in salinity showed a small matrix effect, decreasing the sensitivities of the calibration curves by 8-13% when standard solutions with a salinity of 30‰ were used instead of water. Under optimized conditions with a laser pulse energy of 31 mJ, gate delay of 300 ns, gate width of 5.0 μs, repetition rate of 10 Hz, and accumulation of 500 laser shots, a linear range from 25 to 150 mg L-1 was obtained, with limits of detection of 0.58 and 0.85 mg L-1 for Ca(II) and Sr(II), respectively. The underwater determination of scaling ions in produced water by LIBS provided results that do not significantly differ from those obtained by inductively coupled plasma atomic emission spectroscopy (ICP OES) at a confidence level of 95%, with relative errors of up to 5.2%. These results demonstrate the potential of underwater LIBS measurements as an analytical tool for the determination of alkaline-earth metal ions in produced water, which can help the oil industry to overcome the problems related to scale formation.

Compact Long-Range Single-Photon Underwater Lidar With High Spatial–Temporal Resolution

Oceanic lidar has emerged as a strong technology for oceanic three-dimensional remote sensing. However, most existing oceanic lidars are bulky and high-power consumption, thus difficult to enable underwater operation. Here we present a compact single-photon lidar system for long-range underwater measurement. A single-photon detector was adopted to achieve a high signal-to-noise ratio. Benefiting from the single-photon sensitivity in detection, long-range active detection was realized with a low pulse energy laser at 1 μJ and a small-aperture coupler at 12 mm. Moreover, a narrow linewidth picosecond fiber laser with high repetition rate was employed to guarantee a high spatial resolution and high update rate. A fiber-connected configuration was specially designed for the miniaturized and robust structure in an optical receiver. In an experimental demonstration, the profile of backscattered signal from clean water was obtained over 70 m with high spatial-temporal resolution to demonstrate the capability of this lidar system. The maximum detection distance of the single-photon lidar reaches ~3.6/ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K_d</i> ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K_d</i> , diffuse attenuation coefficient) for waterbody and up to 5.5/ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K_d</i> for a hard target. Furthermore, it exhibits a high update rate capability and realizes the localization and quantification of underwater bubbles up to 26 m away at a high update rate of 100 Hz. These results indicate its potential in a variety of applications including remote sensing of marine biogeochemical parameters, the quantification of seabed gas emissions, and long-range underwater imaging.

Fluorescence for non-contact detection of living salmon lice on salmon skin

This work presents a promising method for automatic, non-contact, detection and counting of salmon lice infested on salmon in an aquacultural farm setting. The method uses fluorescence from chitin in the visual part of spectrum to enhance the contrast between fish skin and salmon lice, and show that the fluorescence is even strong enough to give a real-time view of the digestive and reproduction system in live lice without use of staining dyes. The wavelengths used are compatible with an underwater measurement system.

A multi-refraction-based 3D reconstruction method for underwater measurement with rotating scanning

ABSTRACT This paper presents a novel reconstruction method based on multi-refraction to remedy the non-linear refraction, which recovers 3D geometry from 2D images. To simplify calibration, this paper prove that air calibration can replace underwater calibration. Finally, a multi-line rotating structured is designed to improve the measurement efficiency without additional equipment. The results of the clean water experiment show that the measurement accuracy is 1.91 mm within the 1000–2000 mm depth of field. The qualitative analysis demonstrates our method can reconstruct the surface topography of underwater U and N letter boards. In different Nephelometric Turbidity Unit (NTU) (as NTU = 0.156 mg/ml) environment, a circular plate with a diameter of 220 mm is measured, the shape reconstruction error (whether the reconstructed shape is a circular, square, or other) is 1.74 mm; the diameter measurement result is 218 mm, so the size error of the circular plate is 2 mm.

Measuring Maritime Paint Thickness under Water Using THz Cross-Correlation Spectroscopy

The shipping industry is a major contributor to global greenhouse gas (GHG) emissions, which is why it is important to optimize every aspect of the efficiency of ocean-going vessels. This includes the antifouling paint that ensures hydrodynamic efficiency. Measuring the thickness of the antifouling on top of all other paint layers using THz cross-correlation spectroscopy (THz-CCS) underwater could enable vessel operators to monitor the state of the paint on ship hulls and plan any vessel’s sailing route and maintenance optimally. However, due to the high absorption of water in the THz domain, measuring through any significant amount of water is impossible, making a water removal method necessary. This study shows how a THz-CCS system can be packaged for underwater measurements using a molded silicone contact seal. In combination with a spectroscopic model for data treatment, the thickness of a single paint layer is retrieved underwater. This paves the way for a more advanced system capable of measuring multilayer maritime paint underwater, which will enable shipping companies to continuously monitor the paint layers’ thickness.

Synthesis of novel smart pH-sensitive modified silica nanoparticles for controllable oil-water separation

The newly designed superwettable polymers with external stimuli-responsive behavior have been greatly proposed a great research interest in the oil industry due to their fast and simple mechanism process. Herein, the development of the advanced silica-based pH-responsive materials linked with hydrophilic 3-(aminopropyl)triethoxysilane (APTES) and hydrophobic 1H,1H,2H,2H-perfluorooctyl-triethoxysilane (FOTS) chelating ligands have been designed for oil/water separation by a facile dip-coating process. This smart material can propose an excellent wettability conversion between superhydrophobicity-superoleophilicity and superhydrophilicity-underwater superoleophobicity, favorable stability, and durability, resulting in efficient separation for oil/water mixture. These properties were observed from the well-defined 3D porous surface of the smart materials when they were coated on the fabric surface. Furthermore, characterization described that the material compositions were chemically reacted and distributed throughout the smart fabric surface caused by the robust chemical interaction between the functional groups such as hydroxide, carboxylate, and silane. The water contact angle (pH≈10) was displayed at 167-160° and for acidic water (pH≈2) was ranging from 143-0° over 100 s denoting outstanding superhydrophobic and superhydrophilic features respectively. Underwater measurement pointed out that the smart fabric also has a great superoleophobicity for oil contact angle (150°). Moreover, the series of test results revealed a selective separation for oil/water system with efficacy > 98% and high oil flux ranging from 7200 to 7900 L.h−1.m−2 as well as water flux (5300–6200 L.h−1.m−2). Therefore, these smart materials with fabric and non-fabric preparation have a direct environmental impact on real samples and they can largely dominate various oily wastewater/oil spills clean up.

An Alternative Experimental Method for Determination of Light Beam Attenuation Coefficient in Underwater Wireless Optical Communication

In wireless optical communication systems, the transmission of optical signals via the channel (air or water) is affected by absorption and scattering. These reduce the signal strength (attenuation) and transmission distance of the signals. In pure water, the blue-green region of the visible light gives low attenuation. Some models have been developed to characterize the underwater optical channel such as Beer Lambert’s law, Radiative Transfer equation and Monte Carlo model. In underwater optical communication, optical power meters are an invaluable tool in the determination of attenuation coefficients. However, optical power meters for underwater optical communication are very expensive. There is a need to be able to determine the attenuation in the underwater optical communication channel at a low cost, especially in the absence of underwater optical power meters. In this paper, we present an alternative low-cost experimental method of obtaining the approximate attenuation coefficient of the visible light beam in underwater optical wireless communication without the use of optical power meters. A wireless visible light communication system was set up experimentally for underwater measurements involving an oscilloscope as the only measuring device. The system uses sub-carrier frequency modulation; a white light-emitting diode array for the transmitter, and a solar panel at the receiver front end. A theoretical transmission model was developed from the experimental setup based on the line of sight method in an unbounded medium, Beer Lambert’s law, and the received sub-carrier signals; in order to provide an alternative method of determining approximately the attenuation coefficient of the underwater medium. Experiments were performed in air, clear water and in saline water. The attenuation in the air was used as a reference upon which attenuation in the clear water and the saline water was based. The saline water has a salt concentration of 6.7 g/100 mL by weight and a total dissolved solid of 86.2 ppt. The trend of the measured received sub-carrier signals showed deviation from the developed theoretical model, and the model was therefore adjusted to conform to the experimental results. From the adjusted model, the attenuation coefficient of 0.0007379 cm-1 and 0.02447 cm-1 were obtained for clear water and the saline water respectively. The method is simple, straightforward, easy to set up in a laboratory, low cost and can be applied to visible light of any wavelength.

Methodology for Phytoplankton Taxonomic Group Identification towards the Development of a Lab-on-a-Chip

This paper presents the absorbance and fluorescence optical properties of various phytoplankton species, looking to achieve an accurate method to detect and identify a number of phytoplankton taxonomic groups. The methodology to select the excitation and detection wavelengths that results in superior identification of phytoplankton is reported. The macroscopic analyses and the implemented methodology are the base for designing a lab-on-a-chip device for a phytoplankton group identification, based on cell analysis with multi-wavelength lighting excitation, aiming for a cheap and portable platform. With such methodology in a lab-on-a-chip device, the analysis of the phytoplankton cells’ optical properties, e.g., fluorescence, diffraction, absorption and reflection, will be possible. This device will offer, in the future, a platform for continuous, autonomous and in situ underwater measurements, in opposition to the conventional methodology. A proof-of-concept device with LED light excitation at 450 nm and a detection photodiode at 680 nm was fabricated. This device was able to quantify the concentration of the phytoplankton chlorophyll a. A lock-in amplifier electronic circuit was developed and integrated in a portable and low-cost sensor, featuring continuous, autonomous and in situ underwater measurements. This device has a detection limit of 0.01 µ/L of chlorophyll a, in a range up to 300 µg/L, with a linear voltage output with chlorophyll concentration.

REAL-TIME POSITIONING AND TRACKING FOR VISION-BASED UNMANNED UNDERWATER VEHICLES

Abstract. Unmanned underwater vehicle (UUV) is a key technology for marine resource exploration and ecological monitoring. How to use vision-based active positioning and three-dimensional perception to realize UUV underwater autonomous navigation and positioning is the basis for UUV's underwater operations. The complexity and unstructured characteristics of seawater bring new challenges to vision-based underwater high-precision positioning. Traditional visual localization algorithms mainly include geometric-based visual localization algorithms (such as ORB-SLAM2) and deep learning-based visual localization algorithms (such as DXSLAM). In this paper, based on the typical marine environment (low brightness, dynamic fish interference, underwater light spot, high turbidity), the experimental analysis and comparison of different visual positioning methods of UUV is carried out, which provides a reference for realizing the real-time localization of UUV, and further provides a better solution for UUV underwater measurement and monitoring operations.

Design, development and characterization of a mid-frequency (35 khz) tonpilz transducer array from 0.675PMN-0.325PT piezoceramics

A mid-frequency tonpilz transducer array was designed and analyzed by finite element method using commercial ATILA code and constructed using piezoceramic rings with lead magnesium niobate (PMN)-lead titanate (PT) composition. The morphotropic phase boundary (MPB) 0.675Pb(Mg1/3Nb2/3)O3-0.325PbTiO3 composition was chosen to obtain higher performance from the transducer due to the superior dielectric and electromechanical properties of the MPB compositions. A pure perovskite phase was obtained from the ceramics. Piezoelectric charge coefficient (d33) was measured as 435 pC×N-1. Single tonpilz element and an array consisting of 7-unit array were constructed and later potted in polyurethane for underwater measurements. A second array was also constructed for comparison from commercial hard Pb(Zr,Ti)O3-PZT ceramics. The longitudinal piston mode vibration frequency of the tonpilz transducer was measured around 33 kHz in air and around 35 kHz in water. The PMN-PT based array was found to have a comparable transmit performance and a superior receive performance compared to PZT.

A compact multifunctional fiber sensor for simultaneous underwater measurement of salinity, temperature and strain

In this paper, a compact multifunctional optical fiber sensor based on single mode fiber-photonic crystal fiber-no core fiber-photonic crystal fiber-single mode fiber-polarization maintaining fiber Bragg grating (SPNPS-PMFBG) has been proposed. By monitoring the wavelength and output power of the two resonance peaks of PMFBG, simultaneous measurement of salinity, temperature and strain can be realized. In our experiment under water, the sum output power of the two resonance peaks in 0–7 wt% salinity shows a good linear relationship and repeatability, and the sensitivity is −0.58 dB/wt% with 3 s response time. In addition, the measured temperature wavelength sensitivities at two resonance peaks are 0.042 nm/°C and 0.040 pm/°C, and the output power sensitivities are approximate to 0.079 dB/°C and 0.081 dB/°C, respectively. Further, the strain wavelength sensitivities are 0.535 nm/1000 με, and the output power sensitivities are 0.65 dB/1000 με and 0.70 dB/1000 με.

Calibration of an underwater binocular vision system based on the refraction model.

Underwater measurement based on stereo vision attaches great importance to camera calibration. However, it is challenging to perform accurate calibration due to the significant refraction presented at the interfaces of air and water. To solve this problem, a calibration method for an underwater binocular vision system based on the optimized refractive model is proposed. First, conventional calibration is performed to obtain basic initial camera parameters using checkerboard images collected in the air. Then, an evolutionary multi-objective function is established according to Snell's law, the refractive light propagation path, and checkerboard geometric relationship. Finally, precise camera parameters and involved refraction parameters are both obtained for underwater target positioning and size measurement by the non-dominated sorting genetic algorithm of the reference point. A group of experiments is performed, and the validity and effectiveness of the proposed calibration algorithm is demonstrated.

Underwater 3D Measurements with Advanced Camera Modelling

A novel concept of camera modelling for underwater 3D measurements based on stereo camera utilisation is introduced. The geometrical description of the ray course subject to refraction in underwater cameras is presented under assumption of conditions, which are typically satisfied or can be achieved approximately. Possibilities of simplification are shown, which allow an approximation of the ray course by classical pinhole modelling. It is shown how the expected measurement errors can be estimated, as well as its influence on the expected 3D measurement result. Final processing of the 3D measurement data according to the requirements regarding accuracy is performed using several kinds of refinement. For example, calibration parameters can be refined, or systematic errors can be decreased by subsequent compensation by suitable error correction functions. Experimental data of simulations and real measurements obtained by two different underwater 3D scanners are presented and discussed. If inverse image magnification is larger than about one hundred, remaining errors caused by refraction effects can be usually neglected and the classical pinhole model can be used for stereo camera-based underwater 3D measurement systems.

Accuracy Analysis of an Oblique Underwater Laser Lightsheet Triangulation System

Laser lightsheet triangulation is a well-established optical measurement method, which is frequently used in industrial applications. With some adaptions, the technique can also be used underwater. Placing a green laser line projector and a camera at a fixed base inside a watertight housing enables flexible and accurate underwater measurements at low cost. To achieve near-orthogonal intersections on the object surface, camera and laser need to be placed oblique to the housing interface. Refraction influences have to be considered strictly in geometric modelling, with the light propagating through the air–glass and glass–water interfaces. The measurement and calibration methods presented in the paper utilize a concept of splitting the lightsheet into multiple sub-beams, which are traced, refracted, and intersected with image observations to receive 3D measurements. In a calibration step, the relative orientation between camera, laser and interfaces is determined. In a theoretical accuracy analysis, the error influences caused by the calibrated parameters is estimated with a standard deviation of 0.2 to 0.4 mm in a depth range of up to 15 cm. A prototype triangulation sensor is applied for practical measurements of two test objects with known geometry. The predicted accuracy is validated in stationary single scan measurements. The sensor is furthermore moved along the test objects, using a six-degrees-of-freedom method to determine its exterior orientation. The combination of multiple single scans results in dense point clouds with 0.3 mm standard deviation compared to a reference.

Fluorescence for non-contact detection of salmon lice in fish farms

This work presents a promising method for automatic non-contact detection and counting of salmon lice infested on salmon in an aquacultural farm setting. The method uses fluorescence in the visual part of spectrum to enhance the contrast between fish skin and lice. The wavelengths used are compatible with an underwater measurement system.

Monocular Underwater Measurement of Structured Light by Scanning with Vibrating Mirrors

Monocular Underwater Measurement of Structured Light by Scanning with Vibrating Mirrors

High precision underwater 3D imaging of non-cooperative target with frequency comb

High-precision underwater imaging technology has an important role in underwater navigation and identification, underwater pipeline inspection, sinker docking, and underwater vibration measurement. Due to the different mechanisms of underwater measurement sources, such as 3D gated imaging and streak tube imaging, the accuracy of underwater target imaging can reach the millimeter level. In this paper, we propose a high-precision underwater imaging method based on frequency combs. When measuring the non-cooperative target underwater, the spectrogram is modulated due to the strong dispersion property of water. After that, for the gained spectrogram, we use two algorithms, i.e., center offset frequency and phase slope, to obtain the distance and imaging, respectively. By comparing with reference values, the underwater experimental results combining two algorithms show an agreement within 8 μm in the range of 2.5 m, corresponding to 3.2 × 10−6 relative precision. This work can provide a strong guarantee and support for high-precision underwater applications.