Silvia Matt

Restoration of images degraded by underwater turbulence using structure tensor oriented image quality (STOIQ) metric

Recent advances in image processing for atmospheric propagation have provided a foundation for tackling the similar but perhaps more complex problem of underwater imaging, which is impaired by scattering and optical turbulence. As a result of these impairments underwater imagery suffers from excessive noise, blur, and distortion. Underwater turbulence impact on light propagation becomes critical at longer distances as well as near thermocline and mixing layers. In this work, we demonstrate a method for restoration of underwater images that are severely degraded by underwater turbulence. The key element of the approach is derivation of a structure tensor oriented image quality metric, which is subsequently incorporated into a lucky patch image processing framework. The utility of the proposed image quality measure guided by local edge strength and orientation is emphasized by comparing the restoration results to an unsuccessful restoration obtained with equivalent processing utilizing a standard isotropic metric. Advantages of the proposed approach versus three other state-of-the-art image restoration techniques are demonstrated using the data obtained in the laboratory water tank and in a natural environment underwater experiment. Quantitative comparison of the restoration results is performed via structural similarity index measure and normalized mutual information metric.

Experimental and numerical study of underwater beam propagation in a Rayleigh–Bénard turbulence tank

The propagation of a laser beam through Rayleigh-Bénard (RB) turbulence is investigated experimentally and by way of numerical simulation. For the experimental part, a focused laser beam transversed a 5  m×0.5  m×0.5  m water filled tank lengthwise. The tank is heated from the bottom and cooled from the top to produce convective RB turbulence. The effect of the turbulence on the beam is recorded on the exit of the beam from the tank. From the centroid motion of the beam, the index of refraction structure constant Cn2 is determined. For the numerical efforts RB turbulence is simulated for a tank of the same geometry. The simulated temperature fields are converted to the index of refraction distributions, and Cn2 is extracted from the index of refraction structure functions, as well as from the simulated beam wander. To model the effect on beam propagation, the simulated index of refraction fields are converted to discrete index of refraction phase screens. These phase screens are then used in a split-step beam propagation method to investigate the effect of the turbulence on a laser beam. The beam wander as well as the index of refraction structure parameter Cn2 determined from the experiment and simulation are compared and found to be in good agreement.

Turbulent kinetic energy and temperature variance dissipation in laboratory generated Rayleigh-Bénard turbulence designed to study the distortion of light by underwater microstructure fluctuations

Small-scale variation in temperature and salinity can lead to localized changes in the index of refraction and can distort electro-optical (EO) signal transmission in ocean and atmosphere. This phenomenon is well-studied in the atmosphere and in this context is generally called “optical turbulence”. Less is known about how turbulent fluctuations in the ocean distort EO signal transmissions, an effect that can impact various underwater applications, from diver visibility to active and passive remote sensing. To provide a test bed for the study of the impacts from turbulent flows on EO signal transmission, as well as to examine and mitigate turbulence effects, we set up a laboratory turbulence environment allowing the variation of turbulence intensity. Convective turbulence is generated in a large Rayleigh-Bénard type tank (5m by 0.5m by 0.5m) and the turbulent flow is quantified using a suite of sensors that includes high-resolution Acoustic Doppler Velocimeter profilers (Vectrino Profiler) and fast thermistor probes (PME Conductivity- Temperature probe). These measurements allow the characterization of turbulent kinetic energy and temperature variance dissipation rates in the tank, for different convective strengths. Optical image degradation in the tank is then assessed in relation to turbulence intensity. The turbulence measurements are further complemented by very high-resolution computational fluid dynamics simulations of convective turbulence emulating the tank environment. These numerical simulations supplement the sparse laboratory measurements, providing full fields of temperature and velocity in the tank. The numerical data compared well to the laboratory data and both conformed to the Kolmogorov spectrum of turbulence and the Batchelor spectrum of temperature fluctuations. The numerical model was able to qualitatively reproduce the turbulence fields observed in the laboratory tank. Quantitatively, the numerical simulations are consistent with the observed ε in the tank, but do not fully resolve the temperature gradients and thus underestimate Ξ. The unique approach of integrating optical techniques, turbulence measurements and numerical simulations can help advance our understanding of how to mitigate the effects of turbulence impacts on underwater optical signal transmission, as well as on the use of optical techniques to probe oceanic processes.

Measurements of optical underwater turbulence under controlled conditions

Laser beam propagation underwater is becoming an important research topic because of high demand for its potential applications. Namely, ability to image underwater at long distances is highly desired for scientific and military purposes, including submarine awareness, diver visibility, and mine detection. Optical communication in the ocean can provide covert data transmission with much higher rates than that available with acoustic techniques, and it is now desired for certain military and scientific applications that involve sending large quantities of data. Unfortunately underwater environment presents serious challenges for propagation of laser beams. Even in clean ocean water, the extinction due to absorption and scattering theoretically limit the useful range to few attenuation lengths. However, extending the laser light propagation range to the theoretical limit leads to significant beam distortions due to optical underwater turbulence. Experiments show that the magnitude of the distortions that are caused by water temperature and salinity fluctuations can significantly exceed the magnitude of the beam distortions due to atmospheric turbulence even for relatively short propagation distances. We are presenting direct measurements of optical underwater turbulence in controlled conditions of laboratory water tank using two separate techniques involving wavefront sensor and LED array. These independent approaches will enable development of underwater turbulence power spectrum model based directly on the spatial domain measurements and will lead to accurate predictions of underwater beam propagation.

Velocity fields and optical turbulence near the boundary in a strongly convective laboratory flow

Boundary layers around moving underwater vehicles or other platforms can be a limiting factor for optical communication. Turbulence in the boundary layer of a body moving through a stratified medium can lead to small variations in the index of refraction, which impede optical signals. As a first step towards investigating this boundary layer effect on underwater optics, we study the flow near the boundary in the Rayleigh-Bénard laboratory tank at the Naval Research Laboratory Stennis Space Center. The tank is set up to generate temperature-driven, i.e., convective turbulence, and allows control of the turbulence intensity. This controlled turbulence environment is complemented by computational fluid dynamics simulations to visualize and quantify multi-scale flow patterns. The boundary layer dynamics in the laboratory tank are quantified using a state-of-the-art Particle Image Velocimetry (PIV) system to examine the boundary layer velocities and turbulence parameters. The velocity fields and flow dynamics from the PIV are compared to the numerical model and show the model to accurately reproduce the velocity range and flow dynamics. The temperature variations and thus optical turbulence effects can then be inferred from the model temperature data. Optical turbulence is also visible in the raw data from the PIV system. The newly collected data are consistent with previously reported measurements from high-resolution Acoustic Doppler Velocimeter profilers (Nortek Vectrino), as well as fast thermistor probes and novel next-generation fiber-optics temperature sensors. This multi-level approach to studying optical turbulence near a boundary, combining in-situ measurements, optical techniques, and numerical simulations, can provide new insight and aid in mitigating turbulence impacts on underwater optical signal transmission.

A controlled laboratory environment to study EO signal degradation due to underwater turbulence

Temperature microstructure in the ocean can lead to localized changes in the index of refraction and can distort underwater electro-optical (EO) signal transmission. A similar phenomenon is well-known from atmospheric optics and generally referred to as “optical turbulence”. Though turbulent fluctuations in the ocean distort EO signal transmission and can impact various underwater applications, from diver visibility to active and passive remote sensing, there have been few studies investigating the subject. To provide a test bed for the study of impacts from turbulent flows on underwater EO signal transmission, and to examine and mitigate turbulence effects, we set up a laboratory turbulence environment allowing the variation of turbulence intensity. Convective turbulence is generated in a large Rayleigh- Bénard tank and the turbulent flow is quantified using high-resolution Acoustic Doppler Velocimeter profilers and fast thermistor probes. The turbulence measurements are complemented by computational fluid dynamics simulations of convective turbulence emulating the tank environment. These numerical simulations supplement the sparse laboratory measurements. The numerical data compared well to the laboratory data and both conformed to the Kolmogorov spectrum of turbulence and the Batchelor spectrum of temperature fluctuations. The controlled turbulence environment can be used to assess optical image degradation in the tank in relation to turbulence intensity, as well as to apply adaptive optics techniques. This innovative approach that combines optical techniques, turbulence measurements and numerical simulations can help understand how to mitigate the effects of turbulence impacts on underwater optical signal transmission, as well as advance optical techniques to probe oceanic processes.

Characterizing instabilities in the developed and transitional boundary layer

It is well known that elastic or compliant boundaries can have a stabilizing effect on boundary layer flow leading to a reduction in turbulence and frictional drag. This phenomenon has wide-ranging interdisciplinary applications from the study of energy-efficient propulsion to the study of blood flow through the cardiovascular system. While a substantial body of work exists on the theory of turbulent boundary layers and the transition of laminar to turbulent flow, it is equally important to measure in detail the flow near rigid and compliant boundaries to better understand the dynamics underlying the stabilizing effect and the reduction of turbulence. Recent advances in technology and computational resources have allowed the measurement and numerical simulation of boundary layer instabilities in unprecedented detail. We employ particle image velocimetry as well as high-frequency fiber-optics sensors to visualize and measure velocity and temperature fluctuations under various flow conditions: a laminar flow tank to study the development of Tollmien-Schlichting waves and the laboratory tank of the Simulated Turbulence and Turbidity Environment (SiTTE) to identify boundary layers streaks. The laboratory environments are complemented by computational fluid dynamics representations of the respective setups, implemented as high-resolution large-eddy simulation. The simulations provide spatial and temporal scales of boundary layer instabilities, allow the calculation of turbulence characteristics and add prediction capabilities. The combined approach allows the detailed characterization of boundary layer instabilities for a range of flow conditions, which is critical to improve our understanding of the impact of elastic boundaries, both active and passive, on boundary layer drag.

The impact of optical turbulence on particle image velocimetry

Particle image velocimetry (PIV) is a well-established tool to collect high-resolution velocity and turbulence data in the laboratory. PIV measurements are based on using a laser sheet to illuminate a flow seeded with small particles and taking quick successive images or image pairs of the illuminated particle field with a CCD or CMOS camera. The movement of the particles between images can be used to infer flow field velocities over an image area. During experiments at the Simulated Turbulence and Turbidity Environment (SiTTE) laboratory tank, we observed a marked influence of optical turbulence, i.e. strong temperature gradients leading to changes in the index of refraction, on particle imaging in PIV. The particles look blurred and have a “shooting star” appearance. PIV is routinely used in flows with very high temperature gradients, such as nuclear reactor cooling rods, but the optical path length is typically very short (on the order of cm), and no such effect is generally considered for measurements in liquids. We investigated the effect of optical turbulence on PIV imaging for various optical path lengths (0.5m to 2m) and turbulence strengths. Velocities from the PIV measurements were calculated using the algorithms provided within Dantec’s Dynamic Studio and compared to velocities from concurrent velocity point measurements with a Laser Doppler Velocimetry system. The results indicate that optical turbulence can affect PIV measurements in liquids, and that depending on the strength of the optical turbulence and path length, care needs to be taken to mediate this effect using appropriate post-processing techniques when inferring velocities from PIV data.

Beam wander due to optical turbulence in water (Conference Presentation)

Optical methods to communicate or sense in the ocean environment can be effected inhomogeneities in the index of refraction called optical turbulence. Beam wander introduced by optical turbulence is of particular interest for optical means relying on the propagation of a well-defined laser beam such as free space communication and laser line scan. Here we present a comprehensive study of beam propagation simulations, lab experiments, and field measurements of laser beams propagating through varying degrees of optical turbulence. For the computational part of the investigation a true end to end simulation was performed. Starting with a CFD simulation of Rayleigh–Bénard convection the temperature fields where converted to index of refraction phase screens which then where used to simulate the propagation of a focused Gaussian laser beam via the split-step Fourier method. Lab experiments where conducted using the same parameters as in the simulation using a good quality TEM00 beam and a CCD camera to record data. For the field experiments a Telescoping Ridged Underwater Sensor Structure (TRUSS) was equipped with a transmitter and a receiver capable of analyzing a multitude of laser beams simultaneously. The TRUSS was deployed in the Bahamas to record beam wander under weak optical turbulence conditions above and stronger optical turbulence conditions inside the thermocline. The data from the experimental and lab experiments are compared and the strength of the optical turbulence in terms of the structure parameter Cn2 are extracted. We also extract Cn2 from the TRUSS experiments and in doing so provide, for the first time, a quantitative estimate for the strength of optical turbulence in the ocean.

Assessment of lidar remote sensing capability of Raman water temperature from laboratory and field experiments (Conference Presentation)

Lidar remote sensing based on visible wavelength is one of the only way to penetrate the water surface and to obtain range resolved information of the ocean surface mixed layer at the synoptic scale. Accurate measurement of the mixed layer properties is important for ocean weather forecast and to assist the optimal deployment of military assets. Turbulence within the mixed layer also plays an important role in climate variability as it also influences ocean heat storage and algae photosynthesis (Sverdrup 1953, Behrenfeld 2010). As of today, mixed layer depth changes are represented in the models through various parameterizations constrained mostly by surface properties like wind speed, surface salinity and sea surface temperature. However, cooling by wind and rain can create strong gradients (0.5C) of temperature between the submillimeter surface layer and the subsurface layer (Soloviev and Lukas, 1997) which will manifest itself as a low temperature bias in the observations. Temperature and salinity profiles are typically used to characterize the mixed layer variability (de Boyer Montégut et al. 2004) and are both key components of turbulence characterization (Hou 2009). Recently, several research groups have been investigating ocean temperature profiling with laser remote sensing based either on Brillouin (Fry 2012, Rudolf and Walther 2014) or Raman scattering (Artlett and Pask 2015, Lednev et al. 2016). It is the continuity of promising research that started decades ago (Leonard et al. 1979, Guagliardo and Dufilho 1980, Hirschberg et al. 1984) and can benefit from the current state of laser and detector technology. One aspect of this research that has not been overlooked (Artlett and Pask 2012) but has yet to be revisited is the impact of temperature on vibrational Raman polarization (Chang and Young, 1972). The TURBulence Ocean Lidar is an experimental system, aimed at characterizing underwater turbulence by examining various Stokes parameters. Its multispectral capability in both emission (based on an optical parametric oscillator) and detection (optical filters) provide flexibility to measure the polarization signature of both elastic and inelastic scattering. We will present the characteristics of TURBOL and several results from our laboratory and field experiments with an emphasis on temperature profiling capabilities based on vibrational Raman polarization. We will also present other directions of research related to this activity.