Elena Torrecilla

Derivative analysis of hyperspectral oceanographic data

It has long been recognized that transmission of light in sea water is essential to the productivity of the oceans. It provides the energy necessary for ocean currents, and the majority of marine life is supported by the thin layer of warm water near the oceans surface. Light plays a decisive role in the primary formation of biomass by oceanic chlorophyllbearing marine plants, through the process of photosynthesis, which is the basis of the entire marine food chain. Light transmission is therefore a key factor in the ecology of the upper ocean and biogeochemical cycling, since it has a strong influence on the dynamics of the chemical compounds. In addition, variability of light in the sea is strongly influenced by the distribution of the components in the water column, which varies across horizontal and vertical space and time scales.

Effect of spectral resolution in hyperspectral data analysis

The effect of hyperspectral data resolution on the results obtained using derivative spectroscopy is discussed in this article. A comparison was made between attenuation spectra measured using two different hyperspectral sensors with 3648 and 256 spectral bands, respectively. Smoothing and derivative algorithms were applied to both types of spectral data in order to assess qualitative information from the spectral features of several laboratory samples. In order to make an optimal application of the derivative analysis, a suitable selection of the smoothing and derivating parameters was done according to the resolution of each type of hyperspectral data. Second derivative spectra were obtained and peaks related to the absorption bands of pigments present in the considered samples were identified in both cases. Furthermore, interpolation techniques were applied so as to match spectral resolutions of data collected by the two sensors and negligible variations in the positions of the peaks were achieved in the derivative spectra.

Cost-Effective Hyperspectral Transmissometers for Oceanographic Applications: Performance Analysis

The recent development of inexpensive, compact hyperspectral transmissometers broadens the research capabilities of oceanographic applications. These developments have been achieved by incorporating technologies such as micro-spectrometers as detectors as well as light emitting diodes (LEDs) as light sources. In this study, we evaluate the performance of the new commercial LED-based hyperspectral transmissometer VIPER (TriOS GmbH, Rastede, Germany), which combines different LEDs to emulate the visible light spectrum, aiming at the determination of attenuation coefficients in coastal environments. For this purpose, experimental uncertainties related to the instrument stability, the effect of ambient light and derived temperature, and salinity correction factors are analyzed. Our results identify some issues related to the thermal management of the LEDs and the contamination of ambient light. Furthermore, the performance of VIPER is validated against other transmissometers through simultaneous field measurements. It is demonstrated that VIPER provides a compact and cost-effective alternative for beam attenuation measurements in coastal waters, but it requires the consideration of several optimizations.

Monolithic spectrometer for environmental monitoring applications

Hyperspectral data analysis has been shown to be a reliable technique to identify several water components. Currently, there are numerous commercially available miniature spectrometer systems as well as discrete components that are used by researchers in designing their own systems. We have developed a custom-designed hyperspectral sensor. Experimental results and potential applications are discussed on this article. It can be used as a low cost optical monitoring system to be employed on fish farming and aquaculture facilities.

Hyperspectral remote sensing of phytoplankton assemblages in the ocean: Effects of the vertical distribution

The increasing availability of hyperspectral technology on remote sensing observing platforms provides marine scientists the potential to better map the phytoplankton community composition in the ocean globally. In this study, this approach is examined using unsupervised cluster techniques applied to data sets of hyperspectral remote sensing reflectance obtained by radiative simulations. Different oceanic environments in terms of phytoplankton biodiversity were satisfactorily classified. Furthermore, an assessment of the effect of a variable stratified water column scenario on the remote sensing reflectance points out that vertical distributions of phytoplankton communities along the water column play an essential role in the accurate phytoplankton assemblages mapping by remote sensing.

Towards Cost-Effective Operational Monitoring Systems for Complex Waters: Analyzing Small-Scale Coastal Processes with Optical Transmissometry

The detection and prediction of changes in coastal ecosystems require a better understanding of the complex physical, chemical and biological interactions, which involves that observations should be performed continuously. For this reason, there is an increasing demand for small, simple and cost-effective in situ sensors to analyze complex coastal waters at a broad range of scales. In this context, this study seeks to explore the potential of beam attenuation spectra, c(λ), measured in situ with an advanced-technology optical transmissometer, for assessing temporal and spatial patterns in the complex estuarine waters of Alfacs Bay (NW Mediterranean) as a test site. In particular, the information contained in the spectral beam attenuation coefficient was assessed and linked with different biogeochemical variables. The attenuation at λ = 710 nm was used as a proxy for particle concentration, TSM, whereas a novel parameter was adopted as an optical indicator for chlorophyll a (Chl-a) concentration, based on the local maximum of c(λ) observed at the long-wavelength side of the red band Chl-a absorption peak. In addition, since coloured dissolved organic matter (CDOM) has an important influence on the beam attenuation spectral shape and complementary measurements of particle size distribution were available, the beam attenuation spectral slope was used to analyze the CDOM content. Results were successfully compared with optical and biogeochemical variables from laboratory analysis of collocated water samples, and statistically significant correlations were found between the attenuation proxies and the biogeochemical variables TSM, Chl-a and CDOM. This outcome depicted the potential of high-frequency beam attenuation measurements as a simple, continuous and cost-effective approach for rapid detection of changes and patterns in biogeochemical properties in complex coastal environments.

Spectral range sensitivity analysis to improve hyperspectral remote sensing of phytoplankton biodiversity in the ocean

The increasing availability of hyperspectral oceanographic measurements provides marine scientists the potential to develop algorithms for large-scale remote sensing of phytoplankton biodiversity and dynamics in the ocean. In this study, this approach is examined using unsupervised cluster techniques applied to field data sets of hyperspectral absorption of phytoplankton and remote sensing reflectance collected along the eastern Atlantic Ocean in 2008. Different open ocean environments in terms of phytoplankton biodiversity were successfully classified. Our performance was validated by considering an objective criterion based on the pigment composition of phytoplankton detected at each station. In order to achieve a good discrimination, an assessment of the effect of the spectral range considered in the cluster-based analysis was required and revealed that some optically significant spectral regions play an essential role in mapping oceanic phytoplankton assemblages by remote sensing.

Stray-light correction of in-water array spectroradiometers. Effects on underwater optical measurements

Two miniature hyperspectral sensors are being considered to become part of a marine observatory system. The effect of stray-light radiation on its response is discussed in this article. A new method to correct the unwanted spectral stray-light radiation has been developed and applied to both hyperspectral sensors. The correction method has been proved as effective for both considered miniature spectrographs using different calibration light sources. Furthermore, preliminary results of spectral stray-light corrections applied to underwater light field measurements point out that spectral stray-light correction plays an important role in accurate underwater optical hyperspectral measurements and its later spectral analysis.

Low-cost computationally hyperspectral simulator for highly dynamic marine environments

Hyperspectral optical observations and the development of new processing strategies are key for a better understanding of complex marine ecosystems and space-time distribution of ecological parameters. In this paper, the methodologies to implement a simulator of hyperspectral-resolved optical data corresponding to highly dynamic marine environments are presented. The simulator is based on a coupled radiative transfer and Lagrangian hydrodynamic model, which is organized in four basic blocks: a hydrodynamic model, a particle tracking model, a transformation function and a radiative transfer model. The transformation function is needed to adapt the output of the tracking model (given in number of particles per unit volume) to mass concentration, suitable for the radiative transfer model. The transformation function has been derived considering an allometric relationship between both magnitudes, since it is found in nature. The simulator is finally tested by considering the Alfacs Bay (NW Mediterranean Sea), as a case study site.

Do hyperspectral transmissometers allow us to go one step further on the analysis of particulate matter characteristics of marine samples

Several studies have been carried out to investigate the correlation between the spectral shape features of the beam attenuation coefficient and the particulate matter characteristics in seawater, but little attention has been paid to the spectral resolution of these measurements. For this reason, the potential of the new hyperspectral transmissometer VIPER (TriOS GmbH), with 1.7 nm spectral resolution, has been evaluated in this study and compared with lower resolution and multispectral based approaches (e.g. ac-9 or ac-s-with 4 nm resolution - from WETLabs Inc.) in order to evaluate whether any additional information about water composition can be retrieved from a spectral shape-based assessment. In this way, this study proposes a statistical-based method - a Hierarchical Cluster Analysis (HCA) using the cosine distance as similarity value - which allows discriminating suspended sediment samples with different particle size distribution (PSD) based on the attenuation spectral shape features. Finally, the effects of both particle size and concentration on the spectral shape have been analyzed separately. The results confirmed that the beam attenuation spectral features are in first-order driven by particle concentration, which means that a prior knowledge of particulate matter concentration is required in order to classify sediment samples according to their particle size. This approach based on hyperspectral attenuation measurements to characterize the PSD has been demonstrated a potential alternative compared to the traditional methods such as Coulter Counter or the particle size analyzer LISST 100X, which are much more expensive and time-consuming approaches.