On the history of the Secchi disc (original) (raw)
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Secchi Disk Measurements in Turbid Water
Journal of Geophysical Research: Oceans, 2020
In the classical theory of the Secchi disk depth, diffuse sunlight falling on the disk is reflected back to the observer's eye along the most direct route, as a beam. The disappearance depth, ZSD, of the disk is then expected to vary inversely with the sum of the beam and diffuse attenuation coefficients: c + KD. Observations presented here show that, in the most turbid waters sampled, the Secchi disk is visible at greater depths (by a factor of up to 4) than predicted by this theory. In these conditions, the disk appears blurry, and it seems likely that some of the light reflected by the disk returns to the eye as diffuse light, photons being scattered one or more times on their journey from the disk surface to the observer. We have modified the theory of the Secchi disk in turbid water to allow for a mixture of beamed and diffuse light contributing to disk visibility. The modified theory corrects the under‐estimate of Secchi depths in turbid waters and gives good agreement wit...
Re-evaluation of Padre Secchi's visual method for modern oceanography
Frontiers in Marine Science, 2024
Secchi-disk is an instrument that enables investigating water transparency, being reflective of sunlight to be used to measure the depth of light penetration in the water column. In biological marine research, the device is used to determine the intensity of primary production of planktonic algae. In the present study, we reevaluated the practicality of Secchi's method, developed some 160 years ago, for modern oceanography by using the same locations, methods, objectives, and comparable Secchi disk that were used in his survey in 1865. We show that the Secchi-disk is still a valid scientific method in environmental marine research and therefore defends its place reliably among modern electronical research infrastructure. Observer's subjectivity didn't have a significant effect on measuring but environmental conditions can influence Z SD determination in general. It is therefore justified to follow, for example, the general recommendations for the use of the Secchi net issued by HELCOM. Our results also revealed that Z SD was located about twofold deeper in 1865 than presently which cannot be explained only by annual variation. We show that green(ish) sea areas have today extended to larger sea areas than they were 160 years ago, due to an increase in the turbidity of the water resulting from, e.g., general eutrophication.
Secchi disk depth: A new theory and mechanistic model for underwater visibility
Remote Sensing of Environment, 2015
Secchi disk depth (Z SD) is a measure of water transparency, whose interpretation has wide applications from diver visibility to studies of climate change. This transparency has been explained in the past 60+ years with the underwater visibility theory, the branch of the general visibility theory for visual ranging in water. However, through a thorough review of the physical processes involved in visual ranging in water, we show that this theory may not exactly represent the sighting of a Secchi disk by a human eye. Further, we update the Law of Contrast Reduction, a key concept in visibility theory, and develop a new theoretical model to interpret Z SD. Unlike the classical model that relies strongly on the beam attenuation coefficient, the new model relies only on the diffuse attenuation coefficient at a wavelength corresponding to the maximum transparency for such interpretations. This model is subsequently validated using a large (N = 338) dataset of independent measurements covering oceanic, coastal, and lake waters, with results showing excellent agreement (~18% average absolute difference, R 2 = 0.96) between measured and theoretically predicted Z SD ranging from b 1 m to N30 m without regional tuning of any model parameters. This study provides a more generalized view of visual ranging, and the mechanistic model is expected to significantly improve the current capacity in monitoring water transparency of the global aquatic environments via satellite remote sensing.
Optical Properties of Polish Lakes: The Secchi Disc Transparency
Limnological Review, 2015
The spatial variation of water transparency of the largest Polish lakes was presented on the basis of documentary material collected by the Voivodeship Inspectorates for Environment Protection as well as university geography departments in the period of 1991-2013. Seasonal results of the Secchi disc measurements (spring, summer) for 706 lakes were gathered. This is more than 68% of the total number of Polish lakes with a surface area larger than 50 ha. The mean water transparency of these lakes was 1.87 m, while the median was 1.58 m. Distinct inter-regional differences of the Secchi disc transparency were found, and it was also possible to distinguish three relatively homogeneous groups of macroregions where lakes have different optical characteristics. They can be divided into areas with lakes with high (3.05 m), medium (1.99-2.11 m) and low (1.22-1.52 m) water transparency. The Secchi disc transparency of these lakes was also analysed in relation to concentration values of the ma...
Ocean Science, 2014
The Secchi depth and its relationships to other properties of the sea water in the Oslofjord–Skagerrak area have been investigated. White and black disks of different sizes have been applied, and the Secchi depth has been observed with the naked eye, through colour filters and with a water telescope. Spectral luminances and illuminances have been calculated from recordings of radiance and irradiance, and attenuation coefficients have been determined. A theoretical expression for the Secchi depth based on luminances has been tested against field observations, and it is found that the field results for the product of Secchi depth and attenuation coefficients are on average only 4% less than the predicted value for the white disk. For the Secchi depths observed through colour filters or for the black disk, the average field results are more than 30% smaller than the theoretical estimates. The reduction in the disk diameter from 30 to 10 cm should theoretically reduce the Secchi depths ...
Water transparency measurements in the deep Ionian Sea
Astroparticle Physics, 2010
A long optical base line spectrophotometer designed to measure light transmission in deep sea waters is described. The variable optical path length allows measurements without the need for absolute or external calibration. The spectrophotometer uses eight groups of uncollimated light sources emitting in the range 370-530 nm and was deployed at various depths at two locations in the Ionian Sea that are candidate sites for a future underwater neutrino telescope. Light transmission spectra at the two locations are presented and compared.
Remote Sensing of Environment, 2011
... Maéva Doron a , b , c , low asterisk , E-mail The Corresponding Author , Marcel Babin b , c , Odile Hembise a , Antoine Mangin a and ... Using ocean color data, Prasad et al.(1998) proposed a relationship between the Secchi depth and the ratio of two water-leaving radiances. ...
Progress In Electromagnetics Research, 2002
This study employed water transparent characteristics from the Gulf and archipelago of Finland and the corresponding data sets of optical sensors at the green wavelength band to estimate Secchi disk depth (SDD). The SDD is one major optical measurement of water transparency in the study area, where the coastal waters are dominated by absorption from both dissolved and particulate organic matter since the Gulf is optically dominated by scattering from suspended sediments. Concurrent in situ measurements, Landsat TM and simulated SeaWiFS data were obtained in August 1997. The results show that the SDD from the narrow green bandwidth (20 nm) data of simulated SeaWiFS is slightly better than the SDD from the broad green bandwidth (80 nm) data of TM using the semi-empirical algorithm developed in the study. The study of water transparent characteristics in the area still needs to be further investigated using SeaWiFS, MODIS and MERIS in the future.
Secchi depth analysis using bio-optical parameters measured in the Arabian Sea
Remote Sensing of the Marine Environment, 2006
Secchi depth provides the oceanographer with the first hand information about transparency and penetration of light in the water. Here we present results of the Secchi depth and the optical properties measured in the Arabian Sea. Our analyses show spatial and temporal variability of Secchi depth and their dependence on the optical properties beam attenuation and diffuse attenuation the biological parameter of Chlorophyll. The in-situ measured inherent and apparent optical properties have been used to understand the underwater light properties and their relations to the Secchi depth in various water types. The Secchi depth model is validated using the measured optical properties. We also present an empirical method to determine Secchi depth from the satellite ocean color sensor, and the application of the same to the IRS-P4 OCM is found to provide comparable results to the measured values.