HyperSpectral Imaging
CPI is assisting the Coastal Ocean Remote Sensing Branch of the Naval Research Laboratory (NRL) in developing tools and
technology to exploit hyperspectral imagery in the visible to near-infrared (VNIR) part of the electromagnetic spectrum over the coastal ocean. Hyperspectral imaging in the VNIR is a powerful tool to interrogate both the surface and subsurface
contents of the coastal ocean due to the fact that these wavelengths penetrate the water. Information about the water depth, bottom type, and water column constituents can be determined from the reflectance spectra. Using a spiral development approach to address scientific and technical problems NRL is constantly creating new solutions for the remote sensing needs of the Navy in the coastal zone. In this approach, new sensors are developed then rigorously characterized using
state-of-the-art calibration facilities. Sensors are then tested in the field and validated with ground/water truth data.
Products are created from this field data and evaluated based on the needs of a variety user groups. Finally, sensor
requirements are re-evaluated based on these results and a new stage in development begins.
CPI is contributing substantially to these efforts in several areas. In field testing CPI employees have been involved in numerous sensor deployments both in aircraft, operating the sensors and on the ground/water, as well as collecting the validation data. CPI has provided technical support for product development through modeling efforts using a radiative transfer code to determine the optimum sensor characteristics (spectral resolution, spatial resolution, wavelength range, and sensor sensitivity) and identifying which parameter subsets contribute meaningfully to observed reflectance spectra based on known sensor characteristics. CPI has also evaluated these products through rigorous statistical analysis of product output with the previously mentioned validation data.
Remote Sensing of Wastewater Discharge
An example of such activities is the detection of municipal wastewater discharge into the ocean. Many coastal communities discharge municipal wastewater into open-ocean waters through a submerged discharge pipe, or ocean outfall. The discharge, or effluent, undergoes rapid mixing in the immediate vicinity of the outfall, becoming diluted with ambient seawater as it rises buoyantly toward the sea surface. If the ocean is stratified, there is the possibility the discharge will be trapped below the sea surface. Monitoring of wastewater outfalls has historically relied on in-water sampling, but modern remote sensing techniques now enable the detection of the discharge using airborne spectrographic imagery, thermal infrared imagery, multispectral imagery, and space-borne synthetic aperture radar.
NRL and CPI have investigated a municipal wastewater outfall located off the southeast coast of Florida, USA., using airborne imagery obtained simultaneously with a Compact Airborne Spectrographic Imager (CASI) and a mid-wave infrared (IR) camera. This unique combination of sensors offers potential synergies stemming from different sensing physics. Elevated levels of chromophoric dissolved organic matter plus detrital material (CDOM) can be deduced directly from the CASI data in the upper ~10 m of the water column using an ocean-community algorithm. The IR imagery enables any surface thermal variation due to the discharge to be observed in th upper 30 μm of the water column. By detecting the optical penetration of the offshore coastal waters using vastly different sensing, it is possible to determine whether the wastewater discharge is surfacing. Figure 1 below shows simultaneous CASI and IR imagery of the region where a section of the discharge pipe can be seen (feature 1) where it runs across a <20-m-deep alongshore reef. Amidst this background the plume can be seen (labeled in the figure), which appears relatively dark compared to the immediately surrounding water. There is a similarly shaped plume front in both the CASI and IR images. A time sequence of IR imagery shows the variability of the surface thermal structure within the plume (Figure 2).
Figure 1. Simultaneous CASI (a) and IR (b) imagery from a sampling swath off the southeast coast of Florida. (Imagery courtesy of Naval Research Laboratory).
Figure 2. Time sequence of IR imagery showing variability of surface thermal structure within the plume. (Imagery courtesy of Naval Research Laboratory).
This illustrates the potential of remote sensing for detecting municipal wastewater discharge, and for more fully understanding the physical processes responsible for the mixing and diluting of a surface plume.
STAY CONNECTED