– tank: evaluation of the influence of the slick thickness.
1.3.1.1. Vertical configuration
Eight different HNS (benzene, toluene, xylene, diethyl ether (DEE), rapeseed oil, propanol, methanol and heptane) were released into seawater inside a floating aluminum frame that was installed in the CEDRE pool, as presented in Figure 1.1. For each product, different spill volumes were used from 60 mL up to 5 L, and the seawater was thoroughly cleaned after each spill.
Figure 1.1. (a) Aluminum frame installed in the CEDRE pool. (b) Aerial lift with the three hyperspectral cameras
Three different hyperspectral imaging systems from 0.4 to 12 μm were used during this campaign: two reflective sensors (NEO HySpex cameras) from 0.4 to 1 μm (VNIR) and from 1 to 2.5 μm (SWIR) and a thermal longwave (LWIR) sensor from 8 to 12 μm (Telops Hypercam). Hyperspectral imaging cameras were deployed inside an aerial lift at a height of 12 m above the pool with a nadir-looking geometry and a Bomem MR300 spectroradiometer was placed next to the edge of the pool. The three sensors were pointed towards the expected center of the slick.
The aim was to evaluate how hyperspectral sensors can contribute to the detection of pollutants in a nadir-looking geometry.
1.3.1.2. Horizontal configuration
The sensors used for this configuration were as follows: two reflective sensors (NEO HySpex cameras) from 0.4 to 1 μm (VNIR) and from 1 to 2.5 μm (SWIR), an ASD Fieldspec camera from 0.4 to 2.5 µm, a thermal longwave (LWIR) sensor from 8 to 12 μm (Telops Hypercam), a Bomem MR300 spectroradiometer and a P-iCATSI (Polarized Improved Compact ATmospheric Sounding Interferometer).
Heptane, toluene, xylene, methanol, rapeseed oil, diethyl ether, silicone oil, unleaded gasoline and oil were released (volumes from 2 to 10 L). Diethyl ether and methanol were discharged directly in the basin, whereas the other products were released inside the floating aluminum frame.
Except for the interferometer, which was placed in an aerial lift several meters away from the basin, all the sensors were placed next to the edge of the pool.
1.3.1.3. Tank
The aim of this test was to qualify the impact of the thickness of the slick on the measured spectra. For this configuration, the Bomem MR300 spectroradiometer and the ASD Fieldspec camera were used. Heptane, xylene and rapeseed oil were released at the surface of a black-painted metallic barrel full of seawater.
The cloud coverage during the trial did not enable the realization of the initially planned measurements, and supplementary measures must be realized in the ONERA laboratory.
1.3.2. Evaluation of radar, optical and hyperspectral sensors at sea
1.3.2.1. General presentation of the experiment
The experimentation took place in May 2015 over the French coast in the Mediterranean Sea at two locations: 42°46.8’N/6°2.0’E for the first release and 42°45.5’N/5°48.5’E for the second and third releases. One cubic meter of each of the six chemical products (presented in section 1.2.2) was released at sea and imaged by radar and optical airborne sensors for their evaluation in real conditions.
HNS releases were performed from the salvage, rescue and oil spill response vessel Ailette of the French Navy under the direction of CEPPOL (Centre of Practical Expertise in Pollution Response) and CEDRE. Each chemical product was contained in a one cubic-meter tank, in HDPE (High Density PolyEthylene) for non-aggressive HNS (rapeseed oil, FAME and methanol) and in metal for reactive or corrosive HNS (xylene, heptane and toluene). Each tank was inserted in a metallic structure equipped with two 220 L floaters to ensure the floatability of the system and a lifting strap to manipulate the tanks with the onboard crane (Figure 1.2). The release of the HNS was performed from a dinghy by pulling a rope that activates the opening of the tank. Due to the difference in density between seawater and HNS, the chemicals spread at the sea surface.
In order to follow the drifts of the HNS slicks, two drifting buoys were implemented. Their GPS positions were transmitted by satellite every 15 minutes.
Figure 1.2. One cubic-meter tank. (a) HDPE. (b) Metal
Three aircraft imaged the HNS slicks:
– a Falcon 20 from AVdef company equipped with the experimental pod SETHI, including the ONERA sensors: two radars (SAR) at X and L bands, full polarization (HH, HV, VH, VV), one SWIR hyperspectral camera (HYSPEX), one CamV2 camera (visible range);
– a POLMAR aircraft from DGDDI equipped with IR sensor, UV sensor and SLAR radar at X-band;
– a Cessna C-303 chartered by DRDC equipped with an IR hyperspectral camera (TELOPS).
On board the Ailette, two optical cameras were installed by DRDC: a MWIR hyperspectral camera and a Bomem MR304 LWIR and MWIR spectroradiometer.
In order to characterize the behavior of HNS once released at sea (evaporation, dissolution, emulsification, etc.), sampling of the slicks, the water column and the atmosphere were planned.
1.3.2.2. Experimental releases
HNS releases were planned to be performed two at a time, i.e. two HNS were nearly simultaneously released and imaged by the aircraft. The most dangerous HNS were first released (heptane and toluene), then methanol and xylene for the second release and, finally, FAME and rapeseed oil for the third release.
Table 1.1. Environmental conditions
| Date | Time (UTC) | Wind speed (m/s) | Wind direction (from-deg) | Wave height (m) | Wave direction (from-deg) |
| May 18, 2015 | 17:00 | 8 | 255 | 0.5 | 240 |
| May22, 2015 | 13:00 | 7 | 315 | 2 | 270 |
| May 22, 2015 | 16:00 | 7 | 315 | 1.75 | 270 |
The methodology used to release HNS is ideally suited to good weather conditions and, in particular, for a sea state compatible with the lifting operations. The first release was performed as planned on May 18 thanks to good weather conditions. Then, a strong wind episode (May 19–21, force 8, 35–40 knots) forced us to replan the last two releases on May 22. Sea and weather conditions during the experimentation