3.1. NESZ Analysis
In this section, the NRCS of each polarization channel is plotted to analyze the signal level relative to the NESZ in Figure 4
. The box-plots show the minimum, maximum, upper quartile, median, lower quartile, and outliers of the backscatter values σ0
in sample areas labeled as blue/red/yellow boxes in Figure 2
and Figure 3
, which represent the distribution of the sample data. The median is the value separating an ordered data array into two equal parts; the upper quartile is defined as the middle number between the minimum and median of the ordered array sample data, and the lower quartile is the middle number between the maximum and the median.
For the imagery in Case 1, three groups of sample data were extracted for follow-up analysis, including seawater, thick oil, and the thin oil slick mixed with water at the boundary of the oil slick. For the VV channel, the signal values of seawater are mostly greater than the NESZ, with the exception of some data at large incident angles. In the thick oil region, the data from both sample areas span the NESZ, with more than 50% of the data being lower than the NESZ values. The data range extends from 7.4 dB below to 6.6 dB above the NESZ baseline, and the data are distributed mostly within the area with a lower backscattering value than that of the sea. In the thin oil area, approximately 50–70% of the data lie above the NESZ baseline. The mixture of oil and water is sufficient at the boundary of the oil spill area, especially in the sample area with high incidence angle. The data values range from 12 dB below to 6.1 dB above the NESZ in the VV. Backscatter decreases faster in the HH channel than in the VV channel, so all the sample data in the HH channel are approximately 5 dB lower than those in the VV channel and thus closer to the noise floor (Figure 4
c). As shown in Figure 4
e, high noise contamination was observed in returns from all three surfaces in the cross-polarization (VH) channels.
For the imagery in Case 2, the overall values of the sea area are 1.9–7.2 above the NESZ baseline in the VV channel (Figure 4
b). In the biogenic slick region, all the data values span the NESZ. Approximately 50–75% of the slick data are higher than the NESZ baseline, and most slick data are distributed in the area with a lower backscattering value than that of the sea. In the area with the peanut oil slick, more than 75% of the data are 2.4 dB above the NESZ baseline; the statistical values of peanut oil slick are much lower than those of the sea and are slightly higher than those of the biogenic slick. On the other hand, the mean HH values lie about 1.3–5.2 dB below the mean VV signal in all the samples (Figure 4
d). In the VH channel, all data are contaminated by instrument noise, and fluctuate around the NESZ baseline (Figure 4
Analysis of the two images shows that co-polarization channels (VV/HH) have higher signal-to-noise ratios (SNRs) than the cross-polarization (VH/HV) channels, and the backscatter in the VV polarization channel is lower than that in the HH channel. In addition, the noise floor baseline is higher in Case 1 than in Case 2, which results in the image in Case 1 having a higher SNR.
3.2. Polarization Parameters of Continuous Slow-Release Slick
presents the results for entropy analysis in the two cases. The polarization scattering entropy describes the randomness of various scattering types, which are roll-invariant. As shown in Figure 5
a,b, seawater is dominated primarily by a single scattering target return from surface Bragg scatter, whose entropy value is lower than those of both crude oil and biogenic slicks. Natural oil seeps and biogenic slicks are clearly distinguished from the ambient ocean with significantly higher H, which indicates that the scattering has relatively strong randomness, and the number of identifiable scattering mechanisms is reduced. For Case 1, the data results extracted from the oil sample area (the green boxes in Figure 2
) are shown in Figure 5
c,d. The entropy value within the thick oil region increases significantly, with all values being greater than 0.9. The oil slick has multiple scattering mechanisms, including both Bragg and non-Bragg scattering, and thus presents complex characteristics [31
]. The entropy values within the thin oil region in Case 1 range from 0.8 to 0.9.
The results show the differences in entropy between the slick (0.6–0.95), ambient sea (0.2–0.4), peanut oil (0.75–0.85), and atmospheric front (0.5–0.7) areas in Case 2. The NESZ baseline is lower in the image with a smaller incidence angle (Case 2) than that with larger incidence angle (Case 1), and signals are less contaminated by noise. Hence, the ambient signal level within the image with small incidence angle (Case 2) has a higher SNR. These results may explain why the ocean has higher entropy in images of natural oil seeps (i.e., with larger incidence angle) than that in images of biogenic slicks (i.e., with smaller incidence angle). These findings concur with those reported in the literature [25
depicts the character of the mean scattering angle α, which corresponds to the scattering mechanism of the target area. The effective range of α corresponds to the continuous variation of the scattering mechanism. The primary scattering mechanism is surface scatter, including geometrical optics surface scattering, physical optics surface scattering, specular scattering, and Bragg surface scattering when α is <42.5°. Dipole scattering is dominant when α is in the range from 42.5° to 47.5°, and double-bounce scattering or even-bounce scattering is dominant when α is in the range 47.5–90°. In the ocean area, surface single scattering is the main scattering mechanism. For Case 1, the mean scattering angle α is low (~30°) for clean sea areas, indicating that single-bounce surface scatter is dominant. The range of α is around 55–65° within the thick oil area, and 45–55° within thin oil area, as the presence of oil modifies the scattering mechanism. In addition, the regular ripples caused by wind, waves, and currents on the sea surface also show similar characteristics to the thin oil slick for α. On the other hand, the α value of seawater is <15°, within the range 45–55° for the biogenic slick, and 35–42° for peanut oil in Case 2.
As seen in Figure 7
, modified anisotropy A12
provides a good visual contrast between slicks and sea in both cases. For Case 1, the oil region, which is primarily dominated by the second eigenvalue λ2
, has a lower modified anisotropy value when compared to clean seawater. There is a distinct boundary between the thick oil region and the seawater with substantially different values. The range of A12
within the thick oil area is approximately 0.1–0.25. In the thin oil region (0.3–0.5), A12
has slightly higher values than those in the thick oil region, but still lower than those of ambient sea. A12
values are 0.8–0.95 for the area of clean sea (Figure 7
a). It should be noted that the modified anisotropy A12
is calculated using the two largest eigenvalues, in case the second and third eigenvalues are seriously affected by noise, which differs from the conventional calculation procedure.
Additional results for A12
in Case 2 are presented in Figure 7
values in the clean seawater and biogenic slick area show similar trends to those of Case 1. The range of A12
values within the biogenic slick region is around 0.2–0.3. The molecular density on the edge of the biogenic slick is relatively low, with sparse monomolecular distribution, and A12
range of 0.4–0.6. For the above reasons, the biogenic slicks present as a thinner strip in the image, accompanied by the discontinuous phenomenon. In addition, the peanut oil poured onto the sea surface is regarded as a type of monomolecular slick to simulate biogenic slick. However, it exhibits much less capacity to suppress sea surface roughness than do biogenic slicks, resulting in A12
values within the range of 0.35–0.5.
As shown in Figure 8
, Figure 9
and Figure 10
, the combinations of entropy H
and modified anisotropy A12
, which represent a heuristic modification from the conventional combination of H
, are used to increase the recognition ability of various scattering types of the target area. The modified combinations are also able to highlight some details concerning changes in the various objectives under different combinations, which have similar purpose and significance to the traditional H
combinations, i.e., (1 − H
)*(1 − A12
), (1 − H
*(1 − A12
), and H
and Figure 9
show the combined H
*(1 − A12
) and H
images. In comparison with the traditional H
combination parameters, the result presents an interesting phenomenon in that the entire H
*(1 − A12
) image is relatively smooth. This effectively suppresses noise interference, but part of the internal wave region is still misclassified as a thin oil slick, due to similar data characteristics and range in the natural oil seep image. Some interference data remain, similarly to the thin oil slick. In contrast, the thin region at the biogenic slick boundary exhibits similar parameter values to those of peanut oil, but the atmospheric front shows obviously reduced interference in comparison to the peanut oil area; hence, the signal level of the biogenic slick area retains an obvious contrast with the ambient sea. Additional results for H
are presented in Figure 11
. In the oil spill image, the modified parameters H
produced a map of the oil spill that effectively suppressed the ambient interference information, i.e., internal waves, which can be used for the extraction and classification of the oil spill. The 3-D spatial distribution plots of H
and the H/α plane are shown in Figure 10
and Figure 11
. These plots can help to better interpret the scattering difference between continuous slow-release slicks and seawater [34
displays the VSI results. The VSI in the continuously slow-release slick region is obviously higher than that for seawater in both cases, indicating the occurrence of volume scattering within the continuous slow-release slick layer. For Case 1, the image is obviously affected by noise, and the range of VSI values for seawater fluctuate around 0.25–0.45 (average value ~0.35). The value within the thick oil slick fluctuates around 0.55, compared with 0.45 within the thin oil slick. Case 2 shows a better SNR, with obviously lower levels of fluctuation of the sample data in the image. The VSI values for biogenic slicks range from 0.4 to 0.5, while those of peanut oil and the atmospheric front areas range from 0.35 to 0.4 and 0.25–0.3, respectively.