On the analysis of a time series of X–band TerraSAR–X SAR imagery over oil seepages

Abstract

A large time series of 42 dual-polarimetric co-polarized TerraSAR-X (TSX) StripMap Synthetic Aperture Radar (SAR) measurements are exploited to monitor a well-known oil seep area, i.e., the Taylor Energy site in the Gulf of Mexico. A comprehensive scattering analysis is undertaken to assess the impact of SAR imaging parameters (polarization, angle of incidence-AOI, noise floor) and environmental conditions (wind speed-WS, oil properties) on single-polarization SAR-based sea oil seep observation. The main goal of this study is to evaluate the reliability of the scatteringbased information derived from the time series of TSX SAR imagery. A two-scale backscattering model is considered to give a physical framework that supports a better understanding of the effects of the above-mentioned factors. Experimental results showed that the high TSX noise floor significantly limits a reliable interpretation of the slick-free sea surface and oil seep backscattering at AOIs larger than 34 and 26 , respectively, since they are contaminated by noise. Hence, it is shown that, at larger AOIs, the joint contribution of noisy SAR measurements and low oil backscattering does not result in a larger oil/sea separability. The latter is not remarkably influenced by polarization and WS, under low-to-moderate conditions. Experiments also demonstrate that, when reliable SAR measurements are available, sea oil seep backscattering is affected by the oil's damping properties more than its concentration in the water column. The time variability of the polluted area is also estimated using the time series of TSX imagery and the obtained results agree with independent analysis undertaken on the same test site.

A Sensitivity Analysis of the Standard Deviation of the Copolarized Phase Difference for Sea Oil Slick Observation

“…It can be noted that σ φc success in highlighting the oil-affected area while neither the Pauli phase, nor its standard deviation show any significant hint related to the oil seep. This is likely due to the fact that the Pauli phase is related to the dielectric properties of the scattering scene that, as it was demonstrated in [37], play a marginal role when dealing with sea oil seep backscattering.…”

Mineral Oil Slicks Identification Using Dual Co-polarized Radarsat-2 and TerraSAR-X SAR Imagery

“…According to Equation (19), the average position of the RND parameter changes by~30% when the Bragg wave number is changing on 100% (within the range of 117 to 267 rad/m). These changes in the RND parameter can be related to both changes in the proportions of suppression of resonant ripples and non-resonant wave breaking with changing incidence angle or the carrier frequency of the radio wave, as well as an increase in the relative noise fraction in the data, which occurs with increasing incidence angle, as discussed by Nunziata et al [29]. The RND values of the RSd_E and TSa_E points were obtained at approximately the same SNR slick of 7-8 dB ( Figure 5) and close angles of 28 • -31 • , but at different frequencies that differed by 7% ( Table 3).…”

“…The instrument noise floor, namely the NESZ, which is a measure of the sensitivity of a given SAR, has a great impact on the sea slicks detectability by the SAR [3,14,29]. The NESZ is caused by a number of factors [17,29], e.g., the antenna pattern, the power of the transmitted pulse, the receiver noise, the bandwidth, the analogue-digital converter quantization noise, and, to a negligible extent, processing noise (for TS data, there was a NESZ dependence on the preprocessor version also before the version 4.7 [52]). Considering that the NESZ is an additive noise, the SAR data specifications [17] and the practical use of the low backscattered signal [12,13] require removing the NESZ from the data as follows:…”

Pre-Launch Assessment of RADARSAT Constellation Mission Medium Resolution Modes for Sea Oil Slicks and Lookalike Discrimination

“…Oil films dampen the capillary waves on the sea surface and thus the backscatter, causing slicks to appear as dark spots in gray tones intensity SAR images, while the surrounding slick-free sea remains relatively brighter (Apel and Jackson 2004). The detectability of oil slicks depends on a number of factors, such as the rheological properties of oil slicks, the speckle noise in SAR imagery, the radar incidence angle and the wind speed (Alpers et al 2017;Buono et al 2018;Skrunes et al 2018;Nunziata et al 2019). Oil slicks are not the only phenomenon that produces dark spots in SAR images.…”