Experimental study of C-band microwave scattering characteristics during the emulsification process of oil spills

In this study,oil spill experiments were performed in a water tank to determine changes in the surface scattering characteristics during the emulsification of oil spills.A C-band fully-polarimetric microwave scatterometer and a vector network analyzer were used to observe films of the following oils:crude oil with an asphalt content below3%that is prone to emulsification(type A),fresh crude oil extracted from an oilfield(type B),and industrial crude oil that was dehydrated and purified(type C).The difference in the backscatter results between the emulsified oil film and the calm water surface under C-band microwaves and the influence of the emulsification of the oil film on the backscatter were analyzed in detail.The results demonstrate that under a low-wind and no-waves condition(the maximum wave height was below than 3 mm),the emulsification of crude oil could modulated the backscatter through changes in the surface roughness and the dielectric constant,where the surface roughness had the dominant effect.The surface backscatters of the type B oil were greater than that of the type C oil in both the emulsified and non-emulsified states.In the non-emulsified state,the average differences in the backscatter between the type B and C oils were 2.19 dB,2.63 dB,and 2.21 dB for the polarization modes of VV,HH,and HV/VH,respectively.Smaller corresponding average differences of 0.98 dB,1.49 dB,and 1.5 dB were found for the emulsified state with a 20%moisture constant for the oil film.The results demonstrated that the surface roughness of the different oil films could vary due to the differences in the oil compositions and the oil film properties,which in turn affect the backscatter of the oil film surface.

The microwave scattering characteristics of sea ice in the Bohai Sea

Microwave remote sensing has become the primary means for sea-ice research, and has been supported by a great deal of field experiments and theoretical studies regarding sea-ice microwave scattering. However, these studies have been barely carried in the Bohai Sea. The sea-ice microwave scattering mechanism was first developed for the thin sea ice with slight roughness in the Bohai Sea in the winter of 2012, and included the backscattering coefficients which were measured on the different conditions of three bands（L, C and X）, two polarizations（HH and VV）, and incident angles of 20° to 60°, using a ground-based scatterometer and the synchronous physical parameters of the sea-ice temperature, density, thickness, salinity, and so on. The theoretical model of the sea-ice electromagnetic scattering is obtained based on these physical parameters. The research regarding the sea-ice microwave scattering mechanism is carried out through two means, which includes the comparison between the field microwave scattering data and the simulation results of the theoretical model, as well as the feature analysis of the four components of the sea-ice electromagnetic scattering. It is revealed that the sea-ice microwave scattering data and the theoretical simulation results vary in the same trend with the incident angles. Also, there is a visible variant in the sensitivity of every component to the different bands.For example, the C and X bands are sensitive to the top surface, the X band is sensitive to the scatterers, and the L and C bands are sensitive to the bottom surface, and so on. It is suggested that the features of the sea-ice surfaces and scatterers can be retrieved by the further research in the future. This experiment can provide an experimental and theoretical foundation for research regarding the sea-ice microwave scattering characteristics in the Bohai Sea.

Measurement of time-varying electron density of the plasma generated from a small-size cylindrical RDX explosion by Rayleigh microwave scattering

It is challenging to measure the electron density of the unsteady plasma formed by charged particles generated from explosions in the air,because it is transient and on a microsecond time scale.In this study,the time-varying electron density of the plasma generated from a small cylindrical cyclotrimethylenetrinitramine(RDX)explosion in air was measured,based on the principle of microwave Rayleigh scattering.It was found that the evolution of the electron density is related to the diffusion of the detonation products.The application of the Rayleigh microwave scattering principle is an attempt to estimate the electron density in explosively generated plasma.Using the equivalent radius and length of the detonation products in the bright areas of images taken by a high-speed framing camera,the electron density was determined to be of the order of 10^(20)m^(−3).The delay time between the initiation time and the start of variation in the electron-density curve was 2.77–6.93μs.In the time-varying Rayleigh microwave scattering signal curve of the explosively generated plasma,the electron density had two fluctuation processes.The durations of the first stage and the second stage were 11.32μs and 19.20μs,respectively.Both fluctuation processes increased rapidly to a peak value and then rapidly attenuated with time.This revealed the movement characteristics of the charged particles during the explosion.

Microwaves Scattering by Underdense Inhomogeneous Plasma Column

The scattering characteristics of microwaves （MWs） by an underdense inhomoge- neous plasma column have been investigated. The plasma column is generated by hollow cathode discharge （HCD） in a glass tube filled with low pressure argon. The plasma density in the column can be varied by adjusting the discharge current. The scattering power of X-band MWs by the column is measured at different discharge currents and receiving angles. The results show that the column can affect the properties of scattering wave significantly regardless of its plasma frequency much lower than the incident wave frequency. The power peak of the scattering wave shifts away from 0° to about ±15° direction. The finite-different time-domain （FDTD） method is employed to analyze the wave scattering by plasma column with different electron density distributions. The reflected MW power from a metal plate located behind the column is also measured to investi- gate the scattering effect on reducing MW refiectivity of a metal target. This study is expected to deepen the understanding of plasma-electromagnetic wave interaction and expand the applications concerning plasma antenna and plasma stealth.

A new model for Doppler shift of C-band echoes backscattered from sea surface

Within the framework of the two-scale scattering model,the Doppler shift of C-band radar return signals from the nonlinear sea surface are numerically evaluated.As an analytical approximation method,the Bragg resonance scattering method cannot accurately describe the backscattering field from sea surface.Therefore,in the twoscale scattering model,more accurate scattering coefficient(the normalized radar cross section,NRCS)evaluated by the C-band dual-polarized(HH/VV)empirical geophysical model function(CSAR model)is employed to replace the traditional Bragg NRCS to weight the Doppler shift.The numerical results indicate that there are obvious differences between the Doppler shift weighted by the CSAR NRCS and that weighted by the traditional Bragg NRCS.The hydrodynamic modulation of the large-scale waves is one of the important factors that affect the difference between the Doppler shift predicted in upwind and downwind directions.If the relaxation rate in the hydrodynamic modulation is set to be the angular frequency of the dominant water waves,the Doppler shift predicted by the numerical method can fit the results of the empirical model(C-band empirical geophysical model function,CDOP)well at moderate wind speed.Under low wind condition,the comparison shows that the empirical CDOP model appears to overestimate the Doppler shift.In order to facilitate the application,at the end of this paper a semi-empirical CSAR-DOP model,which is a polynomial fitting formula,is developed for evaluating the Doppler shift of C-band signals from time varying sea surface.

A NUMERICAL STUDY FOR MICROWAVE BACKSCATTERING CROSS-SECTION OF SEA SURFACE

Based on Tian et al. ＇s theoretical model of microwave scattering, by using the wind wave spectrum suggested by Fung et al. , numerical results of polarized back-scattering were calculated. The quantitative relationships between the calculated backscattering cross section and the environmental parameters, such as wind speeds, azimuthal angles and incidence angles, were studied. Compared with the traditional two-scale scattering model, the results are much more coincident with the observations under the condition of moderate incidence angles, which is very useful to make up for the two-scale model. To make the new calculation model to be used conveniently in the practice, its simplified form was proposed.