RU2626233C2 - Method of differentiating anomalies on water surface by multi-frequency microwave radar - Google Patents

Abstract

FIELD: radio engineering, communication.

SUBSTANCE: signals are emitted and received, scattered by the water surface in the microwave bands (LX-bands) on two polarizations (HH and VV) for two frequencies separated by not less than one and a half times at a certain observation angle. Herewith the observation angle is chosen in the range of 50-80° from the vertical. As the characteristics of the received signals, the difference of the measured specific effective scattering areas of the received signals on two polarizations for each of the two frequencies is used. The obtained values of the differences determine the experimental values of the wave intensities at the Bragg wave numbers. At the same time, the average wind speed is measured. For the measured average wind speed, theoretical background wave intensities at Bragg wave numbers are calculated using the model spectrum. Spectral contrasts of waves on the water surface are obtained as the ratio of the theoretical background values of the wave intensities at the Bragg wave numbers to the experimental values of the wave intensities at the Bragg wave numbers for both frequencies for the measured average wind speed. A decision is taken on the presence of an anomaly on the water surface on the basis of the magnitude comparison of the spectral contrast of waves on the water surface at the maximum of the Bragg wave numbers with a certain threshold value. The ratio of the obtained spectral contrasts is calculated. It is concluded that there is a film slip or a calm zone on the water surface based on the position of the contrast ratio value found for the measured value of the average wind speed relative to the semiempirical curve of the contrast ratio relationship to the wind speed at a certain observation angle: if the value is above the curve, the film slip is observed, if it is below - the calm zone is observed.

EFFECT: increasing the accuracy of anomalies on the water surface.

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Description

The invention relates to radar, and in particular to radar methods for studying the water surface in order to identify and determine the nature of the anomalies present on it (areas with reduced wave intensity).

The method allows to distinguish the nature of the anomaly on the water surface, namely to distinguish the region of local (short-term) decrease in wind speed (calm zone) from the region covered by a film of surface-active substance (film slick), according to observations by a multi-frequency radar system, including the emission of signals in the microwave range (LX-ranges) on two LV polarizations (emission and reception on horizontal polarization) and VV (emission and reception on vertical polarization) for two spaced no less than one and a half times an hour by. The method is applicable for observing the water surface in moderate winds (wind speeds up to 7 m / s) at not too small viewing angles (50-80 ° from the vertical) from pile bases or from ships.

Existing methods for locating anomalies on the water surface make it possible to determine the coordinates of the anomaly region, such as a calm zone or film slick, but do not allow to reliably determine the nature of the anomaly. In this case, it is usually assumed by default that the area of decrease in the specific effective scattering areas of the received signals corresponds to a spill of a surfactant, such as oil or oil products, and the probability of false alarm is not evaluated.

The closest in technical essence to the proposed method is a prototype method for detecting oil pollution of the sea surface using a coherent radar (Method of detecting oil spill at the sea by means of an oil spill radar, and such an oil spill radar: GB patent 2511254: IPC G01S 13/34; G01S 13/88; G01S 7/02; G01S 7/41 / Norland Richard; ISPAS AS [NO]. - GB 20140010224; 12.19.2012), including the emission and reception of microwave signals with a frequency from 2 to 18 GHz (S-Ku bands) in two polarization modes (LV and VV) at viewing angles of 60-89.5 ° from the vertical. Fresnel coefficients proportional to the specific effective scattering areas are used as characteristics of the received signals. Find the ratio of the received signals for HH and VV polarizations. The absolute value of the signal-ratio is compared with the theoretical background value responsible for scattering from a clean water surface, in the case of a significant deviation from this value, it is believed that an anomaly is observed at a given point, namely a film slick (or rather, a spot of a surfactant, namely oil). The magnitude of the phase jump of the signal-ratio determines the thickness of the film.

The disadvantage of the prototype method is the lack of accuracy in determining areas of the water surface covered with films of surfactants. The reason for this is that for a clean water surface the absolute value of the signal-ratio will be close to the theoretical background value (the ratio of Fresnel coefficients according to the two-scale scattering model) only if we ignore the contribution to the specific effective scattering area of the non-polarized component. In the prototype method, the polarized component, which is not excluded from the calculations, can make a significant contribution to the value of the specific effective scattering area even for a clean water surface, which will lead to a false conclusion about the presence of an anomaly - film slick. An additional source of error is that a significant change in the signal-ratio at different polarizations can be caused not by a film of a surfactant, but by the presence of a calm region on the water surface. It is known that the appearance of both a calm zone and a film slick leads to a decrease in the specific effective scattering areas of backscattering signals in comparison with the theoretical background value.

The objective of the invention is to create a method that allows to increase the accuracy of detection of films of surface-active substances (film slicks) on the water surface by radar methods by eliminating cases of local decrease in wind speed (calm zones), similarly manifested during radar sensing.

The technical effect is achieved by emitting and receiving signals scattered by the water surface in the microwave range at two polarizations of LV and VV, comparing the characteristics of the received signals for both polarizations with their theoretical background values and, based on the comparison, make a conclusion about the presence or absence of an anomaly.

What is new is that the signals scattered by the water surface in the microwave range (LX ranges) are emitted and received at two polarizations for two frequencies spaced no less than one and a half times at a certain viewing angle, the viewing angle is chosen in the range of 50-80 ° from the vertical , as the characteristics of the received signals, use the difference of the measured specific effective areas of scattering of the received signals at two polarizations for each of the two frequencies, the experimental values of the inte For the measured average wind speed, the theoretical background values of the wave intensities on the Bragg wave numbers are calculated using the model spectrum, and the spectral contrasts of the waves on the water surface are found as the ratios of the theoretical background values of the wave intensities on the Bragg wave numbers numbers to the experimental values of the wave intensities at the Bragg wave numbers for both frequency for the measured average wind speed, decide on the presence of anomalies on the water surface by comparing the magnitude of the spectral contrast of the waves on the water surface at the maximum of the Bragg wave numbers with a certain threshold value, calculate the ratio of the obtained spectral contrasts, and then conclude that the water surface of a film slick or a calm zone based on the position of the value of the found contrast ratio for the measured value of the average wind speed relative to the semi-empirical curve of the relationship of contrasts to wind speed at a certain viewing angle: if the value is higher than the curve, film slick is observed, if lower, the calm zone.

The method is illustrated by the following drawings.

In FIG. Figure 1 shows the frequency dependence of spectral contrast in film slicks (solid curves, the numbers on the curves show the elasticity of the surfactant film) and in calm zones (dashed lines, the numbers on the curves show wind damping as a percentage of the average wind speed).

In FIG. Figure 2 shows an example of a semi-empirical curve of the relationship between contrasts and wind speed for the use of C- and S-sensing ranges at an observation angle of 60 °. The curve indicates the separation of the case of a calm zone when the wind speed drops by 40% and the case of film slick when the elasticity of the surfactant film is more than 3 mN / m.

The method is illustrated as follows. The intensity of small-scale wind waves on the water surface strongly depends on the wind speed and the presence of films of surface-active substances. Hereinafter, we call the calm zone the region of short-term local wind speed reduction of not more than 40%, film slick - the region of the water surface covered with a film of a surfactant with not too low elasticity, namely more than 3 mN / m. With a local and short-term decrease in wind speed, as well as in the presence of films of surface-active substances, the wind wave of the dm-cm range is weakened, forming anomalies - regions with a reduced wave intensity. The degree of attenuation of surface waves with a given wavelength is characterized by spectral contrast (the ratio of the theoretical background value of the wave intensity to the experimental value of the wave intensity in the anomaly region at a given wave number). The nature of the dependence of spectral contrast on the surface wavelength varies significantly for film slicks and calm zones, a typical example is shown in FIG. 1 (Sergievskaya, I. On discrimination between film slicks and "look-alikes" on the sea surface in multifrequency radar images / I. Sergievskaya; SA Ermakov, I. Kapustin // Remote Sensing of the Ocean, Sea Ice, Coastal Waters, and Large Water Regions. Proc. SPIE 9638, 2015. Doi: 10.1117 / 12.2195031; http://dx.doi.org/10.1117/12.2195031).

In this regard, the simultaneous measurement of the characteristics of surface waves of different lengths using multi-frequency radar makes it possible to distinguish a calm zone from film slick. For a film slick, in contrast to the calm zone, significant (several times) changes in spectral contrast occur on the scales of changes in the surface wavelength by 1.5–2 times. Therefore, for measurements, it is preferable that the frequencies of the probing radar radiation are spaced at least one and a half times, which will provide the necessary (easily detectable) difference in contrasts at the analyzed surface wavelengths.

в штилевых зонах с заданным уменьшением скорости ветра ΔV не может превышать некоторую критическую величину A, зависящую от скорости ветра V и волновых чисел k1<k2 волн на водной поверхности:Analysis (Sergievskaya, I. On discrimination between film slicks and "look-alikes" on the sea surface in multifrequency radar images / I. Sergievskaya; SA Ermakov, I. Kapustin // Remote Sensing of the Ocean, Sea Ice, Coastal Waters , and Large Water Regions. Proc. SPIE 9638, 2015. Doi: 10.1117 / 12.2195031; http://dx.doi.org/10.1117/12.2195031) shows that the ratio of contrasts on two different wave numbers of surface waves

in calm zones with a given decrease in wind speed ΔV cannot exceed a certain critical value A, depending on the wind speed V and wave numbers k1 <k2 of waves on the water surface:

. Условие справедливо, если

.where α = 4.62⋅10 ^-5 ± 2⋅10 ^-5 is the empirical coefficient, ν is the viscosity of water,

. The condition is true if

.

Thus, if the ratio of contrasts for two radiation frequencies for the measured value of the average wind speed at a certain viewing angle exceeds a critical value A, we can conclude that there is a film slick, i.e. surfactant films on the surface; otherwise, it can be considered that a calm zone is observed.

The method is as follows. The signals scattered by the water surface radiate and receive signals in the microwave range (LX ranges) at a certain viewing angle at two polarizations VV (radiation and reception on vertical polarization) and LV (radiation and reception on horizontal polarization) for two spaced no less than one and a half times radiation frequencies.

, (где

- частота излучения СВЧ-радиолокатора, а θ - угол наблюдения), которая пропорциональна разности удельных эффективных площадей рассеяния принятых сигналов на поляризациях НН и VV, для каждой частоты (см., например, Kudryavtsev, V. Quad-polarized SAR measurements of ocean currents in C-and L-bands / V. Kudryavtsev, I. Kozlov, B. Chapron, J.A. Johannessen // Proc. Geosci. Remote Sens. Symp (IGARSS), IEEE International, 2015. P. 4212-4215):Find the intensity of the waves on the Bragg wave number

, (where

is the microwave radiation frequency, and θ is the observation angle), which is proportional to the difference in the specific effective scattering areas of the received signals at the HH and VV polarizations, for each frequency (see, for example, Kudryavtsev, V. Quadratic polarized SAR measurements of ocean currents in C-and L-bands / V. Kudryavtsev, I. Kozlov, B. Chapron, JA Johannessen // Proc. Geosci. Remote Sens. Symp (IGARSS), IEEE International, 2015. P. 4212-4215):

where the index i = 1, 2 corresponds to one of the working frequencies.

By finding the difference in the measured specific effective scattering areas of the received signals at two polarizations for each of the two frequencies, an unpolarized scattering component is excluded from further calculations.

At the same time, the average wind speed is measured, the value of which is used to calculate the theoretical background value of the wave intensity at the Bragg wave number F _background (k _braggi ) for each of the frequencies using the model spectrum

disturbances (Elfouhaily, TB A unified directional spectrum for long and short wind-driven waves / TB Elfouhaily, B. Chapron, KB Katsaros, DJ Vandemark // J. Geophys. Res. 1997. V.107. P. 15781-15796) . Then, contrast is calculated for each of the frequencies.

,

,

where F (k _braggi ) is the experimentally obtained value of the intensity of the waves on the Bragg wave number at each moment in time. The decision on the presence of anomalies is made when the contrast exceeds the maximum of the Bragg wave numbers of a certain threshold value, the value of which lies in the range 1.15-1.25 and is specified directly in the measurement process.

в области аномалии, определяют его положение относительно полуэмпирической кривой зависимости отношения контрастов от скорости ветра при определенном угле наблюдения. Кривую рассчитывают на основе проверенной в натурных условиях (Ермаков, Влияние пленок на динамику гравитационно-капиллярных волн / С.А. Ермаков. - Н. Новгород: ИПФ РАН, 2010. - 165 с.) модели локального баланса. Пример такой кривой для случая использования С- и S-диапазонов зондирования при угле наблюдения 60° приведен на фиг. 2. Кривая обозначает разделение случая штилевой зоны при падении скорости ветра на 40% и случая пленочного слика при упругости пленки поверхностно-активного вещества более 3 мН/м.Then find the ratio of spectral contrasts

in the region of the anomaly, determine its position relative to the semi-empirical curve of the relationship of contrasts to wind speed at a certain viewing angle. The curve is calculated on the basis of a model tested in natural conditions (Ermakov, Influence of films on the dynamics of gravitational-capillary waves / S. A. Ermakov. - N. Novgorod: IAP RAS, 2010. - 165 p.) Local balance model. An example of such a curve for the case of using C- and S-sensing ranges at a viewing angle of 60 ° is shown in FIG. 2. The curve indicates the separation of the case of a calm zone when the wind speed drops by 40% and the case of film slick with a film elasticity of the surfactant exceeding 3 mN / m.

If the value of the contrast ratio lies above the curve, then we conclude that a film slick is observed on the water surface (i.e. the water surface in the anomaly region is covered with a film of a surfactant), if lower, then a local calm zone.

The above reasoning can be extended to the case of an increase in the number of microwave sounding frequencies within the L-X ranges, which will improve the reliability of distinguishing between film slicks and calm zones.

Claims (1)

A method for distinguishing anomalies on the water surface by means of multi-frequency microwave radar, in which signals in the microwave range are emitted and received by the water surface scattered by two HH and VV polarizations, the characteristics of the received signals for both polarizations are compared with their theoretical background values and, based on the comparison, they conclude the presence or absence of anomalies, characterized in that they emit and receive signals scattered by the water surface in the microwave range (LX ranges) at two polarizations for two separated no less than one and a half times the frequencies at a certain viewing angle, the viewing angle is selected in the range of 50-80 ° from the vertical, the characteristics of the received signals use the difference of the measured specific effective scattering areas of the received signals at two polarizations for each of the two frequencies, according to the received the values of the differences determine the experimental values of the intensities of the waves at the Bragg wave numbers, at the same time measure the average wind speed, for the measured average wind speed calculate t The theoretical background values of the wave intensities on the Bragg wave numbers using the model spectrum find the spectral contrasts of the waves on the water surface as the ratio of the theoretical background values of the wave intensities on the Bragg wave numbers to the experimental values of the wave intensities on the Bragg wave numbers for both frequencies for the measured average wind speed , decide on the presence of anomalies on the water surface based on a comparison of the spectral of the total contrast of waves on the water surface at the maximum of the Bragg wave numbers with a certain threshold value, the ratio of the obtained spectral contrasts is calculated, and then a conclusion is made on the presence of a film slick or a calm zone on the water surface based on the position of the value of the found contrast ratio for the measured average speed wind relative to the semi-empirical curve of the relationship of contrasts to wind speed at a certain viewing angle: if the value is higher than the curve - film slick is observed, if below - a calm zone.

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