Submarine groundwater discharge (SGD) plays an important role in coastal biogeochemical processes and hydrological cycles, particularly off volcanic islands in oligotrophic oceans. However, the spatial and temporal variations of SGD are still poorly understood owing to difficulty in taking rapid SGD measurements over a large scale. In this study, we used four airborne thermal infrared surveys (twice each during high and low tides) to quantify the spatiotemporal variations of SGD over the entire coast of Jeju Island, Korea. On the basis of an analytical model, we found a linear positive correlation between the thermal anomaly and squares of the groundwater discharge velocity and a negative exponential correlation between the anomaly and water depth (including tide height and bathymetry). We then derived a new equation for quantitatively estimating the SGD flow rates from thermal anomalies acquired at two different tide heights. The proposed method was validated with the measured SGD flow rates using a current meter at Gongcheonpo Beach. We believe that the method can be effectively applied for rapid estimation of SGD over coastal areas, where fresh groundwater discharge is significant, using airborne thermal infrared surveys.
Abstract Oceanic internal waves are known to be important to the understanding of underwater acoustics, marine biogeochemistry, submarine navigation and engineering, and the Earth’s climate. In spite of the importance and increased knowledge of their ubiquity, the wave generation is still poorly understood in most parts of the world’s oceans. Here, we use satellite synthetic aperture radar images, in-situ observations, and numerical models to (1) show that wave energy (having relatively high amplitude) radiates from a shallow sill in the East China Sea in all directions, but with a significant time lag dependent on background conditions, (2) reveal that wave fronts are locally formed with often favorable conditions for re-initiation, and (3) demonstrate the resulting variety of wave patterns. These findings would be the case for any broad shelf having shallow sills with time-varying conditions, and therefore have significant implications on the redistribution of energy and materials in the global as well as regional ocean.
Space-borne Synthetic Aperture Radar (SAR) has been one of the most effective tools for physical oceanographic and air-sea interface research because the direct observation of ocean surface currents response to tropical cyclones such as typhoon and hurricane is limited under various extreme conditions. Relative motions between a sensor and Earth's target causes Doppler shift of radar signal which can be recorded in SAR signal data, and we employ the effect of Doppler shift observed in the raw SAR data to estimate ocean surface velocity. The ocean surface velocity in the Line-of-Sight (LOS) direction can be derived by comparing the anomaly between the prediction of the Doppler centroid using geometry model and the estimation of the Doppler centroid using the Correlation Doppler Estimator (CDE). Moreover, the net Bragg-wave phase velocity, a component of the surface currents, is subtracted from the ocean surface velocity in order to derive the actual ocean surface current velocity. Until recently, studies to characterize and quantify oceanic response to tropical cyclones have been rarely reported because of its difficulty of observation under these extreme conditions. The object of this study is to extract more accurate ocean surface current velocity response to tropical cyclones using RADARSAT-1 ScanSAR raw data.
Doppler velocity can be derived by calculating Doppler shift anomalies between predicted and estimated Doppler centroids. The predicted Doppler centroid is calculated based on a geometric model of satellite assuming that the target is not moving. The estimated Doppler centroid can be directly extracted from the raw SAR signal data by applying the average cross-correlation coefficient method. It is known that wind-generated ocean waves can significantly contribute to Doppler velocity due to the correlation between orbital motions of the waves and (tilt and hydrodynamic) modulated radar cross sections, in addition to what sea surface current contributes. In this study, the characteristics of Doppler velocities under hurricane conditions were investigated using RADARSAT-1 ScanSAR raw data. Five different hurricanes (Hurricane Dean, Hurricane Ivan, Hurricane Kyle, Hurricane Lili, and Typhoon Xangsane) and sequential acquisitions of two hurricanes (Hurricane Kyle and Hurricane Lili) were selected to study the contribution of wind-induced waves to Doppler velocities and compared with in situ measurements of drifting buoys. The results show that hurricane-generated seas and associated winds and waves appear to be different from ordinary sea state. This leads to lower estimates of Doppler velocities than expected and much closer to sea surface current velocities.
Submarine groundwater discharge (SGD) is a widely recognized process that carries considerable amounts of groundwater and dissolved chemicals to the coastal ocean. Despite its importance, a lack of suitable tools to assess SGD's spatial and temporal variability has hampered a complete understanding of the process. Here we report, for the first time, use of an unmanned aerial vehicle (UAV or "drone") to assess SGD variations. An octocopter UAV platform equipped with a thermal infrared (TIR) system was flown along a coastline on Jeju Island, Korea. The UAV clearly captured thermal signatures of SGD plumes and their dynamic temporal fluctuations modulated by tidal variations. Based on a plume area-SGD flux relation we developed by combining aerial and field data, we estimated that the SGD flux of the study site ranged from 33,000 to 54,000 m3 d−1. The drone approach enabled acquisition of time series plume imagery with easy control of spatial resolution, flexible field operations, and remote sensing of SGD at low cost compared to conventional aerial surveys. Combining the UAV-TIR images with on-site sampling enables one to determine fluxes of nutrients and other dissolved species. UAV-TIR mapping can thus serve as a powerful tool for study of SGD and other coastal processes.
Space-borne SAR (Synthetic Aperture Radar) has been one of the most effective tools for physical oceanographic and air-sea interface research. We applied the effect of Doppler shift observed in the raw SAR data to estimate ocean surface velocity. Relative motion between sensor and Earth's target causes Doppler shift of radar signal which can be recorded in SAR signal data. The relative motion can be derived by comparing the difference between target Doppler centroid and reference Doppler centroid. The reference Doppler centroid is a Doppler centroid that corresponds to the stationary area or a fixed target. The reference Doppler centroid can be calculated with the geometry model on the rotating Earth. On the other hand, the target Doppler centroid was estimated by applying the correlation Doppler estimator to the RADARSAT-1 ScanSAR raw data (also known as the Average Cross Correlation Coefficient; ACCC). Target Doppler centroid is caused by the movement of upper ocean waterbody. By comparing of these two Doppler centroids, line-of-sight velocities of ocean surface can be retrieved. Until recently, studies to characterize and quantify oceanic response to tropical cyclones such as typhoon and hurricane have rarely reported because of its difficulty of observation under these extreme conditions. The object of this study is to extract more accurate ocean surface velocity response to tropical cyclones from RADARSAT-1 ScanSAR raw data. In the future, we will be able to investigate oceanic response to tropical cyclones from the estimated ocean surface velocities.
인공위성 Synthetic Aperture Radar(SAR)는 물리해양학적 현상을 정량적으로 관측하는데 가장 유용한 도구 중의 하나이다. SAR의 도플러 편이(Doppler shift) 현상은 센서와 해양표면 유체와의 상대적인 움직임 차이로 인해 발생될 수 있다. 따라서, 단 채널 SAR 원시자료에 기록된 도플러 정보는 해양의 유체 이동속도를 추정하는데 유용하다. 유체의 이동속도는 측정된 도플러 중심주파수(estimated Doppler centroid)와 예측된 도플러 중심주파수(predicted Doppler centroid) 사이의 차이를 측정함으로써 계산될 수 있다. 예측된 도플러 중심주파수는 표적이 움직이지 않는다고 가정했을 때의 중심주파수로서 위성의 궤도, 시선 각, 자세 등과 같은 기하모델을 통해 계산될 수 있고, 측정된 도플러 중심주파수는 실제 SAR 촬영시 표적의 움직임에 해당하는 도플러 중심주파수로서 원시자료에 기록된 정보를 이용하고 평균상관계수법(Average Cross Correlation Coefficient; ACCC)을 적용하여 추출될 수 있다. 이렇게 추출된 도플러 속도에서 브래그 공명을 일으키는 표면 장력파의 위상속도를 제거하여 좀더 정밀한 표층 해류의 속도를 추출하였다. 이러한 기법들을 동해를 촬영한 Envisat ASAR 원시자료에 적용하였으며, 추출된 해류속도를 HF-radar에서 관측한 해류속도와 비교하였다. Space-borne Synthetic Aperture Radar(SAR) has been one of the most effective tools for monitoring quantitative oceanographic physical parameters. The Doppler information recorded in single-channel SAR raw data can be useful in estimating moving velocity of water mass in ocean. The Doppler shift is caused by the relative motion between SAR sensor and the water mass of ocean surface. Thus, the moving velocity can be extracted by measuring the Doppler anomaly between extracted Doppler centroid and predicted Doppler centroid. The predicted Doppler centroid, defined as the Doppler centroid assuming that the target is not moving, is calculated based on the geometric parameters of a satellite, such as the satellite's orbit, look angle, and attitude with regard to the rotating Earth. While the estimated Doppler shift, corresponding to the actual Doppler centroid in the situation of real SAR data acquisition, can be extracted directly from raw SAR signal data, which usually calculated by applying the Average Cross Correlation Coefficient(ACCC). The moving velocity was further refined to obtain ocean surface current by subtracting the phase velocity of Bragg-resonant capillary waves. These methods were applied to Envisat ASAR raw data acquired in the East Sea, and the extracted ocean surface currents were compared with the current measured by HF-radar.
Sea surface current plays an important role in understanding physical oceanography and can provide critical information for preventing natural disasters such as oil spill and ship distress. In this study, two techniques using space-borne Synthetic Aperture Radar (SAR) systems were processed to evaluate their feasibility to extract sea surface current velocities. The first technique employs the Doppler shift algorithm from a single SAR system which is generated when moving targets are imaged in East Sea of Korean Peninsula. The second technique utilizes Along-Track Interferometry (ATI) from dual SAR system to estimate sea surface current velocity in Strait of Korea. Envisat ASAR and TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurement) data were used in each technique, respectively.
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