Diffraction imaging using the two-way imaging condition

Diffraction imaging can increase the spatial resolution of seismic images beyond conventional means to provide interpreters with high-resolution structural and stratigraphic sections. The reflection image of a specular reflector only exists in either the positive- or negative-dip structure image; however, the image of a diffractor appears in the both positive- and negative-dip images. By applying a sample-by-sample multiplication imaging condition to the opposite dip images, the diffraction energy is retained while the reflection energy is significantly attenuated. The two dip images are generated by separating the source and receiver wavefields into different propagation directions. Two challenges are faced when we use the left-downgoing source and receiver wavefields to calculate the positive-dip structure image and the right-downgoing source and receiver wavefields to generate the negative-dip structure image. First, we need to compute the up- and downgoing wavefields by the Hilbert transform with respect to time and depth for the source and receiver wavefields. This operation requires two additional forward modeling calculations with the Hilbert-transformed source and receiver wavefields. Second, the information provided by the upgoing wave is missing if we only use the downgoing wave to generate two opposite dip structure images. To overcome the limitations of the one-way imaging condition, we use the two-way imaging condition of positive- and negative-dip structure images. Impulse response analysis demonstrates that the two-way imaging condition has the ability to image the positive- and negative-dip structure images separately. Without up and downgoing wavefield separation, the upgoing source and receiver wavefields can generate the same structure dip as using the downgoing source and receiver wavefields. Results indicate that using the two-way imaging condition can provide broader illumination and enhance the diffraction image because of the additional contribution from the upgoing wavefield. In addition, it saves almost half the computation cost compared to using the one-way imaging condition.

Change Detection and Flood Water Mapping Using Sentinel-1A Synthetic Aperture Radar Images

Prodigious flooding in the state of Uttar Pradesh, India during the month of August 2017 was induced by heavy rainfall, causing water levels in several rivers to cross the danger mark bringing normal life to a standstill. The peculiar rainfall pattern in India makes it highly vulnerable to floods. Demand for crisis information, for instance, natural disasters like severe flood events has increased. A simple but effective method is proposed in this study to find the areas that are affected due to floods, to detect the changes and for flood mapping. These indicators were derived from the Sentinel-1A Synthetic Aperture Radar (SAR) data by taking the crisis and archive images. An open flood surface can be detected easily in SAR data as it acts as a specular reflector that scatters the energy away from the sensor, causing relatively dark pixels of low backscattered SAR data. In contrast, the surrounding non-water areas usually exhibit a higher return due to surface roughness. Red, Green, Blue (RGB) composite is made for highlighting the flooded areas and for detecting changes by combining both archive and crisis images. Finally the flood map is compared with the optical imagery on the Google earth by integrating the resultant RGB composite image on the Google earth. Identification of the flood-prone areas is crucial to action the appropriate control measures in the flood-affected regions.

Optical Design of Multisource High-Flux Solar Simulators

We present a systematic approach to the design of a set of high-flux solar simulators (HFSSs) for solar thermal, thermochemical, and materials research. The generic simulator concept consists of an array of identical radiation modules arranged in concentric rows. Each module consists of a short-arc lamp coupled to a truncated ellipsoidal specular reflector. The positions of the radiation modules are obtained based on the rim angle, the number of concentric rows, the number of radiation modules in each row, the reflector radius, and a reflector spacing parameter. For a fixed array of radiation modules, the reflector shape is optimized with respect to the source-to-target radiation transfer efficiency. The resulting radiative flux distribution is analyzed on flat and hemispherical target surfaces using the Monte Carlo ray-tracing technique. An example design consists of 18 radiation modules arranged in two concentric rows. On a 60-mm dia. flat target area at the focal plane, the predicted radiative power and flux are 10.6 kW and 3.8 MW m−2, respectively, and the predicted peak flux is 9.5 MW m−2.