Wave-equation diffraction imaging using pseudo dip-angle gather

Diffraction images can directly indicate local heterogeneities such as faults, fracture zones, and erosional surfaces that are of high interest in seismic interpretation and unconventional reservoir development. We propose a new tool called pseudo dip-angle gather (PDAG) for imaging diffractors using the wave equation. PDAG has significantly lower computational cost compared with the classical dip-angle gather (DAG) due to using plane-wave gathers, a fast local Radon transform algorithm, and one-side decomposition assumption. Pseudo dip angle is measured from the vertical axis to the bisector of the plane-wave surface incident angle and scattered wave-propagation angle. PDAG is generated by choosing the zero lag of the correlation of the plane-wave source wavefields and the decomposed receiver wavefields. It reveals similar diffraction and reflection patterns to DAG, i.e. diffractions spreading as a flat event and reflections focused at a spectacular angle, while they may have dissimilar coverage for diffraction and different focused locations for reflection compared with that of DAG. A windowed median filter is then applied to each PDAG for extracting the diffraction energy and suppressing the focused reflection energy. Besides, the stacked PDAG can be used to evaluate the migration accuracy by measuring the flatness of the image gathers. Numerical tests on both synthetic and field data sets demonstrate that our method can efficiently produce accurate results for diffraction images.

Visualization of defects in thin metal plates by a scanning airborne ultrasound source technique using guided waves of different frequencies

A scanning airborne ultrasound source technique was developed to overcome the riskiness of laser ultrasound, which uses an ultrasound source that has a fixed sound wave focusing point and thus requires mechanical motion for sound source scanning. Therefore, the measurement time becomes longer. To solve this problem, we have proposed a method of simultaneously exciting many measurement points in the target using focused ultrasound sources of different frequencies. In this paper, we investigated the visualization of defects in a thin metal plate by the scanning elastic wave source technique using an airborne ultrasound source driven at two frequencies. When the testing was performed using two frequencies, either frequency visualized the defects.

Lensing Magnification Seen by Gravitational Wave Detectors

In this paper, we studied the gravitational lensing of gravitational wave events. The probability that an observed gravitational wave source has been (de-)amplified by a given amount is a detector-dependent quantity which depends on different ingredients: the lens distribution, the underlying distribution of sources and the detector sensitivity. The main objective of the present work was to introduce a semi-analytic approach to study the distribution of the magnification of a given source population observed with a given detector. The advantage of this approach is that each ingredient can be individually varied and tested. We computed the expected magnification as both a function of redshift and of the observedsource luminosity distance, which is the only quantity one can access via observation in the absence of an electromagnetic counterpart. As a case study, we then focus on the LIGO/Virgo network and on strong lensing (μ>1).

The role of the Maritime Continent SST anomalies in maintaining the Pacific–Japan pattern on decadal time scales

The decadal Pacific–Japan (PJ) pattern, the dominant decadal mode of summer vorticity anomaly over East Asia, is characterized as a meridionally arranged wave pattern with one anomalous cyclone located over Taiwan, and two anomalous anticyclones around the South China Sea (SCS) and the Bohai Sea. This pattern can cause wetter and colder conditions in Southeast China and dryer and warmer conditions in North China. Local SST–rainfall relationship reveals that the Maritime Continent (MC) SST can act as an engine to regulate and maintain the decadal PJ pattern. Driven by enhanced convection over the MC, anomalous divergent flows in the upper troposphere move northward, cross the equator and then converge and subside over the SCS. The SCS low-level divergence, maintained by this meridional overturning circulation under the Sverdrup vorticity balance, further works as a Rossby wave source and excites the decadal PJ pattern pointing straight northward. The transhemispheric impacts of the MC SST are well reproduced by both the atmospheric general circulation model and the dry linear baroclinic model, with the former emphasizing the MC’s original forcing role and the latter highlighting the SCS anticyclone’s role in relaying and amplifying those climatic impacts. Thus, our results indicate that SST variations over the MC region can be viewed as a potential source of East Asian decadal climate predictability.

A Series Solution for 2D Scattering of Cylindrical SH-Waves by Surrounding Loose Rock Zone of Underground Tunnel Lining

This paper presents a closed-form series solution of cylindrical SH-wave scattering by the surrounding loose rock zone of underground tunnel lining in a uniform half-space based on the wave function expansion method and the mirror image method. The correctness of the series solution is verified through residual convergence and comparison with the published results. The influence of the frequency of the incident cylindrical SH-wave, the distance between the wave source and the lining, the lining buried depth, and the properties of the surrounding loose rock zone on the dynamic stress concentration of the tunnel lining is investigated. The results show that the incident wave with high frequency always makes the dynamic stress concentration of the tunnel lining obvious. With the increase of the distance between the wave source and the tunnel lining, the stress around the tunnel lining decreases, but the dynamic stress concentration factor around the tunnel lining does not decrease significantly but occasionally increases. The ground surface has a great influence on the stress concentration of the tunnel lining. The amplitude of the stress concentration factor of tunnel lining is highly related to the shear wave velocity of the surrounding loose rock zone. When the property of the surrounding rock (shear wave velocity) changes more, the amplitude of the stress concentration factor is larger, that is, the stress concentration is more significant.

Three-dimensional Shear Wave Elastography Using a 2D Row Column Addressing (RCA) Array

Objective: To develop a 3D shear wave elastography (SWE) technique using a 2D row column addressing (RCA) array, with either external vibration or acoustic radiation force (ARF) as the shear wave source. Impact Statement: The proposed method paves the way for clinical translation of 3D-SWE based on the 2D RCA, providing a low-cost and high volume-rate solution that is compatible with existing clinical systems. Introduction: SWE is an established ultrasound imaging modality that provides a direct and quantitative assessment of tissue stiffness, which is significant for a wide range of clinical applications including cancer and liver fibrosis. SWE requires high frame-rate imaging for robust shear wave tracking. Due to the technical challenges associated with high volume-rate imaging in 3D, current SWE techniques are typically confined to 2D. Advancing SWE from 2D to 3D is significant because of the heterogeneous nature of tissue, which demands 3D imaging for accurate and comprehensive evaluation. Methods: A 3D SWE method using a 2D RCA array was developed with a volume-rate up to 2000 Hz. The performance of the proposed method was systematically evaluated on tissue-mimicking elasticity phantoms. Results: 3D shear wave motion induced by either external vibration or ARF was successfully detected with the proposed method. Robust 3D shear wave speed maps were reconstructed for both homogeneous and heterogeneous phantoms with inclusions. Conclusion: The high volume-rate 3D imaging provided by the 2D RCA array provides a robust and practical solution for 3D SWE with a clear pathway for future clinical translation.

Subwavelength probing of surface plasmons in magnetoplasmonic crystals

In this work, we report on near-field studying of propagating surface plasmons (SPs) in one-dimensional magnetoplasmonic crystals (MPCs) by aperture type scanning near-field optical microscopy (SNOM). Optical near-field around the aperture probe is used to drive SPs in the MPC locally. The SNOM signal represents the scattered intensity caused by the interaction of the SNOM probe near-field with the MPC. Scanning the MPC surface with polarization resolving of the scattered radiation shows decreasing of the intensity due to the SP excitation. The observed polarization dependence of the scattered SNOM signal is associated with the selective coupling of the near-field components of the SNOM probe with SPs. Finite-difference time-domain simulations reproduce the experimental SNOM signal. It is shown the excitation of SPs with symmetric (even parity) field distribution, which is forbidden for plane wave source at normal incidence.

Numerical Bichromatic Wave Generation Using Designed Mass Source Function

A mass source wave-maker method is generalized as the two-wave-source wave-maker method to generate bichromatic waves in the numerical model, whose governing equations are Navier–Stokes equations with the continuity equation. The Fluent software is taken as the calculation platform. In the numerical model, the waves at both the left and right ends of the numerical wave flume are absorbed with the momentum sources added in Navier–Stokes equations. The numerical simulation of bichromatic waves propagation with different frequencies in uniform deep, intermediate, and shallow water has been conducted. The numerical solutions are compared with the theoretical solutions obtained on the basis of Stokes waves theory. The frequency spectrum analyses of the results are conducted and discussed, and the differences between the weakly nonlinear theoretical solutions and the fully nonlinear numerical results are investigated in detail. It is found that the numerical model can effectively simulate the nonlinear effect of bichromatic waves in water with different depths, and the theoretical solutions only adapt the deep and intermediate water. The results indicate that the present numerical model is valuable in the aspect of practical application.

Disturbances of the Thermosphere and the Ionosphere during a Meteorological Storm

Determination of the physical mechanisms of energy transfer of tropospheric disturbances to the ionosphere is one of the fundamental problems of atmospheric physics. This article presents the results of observations carried out using two-wavelength lidar sensing at tropospheric altitudes and satellite GPS measurements during a meteorological storm in Kaliningrad (Russia, 54.7° N, 20.5° E) on 1 April 2016. During lidar sensing, it was found that the amplitudes of variations in atmospheric parameters with periods of acoustic (AWs) and internal gravity (IGWs) waves significantly increased. As a result of numerical modeling using the AtmoSym software package, it was shown that there is a noticeable increase in the period of temperature disturbances from 6–12 min to 10–17 min at altitudes from 150 km up to 230 km during the vertical propagation of acoustic waves and internal gravity waves from the troposphere. Nonlinear and dissipative processes in this layer lead to the formation of sources of secondary waves in the thermosphere with periods longer than those of primary ones. In this case, the unsteady nature of the wave source and the short duration of its operation does not lead to significant heating of the thermosphere. Simultaneous satellite observations demonstrate the response of the ionosphere (total electron content (TEC) disturbance) to tropospheric disturbances. Analysis of the time series of the amplitudes of the reflected lidar signal and TEC made it possible to determine that the response time of the ionosphere to tropospheric disturbances is 30–40 min.