Vertical Subarray Research Articles

Design of multi-channel terahertz beam splitter based on Z-shaped metasurface

A reflective beam splitter is proposed and verified. The unit cell of the beam splitter is composed of a metal pattern, a dielectric substrate, and a metallic ground. Each subarray structure of the device is composed of four unit cells, which are gradually rotated at 45°. The horizontal and vertical subarrays form a 4×4 gradient metasurface supercell. In the operating frequency band, the incident linearly polarized terahertz wave is reflected and divided into four beams of approximately equal power but different propagation directions. The proposed terahertz beam splitter based on metasurface has the advantages of small size, low cost and easy processing, and can be applied to terahertz stealth and imaging.

Detection of regional phases of seismic body waves using an array of three-component sensors

A small-aperture seismic array consisting of seven three-component seismometers carried out continuous measurements of regional seismicity in a selected area of the Nizhni Novgorod nuclear power plant during four months of 2013. Automatic signal detection using beamforming was applied separately for each motion component. Two horizontal components were transformed into radial and transverse components for the given values of the velocity and azimuth of the plane wave front. We have investigated the dependence of the coherence of microseismic noise on frequency, azimuth, and slowness, as well as determining the level of cross-correlation between signals on separate channels in order to estimate expected improvement in the signal-to-noise ratio, which is crucial for signal detection. Most signals detected by the seismic array from regional sources are associated with quarry blasts. Using repetitive explosions at seven quarries, we have quantitatively estimated and compared the increase in detection efficiency of regional seismic phases using a three-component small aperture seismic array and a subarray of vertical sensors. Horizontal sensors showed a higher efficiency in the detection of transverse waves, while the subarray of vertical sensors missed S-waves from certain events. For one of the nearby quarries, the vertical subarray missed up to 25% of events (5 of 20). The results of the investigation point to the need for the use of three-component seismic arrays for the study of regional seismicity.

Filtering Performance of Digital Alternate Array on Target Tracking

For target tracking of agile digital array, we propose a novel model called alternate array. The model means that total array is divided into vertical subarray at the present moment, and vertical subarray is transformed into horizontal subarray at the next moment, then the subarray comes to the first subarray divisions, and so on. Alternate array gives rise to differences of statistical characters of measurement error. Filtering performance is influenced by the feature of measurement data. In the paper, we come up with a new method based on Kalman filter (KF) to solve the difficulty. The new method merges pre-processing and data fusion thought together. Simulation results demonstrate the validity of the proposed algorithm.

Measurement and modeling of sound penetration into sandy sediment: Absolute pressure levels

As part of the SAX99 high-frequency sediment acoustics experiment, a buried array was used to measure the pressure field in a sandy sediment with sources in the water column. The sediment was a medium sand with pronounced directional ripples. The buried array consisted of five vertical subarrays of six elements each spanning a horizontal aperture of approximately 30 cm and a vertical aperture of approximately 45 cm. Sources operating over the frequency range 11–50 kHz were moved to obtain various propagation directions relative to the ripple field with ranges and elevations that provided incident grazing angles both larger and smaller than the 30-deg critical angle. Signal levels measured by the buried hydrophones were compared with calculations based on a fluid sediment model and small-roughness perturbation theory, with both directional ripple and small-scale isotropic roughness. Under the assumption that hydrophone sensitivity in-sediment is the same as the value measured in water, data-model comparisons show similar azimuthal dependence and agree in absolute level to within about 2 dB for above-critical incidence and about 3 dB for below-critical incidence. [Work supported by ONR.]

Effective vertical aperture for three‐dimensional arrays in shallow waters

In shallow waters, the effective vertical aperture of an array composed of horizontal and vertical segments may be greater than the water column depth. The extended vertical aperture leads to better decomposition of bottom‐interacting normal modes and hence better range‐depth localization compared with the performance of the vertical subarray alone. The conditions under which the effective vertical aperture gives improved localization performance over a vertical array spanning the water column are demonstrated. The effect of source frequency, ratio of horizontal to vertical array length, and bottom properties on localization performance are discussed.