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Internal structure imaging of the Earth, along with determining basin structure, can aid in evaluating potential seismic hazards. However, the high operating cost limits the current geophysical exploration methods; moreover, it is difficult to apply these techniques over a large area, which limits our understanding of the Quaternary structure and the development of earthquake prevention science. A combination of dense array observation technology and ambient noise surface wave tomography is being rapidly developed as a high-resolution urban detection method. Here, we report the ambient noise imaging results of a high-density array experiment. In the ambient noise surface wave tomography method (e.g., surface wave tomography; Eikonal tomography), the signal is assumed to be a single mode. However, several multimode signals were detected in this dataset. With the use of traditional methods to measure the dispersion, mode confusion occurs and the extracted dispersion curve jumps. To solve this problem, by combining the advantages of phase-matched filtering and dispersion compensation, we realized the automatic pickup of fundamental group velocity using reference phase velocity. From this, a Rayleigh wave group velocity map was obtained. The regional average phase velocity information was included in the inversion steps to reduce the uncertainty in the inversion of shear wave velocity. Finally, an S-wave velocity structure model was obtained within a depth of 500 m. The velocity structure was roughly layered and grew with depth. In the depth range of 240–320 m, obvious decreases in the S-wave velocity were observed. Compared with geothermal drilling data, this was speculated to be the reflection of a water-rich (confined water) sand layer. This study provides a technical approach for and a processing example of a high-density array, and its velocity model can be used as a reference for urban subsurface structure, underground space utilization, and earthquake disaster prevention and control. [ABSTRACT FROM AUTHOR]

We conducted a spatial coherency analysis of ground motion using earthquake recordings from a hard rock outcrop dense array situated at a nuclear power plant site in Busan, located in the south-eastern coast of Korea. Utilizing data from a total of 16 events occurring from July 2021 to June 2022, we computed the plane-wave, lagged, and unlagged coherency functions of both horizontal and vertical components. We also provided comprehensive comparisons with other empirical functions developed for rock sites. Notably, all reported rock site curves exhibit clear distinctions, emphasizing the site-specific nature of these curves. The observed coherency tends to be larger for harder sites, that happen to have higher Vs30 as well, at separation distances less than 50 m. The Busan array, being the hardest among the available rock site arrays in existing literature, demonstrated the highest coherency at short distances and higher frequencies (e.g. above 25 Hz). This observation could be attributed to the presence of hard rock layers that have relatively higher spatial homogeneity at the Busan array site than the others. However, at larger interstation distances (e.g., above 50 m), relatively lower coherency is observed at Busan array. This could be attributed to the particular shape of the array as the farther away stations in a pair are located on more heterogeneous grounds, and experience direction-dependent, phase-shifted seismic waves. [ABSTRACT FROM AUTHOR]

The feasibility of using a dense seismic array with an airgun source to study the quality factors of shallow media is verified. Data were obtained from 37 stations in the dense seismic array located in Binchuan, Yunnan Province, China, and the amplitude–distance attenuation method and the coda normalization method were applied to calculate the S-wave quality factors in the area. The amplitude–distance attenuation method yielded Q s - 1 =0.0260 ± 0.0103, and the frequency-dependent Q s - 1 calculated by the coda normalization method can be expressed by the power law Q s - 1 (f) ≅ 0.0554 f - 0.5643 . The consistency between the results of these two methods shows that a dense seismic array with an airgun source can be used to study the attenuation characteristics of shallow media. The amplitudes at some points deviate substantially from the fitted curve and thus have a certain influence on the fitting results; hence, we must select high-precision data for the calculation. Given the topography, we speculate that the anomalous stations located on the edge of the Binchuan Basin and in the western hilly area are due to the edge effect of the basin and the weak attenuation of the hilly area and that the anomalous station located in the northern Binchuan depocenter is attributable to local site factors. Compared with the Q s - 1 estimated by previous studies, the Q s - 1 in the Binchuan area is found to lie between those of the hard soil and sedimentary rock and is similar to the Q s - 1 in the North China Basin, corresponding to the shallow velocity structure in this area. [ABSTRACT FROM AUTHOR]

The stochastic estimation of coherency is perceived, in some studies, to be significantly influenced by the time window length under consideration. For the engineering purposes, usually, coherency is estimated from the strong motion, i.e., the shear (S-) wave segment of the recorded time series. However, the identification of a purely S-wave dominated window could be challenging in case of complex wave mixing on the seismograms. Moreover, there has been relatively little research on how much variation, in reality, is introduced in the coherency estimates owing to the choice of signal length. Therefore, the debate about the procedure for selecting a representative time window has remained inconclusive: different authors keep their different practices. The current article is an effort to shed light on the sensitivity of the coherency estimates to the choice of various time windows. The research draws on the dataset gathered from a dense seismic-array deployed at the small size, shallow alluvial valley of Koutavos-Argostoli, situated in Cephalonia Island, Greece. The lagged coherency over interstation distance ranges from 5 to 80 m has been estimated from a set of 46 earthquakes having magnitude 2-5 and epicentral distance 3-200 km, considering different lengths of time windows. The findings revealed that the statistics of coherency estimates derived from many events is only weakly sensitive to the selection of time window lengths provided that the segments include the same energetic pulse. [ABSTRACT FROM AUTHOR]

The term “spatial variability of seismic ground motions” denotes the differences in the amplitude and phase content of seismic motions. The effect of such spatial variability on the structural response is still an open issue. In-situ experiments may be helpful in order to answer the questions regarding both the quantification of the spatial variability of the ground motion within the dimensions of a structure as well as the effect on its dynamic response. The goal of the present study is to quantify the variability of the seismic ground motion accelerations in the shallow sedimentary basin of Argostoli, Greece, and thereafter to identify its effect on the linear and non-linear elasto-plastic response of a single degree of freedom system in terms of spectral displacements. Around 400 earthquakes are used, recorded by the 21-element very dense seismological array deployed in Argostoli with inter-station spacing ranging from 5 to 160 meters. The seismic motion variability, evaluated in terms of spectral accelerations, is found to be significant and to increase with inter-station distance and frequency. Thereafter, the amplitude variability in terms of spectral displacements, which is indeed the linear response of a single degree of freedom (SDOF) system with various fundamental periods, is compared with the amplitude variability of a SDOF with non-linear elasto-plastic response. The variability of the maximum top displacement of the linear single degree of freedom system is estimated to be on average 12% with larger variabilities to be observed within two narrow frequency ranges (between 1.5 and 1.7 Hz and between 3 and 4 Hz). Such high variabilities are caused by locally edge-generated diffracted surface waves. The non-linear perfectly elasto-platic structural response of the SDOF system shows that although the variability has the same trends as in the case of linear response, it is almost constantly increased by 5%. [ABSTRACT FROM AUTHOR]

The spatial variation of the earthquake ground motion over short distances can significantly affect the dynamic response of large and extended engineered structures, especially on sites with inhomogeneity in surface geology and geometry. In current practices, such variation is taken into account in terms of coherency, a function of frequency and distance, established on an essentially empirical basis and difficult to extrapolate at different sites. Hence, a better understanding of its physical significance and its relationship with the underlying ground structure is indispensable. A two-dimensional dense array, deployed at the small and shallow Koutavos-Argostoli valley in Cephalonia, Greece, provided an abundance of data to study the stochastic characteristics of seismic ground motions over very short distances. A set of 46 magnitude 2-5 events at epicentral distances 0-200 km has been selected for the analysis. The lagged coherency of the S-wave dominating seismogram was computed for each station-pair within the array and was averaged over various distance intervals for the whole data set. The results indicate a lack of a clear dependence of the average coherency on the magnitude, back azimuth or site-to-source distance of the event. The most striking result concerns the influence of the site geometry: the coherency is systematically lower for the pairs aligned perpendicular to the axis of the valley (2D) compared to those aligned in the parallel direction. This finding is consistent with the dominance of valley-edge generated surface waves propagating from one edge to the other. The averaged coherency estimates are only weakly represented by the existing parametric models, indicating its strong site dependent nature. [ABSTRACT FROM AUTHOR]

We report on Förster resonance energy transfer in the dense arrays of epitaxial quantum dots (QDs), formed by fractional monolayer CdSe insertions within a ZnSe matrix. In such arrays comprising the QDs of different sizes, the energy transfer can take place between the ground levels of small QDs and the excited levels of large radiating QDs, when these states are in resonance. This mechanism provides directional excitation of a limited number of the large QDs possessing the excited levels. It reveals itself by the shrinkage of photoluminescence (PL) bands and the appearance of the narrow single excitonic lines in micro-PL spectra. The strong shortening of characteristic PL decay times in the energy-donating QDs is observed when the distance between them and the energy-accepting QDs decreases. Photoluminescence excitation spectroscopy demonstrates the switching of the dominant energy transfer mechanism at the energy predicted by theoretical modeling of the excitonic levels in the QD arrays. Our results pave the way for engineering of the architecture of excitonic levels in the QD arrays to realize efficient nano-emitters. [ABSTRACT FROM AUTHOR]

This paper presents results from three-dimensional quantitative modeling on dense, moderately doped [ N ( N) = 5 × 10 cm] short-wave infrared (SWIR) p n and n p HgCdTe double planar heterostructure photodetecting arrays with absorber x = 0.451 and cap x = 0.55. At uniform reverse bias, the competition for minority carriers between closely spaced diodes preserves densities below equilibrium levels throughout the absorber. This carrier suppression has several consequences in addition to suppressing dark current by constraining the minority-carrier gradients at each diode junction. First, the dense arrays maintain volume-average negative net radiative recombination rates (negative luminescence) roughly an order of magnitude larger than comparably biased isolated diodes. Second, the negative excess minority-carrier densities suppress the volume-average net Auger recombination rate by roughly an order of magnitude in dense n-type HgCdTe arrays compared with a single diode. Third, the long minority electron diffusion lengths in the p-type HgCdTe absorber not only suppress lateral diffusion currents, but do so in a manner that provides negative differential resistance. By suppressing intrinsic recombination rates, or lateral diffusion currents, each effect can contribute to increasing R A products in SWIR HgCdTe dense arrays. These effects should be considered when optimizing device structures for pitch, thickness, feature size, doping, and bias points. [ABSTRACT FROM AUTHOR]

This paper is concerned with spatial properties of linear arrays of antennas spaced less than half wavelength. Possible applications are in multiple-input multiple-output (MIMO) wireless links for the purpose of increasing the spatial multiplexing gain in a scattering environment, as well as in other areas such as sonar and radar. With reference to a receiving array, we show that knowledge of the received field can be extrapolated beyond the actual array size by exploiting the finiteness of the interval of real directions from which the field components impinge on the array. This property permits to increase the performance of the array in terms of angular resolution. A simple signal processing technique is proposed allowing formation of a set of beams capable to cover uniformly the entire horizon with an angular resolution better than that achievable by a classical uniform-weighing half-wavelength-spaced linear array. Results are also applicable to active arrays. As the above approach leads to arrays operating in super-directive regime, we discuss all related critical aspects, such as sensitivity to external and internal noises and to array imperfections, and bandwidth, so as to identify the basic design criteria ensuring the array feasibility. [ABSTRACT FROM AUTHOR]