Arrival Azimuth Research Articles

New Method for Determining Azimuths of ELF Signals Associated With the Global Thunderstorm Activity and the Hunga Tonga Volcano Eruption

AbstractA new method is proposed for deriving extremely low frequency (ELF) wave arrival azimuths using the wide range of signal amplitudes, contrary to previously applied high amplitude impulses only. The method is applied to observations from our new magnetic sensor in the Hylaty station with an 18 bit dynamic range and a 3 kHz sampling frequency. We analyzed a day of 15 January 2022, to test the procedure against the ability to extract ELF signals generated during the Hunga Tonga volcano eruption. With complementary filtering of power line 50 Hz signatures, precise azimuth information can be extracted for waves from a multitude of thunderstorms on Earth varying during the day at different azimuths. A phenomenon of successive regular variation—decay or activation—of thunderstorms activity with varying azimuth is observed, possibly due to passing over the solar (day/night) terminator, and signatures of azimuth direction change during this passage can be noted. We also show that the erupting Hunga Tonga volcano associated impulses dispersed due to a long propagation path are clearly revealed in the azimuth distribution with analysis using parameters fitted to measure slowly varying signals, but not for fast varying impulses. We show that the Hunga Tonga related signals arrive from the azimuth ≈10° smaller than the geographic great circle path. The discrepancy is believed to be due to propagation through the polar region and in the vicinity of the solar terminator.

Dynamic V2V Channel Measurement and Modeling at Street Intersection Scenarios

Vehicular communication has been widely concerned by academia and industry. Wireless channel characterization and modeling are the foundation of communication systems. Compared with typical road scenarios such as urban and suburban areas, wireless channel characterization in intersections is a challenging task. In this paper, channel measurements at 5.9 GHz in street intersection scenarios are carried out and provide data for the characterization and modeling of time-varying vehicular channels. This paper proposes a double-slope path loss model, which can accurately reflect the signal propagation loss at intersections. Further, the time-varying spatial characteristics of multipath are extracted and analyzed, and the azimuth and elevation spread of arrival are statistically characterized. The influence of obstacles on multipath spatial characteristics in intersection scenarios is investigated. Then we investigate the spatial stationarity of channels and analyze the possible reasons for the differences in spatial stationarity of different scenarios. In addition, based on multipath extraction, we quantitatively analyze the energy of dense multipath components in different propagation scenarios. The research in this paper can enrich the investigation of vehicular channels and enable the analysis and design of vehicular communication systems.

Fireworks

Once a year, in many parts of the country, transient acoustic sources light up the sky. A fourth-of-July fireworks display provides an excellent opportunity for exploring array-based detection and direction-of-arrival determination. Multiple-boom events are difficult to process; however, less-frequent single-boom events are often sufficiently isolated in time for unambiguous interpretation. Cross-correlations between channels of a four-microphone array provide time delays for angle-of-arrival determination and measures for event detection. The distinct time-domain wave shapes and high signal-to-noise enable straightforward checks of the cross-correlation delays. In addition, a chirp sequence broadcast from a known location provides a ground-truth measurement and opportunities to implement matched filtering and envelope processing. Detection strategies can include received power, rate of change of power, normalized cross-correlation coefficient, or Fisher F-statistic. A least-squares fit by plane-wave approximation gives an estimate of the actual slowness vector, which, in turn, permits estimation of error and of local sound speed. Conversion of the slowness vector to arrival azimuth and elevation angles provides a simple exercise in 3D interpretation. The homework exercise can be as simple as determination of inter-sensor time delays or as complicated as demonstrating a scheme for automatic detection and interpretation.

Rapid Location of Remote Volcanic Infrasound Using 3D Ray Tracing and Empirical Climatologies: Application to the 2011 Cordón Caulle and 2015 Calbuco Eruptions, Chile

AbstractInfrasound (<20 Hz) can propagate thousands of kilometers through the atmosphere, enabling global source location using networks of arrays such as the International Monitoring System infrasound network. However, atmospheric spatiotemporal variability poses a major challenge to locating infrasound sources. Strong horizontal cross‐winds deviate the observed infrasound arrival azimuths, producing source mislocations that can be on the order of hundreds of kilometers. We introduce a method that combines empirical climatologies (HWM14/NRLMSIS2.0) and 3D ray tracing (infraGA) to obtain first‐order, robust, and rapid estimates of the backazimuth deviations for source location procedures. For each trial source node and receiver path, day of the year and time, we perform an automatic iterative search for infrasound ground intercepts around the target station and obtain the corresponding backazimuth deviation. We test the method using IMS infrasound data from stations up to ∼5,000 km range for two similar explosive eruption case studies: 2011 Puyehue‐Cordón Caulle and 2015 Calbuco. We obtain a source mislocation reduction up to ∼84% (242–38.7 km) and ∼75% (366–93.1 km) for Puyehue‐Cordón Caulle and Calbuco eruptions, respectively. To evaluate the approach, we repeat the procedure using more realistic hybrid atmospheric descriptions; we obtain comparable results (up to ∼75% mislocation reduction for both eruptions). Potential applications of the approach include long‐range volcano monitoring in near‐real time by using pre‐computed look‐up tables or large‐scale, multi‐year reanalyses of infrasound waveform archives.

Infrasound Source Localization of Distributed Stations Using Sparse Bayesian Learning and Bayesian Information Fusion

The precise localization of the infrasound source is important for infrasound event monitoring. The localization of infrasound sources is influenced by the atmospheric propagation environment and infrasound measurement equipment in the large-scale global distribution of infrasound arrays. A distributed infrasound source localization method based on sparse Bayesian learning (SBL) and Bayesian information fusion is proposed to reduce the localization error. First, the arrival azimuth of the infrasound source is obtained based on the SBL algorithm. Then, the infrasound source localization result is obtained by the Bayesian information fusion algorithm. The localization error of the infrasound source can be reduced by this infrasound source method, which incorporates the uncertainty of the infrasound propagation environment and infrasound measurement equipment into the infrasound source localization results. The effectiveness of the proposed algorithm was validated using rocket motor explosion data from the Utah Test and Training Range (UTTR). The experimental results show that the arrival azimuth estimation error can be within 2° and the localization distance error is 3.5 km.

A multi‐step method for passive broadband source localisation using a single vector sensor

In the deep ocean, a vector sensor is deployed near the seabed to receive direct waves from the reliable acoustic path (RAP). In the space domain, the vector sensor can estimate the vertical arrival azimuth of the signal, which is sensitive to distance. In the frequency domain, the periodic interference pattern formed by the direct and surface-reflected wave is decided by both the source depth and the vertical arrival azimuth. In this work, a multi-step method for passive broadband source localisation using a single-vector sensor (SVS) is proposed. At first, the depth is estimated by the interference pattern in the frequency domain. Then the ranging problem is solved based on the depth estimated result. The impact of the surface-generated noise is considered. Compared with the scalar vertical line array (VLA) in simulation, the localisation performance of SVS has higher requirements for the signal-to-noise ratio (SNR) but is much smaller in size. In the experiment, Gaussian white noise from 310 to 430 Hz was emitted to simulate the underwater target. Compared with traditional match-field processing (MFP), the multi-step method is much more stable, accurate, and efficient. The computing time is drastically shortened.

Three-dimensional modeling of T-wave generation and propagation from a South Mid-Atlantic Ridge earthquake.

A three-dimensional (3D) hybrid modeling method is used to study the generation and propagation of T waves in the ocean triggered by a Southern Mid-Atlantic Ridge earthquake. First, a finite-element method model named SPECFEM3D is used to propagate seismic waves in the crust and acoustic waves in the ocean for the T-wave generation in a 200 × 50 km area near the epicenter. A 3D parabolic equation (PE) method is then used to propagate the T waves in the ocean for about 850 km further to the hydrophone stations deployed by the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) near Ascension Island. All of the simulations considered the realistic bathymetry and water sound speed profile. The SPECFEM3D results suggest that T waves with clear modal features could be generated by the concentration of reflected head waves in two depressions 40 km away from the epicenter. To compare with the hybrid modeling method for calculating T-wave propagation losses and arrival azimuths at the CTBTO hydrophones, point source simulations using the 3D PE model from the T waves source locations, identified with SPECFEM3D, were also implemented. The advantages and limitations of each approach are discussed.

Search for Infrasound Signals in InSight Data Using Coupled Pressure/Ground Deformation Methods

ABSTRACT The unprecedented quality and sampling rate of seismometer and pressure sensors of the InSight Mars mission allow us to investigate infrasound through its pressure and ground deformation signals. This study focuses on compliance effects induced by acoustic waves propagating almost horizontally close to the surface. The compliance of acoustic waves is first estimated using the compliance estimates from pressure perturbations moving at wind speed. Then, a marker of compliance events is used to detect events of ground deformation induced by pressure variations, in three frequency bands from 0.4 to 3.2 Hz, from InSight sol 180 to 690. Additional selection criteria are imposed on the detected events to focus on acoustic waves and to remove various noise sources (e.g., wind effects or seismometer artifacts). After an automated selection, the visual inspection of the records allows us to validate two infrasound candidates that cannot be related to pressure perturbations moving at wind speed nor to known noise sources. For our highest quality infrasound candidate, the relation between this event and a convective vortex occuring 10 s later is tested. The azimuth of the vortex position at the time of infrasound detection is not consistent with the arrival azimuth of the suspected infrasound inferred from the polarization of seismometer records, thus the link between these two phenomena cannot be demonstrated. Further investigations would require a better understanding of wind-related noise impacting InSight sensors and of the effects of lateral variations of subsurface mechanical properties on the ground deformations induced by atmospheric pressure variations.

ULoc

A myriad of IoT applications, ranging from tracking assets in hospitals, logistics, and construction industries to indoor tracking in large indoor spaces, demand centimeter-accurate localization that is robust to blockages from hands, furniture, or other occlusions in the environment. With this need, in the recent past, Ultra Wide Band (UWB) based localization and tracking has become popular. Its popularity is driven by its proposed high bandwidth and protocol specifically designed for localization of specialized "tags". This high bandwidth of UWB provides a fine resolution of the time-of-travel of the signal that can be translated to the location of the tag with centimeter-grade accuracy in a controlled environment. Unfortunately, we find that high latency and high-power consumption of these time-of-travel methods are the major culprits which prevent such a system from deploying multiple tags in the environment. Thus, we developed ULoc, a scalable, low-power, and cm-accurate UWB localization and tracking system. In ULoc, we custom build a multi-antenna UWB anchor that enables azimuth and polar angle of arrival (henceforth shortened to '3D-AoA') measurements, with just the reception of a single packet from the tag. By combining multiple UWB anchors, ULoc can localize the tag in 3D space. The single-packet location estimation reduces the latency of the entire system by at least 3×, as compared with state of art multi-packet UWB localization protocols, making UWB based localization scalable. ULoc's design also reduces the power consumption per location estimate at the tag by 9×, as compared to state-of-art time-of-travel algorithms. We further develop a novel 3D-AoA based 3D localization that shows a stationary localization accuracy of 3.6 cm which is 1.8× better than the state-of-the-art two-way ranging (TWR) systems. We further developed a temporal tracking system that achieves a tracking accuracy of 10 cm in mobile conditions which is 4.3× better than the state-of-the-art TWR systems.

Sound source localization based on improved adaptive beamforming

In adaptive systems, the least mean square error (LMS) adaptive algorithm is widely used. The variable step size LMS algorithm (SVSLMS) based on the Sigmoid function has greatly improved the convergence speed, and the normalized LMS algorithm is stable Sexual performance has certain advantages. Therefore, improvement is made on this basis, the SVSLMS algorithm is normalized, and the iteration speed is increased by changing the iteration factor, and the stability of the algorithm is also improved. This paper uses MATLAB simulation tools to verify the superiority of the new algorithm. Finally, the improved algorithm is applied to beamforming to estimate the azimuth of arrival of the simulated sound source signal at the partial discharge point of the submarine power cable.

Phased Array Analysis Incorporating the Continuous Wavelet Transform

ABSTRACT Nonlinear block thresholding of the continuous wavelet transform (CWT) of 2D phased array signals offers high time-resolution solutions for analyzing seismograms of local and regional seismic events. An initial denoising step on an array ensemble reveals the regions of the scale-time plane that contain high signal-to-noise arrivals. Individual seismic phase arrivals in ensemble, denoised seismograms can be partitioned using scale-time gating in which CWT wave packets of an individual seismic phase on the scale-time plane for a reference array element are time-correlated with all other elements to find an optimum time shift for the phase across all elements. The seismic phase is then clipped out of the CWT of each array element using this optimal time shift for further analysis. The seismogram can be separated into component seismic waves for a detailed view of wave characteristics such as slowness and arrival azimuth using conventional frequency–wavenumber methods. However, the process can be taken further using the CWT of each phase to construct high time-resolution signal beams over the CWT scale. Local explosion data from the 2016 Incorporated Research Institutions for Seismology (IRIS) Wavefields Community Experiment in northern Oklahoma are used to demonstrate these techniques in separating surface-wave modes and body waves and to examine the scattering of regional phases. High-resolution CWT processing in the 1–3 Hz band for northern Oklahoma reveals horizontal Rayleigh-wave refraction and multipathing. Rayleigh-wave phase-velocity dispersion measurements are used to construct a 1D velocity model under the IRIS experiment. Scale-time gating helps expose near-source surface reflections from a local M 3.6 earthquake that are used to verify the 4 km source depth obtained from a published regional moment tensor solution.

Analytical Nonstationary 3D MIMO Channel Model for Vehicle-to-Vehicle Communication on Slope

Vehicle-to-vehicle communication plays a strong role in modern wireless communication systems, appropriate channel models are of great importance in future research, and propagation environment with slope is one special kind. In this study, a novel three-dimensional nonstationary multiple-input multiple-output channel model for the sub-6 GHz band is proposed. This model is a regular-shaped multicluster geometry-based analytical model, and it combines the line-of-sight component and multicluster scattering rays as the nonline-of-sight components. Each cluster of scatterers represents the influence of different moving vehicles on or near a slope, and scatterers are, respectively, distributed within two spheres around the transmitter and the receiver. In this model, it is considered that the azimuth and elevation angles of departure and arrival are jointly distributed and conform to the von Mises–Fisher distribution, which can easily control the range and concentration of the scatterers within spheres to mimic the real-world situation well. Moreover, the impulse response and the autocorrelation function of the corresponding channel is derived and proposed; then, the Doppler power spectrum density of the channel is simulated and analyzed. In addition, the nonstationary characteristics of the presented channel model are observed through simulations. Finally, the simulation results are compared with measurement data in order to validate the utility of the proposed model.

High resolution location of deep low-frequency tremors beneath the Kii Peninsula, Nankai subduction zone, Japan, using data from a dense seismic array

SUMMARY This study performs detection and location of deep low-frequency tremors beneath the Kii Peninsula, Nankai subduction zone, Japan, using data from a dense seismic array. We set a quantitative threshold for the tremor detection based on the Gaussian distribution of waveform coherence among stations to distinguish between tremor signal and noise. Then, we estimated slownesses and arrival azimuths by applying the Multiple Signal Classification method and performed hypocentre determination by backprojection onto the plate interface. During the analysed period of 2 yrs from July 2012 to July 2014, we detected 25155 tremor events which are approximately 2.2 times more than those in the AIST-tremor catalogue based on the conventional envelope correlation method ( 11475 events). This improvement is due to the detection of even weak amplitude tremors owing to the array analysis scheme, which uses recorded waveform’s phase information. Our high-resolution tremor catalogue leads to the following findings of the tremor activity in the Kii Peninsula. A few tremor episodes show continuous updip migration in the initial stage. The updip migration speed depends on whether the tremor migrates in the along-strike direction after the updip migration: approximately 1.0 km hr –1 for tremors that develop into the along-strike migration, and approximately 0.5 km hr–1 for those that do not. Estimated radiated energies are found to increase gradually with propagation into the updip direction. These findings suggest the depth dependence of the frictional strength on the plate interface. Furthermore, we noticed that accumulated radiated energies are positively correlated with the updip migration speed, and this relationship implies that fluid migration might play a role in the updip migration.

The effect of geologic structure on fracture characterization using azimuthal vertical seismic profiles

Azimuthal vertical seismic profile (AzVSP) surveys have commonly been used for fracture characterization by analyzing the P-to-S converted wave response across fractured zones. Fractured media produce characteristic two-cycle patterns on AzVSP gathers that are disrupted in the presence of complex structures. Aiming to characterize the complexity of the AzVSP response in the presence of geologic structures, we have derived anisotropy parameters for effective horizontal transverse isotropic media, generated AzVSP signatures for the flat-interface case as the baseline, and compared the baseline signatures with AzVSP signatures for the dipping-interface cases. The fracture fill and fracture intensity are varied to capture the effect of the intrinsic fracture parameters on the AzVSP signatures. We find that dry fractures produce a stronger response compared with fluid-filled fractures for the same fracture density. The structural response on AzVSP signatures is isolated by generating synthetic seismograms across structurally equivalent isotropic models. Transverse energy, resulting from structures, could be misinterpreted as evidence for fracturing. The AzVSP signatures for a fractured dipping-interface two-layer model show significant distortion of the fracture response for the 10° dip case compared with the flat-interface signatures. As a possible solution to the structural problem, we use the arrival azimuths of the transmitted P-to-S event in synthetic signatures generated with the isotropic structural model, to orient the radial and transverse components parallel and perpendicular to the isotropic P-to-S event, respectively. Such structurally consistent orientation negates the effect of structure on AzVSP gathers and uncovers the underlying fracture response. This methodology can be extended to complex overburdens that are structurally well constrained.

Pressure Effects on the SEIS‐InSight Instrument, Improvement of Seismic Records, and Characterization of Long Period Atmospheric Waves From Ground Displacements

AbstractMars atmospheric pressure variations induce ground displacements through elastic deformations. The various sensors of the InSight mission were designed in order to be able to understand and correct for these ground deformations induced by atmospheric effects. Particular efforts were made, on one hand, to avoid direct pressure and wind effects on the seismometer and, on the other hand, to have a high performance pressure sensor operating in the same frequency range as the seismometer. As a consequence of these technical achievements and the low background seismic noise of Mars, the InSight mission is opening a new science domain in which the ground displacements can be used to perform atmospheric science. This study presents an analysis of pressure and seismic signals and the relations between them. After a short description of the pressure and seismic sensors, we present an analysis of these signals as a function of local time at the InSight location. Then the coherent signals recorded by both pressure and seismic sensors are described and interpreted in terms of atmospheric signals and ground deformation processes. Two different methods to remove the pressure effects recorded by SEIS sensors are presented, and their efficiency is estimated and compared. These decorrelation methods allow the pressure generated noise to be reduced by a factor of 2 during the active day time period. Finally, an analysis of SEIS signals induced by gravity waves demonstrates the interest of ground displacement measurements to characterize their arrival azimuth.

Single‐Station Lightning Location Using Azimuth and Time of Arrival of Sferics

AbstractLightning strikes produce electromagnetic waves, now referred to as sferics, in the very low frequency (3 kHz–30 kHz) and the extremely low frequency (3 Hz–3 kHz) bands. Within these frequency bands, the Earth and ionosphere form a waveguide in which sferics propagate long distances with low attenuation. The structure of the received sferic waveform is mainly a function of propagation distance and the waveguide's parameters. This suggests that each observed sferic waveform contains information about the distance that this sferic has propagated which can be used to geolocate lightning. There are various approaches for analyzing received sferics, which mostly rely on measurements from multiple stations. However, in these methods, each station imposes an additional cost for building, maintenance, and synchronization. Here we present a novel method to estimate both the emission time and location of lightning, which works by measuring sferics recorded at a single station. We first process the sferic waveforms to obtain the arrival times of the very low frequency and extremely low frequency radiation components which propagate with different speeds. Once these two separate arrival times are determined, we use them to approximate the distance the sferic propagated in the Earth‐ionosphere waveguide. We have used this novel method in combination with a method to find sferic direction to geolocate a significant number of lightning strikes for 4 July 2013. Using this proposed method, the distance of propagation estimates are accurate to within 6.7% of the National Lightning Detection Network‐determined propagation distance, and the direction of propagation estimates are accurate to within 1.3% of the National Lightning Detection Network‐determined direction.

An Optimum Volumetric Array Design Approach for Both Azimuth and Elevation Isotropic DOA Estimation

In this study, a new volumetric array design approach is presented for both azimuth and elevation isotropic direction of arrival (DOA) estimation. The presented method uses a minimum number of additional sensors to extend the given arrays to a volumetric array which has equal values for the Cramér-Rao lower bound (CRLB) for the entire azimuth and elevation angles. This approach can be applied to all arbitrary linear and planar arrays. Moreover, the design approach also takes into account the electromagnetic mutual coupling (MC) effect between the array elements and both the azimuth and elevation angular ambiguity uncertainties in an optimum manner while extending the array. It has been verified both analytically and with simulations that the designed volumetric arrays satisfy the two dimensional (2-D) isotropic DOA estimation conditions. The proposed approach is applied to Uniform Linear Arrays (ULA), Uniform Circular Arrays (UCA), V-shaped Arrays, and an arbitrary planar array to extend to optimum volumetric arrays with considerable results.

Single station lightning location 1 using azimuth and time of arrival of sferics

Single station lightning location 1 using azimuth and time of arrival of sferics

V2V channel characterization and modeling for underground parking garages

As an important part of the intelligent transportation system (ITS), vehicle-to-vehicle (V2V) communication will improve the safety and efficiency of the transportation system by realizing the information transmission between vehicles. The underground parking garage is a typical scenario in V2V communication, but as an indoor environment, it is different from the conventional outdoor road scenarios significantly. Therefore, the purpose of this paper is to analyze and model the V2V channel characteristics of an underground garage. In this paper, a novel channel measurement carried out in the underground garage is first introduced. Considering that there are a lot of obstacles in an underground garage, the non-line-of-sight (NLoS) condition is taken into account during the measurement. Then, channel characteristic parameters, including large-scale fading, delay spread, and K-factor, are analyzed based on the measured data. In addition, we also carry out channel angle of arrival analysis, show the distribution of azimuth of arrival (AOA) and elevation of arrival (EOA) of multipath components (MPC), and conduct a statistical analysis of the angular spread. In the process of analysis and modeling, we focus on the differences between line-of-sight (LoS) and NLoS conditions. The analysis and conclusions presented in this paper will enrich the understanding of V2V channel and benefit the design of V2V communications.

Fast estimation of earthquake arrival azimuth using a single seismological station and machine learning techniques

The objective of this research is to apply a new approach to estimate arrival azimuth of seismic events using seismological records of the “El Rosal” station, near to the city of Bogota – Colombia, by applying support vector machines (SVMs). The algorithm was trained with time signal descriptors of 863 seismic events acquired from January 1998 to October 2008; considering only events with magnitude ≥ 2 ML. The earthquake signals were filtered in order to remove diverse kind of low and high frequency noise not related to such events. During training stages of SVMs, several combinations of kernel function exponent and complexity factor were applied to time signals of 5, 10 and 15 seconds along with earthquake magnitudes of 2.0, 2.5, 3.0 and 3.5 ML. The best classification of SVMs was obtained using time signals of 5 seconds and earthquake magnitudes greater than 3.0 ML with kernel exponent of 10 and complexity factor of 2, showing accuracy of 45.4 degrees. This research is an improvement of previous works related to earthquake arrival azimuth determination from data of one single seismic station employing machine learning techniques.