Multichannel Cross-correlation Research Articles

Upper Mantle Velocity Structure Beneath the Yarlung–Tsangpo Suture Revealed by Teleseismic P-Wave Tomography

We applied teleseismic tomography to investigate the 3D P-wave velocity (Vp) structure of the crust and upper mantle at depths of 50–400 km beneath the Yarlung–Tsangpo suture (YTS), by using 6164 P-wave relative travel-time residuals collected from 495 teleseismic events recorded at 20 three-component broadband seismograms. A modified multi-channel cross-correlation method was adopted to automatically calculate the relative arrival-time residuals of all teleseismic events, which significantly improved the efficiency and precision of the arrival-time data collection. Our results show that alternating low- and high-Vp anomalies are visible beneath the Himalayan and Lhasa blocks across the YTS, indicating that strong lateral heterogeneities exist beneath the study region. A significant high-Vp zone is visible beneath the southern edge of the Lhasa block at 50–100 km depths close to the YTS, which might indicate the rigid Tibetan lithosphere basement. There exists a prominent low-Vp zone beneath the Himalayan block to the south of the YTS extending to ~150 km depth, which might be associated with the fragmentation of the underthrusting Indian continental lithosphere (ICL) and induce localized upwelling of asthenospheric materials from the upper mantle. In addition, significant low-Vp anomalies were observed beneath the Yadong–Gulu rift and the Cona–Sangri rift extending to ~300 km depth, indicating that the tearing of the subducted ICL might provide pathways for the localized asthenospheric materials upwelling, which contributes to the widespread distribution of north–south trending rifts and geothermal activities in southern Tibet.

Time delay estimation via average magnitude differences among multiple microphone signals

Time delay estimation (TDE) plays a significant role in hands-free speech communication systems for localizing and tracking speakers. To boost the robustness of time delay estimators in room acoustic environments, a novel TDE approach is proposed in this paper. This method first exploits the reciprocals of average magnitude difference functions of sound signals captured at a microphone array, instead of cross correlation coefficients, to construct the entries of the parameterized correlation coefficient matrix in the multichannel cross-correlation coefficient algorithm. A multichannel average magnitude difference coefficient is then defined to establish the time delay estimator. Simulation results demonstrate that the proposed TDE strategy can yield better performance as compared to the multichannel cross-correlation coefficient method in noisy and reverberant environment.

Automatic Time Picking for Weak Seismic Phase in the Strong Noise and Interference Environment: An Hybrid Method Based on Array Similarity.

The extraction of travel-time curve of seismic phase is very important for the subsequent inference of the structural properties of underground media in seismology. In recent years, with the increase in the amount of data, manual processing is facing significant challenges, and automatic signal processing has gradually become the mainstream. According to the similarity of array signals and considering the elimination of outliers, we propose an improved multi-channel cross-correlation method using the L1 norm measure to obtain preliminary results, which builds on a new controllable measurement mode. Then, the post-correction step is carried out in combination with the signal gain property of beamforming technique. Based on these two methods, this paper proposes a new scheme of automatic arrival time picking. We apply the scheme to actual data to verify the effects of the two methods step by step. The entire scheme achieves fine results: direct water waves, seismic waves refracted by the crust and seismic waves reflected by the upper mantle are automatically detected. In addition, compared with the two traditional methods, the scheme proposed in this paper has a better overall effect and a reasonable computation cost.

Integrated 2D beam emission spectroscopy diagnostic at the Huan-Liuqi-2A (HL-2A) tokamak.

Two newly developed, eight-channel, integrated Beam Emission Spectroscopy (BES) detectors have been installed at Huan-Liuqi-2A tokamak, which extends the existing 16 single-channel modular BES system with additional 16 spatial channels. The BES collects the Doppler-shifted Balmer Dα emission with a spatial resolution of 1cm (radial) × 1.5cm (poloidal) and a temporal resolution of 0.5 µs to measure long-wavelength (k⊥ρi < 1) density fluctuations. Compared to the modular BES, the dark noise of the integrated BES is reduced by 50%-60% on average. The signal-to-noise ratio of the integrated BES system is optimized by the high light throughput front-end optics, high quantum efficiency photodiodes, high-gain, low-noise preamplifiers, and sufficient cooling capacity provided by the thermoelectric cooling (TEC) units that maintain the detectors at -20 °C. Crosstalk between channels that share the same optical system is found to be negligible. High-quality density fluctuation data enables 2D (radial-poloidal) imaging of turbulence, which allows for multi-channel spectral analysis, multi-channel cross-correlation analysis and velocimetry analysis. Preliminary results show that BES successfully captures the spatiotemporal features of the local turbulence and obtains statistically consistent turbulence characterization results.

Sound Source Localization Based on Multi-Channel Cross-Correlation Weighted Beamforming.

Beamforming and its applications in steered-response power (SRP) technology, such as steered-response power delay and sum (SRP-DAS) and steered-response power phase transform (SRP-PHAT), are widely used in sound source localization. However, their resolution and accuracy still need improvement. A novel beamforming method combining SRP and multi-channel cross-correlation coefficient (MCCC), SRP-MCCC, is proposed in this paper to improve the accuracy of direction of arrival (DOA). Directional weight (DW) is obtained by calculating the MCCC. Based on DW, suppressed the non-incoming wave direction and gained the incoming wave direction to improve the beamforming capabilities. Then, sound source localizations based on the dual linear array under different conditions were simulated. Compared with SRP-PHAT, SRP-MCCC has the advantages of high positioning accuracy, strong spatial directivity and robustness under the different signal–noise ratios (SNRs). When the SNR is −10 dB, the average positioning error of the single-frequency sound source at different coordinates decreases by 5.69%, and that of the mixed frequency sound sources at the same coordinate decreases by 5.77%. Finally, the experimental verification was carried out. The results show that the average error of SRP-MCCC has been reduced by 8.14% and the positioning accuracy has been significantly improved, which is consistent with the simulation results. This research provides a new idea for further engineering applications of sound source localization based on beamforming.

Sound Source Localization Based on Multi-Channel Cross-Correlation Weighted Beamforming

Sound Source Localization Based on Multi-Channel Cross-Correlation Weighted Beamforming

Underwater Target Localization Using Opportunistic Ship Noise Recorded on a Compact Hydrophone Array

In this research, a new application using broadband ship noise as a source-of-opportunity to estimate the scattering field from the underwater targets is reported. For this purpose, a field trial was conducted in collaboration with JASCO Applied Sciences at Duncan’s Cove, Canada in September 2020. A hydrophone array was deployed in the outbound shipping lane at a depth of approximately 71 m to collect broadband noise data from different ship types and effectively localize the underwater targets. In this experiment, a target was installed at a distance (93 m) from the hydrophone array at a depth of 25 m. In this study, a matched field processing (MFP) algorithm is utilized for localization. Different propagation models are presented using Green’s function to generate the replica signal; this includes normal modes in a shallow water waveguide, the Lloyd-mirror pattern for deep water, as well as the image model. We use the MFP algorithm with different types of underwater environment models and a proposed estimator to find the best match between the received signal and the replica signal. Finally, by applying the scatter function on the proposed multi-channel cross correlation coefficient time-frequency localization algorithm, the location of target is detected.

Multichannel Alignment of S Waves

ABSTRACT High-resolution earthquake locations and structural inversions using body waves rely on precise delay-time measurements. Subsample accuracy can be realized for P waves using multichannel cross correlation (MCCC), as developed by VanDecar and Crosson (1990), which exploits redundancy in pairwise cross correlations to determine delays between similar waveforms in studies of mantle structure using teleseismic sources (common source and multiple stations) and regional studies of structure and seismicity (multiple sources and common station). For regional S waves, alignment is complicated by the additional degree of freedom in waveform polarity that is expressed for sources with different moment tensors. Here, we recast MCCC within a principal component framework and demonstrate the equivalence between maximizing waveform correlation and minimization of various singular value–based objective functions for P waves. The singular-value framework is more general and leads naturally to an MCCC linear system for S waves that possesses an order of magnitude greater redundancy than that for P waves. Robust L1 solution of the system provides an effective means of mitigating outliers at the expense of subsample precision. Residual time shifts associated with higher-order singular vectors are employed in an iterative adaptive alignment that achieves subsample resolution. We demonstrate application of the approach on a seismicity cluster within the northern Cascadia crustal fore-arc.

New Methods for Data Stacking and P‐ and S‐wave Arrival Time Determination Using the Deep Moonquake Apollo Recordings

AbstractA new method of simultaneous time domain cross‐correlation of multiple components of motion at multiple seismic stations is developed for P‐ and S‐wave identification for determination of arrival times of deep moonquakes using Apollo Passive Seismic Experiment (APSE) data. Deep moonquakes occur in selenographically isolated clusters, allowing the stacking of a large number of moonquakes in each cluster to improve signal‐to‐noise ratio. The method developed here seeks to improve upon multichannel cross‐correlation (MCCC) of a single component of motion and multiple events used in past work by incorporating all components and stations simultaneously. This multicomponent multichannel cross‐correlation (MCMCCC) method not only maximizes the inherent correlation between the 12 components (three components of motion at each of four stations) across all events in a given cluster, but also establishes representative stacked traces of moonquakes within each cluster. Additionally, the MCMCCC method utilizes components and events with the highest data quality to guide the inclusion of lower quality components and events (or events with fewer components). A closure difference residual time is introduced to identify moonquake events with better data and/or close source locations, to further improve the signal‐to‐noise ratio of stacked traces. A multicomponent short‐term to long‐term average algorithm is also developed to objectively determine P‐ and S‐wave arrivals that best align with their travel time curves based on lunar velocity models, simultaneously for all 12 components of motion. Newly computed deep moonquake cluster stacks and some updated deep moonquake cluster locations are presented, which improve upon past results.

Infrasonic Earthquake Detectability Investigated in Southern Part of Japan, 2019.

The Kochi University of Technology (KUT) Infrasound Sensor Network contains 30 infrasound sensors which are distributed all over Japan especially in Shikoku Island. At all infrasound stations installed with three-axis accelerometers to measure the peak ground acceleration (PGA). Many earthquakes were detected by our system after establishing of the network since 2016. In this study we will focus on all the possibilities for infrasound detection generated from earthquakes using KUT sensor network and International Monitoring system (IMS) stations for the earthquakes which were detected in southern part of Japan during 2019. As for earthquakes with strike-slip mechanisms the P-waves could not be detected by our sensors. In addition, The conversion from seismic to acoustic waves can be happened through the generating of the T-phase from oceanic earthquakes. On 9 May 2019, progressive multi-channel cross correlation (PMCC) method applied infrasound and hydroacoustic waves from two earthquakes happened in west of Kyushu Island as the T-phase was well-recorded at H11N station near Wake Island. Moreover, infrasound propagation modeling is applied to the reconstructed atmosphere profile by Ground to Space Model (AVO-G2S) to confirm the infrasound arrivals, furthermore the 3D ray tracing process and the calculations by using the transmission loss equation with normal modes and parabolic equation methods are investigated. The study confirmed the infrasound generation scenario from the T-phase of oceanic propagation.

Mass-Flow-Rate Measurement of Pneumatically Conveyed Particles Through Acoustic Emission Detection and Electrostatic Sensing

Accurate online mass flow rate measurement of pneumatically conveyed particles is desirable to convert a conventional pulverized fuel-fired power station into a smart thermal power plant. This article presents a novel method for the online measurement of the mass flow rate of pulverized fuel through acoustic emission (AE) detection and electrostatic sensing. An integrated sensing head with an AE probe and three sets of electrostatic sensor arrays is developed. The proposed method determines the particle velocity by multichannel cross correlation of the electrostatic signals and extracts the information about mass flow rate from the AE signal arising from impacts of particles with a waveguide protruding into the flow. An analytical model that relates the energy of the AE signals, the particle velocity, and the mass flow rate is established. The sensing head was mounted on the vertical and horizontal sections of a 72-mm bore laboratory-scale test rig conveying fine silica particles. Experimental tests were conducted under a range of flow conditions and installation orientations to assess the performance of the developed measurement system. The results demonstrate that the sensing head should be installed in any orientation away from the elbow on the vertical section of a pipe, while for installation on a horizontal pipe, the waveguide should be in the horizontal direction. The instrumentation system is capable of measuring the mass flow rate of particles in the vertical pipe with a relative error within ±6.5% regardless of the orientation of the sensing head over the mass flow rate from 7 to 25 kg/h and the particle velocity from 12 to 30 m/s, while on the horizontal pipe, the error is within 5.8% when the sensing head is installed with the waveguide in the horizontal direction under the same flow conditions.

Teleseismic P-wave crustal tomography of the Weifang segment on the Tanlu fault zone: A case study based on short-period dense seismic array experiment

The 1896 high-quality relative travel-time residuals are collected by applying the Multi-Channel Cross-Correlation technique to the waveform data of teleseismic events recorded by a short-period dense seismic array consisting of 302 stations in the Weifang segment of the Tanlu fault zone. Using the Fast Marching Teleseismic Tomography technique, these residuals are inverted for a fine P-wave crustal velocity model under the study region down to 30 km depth with the resolution of ~20 km in horizontal directions and ~30 km in depth. We image obvious high-velocity anomalies to the west of the fault zone from the surface down to a depth of 30 km, and striking low-velocity anomalies appear within the fault zone around 36.5°N where the Changle volcano exists on the surface, whilst to the east velocity anomalies exhibit a complicated pattern. Meanwhile, our tomographic images roughly show a boundary between the fault zone and its surrounding areas. We infer that the fault zone provides a plausible channel for continuous upwelling of hot and wet materials from the mantle to the surface when it intruded into the crust. The process of upwelling of hot and wet materials from the mantle could be associated with mantle dynamics of the big mantle wedge due to the stagnation and dehydration of the subducted Pacific slab in the mantle transition zone under the region. Our present study demonstrates that it is possible that a fine crustal velocity structure can be obtained by teleseismic tomography with a short-period dense seismic array.

Deep structure of the Hellenic lithosphere from teleseismic Rayleigh-wave tomography

SUMMARYThis research provides new constraints on the intermediate depth upper-mantle structure of the Hellenic lithosphere using a three-step Rayleigh-wave tomography. Broadband waveforms of about 1000 teleseismic events, recorded by ∼200 permanent broad-band stations between 2010 and 2018 were acquired and processed. Through a multichannel cross-correlation technique, the fundamental mode Rayleigh-wave phase-velocity dispersion curves in the period range 30–90 s were derived. The phase-velocities were inverted and a 3-D shear velocity model was obtained down to the depth of 140 km. The applied method has provided 3-D constraints on large-scale characteristics of the lithosphere and the upper mantle of the Hellenic region. Highlighted resolved features include the continental and oceanic subducting slabs in the region, the result of convergence between Adria and Africa plates with the Aegean. The boundary between the oceanic and continental subduction is suggested to exist along a trench-perpendicular line that connects NW Peloponnese with N. Euboea, bridging the Hellenic Trench with the North Aegean Trough. No clear evidence for trench-perpendicular vertical slab tearing was resolved along the western part of Hellenic Subduction Zone; however, subcrustal seismicity observed along the inferred continental–oceanic subduction boundary indicates that such an implication should not be excluded. The 3-D shear velocity model supports an N–S vertical slab tear beneath SW Anatolia that justifies deepening, increase of dip and change of dip direction of the Wadati-Benioff Zone. Low velocities found at depths &amp;lt;50 km beneath the island and the backarc, interrelated with recent/remnant volcanism in the Aegean and W. Anatolia, are explained by convection from a shallow asthenosphere.

A class of multichannel sparse linear prediction algorithms for time delay estimation of speech sources

Time delay estimation (TDE), which is also called time difference of arrival estimation, is an important yet challenging problem in room acoustic environments where reverberation and noise coexist. The multichannel cross-correlation coefficient algorithm extends the traditional cross-correlation method from the two- to the multiple-channel cases and exploits the spatial information among all microphones to improve the robustness of TDE with respect to noise. The multichannel spatiotemporal prediction algorithm generalizes the multichannel cross-correlation coefficient algorithm by incorporating both the spatial and temporal information to make TDE robust to reverberation. This multichannel spatiotemporal prediction algorithm, however, is sensitive to noise. In this work, we attempt to improve the robustness of this algorithm by making it robust to both reverberation and noise. Based on the sparsity of the prediction coefficient matrix of speech signals, a class of multichannel sparse linear prediction algorithms, including the multichannel spatiotemporal sparse prediction and the multichannel spatiotemporal group sparse prediction, are developed for TDE. The multichannel cross-correlation coefficient and multichannel spatiotemporal prediction algorithms are unified from a TDE performance perspective via an F/ℓ1-norm (or F/ℓ1,2-norm) optimization model, which is solved by an alternating direction method of multipliers. The two new algorithms also respectively construct a set of time delay estimators, which make different tradeoffs between prewhitening and non-prewhitening by adjusting a regularization parameter.

Improved infrasound array processing with robust estimators

Accurate infrasound source and path characterization rely on high-quality array processing parameter estimates. Physical and statistical assumptions underlying conventional array processing techniques sometimes fail in practice due to propagation effects or station degradation. Unlike conventional least squares regression, robust regression estimators are relatively insensitive to data that deviate from the assumed planar model. We compare two such estimators, M-estimators, and least-trimmed squares (LTS), to conventional array processing methods (frequency-wavenumber beamforming, progressive multi-channel cross correlation, L1 regression, and ordinary least squares) using synthetic and real infrasound data. Synthetic testing suggests that robust estimators are resistant to timing errors and noise contamination. We also present case studies from both International Monitoring System and the Alaska Volcano Observatory infrasound data that demonstrate how these techniques have produced accurate array processing results despite an element polarity reversal, timing error due to the loss of GPS lock, and a deviation from the plane wave assumption. We also evaluate the effectiveness of these techniques to arrays with differing geometries and number of elements, and note that the examination of LTS residuals enables outlying inter-element differential times to be flagged automatically, providing a data quality tool.

Reverberation-Robust Localization of Speakers Using Distinct Speech Onsets and Multichannel Cross Correlations

Many speaker localization methods can be found in the literature. However, speaker localization under strong reverberation still remains a major challenge in the real-world applications. This paper proposes two algorithms for localizing speakers using microphone array recordings of reverberated sounds. To separate concurrent speakers, the first algorithm decomposes microphone signals spectrotemporally into subbands via an auditory filterbank. To suppress reverberation, we propose a novel speech onset detection approach derived from the speech signal and impulse response models, and further propose to formulate the multichannel cross-correlation coefficient of encoded speech onsets in each subband. The subband results are combined to estimate the directions-of-arrival of speakers. The second algorithm extends the generalized cross-correlation phase transform method by using redundant information of multiple microphones to address the reverberation problem. The proposed methods have been evaluated under adverse conditions using not only simulated signals (reverberation time $T_{60}$ of up to $1$ s) but also recordings in a real reverberant room ( $T_{60} \approx 0.65$ s). Comparing with some state-of-the-art localization methods, experimental results confirm that the proposed methods can reliably locate static and moving speakers, in presence of reverberation.

Multi-channel cross-correlation used to estimate time delays

Many marine mammal localization techniques rely on estimating the time of arrival delays of a call between the hydrophones of an array. This is commonly achieved by cross-correlating (some version of) received signals between hydrophone pairs. This process is complicated in cases with noise, multiple animals, and multipath arrivals. In this talk, I explore and demonstrate the potential of a multi-channel cross-correlation technique to help establish time delay estimates. Possible extensions to signal enhancement for detection and classification purposes are also discussed. [This work was supported by ONR Award No. N00014-16-1-2598.]

Upper mantle structure and geodynamics of the Sumatra subduction zone from 3-D teleseismic P-wave tomography

Teleseismic waveforms from a passive array of 63 seismic stations are applied to image the 3-D P-wave velocity structure of the mantle in the Sumatra subduction zone. In this study, 102 teleseisms with good signal-to-noise ratios were selected. In total, 1627 P-wave relative arrivals have been extracted with high accuracy based on multi-channel cross-correlation and adaptive stacking methods. With this dataset, we conducted the 3-D tomographic inversions for the 3-D P-wave velocity structure of the upper mantle in the Sumatra subduction zone. The tomographic results suggest significant lateral variations of P-wave velocity in the mantle. The trench-parallel low-velocity features in the upper mantle reveal that there may be a trench-parallel flow associated with 3-D mantle flow beneath the subducting slab. The subducting Indo-Australian Plate is also clearly imaged as a continuous high-velocity anomalous zone in the mantle along the Sumatra Island. Beneath southern Sumatra, there could have a lateral detachment of high-velocity anomaly from ∼560 km to the deep part of the slab, which may imply the slab tearing. If it is true, we could infer that this tear in the slab may be terminated around 17.5 Ma. The advancing speed of the Indo-Australian Plate increasing from North Sumatra to South Sumatra plays a key role in the slab tear, which suggests likely that the northeastward subducting of the Indo-Australian Plate along the Sunda trench and the southeastward clockwise motion of the Sunda Shelf may form a torque tracing the Sumatra Island that facilitated the clockwise rotation of the fast-moving plate. With the oblique subduction of the Indo-Australian Plate, the asymmetric distribution of stresses along this torque on either side may induce the slab tearing.

Environmental sound event detection in wireless acoustic sensor networks for home telemonitoring

In this paper, we present an approach to improve the accuracy of environmental sound event detection in a wireless acoustic sensor network for home monitoring. Wireless acoustic sensor nodes can capture sounds in the home and simultaneously deliver them to a sink node for sound event detection. The proposed approach is mainly composed of three modules, including signal estimation, reliable sensor channel selection, and sound event detection. During signal estimation, lost packets are recovered to improve the signal quality. Next, reliable channels are selected using a multi-channel cross-correlation coefficient to improve the computational efficiency for distant sound event detection without sacrificing performance. Finally, the signals of the selected two channels are used for environmental sound event detection based on bidirectional gated recurrent neural networks using two-channel audio features. Experiments show that the proposed approach achieves superior performances compared to the baseline.

Geographic variations in lowermost mantle structure from the ray parameters and decay constants of core‐diffracted waves

AbstractWe introduce an array‐based approach for constraining seismic velocity structure in the lowermost mantle by measuring the frequency dependence of the ray parameter p and decay constant γ for core‐diffracted waves (Pdiff and SHdiff). The approach uses an iterative multichannel cross‐correlation algorithm that solves for relative arrival times and amplitudes of core‐diffracted waveforms from multiple peaks in normalized correlograms from pairs of coazimuthal stations. The approach is applied to 60 mb ≥ 5.8 earthquakes for Pdiff and 36 for SHdiff during 2001–2002, with nearly 50,000 unique profile station pairs with epicentral‐distance differences of Δd ≥ 10° and azimuth differences of Δφ ≤ 10°, sampling a significant portion of the lowermost mantle. Cap‐averaging the resulting ray parameter estimates produces geographic variations that are largely consistent with the distribution of lowermost mantle large low‐velocity provinces and seismically fast slab‐accumulation regions seen in seismic tomography models, whose locations also strongly influence the geographic distribution of heat flow out of the core. Geographic variations in station‐pair diffracted‐wave decay constants differ from those of the ray parameters, suggesting that variations in the decay of core‐diffracted waves are more linked to lowermost mantle seismic velocity gradients than absolute values of seismic velocity. The ray parameters and decay constants of core‐diffracted waves are strongly frequency‐dependent, and these frequency variations also vary significantly with geographic location. Combining lateral and vertical seismic‐velocity variations with mineral‐physics data on elasticity and conductivity of lowermost mantle species can provide constraints on D″ composition and CMB heat flux.