Subsequent Waveform Research Articles

The 2023 New Zealand Ground-Motion Database

ABSTRACT This article summarizes the development of the 2023 New Zealand ground-motion database (NZGMDB). A preceding version was formally used as the central ground-motion database in the ground-motion characterization modeling for the 2022 New Zealand (NZ) National Seismic Hazard Model (NSHM) revision. The database contains ground motions for events with a moment magnitude greater than ∼3.0 from the years 2000 to the end of 2022. Several challenges associated with NZ earthquake source metadata are explained, including determination of earthquake location, magnitude, tectonic classification, and finite-fault geometry, among others. The site table leverages the site database developed as a part of the 2022 NZ NSHM revision, and several definitions of source-to-site distance are computed for the propagation path table. The ground-motion quality classification was initially assessed using a neural network. Subsequent waveform quality verification was conducted and additional quality criteria were enforced to ensure a sufficiently high-quality database. Standard processing techniques were applied to the ground motions before intensity measure (IM) calculation. IMs in the database include peak ground acceleration, 5%-damped pseudoacceleration response spectra, smoothed Fourier amplitude spectra, and other cumulative and duration-related metrics. The NZGMDB is publicly available and routinely updated as new and higher quality data become available.

Inference of velocity structures of oceanic crust and upper mantle from surface waveform fitting

SUMMARYInversion for seismological structures of the oceanic lithosphere–asthenosphere system is important to understand the mechanisms of plate tectonics. Previous models of the oceanic upper mantle have been primarily obtained via global tomography using surface waveforms. However, besides scarcity of waveform data in the oceanic regions, difficulties in fitting phases for shorter-period components in the previous global tomography have yielded resultant models that possess poor resolutions above $\sim \, 50$ km depth. Recent developments of broad-band ocean-bottom seismometer (BBOBS) arrays provide larger amount of seismic data with epicentral distances of <20°. In this study, we develop an appropriate method to fully utilize the information contained in the shorter-period components of BBOBS arrays. We first fit the envelopes without phase information to analyse the shorter-period components (8–60 s) which are generally unavailable in the conventional phase fitting. We then use the resultant model as our initial model for waveform inversion of the longer periods (12.5–200 s) to fit the phase, which allows us to infer a continuous structure model from the crust to the asthenosphere. We demonstrate the validity of this combined envelope-fitting and waveform inversion method by analysing the waveform data from a BBOBS array that was deployed in the Northwestern Pacific and has recorded events in the vicinity of the Japan Trench to obtain the average velocity structure between the event and station arrays. We independently resolve the crustal compressional and shear wave velocities, and thickness by analysing the envelopes, which minimizes biases in the subsequent waveform inversion. We also find that the waveform inversion improves the resolution in the asthenosphere. Our results suggest that further extension of this method should improve our knowledge of the oceanic lithosphere–asthenosphere system.

Microwave photonic radar system with ultra-flexible frequency-domain tunability.

Both effective detection demand against diversified development of flight targets and remote sensing providing identifiable and fine target information all call for microwave radar system with flexible and ultra-wideband frequency-domain ability to provide more high-resolution and multi-source information. A microwave photonic radar system with theoretically full-band and ultra-wideband working ability is presented and experimentally demonstrated. An optical frequency operation module is employed in the transmitter to break the frequency-domain limitation on the emitted radar signal, while two types of optical mixing structures are switched to provide the ability to receive target echoes at any frequency band. In the experiments, high-SNR optical frequency operation and subsequent waveform generation at each normal radar band, that is from HF to Ka, are carried out. Good linearity and coherence of the generated waveforms are also demonstrated. Then, multi-band system-level detection experiments are completed to show the full-band working ability. Centimeter-scale or even sub-centimeter-scale resolution are realized at different bands, which identifies the proposed system can handle target detection with flexible performance and support acquiring rich target information at different frequency bands in future.

Ultrasonic Structural Health Monitoring Using Fiber Bragg Grating.

The fiber Bragg grating (FBG) sensor, which was developed over recent decades, has been widely used to measure manifold static measurands in a variety of industrial sectors. Multiple experiments have demonstrated its ability in ultrasonic detection and its potential in ultrasonic structural health monitoring. Unlike static measurements, ultrasonic detection requires a higher sensitivity and broader bandwidth to ensure the fidelity of the ultrasonic Lamb wave that propagates in a plate-like structure for the subsequent waveform analysis. Thus, the FBG sensor head and its corresponding demodulation system need to be carefully designed, and other practical issues, such as the installation methods and data process methods, should also be properly addressed. In this review, the mature techniques of FBG-based ultrasonic sensors and their practical applications in ultrasonic structural health monitoring are discussed. In addition, state-of-the-art techniques are introduced to fully present the current developments.

Reciprocal data acquisition and subsequent waveform matching for integrated onshore–offshore seismic profiling

Reciprocal data acquisition and subsequent waveform matching for integrated onshore–offshore seismic profiling

Construction of Waveform Library in Cognitive Radar

Abstract Based on the thoughts of cognitive radar, Fractional Fourier Transform (FrFT) is used to generate a rotatable waveform libraries of Frank coded/Barker coded waveform in this paper. Then, the ambiguity function is used to analyze the delay resolution, Doppler resolution, delay side-lobe level, and Doppler side-lobe level of the waveform libraries and orthogonality of them is also analyzed. Furthermore, we proved theoretically that there is a fixed coordinate transformation between the waveforms of library and its origin waveform. Therefore, the Cramér-Rao low bound (CRLB) of motion parameters can be computed easily using the waveforms of the libraries, which facilitate the subsequent waveform scheduled work. Simulation results show that the library waveforms can reduce delay resolution to satisfy the different situations and can bring significant benefits for delay resolution, orthogonality and reuse interval.

MULTI-TONE MEASUREMENT SYSTEM WITH THE STATE-OF-THE-ART ACTIVE IF AND RF LOAD-PULL CAPABILITY

This paper reports a refined multi-tone waveform measurement system for the robust characterization and optimization of nonlinear microwave devices when driven by broadband multi-tone stimuli. This enhanced system has the ability to present specific, constant broadband impedances, not only at baseband (IF) frequencies, but also at RF frequencies, particularly, around the carrier and significant harmonics. This functionality is key in studying the effects of out-of-band impedance termination on both short and long-term electrical memory effects. Achieving such comprehensive impedance control across wide modulation bandwidths is also critical in allowing the "emulation" of emerging power amplifier modes and architectures, and the subsequent waveform characterization of devices operating in these complex and often dynamic impedance environments. Initially, the baseband load-pull capabilities of the enhanced measurement system are experimentally demonstrated through the measurement of adjacent channel power (ACP) behavior of a class AB biased 10 W GaN HEMT, in response to a varying baseband load. Complete baseband and RF impedance control is then demonstrated through the emulation and analysis of a modulated Class-J impedance environment, which interestingly highlights the presence of separate optimum baseband impedance conditions necessary for the suppression of individual IM products.

An Enhanced Modulated Waveform Measurement System

Microwave power transistors used in modern day wireless communication systems need to be characterised under complex modulated excitations into variable load impedances for the determination of optimum matching conditions. This paper presents a refined modulated waveform measurement system for the robust characterisation and optimisation of non-linear microwave devices when driven by broadband multi-tone stimuli. This enhanced system has the ability to present specific, constant broadband impedances, not only at baseband frequency, but also at radio frequencies, particularly, around the carrier and significant harmonics. Achieving such comprehensive impedance control across wide modulation bandwidths is critical in allowing the emulation of new power amplifier modes and architectures, and the subsequent waveform characterisation of devices operating in these complex and often dynamic impedance environments. The capability of the enhanced system is experimentally demonstrated through the measured adjacent channel intermodulation power behaviour of a 10 W GaN HEMT biased in class AB and tuned for maximum output peak envelope power.

An enhanced modulated waveform measurement system

Microwave power transistors used in modern day wireless communication systems need to be characterised under complex modulated excitations into variable load impedances for the determination of optimum matching conditions. This paper presents a refined modulated waveform measurement system for the robust characterisation and optimisation of non-linear microwave devices when driven by broadband multi-tone stimuli. This enhanced system has the ability to present specific, constant broadband impedances, not only at baseband frequency, but also at radio frequencies, particularly, around the carrier and significant harmonics. Achieving such comprehensive impedance control across wide modulation bandwidths is critical in allowing the emulation of new power amplifier modes and architectures, and the subsequent waveform characterisation of devices operating in these complex and often dynamic impedance environments. The capability of the enhanced system is experimentally demonstrated through the measured adjacent channel intermodulation power behaviour of a 10 W GaN HEMT biased in class AB and tuned for maximum output peak envelope power.

Waveform design for enhanced detection of extended target in signal-dependent interference

In this study, the authors address the design of optimal waveform for detecting extended target in signal-dependent interference, using the probability of detection as a cost function. The authors model the received signal in frequency domain and derive the optimum Neyman-Pearson (NP) detector. Considering the distribution of test statistic is rather complicated, the authors use a mathematically tractable form to approximate and derive an analytical expression for the probability of detection with regard to the transmitted waveform. This lays the foundation for the subsequent waveform optimisation. Finally, the authors focus on rank-one target, and propose methods for designing optimal, suboptimal and asymptotic optimal waveforms for detecting rank-one target.

Spike sorting with hidden Markov models

The ability to detect and sort overlapping spike waveforms in extracellular recordings is key to studies of neural coding at high spatial and temporal resolution. Most spike-sorting algorithms are based on initial spike detection (e.g. by a voltage threshold) and subsequent waveform classification. Much effort has been devoted to the clustering step, despite the fact that conservative spike detection is notoriously difficult in low signal-to-noise conditions and often entails many spike misses.Hidden Markov models (HMMs) can serve as generative models for continuous extracellular data records. These models naturally combine the spike detection and classification steps into a single computational procedure. They unify the advantages of independent component analysis (ICA) and overlap-search algorithms because they blindly perform source separation even in cases where several neurons are recorded on a single electrode. We apply HMMs to artificially generated data and to extracellular signals recorded with glass electrodes. We show that in comparison with state-of-art spike-sorting algorithms, HMM-based spike sorting exhibits a comparable number of false positive spike classifications but many fewer spike misses.

Doppler assessment of uterine artery blood flow for the prediction of pregnancy after assisted reproduction treatment

Doppler assessment of uterine artery blood flow for the prediction of pregnancy after assisted reproduction treatment

Identification of the neural sources of the pattern-reversal VEP

This study aimed to characterize the neural generators of the early components of the visual-evoked potential (VEP) to pattern-reversal gratings. Multichannel scalp recordings of VEPs and dipole modeling techniques were combined with functional magnetic resonance imaging (fMRI) and retinotopic mapping in order to estimate the locations of the cortical sources giving rise to VEP components in the first 200 ms poststimulus. Dipole locations were seeded to visual cortical areas in which fMRI activations were elicited by the same stimuli. The results provide strong evidence that the first major component of the VEP elicited by a pattern-reversal stimulus (N75/P85) arises from surface-negative activity in the primary visual cortex (area V1). Subsequent waveform components could be accounted for by dipoles that were in close proximity to fMRI activations in the following cortical areas: P95 (area MT/V5), P125/N135 (area V1), N150 (transverse parietal sulcus, TPS), N160 (ventral occipital areas VP, V4v, and V4/V8), and N180 (dorsal occipital areas V3A/V7). These results provide a detailed spatiotemporal profile of the cortical origins of the pattern-reversal VEP, which should enhance its utility in both clinical and basic studies of visual-perceptual processing.

Multiscale imaging of complex structures from multifold wide-aperture seismic data by frequency-domain full-waveform tomography: application to a thrust belt

An application of full-waveform tomography to dense onshore wide-aperture seismic data recorded in a complex geological setting (thrust belt) is presented. The waveform modelling and tomography are implemented in the frequency domain. The modelling part is solved with a finite-difference method applied to the visco-acoustic wave equation. The inversion is based on a local gradient method. Only the P-wave velocity is involved in the inversion. The inversion is applied iteratively to discrete frequency components by proceeding from low to high frequencies. This defines a multiscale imaging in the sense that high wavenumbers are progressively incorporated in images. The linearized waveform tomography requires an accurate starting velocity model that has been developed by first-arrival traveltime tomography. After specific pre-processing of the data, 16 frequency components ranging between 5.4 and 20 Hz were inverted. Ten iterations were computed per frequency component leading to 160 tomographic models. The waveform tomography has successfully imaged southwest-dipping structures previously identified from other geophysical data as being associated with high-resistivity bodies. The relevance of the tomographic images is locally demonstrated by comparison of a velocity–depth function extracted from the waveform tomography models with a coincident vertical seismic profiling (VSP) log available on the profile. Moreover, comparison between observed and synthetic seismograms computed in the (starting) traveltime and waveform tomography models demonstrates unambiguously that the waveform tomography successfully predicts for wide-angle reflections from southwest-dipping geological structures. This study demonstrates that the combination of first-arrival traveltime and frequency-domain full-waveform tomographies applied to dense wide-aperture seismic data is a promising approach to quantitative imaging of complex geological structures. Indeed, wide-aperture acquisition geometries offer the opportunity to develop an accurate background velocity model for the subsequent waveform tomography. This is critical, because the building of the macromodel remains an open question when only near-vertical reflection data are considered.

Inversion of shallow-seismic wavefields: II. Inferring subsurface properties from wavefield transforms

The problem of inferring subsurface properties from shallow-seismic data is solved by a two-stage scheme that fits the full wavefield by its synthetic counterpart. In a first stage (described in a companion paper) I derive Fourier—Bessel expansion coefficients for the recorded data through a wavefield transformation. The present paper describes the joint inversion of these coefficients together with P-wave arrival times to infer subsurface properties. In this way we exploit the full signal-content including the dispersion of higher-modes, leaky-modes, their true amplitudes, and, at least partly, body waves. Owing to the multi-mode character of shallow-seismic field-data conventional techniques of dispersion analysis are not applicable. Since an initial model appropriate for inversion of full seismograms is rarely available in shallow seismics, the direct inversion of waveforms is not feasible. The wavefield transformation removes a remarkable amount of non-linearity from the data. In consequence the proposed method is robust even in the absence of a priori information. In distinction to the inversion of dispersion curves, it does not require the identification of normal-modes prior to inversion. The method performs well when applied to the multi-mode wavefields present in most shallow-seismic data sets. Compared to waveform fitting, it can be more efficient by about a factor of ten, because we need not evaluate the Bessel-expansion and therefore need less calculations of the forward problem. Subsurface properties are derived for the two sets of field-data that were already presented in the first paper. One of them includes a pronounced low-velocity channel. For both we observe a remarkably good resolution of S-velocity down to the bedrock, which is found in 6 and 16 m depth, respectively. In both cases it would not be possible to infer the depth of bedrock from P-wave data alone. Synthetic seismograms calculated from the final model match the recorded waveforms surprisingly well, although no waveform fitting was applied. A subsequent waveform inversion becomes feasible with initial models taken from the results of this method. Finally it is shown by example, that conventional techniques of dispersion-curve fitting are likely to give misleading results when applied to our field-data.

Comparing the inspiral of irrotational and corotational binary neutron stars

We model the adiabatic inspiral of relativistic binary neutron stars in a quasi-equilibrium (QE) approximation, and compute the gravitational wavetrain from the late phase of the inspiral. We compare corotational and irrotational sequences and find a significant difference in the inspiral rate, which is almost entirely caused by differences in the binding energy. We also compare our results with those of a point-mass post-Newtonian calculation. We illustrate how the late inspiral wavetrain computed with our QE numerical scheme can be matched to the subsequent plunge and merger waveform calculated with a fully relativistic hydrodynamics code.

Oblique reflection of a nonlinear P wave from the boundary of an elastic half-space

This paper describes mode conversion effects and subsequent waveform distortion arising when a finite amplitude dilatational (P) wave that has already experienced nonlinear distortion is obliquely incident on a stress-free boundary of an isotropic elastic half-space. A two-term perturbation expansion is first employed to identify the dominant nonlinear effects. The understanding of wave interactions obtained from the perturbation analysis is then exploited to derive a successful solution using the method of characteristics for two-dimensional wave. It is shown that the incident and reflected P waves undergo nonlinear amplitude dispersion along their ray paths. The orientation of the rays for the reflected waves are time dependent, being governed by a modified form of Snell’s law, in which the phase speed incorporates the nonlinear correction for the association particle velocity. The reflection coefficients are shown to resemble those of linear theory, except for the dependence on the variable angles of reflection. The nonlinear propagation and reflection laws are solved to determine temporal waveforms for the reflected P and SV (vertically polarized shear) waves. This requires an iterative procedure in order to trace rays arriving at a specified field point at an arbitrary instant back to their source.

Spectral analysis of canine epicardial electrogram. Short-term variations in the frequency content induced by myocardial ischemia.

Power spectra of epicardial electrograms were studied in 13 anesthetized dogs subjected to occlusion of the left anterior descending coronary artery. Electrograms were obtained from a bipolar electrode placed on the epicardial surface of the left ventricle and recorded before and after coronary occlusion. After digitization, power spectra of the first and every 50th subsequent waveform were evaluated and compared with the power spectrum of the average waveform obtained from the baseline recording. In particular, we examined variations in relative power content in three frequency ranges: 150-250 Hz, previously shown to be directly affected by myocardial ischemia; 40-150 Hz, presumably corresponding to the fine notches and slurs on the body surface QRS; and 2-40 Hz, the low-frequency range. Apart from a mild initial rise during the first 50 heart beats, the power in the high-frequency range gradually decreased, reaching 5% of the control value at wave 500. The power in the mid-frequency range showed a monotonous decrease after the occlusion and reached 16% of the control. The power in the low-frequency range showed a gradual buildup to 140% of the control after 500 heart beats. Therefore, ischemia causes a shift of the high-frequency spectral components of the local electrographic waveform to lower frequencies. Our findings and the fact that body surface ECG is produced by spatial summation of local electric potentials over the different regions of the myocardial tissue may explain two previously described effects of acute myocardial ischemia on the body surface ECG complex. First, local loss of spectral components in the range 150-250 Hz may produce a zone of reduced amplitude within a QRS complex band-pass filtered in this range. Second, displacement of power in the frequency domain may suggest an explanation to the increased incidence of visible notches and slurs on the surface QRS complex, characteristic to various myocardial pathologies.

Statistical validation of new ventricular tachycardia detection schemes

The reliability of three new template matching techniques for detecting ventricular tachycardia (VT) using intracardiac waveform morphology was assessed using nonparametric tolerance intervals. These new techniques, the bin area method (BAM) , the normalized area of difference (NAD), and the derivative area method (DAM), were compared with correlation waveform analysis (CWA), an established method of waveform analysis. All four methods are independent of amplitude fluctuations and produced a bounded similarity measure, H, between 1 and 1. Like CWA, BAM and DAM are independent baseline changes. The BAM adds five consecutive sample points to form bins, removes the mean value, and compares each waveform with a similarly processed template. The NAD is identical to BAM, except that the mean is not subtracted. The DAM uses the zero-crossings of the derivative of the template to partition the template into areas. The same partitioning is then enforced on each subsequent waveform, and partitioned areas are compared to the corresponding areas in the template. The BAM and DAM required only

Cross-correlation technique for arrythmia detection using PR and PP intervals

A combined, cross-correlation procedure for computerized arrythmia detection is presented. It enables the classification of a wide range of arrythmia and is based upon the determination of both P and QRS waves. The electrocardiogram (ECG) signal is recorded from external leads, digitized (10 bits of resolution) at 1.28 kHz and processed with a CDC 6600 computer. A normal beat is stored as a reference signal and the onsets of the P wave and the QRS complex are measured for determining the PR, PP, and RR intervals. The combined cross-correlation procedure is carried out between the template and each successive ECG waveform over two time intervals, the first of which includes the P wave and the second, the QRS complex. Prior to this correlation procedure each of the ECG waveforms is filtered through a nonrecursive digital bandpass filter (10 and 100 Hz for low and high cutoff frequencies, respectively). The cross-correlation functions are then calculated by means of a cross spectrum and Fast Fourier Transform algorithm. The maximum value of the normalized cross-correlation function and the time shift providing that value are searched for, allowing (1) determination of the similarity between the present beat and the normal reference beat, allowing for the discrimination of abnormal P and QRS shapes; (2) accurate measurement of the RR, PP, and PR intervals of this subsequent waveform. Applying this method enables a reliable detection of a wide variety of arrythmia.