High-resolution Waveforms Research Articles

DAS for 2-D MASW imaging: a case study on the benefits of flexible subarray processing

SUMMARY Distributed acoustic sensing (DAS) is a relatively new technology for recording the propagation of seismic waves, with promising applications in both engineering and geophysics. DAS's ability to simultaneously collect high spatial resolution waveforms over long arrays suggests that it is well-suited for near-surface imaging applications such as 2-D multichannel analysis of surface waves (MASWs), which require, at a minimum, long, linear arrays of single-component receivers. The 2-D MASW method uses a large number of sensor subarrays deployed along a linear alignment to produce 1-D shear-wave velocity (VS) profiles beneath each subarray. The 1-D VS profiles are then combined to form a pseudo-2-D VS image beneath the entire linear alignment that can be used for the purpose of identifying and characterizing lateral variations in subsurface layering. Traditionally, 2-D MASW is conducted using arrays consisting of either 24 or 48 geophones. While additional receivers could easily be incorporated into the testing configuration, it is rare for researchers and practitioners to have access to greater numbers of seismographs and geophones. When a limited number of geophones are available for deployment, there is a need to pre-determine the geophone spacing and subarray length prior to field data acquisition. Studies examining how the choice of subarray geometry impacts the resulting pseudo-2-D VS cross-sections have been largely limited to synthetic data. In response, this study utilizes DAS data to examine the effects of using various subarray lengths by comparing pseudo-2-D VS cross-sections derived from active-source waveforms collected at a well-characterized field site. DAS is particularly useful for 2-D MASW applications because the subarray geometry does not need to be determined prior to field data acquisition. We organize the DAS waveforms into multiple sets of overlapping MASW subarrays of differing lengths, ranging from 11 to 47 m, along the same alignment, allowing for direct comparison of the derived pseudo-2-D VS results at the site. We show that the length of the individual MASW subarrays has a significant effect on the resulting VS cross-sections, including the resolved location of a strong impedance contrasts at our study site, and evaluate the results relative to ground truth from invasive testing. Our results suggest that the choice of subarray length is important and should be carefully chosen to meet project-specific goals. Furthermore, analysts may consider using multiple subarray geometries during the data processing stage, as is made possible by DAS, to properly evaluate the uncertainty of 2-D MASW results. This study demonstrates the potential of using DAS to collect data for 2-D MASW in a manner that is efficient and flexible, and can be easily scaled up for use with very long arrays.

고해상도 파형을 사용하는 소형레이다용 주파수합성기의 진동에 의한 위상잡음 악화 분석

High-resolution compact radar in high-speed aerial vehicles can be exposed to vibration, and vibration can degrade the phase noise of crystal oscillators and other RF circuits. In this study, a laboratory experiment was conducted similar to the actual operating environment using a coherent frequency synthesizer, superheterodyne receiver realized by commercial RF circuits, an optical delay line with a maximum 20 km range delay, and a vibrator. After applying vibration to the frequency synthesizer, the phase noise was measured at each node of the receiver using a signal source analyzer. Consequently, it was confirmed that the effect of the phase noise suppression function depends on the range delay. The suppression function not only degrades the phase noise by frequency multiplication, but also degrades it by vibration. On the other hand, it has been shown theoretically and experimentally that the lower limit can be suppressed by the suppression function is determined by the higher level of the phase noise generated from the oscillator to the intermediate frequency signal or thermal noise. Finally, a transceiver architecture that has the advantage of suppressing phase noise is proposed for compact radar using high-resolution waveforms.

RESIDUAL RECURRENT NEURAL NETWORK FOR SPEECH ENHANCEMENT.

Most current speech enhancement models use spectrogram features that require an expensive transformation and result in phase information loss. Previous work has overcome these issues by using convolutional networks to learn the temporal correlations across high-resolution waveforms. These models, however, are limited by memory-intensive dilated convolution and aliasing artifacts from upsampling. We introduce an end-to-end fully recurrent neural network for single-channel speech enhancement. The network structured as an hourglass-shape that can efficiently capture long-range temporal dependencies by reducing the features resolution without information loss. Also, we use residual connections to prevent gradient decay over layers and improve the model generalization. Experimental results show that our model outperforms state-of-the-art approaches in six quantitative evaluation metrics.

Large‐amplitude electric fields associated with bursty bulk flow braking in the Earth's plasma sheet

AbstractWe report observations of large‐amplitude (>50 mV/m) electric fields primarily associated with bursty bulk flow events. These electric fields reach ~500 mV/m, which are some of the largest electric fields (E) observed in the magnetotail. E not only has a larger than expected component perpendicular to the magnetic field but often has an intense parallel component. High time resolution waveforms reveal nonlinear structures such as electron phase‐space holes and double layers, which suggest strong field‐aligned currents or electron beams. Further examination shows that these large‐amplitude electric fields are almost always accompanied by enhanced magnetic field fluctuations. The electric fields are enhanced both above and below the ion cyclotron frequency, whereas the magnetic field fluctuations (δB) are mostly below the ion cyclotron frequency. Analysis of the wave spectra and the Poynting flux suggest that shear Alfvén waves are participating in these events. The Alfvén waves are revealed through the |δE|/|δB| ratio and strong field‐aligned Poynting flux, sometimes reaching nearly 1 mW/m2. This value, when mapped to the low‐altitude auroral region, exceeds 1 W/m2, which is an extreme value for that region. This Alfvénic activity is accompanied by evidence of compressional modes. These observations support a hypothesis whereby intense currents or electron beams, generated by kinetic Alfvénic waves that result from a turbulent cascade in bursty bulk flow (BBF) braking region, may be an energy source for large‐amplitude electric fields. The large‐amplitude electric fields may act as a dissipation mechanism and relax the highly tangled magnetic fields that result from the turbulence. Furthermore, these observations offer strong support that Alfvénic Poynting flux from the BBF braking region can be the energy source for Alfvénic aurora.

Radar observations of individual rain drops in the free atmosphere

Atmospheric remote sensing has played a pivotal role in the increasingly sophisticated representation of clouds in the numerical models used to assess global and regional climate change. This has been accomplished because the underlying bulk cloud properties can be derived from a statistical analysis of the returned microwave signals scattered by a diverse ensemble comprised of numerous cloud hydrometeors. A new Doppler radar, previously used to track small debris particles shed from the NASA space shuttle during launch, is shown to also have the capacity to detect individual cloud hydrometeors in the free atmosphere. Similar to the traces left behind on film by subatomic particles, larger cloud particles were observed to leave a well-defined radar signature (or streak), which could be analyzed to infer the underlying particle properties. We examine the unique radar and environmental conditions leading to the formation of the radar streaks and develop a theoretical framework which reveals the regulating role of the background radar reflectivity on their observed characteristics. This main expectation from theory is examined through an analysis of the drop properties inferred from radar and in situ aircraft measurements obtained in two contrasting regions of an observed multicellular storm system. The observations are placed in context of the parent storm circulation through the use of the radar's unique high-resolution waveforms, which allow the bulk and individual hydrometeor properties to be inferred at the same time.

Analysis of the turbulence observed in the outer cusp turbulent boundary layer

One of the prominent features of the cusp Turbulent Boundary Layer (TBL) is a persistent low frequency electromagnetic turbulence that extends from <1Hz up to the electron cyclotron frequency, accompanied by what appears to be purely electrostatic noise above this frequency range. The Plasma Wave Instrument onboard Polar obtained plasma wave measurements in the cusp TBL in the form of waveform captures simultaneously from 6 different sensors (3 each orthogonal electric and magnetic) in the frequency range 1 Hz up to 25 kHz. This allowed us to directly calculate the phase velocity from the measured ratio of |dE| to |dB| and compare it to theoretical values for various modes. Using this technique, we have gained some insight into the mode of the electromagnetic turbulence that extends in frequency from ∼ 1 Hz up to the electron cyclotron frequency (several hundred Hz to a few kHz) in the TBL. The whistler and kinetic Alfvén wave modes are discussed as the possible modes of this turbulence. By analyzing the high time resolution waveforms, we isolate and identify some of these modes. The electrostatic turbulence above the electron cyclotron frequency is associated with pulses and quasi-sinusoidal waveforms observed in the measured time series. These do not fit any known mode, although work is continuing in this area to show that some of them may be associated with electron holes or with downshifted Langmuir waves produced through a two-stream instability.

Bistatic reverberation from an ocean ridge

Bistatic reverberation from a highly lineated ridge in the mid-Atlantic was measured by two research vessels during the ARSRP Main Acoustics Experiment of 1993. Each vessel used a vertical source array and a horizontal receiving array to collect bistatic and monostatic data over a full range of incident and scattered angles with respect to the ridge axis. High-resolution bathymetry reveals that the ridge protrudes above conjugate depth for roughly 50 km, and is characterized by a number of extended scarps running parallel to the ridge axis. Sufficient excess depth exists around the ridge to isolate it with waterborne propagation paths from 1/2 convergence zone (CZ) range (∼33 km) where low-frequency returns register precisely with steep scarps. Significant variations in received level from a given scarp occur as a function of incident and scattered angle. The most prominent returns register with scarps facing the receiving array. Registration between returns from high-resolution waveforms (∼10 m in range) and fine-scale bathymetry is used to elucidate the fundamental scattering process. Comparisons with monostatic measurements of the same ridge from 1/2 CZ and 2 1/2 CZ indicate how fine-scale structure from a given scarp is smeared in long-range reverberation.

Radar sea-clutter at low grazing angles

An analysis of sea-clutter data obtained in conditions typical of sea state 2 at X-, S-, L- bands, UHF and VHF is presented. Results show that sea-clutter exhibits very different spectral characteristics at higher frequencies compared to those at low frequencies. Experimental sea-clutter coefficients as a function of grazing angle at various frequency bands were obtained for upswell and cross-swell conditions. Most sea-clutter models use the radar look direction relative to the wind direction as an input parameter (e.g. upwind, crosswind, etc.). Hence a direct comparison of sea-clutter magnitude with values calculated from models under different wind conditions was not made. Instead, these results were contrasted with those calculated from two sea-clutter (the GIT and Sittrop) models, assuming an upwind and a crosswind condition. The data were quite dissimilar in the low-grazing-angle regions to those predicted by the two models. Further research on sea-clutter in the low-grazing-angle regions is needed to ascertain and explain the observed differences. Sea-clutter amplitude statistics were found to fit the K-distribution best in the important low probability of false alarm region. The distribution approached that of a Rayleigh model for vertical polarisation and low-resolution waveforms. Non-Rayleigh statistics were frequently observed even for relatively low resolution and vertical polarisation waveforms. For horizontal polarisation and/or high resolution waveforms, the statistics approached those of a lognormal model. The statistical results provided further evidence that the K-distribution can serve as a limiting distribution for sea-clutter. The results of this paper are pertinent to shipboard radar applications where the grazing angle is low. They also provide additional information for sea-clutter modelling in the low-angle region, particu larly at the lower frequency bands.

Computer simulations—an alternative to the use of animals in teaching?

Two computer simulation programs, ‘Nerve physiology’ and ‘Frog heart’, are described in relation to their use as practical alternatives to the use of animals in class experiments. The programs, which are highly interactive, utilize actual experimental data to generate high resolution waveforms of compound nerve action potentials and heart contractions, identical to those which would normally be recorded. These are presented on either an oscilloscope-like display screen or on a continuously scrolling display to simulate a chart recorder. Students thus work through the programs at their own pace and take measurements directly from the monitor screen, as they would in an actual experiment. The programs, which are written for the BBC microcomputer and aimed at A-level students and above, contain introductory text and student assignments and come complete with a comprehensive instruction manual and tutor's notes. The potential of these programs to provide a realistic alternative to conventional laboratory experiments using animals is discussed in relation to the major teaching objectives of such teaching methods.

High-resolution digital sinewave generation

A technique is described for the generation of high-resolution sinusoidal waveforms in digital form, using an economical look-up-table method. Table storage requirements are typically an order of magnitude lower than for conventional look-up-table methods, but are achieved without the need for complex interpolation algorithms.

Decision-directed demodulation of frequency-shift-keyed signals

Coherent demodulation of continuous-phase frequency-shift keyed (f.s.k.) signals is achieved by generating the receiver waveforms according to previous demodulator decisions. Convolution of phase information in the signaling waveform gives rise to the possibility of an error-detection process. A particularly simple process can be implemented for f.s.k. signals with deviation ratios in the range 0.7–0.8. If the error-detection capability is combined with a request for retransmission, the undetected error performance of the system can approach the optimum for binary-data transmission. Analysis of the error-detecting performance of the system, and simulation results for demodulators employing high-resolution waveforms and binary (sliced) waveforms, are given.