Correlation Waveforms Research Articles

The impact of ambient noise sources in subsurface models estimated from noise correlation waveforms

SUMMARY Cross-correlations of seismic ambient noise are frequently used to image Earth structure. Usually, tomographic studies assume that noise sources are uniformly distributed and interpret noise correlations as empirical Green’s functions. However, previous research suggests that this assumption can introduce errors in the estimated models, especially when noise correlation waveforms are inverted. In this paper, we investigate changes in subsurface models inferred from noise correlation waveforms depending on whether the noise source distribution is considered to be uniform. To this end, we set up numerical experiments that mimic a tomographic study in Southern California exploiting ambient noise generated in the Pacific Ocean. Our results show that if the distribution of noise sources is deemed uniform instead of being numerically represented in the wave simulations, the misfit of the estimated models increases. In our experiments, the model misfit increase ranges between 5 and 21 per cent, depending on the heterogeneity of the noise source distribution. This indicates that assuming uniform noise sources introduces source-dependent model errors. Since the location of noise sources may change over time, these errors are also time-dependent. In order to mitigate these errors, it is necessary to account for the noise source distribution. The spatial extent to which noise sources must be considered depends on the propagation distance of the ambient noise wavefield. If only sources near the study area are considered, model errors may arise.

Simulating correlation waveforms of random modulated continuous wave LIDAR

Advanced light detecting and ranging (LIDAR) sensors are the primary sensing modality for autonomous vehicles and are seeing increasing adoption in consumer and commercial vehicles for robust advanced driver assist systems. LIDAR returns from the environment are typically predicted using elastic LIDAR models, which can help emulate the performance of LIDAR sensors in environments with multiple returns or heavy obscurants. We derive the first elastic LIDAR model for a random modulated continuous wave LIDAR system using a homodyne receiver and show good agreement with experimental measurements.

Online Failure Diagnostic in Full-Bridge Module for Optimum Setup of an IGBT-Based Multilevel Inverter

An online failure diagnostic test is essential to ensure the robustness and reliability of high-powered systems. Furthermore, the overall design must comprise diagnostic strategies to detect in-service and high-powered module defects. This paper describes the critical failure mechanisms––cross-conduction, inductive avalanche, second turn-on, VS-undershoot, inrush current, and thermal runaway––that directly affect insulated gate bipolar transistor (IGBT) operation. The constructed inverter contains 18 transformer-based taps (six per phase); however, this work studied a single tap (IGBT-based full-bridge module) to understand the reasons for failure and the routes to mitigate them. Moreover, a cost-effective solution using the IR2127STRPBF driver circuit was implemented to reduce the probability of thermal runaway in case of overcurrent, short-circuit, or avalanche events. For this reason, the electrical current state was adjusted using an FPGA digital resource to perform dynamic PWM control signals. The obtained correlation waveforms are valuable for verifying diagnostic data at the design stage to emphasize the significance of evading premature failure events. The comprehensive study on failure diagnosis enabled successful design strategies to construct a robust 45 kVA three-phase multilevel inverter for a 22 kW eolic-photovoltaic generation plant.

Ultracompact autocorrelator with pulse-width-range switch function integrated on a silicon photonic chip

We demonstrated a fully integrated autocorrelator that can switch the measurable range of the pulse width. The autocorrelator was fabricated on a chip using a silicon photonics process. Autocorrelation was achieved using a two-photon absorption photodiode (TPA-PD) array embedded on a Si waveguide. We integrated multiple TPA-PD array waveguides on a chip to provide multiple pulse width ranges for the extension of the range. The switching of the waveguides was carried out using an integrated optical path switch. We fabricated two parallel waveguides with lengths of 110 and 330 μm, and a path switch consisting of a thermally controlled Mach–Zehnder interferometer. Correlation waveforms of picosecond pulses were appropriately obtained, and range switching was successfully performed. We show that the proposed device is scalable for the increase in the number of autocorrelators and the number of pulse-width ranges.

Optimal processing for seismic noise correlations

SUMMARYA wide spectrum of processing schemes is commonly applied during the calculation of seismic noise correlations. This is intended to suppress large-amplitude transient and monochromatic signals, to accelerate convergence of the correlation process or to modify raw correlations into more plausible approximations of interstation Green’s functions. Many processing schemes, such as one-bit normalization or various other nonlinear normalizations, clearly break the linear physics of seismic wave propagation. This naturally raises the question: To what extent are the resulting noise correlations physically meaningful quantities? In this contribution, we demonstrate that commonly applied processing methods may indeed introduce an unphysical component into noise correlations. This affects not only noise correlation amplitudes but also, to a lesser extent, time-dependent phase information. The profound consequences are that most processed correlations cannot be entirely explained by any combination of Earth structure and noise sources, and that inversion results may thus be polluted. The positive component of our analysis is a new and easily applicable method that allows us to modify any existing processing such that it becomes optimal in the sense of (1) completely avoiding the unphysical component while (2) approximating the result of the original processing as closely as possible. The resulting optimal schemes can be derived purely on the basis of observed noise, without any knowledge of or assumptions on the nature of noise sources. In addition to the theoretical analysis, we present illustrative real-data examples from the Irish National Seismic Network and the Lost Hills array in Central California. We anticipate that optimal processing schemes may be most useful in applications that exploit complete correlation waveforms, amplitudes and weak arrivals, or small (time-dependent) phase shifts.

High-performance on-chip autocorrelator using a rib waveguide loaded with two-photon absorption diodes.

We report an on-chip autocorrelator comprising a Si rib waveguide and two-photon-absorption photodiodes in which two pulses simultaneously propagate in opposite directions, and their correlation is acquired. The rib waveguide's broadband guiding mode and moderately low group index allow the device to operate over a wide wavelength range with high resolution. We confirm that the waveguide has a transmission band of at least 1300-1630nm and observed correlation waveforms for pulses of the order of several hundred femtoseconds to several picoseconds, with a range of at least the entire C band region.

Experimental Results of an X-Band PARIS Receiver Using Digital Satellite TV Opportunity Signals Scattered on the Sea Surface

A bistatic radar at X-band (11 GHz) using opportunity signals (Passive Reflectometry and Interferometry System (PARIS) concept implementing the interferometric technique) has been developed and tested in a coastal-based experimental setup. Digital satellite TV signals broadcast from geostationary orbit have been used as sources of opportunity, and correlation waveforms have been collected at different receiver bandwidths. Calibration algorithms to compensate instrumental errors have improved our observables and allowed us to obtain delay measurements with a precision of about 10 cm for wind speeds in the range of 3 m/s. A statistical analysis of the measurement noise is used to determine the quality of the obtained observables. This paper also discusses the opportunities of the PARIS concept applied at higher frequency band and with stronger signals.

Source and processing effects on noise correlations

Abstract We quantify the effects of spatially heterogeneous noise sources and seismic processing on noise correlation measurements and their sensitivity to Earth structure. Using numerical wavefield simulations and adjoint techniques, we calculate interstation correlations and sensitivity kernels for arbitrarily distributed noise sources where—as in the real Earth—different frequencies are generated in different locations. While both heterogeneous noise sources and processing can have profound effects on noise correlation waveforms, narrow-band traveltime measurements are less affected, in accord with previous analytical studies. Sensitivities to Earth structure depend strongly on the source distribution and the processing scheme, and they reveal exotic frequency dependencies that go beyond the well-known frequency scaling of the Fresnel zone width. Our results indicate that modern full waveform inversion applied to noise correlations is not possible unless one of the following measures is taken: (1) properly account for noise source distribution and processing, or (2) limit measurements to phase or time shifts in narrow frequency bands. Failure to do so can lead to erroneous misfits, tomographic artefacts, and reduced resolution.

Partially Correlation Waveforms Design and Processing for Co-located MIMO Radar

Partially Correlation Waveforms Design and Processing for Co-located MIMO Radar

Two-photon-absorption photodiodes in Si photonic-crystal slow-light waveguides

We demonstrate two-photon-absorption photodiodes in Si photonic-crystal waveguides, which shows wideband low-dispersion slow light. The device was fabricated on SOI substrate by CMOS-compatible process. The responsivity was improved by higher group indexes of slow light up to 0.052 A/W for pulses at wavelengths around 1550 nm with a 2.7 ps width and sub-watt peak powers. We applied this device to an optical correlator and dispersion detector. In the former, the correlation waveforms of 0.7−10 ps pulses were observed with small errors. In the latter, photocurrents inversely proportional to the pulse width were detected.

Airborne GNSS-R Wind Retrievals Using Delay–Doppler Maps

Global navigation satellite system (GNSS) reflectometry has emerged recently as a promising remote sensing tool to retrieve various geophysical parameters of the Earth's surface. GNSS-reflected signals, after being received and processed by the airborne or spaceborne receiver, are available as delay correlation waveforms or as delay-Doppler maps (DDMs). In the case of a rough ocean surface, those characteristics can be related to the rms of the L-band limited slopes of the surface waves and, from there, to the surface wind speed. The raw GNSS-reflected signal can be either processed in real time by the receiver or recorded and stored on board and postprocessed in a laboratory. The latter approach leveraging a software receiver allows more flexibility while processing the raw data. This work analyzes DDMs obtained as a result of processing of the data collected by the Global Positioning System (GPS) data logger/software receiver on board the National Oceanic and Atmospheric Administration Gulfstream-IV jet aircraft. Thereafter, the DDMs were used to retrieve surface wind speed employing several different metrics that characterize the DDM extent in the Doppler frequency-delay domain. In contrast to previous works in which winds have been retrieved by fitting the theoretically modeled curves into measured correlation waveforms, here, we do not rely on any model for the determination. Instead, the approach is based on a linear regression between DDM observables and the wind speeds obtained in simultaneous GPS dropsonde measurements.

Artifact Reduction in Tomographic Images for Nondestructive Testing of Square Billets Using Ultrasonic Computerized Tomography

To reduce the intensity of artifacts in visualized images for detecting defects in square billets by ultrasonic computerized tomography, we propose a method, which utilizes the crosspower-spectrum phase (CSP) function between a measurement signal and a reference signal, to measure the time-of-flight (TOF) difference. The artifacts arise owing to measurement errors of the TOF difference, which are caused by the effects of propagation characteristics and side lobes in correlation waveforms. The effects of propagation characteristics may be reduced using the reference signal, which contains the same characteristics as those of the measurement signal. The side lobes in correlation waveforms may also be decreased using the CSP method. The results obtained by experiment suggest that the intensity of artifacts was reduced because the proposed method reduced the side lobe level by about -14 dB and the number of measurement errors. The visualized images showed that the defects whose diameter was more than 2.0 mm were detected using the input signal whose wavelength was in the range from 2 to 6 mm.

Fabrication of high-speed GaAs photoconductive pulse generators and sampling gates by ion implantation

An ion-implantation technique used to create high-speed photoconductive devices in semi-insulating gallium arsenide (GaAs) is described. The effects of electrical contacts, GaAs substrate material, and various implant parameters on device performance are presented. The best measured performance characteristics of sampled (correlation) waveforms are: full-width-at-half-maximum of 4.5 ps, rise time (10 to 90% of full amplitude) of 3.2 ps, and signal-to-noise ratio of approximately 50 dB (integration time is 10 ms). >