Source-dependent bias of sferics in magnetotelluric responses

Geophysics ◽  
2018 ◽  
Vol 83 (3) ◽  
pp. E161-E171 ◽  
Author(s):  
Lachlan Hennessy ◽  
James Macnae
Keyword(s):  
Time Series ◽  
Apparent Resistivity ◽  
Propagation Model ◽  
Lightning Strike ◽  
Source Information ◽  
Peak Current ◽  
Amplitude Data ◽  
Lower Amplitude ◽  
Bounded Influence ◽  
Normally Distributed

The predominant signals of audio-frequency magnetotellurics (AMT) are called sferics, and they are generated by global lightning activity. When sferic signals are small or infrequent, measurement noise in electric and magnetic fields causes errors in estimated apparent resistivity and phase curves, leading to great model uncertainty. To reduce bias in apparent resistivity and phase, we use a global propagation model to link sferic signals in time series AMT data with commercially available lightning source information including strike time, location, and peak current. We then investigate relationships between lightning strike location, peak current, and the quality of the estimated apparent resistivity and phase curves using the bounded influence remote reference processing code. We use two empirical approaches to preprocessing time-series AMT data before estimation of apparent resistivity and phase: stitching and stacking (averaging). We find that for single-site AMT data, bias can be reduced by processing sferics from the closest and most powerful lightning strikes and omitting the lower amplitude signal-deficient segments in between. We hypothesized that bias can be further reduced by stacking sferics on the assumptions that lightning dipole moments are log-normally distributed whereas the superposed noise is normally distributed. Due to interference between dissimilar sferic waveforms, we tested a hybrid stitching-stacking approached based on clustering sferics using a wavelet-based waveform similarity algorithm. Our results indicate that the best approach to reduce bias was to stitch the closest and highest amplitude data.

scholarly journals Statistical and Criticality Analysis of the Lower Ionosphere Prior to the 30 October 2020 Samos (Greece) Earthquake (M6.9), Based on VLF Electromagnetic Propagation Data as Recorded by a New VLF/LF Receiver Installed in Athens (Greece)

Entropy ◽  
2021 ◽  
Vol 23 (6) ◽  
pp. 676
Author(s):  
Dimitrios Z. Politis ◽  
Stelios M. Potirakis ◽  
Yiannis F. Contoyiannis ◽  
Sagardweep Biswas ◽  
Sudipta Sasmal ◽  
...  
Keyword(s):  
Time Series ◽  
Fresnel Zone ◽  
Lower Ionosphere ◽  
Propagation Path ◽  
Northern Coast ◽  
Analysis Method ◽  
Ionospheric Propagation ◽  
Amplitude Data ◽  
Natural Time ◽  
Criticality Analysis

In this work we present the statistical and criticality analysis of the very low frequency (VLF) sub-ionospheric propagation data recorded by a VLF/LF radio receiver which has recently been established at the University of West Attica in Athens (Greece). We investigate a very recent, strong (M6.9), and shallow earthquake (EQ) that occurred on 30 October 2020, very close to the northern coast of the island of Samos (Greece). We focus on the reception data from two VLF transmitters, located in Turkey and Israel, on the basis that the EQ’s epicenter was located within or very close to the 5th Fresnel zone, respectively, of the corresponding sub-ionospheric propagation path. Firstly, we employed in our study the conventional analyses known as the nighttime fluctuation method (NFM) and the terminator time method (TTM), aiming to reveal any statistical anomalies prior to the EQ’s occurrence. These analyses revealed statistical anomalies in the studied sub-ionospheric propagation paths within ~2 weeks and a few days before the EQ’s occurrence. Secondly, we performed criticality analysis using two well-established complex systems’ time series analysis methods—the natural time (NT) analysis method, and the method of critical fluctuations (MCF). The NT analysis method was applied to the VLF propagation quantities of the NFM, revealing criticality indications over a period of ~2 weeks prior to the Samos EQ, whereas MCF was applied to the raw receiver amplitude data, uncovering the time excerpts of the analyzed time series that present criticality which were closest before the Samos EQ. Interestingly, power-law indications were also found shortly after the EQ’s occurrence. However, it is shown that these do not correspond to criticality related to EQ preparation processes. Finally, it is noted that no other complex space-sourced or geophysical phenomenon that could disturb the lower ionosphere did occur during the studied time period or close after, corroborating the view that our results prior to the Samos EQ are likely related to this mainshock.

scholarly journals Effect of indirect lightning strike to 66kV transmission line in Java-Bali system

2018 ◽  
Vol 197 ◽  
pp. 11001
Author(s):  
Aristo Adi Kusuma ◽  
Putu Agus Aditya Pramana ◽  
Brian Bramantyo S.D.A. Harsono ◽  
Buyung Sofiarto Munir
Keyword(s):  
Transmission Line ◽  
Transmission Lines ◽  
Lightning Strike ◽  
Actual Condition ◽  
Peak Current ◽  
Lightning Strikes ◽  
Detection Systems ◽  
Current Magnitude ◽  
Lightning Detection ◽  
Insulator Flashover

Based on Java-Bali grid disturbance data, the 66kV transmission lines that is close to or intersect with 150kV or 500kV transmission line is often experienced earth fault due to insulator flashover. The insulator flashover can be caused by indirect lightning strike since lightning strikes tend to strike higher structure. Therefore, this paper will determine the effect of indirect lightning strike on 150kV or 500kV transmission line to 66kV transmission line by modeling and simulation using application of transient analysis. Variation of lightning peak current magnitude and gap between 66kV transmission line and transmission line with higher voltage is performed during simulation. The range of peak current magnitude follows the data from lightning detection systems, while the value of gap follows the data from actual condition. It is found that higher current peak and closer gap will cause higher transient overvoltage on insulator of 66kV transmission line thus insulator flashover may occur more frequent. Addition of earth wire on 66kV transmission line and gap between each transmission by organizing the sag of conductor can be performed to minimize the insulator flashover.

Inter- and intra-hemispheric spatiotemporal organization of spontaneous electrocortical oscillations

1996 ◽  
Vol 76 (1) ◽  
pp. 423-437 ◽  
Author(s):  
K. D. MacDonald ◽  
B. Brett ◽  
D. S. Barth
Keyword(s):  
Time Series ◽  
Somatosensory Cortex ◽  
Phase Locking ◽  
Sensory Information ◽  
Gamma Oscillations ◽  
Sensory Cortex ◽  
Electrode Arrays ◽  
Sharp Wave ◽  
Lower Amplitude ◽  
The Right

1. Two 64-channel epipial electrode arrays were positioned on homologous locations of the right and left hemisphere, covering most of primary and secondary auditory and somatosensory cortex in eight lightly anesthetized rats. Array placement was verified with the use of cytochrome oxidase histochemistry. 2. Middle-latency auditory and somatosensory evoked potentials (MAEPs and MSEPs, respectively) and spontaneous oscillations in the frequency range of 20-40 Hz (gamma oscillations) were recorded and found to be spatially constrained to regions of granular cortex, suggesting that both phenomena are closely associated with sensory information processing. 3. The MAEP and MSEP consisted of an initial biphasic sharp wave in primary auditory and somatosensory cortex, respectively, and a similar biphasic sharp wave occurred approximately 4-8 ms later in secondary sensory cortex of the given modality. Averaged gamma oscillations also revealed asynchronous activation of sensory cortex, but with a shorter 2-ms delay between oscillations in primary and secondary regions. Although the long latency shift of the MAEP and MSEP may be due in part to asynchronous activation of parallel thalamocortical projections to primary and secondary sensory cortex, the much shorter shift of gamma oscillations in a given modality is consistent with intracortical coupling of these regions. 4. Gamma oscillations occurred independently in auditory and somatosensory cortex within a given hemisphere. Furthermore, time series averaging revealed that there was no phase-locking of oscillations between the sensory modalities. 5. Gamma oscillations were loosely coupled between hemispheres; oscillations occurring in auditory or somatosensory cortex of one hemisphere were often associated with lower-amplitude oscillations in homologous contralateral sensory cortex. Yet, the fact that time series averaging revealed no interhemispheric phase-locking suggests that the corpus callosum may not coordinate the bilateral gamma oscillations, and that a thalamic modulatory influence may be involved.

scholarly journals Deviations from normal distributions in artificial and real time series: a false positive prescription

2019 ◽  
Vol 489 (2) ◽  
pp. 2117-2129 ◽  
Author(s):  
Paul J Morris ◽  
Nachiketa Chakraborty ◽  
Garret Cotter
Keyword(s):  
Time Series ◽  
False Positive ◽  
Functional Form ◽  
False Positive Rate ◽  
Light Curves ◽  
Artificial Light ◽  
Positive Rate ◽  
Functional Forms ◽  
Distribution Of Power ◽  
Normally Distributed

ABSTRACT Time-series analysis allows for the determination of the Power Spectral Density (PSD) and Probability Density Function (PDF) for astrophysical sources. The former of these illustrates the distribution of power at various time-scales, typically taking a power-law form, while the latter characterizes the distribution of the underlying stochastic physical processes, with Gaussian and lognormal functional forms both physically motivated. In this paper, we use artificial time series generated using the prescription of Timmer & Koenig to investigate connections between the PDF and PSD. PDFs calculated for these artificial light curves are less likely to be well described by a Gaussian functional form for steep (Γ⪆1) PSD indices due to weak non-stationarity. Using the Fermi LAT monthly light curve of the blazar PKS2155-304 as an example, we prescribe and calculate a false positive rate that indicates how likely the PDF is to be attributed an incorrect functional form. Here, we generate large numbers of artificial light curves with intrinsically normally distributed PDFs and with statistical properties consistent with observations. These are used to evaluate the probabilities that either Gaussian or lognormal functional forms better describe the PDF. We use this prescription to show that PKS2155-304 requires a high prior probability of having a normally distributed PDF, $P(\rm {G})~$ ≥ 0.82, for the calculated PDF to prefer a Gaussian functional form over a lognormal. We present possible choices of prior and evaluate the probability that PKS2155-304 has a lognormally distributed PDF for each.

scholarly journals A SIMPLE EFFICIENT INSTRUMENTAL VARIABLE ESTIMATOR FOR PANEL AR(p) MODELS WHEN BOTHNANDTARE LARGE

2009 ◽  
Vol 25 (3) ◽  
pp. 873-890 ◽  
Author(s):  
Kazuhiko Hayakawa
Keyword(s):  
Time Series ◽  
Lower Bound ◽  
Asymptotic Distribution ◽  
Cross Section ◽  
Simulation Study ◽  
Instrumental Variable ◽  
Asymptotic Variance ◽  
The Cross ◽  
Instrumental Variable Estimator ◽  
Normally Distributed

In this paper, we show that for panel AR(p) models, an instrumental variable (IV) estimator with instruments deviated from past means has the same asymptotic distribution as the infeasible optimal IV estimator when bothNandT, the dimensions of the cross section and time series, are large. If we assume that the errors are normally distributed, the asymptotic variance of the proposed IV estimator is shown to attain the lower bound when bothNandTare large. A simulation study is conducted to assess the estimator.

scholarly journals Prediksi Harga Beras Sultan dan Membramo di Kota Manado dengan Menggunakan Model ARIMA

Jurnal MIPA ◽  
2013 ◽  
Vol 2 (1) ◽  
pp. 1
Author(s):  
Varra Wuwung ◽  
Nelson Nainggolan ◽  
Marline Paendong
Keyword(s):  
Time Series ◽  
P Value ◽  
Arima Models ◽  
Result Show ◽  
Normally Distributed

Pada makalah ini diuraikan model ARIMA dari harga beras di kota Manado yang meliputi beras Sultan dan beras Membramo. Data yang diamati adalah data bulanan dari Januari 2007 sampai dengan Maret 2012. Hasil analisis time series menunjukan bahwa untuk beras Sultan diperoleh model ARIMA(1,1,1) dan beras Membramo diperoleh model ARIMA(1,1,0). Hasil diagnosis menunjukan bahwa galat dari model untuk beras Sultan dan beras Membramo sudah berdistribusi normal dengan p-value lebih dari 0,05 yaitu masing-masing 0,15 dan 0,07. Prediksi harga beras untuk tiga periode kedepan untuk beras Sultan berkisar antara Rp. 8.287 sampai Rp. 8.389, dan beras Membramo berkisar antara Rp. 8.482 sampai Rp. 8.593.This paper described ARIMA models of the rice prince in Manado, that is sultan and membramo rice. The observed data is monthly from January-2007 to March-2012. The result show that the models for sultan is ARIMA(1,1,1) and for membramo is ARIMA(1,1,0). The diagnosis results show that the residuals of the models for sultan and membramo is normally distributed with a p-value more than 0,05, that is 0,15 and 0,07 respectively. The prediction of price for the next three periods for sultan from Rp. 8.287,30 to Rp. 8.389,92 and for membramo from Rp. 8.482 to Rp. 8.593.

EXPERIMENTAL STUDY OF LIGHTNING DAMAGE RESISTANCE OF UNPROTECTED AND PROTECTED STITCHED WARP-KNIT CARBON-EPOXY COMPOSITES

2021 ◽  
Author(s):  
DOUNIA BOUSHAB ◽  
KHARI HARRISON ◽  
ANIKET MOTE ◽  
THOMAS E. LACY, JR. ◽  
CHARLES U. PITTMAN, JR.
Keyword(s):  
Carbon Fiber ◽  
Composite Structures ◽  
Close Contact ◽  
Lightning Strike ◽  
Fiber Direction ◽  
Damage Resistance ◽  
Peak Current ◽  
Lightning Current ◽  
Protection Systems ◽  
Carbon Fiber Paper

In-flight lightning strike damage to aircraft composite structures may compromise aircraft airworthiness. Hence, it is crucial to incorporate adequate protection systems to mitigate the lightning current. Most lightning strike protection (LSP) techniques involve bonding a metallic conductive layer to the cured laminate exterior. In this study, a novel LSP integration technique was used to develop unitized panels with three different protection layers: pitch carbon fiber paper (PCFP), graphene paper (GP), and copper mesh (CM). Each LSP layer was overlaid on through-the-thickness VectranHT stitched warp-knit multiaxial dry carbon fabric stacks, resin-infused, and oven-cured. A series of lightning strike tests to protected and unprotected stitched carbonepoxy laminates were conducted at a nominal peak current of 150 kA. Visual inspection was used to investigate each panel’s lightning damage resistance, understand the damage mechanisms, and evaluate the surface morphology at the strike locations. The size and severity of the damaged area depended on several factors: the outermost ply fiber direction, the strike location relative to VectranHT and polyester knitting treads, the lightning peak current, and the conductivity of the protection layers. The CM and GP protection layers effectively dissipated the lightning current in-plane and showed no damage to the underlying composite. The degree of lightning damage on an unprotected laminate was significantly lower than for a similar panel with PCFP protection. The presence of VectranHT structural stitches and polyester warp-knitting threads profoundly reduced the size and severity of lightning damage. These threads appeared to promote close contact between adjacent carbon fiber tows, resulting in better in-plane and through-thickness electrical/thermal conductivities and reduced lightning damage.

Secular variation in the global carbon cycle during the Mesozoic: a new composite carbonate δ13C reference curve calibrated to GTS2020

2020 ◽  
Author(s):  
Ian Jarvis
Keyword(s):  
Time Series ◽  
Carbon Cycle ◽  
Carbon Isotope ◽  
Global Carbon Cycle ◽  
Published Data ◽  
Time Interval ◽  
Reference Curve ◽  
Carbonate Sediments ◽  
Lower Amplitude ◽  
Global Carbon

<p>A new δ<sup>13</sup>C reference curve for the Mesozoic is presented. This has been constructed using in excess of 10,000 published analyses of bulk carbonate sediments extracted from published literature.  Available data from sections world-wide were compiled for each stage and the stratigraphic trends visually compared.  Data sets used to construct the composite reference curve were those offering patterns that are consistent with other sections and offer the highest stratigraphic resolution (close sample spacing), constrained by biostratigraphic first appearance (FAD) and last appearance datum (LAD) levels, magnetostratigraphy, radiometric dates and cyclostratigraphy.  Preference was given to time series that showed the least scatter.  Pelagic carbonates proved most suitable for these purposes but data from hemipelagic and shallow-water carbonate sections were included where necessary. </p><p>Age calibration was achieved using stage boundary ages, biostratigraphic FAD and LAD datums levels, and chron boundary ages derived from the new GTS2020 timescale.  Where possible, data from multiple authors and/or multiple stratigraphic sections were age-calibrated and interleaved to generate composite profiles for each time interval.  Data from individual stages were spliced together with offsets being avoided wherever possible; minor offsets in values were corrected where necessary to generate a continuous smooth time series.  The uneven geographical spread of published data and suitable lithofacies has resulted in source information being derived from different regions for different time intervals.  For example, the Early – Middle Triassic curve is constructed from eastern Paleotethys sections (South China), the Jurassic and Early Cretaceous curves principally from Tethyan areas of Europe and North Africa (Morocco, Portugal, southern France, Switzerland, northern Italy), and the Late Cretaceous curve from the Boreal Sea of northern Europe (England, Denmark).  The global significance of the resulting curves requires further testing.</p><p>The stratigraphic positions and recalibrated ages of positive and negative δ<sup>13</sup>C excursions that define carbon isotope events (CIEs) are presented.  These reflect major perturbation in the global carbon cycle.  Changes in the production and burial of organic matter on land and in the oceans, plus the balance between carbonate versus organic carbon deposition, are the principal mechanisms driving the observed long-term stratigraphic trends and short-term excursions.  These are linked to palaeogeographic and palaeoceanographic change, with climate and sea-level fluctuations driven by orbital forcing, tectonics, and volcanic events.  The emplacement of large igneous plateaus (LIPs) and associated volcanism likely played a major role in driving many of the palaeoenvironmental perturbations reflected in the carbon isotope stratigraphy. </p><p>The most prominent CIEs characterise the Early Triassic with amplitudes exceeding 5‰ δ<sup>13</sup>C<sub>carb </sub>(VPDB), with other notable excursions in the mid-Carnian, mid-Norian and Rhaetian.  The Toarcian negative CIEs are the stand-out feature of the Jurassic, but multiple lower amplitude CIEs occur throughout, notably in the Hettangian, Bajocian Callovian and Oxfordian.  The most prominent Cretaceous CIEs in the Valanginian, Aptian and at the Cenomanian/Turonian boundary are linked to Oceanic Anoxic Events.</p>

Time-dependent variations of transcutaneous PCO2 level in normal subjects

1988 ◽  
Vol 64 (5) ◽  
pp. 1858-1863 ◽  
Author(s):  
D. Pilsbury ◽  
G. Hibbert
Keyword(s):  
Time Series ◽  
Healthy Adult ◽  
Normal Subjects ◽  
Time Dependent ◽  
Pco2 Level ◽  
Arterial Pco2 ◽  
Mean Values ◽  
Respiratory Apparatus ◽  
Apparent Association ◽  
Normally Distributed

Transcutaneous PCO2 (PtcCO2), which is linearly related to arterial PCO2, was continuously recorded in healthy, adult, normal volunteers for 8-h periods. Recording this variable with the apparatus employed permits measurement of changes in the level of ventilation while subjects are freely ambulant and unencumbered by invasive and flow-resistive respiratory apparatus. The time series obtained exhibited marked periodicities. The frequencies and amplitudes varied between subjects. Peak-to-peak variation was 10–20% of mean values. There was no apparent association between fluctuations in PCO2 and activity other than formal exercise. Visual inspection of the time series and preliminary statistical analysis of digitally converted data suggested that the time-dependent changes of PtcCO2 were normally distributed. However, more rigorous statistical examination revealed that in no case was PtcCO2 actually normally distributed.

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