When Clocks Are Not Working: OBS Time Correction

2020 ◽  
Vol 91 (4) ◽  
pp. 2247-2258
Author(s):  
Karina Loviknes ◽  
Zeinab Jeddi ◽  
Lars Ottemöller ◽  
Thibaut Barreyre
Keyword(s):  
Ambient Noise ◽  
Internal Clock ◽  
P Wave ◽  
Small Scale ◽  
Ocean Bottom ◽  
Arrival Times ◽  
Before And After ◽  
Time Drift ◽  
Corrected Data ◽  
Time Correction

Abstract Ocean-bottom seismographs (OBSs) are used to obtain seismic recordings offshore and are an increasingly important tool for investigating the globe. However, because OBS data cannot be time stamped using Global Positioning System (GPS) during deployment, correction for drift of the internal clock is required. This time drift is typically derived by synchronizing the clock before and after deployment. Linear correction is then applied using the timing deviation between GPS and the instrument’s internal clock at recovery, that is, the skew measurement. If synchronization measurements are missing, ambient noise cross-correlation functions (CCFs) are commonly used for time correction. When investigating recordings from a small-scale OBS network located on the Mohn’s mid-ocean ridge, we observed a remaining drift on the skew-corrected data. After recalculating the drift of the raw data using CCFs, we found that the skew-based time correction was incorrect. This was also verified with the observation of teleseismic P-wave arrivals. We describe a method to obtain properly time-corrected data and discuss the OBS timing issues in detail. The results shown were obtained using a software package that we developed for this specific purpose and made available as open-source software. Although we cannot explain the technical reason for the failure of skew correction, this study shows that skew corrections should not be trusted alone, and OBS timing should always be verified by either ambient noise correlations or P-wave arrival times.

scholarly journals Using Teleseismic P-Wave Arrivals to Calibrate the Clock Drift of Autonomous Underwater Hydrophones

2020 ◽  
Author(s):  
Alexey Sukhovich ◽  
Julie Perrot ◽  
Jean-Yves Royer
Keyword(s):  
Drift Rate ◽  
Seismic Velocity ◽  
Velocity Model ◽  
Internal Clock ◽  
P Wave ◽  
Ocean Bottom ◽  
P Waves ◽  
Arrival Times ◽  
Clock Drift ◽  
Common Reference

ABSTRACT Networks of autonomous underwater hydrophones (AUHs) are successfully employed for monitoring the low-level seismicity of mid-oceanic ridges by detecting hydroacoustic phases known as T waves. For a precise localization of a seismic event from T-wave arrival times, all AUHs must be synchronized. To this effect, at the beginning of the experiment, all instrument clocks are set to GPS time, which serves as a common reference. However, during the experiment, the instrument clock often deviates from GPS time, and, because the amount of deviation differs from one instrument to another, the synchronization of the AUHs deteriorates, as the experiment progresses in time. Just after the instrument recovery, the time difference (called “skew”) between the instrument and the GPS clocks is measured. Assuming that the skew varies linearly with time, the correction of a time series for the clock drift is a straightforward procedure. When the final skew cannot be determined, correcting for the clock drift is not possible, and any event localization becomes problematic. In this article, we demonstrate that the clock-drift rate (assumed to be time-independent) can be successfully estimated from arrival times of teleseismic P waves, commonly recorded by AUHs. Using a ray-tracing code, and accounting for the uncertainties in event hypocenter locations, origin times, and the Earth seismic-velocity model, confidence intervals of the estimated drift rates are deduced. The validity of the approach is tested on data from two AUHs with known clock drifts. Our results show that a reliable estimation is possible for skews as small as 4 s per two years (corresponding to a drift rate of about 5.5  ms·day−1). This method can also be applied to correct data of other recording instruments subject to internal-clock drift, such as ocean-bottom seismometers, when the skew is unknown.

Study on the mantle discontinuity structures beneath Northeast China with time–frequency phase-weighted stacks of ambient noise correlations

2019 ◽  
Vol 218 (3) ◽  
pp. 1490-1501 ◽  
Author(s):  
Songlin Li ◽  
Jinli Huang ◽  
Zhikun Liu ◽  
Yi Zhang
Keyword(s):  
Northeast China ◽  
Ambient Noise ◽  
P Wave ◽  
Small Scale ◽  
Study Region ◽  
Time Frequency ◽  
Changbaishan Volcano ◽  
Pacific Slab ◽  
Stagnant Slab ◽  
Frequency Phase

SUMMARY In this study, we use continuous waveforms recorded by 258 seismic stations from permanent and temporary networks in Northeast China from September 2009 to August 2011. Using ambient noise cross-correlations and time–frequency phase-weighted stacking based on the S-transform, we retrieve the P waves reflected on 410-km and 660-km discontinuities, we employ a 3-D P-wave velocity model as references to make time–depth conversion and then obtain the lateral depth variations of the two mantle discontinuities. Our results show that beneath the Changbaishan volcano, the 410-km discontinuity is depressed and is relatively thin, indicating the presence of high-temperature materials above the discontinuity. This implies that the Changbaishan volcano formed by upwelling of mantle melting associated with dehydration of the subducting Pacific slab. In the southeast margin of the study region, the 660-km discontinuity is depressed by ∼21 km, indicating the existence of low-temperature materials above the discontinuity. These results support the hypothesis that the Pacific slab has subducted beneath Northeast China and has stagnated at the bottom of the mantle transition zone. In addition, we determine that the 660-km discontinuity at 123.6°E–126.2°E, to the west of this stagnant slab, is slightly uplifted at a small scale, which may have been caused by upwelling melting formed by vertical tearing of the stagnant slab. Moreover, we preliminarily investigate the discontinuities beneath the northern part of the study region. The results show that the 410-km discontinuity is significantly depressed beneath the northeastern margin of the study region, which we attribute to reflected P-wave signals on the boundary between hydrous and anhydrous wadsleyite. The large-scale depression of the 660-km discontinuity observed beneath the northeastern part of the study region may be related to the existence of the stagnant slab.

Time Correction of Ocean-Bottom Seismometers Using Improved Ambient Noise Cross Correlation of Multicomponents and Dual-Frequency Bands

2021 ◽  
Author(s):  
Jinyu Tian ◽  
Jian Lin ◽  
Fan Zhang ◽  
Min Xu ◽  
Yayun Zhang ◽  
...  
Keyword(s):  
Frequency Band ◽  
Ambient Noise ◽  
Cross Correlation ◽  
Single Frequency ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometer ◽  
Dual Frequency ◽  
General Applicability ◽  
Time Correction ◽  
Noise Cross Correlation

Abstract An effective approach was developed for identifying and correcting ocean-bottom seismometer (OBS) time errors through improving ambient noise cross-correlation function (NCCF) analysis and combination with other methods. Significant improvements were illustrated through analyzing data from a passive-source seismic experiment in the southwestern sub-basin of the South China Sea. A novel method was first developed that can effectively identify errors in the sampling frequency of the OBS instruments. The traditional NCCF method was then expanded by increasing the analyzed data spectrum from a single-frequency band to dual-frequency band pairs, thus doubling the number of available data points and substantially improving the time correction quality. For data with relatively low signal-to-noise ratios, the average time errors were reduced from the original average values of 60–80 ms by the conventional methods to <40  ms using the improved approaches. The new multistep procedure developed in this study has general applicability to analysis of other OBS experiments. The demonstrated significant improvements in the data quality are critical for advancing seismic tomography and other modern marine geophysical studies that require high accuracy in the OBS data.

TO STUDY THE EFFECT OF BMV ON P WAVE DISPERSION AND SHORT TERM PROGNOSTIC IMPACT OF P WAVE DISPERSION IN PREDICTION OF CLINICAL OUTCOME AFTER PERCUTANEOUS BALLOON MITRAL VALVULOPLASTY IN PATIENTS WITH MITRAL STENOSIS AND SINUS RHYTHM

2020 ◽  
Vol 4 (9) ◽  
Author(s):  
Ashish Kumar Agarwal ◽  
Daulat Singh Meena ◽  
Vijay Pathak ◽  
Anoop Jain ◽  
Rakesh Kumar Ola
Keyword(s):  
Clinical Outcome ◽  
Right Ventricular ◽  
Wave Dispersion ◽  
Tissue Doppler ◽  
P Wave ◽  
Short Term ◽  
P Wave Dispersion ◽  
Mitral Valvuloplasty ◽  
Percutaneous Balloon ◽  
Before And After

Background: The aim of the present study was to study the effect of percutaneous balloon mitral  valvuloplasty (PBMV) on P wave dispersion and to test the correlation between P-maximum and  P-dispersion to right ventricular function and pulmonary artery pressure before and after PMBV. Also to study the impact of P-maximum and P-wave dispersion on the short term clinical outcome after successful PBMV in patients with mitral stenosis (MS) and sinus rhythm. Methods: 75 patients undergoing PMBV were enrolled in this study. We evaluated P-maximum, P-minimum and P-wave dispersion before and one month and one year after PBMV . We studied the changes in pulmonary arterial pressure (PAP), left atrial (LA) dimension, mitral diastolic gradient, and mitral valve area, in addition to the changes in right ventricular function utilizing tissue Doppler assessment both before and after PMBV, in addition the role of the P-wave dispersion in prediction of late cardiac events. Results: There were significant decrease in mean diastolic gradient, PAP, and LA size and significant improvement in right ventricular tissue Doppler indices after PMBV. Accompany these hemodynamic changes after PMBV. P-maximum and P-wave dispersion were found to be decreased (P < 0.001). Conclusion: Successful PBMV was associated with a decrease in Pmax and PWD. These simple electrocardiographic indices may predict the success of the procedure immediately after PBMV.  The P-maximum and P-wave dispersion changes were correlated with significant impairment of right dysfunction and the degree of pulmonary artery pressure. Keywords: PBMV.PAP,LA

Study of Diabetic Mellitus and Knowledge of Lotion Foot Care on the Community

2018 ◽  
Vol 9 (3) ◽  
Author(s):  
Hariyadi DM ◽  
Athiyah U ◽  
Hendradi E ◽  
Rosita N ◽  
Erawati T ◽  
...  
Keyword(s):  
Blood Glucose ◽  
T Test ◽  
Small Scale ◽  
Scale Production ◽  
Foot Care ◽  
Diabetic Mellitus ◽  
Glucose Levels ◽  
Blood Glucose Levels ◽  
Before And After ◽  
History Of

The prevention of Diabetic Mellitus (DM) and its complications is the main aim of this study, in addition to the training of lotion foot care application and the development of small scale industry. The research team delivered knowledge in the form of training on Diabetic Mellitus, healthy food, treatment and prevention of complications, and small-scale production of cosmetic products. The aim of this study was to determine the correlation between training on diabetic and lotion foot care application as preventive measures against diabetic complications on the patient's blood glucose levels in the community of residents in Banyuurip Jaya, Surabaya. It was expected from this training that the knowledge of the residents increases and people living with diabetic undergo lifestyle changes and therefore blood sugar levels can be controlled. The parameters measured in this research were blood glucose levels, the anti diabetic drug types consumed, and compliance on diabetics. This study used the data taken from 60 patients with DM over a period of one month. Questionnaires and log books was used to retrieve data and changes in blood glucose levels in diabetic patients. The results showed the demographic data of patients with type 2 diabetic of 85% female and 15% male, with the range of patients aged of 61-70 years of 46.67% and had history of diabetic (90%). The history of drugs consumed by respondents was anti diabetic drugs such as metformin (40%), glimepiride (33.37%) and insulin (6.67%). In addition, the increased knowledge of DM patients after being given the training compared to before training was shown in several questions in the questionnaire. A statistical analysis using t-test analyzed a correlation between training provided in order to enhance understanding of the patient, as well as correlation with blood glucose levels. A paired T-test showed that there was a relationship between the knowledge of trainees before and after training (p less than 0.05). An interesting result was that there was no relationship between blood glucose levels before and after training provided (p> 0.05).

scholarly journals Species and characteristics of volatile organic compounds emitted from an auto-repair painting workshop

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
M. Y. Song ◽  
H. Chun
Keyword(s):  
Volatile Organic Compounds ◽  
Organic Compounds ◽  
Large Scale ◽  
Butyl Acetate ◽  
Small Scale ◽  
Indoor Environments ◽  
Voc Emissions ◽  
Volatile Organic ◽  
Before And After ◽  
Adsorption Systems

AbstractVolatile organic compounds (VOCs) are secondary pollutant precursors having adverse impacts on the environment and human health. Although VOC emissions, their sources, and impacts have been investigated, the focus has been on large-scale industrial sources or indoor environments; studies on relatively small-scale enterprises (e.g., auto-repair workshops) are lacking. Here, we performed field VOC measurements for an auto-repair painting facility in Korea and analyzed the characteristics of VOCs emitted from the main painting workshop (top coat). The total VOC concentration was 5069–8058 ppb, and 24–35 species were detected. The VOCs were mainly identified as butyl acetate, toluene, ethylbenzene, and xylene compounds. VOC characteristics differed depending on the paint type. Butyl acetate had the highest concentration in both water- and oil-based paints; however, its concentration and proportion were higher in the former (3256 ppb, 65.5%) than in the latter (2449 ppb, 31.1%). Comparing VOC concentration before and after passing through adsorption systems, concentrations of most VOCs were lower at the outlets than the inlets of the adsorption systems, but were found to be high at the outlets in some workshops. These results provide a theoretical basis for developing effective VOC control systems and managing VOC emissions from auto-repair painting workshops.

scholarly journals Diurnal variation effect in marine magnetometric surveys: clues from surveys in southeast Brazil

2021 ◽  
Vol 42 (3) ◽  
Author(s):  
Paula Possamai Sergipe ◽  
Yára Regina Marangoni ◽  
Roberto Paulo Zanon dos Santos ◽  
Denise Silva de Moura ◽  
Luigi Jovane
Keyword(s):  
Magnetic Field ◽  
Continental Shelf ◽  
Diurnal Variation ◽  
Brazilian Coast ◽  
Observatory Data ◽  
Before And After ◽  
Corrected Data ◽  
Well Correlation ◽  
The Magnetic Field ◽  
Magnetic Observatories

AbstractThe diurnal variation of the magnetic field cannot be predicted or modeled and for that reason, it is monitored during the magnetic surveys, usually by a stationary magnetometer. However, marine surveys have a practical issue with diurnal monitoring, owing to the distance between the survey, stationary magnetometers, and magnetic observatories. This work aims to verify the use of nearby magnetic observatories to estimate the diurnal variation correction in different marine surveys and evaluate its effectiveness. In this study, we selected surveys at the continental shelf near Santos city (Survey 1), continental slope next to the first survey location (Survey 2), continental shelf near Ubatuba city (Survey 3), and Mamanguá ria in the Paraty city (Survey 4), all southeast to the Brazilian coast. The crossing points were implemented to compare the magnetic field values at different times and days at the same measurement point, before and after the correction. Afterwards, we measure the Pearson’s Correlation of the raw data and the diurnal corrected data in all crossing points of each survey which showed an improvement after correction by the value approximating to 1, which indicates a very well correlation. The Ubatuba and Mamanguá surveys allowed comparing the observatory correction results with the base magnetometer results that were rather similar. Our analyses indicate a satisfactory diurnal correction using the observatory data and the crossing points approach, which can be used for every marine magnetometric survey worldwide placed near the coast (< 280 km) that do not have a stationary magnetometer available.

Effect of Full- and Small-Scale Reeling Simulation on Mechanical Properties of Weldable 13Cr Seamless Line Pipe

2013 ◽  
Author(s):  
Hidenori Shitamoto ◽  
Nobuyuki Hisamune
Keyword(s):  
Mechanical Properties ◽  
Plastic Deformation ◽  
Oil And Gas ◽  
Small Scale ◽  
Line Pipe ◽  
Offshore Pipeline ◽  
Oil And Gas Pipelines ◽  
Before And After ◽  
Offshore Oil And Gas ◽  
Offshore Oil

There are several methods currently being used to install offshore oil and gas pipelines. The reel-lay process is fast and one of the most effective offshore pipeline installation methods for seamless, ERW, and UOE line pipes with outside diameters of 18 inches or less. In the case of the reel-laying method, line pipes are subjected to plastic deformation multiplication during reel-laying. It is thus important to understand the change of the mechanical properties of line pipes before and after reel-laying. Therefore, full-scale reeling (FSR) simulations and small-scale reeling (SSR) simulations are applied as evaluation tests for reel-laying. In this study, FSR simulations were performed to investigate the effect of cyclic deformation on the mechanical properties of weldable 13Cr seamless line pipes. Furthermore, SSR simulations were performed to compare the results obtained by FSR simulations.

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