Tidal Triggering of the Harmonic Noise in Ocean-Bottom Seismometers

2020 ◽  
Vol 91 (2A) ◽  
pp. 803-813 ◽  
Author(s):  
Telluri Ramakrushana Reddy ◽  
Pawan Dewangan ◽  
Lalit Arya ◽  
Pabitra Singha ◽  
Kattoju Achuta Kamesh Raju
Keyword(s):  
Spectral Analysis ◽  
Fundamental Frequency ◽  
Ocean Bottom ◽  
Automated Algorithm ◽  
Tidal Triggering ◽  
Spectral Peaks ◽  
Ocean Bottom Seismometers ◽  
Oceanic Tides ◽  
Harmonic Noise

Abstract We observed a harmonic noise (HN) in DEutscher Geräte-Pool für Amphibische Seismologie ocean-bottom seismometers (OBSs) data recorded from the Andaman–Nicobar region. The HN is characterized by sharp spectral peaks with a fundamental frequency and several overtones occurring at integer multiples of the fundamental frequency. We used an automated algorithm to quantify the occurrence of HN for the entire four-month deployment period (1 January 2014 to 30 April 2014). The algorithm detected more than 23 days of HN for some OBS stations. The spectral analysis of the hourly count of HN shows distinct lunar and solar tidal periodicities at 4.14, 6.1, 6.22, 12, and 12.4 hr as well as 13.66 days. The observed periodicities provide evidence of tidal triggering of HN. The HN is generated by the strumming of head buoys due to seafloor currents initiated by oceanic tides in the Andaman–Nicobar region.

scholarly journals Microearthquake seismicity and focal mechanisms at the Rodriguez Triple Junction in the Indian Ocean using ocean bottom seismometers

2001 ◽  
Vol 106 (B12) ◽  
pp. 30689-30699 ◽  
Author(s):  
Kei Katsumata ◽  
Toshinori Sato ◽  
Junzo Kasahara ◽  
Naoshi Hirata ◽  
Ryota Hino ◽  
...  
Keyword(s):  
Indian Ocean ◽  
Triple Junction ◽  
Focal Mechanisms ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometers ◽  
The Indian Ocean

Comparison of the S/N ratios of low-frequency hydrophones and geophones as a function of ocean depth

1981 ◽  
Vol 71 (5) ◽  
pp. 1649-1659
Author(s):  
Thomas M. Brocher ◽  
Brian T. Iwatake ◽  
Joseph F. Gettrust ◽  
George H. Sutton ◽  
L. Neil Frazer
Keyword(s):  
Nova Scotia ◽  
Continental Margin ◽  
Low Frequency ◽  
Weather Conditions ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometer ◽  
Scotian Shelf ◽  
Ocean Bottom Seismometers ◽  
Particle Velocities ◽  
Refraction Data

abstract The pressures and particle velocities of sediment-borne signals were recorded over a 9-day period by an array of telemetered ocean-bottom seismometers positioned on the continental margin off Nova Scotia. The telemetered ocean-bottom seismometer packages, which appear to have been very well coupled to the sediments, contained three orthogonal geophones and a hydrophone. The bandwidth of all sensors was 1 to 30 Hz. Analysis of the refraction data shows that the vertical geophones have the best S/N ratio for the sediment-borne signals at all recording depths (67, 140, and 1301 m) and nearly all ranges. The S/N ratio increases with increasing sensor depth for equivalent weather conditions. Stoneley and Love waves detected on the Scotian shelf (67-m depth) are efficient modes for the propagation of noise.

scholarly journals Quantitative EEG evaluation in patients with acute encephalopathy

2013 ◽  
Vol 71 (12) ◽  
pp. 937-942 ◽  
Author(s):  
Aline Souza Marques da Silva Braga ◽  
Bruno Della Ripa Rodrigues Assis ◽  
Jamil Thiago Rosa Ribeiro ◽  
Patricia Maria Sales Polla ◽  
Breno Jose Hulle Pereira ◽  
...  
Keyword(s):  
Spectral Analysis ◽  
Spectral Power ◽  
Quantitative Eeg ◽  
Acute Encephalopathy ◽  
Frequency Range ◽  
Alpha Frequency ◽  
Outcome Group ◽  
Spectral Peaks ◽  
Group A ◽  
Group B

Objective To investigate the use of quantitative EEG (qEEG) in patients with acute encephalopathies (AEs) and EEG background abnormalities. Method Patients were divided into favorable outcome (group A, 43 patients) and an unfavorable outcome (group B, 5 patients). EEGLAB software was used for the qEEG analysis. A graphic of the spectral power from all channels was generated for each participant. Statistical comparisons between the groups were performed. Results In group A, spectral analysis revealed spectral peaks (theta and alpha frequency bands) in 84% (38/45) of the patients. In group B, a spectral peak in the delta frequency range was detected in one patient. The remainder of the patients in both groups did not present spectral peaks. Statistical analysis showed lower frequencies recorded from the posterior electrodes in group B patients. Conclusion qEEG may be useful in the evaluations of patients with AEs by assisting with the prognostic determination.

scholarly journals The GITEWS ocean bottom sensor packages

2010 ◽  
Vol 10 (8) ◽  
pp. 1759-1780
Author(s):  
O. Boebel ◽  
M. Busack ◽  
E. R. Flueh ◽  
V. Gouretski ◽  
H. Rohr ◽  
...  
Keyword(s):  
Warning System ◽  
Deep Ocean ◽  
Indian Ocean Tsunami ◽  
False Alarms ◽  
Ocean Bottom ◽  
Bottom Pressure ◽  
Tsunami Early Warning ◽  
Ocean Bottom Seismometers ◽  
First Results ◽  
Sensor Package

Abstract. The German-Indonesian Tsunami Early Warning System (GITEWS) aims at reducing the risks posed by events such as the 26 December 2004 Indian Ocean tsunami. To minimize the lead time for tsunami alerts, to avoid false alarms, and to accurately predict tsunami wave heights, real-time observations of ocean bottom pressure from the deep ocean are required. As part of the GITEWS infrastructure, the parallel development of two ocean bottom sensor packages, PACT (Pressure based Acoustically Coupled Tsunameter) and OBU (Ocean Bottom Unit), was initiated. The sensor package requirements included bidirectional acoustic links between the bottom sensor packages and the hosting surface buoys, which are moored nearby. Furthermore, compatibility between these sensor systems and the overall GITEWS data-flow structure and command hierarchy was mandatory. While PACT aims at providing highly reliable, long term bottom pressure data only, OBU is based on ocean bottom seismometers to concurrently record sea-floor motion, necessitating highest data rates. This paper presents the technical design of PACT, OBU and the HydroAcoustic Modem (HAM.node) which is used by both systems, along with first results from instrument deployments off Indonesia.

Characteristics of Current-Induced Harmonic Tremor Signals in Ocean-Bottom Seismometer Records

2021 ◽  
Author(s):  
David Essing ◽  
Vera Schlindwein ◽  
Mechita C. Schmidt-Aursch ◽  
Celine Hadziioannou ◽  
Simon C. Stähler
Keyword(s):  
Time Series ◽  
Fundamental Frequency ◽  
Volcanic Tremor ◽  
Mode Locking ◽  
Spectral Amplitude ◽  
Bottom Current ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometer ◽  
Natural Sources ◽  
Bottom Currents

Abstract Long-lasting harmonic tremor signals are frequently observed in spectrograms of seismological data. Natural sources, such as volcanoes and icebergs, or artificial sources, such as ships and helicopters, produce very similar harmonic tremor episodes. Ocean-bottom seismometer (OBS) records may additionally be contaminated by tremor induced by ocean-bottom currents acting on the OBS structure. This harmonic tremor noise may severely hinder earthquake detection and can be misinterpreted as volcanic tremor. In a 160-km-long network of 27 OBSs deployed for 1 yr along the Knipovich ridge in the Greenland Sea, harmonic tremor was widely observed away from natural sources such as volcanoes. Based on this network, we present a systematic analysis of the characteristics of hydrodynamically induced harmonic tremor in OBS records to make it distinguishable from natural tremor sources and reveal its generation processes. We apply an algorithm that detects harmonic tremor and extracts time series of its fundamental frequency and spectral amplitude. Tremor episodes typically occur twice per day, starting with fundamental frequencies of 0.5–1.0 Hz, and show three distinct stages that are characterized by frequency-gliding, mode-locking, and large spectral amplitudes, respectively. We propose that ocean-bottom currents larger than ∼5  cm/s cause rhythmical Karman vortex shedding around protruding structures of the OBS and excite eigenvibrations. Head-buoy strumming is the most likely source of the dominant tremor signal, whereas a distinctly different tremor signal with a fundamental frequency ∼6  Hz may be related to eigenvibrations of the radio antenna. Ocean-bottom current velocities reconstructed from the fundamental tremor frequency and from cross correlation of tremor time series between stations match observed average current velocities of 14–20  cm/s in this region. The tremor signal periodicity shows the same tidal constituents as the forcing ocean-bottom currents, which is a further evidence of the hydrodynamic nature of the tremor.

An optimum data editing technique for the reduction of noise in multi-channel geophysical data

1984 ◽  
Vol 74 (3) ◽  
pp. 1059-1078
Author(s):  
P. A. Tyraskis ◽  
O. G. Jensen ◽  
D. E. Smylie ◽  
J. A. Linton
Keyword(s):  
Time Series ◽  
Spectral Analysis ◽  
Seismic Data ◽  
Data Model ◽  
Optimum Interpolation ◽  
Long Period ◽  
Data Editing ◽  
Spectral Peaks ◽  
Data Gaps ◽  
The Individual

Abstract We develop a data editing method, for the optimum interpolation of multichannel time series containing time-coincident data gaps, in one, several, or all channels based upon the autoregressive data model. The method is applied to a set of very long-period seismic data recorded during the 19 August 1977 Indonesian earthquake, which shows several unassociated bursts of noise. Spectral analysis following editing and interpolation of the record indicates existence of systematic signals with periods higher than 1 hr and perhaps as long as 2 hr. The individual spectral peaks in this subseismic band have not been identified.

scholarly journals Analysis of recent polar motion data in view of excitation function and possible seismic causes

1988 ◽  
Vol 128 ◽  
pp. 393-398 ◽  
Author(s):  
H. Lenhardt ◽  
E. Groten
Keyword(s):  
Spectral Analysis ◽  
Excitation Function ◽  
Polar Motion ◽  
Triggering Mechanism ◽  
Motion Data ◽  
Tidal Triggering

Recent BIH-data from September 1979 - September 1986 have been deconvolved. The excitation function has been considered in detail and a spectral analysis indicates that there are atmospheric contributions; moreover, the September 19, 1985 Mexican Earthquake has been related to polar motion and a possible tidal triggering mechanism has been investigated.

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