Noise level variation at norsar and its effect on detectability

1977 ◽  
Vol 67 (2) ◽  
pp. 479-492 ◽  
Author(s):  
F. Ringdal ◽  
H. Bungum
Keyword(s):  
Atmospheric Pressure ◽  
Noise Level ◽  
Amplitude Distribution ◽  
P Wave ◽  
Seasonal Fluctuations ◽  
Long Period ◽  
Short Period ◽  
Noise Data ◽  
Band Pass ◽  
Period Data

Abstract For a 3-year period, noise level measurements of short- and long-period data at NORSAR have been sampled at hourly intervals. Significant seasonal fluctuations in noise level have been found, in particular for long-period data. The noise amplitude distribution is approximately lognormal for band-pass filtered short-period data in the P-wave detection band, while the long-period noise data show a skewness that cannot be represented by a lognormal distribution. Diurnal fluctuations in noise level are quite small, but definitely present both for short and horizontal component long-period data. Cultural sources are found to account for the short-period variability, while the long-period fluctuations are attributed to atmospheric pressure variation. Event detection performance generally follows the noise level trends, with an increase in the number of reported events during summer of about 50 per cent relative to winter.

scholarly journals Capabilities of Satellite-Based Precipitation to Estimate the Spatiotemporal Variation of Flood/Drought Class in Poyang Lake Basin

2013 ◽  
Vol 2013 ◽  
pp. 1-9 ◽  
Author(s):  
Xianghu Li ◽  
Qi Zhang ◽  
Xuchun Ye
Keyword(s):  
Poyang Lake ◽  
Rainfall Data ◽  
Data Series ◽  
Lake Basin ◽  
Long Period ◽  
Poyang Lake Basin ◽  
Short Period ◽  
Period Data ◽  
Temporal And Spatial ◽  
Drought Classes

Poyang Lake basin is one of the most frequently affected areas by a variety of flood or drought events in China. Satellite-based precipitation data have greatly improved their temporal and spatial resolution in recent years, but the short length of records limited their applications in some fields. This paper compared and evaluated the creditability of using a short period data series to estimate the statistics characteristics of long period data series and investigated the usefulness of TRMM rainfall data for monitoring the temporal and spatial distribution of flood/drought classes by theZindex method in Poyang Lake basin. The results show that (1) the 1998–2010 data series are sufficiently robust to depict the statistics characteristics of long period data; (2) the intra-annual distribution and interannual variability of flood/drought classes based on TRMM rainfall data matched well with the results from rain gauges data; (3) the spatial agreement between TRMM and interpolated gauges rainfall varied with the precipitation characteristics; and (4) TRMM rainfall data described the similar spatial pattern of flood/drought classes with the interpolated gauges rainfall. In conclusion, it is suitable and credible for flood/drought classes evaluation based on the TRMM rainfall data in Poyang Lake basin.

scholarly journals The effects of tectonic release on short-period P waves from NTS explosions

1984 ◽  
Vol 74 (3) ◽  
pp. 819-842
Author(s):  
Thorne Lay ◽  
Terry C. Wallace ◽  
Don V. Helmberger
Keyword(s):  
Frequency Component ◽  
Temporal Variations ◽  
P Wave ◽  
High Frequency Component ◽  
Stress Regime ◽  
P Waves ◽  
Underground Nuclear Explosions ◽  
Long Period ◽  
Short Period ◽  
Amplitude Pattern

Abstract The first cycle (ab amplitude) of teleseismic short-period P waves from underground nuclear explosions at Pahute Mesa (NTS) show a systematic azimuthal amplitude pattern that can possibly be explained by tectonic release. The amplitudes vary by a factor of three, with diminished amplitudes being recorded at azimuths around N25°E. This azimuthal pattern has a strong sin(2φ) component and is observed, to varying degrees, for 25 Pahute Mesa events, but not for events at other sites within the NTS. Events that are known to have large tectonic release have more pronounced sin(2φ) amplitude variations. A synthesis of long-period body and surface wave investigations of tectonic release for Pahute Mesa events shows that, in general, the nonisotropic radiation is equivalent to nearly vertical, right-lateral strike-slip faulting trending from N20°W to due north. Long-period P waves at upper mantle distances demonstrate that there is a significant high-frequency component to the tectonic release. Using the long-period constraints on orientation, moment, and frequency content of the tectonic release, the expected short-period P wave effects are predicted. For models in which the downgoing P wave from the explosion triggers tectonic release within a few kilometers below the shot point, a factor of 2.5 amplitude variation with azimuth is predicted for the short-period ab amplitudes, with the lowest amplitudes expected near N25°E. Rather subtle azimuthal variations in the waveforms are expected, particulary for downward propagating ruptures, which is consistent with the absence of strong variations in the data. The occurrence of the azimuthal pattern, albeit with varying strength, for all of the Pahute Mesa events suggests a tectonic release model in which the shatterzone surrounding the explosion cavity is extended preferentially downward by driving a distributed network of faults and joints underlying the Mesa several kilometers beneath the surface. In this model, all events could have a component of tectonic release which would reflect the regional stress regime, although there may be slight spatial and temporal variations in the tectonic release contribution. Some events may trigger slip on larger throughgoing faults as well. While it is shown that tectonic release can affect teleseismic short-period signals significantly, and may contribute to the Pahute Mesa amplitude pattern, other possible explanations are considered.

scholarly journals The terminus of Hubbard Glacier, Alaska

1992 ◽  
Vol 16 ◽  
pp. 151-157 ◽  
Author(s):  
Robert M. Krimmel ◽  
D. C. Trabant
Keyword(s):  
Seasonal Variation ◽  
Tidal Current ◽  
Time Lapse ◽  
Seasonal Fluctuations ◽  
Seasonal Differences ◽  
Long Period ◽  
Short Period ◽  
Length Changes ◽  
Detailed Record ◽  
Seasonal Length

Hubbard Glacier advanced across the entrance of Russell Fiord in May 1986, transforming the fiord into a lake, which began filling with fresh water. The dam failed in October 1986. The calving terminus of Hubbard Glacier has been mapped with increasing frequency for nearly a century. A mapping interval of a few years establishes that the terminus has made a slow, but accelerating, advance. Recently the terminus has been mapped several times per month using vertical photography, ground surveys, and time-lapse cameras. At this frequency of observation, the terminus position is found to fluctuate seasonally. These seasonal fluctuations are compared with those of Columbia Glacier, where a longer detailed record is available. Although Columbia Glacier is now undergoing a drastic retreat, it continues to have seasonal length changes similar to those it experienced before the retreat began. The lengths of both Columbia and Hubbard Glaciers are extended in the spring, and retracted in the fall. The relatively long period of record for Columbia Glacier shows consistent seasonal variation in length and, when compared with the short period of record for Hubbard Glacier, suggests that there are consistent seasonal differences in length at Hubbard also. The lower Hubbard Glacier diverges, feeding ice into Disenchantment Bay and Russell Fiord, and advances and retreats synchronously over most of its width. The tidal current, which passes through the 250–500 m wide entrance to Russell Fiord, does not affect the seasonal advance and retreat in that area significantly.

A real-time digital seismic event detection and recording system for network applications

1982 ◽  
Vol 72 (6A) ◽  
pp. 2339-2348
Author(s):  
Andrew J. Michael ◽  
Stephen P. Gildea ◽  
Jay J. Pulli
Keyword(s):  
Real Time ◽  
Event Detection ◽  
Seismic Event ◽  
Weather Conditions ◽  
Recording System ◽  
Walsh Transform ◽  
Long Period ◽  
Short Period ◽  
Seismic Event Detection ◽  
Period Data

abstract A real-time digital seismic event detection and recording system has been developed for the MIT Seismic Network. The system has been designed specifically for an environment of low natural seismic activity and for surface stations which are often influenced by weather conditions and cultural noise. The system runs on an HP-1000 computer and can handle up to 16 channels of short- and long-period data. The structure of the system centers around the event detectors, one for short-period data and one for long-period data. These detectors base their decisions on a metric computed from the Walsh transform of the data. This allows them to detect changes in the amplitude of the waveform as well as frequency shifts. Detections at several stations are correlated to prevent glitches from triggering the detector. Present operation successfully saves those events that are large enough for analysis and leaves 23 of the computer available for general timesharing use.

Definition and identification of seismic events in the USSR in 1971

1974 ◽  
Vol 64 (3-1) ◽  
pp. 607-636
Author(s):  
Ola Dahlman ◽  
Hans Israelson ◽  
Atle Austegard ◽  
Gunnel Hörnström
Keyword(s):  
Focal Depth ◽  
Seismic Monitoring ◽  
Seismic Events ◽  
Long Period ◽  
The Earth ◽  
Short Period ◽  
Complete Test ◽  
Period Data ◽  
Test Ban ◽  
Test Ban Treaty

abstract Seismic events reported to have occurred in the USSR in 1971 are studied to assess the seismic monitoring problem as it may occur in the context of a complete test-ban treaty. Available epicenter data of a total of 199 events, 180 earthquakes and 19 explosions, are presented. Focal depth estimates reported by the National Oceanic and Atmospheric Administration, U.S., and the Institute of Physics of the Earth, Moscow, are compared. Identification parameters determined using short- and long-period data from Hagfors Observatory and supplementary short-period data from the Yellowknife array station in Canada are presented. To study the combined operative efficiency and applicability of available identification parameters, the reported depth estimates and the identification data are assessed in a defined way.

Climatological monthly characteristics of middle atmosphere gravity waves (10 min-10 h) during 1979-1993 at Saskatoon

1995 ◽  
Vol 13 (3) ◽  
pp. 285-295 ◽  
Author(s):  
N. M. Gavrilov ◽  
A. H. Manson ◽  
C. E. Meek
Keyword(s):  
Gravity Waves ◽  
Middle Atmosphere ◽  
Radar Data ◽  
Internal Gravity Waves ◽  
Annual Variations ◽  
Long Period ◽  
Short Period ◽  
Annual Component ◽  
Band Pass ◽  
Summer Minimum

Abstract. Saskatoon (52° N, 107°W) medium frequency (MF) radar data from 1979 to 1993 have been analyzed to investigate the climatology of irregular wind components in the height region 60-100 km. This component is usually treated in terms of internal gravity waves (IGW). Three different band-pass filters have been used to separate the intensities of IGWs having periods 0.2-2.5; 1.5-6 and 2-10 h, respectively. Height, seasonal and inter-annual variations of IGW intensities, anisotropy and predominant directions of propagation are investigated. Mean over 14 years' seasonal variation of the intensity of long-period IGWs shows a dominant annual component with winter maximum and summer minimum. Seasonal variations of the intensity of short-period waves have a strong semi-annual component as well, which forms a secondary maximum in summer. Predominant azimuths of long-period IGWs are generally zonal, though they vary with season. For short-period IGWs, the predominant azimuth is closer to the meridional direction. Anisotropy of IGW intensity is larger in summer, winter and at lower altitudes. The IGW intensity shows apparent correlation with both solar and geomagnetic activity. In most cases, this correlation appears to be negative. The variations versus solar activity is larger for longer-period IGW. Possible reasons and consequences of the observed climatological variations of IGW intensity are discussed.

Short- and long-period subevents of the 4 February 1965 Rat Islands earthquake

1984 ◽  
Vol 74 (4) ◽  
pp. 1331-1347
Author(s):  
Jim Mori
Keyword(s):  
Wave Velocity ◽  
Fault Plane ◽  
High Stress ◽  
P Wave ◽  
Long Period ◽  
P Wave Velocity ◽  
Short Period ◽  
Main Fault ◽  
The Times ◽  
Stress Drops

Abstract Short- and long-period records of the P wave of the 1965 Rat Islands earthquake were analyzed to locate subevents within the main rupture. Four subevents were identified on the short-period records in the first 100 sec and on the two long-period records in the first 30 sec. The short-period subevents cluster in an area 100 km south of the initial epicenter which appears to be off of the main fault plane, an area in which two larger aftershocks have relatively high stress drops. The long-period subevents are located 90 km west of the initial epicenter. The times and locations of the first short- and long-period subevents indicate they were triggered by a front moving near the P-wave velocity.

scholarly journals Reconciling PP and P′P′ precursor observations of a complex 660 km seismic discontinuity

2013 ◽  
Vol 194 (2) ◽  
pp. 834-838 ◽  
Author(s):  
Elizabeth A. Day ◽  
Arwen Deuss
Keyword(s):  
Phase Change ◽  
Transition Zone ◽  
High Frequency ◽  
Data Types ◽  
Mantle Transition Zone ◽  
Long Period ◽  
Short Period ◽  
Period Data ◽  
Seismic Discontinuity

Abstract High frequency precursors to P′P′ almost invariably observe a narrow 660 km discontinuity, whereas PP precursor studies at long periods struggle to detect a reflection from the ‘660’ despite its apparent sharpness to P′P′. To investigate these contradictory observations we compare PP and P′P′ precursors in the same region. Using short period P′P′ precursors we observe a sharp 660 km discontinuity, which appears to vary in depth substantially. The apparent topography on the ‘660’ is too large to originate solely from thermal variations, regardless of its cause, therefore indicating chemical variations at the base of the mantle transition zone. Long period P′P′ precursors show no ‘660’ as they are sensitive to a larger area and thus average out the apparent topography, in agreement with long period PP precursors. Instead, we see some evidence in both long period data types for a reflection from 720 km depth, which is likely to correspond to a phase change in the garnet system.

DETECTION OF P‐WAVES FROM WEAK SOURCES AT GREAT DISTANCES

Geophysics ◽  
1964 ◽  
Vol 29 (2) ◽  
pp. 197-211 ◽  
Author(s):  
Jon F. Claerbout
Keyword(s):  
Time Series ◽  
Prediction Error ◽  
P Wave ◽  
Pass Band ◽  
Multiple Time ◽  
Multiple Time Series ◽  
P Waves ◽  
Short Period ◽  
Band Pass ◽  
Detection Of Signals

Optimum (Wiener sense) filters for suppression of noise in multiple time series are computed by a new method due to E. A. Robinson. Filters for prediction error and interpolation error have been used to detect P‐wave signals from three teleseismic events. These filters facilitate detection of signals in noise with low signal‐to‐noise ratios. The instrumentation consists of short‐period Benioff seismometers, both three‐component stations and surface arrays of verticals. It was found that microseismic noise in the pass band of these instruments is more accurately termed “Brownian motion of a surface” than “random waveforms with characteristic direction(s) of propagation.” Thus, single time‐series filters work almost as well as multiple time‐series matrix filters. Prediction‐error filters gave results substantially more satisfactory than simple band‐pass filters.

Thermal waves on the Queensland shelf

1983 ◽  
Vol 34 (1) ◽  
pp. 81 ◽  
Author(s):  
JC Andrews
Keyword(s):  
Atmospheric Pressure ◽  
Bottom Water ◽  
Driving Mechanism ◽  
Mean Square ◽  
Thermal Waves ◽  
Root Mean Square Amplitude ◽  
Mean Sea Level ◽  
Long Period ◽  
Short Period ◽  
Break Water

Thermocline waves are found on the continental slope and shelf of Queensland. Slope waves of period near 90 days accompany upwelling and downwelling. Shelf thermal waves are created by onshore surges of shelf- break water, which is 1-4.5�C cooler than lagoon water, depending on the season. The bottom surges are investigated by means of the cross-spectra of temperatures with longshore winds, with atmospheric pressure and with mean sea level. Longshore winds are found to be the driving mechanism and significant coherence is found near periods of 4 and 8 days (summer and winter, respectively) and in a band with periods of 10-70 days, perhaps peaking near 30 days. Correlation and cross-spectral analyses of temperature at sites 100 km apart suggest that the short period thermal waves popagate equatorward wheras the long period waves, which contain most of the thermal covariance, form a standing mode. Cross-spectral analysis of the root mean square amplitude of diurnal thermal variations with the longer-period thermal waves suggests that localized tidal upwelling brings bottom water over reefs.

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