Broad-Band P-Wave Signals and Spectra from Digital Stations

1989 ◽  
pp. 351-374 ◽  
Author(s):  
M. Bezzeghoud ◽  
A. Deschamps ◽  
R. Madariaga
Keyword(s):  
Broad Band ◽  
P Wave

Sharpness of the 410-km discontinuity from the P410s and P2p410s seismic phases

2019 ◽  
Author(s):  
Lev Vinnik ◽  
Yangfan Deng ◽  
Grigoriy Kosarev ◽  
Sergey Oreshin ◽  
Zhou Zhang ◽  
...  
Keyword(s):  
Transition Zone ◽  
Southern Africa ◽  
Amplitude Ratio ◽  
Southern China ◽  
Broad Band ◽  
Receiver Functions ◽  
P Wave ◽  
Seismic Model ◽  
Yangtze Craton ◽  
Zone Water

Summary Sharpness of the 410-km boundary is of interest because it is sensitive to water content in the transition zone. We evaluate the width of the 410-km discontinuity with a new seismic method. Our estimates are inferred from the amplitude ratio of the P2p410s and P410s seismic phases that are detected in P-wave receiver functions. We applied this method to seismic recordings from arrays of broad-band stations deployed in central Fennoscandia, southern Africa and southern China. The obtained estimates of width of the 410-km discontinuity range from 10 to 22 km and always exceed the width of 7 km which is expected for anhydrous conditions. The enlarged width may be interpreted in terms of hydrous conditions, but we have found only one region (the eastern Yangtze Craton in China) where the broad 410-km discontinuity, as expected, is accompanied by a broad transition zone. Water in the transition zone may be a kind of a global phenomenon, but evidence of the enlarged width of the transition zone may be missing in most of our data because the reference seismic model is affected by water, as well.

In-Situ Calibration of Differential Pressure Gauges on OBSIP Ocean Bottom Seismometers

2020 ◽  
Author(s):  
Gabi Laske ◽  
Adrian Doran
Keyword(s):  
Pressure Sensor ◽  
Broad Band ◽  
Pressure Gauge ◽  
Differential Pressure ◽  
P Wave ◽  
Earth Tides ◽  
Release Mechanism ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometer

<p>A standard ocean bottom seismometer (OBS) package of the U.S. OBS Instrument Pool (OBSIP) carries a seismometer and a pressure sensor. For broadband applications, the seismometer typically is a wide-band or broad-band three-components seismometer, and the pressure sensor is a differential pressure gauge (DPG). The purpose of the pressure sensor is manifold and includes the capture of pressure signals not picked up by a ground motion sensor (e.g. the passage of tsunami), but also for purposes of correcting the seismograms for unwanted signals generated in the water column (e.g. p-wave reverberations).<br>Unfortunately, the instrument response of the widely used Cox-Webb DPG remains somewhat poorly known, and can vary by individual sensor, and even by deployment of the same sensor.</p><p>Efforts have been under way to construct and test DPG responses in the laboratory. But the sensitivity and long‐period response are difficult to calibrate as they  vary with temperature and pressure, and perhaps by hardware of the same mechanical specifications.  Here, we present a way to test the response for each individual sensor and deployment in situ in the ocean. This test requires a relatively minimal and inexpensive modification to the OBS instrument frame and a release mechanism that allows a drop of the DPG by 3 inches after the OBS package settled and the DPG equilibrated on the seafloor. The seismic signal generated by this drop is then analyzed in the laboratory upon retrieval of the data. </p><p>The results compare favorably with calibrations estimated independently through post‐deployment data analyses of other signals such as Earth tides and the signals from large teleseismic earthquakes. Our study demonstrates that observed response functions can deviate from the nominal response by a factor of two or greater with regards to both the sensitivity and the time constant. Given the fact that sensor calibrations of DPGs in the lab require very specific and stable environments and are time consuming, the use of in-situ DPG calibration frames pose a reliable and inexpensive alternative. </p>

scholarly journals Design of a P-wave seismic vibrator with advanced performance

GeoArabia ◽  
2008 ◽  
Vol 13 (2) ◽  
pp. 123-136
Author(s):  
Zhouhong Wei
Keyword(s):  
Broad Band ◽  
Harmonic Distortion ◽  
P Wave ◽  
Key Factors ◽  
Servo Valve ◽  
Supply Pressure ◽  
Seismic Resolution ◽  
Force Signal ◽  
Flow Pressure ◽  
Frequency Controller

ABSTRACT For optimal seismic imaging, the vibroseis method requires the vibrator to generate synchronous, repeatable sweeps over a broad frequency range and output the ground-force energy with minimum harmonic distortion. This requires re-evaluating each element of the vibrator system to ensure that it contributes to the success of the method. Key factors that cause the vibrator to suffer from severe harmonic distortion are fluctuations in the hydraulic power supply pressure, flexing of the baseplate, coupling or loading between the baseplate and the ground, nonlinear servo-valve flow-pressure characteristics, and servo-valve characteristics near null. This paper examines these factors and describes the design of a new P-wave vibrator by ION (previously I-O) to improve seismic resolution. Experimental results demonstrate that the newly designed vibrator dramatically reduces harmonic distortion in the ground-force signal under various coupling conditions, particularly on hard and uneven ground. With the high-frequency controller and Pelton DR valve, a broad-band sweep frequency is achieved. Cavitations in the supply pressure, a long-standing problem in vibrator mechanics, are almost completely eliminated.

Seismic source functions and magnitude determinations for underground nuclear detonations

1977 ◽  
Vol 67 (1) ◽  
pp. 135-158
Author(s):  
John R. Murphy
Keyword(s):  
Body Wave ◽  
Broad Band ◽  
Seismic Source ◽  
Source Model ◽  
The Body ◽  
P Wave ◽  
Cube Root ◽  
Motion Data ◽  
Surface Wave Data ◽  
Body Wave Magnitude

abstract A variety of near-regional, regional, and teleseismic ground-motion data have been used to evaluate proposed models of the nuclear seismic source function for underground detonations in tuff/rhyolite emplacement media. It has been found that both the near-regional broad-band seismic data and the teleseismic body-wave magnitude data are consistent with the modified source model proposed by Mueller and Murphy (1971) but not with the simple cube-root of the yield-scaling source model. In particular, the observed linearity and slopes of the body-wave magnitude-yield curves as well as the observed variation of P-wave period with yield have been found to be fully compatible with the modified source model. On the other hand, it has been concluded that the observed long-period surface-wave data are inconsistent with a simple, spherically symmetric source model. The results of a preliminary analysis have suggested that this discrepancy may be related to the spall closure phenomenon.

Integrated teleseismic studies of the southern Alberta upper mantle

2002 ◽  
Vol 39 (3) ◽  
pp. 399-411 ◽  
Author(s):  
J Shragge ◽  
M G Bostock ◽  
C G Bank ◽  
R M Ellis
Keyword(s):  
Upper Mantle ◽  
Velocity Structure ◽  
Broad Band ◽  
Receiver Functions ◽  
P Wave ◽  
Crustal Thickening ◽  
Plate Motion ◽  
Moho Depth ◽  
Hydrous Minerals ◽  
South Central

This paper presents results from a teleseismic experiment conducted across the Hearne Province in south-central Alberta. Data from an array of nine portable broad-band seismographs deployed along a 500 km NW–SE array have been supplemented with recordings from two Canadian National Seismograph Network stations. P-wave delay times from 293 earthquakes have been inverted for upper-mantle velocity structure below the array. The recovered model reveals high velocities beneath much of the southern Hearne Province to depths of 200–250 km, which are interpreted as deep-seated lithospheric structure. Contrary to recent tectonic models, these results suggest that the Hearne lithosphere has remained intact. In particular, it appears unlikely that evidence for extensive, lower crustal melting derives from lithospheric delamination. However, the results admit the possibility that high mantle conductivity, as revealed in magnetotelluric studies, originates through small volumes of connected hydrous minerals or other conductive species introduced during subduction. Decreased upper-mantle velocities at the northern end of the Medicine Hat block also pose challenges for the interpretation of differential subsidence across the region which may manifest distant forcing due to more recent subduction. Multievent SKS-splitting analysis yields an average polarization direction that is broadly consistent with both the orientation of fossil strain fields, related to ~ 1.8 Ga NW–SE shortening, and North American absolute plate motion. Moho depth estimates from receiver functions are fairly uniform (~ 38 km) beneath northern stations but show crustal thickening (>40 km) within the Medicine Hat block to the south and are consistent with values from active-source profiling.

scholarly journals Time functions appropriate for some aftershocks of the point Mugu, California earthquake of February 21, 1973

1975 ◽  
Vol 65 (1) ◽  
pp. 127-132
Author(s):  
David King ◽  
Donald V. Helmberger
Keyword(s):  
Broad Band ◽  
P Wave ◽  
S Wave ◽  
Dislocation Source ◽  
Small Earthquake ◽  
Wave Pulse ◽  
Excellent Opportunity ◽  
Earthquake Records ◽  
Source Models ◽  
Point Dislocation

abstract Broad-band recordings of aftershocks of the Pt. Mugu earthquake at small epicentral distances provided an excellent opportunity to test source models for small earthquakes. Simple events recorded at nearly vertical incidence produced a single P-wave pulse of a duration of about 0.07 sec and a somewhat more complicated S wave with a slightly longer duration. Such events are consistent with a point dislocation source for which Qβ = 100 or for which there is directivity with the fault breaking downward. We attribute the more usual complexities of small earthquake records to multiple events, some of which we observed, layering effects combined with greater epicentral distances, and scattering.

Decoupled equations for reverse time migration in tilted transversely isotropic media

Geophysics ◽  
2012 ◽  
Vol 77 (2) ◽  
pp. T37-T45 ◽  
Author(s):  
Ge Zhan ◽  
Reynam C. Pestana ◽  
Paul L. Stoffa
Keyword(s):  
Broad Band ◽  
Transversely Isotropic ◽  
Rapid Expansion ◽  
P Wave ◽  
Reverse Time ◽  
Reverse Time Migration ◽  
Coupled Equations ◽  
Time Migration ◽  
Tti Media ◽  
Wave Modes

Conventional modeling and migration for tilted transversely isotropic (TTI) media may suffer from numerical instabilities and shear wave artifacts due to the coupling of the P-wave and SV-wave modes in the TTI coupled equations. Starting with the separated P- and SV-phase velocity expressions for vertical transversely isotropic (VTI) media, we extend these decoupled equations for modeling and reverse time migration (RTM) in acoustic TTI media. Compared with the TTI coupled equations published in the geophysical literature, the new TTI decoupled equations provide a more stable solution due to the complete separation of the P-wave and SV-wave modes. The pseudospectral method is the most convenient method to implement these equations due to the form of wavenumber expressions and has the added benefit of being highly accurate and thus avoiding numerical dispersion. The rapid expansion method (REM) in time is employed to produce a broad band numerically stable time evolution of the wavefields. Synthetic results validate the proposed TTI decoupled equations and show that modeling and RTM in TTI media with the decoupled equations remain numerically stable even for models with strong anisotropy and sharp contrasts.

scholarly journals Implementation of Filter Picker Algorithm For Aftershock Identification of Lombok Earthquake 2018

2019 ◽  
Vol 17 (1) ◽  
pp. 25
Author(s):  
A. Ardianto ◽  
Y.M. Husni ◽  
A. D. Nugraha ◽  
M. Muzli ◽  
Z. Zulfakriza ◽  
...  
Keyword(s):  
Arrival Time ◽  
Seismic Station ◽  
Broad Band ◽  
P Wave ◽  
Practical Reasons ◽  
Time Of Arrival ◽  
P Waves ◽  
Short Period ◽  
Minimum Number ◽  
Comparison Of The Results

The ability to identify earthquake events that are consistent, efficient and accurate is increasingly needed along with the increase in the amount of data analyzed. In this paper a filter picker algorithm is implemented to identify aftershock  events and determination of arrival time automatically, especially for the P wave phase. Here modifications are made in determining the uncertainty of arrival time and there are additional criteria in determining the time of arrival used. The additional criteria are that in a certain time span, there are at least 5 stations determined by the time the filter picker arrives. This is done to minimize identification errors due to local noise and other practical reasons, namely the minimum number of stations to determine the location and other seismological analysis. To test the filter picker algorithm, aftershock data from the Lombok earthquake occurred on July 29 (M 6.4), August 5 (M 7), and August 19 (M 6.3 and M 6.9)  2018. The aftershock data were used for 30 days, from August 4, 2018 to September 4, 2018 using local seismic station in Lombok Island. The results of the filter picker algorithm were evaluated by comparing the number of earthquake events detected and the accuracy of determining the P wave arrival time automatically to the results of manually arriving time. In addition, a comparison of the results obtained from a broadband type seismometer with a short period is used to find out how much influence the type of tool has on its performance results. The results of the comparison with the manual arrival time show that more than 85 percent of the results of the automatic arrival time have a difference below 0.2 seconds. Therefore, it can be said that the filter picker algorithm is quite effective for identifying events and determining the arrival time of P waves. In this paper it is also shown that this algorithm can be used for broad band and short period seismometer sensor, even without the prior correction of instruments.

scholarly journals Seismotectonics and 1-D velocity model of the Greater Geneva Basin, France–Switzerland

2020 ◽  
Vol 221 (3) ◽  
pp. 2026-2047 ◽  
Author(s):  
Verónica Antunes ◽  
Thomas Planès ◽  
Jiří Zahradník ◽  
Anne Obermann ◽  
Celso Alvizuri ◽  
...  
Keyword(s):  
Large Scale ◽  
Waveform Inversion ◽  
Broad Band ◽  
Velocity Model ◽  
Strike Slip ◽  
P Wave ◽  
Tectonic Structures ◽  
Stress Regime ◽  
Geothermal Exploration ◽  
Velocity Models

SUMMARY The Greater Geneva Basin (GGB), located in southwestern Switzerland and neighboring France, is enclosed by the rotating northwestern edge of the Alpine front and the Jura mountains chain. Recently, this basin has received increasing attention as a target for geothermal exploration. Historical and instrumental seismicity suggest that faults affecting the basin may still be active. Moderate-magnitude earthquakes have been located along the Vuache fault, a major strike-slip structure crossing the basin. Before geothermal exploration starts, it is key to evaluate the seismic rate in the region and identify possible seismogenic areas. In this context, we deployed a temporary seismic network of 20 broad-band stations (from September 2016 to January 2018) to investigate the ongoing seismic activity, its relationship with local tectonic structures, and the large-scale kinematics of the area. Our network lowered the magnitude of completeness of the permanent Swiss and French networks from 2.0 to a theoretical value of 0.5. Using a new coherence-based detector (LASSIE - particularly effective to detect microseismicity in noisy environments), we recorded scarce seismicity in the basin with local magnitudes ranging from 0.7 to 2.1 ML. No earthquakes were found in the Canton of Geneva where geothermal activities will take place. We constructed a local ’minimum 1-D P-wave velocity model’ adapted to the GGB using earthquakes from surrounding regions. We relocated the events of our catalogue obtaining deeper hypocentres compared to the locations obtained using the available regional velocity models. We also retrieved eight new focal mechanisms using a combination of polarities and waveform inversion techniques (CSPS). The stress inversion shows a pure strike-slip stress regime, which is in agreement with structural and geological data. Combining the background seismicity with our catalogue, we identified seismogenic areas offsetting the basin.

Chronic Laser Exposure Effects on Rabbit Retina

1972 ◽  
Vol 30 ◽  
pp. 54-55
Author(s):  
Burton B. Silver ◽  
Theodore Lawwill
Keyword(s):  
Laser Irradiation ◽  
Laser Light ◽  
Broad Band ◽  
Argon Laser ◽  
Atropine Sulfate ◽  
Ultrastructural Changes ◽  
Laser Exposure ◽  
Rabbit Retina ◽  
Histological Changes ◽  
Threshold Doses

Dutch-belted 1 to 2.5 kg anesthetized rabbits were exposed to either xenon or argon laser light administered in a broad band, designed to cover large areas of the retina. For laser exposure, the pupil was dilated with atropine sulfate 1% and pheny lephrine 10%. All of the laser generated power was within a band centered at 5145.0 Anstroms. Established threshold for 4 hour exposures to laser irradiation are in the order of 25-35 microwatts/cm2. Animals examined for ultrastructural changes received 4 hour threshold doses. These animals exhibited ERG, opthalmascopic, and histological changes consistent with threshold damage.One month following exposure the rabbits were killed with pentobarbitol. The eyes were immediately enucleated and dissected while bathed in 3% phosphate buffered gluteraldehyde.

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