scholarly journals A repeatable seismic source for tomography at volcanoes

1999 ◽  
Vol 42 (3) ◽  
Author(s):  
U. Wegler ◽  
B. G. Lühr ◽  
A. Ratdomopurbo
Keyword(s):  
Seismic Tomography ◽  
Velocity Structure ◽  
Seismic Source ◽  
Velocity Model ◽  
Seismic Events ◽  
Seismic Signals ◽  
Improvement Project ◽  
Site Survey ◽  
3D Velocity Model ◽  
Source Signal

One major problem associated with the interpretation of seismic signals on active volcanoes is the lack of knowledge about the internal structure of the volcano. Assuming a 1D or a homogeneous instead of a 3D velocity structure leads to an erroneous localization of seismic events. In order to derive a high resolution 3D velocity model of<r>Mt. Merapi (Java) a seismic tomography experiment using active sources is planned as a part of the MERAPI (Mechanism Evaluation, Risk Assessment and Prediction Improvement) project. During a pre-site survey in August 1996 we tested a seismic source consisting of a 2.5 l airgun shot in water basins that were constructed in different flanks of the volcano. This special source, which in our case can be fired every two minutes, produces a repeatable, identical source signal. Using this source the number of receiver locations is not limited by the number of seismometers. The seismometers can be moved to various receiver locations while the source reproduces the same source signal. Additionally, at each receiver location we are able to record the identical source signal several times so that the disadvantage of the lower energy compared to an explosion source can be reduced by skipping disturbed signals and stacking several recordings.

Passive seismic tomography using recorded microseismicity: Application to mining-induced seismicity

Geophysics ◽  
2019 ◽  
Vol 84 (1) ◽  
pp. B41-B57 ◽  
Author(s):  
Himanshu Barthwal ◽  
Mirko van der Baan
Keyword(s):  
Seismic Tomography ◽  
Velocity Model ◽  
P Wave ◽  
Double Difference ◽  
Lateral Velocity ◽  
Study Region ◽  
Arrival Times ◽  
3D Velocity Model ◽  
Passive Seismic ◽  
Dip Angle

Microseismicity is recorded during an underground mine development by a network of seven boreholes. After an initial preprocessing, 488 events are identified with a minimum of 12 P-wave arrival-time picks per event. We have developed a three-step approach for P-wave passive seismic tomography: (1) a probabilistic grid search algorithm for locating the events, (2) joint inversion for a 1D velocity model and event locations using absolute arrival times, and (3) double-difference tomography using reliable differential arrival times obtained from waveform crosscorrelation. The originally diffusive microseismic-event cloud tightens after tomography between depths of 0.45 and 0.5 km toward the center of the tunnel network. The geometry of the event clusters suggests occurrence on a planar geologic fault. The best-fitting plane has a strike of 164.7° north and dip angle of 55.0° toward the west. The study region has known faults striking in the north-northwest–south-southeast direction with a dip angle of 60°, but the relocated event clusters do not fall along any mapped fault. Based on the cluster geometry and the waveform similarity, we hypothesize that the microseismic events occur due to slips along an unmapped fault facilitated by the mining activity. The 3D velocity model we obtained from double-difference tomography indicates lateral velocity contrasts between depths of 0.4 and 0.5 km. We interpret the lateral velocity contrasts in terms of the altered rock types due to ore deposition. The known geotechnical zones in the mine indicate a good correlation with the inverted velocities. Thus, we conclude that passive seismic tomography using microseismic data could provide information beyond the excavation damaged zones and can act as an effective tool to complement geotechnical evaluations.

3D seismic refraction traveltime tomography at a groundwater contamination site

Geophysics ◽  
2006 ◽  
Vol 71 (5) ◽  
pp. H67-H78 ◽  
Author(s):  
Colin A. Zelt ◽  
Aron Azaria ◽  
Alan Levander
Keyword(s):  
Groundwater Contamination ◽  
Velocity Structure ◽  
Seismic Refraction ◽  
Vertical Direction ◽  
Velocity Model ◽  
3D Seismic ◽  
Traveltime Tomography ◽  
3D Velocity Model ◽  
First Arrival ◽  
Tomographic Approach

We have applied traveltime tomography to 3D seismic refraction data collected at Hill Air Force Base, Utah, in an approximately [Formula: see text] area over a shallow [Formula: see text] groundwater contamination site. The purpose of this study is to test the ability of 3D first-arrival-time data to characterize the shallow environment and aid remediation efforts. The aquifer is bounded below by a clay aquiclude, into which a paleochannel has been incised and acts as a trap for dense nonaqueous phase liquid (DNAPL) contaminants. A regularized nonlinear tomographic approach was applied to [Formula: see text] first-arrival traveltimes to obtain the smoothest minimum-structure 3D velocity model. The resulting velocity model contains a velocity increase from less than [Formula: see text] in the upper [Formula: see text]. The model also contains a north-south-trending low-velocity feature interpreted to be the paleochannel, based on more than 100 wells in the area. Checkerboard tests show [Formula: see text] lateral resolution throughout most of the model. The preferred final model was chosen after a systematic test of the free parameters involved in the tomographic approach, including the starting model. The final velocity model compares favorably with a 3D poststack depth migration and 2D waveform inversion of coincident reflection data. While the long-wavelength features of the model reveal the primary target of the survey, the paleochannel, the velocity model is likely a very smooth characterization of the true velocity structure, particularly in the vertical direction, given the size of the first Fresnel zone for these data.

The January 2019 (Mw 6.7) Coquimbo Earthquake: Insights from a Seismic Sequence within the Nazca Plate

2019 ◽  
Author(s):  
Sergio Ruiz ◽  
Jean‐Baptiste Ammirati ◽  
Felipe Leyton ◽  
Leoncio Cabrera ◽  
Bertrand Potin ◽  
...  
Keyword(s):  
Moment Tensor ◽  
High Stress ◽  
Normal Fault ◽  
Seismic Source ◽  
Velocity Model ◽  
Seismic Sequence ◽  
Ground Acceleration ◽  
Nazca Plate ◽  
3D Velocity Model ◽  
Macroseismic Intensities

ABSTRACT On 20 January 2019, the Chilean cities of Coquimbo and La Serena were shaken by an intraplate earthquake of Mw 6.7 located at 70 km depth. High peak ground acceleration values and macroseismic intensities were reported. The mainshock was followed by more than 150 aftershocks higher than ML 2.5, a seismic sequence completely recorded by local stations. Using a 3D velocity model, we precisely located the seismicity. The aftershocks were located some 20 km above and shifted from the mainshock but still inside the Nazca plate. We also performed moment tensor inversion of nine events obtaining mostly normal‐fault focal mechanisms and kinematic inversions using the elliptical‐patch approach. We found that the mainshock broke an approximated zone of 6 km by 8 km, propagated upward in the northwest direction and away from the aftershock area. The rupture inverted from accelerograms containing up to 1 Hz was characterized with a high stress drop of 7.51 MPa and a short seismic source time function of only 3 s duration.

scholarly journals 3D Seismic Waves Velocity Structure of West Sumatera Seismic Waves Using Earthquake Data from January 2010-December 2017

2021 ◽  
Vol 873 (1) ◽  
pp. 012003
Author(s):  
Rizki Wulandari ◽  
Tedi Yudistira ◽  
Erlangga Ibrahim Fattah ◽  
Atin Nur Aulia ◽  
Adhi Wibowo
Keyword(s):  
Fault Zone ◽  
Seismic Tomography ◽  
Seismic Waves ◽  
Velocity Structure ◽  
Secondary Data ◽  
Velocity Model ◽  
P Wave ◽  
3D Seismic ◽  
Time Data ◽  
West Sumatra

Abstract This study analyses the P wave velocity (Vp) and Vp/Vs based on 3D seismic tomography in West Sumatra. Seismic tomography is a method of reconstructing the image of the subsurface structure of the earth using travel time data. This research uses secondary data obtained from the Incorporated Research Institutions for Seismology (IRIS) from January 2010 to December 2017. The data obtained were 472 earthquake events and 21 seismic stations that recorded the earthquake events. This research consists of are the hypocenter relocation, which will simultaneously renew the 1D velocity model using VELEST software, and tomographic inversion using SIMULPS12 software. The minimum anomaly of Vp/Vs value is around 1.39, while the maximum anomaly Vp/Vs value is around 2.05. The Vp distribution results have low and high Vp/Vs values around Sumatra Fault Zone and Mentawai Fault Zone. Anomaly results from the tomogram of these areas have an association with saturated pressure sedimentary areas and the presence of fractures which will further contribute to earthquake events.

scholarly journals Upper mantle velocity structure beneath Italy from direct and secondary P-wave teleseismic tomography

1997 ◽  
Vol 40 (1) ◽  
Author(s):  
G. B. Cimini ◽  
P. De Gori
Keyword(s):  
Upper Mantle ◽  
Velocity Structure ◽  
Velocity Model ◽  
P Wave ◽  
Depth Range ◽  
Tyrrhenian Sea ◽  
Tomographic Images ◽  
3D Velocity Model ◽  
Southern Tyrrhenian Sea ◽  
Velocity Anomalies

High-quality teleseismic data digitally recorded by the National Seismic Network during 1988-1995 have been analysed to tomographically reconstruct the aspherical velocity structure of the upper mantle beneath the Italian region. To improve the quality and the reliability of the tomographic images, both direct (P, PKPdf) and secondary (pP,sP,PcP,PP,PKPbc,PKPab) travel-time data were used in the inversion. Over 7000 relative residuals were computed with respect to the IASP91 Earth velocity model and inverted using a modified version of the ACH technique. Incorporation of data of secondary phases resulted in a significant improvement of the sampling of the target volume and of the spatial resolution of the heterogeneous zones. The tomographic images show that most of the lateral variations in the velocity field are confined in the first ~250 km of depth. Strong low velocity anomalies are found beneath the Po plain, Tuscany and Eastern Sicily in the depth range between 35 and 85 km. High velocity anomalies dominate the upper mantle beneath the Central-Western Alps, Northern-Central Apennines and Southern Tyrrhenian sea at lithospheric depths between 85 and 150 km. At greater depth, positive anomalies are still observed below the northernmost part of the Apenninic chain and Southern Tyrrhenian sea. Deeper anomalies present in the 3D velocity model computed by inverting only the first arrivals dataset, generally appear less pronounced in the new tomographic reconstructions. We interpret this as the result of the ray sampling improvement on the reduction of the vertical smearing effects.

scholarly journals Application of Double Difference Tomography Method to Determine The 3D Seismic Wave Velocity Structure of GoLF Geothermal Field

2018 ◽  
Vol 16 (1) ◽  
pp. 27
Author(s):  
Kana N. Naamin ◽  
David P. Sahara ◽  
Andri D. Nugraha ◽  
Irvan Ramadhan
Keyword(s):  
Velocity Structure ◽  
Seismic Velocity ◽  
Correlation Method ◽  
Velocity Model ◽  
Geothermal Field ◽  
Double Difference ◽  
Seismic Wave Velocity ◽  
Recording Time ◽  
3D Velocity Model ◽  
Geophysical Surveys

GoLF geothermal eld is located in South Solok Regency, 150 km SE of Padang city, West Sumatra.Geology, geochemistry and geophysical surveys had been conducted since 2008. Geophysical survey which had been performed including microseismic and magnetotelluric surveys. Seismic velocity structure modelling need to be conducted in order to characterize geothermal reservoir.This study uses microseismic data recorded from 36 seismometers which installed in two time recording time ranges; from September 2010 to April 2011 and from September 2012 to December 2013, with microseismic events recorded respectively 135 and 2692 events. To maximize the result of picking waveform, the data is processed using the Master Event Cross Correlation method to update the catalog data and get more accurate arrival time. Furthermore, the author used TomoDD software to produce hypocenter relocation and the 3D velocity structure under GoLF's geothermal reservoir. The results of the 3D velocity model can be used to determine the structure and phase of the fluid under GoLF geothermal field.

Gaussian beam migration

Geophysics ◽  
1990 ◽  
Vol 55 (11) ◽  
pp. 1416-1428 ◽  
Author(s):  
N. Ross Hill
Keyword(s):  
Gaussian Beam ◽  
Wave Field ◽  
Seismic Source ◽  
Velocity Model ◽  
Downward Continuation ◽  
Gaussian Beams ◽  
Seismic Velocities ◽  
Migration Method ◽  
Gaussian Beam Migration ◽  
Lateral Variations

Just as synthetic seismic data can be created by expressing the wave field radiating from a seismic source as a set of Gaussian beams, recorded data can be downward continued by expressing the recorded wave field as a set of Gaussian beams emerging at the earth’s surface. In both cases, the Gaussian beam description of the seismic‐wave propagation can be advantageous when there are lateral variations in the seismic velocities. Gaussian‐beam downward continuation enables wave‐equation calculation of seismic propagation, while it retains the interpretive raypath description of this propagation. This paper describes a zero‐offset depth migration method that employs Gaussian beam downward continuation of the recorded wave field. The Gaussian‐beam migration method has advantages for imaging complex structures. Like finite‐difference migration, it is especially compatible with lateral variations in velocity, but Gaussian beam migration can image steeply dipping reflectors and will not produce unwanted reflections from structure in the velocity model. Unlike other raypath methods, Gaussian beam migration has guaranteed regular behavior at caustics and shadows. In addition, the method determines the beam spacing that ensures efficient, accurate calculations. The images produced by Gaussian beam migration are usually stable with respect to changes in beam parameters.

scholarly journals Structure-controlled asperities of the 1920 Haiyuan M8.5 and 1927 Gulang M8 earthquakes, NE Tibet, China, revealed by high-resolution seismic tomography

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Quan Sun ◽  
Shunping Pei ◽  
Zhongxiong Cui ◽  
Yongshun John Chen ◽  
Yanbing Liu ◽  
...  
Keyword(s):  
High Resolution ◽  
Seismic Tomography ◽  
High Velocity ◽  
Three Dimensional ◽  
Large Earthquake ◽  
Velocity Model ◽  
Tectonic Stress ◽  
Double Difference ◽  
Source Regions ◽  
Large Earthquakes

AbstractDetailed crustal structure of large earthquake source regions is of great significance for understanding the earthquake generation mechanism. Numerous large earthquakes have occurred in the NE Tibetan Plateau, including the 1920 Haiyuan M8.5 and 1927 Gulang M8 earthquakes. In this paper, we obtained a high-resolution three-dimensional crustal velocity model around the source regions of these two large earthquakes using an improved double-difference seismic tomography method. High-velocity anomalies encompassing the seismogenic faults are observed to extend to depths of 15 km, suggesting the asperity (high-velocity area) plays an important role in the preparation process of large earthquakes. Asperities are strong in mechanical strength and could accumulate tectonic stress more easily in long frictional locking periods, large earthquakes are therefore prone to generate in these areas. If the close relationship between the aperity and high-velocity bodies is valid for most of the large earthquakes, it can be used to predict potential large earthquakes and estimate the seismogenic capability of faults in light of structure studies.

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