scholarly journals The assessment of local site effects and dynamic behaviour in Nicosia, Cyprus

Geofizika ◽  
2021 ◽  
Vol 38 (1) ◽  
pp. 61-80
Author(s):  
Hilmi Dindar ◽  
Mustafa Akgün ◽  
Cavit Atalar ◽  
Özkan Cevdet Özdağ
Keyword(s):  
Seismic Waves ◽  
Eastern Mediterranean ◽  
Seismic Source ◽  
Vulnerability Index ◽  
Peak Period ◽  
Single Station ◽  
Lithospheric Plates ◽  
Masw Method ◽  
Earthquake Zone ◽  
Soil Behaviour

Single-station microtremor measurements were conducted to investigate earthquake and soil behaviour for the first time in Nicosia, Cyprus. Cyprus is located in a tectonically complex area in the Eastern Mediterranean where three plates meet. The study area was chosen to cover the areas to be opened for new development. Nicosia, the capital of Cyprus, is also the island's most important cultural, industrial, commercial, and transportation centre. The study creates base maps for the soil to assess earthquake resistance crucial for construction. Microtremor Method was applied at 100 stations and the Multi-Channel Analysis of Surface Waves (MASW) method was used at 52 stations. Also, RefractionMicrotremor (Re-Mi) and L-Shaped Spatial Autocorrelation (L-SPAC) methods were carried out at 17 stations to substantiate the research. The results of the microtremor method indicate that the predominant soil period values have an average of 1 second and pre-dominant peak period values are generally found between 0.1 to 5 s at the study area. Peak amplitude values are observed between 1 and 2.4. The Vulnerability Index Parameter (Kg) exceeded 20 at the central and the southern stations, and Kg values change between 7 and 54 units. The Kg values were found to be higher than 20 in soils where shear wave velocity is lower than 760 m/s. At the same time, the values of the predominant peak period were greater than 1 second. Cyprus is located in the Alpine Himalayan earthquake zone. The Cyprus Arc is known as the main seismic source of the island, It constitutes the tectonic border among African and Eurasian lithospheric plates in the region. During an earthquake in Nicosia, seismic waves will be amplified by an average of 1.5 times and soil deformation will occur due to the exceeding elastic limits. The results provided important insight into soil behaviour and indicated its reactions in a potential earthquake.

scholarly journals On the point-source approximation of earthquake dynamics

2014 ◽  
Vol 57 (3) ◽  
Author(s):  
Andrea Bizzarri
Keyword(s):  
Point Source ◽  
Seismic Waves ◽  
Single Point ◽  
Seismic Source ◽  
Transition Process ◽  
Point Sources ◽  
Multiple Point ◽  
Earthquake Dynamics ◽  
Planar Fault ◽  
Source Models

<p>The focus on the present study is on the point-source approximation of a seismic source. First, we compare the synthetic motions on the free surface resulting from different analytical evolutions of the seismic source (the Gabor signal (G), the Bouchon ramp (B), the Cotton and Campillo ramp (CC), the Yoffe function (Y) and the Liu and Archuleta function (LA)). Our numerical experiments indicate that the CC and the Y functions produce synthetics with larger oscillations and correspondingly they have a higher frequency content. Moreover, the CC and the Y functions tend to produce higher peaks in the ground velocity (roughly of a factor of two). We have also found that the falloff at high frequencies is quite different: it roughly follows ω<span><sup>−2</sup></span> in the case of G and LA functions, it decays more faster than ω<span><sup>−2</sup></span> for the B function, while it is slow than ω<span><sup>−1</sup></span> for both the CC and the Y solutions. Then we perform a comparison of seismic waves resulting from 3-D extended ruptures (both supershear and subshear) obeying to different governing laws against those from a single point-source having the same features. It is shown that the point-source models tend to overestimate the ground motions and that they completely miss the Mach fronts emerging from the supershear transition process. When we compare the extended fault solutions against a multiple point-sources model the agreement becomes more significant, although relevant discrepancies still persist. Our results confirm that, and more importantly quantify how, the point-source approximation is unable to adequately describe the radiation emitted during a real world earthquake, even in the most idealized case of planar fault with homogeneous properties and embedded in a homogeneous, perfectly elastic medium.</p>

Modeling and interpretation of scattered waves in inter-stage DAS VSP survey

Geophysics ◽  
2020 ◽  
pp. 1-49
Author(s):  
Aleksei Titov ◽  
Gary Binder ◽  
Youfang Liu ◽  
Ge Jin ◽  
James Simmons ◽  
...  
Keyword(s):  
Hydraulic Fracturing ◽  
Seismic Waves ◽  
Seismic Source ◽  
Cost Effective ◽  
Hydraulic Fractures ◽  
Zone Model ◽  
Low Velocity ◽  
Low Velocity Zone ◽  
Field Development ◽  
Velocity Zone

Optimization of well spacings and completions are key topics in research related to the development of unconventional reservoirs. In 2017, a vertical seismic profiling (VSP) survey using fiber-optic-based distributed acoustic sensing (DAS) technology was acquired. The data include a series of VSP surveys taken before and immediately following the hydraulic fracturing of each of 78 stages. Scattered seismic waves associated with hydraulic fractures are observed in the seismic waveforms. Kinematic traveltime analysis and full-wavefield modeling results indicate these scattered events are converted PS-waves. We tested three different models of fracture-induced velocity inhomogeneities that can cause scattering of seismic waves: single hydraulic fracture, low-velocity zone, and tip diffractors. We compare the results with the field observations and conclude that the low-velocity zone model has the best fit for the data. In this model, the low-velocity zone represents a stimulated rock volume (SRV). We propose a new approach that uses PS-waves converted by SRV to estimate the half-height of the SRV and the closure time of hydraulic fractures. This active seismic source approach has the potential for cost-effective real-time monitoring of hydraulic fracturing operations and can provide critical constraints on the optimization of unconventional field development.

Shallow to very shallow, high‐resolution reflection seismic using a portable vibrator system

Geophysics ◽  
1998 ◽  
Vol 63 (4) ◽  
pp. 1295-1309 ◽  
Author(s):  
Ranajit Ghose ◽  
Vincent Nijhof ◽  
Jan Brouwer ◽  
Yoshikazu Matsubara ◽  
Yasuhiro Kaida ◽  
...  
Keyword(s):  
High Resolution ◽  
High Frequency ◽  
Seismic Waves ◽  
Soft Soil ◽  
Geophysical Methods ◽  
Seismic Source ◽  
P Wave ◽  
Nondestructive Technique ◽  
Buried Objects ◽  
Reflection Seismic

In shallow engineering‐geophysical applications, there is a lack of controlled, nondestructive, high‐resolution mapping tools, particularly for the target depth that ground‐penetrating radar cannot reach but which is too shallow for other conventional geophysical methods. For soft soil, this corresponds to a depth of 2 to 30 m. We have developed a portable, high‐frequency P-wave vibrator system that is capable of bridging this gap. As far as the important contribution of the seismic source is concerned, penetration and resolution can be individually controlled through easy modulation of the sweep signal generated by this electromagnetic vibrator. The feasibility of this system has been tested in shallow (10–50 m) to very shallow (0–10 m) applications. Seven field data sets representing varying geology, site conditions, and exploration targets are presented to illustrate the applicability. The first three examples show the potential of this portable vibrator source in shallow applications. Under favorable situations, a maximum resolution of about 20 cm for events located at 15–30 m depth could be achieved. Because high‐frequency seismic waves suffer from severe attenuation in the dry, unsaturated weathered zone, the penetration is relatively limited when the water table is deeper than 4–5 m. The fourth to seventh field examples illustrate very shallow applications at noisy, asphalt‐paved urban sites that are often encountered in civil, geotechnical, and environmental engineering projects. The prospecting targets were thin soil layers or small buried objects. On asphalt, the vibrator can produce high‐frequency energy easily. The fourth example shows high‐resolution delineation of very shallow soil structures. The last three examples present successful location of buried bodies—often small and closely spaced—in soft soil at depths of 0.5 to 5 m. We observe well‐defined reflection events of frequency exceeding 200 Hz. These results suggest that high‐frequency seismic reflection imaging using the portable vibrator system can indeed serve as a powerful, nondestructive technique for shallow to very shallow underground prospecting.

scholarly journals A source-synchronous filter for uncorrelated receiver traces from a swept-frequency seismic source

Geophysics ◽  
2016 ◽  
Vol 81 (5) ◽  
pp. P47-P55 ◽  
Author(s):  
Neal Lord ◽  
Herbert Wang ◽  
Dante Fratta
Keyword(s):  
Seismic Waves ◽  
Seismic Source ◽  
Propagation Delay ◽  
External Noise ◽  
Sweep Rate ◽  
Noise Sources ◽  
Frequency Domain Methods ◽  
Phase Alignment ◽  
Source Signal ◽  
Downhole Array

We have developed a novel algorithm to reduce noise in signals obtained from swept-frequency sources by removing out-of-band external noise sources and distortion caused from unwanted harmonics. The algorithm is designed to condition nonstationary signals for which traditional frequency-domain methods for removing noise have been less effective. The source synchronous filter (SSF) is a time-varying narrow band filter, which is synchronized with the frequency of the source signal at all times. Because the bandwidth of the filter needs to account for the source-to-receiver propagation delay and the sweep rate, SSF works best with slow sweep rates and moveout-adjusted waveforms to compensate for source-receiver delays. The SSF algorithm was applied to data collected during a field test at the University of California Santa Barbara’s Garner Valley downhole array site in Southern California. At the site, a 45 kN shaker was mounted on top of a one-story structure and swept from 0 to 10 Hz and back over 60 s (producing useful seismic waves greater than 1.6 Hz). The seismic data were captured with small accelerometer and geophone arrays and with a distributed acoustic sensing array, which is a fiber-optic-based technique for the monitoring of elastic waves. The result of the application of SSF on the field data is a set of undistorted and uncorrelated traces that can be used in different applications, such as measuring phase velocities of surface waves or applying convolution operations with the encoder source function to obtain traveltimes. The results from the SSF were used with a visual phase alignment tool to facilitate developing dispersion curves and as a prefilter to improve the interpretation of the data.

scholarly journals Ground Water Induced Changes in Velocities of P and S Waves (Vp and Vs) Measured Using Accurately Controlled Seismic Source

2020 ◽  
Author(s):  
Rina Suzuki ◽  
Koshun Yamaoka ◽  
Shuhei Tsuji ◽  
Toshiki Watanabe
Keyword(s):  
Ground Water ◽  
Seismic Waves ◽  
Crack Density ◽  
Seismic Source ◽  
Initial Response ◽  
Vibration Source ◽  
Water Mixture ◽  
S Waves ◽  
Fluid Saturation ◽  
Induced Changes

Abstract We analyzed the temporal variation in the travel times of both the P and S waves (Vp and Vs) for 14 months at Toyohashi (central Japan) with a continuously operating vibration source that could produce both P and S waves efficiently. The seismic waves produced by the source, which is named ACROSS (accurately-controlled routinely-operated signal system), were recorded by three nearby seismic stations, and the travel time variation at each station was calculated using the transfer function calculated from the recorded data. We observed the seasonal variations in the Vp and Vs for all the stations—which can be interpreted using the change in the fluid saturation and crack density of subsurface rocks—are consistent with the variation in the ground water level. The short-term responses to rainfall are observed at the nearest station; the interpretation of the changes in crack density and saturation is inconsistent with the ground water observation partly due the initial response to rainfall. This can be interpreted as an air-water mixture within pores or cracks on a fine scale.

scholarly journals Model for Calculating Seismic Wave Spectrum Excited by Explosive Source

2021 ◽  
Vol 2021 ◽  
pp. 1-15
Author(s):  
Qian Xu ◽  
Zhong-Qi Wang
Keyword(s):  
Seismic Wave ◽  
Seismic Waves ◽  
Wave Spectrum ◽  
Seismic Source ◽  
Source Model ◽  
Detonation Pressure ◽  
Adiabatic Expansion ◽  
Main Frequency ◽  
Explosive Source ◽  
Seismic Source Model

To reveal the characteristics and laws of the seismic wavefield amplitude-frequency excited by explosive source, the method for computing the seismic wave spectrum excited by explosive was studied in this paper. The model for calculating the seismic wave spectrum excited by explosive source was acquired by taking the seismic source model of spherical cavity as the basis. The results of using this model show that the main frequency and the bandwidth of the seismic waves caused by the explosion are influenced by the initial detonation pressure, the adiabatic expansion of the explosive, and the geotechnical parameters, which increase with the reduction of initial detonation pressure and the increase of the adiabatic expansion. The main frequency and the bandwidth of the seismic waves formed by the detonation of the explosives in the silt clay increase by 23.2% and 13.6% compared to those exploded in the silt. The research shows that the theoretical model built up in this study can describe the characteristics of the seismic wave spectrum excited by explosive in a comparatively accurate way.

Repeat well logging using earthquake wave amplitudes measured by distributed acoustic sensors

2020 ◽  
Vol 39 (7) ◽  
pp. 513-517
Author(s):  
Roman Pevzner ◽  
Boris Gurevich ◽  
Anastasia Pirogova ◽  
Konstantin Tertyshnikov ◽  
Stanislav Glubokovskikh
Keyword(s):  
Seismic Waves ◽  
Fiber Optic ◽  
Synthetic Data ◽  
Seismic Source ◽  
Small Scale ◽  
Acoustic Sensors ◽  
Linear Strain ◽  
Subsurface Characterization ◽  
Distributed Acoustic Sensing ◽  
Subsurface Monitoring

Well-based technologies for seismic subsurface monitoring increasingly utilize fiber-optic cables installed in boreholes as distributed acoustic sensing (DAS) systems. A DAS cable allows measuring linear strain of the fiber and can serve as an array of densely spaced seismic receivers. The strain amplitudes recorded by the DAS cable depend on the near-well formation properties (the softer the medium, the larger the strain). Thus, these properties can be estimated by measuring relative variations of the amplitudes of seismic waves propagating along the well. An advantage of such an approach to subsurface characterization and monitoring is that no active seismic source is required. Passive sources such as earthquakes can be utilized. A synthetic data example demonstrates viability of the approach for monitoring of small-scale CO2 injection into an aquifer. Two field DAS data examples based on signal recordings from several distant earthquakes show that the relevant properties of the near-well formation can be estimated with an accuracy of approximately 5%.

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