Wave dispersion in dry granular materials by the distinct element method

Numerical studies are conducted to investigate the existence of wave dispersion in resonant column tests on dry granular soil. To this end, the two-dimensional distinct element method (DEM) in the time domain is employed. The investigations focus on the effect of sample width, voids ratio, viscous damping and wavelength, on propagation velocities of longitudinal harmonic waves in rectangular samples of uniform grains. It is shown that granular materials may exhibit anomalous dispersion that is, wave velocities that increase with increasing excitation frequency. This increase may exceed 20% for squatty samples, but becomes less pronounced for slender samples. Similar findings have been reported in some experiments on granular materials, but have not been systematically explored by numerical means. Results are presented in the form of dimensionless graphs and charts that elucidate the salient features of the problem. Comparisons with findings from gradient elastodynamic and viscoelastic theories are discussed.     

Evaluation of near-source seismic records based on damage potential parameters: Case study: Greece

The study presents the results of an investigation that identifies among the available records in Greece those that reveal near-source characteristics, using a procedure based on damage potential parameters. The findings reaffirm the opinion that earthquakes with magnitude less than M=6.0 present near-source phenomena that can cause severe damage to relatively stiff structures, common in urban areas. Spectra from selected accelerograms of near-source records are compared with the corresponding elastic spectra of the current Greek Seismic Code, (EAK 2000), the impulsive character of the Greek near-source records is ascertained and shortcomings of EAK 2000 to account for near-source effects are demonstrated.Based on records from seismic events in Greece and records from international earthquakes of small-to-moderate magnitude, the study demonstrates that there exists a near-source magnitude-distance region, where the velocity pulses have smaller amplitude and period in comparison with earthquakes of great magnitude. A simplified representation of three pulse types is, also, adopted for near-source events. It is found that the type-A pulse related to permanent displacement phenomena does not characterize Greek records. In addition a simple criterion is developed to identify the most appropriate simplified pulse type for near-source seismic events independently of magnitude.     

A numerical study on deblending of land simultaneous shooting acquisition data via rank-reduction filtering and signal enhancement applications

We propose a workflow of deblending methodology comprised of rank-reduction filtering followed by a signal enhancing process. This methodology can be used to preserve coherent subsurface reflections and at the same time to remove incoherent and interference noise. In pseudo-deblended data, the blending noise exhibits coherent events, whereas in any other data domain (i.e. common receiver, common midpoint and common offset), it appears incoherent and is regarded as an outlier. In order to perform signal deblending, a robust implementation of rank-reduction filtering is employed to eliminate the blending noise and is referred to as a joint sparse and low-rank approximation. Deblending via rank-reduction filtering gives a reasonable result with a sufficient signal-to-noise ratio. However, for land data acquired using unconstrained simultaneous shooting, rank-reduction–based deblending applications alone do not completely attenuate the interference noise. A considerable amount of signal leakage is observed in the residual component, which can affect further data processing and analyses. In this study, we propose a deblending workflow via a rank-reduction filter followed by post-processing steps comprising a nonlinear masking filter and a local orthogonalization weight application. Although each application shows a few footprints of leaked signal energy, the proposed combined workflow restores the signal energy from the residual component achieving significantly signal-to-noise ratio enhancement. These hierarchical schemes are applied on land simultaneous shooting acquisition data sets and produced cleaner and reliable deblended data ready for further data processing.