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Tselentis G.-A.,University of Patras | Martakis N.,LandTech Enterprises | Paraskevopoulos P.,University of Patras | Lois A.,University of Patras
Geophysics | Year: 2011

We have studied using traveltimes of P- and S-waves and initial seismic-pulse rise-time measurements from natural microearthquakes to derive 3D P-wave velocity VP information (mostly structural) as well as P- and S-wave velocity VP/VS and P-wave quality factor QP information (mostly lithologic) in a known hydrocarbon field in southern Albania. During a 12-month monitoring period, 1860 microearthquakes were located at a 50-station seismic network and were used to obtain the above parameters. The data set included earthquakes with magnitudes ranging from -0.1 to 3.0 R (Richter scale) and focal depths typically occurring between 2 and 10 km. Kohonen neural networks were implemented to facilitate the lithological classification of the passive seismic tomography (PST) results. The obtained results, which agreed with data from nearby wells, helped delineate the structure of the reservoir. Two subregions of the investigated area, one corresponding to an oil field and one to a gas field, were correlated with the PST results. This experiment showed that PST is a powerful new geophysical technique for exploring regions that present seismic penetration problems, difficult topographies, and complicated geologies, such as thrust-belt regions. The method is economical and environmentally friendly, and it can be used to investigate very large regions for the optimal design of planned 2D or 3D conventional geophysical surveys. © 2011 Society of Exploration Geophysicists. All rights reserved.


Tselentis G.,University of Patras | Paraskevopoulos P.,University of Patras | Martakis N.,LandTech Enterprises
Geophysical Prospecting | Year: 2010

We determine the attenuation structure of a three-dimensional medium based on first pulse-width measurements from microearthquake data. Ninety-five microearthquakes from a seventy stations local network were considered in this study. Measurements of the first half cycle of the wave, the so-called rise time τ were carried out on high quality velocity seismograms and inverted to estimate the P-waves intrinsic quality factor Qp. The results of this investigation indicate that first pulse width data from a local microearthquake network permit retrieval with sufficient accuracy of the heterogeneous Qp structure. The inferred attenuation variability corresponds to the known geological formations in the region. © 2010 European Association of Geoscientists & Engineers.


Tselentis G.,University of Patras | Martakis N.,LandTech Enterprises | Paraskevopoulos P.,University of Patras | Lois A.,LandTech Enterprises | Sokos E.,University of Patras
SEG Technical Program Expanded Abstracts | Year: 2011

A combined method is proposed for seismic events detection, signal enhancement and automatic P-phase picking. This method is comprised by a Chi-squared based test statistical test for the event detection, filtering in the S-transform domain, for denoising and an automatic picker based on the Kurtosis criterion. The performance of the method is tested and evaluated on both synthetic and real data. © 2011 Society of Exploration Geophysicists.


Serpetsidaki A.,University of Patras | Tselentis G.-A.,University of Patras | Martakis N.,LandTech Enterprises | Sokos E.,University of Patras
Bulletin of the Seismological Society of America | Year: 2012

The Albanides represent a complex orogen made up of a heterogeneous tectonic nappe pile of Paleozoic, Mesozoic, and Cenozoic domains. Albania is tectonically active, and moderate to strong earthquakes have occurred in the past. However, abundant microseismicity has not been monitored and studied by a dense seismic network. During this study, a seismic network of 40 stations was deployed in southern Albania for one year. A total of 2113 microearthquakes were well located. The most accurately located events and 810 focal mechanisms were used in order to define the seismotectonics and the stress pattern in the area. Results indicate that thrust and strike-slip faulting both exist in southwestern Albania, suggesting a continuation of the complex tectonic setting of the neighboring northwestern Greece to the north.


Lois A.,University of Patras | Sokos E.,University of Patras | Martakis N.,LandTech Enterprises | Paraskevopoulos P.,University of Patras | Tselentis G.-A.,University of Patras
Geophysics | Year: 2013

Algorithms that deal with the automatic S-onset time identification problem are a topic of ongoing research. Modern dense seismic networks used for earthquake location, seismic tomography investigations, source studies, early warning, etc., demand accurate automatic S-wave picking. Most of the techniques that have been proposed up to now are mainly based on the polarization features of the seismic waves. We propose a new time domain method for the automatic determination of the S-phase arrival onsets, and present its implementation on local earthquake data. Eigenvalue analysis takes place over small time intervals, and the maximum eigenvalue which is obtained on each step is retained for further processing. In this way, a time series of maximum eigenvalues is formed, which serves as a characteristic function. We obtain a first S-phase arrival time estimation by applying the kurtosis criterion on the derived characteristic function. Furthermore, a multiwindow approach combined with an energy-based weighting scheme is also applied, to reduce the algorithm's dependence on the moving window's length and provide a weighted S-phase onset. Automatic picks were compared against manual reference picks, resulting in mean residual time of 0.051 s. Moreover, the proposed technique was subjected to a noise robustness testand sustained a good performance. The mean residual time remained lower than 0.1 s, for noise levels between -1 up to 8 dB. The proposed method is easy to implement, because it is almost parameter free and demands low computational resources. © 2012 Society of Exploration Geophysicists.


Tselentis G.,University of Patras | Martakis N.,LandTech Enterprises | Paraskevopoulos P.,University of Patras | Lois A.,LandTech Enterprises | Sokos E.,University of Patras
Geophysics | Year: 2012

Small-magnitude seismic events, either natural or induced microearthquakes, have increasingly been used in exploration seismology with applications ranging from hydrocarbon and geothermal reservoir exploration to high-resolution passive seismic tomography surveys. We developed an automated methodology for processing and analyzing continuously recorded, single-channel seismic data. This method comprised a chi-squared-based statistical test for microseismic event detection and denoising filtering in the S-transform domain based on the Otsu thresholding method. An automatic P-phase picker based on higher order statistics criteria was used. The method was used with data from a surface seismic station. The performance of the method was tested and evaluated on synthetic and real data from a microseismic network used in a high-resolution PST survey and revealed a high level of consistency. © 2012 Society of Exploration Geophysicists.

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