Paraskevas Paraskevopoulos

Local high-resolution passive seismic tomography and Kohonen neural networks — Application at the Rio-Antirio Strait, central Greece

A high-resolution passive seismic investigation was performed in a 150 km 2 area around the Rio-Antirio Strait in centralGreeceusingnaturalmicroearthquakesrecordedduringthreemonthsbyadense,temporaryseismicnetworkconsisting of 70 three-component surface stations. This work was part of the investigation for a planned underwater rail tunnel, and it gives us the opportunity to investigate the potential of this methodology. First, 150 well-located earthquake events were selected to compute a minimum 1D velocity model for the region. Next, the 1D model served as the initialmodelfornonlinearinversionfora3DP-andS-velocity crustal structure by iteratively solving the coupled hypocenter-velocity problem using a least-squares method. The retrieved Vp and Vp/Vs images were used as an input to Kohonen self-organizing maps SOMs to identify, systematicallyandobjectively,theprominentlithologiesintheregion. SOMs are unsupervised artificial neural networks that map the input space into clusters in a topological form whose organization is related to trends in the input data. This analysis revealed the existence of five major clusters, one of which may be related to the existence of an evaporite body not shown in the conventional seismic tomography velocity volumes. The survey results provide, for the first time, a 3D model of the subsurface in and around the Rio-Antirio Strait. It is thefirst time that passive seismic tomography is used together with SOM methodologies at this scale, thus revealing themethod’spotential.

Application of a high‐resolution seismic investigation in a Greek coal mine

High‐resolution seismic methods were applied to map the detailed structure and thickness of the Domeniko coal basin (central Greece) and to search for lateral discontinuities, such as pinch‐outs and faults. Extensive tests were performed to optimize recording parameters and equipment. Reflection events which can be attributed to coal layers can be interpreted from depths of approximately 30 to 150 m on CDP stacked and inverted sections. Several low‐throw faults have been interpreted from the sections. Results obtained from the high‐resolution seismic reflection survey combined with drillhole information clearly revealed the 3‐D model of the coal field.Using geostatistical methods, the results of the high‐resolution reflection seismic survey were combined with the information from the borehole program to clearly reveal the 3‐D model of the basin.

High-resolution passive seismic tomography for 3D velocity, Poisson’s ratio ν, and P-wave quality QP in the Delvina hydrocarbon field, southern Albania

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 ...

A method for microseismic event detection and P‐phase picking

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 Stransform 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.

On the Use of Kohonen Neural Networks for Site Effects Assessment by Means of H/V Weak-Motion Spectral Ratio: Application in Rio-Antirrio (Greece)

Abstract The investigated area, located around the Rio-Antirrio Strait, Central Greece, has been the target of a seismic microzonation campaign. Seventy seismic stations have been deployed for a period of 4 months, recording in continuous mode. Despite the high level of urban noise, we compiled a data set of 95 earthquakes recorded at most of the 70 sites. By employing the attributes of self-organizing maps (SOMs), a quality-control and signal-improving method is proposed. A SOM (Kohonen, 1997) is a type of unsupervised neural network. The main property of SOMs utilized is that while the competitive learning algorithm on whom this method is based maps the input data on an n -dimensional grid of neurons, the topological relations (proximity of patterns in input data) are preserved in the output space. SOM is applied to the horizontal-to-vertical spectral ratios (HVSR) of every weak event analyzed for each station separately and allows a better evaluation of the stability of the HVSR.

Ray and finite-difference modelling of CDP seismic sections for shallow lignite deposits

Abstract A numerical experiment carried out to investigate the structural model of the Domenico lignite site is discussed. The model is a 2D structure containing several lignite layers at different depths, and a low-velocity layer at the top of the model. The experiment consists in simulating a measured CDP section by two independent techniques, based on completely different concepts: the finite-difference method and the ray method. Due to the incompleteness of the ray synthetic wave field, as well as to numerical problems of the finite differences at higher frequencies, the agreement between the synthetic seismogram sections for the individual shot points is poor. However, the CDP stacked sections modelled by the ray and finite-difference methods agree rather well. This is because the main differences between the wave fields computed by the two methods are due to the presence of the low-velocity layer (ground roll, head waves, etc.), and just these parts of the wave field can be suppressed by routine data processing such as f–k filtration. Synthetic ray and finite-difference CDP stacks agree relatively well with the observed data. They confirm three lignite seams and a fault in the shallower one. The synthetic data also indicate that many apparent horizons of the measured section may be due to the multiple reflections within the subsurface low-velocity layer.

Evaporite mapping using high resolution passive seismic tomography and Kohonen neural networks

A034 Evaporite Mapping Using High Resolution Passive Seismic Tomography and Kohonen Neural Networks G. Tselentis* (University of Patras Seismological Laboratory) N. Martakis (LandTech Enterprise SA) P. Paraskevopoulos (University of Patras Seismological Laboratory) & S. Kapotas (LandTech Enterprises SA) SUMMARY Passive seismic tomography application for exploration shows great potential. We present two case studies in Greece that successfully use the passive seismic tomography method in combination with Kohonen neural networks in order to better map subsurface features EAGE 69 th Conference & Exhibition — London UK 11 - 14 June 2007 Introduction Evaporite identification from geophysical data is an important task

Saline Water Intrusion Monitoring And ControlAt Guves Prefecture - Crete

This paper addresses the problem of saline water intrusion into the coastal aquifers of Guves Prefecture, Crete. First, after contacting a carefully planned geoelectric survey we mapped all regions of the aquifer possessing the highest saline water intrusion hazard. Next, a system was specially designed which continuously monitored the movement of the saline water front and automatically controlled all nearby pumping stations.