Ionelia Panea

The spatial data-adaptive minimum-variance distortionless-response beamformer on seismic single-sensor data

Coherent noise generated by surface waves or ground roll within a heterogeneous near surface is a major problem in land seismic data. Array forming based on single-sensor recordings might reduce such noise more robustly than conventional hardwired arrays. We use the minimum-variance distortionless-response (MVDR) beamformer to remove (aliased) surface-wave energy from single-sensor data. This beamformer is data adaptive and robust when the presumed and actual desired signals are mismatched. We compute the intertrace covariance for the desired signal, and then for the total signal (desired signal+noise) to obtain optimal weights. We use the raw data of only one array for the covariance of the total signal, and the wavenumber-filtered version of a full seismic single-sensor record for the covariance of the desired signal. In the determination of optimal weights, a parameter that controls the robustness of the beamformer against an arbitrary desired signal mismatch has to be chosen so that the results are optimal. This is similar to stabilization in deconvolution problems. This parameter needs to be smaller than the largest eigenvalue provided by the singular value decomposition of the presumed desired signal covariance. We compare results of MVDR beamforming with standard array forming on single-sensor synthetic and field seismic data. We apply 2D and 3D beamforming and show prestack and poststack results. MVDR beamformers are superior to conventional hardwired arrays for all examples.

Array forming in the presence of phase variations

Seismic reflections are sometimes strongly affected by the phase variations introduced by the intra-array statics, but corrections can be made using single sensors for array forming. In this way, the effect of the phase variations is attenuated by removing the intra-array statics from the single-sensor recordings. Then, the array response can be synthesized, and the amplitude and the shape of the reflection wavelet are preserved.

Scenarios for Local Seismic Effects of Tulcea (Romania) Crustal Earthquakes—Preliminary Approach of the Seismic Risk Characterization for Tulcea City

This paper is a multidisciplinary presentation of the seismogenic area situated in the North-Dobrogea Orogen (Tulcea). This zone is characterized by significant crustal seismic activity, with crustal earthquakes of magnitude Ms ≥ 5.0 on the Richter scale. Geological and geophysical data for the area are presented. The seismicity of the region is presented by making use of the latest catalogues, exemplified with maps and a 3D figure. Focal mechanisms with their parameters for several earthquakes are analyzed together with the observation data (provided by different seismological stations). A brief presentation of the main geological features, which are characteristic of the tectonic units that build up the North-Dobrogean Orogene, outlines the diverse petrographic constitution of the various structural levels. In order to discuss the local seismic site effects two scenarios are considered, both of which take into account the characteristics of the seismogenic area. The first one considers the city exposed to an earthquake (superficial) from the E Vrancea zone and the second one considers the city exposed to a seismic event with magnitude Mw = 5.1 from Sf. Gheorghe fault. The earthquake epicentres are located in very active seismic areas. The important features taken into account are the nonlinear behavior of the upper soil strata, the effect of the bedrock elasticity, and different shear modulus and damping of the linear-equivalent-type system. Additionally, several local amplification functions are presented.

Analysis of the surface waves present on the passive seismic data set recorded in the Mizil area (Romania)

A passive seismic survey in April 2009 had two main goals: the use of seismic interferometry to retrieve the seismic signal (reflected waves) from the background noise and to demonstrate that passive surveys can cover spatial gaps between active profiles. Seismic interferometry works well if the passive records are not affected by surface waves with high amplitudes; therefore, such records are neglected in the application of seismic interferometry.

Geophysical Analysis of Major Geothermal Anomalies in Romania

The Romanian segment of the Eastern Pannonian Basin and the Moesian Platform are known for their geothermal and hydrocarbon-bearing structures. We used seismic, gravity, and geothermal data to analyze the geothermal behavior in the Oradea and Timisoara areas, from the Romanian segment of Eastern Pannonian Basin, and the Craiova–Bals–Optasi area, from the Moesian Platform. We processed 22 seismic reflection data sets recorded in the Oradea and Timisoara areas to obtain P-wave velocity distributions and time seismic sections. The P-wave velocity distributions correlate well with the structural trends observed along the seismic lines. We observed a good correlation between the high areas of crystalline basement seen on the time seismic sections and the high heat flow and gravity-anomaly values. For the Craiova–Bals–Optasi area, we computed a three-dimensional (3D) temperature model using calculated and measured temperature and geothermal gradient values in wells with an irregular distribution on the territory. The high temperatures from the Craiova–Bals–Optasi area correlate very well with the uplifted basement blocks seen on the time seismic sections and high gravity-anomaly values.

The use of seismic modeling for the geologic interpretation of deep seismic reflection data with low signal-to-noise ratios

AbstractWe have performed a deep seismic reflection study, DACIA-PLAN, based on the data recorded along a crooked line across the southeastern Romanian Carpathians. The signal-to-noise ratio (S/N) of these data varies along the seismic profile, and its variation is considered to be an effect of the rough topography, complex subsurface geology, and varying surface conditions encountered during seismic data acquisition. The migrated time section that covers the mountainous area is clear, without visible reflections, making the geologic interpretation very difficult. We used a seismic modeling technique to explain the poor S/N of the recorded data and to generate synthetic seismic sections that can be useful for the geologic interpretation of the field seismic section (migrated time section). We used ray-tracing modeling to obtain the expected seismic expression of horizons of interest. Subsurface illumination modeling indicates that the complex subsurface geology and irregularly deployed sources and receive...

Analysis of crooked-line 2D seismic reflection data recorded in areas with complex surface and subsurface conditions

We analysed a crooked-line two-dimensional (2D) seismic reflection dataset recorded in the northern part of the Eastern Pannonian Basin using a different processing flow than the one typically used in hydrocarbon exploration studies, with the purpose of obtaining high-resolution images for the depth interval of interest. The data acquisition was done using irregular spacing between receivers and sources due to the rough topography and complicated geological structure. As an effect of data acquisition, the midpoints were spread over an area with a width of ~900 m on the central part of the seismic profile. A significant number of field records contained noisy first arrivals on the offset interval used in the computation of static corrections. We defined two types of geometry: one for a 2D crooked-line and another for a pseudo-3D survey; then, we used the same processes and parameters during processing. For the noisy field records, we used refraction interferometry (RI) by cross-correlation and convolution to enhance the first arrivals and to better control picking of the first-arrival traveltimes. We show that the continuity of the supervirtual first arrivals depends on the number of records, with noisy and clear first arrivals used as input to RI. Comparing the pre- and post-stack data, represented by filtered common-depth-point (CDP) gathers and time sections, respectively, we notice that the use of pseudo-three-dimensional (3D) geometry helps us to improve the signal-to-noise ratio (SNR) of pre- and post-stack data, and to image the main geological marker reflections in the investigated area. A crooked-line 2D seismic reflection dataset recorded in the northern part of the Eastern Pannonian Basin was processed to obtain a high-resolution seismic image for the area of interest. Refraction interferometry was used to enhance the first arrivals and to compute static corrections with good accuracy.

Integrated geophysical and geochemical investigations for identifying potable water sources on Ampoi Valley, Romania

The mining activities generate today the biggest pollution problems that the industry is facing. Zlatna mining region, Romania, was declared in the ‘90s the most polluted area in Europe. The primary goal of the study is to analyze the contamination in the studied area at the soil level, underground terrain, hydrological and hydrogeological systems, generated by the ore processing activities. The study is part of a project which further aims to guide the local communities as to which land is more favorable to the practice of agriculture and grazing of animals. The project team will search for solutions for identifying sources of potable water in the context of the polluted aquifers

Array forming and prestack stereotomography

On land seismic reflection data, the surface waves are considered the main type of noise observed. The last decades, the hard-wired arrays were successfully used for their attenuation. Unfortunately, according to some modeling results, the hard-wired arrays can destroy the seismic signal (primary reflected waves), especially in the case of areas with rough topography where, locally, the reflections seen on the array responses are stretched and show very low amplitude; as a result, the signal-to-noise ratio of the responses is lower. The processing of the seismic data characterized by low signal-to-noise ratio can be difficult to perform. For example, the velocity analysis depends on the presence of clear reflections. A very efficient technique used for performing a velocity analysis is the pre-stack stereotomography, but we will show that its results are a function of the quality of input data. Analyzing array responses from an area with rough topography, shows that the responses based on the single-sensor recordings can be successfully used in the pre-stack stereotomography, contrasting with those obtained using hard-wired arrays.