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Selected aspects of modern seismic imaging and near-surface velocity model building in the area of Carpathian fold and thrust belt

Dalętka Andrzej Michał

Geology, Geophysics and Environment

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2021

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Vol. 47, no. 2

71--93

EN

Despite the increasing technological level of the reflection seismic method, the imaging of fold and thrust belts remains a demanding task, and usually leaves some questions regarding the dips, the shape of the subthrust structures or the most correct approach to velocity model building. There is no straightforward method that can provide structural representation of the near-surface geological boundaries and their velocities. The interpretation of refracted waves frequently remains the only available technique that may be used for this purpose, although one must be aware of its limitations which appear in the complex geological settings. In the presented study, the analysis of velocity values obtained in the shallow part of Carpathian orogenic wedge by means of various geophysical methods was carried out. It revealed the lack of consistency between the results of 3D refraction tomography and both the sonic log and uphole velocities. For that reason, instead of the industry-standard utilization of tomography, a novel, geologically-consistent method of velocity model building is proposed. In the near-surface part, the uphole velocities are assigned to the formations, documented by the surface geologic map. Interpreted time-domain horizons, supplemented by main thrusts, are used to make the velocity field fully-compatible with the litho-stratigraphic units of the Carpathians. The author demonstrates a retrospective overview of seismic data imaging in the area of the Polish Carpathian orogenic wedge and discusses the most recent global innovations in seismic methodology which are the key to successful hydrocarbon exploration in fold and thrust regions.

2

Near-surface Structure of the Carpathian Foredeep Marginal Zone in the Roztocze Hills Area

Majdański M., Grzyb J., Owoc B., Krogulec T., Wysocka A.

Acta Geophysica

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2018

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Vol. 66, no. 2

179--189

EN

Shallow seismic survey was made along 1280 m profile in the marginal zone of the Carpathian Foredeep. Measurements performed with standalone wireless stations and especially designed accelerated weight drop system resulted in high fold (up to 60), long offset seismic data. The acquisition has been designed to gather both high-resolution reflection and wide-angle refraction data at long offsets. Seismic processing has been realised separately in two paths with focus on the shallow and deep structures. Data processing for the shallow part combines the travel time tomography and the wide angle reflection imaging. This difficult analysis shows that a careful manual front mute combined with correct statics leads to detailed recognition of structures between 30 and 200 m. For those depths, we recognised several SW dipping tectonic displacements and a main fault zone that probably is the main fault limiting the Roztocze Hills area, and at the same time constitutes the border of the Carpathian Forebulge. The deep interpretation clearly shows a NE dipping evaporate layer at a depth of about 500-700 m. We also show limitations of our survey that leads to unclear recognition of the first 30 m, concluding with the need of joint interpretation with other geophysical methods.

3

Implementation of Elastic Prestack Reverse-Time Migration Using an Efficient Finite-Difference Scheme

Yan H., Yang L., Dai H., Li X. Y.

Acta Geophysica

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2016

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Vol. 64, no. 5

1605--1625

EN

Elastic reverse-time migration (RTM) can reflect the underground elastic information more comprehensively than single-component Pwave migration. One of the most important requirements of elastic RTM is to solve wave equations. The imaging accuracy and efficiency of RTM depends heavily on the algorithms used for solving wave equations. In this paper, we propose an efficient staggered-grid finite-difference (SFD) scheme based on a sampling approximation method with adaptive variable difference operator lengths to implement elastic prestack RTM. Numerical dispersion analysis and wavefield extrapolation results show that the sampling approximation SFD scheme has greater accuracy than the conventional Taylor-series expansion SFD scheme. We also test the elastic RTM algorithm on theoretical models and a field data set, respectively. Experiments presented demonstrate that elastic RTM using the proposed SFD scheme can generate better images than that using the Taylor-series expansion SFD scheme, particularly for PS images. Furthermore, the application of adaptive variable difference operator lengths can effectively improve the computational efficiency of elastic RTM.

4

Application of first-arrival tomography to characterize a quick clay landslide site in southwest Sweden

Adamczyk A., Malinowski M., Malehmir A.

Acta Geophysica

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2013

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Vol. 61, no. 5

1057--1073

EN

First-arrival traveltime tomography was applied to high-resolution seismic data acquired over a known quick-clay landslide scar near the Göta River in southwest Sweden in order to reveal the geometry and physical properties of clay-related normally consolidated sediments. Investigated area proved to be a challenging environment for tomographic imaging because of large P-wave velocity variations, ranging from 500 to 6000 m/s, and relatively steeply-dipping bedrock. Despite these challenges, P-wave velocity models were obtained down to ca. 150 m for two key 2D seismic profiles (each about 500-m long) intersecting over the landslide scar. The models portrait the sandwich-like structure of marine clays and coarse-grained consolidated sediments, but the estimated resolution (20 m) is too small to distinguish thin layers within this structure. Modelled velocity structures match well the results of reflection seismic processing and resistivity tomography available along the same profiles.

5

Forward modeling and reverse-time migration in wavenumbers (kx, kz) domain

Kostecki A., Półchłopek A.

Acta Geophysica Polonica

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2001

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Vol. 49, nr 2

203-212

EN

The paper presents the theoretical foundation of wavefield extrapolation process in inhomogeneous medium with arbitrary variations of velocity. The matrix-like Neumann series in wavenumber and time domain is shown to be one of solutions to the wave equation. The approximation used in seismic migration to reduce the time consuming matrix operations is presented. The synthetic wavefields were used to demonstrate the properties of forward modeling and reverse-time migration.