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High-resolution reflection seismic imaging of the upper crust at Laxemar, southeastern Sweden
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
2002 (English)In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 355, no 1-4, 201-213 p.Article in journal (Refereed) Published
Abstract [en]

A major cost in exploring the upper 1–2 km of crystalline crust with reflection seismics is the drilling required for explosive sources. By reducing the charge size to a minimum, shallow inexpensive shotholes can be drilled with handheld equipment. Here, we present results from a full-scale test using small charges for high-resolution seismic surveying over a nuclear waste disposal study site (not an actual site). Two 2–2.5-km-long crossing profiles were acquired in December 1999 with 10-m shot and geophone spacing in the Laxemar area, near Oskarshamn in southeastern Sweden. After standard processing, including dip moveout (DMO), several subhorizontal to moderately dipping reflections are imaged. Many of the dipping ones can be correlated to fracture zones observed in a ca. 1700-m-deep borehole where the profiles cross and/or to fracture zones mapped on the surface. The imaged fracture zones form a complex 3D pattern illustrating the necessity of having 3D control before interpreting seismic reflection data. Analyses of sonic and density logs from the borehole show that greenstones have significantly higher impedances than the more dominant granite found in the borehole (granite/greenstone reflection coefficient is +0.065). These greenstones may contribute to the reflectivity when associated with fracture zones. In some cases, where they are present as larger subhorizontal lenses, they may be the dominant source of reflectivity. A set of north-dipping (10°) reflectors at 3–3.5-km depth can be correlated to a similar set observed below the island of Ävrö about 3 km to the east.

Place, publisher, year, edition, pages
2002. Vol. 355, no 1-4, 201-213 p.
Keyword [en]
Nuclear waste, 3D structure, Fracture zones, Crystalline rocks
National Category
Earth and Related Environmental Sciences
URN: urn:nbn:se:uu:diva-92680DOI: 10.1016/S0040-1951(02)00142-7OAI: oai:DiVA.org:uu-92680DiVA: diva2:165849
Available from: 2005-02-22 Created: 2005-02-22 Last updated: 2014-02-11Bibliographically approved
In thesis
1. High-Resolution Seismics Methods Applied to Till Covered Hard Rock Environments
Open this publication in new window or tab >>High-Resolution Seismics Methods Applied to Till Covered Hard Rock Environments
2005 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Reflection seismic and seismic tomography methods can be used to image the upper kilometer of hard bedrock and the loose unconsolidated sediments covering it. Developments of these two methods and their application, as well as identifying issues concerning their usage, are the main focus of the thesis. Data used for this development were acquired at three different sites in Sweden, in Forsmark 140 km north of Stockholm, in the Oskarshamn area in southern Sweden, and in the northern part of the Siljan Ring impact crater area.

The reflection seismic data were acquired with long source-receiver offsets relative to some of the targeted depths to be imaged. In the initial processing standard steps were applied, but the uppermost part of the sections were not always clear. The longer offsets imply that pre-stack migration is necessary in order to image the uppermost bedrock as clearly as possible. Careful choice of filters and velocity functions improve the pre-stack migrated image, allowing better correlation with near-surface geological information.

The seismic tomography method has been enhanced to calculate, simultaneously with the velocity inversion, optimal corrections to the picked first break travel times in order to compensate for the delays due to the seismic waves passing through the loose sediments covering the bedrock.

The reflection seismic processing used in this thesis has produced high-quality images of the upper kilometers, and in one example from the Forsmark site, the image of the uppermost 250 meters of the bedrock has been improved. The three-dimensional orientation of reflections has been determined at the Oskarshamn site. Correlation with borehole data shows that many of these reflections originate from fracture zones.

The developed seismic tomography method produces high-detail velocity models for the site in the Siljan impact area and for the Forsmark site. In Forsmark, detailed estimates of the bedrock topography were calculated with the use of the developed tomography method.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2005. viii+66 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 19
Geophysics, High-Resolution, Reflection Seismics, Tomography, Nuclear waste repository, Bedrock, Sediments, Geofysik
National Category
urn:nbn:se:uu:diva-4811 (URN)91-554-6162-X (ISBN)
Public defence
2005-03-18, Axel Hambergsalen, Geocentrum, Villavägen 16, Uppsala, 10:00
Available from: 2005-02-22 Created: 2005-02-22 Last updated: 2016-05-13Bibliographically approved

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