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Seismic tomography studies of cover thickness and near-surface bedrock velocities
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, Geophysics.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
2006 (English)In: Geophysics, ISSN 0016-8033, Vol. 71, no 6, U77-U84 p.Article in journal (Refereed) Published
Abstract [en]

Reflection seismic imaging of the uppermost kilometer of crystalline bedrock is an important component in site surveys for locating potential storage sites for nuclear waste in Sweden. To obtain high-quality images, refraction statics are calculated using first-break travel times. These first-break picks may also be used to produce tomographic velocity images of the uppermost bedrock. In an earlier study, we presented a method applicable to data sets where the vast majority of shots are located in the bedrock below the glacial deposits, or cover, typical for northern latitudes. A by-product of this method was an estimate of the cover thickness from the receiver static that was introduced to sharpen the image. We now present a modified version of this method that is applicable for sources located in or on the cover, the general situation for nuclear waste site surveys. This modified method also solves for 3D velocity structure and static corrections simultaneously in the inversion process. The static corrections can then be used to estimate the cover thickness. First, wetest our tomography method on synthetic data with the shot points in the bedrock below the cover. Next, we develop a strategy for the case when the sources are within the cover. The method is then applied to field data from five crooked-line, high-resolution reflection seismic profiles ranging in length from 2 to 5 km. The crooked-line profiles make the study 2.5 dimensional regarding bedrock velocities. The cover thickness along the profiles varies from 0 to 15 m. Estimated thickness of the cover agrees well with data from boreholes drilled near the profiles. Low-velocity zones in the uppermost bedrock generally correlate with locations where reflections from the stacked sections project to the surface. Thus, the method is functional, both for imaging the uppermost bedrock velocities as well as for estimating the cover thickness.

Place, publisher, year, edition, pages
2006. Vol. 71, no 6, U77-U84 p.
Keyword [en]
Geophysical techniques, Imaging, Radioactive waste repositories, Rocks, Seismic waves, Seismology
National Category
Earth and Related Environmental Sciences
Identifiers
URN: urn:nbn:se:uu:diva-92682DOI: 10.1190/1.2345191ISI: 000242598600057OAI: oai:DiVA.org:uu-92682DiVA: diva2:165851
Available from: 2005-02-22 Created: 2005-02-22 Last updated: 2017-01-25Bibliographically 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.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 19
Keyword
Geophysics, High-Resolution, Reflection Seismics, Tomography, Nuclear waste repository, Bedrock, Sediments, Geofysik
National Category
Geophysics
Identifiers
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
Opponent
Supervisors
Available from: 2005-02-22 Created: 2005-02-22 Last updated: 2016-05-13Bibliographically approved

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Juhlin, Christopher

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