Seismic Exploration of the Deep Continental Crust: Methods and Concepts of DEKORP and Accompanying Projects

370 Pages·1999·18.57 MB·English

by  D. Gajewski

Seismic Exploration of the
Deep Continental Crust
Methods and Concepts ofDEKORP and
Accompanying Projects
Edited by
Dirk Gajewski
Wolfgang Rabbel
1999 Springer Basel AG
Reprint from Pure and Applied Geophysics
(PAGEOPH), Volume 156 (1999), No. 1/2
Editors:
Dirk Gajewski Wolfgang Rabbel
Institute for Geophysics Institute of Geosciences
University of Hamburg University of Kiel
Hamburg Kiel
Germany Germany
email: [email protected] e-mail: [email protected]
A CIP catalogue record for this book is available from the Library of Congress, Washington
D.C., USA
Deutsche Bibliothek Cataloging-in-Publication Data
Seismic exploration of the deep continental crust : methods and concepts of DEKORP and
accompanying projects led. by Dirk Gajewski ; Wolfgang Rabbel. -Basel ; Boston; Berlin:
Birkhiiuser, 1999
(Pageoph topical volumes)
ISBN 978-3-7643-6210-2 ISBN 978-3-0348-8670-3 (eBook)
DOI 10.1007/978-3-0348-8670-3
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© 1999 Springer Basel AG
Originally published by Birkhăuser Verlag in 1999
Printed on acid-free paper produced from chlorine-free pulp
ISBN 978-3-7643-6210-2
987654321
Contents
Foreword: Seismic Exploration of the Deep Continental Crust
D. Gajewski and W. Rabbel
7 Nature of Crustal Reflectivity along the DEKORP Profiles in Germany in
Comparison with Reflection Patterns from Different Tectonic Units World
wide: A Review
R. Meissner and W. Rabbel
29 Scales of Heterogeneities in the Continental Crust and Upper Mantle
M. Tittgemeyer, F. Wenzel, T. Ryberg and K. Fuchs
53 Seismic, Structural and Petrological Models ofthe Subcrustal Lithosphere in
Southern Germany: A Quantitative Revaluation
T. Weiss, S. Siegesmund and T. Bohlen
83 Seismic Shear Waves in Deep Seismic Reflection Surveys: Some Notes on
Problems and Profits
E. Liischen
97 Seismic Velocities and Anisotropy of the Lower Continental Crust: A
Review
T. Weiss, S. Siegesmund, W. Rabbel, T. Bohlen and M. Pohl
123 Recovering Shear-wave Anisotropy of the Lower Crust: The Influence of
Systematic Errors on Travel-time Inversion
T. Bohlen, W. Rabbel, T. Weiss, S. Siegesmund and M. Pohl
139 Realistic Models of Anisotropic Laminated Lower Crust
M. Pohl, F. Wenzel, T. Weiss, S. Siegesmund, T. Bohlen and W. Rabbel
157 3-D Prestack Kirchhoff Migration of the IS089-3D Data Set
S. Buske
173 3-D Prestack-migration of Wide-angle Data from a Variscan Transition
Zone
F. Bleibinhaus, M. Bopp, M. Simon and H. Gebrande
187 Focusing in Prestack Isochrone Migration Using Instantaneous Slowness
Information
M. Til/manns and H. Gebrande
207 An Attempt to Integrate Reflection Seismics and Balanced Profiles
D. Gajewski, A. Lambrecht and O. Oncken
233 Incorporation of the Effects of Absorption into Kirchhoff Migration
C. Toutou
253 Enhancement of Deep Seismic Reflections in Pre-stack Data by Adaptive
Filtering
B. Buttkus and C. Bonnemann
279 Estimation of Seismic Velocities from Deep Reflections in the r-p Domain
C. Bonnemann and B. Buttkus
303 The Use of Wave-field Directivity for Velocity Estimation: Moving Source
Profiling (MSP) Experiments at KTB
J. Muller, M. Janik and H.-P. Harjes
319 Elimination of Noise Caused by Spikes and Bursts in Vibroseis Data
U. P%m
345 How to Remedy Non-optimal Seismic Data by Seismic Processing
J. Fertig, M. Thomas and R. Thomas
©BirkhiluserVerlag,Basel.1999
Pure app!. geophys. 156 (1999) 1-6 I
0033-4553/99/010001-06 $ 1.50+0.20/0 Pure and Applied Geophysics
Foreword
Seismic Exploration of the Deep Continental Crust
The DEKORP Project
When DEKORP, the German continental reflection seismic program, started
its first deep seismic survey in 1984, no one would have expected that this project
would run for 14 consecutive years. During this time it became the major focus
of deep seismic research in Germany. Its open organisational structure, compris
ing separate executive and steering committees, admitted the participation of
many geoscientific institutions and university institutes in the planning and per
forming of research. Scientifically, the DEKORP activities were inspired by simi
lar foreign research programs already in existence at that time, such as COCORP
and BIRPS. DEKORP was funded by the German Federal Ministry of Research
(BMBF) and complemented by special funding programs of the German science
foundation (DFG), dedicated to the lower crust and the Variscan orogenic belt.
In the 1980s DEKORP focussed on deep seismic reflection profiling of the
crystalline crust of the Variscan mountain belt located between the North Ger
man Basin and the Alps. The seismic sections provided fundamental new insight
into the structure of the European continent and the dynamics of continental
formation: "from collision to collapse." They formed the basis for worldwide
comparative studies of orogenic structure. Many of the findings of DEKORP
and other reflection programs have become common geoscientific knowledge to
day, such as thin skinned thrusts and detachment zones, deeply extending frac
ture zones, laminated lower crust and missing mountain roots in the central
European Variscides, evolution models of upper and lower crust, among many
others.
In 1989, in support of the Continental Deep Drilling Program (KTB),
DEKORP performed the first 3-D survey of deep crystalline crust worldwide
imaging the profound complexity of tectonics at the KTB drilling site in the
Oberpfalz (S. Germany). The DEKORP group broadened its spectrum of re
search targets in the 1990s to approach further fundamental questions of conti
nental evolution, such as the formation of intracontinental basins and
subduction. The suite of DEKORP-profiles was finalized, partly in international
cooperation, by spectacular depth images of the Urals, including superdeep
2 Foreword Pure app!. geophys.,
reflections from the earth's mantle, of the Central Andes and the North Ger
man/Danish Basin.
From the outset of DEKORP near vertical reflection profiling was comple
mented by wide-angle reflection and refraction investigations to better determine
velocity depth functions, to improve the imaging of reflections, and to render
petrological interpretation feasible. In the processing of seismic reflection data,
DEKORP basically relied on the CMP-method approved for sedimentary data
but applied with special care to the complex crystalline targets by the processing
centres in Clausthal-Zellerfeld and Potsdam.
The complicated signature of the reflections from the deep crust often resem
bled scattering patterns rather than echos from continuous interfaces. This seis
mic structure as well as low impedance contrasts and low signal-to-noise ratios
indicated that new processing and interpretation techniques must be considered
to better image the crystalline crust and to take into account its particular
characteristics in wave propagation. Therefore, seismic data acquisition and geo
logical interpretation were complemented by research projects aiming at the im
provement of existing and the development of new seismic methods and
interpretation concepts, such as pre-stack migration, 3-D imaging, or the use of
shear waves and seismic anisotropy. Most of these accompanying projects were
directly conducted under the roof of the DEKORP organisation, others closely
related to it but funded by the DFG. The results of some of these projects are
presented in this volume of Pure and Applied Geophysics. The seismic reflection
sections and interpretation of the DEKORP data are published in a number of
Journal articles and most of the reflection data gathered before 1989 can be
viewed in the DEKORP-Atlas (Meissner, R. and Bortfeld, R.K. (eds.):
DEKORP-Atlas, Springer, Berlin, 1990). The first article of the present volume
by Meissner and Rabbel is a review of major results of the DEKORP project
shown in the context of continental reflection profiling worldwide.
This Volume
The articles of this special issue are organized in three topical groups:
(aJ Seismic Structure ofthe Deep Crust and Upper Mantle: Reflectivity,
Anisotropy and Scaling Effects
Meissner and Rabbel provide a review of reflection images associated with
different tectonical processes and compare these observations against each
other.
Vol. 156, 1999 Foreword 3
- Tittgemeyer et al. show that different scale lengths ofseismic heterogeneity exist
at different depth levels ofthe crust and mantle and they demonstrate how scale
length can be assessed quantitatively by seismic profiling and modelling.
- Liischen emphasizes the need for shear-wave investigations as a lithological
discriminator in deep crustal exploration and discusses problems ofS-wave data
acquisition.
- Weiss et al. present a review of the anisotropy of the lower continental crust
where laboratory investigations ofrock samples are considered and compared to
numerical models using the determined elastic parameters.
- In a second article, Weiss ef al. discuss the petrology of the upper mantle
beneath southern Germany and its relation to seismic velocity, anisotropy and
reflection strength on the basis of petrophysical data and numerical modelling.
- Considering a transversely isotropic lower crust, Bohlen et al. study sensitivity of
an iterative linearized 3-D travel-time inversion on two types of systematic
errors: erroneous velocity and/or interface topography.
- Pohl et al. present a modelling study on a laminated anisotropic lower crust
using the reflectivity method. Laboratory measurements of rocks from three
different sections of exposed lower crust served as input models.
(b) New Concepts ofSeismic Migration and Interpretation
Buske applied a Kirchhoff pre-stack depth migration to 3-D seismic data
collected at the site of the German continental deep drill hole. Depth sections
and time slices provide a 3-D image of a steeply dipping major fault zone, the
SEI-reflector, and a bright mid-crustal reflection, the so-called Erbendorf body.
- Bleibinhaus et al. used pre-stack migration of wide-angle data for imaging the
boundary between the Saxothuringian and Moldaunubian terranes.
- Tillmanns and Gebrande present a slowness-driven isochrone migration for 2-D
seismic data. The combination of slowness information with migration leads to
an improved depth image.
- Gajewski et al. present an approach to integrate structural geology and reflection
seismic data based on seismic modelling and geophysical borehole data.
- Toutou incorporates the effects of absorption into Kirchhoff migration. A
complex velocity is introduced into the Helmholtz equation leading to an
anti-dissipation operator which is convolved with the wave field.
(c) Seismic Signal Processing and Velocity Determination
- Buttkusand Bonnemann show that adaptive filtering is an attractive alternative to
conventional Wiener filtering. In order to increase the signal-to-noise ratio three
different adaptive filters involving multichannel seismic data are presented and
applied to DEKORP data.
4 Foreword Pure app!. geophys.,
Bonnemann and Buttkus review the potential of the r-p transform for the
determination of velocities from deep seismic reflection data. Special properties
of the transform are exploited to enhance signal-to-noise ratio.
- Muller et al. use the wave-field directivity to obtain velocity information from a
walk away VSP-experiment at the KTB-site. Local slant-stacks serve as a tool to
extract slowness information.
- Polom developed a new method to detect and eliminate impulse type noise from
vibroseis data before stack.
- Fertig et al. review key elements of digital seismic processing, such as wavelet
processing and coherency filtering, emphasizing their role as a precondition to
reliable interpretation.
In part, the articles open the perspective to new and future research. In part,
they are intended as a documentation of research activity triggered or inspired by
technical and interpretational questions raised by DEKORP field work and profil
ing results. Many of the presented methods can find immediate application in
industrial seismic prospecting, just as for many years the work for and with
DEKORP became an excellent starting point for students and their careers.
Future Perspectives ofDeep Seismic Research
The DEKORP project started as a national initiative to study the structure of
the deep crust of the Variscan mountain belt in central Europe, and to develop
appropriate methods of data processing and interpretation. The final phase was
characterized by international cooperation in science and funding, by the diversifi
cation of geological targets, representing different tectonic processes, and by
including the marine component to the terrestrial acquisition technique.
After DEKORP concluded and based on the results of deep seismic projects
worldwide, we can summarize the status and future perspectives of deep seismic
research by the following statements:
• Seismic reflection imaging, combined with refraction and wide-angle measure
ments, is established as a reliable standard technology of deep crustal research.
The current standard implies the use ofcompressional waves and a 2-D profiling
approach. The aim of future methodical efforts may be to identify new wave
field attributes which improve the definition of seismic subsurface models.
• The complex three-dimensional character ofmany geological key targets and the
need to develop efficient 3-D technology for exploring deep and superdeep
targets has become evident. Such technology will have to combine both high
resolution reflection seismics and passive seismological methods cost-effectively.
Vol. 156, 1999 Foreword 5
• Seismic response of tectonic features and the origin ofreflectivity are still under
discussion. Progress can only be achieved by increased efforts in petrophysics,
shear-wave technology and seismic anisotropy, by modelling, and, ofcourse, by
drilling and borehole investigations such as the International Continental scien
tific Drilling Program (lCDP).
• The dynamic aspects of crustal development, differences in the evolution of
upper and lower crust, the influence of rheology, stress and temperatures of
deformation have shown the need for comprehensive interdisciplinary studies,
preferably in regions which are tectonically active at present. Deep crustal
research in seismically active areas will represent a close link between scientific
and social needs.
• An increasing number of superdeep reflections from the earth's upper mantle is
being reported. Understanding mantle dynamics as the driving force of tectonic
processes will require the enhanced application of both active reflection and
passive seismic methods focussing on reflectivity, anisotropy and the scale length
of seismic heterogeneity.
• Studying crust-mantle interaction requires large-scale research programs which
combine transects of both oceanic and continental lithosphere with a focus on
active and passive continental margins.
• Even the need for classical seismic studies of the crust is far from being
saturated. The repeated occurrence of unexpected crustal features, such as the
bright spots in Tibet, illustrates that the catalogue ofdeep crustal conditions is
incomplete and too small to allow predictions of deep structure and processes
without seismic imaging.
In the framework of international reflection programs, DEKORP has broad
ened our insight into the evolution ofthe continental crust. It eventually led to new
frontiers to be opened by the next generation ofresearch initiatives dedicated to the
dynamics of the earth's interior.
Acknowledgements
The following persons substantially contributed to this volume by serving as
reviewers or as partners in discussing future perspectives in a spontaneous opinion
poll. We are grateful to: G. Bokelmann, Ch. B6nnemann, M. Bopp, H.-J. Brink, S.
Buske, B. Buttkus, N. Ettrich, J. Fertig, E. Fliih, W. Franke, K. Fuchs, H.
Gebrande, J.-H. G6tze, A. Henk, R. Kind, D. Kliischen, M. Korn, Ch. Krawczyk,
P. Krajewski, G. Lambare, E. Liischen, H. Lykke-Andersen, R. Marschal, J.