PreviousNext No AccessSeismic Diffraction4. Diffractions Observed on Seismic DataAuthors: Bruno F. J. KunzF. Steve SchiltSidney KaufmanGeorge H. LongRalph A. DoughertyRalph A. StephenRichard K. SnavelyA. K. M. SarwarP. SadowiakJ. VossR. MeissnerEnru LiuStuart CrampinJohn A. HudsonNathalie Favretto-CristiniEric de BazelaireGilles GrandjeanDonatienne LeparouxP. DiviaccoM. RebescoA. CamerlenghiJoachim PlaceCharles NavilleIsabelle MorettiJianghai XiaJonathan E. NyquistYixian XuMary J. S. RothRichard D. MillerBruno F. J. Kunz, F. Steve Schilt, Sidney Kaufman, George H. Long, Ralph A. Dougherty, Ralph A. Stephen, Richard K. Snavely, A. K. M. Sarwar, P. Sadowiak, J. Voss, R. Meissner, Enru Liu, Stuart Crampin, John A. Hudson, Nathalie Favretto-Cristini, Eric de Bazelaire, Gilles Grandjean, Donatienne Leparoux, P. Diviacco, M. Rebesco, A. Camerlenghi, Joachim Place, Charles Naville, Isabelle Moretti, Jianghai Xia, Jonathan E. Nyquist, Yixian Xu, Mary J. S. Roth, and Richard D. Millerhttps://doi.org/10.1190/1.9781560803188.ch4 SectionsAboutPDF/ePub ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InRedditEmail Abstract Selected papers illustrating diffractions on seismic data are presented in Chapter 4, including the responses to faulting, near surface anomalies, seafloor features, and topography. The seismic data examples illustrate the strong diffraction response of subsurface discontinuity or heterogeneity. Permalink: https://doi.org/10.1190/1.9781560803188.ch4FiguresReferencesRelatedDetails Seismic DiffractionISBN (print):978-1-56080-317-1ISBN (online):978-1-56080-318-8Copyright: 2016 Pages: 832 publication data© 2016 All rights reserved. This book or parts hereof may not be reproduced in any form without permission in writing from the publisher.Publisher:Society of Exploration Geophysicists HistoryPublished in print: 01 Jan 2016 CITATION INFORMATION Bruno F. J. Kunz, F. Steve Schilt, Sidney Kaufman, George H. Long, Ralph A. Dougherty, Ralph A. Stephen, Richard K. Snavely, A. K. M. Sarwar, P. Sadowiak, J. Voss, R. Meissner, Enru Liu, Stuart Crampin, John A. Hudson, Nathalie Favretto-Cristini, Eric de Bazelaire, Gilles Grandjean, Donatienne Leparoux, P. Diviacco, M. Rebesco, A. Camerlenghi, Joachim Place, Charles Naville, Isabelle Moretti, Jianghai Xia, Jonathan E. Nyquist, Yixian Xu, Mary J. S. Roth, and Richard D. Miller, (2016), "4. Diffractions Observed on Seismic Data," Geophysics Reprints Series : 499-653. https://doi.org/10.1190/1.9781560803188.ch4 Plain-Language Summary PDF DownloadLoading ...

In the autumn of 1989 a co-operative experiment involving 12 research institutions in northwestern Europe collected 2268 km of deep seismic reflection profiles in the Gulf of Bothnia and the Baltic Sea. The 121 litre airgun array used for this profiling was also recorded by 62 multicomponent land stations to provide coincident refraction surveys, fan-spreads, and 3-D seismic coverage of much of the Gulf of Bothnia. We thus have potentially both high-resolution impedance contrast images as well as more regional 3-D velocity models in both P- and S-waves. In the Bothnian Bay a south-dipping, non-reflective zone coincides with the conductive Archaean-Proterozoic boundary onshore in Finland. Between the Bothnian Bay and Bothnian Sea observed reflectivity geometries and velocity models at Moho depths suggest structures inherited from a 1.9 Ga subduction zone; the upper crust here appears to have anomalously low velocity. Within the Bothnian Sea, reflectivity varies considerably beneath the metasedimentary/granitoid rocks of the Central Svecofennian Province (CSP) and the surrounding metavolcanic-arc rocks. Numerous dipping reflectors appear throughout the metavolcanic crust, whereas the CSP has little reflectivity. Wide-angle reflections indicate that the metasedimentary crust of the Bothnian Basin is 10 km thicker than the neighbouring Svecofennian subprovinces. Near the Aland archipelago Rapakivi granite plutons exhibit bright reflections, a contrast to the usual non-reflective plutons elsewhere in western Europe. Additional dipping reflections deep in the crust of this area may support models of rifting and crustal thinning during emplacement of the 1.70-1.54 Ga Rapakivi granites. Coeval gabbroic/anorthositic magmatism may explain the high reflectivity and high velocity of these plutons. The c. 1.25 Ga mafic sills and feeder dykes of the Central Scandinavian Dolerite Group also produce clear reflections on both near- and far-offset seismic sections. Continued modelling will produce better velocity models of the crust and better constrained contour maps of crustal thickness in this part of the Baltic shield.

In 1989 the BABEL Working Group collected 2268 km of near-vertical reflection data in the Baltic and Bothnian Seas. As an integrated part of the field survey, the marine airgun shots were recorded by 64 multicomponent land stations. In this paper results are presented from interpretation of profiles B and A in the Baltic Sea, extending from the Aland Archipelago (Finland) into the Bay of Lubeck (Germany). In the shield part of the profiles northeast of the Sorgenfrei-Tornquist Zone, crustal reflectivity is observed at all levels and its termination in depth coincides largely with the crust-mantle boundary. The wide-angle data indicate a three-layer crust with velocities of 6.1-6.4, circa 6.6, and 6.9-7.2 km s-1. The Moho is found between 40-48 km depth, corresponding to 12-15 s TWT. In the northeastern part of profile A and the southern part of profile B, steeply northeast-dipping reflections are found at all crustal levels. The tectonic inversion of the Sorgenfrei-Tornquist Zone is clearly imaged above a thickened, high-velocity lowermost crust (7.1-7.4 km s-1). At depth the Sorgenfrei-Tornquist Zone widens and displays some asymmetry that is believed to be indicative of crustal shortening across the zone. An undulating Moho is observed along profile A where the lateral variability in structure and velocity field primarily is in the lower crust. Beneath the Skurup Basin south of the Sorgenfrei-Tornquist Zone, no intracrustal discontinuities are seen in the wide-angle data and a highly reflecting lowermost crust between 8 and 10 s TWT corresponds to a zone between 25 and 31 km depth with high velocity gradient (6.7-7.1 km s-1). A bright upper mantle reflection at 12 s TWT below the Skurup Basin can be explained by a velocity increase from 7.8 to 8.2 km s-1. Southwest-dipping reflections in the basement of the Mon High, an eastward continuation of the Ringkobing-Fyn basement High, indicate that the Caledonian Deformation Front is located at least 50 km further north than previously believed. The crust below the Mon High is 38 km thick with high velocities (7.1-7.4 km s-1) in the lower crust. In the North German Lowlands, the crystalline crust below the 10 km thick post-Caledonian sedimentary sequence is only 20 km thick and has velocities between 6.0 and 6.9 km s-1. It is hypothesized that during the Caledonian evolution, Baltica's Pre-Cambrian crust protruded into the docking Avalonian terrain as a major crustal flake structure.

Before the deposition of a Proterozoic cover and the repeated Proterozoic reworking of the older rocks, the presently exposed Archaean areas in northern Sweden formed part of a coherent craton. In the present study, we have used SmNd isotopic analyses of Proterozoic granitoids and metavolcanics to delineate the Archaean palaeoboundary. In a regional context, the transition from strongly negative ϵNd(t) values in the northeast to positive values in the southwest is distinct, and approximately defines the border of the old craton. The Archaean palaeoboundary extends in a WNW direction, and is subparallel to the longitudinal axis of the Skellefte sulphide ore district but it is situated ∼ 100 km farther to the north. The ∼ 1.9 Ga old granitoids on the two sides of the palaeoboundary were all formed in compressional environments, but those situated to the north have higher contents of LILE and LREE at similar contents of Si. This indicates that they were generated in an area with thicker crust and supports the location of the Archaean-Proterozoic palaeoboundary. There is no simple correlation between the Archaean palaeoboundary, as defined by the isotopic results, and any of the major fracture systems as interpreted from regional geophysical measurements. Reflection seismic work indicates that juvenile volcanic-arc terrains to the south have been thrust onto the Archaean craton. Possible thrust faults have been identified from aeromagnetic measurements. Rifting of the Archaean craton created a passive margin ∼ 2.0 Ga ago. Spreading shifted to convergence with subduction beneath the Archaean continent ∼ 1.9 Ga ago. Subsequently, the resulting juvenile volcanic arc collided with the old continent, and the Archaean palaeoboundary as existing today was formed by a collision characterized by overthrusting. The boundary then was disturbed by later deformation predominantly along NNE-trending fracture systems.

In 1989 comprehensive seismic studies were carried out at the German deep drilling location (KTB) in the Oberpfalz (NE-Bavaria) by the DEKORP group. The survey is known as Integrated Seismics Oberpfalz 1989 (IS089). The aims were to predict events ahead of the drill bit, to investigate the structures and to connect the more or les small-scale results from the KTB-drillhole with the large-scale geological/tectonical environment, to investigate parameters as seismic velocities and anisotropy and to learn about the nature of seismic-reflections in crystalline area.

The dense network of deep seismic reflection lines in western and central Europe makes a systematic comparative study possible. The reflectivity of the continental crust is not the same but can change significantly from one seismic line to the next. Different seismic reflectivity patterns can be observed and correlations to specific tectonic units are found.

SUMMARYThe terrane concept is understood as an important extension of plate tectonics and is based on the recognition of allochthonous, mobile geological units. The concept is successfully applied to the Variscides with their wide range of collisional belts. It is mainly supported by the dense deep‐seismic network of DEKORP, which reveals certain reflectivity patterns and succeeds in mapping old and new deep fault zones between the terranes. Variscan terranes are rooted in the ductile lower crust and seem to consist of continental crust only, partly exclusively of rigid upper crust. Oceanic terranes, on the other hand, are always rooted in the asthenosphere. The development of continental terranes and their boundaries depends strongly on their thermal and rheological history. In the case of post‐orogenic collapse with heating and extension of the lower crust, seismic lamellae develop and often truncate former thrust faults.

This paper demonstrates that - under favorable conditions - by using multichannel recording and subsequent stacking of adjacent records marine airgun shots have been detected at offset distances up to 700 km, the maximum offset at which we attempted to record data. Besides a powerful airgun array, a low noise environment at the recording site and the elimination of static shifts are the prerequisites to obtain refracted and reflected arrivals from the crust and upper mantle at such large offsets. Primary arrivals detected at offsets between 400 and 700 km image the upper mantle from 70 to about 120 km depth. Stacking of neighbouring shots and/or receivers successfully increases the signal-to-noise ratio, if the traces have been corrected for offset differences, which requires knowledge of the apparent phase velocities. The data presented here were collected in autumn 1989 during the BABEL Project on the Baltic Shield.

A systematic comparative study of the reflectivity of the continental crust is undertaken-made possible by the expanded data basis of deep seismic reflection profiles in Europe. The reflectivity is not at all the same but can change dramatically from one seismic line to the next. Different seismic reflectivity patterns-independent of applied techniques-are observed. Correlations between these seismic patterns and specific tectonic units can be found. While lamellae and bands of reflections in the lower crust are widespread in post-orogenic extensional areas, 'crocodiles' seem to represent compressional zones that are occasionally accompanied by seismic duplex structures. The 'fishbone' pattern-many 'mini-crocodiles'-is characteristic for the old London-Brabant Massif. The 'ramp and flat' structure displays the thin-skinned tectonics of the North Variscan Deformation Front over a length of 2000 km. Diffraction clusters in the lower crust accompanied by a dipping reflection in the upper crust can be observed close to thick-skinned deformation fronts. Diffractions are also present in rift areas, for example in the North Sea. Some regions show a decreasing reflectivity with depth. The reflections are concentrated in the upper crust and no Moho reflections can be observed. These areas can be correlated to areas with Precambrian crust.

The Tornquist Zone is Europe's longest tectonic lineament and bisects the continent in a NW-SE direction from the North Sea (off NW Denmark) to the Black Sea. New deep seismic reflection and coincident refraction data have been collected across its 50 km wide, intensely faulted and inverted NW part. The marine reflection profile in the area north of Bornholm Island shows a tilted block structure in the rigid upper crust, whereas the lower crust seems to be more gently uplifted. A complex transition from the highly reflective lower crust to the mantle is indicated by mantle reflections and a curious wide-angle event recorded by a landstation on Bornholm Island. We suggest that deep-reaching inversion tectonics, induced by Alpine and Carpathian orogeny, were responsible for the development of the gross crust-mantle structure of the Tornquist Zone in our study area, which seems to be similar to that in Poland.

Within the DEKORP project (DEKORP: Deutsches Kontinentales Reflexionsseismisches Programm) a joint deep seismic reflection venture with the BELCORP (Belgian Continental Reflection Seismic Programme) group of the Belgian Geological Survey was carried out in 1987 across the Rhenish Massif, a part of the mid-European Variscides. This orogenic belt developed in the Upper Devonian/Carboniferous. Mostly Devonian rocks crop out at the surface. The Rhenish Massif is bordered by two sedimentary troughs: the sub-Variscan Foredeep in the north and the Permo-Carboniferous Saar-Nahe Basin in the south. In the east-west direction it is subdivided by the axial depression of the Eifel Nord-Sud Zone.The aim of the survey which totals almost 220 km of seismic profiling, was to investigate the crustal structure of the western part of the Rhenish Massif and to compare it with the line DEKORP 2-N which crosses the eastern portion of the massif. The results indicate the presence of NW-vergent tectonics of various styles that can often be traced down to deep parts of the crust. Horizontal Variscan compression plays a dominant role in the northern part while post-Variscan extension seems to dominate in the Saar-Nahe Basin, although even there traces of Variscan compression seem to be preserved in the middle crust. Common characteristics of the pre-Palaeozoic basement, differences between the western and eastern parts of the Rhenish Massif, and the deep extension of the Aachen Thrust (Faille du Midi) have been clearly observed. This prominent thrust in the north with its characteristic ramp and flat structure has been followed over 100 km length down to 15 km depth. It contrasts sharply with the very complex deep fault system in the south, separating the post-orogenic Saar-Nahe Basin from the Hunsruck mountains. The integration of accompanying geophysical and geological studies provides a new synoptic picture of the evolution and tectonics of this key area of Central Europe.

Reflection seismics in compressional belts has revealed the structure of crustal shortening and thickening processes, showing complex patterns of indentation and interfingering of colliding crusts and subcrustal lithospheres. Generally in the upper crust large zones of detachments develop, often showing duplexes and 'crocodile' structures. The lower crust from zones of active collision (e.g. Alps, Pyrenees) is characterized by strongly dipping reflections. The base of the crust with the Moho must be continuously equilibrating after orogenic collapse as areas of former continental collision exhibit flat Mohos and subhorizontal reflections. The depth to the Moho increases during collision and decreases after the onset of post-orogenic extension, until finally the crustal root disappears completely together with the erosion of the mountains.Processes, active during continental collisions and orogenic collapse, create distinct structures which are imaged by reflection seismic profiling. Examples are shown and discussed.

The Seismic reflection profiles of DEKORP (DEutsches KOntinentales ReflexionsSeismisches Programm) in the Federal Republic of Germany to date have been limited to areas of the Variscan orogeny. Nevertheless, the character of their reflections differs considerably and may be correlated to certain Variscan and post-Variscan developments. Lower crust lamellae develop in areas of high heat flow, mostly associated with post-Variscan extensional processes; “crocodile” and nappe tectonics are best preserved in the cores and at the flanks of older massifs which were incorporated into the Variscan orogeny. So far poor reflectivity has been observed only in the area of the London-Brabant Massif which was not involved in any of the Phanerozoic orogenies.

Seismic reflection profiling across strong dip (more than 25°) suture zones between different crustal terranes or provinces reveals a distinct seismic reflectivity pattern of dipping reflections and associated diffractions. Suture zones in areas dominated by thin‐skinned tectonics do not show this pattern. Profiles from southern Germany (Deutsches Kontinentales Reflexionsseismisches Programm), Great Britain (British Institutions Reflection Profiling Syndicate) and North America (Consortium for Continental Reflection Profiling and U.S. Geological Survey) crossing major sutures are examined with respect to dipping reflections in the upper crust and accompanying diffractions in the lower crust. The strong resemblance of seismic patterns suggests a similar origin for these structures.

Plate tectonics provides the linking framework for all tectonic and magmatic activity seen today, but it is not known when plate tectonics first developed on Earth. New deep seismic reflection and coincident refraction profiles across an exposed, 1.89-Gyr-old volcanic arc complex show a 10-km-thick offset in the Moho and bivergent reflectors in the crust, which were most probably created by plate convergence, subduction and accretion during the Early Proterozoic. Hence, plate tectonic models seem to be applicable for at least the second half of Earth's history.