IGCP Project 474
Images of the Earth's Crust & Upper Mantle
contact us: contact@earthscrust.org.au | about us
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IGCP Project 474
Images of the Earth's Crust & Upper Mantle |
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contact us: contact@earthscrust.org.au | about us
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34th International
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Updated: Jun 02, 2016 4th International Symposium on Deep Seismic Profiling of the Continents and Their MarginsBayreuth, Germany, 4-7 September 1990from Geodynamics Series Volume 329, pages 1-450 (1991) (Full copies of these papers may be obtained by purchasing the AGU Geodynamics Series Volume 22. Contact AGU via www.agu.org for details.)
COCORP Seismic Investigations of the Surrency Bright Spot Beneath the Southern Appalachians, USAPages 1-7 Now at: An unusually strong mid-crustal reflector, the Suffency Bright Spot (SBS), was found at a depth of approximately 16 km during earlier COCORP profiling in the southeastern U.S. In addition to being very bright, the SBS is notable for being unusually flat and horizontal for about half of its 4 km length. As these characteristics are similar to those of fluid caused reflections at shallow depths, it has been suggested that the SBS may be caused by in-situ mid-crustal fluids. If caused by fluid enclosed in fracture porosity in solid rock, the reflection would be expected to exhibit a negative polarity from the top of the porous zone as well as showing anomalous amplitude versus offset reflectivity. A series of seismic reflection experiments have therefore been undertaken by COCORP to measure the reflection properties of the SBS. A small reflection seismic experiment with an explosive source was conducted to determine the polarity of the SBS. The experiment consisted of four dynamite shots, each recorded at three receiver stations by Seismic Group Recorders (SGR) borrowed from Amoco Production Company. Comparison of the dynamite records with geophone polarity tests indicate that the SBS is characterized by a positive reflection coefficient at its top. This result itself does not negate the fluid hypothesis a fluid fluid interface could cause the positive reflection as well as the "flat spot" nature of the reflector. However, modelling indicates that the SBS waveform is most simply interpreted as originating from a circa 120 m thick, high-impedance layer with wavelet tuning contributing to the unusually large amplitude. In this light, a fluid model becomes much more ad hoc. Though the fluid hypothesis is not precluded by this experiment, we feel that the SBS is more likely caused by a mafic or ultramafic layer, either tectonically or magmatically emplaced. A much more extensive experiment was undertaken in the summer of 1990 to obtain 3 component CDP and expanding spread (ESP) profiles over the SBS, as well as crossline and three dimensional control. Initial results show that the SBS is well imaged on all of the P-wave data to 36 km offsets and is visible on some of the S-wave data as well. In the third dimension the SBS is subhorizontal and of comparable lateral extent as on the main profile. Deep Seismic Reflection Constraints on Palaeozoic Crustal Structure and Definition of the Moho in the Buried Southern Appalachian OrogenPages 9-20 Now at: Experimental reprocessing of COCORP deep seismic reflection data collected over the buried Southern Appalachian orogen (southeastern USA) provides a more accurate characterisation of middle/lower crustal structure and Moho, thus allowing a quantitative basis for geologic interpretation. Reprocessing and migration of data over the late Palaeozoic suture (Alleghanian Hercynian) between North America and relict west Africa reveal varying north vergent thrust geometries including ramp-and-flat, antiformal, and planar structures. Interpretation of a strike-parallel line over the suture zone implies that the suture is dominated by discrete bands of south-dipping thrusts which are expressed, in strike view, as a "layered" fabric of sub horizontal reflections. The internal structure of the suture zone varies dramatically along strike becoming broader and less steep eastward across the Atlantic Coastal Plain as the amount of crustal "overlap" between relict west African and North American terranes progressively increases. Suture zone reflections are abruptly truncated in the lower crust by a ubiquitous sub horizontal reflection Moho which is anomalously shallow generally at 33-36 km (relative to a much deeper Moho beneath the Blue Ridge and Inner Piedmont to the west). The character of the reflection Moho varies throughout the area and, after deconvolution and frequency filtering, appears either as a simple doublet/triplet or more complex multicycle event. This multicyclic character may be suggestive of magmatic underplating by mafic igneous sills intruded during a regional thermal event brought on by early Mesozoic extension and crustal thinning related to the initial rifting and opening of the North Atlantic. The broad correspondence between a highly reflective Moho underlying thinned crust and the region of early Mesozoic rifting, together with its horizontal truncation of Palaeozoic suture-zone structure, suggests that the Moho is a dynamic boundary that is early Mesozoic in age and was produced by rifting processes. High Resolution P- and S-Wave Deep Crustal Imaging Across the Edge of the Colorado Plateau, USA: Increased Reflectivity Caused by Initiating ExtensionPages 21-29 The Transition Zone bordering the southwestern edge of the Colorado Plateau is a region that is undergoing the early stages of extension, as the Basin and Range province encroaches into non-extended Colorado Plateau crust. A high-resolution seismic reflection profile imaging the southwestern edge of the Colorado Plateau was acquired in 1989 by Stanford University in cooperation with the U.S. Geological Survey 1989 Colorado Plateau seismic refraction program. Three-component recording allowed us to compile a single fold S-wave common midpoint (CMP) section to accompany a P-wave CMP section. The P- and S-wave sections reveal a "transparent" upper crust, underlain by middle crust with a laminated sub horizontal reflective texture. Beginning about 9 s two way travel-time (twtt) (P-wave), a gradual die-out of reflectivity occurs with the Moho located between 12 and 13 s twtt (about 38 to 40 km depth). An increase in middle and lower crustal reflectivity is observed beneath Chino Valley, at the northeastern edge of the Transition Zone, as compared to the northern end of the profile beneath the Colorado Plateau. This increased reflectivity in combination with local seismicity and high heat flow may be an indication of continental crust in the initial stages of extension. The vertical distribution of reflectivity both on the P- and S-wave sections also suggests changes with depth in the way the crust is responding to tectonic extension. If the magnitude of crustal reflectivity in extending regions is an indication of amounts of ductile shear or magmatic intrusion, then we are observing a zone of mid-crustal deformation located 10 20 km above the Moho. Crustal Anisotropy and the Structure of the Mohorovicic Discontinuity in Western Nevada of the Basin and Range ProvincePages 31-38 Near the intersection of perpendicular lines, the 1986 Nevada PASSCAL wide-angle seismic reflection data show strikingly different responses of the deep crust and Moho in each direction. The seismic signature of the crust and Moho in the dip line (east-west) is characterized at normal incidence by strong, subparallel, laterally discontinuous, multicyclic events lacking observable diffractions. In the strike line (north-south), the crust is highly transparent with short events and a high-amplitude, laterally continuous Moho reflection. The PmP phase presents an abrupt high frequency attenuation from subcritical to postcritical offsets characteristic of tunnelling waves. PmS and/or SmP and SmS phases are also identified as lower frequency dispersed wave trains. Forward modelling of amplitudes by the reflectivity algorithm, reflection coefficients estimates, and heat flow data suggest such high velocity contrasts for the Moho structure that interlayering of partial melts with mantel (high .velocity) material at the base of the crust is a strong possibility. Finite difference forward modelling indicates that mid- and/or lower-crustal anisotropy achieved by placing a stack of tectonic lenses (boudins) strongly elongated parallel to the stretching direction (dip_direction), disrupts a strong laterally continuous event to a series of short multicyclic reflection segments. The synthetic seismic record sections parallel and perpendicular to the longer axes of the heterogeneities (i.e.,lenses) closely model the seismic signature observed in the E-W and N-S shot gathers, respectively. Thus deep crust may consists of a series of lenses, highly elongated in the E-W (stretching) direction, overlying a Moho marked by ponded lenses of about 5% partial melt. Seismic Reflectors in High Grade Metamorphic Rocks of the Kapuskasing Uplift: Results of Preliminary Drill Site SurveysPages 39-45 The nature and origin of deep-crustal reflections can be studied in the Kapuskasing uplift of the central Superior Province, where middle to lower crustal rocks breach the surface. High seismic velocities and reflection zones can be traced up dip from near to their normal depths at >20 km to within a few hundred metres of the surface. The gently dipping reflection zones, defined by LITHOPROBE Vibroseis sections, project to a high-grade metamorphic sequence of interlayered mafic and felsic gneiss and tonalite. Physical property studies of representative rock samples from the study area confirm the presence of high densities and high compressional wave velocities at shallow depths. Additional dynamite high resolution reflection profiling was carried out in 1989 as a preliminary site for the Canadian Continental Drilling Program (CCDP). The main goal of the survey was to provide lateral control of the laminated reflections observed at shallow depths on the Vibroseis profiles. The strong reflections seen on the Vibroseis profiles can be traced with confidence into the dynamite profiles. Amplitude-versus-offset (avo) studies of these shallow reflections show an amplitude decay with offset that is compatible with a simple model of interlayered mafic, felsic and tonalitic rocks. Seismic Reflectivity Patterns of the Kapuskasing Structural ZonePages 47-52 In 1987 88 a deep seismic reflection survey was conducted over the Kapuskasing structural zcne (KSZ) in the Canadian shield as a part of the L ITHOPROBE Project. Approximately 360 km of vibroseis data were acquired along 9 profiles which crossed the boundaries between the KSZ and Val Rita Block to the west and between the KSZ and the Abitibi belt to the east. A statistical analysis of the data reveals that are large differences in the reflectivity patterns of the various blocks. The pattern of the KSZ is typical for old shield areas, i.e. a relatively reflective upper crust underlain by a largely non-reflective lower crust with no indication of the Moho. In contrast, the whole crust in the adjacent Val Rita and Abitibi region is highly reflective. A comparison of the reflection and wide-angle refraction data from 1984 Kapuskasing seismic refraction survey further confirms these crustal reflectivity differences. Both data sets also suggest that the Moho is well defined under the Val Rita Block. Our results show that the contrasts may have resulted from the effects of the thrust-like tectonic processes which occurred during the development of the KSZ. These processes could have caused brittle faulting in the upper crust and ductile flow in the lower crust. The results indicate that after tectonic processes have created a reflectivity pattern in the crust, subsequent tectonic periods will not be able to destroy it. The Structure of the Archean Crust in SW Greenland from Seismic Wide-Angle Data: A Preliminary AnalysisPages 53-57 An extensive seismic wide-angle and vertical-incidence experiment was conducted over Archean crust in SW Greenland. 15 PASSCAL REFTEK recorders were placed on 35 locations along the coast and fjords recording 6000 cu.in. air-gun shots from offsets as far as 350 km. 100-150 m shot spacing allows one to identify several unaliased pre- and post-critical coherent phases from throughout the crust and uppermost mantle. The most prominent P-wave phases consist of Pg- and PmP-arrivals, mid- and lower crustal (PIP) reflections, and Pn-arrivals. Excellent quality PIP and PmP phases can be traced from near-vertical to post-critical distances. S-wave arrivals include Sg and SmS (or SIS) -phases, and a strong Sn-phase at offsets between 200 and 280 km. To obtain local one-dimensional estimates of the velocity structure, an extremal inversion method was applied. All receiver gathers were slant-stacked and semblance-filtered. Pre- and post-critical arrivals were picked in the tau-p domain and inverted. Depth bounds show that velocities exceed 6.0 km/s at depths below 4 to 6 km, followed by a low gradient to the bottom of the crust where velocities exceed 7.0 km/s. The Moho depth is estimated to be 30-40 km with increasing depth northward. Depth bounds derived for two of the stations show a major discontinuity appearing between 6 and 8 km above the Moho. Mafic material, possibly accreted during the opening of the Labrador Sea, might be responsible for the high velocities between both reflectors. Post Orogenic Evolution of the European Crust Studied from ECORS Deep Seismic ProfilesPages 59-68 Recent deep seismic profiles acquired by the ECORS project have been compared to former surveys. The Gulf of Lions basement is a segment of the Pyrenees-Provence belt beneath which the crustal root was removed in .the late Eocene. Metamorphism and/or magmatic intrusion played a part in crustal removal as in the Variscan crust. The evolution of the crust beneath the Tertiary rifts of the Upper Rhinegraben, Bresse and the Gulf of Lions is characterized by a small extension of the basement, great crustal thinning, high P-wave velocities in the deep crust and asthenospheric upwelling. Interpretation suggests that interactions between asthenosphere, lithosphere and crust should have occurred in relation to Alpine belt development. The Cretaceous and Triassic-Jurassic basins also show a discrepancy between basement extension and crustal thinning, probably also related to interactions between mantle and crust. In contrast, the layered lower crust, unthinned beneath the Triassic-Jurassic basins, suggests that underplating occurred during or after the crustal thinning which controlled basin subsidence. The present review shows that the Moho and, to a certain extent, the layered lower crust can be adjusted through time. Their changes were governed not only by mechanical deformation but also by transfer of material and modification of the physical properties near the crust-mantle boundary. Tthe DEKORP Surveys: Major Results in Tectonic and Reflective StylesPages 69-76 Looking back on more than five years of DEKORP's activity the great increase of our knowledge on the deep structure and evolution of the Variscan mountain belts in Central Europe is reviewed. Collisions and sutures have been imaged, and especially in the Rhenohercynian a complex interwedging and interfingering of crustal units are observed. Similar processes can still be verified in the Moldanubian/Saxothuringian area of the Oberpfalz on the flanks of the large Bohemian Massif. In contrast, the center part of the Moldanubian and Saxothuringian shows numerous signs of post-Variscan crustal modifications, like flattening and equilibration of the Moho, lower crust lamination, thin mafic lower crusts and many plutons, possibly responsible for the rather transparent upper crust. These and other types of reflectivity and their correlation to tectonic terranes and their boundaries can be explained by different thermal histories and different heat supplies with their strong impact on crustal rheology. Reflectivity of a Proterozoic Shield: Examples from BABEL Seismic Profiles Across FennoscandiaPages 77-86 We present examples of recently acquired marine seismic reflection profiles across the Baltic Shield. Our regional transect across a distance of 1350 km shows a highly variable range of reflectivity patterns in the crust. Some of the data exhibit a clear reflection Moho and bright lower-crustal reflections that have been recognized in the past mainly in Phanerozoic terranes. In other parts of the BABEL sections we see an apparently transparent crust, a diffractive crust, or regions without a clear reflectivity transition from crust to mantle, features that have commonly been recognized in Precambrian areas in other surveys. Though this paper is based only on brute stacks, the excellent quality of the BABEL data clearly shows that it is inappropiate to distinguish generically between Phanerozoic and Precambrian reflectivity patterns. Rather, the variability in reflectivity along the BABEL profiles seems more probably due to differences in structures and lithology than to differences in crustal age. Members of the BABEL Working Group:
Reflectivity Variations of Variscan Terranes in GermanyPages 87-90 Interpretations of the Variscan orogenic domain involve a plate tectonic evolution. They require inter-province oceans, but ophiolites or other relics have been found so far. The widths of the oceans are also debated, and paleomagnetic data do not support any particular interpretation. It is one of the main objectives of the deep reflection seismic projects in Germany to study crustal fine structure and decipher the evolution of the Variscides. Profiles now total more than 2,400 km in length. Line DEKORP 2 (470 km) traverses all Variscan provinces, and exhibits distinctive terrane-related properties, both in terms of structure and distribution of reflections. We apply the method of automatic energy histograms (based on final stacks in digital format on magnetic tape) to more clearly extract information about reflectivity from the data in an unbiased manner. The results yield convincing evidence for the existence of different terranes welded together during the various tectonic compressions. Structure of the Saar Nahe Basin (SW Germany) from DEKORP Profiles 1-C AND 9-NPages 91-95 Two deep seismic reflection profiles have been acquired by the DEKORP program across the northeastern part of the Late Palaeozoic Saar-Nahe-Basin (SW Germany). The geological interpretation reveals a halfgraben structure bounded to the NW by a fault zone with complex movement history consisting of a subvertical fault and a southeast-dipping normal fault. The latter is a detachment fault which delimits the Permo-Carboniferous basin fill. On the basis of DEKORP profiles 1-C and 9-N a model for the evolution of the Saar-Nabe-Basin is developed. Seismic Anisotropy of the Earth's Crust of the Urals and its Possible Relation to Oriented Cracking and to Stress StatePages 97-99 This paper reports on a field experiment and first interpretations of azimuthal anisotropy of seismic waves in the Ural Mountains. Field observations were conducted by the geological organization "Uralgeologiya". We used 25, 48-channel sets of geophones, located on an area of 5000 square km with a spacing of 10 km, on four crossing lines. Every set has 24 vertical geophones, with 100 m spacing on two orthogonal lines of 1100 m length, as well as three-component and twelve-component inclined azimuth-oriented geophones [Gal'perin, 1984], Shots were located so that the investigated area could be examined by seismic rays at all azimuths and up to distances of 60 km. The following observations were made:
3-D Vertical Incidence Seismic Reflection Survey at the KTB Location, OberpfalzPages 101-113 Within the Integrated Seismics Oberpfalz (ISO 89) the 3-D reflection survey attains a key role. The DEKORP Processing Center was charged with the scientific planning and the complete 3-D processing. The survey area is located in the FR of Germany, northeastern Bavaria, its center is the KTB borehole (KTB = Kontinentales Tiefbohr-Programm der Bundesrepublik Deutschland; German Continental Deep Drilling Program). In this paper the following topics will be discussed: planning (aims, theoretical acquisition configuration, field inspection, exact positioning of all locations, full 3-D pre binning), acquisition (transposition of all locations .into the survey area, summary of all actual field parameters), and first results (actual 3-D bin-coverage, concept of 3-D processing, azimuthal velocity determination, preliminary 3-D stacks). The field data comprise 3327 records each with 478 channels with 12 s recording-time at 4 ms sampling rate, which amounts to a total data volume of about 18 GigaBytes covering a 3-D block of about 20 km side length. The processing is not yet finalized, but a few insights into seismic fine structures are already possible in order to facilitate geotectonic interpretation of the KTB data. Investigation of Structures and Anisotropy by a 3-D Expanding-Spread ExperimentPages 115-120 In the scope of the Integrated Seismics Oberpfalz 1989 (ISO89) at the Oberpfalz continental deep drilling (KTB) location, a 3D expanding-spread experiment was carried out by the DEKORP group in 1989. The vibrator signals of the 3D seismic survey were recorded with additonal geophone spreads in such a way that a ~40-fold 2.6 x 3.8 squ km CMP-area centered at the KTB location was obtained. The source-receiver offsets are in the range of 0 24 km and azimuth angles of 0 180 are achieved. The data set consists of 189,460 seismic traces. The configuration and data description from this experiment are presented, and problems in the velocity analysis caused by geology, dipping reflections and anisotropy are discussed. Integrated P- and S-Wave Borehole Experiments at the KTB Deep Drilling Site in the Oberpfalz Area (SE Germany)Pages 121-133 Wolfgang Rabbel Data are presented from integrated special experiments which have been performed in :conjunction with a seismic 3-D reflection survey at the KTB (Continental Deep Drilling Program) site in the Oberpfalz area. The main objective of our investigations was to study the nature of P- and S-wave reflections and velocities, the Poisson's ratio and the seismic anisotropy in a medium of crystalline rocks around the KTB site, where the pilot-borehole has reached a depth of 4000 m. The program described here consists of:
The VSP surveys display steeply dipping reflections, increasing in number below 3000 m depth. Horizontal structures, preferentially seen in the surface profiling, are the exception. Seismic anisotropy revealed by polarization analysis and traveltimes of split shearwaves can be explained by the predominant azimuth and dip of rock foliation. Processing and Analysis of MSP Experiments Within the KTB ProjectPages 135-145 At the KTB drill-site several MSP-surveys were conducted. Their aim was the prediction of discontinuities that should be hit by the main drill hole. By iterative forward and inverse modelling a subsurface image was obtained. In addition to a steeply dipping reflector element at approximately 4500 m depth and a few near-horizontal elements in the depth interval from 6000 m to 8000 m, the analysis suggests a highly shattered, inhomogeneous rock unit down to at least 3.5 s TWT, without discontinuities of considerable spatial extension. 3-D Wide-Angle Investigations in the KTB Surroundings as part of the "Integrated Seismics Oberpfalz 1989 (IS089)", First ResultsPages 147-160 The high reflectivity of the upper and middle crust of the Oberpfalz, both for near vertical and wide-angle reflections, was one reason for selecting this area as the site for the German Continental Deep Drilling Program KTB. A peculiarity in the middle crust beneath the KTB site is the so-called Erbendorf Body (EB) giving rise to extremely strong wide-angle reflections. Its position right on the border between two tectonometamorphic units of the Variscan fold belt, unusual high P-wave velocities (over 7.0 km/s) in its lower part at 11 to 14 km depth, and associated dipping reflectors suggest a possible lower crustal origin of the EB. A specially designed and so far _unique wide-angle 3D-survey was carried out as part of the program "Integrated Seismics Oberpfalz 89" for investigating the spatial extent, the velocity and the internal structure of the EB. The first results show clearly that the EB is not a local phenomenon beneath the DEKORP4 line, but that it exists, yet with remarkable complexities, beneath the whole covered area between the Franconian Line and the Falkenberg granite complex. The wide-angle shots were also recorded by four 3-component geophones in the KTB pilot hole at 3195 to 3295 m depth and provided convincing evidence for S-wave splitting in the upper crust SE of the KTB location. This anisotropy effect seems to be related to the overall strike and dip of rock foliation in the zone of Erbendorf-Vohenstraul3 (ZEV). Crustal Indentation in the Alps - An Overview of Reflection Seismic Profiling in SwitzerlandPages 161-176 Waiter Frei and Paul Valasek Peter Lehner Reflection seismic profiling across the Swiss Alps displays the structural relationship of the junction between the European and the African plates with a wedge of African crust indented into the European one during Neo-Alpine deformation. This structural :configuration clearly postdates the Eo-/Meso-Alpine thrusting of the African plate over the European one and the imbrication of the thinned south European margin. Crustal Reflections Beneath the Alps and the Alpine Foreland: Geodynamic ImplicationsPages 177-183 Jean Michel Marthelot The ECORS-CROP deep seismic profile provides a 330 km long traverse across the western part of the Alpine collision belt. The most characteristic reflections are recorded : 1) in the lower crust, which displays the strong reflectivity typical of the European Hercynian crust beneath the northwestern foreland and the Belledonne Massif; 2) at the boundary between the external and the internal zones, where two southeast dipping reflective zones cross the crust from less than 1 s to 5 s and flatten at this level beneath the metamorphic Pennine zone. The lateral variations in the crustal reflectivity of the .lower crust cannot be correlated to the Alpine shortening. The foreland lithosphere seems to be unaffected by the Alpine deformation and supports a flexure that increases the depth of the Moho from 9 s beneath the Massif Central to 14 s beneath the Belledonne Massif. Thickening of the external part of the Alpine crust is induced by a thin-skinned thrust belt, a foreland basin and an upper crustal imbrication. The southeast dipping reflective zones are linked to major geological features, i.e. the bottom of the sediments resting on the back of the Belledonne Massif and a major Alpine suture including some basic and ultrabasic rocks of the narrow Valais domain. A dense and high-velocity lithospheric heterogeneity, inherited from the floor of this domain, acts as an indenter beneath the metamorphic belt. It forces the European upper crust upward mainly by thrusting with a west vergence, while the lower crust is buried beneath this heterogeneity. Crustal Reflections from the Alpine Orogen: Results from Deep Seismic ProfilingPages 185-193 Lucia Levato Paul Valasek Near vertical deep seismic reflection profiling through the eastern Swiss Alps offers the opportunity to study exhumed crust and to calibrate reflections by downplunge projections. Lithologic layering, including intrusive contacts and relatively thin layers of metamorphosed Mesozoic sedimentary rocks seem to be the primary reason for the observed reflective nature of the upper crust. Reflectivity is strongly controlled by orientation of this layering. Shear zones seem be responsible for reflections only if they are part of thick mylonite belts. Combined partial stacks of varied offsets used in conjunction with complete stacks proved to be a valuable tool to overcome the difficulties associated with 3-D effects of Alpine structure like hummocky surfaces with pronounced cross dip. By combining refraction and near-vertical reflection data it was possible to trace the European crust-mantle boundary into the Adriatic mantle. A wedge-shaped indenter of Adriatic lower crust was apparently forced into the European crust, splitting it apart and delaminating it. Defocussing effects due to internal deformation of the indenter prevented the penetration of seismic energy in near-vertical reflection profiling. The stacking of delaminated slivers of upper crust during the collision resulted in a nappe pile. More or less coeval north- and south-directed thrusting lead to a remarkably symmetrical thrust belt overlying a southvergent subduction zone. Integrated Analysis of Seismic Normal Incidence and Wide Angle Reflection Measurements Across the Eastern Swiss AlpsPages 195-205 (1) Inst. of Geophysics, ETH-Honggerberg, CH 8093 Zurich, Switzerland; Deep crustal seismic data extending along a N-S transect through the eastern Swiss Alps have been compiled to a detailed image of the crustal structure across a portion of this continental collision zone. The data sample the entire Alpine edifice beginning in the Molasse Basin in the north and ending in the Po Plain. The image incorporates both large-scale refraction and wide-angle reflection measurements as well as near-vertical deep-crustal reflection profiles. The reflection surveys are part of the Swiss National Science Foundation Program NFP-20 and are almost coincident with the Alpine segment of the European Geotraverse (EGT). Examples of the two types of data are displayed separately and then combined as a single composite section. The EGT wide-angle reflection data are shown also as NMO corrected profiles. The standard, reduced traveltime sections were used :to select seismic events to base forward modelling by 2D raytracing. The NMO-corrected wide-angle data reproduce the sections in a normal-incidence format which can be compared directly to the deep-crustal reflection profiles. The reflection data from several of the NFP-20 traverses are compiled after migration in a line-drawing format onto a single transect. These migrated reflection data are superimposed on the crustal velocity structure obtained from refraction modelling which includes generalized velocity information from profiles crossing the EGT. Most of the main structural seismic features correlate well between the reflection and refraction data sets. In particular, a remarkable agreement exists between the reflection and refraction Mohos. From the refraction data the base of the European crust is shown to dip from 35 km depth in the north to 56 km beneath the Penninic nappes. A 18 km thick slab of lower crust is subducted further south under the Southern Alps. On the reflection data, this interface correlates well with the base of a 3 6 km thick package of deep reflections which suggests a transitional velocity-depth distribution. Steeply dipping structures such as the Insubric line are less well imaged on the reflection profile which crosses it, but are clearly visible on the more distant high-resolution reflection spreads. Overall, the advantage of this integration is a unified data base whereby the large-scale velocity distribution derived from the refraction measurements is resolved in fine structural details by the reflection data. Seismic Data from the Alps and their Bearing on the Formation of Granulite and Eclocite Nappes.Pages 207-212 Several models for the uplift to the surface of deep-seated masses have been proposed in the past, largely on surface evidence. Geophysical data from the deeper parts of young collision zones had been scant. but recent seismic surveys through Alpine ranges provided important information. Although these data emphasize Neogene tectonics which severely disrupted older structures that contain eclogitic to granulitic bodies, they suggest various ways for the submergence, deepseated metamorphism, and re-emergence of originally surficial rocks. In a section through central Switzerland, three modes of the localized uplift of deep-seated masses may be surmised:
The transpressive scenarios (2) and (3) are probably important for eclogitic rocks from the early Cretaceous when according to plate tectonics models strike-slip predominated. Transpression-induced localized depression and uplift of both oceanic and continental crustal slivers may explain a number of otherwise puzzling features such as the juxtaposition as exotic terranes of eclogitic slivers of Austroalpine, Piemontese, and European affinities. A Deep Seismic Reflection Profile across the Archean Proterozoic Witwatersrand Basin, South AfricaPages 213-224 L. O. Nicolaysen The 112 km, 16 s two-way time trans-Witwatersrand basin seismic reflection profile stretches from the Ventersdorp dome, across the Potchefstroom syncline and terminates in the center of the Vredefort dome. The Potchefstroom syncline rests on Archean basement and contains Archean (Dominion Group and Witwatersrand Supergroup) and early Proterozoic (Ventersdorp Supergroup and Transvaal Sequence) st:rata. The disposition of the strata is clearly imaged on the seismic section - in particular a series of half-grabens on the basin margin, the unconformity between the Witwatersrand and Ventersdorp Supergroups, and the overturned strata forming the collar of the Vredefort dome. The crystalline basement is divided into three domains on the basis of the seismic fabric: the domain stretching from the Ventersdorp dome to Potchefstroom characterized by several distinct zones of strong, sub-horizontal reflections; the relatively transparent domain between Potchefstroom and the collar of the Vredefort dome; and the domain of occasional reflections, within the core of the Vredefort dome. The reflectivity observed within the crystalline basement is probably due to sills intruded during Precambrian extension, or ductile banding. The western portion of the profile shows a change in reflective character at approx. 12 s TWT, which probably marks the crust/mantle transition. The absence of a distinct "reflection Moho" suggests a smooth transition from crust to mantle over a distance of a few kilometers. The fundamental basis for the three domains which display contrasted seismic fabrics in the basement is not understood. Systematic mapping of such basement domains by reflection profiling may provide insights regarding processes responsible for localising stratified basins. Deep-Penetration Seismic Reflection Images across the Southeastern United States Continental MarginPages 225-240 Joseph D. Phillips, James A. Austin, Jr., and Paul L. Stoffa Approximately 1,200 km of deep-penetration reflection (16 s, 240 channels) seismic data acquired over the Southeastern Georgia Embayment along six profiles using a large (10,800 cu.in.), tuned airgun source array and a 6,000 m streamer receiver provide detailed images of basement reflectections associated with Mesozoic rifting and deeper pre-Mesozoic crustal structures. The new, high-quality seismic profiles that result from such seismic data processing techniques as f-k filtering and inside muting to suppress multiples and dip moveout to enhance images indicate: 1) Wedge structures consisting of several seaward-dipping, subaerially-emplaced Jurassic basalt flows characterize the basement hinge zone. 2) The inferred Mesozoic basins which have been defined from previous magnetic and seismic studies below the Brunswick magnetic anomaly (BMA) are not real structures. They result from diffraction artifacts. 3) A. crustal-scale ramp structure interpreted as the suture zone, bounded on its south side by south-dipping intracrustal reflections, is observed at varying distances north of the BMA, suggesting that the BMA is not directly related to the suture. Images of the Plate Boundary Beneath Southern AlaskaPages 241-246 The geometry of the plate boundary beneath Prince William Sound in Southern Alaska has been determined from a combined seismic reflection and wide-angle reflection/refraction experiment. Strong reflections dipping .landward from 6 to 8 s originate from a mid-crustal velocity discontinuity at a depth of 15 to 24 km. Wide-angle diving waves indicate that the reflections originate at the top of a 6.7 km/s layer, which underlies inferred accreted sedimentary rocks having velocities less than 6 km/s. This reflective, high velocity, mid-crustal layer is also highly magnetic; and thus may represent basalts of the underthrust Yakutat terrane. The seismic character of the reflector, its ; subparallel relationship to the subducting oceanic plate, its location near the top of the Wadati-Benioff seismic zone, and its correspondence in dip and possibly depth to the inferred slip plane for the great 1964 earthquake suggests that this prominent mid-crustal reflector represents the interplate decollement between North America and the lower crust of the subducting Yakutat terrane sutured to the top of the Pacific plate. Lithoprobe Reflection Transect of Southwestern Canada: Mesozoic Thrust and Fold Belt to Mid Ocean RidgePages 247-255 Ronald M. Clowes Deep seismic reflection profiles recorded between 1984 and 1989 provide a 1200 km long section that crosses the entire southern Canadian Cordillera, the active Cascadia subduction zone, the oceanic Juan de Fuca plate, .the Juan de Fuca ridge, and ends on the Pacific plate. On the east, the cross section begins in the Mesozoic Rocky Mountain thrust and fold belt where the reflectivity is dominated by upper crustal sediments deformed during east-directed thin-skin thrusting. The profiles progress westward across the central Cordillera where Mesozoic terrane accretion and basement-involved crustal imbrication were followed by Eocene regional crustal extension. Throughout this region, the seismic data show reflectivity at all levels in the crust, a well defined Moho that decreases in travel time from 12.0 s on the east to about 10.5 s on the west over a distance of about 250 km, and crustal scale antiforms associated with exposures of gneiss complexes. On the western side of the mainland, the profiles cross Mesozoic accreted terranes and plutonic complexes of the Coast Mountains, including the recent Garibaldi/Pemberton volcanic belt. In this region, faults associated with at least two stages of accretion are observed in the upper, middle, and lower crust, and the Moho is commonly seen at about 11.0 11.5 S. Near the coast, the data exhibit reflections to about 16.0 s, some of which are downdip from reflections observed beneath Vancouver Island and are likely associated with the Cascadia subduction complex. Offshore beneath the continental shelf and slope, the data image terranes underthrust beneath the pre Tertiary margin, structural deformation associated with the accretionary wedge, and the top of the subducting oceanic crust. Across the Juan de Fuca plate, the igneous crust produces prominent continuous reflections from its upper surface, from the Moho, and from a newly discovered seamount that may exhibit subcrustal magmatic underplating. The oceanic Moho is imaged to within 5 km of the east side of the Juan de Fuca ridge, is unclear beneath the ridge, and then reappears west of the ridge where the section ends. Seismic Structure of the Northern Cascadia Accretionary Prism: Evidence from New Multichannel Seismic Reflection DataPages 257-263 R.D. Hyndman, E.E. Davis and C.J. Yorath Within the Cascadia accretionary prism west of Vancouver Island, new marine seismic reflection profiles totalling 722 km provide improved images which enable the three-dimensional variation of structures along the margin to be determined. At the deformation front, landward dipping thrusts spaced roughly 5 km apart are commonly observed which may penetrate to near the top of the subducting oceanic crust. Along lines separated by 3 km, the amount of displacement on a given fault is seen to vary significantly along the margin. Seaward dipping faults occasionally develop, propagating upward from the frontal thrust. In one region above gently-dipping oceanic crust, a shallow taper sediment wedge has formed, implying high pore fluid pressures along the detachment surface at the top of the oceanic crust. The detachment exhibits a strong reflection probably indicative of high fluid pressure. The accretionary prism is bounded by a landward dipping continental backstop, formed by the marine volcanic Crescent Terrane. The base of this terrane is imaged extending down to near the top of the subducting oceanic crust, so that little sediment is available for deeper subduction or underplating. The seaward part of the Crescent Terrane appears to have been uplifted, probably as a response to the accretion of prism sediments. First Deep Seismic Reflection Transect from the Gulf of Lions to Sardinia (ECORS Crop Profiles in Western Mediterranean)Pages 265-274 A 500 km long ECORS-CROP profile provides the first continuous transect across the Western Mediterranean Basin, between the Gulf of Lions and West Sardinian margins. In the Gulf of Lions, a 70 km wide zone where crustal thickness is reduced from about 25 km to less than 5 km separates the shelf from a deep water domain where the crust is 4 5 km thick. The zone where most of the crustal thinning occurs is characterized by lower crustal and possibly Moho reflections, observed nowhere else on the magin. Half grabens bounded by listric faults are observed across the Provencal shelf. At the lower part of the slope, tilted blocks are visible above a continuous, subhorizontal reflector, suggesting detachment faulting, though a volcanic origin for part of this relief cannot be excluded. In the deep water domain, lateral variations in basement topography and in sediment thicknesses are observed. The central part of the basin is a structural low, characterized by significant structures within basement coinciding with a central magnetic anomaly. The structures of the eastern part of the basin are taken as evidence for post-opening tectonic activity, already documented in western Sardinia. Structure and Evolution of Adjoining Segments of the West African Margin Determined from Deep Seismic ProfilingPages 275-289 Jean Claude Icart and Jean Ravat Previous geological and geophysical work has established that the West African continental margin between the Douala Basin (Cameroon) and the mouth of the Congo River is divided into three segments separated by major transverse faults. Each segment has its own specific geodydynamic evolution, whose mechanisms were previously hard to assess. A deep seismic survey in 1988 revealed the crustal structure of the central and southern segments as well as of the faulted zone separating them. Differences in the rate of crustal synrift thinning were revealed between these two segments, which can be explained by comparison with the Brazilian conjugate margin during the pre-opening stage. From this we consider that the fault zones separating the segments are transfer zones involving no intraplate discontinuity. Reflectivity Caused by Texture-Induced Anisitropy in MylonitesPages 291-298 M. Fritzsche G. Braun The directional dependence of P- and S-wave velocities from mid-crustal mylonites in high-pressure situations has been investigated. From crystallographic preferred orientations of mineral components and the respective elastic constants, the Voigt average was used to determine the complete 3-D elasticity tensor. With increasing preferred orientation, due to progressive mylonitization, the anisotropy of Vp increases from about 2% (protolith) to about 11% at the ultramylonitic stage. Shear wave splitting is at a minimum (< 1%) in the nearly isotropic parent rock; whereas, in the ultramylonite, it is around 8%. Synthetic seismograms were calculated, using the reflectivity method, for models consisting of horizontal layers with increasing with depth, in a first attempt to model the seismic response of an anisotropic medium with realistically derived elasticity tensors. Correlating Crustal Reflections with Geologic Outcrops: Seismic Modeling Results from the Southwestern USA and the Swiss AlpsPages 299-305 R. H. Marchant 0. A. Pfiffner E. C. Hauser Outcropping geology can often provide direct and detailed information concerning the relationship of seismic reflections to specific geologic features, a crucial but difficult aspect of deep seismic interpretation. In this study we present two very different examples in which fortuitous geologic circumstances enable us to anticipate the nature and geometry of reflectors to substantial depths. The approach used is to construct geologic models based on surface mapping, for which synthetic seismograms are calculated and then compared to nearby deep seismic lines. In these cases, the geologic features observed at the surface lead to reflection patterns quite similar to those actually observed on the data. In the Buck Mts. of Arizona, southwestern USA, rotation above a Tertiary detachment fault has exposed an upper crustal block which now represents a virtual cross-section of pre Tertiary crust. Seismic modelling based on this exposure reveals that the numerous (originally) subhorizontal Proterozoic diabase intrusions observed in the Buck Mts. generate strong synthetic reflections that are very similar in character, continuity, and distribution to the anomalous Bagdad Reflection Sequence (BRS) imaged by nearby COCORP and industry seismic lines. We infer that the BRS, which permeates the upper crust for distances of some 100 km and corresponds to depths of up to 15 km, is due to a series of subhorizontal mafic intrusions similar to those in the Buck Mts. Another area where surface geology is especially helpful in interpreting deep reflections is in the Penninic Alps of eastern Switzerland, where the combination of relatively steep dips, high relief, good exposures, and extensive fieldwork has permitted the construction of contour maps of several Penninic nappe surfaces. Extrapolating these contours to depth enables construction of a 3-D model of the upper crust beneath the south end of Swiss seismic line NFP20-East. Results of raytracing indicate that interfaces between nappe surfaces and .metasedimentary cover rocks can account for most reflections in the upper six seconds, thereby constraining the 3-D structure of the upper crust in this area. In both examples, reflections can be explained as caused by simple lithologic contrasts: between mafic and silicic rocks (in Arizona), and between silicic basement, metasediments, and ophiolites (in Switzerland). These results emphasize the importance of lithologic heterogeneity in producing deep crustal reflections, and provide possible analogs for reflection patterns in areas where geologic control is less certain. Reflections from a Relic Moho in Sotland?Pages 307-313 Prominent reflectors are observed at about 30 and 50 km depths throughout much of northern Scotland. The deeper reflector results from impedance contrasts here attributed to metasomatic intrusions: high-pressure, relatively low density minerals such as phlogopite or kyanite in a local zone that has survived due to a decreased geothemal gradient following the Caledonian orogen. This metasomatism may be localized by a shear zone, a delamination scar or by a former base to the crust. The position of the Moho is here assumed to be controlled largely by isostatic forces driven by heat. Garnet formation in the gabbroic, deep lower crust of Caledonian Scotland produced eclogitic metamorphic rocks with seismic velocities of 8 km/s. These rocks are part of the Scottish lithosphere between 28 and 50 km depths that today forms the seismic uppermost mantle, capped by the Moho at 28-30 km. The relic metasomatic zone at 50 km depth marks the former Caledonian Moho, the boundary between the gabbroic/eclogitic mid-lithosphere and the peridotitic uppermost mantle lithosphere in this alternative model. The Reflection Moho Along the COCORP Northwest U.S. TransectPages315-322 Marked variations in the seismic reflection character of the continental Moho occur on the COCORP deep seismic reflection transect of the northwestern U.S. These variations correlate with the four major tectonic provinces crossed by the transect, the extended Cordilleran interior, the Rocky Mountain thrust belt, the Precambrian craton, and the western Williston basin. Variations in Moho structure and reflectivity observed along this transect provide an important constraint on processes and structures at the base of the crust. The prominent reflectivity beneath both the Cordilleran interior and Williston basin edge suggests that magmatic underplating may have played an important role in the development of the Moho in both regions. Upper Crustal Poisson's Ratios in the Colorado Plateau from Multicomponent Wide Angle Seismic RecordingPages 323-328 Vertical- and horizontal- component recording of wide-angle seismic data on the Colorado Plateau ~60 km NNW of Flagstaff, Arizona, indicates that upper crustal shear wave velocities are high with respect to compressional-wave velocities. Calculations of Poisson's ratios using velocities determined from plane-wave decomposition (slant stacking) yield values that are significantly lower than 0.25. Poisson's ratio in the upper 3-4 km of the crystalline basement (beneath Phanerozoic sedimentary rocks) is 0.24, but decreases to 0.20 between about 6 and 15 km depth in the upper crust. Significant decrease in Poisson's ratio with depth can be explained by appropriate juxtapositions of wet granite, granitic gneiss or felsic schist and closure of microcracks with increasing depth. Elevated subsurface temperatures may play a role in lowering Poisson's ratio. Evidence for anomalously high fluid pressures, which would significantly increase Poisson's ratio, is not found. Velocity-Resistivity Correlations in the Deep CrustPages 329-333 Roy D. Hyndman A compilation of approximately coincident refraction velocity and magnetotelluric resistivity data shows a general correlation of lower velocity with lower resistivity in the deep crust. The data fit a range of models where both velocity and resistivity are mainly a function of fluid porosity. Reduction of velocity from an expected zero porosity mafic rock value of about 7.2 km/s to commonly observed values of about 6.7 km/s and explanation of the low resistivities require between 0.3 to 3% porosity, depending on the pore geometry. Poisson's ratio data may provide an important additional constraint on the pore geometries. Determination of the Fine Structure of Seismic Crustal Boundaries from their Reflected PhasesPages 335-342 The characteristics of the wave form of a particular event in a reflection (normal incidence and/or wide angle) record section are due to complex interference of individual wave fields generated by different physical phenomena:
Spectral ratios and/or qualitative comparisons of the amplitude spectrum of different phases are often used in studying intrinsic attenuation and crustal structure. However, this approach disregards all the information contained in the phase spectra. Thus the deconvolution (spectral ratio) of the target event (SmS, PmP, etc) by an immediately preceding arrival removes the contaminating effects due to the interfering processes that can be regarded as filters and provides estimates of the reflection coefficients and/or impedance contrasts. This can be done through O'Doherty-Anstey formula or by considering mathematical developments that take into consideration the reflection and transmission properties of the interfaces. These estimates place important constraints on the determination of the structure and geological nature of crustal discontinuities. Lower Crustal Lithology from Shear Wave Seismic Reflection DataPages 343-349 Mike Warner WISPA (Weardale Integrated S and P-wave Analysis) was BIRPS' first major venture on land. The aim of this experiment was to image the lower crust using both P and S-waves at near-normal incidence, and to measure the variation of Poisson's ratio with depth. Inital results were excellent, with raw S-wave shot gathers showing clear lower crustal layering corresponding to the P-wave reflectivity. Conventional CDP stacking of shear-wave data proved difficult because of severe S-wave statics problems. The data are of sufficient quality to allow direct and accurate estimation of Poisson's ratio. We obtain an average value of 0.23 (Vp:Vs = 1.70) for the whole crust. However, this figure disguises a clear and systematic change of Poisson's ratio with depth through crust; in the upper crystalline crust values are low, around 0.20 (Vp:Vs = 1.64), while in the layered lower crust values rise to an average of 0.29 (Vp:Vs = 1.85). These values are consistent with a geological model in which the upper crust is granitic, becoming increasingly mafic with depth, leading a lower crust of mafic amphibolite. Such a model is consistent with shear-wave data at wide-angle, which may be explained by a layered sequence in which the average S-wave velocity gradient is slightly negative with depth, but the P-wave velocity gradient is strongly positive. The combined normal incidence and wide-angle shear-wave data sets are not consistent with a granulitic lower crust, with a lower crust of intermediate or acidic composition, nor do they require that S-wave layering is produced by aligned amphiboles. Layer Thicknesses in the Lower Crust: Modelling and Spectral Analysis of BIRPS DataPages 351-357 Richard Hobbs
BIRPS, Bullard Laboratories, Most of the in situ velocity models proposed for the reflective lower crust of W-Europe include thin layers with thicknesses concentrated in a very narrow range: 50 200m. Within this hypothesis, constructive interference should induce a strong of reflected energy in narrow frequency bands at normal incidence. Based on this argument, a simple method is proposed to evaluate thicknesses of lower crust lamellae from detailed spectral analysis of the reflections in the time-frequency domain. Its resolution is tested on synthetic shotpoint gathers computed from complex crustal models. The applicability to real data tested on more than 200 shotpoints from the WAM profile and WISPA experiment of the BIRPS dataset. In WAM, the spectra of a group of reflections show focussing of reflected energy around 17 Hz, suggesting most of the associated reflecting boundaries have a nearly constant spacing of 90 to 110 m. However the frequency spectra show clear variations with both traveltime (i.e. depth) in the lower crust and distance along the profile. This suggests that the structure of the lower crust is more disorganized than the velocity models proposed in previous studies. In WISPA, some focussing of energy could also be detected for a few reflections. But since the frequency bandpass is very different from WAM, the frequency of resonance appears to be shifted towards higher frequencies (28-.32 Hz). This difference has two possible origins: 1) the dominant thickness is smaller in WISPA (i.e. 50 70 m) and interference is induced at 30Hz, 2) it is greater (150-170 m) and focussing is observed at the first harmonic of the frequency of resonance (9-10 Hz). This ambiguity shows that the absence of low frequencies in deep seismic profiling is a critical limitation to such analyses. Constructive Interference Geophysical Mythology Re-ExaminedPages 359-362 It is almost universally assumed by deep reflection seismologists, that a multi-layered sequence of rocks, with only modest acoustic impedance contrasts, can produce large apparent reflection coefficients by constructive interference. It is also generally believed that a quarter wavelength layer produces a factor of two increase in amplitude, and that many fine layers can produce large effects. For the effective seismic wavelets used in deep marine profiling none of these assumptions is correct, and constructive interference is much less important than is generally assumed. Computer modelling using simple one-dimensional layered models, including the effects of transmission losses and multiple reflections, has been used to test the effects of constructive interference with a variety of wavelets. A wide range of models has been tested, including both deterministic and random models, in which variable layer spacing, variable reflection coefficients and variable acoustic impedance contrasts were generated. The effectiveness of constructive interference depends strongly upon the wavelet used; it is greatest for a long multi-cyclic wavelet, and is least effective for a minimum-phase short-duration wavelet of the type in marine profiling. For a quarter wavelength layer, a typical marine wavelet produces only a factor of 1.5 increase in apparent reflection coefficient, compared with a factor of 2 appropriate for a sine or for the zero phase Ricker wavelet. Using marine wavelets on the suite of multi-layered models, the largest increase in amplitude we able to obtain was just over a factor of 2. Constructive interference from a plane layered model, using a realistic wavelet, can never produce the large increases in amplitude often stated or assumed in seismic literature. Seismic Reflectivity Patterns: Comparative Investigations of Europe and North AmericaPages 371-375 Reflectivity maps of Europe and the United States of America have been compiled to summarize major variations in the reflection character of the crust. Significant differences in reflectivity largely independent of seismic technique are observed and systematic correlations with tectonic setting are found. Reflectivity, concentrated in the lower crust as bands or lamellae, is predominant in poBt orogenic extensional areas, i.e. regions of orogenic collapse. Stable crustal units of Precambrian age seem to be characterized by more diffuse reflectivity, mostly decreasing with depth, and by frequent diffractionlike events. Compressional areas are marked by ramp and flat structures (thin skinned tectonics) or special reflection diffraction patterns (thick-skinned tectonics), further by seismic "crocodiles" and seismic duplexes. Major shear zones are characterized by deep reaching, dipping reflection zones. Crustal Suture Zones: Seismic Signature and Structural InterpretationPages 371-375 Collision of continents always results in a juxtaposition of different terranes. At the surface the prominent sutures can be easily recognized by contrasting geology and tectonics on either side. At depth only indirect mapping is possible. We inspected suture-crossing near-vertical reflection seismic profiles from three continents. The strong similarity of seismic patterns leads us to the conclusion that the typical pattern may be a diagnostic tool for identifying continent-continent collisions in areas where outcrops are hidden by e.g., post-collision sediments. The pattern is not observed in association with thin-skinned tectonics. We discuss various models for the interpretation of the observed seismic signature across thick-skinned sutures. Deep Seismic Profiling and Continental EvolutionPages 377-382 Key results from continental deep seismic reflection and refraction profiling can be incorporated into a generalized uniformitarian theory of continental evolution [Nelson, 1991, Geophysical Journal International, 105 25-35]. This theory provides a framework within which to view variations in the reflection character of the continental crust and Moho, variations in crustal thickness and mean crustal velocity, and the relationship of each of these parameters to tectonic province. Multiple Supression in Deep WaterPages 383-389 Richard W. Hobbs The quality of marine deep seismic data is degraded by shot-generated noise and multiple reflections. In water greater than 300m deep, such noise is often of greater amplitude than that of the lower crustal targets. Here we illustrate the effectiveness of a formalized strategy for multiple suppression using reprocessed portions of the BIRPS WAM and WIRE surveys. Several dip-filtering techniques are successfully used for the suppression of water-layer multiples. Adaptive predictive deconvolution and wave-equation modelling are used to suppress peg-leg multiples, with varying degrees of success. A Complete Waveform Inversion and its Application to ECORS DataPages 383-389 Michel Dietrich Francois Chapel A normal procedure in seismic waveform inversion is to minimize the misfit between observed and calculated wavefields. As this misfit function is highly non-linear, the only way to be sure of arriving at the global solution is to use a Monte-Carlo search. This is feasible if the number of unknown parameters is not too large. If it is large, as is the case with seismic inverse problems, it is extremely expensive to use the Monte-Carlo search to optimize for a single misfit function over the whole model space. However, if one uses a number of misfit functions depending upon the sensitivity of the model parameters on the data, one can implement the Monte-Carlo method efficiently. Fortunately, this is possible as the different wavelengths of the model parameters influence the different parts of the data. Here, we propose a three-step strategy for seismic waveform inversion. In the first step, we explore the complete model space for a certain number of parameters using the zeroth-order relationship between model parameters and data. The model so obtained is in the region of the global minimum, but may be far from the global solution. We then define upper and lower bounds on this solution which are obtained from the uncertainty in the solution. Then, for a certain number of parameters, which should represent the gross features of the model, we explore the bounded model space using a Monte-Carlo search. The data are related to the model parameters with a first-order approximation. Finally, we use a local non-linear waveform inversion to arrive at the final solution. The data are now related to model parameters with a second-order approximation. To demonstrate the theory, we treat a one-dimensional problem in the (tau, p) domain. The numerical example of synthetic data supports the theory. Finally, the last step of the inversion scheme is applied to marine reflection data collected by ECORS group from the Bay of Biscay, France. Inversion result predicts a laminated lower crust with velocity contrast varying from 50 m/s to 500 m/s of 100 m to 500 m thick layers. Geometrical Migration of Line Drawings: A Simplified Method Applied to ECORS DataPages 401-407 The geometrical migration of line-drawings can be an intermediate solution to the problem of migrating vertical-reflection data. Our method, based on the principle of the common tangent, quickly produces a depth-migrated line drawing that is readily usable for interpretation. Energy- and Power-Sections in Seismic InterpretationPages 409-415 In deep reflection seismics, scattering and diffraction of seismic waves are often observed. These phenomena cause, for example, offset dependent reflectivity pattern and phase distortions which cause additional processing problems. In this context it is known that the phase- and group-characteristics of reflected/diffracted arrivals may deviate from one another. Compared to the phase behaviour, the group- or energy-pattern often represents the more stable average reflection response of an interface. Using data from the DEKORP 2S profile (South-Germany) we illustrate above problems and aspects of enhancing group characteristics as an additional tool for seismic processing and interpretation. Three algorithms are shown which focus on the energy and on temporal changes of the energy density of a seismogram. The corresponding types of sections are:
In comparison to conventional stacked sections, the resulting section types have lower resolution. But they are, on the other hand, not so sensitive to details of velocity structure and small static time shifts. Reliable information on the energy pattern of a seismic section can, therefore, be obtained even in early stages of processing. Focussing on zones of changing energy level, power-, perigram- or energy-sections enhance the dominant features of the seismic image of the subsurface and thus contribute to simplifying the interpretation. Improved Interpretation of Kapuskasing Crustal Reflection Data by Selection of the Most Effective Reprocessing Sequences.Pages 417-433 G.F. West Reprocessed and migrated seismic sections of lines 1 and 6 of the Kapuskasing reflection experiment displayed a highly reflective upper crust with a significant number of previously unrecognized laterally continuous reflectors. Five of these reflectors imaged major low angle thrust faults with geometry revealing a ramp-and-flat style of deformation. Three of these events are representing the imbricate fault system of the Kapuskasing uplift. The other two underlying subparallel reflections and an equivalent number of major near-vertical faults were detected at mid-crustal depth. The combined structural attitudes of these features established a local subsidence framework which affected the entire crust under the central region of the Kapuskasing structural zone. Comparison of Coincident High-Resolution Wide-Aperture and CDP Profiling Along the Southwest Coast of NorwayPages 435-442 Comparison of coincident wide-aperture (WA) and CDP data recorded in southwestern Norway shows that distinct crustal features defined by each data set may have little correspondence. In some areas where the upper crust is seismically transparent in near vertical incidence images wide aperture data indicate there are distinct and extensive velocity heterogeneities. A pervasive upper-mid crustal velocity boundary interpreted from WA reflections generally does not correlate with CDP reflection horizons. Conversely, areas of dense reflectivity in CDP profiles may not be imaged at wide angle. Moho interpreted from WA reflections can correspond with an abrupt lower crustal reflectivity boundary within several hundred metres or less over broad (100 km) areas but clearly does not always coincide so closely, particularly in areas with structural complexities in the Moho. Source-Generated Noise in Marine Seismic Profiles: The Limits of Reflection Detectability in the Upper CrustPages 443-450 Experiments demonstrate that source-generated noise in marine seismic profiles may mask reflections in the upper crystalline crust and limit the completeness and reliability of some deep seismic data. The level of source-generated noise depends on the size of the acoustic contrasts in the near-surface geology. Areas in which source-generated noise is important are not limited to the well-known hard-bottom situations and may be common in marginal platform areas such as those of northwest Europe. The standard processing sequence commonly applied to marine deep seismic data has limited effectiveness for attenuating source-generated noise. In the experiments, reflections from acoustic impedance contrasts commonly encountered in crystalline rocks (0.025 0.10) are not imaged or poorly imaged in the first 6 to 8 km of the crust but are better imaged with increasing depth. These experiments suggest that the commonly observed transparent upper crust and onset of reflections in the middle crust may be the result of limitations of the seismic reflection technique rather than crustal geology. |
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