Publications
113 results found
Agudo OC, da Silva NV, Warner M, et al., 2017, Addressing viscous effects in acoustic full-waveform inversion, Publisher: SOC EXPLORATION GEOPHYSICISTS, Pages: R611-R628, ISSN: 0016-8033
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- Citations: 6
Morgan JV, Gulick SPS, Bralower T, et al., 2016, The formation of peak rings in large impact craters, Science, Vol: 354, Pages: 878-882, ISSN: 0036-8075
Large impacts provide a mechanism for resurfacing planets through mixing near-surface rocks with deeper material. Central peaks are formed from the dynamic uplift of rocks during crater formation. As crater size increases, central peaks transition to peak rings. Without samples, debate surrounds the mechanics of peak-ring formation and their depth of origin. Chicxulub is the only known impact structure on Earth with an unequivocal peak ring, but it is buried and only accessible through drilling. Expedition 364 sampled the Chicxulub peak ring, which we found was formed from uplifted, fractured, shocked, felsic basement rocks. The peak-ring rocks are cross-cut by dikes and shear zones and have an unusually low density and seismic velocity. Large impacts therefore generate vertical fluxes and increase porosity in planetary crust.
Morgan JV, Warner MR, Arnoux G, et al., 2016, Next-generation seismic experiments – II: wide-angle, multi-azimuth, 3-D, full-waveform inversion of sparse field data, Geophysical Journal International, Vol: 204, Pages: 1342-1363, ISSN: 1365-246X
3-D full-waveform inversion (FWI) is an advanced seismic imaging technique that has been widely adopted by the oil and gas industry to obtain high-fidelity models of P-wave velocity that lead to improvements in migrated images of the reservoir. Most industrial applications of 3-D FWI model the acoustic wavefield, often account for the kinematic effect of anisotropy, and focus on matching the low-frequency component of the early arriving refractions that are most sensitive to P-wave velocity structure. Here, we have adopted the same approach in an application of 3-D acoustic, anisotropic FWI to an ocean-bottom-seismometer (OBS) field data set acquired across the Endeavour oceanic spreading centre in the northeastern Pacific. Starting models for P-wave velocity and anisotropy were obtained from traveltime tomography; during FWI, velocity is updated whereas anisotropy is kept fixed. We demonstrate that, for the Endeavour field data set, 3-D FWI is able to recover fine-scale velocity structure with a resolution that is 2–4 times better than conventional traveltime tomography. Quality assurance procedures have been employed to monitor each step of the workflow; these are time consuming but critical to the development of a successful inversion strategy. Finally, a suite of checkerboard tests has been performed which shows that the full potential resolution of FWI can be obtained if we acquire a 3-D survey with a slightly denser shot and receiver spacing than is usual for an academic experiment. We anticipate that this exciting development will encourage future seismic investigations of earth science targets that would benefit from the superior resolution offered by 3-D FWI.
Agudo OC, da Silva NV, Warner M, et al., 2016, Acoustic full-waveform inversion in an elastic world, Pages: 1058-1062, ISSN: 1052-3812
Despite the elastic nature of the earth, wave propagation in the subsurface is normally modeled using the acoustic anisotropic wave equation, in part due to the requirement to be efficient when dealing with large 3D datasets. This simplification has a negative effect on the quality of recovered P-wave models, as it means that amplitude information in the observed data cannot be fully utilized when applying full-waveform inversion (FWI) (Warner et al., 2013). We examine the consequences of using an acoustic wave propagator in two synthetic examples, and we propose a method to mitigate elastic effects in acoustic FWI based on matching filters. We find that our proposed approach is successful: the recovered P-wave models are better resolved than those obtained using conventional acoustic FWI.
Silverton A, Warner M, Morgan J, et al., 2015, Offset-variable density improves acoustic full-waveform inversion: a shallow marine case study, Geophysical Prospecting, Vol: 64, Pages: 1201-1214, ISSN: 0016-8025
We have previously applied three-dimensional acoustic, anisotropic, full-waveform inversion to a shallow-water, wide-angle, ocean-bottom-cable dataset to obtain a high-resolution velocity model. This velocity model produced: an improved match between synthetic and field data, better flattening of common-image gathers, a closer fit to well logs, and an improvement in the pre-stack depth-migrated image. Nevertheless, close examination reveals that there is a systematic mismatch between the observed and predicted datafrom this full-waveform inversion model, with the predicted data being consistently delayed in time. We demonstrate that this mismatch cannot be produced by systematic errors in the starting model, by errors in the assumed source wavelet, by incomplete convergence, or by the use of an insufficiently fine finite-difference mesh. Throughout these tests, the mismatch is remarkably robustwith the significant exception that we do not see an analogous mismatch when inverting synthetic acoustic data. We suspect therefore that the mismatch arises because of inadequacies in the physics that are used during inversion. For ocean-bottom-cabledata in shallow water at low frequency, apparent observed arrival times, in wide-angle turning-ray data, result from the characteristics of the detailed interference pattern between primary refractions, surface ghosts, and a large suite of wide-angle multiple reflected and/or multiple refracted arrivals. In these circumstances, the dynamics of individual arrivals can strongly influence the apparent arrival times of the resultant compound waveforms. In acoustic full-waveform inversion, we do not normally know the density of the seabed, and we do not properly account for finite shear velocity, finite attenuation, and fine-scale anisotropy variation, all of which can influence the relative amplitudes of different interfering arrivals, which in their turn influence the apparent kinematics. Here, wedemonstrate that the introduction of
Belcher CM, Hadden RM, Rein G, et al., 2015, An experimental assessment of the ignition of forest fuels by the thermal pulse generated by the Cretaceous-Palaeogene impact at Chicxulub, JOURNAL OF THE GEOLOGICAL SOCIETY, Vol: 172, Pages: 175-185, ISSN: 0016-7649
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- Citations: 21
Bray VJ, Collins GS, Morgan JV, et al., 2014, Hydrocode simulation of Ganymede and Europa cratering trends - How thick is Europa's crust?, ICARUS, Vol: 231, Pages: 394-406, ISSN: 0019-1035
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- Citations: 42
Silverton A, Warner M, Umpleby A, et al., 2014, Non-physical water density as a proxy to improve data fit during acoustic FWI, Pages: 4135-4139
Major uplift in imaging is evident when migration is performed with a FWI velocity model for a North Sea hydrocarbon field. However, a small but significant, systematic mismatch in travel-time remains between the field data and synthetic data predicted using the final FWI model. However we perturb the model, the source, the number of iterations, the end result invariably returns to give the same final mismatch in which the predicted data are late. We know that both the synthetic and field data contain strong water-bottom multiples, and these affect the duration, bandwidth and amplitude decay of the coda. However, the finitedifference representation of the velocity model does not contain the seabed explicitly. We propose changing the assumed density of the water layer, which changes the seabed reflection amplitudes without affecting other aspects of the data, thereby properly modelling the seabed reflectivity. The wave-train is in reality an interference pattern between several arrivals, and as the relative strength of those arrivals changes, the interference pattern changes, thereby better fitting the travel-times. We find that decreasing the density of seawater improves the fit to the field data, and that we have to reduce the density by a greater factor as offset increases.
Morgan J, Artemieva N, Goldin T, 2013, Revisiting wildfires at the K‐Pg boundary, Journal of Geophysical Research: Biogeosciences, Vol: 118, Pages: 1508-1520, ISSN: 2169-8953
<jats:title>Abstract</jats:title><jats:p>The discovery of large amounts of soot in clays deposited at the Cretaceous‐Paleogene (K‐Pg) boundary and linked to the ~65 Ma Chicxulub impact crater led to the hypothesis that major wildfires were a consequence of the asteroid impact. Subsequently, several lines of evidence, including the lack of charcoal in North American sites, were used to argue against global wildfires. Close to the impact site fires are likely to be directly ignited by the impact fireball, whereas globally they could be ignited by radiation from the reentry of hypervelocity ejecta. To‐date, models of the latter have yet to take into account that ejection—and thus the emission of thermal radiation—is asymmetric and dependent on impact angle. Here, we model: (1) the impact and ejection of material, (2) the ballistic continuation of ejecta around a spherical Earth, and (3) the thermal pulse delivered to the Earth's surface when ejecta reenters the atmosphere. We find that thermal pulses in the downrange direction are sufficient to ignite flora several thousand kilometers from Chicxulub, whereas pulses at most sites in the uprange direction are too low to ignite even the most susceptible plant matter. Our analyses and models suggest some fires were ignited by the impact fireball and ejecta reentry, but that the nonuniform distribution of thermal radiation across the surface of the Earth is inconsistent with the ignition of fires globally as a direct and immediate result of the Chicxulub impact. Instead, we propose that the desiccation of flora by ejecta reentry, as well as the effects of postimpact global cooling/darkness, left much of the terrestrial flora prone to fires, and that the volume of soot in the global K‐Pg layer is explained by a combination of syn‐ and postimpact wildfires.</jats:p>
Morgan JV, Warner MR, Bell R, et al., 2013, Next-generation seismic experiments: wide-angle, multi-azimuth,three-dimensional, full-waveform inversion, Geophysical Journal International, Vol: in press
Full-waveform inversion (FWI) is an advanced seismic imaging technique that has recentlybecome computationally feasible in three dimensions, and that is being widely adopted andapplied by the oil and gas industry. Here we explore the potential for 3-D FWI, when combinedwith appropriate marine seismic acquisition, to recover high-resolution high-fidelity P-wavevelocity models for subsedimentary targets within the crystalline crust and uppermost mantle.We demonstrate that FWI is able to recover detailed 3-D structural information within aradially faulted dome using a field data set acquired with a standard 3-D petroleum-industrymarine acquisition system. Acquiring low-frequency seismic data is important for successfulFWI; we show that current acquisition techniques can routinely acquire field data from airgunsat frequencies as low as 2 Hz, and that 1 Hz acquisition is likely to be achievable using oceanbottomhydrophones in deep water. Using existing geological and geophysical models, weconstruct P-wave velocity models over three potential subsedimentary targets: the Soufri`ereHills Volcano on Montserrat and its associated crustal magmatic system, the crust and uppermostmantle across the continent–ocean transition beneath the Campos Basin offshore Brazil,and the oceanic crust and uppermost mantle beneath the East Pacific Rise mid-ocean ridge.Weuse these models to generate realistic multi-azimuth 3-D synthetic seismic data, and attempt toinvert these data to recover the original models.We explore resolution and accuracy, sensitivityto noise and acquisition geometry, ability to invert elastic data using acoustic inversion codes,and the trade-off between low frequencies and starting velocity model accuracy.We show thatFWI applied to multi-azimuth, refracted, wide-angle, low-frequency data can resolve featuresin the deep crust and uppermost mantle on scales that are significantly better than can beachieved by any other geophysical technique, and that these results ca
Gulick S, Christeson G, Barton P, et al., 2013, Geophysical Characterization of the Chicxulub Impact Crater, Reviews of Geophysics
Geophysical data indicate that the 65.5 Ma Chicxulub impact structure is a multi-ring basin, with three sets of semi-continuous, arcuate ring faults and a topographic peak ring. Slump blocks define a terrace zone, which steps down from the inner rim into the annular trough. Fault blocks underlie the peak ring, which exhibits variable relief, due to target asymmetries. The central structural uplift is >10 km and the Moho is displaced by 1-2 km. The working hypothesis for the formation of Chicxulub is: a 50 km radius transient cavity, lined with melt and impact breccia, formed within 10s of seconds of the impact and within minutes, weakened rebounding crust rose kilometers above the surface, the transient crater rim underwent localized deformation and collapsed into large slump blocks, resulting in a inner rim at 70-85 km radius, and outer ring faults at 70-130 km radius. The over-heightened structural uplift collapsed outwards, buried the inner slump blocks, and formed the peak ring. Most of the impact melt was ultimately emplaced as a coherent <3-km thick melt sheet within the central basin that shallows within the inner regions of the peak ring. Smaller pockets of melt flowed into the annular trough. Subsequently, slope collapse, ejecta, ground surge, and tsunami waves infilled the annular trough and annular basin with sediments up to 3 km and 900 m thick, respectively. Testing this working hypothesis requires direct observation of the impactites, within and adjacent to the peak ring and central basin.
Warner M, Ratcliffe A, Nangoo T, et al., 2013, Anisotropic 3D full-waveform inversion, Geophysics, Vol: 78, Pages: R59-R80
We have developed and implemented a robust and practical scheme for anisotropic 3D acoustic full-waveform inversion (FWI). We demonstrate this scheme on a field data set, applying it to a 4C ocean-bottom survey over the Tommeliten Alpha field in the North Sea. This shallow-water data set provides good azimuthal coverage to offsets of 7 km, with reduced coverage to a maximum offset of about 11 km. The reservoir lies at the crest of a high-velocity antiformal chalk section, overlain by about 3000 m of clastics within which a low-velocity gas cloud produces a seismic obscured area. We inverted only the hydrophone data, and we retained free-surface multiples and ghosts within the field data. We invert in six narrow frequency bands, in the range 3 to 6.5 Hz. At each iteration, we selected only a subset of sources, using a different subset at each iteration; this strategy is more efficient than inverting all the data every iteration. Our starting velocity model was obtained using standard PSDM model building including anisotropic reflection tomography, and contained epsilon values as high as 20%. The final FWI velocity model shows a network of shallow high-velocity channels that match similar features in the reflection data. Deeper in the section, the FWI velocity model reveals a sharper and more-intense low-velocity region associated with the gas cloud in which low-velocity fingers match the location of gas-filled faults visible in the reflection data. The resulting velocity model provides a better match to well logs, and better flattens common-image gathers, than does the starting model. Reverse-time migration, using the FWI velocity model, provides significant uplift to the migrated image, simplifying the planform of the reservoir section at depth. The workflows, inversion strategy, and algorithms that we have used have broad application to invert a wide-range of analogous data sets.
Morgan J, Rebolledo-Vieyra M, 2013, Geophysical studies of impact craters, Impact Cratering, Editors: Osinski, Pierazzo, Publisher: Wiley-Blackwell, Pages: 211-222, ISBN: 9781405198295
Warner M, Nangoo T, Shah N, 2013, Full-waveform inversion of cycle-skipped seismic data by frequency down-shifting, Pages: 903-907, ISSN: 1052-3812
Full-waveform inversion can be severely compromised by problems of cycle skipping; this occurs when predicted and observed data differ by more than half a cycle, and it leads the inversion to recover a local rather than the global minimum model. Overcoming cycle skipping normally requires both a good starting model and low-frequency content in the field data. Here we present a scheme that uses a non-linear extrapolation to add missing low-frequencies into the field data. We demonstrate the scheme using a 3D OBC field dataset, and show that it can invert to recover the global minimum model even when the original un-extrapolated field dataset is significantly cycle skipped.
Nangoo T, Warner M, O'Brien GS, et al., 2013, The application of full waveform seismic inversion to a narrow-azimuth marine dataset, Pages: 4422-4426
We apply 3D anisotropic acoustic full-waveform inversion to a North Sea narrow-azimuth, marine-streamer dataset. We use a windowed strategy, with 3 stages, first focusing on mainly refracted arrivals with offsets up to (a) 1 km, (b) 2 km and then (c) 3 km with increasing iterations. We demonstrate that our recovered velocity model is realistic.
Christeson GL, Morgan JV, Warner MR, 2012, Shallow oceanic crust: Full waveform tomographic images of the seismic layer 2A/2B boundary, Journal of Geophysical Research, Vol: 117
We present results of full-waveform tomographic inversions of four profiles acquired over young intermediate- and fast spreading rate oceanic crust. The mean velocity-depth functions from our study include a 0.25–0.30 km-thick low-velocity, low-gradient region beneath the seafloor overlying a 0.24–0.28-km-thick high-gradient region; together these regions compose seismic layer 2A. Mean layer 2A interval velocities are 3.0–3.2 km/s. The mean depth to the layer 2A/2B boundary is 0.49–0.54 km, and mean velocities within the upper 0.25 km of layer 2B are 4.7–4.9 km/s. Previous velocity analyses of the study areas using 1-D ray tracing underestimate the thickness of the high-gradient region at the base of layer 2A. We observe differences in the waveform inversion velocity models that correspond to imaging of the layer 2A event; regions with a layer 2A event have higher velocity gradients at the base of layer 2A. Intermittent high velocities, which we interpret as massive flows, are observed in the waveform inversion velocity models at 0.05–0.10 km below the seafloor (bsf) over 10–25% of the intermediate-spreading profiles and 20–45% of the fast spreading profiles. The high-gradient region located 0.25–0.54 km bsf at the base of layer 2A may be associated with an increased prevalence of massive flows, the first appearance of dikes (lava-dike transition zone), or with increased crack sealing by hydrothermal products. The upper portion of layer 2B, which begins at 0.49–0.54 km bsf, may correspond to sheeted dikes or the top of the transition zone of lavas and dikes.
Nangoo T, Warner M, Morgan J, et al., 2012, Full-waveform seismic inversion at reservoir depths, Pages: 1117-1121
We apply 3D anisotropic acoustic full-waveform tomographic seismic inversion to a North Sea wide-Angle OBC dataset, and demonstrate that our recovered velocity model is realistic within the chalk reservoir sequence to depths of up to 4000 m. Because we use predominantly wide-Angle refracted arrivals in the inversion, we are able to undershoot and image within the otherwise seismic obscured area beneath a shallow gas cloud that overlies the reservoir. Wide-Angle FWI techniques using similar datasets have not previously been used to image successfully at these depths.
Vieira Da Silva N, Morgan J, Warner M, et al., 2012, 3D constrained inversion of CSEM data with acoustic velocity using full waveform inversion, Pages: 605-609
The marine Controlled Source Electromagnetic Method (CSEM) has been used as exploration technology in the oil and gas industry. Inversion is the most widely used technique for interpretation of electromagnetic data. Nonetheless, resistivity images obtained from CSEM data generally have low resolution creating difficulties in the accurate characterization of geological reservoirs (containing hydrocarbons or for CO2 storage for example). Here is shown that by integrating a velocity field to guide the CSEM inversion leads to better focused resistivity images reducing ambiguities and source-receiver footprint. The proposed method integrates velocity models obtained through seismic full waveform inversion, in a constrained inversion algorithm for controlled source electromagnetic data. A synthetic example using a modified version of the Marmousi model is presented to validate the proposed methodology.
Guasch L, Warner M, Nangoo T, et al., 2012, Elastic 3D full-waveform inversion, Pages: 2567-2571
We demonstrate the application of elastic 3D full-waveform inversion (FWI) to a field dataset. We first analyze and validate its performance using synthetic data generated from a 3D version of the Marmousi model. We show that elastic FWI can recover both p-wave and shear-wave velocity models using only single-component hydrophone data. We then apply elastic FWI to a 3D multi-component OBC air-gun dataset from the North Sea, and show that it successfully recovers both the p-wave and shear-wave velocity structure of shallow channels.
Shah N, Warner M, Nangoo T, et al., 2012, Quality assured full-waveform inversion: Ensuring starting model adequacy, Pages: 2562-2566
Successful full-waveform inversion (FWI) of 3D seismic data typically requires low-frequency content in the field data coupled with an accurate starting velocity model. In these circumstances, two fundamental questions always arise: (1) is the starting model sufficiently accurate given the data that are available, and (2) will the inversion iterate towards the global minimum, or will it instead become trapped locally leading to an erroneous final model? We present a robust and objective means to answer both these questions. The diagnostic feature that we use to achieve this is the spatial continuity of the phase difference between the predicted and observed field data, extracted at a single low frequency, after windowing in time around early arrivals. We show proof of principle on a simple 2D synthetic example, and demonstrate the application of quality assured full-waveform inversion (QA-FWI) to a full 3D field dataset that shows significant velocity anisotropy.
Warner M, Morgan J, Umpleby A, et al., 2012, Which physics for full-wavefield seismic inversion?, Pages: 2994-2998
Full waveform seismic inversion, as currently commercially available in 3D, uses the acoustic approximation to the wave equation, and generally ignores the effects of elasticity, attenuation and anomalous density variations. We examine the consequences of these practical compromises by inverting 3D synthetic seismic data using different approximations to the physics of wave propagation in the forward and inverse modelling. We also invert using different portions of the data, and examine the effects of data amplitudes. We conclude that current FWI practice works well if amplitudes and reflected energy are suppressed in the inversion, but that a more-complete description of the physics is required in order to extract quantitative physical properties from amplitudes and reflections.
Vieira da Silva N, Morgan JV, MacGregor L, et al., 2012, A finite element multifrontal method for 3D CSEM modeling in the frequency domain, Geophysics, Vol: 77, Pages: E101-E115
There has been a recent increase in the use of controlled-source electromagnetic (CSEM) surveys in the exploration for oil and gas. We developed a modeling scheme for 3D CSEM modeling in the frequency domain. The electric field was decomposed in primary and secondary components to eliminate the singularity originated by the source term. The primary field was calculated using a closed form solution, and the secondary field was computed discretizing a second-order partial differential equation for the electric field with the edge finite element. The solution to the linear system of equations was obtained using a massive parallel multifrontal solver, because such solvers are robust for indefinite and ill-conditioned linear systems. Recent trends in parallel computing were investigated for their use in mitigating the computational overburden associated with the use of a direct solver, and of its feasibility for 3D CSEM forward modeling with the edge finite element. The computation of the primary field was parallelized, over the computational domain and the number of sources, using a hybrid model of parallelism. When using a direct solver, the attainment of multisource solutions was only competitive if the same factors are used to achieve a solution for multi right-hand sides. This aspect was also investigated using the presented methodology. We tested our proposed approach using 1D and 3D synthetic models, and they demonstrated that it is robust and suitable for 3D CSEM modeling using a distributed memory system. The codes could thus be used to help design new surveys, as well to estimate subsurface conductivities through the implementation of an appropriate inversion scheme.
Viera da Silva N, Morgan JV, MacGregor L, et al., 2012, A finite element multifrontal method for 3D CSEM modeling in the frequency domain, Geophysics, Vol: 77, Pages: E101-E115
There has been a recent increase in the use of controlled-source electromagnetic (CSEM) surveys in the exploration for oil and gas. We developed a modeling scheme for 3D CSEM modeling in the frequency domain. The electric field was decomposed in primary and secondary components to eliminate the singularity originated by the source term. The primary field was calculated using a closed form solution, and the secondary field was computed discretizing a second-order partial differential equation for the electric field with the edge finite element. The solution to the linear system of equations was obtained using a massive parallel multifrontal solver, because such solvers are robust for indefinite and ill-conditioned linear systems. Recent trends in parallel computing were investigated for their use in mitigating the computational overburden associated with the use of a direct solver, and of its feasibility for 3D CSEM forward modeling with the edge finite element. The computation of the primary field was parallelized, over the computational domain and the number of sources, using a hybrid model of parallelism. When using a direct solver, the attainment of multisource solutions was only competitive if the same factors are used to achieve a solution for multi right-hand sides. This aspect was also investigated using the presented methodology. We tested our proposed approach using 1D and 3D synthetic models, and they demonstrated that it is robust and suitable for 3D CSEM modeling using a distributed memory system. The codes could thus be used to help design new surveys, as well to estimate subsurface conductivities through the implementation of an appropriate inversion scheme.
Morgan JV, Warner MR, Collins GS, et al., 2011, Full waveform tomographic images of the peak ring at the Chicxulub impact crater, Journal of Geophysical Research, Vol: 116
Peak rings are a feature of large impact craters on the terrestrial planets and are generally believed to be formed from deeply buried rocks that are uplifted during crater formation. The precise lithology and kinematics of peak ring formation, however, remains unclear. Previous work has revealed a suite of bright inward-dipping reflectors beneath the peak ring at the Chicxulub impact crater and that the peak ring was formed from rocks with a relatively low seismic velocity. New 2D full-waveform tomographic velocity images show that the uppermost lithology of the peak ring is formed from a thin (~100-200 m thick) layer of low-velocity (~3000-3200 m/s) rocks. This low-velocity layer is most likely to be composed of highly porous, allogenic impact breccias. Our models also show that the change in velocity between lithologies within and outside the peak ring is more abrupt than previously realized and occurs close to the location of the dipping reflectors. Across the peak ring, velocity appears to correlate well with predicted shock pressures from a dynamic model of crater formation, where the rocks that form the peak ring originate from uplifted basement that has been subjected to high shock pressures (10-50 GPa), and lie above downthrown sedimentary rocks that have been subjected to shock pressures of < 5 GPa. These observations suggest that low-velocities within the peak ring may be related to shock effects and that the dipping reflectors underneath the peak ring might represent the boundary between highly-shocked basement and weakly-shocked sediments.
Kamo S, Lana C, Morgan J, 2011, U–Pb ages of shocked zircon grains link distal K–Pg boundary sites in Spain and Italy with the Chicxulub impact, Earth and Planetary Science Letters, Vol: 310, Pages: 401-408
The U–Pb ages of shocked zircon crystals from the Chicxulub impact crater and Cretaceous-Paleogene (K-Pg) boundary sites in Haiti, the USA, and Canada, and the pattern of decreasing particle size with paleodistance from the crater, have been used as evidence of a genetic link between Chicxulub and the K–Pg boundary. Despite this, the inference that the K–Pg boundary layer formed as a direct consequence of the Chicxulub impact has been repeatedly questioned. Here we present U–Pb (ID-TIMS) ages and textural evidence of shock metamorphosed zircon grains from the K–Pg boundary at Caravaca, Spain, and Petriccio, Italy, that establish a causal connection between the impact and formation of the K–Pg boundary layer. The shocked zircon grains give data that produce a characteristic age pattern, which indicates a primary source age of 549.5 ± 5.7 Ma and a secondary event at the approximate time of impact at 66 Ma. The intensity of the shock features is proportional to the degree of isotopic resetting, and all textural features and ages are analytically identical to those of previously analyzed zircon from Chicxulub and K–Pg boundary sites in North America. Caravaca and Petriccio were > 8000 km from Chicxulub at the time of impact, and are therefore the farthest K–Pg sites identified that can be linked to Chicxulub through the dating of individual shocked zircon grains. We conclude that the combined age data and textural observations provide unambiguous evidence that ejecta from the Chicxulub impact formed the global K–Pg boundary layer. These data cannot be explained by the alternative scenario that the Chicxulub impact occurred ~ 300 ka prior to the K–Pg boundary.
Barton PJ, RAF G, Morgan JV, et al., 2010, Seismic images of Chicxulub impact melt sheet and comparison with the Sudbury structure, Large Meteorite Impacts and Planetary Evolution IV, Editors: Reimold, Gibson, Publisher: Geological Society of America, Pages: 103-114, ISBN: 9780813724652
Schulte P, Alegret L, Arenillas I, et al., 2010, Response - Cretaceous Extinctions, SCIENCE, Vol: 328, Pages: 975-976, ISSN: 0036-8075
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- Citations: 9
Grieve RAF, Ames DE, Morgan JV, et al., 2010, The evolution of the Onaping Formation at the Sudbury impact structure, METEORITICS & PLANETARY SCIENCE, Vol: 45, Pages: 759-782, ISSN: 1086-9379
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- Citations: 81
Schulte P, Alegret L, Arenillas I, et al., 2010, The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary, SCIENCE, Vol: 327, Pages: 1214-1218, ISSN: 0036-8075
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- Citations: 886
Morgan J, Christeson G, Warner M, 2010, Deep ocean 3D tomography on field data, Pages: 232-236
We have run a suite of 2D and 3D wavefield inversions to recover fine-scale velocity structure in upper oceanic crust. The data were acquired on a plateau, close to a transform fault in deep water in the Pacific ocean. At the fault, a vertical section of oceanic crust is exposed and has been mapped using submersibles, hence our inverted velocity models can be directly compared with adjacent outcrop data. Synthetic tests using the 2D inversion code suggest that the inverted velocity structure may contain artefacts caused by offline arrivals and feathering of the streamer. Synthetic tests using the 3D inversion code show that true velocity structure can be recovered, and 3D inversions of the real data suggest that there is a velocity inversion in the upper oceanic crust in the area modelled.
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