3-D velocity model for Norway on-shore and off-shore

Geophysical Journal International, 2014

New insights in crustal structure in southern Norway are given by combining stacking techniques and traveltime tomography of 3-D wide-angle reflection/refraction data. The Magnus Rex crustal scale wide-angle refraction/reflection data set in Southern Norway covers an area of 400 km × 430 km where 716 receivers on three profiles recorded seismic waves from 26 explosive sources. Previous data analysis focused on 2-D interpretation along the profiles. Here we extract additional P-wave velocity information by inverting inline and cross-line data simultaneously. We combine stacking routines, traveltime tomography, and interpolation algorithms to the high quality inline and cross-line data. A smooth 3-D crustal velocity model is inverted from traveltimes of diving Pg waves with similar results for two initial models. Initial models include a 1-D average model and an interpolated 3-D model based on robust, local 1-D velocity-depth functions derived from CMP-sorted and stacked records. The depth to Moho is determined from reflected waves (PmP) by traditional exploration seismology processing routines (CMP sorting, NMO correction, stacking, depth conversion). We find that this combination of stacking methods and traveltime tomography is well suited to exploit sparse 3-D wide-angle data. The results along the profiles are similar to the earlier 2-D models and the 3-D velocity model shows little lateral variation. The crust in SW Norway is generally 35-40 km thick and has relatively low average velocity, as it lacks the characteristic high-velocity lower crust, otherwise observed in the Baltic Shield. However, the Oslo Graben is characterized by high crustal velocities and a slightly elevated Moho. Our results suggest that this crustal structure continues towards the north along the strike of the graben.

27th Seismic Research Review: Ground-Based Nuclear Explosion Monitoring Technologies, 2005

We have compiled a 3D seismic velocity model for the crust and upper mantle in the greater Barents Sea region including northern Scandinavia, Svalbard, Novaya Zemlya, the Kara Sea, and the Kola-Karelia regions. While the general motivation for developing this model is basic geophysical research, a more specific goal is to create a model for research on the identification and location of small seismic events in the study region, and for operational use in locating and characterizing seismic events in the study region. The observational basis for the velocity model are previous, crustal-scale 2D seismic reflection and refraction profiles, and passive seismological recordings, supplemented by potential field data to provide additional constraints on the crustal structure. The model is defined at grid tiles spaced every 50 km, and each tile is represented by up to two sedimentary and three crystalline crustal layers (plus water and ice). For crustal regions not constrained by primary velocity data, we developed an interpolation scheme based on several defined geological provinces that are characterized by individual tectono-sedimentary histories. The interpolation utilizes the observed strong correlation between sediment and crystalline crustal thickness within continental provinces. For comparison, an alternative interpolation approach applies a continuous curvature gridding algorithm within each of the provinces. To provide a complete lithospheric model, we complemented the crustal model with an upper mantle velocity model based on surface wave inversion, thereby covering depths essential for Pn and Sn travel time modeling. As an extension to the previously existing data set, we recently retrieved a large amount of surface wave data recorded or excited in the European Arctic during the last three decades. The merged surface wave data set will enable us to refine the upper mantle velocity structure in the study region significantly. Preliminary group velocity maps for Rayleigh and Love waves reflect large-scale geological structures and demonstrate lateral velocity variations in the mantle. Validation of our velocity model includes travel time modeling and relocation of seismic events. For this purpose we compiled a set of Ground Truth (GT) events comprising chemical and nuclear explosions, and natural earthquakes. Phase arrival times of multiple events at some sites provide timing error estimates at some stations. With the GT events we obtain a rather good Pn and Sn ray coverage in the main target region. Besides the comparison of observed and modeled travel times along selected transects, we have computed source-specific station corrections (SSSCs) from our 3D model. The crustal velocity models are also evaluated by comparison of predicted gravity fields with the observed free-air gravity. To model the gravity field, we used standard velocity-density relationships for crustal rock types and the density structure of the upper mantle from previous studies. The inferred gravity fields both reflect and exaggerate the basic geological features. Accomplishments so far have been concerned with implementation of a forward modeling procedure and software development needed to support the complex 3D model structure. The forward modelling is done in order to reduce the misfit between observed and modelled gravity and finally to supplement our crustal velocity model with a density distribution.

Geophysical Journal …, 2002

Using local earthquakes and explosions recorded on the 3-D seismic network operating in Southwest Iceland, the 3-D velocity structure has been modelled to depths of 10-15 km. The tomography algorithm simultaneously inverts for both P-and S-wave velocities and hypocentral locations. Major tectonic features within the 224 × 112 km 2 rectangular study region include the South Iceland Seismic Zone, the Hengill volcanic system and the Reykjanes Volcanic Zone. Reduced velocities from the surface to as deep as we can resolve, or about 9 km, are associated with the Hengill central volcano. As low Vp/Vs ratios prevail in the entire anomalous region, we suggest supercritical fluids within the volcanic fissure system cause the reduced velocities, rather than any large regions of partial melt. Along the Reykjanes Volcanic Zone, relatively low velocities down to depths of 6-8 km are observed in the centre of the zone. Normal velocities are observed in the South Iceland Seismic Zone with a slight reduction in Vp/Vs ratio. The thickness of the brittle crust, defined as the depth above which 90 per cent of the earthquakes occur, increases from about 5 km in the relatively young crust of the Reykjanes Volcanic Zone to about 12 km in the eastern end of the South Iceland Seismic Zone. These depths correspond to temperatures in the range of 580-750 • C, estimated from borehole heat flow measurements.

SEG Technical Program Expanded Abstracts 2008, 2008

One of the fields in the Norwegian Sea has been imaged several times over the past decades, both with conventional narrow azimuth seismic surveys as well as with ocean bottom seismic. The extensively faulted structure of the field and the possible presence of a salt diapir cause imaging problems in some areas. Therefore, a simulation study has been initiated to judge whether or not a marine full azimuth acquisition geometry improves the image of the subsurface. For this simulation study, simplified velocity and density models of the field were created, containing the main features characterizing it, as well as the problem areas. The simulation was done with 3D finite difference (FD) modeling. Data sets with and without free surface multiples were generated, and imaging from a full azimuth acquisition geometry was compared with imaging from a conventional narrow azimuth geometry. FD modeling shows that the full azimuth design generally leads to a better suppression of noise in the data, mainly due to increased fold. Depth slices show that fault edges are imaged sharper in a full azimuth geometry. Also, the image of and below the salt/limestone structure is improved. However, attenuation of multiple energy due to increased cross line fold is less than expected, except for the first seabed multiple. The low maximum frequency used in FDmodeling may have limited the increase in image quality with the full azimuth modeling.

Geophysical Journal International, 1992

A method of seismic traveltime inversion for simultaneous determination of 2-D velocity and interface structure is presented that is applicable to any type of body-wave seismic data. The advantage of inversion, as opposed to trial-and-error forward modelling, is that it provides estimates of model parameter resolution, uncertainty and non-uniqueness, and an assurance that the data have been fit according to a specified norm. In addition, the time required to interpret data is significantly reduced. The inversion scheme is iterative and is based on a model parametrization and a method of ray tracing suited to the forward step of an inverse approach. The number and position of velocity and boundary nodes can be adapted to the shot-receiver geometry and subsurface ray coverage, and to the complexity of the near-surface. The model parametrization also allows ancillary amplitude information to be used to constrain model features not adequately resolved by the traveltime data alone. The method of ray tracing uses an efficient numerical solution of the ray tracing equations, an automatic determination of take-off angles, and a simulation of smooth layer boundaries that yields more stable inversion results. The partial derivatives of traveltime with respect to velocity and the depth of boundary nodes are calculated analytically during ray tracing and a damped least-squares technique is used to determine the updated parameter values, both velocities and boundary depths simultaneously. The stopping criteria and optimum number of velocity and boundary nodes are based on the trade-off between RMS traveltime residual and parameter resolution, as well as the ability to trace rays to all observations. Methods for estimating spatial resolution and absolute parameter uncertainty are presented. An example using synthetic data demonstrates the algorithm's accuracy, rapid convergence and sensitivity to realistic noise levels. An inversion of refraction and wide-angle reflection traveltimes from the 1986 IRISPASSCAL Nevada, USA (Basin and Range province) seismic experiment illustrates the methodology and practical considerations necessary for handling real data. A comparison of our final 2-D velocity model with results from studies using other 1-D and 2-D forward and inverse methods serves as a check on the validity of the inversion scheme and provides estimates of parameter uncertainties that account for the bias introduced by the modelling approach and the interpreter.

Geophysical Journal International, 1999

We have analysed the fundamental mode of Love and Rayleigh waves generated by 12 earthquakes located in the mid-Atlantic ridge and Jan Mayen fracture zone. Using the multiple filter analysis technique, we isolated the Rayleigh and Love wave group velocities for periods between 10 and 50 s. The surface wave propagation paths were divided into five groups, and average group velocities calculated for each group. The average group velocities were inverted and produced shear wave velocity models that correspond to a quasi-continental oceanic structure in the Greenland-Norwegian Sea region. Although resolution is poor at shallow depth, we obtained crustal thickness values of about 18 km in the Norwegian Sea area and 9 km in the region between Svalbard and Iceland. The abnormally thick crust in the Norwegian Sea area is ascribed to magmatic underplating and the thermal blanketing effect of sedimentary layers. Maximum crustal shear velocities vary between 3.5 and 3.9 km s−1 for most paths. An average lithospheric thickness of 60 km was observed, which is lower than expected for oceanic-type structure of similar age. We also observed low shear wave velocities in the lower crust and upper mantle. We suggest that high heat flow extending to depths of about 30 km beneath the surface can account for the thin lithosphere and observed low velocities. Anisotropy coefficients of 1-5 per cent in the shallow layers and >7 per cent in the upper mantle point to the existence of polarization anisotropy in the region.

Geophysical Journal International, 2011

We process seismic broad-band data from southern Norway by cross correlation of ambient seismic noise in view of getting a better image of the crustal structure in the area. The main data set sterns from the temporary MAGNUS network which operated continuously from 2006 September to 2008 June. Additionally, data from permanent stations of the National Norwegian Seismic Network, the NORSAR array and GSN stations in the region are used. We compute vertical component cross-correlation functions using 41 receivers for 3-month time windows. Evaluation of the azimuthal and temporal variation of signal-to-noise ratios (SNRs) and f -k analysis of data from NORSAR array between 3 and 25 s period shows that the dominant source areas of seismic noise are located to the west and north of the network during most of the measurement time, which corresponds well to the Norwegian coast line. During summer months, the SNRs decrease but the azimuthal distribution becomes more uniform between 7 and 12 s period, suggesting a more diffuse character of the wavefield. Primary ocean microseisms above 12 s show different dominant source azimuths during this time period compared to the winter months. Time-frequency analysis is applied to measure Rayleigh wave group velocity dispersion curves between each station pair for each 3-month correlation stack and the mean and variance of all dispersion curves is computed for each path. After rejection of low-quality data, a careful analysis shows that the group velocities are not biased by noise directionality. We invert the data for group velocity maps at period bands between 3 and 25 s. At short periods, we find an average Rayleigh wave group speed of about 3 km s −1 and velocity anomalies that correlate very well with local surface geology. While higher velocities (+5 per cent) can be associated with the Caledonian nappes in the central part of southern Norway, the Oslo Graben is reflected by negative velocity anomalies (−3 to −5 per cent). At longer periods, group velocities correlate well with the variation of Moho depths beneath southern Norway.

A 2-D seismic line using four-component (4-C) receivers-a 3-C geophone and hydrophone-laid on the sea bottom was acquired in 1996 by PGS. The survey was undertaken by Amoco and its partners over the Valhall Field, offshore Norway. The main objective of the survey was to provide a better image of a chalk reservoir. Converted (P-S) waves are used, as P-P waves are strongly attenuated and scattered due to the presence of gas in the layers over the reservoir. The vertical component of the geophone and hydrophone showed a largely reflection-free zone for the target at the middle of the section. Four processing flows were applied to the radial receiver component: conventional CDP processing (for possible P-S-S events, i.e., a conversion at the sea bottom), common conversion point (CCP) asymptotic binning, P-S DMO, and equivalent offset migration (EOM). We did not find convincing evidence of a P-S-S event. The final result for the radial component processed for P-S events was good, as a continuous image for the target was obtained. A very good overall section was generated using the asymptotic binning method. The EOM method gave better results than converted-wave DMO at practically the same CPU time. The transverse component has reflections for the same events mapped by the radial component, but with much lower continuity.

Geophysical Journal International, 2007

BARENTS50, a new 3-D geophysical model of the crust in the Barents Sea Region has been developed by the University of Oslo, NORSAR and the U.S. Geological Survey. The target region comprises northern Norway and Finland, parts of the Kola Peninsula and the East European lowlands. Novaya Zemlya, the Kara Sea and Franz-Josef Land terminate the region to the east, while the Norwegian-Greenland Sea marks the western boundary. In total, 680 1-D seismic velocity profiles were compiled, mostly by sampling 2-D seismic velocity transects, from seismic refraction profiles. Seismic reflection data in the western Barents Sea were further used for density modelling and subsequent density-to-velocity conversion. Velocities from these profiles were binned into two sedimentary and three crystalline crustal layers. The first step of the compilation comprised the layer-wise interpolation of the velocities and thicknesses. Within the different geological provinces of the study region, linear relationships between the thickness of the sedimentary rocks and the thickness of the remaining crystalline crust are observed. We therefore, used the separately compiled (area-wide) sediment thickness data to adjust the total crystalline crustal thickness according to the total sedimentary thickness where no constraints from 1-D velocity profiles existed. The BARENTS50 model is based on an equidistant hexagonal grid with a node spacing of 50 km. The P-wave velocity model was used for gravity modelling to obtain 3-D density structure. A better fit to the observed gravity was achieved using a grid search algorithm which focussed on the density contrast of the sediment-basement interface. An improvement compared to older geophysical models is the high resolution of 50 km. Velocity transects through the 3-D model illustrate geological features of the European Arctic. The possible petrology of the crystalline basement in western and eastern Barents Sea is discussed on the basis of the observed seismic velocity structure. The BARENTS50 model is available at http://www.norsar.no/seismology/barents3d/.

2015

The Scandinavian Caledonides provide a well-preserved example of a Palaeozoic continentcontinent collision, where surface geology in combination with geophysical data provides information about the geometry of parts of the Caledonian structure. The project COSC (Collisional Orogeny in the Scandinavian Caledonides) investigates the structure and physical conditions of the orogen units and the underlying basement with two approximately 2.5 km deep cored boreholes in western Jämtland, central Sweden. In 2014, the COSC-1 borehole was successfully drilled through a thick section of the Seve Nappe Complex. This tectonostratigraphic unit, mainly consisting of gneisses, belongs to the so-called Middle Allochthons and has been ductilely deformed and transported during the collisional orogeny. After the drilling, a major seismic survey was conducted in and around the COSC-1 borehole with the aim to recover findings on the structure around the borehole from core analysis and downhole logging. The survey comprised both seismic reflection and transmission experiments, and included zero-offset and multiazimuthal walkaway Vertical Seismic Profile (VSP) measurements, three long offset surface lines centred on the borehole, and a limited 3-D seismic survey. In this study, the data from the multiazimuthal walkaway VSP and the surface lines were used to derive detailed velocity models around the COSC-1 borehole by inverting the first-arrival traveltimes. The comparison of velocities from these tomography results with a velocity function calculated directly from the zero-offset VSP revealed clear differences in velocities for horizontally and vertically travelling waves. Therefore, an anisotropic VTI (transversely isotropic with vertical axis of symmetry) model was found that explains first-arrival traveltimes from both the surface and borehole seismic data. The model is described by a vertical P-wave velocity function derived from zero-offset VSP and the Thomsen parameters = 0.03 and δ = 0.3, estimated by laboratory studies and the analysis of the surface seismic and walkaway VSP data. This resulting anisotropic model provides the basis for further detailed geological and geophysical investigations in the direct vicinity of the borehole.