ALBERT

Description: This paper reports a comparison of three different rheological models used to characterize receiver coupling to the seafloor. We used a finite-element simulation tool to simulate the mechanical receiver coupling to the seafloor as a viscoelastic system with a combination of linear elastic springs and linear viscous dashpots, known as rheological models . Three models cover most of all mechanic coupling systems, the most commonly applied Kelvin-Voigt model (KVM), the Maxwell model (MM), and the standard linear solid (SLS) model. The models differ in behavior for different coupling aspects such as oscillation, creeping, stress relaxation, and their combinations. We tested these models’ ability and relevance for use in modeling seismic receiver coupling to the seafloor. For that purpose, we used an optimized mathematical approach to simulate coupling behavior under various coupling conditions. We found how receiver coupling will affect P- and S-waves for all three models and provided some insight into which model is most suitable to describe coupling under different circumstances. We found that the SLS model represents a general description of most of the coupling effects to the seafloor and should be used when the coupling acts as a viscoelastic system. The KVM and MM are applicable in extreme cases, such as for elastic waves in consolidated sediments (KVM) and dominant creeping effects, as in very soft biosediment (MM).

Description: Inconsistent horizontal receiver coupling to the seafloor causes measured signal differences on both horizontal receiver components. To explain this inconsistency, we considered distinct coupling parameters, the damping ratio and resonance frequency, for the receiver inline and crossline directions. Our approach combined these coupling parameters with the azimuth angle between an airgun shot and the receiver geometrically and used two visualization methods to show spatially dependent receiver coupling, based on correlation and root-mean-square amplitudes. We developed finite-element method simulation results together with field data from one ocean bottom cable (OBC) in very soft biosediment. The simulations provided an insight to the difference between perfectly coupled ideal receiver response and poor coupling. From the field data, we compared OBC receiver coupling for trenched and untrenched cable. Our results revealed that the field data had an azimuth-dependent response pattern with amplitude decay and time shift on the untrenched inline component, which we can reproduce with our simulations. Azimuth-dependent receiver coupling indicated that the inline and crossline receiver components were connected by the direction of the traveling wave, and trenching the cable will reduce the azimuth-dependent coupling effects.

Description: Seismic experiments in which the number of sources is considerably larger than the number of receivers occur regularly. An important example is the collection of crustal scale seismic data using ocean bottom seismometers and marine sources. We describe a method to accurately and efficiently compute synthetic seismograms for such experiments by using finite differences and reciprocity. We show numerically how to decompose an explosive source into its equivalent body force components using the staggered-grid finite-difference technique with a fourth-order approximation for the spatial derivative and a second-order approximation for the temporal derivative. This decomposition results in a source configuration where the equivalent body forces are defined in 12 points around the point where the ex-plosive source is applied. We then use the derived equivalent body forces for the explosive source and seismic reciprocity theorems to convert the common receiver gather to a common shot gather. The method is tested on a structurally complex elastic model of the crust and the results show that it is accurate within floating point precision. The synthetic data are compared to data from a real ocean bottom seismometer experiment conducted across a continent-ocean transition zone. A good fit in terms of traveltime is observed for many of the prominent seismic phases. The amplitude fit of these arrivals is not always as good as the traveltime fit. This indicates that full-waveform modeling of such data can provide useful information about the subsurface that cannot be obtained from traveltime modeling. If enough data are available, the modeling method can be used in full-waveform inversion.

Description: A dense grid of 2D multichannel seismic data was used for the interpretation of sub-sea-floor structures in the area of Isfjorden in western Spitsbergen. West Spitsbergen underwent Eocene transpressional deformation that resulted in formation of the West Spitsbergen Fold-and-Thrust Belt. Three horizons were defined for the seismic interpretation as well-expressed and continuous reflections: (1) the top of the metamorphic basement; (2) the base of the upper Carboniferous Nordenskiöldbreen Formation; (3) the base of the Lower Cretaceous Helvetiafjellet Formation. Time–structure maps and analysis of the sub-bottom structural trends were generated for each horizon. The top of the metamorphic basement displays north–south-trending graben structures, apparently representing continuation of the Devonian grabens from northern Spitsbergen. The tectonostratigraphic unit bounded by the base of the upper Carboniferous Nordenskiöldbreen Formation and base of the Helvetiafjellet Formation encloses the fold-and-thrust belt and is affiliated with WSW–ENE shortening involving three décollement levels. Within this unit the strata between the middle (Triassic shales) and upper (Upper Jurassic shales) décollements have undergone the most intense strain, whereas sediments situated between the basal (lower Permian evaporites) and middle décollements underwent a relatively mild deformation. The strata above the base of the Helvetiafjellet Formation are characterized by minor Tertiary deformation only.

Description: The current effort represents a systematic regional study of the vast and poorly sampled area, linking the Barents Sea and the Arctic Ocean. The deep structure of the Northern Barents Sea was examined by means of integration various geophysical techniques, including numerical geodynamic modeling. Ocean Bottom Seismometers data have been acquired east of Svalbard and processed using a seismic refraction/reflection tomography method. A series of crustal-scale geotransects, illustrating the architecture of the Cenozoic Northern Barents Sea margin were constructed using gravity modeling, sparse seismic reflection profiles and depth to magnetic sources estimates. The structure of the Mesozoic passive margin, facing to the Amerasia Basin, was inferred based on a similar technique, involving plate reconstructions. Numerical simulations of the lithosphere extension, leading to formation of the Eurasia Basin, was performed using the finite element method. The velocity structure east of Svalbard exhibits evidences of Cretaceous magmatism. In particular, funnel-shaped high-velocity anomalies, reaching 10% relative to the 1D background model, are interpreted as Early Cretaceous magmatic intrusions. Further to the north, a narrow and steep continent-ocean transition was observed. The conjugate northern (and eastern) Barents Sea - Lomonosov Ridge margins are symmetric and narrow whereas the continent-ocean transition on the Podvodnikov Basin's side of the Lomonosov Ridge is broad. On the continental side, the Northern Barents Sea margin is underlain by Paleozoic-Early Mesozoic deep sedimentary basins separated from the oceanic side by the marginal basement uplift. The Northern Barents Sea, including Svalbard, was not affected by the major Late Jurassic - Early Cretaceous rifting which gave rise to deep basins in the South Western Barents Sea. However, the area experienced widespread Early Cretaceous magmatism. The emplacement of mafic magmas was controlled by Paleozoic rift structures which were reactivated in the Early Cretaceous. The magmatism east of Svalbard developed without significant crustal thinning, but was probably triggered by localized lithospheric weakness zones. The Mesozoic passive margin, originated due to the opening of the Podvodnikov Basin, was subjected to significant crustal thinning. The Northern Barents Sea region together with the Lomonosov Ridge was standing high during most of the Late Cretaceous. The regional uplift sourced from the Alpha Ridge area. The Eurasia Basin's breakup in the Paleocene preceded the opening of the Norwegian Sea, implying a connection to the Labrador Sea. A short-lived lithosphere-scale shear zone has likely facilitated to the detachment of the Lomonosov Ridge microcontinent and onset of seafloor spreading. Shear heating in the mantle lithosphere accompanied the development of the proposed shear zone and served as a mechanism for strain localization.

Description: The outer mid-Norwegian margin is characterized by strong breakup magmatism and has been extensively surveyed. The crustal structure of the inner continental shelf, however, is less studied, and its relation to the onshore geology, Caledonian structuring, and breakup magmatism remains unclear. Two Ocean Bottom Seismometer profiles were acquired across the Trøndelag Platform in 2003, as part of the Euromargins program. Additional-land stations recorded the marine shots. The P-wave data were modeled by ray-tracing, supported by gravity modeling. Older multi-channel seismic data allowed for interpretation of stratigraphy down to the top of the Triassic. Crystalline basement velocity is ~6 km s-1 onshore. Top basement is difficult to identify offshore, as velocities (5.3-5.7 km s-1) intermediate between typical crystalline crust and Mesozoic sedimentary strata appear 50-80 km from the coast. This layer thickens towards the Klakk-Ytreholmen Fault Complex and predates Permian and later structur-ing. The velocities indicate sedimentary rocks, most likely Devonian. Onshore late- to post-Caledonian detachments have been proposed to extend offshore, based on the magnetic anomaly pattern. We do not find the expected correlation between upper basement velocity structure and detachments. However, there is a distinct, dome-shaped lower-crustal body with a velocity of 6.6-7.0 km s-1. This is thickest under the Froan Basin, and the broad magnetic anomaly used to delineate the detachments correlates with this. The proposed offshore continuation of the detachments thus appears- unreliable. While we find indications of high density and velocity (~7.2 km s-1) lower crust under the Rås Basin, similar to the proposed igneous underplating of the outer margin, this is poorly constrained near the end of our profiles. The gravity field indicates that this body may be continuous from the pre-breakup basement structures of the Utgard High to the Frøya High, suggesting that it could be an island arc or oceanic terrane-accreted during the Caledonian orogeny. Thus, we find no clear evidence of early Cenozoic igneous underplating of the inner part of the shelf.