Per Digranes

Crustal structure of the northern part of the Vøring Basin, mid-Norway margin, from wide-angle seismic and gravity data

Abstract Three regional Ocean Bottom Seismograph (OBS) profiles with a total of fifteen recovered OBSs were acquired in the northern part of the Voring Basin, mid-Norway margin, in 1996. The data have been modelled by inversion and forward modelling of the OBS vertical components (P-waves). The velocity is found to increase downwards within the sedimentary layers, due to increasing depth of burial (confining pressure). Within the two deepest sedimentary layers there seems to be an increase of 0.5–1 km/s in velocity northwestwards. This increase is most likely caused by high-velocity sill-intrusions in the sedimentary rocks, emplaced during the rift episode leading to Early Eocene opening of the NE Atlantic. The presence of significant amounts of sills seems to terminate close to the Nyk High along profile 5 (dip-profile), and close to a dome northwest of the shelf edge along profile 6 (dip-profile). The velocity in the top of this dome is anomalously high (3.4 km/s at only 1 km depth beneath the seafloor), suggesting that the high consists of volcanic rocks, or heavily intruded sedimentary rocks. The relatively low velocities derived from within the upper crystalline crust (5.9–6.2 km/s) confirms that the crust is of continental origin. Within the lower crust there seems to be a clear increase in velocity northwestwards (from about 7.0 km/s to about 7.4 km/s), suggesting that the amount of high-velocity intrusions (underplating) in the lower crust decreases landwards. The modelling of the strike-profile (profile 7) suggests that the transition from a lower crust dominated by magmatic intrusions to a more undisturbed continental lower crust is located approximately beneath this profile. The model for profile 7 ties closely southwestwards to the models derived from OBS data acquired in 1992, and northeastwards to Lofoten (OBS data acquired in 1988). The models indicate that the thick magmatic intrusions in the sedimentary rocks and lower crust extend further landwards in the area of the 1992 survey, and that the transition zone is related to the Surt Lineament. The distribution of magmatic rocks thus seems to correlate strongly with pre-breakup structures. In addition to the P-wave modelling, gravity modelling has been performed along all profiles. The gravity modelling confirmed all main aspects of the OBS models, and provided constraints on the crustal structures towards the ends of the profiles.

Crustal structure of the outer Vøring Plateau, offshore Norway, from ocean bottom seismic and gravity data

Four ocean bottom seismograph (OBS) profiles acquired across the continent-ocean transition zone of the outer Voring Plateau have been modeled by inversion and forward modeling of the OBS vertical component traces (P waves). The thickness of the sedimentary layers deposited since the early Eocene continental breakup varies from up to 2.2 km above the basaltic inner flows (IF) to a minimum of 0.2 km about 20 km seaward of the Voring Escarpment (VE), with a velocity of 1.6–2.55 km/s. The thickness of the uppermost layer of basalt varies from 0.5 to 4.5 km, and its velocity ranges from 3.4 to 5 km/s. The modeling of the two dip profiles suggests that strongly intruded preopening sedimentary rocks extend ∼40 km seaward of the VE. Farther seaward, another layer of extrusives/intrusives is found beneath the uppermost layer of basalt. The lateral variations in velocity in the main crustal layer along the dip profiles indicate that the continent/ocean transition extends over a 30–50 km wide zone in between (strike) profile 3 (oceanic; 6.6–7.35 km/s) and (strike) profile 4 (continental; 6.1–7.0 km/s). An up to 7 km thick lower crustal high-velocity body (7.1–7.4 km/s) is interpreted in terms of intrusions in the lower crust beneath and landward of the continent/ocean transition. The total thickness of the normal oceanic crust is estimated to be 7–9 km, the anomalously thick oceanic crust reaches a maximum thickness of about 25 km, and the crustal thickness is estimated to be 17–22 km beneath the IF. Gravity modeling of the profiles confirmed all main aspects of the velocity models and provided constraints on the crustal structures toward the ends of the profiles where the ray coverage was low.

Vp/Vs ratio along the Vøring Margin, NE Atlantic, derived from OBS data: implications on lithology and stress field

A total of 13 regional Ocean Bottom Seismograph (OBS) profiles with an accumulated length of 2207 km acquired on the Voring Margin, NE Atlantic have been travel time modelled with regards to S-waves. The Vp/Vs ratios are found to decrease with depth through the Tertiary layers, which is attributed to increased compaction and consolidation of the rocks. The Vp/Vs ratio in the intra-Campanian to mid-Campanian layer (1.75–1.8) in the central Voring Basin is significantly lower than for the layers above and beneath, suggesting higher sand/shale ratio. This layer was confirmed by drilling to represent a layer of sandstone. This mid-Cretaceous ‘anomaly’ is also present in the northern Voring Basin, as well as on the southern Lofoten Margin further north. The Vp/Vs ratio in the extrusive rocks on the Voring Plateau is estimated to be 1.85, conformable with mafic (basaltic) rocks. Landward of the continent/ocean transition (COT), the Vp/Vs ratio in the layer beneath the volcanics is estimated to be 1.67–1.75. These low values suggest that this layer represents sedimentary rocks, and that the sand/shale ratio might be relatively high here. The Vp/Vs ratio in the crystalline basement is estimated to be 1.67–1.75 in the basin and on the landward part of the Voring Plateau, indicating the presence of granitic/granodioritic continental crust. In the lower crust, the Vp/Vs ratio in the basin decreases uniformly from southwest to northeast, from 1.85–1.9 to 1.68–1.73, suggesting a gradual change from mafic (gabbroic) to felsic (granodioritic) lower crust. Significant (3–5%) azimuthal S-wave anisotropy is observed for several sedimentary layers, as well as in the lower crust. All these observations can be explained by invoking the presence of liquid-filled microcracks aligned vertically along the direction of the present day maximum compressive stress (NW–SE).

Modelling shear waves in OBS data from the Vøring basin (Northern Norway) by 2-D ray-tracing

Three component recordings from an array of five ocean bottom seismographs in the northwestern part of the Vøring basin have been used to obtain a 2-D shear-wave (S-wave) velocity-depth model. The shear waves are identified by means of travel-time differences compared to the compressional (P) waves, and by analyzing their particle motions. The model has been obtained by kinematic (travel-time) ray-tracing modelling of the OBS horizontal components.The shear-wave modelling indicates that mode conversions occur at several high velocity interfaces (sills) in the 4–10 km depth range, previously defined by a compressional-wave velocity-depth model using the same data set.An averageVp/Vsratio of 2.1 is inferred for the layers above the uppermost sill, indicative of both poorly consolidated sediments and a low sand/shale ratio. A significant decrease in theVp/Vsratio (1.7) below the first sill may in part be atributed to well consolidated sediments, and to a change in lithology to more sandy sediments. This layer is interpreted to lie within the lower Cretaceous sequence. At 5–10 km depthVp/Vsratios of 1.85 indicate a lower sand/shale ratio consistent with the expected lithologies. The averageVp/Vsratio inferred for the crust is 1.75, which is consistent with values obtained north of Vøring, in the Lofoten area. An eastward thinning of the crystalline basement is supported by the shear-wave modelling.

Vertical seismic profile results from the Kola Superdeep Borehole, Russia

Multi-offset vertical seismic profiles (VSPs) from the Kola Superdeep Borehole (SG-3), as part of a larger seismic study of the Kola region conducted during the spring of 1992, sample the dipping Pechenga complex from 2175 m to 6000 m and contribute to the understanding of reflectivity in crystalline and Precambrian environments. From the surface to 6000 m, the SG-3 borehole penetrates interlayered Proterozoic metavolcanic and metasedimentary units and a mylonitic shear zone ranging from greenschist to amphibolite metamorphic grade, respectively. The Kola VSPs display a 6% velocity decrease which coincides to a mylonitic shear zone located between 4500 m and 5100 m within the SG-3 borehole. Seismic interfaces are identified by mode-converted energy (PS, and SP transmissions and reflections) in addition to primary seismic phases. The VSP shear wave energy is generated at or near the source by vertical vibrators. P-wave and S-wave reflections are generally detected from the same reflecting horizons, but increases in relative S-wave and SP reflection amplitudes originate at 1900 m, 3800 m, 4500 m, and 5100 m depths. These depths coincide with zones of elevated Vo/Vs and may support the presence of free pore fluid which is reported from initial drilling. For the Proterozoic lithologies sampled by the VSP, reflection events result from five mylonitic shear zones and three lithologic contrasts.

Application of the Single-Bubble Airgun Technique for OBS-Data Acquisition Across the Jan Mayen Ridge, North Atlantic

A 140 km long wide-angle seismic profile has been acquired by use of 6 Ocean Bottom Seismographs across the Jan Mayen Ridge, North Atlantic. The profile was acquired twice; once with a traditionally tuned standard source and secondly with a somewhat smaller source tuned on the first bubble pulse. Analysis of the frequency content of the data reveals that the single-bubble source within the 10-15 Hz frequency range generates a signal with a level about 5 db above that of the standard source. These differences can partly be related to differences in airgun depth. The higher output level for these frequencies enables the single-bubble source to resolve intra-crustal structures with a higher degree of certainty, when compared to the data acquired by use of the standard source array. The standard source seems to generate slightly more energy for frequencies around 6 Hz, probably due to the use of a large 1200 in/sup3 gun in this array. These low frequencies a re of importance for mapping of lower crustal and upper mantle structures, and it is recommended that this is taken into account when seismic sources for mapping of deep crustal and upper mantle structures are designed.

Use of mode-converted waves in marine seismic data to investigate the lithology of the sub-bottom sediments in Isfjorden, Svalbard

AbstractsThe compression wavefield is efficiently converted to shear-wave energy at post-critical angles in areas of high impedance contrast at the sea floor. We have analysed mode-converted shear waves in a data set acquired with a hybrid marine/land geometry in Isfjorden, Svalbard. Through a kinematic 2D ray-tracing modellingVp/Vs ratios for part of the uppermost 5km of the crust are obtained. Low values (Vp/Vs=1.65) are tentatively associated with the section of Devonian sandstones which appears to attain a minimum thickness of 1.5km below 3 km depth about 10km west of Kapp Thorden.

Regional and semi-regional modelling of wide-angle shear waves in OBS data from the Vøring Basin, N. Norway—a comparison

Shear wave refractions and wide-angle reflections recorded on densely spaced Ocean Bottom Seismometers (OBSs) in the Vøring Basin have been modelled by use of 2-D kinematic ray-tracing. Twenty-five three component OBSs were deployed along a 120 km long profile, and the semi-regional shear-wave velocity model derived from modelling the horizontal components is compared with a regional model from 5 OBSs along the same profile. The overall shear-wave velocity distribution of the two models are fairly similar, proving that the regional procedure with large OBS spacing provides a reliable regional shear-wave velocity model. The semi-regional model is more detailed than the regional model due to the much closer receiver spacing. The vertical resolution is improved, especially in the shallow sediments, and this enables a more detailed lithological interpretation to be carried out. Intermediate and deep sediments are also better resolved in the semi-regional model, indicating both vertical and lateral changes in the lithology. The Vp/Vs ratio inferred for the crystalline basement is 1.75 in both models, considered to be typical for the continental crust. An increase in the Vp/Vs ratio in the lower crust is observed both in the regional and in the semi-regional model. This indicates a more mafic composition in the lower crust, supporting the interpretation that the lower crust is a “mixing” of continental crust and magmatic underplating.