Christopher Juhlin

3D baseline seismics at Ketzin, Germany : The CO2SINK project

A 3D 25-fold seismic survey with a bin size of 12 by 12 m and about 12 km(2) of subsurface coverage was acquired in 2005 near a former natural gas storage site west of Berlin, as part of the fiv ...

Interpretation of the reflections in the Siljan Ring area based on results from the Gravberg-1 borehole

Abstract During 1985 about 80 km of surface multichannel Seismic reflection data were collected across the meteorite impact in the Siljan Ring area in central Sweden. The area consists mainly of granitic and gneissic rock ranging in age from 1400 to 2000 million years with remnants of Palaeozoic sedimentary rocks preserved by downfaulting after a meteorite impact at 360 Ma, thereby forming a circular ring about 40 km in diameter. Dolerite intrusions ranging in age from 850 to 1700 million years are also present. The Seismic data revealed several high-amplitude, laterally continuous, sub-horizontal reflections in the northern part of the structure. The high-amplitude reflections and a possible intermediate low-velocity zone were contributing factors in choosing the site for the Gravberg-1 Deep Earth Gas test well. Drilling and vertical Seismic profiling (VSP) found that the reflectors were associated with dolerite sills which had intruded into the granite and which range in thickness from a few metres up to 60 m and with a pre-impact area extent of at least 800 km 2 . Studies of amplitude and frequency versus offset (AFVO) show the observations are compatible with a model of simple granite/dolerite/granite layering.

CRUSTAL-SCALE STRUCTURE AND EVOLUTION OF AN ARC-CONTINENT COLLISION ZONE IN THE SOUTHERN URALS, RUSSIA

The outcropping geology of the southern Urals contains a well-preserved accretionary complex related to the Paleozoic collision that took place between the Magnitogorsk arc and the former East European Craton. The crustal-scale structure of the accretionary complex has been determined from outcropping field geology that is integrated with three reflection seismic profiles. The reflection profiles show the accretionary complex to be highly reflective, allowing direct comparison of many reflections with surface geological features. We interpret the accretionary complex to be a thrust stack that is composed of shallowly subducted continental shelf and rise material, syncollisional sediments derived from the arc, deeply subducted high-pressure gneisses that are intercalated with eclogites and blueschist, and, at the highest structural level, ophiolite complexes. It is bound at the base by a thrust and at the rear by a highly deformed zone (the Main Uralian fault) adjacent to the backstop (the Magnitogorsk arc). Deposition of the Late Devonian volcaniclastic sediments of the Zilair Formation appears to be related to collision, uplift, and erosion of the arc, possibly following the arrival of the full thickness of the East European Craton continental crust at the subduction zone. With the arrival of the continental crust at the subduction zone, offscraping and underplating of Paleozoic slope and platform material took place at the base of the accretionary complex. Uplift of the arc was followed by its collapse and the unconformable deposition of Lower Carboniferous shallow water carbonates on top of it. A time lag of 10 – 15 Myr occurred between the high-pressure metamorphism and the subsequent arrival of the East European Craton at the subduction zone.

Collisional Orogeny in the Scandinavian Caledonides (COSC)

The COSC project is focused on the mid Paleozoic Caledonide Orogen in Scandinavia in order to better understand orogenic processes, both in the past and in todays active mountain belts. It relates to two of ICDPs main themes – the fundamental physics of plate tectonics and heat, mass and fluid transfer through Earths crust, and on improving interpretation of geophysical data used to determine the structure and properties of the Earths crust. Lateral transport of Caledonian allochthons over distances of several hundreds of kilometers in the Scandes, by a combination of thrusting and ductile extrusion, is comparable to that recognized in the Himalayas. The Caledonides in Scandinavia provide special opportunities for understanding Himalayan-type orogeny and the Himalayan Orogen itself, thanks to the deep level of erosion and the paucity of superimposed post-Paleozoic deformation. The surface geology in combination with the seismic, magnetotelluric, magnetic and gravity data provide control of the geometry of the Caledonian structure, both of the allochthon and the underlying parautochthon-autochthon, and define the locations for drilling. The latter will investigate both the high-grade, ductile Caledonian nappes and the underlying allochthons and basement, with two c. 2.5 km deep boreholes, located near Åre and Järpen in western Jämtland. The two boreholes will also provide unique information about other important aspects of the Scandinavian bedrock, including the heat flow and potential for geothermal energy, mineralization in the Seve nappes and alum shales, the uplift history of the Scandes, the Holocene paleoclimatological changes and the deep biosphere.

Imaging of fracture zones in the Finnsjon area, central Sweden, using the seismic reflection method

In 1987 the Swedish Nuclear Fuel and Waste Management Co. (SKB) funded the shooting of a 1.7-km long, high-resolution seismic profile over the Finnsjon study site using a 60-channel acquisition system with a shotpoint and geophone spacing of 10 m. The site is located about 140 km north of Stockholm and the host rocks are mainly granodioritic. The main objective of the profile was to image a known fracture zone with high hydraulic conductivity dipping gently to the west at depths of 100 to 400 m. The initial processing of the data failed to image this fracture zone. However, a steeply dipping reflector was imaged indicating the field data were of adequate quality and that the problem lay in the processing. These data have now been reprocessed and a clear image of the gently dipping zone has been obtained. In addition, several other reflectors were imaged in the reprocessed section, both gently and steeply dipping ones. Correlations with borehole data indicate that the origin of these reflections are also fracture zones. The improvement over the previous processing is caused mainly by (1) refraction statics, (2) choice of frequency band, (3) F-K filtering, and (4) velocity analyses.In addition to reprocessing the data, some further analyses were done including simulation of acquisition using only the near-offset channels (channels 1-30) and the far-offset channels (channels 31-60), and determining the damping factor Q in the upper few hundred meters based upon the amplitude decay of the first arrivals. The data acquisition simulation shows the far-offset contribution to be significant even for shallow reflectors in this area, contrary to what may be expected. A Q value of 10, determined from observed amplitude decay rates, agrees well with theoretical ones assuming plane wave propagation in an attenuating medium.

Reflection seismic investigations in the Dannemora area, central Sweden: Insights into the geometry of polyphase deformation zones and magnetite‐skarn deposits

The Bergslagen region is one of the most ore prospective districts in Sweden. Presented here are results from two nearly 25 km long reflection seismic profiles crossing this region in the Dannemora ...

CRUSTAL STRUCTURE OF THE MIDDLE URALS : RESULTS FROM THE (ESRU) EUROPROBE SEISMIC REFLECTION PROFILING IN THE URALS EXPERIMENTS

As a contribution to Europrobes seismic reflection profiling in the Urals (ESRU) project, three overlapping seismic reflection data sets were acquired in the Middle Urals. A 56 km long profile was registered over the Europe-Asia suture, two 25 km long intersecting profiles were collected over the Urals superdeep borehole (SG4), and an 80 km long profile was recorded eastward extending east toward the West Siberian Basin. Reflections on the seismic sections delineate several major middle to late Paleozoic thrust zones in the upper crust. These thrust zones have a bivergent geometry with westerly vergence west of the Uralian orogenic axis and easterly vergence to the east. The principal terrane boundaries are the Main Uralian Thrust Fault in the west and the Trans-Uralian Thrust Zone in the east. Normal faults are spatially associated with former thrust faults, or they crosscut them. The thrust and normal faults can be confidently correlated with surface geological features. Near-vertical and wide-angle seismic reflection profiling reveals thickening of the crust from about 45 km to approximately 53 km below the central axis of the Urals. East and west of the root zone, the lower crust is reflective, particularly toward the West Siberian Basin. We interpret the reflectivity of the crust below the East European Craton as pre-Uralian, whereas that toward the West Siberian Basin is interpreted as late orogenic. Although the principal tectonic features imaged by the seismic sections are probably of Paleozoic age, a post-Paleozoic origin for the lower crustal reflectivity in the east cannot be ruled out.

Crustal evolution of the Middle Urals based on seismic reflection and refraction data

Abstract Integration of seismic refraction and deep reflection data from the Middle Urals of central Russia provides important new constraints on the structure of the Uralian crust. Re-analysis of the GRANIT refraction profile and comparison with coincident reflection data from the ESRU profile shows a high-velocity (7.6–7.8 km/s) root zone from c. 45 to 55 km with low reflectivity beneath the Urals. We interpret this interval as crustal material, consistent with previous Russian interpretations of this velocity anomaly. Above this crustal root, one of the principle features imaged on the ESRU profile is a thick zone (3–4 s TWT) of relatively strong reflectivity which may characterize the lower crust of the East European platform, but is considerably shallower to the east at 8–12 s TWT (25–37 km) than in the west at 10–14 s TWT (31–43 km). Reprocessing of 20 s records from the R-17 Profile in the West Siberian basin, ∼315 km northeast of the ESRU profile, reveals a similar pronounced lower-crustal reflectivity between 11–14 s TWT (34–43 km), in the hinterland of the Middle Urals. This lower-crustal reflection fabric may represent a feature developed during collisional orogenesis, or a younger property imparted through post-orogenic extension. Future deep reflection profiling will be critical to address the continuity, and accordingly, the tectonic significance of this lower-crustal reflectivity in the Urals.

High-resolution seismic traveltime tomography incorporating static corrections applied to a till-covered bedrock environment

A major obstacle in tomographic inversion is near-surface velocity variations. Such shallow velocity variations need to be known and correctly accounted for to obtain images of deeper structures with high resolution and quality. Bedrock cover in many areas consists of unconsolidated sediments and glacial till. To handle the problems associated with this cover, we present a tomographic method that solves for the 3D velocity structure and receiver static corrections simultaneously. We test the method on first-arrival picks from deep seismic reflection data acquired in the mid- late to 1980s in the Siljan Ring area, central Sweden. To use this data set successfully, one needs to handle a number of problems, including time-varying, near-surface velocities from data recorded in winter and summer, several sources and receivers within each inversion cell, varying thickness of the cover layer in each inversion cell, and complex 3D geology. Simultaneous inversion for static corrections and velocity produces a much better image than standard tomography without statics. The velocity model from the simultaneous inversion is superior to the velocity model produced using refraction statics obtained from standard reflection seismic processing prior to inversion. Best results using the simultaneous inversion are obtained when the initial top velocity layer is set to the near-surface bedrock velocity rather than the velocity of the cover. The resulting static calculations may, in the future, be compared to refraction static corrections in standard reflection seismic processing. The preferred final model shows a good correlation with the mapped geology and the airborne magnetic map.

Contrasting tectonic history of the arc–continent suture in the Southern and Middle Urals: implications for the evolution of the orogen

The Main Uralian Fault has been considered the original arc–continent suture for 2000 km along the Uralide orogen. The symmetry of the tectonic units across it suggested a consistent east-dipping polarity for the palaeosubduction zone, which, together with its topographic and aeromagnetic signature, supported the idea of a single suture. However, several characteristics vary at different latitudes. In the Middle Urals, it is a strike-slip fault zone with moderately deformed and metamorphosed volcanic arc fragments in its hanging wall, and low-grade metamorphic rocks of the East European Craton in its footwall. Here, it has a prominent NNW-trending magnetic signature which cross-cuts north-trending anomalies in its hanging wall, and a pronounced reflection seismic signature that can be traced to the top of the middle crust at c. 5 s. TWT. In the Southern Urals, it is a serpentinite mélange zone of ambiguous kinematics, with a weakly deformed and metamorphosed volcanic arc in its hanging wall, and moderately metamorphosed to high pressure rocks of the East European Craton in its footwall. In this part of the orogen, it has a weak reflection seismic character, and a magnetic signature that parallels that of its hanging wall. On the basis of an integrated analysis of these different data sets, we suggest that the Main Uralian Fault, as it is currently defined, is not a single entity, but rather the original arc–continent suture in the south, and the western strand of a strike-slip fault system that reworked the original suture in the Middle Urals.