Seismic on floating ice: data acquisition versus flexural wave noise     

Tor Arne Johansen  Bent Ole Ruud  Ronny Tømmerbakke  Kristian Jensen 《Geophysical Prospecting》2019,67(3):532-549

Geophysical surveying of the Arctic will become increasingly important in future prospecting and monitoring of the terrestrial and adjacent areas in this hemisphere. Seismic data acquired on floating ice are hampered with extensive noise due to ice vibrations related to highly dispersive ice flexural waves generated by the seismic source. Several experiments have been conducted on floating ice in van Mijenfjorden in Svalbard in the Norwegian Arctic to specifically analyse the extent of flexural waves recorded with various seismic receivers and sources deployed both on top of ice and in the water below. The data show that flexural waves are severely damped at 5 m or deeper below the ice and hydrophone data suffer less from these vibrations compared with data recorded on the ice. Aliasing of single receiver hydrophone data can to some extent be suppressed using an in-line line source of detonating cord. Experiments on ice on shallow water show prominent guided wave modes often referred to as Scholte waves propagating along the seabed. In this case, both flexural and Scholte waves interfere and make a complicated pattern of coherent noise. On shallow water, the positioning and type of the seismic source must be evaluated with respect to the coherent noise generated by these waves. Geophone strings of 25 m effectively suppress both flexural and Scholte waves due to their relative short wavelengths. An airgun generates relative more low-frequency energy than a surface source of detonating cord. Accordingly, seismic mapping of deep seismic horizons seem to be best achieved using geophone strings of such length and an airgun source. For shallow targets, the use of hydrophones in combination with detonating cord is an appropriate solution. Seismic surveying in the Arctic always have to follow environmental restrictions of not disturbing or harming wildlife and not causing permanent footprints into the vulnerable tundra, which implies that the choice of seismic acquisition strategy might occur as a trade-off between optimum data quality and environmental constraints.  相似文献   

Shocked quartz grains in the early Cambrian Vakkejokk Breccia,Sweden—Evidence of a marine impact     

Carl Alwmark  Jens Orm  Arne T. Nielsen 《Meteoritics & planetary science》2019,54(3):609-620

Here we present a study of the abundance and orientation of planar deformation features (PDFs) in the Vakkejokk Breccia, a proposed lower Cambrian impact ejecta layer in the North‐Swedish Caledonides. The presence of PDFs is widely accepted as evidence for shock metamorphism associated with cosmic impact events and their presence confirms that the Vakkejokk Breccia is indeed the result of an impact. The breccia has previously been divided into four lithological subunits (from bottom to top), viz. lower polymict breccia (LPB), graded polymict breccia (GPB), top sandstone (TS), and top conglomerate (TC). Here we show that the LPB contains no shock metamorphic features, indicating that the material derives from just outside of the crater and represents low‐shock semi‐autochthonous bombarded strata. In the overlying, more fine‐grained GPB and TS, quartz grains with PDFs are relatively abundant (2–5% of the grain population), and with higher shock levels in the upper parts, suggesting that they have formed by reworking of more distal ejecta by resurge of water toward the crater in a marine setting. The absence of shocked quartz grains in the TC indicates that this unit represents later slumps associated with weathering and erosion of the protruding crater rim. Sparse shocked quartz grains (<0.2%) were also found in sandstone beds occurring at the same stratigraphic level as the Vakkejokk Breccia 15–20 km from the inferred crater site. It is currently unresolved whether the sandstone at these distal sites is related to the impact or just contains rare reworked quartz grains with PDFs.  相似文献   

Validation of automated supervised segmentation of multibeam backscatter data from the Chatham Rise,New Zealand     

Jess I. T. Hillman  Geoffroy Lamarche  Arne Pallentin  Ingo A. Pecher  Andrew R. Gorman  Jens Schneider von Deimling 《Marine Geophysical Researches》2018,39(1-2):205-227

Using automated supervised segmentation of multibeam backscatter data to delineate seafloor substrates is a relatively novel technique. Low-frequency multibeam echosounders (MBES), such as the 12-kHz EM120, present particular difficulties since the signal can penetrate several metres into the seafloor, depending on substrate type. We present a case study illustrating how a non-targeted dataset may be used to derive information from multibeam backscatter data regarding distribution of substrate types. The results allow us to assess limitations associated with low frequency MBES where sub-bottom layering is present, and test the accuracy of automated supervised segmentation performed using SonarScope® software. This is done through comparison of predicted and observed substrate from backscatter facies-derived classes and substrate data, reinforced using quantitative statistical analysis based on a confusion matrix. We use sediment samples, video transects and sub-bottom profiles acquired on the Chatham Rise, east of New Zealand. Inferences on the substrate types are made using the Generic Seafloor Acoustic Backscatter (GSAB) model, and the extents of the backscatter classes are delineated by automated supervised segmentation. Correlating substrate data to backscatter classes revealed that backscatter amplitude may correspond to lithologies up to 4 m below the seafloor. Our results emphasise several issues related to substrate characterisation using backscatter classification, primarily because the GSAB model does not only relate to grain size and roughness properties of substrate, but also accounts for other parameters that influence backscatter. Better understanding these limitations allows us to derive first-order interpretations of sediment properties from automated supervised segmentation.  相似文献   

Amorphous silicates as a record of solar nebular and parent body processes—A transmission electron microscope study of fine‐grained rims and matrix in three Antarctic CR chondrites     

Christian Vollmer  Mandy Pelka  Jan Leitner  Arne Janssen 《Meteoritics & planetary science》2020,55(7):1491-1508

Renazzo‐type (CR) carbonaceous chondrites belong to one of the most pristine meteorite groups containing various early solar system components such as matrix and fine‐grained rims (FGRs), whose formation mechanisms are still debated. Here, we have investigated FGRs of three Antarctic CR chondrites (GRA 95229, MIL 07525, and EET 92161) by electron microscopy techniques. We specifically focused on the abundances and chemical compositions of the amorphous silicates within the rims and matrix by analytical transmission electron microscopy. Comparison of the amorphous silicate composition to a matrix area of GRA 95229 clearly shows a compositional relationship between the matrix and the fine‐grained rim, such as similar Mg/Si and Fe/Si ratios. This relationship and the abundance of the amorphous silicates in the rims strengthen a solar nebular origin and rule out a primary formation mechanism by parent body processes such as chondrule erosion. Moreover, our chemical analyses of the amorphous silicates and their abundance indicate that the CR rims experienced progressive alteration stages. According to our analyses, the GRA 95229 sample is the least altered one based on its high modal abundance of amorphous silicates (31%) and close‐to‐chondritic Fe/Si ratios, followed by MIL 07525 and finally EET 92161 with lesser amounts of amorphous silicates (12% and 5%, respectively) and higher Fe/Si ratios. Abundances and chemical compositions of amorphous silicates within matrix and rims are therefore suitable recorders to track different alteration stages on a submicron scale within variably altered CR chondrites.  相似文献   

The German Bight (North Sea) is a nursery area for both locally and externally produced sprat juveniles     

Hannes Baumann  Arne M. Malzahn  Rudi Voss  Axel Temming 《Journal of Sea Research》2009,61(4):234-243

To better understand the role of the German Bight (GB) as a nursery area for juvenile North Sea sprat Sprattus sprattus we sought to determine whether the area may receive only locally or also externally produced offspring. We sampled juveniles during 3 trawl surveys in the GB in August, September, and October 2004 and applied otolith microstructure analysis in order to reconstruct their distributions of the day-of-first-increment-formation (dif). These were contrasted with spatial and seasonal patterns of sprat egg abundance in the GB and its adjacent areas, observed during 6 monthly plankton surveys. It was found that the majority of juveniles originated mainly from April/May 2004, coinciding with high spawning activity west of the GB, whereas spawning and larval production inside the GB peaked notably later, in May/June. This indicated that a large proportion of juveniles was produced outside the GB and transported subsequently into it through passive and/or active migration. Shifts to later mean difs from one survey to the next and length distributions indicative of the simultaneous presence of multiple cohorts, supported the notion that the GB is a complex retention and nursery area for sprat offspring from different North Sea spawning grounds and times. Later born juveniles had significantly faster initial growth rates than earlier born conspecifics, which was likely temperature-mediated, given the strong correlation between back-calculated growth histories and sea surface temperature as a proxy for thermal histories of juveniles (r² = 0.52).  相似文献   

‘Head for my tail’: a new hypothesis to explain how venomous sea snakes avoid becoming prey     

Arne Redsted Rasmussen  Johan Elmberg 《Marine Ecology》2009,30(4):385-390

Sea snakes are widespread and conspicuous inhabitants of shallow waters in the Indian and Pacific Oceans. They are agile top predators and possess extremely potent venom, but they are still susceptible to predation by large fish, e.g. sharks, and other vertebrates. We describe how crevice‐probing and temporarily non‐vigilant Yellow‐lipped Sea Kraits Laticauda colubrina twist the tail around their length axis so that the tail tip’s lateral aspect corresponds to the dorsal view of the head. In doing so, coloration and pattern in combination with tail movement and posture make the tail appear very similar to the (non‐visible and foraging) head. We examined 98 Laticauda spp. sea snakes in three major museum collections and reviewed the literature to assess the generality and implications of our field observations. This leads us to hypothesize that a combination of: (i) head and tail being similarly coloured and patterned, and (ii) the tail being motioned to resemble the head, is a hitherto overlooked mimetic and ‘prophylactic’ anti‐predator adaptation in the L. colubrina complex, and possibly in other species of sea snake. We propose this is a concerted behavioural–morphological adaptation, and we briefly speculate about its possible fitness trade‐offs as well as its origin. Explicit and testable predictions derived from the hypothesis are presented.  相似文献