High-resolution Acoustic Profiler Research Articles

Controls on shallow gas distribution, migration, and associated geohazards in the Yangtze subaqueous delta and the Hangzhou Bay

Shallow gas is generally extensively distributed in the Holocene muddy sediments and gas seepage has been increasingly reported to induce geohazards in coastal seas, but controls on gas distribution and migration remain elusive. This study explores gas distribution and migration in the Yangtze subaqueous delta and the Hangzhou Bay using high-resolution acoustic profiles and core data. Shallow gas is widely detected by the common presence of acoustic anomalous reflections including enhanced reflection, gas chimney, bright spot, acoustic blanking, and acoustic turbidity. The gas front depth is generally less than 17.5 m, and is meanly shallower in the Hangzhou Bay than in the Yangtze subaqueous delta because of relatively shallower water depth and coarser Holocene sediments in the Hangzhou Bay. Shallow gas is inferred to be a biogenic product, and its distribution is highly contingent on the Holocene stratal thickness and water depth. Active gas migration and seepages are evident, and recently increasing occurrences of gas seepage can be ascribed to global warming and seabed erosion due to sediment deficit. The findings warn us to pay more attention to the positive feedback loops of gas seepages with global warming and seabed erosion for the associated geohazard prediction and reduction, typically in the highly developed coastal regions.

CAGE13-5 Cruise Report: Investigation of glacial geomorphology in the Storfjordrenna.

The course GEO‐8144/3144 Marine geology and geophysics cruise, taught at the University of Tromsø, is part of the obligatory courses to be completed by PhD students affiliated with the AMGG research school and yields 5 credits (ECTS). Participants include scientific staff and students.
 This cruise was conducted within the framework of
 
 the Norwegian Science Council (NFR) project Glaciations in the Barents Sea area (GlaciBar) and
 the trainee school in Arctic Marine Geology and Geophysics (AMGG) financed by the University of Tromsø. The cruise was funded by AMGG.
 
 The main target areas are the Storfjordbanken and Storfjordrenna south of Svalbard. The cruise addressed marine glacigenic processes of the areas, with a focus on the last glacial-interglacial cycle in order to reconstruct in a more detail way the retreat of the Barents Sea–Svalbard Ice Sheet.
 To collect data have been used different methods and instruments, which will be described in a more detailed way in chapter 3. Below here there is only an overview:
 
 Multibeam echo sounder: Sound-wave beams are transmitted in a fan perpendicular to the ship track from a hull-mounted echo sounder. Produces high-resolution seafloor bathymetry.
 Chirp: High-resolution acoustic profiling.
 CTD (Conductivity, Temperature, Depth): Sensors installed in a frame measure CTD properties of the water column. Generally used for oceanographic studies, but here mainly to calibrate the multibeam data from calculated velocities.
 2D seismic: GI airguns and hydrophone cable (streamer).
 Sediment coring: Gravity coring.
 Gas sample collection.
 
 The targeted study area is under-explored compared to similar areas (Bjørnøyrenna) in the SW Barents Sea. The study areas in the Storfjordrenna and Stordfjorbanken are shown. Several different features have been identified on the seafloor, like MSGLs, grounding zone wedges, flares, pockmarks and ploughmarks. All these features will be described in detail in chapter 4.
 In addition, 9 gravity cores were acquired in areas likely to provide information on the timing of the formation of the sediment accumulation. The specific core locations were chosen based on information from multibeam swath bathymetry and chirp data. Furthermore, gas samples were collected from the cores from pockmarks and will be analyzed later.
 The new results add new detailed data regarding the deglaciation of the Storfjorden trough. Glacial geomorphologic features similar to the ones in Bjørnøyrenna were discovered. The observation of gas flares were the first such discovery in the northern Barents Sea. The discovery of flares and pockmarks might have a big impact on future petroleum exploration activities in the region.
 The cruise may be known as: HH_2013_GEO_8144_3144

On sedimentary waves at the western slope of the Middle Caspian Sea

Based on data of high-resolution acoustic profiling performed in the upper part of the sedimentary succession at the western continental slope of the Middle Caspian Sea (at a depth range from 100 to 400 m), we revealed specific accumulative forms of a running wave type. Examination of their morphology and inner structure allowed us to suggest two mechanisms of their formations: (a) by means of flank growth due to migration of channels of sedimentary flow associated with the Samur and other rivers and (b) due to flows of sedimentary material forming sedimentary waves.

Eivissa slides, western Mediterranean Sea: morphology and processes

After obtaining full-coverage swath bathymetry data in 1995 and very high-resolution acoustic profiles in 2002, four slides at the Balearic Margin of the Eivissa Channel in the western Mediterranean Sea were revisited in 2004 when side-scan sonar data were collected using a MAK-1M deep-towed acoustic system. These new findings, higher in resolution than those for the swath bathymetry, show two main features previously undetected within these submarine landslides: (1) a series of step-forming inclined and detached slabs oriented perpendicular to the slide movement and located in the uppermost part of the slides, and (2) arcuate regular positive ridges oriented also normal to the slide movement and located in the depositional lobes of some of the slides. The former are interpreted as extensional ridges, suggesting a retrogressive post-failure evolution of the slides. The latter are interpreted as compression ridges, related to plastic deformation of the sediment before movement freezing. Moreover, the new data show that fluid escape features are even more widespread in the Eivissa Channel than previously thought, dozens of new pockmarks less than 20 m in diameter having been identified.

Reconstructions of western Ross Sea palaeo‐ice‐stream grounding zones from high‐resolution acoustic stratigraphy

This study analyses acoustic profiles of two mid‐shelf troughs, the JOIDES Basin and Pennell Trough in the western Ross Sea, Antarctica. These troughs are subglacial erosion features formed by repeated advance of streaming ice onto the Ross Sea continental shelf. Sediment wedge formations, interpreted to have been deposited at the most seaward grounding zone during the Last Glacial Maximum (LGM), are observed within the mid‐shelf region of these troughs. By correlating high‐resolution acoustic profiles with core samples, we resolve the stratigraphy of these wedge formations to assess the relationships between bathymetry, subglacial sediment distribution and palaeo‐ice‐stream configuration. The grounding zone wedge geometries were controlled by the pre‐existing topography. The JOIDES and Pennell Troughs were only partially infilled during the LGM. Axial diamict progradation from the landward margin of the troughs indicates enhanced flow (debris supply) along the axis of ice flow. Differences in grounding zone geometries indicate a decrease in basal debris deposition and/or supply during ice recession. The lack of a recessional ice‐shelf facies indicates that either the ice shelf was absent during retreat, or that there was no melt‐out of basal debris.

High-resolution sediment echosounding off Peru: Late Quaternary depositional sequences and sedimentary structures of a current-dominated shelf

About 6000 km of both bathymetric and high-resolution acoustic profiles were acquired on the shelf and upper slope offshore Peru between 9° S and 14° S. Two new sediment echosounder systems – SEL-96 and SES-2000DS – provided details of the sedimentary structures of the Quaternary sequences within the Sechura-Salaverry, Huacho and Pisco Basins. To a great extent, the poleward undercurrent determines the distribution of sediments. The undercurrent has generated numerous erosional unconformities, it has winnowed hardgrounds and has created mudwaves common between 250 m and 400 m water depth. Distinct subbottom reflectors within sedimentary units represent hiatuses due to periods of intensified winnowing or non-deposition. Erosional unconformities usually marked by pronounced reflectors suggest shifts of the undercurrent system related to climatic changes and eustatic variations of sea level. On a larger scale, the stacked prograding depositional sequences reflect the sea-level cycles of the Middle Pleistocene to the Holocene. Based on the stratigraphy of our piston cores and that of Ocean Drilling Program (ODP) Site 680, the depositional sequences limited by extended unconformities were assigned to oxygen isotope stages 1 to 7. Other sedimentary structures are small straight channels that were conduits for downslope sediment transport. Deformed sediments associated with synsedimentary normal faults result from creep movements indicating beginning slope failure.

Lowermost Amazon River: morphology and shallow seismic characteristics

The riverbed morphology of the lowermost Amazon River was characterized by high-resolution acoustic profiling in 21 areas. This technique has been used for the first time in this area, and allowed the discovery of a variety of reflectors, which delineate areas either of sediment transport or of deposition on an older river bottom. Records obtained with the Pinger system made it possible to distinguish three main kinds of reflectors, both surface and sub-bottom: (1) flat, (2) wavy, and (3) irregular reflection patterns, which can be interpreted as strata deposited in a variable, mostly high energy setting, or reworked structures of initially flat or wavy strata that have been changed. Mapping of these different reflectors shows that they are not randomly distributed. The constant association of a given reflector with a particular morphological setting is indicative of its lithological significance (supported by riverbed sampling), and erosional/depositional processes. Depositional areas are characterized by transparent units consisting of soft silt overlying a highly reflective sub-bottom, related to old substrates (Alter do Chão or Barreiras Formation). This may result from inundation and incision caused by Quaternary sea-level changes. Previously eroded areas are covered by a wavy layer consisting of very fine to fine sand (unit FS), which can change locally to medium–coarse sand (unit MS). Reflectors are more abundant downstream. A minimum of 3 units were indicated by seismic records and could be related to sea level fluctuations and changes in the river discharge. Moreover, the detection of fault scarps, interpreted to result from neotectonic activity, shows that neotectonic features can be important locally and should not be ignored as a possible reason for modern river instability. The dynamic setting of the lowermost Amazon area results in patterns of deposition and erosion probably controlled by complex interactions of fluvial and oceanic processes. The effect of the Gurupa arch may result in a zone of sediment bypass in the central part and a zone of increased deposition in the distal downstream part.

Glaciomarine sedimentation in a modern fjord environment, Spitsbergen

By means of high resolution acoustic profiling and correlation of echo character and sediment lithology, fjords in western and northern Spitsbergen are shown to be blanketed by a 5-20 m layer of acoustically transparent sediments consisting mainly of soft homogeneous mud with ice rafted clasts. Acoustically semi-transparent material is found on slopes and sills reflecting their coarser composition. The areal average depositional rate in the outer fjord is in the range of from 0.1 to l.Omm/year, increasing towards the glaciers. In Kongsfjorden, 5&100mm/year of muddy sediments is deposited at a distance of lOkm from the calving Kongsvegen glacier. Close to the ice front ( 4 . 5 km) coarser grained, interbedded (sanqmud) sediments are deposited. The main sediment sources are from settlement out of the turbid surface sediment plume, combined with various types of gravity flow (sediment creep, minor slides, and slumping). Material deposited from turbidity current is probably of minor importance. On shallow sills the sediments are remobilized by icebergs. The sediment adjacent to the ice front is reworked and compacted during surges, a common form of glacial advance for Spitsbergen glaciers. During the surge considerable amounts of coarse-grained sediment are deposited by meltwater in front of the ice margin.

Sedimentary Processes Active on Slopes of California Borderland: ABSTRACT

The slopes of the California borderland are an important pathway for sediments transported from the mainland shelf, bank tops, and island platforms to the adjacent basin floor. Sedimentologic conditions on the slope are governed by a complex interplay of depositional and mass movement processes which are controlled by driving forces and stabilizing factors. Driving forces are predominantly external to the sediment mass and include oceanographic, biologic, and tectonic factors. Sedimentologic and geotechnical properties of sediments, as well as certain environmental parameters such as declivity, may provide a stabilizing influence to the sediments. These same factors, however, may be an influence in decreasing the stability and enhancing the influence of the driving mechan sms. Declivity, an environmental property commonly considered a major factor controlling failure, is less important than either proximity to active sediment sources or the influence of external driving forces. We have conducted field studies at different scales of examination including high resolution acoustic profiling, sediment sampling, bottom photography, and observations from manned submersibles. These data show that failures themselves are greatly varied in scale, ranging from large features kilometers on a side and approximately 50 m thick, through smaller scale failures tens to hundreds of meters on a side, and about 1 m thick, to very small displacements composed of locally contorted and deformed sediment layers only a few centimeters thick. We regard these small displacements as being more important in the basin filling process than has been previously recognized. In a detailed study of an acoustically defined failure zone on the mainland slope off San Mateo Point, we observed the zone itself to be composed of numerous narrow slumps. Cores from these deposits contain combinations of hemipelagic sediments and small-scale mass flow deposits exhibiting internal plastic deformation and basal scour. Vertical stacking of these sediment packets show that small-scale displacements can occur repeatedly at a given locality. Continued, episodic loading of the sediments through time produces locally inhomogeneous, weakened sediment masses which, in conjunction with driving forces, may contribute to the generation of large-scale fail res. Such large-scale features are those commonly identified by conventional acoustic techniques. End_of_Article - Last_Page 442------------