Seafloor Interaction Research Articles

Crevasse refreezing and signatures of retreat observed at Kamb Ice Stream grounding zone

Ice streams flowing into Ross Ice Shelf are presently responsible for around 10% of the mass flux from West Antarctica, with the noteworthy exception of Kamb Ice Stream, which stagnated in the late 1800s. The subsequent reduction in ice supply led to grounding-line retreat at the coastal margin where Kamb transitions into the floating Ross Ice Shelf. Grounding-line migration is linked to broader changes in ice-sheet mass balance and sea level, but our understanding of related ice, ocean and seafloor interactions is limited by the difficulty in accessing these remote regions. Here we report in situ observations from an underwater vehicle deployed at Kamb that show how fine-scale variability in ice and ocean structure combine to influence a diversity of ice–ocean interactions. We found a stratified water column within a tenth of a degree of freezing at the ice base and mapped basal crevasses with supercooled water and active marine ice formation. At the seafloor, we interpret parallel ridges as crevasse impressions left as the ice lifted off during grounding-line retreat. These observations from a recently ungrounded sub-shelf environment illuminate both the geomorphological signatures of past grounding-line retreat and the fine-scale sensitivity of ongoing ice–ocean interactions to ice topography.

Past ice sheet–seabed interactions in the northeastern Weddell Sea embayment, Antarctica

Abstract. The Antarctic ice sheet extent in the Weddell Sea embayment (WSE) during the Last Glacial Maximum (LGM; ca. 19–25 calibrated kiloyears before present, ka cal BP) and its subsequent retreat from the shelf are poorly constrained, with two conflicting scenarios being discussed. Today, the modern Brunt Ice Shelf, the last remaining ice shelf in the northeastern WSE, is only pinned at a single location and recent crevasse development may lead to its rapid disintegration in the near future. We investigated the seafloor morphology on the northeastern WSE shelf and discuss its implications, in combination with marine geological records, to create reconstructions of the past behaviour of this sector of the East Antarctic Ice Sheet (EAIS), including ice–seafloor interactions. Our data show that an ice stream flowed through Stancomb-Wills Trough and acted as the main conduit for EAIS drainage during the LGM in this sector. Post-LGM ice stream retreat occurred stepwise, with at least three documented grounding-line still-stands, and the trough had become free of grounded ice by ∼10.5 ka cal BP. In contrast, slow-flowing ice once covered the shelf in Brunt Basin and extended westwards toward McDonald Bank. During a later time period, only floating ice was present within Brunt Basin, but large “ice slabs” enclosed within the ice shelf occasionally ran aground at the eastern side of McDonald Bank, forming 10 unusual ramp-shaped seabed features. These ramps are the result of temporary ice shelf grounding events buttressing the ice further upstream. To the west of this area, Halley Trough very likely was free of grounded ice during the LGM, representing a potential refuge for benthic shelf fauna at this time.

Effect of Surface Waves on Submarine Pipeline during Lying

Submarine pipelines are essentially used for the transmission of gas and oil across oceans between countries or for transport between shore and offshore installations. The pipeline applications were studied to be installed in deep water, which exposed to different loads such as currents and waves in various directions, barge movements, seafloor interaction, etc. This paper developed a dynamic analysis of the J-lay suspended submarine pipeline during laying taking into account the effect of water depth, the direction of the wave heading, and sea state without vessel movement. The finite element program ANSYS R17.2 is used for modeling and analysis of the pipelines. The random sea state is modeled using the JONSWAP spectrum. It was found that the effect of the direction of wave heading on the bending moment from dynamic analysis of pipeline is obvious in a depth of (2 m) below water surface, and then gradually decreases until it disappears in depth of (100 m). Whereas the effect of wave height is obvious in a depth of (2 m) and then gradually decreases until it disappears in depth of (120 m).

Diffraction and scattering at the seafloor

In bottom-interacting ocean acoustics discrete, deterministic arrivals with significant energy are returned from the seafloor at angles that are not predicted by Snell’s law. This observation has led to some confusion between the terms “diffraction” and “scattering” as used in ocean acoustics. The common dictionary definitions of diffraction, as the process by which a beam is spread out or bent after passing the edge of an obstacle, and of scattering, as deflecting waves in a more or less random fashion, do not describe the observed seafloor interaction where a pulse of energy is returned deterministically, not randomly, from the seafloor in a non-Snell’s law direction. In ocean acoustics and marine seismology, it is rare to consider “bending diffraction” but it is quite common to see “scattering diffraction,” for example, as diffraction hyperbolas on bottom profiling and seismic records. For impulsive sources, in addition to specular reflections and random scattering from the seafloor, a significant aspect of bottom-interacting acoustics in deep water is diffraction of energy from discrete seafloor locations along repeatable paths in three-dimensions. [Work supported by ONR.]

From catastrophic collapse to multi-phase deposition: Flow transformation, seafloor interaction and triggered eruption following a volcanic-island landslide

The current understanding of tsunamis generated by volcanic-island landslides is reliant on numerical models benchmarked against reconstructions of past events. As the largest historical event with timed tsunami observations, the 1888 sector collapse of Ritter Island, Papua New Guinea provides an outstanding opportunity to better understand the linked process of landslide emplacement and tsunami generation. Here, we use a combination of geophysical imaging, bathymetric mapping, seafloor observations and sampling to demonstrate that the Ritter landslide deposits are spatially and stratigraphically heterogeneous, reflecting a complex evolution of mass-flow processes. The primary landslide mass was dominated by well-bedded scoriaceous deposits, which rapidly disintegrated to form an erosive volcaniclastic flow that incised the substrate over much of its pathway. The major proportion of this initial flow is inferred to have been deposited up to 80 km from Ritter. The initial flow was followed by secondary failure of seafloor sediment, over 40 km from Ritter. The most distal part of the 1888 deposit has parallel internal boundaries, suggesting that multiple discrete units were deposited by a series of mass-flow processes initiated by the primary collapse. The last of these flows was derived from a submarine eruption triggered by the collapse. This syn-collapse eruption deposit is compositionally distinct from pre- and post-collapse eruptive products, suggesting that the collapse immediately destabilised the underlying magma reservoir. Subsequent eruptions have been fed by a modified plumbing system, constructing a submarine volcanic cone within the collapse scar through at least six post-collapse eruptions. Our results show that the initial tsunami-generating landslide at Ritter generated a stratigraphically complex set of deposits with a total volume that is several times larger than the initial failure. Given the potential for such complexity, there is no simple relationship between the volume of the tsunamigenic phase of a volcanic-island landslide and the final deposit volume, and deposit area or run-out cannot be used to infer primary landslide magnitude. The tsunamigenic potential of prehistoric sector-collapse deposits cannot, therefore, be assessed simply from surface mapping, but requires internal geophysical imaging and direct sampling to reconstruct the event.

Seismic receiver coupling to the seafloor

ABSTRACTThe presence of geophysical receivers on the seafloor changes the local wave field due to the receiver seafloor interaction. The resulting PP‐ and PS‐wave distortion of the wave field is often referred to as receiver coupling to the seafloor and can make data processing challenging and sometimes impossible. This paper provides an overview of the mathematical approaches to describe receiver coupling, how to estimate receiver coupling and what the difficulties and possible solutions are. The first section shows how the mathematical approach developed from a simple model considering only the vertical receiver component to include all three receiver components and their interactions with the seafloor. In the second section, I show how receiver coupling can be measured and how it can be improved using mathematical and data‐driven approaches.

Evaluating off-bottom sweeps of a U.S. West Coast groundfish bottom trawl: Effects on catch efficiency and seafloor interactions

In the U.S. West Coast groundfish bottom trawl fishery, lengthy sweeps (>85 m) that maintain seafloor contact are traditionally used. While these sweeps are effective at herding groundfishes, their bottom tending characteristics increase the potential to cause seafloor disturbances, and injury and unobserved mortality to benthic organisms. In this study, we examined if changing from conventional to modified sweeps (with sections elevated 6.5 cm off bottom) would affect catch efficiency of target groundfishes and seafloor interactions. We used a DIDSON imaging sonar to observe how each sweep configuration interacted with the seafloor. An altimeter was periodically placed on the modified sweep to measure height off bottom. Results detected no significant catch efficiency effect of changing from conventional to modified sweeps. The DIDSON and altimeter data showed the modified sweeps exhibit elevated sections where infaunal and lower-profile epifaunal organisms can pass under without disturbance. Results demonstrate that seafloor interactions can be substantially reduced using elevated sweeps in this fishery without impacting catch efficiency. Further, findings from this research could be potentially applicable to other fisheries nationally and internationally.

A practical approach towards a timely seafloor interaction database.

A seafloor database of physics-based parameters may be considered an ideal resource. However, geoacoustic inversion to a model accurately describing the sediment structure, sub-bottom inhomogeneities, interface roughness, and anisotropy is still a very challenging research topic. While such geoacoustic inversion challenges are resolved, a more practical approach toward the generation of a timely seafloor interaction database is sorely needed. This practical approach must offer the flexibility to accommodate any physics-based and empirical model, direct measurement, and geoacoustic inversion from all research and operational sonars. It must also offer consistency with other seafloor-related databases, sufficiency to be useful to any sonar model, and completeness to model sonar systems across a wide frequency range. Empirical seafloor scattering models have a limited number of parameters to fit all measurements across grazing angle and frequency, while the most complete physics-based models have too many parameters for high-confidence geoacoustic inversion. Therefore, a more practical worldwide seafloor database must also offer enough flexibility to accurately reconstruct the seafloor reflection and scattering functions with a minimum number of parameters. Finally, this approach must quickly arrive at an unambiguous unique solution. Such an approach will be described and demonstrated. [This effort was supported by SPAWAR.]

Perth‐Bermuda (1960) revisited global scale acoustic propagation.

A recent paper in Geophysical Research Letter [Dushaw, Geophys. Res. Lett. 35, L08601 (2008)] commented on the Perth‐Bermuda Sound Propagation Experiment (1960): An Adiabatic Mode Interpretation JASA 95, November 1991, by Kevin D. Heaney, William S. Kuperman, and B. E. McDonald. Dushaw raises questions about the validity of the approach and conclusions of Heaney et al. In particular, the validity of the adiabatic mode assumption for long‐range propagation in the presence of internal wave scattering and seafloor interaction is challenged. Dushaw finds no other explanation for the observed double pulse arrival in the original 1960 experiment. The approach and conclusions of Heaney et al. will be presented. The range of validity of the adiabatic mode approximation in global scale acoustics will be addressed. The original computations of Heaney et al. were for a single mode at 10 Hz due to the computational power available at the time. Broadband results (10–50 Hz) using multiple modes summed coherently will be presented.

Single parameter description of seafloors for shallow oceans

For a shallow ocean, sound transmission beyond short range is dominated by seafloor interactions at small grazing angles, for which the loss in dB on each bottom reflection may be approximated by a function F dB/radian which is linear with grazing angle. Acoustic inversion techniques have been shown to be able to obtain the value of F for a particular frequency. The suitability of this single parameter F as a seafloor descriptor has now been studied for an extensive range of seafloor types. It is shown that the phase-incoherent transmission loss (TL) obtained using the parameter F is not greatly different to TL predicted using complete knowledge of a particular seafloor, for a shallow ocean. Further, if the phase angle for a seafloor reflection is linked to the parameter F via a simple approximation, the phase-coherent properties of the shallow water interference field may be estimated to an accuracy which is surprising. This paper reviews the relevant theory and presents comparisons between TL predicted using full geoacoustic parameters versus TL based on the single parameter, for both uniform half-space and layered seafloor types.

Basin Acoustic Seamount Scattering Experiment (BASSEX 04) II. Downslope propagation

During the Basin Acoustic Seamount Scattering EXperiment (BASSEX04), the Five-Octave Research Array (FORA) was deployed near the North Pacific Acoustics Laboratory (NPAL) bottom mounted tomography source just off the coast of Kauai. Receptions from the Kauai source were taken at ranges from 2 km out to 500 km. The data was taken to examine the affects of seafloor interaction on downslope propagation (propagation down both steep and shallow slopes), shallow water range-dependent 3-D propagation and conversion from shallow water propagation to deep water propagation. Conventional and adaptive beamforming results will be presented as well as comparisons between measured responses and PE modeled responses. Geo-acoustic inversions will be performed with the broadband source in end-fire and broadside orientations of the array. [Work supported by ONR Ocean Acoustics.]

Mid-infrared emission prior to strong earthquakes analyzed by remote sensing data

Solid Earth–atmosphere interactions and possibly solid Earth–seafloor interactions prior to major earthquakes are the subject of this study. Using data from Moderate Resolution Imaging Spectroradiometer (MODIS) onboard National Aeronautical Space Agency (NASA) Terra and Aqua satellites, we have begun to analyze infrared (IR) emissivity, land surface temperature (LST) and sea surface temperature (SST) data. Specifically, we look for correlations between solid Earth processes and atmosphere/ocean dynamics prior to strong earthquakes, selecting examples from 2001 and 2000. We find evidence for such correlations, specifically for a thermal anomaly LST pattern that is apparently related to pre-seismic activity. A new mechanism has recently been proposed based on positive hole-type charge carriers in rocks, which become activated when rocks are subjected to high levels of stress prior [J. Geodyn. 33 (2002) 545–572]. The appearance of these electronic charge carriers within a finite rock volume leads to enhanced IR emission from the rock surface in the 8–12 μm region.

The acoustic transient recording buoy (ATRB): system description and initial measurement results

An acoustic transient recording buoy (ATRB) developed to provide improved dynamic range and recording capacity in a reconfigurable manner is described. This digital system can acquire and record up to 16 h of broadband wide dynamic range ( approximately=80 dB) acoustic data from eight hydrophones. A unique feature is the use of two inexpensive video cassette recorders to obtain up to 10 Gb of data storage capacity. The system is self-contained and capable of unattended bottom-moored operation. An experiment designed and conducted using a single ship and this system to obtain simultaneous measurements of sea surface forward scatter, propagation loss, and sea floor interaction is reported. Data obtained demonstrate the utility of this system for ocean acoustic experiments. Explosive charge source levels using direct path measurements agreed with previous measurements. Surface reflected data exhibited a frequency dependence attributed to sea surface swell and roughness.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>