Velocity field in the McMurdo shear zone from annual ground penetrating radar imaging and crevasse matching

The McMurdo shear zone (MSZ) is strip of heavily crevassed ice oriented in the south-north direction and moving northward. Previous airborne surveys revealed a chaotic crevasse structure superimposed on a set of expected crevasse orientations at 45 degrees to the south-north flow (due to shear stress mechanisms). The dynamics that produced this chaotic structure are poorly understood. Our purpose is to present our field methodology and provide field data that will enable validation of models of the MSZ evolution, and here, we present a method for deriving a local velocity field from ground penetrating radar (GPR) data towards that end. Maps of near-surface crevasses were derived from two annual GPR surveys of a 28 km² region of the MSZ using Eulerian sampling. Our robot-towed and GPS navigated GPR enabled a dense survey grid, with transects of the shear zone at 50 m spacing. Each survey comprised multiple crossings of long (> 1 km) crevasses that appear in echelon on the western and eastern boundaries of the shear zone, as well as two or more crossings of shorter crevasses in the more chaotic zone between the western and eastern boundaries. From these maps, we derived a local velocity field based on the year-to-year movement of the same crevasses. Our velocity field varies significantly from fields previously established using remote sensing and provides more detail than one concurrently derived from a 29-station GPS network. Rather than a simple velocity gradient expected for crevasses oriented approximately 45 degrees to flow direction, we find constant velocity contours oriented diagonally across the shear zone with a wavy fine structure. Although our survey is based on near-surface crevasses, similar crevassing found in marine ice at 160 m depth leads us to conclude that this surface velocity field may hold through the body of meteoric and marine ice. Our success with robot-towed GPR with GPS navigation suggests we may greatly increase our survey areas.

Bayesian inference of ion velocity distribution function from laser-induced fluorescence spectra

The velocity distribution function is a statistical description that connects particle kinetics and macroscopic parameters in many-body systems. Laser-induced fluorescence (LIF) spectroscopy is utilized to measure the local velocity distribution function in spatially inhomogeneous plasmas. However, the analytic form of such a function for the system of interest is not always clear under the intricate factors in non-equilibrium states. Here, we propose a novel approach to select the valid form of the velocity distribution function based on Bayesian statistics. We formulate the Bayesian inference of ion velocity distribution function and apply it to LIF spectra locally observed at several positions in a linear magnetized plasma. We demonstrate evaluating the spatial inhomogeneity by verifying each analytic form of the local velocity distribution function. Our approach is widely applicable to experimentally establish the velocity distribution function in plasmas and fluids, including gases and liquids.

Scale-invariant spins and tumbles in turbulence

A Lagrangian perspective has yielded many new insights in our quest to reveal the intricacies of turbulent flows. Much of this progress has been possible by following the trajectories of idealised, inertialess objects (tracers) traversing through the flow. Their spins and tumbles provide a glimpse into the underlying local velocity gradients of the turbulent field. While it is known that the spinning and tumbling rates of anisotropic particles are modified in turbulence – compared with those in a random flow field – a quantitative explanation for this has remained elusive. Now, Pujara et al. (J. Fluid Mech., vol. 922, 2021, R6) have made an attempt to predict the split between spinning and tumbling rates by accessing the particle's alignment with the local vorticity. Their analysis of filtered turbulent fields reveals a Lagrangian scale invariance, whereby key quantities relating to the particle's rotational statistics are preserved from the dissipative to the integral scale.

Micro-PIV Measurements of Multiphase Flow in Deforming Porous Media Subject to Mineral Dissolution

Mineral dissolution is studied in novel calcite-based porous micromodels under single- and multiphase conditions, with a focus on the interactions of mineral dissolution with pore flow. Microscopic particle image velocimetry (PIV) was utilized to simultaneously characterize the local velocity field and the instantaneous shapes of the dissolving grains. The preliminary results provide a unique view of the coupled dynamics between pore flow and mineral dissolution.

Load Coefficients and Dimensions of Raschel Knitted Netting Materials in Fish Farms

New types of fish farms are often larger and structurally more complex than conventional fish farming structures, and associated challenges concerning safety and costs increase correspondingly. Thus, increased precision in structural design is required, with estimation of hydrodynamic loads on nets as an important topic. Today, both load coefficients for nets and measured netting dimensions are given with relatively high uncertainties. New knowledge for netting materials with high solidities as well as scaled netting commonly applied in model tests are included in the presented study. Results from towing tests and the development of a new mathematical expression for local drag coefficients (for netting twines) indicate that drag coefficients are not only dependent on solidity and Reynolds number, but may also be affected by the velocity reduction and the local velocity at the twines.

Micro-seismicity, seismic-wave velocity model and earthquake clustering in the Akarnanian region (western Greece)

<p>Characterized for the first time in 2013, the Island Akarnanian Block (IAB) is a micro-plate located in the western Greece. This micro-plate accommodates the deformation in between larger scale tectonic structures as the Gulf of Corinth (South-East), the Hellenic subduction (South) and the Apulian Collison (West).</p><p><span>W</span><span>e </span><span>start</span><span>ed a micro-seismic </span><span>survey</span><span> (MADAM) at the end of 2015 with a dense seismological network </span><span>over the area, between the Gulf of Patras and the Gulf of Amvrakikos. </span><span>In order t</span><span>o obtain precise locations of the recorded events, we better constrained the local velocity model. In fact, </span><span>s</span><span>everal velocity models </span><span>(local or regional) </span><span>have been proposed for this area. </span><span>H</span><span>owever,</span><span> the velocity model generally used by the scientific community remains the Hasslinger 98 velocity model. This model, nevertheless, raises some questions about its physical meaning, mainly due to a low velocity layer bet</span><span>w</span><span>ee</span><span>n </span><span>4 and 7 km-depth. </span></p><p><span>Thanks to our seismological network and permanent networks of the Corinth Rift Laboratory and the Hellenic Unified Seismic Network, w</span><span>e colle</span><span>c</span><span>ted and anal</span><span>y</span><span>s</span><span>ed</span><span> a huge quantity of data </span><span>a</span><span>c</span><span>quired </span><span>between </span><span>O</span><span>ctober 2015 and </span><span>D</span><span>ecember 2017. </span><span>Those analyses of </span><span>more than </span><span>10,000</span><span> events allowed us to </span><span>develop </span><span>a new </span><span>and robust </span><span>local velocity model, </span><span>wh</span><span>ich</span><span> is consi</span><span>s</span><span>tent with the seismic data and </span><span>the </span><span>geophysical obse</span><span>r</span><span>vations</span><span>.</span></p><p>The observed seismic activity is characterized by the presence of numerous clusters. The clusters are analysed in detail by relative relocations in order to appraise their physical processes and their possible implications in the fault activity to finally have a better understanding of the deformation mode(s) of the IAB micro-plate.</p>