AN INVESTIGATION OF SEISMIC ATTENUATION IN MARINE SEDIMENTS

There have been relatively few investigations into the attenuation properties of unconsolidated sediments using marine surface seismic data. Several methods of measuring attenuation were assessed for reliability in a noise-free case and with the addition of noise using a set of synthetically absorbed and dispersed wavelets. Wavelet modelling proved to be superior to the other techniques, followed by spectrum modelling and the spectral ratios method. Complex trace analysis using the analytical signal proved to be unreliable for non-sinusoidal wavelets, whilst the risetime method was found to be very susceptible to noise for practical purposes. Numerical modelling was carried out to assess the spectral effects of layering on a propagating pulse. The thin layer / peg-leg phenomenon has varying filtering effects on the propagating pulse. In particular, layers which are less than the "tuning thickness" of the propagating pulse have a low-pass effect. The quality factor, Q, was measured in two case studies. In the first, the mean Q was determined from wavelet and spectrum modelling and found to be 60 for fine sands and 47 for coarse sands in the 1 kHz to 3 kHz frequency band. In the second, Q was determined as 59 for poorly sorted sandy diamicts in the 100 Hz to 240 Hz frequency band. The close fit between synthesised spectra and wavelets and observed data showed that a constant- Q mechanism would account for the spectral changes between the seabed and the deeper target reflection events in the two case studies. The spectra of the target reflection events in both case studies were lacking in low frequencies which is likely to be due to low-pass filtering from composite reflection events due to thin bed layering. For practical purposes, the determination of Q from a mean normalised seismic trace yielded the same result as measuring a mean Q from individual traces. In a third case study, the seabed multiple was compared to the seabed reflection using wavelet and spectrum modelling. A lack of low frequencies in the seabed multiple showed that the seabed can act as a low-pass filter to an incident pulse. As the numerical methods rely on the seabed as having a white reflection and transmission response, the low-pass effect will result in erroneous estimates of the quality factor, Q

Quantifying Eulerian Eddy Leakiness in an Idealized Model

An idealized eddy‐resolving ocean basin, closely resembling the North Pacific Ocean, is simulated using MITgcm. We identify rotationally coherent Lagrangian vortices (RCLVs) and sea surface height (SSH) eddies based on the Lagrangian and Eulerian framework, respectively. General statistical results show that RCLVs have a much smaller coherent core than SSH eddies with the ratio of radius is about 0.5. RCLVs are often enclosed by SSH anomaly contours, but SSH eddy identification method fails to detect more than half of RCLVs. Based on their locations, two types of eddies are classified into three categories: overlapping RCLVs and SSH eddies, nonoverlapping SSH eddies, and nonoverlapping RCLVs. Using Lagrangian particles, we examine the processes of leakage and intrusion around SSH eddies. For overlapping SSH eddies, over the lifetime, the material coherent core only accounts for about 25% and about 50% of initial water leak from eddy interior. The remaining 25% of water can still remain inside the boundary, but only in the form of filaments outside the coherent core. For nonoverlapping SSH eddies, more water leakage (about 60%) occurs at a faster rate. Guided by the number and radius of SSH eddies, fixed circles and moving circles are randomly selected to diagnose the material flux around these circles. We find that the leakage and intrusion trends of moving circles are quite similar to that of nonoverlapping SSH eddies, suggesting that the material coherence properties of nonoverlapping SSH eddies are not significantly different from random pieces of ocean with the same size

LES modelling of nitric oxide (NO) formation in a propane-air turbulent reacting flame

Large Eddy Simulation (LES) technique is applied to investigate the nitric oxide (NO) formation in the propane-air flame inside a cylindrical combustor. In LES a spatial filtering is applied to the governing equations to separate the flow field into large scale eddies and small scale eddies. The large scale eddies which carry most of the turbulent energy are resolved explicitly while the unresolved small scale eddies are modelled. A Smagorinsky model with model constant Cs = 0.1 as well as a dynamic model has been employed for modelling of the sub-grid scale eddies, while the nonpremixed combustion process is modelled through the conserved scalar approach with laminar flamelet model. In NO formation model, the extended Zeldovich (thermal) reaction mechanism is taken into account through a transport equation for NO mass fraction. The computational results are compared with those of the experimental results investigated by Nishida and Mukohara [1] in co-flowing turbulent flame

Dissipative inertial transport patterns near coherent Lagrangian eddies in the ocean

Recent developments in dynamical systems theory have revealed long-lived and coherent Lagrangian (i.e., material) eddies in incompressible, satellite-derived surface ocean velocity fields. Paradoxically, observed drifting buoys and floating matter tend to create dissipative-looking patterns near oceanic eddies, which appear to be inconsistent with the conservative fluid particle patterns created by coherent Lagrangian eddies. Here we show that inclusion of inertial effects (i.e., those produced by the buoyancy and size finiteness of an object) in a rotating two-dimensional incompressible flow context resolves this paradox. Specifically, we obtain that anticyclonic coherent Lagrangian eddies attract (repel) negatively (positively) buoyant finite-size particles, while cyclonic coherent Lagrangian eddies attract (repel) positively (negatively) buoyant finite-size particles. We show how these results explain dissipative-looking satellite-tracked surface drifter and subsurface float trajectories, as well as satellite-derived \emph{Sargassum} distributions.Comment: Submitted to \emph{Chaos} Focus Issue on Objective detection of Lagrangian Coherent Structures. Revised 23-Feb-1

Coherent water transport across the South Atlantic

The role of mesoscale eddies in transporting Agulhas leakage is investigated using a recent technique from nonlinear dynamical systems theory applied on geostrophic currents inferred from the over two-decade-long satellite altimetry record. Eddies are found to acquire material coherence away from the Agulhas retroflection, near the Walvis Ridge in the South Atlantic. Yearly, 1 to 4 coherent material eddies are detected with diameters ranging from 40 to 280 km. A total of 23 eddy cores of about 50 km in diameter and with at least 30\pct of their contents traceable into the Indian Ocean were found to travel across the subtropical gyre with minor filamentation. Only 1 eddy core was found to pour its contents on the North Brazil Current. While ability of eddies to carry Agulhas leakage northwestward across the South Atlantic is supported by our analysis, this is more restricted than suggested by earlier ring transport assessments.Comment: In pres

A visible record of eddies in the southern Mozambique Channel

The flows around Madagascar feed into the Agulhas Current, but there have been few hydrographic studies of the flow within the Mozambique Channel. Some cruise and altimetric data point to this being a region of high mesoscale activity, with eddies migrating through the area. Here we show how ocean colour data throw light on the behaviour of eddies in the southern Mozambique Channel

Polarization of superfluid turbulence

We show that normal fluid eddies in turbulent helium II polarize the tangle of quantized vortex lines present in the flow, thus inducing superfluid vorticity patterns similar to the driving normal fluid eddies. We also show that the polarization is effective over the entire inertial range. The results help explain the surprising analogies between classical and superfluid turbulence which have been observed recently.Comment: 3 figure