Search

The classical Backus–Gilbert method seeks localized Earth-structure averages at the shortest length scales possible, given a data set, data errors, and a threshold for acceptable model errors. The resolving length at a point is the width of the local averaging kernel, and the optimal averaging kernel is the narrowest one such that the model error is below a specified level. This approach is well suited for seismic tomography, which maps 3-D Earth structure using large sets of seismic measurements. The continual measurement-error decreases and data-redundancy increases have reduced the impact of random errors on tomographic models. Systematic errors, however, are resistant to data redundancy and their effect on the model is difficult to predict. Here, we develop a method for finding the optimal resolving length at every point, implementing it for surface-wave tomography. As in the Backus–Gilbert method, every solution at a point results from an entire-system inversion, and the model error is reduced by increasing the model-parameter averaging. The key advantage of our method stems from its direct, empirical evaluation of the posterior model error at a point. We first measure inter-station phase velocities at simultaneously recording station pairs and compute phase-velocity maps at densely, logarithmically spaced periods. Numerous versions of the maps with varying smoothness are then computed, ranging from very rough to very smooth. Phase-velocity curves extracted from the maps at every point can be inverted for shear-velocity (V S) profiles. As we show, errors in these phase-velocity curves increase nearly monotonically with the map roughness. We evaluate the error by isolating the roughness of the phase-velocity curve that cannot be explained by any Earth structure and determine the optimal resolving length at a point such that the error of the local phase-velocity curve is below a threshold. A 3-D V S model is then computed by the inversion of the composite phase-velocity maps with an optimal resolution at every point. The estimated optimal resolution shows smooth lateral variations, confirming the robustness of the procedure. Importantly, the optimal resolving length does not scale with the density of the data coverage: some of the best-sampled locations display relatively low lateral resolution, probably due to systematic errors in the data. We apply the method to image the lithosphere and underlying mantle beneath Ireland and Britain. Our very large data set was created using new data from Ireland Array, the Irish National Seismic Network, the UK Seismograph Network and other deployments. A total of 11 238 inter-station dispersion curves, spanning a very broad total period range (4–500 s), yield unprecedented data coverage of the area and provide fine regional resolution from the crust to the deep asthenosphere. The lateral resolution of the 3-D model is computed explicitly and varies from 39 km in central Ireland to over 800 km at the edges of the area, where the data coverage declines. Our tomography reveals pronounced, previously unknown variations in the lithospheric thickness beneath Ireland and Britain, with implications for their Caledonian assembly and for the mechanisms of the British Tertiary Igneous Province magmatism. [ABSTRACT FROM AUTHOR]

There is little disagreement that entering floodwater is risky, whether in a vehicle or on foot. There is usually little or no visibility of what is under the surface and even shallow water with moderate flow can make vehicles unstable or sweep people off their feet. In addition, floodwater will often contain contaminants and debris. Therefore, the safest course of action is to avoid entering floodwater. Indeed, the most pervasive risk communication message is 'If it's flooded, forget it'. This clear, unambiguous message is good at getting people's attention, but it is unclear whether it actually changes behaviour. Research with Australian public, floodplain management professionals, State Emergency Services (SES) personnel, and other emergency management experts has identified a number of challenges to floodwater safety and risk communications. Using a combination of research evidence and expert opinion, this paper discusses four pressing challenges and highlights some of the approaches being taken to help address them. [ABSTRACT FROM AUTHOR]

International Fusion Energy Organization (ITER) is one of the most ambitious energy projects in the world today. The poloidal field (PF) coil is cooled down to the required operating temperature (4.2 K) by a forced flow of supercritical helium, which is introduced through the helium inlet. The mechanical reliability requirement on the irregular structure of helium inlet due to the introduction of orifice make the helium inlet one of the critical components of the PF coil cooling system. This paper focuses on the fatigue tests on ITER PF6 coil full-size helium inlet samples to validate whether the fatigue performance of the conduit with the welded inlet can withstand the lifetime of PF6 coil close to the working condition. According to the ITER requirement, the conductor jacket shall withstand a cyclic longitudinal strain (0.95 to 9.5) × 10-4 during 600 000 cycles at 77K or (1.9-19) × 10-4 during 30 000 cycles at 77 K, and the equivalent nominal tensile load range we calculated is about (33.5-335) kN. Two samples were designed and prepared to assess the structural design and weld quality. The fatigue test of the sample 1 was carried out with a load range of (33.5-335) kN at 4 Hz and 77 K, and it failed after 593 000 cycles. The scanning electron microscope images and finite element analysis showed that crack initiation area of the fractured sample 1 was at the bottom of the hole. The main reasons were that there were some stress concentrations inside the hole when the tensile load was provided, and the residual burrs assisted the crack initiation. Finally, the modified sample 2 with the completely removed burrs succeed after 600 000 cycles with a nominal load range of (33.5-335) kN. Therefore, it is concluded that the present optimized ITER PF6 helium inlet sample has good fatigue performance and meets the requirements of ITER, which can be utilized for practical applications. [ABSTRACT FROM AUTHOR]