Souppez, Jean-Baptiste R. G. (Warsash School of Maritime Science and Engineering, Solent University) | Arredondo-Galeana, Abel (University of Edinburgh) | Viola, Ignazio Maria (University of Edinburgh)
While sailing downwind has benefited from millennia of evolution, the very first instance of a highly cambered and dedicated downwind sail, termed spinnaker, did not occur until 1865, as reported by King (1981), and was not popularized until the 1970s and 1980s; primarily thanks to the development of symmetric spinnakers for the America's Cup. Asymmetric spinnakers were then introduced in the 1980s in the 18ft fleet in Sydney, before being popularised on offshore racing yachts in the 1990s. These new sails were promptly adopted in many significant sailing events; firstly in offshore races such as the Vendée Globe and the Whitbread 60, and later in the America's Cup (Fallow, 1996; Richards et al, 2001; Viola & Flay, 2009). The significant advances made in terms of spinnaker design and analysis during this particular decade can be related to the greater part that downwind legs took in the 1995 America's Cup (Fallow, 1996), thus motivating further research and development. The 1990s also coincide with a fast increase in accessible computational power, allowing advanced numerical methods to be used in sail design (Hedges, 1993; Hedges et al, 1996), particularly for downwind sails.
da Costa Filho, Carlos Alberto (University of Edinburgh) | Tant, Katherine (University of Strathclyde) | Curtis, Andrew (University of Edinburgh) | Mulholland, Anthony (University of Strathclyde) | Moran, Carmel M. (University of Edinburgh)
It has been employed to diminish the effects of multiples in seismic data. Several such methods rely on an absolute scaling of the data; this is usually considered to be known in synthetic experiments, or is estimated using heuristic methods in real data. Here, we show using real ultrasonic laboratory data that the most common of these methods may be ill suited to the task, and that reliable ways to estimate scaling remains unavailable. Marchenko methods which rely on adaptive subtraction may therefore be more appropriate. We present two adaptive Marchenko methods: one is an extension of a current adaptive method, and the other is an adaptive implementation of a nonadaptive method. Our results show that Marchenko methods improve imaging compared to reverse-time migration, but less so than expected. This reveals that some Marchenko assumptions were violated in our experiment and likely are also in seismic data, showing that laboratory experiments contribute critical information to the development and testing of Marchenko-based methods. INTRODUCTION Seismic processors have long been concerned with the presence of multiples (waves that reflect multiple times in the subsurface before being recorded at the receiver array) in seismic data.
SUMMARY We apply an acoustic multiple attenuation method to a marine OBC dataset from the Volve field in the North Sea. The method consists of estimating surface-to-subsurface virtual responses using the Marchenko method, and using interferometric integrals to compute prestack multiple estimates from these virtual responses. The prestack multiples are adaptively subtracted from the original reflection data, and then images are created. We provide detailed comparisons between the image obtained from the multiples-attenuated image, the migrated original dataset, and an image obtained by migrating only the multiples. We show that several structures are attenuated in our new images, evidencing that our multiple attenuation method is effective in field data.
We present an inversion scheme for surface waves that jointly inverts point-receiver data for both a densely reconstructed wave field and an estimate of seismic velocities. The formulation is posed as a partial differential equation (PDE) constrained inverse problem. We use the dispersive Helmholtz equation to approximate the far-field behavior of surface waves as two dimensional wave propagation through a phase velocity map. The Helmholtz equation does not accurately describe surface wave propagation near sources so we mask source regions from the PDE constraint. This leads to excellent wave field reconstruction and medium velocity estimates. The new theory and algorithm are supported by a numerical example with simulated elastic data. The phase velocity model obtained is verified by frequency-wavenumber dispersion analysis.
Presentation Date: Tuesday, October 16, 2018
Start Time: 8:30:00 AM
Location: 204A (Anaheim Convention Center)
Presentation Type: Oral
The Marchenko method allows seismic images to be constructed with reduced artifact contamination due to internal multiples. In this article we apply the method to a synthetic subsurface model containing vertical and horizontal features. We show that standard Marchenko methods fail to reconstruct wavefield components caused by vertical interfaces. We present a new way to incorporate VSP data into the Marchenko method that produces accurate subsurface images, even of vertical interfaces.
Presentation Date: Wednesday, October 17, 2018
Start Time: 1:50:00 PM
Location: 211A (Anaheim Convention Center)
Presentation Type: Oral
Bond, Alexander (Quintessa Ltd.) | Chittenden, Neil (Quintessa Ltd.) | Fedors, Randall (United States Nuclear Regulatory Commission) | Lang, Philipp (Imperial College London) | McDermott, Christopher (University of Edinburgh) | Neretnieks, Ivars (KTH) | Pan, Peng-Zhi (Chinese Academy of Sciences) | Sembera, Jan (Technical University of Liberec) | Brusky, I. (Technical University of Liberec) | Watanabe, Norihiro (Helmholtz Centre for Environmental Research - UFZ) | Lu, R. (Helmholtz Centre for Environmental Research - UFZ) | Yasuhara, H. (Ehime University)
The evolution of fracture permeability can have important impacts for the resaturation of the facility and long-term transport of any radionuclides that escape the immediate area of disposal. Whereas such systems have been looked at both within the DEvelopment of COupled models and their VALidation against EXperiments (DECOVALEX) project and elsewhere, attempts to model a fully-coupled THMC system on a single fracture have been limited. Examples where THMC analysis in fracture rock has been addressed include Yasuhara and Elsworth (2006), Taron et al. (2009) and Zhang (2012), but with the exception of Yasuhara and Elsworth (2006), the emphasis has been largely on theoretical studies, with no direct comparison against well-constrained small-scale experimental data. There is however a large body of knowledge concerning THM behavior with non-reactive transport in fractures (e.g. Berkowitz, 2002; Neuman, 2005) and a wide range of work examining chemical interactions in fractured systems (e.g. Watson et al., 2016) but modelling efforts incorporating THM and C processes for single fractures are rare. The objective of this Task (Task C1: one of 5 Tasks in the previous phase of DECOVALEX; please see www.decovalex.org for more information on DECOVALEX including numerous examples of this type of collaborative research) is to use the experimental data of Yasuhara et al., 2006 and 2011 to model evolving single fractures incorporating coupled THMC effects for novaculite (quartzite) and granite fractures. This work is not focused on blind prediction, rather it is concerned with building experience and understanding of the physical processes in operation in single fractures on the basis of experimental data and to understand how to represent such processes through numerical and/or semi-analytical models. The Task has had significant technical contributions from six teams (abbreviations, where used, are shown in bold), as well as input from Neretnieks, 2014 and Sandia National Laboratory: - BGR/UFZ - Germany-Federal Institute for Geosciences and Natural Resources and the Helmholtz Centre for Environmental Research.
Results obtained from numerical modelling of wave and tidal currents and the resulting turbulence parameters at tidal energy sites in the Fall of Warness, which is a region consented by the Crown Estate for deployment of tidal stream devices in the Orkney Islands, Scotland, are reported in this paper. The software suite MIKE 21/3, which is a coupled wave-tidal flow model, has been used for this purpose. The coupled wave-current model is driven by boundary inputs of spatially and temporally varying wind, wave and tidal elevations. Turbulence closure is achieved using a two-equation (k-ε) turbulence model. The coupled model has been calibrated and validated with field measurements of waves and tidal currents acquired by Acoustic Doppler and Current Profilers (ADCPs) deployed in the Fall of Warness. The results indicate that the coupled model works well and the predicted wave-current parameters provide very good match to site measurements at different depths of the water column. The model outputs such as significant wave height, peak wave period, mean wave direction, tidal current speed and its direction, Turbulent Kinetic Energy (TKE) and its dissipation rate and Eddy viscosities are presented and discussed. These parameters will find its use in the design of tidal turbine components and its supporting structures.
Hydrodynamic loads on both fixed and floating tidal stream turbine components, e.g., rotor, supporting structures and moorings etc, need to be carefully determined when the machines are designed to operate in conditions where both waves and tidal currents co-exists. Unsteady flow due to turbulence, wave-current interactions, and variation of flow characteristics with depth can cause unsteady blade loading, resulting in fatigue. High-end computational modelling tools such as CFD software packages may have the ability to simulate wave-current interactions, however, its application to realistic site conditions with complex bathymetry and large computational domains covering several square kilometres, may not be feasible due to computational expenses and it may not even fully reproduce wave-current scenarios, especially when the interaction of directional waves, currents, and turbulences are to be modelled. It is well established that wave loads combined with different flow turbulent intensities, originating from wave-current-turbulent interactions, are the main contributors to fatigue failures of turbine blades. Wave-current induced turbulence affects tidal turbine power production in several ways, specifically through power performance effects, impacts on turbine loads, fatigue and wake effects, and noise propagation; it is therefore important to develop an enhanced understanding of wave-current-turbulence interactions in tidal energy research.
Hodge, Caitlin Worden (Industrial Doctoral Centre for Offshore Renewable Energy (IDCORE) & Zyba) | Bateman, Will (Industrial Doctoral Centre for Offshore Renewable Energy (IDCORE) & Zyba) | Yuan, Zhiming (University of Exeter) | Thies, Philipp R. (University of Strathclyde) | Bruce, Tom (University of Edinburgh)
The mechanical motion of a wave energy converter (WEC) is converted by the power take-off (PTO) system into electricity, but these two systems are not independent, as they have been treated in WEC modelling. Treating them as such leads to inaccuracies in prediction of power output and reliability, and can erode confidence in numerical modelling tools. This paper presents a methodology for the two-way coupling of high fidelity modelling of WEC hydrodynamics with a more accurate representation of the PTO and investigates the impact of using simplified PTO models. Simplified models represent the full PTO with a single parameter which is in itself difficult to choose and may require a number of iterations. These different methods were used to assess the behaviour of the CCell WEC in a regular wave, with the calculations for mean power varying considerably in different wave conditions and the range of motion consistently under predicted by the simpler models. The coupled model increased the computational requirement for the simulation, however it provided the developer a better understanding of the impact on and utilisation of different hydraulic components.
WECs are designed to convert the energy from waves into a mechanical motion which is then converted into electrical power through the power take-off (PTO) system. Oscillating wave surge converters, such as the CCell device, Fig. 1, have generally evolved as buoyant bottom-hinged flap WECs, which pitch back and forth from sea to shore under the influence of the horizontal motion of waves (Cameron, L, 2010). This pitching motion is transformed into useful energy usually through a hydraulic piston, which draws on the robustness and high power density that hydraulic circuits offer. Similar systems have also commonly been used in heaving buoys (Cargo, C, 2012).
Numerical modelling has become a valuable toolbox for WEC developers as it allows rapid modifications to a WEC design, without the additional manufacture and testing costs, or scaling issues. It can build up a picture of the Mean Annual Energy Production (MAEP) and the load estimates on the device which can aid design decisions and inform the required O&M procedures. However, numerical simulations can be slow to compute without adequate computer power and some simplifications must be introduced for efficiency, especially regarding the modelling of the power take-off system and/or the hydrodynamics.
Ziolkowski, Anton (University of Edinburgh)
Using the cube-root scaling law for explosive sources, I estimate the source time functions and yields of explosions directly from seismograms for two explosions in the same location. The path effect between source and receiver is eliminated by finding a ratio filter that shapes the seismogram of the smaller event to the seismogram of the larger. If the noise is small, the convolution of the filter with the source time function of the smaller event yields the source time function of the larger event. The two source time functions are also related by the well-known scaling law in which the injected volume is proportional to the yield and the time constant is proportional to the cube-root of the yield. These two independent equations are solved for the two source time functions, and the seismograms are then deconvolved to recover two estimates of the earth impulse response. The method is applied to seismograms from the North Korean (NK) underground nuclear tests, using measurements of Nevada Test Site tests for calibration, giving estimates of the source time functions and yields for all five events. The method can readily be applied to land seismic exploration with dynamite, for which the data processing is easier, because the locations, origin times and yields of the explosions are known.
Presentation Date: Tuesday, September 26, 2017
Start Time: 3:30 PM
Presentation Type: ORAL
We pose full wave field inversion of ambient seismic noise as a partial differential equation (PDE) constrained inverse problem resulting in a joint estimation of both the wave field (noise free) and the velocity parameters. Because the ambient seismic field is usually dominated by fundamental mode surface waves, we elect to impose a dispersive Helmholtz equation as a PDE constraint and attempt to retrieve the surface wave dispersion while estimating the wave field that best fits the data. The character of the ambient seismic field is irrelevant, as long as the recordings are non-zero and we can assume the wave field sources to be negligible inside the domain of interest. The boundary conditions of the wave field are explicitly omitted from the PDE constraint and recovered during the inversion. We support the theory with two numerical examples.
Presentation Date: Thursday, September 28, 2017
Start Time: 9:20 AM
Presentation Type: ORAL