With the proliferation of HVAC armoured cables in offshore wind applications, the provision of methods for the calculation of both magnetic and electric field effects from HV cables is of high importance for consenting processes associated with offshore wind development and biological experiments. Despite electro-reception of marine animals being a known phenomenon, the electric fields present outside of HVAC cables, mostly induced as a result of the time-varying magnetic vector potential, have not been thoroughly analysed in literature. The available methods for the calculation of magnetic fields are often complex and require an expert user. In this work, this knowledge gap is addressed through the development of an analytical method, with some parameters derived from simple numerical simulations, that can be applied to both magnetic and electric fields and by provision of MATLAB code for calculation of both effects. The presented method includes the impact of metallic sheaths, cable twist, and magnetic armour. For the development of electric field expressions, the impact of the insulating layer at the cable outer boundary is considered. The presented results are within 3% of the numerical solution for the magnetic and electric field norm and within 5% for the individual components of the fields.

This paper presents a superposition-based thermal model that estimates the transient radial temperature distribution in high voltage direct current (HVDC) cable insulation from load current. The total temperature is decomposed into an undisturbed ground component from a one-dimensional diffusion model and a cableinduced perturbation solved by a 32-state radial resistor-capacitor (RC) network with temperature-dependent conductivity and fielddependent dielectric loss. Validated over 300 days against finite element analysis (FEA) (conductor root mean squared error (RMSE) $0. 2 6^{\circ} \mathrm{C}$) and continuous distributed temperature sensing (DTS) measurements (RMSE 0.33°C) from a 600 kV bipole submarine cable, the model is further compressed via Proper Orthogonal Decomposition (POD)- Discrete Empirical Interpolation Method (DEIM) to a 4-state system (reduced order model - ROM) that reproduces the insulation temperature drop to within $0. 1 3^{\circ} \mathrm{C}$ RMSE of 32-states (full order model - FOM), with the required number of modes independent of mesh resolution.

With the raised investment in interconnectors and offshore wind farms located further from the shore, there has been an increased need for the licensing and environmental assessment of high voltage direct current (HVDC) submarine cables. The motionally induced electric fields due to HVDC submarine cables have not been previously modeled in the literature. In this article, a methodology for modeling this phenomenon is outlined. Factors, such as geographical location of the cable system, electrical conductivities of the media, reference frames, and water velocity profile, are included in the analysis. The results are compared with an approximation adopted in other publications. The resulting electric fields and their spatial distributions are presented and discussed for different cable systems, cable orientations and reference frames. The dependency of the local electric fields on the geographical location is shown. The investigation demonstrates the sensitivity of electric fields to variations in seabed and water conductivities. Specifically, the most extreme combination yielded an 85% increase compared to the case with equal conductivities, which emphasises the substantial impact of media conductivities on local electric field prediction. The methodology outlined in this paper can provide a basis for future empirical validation, and inform biological experiments and licensing processes.

Determining reliable cable ampacities for marine High Voltage Cables is currently the subject of significant industry and academic reassessment in order to optimize (maximizing load while maintaining safe operating temperatures) design and reduce costs. Ampacity models can be elaborate, and inaccuracies are increasingly predicated on the uncertainty in environmental inputs. A stark example is the role of ambient temperature at cable depth, which, due to the scale of cables and the inaccessibility of the seafloor, is commonly estimated at 15 °C. Oceanographic models incorporating ocean bottom temperature are increasingly available, and they achieve coverage and spatiotemporal resolutions for cable applications without the requirement for project specific measurements. Here, a rudimental validation of the AMM15 and AMM7 mean monthly ocean bottom temperature models for the NW European Shelf indicates encouraging accuracies (MBE ≤ 1.48 °C; RMSE ≤ 2.2 °C). A series of cable case studies are used to demonstrate that cable ratings can change between −4.1% and +7.8% relative to ratings based on a common static (15 °C) ambient temperature value. Consideration of such variations can result in both significant ratings (and hence capital expenditure and operating costs) gains and/or the avoidance of cable overheating. Consequently, validated modelled ocean bottom temperatures are deemed sufficiently accurate, providing incomparable coverage and spatiotemporal resolutions of the whole annual temperature signal, thereby facilitating much more robust ambient temperatures and drastically improving ampacity estimates.

The ampacity of buried cables is significantly influenced by the thermal properties of the burial environment. When these thermal properties are not homogeneous it is usually necessary to utilize simulations with a relatively high computational cost that may also use commercial software. In this paper an alternative approach is proposed using conformal maps. Temperature is calculated in an annular domain which is a conformal mapping of the half plane space. Circumferential dependence is captured by expanding temperature as a Fourier series, a finite difference solver then determines temperature components radially. The model is as flexible as any two-dimensional slice model of heat transfer through thermal conduction only. Two case studies are considered: three land-based cables in planar configuration and a submarine export cable. The thermal properties of both burial environments are based on conditions which may be encountered in the field and exhibit a high level of stratification. Using a finite element analysis simulation as a benchmark, typical percentage differences in cable ampacities were 0.5%−1%. In addition to accuracy and flexibility the low computational cost of the proposed approach allows for large parameter sweeps, which may be required in a design phase, without requiring commercial software.

Determining the geotechnical seafloor properties needed to plan subsea infrastructure is time consuming and expensive as it requires soil sampling or in-situ contact measurements to be made using Remotely Operated Vehicles or ship based systems. To increase the efficiency of such surveys, we introduce a predictive framework for autonomous underwater vehicles (AUV) to determine locations where they can land and make contact measurements. We introduce a geotechnical measurability index that is computed using high, cm-resolution AUV observations. To address the small footprint of high-resolution AUV observations, our method infers the distribution of measurability onto more widely available remote sensed bathymetry that has resolutions of tens of centimeters to metres. Features are extracted from these low-resolution priors using an unsupervised Location-Guided Autoencoder. Geotechnical measurability maps are generated using a Bayesian Neural Network that combines these features with the geotechnical measurability calculated from high-resolution AUV observations to infer the measurability over a wide area. The framework is demonstrated using AUV structured light mapping data that was obtained from a $420\times 120m$ region of the Takuyo Daigo seamount. The data was artificially down sampled to simulate low resolution priors with sub-regions observed at high-resolution. The geotechnical measurability maps generated using the predictive framework preserve details that would otherwise be lost if the measurability index was calculated directly based on low resolution priors.

This paper demonstrates the impact of using realistic wind power generation profiles, time-varying ocean bottom temperatures and hypothetical wind farm over-planting scenarios on export cable capacity optimisation. Given the inherent risk in over-planting, a novel hour ahead thermal risk estimation method was developed to foresee and mitigate cable temperature exceedance, employing a preventive curtailment. Two offshore wind farm locations L1(North-west European Shelf) and L2(Australian Shelf) have been chosen for testing but utilising real wind and ocean bottom temperature data. These simulated results demonstrate a 10% rating increment over the static rating in L1 resulted in a 13% increment in the amount of energy delivered over a year (MWh/year) without any risk or instances of thermal overheating. Similarly, a 9.8% rating increment in L2 resulted in a 13.6% increment in annual energy transmission (MWh/year). The financial increment for both over-planting scenarios was approximately 9 pound million/year for the studied cases.

The ampacity of submarine cable circuits is strongly influenced by heat transfer in the marine environment surrounding the cable. It has been demonstrated in previous work that for high permeability sediments convective heat transfer can play a significant roleusing both bespoke two dimensional models and experimental investigations [T. J. Hughes, 2016, C. J. Emeana, 2016]. This paper introduces a one dimensional model which is capable of calculating cable temperatures within both convective and conductive sediments. Agreement between the one dimensional model and a two dimensional simulation was found to be within 1.5 degrees C. The model is used to demonstrate that the ampacity of power cables may be significantly increased due to convective heat transfer. Further, the one dimensional model offers significant savings in computational time and cost compared to the two dimensional equivalent model. This allows the analysis of large DTS data sets in order to calculate: dynamic ratings; burial depths; and the long-term (annual to decadal) performance of the cable.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Summary The last glacial cycle covers ~128kaBP to present. On the NW Australian shelf extensive seismic coverage has allowed geomorphological interpretation of an evolving coastal and fluvial landscape for an area encompassing the Bonaparte Gulf. It is possible to infer coastal positions and landscapes over the last glacial period denoted by Marine Isotope Stages (MIS) 5e-1. A key focus for the present study is MIS 4 where a lowstand relative sea level minimum of approximately 77-80m below present day developed ~63kaBP based on SE Australia measurements. This time range is consistent with some of the earliest recorded archaeology in Australia supporting human occupation of northern Australia by 65kaBP. In order for human occupation of Australia to have occurred by this time a sea crossing would have been required. The interpretation of geomorphological features from the seismic database has allowed prediction of most likely coastal position and possible tidal ranges during the MIS 4 lowstand in addition to contemporaneous depositional environments associated with reef, beach, estuarine, lagoonal and fluvial facies in an emergent landscape. The predicted palaeocoastline models are now being used to run simulations of tidal and ocean currents to inform possible maritime transit routes from Sunda to Sahul.

Time-lapse (4D) seismic imaging is now widely used as a tool to map and interpret changes in deep reservoirs as well as investigate dynamic, shallow hydrological processes in the near surface. However, there are very few examples of time-lapse analysis using ultra-high-frequency (UHF; kHz range) marine seismic reflection data. Exacting requirements for navigation can be prohibitive for acquiring coherent, true-3D volumes. Variable environmental noise can also lead to poor amplitude repeatability and make it difficult to identify differences that are related to real physical changes. Overcoming these challenges opens up a range of potential applications for monitoring the subsurface at decimetric resolution, including geohazards, geologic structures, as well as the bed-level and subsurface response to anthropogenic activities. Navigation postprocessing was incorporated to improve the acquisition and processing workflow for the 3D Chirp subbottom profiler and provide stable, centimeter-level absolute positioning, resulting in well-matched 3D data and mitigating 4D noise for data stacked into [Formula: see text] common-midpoint bins. Within an example 4D data set acquired on the south coast of the UK, interpretable differences are recorded within a shallow gas blanket. Reflections from the top and bottom of a gas pocket are imaged at low tide, whereas at high tide only the upper reflection is imaged. This case study demonstrates the viability of time-lapse UHF 3D seismic reflection for quantitative mapping of decimeter-scale changes within the shallow marine subsurface.

The earliest human migration from Sunda (South-East Asian archipelago) to Sahul (Australia and New Guinea) is still heavily debated with proposed timings between c.65-45kaBP depending on the evidence base and interpretation of the data. As part of the EU funded ACROSS project, focused on the mode and route of early migration in to SAHUL, we are undertaking an integrated interpretative study of the evolving submerged landscapes for the Late Pleistocene of the NW Australian Shelf. Oil and gas industry 3D and 2D seismic data, with some core/borehole data, are being used to determine lowstand palaeo-environments and shoreline positions. This information is informing modelling of ocean tide and current patterns that may have been influenced. The seismic is being interpreted supplemented by using time-slices on relative impedance inverted post-stack data. Layer stripping, seismic geomorphology, sequence boundary and depth analysis are being applied to datasets in the Bonaparte Basin, Kimberley Shelf and Arafura regions of Australia’s North-West Shelf area. Interpretation of the seismic data is constrained by dated stratigraphy in shallow cores with lower bounds determined from oil/gas well bores. MIS stages 1-4 are identified, however, the seismic response is a composite of time periods due to varying sedimentation rates, non-depositional hiatuses and minimal vertical seismic travel time covering this interval which limits the analysis to the top 50ms TWT (c. 40-45 m) of events below the seabed. This paper reviews the workflows that have been developed to maximise the fine scale detail that can be recovered for a range of terrestrial and marine environments. Procedures include inverse-Q, impedance inversion, spectral decomposition and time-slicing relative to seabed. High resolution 2D seismic data is also being used to augment and inform the interpretation of the conventional oil/gas 3D seismic data. Data examples will be presented showing the geomorphological characteristics (river channels, avulsions, levees, drainage channels, dunes and near shore carbonate reefs) of the lowstand and transgressive landscapes during this period. The palaeo-reconstructions are now being developed from the interpreted seismic geomorphology for the specific consideration of human seaborne travel.

Summary The life time performance of both HV cables (ORE inter -array and export cables and cross-continental shelf interconnectors) and oil and gas pipelines are limited by the physical properties of the sediment in which the cable/pipeline is buried. In the case of HV cables the burial material and burial depth have implications for heat dissipation from the cable, which in turn plays a primary role in cable rating and its lifetime operation and maintenance. For a pipeline changes in the density and strength of the overburden material can impact on buckling potential once in operation. Our current understanding of the key physical parameters of the sediment (e.g. grain size, porosity, permeability, thermal conductivity, relative density and strength) are based on in situ measurements of the ambient condition and rarely take account of physical property changes during the trenching process. We provide initial acoustic inversion results from high resolution 3D Chirp volumes from both a prototype scale, CPT calibrated, tank experiment and in situ trenched cables in a range of substrates. We shall demonstrate the potential of acoustic inversion to non-destructively quantify trench disturbance in this critical engineering scenarios.

Distributed Temperature Sensing (DTS) and Distributed Acoustic Sensing (DAS) measurements were acquired over a 40 km section of the 137 km long Skagerrak 4 (SK4) subsea cable before and during a retrenching operation. Five sections of the cable between 140 m and 280 m in length had been identified for reburial using conventional geophysical and video techniques. DTS data taken over a three week period prior to the reburial clearly showed cold spots at all of the planned reburial sites. Absolute temperature at these locations fluctuated by over 8 °C, which is interpreted as being in response to temporally and spatially varying bottom water temperatures. The reburial process itself was accurately monitored in real-time using DAS. After reburial, consistent DTS temperatures within ± 1 °C were measured at all reburial sites, comparable to the adjacent buried sections of the cable and thereby confirming reburial was successful.

Summary High-resolution seismic data provides information for many applications including offshore engineering work, where an accurate characterisation of the shallow marine subsurface is essential. However, single 3D volumes only provide a temporal snapshot and do not fully capture the highly dynamic nature of shallow water environments. Although changes at the seabed can be interpreted from repeat bathymetry, only very limited information about the substrate below. Here, we discuss the application of multiple, collocated, ultra-high-resolution (kHz-range) 3D seismic surveys as a tool to investigate changing processes in the marine subsurface. Examining data acquired with the 3D Chirp sub-bottom profiler, two case study examples will be presented. Results illustrate the capability for quantitative mapping of subsurface differences at decimetre-scale resolution using bin sizes of 0.25 cm and smaller.

Summary Time-lapse seismic imaging has improved our capability to measure and understand dynamic processes in the subsurface. However, there are very few examples using ultra-high-frequency (kHz-range) seismic data. Exacting requirements for navigation can be prohibitive for acquiring coherent, true-3D volumes and residual errors manifest as noise in time-lapse differences making it challenging to interpret real subsurface changes. By updating the acquisition and processing workflows for the 3D Chirp, an ultra-high-frequency sub-bottom profiler, initial results illustrate high amplitude and navigation repeatability. Post-processing was used to improve the capability and performance of real-time kinematic GPS, coupled with high-accuracy inertial measurements to yield centimetre-level absolute positioning in a range of operating conditions. A comparison of seismic reflection volumes acquired over the same area at high and low tide reveals a normalized RMS difference of 16.1% and demonstrates the capability for direct quantitative monitoring of the shallow subsurface at decimeter-resolution.

Often cultural conservators are asked to assess the preservation state of waterlogged wooden artefacts whose identity and rarity place an ethical barrier on the use of destructive analysis techniques. In addition, conservators are continually being challenged to find new ways of assessing the preservation of the underwater heritage, such as wooden shipwrecks, whilst in situ, and thus assist the process of managing such sensitive archaeological sites. Ultrasound compressional (p-) wave velocity has been researched in the past as a potential tool for estimating the preservation state of wooden artefacts and timbers. Its non-invasive principal complies with conservators' working ethics, while it has shown the potential of mapping and imaging submerged wooden archaeological heritage objects, as well as estimating the in situ preservation state. The aims of this paper are to present a viable non-destructive assessment method for cultural conservators for working on laboratory samples of waterlogged wood and to provide data for the analysis of in situ sites. This paper outlines the approach for the preparation of samples; the generation of controlled test -pieces for systematically quantitatively assessing the relationship between mass loss expressed as basic density and p -wave measurements; acoustic measurement; and the initial empirical results. Mass loss is achieved in a controlled and reproducible way for testing with ultrasound. The process incorporates a set of increasing wood degradation levels by gradually removing wood mass from waterlogged oak and pine test -pieces via drilling holes along the grain (longitudinal wood growth axis). This is followed by a chemical treatment with alkaline of the fully drilled wood test -pieces. The same test -pieces are used from zero to maximum degradation. This allows consistent observations, restricts variability and enhances interpretation of the results. The study considers wood both as a raw material and an artefact, here exemplified as the hull components of ancient wooden ships. Dimensions and cutting orientations of the test -pieces respect those noted in archaeological records. The focus is set on the RL and TL planes (radial and tangential axis respectively) and TL (tangential axis) planes, the main planes expected to be insonified with ultrasound considering timber conversion techniques in ancient shipbuilding. Ultrasound testing is performed within a reinforced polyethylene water tank, with the wood test -piece placed in between the transmitter and the receiver in good alignment. Using the trough -transmission immersion technique the time it takes a p -wave to travel through the test -piece together with the latter's thickness, are used to calculate the propagation velocity. Results demonstrate that ultrasound waves travel faster in the radial than in the tangential direction; although advancing the degradation, wood becomes more isotropic across the grain as indicated by the reduction of VRadial/VTangennal ratio. Ultrasound velocity is unaffected by the structural differences between ring -porous oak and pine allowing quantitative results for a density range between 0.567 gcm-3 (fresh) and 0.292 gcm-3 (degraded) irrespective of wood species used. Two significant empirically derived equations can be used by the cultural conservator to derive a wood density level, a common bench mark for assessing archaeological wood degradation level. (C) 2018 Elsevier Masson SAS. All rights reserved.

Four hundred square kilometres of 1 m binned, full coverage swath bathymetry data, integrated with similar resolution onshore topography, have been used to generate a seamless onshore to offshore bedrock map covering an extensive area adjacent to the ‘Jurassic Coast’ World Heritage site. Analysis of these data provides new insights into the structural development of the Purbeck Monocline Cenozoic inversion structure; in particular, variations in the expression of strain between the hanging-wall block and the fault inversion zone. The footwall to the basin-bounding faults compartmentalized deformation and uplift, and acted as a buttress to compression. The data also show a limited thickness changes within the major lithostratigraphical divisions, and a notable absence of basin-related extensional faulting in the offshore area that is in marked contrast to the more extensively studied onshore region. This indicates that prior to inversion, the basin evolved by intermittent activity on a few major extensional faults. This improved understanding of the development of the basin and inversion structures results from our ability to integrate and quantitatively manipulate these high-resolution and spatially extensive offshore and onshore datasets.

Near surface sediment stratigraphy associated with ancient human settlements can potentially reveal the complex history of human impact. This study explores such impacts in the area around ancient Caesarea, a well-known Roman to Early Islam period metropolis in the central coastal plain of Israel, with analysis of human-induced macro-features and microscopic remains found in buried sediments. We retrieved these anthropogenic markers through boreholes and analysed them with sedimentological and radiometric dating techniques, integrated with archaeological and historical records. The analysis identified a refuse deposit comprising two grey loamy sand artefact-bearing facies bedded between late Holocene aeolian sand. One anthropogenic facies represents an urban garbage mound and the other may be an agricultural pedo-sediment, both dated to the Roman to Early Islamic periods. The grey pedo-sediment, contained in three boreholes in the lowlands south of Caesarea, covers an area of at least 1.4km2. Apparently improved in terms of soil fertility, we postulate that the pedo-sediment is the outcome of composting enrichment of the soil for agriculture. Taking advantage of the high coastal freshwater aquifer in the study area, we propose that the pedo-sediment represents buried agricultural plots. The comprehensive, multi-disciplinary approach demonstrated in this study of cored sediments outside ancient human settlements is among the few in the coastal area of the southern Levant. It could be relevant to other archaeological sites in the Mediterranean and elsewhere around the world.