Multi-scale Quantitative Risk Analysis of Seabed Minerals: Principles and Application to Seafloor Massive Sulfide Prospects

The potential for mining hydrothermal mineral deposits on the seafloor, such as seafloor massive sulfides, has become technically possible, and some companies (currently not many) are considering their exploration and development. Yet, no present methodology has been designed to quantify the ore potential and assess the risks relative to prospectivity at prospect and regional scales. Multi-scale exploration techniques, similar to those of the play analysis that are used in the oil and gas industry, can help to fulfill this task by identifying the characteristics of geologic environments indicative of ore-forming processes. Such characteristics can represent a combination of, e.g., heat source, pathway, trap and reservoir that all dictate how and where ore components are mobilized from source to deposition. In this study, the understanding of these key elements is developed as a mineral system, which serves as a guide for mapping the risk of the presence or absence of ore-forming processes within the region of interest (the permissive tract). The risk analysis is carried out using geoscience data, and it is paired with quantitative resource estimation analysis to estimate the in-place mineral potential. Resource estimates are simulated stochastically with the help of available data (bathymetric features in this study), conventional grade–tonnage models and Monte Carlo simulation techniques. In this paper, the workflow for a multi-scale quantitative risk analysis, from the definition to the evaluation of a permissive tract and related prospect(s), is described with the help of multi-beam data of a known hydrothermal vent site.

     

Evolution of the western Barents Sea

Information from multichannel seismic reflection data complemented by seismic refraction, gravity and magnetics forms the basis for a regional structural and evolutionary model of the western Barents Sea during post-Caledonian times. The western Barents Sea contains a thick succession, locally > 10 km, of Upper Paleozoic to Cenozoic sedimentary rocks covering a basement of probably Caledonian origin. The area is divided into three regional geological provinces: (1) an east-west trending basinal province between 74°N and the coast of Norway; (2) an elevated platform area to the north towards Svalbard; and (3) the western continental margin. Several structural elements of different origin and age have been mapped within each of these provinces. The main stratigraphic sequence boundaries have been tentatively dated from available well information, correlation with the geology of adjacent areas, and correlation with the interregional unconformities caused by relative changes of sea level. The main structural elements were developed during three major post-Caledonian tectonic phases: the Svalbardian phase in Late Devonian to Early Carboniferous times, the Mid and Late Kimmerian phase in Mid Jurassic to Early Cretaceous times and Cenozoic tectonism related to the progressive northward opening of the Norwegian-Greenland Sea. The sediments are predicted to be of mainly clastic origin except for a thick sequence of Middle Carboniferous — Lower Permian carbonates and evaporites. Salt diapirs have developed in several sub-basins, especially in the Nordkapp Basin where they form continuous salt walls that have pierced through > 7 km of sediments.     

Development of a laboratory exposure system using marine fish to carry out realistic effect studies with produced water discharged from offshore oil production

A biotest system for environmentally realistic exposure of fish to produced water (PW) was developed and tested. Authentic PW was collected at an oil production platform in the North Sea and preserved by freezing in multiple aliquots a 25 L. After transport to the test laboratory onshore, daily PW aliquots were thawed, homogenised and administered to the test fish, Atlantic cod (Gadus morhua), in two diluted exposure concentrations, 0.1% and 0.5%, during a 15 d period, using a continuous flow-through exposure setup. Positive control groups were exposed to two crude oil treatments for comparison. Chemical analyses showed that alkylphenol (AP) and PAH concentrations in PW exposure waters were very low. Observations of significantly increased AP and PAH metabolite levels in PW exposed fish demonstrated the suitability of the biotest system for its use in biological exposure/effect studies of PW, and it also demonstrated the sensitivity of bile metabolites as PW exposure markers in fish. The relevance of the biotest system for PW effect studies and for validating modelled environmental risk estimates of PW dischargers from offshore oil production is discussed.     

Discussion of ‘Soil creep effects on ground lateral deformation and pore water pressure under embankments’

Settlement analysis of field cases is normally studied based on parameters interpreted from laboratory samples influenced by varying degrees of sample disturbance. Such disturbance is more pronounced in natural soft clays and could significantly affect the engineering properties of the soil, e.g. the over consolidation ratio (OCR) and compressibility index (C_c). Hence, it is vital to understand the role of sample quality in relation to soil characterisation for long-term settlement analyses. In this work, this is numerically illustrated by use of a simple creep model along with realistic parameter selection. This work takes its starting point on critical discussion of the work presented by Fatahi et al. (2013) and uses the opportunity to further clarify some important aspects of settlement/creep analyses in light of sample quality and parameter interpretation valid for the corresponding constitutive model.     

Formulation of non-coaxial plastic dissipation and stress–dilatancy relations for geomaterials

Acta Geotechnica - Stress–dilatancy theories play a central role in the modeling of the plastic dissipation of geomaterials. There exist several mathematical frameworks for describing the...     

High-resolution P-Cable 3D seismic imaging of gas chimney structures in gas hydrated sediments of an Arctic sediment drift

The newly developed P-Cable 3D seismic system allows for high-resolution seismic imaging to characterize upper geosphere geological features focusing on geofluid expressions (gas chimneys), shallow gas and gas hydrate reservoirs. Seismic imaging of a geofluid system of an Arctic sediment drift at the Vestnesa Ridge, offshore western Svalbard, provides significantly improved details of internal chimney structures from the seafloor to ∼500 m bsf (below seafloor). The chimneys connect to pockmarks at the seafloor and indicate focused fluid flow through gas hydrated sediments. The pockmarks are not buried and align at the ridge-crest pointing to recent, topography-controlled fluid discharge. Chimneys are fuelled by sources beneath the base of gas hydrate stability zone (GHSZ) that is evident at ∼160–170 m bsf as indicated by a bottom-simulating reflector (BSR). Conduit centres that are not vertically straight but shift laterally by up to 200 m as well as discontinuous internal chimney reflections indicate heterogeneous hydraulic fracturing of the sediments. Episodically active, pressure-driven focused fluid flow could explain the hydro-fracturing processes that control the plumbing system and lead to extensive pockmark formation at crest of the Vestnesa Ridge. High-amplitude anomalies in the upper 50 m of the chimney structures suggest formations of near-surface gas hydrates and/or authigenic carbonate precipitation. Acoustic anomalies, expressed as high amplitudes and amplitude blanking, are irregularly distributed throughout the deeper parts of the chimneys and provide evidence for the variability of hydrate and/or carbonate formation in space and time.     

Mid-winter hydroxyl night airglow emission intensities in the northern polar region

Ground-based spectrophotometric measurements of night airglow OH (8-3) band absolute intensities in the polar cap region (78.4°N) during winter solstice are reported. A mean value of 425 ± 40 R is found for the absolute intensity of the OH (8-3) band. Maximum and minimum daily mean values were 770 and 320 R respectively with hourly mean values ranging from 180 to 1020 R. Neither a winter solstice minimum or maximum in the intensity is obvious from the data. No consistent correlation was found between the absolute intensity and geomagnetic and solar activity. A mean transport of O and O₃ into the polar cap region corresponding to a meridional wind speed of at least 20 m s^?1 at 90 km height seems necessary to maintain the observed intensity. A dominant semidiurnal tide component is found in the intensity data, both on a 20-day and a 3-day time scale.     

Marine C reservoir ages for 19th century whales and molluscs from the North Atlantic

In order to compare radiocarbon dates on marine and terrestrial samples the former have to be corrected for a reservoir age. We present reservoir ages from dating 21 whales collected 1860–1901 and recalculating dates of 23 molluscs collected 1857–1926. Most of the whales were caught along the coast of Norway, but one is from France and one from Iceland. We assume the former mainly lived in the North and equatorial Atlantic and in the Norwegian Sea. Whales feed only on pelagic organisms and will provide the reservoir age for the open ocean surface water. However, they travel long distances and will integrate the reservoir ages of the different water masses along their way. Molluscs (dated from Norway, Spitsbergen and Arctic Canada) are stationary and monitor the sea water passing their dwelling site, but some also take up carbon from particulate food or sediment pore water. Coastal water also often contains some continental carbon. We present two different views on how to analyze and interpret the data. Mangerud recommends to use reservoir ages based on a combination of the whale and mollusc dates, i.e. 380±30 and 360±30 yr relative to Intcal04 and British oak, respectively, and a ΔR value of 20±30 for the surface water in the N-Atlantic and Norwegian Sea. Bondevik and Gulliksen maintain that the reservoir age—and ΔR—along the Norwegian coast is latitude dependant, with ΔR-values increasing from −3±22 in the South to 105±24 at Spitsbergen. Whales, reflecting North Atlantic open ocean surface water have lower ΔR (7±11) than most molluscs.     

Experimental study of strain localization in sensitive clays

For evaluation of slope stability in materials displaying strain-softening behavior, knowledge concerning the failed state material response is of importance. Here, soft sensitive clay is studied. Such clays behave contractant at failure, which for undrained conditions yields a strain-softening behavior governed by the generation of excess pore water pressure. Strain softening is further linked with material instability and the phenomenon of strain localization. In the case of shear band formation, internal pore pressure gradients are then expected to be present for globally undrained conditions in the sensitive clay due to its low permeability. In the present study, this hypothesis and its implications on the global response and shear band properties are investigated. Utilizing an experimental setup with a modified triaxial cell allowing for shear band formation, the effect of varying the displacement rate is studied. Onset of strain localization is interpreted to occur just before or at the peak shear strength. A strong rate dependency of the softening response is observed. Increasing displacement rates give raised brittleness in terms of the slope of the global softening curve due to accumulating pore pressure. Also, reduced shear band thickness and a shear band inclination approaching 45° are obtained for increasing rates. In the context of slope failure in such materials, the rate dependency in the post-peak state opens up for a large variation in behavior, all depending on time as an important factor.