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.
Very early times in the order of 2–3 μs from the end of the turn‐off ramp for time‐domain electromagnetic systems are crucial for obtaining a detailed resolution of the near‐surface geology in the depth interval 0–20 m. For transient electromagnetic systems working in the off time, an electric current is abruptly turned off in a large transmitter loop causing a secondary electromagnetic field to be generated by the eddy currents induced in the ground. Often, however, there is still a residual primary field generated by remaining slowly decaying currents in the transmitter loop. The decay disturbs or biases the earth response data at the very early times. These biased data must be culled, or some specific processing must be applied in order to compensate or remove the residual primary field. As the bias response can be attributed to decaying currents with its time constantly controlled by the geometry of the transmitter loop, we denote it the ‘Coil Response’. The modelling of a helicopter‐borne time‐domain system by an equivalent electronic circuit shows that the time decay of the coil response remains identical whatever the position of the receiver loop, which is confirmed by field measurements. The modelling also shows that the coil response has a theoretical zero location and positioning the receiver coil at the zero location eliminates the coil response completely. However, spatial variations of the coil response around the zero location are not insignificant and even a few cm deformation of the carrier frame will introduce a small coil response. Here we present an approach for subtracting the coil response from the data by measuring it at high altitudes and then including an extra shift factor into the inversion scheme. The scheme is successfully applied to data from the SkyTEM system and enables the use of very early time gates, as early as 2–3 μs from the end of the ramp, or 5–6 μs from the beginning of the ramp. Applied to a large‐scale airborne electromagnetic survey, the coil response compensation provides airborne electromagnetic methods with a hitherto unseen good resolution of shallow geological layers in the depth interval 0–20 m. This is proved by comparing results from the airborne electromagnetic survey to more than 100 km of Electrical Resistivity Tomography measured with 5 m electrode spacing. 相似文献
Low‐temperature specific heat capacities of meteorites provide valuable data for understanding the composition and evolution of meteorites and modeling the thermal behavior of their source asteroids. By liquid nitrogen immersion, we measured average low‐temperature heat capacities for 60 ordinary chondrite falls from the Vatican collection. We further characterized the temperature dependence of ordinary chondrite by direct measurement of Cp(T) over the range 5–320 K for five OC falls, coupled by composition‐based models for 94 ordinary chondrites. We find that the heat capacity as a function of temperature for typical ordinary chondrites can be closely approximated by a third‐order polynomial in temperature. Furthermore, those polynomial coefficients can be estimated from the single‐value average heat capacity measurement. These measurements have important implications for the orbital and spin evolution of S‐ and Q‐type asteroids via the various Yarkovsky effects and the thermal evolution of meteorite parent bodies. 相似文献
Chemical mass transfer was quantified in a metacarbonate xenolith enclosed within the granodiorite of the Quérigut massif
(Pyrenees, France). Mass balance calculations suggest a strong decrease of CaO, SrO and CO2 contents (up to −90%), correlated with a decrease of modal calcite content as the contact is approached. Most other chemical
elements behave immobile during metasomatism. They are therefore passively enriched. Only a small increase of SiO2, Al2O3 and Fe2O3 contents occurs in the immediate vicinity of the contact. Hence, in this study, skarn formation is characterized by the lack
of large chemical element influx from the granitoid protolith. A large decrease of the initial carbonate volume (up to −86%)
resulted from a combination of decarbonation reactions and loss of CaO and CO2. The resulting volume change has potentially important consequences for the interpretation of stable isotope profiles: the
isotope alteration could have occured over greater distances than those observed today. 相似文献