A contribution to aeromagnetic deculturing in populated areas

Modern regional airborne magnetic datasets, when acquired in populated areas, are inevitably degraded by cultural interference. In the United Kingdom context, the spatial densities of interfering structures and their complex spatial form severely limit our ability to successfully process and interpret the data. Deculturing procedures previously adopted have used semi‐automatic methods that incorporate additional geographical databases that guide manual assessment and refinement of the acquired database. Here we present an improved component of that procedure that guides the detection of localized responses associated with non‐geological perturbations. The procedure derives from a well‐established technique for the detection of kimberlite pipes and is a form of moving‐window correlation using grid‐based data. The procedure lends itself to automatic removal of perturbed data, although manual intervention to accept/reject outputs of the procedure is wise. The technique is evaluated using recently acquired regional United Kingdom survey data, which benefits from having an offshore component and areas of largely non‐magnetic granitic response. The methodology is effective at identifying (and hence removing) the isolated perturbations that form a persistent spatial noise background to the entire dataset. Probably in common with all such methods, the technique fails to isolate and remove amalgamated responses due to complex superimposed effects. The procedure forms an improved component of partial automation in the context of a wider deculturing procedure applied to United Kingdom aeromagnetic data.     

Continuous gas monitoring for the assessment of outburst-proneness at a mining face

Summary Continuous gas monitoring has been performed in three development panels at No. 2 Mine, Collinsville, Queensland, Australia to assess seam gassiness at the mining face as an indication of outburst-proneness. The first panel monitored encountered two outbursts during mining of virgin conditions, and a third outburst occurred on the fringe of a gas-drained block, adjacent to a thrust fault. The second panel successfully mined through the thrust fault without outbursting and finally halted due to high gas emissions. The third panel mined through a gas-drained block of coal, where an overall substantial reduction in gas emission was observed.The results from all three panels have been analysed and compared to establish parameters for outburst-proneness assessment, based on a face cut-out cycle (FCC) CO₂ emission. For FCC CO₂ emissions less than 2.6 m³ t^–1, no outbursting occurs, and normal mining proceeds. Up to 3.8 m³ t^–1, low-medium outburst-prone conditions exist and mining takes place under controlled procedures. When values exceed 3.8 m³ t^–1, high outburst-prone conditions exist and mining is halted.     

Induction in arbitrarily shaped oceans — IV. Sq for a simple case

Summary. World curves are presented of electric currents induced in electrically insulated oceans by Sq using a perfectly conducting sphere to simulate the underlying mantle. The curves are valid everywhere away from those coastal regions at which the sea bed rises steeply. In such cases an edge correction may be needed. The numerical computations are for periods of 6, 8, 12 and 24 hr and comparison is made with results obtained by others for a 24-hr period. Circulation conditions around the island are ignored in this note.     

A high resolution study of continuous pulsations in the European sector

Complex demodulation has been described in detail and applied to Pi2 pulsations in a previous paper by Beamish et al. (1979). The technique is now extended to demonstrate spatiotemporal variations in the fundamental characteristics of Pc3 and Pc4 pulsations along a meridional profile extending from the U.K. to Iceland. With the exception of a high latitude Pc4 coupled resonance the results are consistent with a ?90° Hughes rotation (introduced by the ionosphere) of magnetospheric toroidal line resonances. Furthermore, the ionosphere appears capable of smoothing away the polarisation reversal which would be expected across such amplitude maxima within the plasmasphere. However, a toroidal line resonance in the Pc3 period range about which a sense of polarisation reversal is clearly observed on the ground is suggested as occurring at the plasmapause. This is accounted for in terms of the width of the resonance structure.     

A critical size and period hypothesis to explain natural regulation of salmon abundance and the linkage to climate and climate change

We hypothesise that salmon year class strength is determined in two stages during the first year in the ocean. There is an early natural mortality that is mostly related to predation, which is followed by a physiologically-based mortality. Juvenile salmon that fail to reach a critical size by the end of their first marine summer do not survive the following winter. In this study we describe our initial tests of this critical size and critical period hypothesis using data from ocean surveys of juvenile salmon and from experimental feeding studies on coho. Conservative swept volume abundance estimates for juvenile coho, and possibly chinook, indicate that there is high mortality in fall and winter during their first year in the sea. Studies of otolith weight show that the length and otolith-weight relationship for young coho changes in the early fall of their first ocean year. Studies of growth and associated hormone levels in feeding studies show that slow growing juvenile coho are stunted and deficient in an insulin-like growth factor-I (IGF-I). Juvenile coho sampled in September had low IGF-I values, indicative of poor growth. The results of these studies provide evidence for the general hypothesis that growth-related mortality occurs late in the first marine year and may be important in determining the strength of the year class (brood year). The link between total mortality and climate could be operating via the availability of nutrients regulating the food supply and hence competition for food (i.e. bottom–up regulation).     

Have there been recent changes in climate? Ask the fish

It is generally accepted that a climate shift occurred about 1977 that affected the dynamics of North Pacific marine ecosystems. Agreement on the possibility of further climate shifts in 1989 and the late 1990s is yet to be achieved. However, there have been changes in the dynamics of key commercial fishes that indicate changes in their environment occurred in the early 1990s, and possibly around 1998. One method of measuring climate change is to observe the dynamics of species that could be affected.Several studies have described decadal-scale changes in North Pacific climate–ocean conditions. Generally, these studies focus on a single index. Using principal components analysis, we use a composite index based on three aspects of climate ocean conditions: the Aleutian Low Pressure Index, the Pacific Atmospheric Circulation Index and the Pacific Interdecadal Oscillation Index. We link this composite index (Atmospheric Forcing Index) to decadal-scale changes in British Columbia salmon and other fish populations. Around 1989 there was a change from intense Aleutian Lows (above average south-westerly and westerly circulation patterns and warming of coastal sea surface temperatures) to average Aleutian Lows (less frequent south-westerly and westerly circulation and slightly cooler coastal sea surface temperatures in winter). These climate–ocean changes were associated with changes in the abundance and ocean survival of salmon (Oncorhynchus spp.), distribution and spawning behaviour of hake (Merluccius productus) and sardines (Sardinops sagax) and in recruitment patterns of several groundfish species.     

Longevity overfishing

Overfishing is generally considered to be a reduction in biomass below some critical level such that the remaining fish are not able to replenish the population. We propose that the removal of large numbers of older age groups by fishing is also a form of overfishing, which we identify as longevity overfishing. Longevity overfishing is a potentially important consideration for the commercial fisheries off Canada’s Pacific coast that are dominated by species that have maximum ages of 30 years or longer. Sablefish is one of the key long-lived species that is managed for biomass and not longevity. An age structured model showed that if younger fish do not have the same productivity per unit biomass as older fish, the population depleted of older fishes would not recover after a shift of carrying capacity from a prolonged period of poor productivity to a more productive ocean ecosystem. Current management of long-lived species implicitly assumes that young fish will have the same productivity as older fishes, an assumption that is not supported by a sparse literature, and is thus not precautionary. We propose that the evolved age structure is an indication that long-lived species must be managed for longevity as well as biomass, which requires a management time frame that is decades and not annual.