Effects of laser beam alignment tolerance on lidar accuracy

One of the major lidar error sources not yet analyzed in the literature is the tolerance of the laser beam alignment with respect to the scanning mirror. In this paper, the problem of quantifying these errors is solved for rotating polygon mirror type lidar systems. An arbitrary deviation of the beam from its design direction–the vector of beam misalignment–can be described by two independent parameters. We choose these as horizontal and vertical components of the misalignment vector in the body frame. Either component affects both, horizontal and vertical lidar accuracy. Horizontal lidar errors appear as scan line distortions—along and across track shifts, rotations and scaling. It is shown that the horizontal component of misalignment results in a scan line first being shifted across the track and then rotated around the vertical at the new center of the scan line. Resulting vertical lidar error, being a linear function of the scan angle, is similar to that produced by a roll bias. The vertical component of the beam misalignment causes scan line scaling and an along track shift. The corresponding vertical error is quadratic with respect to the scan angle. The magnitude of these effects is significant even at tight alignment tolerances and cannot be realistically accounted for in the conventional calibration model, which includes only range, attitude and GPS biases. Therefore, in order to attain better accuracy, this model must be expanded to include the beam misalignment parameters as well. Addition of new parameters into the model raises a question of whether they can be reliably solved for. To give a positive answer to this question, a calibration method must utilize not only ground control information, which is typically very limited, but also the relative accuracy information from the overlapping flight lines.     

Influence of anthropogenic pressure and seasonal upwelling on coral reefs in Nha Trang Bay (Central Vietnam)

The growing coastal development, dredging and dumping activities, overfishing and expansion of marine cage culture in Nha Trang Bay (NTB) of Central Vietnam since the beginning of the 2000s have resulted in a dramatic decrease of live coral cover. Surveys conducted in April–May 2013 and the same period in 2014 revealed that with an increase in distance from the outer part of the bay towards the mainland, the rivers’ influxes and dredged areas, coral cover decreased from 75% to 0.6% and species richness from 63 to 5, while the abundance of macroalgae increased from 0% to 56%. These changes correlate with differences in the concentration of suspended sediments on the same gradient. The abundance of the crown‐of‐thorns starfish Acanthaster planci and of the echinoid Diadema setosum significantly increased between the first estimation in 1998 and the survey in 2014, from 0 to 1.7 individuals (ind.) per 100 m^?2 and from 50.8 to 94.5 ind. per 100 m^?2, respectively, contributing to coral loss and intensive bioerosion of the reef framework in the bay. The large sizes of adult colonies of tabulate Acropora on the remote stations with negligible sedimentation and eutrophication loads were inconsistent with the assumptions that temperature‐induced coral bleaching or cyclones could be the major impacts in Nha Trang Bay. Analysis of the 16‐year thermal history of the bay did not reveal any instances in which the coral thermal bleaching threshold had been exceeded up to the present study. Seasonal upwelling, which occurs annually in the vicinity of Central Vietnam, may contribute to mitigation of thermal anomalies within NTB and to the maintenance of healthy coral communities on the remote reefs with relatively low anthropogenic impact.     

New insights into precious metal enrichment on the Isle of Rum,Scotland

The Rum Layered Suite (NW Scotland) is generally regarded as one of a handful of classic examples of open‐system layered mafic‐ultramafic intrusions, or ‘fossilized’ basaltic magma chambers, world‐wide. The eastern portion of the Rum intrusion is constructed of sixteen repeated, coupled, peridotite–troctolite units. Each major cyclic unit has been linked to a major magma replenishment event, with repeated settling out of ‘crops’ of olivine and plagioclase crystals to form the cumulate rocks. However, there are variations in the lithological succession that complicate this oversimplified model, including the presence of chromitite (>60 vol. percent Cr‐spinel) seams. The ～2 mm thick chromitite seams host significant platinum‐group element (PGE) enrichment (e.g. ～2 ppm Pt) and likely formed in situ, i.e. at the crystal mush–magma interface. Given that the bulk of the world's exploited PGE come from a layered intrusion that bears remarkable structural and lithological similarities to Rum, the Bushveld Complex (South Africa), comparisons between these intrusions raise intriguing implications for precious metal mineralization in layered intrusions.     

Hydrochemical investigation and quality assessment of ground water in rural areas of Delhi,India

The suitability of groundwater quality for drinking and agricultural purposes was assessed in the rural areas of Delhi based on the various water quality parameters. A total of 50 ground water samples were collected randomly from different sources viz. hand pump, tube well, boring and analyzed for major ion chemistry to understand the operating mechanism of geochemical processes for ground water quality. The quality analysis is performed through the estimation of pH, EC, TDS, total hardness, total alkalinity, Na, K, Cl, NO₃, SO₄, DO, BOD, Cu, Cr, Cd, Ni, Zn and Pb. Hydrochemical facies were identified using Piper, Durov and Chadha diagram. Chemical data were also used for mathematical calculations (SAR, %Na, RSC, PI, KI, and chloroalkaline indices) for better understanding the suitability of ground water for irrigation purposes. The results of saturation index shows that all the water samples were supersaturated to undersaturated with respect to carbonate minerals and undersaturated with respect to sulphate and chloride minerals. According to USSL diagram, most of the samples fall in the field of C₃S₁, indicating medium salinity and low sodium water which can be used for almost all types of soil with little danger of exchangeable sodium. Assessment of water samples from various methods indicated that majority of the ground water in the study area is chemically suitable for drinking and agricultural uses.     

Infiltration metasomatism in layered intrusions revisited: a reinterpretation of compositional reversals at the base of cyclic units

Summary Irvine (1980) proposed an elegant mechanism termed “infiltration metasomatism” to explain upward displacement of discontinuities in mineral compositional trends relative to contacts between cyclic units in the Muskox intrusion. It was concluded that the offsets of Mg/(Mg + Fe) discontinuities in olivine and chromite are a secondary postmagmatic feature that resulted from reaction between the cumulus minerals and intercumulus liquid that was frontally displaced upwards from the underlying crystal pile as a result of compaction. We reinterpret this feature in the Muskox and other layered intrusions as basal reversals that arise from a temperature gradient-driven flux of low melting point components from the hot magma parental to cyclic units towards a relatively cold cumulate floor. In this interpretation basal reversals are a primary magmatic feature that does not involve intercumulus liquid migration. Authors’ addresses: R. M. Latypov, S. Yu. Chistyakova, Kola Science Centre, Geological Institute, Fersman Str. 16, 184200, Apatity, Russia; Present address: Department of Geosciences, University of Oulu, Oulu, P.O. Box 3000, FIN-90014, Finland; T. T. Alapieti, Department of Geosciences, University of Oulu, Oulu, P.O. Box 3000, FIN-90014, Finland     

Towards a model for the in situ origin of PGE reefs in layered intrusions: insights from chromitite seams of the Rum Eastern Layered Intrusion, Scotland

The current debate on the origin of platinum-group element (PGE) reefs in layered intrusions centres mostly on gravity settling of sulphide liquid from overlying magma versus its introduction with interstitial melt/fluids migrating upward from the underlying cumulate pile. Here, we show that PGE-rich chromitite seams of the Rum Eastern Layered Intrusion provide evidence for an alternative origin of such deposits in layered intrusions. These laterally extensive 2-mm-thick chromitite seams occur at the bases of several cyclic mafic–ultramafic units and show lithological and textural relationships suggesting in situ growth directly at a crystal–liquid interface. This follows from chromitite development along the edges of steeply inclined culminations and depressions at unit boundaries, even where these are vertically oriented or overhanging. High concentrations of PGE (up to 2–3 ppm Pd + Pt) are controlled by fine-grained base-metal sulphides, which are closely associated with chromitite seams. The following sequence of events explains the origin of the PGE-rich chromitite seams: (a) emplacement of picritic magma that caused thermal and mechanical erosion of underlying cumulate, followed by in situ growth of chromite against the base, (b) precipitation of sulphide droplets on chromite grains acting as favourable substrate or catalyst for sulphide nucleation, (c) the scavenging of PGE by sulphide droplets from fresh magma continuously brought towards the base by convection. Since the rate of magma convection is 10⁵–10⁷ times higher than that of the solidification (km/year to km/day versus 0.5–1.0 cm/year), the in situ formed sulphide droplets can equilibrate with picritic magma of thousands to million times their own volume. As a result, the sulphide-bearing rocks are able to reach economic concentrations of PGE (several ppm). We tentatively suggest that the basic principles of our model may be used to explain the origin of PGE-rich chromitites and classical PGE reefs in other layered mafic–ultramafic intrusions.     

Fine-grained mafic bodies as preserved portions of magma replenishing layered intrusions: the Nadezhda gabbronorite body, Lukkulaisvaara intrusion, Fennoscandian Shield, Russia

Summary The 100 m thick and 700 m long Nadezhda body in the Lukkulaisvaara layered intrusion exhibits concentric zonation with an inward progression from a 0.5 to 1.0 m thick marginal layer of medium- to coarse-grained norites and gabbronorites that abruptly give way to fine-grained oikocrystic gabbronorites composing the rest of the body. The concentric zonation is additionally emphasized by well-developed alignment of plagioclase laths and orthopyroxene oikocrysts parallel to the outer contacts of the body, pegmatitic gabbronorite segregations in the centre of the body and slight inward decrease in whole-rock Mg# and Cr and increase in incompatible elements. The body has distinctly higher whole-rock Mg# and lower concentrations of all incompatible components than its host rocks. It is enveloped by highly altered marginal anorthosites belonging to host norites and gabbronorites. We interpret the Nadezhda body as a portion of high Mg# (∼75%) and incompatible element-poor (∼20 ppm, Zr; ∼10 ppm, total REE; ∼0.20 wt%, TiO₂) magma that replenished the evolving chamber and became trapped within the cumulate pile. Recrystallization of adjacent rocks by volatiles exsolved from the magma upon emplacement resulted in formation of marginal anorthosites. Upon cooling the magma started to crystallize medium- to coarse-grained norites along its margins, but subsequent decompression and loss of volatiles led to rapid crystallization of magma into fine-grained oikocrystic gabbronorites. Solidification of the remaining residual liquid gave rise to pegmatitic gabbronorite segregations. Supplementary material to this paper is available in electronic form at Tables 3–6 available as electronic supplementary material Authors’ addresses: R. M. Latypov, S. Yu. Chistyakova, Kola Science Centre, Geological Institute, Fersman Str. 16, 184200 Apatity, Russia; Present address: Department of Geosciences, University of Oulu, P.O. Box 3000, Oulu, FIN-90014, Finland; T. T. Alapieti, Department of Geosciences, University of Oulu, P.O. Box 3000, Oulu, FIN-90014, Finland