3D Modelling of the electromagnetic response of geophysical targets using the FDTD method1

A publicly available and maintained electromagnetic finite-difference time domain (FDTD) code has been applied to the forward modelling of the response of 1D, 2D and 3D geophysical targets to a vertical magnetic dipole excitation. The FDTD method is used to analyse target responses in the 1 MHz to 100MHz range, where either conduction or displacement currents may have the controlling role. The response of the geophysical target to the excitation is presented as changes in the magnetic field ellipticity. The results of the FDTD code compare favourably with previously published integral equation solutions of the response of 1D targets, and FDTD models calculated with different finite-difference cell sizes are compared to find the effect of model discretization on the solution. The discretization errors, calculated as absolute error in ellipticity, are presented for the different ground geometry models considered, and are, for the most part, below 10% of the integral equation solutions. Finally, the FDTD code is used to calculate the magnetic ellipticity response of a 2D survey and a 3D sounding of complicated geophysical targets. The response of these 2D and 3D targets are too complicated to be verified with integral equation solutions, but show the proper low- and high-frequency responses.     

Calcification, photosynthesis and growth of the bloom-forming coccolithophore, Emiliania huxleyi

Measurements were made of photosynthesis and calcification in a bloom of coccolithophores in the Gulf of Maine as well as in laboratory batch cultures of the Emiliania huxleyi strain 88E. The¹⁴C technique for measuring calcification was evaluated with calcifying and noncalcifying coccolithophore strains. Shipboard bloom observations showed physiological evidence of stratification between surface and deep populations within the surface mixed layer. Photosynthesis and calcification rates were enhanced within hours of addition of KNO₃ or nitrate-rich deep water (to the same final concentration). Such rates were strongly temperature dependent in laboratory cultures, decreasing over a factor of two from 15 to 20°C. Calcification was considerably less light-dependent than photosynthesis and consistently had a measurable dark rate. In culture, maximum calcification rates and degree of cell plating were limited to logarithmic growth phase. This was followed by rapid increase in the coccolith shedding rate 1 day prior to the onset of stationary phase which continued several days after cell division stopped. The regional significance of these features to total organic and inorganic carbon production as well as calcite burial is discussed.     

Compositional characteristics of olivines from Apollo 12 samples

The olivine phenocrysts of four basalts (12004, 12008, 12009 and 12022) are concentrically zoned and have core compositions about as magnesian as experimentally produced liquidas olivines, features which suggest fractional crystallization and absence of Fe-Mg reequilibration. In the magnesium- and olivine-rich granular basalt 12035, the olivines are either unzoned or are zoned toward adjacent grains and have compositions more iron-rich than either cumulus olivines or liquidus olivines (should the rock represent the composition of a melt), features which suggest extensive Fe-Mg re-equilibration.     

Organic carbon isotope ratios in Quaternary cores from the Gulf of Mexico

All of the major deep-water sedimentary provinces of the Gulf of Mexico were sampled with 48 piston cores, representative of the late Quaternary. The amount (per cent) and δ5C¹³ of the organic carbon in the sediment was measured at intervals within each core.Graphs of δC¹³ versus depth for each core give an indication of the sedimentological history of the Gulf. They show the extent of terrestrial influence on the Gulf during the late Pleistocene.Changes in δC¹³ of up to 6.0%.(from ~ ?19%.to ?25%. vs NBS-20) were measured across the Pleistocene-Holocene boundary in cores from the abyssal plain. These changes are consistent with a model wherein varying amounts of land-derived organic carbon were transported to the Gulf basin during glacial periods.By comparing graphs for cores from different areas, it was concluded that the major parameter affecting the δC¹³ values of organic carbon from marine sediments is the relative amount of terrestrial material present in the sediment. The maximum possible effect of the Pleistocene-Holocene temperature change in the Gulf was determined to be ~1.0%, if such an effect occurs at all.     

HOUSING INTEGRATION AND RENT SUPPLEMENTS TO EXISTING HOUSING*

This work examines the use of an annually updated metropolitan data system as a tool for evaluating benign racial location policies during the first year of the Section 8 Existing Rent Supplement program. Case-study evidence shows that the rent supplement program (with benign locational policies) is ineffective in reducing the isolation faced by minority groups, and that the information used to monitor compliance of the program with benign locational policies lags badly behind the true state of affairs. As a result, new areas of racial concentration have been encouraged and augmented.     

Was a dacite dome implicated in the 9,500 <Emphasis Type="SmallCaps">b.p.</Emphasis> collapse of Mt Ruapehu? A palaeomagnetic investigation

Constraining the process by which volcanoes become unstable is difficult. Several models have been proposed to explain the driving forces which cause volcanic edifices to catastrophically collapse. These include models for destabilisation of volcanic flanks by wedging due to dyke intrusion and the weakening of mechanical properties by pressurisation of pore fluids. It is not known which, if any, of the models are relevant to particular sector collapse events. Recent developments in the palaeomagnetic estimation of emplacement temperatures of volcaniclastic rocks have shown that even relatively low emplacement temperatures can be recorded by volcaniclastics with high fidelity. We have carried out a palaeomagnetic study of emplacement temperatures to investigate the role of igneous activity in the initiation of the 9,500 b.p. Murimotu sector collapse of Mt Ruapehu, New Zealand. This debris avalanche deposit has three fades which are stratigraphically superimposed, and the lowermost fades contains three lithological assemblages representing different segments of the edifice which were transported with little internal mixing within the flow. We have determined that some of the dacite-bearing assemblage 1, fades 1 was hot (∼350 °C) during transport and emplacement, whereas none of the other lithological assemblages of fades contained hot material. Our interpretation is that a dacite dome was active on the ancient Ruapehu edifice immediately prior to the Murimotu sector collapse. The partially cooled carapace of the dome and material shed from this part was incorporated into the avalanche deposit, along with cold lavas and volcaniclastics. We have not found evidence for incorporation of material at or close to magmatic temperatures, at least in the sampled locations. Our palaeomagnetic work allows us to develop a comprehensive, new palaeomagnetic classification of volcaniclastics. Published online: 25 January 2003 Editorial responsibility: D. Dingwell     

Geochemistry of ice-covered, meromictic Lake A in the Canadian High Arctic

The geochemical processes occurring within meromictic Lake A (maxdepth 120 m) on northern Ellesmere Island, Canada, were investigated to determinethe history of the lake and to provide a baseline for future studies. The lake contained seawaterdiluted by freshwater input that had been mixed prior to the lake's isolation from tidalaction. Input of freshwater after isolation of the lake created vertical stratification resultingin the creation of distinct oxic, suboxic and anoxic zones. Dissolved oxygen was present to 13 m,and sulphide beneath 32 m. Manganese and iron cycling dominated the redox chemistrybetween these depths. Total manganese concentrations reached 176 M, higher thanin most other natural stratified lake or marine environments.     

Construction and analysis of higher order finite difference schemes for the 1D wave equation

In this article, we propose to: 1. Establish most of the properties conjectured in [2] about the higher order finite difference approximation of the 1D Laplace operator. 2. Generalize to any order the fourth-order accurate scheme in space and time of Shubin and Bell [20] and Cohen [6]. For this new family of 2m–2m schemes, we establish, via elementary mathematics, various stability and dispersion results that are helpful to compare these schemes to the 2–2m schemes of Anné et al. [2]. This revised version was published online in July 2006 with corrections to the Cover Date.