Geomagnetic effects on meteor train diffusion     

T.H. Webb 《Planetary and Space Science》1981,29(4):415-424

The diffusion of under-dense radio-meteors has been measured and found to be influenced by the Earth's magnetic field above 95 km. The observed behaviour is shown to be consistent with that expected theoretically. Measured meteor train drifts show no magnetic effect. Results of previous experiments on over-dense meteors also seem to confirm current theoretical models.  相似文献   

Re-inventing government: Emerging geographies in the aftermath of the 1984 reforms in New Zealand     

Gary L. Hermansson  Susan B. Webb 《GeoJournal》2004,59(2):91-92

Introduction

Re-inventing government: Emerging geographies in the aftermath of the 1984 reforms in New Zealand  相似文献   

Geophysical monitoring and evaluation of coastal plain aquifers   总被引：1，自引：0，他引：1  

Land LA  Lautier JC  Wilson NC  Chianese G  Webb S 《Ground water》2004,42(1):59-67

We use time domain electromagnetic (TDEM) soundings to monitor ground water conditions beneath the coastal plain in eastern North Carolina. The TDEM method measures the earth's response to an induced electromagnetic field. The resulting signal is converted, through a complex inversion process, to apparent resistivity values, which can be directly correlated to borehole resistivity logs. TDEM soundings are used to map the interface between fresh and salt water within coastal aquifers, and estimate depth to basement when siting new monitoring wells. Focused TDEM surveys have identified areas of salt water encroachment caused by high volumes of discharge from local supply wells. Electromagnetic sounding, when used in tandem with the state's network of monitoring wells, is an accurate and inexpensive tool for evaluating fresh water/salt water relationships on both local and regional scales within coastal plain aquifers.  相似文献   

Spectral analyses of solar photospheric fluctuations     

Frank N. Edmonds Jr.  Carol J. Webb 《Solar physics》1972,22(2):276-296

The application of the fast-Fourier-transform (FFT) algorithm to calculating one-dimensional and bi-dimensional (temporal and spatial), power and cross-power (coherence and phase) spectra is examined for solar photospheric fluctuations. Alternative methods for smoothing raw spectra, direct averaging (employing various weights) and indirect truncation of the correlation function, are compared, and indirect smoothing is compared with spectra calculated by mean-lagged-product (MLP) methods. Besides providing the raw spectrum, FFT techniques easily allow computing a series of spectra with varying amounts of smoothing. From these spectra a range of satisfactory compromise between resolution and stability can be determined which helps in the interpretation of spectral trends, and in identifying more clearly the existence and significance of spectral features. For bi-dimensional spectra presented as contour plots, this range of satisfactory smoothing can be restricted, particularly when spectral trends must be represented by small-scale contours. Equivalent spectra (i.e. comparable equivalent degrees of freedom) computed or smoothed by different methods have minor, but not negligible, differences. Examination of these differences favors computing of FFT spectra smoothed by averaging for photospheric fluctuations.  相似文献   

A Comparison of Charcoal and Archaeological Information to Address the Influences on Holocene Fire Activity in the Sydney Basin     

S. D. Mooney  M. Webb  V. Attenbrow 《The Australian geographer》2007,38(2):177-194

The influence of Aboriginal people on fire activity and hence the vegetation of Australia has long been debated. This study aimed to document the local fire activity of the Holocene in the catchment of a small freshwater reed swamp located in the Sydney Basin and to compare this with nearby archaeological evidence; including artefact discard rates and the number of base camps and activity locations used through time. This archaeological evidence was used as an index of human activity through time to assess anthropogenic influences on fire activity. Charcoal (>250 µm) was quantified in a radiocarbon-dated sediment core from Griffith Swamp covering ～6000 calibrated years BP. A substantial increase in fire activity was found from ～3000 years BP and a lesser increase approximately 700 years ago. The change in fire activity at ～3000 years BP was approximately coeval with changes in archaeological evidence from Upper Mangrove Creek, suggesting either greater human presence in the landscape or altered subsistence and land-use strategies. Fire frequency in the catchment of Griffith Swamp peaked at about eight episodes per century, perhaps in response to environmental change that promoted both increased human activity and a higher natural fire frequency. This study provides an extended temporal perspective on fire and humans in this landscape, demonstrating how palaeoecology can provide practical information for the contemporary management of such fire-prone ecosystems.  相似文献   

Calcite-filled borings in the most recently deposited skeleton in live-collected Porites (Scleractinia): Implications for trace element archives     

Luke D. Nothdurft  Gregory E. Webb  Llew Rintoul 《Geochimica et cosmochimica acta》2007,71(22):5423-5438

Skeletons of the scleractinian coral Porites are widely utilized as archives of geochemical proxies for, among other things, sea surface temperature in paleoclimate studies. Here, we document live-collected Porites lobata specimens wherein as much as 60% of the most recently deposited skeletal aragonite, i.e., the part of the skeleton that projects into the layer of living polyps and thus is still in direct contact with living coral tissue, has been bored and replaced by calcite cement. Calcite and aragonite were identified in situ using Raman microspectroscopy. The boring-filling calcite cement has significantly different trace element ratios (Sr/Ca_(mmol/mol) = 6.3 ± 1.4; Mg/Ca_(mmol/mol) = 12.0 ± 5.1) than the host coral skeletal aragonite (Sr/Ca_(mmol/mol) = 9.9 ± 1.3; Mg/Ca_(mmol/mol) = 4.5 ± 2.3). The borings appear to have been excavated by a coccoid cyanobacterium that dissolved aragonite at one end and induced calcite precipitation at the other end as it migrated through the coral skeleton. Boring activity and cement precipitation occurred concomitantly with coral skeleton growth, thus replacing skeletal aragonite that was only days to weeks old in some cases. Although the cement-filled borings were observed in only ∼20% of sampled corals, their occurrence in some of the most recently produced coral skeleton suggests that any corallum could contain such cements, irrespective of the coral’s subsequent diagenetic history. In other words, pristine skeletal aragonite was not preserved in parts of some corals for even a few weeks. Although not well documented in coral skeletons, microbes that concomitantly excavate carbonate while inducing cement precipitation in their borings may be common in the ubiquitous communities that carry out micritization of carbonate grains in shallow carbonate settings. Thus, such phenomena may be widespread, and failure to recognize even very small quantities of early cement-filled borings in corals used for paleoclimate studies could compromise high resolution paleotemperature reconstructions. The inability to predict the occurrence of cement-filled borings in coralla combined with the difficulty in recognizing them on polished blocks highlights the great care that must be taken in vetting samples both for bulk and microanalysis of geochemistry.  相似文献   

Using Field-Portable XRF to Assess Geochemical Variations Within and Between Dolerite Outcrops of Preseli, South Wales   总被引：1，自引：0，他引：1  

Michael C. Jones  Olwen Williams-Thorpe  Philip J. Potts  Peter C. Webb 《Geostandards and Geoanalytical Research》2005,29(3):251-269

Eight dolerite outcrops in Preseli, south Wales were measured in situ using field-portable XRF analysis, in order to compare two different analysis ("sampling") strategies, and to investigate geochemical variability within and between outcrops. A sampling strategy of two (neighbouring but independent) measurements at each of a maximal number of locations dispersed over an outcrop was the more effective in indicating the overall chemical variance of that outcrop. Analysis of variance indicated that much of the observed variance within individual outcrops stems from real geochemical variability rather than from sampling and analytical factors. Standard ANOVA F tests showed that several of the studied outcrops are heterogeneous at the 5% significance level for one or more elements. Geochemical distinctions between some outcrops were demonstrated using discriminant analysis. PXRF analysis offers an alternative approach to conventional characterisation of outcrops, which is often based on laboratory analysis of small numbers of samples. However, PXRF data are affected by rock weathering and may require correction for this if they are to be compared with analyses of fresh rock.  相似文献   

Past Global Changes and Their Significance for the Future   总被引：1，自引：0，他引：1  

Thompson Webb II 《Journal of Paleolimnology》2001,26(2):227-229

  相似文献   

Combining ERBE and ISCCP data to assess clouds in the Hadley Centre, ECMWF and LMD atmospheric climate models   总被引：3，自引：2，他引：3  

M. Webb  C. Senior  S. Bony  J.-J. Morcrette 《Climate Dynamics》2001,17(12):905-922

 This study compares radiative fluxes and cloudiness fields from three general circulation models (the HadAM4 version of the Hadley Centre Unified model, cycle 16r2 of the ECMWF model and version LMDZ 2.0 of the LMD GCM), using a combination of satellite observations from the Earth Radiation Budget Experiment (ERBE) and the International Satellite Cloud Climatology Project (ISCCP). To facilitate a meaningful comparison with the ISCCP C1 data, values of column cloud optical thickness and cloud top pressure are diagnosed from the models in a manner consistent with the satellite view from space. Decomposing the cloud radiative effect into contributions from low-medium- and high-level clouds reveals a tendency for the models' low-level clouds to compensate for underestimates in the shortwave cloud radiative effect caused by a lack of high-level or mid-level clouds. The low clouds fail to compensate for the associated errors in the longwave. Consequently, disproportionate errors in the longwave and shortwave cloud radiative effect in models may be taken as an indication that compensating errors are likely to be present. Mid-level cloud errors in the mid-latitudes appear to depend as much on the choice of the convection scheme as on the cloud scheme. Convective and boundary layer mixing schemes require as much consideration as cloud and precipitation schemes when it comes to assessing the simulation of clouds by models. Two distinct types of cloud feedback are discussed. While there is reason to doubt that current models are able to simulate potential `cloud regime' type feedbacks with skill, there is hope that a model capable of simulating potential `cloud amount' type feedbacks will be achievable once the reasons for the remaining differences between the models are understood. Received: 23 January 2000 / Accepted: 24 January 2001  相似文献   

Book reviews     

Gregory E. Webb 《Sedimentology》2001,48(6):1434-1438

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