A canonical theory for short GPS baselines. Part IV: precision versus reliability

This contribution is the last of four parts and deals with the link between baseline precision and ambiguity reliability. It is shown analytically how and to what extent the baseline-ambiguity correlation is related to the gain in baseline precision, to the volume of the ambiguity search space, and to the impact of potential integer ambiguity biases. Also, an ambiguity DOP measure is introduced together with its closed-form formulae for the three different single-baseline models. Received: 16 July 1996 / Accepted: 14 November 1996     

EMPLOYMENT DATA AND THE CLASSIFICATION OF URBAN SETTLEMENTS

Classifications of urban settlements have traditionally used total employment data in place of basic (export-oriented) employment data. While urban analysis agree upon the superiority of basic employment data for these classifications, usually only total employment data are available. This paper uses the Arizona Community Data Set, a unique survey-generated body of information that describes the major economic characteristics of numerous Southwestern communities and includes both basic and total employment data, to explore the extent to which misclassification results when using total employment data.     

Late Quaternary sea‐level change and evolution of Belfast Lough,Northern Ireland: new offshore evidence and implications for sea‐level reconstruction

The interplay of eustatic and isostatic factors causes complex relative sea‐level (RSL) histories, particularly in paraglacial settings. In this context the past record of RSL is important in understanding ice‐sheet history, earth rheology and resulting glacio‐isostatic adjustment. Field data to develop sea‐level reconstructions are often limited to shallow depths and uncertainty exists as to the veracity of modelled sea‐level curves. We use seismic stratigraphy, 39 vibrocores and 26 radiocarbon dates to investigate the deglacial history of Belfast Lough, Northern Ireland, and reconstruct past RSL. A typical sequence of till, glacimarine and Holocene sediments is preserved. Two sea‐level lowstands (both max. ?40 m) are recorded at c. 13.5 and 11.5k cal a bp . Each is followed by a rapid transgression and subsequent periods of RSL stability. The first transgression coincides temporally with a late stage of Meltwater Pulse 1a and the RSL stability occurred between c. 13.0 and c. 12.2k cal a bp (Younger Dryas). The second still/slowstand occurred between c. 10.3 and c. 11.5k cal a bp . Our data provide constraints on the direction and timing of RSL change during deglaciation. Application of the Depth of Closure concept adds an error term to sea‐level reconstructions based on seismic stratigraphic reconstructions.     

The effect of weathering on the distribution of strontium isotopes in weathering profiles

The strontium isotope ratio in sea water has varied through geologic time owing to the input of strontium from rock weathering. To evaluate the possibility that $\text{Sr}{}^{87}\text{Sr}{}^{86}$ ratios might be altered during weathering, seven weathering profiles developed on Mesozoic arkoses located along the length of New Zealand were investigated. The rubidium-strontium-strontium isotope relations in these profiles give ‘isochron’ ages less than the ages of deposition of the arkoses. These ages appear to result from the weathering of a homogeneous source rock. The age calculated from the rubidium-strontium system (t′) is related to the original age (t) by the equation t′/t = (n ? 1)/n, where n is the ratio of the amounts of common and radiogenic strontium leached from the rock. Shales formed by the accumulation of these residual solids may inherit misleading isochron relationships which may not be erased during deposition or early diagenesis. The strontium which goes into solution and is transported to the sea is slightly less radiogenic than the strontium in the unweathered rock, while the residual clays may be much more radiogenic.     

Variable vegetation cover and episodic sand movement on longitudinal desert sand dunes

Longitudinal (linear) sand dunes of the Simpson and Strzelecki dunefields in eastern central Australia present a paradox. Low levels of activity today stand in contrast to luminescence dating which has repeatedly shown deep deposits of sand on dune crests dating to within the late Holocene. In order to investigate the nature of dune activity in the Simpson–Strzelecki dunefield, vegetation and sand mobility were investigated by detailed vegetation survey and measurement of rippled area and loose sand depth of dunes at three sites along a climatic gradient. The response of both vegetation and sand movement to inter-annual climate variability was examined by repeat surveys of two sites in drought and non-drought conditions. Projected plant cover and plant + crust cover were found to have inverse linear relationships with rippled area and the area of deep loose sand. No relationship was found between these measures of sand movement and the plant frontal area index. A negative exponential relationship between equivalent mobile sand depth on dune surfaces and both vascular plant cover and vascular + crust cover was also found. There is no simple threshold of vegetation cover below which sand transport begins. Dunes with low perennial plant cover may form small dunes with slip faces leading to a positive feedback inhibiting ephemeral plant growth in wet years and accelerating sand transport rates. The linear dunefields are largely within the zone in which plant cover is sufficient to enforce low sand transport rates, and in which there is a strong response of vegetation and sand transport to inter-annual variation in rainfall. Both ephemeral plants (mostly forbs) and crust were found to respond rapidly to large (> 20 mm/month) rainfall events. On millennial time-scales, the level of dune activity is controlled by vegetation cover and probably not by fluctuations of wind strength. Land use or extreme, decadal time-scale, drought may destabilise dunes by removing perennial plant cover, accelerating wind erosion.     

Kimberlites: Their relation to mantle hotspots

Available age data support the hypothesis that kimberlite intrusions are formed by mantle hotspots. The hypothesis has been tested by inverting the volcanic traces formed by three hotspots to determine the post-Triassic motions of Africa, South America, and North America relative to these hotspots. Then, using these motions, the kimberlites intruded on these continents within the last 150 m.y. are relocated to their place of origin in the present hotspot reference frame. The result indicates that a majority of the kimberlites formed within 5° of a mantle hotspot. Statistical analysis shows that this kimberlite/hotspot correlation is significant at above the 90% level.     

Geoelectric section of the Central Tien Shan: Sequential inversion of the magnetovariational and magnetotelluric data along the Naryn Line

The paper presents the results of 2D inversion of deep magnetotelluric (MT) and magnetovariational (MV) soundings along the Naryn Line. The method of partial (sequential) inversions is used. According to this method, at the first stage, magnetovariation responses are used for the localization of deep anomalies of electrical conductivity, and then the magnetotelluric sounding data are invoked to refine the structure of the host medium and the structural details in the upper part of the section. It is shown that this approach enables one to estimate the informativeness of separate components of the electromagnetic field, to reduce the distorting influence of the near-surface geoelectric inhomogeneities, and to increase the stability of the final solution of the inverse problem.     

The arrow of time

Why does time apparently fly one way, when the laws of physics are actually time-symmetrical? Paul Davies proposes a quantum solution, in the Whitrow Lecture 2004, given on 8 October 2004.