Analysis of gravity source moment inversion. Part II: Usinga priori information about the source

The method of moment inversion, based on the approximation of the gravity anomaly by thetruncated series obtained from its multipole expansion, uses, implicitly,a priori information about the anomalous body. The series truncation imposes a regularizing condition on the equipotential surfaces (produced by the anomalous body), allowing the unique determination of some moments and linear combinations of moments that are the coefficients of the basis functions in the multipole expansion series. These moments define a class of equivalent distributions of mass. The equivalence criterion is based on the misfit between the observations and the field produced by the series truncated at a prefixed maximum order for the moments. The estimates of the moments of the equivalent distribution are shown to compose the stationary solution of a system of first-order linear differential equations for which uniqueness and asymptotic stability are guaranteed. Specifically for the series retaining moments up to second order, the implicita priori information introduced requires that the source have finite volume, be sufficiently distant from the measurement plane and that its spatial distribution of mass present three orthogonal planes of symmetry intersecting at the center of mass. Subject to these hypotheses, it is possible to estimate uniquely and simultaneously the total excess of mass, the position of the center of mass and the directions of the three principal axes of the anomalous body.     

Downscaling GCM information to regional scales: a non-parametric multivariate regression approach

The multivariate adaptive regression splines (MARS) model was used to capture the observed relationships between sea level pressure (SLP) anomalies over the Euro-Atlantic sector and the winter time (December–February) monthly rainfall at eight sites in Portugal; possible anthropogenic changes of the rainfall in a perturbed future climate were then estimated by using both the observed SLP-rainfall relationships, described by MARS models, and the GCM simulated SLP, taken from the output of the Hadley Centre Transient Climate Change Experiment (UKTR). Also, principal component analysis was carried out to reduce the dimensionality of the SLP data, and to assess the ability of the GCM in simulating the large-scale circulation; the first six principal components were retained as predictors in the MARS model. The MARS model were built up by using the data for 1946–1991 as the training set and that for 1901–1943 as the testing set, showing satisfactory prediction skills. It is concluded that the UKTR control simulation successfully reproduced main features of the large-scale circulation, but the observed relationship between SLP and the regional rainfall was not well preserved. With respect to the 54-year experiment of perturbed run, the MARS estimation of rainfall and the relevant direct GCM rainfall output possess similar multi-annual variations; however, there are substantial differences regarding details; the change in the area mean of winter time mean monthly rainfall in Portugal estimated by MARS (indirect GCM output) is about –12.7 mm per 54-year, and the relevant direct GCM output is –16.9 mm/54-year. This reduction tendency is consistent with previously reported findings respecting rainfall in the Iberian Peninsula, which were based on the MPI (Max-Planck Institute for Meteorology) transient simulations.     

Subduction erosion and accretion in the Solomon Sea region

The Solomon Sea region is an area of intense tectonic activity characterized by structural complexity, a high level of seismicity and volcanism, and rapid evolution of plate boundaries. There is little accretion in the eastern New Britain Trench. Accretion gradually increases westward with thick accretion in the western New Britain Trench and in the Trobriand Subduction System. The thick accretion in the western part of the New Britain Trench may be a result of collision from the north of Finisterre-Huon block with New Guinea mainland. The present boundary of the collision is along the Ram-Markham fault. Deformation structures and present day seismicity suggest that the northern block is under compression.

Accretion has occurred in the sediment filled trenches in the Solomon Sea. The scale of the accretionary wedge depends on the amount of trench-fill sediment available. It is unlikely that there is no sediment supply to the eastern part of the New Britain Trench where no accretion is observed and subduction erosion may be occurring. There are two possible mechanisms for subduction erosion of sediment; either a rapid rate of subduction relative to the supply of sediment inhibiting sediment accumulation in the trench; or horizontal tensional force superimposed on both the forearc and backarc regions of the arc. Seafloor spreading in both the Manus and Woodlark basins is fan-like with nearby poles in the western margins of the basins. This may be a reflection of a horizontally compressional field in the western part and a tensional field in the eastern part of the Solomon Sea. Therefore it is possible to conclude that the consumption of sediment in the eastern New Britain Trench is related to the horizontal tensional field superimposed on both the forearc and backarc regions of the subduction system.

Imbricated thrust and overthrust faults in the western New Britain Trench and Trobriand Trough are not linear over long distance, but form wavy patterns in blocks with unit distance of approximately 10 km.     

Climatic controls on Holocene lake-level changes in Europe

Sensitivity experiments with a simple water-balance model were used to constrain the possible climatic causes of distinct Holocene patterns of lake-level variation in different regions of Europe. Lakes in S Sweden were low at 9 ka, high around 6.5 ka, low again around 4 ka and are high now. Lakes in Estonia show similar but weaker trends. Lakes in S France were highest around 9 ka, lowest around 4 ka, intermediate now. Lakes in Greece were also high around 9 ka but continued rising until 7.5 ka, then fell gradually from 5 ka with a brief high phase around 3 ka, and are low now. The model was forced with insolation anomalies deduced from orbital variations, temperature anomalies inferred from the pollen record and cloudiness anomalies derived from changes in the position of the subtropical anticyclone (inferred from reconstructed changes in the equator-to-pole temperature gradient), in order to evaluate the effects of resultant evaporation changes on catchment water balance. The resulting simulated changes in runoff (precipitation minus actual evapotranspiration) were slight, and frequently opposite to the observed trends. Larger changes in precipitation are plausible and are required to explain the data. The required precipitation increase in N Europe from 9 ka (low) to 6 ka (high) is suggested by GCM experiments to have been a consequence of interacting insolation and residual ice-sheet effects on the atmospheric circulation over the North Atlantic. The explanation of other observed changes, including the drying trend during the Holocene in S Europe, has not been provided by GCM experiments to date. Explanations may lie in changes in mesoscale circulation, sea-surface temperature patterns and the coupling between these phenomena that may not follow orbital changes in any simple way. Correspondence to: SP Harrison     

Women in the Field: Critical Feminist Methodologies and Theoretical Perspectives*

Feminist scholars have traditionally emphasized the importance of incorporating “the everyday” worlds of women into the historically masculinist theoretical and empirical foundations of the social sciences. Such emphases have commonly resulted in smaller-scale research projects and more interactive kinds of research methods and methodologies. Feminist geographers have uniquely contributed to the body of feminist scholarship through drawing out the importance of place in everyday constructions of gender and, more recently, sexuality. Critical field-based research has therefore from the beginning been the mainstay of subdisciplinary research. Like the discipline as a whole, however, little explicit attention has been given in publications or pedagogically to the politics of fieldwork (including how a “field” is defined and the politics involved in choosing and working in a particular “field”) or the politics of representation (which includes considerations of the partiality of knowledge and how and to whom we represent our work, ourselves and others in various kinds of texts). These “Opening Remarks” show how these issues are addressed in the papers that follow and how feminist geography has much to contribute to critical analyses of global and multinational processes, including patriarchy, capitalism, and racism.     

“Ghana must Progress,but we are Really Suffering”: Bui Dam,Antipolitics Development,and the Livelihood Implications for Rural People

In late-2013, Bui Dam was commissioned on Ghana’s Black Volta River. The hydroelectric project inundated 444?km², flooding communities, forests, crops, and a national park. State elites promoted the dam through nationalistic discourses while simultaneously framing rural people responsible for accessing its trickle-down economic benefits. Drawing from critical development literature within a political ecology framework, this article examines tensions between discourses underpinning construction of Bui Dam and the lived experiences of rural people. Drawing from data collected in 82 interviews, the dam’s implications for resettlement, food security, mental health, agricultural production, and fishing livelihoods are detailed. I argue interlocking discourses, laws and processes—from eminent domain and compensation metrics to party politics—combined to work as an “antipolitics machine.” Hydroelectric development renders invisible structural processes shaping inequality and creates new injustices. As Bui Power Authority, the corporation managing the dam and surrounding land-uses has a new CEO, the article concludes with several practical suggestions for improving community members’ everyday lives.     

A Chart for Judging Optimal Sample Spacing for Ore Grade Estimation

Mineral deposit grades are usually estimated using data from samples of rock cores extracted from drill holes. Commonly, mineral deposit grade estimates are required for each block to be mined. Every estimated grade has always a corresponding error when compared against real grades of blocks. The error depends on various factors, among which the most important is the number of correlated samples used for estimation. Samples may be collected on a regular sampling grid and, as the spacing between samples decreases, the error of grade estimated from the data generally decreases. Sampling can be expensive. The maximum distance between samples that provides an acceptable error of grade estimate is useful for deciding how many samples are adequate. The error also depends on the geometry of a block, as lower errors would be expected when estimating the grade of large-volume blocks, and on the variability of the data within the region of the blocks. Local variability is measured in this study using the coefficient of variation (CV). We show charts analyzing error in block grade estimates as a function of sampling grid (obtained by geostatistical simulation), for various block dimensions (volumes) and for a given CV interval. These charts show results for two different attributes (Au and Ni) of two different deposits. The results show that similar errors were found for the two deposits, although they share similar features: sampling grid, block volume, CV, and continuity model. Consequently, the error for other attributes with similar features could be obtained from a single chart.     

Tectonic evolution of an intraplate basin: the Lower Tagus Cenozoic Basin,Portugal

This article focuses on the reinterpretation of well, seismic reflection, magnetic, gravimetric, surface wave and geological surface data, together with the acquisition of seismic noise data to study the Lower Tagus Cenozoic Basin tectono‐sedimentary evolution. For the first time, the structure of the base of the basin in its distal and intermediate sectors is unravelled, which was previously only known in the areas covered by seismic reflection data (distal and small part of intermediate sectors). A complex geometry was found, with three subbasins delimited by NNE‐SSW faults and separated by WNW‐ESE to NW‐SE oriented horsts. In the area covered by seismic reflection data, four horizons were studied: top of the Upper Miocene, Lower to Middle Miocene top, the top of the Palaeogene and the base of Cenozoic. Seismic data show that the major filling of the basin occurred during Upper Miocene. The fault pattern affecting Neogene and Palaeogene units derived here points to that of a polyphasic basin. In the Palaeogene, the Vila Franca de Xira (VFX) and a NNE‐SSW trending previously unknown structure (ABC fault zone) probably acted as the major strike‐slip fault zones of the releasing bend of a pull‐apart basin, which produced a WNW‐ESE to NW‐SE fault system with transtensional kinematic. During the Neogene, as the stress regime rotated anticlockwise to the present NW‐SE to WNW‐ESE orientation, the VFX and Azambuja fault zones acted as the major transpressive fault zones and Mesozoic rocks overthrusted Miocene sediments. The reactivation of WNW‐ESE to NW‐SE fault systems with a dextral strike‐slip component generated a series of horsts and grabens and the partitioning of the basin into several subbasins. Therefore, we propose a polyphasic model for the area, with the formation of an early pull‐apart basin during the Palaeogene caused by an Iberia–Eurasia plates collision that later evolved into an incipient foreland basin along the Neogene due to a NW‐SE to WNE‐ESE oriented Iberia–Nubia convergence. This convergence is producing uplift in the area since the Quaternary except for the Tagus estuary subbasin around the VFX fault, where subsidence is observed. This may be due to the locking or the development of a larger component of strike‐slip movement of the NNE‐SSW to N‐S thrust fault system with the exception of the VFX fault, which is more favourably oriented to the maximum compressive stress.     

Assessing homogeneous regions relative to drought class transitions using an ANOVA-like inference. Application to Alentejo, Portugal

In the present study, an ANOVA-like inference technique is used aiming at to assess if Alentejo, southern Portugal, could be considered a homogeneous region for drought management purposes. First, Alentejo was divided into four sub-regions according to latitude (north and south), and longitude (west and east). Inside each sub-region, 10 weather stations were considered. The time series of the Standardized Precipitation Index (SPI) were obtained for these stations using precipitation data for the period 1932–1999 (67 years). Contingency tables for the transitions between SPI drought classes were obtained for these time series. Loglinear models were fitted to these contingency tables to estimate the probabilities for drought class transitions. An ANOVA-like inference was applied considering the four sub-regions like treatments of a two way layout with two factors, latitude and longitude, each one with two levels, north and south, and west and east respectively. The weather stations of each sub-region were treated as replicates. Significant differences between west and east were found, that allowed to consider that Alentejo could be composed by two sub-regions.     

Geochemical evidence of hydrothermal recharge in Lake Baringo,central Kenya Rift Valley

Lake Baringo, a freshwater lake in the central Kenya Rift Valley, is fed by perennial and ephemeral rivers, direct rainfall, and hot springs on Ol Kokwe Island near the centre of the lake. The lake has no surface outlet, but despite high evaporation rates it maintains dilute waters by subsurface seepage through permeable sediments and faulted lavas. New geochemical analyses (major ions, trace elements) of the river, lake, and hot spring waters and the suspended sediments have been made to determine the main controls of lake water quality. The results show that evaporative concentration and the binary mixing between two end members (rivers and thermal waters) can explain the hydrochemistry of the lake waters. Two zones are recognized from water composition. The southern part of the lake near sites of perennial river inflow is weakly influenced by evaporation, has low total dissolved species (TDS), and has a seasonally variable load of mainly detrital suspended sediments. In contrast, waters of the northern part of the lake show evidence for strong evaporation (TDS of up to eight times inflow). Authigenic clay minerals and calcite may be precipitating from those more concentrated fluids. The subaerial hot‐spring waters have a distinctive chemistry and are enriched in some elements that are also present in the lake water. Comparison of the chemical composition of the inflowing surface waters and lake water shows (1) an enrichment of some species (HCO₃^?, Cl, SO₄^2?, F, Na, B, V, Cr, As, Mo, Ba and U) in the lake, (2) a depletion in SiO₂ in the lake, and (3) a possible hydrothermal origin for most F. The rare earth element distribution and the F/Cl and Na/Cl ratios give valuable information on the rate of mixing of the river and hydrothermal fluids in the lake water. Calculations imply that thermal fluids may be seeping upward locally into the lake through grid‐faulted lavas, particularly south of Ol Kokwe Island. Copyright © 2006 John Wiley & Sons, Ltd.