A revised estimate of the life span for Lake Nasser

A number of estimates of the potential life span of Lake Nasser located in Egypt and Sudan have been made in the past several years. Published estimates range from as little as 20 years (Sterling 1970) to over 1500 years (Makary 1982). The wide range of differing values is a function of many variables, including computation method, input data, and theoretical assumptions underlying the mathematical approach taken. Such a broad range of values is typical of sedimentation studies performed for areas that lack an adequate historic data base.One of the chief constraints to development of a reliable estimate the Lake Nasser's case has been lack of sufficient historical data on which to construct a useful data base. Enough information of this type has been collected since the reservoir's formation in 1964 to permit a realistic assessment of its life span. Granted the obvious need to determine the useful lifetime of this or any other reservoir, it is now possible to assess the reliability of the current official estimate for Lake Nasser. By applying appropriate modifications and additional data to the official estimate, an improved estimate was made.The time forecasted for filling of the reservoir by the official estimate is 362 years, somewhat less than the original design life of 500 years. By taking into account changes in the hydrological regime of the Nile after 1964 and another compaction factor, an estimate of 535 years was made. The additional time before filling estimated in the revision is significant for decision rules governing operation of the Aswan High Dam.     

Geophysical exploration for epithermal gold deposits

Epithermal gold deposits are highly variable in form, ranging from thin quartz veins to large disseminated deposits, and are located in a variety of geological environments. Consequently, they exhibit a wide range of geophysical signatures. Hydrothermal alteration accompanying these deposits causes pronounced changes in the physical properties of the rocks. Magnetic susceptibility and remanence decrease; the potassium content commonly increases causing an increase in radioactivity; the electrical resistivity changes by up to two orders of magnitude; and the density increases or decreases depending on the nature of the host rock and alteration processes.Airborne magnetic surveys are effective in delineating major structures which may control the locations of epithermal gold deposits, and also in detecting magnetic lows or “flat zones” resulting from the destruction of magnetite by hydrothermal alteration. Radiometric surveys may detect potassium enrichment accompanying this alteration. Filtering and image-processing techniques are particularly useful in enhancing magnetic and radiometric data to reveal subtle structures and alteration systems.Ground geophysical techniques can play an important in defining drill targets. Well controlled gravity surveys may assist in the delineation of major structures, basement highs and alteration zones. Buried conductive alteration systems can be defined by resistivity, electromagnetic and magnetotelluric methods. Resistive gold-bearing silicified zones and shallow quartz vein systems are commonly detectable by conventional galvanic or inductive resistivity techniques.Geophysical surveys over a number of epithermal gold deposits are presented, including Bimurra-Conway (northeast Queensland, Australia), Gold Ridge (Solomon Islands), Ohui (Coromandel Peninsula, New Zealand), McLaughlin (California, USA), Rhyolite Creek (Victoria, Australia) and Mt Aubrey (New South Wales, Australia).Provided there is a clear appreciation of survey objectives and a good understanding of the changes in physical properties which rocks undergo during hydrothermal alteration, geophysics can make a substantial contribution to epithermal gold exploration.     

Inversion of electromagnetic data: An overview of new techniques

This paper explores some of the newer techniques for acquiring and inverting electromagnetic data. Attention is confined primarily to the 2d magnetotelluric (MT) problem but the inverse methods are applicable to all areas of EM induction. The basis of the EMAP technique of Bostick is presented along with examples to illustrate the efficacy of that method in structural imaging and in overcoming the deleterious effects of near-surface distortions of the electric field. Reflectivity imaging methods and the application of seismic migration techniques to EM problems are also explored as imaging tools. Two new approaches to the solution of the inverse problem are presented. The AIM (Approximate Inverse Mapping) inversion of Oldenburg and Ellis uses a new way to estimate a perturbation in an iterative solution which does not involve linearization of the equations. The RRI (Rapid Relaxation Inverse) of Smith and Booker shows how approximate Fréchet derivatives and sequences of 1d inversions can be used to develop a practical inversion algorithm. The overview is structured to provide insight about the latest inversion techniques and also to touch upon most areas of the inverse problem that must be considered to carry out a practical inversion. These include model parameterization, methods of calculating first order sensitivities, and methods for setting up a linearized inversion.     

Heat flow in the Ozark Plateau, Arkansas and Missouri: relationship to groundwater flow

Heat flow values were calculated from direct measurements of temperature and thermal conductivity at thirteen sites in the Arkansas-Missouri Ozark Plateau region. These thirteen values are augmented by 101 estimates of heat flow, based on thermal conductivity measurements and temperature gradients extrapolated from bottom-hole temperatures. The regional heat flow profile ranges from 9 mW m⁻² to over 80 mW m⁻², but at least two distinct thermal regimes have been identified. Seven new heat flow determinations are combined with three previously published values for the St. Francois Mountains (SFM), a Precambrian exposure of granitic and rhyolitic basement rocks, average 47 mW m⁻². Radioactive heat production of 76 samples of the exposed rocks in the SFM averages 2.4 μW m⁻² and a typical continental basement contribution of 14 mW m⁻² is implied. Conversely, the sedimentary rock sequence of the plateau is characterized by an anomalously low heat flow, averaging approximately 27 mW m⁻². Groundwater transmissivity values that are based on data from 153 wells in deep regional aquifers demonstrate an inverse relationship to the observed heat flow patterns. The areas of high transmissivity that correspond to areas of low total heat flux suggest that the non-conservative vertical heat flow within the Ozark sedimentary sequence can be attributed to the effects of groundwater flow.     

The application of cellular automata to the erosion of landforms

Two approaches to modelling surface erosion are described which can be used to explain the development of geomorphological features. The first approach uses non-linear wave theory and shows how surface gradient discontinuities can arise as the erosion of an initially smooth surface proceeds. For isotropic erosion, surface shapes may be predicted by a simple application of the Huyghens wavefront construction and Snell's law at media boundaries. A second approach uses a computational cellular method. The general shapes predicted by the computer simulations are shown to agree with those determined from the wave approach. The application of the cellular method to problems of geomorphological interest are discussed.     

Rainfall estimation using raingages and radar — A Bayesian approach: 1. Derivation of estimators

Procedures for estimating rainfall from radar and raingage observations are constructed in a Bayesian framework. Given that the number of raingage measurements is typically very small, mean and variance of gage rainfall are treated as uncertain parameters. Under the assumption that log gage rainfall and log radar rainfall are jointly multivariate normal, the estimation problem is equivalent to lognormal co-kriging with uncertain mean and variance of the gage rainfall field.The posterior distribution is obtained under the assumption that the prior for the mean and inverse of the variance of log gage rainfall is normal-gamma 2. Estimate and estimation variance do not have closed-form expressions, but can be easily evaluated by numerically integrating two single integrals. To reduce computational burden associated with evaluating sufficient statistics for the likelihood function, an approximate form of parameter updating is given. Also, as a further approximation, the parameters are updated using raingage measurements only, yielding closed-form expressions for estimate and estimation variance in the Gaussian domain.With a reduction in the number of radar rainfall data in constructing covariance matrices, computational requirements for the estimation procedures are not significantly greater than those for simple co-kriging. Given their generality, the estimation procedures constructed in this work are considered to be applicable in various estimation problems involving an undersampled main variable and a densely sampled auxiliary variable.     

地面对晴空大气热辐射结构的影响

本文利用1987年美国FIFE资料及1986年6月TIPMEX资料,并利用大气长、短波一维辐射传输数值模式,计算了晴空大气状态下地面辐射参数(地面热辐射温度、地表反照率等)对大气长波冷却率、短波加热率廓线的影响。结果表明,地球表面对大气边界层内的热辐射结构影响很大,特别是在高原地区这种影响尤其显著。     

Kinematic interpretation of remnant en-echelon rift segments of the southwestern Japan Sea

The southwestern Japan Sea and the margin of southwestern Japan feature parallel elongate extensional sub-basins and grabens. These structures are oblique to the trend of southwestern Japan and are interpreted as the remnant of an en-echelon array. Such a right-stepping remnant en-echelon pattern indicates sinistral rifting of the southwestern Japan block from Asia. The major extensional fault trends are parallel over a distance of 600 km. Infinitesimal kinematic relations show that it is unlikely that these faults initiated in response to rotation about the nearby pole determined from paleomagnetic studies. Rather, an earlier stage of rifting about a distant pole is indicated. The rotational pole of this early rifting was probably located at least 6000 km away from the rift and the pole of rotation was offset from the rift trend by a similar distance. Rifting about this distant rotational pole loosened the southwestern Japan continental block from the Asian mainland making it susceptible to a large rotation about a nearby pole.     

A note on the characteristics and possible origins of desert varnishes from Southeast Morocco. A reply to comments by R. I. Dorn

This reply to Dorn (1989) argues that a biotic origin is not the only way in which desert varnishes may be formed. It is suggested that a variety of initial conditions may result in similar end products. It is agreed that the dissimilar views of the authors may result from their different philosophies concerning the classification of Mn/Fe varnishes.