influence de l effet d echelle de l heterogeneite et de la connexite des aquiferes fissures sur le comportement des forages en zone de socle afrique de l ouest abstract pumping tests executed at an experimental station in ivory coast equipped with drill holes in fresh rock 37 61 m and 23 in the weathered zone at different depths 5 15 m permitted the evaluation of geometrical and hydraulical parameters of a fissured aquifer in granite the interpretation of these results by methods referring to equivalent porous media or single fracture models showed the improtance of the scale effect in relation with fracture heterogeneity in discontinous media and the interest of the representative elementary volume r e v concept to explain the observed phenomena

the effect of the subglacial water pressure on the sliding velocity of a glacier in an idealized numerical model in order to interpret observed short term variations of the sliding velocity of a glacier the effect of a variable subglacial water pressure on the sliding velocity has been studied using an idealized numerical model in particular the transient stages of growing or shrinking water filled cavities at the ice bedrock interface were analysed it was found that the sliding velocity was larger when cavities were growing than when they had reached the steady state size for a given water pressure the smallest sliding velocities occurred while cavities were shrinking when cavitation is substantial a small drop of water pressure below the steady state value e g by 0 5 bar can temporarily cause backward sliding a limiting water pressure at which sliding becomes unstable is derived the consequences of more realistic assumptions than those of the model are discussed

the duration of the active phase on surge type glaciers contrasts between svalbard and other regions many glaciers in svalbard and in other glacierized areas of the world are known to surge however the time series of observations required to assess the duration of fast motion is very restricted data on active phase duration in svalbard come from aerial photographs satellite imagery field surveys and airborne reconnaissance evidence on surge duration is available for eight svalbard ice masses varying from 3 to 1250 km 2 worldwide active phase duration is recorded for less than 50 glaciers few observations are available on high polar ice masses the duration of the active phase is significantly longer for svalbard glaciers than for surge type glaciers in other areas from which data are available in svalbard the active phase may last from 3 to 10 years by contrast a surge duration of 1 2 years is more typical of ice masses in northwest north america iceland and the pamirs ice velocities during the protracted active phase on svalbard glaciers are considerably lower than those for many surge type glaciers in these other regions mass is transferred down glacier more slowly but over a considerably longer period svalbard surge type glaciers do not exhibit the very abrupt termination of the active phase over periods of a few days observed for several alaskan glaciers the duration of the active phase in svalbard is not dependent on parameters related to glacier size the quiescent phase is also relatively long 50 500 years for svalbard ice masses detailed field monitoring of changing basal conditions through the surge cycle is required from surge type glaciers in svalbard in order to explain the significantly longer length of the active phase for glaciers in the archipelago which may also typify other high polar ice masses the finding that surge behaviour in the form of active phase duration shows systematic differences between different regions and their environments has important implications for understanding the processes responsible for glacier surges

a digital simulation model for the unsteady state radial flow to a dug well abstract a digital simulation model has been presented for the solution of the unsteady state radial flow to a large diameter dug well perforating the full saturated thickness of an unconfined aquifer the numerical solution is based on the finite difference approach the computational algorithm is an iterative version of the alternating direction implicit method the time variant discharge from the aquifer and the reduction in saturated thickness due to water withdrawal during both the abstraction and recovery phases have been suitably incorporated in the simulation model a sensitivity analysis has been carried out on the model parameters of the aquifer namely lateral permeability anisotropy specific storage and specific yield the results indicate that the model response of hydraulic heads in the aquifer is mainly sensitive to variations in lateral permeability and specific yield alone the simulation model has also been applied to analyse data from well pumping tests the particular combination of parameters characterizing the aquifer at the test site have been identified by a parameter adjustment procedure

transient flow to a large diameter well in an aquifer with storative semiconfining layers the hantush theory of leaky aquifers with storge in the semiconfining layers is combined with large diameter well theory to produce equations that can be used in the analysis of pumped well and observation well data for stratified formations included in the equations are storage in the pumped well and a linear resistance to flow at the sand face or well bore skin three cases proposed by hantush are considered these depend upon whether the upper boundary of the overlying semiconfining layer or the lower boundary of the underlying semiconfining layer are constant head or no flow boundaries laplace transform solutions valid for the complete time domain are given for each of the three cases for the hydraulic head in the pumped well the aquifer and each of the semiconfining layers type cures obtained by numerical inversion are selected to illustrate the effects of well bore storage well bore skin and leakage although several dimensionless parameters are involved these parameters tend to influence the character of different portions of the type curves suggesting that unique matches are possible the type curves show that well bore storage in a large diameter well may completely obliterate effects of leakage derived from compressible storage in semiconfining layers for the purposes of aquifer testing it may be possible to reduce the magnitude of well bore storage in a large diameter well and thus reveal the presence of leaky semiconfining layers this may help to prevent erroneous interpretation of the well test data and incorrect evaluation of the aquifer parameters

rosetta s comet 67p churyumov gerasimenko sheds its dusty mantle to reveal its icy nature the rosetta spacecraft has investigated comet 67p churyumov gerasimenko from large heliocentric distances to its perihelion passage and beyond we trace the seasonal and diurnal evolution of the colors of the 67p nucleus finding changes driven by sublimation and recondensation of water ice the whole nucleus became relatively bluer near perihelion as increasing activity removed the surface dust implying that water ice is widespread underneath the surface we identified large 1500 square meters ice rich patches appearing and then vanishing in about 10 days indicating small scale heterogeneities on the nucleus thin frosts sublimating in a few minutes are observed close to receding shadows and rapid variations in color are seen on extended areas close to the terminator these cyclic processes are widespread and lead to continuously slightly varying surface properties

evidence for extreme variations in the permeability of laterite from a detailed analysis of well behaviour in nigeria laterite soils are widespread in tropical africa and have a large impact on the hydrology of the areas they cover the permeability of laterite helps determine the partitioning of runoff and interflow and regulates groundwater recharge to underlying bedrock groundwater within laterite also forms a widespread source of drinking water typically from unimproved hang dug wells despite its importance there is little published information on laterite aquifer properties in this study data from a 6 m deep well in nigeria have been analysed to characterise the hydraulic conductivity of the laterite from repeated pumping tests transmissivity measurements from 40 tests spread out across a hydrological year varied from 0 1 to 1000 m2 d further interpretation of the data demonstrate a strong non linear decrease in horizontal hydraulic conductivity with depth characterised by an upper horizon of extreme permeability 400 m d and a much lower permeability profile beneath 0 1 m d these data are substantiated with observations from other wells throughout the area this non linear permeability structure has several implications the upper laterite can facilitate rapid lateral throughflow in the wet season enabling contaminants to be transported significant distances up to 1 km natural groundwater levels are restricted to a narrow range for much of the year and in the dry season the lower permeability of the deeper laterite restricts the amount of water which can be abstracted from shallow wells leading to well failure the work highlights the need for a wider study to better understand laterite soils and the role they play in regional hydrology 2013 natural environment research council hydrological processes published by john wiley sons ltd

flow regime associated with partially penetrating large diameter wells in hard rocks abstract in case of a large diameter well which penetrates an aquifer only partially significant vertical flows may set in as a result of withdrawal from the well provided of course the vertical permeability below the well bottom is appreciable a finite difference model of groundwater flow regime associated with a partially penetrating well in a hard rock terrain has been presented use of this model in the estimation of hydrogeological parameters in such a situation has been illustrated by a field example the analysis of pump test data shows that parameter estimation will be in error if the vertical flows are neglected the same model has also been used for prognosing the efficacy of a borewell in a large diameter partially penetrating well to increase the yield

the evolution of gently sloping mantled deposits on comet 67p churyumov gerasimenko abstract we investigated the role of local illumination and material parameters that can be important in determining evolution of a gently inclined slopes on the nucleus of comet the considered material parameters are size of ice grains agglomerates and pores in between thermal conductivity of dust mantle size of pores in the dust mantle presence of organic admixtures in water ice difference of bonds between agglomerates composing the dust mantle and bonds in the underlying ice dust material thickness of the layer of dust the simulations were performed for the selected locations in the north part of the small lobe of the comet we have found that the rate of erosion is sensitive to the granulation of material that is not the case in the region hatmehit kossacki and czechowski 2018 the erosion may lead to the formation of small scale pits

numerical analysis of large diameter wells with a seepage face abstract a methodology is outlined for estimating the aquifer parameters from pumping tests in large diameter wells taking into account the dynamic seepage face a general expression for the seepage face which is the difference between the well water level and the phreatic surface at the well face is deduced from experimental observations a numerical model is used to simulate both the pumping and recovery phases of the test both the drawdown in the pumped well and the flow from the aquifer to the well are used in matching the model and field results examples are used which show that if the seepage face is ignored in the analysis the value of the hydraulic conductivity is underestimated

analytical solution for transient flow into a hemispherical auger hole abstract an explicit analytic solution for a transient groundwater flow to an auger hole of hemispherical shape is found on the base of ideas and under the same assumptions as in the original paper of kirkham and van bavel 1948 theory of seepage into auger holes soil sci soc am proc 13 75 82 the legendre polynomial expansions are used to solve the laplace equation with a combination of seepage face and equipotential conditions along the hole contour a type curve for increasing water level in an initially empty hole is plotted which allows for a fast determination of the hydraulic conductivity from the observation of filling of a small hole tapping a shallow water table time decreasing total flow rates velocities particle trajectories a capture zone of the hole are calculated using the wolfram mathematica software

recovery in large diameter wells a closed form approximate analytical solution for recovery in fully penetrating large diameter wells tapping a confined aquifer is presented the solutions are also applicable for unconfined flow under conditions of small drawndown the analysis predicts exponential decreases of inflow rate a transient logarithmic recovery response and an asymptotic increase of comulative recovery with time the solution is verified against the field recovery response in a wide range of geological aquifer formations e g fissured rock lateritic as well as sandy alluvial deposits the predicted response and the actual field response for all the types of aquifers mentioned are found to match extremely well the available theoretical time recovery relationship for unconfined flow conditions approaches to the present theory of confined aquifer under smaller drawdown

evolution of variegated glacier alaska u s a prior to its surge on presente l evolution saison apres saison des changements dans la geometrie et la vitesse du glacier et on tente de rattacher ces variations a la deformation de la glace et au glissement basal

analysis of pump test data for a large diameter well near a hydrogeological boundary abstract a numerical method is presented for the interpretation of pump test data for a large diameter well situated near such a boundary as a river a lake or a dyke which commonly occur in hard rock terrains the technique considers the time drawdown data for the pumping as well as for the recovery phase a field example is presented to illustrate the use of the technique the computer code for numerical modeling is given in basic language which can easily be implemented on a portable microcomputer for the estimation of aquifer parameters in the field

pumping tests using large diameter observation wells abstract the drawdown in a large diameter observation well during a pumping test using a production well of negligible diameter is shown to be identical to the drawdown that would be observed if the roles of the wells were reversed the drawdown is therefore given by the well function for a large diameter well

hydrogeological parameter estimation from pump tests on a large diameter well abstract a versatile and easy method using numerical modeling is presented for the estimation of aquifer parameters from pump tests on a large diameter well the method is applicable even when the abstraction rate is not constant during the pump test all observational data during the pumping and recovery phases are used in the estimation of the aquifer parameters a field example of the pump test carried out on a large diameter well in granitic terrain is presented to illustrate the method the computer program written in basic is also given it may be executed on a hand held calculator and aquifer characteristics can be determined in the field itself

the morphological diversity of comet 67p churyumov gerasimenko images of comet 67p churyumov gerasimenko acquired by the osiris optical spectroscopic and infrared remote imaging system imaging system onboard the european space agency s rosetta spacecraft at scales of better than 0 8 meter per pixel show a wide variety of different structures and textures the data show the importance of airfall surface dust transport mass wasting and insolation weathering for cometary surface evolution and they offer some support for subsurface fluidization models and mass loss through the ejection of large chunks of material

slug tests in fissured aquifers a simple model of a slug test in a fissured aquifer has been studied in order to gain insight into the difficulties of interpreting data from such tests the model is used to estimate the errors that would result from applying the standard type curve analysis to fissured aquifers it is found that the derived aquifer transmissivity will always be overestimated but by a factor which is unlikely to exceed 3 by contrast the derived storage coefficient can be in error by a factor ranging from 10 6 to 105 the volume of the aquifer influenced by a slug test is investigated and a simple formula for estimating the range of a test is suggested

estimation of hydraulic conductivity in large diameter hand dug wells using slug test methods abstract to demonstrate the viability of using slug test analysis techniques to interpret recovery data in large diameter hand dug wells 37 wells representing a range of hydraulic conductivity and located in unconfined and weathered chalk and marl and confined alluvium in north central texas were tested recovery data were analyzed by means of a numerical model two semiempirical methods developed especially for interpreting recovery data from large diameter hand dug wells and different slug test analysis methods that have different shape factors describing well geometry most hydraulic conductivity values estimated by slug test methods are within an order of magnitude of values that are estimated by using a numerical model or methods derived specifically for hand dug wells because of the unique geometry of hand dug wells diameters on the same order as well lengths care must be taken to ensure that the proper shape factors are used with slug test methods because of the substantial well storage in hand dug wells considerable time may be required to lower the water level to the desired position especially if pump capacity is limited this pumping time results in a noninstantaneous withdrawal of water from the aquifer which violates one of the assumptions of slug test interpretation methods however in low permeability environments reasonable results can still be attained if water removed from the well sources from well storage or if recovery time is considerably longer than pumping time

drawdown at a large diameter observation well abstract a generalized discrete kernel approach has been used to analyse the effect of both production well storage and observation well storage on drawdown at any point in the aquifer during pumping and recovery non dimensional time drawdown plots have been presented for four different combinations of a production well and an observation well which may or may not have storage the non dimensional time drawdown plots include the response of the aquifer during the recovery phase the contribution from the observation well storage to the aquifer during pumping and the replenishment of the observation well storage during recovery have been presented for specific cases

the determination of glacier speed by time lapse photography under unfavorable conditions two practical problems in the use of time lapse photography for the measurement of speed were encountered during the recent surge of west fork glacier in the central alaska range alaska u s a the first is severe rotational camera instability we show how natural unsurveyed features of the valley wall can be used to make the necessary corrections the second problem is the computation of absolute speed when many different unsurveyed glacier surface features are used as targets