stress magnitude and orientation in the potiguar basin brazil implications on faulting style and reactivation 1 we analyzed borehole breakout data and drilling induced tensile fractures derived from resistivity image logs run at 10 oil wells to derive the orientation of the maximum horizontal stress shmax from the potiguar basin in the continental margin of brazil stress magnitudes are derived from density logs for the vertical stress mini frac tests for the minimum horizontal stress shmin and rock strength laboratory analysis to estimate the shmax magnitudes we compared these results with the stress regime and shmax orientation derived from nine earthquake series located in the crystalline basement where seismicity is concentrated and previous breakout data from the basin in the basin the shmax gradient is 20 0 mpa km and the shmax shmin ratio is 1 154 indicating a normal tectonic stress regime from 0 5 to 2 0 km whereas the shmax gradient of 24 5 mpa km and shmax shmin ratio of 1 396 indicate a transition from a normal to strike slip stress regime at 2 5 to 4 0 km the deeper stress regime in the basin is similar to that in the basement at 1 12 km depth this transition of the tectonic stress regime is consistent with an incipient tectonic inversion in the basin we note that the shmax orientation rotates from nw se in the western part of the potiguar basin to e w in its central and eastern parts roughly following the shoreline geometry indicating that local features such as flexural stresses influence the local scale 100 km stress pattern we also conclude that the basement is critically stressed but not the basin

effects of geomechanical changes on the validity of a discrete fracture network representation of a realistic two dimensional fractured rock abstract this paper aims to examine the validity of the discrete fracture network dfn method in representing a realistic two dimensional fractured rock in terms of their geomechanical response to in situ stresses and hydraulic behaviour in a steady state fluid field first a real fracture network is extracted from the geological map of an actual rock outcrop which is termed the analogue fracture network afn multiple dfn realisations are created using the statistics of the analogue pattern a conductivity parameter that was found to have a linear relationship with the conductivity of 2d fracture networks is included to further enhance network similarity a series of numerical experiments are designed with far field stresses applied at a range of angles to the rock domains and their geomechanical response is modelled using the combined finite discrete element method femdem a geomechanical comparison between the afn and its dfn equivalents is made based on phenomena such as heterogeneity of fracture dependent stress contours sliding between pre existing fracture walls coalescence of propagating fractures and variability of aperture distribution furthermore an indirect hydro mechanical hm coupling is applied and the hydraulic behaviour of the porous rock models is investigated using the hybrid finite element finite volume method fefvm a further comparison is conducted focusing on the hydraulic behaviour of the afn and dfns under the effects of geomechanical changes the results show that although dfns may represent an afn quite well for fixed mechanical conditions such a representation may not be dependable if mechanical changes occur

do swarms of migrating barchan dunes record paleoenvironmental changes a case study spanning the middle to late holocene in the pampa de jaguay southern peru swarms of barchan dunes are common in arid environments unlike immobile or slowly moving dunes whose stratigraphy can be used to reconstruct paleoenvironmental changes the high migration rate of barchans seems to prohibit their use as geoarchives in this paper a possible use of barchan swarms for paleoenvironmental reconstructions is presented the approach is based on dune migration in a case study based on 500 dunes in the pampa de jaguay aeolian transport corridor in coastal southern peru middle to late holocene changes in the supply of aeolian material are inferred from calculating the time of dune initiation at the coastline limitations posed by the assumptions underlying this approach are discussed

a simple model for the skewness of global sea surface winds abstract a strong linear relationship between the mean and skewness of global sea surface winds both zonal and meridional is shown to exist such that where the wind component is on average positive it is negatively skewed and vice versa this relationship is observed in reanalysis satellite and buoy data this relationship between the mean and skewness fields of sea surface winds follows from the nonlinear surface drag predicted for a turbulent boundary layer by monin obukhov similarity theory since forcing perturbations speeding the wind up are subject to a stronger drag force than perturbations slowing it down furthermore it is demonstrated that the results of an empirical fit of observed surface winds to a stochastic differential equation presented in a recent study by sura are consistent with the white noise limit of the momentum equations for a turbulent boundary layer subject to fluctuating forcing albeit with a somewhat different physical interpretation

practical approaches to identifying sealed and open fractures for one essential ingredient of permeable fracture networks degree of fracture pore space preservation in large fractures i show how the characterization challenge presented by sparse fracture sampling can be overcome by measuring a surrogate the abundance of rock mass cement that precipitated after fractures ceased opening sampling limitations are overcome because the surrogate is readily measured in small rock samples including sidewall cores and cuttings permitting site specific diagnosis of the capacity of fractures to transmit fluid over a wider range of sample depths than conventional methods allow a diverse core database shows that this surrogate correctly predicts where large fractures are sealed information on timing of fracture opening relative to cement sequence can be obtained in two ways first evidence of fracture movement history and cement sequences in sparse large fractures can be extrapolated to areas having only cement data alternately evidence of fracture timing can be acquired from sealed micrometer scale fractures distribution of porosity reducing cement is commonly heterogeneous from bed to bed and location to location in siliciclastic and carbonate rocks however because patterns of sealed or open fractures cannot be delineated using fracture observations alone surrogates have practical value for production fairway mapping and other applications in which identifying open fractures is essential this study highlights the vital interplay among structural and diagenetic processes for fracture porosity preservation or destruction

morphological characteristics and topographic thresholds of gullies in different agro ecological environments abstract gully erosion is an important sediment source and causes severe environmental degradation particularly in the drought prone regions of ethiopia we investigated the morphological characteristics of gullies the topographic thresholds of gully formation and estimated headcut retreat rates in three agro ecologies of the upper blue nile basin ethiopia highland guder midland aba gerima and lowland dibatie gully morphological characteristics were analyzed using data measured in the field whereas the topographic thresholds were estimated using slope and upslope drainage area average annual linear rl and volumetric ve headcut retreat rates were estimated by visual interpretation of very high resolution 0 5 1 5 m satellite images quickbird ikonos spot 7 and pleiades in a gis environment a significant power relationship fitted between gully volume v and length l for the three sites is v 8 097 l1 032 r2 0 902 n 94 the average annual rl ve and soil loss rates were estimated at 0 76 m yr 1 6 77 m3 yr 1 and 8 73 t ha 1 yr 1 in guder 2 09 m yr 1 19 58 m3 yr 1 and 20 76 t ha 1 yr 1 in aba gerima and 3 42 m yr 1 42 16 m3 yr 1 and 49 33 t ha 1 yr 1 in dibatie the higher gully headcut retreat rate in dibatie is mainly related to the highly erodible nature of vertisols the coefficients of the slope s drainage area a relationship s aa b were a 0 219 and b 0 139 in guder a 0 133 and b 0 234 in aba gerima and a 0 113 and b 0 216 in dibatie indicating that topographic thresholds for gully initiation varied among the agro ecologies the results of this study can be used to estimate gully erosion rates and identify areas for gully initiation thereby supporting the planning of appropriate gully control measures in the study sites and other areas with similar environmental settings

dust raising in the dustiest place on earth the bodele in northern chad is the dustiest place on earth it is dusty because of the co location of strong near surface winds and a large exposure of low density friable diatomite data are reported from three kinds of observation on dust generation in the bodele a two automatic weather stations and pilot balloon tracking complemented with reanalysis data measuring and estimating wind velocities b sun photometers measuring atmospheric dust concentrations c a vacuum pump dust sampler and an array of mwac sediment samplers measuring in situ sediment transport with these data we develop four hypotheses about dust generation 1 the pulsing of dust output from the bodele depends primarily on the varying strength of a low level jet llj i e at this scale dust production is transport limited 2 the most prolific mechanism of dust production is the breakdown of saltating diatomite flakes as they collide with each other and with the surface 3 disturbance of flow around the large barchans particularly turbulence and convergence on their horns is a major contributor to dust production i e the dunes are dust mills 4 in consequence dust production is localised within the main exposure of diatomite to places where there are the most dunes as on the eastern and northern fringes of the exposure

extent of power law scaling for natural fractures in rock new data sets from natural faults and extension fractures exhibit simple power law scaling across 3 4 4 9 orders of magnitude regardless of rock type or movement mode the data show no evidence of natural gaps or scaling changes each data set consists of independent measurements made at different observational scales a power law regression to the subset of smaller fractures in each case provides an extrapolation that accurately predicts associated larger fractures consequently data representing a limited range of fracture sizes may be used to characterize a much broader spectrum of fracture sizes

imaging 3 d fracture networks around boreholes a new method has been developed for building and visualizing three dimensional 3 d subsurface natural fracture networks in rocks surrounding boreholes using image logs such as cast emi fmi fms rosi and others the method correlates fracture patterns with different stages of fracture network development for individual sets of fractures and extrapolates fracture density and connectivity from one dimension 1 d in boreholes to 3 d in the surrounding rocks the application of this work is 3 d visualizations of fracture distributions in volumes close to the boreholes for well planning reservoir scale fracture model building reservoir flow simulation and hydraulic fracture control

pattern classification of typhoon tracks using the fuzzyc means clustering method abstract a fuzzy c means clustering method fcm is applied to cluster tropical cyclone tc tracks fcm is suitable for the data where cluster boundaries are ambiguous such as a group of tc tracks this study introduces the feasibility of a straightforward metric to incorporate the entire shapes of all tracks into the fcm that is the interpolation of all tracks into equal number of segments four validity measures e g partition coefficient partition index separation index and dunn index are used objectively to determine the optimum number of clusters this results in seven clusters from 855 tcs over the western north pacific wnp from june through october during 1965 2006 the seven clusters are characterized by 1 tcs striking the korean peninsula and japan with north oriented tracks 2 tcs affecting japan with long trajectories 3 tcs hitting taiwan and eastern china with west oriented tracks 4 tcs passing the east of japan with early recurving tracks 5 tcs traveling the easternmost re

assessment of gully erosion in eastern ethiopia using photogrammetric techniques abstract eastern ethiopia is severely affected by soil erosion as it is revealed by the commonly occurring deep and wide gullies these gullies are dynamically expanding into agricultural lands at an alarming rate they are also the chief sources of sediment causing siltation of lakes that are valuable sources of both drinking and irrigation water in the alemaya and harar areas a quantitative assessment of gully dynamics and the associated soil loss and a prognosis of areas susceptible to further gullying were deemed crucial aerial photographs from two different dates 1966 and 1996 with approximate scales of 1 45 000 were used to study the development and dynamics of gullies as indicated by volume changes between the two dates they were digitized at a pixel size of 20 m digital elevation models dem with grid widths of 20 and 2 m were derived from stereo image pairs separately gully breaklines and borderlines were measured for the identified gully systems gully profiles were positioned at regular intervals of 20 m along the gully length giving rise to gully sections that are portions of a gully between neighbouring profiles gully volume was computed for every gully section soil loss was estimated from the computed gully volumes using soil bulk density it showed that soil loss per unit of gully surface is very high 1 7 tons m 2 of gully surface over the 30 years time period indicating the severity of gully erosion in the subcatchment there was significant agreement between breakline measured and field measured gully depth and width for selected points this is an indication of the potential of the method for reliable gully volume estimation and hence quantification of soil loss due to gully erosion the possibility of predicting hill slope areas susceptible to further gullying based on the topographic attributes such as specific contributing area and local slope from an interpolated detailed dem with 2 m resolution was tested it was found that wetness index alone or a combination of the wetness index and the compound topographic index at the threshold values established in literature would give better results as viewed from better connectivity of flow lines and higher conformity of the predicted susceptible areas to natural gully headcut and expansion in a landscape reality

mechanical constraints on the chronology of fracture activation in folded devonian sandstone of the western moroccan anti atlas the three dimensional meter scale fracture networks observed on exposed folds between the towns of tata and akka western moroccan anti atlas consist mostly of planar discontinuities which are sub perpendicular to the bedding and partitioned in three main sets the chronology of their activation is proposed in five stages since the hercynian orogeny stage 1 predates folding and involves the horizontal compression of the emsian sandstone it involves fracture set i composed of systematic joints parallel to the direction of compression stages 2 4 correspond to the folding and are marked in the outer arc by the activation of fracture set ii composed mainly of joints parallel to the fold axial plane stage 5 is a regional shear event during which sets i and iii separated by an angle close to 608 are activated in a conjugate manner to throw light on the recurrent difficulty in discriminating between activation of inherited and new fractures an elasto plastic model is used to construct a stress path in the pervasively fractured medium idealized as a continuum each fracture set obeys the mohr coulomb criterion truncated in tension to describe both sliding and opening activations finite element simulations of a simple buckling event accounting for the field fracture sets are presented it is shown that set i cannot be generated by folding and thus does belong to stage 1 set ii is activated at a later stage of folding than expected from the field interpretation set iii cannot be activated during stage 2 confirming its role in stage 5 the advantages and limitations of the proposed modeling are finally discussed q 2002 elsevier science ltd all rights reserved

inter well scale natural fracture geometry and permeability variations in low deformation carbonate rocks regional natural fracture networks often show variations on a scale below that captured by seismic reflection data this variability is not considered in most reservoir models but likely impacts uncertainties in permeability we quantify this uncertainty using a database of 13 000 fractures in nine outcrops digitised in the carbonate jandaira formation potiguar basin brazil distance between outcrops is on average 11 km with a minimum of 300 m which is comparable to the distance between wells in naturally fractured reservoirs in between outcrops significant variations exist in orientation intensity length and topology using discrete fracture matrix flow models we model the permeability of each deterministic pattern and find that small changes in geometry and topology result in permeability variations that are not captured by connectivity based analyses such as percolation probabilities particularly when the matrix is permeable the permeability variations associated with subseismic scale fracture variability are not captured in conventional stochastic models but can be captured using deterministic outcrop models with flow through discrete fractures the deterministic models provide a permeability range associated with subseismic fracture variability that can be assigned to grid cells of fractured reservoir flow models as an alternative to assuming constant permeability in the absence of subseismic scale deformation

is enso a cycle or a series of events 1 after early ideas that saw el ninos as isolated events the advent of coupled models brought the conception of enso as a cycle in which each phase led to the next in a self sustained oscillation twenty two years of observations that represent the el nino and la nina peaks east pacific sst and the memory of the system zonal mean warm water volume suggest a distinct break in the cycle in which the coupled system is able to remain in a weak la nina state for up to two years so that memory of previous influences would be lost similarly while the amplitude of anomalies persists from the onset of a warm event through its termination there is no such persistence across the la nina break these observations suggest that el ninos are in fact event like disturbances to a stable basic state requiring an initiating impulse not contained in the dynamics of the cycle itself

a scale independent approach to fracture intensity and average spacing measurement fracture intensity the number of fractures per unit length along a sample line is an important attribute of fracture systems that can be problematic to establish in the subsurface lack of adequate constraints on fracture intensity may limit the economic exploitation of fractured reservoirs because intensity describes the abundance of fractures potentially available for fluid flow and the probability of encountering fractures in a borehole traditional methods of fracture intensity measurement are inadequate because they ignore the wide spectrum of fracture sizes found in many fracture systems and the consequent scale dependence of fracture intensity an alternative approach makes use of fracture size distributions which allow more meaningful comparisons between different locations and allow microfractures in subsurface samples to be used for fracture intensity measurement comparisons are more meaningful because sampling artifacts can be recognized and avoided and because common thresholds of fracture size can be enforced for counting in different locations additionally quantification of the fracture size distribution provides a mechanism for evaluation of uncertainties estimates of fracture intensity using this approach for two carbonate beds in the sierra madre oriental mexico illustrate how size cognizant measurements cast new light on widely accepted interpretation of geologic controls of fracture intensity

natural fractures in tight oil sandstones a case study of the upper triassic yanchang formation in the southwestern ordos basin china natural fractures are important storage spaces and fluid flow channels in tight oil sandstones intraformational open fractures are the major channels for fluid flow in tight oil sandstones small faults may provide fluid flow channels across different layers according to analogous outcrops cores and borehole image logs small faults and intraformational open fractures are developed in the tight oil sandstones of the upper triassic yanchang formation in the southwestern ordos basin china among them high dip angle intraformational open fractures are the most abundant northeast southwest trending fractures are the principal fractures for fluid flow because that is the present day maximum horizontal compressive stress direction combined with production data horizontal wells striking normal to or at a large angle relative to the major flow pathways are beneficial for tight oil production improvement fractures with high dip angles are the main factor that influences initial oil production linkage and tip damage zones are more favorable for oil production improvement than wall damage zones this study provides an example of natural fracture characterization and unravels fracture contributions to reservoir physical properties and oil production of tight oil sandstones which could provide a geological basis for oil exploration and development in tight sandstones

estimation of the highest potential transmissivity of discrete shear fractures using the ductility index abstract previous studies indicated that the highest potential transmissivities of fractures in fault zones might be well estimated by a mechanical indicator the ductility index di the di is defined as the effective mean stress normalized to the tensile strength of the intact rock mass the mechanism of formation and preservation of pore structures in fault zone fractures can be explained by local shear induced tensile stresses and pore structures in discrete shear fractures may form in the same way i investigated therefore whether the di model can correctly predict the highest transmissivities of shear fractures recorded in fractured neogene diatomaceous mudstone koetoi formation where fault zones and joints are rare but discrete shear fractures without mineral fillings are abundant analyses of the shear fractures reveal that the detected states of paleostress are nearly equal to the current stress state implying the shear fractures have potentially been tectonically active thus the highest transmissivities detected at fractures by flowing fluids electric conductivity ffec borehole logging may potentially be treated as the highest potential transmissivities in terms of the given dis relationships between the transmissivities of flow anomalies and dis fit the existing di model well which suggests the model is applicable to discrete shear fracture systems as well as fault zones

fracture aperture size frequency spatial distribution and growth processes in strata bounded and non strata bounded fractures cambrian meson group nw argentina abstract in cambrian meson group nw argentina small faults and three opening mode fracture sets defined by orientation and cement texture sets 1 3 formed sequentially in sandstone that most likely had constant mechanical properties throughout deformation yet the opening mode sets display contrasting fracture aperture size distributions spacing patterns and tendency to be bed bounded set 1 fractures are quartz filled or lined opening mode fractures with crack seal texture having a wide range of opening displacement kinematic aperture sizes they are irregularly spaced and non strata bounded fractures set 1 macro and microfracture opening displacement sizes are well described by a power law with slope 0 8 set 2 fractures are microscopic mostly quartz filled and have characteristic aperture sizes are probably not bed bounded and have either a near random or clustered spatial distribution set 3 fractures are quartz lined opening mode fractures with extensive open pore space having a narrow characteristic opening displacement size distribution they are regularly spaced and stratabounded differences between sets 1 and 3 can be accounted for by quartz deposition resisting fracture reopening to a greater extent for set 1 during repeated episodic growth where crack seal texture is present in fracture spanning quartz in contrast set 3 fractures are nearly barren with only trace cement deposits that did not resist opening power law opening displacement size distributions may be favored in cases where fracture growth is unequally partitioned amongst variably cemented fractures whereas a characteristic size is favored where growth is unaffected by cementation results imply that thermal history and diagenesis are important for fracture size distribution patterning

are open fractures necessarily aligned with maximum horizontal stress abstract fluid flow in fractured rock is an increasingly central issue in recovering water and hydrocarbon supplies and geothermal energy in predicting flow of pollutants underground in engineering structures and in understanding large scale crustal behaviour conventional wisdom assumes that fluids prefer to flow along fractures oriented parallel or nearly parallel to modern day maximum horizontal compressive stress or s hmax the reasoning is that these fractures have the lowest normal stresses across them and therefore provide the least resistance to flow for example this view governs how geophysicists design and interpret seismic experiments to probe fracture fluid pathways in the deep subsurface contrary to these widely held views here we use core stress measurement and fluid flow data to show that s hmax does not necessarily coincide with the direction of open natural fractures in the subsurface 3 km depth consequently in situ stress direction cannot be considered to predict or control the direction of maximum permeability in rock where effective stress is compressive and fractures are expected to be closed chemical alteration dictates location of open conduits either preserving or destroying fracture flow pathways no matter their orientation

a 48 m y history of fracture opening temperature and fluid pressure cretaceous travis peak formation east texas basin quartz cement bridges across opening mode fractures of the cretaceous travis peak formation provide a textural and fluid inclusion record of incremental fracture opening during the burial evolution of this low porosity sandstone incremental crack seal fracture opening is inferred based on the banded structure of quartz cement bridges consisting of up to 700 cement bands averaging 5 m in thickness as observed with scanning electron microscope cathodoluminescence crack seal layers contain assemblages of aqueous two phase fluid inclusions based on fluid inclusion microthermometry and raman microprobe analyses we determined that these inclusions contain methane saturated brine trapped over temperatures ranging from 130 c to 154 c using textural crosscutting relations of quartz growth increments to infer the sequence of cement growth we reconstructed the fluid temperature and pore fluid pressure evolution during fracture opening in combination with published burial evolution models this reconstruction indicates that fracture opening started at ca 48 ma and above hydrostatic pore fluid pressure conditions and continued under steadily declining pore fluid pressure during partial exhumation until present times individual fractures opened over an 48 m y time span at rates of 16 23 m m y these rates suggest that fractures can remain hydraulically active over geologically long times in deep basinal settings

factors controlling the morphology and volume v length l relations of permanent gullies in the northern ethiopian highlands small scale aerial photographs and high resolution satellite images available for ethiopia since the second half of the twentieth century as for most countries allow only the length of gullies to be determined understanding the development of gully volumes therefore requires that empirical relations between gully volume v and length l are established in the field so far such v l relations have been proposed for a limited number of gullies environments and were especially developed for ephemeral gullies in this study v l relations were established for permanent gullies in northern ethiopia having a total length of 152 km in order to take the regional variability in environmental characteristics into account factors that control gully cross sectional morphology were studied from 811 cross sections this indicated that the lithology and the presence of check dams or low active channels were the most important controls of gully cross sectional shape and size cross sectional size could be fairly well predicted by their drainage area the v l relation for the complete dataset was v 0 562 l 1 381 n 33 r2 0 94 with 34 9 of the network having check dams and or being low active producing such relations for the different lithologies and percentages of the gully network having check dams and or being low active allows historical gully development from historical remote sensing data to be assessed in addition gully volume was also related to its catchments area a and catchment slope gradient sc this study demonstrates that v l and v a sc relations can be very suitable for planners to assess gully volume but that the establishment of such relations is necessarily region specific copyright 2013 john wiley sons ltd

strong influence of westerly wind bursts on el nino diversity el nino diversity and its genesis are debated an overview of existing work along with a fuzzy clustering analysis and simulations suggest that the asymmetry irregularity and extremes of el nino result from westerly wind bursts

an advective reflective conceptual model for the oscillatory nature of the enso recent findings about zonal displacements of the pacific warm pool required a notable modification of the delayed action oscillator theory the current leading theory for the el nino southern oscillation enso simulations with a linearized coupled ocean atmosphere model resulted in 3 to 6 year enso like oscillations with many of the variable model parameters found to be very close to their observed values this simple model suggests that ocean processes that are ignored or underestimated in the delayed action oscillator theory such as zonal current convergence zonal advection of sea surface temperature and equatorial wave reflection from the eastern ocean boundary are fundamental to the development of the enso in particular to its manifestations in the central equatorial pacific

neural network modelling of planform geometry of headland bay beaches abstract the shoreline of beaches in the lee of coastal salients or man made structures usually known as headland bay beaches has a distinctive curvature wave fronts curve as a result of wave diffraction at the headland and in turn cause the shoreline to bend the ensuing curved planform is of great interest both as a peculiar landform and in the context of engineering projects in which it is necessary to predict how a coastal structure will affect the sandy shoreline in its lee a number of empirical models have been put forward each based on a specific equation a novel approach based on the application of artificial neural networks is presented in this work unlike the conventional method no particular equation of the planform is embedded in the model instead it is the model itself that learns about the problem from a series of examples of headland bay beaches the training set and thereafter applies this self acquired knowledge to other cases the test set for validation twenty three headland bay beaches from around the world were selected of which sixteen and seven make up the training and test sets respectively as there is no well developed theory for deciding upon the most convenient neural network architecture to deal with a particular data set an experimental study was conducted in which ten different architectures with one and two hidden neuron layers and five training algorithms 50 different options combining network architecture and training algorithm were compared each of these options was implemented trained and tested in order to find the best performing approach for modelling the planform of headland bay beaches finally the selected neural network model was compared with a state of the art planform model and was shown to outperform it

linking long term gully and river channel dynamics to environmental change using repeat photography northern ethiopia abstract in the highlands of northern ethiopia gully occurrence is linked to poverty driven unsustainable use of the land in a vulnerable semi arid and mountainous environment where intensive rainfall challenges the physical integrity of the landscape trends in gully and river channel erosion and their relation to triggering environmental changes can proffer valuable insights into sustainable development in northern ethiopia in order to assess the region wide change in gully and river channel morphology over 140 years a set of 57 historical photographs taken in tigray and clearly displaying gully cross sections were precisely repeated from 2006 till 2009 ninety two percent of the gully and river sections n 38 increased in cross sectional area during the studied period especially after 1975 two repeatedly photographed catchments of lake ashenge and atsela allowed a detailed study of gully development from 1936 until 2009 a conceptual hydrogeomorphic model was devised for these catchments and validated for the northern ethiopian highlands three major phases can be distinguished in the hydrological regime of the catchments in the first phase between 1868 or earlier and ca 1965 the relatively stable channels showed an oversized morphology inherited from a previous period when external forcing in environmental conditions had caused the channels to shape in the second phase ca 1965 ca 2000 increased aridity and continued vegetation clearance accelerated the channel dynamics of the gully and river system the third phase ca 2000 present started after the large scale implementation of soil and water conservation measures in 2009 23 of the gully and river sections were stabilizing this paper validates previous research indicating severe land degradation in the second half of the 20th century additionally it demonstrates that the recent erosive cycle started around 1965 and that at the present time improved land management stabilizes headwater streams

non linear growth kinematics of opening mode fractures abstract fracture aperture is commonly assumed to be a linear function of fracture length stress and elastic material properties under constant stress fracture opening displacement may change without concurrent length or height growth if the material effectively weakens after initial linear elastic fracture growth by changes in elastic properties or by non elastic deformation processes to investigate the kinematics of fracture opening and its dependence on length and height growth we reconstructed the opening history of three opening mode fractures that are bridged by crack seal quartz cement similar crack seal cement bridges had been interpreted to form by repeated incremental fracture opening and subsequent precipitation of quartz cement using scanning electron microscope cathodoluminescence imaging we determined the thickness and number of crack seal cement layers as a function of position along fracture length and height observed trends in crack seal cement layer thickness and number of fracture opening increments are consistent with non linear fracture growth kinematics consisting of an initial stage of fast fracture propagation relative to aperture growth followed by a stage of slow propagation and pronounced aperture growth consistent with earlier fluid inclusion observations indicating fracture opening and propagation occurring over 40 50 m y we interpret the second stage of pronounced aperture growth and slow propagation to result from fracture opening strain accommodated by solution precipitation creep and concurrent slow possibly subcritical fracture propagation

multi scale fracture network analysis from an outcrop analogue a case study from the cambro ordovician clastic succession in petra jordan abstract a regional multi scale fracture network is prominently exposed in the cambro ordovician sedimentary succession in south western jordan this succession consists of marine deposits overlain by heterogeneous deltaic sediments culminating in 500 m of massive to cross bedded fluvial sandstones a data set acquired from high resolution satellite imagery and field measurements enables the quantification of the geometry and spatial characteristics of the fracture network in three dimensions five directional fracture sets are identified from the satellite data with mean strikes of 000 040 065 120 and 145 large fracture zones longer than 600 m composed of tightly spaced joints confined in fracture swarms exhibit a log normal trace length distribution and the fracture swarms are regularly spaced the trace length distribution of intermediate fractures 40 600 m long exhibits power law characteristics and the fractures are clustered reflecting different fracture system styles and stratigraphic controls as a function of scale the small scale fracture network is strongly dependent on the distribution of bedding plane discontinuities throughout different parts of the succession the multi scale fracture network is categorized in a three tier hierarchical fracture model that reflects underlying stratigraphic controls in the study area this type of model can be used to improve the representation of fracture networks in fluid flow simulations of fractured reservoirs with hybrid models that combine elements of discrete fracture networks and continuum models

quantitative structural analysis using remote sensing data kurdistan northeast iraq the use of remote sensing data increases the efficiency of field mapping especially in areas with difficult access or where geologic fieldwork is expensive or hazardous this study presents a newly developed software tool for interactively mapping and measuring the spatial orientation i e dip angle and dip direction of finite planar geologic structure from digital elevation models dems the orientations of planar data e g sedimentary bedding or fault planes are derived by approximating a virtual plane to the intersection of the planar feature with the dem topography to increase the informative value of the dem satellite images can be draped onto the topographic data set the software tool was tested in the zagros fold and thrust belt northeast of erbil kurdistan northeast iraq where the stratigraphy has been deformed into subcylindrical fold trains computed orientations have been compared with actual dip angles and directions measured in the field under favorable conditions moderately dipping planes strong competence contrast between stratigraphic boundaries intersection with a rugged topography low vegetation statistical comparison of computed data with the field measurements demonstrates that the spatial data set can be reproduced from the dem within an average error of approximately 10 the strength of the method is demonstrated by integrating field data with computed values from inaccessible areas resulting in a reasonably well constrained balanced geologic cross section

natural fractures in the barnett shale and their importance for hydraulic fracture treatments gas production from the barnett shale relies on hydraulic fracture stimulation natural opening mode fractures reactivate during stimulation and enhance efficiency by widening the treatment zone knowledge of both the present day maximum horizontal stress which controls the direction of hydraulic fracture propagation and the geometry of the natural fracture system which we discuss here is therefore necessary for effective hydraulic fracture treatment design we characterized natural fractures in four barnett shale cores in terms of orientation size and sealing properties we measured a mechanical rock property the subcritical crack index which governs fracture pattern development natural fractures are common narrow 0 05 mm 0 002 in sealed with calcite and present in en echelon arrays individual fractures have high length width aspect ratios 1000 1 they are steep 75 and the dominant trend is west northwest other sets trend north south the narrow fractures are sealed and cannot contribute to reservoir storage or enhance permeability but the population may follow a power law size distribution where the largest fractures are open the subcritical crack index for the barnett shale is high indicating fracture clustering and we suggest that large open fractures exist in clusters spaced several hundred feet apart these fracture clusters may enhance permeability locally but they may be problematic for hydraulic fracture treatments the smaller sealed fractures act as planes of weakness and reactivate during hydraulic fracture treatments because the maximum horizontal stress trends northeast southwest and is nearly normal to the dominant natural fractures reactivation widens the treatment zone along multiple strands

a geometrically based method for predicting stress induced fracture aperture and flow in discrete fracture networks modeling of fluid flow in naturally fractured reservoirs is often done through modeling and upscaling of discrete fracture networks dfns the two dimensional fracture geometry required for dfns is obtained from subsurface and outcropping analog data however these data provide little information on subsurface fracture aperture which is essential for quantifying porosity and permeability apertures are difficult to obtain from either outcropping or subsurface data and are therefore often based on fracture size or scaling relationships but these do not consider the orientation and spatial distribution of fractures with respect to the in situ stress field using finite element simulations mechanical aperture can be modeled explicitly but because changes in fracture geometry require renewed meshing and simulating this approach is not easily integrated into subsurface dfn modeling workflows we present a geometrically based method for calculating the shear induced hydraulic aperture that is an aperture of up to 0 5 mm 0 02 in that can result from shear displacement along irregular fracture walls the geometrically based method does not require numerical simulations but it can instead be directly applied to dfns using the fracture orientation and spacing distributions in combination with an estimate of the regional stress tensor and orientation the frequency distribution of hydraulic aperture from the geometrically based method is compared with finite element models constructed from five real fracture networks digitized from outcropping pavements these networks cover a wide range of possible geometries and spatial distributions the geometrically based method predicts the average hydraulic aperture and equivalent permeability of fractured porous media with error margins of less than 5

human impact on the environment in the ethiopian and eritrean highlands a state of the art abstract this review analyses the environmental evolution of the ethiopian highlands in the late quaternary the late pleistocene 20 000 12 000 14c years bp was cold and dry with 1 low lake levels in the rift valley 2 large debris fans on the flanks of lake abhe basin and 3 the blue nile transporting coarse bedload then a period with abundant and less seasonal rains existed between 11 500 and 4800 14c years bp as suggested by increased arboreal pollen high river and lake levels low river turbidities and soil formation around 5000 4800 14c years bp there was a shift to more arid conditions and more soil erosion many phenomena that were previously interpreted as climate driven might however well be of anthropic origin thick sediment deposits on pediments as well as an increase of secondary forest scrub and ruderal species in pollen diagrams are witnesses of this human impact one important aspect of the late quaternary palaeoenvironment is unclear changes in nile flow discharges and rift valley lake levels have been linked to changes in precipitation depth most authors do not take into account changes in land use in the highlands nor changes in the seasonality of rain both of which can lead to a change in runoff coefficients tufa and speleothem deposition around 14 000 years ago tend to indicate that at the end of the last glacial maximum lgm conditions might have been wetter than generally accepted the most important present day geomorphic processes are sheet and rill erosion throughout the country gullying in the highlands and wind erosion in the rift valley and the peripheral lowlands based on existing sediment yield data for catchments draining the central and northern ethiopian highlands an equation was developed allowing to assess area specific sediment yields sy 2595a 0 29 n 20 r 2 0 59 where sy area specific sediment yield t km 2 year 1 and a drainage area km2 with respect to recent environmental changes temporal rain patterns apart from the catastrophic impact of dry years on the degraded environment cannot explain the current desertification in the driest parts of the country and the accompanying land degradation elsewhere causes are changing land use and land cover which are expressions of human impact on the environment deforestation over the last 2000 3000 years was probably not a linear process in ethiopia studies on land use and land cover change show however a tendency over the last decades of increasing removal of remnant vegetation which is slowed down or reversed in northern ethiopia by a set aside policy ongoing land degradation requires urgent action at different levels of society soil and water conservation swc structures are now widely implemented local knowledge and farmer s initiatives are integrated with introduced swc techniques at various degrees impact assessments show clear benefits of the soil conservation measures in controlling runoff and soil erosion in high rain areas runoff management requires greater emphasis during the design of soil conservation structures in such areas investment in swc might not be profitable at farm level although benefits for society are positive this pleads in favour of public support the present land degradation in the ethiopian highlands has a particular origin which includes poverty and lack of agricultural intensification causes of these are to be found in the nature of past and present regional social relations as well as in international unequal development this review strengthens our belief that under improved socio economic conditions land husbandry can be made sustainable leading to a reversal of the present desertification and land degradation of the ethiopian highlands

a note on the angular correction applied to fracture intensity profiles along drill core we derive the angular correction that has to be applied to borehole fracture intensities to recover the actual three dimensional distribution when the fracture networks have a length distribution we assume that the fracture intensity is calculated from the fractures that fully transect the core because of the length distribution the classical terzaghi correction which involves the cosine of fracture dip is no longer valid solutions have been calculated in the specific case of fractal fracture networks with power law length distribution we show that the terzaghi correction may significantly overestimate the frequency of fractures subparallel to the borehole the correction procedure proposed here was tested on a fracture database recorded at a study site for a repository of spent nuclear fuel located in se sweden with three outcrop maps and three boreholes a consistency between the dip distributions of outcrops and boreholes was achieved when applying the angular correcting term calculated with the power law length distribution deduced from outcrop maps

from outcrop to flow simulation constructing discrete fracture models from a lidar survey terrestrial light detection and ranging lidar surveys offer potential enrichment of outcrop based research efforts to characterize fracture networks and assess their impact on subsurface fluid flow here we explore two methods to extract the three dimensional 3 d positions of natural fractures from a lidar survey collected at a roadcut through the cretaceous austin chalk 1 a manual method using the university of california davis keck center for active visualization in the earth sciences and 2 a semiautomated method based on mean normal and gaussian curvature surface classification each extraction method captures the characteristic frequencies and orientations of the primary fracture sets that we identified in the field yet they extract secondary fracture sets with varying ability after making assumptions regarding fracture lengths and apertures the extracted fractures served as a basis to construct a discrete fracture network dfn that agrees with field observations and a priori knowledge of fracture network systems using this dfn we performed flow simulations for two hypothetical scenarios with and without secondary fracture sets the results of these two scenarios indicate that for this particular fracture network secondary fracture sets marginally impact 10 change the breakthrough time of water injected into an oil filled reservoir our work provides a prototype workflow that links outcrop fracture observations to 3 d dfn model flow simulations using lidar data an approach that offers some improvement over traditional field based dfn constructions in addition the techniques we used to extract fractures may prove applicable to other outcrop studies with different research goals

ravine formation on steep slopes forward versus regressive erosion some case studies from rwanda summary observations on steep slopes in rwanda reveal that human interventions on surficial drainage can introduce gullying on places where runoff normally leads to diffuse or ephemeral rill erosion in such cases an initial runoff incision or slide scar respectively due to an artificial concentration of runoff or underflow grows in downslope direction mainly by runoff incision this type of progressive or forward gully development contrasts with the more classical regressive development here only observed in places where the flow gradient is less than 7 8

characteristics of the response of sea surface temperature in the central pacific associated with warm episodes of the southern oscillation abstract the zonal distribution of sea surface temperature sst in the equatorial pacific 4 n 4 s 120 e 80 w associated with el nino southern oscillation enso has been studied by using the seasonal mean file of the comprehensive ocean atmosphere data set coads for the period 1940 to 1983 although the warmest ocean area in the western central pacific exhibits very little annual variation it is very sensitive to the enso displaying large variability during such episodes the eastward migration of this warmest area 28 5 c isotherm is used here as a criterion related to strong tropical convection and heavy rainfall is a common feature in the developing stage of almost all enso events since 1940 not only for the event of 1982 the extent of its eastward migration varies from event to event and represents a large contribution to the interannual variability of zonal sst distribution in the equatorial pacific comparable to the behavior of the equatorial cold tongue in the east the spread of the war

two types of el nino events cold tongue el nino and warm pool el nino abstract in this study two types of el nino events are classified based on spatial patterns of the sea surface temperature sst anomaly one is the cold tongue ct el nino which can be regarded as the conventional el nino and the other the warm pool wp el nino the ct el nino is characterized by relatively large sst anomalies in the nino 3 region 5 s 5 n 150 90 w while the wp el nino is associated with sst anomalies mostly confined to the nino 4 region 5 s 5 n 160 e 150 w in addition spatial patterns of many atmospheric and oceanic variables are also distinctively different for the two types of el nino events furthermore the difference in the transition mechanism between the two types of el nino is clearly identified that is the discharge process of the equatorial heat content associated with the wp el nino is not efficient owing to the spatial structure of sst anomaly as a result it cannot trigger a cold event it is also demonstrated that zonal advective feedback i e zonal adve

spatial distribution of gully head activity and sediment supply along an ephemeral channel in a mediterranean environment abstract in this study we examined the factors that control the spatial distribution of bank gully heads along a reach of an ephemeral river rambla salada in an area threatened by desertification in southeast spain the activity of 458 gully heads was assessed in the field by pre defined criteria such as sharp edges presence of a plunge pool tension cracks recent deposited sediments flow marks and vegetation re growth the results showed that land use has a significant impact on bank gully head activity recent land use changes involving the extension of almond cultivation appears to intensify bank gully head activity also lithology has a clear impact on the bank gully extension it was further investigated whether the gully heads were important sediment sources that contributed to reservoir sedimentation the density of very active bank gully heads along the study reach was one per 17 m of channel length average annual retreat volumes were derived from measurements at 46 active gully heads 4 0 m3 y 1 by selecting all the channel sections in the catchment of the puentes reservoir with a similar pattern of bank gullies using aerial photographs an estimate of basin wide sediment production of bank gully heads was established it was estimated that the retreat of active bank gully heads alone in the 12 760 ha study area representing 12 of the total catchment area of the puentes reservoir produced 6 of the sediment filling up the reservoir considering that the sediment is also derived from other sources such as channel walls channel beds and hillslopes the overall conclusion is that bank gully expansion in southeast spain is a major point source of sediment and therefore a major process of land degradation

spatial modelling of gully erosion in mazandaran province northern iran abstract gully erosion is one of the most severe environmental problems in large areas of iran the spatial distribution of gully erosion and its susceptibility zonation was studied using different bivariate statistical models such as frequency ratio fr weights of evidence wofe and index of entropy iofe for this purpose 109 gully erosion locations were identified and divided into training 70 and validating 30 datasets effective factors including elevation slope aspect slope degree slope length ls topographical wetness index twi plan curvature profile curvature land use lithology distance from river drainage density and distance from road were selected to develop maps of gully erosion susceptibility the spatial relationship between gully erosion and each effective factor was calculated by the mentioned models the relative operating characteristic roc curve was implemented for evaluating the accuracy of the applied predictive models results indicated that frequency ratio model had better performance 80 4 than the weight of evidence 79 5 and index of entropy 79 models the produced gully erosion susceptibility maps can be helpful to make decisions for soil and water planning and management and finally sustainable development in the valasht watershed

gully head retreat rates in the semi arid highlands of northern ethiopia due to overgrazing and agricultural intensification gully erosion severely affects sub saharan countries however insufficient quantitative studies exist for this part of the world this paper presents data on gully head retreat rates in northern ethiopia and relates these rates to gully and environmental characteristics the monitoring of headcuts over one rainy season 2010 revealed that present day retreat rates are low with average annual linear rl areal ra and volumetric ve retreat rates of 0 34 m y 1 1 70 m2 y 1 and 5 2 m3 y 1 respectively these results express the positive effects of recent soil and water conservation practices on gully stabilization significantly higher values of rl up to 1 93 m y 1 occurred in the vertisol areas affected by soil piping when considering the medium to long time scale 1 47 years using archival terrestrial and aerial photographs headcut retreat rates proved to be significantly higher than those in the short term the averages for rl ra and ve are 3 8 m y 1 31 5 m2 y 1 and 47 7 m3 y 1 respectively retreat rates are up to 10 times higher after road construction for the medium to long term headcut retreat rates were positively related to the catchment area a a power relationship that best describes the relation between ve and a is ve 0 53 a0 31 r2 0 27 n 18 compared to other areas worldwide regressive erosion has been rapid in ethiopia as a result of the degraded and steep landscape combined with erosive rains and the occurrence of vertisols in vertisols headcut retreat is controlled by soil piping because no adequate techniques exist to control gully initiation and development in vertisols alternative techniques should be developed that deactivate soil pipes

upscaling two phase flow in naturally fractured reservoirs simulation grid blocks of naturally fractured reservoirs contain thousands of fractures with variable flow properties dimensions and orientations this complexity precludes direct incorporation into field scale models macroscopic laws capturing their integral effects on multiphase flow are required numerical discrete fracture and matrix simulations show that ensemble relative permeability as a function of water saturation kri sw water breakthrough and cut depend on the fraction of the cross sectional flux that occurs through the fractures this fracture matrix flux ratio qf qm can be quantified by steady state computation here we present a new semianalytical model that uses qf qm and the fracture related porosity f to predict kri sw capturing that shortly after the first oil is recovered the oil relative permeability kro becomes less that that of water krw and krw kro approaches qf qm as soon as the most conductive fractures become water saturated to include a capillary driven fracture matrix transfer into our model we introduce the nonconventional parameter af w sw the fraction of the fracture matrix interface area in contact with the injected water for any grid block average saturation the af w sw is used to scale the capillary transfer modeled with conventional transfer functions and expressed in terms of a rate and capillary pressure dependent kro all predicted parameters can be entered into conventional reservoir simulators we explain how this is accomplished in both single and dual continua formulations the predicted grid block scale fractional flow fi sw is convex with a near infinite slope at the initial saturation the upscaled flow equation therefore does not contain an sw shock but a long leading edge capturing the progressively widening saturation fronts observed in numerical experiments published previously

geomorphological investigation on gully erosion in the rift valley and the northern highlands of ethiopia abstract gully erosion phenomena are very common in ethiopia they affect large areas with different morphological pedological and climatic characteristics the amount of soil loss due to gullying has become a very serious problem in the recent decades as it was associated to remarkable depletion of cultivated land field investigations on gully morphology and its genetic processes were carried out in two study areas of ethiopia representative of different geo environmental conditions the lakes region in the rift portion north of shashamene and the area surrounding the town of mekele in tigray two main types of gullies were identified on the basis of their morphological and hydraulic geometry characteristics 1 discontinuous gullies which generally develop on low gradient slopes 1 5 on average and the hydraulic radius of which increases from an upstream minimum to a maximum at approximately their mid length and decreases again to a relative minimum at their downstream end and 2 stream gullies formed by deep erosion processes typically migrating upslope in order to investigate the main causes originating the different types of gullies shear stress data were collected in the field from their hydraulic geometry hypotheses on the mechanisms responsible for both discontinuous and stream gullies development and for their different characteristics are discussed considering the pattern of shear stress variation in the downstream direction

effects of westerly wind bursts on el nino a new perspective daily observations from 1971 to 2010 reveal that every el nino during this period was accompanied by congregated westerly wind bursts suggesting a close relationship of these bursts with both cold tongue and warm pool el nino events with the addition of burst like multiplicative noise to an intermediate ocean atmosphere coupled model it is shown that westerly wind bursts by generating eastward equatorial surface currents and downwelling kelvin waves could be responsible for the existence of the warm pool el nino and for the irregularity and extremes of the cold tongue el nino whether these bursts give rise to one type of el nino or the other depends on the timing of their occurrence relative to the phase of the recharge discharge cycle of the equatorial upper ocean heat content

accuracy assessment of georectified aerial photographs implications for measuring lateral channel movement in a gis aerial photographs are commonly used to measure planform river channel change we investigated the sources and implications of georectification error in the measurement of lateral channel movement by testing how the number 6 30 and type human versus natural landscape features of ground control points gcps and the order of the transformation polynomial first second and third order affected the spatial accuracy of a typical georectified aerial photograph error was assessed using the root mean square error rmse of the gcps as well as error in 31 independent test points the rmse and the mean and median values of test point errors were relatively insensitive to the number of gcps above eight but the upper range of test point errors showed marked improvement i e the number of extreme errors was reduced as more gcps were used for georectification using more gcps thus improved overall georectification accuracy but this improvement was not indicated by the rmse suggesting that independent test points located in key areas of interest should be used in addition to rsme to evaluate georectification error the order of the transformation polynomial also influenced test point accuracy the second order polynomial function yielded the best result for the terrain of the study area gcp type exerted a less consistent influence on test point accuracy suggesting that although hard edged points e g roof corners are favored as gcps some soft edged points e g trees may be used without adding significant error based upon these results we believe that aerial photos of a floodplain landscape similar to that of our study can be consistently georectified to an accuracy of approximately f5 m with 10 chance of greater error the implications of georectification error for measuring lateral channel movement are demonstrated with a multiple buffer analysis which documents the inverse relationship between the size of the buffers applied to two channel centerlines and the magnitude of change detected between them this study demonstrates the importance of using an independent test point analysis in addition to the rsme to evaluate and treat locational error in channel change studies d 2005 published by elsevier b v

catastrophic landslide induced by typhoon morakot shiaolin taiwan abstract typhoon morakot induced the catastrophic and deadly shiaolin landslide in southern taiwan on 9 august 2009 resulting in more than 400 casualties we undertook a geological and geomorphological investigation with the aim of reconstructing the events leading up to this landslide and to clarify factors that contributed to its development cumulative rainfall reached up to 1676 5 mm in about three days under the influence of the typhoon and the shiaolin landslide with a volume of 25 106 m3 occurred one day after the peak in rainfall intensity the landslide occurred on a dip slope overlying late miocene to early pliocene sedimentary rocks consisting of silty shale massive mudstone and sandstone it started as a rockslide in the upper third of the landslide area and transformed into a rock avalanche that crossed a series of terraces and displaced or buried the village below it buried the riverbed of the chishan river and ran up the opposite slope creating a landslide dam 60 m high which was breached about 1 h and 24 min later flooding the village the velocity of the landslide is estimated to have been 20 4 to 33 7 m s 1 and its apparent friction angle was 14 which indicates its high mobility the detachments in the source area consist of combinations of bedding planes and joints or faults the landslide was preceded by gravitational deformation which appeared as hummocky landforms before the landslide and as buckle folds exposed after the event the landslide deposits consist of fragments of mudstone shale and sandstone as well as clayey material at its base this clayey material consisting of illite chlorite quartz feldspar and calcite is assumed to have strongly influenced the long rapid runout

aperture size scaling variations in a low strain opening mode fracture set cozzette sandstone colorado the aperture size distribution of a set of opening mode fractures in the cozzette sandstone is complex and cannot be described uniquely by a single equation this study of horizontal core includes aperture size data of macrofractures measured using a hand lens and microfractures measured using scanning electron microscope based cathodoluminescence sem cl macrofractures are poorly cemented and total fracture strain is an order of magnitude lower than the smallest strain values of most previously described vein sets macrofractures in this set therefore resemble typical joints as opposed to veins in most respects although intragranular microfractures compose about 98 of the microfracture population orientation fracture fill and fracture trace characteristics suggest that only transgranular microfractures can be shown to be genetically related to macrofractures aperture size data below 0 012 mm and above 1 mm follow a log normal distribution intermediate size data though relatively sparse suggest a power law aperture size distribution these results support the notion that this fracture set is an intermediate case between typical veins and joints the results also suggest that processes leading to size distributions commonly associated with veins i e power laws may be present in joint sets but over a more limited range bound at small size scales by grain scale effects and above by mechanical layering effects

triggering of el nino by westerly wind events in a coupled general circulation model two ten members ensemble experiments using a coupled ocean atmosphere general circulation model are performed to study the dynamical response to a strong westerly wind event wwe when the tropical pacific has initial conditions favourable to the development of a warm event in the reference ensemble cref no wind perturbation is introduced whereas a strong westerly wind event anomaly is introduced in boreal winter over the western pacific in the perturbed ensemble cwwe our results demonstrate that an intense wwe is capable of establishing the conditions under which a strong el nino event can occur first it generates a strong downwelling kelvin wave that generates a positive sea surface temperature sst anomaly in the central eastern pacific amplified through a coupled ocean atmosphere interaction this anomaly can be as large as 2 5 c 60 days after the wwe secondly this wwe also initiates an eastward displacement of the warm pool that promotes the occurrence of subsequent wwes in the following months these events reinforce the initial warming through the generation of additional kelvin waves and generate intense surface jets at the eastern edge of the warm pool that act to further shift warm waters eastward the use of a ten members ensemble however reveals substantial differences in the coupled response to a wwe whereas four members of cwwe ensemble develop into intense el nino warming as described above four others display a moderate warming and two remains in neutral conditions this diversity between the members appears to be due to the internal atmospheric variability during and following the inserted wwe in the four moderate warm cases the warm pool is initially shifted eastward following the inserted wwe but the subsequent weak wwe activity when compared to the strong warming cases prevents to further shift the warm pool eastwards the seasonal strengthening of trade winds in june july can therefore act to shift warm waters back into the western pacific reducing the central eastern pacific warming this strong sensitivity of the coupled response to wwes may therefore limit the predictability of el nino events as the high frequency wind variability over the warm pool region remains largely unpredictable even at short time lead

gully erosion impacts factors and control gully erosion attracts increasing attention from scientists as reflected by two recent international meetings poesen and valentin eds catena 50 2 4 87 564 li et al 2004 gully erosion under global change sichuan science technology press chengu china 354 pp this growing interest is associated with the increasing concern over off site impacts caused by soil erosion at larger spatial scales than the cultivated plots the objective of this paper is to review recent studies on impacts factors and control of gully erosion and update the review on dgully erosion and environmental change importance and research needst poesen et al 2003 catena 50 2 4 91 134 for the farmers the development of gullies leads to a loss of crop yields and available land as well as an increase of workload i e labour necessary to cultivate the land gullies can also change the mosaic patterns between fallow and cultivated fields enhancing hillslope erosion in a feedback loop in addition gullies tend to enhance drainage and accelerate aridification processes in the semi arid zones fingerprinting the origin of sediments within catchments to determine the relative contributions of potential sediment sources has become essential to identify sources of potential pollution and to develop management strategies to combat soil erosion in this respect tracers such as carbon nitrogen the nuclear bomb derived radionuclide 137 cs magnetics and the strontium isotopic ratio are increasingly used to fingerprint sediment recent studies conducted in australia china ethiopia and usa showed that the major part of the sediment in reservoirs might have come from gully erosion gullies not only occur in marly badlands and mountainous or hilly regions but also more globally in soils subjected to soil crusting such as loess european belt chinese loess plateau north america and sandy soils sahelian zone north east thailand or in soils prone to piping and tunnelling such as dispersive soils most of the time the gullying processes are triggered by

quantifying long term changes in gully networks and volumes in dryland environments the case of northern ethiopia abstract understanding historical and present gully development is essential when addressing the causes and consequences of land degradation especially in vulnerable dryland environments for northern ethiopia several studies exist on the severity of gully erosion yet few have quantified gully development in this study gully network and volume development were quantified over the period 1963 2010 for an area of 123 km2 representing the regional variability in environmental characteristics gully networks were mapped from small scale aerial photographs and high resolution satellite images for the latter visualizing google earth images in 3d proved to be very suitable to investigate gully erosion from the changes in networks and volumes over the period 1963 2010 the occurrence of one cut and fill cycle is apparent from a largely low dynamic gully system in the 1960s network expansion and increased erosion rates in the 1980s and 1990s caused the drainage density and volume to peak in 1994 the average gully density dtotal was then 2 52 km km 2 and the area specific gully volume va 60 103 m3 km 2 this coincides with soil losses by gully erosion slg of 17 6 t ha 1 y 1 over the period 1963 1994 by 2010 improved land management and the region wide implementation of soil and water conservation measures caused 25 of the gully network to stabilize resulting in a net infilling of the gully channels over the period 1994 2010 the study validates previous findings that land degradation by gullying was indeed severe in northern ethiopia in the second half of the 20th century but also shows that when proper land management is applied a gully can be transformed into a linear oasis which increases the resistance of gullies to further erosion

regional fractures i a mechanism for the formation of regional fractures at depth in flat lying reservoirs 1 regional fractures may form slowly at depth in extension and parallel to regional horizontal tectonic compression such fracture sets can be common in reservoirs despite the absence of flexure of the strata and are commonly important contributors to reservoir permeability such fractures make good targets for deviated wells high formation pore pressure is crucial to creating low effective confining stresses and brittle rock properties in the reservoirs conditions conducive to fracturing despite deep burial but pore pressure does not exceed the least principal stress and does not cause tensile fracturing the mechanism of natural hydraulic fracturing is incompatible with the characteristics of most regional fracture sets rather many regional fractures form duri g far field compression initiate at locally induced tensile stresses caused by flaws in the rock and propagate in the plane of the maximum and intermediate compressive stresses the differential stress required for initiation and propagation of regional fractures is well below that necessary for shear failure in the presence of high pore pressure fractures can be held open at depth by a tectonic basinwide dilatancy of the strata and the open void space commonly becomes mineralized

evaluation of sampling methods for fracture network characterization using outcrops outcrops provide valuable information for the characterization of fracture networks sampling methods such as scanline sampling window sampling and circular scanline and window methods are available to measure fracture network characteristics in outcrops and from well cores these methods vary in their application the parameters they provide and therefore have advantages and limitations we provide a critical review on the application of these sampling methods and apply them to evaluate two typical natural examples 1 a large scale satellite image from the oman mountains oman 120 000 m2 1 291 669 ft2 and 2 a small scale outcrop at craghouse park united kingdom 19 m2 205 ft2 the differences in the results emphasize the importance to 1 systematically investigate the required minimum number of measurements for each sampling method and 2 quantify the influence of censored fractures on the estimation of fracture network parameters hence a program was developed to analyze 1300 sampling areas from 9 artificial fracture networks with power law length distributions for the given settings the lowest minimum number of measurements to adequately capture the statistical properties of fracture networks was found to be approximately 110 for the window sampling method followed by the scanline sampling method with approximately 225 these numbers may serve as a guideline for the analyses of fracture populations with similar distributions furthermore the window sampling method proved to be the method that is least sensitive to censoring bias reevaluating our natural examples with the window sampling method showed that the existing percentage of censored fractures significantly influences the accuracy of inferred fracture network parameters

development of badlands and gullies in the sneeuberg great karoo south africa abstract the study aims to examine the origin and development of land degradation with particular emphasis on badland and gully systems in the sneeuberg uplands of the great karoo this is an area of semiarid extensive stock farming where land degradation in the form of rill and gully erosion has accompanied the replacement of grassland by shrub vegetation species diversity has declined and ground cover has been reduced leading to a positive feedback loop which exacerbates the degradation many foot slopes developed in shales clays and colluvium have extensive incipient badland development with closely spaced gullying up to 1 5 m deep in valley bottom and valley side depression locations gullies up to 8 m deep have developed usually cut to bedrock through valley fills of mainly holocene colluvium both badlands and gullies appear to have developed since european settlement and to be part of the same hydrological system with extensive areas of bare ground badlands feeding water to incising gullies experiments using simulated rainfall throw some light on current processes badland areas are active under high frequency low magnitude rainfall events major gullies are likely to be the result of occasional high magnitude events but these have not been observed overgrazing in the past is the most likely cause of the degradation

geomorphic change detection using historic maps and dem differencing the temporal dimension of geospatial analysis abstract the ability to develop spatially distributed models of topographic change is presenting new capabilities in geomorphic research high resolution maps of elevation change indicate locations processes and rates of geomorphic change and provide a means of calibrating temporal simulation models methods of geomorphic change detection gcd based on gridded models may be applied to a wide range of time periods by utilizing cartometric remote sensing or ground based topographic survey data to measure volumetric change advantages and limitations of historical dem reconstruction methods are reviewed with a focus on coupling them with subsequent dems to construct dems of difference dod which can be created by subtracting one elevation model from another to map erosion deposition and volumetric change the period of dod analysis can be extended to several decades if accurate historical dems can be generated by extracting topographic data from historical data and selecting areas where geomorphic change has been substantial the challenge is to recognize and minimize uncertainties in data that are particularly elusive with early topographic data this paper reviews potential sources of error in digitized topographic maps and dems although the paper is primarily a review of methods three brief examples are presented at the end to demonstrate gcd using dods constructed from data extending over periods ranging from 70 to 90 years

the impact of in situ stress and outcrop based fracture geometry on hydraulic aperture and upscaled permeability in fractured reservoirs abstract aperture has a controlling impact on porosity and permeability and is a source of uncertainty in modeling of naturally fractured reservoirs this uncertainty results from difficulties in accurately quantifying aperture in the subsurface and from a limited fundamental understanding of the mechanical and diagenetic processes that control aperture in the absence of cement bridges and high pore pressure fractures in the subsurface are generally considered to be closed however experimental work outcrop analyses and subsurface data show that some fractures remain open and that aperture varies even along a single fracture however most fracture flow models consider constant apertures for fractures we create a stress dependent heterogeneous aperture by combining finite element modeling of discrete fracture networks with an empirical aperture model using a modeling approach that considers fractures explicitly we quantify equivalent permeability i e combined matrix and stress dependent fracture flow fracture networks extracted from a large outcropping pavement form the basis of these models the results show that the angle between fracture strike and 1 has a controlling impact on aperture and permeability where hydraulic opening is maximum for an angle of 15 at this angle the fracture experiences a minor amount of shear displacement that allows the fracture to remain open even when fluid pressure is lower than the local normal stress averaging the heterogeneous aperture to scale up permeability probably results in an underestimation of flow indicating the need to incorporate full aperture distributions rather than simplified aperture models in reservoir scale flow models

pacific meridional mode and el nino southern oscillation 1 we present intriguing evidence that the majority of el nino events over the past four decades are preceded by a distinctive sea surface warming and southwesterly wind anomaly in the vicinity of the intertropical convergence zone itcz during the boreal spring this phenomenon known as the meridional mode mm is shown to be intrinsic to the thermodynamic coupling between the atmosphere and ocean the mm effectively acts as a conduit through which the extratropical atmosphere influences enso modeling results further suggest that the mm plays a vital role in the seasonal phase locking behavior of enso the findings provide a new perspective for understanding the important role of thermodynamic ocean atmosphere feedback in enso and may have profound implications for enso prediction particularly the unresolved issue of the spring predictability barrier citation chang p l zhang r saravanan d j vimont j c h chiang l ji h seidel and m k tippett 2007 pacific meridional mode and el nino southern oscillation geophys res lett 34 l16608 doi 10 1029 2007gl030302

complex fracture development related to stratigraphic architecture challenges for structural deformation prediction tensleep sandstone at the alcova anticline wyoming fracture prediction in subsurface reservoirs is critical for exploration through exploitation of hydrocarbons methods of predicting fractures commonly neglect to include the stratigraphic architecture as part of the prediction or characterization process this omission is a critical mistake we have documented a complex heterogeneous fracture development within the eolian tensleep sandstone in wyoming which arguably is one of the least complex reservoir facies fractures develop at four scales of observation lamina bound facies bound sequence bound and throughgoing fractures that span the formation we documented a detailed facies and fracture intensity model using lidar scanned outcrops located at the alcova anticline in central wyoming through this characterization we reveal the existence of a striking variability in fracture intensity caused by original depositional architecture overall structural deformation and diagenetic alteration of the host rock

late quaternary changes in climate and land cover in the northern horn of africa and adjacent areas abstract most studies of landscape dynamics in the ethiopian highlands offer linear relations of population pressure and periodic droughts as causes of environmental degradation such views rely on limited historical information of one or two centuries which is inadequate for understanding the longer term effects of climatic shifts on landscape dynamics and environmental change in the highlands in this study we conducted a meta analysis of long term climate and biogeographic change and landscape dynamics during the holocene using multi proxy evidence from lacustrine n 10 marine n 5 geomorphic n 13 and vegetation n 7 records from the highlands and the wider horn of africa and western indian ocean region based on the datasets we discern late pleistocene dry phases in the region during the last glacial maximum the heinrich 1 event and the younger dryas additionally we recognize at least six dry phases during the holocene after 6500 bce 4900 bce 3700 bce 1000 bce 450 ce and 1750 ce the hydroclimatic shifts seem associated with changing equatorial atlantic and western indian ocean sea surface temperatures further the results show nonlinear relations between major holocene climate shifts and grassland expansions in the highlands our findings challenge conventional cause effect explanations of human population growth and land degradation and point instead to potentially strong but unexplored feedbacks between climate changes human activity and landscape dynamics over longer and short term periods

formation and growth of extensional fracture sets naturally occurring extension fractures including some joints and veins form sets consisting of numerous subparallel fractures each with well defined ends in order to correctly interpret these fractures in the field it is necessary to determine which factors control the geometries of fracture sets this requires consideration of the conditions leading to both the initiation and the cessation of fracture propagation a fracture begins to propagate when the crack extension force reaches a critical value which is a property of the rock and of environmental conditions after propagation initiates the crack extension force varies owing to 1 increase in crack length 2 change in remote strain 3 elastic interaction with nearby cracks 4 change in internal fluid pressure and 5 stress relaxation due to growth of cracks in the surrounding rock each fracture continues to propagate as long as its crack extension force exceeds the critical value and terminates when this condition is no longer met neglecting fluid effects the final geometry of the fracture set is a function of the remote strain history and the initial crack geometry which can be characterized by the initial crack density for low initial densities large increases in crack length occur with little or no increase in remote strain however for large initial densities crack lengths increase only if the applied strain increases this analysis is used to estimate the variation in remote strain during formation of a fracture set in granitic rock from observations of the final fracture geometry

enso regimes reinterpreting the canonical and modoki el nino 1 we propose that the first two empirical orthogonal function eof modes of tropical pacific sea surface temperature sst anomalies do not describe different phenomena i e el nino southern oscillation enso and el nino modoki but rather the nonlinear evolution of enso we introduce two new uncorrelated indices e and c based on the leading eofs that respectively account for extreme warm events in the eastern and cold moderate warm events in the central equatorial pacific corresponding to regimes with different evolution recent trends in enso can be described as an increase in the central pacific c variability that is associated with stronger cold events as well as a reduction in the eastern pacific e variability within the cold moderate warm regime consistent with model projections however little can be said observationally with respect to the extreme warm regime