effective shear stress of graded sediments 1 a meta analysis of fractional mobilization data from 14 sets of experiments totaling 103 different mixed sand and gravel beds and flow conditions has been carried out in order to identify an expression for effective shear stress here defined as the component of bed shear stress that is directly involved in transporting each grain size fraction in graded sediment in doing so we test the assumption that excess stress should be defined solely in terms of a critical stress rather than effective stress which exhibits sensitivity to the flow stage in contrast to the approach which evaluates the size distribution effects on motion threshold by comparing inferred transport rates an alternative approach is utilized which is based on the skill of reproducing the measured mobilized particle size distribution a simple equation is developed for mobilization of sediment mixtures based on a well established transport law and employing a classical hiding function approach to the problem of mitigating the bias toward mobilizing fine material in the mixture we use inverse methods to find the optimal form of the hiding function which provides the best fit with the data we find that the hiding function is indeed sensitive to the flow and bed composition on this basis a simple deterministic equation is proposed for fraction specific effective stress which outperforms the next best existing formula based on critical stress by 34 on aggregate

fluid and sediment dynamics of upper stage plane beds to understand more fully the fluid and sediment dynamics of upper stage plane beds laboratory experiments were conducted using mobile and fixed beds where turbulent motions of fluid and sediment were measured using laser anemometry bed elevation fluctuations on mobile upper stage plane beds reveal millimeter high bed waves vertical profiles of flow velocity mixing length and eddy viscosity diffusivity are represented well by the law of the wall for the mobile bed von karman s 0 33 and equivalent sand roughness to mean bed grain size varies from 9 to 17 because of the presence of bed load and low relief bed waves for fixed beds with no sediment transport 0 41 and equivalent sand roughness is equal to the mean bedgrain size the decrease in for mobile beds is related to the relative motion of grains and fluid mobile bed turbulence intensities are greater than those for sediment free fixed beds because of enhanced wake formation from the lee side of near bed grains and low relief bed waves sediment diffusivities es calculated in a similar way to fluid diffusivities e indicate that es e sediment diffusivities calculated using the equilibrium balance between upward diffusion and downward settling of sediment are similar to e in near bed regions y d 0 3 suggesting that larger more energetic turbulent eddies are responsible for sediment suspension higher in the flow

size fractionation by suspension transport a large scale flume experiment with shoaling waves abstract laboratory simulation of wave shoaling over a medium sand bed mean size 250 m for a range of wave conditions a natural spectrum symmetrical and asymmetrical wave groups showed that a distinct horizontal fractionation of sediment by size occurred sediment became progressively coarser down wave onshore from the test section and progressively finer up wave offshore from the same source this spatial sorting is explained by suspension transport alone and controlled by two factors a a vertical profile of grain size in the suspension in which there is a distinct increase in the proportion of fines with elevation and a decrease in the mean size by approximately 18 between 0 04 and 0 24 m and b a vertical profile of the mean mass transport velocity which revealed a down wave onshore directed current close to the bed and a current reversal at higher elevations the average elevation for the reversal was 0 14 m under the range of wave conditions simulated h s 0 22 0 78 m t pk 2 25 3 78 s these two factors were enhanced by a frequency dependent transport in the wave field the primary waves produced a maximum net transport close to the bed which was directed down wave at higher elevations as a response to a change in the phase coupling between concentration and velocity the net transport was reversed in contrast the net transport associated with the group bound long wave was directed up wave at all elevations and the decay rate with elevation was significantly less than that associated with the primary waves thus near the bed transport by primary and secondary waves was roughly equal and opposite resulting in little wave induced net transport at higher elevations an up wave transport by the secondary wave was dominant complementing the net transport due to the mean current

vertical and horizontal structure is suspended sand concentrations and wave induced fluxes over bedforms abstract high resolution measurements of suspended sand concentrations have been made using a multi transducer acoustic back scatter sensor over both steep and flat bedforms under low energy swell conditions with weak currents present on a micro tidal beach in the southwest of england similar measurements were made over steep bedforms in a large scale laboratory wave basin using a wave record simulated from field data a detailed interpretation of the suspension process associated with steep vortex type ripples was made possible with knowledge of the position of the acoustic sensors relative to the bedform geometry analysis of time averaged wave ensemble averaged suspended sediment concentrations and velocity concentration phase angles indicates that the bedforms are an important control on the suspension patterns produced by wave induced flows steep asymmetric ripples under shoaling waves produces greater amounts of suspension than low steepness ripples due to the effects of vortex ejection associated with the steep ripples over rippled beds we find evidence for a significant phase couplined between the resuspended sediment and the bedforms in the near bed region

interpretation of non specular radar meteor trails 1 radar data of non specular meteor trails shows two clear and consistent features 1 non specular meteor trails are observed from a narrower altitude range than are head echoes and 2 an approximately 20 ms delay between meteor head echoes and trail radar scatter this paper shows that both features can result from meteor trail plasma instability simulations have demonstrated that trails often develop farley buneman gradient drift fbgd waves which become turbulent and generate field aligned irregularities fai plasma stability analysis shows that trails are only unstable within a limited altitude range matching the observed altitudes of non specular trails to within 1 2 km the simulations show that instability develops into turbulence in 20 ms and appears to be the only meteor trail process that can explain both the observed delay between head and trail echoes and generate coherent scatter at both uhf and vhf wavelengths

microseismic hydraulic fracture imaging in the marcellus shale using head waves we studied microseismic data acquired from a geophone array deployed in the horizontal section of a well drilled in the marcellus shale near susquehanna county pennsylvania head waves were used to improve event location accuracy as a substitution of the traditional p wave polarization method we identified that resonances due to poor geophone to borehole coupling hinder arrival time picking and contaminate the microseismic data spectrum the traditional method had substantially greater uncertainty in our data due to the large uncertainty in p wave polarization direction estimation we also identified the existence of prominent head waves in some of the data these head waves are refractions from the interface between the marcellus shale and the underlying onondaga formation the source location accuracy of microseismic events can be significantly improved by using both the p s wave direct arrival times and the head wave arrival times based on the improvement we propose a new acquisition geometry and

meteor trail diffusion and fields 1 simulations 1 a meteoroid penetrating the earth s atmosphere leaves behind a trail of dense plasma embedded in the lower e upper d region ionosphere while radar measurements of meteor trail evolution have been collected and used to infer meteor and atmospheric properties since the 1950s no accurate quantitative model of trail fields and diffusion exists this paper describes finite element simulations of trail plasma physics applicable to the majority of small meteors unlike earlier research our simulations resolve both the trail and a vast current closure area in the background ionosphere this paper also summarizes a newly developed analytical theory of meteor electrodynamics and shows that our simulations and theory predict nearly identical fields and diffusion rates this study should enable meteor and atmospheric researchers to more accurately interpret radar observations of specular and nonspecular meteor echoes

cross shore distribution of sediment texture under breaking waves along low wave energy coasts abstract sediment samples were collected with streamer traps at different elevations in the water column and across the surf zone beach profiles and breaking waves were measured together with the sediment sampling the experiments were conducted on beaches with various sediment composition ranging from well sorted fine sand to poorly sorted gravel and shell debris the cross shore variation of sediment mean grain size ranged from less than 1 to significant variation of up to 3 5 the resultant database contains 99 vertical grain size profiles composed of 99 bottom samples and 552 trap samples taken throughout the water column and at 29 different locations along the southeast coast of the united states and the gulf coast of florida a homogeneous vertical profile of mean grain size and grain size distribution pattern was found on most of the beaches with a wide range of sediment sizes the homogeneous vertical profile representing 92 of the measurements was found on all morphological features swash zone breaker line mid surf zone trough and bar a homogeneous distribution indicates that the vertical mixing mechanism in the water column of the surf zone is independent of sediment size ranging from fine sand to fine pebbles bottom sediment represented by an 8 cm core sample was generally coarser than the sediment trapped in the water column

a microseismic study in the western part of the gulf of corinth greece implications for large scale normal faulting mechanisms summary we present the results of a dense seismological experiment in the western part of the gulf of corinth psathopyrgos aigion area one of the most active rifts in the aegean region for which we have precise tectonic information the network included 51 digital stations that operated during july and august 1991 covering a surface of 40 x 40 km2 among the 5000 recorded events with ml ranging between 1 0 and 3 0 we precisely located 774 events we obtained 148 well constrained focal mechanisms using p wave first motions of these 60 also have mechanisms obtained by combining the p wave first motions with the s wave polarization directions the observed seismicity is mainly located between 6 and 11 km depth most of the fault plane solutions correspond to e w striking normal faulting in agreement with the geological evidence most of the well determined mechanisms indicate a nodal plane dipping 10 25 due north and a steep south dipping plane a similar asymmetry is also seen in the seismicity distribution and in the overall geological structure of the corinth rift we discuss this evidence and the inference of a deep detachment zone a structure where the major faults seen at the surface appear to root a large part of the microseismic activity appears to cluster in regions near the junctions of the main faults with the proposed detachment zone this feature of the microseismicity is interpreted in terms of stress transfer and stress concentration in regions of probable nucleation of future large earthquakes

beyond the dots in the box microseismicity constrained fracture models for reservoir simulation summary monitoring of induced microseismic events usually results in locations for these events and a geometrical interpretation of these dots in the box in this study we show how additional information obtained from observed microseismic events namely the source mechanisms were used to generate a discrete fracture network using the wide aperture of a surface star like array fracstar allows inversion for both shear and non shear source mechanisms both volumetric and shear only source mechanism inversion was carried out on microseismic events from the treatment of a shale gas reservoir in the continental us during the same hydraulic fracture stimulation treatment both dip slip and reverse faulting sources were active in this reservoir the source mechanisms revealed fracture orientations more accurately than could be inferred from microseismic event locations alone the activity associated with different mechanisms is interpreted as indicating reactivation of existing fractures in the rock as well as suggesting generation of new fractures failure analysis using source mechanisms on individual events allows an integrated understanding of the complex fracture interactions taking place in the reservoir and also provides a more complete understanding of the stress conditions in the reservoir during the treatment fracture orientations locations and failure mechanisms are translated into discrete fracture network dfn models that can be used to verify the extent and character of the fractures created or reactivated during the fracture treatment and may ultimately be used to generate fracture flow properties for reservoir simulation

wavefield characterization of perforation shot signals in a shale gas reservoir abstract signals of perforation or string shots which are usually used for calibrating velocity models and estimating the orientation of 3 component down hole receivers during microseismic monitoring are occasionally hard to identify due to poor signal to noise ratios snr or confusion with the events induced during fracturing in adjacent wells a significant feature for distinguishing perforation signals from hydraulic fracturing events is the tube wave which is generated in the treatment well and received in the monitoring well we analyze seismic wavefields from perforations during a hydraulic fracturing operation on a pad well a group of horizontal wells with the wellheads at a same small surface area in a shale gas reservoir to understand the wave propagation phenomena including attenuation and the identification of tube waves guided wave in borehole and their conversions since they are dominated by high frequencies and lack energy at low frequencies the p and s wave arrivals of perforation shots decrease much more rapidly with propagation distance than that of induced events we identify six modes within the wavefields of the perforation or string shots recorded in a nearby well that are related to the tube waves the six modes include p s waves converted from tube waves in the shot treatment well at plugs or the well bottom an up going tube wave in the monitoring well generated by the tube wave in the nearby treatment well a down going tube wave from the treatment wellhead and the multiple a scatting body wave activated by tube waves in the treatment well these wave modes all originate from the waves radiated at the perforation point in the treatment well as tube waves and are then radiated into the formation and received by geophones in the nearby borehole as p s or tube waves if energy from the perforation shot is strong enough the tube wave will turn back from the surface and be reflected at the well bottom or plugs which would excite the further waves based on the features of the wavefield we identify the perforation events and facilitate better use of calibration from the perforation seismic recording for microseismic data processing we also evaluate attenuation of the signals with propagation distance and propose a procedure for developing a plan for monitoring of perforation and string shot signals

intrawave observations of sediment entrainment processes above sand ripples under irregular waves 1 measurements of intrawave sediment entrainment processes are reported above ripples under irregular waves generated in a large scale flume facility the data consist of substantive observations of small scale processes collected at high spatial and temporal resolution under irregular wave forcing typical of coastal environments acoustic measurements were made of water velocities bed forms and suspended sediment concentration over plane and rippled beds during each rippled bed experiment measurements were taken over the crests of steep sided ripples above which flow separation was considered likely to occur above the ripple crests a strong intrawave variation in the near bed suspended sediment indicative of vortex formation and shedding was observed to dominate wave half cycles where the orbital diameter was greater than 1 2 times the ripple wavelength l the irregular waves were parameterized with a significant wave height hs and a significant orbital diameter d0s thus during irregular wave conditions and over steep ripples where d0s l 1 2 vortex shedding was deemed as the dominant sediment entrainment process furthermore the percentage of wave half cycles where vortex shedding was observed was found to scale as 26 d0s l these results demonstrate for under irregular waves the existence of a clear suspended sediment structure consistent with vortex shedding previously observed under regular waves citation o hara murray r b p d thorne and d m hodgson 2011 intrawave observations of sediment entrainment processes above sand ripples under irregular waves j geophys res 116 c01001 doi 10 1029 2010jc006216

entrainment and distribution of different sand sizes under water waves abstract analysis of the bed sediment and suspended sediment collected at different elevations over wave ripples shows that for grain sizes larger than 0 5 mm the likelihood of entrainment into suspension decreases with increasing grain size whereas for the finer material diameter smaller than 0 5 mm the amount of suspended material is proportional to the amount available in the bed material the concentration profiles for individual sieve fractions very similar upwards from about 4 cm above the ripple crest so that the grain size distribution for suspended sand above this level varies very little with elevation all of this taken together indicates that the finer material that goes into suspension is not going to be sorted whereas the coarsest material will be singled out

low altitude meteor trail echoes 1 we present here the first observations of a new type of meteor echo these low altitude trail echoes late observed almost exclusively below 90 km altitude using the jicamarca research observatory 50 mhz radar exhibit very different features and characteristics from the other well studied and reported meteor trails the specular meteor trails and the range spread trail echoes some of these features include strong altitude dependence low aspect sensitivity and no gap between the head echo and the trail perhaps most importantly almost 80 of the head echoes below 85 km altitude in our data set exhibit late trails the study of these echoes provides valuable insights into the role of diffusion and plasma instabilities in the formation and evolution of meteor trails

selective entrainment of sediment graded by size and density under waves the influence of near bed sorting processes on heavy mineral content in suspension is discussed sediment concentrations above a rippled bed of mixed quartz and heavy mineral sand were measured under regular nonbreaking waves in the laboratory using the traditional gradient diffusion process settling velocity would be expected to strongly affect sediment distribution this was not observed during present trials in fact the vertical gradients of time averaged suspension concentrations were found to be similar for the light and heavy minerals despite their different settling velocities this behavior implies a convective rather than diffusive distribution mechanism between the nonmoving bed and the lowest suspension sampling point light and heavy mineral concentration differs by two orders of magnitude this discrimination against the heavy minerals in the pickup process is due largely to selective entrainment at the ripple face bedform dynamics and the nature of quartz suspension profiles are found to be little affected by the trialed proportion of overall heavy minerals in the bed 3 8 22 1

opening mode fracture in siliceous mudstone at high homologous temperature effect of surface forces in analogy to high temperature sintering of ceramics and metal powder compacts the formation of opening mode fractures in siliceous mudstone during natural in situ combustion of hydrocarbons is attributed to contractile surface forces between mineral grains and an interstitial melt phase a comparison between bulk density increase during sintering and created fracture space indicates that fracturing resulted from contraction of the rock matrix due to porosity reduction grain scale mass transfer and high temperature mineral formation it is suggested that contractile surface forces between mineral grains and between mineral grains and pore fluid contribute to subcritical fracture formation under a wide range of subsurface conditions

meteor induced ridge and trough formation and the structuring of the nighttime e region ionosphere 1 when meteor generated plasma trails diffuse into the ionosphere they create large ambipolar electric fields mainly perpendicular to the earth s geomagnetic field b and extending many kilometers from the trail along b these fields will cause ionospheric plasma to collect into ridges extending from the trail along b enhancing the density by as much as a factor of 2 and they will also remove up to 90 of the plasma on each side of the ridge we predict that meteor induced density perturbations may fill as much as 20 of the ionosphere between 95 and 120 km altitude this paper presents simulations and theory to show how meteors produce plasma ridges and troughs we estimate the extent of these as a function of altitude and meteor density this process may explain observations of extensive nighttime e region density structures made by rockets and radars

water and sediment fluxes on the ebro delta shoreface on the role of low frequency currents abstract field measurements of water and sediment fluxes were acquired on the ebro delta shoreface at depths of 8 5 and 12 5 m during winter this period was characterised by high river discharges a dominant nw wind and the presence of six wave storms the survey can be considered as being typical of high energy conditions water fluxes on the shoreface were jointly dominated by mesoscale shelf processes anticyclonic gyre on the ebro shelf inducing a dominant northward flow and by local meteorological forcing n ne winds inducing southward current reversals which were coupled with the presence of e wave storms the concentration of suspended sediment in the water column was associated with wave activity i e wave induced near bottom velocity moreover a lag of 6 h was found between the concentration time series at both locations which was associated with a slow seaward advection of fine sediment as a result of this advection an increase of 30 in the content of fines of the bottom sediment at the deepest location was detected during the survey period measured sediment fluxes presented a pulsating structure related to mean current fluctuations and fluctuations in sediment concentration induced by waves longshore transport rates were dominant over cross shore rates when time integrated the net cross shore fluxes were offshore at the outer site and onshore at the inner site due to differences in current direction since it shifted towards the coast at the shallower location net longshore transport was almost nil when current reversals were present and when disappeared transport at the inner shelf was northward

a comparison of lower thermospheric winds derived from range spread and specular meteor trail echoes 1 interferometry measurements of range spread meteor trail echoes rstes also known as nonspecular echoes have provided new insights into both the irregularity structures in meteor trails and lower thermospheric winds ltws in this study we used trail echoes observed with the newly installed sanya 18 4 n 109 6 e 47 5 mhz vhf coherent radar and the sanya all sky meteor radar to estimate instantaneous zonal and hourly averaged meridional winds from rstes and hourly averaged zonal and meridional winds from large numbers of specular meteor echoes the mean height variations in both the zonal and meridional winds estimated from the rstes were generally consistent with those estimated from specular meteor echoes below 96 km this gives validity to the technique proposed recently by oppenheim et al 2009 and suggests that rste measurements made with a small radar can be used to investigate ltws whereas this had previously been limited to larger radars such as the jicamarca radar however some observations show significant differences in wind magnitude at individual heights at times the results of rste measurements show the presence of an intense westward wind with a speed near 100 ms 1 in contrast the specular meteor zonal winds were generally less than 50 ms 1 on the other hand the meridional drift of rstes derived from the meridional doppler velocity at higher altitudes shows a very poor correlation with the specular meteor meridional wind potential causes for the discrepancy in wind estimates obtained from rste and specular meteor trail echoes are discussed

modelling cross shore intensive sand transport and changes of bed grain size distribution versus field data abstract the study is focused on the cross shore transport of size graded sediment under combined wave current flow in the intensive flow regime the mathematical method for the description of hydrodynamic processes and sediment transport is adapted from the approach of kaczmarek and ostrowski coast eng 45 2002 1 for non graded sediment the theoretical model of vertical structure of sediment concentration as well as vertical mixing and sorting is presented the model of transport of graded sediment accounts for two directional cross shore sand flux the first component of this flux is directed onshore and results from wave asymmetry effects while the second one is directed seawards and is caused by undertow the vertical structure of this two directional sediment flux and its variability along a cross shore profile is a reason for segregation of grains in a horizontal direction this horizontal sorting of sand is theoretically modelled using the present approach a concept of the carpet of sorted sediments is implemented to reproduce the segregation of sediment in time and to assess the influence of grain size changes on short term evolution of the cross shore profile the results of computations are compared with the large scale laboratory data from the literature and with field measurements carried out by the authors the agreement between the model results and the experimental data is found to be very good particularly it has been theoretically shown that the observed in situ bimodal grain size distributions result from two directional sediment fluxes on the cross shore profile

day to night variation in meteor trail measurements evidence for a new theory of plasma trail evolution 1 a recent theory of meteor trail plasma diffusion made the prediction that meteors will generate more and longer lasting non specular echoes at night than during the day this letter presents the first evidence of a dramatic day to night difference in non specular meteor trail occurrence rates and their duration these observations were made by the 50mhz radar at the jicamarca radio observatory jro in peru in one 20 minute period starting 95 minutes before sunrise this radar detected 1288 head echoes and 341 trails while a similar time after dawn it measured 1240 head echoes but only 50 trails also the duration of the nighttime trails greatly exceeded the daytime ones this pattern was confirmed by a second experiment in july 2007 this data provides strong evidence that it is necessary to account for the effect of the ionospheric plasma density to explain meteor diffusion

nonspecular meteor trail altitude distributions and durations observed by a 50 mhz high power radar 1 high power large aperture hpla radars frequently observe nonspecular meteor trail echoes that result from plasma turbulence driven by the intense pressure gradients on the trail edges this paper analyzes the altitude range duration and dependence on head echo strength of nonspecular trails using two large data sets from the 50 mhz hpla radar at the jicamarca radio observatory over 2100 trails were used to build altitude profiles that extend from 86 km to 120 km altitude with 97 of events occurring between 90 km and 110 km longer duration trails tend to form at lower altitudes than shorter duration echoes the peak of the altitude distribution of trails lasting at least 5 s can be up to 12 km lower than the peak of the distribution of trails shorter than 5 s further the data show a clear power law relationship for the frequency of both head echo and nonspecular trail power an improved knowledge of nonspecular trails will allow researchers to better understand meteor trail evolution and their use in monitoring lower thermospheric winds

petroleum reservoir characterization using downhole microseismic monitoring imaging of microseismic data is the process by which we use information about the source locations timing and mechanisms of the induced seismic events to make inferences about the structure of a petroleum reservoir or the changes that accompany injections into or production from the reservoir a few key projects were instrumental in the development of downhole microseismic imaging most recent microseismic projects involve imaging hydraulic fracture stimulations which has grown into a widespread fracture diagnostic technology this growth in the application of the technology is attributed to the success of imaging the fracture complexity of the barnett shale in the fort worth basin texas and the commercial value of the information obtained to improvecompletions and ultimately production in the field the use of commercial imaging in the barnett is traced back to earlier investigations to prove the technology with the cotton valley imaging project and earlier experiments at the m site in the piceance

remote sensing lower thermosphere wind profiles using non specular meteor echoes 1 this article describes a new method of measuring wind velocity profiles between 93 km and 110 km altitude by tracking non specular meteor echoes as neutral winds transport the plasma trails this requires a large vhf radar with interferometric capability able to point nearly perpendicular to the geomagnetic field a small data sample from the jicamarca radio observatory allows the measurement of horizontal wind speeds and directions with a range resolution of a few hundred meters these observations show speeds reaching 150 m s and sometimes changing by as much as 100 m s over a 6 km altitude range at the best times these measurements can be made with only a few minutes of data with some refinement of the data collection and analysis techniques this technique should produce high resolution images of lower thermospheric winds as they change in both altitude and time

locating microearthquakes induced by hydraulic fracturing in crystalline rock microearthquakes induced by hydraulic fracturing in crystalline rock at a depth of 3 5 km were located with a precision of better than 30 m to obtain information about the geometry and dimensions of the fracture system produced the induced microseismicity was monitored by a network of five vorehole seismic stations a total of about 800 induced events were reliably located from arrival times event locations show a tabular distribution that strikes 350 sup 0 and dips 65 sup 0 east subparallel to the injection well the injection was intended to produce a fracture system that would hydraulically connect two subparallel wells a lack of fluid communication between them is consistent with a lack of induced microearthquakes near the target wellbore the 150 m thickness of the zone of seismicity far exceeds the relative locational uncertainties the injected fluid appears to have stimulated a zone of rock rather than simply a single fracture the distribution of microseismic events presumably envelopes the zone of fluid paths created by the fracture experiment