anisotropic inversion of refracted waves in vertical cable data in the presence of dip refracted arrivals are analysed to estimate the near surface anisotropy of marine sediments using a vertical cable vc configuration in the presence of dip the horizontal and vertical ray slownesses are obtained from the observed apparent slownesses in the up and downdip directions using a sum or difference at each azimuth the multiple azimuths generated by a vc geometry permit the ray slowness distribution of the marine sediments to be determined an inversion procedure is developed to provide dip and anisotropy parameters for refractive layers from the measured refraction traveltimes in multilayered azimuthally isotropic and anisotropic media two sets of transversely isotropic models are used to analyse the azimuthal variations of apparent and ray slownesses in the first set we fix the anisotropic parameters of the models but vary the dip 0 5 and 10 to test the effects of the presence of dip in the second set we vary the p wave anisotropy strength 5 2 10 3 15 8 and 22 0 to examine the sensitivity and accuracy of ray slowness approximations which are independent of dip we test this inversion procedure on synthetic p wave vc data calculated for six different models by a finite difference method the results of applications to real vc data acquired from the north sea are also presented

sediment transport and topographic evolution of a coupled river and river plume system an experimental and numerical study sediment transfer from rivers to the ocean is the fundamental driver of continental sedimentation with implications for carbon burial land use dynamics and unraveling global climate change and earth history from sedimentary strata coastal rivers are dynamically coupled to their offshore plumes at the river mouth creating regions of nonuniform flow that can dictate patterns of erosion and deposition both onshore and offshore however there are limited experimental and modeling studies on sediment transport and morphodynamics of coupled river and river plume systems and their response to multiple flood events to address this knowledge gap we developed a quasi 2 d morphodynamic numerical model and conducted exploratory flume experiments in a 7 5 m long flume where a 10 cm wide river channel was connected to a 76 cm wide ocean basin both the numerical model and the flume results demonstrate that 1 during low discharge flows backwater hydrodynamics cause spatial flow deceleration and deposition in the river channel and the offshore plume area and 2 during high flows the water surface is drawn down to sea level resulting in spatial flow acceleration and bed scour during high discharge flows we also found that the offshore river plume self channelized owing to both levee formation and bed scour our study suggests that coastal rivers may be in a perpetual state of morphodynamic adjustment and highlights the need to link rivers and river plumes under a suite of flow discharges to accurately predict fluvio deltaic morphodynamics and connectivity between fluvial sediment sources and marine sinks

analysis of multiazimuthal vsp data for anisotropy and avo multioffset vertical seismic profile vsp experiments commonly referred to as walkaways enable anisotropy to be measured reliably in the field the results can be fed into modeling programs to study the impact of anisotropy on velocity analysis migration and amplitude versus offset avo properly designed multioffset vsps can also provide the target avo response measured under optimum conditions since the wavelet is recorded just above the reflectors of interest with minimal reflection point dispersal in this paper the multioffset vsp technique is extended to include multioffset azimuths and a multiazimuthal multiple vsp data set acquired over a carbonate reservoir is analyzed for p wave anisotropy and avo direct arrival times down to the overlying shale and reflection times and amplitudes from the carbonate are analyzed data analysis involves a three term fit to account for nonhyperbolic moveout dip and azimuthal anisotropy results indicate that the overlying shale is transversely isotropi

3d 9c seismic modeling and inversion of weyburn field data abstractinversion of 3d 9c wide azimuth vertical seismic profiling vsp data from the weyburn field for 21 independent elastic tensor elements was performed based on the christoffel equation using slowness and polarization vectors measured from field data to check the ability of the resulting elastic tensor to account for the observed data simulation of the 3c particle velocity seismograms was done using eighth order staggered grid finite differencing with the elastic tensor as input the inversion and forward modeling results were consistent with the anisotropic symmetry of the weyburn field being orthorhombic it was dominated by a very strong tranverse isotropy with a vertical symmetry axis superimposed with minor near vertical fractures with azimuth 55 from the inline direction the predicted synthetic seismograms were very similar to the field vsp data the examples defined and provided a validation of a complete workflow to recover an elastic tensor from 9c data the number and values of

modeling framework for sediment deposition storage and evacuation in the floodplain of a meandering river theory 1 a theory for the movement of suspendible sediment through the floodplain of a simple actively migrating river is developed the floodplain is represented as a series of sediment storage reservoirs each incorporating multiple bends of a river independent models for floodplain sediment deposition and for subsequent removal of sediment by lateral channel migration allow the volume of sediment stored in the floodplain and the associated average floodplain elevation to change over time and down the valley at a given quasi steady discharge deposition depends on a simple one dimensional partition between channel and floodplain discharge and on suspended sediment concentration one size class which varies along the reach depending on lateral supply and floodplain history sediment supply is specified at the upstream end of the reach using a suspended sediment rating curve long term average overbank deposition rates are approximated by weighting multiple steady state solutions using a flow duration curve the channel bed elevation of the system is assumed to be fixed and tributaries are handled using simple lateral source terms the interaction between the submodels is such that under constant boundary conditions the channel evolves toward a state of geomorphic equilibrium where the volume of sediment added to the floodplain by overbank deposition just balances the volume removed by channel migration the model is easily adapted to track conservative suspended sediment tracer material concentration as such material moves through the channel floodplain system

net local removal of floodplain sediment by river meander migration abstract erosion from the banks of meandering rivers causes a local influx of sediment to the river channel most of the eroded volume is usually replaced in nearby point bars however the typical geometry of river bends precludes the local replacement of all eroded material because i point bars tend to be built to a lower elevation than cutbanks and ii point bars tend to be shorter than the eroding portion of cutbanks because of channel curvature in a floodplain that is in equilibrium i e neither increasing nor decreasing in volume sediment eroded from cut banks must be replaced elsewhere on the floodplain the local imbalance caused by differences in bank height should be balanced primarily by overbank deposition while the local imbalance caused by curvature should be balanced primarily by deposition in abandoned stream courses or oxbow lakes estimates of these local imbalances based on remotely sensed measurements of bank geometry and channel migration have been made on four systems in the united states a 91 km reach of the pearl river in louisiana and mississippi a 62 km reach of the bogue chitto river in louisiana a 35 km reach of the neuse river in north carolina and a 2 7 km reach of the vermillion river in minnesota for these systems the total local imbalance integrated over many bends ranged from 0 36 to 2 27 m 3 yr m of valley length 0 15 to 1 32 m 3 yr m of channel length or 7 to 45 of gross cutbank erosion with a typical value of about 17 when compared with gauged suspended sediment data the measurements provide estimates of the relative importance of floodplains for storing material transported by a river system the data suggest that even if the studied systems are near long term mass balance equilibrium as opposed to undergoing net deposition or erosion almost all of the sand and in some cases much of the silt and clay in transit through these systems is likely to have spent some time stored in an upstream floodplain

inversion of multicomponent multiazimuth walkaway vsp data for the stiffness tensor vertical seismic profiling vsp an established technique can be used for estimating in situ anisotropy that might provide valuable information for characterization of reservoir lithology fractures and fluids the p wave slowness components conventionally measured in multiazimuth walkaway vsp surveys allow one to reconstruct some portion of the corresponding slowness surface a major limitation of this technique is that the p wave slowness surface alone does not constrain a number of stiffness coefficients that may be crucial for inferring certain rock properties those stiffnesses can be obtained only by combining the measurements of p waves with those of s or ps modes here we extend the idea of horne and leaney who proved the feasibility of joint inversion of the slowness and polarization vectors of p and sv waves for parameters of transversely isotropic media with a vertical symmetry axis vti symmetry we show that there is no need to assume a priori vti symmetry or any other specific typ

changes in river regime after the construction of upstream reservoirs this article presents and analyses many years of investigations in china on the fluvial processes downstream of impounding and detention reservoirs the study covers the change in hydrograph the recovering of sediment concentration along the river course the degradation of stream bed the adjustment of longitudinal profile the coarsening of bed material the change in channel width and the trend of channel pattern variation for alluvial streams downstream of impounding reservoirs without confluence of major tributaries the degradation may extend to a great distance below the dam in the process of reducing the sediment carrying capacity of the flow to match the diminished sediment supply the coarsening of bed material is a factor of equal if not greater importance as compared with the flattening of channel gradient in places where the flow has not been sufficiently cut down and the bank is erosive non resistant a receding of banklines may take place in concurrence with the deepening of the river bed below detention reservoirs even if the total runoff and sediment supply remain essentially unchanged the modification of the hydrograph is sufficient to enhance the deterioration of the downstream channel

geomorphic signature of a dammed sandy river the lower trinity river downstream of livingston dam in texas usa abstract reservoirs behind dams act as deposition sites for much of the sediment being transported by rivers as a result the downstream river flow can be well below the transport capacity for bed material this promotes bed erosion and other geomorphic changes over some length of river located immediately downstream from a dam these adjustments have been characterized for the trinity river tx downstream of livingston dam field measurements and results from a 1d numerical model define a 50 60 river kilometer segment of river undergoing bed erosion as the transport capacity for bed material is reestablished consequences of this erosion include lowering of the channel bed reduction in the sediment volume of channel bars coarsening of sediment on bar tops steepening of channel banks and reduction in lateral migration rates of river bends repeat surveys of the river long profile reveals that 40 yr of dam closure has produced up to seven meters of channel bottom incision downstream of the dam transforming an initially linear profile into a convex up long profile the model output matches this observed change providing confidence that calculated estimates for spatial and temporal changes in bed material sediment flux can be used to explore the long term signature of dam influence on the geomorphology of a sand bed channel measurements of channel geometry profile lateral migration and grain size of the lower trinity river with distance downstream define both the trend and expected variability about the trend associated with the disruption to the bed material load

quantifying downstream impacts of impoundment on flow regime and channel planform lower trinity river texas abstract as human population worldwide has grown so has interest in harnessing and manipulating the flow of water for the benefit of humans the trinity river of eastern texas is one such watershed greatly impacted by engineering and urbanization draining the dallas fort worth metroplex just under 30 reservoirs are in operation in the basin regulating flow while containing public supplies supporting recreation and providing flood control lake livingston is the lowest as well as largest reservoir in the basin a mere 95 km above the trinity s outlet near galveston bay this study seeks to describe and quantify channel activity and flow regime identifying effects of the 1968 closure of livingston dam using historic daily and peak discharge data from usgs gauging stations flow duration curves are constructed identifying pre and post dam flow conditions a digital historic photo archive was also constructed using six sets of aerial photographs spanning from 1938 to 1995 and three measures of channel activity applied using a gis results show no changes in high flow conditions following impoundment while low flows are elevated however the entire post dam period is characterized by significantly higher rainfall which may be obscuring the full impact of flow regulation channel activity rates do not indicate a more stabilized planform following dam closure rather they suggest that the trinity river is adjusting itself to the stress of livingston dam in a slow gradual process that may not be apparent in a modern time scale

dams and geomorphology research progress and future directions dams impose changes of flow and sediment transfer that drive changes of channel form along the downstream regulated river these changes have been described for more than 50 years but process form relationships have only been advanced with the establishment of a conceptual framework during the 1970s and then the extension of monitoring data and advancement of remote sensing technologies particularly over the past 20 years this paper reviews these developments and identifies three influential themes i channel dynamics ii the role of riparian vegetation and iii channel change as the driver of ecological change changes can be rapid in semi arid regions but elsewhere relaxation periods may extend to millenia in these latter cases regime or steady state models should be replaced by models of transient states applied to the reach scale in order to respond to the needs of river managers over decadal timescales

alternate bar response to sediment supply termination 1 sediment supply is widely held to be one of the primary controls on bar topography in alluvial channels yet quantitative linkages between sediment supply and bar topography are not well developed we explore the conditions under which alternate bars form and how they respond to the elimination of sediment supply in two linked laboratory experiments the first set of experiments was conducted in a 28 m long 0 86 m wide flume channel using a unimodal sand gravel mix the second set of experiments was conducted at field scale in a 55 m long 2 74 m wide channel using a unimodal gravel mixture in both experiments alternate bars and patchy surface grain size distributions developed under steady flow and sediment supply conditions the cessation of the sediment supply induced a reduction in the surface grain size heterogeneity and the bars were eliminated in both flumes mean boundary shear stress had declined but were capable of moving sediments after the bars disappeared albeit at relatively small rates compared to when the bars were present in the smaller flume the previously stationary bars migrated out of the flume and were not replaced with new bars a nearly featureless bed formed with limited surface grain size heterogeneity a slightly coarsened surface and a slightly reduced slope in the larger flume the formation of alternate bars was induced by an imposed upstream flow constriction and as such the bars did not migrate termination of sediment supply led to progressive erosion of bed topography and loss of the bars coarsening of the bed surface loss of bed texture patchiness and significant slope reduction the original alternate bar topography redeveloped when the sediment supply was restored once sufficient deposition had occurred to reconstruct the original channel slope this shows that the bar loss was reversible by establishing the previous conditions and highlights the importance of sediment supply for bar formation the role of sediment supply in bar formation and stability is not often recognized in stream restoration our results suggest that the loss of sediment supply can significantly affect alternate bar topography and that considerable volumes of sediment may be needed restore channel bars

downstream trends in discharge slope and stream power in a lower coastal plain river conceptual models of river estuary interaction are typically based on a notion of systematic downstream change in the intensity of fluvial processes low slopes backwater effects and effects of antecedent topography and landforms may complicate downstream tends in water and sediment flux in coastal plain rivers an analysis of the lower trinity river texas shows no consistent downstream pattern of increases or decreases in the discharge stream power or water surface slope flows may decrease downstream due to coastal backwater effects in the lowermost reaches and due to diversion of flow into valley bottom depressions during high flows in both the fluvial and fluvial estuarine transition reaches in general however stream power and slope decrease in the lower reaches consistent with earlier findings of limited fluvial sediment delivery to the coastal zone some tributaries may become distributaries at high but sub bankfull flows as backwater effects reverse flows into depressions associated with paleomeanders the paleomeanders and possibly the locations of these reversible channels are related to antecedent topography associated with aggradation degradation cycles over the past 100 ka low gradient coastal plain rivers may not function as simple conduits from land to sea further the transition from fluvial to coastal dominance may be variable along the river with the variability controlled not just by the relative magnitude of river and tidal or backwater forcing but also by valley topography controlled in part by antecedent landforms

determination of anisotropic velocity models from walkaway vsp data acquired in the presence of dip wide aperture walkaway vertical seismic profile vsp data acquired through transversely isotropic horizontal layers can be used to determine the p phase slowness surface local to a receiver array in a borehole in the presence of dip errors in the slowness surface may occur if the medium is assumed to be layered horizontally if the acquisition plane is oriented parallel to the dip direction the derived slowness is too large for sources offset from the well in the down dip direction and too small for sources offset from the well in the up dip direction for acquisition parallel to the strike of the layers the recovery of the p phase slowness in the vicinity of the receiver array is excellent it is therefore preferable to orient the walkaway vsp in the strike direction to estimate the anisotropic parameters of the medium in the vicinity of a receiver array however this may not be possible if the dip direction of all layers has the same azimuth the variation of walkaway traveltimes with azimuth ha

joint inversion of geophysical data summary by jointly inverting several different kinds of geophysical measurements at a site we avoid some of the ambiguity inherent in the individual methods we show how this can be done for the combination of dc resistivity and magnetotelluric measurements on a layered medium by considering a simple 3 layer model the combination resolves the resistivity of the thin resistive second layer even though neither of the two methods can do so alone the method is then applied to field data from a shallow sedimentary basin a blind zone occurs beneath a thick near surface conductive shale by a study of the eigenvalue structure of the model it can be seen that resolution in this zone would be slightly enhanced by higher frequency magnetotelluric data but additional dc data at larger spacing would yield no improvement

an interpretation of episodic slip on the calaveras fault near hollister california data from a precision multi wavelength distance measuring mwdm instrument located near hollister california have been analyzed in terms of strike slip faulting in the region covered by the network four episodes of deformation that are readily identifiable in the mwdm data for the year following september 1975 can be modeled as slip on the calaveras fault creepmeter data tend to support this interpretation since the creep events and the observed episodes of deformation appear to be correlated in a temporal sense lower bounds on the moment for each episode of slip can be determined from the mwdm data by the technique of linear programing the results indicate that aseismic slip is the dominant mechanism for strain release since the combined moment of 1 16 1024 dyn cm for the four episodes far exceeds the moment of all of the earthquakes that occurred during the year creepmeter data and geologic evidence that are taken in conjunction with the lower bound of the moment indicate that the depth of aseismic slip for some of the episodes of fault slip could extend to below the greatest depth of earthquake hypocenters for this region of central california

sediment transport part iii bed forms and alluvial roughness a method is presented that makes the classification of bed forms the prediction of the bed form dimensions and the effective hydraulic roughness of the bed forms feasible the proposed relationships are based on the analysis of reliable flume and field data a verification analysis using about 1 500 alternative reliable flume and field data shows good results in predicting the hydraulic roughness friction factor for field conditions the proposed method yields considerably better results than previously proposed methods which are reviewed here the proposed method has also been used to predict the flow depth and the total bed material load

model for calculating bed load transport of sediment a bed load transport model based on the mechanics of sediment moving by saltation yields predicted values of bed load flux as a function of boundary shear stress grain diameter and density the parameters required to calculate bed load transport particle velocity bed load sediment concentration and the height of the bed load layer can all be determined from our model which computes sequences of trajectories of individual saltating grains as well as the concentration of moving grains that the flow can support the latter is related to the momentum the accelerating grains extract from the flow predicted curves of bed load transport vs boundary shear stress agree well with transport data collected by gilbert meyer peter iet al n and the waterways experimental station measured shear stress was corrected for pressure drag when bed forms were present comparison of predicted bed load transport with common bed load equations reveals considerable similarity among the relationships the best agreement with the data is produced by relationships in which the transport rate vanishes as the shear stress approaches the critical shields number as in our model and yalin s equation

sediment supply and the development of the coarse surface layer in gravel bedded rivers the bed surface of most gravel rivers is considerably coarser than the sub surface or the gravel load transported over it a phenomenon affecting river dynamics morphology and ecology the coarse surface layer often called an armour or pavement has been attributed to an inherent tendency for small grains to settle between larger ones during active transport of all sizes1 2 and to selective erosion or trapping of finer particles when the coarse grains are relatively immobile3 4 where bedload supply is cut off below dams selective erosion causes coarsening5 6 using a simple quantitative model we propose that surface coarsening develops in gravel bedded rivers when local bedload supply from upstream is less than the ability of the flow to transport that load we present laboratory results which support this hypothesis and show that supply reduction causes changes in bedforms and progressive confinement of active bedload transport to a narrow band of finer bed surface bordered by a coarse less active bed it may therefore be possible to relate the degree of river bed surface coarsening to sediment supply resulting from land use

backwater controls of avulsion location on deltas river delta complexes are built in part through repeated river channel avulsions which often occur about a persistent spatial node creating delta lobes that form a fan like morphology predicting the location of avulsions is poorly understood but it is essential for wetland restoration hazard mitigation reservoir characterization and delta morphodynamics following previous work we show that the upstream distance from the river mouth where avulsions occur is coincident with the backwater length i e the upstream extent of river flow that is affected by hydrodynamic processes in the receiving basin to explain this observation we formulate a fluvial morphodynamic model that is coupled to an offshore spreading river plume and subject it to a range of river discharges results show that avulsion is less likely in the downstream portion of the backwater zone because during high flow events the water surface is drawn down near the river mouth to match that of the offshore plume resulting in river bed scour and a reduced likelihood of overbank flow furthermore during low discharge events flow deceleration near the upstream extent of backwater causes enhanced deposition locally and a reduced channel fill timescale there both mechanisms favor preferential avulsion in the upstream part of the backwater zone these dynamics are fundamentally due to variable river discharges and a coupled offshore river plume with implications for predicting delta response to climate and sea level change and fluvio deltaic stratigraphy

sand as a stable and sustainable resource for nourishing the mississippi river delta dams have starved the lower mississippi river of sediment over recent decades suggesting that the drowning of the delta is inevitable analysis of the rivers suspended sediment load and morphodynamic modelling suggest that the amount of sand essential for land building has not significantly decreased since dam construction with sand remaining available for several centuries

electromagnetic soundings in the sedimentary basin of southern ontario a case history the results of electrical and electromagnetic em soundings conducted in the sedimentary basin of southern ontario are presented the sounding sites are located strategically to take advantage of the dipping nature of the sediments a progressive study being undertaken from shallow to deep sections the vertical magnetic field transfer function estimated with a pseudonoise source em system and conventional schlumberger apparent resistivity are jointly inverted for each site the layered earth model containing the minimum number of layers is fitted to the data the joint inversion enables up to eight distinct layers to be identified at some sites the nonuniquenesses or ambiguities in each model for example s conductivity thickness product equivalence of a deep thin conductive layer are revealed through an eigensolution analysis the intrinsic ambiguities of the models of the deeper sections are resolved by a systematic progressive site to site correlation of electrical units across the basin for example the conductivity of a conductive unit is measurable when it is at a shallow depth in a deeper section if the unit appears as a thin layer it is assigned this value of conductivity removing the s equivalence ambiguity the individual inversions combined with the site to site correlation produce an overall electrical model of the basin consistent with the known geologic section

joint interpretation of gravity and magnetic data over axial symmetric bodies with application to the darnley bay anomaly nwt canada a method is presented for determining bounds of the properties of axial symmetric bodies from a finite number of gravity and magnetic observations based on parker s theory of ideal bodies bounds on the density contrast and the intensity of magnetization are calculated as a function of depth to the top of the anomalous source restricting the range of smallest possible solutions to fit the data the model studied is approximated by an array of vertical annuli cylinders each of uniform density and magnetization linear programming algorithms based on the ideal body theory were used to calculate the distribution of these parameters within the body simultaneous inversion of gravity and magnetic data is performed assuming a constant ratio between the density contrast and the intensity of magnetization and that a common body is responsible for both observed fields the parameter kappa absolute value of j delta rho provides information about the rock type of the structure interpretation of gravity and aeromagnetic data from darnley bay nwt canada indicated the presence of a shallow ultrabasic intrusion