analysis of a guided wave along a conducting structure in a borehole a guided wave along a borehole is often observed in borehole radar measurements these guided waves deform the antenna pattern and can cause artefacts in radar measurements a water filled borehole or a conducting logging cable can function as a waveguide for electromagnetic waves under some conditions we describe the theoretical characteristics of such a guided wave in a borehole and compare them with our experiments the measured signal discussed was obtained with a directional borehole radar this radar uses a cylindrical conformal array antenna as receiver and is a model of a conducting structure in a borehole the induced field around the borehole was compared with the theory the most fundamental symmetrical and asymmetrical modes were tm 01 and he 11 and they were identified in the measured signals using time frequency distribution analysis and by observation of the azimuthal field distribution of the magnetic field

application of k ϵ turbulence models to enclosed basins the role of internal seiches 1 a numerical model was developed for the prediction of the density stratification of lakes and reservoirs it combines a buoyancy extended k model with a seiche excitation and damping model to predict the diffusivity below the surface mixed layer the model was applied to predict the seasonal development of temperature stratification and turbulent diffusivity in two medium sized lakes over time periods ranging from 3 weeks to 2 years depending on the type of boundary condition for temperature two or three model parameters were optimized to calibrate the model the agreement between the simulated and the observed temperature distributions is excellent in particular if lake surface temperatures were prescribed as surface boundary condition instead of temperature gradients derived from heat fluxes comparison of different model variants revealed that inclusion of horizontal pressure gradients and or stability functions is not required to provide good agreement between model results and data with the aid of uncertainty analysis it is shown that the depth of the mixed surface layer during the stratified period could be predicted accurately within 1 m the sensitivity of the model to several parameters is discussed index terms 4211 oceanography general benthic boundary layers 4255 oceanography general numerical modeling 4568 oceanography physical turbulence diffusion and mixing processes keywords lake turbulence model seiche stratification simulation turbulence kinetic energy

reverse time migration a prospect of seismic imaging methodology abstract reverse time migration rtm is a seismic imaging method to map the subsurface reflectivity using recorded seismic waveforms the practice in exploration seismology has long established a two fold approach of seismic imaging using velocity modeling building to establish the long wavelength reference velocity models and using seismic migration to map the short wavelength reflectivity structures among various seismic migration methods for different situations rtm is the only method that is capable to use all seismic wave types that can be computed numerically being initiated in early 1980 s rtm seeks an image of the subsurface reflectivity as the best match in an image space between the extrapolation of time reversed waveform data and the prediction based on estimated velocity model and source parameters judging the image quality in the same space of forming the images is more advantageous than the approaches of modeling and inversion which seek the solution in the model space but judge its fitness in data space considering that most seismic migration applications today still use primary reflection as the only signal the capability of rtm to use all computable wave types is unique and helpful reducing the imaging artifacts due to mistaking non primary waves as primary reflections hence we refer to those rtm algorithms using only primary reflections as the first generation rtm methods and the rtm algorithms making a full use of primary reflections multiple reflections and other non primary waveform data as the second generation rtm methods this paper reviews the development history of the rtm along with its major challenges current solutions and future perspectives

three dimensional modeling of negatively buoyant flow in diverging channels diverging channels also known as diffusers represent common natural and industrial outlets to lakes reservoirs and rivers if the outflow in a diffuser has a larger density than the ambient water the inflow may plunge and form a density underflow in this paper a three dimensional numerical study was conducted to gain insight into the mechanism of negatively buoyant flows in diffusers with a sloping bottom of particular interest is the formation of separated flows such as wall jet and free jet flows various cases of plunging and the associated density current in a diffuser with different divergence angles and inflow densimetric froude numbers are considered the model successfully simulates the formation of attached flow wall jets and free jets in a negatively buoyant environment the onset evolution and stabilization of a stall and the subsequent development of a wall jet in a negatively buoyant flow are investigated in detail computed results also show favorable agreement with some published experimental data on density current generated by the plunging of cold water in ambient warm water in a diverging channel

subcritical junction flow the main flow features of subcritical junction flows are explored based on an extended hydraulic model study the flow conditions chosen resulted in flows that are governed by the froude similarity law so called simple junctions with junction angles of 30 60 and 90 were tested the main emphasis was made in determining the characteristics of the lateral flow and the flow contraction in the tailwater branch further expressions for the momentum correction coefficients the lateral wall pressure force and the ratio of flow depths in the lateral and upstream branches were provided a rational approach for the momentum contribution of the lateral branch is presented and applied for the prediction of the backwater effect across a simple junction the complex flow pattern is further documented with selected photographs such that a rather complete description of junction flow is now available

the influence of flow inertia buoyancy wind and flow unsteadiness on mixing at the asymmetrical confluence of two large rivers summary the rates and patterns of mixing of two large rivers with large density differences at a strongly asymmetrical confluence in northern spain are analyzed we assess the factors controlling the site where the denser river plunges and the mixing rates between the rivers in particular we focus on the interaction between inertial and buoyancy forces the effect of wind forcing and the unsteady nature of the hydraulic forcing the steady state location of the plunge line is shown to be controlled by an inertia buoyancy balance which accounts for the relative magnitude of the buoyancy forcing associated with density differences between the confluent rivers and the magnitudes of both the main stream and the side flow tributary inertia the plunge line moves to upstream locations as the inertia of the tributary increases for low tributary inertia and or the density contrast between the rivers increases this has important consequences for river mixing since mixing rates increase as the plunging occurs at the confluence the high mixing rates in this case occur as a result of a large mixing interface surface area and high diffusivities as the plunging area moves upstream or downstream of the confluence vertical diffusivities or the area of contact available for mixing decrease and constrain mixing rates wind forcing depending on its velocity and direction affects mixing rates through 1 altering the buoyancy inertia equilibrium and so changing the location of the plunge line 2 altering the pattern of secondary circulation within the confluence and or 3 increasing shear at the confluence flow unsteadiness can lead to changes in the location of the plunge line through time and thus can strongly modify mixing rates at the confluence the downstream movement of the plunge line is advection dominated while its upstream movement seems to respond to a baroclinic response of the confluence

bed morphology and sedimentology at the confluence of unequal depth channels abstract spatial and temporal variations in the bed geometry and bed material size of a sand bed river confluence with unequal depth channels were monitored during a sequence of floods additionally two other sand bed confluences were surveyed to test the replicability of these observations the confluences studied here have only one avalanche face which corresponds to the front of a tributary mouth bar in the shallower channel and do not exhibit a marked scour zone three distinct morpho sedimentological zones are present 1 an area around the upstream corner of the junction where the sediments are generally finer than the mean 2 a maximum depth zone with coarser than average particles and 3 a bar at the downstream junction corner where grain size is finer than the mean and decreases slightly downstream changes in relative discharge between the two channels had little effect on the grain size of the downstream junction corner bar but exerted a strong influence on the position of the maximum depth zone and the front of the tributary mouth bar the downstream junction corner bar showed little evidence of the separation zone which is commonly observed at the confluences of laboratory channels the contrasting depths of the approach channels at the sites studied here may be partly responsible for the absence of the separation zone

on the mixing of rivers with a difference in density the case of the negro solimoes confluence brazil abstract confluences are common components of all riverine systems and characterized by converging streamlines and potential mixing of separate flows the fluid dynamics of confluences possess a highly complex structure with several common types of flow features observed an investigation was carried out in both low and relatively high flow conditions at the confluence of the negro and solimoes rivers brazil which ranks among the largest river junctions on earth during this field research acoustic doppler velocity profiling adcp and water quality sampling were applied to investigate hydrodynamics sediment transport and mixing characteristics at this confluence it was found that the location and the width of the mixing interface were closely related to changes of the discharge ratio between the tributaries due to both seasonal and annual variations second a timescales analysis was applied to comparatively identify the contribution to mixing at the negro solimoes confluence of four processes difference in 1 velocity and 2 density between the rivers 3 bed friction including form drag and 4 change in channel width the analysis demonstrated that adjustments of confluence hydrodynamics and morphodynamics can modify the relative importance of each contribution immediately downstream of the junction the effects of differences in velocity and density were comparable while farther downstream the latter was predominant and the role of bed friction was significant at the end this study suggests that mixing at the negro solimoes confluence can be explained as a combination of the four hydrologic and morphologic processes

elastic reverse time migration elastic prestack reverse time finite difference migration of two component seismic surface data requires data extrapolation and application of an imaging condition data extrapolation involves synchronous driving of the vertical component and horizontal component finite difference meshes with the time reverse of the recorded vertical and horizontal traces respectively extrapolation uses the coupled elastic wave equation for variable velocity solved with a second order explicit finite difference scheme the imaging condition at any point in the grid is the one way traveltime from the source to that point elastic migrations of both synthetic test data and real two component common source gathers produce simpler images than acoustic migrations because of the coalescing of double reflections compressional waves and shear waves into single loci

causes of rapid mixing at a junction of two large rivers rio parana and rio paraguay argentina 1 airborne and satellite observations show that when large rivers join they can take hundreds of kilometers to mix completely but on occasion may mix very rapidly application of established semitheoretical analyses shows that long mixing lengths should be expected the first measurements of mixing processes at a large river junction rio parana and rio paraguay argentina combined width 2 8 km are presented at two occasions first when they mix in 400 km and second when mixing is complete in only 8 km downstream of the junction for the case of slower mixing at a point surveys showed that mixing driven by turbulent shear associated with a near vertical shear layer was restricted to close to the junction to 0 272 multiples of the postconfluence width downstream transect surveys showed penetration of more turbid water from the rio paraguay underneath the rio parana but this was insufficient to promote more rapid mixing there was no clear channel scale circulation present and slow mixing was compounded by reverse topographic forcing on the mainstream rio parana side of the river this kept more turbid water on the rio paraguay side of the river close to the bed in the case of rapid mixing we found clear channel scale circulation the momentum ratio between the combining flows reinforced the effects of the discordance in bed height between the tributaries at the confluence and allowed penetration of more turbid rio paraguay water further across the channel width deeper within the flow the importance of the interaction between momentum ratio and bed morphology at channel junctions makes mixing rates at the confluence dependent upon basin scale hydrological response which is more likely to differ between large confluent rivers than small rivers as a result of the different climatic topographic zones that they may capture

coal top detection by conductively guided borehole radar wave imaging for open cut blast hole drilling abstract damage to the top of coal seams caused by incorrect blast stand off distances results in coal losses of up to 10 15 to the australian open cut coal mining operations this is a serious issue to be addressed here we propose to use a new forward looking imaging technique based on the borehole radar technology to predict the coal seam top in real time while drilling blast holes this is achieved by coupling the conventional borehole radar waves on to a steel drill rod to induce a guided wave along the axial drill rod the drill rod ahead of the borehole radar behaves as a forward looking antenna for the guided waves both numerical modelling and field trials simulating a drill rod as an antenna are used to investigate the feasibility of the proposed technique for prediction of the coal top under typical open cut environments numerical modelling demonstrated that conductivity of the overburden is the most important factor affecting our ability to see coal seams ahead of the drill bit the guided borehole radar waves could be used for top coal prediction and a theoretical prediction error less than 10 cm and a forward looking capability of 4 6 m can be achieved field trials at australian open cut coal mines also demonstrated that guided borehole radar waves can be observed and used for prediction of coal top ahead of drill bit during blast hole drilling in resistive open cut environments the average resistivity should be higher than 75 m

reply to discussion of imran et al on the orientation of helical flow in curved channels by corney et al sedimentology 53 249 257

ground roll attenuation using the s and x f k transforms ground roll which is characterized by low frequency and high amplitude is an old seismic data processing problem in land based seismic acquisition common techniques for ground roll attenuation are frequency filtering f k or velocity filtering and a type of f k filtering based on the time offset windowed fourier transform these techniques assume that the seismic signal is stationary in this study we utilized the s x f k and t f k transforms as alternative methods to the fourier transform the s transform is a type of time frequency transform that provides frequency dependent resolution while maintaining a direct relationship with the fourier spectrum application of a filter based on the s transform to land seismic shot records attenuates ground roll in a time frequency domain the t f k and x f k transforms are approaches to localize the apparent velocity panel of a seismic record in time and offset domains respectively these transforms provide a convenient way to define offset or time varying reject zones on the separate f k panel at different offsets or times

noise attenuation by vibroseis whitening vsw processing reflection seismic data recorded on the atlantic coastal plain to examine suspected faulting of the shallow 200 m basement were contaminated by severe ground roll noise with an apparent velocity of 587 m sec 25 hz dominant frequency and 23 m apparent wavelength a receiver interval of 5 m and a total spread length of 285 m were used to obtain 24 fold coverage although the noise problem was severe overlapping source and or receiver arrays on the surface were not used for the recording geometry in order to avoid a decrease in resolution caused by the smoothing effect of overlapping subsurface coverage instead the large amplitude surface waves were attenuated by a process called vibroseis whitening vsw vsw is based on the application of time varying amplitude scaling before vibroseis crosscorrelation a conventional method of automatic digital gain control was found to be effective for this purpose this scaling results in a signal to noise ratio improvement equal to the gain expected from crossc

estimation of azimuthal anisotropy from vsp data using multicomponent s wave velocity analysis observation of azimuthal shear wave anisotropy can be useful for characterization of fractures or stress fields shear wave anisotropy is often estimated by measuring splitting of individual shear wave events in vertical seismic profile vsp data however this method may become unreliable for zero offset marine vsp where the seismogram often contains no strong individual shear events such as direct downgoing shear wave but often contains many low amplitude ps mode converted waves we have developed a new approach for estimation of the fast and slow shear wave velocities and orientation of polarization planes based on the multicomponent linear traveltime moveout velocity analysis this technique is applicable to zerooffset vsp data and should take advantage of the presence of a large number of shear wave events with the same apparent velocity which for a horizontally layered medium should be close to the interval velocity the approach assumes that the vsp data are acquired in a vertical well drilled in an orthorhombic medium with a horizontal symmetry plane including horizontal transverse isotropy the main idea is to estimate the dominant apparent velocity for a given polarization direction by measuring the coherency of the seismic signal of a large number of events as a function of the apparent velocity the algorithm was tested on marine three component 3c vsp acquired in the north west shelf of australia and on land 3c vsp acquired with different sources in the same borehole located in otway basin victoria these tests show good agreement between anisotropy parameters magnitude and orientation derived from the vsp and cross dipole sonic log data

fracture evaluation using reflected stoneley wave arrivals we use the low frequency reflected stoneley wave mode to locate permeable fractures intersecting a borehole and to estimate their effective apertures assuming a model in which the average aperture of the fracture is roughly constant theoretical work relates the magnitude of the stoneley wave reflectivity to an effective fracture width we treat both the case of a horizontal fracture and the case of a fracture crossing the borehole at an angle laboratory experiments verify the analytic solution for the case of a horizontal fracture full waveform array sonic data were also acquired in a wellbore with a long recording time 25 5 ms in order to capture the late stoneley wave arrivals the data processing involves computation of the stoneley wave reflectivity response using the measured direct and reflected stoneley wave arrivals a least squares fit to the arrival time of the reflected wave arrivals is used to estimate the locations of permeable fractures and the effective width of the fractures is esti

secondary currents in a curved stratified estuarine channel this paper presents a study of secondary circulation in a curved stratified channel in northern san francisco bay over a 12 5 hour tidal cycle secondary currents were strong at times varying by up to 35 cm s from top to bottom but relatively transient as the balance between centrifugal and lateral baroclinic forcing changed over time the short travel time around the bend did not allow a steady state balance to develop between centrifugal and lateral baroclinic forcing during the flood tide the confluence of two streams with different velocities produced a strong lateral gradient in streamwise velocity as a result lateral advection was a significant term in the streamwise momentum balance having the same order of magnitude as the barotropic and baroclinic pressure gradients and the frictional terms during the first part of the ebb secondary currents were induced by lateral baroclinic forcing the direction of the secondary circulation reversed later in the ebb as the baroclinic forcing became weaker than the centrifugal acceleration the gradient richardson number showed that stratification was stable over most of the tidal cycle decreasing the importance of friction and allowing secondary currents to persist

split step fourier migration the split step fourier method is developed and applied to migrating stacked seismic data in two and three dimensions this migration method which is implemented in both the frequency wavenumber and frequency space domains takes into account laterally varying velocity by defining a reference slowness reciprocal of velocity as the mean slowness in the migration interval and a perturbation term that is spatially varying the mean slowness defines a reference vertical wavenumber which is used in the frequency wavenumber domain to downward continue the data across a depth interval as in constant velocity phase shift migration the perturbation term is used to define a source contribution that is taken into account by the application of a second phase shift in the frequency space domain since the method does not include the effects of second and higher order spatial derivatives of the slowness field the method theoretically is accurate only when there are no rapid lateral slowness variations combined wi

reverse time migration migration of stacked or zero offset sections is based on deriving the wave amplitude in space from wave field observations at the surface conventionally this calculation has been carried out through a depth extrapolation we examine the alternative of carrying out the migration through a reverse time extrapolation this approach may offer improvements over existing migration methods especially in cases of steeply dipping structures with strong velocity contrasts this migration method is tested using appropriate synthetic data sets

paraxial ray kirchhoff migration a rapid nonrecursive prestack kirchhoff migration is implemented for 2 d or 2 5 d media by computing the green s functions both traveltimes and amplitudes in variable velocity media with the paraxial ray method since the paraxial ray method allows the green s functions to be determined at points which do not lie on the ray two point ray tracing is not required the green s functions between a source or receiver location and a dense grid of thousands of image points can be estimated to a desired accuracy by shooting a sufficiently dense fan of rays for a given grid of image points the paraxial ray method reduces computation time by one order of magnitude compared with interpolation schemes the method is illustrated using synthetic data generated by acoustic ray tracing application to vsp data collected in a borehole adjacent to a reef in michigan produces an image that clearly shows the location of the reef

mixing and circulation at the confluence of two rivers entering a meandering reservoir 1 a field data set collected under different conditions is analyzed to characterize the spatial arrangement of two large inflows ebro and segre with distinct physical chemical characteristics as they join at the upstream end of ribarroja reservoir in northern spain given the short average residence time of water in the reservoir the spatial arrangement of the rivers at their confluence and their mixing rates are likely the drivers of the stratification patterns observed near the dam in winter inflows have similar densities 0 o 10 5 and their spatial distribution is largely determined by inertial forces and in particular by the discharge ratio downstream of the confluence both rivers flow side by side and largely unmixed over long distances in summer with 0 of o 10 3 the flow fields at the confluence are largely controlled by buoyancy forces atmospheric forcing during strong wind events and centrifugal forces caused by the meandering shape of the reservoir induce significant tilting of the isotherms leading to localized high mixing rates mixing in general though is weak at this time of the year in fall and early winter density differences are largely controlled by conductivity differences between the incoming flows the warmer ebro water with larger thermal inertia flows beneath the colder segre water the spatial arrangement of the inflows is largely controlled by the discharge ratio and mixing between sources is strong likely as a result of mixed water being denser than either of the incoming flows

mixing processes in the amazon river at the confluences of the negro and solimoes rivers encontro das aguas manaus brazil the confluence of the rivers negro and solimoes gives birth to the amazon river near manaus brazil at their confluence these two rivers with their very different physical and geochemical characteristics provide an interesting example of the mixing of waters along a reach of approximately one hundred kilometres the purpose of this article is to describe and explain the spatial configuration of the solimoes negro rivers mixing zone based on a systematic sampling survey carried out in september 1997 using a special sampler and an acoustic doppler current profiler acoustic doppler current profiler the waters of the negro river differ from those of the solimoes river in their lower velocity 0 3 vs 1 m s 1 conductivity 8 vs 80 s cm 1 at 25 c turbidity 5 vs 80 ntu ph 5 5 vs 7 0 and higher temperatures by 1 c due to their higher density the waters of the solimoes river slide under those of the negro river and consequently negro river waters are located at the surface close to the left bank and solimoes river waters are located at the bottom close to the right bank the contact between the waters of the two rivers changes from a clearly defined vertical boundary to a diffuse horizontal boundary as they move downstream the complete mixing process takes more than 30 h and 100 km copyright 2009 john wiley sons ltd

anisotropic frequency dependent spreading of seismic waves from first arrival vertical seismic profile data analysis abstracta model of first arrival amplitude decay combining geometric spreading scattering and inelastic dissipation is derived from a multioffset 3d vertical seismic profile data set unlike the traditional approaches the model is formulated in terms of path integrals over the rays and without relying on the quality factor q for rocks the inversion reveals variations of geometric attenuation wavefront curvatures and scattering and the effective attenuation parameter with depth both of these properties are also found to be anisotropic scattering and geometric spreading focusing and defocusing significantly affect seismic amplitudes at lower frequencies and shallower depths statistical analysis of model uncertainties quantitatively measures the significance of these results the model correctly predicts the observed frequency dependent first arrival amplitudes at all frequencies this and similar models can be applied to other types of waves and should be useful for true amplitude studi

an application of reflected guided borehole radar waves to detection of subhorizontal structures borehole radar bhr waves will be severely attenuated when a borehole is filled with high conductivity hc media as a result bhr side reflections are significantly weakened i e the capability to detect the targets far from the borehole is restricted however v shaped reflected guided waves rgws are commonly observed in bhr data from such hc boreholes they are often regarded as unwanted contamination to conventional bhr surveys and should be avoided or suppressed however we advocate that the rgws as a type of reflected wave traveling along the borehole contain geological information about the surroundings of the borehole and can be used to locate and recognize geological boundaries such as lithological interfaces and fractures that intersect the borehole unlike conventional bhr surveys mainly imaging the structures near parallel to the borehole the rgws can effectively sense near normal ones in addition we can use rgws to distinguish between echoes from fractures and those from bedding planes this is achieved by recognizing different phase characteristics of rgws for these structures the rgws from both sides of a fracture have the same phase whereas the ones from a stratigraphic interface have opposite phases synthetic and real data are used to demonstrate the feasibility of detecting subhorizontal structures by rwgs rgws extend bhr measurements and allow a better understanding of subsurface structures they help to predict and reduce mining risks and monitor potential geological hazards

three dimensional tidal flow around headlands field measurements of tidal flow around a headland indicate secondary circulation induced by flow curvature the secondary flow defined to be the flow in the plane normal to the direction of the vertically averaged current is directed toward the headland near the bottom and seaward near the surface consistent with theoretical predictions the strength of the secondary flow varies from 5 to 15 of the streamwise flow it is strongest when the water column is stratified because of both the enhanced shear of the streamwise flow and the reduced frictional damping of the secondary flow the transverse exchange accomplished by the secondary flow significantly influences the structure of the streamwise flow causing a broadening of the transverse shear and changing the horizontal and vertical distributions of momentum it may also have a significant influence on horizontal dispersion in the vicinity of headlands

the importance of aspiration and channel curvature in producing strong vertical mixing over a sill on the basis of observations of the time dependent tidally forced flow over a long sill we find that aspiration and channel curvature set the flow structure and condition the flow to allow intense vertical mixing aspiration reduces the potential energy of the water column by thinning it while maintaining its density contrast channel curvature induces a cross channel circulation that can rapidly overturn a stratified flow eighteen along channel sections of density velocity and dissipation rate of turbulent kinetic energy were collected in and around the tacoma narrows of puget sound a site suspected of driving a strong vertical circulation in adjoining main basin rapid inflow to the narrows on flood from one channel of a triple junction reduces dynamic pressure allowing dense water from below sill depth to be uplifted or aspirated into the narrows we estimate water from below 150 m 3 times the sill depth is drawn into the narrows on a 3 m flood tide once in the narrows the flow remains stratified until it passes a 50 bend where a strong secondary circulation overturns the 50 m deep water column and generates intense turbulent mixing cross channel velocities of up to 0 4 m s 1 are observed and maximum values of exceed 10 3 w kg 1 upon leaving the sill stratification is reestablished and turbulence decays a similar set of sequences occurs on ebb except that the outflow bypasses the flood inflow channel and instead discharges into colvos passage the third branch of the triple junction colvos passage ultimately discharges the ebb effluent back into main basin enhancing the impact of mixing at the narrows by discharging the mixed product far from the source scaling of the cross channel momentum equation suggests that below a threshold value of along channel velocity stratification should suppress secondary circulation for a given vertical shear radius of curvature and channel width above the threshold velocity the magnitude of the cross channel velocity is roughly consistent with predictions for unstratified flow we estimate the maximum effective eddy diffusivity that aspiration and mixing in the narrows can produce in main basin to be 10 3 m2 s 1

experiments on flow at a 90 open channel junction although open channel junctions are common in many hydraulic structures no comprehensive data set has been compiled that describes the 3d flow field within the junction itself this physical model study examined a 90 sharp edged open channel junction for channels of equal width depth measurements were made using a point gauge while velocity measurements were taken using an acoustic doppler velocimeter over a grid defined throughout the junction region the average velocity and turbulence intensity were calculated from a time series of velocities that was recorded at each location in addition a 2d mapping of the water surface was performed on a 76 2 mm square grid throughout the channel junction this paper presents the details of the experimental procedure and the general flow characteristics observed the full data set generated during this experimental work is available for downloading on the internet using a small portion of the data recorded an evaluation of several previously proposed theori

stable q analysis on vertical seismic profiling data abstractvertical seismic profiling vsp provides a direct observation of seismic waveforms propagating to various depths within the earth s subsurface the q analysis or attenuation 1 q analysis based on direct comparison between individual waveforms at different depths however suffers from the problem of instability commonly due to fluctuations inherent in the frequency spectrum of each waveform to improve the stability we considered frequency and time variations and conducted q analysis on an integrated observation first we transformed the time or depth frequency domain spectrum to a 1d attenuation measurement with respect to a single variable the product of time and frequency although this 1d measurement has a higher signal to noise ratio than the 2d spectrum in the time frequency domain it can also be used to further generate a stabilized compensation function then we implemented two q analysis methods by data fitting in a least squares sense to either the attenuation measurement o

3d reverse time migration using the acoustic wave equation an experience with the seg eage data set kirchhoff is the most commonly used 3d prestack migration algorithm because of its speed and other economic advantages but it uses a high frequency ray approximation to the wave equation and therefore has difficulties in imaging complex geologic structures where multipathing occurs e g beneath rugose horizons such as faulted salt domes where traveltime calculations become difficult

three dimensional structure of flow at a confluence of river channels with discordant beds this paper presents three dimensional data of the mean and turbulent structure of flow collected at a natural confluence of rivers with discordant beds to 1 describe the three dimensional flow field of a natural junction of channels 2 assess the role of changes in bed morphology occurring during transport effective events on the structure of flow at a confluence and 3 examine how the three dimensional structure of flow varies with changes in the ratio of momentum flux between the two confluent streams three dimensional measurements of velocity were reconstructed from the measurements obtained with an array of four two component electromagnetic current meters six detailed velocity profiles were taken at five cross sections in a wide range of flow conditions the mean field of flow is characterised by 1 the acceleration of flow in the downstream portion of the post confluence channel but by lower velocities upstream in the mixing layer area 2 a stagnation zone at the apex of the junction 3 a zone of flow deviation and strong fluid upwelling close to the avalanche face and at the margin of the tributary mouth bar and 4 reduced velocities over the depositional bar at the downstream junction corner the position and extent of these zones vary with changes in the ratio of momentum flux very high intensity of turbulence peaks up to 50 and turbulent kinetic energy were observed in the mixing layer region distortion of the mixing layer characteristic of flow where bed discordance is present between the two tributary channels was evident from mean and turbulent flow data this field study suggests that the effects of bed discordance on flow sediment transport and the resultant bed morphology must be incorporated into conceptual and numeric models of these sites of complex flow

mixing dynamics at the confluence of two large rivers undergoing weak density variations simulations of tracer experiments conducted with a three dimensional primitive equation hydrodynamic and transport model are used to understand the processes controlling the rate of mixing between two rivers ebro and segre with distinct physical and chemical properties at their confluence upstream of a meandering reservoir ribarroja reservoir mixing rates downstream of the confluence are subject to hourly scale oscillations driven partly by changes in inflow densities and also as a result of turbulent eddies that develop within the shear layer between the confluent rivers and near a dead zone located downstream of the confluence even though density contrasts are low at most o 10 1 kg m 3 difference among sources and almost negligible from a dynamic point of view compared with inertial forces they are important for mixing mixing rates between the confluent streams under weakly buoyant conditions can be of up to 40 larger than those occurring under neutrally buoyant conditions the buoyancy effects on mixing rates are interpreted as the result of changes in the contact area available for mixing distortion of the mixing layer for strong density contrasts though when the contact area between the streams becomes nearly horizontal larger density differences between streams will lead to weaker mixing rates as a result of the stabilizing effect of vertical density gradients

hybrid fourier finite difference 3d depth migration for anisotropic media when a subsurface is anisotropic migration based on the assumption of isotropy will not produce accurate migration images we develop a hybrid wave equation migration algorithm for vertical transversely isotropic vti media based on a one way acoustic wave equation using a combination of fourier finite difference ffd and finite difference fd approaches the hybrid method can suppress an additional solution that exists in the vti acoustic wave equation and it offers speed and other advantages over conventional ffd or fd methods alone the algorithm is tested on a synthetic model involving log data from onshore eastern saudi arabia including estimates of both intrinsic and layer induced vti parameters results indicate that vti imaging in this region offers some improvement over isotropic imaging primarily with respect to subtle structure and stratigraphy and to image continuity these benefits probably will be overshadowed by perennial land seismic data issues such as near surface distortions and multiples

heavy metals in oka river sediments urdaibai national biosphere reserve northern spain lithogenic and anthropogenic effects the oka river basin is located in the urdaibai national biosphere reserve north spain in order to obtain a preliminary view of its environmental conditions nitric acid extractable fe mn zn pb cu cr ni and co organic matter content l o i geochemical fractionation of heavy metals and mineralogical composition were analysed in surficial sediments fraction 63 m collected in october 1991 relatively high concentrations of metals occurred in a sampling site upstream due to a local natural enrichment controlled by weathering of volcanic rocks geochemical fractionation confirmed the lithogenic origin of metals which were mainly associated to the detrital phase the significant increase of heavy metals found in some samples collected in the municipal area of gernika suggested a pollution effect related to anthropogenic wastes large amounts of metallic slags have been found mixed with the sediments upstream from this locality nevertheless in view of their composition it is unlikely that these slags constitute a significant source of heavy metals in the studied area in a second sampling in october 1997 an apparent decreasing trend in some heavy metal levels was noted particularly for pb and cu nowadays enrichment factors relative to background values confirm moderate anthropogenic influence on zn pb cu cr and ni concentrations however heavy metal levels in the oka river sediments are much lower than those detected in other major rivers of the basque country nervion urumea deba

rates and patterns of thermal mixing at a small stream confluence under variable incoming flow conditions confluences are important locations for river mixing within drainage networks yet few studies have examined in detail the dynamics of mixing within confluences this study examines the influence of momentum flux ratio the scale of the flow cross sectional area and the density differences between incoming flows on thermal mixing at a small stream confluence results reveal that rates and patterns of thermal mixing depend on event specific combinations of the three factors the mixing interface at this confluence is generally distorted towards the mouth of the lateral tributary by strong helical motion associated with curvature of flow from the lateral tributary as it aligns with the downstream channel as the momentum flux from the lateral tributary increases mixing is enhanced because helical motion from the curving tributary flow expands over the width of the downstream channel the cross sectional area of the flow is negatively correlated with mixing rates suggesting that the amount of mixing over a fixed distance downstream of the confluence is inversely related to the scale of the flow density differences are not strongly related to rates of mixing results confirm that mixing rates within the region of confluent flow interaction can be highly variable among flow events with different incoming flow conditions but that in general length scales of mixing are short and rates of mixing are high at this small confluence compared with those typically documented at large river confluences copyright 2015 john wiley sons ltd

inversion of p and sv waves from multicomponent offset vertical seismic profiles downgoing waves in multicomponent vsp experiments are used to obtain seismic p and s wave velocities as a function of depth and angle of incidence if p and sv waveforms do not overlap in time at the depth of interest local velocities of the medium are obtained by separate analysis of these events the apparent velocity of the event p or sv is computed from the moveout across several neighboring depth locations the angle of incidence of the same event is computed from the particle motion hodogram within an appropriately chosen time window then the local medium velocity p wave or s wave depending on the chosen event is given by the apparent velocity multiplied by the cosine of the angle of incidence layer interfaces with reasonably sharp velocity contrasts are efficient p wave to sv wave converters even at moderate angles of incidence in offset vsp experiments converted sv waves are generated with varying strengths at practically all depths consequently the converted sv waveforms partially o

hydraulic geometry and changes in flow velocity at a river confluence with coarse bed material a survey of flows was conducted at a river confluence with coarse bed material bridges were installed on both tributaries at the confluence and farther downstream on the receiving stream at these stations flow velocities were measured over a dense grid for seven conditions ranging from very low flows to the bankfull stage hydraulic geometry relationships established at all four stations revealed that flow is accelerated through the confluence as stage rises at bankfull discharge average velocity is 1 6 times higher at the confluence than on either tributary flow acceleration occurs at and above intermediate flow stages and is concentrated at the centre of a linear pool located at the confluence the development of a zone of high shear stress is also associated with the cell of high flow velocity flow acceleration is dissipated at the exit of the pool where water surges over boulder ribs the acceleration is not related to the development of flow separation zones as observed by best and reid 1984 for wide junction angles nor is it explained by the reduction of the friction exerted by the banks acceleration is associated with the plan geometry of the confluence with the lateral slopes which permit water to converge and with a reduction in grain roughness at the confluence owing to the curvature of the tributary and to the acute angle of entry relative power losses through the confluence decrease with increasing stages

time averaged flow structure in the central region of a stream confluence previous process oriented field studies of stream confluences have focused mainly on fluvial dynamics at or immediately downstream of the location where the confluent flows enter the downstream channel this study examines in detail the spatial evolution of the time averaged downstream velocity cross stream velocity and temperature fields between the junction apex where the flows initially meet and the entrance to the downstream channel a well defined vertically oriented mixing interface exists within this portion of the confluence suggesting that lateral mixing of the incoming flows is limited the downstream velocity field near the junction apex is characterized by two high velocity cores separated by an intervening region of low velocity or recirculating fluid in the downstream direction the high velocity cores move inwards towards the mixing interface and high velocity fluid progressively extends downwards into a zone of scour resulting in an increase in flow velocity in the centre of the confluence the cross stream velocity field is dominated by flow convergence but also includes a component associated with a consistent pattern of secondary circulation this pattern is characterized by two surface convergent helical cells one on each side of the mixing interface the helical cells appear to be the mechanism by which high momentum fluid near the surface is advected downwards into the zone of scour for transport ineffective flows the dimensions and intensities of the cells are controlled by the momentum ratio of the confluent streams and by the extant bed morphology within the confluence although the flow structure of formative events was not measured directly in this study documented patterns of erosion and deposition within the central region of the confluence suggest that these events are dynamically similar to the measured flows except for the fact that formative flows are not constrained by but can reshape the bed morphology the results of this investigation are consistent with and augment previous findings on time averaged flow structure in the downstream portion of the confluence 1998 john wiley sons ltd

effect of bed morphology on flow mixing length at river confluences mixing processes at river confluences have an important bearing on problems such as pollutant dispersal and the management of river systems but remain poorly understood previous studies1 11 have indicated that flow mixing downstream of a confluence is a slow process typically completed at distances greater than 100 times the channel width bed morphology is now emerging as a critical factor flume experiments have shown that the width to depth ratios12 and height discordance13 14 between the confluent channels should influence mixing rates12 14 but the magnitude of these effects in real rivers is not known here we report measure ments from three river confluences of moderate size which show that bed discordance can markedly increase mixing rates for the rivers studied mixing is complete at distances five to ten times shorter than those reported previously

migration by extrapolation of time dependent boundary values migration of an observed zero offset wavefield can be performed as the solution of a boundary value problem in which the data are extrapolated backward in time this concept is implemented through a finite difference solution of the two dimensional acoustic wave equation all depths are imaged simultaneously at time 0 the imaging condition and all dips right up to vertical are correctly migrated numerical examples illustrate this technique in both constant and variable velocity media

radon transform wave field separation for vertical seismic profiling data the data obtained by the vertical seismic profiling vsp method require unique processing before interpretation an important step is the separation of upgoing and downgoing signals by application of an f k velocity filter and muting of the direct wave similarly if the vsp section is transformed to the p plane the upgoing and downgoing wave fields become separable because they map to different p quadrants according to their dips this allows either wave field to be reconstructed by application of the inverse radon transform after windowing in the p plane results obtained by the radon transform wave field separation rtws method are similar to those obtained by f k velocity filtering however by implementing an amplitude ratio testing filter in the forward slant stack the quality of the reconstructed wave field is much improved for a specific range of p stack values in the forward transform this filter sorts and removes the unwanted events from the desired ones by testing their consistency

numerical investigation of plunging density current when a buoyant inflow of higher density enters a reservoir it sinks below the ambient water and forms an underflow downstream of the plunge point the flow becomes progressively diluted due to the fluid entrainment the entrainment rate is strongly dependent on the richardson number and reaches a constant value well downstream of the plunge point this study is concerned with the analysis of the plunging phenomenon and the determination of the entrainment a k e model including buoyancy effects both in a sloping and a diverging channel is used to reproduce the main flow characteristics a relation between the depth at the plunge point in a channel of constant width and in a diverging channel is established and theoretical results for the calculation of the dense layer thickness are provided the latter indicates that the spreading rate of the dense layer in a diverging channel is a function of both the entrainment rate and the channel width the predictions of the plunge line location are in agreemen

generalizations of the wedderburn number parameterizing upwelling in stratified lakes the effect of lake geometry on wind induced upwelling in a two layer stratified lake with a variable bottom slope and generic planar shape is investigated 1 the traditional linearized classification parameter for upwelling the wedderburn number is extended to include finite amplitude effects in a rectangular basin this extension is important when the wedderburn number is 3 2 the wedderburn number is generalized to incorporate the influence of a bottom slope and a variable width basin for both the linear large wedderburn number and nonlinear small wedderburn number assumptions laboratory and numerical experiments of a two layer stratification were performed and used to validate these extensions 3 both extensions are combined to derive a methodology to estimate the interface displacement in real lakes where both geometry effects may act together and this methodology is used to discuss the response of the thermocline to the onset of wind in lake como and mundaring weir reservoir in most cases symmetry of the basin geometry along the fetch is the key characteristic that defines tilting of the interface for real lakes asymmetries in terms of bottom slopes induce shortening or stretching of the effective internal fetch while a larger excursion of the interface results in narrower upwind areas of the lake due to horizontal redistribution of displaced water volumes this paper gives the correct definition of the length scales to define the wedderburn number for any stratified basin

heavy metal distribution and environmental status of doon valley soils outer himalaya india doon valley is surrounded by two major river systems ganga and yamuna on either side with a water divide passing nearly across the centre of the valley and is sandwiched between two mountain ranges in the fragile ecological systems of the himalayan foothills in total 398 soil samples were collected from the valley in a grid pattern 1 sample per 2 km2 and investigated for their heavy metal cr cu ni pb and zn abundances that are environmentally sensitive comparison of the heavy metal abundances with the contamination threshold values ctv revealed that most of these elemental abundances in doon valley soils fall well within the range of the uncontaminated to slightly contaminated category in the case of cr and ni a sizeable number of samples exceeded the ctv 250 and 100 mg kg 1 respectively with an overall background value of 109 and 52 mg kg 1 respectively sites of high cr and ni mostly occur in the ganga catchment gc sector that includes even relatively undisturbed forestland the source of this contamination is attributed to geological factors which indicate contribution from the mafic volcanics of the lesser himalaya this is also consistent with the distribution pattern of mn and fe though their abundance levels are not alarming the background concentration of pb is low 22 mg kg 1 in doon valley soils however signs of gradual pb contamination are palpable in and around the centre of the dehra dun city and along the highways aluminium normalized heavy metal ratios were found to exhibit narrow variability in the case of cu ni and cr and had good correlation with al indicating their affinity and association with the clay minerals on the other hand pb and zn seem to be associated with non silicate sources

turbulent mixing in the amazon river the isotopic memory of confluences rivers continuously discharge dissolved material to the oceans dissolved compounds partially result from water rock interactions which produce a large range of water chemical and isotopic compositions these waters are collected by rivers that are commonly assumed to be well mixed with regard to their different tributaries as a result of turbulent dispersion in this paper we test this hypothesis on the solimoes river at manacapuru the largest tributary of the amazon river by analyzing the sodium concentration and strontium isotopic composition of river water on a transverse section at different depths high precision measurements reveal lateral heterogeneities this reflects poor mixing between two main river masses that have distinct chemical and isotopic signatures a hundred kilometers downstream from their confluence the solimoes mainstream and the purus river using sodium concentration data the transverse dispersion coefficient is estimated for the studied solimoes reach the earth s largest river on which such an estimate now exists and is found to be 1 8 0 2 m2 s comparison with previously reported data highlights the potential role of bed morphology and islands in the efficiency of lateral mixing in large rivers we finally demonstrate that the characteristic length of lateral mixing downstream from confluences in large rivers is at least of several tens of kilometers

distribution and assessment of sediment toxicity in tamaki estuary auckland new zealand heavy metal levels in surface sediments from tamaki estuary demonstrate significant up estuary increases in cu pb zn cd and mud concentrations increased metal levels towards the head of the estuary are linked to local catchment sources reflecting the historical development industrialisation and urbanisation of catchment areas surrounding the upper estuary the relatively narrow constriction in the middle estuary panmure area makes it susceptible to accumulation of upper estuary pollutants since the constriction reduces circulation and extends the time required for fine waterborne sediments in the upper estuary to exchange with fresh coastal water as a result fine fraction sediments trapped in the upper estuary facilitate capture and retention of pollutants at the head of the estuary the increase in sandy mud poor sediments towards the mouth of the estuary is associated with generally low metal concentrations the estuary s geomorphic shape with a mid estuary constriction sediment texture and mineralogy and catchment history are significant factors in understanding the overall spatial distribution of contaminants in the estuary bulk concentration values for cu pb zn and cd in all the studied surface samples occur below anzecc isqg h toxicity values cd and cu concentrations are also below the isqg l toxicity levels for these elements however pb and zn concentrations do exceed the isqg l values in some of the surface bulk samples in the upper estuary proximal to long established sources of catchment pollution

sediment bound metallic contaminants in sydney s estuaries and adjacent offshore australia abstract almost one quarter of australia s population of 17 million and a large proportion of its industry are concentrated in the restricted area of greater metropolitan sydney heavy metal loadings of the surficial sediment which mantles the four deeply incised estuaries dissecting this region and the adjacent continental shelf are being investigated to determine potential sources and to understand the processes of dispersion and accumulation the parramatta river port jackson and georges river botany bay estuaries have been major repositories for urban industrial waste and are heavily impacted by anthropogenic metalliferous cu zn ni pb loadings the majority of estuarine areas are greatly in excess of background values the most elevated regions are the upper reaches of estuaries and bay ends generally the central sections of these estuaries are less elevated but still above background values whereas for some metals the lower reaches and mouth areas approach background values in limited parts of the inner shelf most of these metals are above background values but for the majority of the continental shelf values are close to background the strong down estuary decline in metals and lower metal levels in the marine environment is probably source related rather than due to a dilution by less enriched coarse sediment the source of offshore metal enrichment is coastal sewage outfalls and or dissolved and suspended material escaping the estuaries marginally elevated metal concentrations on the inner shelf in excess of that observed in the lower estuary are probably indicative of some sourcing from sewage outfalls in terms of public health the majority of the parramatta river port jackson estuary is in excess of guidelines and tributaries and bay ends of both estuaries are markedly in excess of these levels for all elements offshore surficial sediments are mostly within guideline levels

entrainment and turbulence in saline underflow in lake ogawara under certain tidal conditions a saline underflow originating in the pacific ocean moves into lake ogawara japan the underflow consists of a uniform saline bottom layer that is slightly warmer than the ambient and an interfacial shear layer in which the velocity and density are decreased within the experimental area the underflow is confined to a channel approximately 1 km wide and is essentially two dimensional the underflow had a bulk richardson number defined in terms of the mean properties between 1 and 2 the rate of entrainment into the bottom layer was calculated using two distinct methods the first method used the change in the maximum salinity of the underflow measured at two stations along the path of the underflow to infer the amount of ambient water entrained the second method made direct measurements of vertical mass fluxes with a profiler the agreement between the two methods was excellent the measured entrainment coefficients were consistent with the derived entrainment law the turbulent structure of the flow was mapped for a 3 h quasi steady period of the flow turbulence is predominantly generated on the bottom boundary and is transported vertically to the density interface where it leads to mixing

flow and particle paths at a natural river confluence with coarse bed material abstract an investigation of flow and particle trajectories was conducted at a gravel bed confluence where the junction angle is gradually reduced from 60 to 20 as the tributary joins the main channel a shallow and elongated scour zone is present at the mouth of the tributary three flow stages ranging from 1 7 to 1 2 bankfull were recorded flow velocity vectors were measured from five bridges installed across the channels sediment transport paths were determined by following the progression of seeded marked particles through the confluence from may to october 1986 the results show that flow structure is responsive to stages at lower stages less than 1 3 bankfull flow velocity vectors are controlled by local bed topography and specifically by the steeper slopes around the scour zone at higher stages velocity vectors become aligned with the plan geometry of the confluence for all stages surface flow velocities are higher in the scour zone bed shear stress computed from the law of the wall for the deepest flow is higher at the bottom of the scour zone compared to its sides head and exit the trajectories of particles seeded on the tributary pass through the scour zone particles coming from the main channel displayed a dual pattern first moving laterally towards the scour zone early in the season then following paths that are consistent with the plan geometry of the confluence this shift in pattern reflects an increase in the peak discharges during the survey period these results highlight the role of stage in controlling the flow dynamics and particle paths at the confluence

the influence of tributary flow density differences on the hydrodynamic behavior of a confluent meander bend and implications for flow mixing abstract the goal of this study is to evaluate the influence of tributary flow density differences on hydrodynamics and mixing at a confluent meander bend a detailed field characterization is performed using an acoustic doppler current profiler adcp for quantification of the 3d flow field flow discharge and bathymetry as well as ctd measurements conductivity temperature depth to characterize the patterns of mixing satellite images of the confluence taken at complementary times to the field surveys were analyzed to evaluate the confluence hydrodynamics at different flow conditions the results illustrate the differences in hydrodynamics and mixing length in relation to confluences with equal density tributaries at low density differences and higher discharge ratio qr between the two rivers the flow is similar to equi density confluent meander bends in contrast at high density differences low qr the tributary flow is confined to near the confluence but the density difference causes the flow to move across channel in this case the density difference causes the lateral spread of the tributary flow to be greater than at a greater qr when the density difference is less these results illustrate the potential importance of density differences between tributaries in determining the rate and spatial extent of mixing and sediment dispersal at confluent meander bends

seismic migration problems and solutions historically seismic migration has been the practice science technology and craft of collapsing diffraction events on unmigrated records to points thereby moving migrating reflection events to their proper locations creating a true image of structures within the earth over the years the scope of migration has broadened what began as a structural imaging tool is evolving into a tool for velocity estimation and attribute analysis making detailed use of the amplitude and phase information in the migrated image with its expanded scope migration has moved from the final step of the seismic acquisition and processing flow to a more central one with links to both the processes preceding and following it in this paper we describe the mechanics of migration the algorithms as well as some of the problems related to it such as algorithmic accuracy and efficiency and velocity estimation we also describe its relationship with other processes such as seismic modeling our approach is tutorial we avoid presenting the finest details of either the migration algorithms themselves or the problems to which migration is applied rather we focus on presenting the problems themselves in the hope that most geophysicists will be able to gain an appreciation of where this imaging method fits in the larger problem of searching for hydrocarbons

migration of shallow seismic reflection data we present an analysis of migration effects on seismic reflection images of very shallow targets such as those that are common objectives of engineering groundwater and environmental investigations we use an example of seismic reflection data from depths of 5 to 15 m that show negligible effect from migration despite the apparent steep dip on the seismic section our analysis of the question of when to migrate shallow reflection data indicates it is critical to take into account the highly variable near surface velocities and the vertical exaggeration on the seismic section a simple set of calculations is developed as well as a flow chart based on the migrator s equation that can predict whether migration of an arbitrary shallow seismic section is advisable because shallow reflection data are often processed on personal computers unnecessary migration of a large data set can be prohibitively time consuming and wasteful