multifrequency full waveform sonic logging in the screened interval of a large diameter production well abstracta set of field experiments using multiple transmitter center frequencies was completed to test the application potential of low frequency full waveform sonic logging in large diameter production wells wireline logs were acquired in a simple open drillhole and a high yield large diameter production well completed with wire wound sand screens at an aquifer storage and recovery site in perth western australia phase shift transform methods were applied to obtain phase velocity dispersion images for frequencies of up to 4 khz a 3d representation of phase velocity dispersion was developed to assist in the analysis of possible connections between low frequency wave propagation modes and the distribution of hydraulic properties for sandstone intervals in the test well the highest hydraulic conductivity intervals were typically correlated with the lowest phase velocities the main characteristics of dispersion images obtained from the sand screened well were highly comparable with those obtained at t

measurement of the spatial distribution of fluvial bedload transport velocity in both sand and gravel maps are presented of the spatial distribution of two dimensional bedload transport velocity vectors bedload velocity data were collected using the bottom tracking feature of an acoustic doppler current pro ler adcp in both a gravel bed reach and a sand bed reach of fraser river british columbia block averaged bedload velocity vectors and bedload velocity vectors interpolated onto a uniform grid revealed coherent patterns in the bedload velocity distribution concurrent helley smith bedload sampling in the sand bed reach corroborated the trends observed in the bedload velocity map contemporaneous 2d vector maps of near bed water velocity velocity in bins centered between 25 cm and 50 cm from the bottom and depth averaged water velocity were also generated from the adcp data using a vector correlation coef cient which is independent of the choice of coordinate system the bedload velocity distribution was signi cantly correlated to the near bed and depth averaged water velocity distributions the bedload velocity distribution also compared favorably with variations in depth and estimates of the spatial distribution of shear stress published in 2004 by john wiley sons ltd

estimating phase velocity and attenuation of guided waves in acoustic logging data in acoustic logging the guided waves which propagate along the borehole are affected by the properties of the formation for acoustic logging tools with many receivers array processing methods can be used to calculate the phase velocity and attenuation of these waves this information is crucial in estimating formation properties like permeability transverse isotropy and s wave velocity

flow structure caused by a local cross sectional area increase and curvature in a sharp river bend horizontal flow recirculation is often observed in sharp river bends causing a complex three dimensional flow structure with large implications for the morphological and planimetric development of meanders several field observations in small scale systems show that sharp bends are often found in association with a strong increase in cross sectional area the deposition of outer bank benches and reattachment bars near the inner bank recent studies show that these bends can also occur in large scale systems in this study we present field measurements of a sharp bend in the mahakam river east kalimantan indonesia the cross sectional area increases by a factor of 3 compared with the reach averaged cross sectional area along a river reach of about 150 km cross sectional area correlates strongly with curvature the field measurements are analyzed together with the results from numerical simulation with a three dimensional finite element model which yields a comprehensive view of the intricate flow structure in turn the model is used to validate a new equation that captures the water surface topography dependence on cross sectional area variation and curvature the results show the importance of the increase in cross sectional area in the development of horizontal recirculation vertical acceleration of the flow into the scour causes the pressure to deviate from a hydrostatic pressure distribution strong downflow up to 12 cm s 1 advects longitudinal momentum toward the bed causing the flow to concentrate in the lower part of the cross section this increases the velocity magnitude throughout the cross section which is expected to maintain the large scour depth found in several bends along the mahakam river

inversion of borehole guided wave amplitudes for formation shear wave attenuation values formation shear wave attenuation q 1 and borehole fluid attenuation q 1 values are estimated from the guided wave arrivals in full waveform acoustic logs the use of guided waves avoids the problem of head wave geometrical spreading losses attenuation estimates are obtained from the inversion of guided wave spectral ratios using analytic partition coefficient expressions the guided wave amplitude ratios are measured at uniformly spaced frequencies within a selected frequency window containing the pseudo rayleigh and stoneley waves inversion of synthetic data from an open hole geometry results in q estimates within 2 3 of the actual values inversion of cased hole synthetic data gives fluid q and cement layer q values within 2 3 of the actual values but the formation shear wave q estimate is in error by about 25 due to difficulty in identifying the pseudo rayleigh wave cutoff frequency from the amplitude spectra synthetic data results also indicate that the q estimates are very sensitive to errors in any of the parameters that affect the partition coefficient calculations such as the borehole fluid velocity or borehole radius application of the method to real data recorded in open boreholes provides reasonable results but noise increases the variance of the q estimates

suppression of near surface scattered body to surface waves a steerable and nonlinear filtering approach we present an approach based on local slope estimation for the separation of scattered surface waves from reflected body waves the direct and scattered surface waves contain a significant amount of seismic energy they present great challenges in land seismic data acquisition and processing particularly in arid regions with complex near surface heterogeneities e g dry river beds wadis large escarpments and karst features the near surface scattered body to surface waves which have comparable amplitudes to reflections can mask the seismic reflections these difficulties added to large amplitude direct and back scattered surface rayleigh waves create a major reduction in signal to noise ratio and degrade the final sub surface image quality removal of these waves can be difficult using conventional filtering methods such as an f k filter without distorting the reflected signal the filtering algorithm we present is based on predicting the spatially varying slope of the noise using steerable filters and separating the signal and noise components by applying a directional nonlinear filter oriented toward the noise direction to predict the noise and then subtract it from the data the slope estimation step using steerable filters is very efficient it requires only a linear combination of a set of basis filters at fixed orientation to synthesize an image filtered at an arbitrary orientation we apply our filtering approach to simulated data as well as to seismic data recorded in the field to suppress the scattered surface waves from reflected body waves and we demonstrate its superiority over conventional f k techniques in signal preservation and noise suppression

the interaction of tube waves with borehole fractures part ii analytical models we solve numerically the equations of elastodynamics that govern the propagation of waves in a fluid filled borehole intersected by one or more fluid filled fractures thus extending earlier work on formations presumed to be rigid the model is axis symmetric and it allows for arbitrary radial and vertical variations in the elastic properties of the formation we have developed a novel gridding scheme that takes advantage of the assumed thinness of the fracture compared to any relevant wavelength this technique obviates the necessity for a mesh within the fracture itself and thereby saves computational time and memory we illustrate our technique by means of examples with single and double fractures with or without washouts as well as examples with variable width fractures we compute the frequency dependent reflection and transmission coefficients of tube waves by fractures from the time waveforms generated by the present method major conclusions of this work are 1 reflections of tube waves by fr

a modelling study of open hole single well seismic imaging single well or uni well imaging uses an acoustic source and an array of receivers located in the same borehole to image local geological structure due to the intrinsic attenuation of the formation it is likely that a source emitting frequencies in the typical cross well range would be necessary to illuminate structure at distances above 100 m from the borehole at these frequencies a significant proportion of the source energy is converted into tube waves from which are for the purpose of these surveys noise this paper reports the results of a modeling study designed to assess the feasibility of using existing cross well hardware i e a piezo electric source and hydrophone array modified to run in a single borehole to perform single well surveys in particular the authors study the case of an open borehole in a gas filled low permeability sandstone reservoir the results suggest that the amplitudes of the tube wave reverberations generated by calliper variations are such that reflections of interest arriving in the time window after the first tube wave arrival will not be visible however reflections may be visible in the time window preceding the first tube wave arrival provided tube waves from previous shots are not still present and the long source receiver offsets requiredmore to make observations in this window can be incorporated into the tool design less

preferential dust sources a geomorphological classification designed for use in global dust cycle models received 11 april 2011 revised 28 september 2011 accepted 4 october 2011 published 24 december 2011 1 we present a simple theoretical land surface classification that can be used to determine the location and temporal behavior of preferential sources of terrestrial dust emissions the classification also provides information about the likely nature of the sediments their erodibility and the likelihood that they will generate emissions under given conditions the scheme is based on the dual notions of geomorphic type and connectivity between geomorphic units we demonstrate that the scheme can be used to map potential modern day dust sources in the chihuahuan desert the lake eyre basin and the taklamakan through comparison with observed dust emissions we show that the scheme provides a reasonable prediction of areas of emission in the chihuahuan desert and in the lake eyre basin the classification is also applied to point source data from the western sahara to enable comparison of the relative importance of different land surfaces for dust emissions we indicate how the scheme could be used to provide an improved characterization of preferential dust sources in global dust cycle models

applications of perturbation theory to acoustic logging to study the normal modes generated during acoustic logging the authors have developed a method to calculate 1 phase velocities when the formation has slight general anisotropy 2 partial derivatives of either the wave number or frequency with respect to either an elastic modulus or density 3 group velocities and 4 phase velocities when the cross section of the borehole is slightly irregular the method which is based upon perturbation theory relates first order perturbations in frequency wave number elastic moduli densities and locations of interfaces for a general model with many fluid and solid layers which have any cross sectional shape and any type of anisotropy to demonstrate the use of this method formulas for the four applications are developed for a relatively simple model consisting of a fluid filled borehole through a transversely isotropic solid with its symmetry axis parallel to the borehole and some sample calculations are performed using these examples formulas for more complicated borehole models that may be appropriate for some field situations could readily be developed the significance of this work is that three applications can be used to study the normal modes when the borehole environment is complicated and the other can be used in anmore inversion for formation properties less

links between topography wind deflation lakes and dust the case of the bodele depression chad the bodele depression chad is the planet s largest single source of dust deflation from the bodele could be seen as a simple coincidence of two key prerequisites strong surface winds and a large source of suitable sediment but here we hypothesise that long term links between topography winds deflation and dust ensure the maintenance of the dust source such that these two apparently coincidental key ingredients are connected by land atmosphere processes with topography acting as the overall controlling agent we use a variety of observational and numerical techniques including a regional climate model to show that 1 contemporary deflation from the bodele is delineated by topography and a surface wind stress maximum 2 the tibesti and ennedi mountains play a key role in the generation of the erosive winds in the form of the bodele low level jet llj 3 enhanced deflation from a stronger bodele llj during drier phases for example the last glacial maximum was probably sufficient to create the shallow lake in which diatoms lived during wetter phases such as the holocene pluvial winds may therefore have helped to create the depression in which erodible diatom material accumulated instead of a simple coincidence of nature dust from the world s largest source may result from the operation of long term processes on paleo timescales which have led to ideal conditions for dust generation in the world s largest dust source similar processes plausibly operate in other dust hotspots in topographic depressions

effects of in situ permeability on the propagation of stoneley tube waves in a borehole we investigated the theoretical relationship between propagation characteristics of stoneley tube waves in a borehole and in situ permeability by using a modified formulation of a borehole model with a formation that behaves as a biot porous medium we found that stoneley wave attenuation and phase velocity dispersion increased with increasing permeability and porosity and decreased with increasing frequency in rocks with low to medium permeabilities less than 100 md variations in formation velocity and attentuation were major contributors to variations in stoneley wave properties at normal logging frequencies however in high permeability rocks greater than 100 md coupling between the borehole and pore fluids associated with in situ permeability was more important than lithological changes in controlling stoneley wave properties pore fluid viscosity had an effect on stoneley wave propagation equal but opposite to permeability and hence must he taken into account we compared our theoretical

flow in bedrock canyons a survey along the fraser canyon in canada reveals complex flow dynamics involving velocity inversions and upwelling which suggests ways to improve flow and bedrock incision modelling canyons carved into the bedrock in areas of actively uplifting terrain are important for our understanding of the link between tectonics climate and topography fluid flow and sediment transport are processes associated with canyon bedrock incision in models but to date there are no field observations of the flow structure in bedrock canyons that can be used to test the model components that represent fluid flow this paper describes a survey along the fraser canyon in canada that provides detailed measurements of flow in a narrow bedrock canyon the data reveal a more complex flow structure than currently assumed in flow models implying that a revision in our understanding of bedrock canyon incision is required bedrock erosion in rivers sets the pace of landscape evolution influences the evolution of orogens and determines the size shape and relief of mountains1 2 a variety of models link fluid flow and sediment transport processes to bedrock incision in canyons the model components that represent sediment transport processes are increasingly well developed3 4 5 in contrast the model components being used to represent fluid flow are largely untested because there are no observations of the flow structure in bedrock canyons here we present a 524 kilometre continuous centreline acoustic doppler current profiler survey of the fraser canyon in western canada which includes 42 individual bedrock canyons our observations of three dimensional flow structure reveal that as water enters the canyons a high velocity core follows the bed surface causing a velocity inversion high velocities near the bed and low velocities at the surface the plunging water then upwells along the canyon walls resulting in counter rotating along stream coherent flow structures that diverge near the bed the resulting flow structure promotes deep scour in the bedrock channel floor and undercutting of the canyon walls this provides a mechanism for channel widening and ensures that the base of the walls is swept clear of the debris that is often deposited there keeping the walls nearly vertical these observations reveal that the flow structure in bedrock canyons is more complex than assumed in the models presently used fluid flow models that capture the essence of the three dimensional flow field using simple phenomenological rules that are computationally tractable are required to capture the dynamic coupling between flow bedrock erosion and solid earth dynamics

acoustic modes of propagation in the borehole and their relationship to rock properties acoustic waveform measurements in boreholes have important applications in fracture hydrology and radioactive waste disposal but ambiguities in existing interpretation techniques remain a problem we have addressed the problem by using residue theory to predict the relative excitation of various modes contained in experimental waveforms a plane geometry model involving a layer of fluid between two elastic half spaces is shown to provide velocity dispersion curves for propagating modes that are very similar to those for the fluid filled borehole we use the plane geometry model to illustrate the effects of the confined borehole fluid on surface and body waves traveling along the borehole in the elastic solid we also computed excitation functions for some of the lowest order symmetric modes calculated the time domain response of the trapped modes following the shear head waves and compared them to waveforms recorded in boreholes through several homogeneous formations the insight into the mode composit

calibration of a 3d hydrodynamic meandering river model using fully spatially distributed 3d adcp velocity data abstractthis study used fully spatially distributed three dimensional 3d velocity data for the improved calibration of a 3d hydrodynamic model the conventional calibration methods do not account

finite difference and frequency wavenumber modeling of seismic monopole sources and receivers in fluid filled boreholes in borehole seismic experiments the presence of the borehole has a significant effect on observations unfortunately including boreholes explicitly in modeling schemes excludes the use of some methods e g frequency wavenumber and adds prohibitively to the cost of others e g finite difference to overcome this problem we use the concept of an effective source receiver array to replace the explicit representation of the borehole by a distributed seismic source receiver this method mimics the presence of the borehole at seismic frequencies under a wide variety of conditions without adding a significant computational cost it includes the effects of dispersive and attenuative tube wave propagation the generation of secondary sources at interfaces and caliper changes and the generation of conical waves in low velocity layers comparison with a finite difference scheme with an explicit borehole representation validates the approach the modeling method applied to a continuity logging geometry demon

suppressing near receiver scattered waves from seismic land data upgoing body waves that travel through a heterogeneous near surface region can excite scattered waves when the scatteringtakesplaceclosetothereceivers secondarywaves interferewiththeupcomingreflections diminishingthecontinuity of the wavefront we estimate a near surface scattering distribution from a subset of a data record and use this scattering distribution to predict the secondary waves of the entire data record with a wave theoretical model for near receiver scattering we then subtract the predicted scattered waves from the record to obtain the wavefield that would have been measured in the absence of near surface heterogeneities we apply this method to part of a field data set acquired in an area with significant near surface heterogeneity themainresultofourprocessingschemeisthatweeffectively remove near surface scattered waves this in turn increases trace to trace coherence of reflection events moreover applicationofourmethodimprovestheresultsobtained from just an application of a dip filter because we remove parts of the scattered wave with apparent velocities that are typically accepted by the pass zone of the dipfilter based on these results we conclude that our method for suppressing near receiver scattered waves works well on densely sampled land data collected in areas with strong near surface heterogeneity

downslope winds and dust storms in the salton basin abstractthe salton basin is a closed subsea level basin located in extreme southeastern california at the center of the basin lies the salton sea the state s largest inland lake which is surrou

modelling free forming meander evolution in a laboratory channel using three dimensional computational fluid dynamics abstract the present study investigated the use of computational fluid dynamics cfd in predicting the formation development and migration of free forming meander bends the three dimensional cfd model computed water flow and sediment transport in alluvial channels and predicted vertical and horizontal bed changes different algorithms and parameters were tested to provide an insight into the application range of cfd when modelling free forming meander formation the computational domain was discretized by an unstructured grid a control volume method was used for the discretization of the navier stokes equations for the flow calculation and of the convection diffusion equation for the sediment transport calculation turbulence was modelled by the k e turbulence model the simulation was started from an initially straight grid with neither sediment feed nor any perturbation at the inflow boundary the model computed the river bed evolution over a real time period of 3 d results were compared with laboratory experiments and showed that the cfd model can predict many of the characteristics of the alluvial meander formation and migration however some limitations and uncertainties exist that have to be clarified in future investigations

tube wave suppression in single well seismic acquisition single well seismic imaging is significantly hampered by the presence of borehole tube waves a tube wave suppressor has been tested using single well seismic equipment at the lost hills california oil field the suppressor uses a gas filled bladder kept slightly above borehole fluid pressure field tests show a measurable reduction in tube wave energy as compared to body waves propagating in the surrounding reservoir rock when using a high frequency 500 4000 hz piezoelectric source the p wave tube wave amplitude ratio was increased by 33 db when using a lower frequency 50 350 hz orbital vibrator source the s wave tube wave amplitude ratio was increased by 21 db while the p wave tube wave amplitude ratio was increased by 23 db these reductions in tube wave amplitudes significantly improve single well data quality

analysis and removal of multiply scattered tube waves because of irregularities in or near the borehole vertical seismic profiling vsp or crosswell data can be contaminated with scattered tube waves these can have a large amplitude and can interfere with weaker upcoming reflections destroying their continuity this type of organized noise cannot always be removed with filtering methods currently in use we propose a method based on modeling the scattered tube wave field and then subtracting it from the total data set we assume that the scattering occurs close to the borehole axis and therefore use a 1 d impedance function to characterize borehole irregularities estimation of this impedance function is one of the first steps our method also accounts for multiply scattered tube waves we apply the method to an actual vsp data set and conclude that the continuity of reflected upcoming events improves significantly in a washout zone

propagation of tube waves generated by an external source in layered permeable rocks summary tube wave characteristics generated in a fluid filled borehole may be used to determine enclosing media parameters and in particular to localize petroliferous strata if any and estimate their permeabilities as a rule tube waves are generated by an external seismic source of explosive type located some distance away from a borehole embedded in a layered medium the modelling of tube wave excitation and propagation is a complicated problem whose solution is necessary for solving the inverse problem in the present study an efficient numerical algorithm is presented to compute the tube waves propagation caused by external explosive perturbations in a fluid filled borehole located in a layered enclosing medium fluid flow between the borehole interior and the enclosing saturated porous rocks can easily be taken into account within the framework of the algorithm the solution is obtained by sommerfeldintegral representation of external field potentials on the borehole axis and subsequent analytical integration of the external field along the borehole intervals of homogeneity the resultant tridiagonal amplitude matrix is solved numerically the method was tested in detail numerical results for simple medium configurations surrounding a fluid filled borehole have been obtained the modelling of tube waves caused by the incident p reflected pp ps and the rayleigh wave in a fluid filled borehole of finite length located in an elastic half space adequately describes the main features of tube wave propagation tube wave modelling that includes the presence of a layer with hydraulic conductivity shows a high efficiency of tube wave generation at the layer boundaries and significant amplitude attenuation inside it it follows from the results of testing that the algorithm developed can be used for adequate modelling of vsp data

full wave acoustic logging in an irregular borehole a new boundary integral equation formulation for wave propagation in a borehole of irregular cross section represents the wave field diffracted at the borehole rock interface by the radiation from a distribution of surface sources applied along the borehole wall the wave fields in the borehole fluid and in the elastic rock are then expressed using the discrete wavenumber method application of boundary conditions at discretized locations along the borehole wall leads to a linear system of equations whose inversion yields the required source distribution we have used the method to investigate the effect of changes in borehole diameter on the pressure wave field inside the borehole when the change is smooth records obtained ahead of the discontinuity location are not affected by its presence in the case of a steep variation however a significant amount of the stoneley wave energy is reflected when the borehole diameters are different at the source and receiver levels the microseismograms obtained

numerical computation of individual far field arrivals excited by an acoustic source in a borehole in this paper i model the acoustic logging problem and numerically compute individual arrivals at far field receivers the ability to compute individual arrivals is useful for examining the sensitivities of each arrival to various factors of interest as opposed to examining the full waveform as a whole while the numerical computation of the mode arrivals peterson 1974 and the numerical computation of the first head waves tsang and rader 1979 have been previously reported the numerical computation of the entire set of head wave arrivals is new and is the major contribution of this paper following roever et al 1974 and others the full wave field is represented as a sum of contributions from both poles and branchcuts in the complex wavenumber plane the pole contributions correspond to mode arrivals while the branch cuts are associated with the body waves i e head waves both the pole and branch cut contributions are computed numerically and results are presented for the cases of a slow an

modeling of low frequency stoneley wave propagation in an irregular borehole a fast modeling method is formulated for low frequency stoneley wave propagation in an irregular borehole this fast modeling method provides synthetic waveforms which include the effects of two borehole irregularities diameter changes washout and formation property changes the essential physics of the low frequency stoneley waves are captured with a simple 1 d model a mass balance boundary condition and a propagator matrix are used to express stoneley wave interactions with the borehole irregularities the accuracy of the proposed method was confirmed through comparison with existing finite difference and boundary integral modeling methods that yielded cross correlations greater than 0 98 comparison of synthetic records calculated for an actual borehole with field records showed qualitative agreement in the major reflections because of the washout zones but showed some disagreements in the reflections caused by the fractures since the synthetic records include only information relating to the borehole geometry and the elastic properties of formation the reflection caused by the fracture will appear only in the field record these results suggest the possibility of distinguishing stoneley wave reflections caused by fractures from those caused by borehole irregularities further the fast computational speed of this method over 300 times faster than either boundary integral or finite difference methods makes it quite suitable for field application

the importance of a small wall deformation in the three dimensional acoustic logging results summary a variety of seismic techniques use fluid filled boreholes to host detectors and seismic sources the complexity of the wavefield generated depends on the position of the receivers and the source frequency excited type of formation and type of source in the present paper the boundary element method is used to study the wavefield elicited by monopole or dipole sources within a fluid filled cylindrical cavity drilled through an unbounded elastic formation this model is used to assess the effects of the existence of a small localized wall deformation on the response such a deformation may occur through the mechanical action of the drill string rock failure adjacent to a drilled borehole plastic deformation and washing out of the borehole in soft or poorly consolidated rocks in addition the effect of a source receiver tool placed in an off centre and or tilted position inside the fluid filled borehole on the propagation of both axisymmetric and non axisymmetric wave modes are studied

two dimensional velocity distribution modeling for natural river based on uhf radar surface current abstract non contact flow measurement method plays an increasingly important role in river discharge measurement however most channel cross section velocity distribution models proposed by researchers have certain limitations in application underwater velocities are always necessary when applied in narrow channels which makes it challenging to calculate natural river discharge using water surface velocities measured by remote sensing instruments this letter takes water surface velocities detected by uhf radar as boundary conditions and a new velocity distribution model is derived based on the reynolds averaged navier stokes rans equations uhf radar measures surface velocities of the cross section and velocities on the river bed of the cross section are considered to be 0 the rans equations are simplified to be a two dimensional poisson equation and the cross section velocity distribution is solved by using the closed boundary conditions some field experiments were conducted in the hanjiang river at xiantao hubei china in september 2017 and april july 2018 the processing results of which show that the proposed model is superior to the power law and the log law models especially near the side wall narrow channel region river discharges calculated by this method are highly consistent with those provided by hubei xiantao hydrologic station which verifies the reliability of the model and the feasibility of using uhf radar to measure natural river discharge continuously and automatically

local and global fluid effects on sonic wave modes abstractmost subsurface formations of value to exploration contain a heterogeneous fluid filled pore space where local fluid pressure effects can significantly change the velocities of passing seismic waves to better understand the effect of these local pressure gradients on borehole wave propagation we combined chapman s squirt flow model with biot s poroelastic theory we applied the unified theory to a slow and fast formation with permeable borehole walls containing different quantities of compliant pores these results are compared with those for a formation with no soft pores the discrete wavenumber summation method with a monopole point source generates the wavefields consisting of the p s leaky p stoneley and pseudo rayleigh waves the resulting synthetic wave modes are processed using a weighted spectral semblance wss algorithm we found that the resulting wss dispersion curves closely matched the analytical expressions for the formation compressional velocity and solutions to the peri

finite difference synthetic acoustic logs the finite difference method is a powerful technique for studying the propagation of elastic waves in boreholes even for the simple case of an open borehole with vertical homogeneity the snapshot format of the method displays clearly the interaction between the borehole and the rock and the origin and evolution of phases we present an outline of the finite difference method applied to the acoustic logging problem including a boundary condition formulation for liquid solid cylindrical interfaces which is correct to second order in the space increments absorbing boundaries based on the formulations of reynolds 1978 and clayton and engquist 1977 were used to reduce reflections from the grid boundaries results for a vertically homogeneous sharp interface model are compared with the discrete wavenumber method and excellent agreement is obtained the technique is also demonstrated by considering sharp and continuous transitions damaged zones at the borehole wall and by considering the effects of wa

hydrophone vsp imaging at a shallow site we evaluated the capabilities of vertical seismic profiling vsp for imaging the complex heterogeneous unconsolidated sedimentary structures at a shallow site we deployed a 24 level hydrophone array with 0 5 m level spacing down a preexisting poly vinyl chloride pvc cased well data acquisition time was quick only 15 multioffset shot points using a hammer on plate source were needed to acquire reflection data between the water table at 3 m and the bedrock at 35 m to produce a depth section image this image extended 9 m from the receiver well yielding resolutions between fresh water bearing sands and impermeable muds and clays of better than 1 m depth accuracy of the image was confirmed by good correlation with cone penetrometer logs we used conventional wavefield separation and vsp cdp mapping techniques to image the data tube waves created by seismic arrivals at cross sectional area changes in the borehole fluid column were the primary source of coherent noise in the data the tube wave arriv

joint cross well and single well seismic studies of co2 injection in an oil reservoir a series of time lapse seismic cross well and single well experiments were conducted in a diatomite reservoir to monitor the injection of co2 into a hydrofracture zone based on p and s wave data a high frequency piezo electric p wave source and an orbital vibrator s wave source were used to generate waves that were recorded by hydrophones as well as 3 component geophones during the first phase the set of seismic experiments was conducted after the injection of water into the hydrofractured zone the set of seismic experiments was repeated after a time period of seven months during which co2 was injected into the hydrofractured zone the questions to be answered ranged from the detectability of the geological structure in the diatomic reservoir to the detectability of co2 within the hydrofracture furthermore it was intended to determine which experiment cross well or single well is best suited to resolve these features during the pre injection experiment the p wave velocities exhibited relatively low values between 1700 and 1900 m s which decreased to 1600 1800 m s during the post injection phase 5 the analysis of the pre injection s wave data revealed slow s wave velocities between 600 and 800 m s while the post injection data revealed velocities between 500 and 700 m s 6 these velocity estimates produced high poisson s ratios between 0 36 and 0 46 for this highly porous 50 material differencing post and pre injection data revealed an increase in poisson s ratio of up to 5 both velocity and poisson s ratio estimates indicate the dissolution of co2 in the liquid phase of the reservoir accompanied by an increase in pore pressure the single well data supported the findings of the cross well experiments p and s wave velocities as well as poisson s ratios were comparable to the estimates of the cross well data the cross well experiment did not detect the presence of the hydrofracture but appeared to be sensitive to overall changes in the reservoir and possibly the presence of a fault in contrast the single well reflection data revealed an arrival that could indicate the presence of the hydrofracture between the source and receiver wells while it did not detect the presence of the fault possibly due to out of plane reflections

seismic waveform inversion in the frequency domain part 2 fault delineation in sediments using crosshole data a crosshole experiment was carried out in a layered sedimentary environment in which a normal fault is known to cut through the section initial traveltime inversions produced stable but low resolution images from which the fault could be only vaguely inferred to image the fault wavefield inversion was used to produce a velocity model consistent with the detailed phase and amplitude of the data at a number of frequencies our wavefield inversion scheme uses a classical descent type algorithm for decreasing the data misfit by iteratively computing the gradient of this misfit by repeated forward and backward propagations our propagator is a full wave equation frequency domain acoustic finite difference method the use of the frequency space domain yields computational advantages for multisource data and allows an easy incorporation of viscous effects by running wavefield inversion on the field data a quantitative velocity image was produced that yielded a significantly improved image of the fault when compared with the original traveltime inversions because the original field data were noisy and contained a high degree of multiple scattering from the layering of the sediments the transmitted arrivals were selectively windowed to enhance the image the sediments at the site were strongly attenuating we therefore used a viscoacoustic model during the modeling and the inversion that correctly simulated the observed decrease in amplitude with increasing frequency and source receiver offset furthermore since the traveltime inversion indicated a high degree of anisotropy at the site a fixed homogeneous level of anisotropy was used during the inversion tests at varying levels of anisotropy confirmed the improvement in image quality and in data fit when anisotropy was incorporated the final image was verified by examining the distribution of the residuals in the frequency domain by comparing time domain modeled wavefields with the observed data and by direct comparison with borehole logs

fracture detection using crosswell and single well surveys we recorded high resolution 1 to 10 khz crosswell and single well seismic data in a shallow 15 to 35 m water saturated fractured limestone sequence at conoco s borehole test facility near newkirk oklahoma our objective was to develop seismic methodologies for imaging gas filled fractures in naturally fractured gas reservoirs the crosswell 1 4 m receiver spacing 50 to 100 m well separation surveys used a piezoelectric source and hydrophones before during and after an air injection that we designed to displace water from a fracture zone our intent was to increase the visibility of the fracture zone to seismic imaging and to confirm previous hydrologic data that indicated a preferred pathway for the single well seismic imaging a piezoelectric source and an eight element hydrophone array at 1 4 m spacing we also recorded data before and after the air injection the crosswell results indicate that the air did follow a preferred pathway that was predicted by hydrologic modeling in addition

determination of in situ attenuation from full waveform acoustic logs method for the determination of in situ p and s wave attenuation from full waveform acoustic logs are developed for p waves the peak amplitude ratios of the refracted p waves from two different receivers can be used with geometrical spreading taken into account for s waves owing to the contamination by the guided waves its attenuation cannot be determined directly instead s wave attenuation is determined from the attenuation of the guided waves using the partition coefficients normalized partial derivatives of the phase velocity with respect to the body wave velocities analytical forms of these partition coefficients are presented here along with examples for a number of different rock formations granite limestone sandstone and soft sediments the results show that in high velocity rocks the fluid attenuation controls the guided wave attenuation except near the cut off frequency of the pseudo rayleigh wave for low velocity rock formations especially in the case where the s wave velocity is lower than the fluid velocity the s wave attenuation is the main contributor to the guided wave attenuation synthetic microseisgmogram calculated with the measured body wave attenuation agrees well with the actual microseismograms

adcp measurements of gravity currents in the chicago river illinois a unique set of observations of stratified flow phenomena in the chicago river was made using an upward looking acoustic doppler current profiler adcp during the period november 20 2003 to february 1 2004 water density differences between the chicago river and its north branch nb seem to be responsible for the development of gravity currents with the objective of characterizing the occurrence frequency and evolution of such currents the adcp was configured to continuously collect high resolution water velocity and echo intensity profiles in the chicago river at columbus drive during the observation period 28 gravity current events were identified lasting a total of 77 of the time sixteen of these events were generated by underflows from the nb and 12 of these events were generated by overflows from the nb on average the duration of the underflow and overflow events was 52 3 and 42 1 h respectively a detailed analysis of one underflow event which started on january 7 2004 and lasted

a numerical investigation of head waves and leaky modes in fluid filled boreholes although synthetic borehole seismograms can be computed for a wide range of borehole conditions the physical nature of shear and compressional head waves in fluid filled boreholes is poorly understood this paper presents a series of numerical experiments designed to explain the physical mechanisms controlling head wave propagation in boreholes these calculations demonstrate the existence of compressional normal modes equivalent to shear normal modes or pseudo rayleigh waves with sequential cutoff frequencies spaced between the cutoff frequencies for the shear normal modes major contributions to head wave spectra occur in discrete peaks at frequencies just below mode cutoff for both compressional and shear modes this result is confirmed by calculations with synthetic waveforms at frequencies corresponding to mode cutoff and by branch cut integrals designed to yield independent spectra for the compressional mode for soft formations where shear velocity falls below acoustic velocity in the borehole

3 d flow and scour near a submerged wing dike adcp measurements on the missouri river 1 detailed mapping of bathymetry and three dimensional water velocities using a boat mounted single beam sonar and acoustic doppler current profiler adcp was carried out in the vicinity of two submerged wing dikes located in the lower missouri river near columbia missouri during high spring flows the wing dikes become submerged creating a unique combination of vertical flow separation and overtopping plunging flow conditions causing large scale three dimensional turbulent flow structures to form on three different days and for a range of discharges sampling transects at 5 and 20 m spacing were completed covering the area adjacent to and upstream and downstream from two different wing dikes the objectives of this research are to evaluate whether an adcp can identify and measure large scale flow features such as recirculating flow and vortex shedding that develop in the vicinity of a submerged wing dike and whether or not moving boat single transect data are sufficient for resolving complex three dimensional flow fields results indicate that spatial averaging from multiple nearby single transects may be more representative of an inherently complex temporally and spatially variable three dimensional flow field than repeated single transects results also indicate a correspondence between the location of calculated vortex cores resolved from the interpolated three dimensional flow field and the nearby scour holes providing new insight into the connections between vertically oriented coherent structures and local scour with the unique perspective of flow and morphology in a large river

borehole stoneley wave propagation across permeable structures this study investigates the propagation of borehole stoneley waves across permeable structures by modeling the structure as a zone intersecting the borehole a simple 1d theory is formulated to treat the interaction of the stoneley wave with the structure this is possible because the stoneley wave is a guided wave with no geometric spreading as it propagates along the borehole the interaction occurs because the zone and the surrounding formation possess different stoneley wavenumbers given appropriate representations of the wavenumber the theory can be applied to treat a variety of structures including a fluid filled fracture of special interest are the cases of permeable porous zones and fracture zones the results show that while stoneley wave reflections are generated strong stoneley wave attenuation is produced across a very permeable zone this result is particularly important in explaining the observed strong stoneley wave attenuation at major fractures where it has been difficult to explain the attenuation in terms of the single planar fracture theory in addition by using a simple and sufficiently accurate theory to model the effects of the permeable zone a fast and efficient method is developed to characterize the fluid transport properties of a permeable fracture zone

modeling of borehole stoneley waves in the presence of skin effects an important aspect of oil production from a geological formation is the process of flow into a well penetrating the formation recent laboratory experiments indicate that well inflow can be significantly reduced by a skin effect caused by the presence of a thin layer of low permeability around the wellbore this impairment layer is drilling induced and differs from the mudcake that is usually encountered when logging a well whereas a mudcake may have built up on the borehole wall during drilling the drawdown pressure often removes it when oil is produced from the well in contrast with such a temporary mudcake the impairment layer is believed to form a permanent flow impedance throughout the lifetime of a well unless extensive cleaning measures are taken a simple poro elastic model is constructed to investigate the propagation of stoneley waves in a borehole with a thin impairment layer if the impairment layer affects stoneley wave propagation it may be possible to acoustically detect this layer with sonic logging tools using realistic values for the impairment flow impedance model calculations for a typical sandstone reservoir of 20 porosity indicate that the impairment layer changes the dispersion characteristics of borehole stoneley waves the magnitude of these changes increases with increasing mobility of the reservoir pore fluid

interferometric assessment of clamping quality of borehole geophones borehole arrays are often preferred over surface installations for hydraulic fracture monitoring of deep experiments due to their proximity to the treatment zone borehole geophone strings are typically clamped to the observation wellbore wall using electromechanical or magnetic devices in order for them to be in close contact with the surrounding formations and record the background noise and propagating wavefields related to the microseismic experiments this contact needs to be maintained throughout the recording time we have used seismic interferometry to assess the clamping quality of borehole geophone arrays we determined that the characteristics of the retrieved crosscorrelation functions between a reference receiver and other receivers in an array are indicative of the clamping quality of the former geophone to the borehole wall we have also defined the concept of separation frequency or emergence frequency as the frequency below which direct body waves propagating along the receiver line are clearly observed on the crosscorrelation gathers the crosscorrelation gathers associated with poorly clamped geophones show predominantly tube waves or incoherent waveforms body waves only emerge below very low separation frequencies the crosscorrelation gathers of relatively better coupled geophones on the other hand have higher separation frequencies we have applied this method to four different borehole microseismic data sets labeled here as a b c and d of which data set d was previously known to suffer from some clamping issues data sets b and c with inferred better coupling had separation frequencies of approximately 60 hz whereas the other two data sets are characterized by lower separation frequencies 15 hz for data set a and 20 hz for data set d suggesting relatively poorer coupling

elastic wave propagation in a fluid filled borehole and synthetic acoustic logs the propagation and dispersion characteristics of guided waves in a fluid filled borehole are studied using dispersion curves and modeling full wave acoustic logs by synthetic microseismograms the dispersion characteristics of the pseudo rayleigh reflected and stoneley waves in a borehole with and without a tool in the center are compared effects of different tool properties are calculated the effect of a rigid tool is to make the effective borehole radius smaller as an approximation dispersion characteristics of the guided waves in a borehole with a tool can be calculated as a purely fluid filled borehole with a smaller effective radius theoretical waveforms microseismograms of elastic waves propagating in a borehole are calculated using a discrete wavenumber integration with an appropriate choice of parameters our results look similar to the acoustic waveforms recorded in a limestone and a shale formation several factors affect the shape of an acoustic log microseismogram the effective rad

permeability inversion from low frequency seismoelectric logs in fluid saturated porous formations in this paper a method is proposed to invert permeability from seismoelectric logs in fluid saturated porous formations from the analysis of both the amplitude and the phase of simulated seismoelectric logs we find that the stoneley wave amplitude of the ratio of the converted electric field to the pressure rep is sensitive to porosity rather than permeability while the tangent of the rep s phase is sensitive to permeability the rep s phase reflects the phase discrepancy between the electric field and the pressure at the same location in the borehole we theoretically derive the frequency dependent expression of the rep of the low frequency stoneley wave and find that the tangent of the rep s argument is approximately in inverse proportion to permeability we obtain an inversion formula and present the permeability inversion method by using the tangent of the rep s phase to test this method the permeabilities of different sandstones are inverted from the synthetic full waveform data of seismoelectric logs a modified inversion process is proposed based on the analysis of inversion errors by which the relative errors are controlled below 25 and they are smaller than those of the permeability inversion from the stoneley wave of acoustic logs

physical model for the downhole orbital vibrator dov i acoustic and borehole seismic radiation summary the downhole orbital vibrator dov applies the vibroseis principle to borehole seismic sourcing accelerations by an internal rotating eccentric mass excite cylindrical pressure waves converted at the wellbore to seismic waves essentially within 20 30 of the plane normal to the borehole dov pressure waves in open water are quantitatively described by a rotating point force radiating acoustic waves with displacement amplitude u r 1 2 u0 r1 r where r1 2 dov watera2l 2 1 mm is the dov effective size for dov radius a 5 cm length l 1 m and acoustic radiation wavelength 10 m and u0 1 m and x are respectively the frequency independent dov displacement amplitude and the direction cosine of the observer relative to the instantaneous point force axis in the plane of the rotating point force crosswell seismic radiation amplitude spectrum and angular dependence are quantitatively described by acoustic wave diffraction at a slit with dov axial cross section 2al followed by conversion to seismic waves at the wellbore seismic wave displacement amplitudes u r u0 r2 r scale with effective radius r2 2al 1 cm the frequency dependence of r2 is observed as linear frequency enhancement of the seismic wavelet spectrum relative to the source wavelet spectrum dov borehole p and s wave production peaks strongly in the plane of dov rotation with converted s waves both parallel to and transverse to the borehole axis the small effective source sizes r1 1 mm and r2 1 cm at operational frequencies 50 350 hz imply that dov motion in a borehole is dynamically decoupled from the borehole wall dynamic decoupling allows dov borehole seismic correlation wavelets to be quantitatively modelled in terms of a stable kinematic relation between source and sensor motion acoustic and seismic data rule out dipole source action associated with claims for shear traction and primary s wave radiation from boreholes the stable linear kinetics of dov acoustic action in borehole fluids produces i useful crosswell seismic signals at offsets to 650 800 m ii negligible tube waves and iii stable seismic wavelets suited to in situ time lapse seismic imaging of fluid migration fronts in crustal reservoirs

importance of frictional effects and jet instability on the morphodynamics of river mouth bars and levees 1 in this work extensive numerical simulations have been performed to assess the hydrodynamic and morphodynamic behavior of a river jet debouching in a large quiescent water body a refined three dimensional grid has been used to capture the transition zone between a stable jet and an unstable meandering jet the model results show that the stability number s which is a function of friction and river mouth aspect ratio and the mouth reynolds number are the two parameters that describe the stable unstable character of the jet from a morphodynamic point of view a stable jet always tends to form a mouth bar however a decrease of the stability number together with jet instability increase the delivery of sediments to the jet margins favoring the formation of subaerial levees and elongated channels frictional effects play a major role to set the distance at which the mouth bar becomes stagnant the importance of the stability number in setting depositional patterns at the river mouth is larger than other variables i e momentum of the jet and potential vorticity and therefore should be considered in the design of restoration schemes for deltaic land

linking the spatial distribution of bed load transport to morphological change during high flow events in a shallow braided river this paper provides novel observations linking the connections between spatially distributed bed load transport pathways hydraulic patterns and morphological change in a shallow gravel bed braided river these observations shed light on the mechanics of braiding processes and illustrate the potential to quantify coupled material fluxes using remotely sensed methods the paper focuses upon a 300 m long segment of the rees river new zealand and utilizes spatially dense observations from a mobile acoustic doppler current profiler adcp to map depth velocity and channel topography through a sequence of high flow events apparent bed load velocity is estimated from the bias in adcp bottom tracking and mapped to indicate bed load transport pathways terrestrial laser scanning tls of exposed bar surfaces is fused with the adcp surveys to generate spatially continuous digital elevation models which quantify morphological change through the sequence of events results map spatially distributed bed load pathways that were likely to link zones of erosion and deposition the coherence between the channel thalweg zone of maximum hydraulic forcing and maximum apparent bed load pathways varied this suggests that in places local sediment supply sources exerted a strong control on the distribution of bed load distinct from hydraulic forcing the principal braiding mechanisms observed were channel choking leading to subsequent bifurcation results show the connection between sediment sources pathways and sinks and their influence on channel morphology and flow path directions the methodology of coupling spatially dense adcp surveys with tls has considerable potential to understand connections between processes and morphological change in dynamic fluvial settings

homomorphic processing of the tube wave generated during acoustic logging we have developed a new method to process the tube wave which is generated during acoustic logging to obtain estimates for its wavenumber attenuation coefficient amplitude and phase at every frequency to improve the accuracy of the estimates the method can use data from multiple sources and data collected at successive depths in the borehole this new method has several advantages over other methods that are currently used to process acoustic logging data the new method can obtain accurate estimates of the wavenumber and amplitude from only a few receivers the receivers can be irregularly spaced and no spurious estimates are generated nonetheless this new method has one disadvantage compared to others it can only estimate the parameters for one high amplitude wave like the tube wave also like all other existing methods the new method obtains only reasonable estimates for the attenuation coefficient when data from many receivers are processed

quantifying dust emissions from desert landforms eastern mojave desert usa abstract the measurement of natural dust emissions from desert landforms is crucial in environmental hazard assessment and field checking the accuracy of global dust models more than 500 individual dust measurements from eight common desert landforms in southern california were collected using the pi swerl portable in situ wind erosion lab the largest emitters of dust are dry washes 13 787 to 0 007 mg m 2 s 1 dunes playa margins distal alluvial fans and lacustrine beaches low emitters include salt crusted playas 0 692 to 0 002 mg m 2 s 1 silt clay crusted playas and desert pavements high emissions are a function of saltating sand that bombards the surface liberating dust sized particles for entrainment low dust emissions are primarily a function of surface crusting gravel armoring and vegetation density pi swerl measurements reveal that emission rates can vary by at least three orders of magnitude reflecting local variability in soil texture and continuity of surface crusts shear stress partitioning models can be applied to dust data measured by the pi swerl to account for large surface roughness features such as vegetation the results presented here give an approximation of the contributions to atmospheric dust loading by landforms in the mojave desert and can potentially be used to improve atmospheric dust models

hydraulic properties of closely spaced dipping open fractures intersecting a fluid filled borehole derived from tube wave generation and scattering fluid filled fractures and fissures often determine the pathways and volume of fluid movement they are critically important in crustal seismology and in the exploration of geothermal and hydrocarbon reservoirs we introduce a model for tube wave scattering and generation at dipping parallel wall fractures intersecting a fluid filled borehole a new equation reveals the interaction of tube wavefield with multiple closely spaced fractures showing that the fracture dip significantly affects the tube waves numerical modeling demonstrates the possibility of imaging these fractures using a focusing analysis the focused traces correspond well with the known fracture density aperture and dip angles testing the method on a vsp data set obtained at a fault damaged zone in the median tectonic line japan presents evidences of tube waves being generated and scattered at open fractures and thin cataclasite layers this finding leads to a new possibility for imaging characterizing and monitoring in situ hydraulic properties of dipping fractures using the tube wavefield

vertical seismic profiling vertical seismic profiling is one of the rapidly developing areas of geophysical technology for exploring in mature basins the measurement basically involves recording the total upgoing and downgoing seismic wave fields propagating through a stratigraphic section by means of geophones clamped to the wall of a drilled well