the 1983 hydraulic jump in crystal rapid implications for river running and geomorphic evolution in the grand canyon for the last 1 000 to 10 000 years dozens of large debris fans have severely constricted the path of the colorado river in the grand canyon arizona at most of these fans the narrowest part of the channel eroded by the river is 0 5 of the upstream width at crystal creek a debris fan was emplaced in 1966 constricting the channel of the colorado river to about 0 25 of its upstream width between 1967 and 1983 forming a major rapid in this paper the hydraulics of crystal creek rapid are described and an analysis is presented to support the hypothesis that the major wave in the rapid was a normal wave one type of hydraulic jump hydraulic jumps rarely occur in natural river channels with erodible beds but one was present at crystal rapid because of the unusually severe constriction of the colorado river by the 1966 debris fan observations on the hydraulics of the river during this time including mid 1983 when progressively higher discharges culminated in excess of 96 000 cubic feet per second h

kirchhoff migration using eikonal equation traveltimes the use of ray shooting followed by interpolation of traveltimes onto a regular grid is a popular and robust method for computing diffraction curves for kirchhoff migration an alternative to this method is to compute the traveltimes by directly solving the eikonal equation on a regular grid without computing raypaths solving the eikonal equation on such a grid simplifies the problem of interpolating times onto the migration grid but this method is not well defined at points where two different branches of the traveltime field meet also computational and data storage issues that are relatively unimportant for performance in two dimensions limit the applicability of both schemes in three dimensions a new implementation of a gridded eikonal equation solver has been designed to address these problems a 2 d version of this algorithm is tested by using it to generate traveltimes to migrate the marmousi synthetic data set using the exact velocity model the results are compared with three other images an f x migration a standard for comparison a kirchhoff migration using ray tracing and a kirchhoff migration using traveltimes generated by a commonly used eikonal equation solver the f x migrated image shows the imaging objective more clearly than any of themore kirchhoff migrations and the authors advance a heuristic reason to explain this fact of the kirchhoff migrations the one using ray tracing produces the best image and the other two are of comparable quality less

debris fan constrictions and flood hydraulics in river canyons some implications from two dimensional flow modelling rapids in river canyons are frequently found at sites where debris fans constrict flow along the channel whereas some fans may have persisted in the same location with unchanging geometry for centuries to millennia others have changed in response to flow conditions imposed by successive floods such a change in boundary conditions may alter local flow hydraulics this paper utilizes two dimensional flow modelling to compare flood hydraulics along two alternative versions of an idealized reach of a river canyon one with uniform width gradient and cross section and a second perturbed by a prominent debris fan along the valley wall the flow pattern along the reach with the fan is far more complex than the pattern along the uniform reach maximum velocity along the debris fan reach is up to 50 per cent higher than along the uniform reach maximum bed shear stress is up to three or four times higher and an area of supercritical flow is predicted extending from the nose of the fan into the zone of flow expansion immediately downstream comparison of model output along longitudinal profiles of the two reaches indicates that the backwater effect of the fan extends several valley widths upstream predicted flows based on the same stage are as much as 190 to 230 per cent greater along the uniform reach than along the debris fan reach reconstruction of palaeoflood discharge based on remnant flood marks in the vicinity of the fan would be sensitive to assumptions about boundary conditions that existed in the past this effect relaxes over a longitudinal distance of several hundred metres furthermore there are significant cross stream gradients that change slope and direction several times in the vicinity of the fan calling into question the utility of one dimensional step backwater hydraulic models for predicting high water marks in areas of complex valley morphology

investigation into the use of 2d elastic waveform inversion from look ahead walk away vsp surveys pore pressure in sediments beneath salt in the gulf of mexico varies widely creating a potentially dangerous and difficult drilling challenge estimating elastic parameters of sediments beneath salt is key to the prediction of pore pressure and reducing the drilling risk in exiting the base of the salt in this paper we investigate the ability of 2d waveform inversion to recover the elastic parameters in the sedimentary layer beneath the salt from a walk away vsp vertical seismic profile carried out with the receivers in the salt with the objective of estimating pore pressure at the base of the salt to be estimated using traditional methods we propose an effective method for performing the inversion and apply this method to a blind test of a large and realistic synthetic dataset to facilitate the design of a vsp survey suitable to this type of inversion we also present an analysis into the effects of the receiver array receiver spacing it is shown that the resulting velocity estimates are sufficiently accurate to predict the pore pressure within the limits required for drilling

interferometric daylight seismic imaging summary claerbout s daylight imaging concept is generalized to a theory of interferometric seismic imaging ii interferometric seismic imaging is defined to be any algorithm that inverts correlated seismic data for the reflectivity or source distribution as examples we show that ii can image reflectivity distributions by migrating ghost reflections in passive seismic data and generalizes the receiver function imaging method used by seismologists interferometric seismic imaging can also migrate free surface multiples in common depth point cdp data and image source distributions from passive seismic data both synthetic and field data examples are used to illustrate the different possibilities of ii the key advantage of ii is that it can image source locations or reflectivity distributions from passive seismic data where the source position or wavelet is unknown in some cases it can mitigate defocusing errors as a result of statics or an incorrect migration velocity the main drawback with ii is that severe migration artefacts can be created by partial focusing of virtual multiples

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

mitigation of defocusing by statics and near surface velocity errors by interferometric least squares migration with a reference datum abstractimaging seismic data with an erroneous migration velocity can lead to defocused migration images to mitigate this problem we first choose a reference reflector whose topography is well known from the well logs for example reflections from this reference layer are correlated with the traces associated with reflections from deeper interfaces to get crosscorrelograms interferometric least squares migration ilsm is then used to get the migration image that maximizes the crosscorrelation between the observed and the predicted crosscorrelograms deeper reference reflectors are used to image deeper parts of the subsurface with a greater accuracy results on synthetic and field data show that defocusing caused by velocity errors is largely suppressed by ilsm we have also determined that ilsm can be used for 4d surveys in which environmental conditions and acquisition parameters are significantly different from one survey to the next the limitations of ilsm are that it requires prior knowledge of

valley width variation influences riffle pool location and persistence on a rapidly incising gravel bed river abstract decades of research have demonstrated that gravel bed rivers exhibit multiple spatial scales of landform nonuniformity landform features such as valley wall oscillations alluvial fans terraces river meanders point bars boulder clusters and logjams contribute to river corridor heterogeneity further studies have shown that the locations of riffles and pools in gravel bed rivers are closely associated with the spatial pattern of river corridor morphological variability these studies imply but do not directly test that persistence of riffles and pools through time is also conditioned by landform variability mechanistically it may be hypothesized that overbank flows interact hydraulically with oscillating valley walls by way of a mechanism known as flow convergence routing in such circumstances can variation in valley width influence channel morphology forcing riffle pool location and persistence to address this question research was performed on a valley confined wandering gravel bed reach of the regulated lower yuba river california usa that is incising from reduced sediment supply and flow regulation after a historic period of mining related valley sedimentation this study quantified stream planform change 1984 2006 and elevation change 1999 2006 using eight aerial photo sets and two digital elevation models and it qualitatively assessed channel change in older aerial photos going back to 1937 analysis revealed that the river has undergone rapid incision 0 11 m year and planform change in response to frequent floods yet eight riffles persisted in the same locations which coincided with the locally wide areas of the valley longitudinal profiles revealed that deep pools were associated with long valley constrictions geomorphic analysis and design need to explicitly account for multiple scales of landform nonuniformity in order to rehabilitate natural fluvial processes

velocity reversals and sediment sorting in pools and riffles controlled by channel constrictions abstract keller keller e a 1971 areal sorting of bed load material the hypothesis of velocity reversal geological society of america bulletin 82 753 756 hypothesized that at high flow near bed velocities in pools exceed velocities in riffles and create pool scour pools however typically have larger cross sectional areas of flow at bankfull discharge this condition raises an inconsistency with keller s velocity reversal hypothesis and the one dimensional continuity of mass equation to address this problem a model of pool maintenance and sediment sorting is proposed that relies on constriction of flow by recirculating eddies and flow divergence over the exit slopes of pools according to the model a narrow zone of high velocity occurs in the center of pools creating scour along the downstream end of pools an uphill climb of particles up the pool exit slope promotes sediment deposition the model is tested with field and flume measurements of velocity water surface elevation and size of bed sediments in recirculating eddy influenced pools local reversals of the water surface gradient were measured in the field and a velocity reversal was created in the flume the reversals that were measured indicate higher gradients of the water surface over the upstream portions of pools and higher velocities in pools at high flow the distribution of bed sediments collected in the field also support the proposed model of pool maintenance

paleohydrology of pool and riffle pattern development boulder creek utah the low flow channel morphology of boulder creek is characterized by a well developed pool and riffle pattern the riffles consist of accumulations of basaltic boulders deposited from upstream source areas during extremely large flows paleoflood water surface profiles defined by high water indicators such as slack water sediments and silt lines indicate that discharges of up to 400 50 m 3 s have affected the lower reaches of this bedrock stream system stratigraphic relationships and archaeologic and radiometric age constraints indicate that at least four large magnitude low frequency flow events have occurred within the past 500 to 1 000 radiocarbon years b p step backwater hydraulic reconstructions of these large flows suggest that the positions of the boulder comprised riffles are controlled by spatial variations in large flow stream power boulder deposition occurs where channel stream power drops below critical power thresholds necessary for boulder transport high discharge stream power minima occur in reaches immediately upstream of canyon bends and constrictions and downstream of canyon expansions the low flow riffles occur at these sites comparison of calculated stream power values and measured boulder sizes with established coarse particle transport relationships indicates that a 400 m 3 s flow is approximately the minimum discharge competent to affect this pool and riffle pattern

superwide angle one way wave propagator and its application in imaging steep salt flanks one way wave propagators have been used extensively in forward and inverse problems the advantages of one way wave propagators are the fast speed of computation and the large savings in computer resources however regular one way wave propagators have inherent angle limitation to within 90 of the preferred direction e g z direction we propose to overcome the limitation by a superwide angle one way method which combines and interpolates two orthogonally propagated one way wavefields in the 2d case a weight function determined by the propagation angle is employed to combine the two one way wavefields in the current implementation we use the wavefield gradient to estimate the propagation angle numerical experiments show that the proposed method has accurate superwide angle 90 propagation and can model turning waves well by applying the superwide angle one way wave propagator on several synthetic model data sets we can demonstrate the validity of the method in imaging steep and overhanging

paleohydrology of southwestern texas current statistical methods may be unable to accurately predict recurrence intervals of rare large magnitude floods especially in semiarid regions having positively skewed annual flood distributions great hydrologic variability and widely spaced gaging stations current approaches rely on historical data but catastrophic floods may have recurrence intervals far greater than the length of historical records in the lower pecos and devils rivers of southwestern texas paleoflood discharge and frequency estimates are extended over 10 000 years by the study of slack water flood sediments slack water deposits are typically fine grained sand and silt that accumulate during floods in areas where current velocity is reduced i e in back flooded tributary mouths channel expansions downstream from bedrock spurs and or slump blocks and in shallow caves along bedrock walls radiocarbon dating of organic detritus in slack water deposits establishes the flood chronology while paleoflood discharges can be estimated by slope area techniques paleoflood information extracted from slack water sediments can greatly extend flood records these floods may be weighted like historical data in log pearson type 3 calculations of flood frequency our morphostratigraphic approach combines recorded data with geomorphic evidence to derive estimates of flood frequency this technique offers an inexpensive and rapid way to assess catastrophic flood risk

the characteristics of turbulence in a shear zone downstream of a channel constriction in a coarse grained forced pool a field based project was initiated to characterize the influence of varying discharge and bed topography along a pool exit slope on the strength of turbulence generated by vortex shedding velocities were measured with an adv downstream of a boulder constriction within a shear zone of a large pool measurements were repeated for four flows that varied from 20 to 50 of bankfull discharge an additional longitudinal profile was conducted along the pool thalweg plots of velocity demonstrate high near bed velocities and turbulence levels in the pools turbulence levels were highest closer to the constriction in the shear zone turbulent kinetic energy decayed in the downstream direction in response to the bed topography and at lower discharges instantaneous velocities are large enough to temporarily lift pebbles at this depth it appears that instantaneous forces created by vortex shedding may play an important role in scouring pools downstream of constrictions the turbulence may also respond to changes in bed topography in a manner that encourages feedbacks among bed topography characteristics of turbulence and sediment transport

imaging of offset vsp data acquired in complex areas with modified reverse time migration a modified reverse time migration algorithm for offset vertical seismic profiling data is proposed this algorithm performs depth imaging of target areas in the borehole vicinity without taking into account the overburden originally recorded seismograms are used reliable results can be obtained using only the velocity profile obtained along the well the downgoing wavefield emitted from a surface source is approximated in the target area using the transmitted p wave recorded by the receivers deployed in the well this is achieved through a reverse time extrapolation of the direct transmitted p wave into the target area after its separation in offset vertical seismic profiling seismograms generated using a finite difference scheme for the solution of the scalar wave equation the proposed approach produces kinematically reliable images from reflected pp and ps waves and furthermore can be applied as a salt proximity tool for salt body flank imaging based on the transmitted ps waves our experiments on synthetic data demonstrate that the modified reverse time migration provides reliable depth images based on offset vertical seismic profiling data even if only the velocity profile obtained along the borehole is used

side valley tributary fans in high energy river floodplain environments sediment sources and depositional processes navarro river basin california side valley tributary fans within the main axial valley river floodplain system are integral components of the sediment assemblage characterizing floodplains in the high energy navarro river basin california laboratory analyses of subsurface core and auger samples from four study sites reveal that sediment size distributions within the floodplain are controlled by spatial and temporal variation in sediment sources and depositional processes over 75 of sediment within individual lower fan strata are poorly sorted and include gravel clasts inferred to be contributions from the side valley tributary whereas remaining strata are composed of fine sediment less than 2 mm without gravel high water marks indicate that portions of the side valley tributary fans are episodically inundated by axial valley overbank flow a factor that influences fan relief and profile thus two dominant sources supply sediment to this transitional environment side valley tributaries and the axial valley river the transport and depositional processes that supply sediment to the fans operate under three conditions 1 tributary debris or fluvial flows and selective entrainment of fines by fan surface flows deposit sediment when there is no overbank flow from the axial channel 2 overbank flow deposits sediment derived from the axial valley river when there is no tributary input and 3 tributary flows deposit sediment while the floodplain is inundated by floods from the axial valley river the mixing and interlayering of sediment from these disparate sources within the active floodplain illustrate the significance of both hillslope floodplain and channel flood plain interactions in high energy fluvial systems with floodplains

local vertical seismic profiling vsp elastic reverse time migration and migration resolution salt flank imaging with transmitted p to s waves to avoid the defocusing effects of propagating waves through salt and overburden with an inaccurate overburden velocity model we introduce a vertical seismic profiling vsp local elastic reverse time migration rtm method for salt flank imaging by transmitted p to s waves this method back projects the transmitted ps waves using a local velocity model around the well until they are in phase with the back projected pp waves at the salt boundaries the merits of this method are that it does not require the complex overburden and salt body velocities and it automatically accounts for source side statics in addition the method accounts for kinematic and dynamic effects including anisotropy absorption and all other unknown rock effects outside of this lo cal subsalt velocity model numerical tests on an elastic salt model and offset 2d vsp data in the gulf of mexico using a finite difference time domain staggered grid rtm scheme partly demonstrate the effectiveness of this method over interferometry

3 d prestack kirchhoff beam migration for depth imaging a beam implementation is presented for efficient full volume 3 d prestack kirchhoff depth migration of seismic data unlike conventional kirchhoff migration in which the input seismic traces in time are migrated one trace at a time into the 3 d image volume for the earth s subsurface the beam migration processes a group of input traces a supergather together the requirement for a supergather is that the source and receiver coordinates of the traces fall into two small surface patches the patches are small enough that a single set of time maps pertaining to the centers of the patches can be used to migrate all the traces within the supergather by taylor expansion or interpolation the migration of a supergather consists of two major steps stacking the traces into a p beam volume and mapping the beams into the image volume since the beam volume is much smaller than the image volume the beam migration cost is roughly proportional to the number of input supergathers the computational speedup of be

reactive transport under stress permeability evolution in deformable porous media abstract we study reactive transport in a stressed porous media where dissolution of the solid matrix causes two simultaneous competing effects pore enlargement due to chemical deformation and pore compaction due to mechanical weakening we use a novel mechanistic pore scale model to simulate flooding of a sample under fixed confining stress our simulations show that increasing the stress inhibits the permeability enhancement increasing the injected volume required to reach a certain permeability in agreement with recent experiments we explain this behavior by stress concentration downstream in the less dissolved hence stiffer outlet region as this region is also less conductive even its small compaction has a strong bottleneck effect that curbs the permeability our results also elucidate that the impact of stress depends on the dissolution regime under wormholing conditions slow injection i e high damkohler number da the development of a sharp dissolution front and high porosity contrast accentuates the bottleneck effect this reduces transport heterogeneity promoting wormhole competition once the outlet starts eroding the extreme focusing of transport and hence dissolution characteristic of wormholing becomes dominant diminishing the bottleneck effect and hence the impact of stress at breakthrough in contrast at high flow rates low da incomplete reaction upstream allows some of the reactant to traverse the sample causing a more uniform dissolution the continuous dissolution and its partial counteraction by compaction at the outlet provides a steady gradual increase in the effect of stress consequently the impact of stress is more pronounced at high da during early stages low permeability and at low da close breakthrough our work promotes understanding of the interplay between dissolution and compaction and its effect on the hydromechanical property evolution with important implications for processes ranging from diagenesis and weathering of rocks to well stimulation and carbon sequestration

flow reversal over a natural pool riffle sequence a computational study a computational study is presented on the hydraulics of a natural pool riffle sequence composed of mixed cobbles pebbles and sand in the river lune northern england a depth averaged two dimensional numerical model is employed calibrated with observed data at the field site from the computational outputs the occurrence of longitudinally double peak zones of bed shear stress and velocity is found in particular at low discharge there exists a primary peak zone of bed shear stress and velocity at the riffle tail in line with the local maximum energy slope in addition to a secondary peak at the pool head as discharge increases the primary peak at the riffle tail at low flow moves toward the upstream side of the riffle along with the maximum energy slope showing progressive equalization to the surrounding hydraulic profiles concurrently the secondary peak due to channel constriction appears to stand at the pool head with its value increasing with discharge and approaching or exceeding the primary peak over the riffle the existence of flow reversal is demonstrated for this specific case which is attributable to channel constriction at the pool head a dynamic equilibrium model is presented to reconstruct the pool riffle morphology a series of numerical modelling exercises demonstrates that the pool riffle morphology is more likely produced by shallow flows concentrated with coarse sediments than deep flows laden with low concentrations of fine sediments it is concluded that channel constriction can but may not necessarily lead to competence reversal depending on channel geometry flow discharge and sediment properties

vsp beyond time to depth vsp or vertical seismic profile was originally designed and is currently primarily used to give us time to depth for seismic well ties beyond time to depth a number of possibilities exist recently there has been considerable interest in vsp imaging ray et al 2003 paulsson et al 2004 hornby et al 2004 hornby et al 2005a with extensive surveys being acquired both on land and offshore modeling studies using full waveform finite difference method fdm payne et al 1994 van gestel et al 2003 show us what we can image for a particular acquisition geometry and geology with best image results seen with 3d vsp surveys incorporating a large vsp array in the well and a 2d source pattern acquired using a surface seismic shooting vessel traditionally vsp imaging has been implemented using surface seismic processing algorithms however the vsp geometry poses its own challenges and unique opportunities in this article we explore some imaging methods to attempt to take advantage of the vsp g

imaging above an extended reach horizontal well using converted shear waves and a rig source abstractwe evaluated a method for using 3c vertical seismic profile data to image acoustic interfaces located between the surface source and a downhole receiver array the approach was based on simple concepts adapted from whole earth seismology in which observed compressional and shear wavefields are traced back to a common origin however unlike whole earth and passive seismology in which physical sources are imaged we used the observed compressional and shear wavefields to image secondary sources scatterers situated between the surface source and the downhole receiver array the algorithm consisted of the following steps first estimating the receiver compressional wavefield second using polarization to estimating the shear wavefield third deconvolving the shear wavefield using estimates of the source wavelet obtained from the direct compressional wave fourth the compressional and shear wavefields were back projected into the volume between the source and receivers where finally an imag

palaeoflood hydrology of the salt river arizona geomorphic evidence along bedrock confined reaches of the salt river in east central arizona provides a record of the river s largest flood events fine grained flood slackwater deposits accumulated at channel margin irregularities several metres above the low flow channel discharges associated with flow events responsible for the deposits were estimated by computer flow modelling these estimates document flood magnitudes in excess of gauged historic streamflows relative and radiocarbon dating suggest that a flood record in excess of 600 y is preserved in the slackwater sequences a prominent flood scar cut into grussy hillslope soils allows the extension of the prehistoric flood record to several thousand years a maximum discharge estimate of 4600 m3s 1 affixed to the flood scar represents the largest flood event in the record and is given a minimum recurrence interval of 1000 2000 y the 1952 flood is the largest historic flow event experienced along the study reach and is estimated at 2900 m3s 1 two palaeoflood events preserved in the slackwater stratigraphy exceed the 1952 event and are given recurrence intervals of 300 and 600 y the latter flood event had an estimated discharge of 3200 m3s 1 it is apparent that discharge estimates affixed to these infrequent large magnitude flood events approach a maximum with decreased probabilities large recurrence intervals this suggests that a physical limit on discharge may exist within the salt river drainage basin and is perhaps directly related to drainage basin size

water velocity and the nature of critical flow in large rapids on the colorado river utah 1 rapids are an integral part of bedrock controlled rivers influencing aquatic ecology geomorphology and recreational value flow measurements in rapids and high gradient rivers are uncommon because of technical difficulties associated with positioning and operating sufficiently robust instruments in the current study detailed velocity water surface and bathymetric data were collected within rapids on the colorado river in eastern utah with the water surface survey it was found that shoreline based water surface surveys may misrepresent the water surface slope along the centerline of a rapid flow velocities were measured with an adcp and an electronic pitot static tube integrating multiple measurements the adcp returned velocity data from the entire water column even in sections of high water velocity the maximum mean velocity measured with the adcp was 3 7 m s the pitot static tube while capable of only point measurements quantified velocity 0 39 m below the surface the maximum mean velocity measured with the pitot tube was 5 2 m s with instantaneous velocities up to 6 5 m s analysis of the data showed that flow was subcritical throughout all measured rapids with a maximum measured froude number of 0 7 in the largest measured rapids froude numbers were highest at the entrance of a given rapid then decreased below the first breaking waves in the absence of detailed bathymetric and velocity data the froude number in the fastest flowing section of a rapid was estimated from near surface velocity and depth soundings alone

flow convergence routing hypothesis for pool riffle maintenance in alluvial rivers 1 the velocity reversal hypothesis is commonly cited as a mechanism for the maintenance of pool riffle morphology although this hypothesis is based on the magnitude of mean flow parameters recent studies have suggested that mean parameters are not sufficient to explain the dominant processes in many pool riffle sequences in this study two and three dimensional models are applied to simulate flow in the pool riffle sequence on dry creek california where the velocity reversal hypothesis was first proposed these simulations provide an opportunity to evaluate the hydrodynamics underlying the observed reversals in near bed and section averaged velocity and are used to investigate the influence of secondary currents the advection of momentum and cross stream flow variability the simulation results support the occurrence of a reversal in mean velocity and mean shear stress with increasing discharge however the results indicate that the effects of flow convergence due to an upstream constriction and the routing of flow through the system are more significant in influencing pool riffle morphology than the occurrence of a mean velocity reversal the hypothesis of flow convergence routing is introduced as a more meaningful explanation of the mechanisms acting to maintain pool riffle morphology

interferometric imaging of a salt flank using walkaway vsp data vsp has long been a tool for investigating the location of salt flanks traditionally a salt proximity survey has been used to estimate the location of salt boundaries relative to a wellbore however this technique depends on accurate knowledge of the shape of the salt and the local sediment velocities in addition this method does not work well with complicated 3d salt structure beyond the traditional salt proximity survey many researchers have developed various methods and strategies for improving the locating and or imaging of salt flanks o brien et al 2002 brandsberg dahl et al 2003 zhao et al 2006 with these methods the quality of the resultant image depends on a prior knowledge of the possibly anisotropic overburden velocity field for very long offsets and also is concerned with multi pathing through salt invalidating a 2d assumption in looking for a simpler approach to this problem a single well imaging concept may be attractive using downhole sources and receivers it is possi

subsalt 3d vsp imaging at deimos field in the deepwater gulf of mexico at deimos field in the deepwater gulf of mexico surface seismic imaging cannot fully image sedimentary structure near the steeply dipping base of salt 3d vertical seismic profile vsp imaging was introduced to the deepwater gom to improve seismic imaging of complex structural and stratigraphic features ray et al 2003 hornby et al 2006 and recently has been used for imaging blind spots in surface seismic coverage hornby et al 2005 hornby et al 2007 with this method 3d surveys are conducted using a surface source vessel and downhole geophones here we investigate the use of 3d vsp imaging to complement the surface results with additional coverage beyond that of the surface seismic adjacent to the complex salt body

transient eddy formation around headlands eddies with length scales of 1 10 km are commonly observed in coastal waters and play an important role in the dispersion of water borne materials the generation and evolution of these eddies by oscillatory tidal flow around coastal headlands is investigated with analytical and numerical models using shallow water depth averaged vorticity dynamics eddies are shown to form when flow separation occurs near the tip of the headland causing intense vorticity generated along the headland to be injected into the interior an analytic boundary layer model demonstrates that flow separation occurs when the pressure gradient along the boundary switches from favoring accelerating to adverse decelerating and its occurrence depends principally on three parameters the aspect ratio b a where b and a are characteristic width and length scales of the headland h cda where h is the water depth cd is the depth averaged drag coefficient and uo a where uo and are the magnitude and frequency of the far field tidal flow simulations with a depth averaged numerical model show a wide range of responses to changes in these parameters including cases where no separation occurs cases where only one eddy exists at a given time and cases where bottom friction is weak enough that eddies produced during successive tidal cycles coexist interacting strongly with each other these simulations also demonstrate that in unsteady flow a strong start up vortex forms after the flow separates leading to a much more intense patch of vorticity and stronger recirculation than found in steady flow

modelling floodplain flows using a two dimensional finite element model a prototype two dimensional finite element flow model for depth averaged free surface flows was developed for floodplain environments limited refinement of the model s physical representation was undertaken and the enhanced scheme applied to an 11 km river channel floodplain reach in the u k preliminary model results indicate that this modelling approach can be used to identify dynamic variations in the flow field parameters over length scales of the order of 10 100 m potentially such data have the ability to permit detailed analysis of short term floodplain sedimentary dynamics

beyond laboratory scale prediction for channeling flows through subsurface rock fractures with heterogeneous aperture distributions revealed by laboratory evaluation the present study evaluates aperture distributions and fluid flow characteristics for variously sized laboratory scale granite fractures under confining stress as a significant result of the laboratory investigation the contact area in fracture plane was found to be virtually independent of scale by combining this characteristic with the self affine fractal nature of fracture surfaces a novel method for predicting fracture aperture distributions beyond laboratory scale is developed validity of this method is revealed through reproduction of the results of laboratory investigation and the maximum aperture fracture length relations which are reported in the literature for natural fractures the present study finally predicts conceivable scale dependencies of fluid flows through joints fractures without shear displacement and faults fractures with shear displacement both joint and fault aperture distributions are characterized by a scale independent contact area a scale dependent geometric mean and a scale independent geometric standard deviation of aperture the contact areas for joints and faults are approximately 60 and 40 changes in the geometric means of joint and fault apertures m em joint and em fault with fracture length m l are approximated by em joint 1 102 l0 1 and em fault 1 103 l0 7 whereas the geometric standard deviations of both joint and fault apertures are approximately 3 fluid flows through both joints and faults are characterized by formations of preferential flow paths i e channeling flows with scale independent flow areas of approximately 10 whereas the joint and fault permeabilities m2 kjoint and kfault are scale dependent and are approximated as kjoint 1 10 12 l0 2 and kfault 1 10 8 l1 1

influence of a dam on fine sediment storage in a canyon river glen canyon dam has caused a fundamental change in the distribution of fine sediment storage in the 99 km reach of the colorado river in marble canyon grand canyon national park arizona the two major storage sites for fine sediment i e sand and finer material in this canyon river are lateral recirculation eddies and the main channel bed we use a combination of methods including direct measurement of sediment storage change measurements of sediment flux and comparison of the grain size of sediment found in different storage sites relative to the supply and that in transport in order to evaluate the change in both the volume and location of sediment storage the analysis shows that the bed of the main channel was an important storage environment for fine sediment in the predam era in years of large seasonal accumulation approximately 50 of the fine sediment supplied to the reach from upstream sources was stored on the main channel bed in contrast sediment budgets constructed for two short duration high experimental releases from glen canyon dam indicate that approximately 90 of the sediment discharge from the reach during each release was derived from eddy storage rather than from sandy deposits on the main channel bed these results indicate that the majority of the fine sediment in marble canyon is now stored in eddies even though they occupy a small percentage similar to 17 of the total river area because of a 95 reduction in the supply of fine sediment to marble canyon future high releases without significant input of tributary sediment will potentially erode sediment from long term eddy storage resulting in continued degradation in marble canyon

gaussian beam migration just as synthetic seismic data can be created by expressing the wave field radiating from a seismic source as a set of gaussian beams recorded data can be downward continued by expressing the recorded wave field as a set of gaussian beams emerging at the earth s surface in both cases the gaussian beam description of the seismic wave propagation can be advantageous when there are lateral variations in the seismic velocities gaussian beam downward continuation enables wave equation calculation of seismic propagation while it retains the interpretive raypath description of this propagation this paper describes a zero offset depth migration method that employs gaussian beam downward continuation of the recorded wave field the gaussian beam migration method has advantages for imaging complex structures like finite difference migration it is especially compatible with lateral variations in velocity but gaussian beam migration can image steeply dipping reflectors and will not produce unwanted reflectio

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

imaging salt with turning seismic waves turning seismic waves which first travel downward and then upward before and after reflection have been recorded in a 3 d seismic survey conducted over an overhanging salt dome careful processing of these turning waves enables the imaging of the underside of the salt dome and of intrusions of salt into vertical faults radiating from the dome when seismic wave velocity increases with depth waves that initially travel downward are reflected and may turn so as to travel upward before reflection a simple geometrical argument suggests that these turning waves are likely to exhibit abnormal moveout in com mon midpoint cmp gathers in that reflection time decreases with increasing source receiver offset this abnormal moveout and the attenuation of turning waves by most migration methods suggest that conventional seismic processing does not properly image turning waves the most important step in imaging turning waves assuming that they have been recorded is the migration process simple and inexpensive modifications to the conventional phase shift migration method enable turning waves to be imaged for little additional computational cost the examples provided in this paper suggest that these and other such modifications to conventional processing should be used routinely when imaging salt domes

elastic reverse time migration of marine walkaway vertical seismic profiling data walkaway vertical seismic profiling vsp acquisition with three component geophones allows for direct measurement of compressional as well as shear energy this makes full elastic reverse time migration an attractive alternative for imaging data we present results from elastic reverse time migration of a marine walkaway vsp acquired offshore norway the reverse time migration scheme is based on a high order finite difference solution to the two way elastic wave equation depth images of the subsurface are constructed by correlation of forward and back propagated elastic wavefields in the walkaway vsp configuration the number of shots is much larger than the number of geophone levels using processing methods operating in the shot receiver domain it is advantageous to use the reciprocal relationship between the walkaway vsp and the reverse vsp configurations we do this by imaging each component of each geophone level as a reverse vsp common shot gather the final images are constructed by stacking partial images from each level the depth images obtained from the vertical components reveal the major characteristics of the geological structure below geophone depth a graben in the base cretaceous unconformity and a faulted coal layer can be identified the horizontal components are more difficult to image compared to the vertical components the horizontal component images are more corrupted by migration artifacts this is because the horizontal component images are more sensitive to aperture effects and to the shear wave velocity macromodel when converted to two way time the migration results tie well with the surface seismic section comparison of fully elastic and acoustic reverse time migration shows that the vertical component is dominantly pp reflected events whereas the horizontal components get important contributions from ps converted energy the horizontal components also provide higher resolution because of the shorter wavelength of the shear waves

tracer transport in fractured crystalline rock evidence of nondiffusive breakthrough tailing extended tailing of tracer breakthrough is often observed in pulse injection tracer tests conducted in fractured geologic media this behavior has been attributed to diffusive exchange of tracer between mobile fluids traveling through channels in fractures and relatively stagnant fluid between fluid channels along fracture walls or within the bulk matrix we present a field example where tracer breakthrough tailing apparently results from nondiffusive transport tracer tests were conducted in a fractured crystalline rock using both a convergent and weak dipole injection and pumping scheme deuterated water bromide and pentafluorobenzoic acid were selected as tracers for their wide range in molecular diffusivity the late time behavior of the normalized breakthrough curves were consistent for all tracers even when the pumping rate was changed the lack of separation between tracers of varying diffusivity indicates that strong breakthrough tailing in fractured geologic media may be caused by advective transport processes this finding has implications for the interpretation of tracer tests designed to measure matrix diffusion in situ and the prediction of contaminant transport in fractured rock

random controls on semi rhythmic spacing of pools and riffles in constriction dominated rivers average pool spacing between five and seven bankfull widths has been documented in environments throughout the world but has limited theoretical justification in coarse bedded and bedrock environments pool formation in coarse bedded and bedrock channels has been attributed to bedrock and boulder constrictions because the spacing of these constrictions may be irregular in nature it is difficult to reconcile pool formation processes with the supposedly rhythmic spacing of pools and riffles to address these issues a simulation model for pool and riffle formation is used to demonstrate that semi rhythmic spacing of pools with an approximate spacing of five to seven bankfull widths can be recreated from a random distribution of obstructions and minimum pool and riffle length criteria it is assumed that a pool riffle couplet will achieve a minimum length based on dominant discharge conditions values for the minimum length assumption are based on field data collected in new england and california while the theoretical basis relies on the demonstrated hydraulic response of individual pools to elongation results from the simulations show that the location of pools can be primarily random in character but still assume an average spacing between four and eight bankfull widths for a variety of conditions field verification data generally support the model but highlight a highly skewed distribution of pool forming elements and pool spacing the relation between pool spacing and bankfull widths is attributed to the common geometric response of these features to dominant discharge conditions copyright 2001 john wiley sons ltd

euclidean and fractal models for the description of rock surface roughness accurate description of the topography of rock surfaces is essential because surface roughness affects frictional strength the flow of fluids in joints and fractures the seismic behavior of faults and the formation of gouge and breccia in fault zones real rock surfaces can be described using self similar and self affine fractal models of surface roughness if a surface is self similar a small portion of the surface when magnified isotropically will appear statistically identical to the entire surface if a surface is self affine a magnified portion of the surface will only appear statistically identical to the entire surface if different magnifications are used for the directions parallel and perpendicular to the surface at least two parameters are required to describe a fractal model one parameter typically describes how roughness changes with scale while the other specifies the variance or surface slope at a reference scale the divider method and the spectral method are in common use to determine the best fit fractal model from surface profile data power spectra from self similar surfaces have slopes of 3 on log log plots of power spectral density versus spatial frequency while spectra from self affine surfaces have slopes other than 3 power spectra can be interpreted with greater facility if dimensionless amplitude to wavelength ratios are contoured on plots of power spectral density versus frequency the topography of many natural rock surfaces including both fractures and faults is approximately self similar within the 6 5 order of magnitude wide wavelength band of 10 m to 40 m within smaller wavelength bands natural rock surfaces may exhibit self affine behavior

recirculating flow and sedimentation in the colorado river in grand canyon arizona debris fans debouching into the bottom of grand canyon create rapids and flow separation in the colorado river the patterns of flow and the behavior of recirculation zones formed by flow separation are consistent throughout the canyon s length zones of recirculating flow occur along the margin of channel expansions recirculation zones are comprised of one primary eddy secondary eddies and areas of unorganized low velocity may exist upstream from the primary eddy the longest recirculation zones are formed by channel constrictions of low width to depth ratio recirculation zones increase in length with increasing discharge sand bars form beneath recirculation zones especially near separation and reattachment points reattachment bars project upstream from the reattachment point and underlie primary eddies separation bars mantle the downstream parts of the debris fans and form beneath secondary eddies and low velocity areas sediment that forms reattachment bars is dominated by sizes characteristic o

discrete fracture network interpretation of field tracer migration in sparsely fractured rock field tracer migration in sparsely fractured rock is analyzed with a discrete fracture network model that has been calibrated on fracture and flow data from the stripa 3d experiment in sweden comparison between transport simulations in the flow calibrated discrete model and tracer tests of the field experiment confirms the fracture network parameters obtained from the flow analysis except for the variance of the fracture conductivities which had to be increased the discrete model reproduces the uneven spatial distribution of flow and tracers the complex dispersive behavior and channeling effects that have been observed in the field experiment as a result of flow channeling transport occurs along preferential paths whose transport properties may substantially deviate from the medium average properties for example porosity and wetted fracture surface area available to sorption may be reduced by almost 2 orders of magnitude compared to the medium average values

emergence of anomalous transport in stressed rough fractures abstract we report the emergence of anomalous non fickian transport through a rough walled fracture as a result of increasing normal stress on the fracture we show that the origin of this anomalous transport behavior can be traced to the emergence of a heterogeneous flow field dominated by preferential channels and stagnation zones as a result of the larger number of contacts in a highly stressed fracture we show that the velocity distribution determines the late time scaling of particle spreading and velocity correlation determines the magnitude of spreading and the transition time from the initial ballistic regime to the asymptotic anomalous behavior we also propose a spatial markov model that reproduces the transport behavior at the scale of the entire fracture with only three physical parameters our results point to a heretofore unrecognized link between geomechanics and particle transport in fractured media

single fractures under normal stress the relation between fracture specific stiffness and fluid flow abstract fracture specific stiffness and fluid flow through a single fracture under normal stress are implicitly related through the geometry of the void space and contact area that comprise the fracture data from thirteen different rock samples each containing a single fracture show that relationships between fracture specific stiffness and fluid flow through a fracture fall into two general classes of behavior fractures either fall on a loosely defined universal curve relating fluid flow to fracture specific stiffness or else the flow is weakly dependent on fracture specific stiffness the second relationship shows that flow decreases slowly with increasing fracture specific stiffness the first relationship shows that flow decreases rapidly for increases in fracture specific stiffness to understand this behavior computer simulations on simulated single fractures were performed to calculate fluid flow fracture displacement and fracture specific stiffness as a function of normal stress simulated fractures with spatially correlated and uncorrelated aperture distributions were studied fractures with spatially uncorrelated aperture distributions tend to exhibit a weak dependence of fluid flow on fracture specific stiffness because these fractures tend to have multiple connected paths across the sample which can support flow with uniformly distributed contact area thus an increment in stress will increase the stiffness of the fracture without greatly reducing the amount of fluid flow on the other hand fractures with spatially correlated aperture distributions tend to belong to the universal relationship because correlated fractures tend to have only one or two dominant flow paths and the contact area is limited to a few regions resulting in a compliant fracture thus an increment in stress on a spatially correlated fracture will result in an increase in stiffness and rapid decrease in fluid flow these spatial correlations in fracture void geometry can be differentiated in the laboratory based on the observed fracture specific stiffness fluid flow relationship for a single fracture under normal loading

direct measurement of contact area and stress dependence of anisotropic flow through rock fracture with heterogeneous aperture distribution abstract direct measurement of a contact area using pressure sensitive film was performed for artificially generated granite tensile fractures with different shear displacements of 2 3 and 5 mm and at various normal stresses of up to approximately 100 mpa to clarify the relationship between anisotropic flow and normal stress in a rock fracture with shear displacement aperture distributions with contact areas were then numerically generated using fracture surface geometry data and anisotropic flow was evaluated through a bi directional flow simulation the directional distributions of contacting asperities in the fracture plane were clearly observed for every given condition and the contacting asperities tended to align in the direction perpendicular to shear displacement generally the contact area increased and the anisotropic distribution was enhanced with increasing normal stress however the contact areas for 3 mm shear displacement were less stress dependent anisotropic flow with higher permeability in the perpendicular direction was always observed for aperture distributions with an anisotropic distribution of the contacting asperities flow paths created in the aperture distributions seemed less tortuous in the perpendicular direction due to the distribution of the contacting asperities the anisotropic flow was enhanced with increasing normal stress to the fracture and was more evident for the smaller shear displacements of 2 mm due to the high sensitivity of the contact area to the normal stress the results suggested that the anisotropic flow was likely for a wide range of normal stresses and correspondingly at varying depths in the earth s crust at which anisotropic flow was enhanced with increasing normal stress based on the stress dependence of the contact area in addition the stress dependent anisotropic flow is expected for much higher normal stresses because complete fracture closure with 100 contact area may be unexpected under the crustal stress conditions at depths of several to ten thousand meters

virtual source applications to imaging and reservoir monitoring the virtual source method is a breakthrough that allows us to image and monitor the subsurface in cases where surface seismic or vsp fail to deliver in settings where the overburden is complex or changing traditional time lapse signals are weak or nonrepeatable leading to ineffective seismic input to reservoir modeling and little of value generated by repeated seismic surveys on the other hand the virtual source method allows us to image under complex overburden yields repeatable data for reservoir monitoring enables shear seismic and may help us look ahead as we drill

large scale floodplain modelling a two dimensional horizontal finite element numerical model rma 2 was applied to a 24 km river channel floodplain reach in west germany initial results indicate that finite element schemes may successfully estimate inundation in large scale floodplain applications potentially the resulting detailed velocity vector distributions and identification of inundation zones throughout storm events could provide an insight into the present day sedimentary environment on the floodplain

fluvial response to debris associated with mass wasting during extreme floods evolution of channels and bottomlands in mountain valleys of the central appalachians is strongly influenced by debris supplied to stream channels from mass wasting during extreme storms the type of change observed varies with basin scale and storm characteristics along channels receiving coarse sediment from debris avalanches or debris flows during hurricane camille in 1969 pure scour occurred in drainage areas less than 1 km 2 and gradients steeper than 0 1 in hurricane camille and in the june 1949 storm mixed erosion and deposition with continuous reworking of the valley floor was observed along streams with drainage areas up to 65 km 2 in basins larger than 100 km 2 valley floor reworking associated with influx of debris during both storms was localized and discontinuous in the south branch potomac river basin in west virginia intense precipitation within a small contributing area generated scores of debris slides and avalanches in june 1949 debris transported by tributaries to main valleys exceeded the competence of the larger channels and formed new bottomland long duration moderate intensity precipitation in november 1985 generated fewer debris avalanches flood peaks associated with a larger contributing area along the main valleys were 80 to 190 larger than in 1949 and caused extensive channel and flood plain erosion including truncation and removal of 1949 deposits at some locations relict debris deposits may have influenced hydraulic conditions and affected patterns of erosion and deposition during the 1985 storm sequential occurrence of extreme storms with different hydrologic characteristics creates a bottomland mosaic of surfaces with varying elevations and textures

the challenge of modeling pool riffle morphologies in channels with different densities of large woody debris and boulders studies on pool morphologies include reports of over 80 or 90 of pools being associated with structural controls and large obstructions that include boulders bedrock outcrops and large woody debris lwd a monte carlo simulation approach and developmental computer model was created to predict pool formation spacing and the percentage length covered by pools riffles scour holes and runs based on input data that include channel slope width the number of small and large boulders and the number of 10 30 cm 30 60 cm and 60 cm pieces of wood the statistical empirical model is founded on the idea that boulders bedrock outcrops and large woody debris provide a physical framework that then controls local water surface slopes velocity patterns and the locations of pools and riffles the spacing values of individual types and sizes of obstructions are modeled as log normal distributions with separate distributions for each obstruction type pools are assigned different probabilities of development depending on the obstruction type pool and riffle lengths used to create the subsequent morphology follow their own slope dependent log normal trends a minimum distance develops between successive pools because of the backwater and turbulent conditions needed for pool formation the total number and spacing of pools riffles and scour holes thus reflects the number and locations of obstructions and characteristics of the pool riffle couplet the simulation model accurately captures the number of pools in the modeled data range at 65 of all the verification field sites and 86 of the verification field sites with a more limited range of width and slope characteristics lower levels of prediction capabilities are associated with modeled numbers of scour holes and log jams the model accurately mimics some statistical attributes of pool spacing and future versions of the model could be developed to improve overall predictive capabilities copyright 2011 john wiley sons ltd

gaussian beam migration of common shot records gaussian beam migration is a depth migration method whose accuracy rivals that of migration by wavefield extrapolation so called wave equation migration and whose efficiency rivals that of kirchhoff migration this migration method can image complicated geologic structures including very steep dips in areas where the seismic velocity varies rapidly however applications of prestack gaussian beam migration either have been limited to common offset common azimuth data volumes and thus are inflexible or suffer from multiarrival inaccuracies in a common shot implementation in order to optimize both the flexibility and accuracy of gaussian beam migration i present a common shot implementation that handles multipathing in a natural way this allows the migration of data sets that can include a variety of azimuths and it allows a simplified treatment of near surface issues application of this method to model data typical of canadian foothills structures and to model data that includes a complicate

imaging of near borehole structure using full waveform sonic data the full waveforms recorded by an array of receivers in a modern borehole sonic tool contain secondary arrivals that are reflected from near borehole structural features these arrivals are used to form an image of the near borehole structural features in a manner similar to seismic migration possible uses of this technique include horizontal well logging structural dip and contour determination fault salt dome pinnacle reef and fracture zone imaging and eor steam flood monitoring since both the source and the receivers pass through structures that cross the borehole the downdip structure and the updip structure can be imaged separately the technique involves a backprojection of the recorded data into a matrix of accumulation bins representing distances radially out from the borehole and along the borehole axis separate matrices are formed for the updip and for the downdip raypaths the basic technique is illustrated with synthetic data generated to approximate the case of a sonic tool logging

simple mathematical model of a rough fracture a simple mathematical model of rough walled fractures in rock is described which requires the specification of only three main parameters the fractal dimension the rms roughness at a reference length scale and a length scale describing the degree of mismatch between the two fracture surfaces fractured samples collected from natural joints and laboratory specimens have been profiled to determine the range of these three parameters in nature it is shown how this surface roughness model can be implemented on a computer allowing future detailed study of the mechanical and transport properties of single fractures and the scale dependence of these properties

pool fills a window to palaeoflood history and response in bedrock confined rivers channel scale sedimentary units associated with bedrock controlled riffle pool morphology are examined in detail along sandy creek gorge an ephemeral stream in arid south eastern central australia pool fills comprise cut and fill assemblages of poorly sorted sediments ranging in texture from muds to boulders five unit types are defined based on particle size sedimentary structures geometry and bounding surface character 1 coarse grained bar platform 2 fine grained bar supraplatform 3 fine grained pool fill 4 fine grained bench and 5 modern pool fill the last coarse grained unit currently lining the pools suggests an altered sedimentation style over the post settlement period post ad 1860s situated at bedrock valley constrictions pool fills are compared with other sedimentary units associated with recirculating currents eddy bars and slackwater deposits but only the fine grained bench units reflect eddy recirculation the pool fills are principally forced bars associated with bedrock controlled or forced riffle pool morphology a late holocene palaeoflood history is proposed based on radiocarbon ages from the pool fills multiple phases of cut and fill activity were preceded by a superflood 3400 1900 years ago that eroded the pool fills to bedrock the resilience of the pool fills was illustrated by the passage of a 1 in 100 year flood in 1992 which caused only minor erosion the presence of pool fills may provide a window to past phases of river activity that cannot be extracted from either historical records observations or palaeoflood slackwater sediment analyses the formation and sedimentary preservation potential of these landforms reflect a combination of hydraulic and structural influences but the occurrence of high magnitude floods exerts the dominant control

synthetic rough fractures in rocks natural fractures serve a very important function in the transport of fluid through rocks as well as in the flow of electrical charge and heat the creation of numerically synthesized fractures can aid the study of the physical processes connected with fractures synthetic fracture is the term used to describe fractures that are created numerically in such a way that they share the same mean geometrical characteristics as specific natural fractures measured by profiling by a process known as tuning we have modified methods for producing synthetic rough surfaces whose geometric properties are tuned to mimic natural fracture surfaces in rocks in order to create synthetic fractures that are statistically identical to those found in rocks one important such modification has been the incorporation of a method that allows the surfaces to be matched at long wavelengths and unmatched at short wavelengths with the degree of matching varying smoothly in between as it does for real fractures we have compared numerically synthetic fractures created using the new method with fractures created using an existing technique that uses a mismatch wavelength as a sudden discontinuity between matched and unmatched behavior as well as data from fractures in real rocks this comparison has shown that the new technique provides much more realistic numerically synthesized fractures than previous methods synthetic fractures created with the new method have been used in normal closure and fluid flow modeling and the results are reported in a companion paper

redatuming through a salt canopy and target oriented salt flank imaging we describe a new shortcut strategy for imaging the sediments and salt edge around a salt flank through an overburden salt canopy we tested its performance and capabilities on 2d synthetic acoustic seismic data from a gulf of mexico style model we first redatumed surface shots using seismic interferometry from a walkaway vertical seismic profile survey as if the source and receiver pairs had been located in the borehole at the positions of the receivers this process creates effective downhole shot gathers by completely moving surface shots through the salt canopy without any knowledge of overburden velocity structure after redatuming we can apply multiple passes of prestack migration from the reference datum of the bore hole in our example first pass migration using only a simple vertical velocity gradient model reveals the outline of the salt edge a second pass of reverse time prestack depth migration using full two way wave equation was performed with an updated velocity model that consisted of the velocity gradient and salt dome the second pass migration brings out dipping sediments abutting the salt flank because these reflectors were illuminated by energy that bounced off the salt flank forming prismatic reflections in this target oriented strategy the computationally fast redatuming process eliminates the need for the traditional complex process of velocity estimation model building and iterative depth migration to remove effects of the salt canopy and surrounding overburden this might allow this strategy to be used in the field in near real time