seismic tomography a window into deep earth abstract the goal of this paper is to provide an overview of the current state of the art in seismic tomography and trace its origins from pioneering work in the early 1970s to its present status as the pre eminent tool for imaging the earth s interior at a variety of scales due to length limitations we cannot hope to cover every aspect of this diverse topic or include mathematical derivations of the underlying principles rather we will provide a largely descriptive coverage of the methodology that is targeted at readers not intimately familiar with the topic the relative merits of local versus global parameterization ray tracing versus wavefront tracking backprojection versus gradient based inversion and synthetic testing versus model covariance are explored a variety of key application areas are also discussed including body wave traveltime tomography surface wave tomography attenuation tomography and ambient noise tomography established and emerging trends many of which are driven by the ongoing rapid increases in available computing power will also be examined including finite frequency tomography full waveform tomography and joint tomography using multiple datasets several practical applications of seismic tomography including body wave traveltime attenuation and surface waveform are presented in order to reinforce prior discussion of theory

a quantitative method for the characterisation of karst aquifers based on spring hydrograph analysis this paper presents a method for characterizing flow systems in karst aquifers by acquiring quantitative information about the geometric and hydraulic aquifer parameters from spring hydrograph analysis numerical sensitivity analyses identified two fundamentally different flow domains depending on the overall configuration of aquifer parameters these two domains have been quantitatively characterized by deducing analytical solutions for the global hydraulic response of simple two dimensional model geometries during the baseflow recession of mature karst systems the hydraulic parameters of karst conduits do not influence the drainage of the low permeability matrix in this case the drainage process is influenced by the size and hydraulic parameters of the low permeability blocks alone this flow condition has been defined as matrix restrained flow regime mrfr during the baseflow recession of early karst systems and fissured systems as well as the flood recession of mature systems the recession process depends on the hydraulic parameters and the size of the low permeability blocks conduit conductivity and the total extent of the aquifer this flow condition has been defined as conduit influenced flow regime cifr analytical formulae demonstrated the limitations of equivalent models while equivalent discrete continuum models of early karst systems may reflect their real hydraulic response there is only one adequate parameter configuration for mature systems that yields appropriate recession coefficient consequently equivalent discrete continuum models are inadequate for simulating global response of mature karst systems the recession coefficient of equivalent porous medium models corresponds to the transition between matrix restrained and conduit influenced flow consequently equivalent porous medium models yield corrupted hydrographs both in mature and early systems and this approach is basically inadequate for modelling global response of karst aquifers

tomographic modeling of a cross borehole data set in this paper we describe a methodology for estimating p wave velocities from a cross borehole seismic survey that uses straight ray tomography ray tracing and finite difference wave equation modeling to produce velocity models that fit the first break traveltimes after a starting model is established by straight ray tomography the velocity model is checked by ray tracing and wave equation modeling since the models for each procedure show consistent results and the modeled traveltimes closely match those traveltimes from the actual data we felt our interpretation was confirmed however the fitting of cross well first break traveltimes is only a necessary validity check and is not sufficient to guarantee that the true solution has been found two wells were drilled through the areas that were anomalous on the derived tomogram and check shot velocity surveys were run due primarily to a lateral ambiguity in velocity estimation caused by too few near vertical raypaths the check shot surveys did not agree with the tomogram velocities however subsequently the check shot traveltimes were used to place bounds on velocity in a constrained least squares procedure the combined modeling of uphole and cross well rays produced an optimum velocity model which satisfies all available data

surface climate variations over the north atlantic ocean during winter 1900 1989 abstract the low frequency variability of the surface climate over the north atlantic during winter is described using 90 years of weather observations from the comprehensive ocean atmosphere data set results are based on empirical orthogonal function analysis of four components of the climate system sea surface temperature sst air temperature wind and sea level pressure an important mode of variability of the wintertime surface climate over the north atlantic during this century is characterized by a dipole pattern in ssts and surface air temperatures with anomalies of one sign cast of newfoundland and anomalies of the opposite polarity off the southeast coast of the united states wind fluctuations occur locally over the regions of large surface temperature anomalies with stronger than normal winds overlying cooler than normal ssts this mode exhibits variability on quasi decadal and biennial time scales the decadal fluctuations are irregular in length averaging 9 years before 1945 and 1

a quantitative approach to spring hydrograph decomposition summary a combined analytical numerical study for the characterization of spring hydrographs is presented two dimensional analytical solutions for diffusive flux from rectangular blocks of arbitrary shape facilitate a quantitative characterization of exponential hydrograph components together with analytical solutions for block discharge a systematic analysis of numerically simulated spring hydrographs of synthetic karst systems provides an insight into karst hydrodynamics different hydrograph components do not represent different classes of rock permeability hydrographs of individual homogeneous blocks can be decomposed into several exponential components discharge hydrographs of symmetric rectangular blocks can be reconstructed by the sum of only three exponential components increasing block asymmetry results in an increasing number of exponential components contributing significantly to total discharge spring hydrographs represent a sum of individual block discharges originating from diffuse infiltration and conduit discharge originating from concentrated recharge beyond the inflection of the recession limb a spring hydrograph can be decomposed in a similar manner to that of individual homogeneous blocks the presented hydrograph analytical method facilitates the estimation of hydraulic and geometric parameters of karst hydrogeological systems

a new exploration tool quantitative core characterization we will describe a new laboratory system which was designed to be highly automated and portable while maintaining quality driving this design was the recognition of the temporal dependence of physical properties it becomes apparent that some sedimentary rocks particularly shales degrade and disaggregate so completely that mechanical or elastic properties cannot be measured this temporal dependence displays a time scale much shorter than normal weathering but greater than the time for stress relief a system was designed to permit field characterization of freshly recovered core material a benefit of automation and portability is a marked increase in measurement efficiency the attributes of this system permit rapid characterization of a large number of fresh cores in remote frontier exploration areas this feature can significantly reduce prospect evaluation time statistically significant rock property databases can be created in a short period of time

borehole seismic surveys for fault delineation in the dutch north sea a significant share of gas production in the dutch sector of the southern north sea basin comes from permian rotliegend fault blocks precise knowledge of the positions of these faults is necessary for efficient exploitation of the reservoir structures and for future field development strategies two areas are presented where the lateral resolution of the surface seismic data was not sufficient to determine positions of major fault block boundaries accurately walkaway borehole seismic profiles were shot over each of these areas with the objective of illuminating the fault boundaries to obtain an image with a higher resolution the images were generated using borehole seismic reflection point mapping and migration techniques large aperture migrations tend to produce unacceptable migration smiles while reflection point mapping is a model dependent process a hybrid approach to these processes was necessary to avoid problems associated with the limited angular illumination permitted by the field acquisiti

evaluation of aquifer thickness by analysing recession hydrographs application to the oman ophiolite hard rock aquifer for more than a century hydrologists and hydrogeologists have been investigating the processes of stream and spring baseflow recession for obtaining data on aquifer characteristics the maillet formula librairie sci a hermann paris 1905 218 an exponential equation widely used for recession curve analysis is an approximate analytical solution for the diffusion equation in porous media whereas the equation proposed by boussinesq c r acad sci 137 1903 5 j math pure appl 10 1904 5 that depicts baseflow recession as a quadratic form is an exact analytical solution other formulas currently used involve mathematical functions with no basis on groundwater theory only the exact analytical solutions can provide quantitative data on aquifer characteristics the efficiency of the two methods was compared on the basis of recession curves obtained with a 2d cross sectional finite differences model that simulates natural aquifers simulations of shallow aquifers with an impermeable floor at the level of the outlet show that their recession curves have a quadratic form thus the approximate maillet solution largely overestimates the duration of the influenced stage and underestimates the dynamic volume of the aquifer moreover only the boussinesq equations enable correct estimates of the aquifer parameters numerical simulations of more realistic aquifers with an impermeable floor much deeper than the outlet proves the robustness of the boussinesq formula even under conditions far from the simplifying assumptions that were used to integrate the diffusion equation the quadratic form of recession is valid regardless of the thickness of the aquifer under the outlet and provides good estimates of the aquifer s hydrodynamic parameters nevertheless the same numerical simulations show that aquifers with a very deep floor provide an exponential recession thus in that configuration the maillet formula also provides a good fit of recession curves even if parameter estimation remains poor in fact the recession curve appears to be closer to exponential when flow has a very important vertical component and closer to quadratic when horizontal flow is dominant as a consequence aquifer permeability anisotropy also changes the recession form the combined use of the two fitting methods allows one to quantify the thickness of the aquifer under the outlet

internal variability and external forcings in the ocean atmosphere multidecadal oscillator over the north atlantic in this paper we generalize the concept of external forcing to include any mechanism that modulates the long term evolution of a meteorological variable but is not directly related to the internal variability of the climate system applying this concept the corresponding external forcings are removed from several long record datasets of oceanic and atmospheric variables at the surface in the north atlantic we perform a multivariate analysis in the frequency domain over both the original data fields and the new internal variability fields this multivariate analysis is based on a multitaper method singular value decomposition mtm svd it is noteworthy that after the removal of the external forcings there is an almost perfect alignment of the main multidecadal oscillatory band f 0 21 cycles decade with all the spectra of the analysed fields this alignment was not observed before the external forcings were removed particularly striking is the case of the sea level pressure slp which shows a notable variation in its oscillation period despite the fact that this variable has traditionally been considered to be unaffected at global scale by any external forcing the external forcing in the slp records is very probably caused by the scarcity of the observed data during the first hundred years of the record most evident during the earliest decades by the spatial distribution of those observations and possibly by the assimilation model employed to build those long record datasets when we analysed the relationship between the ocean and the atmosphere using this approach we found strong evidence of a cyclic behaviour in which oceanic conditions modulate the atmospheric variability with a lead time of up to about 10 years

investigation of young water inflow in karst aquifers using sf6 cfc 3h he 85kr 39ar and stable isotope components abstract karst aquifers are known for their wide distribution of water transfer velocities from this observation a multiple geochemical tracer approach seems to be particularly well suited to provide a significant assessment of groundwater flows but the choice of adapted tracers is essential in this study several common tracers in karst aquifers such as physicochemical parameters major ions stable isotopes and 13c to more specific tracers such as dating tracers 14c 3h 3h 3he cfc 12 sf6 and 85kr and 39ar were used in a fractured karstic carbonated aquifer located in burgundy france the information carried by each tracer and the best sampling strategy are compared on the basis of geochemical monitoring done during several recharge events and over longer time periods months to years this study s results demonstrate that at the seasonal and recharge event time scale the variability of concentrations is low for most tracers due to the broad spectrum of groundwater mixings the tracers used traditionally for the study of karst aquifers i e physicochemical parameters and major ions efficiently describe hydrological processes such as the direct and differed recharge but require being monitored at short time steps during recharge events to be maximized from stable isotopes tritium and cl contents the proportion of the fast direct recharge by the largest porosity was estimated using a binary mixing model the use of tracers such as cfc 12 sf6 and 85kr in karst aquifers provides additional information notably an estimation of apparent age but they require good preliminary knowledge of the karst system to interpret the results suitably the cfc 12 and sf6 methods efficiently determine the apparent age of baseflow but it is preferable to sample the groundwater during the recharge event furthermore these methods are based on different assumptions such as regional enrichment in atmospheric sf6 excess air and flow models among others 85kr and 39ar concentrations can potentially provide a more direct estimation of groundwater residence time conversely the 3h 3he method is inefficient in the karst aquifer for dating due to 3he degassing

nao implicated as a predictor of northern hemisphere mean temperature multidecadal variability 1 the twentieth century northern hemisphere mean surface temperature nht is characterized by a multidecadal warming cooling warming pattern followed by a flat trend since about 2000 recent warming hiatus here we demonstrate that the north atlantic oscillation nao is implicated as a useful predictor of nht multidecadal variability observational analysis shows that the nao leads both the detrended nht and oceanic atlantic multidecadal oscillation amo by 15 20 years theoretical analysis illuminates that the nao precedes nht multidecadal variability through its delayed effect on the amo due to the large thermal inertia associated with slow oceanic processes an nao based linear model is therefore established to predict the nht which gives an excellent hindcast for nht in 1971 2011 with the recent flat trend well predicted nht in 2012 2027 is predicted to fall slightly over the next decades due to the recent nao decadal weakening that temporarily offsets the anthropogenically induced warming

vertical seismic profiling separation of upgoing and downgoing acoustic waves in a stratified medium one of the essential steps in the processing of a vertical seismic profile is the separation of upgoing and downgoing signals with this perspective in mind seismic data recorded in a borehole are modeled in terms of these waves and a mathematically optimal least squares technique for extracting them is derived the method imposes practically no constraints on the spacing between recording levels and allows almost perfect rejection of a coherent downgoing signal the exact formulation of the one dimensional model requires that acoustic impedance information be included but a reasonable and realistic approximation can neglect impedance we derive frequency wavenumber response plots for the two limiting cases of even and randomly spaced levels and compare these to the response of a conventional velocity filtering technique by a careful study of available logs recording levels can be chosen to optimize geophone coupling rather than insisting on uniform spatial sampling data editing normalization a

crosshole seismic tomography many tomographic interpretations of crosshole seismic traveltimes have approximated the raypaths with straight lines connecting the source and receiver this approximation is valid where the velocity does not vary greatly but in many regions of interest velocity variations of 10 20 percent or more are observed causing significant ray curvature other work has taken this nonlinear effect into account but there do not appear to be many cases of demonstrated success in its application to the crosshole seismic problem we present here an iterative inversion scheme based on two dimensional ray tracing and its successful application to field data the interpretation method iteratively ray traces and then updates the velocity model within each iteration the differences between the data and the current model traveltimes obtained by ray tracing are related to the unknown velocity perturbations through a system of linear equations a damped least squares method solves for the velocity perturbations which updat

modelling the hydrologic functions of a karst aquifer under active water management the lez spring a rainfall discharge model is applied to an aquifer characterized by active groundwater management the lez karst aquifer in the south of france groundwater is withdrawn by pumping in the main drain 400 m upstream from the main spring and 48 m below the aquifer overflow level a reservoir model was developed to characterize the functioning of this aquifer it is based on a conceptual model of the karst aquifer that takes into account flow through both the saturated zone and the unsaturated infiltration zone both rapid and slow flow in the infiltration zone and flow in the saturated zone are modelled since pumping withdraws groundwater from the main drain of the saturated zone modules characterizing various pumping practices are taken into account in the saturated zone reservoir the model simultaneously simulates spring discharge and water levels in the drain which characterize the saturated zone of the karst the groundwater reserves stored in the saturated zone can then be estimated the model can be used for water management since runoff and overflow from the aquifer feeding the lez river depend on the degree of saturation of the aquifer the model could be used for the flood management

investigating transport properties and turbidity dynamics of a karst aquifer using correlation spectral and wavelet analyses in many places throughout the world drinking water is frequently contaminated by turbidity such turbidity however as representative of particle transport can be used as to trace certain features of particle transport properties in order to investigate the relation between particle and dissolved species transport and hydrodynamics in karst systems correlation and spectral analyses were performed on time series of rainfall input signal and water level specific conductance and turbidity output signals at a karst spring system in the chalk aquifer of the lower seine valley france this system is composed of a spring connected to a sinkhole on the chalk plateau where a small creek enters the subsurface the autocorrelation functions for water level and turbidity showed a short memory effect demonstrating the short duration of the influence of flood events on these two parameters whereas specific conductance representing less mineralized storm derived water had a much longer memory effect these results were interpreted as reflecting the rapid reactivity of the spring to rain events with storage of water in the fissured chalk explaining the longer memory effect for specific conductance than for particles energy spectra computed by fast fourier transform of autocorrelation functions showed a strong structure in the output signals whereas the input signal rainfall was random thus allowing assessment and comparison of system behaviour regarding dissolved and solid transport as well as hydraulics cross correlation functions which allow an assessment of impulse response functions confirmed the low inertia of the system for water level and turbidity and the much higher inertia for specific conductance in addition to the main peak two secondary peaks in the cross correlation functions suggest the existence of additional flowpaths that might involve the contribution of other point source recharge and or delayed infiltration through the clayey surficial formation and solution pipes which could act as epikarst analysis using continuous wavelet spectra allowed a better investigation of the rainfall signal structure and highlighted a strong relationship between turbidity and rainfall which could not be deduced from the energy spectra specific spectral components in water level were isolated and a time domain reconstruction using inverse wavelet transform allowed separation of quick flow and slow flow components

oceanic control of multidecadal variability in an idealized coupled gcm idealized ocean models are known to develop intrinsic multidecadal oscillations of the meridional overturning circulation moc here we explore the role of ocean atmosphere interactions on this low frequency variability we use a coupled ocean atmosphere model set up in a flat bottom aquaplanet geometry with two meridional boundaries the model is run at three different horizontal resolutions 4 2 and 1 in both the ocean and atmosphere at all resolutions the moc exhibits spontaneous variability on multidecadal timescales in the range 30 40 years associated with the propagation of large scale baroclinic rossby waves across the atlantic like basin the unstable region of growth of these waves through the long wave limit of baroclinic instability shifts from the eastern boundary at coarse resolution to the western boundary at higher resolution increasing the horizontal resolution enhances both intrinsic atmospheric variability and ocean atmosphere interactions in particular the simulated atmospheric annular mode becomes significantly correlated to the moc variability at 1 resolution an ocean only simulation conducted for this specific case underscores the disruptive but not essential influence of air sea interactions on the low frequency variability this study demonstrates that an atmospheric annular mode leading moc changes by about 2 years as found at 1 resolution does not imply that the low frequency variability originates from air sea interactions

interdecadal changes of 30 yr sst normals during 1871 2000 abstract sst predictions are usually issued in terms of anomalies and standardized anomalies relative to a 30 yr normal climatological mean cm and standard deviation sd the world meteorological organization wmo suggests updating the 30 yr normal every 10 yr in complying with the wmo s suggestion a new 30 yr normal for the 1971 2000 base period is constructed to put the new 30 yr normal in historical perspective all the 30 yr normals since 1871 are investigated starting from the beginning of each decade 1871 1900 1881 1910 1971 2000 using the extended reconstructed sea surface temperature ersst on a 2 grid for 1854 2000 and the hadley centre sea ice and sst dataset hadisst on a 1 grid for 1870 1999 eleven 30 yr normals are calculated and the interdecadal changes of seasonal cm seasonal sd and seasonal persistence p are discussed the interdecadal changes of seasonal cm are prominent 0 3 0 6 in the tropical indian ocean the midlatitude north pacific the midlatitude no

study of traveltime and amplitude time lapse tomography using physical model data in seismic tomography the observed traveltimes or amplitudes of direct waves are inverted to obtain an estimate of seismic velocity or absorption of the section surveyed there has been much recent interest in using cross well traveltime tomography to observe the progress of fluids injected into the reservoir rocks during enhanced oil recovery eor processes if repeated surveys are carried out then eor processes may monitored over a period of time this paper describe the results of simulated time lapse tomography experiment to image the flood zone in an eor process two physical models were made out of epoxy resins to simulate an essentially plane layered sedimentary sequence containing a reservoir layer and simple geological structure the models differed only in the reservoir layer which was uniform in the pre flood model and contained a flood zone of known geometry in the post flood model data sets were acquired from each model using a cross well survey geometry traveltime and amplitude tomographic imaging techniques have been applied to these data in an attempt to locate the extent of the flood zone traveltime tomography locates the flood zone quite accurately amplitude tomography shows the flood zone as a region of higher absorption but does not image itsmore boundaries as precisely this is primarily because of multipathing and diffraction effects which are not accounted for by the ray based techniques for inverting seismic amplitudes less

imaging low velocity anomalies with the aid of seismic tomography abstract theoretical considerations snell s law suggest that low velocity fanomalies are undersampled and therefore should be poorly resolved by inversion schemes based on ray tracing methods a synthetic study considering a 40 20 m low velocity anomaly 300 m s placed at the center of a 400 50 m block with gradient background velocity model from 3000 m s at the surface to 4000 m s at the base indicates that the low ray density in ray tracing coverage diagrams of tomographic inversion studies can be used as evidence for the existence of low velocity anomalies combined normal incidence seismic reflection images and the velocity models obtained by tomographic inversions of first arrival travel times form an efficient scheme to resolve low velocity anomalies such as fracture zones furthermore the velocity models derived from tomographic inversions are used in a wave equation datuming algorithm to account for statics caused by a strongly laterally variable shallow surface weathering layer and provide seismic reflection images of fracture zones low velocity anomaly within a granitic pluton

hydrograph separation using isotopic chemical and hydrological approaches strengbach catchment france the streamflow components were determined in a small catchment located in eastern france for a 40 mm rain event using isotopic and chemical tracing with particular focus on the spatial and temporal variations of catchment sources precipitation soil solution springwater and streamwaters were sampled and analysed for stable water isotopes 18o and 2h major chemical parameters so42 no3 cl na k ca2 mg2 nh4 h h4sio4 alkalinity and conductivity dissolved organic carbon doc and trace elements al rb sr ba pb and u 18o si doc ba and u were finally selected to assess the different contributing sources using mass balance equations and end member mixing diagrams isotopic hydrograph separation shows that the pre event water only contributes to 2 at the beginning of the stormflow to 13 at the main peak flow doc associated to si and u to ba allow to identify the different contributing areas upper layers of the saturated areas deep layers of the hillslope and rainwater the streamflow 70 originates from the deep layers of the hillslope the remaining being supplied by the small saturated areas the combination of chemical both trace and major elements and isotopic tracers allows to identify the origin of water pathways during the first stage of the storm event a significant part of the runoff 30 39 comes from the small extended saturated areas located down part of the basin overland runoff then groundwater ridging during the second stage the contribution of waters from the deep layers of the hillslope in the upper subcatchment becomes more significant the final state is characterised by a balanced contribution between aquifers located in moraine and downslopes indeed this study demonstrates the interest of combining a variety of hydrometric data geochemical and isotopic tracers to identify the components of the streamwater in such conditions

analytical 1d dual porosity equivalent solutions to 3d discrete single continuum models application to karstic spring hydrograph modelling abstract one dimensional analytical porosity weighted solutions of the dual porosity model are derived providing insights on how to relate exchange and storage coefficients to the volumetric density of the high permeability medium it is shown that porosity weighted storage and exchange coefficients are needed when handling highly heterogeneous systems such as karstic aquifers using equivalent dual porosity models the sensitivity of these coefficients is illustrated by means of numerical experiments with theoretical karst systems the presented 1d dual porosity analytical model is used to reproduce the hydraulic responses of reference 3d karst aquifers modelled by a discrete single continuum approach under various stress conditions simulation results show the relations between the dual porosity model coefficients and the structural features of the discrete single continuum model the calibration of the equivalent 1d analytical dual porosity model on reference hydraulic responses confirms the dependence of the exchange coefficient with the karstic network density the use of the analytical model could also point out some fundamental structural properties of the karstic network that rule the shape of the hydraulic responses such as density and connectivity

high resolution crosswell imaging of a west texas carbonate reservoir using crosswell data collected at a depth of about 3000 ft 900 m in west texas carbonates one of the first well to well reflection images of an oil reservoir was produced the p and s brute stack reflection images created after wavefield separation tied the sonic logs and exhibited a vertical resolution that was comparable to well log resolution both brute stacks demonstrated continuity of several reflectors known to be continuous from log control and also imaged an angular unconformity that was not detected in log correlations or in surface seismic profiling the brute stacks particularly the s wave reflection image also exhibited imaging artifacts we found that multichannel wavefield separation filters that attenuated interfering wavemodes were a critical component in producing high resolution reflection images in this study the most important elements for an effective wavefield separation were the time alignment of seismic arrivals prior to filter application and the implementation of wavefield separation filters in multiple domains particularly in common offset domain the effectiveness of the multichannel filtering was enhanced through the use of extremely fine wellbore sampling intervals in this study 2 5 ft 0 76 m vertical sampling intervals for both source and receiver were used whereas most previous crosswell data sets were collected with much coarser sampling intervals resulting in spatial aliasing and limiting the utility of the data for reflection processing the wavefield separation techniques employed in this study used data volumes and associated filtering operations that were several orders of magnitude larger than those encountered in conventional vsp data analysis

stochastically forced multidecadal variability in the north atlantic a model study observations show a multidecadal signal in the north atlantic ocean but the underlying mechanism and cause of its timescale remain unknown previous studies have suggested that it may be driven by the north atlantic oscillation nao which is the dominant pattern of winter atmospheric variability to further address this issue the global ocean general circulation model nucleus for european modelling of the ocean nemo is driven using a 2 000 years long white noise forcing associated with the nao focusing on key ocean circulation patterns we show that the atlantic meridional overturning circulation amoc and sub polar gyre spg strength both have enhanced power at low frequencies but no dominant timescale and thus provide no evidence for a oscillatory ocean only mode of variability instead both indices respond linearly to the nao forcing but with different response times the variability of the amoc at 30 n is strongly enhanced on timescales longer than 90 years while that of the spg strength starts increasing at 15 years the different response characteristics are confirmed by constructing simple statistical models that show amoc and spg variability can be related to the nao variability of the previous 53 and 10 winters respectively alternatively the amoc and the spg strength can be reconstructed with auto regressive ar models of order seven and five respectively both statistical models reconstruct interannual and multidecadal amoc variability well while on the other hand the ar 5 reconstruction of the spg strength only captures multidecadal variability using these methods to reconstruct ocean variables can be useful for prediction and model intercomparision

modeling spatially and temporally varied hydraulic behavior of a folded karst system with dominant conduit drainage at catchment scale hochifen gottesacker alps summary karst aquifers are important for freshwater supply but difficult to manage due to highly variable water levels and spring discharge rates conduits are crucial for groundwater flow in karst aquifers but their location is often unknown thus limiting the applicability and validity of numerical models we have applied a conduit model swmm to simulate highly variable flow in a folded alpine karst aquifer system where the underground drainage pattern is comparatively well known from previous tracer studies the conduit model was coupled with a reservoir model representing recharge storage and transfer of water in the epikarst and unsaturated zone the global optimization approach ga was applied to achieve an efficient model calibration it was possible to simultaneously simulate the highly variable discharge characteristics of an estavelle and overflow spring and a permanent spring draining the conduit system the model allowed for the collection of spatially differentiated information on recharge rapid flow and slow flow in four individual sub catchments the formation of backwater upgradient from conduit restrictions turned out to be a key process in activating overflow springs the proposed modeling approach appears to be transferrable to other karst systems with predominant conduit drainage but requires previous knowledge of the configuration of the conduit system

interdecadal variations in north atlantic sea surface temperature and associated atmospheric conditions abstract evidence is presented for a distinct pattern of ocean atmosphere relationship associated with interdecadal variability in the north atlantic region using a century of surface marine observations it is shown that middle and high latitude sea surface temperature sst display a long term fluctuation with negative anomalies before 1920 and during the 1970s and 1980s positive sst conditions prevailed from about 1930 to 1960 the pattern of interdecadal sst variability is constructed by subtracting the average field during 15 cold years from that during a similar interval of warm years the early century warming and the more recent cooling display a similar spatial pattern in both cases the pattern is basin scale and largely of one polarity with maxima in the vicinity of iceland in the labrador sea and northeast of bermuda the corresponding differences in surface atmospheric conditions are determined by averaging and subtracting in the same manner fields of sea level pressure slp and surf

reverse time migration of offset vertical seismic profiling data using the excitation time imaging condition to apply reverse time migration to prestack finite offset data from variable velocity media the standard time zero imaging condition must be generalized because each point in the image space has a different image time or times this generalization is the excitation time imaging condition in which each point is imaged at the one way traveltime from the source to that point reverse time migration with the excitation time imaging condition consists of three elements 1 computation of the imaging condition 2 extrapolation of the recorder wave field and 3 application of the imaging condition computation of the imaging condition for each point in the image is done by ray tracing from the source point this is equivalent to extrapolation of the source wave field through the medium extrapolation of the recorded wave field is done by an acoustic finite difference algorithm imaging is performed at each step of the finite difference extrapolation by extracting from the propagating wave field the

multiborehole seismic imaging in steam injection heavy oil recovery projects crosshole seismic measurements before and after steam injection were obtained for the purpose of delineating the fluid invaded zone the experiment despite its limited temporal and spatial sampling showed significant changes in seismic signature after steam injection a single chemical explosive seismic source was fired repeatedly at a fixed depth within the source well for various positions of a receiver clamped within the receiver well the seismic records of the before and after steam flood experiments were normalized by using the tube waves important results were obtained from power spectral studies which show significant changes after fluid injection for higher resolution and increased constraints on the determination of the steam invaded zone a more sophisticated cross hole experiment has been designed computer simulation of the proposed experiment has been carried out assuming an asymmetrical shape for the steam zone inversion via a tomographic approach by solution of an overdetermine

a decade of shear wave splitting in the earth s crust what does it mean what use can we make of it and what should we do next it is 10 years since shear wave splitting throught to be diagnostic of some form of seismic anisotropy was first positively identified in the earth s crust from the beginning it was argued that the splitting was probably associated with the presence of stress aligned cracks inclusions in the crust and that this would provide the opportunity for monitoring the in situ geometry of cracks and stress in a variety of different circumstances and in a variety of different applications

seismic imaging of the naga thrust using multiscale waveform inversion this case study images the structural features related to the naga thrust fault northeast india using a combination of multiscale waveform inversion and prestack depth migration psdm the waveform model and the psdm image complement each other the former provides a physical property map p wave velocity model and the latter provides a structural image the velocity model encompassing the starting model for waveform inversion is constructed using joint inversion of first and reflected traveltimes the 4 8 hz data are inverted consecutively in 1 hz bandwidths to yield the final waveform model which in turn is used for psdm the psdm image and the waveform model are consistent with the lithological interpretation of an inline exploratory well when interpreted jointly the psdm image and the waveform model reveal the presence of a conjugate fault system in the naga thrust and fold belt

numerical simulation of first order backscattered p and s waves for time lapse seismic imaging in heterogeneous reservoirs summary first order elastic wave scattering theory validated in general features by deep borehole seismic coda wave observations is used to numerically simulate time lapse borehole seismic data aimed at monitoring oil water substitution in heterogeneous hydrocarbon reservoirs a first order perturbation solution of the vector wave equation leads to p p and s s backscattered displacement vector motion expressed as the angular summation of the second radial derivative of p and s wave velocity and density fluctuations over expanding spherical wave fronts of radii t 2 and t 2 p and s wave velocity and density spatial heterogeneity is modelled as long range correlated random fluctuations consistent with the power law scaling character of crustal rock physical properties measured by borehole logs the spectra of model scattering displacement seismograms for a power law scaling volumetric noise distribution duplicate the frequency enrichment observed in the spectra of broadband earthquake coda waves recorded in a deep well modelling of time lapse scattering in power law scaling permeability structures suggests that a stable borehole seismic source can locate oil water substitution in reservoir volumes tens of metres on a side at distances of up to a few hundred metres from an observation well using currently available borehole seismic technology time lapse tracking of oil water substitution and the monitoring of reservoir stress conditions can lead to spatially well constrained reservoir models despite large scale large amplitude correlated random heterogeneity

comment on quantitative measurements of shear wave polarizations at the anza seismic network southern california implications for shear wave splitting and earthquake prediction by richard c aster peter m shearer and jon berger

crosshole seismology and seismic imaging in crystalline rocks a borehole to borehole audiofrequency seismic system using piezoelectric transducers as active elements has been developed and used to investigate crystalline rocks in canada high quality seismograms have been recorded for transmitter receiver distances of up to 230 meters it has been found that both travel times and signal amplitudes provide diagnostic information about rock quality two dimensional images of seismic velocity and transparency distributions between two boreholes derived from observed arrival times and amplitudes by simple back projection tomography reflect lithology and geological structure the results indicate the potential usefulness of crosshole seismic imaging as a tool for rock mass characterization

high resolution crosswell imaging of a west texas carbonate reservoir part 4 reflection imaging reliable crosswell reflection imaging is a challenging task even after the data have been wavefield separated in the time domain residual strong coherent noise is still present in the data stacking is complicated by the wide range of reflection incidence angles available for imaging with wavelengths of a few feet small misalignments as a result of velocity or geometric errors produce destructive interference and degrade the quality of the stacked image we present an imaging sequence that addressed these complications and allowed us to produce high quality stacked images for both p and s waves from a large volume crosswell data set a very good tie was achieved at both wells heterogeneities imaged from well to well included very thin beds less than 5 ft 1 5 m thick within the reservoir pinchouts and a major angular unconformity the grayburg san andres that could not be observed reliably with any other technique log correlation surface seismic imaging or tomography in fact the produced crosswell reflection images exhibit dramatically higher resolution and continuity than the p wave traveltime tomogram

energetics of multidecadal atlantic ocean variability abstractoscillatory behavior of the atlantic meridional overturning circulation moc is thought to underlie atlantic multidecadal climate variability while the energy sources and sinks driving the mean moc have received intense scrutiny over the last decade the governing energetics of the modes of variability of the moc have not been addressed to the same degree this paper examines the energy conversion processes associated with this variability in an idealized north atlantic ocean model in this model the multidecadal variability arises through an instability associated with a so called thermal rossby mode which involves westward propagation of temperature anomalies applying the available potential energy ape framework from stratified turbulence to the idealized ocean model simulations the authors study the multidecadal variability from an energetics viewpoint the analysis explains how the propagation of the temperature anomalies leads to changes in ape which are subsequently converted into t

acoustic tomography for monitoring enhanced oil recovery in 1984 professor amos nur of stanford university presented a paper at the seg annual international meeting in which he reported results from work carried out by his group to measure compressional and shear wave velocities as well as attenuation in samples of heavy oil sands from several reservoirs although the explanation for the observed effect was not clear the results suggested that acoustic velocities and attenuation in sands saturated with heavy oils are strongly dependent on the degree of heating of the sample to many who heard that presentation it was apparent that these observations might provide the basis for applying geophysical technology to the problem of monitoring the progression of flood fronts in thermal enhanced oil recovery eor projects in heavy oil reservoirs since that time a number of important projects have been carried out to test the theory and to attempt to verify the results predicted by the laboratory experiments it is reasonable to say that results to date have b

inverse theory applied to multi source cross hole tomography part 1 acoustic wave equation method1 frequency domain methods are well suited to the imaging of wide aperture cross hole data however although the combination of the frequency domain with the wavenumber domain has facilitated the development of rapid algorithms such as diffraction tomography this has also required linearization with respect to homogeneous reference media this restriction and association restrictions on source receiver geometries are overcome by applying inverse techniques that operate in the frequency space domain in order to incorporate the rigorous modelling technique of finite differences into the inverse procedure a nonlinear approach is used to reduce computational costs the method of finite differences is applied directly to the frequency domain wave equation the use of high speed high capacity vector computers allow the resultant finite difference equations to be factored in place in this way wavefields can be computed for additional source positions at minimal extra cost allowing inversions to be generated using data from a very large number of source positions synthetic studies show that where weak scatter approximations are valid diffraction tomography performs slightly better than a single iteration of non linear inversion however if the background velocities increase systematically with depth diffraction tomography is ineffective whereas non linear inversion yields useful images from one frequency component of the data after a single iteration further synthetic studies indicate the efficacy of the method in the time lapse monitoring of injection fluids in tertiary hydrocarbon recovery projects

multidomain analysis and wavefield separation of cross well seismic data while cross well traveltime tomography can be used to image the subsurface between well pairs the use of cross well reflections is necessary to image at or below the base of wells where the reservoir unit is often located one approach to imaging cross well reflections is to treat each cross well gather as an offset vsp and perform wavefield separation of direct and reflected arrivals prior to stacking or migration wavefield separation of direct and reflected arrivals in vsp is accomplished by separating the total wavefield into up and downgoing components since reflectors can exist both above and below the borehole wavefield separation of cross well data into up and downgoing components does not achieve separation of direct and reflected arrivals in our technique we use moveout filters applied in the domain of common vertical source receiver offset to extract reflected arrivals from the complex total wavefield of a cross well seismic data set the multiple domains available for filtering and analysis make cross well data more akin to multifold surface seismic data which can also be filtered in multiple domains rather than typical vsp data where there is only one domain common source in which to filter wavefield separation of cross well data is shown to be particularly effective against multiples when moveout filters are applied in common offset space

modelling of the functioning of karst aquifers with a reservoir model application to fontaine de vaucluse south of france summary this work deals with a rainfall discharge model applied to a well known karst aquifer a new approach is developed in order to minimize the fitting parameters here some of the model parameters do not result from a simple fitting as it was the case with earlier models i e some of them were assessed from the hydrograph analysis the conceptual model of the functioning is based on a production function based on a simple calculation of effective rainfall and a transfer function consisting of two reservoirs a slow discharge reservoir transfers the low flow and a rapid discharge reservoir feeds the high flow the model has three fitted parameters plus one for its initialisation three parameters are deduced from the hydrograph analysis over the entire time series for example the recession coefficient of the slow discharge reservoir is determined from the hydrodynamic analysis of the recession mangin a 1975 contribution a l etude hydrodynamique des aquiferes karstiques 3eme partie constitution et fonctionnement des aquiferes karstiques annales de speleologie 30 1 210 124 this model was tested over a ten years period on the fontaine de vaucluse french karst system the first hydrological year is used for fitting the model the nine other cycles validate the modelling the good quality of the model is proved by the nash criterion of 92 3 on the validation period moreover the simulation results were validated by a statistical analysis of measured and simulated time series the model successfully simulates both the high and low flow at the same time also it estimates the water volumes available in the different parts of the aquifer and it proposes a management tool capable of predicting the evolution of the discharge in different climate conditions

channel waves in cross borehole data layered geological formations with large seismic velocity contrasts can effectively create channel waves in cross borehole seismic data the existence of channel waves for such waveguides can be confirmed by ray tracing wave equation modeling and modal analysis channel wave arrivals are identified in cross borehole data recorded at conoco s newkirk test facility for these data where velocity contrasts are about 2 to 1 tomography based on first arrival traveltimes is limited due to problems with extreme ray bending and seismic shadow zones however it may be possible to extract geological information using channel wave information the seismometer differencing method appears to be a promising approach for detecting waveguide boundaries by use of cross borehole data

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

impact of fracture coatings on fracture matrix flow interactions in unsaturated porous media groundwater flow in unsaturated fractured rock is often assumed to be dominated by the porous matrix component this is frequently the result of reasoning that water flowing in the fractures is rapidly imbibed into the rock matrix by capillary suction forces with negligible resistance to uptake at the fracture matrix interface however the existence of a low permeability mineralized layer or coating at this interface may substantially reduce matrix imbibition and could consequently result in fracture dominated flow to test this idea four tuff samples containing natural fractures were obtained from tuff formations in southern nevada by performing imbibition experiments into the matrix rock across a mineralized fracture face and then across a fresh uncoated fracture face water uptake as a function of time and coating was measured a model combining darcy s law and the washburn equation has been used to describe the imbibition behavior before testing the tuff samples the experimental technique and imbibition model were first tested using ordered sphere packings with known pore size and structure from the tuff imbibition data the ratio of the permeability through the mineralized face to the permeability through the uncoated face was found to be 0 3 relatively little effect of the mineralized layer for two samples 0 01 for one sample and 10 7 uptake strongly restricted by the mineralized layer for one sample using a simple fracture flow model and the imbibition model mentioned above to simulate matrix uptake from the fracture numerical simulations indicate that the existence of fracture coatings could significantly decrease the travel time and increase the travel depth of water flowing through fractures

high resolution cross well reflection imaging potential and technical difficulties the case study presented in this paper demonstrates that reflections can be found in high frequency cross well data these reflections can be enhanced by a processing sequence similar to the one applied to vsp data to produce very high resolution images vertical resolution of the order of 3 5 ft 1 1 5 m for well distances of a few hundred feet is potentially feasible although this study produced a good tie between the cross well reflection image and the sonic log our research also illustrated some of the technical difficulties associated with cross well reflection imaging specifically we found that the signal to noise ratio of the image decreases as the distance between the imaged reflection points and the wells increases and that aliased tube waves as well as shear and converted waves were the main components of the noise we also observed that interpretation of wide angle reflections might be misleading since these events are significantly stretched both horizontally and vertically during ima

combining wave equation imaging with traveltime tomography to form high resolution images from crosshole data traveltime tomography is an appropriate method for estimating seismic velocity structure from arrival times however tomography fails to resolve discontinuities in the velocities wave equation techniques provide images using the full wave field that complement the results of traveltime tomography we use the velocity estimates from tomography as a reference model for a numerical propagation of the time reversed data these backpropagated wave fields are used to provide images of the discontinuities in the velocity field the combined use of traveltime tomography and wave equation imaging is particularly suitable for forming high resolution geologic images from multiple source multiple receiver data acquired in borehole to borehole seismic surveying in the context of crosshole imaging an effective implementation of wave equation imaging is obtained by transforming the data and the algorithms into the frequency domain this transformation allows the use of efficient frequency domain numerical propagat

high resolution crosswell imaging of a west texas carbonate reservoir part 3 wavefield separation of reflections using crosswell data collected at a depth of about 3000 ft 900 m in west texas carbonates one of the first well to well reflection images of an oil reservoir was produced the p and s brute stack reflection images created after wavefield separation tied the sonic logs and exhibited a vertical resolution that was comparable to well log resolution both brute stacks demonstrated continuity of several reflectors known to be continuous from log control and also imaged an angular unconformity that was not detected in log correlations or in surface seismic profiling the brute stacks particularly the s wave reflection image also exhibited imaging artifacts we found that multichannel wavefield separation filters that attenuated interfering wavemodes were a critical component in producing high resolution reflection images in this study the most important elements for an effective wavefield separation were the time alignment of seismic arrivals prior to filter application and the implementation of wavefield separation filters in multiple domains particularly in common offset domain the effectiveness of the multichannel filtering was enhanced through the use of extremely fine wellbore sampling intervals in this study 2 5 ft 0 76 m vertical sampling intervals for both source and receiver were used whereas most previous crosswell data sets were collected with much coarser sampling intervals resulting in spatial aliasing and limiting the utility of the data for reflection processing the wavefield separation techniques employed in this study used data volumes and associated filtering operations that were several orders of magnitude larger than those encountered in conventional vsp data analysis

a new quasi 3d unsaturated saturated hydrogeologic model of the plateau de saclay france summary a new hydrogeologic model is developed for the plateau de saclay 20 km south west of paris france which covers an area of 74 km 2 it is based on a 2d model of the fontainebleau sands aquifer combined with a 1d model of the unsaturated zone taking into account the spatial variability of the groundwater permeability field and the thickness of the unsaturated zone the paper focuses on the estimation of a triplet of parameters permeability infiltration and effective porosity based on transient flow simulations first the permeability is obtained by inversion of mean hydraulic head data for different values of infiltration then infiltration and effective porosity are determined by using transient flow simulations and fitting the hydraulic head measurements at several piezometers over the 1970 2014 period the infiltration is estimated at between 125 and 150 mm yr the mean permeability between 2 10 5 and 6 10 5 m s and the effective porosity between 20 and 30 furthermore the role of the unsaturated zone is quantified and the induced delay is estimated at about 20 years in the case of the transport of a tracer

interdecadal changes of surface temperature since the late nineteenth century we present global fields of decadal annual surface temperature anomalies referred to the period 1951 1980 for each decade from 1881 1890 to 1981 1990 and for 1984 1993 in addition we show decadal calendar seasonal anomaly fields for the warm decades 1936 1945 and 1981 1990 the fields are based on sea surface temperature sst and land surface air temperature data the ssts are corrected for the pre world war ii use of uninsulated sea temperature buckets and incorporate adjusted satellite based ssts from 1982 onward our results extend those published in the 1990 intergovernmental panel on climate change scientific assessment and its 1992 supplement we assess the impact of various sources of error in the fields despite poor data coverage initially and around the two world wars the generally cold end of the nineteenth century and start to the twentieth century are confirmed together with the substantial warming between about 1920 and 1940 slight cooling of the northern hemisphere took place between the 1950s and the mid 1970s although slight warming continued south of the equator recent warmth has been most marked over the northern continents in winter and spring but the 1980s were warm almost everywhere apart from greenland the northwestern atlantic and the midlatitude north pacific parts of the middle to high latitude southern ocean may also have been cool in the 1980s but in this area the 1951 1980 climatology is unreliable the impact of the satellite data is reduced because the record of blended satellite and in situ sst is still too short to yield a climatology from which to calculate representative anomalies reflecting climatic change in the southern ocean however we propose a method of using existing satellite data in a step toward this target the maps are condensed into global and hemispheric decadal surface temperature anomalies we show the sensitivity of these estimated anomalies to alternative methods of compositing the spatially incomplete fields running decadal zonal means and annual global and hemispheric time series are also shown finally we discuss some salient features in terms of observed atmospheric circulation changes and of the results of climate model integrations with increasing atmospheric greenhouse gases

seismic envelope inversion reduction of local minima and noise resistance waveform inversion met severe challenge in retrieving long wavelength background structure we have proposed to use envelope inversion to recover the large scale component of the model using the large scale background recovered by envelope inversion as new starting model we can get much better result than the conventional full waveform inversion by comparing the configurations of the misfit functional between the envelope inversion and the conventional waveform inversion we show that envelope inversion can greatly reduce the local minimum problem the combination of envelope inversion and waveform inversion can deliver more faithful and accurate final result with almost no extra computation cost compared to the conventional full waveform inversion we also tested the noise resistance ability of envelope inversion to gaussian noise and seismic interference noise the results showed that envelope inversion is insensitive to gaussian noise and to a certain extent insensitive to seismic interference noise this indicates the robustness of this method and its potential use for noisy data

double porosity models for a fissured groundwater reservoir with fracture skin theories of flow to a well in a double porosity groundwater reservoir are modified to incorporate effects of a thin layer of low permeability material or fracture skin that may be present at fracture block interfaces as a result of mineral deposition or alteration the commonly used theory for flow in double porosity formations that is based upon the assumption of pseudo steady state block to fissure flow is shown to be a special case of the theory presented in this paper the latter is based on the assumption of transient block to fissure flow with fracture skin under conditions where fracture skin has a hydraulic conductivity that is less than that of the matrix rock it may be assumed to impede the interchange of fluid between the fissures and blocks resistance to flow at fracture block interfaces tends to reduce spatial variation of hydraulic head gradients within the blocks this provides theoretical justification for neglecting the divergence of flow in the blocks as required by the pseudo steady state flow model coupled boundary value problems for flow to a well discharging at a constant rate were solved in the laplace domain both slab shaped and sphere shaped blocks were considered as were effects of well bore storage and well bore skin resultsmore obtained by numerical inversion were used to construct dimensionless type curves that were applied to well test data for a pumped well and for an observation well from the fractured volcanic rock terrane of the nevada test site less

improved extended reconstruction of sst 1854 1997 abstract an improved sst reconstruction for the 1854 1997 period is developed compared to the version 1 analysis in the western tropical pacific the tropical atlantic and indian oceans more variance is resolved in the new analysis this improved analysis also uses sea ice concentrations to improve the high latitude sst analysis and a modified historical bias correction for the 1939 41 period in addition the new analysis includes an improved error estimate analysis uncertainty is largest in the nineteenth century and during the two world wars due to sparse sampling the near global average sst in the new analysis is consistent with the version 1 reconstruction the 95 confidence uncertainty for the near global average is 0 4 c or more in the nineteenth century near 0 2 c for the first half of the twentieth century and 0 1 c or less after 1950

cross borehole compressional wave imaging of unconsolidated sediments cross borehole elastic wave tomography offers great potential for nondestructive imaging of near surface sediments fluids and soils this method was used to create a high resolution tomographic image of velocity structure from simulated sedimentary layers obtained using compressional wave travel times the image shows three distinct layers and a water filled container located in the center of the middle layer an image obtained from computer simulated travel times shows good agreement with the image from laboratory data the high quality data and the resulting image were due in part to a novel transmitter and receiver design these were composed of cylindrical piezoelectric transducers individually sealed in oil filled teflon containers and stacked to form stringers the transmitters were extremely efficient at coupling energy into the medium the transmitter frequency was 135 khz velocities ranged from 1 4 to 1 8 km s and thus wavelengths were 1 0 1 4 cm the distance between boreholes was 30 cm with about 60 cm of vertical coverage

a study of methods for tomographic velocity estimation in the presence of low velocity zones this paper deals with the problem of seismic velocity estimation from first arrival traveltimes in a two dimensional 2 d cross hole geometry where explosions are detonated in one borehole while recordings are made in another borehole and on the surface standard tomographic procedures are based on decomposition of the cross hole area into a number of cells and a simplifying assumption of straight raypaths in the presence of significant low velocity zones the resulting images may be contaminated different ways of performing tomographic inversion are tested on a number of synthetic examples images obtained by direct unrestricted least squares inversion are often seriously distorted however methods using more cells and some kind of damping often give more satisfactory results because the risk of distorted images is always present in inversion procedures comparison with synthetic data forward modeling is a valuable tool in the interpretation process with a reasonably good initial solution impro

2 d and 3 d resistivity image reconstruction using crosshole data theoretical changes in the distribution of electrical potential near subsurface resistivity anomalies have been studied using two resistivity models the results suggest that the greatest response from such anomalies can be observed with buried electrodes and that the resistivity model of a volume between boreholes can be accurately reconstructed by using crosshole data the distributive properties of crosshole electrical potential data obtained by the pole pole array method have also been examined using the calculated partial derivative of the observed apparent resistivity with respect to a small cell within a given volume the results show that for optimum two dimensional 2 d and three dimensional 3 d target imaging in line data and crossline data should be combined and an area outside the zone of exploration should be included in the analysis in this paper the 2 d and 3 d resistivity images presented are reconstructed from crosshole data by the combination of two inversion algorithms the first algorithm uses the alpha center method for forward modeling and reconstructs a resistivity model by a nonlinear least squares inversion alpha centers ex press a continuously varying resistivity model and the distribution of the electrical potential from the model can be calculated quickly an initial general model is determined by the resistivity backprojection technique rbpt prior to the first inversion step the second process uses finite elements and a linear inversion algorithm to improve the resolution of the resistivity model created by the first step simple 2 d and 3 d numerical models are discussed to illustrate the inversion method used in processing data from several field studies are also presented to demonstrate the capabilities of using crosshole resistivity exploration techniques the numerical experiments show that by using the combined reconstruction algorithm thin conductive layers can be imaged with good resolution for 2 d and 3 d cases the integration of finite element computations is shown to improve the image obtained by the alpha center inversion process for 3 d applications the first field test uses horizontal galleries to evaluate complex 2 d features of a zinc mine the second field test illustrates the use of three boreholes at a dam site to investigate base rock features and define the distribution of an altered zone in three dimensions

pitfalls in crosshole seismic interpretation as a result of 3 d effects in surface seismics much of the reflection data originate from more or less stratified structures with large lateral extensions consequently the data can often be interpreted without introducing serious errors by treating the earth as a 2 d medium in crosshole reservoir tomography however the target of interest is a finite object with geometric features comparable in size to the dominant wavelength of the source signal under these conditions a significant amount of energy can bypass the object under investigation as a result of diffraction when the velocity of the anomalous body is less than that of the surrounding medium the bypassed energy can appear as the first arrival in the receiver borehole in accordance with fermat s principle crosshole seismic response of a number of simple 3 d reservoirs is investigated by finite difference models based on the acoustic wave equation the nature of the signals computed along the receiver borehole confirms the presence of both the transmitted and the by

a double medium approach to simulate groundwater level variations in a fissured karst aquifer abstract karst aquifers are known for their complex flow mechanisms produced by the high spatial heterogeneity of void distribution this study aims at confirming and enhancing the understanding of these flow mechanisms in a fissured karst aquifer using flow simulation with a double medium approach groundwater level gwl variations of the studied site a 17 km2 topographic hill located in burgundy france are monitored hourly through 18 boreholes 3 karstic resurgences and the outlet river discharge of the catchment basin 40 km2 the gerke and van genuchten double porosity model is implemented in the metis code and used to simulate gwl variations with a matrix and a fracture medium model parameters are calibrated by fitting a simulated fracture medium gwl to measured gwl for four boreholes that are representative of the topographic hill hydrodynamics the double medium approach succeeds in reproducing measured fast gwl responses after recharge events that are not reproduced by a single medium approach investigating the model parameters impact on simulated gwl variations and the exchange flow between the two media allows the conceptualization of the physical structure and the hydrodynamic mechanisms of the fissured karst aquifer the spatial organization of parameters is observed and indicates areas of higher fracture density producing rapid gwl responses in contrast to lower fracture density areas which produce slower and higher gwl responses the fracture medium hydraulic parameters and the exchange parameter controlling the matrix fracture interface properties are the most influential parameters for the simulated fracture medium gwl and the exchange flow between the matrix and the fracture medium during a recharge event calibrated fracture medium hydraulic conductivity and effective porosity vary respectively from 6 3e to 05 m s 1 and 0 3 to 3 2e 04 m s 1 and 2 4 and the exchange parameter varies from 2 9e 10 m 1 s 1 to 6 5e 10 m 1 s 1 matrix medium hydraulic conductivity variations have little impact on the simulation results while matrix medium effective porosity affects slightly simulated gwl values vary from 11 to 17 finally this study shows the gerke and van genuchten double porous medium validity for the simulation of groundwater level in a fissured karst aquifer

seismic fracture anisotropy in the earth s crust an overview fluid filled fractures and microcracks are the most stress sensitive component of crustal rock and often determine the pathways and volume of crustal fluid movement if we are to comprehend the role of fractures and fluids in tectonic processes to formulate an accurate hypothesis for phenomena precursory to catastrophic seismic failure or to monitor hydrocarbon and geothermal reservoirs for the presence or absence of major fluid pathways we must understand how seismic waves interact with the fracture crack and microcrack structures within the rock mass a salient feature of fracture related rock structure is the effective seismic anisotropy of aligned fractures and microcracks with the advent of large volume digital seismic data sets it is feasible to use polarized shear waves to explore the anisotropic fracture properties of the crust crustal fracture structures which may be monitored by polarized shear waves range in dimensions from the 10 100 km scale of crustal stress orientation and preseismic stress buildup through the 100 1000 m scale of fluid reservoir structure and characterization and the 1 10 scale of mining and geoengineering rock strain monitoring to millimeter and micrometer or possibly smaller fluid filled inclusions determining the elastic response of the intact rock mass to focus developments in seismic anisotropic studies of crustal fracture phenomena the american geophysical union and the society of exploration geophysicists sponsored a chapman conference on seismic anisotropy in the earth s crust held may 31 to june 4 1988 at the lawrence berkeley laboratory of the university of california this was the third international workshop on seismic anisotropy 3iwsa nineteen conference papers are presented in this special section of the journal of geophysical research

diffraction tomography and multisource holography applied to seismic imaging seismic tomography is emerging as an imaging method for determining subsurface structure when the view angle coverage is limited and the scale of the medium inhomogeneities is comparable with the wavelength as is often true in geophysical applications the performance of ordinary ray tomography becomes poor other tomographic methods are needed to improve the imaging process here we study diffraction tomography and multisource holography and evaluate their performances for surface reflection profiling srp vertical seismic profiling vsp and cross hole measurements theoretical formulations are derived for two dimensional geometry in terms of line sources along a source line and line receivers along a receiver line the theory for diffraction tomography is based on the born or rytov approximation the performances of diffraction tomography and multisource holography are evaluated by examining the information coverage in the spatial frequency domain and by numerical examples multisource holography

applications of seismic travel time tomography summary this paper describes the application of tomography to seismic travel time inversion there are various implementations of travel time tomography in reflection tomography sources and receivers are on the surface of the earth and the principal seismic events are reflections from subsurface velocity discontinuities in transmission tomography sources and or receivers may be buried beneath the surface and the events correspond to direct or unreflected arrivals this is the analogue of medical tomography there are also cases in which both direct as well as reflected arrivals are important such as in vertical seismic profiling the latter is a direct application of the first two but is not discussed in any detail here it is also shown how the iterative use of travel time tomography and depth migration can produce much enhanced subsurface images examples of both transmission tomography and reflection tomography combined with depth migration illustrate the methods

ocean atmosphere dynamical coupling fundamental to the atlantic multidecadal oscillation abstractthe north atlantic has shown large multidecadal temperature shifts during the twentieth century there is ongoing debate about whether this variability arises primarily through the influenc

spatial organization of the impulse response in a karst aquifer summary karst aquifers are characterized by a strong heterogeneity in their physical properties the purpose of the study is the spatial variability of water transfers in a carbonated karstic aquifer to this end a high spatial density of information about the water transfer is needed the characteristics of the site a topographic hill of 13 km 2 with eight boreholes which was monitored hourly over four years allows the study of the spatial variability of water transfers the variability of the impulse response of the system is studied using autocorrelation and cross correlation analysis between the rainfall and piezometric level time series the shapes of the autocorrelation and cross correlation functions vary according to the geographical location of the boreholes that proves a spatial organization of the groundwater transfer the response time varies depending on the thickness of the unsaturated zone by an unusual inverse correlation in this case the water level signal spatially integrates the signal transfer of the unsaturated zone and the signal transfer of the saturated part of the aquifer consequently inertia and response time increased with the distance between the borehole and the top of piezometric dome this description supports highly organized fast transfers in this karst aquifer and a highly connected fracture network

temporal variability of karst aquifer response time established by the sliding windows cross correlation method summary we study the temporal variability of water transfer through the infiltration zone of a karst aquifer by estimating the impulse response of the system using cross correlogram analyses between rainfall and piezometric level time series we apply a sliding window cross correlation method which calculates cross correlograms on partially superposed short time series windows we apply this method for rainfall and piezometric level time series at six boreholes in a fractured karstic aquifer located in burgundy france based on cross correlogram functions we obtain a time series of response time at most of the boreholes the cross correlation functions change over time and the response times vary seasonally being shorter during the summer this unusual structure can be partly explained by the seasonal variability in rainfall intensity which is higher during the summer may september inducing the seasonal behaviour of the epikarst during the summer when rainfall intensity is higher the epikarst is more easily and quickly saturated this induces an increase in lateral water transfer within the epikarst and an increase in concentrated fast flows we also show that the response time seems to tend towards a limit which represents the maximum saturation of the epikarst