linearized inversion of seismic reflection data this is the first of a series of papers giving the solution of the inverse problem in seismic exploration the acoustic approximation is used together with the assumption that the velocity field has the form the forward problem is then linearized thus neglecting multiple reflected waves and the inverse problem of estimating is set up its rigorous solution can be obtained using an iterative algorithm each step consisting of a classical kirchhoff migration hyperbola summation plus a classical forward modeling step circle summation

on the lateral resolution achieved by kirchhoff migration i show that the implementation of the kirchhoff summation migration technique is equivalent to the generation of a synthetic focused array the focused array theory which is well developed in underwater acoustics is used to deal with the factors limiting lateral resolution achieved when carrying out point diffractor imaging by kirchhoff summation lateral resolution of a focused array defined as the horizontal distance between two identical point diffractors which are just resolvable is obtained from the width of the array response at the 0 35 amplitude points the focused array theory approach should help in obtaining more insights into the practical aspects of kirchhoff summation imaging technique namely choice of aperture and spatial sampling interval and the influence of velocity errors and seismic frequency examples given show how the lateral resolution can be improved by either increasing the array aperture or by using higher seismic frequency i use the concept of the depth of focus to demon

spatiotemporal characterization of land subsidence and uplift in phoenix using insar time series and wavelet transforms the effects of land subsidence pose a significant hazard to the environment and infrastructure in the arid alluvial basins of phoenix arizona improving our understanding of the source and mechanisms of subsidence is important for planning and risk management here we employ multitemporal interferometric analysis of large synthetic aperture radar data sets acquired by ers and envisat satellites to investigate ground deformation the ers data sets from 1992 to 1996 and envisat 2003 2010 are used to generate line of sight los time series and velocities in both the ascending and descending tracks the general deformation pattern is consistent among data sets and is characterized by three zones of subsidence and a broad zone of uplift the multitrack envisat los time series of surface deformation are inverted to obtain spatiotemporal maps of the vertical and horizontal deformation fields we use observation wells to provide an in situ independent data set of hydraulic head levels then we analyze vertical interferometric synthetic aperture radar and hydraulic head level time series using continuous wavelet transform to separate periodic signal components and the long term trend the isolated signal components are used to estimate the elastic storage coefficient the inelastic skeletal storage coefficient and compaction time constants together these parameters describe the storage response of an aquifer system to changes in hydraulic head and surface elevation understanding aquifer parameters is useful for the ongoing management of groundwater resources

density versus depth models from multimode surface waves multimode love and rayleigh wave data related to the pacific ocean and to north america are inverted using two sets of parameters 1 the isotropic p and s velocities plus the density and 2 five elastic parameters describing a transversely isotropic medium plus the density it is found that the assumption of isotropy leads to very unrealistic upper mantle models while much more realistic models are obtained under the assumption of transverse isotropy the lateral variations in density between the pacific ocean and north america are then less than 1 5 and in both regions a large density gradient is well resolved at a depth near 360 km

least squares reverse time migration in a matrix based formulation this paper describes least squares reverse time migration the method provides the exact adjoint operator pair for solving the linear inverse problem thereby enhancing the convergence of gradient based iterative linear inversion methods in this formulation modified source wavelets are used to correct the source signature imprint in the predicted data moreover a roughness constraint is applied to stabilise the inversion and reduce high wavenumber artefacts it is also shown that least squares migration implicitly applies a deconvolution imaging condition three numerical experiments illustrate that this method is able to produce seismic reflectivity images with higher resolution more accurate amplitudes and fewer artefacts than conventional reverse time migration the methodology is currently feasible in 2 d and can naturally be extended to 3 d when computational resources become more powerful

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

illumination based normalization for wave equation depth migration illumination problems caused by finite recording aperture and lateral velocity lensing can bias amplitudes in migration results in this paper i develop a normalization scheme appropriate for wave equation migration algorithms that compensates for irregular illumination i generate synthetic seismic data over a reference reflectivity model using the adjoint of wave equation shot profile migration as the forward modeling operator i then migrate the synthetic data with the same shot profile algorithm the ratio between the synthetic migration result and the initial reference model is a measure of seismic illumination dividing the true data migration result by this illumination function mitigates the illumination problems the methodology can take into account reflector dip as well as both shot and receiver geometries and because it is based on wave equation migration it naturally models the finite frequency effects of wave propagation the reference model should be as close to the true model as possi

geothermal point sources identified in a fumarolic ice cave on erebus volcano antarctica using fiber optic distributed temperature sensing 1 degassing of co2 on the flanks of the active erebus volcano is thought to occur mainly through fumarolic ice caves fic and associated fumarolic ice towers there is also minor co2 degassing from isolated areas of warm ground the mechanism supplying heat and co2 gas into the fic is poorly understood to investigate this system a fiber optic distributed temperature sensing dts system was deployed in a fic to obtain temperature measurements every meter the dts data reveal that localized gas vents gv supply heat to the fic air mass and are an important component of the fic microclimate fic temperature is anti correlated with local atmospheric pressure indicating barometric pumping of the gv these results enable the use of fic temperature as a proxy for flank degassing rate on erebus and represent the first application of dts for monitoring an active volcano

a parametric analysis of two dimensional elastic full waveform inversion of teleseismic data for lithospheric imaging the development of dense networks of broad band seismographs makes teleseismic data amenable to full waveform inversion fwi methods for high resolution lithospheric imaging compared to scattered field migration fwi seeks to involve the full seismic wavefield in the inversion we present a parametric analysis of 2 d frequency domain fwi in the framework of lithospheric imaging from teleseismic data to identify the main factors that impact on the quality of the reconstructed compressional p wave and shear s wave speed models compared to controlled source seismology the main adaptation of fwi to teleseismic configuration consists of the implementation with a scattered filed formulation of plane wave sources that impinge on the base of the lithospheric target located below the receiver network at an arbitrary incidence angle seismic modelling is performed with a hp adaptive discontinuous galerkin method on unstructured triangular mesh a quasi newton inversion algorithm provides an approximate accounting for the hessian operator which contributes to reduce the footprint of the coarse acquisition geometry in the imaging a versatile algorithm to compute the gradient of the misfit function with the adjoint state method allows for abstraction between the forward problem operators and the meshes that are during seismic modelling and inversion respectively an approximate correction for obliquity is derived for future application to real teleseismic data under the two dimension approximation comparisons between the characteristic scales involved in exploration geophysics and in teleseismic seismology suggest that the resolution gain provided by full waveform technologies should be of the same order of magnitude for both applications we first show the importance of the surface reflected wavefield to dramatically improve the resolving power of fwi by combining tomography like and migration like imaging through the incorporation of the forward scattered and the backscattered wavefields in the inversion the resolution of fwi is assessed through checkerboard tests and confirms a resolution of the order of the wavelength for both the p and s speeds when the full wavefield is incorporated in the inversion secondly we show that computationally efficient strategies which consist of decimating the number of frequency components involved in the inversion do not apply to teleseismic acquisitions because the scattering angle bandwidth sampled by plane wave sources can be narrow and coarsely sampled compared to that provided by dense profiles of point sources in exploration seismology the waveform inversion is less sensitive to the band of incidence angles spanned by the plane wave sources and to the sampling of this band however the deficit of vertically propagating plane waves hampers the vertical resolution of planar layers aliasing artefacts created by coarse arrays of receivers are illustrated we show how taking into account the hessian in the inversion and the suitable management of frequencies in the inversion help to mitigate these artefacts acceptable reconstructions are shown for both the p and s wave speeds for a receiver spacing of up to 20 km in the 0 1 0 4 hz frequency range building a reliable initial model for fwi is a highly non linear problem in exploration seismology we show how the low frequency content of the teleseismic sources allow us to build accurate p and s wave speed models starting from simple vertical gradient velocity models all of these results are derived using a 2 d realistic synthetic experiment that was performed with noise free data most of conclusions of this parametric study should apply in three dimensions the impact of noise and the footprint of other experimental parameters such as the estimation of the temporal plane wave signature and the estimation of the incidence angle of the impinging plane waves need however to be assessed in the future

release of arsenic to deep groundwater in the mekong delta vietnam linked to pumping induced land subsidence deep aquifers in south and southeast asia are increasingly exploited as presumed sources of pathogen and arsenic free water although little is known of the processes that may compromise their long term viability we analyze a large area 1 000 km2 of the mekong delta vietnam in which arsenic is found pervasively in deep pliocene miocene age aquifers where nearly 900 wells at depths of 200 500 m are contaminated there intensive groundwater extraction is causing land subsidence of up to 3 cm y as measured using satellite based radar images from 2007 to 2010 and consistent with transient 3d aquifer simulations showing similar subsidence rates and total subsidence of up to 27 cm since 1988 we propose a previously unrecognized mechanism in which deep groundwater extraction is causing interbedded clays to compact and expel water containing dissolved arsenic or arsenic mobilizing solutes e g dissolved organic carbon and competing ions to deep aquifers over decades the implication for the broader mekong delta region and potentially others like it across asia is that deep untreated groundwater will not necessarily remain a safe source of drinking water

land subsidence at aquaculture facilities in the yellow river delta china 1 differential interferometric synthetic aperture radar is applied to the coast of the yellow river delta yrd in china like many deltas the coastline of the yrd is dominated by aquaculture advanced land observation satellite phased array l band synthetic aperture radar sar and envisat advanced sar data acquired between 2007 and 2011 show that subsidence rates are as high as 250 mm y at aquaculture facilities likely due to groundwater pumping these rates exceed local and global average sea level rise by nearly 2 orders of magnitude and suggest that subsidence and associated relative sea level rise may present a significant hazard for asian megadeltas

radar interferometry and its application to changes in the earth s surface geophysical applications of radar interferometry to measure changes in the earth s surface have exploded in the early 1990s this new geodetic technique calculates the interference pattern caused by the difference in phase between two images acquired by a spaceborne synthetic aperture radar at two distinct times the resulting interferogram is a contour map of the change in distance between the ground and the radar instrument these maps provide an unsurpassed spatial sampling density 100 pixels km 2 a competitive precision 1 cm and a useful observation cadence 1 pass month 1 they record movements in the crust perturbations in the atmosphere dielectric modifications in the soil and relief in the topography they are also sensitive to technical effects such as relative variations in the radar s trajectory or variations in its frequency standard we describe how all these phenomena contribute to an interferogram then a practical summary explains the techniques for calculating and manipulating interferograms from various radar instruments including the four satellites currently in orbit ers 1 ers 2 jers 1 and radarsat the next chapter suggests some guidelines for interpreting an interferogram as a geophysical measurement respecting the limits of the technique assessing its uncertainty recognizing artifacts and discriminating different types of signal we then review the geophysical applications published to date most of which study deformation related to earthquakes volcanoes and glaciers using ers 1 data we also show examples of monitoring natural hazards and environmental alterations related to landslides subsidence and agriculture in addition we consider subtler geophysical signals such as postseismic relaxation tidal loading of coastal areas and interseismic strain accumulation we conclude with our perspectives on the future of radar interferometry the objective of the review is for the reader to develop the physical understanding necessary to calculate an interferogram and the geophysical intuition necessary to interpret it

resolution in seismic imaging is it all a matter of perspective classical wave equation migration of seismic data combines both acoustic wave propagation and an imaging condition to produce a subsurface section integral methods implicitly combine high frequency ray asymptotics of the wave equation with a ray dependent imaging condition the inherent limits on subsurface resolution imposed by acoustic wave generation e g source bandwidth subsurface propagation e g attenuation and recording e g receiver arrays and aperture have been studied extensively this article studies how or whether the choice of imaging condition affects resolution we will see in particular that the phenomenon of migration wavelet stretch wherein the vertical resolution of steeply dipping reflectors is preferentially lowered by migration is neither an artifact of an imaging condition nor a reduction of resolution on steeply dipping reflectors in fact we will discover that in seismic imaging of prestack data steeply dipping reflectors are ultimately better resolved than hori

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

an overview of depth imaging in exploration geophysics prestack depth migration is the most glamorous step of seismic processing because it transforms mere data into an image and that image is considered to be an accurate structuraldescriptionoftheearth thus ourexpectationsofitsaccuracy robustness and reliability are high amazingly seismic migration usually delivers the past few decades have seen migration move from its heuristic roots to mathematically sound techniques that using relatively few assumptions render accurate pictures of the interior of the earth interestingly theearthandthesubjectswewanttoimageinside itarevariedenoughthat sofar nosinglemigrationtechnique has dominated practical application all techniques continually improve and borrow from each other so one technique mayneverdominate despitetheprogressinstructuralimaging wehavenotreachedthepointwhereseismicimagesprovide quantitatively accurate descriptions of rocks and fluids nor have we attained the goal of using migration as part of a purelycomputationalprocesstodeterminesubsurfacevelocity in areas where images have the highest quality we might be nearing those goals collectively called inversion where data are more challenging the goals seem elusive we describe the progress made in depth migration to the present and the most significant barriers to attaining its inversion goalsinthefuture wealsoconjectureonprogresslikelytobe made in the years ahead and on challenges that migration mightnotbeabletomeet

sensing the ups and downs of las vegas insar reveals structural control of land subsidence and aquifer system deformation land subsidence in las vegas nevada united states between april 1992 and december 1997 was measured using spaceborne interferometric synthetic aperture radar the detailed deformation maps clearly show that the spatial extent of subsidence is controlled by geologic structures faults and sediment composition clay thickness the maximum detected subsidence during the 5 75 yr period is 19 cm comparison with leveling data indicates that the subsidence rates declined during the past decade as a result of rising ground water levels brought about by a net reduction in ground water extraction temporal analysis also detects seasonal subsidence and uplift patterns which provide information about the elastic and inelastic properties of the aquifer system and their spatial variability

iterative resolution estimation in least squares kirchhoff migration we apply iterative resolution estimation to least squares kirchhoff migration reviewing the theory of iterative optimization uncovers the common origin of different optimization methods this allows us to reformulate the pseudo inverse model resolution and data resolution operators in terms of effective iterative estimates when applied to kirchhoff migration plots of the diagonal of the model resolution matrix reveal low illumination areas on seismic images and provide information about image uncertainties synthetic and real data examples illustrate the proposed technique and confirm the theoretical expectations

synthetic inversions for density using seismic and gravity data density variations drive mass transport in the earth from plate tectonics to convection in the mantle and core nevertheless density remains poorly known because most geophysical measurements used to probe the earth s interior either have little sensitivity to density suffer from trade offs or from non uniqueness with the ongoing expansion of computational power it has become possible to accurately model complete seismic wavefields in a 3 d heterogeneous earth and to develop waveform inversion techniques that account for complicated wavefield effects this may help to improve resolution of density here we present a pilot study where we explore the extent to which waveform inversion may be used to better recover density as a separate independent parameter we perform numerical simulations in 2 d to investigate under which conditions and to what extent density anomalies may be recovered in the earth s mantle we conclude that density can indeed be constrained by seismic waveforms mainly as a result of scattering effects at density contrasts as a consequence the low frequency part of the wavefield is the most important for constraining the actual extent of anomalies while the impact of density heterogeneities on the wavefield is small compared to the effects of velocity variations it is likely to be detectable in modern regional to global scale measurements we also conclude that the use of gravity data as additional information does not help to further improve the recovery of density anomalies unless strong a priori constraints on the geometry of density variations are applied this is a result of the inherent physical non uniqueness of potential field inverse problems finally in the limited numerical setup that we employ we find that the initially supplied anomalies in s and p velocity models are of minor importance

fiber optic network observations of earthquake wavefields author s lindsey nj martin er dreger ds freifeld b cole s james sr biondi bl ajo franklin jb abstract 2017 american geophysical union all rights reserved our understanding of subsurface processes suffers from a profound observation bias seismometers are sparse and clustered on continents a new seismic recording approach distributed acoustic sensing das transforms telecommunication fiber optic cables into sensor arrays enabling meter scale recording over tens of kilometers of linear fiber length we analyze cataloged earthquake observations from three das arrays with different horizontal geometries to demonstrate some possibilities using this technology in fairbanks alaska we find that stacking ground motion records along 20nm of fiber yield a waveform that shows a high degree of correlation in amplitude and phase with a colocated inertial seismometer record at 0 8 1 6nhz using an l shaped das array in northern california we record the nearly vertically incident arrival of an earthquake from the geysers geothermal field and estimate its backazimuth and slowness via beamforming for different phases of the seismic wavefield lastly we install a fiber in existing telecommunications conduits below stanford university and show that little cable to soil coupling is required for teleseismic p and s phase arrival detection

estimating the model resolution matrix for large seismic tomography problems based on lanczos bidiagonalization with partial reorthogonalization summary the propack package developed by larsen is able to efficiently and accurately estimate singular values and vectors for large matrices based on the lanczos bidiagonalization with partial reorthogonalization we incorporate the pavtial reorthogonalization package propack package into the double difference seismic tomography code tomodd and estimate the model resolution matrix for large seismic tomography problems compared to previous least squares qr lsqr based methods for estimating the model resolution matrix the propack based method calculates the full resolution matrix and thus gives a complete description of how well the model is resolved several observations are drawn from the application to data from the 2001 eruption of mt etna for this example it is reasonable to use ray sampling density information to characterize the model resolution qualitatively the model resolution resulting from just velocity inversion bears a close linear relationship to that from simultaneous inversion but always overestimates resolution and the inversion system using differential times has a greater ability to resolve the source region structure than the system using absolute times

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

vertical oxygen minimum zone oscillations since 20 ka in santa barbara basin a benthic foraminiferal community perspective 1 here we present a history of deoxygenation of upper intermediate waters during the last deglaciation from santa barbara basin sbb based on quantitative analyses of benthic foraminiferal assemblages from a new shallow piston core above basin sill depth mv0811 15jc 418m and previously described sequences in the deeper basin md02 2504 481m and md02 2503 570m we document a 152m depth transect of benthic foraminiferal assemblages to extract changing community structure density diversity and evenness and improve paleoenvironmental interpretation of late quaternary vertical oscillations in the upper boundary of the oxygen minimum zone omz close interaction between changes in open margin omz and that of the restricted sbb is documented using these quantitative techniques mv0811 15jc while being unlaminated contains strongly hypoxic foraminiferal assemblages including species bolivina tumida and nonionella stella coeval with preserved sediment laminations in the deeper cores last glacial maximum lgm assemblages across this transect contained oxic fauna and high diversity at 14 7ka glacial termination ia hypoxic benthic fauna appeared across the transect recording hypoxic waters 0 5mll 1 300m from the ocean surface bolling allerod b a assemblages uniquely stand out in the record exhibited by low density diversity and evenness and taxonomic composition reflecting extreme and stressful hypoxia and methane rich environments younger dryas assemblages were diverse and composed of oxic fauna similar to lgm assemblages termination ib initiated another deoxygenation shift followed by omz associated faunal and density patterns this analysis strengthens the quantitative assessment of oxygen concentrations involved in deglacial omz change and reveals the unexpected remarkable shallowness of omz influence during the b a

waveform inversion for three dimensional density and s wave structure long period seismic data longer than about 100 s are sensitive to density as well as p wave and s wave structure in the earth seismograms containing overtone and fundamental mode signals are inverted directly for density and s wave velocity structure a simple relation between p wave and s wave velocity perturbations is assumed in order to reduce the number of unknowns a model is parameterized by spherical harmonics up to degree and order 8 and by five knots in depth at 30 220 450 670 and 882 km with linear interpolation between adjacent knots density resolution is good only at shallow depths in fact only the average in the upper 200 km is marginally resolved s wave resolution is generally good throughout the depth range s wave results show good correlation to surface geological features and are similar to previous studies density results show quite different patterns from s wave maps high density regions appear along the subduction zones surrounding the pacific ocean and low density regions appear in most major continents most continents have low density and high s wave velocity which suggests that continental mantle consists of compositionally different materials since thermal effects can never produce such perturbations high s wave velocity also suggests a relatively cold geotherm for continents and thus continents should consist of significantly low density material such as depleted mantle rocks as proposed by many researchers before the density map also correlates well with a geoid map for spherical harmonic components l 4 6 but not for l 2 3 it suggests that sources for these geoid undulations l 4 6 are in the upper mantle probably in the upper 300 400 km but those for l 2 3 are in deeper regions

evolution of long term land subsidence near mexico city review field investigations and predictive simulations 1 aquitard consolidation in the chalco plain is the most recent of a series of major land subsidence problems near mexico city caused by leaky aquifer pumping and involving a complex distribution of basalt flows within a lacustrine sequence this study first conducted a ground magnetic survey combined with lithologic logs to map the extension of basalts then it assessed the evolution of ground surface elevations and updated hydraulic heads in the aquifer and aquitard in order to verify the accuracy of previous simulations and develop new predictions on land subsidence employing a one dimensional nonlinear groundwater flow consolidation model results show the presence of shallow basalts that extend from sierra santa catarina into the chalco plain causing a differential consolidation that controls both the distribution of large scale fractures in the aquitard and the shape of a new lake cumulative land subsidence in the center of the chalco plain reached 13 m in 2006 thus closely matching previous numerical estimations since 1985 the ground surface decline has continued at a rate of 0 40 m yr while the potentiometric surface decline in the aquifer proceeds at an average rate of 1 5 m yr indicating that the flow system has not yet reached steady state conditions numerical predictions show that under current pumping rates where the aquitard is 300 m total land subsidence will reach 19 m by the year 2020 while where the aquitard is 140 m thick total land subsidence will reach 12 m and increase the risk of flooding and aquitard fracturing for nearby urban centers

land subsidence in china land subsidence in china occurs in different regions it is primarily caused by excessive groundwater withdrawal other reasons for the subsidence include the oil warm groundwater withdrawal and the neotectonic movement the common characteristics of land subsidence in china are slow accumulative irreversible and other unique properties the range of subsidence still keeps extending and the accumulative subsidence increasing though some measures taken adjustment of the aquifer exploitation practice is a subsidiary way to control land subsidence but it cannot solve this problem completely in a specfic way of groundwater changing the contribution of a certain soil layer to the total subsidence depends on its compressibility and thickness besides the elasticity both cohesive soil layers aquitards and sand layers aquifers are observed to be plastic and creep when the groundwater level fluctuates in a specific way which often leads to subsidence delay

least squares migration of incomplete reflection data a least squares migration algorithm is presented that reduces the migration artifacts i e recording footprint noise arising from incomplete data instead of migrating data with the adjoint of the forward modeling operator the normal equations are inverted by using a preconditioned linear conjugate gradient scheme that employs regularization the modeling operator is constructed from an asymptotic acoustic integral equation and its adjoint is the kirchhoff migration operator we tested the performance of the least squares migration on synthetic and field data in the cases of limited recording aperture coarse sampling and acquisition gaps in the data numerical results show that the least squares migrated sections are typically more focused than are the corresponding kirchhoff migrated sections and their reflectivity frequency distributions are closer to those of the true model frequency distribution regularization helps attenuate migration artifacts and provides a sharper better frequency distribution of estimated reflectivity the least squares migrated sections can be used to predict the missing data traces and interpolate and extrapolate them according to the governing modeling equations several field data examples are presented a ground penetrating radar data example demonstrates the suppression of the recording footprint noise due to a limited aperture a large gap and an undersampled receiver line in addition better fault resolution was achieved after applying least squares migration to a poststack marine data set and a reverse vertical seismic profiling example shows that the recording footprint noise due to a coarse receiver interval can be suppressed by least squares migration