passive seismic imaging in the presence of white noise sources passive seismic imaging is based on the relation between the reflection and the transmission responses of the subsurface let one have the transmission responses measured at surface points a and b of a 3d inhomogeneous medium in the presence of white noise sources in the subsurface when these transmission responses are cross correlated one obtains the reflection response of the same medium as if measured at point a in the presence of an impulsive source at point b the quality of the simulated reflection response strongly depends on the whiteness and the distribution of the noise sources reflectors present below the sources cause the appearance of some ghost events random distribution of the noise sources will however weaken these ghost events

extracting time domain green s function estimates from ambient seismic noise 1 it has been demonstrated experimentally and theoretically that an estimate of the green s function between two seismic stations can be obtained from the time derivative of the long time average cross correlation of ambient noise between these two stations this tdgf estimate from just the noise field includes all tensor components of the green s function and these green s function estimates can be used to infer earth structure we have computed cross correlations using 1 to 30 continuous days of ambient noise recorded by over 150 broadband seismic stations located in southern california the data processing yielded thousands of cross correlation pairs for receiver separations from 4 500 km which clearly exhibit coherent broadband propagating dispersive wavetrains across frequency band 0 1 2 hz

3d wave equation interferometric migration of vsp free surface multiples interferometric migration of free surface multiples in vertical seismic profile vsp data has two significant advantages over standard vsp imaging 1 a significantly larger imaging area compared to migrating vsp primaries and 2 less sensitivity to velocity estimation and static errors than other methods for migration of multiples in this paper we present a 3d wave equation interferometric migration method that efficiently images vsp free surface multiples synthetic and field data results confirm that a reflectivity image volume comparable in size to a 3d surface seismic survey around the well can be computed economically the reflectivity image volume has less fold density and lower signal to noise ratio than that obtained by a conventional 3d surface seismic survey because of the relatively weak energy of multiples and the limited number of geophones in the well however the efficiency of this method for migrating vsp multiples suggests that it might sometimes be a useful tool for 4d seismic monitoring where reflectivity images can be computed quickly for each time lapse survey

surface wave tomography of the western united states from ambient seismic noise rayleigh and love wave phase velocity maps summary we present the results of rayleigh wave and love wave phase velocity tomography in the western united states using ambient seismic noise observed at over 250 broad band stations from the earthscope usarray transportable array and regional networks all available threecomponent time series for the 12 month span between 2005 november 1 and 2006 october 31 have been cross correlated to yield estimated empirical rayleigh and love wave green s functions the love wave signals were observed with higher average signal to noise ratio snr than rayleigh wave signals and hence cannot be fully explained by the scattering of rayleigh waves phase velocity dispersion curves for both rayleigh and love waves between 5 and 40 speriod were measured for each interstation path by applying frequency time analysis the average uncertainty and systematic bias of the measurements are estimated using a method based on analysing thousands of nearly linearly aligned station triplets we find that empirical green s functions can be estimated accurately from the negative time derivative of the symmetric component ambient noise cross correlation without explicit knowledge of the source distribution the average traveltime uncertainty is less than 1 s at periods shorter than 24 s we present rayleigh and love wave phase speed maps at periods of 8 12 16 and 20 s the maps show clear correlations with major geological structures and qualitative agreement with previous results based on rayleigh wave group speeds

transmission to reflection transformation of teleseismic wavefields 1 in this paper we review the transformation of p and sv transmission green s functions into the corresponding reflection quantities for one dimensional elastic media at precritical slownesses to obtain estimates of the transmission green s functions from observed data we apply a recently developed approach that exploits the minimum phase nature of the direct waves and employs autospectra and cross spectra of raw component seismograms representing multiple sources recorded at a single station to accomplish transformation to reflection green s functions we outline a practical recipe that involves amplitude balancing energy normalization cross correlation and removal of free surface effects we assess its performance through application to both synthetic seismograms and data from station hyderabad in all cases we are able to identify the first order scattering contributions from the continental moho the transmission to reflection transformation has potential applications for imaging hitherto elusive shallow mantle discontinuities since interfering forward and back scattering contributions are effectively separated by component in the resulting reflection green s functions

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

tutorial on seismic interferometry part 1 basic principles and applications seismic interferometry involves the crosscorrelation of responses at different receivers to obtain the green s function between these receivers for the simple situation of an impulsive plane wave propagating along the x axis the crosscorrelation of the responses at two receivers along the x axis gives the green s function of the direct wave between these receivers when the source function of the plane wave is a transientas in exploration seismology or a noise signalas in passive seismology then the crosscorrelation gives the green s function convolved with the autocorrelation of the source function direct wave interferometry also holds for 2d and 3d situations assuming the receivers are surrounded by a uniform distribution of sources in this case the main contributions to the retrieved direct wave between the receivers come from sources in fresnel zones around stationary points the main application of direct wave interferometry is the retrieval of seismic surface wave responses from ambient noise and the subsequent tomographic determination of the surfacewave velocity distribution of the subsurface seismic interferometry is not restricted to retrieving direct waves between receivers in a classic paper claerbout shows that the autocorrelation of the transmission response of a layered medium gives the plane wave reflection response of that medium this is essentially 1d reflected wave interferometry similarly the crosscorrelation of the transmission responses observed at two receivers of an arbitrary inhomogeneous medium gives the 3d reflection response of that medium one of the main applications of reflected wave interferometry is retrieving the seismic reflection response from ambient noise and imaging of the reflectors in the subsurface a common aspect of direct and reflected wave interferometry is that virtual sources are created at positions where there are only receivers without requiring knowledge of the subsurface medium parameters or of the positions of the actual sources

multiparameter two dimensional inversion of scattered teleseismic body waves 3 application to the cascadia 1993 data set this is the third paper in a three part series that examines formal inversion of the teleseismic p wave coda for discontinuous two dimensional 2 d variations in elastic properties beneath dense arrays of three component broadband seismometers in this paper the method is applied to data from the incorporated research institutions for seismology program for array seismic studies of the continental lithosphere iris passcal cascadia 1993 experiment undertaken across central oregon two major features are imaged in the resulting model the continental moho becomes evident 150 km from the coast beneath the western cascades and extends through the eastern end of the profile at 35 40 km depth in the western portion of the model oceanic crust of the subducting juan de fuca plate dips shallowly 12 at the coast and more steeply 27 below the willamette valley and is evident to depths of 100 km beneath the high cascades the abrupt increase in plate dip at 40 km depth coincides with an apparent thickening of the oceanic crust followed by a diminution in its signature building on previous work we argue that these results are consistent with the consequences of prograde metamorphic reactions occurring within the oceanic crust progressive dehydration at lower grade facies conditions culminates in the transformation to eclogite producing a pronounced increase in the seismic velocity density and dip of the subducting plate and structural complexity in the overlying wedge

shear wave seismic interferometry for lithospheric imaging application to southern mexico seismic interferometry allows for the creation of new seismic traces by cross correlating existing ones with sufficient sampling of remote source positions it is possible to create a virtual source record by transforming a receiver location into a virtual source the imaging technique developed here directly retrieves reflectivity information from the subsurface other techniques namely receiver function and tomography rely on mode converted energy and perturbations in a velocity field respectively to make inferences regarding structure we select shear phases as an imaging source because of their lower propagation velocity sensitivity to melt and ability to treat vertical shear and horizontal shear wavefields independently teleseismic shear phases approximate a plane wave due to the extent of wavefront spread compared to a finite receiver array located on the free surface the teleseismic shear phase transmission responses are used as input to the seismic interferometry technique we create virtual shear source records by converting each receiver in the array into a virtual source by cross correlating the received signals the complex source character of distant earthquakes is imprinted on the virtual source records as the average autocorrelation of individual source time functions we demonstrate a technique that largely removes this imprint by filtering in the common offset domain a field data set was selected from the meso america subduction experiment despite the suboptimal remote source sampling an image of the lithosphere was produced that confirms features of the subduction zone that were previously found with the receiver function technique

autocorrelation of the seismic wavefield at newberry volcano reflections from the magmatic and geothermal systems we show that seismic autocorrelations provide new depth constraints on upper crustal magmatic systems autocorrelations of both ambient noise recorded on seismometers and geophones and teleseismic earthquake coda recorded on seismometers elucidate the structure of newberry volcano these autocorrelations result in the two way body wave green s function beneath a station within the caldera a reproducible coherent p wave reflection is inferred to come from the top of a magma body at 2 5 km depth and maps with the lateral extent of an anomalously low velocity body imaged tomographically on the west flank of the volcano a reflection that deepens with distance from the caldera is inferred to result from a temperature dependent change in metamorphic facies and may map the thermal structure of the edifice our results show that the autocorrelation of diffuse seismic energy reconstructs reflections from seismically sharp boundaries associated with the upper crustal magmatic structure

dynamics of piton de la fournaise volcano observed by passive image interferometry with multiple references abstract activity of piton de la fournaise pdf volcano in la reunion island modifies the seismic velocities within the edifice using the 2010 and 2011 data from a network of 21 seismic stations in the vicinity of pdf changes of seismic velocities are investigated using passive image interferometry i e interferometry of seismic noise correlations as noise correlations change significantly over time in response to volcanic activity a method is presented that allows us to measure continuous long term velocity changes with high and constant accuracy by using multiple periods as reference a long term velocity increase is found that averages about 0 25 per year this trend is superimposed by short term changes that exhibit a clear connection with summit seismo tectonic earthquakes indicating the effect of volcanic activity characteristic signatures of velocity changes are identified for post eruptive periods of deflation that show an increase of velocity associated with subsidence observed by gps periods of pre eruptive inflation are characterized by decreasing velocity seismic crises can be associated with either increasing or decreasing velocity depending on whether the magma movement leads to deflation due to an eruption emptying the shallow plumbing system or to inflation caused by a non eruptive intrusion with a simple assumption about the spatial sensitivity of the measurements both processes are found to have the strongest effect in the central summit area of the volcano which also shows the strongest surface displacements during the time investigated here we do not observe a dependence of the velocity change on the location of the erupting fissures instead the distribution of changes for the three inflation periods and the two eruptions are similar indicating that the velocity changes observed here reflect the dynamics of a shallow magma reservoir rather than the effect of the eruption at the surface

multiparameter two dimensional inversion of scattered teleseismic body waves 2 numerical examples in this paper we investigate the formal inversion of synthetic teleseismic p coda waves for subsurface elastic properties using a ray theoretic approach which assumes single scattering bostock et al this issue we consider a model comprising an idealized lithospheric suture zone whose geometrical configuration is drawn from previous deep crustal seismic studies two dimensional pseudospectral synthetic seismograms representing plane waves propagating through this model are preprocessed to extract an estimate of the scattered wave field generated by short wavelength structure these data are employed in a series of numerical simulations which examine the dependence of multiparameter inversion results on a range of input parameters in particular we demonstrate 1 the contrasting sensitivity which forward and backscattered waves display to structural recovery 2 the diminution of the problem null space accompanied by increased source coverage 3 improvements in model reconstruction achieved through simultaneous treatment of multiple scattering modes and 4 the robustness of the method for data sets with noise levels and receiver geometries that approach those of field experiments

receiver structure from teleseisms autocorrelation and cross correlation we present a way of characterizing the structure beneath a seismic station by exploiting stacked correlograms of three component records from teleseismic events this seismic daylight imaging approach exploits the extraction of reflection and conversion information from teleseismic coda via tensor autocorrelation the approach is illustrated for a number of australian stations in a variety of tectonic environments using hundreds of teleseismic events to extract p and s reflectivity and converted ps and sp information the results show a very good agreement with prior knowledge across australia compared with the classical receiver function the broader frequency band of 0 5 4 0 hz provides additional information on finer scale structure

seismic interferometry using non volcanic tremor in cascadia 1 seismic interferometry to date has focussed on surface waves due to paucity of deep high frequency noise sources here we investigate use of non volcanic tremor on the cascadia subduction zone as a noise source to recover scattered body wave contributions to the green s function tremor data for 2004 2005 recorded at polaris bc stations twkb mgcb and lzb were filtered and cross correlated twkb and mgcb correlations generate a highly reproducible arrival at 4 5 s for combinations of vertical with radial or transverse components twkb and lzb correlations are less reproducible but yield a strong arrival at 3 5 s for 2005 upon consideration of source receiver geometry polarity and timing we interpret these arrivals to represent reflection conversions arrival times imply that these signals originate at depths between 9 and 12 km coincident with an interval of strong reflectivity imaged in the 1987 lithoprobe southern cordillera transect

seismic interferometry turning noise into signal turning noise into useful data every geophysicist s dream and now it seems possible the field of seismic interferometry has at its foundation a shift in the way we think about the parts of the signal that are currently filtered out of most analyses complicated seismic codas the multiply scattered parts of seismic waveforms and background noise whatever is recorded when no identifiable active source is emitting and which is superimposed on all recorded data those parts of seismograms consist of waves that reflect and refract around exactly the same subsurface heterogeneities as waves excited by active sources the key to the rapid emergence of this field of research is our new understanding of how to unravel that subsurface information from these relatively complex looking waveforms and the answer turned out to be rather simple this article explains the operation of seismic interferometry and provides a few examples of its application

a novel application of time reversed acoustics salt dome flank imaging using walkaway vsp surveys in this paper we present initial results of applying time reversed acoustics tra technology to saltdome flank seismic imaging we created a set of synthetic traces representing a multilevel walkaway vsp for a model composed of a simplified gulf of mexico vertical velocity gradient and an embedded salt dome we first applied the concepts of tra to the synthetic traces to create a set of redatummed traces without having to perform velocity analysis moveout corrections or complicated processing each redatummed trace approximates the output of a zero offset downhole source and receiver pair to produce the final salt dome flank image we then applied conventional poststack depth migration to the zero offset section our results show a very good image of the salt when compared to an image derived using data from a downhole zero offset source and receiver pairs the simplicity of our tra implementation provides a virtually automated method to estimate a zero offset seismic section as if it had been collected from the reference frame of the borehole containing the vsp survey

teleseismic shot profile migration the shot profile migration approach of wave equation migration generates subsurface images using the interferometric principle of crosscorrelating two passive wavefields these wavefields are typically a source wavefield containing energy from an excited source and a receiver wavefield comprised of scattered source wavefield energy by the discontinuous earth structure shot profile migration can be recast as a novel way of imaging the earth s lithosphere using teleseismic wavefield data where the source wavefield is the directly arriving wavefront and the receiver wavefield is the following wavefield coda we demonstrate that the shot profile technique can be tailored to suit teleseismic acquisition geometry and wavefields assuming an acoustic framework and 2 5d experimental geometry we develop procedures that enable kinematic and structural imaging migration using both transmission and free surface reflected passive wavefields experiments with synthetic data demonstrate the method s applicability a

crosscorrelogram migration of inverse vertical seismic profile data we present the theory of crosscorrelogram migration of ghost reflections also known as interferometric imaging to delineate reflector geometries from inverse vertical seismic profile data the theory includes the equations for forward modeling migration asymptotic inversion and model resolution of crosscorrelgrams rather than using primary reflections crosscorrelogram migration can use ghost reflections to reconstruct the reflector geometry other multiples can be used as well including pegleg multiples and higher order multiples its main advantages over conventional kirchhoff migration are 1 both source location e g drill bit position and source wavelet need not be known such as when using a drill bit as a source in a deviated well 2 it is insensitive to source related static errors and 3 it has wider subsurface illumination than conventional kirchhoff migration of primary reflections crosscorrelation migration can effectively widen the source receiver aperture by more than 50 co

seismic imaging of the site response using microearthquake recordings part i method abstract microearthquake signals from widely distributed geographical regions have been noticed repeatedly to be strongly affected by local site effects the corresponding seismograms contain important information on the individual propagation path a single station inversion method is presented which allows the imaging of the subsurface impedance structure from locally recorded sh waves records the technique is based on the kunetz claerbout equation which in its original form states the relationship between the transmission response and the reflection response for p waves in a horizontally layered medium and vertical incidence claerbout 1968 by adapting claerbout9s formulation to the transmission problem for sh waves under oblique incidence microearthquake recordings can be used to calculate corresponding pseudo reflection seismograms which in turn can be inverted for the impedance structure by exploiting levinson recursion limitations in the high frequency content of the signals due to the combined effects of absorption recording system and bandlimited source signals as well as the influence of noise drastically reduce the resolutional power of the inversion procedure a number of different deconvolution techniques have been tested with synthetic seismograms for the applicability to reduce these effects good results have been obtained with the blind deconvolution technique of scherbaum and stoll 1985 utilizing the minimum phase property of the green9s function from the analysis of synthetic data it can be expected that for realistic source receiver situations and short period recording systems the main reflectors down to a depth of several hundreds of meters could be resolved

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

characterization of shallow geology by high frequency seismic noise tomography summary to study the applicability of the passive seismic interferometry technique to near surface geological studies seismic noise recordings from a small scale 2 d array of seismic stations were performed in the test site of nauen germany rayleigh wave green s functions were estimated for different frequencies a tomographic inversion of the traveltimes estimated for each frequency from the green s functions is then performed allowing the laterally varying 3 d surface wave velocity structure below the array to be retrieved at engineering geotechnical scales furthermore a 2 d s wave velocity cross section is obtained by combining 1 d velocity structures derived from the inversion of the dispersion curves extracted at several points along a profile where other geophysical analyses were performed it is shown that the cross section from passive seismic interferometry provides a clear image of the local structural heterogeneities that are in excellent agreement with georadar and geoelectrical results such findings indicate that the interferometry analysis of seismic noise is potentially of great interest for deriving the shallow 3 d velocity structure in urban areas

in situ assessment of the g γ curve for characterizing the nonlinear response of soil application to the garner valley downhole array and the wildlife liquefaction array abstract we analyze the nonlinear and near surface geological effects of two network for earthquake engineering simulation at university of california santa barbara nees ucsb instrumented sites the garner valley downhole array gvda and the wildlife liquefaction array wla the seismic interferometry by deconvolution method is applied to earthquake data recorded by the multisensor vertical array between january 2005 and september 2013 along the cross section local shear wave velocity is extracted by estimating travel time between sensors the s wave velocity profiles are constructed and compared with classical in situ geophysical surveys we show that velocity values change according to the amplitude of the ground motion and we find anisotropy between east west and north south directions at the gvda site the ratio between average peak particle velocity v and local s wave velocity between two boreholes is tested as a deformation proxy using average peak particle acceleration a the a versus curve is used to represent the stress strain curve for observing the site s nonlinear responses under different levels of excitation nonlinearity is observed from quite low shear strain levels 1 10 5 and a classic hyperbolic model is derived proves to be a good deformation proxy finally the shear modulus degradation curves are constructed for each depth and test site and they are similar to previous laboratory measurements or in situ geophysical surveys a simple comparison regarding nonlinear behavior between gvda and wla is performed

body wave passive seismic interferometry revisited mining exploration using the body waves of local microearthquakes abstract as the global need for mineral resources is constantly rising and the exploitable concentrations of these resources tend to become increasingly complex to explore and exploit the mining industry is in a constant quest for innovative and cost effective exploration solutions in this context and in the framework of the smart exploration action an integrated passive seismic survey was launched in the gerolekas bauxite mining site in central greece a passive seismic network consisting of 129 three component short period stations was installed and operated continuously for 4 months the acquired data permitted detection of approximately 1000 microearthquakes of very small magnitude duration magnitude ranging between 1 5 and 2 0 located within or at a very close distance from the study area we use this microseismicity as input for the application of passive seismic interferometry for reflection retrieval using the body waves p and s wave coda of the located microearthquakes we retrieve by autocorrelation zero offset virtual reflection responses per component below each of the recording stations we process the acquired results using reflection processing techniques to obtain zero offset time and depth sections both for p and for s waves in the context of the present work we evaluate one of the acquired depth sections using an existing seismic line passing through the gerolekas passive seismic network and we perform forward modelling to assess the quality and value of the acquired results we confirm that passive seismic reflected wave interferometry could constitute a cost effective and environmentally friendly innovative exploration alternative especially in cases of difficult exploration settings

interferometric ground roll removal attenuation of scattered surface waves in single sensor data land seismic data are contaminated by surface waves or ground roll these surface waves are a form of source generated noise and can be strongly scattered by near surface heterogeneities the resulting scattered ground roll can be particularly difficult to separate from the desired reflection data especially when this scattered ground roll propagates in the crossline direction we have used seismic interferometry to estimate scattered surface waves recorded during an exploration seismic survey between pairs of receiver locations where sources and receivers coincide these interreceiver surface wave estimates were adaptively subtracted from the data this predictive subtraction process can successfully attenuate scattered surface waves while preserving the valuable reflected arrivals forming a new method of scattered ground roll attenuation we refer to this as interferometric ground roll removal

on bias and noise in passive seismic data from finite circular array data processed using spac methods the finite nature of typical small seismic arrays used in conjunction with spatial autocorrelation spac processing for observing the microtremor wavefield causes predictable perturbationsofthespacspectrumwhensourcesofseismic noiseareconfinedtoarestrictedrangeofazimuths suchperturbations are especially evident at higher frequencies where wavelengths are on the order of the array radius the effects arereadilymodeledandshowthatthetriangulararraygeometries commonly used for microtremor studies require azimuthal distributions of wave energy on the order of 60 or greatertohaveahighprobabilityofbeingfreeofsuchperturbations the imaginary component of the spac spectrum which is ideally zero for a sufficiently dense circular array and or a sufficiently isotropic wavefield is in practice often nonzero and provides three quality control indicators 1 an indication of insufficient spatial averaging 2 an empirical measure of the level of statistical uncertainty in spac spectralestimates and3anindicationofdeparturesfromplanewavestationarityoftheseismicnoisewavefield

monitoring velocity changes caused by underground coal mining using seismic noise we use passive seismic interferometry to monitor temporal variations of seismic wave velocities at the area of underground coal mining named jas mos in poland ambient noise data were recorded continuously for 42 days by two three component broadband seismometers deployed at the ground surface the sensors are about 2 8 km apart and we measure the temporal velocity changes between them using cross correlation techniques using causal and acausal parts of nine component cross correlation functions ccfs with a stretching technique we obtain seismic velocity changes in the frequency band between 0 6 and 1 2 hz the nine component ccfs are useful to stabilize estimation of velocity changes we discover correlation between average velocity changes and seismic events induced by mining especially after an event occurred between the stations the velocity decreased about 0 4 based on this study we conclude that we can monitor the changes of seismic velocities which are related to stiffness effective stress and other mechanical properties at subsurface caused by mining activities even with a few stations

convergence of the two point correlation function toward the green s function in the context of a seismic prospecting data set when considering direct waves in the correlation process the green s function is reconstructed when using an even distribution of seismic sources or when the source distribution is restricted to the direction close to the alignment of the sensors on the other hand when considering records of coda waves the convergence is achieved for any source distribution as expected theoretically we extract the expected amplitude decay along a seismic profile from the correlation functions when an even distribution of sources is considered or when the time window includes scattered waves

surface wave array tomography in se tibet from ambient seismic noise and two station analysis i phase velocity maps summary empirical green s functions egfs between pairs of seismographs can be estimated from the time derivative of the long time cross correlation of ambient seismic noise these egfs reveal velocity dispersion at relatively short periods which can be used to resolve structures in the crust and uppermost mantle better than with traditional surface wave tomography we combine rayleigh wave dispersion estimates from egfs and from traditional two station ts analysis into a new approach to surface wave array tomography with data from dense receiver arrays we illustrate the methodology with continuous broad band recordings from a temporary seismographic network on the southeastern part of the tibetan plateau in sichuan and yunnan provinces sw china the egfs are robust under temporal changes in regional seismicity and the use of either ambient noise approximated by records without signal from events with magnitude mb 5 or 4 or surface wave coda produces similar results the egfs do not strongly depend on the presence of large earthquakes but they are not reciprocal for stations aligned in the n s direction this directionality reflects the paucity of seismicity to the north of the array using a far field representation of the surface wave green s function and an image transformation technique we infer from the egfs the rayleigh wave phase velocity dispersion in the period band from 10 30 s a classical ts approach is used to determine rayleigh wave phase velocity dispersion between 20 120 s together they constrain phase velocity variations for t 10 120 s which can be used to study the structure from the crust to the upper mantle beneath se tibet short and intermediate period 10 80 s phase velocities are prominently low suggesting that the crust and upper mantle beneath se tibet is characterized by slow shear wave propagation

the manifestation of the younger dryas event in the east asian summer monsoon margin new evidence from carbonate geochemistry of the dali lake sediments in northern china the processes and mechanisms of the younger dryas yd event in the modern northern margin of the east asian summer monsoon easm are still heatedly debated this study presents new high resolutio

studying co2 storage with ambient noise seismic interferometry a combined numerical feasibility study and field data example for ketzin germany seismic interferometry applied to ambient noise measurements allows the retrieval of the seismic response between pairs of receivers we studied ambient noise seismic interferometry ansi to retrieve time lapse reflection responses from a reservoir during co2 geologic sequestration using the case of the experimental site of ketzin germany we applied ansi to numerically modeled data to retrieve base and repeat reflection responses characterizing the impedances occurring at the reservoir both with and without the injection of co2 the modeled data represented global transmission responses from band limited noise sources randomly triggered in space and time we found that strong constraints on the spatial distribution of the passive sources were not required to retrieve the time lapse signal as long as sufficient source location repeatability was observed between the base and the repeat passive survey to illustrate the potential of the technique ansi was applied to three days of passive field data recorded in 2012 at ketzin comparison with the modeled results illustrated the potential to retrieve key reflection events using ansi on field data from ketzin this study supports the idea that the geologic setting and characteristics of ambient noise at ketzin may be opportune to monitor co2 sequestration

passive seismic imaging using microearthquakes we have developed a method of processing seismic signals generated by microearthquakes to image local subsurface structure beneath a recording station this technique uses the autocorrelation of the vertically traveling earthquake signals to generate pseudoreflection seismograms that can be interpreted for subsurface structure processed pseudoreflection data from microearthquakes recorded in the island of hawaii show consistent reflectivity patterns that are interpreted as near surface horizontal features forward modeling of the pseudoreflection data results in a p wave velocity model that shows reasonable agreement with the velocity model derived from a refraction study in the region usable signal to noise ratio is obtained down to 2 s a shear wave velocity model was also generated by applying this technique to horizontal component data

improved green s functions for passive source structural studies over the past two decades teleseismic receiver functions have proved to be a useful tool to investigate crustal structure because they represent a first order approximation to the s wave component of the teleseismic p green s function receiver functions provide valuable information on physical properties related to shear modulus however the implicit use of the p component seismogram as a proxy for the source precludes the recovery of information on discontinuous structure involving contrasts in compressional modulus by deconvolving improved estimates of complex source time functions generated by earthquakes one may move beyond the conventional receiver function paradigm to a more accurate approximation of the earth s green s function using a new deconvolution method we present estimates of the p component of the teleseismic p green s functions at several stations of the canadian national seismic network cnsn that clearly show the receiver side pure p wave crustal multiple the identificat

feasibility of borehole ambient noise interferometry for permanent reservoir monitoring the analysis of seismic ambient noise acquired during temporary or permanent microseismic monitoring campaigns e g improved enhanced oil recovery monitoring surveillance of induced seismicity is potentially well suited for time lapse studies based on seismic interferometry no additional data acquisition required ambient noise processing can be automatized to a high degree and seismic interferometry is very sensitive to small medium changes thus there is an opportunity for detection and monitoring of velocity variations in a reservoir at negligible additional cost and effort data and results are presented from an ambient noise interferometry study applied to two wells in a producing oil field in romania borehole microseismic monitoring on three component geophones was performed for four weeks concurrent with a water flooding phase for improved oil recovery from a reservoir in ca 1 km depth both low frequency 2 hz 50 hz p and s waves propagating through the vertical borehole arrays were reconstructed from ambient noise by the virtual source method the obtained interferograms clearly indicate an origin of the ambient seismic energy from above the arrays thus suggesting surface activities as sources it is shown that ambient noise from time periods as short as 30 seconds is sufficient to obtain robust interferograms sonic log data confirm that the vertical and horizontal components comprise first arrivals of p wave and s waves respectively the consistency and high quality of the interferograms throughout the entire observation period further indicate that the high frequency part up to 100 hz represents the scattered wave field the temporal variation of apparent velocities based on first arrival times partly correlates with the water injection rate and occurrence of microseismic events it is concluded that borehole ambient noise interferometry in production settings is a potentially useful method for permanent reservoir monitoring due to its high sensitivity and robustness

interferometric obs imaging for wide angle seismic data abstractmarine wide angle seismic data obtained using air guns and ocean bottom seismographs obss are effective for determining large scale subseafloor seismic velocities but they are ineffective for imaging details of shallow seismic reflection structures because of poor illumination surface related multiple reflections offer the potential to enlarge the obs data illumination area we have developed a new seismic imaging method for obs surveys applying seismic interferometry a technique that uses surface related multiples similarly to mirror imaging seismic interferometry can use higher order multiple reflections than mirror imaging which mainly uses first order multiple reflections a salient advantage of interferometric obs imaging over mirror imaging is that it requires only single component data whereas mirror imaging requires vertical geophone and hydrophone components to separate upgoing and downgoing wavefields we applied interferometric obs imaging to actual 175 km long wide angle obs da

a shallow seismic velocity model for the groningen area in the netherlands the province of groningen in the netherlands holds one of the world s largest natural gas fields and it has been an important source of energy for western europe for many decades the seismicity in recent years called for a better understanding of the local subsurface and therefore a dense network of 70 boreholes was installed in early 2015 each borehole is equipped with four geophones and a surface accelerometer in this study data from this network are used to determine the shallow velocity structure that is important for the quantification of the seismic hazard and accurate source localizations compressional and shear wave velocity profiles with uncertainties are derived for each of the 200 m deep boreholes using passive seismic interferometry applied to local event data the resulting seismic velocity distributions are presented as contour maps for 50 m depth intervals the maps show strong lateral variations where areas of low vp vs ratio correspond to regions of sedimentary infill the shear wave velocities were derived using the transverse component seismograms because the sensor orientations of the borehole geophones were unknown they had to be determined first this was done using a novel method based on cross correlations between the geophones and their colocated surface accelerometer in addition by extensive cross correlation analysis over the network several installation inconsistencies were identified and resolved

surface wave tomography from ambient seismic noise of accelerograph networks in southern korea 1 an efficient and robust cross correlation technique for estimation of surface wave green s function between two locations is applied to a set of ambient noise data recorded by accelerograph networks in the southern korea we have estimated 1843 group velocities of rayleigh waves in the period band of 2 4 seconds from correlation pairs of 91 accelerograph stations relatively dense and uniform coverage of the pairs provides us with high resolution tomographic image of surface wave group velocities in the region the resulting group velocity distribution correlates well with the characteristic features of geological map of the region displaying low velocities for the sedimentary basins and regions of volcanic and metasedimentary rocks and high velocities for the regions where granitoid basement rocks outcrop the short period information from the accelerograph network can enhance resolution of the subsurface structures at depths within a few kilometers which may be predominantly responsible for simulation results of strong ground motions

predicting nonlinear site response using spectral acceleration vs pgv vs30 a case history using the volvi test site in this study we analyze the efficiency of the ratio between particle velocity and shear wave velocity as a strain proxy for evaluating the nonlinear seismic response of sediments the in situ stress strain relationships are derived from accelerometric vertical array recordings at the tst site in volvi thessaloniki greece first the shear wave velocity between two successive sensors was computed by seismic interferometry and strain was computed as the velocity ratio or the relative displacement between sensors the shear wave velocity profile and in situ shear modulus degradation curve with strain were compared with previous studies performed at the tst site finally the stress strain relationships were derived from data recorded at the surface by extending the strain proxy and stress values to the ratio between peak ground velocity and the vs30 parameter used for site classification i e without requiring the accelerometric vertical array our model captures the in situ nonlinear response of the site without consideration of azimuth or distance of the earthquakes in conclusion the acceleration stress values based on the accelerometric response spectra instead of peak ground acceleration compared with the deformation strain proxy provide an effective model of the in situ nonlinear response providing information that can be integrated into ground motion prediction equations

on the use of microtremors for hydrocarbon detection the link of spectral anomalies of microtremors to underlying hydrocarbon reservoirs is very controversial as field experiments support both positive and negative opinions and there is not a solid theory supporting this work hypothesis we conducted field tests at different sites with and without oil and gas presence to add new experimental data to the ongoing studies microtremor information may become repeatable and so physically meaningful only when the observation duration exceeds a few days but even in this case factors such as topography and active faults may severely bias the signal ocean waves impinging the coasts provide natural background noise which stands out clearly when the observation time exceeds a dozen days or so in such a way that human noise is stacked out statistically over time microtremors recorded in relatively deep wells may provide useful information about ongoing production in a reservoir and may link well data and seismic surveys as their interferometric analysis can provide information comparable to vertical seismic profiles

surface wave array tomography in se tibet from ambient seismic noise and two station analysis ii crustal and upper mantle structure summary we determine the 3 d shear wave speed variations in the crust and upper mantle in the southeastern borderland of the tibetan plateau sw china with data from 25 temporary broad band stations and one permanent station interstation rayleigh wave phase velocity dispersion curves were obtained at periods from 10 to 50 s from empirical green s function egf derived from ambient noise interferometry and from 20 to 150 s from traditional two station ts analysis here we use these measurements to construct phase velocity maps from 10 to 150 s using the average interstation dispersion from the egf and ts methods between 20 and 50 s and estimate from them with the neighbourhood algorithm the 3 d wave speed variations and their uncertainty the crust structure parametrized in three layers can be well resolved with a horizontal resolution about of 100 km or less because of the possible effect of mechanically weak layers on regional deformation of particular interest is the existence and geometry of low shear velocity layers lvls in some regions prominent lvls occur in the middle crust in others they may appear in the lower crust in some cases the lateral transition of shear wave speed coincides with major fault zones the spatial variation in strength and depth of crustal lvls suggests that the 3 d geometry of weak layers is complex and that unhindered crustal flow over large regions may not occur consideration of such complexity may be the key to a better understanding of relative block motion and patterns of seismicity

broadband ambient noise surface wave tomography across the united states 1 this study presents surface wave dispersion maps across the contiguous united states determined using seismic ambient noise two years of ambient noise data are used from march 2003 through february 2005 observed at 203 broadband seismic stations in the us southern canada and northern mexico cross correlations are computed between all station pairs to produce empirical green functions at most azimuths across the us coherent rayleigh wave signals exist in the empirical green functions implying that ambient noise in the frequency band of this study 5 100 s period is sufficiently isotropically distributed in azimuth to yield largely unbiased dispersion measurements rayleigh and love wave group and phase velocity curves are measured together with associated uncertainties determined from the temporal variability of the measurements a sufficient number of measurements 2000 is obtained between 8 and 25 s period for love waves and 8 and 70 s period for rayleigh waves to produce tomographic dispersion maps both phase and group velocity maps are presented in these period bands resolution is estimated to be better than 100 km across much of the us from 8 40 s period for rayleigh waves and 8 20 s period for love waves which is unprecedented in a study at this spatial scale at longer and shorter periods resolution degrades as the number of coherent signals diminishes the dispersion maps agree well with each other and with known geological and tectonic features and in addition provide new information about structures in the crust and uppermost mantle beneath much of the us

seismic imaging of the site response using microearthquake recordings part ii application to the swabian jura southwest germany seismic network the subsurface site structure of the swabian jura southwest germany seismic network has been modeled from the inversion of locally recorded sh wave microearthquake seismograms the observed records have been deconvolved from the effects of apparent source pulse broadening due to the combined influence of frequency bandlimited source signals absorption peg leg multiple reflections and the recording system following the procedure of scherbaum and stoll 1985 corresponding pseudo reflection seismograms were calculated based on the kunetz claerbout relation for sh waves and inverted using a levison recursion algorithm the final structural models have been optimized in terms of numbers of layers and depth resolution using ferber s 1985 noise stabilization technique the resulting impedance models for the swabian jura southwest germany seismic network show some strongly correlated reflectors in the uppermost layers which are well understood in terms of the local stratigraphy the comparison of the inverted impedance model hsn with the p wave velocity log from a nearby exploration drilling trochtelfingen beb shows a high degree of similarity of the dominating reflectors down to a depth of several hundred meters as a consequence the coda generating process for the early part of the coda of the observed swabian jura microearthquake seismograms wood anderson magnitude mwa 3 0 can be completely interpreted as an effect of the local subsurface structure

constructing new seismograms from old earthquakes retrospective seismology at multiple length scales if energy emitted by a seismic source such as an earthquake is recorded on a suitable backbone array of seismometers source receiver interferometry sri is a method that allows those recordings to be projected to the location of another target seismometer providing an estimate of the seismogram that would have been recorded at that location since the other seismometer may not have been deployed at the time at which the source occurred this renders possible the concept of retrospective seismology whereby the installation of a sensor at one period of time allows the construction of virtual seismograms as though that sensor had been active before or after its period of installation here we construct such virtual seismograms on target sensors in both industrial seismic and earthquake seismology settings using both active seismic sources and ambient seismic noise to construct sri propagators and on length scales ranging over 5 orders of magnitude from 40 m to 2500 km in each case we compare seismograms constructed at target sensors by sri to those actually recorded on the same sensors we show that spatial integrations required by interferometric theory can be calculated over irregular receiver arrays by embedding these arrays within 2 d spatial voronoi cells thus improving spatial interpolation and interferometric results the results of sri are significantly improved by restricting the backbone receiver array to include approximately those receivers that provide a stationary phase contribution to the interferometric integrals finally we apply both correlation correlation and correlation convolution sri and show that the latter constructs fewer nonphysical arrivals

deep structure of the nojima fault southwest japan estimated from borehole observations of fault zone trapped waves abstract to estimate the deep structure of the southern part of the nojima fault southwest japan without the influence of near surface structures we analyzed the love wave type fault zone trapped waves ltws recorded by a borehole seismometer at 1800 m depth we examined the polarization dispersion and dominant frequency of the wavetrain following the direct s wave in each seismogram to identify the ltw we selected eight candidates for typical ltws from 462 records because the duration of the ltw increases with hypocentral distance we infer that the low velocity fault zone of the nojima fault continues towards the seismogenic depth in addition since the duration of the ltw increases nonlinearly with hypocentral distance we infer that the s wave velocity of the fault zone increases with depth the location of events showing the ltw indicates that the fault zone dips to the southeast at 75 and continues to a depth of approximately 10 km we assumed a uniform low velocity waveguide to estimate the average structure of the fault zone we estimated the average width s wave velocity and q s of the fault zone by comparing an analytical solution of the ltw with measured data the average width s wave velocity and q s of the fault zone are 150 to 290 m 2 5 to 3 2 km s and 40 to 90 respectively hence the fault zone structure with a larger width and smaller velocity reduction than the fault zone model estimated by previous surface observation is more suitable to represent the average fault zone structure of the nojima fault the present study also indicated that the shallow layers and or a shallow fault zone structure drastically changes the characteristics of the ltw recorded at the surface and therefore cause a discrepancy in the fault zone model between the borehole observation and surface observation

application of seismic interferometry to extract p and s wave propagation and observation of shear wave splitting from noise data at cold lake alberta canada we extract downward propagating p and s waves from industrial noise generated by human and or machine activity at the surface propagating down a borehole at cold lake alberta canada and measure shear wave splitting from these data the continuous seismic data are recorded at eight sensors along a downhole well during steam injection into a 420 470 m deep oil reservoir we crosscorrelate the waveforms observed at the top sensor and other sensors to extract estimates of the direct p and s wave components of the green s function that account for wave propagation between sensors fast high frequency and slow low frequency signals propagating vertically from the surface to the bottom are found for the vertical and horizontal components of the wave motion which are identified with p and s waves respectively the fastest s wave polarized in the east northeast west southwest direction is about 1 9 faster than the slowest s wave polarized in the northwest southeast direction the direction of polarization

salt flank delineation by interferometric imaging of transmitted p to s waves we describe how vertical seismic profile vsp interferometric imaging of transmitted p to s ps waves can be used to delineate the flanks of salt bodies unlike traditional migration methods interferometric ps imaging does not require the migration velocity model of the salt and or upper sediments in order to image the salt flank synthetic elastic examples show that ps interferometric imaging can clearly delineate the upper and lower boundaries of a realistic salt body model results also show that ps interferometric imaging is noticeably more accurate than conventional migration methods in the presence of static shifts and or migration velocity errors however the illumination area of the ps transmitted waves is limited by the width of the shot and geophone aperture which means wide shot offsets and deeper receiver wells are needed for comprehensive salt flank imaging interferometric imaging results for vsp data from the gulf of mexico demonstrate its superiority over the traditional migration method we also discuss other arrivals that can be used for interferometric imaging of salt flanks for comparison reduced time migration results are presented which are similar in quality to those obtained for interferometric imaging we conclude that ps interferometric imaging of vsp data provides the geophysicist with a new tool by which salt flanks can be viewed from both above and below vsp geophone locations

reflection images from ambient seismic noise one application of seismic interferometry is to retrieve the impulse response green s function from crosscorrelation of ambient seismic noise various researchers show results for retrieving the surface wave part of the green s function however reflection retrieval has proven more challenging we crosscorrelate ambient seismic noise recorded along eight parallel lines in the sirte basin east of ajdabeya libya to obtain shot gathers that contain reflections we take advantage of geophone groups to suppress part of the undesired surface wave noise and apply frequency wavenumber filtering before crosscorrelation to suppress surface waves further after comparing the retrieved results with data from an active seismic exploration survey along the same lines we use the retrieved reflection data to obtain a migrated reflection image of the subsurface

seismic interferometry with a tbm source of transmitted and reflected waves we discuss the use of autocorrelogram interferometry by using noise from the tunnel boring machine tbm the tbm provides seismic signals waves while drilling in a tunnel tswd the tunnel geometry unlike a reverse vertical seismic profile rvsp using a drill bit makes it possible to record the waves reflected from the region between the tunnel face and the projected tunnel exit and those transmitted ahead of the tunnel face we processed the waves recorded at back positions with respect to the tbm in a manner similar to a rvsp data set obtained by conventional reference correlation techniques we processed the waves transmitted ahead of the tbm using autocorrelogram interferometry techniques using these wavefields offers advantages over conventional borehole drill bit vertical seismic profiles vsps the most important advantage is getting reflections from the transmitted front wavefield by utilizing kunetz s equation and reversed time traces the approach also improves the analysis of the trans

surface wave tomography from microseisms in southern california received 5 april 2005 revised 23 may 2005 accepted 9 june 2005 published 26 july 2005 1 since it has already been demonstrated that point topoint seismic propagation green functions can be extracted from seismic noise it should be possible to image earth structure using the ambient noise field seismic noise data from 148 broadband seismic stations in southern california were used to extract the surface wave arrival times between all station pairs in the network the seismic data were then used in a simple but densely sampled tomographic procedure to estimate the surface wave velocity structure within the frequency range of 0 1 0 2 hz for a region in southern california the result compares favorably with previous estimates obtained using more conventional and elaborate inversion procedures this demonstrates that coherent noise field between station pairs can be used for seismic imaging purposes citation sabra k g p gerstoft p roux w a kuperman and m c fehler 2005 surface wave tomography from microseisms in southern california geophys res lett 32 l14311 doi 10 1029 2005gl023155

retrieval of local surface wave velocities from traffic noise an example from the la barge basin wyoming abstract in regions where active source seismic exploration is constrained by limitations of energy penetration and recovery cost and logistical concerns or regulatory restrictions analysis of natural source seismic data may provide an alternative in this study we investigate the feasibility of using locally generated seismic noise in the 2 6 hz band to obtain a subsurface model via interferometric analysis we apply this technique to three component data recorded during the la barge passive seismic experiment a local deployment in south western wyoming that recorded continuous seismic data between november 2008 and june 2009 we find traffic noise from a nearby state road to be the dominant source of surface waves recorded on the array and observe surface wave arrivals associated with this source up to distances of 5 kms the orientation of the road with respect to the deployment ensures a large number of stationary points leading to clear observations on both in line and cross line virtual source receiver pairs this results in a large number of usable interferograms which in turn enables the application of standard active source processing methods like signal processing common offset stacking and traveltime inversion we investigate the dependency of the interferograms on the amount of data on a range of processing parameters and on the choice of the interferometry algorithm the obtained interferograms exhibit a high signal to noise ratio on all three components rotation of the horizontal components to the radial transverse direction facilitates the separation of rayleigh and love waves though the narrow frequency spectrum of the surface waves prevents the inversion for depth dependent shear wave velocities we are able to map the arrival times of the surface waves to laterally varying group and phase velocities for both rayleigh and love waves our results correlate well with the known geological structure we outline a scheme for obtaining localized surface wave velocities from local noise sources and show how the processing of passive data benefits from a combination with well established exploration seismology methods we highlight the differences with interferometry applied to crustal scale data and conclude with recommendations for similar deployments

time distance helioseismology the application of seismology to the study of the solar interior1 2 helioseismology has advanced almost solely by the prediction and measurement of the sun s frequencies of free oscillation or normal modes direct measurement of the travel times and distances of individual acoustic waves the predominant approach in terrestrial seismology3 would appear to be more difficult in view of the number and stochastic nature of solar seismic sources here however we show that it is possible to extract time distance information from temporal cross correlations of the intensity fluctuations on the solar surface this approach opens the way for seismic studies of local solar phenomena such as subsurface in homogeneities near sunspots and should help to refine global models of the internal velocity stratification in the sun

temporal variations of seismic velocity at paradox valley colorado using passive image interferometry in our recent article ugalde et al 2014 we applied the passive image interferometry technique of sens schonfelder and wegler 2006 to monitor seismic velocity changes in paradox valley southwestern colorado for this purpose we computed the autocorrelation

ambient noise rayleigh wave tomography in western sichuan and eastern tibet abstract we perform rayleigh wave tomography in western sichuan and eastern tibet by applying the ambient noise method to broadband data recorded at 49 stations from the sichuan digital seismic network cross correlations of vertical component ambient noise data are computed in one hour segments and stacked over 7 months from june to december 2007 then rayleigh wave group dispersion curves from 6 to 48 s were measured using the multiple filter analysis method group velocity maps at periods from 8 to 44 s were inverted the resulting group velocity maps generally show good correlations with tectonic features the sichuan basin is clearly resolved with lower group velocities at the short periods 8 to 20 s due to thick sedimentary layers and a prominent low velocity zone exhibits in the middle crust beneath the eastern margin of the tibetan plateau the longmen shan thrust belt marks not only the rapid lateral velocity variation between the topographically high tibetan plateau and low elevation sichuan basin but also the transition from the thick crust beneath the tibetan plateau to the normal continental crust beneath the sichuan basin

lunar subsurface investigated from correlation of seismic noise by correlating seismic noise recorded by four sensors placed on the moon during the apollo 17 mission we have retrieved a well defined dispersed rayleigh wave pulse inversion of its group velocity provides new constraints on the lunar subsurface structure the estimated signal to noise ratio snr of the retrieved rayleigh wavetrain is strongly dependent on solar illumination effectively making solar heating a source of seismic noise on the moon this result suggests that in future planetary missions it is feasible to extract information on the internal structure of extraterrestrial objects by correlating seismic noise even when natural quakes are absent

testing community velocity models for southern california using the ambient seismic field abstract we correlate the vertical component of ambient seismic noise data recorded on 56 broadband stations with dense coverage in the greater los angeles area to determine station to station green s functions these green s functions provide an important test of community velocity models southern california earthquake center scec cvm 4 0 and cvm h 5 2 used for strong ground motion prediction for future scenario earthquakes in southern california comparisons of the ambient noise green s functions for nearly 300 paths with those calculated by the finite element method in the community velocity models reveal a strong waveform similarity for the dominant surface waves between 0 1 and 0 2 hz we find a mean correlation coefficient between the ambient noise and finite element green s functions of 0 62 for the cvm 4 0 and 0 49 for the cvm h 5 2 indicating stronger waveform similarity for cvm 4 0 we also find that for 77 of the paths the surface waves in the finite element green s functions for cvm 4 0 arrive early suggesting that the cvm 4 0 has velocities in the upper 10 km that are too fast along these paths the same bias is evident for cvm h 5 2 but is substantially smaller with only 61 of the paths too fast for 67 of the paths cvm 4 0 has velocities faster than cvm h 5 2 the time lags we obtain between the ambient noise and finite element green s functions provide key information for improving future community velocity models

correlation migration using gaussian beams of scattered teleseismic body waves correlation migration for structural imaging using gaussian beams is described for the inversion of passively recorded teleseismic waves gaussian beam migration is based on an overcomplete frame based representation of the seismic wave field and uses localized slant stack windows of the data paraxial gaussian beams are then utilized for the backpropagation of the seismic waves into the me dium the method can provide stable imaging of seismic data in smoothly varying background media where caustics and triplicated arrivals exist we develop gaussian beam migration for structural imaging which allows for incident teleseismic waves from beneath the structure using directly scattered or surface reflected phases the method is applied to synthetic data computed for a collisional zone model and the inversions show that the gaussian beam migration can image structure using passive teleseismic data the method is next applied to synthetic data that have been con volved with an observed pulse from the 1993 cascadia experiment the autocorre lation is computed and the autocorrelated data are migrated by using gausian beams for direct p to s scattered waves and surface reflected waves although the migra tions of the autocorrelation data with the source pulse included have more noise than the migrations of the impulsive source data the subduction zone structure can still be clearly identified in the migrated results without removal of the source pulses

coseismic and postseismic elastic wave velocity variations caused by the 2008 iwate miyagi nairiku earthquake japan 1 we analyze coseismic and postseismic velocity variations caused by the june 13 2008 iwate miyagi nairiku earthquake mw 6 9 using passive image interferometry pii seismic noise is correlated in order to reconstruct the green s function between two sensors shear wave velocity changes are determined by relating the coda parts of the daily green s functions to a long term reference green s function our study extends from january 2008 to august 2010 and includes the correlations for 190 station pairs in three different frequency ranges from 0 125 to 1 0 hz we show that combining the 9 different component cross correlation functions stabilizes the velocity change estimation and increases analysis resolution the observed velocity change curves can be fitted by model time series consisting of a coseismic velocity drop followed by logarithmic postseismic recovery and seasonal velocity variations the coseismic velocity drops are stronger at higher frequencies and are concentrated in the southern part of the fault zone a tomography algorithm was developed to reproject the observed velocity variations of the different sensor pairs onto the single sensors the depth distribution of coseismic changes was modeled for the three stations with the largest velocity drops at two stations the coseismic velocity changes are located in the upper several hundred meters the third station shows indications for deeper changes in the order of kilometers postseismic recovery takes significantly longer than the analyzed two year period seasonal velocity variations with periods of one year are observed at all analyzed frequencies for most station pairs

seismic interferometry of teleseicmic s wave coda for retrieval of body waves an application to the philippine sea slab underneath the japanese islands summary the reconstruction of surface waves from the cross correlation of random wavefields has recently been extensively inspected by theoretical and experimental approaches however the reconstruction of body waves has not been extensively studied in this study we present a method for extracting body waves that is direct p and s waves and reflected waves from the philippine sea slab we use the cross correlation of the wavefield generated by the teleseismic s waves observed by hi net tiltmeters with a passband of 0 07 0 5 hz to enhance the contribution of the s coda we reduced the source time function and the deterministic phases ss scs and ss and the surface wave in s coda the records of the processed s coda observed at every station pair are cross correlated for each teleseismic event the cross correlation functions ccfs of different earthquakes are stacked to retrieve wavefields propagating between the station pairs as a result direct p s and the reflected waves could be extracted when the stacked ccfs are aligned as a function of distance of separation between two stations the gradients of the traveltime curves for the direct p and s waves are approximately 5 7 and 3k m s 1 respectively these velocities correspond to the seismic velocities of the crust or the p wave velocity of the uppermost mantle in order to enhance the reflected waves we searched for the reflection points by assuming that the later phases in the ccfs are ss reflections and map the amplitudes onto the depth sections as a result the negative phases dipping to the north can be traced right below the region of hypocentre distribution these negative phases probably correspond to the oceanic moho within the philippine sea slab we also show that the oceanic moho can also be traced by assuming pp reflections these results indicate that the ccfs plausibly contain information regarding both p and s waves propagating between the two receivers and are capable of detecting reflected phases in addition to the direct waves

the virtual refraction useful spurious energy in seismic interferometry seismic interferometry is rapidly becoming an established technique to recover the green s function between receivers but practical limitations in the source energy distribution inevitably lead to spurious energy in the results instead of attempting to suppress all such energy we use a spurious wave associated with the crosscorrelation of refracted energy at both receivers to infer estimates of subsurface parameters we named this spurious event the virtual refraction illustrated by a numerical two layer example we found that the slope of the virtual refraction defines the velocity of the faster medium and that the stationary phase point in the correlation gather provides the critical offset with the associated critical time derived from the real shot record this approach includes all of the necessary information to estimate wave speeds and interface depth without the need of inferences from other wave types

green s functions extraction and surface wave tomography from microseisms in southern california we use crosscorrelations of seismic noise data from 151 stations in southern california to extract the group velocities of surface waves between the station pairs for the purpose of determining the surface wave velocity structure we developed an automated procedure for estimating the green s functions and subsequent tomographic inversion from the 11 325 station pairs based on the characteristics of the noise field we eliminate specific events by a procedure that does not introduce any spurious spectral distortion in the band of interest 0 05 0 2 hz further we only used the emerging arrival structure above a threshold signal to noise ratio the result is that mostly station pairs with their axes oriented toward the sea are used consistent with the noise having a microseism origin finally it is the time derivative of the correlation function that is actually related to the green s function the emergence of the time domain green s function is proportional to the square root of the recording time and inversely proportional to the square root of the distance between stations the tomographic inversion yields a surface wave velocity map that compares favorably with more conventional and elaborate experimental procedures

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

migration methods for imaging different order multiples multiples contain valuable information about the subsurface and if properly migrated can provide a wider illumination of the subsurface compared to imaging with vsp primary reflections in this paper we review three different methods for migrating multiples the first method is model based and it is more sensitive to velocity errors than primary migration the second method uses a semi natural green s function for migrating multiples where part of the traveltimes are computed from the velocity model and part of the traveltimes i e natural traveltimes are picked from the data to construct the imaging condition for multiples the third method uses cross correlation of traces the last two methods are preferred in the sense that they are significantly less sensitive to velocity errors and statics because they use natural data to construct part of the migration imaging conditions compared with the interferometric i e crosscorrelation imaging method the semi natural green s function method is more computationally efficient and is sometimes less prone to migration artifacts numerical tests with 2 d and 3 d vsp data show that a wider subsurface coverage higher fold and more balanced illumination of the subsurface can be achieved with multiple migration compared with migration of primary reflections only however there can be strong interference from multiples with different orders or primaries when multiples of high order are migrated one possible solution is to filter primaries and different orders of multiples before migration and another possible solution is least squares migration of all events a limitation of multiple migration is encountered for subsalt imaging here the multiples must pass through the salt body more than twice which amplifies the distortion of the image