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

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

drill bit seismic interferometry with and without pilot signals we use different interferometry approaches to process the seismic signals generated by a drill bit source in one well and recorded by seismic receivers located both in a second borehole and at the surface near the source well we compare the standard interferometry results obtained by using the raw drill bit data without a pilot signal with the new interferometry results obtained by using the drill bit seismograms correlated with a reference pilot signal the analysis of the stationary phase shows that the final results have different s n levels and are affected by the coherent noise in the form of rig arrivals the interferometry methods are compared by using different deconvolution approaches the analysis shows that the results agree with the conventional drill bit seismograms and that using the reference pilot signal improves the quality of the drill bit wavefields redatumed by the interferometry method

using stable isotopes as natural tracers to delineate hydrological drainage basins on the greenland ice sheet margin abstract stable isotope studies of surface ice and water samples collected in the ablation zone of the greenland ice sheet margin at paakitsup akliarusersua 40 km northeast of ilulissat jakobshavn west greenland show that there is a large system atic spatial variation of the 18 o 16 o ratio 18 o over the ablation area 18 o can therefore be used as a natural tracer to locate the origin of the melt water runoff a simple model is set up for simulating the 18 o variation in the melt water discharge from the area simulations are made with a drainage basin obtained by modelling the subglacial drainage conditions which requires costly and time consuming mapping of the subglacial topography the simulated values agree well with observed values of water samples from three pro glacial lakes which receive the melt water discharge this suggests that the stable isotope method can be used to give an initial estimate of melt water drainage basin boundaries on the greenland ice sheet margin

finite difference modeling experiments for seismic interferometry in passive seismic interferometry new reflection data can be retrieved by crosscorrelating recorded noise data the quality of the retrieved reflection data is among others dependent on the duration and number of passive sources present during the recording time the source distribution and the source strength to investigate these relations we set up several numerical modeling studies to carry out the modeling in a feasible time we design a finite difference algorithm for the simulation of long duration passive seismic measurements of band limited noise signatures in the subsurface novel features of the algorithm include the modeling of thousands of randomly placed sources during one modeling run the modeling experiments explore the dependency relation between the retrieved reflections and source signature length source positions number of sources and source amplitude variations from these experiments we observed that the positions of the passive sources and the length of the source signals are of direct influence on the quality of the retrieved reflections random amplitude variations among source signals do not seem to have a big impact on the retrieved reflections

s wave velocity images of the dead sea basin provided by ambient seismic noise abstract based on passive seismic interferometry applied to ambient seismic noise recordings between station pairs belonging to a small scale array we have obtained shear wave velocity images of the uppermost materials that make up the dead sea basin we extracted empirical green s functions from cross correlations of long term recordings of continuous data and measured inter station rayleigh wave group velocities from the daily correlation functions for positive and negative correlation time lags in the 0 1 0 5 hz bandwidth a tomographic inversion of the travel times estimated for each frequency is performed allowing the laterally varying 3 d surface wave velocity structure below the array to be retrieved subsequently the velocity frequency curves are inverted to obtain s wave velocity images of the study area as horizontal depth sections and longitude and latitude depth sections the results which are consistent with other previous ones provide clear images of the local seismic velocity structure of the basin low shear velocities are dominant at shallow depths above 3 5 km but even so a spit of land with a depth that does not exceed 4 km is identified as a salt diapir separating the low velocities associated with sedimentary infill on both sides of the lisan peninsula the lack of low speeds at the sampling depth of 11 5 km implies that there are no sediments and therefore that the basement is near 10 11 km depth but gradually decreasing from south to north the results also highlight the bowl shaped basin with poorly consolidated sedimentary materials accumulated in the central part of the basin the structure of the western margin of the basin evidences a certain asymmetry both whether it is compared to the eastern margin and it is observed in north south direction infill materials down to 8 km depth are observed in the hollow of the basin unlike what happens in the north and south where they are spread beyond the western dead sea shore

drill bit signal separation for rvsp using statistical independence drill bit noise and rvsps time and depth migration of seismic data usually is based on velocities estimated from surface measurements the actual velocities which calibrate the depth migrated seismic to the well data are determined after the well is drilled using check shots and vsps by then it may be too late to correct for seismic migration velocity imprecision and impact the well s success the earlier we can obtain such velocity information the more time will be available for reprocessing remigration of either the seismic or any depth maps if necessary it is therefore very useful to determine the velocities to calibrate seismic and well velocities while drilling this allows us to better locate the drill bit in time on the seismic section to redirect the well if necessary and to gather as much further information as possible on the rocks ahead of the bit

the effect of a low viscosity layer on convection in the mantle summary in this paper we attempt to find a solution of the following problem it appears reasonable to expect that if thermal convection occurs in the earth s mantle it may also occur within the moon and mars the dimensions of these latter two bodies are comparable to the thickness of the earth s mantle presumably the amount of radioactive heat generated per unit mass is similar in all three bodies yet the surface morphology of the earth which many scientists believe arises ultimately from mantle convection differs markedly from that of the moon or mars the explanation advanced here for this difference is based on the effect produced on convection in the mantle by the presence of a low viscosity or low creep strength layer it is assumed that the low velocity layer of the mantle is such a low creep strength layer a low viscosity layer changes the amount of coupling between the outer crust and mantle convection the crust is coupled if mantle convection produces stresses in the crust which are large enough to deform it plastically the crust is decoupled from the mantle if these stresses are insufficient to produce plastic deformation the theory assumes that the viscosity or creep strength is essentially zero in the low viscosity layer the analysis is similar to that developed earlier for the calculation of stresses within the mantle we find that the deeper within the mantle lies the low viscosity layer the greater is the coupling of the outer crust to the mantle convection currents if the low creep strength layer lies close to the surface the outer crust is decoupled from the interior according to the literature the depth of the low velocity layer is determined by the temperature and pressure profiles within the mantle if the temperature profile were kept constant but the pressure reduced by changing g the gravitational acceleration the low velocity zone would be moved to a level closer to the surface it is proposed that because of this interdependence between temperature pressure and depth of the low velocity layer the low velocity layers in mars and the moon if such layers exist in these bodies lie much closer to the surface than does the corresponding layer in the earth hence the outer crusts of mars and the moon are more nearly decoupled from the interior than is the earth s crust

thermal convection and the origin of ice streams ice streams are a fact of ice sheet dynamics draining up to 90 of the ice thermal convection in ice below the density inversion is a speculation an attempt is made to meld the two in such a way that the speculation becomes an explanation for the fact

seismic exploration scale velocities and structure from ambient seismic noise 1 hz the successful surface waves retrieval in solid earth seismology using long time correlations and subsequent tomographic images of the crust have sparked interest in extraction of subsurface information from noise in the exploration seismology subsurface information in exploration seismology is usually derived from body wave reflections 1 hz which is challenging for utilization of ambient noise we use 11 h of noise recorded in the sirte basin libya first we study the characteristics of the noise we show that the bulk of the noise is composed of surface waves at frequencies below 6 hz some noise panels contain nearly vertically traveling events we further characterize these events using a beamforming algorithm from the beamforming we conclude that these events represent body wave arrivals with a fairly rich azimuthal distribution having body wave arrivals in the noise is a prerequisite for body wave reflections retrieval we crosscorrelate and sum the recorded ambient noise panels to retrieve common source gathers following two approaches using all the noise and using only noise panels containing body wave arrivals likely to contribute to the reflections retrieval comparing the retrieved gathers with active seismic data we show that the two way traveltimes at short offsets of several retrieved events coincide with those of reflections in the active data and thus correspond to apexes of reflections we then compare retrieved stacked sections of the subsurface from both approaches with the active data stacked section and show that the reflectors are consistent along a line the results from the second approach exhibit the reflectors better

virtual seismometers in the subsurface of the earth from seismic interferometry earthquakes often occur in areas that lack an array of seismometers resulting in a scarcity of local measurements from some regions of great geological interest in such regions some earthquakes themselves may be turned into virtual seismometers that are capable of measuring strain caused by passing waves from other earthquakes

in situ observations of velocity changes in response to tidal deformation from analysis of the high frequency ambient wavefield we report systematic seismic velocity variations in response to tidal deformation measurements are made on correlation functions of the ambient seismic wavefield at 2 8 hz recorded by a dense array at the site of the pinon flat observatory southern california the key observation is the dependence of the response on the component of wave motion and coda lapse time measurements on the vertical correlation component indicate reduced wave speeds during periods of volumetric compression whereas data from horizontal components show the opposite behavior compatible with previous observations these effects are amplified by the directional sensitivities of the different surface wave types constituting the early coda of vertical and horizontal correlation components to the anisotropic behavior of the compliant layer the decrease of the velocity volumetric strain sensitivity s with indicates that this response is constrained to shallow depths the observed velocity dependence on strain implies nonlinear behavior but conclusions regarding elasticity are more ambiguous the anisotropic response is possibly associated with inelastic dilatancy of the unconsolidated low velocity material above the granitic basement however equal polarity of vertical component velocity changes and deformation in the vertical direction indicate that a nonlinear poisson effect is similarly compatible with the observed response pattern peak relative velocity changes at small are 0 03 which translates into an absolute velocity strain sensitivity of s 5 10 3 and a stress sensitivity of 0 5 mpa 1 the potentially evolving velocity strain sensitivity of crustal and fault zone materials can be studied with the method introduced here

tracer transport in the greenland ice sheet three dimensional isotopic stratigraphy abstract polar ice cores can provide both a record of climate history and a sharp test of the performance of numerical ice dynamics models the stratigraphic structure of an ice sheet is an expression of its full depositional and dynamic history and thus presents a greater challenge to computer models than merely matching the contemporary ice thickness and areal extent we describe a coupled model of ice and tracer dynamics that is realized by adding a semi lagrangian tracer transport scheme to a conventional thermomechanical ice dynamics model model skill is demonstrated by using ice core data from the grip site near summit greenland to successfully predict the isotopic stratigraphy of ice cores at other deep drilling sites the success of this effort indicates that when compensated for the effects of ice flow and elevation all the deep cores relate a coherent glacial history over the past 120 000 years according to the simulation results the oldest greenland ice lies beneath the grip gisp2 and northgrip sites although comparably old ice may also be found in north greenland and east central greenland

acoustical properties of drill strings the recovery of petrochemical and geothermal resources requires extensive drilling of wells to increasingly greater depths real time collection and telemetry of data about the drilling process while it occurs thousands of feet below the surface is an effective way of improving the efficiency of drilling operations unfortunately due to hostile down hole environments telemetry of this data is an extremely difficult problem currently commercial systems transmit data to the surface by producing pressure pulses within the portion of the drilling mud enclosed in the hollow steel drill string transmission rates are between 2 and 4 data bits per second any system capable of raising data rates without increasing the complexity of the drilling process will have significant economic impact one alternative system is based upon acoustical carrier waves generated within the drill string itself if developed this method would accommodate data rates up to 100 bits per second unfortunately the drill string is a periodic structure of pipe and threaded tool joints the transmission characteristics are very complex and exhibit a banded and dispersive structure over the past 40 years attempts to field systems based upon this transmission method have resulted in little success this article examines this acoustical transmission problem in great detail the basic principles of acoustic wave propagation in the periodic structure of the drill string are examined through theory laboratory experiment and field test the results indicate the existence of frequency bands that are virtually free of attenuation and suitable for data transmission at high bit rates

retrieving virtual reflection responses at drill bit positions using seismic interferometry with drill bit noise in the field of seismic interferometry researchers have retrieved surface waves and body waves by cross correlating recordings of uncorrelated noise sources to extract useful subsurface information the retrieved wavefields in most applications are between receivers when the positions of the noise sources are known inter source interferometry can be applied to retrieve the wavefields between sources thus turning sources into virtual receivers previous applications of this form of interferometry assume impulsive point sources or transient sources with similar signatures we investigate the requirements of applying inter source seismic interferometry using non transient noise sources with known positions to retrieve reflection responses at those positions and show the results using synthetic drilling noise as source we show that if pilot signals estimates of the drill bit signals are not available it is required that the drill bit signals are the same and that the phases of the virtual reflections at drill bit positions can be retrieved by deconvolution interferometry or by cross coherence interferometry further for this case classic interferometry by cross correlation can be used if the source power spectrum can be estimated if pilot signals are available virtual reflection responses can be obtained by first using standard seismic while drilling processing techniques such as pilot cross correlation and pilot deconvolution to remove the drill bit signatures in the data and then applying cross correlation interferometry therefore provided that pilot signals are reliable drill bit data can be redatumed from surface to borehole depths using this inter source interferometry approach without any velocity information of the medium and we show that a well positioned image below the borehole can be obtained using interferometrically redatumed reflection responses with just a simple velocity model we discuss some of the practical hurdles that restrict the application of the proposed method offshore

retrieval of moho reflected shear wave arrivals from ambient seismic noise theoretical studies on ambient seismic noise asn predict that complete green s function between seismic stations can be retrieved from cross correlation however only fundamental mode surface waves emerge in most studies involving real data here we show that moho reflected body wave sms and its multiples can be identified with asn for station pairs near their critical distances in the short period band 1 5 s we also show that an uneven distribution of noise sources such as mining activity and wind topography interaction can cause surface wave precursors which mask weaker body wave phases

shear wave imaging from traffic noise using seismic interferometry by cross coherence we apply the cross coherence method to the seismic interferometry of traffic noise which originates from roads and railways to retrieve both body waves and surface waves our preferred algorithm in the presence of highly variable and strong additive random noise uses cross coherence which uses normalization by the spectral amplitude of each of the traces rather than crosscorrelation or deconvolution this normalization suppresses the influence ofadditive noise and overcomes problems resulting from amplitudevariations among input traces by using only the phase information and ignoring amplitude information the method effectively removes the source signature from the extracted response and yields a stable structural reconstruction even in the presence of strong noise this algorithm is particularly effective where the relative amplitude among the original traces is highly variable from trace to trace we use the extracted reflected shear waves from the traffic noise data to construct a stackedandmigratedimage andweusetheextractedsurfacewaves love waves to estimate the shear velocity as a function of depth this profile agrees well with the interval velocity obtained from the normal moveout of the reflected shear waves constructed by seismic interferometry these results are useful in a wide range of situations applicable to both geophysics and civil engineering

a comparison of coherency measurement using semblance and multiple signal classification from a seismic while drilling perspective abstracta diamond drill bit is usually considered to be an inadequate seismic vibration source to detect and use weak drill bit generated seismic wavefields in hard rock drilling we compared different coherency measures between a conventional method of semblance and a generalized multiple signal classification music algorithm we tested the detectability and resolution differences between semblance and music with synthetic examples music coherency has the advantage of higher resolution over semblance measurement when the source wavefronts are accurately predicted in addition we applied both methods to detect the coherent moveout of a diamond drill bit signal from a hard rock seismic while drilling experiment at hillside south australia we used the coherent moveout to estimate the overburden velocity around the borehole we also performed interferometry migration using the coherency measurements to image the drill bit position our analysis determined that the direct waves generated from a diamond

comparison between greenland ice margin an ice core oxygen 18 records old ice for paleoenvironmental studies retrieved by deep core drilling in the central regions of the big ice sheets can also be retrieved from the ice sheet margins the d18o content of the surface ice was studied at 15 different greenland ice margin locations at some locations two or more records were obtained along closely spaced parallel sampling profiles showing good reproducibility of the records we present ice margin d18o records reaching back into the pleistocene many of the characteristic d18o variations known from greenland deep ice cores can be recognized allowing an approximate time scale to be established along the ice margin records a flow line model is used to determine the location on the ice sheet where the margin ice was originally deposited as snow the pleistocene holocene d18o change at the deposition sites is determined by comparing the d18o values in the ice margin record to the present 18o values of the surface snow at the deposition sites on the northern slope of the greenland ice sheet the pleistocene holocene d18o change is c 10 per mil in contrast to a change of 6 7 per mil at locations near the central ice divide this is in accordance with deep ice core results we conclude that d18o records measured on ice from the greenland ice sheet margin provide useful information about past climate and dynamics of the ice sheet and thus are important and cheap supplements to deep ice core records

radiation pattern and seismic waves generated by a working roller cone drill bit the seismic body wave radiation pattern of a working roller cone drill bit can be characterized by theoretical modeling and field data examples our model of drill bit signal generation is a pseudo random series of bit tooth impacts that create both axial forces and tangential torques about the borehole axis each drill tooth impact creates an extensional wave that travels up the drill string and body waves that radiate into the earth the model predicts that p waves radiate primarily along the axis of the borehole and shear waves radiate primarily perpendicular to the borehole axis in a vertical hole the largest p waves will be recorded directly above and below the drill bit whereas the largest shear waves will be recorded in a horizontal plane containing the drill bit in a deviated borehole the radiation patterns should be rotated by the inclination angle of the drill bit this proposed seismic body wave radiation pattern is investigated with field data examples the presence of the drill string

the use of drill bit energy as a downhole seismic source a new wellbore seismic technique uses the vibrations produced by a drill bit while drilling as a downhole seismic energy source the technique is described as inverse vsp because the source and receiver positions of conventional vsp are reversed no downhole instrumentation is required to obtain the data and the data recording does not interfere with the drilling process these characteristics offer a method by which borehole seismic data can be acquired processed and interpreted while drilling interchanging the conventional vsp source and receiver positions improves the efficiency of recording multioffset surveys for imaging a 3 d data volume in the borehole vicinity the continuous signals generated by the drill bit are recorded by a pilot sensor attached to the top of the drillstring and by receivers located at selected positions around the borehole the pilot signal is crosscorrelated with the receiver signals to compute traveltimes of the arrivals and to attenuate incoherent noise deconvolution and time shifts of the pilot signal compensate for the effects of propagation from the drill bit to the top of the drillstring by repeating this process for an interval of the well a vsp equivalent data set is generated results from a test well demonstrate that the processed drill bit data are comparable to conventional vsp data

mantle heterogeneities and the scec reference three dimensional seismic velocity model version 3 we determine upper mantle seismic velocity heterogeneities below southern california from the inversion of teleseismic travel time residuals teleseismic p wave arrival times are obtained from three temporary passive experiments and southern california seismic network scsn stations producing good raypath coverage the inversion is performed using a damped least squares conjugate gradient method lsqr the inversion model element spacing is 20 km before the inversion the effects of crustal velocity heterogeneities represented by the southern california earthquake center scec seismic velocity model version 2 are removed from the teleseismic travel times the p wave inversion produces a variance reduction of 43 s wave velocities are determined from laboratory vp vs ratios the most prominent features imaged in the results are high p wave velocities 3 in the uppermost mantle beneath the northern los angeles basin and the previously reported tabular high velocity anomaly 3 to depths of 200 km beneath the transverse ranges crosscutting the san andreas fault we incorporate the upper mantle seismic velocity heterogeneities into the scec southern california reference seismic velocity model the prior accounting for the crustal velocity heterogeneity demonstrates the utility of the top down method of the scec seismic velocity model development

inter source body wave propagations derived from seismic interferometry summary seismicinterferometrycombinedwithsource receiverreciprocityhasrecentlybeenemployed to extract inter source wave propagations in this study we applied the technique to seismograms of earthquakes that occurred at depths of 300 500 km beneath the bonin islands the wavelets of direct p and s waves generated by the deep earthquakes were examined to compute cross correlation functions ccfs we stacked the ccfs observed at receivers around the stationary point whose location could be estimated by the ray tracing technique because the phases in such ccfs potentially emerge at similar lag times that is the stationary phases the results demonstrate the retrieval of direct p and s waves propagating between pairs of deep earthquakes in particular direct p waves are reconstructed for most event pairs the following points should be considered to effectively retrieve the propagation of body waves 1 it is important to design a geometry in which the stationary phase derived from two earthquakes can be observed at the employed stations 2 the lag time difference of the stationary phases that emerge in the employed ccfs should be as small as possible and 3 the direct waves associated with deep earthquakes have a large amplitude at stations employed for cross correlation which is accomplished by both low noise level at the stations and the radiation pattern of the sources

reconstruction of subsurface structure from ambient seismic noise an example from ketzin germany summary passive seismic interferometry is a new promising methodology for seismic exploration interferometry allows information about the subsurface structure to be extracted from ambient seismic noise in this study we apply the cross correlation technique to approximately 25 hr of recordings of ambient seismic noise at the ketzin experimental co2 storage site germany common source gathers were generated from the ambient noise for all available receivers along two seismic lines by cross correlation of noise records this methodology isolates the interstation green s functions that can be directly compared to active source gathers we show that the retrieved response includes surface waves refracted waves and reflected waves we use the dispersive behaviour of the retrieved surface waves to infer geological properties in the shallow subsurface and perform passive seismic imaging of the subsurface structure by processing the retrieved reflected waves

steady state characteristics of the greenland ice sheet under different climates the greenland ice sheet is modelled to simulate its extent and volume in warmer climates and also to find out whether the ice sheet would re form on theice free bedrock under present climatic conditions the ice sheet model is a three dimensional thermo mechanical model with a fine resolution grid thebedrock surface beneath the ice sheet was mapped using radio echo sounding measurements by the electromagnetic institute copenhagen the modelexperiments show that increased temperature will result in ice margin retreat but the ice sheet is relatively stable it takes a rather high temperature rise of atleast 6ic for the ice sheet to disappear completely which indicates that the ice sheet probably survived the last interglacial also it appears that the greenlandice sheet is not a mere relict ice mass from a previously colder climate but that the ice sheet will still re form on the bare bedrock under the present or evenslightly warmer climatic conditions

positioning drill bit and look ahead events using seismic traveltime data we study seismic traveltime measurements acquired in the borehole including vertical seismic profiling seismic measurements while drilling and drill bit noise generated data these traveltime data are used to assess informative parameters including drill bit position distance to drilling target and parameters of the velocity model first we analyze seismic traveltime data using a simple hyperbolic traveltime equation for rays between surface sampling locations and the drill bit second we describe a model for estimating both the position of the drill bit and the relative distance to geologic interfaces ahead of the bit finally we present a dynamic bayesian strategy for real time prediction of drill bit positions velocity parameters and distances to geologic markers walk away vertical seismic profiling data from the norwegian sea are used to demonstrate our methods for this data set we pick five key reflectors ahead of the drill bit the deepest reflector is estimated to be 207 5 m ahead of

drill bit seismic monitoring while drilling by downhole wired pipe telemetry abstract drill bit seismic while drilling provides reverse vertical seismic profiles with large configurations of surface seismic sensors this seismic while drilling technique makes it possible to predict the formation changes ahead of the bit and to image 2d and 3d structures without interfering with the drilling activity the method is based on the recording of reference pilot signals which enable us to recognize and process the signal of the downhole drill bit source thus obtaining impulsive seismograms after the crosscorrelation and deconvolution of the pilot signals and the seismic data recorded by surface or crosswell geophones one issue in the application of this methodology can be the loss of the transmitted energy for the reference signal propagated from the bit to the surface through the drill string when the pilot signals are recorded at the surface at the top of the drill string it is well known that with polycrystalline diamond compact bits the pilot signal recorded at the surface may be weak and consequently the seismic while drilling results are poor this may also happen during drilling in the sliding mode without pipe rotation and in highly deviated or horizontal wells a solution to improve the drill bit seismic while drilling method in these conditions is to record the reference signal in the proximity of the bit source using downhole near bit tools to get good quality measurements of the pilot signal we present drill bit seismic while drilling results obtained in a drilling test using wired pipes for high rate communication from bottom hole to the surface the results demonstrate the applicability of this integrated approach as a standard procedure the advantage of this is to provide real time synchronized reverse vertical seismic profiles as well as high resolution and good quality data in terms of s n and high frequency content the method improves the use of the working drill bit as a downhole seismic source with different types of bit and drilling conditions

seismic while drilling by using dual sensors in drill strings drill string waves can be successfully used as reference pilot signals for drill bit seismic while drilling swd purposes the seismic signals obtained by correlating pilot and geophone measurements are disturbed by the drill string reverberations because the pilot waves are reflected at each interface between string sections with different acoustic impedances inverse filtering of these reflections using a reference pilot deconvolution calculated in the presence of additional noise may cause signal distortion to overcome this problem we consider using dual sensor measurements in the drill string to remove the reflections of the drill bit waves in the acquisition phase and to improve pilot deconvolution we measure acceleration and strain of drill string dual fields which have opposite reflection coefficients and in a string of constant elastic properties the same transmission coefficients these quantities are scaled to fit the amplitude of the direct arrivals summed to remove the reflections in the drill string and in the rig and may be deconvolved by einstein deconvolution to characterize the reflection coefficient between the drill bit and the formation synthetic numerical examples and real measurements acquired downhole in a location close to the bit show that upgoing and downgoing drill string pilots can be separated using dual fields and jointly used to improve the swd seismograms

reconstruction of the glacial ice covers of greenland and the canadian arctic islands by three dimensional perfectly plastic ice sheet modelling a three dimensional perfectly plastic ice sheet model developed for determining the surface elevations and the flow pattern of an ice sheet with given bottom topography and ice margin positions is applied to the reconstruction of the glacial ice covers of greenland and the canadian arctic islands in the northern regions two different reconstructions have been performed with ice margins along the present 600 and 200 m sea depth contours respectively in central greenland the ice margin is considered to be at the outermost ice margin deposits on the coastal shelf to the west and at the present 200 m sea depth contour to the east the main conclusions to be drawn from the reconstructions are 1 the flow pattern of the glacial ice cover of greenland shows a great resemblance to the present one the central ice divide being displaced less than 50 km from its present position and being no more than 200 m higher than today 2 the main ice divide of the ice sheet covering the canadian arctic islands the innuitian ice sheet was located over the highlands of eastern ellesmere island with local domes positioned over the present ice caps indicating that even the deep ice of wisconsin age in these ice caps is of local origin this is also the case for the devon island ice cap 3 even in the not very likely case of a rather extensive glacial ice cover in north west greenland the ice flow pattern upstream of the camp century deep drill site would not have changed radically compared to the present flow pattern thus it is concluded that even advanced ice margins in late wisconsin time could at most have resulted in an elevation of the deposition site of the late wisconsin ice at camp century 600 m higher than at present the consequences of this conclusion are discussed

seismic interferometry experiment in a shallow cased borehole using a seismic vibrator source we present the results of a seismic interferometry experiment in a shallow cased borehole the experiment is an initial study for subsequent borehole seismic surveys in an instrumented well site where we plan to test other surface borehole seismic techniques the purpose of this application is to improve the knowledge of the reflectivity sequence and to verify the potential of the seismic interferometry approach to retrieve high frequency signals in the single well geometry overcoming the loss and attenuation effects introduced by the overburden we used a walkaway vertical seismic profile vsp geometry with a seismic vibrator to generate polarized vertical and horizontal components along a surface seismic line and an array of 3c geophones cemented outside the casing the recorded traces are processed to obtain virtual sources in the borehole and to simulate single well gathers with a variable sourcereceiver offset in the vertical array we compare the results obtained by processing the field data with synthetic signals calculated by numerical simulation and analyse the signal bandwidth and amplitude versus offset to evaluate near field effects in the virtual signals the application provides direct and reflected signals with improved bandwidth after vibrator signal deconvolution clear reflections are detected in the virtual seismic sections in agreement with the geology and other surface and borehole seismic data recorded with conventional seismic exploration techniques

reconstruction of basal boundary conditions at the greenland ice sheet margin from gas composition in the ice abstract modelling the response of the greenland ice sheet to a temperature increase is an essential step towards estimating the climatic changes that could affect a large area of the northern hemisphere in the near future and is heavily dependent on accurate evaluation of the boundary conditions at the ice bedrock interface here we use a study of gas composition in the basal ice from west greenland this study shows that changes in basal thermal zones occur towards the border in the marginal zone of the ice sheet first slight melting at crystal boundaries and vein water squeezing occurs when the ice could still be below the pressure melting point then where the melting point is reached and more meltwater is produced sliding by regelation occurs closer to the margin partial freezing and ice accretion takes place the melting point being maintained at the bed because of the latent heat release pressure effects induce a certain amount of decoupling such changes in basal flow conditions can in turn promote ice sheet thinning at the margin a temperature increase of the air along the edge of some ice sheet areas in west greenland where decoupling occurs would thus give rise to higher ablation rates and would be likely to lead to fast ice retreat

prediction ahead of the bit by using drill bit pilot signals and reverse vertical seismic profiling rvsp surface drillstring axial pilot signals are used to predict reflections ahead of the drill bit we show that part of the drill bit signal propagates downward in the formation reflects upward by a seismic interface and is then transmitted to the drillstring and the surface pilot sensors these reflections are interpreted in drill bit pilot signals by means of a numerical model of the drillstring coupled to the formation at the bit rock contact the result is an additional low cost reverse vsp rvsp in the zero offset approximation these while drilling results are integrated with conventional drill bit rvsp measurements and compared with other geophysical and well results

extracting coherent wave fronts from acoustic ambient noise in the ocean a method to obtain coherent acoustic wave fronts by measuring the space time correlation function of ocean noise between two hydrophones is experimentally demonstrated though the sources of ocean noise are uncorrelated the time averaged noise correlation function exhibits deterministic waveguide arrival structure embedded in the time domain green s function a theoretical approach is derived for both volume and surface noise sources shipping noise is also investigated and simulated results are presented in deep or shallow water configurations the data of opportunity used to demonstrate the extraction of wave fronts from ocean noise were taken from the synchronized vertical receive arrays used in the frame of the north pacific laboratory npal during time intervals when no source was transmitting

energy balance of a drill bit seismic source part 2 drill bit versus conventional seismic sources the radiation properties of a downhole drill bit seismic source are related to the amplitude and frequency of the forces exerted by the working bit the main vibration modes of roller cone and polycrystalline diamond compact pdc bits are investigated under different drilling conditions the analysis includes vibrations produced by teeth indention multilobed patterns bouncing with periodic and random effects single cutter forces stick slip and whirling effects mud pressure modulation forces and bit wear drill bit radiation properties are calculated using the results obtained in part 1 of this paper and are numerically compared to the radiation of conventional vertical seismic profiling vsp sources

combination of surface and borehole seismic data for robust target oriented imaging a novel application of seismic interferometry si and marchenko imaging using both surface and borehole data is presented a series of redatuming schemes is proposed to combine both data sets for robust deep local imaging in the presence of velocity uncertainties the redatuming schemes create a virtual acquisition geometry where both sources and receivers lie at the horizontal borehole level thus only a local velocity model near the borehole is needed for imaging and erroneous velocities in the shallow area have no effect on imaging around the borehole level by joining the advantages of si and marchenko imaging a macrovelocity model is no longer required and the proposed schemes use only single component data furthermore the schemes result in a set of virtual data that have fewer spurious events and internal multiples than previous virtual source redatuming methods two numerical examples are shown to illustrate the workflow and to demonstrate the benefits of the method one is a synthetic model and the other is a realistic model of a field in the north sea in both tests improved local images near the boreholes are obtained using the redatumed data without accurate velocities because the redatumed data are close to the target

numerical modeling and interpretation of drillstring waves drill bit seismic and pilot seismograms contain drillstring periodicities from signal and environmental noise sources these coherent components are similar in seismic while drilling correlations and their joint contribution may cause distortions in signal processing numeric modeling of the drillstring transmission line is used to correctly interpret the axial and torsional events with similar propagating modes of signal and noise the reflection coefficients are computed for drillstrings of arbitrary materials and their average mechanical features are related to the axial and torsional group velocities obtained by the dispersion equations fitting of the periodical components in the real seismograms is used to automatically correct the drill bit pilot delays and to estimate the surface and downhole boundary conditions

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

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

interferometry by deconvolution part 2 theory for elastic waves and application to drill bit seismic imaging deconvolution interferometry successfully recovers the impulse response between two receivers without the need for an independent estimate of the source function here we extend the method of interferometry by deconvolution to multicomponent data in elastic media as in the acoustic case elastic deconvolution interferometry retrieves only causal scattered waves that propagate between two receivers as if one acts as a pseudosource of the point force type interferometry by deconvolution in elastic media also generates artifacts because of a clamped point boundary condition imposed by the deconvolution process in seismic while drilling swd practice the goal is to determine the subsurface impulse response from drill bit noise records most swd technologies rely on pilot sensors and or models to predict the drill bit source function whose imprint is then removed from the data interferometry by deconvolution is of most use to swd applications in which pilot records are absent or provide unreliable estimates of bit excitation with a numerical swd subsalt example we show that deconvolution interferometry provides an image of the subsurface that cannot be obtained by correlations without an estimate of the source autocorrelation finally we test the use of deconvolution interferometry in processing swd field data acquired at the san andreas fault observatory at depth safod because no pilot records were available for these data deconvolution outperforms correlation in obtaining an interferometric image of the san andreas fault zone at depth

placers of cosmic dust in the blue ice lakes of greenland a concentration process occurring in the melt zone of the greenland ice cap has produced the richest known deposit of cosmic dust on the surface of the earth extraterrestrial particles collected from this region are well preserved and are collectable in large quantities the collected particles are generally identical to cosmic spheres found on the ocean floor but a pure glass type was discovered that has not been seen in deep sea samples iron rich spheres are conspicuously rare in the collected material

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

the basal ice layer of glaciers and ice sheets abstract in many glaciers and ice sheets there is a basal ice layer bil in which the ice is conditioned primarily by processes operating at the bed the bil is chemically and physically distinctive and is characterised by a component of basally derived sediment the bil is a rheological control on ice sheet dynamics an indicator of subglacial conditions and processes an agent of subglacial geologic processes the source of a substantial proportion of glacial sediments a limit to the downward extension of the climate record from deep ice cores if debris characteristics of the bil are preserved in glacial sediments former glacier conditions can be inferred analysis of stable isotopes gas and cation composition ice crystallography debris composition and structural glaciology has elucidated many mechanisms for bil formation common elements can be identified from a range of locations key issues are entrainment of ice and debris from the bed formation of new ice metamorphism of existing ice in the bil and thickening of the bil by deformation a distinction can be made between basal ice formed by accretion of material from the bed stratified facies and basal ice formed by metamorphic processes within the ice close to the bed dispersed facies

the greenland ice sheet margin a mine of ice for paleo environmental studies abstract stable isotope analysis of surface ice samples from three locations on the margin of the greenland ice sheet have provided 18 o records dating back into the wisconsinan the samples were collected in profiles transverse to the ice margin on a location about 40 km northeast of jakobshavn west greenland and at two locations in warming land north greenland the holocene wisconsinan transition as detected by a large aburpt shift was found between 470 and more than 850 m from the ice edge the magnitude of the shift was about 8 and 12 in the west and north greenland records respectively the west greenland record which is the more detailed documents that some variations in the wisconsinan are apparently preserved in the ice margin isotopic records by linking locations in the accumulation and ablation zones with the same observed value the surface ice of the ablation zone can be dated back to at least the wisconsinan holocene transition if the velocity of the ice motion is measured or estimated by ice flow modelling this means that dated records of ice constituents gas composition chemistry content of microparticles micro meteorites etc can be established by mining the easily accessible surface ice of the ablation zone

walk away vsp using drill noise as a source we describe a method for extracting and deconvolving a signal generated by a drill bit and collected by an array of surface geophones the drill noise signature is reduced to an effective impulse by means of a multichannel wiener deconvolution technique producing a walk away reverse vertical seismic profile vsp sampled almost continuously in depth we show how the multichannel technique accounts for noise and for internal drill string reflections automatically limiting the deconvolved data to frequencies containing significant energy we have acquired and processed a data set from a well in germany while drilling at a depth of almost 4000 m the subsurface image derived from these data compares well with corresponding images from a 3 d surface seismic survey a zero offset vsp survey and a walk away vsp survey acquired using conventional wireline techniques the effective bandwidth of the deconvolved drill noise data is comparable to the bandwidth of surface seismic data but significantly smaller tha

seismic while drilling technology use and analysis of the drill bit seismic source in a cross hole survey multicomponent seismic while drilling data were analysed in order to define better the different wavefield components and the energy radiation pattern of a tricone drill bit a unique cross hole acquisition scheme from one of eni agip s special test sites was used direct measurements of high quality p and s waves allowed the determination of the radial horizontal component hr and of the corresponding transverse horizontal component ht subsequently analysis of the vertical transverse component t derived by combining z and hr led to the computation of simplified radiation pattern for the sv component of the drill bit signal a large time splitting between horizontally and vertically polarized shear waves influenced by the acquisition geometry and by the geological setting was analysed an unconventional sv velocity tomographic reconstruction from the drill bit signal is also derived and discussed

drill bit displacement source model source performance and drilling parameters the emission properties of a drill bit source and the signature of the drill bit seismograms depend on the dynamic action of the drill bit which in turn is a function of the geological conditions rock properties and drilling parameters in fact drilling is a variable dynamic process in which the vibrations produced by the drill bit are transmitted into the drillstring and formation with a partition of energy determined by the impedances at the bit in this analysis we study drill bit performance and calculate the forces produced by an ideal drill bit to determine the variations in the signal signature and ultimately the conditions of source repeatability as a function of average drilling parameters and signal frequency the model includes near field effects and assumes the drill bit acts as a displacement source producing axial bit forces with relative bit formation displacement and vertical penetration in the formation the analysis shows that the expected bit signature which is equivalent to the ground force in vibroseis depends on average tooth impact area percussion frequency rate of penetration formation strength bit wear and downhole pressure in particular the results show that larger amplitudes of the axial forces are expected at a lower rate of penetration higher formation strength and lower formation pressure

the last interglacial as recorded in the greenland ice sheet and canadian arctic ice caps abstract the greenland ice sheet and canadian arctic ice cap record of the last interglacial undertood broadly as marine isotopic stage 5 is critically reviewed lack of age control on the deep ice core records leaves open two largely different interpretations of the climate in marine isotopic stage 5 either most of stage 5 with the exception of substage 5e experienced a cold climate and the main change from interglacial to glacial conditions took place at the boundary between substages 5e and 5d or during much of stage 5 the climate was integracial type i e as warm as or warmer than at present with substage 5d as the only period with a glacial type climate it is concluded that most of the hard evidence is in favour of the latter scenario ice sheet dynamic model studies using the warm marine isotopic stage 5 scenario indicate that the greenland ice sheet in the warmest periods of stage 5 had split up into a main ice sheet covering central and northern greenland and a much smaller ice cap over the southeastern highlands the greenland record of marine isotopic stage 5 is then discussed in relation to other paleoclimatic and paleoenvironmental records the possibility of major inter hemispheric differences of the climate in mid and late marine isotopic stage 5 is discussed it is concluded that it is important to study the entire marine isotopic stage 5 not only substage 5e in order to understand possible differences in the global climatic responses to nearly indentical peaks in orbitally driven summer insolation changes in the northern hemisphere

three dimensional s wave velocity structure in southern california 1 the three dimensional s wave velocity structure was constructed from rayleigh wave phase velocities measured using teleseismic trinet data the s wave velocity maps show some features that are much more distinct than previous tomographic results in southern california there is a clear seismic velocity contrast across the san andreas fault the north american plate side is systematically slower than the pacific plate side under the eastern california shear zone there are distinct slow velocity anomalies in the crust suggesting a close connection to lower viscosity crust previously concluded from observation of the post seismic deformation after the landers earthquake major upper mantle features include a fast velocity root under the transverse ranges which is the dominant feature in the upper mantle and a slow velocity anomaly under the salton trough our results support that the velocity contrast across the san andreas fault extends below the moho

carbonaceous particles reveal that late holocene dust causes the dark region in the western ablation zone of the greenland ice sheet a dark region in the western ablation zone of the greenland ice sheet is caused by outcropping ice layers that contain more dust than the surrounding brighter ice these higher amounts of dust were deposited in the accumulation zone of the ice sheet and travelled with the ice to the ablation zone to deduce the period and the causes of this higher dust deposition carbonaceous particles in ice samples from the dark region and from brighter reference ice were analysed and used for dating samples including ice from directly below the surface contain high amounts of modern organic carbon probably from microorganisms on the ice surface deeper samples reveal low amounts of carbonaceous particles which are originally deposited in the accumulation zone the amount of outcropping carbonaceous particles in the dark region seems significantly higher than in the reference ice one of the samples that contained material initially deposited in the accumulation zone was dated and revealed late holocene ages coinciding with a period of enhanced eolian activity in the nearby tundra therefore variable eolian activity during the holocene effected dust fluxes towards the ice and hence leads to albedo variations in the present ablation zone of the ice sheet

retrieval of reflections from ambient noise recorded in the mizil area romania we applied seismic interferometry si by crosscorrelation to ambient noise panels recorded in the mizil area romania aiming to retrieve body wave reflections to achieve this goal surface waves in the noise panels input to si should be suppressed we did this by selecting for input to si only noise panels that are not dominated by surface waves the selection was either after visual inspection in the time domain or after automatic slowness evaluation on crosscorrelated panels the latter used the slowness of arrivals passing through the virtual source position at time 0 s we discovered that the automatic slowness evaluation method allows better retrieval of reflections from the retrieved reflection gathers we obtained stacked images of the subsurface comparing the si images to a stacked image from active source data we concluded that some retrieved events correspond to reflectors in the active seismic section including known geologic markers in a previous application of si to ambient noise the retrieved reflections exhibited frequency content lower than that of active data in our results the frequency content of the si retrieved data was comparable to the one of the active data