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

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

fabric and texture at siple dome antarctica preferred c axis orientations are present in the firn at siple dome west antarctica and recrystallization begins as shallow as 200 m depth in ice below 20 c based on digital analysis of c axis fabrics grain sizes and other characteristics of 52 vertical thin sections prepared in the field from the kilometer long siple dome ice core the shallowest section analyzed from 22 m shows clustering of c axes toward the vertical by 200 m depth girdle fabric and other features of recrystallized ice are evident in layers or regions separated by layers regions of typically finer grained ice lacking evidence of recrystallization ice from about 700 780m depth which was deposited during the last ice age is especially fine grained with strongly vertical c axes but deeper ice shows much larger crystals and strong evidence of recrystallization azimuthal asymmetry of some c axis fabrics trends in grain size and other indicators reveal additional information on processes and history of ice flow at siple dome

summit of the east antarctic ice sheet underlain by thick ice crystal fabric layers linked to glacial interglacial environmental change ice cores in antarctica and greenland reveal ice crystal fabrics that can be softer under simple shear compared with isotropic ice owing to the sparseness of ice cores in regions away from the ice divide we currently lack information about the spatial distribution of ice fabrics and its association with ice flow radio wave reflections are influenced by ice crystal alignments allowing them to be tracked provided reflections are recorded simultaneously in orthogonal orientations polarimetric measurements here we image spatial variations in the thickness and extent of ice fabric across dome a in east antarctica by interpreting polarimetric radar data we identify four prominent fabric units each several hundred metres thick extending over hundreds of square kilometres by tracing internal ice sheet layering to the vostok ice core we are able to determine the approximate depth age profile at dome a the fabric units correlate with glacial interglacial cycles most noticeably revealing crystal alignment contrasts between the eemian and the glacial episodes before and after the anisotropy within these fabric layers has a spatial pattern determined by ice flow over subglacial topography

impact of soil moisture and convectively generated waves on the initiation of a west african mesoscale convective system a mesoscale convective system mcs case study was observed over northeast mali as part of the african monsoon multidisciplinary analysis amma on 31 july 2006 observations of this case suggest that the soil moisture heterogeneity and atmospheric gravity waves emitted from a parent mcs were important trigger mechanisms for this system this study uses high resolution met office unified model metum simulations to assess the importance of the synoptic circulation land surface and gravity waves in the initiation and development of the mcs during the early afternoon shallow convection developed over a region of dry soil within a synoptic scale convergence zone which was caused by the confluence of the southerly monsoon flow with winds associated with the circulation around the saharan heat low two pronounced waves were emitted from a nearby parent storm and propagated towards the convergence zone when the second wave reached the location of the shallow convection deep convection was immediately initiated further convective cells developed later in the afternoon over dry soil many adjacent to strong soil moisture gradients these aggregated with the main storm which later developed into the case study mcs a comparison of model simulations with without the soil moisture heterogeneity and gravity waves shows that the synoptic scale circulation and convergence zones specified by the atmospheric analysis were the most important factors for the successful simulation of the mcs if the location of the initiation of the system is to be forecast accurately the land surface that is the soil moisture must be represented adequately in order to reproduce the timing of the secondary initiation of convection correctly the model must be able to capture gravity waves that are emitted by existing systems

acoustic velocity of seismic while drilling swd borehole guided waves borehole guided waves other than the extensional waves traveling through the drillstring can be used as pilot signals to obtain seismic while drilling swd seismograms and information about the drilling conditions this occurs when there is no rotation of the drillstring while drilling and when the contacts between the drillpipe and the borehole wall preclude the propagation of the extensional wave to obtain the velocity of the guided waves we model the compressional wave velocity of drilling mud with different compositions of low and high gravity solids corresponding to a given drilling plan and taking into account the presence of formation cuttings and in situ conditions then we compute the velocities of the tube wave with and without casing and the guided wave traveling in the mud inside the drillstring pipe wave the results indicate the pipe wave constitutes a reliable pilot signal in the absence of drillpipe rotation this allows us to obtain a complete depth reverse vertical seismic pro

interface prediction ahead of the excavation front by the tunnel seismic while drilling tswd method predicting geologic interfaces ahead of a tunnel front is of major importance when boring tunnels unexpected variations in ground properties can cause problems for tunnel boring advance and risk for human safety the tunnel seismic while drilling tswd method utilizes noise produced during mechanical excavation to obtain interpretable seismic data this passive method uses accelerometers mounted on the advancing tunnel boring machine reference signals together with seismic sensors located along and outside the tunnel data recorded by fixed sensors are crosscorrelated with the reference signal and sorted by offset similar to reverse vertical seismic profiling crosscorrelated tswd data are processed to extract the reflected wavefield during mechanical excavation of a 950 m tunnel through upper triassic dolomite a survey was performed to predict geologic interfaces faults intersecting the tunnel were observed on seismic tswd data and later were confirmed by geostructural inspection p and s wave i

evidence of dynamic recrystallization in polar firn 1 microstructural analyses have been performed on polar firn from the european project for ice coring in antarctica drilling site in dronning maud land antarctica the results derived from images of the firn structure in microscopic resolution indicate that dynamic recrystallization is active in firn at all depths and it dominates the evolution of the microstructure when the firn density exceeds a critical value of 730 kg m3 overburden snow load 0 2 mpa at the firn ice transition density 820 kg m3 the microstructure is characterized by many small grains and bulged or irregularly shaped grain boundaries more than half of all grains show subgrain boundaries thus strain induced boundary migration is an essential feature to describe the irregular grain structure in agreement with previous studies significant grain growth has been observed with depth for the largest grains in the samples however our microscopic analysis reveals that the grain growth with depth in fact vanishes if all grains larger than 65 m in diameter are taken into account this result reflects the fact that the growth of the largest grains is counteracted by grain size reduction by shrinking and subdivision of old grains as well as production of new grains consequently previous conclusions that grain growth in polar firn is essentially analogous to normal grain growth in metallic and ceramic sinters and that the stored strain energy is small in comparison with grain boundary energy can no longer be supported additionally our observations show that the incipience of dynamic recrystallization in polar ice sheets is not as sensitive to temperature as supposed so far a discussion of the change of the mean grain size due to the measuring technique is imperative

seismic while drilling by using tunnel boring machine noise the tunnel boring machine tbm is used extensively to mechanically excavate tunnels to optimize the mechanical drilling and work safely an estimate of the geology to be drilled is necessary we consider using the elastic waves produced by the tbm cutting wheel to obtain seismic while drilling swd information for predicting the geology ahead of the drilling front this method uses accelerometers mounted on the tbm together with geophones located along and outside the tunnel similar to the technique successfully used to drill oil and geothermal wells study of noise and the resolution of the signal produced by the large diameter cutting head shows that nonstationary noise separation can be achieved by locating sensors at the front and rear ends of the tunnel the higher resolution in front of the tbm is limited by pilot delays while the lower lateral resolution is limited by the radial dimension of the tbm analysis of seismic data acquired in a field test shows that p and s wave arrivals have a wide frequency band and high amplitude in seismic traces measured 700 m away from the drilling front in comparison with swd applications in wells tunnel swd technology has the advantage of allowing direct access to the tunnel front which makes it easy to connect the tbm reference sensors for while drilling monitoring this method can be successfully applied without interfering with drilling activity to monitor tunnel excavation continuously reduce risks and optimize drilling

deformation in the vicinity of ice divides calculs numeriques aux elements finis de la variation de la vitesse horizontale avec la profondeur au voisinage d une difference de la glace symetrique isotherme et ou glissement sur un lit plat

the crystal fabric of ice from full waveform borehole sonic logging 1 in an ice sheet a preferred crystal orientation fabric affects deformation rates because ice crystals are strongly anisotropic shear along the basal plane is significantly easier than shear perpendicular to the basal plane the effect of fabric can be as important as temperature in defining deformation rates fabric is typically measured using analysis of thin sections under the microscope with co polarized light due to the time consuming and destructive nature of these measurements however it is difficult to capture the spatial variation in fabric necessary for evincing ice sheet flow patterns because an ice crystal is similarly elastically anisotropic the speed of elastic waves through ice can be used as a proxy for quantify anisotropy we use borehole sonic logging measurements and thin section data from dome c east antarctica to define the relations between apparent fabric and borehole measured elastic speeds compressionalvp and vertically polarized shear vsv these relations valid for single maximum fabrics allow in situ depth continuous fabric estimates of unimodal fabric strength from borehole sonic logging we describe the single maximum fabric usinga1 the largest eigenvalue of the second order orientation tensor for ice at 16 c anda1in the 0 7 1 range the relations arevp 248 a13 7 3755 m s 1 and vsv 210a17 3 1968 m s 1

metamorphism of stratified firn at dome fuji antarctica a mechanism for local insolation modulation of gas transport conditions during bubble close off 1 the evolution of the structure of a 112 59 m long firn core recovered at dome fuji east antarctica was investigated in order to improve understanding of firn densification and bubble formation processes which are important for interpreting local insolation proxies used for astronomical dating of deep ice cores using selected samples we measured physical properties including 1 the relative dielectric permittivities in both the vertical and horizontal planes 2 the bulk density at a resolution of millimeters 3 the three dimensional geometric structure of pore space and 4 crystal orientation fabrics we found that the firn at dome fuji contains horizontal strata with thicknesses of several centimeters near the surface of the ice sheet these strata are characterized by contrasting bulk density earlier field studies suggest that summer insolation causes densification of surface firn down to 30 m density maxima exhibited a clear positive correlation with the strength of structural anisotropy and c axis clustering around the vertical in contrast the correlation is negative in deeper firn confirming previous findings that initially less dense firn became denser than initially dense firn in addition numerous examples of textures indicating that deformation preferentially occurred in weaker layers were found moreover the initially dense firn layers were more permeable for air near the bottom of firn we propose a model linking firn properties with conditions for the gas transport processes near the bottom of firn the model explains how stronger insolation can lead to bulk ice with a lower o2 n2 ratio and smaller total gas content

strain rate enhancement at dye 3 greenland ice at depth in ice sheets can be softer in bed parallel shear than glen s flow law predicts for example at dye 3 greenland enhancement factors of 3 4 are needed in order to explain the rate of borehole tilting previous authors have identified crystal fabric as the dominant contributor but the role of impurities and crystal size is still incompletely resolved here we use two formulations of anisotropic flow laws for ice azuma s and sachs models to account for the effects of anisotropy and show that the measured anisotropy of the ice at dye 3 cannot explain all the detailed variations in the measured strain rates the jump in enhancement across the holocene wisconsin boundary is larger than expected from the measured fabrics alone dust and soluble ion concentration divided by crystal size correlates well with the residual enhancement indicating that most of the excess deformation may be due to impurities or crystal size while the major features of the deformation at dye 3 are explained by anisotropy and temperature results also suggest that further research into the role of impurities and crystal size is warranted

performance and applications of an automated c axis ice fabric analyzer a completely automated instrument for ice fabric analysis has been built tested and applied the analysis of a thin section is performed in two steps first the section is loaded into the machine and is automatically imaged in about 16 min then at a later time and convenient location the grains in the section are identified from a stored image either automatically or by the user and the c axis orientation of each grain is calculated at a rate of 1200 grains h 1 comparison of automated results with manual rigsby stage results shows excellent agreement diagnostics indicate that the accuracy of the new technique is 0 5 in the c axis orientation two specific applications are demonstrated 1 the ability to measure internal grain orientations that may reveal internal stress and 2 the ability to measure very thin thin sections the latter combined with the ability to zoom in on small regions of section will be useful for examining the fabric of fine grained 1 mm size grains ice

high resolution quantitative seismic imaging of a strike slip fault with small vertical offset in clay rocks from underground galleries experimental platform of tournemire france imaging tectonic faults with small vertical offsets in argilittes clay rock using geophysical methods is challenging in the context of deep radioactive waste disposals the presence of such faults has to be assessed since they can modify the rock confining properties in the tournemire experimental platform tep france fault zones with small vertical offsets and complex shape have been identified from underground works however 3d high resolution surface seismic methods have shown limitations in this context that led us to consider the detection and characterization of the faults directly from underground works we investigate here the potential of seismic full waveform inversion fwi applied in a transmission configuration to image the clay rock medium in a horizontal plane between galleries and compare it with first arrival traveltime tomography fatt our objective is to characterize seismic velocities of a block of argilittes crossed by a subvertical fault zone with a small vertical offset the specific measurement configuration allows us to neglect the influence of the galleries on the wave propagation and to simplify the problem by considering a 2d isotropic horizontal imaging domain our fwi scheme relies on a robust adaptation of early arrival waveform tomography the results obtained with fatt and fwi are in accordance and both correlate with the geological observations from the gallery walls and boreholes we show that even though various simplifications are done in the inversion scheme and only a part of the data is used fwi allows to get higher resolution images than fatt and is especially less sensitive to the incomplete illumination as it uses also diffracted energy the results provided in this study highlight the complexity of the fault zone showing a complex interaction of the main fault system with a secondary system composed of decimetric fractures associated with the presence of water

mesoscale convective systems in weakly forced large scale environments part iii numerical simulations and implications for operational forecasting abstract during a 24 h period beginning 1200 utc 11 may 1982 a series of mesoscale convective systems developed within a weakly forced large scale environment two of these systems had a large component of motion against the midtropospheric flow and propagated in a direction nearly opposite to that of the travelling upper level disturbances this evolution of convection is very different from traditional ones in which convection develops and moves more or less in phase with traveling upper level disturbances it presents a tremendous challenge for three dimensional numerical models since the initiation and evolution of convection are tied to mesoscale features that are not well observed by the conventional upper air network and may not be well approximated in the model parameterization schemes mesoscale model simulations are conducted to evaluate the ability of the model to reproduce this complex event and to examine the model sensitivities to differences in the convective trigger function and model i

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

a flow law for anisotropic ice and its application to ice sheets a new formulation of the flow law of polycrystalline ice was derived for an rbitrary c axis orientation fabric pattern the rate factor of the flow law is given by a geometrical factor that can be calculated with c axis fabric data and stress condition a theoretical calculation combining this flow law with a lattice rotation model by intracrystalline slip in the constituent grains predicts the development of characteristic fabric patterns observed in many laboratory experiments and in the field according to this flow model of polycrystalline ice if significant recrystallization does not occur uniaxial compression makes the ice body harder with increasing strain whereas simple shear makes the ice body softer with strain this may significantly affect ice sheet dynamics

rate controlling processes in the creep of polar ice influence of grain boundary migration associated with recrystallization abstract information on the deformation modes and recrystallization processes in polar ice sheets was obtained thanks to the analysis of the ice structure along two deep ice cores from greenland and antarctica it is shown that the deformation of polar ice at low stresses is produced by intracrystalline slip accommodated by grain boundary migration gbm a deformation model based on an equilibrium between work hardening and recovery processes has been developed the decrease in dislocation density is due to both gbm associated with grain growth and the formation of boundaries by recrystallization the value of the dislocation density along the byrd and grip ice cores obtained thanks to the model is about 1 10 11 m 2 this value is in agreement with data from synchrotron x ray diffraction on samples taken along the core this deformation model can account for the deformation of polar ice at low stresses it is shown that the flow law with a stress exponent lower than 2 can be related to the efficiency of gbm as a recovery process

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

autocorrelogram migration ivspwd test we present the results of applying autocorrelogram migration to inverse vertical seismic while drilling ivspwd profile data collected in the austin chalk formation the seismic reflections were excited by a drill bit moving along a horizontal well at a depth of 2800 m the data were recorded by a receiver array on the surface there were 609 twenty second traces recorded at each of 10 three component stations after preprocessing the traces were autocorrelated and migrated two imaging conditions were examined the ghost reflection imaging condition produced a reflectivity section that agreed with a nearby common depth point section the migration section obtained with a primary reflection imaging condition produced a reflectivity section inferior in quality to that of the ghost image a possible explanation is that the drill bit location was not precisely known which can be shown to induce weak errors in the ghost imaging condition but stronger errors in the primary reflection imaging condition ghos

multiscale convective overturning in mesoscale convective systems reconciling observations simulations and theory abstract an analysis of how parameterized convection interacts with hydrostatic explicitly resolved precipitation processes to represent multiscale convective overturning in a mesoscale resolution numerical simulation is presented critically important ingredients of the successful simulation are identified and the degree to which simulations are consistent with observations and theoretical considerations is examined of particular concern is how convective parameterization routines reconcile the deep moist absolutely unstable structures that form in mesoscale convective systems it is found that these structures arise primarily from resolvable scale vertical motion and that the model responds to these structures not only by maintaining parameterized convection but also by developing a hydrostatic manifestation of convective overturning on its smallest resolvable horizontal scales the strongest and most distinctive mesoscale perturbations develop in regions where the mesoscale contribution to convectiv

numerical simulation of tornadogenesis in a high precipitation supercell part i storm evolution and transition into a bow echo abstract a nested grid primitive equation model rams version 3b was used to simulate a high precipitation hp supercell which produced two weak tornadoes six telescoping nested grids allowed atmospheric flows ranging from the synoptic scale down to the tornadic scale to be represented in the simulation all convection in the simulation was initiated with resolved vertical motion and subsequent condensation latent heating from the model microphysics no warm bubbles or cumulus parameterizations were used part i of this study focuses on the simulated storm evolution and its transition into a bow echo the simulation initially produced a classic supercell that developed at the intersection between a stationary front and an outflow boundary as the simulation progressed additional storms developed and interacted with the main storm to produce a single supercell this storm had many characteristics of an hp supercell and eventually evolved into a bow echo with a rotating comma head structure an analysi

visual stratigraphy of the north greenland ice core project northgrip ice core during the last glacial period 1 a continuous high resolution record of digital images has been obtained from the north greenland ice core project northgrip ice core 75 1 n 42 3 w in the depth interval from 1330 m to the bedrock at 3085 m the ice core stratigraphy is clearly visible throughout the glacial period with the most frequent and brightest visible layers appearing during the coldest events down to a depth of 2600 m the horizontal layering is very regular below this depth small irregularities in the layering start to appear and below 2800 m the visual stratigraphy becomes more uncertain perhaps because of penetration into climatically warmer ice comparison of the visual stratigraphy with high resolution continuous records of chemical impurities and dust reveals a high degree of correlation which indicates that the visible layers are caused by these impurities a new approach is used to automatically determine annual layer thicknesses from the visual stratigraphy record by carrying out a frequency analysis of the most prominent visible layers in the profile the result gives strong support for the northgrip timescale model

grain growth in polar ice i theory many observations regarding grain growth in ice sheets are glaciologically interesting but imperfectly understood here we develop the theory of grain growth in ice that is not deforming rapidly and in the succeeding paper we use this theory to explain observations from glacial ice in the absence of significant strain energy the driving force for grain growth arises from grain boundary curvature grain growth is slowed by the interaction of grain boundaries with extrinsic materials microparticles bubbles and dissolved impurities if the driving force for growth is not large enough to cause boundaries to separate from an extrinsic material then the grain boundary velocity is determined by the velocity characteristic of the extrinsic material low velocity regime if the driving force is large enough to cause separation then boundaries migrate more rapidly than the extrinsic material high velocity regime but the net driving force is reduced through transient pinning by the extrinsic material polar ice is typically in the low velocity regime relative to dissolved impurities and the high velocity regime relative to microparticles and bubbles cross sectional area of grains is predicted to increase linearly with time under most but not all circumstances

a polarimetric coherence method to determine ice crystal orientation fabric from radar sounding application to the neem ice core region ice crystal orientation fabric cof records information about past ice sheet deformation and influences the present day flow of ice polarimetric radar sounding provides a means to infer anisotropic cof patterns due to the associated birefringence of polar ice here we develop a polarimetric coherence phase based method to determine horizontal properties of the cof the method utilizes the azimuth and depth dependence of the vertical gradient of the hhvv coherence phase to infer the dielectric principal axes and birefringence which are then related to the second order fabric orientation tensor specifically under the assumption that one of the orientational eigenvectors is vertical we can determine the horizontal eigenvectors and the difference between the horizontal eigenvalues a measure of horizontal fabric asymmetry the method exploits single polarized data acquired with varying antenna orientation it applies to ground based multi polarization surveys and is demonstrated using data acquired by center for remote sensing of ice sheets cresis using multi channel coherent radar depth sounder mcrds from the north greenland eemian ice drilling neem ice core region in greenland the analysis is validated using a combination of polarimetric matrix backscatter simulations and comparison with cof data from the neem ice core the results are consistent with a conventional model of ice deformation at an ice divide where a lateral tension component is present with minor horizontal cof asymmetry and the greatest horizontal concentration of crystallographic axes orientated near parallel to the ice divide

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

on the severe convective storm of 29 october 2013 in the balearic islands observational and numerical study on 29 october 2013 a severe squall line developed to the west of the balearic islands spain ahead of an advancing cold front and then crossed the archipelago we first provide an observational characterization of the event based on surface reports remote sensing products radiosoundings and synoptic information we also achieve by means of numerical experiments new insights into the kinematic and thermodynamic factors that governed the genesis and evolution of the linear convective system radar and satellite images confirm the fast movement and linear shape of the system with an indication of a possible transition into a bow echo structure during its later stages the synoptic setting at mid upper tropospheric levels was dominated by a cold trough extended over western europe associated with a jet stream located downstream convection evolved under the right entrance region of the jet and initiated under the crucial influence of a surface low that developed over the mediterranean sea ahead of the cold front the low not only cooperated with the upper level dynamical forcing to erode a capping inversion initially present over the balearics and moisten the atmospheric column above but also shaped and enhanced a convergence line along which the first convective cells grew and self aggregated this scenario is confirmed by numerical simulations of the case which also emphasize the important role of regional topography in the production of the aforementioned maritime convergence through the mesoscale modulation of the low level flow additional simulations show that i the destabilization of the low level air mass necessary for triggering and feeding an organized convective system on 29 october 2013 was attributable to prior evaporation from the mediterranean and ii sea surface temperatures appear to be critical for a successful fine grid numerical forecast of the mode degree of severity timing and track of the convective precipitation system

physical properties of the wais divide ice core the wais west antarctic ice sheet divide deep ice core was recently completed to a total depth of 3405 m ending 50 m above the bed investigation of the visual stratigraphy and grain characteristics indicates that the ice column at the drilling location is undisturbed by any large scale overturning or discontinuity the climate record developed from this core is therefore likely to be continuous and robust measured grain growth rates recrystallization characteristics and grain size response at climate transitions fit within current understanding significant impurity control on grain size is indicated from correlation analysis between impurity loading and grain size bubble number densities and bubble sizes and shapes are presented through the full extent of the bubbly ice where bubble elongation is observed the direction of elongation is preferentially parallel to the trace of the basal 0001 plane preferred crystallographic orientation of grains is present in the shallowest samples measured and increases with depth progressing to a vertical girdle pattern that tightens to a vertical single maximum fabric this single maximum fabric switches into multiple maxima as the grain size increases rapidly in the deepest warmest ice a strong dependence of the fabric on the impurity mediated grain size is apparent in the deepest samples

representing convective organization in prediction models by a hybrid strategy the mesoscale organization of precipitating convection is highly relevant to next generation global numerical weather prediction models which will have an intermediate horizontal resolution grid spacing about 10 km a primary issue is how to represent dynamical mechanisms that are conspicuously absent from contemporary convective parameterizations a hybrid parameterization of mesoscale convection is developed consisting of convective parameterization and explicit convectively driven circulations this kind of problem is addressed for warm season convection over the continental united states although it is argued to have more general application a hierarchical strategy is adopted cloud systemresolving model simulations represent the mesoscale dynamics of convective organization explicitly and intermediate resolution simulations involve the hybrid approach numerically simulated systems are physically interpreted by a mechanistic dynamical model of organized propagating convection this model is a formal basis for approximating mesoscale convective organization stratiform heating and mesoscale downdraft by a first baroclinic heating couplet the hybrid strategy is implemented using a predictor corrector strategy explicit dynamics is the predictor and the first baroclinic heating couplet the corrector the corrector strengthens the systematically weak mesoscale downdrafts that occur at intermediate resolution when introduced to the betts miller janjic convective parameterization this new hybrid approach represents the propagation and dynamical structure of organized precipitating systems therefore the predictor corrector hybrid approach is an elementary practical framework for representing organized convection in models of intermediate resolution

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

physical properties crystalline textures and c axis fabrics of the siple dome antarctica ice core the quality of the ice core from siple dome west antarctica varied widely with significant fracturing below 400 m bubbly ice persisted to the ice rock interface at 1004 m and constituted the brittle zone the core has undergone minimal relaxation and has remained brittle and prone to fracturing more than 5 years after it was drilled this behavior is attributed to unrelieved stresses from kamb and bindschadler ice streams former ice streams c and d flanking the dome melt layers were identified sporadically throughout the core as were inclined layers tilted at angles that occasionally exceeded 10 structurally the ice was characterized by extensive recrystallization including grain size changes from 0 074 cm 2 at 59 m to 50 cm 2 at 992 m and major transitions in c axis fabrics unusual fabrics included vertical c axis clusters superimposed on vertical girdles that may reflect vertical compression acting in conjunction with horizontal tension the sudden appearance of a shear type fabric at 700 800 m appears closely linked to the occurrence of abundant tephra particles embedded in the ice the occurrence of dispersed sediment in the bottom 2 m is attributed to freeze on of basal meltwater

polar ice structure and the integrity of ice core paleoclimate records abstract polar ice is a unique archive of the climatic conditions in the past however ice sheets flow and this flow may affect the integrity of paleoclimate records a useful method to analyze the effect of ice flow upon climate records is the combination of microstructure mapping with ice core line scanning microstructure and stratigraphy have been mapped along the entire epica dml ice core with this combined method on the macroscale the stratigraphy seems perfectly preserved down to ca 1700 m depth mis4 below which minor undulations start to develop layers inclined up to 15 and millimeter scale z folds are observed below 2050 m depth notwithstanding the epica dml climate record appears not seriously disturbed down to ca 2400 m depth which marks the climatic transition from the last interglacial mis5e to the mis6 glacial period below this depth the synchronization with the epica dome c record is lost and stratigraphic disturbances appear up to the meter scale on the microscopic scale we observe dynamic recrystallization already in deep firn leading to substantial microstructural changes prior to bubble close off the concentration of visible micro inclusions in the holocene part of the core seems to increase with depth which could possibly indicate post depositional formation of salts in glacial period ice the concentration of visible micro inclusions in certain layers is so high that these strata appear as light scattering bands often called cloudy bands another interesting stratigraphic feature is what we call bubble free bands viz millimeter thick strata deprived of bubbles and rich in clathrate hydrates which are found within the bubble hydrate transition zone 800 1200 m depth and are probably caused by anomalously fast clathration while there is hardly any interaction between visible micro inclusions and grain boundaries down to 2500 m depth in the deeper ice warmer than 10 c many grain boundaries seem to harvest micro inclusions here we discuss these observations and their relevance for the interpretation of climate records

synchronous climate changes in antarctica and the north atlantic central greenland ice cores provide evidence of abrupt changes in climate over the past 100 000 years many of these changes have also been identified in sedimentary and geochemical signatures in deep sea sediment cores from the north atlantic confirming the link between millennial scale climate variability and ocean thermohaline circulation it is shown here that two of the most prominent north atlantic events the rapid warming that marks the end of the last glacial period and the bolling allerod younger dryas oscillation are also recorded in an ice core from taylor dome in the western ross sea sector of antarctica this result contrasts with evidence from ice cores in other regions of antarctica which show an asynchronous response between the northern and southern hemispheres

observable surface anomalies preceding simulated isolated convective initiation abstractidealized ensemble simulations of isolated convective initiation ci are analyzed to identify storm scale features in surface weather fields that precede initiation in a variety of background environments and the observations that would be needed to resolve these features precipitating storms are identified with an object based method and composites of surface anomalies are generated for the variables of interest surrounding times and locations of initiation correlation length scales and anomaly magnitudes throughout the ci process are examined in detail with the latter comparing favorably to anomaly estimates obtained from previous observational and modeling studies negative temperature anomalies due to cloud shadowing are found to be the most prominent storm scale feature prior to initiation significant spatial correlations are shown to extend from the surface throughout the boundary layer and even into the cloud bearing layer once deep convective clouds become established the findings are

non basal dislocations should be accounted for in simulating ice mass flow prediction of ice mass flow and associated dynamics is pivotal at a time of climate change ice flow is dominantly accommodated by the motion of crystal defects the dislocations in the specific case of ice their observation is not always accessible by means of the classical tools such as x ray diffraction or transmission electron microscopy tem part of the dislocation population the geometrically necessary dislocations gnds can nevertheless be constrained using crystal orientation measurements via electron backscattering diffraction ebsd associated with appropriate analyses based on the nye 1950 approach the present study uses the weighted burgers vectors a reduced formulation of the nye theory that enables the characterization of gnds applied to ice this method documents for the first time the presence of dislocations with non basal c or burgers vectors these c or dislocations represent up to 35 of the gnds observed in laboratory deformed ice samples our findings offer a more complex and comprehensive picture of the key plasticity processes responsible for polycrystalline ice creep and provide better constraints on the constitutive mechanical laws implemented in ice sheet flow models used to predict the response of earth ice masses to climate change

theory of metamorphism of dry snow the growth of ice particles in dry seasonal snow is caused by vapor diffusion among particles due to temperature gradients imposed on the snow cover the diffusion is calculated by using the potential field solutions for electrostatically charged particles the stereography of snow is represented by using a log normal distribution function for a geometrical enhancement factor defined here reasonable crystal growth rates and supersaturations are found the transition between the growth of highly faceted crystals and the growth of highly rounded crystals is determined by the critical supersaturation for the onset of dislocation aided growth thermal convection and the continuous movement of vapor around the particles due to the imposed temperature gradient are accommodated in the theory although both have little effect relative to the interparticle diffusion the growth of a layer of faceted crystals just below semi permeable crusts is explained by showing that crusts can cause the local supersaturation to exceed the critical value faceted crystals are shown to grow most rapidly in the lower warmer portions of the snow cover because of temperature effects on growth rate

a comparison of radiated energy from diamond impregnated coring and reverse circulation percussion drilling methods in hard rock environments abstractwe compared and analyzed hard rock drilling elastic energy emission between diamond impregnated coring and reverse circulation rc percussion drilling methods from an experiment at brukunga south australia the two drilling mechanisms generated very different seismic wavefields this comparison emphasized their energy radiation differences and signal characteristics from the field data by investigating the raw data energy frequency analysis and crosscorrelation tests the seismic records from percussive rc drilling provided a strong indication that the drill bit energy could be suitable for drill bit seismic imaging purposes the energy radiation from the percussive rc bit could also provide high resolution images with borehole seismic acquisition in contrast at comparable drilling conditions the coring drilling using a diamond impregnated coring bit was quiet the radiated acoustic energy from this drilling mechanism was difficult to detect by a surface receiver array and there was no co

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

tomographic imaging of rock conditions ahead of mining using the shearer as a seismic source a feasibility study roof falls due to poor rock conditions in a coal longwall panel may threaten miner s life and cause significant interruption to mine production there has been a requirement for technologies that are capable of imaging the rock conditions in longwall coal mining ahead of the working face and without any interruption to production a feasibility study was carried out to investigate the characteristics of seismic signals generated by the continuous coal cutter shearer and recorded by geophone arrays deployed ahead of the working face for the purpose of seismic tomographic imaging of roof strata condition before mining two experiments were conducted at a coal mine using two arrays of geophones the experiments have demonstrated that the longwall shearer generates strong and low frequency 40 hz seismic energy that can be adequately detected by geophones deployed in shallow boreholes along the roadways as far as 300 m from the face using noise filtering and signal cross correlation techniques the seismic arrival times associated with the shearer cutting can be reliably determined it has proved the concept that velocity variations ahead of the face can be mapped out using tomographic techniques while mining is in progress

the response of fabric variations to simple shear and migration recrystallization the observable microstructures in ice are the result of many dynamic and competing processes these processes are influenced by climate variables in the firn layers deposited in different climate regimes may show variations in fabric which can persist deep into the ice sheet fabric may remember these past climate regimes we model the evolution of fabric variations below the firn ice transition and show that the addition of shear to compressive stress regimes preserves the modeled fabric variations longer than compression only regimes because shear drives a positive feedback between crystal rotation and deformation even without shear the modeled ice retains memory of the fabric variation for approx 200 unit ka in typical polar ice sheet conditions our model shows that temperature affects how long the fabric variation is preserved but only affects the strain integrated fabric evolution profile when comparing results straddling the thermal activation energy threshold approx 10 degr unit c even at high temperatures migration recrystallization doesn t eliminate the modeled fabric s memory under most conditions high levels of nearest neighbor interactions will however eliminate the modeled fabric s memory more quickly than low levels of nearest neighbor interactions ultimately our model predicts that fabrics will retain memory of past climatic variations when subject to a wide variety of conditions more found in polar ice sheets less

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

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

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

the role of crystal fabric in flow near an ice divide polycrystalline ice near an ice divide typically shows a crystal fabric crystal preferred orientation with c axes clustered vertically we explore the effect of this fabric on the large scale flow pattern near an ice divide we incorporate an analytical formulation for anisotropy into a non linear flow law within a finite element ice sheet flow model with four different depth profiles of crystal fabric we find that the effect of fabric is significant only when a profile has a minimum cone angle of less than 25 for a steady state divide the shape and size of the isochrone arch can depend as much on the crystal fabric as it does on the non linearity of ice flow a vertically oriented fabric tends to increase the size of the isochrone arch never to reduce it also non random fabric has little effect on the ice divide flow pattern when ice is modeled as a linear newtonian fluid finally when we use a crystal fabric profile that closely approximates the measured profile for siple dome west antarctica the model predicts concentrated bed parallel shearing 300 m above the bed

prediction of convective initiation and storm evolution on 12 june 2002 during ihop_2002 part i control simulation and sensitivity experiments abstract the 12 13 june 2002 convective initiation case from the international h2o project ihop 2002 field experiment over the central great plains of the united states is simulated numerically with the advanced regional prediction system arps at 3 km horizontal resolution the case involves a developing mesoscale cyclone a dryline extending from a low center southwestward with a cold front closely behind which intercepts the midsection of the dryline and an outflow boundary stretching eastward from the low center resulting from earlier mesoscale convection convective initiation occurred in the afternoon at several locations along and near the dryline or near the outflow boundary but was not captured by the most intensive deployment of observation instruments during the field experiment which focused instead on the dryline outflow boundary intersection point standard and special surface and upper air observations collected during the field experiment are assimilated into the arps at hourly inte

dynamic recrystallization and texture development in ice as revealed by the study of deep ice cores in antarctica and greenland the preferred c axis orientation of ice from polar ice sheets develops essentially as a result of intracrystalline slip but dynamic recrystallization appears to alter the kinetics of the development of deformation textures and is at high temperature at the origin of recrystallization textures the purpose of this work is to obtain a better understanding of recrystallization processes that occur in polar ice sheets and to clarify the relationship between dynamic recrystallization and textures the study was based on two deep ice cores from greenland and antarctica the greenland ice core project grip and vostok ice cores the structure along the grip core displays normal grain growth in the first 100 m of the ice sheet and rotation recrystallization and migration recrystallization near the bottom only grain growth and rotation recrystallization appear to occur in the vostok ice core the transition between these recrystallization regimes was studied estimating for interglacial ice the evolution with depth of the dislocation density this calculation has shown the efficiency of grain boundary migration for the absorption of dislocations at vostok the highest value of the dislocation density is found at a depth of about 1000 m and the continuous decrease in the dislocation density below this depth is related to the increase of the grain boundary migration rate it is shown that the driving force required to initiate migration recrystallization is not reached in interglacial ice at vostok the observed textures were compared with those predicted by the self consistent approach recrystallization textures are interpreted by assuming that the less stressed grains i e the best oriented for basal slip are favored by the size advantage of subgrains the recrystallization textures are compared with those of other materials

the evolution of crystal fabric in ice sheets and its link to climate history the evolution of preferred crystal orientation fabrics is strongly sensitive to the initial fabric and texture a perturbation in climate can induce variations in fabric and texture in the firn feedbacks between fabric evolution and ice deformation can enhance these variations through time and depth in an ice sheet we model the evolution of fabric under vertical uniaxial compression and pure shear regimes typical of ice divides using an analytic anisotropic flow law applied to an aggregate of distinct ice crystals the model evolves the fabric and includes parameterizations of crystal growth polygonization and migration recrystallization stress and temperature history drive the fabric evolution using this model we explore the evolution of a subtle variation in near surface fabric using both constant applied stress and a stress temperature history based on data from taylor dome east antarctica our model suggests that a subtle variation in fabric caused by climate perturbations will be preserved through time and depth in an ice sheet the stress history and polygonization rate primarily control the magnitude of the preserved climate signal these results offer the possibility of extracting information about past climate directly from ice fabrics

seeing coal seam top ahead of the drill bit through seismic while drilling blast damage to the tops of coal seams due to incorrect blast standoff distances is a serious issue costing the industry in australia about one open cut mine for every ten operating mines the current approach for mapping coal seam tops is through drilling and pierce point logging to provide appropriate depth control with accuracy of 0 2 m for blast hole drilling it is typically necessary to drill deep reconnaissance boreholes on a 50 m x 50 m grid well in advance of overburden removal pierce point mapping is expensive and can be inaccurate particularly when the seam is disturbed by rolls faults and other obstacles numerical modelling and prototype field testing are used in this paper to demonstrate the feasibility of two seismic while drillingbased approaches for predicting the approach to the top of coal during blast hole drilling i reverse walk away vertical seismic profiling recording in which the drill bit vibration provides the source signal and the geophones are planted on the surface near the drill rig and ii in seam seismic recording in which channel waves driven by the coupling to the coal of the seismic signal emitted by the approaching drill bit are guided by the seam to geophones located within the seam in nearby or remote boreholes

a user friendly anisotropic flow law for ice sheet modelling for accurate ice sheet flow modelling the anisotropic behaviour of ice must be taken fully into account however physically based micro macro m m models for the behaviour of an anisotropic ice polycrystal are too complex to be implemented easily in large scale ice sheet flow models an easy and efficient method to remedy this is presented polar ice is assumed to behave as a linearly viscous orthotropic material whose general flow law golf depends on six parameters and its orthotropic fabric is described by an orientation distribution function odf depending on two parameters a method to pass from the odf to a discrete description of the fabric and vice versa is presented considering any available m m model the parameters of the golf that fit the response obtained by running this m m model are calculated for any set of odf parameters it is thus possible to tabulate the golf over a grid in the space of the odf parameters this step is performed once and for all ice sheet flow models need the general form of the golf to be implemented in the available code once then during each individual run to retrieve the golf parameters from the table by interpolation as an example author i have changed from application ok the golf is tabulated using three different m m models and used to derive the rheological properties of ice along the grip ice core

grain area as a statistical weight for polycrystal constituents by using recently developed automatic instruments for fabric and texture measurements on ice both the c axis orientation and area of the individual crystals can be determined each grain can then be associated with its volume fraction defined as a function of its measured cross sectional area to describe the microstructure of a a polycrystal the relevance of this approach is studied using a three dimensional microstructure obtained from the potts model in particular the area weighting is compared to the classical implicit equal weighting used by glaciologists which assumes that all the grains have the same volume fraction discrete uniform distribution then using the measurements of c axis orientation and crystal size performed on the north greenland icecore project northgrip ice core we compare area weighted and equal weighted fabrics all these comparisons are made with respect to the orientation tensor according to the ability of the potts model to reproduce the ice microstructure it is shown that using the grain cross sectional area to infer its volume fraction improves the description of the actual polycrystal fabric

flow law hypotheses for ice sheet modeling ice flow modeling requires a flow law relating strain rates to stresses in situ but a flow law cannot be measured directly in ice sheets microscopic processes such as dislocation glide and boundary diffusion control both the flow law for ice and the development of physical properties such as grain size and c axis fabric these microscopic processes can be inferred from observations of the physical properties and the flow law can then be estimated from the microscopic processes a review of available literature shows that this approach can be imperfectly successful interior regions of large ice sheets probably have depth varying flow law constants with the stress exponent n for power law creep less than 3 in upper regions and equal to 3 only in deep ice n probably equals 3 through most of the thickness of ice shelves and ice streams

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

ice flow structure and ice dynamic changes in the weddell sea sector of west antarctica from radar imaged internal layering recent studies have aroused concerns over the potential for ice draining the weddell sea sector of west antarctica to figure more prominently in sea level contributions should buttressing from the filchner ronne ice shelf diminish to improve understanding of how ice stream dynamics there evolved through the holocene we interrogate radio echo sounding res data from across the catchments of institute and moller ice streams iis and mis focusing especially on the use of internal layering to investigate ice flow change as an important component of this work we investigate the influence that the orientation of the res acquisition track with respect to ice flow exerts on internal layering and find that this influence is minimal unless a res flight track parallels ice flow we also investigate potential changes to internal layering characteristics with depth to search for important temporal transitions in ice flow regime our findings suggest that ice in northern iis draining the ellsworth subglacial highlands has retained its present ice flow configuration throughout the holocene this contrasts with less topographically constrained ice in southern iis and much of mis whose internal layering evinces spatial changes to the configuration of ice flow over the past 10 000 years our findings confirm siegert et al s 2013 inference that fast flow was diverted from bungenstock ice rise during the late holocene and suggest that this may have represented just one component of wider regional changes to ice flow occurring across the iis and mis catchments as the west antarctic ice sheet has thinned since the last glacial maximum

an analytical approach to deformation of anisotropic ice crystal aggregates deformation rates of single hexagonal crystals deforming by glide on the basal plane are described as a function of stress state and crystal orientation these results are used to infer the deformation rate of crystal aggregates assuming that the stress distribution within the crystal aggregate is homogeneous analytical equations for the deformation rate of anisotropic ice aggregates are derived for vertically symmetric girdle fabric this type of fabric is approximated by a uniform distribution of c axis orientations between a cone angle and a smaller girdle angle relative to the symmetry axis for simple shear stress acting on a single maximum fabric there is a slight de enhancement for cone angles of 60 90 in uniaxial compression a maximum enhancement of 1 7 occurs at a cone angle of 57 a pure shear stress state has similar features with the additional complication that it causes a non zero transverse strain rate except for perfect vertical alignment of crystals and isotropic fabric in combined states of stress the contribution of each stress component to the strain rate depends on fabric a single enhancement factor is not adequate to describe the effects of anisotropy for complex stress states

simulation of a serial upstream propagating mesoscale convective system event over southeastern south america using composite initial conditions abstract serial upstream propagating mesoscale convective system mcs events over southeastern south america are important contributors to the local hydrologic cycle as they can provide roughly half of the total monthly summer precipitation however the mechanisms of upstream propagation for these events have not been explored to remedy this situation a numerical simulation of the composite environmental conditions from 10 observed serial mcs events is conducted results indicate that the 3 day simulation from the composite yields a reasonable evolution of the large scale environment and produces a large region of organized convection in the warm sector over an extended period as seen in observations upstream propagation of the convective region is produced and is tied initially to the development and evolution of untrapped internal gravity waves however as convective downdrafts develop and begin to merge and form a surface cold pool in the simulation the cold pool and its interaction with the env

vertical seismic profiling vertical seismic profiling is one of the rapidly developing areas of geophysical technology for exploring in mature basins the measurement basically involves recording the total upgoing and downgoing seismic wave fields propagating through a stratigraphic section by means of geophones clamped to the wall of a drilled well