processing of seismic data in the presence of anisotropy processing techniques for extracting the polarization directions of the fast and slower split shear waves together with their traveltime splitting from vector wavefield data form the basis of prospective procedures for imaging details of the internal structure of subsurface rocks a recently developed technique involving the independent rotation of the source polarization direction and geophone axes provides an effective processing tool for extracting the effects of anisotropy from shear wave data it is theoretically capable of handling nonorthogonal polarization directions of split shear waves it is further developed and its performance examined using vertical seismic profiling vsp shear wave datasets for different anisotropic earth models and computed using full wave modeling the procedure works well for shear waves propagating through an earth model with a uniform crack strike with depth but produces inaccurate estimates of polarization and time delay when the crack strike varies abruptly

prediction of lateral variability in fracture intensity using multicomponent shear wave surface seismic as a precursor to horizontal drilling in the austin chalk summary a method of multicomponent multisource shear wave interpretation utilizing the output of the alford rotation for separating the fast and slow particle motion stacks in azimuthally anisotropic media yielded a qualitative prediction of lateral fracture intensity variation prior to horizontal drilling the method exploits a fundamental difference between the two particle motion stacks when vertical aligned fractures are the source of the shear wave splitting the fast shear wave s1 section is insensitive to the lateral variation in fracture intensity since its particle motion is parallel to the fractures hence reflectivity on the s1 cdp stacked section should be likewise insensitive to fractures the slow shear wave s2 section however possesses particle motion perpendicular to the fracture planes so variation in lateral fracture intensity will affect velocity and reflectivity examination of the reflectivity on the two sections for a seismic line reveals laterally consistent reflector strength on the s1 section for the austin chalk the horizon of interest but laterally inconsistent reflector strength for the same horizon on the s2 section in the study area reflector dimming on the s2 section was interpreted as indicating fracture intensity maxima horizontal drilling confirmed both fracture azimuth and intensity predictions and resulted in significantly higher initial hydrocarbon production than for surrounding wells

twelve years of vertical birefringence in nine component vsp data since 1986 when industry scientists first publicly showed data supporting the presence of azimuthal anisotropy in sedimentary rock we have studied vertical shear wave s wave birefringence in 23 different wells in western north america the data were from nine component vertical seismic profiles vsps supplemented in recent years with data from wireline crossed dipole logs this paper summarizes our results including birefringence results in tabular form for 54 depth intervals in 19 of those 23 wells in the appendix we present our conclusions about how to record vsp data optimally for study of vertical birefringence we arrived at four principal conclusions about vertical s wave birefringence first birefringence was common but not universal second birefringence ranged from 0 21 but values larger than 4 occurred only in shallow formations 1200 m within 40 km of california s san andreas fault third at large scales birefringence tended to be blocky that is both the birefringence magnitude

special section on azimuthal dependence of p wave seismic signatures introduction this special issue is based on papers presented at the postconvention seg workshop on azimuthal dependence of p wave signatures held in dallas in 1997 the main motivation for analyzing the azimuthal variation of seismic traveltimes amplitudes attenuation etc is to obtain reliable information about azimuthal anisotropy in the subsurface another potential application of multiazimuth techniques is in finding andmappingdesirablelateralheterogeneitiesthatcould masquerade as azimuthal anisotropy the last topic has not yet been fully discussed and is not addressed in the special issue butseveralexplorationanddevelopmentscenarioscontainoriented lateral heterogeneities sand channels etc that at the right scale length could be highly visible in properly processed wide azimuth 3 d data themostcommonphysicalreasonsforazimuthalanisotropy are systems of vertical or steeply dipping fractures or tilted transversely isotropic layers both types of azimuthally anisotropic models are discussed here with an emphasis on issues related to fracture detection and characterization in tight formations the extraction offluids is often impossible without exploiting the increased drainage provided by fracture networks therefore geophysicalcharacterizationoffracturedreservoirs is an extremely important problem from an economic standpoint sweet spots of high fracture intensity are not always correlated with faults and other structural features and are difficult to detect using conventional exploration techniques effective exploitation of fractured reservoirs also requires determination of the orientation and connectivity of fractures as well as their spatial relationships to other reservoir heterogeneities most existing seismic methods for fracture characterization are based on analysis of shear wave splitting e g crampin 1985 thomsen 1988 winterstein and meadows 1991 mueller 1992 since split s waves travel along essentially the same raypath the difference between their velocities andreflectionamplitudesdependsprimarilyontheanisotropic properties of the medium rather than on lateral heterogeneity theshear wavetechnology however hasknownshortcomings

interpretation of total wave field data over lost hills field kern county california approximately 5 miles 8 km of total wave field data were acquired by production geophysical services then kim tech inc using the omnipulse multi mode shear wave generator source over the southern end of lost hills field kern county california the quality of the shear wave sections was excellent they represent a significant improvement over conventional p wave sections from the area in that they provide better reflection continuity and imaging of the lost hills anticline a multicomponent vsp which was acquired close to the line provided crucial p wave to s wave correlation as well as fracture information vp vs ratios computed from interval times ranged from 2 79 to 1 63 an anomalously low vp vs ratio of 1 65 in the zone of interest lower reef ridge to mcdonald shale confirmed by multicomponent vsp data corresponds to the producing interval evidence of shear wave splitting due to azimuthal anisotropy was observed so the sv wave and sh wave data sets were rotated into principal compo

identification of fractured carbonate vuggy reservoirs in the s48 well area using 3d 3c seismic technique a case history from the tarim basin

pore fluid effects on seismic velocity in anisotropic rocks a simple new technique predicts the high and low frequency saturated velocities in anisotropic rocks entirely in terms of measurable dry rock properties without the need for idealized crack geometries measurements of dry velocity versus pressure and porosity versus pressure contain all of the necessary information for predicting the frequency dependent effects of fluid saturation furthermore these measurements automatically incorporate all pore interaction so there is no limitation to low crack density the velocities are found to depend on five key interrelated variables frequency the distribution of compliant crack like porosity the intrinsic or noncrack anisotropy fluid viscosity and compressibility and effective pressure the sensitivity of velocities to saturation is generally greater at high frequencies than low frequencies the magnitude of the differences from dry to saturated and from low frequency to high frequency is determined by the compliant or crack like porosity predictions of s

daytime rock surface temperature variability and its implications for mechanical rock weathering tenerife canary islands summary summer and winter rock surface temperature measurements are presented from three sites along an altitudinal gradient significant between and within site variability is recognised related to insolation cloud cover and air temperature differences maritime influences and time of year the variability argues against the use of a single rock temperature regime to characterise hot desertic environments in for example weathering simulation studies superimposed upon diurnal patterns of heating and cooling are numerous short 3 15 minutes and medium term 1 2 hours fluctuations related to wind speed and cloud cover variations rates of surface temperature change are very high during these fluctuations 3 c minute and it is proposed that they may enhance the operation of a number of mechanical rock weathering mechanisms

estimation of reservoir fracturing from marine vsp using local shear wave conversion a marine vsp is designed to estimate the orientation and density of fracturing within a gas producing dolomite layer in the southern north sea the overburden anisotropy is firstly estimated by analysing shear waves converted at or just below the sea bed from airgun sources at four fixed offset azimuths full wave modelling helps confirm that the background has no more than 3 vertical birefringence originating from tih anisotropy with a symmetry axis orientated perpendicular to the maximum horizontal compressive stress of nw se this finding concurs with current hypotheses regarding the background rock matrix in the upper crust more detailed anisotropy estimates reveal two thin zones with possible polarization reversals and a stronger anisotropy the seismic anisotropy of the dolomite is then determined from the behaviour of locally converted shear waves providing a direct link with the physical properties of its fractures it is possible to utilize this phenomenon due to the large seismic velocity contrast between the dolomite and the surrounding evaporites two walkaway vsps at different azimuths recorded on three component receivers placed inside the target zone provide the appropriate acquisition design to monitor this behaviour anisotropy in the dolomite generates a transverse component energy which scales in proportion to the degree of anisotropy the relative amplitudes for this component between the different walkaway azimuths relate principally to the orientation ofthe anisotropy full wave modelling confirms that a 50 vertical birefringence from tih anisotropy with a similar orientation to the overburden is required to simulate the field observations this amount of anisotropy is not entirely unexpected for a fine grained brittle dolomite with a potentially high fracture intensity particularly if the fractures contain fluid which renders them compliant to the shear wave motion

misalignment of the orientation of fractures and the principal axes for p and s waves in rocks containing multiple non orthogonal fracture sets summary azimuthal anisotropy in rocks can result from the presence of one or more sets of partially aligned fractures with orientations determined by the stress history of the rock a shear wave propagating in an azimuthally anisotropic medium splits into two components with diverent polarizations if the source polarization is not aligned with the principal axes of the medium in the presence of two or more non orthogonal sets of vertical fractures the symmetry of the medium may be approximated as monoclinic with a horizontal plane of mirror symmetry if in the absence of fractures the rock is transversely isotropic ti with the symmetry axis perpendicular to the bedding plane for such a medium the fast and slow polarization directions for vertically propagating shear waves are not parallel or perpendicular to any of the fracture planes but lie in directions given by the principal axes of a second rank fracture compliance tensor this tensor is independent of the normal compliance of the fractures it follows that for vertical propagation of shear waves in a vertically fractured ti medium any number of arbitrarily oriented vertical fracture sets is equivalent to two mutually perpendicular fracture sets provided that the seismic wavelength is large compared to the size of the fractures for ovsets typical of surface seismic acquisition the p wave velocity at fixed ovset varies with azimuth as cos 2 w w 0 where w is the azimuth measured with respect to the fast polarization direction for a vertically polarized shear wave w 0 depends on both the normal and the shear compliance of the fractures and may diver from zero if the ratio of the normal to shear compliance of the fractures varies significantly between fracture sets if this ratio is similar for all fractures w 0 0 and the principal axes of the variation in p wave velocity with azimuth for fixed ovset are determined by the principal axes of the second rank fracture compliance tensor

why the western pacific subtropical high has extended westward since the late 1970s the western pacific subtropical high wpsh is closely related to asian climate previous examination of changes in the wpsh found a westward extension since the late 1970s which has contributed to the inter decadal transition of east asian climate the reason for the westward extension is unknown however the present study suggests that this significant change of wpsh is partly due to the atmosphere s response to the observed indian ocean western pacific iwp warming coordinated by a european union s sixth framework programme understanding the dynamics of the coupled climate system dynamite five agcms were forced by identical idealized sea surface temperature patterns representative of the iwp warming and cooling the results of these numerical experiments suggest that the negative heating in the central and eastern tropical pacific and increased convective heating in the equatorial indian ocean maritime continent associated with iwp warming are in favor of the westward extension of wpsh the sst changes in iwp influences the walker circulation with a subsequent reduction of convections in the tropical central and eastern pacific which then forces an enso gill type response that modulates the wpsh the monsoon diabatic heating mechanism proposed by rodwell and hoskins plays a secondary reinforcing role in the westward extension of wpsh the low level equatorial flank of wpsh is interpreted as a kelvin response to monsoon condensational heating while the intensified poleward flow along the western flank of wpsh is in accord with sverdrup vorticity balance the iwp warming has led to an expansion of the south asian high in the upper troposphere as seen in the reanalysis

surface deformation associated with a dipping hydrofracture crustal deformation associated with hydrofracture is modeled by a dipping rectangular dislocation beneath the surface of an elastic half space the burger s vector is taken normal to the rectangular surface analytical expressions for vertical displacements are obtained from integration of volterra s equation by using mindlin s point force solutions for the elastic halfspace in the limits of 0 and 90 dip the results agree with those obtained by maruyama 1964 for poisson s ratio 0 25 expressions for arbitrary poisson s ratio are given for elongate dislocations the displacement field agrees with curves given by pollard and holzhausen 1979 for two dimensional near surface fractures at various dips at dip 0 a deep square dislocation gives displacements indistinguishable from those of sun s 1969 model of a horizontal penny shaped crack beneath the surface of an elastic half space modifying his result for shallow depths shows that they correspond to a dislocation of semi elliptical cross section with the surface displacement field asymptotically approaching a semi elliptical shape as depth decreases to zero similarly the surface displacement field of the square dislocation approaches that of its rectangular cross section nonlinear inversion of surface tilt fields associated with hydrofracture of deep boreholes gives estimates of fracture geometry position and orientation along with their uncertainties two examples are presented one of a near horizontal and one of a near vertical fracture

layer stripping of shear wave splitting in marine ps waves summary the properties of split s waves can be used to infer 1 the state of stress and strain in the earth 2 the directional dependence of hydraulic conductivity and 3 small changes in pore fluid pressure in the rock mass that occur in response to dynamic processes such as the earthquake cycle measurements of split s waves are particularly useful in shallow 1000 m subseabed marine sediments where s wave splitting from an azimuthal elastic anisotropy is typically produced by the presence of near vertical aligned cracks here we present a method of measuring small amounts of s wave splitting in marine p to s mode converted waves and illustrate the technique with data from an ocean bottom seismometer obs deployed on the west svalbard continental slope the analysis applies a modified version of the alford rotation and layer stripping technique developed for zero offset s wave sources and treats ps waves that undergo mode conversion at reflectors that are close to the seabed in comparison with the overlying water depth when the seismic record contains coherent signal on both the in plane and out of plane components the layer stripping technique is capable of decoupling the swave splitting from the effects of p wave velocity anisotropy and reflector dip that influence the downgoing p wave part of the ray path the amount of s wave splitting in the data is small however and we find that this causes a greater practical problem for the analysis than the known theoretical limitations of the layer stripping theory such as use of a finite offset source for the analysis of the example data we develop a number of procedures that are necessary to mitigate the low signal to noise levels these include using a wide range of shot receiver azimuths to generate data redundancy methods of identifying and rejecting poor measurements and a predictive layer stripping approach that minimizes the propagation of errors through the analysis that arise from scatter in the layer by layer results with the ps waves of the example data which have a dominant period about 30 ms we find the technique is capable of measuring s wave splitting to a precision of about 0 5 ms for 4 or 5 layers the number of layers successfully treated would increase if the amount of s wave splitting were larger than in these data for which the total cumulative s wave splitting was about 10 ms over a 450 m depth interval the orientation of the fast split s wave was measured with a precision of about 15 our results give an s wave velocity anisotropy of 1 2 per cent in the shallowest 25 30 m subseabed that implies the presence of a differential horizontal stress at or very close to the seabed the s wave splitting accumulated throughout the investigated section at a rate that was consistent with predictions made for a single set of parallel fluid filled cracks with crack density about 0 015 the fast s wave was found to be oriented at 75 15 for the uppermost 150 m before drifting clockwise to an azimuth of 190 210 the clockwise drift in the fast s wave polarization direction implies a transfer of dominance between one set of cracks in the near surface probably related to the slope of the seabed and a different set in deeper sediments of tectonic origin a zone of azimuthal isotropy where the mechanisms that produce the elastic anisotropy may cancel each other out occupies the interval between the two almost orthogonal sets of cracks from interpretation

rock block monitoring of rapid salt weathering in southern tunisia rectangular blocks of york stone and of concrete placed on a sodium chloride sabkha in southern tunisia for six years suffered very severe breakdown thereby indicating the power of salt weathering as a process in sabkha environments

linking reservoir geomechanics and time lapse seismics predicting anisotropic velocity changes and seismic attributes seismic technology has been used successfully to detect geomechanically induced signals in repeated seismic experiments from more than a dozen fields to explain geomechanically induced time lapse 4d seismic signals we use results from coupled reservoir and geomechanical modeling the coupled simulation yields the 3d distribution over time of subsurface deformation and triaxial stress state in the reservoir and the surrounding rock predicted changes in triaxial stress state are then used to compute changes in anisotropic p and s wave velocities employing a stress sensitive rock physics transform we predict increasing vertical p wave velocities inside the reservoir accompanied by a negative change in p wave anisotropy e 0 a stress sensitive rock physics transform that predicts a

depth dependent crustal anisotropy at midwestern honshu japan aligned cracks possibly induced by tectonic stress produce anisotropy in the earth s crust permeability and state of stress at several depths in the crust can be estimated from the state of crack distribution we present clear evidence that crustal anisotropy changes with depth at the inagawa earthquake swarm area mid western honshu japan the observed fast shear wave directions are parallel to the maximum horizontal compressional axes obtained from a hydro fracturing test at a depth of 0 80 km and focal mechanism solution at depths 4 8 km in the same area these observations provide us the evidence that the crustal anisotropy in this region is controlled by the tectonic stress we estimated the degree of anisotropy that is the ratio of the difference between fast and slow s wave velocities as a function of depth using the layer stripping method the obtained value is 2 or less at most depths but anisotropy of 11 is observed at depths 6 8 km this high anisotropy layer has a local gradient in seismicity rates we infer that vertical fluid flow from the high anisotropy layer would be trapped in the low anisotropy layer above pore pressure would increase and it could generate earthquakes this correspondence suggests that the degree of anisotropy in the upper crust changes with depth and has some relation to seismicity

earthquake prediction a new physical basis summary subcritical crack growth in the laboratory occurs slowly but progressively in solids subjected to low stresses at low strain rates the cracks tend to grow parallel to the maximum compressive stress so that when this stress is aligned over a large enough region the cracks will also be aligned and possess effective seismic anisotropy it is suggested that such subcritical crack growth produces extensive dilatancy anisotropy eda throughout earthquake preparation zones this process is a possible driving mechanism for earthquake precursors observed at substantial distances from impending focal zones and provides in the shear wave splitting which has been observed in several seismic regions a possible technique for monitoring the build up of stress before earthquakes

seismic detection of a hydraulic fracture from shear wave vsp data at lost hills field california a shear wave s wave vsp experiment was performed at lost hills field california in an attempt to detect hydraulic fractures induced in a nearby well the hydrofrac well was located between an impulsive s wave source on the surface and a receiver well containing a clamped three component geophone both direct and scattered waves were detected immediately after shut in when the hydraulic pumps were shut off and recording started the scattered energy disappeared within about an hour which is consistent with other measurements that indicate some degree of fracture closure and leak off within that period although s wave splitting was evident no change was detected in the fast wave polarized parallel to the fracture however the slow wave polarized perpendicular to the fracture did change over a period of about an hour after which the prehydrofrac wavelet shape was recovered the fact that only the wave polarized perpendicular to the fracture was affected is a dramatic confirmation of both theoretical predictions and laboratory observations of s wave behavior in a fractured medium subtracting the prehydrofrac wavelet from the wavelets recorded within the first hour after shut in revealed scattered wavelets that were diminished and phase rotated versions of the incident prehydrofrac wavelet arrival times of the direct and scattered waves were matched by ray tracing we accounted for the scattered wave amplitudes by using numerical solutions of s wave diffractions off of ribbon shaped fractures amplitudes derived from full wavefield born scattering however did not match recorded amplitudes the phase of the scattered wavelets was matched very well by born scattering when the incident wavelet was input but only for fracture lengths no larger than half those predicted from fracture simulator models these results show that a carefully controlled experiment combined with accurate modeling can provide important information about the geometry of induced fractures

continuation of a deep borehole stress measurement profile near the san andreas fault 2 hydraulic fracturing stress measurements at black butte mojave desert california hydraulic fracturing stress measurements were obtained in the black butte drill hole 18 km northeast of the san andreas fault in the mojave desert at depths from 251 to 635 m in all tests the least and greatest horizontal principal stresses s h and s h respectively exceeded the vertical stress s indicating a thrust faulting stress regime a single good quality hydraulic fracture impression from 309 m depth indicates an s h direction of n41 e 10 this s h direction should be interpreted with caution because it is based on only one observation this orientation is fairly compatible with nearby surface stress measurements but is incompatible with most of the hydraulic fracturing stress orientations reported from comparable depths in the mojave desert and is not favorable for right lateral slip on either the san andreas fault or nw striking faults present farther to the east the stress regime measured in the black butte hole is comparable to that measured at nearby shallow depths but differs from the strike slip or transitional strike slip to thrust faulting stress regime present at similar depths in two nearby holes crystallaire 4 km northeast of fhe san andreas fault and hi vista 32 km northeast of the san andreas fault the s h direction measured in these holes is approximately 60 counterclockwise of that observed in the black butte hole the differences in stress magnitudes and orientation among these holes substantiate previous indications of local variations in stress in the upper kilometer of the crust in this area and cast doubt on the validity of linear elastic models in which the effects of the san andreas fault dominate the stress field in the western mojave desert

drought reconstructions for the continental united states abstract the development of a 2 lat 3 long grid of summer drought reconstructions for the continental united states estimated from a dense network of annual tree ring chronologies is described the drought metric used is the palmer drought severity index pdsi the number of grid points is 154 and the reconstructions cover the common period 1700 1978 in producing this grid an automated gridpoint regression method called point by point regression was developed and tested in so doing a near optimal global solution was found for its implementation the reconstructions have been thoroughly tested for validity using pdsi data not used in regression modeling in general most of the gridpoint estimates of drought pass the verification tests used in addition the spatial features of drought in the united states have been faithfully recorded in the reconstructions even though the method of reconstruction is not explicitly spatial in its design the drought reconstructions show that the 1930s dust bow

fractured reservoir delineation using multicomponent seismic data the characteristic seismic response to an aligned fracture system is shear wave splitting where the polarizations time delays and amplitudes of the split shear waves are related to the orientation and intensity of the fracture system this offers the possibility of delineating fractured reservoirs and optimizing the development of the reservoirs using shear wave data however such applications require carefully controlled amplitude processing to recover properly and preserve the reflections from the target zone here an approach to this problem is suggested and is illustrated with field data the proposed amplitude processing sequence contains a combination of conventional and specific shear wave processing procedures assuming a four component recording the split shear waves can be simulated by an effective eigensystem and a linear transform technique can be used to separate the recorded vector wavefield into two principal scalar wavefields representing the fast and slow split shear waves conventional scalar processing methods designed for processing p waves including noise reduction and stacking procedures may be adapted to process the separated scalar wavefields an overburden operator is then derived from and applied to the post stacked scalar wavefields a four component seismic survey with three horizontal wells drilled nearby was selected to illustrate the processing sequence the field data show that vectormore wavefield decomposition and overburden correction are essential for recovering the reflection amplitude information in the target zone the variations in oil production in the three horizontal wells can be correlated with the variations in shear wave time delays and amplitudes and with the variations in the azimuth angle between the horizontal well and the shear wave polarization dim spots in amplitude variations can be correlated with local fracture swarms encountered by the horizontal wells less

spatial expression of enso drought and summer monsoon in seasonal δ13c of ponderosa pine tree rings in southern arizona and new mexico 1 major climatic impacts from enso and the north american monsoon in the u s southwest provide fortuitous circumstances for exploring the suitability of stable carbon isotope 13c composition of tree rings to represent and reconstruct interannual climate variability we sampled ponderosa pine tree rings at eight sites in seven mountain ranges over a 500 km transect in southern arizona and new mexico we developed 13c chronologies for 1985 1995 by subdividing rings into pre and post monsoon onset segments based on the presence of a false latewood band in each ring induced by the arid period immediately preceding the summer monsoon pooling these segments from multiple trees at each site and analyzing the cellulose component chronologies from two subsets of trees from the same site and separate chronologies from two different sites in the same mountain range indicate strong coherence and signal reproducibility additionally the seasonal 13c patterns showed remarkable coherence across the transect with the spatially disjunct easternmost chronology tending to be most frequently dissimilar the 13c values of ring subdivisions before the false latewood band are generally positively correlated with winter southern oscillation indices soi resulting from the enso teleconnection with southwestern winter early spring precipitation the 13c from these sites correlates with monthly palmer drought severity indices pdsi with the strongest correlations occurring in progressively later months from first formed to last formed subdivisions the post false latewood subdivision 13c was particularly strongly correlated with summer precipitation amount these results confirm the strong influence of moisture on tree ring 13c and support their use in both temporal and spatial modes to infer past climate variability

75 plus years of anisotropy in exploration and reservoir seismics a historical review of concepts and methods the idea that the propagation of elastic waves can be anisotropic i e that the velocity may depend on the direction is about 175 years old the first steps are connected with the top scientists of that time people such as cauchy fresnel green and kelvin for most of the 19th century anisotropic wave propagation was studied mainly by mathematical physicists and the only applications were in crystal optics and crystal elasticity during these years important steps in the formal description of the subject were made at the turn of the 20th century rudzki stressed the significance of seismic anisotropy he studied many of its aspects but his ideas were not applied research in seismic anisotropy became stagnant after his death in 1916 beginning about 1950 the significance of seismic anisotropy for exploration seismics was studied mainly in connection with thinly layered media and the resulting transverse isotropy very soon it became clear that the effect of layer induced anisotropy on data ac

estimation of fracture parameters from reflection seismic data part iii fractured models with monoclinic symmetry existing geophysical and geological data indicate that orthorhombic media with a horizontal symmetry plane should be rather common for naturally fractured reservoirs here we consider two orthorhombic models one that contains parallel vertical fractures embedded in a transversely isotropic background with a vertical symmetry axis vti medium and the other formed by two orthogonal sets of rotationally invariant vertical fractures in a purely isotropic host rock using the linear slip theory we obtain simple analytic expressions for the anisotropic coefficients of effective orthorhombic media under the assumptions of weak anisotropy of the background medium for the first model and small compliances of the fractures all effective anisotropic parameters reduce to the sum of the background values and the parameters associated with each fracture set for the model with a single fracture system this result allows us to eliminate the influence of the vti background by evaluating the differences between t

individual and pooled tree ring stable carbon isotope series in chinese pine from the nan wutai region china common signal and climate relationships abstract to investigate the differences in the climatic signals in stable carbon isotopic composition captured by averaging series from individual trees versus raw wood of trees pooled prior to analysis we analyzed two groups of chinese pine pinus tabulaeformis carr from the nan wutai region of the qinling mountains china one group included three trees that were analyzed separately and the other group comprised four other trees that were pooled prior to preparation and analysis all 13 c series were positively correlated r 0 50 0 58 p 13 c of atmospheric co 2 correlations between the meteorological data and all individual and pooled discrimination 13 c series revealed significant negative responses to temperature for several specific months and for mean january to september t j s temperature we used a numerical mix method nmm equivalent to unweighted mean by averaging individual 13 c series nps1 nps2 nps3 to generate a new series that more strongly correlated to climate series t j s r 0 67 p j s is significant for plant growth and is consistent with the tree physiology in this region our results suggest that the numerical mix method with tree ring stable isotope data from three trees provides a series quite satisfactory for climatic reconstruction the relationship of the numerical mix model 13 c with temperature was stronger than for the pooled series suggesting numerical mixing of series can be more effective than raw wood sample pooling at least according to the trees in this study

practical aspects of wavefield separation of two component surface seismic data based on polarization and slowness estimates the processing of multicomponent seismic data carried out individually on the different wavetypes p s and converted waves should result in an improved image of the subsurface we examine the wavefield separation method proposed by cho and spencer we discuss practical aspects related to the separation of interfering waves on two component surface seismic data and illustrate these using synthetic data a sliding spatial window is used for analysis the choice of its width represents a trade off between stabilizing the method in the presence of random noise and ensuring a good spatial resolution no a priori knowledge of the subsurface is required but locally the characteristic parameters of the waves i e horizontal slowness and polarization are assumed to be constant inside the analysis window these parameters are estimated at each frequency but a statistical analysis provides a more robust estimate especially in the presence of random noise this approach also solves the problem of eigenvalue sharing and switching additional smoothing of the estimates according to a model may further improve the results the width of the analysis window may be decreased if the waves inside the data window differ significantly in amplitude the dominant wave in each case is separated from the lower amplitude waves and subtracted from the data this novel iterative approach thereby allows for the isolation of low amplitude events

changes in shear wave polarization azimuth with depth in cymric and railroad gap oil fields shear wave s wave polarization azimuths although consistent over large depth intervals changed abruptly and by large amount of various depths in nine component vertical seismic profiling vsp data from the cymric and railroad gap oil fields of the southwest san joaquin basin a simple layer stripping technique made it possible to follow the polarization changes and determine the s wave birefringence over successive depth intervals because the birefringence and polarization azimuth are related to in situ stresses and fracture information from such analysis could be important for reservoir development near offset vsp data from cymrix indicated that the subsurface could be appproximated roughly as two anisotropic layers the upper layer from the surface to 800 ft 240 m had vertical s wave birefringence as large was about 6 percent down to 1300 ft 400 m in the upper layer the polarization azimuth of the fast s wave was n 60 e while in the lower layer it was about n 10 e refinement of the laye

pre seismic geomagnetic anomaly and earthquake location article i nfo many researchers studied the relationships between appearances of geomagnetic anomalies and their distances to earthquake epicenters or faults yet occasionally some magnetometer stations located nearby earthquake epicenters and or faults do not observe geomagnetic anomalies in this paper a new hybrid system which simultaneously takes the hypocenter and fault plane solution into account is constructed to examine 38 earthquakes interpreted to be associated with geomagnetic anomalies during the period 1988 2001 in taiwan the surface magnetic anomaly reference tip smart of the new system is used instead of the epicenter or the fault to investigate statistically the distance relationship between the anomalies and the earthquake parameters results show that the anomalies gather along the fault and in the belt zone to the smart possible mechanisms causing the anomalies in the two zones are proposed and discussed characteristics of the anomaly might shed some light on locations of faults before earthquake occurrences

reflection shear wave data collected near the principal axes of azimuthal anisotropy the presence of vertically oriented fractures and or unequal horizontal stresses has created an azimuthally anisotropic earth in which shear wave sh data collected along the principal axes of the anisotropy display time and reflection amplitude anomalies amoco shot two crossing shear wave sh lines that were approximately parallel to the orthogonal principal axes of the azimuthal anisotropy at the tie point these crossing sh lines display a time variant mis tie the tie point also displays reflection coefficient anomalies attributable to azimuthally dependent shear wave velocities field mapping documented a set of fractures striking n69e which are approximately parallel to the line that exhibited greater traveltimes time variant mis ties and reflection coefficient anomalies are two of the seismic responses theoretically expected of an azimuthally anisotropic earth i e one in which the shear wave velocity depends upon the polarization azimuth of the shear wave

regional hydroclimate and precipitation δ18o revealed in tree ring cellulose δ18o from different tree species in semi arid northern china abstract oxygen isotopic ratios 18o of tree ring cellulose grown from 1954 to 2003 in semi arid northern china demonstrate a common inter annual variability despite coming from three different species in two distinct growth environments an alpine meadow and a rocky ridge the variability was significantly negative correlated with precipitation and relative humidity during the growing season this suggests that the past summer hydroclimate can be inferred from tree ring cellulose 18o from various kinds of trees growing in semi arid northern china in addition we evaluated past changes in 18o of precipitation from the tree ring cellulose 18o and relative humidity using the mechanistic model for tree ring cellulose 18o in roden et al 2000 by fixing the species dependent exchange rate of oxygen between carbohydrate and xylem water for larix principis rupprechtii and picea koraiensis we could also reconstruct the variations in precipitation 18o from the different tree species which are similar to the observed 18o of precipitation during 1985 2002 although the reconstructed 18o of precipitation does not have any significant relation to local temperature or precipitation during 1954 2003 its long term variation pattern is similar to that of the asian summer monsoon indices and 18o of stalagmite in the heshang cave 30 27 n 110 25 e fig 1 suggesting that 18o of precipitation is not controlled by local meteorology but is influenced by large scale atmospheric circulation

monsoon precipitation variation recorded by tree ring δ18o in arid northwest china since ad 1878 abstract the east asian summer monsoon easm precipitation from may to august over the helan mountains located in the northwest margin of the easm has been investigated through tree ring 18o for the last 120 years the dry and wet intervals derived from the 18o series can be verified by the tree ring width based may july precipitation reconstruction most of the time the 18o series and ring width derived precipitation reconstruction vary synchronously at low frequency high values of 18o correspond to high precipitation periods and vice versa since the 1970s the east asian summer monsoon precipitation has been increasing which was confirmed by other records and documents our results show that tree ring 18o can serve as an indicator of the strength of the east asian summer monsoon variation in the past we also found that tree ring 18o varies inversely to 13c in the study area

multicomponent near surface correction for land vsp data multicomponent seismic data collected using directional sources are degraded by the wave excitation process due to inaccurate control of the ground motion unequal activation strengths or ground couplings between differently oriented sources and misalignment of the pad these acquisition uncertainties are exacerbated by the complicated near surface scattering present in most seismic areas neither group of effects should be neglected in multicomponent analyses that make use of relative wavefield attributes derived from compressional and shear waves these effects prevent analysis of the direct and reflected waves using procedures based on standard scalar techniques or a prima facia interpretation of the vector wavefield properties even for the seemingly straightforward case of a near offset vertical seismic profile vsp near surface correction using a simple matrix operator designed from the shallowest recordings alleviates many of these interpretational difficulties in near offset vsp data results from application of this technique to direct waves from a nine component vsp shot at the conoco test site facility oklahoma are encouraging the technique corrects for unexpected compressional wave energy from shear wave vibrators and collapses near surface multiples thus facilitating further processing for the upgoing wavefield the method provides a simple and effective processing step for routine application to near offset vsp analyses

inversion for subsurface anisotropy using estimates of shear wave splitting summary inversion of the parameters characterizing shear wave splitting polarizations time delays and differential attenuation provides a way of obtaining a good starting model for full wave anisotropic modelling and improves the convergence to a final waveform match this is necessary as the shear waves behave in a highly non linear fashion with respect to anisotropy here we discuss the extent to which polarization and time delay information estimated from split shear waveforms may supply details of this starting model 3 d patterns of these parameters for different anisotropic models are considered to investigate the potential of the inversion scheme guidelines are formed for the best arrangements of sources and geophones to provide optimum resolution of an anisotropic model the study indicates that in most cases different anisotropic models cannot be adequately distinguished using vertical seismic profiles vsps reflection profiles and cross hole surveys in which separate polarization or time delay measurements at one azimuth are made better results are obtained by combining polarization and time delays estimates preferably from more than one azimuth the type of model parameters which can be resolved depends crucially upon the geometry of the acquisition system and to a lesser extent on the background velocity structure individual model parameters are more difficult to extract but some details appear in principle to be resolvable for certain types of survey vsps are most suitable to monitor crack strike and density of parallel vertical aligned cracks and the dip of the fine layering contributing to layer induced anisotropy cross hole experiments are best for measuring the dip of the cracks the resolution of the crack content and aspect ratio are determined principally by the position of line singularities and either an offset vsp or an adapted cross hole survey may be adequate for resolving these properties if the line is shot at an appropriate azimuth with respect to the strike of the cracks the ratio of crack induced anisotropy to matrix anisotropy in sedimentary basins can be evaluated by measuring the positions of the point singularities in recordings of polarization and time delay patterns using a near offset vsp

groundwater strain coupling before the 1999 mw 7 6 taiwan chi chi earthquake summary the coupling of pre earthquake anomalous phenomena between long term groundwater levels recorded at 42 monitoring stations and time varying surface strain derived from 16 gps stations was found in the choshuichi alluvial fan before the 1999 m w 7 6 chi chi earthquake in taiwan the noise free groundwater level anomalies consistently comprised by a sequence of decrease rise and flat phases which agree very well with changes in strain rates computed from the gps stations these coupling agreements show that in addition to compression tension can be generated before a thrust earthquake occurrence as well this case demonstrates that short term surface deformation as signals against noise and accuracy of pre earthquake anomalous phenomena can be simultaneously examined by using multiple parameter crosscheck for significantly reducing the uncertainty of earthquake precursory evaluation

april september mean maximum temperature inferred from hailar pine pinus sylvestris var mongolica tree rings in the hulunbuir region inner mongolia back to 1868 ad two robust tree ring width chronologies were developed for the hulunbuir region inner mongolia northeast china both chronologies were significantly correlated with each other and were arithmetically averaged to build a regional chronology nbstd nbstd showed significant and negative correlations with the growing season maximum temperature from april to september based on this relationship the mean maximum temperature of april to september was reconstructed for the period from 1868 to 2008 the percentage of variance in the data explained by the reconstruction was 40 3 during the calibration period of 1953 2008 five severe warm events of two years or more were found in 1905 1909 1996 1997 2000 2001 2003 2005 and 2007 2008 five cold events occurred during the periods 1880 1881 1897 1900 1948 1949 1955 1960 and 1962 1964 on a decadal scale several warm intervals above the 1868 2008 mean were 1904 1913 1920 1933 1968 1975 and 1990 2003 cold intervals below the mean were 1893 1901 1950 1963 and 1976 1989 warmer events and intervals corresponded to dry periods colder events and intervals coincided with wet or moist conditions in and near the study region spatial correlation patterns revealed that our reconstruction represents a regional temperature and drought signal for the central and eastern mongolian plateau comparison with other tree ring based temperature reconstructions from surrounding areas provides a high degree of confidence in our reconstruction power spectrum and wavelet analyses suggest that the reconstructed mean maximum temperature variation may be related to large scale atmospheric oceanic variability such as the pacific decadal oscillation pdo el nino southern oscillation enso and sunspot activity c 2011 elsevier b v all rights reserved less

potential relationships between seismo deformation and seismo conductivity anomalies abstract this study examines the relationships between seismo deformation and seismo conductivity anomalies during two m6 earthquakes that occurred on march 27th and june 2nd 2013 in taiwan the hilbert huang transform is applied on surface displacement data to remove the effects of noise semi annual and annual cycles and the long term plate movements the residual displacements have similar orientations when earthquake related stress accumulates in the crust once the accumulated stress approaches the threshold for fault rupture the orientations of the residual displacements generally become random except in a small region near the epicenter interestingly high conductivity anomalies which can be detected from the 3 component magnetic data via the magnetic transfer function exist in places very close to this small region near the epicenter spatial and temporal correlations between the high conductivity anomalies and the small region of seismo deformation anomalies suggest that electric charges may migrate and become trapped in the region during seismogenic processes due to differential stress accumulation these electric charges form a high conductivity material that affects the parkinson vector of the geomagnetic field

fluid dependent shear wave splitting in fractured media azimuthal anisotropy in rocks can result from the presence of one or more sets of partially aligned fractures with orientations determined by the stress history of the rock a shear wave propagating in an azimuthally anisotropic medium splits into two components with different polarizations if the source polarization is not aligned with the principal axes of the medium for vertical propagation of shear waves in a horizontally layered medium containing vertical fractures the shear wave splitting depends on the shear compliance of the fractures but is independent of their normal compliance if the fractures are not perfectly vertical the shear wave splitting also depends on the normal compliance of the fractures the normal compliance of a fluid filled fracture decreases with increasing fluid bulk modulus for dipping fractures this results in a decrease in shear wave splitting and an increase in shear wave velocity with increasing fluid bulk modulus the sensitivity of the shear wave splitting to fluid bulk modulus depends on the interconnectivity of the fracture network the permeability of the background medium and on whether the fracture is fully or partially saturated

avd an emerging new marine technology for reservoir characterization acquisition and application several significant developments in marine technologies in the past few years have resulted in the creation of acquisition techniques suited to azimuthal anisotropy analysis in the offshore environment the developments have parallelled the evolution in the theory underlying the use of p p and p s amplitude versus direction avd for seismic anisotropy estimation the demands of such avd methods for a wide azimuthal coverage have only recently been met to guide future work the avd method has been assessed using data from intersecting streamer lines application of the method in this example permits an identification of the strike direction of hydrocarbon filled fractures within a chalk formation in the central north sea the results of this study provide confidence that the method is sufficiently sensitive to fractures and can help guide future analyses the new generation of vertical cables seabed seismic sensors and walk away and or 3 d vertical seismic profiles will eventually lead to high resolution anisotropy estimation in the offshore environment using this approach

shear wave polarizations and subsurface stress directions at lost hills field 2 2 s wave data matrix accomplished by computationally rotating sources and receivers although polarization directions obtained by assuming a homogeneous subsurface were moderately consistent with depth considerable improvement in consistency resulted from analytically stripping off a thin near surface layer whose fast s wave polarization direction was about n 6 e s wave birefringence for vertical travel averaged 3 percent in two zones 200 700 ft and 1200 2100 ft 60 210 m and 370 640 m which had closely similar s wave polarizations between those zones the polarization direction changed and the birefringence magnitude was not well defined s wave polarizations from two concentric rings of offset vsps were consistent in azimuth with one another and with polarizations of the near offset vsp this consistency argues strongly for the robustness of the s wave polarization technique as applied in this area the s wave polarization pattern in offset data fits a model of vertical cracks striking n 55 e

the rate of iron oxide enrichment in arid zone alluvial fan soils tunisian southern atlas measured by mineral magnetic techniques abstract rates of iron oxide enrichment of soils on a segmented alluvial fan in southern tunisia are examined mineral magnetic and chemical characteristics of soils developed on two dated relict segments and on contemporary alluvium suggest that the rate of fe oxide enrichment was more rapid in the holocene 2 6 10 8 g g 1 yr 1 over the last 5000 years than in the pleistocene 1 91 10 9 g g 1 yr 1 over the previous 45 000 years thus fe oxide concentration may be used to determine an ordinal chronology of some geomorphological surfaces in drylands but rates of enrichment cannot be assumed to be uniform over time

estimating interval shear wave splitting from multicomponent virtual shear check shots measuring shear wave splitting from vertical seismic profilingvspdata can benefit fracture and stress characterization as well as seismic processing and interpretation the classic approach to measuring azimuthal anisotropy at depth involves layer stripping its inherent weakness is the need to measure and undo overburden effects before arriving at an anisotropy estimate at depth that task is challenging when the overburden is complex and varies quickly with depth moreover vsp receivers are rarely present all the way from the surface to the target that necessitates the use of simplisticassumptionsabouttheuninstrumentedpartoftheoverburdenthatlimitthequalityoftheresult weproposeanewtechnique for measuring shear wave splitting at depth that does notrequireanyknowledgeoftheoverburden itisbasedona multicomponent version of the virtual source method in whicheachtwo component2 cvspreceiveristurnedinto a2 cshearsourceandrecordedatdeepergeophones theresultingvirtualdatasetisaffectedonlybythepropertiesofthe medium between the receivers a simple alford rotation transforms the data set into fast and slow shear virtual check shots from which shear wave splitting can be measured easilyandaccuratelyunderarbitrarilycomplexoverburden

developments in a non destructive method of determining rock strength material strength is an important variable for any study of the relationships between rock mass geomechanical characteristics and landform development standard field and laboratory tests for measuring strength present a variety of problems to the geomorphologist recent studies allison 1988 have described a laboratory based non destructive method for indirectly determining material strength by measuring dynamic young s modulus data presented here suggest that the same apparatus can be used as a field technique tests have been conducted on the devonian limestone of the napier range western australia laboratory results obtained using the non destructive ultrasonic apparatus have a high correlation with triaxial hoek cell tests results from on site tests have much greater accuracy than data collected using the schmidt hammer which is currently the most widely used geomorphological field technique for determining rock strength

interpreting data matrix asymmetry in near offset shear wave vsp data poor experimental control in shear wave vsps may contribute to unreliable estimates of shear wave splitting and possible misinterpretation of the medium anisotropy to avoid this the acquisition and processing of multicomponent shear wave data needs special care and attention measurement of asymmetry in the recorded data matrix using singular value decomposition svd provides a useful way of examining possible acquisition inaccuracies and may help guide data conditioning and interpretation to ensure more reliable estimates of shear wave polarization azimuth three examples demonstrate how variations in shear wave polarization and acquisition inaccuracies affect the svd results in different ways in the first example analysis of synthetic seismograms with known depth changes in the polarization azimuth show how these may be detected in the second example a known source re orientation and polarity reversal is detected by applying svd to near offset shear wave vsp data recorded in the romashkino fiel

inversion of normal moveout for monoclinic media1 multiple vertical fracture sets possibly combined with horizontal fine layering produce an equivalent medium of monoclinic symmetry with a horizontal symmetry plane although monoclinic models may be rather common for fractured formations they have hardly been used in seismic methods of fracture detection due to the large number of independent elements in the stiffness tensor here we show that multicomponent wide azimuth reflection data combined with known vertical velocity or reflector depth or multi azimuth walkaway vsp surveys provide enough information to invert for all but one anisotropic parameters of monoclinic media in order to facilitate the inversion procedure we introduce a thomsen style parametrization for monoclinic media that includes the vertical velocities of the p wave and one of the split s waves and a set of dimensionless anisotropic coefficients our notation defined for the coordinate frame associated with the polarization directions of the vertically propagating shear waves captures the combinations of the stiffnesses responsible for the normal moveout nmo ellipses of all three pure modes the first group of the anisotropic parameters contains seven coefficients e 1 2 1 2 3 and 1 2 analogous to those defined by tsvankin for the higher symmetry orthorhombic model the parameters e 1 2 1 2 and 1 2 are primarily responsible for the pure mode nmo velocities along the coordinate axes x 1 and x 2 i e in the shear wave polarization directions the remaining coefficient 3 is not constrained by conventional spread reflection traveltimes in a horizontal monoclinic layer the second parameter group consists of the newly introduced coefficients 1 2 3 which control the rotation of the p s 1 and s 2 wave nmo ellipses with respect to the horizontal coordinate axes misalignment of the p wave nmo ellipse and shear wave polarization directions was recently observed on field data by perez et al our parameter estimation algorithm based on nmo equations valid for any strength of the anisotropy is designed to obtain anisotropic parameters of monoclinic media by inverting the vertical velocities and nmo ellipses of the p s 1 and s 2 waves a dix type representation of the nmo velocity of mode converted waves makes it possible to replace the pure shear modes in reflection surveys with the ps 1 and ps 2 waves numerical tests show that our method yields stable estimates of all relevant parameters for both a single layer and a horizontally stratified monoclinic medium

seismic imaging of the shallow subsurface shear wave case histories obtaining high resolution images of the geology and hydrogeology of the subsurface in the depth range from ground level to 50 m is one of the major challenges of modern geophysics the methods which are commonly used such as compressional wave surveys and ground penetrating radar often suffer from adverse effects caused by the near surface conditions changes in water saturation and various sources of noise this paper demonstrates some of the advantages offered by the use of shear wave seismology and by the combination of shear and compressional wave seismic methods in shallow subsurface investigations multicomponent shallow seismic tests were carried out at four different sites to examine the effectiveness of different acquisition geometries under a variety of near surface geological conditions near surface conditions encountered at the sites included thick clays clay sand sequences overlying chalk mudstone overlying granodiorite bedrock and landfill material under all conditions shear wave data acquisition was found to have advantages over compressional wave acquisition for the investigation of the shallow subsurface shear head waves being unaffected by water saturation achieved penetration to greater depths at a site in crewkerne dorset where compressional head wave penetration was limited to the near surface layers better vertical resolution was achieved at shallow depths using shear wave reflection energy at a landfill site shear wave reflections from shallow interfaces were in some cases less affected by noise compared with the equivalent compressional wave reflections combinations of shear and compressional wave data recording allowed the measurement of a poisson s ratio log and gave indications of seismic anisotropy at two sites where dipping clay layers were present

physical modeling of overburden effects reservoir properties can be inferred from the amount of anisotropy estimated from seismic data unfortunately irregularities in the formations above the reservoir unit can mask or overprint the true seismic anisotropy of the reservoir unit this overburden effect subjects the measured reservoir seismic anisotropy to a high degree of uncertainty we investigate this overburden effect on p waves with a three layer ultrasonic laboratory scale model whose middle layer contains localized gas filled vertical fractures we analyze the reflection amplitudes and traveltimes of a p wave reflection event from below the overburden to understand the overburden effect on anisotropy analysis and imaging our study shows that steps must be taken to reduce the p wave overburden effect when significant irregularities occur in the formations above the reservoir unit

shear wave polarization anisotropy in the upper mantle beneath honshu japan shear wave polarization anisotropy in the wedge portion of the upper mantle between a subducting plate and the earth s surface is investigated using three component seismograms of intermediate depth and deep earthquakes recorded at 14 local stations in honshu japan eighty nine high quality seismograms were selected from a period of 3 years the data used in this study are restricted such that incidence angles are smaller than the critical angle of 30 to the earth s surface in order to avoid phase shifts in the shear wave train to find directions of the maximum and minimum velocities in split shear waves where shear waves are resolved into two phases with the maximum time separation each set of the two horizontal component seismograms is rotated in the horizontal plane the split shear waves thus obtained are again recombined after the correction of anisotropy and the anisotropy corrected particle motion is compared with the focal mechanism for a cross check of the observed anisotropy directions of the maximum axes are plotted on azimuthincidence angle stereograms at each station the stereograms and the cross sections of seismic ray paths show that 1 the anisotropic material is distributed at intermediate location between earthquake sources and receiving stations and 2 the anisotropic region is separated into two parts one in the north of the present study area with the polarization of the maximum velocity shear wave trending 0 to 30 from the north north anisotropy and the other in the south with it trending 90 to 120 south anisotropy the maximum time delays between the two shear waves along a vertical seismic ray is about l s for both the anisotropic regions the horizontal extent of the anisotropic area in the north is 50 km at depths of 50 to 150 km perhaps prevalent in west honshu however the depth of anisotropic material is not well constrained because of insufficient coverage of seismic ray paths and angles if we assume the vertical extent of the anisotropic material to be 100 km the maximum velocity contrast would be 4 if we adopt a crack alignment model for the observed anisotropy cracks are inferred to be distributed vertically trending 0 to 30 in the north and 90 to 120 in the south if we assume an olivine alignment model olivine crystals are inferred to be aligned in the north with the a axis of the slip direction trending 0 to 30 and the slip plane normal to the b axis being vertical and in the south anisotropy with the axis trending 90 to 120 and the slip plane being horizontal

the weathering effects of the lichen lecidea aff sarcogynoides koerb on magaliesberg quartzite the effect of the weathering processes generated by lecidea aff sarcogynoides koerb on the substrate was studied by means of a scanning electron microscope the elements present in the substrate magaliesberg quartzite and in the lichen thallus were determined by x ray fluorescence spectrometry for the purpose of comparison the elements present were mostly similar although a few were present in the thallus which were not observed in the quartzite it is possible that those elements present in the lichen thallus which were not present in the substrate may have been extracted from the atmosphere the occurrence of small hollows weathering pits in which the early stages of plant development occurs and the disintegration of the rock indicate that lecidea aff sarcogynoides koerb contributes to the chemical weathering processes by chelation and mechanically by the penetration and expansion of hyphae a model is proposed in which a possible mechanism for these weathering processes is suggested

a case example of near surface correction for multicomponent vsps a deterministic near surface correction procedure is developed for multicomponent vsp data shot using directional sources and recorded using three component receivers the method is capable of removing unwanted effects of acquisition such as unequal source strengths or misorientations but may also remove near surface multiples and anisotropy this is of considerable benefit for obtaining accurate and consistent estimates of subsurface anisotropy from different source combinations application of the technique is illustrated using a dataset from the romashkino reservoir in russia where three or four different directional sources are used at the same source locations the technique corrects for the large discrepancies which exist between the estimates obtained using different source combinations application of the technique to three wells in the survey region reveals a nearly isotropic subsurface except for a few isolated zones of moderate to high 2 to 8 anisotropy which lie close to the expected depth for the reservoir although there is no significant correlation with the production figures for each well the qs1 polarization azimuth within the reservoir does vary at each well location suggesting that this may be a more sensitive indicator of reservoir drocess

correlation analysis of element contents and mechanical characteristics of shale reservoirs a case study in the cen gong block south china abstract the mechanical parameters of shale are some of the most significant assessment indices for shale gas exploitation the development of element logging offers an opportunity to study the correlations between the elemental concentrations and the mechanical characteristics of shale using the lower cambrian shale in the cen gong area guizhou province china as an example 490 data sets are extracted using formation element and mineralogy fem and cross multipole array acoustic xmac logging through spearman correlation analysis the correlations between the major elements si ca fe s ti mn mg al and k and the mechanical parameters young s modulus poisson s ratio shear strength shear modulus and brittleness index are studied the effects of the si content on the mechanical parameters positive negative correlations magnitude and significance of the correlations are discussed based on x ray diffraction analyses of the minerals in 146 samples when 1 84 si si content si si

influences of eolian and pedogenic processes on the origin and evolution of desert pavements well developed desert pavements are present above eolian deposits that mantle flows of the cima volcanic field located in the mojave desert california soil stratigraphic data and geochemical data demonstrate that eolian and pedogenic processes play major roles in the evolution of these pavements eolian dust 1 accelerates mechanical fragmentation of flow rock providing the source material for pavements and 2 accumulates slowly below basaltic colluvium in flow depressions eventually promoting development of cumulate soils below the evolving stone pavement an increase in dust flux during the holocene has raised ancient pleistocene pavements as much as 20 cm above the former land surface the results of our studies demonstrate for the first time that most desert pavements do not form by deflation by overland flow or by upward migration of stones through a slowly formed clayey argillic horizon desert pavements are born and maintained at the surface

s wave velocities and anisotropy in sediments entering the nankai subduction zone offshore japan summary p and mode converted s waves from airgun shots recorded along a line of ocean bottom seismometers were used to construct a 2 d velocity section across the nankai trough and the toe of the accretionary complex offshore southeast japan the site was chosen because of plans to drill into the seismogenic zone of the main subduction thrust which has a history of m 8 earthquakes the section is constrained by well log data from three ocean drilling program boreholes s wave velocities of the sediments over the entire section down to 1 1 km below seafloor range from 107 to 915 m s 1 shear wave splitting indicates anisotropy in all the sediments except possibly the distal trench fill if interpreted conventionally as being due to vertical stress aligned cracks the anisotropy in both the trench and the toe of the accretionary wedge is consistent with crack orientation nw se parallel to the direction of maximum tectonic compression crack density is 0 01 0 03 consistent with closeness to fracture criticality

late quaternary climatic reconstruction for the maghreb north africa abstract the paleoclimatic evolution of the maghreb since 40 000 yr b p is not yet known in great detail for that purpose data from more than 35 sites have been collected although only about 11014c dates and a few prehistoric remains can be used to correlate these data however the sites are well distributed over the maghreb and the convergent results from paleosoils paleolakes alluvian or eolian deposits pollen analysis etc induce one to draw up a certain pattern in climatic evolution for the period between 40 000 and 20 000 yr b p the data are still fragmentary but are concordant in indicating wetter environments generalized as far as the northern sahara the effect of dropping temperatures on the increasing water budget and soil moisture is not yet clear but rainfall could have been more regular and more abundant as a consequence of the shifting of middle latitude cyclone paths towards the south in both summer and winter the environmental indicators show that between about 20 and 18 000 yr b p rainfall progressively became more irregular however proofs of aridity in the south maghreb and the northern sahara is evident only for the period between 12 10 000 yr b p at that time the saharan high pressure belt was reinforced and well established at higher latitudes than at present this dry climate lasted more or less until the thermal optimum after 6000 yr b p there is some evidence that the climate became wetter again but the environmental conditions were rather different from those of before 20 000 yr b p for instance the middle holocene dark humic sois are the result of a pedogenesis different from that of the late pleistocene reddish paleosoils for the last two millennia it is not easy to distinguish between a natural trend towards aridification and desertification through man s intervention

techniques for measuring rock weathering application to a dated fan segment sequence in southern tunisia there are a number of techniques for estimating the amount of weathering a clast has undergone these usually have the objective of establishing an ordinal chronology of geomorphological surfaces or investigation of site specific conditions affecting weathering rates three such techniques are applied to a dated sequence of alluvial fan segments in southern tunisia two of these techniques depend on measuring surface roughness the micro roughness meter and a displacement approach and one uses the structural weakening of the rock fabric schmidt hammer the micro roughness meter enables calculation of standard deviation of surface height variation root mean square roughness and surface autocorrelation function of these techniques schmidt hammer rebound values standard deviation root mean square roughness and the displacement technique show systematic changes on the three fan segments which are statistically significant at the 0 05 level however the amount of variance in all datasets is very large indicating the need for caution in application of these techniques for relative dating 1998 john wiley sons ltd

ground surface temperature reconstruction for the jinggangshan mountains interpreting the hydro thermal coupling pattern in southeastern china abstract understanding the spatial temporal variations of climatic factors and hydro thermal coupling patterns htcp is crucial to forecasting and preparing for future climate change based on accurately dated tree rings we developed the first dekad 10 days 6 to dekad 27 minimum ground surface temperature gstmin6 27 reconstruction for the jinggangshan mountains jgsm in southeastern china sc spanning the period 1791 2014 ad this reconstruction explains 57 9 of observed temperature during the instrumental period 1960 2014 ad our record exhibits a slow cooling trend before 1834 ad followed by drastic fluctuations between 1835 and 1850 ad and progressive warming r2 0 31 thereafter the highest gstmin6 27 variance occurs during the 1820s 1840s and from the 1900s to the early 1930s our reconstruction not only captures the characteristics of temperature variability over a large portion of sc but also reveals decadal scale temperature fluctuations that are approximately synchronous with conditions across north china and the northern hemisphere this research suggests that the western pacific subtropical high wpsh was likely the primary control of gstmin6 27 variability in the jgsm while the pacific decadal oscillation pdo tropical indian and western pacific ocean sea surface temperatures along with the north atlantic oscillation nao indirectly influenced decadal scale temperature variations based on our new reconstruction we identified regional decadal scale differences in the htcp over sc with an insignificant warm dry cold wet coupling pattern dominating the jgsm and a significant warm wet cold dry pattern in coastal areas

differential weathering of feldspar and pyroxene in an arctic alpine environment the relative relief of adjacent plagioclase and pyroxene minerals was accurately measured on a recently exhumed outcrop of gabbroic rocks in leirdalen southern norway above the level of the former soil surface the feldspars protrude above the pyroxenes whereas below that level the pyroxenes are higher differential relief declines with increasing depth of burial down to 80 cm on exposed surfaces the mean loss of pyroxene relative to feldspar is 103 cm3 m 2 of rock surface with burial down to 50 cm in an arctic alpine brown soil the mean loss of feldspar relative to pyroxene is 179 cm3 m 2 these figures represent minimum amounts of weathering over about 9000 years the results confirm the importance of chemical weathering in arctic alpine environments and the techniques may provide useful physical measures of degree of weathering for use in rate studies and relative dating

age related trends in subfossil tree ring δ13c data abstract long stable isotope chronologies are increasingly constructed from tree rings averaging the mean chronologies necessitates that the contributing series contain no age related trends therefore any set of dendroisotope data needs to be tested for age dependency in isotope values before the chronology is compared with climate or other environmental records such age trends are common to tree ring width and density data but corresponding evidence on stable isotope records is yet relatively limited moreover there is contrasting evidence suggesting that while a number of dendroisotope datasets appear to portray no age related trends such trends may be found in different regions and tree species here we analyze a collection of 182 subfossil tree ring 13c series from a species pinus sylvestris and a region northern fennoscandia where previous investigations have found no support for age related trends beyond the juvenile effect in the tree ring 13c data our 13c dataset benefits from a large sample size of analysed tree rings representing a wide range of calendar years 5630 bc through ad 1930 formed under varying climate conditions our results indicate that the tree ring 13c series do contain age related trends throughout the trees lifespan this age dependence corresponded to an ascending linear change of ca 0 4 per 100 years illustrating the mean 13c curve as a function of trees biological age demonstrated that the age dependence may alternatively be modelled with non linear trends with varying curvature there were also time variant changes in the 13c trend curvature of subfossil tree ring data increasing and diminishing age trend curvatures coincided with palaeoecological indications of denser and thinner pine populations respectively over the study period overall our results highlight the hardships of detecting age related trends in dendroisotope data in comparison to records of tree ring widths and densities the possibility that age related trends are present in dendroisotope data in regions and tree species where such trends may not yet have been detected is underlined

processing of a nine component near offset vsp for seismic anisotropy a convolutional sequence of matrix operators is offered as a convenient deterministic scheme for processing a multicomponent vertical seismic profile vsp this sequence is applied to a nine component near offset vsp recorded at the conoco borehole test facility kay county oklahoma these data are corrected for tool spin and near surface anisotropy together with source coupling or imbalance after wave field separation using a standard f k filter each source and receiver pair for the upgoing waves is adjusted to a common reference depth using a matrix operator based on the downgoing wavefield the up and downgoing waves are then processed for anisotropy by a similarity transformation to separate the qs1 and qs2 waves from which the anisotropic properties are estimated these estimates reveal a strong apparent vertical birefringence in the near surface but weak or moderate values for the majority of the subsurface the target zone consists of a thin sandstone and deeper shale layer both of which possess a strong vertical birefringence the sandstone corresponds to a zone of known fluid flow an observed qs2 attenuation and polarization change in the shale suggest it contains large fractures

systematic analysis of shear wave splitting in the aftershock zone of the 1999 chi chi taiwan earthquake shallow crustal anisotropy and lack of precursory variations we analyze shear wave splitting sws in a high quality waveform data set recorded at surface and downhole 0 2 km seismometers in a region around the 20 september 1999 m w 7 6 chi chi taiwan earthquake sequence the data set was generated by events in a 5 year period before during and after the mainshock the purpose is to investigate the depth extent of the crustal anisotropy and its possible temporal evolution in relation to the occurrence of large earthquakes results from downhole records show a stable polarization direction of the fast shear wave that matches well the local global positioning system gps velocity field a slightly different polarization direction of the fast shear wave is obtained from surface data this suggests a possible anisotropy change between the top 0 2 km structure and the deeper section of the crust measured time delays below the downhole station have an average value of 0 16 sec without systematic changes for sources from about 8 km to 20 km in depth estimates of time delays in the top 0 2 km of the crust based on shear waves reflected from the free surface give a constant 0 04 sec a likely depth distribution inferred from these two types of measurements and an s velocity model indicates that the crustal anisotropy in the region is dominated by the top 2 to 3 km the measured polarization directions and time delays give essentially constant values over the 2 7 year premainshock and 2 3 year postmainshock periods in the region adjacent to the chi chi rupture and within 10 km from the epicentral region of its two large m 6 0 aftershocks analysis of sws in waveforms produced by earthquake multiplets confirms further the lack of precursory temporal variations of crustal anisotropy in the immediate neighborhood of the chi chi earthquake sequence the results raise doubts on the general usefulness of sws measurements for earthquake forecasting an apparent coseismic increase in anisotropy time delay of approximately 10 is observed for depths 0 2 km however this value is clearly affected by spatial changes associated with different event locations before and after the chi chi mainshock

rock surface roughness as an indicator of degree of rock surface weathering rock surface weathering often leads to increased rock surface roughness but roughness has proved difficult to quantify several instruments are available for micro mapping and recording rock surface profiles but the most appropriate for most purposes is the simple profile gauge short profiles can be recorded quickly and accurately a range of roughness indices has been proposed in other areas of geomorphology and their efficacy as measures of roughness at scales of interest in studies of weathering is assessed some are too complex or labour intensive and others are too sensitive to the scale of roughness to provide reliable measures of magnitude the most appropriate indicator of both the scale and magnitude of roughness is the standard deviation of the differences between height values at a range of set horizontal intervals along a profile the deviogram varying the measurement interval records roughness at different scales a regression approach root mean square roughness provides a reliable measure of the magnitude of roughness at the maximum scale present three case studies confirm the efficacy of these approaches to studies of weathering of different rocks in different environments software is supplied which automates the calculation of roughness indices from gauge profiles

short lived juvenile effects observed in stable carbon and oxygen isotopes of uk oak trees and historic building timbers abstract stable carbon 13 c and oxygen 18 o isotope ratios were measured on the latewood cellulose of individual oak quercus robur l q petraea liebl samples from living trees and historic building timbers this represents the type of material available to produce long tree ring chronologies for north western europe including the uk and ireland results from the juvenile rings those located closest to the pith were compared with results from equivalent sections representing the same calendar years from independent master isotope chronologies that do not contain any juvenile wood allowing any juvenile offsets and trends to be separated from those caused by environmental change oak timbers from archaeological sources are often relatively short pinus sp would severely constrain the material available for chronology construction the aim of this study was to determine the magnitude and duration of juvenile effects including the detection of trends offsets and their influence upon signal strength the results show clearly that juvenile effects for oak from central england are very small and short lived and that removing merely the first five rings closest to the pith is sufficient to avoid them this result greatly increases the potential for building long and well replicated stable isotope chronologies using archived oak samples from historic building timbers allowing high resolution climate reconstructions to be produced for the highly populated regions where oak is abundant and which are currently under represented in regional palaeoclimate reconstructions

converted wave seismic exploration applications converted seismic waves specifically downgoing p waves that convert on reflection to upcoming s waves are increasingly being used to explore for subsurface targets rapid advancements in both land and marine multicomponent acquisition and processing techniques have led to numerous applications for p s surveys uses that have arisen include structural imaging e g seeing through gas bearing sediments improved fault definition enhanced near surface resolution lithologic estimation e g sand versus shale content porosity anisotropy analysis e g fracture density and orientation subsurface fluid description and reservoir monitoring further applications of p s data and analysis of other more complicated converted modes are developing

tree ring stable carbon isotope based may july temperature reconstruction over nanwutai china for the past century and its record of 20th century warming growth anomaly of trees in some regions was detected under current episode of rapid warming thisraises a dilemma for temperature reconstructions by using tree ring width which is believed to be themost important proxy on inter annual temperature reconstruction during the past millenniums here weemployed the tree ring d13c to reconstruct temperature variations for exploring their potential oncapturing signals of rapid warming and to test how its difference with the tree ring width basedreconstruction in this study the mean mayejuly temperature tmej was reconstructed over the pastcentury by tree ring d13c of chinese pine trees growing in the nanwutai region the explained varianceof the reconstruction was 43 3 42 1 after adjusting the degrees of freedom compared to a ring widthtemperature reconstruction mayejuly from the same site the tree ring d13c based temperaturereconstruction offered two distinct advantages 1 it captured a wider range of temperature variability i e at least mayejuly even over a longer part of the year januaryeseptember and 2 the reconstructionpreserved more low frequency climate information than that of ring width did the 20th century warming was well represented in the nanwutai tree ring d13c temperature reconstruction which implied that stable carbon isotope of tree rings potentially represents temperaturevariations during historical episodes of rapid warming a spatial correlation analysis showed that ourtemperature reconstruction represented climate variations over the entire loess plateau in north centralchina significant positive correlations p 0 1 were found between the temperature reconstruction andenso as well as ssts in the pacific and indian oceans the reconstruction showed the periodicities of22 78 4 16 3 45e3 97 and 2 04e2 83 year quasi cycles at a 95 confidence level our results suggestedthat temperature variability in the nanwutai region may be linked to pacific and indian ocean sstvariations and solar activity less

processing modelling and predicting time lapse effects of overpressured fluid injection in a fractured reservoir summary time lapse seismology is important for monitoring subsurface pressure changes and fluid movements in producing hydrocarbon reservoirs we analyse two 4 d 3c onshore surveys from vacuum field new mexico usa where the reservoir of interest is a fractured dolomite in phase vi a time lapse survey was acquired before and after a pilot tertiary recovery programme of overpressured co2 injection which altered the fluid composition and the pore fluid pressure phase vii was a similar time lapse survey in the same location but with a different lowerpressure injection regime applying a processing sequence to the phase vi data preserving normal incidence shearwave anisotropy time delays and polarization and maximizing repeatability interval time analysis of the reservoir interval shows a significant 10 per cent change in shear wave velocity anisotropy and 3 per cent decrease in the p wave interval velocities a 1 d model incorporating both saturation and pressure changes is matched to the data the saturation changes have little effect on the seismic velocities there are two main causes of the time lapse changes any change in pore fluid pressures modifies crack aspect ratios additionally when there are overpressures as there are in phase vi there is a 90 change in maximum impedance directions and the leading faster split shear wave instead of being parallel to the crack face as it is for low pore fluid pressures becomes orthogonal to the crack face the anisotropic poro elasticity ape model of the evolution of microcracked rock calculates the evolution of cracked rock to changing conditions ape modelling shows that at high overburden pressures only nearly vertical cracks to which normal incidence p waves are less sensitive than s waves remain open as the pore fluid pressure increases ape modelling matches the observed time lapse effects almost exactly demonstrating that shear wave anisotropy is a highly sensitive diagnostic of pore fluid pressure changes in fractured reservoirs in this comparatively limited analysis ape modelling of fluid injection at known pressure correctly predicted the changes in seismic response particularly the shear wave splitting induced by the high pressure co2 injection in the phase vii survey ape modelling also successfully predicted the response to the lower pressure injection using the same phase vi model of the cracked reservoir the underlying reason for this remarkable predictability of fluid saturated reservoir rocks is the critical nature and high crack density of the fluid saturated cracks and microcracks in the reservoir rock which makes cracked reservoirs critical systems

relative age dating of inorganic deposits the need for a more critical approach relative age dating rd techniques rely on indices of post depositional change principally degree of rock surface weathering or soil development in the chronosequence approach where a measured parameter displays a trend in values that parallels a trend in surface age the parameter is commonly assumed to be age dependent such an assumption may be invalid this is demonstrated with particular reference to glacier forelands where environmental and sedimentological factors that can influence commonly used rd parameters may exhibit systematic variations in parallel with but independent of surface age where some independent dating control is available regression analysis may be used to define chronofunctions providing numerical ages based on rd parameters however the error limits of such ages are often underestimated the use of modal rather than mean values is particularly problematic a more critical approach is suggested involving deductive experiments to examine the influence of non tempo

observation of shallow anisotropy on local earthquake records at the garner valley southern california downhole array abstract we study very shallow s wave anisotropy effects on local earthquake seismograms recorded by a vertical array of accelerometers at garner valley southern california using seismic events recorded by sensors buried at depth in the crystalline bedrock 220 m in weathered crystalline rocks 22 m and in sediments 15 6 and 0 m we analyze the s wave vertical travel times and polarizations at different depths hodograms plotted for horizontal components at s onset exhibit a clear linear and constant polarization independent of both earthquake location and mechanism this alignment varies with depth and is n 325 13 at 220 m while n 25 20 at 22 15 6 and 0 m a for the deep propagation 220 m the s wave direction of polarization at 220 m is identical to the observation obtained on a crystalline outcrop at the knw usgs stations 7 km away studies conducted on this site concluded to a rock fabric anisotropy due to the alignment of minerals and or microcracks along the lineation direction b for the shallow s fast s slow travel time delays roughly correspond to half the dominant period of the signal and the subsurface linear polarization is interpreted as a degenerated ellipse the north 20 anisotropy fast axis coincides with the maximum horizontal compressive stress associated with the san andreas fault system the shallow anisotropy extension corresponds to slow velocities associated with altered granite it suggests either a stress induced anisotropy due to pore crack deformation in an altered e g hydrothermally medium or the superposition of two crystalline units having different lineation directions these data show that in presence of slow 0 to 200 m subsurface layers shallow anisotropy strongly affects the earthquake surface records in the usual frequency range below 30 hz used in short period seismology

are pooled tree ring δ13c and δ18o series reliable climate archives a case study of pinus nigra spp laricio corsica france abstract stable carbon and oxygen isotopes in tree rings are considered as reliable climate archives as they provide past environmental information with high resolution however recent studies have shown that isotope chronologies may reveal a long term age trend and be influenced by non climatic factors as other tree ring parameters as well these trends can only be identified in chronologies measured on individual trees but not on pooled sample chronologies consisting of several trees in order to test whether pooled chronologies from pine trees from the island of corsica mediterranean france can be used for climatic reconstructions we compared calculated mean values from 5 individual trees with pooled chronologies from the same 5 individuals carbon and oxygen isotope chronologies for a 50 year interval with annual resolution and 400 years with decadal resolution were analysed in order to document secular changes in inter tree variability and to test for age related trends in the isotope ratios pooled carbon and oxygen isotope series correspond well to chronologies based on mean values calculated from analyses of individual trees inter tree variability in oxygen isotope ratios is higher than in carbon isotope ratios but remains relatively constant over time similarities between the isotope series of individual trees are stronger over the 400 year time scale documenting a common long term signal in the isotope values no long term age related trends are observed oxygen isotope values of the juvenile phase are characterised by a remarkable decrease over 40 50 years interpreted to be related to the less developed root system of the young trees this age effect can be avoided by not considering the first 50 years from the chronologies our results confirm that pooled carbon and oxygen isotope chronologies from pinus nigra can be used for environmental reconstructions without statistical detrending

detection and analysis of naturally fractured gas reservoirs multiazimuth seismic surveys in the wind river basin wyoming multiazimuth binning of 3 d p wave reflection data is a relatively simple but robust way of characterizing the spatial distribution of gas producing natural fractures in our survey data were divided into two volumes by ray azimuth approximately perpendicular and parallel 45 to the dominant fracture strike and separately processed azimuthal differences or ratios of attributes provided a rough measure of anisotropy improved imaging was also attained in the more coherent fracture parallel volume a neural network using azimuthally dependent velocity reflectivity and frequency attributes identified commercial gas wells with greater than 85 success furthermore we were able to interpret the physical mechanisms of most of these correlations and so better generalize the approach the apparent velocity anisotropy was compared to that derived from other p and s wave methods in an inset three component survey prestack determination of the azimuthal moveout ellipse will best quantify velocity anisotro

a global merged land air sea surface temperature reconstruction based on historical observations 1880 1997 abstract a merged land air sea surface temperature reconstruction analysis is developed for monthly anomalies the reconstruction is global and spatially complete reconstructed anomalies damp toward zero in regions with insufficient sampling error estimates account for the damping associated with sparse sampling and also for bias uncertainty in both the land and sea observations averages of the reconstruction are similar to simple averages of the unanalyzed data for most of the analysis period for the nineteenth century when sampling is most sparse and the error estimates are largest the differences between the averaged reconstruction and the simple averages are largest sampling is always sparse poleward of 60 latitude and historic reconstructions for the polar regions should be used with caution

the fracture criticality of crustal rocks summary the shear wave splitting observed along almost all shear wave ray paths in the earth s crust is interpreted as the effects of stress aligned fluid filled cracks microcracks and preferentially oriented pore space once away from the free surface where open joints and fractures may lead to strong anisotropy of 10 per cent or greater intact ostensibly unfractured crustal rock exhibits a limited range of shear wave splitting from about 1 5 to 4 5 per cent differential shear wave velocity anisotropy interpreting this velocity anisotropy as normalized crack densities a factor of less than two in crack radius covers the range from the minimum 1 5 per cent anisotropy observed in intact rock to the 10 per cent observed in heavily cracked almost disaggregated near surface rocks this narrow range of crack dimensions and the pronounced effect on rock cohesion suggests that there is a state of fracture criticality at some level of anisotropy between 4 5 and 10 per cent marking the boundary between essentially intact and heavily fractured rock when the level of fracture criticality is exceeded cracking is so severe that there is a breakdown in shear strength the likelihood of progressive fracturing and the dispersal of pore fluids through enhanced permeability the range of normalized crack dimensions below fracture criticality is so small in intact rock that any modification to the crack geometry by even minor changes of conditions or minor deformation particularly in the presence of high pore fluid pressures may change rock from being essentially intact below fracture criticality to heavily fractured above fracture criticality this recognition of the essential compliance of most crustal rocks and its effect on shear wave splitting has implications for monitoring changes in any conditions affecting the rock mass these include monitoring changes in reservoir evolution during hydrocarbon production and enhanced oil recovery and in monitoring changes before and after earthquakes amongst others

shear wave polarizations near the north anatolian fault i evidence for anisotropy induced shear wave splitting summary the turkish dilatancy projects tdp1 in 1979 and tdp2 in 1980 recorded small earthquakes near the north anatolian fault with closely spaced networks of three component seismometers in order to investigate the possibility of diagnosing dilatancy from its effects of shear wave propagation this paper examines the polarizations of shear wavetrains recorded in the shear wave window immediately above the earthquake foci abrupt changes in the orientation and or ellipticity of the shear wave polarizations are almost always observed during the first few cycles following the initial shear wave arrival on each seismogram the horizontal projections of the polarizations of the first shear wave arrivals at any given station show nearly parallel alignments with approximately the same orientations at each of the recording sites with one exception it is difficult to explain this uniform alignment over a wide area in terms of scattering at the irregular surface topography or by earthquake focal mechanisms we demonstrate that the shear wave splitting is likely to be the result of anisotropy in the region above the earthquake foci which could produce polarizations displaying the observed alignments the temporal change of the azimuth of alignment observed at one locality between 1979 and 1980 may be due to the release of a local stress anomaly by a very near earthquake

full wavefield inversion to estimate impact source orientation from multicomponent land seismic data orientation of an impact source is estimated from multicomponent land data the seismic wavefield produced by an arbitrarily oriented impact source is represented by the linear combination of the nine independent wavefield components green s functions for a full vector source coefficients of this linear combination are expressed in terms of the parameters that define source orientation the parameters are an azimuthal angle a vertical angle and source strength which are the unknowns of the inverse problem once the green s functions are computed source parameters are estimated by full wavefield inversion using a grid search over the source parameters this inverse procedure also provides information about the uniqueness and resolution of the solution information on the source orientation is dominated by the relative amplitudes between components so the inversion can be performed even when the source time wavelet is not well known the approach is tested using a synthetic common source gather res