the influence of fluid type on elastic wave velocity and attenuation in volcanic rocks abstract volcano seismic signatures are the convolution of source and path effects the source signature provides important information about the dynamic rock deformation and fluid resonance processes within the volcano but path effects are the result of the volcanic plumbing the edifice physical conditions and the complex geological structure complicating the inversion of seismic data further is the influence of fluid properties here we use ultrasonic experiments to understand the effect of variable fluid viscosity and bulk modulus on p wave attenuation and velocity for volcanic rocks from white island volcano whakaari new zealand our experimental conditions for high porosity ash tuff and low porosity lava resemble shallow and deep volcano facies respectively p wave velocity vp and quality factor qp increase with the viscosity and bulk modulus of the saturating fluid and decrease with porosity and fracture volume tuffs are highly sensitive to saturating fluid with variations in vp of up to 67 and qp of 383 vp and qp in fractured lavas are slightly less sensitive to fluids 24 and 296 respectively but vp can be used to identify lithology for these tuff vs lavas comparison of the experimental data with rock physics models suggests that squirt flow is the dominant attenuation mechanism for viscosities greater than that of water while scattering losses may be important in gas saturated rocks our results have significant implications for the interpretation of seismic tomography studies and for volcano hazard monitoring based on the characteristics of detected seismicity

imaging permeability structure within the highly permeable carbonate earth inverse theory and experiment in most oil reservoir rocks the squirt flow characteristic frequency fsq range of 10 100 khz is much less than the biot characteristic frequency f0 range of 1 mhz 1 ghz in contrast carbonate aquifers have very high permeability 10 3000 d and the two characteristic frequencies are reversed f0 from 10 hz to 10 khz and fsq from 1 mhz to 100 mhz the principal objective of this paper is to develop a pilot inverse method for robust imaging of the high permeability structure within carbonate aquifers an analytical approximate model called the super k model is developed the super k model coincides numerically with the combined biot and squirt flow mechanism model when permeability is higher than 100 md and the frequency is lower than 100 khz the super k regime in the super k regime the pore fluid is always relaxed so that the attenuation due to the biot mechanism is roughly four times larger than that of the biot model also empirical equations are developed that relate the stiffness and rigidit

timing of pleistocene glacial events in new york state this review of age data for the pleistocene of new york identifies both strengths and weaknesses in the temporal framework relating the glacial chronology of the great lakes region to that of the middle atlantic seaboard the pre wisconsinan record involves saprolith and till in the adirondack mountains marine clay on long island multiple tills at fernbank otto and gowanda and major drainage derangement of the allegheny river middle wisconsinan ice spread into the allegheny plateau damming high level lakes in cayuga trough and southern ontario long island pollen data show late middle wisconsinan warming the plum point interstade glacially overridden organic matter at rush creek lord hill and st davids shows that this episode ended by 24 000 bp maximum late wisconsinan glaciation occurred during the nissouri stade 21 750 18 570 bp the concept of an erie interstade implies that ice recession 15 000 16 000 bp permitted lake drainage across new york new york evidence allows this interpr

catastrophic meltwater discharge down the hudson valley a potential trigger for the intra allerod cold period glacial freshwater discharge to the atlantic ocean during de glaciation may have inhibited oceanic thermohaline circulation and is often postulated to have driven climatic fluctuations yet attributing meltwater discharge events to particular climate oscil lations is problematic because the location timing and amount of meltwater discharge are often poorly constrained we present ev idence from the hudson valley and the northeastern u s conti nental margin that establishes the timing of the catastrophic drain ing of glacial lake iroquois which breached the moraine dam at the narrows in new york city eroded glacial lake sediments in the hudson valley and deposited large sediment lobes on the new york and new jersey continental shelf ca 13 350 yr b p excess 14 c in cariaco basin sediments indicates a slowing in thermohaline circulation and heat transport to the north atlantic at that time and both marine and terrestrial paleoclimate proxy records around the north atlantic show a short lived 400 yr cold event intra allerod cold period that began ca 13 350 yr b p the meltwater discharge out the hudson valley may have played an important role in triggering the intra allerod cold period by diminishing thermohaline circulation

robust satellite techniques rst for monitoring earthquake prone areas by satellite tir observations the case of 1999 chi chi earthquake taiwan abstract for more than 13 years a multi temporal data analysis method named robust satellite techniques rst has been being applied to satellite thermal infrared tir monitoring of seismically active regions it gives a clear definition of a tir anomaly within a validation confutation scheme devoted to verify if detected anomalies can be associated or not to the time and location of the occurrence of major earthquakes in this scheme the confutation part i e verifying if similar anomalies do not occur in the absence of a significant seismic activity assumes a role even much important than the usual validation component devoted to verify the presence of anomalous signal transients before or in association with specific seismic events since 2001 rst approach has been being used to study tens of earthquakes with a wide range of magnitudes from 4 0 to 7 9 occurred in different continents and in various geo tectonic settings in this paper such a long term experience is exploited in order to give a quantitative definition of a significant sequence of tir anomalies ssta in terms of the required space time continuity constraints persistence identifying also the different typologies of known spurious sequences of tir anomalies that have to be excluded from the following validation steps on the same basis taking also into account for the physical models proposed for justifying the existence of a correlation between tir anomalies and earthquakes occurrence specific validation rules in line with the ones used by the collaboratory for the study of earthquake predictability csep project have been defined to drive the validation process in this work such an approach is applied for the first time to a long term dataset of night time gms 5 vissr geostationary meteorological satellite visible and infrared spin scan radiometer tir measurements comparing sstas and earthquakes with m 4 which occurred in a wide area around taiwan in the month of september of different years from 1995 to 2002 in this dataset the chi chi earthquake mw 7 6 which occurred on september 20 1999 represents the major but not unique event the analysis shows that all identified sstas occur in the pre fixed space time window around in terms of time and location earthquakes with m 4 the false positive rate remains zero even if only earthquakes with m 4 5 are considered in the case of the chi chi earthquake 3 sstas were identified all within the established space time correlation window one of them appearing about 2 weeks before and very close to the epicentre of the earthquake just along the associated tectonic lineaments the wide considered space time window together with the high seismicity of the considered area surely positively conditioned the achieved results so that further analyses should be carried out by using longer datasets and different geographic areas however also considering the coincidence with other possible precursor phenomena independently reported particularly within the istep project at the time of the chi chi earthquake achieved results seem already sufficient at least to qualify tir anomalies identified by rst among the parameters to be considered in the framework of a multi parametric approach to a time dependent assessment of seismic hazard t dash

four dimensional seismology and true direct detection of hydrocarbons the petrophysical basis figure 1 published by m king hubbert in 1962 and again in 1969 shows the exploitation of fossil fuels in a historical perspective according to this graph about half of the fossil energy on earth is being used up in a period of about 200 years half of which is by now over

a generalized biot gassmann model for the acoustic properties of shaley sandstones1 we obtain the wave velocities of clay bearing sandstones as a function of clay content porosity and frequency unlike previous theories based simply on slowness and or moduli averaging or two phase models we use a biot type three phase theory that considers the existence of two solids sand grains and clay particles and a fluid the theory which is consistent with the critical porosity concept uses three free parameters that determine the dependence of the dry rock moduli of the sand and clay matrices as a function of porosity and clay content testing of the model with laboratory data shows good agreement between predictions and measurements in addition to a rock physics model that can be useful for petrophysical interpretation of wave velocities obtained from well logs and surface seismic data the model provides the differential equation for computing synthetic seismograms in inhomogeneous media from the seismic to the ultrasonic frequency bands

progressive multiple event location pmel abstract this paper introduces a new method of using earthquake arrival times to simultaneously determine earthquake hypocenters and station corrections progressive multiple event location pmel forms a solution in three major steps 1 hypocenters are located by conventional single event methods using current station correction estimates 2 an annulled data set pavlis and booker 1980 is formed and accumulated using householder transformations and 3 station correction perturbations are calculated from a pseudoinverse solution of the annulled data set in practice pmel converges rapidly we analyze the non uniqueness of pmel estimates in terms of resolution and covariance matrices one of the most important features of pmel is the computer algorithm that we describe which requires only a modest amount of computer memory that is independent of the number of earthquakes to be processed consequently this algorithm can process a virtually unlimited number of earthquakes we apply pmel to a set of 8049 arrival times from 652 earthquakes and 4 explosions recorded by a network in the coso range inyo county california pmel converges after four adjustments to the station corrections the final rms misfit of 0 05 sec is remarkably good and the station correction estimates correlate strongly with surface geology

slow waves in fractures filled with viscous fluid stoneley guided waves in a fluid filled fracture generally have larger amplitudes than other waves and therefore their properties need to be incorporated in more realistic models in this study a fracture is modeled as an infinite layer of viscous fluid bounded by two elastic half spaces with identical parameters for small fracture thickness i obtain a simple dispersion equation for wave propagation velocity this velocity is much smaller than the velocity of a fluid wave in a biot type solution in which fracture walls are assumed to be rigid at seismic prospecting frequencies and realistic fracture thicknesses the stoneley guided wave has wavelengths on the order of several meters and an attenuation q factor exceeding 10 which indicates the possibility of resonance excitation in fluid bearing rocks the velocity and attenuation of stoneley guided waves are distinctly different at low frequencies for water and oil the predominant role of fractures in fluid flow at field scales is supported by permeability data showing an increase of several orders of magnitude when compared to values obtained at laboratory scales these data suggest that stoneley guided waves should be taken into account in theories describing seismic wave propagation in fluid saturated rocks

wave velocities and attenuation of shaley sandstones as a function of pore pressure and partial saturation we obtain the wave velocities and quality factors of clay bearing sandstones as a function of pore pressure frequency and partial saturation the model is based on a biot type three phase theory that considers the coexistence of two solids sand grains and clay particles and a fluid mixture additional attenuation is described with the constant q model and viscodynamic functions to model the high frequency behaviour we apply a uniform gas fluid mixing law that satisfies the wood and voigt averages at low and high frequencies respectively pressure effects are accounted for by using an effective stress law by fitting a permeability model of the kozeny carman type to core data the model is able to predict wave velocity and attenuation from seismic to ultrasonic frequencies including the effects of partial saturation testing of the model with laboratory data shows good agreement between predictions and measurements

anelasticity and dispersion in dry unconsolidated sands laboratory measurements reveal frame modulus dispersion and creeping behavior in room dry samples of unconsolidated quartzo feldspathic upper miocene turbidites from the wilmington field ca a single standard linear solid model predicts both the dispersion and the creep response similar behavior is observed in ottawa sand samples when mixed with wetted montmorillonite in this case the magnitude of dispersion and creep increase with increasing clay content we suggest that the anelastic behavior of the wilmington sands is due to the presence of micas and a small amount of clay the transition frequency of 2 10 4hz i e between high and low frequency behavior suggests that seismic sonic and ultrasonic measurements of rock moduli are quite similar in the absence of fluids the static frame modulus should be used to model tectonic processes and long term production related changes in oil field environments

full frequency range transient solution for compressional waves in a fluid saturated viscoacoustic porous medium1 an analytical transient solution is obtained for propagation of compressional waves in a homogeneous porous dissipative medium the solution based on a generalization of biot s poroelastic equations holds for the low and high frequency ranges and includes viscoelastic phenomena of a very general nature besides the biot relaxation mechanism the viscodynamic operator is used to model the dynamic behaviour associated with the relative motion of the fluid in the pores at all frequency ranges viscoelasticity is introduced through the standard linear solid which allows the modelling of a general relaxation spectrum the solution is used to study the influence of the material properties such as bulk moduli porosity viscosity permeability and intrinsic attenuation on the kinematic and dynamic characteristics of the two compressional waves supported by the medium we also obtain snapshots of the static mode arising from the diffusive behaviour of the slow wave at low frequencies lntroduction the acoustics of porous media within the framework of petroleum geophysics have been receiving more attention in recent years regional exploration seismology aimed at the discovery of hydrocarbon reservoirs is based on simplified rheological models and ray representations of the wavefield in the past these approximations were sufficient since the degree of resolution required was not high in the present time when most reservoirs are delimited the need to increase the resolution and use not only the traveltimes but also the information contained in the amplitude and phase of the seismic wavefield has gradually emerged the new exploration scenario is confined to reservoir areas and involves the presence of oil wells and data from seismic logs and well seismics which have enough resolution to see the effects of bulk properties porosity permeability fluid content and fluid solid interaction on the seismic pulse in this case the correct

earthquake locations and three dimensional crustal structure in the coyote lake area central california previous work on the simultaneous inversion method has been improved and extended to incorporate iterative solution for earthquake locations and laterally heterogeneous structure approximate ray tracing and parameter separation are important elements of the improved method application of the method to p wave arrival time data recorded by stations of the u s geological survey central california network yields a three dimensional model for the velocity structure of the upper crust in an area encompassing the rupture zone of the coyote lake earthquake of august 1979 very strong correlations between the velocity model and the geology and gravity and magnetic anomalies are observed improved estimates of the locations of earthquakes in the study area are also determined the relocation of explosions indicates epicentral accuracies of the order of a kilometer or better based on the revised hypocentral locations it is concluded that the san andreas fault is vertical in this area with no actual offset between the epicenters and the fault trace in contrast the calaveras has two or more active fault surfaces one nearly vertical and another dipping 75 to the northeast

unique problems associated with seismic analysis of partially gas saturated unconsolidated sediments abstract gas hydrate stability conditions restrict the occurrence of gas hydrate to unconsolidated and high water content sediments at shallow depths because of these host sediments properties seismic and well log data acquired for the detection of free gas and associated gas hydrate bearing sediments often require nonconventional analysis for example a conventional method of identifying free gas using the compressional shear wave velocity v p v s ratio at the logging frequency will not work unless the free gas saturations are more than about 40 the p wave velocity dispersion of partially gas saturated sediments causes a problem in interpreting well log velocities and seismic data using the white j e 1975 computed seismic speeds and attenuation in rocks with partial gas saturation geophysics 40 224 232 model for partially gas saturated sediments the difference between well log and seismic velocities can be reconciled the inclusion of p wave velocity dispersion in interpreting well log data is therefore essential to identify free gas and to tie surface seismic data to synthetic seismograms

illuminating and optimising a three dimensional model of an oil shale seam and its volume distribution using the transient electromagnetic induction method central part of jordan the oil shale exploration program in jordan is undertaking great activity in the domain of applied geophysical methods to evaluate bitumen bearing rock in the study area the bituminous marl or oil shale exhibits a rock type dominated by lithofacies layers composed of chalky limestone marls clayey marls and phosphatic marls the study aims to present enhancements for oil shale seam detection using progressive interpretation from a one dimensional inversion to a three dimensional modelling and inversion of ground based transient electromagnetic data at an area of stressed geological layers the geophysical survey combined 58 transient electromagnetic sites to produce geoelectrical structures at different depth slices and cross sections were used to characterise the horizon of the most likely sites for mining oil shale the results show valuable information on the thickness of the oil shale seam at 3 7 m which is correlated to the geoelectrical layer between 2 ms and 4 ms transient time delays and at depths ranging between 85 m and 105 m the 300 m penetrated depth of the transient electromagnetic soundings allows the resolution of the main geological units at narrow resistivity contrast and the distinction of the main geological structures that constrain the detection of the oil shale seam this geoelectrical layer at different depth slices illustrates a localised oil shale setting and can be spatially correlated with an area bounded by fold and fault systems also three dimensional modelling and inversion for synthetic and experimental data are introduced at the faulted area the results show the limitations of oil shale imaging at a depth exceeding 130 m which depends on the near surface resistivity layer the low resistivity contrast of the main lithological units and the degree of geological detail achieved at a suitable model s misfit value this article is protected by copyright all rights reserved

wave propagation in heterogeneous porous media a velocity stress finite difference method a particle velocity stress finite difference method is developed for the simulation of wave propagation in 2 d heterogeneous poroelastic media instead of the prevailing second order differential equations we consider a first order hyperbolic system that is equivalent to biot s equations the vector of unknowns in this system consists of the solid and fluid particle velocity components the solid stress components and the fluid pressure a maccormack finite difference scheme that is fourth order accurate in space and second order accurate in time forms the basis of the numerical solutions for biot s hyperbolic system an original analytic solution for a p wave line source in a uniform poroelastic medium is derived for the purposes of source implementation and algorithm testing in simulations with a two layer model additional slow compressional incident transmitted and reflected phases are recorded when the damping coefficient is small this slow compressional wave is highly attenuated in porous media saturated by a viscous fluid from the simulation we also verified that the attenuation mechanism introduced in biot s theory is of secondary importance for fast compressional and rotational waves the existence of seismically observable differences caused by the presence of pores has been examined through synthetic experiments that indicate that amplitude variation with offset may be observed on receivers and could be diagnostic of the matrix and fluid parameters this method was applied in simulating seismic wave propagation over an expanded steam heated zone in cold lake alberta in an area of enhanced oil recovery eor processing the results indicate that a seismic surface survey can be used to monitor thermal fronts

evidence from the rio bayo valley on the extent of the north patagonian icefield during the late pleistocene holocene transition abstract this paper presents data on the extent of the north patagonian icefield during the late pleistocene holocene transition using cosmogenic nuclide exposure age and optically stimulated luminescence dating we describe geomorphological and geochronological evidence for glacier extent in one of the major valleys surrounding the north patagonian icefield the rio bayo valley geomorphological mapping provides evidence for the existence of two types of former ice masses in this area i a large outlet glacier of the north patagonian icefield which occupied the main rio bayo valley and ii a number of small glaciers that developed in cirques on the slopes of the mountains surrounding the valley cosmogenic nuclide exposure age dating of two erratic boulders on the floor of the rio bayo valley indicate that the outlet glacier of the icefield withdrew from the rio bayo valley after 10 900 1000 yr the mean of two boulders dated to 11 400 900 yr and 10 500 800 yr single grain optically stimulated luminescence osl dating of an ice contact landform constructed against this glacier indicates that this ice mass remained in the valley until at least 9700 700 yr the agreement between the two independent dating techniques osl and cosmogenic nuclide exposure age dating increases our confidence in these age estimates a date obtained from a boulder on a cirque moraine above the main valley indicates that glaciers advanced in cirques surrounding the icefield some time around 12 500 900 yr this evidence for an expanded north patagonian icefield between 10 900 1000 yr and 9700 700 yr implies cold climatic conditions dominated at this time

the effect of thermal transfer on the zeroing of the luminescence of quartz from recent glaciofluvial sediments abstract the natural optically stimulated luminescence osl response of quartz grains extracted from a range of recent glaciofluvial and other glaciogenic sediments from western greenland is presented all samples were deposited within the last 10 years many only days or hours before collection consequently we would expect to measure zero or very small d e values had optical resetting of the osl signal been complete in the case of most samples d e values in the range 0 5 10 gy are observed while higher values are observed for a handful of samples however the regenerated osl growth curves constructed after daylight exposure of at least several hours also show large apparent d e values negative x axis intercepts corresponding samples from four different locations in the indian himalaya display similar osl characteristics the measurement of several regenerated osl growth curves omitting the preheat procedure which normally precedes osl measurement confirms that a significant degree of thermal transfer is occurring during the preheating it is this thermal transfer of charge during the preheat which is responsible for a significant contribution to the large apparent d e values observed for the regenerated osl growth in this paper we describe combined osl and tl measurements which have been used to monitor the charge transfer in detail for one sample an attempt is made to describe the charge transfer behaviour and we discuss methods appropriate to eliminating the unwanted effects of this phenomenon during osl dating preliminary results from the application of these techniques are presented and the implications of these observations for the osl dating of glaciogenic sediments are discussed

seismic wave properties in time dependent porosity homogeneous media summary it is quantified the properties of seismic waves in fully saturated homogeneous porous media within the framework of sahay s modified and reformulated poroelastic theory the computational results comprise amplitude attenuation velocity dispersion and seismic waveforms they show that the behaviour of all four waves modelled as a function of offset frequency porosity fluid viscosity and source bandwidth depicts realistic dissipation within the sonic ultrasonic band therefore it appears that there is no need to include material heterogeneity to model attenuation by inference it is concluded that the fluid viscosity effects may be enhanced by dynamic porosity

variations of p wave travel time residuals before and after the 1999 chi chi taiwan earthquake p wave velocity variations may provide precursory information before large earthquakes we present results from a simple study on variations of p wave travel time residuals before and after the 1999 chi chi taiwan earthquake the short period seismic network deployed in taiwan by the central weather bureau cwb is routinely used for earthquake location the accuracy of earthquake location is often judged by the travel time residual of seismic waves because the velocity model used by the cwb was one dimensional the travel time residuals will show lateral inhomogeneity of the velocity structures and unmodeled vertical structures also the travel time residuals may change with time if the velocity structure changes temporally we used the data of the central weather bureau seismic network cwbsn from 1991 to 2002 to obtain the difference between observed and calculated p wave travel times which is defined as the p wave travel time residual the purpose of this study is to find the mean value of the p wave travel time residuals at each station and its possible temporal changes at stations near the chelungpu fault our results show that the mean p wave travel time residuals at stations 40 km east of the chelungpu fault changed very little before and after the chi chi earthquake but the mean p wave travel time residuals at the stations immediately west of the chelungpu fault clearly changed before and after the chi chi earthquake the anomalous zone is bounded by stations nsy twq1 tcu wnt wgk chn5 yus wdt and whf these temporal changes of p wave travel time residuals can be attributed to changes in the velocity structure which in turn might be caused by crustal deformation east of the chelungpu fault beginning about 6 years before the chi chi earthquake in other words before the chi chi earthquake there was a long term 6 year precursor of p wave velocity decrease possibly due to dilatancy or development of cracks the variations of p wave travel time residuals with time at the anomalous stations are also confirmed by statistical tests that show clear significance this method can be used to monitor and identify variations of p wave travel time residuals which may be precursors to a large earthquake

optically stimulated luminescence dating of sediments over the past 200 000 years optical dating of sediment using optically stimulated luminescence has become important for studying earth surface processes and this technique continues to develop rapidly a group of closely linked luminescence methods can be used to estimate the time since grains of quartz and feldspar were last exposed to daylight by detecting their subsequent response to environmental ionizing radiation exposure the technique is well suited to the dating of deposits as young as one year to several hundred thousand years recent technical developments have established a dating protocol with improved precision a high degree of reliability and an in built means to detect incomplete signal removal during deposition this approach has been extended to age estimation for single grains opening up a wider range of potential environments and new possibilities for understanding postdepositional grain movement ongoing research offers the possibility of significant age range extension and novel applications including low t

acoustic measures of partial gas saturation in tight sandstones the acoustic properties of tight sandstones are very sensitive to partial gas water saturation measurements at 5 khz show that compressional velocity drops sharply with undersaturation extensional and shear losses at 5 khz are large in both fully and partially saturated sands the shear loss qs 1 declines linearly with increasing gas content the extensional loss qe 1 peaks at 0 80 water saturation q 1 is frequency dependent the attenuation is well explained by hydrodynamic relaxation in the grain contacts the measurements indicate velocity dispersions consistent with this mechanism ultrasonic velocities at 500 khz are roughly 10 25 higher than acoustic velocities at 5 khz the ultrasonic velocities exhibit a weak dependence on saturation frame moduli are unrelaxed at these frequencies

measurement of equivalent doses in quartz from contemporary water lain sediments using optically stimulated luminescence the single aliquot technique and its derivatives are very attractive for the measurement of the very small doses found in young materials because they inherently avoid concerns about low dose non linearity and can offer high precisions a modification is outlined to the single aliquot regeneration and added dose sara protocol mejdahl and botter jensen 1994 quaternary geochronology quaternary science reviews 13 551 5541 to permit its application to sedimentary quartz using the optically stimulated luminescence signal this revised protocol is then used to determine the doses found in several contemporary fluvial sediments and one coastal beach sand thought to be deposi ted by recent wave action two over bank river deposits were both homogeneously well bleached with doses of 0 4 gy although most of the grains in three channel deposits from a range of catchment sizes were also well bleached contamination by a small proportion of very poorly bleached quartz grains prevented the measurement of the dose in the majority the beach deposit was homogeneously bleached and contained a dose of about 0 02 gy it is concluded that over bank sites may provide accurate dates for samples older than a few hundred years but that the heterogeneity of the channel sites limits the accuracy of dates less than a few thousand years

analytic simulation of the elastic waves propagation in the neighborhood of fluid filled wells with monopole sources an analytic formulation to understand the scattering diffraction and attenuation of elastic waves at the neighborhood of fluid filled wells is presented an important and not widely exploited technique to carefully investigate the wave propagation in exploration wells is the logging of sonic waveforms fundamental decisions and production planning in petroleum reservoirs are made by interpretation of such recordings nowadays geophysicists and engineers face problems related to the acquisition and interpretation under complex conditions associated with conducting open hole measurements a crucial problem that directly affects the response of sonic logs is the eccentricity of the measuring tool with respect to the center of the borehole even with the employment of centralizers this simple variation dramatically changes the physical conditions on the wave propagation around the well recent works in the numerical field reported advanced studies in modeling and simulation of acoustic wave propagation around wells including complex heterogeneities and anisotropy however no analytical efforts have been made to formally understand the wireline sonic logging measurements acquired with borehole eccentered tools in this paper the graf s addition theorem was used to describe monopole sources in terms of solutions of the wave equation the formulation was developed from the three dimensional discrete wave number method in the frequency domain the cylindrical bessel functions of the third kind and order zero were re derived to obtain a simplified set of equations projected into a bi dimensional plane space for displacements and stresses this new and condensed analytic formulation allows the straightforward calculation of all converted modes and their visualization in the time domain via fourier synthesis the main aim was to obtain spectral surfaces of transfer functions and synthetic seismograms that might be useful to understand the wave motion produced by the eccentricity of the source and explain in detail the new arising borehole propagation modes finally time histories and amplitude spectra for relevant examples are presented and the validation of time traces using the spectral element method is reported

delayed deglaciation or extreme arctic conditions 21 16 cal kyr at southeastern laurentide ice sheet margin 1 the conventionally accepted ages of the last glacial maximum lgm retreat of the southeastern laurentide ice sheet lis are 26 21 cal kyr derived from bulk sediment radiocarbon ages and 28 23 cal kyr varve estimates utilizing accelerator mass spectrometry ams 14c dating of earliest macrofossils in 13 lake bog inorganic clays we find that vegetation first appeared on the landscape at 16 15 cal kyr suggesting ice had not retreated until that time the gap between previous age estimates and ours is significant and has large implications for our understanding of ocean atmosphere linkages older ages imply extreme arctic conditions for 9 5 cal kyr a landscape with no ice yet no deposition in lakes our new ams chronology of lis retreat is consistent with marine evidence of deglaciation from the n atlantic showing significant freshwater input and sea level rise only after 19 cal kyr with a cold meltwater lid perhaps delaying ice melt

acoustic and elastic characterization of marine sediments by analysis modeling and inversion of ultrasonic p wave transmission seismograms ultrasonic p wave transmission seismograms recorded on sediment cores have been analyzed to study the acoustic and estimate the elastic properties of marine sediments from different provinces dominated by terrigenous calcareous and diatomaceous sedimentation instantaneous frequencies computed from the transmission seismograms are displayed as gray shaded images to give an acoustic overview of the lithology of each core centimeter scale variations in the ultrasonic waveforms associated with lithological changes are illustrated by wiggle traces in detail cross correlation multiple filter and spectral ratio techniques are applied to derive p wave velocities and attenuation coefficients s wave velocities and attenuation coefficients elastic moduli and permeabilities are calculated by an inversion scheme based on the biot stoll viscoelastic model together with porosity measurements p and s wave scatter diagrams are constructed to characterize different sediment types by their velocity and attenuation porosity relationships they demonstrate that terrigenous calcareous and diatomaceous sediments cover different velocity and attenuation porosity ranges in terrigenous sediments p wave velocities and attenuation coefficients decrease rapidly with increasing porosity whereas s wave velocities and shear moduli are very low calcareous sediments behave similarly at relatively higher porosities foraminifera skeletons in compositions of terrigenous mud and calcareous ooze cause a stiffening of the frame accompanied by higher shear moduli p wave velocities and attenuation coefficients in diatomaceous ooze the contribution of the shear modulus becomes increasingly important and is controlled by the opal content whereas attenuation is very low this leads to the opportunity to predict the opal content from nondestructive p wave velocity measurements at centimeter scale resolution

ages for the big stone moraine and the oldest beaches of glacial lake agassiz implications for deglaciation chronology glacial lake agassiz has been implicated as the trigger for numerous episodes of abrupt climate change at the close of the last ice age yet the beginning age of the lake has never been determined here we report the fi rst numerical age data on the big stone moraine and the oldest beaches of glacial lake agassiz organic remains from lakes bogs and channels distal to and inset to the big stone moraine require that glacial activity at this moraine ceased prior to 12 000 14 c yr b p 13 950 cal calendar yr a site near new effi ng ton south dakota united states implies full glacial recession north of the topographic divide prior to 11 810 14 c yr b p 13 670 cal yr synchronous with the beginning of glacial lake agassiz lake agassiz shorelines inset to the moraine yield optically stimulated luminescence osl ages from 14 200 12 600 yr cal lower strandlines are younger but the similarity of ages suggests that initial lake lowering was faster than osl ages can currently resolve nevertheless the osl ages represent the fi rst numerical age assignments for the herman norcross and upham beach ridges setting the stage for future numerical age assignments within the lake agassiz basin these two dating methods yield strongly consistent results within stated uncertainties the age of the big stone moraine implies an interval of rapid retreat for the des moines lobe of the laurentide ice sheet during the bolling allerod warm interval the overlapping ages for the uppermost beach levels and abandonment of the highest lake agassiz spillway indicate a rapidly evolving lake until at least 13 500 yr cal

on the propagation of a disturbance in a heterogeneous deformable porous medium saturated with two fluid phases the coupled modeling of the flow of two immiscible fluid phases in a heterogeneous elastic porous material is formulated in a manner analogous to that for a single fluid phase an asymptotic technique valid when the heterogeneity is smoothly varying is used to derive equations for the phase velocities of the various modes of propagation a cubic equation is associated with the phase velocities of the longitudinal modes the coefficients of the cubic equation are expressed in terms of sums of the determinants of 3 3 matrices whose elements are the parameters found in the governing equations in addition to the three longitudinal modes there is a transverse mode of propagation a generalization of the elastic shear wave estimates of the phase velocities for a homogeneous medium based upon the formulas in this paper agree with previous studies furthermore predictions of longitudinal and transverse phase velocities made for the massilon sandstone containing varying amounts of air and water are compatible with laboratory observations

rock anelasticity due to patchy saturation and fabric heterogeneity a double double porosity model of wave propagation heterogeneity of rock s fabric can induce heterogeneous distribution of immiscible fluids in natural reservoirs since the lithological variations mainly permeability may affect fluid migration in geological time scales resulting in patchy saturation of fluids therefore fabric and saturation inhomogeneities both affect wave propagation to model the wave effects attenuation and velocity dispersion we introduce a double double porosity model where pores saturated with two different fluids overlap with pores having dissimilar compressibilities the governing equations are derived by using hamilton s principle based on the potential energy kinetic energy and dissipation functions and the stiffness coefficients are determined by gedanken experiments yielding one fast p wave and four slow biot waves three examples are given namely muddy siltstones clean dolomites and tight sandstones where fabric heterogeneities at three different spatial scales are analyzed in comparison with experimental data in muddy siltstones where intrapore clay and intergranular pores constitute a submicroscopic double porosity structure wave anelasticity mainly occurs in the frequency range 104 107 hz while in pure dolomites with microscopic heterogeneity of grain contacts and tight sandstones with mesoscopic heterogeneity of less consolidated sands it occurs at 103 107 hz and 101 103 hz seismic band respectively the predicted maximum quality factor of the fast compressional wave for the sandstone is the lowest approximately 8 and that of the dolomite is the highest the results of the diffusive slow waves are affected by the strong friction effects between solids and fluids the model describes wave propagation in patchy saturated rocks with fabric heterogeneity at different scales and the relevant theoretical predictions agree well with the experimental data in fully and partially saturated rocks

geochronology and geomorphology of the jones point glacial landform in lower hudson valley new york insight into deglaciation processes since the last glacial maximum abstract the glacial deposits at jones point located on the western side of the lower hudson river new york were investigated with geologic geophysical remote sensing and optically stimulated luminescence osl dating methods to build an interpretation of landform origin formation and timing osl dates on eight samples of quartz sand seven single aliquot and one single grain of quartz yield an age range of 14 27 ka for the proglacial and glaciofluvial deposits at jones point optical age results suggest that jones point deposits largely predate the glacial lake albany drainage erosional flood episode in the hudson river valley ca 15 13 ka based on this data we conclude that this major erosional event mostly removed valley fill deposits leaving elevated terraces during deglaciation at the end of the last glacial maximum lgm our analysis supports the earlier 15 ka date for the flood occurrence and suggests that more recent overlying glaciofluvial deposits formed in the lee side of the jones point promontory

local and global fluid effects on sonic wave modes abstractmost subsurface formations of value to exploration contain a heterogeneous fluid filled pore space where local fluid pressure effects can significantly change the velocities of passing seismic waves to better understand the effect of these local pressure gradients on borehole wave propagation we combined chapman s squirt flow model with biot s poroelastic theory we applied the unified theory to a slow and fast formation with permeable borehole walls containing different quantities of compliant pores these results are compared with those for a formation with no soft pores the discrete wavenumber summation method with a monopole point source generates the wavefields consisting of the p s leaky p stoneley and pseudo rayleigh waves the resulting synthetic wave modes are processed using a weighted spectral semblance wss algorithm we found that the resulting wss dispersion curves closely matched the analytical expressions for the formation compressional velocity and solutions to the peri

finite frequency kernels for wave propagation in porous media based upon adjoint methods s u m m a r y we determine finite frequency kernels for wave propagation in porous media based upon adjoint methods these sensitivity kernels may be obtained based upon two numerical simulations for each source one calculation for the current model and a second adjoint calculation that uses time reversed signals at the receivers as simultaneous fictitious sources the adjoint equations for a porous medium are identical to the usual biot equations with the exception of the adjoint source term which involves time reversed measurements of the differences between simulations and data isotropic poroelastic wave propagation is governed by eight primary model parameters which appear in the original biot equations these parameters include four moduli the shear modulus of the frame plus three bulk moduli we consider two alternative parameterizations one involving density normalized moduli corresponding to squared wave speeds and a second involving the poroelastic shear and compressional wave speeds the alternative parameterizations lead to density sensitivity kernels that are small reflecting the fact that the traveltime of a poroelastic wave is governed by wave speed not density we systematically investigate and illustrate the sensitivity of the fast and slow compressional waves and the shear wave to the poroelastic model parameters using a 2 d spectral element method the poroelastic sensitivity kernels presented herein form the basis of tomographic imaging and inversion in porous media

the squirt flow mechanism macroscopic description we introduce a poroelasticity model that incorporates the two most important mechanisms of solid fluid interaction in rocks the biot mechanism and the squirt flow mechanism this combined biot squirt bisq model relates compressional velocity and attenuation to the elastic constants of the drained skeleton and of the solid phase porosity permeability saturation fluid viscosity and compressibility and the characteristic squirt flow length squirt flow length is a fundamental rock property that does not depend on frequency fluid viscosity or compressibility and is determined experimentally we find that the viscoelastic response of many sandstones is dominated by the squirt flow component of the bisq mechanism and that the viscoelastic properties of these rocks can be expressed through a single dimensionless parameter omega r 2 kappa where omega is angular frequency r is the characteristic squirt flow length and kappa is hydraulic diffusivity the biot mechanism alone does not give an adequate explanation of the observed velocity dispersion and attenuation and the viscoelastic behavior of many sandstones

relating p wave attenuation to permeability to relate p wave attenuation to permeability we examine a three dimensional 3 d theoretical model of a cylindrical pore filled with viscous fluid and embedded in an infinite isotropic elastic medium we calculate both attenuation and permeability as functions of the direction of wave propagation attenuation estimates are based on the squirt flow mechanism permeability is calculated using the kozeny carman relation we find that in the case when a plane p wave propagates perpendicular to the pore orientation q 90 1 attenuation is always higher than when a wave propagates parallel to this orientation q 0 1 the ratio of these two attenuation values q 90 1 q 0 1 increases with an increasing pore radius and decreasing frequency and saturation by changing permeability varying the radius of the pore we find that the permeability attenuation relation is characterized by a peak that shifts toward lower permeabilities as frequency decreases therefore the attenuation of a low frequency wave d

seismological studies at parkfield iii microearthquake clusters in the study of fault zone dynamics abstract more than half of the microearthquakes that occur near parkfield california when located with high resolution methods are seen to define some 80 small clusters of 2 to 12 similar events each cluster occupies a patch typically 100 to 200 m in length within the fault zone cluster members have nearly identical waveforms correlation coefficient of 0 9 or greater to frequencies of 50 to 100 hz as recorded by the borehole installed seismographic network the clusters are distributed throughout the fault zone around the presumed nucleation region in the locked section to the se and in the creeping part of the nw of the previous m 6 hypocenter they are also found in the salinian block several kilometers sw of the fault zone the total area occupied by all of the clusters constitutes only a small fraction 1 to 2 of the fault zone approaching failure at parkfield such clusters provide insight into the dynamics of the failure process through their spatial temporal characteristics and their mechanisms they also serve as highly repetitive sources distributed throughout the fault zone suitable for monitoring the nucleation zone for possible precursory changes in physical properties that affect wave propagation in this article we demonstrate these applications with a detailed analysis of selected clusters

optical dating of sediments a new method for dating sediments is proposed which determines the time since the sediment was last exposed to sunlight an argon ion laser is used to excite electrons from thermally stable light sensitive traps and the subsequent luminescence used as a measure of the past radiation dose two sample sequences spanning the periods 0 700 kyr and 0 6 kyr show steadily increasing luminescence with age an age of 62 8 kyr is obtained for a silt radiocarbon dated at 58 8 0 3 kyr problems were found with two much younger samples

luminescence dating of late pleistocene proximal glacial sediments in the olympic mountains washington abstract late pleistocene glacial sediments from the south fork hoh river valley in the olympic mountains washington usa were dated using optically stimulated luminescence osl on quartz sand high sediment supply typical of glacial environments short transport distances and sediment newly eroded from bedrock sources were expected to pose problems for luminescence dating samples were collected from five distinct sedimentary facies using approximated distances from the ice front to assess how luminescence results varied due to these factors and to determine which samples produced the most reliable age estimates results from the south fork hoh highlight the importance of transport environment and sedimentary facies on solar resetting sediments from better sorted ice distal environments were determined to be more completely bleached and more reliable for osl dating samples were collected as part of larger research goals to improve understanding of the glacial history of the south fork hoh river valley

seismic detection of transient changes beneath black rapids glacier alaska u s a ii basal morphology and processes using changes observed in daily seismic reflections we have investigated the basal morphology of black rapids glacier alaska u s a the englacial drainage of ice marginal lakes caused significant changes in the daily reflections as well as dramatic increases in basal motion changes in reflection arrival times and amplitudes indicate that there is a basal till layer at least 5 m thick at some locations beneath this surge type glacier rapid changes in the observed reflection coefficients during the drainage events indicate that changes in till properties must occur throughout the entire 5 m thick layer they must last for several days following the lake drainages and they must be completely reversible over as little as 36 min our seismic analysis shows that changes in effective pressure of the till are unlikely to cause the required changes in the reflection coefficients but that a decrease in till saturation is likely we therefore interpret the cause ofthe seismic anomalies as being a temporary decrease in saturation as water is input to the subglacial hydraulic system and propose that such a change may occur quickly and reversibly by a redistribution ofoverburden pressure higher water pressures within the hydraulic system cause that region to support more of the glacier s weight leaving the remaining areas to support less any till within these areas of decreased normal stress would experience a consequent decrease in pore water pressure causing gas to exolve thus decreasing saturation this decrease in saturation would cause a change in the strength of the basal layer and may affect basal dynamics

towards successful osl sampling strategies in glacial environments deciphering the influence of depositional processes on bleaching of modern glacial sediments from jostedalen southern norway the optically stimulated luminescence osl signals of quartz and k feldspar are known to bleach poorly within some glacial settings and can present a major challenge to dating applications however because the osl signal is extremely sensitive to sunlight exposure history the residual luminescence signals of modern glacial sediments also encode information about transport and depositional processes through examination of the residual luminescence properties equivalent dose de and overdispersion values of a suite of modern glacial sediments from different depositional settings sandar proglacial delta and main meltwater channel this study provides insights not only into which sediments are likely to be fully bleached within glacial settings but also into how osl can be used to trace different depositional processes across sedimentary landforms improved understanding of the processes of sediment bleaching will enable better sample selection and may improve the accuracy and precision of osl dating of glacial sediments the luminescence signals of both coarse grained quartz and k feldspar with similar sediment sources are found to be sensitive to both depositional process and specific depositional setting whereas modern braid bar head deposits from the nigardsdalen ice proximal proglacial delta typically have ages of 3 ka similar depositional features from the fabergstolsgrandane sandur have residual ages of 26 ka exploration of changing residual luminescence signals across individual sandur and proglacial delta braid bar features shows that braid bar head deposits can retain large residual d e values while the partner braid bar tail deposits are almost completely bleached the quartz osl signal and k feldspar irsl50 and post ir irsl250 signals are shown to bleach at the same rate across the same bar feature and the irsl50 k feldspar signal is also shown to be completely bleached for bar tail deposits in nigardsdalen therefore the irsl50 k feldspar signal is suitable for dating some glacial deposits circumventing the challenges associated with dim quartz signals 2014 the authors published by elsevier ltd this is an open access article under the cc by license

luminescence dating of glaciofluvial deposits a review abstract for glacigenic sediments optically stimulated luminescence osl dating offers an opportunity to date the time of deposition of quartz and feldspar minerals that are ubiquitous within the sediment matrix rather than relying upon the chance occurrence of organic material for radiocarbon dating the osl dating signal or charge accumulates in crystal defects of individual quartz and feldspar minerals through exposure to environmental radiation within their depositional setting the osl signal within individual grains can be reset or bleached through exposure to daylight during transportation processes thus osl dating of sediments attempts to determine the time elapsed since burial glacigenic sediments present considerable challenges for osl dating not only in terms of poor bleaching of the osl signal during the transport and deposition cycle but also poor quartz luminescence characteristics which are often related to sediment provenance the nature of the bedrock or source material and the freshly eroded nature of many such deposits this paper reviews luminescence dating techniques as applied to glaciofluvial sediments and explores the challenges that such deposits present to the technique successful application of osl techniques can be judged in a number of ways comparison with the independent chronologies is prevalent in the literature but recently the movement towards measurement of large numbers of small aliquots and single grains allows a more robust assessment of luminescence properties and behaviour for individual samples and ultimately more precise luminescence ages to be determined for glaciofluvial sediments in particular it is important to investigate the depositional sub environment of each sample in relation to the chance of sufficient bleaching during transport and deposition a lithofacies based approach to sampling for optically stimulated luminescence dating of these sediments is suggested whereby hypothetically well bleached deposits are targeted after a detailed investigation into possible environments of deposition this approach together with small aliquot single grain analysis methodologies should further enhance the likelihood of obtaining a sufficiently bleached sample for osl dating

velocity anisotropy and attenuation of shale in under and overpressured conditions the seismic velocity and attenuation of fully saturated shales were measured for the first time under overpressured conditions using the ultrasonic reflection technique shale cores from naturally overpressured horizons in the north sea were tested in the laboratory at confining and pore pressures relevant to in situ conditions a single frequency tone burst pulse wave was used to determine the seismic wave velocities and quality factors of the shale samples with errors less than 0 3 and 0 1 db cm respectively at a frequency of 0 75 mhz sample length changes with varying confining and pore pressure were measured and the pore pressure equilibration time was monitored for each sample the anisotropy of the seismic attributes vp vs qp and qs was determined over a range of differential pressures from 5 to 60 mpa with respect to the predominant foliation the ultrasonic velocity data followed a transversely isotropic pattern depending on the direction of wave propagation with respect to the laminations the poisson s ratio was found to rise by 5 as the shale material progressed from a normally pressured to an overpressured state the quality factor q characteristics were interpreted in terms of pore geometry and connectivity as well as the directional permeability of the transversely isotropic shale material the results were converted to bulk and shear loss modulus defects and a positive bulk loss was observed for waves propagating perpendicular to the lamination plane even above differential pressures of 20 mpa this indicates different levels of biot flow and squirt flow attenuation mechanisms acting within the shale structure depending on the wave propagation and vibration directions

lithofacies types and vertical profile models an alternative approach to the description and environmental interpretation of glacial diamict and diamictite sequences abstract increased knowledge of modern glacial depositional environments has resulted in rapidly evolving classifications of glacial tills these are based to a large degree on theoretical considerations of likely depositional processes the classifications are sophisticated and more advanced than the establishment of simple field criteria whereby individual till facies can be identified in quaternary and pre quaternary successions this situation is compounded in many quaternary terrains by the continued description of tills in terms of laboratory derived analytical data only reflecting a traditional interest in stratigraphic correlation rather than reconstruction of depositional environment detailed sedimentological logging of lithofacies is rarely undertaken there is thus considerable confusion as to what is being described or sampled when analytical data are presented for many pleistocene tills the same remarks apply to pre pleistocene tillites a lithofacies code is presented here for the rapid description and visual appraisal of field sequences or drill cores containing unconsolidated diamicts or lithified diamictites the term till is not used as it has a strict genetic definition referring to direct aggregation and deposition by glacier ice use of a four part code in conjunction with codes already published for fluvial sediments allows fundamental field properties to be depicted independent of genetic terminology and provides a firm basis for subsequent environmental interpretation and analytical work the value of this approach is illustrated by comparing a representative suite of vertical profiles of diamict assemblages deposited by modern grounded glaciers with a classic late pleistocene glacigenic sequence at scarborough bluffs ontario

experimental observation of water saturation effects on shear wave splitting in synthetic rock with fractures aligned at oblique angles fractured rocks are known to exhibit seismic anisotropy and shear wave splitting sws sws is commonly used for fractured rock characterization and has been shown to be sensitive to fluid type the presence of partial liquid gas saturation is also known to affect the elastic properties of rocks the combined effect of both fractures and partial liquid gas saturation is still unknown using synthetic silica cemented sandstones with aligned penny shaped voids we conducted laboratory ultrasonic experiments to investigate the effect fractures aligned at an oblique angle to wave propagation would have on sws under partial liquid gas saturation conditions the result for the fractured rock shows a saturation dependence which can be explained by combining a fractured rock model and a partial saturation model at high to full water saturation values sws decreases as a result of the fluid bulk modulus effect on the quasi shear wave this bulk modulus effect is frequency dependent as a result of wave induced fluid flow mechanisms which would in turn lead to frequency dependent sws this result suggests the possible use of sws for discriminating between full liquid saturation and partial liquid gas saturation

acoustic signatures of partial saturation the relationship between elastic wave velocities and water saturation in a water gas reservoir depends strongly on whether saturation is heterogeneous patchy or homogeneous heterogeneity in saturation may result from lithologic heterogeneity because under conditions of capillary equilibrium different lithologies within a reservoir can have different saturations depending on their porosities and permeabilities we investigate this phenomenon by generating models of a reservoir in which we control the distribution of lithologic units and theoretically determine the corresponding velocity saturation relationship we assume a state of capillary equilibrium in the reservoir and determine the saturation level of each region within the reservoir from the corresponding capillary pressure curve for the lithologic unit at that location the velocities we calculate for these models show that saturation heterogeneity caused by lithologic variation can lead to a distinct dependence of velocity on saturation in a water gas saturated reservoir a patchy distribution of the different lithologic units is found to cause p wave velocity to exhibit a noticeable and almost continuous velocity variation across the entire saturation range this is in distinct contrast to the response of a homogeneous reservoir where there is only a large change in velocity at water saturations close to 100

facies characteristics of middle pleistocene saalian ice margin subaqueous fan and delta deposits glacial lake leine nw germany abstract the blocking of major river valleys in the leinebergland area by the early saalian scandinavian ice sheet led to the formation of a large glacial lake referred to as glacial lake leine where most of the sediment was deposited by meltwater at the initial stage the level of glacial lake leine was approx 110 m a s l the lake level then rose by as much as 100 m to a highstand of approx 200 m a s l two genetically distinct ice margin depositional systems are described that formed on the northern margin of glacial lake leine in front of the retreating scandinavian ice sheet the bornhausen delta is up to 15 m thick and characterized by a large scale tangential geometry with dip angles from 10 28 reflecting high angle foreset deposition on a steep delta slope foreset beds consist of massive clast supported gravel and pebbly sand alternating with planar parallel stratified pebbly sand deposited from cohesionless debris flows sandy debris flows and high density turbidity flows the finer grained sandy material moved further downslope where it was deposited from low density turbidity currents to form massive or ripple cross laminated sand in the toeset area the freden ice margin depositional system shows a more complex architecture characterized by two laterally stacked sediment bodies the lower part of the section records deposition on a subaqueous ice contact fan the upper part of the freden section is interpreted to represent delta slope deposits beds display low to high angle bedding 3 30 and consist of planar and trough cross stratified pebbly sand and climbing ripple cross laminated sand the supply of meltwater transported sediment to the delta slope was from steady seasonal flows during higher energy conditions 2 d and 3 d dunes formed migrating downslope and passing into ripples during lower energy flow conditions thick climbing ripple cross laminated sand beds accumulated also on higher parts of the delta slope

investigation of the temporal change in attenuation within the ruptured fault zone of the 1999 mw7 3 chi chi taiwan earthquake a decrease in q s values within the hanging wall of the ruptured chelungpu fault two years following the 1999 chi chi earthquake was revealed by q s tomography images and an analysis of single path q s the synthetic and sensitivity tests of the q s determination were carried out accordingly to justify the temporal variation a q s value within the hanging wall above the hypocenter was determined to be 157 18 two years following the chi chi earthquake which is significantly lower than the q s tomography values of 238 17 and 289 13 prior to and two years after the main shock respectively similar values using a signal path q s analysis from events within the ruptured fault zone to stations along the fault were obtained the corresponding q s values were 247 85 prior to the chi chi earthquake after the earthquake we obtained q s values of 158 75 and 318 80 for 2 years following and 2 years after the earthquake respectively considering the two independent methods in determination of q s the reduction in q s by 89 two years following the chi chi earthquake in both methods is significant along with 1 v s reduction revealed by the analysis of repeating earthquakes our studies suggested possible reduction both in v s and q s values within the fault zone after the chi chi earthquake temporal changes in q s after the chi chi earthquake imply variations of pore fluid saturation in the ruptured fault zone the reduction in q s two years following the chi chi earthquake indicates high pore fluid saturation within the fractured fault zone due to the postseismic fluid redistribution

application of the bisq model to clay squirt flow in reservoir sandstones 1 an understanding of the variations of compressional wave velocity and attenuation in sandstones as a function of pore fluid properties is important for the identification and monitoring of hydrocarbon reservoirs in order for seismic inversion to be well constrained a rock physics model that describes accurately the observed behavior is required a number of such models have been proposed but have not been extensively tested against laboratory data the unified biot and squirt flow model was applied to data obtained from ultrasonic experiments conducted on a wide range of sandstone types when saturated with different pore fluids with viscosities ranging from 0 3 to 1000 cp this model was seen to match well all the observed velocity and attenuation values as opposed to the biot model which was only successful for clean sandstones saturated with low viscosity fluids this has led to the conclusion that a squirt flow mechanism is important at the grain scale in addition an increase in the dynamic uniaxial strain modulus of the dry frame with increasing pore fluid viscosity was observed in the absence of free gas or grain contact microcracks the laboratory data were collected at 60 mpa effective pressure this can be linked to decreasing local flow and decreasing relaxation in the soft pore space of the sandstone the extent of which is in turn connected to the clay content and pore structure of the sandstone these results imply that at ultrasonic frequencies the squirt flow is due to the presence of clay

modelling orthorhombic anisotropic effects for reservoir fracture characterization of a naturally fractured tight carbonate reservoir onshore texas usa in this study we present a step by step theoretical modelling approach using established seismic wave propagation theories in anisotropic media to generate unique anisotropic reflection patterns observed from three dimensional pure mode pressure 3d pp full azimuth and full offset seismic reflection data acquired over a naturally fractured tight carbonate field onshore texas usa our aim is to gain an insight into the internal structures of the carbonate reservoir responsible for the observed anisotropic reflection patterns from the generated model we were able to establish that the observed field seismic reflection patterns indicate azimuthal anisotropy in the form of crack induced shear wave splitting and variation in p wave velocity with offset and azimuth amplitude variation with azimuth avaz analysis also confirmed multi crack sets induced anisotropy which is characteristic of orthorhombic symmetry evident as multiple bright and dim amplitude azimuth directions as well as complete reversal of bright amplitude to dim amplitude azimuth direction as the angle of incidence increases from near 15 to mid 30 offsets finally we fitted the generated p wave velocity into an ellipse to determine the intensity and orientation n26e of the open crack set as well as the direction of the minimum in situ stress axis n116e within the reservoir the derived information served as an aid for the design of horizontal well paths that would intercept open fractures and ensure production optimization of the carbonate reservoir which was on production decline despite reservoir studies that indicate un depleted reserves

assessment of the structural representativeness of sample data sets for the mechanical characterization of deep formations accurate characterization of the mechanical behavior of geomaterials at depth is a fundamental need for geologic and engineering purposes laboratory tests on samples from well cores provide the material characterization in terms of mechanical response and other relevant properties representativeness of a sample data set with respect to the in situ conditions at depth is a key issue which needs to be addressed to extrapolate the laboratory response to the whole rock mass we have developed a procedure aimed at quantitatively evaluating the representativeness of laboratory samples the methodology is based on joint processing of laboratory ultrasonic tests and wellbore sonic logs a structural index is used to quantify the difference between the average structure of the laboratory sample and the structure of the formation at the wellbore scale this index could be used to identify different causes of discrepancies between the behavior of the cored samples and the behavior of the rock formation as documented by well logs then it could also be used to integrate laboratory data for the construction of a reliable geomechanical model with reference to the real in situ state the methodology was applied to three different experimental data sets showing the effectiveness of the method

seismic reversal pattern for the 1999 chi chi taiwan mw 7 6 earthquake we in this study have calculated the standard normal deviate z value to investigate the variations in seismicity patterns in the taiwan region before and after the chi chi earthquake we have found that the areas with relatively high seismicity in the eastern taiwan became abnormally quiet before the chi chi earthquake while the area in the central taiwan with relatively low seismicity showed unusually active such a spatially changing pattern in seismicity strikingly demonstrates the phenomenon of seismic reversal and we here thus present a complete representative cycle of seismic reversal embedding in the changes of seismicity patterns before and after the chi chi earthquake

a cracked porous medium elastic wave theory and its application to interpreting acoustic data from tight formations rocks in the earth s crust usually contain pores and cracks typical examples include tight sandstone and shale rocks that have low porosity but contain abundant microcracks by extending the classic biot s poroelastic wave theory to include the effects of cracks we obtain an elastic wave theory for porous rocks containing cracks adding crack density and aspect ratio as two important parameters to the original theory because the flat or narrow shaped cracks can easily deform under acoustic wave excitation the acoustic property of a cracked porous rock is quite different for different saturation conditions the predicted fluid sensitivity is used to interpret acoustic velocity log data from tight sand and shale gas formations in both scenarios the new theory correctly predicts the trend of velocity variation with gas saturation the results confirm that the presence of cracks in tight rocks can give rise to significant hydrocarbon signature in the acoustic measurement data allowing for identifying hydrocarbons from the data

gas generation and overpressure effects on seismic attributes drilling of deep gas resources is hampered by high risk associated with unexpected overpressure zones knowledge of pore pressure using seismic data as for instance from seismic while drilling techniques will help producers plan the drilling process in real time to control potentially dangerous abnormal pressures we assume a simple basin evolution model with a constant sedimentation rate and a constant geothermal gradient oil gas conversion starts at a given depth in a reservoir volume sealed with faults whose permeability is sufficiently low so that the increase in pressure caused by gas generation greatly exceeds the dissipation of pressure by flow assuming a first order kinetic reaction with a reaction rate satisfying the arrhenius equation the oil gas conversion fraction is calculated balancing mass and volume fractions in the pore space yields the excess pore pressure and the fluid saturations this excess pore pressure determines the effective pressure which in turn determines the skeleton bulk moduli if the generated gas goes into solution in the oil this effect does not greatly change the depth and oil gas conversion fraction for which the hydrostatic pressure approaches the lithostatic pressure the seismic velocities versus pore pressure and differential pressure are computed by using a model for wave propagation in a porous medium saturated with oil and gas moreover the velocities and attenuation factors versus frequency are obtained by including rock frame fluid viscoelastic effects to match ultrasonic experimental velocities for the basin evolution model used here pore pressure is seismically visible when the effective pressure is less than about 15 mpa and the oil gas conversion is about 2 5 percent

finite difference modeling of wave propagation and diffusion in poroelastic media numerical modeling of seismic waves in heterogeneous porous reservoir rocks is an important tool for interpreting seismic surveys in reservoir engineering various theoretical studies derive effective elastic moduli and seismic attributes from complex rock properties involving patchy saturation and fractured media to confirm and further develop rock physics theories for reservoir rocks accurate numerical modeling tools are required our 2d velocity stress finite difference scheme simulates waves within poroelastic media as described by biot s theory the scheme is second order in time contains high order spatial derivative operators and is parallelized using the domain decomposition technique a series of numerical experiments that are compared to exact analytical solutions allow us to assess the stability conditions and dispersion relations of the explicit poroelastic finite differ ence method the focus of the experiments is to model wave induced flow accurately in the vicinity of mesoscopic hete

preseismic deformation and coseismic displacements associated with the 1999 chi chi taiwan earthquake the destructive 1999 chi chi taiwan earthquake m w 7 5 produced an approximately 100 km long surface rupture mostly along the previously recognized north south trending chelungpu fault preseismic deformation in central taiwan is realized from annually repeated global positioning system gps data acquired during the 1992 1999 period the total wnw ese shortening rate in the vicinity of the epicentral region that is from the west coast to the western boundary of the central range is up to 25 mm yr the crustal deformation before the chi chi earthquake was essentially a uniaxial compressional strain of 0 36 strain yr in the direction of 114 the gps measurements taken 0 2 2 7 yr before and within 3 months after the mainshock were utilized to estimate the coseismic displacements horizontal movements of 1 1 9 1 m in the nw nnw directions are observed on the hanging wall eastern side of the fault there is a northward increasing trend in the magnitude of the displacement vectors and a dramatic change in the direction of about 50 toward the east along the fault strike in contrast much smaller se see movements of 0 1 1 5 m are found on the footwall western side of the fault the gps data show 2 4 10 1 m of total horizontal offsets across the chelungpu fault vertical offsets of 1 2 4 4 m with the eastern side up are also observed along the surface rupture the uplift on the hanging wall decreases rapidly toward the east it becomes subsidence at sun moon lake and in the puli wushe area the stations on the footwall show subsidence of 0 02 0 26 m the width of the uplift zone increases from about 10 km in the south to approximately 30 km in the north manuscript received 13 october 2000

full waveform ultrasonic transmission seismograms a fast new method for the determination of physical and sedimentological parameters of marine sediment cores detailed information about the sediment properties and microstructure can be provided through the analysis of digital ultrasonic p wave seismograms recorded automatically during full waveform core logging the physical parameter which predominantly affects the elastic wave propagation in water saturated sediments is the p wave attenuation coefficient the related sedimentological parameter is the grain size distribution a set of high resolution ultrasonic transmission seismograms 50 500 khz which indicate downcore variations in the grain size by their signal shape and frequency content are presented layers of coarse grained foraminiferal ooze can be identified by highly attenuated p waves whereas almost unattenuated waves are recorded in fine grained areas of nannofossil ooze color encoded pixel graphics of the seismograms and instantaneous frequencies present full waveform images of the lithology and attenuation a modified spectral difference method is introduced to determine the attenuation coefficient and its power law a kf applied to synthetic seismograms derived using a constant q model even low attenuation coefficients can be quantified a downcore analysis gives an attenuation log which ranges from 700 db m at 400 khz and a power of n 1 2 in coarse grained sands to few decibels per meter and n s 0 5 in fine grained clays a least squares fit of a second degree polynomial describes the mutual relationship between the mean grain size and the attenuation coefficient when it is used to predict the mean grain size an almost perfect coincidence with the values derived from sedimentological measurements is achieved

relationships among compressional wave attenuation porosity clay content and permeability in sandstones anelastic attenuation is the process by which rocks convert compressional waves into heat and thereby modify the amplitude and phase of the waves understanding the causes of compressional wave attenuation is important in the acquisition processing and interpretation of high resolution seismic data and in deducing the physical properties of rocks from seismic data we have measured the attenuation coefficients of compressional waves in 42 sandstones at a confining pressure of 40 mpa equivalent to a depth of burial of about 1 5 km in a frequency range from 0 5 to 1 5 mhz the compressional wave measurements were made using a pulse echo method in which the sample 5 cm diameter 1 8 cm to 3 5 cm long was sandwiched between perspex lucite buffer rods inside the high pressure rig the attenuation of the sample was estimated from the logarithmic spectral ratio of the signals corrected for beam spreading reflected from the top and base of the sample the results show that for these samples compressio

a series of large late wisconsinan meltwater floods through the champlain and hudson valleys new york state usa abstract the champlain valley in northeastern new york lies at the junction of two important late wisconsinan proglacial outflow routes the hudson valley to the south and the st lawrence valley to the northeast freshwater outflow from proglacial lakes in the hudson champlain valleys glacial lake albany vermont and the combined ontario and st lawrence valleys glacial lake iroquois may have affected ocean circulation and thereby altered climate during the last deglaciation we have estimated steady state and three flood pulse discharges from these large meltwater reservoirs into the north atlantic using channel geometry and a high resolution digital elevation model of the late wisconsinan paleogeography of the region we estimate the steady state meltwater discharge into the north atlantic to be 0 3 0 6 sv the first flood event was a combined iroquois vermont outflow at around 10 900 14 c yr bp that released 700 km 3 of meltwater into the north atlantic through the hudson valley with an estimated discharge of 1 1 sv a second outflow event released 2500 km 3 through the hudson valley shortly after the first event finally approximately 1500 km 3 was released to the north atlantic through the gulf of st lawrence at the incursion of the champlain sea about 150 300 years later

sensitivity of 3d 3c synthetic seismograms to anisotropic attenuation and velocity in reservoir models abstractanisotropic attenuation in fluid saturated reservoirs with high fracture density may be diagnostic for reservoir characterization wave induced mesoscale fluid flow is considered to be the major cause of intrinsic attenuation at exploration seismic frequencies we perform tests of the sensitivity of anisotropic attenuation and velocity to reservoir properties in fractured hti media based on the mesoscale fluid flow attenuation mechanism the viscoelastic t matrix a unified effective medium theory of global and local fluid flow mechanisms is used to compute frequency dependent anisotropic attenuation and velocity for ranges of reservoir properties including fracture density orientation fracture aspect ratio fluid type and permeability the 3d 3c staggered grid finite difference anisotropic viscoelastic modeling with a crank nicolson scheme is used to generate seismograms using the frequency dependent velocity and attenuation computed by the viscoelastic t matrix a standard linear solid mo

dynamic poroelasticity a unified model with the squirt and the biot mechanisms the velocities and attenuation of seismic and acoustic waves in rocks with fluids are affected by the two most important modes of fluid solid interaction 1 the biot mechanism where the fluid is forced to participate in the solid s motion by viscous friction and inertial coupling and 2 the squirt flow mechanism where the fluid is squeezed out of thin pores deformed by a passing wave traditionally both modes have been modeled separately with the biot mechanism treated in a macroscopic framework and the squirt flow examined at the individual pore level we offer a model which treats both mechanisms as coupled processes and relates p velocity and attenuation to macroscopic parameters the biot poroelastic constants porosity permeability fluid com pressibility and viscosity and a newly introduced microscale parameter a characteristic squirt flow length the latter is referred to as a fundamental rock property that can be determined experimentally we show that the squirt flow mechanism dominates the biot mechanism and is responsible for measured large velocity dispersion and attenuation values the model directly relates p velocity and attenuation to measurable rock and fluid properties therefore it allows one to realistically interpret velocity dispersion and or attenuation in terms of fluid properties changes e g viscosity during thermal enhanced oil recovery eor or to link seismic measurements to reservoir properties as an example of the latter transformation we relate permeability to attenuation and achieve good qualitative correlation with experimental data

application of the jhd technique to the loma prieta california mainshock aftershock sequence and implications for earthquake location relocation of the loma prieta mainshock aftershock sequence using the jhd technique places the events an average of 1 km to the southwest of the locations determined by other authors the new locations show a concentration of seismic activity in the wedge between the san andreas and sargent faults furthermore there seems to be little seismic activity to the east of the sargent fault these features of the seismicity are not observed in other published locations determined using similar data sets a depth cross section perpendicular to the sargent fault near its intersection with the san andreas fault shows two clusters of events along a line segment between 3 and 10 km that dips to the ne and that intersects the main sw dipping plane these clusters may correspond to secondary faults postulated by other authors the jhd station corrections vary between 0 6 and 0 6 sec and indicate the presence of large velocity variations across and along the san andreas fault these corrections are very close to the weighted average of the station residuals obtained when the events are located singly the locations obtained with a single event location program and using the average residuals as station corrections give locations almost coincident with the jhd locations this high similarity of locations is very unusual and is interpreted as the result of quasi random lateral velocity variations in the loma prieta area to estimate the effect of these variations on the location of events synthetic travel times were computed for a realistic although preliminary three dimensional 3d velocity model these synthetic data were then treated as the actual data the corresponding results show that the jhd technique is able to locate events with an average error of 0 5 km in epicenter depths on the other hand are consistently deeper by about 1 km in addition the station corrections are close to the observed corrections additional tests with synthetic data also show that the observed station corrections suggest velocity variations of up to 40 in the top 3 km of the crust although these variations may seem extremely large they are within the range of variation found by other authors and most importantly are required by the observations

joint hypocentral location in media with lateral velocity variations and interpretation of the station corrections abstract the application of a new implementation of the joint determination of hypocenters origin times and station correction terms jhd technique to data from the arkansas swarm usa the campi flegrei italy and the nazca plate argentina throws new light onto the problem of earthquake location in all these cases good quality p and s wave arrival times were available the main results are as follows 1 large lateral velocity variations llvv are generally associated with relatively large station corrections these corrections in turn are usually much larger than the average of the station residuals determined when the same set of events is located using a single event location sel method 2 the locations determined by the jhd and sel methods may be substantially different 3 since the effect of llvv on jhd locations is not well established simple 3 d velocity models are constructed so that the jhd technique applied to synthetic data generated using these models produce station corrections similar to those observed 4 the analysis of the synthetic data also shows that the locations determined by the jhd method are much closer to the true locations than the locations determined with the sel method the use of the weighted average of the sel station residuals as station corrections does not give better located events although there is a large decrease in the r m s residual computed for each event therefore the use of r m s residuals to estimate the quality of event locations is not always warranted in view of these results it is possible to conclude that the jhd station corrections can be used to detect lateral velocity variations and to produce locations substantially less affected by these variations than the locations determined by the sel method furthermore under appropriate conditions the jhd station corrections can also be used in a forward modeling scheme to get a semiquantitative velocity model such a model may be the only information that can be obtained from a data set when the station and event distributions are not adequate for 3 d velocity inversion

synthetic seismograms for marine sediments and determination of porosity and permeability we present numerical simulations of vertical seismic profiles vsps of marine sediments the theoretical seismograms which are computed for vertically incident waves in flat layered poroelastic media include the effects of dispersion and attenuation predicted by biot theory according to biot theory fast and slow compressional waves are excited and there is mode conversion at the interfaces we include this effect in the calculation of reflection and transmission coefficients as an energy loss mechanism through the slow compressional waves finally we examine the spectral ratio method for determining porosity and permeability from synthetic seismogram data analytical expressions for the velocity and specific attenuation are found based on the weak frame approximation once the frequency dependent velocity and specific attenuation are calculated the porosity and permeability of marine sediments can be determined by using the proposed weak frame approximations spectral ratio calculations on syntheti

a geostatistical framework for incorporating seismic tomography auxiliary data into hydraulic conductivity estimation abstract a geostatistical approach is presented for the inclusion of seismic tomography data and sonic log data into the estimation of hydraulic conductivity the procedure accounts for the errors in seismic tomography inversion and for the correlation of such errors the proposed methodology consists of two steps 1 the cross variogram inference is carried out using only the data at the wellbore both hydraulic and seismic 2 a co kriging procedure takes the cross well data into account to interpolate between boreholes no postulated a priori relationship is needed in order to illustrate the methodology a synthetic data set is generated on the basis of evidence from published case studies and simplified physical considerations the numerical experiments show that the choice of the excitation frequency is critical a trade off exists between the need for a high resolution survey asking for higher frequencies and the need for a good correlation between hydraulic conductivity and seismic properties asking for frequencies below the squirt frequency of the medium in the simulation using seismic data with the best excitation frequency 1 khz in this case the mean squared error of the hydraulic conductivity estimate is two thirds lower than using hydraulic data alone it is important to note also that only a part of the interwell region is adequately sampled by the tomographic experiment such a region can be readily identified by calculating the energy of the quasi null space through singular value decomposition of the tomographic matrix in planning this type of experiments it is necessary to carefully verify case by case whether the adopted range of high frequencies does not prevent the seismic energy from propagating effectively from sources to receivers

3d velocity structure around the source area of the 1999 chi chi taiwan earthquake before and after the mainshock 3d v p and v p v s models for the source area before and after the chi chi earthquake are derived from a tomographic inversion of seismic travel time data from the taiwan central weather bureau seismic network cwbsn resulting 3d velocity models reveal a strong lateral variation in the crust particularly at a depth range of 5 to 25 km along the trend of the dipping chelungpu thrust in general low v p and high v p v s anomalies are found in and around the source volume of the chi chi earthquake a volume centered around the entire 100 km 40 km chelungpu rupture surface we pay special attention to the crustal properties and their changes due to the chi chi event and the association of the crustal anomalies with seismicity combining the results with those of other geological and geophysical surveys the crustal anomalies are interpreted as images of a highly fractured and fluid filled chi chi source region fluid overpressure is known to reduce the strength of the fault zone and in this case may have initiated the nucleation of the chi chi earthquake the v p models obtained before and after the chi chi event show some interesting heterogeneities regions of low v p anomaly appear to have further expanded with the occurrence of the chi chi earthquake the v p v s model shows high anomalies near the source rupture especially near where the rupture is believed to have nucleated after the chi chi event the v p v s model shows a substantial change that may reflect an outward shift of the fluid filled fractured source region this shift is shown by a general expansion in v p v s anomalies to a broader region e g where two intensive aftershock clusters occurred and a significant v p v s reduction to more normal values near the chelungpu rupture we attribute these changes in v p and v p v s anomalies as well as the expansion and shift of the anomalous regions to be a response to the localized stress change caused by the slip of the chi chi rupture the location of the seismic events is correlated with that of the crustal v p and v p v s anomalies large events appear to occur in the transition zones of both v p and v p v s anomalies this finding may be of use in the identification of weakened crustal regions in a seismic zone thus suggesting where impending large earthquakes are likely to occur all input data and initial and final models of this article are given as electronic files in the cd attached to the end of this issue

drainage of late wisconsin glacial lakes and the morphology and late quaternary stratigraphy of the new jersey southern new england continental shelf and slope abstract we propose that late wisconsin deposition and erosion hudson shelf and block island valleys on the shelf and slope from new jersey to southern new england were a consequence of the catastrophic drainage of glacial lakes behind terminal moraine systems and the huge volume of water stored beneath the laurentian ice sheet and subsequent erosion of the lake sediments by flash floods the morphology imparted by glaciation regulated the discharge associated with the ablation of the glaciers associated with the deposits west of hudson shelf valley are the remains of mammoth and mastodon which were transported from their living habitats along the lake shores to their present burial sites on the shelf the floods also triggered gravity flows on the upper continental slope which made possible the transportation of coarse debris over hundreds of km into the deep sea that these catastrophic flood morphologies can still be recognized on the middle to outer shelf suggest that much of its surface was little modified during the late pleistocene holocene transgression thus the late pleistocene holocene transgression may have been characterized by short periods when sea level rose rapidly allowing for the preservation of relict features

on the propagation of a disturbance in a smoothly varying heterogeneous porous medium saturated with three fluid phases abstractfrom the equations governing the deformation of a porous medium containing three fluid phases i derive expressions for the phase velocity of the various modes of displacement these expressions are valid for a medium with smoothly varying heterogeneity there is a single mode of transverse displacement similar in nature to an elastic shear wave the four phase velocities of the longitudinal modes of displacement are derived from the solutions of a quartic equation the coefficients of the polynomial equation are written as linear sums of the determinants of 4 4 matrices the matrices contain various combinations of the parameters from the governing equations the three phase expressions are compared to two phase estimates for the case in which one of the fluid saturations vanishes also in a numerical illustration velocity variations of around 10 are associated with the cyclic injection of carbon dioxide and water into an oil saturated reservoir

numerical simulation of seismic responses of poroelastic reservoirs using biot theory biot theory prosvides a framework for computing the seismic response of fluid saturated reservoirs numerical implementation by 2 d finite differences allows investigation of the effects of spatial variations in porosity permeability and fluid viscosity on seismic displacements of the solid frame and of the fluids oil gas and or water in the reservoir the porosity primarily influences wave velocities the viscosity to permeability ratio primarily influences amplitudes and attenuation synthetic crosswell vsp and surface survey seismograms for representative reservoir models contain primary and converted reflections from fluid as well as lithologic contacts and they illustrate the distribution of information available for describing a reservoir

tomographic imaging of lithospheric structures under taiwan abstract tomographic images of the crustal and mantle velocity structures under taiwan are obtained by simultaneous inversion of local earthquake p wave arrival times for hypocenters and p wave velocity structures in northern taiwan a high velocity zone coinciding with the wadati benioff zone can readily be identified as the subducted philippine sea plate the imaged zone dips toward the north at an angle of 40 from a depth range of 20 55 to 100 130 km an upper mantle low velocity wedge ranging from 40 to 80 km in depth exists above the subducted slab above this wedge is the ilan plain of northern taiwan which lies at the west end of the okinawa trough a well recognized back arc basin the crustal velocities under the plain are also relatively low the well defined high velocity zone and the low velocity wedge provide some constraints on the thermal structures of the subduction system under northern taiwan in tomographic images across the central section of taiwan thickening of the crust and up arching of the lower crustal materials under the central range are commonly observed the crust under the western foothills region is clearly thinner and the near surface low velocity layers are well developed the structures under the central range show that although the taiwan orogeny is quite young a root deeper in the north and shallowing to the south has formed the results of our tomography show that a significant portion of the lithosphere is involved in the taiwan orogeny