monte carlo analysis of seismic reflections from moho and the w reflector near normal incidence reflections have been used to image the moho and the w reflector structure in the lithosphere offshore northern scotland to determine the impedance variations at these reflectors we use a monte carlo technique which allows incorporation of geologically realistic a priori information as well as an extensive exploration of the model space after testing it on a synthetic data set the method is based on bayesian inversion theory the modeled moho consists of a series of layers with a total thickness of 2 4 0 3 km with an overall positive impedance contrast inversion of the w reflector results in a model of five seven layers with a total thickness of about 3 7 0 6 km and mostly nonpositive impedance contrasts the implied fine scale impedance structure of the moho is consistent with the broader velocity structure determined from previous wide angle reflection refraction profiles however the overall nonpositive impedance contrast at the w reflector requires a structure which is overlain or underlain by a broad increase in velocity in order to match amplitudes of reflected phases observed at large offsets interpreted previously to originate at a similar depth

anisotropic velocity analysis for lithology discrimination a new velocity analysis technique is presented for analyzing moveout of signals on multichannel surface seismic or vsp data an approximate skewed hyperbolic moveout formula is derived for horizontally layered transversely isotropic media this formula involves three measurement parameters the average vertical velocity and horizontal and skew moveout velocities by extending dix type hyperbolic moveout analysis we obtain improved coherence over large source geophone offsets for more accurate moveout correction compared with the stacking velocity obtained by simple hyperbolic analysis methods the three velocity parameters estimated by this technique contain more physically meaningful geologic information regarding the anisotropy and or velocity heterogeneity of the subsurface synthetic p wave model experiments demonstrate that the skewed hyperbolic moveout formula yields an excellent fit to time distance curves over a wide range of ray angles consequently the measurement parameters are shown to re

δ18o and δd systematics in the surficial waters of the gaula river catchment area kumaun himalaya india abstract the 18 o and d measurements of surface waters streams springs lakes and precipitation from the gaula river catchment area of kumaun himalaya are reported the d and 18 o values are positively correlated as has been the case with waters from several other regions the lake waters are found to be enriched in 18 o and d compared with the neighbouring regions owing to evaporation estimates of mean total evaporation during the dry season range from 10 to 25 spring water samples collected from various altitudes from 915 m to 2150 m display variations in values showing an altitude effect of 0 1491 per 100m elevation

relationships between compressional wave and shear wave velocities in clastic silicate rocks new velocity data in addition to literature data derived from sonic log seismic and laboratory measurements are analyzed for clastic silicate rocks these data demonstrate simple systematic relationships between compressional and shear wave velocities for water saturated clastic silicate rocks shear wave veloci ty is approximately linearly related to compressional wave velocity and the compressional to shear velocity ratio decreases with increasing compressional velocity laboratory data for dry sandstones indicate a nearly constant compressional to shear velocity ratio with rigidity approximately equal to bulk modulus ideal models for regular packings of spheres and cracked solids exhibit behavior similar to the observed water saturated and dry trends for dry rigidity equal to dry bulk modulus gassmann s equations predict velocities in close agreement with data from the water saturated

transdimensional change point modeling as a tool to investigate uncertainty in applied geophysical inference an example using borehole geophysical logs recently developed methods for inferring abrupt changes in data series enable such change points in time or space to be identified and also allow us to estimate noise levels of the observed data the inferred probability distributions of these parameters provide insights into the capacity of the observed data to constrain the geophysical analysis and hence the magnitudes and likely sources of uncertainty we carry out a change point analysis of sections of four borehole geophysical logs density neutron absorption sonic interval time and electrical resistivity using transdimensional bayesian markov chain monte carlo to sample a model parameter space the output is an ensemble of values which approximate the posterior distribution of model parameters we compare the modeled change points borehole log parameters and the variance of the noise distribution of each log with the observed lithology classes down the borehole to make an appraisal of the uncertainty characteristics inherent in the data our two examples one with well defined lithology changes and one with more subtle contrasts show quantitatively the nature of the lithology contrasts for which the geophysical borehole log data will produce a detectable response in terms of inferred change points we highlight the different components of variation in the observed data due to the geologic process dominant lithology changes that we hope to be able to infer geologic noise due to variability within each lithology and analytical noise due to the measurement process this inference process will be a practical addition to the analytical tool box for borehole and other geophysical data series it reveals the level of uncertainties in the relationships between the data and the observed lithologies and would be of great use in planning and interpreting the results of subsequent routine processing

joint avo inversion wavelet estimation and noise level estimation using a spatially coupled hierarchical bayesian model the main objective of the avo inversion is to obtain posterior distributions for p wave velocity s wave velocity and density from specified prior distributions seismic data and well log data the inversion problem also involves estimation of a seismic wavelet and the seismic noise level the noise model is represented by a zero mean gaussian distribution specified by a covariance matrix a method for joint avo inversion wavelet estimation and estimation of the noise level is developed in a bayesian framework the stochastic model includes uncertainty of both the elastic parameters the wavelet and the seismic and well log data the posterior distribution is explored by markov chain monte carlo simulation using the gibbs sampler algorithm the inversion algorithm has been tested on a seismic line from the heidrun field with two wells located on the line the use of a coloured seismic noise model resulted in about 10 lower uncertainties for the p wave velocity s wave velocity and density compared with a white noise model the uncertainty of the estimated wavelet is low in the heidrun example the effect of including uncertainty of the wavelet and the noise level was marginal with respect to the avo inversion results

spatio temporal variations of double couple aftershock mechanisms and possible volumetric earthquake strain 1 we analyze spatio temporal patterns in rotation angles of double couple constrained mechanisms of aftershocks of the 1992 landers earthquake the rotation angles provide information on the distribution of source geometries in different regions of space and time with respect to the mainshock focal mechanism the results indicate that the mechanisms of the early aftershocks are more scattered and less aligned with the mainshock than those of the long term events this is most pronounced around the northern end of the landers rupture least pronounced around the central section and intermediate around the southern end of the rupture the relatively large scatter and misalignment of the mean rotation angles of the early focal mechanisms around the edges of the landers rupture suggest possible volumetric earthquake strain in these regions the results may reflect isotropic source terms produced by dynamic generation of rock damage synthetic tests indicate that the observed differences in the rotation distributions of the early and long term events around the end regions of the landers rupture can result from neglecting in the inversion process isotropic components that are 0 03 0 15 of the total event moments

solution of the vsp one dimensional inverse problem the acoustical impedance distribution of the substratum or equivalently the reflection coefficient sequence is determined from vsp data this nonlinear inverse problem is solved by a least squares method as the wavelet is unknown the impedance distribution and the neumann boundary condition which characterizes the excitation of the medium are simultaneously identified the inversion method is applied to synthetic and field vsp s the result is satisfactory even when strong noise corrupts the data provided that a suitable constraint on the impedance distribution is introduced in order to ensure the stability of the inverse problem the reliability of the inversion result in the case of field vsp is confirmed some ways in which this result may be used are illustrated calibration of the seismic surface data multiple identification prediction ahead of the bit

looking ahead with vertical seismic profiles several operations enhance our ability to predict the subsurface below the bottom total depth td of the well when applied to zero offset vertical seismic profiling vsp data other key issues regarding the use of vsp data in this fashion are resolution and look ahead distance an impedance log is the most useful form for presenting vsp data to look ahead of the drill bit the vsp composite trace must first tie reliably to the surface seismic section and to the well log synthetic seismogram the impedance log is obtained by inverting this vsp composite trace however before performing the inversion we need to 1 correct the composite trace for attenuation effects below td and 2 input velocities to provide low frequency information an exponential gain function applied to the vsp data below td adequately compensates for the loss of amplitude caused by attenuation a calibration of the seismically derived velocities with vsp velocities yields the necessary low frequency information these concepts ar

modern precipitation stable isotope vs elevation gradients in the high himalaya comment on a new approach to stable isotope based paleoaltimetry implications for paleoaltimetry and paleohypsometry of the high himalaya since the late miocene by david b rowley et al earth planet sci lett 188 2001 253 268

seismic versus sonic velocities a vertical seismic profiling study vertical seismic profiling vsp techniques provide a method to measure accurately the seismic velocity and lithologic structure near the borehole the analysis of a vsp survey can also provide insight into seismic wave propagation especially when related to sonic measurements but vsp and sonic log velocities or traveltimes are often found to disagree recent field evidence of these differences suggests that the vsp traveltimes are delayed with respect to the integrated sonic times especially in the deep section 3000 ft by about 2 0 ms 1000 ft on the average the vsp has numerous applications in exploration geophysics such as calibrating the sonic log it is thus important to understand why the two measurements differ differences in the geometries source frequencies and instrumental errors of the two surveys are reviewed more detailed analysis of seismic wave propagation in the vsp shows that short path multiples and velocity dispersion can have a significant delaying effect on the seismic tr

oxygen 18 salinity relationship simulated by an oceanic general circulation model the distribution of h218o in the ocean is simulated using an oceanic general circulation model gcm surface isotopic fluxes prescribed to this model are derived from an atmospheric gcm which ensures their consistency the oceanic gcm is initialized with a homogeneous composition and redistributes isotopic gradients so as to equilibrate with the atmosphere a good agreement is found between the observations and the simulation concerning i the interbasin h218o gradients especially the atlantic being more enriched than the pacific as well as the meridional gradients and ii the h218o salinity relationship

borehole guided avo analysis of p p and p s reflections quantifying uncertainty on density estimates seismic properties of isotropic elastic formations are characterized by the three parameters acoustic impedance poisson s ratio and density whilst the first two are usually well estimated by analysing the amplitude variation with angle ava of reflected p p waves density is known to be poorly resolved however density estimates would be useful in many situations encountered in oil and gas exploration in particular for minimizing risks in looking ahead while drilling we design a borehole seismic experiment to investigate the reliability of ava extracted density receivers are located downhole near the targeted reflectors and record reflected p p and converted p s waves a non linear wide angle based bayesian inversion is then used to access the a posteriori probability distributions associated with the estimation of the three isotropic elastic parameters the analysis of these distributions suggests that the angular variation of reflected p s amplitudes provides additional substantial information for estimating density thus reducing the estimate uncertainty variance by more than one order of magnitude compared to using only reflected p waves

monte carlo bayesian look ahead inversion of walkaway vertical seismic profiles we describe a method to invert a walkaway vertical seismic profile vsp and predict elastic properties p wave velocity s wave velocity and density in a layered model looking ahead of the deepest receiver starting from bayes s rule we define a posterior distribution of layered models that combines prior information on the overall variability of and correlations among the elastic properties observed in well logs with information provided by the vsp data this posterior distribution of layered models is sampled by a monte carlo method the sampled layered models agree with prior information and fit the vsp data and their overall variability defines the uncertainty in the predicted elastic properties we apply this technique first to a zero offset vsp data set and show that uncertainty in the long wavelength p wave velocity structure results in a sizable uncertainty in the predicted elastic properties we then use walkaway vsp data which contain information on the long wavelength p wave velocity in the reflection moveout and on s wave velocity and density contrasts in the change of reflectivity with offset the uncertainty of the look ahead prediction is considerably decreased compared with the zero offset vsp and the predicted elastic properties are in good agreement with well log measurements

orbital tuning as an inverse problem chronology of the early aptian oceanic anoxic event 1a selli level in the cismon apticore 1 orbital tuning the process of fitting sedimentary cycles to orbital periodicities can estimate with high resolution the timing and duration of key events in the geological record we formulate here orbital tuning as the inverse problem of finding the variation in sedimentation rate that matches sediment cycles with orbital periodicities instead of obtaining a single best estimate we apply a bayesian formulation and define a probability distribution of sedimentation rate variations that result in powerful orbital periodicities by sampling this distribution with a monte carlo method we quantify the uncertainty in the inferred sedimentation rates due to uncertainties in the tuning periods and to components of the sedimentary signal that are unrelated to orbital cycles the method is applied to the chronology of a 30 m interval in the cismon apticore borehole southern alps italy that includes the early aptian oceanic anoxic event 1a selli level estimated to last 1 11 0 11 ma 95 interval the 13c record shows a sudden negative shift of about 1 at the base of the selli level 22 47 ka followed by a recovery to preshift values in 240 ka consistent with a scenario where light carbon is quickly added and then flushed out of the ocean atmosphere system

anisotropic velocity analysis1 results from walkaway vsp and shale laboratory experiments show that shale anisotropy can be significantly anelliptic heterogeneity and anellipticity both lead to non hyperbolic moveout curves and the resulting ambiguity in velocity analysis is investigated for the case of a factorizable anisotropic medium with a linear dependence of velocity on depth more information can be obtained if there are several reflectors the method of dellinger et al for anisotropic velocity analysis in layered transversely isotropic media is examined and is shown to be restricted to media having relatively small anellipticity a new scheme based on an expansion of the inverse squared group velocity in spherical harmonics is presented this scheme can be used for larger anellipticity and is applicable for horizontal layers having monoclinic symmetry with the symmetry plane parallel to the layers the method is applied to invert the results of anisotropic ray tracing on a model sand shale sequence for transversely isotropic media with small anisotropy the scheme reduces to the method of byun et al and byun and corrigan the expansion in spherical harmonics allows the p phase slowness surface of each layer to be determined in analytic form from the layer parameters obtained by inversion without the need to assume that the anisotropy is weak

bayesian estimation in seismic inversion part i principles1 this paper gives a review of bayesian parameter estimation the bayesian approach is fundamental and applicable to all kinds of inverse problems its basic formulation is probabilistic information from data is combined with a priori information on model parameters the result is called the a posteriori probability density function and it is the solution to the inverse problem in practice an estimate of the parameters is obtained by taking its maximum well known estimation procedures like least squares inversion or l 1 norm inversion result depending on the type of noise and a priori information given due to the a priori information the maximum will be unique and the estimation procedures will be stable except in theory for the most pathological problems which are very unlikely to occur in practice the approach of tarantola and valette can be derived within classical probability theory the bayesian approach allows a full resolution and uncertainty analysis which is discussed in part ii of the paper

preliminary results from the chemical records of an 80 4 m ice core recovered from east rongbuk glacier qomolangma mount everest himalaya high resolution chemical records from an 80 4m ice core from the central himalaya demonstrate climatic and environmental changes since 1844 the chronological net accumulation series shows a sharp decrease from the mid 1950s which is coincident with the widely observed glacier retreat a negative correlation is found between the ice core 18o record and the monsoon precipitation for indian region 7 the temporal variation of the terrestrial ions ca2 and mg2 is controlled by both the monsoon precipitation for indian regions 3 7 and 8 located directly south and west of the himalaya and the dust storm duration and frequency in the northern arid regions such as the taklimakan desert china the nh4 profile is fairly flat until the 1940s then substantially increases until the end of the 1980s with a slight decrease during the 1990s which may reflect new agricultural practices the so4 2 and no3 profiles show an apparent increasing trend especially during the period 1940s 80s moreover so4 2 concentrations for the east rongbuk glacier core are roughly double that of the nearby dasuopu core at xixabangma himalaya due to local human activity including that of climbing teams who use gasoline for cooking energy and transport