la sf icp ms zircon u pb geochronology of granitic rocks from the central bundelkhand greenstone complex bundelkhand craton india abstract the central bundelkhand greenstone complex in bundelkhand craton northern india is one of the well exposed archaean supracrustal amphibolite banded iron formation bif and felsic volcanic rocks fv and associated with grey and pink porphyritic granite tonalite trondhjemite granodiorite ttg here we present high precision zircon u pb geochronological data for the pinkish porphyritic granites and ttg the zircons from the grey pinkish porphyritic granite show three different concordia ages of 2531 21 ma 2516 38 ma and 2514 13 ma which are interpreted as the best estimate of the magmatic crystallization age for the studied granites we also report the concordia age of 2669 7 4 ma for a trondhjemite gneiss sample which is so far the youngest u pb geochronological data for a ttg rock suite in the bundelkhand craton this ttg formation at 2669 ma is also more similar to precambrian basement ttg gneisses of the aravalli craton of north western india and suggests that crust formation in the bundelkhand craton occurred in a similar time frame to that recorded from the aravalli craton of the north western india

use of avoa data to estimate fluid indicator in a vertically fractured medium microstructural attributes of cracks and fractures such as crack density aspect ratio and fluid infill determine the elastic properties of a medium containing a set of parallel vertical fractures although the tangential weakness t of the fractures does not vary with the fluid content the normal weakness n exhibits significant dependence on fluid infill based on linear slip theory we used the ratio g n t termed the fluid indicator as a quantitative measure of the fluid content in the fractures with g representing the square of the ratio of s and p wave velocity in the unfractured medium we used a born formalism to derive the sensitivity to fracture weakness of pp and ps reflection coefficients for an interface separating an unfractured medium from a vertically fractured medium our formulae reveal that the pp reflection coefficient does not depend on the 2d microcorrugation surface roughness with ridges and valleys parallel to the fracture strike whereas the ps reflection coefficien

estimation of modified fluid factor and dry fracture weaknesses using azimuthal elastic impedance abstractwe consider the problem of fluid identification and fracture detection in unconventional reservoir tight gas sand and shale gas characterization we begin with a simplification of the stiffness parameters and the derivation of a linearized reflection coefficient and azimuthal elastic impedance ei the accuracy of the simplification is confirmed in application to gas bearing fractured rocks with low porosity and small fracture density we have developed a modified fluid factor that is more sensitive to fluid type and less influenced by porosity a two step inversion workflow is evaluated based on the derived linearized reflection coefficient and azimuthal ei including 1 a damped least squares inversion for azimuthal ei constrained by an initial model and 2 a bayesian markov chain monte carlo inversion for the modified fluid factor and dry fracture weaknesses stability and accuracy are examined with synthetic data from which we conclude that the modified fluid factor and dry fracture we

seismic dispersion and attenuation in saturated porous rocks with aligned fractures of finite thickness theory and numerical simulations part 1 p wave perpendicular to the fracture plane when a seismic wave travels through a fluid saturated porous reservoir containing aligned fractures it induces oscillatory fluid flow between the fractures and the embedding background medium although there are numerous theoretical models for quantifying the associated seismic attenuation and velocity dispersion they rely on certain assumptions such as infinitesimal fracture thickness and dilute fracture concentration which usually do not hold in real reservoirs the objective of this work is to overcome some of these limitations and therefore improve the applicability of the available theoretical models to do so we extend existing models to the finite fracture thickness case for p waves propagating perpendicular to the fracture plane using the so called branching function approach we consider three types of fractures namely periodically and randomly spaced planar fractures as well as penny shaped cracks the extended unified model is then tested by comparing with corresponding numerical sim

qvoa analysis p wave attenuation anisotropy for fracture characterization this paper investigates q anisotropy for characterizing fractured reservoirs specifically the variation of the seismic quality factor q versus offset and azimuth qvoa w e derive an analytical expression for p wave attenuation in a transversely isotropic medium with horizontal symmetry axis hti and provide a method qvoa for estimating fracture direction from azimuthally varying q in pp wave reflection data the qvoa formula is similar to ruger s approximation for pp wave reflection coefficients the theoretical basis for amplitude variation with angle offset avoa analysis the technique for qvoa analysis is similar to azimuthal avo analysis we introduce two new seismic attributes q versus offset qvo gradient and intercept qvo gradient inversion not only indicates fracture orientation but also characterizes q anisotropy we relate the q anisotropy parameter q to fractured medium parameters and invert the qvo gradient to estimate q the attenuation parameter q and thomsen style anisotropy parameter v are found to be interdependent the attenuation anisotropy magnitude strongly depends on the host rock s vs vp parameter whereas the dependence on fracture parameters is weak this complicates the qvo gradient inversion for the fracture parameters this result is independent of the attenuation mechanism to illustrate the qvoa method in synthetic data we use hudson s first order effective medium model of a dissipative fractured reservoir with fluid flow between aligned cracks and random pores as a possible mechanism for p wave attenuation

seismic wave propagation in cracked porous media summary the movement of interstitial fluids within a cracked solid can have a significant effect on the properties of seismic waves of long wavelength propagating through the solid we consider three distinct mechanisms of wave induced fluid flow flow through connections between cracks in an otherwise non porous material fluid movement within partially saturated cracks and diffusion from the cracks into a porous matrix material in each case the cracks may be aligned or randomly oriented leading respectively to anisotropic or isotropic wave speeds and attenuation factors in general seismic velocities exhibit behaviour that is intermediate between that of empty cracks and that of isolated liquid filled cracks if fluid flow is significant in the range of frequencies for which considerable fluid flow occurs there is high attenuation and dispersion of seismic waves fluid flow may be on either a wavelength scale or a local scale depending on the model and whether the cracks are aligned or randomly oriented resulting in completely different effects on seismic wave propagation a numerical analysis shows that all models can have an effect over the exploration seismic frequency range

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

elastic wave propagation in media with parallel fractures and aligned cracks a model of parallel slip interfaces simulates the behaviour of a fracture system composed of large closely spaced aligned joints the model admits any fracture system anisotropy triclinic the most general monoclinic orthorhombic or transversely isotropic and this is specified by the form of the 3 3 fracture system compliance matrix the fracture system may be embedded in an anisotropic elastic background with no restrictions on the type of anisotropy to compute the long wavelength equivalent moduli of the fractured medium requires at most the inversion of two 3 3 matrices when the fractures are assumed on average to have rotational symmetry transversely isotropic fracture system behaviour and the background is assumed isotropic the resulting equivalent medium is transversely isotropic and the effect of the additional compliance of the fracture system may be specified by two parameters in addition to the two isotropic parameters of the isotropic background dilute systems of flat aligned microcracks in an isotropic background yield an equivalent medium of the same form as that of the isotropic medium with large joints i e there are two additional parameters due to the presence of the microcracks which play roles in the stress strain relations of the equivalent medium identical to those played by the parameters due to the presence of large joints thus knowledge of the total of four parameters describing the anisotropy of such a fractured medium tells nothing of the size or concentration of the aligned fractures but does contain information as to the overall excess compliance due to the fracture system and its orientation as the aligned microcracks which were assumed to be ellipsoidal with very small aspect ratio are allowed to become non fiat i e have a growing aspect ratio the moduli of the equivalent medium begin to diverge from the standard form of the moduli for flat cracks the divergence is faster for higher crack densities but only becomes significant for microcracks of aspect ratios approaching 0 3

reconstruction of the layer anisotropic elastic parameters and high resolution fracture characterization from p wave data a case study using seismic inversion and bayesian rock physics parameter estimation wide azimuth seismic data can be used to derive anisotropic parameters on the subsurface by observing variation in subsurface seismic response along different azimuths layer based high resolution estimates of components of the subsurface anisotropic elastic tensor can be reconstructed by using wide azimuth p wave data by combining the kinematic information derived from anisotropic velocity analysis with dynamic information obtained from amplitude versus angle and azimuth analysis of wide azimuth seismic data interval p impedance s impedance and anisotropic parameters associated with anisotropic fracture media are being reconstructed using linearized analysis assuming horizontal transverse anisotropy symmetry in this paper it is shown how additional assumptions such as the rock model can be used to reduce the degrees of freedom in the estimation problem and recover all five anisotropic parameters because the use of a rock model is needed the derived elastic parameters are consistent with the rock model and are used to infer fractured rock properties using stochastic rock physics inversion the inversion is based on stochastic rock physics modelling and maximum a posteriori estimate of both porosity and crack density parameters associated with the observed elastic parameters derived from both velocity and amplitude versus angle and azimuth analysis while the focus of this study is on the use of p wave reflection data we also show how additional information such as shear wave splitting and or anisotropic well log data can reduce the assumptions needed to derive elastic parameter and rock properties

estimating p and s wave inverse quality factors from observed seismic data using an attenuative elastic impedance p and s wave inverse quality factors quantify seismic wave attenuation which are related to several key reservoir parameters porosity saturation viscosity etc estimating the inverse quality factors from observed seismic data provides additional and useful information during gas bearing reservoir prediction we first propose an approximate reflection coefficient and attenuative elastic impedance in terms of the inverse quality factors and then we establish an approach to estimate elastic properties p and s wave impedances and density and attenuation p and s wave inverse quality factors from seismic data at different incidence angles and frequencies the approach is implemented as a two step inversion a model based and damped least squares inversion for attenuative elastic impedance and a bayesian markov chain monte carlo inversion for the inverse quality factors synthetic data tests confirm that p and s wave impedances and inverse quality factors are reasonably estimated in the case of

squirt flow in fully saturated rocks we estimate velocity frequency dispersion and attenuation in fully saturated rocks by employing the squirt flow mechanism of solid fluid interaction in this model pore fluid is squeezed from thin soft cracks into the surrounding large pores information about the compliance of these soft cracks at low confining pressures is extracted from high pressure velocity data the frequency dependence of squirt induced pressure in the soft cracks is linked with the porosity and permeability of the soft pore space and the characteristic squirt flow length these unknown parameters are combined into one expression that is assumed to be a fundamental rock property that does not depend on frequency the appropriate value of this expression for a given rock can be found by matching our theoretical predictions with the experimental measurements of attenuation or velocity the low frequency velocity limits as given by our model are identical to those predicted by gassmann s formula the high frequency limits may sign

fluid pressure diffusion effects on the excess compliance matrix of porous rocks containing aligned fractures the presence of sets of open fractures is common in most reservoirs and they exert important controls on the reservoir permeability as fractures act as preferential pathways for fluid flow therefore the correct characterization of fracture sets in fluid saturated rocks is of great practical importance in this context the inversion of fracture characteristics from seismic data is promising since their signatures are sensitive to a wide range of pertinent fracture parameters such as density orientation and fluid infill the most commonly used inversion schemes are based on the classical linear slip theory lst in which the effects of the fractures are represented by a real valued diagonal excess compliance matrix to account for the effects of wave induced fluid pressure diffusion fpd between fractures and their embedding background several authors have shown that this matrix should be complex valued and frequency dependent however these approaches neglect the effects of fpd on the coupling between orthogonal deformations of the rock with this motivation we considered a fracture model based on a sequence of alternating poroelastic layers of finite thickness representing the background and the fractures and derived analytical expressions for the corresponding excess compliance matrix we evaluated this matrix for a wide range of background parameters to quantify the magnitude of its coefficients not accounted for by the classical lst and to determine how they are affected by fpd we estimated the relative errors in the computation of anisotropic seismic velocity and attenuation associated with the lst approach our analysis showed that in some cases considering the simplified excess compliance matrix may lead to an incorrect representation of the anisotropic response of the probed fractured rock

azimuthally anisotropic elastic impedance inversion for fluid indicator driven by rock physics the normal to tangential fracture compliance ratio is usually used as a fracture fluid indicator ffi for fluid identification in fractured reservoirs with a new parameterization for fracture weaknesses we define a new ffi based on azimuthally anisotropic elastic impedance ei inversion and fractured anisotropic rock physics models first we derived a new azimuthally anisotropic ei equation with a similar expression for the isotropic and anisotropic ei parts to remove the exponential correction of ei which is attributable to weak anisotropy then we built a fractured anisotropic rock physics model used for the estimation of well log parameters for the normal and tangential fracture weaknesses which builds the initial background low frequency trend of fracture weaknesses finally based on the azimuthally anisotropic ei inversion method with the cauchy sparse and low frequency information regularization we estimated a ffi applied to fluid identification in fractured reservoirs tests on both synthe

seismic anisotropy of fractured rock a simple method for including the effects of geologically realistic fractures on the seismic propagation through fractured rocks can be obtained by writing the effective compliance tensor of the fractured rock as the sum of the compliance tensor of the unfractured background rock and the compliance tensors for each set of parallel fractures or aligned fractures the compliance tensor of each fracture set is derivable from a second rank fracture compliance tensor for a rotationally symmetric set of fractures the fracture compliance tensor depends on only two fracture compliances one controlling fracture compliance normal the other tangential to the plane of the fractures the stiffness tensor which is more useful in the consideration of elastic wave propagation through rocks can then be obtained by inversion the components of the excess fracture compliance tensor represent the maximum amount of information that can be obtained from seismic data if the background rock is isotropic and the normal a

atmospheric bromine flux from the coastal abu dhabi sabkhat a ground water mass balance investigation 1 a solute mass balance study of ground water of the 3000 km2 coastal sabkhat salt flats of the emirate of abu dhabi united arab emirates documents an annual bromide loss of approximately 255 metric tons 0 0032 gmoles or 85 kg km2 this value is an order of magnitude greater than previously published direct measurements from the atmosphere over an evaporative environment of a salar in bolivia laboratory evidence consistent with published reports suggests that this loss is by vapor transport to the atmosphere if this bromine flux to the atmosphere is representative of the total earth area of active salt flats then it is a significant and generally under recognized input to the global atmospheric bromide flux

frequency dependent anisotropy due to meso scale fractures in the presence of equant porosity fractured rock is often modelled under the assumption of perfect fluid pressure equalization between the fractures and equant porosity this is consistent with laboratory estimates of the characteristic squirt flow frequency however these laboratory measurements are carried out on rock samples which do not contain large fractures we consider coupled fluid motion on two scales the grain scale which controls behaviour in laboratory experiments and the fracture scale our approach reproduces generally accepted results in the low and high frequency limits even under the assumption of a high squirt flow frequency we find that frequency dependent anisotropy can occur in the seismic frequency band when larger fractures are present shear wave splitting becomes dependent on frequency with the size of the fractures playing a controlling role in the relationship strong anisotropic attenuation can occur in the seismic frequency band the magnitude of the frequency dependence is influenced strongly by the extent of equant porosity with these results it becomes possible in principle to distinguish between fracture and microcrack induced anisotropy or more ambitiously to measure a characteristic fracture length from seismic data

on attenuation of seismic waves associated with flow in fractures heterogeneity of porous media induces a number of fluid flow mechanisms causing attenuation of seismic waves attenuation induced by squirt type mechanisms has previously been analyzed for aspect ratios smaller or equal to 10 3 using a hybrid dimensional modeling approach particularly apt for large aspect ratio conduits we numerically simulated deformation induced fluid flow along two intersecting fractures to investigate the physics of attenuation related to the interaction of fracture induced fluid flow and to leak off attenuation related to fracture flow increases in magnitude with increasing geometrical aspect ratio of the fracture the inherent time scales of both flow mechanisms do not influence each other but the faster process is associated with stronger attenuation than the slower process models relying on simple diffusion equations have rather limited potential for approximation of pressure transients