implication of seismic attenuation for gas hydrate resource characterization mallik mackenzie delta canada wave attenuation is an important physical property of hydrate bearing sediments that is rarely taken into account in site characterization with seismic data we present a field example showing improved images of hydrate bearing sediments on seismic data after compensation of attenuation effects compressional quality factors estimated from zero offset vertical seismic profiling data acquired at mallik northwest territories canada demonstrate significant wave attenuation for hydrate bearing sediments these results are in agreement with previous attenuation estimates obtained from sonic logs and crosshole data at different frequency intervals the application of an inverse q filter to compensate attenuation effects of permafrost and hydrate bearing sediments improved the resolution of surface 3d seismic data and its correlation with log data particularly for the shallowest gas hydrate interval compensation of the attenuation effects of the permafrost likely explains most of the improvements for the shallow gas hydrate zone our results show that characterization of the mallik gas hydrates with seismic data not corrected for attenuation would tend to overestimate thicknesses and lateral extent of hydrate bearing strata and hence the volume of hydrates in place

attenuation of seismic waves in methane gas hydrate bearing sand compressional wave p wave and shear wave s wave velocities vp and vs respectively from remote seismic methods have been used to infer the distribution and volume of gas hydrate within marine sediments recent advances in seismic methods now allow compressional and shear wave attenuations q1p and q1s respectively to be measured however the interpretation of these data is problematic due to our limited understanding of the effects of gas hydrate on physical properties therefore a laboratory gas hydrate resonant column was developed to simulate pressure and temperature conditions suitable for methane gas hydrate formation in sand specimens and the subsequent measurement of both q1p and q1s at frequencies and strains relevant to marine seismic surveys 13 dry gas saturated sand specimens were investigated with different amounts of methane gas hydrate evenly dispersed throughout each specimen the results show that for these dry specimens both q1p and q1s are highly sensitive to hydrate saturation with unexpected peaks observed between 3 and 5 per cent hydrate saturation it is thought that viscous squirt flow of absorbed water or free gas within the pore space is enhanced by hydrate cement at grain contacts and by the nanoporosity of the hydrate itself these results show for the first time the dramatic effect methane gas hydrate can have on seismic wave attenuation in sand and provide insight into wave propagation mechanisms these results will aid the interpretation of elastic wave attenuation data obtained using marine seismic prospecting methods

assessment of radiometric correction techniques in analyzing vegetation variability and change using time series of landsat images abstract the homogeneity of time series of satellite images is crucial when studying abrupt or gradual changes in vegetation cover via remote sensing data various sources of noise affect the information received by satellites making it difficult to differentiate the surface signal from noise and complicates attempts to obtain homogeneous time series we compare different procedures developed to create homogeneous time series of landsat images including sensor calibration atmospheric and topographic correction and radiometric normalization two seasonal time series of landsat images were created for the middle ebro valley ne spain covering the period 1984 2007 different processing steps were tested and the best option selected according to quantitative statistics obtained from invariant areas simultaneous medium resolution images and field measurements the optimum procedure includes cross calibration between landsat sensors atmospheric correction using complex radiative transfer models a non lambertian topographic correction and a relative radiometric normalization using an automatic procedure finally three case studies are presented to illustrate the role of the different radiometric correction procedures when analyzing and explaining gradual and abrupt temporal changes in vegetation cover as well as temporal variability we have shown that to analyze different vegetation processes with landsat data it is necessary to accurately ensure the homogeneity of the multitemporal datasets by means of complex radiometric correction procedures failure to follow such a procedure may mean that the analyzed processes are non recognizable and that the obtained results are invalid

bottom simulating reflectors seismic velocities and avo effects we obtain the wave velocities of ice and gas hydrate bearing sediments as a function of concentration and temperature unlike previous theories based on simple slowness and or moduli averaging or two phase models we use a biot type three phase theory that considers the existence of two solids grain and ice or clathrate and a liquid water and a porous matrix containing gas and water for consolidated berea sandstone the theory underestimates the value of the compressional velocity below 0 c including grain ice interactions and grain cementation yields a good fit to the experimental data strictly speaking water proportion and temperature are closely related fitting the wave velocity at a given temperature allows the prediction of the velocity throughout the range of temperatures provided that the average pore radius and its standard deviation are known the reflection coefficients are computed with a viscoelastic single phase constitutive model the analysis is carried out for the top and botto

radiometric normalization and image mosaic generation of aster thermal infrared data an application to extensive sand sheets and dune fields abstract data from the advanced spaceborne thermal emission and reflection radiometer aster have a significant advantage over previous datasets because of the combination of high spatial resolution 15 90 m and enhanced multispectral capabilities particularly in the thermal infrared tir atmospheric window 8 12 m of the earth where common silicate minerals are more easily identified however the 60 km swath width of aster can limit the effectiveness of accurately tracing large scale features such as eolian sediment transport pathways over long distances the primary goal of this paper is to describe a method for generating a seamless and radiometrically accurate aster tir mosaic of atmospherically corrected radiance and from that extract surface emissivity for arid lands specifically sand seas the gran desierto in northern sonora mexico was used as a test location for the radiometric normalization technique because of past remote sensing studies of the region its compositional diversity and its size a linear approach was taken to transform adjacent image swaths into a direct linear relationship between image acquisition dates pseudo invariant features pifs were selected using a threshold of correlation between radiance values and change pixels were excluded from the linear regression used to determine correction factors the degree of spectral correlation between overlapping pixels is directly related to the amount of surface change over time therefore the gain and offsets between scenes were based only on regions of high spectral correlation the result was a series of radiometrically normalized radiance at surface images that were combined with a minimum of image edge seams present these edges were subsequently blended to create the final mosaic the advantages of this approach for tir radiance as opposed to emissivity data include the ability to 1 analyze data acquired on different dates with potentially very different surface temperatures as one seamless compositional dataset 2 perform decorrelation stretches dcs on the entire dataset in order to identify and discriminate compositional units and 3 separate brightness temperature from surface emissivity for quantitative compositional analysis of the surface reducing seam line error in the emissivity mosaic the approach presented here is valid for any aster related study of large geographic regions where numerous images spanning different temporal and atmospheric conditions are encountered

seismic wave attenuation in a methane hydrate reservoir elastic wave data collected around the world in sediments with methane hydrate point to significant velocity increases due to the presence of the hydrate in the pores this effect can be easily understood if we recall that gas hydrate is a solid as opposed to brine or gas by filling the pore space gas hydrate acts to reduce the porosity available to the pore fluid and increase the elastic moduli of the solid frame however it is difficult to reconcile this effect with more recent observations that the attenuation of elastic waves grows with increasing gas hydrate concentration indeed intuitively one would expect that the stiffer the rock the smaller the relative elastic energy losses per cycle and the smaller the attenuation measurements in many sediments support this intuition for example klimentos and mccann 1990 show that attenuation increases with increasing porosity and clay content while the velocity behaves in an opposite way recent results by koesoemadinata and mcmechan 2001 who statistically generalized many experimental data point to the same fact this inference combined with quantitative modeling led dvorkin et al 2003 to suggest reduced absorption as a possible seismic attribute for methane hydrate detection however the facts are persistent unexpectedly large attenuation in sediments with gas hydrates has recently been observed at different geographical locations in different depositional environments and at different frequencies in 1999 guerin et al presented qualitative evidence of dipole waveform attenuation in the hydrate bearing sediments in the outer blake ridge off the u s east coast sakai 1999 noted that the shear wave vsp signal may be strongly attenuated in a mallik well in northern canada within the methane hydrate interval wood et al 2000 observed increased attenuation of seismic waves at the same location guerin and goldberg 2002 used monopole and dipole waveforms to quantify compressional and shear wave

elasticity of marine sediments rock physics modeling we offer an effective medium model for the elastic moduli of high porosity ocean bottom sediments the elastic constants of the dry sediment frame depend on porosity elastic moduli of the solid phase and effective pressure the model connects two end points in the elastic modulus porosity plane the hertz mindlin modulus of a dense elastic sphere pack at critical porosity and zero at 100 porosity the elastic moduli of saturated sediment are calculated from those of the dry frame using gassmann s equation unlike the suspension model our model assigns non zero elastic constants to the dry sediment frame and can predict the shear wave velocity unlike various modifications of the travel time average equation it is first principle based and contains only physical parameters we justify this model by matching sonic data in shallow marine sediments and in an odp well

influence of gas hydrate morphology on the seismic velocities of sands this paper reports the results of a series of resonant column tests on specimens where gas hydrate has been formed in sands using an excess water technique in these specimens the amount of hydrate formed is restricted by the amount of gas in the specimen and with an excess of water being present in the pore space results of resonant column tests carried out to determine compressional and shear wave velocities suggest that gas hydrate formed in this way are frame supporting in contrast the behavior observed in sands where the hydrate is formed from finite water where the remaining pore space is saturated with methane gas termed in this paper the excess gas method exhibits a cementing behavior while tetrahydrofuran hydrate sands or where the hydrate is formed from dissolved methane within the pore water exhibit a pore filling behavior for hydrate saturations less than 40 for sands where the hydrate is formed using the excess water method much larger volumes of hydrate are required before a significant increase in shear wave velocity occurs although increases in compressional wave velocity are seen at lower hydrate contents these results suggest that hydrate interaction with the sediment is strongly dependent on morphology and that natural hydrate may exhibit contrasting seismic signatures depending upon the geological environment in which it forms

acoustics of marine sediment under compaction binary grain size model and viscoelastic extension of biot s theory this paper presents a model of acoustic wave propagation in unconsolidated marine sediment including compaction using a concept of a simplified sediment structure modeled as a binary grain size sphere pack compressional and shear wave velocities and attenuation follow from a combination of biot s model used as the general framework and two viscoelastic extensions resulting in complex grain and frame moduli respectively an effective grain model accounts for the viscoelasticity arising from local fluid flow in expandable clay minerals in clay bearing sediments a viscoelastic contact model describes local fluid flow at the grain contacts porosity density and the structural biot parameters permeability pore size structure factor as a function of pressure follow from the binary model so that the remaining input parameters to the acoustic model consist solely of the mass fractions and the known mechanical properties of each constituent e g carbonates sand clay and expandable clay of the

biot gassmann theory for velocities of gas hydrate bearing sediments elevated elastic velocities are a distinct physical property of gas hydrate bearing sediments a number of velocity models and equations e g pore filling model cementation model effective medium theories weighted equations and time average equations have been used to describe this effect in particular the weighted equation and effective medium theory predict reasonably well the elastic properties of unconsolidated gas hydrate bearing sediments a weakness of the weighted equation is its use of the empirical relationship of the time average equation as one element of the equation one drawback of the effective medium theory is its prediction of unreasonably higher shear wave velocity at high porosities so that the predicted velocity ratio does not agree well with the observed velocity ratio to overcome these weaknesses a method is proposed based on biot gassmann theories and assuming the formation velocity ratio shear to compressional velocity of an unconsolidated sediment is related to the

radiometric normalization of multitemporal high resolution satellite images with quality control for land cover change detection the radiometric normalization of multitemporal satellite optical images of the same terrain is often necessary for land cover change detection e g relative differences in previous studies ground reference data or pseudo invariant features pifs were used in the radiometric rectification of multitemporal images ground reference data are costly and difficult to acquire for most satellite remotely sensed images and the selection of pifs is generally subjective in addition previous research has been focused on radiometric normalization of two images acquired on different dates the problem of conservation of radiometric resolution in the case of radiometric normalization between more than two images has not been addressed this article reports on a new procedure for radiometric normalization between multitemporal images of the same area the selection of pifs is done statistically with quality control principal component analysis pca is used to find linear relationships between multitemporal images of the same area the satellite images are normalized radiometrically to a common scale tied to the reference radiometric levels the procedure ensures the conservation of radiometric resolution for the multitemporal images involved the new procedure is applied to three landsat 5 thematic mapper tm images from three different years august 1986 1987 and 1991 and of the same area quality control measures show that the error in radiometric consistency between the multitemporal images is reduced effectively the normalized difference vegetation index ndvi is calculated using the radiometrically normalized multitemporal imagery and assessed in the context of land cover change analysis

the effect of methane hydrate morphology and water saturation on seismic wave attenuation in sand under shallow sub seafloor conditions a better understanding of seismic wave attenuation in hydrate bearing sediments is needed for the improved geophysical quantification of seafloor methane hydrates important for climate change geohazard and economic resource assessment hence we conducted a series of small strain 10 6 seismic frequency 50 550 hz laboratory resonant column experiments on synthetic methane hydrate bearing sands under excess water seafloor conditions the results show a complex dependence of p and s wave attenuation on hydrate saturation and morphology p and s wave attenuation in excess water hydrate bearing sand is much higher than in excess gas hydrate bearing sand and increases with hydrate saturation between 0 and 0 44 the experimental range theoretical modelling suggests that load bearing hydrate is an important cause of heightened attenuation for both p and s waves in gas and water saturated sands while pore filling hydrate also contributes significantly to p wave attenuation in water saturated sands a squirt flow attenuation mechanism related to microporous hydrate and low aspect ratio pores at the interface between sand grains and hydrate is thought to be responsible for the heightened levels of attenuation in hydrate bearing sands at low hydrate saturations 0 44

seismic attenuation and pore fluid viscosity in clay rich reservoir sandstones the frequency dependence of seismic attenuation in a suite of clay rich reservoir sandstones was investigated in the laboratory compressional and shear wave velocities vp and vs and quality factors qp and qs were measured as functions of pore fluid viscosity at an effective pressure of 50 mpa and at an experimental frequency of about 0 8 mhz using the pulse echo technique the experimental viscosity ranged from 0 3 to 1000 centipoise which gives equivalent frequencies for a water saturated sandstone of 2 6 mhz to 780 hz assuming a global flow loss mechanism two types of behavior were observed high permeability greater than 100 millidarcies sandstones tend to show variable qp and qs which are similar in magnitude to those predicted by the biot theory over the viscosity range 0 3 to about 20 centipoise equivalent frequency range 2 6 mhz to about 39 khz low permeability less than 50 millidarcies sandstones tend to show almost constant qp and qs over the experimental viscosity range that are

analysis of velocity dispersion using full waveform multichannel sonic logging data a case study seismic attenuation and velocity dispersion are potentially able to reveal the rock physical properties of the subsurface conventionally a frequency independent quality factor q is measured this q is equivalent to the total velocity dispersion in a seismic record and is inadequate for analysing the attenuation mechanism or rock physical properties here a new method is proposed to extract the velocity dispersion curves so that more attributes can be obtained from full waveform multichannel sonic logging data especially the critical frequency fc if it is within the bandwidth of the data this method first decomposes the seismic data into a series of frequency components computes the semblance of each frequency component for different velocity values cross correlates the semblance matrices of adjacent frequency components to get the velocity gradients and finally integrates to obtain a velocity dispersion curve results of this method are of satisfactory accuracy and robustness this method is applied to the data acquired in mallik 5l 38 gas hydrate research well in mackenzie delta northwest territories canada the observed p wave velocity dispersion compares well with the geological setting in the gas hydrate zone about 900 m 1100 m high concentration of gas hydrate causes very strong velocity dispersion and a distinct fc at about 15 khz likely due to strong scattering of centimetre scale inclusions of gas hydrate concurrently water flow in connected cracks in some ranges of this zone adds a large part of velocity dispersion and a dimmer fc at about 9 5 khz immediate underneath the gas hydrate zone abundant free water in weakly laminated sediments causes quite strong velocity dispersion and an fc at about 6 5 khz velocity dispersion is mild and without an obvious fc in sediments above the gas hydrate zone

sonic waveform attenuation in gas hydrate bearing sediments from the mallik 2l 38 research well mackenzie delta canada 1 the mallik 2l 38 research well was drilled to 1150 m under the mackenzie delta canada and penetrated a subpermafrost interval where methane hydrate occupies up to 80 of the pore space a suite of high quality downhole logs was acquired to measure in situ the physical properties of these hydrate bearing sediments similar to other hydrate deposits resistivity and compressional and shear sonic velocity data increase with higher hydrate saturation owing to electrical insulation of the pore space and stiffening of the sediment framework in addition sonic waveforms show strong amplitude losses of both compressional and shear waves in intervals where methane hydrate is observed we use monopole and dipole waveforms to estimate compressional and shear attenuation comparing with hydrate saturation values derived from the resistivity log we observe a linear increase in both attenuation measurements with increasing hydrate saturation which is not intuitive for stiffening sediments numerical modeling of the waveforms allows us to reproduce the recorded waveforms and illustrate these results we also use a model for wave propagation in frozen porous media to explain qualitatively the loss of sonic waveform amplitude in hydrate bearing sediments we suggest that this model can be improved and extended allowing hydrate saturation to be quantified from attenuation measurements in similar environments and providing new insight into how hydrate and its sediment host interact

effect of methane hydrate morphology on compressional wave velocity of sandy sediments analysis of pressure cores obtained in the eastern nankai trough abstract sediment cores containing methane hydrate were obtained under pressure from the eastern nankai trough offshore japan and they have been analyzed to investigate the relationship between compressional wave velocity p wave velocity methane hydrate saturation and pore space hydrate morphology p wave velocities of pressure cores were measured at near in situ pressures thus preventing hydrate dissociation after the measurement of p wave velocity the cores were cut under pressure into separate p wave velocity intervals each core interval was depressurized while measuring the evolved gas volume to quantify methane hydrate saturation the results show that p wave velocity correlates well with hydrate saturation the p wave velocity varied from less than 1700 m s in the hydrate free section to greater than 2300 m s in the section with the highest hydrate saturation of 72 the measured p wave velocities were correctly reproduced by the sediment frame component model by adjusting model parameters such as sand clay ratio and effective stress it was found that all core data plotted within the model predictions assuming zero effective stress and assuming in situ effective stress this may indicate that the cores were in the process of relaxing from their in situ effective stress at the time of measurement by using pressure cores and pressure core analysis technology the relationship between p wave velocity and methane hydrate saturation has been directly obtained nondestructively the observed relationship in high resolution core scale specimens enables estimation of the hydrate morphology and is expected to be more accurate than cross plot data in well logging

across path dn gradient in landsat tm imagery of amazonian forests a challenge for image interpretation and mosaicking abstract an analysis of tropical rain forest covering amazonian lowlands has highlighted a systematic across path east west radiometric gradient within landsat tm imagery visual assessment of 45 and quantitative analysis of 20 amazonian landsat 4 and 5 tm scenes show that the gradient is band dependent and pronounced in visible light bands 1 to 3 but significant also in ir bands 4 to 7 the results show that the scan line location of a pixel explains a considerable amount of the dn variation of forests in the width of the entire scene b1 70 b2 52 b3 44 b4 34 b5 46 b7 39 in digital numbers the difference between east and west side of a scene may be small 9 4 3 10 9 3 respectively but these differences become significant if the images are to be mosaicked or the data are used for mapping relatively subtle differences of natural forests apparently the gradient is a result of at least three factors 1 shadows caused by the undulating terrain 2 anisotropic reflectance of the varying surfaces and 3 atmospheric scattering the phenomenon becomes more significant when the sun is high and the scanning line is close to solar azimuth direction a condition more easily encountered in lower latitudes of the earth

elastic wave velocity characterization of gas hydrate bearing fractured reservoirs in a permafrost area of the qilian mountain northwest china abstract there are two types of gas hydrate bearing reservoirs in the permafrost area of qilian mountain most of the gas hydrates occur mainly in the fractured mudstone reservoirs and rarely in the pores of the sandstone reservoirs in this study for the acoustic velocity characterization of the fractured gas hydrate reservoirs of the qilian mountain permafrost area some mudstone core samples were collected for physical rock experiments such as the acoustic experiment and the porosity and permeability experiment an acoustic velocity numerical simulation of gas hydrate reservoirs was performed according to the biot theory and the differential effective medium theory with the conditions of multiple gas hydrate occurrence models including the suspension model the semi cementation model and the cementation model and considering both infinite and penny shaped cracks fracture porosity was added to the core samples that only contain matrix porosity with fracture porosity ranging from 0 01 to 5 the variation laws between acoustic velocity with fractured porosity and hydrate saturation are obtained 1 in the case of an infinite crack if the fractured porosity is 0 01 1 the p wave velocity decreases rapidly in the case of the three occurrence models if the fractured porosity is higher than 1 the acoustic velocity decreases gradually if the crack shape is a penny shaped crack the p wave velocity decreases almost linearly with increasing fracture porosity 2 if the hydrate occurrence model is the suspension model the p wave velocity increases slightly with increasing hydrate saturation if the occurrence model is the semi cementation model or the cementation model when the gas hydrate saturation of the infinite crack ranges from 0 to 80 the acoustic velocity increases approximately linearly whereas when the gas hydrate saturation ranges from 80 to 100 the velocity increases rapidly if the crack is a penny shaped crack the velocity increases almost linearly with increasing gas hydrate saturation from 0 to 100 3 it is found that the fractured gas hydrate reservoirs of the qilian mountain permafrost area contain both penny shaped and infinite cracks of which the infinite crack is the main crack shape the gas hydrate occurrence in the qilian mountain permafrost area mainly follows the suspension model this has significance for the seismic exploration and log evaluation of gas hydrate bearing fractured reservoirs in the permafrost area of the qilian mountain in studying the acoustic velocity characterization the crack shapes and occurrence models of gas hydrate reservoirs in the study area

seismic attenuation due to wave induced flow analytical expressions for three p wave attenuation mechanisms in sedimentary rocks are given a unified theoretical framework two of the models concern wave induced flow due to heterogeneity in the elastic moduli at mesoscopic scales scales greater than grain sizes but smaller than wavelengths in the first model the heterogeneity is due to lithological variations e g mixtures of sands and clays with a single fluid saturating all the pores in the second model a single uniform lithology is saturated in mesoscopic patches by two immiscible fluids e g air and water in the third model the heterogeneity is at microscopic grain scales broken grain contacts and or micro cracks in the grains and the associated fluid response corresponds to squirt flow the model of squirt flow derived here reduces to proper limits as any of the fluid bulk modulus crack porosity and or frequency is reduced to zero it is shown that squirt flow is incapable of explaining the measured level of loss 10 sup 2 q sup 1 10 sup 1 within the seismic band of frequencies 1 to 10 sup 4 hz however either of the two mesoscopic scale models easily produce enough attenuation to explain the field data

an effective medium inversion algorithm for gas hydrate quantification and its application to laboratory and borehole measurements of gas hydrate bearing sediments the presence of gas hydrate in marine sediments alters their physical properties in some circumstances gas hydrate may cement sediment grains together and dramatically increase the seismic p and s wave velocities of the composite medium hydrate may also form a load bearing structure within the sediment microstructure but with different seismic wave attenuation characteristics changing the attenuation behaviour of the composite here we introduce an inversion algorithm based on effective medium modelling to infer hydrate saturations from velocity and attenuation measurements on hydrate bearing sediments the velocity increase is modelled as extra binding developed by gas hydrate that strengthens the sediment microstructure the attenuation increase is modelled through a difference in fluid flow properties caused by different permeabilities in the sediment and hydrate microstructures we relate velocity and attenuation increases in hydrate bearing sediments to their hydrate content using an effective medium inversion algorithm based on the self consistent approximation sca differential effective medium dem theory and biot and squirt flow mechanisms of fluid flow the inversion algorithm is able to convert observations in compressional and shear wave velocities and attenuations to hydrate saturation in the sediment pore space we applied our algorithm to a data set from the mallik 2l 38 well mackenzie delta canada and to data from laboratory measurements on gas rich and water saturated sand samples predictions using our algorithm match the borehole data and water saturated laboratory data if the proportion of hydrate contributing to the load bearing structure increases with hydrate saturation the predictions match the gas rich laboratory data if that proportion decreases with hydrate saturation we attribute this difference to differences in hydrate formation mechanisms between the two environments

compressional and shear wave velocities in uncemented sediment containing gas hydrate 1 the competing hypotheses for gas hydrate formation at the particle scale in sediments describe processes of pore filling frame building or cementation new measurements of compressional vp and shear wave vs velocities in fine grained sands subjected to low confinement and monitored during formation of tetrahydrofuran hydrate indicate that hydrate nucleates in the pore space presumably at grain boundaries and grows with limited impact on the sediment shear stiffness vp and vs until crystals begin to interact with the granular skeleton at 40 hydrate concentration vs increases significantly more than vp at higher hydrate concentrations reflecting larger changes in the specimen s shear stiffness than its bulk stiffness the results indicate that seismic velocities and or their ratio vp vs have limited capability for locating hydrate or constraining hydrate concentrations

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

attenuation in fine grained marine sediments extension of the biot stoll model by the effective grain model egm it is postulated that the presence of swelling clay minerals in unconsolidated fine grained saturated marine sediments leads to deviations from the normally assumed ideal elasticity of the solid phase in the proposed effective grain model egm the elastic grain material is consequently replaced by an effective medium made up of a homogeneous elastic mineral phase that is isotropically interspersed with cylindrical penny shaped inclusions of low aspect ratio representing the intracrystalline water layers in the swelling clay minerals the two phase nature of the grain material thus specified results in a wave energy consuming squirt flow process from the inclusions into the pore space introducing the egm into the biot stoll model bsm via the complex bulk modulus of the dissipative grain material leads to a better fit to literature data on the attenuation of compressional waves than does the original bsm alone since swelling clay minerals occur in nearly all clay bearing sediments it is conclud

a laboratory investigation into the seismic velocities of methane gas hydrate bearing sand remote seismic methods which measure the compressional wave p wave velocity vp and shear wave s wave velocity vs can be used to assess the distribution and concentration of marine gas hydrates in situ however interpreting seismic data requires an understanding of the seismic properties of hydrate bearing sediments which has proved problematic because of difficulties in recovering intact hydrate bearing sediment samples and in performing valid laboratory tests therefore a dedicated gas hydrate resonant column ghrc was developed to allow pressure and temperature conditions suitable for hydrate formation to be applied to a specimen with subsequent measurement of both vp and vs made at frequencies and strains relevant to marine seismic investigations thirteen sand specimens containing differing amounts of evenly dispersed hydrate were tested the results show a bipartite relationship between velocities and hydrate pore saturation with a marked transition between 3 and 5 hydrate pore saturation for both vp and vs this suggests that methane hydrate initially cements sand grain contacts then infills the pore space these results show in detail for the first time using a resonant column how hydrate cementation affects elastic wave properties in quartz sand this information is valuable for validating theoretical models relating seismic wave propagation in marine sediments to hydrate pore saturation

upper ocean temperature and the baroclinic transport stream function relationship in drake passage repeat hydrographic sections across the antarctic circumpolar current acc in drake passage are used to derive an empirical relationship between upper ocean temperature and the baroclinic transport stream function cross validation shows this relationship can be used to infer baroclinic transport above and relative to 2500 m from expendable bathythermograph xbt temperature measurements with an error of a few per cent transport errors of less than 2 sv are obtained if temperature at depths between 600 and 1600 m is used to define the relationship temperature at depths above 300 m provides an unreliable index of transport because of variability in temperature salinity t s properties produced by air sea interaction the scatter in the relationship between temperature and stream function from repeat observations along a single line is similar in magnitude to the scatter observed when data from the broader drake passage area are considered in both cases variability about the mean temperature stream function relationship reflects advection of water with anomalous t s properties the tight relationship between temperature and stream function in drake passage and south of australia suggests baroclinic transports can be inferred from xbt temperatures with high accuracy in the southern ocean providing a cost effective means of monitoring acc variability however care must be taken at the end points particularly in the drake passage where the strong flow of the subantarctic front sometimes lies over the continental slope

seismic wave attenuation in methane hydrate bearing sediments vertical seismic profiling data from the nankai trough exploratory well offshore tokai central japan 1 data from two vertical seismic profiles vsps from the nankai trough exploratory well offshore tokai central japan are used to estimate compressional attenuation in methane hydrate mh bearing sediments at seismic frequencies of 30 110 hz we compare spectral ratio and centroid frequency shift methods for measuring attenuation to isolate intrinsic attenuation from total attenuation attenuation is computed from multiples using one dimensional synthetic vsp data from sonic and density logs the use of two different measurement methods and two vsps recorded at just 100 m separation provides an opportunity to validate the attenuation measurements no significant compressional attenuation was observed in mh bearing sediments at seismic frequencies macroscopically the peaks of highest attenuation in the seismic frequency range correspond to low saturation gas zones in contrast high compressional attenuation zones in the sonic frequency range 10 20 khz are affected by the presence of methane hydrates in the same well locations thus this study demonstrated the frequency dependence of attenuation in mh bearing sediments mh bearing sediments cause attenuation in the sonic frequency range rather than the seismic frequency range

estimation of seismic attenuation of gas hydrate bearing sediments from multi channel seismic data a case study from krishna godavari offshore basin seismic attenuation of gas hydrate bearing sediments is estimated from the multi channel seismic data acquired in krishna godavari kg offshore basin in the vicinity of site nghp 01 10 where 128 m of fracture filled hydrate is confirmed by drilling coring to understand the effect of gas hydrate on the seismic attenuation the seismic quality factor q which is the inverse of seismic attenuation is estimated from two methods first the spectral ratio method which utilizes the logarithmic ratio of the seafloor and bottom simulating reflector bsr spectra and second the peak frequency method which depends upon the variations of the peak frequency of the bsr spectrum as a function of traveltime in the study area the complex fault system has perturbed the seafloor resulting in noisy seafloor spectrum as a result the logarithm of the spectral ratio was also noisy and the q values obtained from spectral ratio method were found to be unstable on the other hand the peak frequency method is found to be relatively stable as it depends only on the peak frequency of the bsr which can be reliably obtained by conventional processing this method yields an effective q value of bsr which represents a weighted average of the interval q values of the water column and the gas hydrate bearing sediments since the q value of water column can be treated as a constant the variations in effective q value can be interpreted in terms of the variations of q in the gas hydrate bearing sediments for a constant thickness of water column the estimated effective q values along the inline and crossline seismic profiles depend on several factors such as gas hydrate free gas and the complex fault system the combined interpretation of the quality factor and the interval velocity model helps to understand these effects in different zones of the seismic data the attenuation of the background marine sediments is estimated from the analysis of crossline seismic profile the gas hydrate bearing sediments as inferred from the velocity model show low attenuation as compared to that of the background sediments in contrast active migration of free gas across the fault system leads to increase in seismic attenuation within the gas hydrate stability zone the complex fault zone may also lead to an increase in seismic attenuation the blanked seismic zone shows a marginal decrease in seismic attenuation probably related to loadbearing form of gas hydrate our attenuation results are compared against the results obtained from waveform inversion albeit with a higher resolution the relative variation of q estimated from the proposed method is close to the average q values estimated from full waveform inversion for most of the cdps the proposed methodology can be a useful technique to estimate seismic attenuation from multi channel seismic data

quantitative characterization of gas hydrate bearing sediment using elastic electrical rock physics models abstract successful evaluation of reservoir resources in gas hydrate bearing sediments depends on the accurate estimation of reservoir parameters such as hydrate saturation porosity and clay content elastic and electrical measurements are affected by several rock and fluid properties and their joint interpretation can provide complementary information and improve the accuracy of reservoir characterization results in this work we propose a new rock physics inversion workflow based on 3d rock physics template of elastic and electrical attributes for the direct estimation of reservoir parameters we first conduct a quantitative analysis of the available well log data and core data to understand the morphology of gas hydrate within the marine sediment and choose the appropriate elastic rock physics model then we construct the 3d rock physics template by discretizing the 3d space of reservoir parameters and implementing the suitable elastic and electrical models to calculate the poisson s ratio p wave impedance and electrical resistivity for each combination of values of rock and fluid properties finally we superimpose the measurements on the templates and use a grid searching method to find the nearest gridded node to the data point and minimize the objective function of the mismatch between data and model predictions to investigate the feasibility of our proposed inversion scheme we test a rock physics model based on effective medium theory and a modified archie s equation and invert for reservoir parameters of gas hydrate bearing sediments at well 1250f and 1247b in hydrate ridge the good match between inversion results and core data suggests that the proposed inversion scheme is applicable to real data and allows combining elastic and electrical data for reservoir parameter estimation of gas hydrate bearing sediments

characterization of seismic wave velocity and attenuation and interpretation of tetrahydrofuran hydrate bearing sand using resonant column testing abstract understanding the geophysical properties of hydrate bearing sediments is critical for hydrate utilization such as prediction of hydrate distribution and concentration as well as safety evaluation during hydrate production here a customized resonant column is employed to test tetrahydrofuran thf hydrate bearing sandy specimens the results show that wave velocities of all tested specimens increase exponentially with increasing effective confining stress v whereas wave velocity evolution becomes almost stress independent when critical hydrate saturation sh 0 53 in this study is reached which transforms the hydrate morphology from non cementing to cementing type in addition regardless of structure differences between tested specimens an increased factor and decreased exponent follows a uniform equation 0 61 0 2 log m s measured wave velocity differences between hydrate bearing and hydrate free specimens confirm the coexistence of various hydrate morphologies based on matching results of theoretical predictions from rock physics models and interpretation of volumetric proportions of hydrates with different morphologies in tested specimens wave attenuation of tested specimens generally decreases with increasing effective confining stress and increase with increasing hydrate saturation the relationship between wave attenuation and hydrate saturation qs 1 0 015 0 17 sh is confirmed to be a hydrate saturation indicator considering data from field seismic surveys this type of monotonous relationship appears to be less sensitive for hydrate morphology nevertheless through the combined interpretation of wave velocities and attenuations it is theoretically feasible to not only predict hydrate saturation but also illustrate the evolution of hydrate morphology our study contributes to a better understanding of stress related geophysical properties of hydrate bearing sediments which can be useful for the characterization of hydrate reservoirs

sediments with gas hydrates internal structure from seismic avo we interpret amplitude variation with offset avo data from a bottom simulating reflector bsr offshore florida by using rock physics based synthetic seismic models a previously conducted velocity and avo analysis of the in situ seismic data showed that the bsr separates hydrate bearing sediments from sediments containing free methane the amplitudes at the bsr are increasingly negative with increasing offset this behavior was explained by p wave velocity above the bsr being larger than that below the bsr and s wave velocity above the bsr being smaller than that below the bsr we use these avo and velocity results to infer the internal structure of the hydrated sediment to do so we examine two micromechanical models that correspond to the two extreme cases of hydrate deposition in the pore space 1 the hydrate cements grain contacts and strongly reinforces the sediment and 2 the hydrate is located away from grain contacts and does not affect the stiffness of the sediment frame only the second model can qualitatively reproduce the observed avo response thus inferred internal structure of the hydrate bearing sediment means that 1 the sediment above the bsr is uncemented and thereby mechanically weak and 2 its permeability is very low because the hydrate clogs large pore space conduits the latter explains why free gas is trapped underneath the bsr the seismic data also indicate the absence of strong reflections at the top of the hydrate layer this fact suggests that the high concentration of hydrates in the sediment just above the bsr gradually decreases with decreasing depth this effect is consistent with the fact that the low permeability hydrated sediments above the bsr prevent free methane from migrating upwards

the effects of hydrate cement on the stiffness of some sands cementing of sediment occurs naturally in many soils and weak rocks during both the early and late stages of diagenesis this paper reports the results of a series of resonant column tests carried out on a range of sand sized geomaterials to explore the effects of different hydrate cement morphologies on the very small strain stiffness of the materials it is shown that the proportion of void space filled by hydrate cement cement location sand size and grain shape all have a significant effect on shear modulus the change in stiffness between different host materials is affected by the density specific surface and grading of the geomaterial

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

quantification of gas hydrate saturation and morphology based on a generalized effective medium model abstract numerous models have been developed for prediction of gas hydrate saturation based on the microstructural relationship between gas hydrates and sediment grains however quantification of hydrate saturation and morphology from elastic properties has been hindered by failing to account for complex hydrate distributions here we develop a generalized effective medium model by applying the modified hashin shtrikman bounds to a newly developed cementation theory this model is validated by experimental data for synthetic methane and tetrahydrofuran hydrates good comparison of model predictions with experimental measurements not only reveals its ability to merge the results of contact cementation theory and effective medium theory but also indicates its feasibility for characterizing complex morphologies moreover the results of inverting acoustic measurements quantitatively confirm that for synthetic samples in excess gas condition gas hydrates mainly occur as a hybrid cementing morphology with a low percentage of pore filling morphology whereas for pressure core hydrate bearing sediments in natural environments they exist as matrix supporting and pore filling morphologies with a very low percentage of hybrid cementing morphology the hydrate saturations estimated from sonic and density logs in several regions including northern cascadia margin integrated ocean drilling program expedition 311 hole u1326d and hole u1327e alaska north slope mount elbert test well and mackenzie delta mallik 5l 38 are comparable to the referenced hydrate saturations derived from core data and resistivity and or nuclear magnetic resonance log data confirming validity and applicability of our model furthermore our results indicate that 8 hybrid cementing 33 matrix supporting and 59 pore filling hydrates may coexist in the fine grained and clay rich marine sediments on the northern cascadia margin whereas 10 hybrid cementing 54 matrix supporting and 36 pore filling hydrates may coexist in the coarse grained and sand dominated terrestrial sediments of the alaska north slope and mackenzie delta

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

modeling of acoustic wave dissipation in gas hydrate bearing sediments 1 recent sonic and seismic data in gas hydrate bearing sediments have indicated strong waveform attenuation associated with a velocity increase in apparent contradiction with conventional wave propagation theory understanding the reasons for such energy dissipation could help constrain the distribution and the amounts of gas hydrate worldwide from the identification of low amplitudes in seismic surveys a review of existing models for wave propagation in frozen porous media all based on biot s theory shows that previous formulations fail to predict any significant attenuation with increasing hydrate content by adding physically based components to these models such as cementation by elastic shear coupling friction between the solid phases and squirt flow we are able to predict an attenuation increase associated with gas hydrate formation the results of the model agree well with the sonic logging data recorded in the mallik 5l 38 gas hydrate research well cementation between gas hydrate and the sediment grains is responsible for the increase in shear velocity the primary mode of energy dissipation is found to be friction between gas hydrate and the sediment matrix combined with an absence of inertial coupling between gas hydrate and the pore fluid these results predict similar attenuation increase in hydrate bearing formations over most of the sonic and seismic frequency range

rock physics of a gas hydrate reservoir gas hydrates are solids composed of a hydrogen bonded water lattice with entrapped guest molecules of gas there are convincing arguments that vast amounts of methane gas hydrate are present in sediments under the world s oceans as well as in onshore sediments in the arctic this hydrate is possibly the largest carbon and methane pool on earth as such methane hydrate may be the principal factor in global climate balancing one may also treat this methane pool as a potential energy source these considerations ignite the scientific and business community s interest in quantifying the amount of methane hydrate in the subsurface gas hydrate reservoir characterization is in principle no different from the traditional hydrocarbon reservoir characterization similar and well developed remote sensing techniques can be used seismic reflection profiling being the dominant among them seismic response of the subsurface is determined by the spatial distribution of the elastic properties by mapping the elastic contrast the geophysicist can illuminate tectonic features and geobodies hydrocarbon reservoirs included to accurately translate elastic property images into images of lithology porosity and the pore filling phase quantitative knowledge is needed that relates the rock s elastic properties to its bulk properties and conditions specifically to quantitatively characterize a natural gas hydrate reservoir we must be able to relate the elastic properties of the sediment to the volume of gas hydrate present and if at all possible the permeability one way of achieving this goal is through rock physics effective medium modeling several attempts to construct a relation between hydrate concentration and the compressional velocity in sediments have followed the path of modifying the popular wyllie s time average equation which states that total traveltime through rock is the volume weighted sum of traveltimes through the solid phase and the fluid phase considered independently of each other i e v p 1 1 v

evidence of a changing size frequency distribution of landslides in the kyrgyz tien shan central asia there is a strong possibility that environmental change whether climate or land use will be manifest as changes in the size frequency distribution of landslides here evidence is presented for this from western kyrgyzstan central asia remote sensing and spatial analysis have been applied to map mass movements in the central part of the maily say valley and to detect recent landslide activations the evolution of landslide activity over the past 50 years has been analysed on the basis of pre existing landslide maps and new analyses of aerial photographs as well as quickbird images five inventories were produced for the years 1962 based on the existing map of 1962 and aerial photographs of 1962 1984 based on the existing map of 1977 and aerial photographs of 1984 1996 based on aerial photographs of 1996 2002 based on the existing map of 2003 and quickbird imagery of 2002 and 2007 based on quickbird imagery of 2007 the geomorphologic features contained in the catalogues represent the landslide bodies observed from remote imagery of the corresponding year mapped landslides are generally considered as the result of a series of slope failure events size frequency analyses applied to the five landslide inventories show that both the number and size of unstable slopes increased from 1962 162 objects to 2007 208 objects and the power law exponent decreased over time this changing power law exponent may indicate that landslide related hazards are increasing this tendency is documented in more detail for two active landslide zones one in the main valley and one located to the west of it landslide detection methods were used to assist the evolution of slope instabilities choosing appropriate thresholds the image subtraction method based on normalized difference vegetation index ndvi allowed accurate detection of new sliding activation in these two zones this confirmed the results of the more extensive survey that there is a systematic shift in power law exponents and size frequency distributions for central asian landslides copyright 2011 john wiley sons ltd

seismic modeling of multidimensional heterogeneity scales of mallik gas hydrate reservoirs northwest territories of canada 1 in hydrate bearing sediments the velocity and attenuation of compressional and shear waves depend primarily on the spatial distribution of hydrates in the pore space of the subsurface lithologies recent characterizations of gas hydrate accumulations based on seismic velocity and attenuation generally assume homogeneous sedimentary layers and neglect effects from large and small scale heterogeneities of hydrate bearing sediments we present an algorithm based on stochastic medium theory to construct heterogeneous multivariable models that mimic heterogeneities of hydrate bearing sediments at the level of detail provided by borehole logging data using this algorithm we model some key petrophysical properties of gas hydrates within heterogeneous sediments near the mallik well site northwest territories canada the modeled density and p and s wave velocities used in combination with a modified biot gassmann theory provide a first order estimate of the in situ volume of gas hydrate near the mallik 5l 38 borehole our results suggest a range of 528 to 768 106 m3 km2 of natural gas trapped within hydrates nearly an order of magnitude lower than earlier estimates which did not include effects of small scale heterogeneities further the petrophysical models are combined with a 3 d finite difference modeling algorithm to study seismic attenuation due to scattering and leaky mode propagation simulations of a near offset vertical seismic profile and cross borehole numerical surveys demonstrate that attenuation of seismic energy may not be directly related to the intrinsic attenuation of hydrate bearing sediments but instead may be largely attributed to scattering from small scale heterogeneities and highly attenuate leaky mode propagation of seismic waves through larger scale heterogeneities in sediments

physical properties of hydrate bearing sediments 1 methane gas hydrates crystalline inclusion compounds formed from methane and water are found in marine continental margin and permafrost sediments worldwide this article reviews the current understanding of phenomena involved in gas hydrate formation and the physical properties of hydrate bearing sediments formation phenomena include pore scale habit solubility spatial variability and host sediment aggregate properties physical properties include thermal properties permeability electrical conductivity and permittivity small strain elastic p and s wave velocities shear strength and volume changes resulting from hydrate dissociation the magnitudes and interdependencies of these properties are critically important for predicting and quantifying macroscale responses of hydrate bearing sediments to changes in mechanical thermal or chemical boundary conditions these predictions are vital for mitigating borehole local and regional slope stability hazards optimizing recovery techniques for extracting methane from hydrate bearing sediments or sequestering carbon dioxide in gas hydrate and evaluating the role of gas hydrate in the global carbon cycle

theoretical modeling insights into elastic wave attenuation mechanisms in marine sediments with pore filling methane hydrate the majority of presently exploitable marine methane hydrate reservoirs are likely to host hydrate in disseminated form in coarse grain sediments for hydrate concentrations below 25 40 disseminated or pore filling hydrate does not increase elastic frame moduli thus making impotent traditional seismic velocity based methods here we present a theoretical model to calculate frequency dependent p and s wave velocity and attenuation of an effective porous medium composed of solid mineral grains methane hydrate methane gas and water the model considers elastic wave energy losses caused by local viscous flow both i between fluid inclusions in hydrate and pores and ii between different aspect ratio pores created when hydrate grows the inertial motion of the frame with respect to the pore fluid biot s type fluid flow and gas bubble damping the sole presence of pore filling hydrate in the sediment reduces the available porosity and intrinsic permeability of the sediment affecting biot s type attenuation at high frequencies our model shows that attenuation maxima due to fluid inclusions in hydrate are possible over the entire frequency range of interest to exploration seismology 1 106 hz depending on the aspect ratio of the inclusions whereas maxima due to different aspect ratio pores occur only at sonic to ultrasound frequencies 104 106 hz this frequency response imposes further constraints on possible hydrate saturations able to reproduce broadband elastic measurements of velocity and attenuation our results provide a physical basis for detecting the presence and amount of pore filling hydrate in seafloor sediments using conventional seismic surveys

modelling post fire soil erosion hazard using ordinal logistic regression a case study in south eastern spain treatments that minimize soil erosion after large wildfires depend among other factors on fire severity and landscape configuration so that in practice most of them are applied according to emergency criteria therefore simple tools to predict soil erosion risk help to decide where the available resources should be used first in this study a predictive model for soil erosion degree based on ordinal logistic regression has been developed and evaluated using data from three large forest fires in south eastern spain the field data were successfully fit to the model in 60 of cases after 50 runs i e agreement between observed and predicted soil erosion degrees using slope steepness slope aspect and fire severity as predictors north facing slopes were shown to be less prone to soil erosion than the rest

quantifying uncertainties in attenuation estimation at methane hydrate bearing zones using sonic waveform logs abstractapplication of seismic attenuation estimation using sonic waveform data is limited because the estimation methods have not yet been fully developed although the median frequency shift method is considered to be effective and robust compared to the conventional spectral ratio method we demonstrated that the median frequency shift methods strongly depend on reference data under lower signal to noise ratios we modified an existing median frequency shift method not to depend on arbitrarily choosing a reference value and to quantify the uncertainties in attenuation estimation furthermore we implied the optimum selection of receiver pairs used for more stable attenuation analysis our numerical experiments supported the advantage of the proposed method although our main findings by applying the proposed methods in methane hydrate bearing sediments are almost consistent with past field sonic logging measurements we find some differences in the magnitude of attenuation values compared to existing s

estimating the amount of gas hydrate and free gas from marine seismic data marine seismic data and well log measurements at the blake ridge offshore south carolina show that prominent seismic bottom simulating reflectors bsrs are caused by sediment layers with gas hydrate overlying sediments with free gas we apply a theoretical rock physics model to 2 d blake ridge marine seismic data to determine gas hydrate and free gas saturation high porosity marine sediment is modeled as a granular system where the elastic wave velocities are linked to porosity effective pressure mineralogy elastic properties of the pore filling material and water gas and gas hydrate saturation of the pore space to apply this model to seismic data we first obtain interval velocity using stacking velocity analysis next all input parameters to the rock physics model except porosity and water gas and gas hydrate saturation are estimated from geologic information to estimate porosity and saturation from interval velocity we first assume that the entire sediment does not contain gas hydrate o

landsat imagery based visualization of the geomorphological development at the terminus of a dryland river system abstract in this remote sensing based study we present the analysis of the geomorphological development at the low gradient terminus of the modern rio colorado dryland river system in the endorheic altiplano basin bolivia changes in the river morphology occur after short periods of catastrophic peak discharge which result in the expansion of existing crevasse splays formation of new crevasse splays and in river path avulsion episodic peak discharge events in the study area were pinpointed and quantified by combining daily precipitation records from gauging stations in the vicinity with catchment area analysis from aster global dem gdem remote sensing data a time series of landsat imagery for the period 1975 2001 was then used to analyze the river morphology changes after major peak discharge events compensational stacking of crevasse splays in combination with river avulsion produced a thin but aerially extensive connected sand sheet at the terminus of the fluvial system

estimation of gas hydrate concentration from multi component seismic data at sites on the continental margins of nw svalbard and the storegga region of norway high resolution seismic experiments employing arrays of closely spaced four component ocean bottom seismic recorders were conducted at a site off western svalbard and a site on the northern margin of the storegga slide off norway to investigate how well seismic data can be used to determine the concentration of methane hydrate beneath the seabed data from p waves and from s waves generated by p s conversion on reflection were inverted for p and s wave velocity vp and vs using 3d travel time tomography 2d ray tracing inversion and 1d waveform inversion at the nw svalbard site positive vp anomalies above a sea bottom simulating reflector bsr indicate the presence of gas hydrate a zone containing free gas up to 150 m thick lying immediately beneath the bsr is indicated by a large reduction in vp without significant reduction in vs at the storegga site the lateral and vertical variation in vp and vs and the variation in amplitude and polarity of reflectors indicate a heterogeneous distribution of hydrate that is related to a stratigraphically mediated distribution of free gas beneath the bsr derivation of hydrate content from vp and vs was evaluated using different models for how hydrate affects the seismic properties of the sediment host and different approaches for estimating the background velocity of the sediment host the error in the average vp of an interval of 20 m thickness is about 2 5 at 95 confidence and yields a resolution of hydrate concentration of about 3 if hydrate forms a connected framework or about 7 if it is both pore filling and framework forming at nw svalbard in a zone about 90 m thick above the bsr a biot theory based method predicts hydrate concentrations of up to 11 of pore space and an effective medium based method predicts concentrations of up to 6 if hydrate forms a connected framework or 12 if hydrate is both pore filling and framework forming at storegga hydrate concentrations of up to 10 or 20 were predicted depending on the hydrate model in a zone about 120 m thick above a bsr with seismic techniques alone we can only estimate with any confidence the average hydrate content of broad intervals containing more than one layer not only because of the uncertainty in the layer by layer variation in lithology but also because of the negative correlation in the errors of estimation of velocity between adjacent layers in this investigation an interval of about 20 m thickness equivalent to between 2 and 5 layers in the model used for waveform inversion was the smallest within which one could sensibly estimate the hydrate content if lithological layering much thinner than 20 m thickness controls hydrate content then hydrate concentrations within layers could significantly exceed or fall below the average values derived from seismic data