drav model and its application in assessing groundwater vulnerability in arid area a case study of pore phreatic water in tarim basin xinjiang northwest china according to the characteristics of groundwater in arid area this paper proposes drav model for groundwater vulnerability assessment where d is groundwater depth r is the net recharge of aquifer a is the aquifer characteristics and v is the lithology of vadose zone as a case study the paper assesses the vulnerability of pore phreatic water in tarim basin of xinjiang china by using the drav model the results indicate that the areas of phreatic water with vulnerability index ranges of 2 4 4 6 6 8 and 8 accounting for 10 1 80 4 9 2 and 0 2 of the total plain area of the tarim basin respectively and the areas with the latter two vulnerability ranges 6 8 and 8 are mainly located in the irrigation districts with thin soil layer 20 30 cm thick surface soil of vadose zone mainly with underlying sandy gravel and with silty and fine sand layer such vadose zone generally lacks sandy loam and clayey soil and has larger recharge by infiltration of irrigation water

measuring velocity dispersion and attenuation in the exploration seismic frequency band no perfectly elastic medium exists in the earth in an anelastic medium seismic waves are distorted by attenuation and velocity dispersion velocity dispersion depends on the petrophysical properties of reservoir rocks such as porosity fractures fluid mobility and the scale of heterogeneities however velocity dispersion usually is neglected in seismic data processing partly because of the insufficiency of observations in the exploration seismic frequency band 5 through 200 hz the feasibility of determining velocity dispersion in this band is investigated four methods are used in measuring velocity dispersion from uncorrelated vibrator vertical seismic profile vsp data the moving window crosscorrelation mwcc method instantaneous phase method time frequency spectral decomposition method and cross spectrum method the mwcc method is a new method that is satisfactorily robust accurate and efficient in measuring the frequency dependent traveltime in uncorrelated vibrator records the mwc

a new exploration tool quantitative core characterization we will describe a new laboratory system which was designed to be highly automated and portable while maintaining quality driving this design was the recognition of the temporal dependence of physical properties it becomes apparent that some sedimentary rocks particularly shales degrade and disaggregate so completely that mechanical or elastic properties cannot be measured this temporal dependence displays a time scale much shorter than normal weathering but greater than the time for stress relief a system was designed to permit field characterization of freshly recovered core material a benefit of automation and portability is a marked increase in measurement efficiency the attributes of this system permit rapid characterization of a large number of fresh cores in remote frontier exploration areas this feature can significantly reduce prospect evaluation time statistically significant rock property databases can be created in a short period of time

static and dynamic young s moduli of chalk from the north sea we present results from a study of dynamic and static young s moduli of north sea chalk based on laboratory tests on both dry and water saturated chalk we obtained static moduli by using both strain gauge and linear voltage displacement transducer lvdt measurements we investigated the influence of pore fluid on static and dynamic young s moduli and evaluated the two methods for obtaining static young s modulus we obtained good agreement between dynamic and static young s moduli from strain gauge measurements on dry chalk but for water saturated chalk the dynamic young s modulus was larger than the measured static young s modulus this difference may be caused in part by the influence of the difference in frequencies of static and dynamic measurements another reason for the observed difference may be a practical experimental problem that causes the measured static young s modulus for water saturated chalk to be lower than the true modulus when we compared dynamic young s modulus for dry chalk with that for water saturated chalk the dry modulus was larger than the water saturated modulus probably owing to shear weakening of the chalk young s modulus from lvdt measurements does not relate to dynamic young s modulus for dry or water saturated rock because the lvdt is not able to accurately measure the small deformations of the samples during loading at relatively low stresses

comparative precambrian stratigraphy and structure along the mid continent rift the mid continent rift is a geophysically identified tectonic structure that has been traced from kansas northeastward into the lake superior district a related arm has been identified by gravity as extending from eastern lake superior southeastward into the lower peninsula of michigan this rift is of precambrian keweenawan age and began developing approximately 1 1 b y b p for most of its extent this continental class rift system is buried below phanerozoic strata outcrops are found only in east central minnesota northwestern wisconsin and the upper peninsula of michigan the most accepted tectonic model of the mid continent rift is that of a central horst partly covered by clastic rocks bounded by high angle faulting and flanked by basins filled with clastic rocks this model was developed in conjunction with field studies in the southwestern lake superior area and generally has been adopted for minnesota iowa nebraska and kansas a comparable model has yet to be presented for the related arm extending into southern michigan in the lake superior outcrop belt the oldest recognized rifted rocks are layered basalts of middle keweenawan age overlying these basalts is an upper keweenawan thick sedimentary sequence composed of the oronto older and bayfield younger groups these sedimentary rocks are almost entirely clastic with texture ranging from conglomerate to shale mudstone rock colors are principally reds and browns with secondary shades of green and white the oronto group is more immature mineralogically and texturally and more complex structurally than the bayfield group the oronto group also is differentiated by one of its units the nonesuch formation which contains organic shales that bleed liquid hydrocarbons from subsurface fractures and vugs in a michigan copper mine conditions of eposition for both groups are closely allied with an aquatic continental environment specific paleogeography ranged from alluvial fan and plain to deltaic flood plain fluvial and lacustrine southwest and southeast of the lake superior outcrop belt stratigraphic relationships are known principally from fewer than a dozen significant wells drilled into the precambrian within the mid continent rift trend analysis of the proposed upper keweenawan red clastic sedimentary rock column within these wells suggests that the lake superior stratigraphy can be provisionally correlated throughout the mid continent rift trend

estimates of velocity dispersion between seismic and ultrasonic frequencies it is generally accepted that acoustic velocities in fluid saturated rocks vary with frequency evidence comes from experimental measurements and from theoretical causality arguments we have developed a simple analysis technique that gives estimates of total velocity dispersion between zero frequency and any measurement frequency the technique requires compressional p and shear s wave velocity measurements on dry and fully saturated rock assuming that the dry velocities are independent of frequency the biot gassmann equations are used to calculate the zero frequency velocities in the fully saturated rock any difference between the measured velocities and the calculated zero frequency velocities is interpreted as evidence of dispersion application of this analysis technique to a variety c ultrasonic data sets gives consistent results in many rocks dispersion between zero frequency and ultrasonic frequencies is on the order of 10 percent at low effective stress and it decreases to only a few pe

the north american midcontinent rift beneath lake superior from glimpce seismic reflection profiling the midcontinent rift system is a 1 1 b y old structure extending from kansas through the lake superior region and into southern michigan the rift is filled with thick sequences of basaltic volcanic rocks and clastic sediments for most of its extent it is buried beneath paleozoic rocks but can be traced by its strong gravity and magnetic anomalies the rocks of the rift system are exposed only in the lake superior region and comprise the keweenawan supergroup much of the geology of the keweenawan is beneath lake superior and has only been inferred from potential field studies and seismic refraction studies and extrapolation from on shore geology seismic reflection surveys by the great lakes international multidisciplinary program on crustal evolution in 1986 imaged much of the deep structure of the rift beneath the lake in detail the reflection profiles across the rift reveal a deep asymmetrical central graben whose existence and magnitude was not previously documented they show that in addition to crustal sagging documented by previous investigations normal faulting played a major role in subsidence of the axial region of the rift a sequence of volcanic and sedimentary rocks in places greater than 30 km thick fills the graben thinner volcanic and sedimentary units lie on broad flanks of the rift outside of the graben near the axis the prerift crust is thinned to about one fourth of its original thickness apparently by a combination of low angle extensional faulting and ductile stretching or distributed shear the sense of asymmetry of the central graben changes along the trend of the rift documenting the segmented nature of the structure and suggesting the existence of accommodation zones between the segments the location of the accommodation zones is inferred from abrupt disruptions in the bouguer gravity signature of the rift uplift of the central graben occurred when the original graben bounding normal faults were reactivated as high angle reverse faults with throws of 5 km or more in places the midcontinent rift has some striking similarities to some younger passive continental margins we propose that it preserves a record of nearly complete continental separation which had it not been arrested would have created a middle proterozoic ocean basin

collision induced rifting the grenville orogeny and the keweenawan rift of north america abstract we propose that the keweenawan rift formed as a result of convergence related to the synchronous grenville orogeny this proposal is based on 1 1 isotopic ages in the keweenawan rift and the grenville province are similar according to the best u pb and rb sr dates the grenville orogeny existed from 1150 to 1100 ma and keweenawan rifting occurred 1120 1100 ma 2 2 a similar modern orogenic belt and rift relationship in the middle east serves as an actualistic model the red sea gulf of aden rift system has formed as a continuous series of coalesced pull apart basins and is opening parallel to the orogenic front of the zagros and bitlis mountains rifting began in the early miocene while the collision began in the eocene suggesting that rifting is related to the collision 3 3 many other ancient rift collision pairs have a similar age relationship and geometrical configuration based on comparisons of these tectonic regimes we have derived a model whereby these rifts formed as a result of strike slip faults forming in either the foreland or the hinterland of orogenic zones

experimental mechanical compaction of reconstituted shale and mudstone aggregates investigation of petrophysical and acoustic properties of sw barents sea cap rock sequences abstract this study investigates petrophysical and acoustic properties of experimentally compacted reconstituted samples of seal sequences from the southwestern barents sea the aggregates were collected from drill cuttings of mudstone and shale formations of two exploration wells 7220 10 1 salina discovery and 7122 7 3 goliat field the washed and freeze dried samples were characterized for grain size distributions geochemical analyses and mineralogical compositions a total of 25 compaction tests 12 dry and 13 brine saturated were performed with a maximum effective vertical stress of 50 mpa the laboratory measurements demonstrated that petrophysical and acoustic properties of argillaceous sediments can change within a sedimentary basin and even within a given formation the results show that the collected aggregates from goliat field are compacted more compared to salina discovery the maximum and minimum compaction are measured in samples collected from snadd and fuglen formations respectively the final porosity of brine saturated specimens varies between 5 and 22 the ultrasonic velocity measurements depict that samples with the same porosity values can have a broad range of velocity values the resulting compaction trends in this study were compared to published compaction curves for synthetic mixtures of quartz and clay all compaction trends show higher porosity reduction than the silt fraction with 100 quartz comparison of experimental compaction result of each mudstone and shale aggregate with its corresponding acquired well log data helps to delineate the burial history and exhumation in the study area a net exhumation of 950 m and 800 m is estimated at salina and goliat wells respectively the outcomes of this study can provide insights for hydrocarbon prospect discovery in a pre mature sedimentary basin in terms of exploration and production and also for geological co2 storage sites the experimental results may provide information for well log and seismic interpretation basin modeling and seal integrity of investigated horizons

a laboratory study of the dependence of elastic wave velocities on pore scale fluid distribution laboratory data have been collected during a continuous imbibition drainage experiment that show a clear dependence of elastic wave velocities on the details of the pore scale distribution of water and air in a sandstone compressional wave velocity vp was measured at a frequency of 1 mhz shear wave velocity vs was measured at a frequency of 600 khz during the experiment vp showed little variation with the level of water saturation sw during increasing sw through imbibition until sw 0 80 at which point vp increased dramatically when sw was decreased pronounced saturation induced hysteresis was observed in the region 03 0 4 as a simple model we consider the imbibition process as producing a partially saturated state in all pores i e all pores contain both air and water the drainage process in contrast favors the existence of either air filled or water filled pores as elastic wave velocities are very sensitive to the saturation state in the smaller crack like pores these variations in fluid distribution cause related variations in velocities

in situ seismic measurements in claystone at tournemire france compressional and shear wave seismic measurements were performed in an old railway tunnel and in galleries excavated in a 250 m thick toarcian claystone formation in the tournemire experimental station france three component 3c geophones and three orthogonal orientations of the vibroseismic force source were used additionally vertical seismic profiling vsp measurements were recorded with a 3c borehole geophone a hydrophone and a microphone in a 159 m deep borehole id180 in the tunnel the seismic data show that toarcian claystone has strong transverse isotropy ti with a vertical symmetry axis the qp sh and qsv wave propagation velocities in horizontal directions the plane of isotropy of the ti medium are measured as 3550 1850 and 1290 m s 1 respectively the zero offset vsp reveals that only one shear wave propagates in the vertical depth direction and the p and s wave velocities are 3100 and 1375 m s 1 respectively four elastic moduli of the ti medium are determined from the seismic velocities and from the bulk density of 2 53 g cm 3 c11 31 9 gpa c33 24 3 gpa c44 4 5 gpa and c66 8 7 gpa a walkaway vsp with the borehole geophone at 50 m depth in borehole id180 and shot points in the galleries leads to oblique seismic ray paths which allow us to determine the fifth elastic modulus of the ti medium to c13 16 gpa the tube wave recorded by a hydrophone in the water filled lower part of the borehole propagates with 1350 m s 1 which confirms the estimate of the elastic constant c66 the analysis of body wave and surface wave data from a seismic experiment in galerie est shows reflections from several fracture zones in the gallery floor the thickness of the excavation damaged zone edz in the floor of galerie est is estimated to 0 7 m the authors 2014 published by oxford university press on behalf of the royal astronomical society

high vp vs ratio saturated cracks or anisotropy effects 1 we measured vp vs ratios of thermally cracked westerly granite thermally cracked carrara marble and 4 porosity fontainebleau sandstone for an effective mean pressure ranging from 2 to 95 mpa samples were fluidsaturated alternatively with argon gas and water 5 mpa constant pore pressure the experimental results show that at ultrasonic frequencies vp vs ratio of water saturated specimen never exceeded 2 15 even at effective mean pressure as low as 2 mpa or for a lithology for which the poisson s ratio of minerals is as high as 0 3 calcite in order to check these results against theoretical models we examine first a randomly oriented cracked medium with dispersion but without anisotropy and second a medium with horizontally aligned cracks with anisotropy but without dispersion the numerical results show that experimental data agree well with the first model at high frequency vp vs ratios range from 1 6 to 1 8 in the dry case and from 1 6 to 2 2 in the saturated case the second model predicts both vp sv and vp sh to vary from 1 2 to 3 5 depending on the raypath angle relative to the crack fabric in addition perpendicular to the crack fabric a high vp vs ratio is predicted in the absence of shear wave splitting from these results we argue the possibility that high vp vs ratio 2 2 as recently imaged by seismic tomography in subduction zones may come from zones presenting important crack anisotropy the cumulative effects of crack anisotropy and high pore fluid pressure are required to get vp vs ratios above 2 2 citation wang x q a schubnel j fortin e c david y gueguen and h k ge 2012 high vp vs ratio saturated cracks or anisotropy effects geophys res lett 39 l11307 doi 10 1029 2012gl051742

vlda model and its application in assessing phreatic groundwater vulnerability a case study of phreatic groundwater in the plain area of yanji county xinjiang china based on the god method and drastic model for groundwater vulnerability assessment in this paper vlda model for assessing phreatic groundwater vulnerability is proposed where v is lithology of vadose zone l is the pattern of land use d is groundwater depth and a is the aquifer characteristics as a case study the paper assesses the vulnerability of phreatic groundwater in yanji county of xinjiang by using the vlda model as the tool and gis as the platform and matches the vulnerability with nitrate content of the phreatic groundwater in september 2009 the results indicate that areas with highly vulnerable pore phreatic groundwater account for 40 83 of the total inland area of yanji county including most of the northwestern area such as the groundwater wellfield in korla city and yanji town it is also discovered that areas with phreatic groundwater of higher vulnerability essentially coincide with areas with higher nitrate content indicating the feasibility of using vlda model to assess groundwater vulnerability

modelling frequency dependent seismic anisotropy in fluid saturated rock with aligned fractures implication of fracture size estimation from anisotropic measurements measurements of seismic anisotropy in fractured rock are used at present to deduce information about the fracture orientation and the spatial distribution of fracture intensity analysis of the data is based upon equivalent medium theories that describe the elastic response of a rock containing cracks or fractures in the long wavelength limit conventional models assume frequency independence and cannot distinguish between microcracks and macrofractures the latter however control the fluid flow in many subsurface reservoirs therefore the fracture size is essential information for reservoir engineers in this study we apply a new equivalent medium theory that models frequency dependent anisotropy and is sensitive to the length scale of fractures the model considers velocity dispersion and attenuation due to a squirt flow mechanism at two different scales the grain scale microcracks and equant matrix porosity and formation scale fractures the theory is first tested and calibrated against published laboratory data then we present the analysis and modelling of frequency dependent shear wave splitting in multicomponent vsp data from a tight gas reservoir we invert for fracture density and fracture size from the frequency dependence of the time delay between split shear waves the derived fracture length matches independent observations from borehole data

geologic and geochemical evidence for the nature and development of the middle proterozoic keweenawan midcontinent rift of north america abstract general models of active continental rifting must take into account the large plateau basalt provinces many of which have been precursors of or associated with continental breakup the keweenawan middle proterozoic midcontinent rift mcr of north america 2300 km long is one such major continental rift but it aborted before significant crustal separation was achieved these plateau basalt rifts including the mcr differ from the east african type in being dominated by broad subsidence rather than rift valleys tholeiitic flood basalts rather than alkalic central volcanoes and large positive gravity anomalies geologic and geophysical evidence is reviewed which contradicts some recent models which have been proposed for the development of the mcr the basic causes of the pre rift subsidence the u shaped nature of the structure and the abrupt termination of its activity are obscure a triple junction in the usual sense does not appear to have been formed

sensitivity of flow and elastic properties to fabric heterogeneity in carbonates abstract carbonate rocks are heterogeneous at various levels from deposition to diagenesis any existing depositional heterogeneity becomes more complex when carbonate rocks are in contact with polar fluids our experiments on carbonate rocks show that change in textural heterogeneity leads to heterogeneity in the distribution of storage and flow properties that may govern changes in saturation patterns this would be akin to any carbonate reservoir with a mix of heterogeneous and homogeneous facies within a formation and their control on saturation distribution in response to a standard imbibition process associated with the saturation pattern heterogeneities the resultant elastic property distributions also change we quantify this heterogeneity and its effects on flow and seismic properties based on a few textural extremes of fabric heterogeneity in samples that can exist in any typical carbonate reservoir system our measurements show that textural heterogeneity can lead to anisotropy in permeability and in acoustic velocities permeability anisotropy measurements varied between 40 and 100 while acoustic velocity anisotropy measurements varied between 8 and 30 with lower values for homogeneous samples respectively under similar conditions of the saturation experiment spontaneous imbibition at the benchtop and undrained pressure imbibition at 1000 psi the imbibing brine replaced 97 of the pore volume in a homogeneous sample porosity 20 and permeability 2 6 md compared to 80 pore volume in a heterogeneous sample porosity 29 and permeability 23 4 md furthermore after pressure saturation a change of 79 in the bulk modulus and 11 in the shear modulus is observed for homogeneous samples in comparison to 34 in the bulk modulus and 1 in the shear modulus for heterogeneous samples with respect to the dry state moduli values of the samples we also examined the uncertainties associated with gassmann models of elastic properties due to variations in fluid saturations our results provide significant information on the saturation and with it modulus variations that are often ignored during fluid substitution modelling in time lapse seismic studies in carbonate reservoirs we show that the bulk modulus could vary by 45 and the shear modulus by 10 between homogeneous and heterogeneous patches of a reservoir after a water flooding sequence for secondary recovery our findings demonstrate the need to incorporate and couple such fabric controlled saturation heterogeneities in reservoir simulation and in seismic fluid substitution models

prediction of elastic wave velocities in sandstones using structural models elastic wave propagation in fluid saturated sandstones depends upon two sets of rock features 1 the volume fractions and elastic constants of the rock constituents quartz clay water etc and 2 microstructural geometry grain contacts pore aspect ratios while the former data are usually obtainable the latter are relatively inaccessible we present a new method for determining microstructural data using idealized but physically representative models of sandstone the key to the method is the simulation of certain depositional and diagenetic processes in a manner that completely specifies the geometry of the resulting models hence the geometric features of the grain space and void space required for various theories of elastic propagation can be calculated directly from the models we find good agreement between predictions and measurements of compressional and shear wave velocities in both clean and clay bearing saturated sandstones in contrast with previous efforts at predicting velocitie

attenuation behaviour of tuffaceous sandstone and granite during microfracturing summary laboratory measurements of ultrasonic wave propagation in tuvaceous sandstone kimachi japan and granite iidate japan were performed during increasing fracturing of the samples the fracturing was achieved by unconfined uniaxial compression up to and beyond the point of macrofracture of the specimen using a constant low strain rate the observed variation of wave velocity up to 40 per cent due to the development of micro and macrofractures in the rock is interpreted by rock models relating velocity changes to damage and crack density the calculated density of the newly formed cracks reaches higher values for the sandstone than for the granite using the estimated crack densities the attenuation behaviour is interpreted in terms of diverent attenuation mechanisms that is friction and scattering rayleigh scattering as described by the model of hudson 1981 may explain the attenuation qualitatively if the largest plausible crack dimensions are assumed in modelling

acoustic evidence for a broad hydraulically active damage zone surrounding the alpine fault new zealand abstract we present an analysis of the wireline sonic logs acquired through alpine fault hanging wall rocks during the second phase of the deep fault drilling project dfdp 2 due to inaccuracy in the initial tool processed slowness logs a more robust reprocessing of the data using the slowness time coherence method was required the reprocessing of the data yielded slowness logs that provide a significantly better estimate of compressional and shear slowness the average slowness values of compressional and shear waves are 68 s ft and 110 s ft respectively corresponding to velocities of 4 5 km s and 2 8 km s with some excursions related to fractures borehole conditions and tool settings but no systematic downhole trends the log values are compared to laboratory measurements of p and s wave velocity for samples of alpine schist and mylonitic rocks to examine the scale dependence of the measurement the average log velocity is 2 to 18 slower than the laboratory measurements due to the presence of mesoscale fractures that are present in situ but are not sampled at the laboratory scale using effective medium theory we estimate the crack density in the hanging wall host rock to be 0 4 comparison to field scale seismic studies indicates that there is a broad 500 600 m wide outer damage zone surrounding the intensely deformed inner damage zone of the alpine fault this fractured outer zone facilitates fluid flow through the hanging wall and around the alpine fault

modulus decomposition of compressional and shear velocities in sand bodies the advent of borehole shear slowness measurements in sonically slow formations has lead to breakthroughs in the subsurface profiling of geological bodies in sand bodies compressional and shear velocities depend predictably on porosity mineralogy grain contacts and fluid saturation an interpretation is best performed by decomposing the velocities into moduli that are intrinsic measures of the rock frame and pore fluid compressibilities careful experiments on pure materials i e pure quartz sandstones demonstrate two simplifying constitutive relationships first the bulk and shear frame moduli are simple functions of the porosity a comparison of the measured shear frame modulus to the prediction for the pure material distinguishes sand from shale second the ratio of the bulk and shear frame moduli is a constant 0 9 independent of the porosity the measured velocities are directly inverted to yield the bulk modulus of the pore fluid the fluid saturation effects are so dramatic at high porosit

variation of groundwater hydrochemical characteristics in the plain area of the tarim basin xinjiang region china groundwater hydrochemistry could reveal the interaction mechanism between groundwater and the environment which provides a scientific basis for environmental resources management in this study shukaliefu s classification method and piper diagram were adopted to determine the hydrochemical types of groundwater in the tarim basin of xinjiang china the method of one vote veto was applied to evaluate the quality of groundwater phreeqc software was used to calculate the saturation indices of calcite and fluorite in groundwater by comparing groundwater quality data of 2003 and 2011 we characterized the variations in hydrochemical types and water quality types salinization of groundwater and fluoride geochemistry of the plain area of the tarim basin results show that the primary anion in phreatic water in the plain area of the tarim basin changed from hco3 to so4 2 or cl on the contrary the primary anion in confined water changed from so4 2 or cl to hco3 in 2003 63 1 of the sampling points in the study area exceeded the class iii water quality standard of china in 2011 the proportion increased to 82 5 in addition severe groundwater salinization was found at 19 7 of the sampling points some of the deep groundwater samples were salinized as well in the aksu area at the north west part of the tarim basin f concentration exceeding the standard limit 1 mg l was found to be 55 0 of the groundwater samples tested based on these findings it is concluded that the phreatic water in the study area was severely influenced by the industrial wastewater and domestic sewage related to human activities while the confined water was less affected the general quality of groundwater was in an aggravation trend and the groundwater salinization was in a severe condition in this area the ca2 na ionic exchange the unsaturated fluorite and oversaturated calcite in the aquifer of the aksu area are proposed to cause f enrichment in groundwater of this area

changes in dynamic shear moduli of carbonate rocks with fluid substitution to assess saturation effects on acoustic properties in carbonates we measure ultrasonic velocity on 38 limestone samples whose porosity ranges from 5 to 30 under dry and water saturated conditions complete saturation of the pore space with water causes an increase and decrease in compressional and shear wave velocity as well as significant changes in the shear moduli compressional velocities of most water saturated samples are up to 500 m s higher than the velocities of the dry samples some show no change and a few even show a decrease in velocity shear wave velocity vs generally decreases but nine samples show an increase of up to 230 m s water saturation decreases the shear modulus by up to 2 gpa in some samples and increases it by up to 3 gpa in others the average increase in the shear modulus with water saturation is 1 23 gpa the average decrease is 0 75gpa the vp vs ratio shows an overall increase with water saturation in particular rocks displaying shear weakening have disti

seismic wave attenuation and modulus dispersion in sandstones abstractwe have conducted laboratory experiments over the 1 200 hz band to examine the effects of viscosity and permeability on modulus dispersion and attenuation in sandstones and also to examine the effects of partial gas or oil saturation on velocities and attenuations our results have indicated that bulk modulus values with low viscosity fluids are close to the values predicted using gassmann s first equation but with increasing frequency and viscosity the bulk and shear moduli progressively deviate from the values predicted by gassmann s equations the shear moduli increase up to 1 gpa or approximately 10 with high viscosity fluids the p and s wave attenuations qp 1 and qs 1 and modulus dispersion with different fluids are indicative of stress relaxations that to the first order are scaling with frequency times viscosity by fitting cole cole distributions to the scaled modulus and attenuation data we have found that there are similar p wave shear and bulk relaxations and attenuation pe

consortium for continental reflection profiling michigan surveys reprocessing and results reprocessing of consortium for continental reflection profiling data from central michigan with emphasis on deconvolution and velocity filtering before stack and more detailed velocity analysis has resulted in greatly improved imaging of a previously reported keweenawan rift buried beneath the paleozoic of the michigan basin the seismic stratigraphy of the sub paleozoic units can now be related to the basin filling middle keweenawan volcanic sequence and the upper keweenawan clastic assemblage in greater detail important structural and stratigraphic features can be seen for the first time for example reprocessing has imaged an unconformity within the upper keweenawan it is possible some of these newly imaged features are associated with a late keweenawan compressive episode furthermore reprocessing also reveals previously unrecognized sub keweenawan structures including a deeply penetrating reflection which may represent a major crustal fault and possible weak reflections from midcrustal 25 km and moho 43 km depths

crustal structure of the midcontinent rift system results from glimpce deep seismic reflection profiles interpretation of great lakes international multidisciplinary program on crustal evolution glimpce seismic reflection profiles indicates that the midcontinent keweenawan 1100 ma rift system of volcanic rocks and postvolcanic and interbedded sedimentary rocks extends to depths as great as 32 km about 10 5 s reflection time along profiles crossing western central and eastern lake superior and the northern end of lake michigan this area may overlie the greatest thickness of intracratonic rift deposits on earth times to moho reflections vary along strike from 11 5 to 14 s about 37 46 km depth in the west to 17 s about 55 km in the center and 13 to 15 s about 42 49 km in the eastern end of lake superior the prerift crust however was thinned 25 30 km beneath the central rift compared with its flanks providing evidence for crustal extension by factors of about 3 4 the midcontinent rift system differs from phanerozoic rifts in having total crustal thicknesses equal to or greater than the surrounding presumably unextended regions

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

shear wave velocity estimation in porous rocks theoretical formulation preliminary verification and applications shear wave velocity logs are useful for various seismic interpretation applications including bright spot analyses amplitude versus offset analyses and multicomponent seismic interpretations measured shear wave velocity logs are however often unavailable we developed a general method to predict shear wave velocity in porous rocks if reliable compressional wave velocity lithology porosity and water saturation data are available the precision and accuracy of shear wave velocity prediction are 9 and 3 respectively the success of our method depends on 1 robust relationships between compressional and shear wave velocities for water saturated pure porous lithologies 2 nearly linear mixing laws for solid rock constituents 3 first order applicability of the biot gassmann theory to real rocks we verified these concepts with laboratory measurements and full waveform sonic logs shear wave velocities estimated by our method can improve formation evaluation our method has been successfully tested with data from several locations

detachment faulting and the evolution of passive continental margins major detachment faults play a key role in the lithospheric extension process in the basin and range province and may also be important in other continental extension terranes such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift subsidence patterns detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins we therefore suggest the existence of upper plate and lower plate passive margins these give rise to a complementary asymmetry of opposing margins after continental breakup transfer faults offset marginal features and allow margins to switch from upper plate to lower plate characteristics along strike

mantle heterogeneities and their significance results from lithoprobe seismic reflection and refraction wide angle reflection studiesthis article is one of a series of papers published in this special issue on the theme lithoprobe parameters processes and the evolution of a continent lithoprobe contribution 1486 within lithoprobe s 10 transects data from more than 20 000 km of multichannel seismic mcs reflection profiling and 12 refraction wide angle reflection r war surveys were acquired while the

a comparison of theoretical rock models with laboratory data abstract a number of rock modelling theories based on single crystal data were tested in order to assess their effectiveness in modelling actual laboratory rock data these theories included the voigt reuss and hashin shtrikman bounds for isotropic single phase pore free aggregates and composites and the self consistent inclusion theories of hashin shtrikman wu and walsh and thomsen s biot consistent approach for porous materials the results for very low porosity rocks effectively pore free indicate that models based on hashin shtrikman theory yield narrow bounds which accurately predict mean laboratory p and s wave velocities however meaningful comparisons require careful and accurate rock characterization in terms of mineral composition and texture the theories for porous rock models yielded s wave velocities which were not as consistent as their counterpart p wave velocities in matching the laboratory data moreover the discrepancies increase markedly as rock porosity increases this result reflects primarily differences in internal microcrack structure and composition between that assumed in the theoretical modelling procedures and that present in the actual rock samples saturated and dry rocks containing cracks were most successfully modelled by the biot consistent theory proposed recently by thomsen 1985

stress associated intrinsic and scattering attenuation from laboratory ultrasonic measurements on shales seismic attenuation is sensitive to stress induced subtle changes in the physical state of rocks in this study the stress and frequency associated attenuation is quantified through ultrasonic measurements on three differently oriented cylindrical shale samples under various axial stresses as an improvement to the single scattering model the elastic monte carlo method is employed to investigate multiple scattering attenuations by incorporating the boundary reflections and wave conversions our results show that as the axial stress increases the intrinsic attenuation decreases in all directions while the scattering attenuation decreases slightly in the direction perpendicular to the bedding but increases largely and nonlinearly in other directions these discrepancies result from different attenuation mechanisms both the intrinsic and scattering attenuation are found to be largest in the direction 45 to the bedding but least in the perpendicular direction the s wave attenuation is larger and more sensitive to stress changes than p wave attenuation due to its shorter wavelength as expected from sandstone examples the scattering attenuation in shales is significantly larger and more sensitive to stress changes than the intrinsic attenuation the frequency dependence of scattering attenuation suggests that the peak frequency with the maximum scattering attenuation is independent of axial stresses but varies in different directions of an individual rock with different heterogeneity and anisotropy scales the peak frequency of s coda is smaller and its peak scattering attenuation is larger than p coda in conclusion the stress and frequency dependence of ultrasonic attenuations in shales differ largely in various directions indicating significant anisotropy and heterogeneity

ultrasonic properties of sedimentary rocks effect of pressure saturation frequency and microcracks seismic velocities and attenuations are influenced by lithology porosity and permeability as well as the kind and quantity of the pore fluids the microstructure of rocks is another important factor influencing seismic properties this influence can be used to investigate the pressure dependent closure of microcracks for this purpose velocities and attenuations of p and s waves have been determined at ultrasonic frequencies in three different rocks vacuum dry and partially saturated with water as a function of hydrostatic pressure up to 200 mpa a new combined model has been developed with this model an explicit description of the microstructure local fluid flow lf and macrostructure gassmann effect global fluid flow is possible assuming a patchy saturation i e inhomogeneous distribution of the fluid in the pores and modulus reduction both the saturation dependent measurements and the pressure dependent data can be explained the modelling and the phenomenological interpretation of the data gained under increasing hydrostatic pressure yields consistent results concerning the amount of crack closure as well as the range in which crack closure appears in high porosity sandstones bentheimer and obernkirchner sandstone a strong closure of cracks already with a small increase of pressure must be concluded in a low porosity harzer greywacke a significantly lower closure of cracks can be concluded within the pressure range of 0 100 mpa it is shown however that there must be open cracks in all rocks even under elevated hydrostatic pressure resulting in a measurable effect due to lf

mechanisms affecting ultrasonic wave propagation in fluid containing sandstones under high hydrostatic pressure the elastic and anelastic parameters of sandstones at ultrasonic frequencies under low uniaxial pressure are strongly influenced by the fluid content the two main mechanisms controlling the rock behavior are based on 1 local fluid flow and 2 a grain surface effect which results in a modulus reduction both mechanisms depend fundamentally on the microstructure the critical factor controlling seismic wave velocity and attenuation in rocks under low pressure ultrasonic measurements have been performed on a fluid containing sandstone under hydrostatic pressure up to 200 mpa to investigate the mechanisms affecting rock behavior under in situ pressure conditions at high hydrostatic pressure the measured p wave velocities indicate a change in the rock microstructure and in the influence of the fluid by means of two models the effects of the mechanisms of local fluid flow murphy et al 1986 and global fluid flow biot 1956a b were calculated and the loss mechanisms were compared in their capability of explaining the experimental data the experimental and theoretical results show that for the investigated fluid containing sandstone both the local fluid flow mechanism and in the case of polar fluids the modulus reduction are still effective at high external pressure

dispersion and anisotropy of elastic waves in cracked rocks 1 rocks in the earth s crust contain variable amount of cracks depending on the deviatoric and confining stress levels pore pressure and temperature conditions crack damage results in effects that have been investigated for a long time in particular a decrease in elastic wave velocity and the development of anisotropy in this paper we focus on cracked rocks and develop a method to calculate both the elastic wave anisotropy and the dispersion in a fluid saturated cracked rock we show that analytic expressions can be derived for both properties in terms of the crack density tensor components two fundamental theoretical tools are used in this purpose the first one is the effective medium theory and the second one is the anisotropic poroelastic theory using both it is possible from first principles to predict anisotropy and dispersion results are shown to be valid up to a total crack density of 0 5 the failure threshold appears close to a total crack density of 1 between 0 5 and 1 the model still gives reasonably good results anisotropy can increase up to 60 and dispersion up to 30 depending on the crack distribution these results point out important differences that could exist in fluid saturated rocks between laboratory high frequency data and field low frequency results

convective partial melting 1 a model for the formation of thick basaltic sequences during the initiation of spreading stretching and thinning of continental lithosphere during rifting and initiation of seafloor spreading cause upwelling of mantle rocks into a pressure temperature environment where partial melting occurs the amount of melt liberated is however insufficient to account for the thickness of igneous crust emplaced near the continent ocean boundary at some passive margins these igneous structures are known from seismic reflection and deep refraction studies on numerous passive margins worldwide seismic studies also suggest that the transition from continental to oceanic crust on these volcanic margins is relatively abrupt their evolution apparently does not include a prolonged extensional phase preceding the inception of spreading to account for these observations we propose a model for the generation of partial melt in which the lateral temperature contrasts evoked by rifting and asthenospheric upwelling drive convection in the upper mantle deep hot mantle material is transported upward by the convection and hence much larger volumes flow through the region where partial melting occurs than during passive upwelling numerical calculations show that convective partial melting can provide substantial additions to melting due to passive upwelling alone using this model we propose an evolutionary scheme for volcanic passive margins emphasize the contrast with nonvolcanic margins and describe conditions which might lead to the development of both types

paleomagnetism and upb geochronology of volcanic rocks from michipicoten island lake superior canada precise calibration of the keweenawan polar wander track abstract paleomagnetic poles have been determined for normally magnetized samples of late proterozoic volcanic rocks from 23 sites on michipicoten island lake superior canada the results are in general accord with an earlier less precise study by one of the authors and indicate a small polar shift between rocks of the mamainse point formation 186 e 38 n and the michipicoten island formation 175 e 26 n a quartz feldspar porphyry intruding the lower mamainse point formation but unconformably overlain by the michipicoten island formation was sampled for u pb zircon dating the zircons define an age of emplacement of 1086 5 1 3 3 0 ma this age and the pole positions are consistent with and supplement ages previously determined for the keweenawan paleomagnetic polar wander curve these data define an apparent polar wander rate of 14 cm a 1 this age combined with previously published u pb ages indicates a time span of 22 ma for most keweenawan igneous activity in addition the time of the keweenawan paleomagnetic reversal is constrained to be between 1086 5 1 3 3 0 and 1097 6 3 6 ma the age of the upper part of the reversely magnetized osler group the zircon population contained an inherited component from a source 1403 50 ma old this suggests that the porphyry magma was at least partly derived by melting of an anorogenic pluton emplaced during the mid proterozoic

seismic monitoring of fluid fronts an experimental study seismic signatures of time dependent reservoir processes necessary for the interpretation of 4 d seismic data are still insufficiently described this experiment was designed to monitor fluid front movements and saturation changes and to identify the related seismic signatures ultrasonic p and s wave transmission and reflection measurements were used to monitor the waterflooding of a porous sandstone the sandstone was flooded in steps by filling a tank in which the room dry cubic 50 cm side block of rock was placed waterflooding caused the velocity amplitude and frequency of the transmitted waves to diminish significantly however the changes were reversible by drying the maximum reduction of the velocities was 7 and 12 for p and s waves respectively the velocity and amplitude behavior can be explained by the biot gassmann s theory local fluid flow and grain surface effects the correct interpretation of seismic signatures of fluid processes in reservoirs thus involves a knowledge of rock

magmatism at rifted continental margins the intense volcanism and uplift observed on many rifted continental margins forming basaltic seaward dipping reflector sequences is accompanied by the emplacement of a thick igneous section at depth partial melting by decompression of passively upwelling asthenosphere that is hotter than normal because it is near a hotspot explains both the thickness of the new igneous crust and the initial elevation of the rifted margins

laboratory measurements of ultrasonic wave velocities in rocks from the campi flegrei volcanic system and their relation to other field data the campi flegrei caldera located near the city of naples italy has shown signs of unrest characterized by large ground deformation seismic activity and changes in the hydrothermal system since 1970 any attempt to model the processes giving rise to these phenomena requires some knowledge of the subsurface structure and of the physical properties of the rocks at depth in this study we report the results of laboratory measurements of ultrasonic compressional and shear wave velocities in rock samples cored in 4 different geothermal wells reaching a maximun depth of 3 km the wells are located in the san vito sv and mofete mf areas at about 3 km north and 4 km west respectively from the center of the caldera we selected 10 core samples 7 from the sv wells and 3 from the mf wells the attention being focused on the most important rock formations underneath campi flegrei and on the lower sections of the wells where data had been lacking the measurements involved the transmission of ultrasonic waves along three orthogonal directions through dry and water saturated rocks at room conditions the samples are only moderately anisotropic 13 for one sv sample and 8 for all the other samples wave velocities generally increase with the depth of the sample the range of velocities is 3 5 4 km s 1 for p waves and 1 5 2 9 km s 1 for s waves for the sv samples water saturation generally implies slightly larger 10 vp values and smaller 20 vs values as compared with dry velocities for the mf samples water saturation implies both higher vp and vs velocities up to 35 and 18 respectively these results may be explained by the different type of porosity of the sv samples vesicular and the mf samples fissurai the ultrasonic measurements on water saturated samples are slightly higher than the available in situ vp sonic measurements 25 for one sv sample and 15 for all the other samples these differences can be explained by velocity dispersion between ultrasonic and sonic frequencies and by the different pressure temperature conditions existing at depth

waveform inversion of seismic velocities and attenuation from low frequency waves in cylindrical bars a new technique for laboratory measurement of seismic wave velocities and attenuation in the frequency range of 10 150 khz consists of measuring extensional waveforms using two cylindrical bars of the same material but unequal length based on the dispersion equation of the bar and rough estimates of compressional and shear velocities of the bar material the waveform measured within the shorter bar is theoretically continued to the length of the longer bar to match with the waveform measured there an inversion is then performed to minimize the phase difference between the two waveforms the velocities are obtained when the phase difference reaches a minimum at which the two waveforms attain the optimum phase match after the phase match a further inversion is performed to minimize the amplitude difference between the two waveforms to derive the extensional wave attenuation within the bar by this inversion procedure wave velocities and attenuation can be jointly determined at frequencies much lower t

low frequency seismic properties of thermally cracked and argon saturated granite torsional forced oscillation techniques have been used to measure the shear modulus and strain energy dissipation on cylindrical specimens of a fine grained granite delegate aplite the specimens were subjected to thermal cycling and associated microcracking under varying conditions of confining pressure pc and argon pore fluid pressure pf within the low frequency saturated isobaric regime complementary transient flow studies of in situ permeability and volumetric measurements of connected crack porosity allowed the modulus measurements to be interpreted in terms of the density and interconnectivity of the thermally generated cracks the modulus measurements indicate that newly generated thermal cracks are closed by a differential pressure pc pf which ranges from 120 to 160 mpa for temperatures of 300 600 c this suggests crack aspect ratios on the order of 10 3 the covariation of in situ permeability k and thermal crack density e that we infer from the modulus deficit is consistent with percol

investigation of core data reliability to support time lapse interpretation in campos basin brazil quantitative interpretation of time lapse seismic data requires knowledge of saturation and pressure effects on seismic velocities although the former can usually be modeled adequately using the gassmann equation the latter is obtained mainly by laboratory measurements which can be affected by core damage we investigate the magnitude of this effect on compressional wave velocities by comparing laboratory experiments and log measurements we use gassmann fluid substitution to obtain low frequency saturated velocities from dry core measurements thus mitigating the dispersion effects taken at reservoir pressure the analysis is performed for an unusual densely cored well from which 43 cores were extracted over a 45 m thick turbidite reservoir these computed velocities show very good agreement with the sonic log measurements this confirms that for this particular region the effect of core damage on ultrasonic measurements is less than the measurement error consequently stress sensitivity of elast

structure of the southern keweenawan rift from cocorp surveys across the midcontinent geophysical anomaly in northeastern kansas this is the published version copyright 1984 american geophysical union all rights reserved

rift wide correlation of 1 1 ga midcontinent rift system basalts implications for multiple mantle sources during rift development magmatism that accompanied the 1 1 ga midcontinent rift system mrs is attributed to the upwelling and decompression melting of a mantle plume beneath north america five distinctive flood basalt compositions are recognized in the rift related basalt succession along the south shore of western lake superior based on stratigraphically correlated major element trace element and nd isotopic analyses these distinctive compositions can be correlated with equivalent basalt types in comparable stratigraphic positions in other mrs localities around western lake superior four of these compositions are also recognized at mamainse point more than 200 km away in eastern lake superior these regionally correlative basalt compositions provide the basis for determining the sequential contribution of various mantle sources to flood basalt magmatism during rift development extending a model originally developed for eastern lake superior in this refined model the earliest basalts were derived from small degrees of partial melting at great depth of an enriched ocean island type plume mantle source e value of about 0 followed by magmas representing melts from this plume source and interaction with another mantle source most likely continental lithospheric mantle e 0 the

the effect of systematic diagenetic changes on the mechanical behavior of a quartz cemented sandstone abstracta key goal of petrophysical studies of sandstones is to relate common field measurements particularly seismic or sonic velocities to parameters defining the rock s mechanical and hydrologic characteristics these include elastic and inelastic mechanical properties porosity and permeability we explored relationships among these properties in variably quartz cemented mature arenites of the st peter sandstone with porosities ranging from 9 to 25 in a previous paper we described microstructural changes accompanying progressive quartz cementation and related porosity and permeability reduction in this sample suite here we report ultrasonic velocities ultrasonic velocities dynamic and static elastic properties confined compressive strength and tensile strength analyses of these data demonstrated that factors controlling permeability also fundamentally determined the elastic and inelastic mechanical properties we found that the number of grain contacts or bonds per number of grains vi

rift structure beneath the michigan basin from cocorp profiling cocorp consortium for continental reflection profiling surveys across the mid michigan gravity anomaly near the mcclure sparks no 1 deep well delineate a thick layered sequence of relatively strong reflectors that define a structural trough within the precambrian beneath the michigan basin these deep reflections which correlate spatially with the mid michigan gravity and magnetic high are interpreted to be a pile of basaltic extrusive rocks intercalated with clastic rocks and correlative with keweenawan volcanics exposed in the lake superior region to the northwest a reflection poor zone between the base of the paleozoic as found in the deep well and the top of the deep layered sequence probably corresponds to the upper keweenawan clastic assemblage direct evidence of faulting within the inferred volcanic sequence is lacking although sharp flexures structural benches and dips of as much as 10 suggest subsidence controlled by fault block motions reflections from beneath the inferred volcanic pile are rare the deepest clear event being at about 18 km there is no unequivocal evidence on these sections to indicate whether precambrian structures were reactivated to affect the overlying paleozoic strata of the michigan basin proper although data quality may be insufficient to detect such effects

distribution of groundwater arsenic in xinjiang p r china although potential contamination of groundwater as is expected to occur in xinjiang p r china few data are available for the regional distribution of groundwater as in this study the spatial distribution of groundwater as was investigated in the chepaizi cpz n and shihezi shz n areas of northern xinjiang the balikun yiwu basin by e in eastern xinjiang and the tarim trm s and yanqi yq s basins in southern xinjiang arsenic concentrations greater than 10 g l were found in 12 of analyzed groundwaters all groundwater samples collected in cpz n had as concentrations greater than 10 g l 25 185 g l 30 in shz n 10 g l was found in the eastern and southern trm s and yq s distribution of groundwater as showed a tremendous spatial variability which greatly varied over a short distance horizontally arsenic concentration generally increased with increasing sampling depth the spatial distribution of groundwater as would be regulated by as source and hydrogeochemical processes higher ph and or lower orp values were generally observed in high as groundwater 10 g l in comparison with low as groundwater 10 g l arsenic mobility in shz n and cpz n may result from as desorption under relatively high ph conditions and more tentatively from reductive dissolution of fe iii oxides in by e and trm s however detailed mechanisms of as mobilization in these regions need further investigation

dispersion of elastic moduli in a porous cracked rock theoretical predictions for squirt flow abstract crustal rocks contain variable amount of both cracks and equant pores depending on tectonic and thermal stresses but also on their geological origin crack damage and porosity change result in effects on elastic waves velocities when rocks are fluid saturated dispersion of the p and s waves should be taken into account this paper deals with frequency dispersion of elastic moduli in a fluid saturated porous and cracked rock with the assumption that squirt flow is the dominant process we develop a theoretical approach to calculate both high hf and low lf frequency bulk and shear moduli the hf moduli are derived from a new effective medium model called cpem with an isotropic distribution of pores or cracks with idealized geometry respectively spheres and ellipsoids lf moduli are obtained by taking hf dry moduli from the cpem and substituting into gassmann s equations in the case of a porosity only supported by equant pores the calculated dispersion in elastic moduli is equal to zero in the case of a crack porosity no bulk dispersion is predicted but a shear dispersion appears finally in the general case of a mixed porosity pores and cracks dispersion in bulk and in shear is predicted our results show that the maximum dispersion is predicted for a mixture of pores and spheroidal cracks with a very small aspect ratio 10 3 our theoretical predictions are compared to experimental data obtained during hydrostatic experiment performed on a basaltic rock and a good agreement is observed we also used our theoretical model to predict elastic waves velocities and vp vs ratio dispersion we show that the p waves dispersion can reach almost 20 and the vp vs dispersion a maximum value of 9 for a crack porosity of about 1 since laboratory data are ultrasonic measurements and field data are obtained at much lower frequencies these results are useful for geophysicists to interpret seismic data in terms of fluid and rock interactions

transmission of seismic waves across single natural fractures fractures and other nonwelded contacts are important mechanical and hydrological features of rock masses their effects on seismic wave propagation can be modeled as a boundary condition in the seismic wave equation seismic stress is continuous across such a boundary but seismic particle displacement and seismic particle velocity are not the complete solutions for seismic wave reflection conversion and transmission across a displacement and velocity discontinuity between two half spaces with different densities and elastic properties are derived for all angles of the incident wave the ratio between the seismic stress across this boundary and the seismic particle displacement and velocity are described by a specific stiffness and a specific viscosity respectively a displacement discontinuity results in frequency dependent reflection and transmission coefficients and a frequency dependent group time delay the velocity discontinuity results in frequency independent coefficients and zero delay results of laboratory experiments on compressional and shear wave transmission across three different natural fractures in a quartz monzonite are described measurements were made at different effective stresses under dry and saturated conditions at room temperature it is shown that the effect of these fractures on the spectral amplitudes for compressional and shear pulses transmitted across these fractures are described well by a displacement discontinuity for compressional pulses under dry and saturated conditions and by a combined displacement and velocity discontinuity for shear wave pulses under dry and saturated conditions values of specific stiffness and specific viscosity vary between fractures and increase with increasing effective stress as does the static specific stiffness of these fractures changes in the spectral amplitudes of transmitted pulses are also analyzed in terms of attenuation using the seismic quality factor q which is found to be a function of frequency

empirical estimation of viscoelastic seismic parameters from petrophysical properties of sandstone viscoelastic seismic parameters are expressions of underlying petrophysical properties theoretical and empirically derived petrophysical seismic relations exist but each is limited in the number and the range of values of the variables used to provide a more comprehensive empirical model we combined lab measurements from 18 published data sets and well log data for sandstone samples and determined least squares coefficients across them all the dependent variables are the seismic parameters of bulk density compressional and shear wave velocities vp and vs and compressional and shear wave quality factors qp and qs the independent variables are effective pressure porosity clay content water saturation permeability and frequency as the derived expressions are empirical correlations no causal relations should be inferred prediction of is based on volumetric mixing of the constituents for vp and vs predictions separate sets of coefficients are fitted for three water saturation condi

the midcontinent rift system one of the most prominent features on gravity and aeromagnetic maps of the united states is a series of major generally linear anomalies that extend from central kansas to lake superior and then turn southward into central michigan figures 1 2 this system of anomalies clearly reflects major geological features and as such it must represent a very significant episode in the history of the north american continental lithosphere the geologic features associated with these anomalies particularly axial basins filled with basalt and immature clastic rocks along with evidence of crustal extension indicate that the episode was probably one of incipient rifting