empirical relationships among seismic velocity effective pressure porosity and clay content in sandstone we use a multivariate analysis to investigate the influence of effective pressure pe porosity and clay content c on the compressional velocity vp and shear velocity vs of sandstones laboratory measurements on water saturated samples of 64 different sandstones provide a large data set that was analyzed statistically for each sample relationships between effective pressure and vp and vs have been determined all samples were well fit by relationships that have an exponential increase in velocity at low pe tapering to a linear increase with pe for pe greater than 0 2 kbar there are differences in the pressure dependences of velocity for different rocks particularly at very low pressures however the differences cannot be attributed to or c for the combined set of measurements from all samples the best fitting formulations are vp 5 77 6 94 1 73c 0 446 pe e 16 7pe and vs 3 70 4 94 1 57c 0 361 pe e 16 7pe while this is admittedly a very simplified parameterization it is remarkable how well

creep of simulated reservoir sands and coupled chemical mechanical effects of co2 injection 1 geological storage of co2 in clastic reservoirs and aquifers is expected to have a variety of coupled chemical mechanical effects to investigate the effects of co2 injection on creep phenomena we performed uniaxial compaction experiments on granular aggregates of quartz and feldspar under both wet and dry control conditions the experiments were performed in constant stress mode grain size temperature co2 partial pressure and effective stress were varied in order to determine their individual effect pore fluid ph was varied by the injection of co2 and by addition of acidic and alkaline additives pore fluid salinity was increased by the addition of nacl wet samples showed instantaneous compaction upon load application followed by time dependent creep from the mechanical data and microstructures the main compaction mechanism was inferred to be chemically enhanced microcracking in both quartz and feldspar with subcritical crack growth i e stress corrosion cracking controlling deformation in the creep stage the injection of co2 and the concomitant acidification of the pore fluid inhibited microcracking in both the quartz and feldspar samples in line with known effects of ph on stress corrosion cracking we infer that the injection of co2 into quartz and plagioclase bearing sandstones will inhibit grain scale microcracking process and that related geomechanical effects such as reservoir compaction and surface subsidence will be negligible compared with the poroelastic response

laboratory measurements of elastic properties of carbonate rocks during injection of reactive co2 saturated water 1 to investigate the coupled physical and chemical effects of injecting co2 rich water into carbonate rock samples we monitor the elastic and transport properties of six carbonate rocks along with the calcium content and ph of the pore fluid under constant confining pressure carbonate samples range from calcite limestones containing dolomite to pure calcite mudstones which allow us to study how mineral composition and microstructure affect the magnitude of the observed changes the elastic properties of both the saturated and dry rock i e rock frame show a gradual loss of sstrength upon injection as testified by the continuous decrease in the dry p and s wave velocity and by proxy the bulk and shear moduli the magnitude of the observed changes in the elastic properties varies among the samples with a maximum relative change of 24 7 871 m s and 21 2 443 m s in p and s wave velocities respectively the decrease is also accompanied by a relative increase in permeability up to 495 and porosity up to 19 the observed changes likely derive from a change in microstructure which is monitored over time via scanning electron microscopy the variation in porosity results from two competing and interdependent processes the chemical dissolution of calcite and the resulting mechanical compaction under pressure the results of this study show that upon injection of fluids that are in chemical disequilibrium with the hosting rock the variation of the elastic properties cannot be described by existing rock physics models

physics and seismic modeling for monitoring co2 storage we present a new petro elastical and numerical simulation methodology to compute synthetic seismograms for reservoirs subject to co2 sequestration the petro elastical equations model the seismic properties of reservoir rocks saturated with co2 methane oil and brine the gas properties are obtained from the van der waals equation and we take into account the absorption of gas by oil and brine as a function of the in situ pore pressure and temperature the dry rock bulk and shear moduli can be obtained either by calibration from real data or by using rock physics models based on the hertz mindlin and hashin shtrikman theories mesoscopic attenuation due to fluids effects is quantified by using white s model of patchy saturation and the wet rock velocities are calculated with gassmann equations by using an effective fluid modulus to describe the velocities predicted by white s model the simulations are performed with a poro viscoelastic modeling code based on biot s theory where viscoelasticity is described by generalizing the solid fluid coupling modulus to a relaxation function using the pseudo spectral method which allows general material variability a complete and accurate characterization of the reservoir can be obtained a simulation that considers the utsira sand of the north sea illustrates the methodology

case history using time lapse vertical seismic profiling data to constrain velocity saturation relations the frio brine pilot co2 injection author s al hosni m caspari e pevzner r daley tm gurevich b abstract 2016 european association of geoscientists a engineers co2 sequestration projects benefit from quantitative assessment of saturation distribution and plume extent for field development and leakage prevention in this work we carry out quantitative analysis of time lapse seismic by using rock physics and seismic modelling tools we investigate the suitability of gassmann s equation for a co2 sequestration project with 1600 tons of co2 injected into high porosity brine saturated sandstone we analyze the observed time delays and amplitude changes in a time lapse vertical seismic profile dataset both reflected and transmitted waves are analyzed qualitatively and quantitatively to interpret the changes obtained from the vertical seismic profile we perform a 2 5d elastic finite difference modelling study the results show a p wave velocity reduction of 750 m s in the proximity of the injection well evident by the first arrivals travel time delays and amplitude change and reflected wave amplitude changes these results do not match with our rock physics model using gassmann s equation predictions even when taking uncertainty in co2 saturation and grain properties into account we find that time lapse vertical seismic profile data integrated with other information e g core and well log can be used to constrain the velocity saturation relation and verify the applicability of theoretical models such as gassmann s equation with considerable certainty the study shows that possible nonelastic factors are in play after co2 injection e g co2 brine rock interaction and pressure effect as gassmann s equation underestimated the velocity reduction in comparison with field data for all three sets of time lapse vertical seismic profile attributes our work shows the importance of data integration to validate the applicability of theoretical models such as gassmann s equation for quantitative analysis of time lapse seismic data

co2 percolation experiment through chlorite zeolite rich sandstone pretty hill formation otway basin australia underground co2 sequestration is highly recommended as an effective means of significantly decreasing co2 concentration in the atmosphere mineral storage is the more secure technology but requires the presence of high concentration of divalent cations in the pore fluid results from co2 percolation experiments through chlorite zeolite rich sandstone samples from the pretty hill formation otway basin australia are presented the dissolution of the laumontite 7 wt and chamosite 7 wt are the potential sources of calcium iron and magnesium required for carbonate precipitation the percolation experiment was setup to reproduce at laboratory scale the in situ temperature and pressure conditions t 95 degrees c and p 10 mpa the fluid injected at constant flow rate is a rock equilibrated brine subsequently enriched in co2 up to partial pressure of 6 mpa we observe feldspars laumontite and chamosite dissolution kaolinite and silica precipitation and a noticeable sink of co2 in the sample which is attributed to the precipitation of both amorphous carbon due to the reduction of co2 and fe rich carbonate permeability decreases of about one order of magnitude due to the localization of the kaolinite precipitation in the main flow paths while porosity increases the high reactivity of this sandstone makes this reservoir a valuable target for co2 mineralization but the associated permeability decrease may limit the injection rate and the spreading of the co2 in the reservoir

crosswell imaging in a shallow unconsolidated reservoir it has long been recognized by the oil industry that its future will rely more and more on the recovery of products from existing fields the need for more efficient recovery drives the desire to know more about the reservoir structure and composition crosswell tomography is among the most promising of our geophysical utensils for reservoir characterization as evidenced by its ever increasing application in eor monitoring although it is a relatively new method the literature shows that crosswell tomography works quite well in highly consolidated lithology where good data quality in terms of signal to noise ratio and raypath coverage is achievable the case to be addressed here however is the one that occurs under more hostile conditions our intention is to show the ability of crosswell traveltime tomography in circumstances where data quality is lower than usual

spatiotemporal changes of seismic attenuation caused by injected co2 at the frio ii pilot site dayton tx usa a continuous active source seismic monitoring cassm dataset was collected with crosswell geometry during co2 injection at the frio ii brine pilot near liberty tx previous studies have shown that spatio temporal changes in the p wave first arrival time reveal the movement of the injected co2 plume in the storage zone to further constrain the co2 saturation particularly at higher saturation levels we investigate spatial temporal changes in the seismic attenuation of the first arrivals the attenuation changes over the injection period are estimated by the amount of the centroid frequency shift computed by local time frequency analysis we observe that 1 at receivers above the injection zone seismic attenuation does not change in a physical trend 2 at receivers in the injection zone attenuation sharply increases following injection and peaks at specific points varying with distributed receivers which is consistent with observations from time delays of first arrivals then 3 attenuation decreases over the injection time the attenuation change exhibits a bell shaped pattern during co2 injection under frio ii field reservoir conditions white s patchy saturation model can quantitatively explain both the p wave velocity and attenuation response observed we have combined the velocity and attenuation change data in a cross plot format that is useful for model data comparison and determining patch size our analysis suggests that spatial temporal attenuation change is not only an indicator of the movement and saturation of co2 plumes even at large saturations but also can quantitatively constrain co2 plume saturation when used jointly with seismic velocity

earthquake swarm activity revealed from high resolution relative hypocenters clustering of microearthquakes abstract waveform similarity in an earthquake swarm enables precise arrival time differences of p and s waves through cross spectral methods to be obtained a numerical simulation guarantees the time resolution of the order of 1 ms for the time differences this precise arrival time difference is used to determine relative hypocenters of microearthquakes occurring in a swarm source region with a spatial resolution of the order of 10 m this precise hypocenter determination millisecond hypocenter determination is applied to the data using temporary seismic observation in the ashio region japan where swarm activities are especially high relative hypocenters of 153 events are determined with high spatial resolution it is revealed that the swarm activities consist of several clusters of events a cluster is a group of events concentrated within a small area in a short time interval the cluster activities in one swarm never overlap one another sharing some part of the swarm source region of about 100 m 1 km in length one cluster forms a fault like planar area with a length of about 40 150 m the event cluster swarm activity would be a manifestation of the hierarchy structure of earthquake sources

reservoir rock chemo mechanical alteration quantified by triaxial tests and implications to fracture reactivation abstract co 2 injection into geological formations shifts the geochemical equilibrium between the minerals and resident brine the induced mineral brine co 2 reactions can alter the reservoir rock strength and deformational behavior which subsequently affects co 2 storage mechanical integrity this study attempts to investigate quantitatively the effect of mineral cement and particle dissolution through numerical modeling and validation of triaxial tests performed on unaltered and geologically altered entrada sandstone we utilize a numerical model that couples the discrete element method dem and the bonded particle model bpm to perform simulations of triaxial tests on synthetic rocks that mimic tested rock samples under various confining stresses numerical results in agreement with experimental evidence show that both cement and particle dissolution significantly contribute to rock weakening in sandstone samples with carbonate hematite cements and pore filling carbonate sensitivity analyses show that the brittleness of dem numerical samples is mostly conditioned by the cement bond size among all cement microscopic parameters an alteration path that mimics the mineral dissolution under subsurface boundary conditions leads to 1 vertical compaction and horizontal stress relaxation in the reservoir rock and 2 transfer of stresses to adjacent strata that results in bond breakage and potentially natural fracture reactivation at a large scale

rock physics analysis and time lapse rock imaging of geochemical effects due to the injection of co2 into reservoir rocks the fundamental concept of time lapse seismic monitoring is that changes in physical parameters such as saturation pore fluid pressure temperature and stress affect rock and fluid properties which in turn alter the seismic velocity and density increasingly however time lapse seismic monitoring is called upon to quantify subsurface changes due in part to chemical reactions between injected fluids and the host rocks this study springs from a series of laboratory experiments and high resolution images assessing the changes in microstructure transport and seismic properties of fluid saturated sandstones and carbonates injected with co2 results show that injecting co2 into a brine rock system induces chemo mechanical mechanisms that permanently change the rock frame injecting co2 into brine saturated sandstones induces salt precipitation primarily at grain contacts and within small pore throats in rocks with porosity lower than 10 salt precipitation reduces permeability and increases p and s wa

enhanced deformation of limestone and sandstone in the presence of high fluids 1 geological repositories subject to the injection of large amounts of anthropogenic carbon dioxide will undergo chemical and mechanical instabilities for which there are currently little experimental data this study reports on experiments where low and high 8 mpa aqueous fluids were injected into natural rock samples the experiments were performed in flow through triaxial cells where the vertical and confining stresses temperature and pressure and composition of the fluid were separately controlled and monitored the axial vertical strains of two limestones and one sandstone were continuously measured during separate experiments for several months with a strain rate resolution of 10 11 s 1 fluids exiting the triaxial cells were continuously collected and their compositions analyzed the high fluids induced an increase in strain rates of the limestones by up to a factor of 5 compared to the low fluids injection of high fluids into the sandstone resulted in deformation rates one order of magnitude smaller than the limestones the creep accelerating effect of high fluids with respect to the limestones was mainly due to the acidification of the injected fluids resulting in a significant increase in solubility and reaction kinetics of calcite compared to the limestones the much weaker response of the sandstone was due to the much lower solubility and reactivity of quartz in high fluids in general all samples showed a positive correlation between fluid flow rate and strain rate x ray tomography results revealed significant increases in porosity at the inlet portion of each core the porosity increases were dependent on the original lithological structure and composition the overall deformation of the samples is interpreted in terms of simultaneous dissolution reactions in pore spaces and intergranular pressure solution creep

recurrence models and parkfield california earthquakes main shocks of the earthquake sequences that occurred on the parkfield section of the san andreas fault in central california in 1922 1934 and 1966 are characterized by southeast rupture expansion over the same 20 to 30 km long section of the fault whereas the seismic moments for the 1922 and 1934 events are identical to within a precision of 10 the seismic moment for 1966 is 20 greater than for the earlier events to within a precision of 20 the parkfield area seismicity in general seems well described by a recurring moderate size characteristic earthquake repeating the same epicenter magnitude seismic moment rupture area and southeast direction of rupture expansion an unexplained 10 year advance of the 1934 event is the only discrepancy in the hypothesis that the parkfield earthquakes in 1857 1881 1901 1922 1934 and 1966 represent a strictly periodic process assuming the strictly periodic model and the absence since 1966 of the perturbations hypothesized for the 1922 to 1934 period the next characteristic parkfield earthquake should occur between 1983 and 1993

recent advances in time lapse laboratory rock physics for the characterization and monitoring of fluid rock interactions abstractmonitoring thermo chemo mechanical processes geophysically e g fluid disposal or storage thermal and chemical stimulation of reservoirs or natural fluids simply entering a new system raises numerous concerns because of the likelihood of fluid rock chemical interactions and our limited ability to decipher the geophysical signature of coupled processes one of the missing links is understanding the evolution of seismic properties together with reactive transport because rock properties evolve as a result of chemical reactions and vice versa capturing this coupling experimentally is one of the missing elements in the existing literature this paper describes recent advances in rock physics experiments to understand the effects of dissolution induced compaction on acoustic velocity porosity and permeability this paper has a dual aim understanding the mechanisms underlying permanent modifications to the rock microstructure and providing a richer set of experimental information to inform th

estimation of rock frame weakening using time lapse crosswell the frio brine pilot project abstractco2 injection into subsurface reservoirs leads to pressure and saturation changes furthermore co2 brine minerals interaction could result in dissolution or reprecipitation of rock frame forming minerals observed time lapse seismic associated with co2 injection into poorly consolidated sandstone at the frio co2 injection site texas usa could not be predicted using classical rock physics models i e models involving elastic changes in the rock frame due to saturations and or pressures changes only and assuming no changes in the rock microstructure that and the changes in the fluid chemistry after co2 injection suggests that the assumption of a constant rock microstructure might be violated using high resolution time lapse crosswell data we have developed a methodology for estimating changes in the rock frame by quantifying the rock frame drained moduli before and after co2 injection based on rock microstructure diagnostics we found that the changes in the drained frame elastic prope

the combined effects of pressure and cementation on 4d seismic data abstracttime lapse seismology has proven to be a useful method for monitoring reservoir fluid flow identifying unproduced hydrocarbons and injected fluids and improving overall reservoir management decisions the large magnitudes of observed time lapse seismic anomalies associated with strong pore pressure increases are sometimes not explainable by velocity pressure relationships determined by fitting elastic theory to core data this can lead to difficulties in interpreting time lapse seismic data in terms of physically realizable changes in reservoir properties during injection it is commonly assumed that certain geologic properties remain constant during fluid production injection including rock porosity and grain cementation we have developed a new nonelastic method based on rock physics diagnostics to describe the pressure sensitivity of rock properties that includes changes in the grain contact cement and we applied the method to a 4d seismic data example from offshore australia we found that

quantitative measurements of shear wave polarizations at the anza seismic network southern california implications for shear wave splitting and earthquake prediction we analyze shear wave polarizations from local earthquakes recorded by the anza network in southern california using an automated method which provides unbiased and quantitative measurements of the polarization and the duration of linear motion following the shear wave arrival the linearity interval initial shear wave particle motions are strongly aligned at four stations a feature that is not predicted by focal mechanisms the particle motion alignment is most likely caused by shear wave splitting due to anisotropy beneath these stations a result supported by the clear shear wave splitting seen in a borehole recording near one of the anza stations these results are consistent with an earlier analysis of these data by peacock et al 1988 however our analysis does not support claims by crampin et al 1990 that shear wave splitting delay times at station knw exhibit temporal variations which can be correlated with the occurrence of the north palm springs earthquake ml 5 6 of july 8 1986 automatically determined linearity intervals scatter widely from 0 02 to 0 15 s and exhibit no clear temporal trends we find a correlation between earthquake moment and the linearity interval possibly a result of longer effective source time functions for the larger events the inability to identify a distinct slow quasi shear wave pulse for the vast majority of these events indicates that scattering strongly affects the particle motion even in the very early shear wave coda analysis of earthquake clusters with similar waveforms recorded at knw shows that seismic green functions are stable throughout the observational period and that most linearity interval variation is due to source and ray path differences between events if shear wave splitting is causing the observed delay times between horizontal components the waveform stability for events in these clusters restricts any temporal changes in shear wave splitting delay times to less than 5 10

rock physics diagnostic of north sea sands link between microstructure and seismic properties velocity in high porosity sands strongly depends on the position and amount of intergranular material velocity is high if the original grains are cemented at their contacts it is low if the pore filling material is placed away from the contacts the amount and type of intergranular material can be theoretically determined from velocity and porosity data by adjusting an effective medium theoretical model curve to a trend in the data and then assuming that the microstructure of the sediment is such as used in the model we apply this diagnostic method to clean sands in the heimdal formation the north sea the results show that the reservoir zone sand in one well under examination has slight contact cementation while it is completely uncemented and friable in another well this conclusion is supported by microscopic grain images the rock physics diagnostic is important for establishing a velocity porosity trend consistent with local geology such trends can improve the accuracy of porosity prediction from seismic data

critical porosity a key to relating physical properties to porosity in rocks discovering accurate relations between porosity and seismic wave velocities in porous rocks has been an important topic of rock physics for decades such relations if they actually exist are critical when seismic and or sonic data are to be used to infer porosity and porosity related properties e g permeability strength saturation in situ these inferences became especially important in recent years with the rapid development of seismic and sonic methods which are applied to the characterization of heterogeneous reservoirs and recovery monitoring

site characterization for co 2 geologic storage and vice versa the frio brine pilot texas usa as a case study careful site characterization is critical for successful geologic storage of carbon dioxide co2 because of the many physical and chemical processes impacting co2 movement and containment under field conditions traditional site characterization techniques such as geological mapping geophysical imaging well logging core analyses and hydraulic well testing provide the basis for judging whether or not a site is suitable for co2 storage however only through the injection and monitoring of co2 itself can the coupling between buoyancy flow geologic heterogeneity and history dependent multi phase flow effects be observed and quantified co2 injection and monitoring can therefore provide a valuable addition to the site characterization process additionally careful monitoring and verification of co2 plume development during the early stages of commercial operation should be performed to assess storage potential and demonstrate permanence the frio brine pilot a research project located in dayton texas usa is used as a case study to illustrate the concept of an iterative sequence in which traditional site characterization is used to prepare for co2 injection and then co2 injection itself is used to further site characterization efforts constrain geologic storage potential and validate understanding of geochemical and hydrological processes at the frio brine pilot in addition to traditional site characterization techniques co2 movement in the subsurface is monitored by sampling fluid at an observation well running co2 saturation sensitive well logs periodically in both injection and observation wells imaging with crosswell seismic in the plane between the injection and observation wells and obtaining vertical seismic profiles to monitor the co2 plume as it migrates beyond the immediate vicinity of the wells numerical modeling plays a central role in integrating geological geophysical and hydrological field observations

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

an experimental study of solid matrix weakening in water saturated savonnieres limestone petrophysical properties of carbonate reservoirs are less predictable than that of siliciclastic reservoirs one of the main reasons for this is the physical and chemical interactions of carbonate rocks with pore fluids such interactions can significantly change the elastic properties of the rock matrix and grains making the applicability of gassmann s fluid substitution procedure debatable this study is an attempt to understand the mechanisms of fluid rock interactions and the influence of these interactions on elastic parameters of carbonates we performed precise indentation tests on savonni eres limestone at a microscale level under dry distilled water and n decane saturated conditions our experiments display softening of the rock matrix after water saturation we have found that mainly the ooid cortices peloid nuclei and prismatic intergranular cement are affected by water flooding we also observed a shear modulus reduction in savonni eres limestone in an experiment performed at ultrasonic frequencies one of the most important results obtained in our experimental study is that the gassmann fluid substitution theory might not always be applicable to predict the elastic moduli of fluid saturated limestones

tomographic images of p wave velocity variation at parkfield california tomographie inversion is applied io delay times from local earthquakes to image three dimensional velocity variations near parkfield california the 25 20 square km region is represented by nearly cubic blocks of 0 5 km per side arrival times of p waves from 551 local earthquakes with depths of 0 to 15 km were used as sources producing 3135 rays covering the target region the data were recorded on low noise downhole seismographs a conjugate gradient method is used to invert the resulting sparse system of simultaneous equations to diminish the effects of noisy data the laplacian of the model parameters is constrained to be small within horizontal layers providing smoothing of the model the resolution of the model is estimated by calculating point spread functions at blocks of interest estimates of standard errors of the model parameters are calculated by the jackknife statistical procedure the results of the inversion show correlation with some of the local geological and geophysical features station corrections removed the long wavelength anomaly associated with the contrast of the salinian block southwest of the san andreas fault versus the franciscan to the northeast a velocity low located a few kilometers northwest of parkfield depth 2 5 3 5 km appears to lie along the gradient of the large bouguer gravity anomaly associated with the parkfield syncline the south southeastward extension of the low velocities may relate to reflections observed on the parkfield consortium for continental reflection profiling cocorp lines we speculate on the geological meaning of these features and interpret them either as part of the local strike slip tectonics or a shallow crustal detachment the correlation of higher velocity features and seismic activity may indicate that earthquakes are occurring in more competent zones while aseismic slip takes place in zones of lower velocity less competent rocks

high precision relative locations of earthquakes at mount st helens washington several earthquake sequences have occurred at mount st helens as clusters of shallow events with nearly identical waveforms multiplets we present a technique that allows the relative relocation of events within a multiplet to at least an order of magnitude higher precision than is typically possible with traditional techniques a multiplet is studied in pairs of events called doublets each doublet is made up of a reference event and one other event from the multiplet the cross spectrum of a short window beginning just before the p arrival is used to compute differences in arrival times between seismograms from the same station for each doublet these time differences computed with a precision of 1 2 ms are then used to locate each earthquake relative to the reference event engineering explosions with well known locations are used to test the relocation technique explosions can be relocated with accuracy better than 20 m for events within 250 m of the reference explosion the relocation technique is applied to earthquakes preceding the september 10 1984 and the may june 1985 dome building eruptions of mount st helens forty events that occurred during a 12 hour period in september 1984 are located in a volume approximately 30 m in diameter beneath the lava dome the earthquakes following the extrusion do not occur in multiplets and appear to be distributed over a much larger volume in the may june 1985 eruption we recognize three multiplet sets that took place successively in time the multiplets are interpreted as repeated failure or slip within a relatively small volume due to high strain rates around the magma supply conduit

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

relations among fault behavior subsurface geology and three dimensional velocity models the development of three dimensional p wave velocity models for the regions surrounding five large earthquakes in california has lead to the recognition of relations among fault behavior and the material properties of the rocks that contact the fault at seismogenic depths regions of high moment release appear to correlate with high seismic velocities whereas rupture initiation or termination may be associated with lower seismic velocities these relations point toward a physical understanding of why faults are divided into segments that can fail independently an understanding that could improve our ability to predict earthquakes and strong ground motion

uranium series disequilibrium studies of granitic rocks abstract igneous plutons of the canadian shield which are to be considered as possible storage sites for radioactive waste should be free from any evidence of past mobility of chemical species through the rock however cores taken from some plutons to depths of up to 1 km reveal the presence of hydrous low temperature alteration along joints and fractures and within the body of the rock to test whether such rocks or any others have been subjected to chemical transport in geologically recent times we may study the state of radioactive equilibrium between 238 u 235 u and 232 th and their short lived daughterisotopes members of the decay series of these parent isotopes have half lives ranging from a few hours to 0 25 ma also the chemical characteristics of the various daughters vary significantly allowing us to test the mobility of isotopes of similar half life but differing chemistry if the rock has been chemically undisturbed for 10 ma or more then the activity in d p m g of each daughter in these decay schemes will be equal to that of its parent isotope conversely if the rock was chemically disturbed 10 n a ago then some daughter isotopes with half lives t 10 n a will be out of equilibrium with their parents preliminary results are presented showing disequilibrium effects in samples from atikokan ontario

seismic velocities and poisson s ratio of shallow unconsolidated sands we determined p and s wave velocity depth profiles in shallow unconsolidated beach sand by analyzing three component surface seismic data p and s wave velocity profiles were calculated from traveltime measurements of vertical and tangential component seismograms respectively the results reveal two discrepancies between theory and data whereas both velocities were found to be proportional to the pressure raised to the power of 1 6 as predicted by the hertz mindlin contact theory the actual values of the velocities are less than half of those calculated from this theory we attribute this discrepancy to the angularity of the sand grains assuming that the average radii of curvature at the grain contacts are smaller than the average radii of the grains we modify the hertz mindlin theory accordingly we found that the ratio of the contact radius to the grain radius is about 0 086 the second disparity is between the observed poisson s ratio of 0 15 and the theoretical value 0 008 for random pack of quartz spheres this discrepancy can be reconciled by assuming slip at the grain contacts because slip decreases the shearing between grains poisson s ratio increases

gas water rock interactions in frio formation following co2 injection implications for the storage of greenhouse gases in sedimentary basins to investigate the potential for the geologic storage of co2 in saline sedimentary aquifers 1600 t of co2 were injected at 1500 m depth into a 24 m thick sandstone section of the frio formation a regional brine and oil reservoir in the u s gulf coast fluid samples obtained from the injection and observation wells before co2 injection showed a na cacl type brine with 93 000 mg l total dissolved solids tds at near saturation with ch4 at reservoir conditions following co2 breakthrough samples showed sharp drops in ph 6 5 5 7 pronounced increases in alkalinity 100 3000 mg l as hco3 and fe 30 1100 mg l and significant shifts in the isotopic compositions of h2o dissolved inorganic carbon dic and ch4 geochemical modeling indicates that brine ph would have dropped lower but for the buffering by dissolution of carbonate and iron oxyhydroxides this rapid dissolution of carbonate and other minerals could ultimately create pathways in the rock seals or well cements for co2 and brine leakage dissolution of minerals especially iron oxyhydroxides could mobilize toxic trace metals and where residual oil or suitable organics are present the injected co2 could also mobilize toxic organic compounds environmental impacts could be major if large brine volumes with mobilized toxic metals and organics migrated into potable groundwater the d 18 o values for brine and co2 samples indicate that supercritical co2 comprises 50 of pore fluid volume 6 mo after the end of injection postinjection sampling coupled with geochemical modeling indicates that the brine gradually will return to its preinjection composition

time lapse crosswell seismic and vsp monitoring of injected co2 in a brine aquifer seismic surveys successfully imaged a small scale co2 injection 1 600 ton conducted in a brine aquifer of the frio formation near houston texas these time lapse borehole seismic surveys crosswell and vertical seismic profile vsp were acquired to monitor the co2 distribution using two boreholes the new injection well and a pre existing well used for monitoring which are 30 m apart at a depth of 1 500 m the crosswell survey provided a high resolution image of the co2 distribution between the wells via tomographic imaging of the p wave velocity decrease up to 500 m s the simultaneously acquired s wave tomography showed little change in s wave velocity as expected for fluid substitution a rock physics model was used to estimate co2 saturations of 10 20 from the p wave velocity change the vsp survey resolved a large 70 change in reflection amplitude for the frio horizon this co2 induced reflection amplitude change allowed estimation of the co2 extent beyond the monitor well and on three azimuths the vsp result is compared with numerical modeling of co2 saturations and is seismically modeled using the velocity change estimated in the crosswell survey

implications of pore microgeometry heterogeneity for the movement and chemical reactivity of co2 in carbonates abstractwe studied the heterogeneity of natural rocks with respect to their pore size distribution obtained from mercury intrusion capillary pressure micp tests at a scale about one fifth of the standard plug size 2 5 cm we investigated two fontainebleau sandstone and two limestone samples we found that at the scale of the micp tests heterogeneities are practically nonexistent still there are large differences in the capillary curves from one rock type to another also carbonate rocks unlike fontainebleau sandstone show heterogeneities at a scale smaller than the scale used in micp tests as seen by the complexity in the mercury saturation versus pressure curves we used this diversity between the capillary curves and this complexity within a single capillary curve to obtain information about the movement and chemical reactivity of co2 in carbonates the method consists of three steps first subdividing the carbonate pore system into microstructural facies each of them having a specific ra

seismological studies at parkfield i simultaneous inversion for velocity structure and hypocenters using cubic b splines parameterization a model for the 3d p and s wave velocity structure at parkfield has been developed by the systematic inversion of arrival times for microearthquakes and surface vibroseis sources recorded by nearby calnet stations and by the local borehole installed 10 station high resolution seismic network there a modification of the linear b splines model parameterization method of thurber 1983 to a cubic b splines formulation through tests inverting synthetic data sets for the parkfield configuration is found to produce smooth velocity distributions that still retain some small scale features application of the method to 953 p wave and 288 s wave arrivals from 111 microearthquakes and 8 vibroseis locations throughout the study area yields a velocity structure having variable resolution a low velocity zone appears to coincide with the creeping segment of the san andreas fault zone northwest of the 1966 parkfield earthquake epicenter an increase of about 15 per cent in seismic velocities is seen from the northeast to southwest across this segment of the fault zone the higher velocities to the southwest apparently correspond to the granitic basement the contrast across the fault is subdued to the southeast along the 1966 rupture zone a high vp vs ratio of 1 9 is seen in a 2 km wide volume near the location of the 1966 main shock hypocenter presumably the nucleation zone for the expected m 6 parkfield earthquake

earthquake travel time tomography of the southern santa cruz mountains control of fault rupture by lithological heterogeneity of the san andreas fault zone the 1989 loma prieta earthquake occurred along the stretch of the san andreas fault zone within the southern santa cruz mountains that last failed as a major earthquake in 1906 the southeastern end of the 1989 rupture marks the transition from stable aseismic slip on the central creeping section of the san andreas fault to unstable failure on the locked 1906 segment we investigate this transition and the rupture characteristics of the 1989 earthquake using a three dimensional p wave velocity model of the southern santa cruz mountains section of the fault zone the velocity model was determined by joint progressive inversion of 5422 p wave arrival times from 173 earthquakes which included aftershocks of the 1989 earthquake and premainshock background events recorded by the u s geological survey central california network this velocity model was used to relocate 424 background earthquakes the loma prieta mainshock and 292 aftershocks the velocity model is calibrated in terms of lithology using laboratory data for california coast ranges rock types and available seismic refraction profiles and is shown to be in good agreement with the surface geology the model images a large anomalous high velocity body at midcrustal depths within the rupture zone of the 1989 earthquake that the available evidence suggests might have gabbroic or other mafic composition on the basis of the relationship of the lithological features interpreted from the velocity model to the seismicity and surface creep we propose a model in which the high velocity body is primarily responsible for the transition from stable to unstable fault slip at pajaro gap the active plane of the san andreas fault cuts throughout the body the fault system attempts to circumvent this barrier by transferring slip to secondary faults including splay faults that have propagated along the frictionally favorable contact between the high velocity rock mass and franciscan country rocks however the near arrest of stable sliding causes stress to concentrate within the body and the high strength unstable contact within it evolves from a barrier to the asperity that failed in the 1989 earthquake the general features of the 1989 rupture predicted by this asperity model agree with several rupture histories computed for the earthquake the model implies that as proposed by other workers the loma prieta earthquake did not involve a repeat of the 1906 slip which has an important bearing on earthquake recurrence estimates for the santa cruz mountains segment of the fault