dynamic recrystallization and texture development in ice as revealed by the study of deep ice cores in antarctica and greenland the preferred c axis orientation of ice from polar ice sheets develops essentially as a result of intracrystalline slip but dynamic recrystallization appears to alter the kinetics of the development of deformation textures and is at high temperature at the origin of recrystallization textures the purpose of this work is to obtain a better understanding of recrystallization processes that occur in polar ice sheets and to clarify the relationship between dynamic recrystallization and textures the study was based on two deep ice cores from greenland and antarctica the greenland ice core project grip and vostok ice cores the structure along the grip core displays normal grain growth in the first 100 m of the ice sheet and rotation recrystallization and migration recrystallization near the bottom only grain growth and rotation recrystallization appear to occur in the vostok ice core the transition between these recrystallization regimes was studied estimating for interglacial ice the evolution with depth of the dislocation density this calculation has shown the efficiency of grain boundary migration for the absorption of dislocations at vostok the highest value of the dislocation density is found at a depth of about 1000 m and the continuous decrease in the dislocation density below this depth is related to the increase of the grain boundary migration rate it is shown that the driving force required to initiate migration recrystallization is not reached in interglacial ice at vostok the observed textures were compared with those predicted by the self consistent approach recrystallization textures are interpreted by assuming that the less stressed grains i e the best oriented for basal slip are favored by the size advantage of subgrains the recrystallization textures are compared with those of other materials

waterfall ice mechanical stability of vertical structures we present a study of the mechanical in stability of the ephemeral waterfall ice structures that form from the freezing of liquid water seeping on steep rock three vertical structures were studied two near glacier d argentiere france and one in the valsavarenche valley northern italy the generation of internal stresses in the ice structure in relation to air and ice temperature conditions is analyzed from pressure sensor records their role in the mechanical instability of the structures is discussed from a photographic survey of these structures the main result is that dramatic air cooling several 8 8c h 1 over several hours and low temperatures 108c generating tensile stresses and brittleness can trigger a spontaneous or climber induced mechanical collapse leading to unfavorable climbing conditions ice internal pressure fluctuations are also associated with episodes of marked diurnal air temperature cycle with mild days few 8 8c above 08 8c and cool nights few 8 8c below 08 8c through the occurrence of water ice phase transitions within the structure these ice internal stress fluctuations seem however to have a local influence are associated with warm near 08 8c wet and therefore particularly soft ice and do not trigger a collapse of the structure

fresnel volume migration of multicomponent data if the aperture of a seismic reflection experiment is strongly limited kirchhoff migration suffers from strong artifacts attributable to incomplete summation this can be overcome by restricting the migration operator to the region that physically contributes to a reflection event examples of such limited aperture experiments include data acquisition in boreholes tunnels and mines we present an extension to three component 3c kirchhoff prestack depth migration where the migration operator is restricted to the fresnel volume of the specular reflected raypath we use the measured polarization direction at a 3c receiver to determine points of specular reflection in homogeneous media the polarization angle of 3c data can be used directly to decide whether a certain image point belongs to the fresnel volume of a specular reflection in heterogeneous media the fresnel volume around an image point is approximated by means of paraxial ray tracing the method is tested on a synthetic vertical seismic profiling experiment with strongly limited aperture migration artifacts and crosstalk effects from converted waves are strongly reduced compared with standard migration schemes the method is successfully applied to seismic data acquired in a tunnel

fresnel volume migration of single component seismic data standard implementations of kirchhoff prestack depth migration psdm distribute the recorded wavefield along two way traveltime isochrons and an image is generated by constructive interference of these isochrons along the actual reflector elements beside the recent developments of wave equation based approaches kirchhoff psdm is still considered widely as a state of the art technique in obtaining high quality images of the subsurface particularly for highly irregular survey layouts and target oriented imaging tasks however for sparse sampling or limited aperture the resulting image is affected by significant migration noise as a result of limited constructive interference of the back propagated wavefield some modifications have been proposed to reduce these artifacts these modifications include constructing a specular path of wave propagation derived from estimates of the emergent angle of coherent phases in the seismogram section and the mainly heuristic restriction of the imaging operator to the neighborhood of that wavepath our approach uses fresnel volumes to restrict the migration operator in a physically frequency dependent way using the emergent angle at the receiver determined by a local slowness analysis a ray is propagated into the subsurface the back propagation of the wavefield is restricted to the vicinity of this ray according to its approximated fresnel volume this so called fresnel volume migration approach enhances image quality significantly compared with standard kirchhoff psdm because of the inherent focusing and the restriction of the back propagation to the region around the actual reflection point

ice cascade growth and decay a thermodynamic approach the ice volume evolution of a frozen waterfall or ice cascade was studied using a thermodynamic model the model was developed from meteorological data collected in the vicinity of the waterfall and validated from ice volume measurements estimated from terrestrial lidar images the ice cascade forms over a 45 m high rock wall located in northern gaspesie quebec canada two stages of formation were identified during the first stage the growth is mainly controlled by air convection around the flowing and free falling water the ice cascade growth rate increases with decreasing air temperature below 0 c and when the water flow reaches its lowest level during the second stage the ice cascade covers the entire rock wall surface water flow is isolated from the outside environment and ice volume increases asymptotically heat is evacuated from the water flow through the ice cover by conduction the growth is controlled mainly by the conductive heat loss through the ice cover but also by the longwave radiation emitted at the ice surface during the night in spring melting of the ice cascade is dependent on the air convection over the ice surface but also on the sensible heat carried by the increasing water flow and the solar radiation received during the day

microseismic reflection imaging and its application to the basel geothermal reservoir we have developed a new approach for reflection seismic imaging using microseismic waveform data we took the specific character of microseismic downhole data into account and were able to image regions in the direct vicinity or inside of a seismically active zone we used a directional migration method that reduced imaging artifacts caused by the low aperture of the data but required estimation of the wavefield polarization we also used hodogram analysis to identify parts of data that provided reliable polarization estimates only these parts were then used for imaging whereas the rest of the data are excluded the approach was successfully applied to microseismic waveforms recorded during and after the stimulation of the enhanced geothermal system in basel switzerland this data set included more than 2000 events recorded at five sparsely distributed borehole sensors the reflection images revealed a distinct network of reflectors in the vicinity of the open hole section within the injection well the results were compared with the injection borehole ultrasonic image revealing a quite good agreement combined analysis of imaged reflectors event locations and event focal mechanisms indicated that the approach resolved mostly aseismic highly permeable fluid pathways whereas most of the seismicity occurred in the second order fractures

seismic imaging using microearthquakes induced by hydraulic fracturing abstractseismic imaging using microearthquakes induced by hydraulic fracturing produces a three dimensional 3 d s wave velocity model of the fractured zone improves the calculated locations of the microearthquakes and may lead to better estimates of fractureplane orientations fracture density and water flow paths such information is important for predicting the amount of heat energy that may be extracted from geothermal reservoir a fractured zone was created at the los alamos hot dry rock reservoir in north central new mexico within otherwise impermeable basement rock by injecting 21000 m3 of water into a borehole under high pressure at a depth of 3 5 km induced microearthquakes were observed using four borehole seismometers the p wave and s wave arrival times have been inverted to find the 3 d velocity structures and the microearthquake locations and origin times the inversion was implemented using the separation of parameters technique and constraints were incorporated to require smooth vel

high resolution microseismic imaging abstract we examine the feasibility of high resolution microseismic imaging of unconventional reservoirs given frequencies in typical downhole microseismic data that are almost an order of magnitude higher than in seismic reflection data a comparable increase in image resolution might be expected bringing potential resolution of downhole microseismic images to a few meters we demonstrate that such a resolution is indeed achievable and present two case studies illustrating the reservoir features that can be imaged our first example constructed with p waves recorded in the woodford field reveals internal fabric of the woodford reservoir and formations surrounding it allowing us to discriminate the stimulated and unstimulated zones of the lower woodford shale encouraged by those results in our second example this time from the bakken field we use the slow shear waves which are more sensitive to fluids than p waves to find out whether hydraulic fractures themselves could be imaged our seismic v

waterfall ice formation structure and evolution for the first time to our knowledge a scientific study of the formation and evolution of waterfall ice the ephemeral ice structures that form from the freezing of liquid water seeping on steep rock was performed we surveyed and analysed three waterfall ice structures near glacier d argentiere mont blanc massif france between winter 2007 and spring 2009 we reconstruct the global evolution of two vertical ice structures using automatic digital cameras while the internal ice microstructure was analysed using ice coring and sampling macro and microstructural observations are considered along with temperature conditions recorded at a nearby meteorological station and directly within the ice structure they reveal that vertical structures initially grow rapidly from the aggregation of stalactites with microstructures indicative of temperature conditions during their crystallization after this initial stage the volume of the ice structure reaches an asymptotic value as water continues to flow inside the structure isolated from the outside cold ice the outer surface remains dry at the end of the season the collapse of the free standing structure does not occur by progressive melting but is initiated by a horizontal crack propagation at the top the initiation of this crack seems to be triggered by a drastic temperature decrease

paleocave carbonate reservoirs origins burial depth modifications spatial complexity and reservoir implications paleocave systems form an important class of carbonate reservoirs that are products of near surface karst processes and later burial compaction and diagenesis features and origins of fractures breccias and sediment fills associated with paleocave reservoirs have been studied in modern and ancient cave systems information about such cave systems is used in this paper to reconstruct the general evolution of paleocave reservoirs and their associated scale pore networks and spatial complexities spatial complexities in paleocave reservoirs result from near surface and burial processes near surface processes include dissolutional excavation clastic sedimentation chemical precipitation and localized fracturing brecciation and collapse of cave walls and ceilings burial processes begin as cave systems subside into the subsurface remaining cave passages commonly collapse and early formed breccia clasts are rebrecciated differential compaction of strata around and over collapsed passages produces fractures crackle breccias and mosaic breccias near surface and burial processes combine to produce typically complex reservoirs with several scales of heterogeneity hydrocarbon reservoirs of paleocave origin are commonly the product of coalesced collapsed paleocave systems the coalescing of passages in a cave system into larger connected porosity zones results from a combination of multiple cave forming episodes at composite unconformities and from the collapse of cave systems during burial where surrounding host strata are brecciated and fractured this combination of processes creates spatially complex reservoirs that can be hundreds to several thousands of meters across commonly forming large exploration targets final size pore network types and spatial complexities of coalesced collapsed paleocave systems are products of their evolution from near surface development through burial into the deeper subsurface the coalesced collapsed paleocave reservoir hypothesis explains the scale of reservoirs observed and the spatial complexities involved

microseismic imaging of a geothermal reservoir stimulation exploitation of geothermal heat energy from rocks at depths of several km has been the goal of vari ous projects since the mid 1970s to exploit the energy in these deep rocks poses technical challenges in drilling well completion creation of the heat exchange reservoir at depth and control of seismicity in spite of the difficulties what are now known as enhanced engineered geothermal systems egs offer almost limitless clean energy so it is not surprising that this technology continues to attract investors however commercial energy production from the greatest potential resource hot rock at depths 5 km or temperatures 200 c remains elusive

azimuthal anisotropy in microseismic monitoring a bakken case study abstracthydraulic fracturing routinely applied for enhancing the permeability of unconventional oil and gas reservoirs is one of the possible causes for azimuthal anisotropy of the treated formations accounting for both naturally occurring and completion induced azimuthal anisotropy leads to marked improvements in the results of microseismic data processing as illustrated on a data set acquired in the bakken field north dakota usa those improvements include the possibility of modeling the observed shear wave splitting reduction of misfit between the picked and modeled traveltimes of microseismic events and relocation and tightening of the spatial distribution of the event hypocenters in addition and perhaps most importantly for the development of microseismic technology the feasibility of joint inversion of field microseismic data for the event locations and azimuthally anisotropic velocity model containing triclinic layers is demonstrated

microseismic joint location and anisotropic velocity inversion for hydraulic fracturing in a tight bakken reservoir abstractto improve the accuracy of microseismic event locations we developed a new inversion method with double difference constraints for determining the hypocenters and the anisotropic velocity model for unconventional reservoirs we applied this method to a microseismic data set monitoring a middle bakken completion in the beaver lodge area of north dakota geophone arrays in four observation wells improved the ray coverage for the velocity inversion using an accurate anisotropic velocity model is important to correctly assess the height growth of the hydraulically induced fractures in the middle bakken our results showed that 1 moderate to strong anisotropy exists in all studied sedimentary layers especially in the upper and lower bakken shale formations where the thomsen parameters and can be greater than 0 4 2 all the events selected for high signal to noise ratio and used for the joint velocity inversion are located in the bakken and overlying lodgepole formations i e no events a

calcite fabrics growth mechanisms and environments of formation in speleothems from the italian alps and southwestern ireland five fabrics were identified in alpine and irish caves on the basis of morphological and microstructural characteristics and re lated to growth mechanisms and growth environment columnar and fibrous fabrics grow when speleothems are continuously wet and from fluids at near equilibrium conditions low supersaturation sicc 0 35 through the screw dislocation mechanism the highly defective microcrystalline fabrics form at the same supersaturation range as co lumnar fabric but under variable discharge and the presence of growth inhibitors dendritic fabrics which have the highest density of crystal defects develop in disequilibrium conditions high supersaturation un der periodic very low flow regime periods that result in prolonged out gassing cave calcareous tufa forms in disequilibrium conditions only the calcite crystals of fabrics formed at low supersaturation seem to precipitate near isotopic equilibrium conditions