ice pumps and their rates an ice pump is a heat engine driven by the change of freezing point with pressure which will melt ice at depth in the ocean and deposit it at a shallower location it is self starting calculations of the maximum magnitude of this effect are made which show good agreement with field data available for sea and lake ice the discussion is applied to the general case of a moving pack ice sheet with a well mixed surface layer and to floating ice shelves the rate of melt from an 11 m deep pressure ridge keel due to ice pumping is estimated as 26 cm year and that from the front of the ross ice shelf at mcmurdo sound antarctica is estimated as 5 m year for the level of water movement noted in the authors field observations far from the ice front pumping between shelf areas of different thickness will still occur with tidal motion providing the necessary water exchange but its magnitude is now limited by the ability to remove the potentially stable layer of melt water out of the system it is important to realize that the pumping does not depend on the availability of sensible heat in the water column and its effects are additional to any melting caused by the advection of warmer water to the ice water interface

structure of the california coast ranges and san andreas fault at safod from seismic waveform inversion and reflection imaging 1 a seismic reflection and refraction survey across the san andreas fault saf near parkfield provides a detailed characterization of crustal structure across the location of the san andreas fault observatory at depth safod steep dip prestack migration and frequency domain acoustic waveform tomography were applied to obtain highly resolved images of the upper 5 km of the crust for 15 km on either side of the saf the resulting velocity model constrains the top of the salinian granite with great detail steep dip reflection seismic images show several strong amplitude vertical reflectors in the uppermost crust near safod that define an 2 km wide zone comprising the main saf and two or more local faults another prominent subvertical reflector at 2 4 km depth 9 km to the northeast of the saf marks the boundary between the franciscan terrane and the great valley sequence a deep seismic section of low resolution shows several reflectors in the salinian crust west of the saf two horizontal reflectors around 10 km depth correlate with strains of seismicity observed along strike of the saf they represent midcrustal shear zones partially decoupling the ductile lower crust from the brittle upper crust the deepest reflections from 25 km depth are interpreted as crust mantle boundary

estimation of fracture height using microseismicity associated with hydraulic fracturing the ratio of horizontal to vertical h v particle velocity in background microseismic radiation associated with hydraulic fracturing is substantially higher in the dilatant low velocity fractured zone than it is outside this provides a useful diagnostic for determining the height of the fractured zone numerical synthesis of guided wave phenomena within the low velocity fractured zone accounts for much of the observed behavior but measured h v patterns are not totally consistent with either pure tensile or pure shear sources a composite model containing both tensile compressional sources and asperity shear failures appears to satisfy the main observations better than either source type does alone this composite is consistent with current models of earthquake aftershock sequences which also have different mechanisms at the edges and in the interior of a fracture zone tensile and shear respectively the h v phenomenon is consistent with a predominance of energy with shear wave polarization traveli

special section on azimuthal dependence of p wave seismic signatures introduction this special issue is based on papers presented at the postconvention seg workshop on azimuthal dependence of p wave signatures held in dallas in 1997 the main motivation for analyzing the azimuthal variation of seismic traveltimes amplitudes attenuation etc is to obtain reliable information about azimuthal anisotropy in the subsurface another potential application of multiazimuth techniques is in finding andmappingdesirablelateralheterogeneitiesthatcould masquerade as azimuthal anisotropy the last topic has not yet been fully discussed and is not addressed in the special issue butseveralexplorationanddevelopmentscenarioscontainoriented lateral heterogeneities sand channels etc that at the right scale length could be highly visible in properly processed wide azimuth 3 d data themostcommonphysicalreasonsforazimuthalanisotropy are systems of vertical or steeply dipping fractures or tilted transversely isotropic layers both types of azimuthally anisotropic models are discussed here with an emphasis on issues related to fracture detection and characterization in tight formations the extraction offluids is often impossible without exploiting the increased drainage provided by fracture networks therefore geophysicalcharacterizationoffracturedreservoirs is an extremely important problem from an economic standpoint sweet spots of high fracture intensity are not always correlated with faults and other structural features and are difficult to detect using conventional exploration techniques effective exploitation of fractured reservoirs also requires determination of the orientation and connectivity of fractures as well as their spatial relationships to other reservoir heterogeneities most existing seismic methods for fracture characterization are based on analysis of shear wave splitting e g crampin 1985 thomsen 1988 winterstein and meadows 1991 mueller 1992 since split s waves travel along essentially the same raypath the difference between their velocities andreflectionamplitudesdependsprimarilyontheanisotropic properties of the medium rather than on lateral heterogeneity theshear wavetechnology however hasknownshortcomings

structural characteristics of congelation and platelet ice and their role in the development of antarctic land fast sea ice ice cores were obtained in january 1990 from the land fast ice in mcmurdo sound for a study of variations in texture fabric sub structure composition and development two primary ice types were observed congelation and platelet with a minor amount of frazil ice congelation ice growth precedes platelet ice accretion congelation ice fabrics show frequent moderate to strong alignments a phenomenon believed to be due to water current control of selective ice crystal growth platelet ice originates at the base of the congelation ice initially as a porous latticework of tabular ice crystals which subsequently consolidate by congelation of the interstitial water interstitial congelation ice fabrics generally have little or no alignment indicating the reduced effect of currents within the platelet latticework prior to solidification platelet crystal textures range from small wavy edged forms to large blade like forms platelet crystal fabrics indicate that in addition to being randomly oriented the platelet latticeworks commonly include many crystals with their flat 0001 faces oriented both parallel and normal to the base of the overlying ice plate width data suggest that the interstitial congelation ice growth rates remain similar to those of the overlying congelation ice this effective increase in growth rates probably happens because the latticework of accumulating platelets ahead of the freezing interface ensures that the water within the platelet layer is at the freezing point and less heat has to be removed from platelet rich water than from platelet free water for a given thickness of congelation ice to grow the negative oceanic heat flux associated with platelet ice formation in mcmurdo sound explains why mcmurdo sound fast ice is thicker than ross sea pack ice and also why it reaches a greater thickness than arctic fast ice grown in a similar polar marine climate plate widths in the mcmurdo sound congelation ice suggest however that it grows no faster than arctic congelation ice

break up of the largest arctic ice shelf and associated loss of an epishelf lake 1 field observations and radarsat imagery of the ward hunt ice shelf lat 83 n long 74 w nunavut canada show that it broke in two over the period 2000 to 2002 with additional fissuring and further ice island calving the fracturing caused the drainage of an ice dammed epishelf lake disraeli fiord a rare ecosystem type reductions in the freshwater volume of disraeli fiord occurred from 1967 to the present and accompanied a significant rise in mean annual air temperature over the same period in this far northern region the recent collapse of ice shelves in west antarctica has been interpreted as evidence of accelerated climate change in that region similarly the inferred thinning and observed fragmentation of the ice shelf plus the drainage of the epishelf lake are additional evidence for climate change in the high arctic

evidence for platelet ice accretion in arctic sea ice development moderate quantities of platelet ice are reported in ice cores obtained from floes in the central and western beaufort sea arctic ocean the crystal structure characteristics include a fabric of long thin ragged edged crystals with randomly oriented c axes the cellular substructure that is characteristic of congelation ice is not evident in individual platelet ice crystals but they do occur together with congelation ice in layers ranging from 0 05 to 0 6 m thick platelet ice crystals are observed primarily in second year and multiyear floes the structural characteristics of arctic platelet ice are similar to those of antarctic platelet ice the platelet ice layers have a minimum 18o value of 5 3 and in some cases the platelet ice layers have more negative 18o values than the ice layers above and below them the salinity of platelet ice layers is not significantly different from other ice layers a number of platelet ice growth mechanisms and sources are discussed processes associated with the ellesmere ice shelves nearshore processes such as anchor ice growth in shallow waters off the north coast of alaska and ice growth in freshwater impounded behind grounded ice dams might be locally important but are unlikely to contribute significant quantities of platelet ice to pack ice floes two processes probably account for the presence of platelet ice the first is growth in underice melt ponds which form in inverted depressions at the base of floes when meltwater floes off the ice surface the second is the operation of ice pumps in which ice that is melted off the deeper parts of floes and ridges is deposited as platelet ice at a higher level in either case initially loose accumulations of platelet crystals are entrained in the floes by the growth of congelation ice the widespread occurrence of platelet ice in the central and western beaufort sea suggests that ice growth in both underice melt ponds and associated with the operation of ice pumps might be more common than was previously realized

active seismic monitoring of hydraulic fractures in laboratory experiments in scaled laboratory tests we perform acoustic measurements in a time lapse sequence to separate the fracture response from the background signal using both compressional and shear waves that are very sensitive to fluid filled fractures we can not only detect the hydraulic fracture but also characterize its shape and geometry during its growth we show the application of the technique to propagation and reopening of hydraulic fractures during fracture growth the acoustic waves excite diffractions at the tip of the fracture depending on the acquisition geometry we detect many events related to surface waves propagating along the fracture also we observe that shear waves detect the migration of the fluid front during reopening of a pre existing hydraulic fracture in contrast with the compressional waves which are insensitive to the fluid front during reopening

monitoring the width of hydraulic fractures with acoustic waves during scaled hydraulic fracturing experiments in our laboratory the fracture growth process is monitored in a time lapse experiment with ultrasonic waves we observe dispersion of compressional waves that have propagated across the hydraulic fracture this dispersion appears to be related to the width of the hydraulic fracture this means that we can apply the dispersion measurements to monitor the width of the hydraulic fracture in an indirect manner for a direct determination of the width the resolution of the signal is required to distinguish the reflections that are related with two distinct fluid solid interfaces delimiting the hydraulic fracture from its solid embedding to make this distinction the solid fluid interfaces must be separated at least one eighth of a wavelength and represent sufficient impedance contrast the applicability of the indirect dispersion measurement method however extends to a fracture width that is in the order of 1 of the incident wavelength the time lapse ultraso

diffraction of seismic waves by cracks with application to hydraulic fracturing the authors describe a method of modeling seismic waves interacting with single liquid filled large cracks based on the kirchhoff approximation and then apply it to field data in an attempt to estimate the size of a hydraulic fracture they first present the theory of diffraction of seismic waves by fractures using a green s function representation and then compute the scattered radiation patterns and synthetic seismograms for fractures with elliptical and rectangular shapes of various dimensions it is shown that the characteristics of the diffracted wavefield from single cracks are sensitive to both crack size and crack shape finally they compare synthetic waveforms to observed waveforms recorded during a hydraulic fracturing experiment and are able to predict successfully the size of a hydraulically induced fracture length and height in contrast to previously published work based on the born approximation the authors model both phases and amplitudes of observed diffracted waves the modeling has resulted in an estimation of a crack length 1 1 to 1 5 times larger than previously predicted whereas the height remains essentially the same as that derived using other techniques this example demonstrates that it is possible to estimate fracture dimensions by analyzing diffracted waves

joint sets that enhance production from middle and upper devonian gas shales of the appalachian basin the marine middle and upper devonian section of the appalachian basin includes several black shale units that carry two regional joint sets j1 and j2 sets as observed in outcrop core and borehole images these joints formed close to or at peak burial depth as natural hydraulic fractures induced by abnormal fluid pressures generated during thermal maturation of organic matter when present together earlier j1 joints are crosscut by later j2 joints in outcrops of black shale on the foreland northwest side of the appalachian basin the east northeast trending j1 set is more closely spaced than the northwest striking j2 set however j2 joints are far more pervasive throughout the exposed devonian marine clastic section on both sides of the basin by geological coincidence the j1 set is nearly parallel the maximum compressive normal stress of the contemporary tectonic stress field shmax because the contemporary tectonic stress field favors the propagation of hydraulic fracture completions to the east northeast fracture stimulation from vertical wells intersects and drains j2 joints horizontal drilling and subsequent stimulation benefit from both joint sets by drilling in the north northwest south southeast directions horizontal wells cross and drain j1 joints whenever present then staged hydraulic fracture stimulations if necessary run east northeast i e parallel to the j1 set under the influence of the contemporary tectonic stress field thereby crosscutting and draining j2 joints

the direct current response of electrically conducting fractures excited by a grounded current source abstracthydraulic fracture stimulation of low permeability reservoir rocks is an established and cross cutting technology for enhancing hydrocarbon production in sedimentary formations and increasing heat exchange in crystalline geothermal systems whereas the primary measure of success is the ability to keep the newly generated fractures sufficiently open long term reservoir management requires a knowledge of the spatial extent morphology and distribution of the fractures knowledge primarily informed by microseismic and ground deformation monitoring to minimize the uncertainty associated with interpreting such data we investigate through numerical simulation the usefulness of direct current dc resistivity data for characterizing subsurface fractures with elevated electrical conductivity by considering a geophysical experiment consisting of a grounded current source deployed in a steel cased borehole in doing so the casing efficiently energizes the fractures with steady current finite element

solar heating of a stratified ocean in the presence of a static ice cover conductivity temperature and depth measurements were carried out in an isolated transverse lead in static shorefast ice in mould bay prince patrick island northwest territories during a 3 week period at the height of the melt season currents beneath the ice appeared to be weak and largely tidal in nature initially the water was vertically uniform and at the salinity determined freezing point down to a depth of at least 20 m by the end of the experiment the water column consisted of three distinct layers a well mixed nearly fresh surface meltwater layer a very stable half meter thick halocline centered somewhat below the bottom of the ice and a thermally stratified layer of constant salinity extending down to at least 25 m the halocline was characterized by a temperature maximum that was about 2 c warmer than the surrounding water this temperature maximum in the pycnocline effectively trapped shortwave energy absorbed in the lower layer and prevented it from melting the overlying ice theoretical calculations demonstrate that the thermal structure observed beneath the pycnocline was controlled by the input of shortwave radiation and that vertical heat transport was largely the result of diffusive processes the presence of leads drastically increases the amount of energy stored in the water in regions where leads are common it is likely that this energy will significantly accelerate the decay and removal of the ice once it becomes mobile and once the pycnocline is erased

supercooled water in an arctic polynya observations and modeling in situ field measurements of an active polynya in storfjorden svalbard during april 2006 are presented a surface heat flux estimated to be 400 w m 2 produced frazil ice that was advected away from the fast ice edge during the end of a polynya event driven by cold winds from the northeast conductivity temperature and depth casts from the fast ice edge of the polynya were calibrated by accompanying water samples and reveal a supercooling event that lasted for 3 days in a 5 m deep water column surface salinity reached 35 9 psu from brine release during ice growth the maximum supercooling measured was 0 037 0 005 c below the in situ freezing point near the surface and 0 016 0 005 c at the bottom the mean supercooling gradient was 0 020 0 005 c between the surface and the bottom these measurements are consistent with results from a one dimensional frazil ice model confirming that such supercooling levels can be expected frazil ice concentrations in the water were modeled to be lower than 0 02 g l 1 due to advection in the surface layer seven frazil grease ice samples taken from a place where advection was blocked along the fast ice edge showed a mean salinity of 26 2 psu indicating 25 frazil ice and 75 sea water in the grease ice the water column salinity decreased during the measurement period due to less saline water replacing newly formed brine enriched shelf water flowing down to deeper parts of storfjorden the supercooling ceased when the wind direction turned to the east with higher air temperatures and warmer and less saline water being pushed into storfjorden by the northward ekman transport these are the first in situ observations from an active arctic polynya with concurrent sampling of hydrography and frazil ice and the supercooling is the maximum observed in recent years with modern and accurate instrumentation

the freshwater budget and under ice spreading of mackenzie river water in the canadian beaufort sea based on salinity and 18o 16o measurements in water and ice observations of salinity and oxygen isotope composition 18o were made for the beaufort shelf mackenzie estuary waters in september 1990 just prior to ice formation and for both the water column and ice in april may 1991 at the end of winter these measurements are used to determine the apportioning of fresh water in the estuary between its two main sources runoff and sea ice melt changes in disposition of water between seasons and amounts frozen into the growing ice sheet are also derived two domains are considered in order to construct a freshwater budget for the mackenzie shelf the nearshore within which landfast ice grows in winter and the outer shelf most of the winter inflow from the mackenzie river appears to remain impounded as liquid under the ice within the landfast zone at the end of winter and about 15 of it is incorporated into the landfast ice oxygen isotopes 18o in ice cores collected from across the shelf record the progress beneath the ice of new mackenzie inflow as it invades the nearshore throughout winter rates of spreading are about 0 2 cm s 1 away from the coast and 1 3 cm s 1 along the coast as this inflow spreads across the shelf it progressively shuts off convection driven by brine production at locations within the landfast ice salinity and 18o in the offshore water column suggest that about 3 m of sea ice was formed in the outer shelf domain since both brine and newly formed sea ice can be advected off the shelf a complete budget for brine or sea ice production cannot be established without first measuring the advection of one of these two components

image based near surface modeling for statics corrections the near surface is usually defined as the depth interval below the topography composed of a low velocity soil column and weathered rock layer in contrast with the near surface the subsurface is composed of relatively higher velocity rock layers this means that the interface between the near surface and the subsurface often gives rise to a strong shallow reflection the image based method described in this paper makes use of the strong reflection at the base of the near surface to estimate a model for the near surface for statics corrections the method is based on construction of constant velocity migration volume by prestack time migration of shot records the near surface rms velocities associated with the strong reflector at the base of the near surface are picked from the semblance spectra to create the near surface rms velocity field this laterally varying but vertically invariant velocity field actually is equivalent to the near surface interval velocity field which can be used to perform p

seismic monitoring of diffraction images for detection of local heterogeneities diffracted waves contain valuable information regarding both the structure and composition of the media they are in in seismic data processing however these waves are usually regarded as noise in this paper we present an attempt to use scattered diffracted waves for the detection of local heterogeneities the method is based on the detection of diffracted waves by concentrating the signal amplitudes from diffracting points on the seismic section this is done using a correlation procedure that enhances the amplitude of the seismic signal at the location of the diffractors on the common diffraction point section d section the new local time correction for diffraction traveltime curve parameterization is based on the radius of curvature of the diffracted wavefront and near surface velocity we use the idea of seismic monitoring for detection and delineating local objects which may occur within the subsurface resulting from human activity or fast geological processes the method consists of continuou

seismic imaging using microseismic events results from the san andreas fault system at safod 1 we have developed a new passive seismic imaging approach that consists of two steps first the hypocenter of the microseismic event is precisely located second this event is treated as a pseudo active seismic source and the reflections within the recorded wavefield are processed by using a directional migration algorithm in order to construct a high resolution image of the illuminated subsurface region in this paper we demonstrate the application of our approach to a number of microseismic events recorded by a borehole array in the san andreas fault observatory at depth main hole results obtained were high resolution 3 d images of different se nw oriented reflectors related to the san andreas fault saf system in the close vicinity of the borehole to support our approach we compared our findings with other active and passive seismic images and analyzed the correlation with borehole lithology we revealed a predominantly satisfactory agreement for both juxtapositions furthermore the stacked image of several microearthquakes provides a spatial characterization of the complex internal structure of the saf with much higher resolution than can be obtained from surface seismic reflection data

active and passive imaging of hydraulic fractures hydraulic fracturing is a method commonly used to stimulate production in low permeability reservoirs with this method a viscous polymer based fluid is injected into the formation opening a fracture perpendicular to the direction of least principal stress this is followed by injection of a suspension of sand in liquid proppant which prevents the fracture from closing after the fluid pressure is allowed to bleed off after a fracture treatment the reservoir can be produced from the vicinity of a fracture surface which is substantially larger than the surface of the borehole

beyond receiver functions passive source reverse time migration and inverse scattering of converted waves 1 we present a wave equation prestack depth migration to image crust and mantle structures using multi component earthquake data recorded at dense seismograph arrays transmittedp and swaves recorded on the surface are back propagated using an elastic wave equation solver the wave modes are separated after the reverse time continuation of the wavefield from the surface and subjected to a cross correlation type imaging condition forming an inverse scattering transform reverse time migration rtm does not make assumptions about the presence or properties of interfaces notably it does not assume that interfaces are locally horizontal with synthetic experiments and different background models we show that passive source rtm can reconstruct dipping and vertically offset interfaces even in the presence of complex wave phenomena such as caustics and point diffraction and that its performance is superior to traditional receiver function analysis e g common conversion point ccp stacking in complex geological environments

development and properties of sea ice in the coastal regime of the southeastern weddell sea from october to december 1986 a program consisting of sea ice core analysis in combination with sea ice observations was carried out from the icebreaker r v polarstern as part of the winter weddell sea project the ship operated in the central and southeastern weddell sea with interests focusing on the ice shelf front between 70 s and 77 s where a system of coastal polynyas extended for several tens to hundreds of kilometers opened and maintained despite temperatures well below freezing by offshore wind forcing three wind controlled ice formation regimes may be distinguished 1 for wind speeds 10 m s the ice cover is heavily ridged and rafted 50 and formation of granular ice predominates of the total length of cores sampled in the coastal regime 32 consisted of columnar and 35 of granular ice in addition we observed two unique ice classes along the coast platelet ice formed through consolidation of underwater ice platelets underneath the solid ice cover contributed 11 to our core samples another 22 consisted of intermediate columnar granular ice which originates under the dynamic hydrographic conditions of the near ice shelf region

interdecadal changes in snow depth on arctic sea ice snow plays a key role in the growth and decay of arctic sea ice in winter it insulates sea ice from cold air temperatures slowing sea ice growth from spring to summer the albedo of snow determines how much insolation is absorbed by the sea ice and underlying ocean impacting ice melt processes knowledge of the contemporary snow depth distribution is essential for estimating sea ice thickness and volume and for understanding and modeling sea ice thermodynamics in the changing arctic this study assesses spring snow depth distribution on arctic sea ice using airborne radar observations from operation icebridge for 2009 2013 data were validated using coordinated in situ measurements taken in march 2012 during the bromine ozone and mercury experiment bromex field campaign we find a correlation of 0 59 and root mean square error of 5 8 cm between the airborne and in situ data using this relationship and icebridge snow thickness products we compared the recent results with data from the 1937 1954 1991 soviet drifting ice stations the comparison shows thinning of the snowpack from 35 1 9 4 to 22 2 1 9 cm in the western arctic and from 32 8 9 4 to 14 5 1 9 cm in the beaufort and chukchi seas these changes suggest a snow depth decline of 37 29 in the western arctic and 56 33 in the beaufort and chukchi seas thinning is negatively correlated with the delayed onset of sea ice freezeup during autumn

image processing and roughness analysis of exposed bedrock fault planes as a tool for paleoseismological analysis results from the campo felice fault central apennines italy abstract morphologic investigations along the campo felice cf fault central apennines italy have been made in order to develop a procedure for the paleoseismological analysis of bedrock fault scarps the cf fault has been responsible for the formation of an impressive limestone fault scarp geomorphologic work on the cf basin and related fault indicated that the scarp originated from tectonic fault displacements three morphologic units have been distinguished along the fault scarp and defined as morphosome m1 lowest part of the scarp m2 and m3 the uppermost part these units display different karstic features which are the result of their different duration of exposure to weathering micromorphologic analyses focused on the morphosome m1 along which the cf fault plane is exposed for a height ranging between 4 and 7 m these analyses were aimed at defining differently weathered bands located at various heights and parallel to the fault scarp top and base the presence of these bands suggests repeated fault movements the exposed fault surface displays a low grade biokarstic weathering due to the action of epilithic and endolithic organisms the biokarst distribution is however inhomogeneous and conditioned by the presence of nourishing elements moisture and by light intensity an area preferentially affected by the biokarstic processes develops as a band at the bedrock soil contact at the base of the scarp roughness and colour analyses were made to identify uplifted bands which previously formed at the bedrock soil contact the roughness analysis was made using a microroughness meter along 20 cm long horizontal transects repeated each 20 cm of fault height for the entire morphosome m1 at various sites along the scarp the roughness variance data plotted vs the fault height failed to identify differently weathered bands of paleoseismological interest this result is probably due to the complex distribution of biokarst along the investigated fault plane more reliable results have been obtained by areal analysis of the variation of the colour rendering of the rocks exposed along the fault plane at different sites photographic images of large portions of fault surfaces have been processed with standard graphic computer programs the variations of colour indicated the presence of bands at various heights along the fault plane two uplifted bands have been recognised at all the investigated sites suggesting two displacement events e1 and e2 a preliminary chronological framework for these two events the youngest of which affected the cf fault can be derived from the paleoseismological data available for the southernmost branch of the regional fault system that includes the cf fault according to these data e1 may have occurred between 860 and 1300 ad while e2 may have occurred at about 1900 bc work is in progress to define surface exposure ages of different parts of the fault plane by means of in situ produced cosmogenic 36 cl this procedure will give further chronological constraints for the age of e1 and e2 and will also permit to test the validity of the micromorphologic analysis of bedrock fault scarps for paleoseismological aims

double difference tomography the method and its application to the hayward fault california we have developed a double difference dd seismic tomography method that makes use of both absolute and relative arrival times by reducing systematic errors using the more accurate relative arrival times the method produces an improved velocity model simultaneously it yields event locations of a quality equivalent to those of the dd location method while avoiding simplifying assumptions of that method we test this method on a synthetic dataset and find that it produces a more accurate velocity model and event locations than standard tomography we also test this method on a hayward fault california earthquake dataset spanning 1984 1998 the earthquakes relocated by this method collapse to a thin line along the fault trace consistent with previous results the dd velocity model has sharper velocity contrasts near the source region than the standard tomography model

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

thermal evolution of permeability and microstructure in sea ice 1 the fluid permeability k of sea ice constrains a broad range of processes such as the growth and decay of seasonal ice the evolution of summer ice albedo and biomass build up such processes are critical to how sea ice and associated ecosystems respond to climate change however studies of k and its dependence on brine porosity and microstructure are sparse here we present a multifaceted theory for k which closely captures laboratory and field data x ray computed tomography provides an unprecedented look at the brine phase and its connectivity we find that sea ice displays universal transport properties remarkably similar to crustal rocks yet over a much narrower temperature range our results yield simple parameterizations for fluid transport in terms of temperature and salinity and permit more realistic representations of sea ice in global climate and biological models

observation and modeling of platelet ice fabric in mcmurdo sound antarctica 1 during the annual growth of landfast ice in mcmurdo sound antarctica an episodic flux of platelet ice crystals from the ocean contributes to the build up of a porous subice platelet layer which is steadily incorporated into the sea ice cover as it thickens over winter in november 2007 we examined the spatial variability of these processes by collecting sea ice cores with simultaneous oceanographic observations along an east west transect in the sound previously identified draped and bladed platelet ice types were observed in addition we identify resumed columnar growth which appears to be a result of geometric selection from the subice platelet layer after the arrival of new platelet crystals from the ocean has ceased a numerical model of mechanical platelet ice processes is developed that predicts crystal texture and c axis distributions producing virtual incorporated platelet ice with known growth history this model demonstrates how a disordered subice platelet layer arises from an initially flat interface and suggests that such a layer is more likely to form later in the growth season the model also suggests how the grain boundary density in incorporated platelet ice responds to changes in the flux of loose platelet crystals from the ocean application of this result to our 2007 platelet ice observations indicates that sea ice in western mcmurdo sound is subject to larger and more persistent platelet fluxes than the ice in the east this is consistent with the pattern of in situ supercooling just beneath the ocean surface

thin and thinner sea ice mass balance measurements during sheba 1 as part of a large interdisciplinary study of the surface heat budget of the arctic ocean sheba we installed more than 135 ice thickness gauges to determine the sea ice mass balance while installing these gauges during the fall of 1997 we found that much of the multiyear ice cover was only 1 m thick considerably thinner than expected over the course of the yearlong field experiment we monitored the mass balance for a wide variety of ice types including first year ice ponded ice unponded ice multiyear ice hummocks new ridges and old ridges initial ice thicknesses for these sites ranged from 0 3 to 8 m and snow depths varied from a few centimeters to more than a meter however for all of their differences and variety these thickness gauges sites shared a common trait at every site there was a net thinning of the ice during the sheba year the thin ice found in october 1997 was even thinner in october 1998 the annual cycle of ice thickness was also similar at all sites there was a steady increase in thickness through the winter that gradually tapered off in the spring this was followed by a steep drop off in thickness during summer melt and another tapering in late summer and early fall as freeze up began maximum surface melting was in july while bottom ablation peaked in august combining results from the sites we found an average winter growth of 0 51 m and a summer melt of 1 26 m which consisted of 0 64 m of surface melt and 0 62 m of bottom melt there was a weak trend for thicker ice to have less winter growth and greater net loss for the year however ice growth was also impacted by the snow depth considerable variability was observed between sites in both accretion and ablation the total accretion during the 9 month growth season ranged from zero for thick ridged ice to more than a meter for young ice ponds tended to have a large amount of surface melting while ridges had considerable bottom ablation

further testing of the bedding plane slip model for hydraulic fracture opening using moment tensor inversions abstractmoment tensors are calculated by using the p wave first motion peak amplitudes of 59 microseismic events with high signal to noise ratio these events are from a surface microseismic data set gathered during hydraulic fracture stimulation of the marcellus gas shale in washington county pennsylvania usa the majority of these 59 events have a horizontal nodal plane a few degrees characteristic of a dip slip horizontal slip moment tensor if the horizontal nodal plane is an auxiliary the vertical nodal plane has a pure dip slip motion which is inconsistent with the opening motion for vertical hydraulic fractures that enables proppant loading this points to slip on horizontal nodal planes with the auxiliary vertical nodal planes aligned with the local maximum horizontal stress orientation as indicated by drilling induced fractures in nearby vertical wells these 59 microseismic events are caused by slip on horizontal mechanical discontinuities such as bedding planes during the opening of ver

origin of an anhydrite sheath encircling a salt diapir and implications for the seismic imaging of steep sided salt structures egersund basin northern north sea the ingress of meteoric fluids into the crests of salt structures typically results in the formation of anhydrite dominated caprock the migration of connate fluids up the margins of salt structures has also been documented although the products related to this are rarely sampled in the deep subsurface we use 3d seismic reflection and borehole data to document the geometry and lithology of a salt diapir seismic data suggest that a weld is developed along the diapir stem although borehole data indicate that the stem consists of an inner c 1500 m thick halite dominated zone and an outer c 250 m thick anhydrite dominated sheath the anhydrite may have formed early in the basin history as a depositional anhydrite or crestal caprock an alternative interpretation is that the anhydrite represents lateral caprock which formed late in the basin history in response to the migration of nacl poor fluid up the margins of the diapir and dissolution of halite the possibility that lateral caprock may form adjacent to salt structures has implications for understanding the patterns and vigour of groundwater flow in sedimentary basins furthermore our study shows the margins of steep sided salt structures may be misidentified by several hundred metres if time migrated seismic reflection data are used

fast location of seismicity a migration type approach with application to hydraulic fracturing data we propose a new approach for the location of seismic sources using a technique inspired by gaussian beam migration of three component data this approach requires only the preliminary picking of time intervals around a detected event and is much less sensitive to the picking precision than standard location procedures furthermore this approach is characterized by a high degree of automation the polarization information of three component data is estimated and used to perform initial value ray tracing by weighting the energy of the signal using gaussian beams around these rays the stacking is restricted to physically relevant regions only event locations correspond to regions of maximum energy in the resulting image we have successfully applied the method to synthetic data examples with 20 30 white noise and to real data of a hydraulic fracturing experiment where events with comparatively small magnitudes 0 were recorded

scattering by hydraulic fractures finite difference modeling and laboratory data reservoir production can be stimulated by creating hydraulic fractures that effectively facilitate the inflow of hydrocarbons into a well considering the effectiveness and safety of the operation it is desirable to monitor the size and location of the fracture in this paper we investigate the possibilities of using seismic waves generated by active sources to characterize the fractures first we must understand the scattering of seismic waves by hydraulic fractures for that purpose we use a finite difference modeling scheme we argue that a mechanically open hydraulic fracture can be represented by a thin fluid filled layer the width or aperture of the fracture is often small compared to the seismic wavelength which forces us to use a very fine grid spacing to define the fracture based on equidistant grids this results in a large number of grid points and hence computationally expensive problems we show that this problem can be overcome by allowing for a variation in grid spacing in the finite

diffraction imaging by focusing defocusing an outlook on seismic superresolution diffractions always need more advertising it is true that conventional seismic processing and migration are usually successful in using specular reflections to estimate subsurface velocities and reconstruct the geometry and strength of continuous and pronounced reflectors however correct identification of geological discontinuities such as faults pinch outs and small size scattering objects is one of the main objectives of seismic interpretation the seismic response from these structural elements is encoded in diffractions and diffractions are essentially lost during the conventional processing migration sequence hence we advocate a diffraction based data oriented approach to enhance image resolution as opposed to the traditional image oriented techniques which operate on the image after processing and migration even more it can be shown that at least in principle processing of diffractions can lead to superresolution and the recovery of details smaller than the seismic wavelength the so called reflection stack is capable of effectively separating diffracted and reflected energy on a prestack shot gather by focusing the reflection to a point while the diffraction remains unfocused over a large area muting the reflection focus and defocusing the residual wavefield result in a shot gather that contains mostly diffractions diffraction imaging applies the classical isotropic diffraction stack to these diffraction shot gathers this focusingmuting defocusing approach can successfully image faults small size scattering objects and diffracting edges it can be implemented both in model independent and modeldependent contexts the resulting diffraction images can greatly assist the interpreter when used as a standard supplement to full wave images

geomechanical modeling of stresses adjacent to salt bodies part 1 uncoupled models we compare four approaches to geomechanical modeling of stresses adjacent to salt bodies these approaches are distinguished by their use of elastic or elastoplastic constitutive laws for sediments surrounding the salt as well as their treatment of fluid pressures in modeling we simulate total stress in an elastic medium and then subtract an assumed pore pressure after calculations are complete simulate effective stress in an elastic medium and use assumed pore pressure during calculations simulate total stress in an elastoplastic medium either ignoring pore pressure or approximating its effects by decreasing the internal friction angle and simulate effective stress in an elastoplastic medium and use assumed pore pressure during calculations to evaluate these approaches we compare stresses generated by viscoelastic stress relaxation of a salt sphere in all cases relaxation causes the salt sphere to shorten vertically and expand laterally producing extensional strains above and below the sphere and shortening against the sphere flanks deviatoric stresses are highest when sediments are assumed to be elastic whereas plastic yielding in elastoplastic models places an upper limit on deviatoric stresses that the rocks can support so stress perturbations are smaller these comparisons provide insights into stresses around salt bodies and give geoscientists a basis for evaluating and comparing stress predictions

aerobic biomineralization of mg rich carbonates implications for natural environments we studied the formation of mg rich carbonate in culture experiments using different aerobic bacterial strains and aqueous mg ca ratios 2 to 11 5 at earth surface conditions these bacteria promoted the formation of microenvironments that facilitate the precipitation of mineral phases dolomite huntite high mg calcite and hydromagnesite that were undersaturated in the bulk solution or kinetically inhibited dolomite huntite high mg calcite hydromagnesite and struvite precipitated in different proportions and at different times depending on the composition of the medium the mg content of dolomite and calcite decreased with an increasing ca concentration in the medium the stable carbon isotope composition of the mg rich carbonate precipitates reflected the isotope composition of the organic compounds present in the media suggesting that microbial metabolism strongly influenced the carbon isotope composition of biomediated carbonates we observed that ca enriched carbonate precipitates have relatively low carbon isotope composition these results provide insights into the mechanism s of carbonate formation in natural systems and they are of fundamental importance for understanding modern environments in which carbonate minerals form as a window into the geologic past

three dimensional passive seismic waveform imaging around the safod site california using the generalized radon transform 1 we apply a three dimensional 3d generalized radon transform grt to scattered p waves from 575 local earthquakes recorded at 68 temporary network stations for passive source imaging of near vertical structures close to the san andreas fault observatory at depth safod site the grt image profiles through or close by the safod site reveal near vertical reflectors close to the fault zone as well as in the granite to the southwest and the franciscan melange to the northeast of the main fault although slightly lower in resolution these structures are generally similar to features in 2d images produced with steep dip prestack seismic migration of data from active source seismic reflection and refraction surveys our grt images however also reveal several vertical reflectors to the northeast of the saf that do not appear in the migration images but which are consistent with local geology these results suggest that in a seismically active area inverse scattering of earthquake data for instance with the grt can be a viable and in 3d economic alternative to an active source survey

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

self potential observations during hydraulic fracturing 1 the self potential sp response during hydraulic fracturing of intact sierra granite was investigated in the laboratory excellent correlation of pressure drop and sp suggests that the sp response is created primarily by electrokinetic coupling for low pressures the variation of sp with pressure drop is linear indicating a constant coupling coefficient cc of 200 mv mpa however for pressure drops 2 mpa the magnitude of the cc increases by 80 in an exponential trend this increasing cc is related to increasing permeability at high pore pressures caused by dilatancy of microcracks and is explained by a decrease in the hydraulic tortuosity resistivity measurements reveal a decrease of 2 prior to hydraulic fracturing and a decrease of similar to 35 after fracturing an asymmetric spatial sp response created by injectate diffusion into dilatant zones is observed prior to hydraulic fracturing and in most cases this sp variation revealed the impending crack geometry seconds before failure at rupture injectate rushes into the new fracture area where the zeta potential is different than in the rock porosity and an anomalous sp spike is observed after fracturing the spatial sp distribution reveals the direction of fracture propagation finally during tensile cracking in a point load device with no water flow a sp spike is observed that is caused by contact electrification however the time constant of this event is much less than that for transients observed during hydraulic fracturing suggesting that sp created solely from material fracture does not contribute to the sp response during hydraulic fracturing

seismic monitoring of the growth of a hydraulic fracture zone at fenton hill new mexico the hydraulic fracturing technique is an important method for enhancing hydrocarbon recovery geothermal energy extraction and solid waste disposal determination of the geometry and growth process of a hydraulic fracture zone is important for monitoring and assessing subsurface fractures a relative source location approach based on a waveform correlation and a grid search method has been developed to estimate relative hypocenter locations for a cluster of 157 microearthquakes induced by hydraulic fracturing at the los alamos hot dry rock hdr geothermal site among the 157 events 147 microearthquakes occurred in a tight cluster with a dimension of 40 m roughly defining a vertical hydraulic fracture zone with an orientation of n40 w the length height and width of the hydraulic fracture zone are estimated to be 40 35 and 5 m respectively analysis of the spatial temporal pattern of the induced microearthquakes reveals that the fracture zone grew significantly averaging 0 2 m minute in a two h

seasonal evolution of an ice shelf influenced fast ice regime derived from an autonomous thermistor chain ice shelves strongly interact with coastal antarctic sea ice and the associated ecosystem by creating conditions favorable to the formation of a sub ice platelet layer the close investigation of this phenomenon and its seasonal evolution remains a challenge due to logistical constraints and a lack of suitable methodology in this study we characterize the seasonal cycle of antarctic fast ice adjacent to the ekstrom ice shelf in the eastern weddell sea we used a thermistor chain with the additional ability to record the temperature response induced by cyclic heating of resistors embedded in the chain vertical sea ice temperature and heating profiles obtained daily between november 2012 and february 2014 were analyzed to determine sea ice and snow evolution and to calculate the basal energy budget the residual heat flux translated into an ice volume fraction in the platelet layer of 0 18 0 09 which we reproduced by a independent model simulation and agrees with earlier results manual drillings revealed an average annual platelet layer thickness increase of at least 4 m and an annual maximum thickness of 10 m beneath second year sea ice the oceanic contribution dominated the total sea ice production during the study effectively accounting for up to 70 of second year sea ice growth in summer an oceanic heat flux of 21 w m 2 led to a partial thinning of the platelet layer our results further show that the active heating method in contrast to the acoustic sounding approach is well suited to derive the fast ice mass balance in regions influenced by ocean ice shelf interaction as it allows subdiurnal monitoring of the platelet layer thickness

single fractures under normal stress the relation between fracture specific stiffness and fluid flow abstract fracture specific stiffness and fluid flow through a single fracture under normal stress are implicitly related through the geometry of the void space and contact area that comprise the fracture data from thirteen different rock samples each containing a single fracture show that relationships between fracture specific stiffness and fluid flow through a fracture fall into two general classes of behavior fractures either fall on a loosely defined universal curve relating fluid flow to fracture specific stiffness or else the flow is weakly dependent on fracture specific stiffness the second relationship shows that flow decreases slowly with increasing fracture specific stiffness the first relationship shows that flow decreases rapidly for increases in fracture specific stiffness to understand this behavior computer simulations on simulated single fractures were performed to calculate fluid flow fracture displacement and fracture specific stiffness as a function of normal stress simulated fractures with spatially correlated and uncorrelated aperture distributions were studied fractures with spatially uncorrelated aperture distributions tend to exhibit a weak dependence of fluid flow on fracture specific stiffness because these fractures tend to have multiple connected paths across the sample which can support flow with uniformly distributed contact area thus an increment in stress will increase the stiffness of the fracture without greatly reducing the amount of fluid flow on the other hand fractures with spatially correlated aperture distributions tend to belong to the universal relationship because correlated fractures tend to have only one or two dominant flow paths and the contact area is limited to a few regions resulting in a compliant fracture thus an increment in stress on a spatially correlated fracture will result in an increase in stiffness and rapid decrease in fluid flow these spatial correlations in fracture void geometry can be differentiated in the laboratory based on the observed fracture specific stiffness fluid flow relationship for a single fracture under normal loading

a major increase in winter snowfall during the middle holocene on western greenland caused by reduced sea ice in baffin bay and the labrador sea precipitation is predicted to increase in the arctic as temperature increases and sea ice retreats yet the mechanisms controlling precipitation in the arctic are poorly understood and quantified only by the short sparse instrumental record we use hydrogen isotope ratios 2h of lipid biomarkers in lake sediments from western greenland to reconstruct precipitation seasonality and summer temperature during the past 8 kyr aquatic biomarker 2h was 100 more negative from 6 to 4 ka than during the early and late holocene which we interpret to reflect increased winter snowfall the middle holocene also had high summer air temperature decreased early winter sea ice in baffin bay and the labrador sea and a strong warm west greenland current these results corroborate model predictions of winter snowfall increases caused by sea ice retreat and furthermore suggest that warm currents advecting more heat into the polar seas may enhance arctic evaporation and snowfall

reconstructing hydrographic change in petersen bay ellesmere island canada inferred from sar imagery abstract synthetic aperture radar sar satellite imagery was used to reconstruct the change in limnological conditions adjacent to an arctic ice shelf by examining the backscatter values of coastal ice in mid winter scenes high sar backscatter values 6 db suggest that an ice dammed lake was present adjacent to the south coast of petersen bay from 1992 until 2005 following a large calving event of the adjacent petersen ice shelf 8 07 km2 in august 2005 the lake drained through a region where the ice shelf had separated from the coastline this loss of freshwater and replacement of lake ice with sea ice along the southern coast of petersen bay were confirmed from analyses of ice core samples and conductivity temperature depth ctd profiles the exception to this pattern was one distinct area where terrestrial streams entered the edge of petersen bay and freshwater continued to collect from 2006 to 2008 however this ephemeral area of freshwater has not reformed since 2009 due to the persistence of open water events in petersen bay observed in optical satellite imagery which likely facilitated mixing of freshwater with sea water based on the continued break ups of petersen ice shelf and the frequency of open water events it is unlikely that this ice dammed lake will reform the results of this study underscore the utility of sar for reconstructing past hydrographic conditions in the water column below

platelet ice under arctic pack ice in winter the formation of platelet ice is well known to occur under antarctic sea ice where subice platelet layers form from supercooled ice shelf water in the arctic however platelet ice formation has not been extensively observed and its formation and morphology currently remain enigmatic here we present the first comprehensive long term in situ observations of a decimeter thick subice platelet layer under free drifting pack ice of the central arctic in winter observations carried out with a remotely operated underwater vehicle rov during the midwinter leg of the mosaic drift expedition provide clear evidence of the growth of platelet ice layers from supercooled water present in the ocean mixed layer this platelet formation takes place under all ice types present during the surveys oceanographic data from autonomous observing platforms lead us to the conclusion that platelet ice formation is a widespread but yet overlooked feature of arctic winter sea ice growth

local migration with extrapolated vsp green s functions we have developed a novel vertical seismic profile vsp imaging method that requires only a local velocity model around the well this method is denoted as a local migration where the model based forward propagation and backward propagation operators are computed using a local velocity model between the well and the target body the velocity model for the complex overburden and salt body is not needed and the source side statics are automatically accounted for in addition kinematic and dynamic effects including anisotropy absorption and other unknown undefined rock effects outside of this local velocity model are mostly accounted for numerical tests on an acoustic 2d sigsbee vsp data set an elastic salt model and offset 2d vsp data in the gulf of mexico partly validate the effectiveness of this method by accurately imaging the sediments next to the vertical well

physical and morphological properties of sea ice in the chukchi and beaufort seas during the 2010 and 2011 nasa icescape missions abstract physical and morphological properties of sea ice in the chukchi and western beaufort seas were measured during the 2010 and 2011 june july icescape impacts of climate on the eco systems and chemistry of the arctic pacific environment missions aboard the uscgc healy observations of ice conditions including ice thicknesses types and concentrations of primary secondary and tertiary categories were reported at 2 h intervals while the ship was in transit using the antarctic sea ice processes and climate aspect protocol on ice surveys of ice thickness melt pond depth and ice properties including profiles of internal temperature salinity and isotopic composition were conducted at 21 total ice stations 12 in 2010 and 9 in 2011 comparison to historical ship based observations confirms a multi decadal transition from a multiyear to thinner first year dominated seasonal sea ice pack in the region with much earlier ice retreat than in past decades the ice encountered was predominantly 98 first year with un deformed ice thicknesses ranging from 0 73 to 1 2 m pond coverage was extensive averaging 29 in 2010 and 19 in 2011 resulting in considerable light absorption in the ice and transmission of light to the ocean enhanced melting near the ice edge is consistent with transport of pacific origin heat and or ice albedo feedbacks sediment entrainment was visible in 7 5 and 10 9 of the ice in 2010 and 2011 respectively overall the results indicate a regime shift in the characteristics of the ice cover has occurred in the region over recent decades with substantive implications for thermodynamic forcing light availability in the upper ocean and biological and biogeochemical processes in the ice and water column beneath the results presented have applications for the interpretation of optical and biological measurements in the chukchi sea and serve as record of ice conditions for assessing long term change

applications of bacterial carbonatogenesis to the protection and regeneration of limestones in buildings and historic patrimony abstract the ability of the so called carbonatogenic bacteria may be used for producing surficial protecting coatings biocalcin on limestones buildings monuments and statuary this is the concept of biomineralisation laboratory experiments were undertaken in order to choose the most active bacterial strains and to improve the composition of the nutritional media suitable for stimulating the nitrogen cycle metabolic pathways which are the only ones to be used in aerobic and or microaerophilic conditions life size experiments subsequent measurements and observations show that this natural process of biomineralisation is feasible within the frame of industrial constraints this concept of further developments includes the creation of patinas and the manufacturing of biological mortars and cements which were also improved in the laboratory the biomineralisation recreates a material that is remarkably similar to the limestone substrate because it uses natural microbial mediation which follows the same natural processes that formed many limestones

global postseismic rebound of a viscoelastic earth theory for finite faults and application to the 1964 alaska earthquake a spherically symmetric earth model with viscoelastic rheology is used to study the postseismic rebound associated with finite lithospheric dislocations we perform a systematic study of surface deformations due to sources characterized by two and three dimensional faults modeled by a linear and planar distribution of point sources our approach is based on the normal mode technique for a layered earth with linear viscoelastic rheology and allows for a self consistent description of the time evolution of postseismic displacements due to strike and dip slip faults as a case study we compare the predicted horizontal displacements due to the 1964 alaska earthquake with very long baseline interferometry vlbi baselines changes observed in this region in the period 1984 1989 although subduction and postglacial isostatic adjustment are expected to contribute significantly to present day velocities our results show that the postseismic rebound due to the 1964 alaska earthquake plays a relevant role in the changes of several vlbi baselines the last section is devoted to the analysis of the evolution of other north american baselines as suggested by recent investigations based on forward approaches isostatic readjustment of the earth s crust in response to the melting of the last pleistocene ice sheets is a major contributor to baseline variations in this area our results indicate that a correct interpretation of vlbi baseline changes in north america should account also for the effects of postseismic deformation

evidence for basal marine ice in the filchner ronne ice shelf the filchner ronne ice shelf which drains most of the marine based portions of the west antarctic ice sheet is the largest ice shelf on earth by volume the origin and properties of the ice that constitutes this shelf are poorly understood because a strong reflecting interface within the ice and the diffuse nature of the ice ocean interface make seismic and radio echo sounding data difficult to interpret1 2 ice in the upper part of the shelf is of meteoric origin but it has been proposed2 5 that a basal layer of saline ice accumulates from below here we present the results of an analysis of the physical and chemical characteristics of an ice core drilled almost to the bottom of the ronne ice shelf we observe a change in ice properties at about 150 m depth which we ascribe to a change from meteoric ice to basal marine ice the basal ice is very different from sea ice formed at the ocean surface and we propose a formation mechanism in which ice platelets in the water column accrete to the bottom of the ice shelf

ca carbonates precipitation and limestone genesis the microbiogeologist point of view abstract experiments show that the production of carbonate particles by heterotrophic bacteria follows different ways in heterotrophy the passive carbonatogenesis is generated by modifications of the medium that lead to the accumulation of carbonate and bicarbonate ions and to the precipitation of solid particles it is induced by several metabolic pathways of the nitrogen cycle ammonification of amino acids degradation of urea and uric acid dissimilatory reduction of nitrates and of the sulphur cycle dissimilatory reduction of sulphates the active carbonatogenesis is independent of the mentioned metabolic pathways the carbonate particles are produced by ionic exchanges through the cell membrane following still poorly known mechanisms in autotrophy non methylotrophic methanogenesis and cyanobacterial photosynthesis also may contribute to the precipitation of carbonates autotrophic carbonates as carbonatogenesis is neither restricted to particular taxonomic groups of bacteria nor to specific environments it has been an ubiquitous phenomenon since precambrian times carbonatogenesis is the response of heterotrophic bacterial communities to an enrichment of the milieu in organic matter after a phase of latency there is an exponential increase of bacterial numbers together with the accumulation of metabolic end products these induce a ph increase and an accumulation of carbonate and hydrogenocarbonate ions in the medium this phase ends into a steady state when most part of the initial enrichment is consumed and there is a balance between death and growth in bacterial populations particulate carbonatogenesis occurs during the exponential phase and ends more or less after the beginning of the steady state the active carbonatogenesis seems to start first and to be followed by the passive one which induces the growth of initially produced particles in eutrophic conditions the first solid products are patches that appear on the surface of the bacterial bodies and coalesce until forming a rigid coating and or particles excreted from the cell all these tiny particles assemble into biomineral aggregates which often display precrystalline structures these aggregates grow and form biocrystalline build ups which progressively display more crystalline structures with growth in oligotrophic conditions the primary solid products are rapidly smoothed in the crystalline structure and leave no trace in present aqueous environments apart from deep ocean the potential efficiency of heterotrophic bacterial carbonatogenesis in ca carbonate sedimentation is much higher than autotrophic or abiotic processes it much more likely accounts for extensive apparently abiotic limestone formation than any of the latter as far as biodetrital particles are concerned it may be observed that the shells and tests of organisms are built from the activity of cellular organites which are nowadays considered by a number of biologists as endosymbiotic bacteria thus apart from probably mythical purely evaporitic and autotrophic ones most limestones must be considered as principally of heterotrophic bacterial origin as the carbon of limestones is issued from organic matter bacterial heterotrophic carbonatogenesis appears as a fundamental phenomenon in the relationships between atmosphere and lithosphere during the biogeological evolution of the earth

signatures of supercooling mcmurdo sound platelet ice near ice shelves around antarctica the ocean becomes supercooled and has been observed to carry small suspended ice crystals our measurements demonstrate that these small crystals are persistently present in the water column beneath the winter fast ice and when incorporated in sea ice they reduce the mean grain size of the sea ice cover by midwinter larger ice crystals below the ice water interface are observed to form a porous sub ice platelet layer with an ice volume fraction of 0 25 0 06 the magnitude and direction of the oceanic heat flux varied between 5 6 wm 2 upwards and 15 10 wm 2 downwards in may but by september it settled between 6 2 and 11 2 w m 2 the negative values imply that the ocean acts as a heat sink which is responsible for the growth of 12 of the ice thickness between june and september this oceanic contribution should not be ignored in models of antarctic sea ice thickness close to an ice shelf

reflection technique in time frequency domain using multicomponent acoustic emission signals and application to geothermal reservoirs we have developed a reflection technique for estimating deep geothermal reservoir structures using acoustic emission signals as a source which is useful when there is no proper estimating technique because of high temperature high pressure and great depth because its resolution is not high enough for comparison with methods such as well logging we have enhanced the technique by developing a time frequency domain analysis of multicomponent acoustic emission signals using a wavelet transform the reflected wave is separated from an incoherent coda by analyzing the shape of a 3 d hodogram a linear shape indicates the arrival of a coherent signal such as a reflected wave and an incoherent signal such as a coda makes a spherical shape we construct a spectral matrix of 3 d particle motion using a wavelet transform as is done in a time frequency domain we evaluate the linearity of the 3 d hodogram for each time and frequency by using the eigenvalues of the spectral matrix three dimensional inversion o

the inferred formation of a subice platelet layer below the multiyear landfast sea ice in the wandel sea ne greenland induced by meltwater drainage

the effect of ocean heat flux on seasonal ice growth in young sound northeast greenland the seasonal ice cover plays an important role in the climate system limiting the exchange of heat and momentum across the air water interface among other factors sea ice is sensitive to the ocean heat flux in this study we use in situ oceanographic sea ice and meteorological data collected during winter 2013 2014 in young sound ys fjord in northeast greenland to estimate the ocean heat flux to the landfast ice cover during the preceding ice free summer incident solar radiation caused sea surface temperatures of up to 5 6 c subsequently this heat was transferred down to the intermediate depths but returned to the surface and retarded ice growth throughout winter two different approaches were used to estimate the ocean heat fluxes i a residual method based on a 1 d thermodynamic ice growth model and ii a bulk parameterization using friction velocities and available heat content of water beneath the ice the average heat flux in the inner ys varied from 13 w m 2 in october december to less than 2 w m 2 in january may an average heat flux of 9 w m 2 was calculated for the outer ys moreover we show that the upward heat flux in the outer fjord is strongly modulated by surface outflow which produced two maxima in heat flux up to 18 24 w m 2 during 26 december to 27 january and from 11 february to 14 march by may 2014 the upward ocean heat flux reduced the landfast ice thickness by 18 and 24 in the inner and outer ys respectively

tracer studies of pathways and rates of meltwater transport through arctic summer sea ice 1 at the surface heat budget of the arctic ocean sheba program s field site in the northern chukchi sea snow and ice meltwater flow was found to have a strong impact on the heat and mass balance of sea ice during the summer of 1998 pathways and rates of meltwater transport were derived from tracer studies h218o 7be and release of fluorescent dyes complemented by in situ sea ice permeability measurements it was shown that the balance between meltwater supply at the surface averaging between 3 5 and 10 5 mm d 1 and ice permeability between 10 9 m2 determines the retention and pooling of meltwater which in turn controls ice albedo we found that the seasonal evolution of first year and multiyear ice permeability and surface morphology determine four distinct stages of melt at the start of the ablation season stage 1 ponding is widespread and lateral melt flow dominates several tens of cubic meters of meltwater per day were found to drain hundreds to thousands of square meters of ice through flaws and permeable zones significant formation of underwater ice composed between 50 of meteoric water formed at these drainage sites complete removal of snow cover increase in ice permeability and reductions in hydraulic gradients driving fluid flow mark stage 2 concurrent with a reduction in pond coverage and albedo during stage 3 maximum permeabilities were measured with surface meltwater penetrating to 1 m depth in the ice and convective overturning and desalination found to dominate the lower layers of first year and thin multiyear ice enhanced fluid flow into flaws and permeable zones was observed to promote ice floe breakup and disintegration concurrent with increases in pond salinities and 7be advective heat flows of several tens of watts per square meter were derived promoting widening of ponds and increases in pond coverage stage 4 corresponds to freeze up roughly 40 of the total surface melt was retained by the ice cover within the ice matrix as well as in surface and under ice ponds with a total net retention of 15 based on this work areas of improvement for fully prognostic simulations of ice albedo are identified calling for parameterizations of sea ice permeability and the integration of ice topography and refined ablation schemes into atmosphere ice ocean models

three dimensional gravity inversion using simulated annealing constraints on the diapiric roots of allochthonous salt structures in the northern continental slope of the gulf of mexico large oil and gas reservoirs are often found beneath sheetlike allochthonous salt structures that are laterally extensive some of these salt structures retain their diapiric feeders or roots beneath them these hidden roots are difficult to image seismically in this study we develop a method to locate and constrain the geometry of such roots through 3 d inverse modeling of the gravity anomalies observed over the salt structures this inversion method utilizes a priori information such as the upper surface topography of the salt which can be delineated by a limited coverage of 2 d seismic data the sediment compaction curve in the region and the continuity of the salt body the inversion computation is based on the simulated annealing sa global optimization algorithm the sa based gravity inversion has some advantages over the approach based on damped least squares inversion it is computationally efficient can solve underdetermined inverse problems can more easily implement complex a priori information and does not introduce smoothing effects in the final density structure model we test this inversion method using synthetic gravity data for a type of salt geometry that is common among the allochthonous salt structures in the gulf of mexico and show that it is highly effective in constraining the diapiric root we also show that carrying out multiple inversion runs helps reduce the uncertainty in the final density model

ice platelets below weddell sea landfast sea ice basal melt of ice shelves may lead to an accumulation of disc shaped ice platelets underneath nearby sea ice to form a sub ice platelet layer here we present the seasonal cycle of sea ice attached to the ekstrom ice shelf antarctica and the underlying platelet layer in 2012 ice platelets emerged from the cavity and interacted with the fast ice cover of atka bay as early as june episodic accumulations throughout winter and spring led to an average platelet layer thickness of 4m by december 2012 with local maxima of up to 10 m the additional buoyancy partly prevented surface flooding and snow ice formation despite a thick snow cover subsequent thinning of the platelet layer from december onwards was associated with an inflow of warm surface water the combination of model studies with observed fast ice thickness revealed an average ice volume fraction in the platelet layer of 0 25 0 1 we found that nearly half of the combined solid sea ice and ice platelet volume in this area is generated by heat transfer to the ocean rather than to the atmosphere the total ice platelet volume underlying atka bay fast ice was equivalent to more than one fifth of the annual basal melt volume under the ekstrom ice shelf

tube wave to shear wave conversion at borehole plugs abstract in hydraulic fracturing experiments perforation shots excite body and tube waves that sample and thus can be used to characterize the surrounding medium while these waves are routinely employed in borehole operations their resolving power is limited by the experiment geometry the signal to noise ratio and their frequency content it is therefore useful to look for additional complementary signals that could increase this resolving power tube to body wave conversions scattering of tube to compressional or shear waves at borehole discontinuities are one such signal these waves are not frequently considered in hydraulic fracture settings yet they possess geometrical and spectral attributes that greatly complement the resolution afforded by body and tube waves alone here we analyze data from the jonah gas field wyoming usa to demonstrate that tube to shear wave conversions can be clearly observed in the context of hydraulic fracturing experiments these waves are identified primarily on the vertical and radial components of geophones installed in monitoring wells surrounding a treatment well they exhibit a significantly lower frequency content 10 100 hz than the primary compressional waves 100 1000 hz tapping into such lower frequencies could help to better constrain velocity in the formation thus allowing better estimates of fracture density porosity and permeability moreover the signals of tube to shear wave conversion observed in this particular study provide independent estimates of the shear wave velocity in the formation and of the tube wave velocity in the treatment well

observed platelet ice distributions in antarctic sea ice an index for ocean ice shelf heat flux antarctic sea ice that has been affected by supercooled ice shelf water isw has a unique crystallographic structure and is called platelet ice in this paper we synthesize platelet ice observations to construct a continent wide map of the winter presence of isw at the ocean surface the observations demonstrate that in some regions of coastal antarctica supercooled isw drives a negative oceanic heat flux of 30 wm 2 that persists for several months during winter significantly affecting sea ice thickness in other regions particularly where the thinning of ice shelves is believed to be greatest platelet ice is not observed our new data set includes the longest ice ocean record for antarctica which dates back to 1902 near the mcmurdo ice shelf these historical data indicate that over the past 100 years any change in the volume of very cold surface outflow from this ice shelf is less than the uncertainties in the measurements

platelet ice the southern ocean s hidden ice a review basal melt of ice shelves is not only an important part of antarctica s ice sheet mass budget but it is also the origin of one of the most peculiar types of sea ice found in the polar oceans platelet ice in many regions around coastal antarctica tiny ice crystals form and grow in supercooled plumes of ice shelf water releasing heat into the surrounding ocean they usually rise towards the surface eventually becoming trapped under an ice shelf as marine ice frequently masses of those crystals are advected out of the ice shelf cavity and accumulate below a solid sea ice cover to form a semiconsolidated layer when the overlying sea ice grows into this so called sub ice platelet layer the loose crystals are consolidated adding additional thickness to the sea ice these phenomena are generally referred to as platelet ice although confusion about the terminology is widespread in the literature the presence of platelet ice has a profound impact on sea ice properties and processes in several regions of antarctica with numerous implications for the local polar marine biosphere most notably sub ice platelet layers provide a stable sheltered nutrient and food rich habitat which usually results in a highly productive and uniquely adapted ecosystem it has also been hypothesised that platelet ice may be an indicator of the state of an ice shelf although comprehensive time series are limited to the ross sea this paper clears up the terminology by providing exact definitions of the relevant terms we review platelet ice formation observational methods as well as geographical and seasonal occurrence the physical properties and ecological implications are merged in a way understandable for physicists and biologists alike to lay the foundation for the interdisciplinary research that is necessary to tackle the current knowledge gaps

numerical modelling of seismic waves scattered by hydrofractures application of the indirect boundary element method summary we use a numerical method that can model the seismic waveforms scattered from an arbitrary number of fractures that are either empty or contain elastic or fluid material the indirect boundary element method bem is capable of generating the full elastic wavefield and is programmed in 2 d the governing equations and discrete implementation of the technique are described we explain in detail a new approach for evaluating the improper boundary integrals the method is shown to be highly accurate from a comparison with mode summation subsequently the bem is applied to modelling hydrofractures synthetic examples calculated for cross well and single well geometries demonstrate the effects of crack length opening and infill on recorded displacements it is shown that diffractions from the tips can in principle be used to locate and determine the hydrofracture size these arrivals depart from ray theoretical travel times due to defocusing over a fresnel zone s wave diffractions generally have a larger amplitude than the diffracted p waves and so may provide a better indication of fracture size energy which is converted to interface waves and subsequently diffracted from the crack tips is also observed the presence of water allows energy to pass through the fracture this is clearly evident on the cross well seismograms closure of the fracture causes a further increase in transmission amplitude as less energy is attenuated through internal multiples