freeboard and mass extraction of the disintegrated mertz ice tongue with remote sensing and altimetry data abstract in february 2010 the mertz ice tongue mit collapsed and generated a giant iceberg however parameters about this iceberg have not been calculated and published in detail in this study the freeboard map of this iceberg was generated for the first time using a time series icesat glas data methods for producing the freeboard map of this iceberg are suggested field data for ice velocity were used to relocate the footprints collected by different campaigns cross validation was conducted with freeboards extracted from crossovers observed within 30 days of each other the precision of the freeboard extraction is approximately 0 50 m when taking one standard deviation as the precision the freeboard varied from 23 m to 59 m with the mean of 41 m with assumption of hydrostatic equilibrium assuming a snow layer depth of 1 m a snow density of 360 kg m 3 an ice density of 915 kg m 3 and a sea water density of 1024 kg m 3 the minimum maximum and average ice thickness were calculated as 210 m 550 m and 383 m respectively the total ice loss is approximately 8 96 10 11 tons over an area 34 km in width and 75 km in length or approximately 2560 5 km 2 these parameters extracted from remote sensing and altimetry data will provide additional information for studies of the evolution of iceberg especially in iceberg tracking system

mean dynamic topography of the arctic ocean 1 icesat and envisat altimetry data provide measurements of the instantaneous sea surface height ssh across the arctic ocean using lead and open water elevation within the sea ice pack first these data were used to derive two independent mean sea surface mss models by stacking and averaging along track ssh profiles gathered between 2003 and 2009 the icesat and envisat mss data were combined to construct the high resolution icen mss second we estimate the 5 5 year mean dynamic topography mdt of the arctic ocean by differencing the icen mss with the new goco02s geoid model derived from grace and goce gravity using these satellite only data we map the major features of arctic ocean dynamical height that are consistent with in situ observations including the topographical highs and lows of the beaufort and greenland gyres respectively smaller scale mdt structures remain largely unresolved due to uncertainties in the geoid at short wavelengths

sea surface height determination in the arctic ocean from ers altimetry 1 accurate sea surface height measurements have been extracted from ers altimeter data in sea ice covered regions for the first time the data have been used to construct a mean sea surface of the arctic ocean between the latitudes of 60 degreesn and 81 5 degreesn based on 4 years of ers 2 data an rms value for the crossover differences of mean sea surface profiles of 4 2 cm was observed in the ice covered canada basin compared with 3 8 cm in the ice free greenland iceland norwegian seas comparisons are made with an existing global mean sea surface osumss95 highlighting significant differences between the two surfaces in permanently ice covered seas in addition we present the first altimeter derived sea surface height variability map of the arctic ocean comparisons with a high resolution coupled ocean sea ice general circulation model reveal a good qualitative agreement in the spatial distribution of variability quantitatively we found that the observed variability was on average a factor of 3 4 greater than model predictions

analysis of borehole breakouts boreholes drilled into rock which is subjected to stresses that amount to a significant fraction of the strength of the rock may cause the rock to fail adjacent to the borehole surface often this results in the elongation of the cross section of the borehole in the direction of the minimum principal compressive stress orthogonal to the borehole axis such breakouts are valuable indicators of the direction of the minimum compressive stress orthogonal to the axis of the borehole their shapes may provide information about the magnitudes of both the maximum and minimum stresses relative to the strength of the rock borehole breakouts also may be impediments to drilling and to in situ measurement techniques such as hydraulic fracturing observations and analyses of borehole breakouts raise three important questions first how does the shape of the borehole breakout evolve second why are breakout shapes stable despite the very high compressive stress concentrations that they produce third how is the shape of the breakout related to the magnitudes of the stresses in the rock in this paper extensile splitting of rock in unconfined plane strain compression is assumed to be the process of rock failure adjacent to the circumference of the borehole by which a breakout forms to simulate the evolution of a borehole breakout this process is combined with a numerical boundary element analysis of the stresses around a borehole as its cross section evolves from the originally circular shape to that of a stable breakout the tangential stresses around a stable breakout cross section are found to be everywhere less than the unconfined plane strain tensile or compressive strength of the rock the stresses outside the stable breakout are found to be everywhere less than the limiting values of shear strength given by a mohr coulomb criterion in the regions of great stress concentrations at the ends of a breakout cross section which have a pointed shape the state of stress approaches that of equal biaxial compression in plane strain as it does ahead of a mathematical crack or notch the fact that the stresses around a breakout are less than the relevant strength establishes both the stability of the final breakout cross section and the appropriateness of an elastic analysis of the stresses according to this model the cross sectional shapes of stable breakouts are not related uniquely to the state of stress and the strength of the rock for example stable breakouts created instantly in rock already subjected to stress are much larger than stable breakouts created in the same rock with a preexisting borehole by subsequently increasing the stresses to the same values the results of drilling into an actual rock probably lie between these extremes modest changes in borehole cross section as a result of breakout do not alter significantly the minimum tangential tensile stress around a borehole with internal pressure from that given by the kirsch solution for a circular hole subjected to the same stresses therefore hydraulic fracturing interpretations based on the kirsch solution give the correct values for the far field stresses

gravity of the arctic ocean from satellite data with validations using airborne gravimetry oceanographic implications 1 precise mappings of sea surface topography slope and gravity of the arctic ocean are derived from altimeter data collected by envisat and icesat both altimeters measured instantaneous sea surface height at leads in the sea ice to reduce contamination by ice freeboard signal and tracker noise in envisat height data a retracking of the waveform data was performed analogous reprocessing of icesat data was also done arctic mean sea surfaces msss were computed from envisat data spanning 2002 2008 and icesat data spanning 2003 2009 farrell et al 2012 used these icen msss to estimate mean dynamic topography mdt these same envisat and icesat data are used in sea surface slope form to compute the arctic satellite only arcs 2 altimetric marine gravity field arcs 2 extends north to 86 n and uses grace goce gravity data goco02s for its long wavelength 260 km components use of envisat data improves the spatial resolution over that of existing arctic marine gravity fields in many areas arcs 2 s spatial resolution aids in tracing tectonic fabric e g extinct plate boundaries over broad areas of the arctic basin whose tectonic origin remains a mystery arcs 2 s precision is validated using nasa 2010 2011 operation icebridge oib airborne gravimetry arcs 2 and oib gravity along with icen mss results are employed to locate short wavelength errors approaching 1 m in current arctic marine geoids egm2008 precise oib airborne gravity corroborates that such errors in current geoid gravity models are widespread in arctic areas lacking accurate surface gravity data these geoid errors limit the spatial resolution at which mdt can be mapped

sea surface height and dynamic topography of the ice covered oceans from cryosat 2 2011 2014 we examine 4 years 2011 2014 of sea surface heights ssh from cryosat 2 cs 2 over the ice covered arctic and southern oceans results are from a procedure that identifies and determines the heights of sea surface returns along 25 km segments of satellite ground tracks variability in the retrieved sshs is between 2 and 3 cm standard deviation in the arctic and is slightly higher 3 cm in the summer and the southern ocean average sea surface tilts along these 25 km segments are 0 01 3 8 cm 10 km in the arctic and slightly lower 0 01 2 0 cm 10 km in the southern ocean intra seasonal variability of cs 2 dynamic ocean topography dot in the ice covered arctic is nearly twice as high as that of the southern ocean in the arctic we find a correlation of 0 92 between 3 years of dot and dynamic heights dh from hydrographic stations further correlation of 4 years of area averaged cs 2 dot near the north pole with time variable ocean bottom pressure from a pressure gauge and from grace yields coefficients of 0 83 and 0 77 with corresponding differences of 3 cm rms these comparisons contrast the length scale of baroclinic and barotropic features and reveal the smaller amplitude barotropic signals in the arctic ocean broadly the mean dot from cs 2 for both poles compares well with those from the icesat campaigns and the dot2008a and dtu13mdt fields short length scale topographic variations due to oceanographic signals and geoid residuals are especially prominent in the arctic basin but less so in the southern ocean

borehole breakouts and compaction bands in two high porosity sandstones we compare the shape and mechanism of failure around vertical boreholes drilled in blocks of two high porosity sandstones subjected to unequal far field principal stresses tablerock sandstone has a porosity of 28 and is composed of 55 quartz and 37 weaker feldspar grains grain cementation is substantial through microcrystalline quartz critical far field stresses induce failure around boreholes in the form of v shaped dog eared breakouts the result of dilatant intra and trans granular microcracking subparallel to both the maximum horizontal far field stress and to the borehole wall on the other hand boreholes in mansfield sandstone which has similar porosity 26 but contains mainly quartz grains 90 held together primarily by spot sutured contacts fail by developing fracture like breakouts we show evidence suggesting that mansfield sandstone breakouts are preceded by the development of a narrow several grain diameters localized compaction zone along the direction of the least horizontal stress which is where the maximum compressive tangential stress concentration occurs failure mechanism here is simply the removal by the circulating drilling fluid of mainly intact grains loosened during the formation of the compaction band the type of cementation mineral homogeneity grain strength and sphericity appear to be major factors in the formation of compaction bands some breakout dimensions in both rocks are correlatable to the far field principal stresses and can be potentially used in conjunction with other information as indicators of their magnitudes

preface to special section on arctic ocean model intercomparison project aomip studies and results more than 130years ago theaustrian arctic explorer carl weyprecht weyprecht and ihne 1913 who discov ered franz josef land an archipelago north of russia andwho advanced a successful scheme for international coop eration in polar science the international polar year con cept postulated that a number of fundamental problems ofmeteorology and geophysics could be solved near theearth spoles thishypothesisisstillvalidinthe21stcenturybecause the arctic is recognized as a region where globalclimate change signals are amplified the model basedconclusion of manabe and stouffer 1994 that the firstsigns of climate change could be detected in the arctic hasbeen corroborated by numerous other model results e g holland and bitz 2003 symon et al 2005 on the otherhand it has been demonstrated that model conclusions maybe highly uncertain and that model estimates of futureclimate change differ significantly from model to model in order to reduce these uncertainties it is important tovalidate and to improve models the latter is the first majorgoal of the arctic ocean model intercomparison project aomip the second major aomip goal is to investigatevariability of the arctic ocean and sea ice at seasonal todecadal timescales and identify mechanisms responsible forthe observed changes some of the latest aomip activitiesassociated with these project goals arereflected inthe papersof this special section

sea ice circulation around the beaufort gyre the changing role of wind forcing and the sea ice state sea ice drift estimates from feature tracking of satellite passive microwave data are used to investigate seasonal trends and variability in the ice circulation around the beaufort gyre over the multi decadal period 1980 2013 our results suggest an amplified response of the beaufort gyre ice circulation to wind forcing especially during the late 2000s we find increasing anticyclonic ice drift across all seasons with the strongest trend in autumn associated with increased ice export out of the southern beaufort sea into the chukchi sea a flux gate analysis highlights consistency across a suite of drift products despite these seasonal anticyclonic ice drift trends a significant anticyclonic wind trend occurs in summer only driven in part by anomalously anticyclonic winds in 2007 across all seasons the ice drift curl is more anticyclonic than predicted from a linear relationship to the wind curl in the 2000s compared to the 1980s 1990s the strength of this anticyclonic ice drift curl amplification is strongest in autumn and appears to have increased since the 1980s up to 2010 in spring and summer the ice drift curl amplification occurs mainly between 2007 and 2010 these results suggest non linear ice interaction feedbacks e g a weaker more mobile sea ice pack enhanced atmospheric drag and or an increased role of the ocean the results also show a weakening of the anticyclonic wind and ice circulation since 2010 this article is protected by copyright all rights reserved

dynamic topography of the ice covered arctic ocean from icesat 1 we construct the dynamic ocean topography dot of the arctic ocean for five icesat campaigns winter of 2004 2008 using sea surface height estimates in open leads results show that the mean winter dot over the arctic ocean varies by 1 m and features a distinct dome of 40 cm over the beaufort sea standard deviation of the mean field is 20 cm spatial coherence between the five winter dots is consistently high 0 9 whereas the coherence between the dots and the winter djfm sea level pressure fields over the arctic basin is variable this suggests persistence of the underlying hydrodynamic processes at interannual time scales compared to seasonal atmospheric forcing comparison of dynamic heights dh from hydrographic surveys and the dot in 2008 shows a remarkable correlation of 0 92 the geostrophic velocity fields computed from the dot and interpolated dh fields highlight the smaller scale oceanographic features in the satellite estimates

combined airborne laser and radar altimeter measurements over the fram strait in may 2002 knowledge of sea ice thickness is critical for the prediction of future climate and for assessing the significance of changes in thickness sea ice thickness can be calculated from radar or laser satellite altimetry measurements of freeboard however a lack of knowledge of snow depth introduces significant uncertainties into these calculations this paper compares the first coincident airborne laser and radar altimetry data over sea ice collected during the laser radar altimetry lara field campaign lara was a flight of opportunity that provided valuable data to explore techniques to validate satellite measurements of ice freeboard and the possibility of combining laser and radar measurements over snow covered sea ice to calculate the snow depth two new methods were created to analyse these data sets a new radar retracker and a radar power simulator which models radar returns from the laser data we present the first quantitative analysis of data from the lara laser and radar altimeters and demonstrate the potential of combining laser and radar altimetry to estimate snow depth lara elevation estimates compare well with elevations from the radar altimeter onboard ers 2 at the sub meter level and the study provides lessons for future validation of satellite altimetry data over sea ice laser elevations are consistently higher than the radar elevations over snow covered sea ice as lara was a flight of opportunity no coincident in situ measurements were available nevertheless the difference between the reflecting surface of the laser and radar is consistent with snow depth from climatology and the analysis techniques developed in this paper will be useful for future radar and laser altimetry comparisons

borehole instability in high porosity berea sandstone and factors affecting dimensions and shape of fracture like breakouts abstract stress induced breakouts in vertical boreholes are failure zones caused by excessive compressive stress concentration at the borehole wall along the springline of the least horizontal far field stress wellbores are sometimes drilled into aquifers or oil reservoirs that are weak poorly consolidated and highly porous sandstone formations which are often conducive to breakout formation breakouts are an expression of borehole instability and a potential source of sand production on the other hand the breakout phenomenon can be used advantageously in obtaining an estimate of the in situ stress condition the average orientation of breakouts as identified by borehole geophysical logging is a reliable indicator of in situ stress directions it has also been suggested that breakout dimensions could potentially be used as indicators of in situ stress magnitudes the research reported here has concentrated on the unique type of breakouts observed for the first time in high porosity berea sandstone drilling experiments in rock blocks subjected to critical far field true triaxial stress regimes simulating in situ conditions induced breakouts that were unlike the dog ear ones previously observed in granites limestones and low porosity sandstones the newly observed breakouts were thin tabular and very long resembling fractures that counterintuitively extended perpendicular to the maximum principal stress we found that a narrow zone ahead of a fracture like breakout tip underwent apparent localized grain debonding and compaction in the field such zones have been termed compaction bands and are a source of concern because in oil fields and aquifers they constitute curtains of low permeability that can impede the normal flow of oil or water in order to determine whether a correlation exists between fracture like breakouts and in situ stress we conducted several series of tests in which the minimum horizontal and vertical stresses were held constant and the maximum horizontal stress h was increased from test to test these tests showed strong dependence of the breakout length on far field stress signaling that potentially the ability to assess fracture like breakout length in the field could be used to estimate in situ stress magnitudes in conjunction with other indicators another series of tests revealed that breakout length increased substantially when borehole diameter was enlarged this result suggested that in the field where wellbore size is considerably larger fracture like breakout could extend to sizable distances creating a sand production hazard two series of tests one to evaluate the effect of drill bit penetration rate and the other to verify the drilling fluid flow rate effect on breakout formation and dimensions yielded inconsistent results and showed no unique trends remarkably fracture like breakouts maintained a consistent narrow width of about 5 10 grain diameters irrespective of the test conditions this characteristic supports the suggestion that fracture like breakouts are emptied compaction bands

a new longitudinal approach to assess hydrologic connectivity embanked floodplain inundation along the lower mississippi river hydrologic connectivity is fundamental to understanding floodplain processes along meandering river corridors this study contributes to understanding hydrologic connectivity by utilizing a high resolution light detection and ranging dem with a new gis based approach for identifying the precise elevation of flood stage the method created a high resolution longitudinal channel bank profile enabling a detailed examination of embanked floodplain hydrologic connectivity a simple channel cross section approach is likely to result in a large underestimation of floodplain inundation and hydrologic connectivity along meandering river floodplains because of differences in the elevation of the natural levee surface relative to floodplain bottoms and the considerable variability in the elevation of the channel bank profile there is a large disparity in discharge duration associated with floodplain inundation in comparison to river channel bank inundation a discharge duration of 10 is associated with inundation of 87 of the floodplain surface whereas only 53 of the channel bank profile is overtopped an apparent threshold in inundation occurs as the discharge duration decreases below about 35 while a duration of 25 results in very little of the channel bank being overtopped it inundates 50 of the floodplain surface floodplain borrow pits which are created in association with dike construction represent a constant anthropogenic influence on the lower mississippi embanked floodplain morphology and are inundated by low discharge magnitudes the results of the investigation shed new light on the topic of hydrologic connectivity for an important embanked fluvial system that has previously received little attention concerning its physical floodplain processes copyright 2013 john wiley sons ltd

comparison of rock failure criteria in predicting borehole shear failure abstract selection of the appropriate rock failure criteria is one of the key steps in determining minimum required mud weight in wellbore stability analysis numerous failure criteria have been used for rock failure analysis but there is no common agreement of which failure criterion to select in this paper thirteen failure criteria used in predicting borehole shear failure were evaluated for four field cases in a comparison of the results with actual field failure cases tresca von mises and inscribed drucker prager overestimated the rock breakout and predicted the highest required minimum required mud weight for all cases also the results of these criteria are significantly higher than the actual borehole shear failure circumscribed drucker prager underestimated the rock breakout and predicted the lowest bound of the minimum required mud weight in most cases which is mainly less than actual onset of borehole breakout the minimum required mud weights determined by modified lade modified wiebols cook and mogi coulomb is above but close to the onset of breakout based on the field reported failure cases this means that using of any of these three criteria in wellbore stability analysis could be a safe approach furthermore modified lade modified wiebols cook and mogi coulomb provided similar results for all studied cases so these failure criteria may be used interchangeably

stress induced borehole elongation a comparison between the four arm dipmeter and the borehole televiewer in the auburn geothermal well the nature and origin of borehole elongation recorded by the four arm dipmeter calipers is studied utilizing information obtained from hydraulic fracturing stress measurements and borehole televiewer data taken in a well located in auburn new york a preferred orientation n10 w s10 e 10 and a less prominant e w orientation of borehole elongation was observed on two runs of the dipmeter comparisons of borehole geometry determined using the televiewer and the dipmeter show that both tools give the same orientation of borehole elongtion provided that the zone of elongation is longer than 30 cm comparisons of dipmeter caliper data with orientation of in situ stress and natural fractures obtained from hydrofracturing tests and televiewer data show that the n10 w s10 e borehole elongations 1 are axisymmetric 2 are aligned with the minimum horizontal stress sh and 3 are not associated with natural fractures intersecting the well these elongations are interpreted as stress induced well bore breakouts the e w elongation direction is characterized by an asymmetric borehole cross section in thinly bedded rocks and is not caused by breakouts this asymmetric geometry can be discriminated from breakouts using the oriented electric measurements provided by the dipmeter this study demonstrates that the dipmeter can be used to determine the orientation of sh by mapping breakouts confirming the results of earlier less detailed studies and provides a firm basis for mapping regional stress patterns using existing dipmeter data

icesat observations of seasonal and interannual variations of sea ice freeboard and estimated thickness in the weddell sea antarctica 2003 2009 sea ice freeboard heights for 17 icesat campaign periods from 2003 to 2009 are derived from icesat data freeboard is combined with snow depth from advanced microwave scanning radiometer for earth observing system amsr e data and nominal densities of snow water and sea ice to estimate sea ice thickness sea ice freeboard and thickness distributions show clear seasonal variations that reflect the yearly cycle of growth and decay of the weddell sea antarctica pack ice during october november sea ice grows to its seasonal maximum both in area and thickness the mean freeboards are 0 33 0 41 m and the mean thicknesses are 2 10 2 59 m during february march thinner sea ice melts away and the sea ice pack is mainly distributed in the west weddell sea the mean freeboards are 0 35 0 46 m and the mean thicknesses are 1 48 1 94 m during may june the mean freeboards and thicknesses are 0 26 0 29 m and 1 32 1 37 m respectively the 6 year trends in sea ice extent and volume are 0 023 0 051 x 10 exp 6 sq km a 0 45 a and 0 007 1 0 092 x 10 exp 3 cu km a 0 08 a however the large standard deviations indicate that these positive trends are not statistically significant

in situ stress magnitudes at the toe of the nankai trough accretionary prism offshore shikoku island japan quantifying the orientation and magnitude of tectonic stresses is essential toward understanding deformation and faulting in subduction zones however constraints on in situ horizontal stress magnitudes shmin and shmax are rare we estimate shmin and shmax at ocean drilling program site 808 at the toe of the nankai accretionary prism offshore japan using coupled constraints from 1 the width of wellbore breakouts together with estimates of rock strength and a model describing stress redistribution at the borehole wall and 2 limits on regional differential stress defined by failure on preexisting faults our analysis extends from 175 to 915 m below seafloor mbsf and spans the active frontal thrust for an upper bound on rock unconfined compressive strength ucs and assuming hydrostatic formation pore pressure shmin and shmax referenced to the seafloor increase from 6 5 mpa at 175 mbsf to 17 4 mpa at 915 mbsf with the stress state gradually transitioning from a thrust or strike slip faulting regime above 800 mbsf to a normal faulting regime below for cases with higher formation pore pressure horizontal stresses are slightly lower but follow a similar pattern we show that estimated shmin and shmax are strongly dependent on ucs breakout width and friction coefficient all of which are characterized by uncertainty our results suggest that the prism is near thrust failure in the upper 300 mbsf but far from failure below this may be reconciled with active thrusting if thrust faults are locally weaker than the surrounding rock or if shmax fluctuates during the seismic cycle

high resolution dynamic ocean topography in the southern ocean from goce a mean dynamic ocean topography mdt has been computed using a high resolution goce gravity field and steady state ocean circulation explorer gravity model and a new mean sea surface obtained from a combination of satellite altimetry covering the period 1992 october till 2010 april the considered gravity model is go cons gcf 2 tim r3 which computes geoid using 12 months of goce gravity field data the goce gravity data allow for more detailed and accurate estimates of mdt this is illustrated in the southern ocean where the commission error is reduced from 20 to 5cm compared to the mdt computed using the grace gravity field model itg grace2010 as a result of the more detailed and accurate mdt the calculation of geostrophic velocities from the mdt is now possible with higher accuracy and spatial resolution and the error estimate is about 7 cms 1 for the southern ocean

estimation of maximum horizontal principal stress magnitude from stress induced well bore breakouts in the cajon pass scientific research borehole in this paper we utilize the mechanical properties of core samples detailed observations of stress induced well bore breakouts and estimates of the magnitude of shmin from hydraulic fracturing experiments to construct a vertical profile of the maximum horizontal principal stress shmax to 3 5 km depth in the cajon pass borehole as in essentially all other boreholes the hydraulic fracturing stress measurements in the cajon pass borehole yielded appreciably more and more reliable data on the magnitude of the least principal horizontal stress shmin than the maximum principal horizontal stress shmax to utilize the breakout observations to constrain the magnitude of shmax a brittle failure criterion was used that is based on the effective strain energy concept this failure criterion also allows us to account for the polyaxial stress around the borehole and to incorporate the role of pore fluid pressure on rock strength in various ways a comparison of the shmax profile estimated from the breakouts with estimates of shmax obtained from the hydraulic fracturing tests indicates that they compare fairly well for the case when the effect of pore pressure on hydraulic fracture initiation is negligible the overall stress state to a depth of 3 5 km in the well corresponds to a normal strike slip faulting regime the shmax profile indicates several marked decreases in stress magnitude associated with major fault zones an observation consistent with the shmin profile obtained from hydraulic fracturing tests

a combined approach for estimating horizontal principal stress magnitudes from borehole breakout data via artificial neural network and rock failure criterion abstract in this paper a newly proposed approach on horizontal stress estimation from borehole breakout data is presented in a previous study a machine learning model was developed capable of estimating maximum horizontal stress h from breakout data accurately however due to the limitation in experimental data it was difficult to obtain the minimum horizontal stress h reliably in this study a series of breakout tests on hydrostone tb specimens was carried out to investigate the influence of h and vertical stress v on breakout geometries as these two parameters were rarely studied previously results revealed that both breakout angular span and depth decrease with increasing h or v although the influence of h is more significant based on experimental results from this paper as well as the literature nine failure criteria were examined on the prediction accuracy of h providing the magnitude of h except for one model all the other eight failure criteria consider the influence of v as indicated in experimental findings however none of the failure criteria yielded reasonable h estimations to overcome this problem an artificial neural network ann model was developed from the experimental dataset once the model was constructed it was examined against twenty three field data and yielded an acceptable average error rate of 15 88 on h considering the easily accessible breakout data then a comparative analysis on h estimation was performed via a number of approaches namely kriging ann and constitutive modeling results revealed that the use of the mogi coulomb failure criterion is the most reliable approach for h estimation with an average error rate of 6 82 overall this newly presented ann mogi coulomb approach to horizontal stress estimation shows reasonable prediction results which is expected to be improved in future studies by including additional data

polyaxial strength criteria and their use in estimating in situ stress magnitudes from borehole breakout dimensions abstract we conducted laboratory simulation tests of borehole breakouts and investigated their potential use as an indicator of in situ stress magnitudes in westerly granite and berea sandstone we also carried out simple triaxial tests and used the results to derive several strength criteria for these rocks using the assumption that the state of stress at the breakout boundary on the borehole wall is in a state of limit equilibrium with the rock strength we attempted to determine the appropriate strength criterion for each of the two rocks we concluded that the well known mohr coulomb criterion is not compatible with the condition of stress at breakout failure on the other hand truly triaxial polyaxial strength criteria which incorporate the effect of the intermediate principal stress on failure are much more in agreement with the stress at the breakout boundary one such criterion due to nadai and another due to mogi appear suitable for determining breakout failure in the sandstone and the granite respectively thin section analysis suggests that breakout failure mechanism may play an important role in determining the appropriate strength criterion for a given rock type utilizing the appropriate strength criterion for the corresponding rock type and having knowledge of the breakout span at the borehole wall and the minimum horizontal and vertical principal stresses we were able to estimate the maximum far field principal stress within an average of 14 error

in situ stress state in the nankai accretionary wedge estimated from borehole wall failures we constrain the orientations and magnitudes of in situ stress tensors using borehole wall failures borehole breakouts and drilling induced tensile fractures detected in four vertical boreholes c0002 c0001 c0004 and c0006 from nw to se drilled in the nankai accretionary wedge the directions of the maximum horizontal principal stress shmax indicated by the azimuths of borehole wall failures are consistent in individual holes but those in c0002 margin parallel shmax are nearly perpendicular to those in all other holes margin normal shmax constrained stress magnitudes in c0001 and c0002 using logged breakout widths combined with empirical rock strength derived from sonic velocity as well as the presence of the drilling induced tensile fractures suggest that the stress state in the shallow portion of the wedge fore arc basin and slope sediment formations is predominantly in favor of normal faulting and that the stress state in the deeper accretionary prism is in favor of probable strike slip faulting or possible reverse faulting thus the stress regime appears to be divided with depth by the major geological boundaries such as unconformities or thrust faults the margin perpendicular tectonic stress components in the two adjacent sites c0001 and c0002 are different suggesting that tectonic force driven by the plate pushing of the philippine sea plate does not uniformly propagate rather the stress field is inferred to be influenced by additional factors such as local deformation caused by gravitation driven extension in the fore arc and thrusting and bending within individual geologic domains

intensification of geostrophic currents in the canada basin arctic ocean abstractcontinuous sampling of upper ocean hydrographic data in the canada basin from various sources spanning from 2003 through 2011 provides an unprecedented opportunity to observe changes occurring in a major feature of the arctic ocean in a 112 km radius circle situated near the center of the traditional beaufort gyre geopotential height referenced to 400 dbar increased by about 0 3 gpm from 2003 to 2011 and by the end of the period had increased by about 65 from the climatological value near the edges of the domain considered the anomalies in dynamic height are much smaller indicating steeper gradients a rough dynamic topography constructed from profiles collected between 2008 and 2011 shows the center of the gyre to have shifted south by about 2 in latitude along the 150 w meridian geostrophic currents are much stronger on the periphery of the gyre reaching amplitudes 5 6 times higher than climatological values at grid points just offshore from the beaufort and chukchi shelf slopes esti

time dependent brittle creep as a mechanism for time delayed wellbore failure abstract by drilling a wellbore cavity high stresses arise at the wellbore wall leading to the formation of breakouts which enlarge the hole to an elongated shape oriented along the direction of the minimum principal stress if formation of breakouts is delayed rock debris falls on the drill bit which may lead to stuck pipe problems or even abandonment of the drill string reasons for such time delayed failure of the wellbore may be due to chemical fluid rock interaction especially in swelling clays however such delayed instabilities have also been observed e g in gneiss formations at the ktb borehole germany that are not known to exhibit a swelling behavior we propose to explain observations of delayed wellbore failure by time dependent brittle creep which has been observed for many types of rocks following this approach rock fails under loads less than their short time strength but after a long enough time span this time is in exponential relation to the load applied to the rock we implement a model developed for the creation of shear bands on the basis of time dependent brittle creep by amitrano and helmstetter 2006 here progressive damage of the formation is captured by a damage parameter d and the time to failure ttf young s modulus e is decreased by a factor every time ttf is expired i e when failure is reached subsequently stresses are redistributed according to the new distribution of e in the formation using this approach we obtain closure of the well with primary and secondary creep phases wellbore breakouts are formed progressively with deepening and widening of the initially damaged zone after a certain time the formation of breakouts comes to an end with an omori like decay of failure approaching a steady state

large scale ocean circulation from the grace ggm01 geoid 1 the grace gravity model 01 ggm01 computed from 111 days of grace k band ranging kbr data is differenced from a global mean sea surface mss computed from a decade of satellite altimetry to determine a mean dynamic ocean topography dot as a test of the ggm01 gravity model large scale zonal and meridional surface geostrophic currents are computed from the topography and are compared with those derived from a mean hydrographic surface reduction in residual rms between the two by 30 60 and increased correlation indicates that the ggm01 geoid represents a dramatic improvement over older geoid models which were developed from multiple satellite tracking data altimetry and surface gravity measurements for the first time all major current systems are clearly observed in the dot from space based measurements

wintertime variability of the beaufort gyre in the arctic ocean derived from cryosat 2 siral observations we processed the sea surface height measured by the sar synthetic aperture radar interferometric radar altimeter siral on board cryosat 2 cs 2 and successfully estimated the monthly dynamic ocean topography dot of the arctic ocean the cs 2 monthly dot showed the interannual and monthly variability of the beaufort gyre bg during winter between 2010 2011 and 2014 2015 the northward flow at the western edge of the bg was primarily estimated over the chukchi borderland cbl however the bg extended across the cbl and the northward flow was estimated over the mendeleev ridge in the winter of 2012 2013 our analyses revealed a significantly variable bg in response to changes in the sea surface stress field our analysis indicated that 1 sea ice motion driven by wind fields acts as a driving force for the bg when sea ice motion was intensified during winter and 2 sea ice motion can also act as an inhibiting force for the bg when sea ice motion is weakened during winter in addition the relationship between the dot steric height and ocean bottom pressure implied that the dot during winter responded to varying wind stresses through baroclinic and barotropic adjustments according to a tracer experiment we inferred that in the winter of 2012 2013 the pacific origin water carried into the bg through the barrow canyon was transported to the northern shelf and shelf break of the chukchi sea rather than the cbl which is where the pacific origin water had been transported in the other years of the observation period

using the interferometric capabilities of the esa cryosat 2 mission to improve the accuracy of sea ice freeboard retrievals a significant source of error in the retrieval of sea ice freeboard from pulse limited radar altimeters arises when scattering from off nadir leads dominates the power echo causing the onboard tracker to snag and overestimate the two way travel time this range overestimate translates into an ocean elevation underestimate relative to the ice surface and an overestimate of the sea ice freeboard we demonstrate using interferometric cryosat 2 data that it is possible to infer the across track angle of return to off nadir leads and their location in the cryosat 2 footprint and hence correct for the associated range error for the first time it is found that specular scattering from leads can dominate the radar echo some 1530 m off nadir over the region studied the mean ocean elevation bias is closely associated with the pulse peakiness pp parameter used for identifying specular waveforms considering only the most specular waveforms the elevation bias was measured to be 1 21 0 93 cm however lowering the pp threshold includes lower power waveforms originating from higher off nadir angles and the elevation bias becomes 4 06 1 66 cm unaccounted for these biases represent an 10 40 cm overestimate in ice thickness despite the relatively large error on the mean bias correcting for off nadir ranging contributes only a small amount to the elevation uncertainty when combined with range and orbit determination uncertainties we found that making use of cryosat 2 s interferometric mode over sea ice ultimately decreases the uncertainty on the area averaged ocean elevation by allowing the inclusion of more waveforms in the analysis

the ice cloud and land elevation satellite 2 icesat 2 science requirements concept and implementation abstract the ice cloud and land elevation satellite icesat mission used laser altimetry measurements to determine changes in elevations of glaciers and ice sheets as well as sea ice thickness distribution these measurements have provided important information on the response of the cryopshere earth s frozen surfaces to changes in atmosphere and ocean condition icesat operated from 2003 to 2009 and provided repeat altimetry measurements not only to the cryosphere scientific community but also to the ocean terrestrial and atmospheric scientific communities the conclusive assessment of significant ongoing rapid changes in the earth s ice cover in part supported by icesat observations has strengthened the need for sustained high accuracy repeat observations similar to what was provided by the icesat mission following recommendations from the national research council for an icesat follow on mission the icesat 2 mission is now under development for planned launch in 2018 the primary scientific aims of the icesat 2 mission are to continue measurements of sea ice freeboard and ice sheet elevation to determine their changes at scales from outlet glaciers to the entire ice sheet and from 10s of meters to the entire polar oceans for sea ice freeboard icesat carried a single beam profiling laser altimeter that produced 70 m diameter footprints on the surface of the earth at 150 m along track intervals in contrast icesat 2 will operate with three pairs of beams each pair separated by about 3 km cross track with a pair spacing of 90 m each of the beams will have a nominal 17 m diameter footprint with an along track sampling interval of 0 7 m the differences in the icesat 2 measurement concept are a result of overcoming some limitations associated with the approach used in the icesat mission the beam pair configuration of icesat 2 allows for the determination of local cross track slope a significant factor in measuring elevation change for the outlet glaciers surrounding the greenland and antarctica coasts the multiple beam pairs also provide improved spatial coverage the dense spatial sampling eliminates along track measurement gaps and the small footprint diameter is especially useful for sea surface height measurements in the often narrow leads needed for sea ice freeboard and ice thickness retrievals the icesat 2 instrumentation concept uses a low energy 532 nm green laser in conjunction with single photon sensitive detectors to measure range combining icesat 2 data with altimetry data collected since the start of the icesat mission in 2003 such as operation icebridge and esa s cryosat 2 will yield a 15 year record of changes in ice sheet elevation and sea ice thickness icesat 2 will also provide information of mountain glacier and ice cap elevations changes land and vegetation heights inland water elevations sea surface heights and cloud layering and optical thickness

well bore breakouts and in situ stress the detailed cross sectional shape of stress induced well bore breakouts has been studied using specially processed ultrasonic borehole televiewer data we show breakout shapes for a variety of rock types and introduce a simple elastic failure model which explains many features of the observations both the observations and calculations indicate that the breakouts define relatively broad and flat curvilinear surfaces which enlarge the borehole in the direction of minimum horizontal compression this work supports the hypothesis that breakouts result from shear failure of the rock where the compressive stress concentration around the well bore is greatest and that breakouts can be used to determine the orientation of the horizontal principal stresses in situ

combined satellite gravity field model goco01s derived from goce and grace 1 the satellite only gravity field model goco01s is a combination solution based on 61 days of goce gravity gradient data and 7 years of grace gps and k band range rate data resolved up to degree order 224 of a harmonic series expansion the combination was performed consistently by addition of full normal equations and stochastic modeling of goce and grace observations the model has been validated against external global gravity models and regional gps leveling observations while low to medium degrees are mainly determined by grace significant contributions by the new measurement type of goce gradients can already be observed at degree 100 beyond degree 150 goce becomes the dominant contributor correspondingly with goco01s a global gravity field model with high performance for the complete spectral range up to degree order 224 is now available this new gravity model will be beneficial for many applications in geophysics oceanography and geodesy

dnsc08 mean sea surface and mean dynamic topography models 1 the danish national space center data set dnsc08 mean sea surface mss is a new enhanced mapping of the mean sea surface height of the worlds oceans derived from a combination of 12 years of satellite altimetry from a total of eight different satellites covering the period 1993 2004 it is the first global mss without a polar gap including all of the arctic ocean by including laser altimetry from the icesat mission the mean dynamic topography mdt is the quantity that bridges the geoid and the mean sea surface constraining large scale ocean circulation here we present a new high resolution 1 min global mdt called dnsc08 mdt derived from the slightly smoothed difference between the dnsc08 mss and the egm2008 geoid the derivation and quality control of the new dnsc08 mss and dnsc08 mdt is presented in this paper along with suggestions for time period standardization of the mss and mdt models this way a consistent modeling of the interannual sea level variability is carried out before different mss and mdt models are compared altimetric derived physical mss can be converted into an inverse barometer corrected mss by correcting the altimeter range for the inverse barometer effect of the atmosphere on the sea surface height it is demonstrated that it is important to choose the right version of the mss when comparing with hydrodynamic models and gps measured tide gauges

quality assessment of a satellite altimetry data product in the nordic barents and kara seas 1 satellite altimetry provides high quality sea surface height data that have been successfully used to study the variability of sea level and surface geostrophic circulation at different spatial and temporal scales however the high latitude regions have traditionally been avoided due to the persistent sea ice cover most of the validation studies have focused on the areas below the polar circles in this paper we examine the quality and performance of a gridded satellite altimetry product in the nordic barents and kara seas the altimetric sea level in coastal areas is validated using available tide gauge records we show that at most locations in the nordic seas the altimetry and tide gauge measurements are in a good agreement in terms of the root mean square differences and the amplitudes and phases of the seasonal cycle the agreement deteriorates in the shallow areas of the barents and kara seas subject to the seasonal presence of sea ice and where the altimetry data are contaminated by the residual aliasing of unresolved high frequency signals the comparison of linear trends at the locations of tide gauges reveals discrepancies that need to be taken into account when interpreting long term changes of sea level in the region away from the coast the altimetry data are compared to drifter trajectories corrected for ekman currents the drifter trajectories are found consistent with the mesoscale variability of the altimetric sea level this study provides the first comprehensive validation of a gridded satellite altimetry data product in the high latitude seas

hydraulic fracturing stress measurements at yucca mountain nevada and relationship to the regional stress field hydraulic fracturing stress measurements and acoustic borehole televiewer logs were run in holes usw g 1 and usw g 2 at yucca mountain as part of the nevada nuclear waste storage investigations for the u s department of energy eight tests in the saturated zone at depths from 646 to 1288 m yielded values of the least horizontal stress s h that are considerably lower than the vertical principal stress s v in tests for which the greatest horizontal principal stress s h could be determined it was found to be less than s v indicating a normal faulting stress regime the borehole televiewer logs showed the presence of long in excess of 10 m vertical drilling induced fractures in the first 300 m below the water table these are believed to form by the propagation of small preexisting cracks under the excess downhole fluid pressures up to 5 2 mpa applied during drilling the presence of these drilling induced hydrofractures provides further confirmation of the low value of the least horizontal stresses a least horizontal principal stress direction of n60 w n65 w is indicated by the orientation of the drilling induced hydrofractures n25 e n30 e and the orientation of stress induced well bore breakouts in the lower part of usw g 2 n65 w this direction is in good agreement with indicators of stress direction from elsewhere at the nevada test site the observed stress magnitudes and directions were examined for the possibility of slip on preexisting faults using these data the coulomb criterion for frictional sliding suggests that for coefficients of friction close to 0 6 movement on favorably oriented faults could be expected for coefficients of friction of 1 0 preexisting faults of all orientations should be stable laboratory studies on the yucca mountain tuffs reported elsewhere yield coefficients of friction ranging from 0 6 to 0 9

utilization of observations of well bore failure to constrain the orientation and magnitude of crustal stresses application to continental deep sea drilling project and ocean drilling program boreholes

evolution of stress induced borehole breakout in inherently anisotropic rock insights from discrete element modeling the aim of this study is to better understand the mechanisms controlling the initiation propagation and ultimate pattern of borehole breakouts in shale formation when drilled parallel with and perpendicular to beddings a two dimensional discrete element model is constructed to explicitly represent the microstructure of inherently anisotropic rocks by inserting a series of individual smooth joints into an assembly of bonded rigid discs both isotropic and anisotropic hollow square shaped samples are generated to represent the wellbores drilled perpendicular to and parallel with beddings at reduced scale the isotropic model is validated by comparing the stress distribution around borehole wall and along x axis direction with analytical solutions effects of different factors including the particle size distribution borehole diameter far field stress anisotropy and rock anisotropy are systematically evaluated on the stress distribution and borehole breakout propagation simulation results reveal that wider particle size distribution results in the local stress perturbations which cause localization of cracks reduction of borehole diameter significantly alters the crack failure from tensile to shear and raises the critical pressure rock anisotropy plays an important role on the stress state around wellbore which lead to the formation of preferred cracks under hydrostatic stress far field stress anisotropy plays a dominant role in the shape of borehole breakout when drilled perpendicular to beddings while a secondary role when drilled parallel with beddings results from this study can provide fundamental insights on the underlying particle scale mechanisms for previous findings in laboratory and field on borehole stability in anisotropic rock

icesat measurements of sea ice freeboard and estimates of sea ice thickness in the weddell sea 1 sea ice freeboard heights in the weddell sea of antarctica are derived from the ice cloud and land elevation satellite icesat laser altimeter measurements which have a unique range precision to flat surfaces of 2 cm within 70 m footprints spaced at 172 m along track although elevations of flat surfaces can be obtained to an accuracy of 10 cm 1s per footprint direct determination of freeboard heights is precluded by errors in knowledge of the geoid and temporal variability of the ocean surface therefore freeboards are determined relative to an ocean reference level detected over areas of open water and very thin ice within the sea ice pack using an along track filtering method the open water thin ice selections show good agreement in the combined analysis of icesat segments and envisat synthetic aperture radar sar imagery the average residual between the icesat measured ocean level and the egm96 geoid is 1 4 m estimates of snow depth on the sea ice from amsr e passive microwave data along with nominal densities of snow water and sea ice are used to estimate sea ice thickness four periods of icesat data in may june fall and october november late winter of 2004 and 2005 between longitudes 298e and 360e are analyzed in the fall the mean freeboards are 0 28 m in 2004 and 0 29 m in 2005 and the mean thicknesses are 1 33 m in 2004 and 1 52 m in 2005 in late winter the freeboards grew to 0 37 m in 2004 and 0 35 in 2005 and the thicknesses grew to 2 23 m in 2004 and 2 31 m in 2005 the interannual differences in freeboard are small and the larger interannual change in estimated thickness mainly represents differences in the snow depth estimates seasonal changes in the spatial patterns of freeboard and thickness over the 4 months correlate with the expected circulation of sea ice in the weddell sea as indicated by sea ice velocity fields

mass gains of the antarctic ice sheet exceed losses during 2003 to 2008 the mass gain of the antarctic ice sheet from snow accumulation exceeded the mass loss from ice discharge by 49 gt yr 2 5 of input as derived from icesat laser measurements of elevation change the net gain 86 gt yr over the west antarctic wa and east antarctic ice sheets wa and ea is essentially unchanged from revised results for 1992 to 2001 from ers radar altimetry imbalances in individual drainage systems ds are large 68 to 103 of input as are temporal changes 39 to 44 the recent 90 gt yr loss from three ds pine island thwaites smith and marie bryd coast of wa exceeds the earlier 61 gt yr loss consistent with reports of accelerating ice flow and dynamic thinning similarly the recent 24 gt yr loss from three ds in the antarctic peninsula ap is consistent with glacier accelerations following breakup of the larsen b and other ice shelves in contrast net increases in the five other ds of wa and ap and three of the 16 ds in east antarctica ea exceed the increased losses alternate interpretations of the mass changes driven by accumulation variations are given using results from atmospheric model re analysis and a parameterization based on 5 change in accumulation per degree of observed surface temperature change a slow increase in snowfall with climate warming consistent with model predictions may be offsetting increased dynamic losses

an assessment of state of the art mean sea surface and geoid models of the arctic ocean implications for sea ice freeboard retrieval state of the art arctic ocean mean sea surface mss models and global geoid models ggms are used to support sea ice freeboard estimation from satellite altimeters as well as in oceanographic studies such as mapping sea level anomalies and mean dynamic ocean topography however errors in a given model in the high frequency domain primarily due to unresolved gravity features can result in errors in the estimated along track freeboard these errors are exacerbated in areas with a sparse lead distribution in consolidated ice pack conditions additionally model errors can impact ocean geostrophic currents derived from satellite altimeter data while remaining biases in these models may impact longer term multi sensor oceanographic time series of sea level change in the arctic this study focuses on an assessment of five state of the art arctic mss models ucl13 04 dtu15 13 10 and a commonly used ggm egm2008 we describe errors due to unresolved gravity features inter satellite biases and remaining satellite orbit errors and their impact on the derivation of sea ice freeboard the latest mss models incorporating cryosat 2 sea surface height measurements show improved definition of gravity features such as the gakkel ridge the standard deviation between models ranges 0 03 0 25 m the impact of remaining mss ggm errors on freeboard retrieval can reach several decimeters in parts of the arctic while the maximum observed freeboard difference found in the central arctic was 0 59 m ucl13 mss minus egm2008 ggm the standard deviation in freeboard differences is 0 03 0 06 m

avulsion regimes in southeast texas rivers avulsions relatively sudden changes in course or establishment of new anabranches are an important process in alluvial rivers their key role in floodplain construction and alluvial architecture and the general conditions favouring avulsions are well known however avulsion processes and evolution and the factors controlling avulsion regimes are poorly understood in the southeast texas coastal plain where avulsions are common features of the river valleys avulsions were studied on the lower brazos navasota trinity neches and sabine rivers using a combination of aerial imagery digital elevation models and field surveys avulsions have important influences on the surface morphology and contemporary processes in all five rivers features associated with avulsions are active and distinct throughout the study area and all the rivers have experienced geologically if not historically recent avulsions however no two of the study rivers have the same contemporary avulsion regime first order differences in avulsion style are controlled by the stage of valley filling and within the three rivers characterized by an unfilled incised valley antecedent morphology associated with late quaternary and holocene coastal and fluvial deltaic processes accounts for the major differences in the navasota 27 avulsions in 185 km and neches 21 in 340 km rivers subchannels associated with avulsions exist in all stages of development from active to infilled and some have occurred in recent decades the other rivers have fewer avulsions but both the sabine and trinity have experienced historic channel shifts only the brazos river has experienced no avulsions within the past c 300 years results show that even within a region of similar environmental controls and geological history local variations in inherited morphology can result in different avulsion regimes copyright 2008 john wiley sons ltd