hydraulic fracture reopening pressure and the estimation of maximum horizontal stress abstract in hydrofracture stress measurements the magnitude of the maximum horizontal stress s h is commonly estimated from the borehole pressure required to reopen an induced axial crack examination of the processes which govern the borehole pressure history recorded during the reopening cycle of such tests indicates two sources of error in the estimates of s h derived using the conventional method proposed by bredehoeft et al bredehoeft jd wolff rg keys ws and shutter e 1976 colorado geol soc amer bull 87 250 8 the first arises from the failure to include a term arising from pressure penetration into the crack prior to reopening in the force balance acting across the mouth of the induced axial cracks the problem can be remedied by using a modified reopening equation which includes pressure penetration of the crack the second source of error is more problematic and concerns the correct identification of the true reopening pressure from the borehole pressure records analysis of the process of reopening aided by numerical simulations shows that the true reopening pressure is generally less than the apparent i e that detected reopening pressure the discrepancy between true and apparent reopening pressures increases with larger hydraulic compliance of the test equipment the compliance in question refers to that of the fluid volume between the flow meter and the crack mouth s simulation of a pair of 1 m high axial cracks with 2 m residual hydraulic aperture in a 100 mm borehole indicates that the system compliance must be reduced to 5 10 7 m 3 mpa to enable the true reopening pressure to be estimated to better than 10 at flow rate is less than 10 4 m 3 s this is several orders of magnitude less than applies to conventional hydrofracture systems but is attainable for tests in small holes at shallow depth by making relatively minor system modifications tests at greater depth however would seem to require downhole measurement of flow at the packers we validate our assertions with a field test in which reopening pressure was determined mechanically and hydraulically

stress directions in eastern north america determined to 4 5 km from borehole elongation measurements stress directions have been determined to depths of 4 5 km in eastern north america from borehole elongation measured by dipmeter calipers in 47 wells the average maximum horizontal stress directions are eastern canada n54 e 7 appalachian basin n58 e 8 and the illinois basin n89 w 5 stress directions determined for the illinois and appalachian basins are in agreement with published interpretations from hydraulic fractures and earthquake fault plane solutions new results from eastern canada and the appalachian basin show that stress directions are everywhere consistent with the midcontinent stress province these data suggest that the maximum horizontal stress does not rotate from the ene midcontinent trend to nw near the atlantic coast a directional analysis of hydraulic fractures petal centerline fractures and natural fractures from the appalachian basin shows that borehole elongation is systematically aligned perpendicular to the directions of hydraulic fractures centerline fractures and northeast striking natural fractures but is not systematically aligned with northwest striking natural fractures these observations indicate that borehole elongations provide reliable stress orientations

an analysis of horizontal fracture initiation during hydrofrac stress measurements in granite at north conway new hampshire summary the in situ state of stress was investigated using the hydro fracturing technique in a cored borehole in granite at north conway new hampshire usa a total of 21 successful tests were made in fracture free intervals to a depth of 579m of the 18 tests in which oriented impressions were taken of induced fractures 16 were found to be sub horizontal laboratory and finite element simulations show that these fractures could not have been a result of stresses induced by the packer system although a conventional analysis in which the borehole wall is assumed to be a smooth continuum would not admit the possibility of horizontal fracturing we present a simple analysis based on the experimental result of jaeger 1963 and the theoretical work of haimson 1968 that shows that if the fluid intrudes micro cracks in the borehole wall horizontal fractures can be initiated where the least horizontal stress is only modestly greater than the vertical stress for rocks of microdarcy permeability the wellbore pressure must be raised to breakdown levels in less than ten seconds if the effect is to be suppressed the shut in pressures provided an unusual opportunity to measure the vertical stresses the results show that the vertical stress closely followed the calculated overburden to a depth of 400 m three measurements below 415 m however yielded vertical stresses which are distinctively sub lithostatic

two methods for hydraulic fracturing stress measurements needless the ambiguous reopening pressure abstract two new methods were presented for stress measurements based on the hydraulic fracturing technique these are the methods i and ii which do not use the reopening pressure i e i the method using only the shut in pressures for transverse cracks and ii the method using the actual reopening pressure of a longitudinal crack which coincides actually with the reopening of the crack lip at the borehole wall the former method can be estimated the range of error in the evaluated magnitudes and orientations of the remote stresses these methods were applied to the results of hydraulic fracturing experiments performed at the kamaishi mine in japan the reliability of the evaluated stress field was confirmed by comparing the stress field with that evaluated by the over coring method

crustal stress determination from boreholes and rock cores fundamental principles abstract obtaining quantitative information on the state of stress in the crust can only be accomplished by accessing the zone of interest by drilling the borehole cavity however disrupts the virgin stress state by concentrating stresses in predictable patterns with a number of consequences at sufficiently low stress magnitudes the concentrated stresses amplify the elastic anisotropy azimuthally around the borehole because the elastic properties of most rocks depend nonlinearly on stress at higher levels the stresses damage then fracture the rock near the wellbore wall or within a growing core stub indeed almost all of the borehole techniques indirectly measure stress through these manifestations of the concentrated stresses this contribution reviews at a fundamental level the concentration of stresses by the borehole the effects on the materials in the borehole s vicinity and how these are used to infer stress states stress concentrations applied to rocks which are generally nonlinear elastic materials induce both radial and azimuthal variations in elastic wave speeds near the borehole that can be used to infer stress directions from advanced acoustic logging methods hydraulic fractures initiate propagate and remain propped open by measured pressures that can be interpreted with knowledge of the stress concentrations to obtain quantitative stress magnitudes at higher stress levels the rock fails producing borehole breakouts or drilling induced tensile fractures both of which are indicative of stress directions and can be used to constrain stress magnitudes similarly the various styles of drilling induced core fractures indicate both stress directions and the faulting environment encountered unfortunately no method currently allows for complete determination of the stress tensor using a number of different but complementary techniques is necessary to best constrain in situ stress states

appalachian stress study 1 a detailed description of in situ stress variations in devonian shales of the appalachian plateau we describe an experiment to measure variations in the state of stress within a horizontally bedded devonian shale sandstone limestone sequence in western new york a total of 75 stress measurements were made in three wells a kilometer or so apart using a wireline supported hydraulic fracturing system the stress profiles indicate that a major drop in horizontal stress level occurs in the generally massive shales this drop occurs principally across the lowermost member of a group of sand beds and corresponds to an offset in sh and sh of 3 5 and 9 mpa respectively above the sands thrust regime conditions prevail although the amount by which sh exceeds sv is undetermined since instantaneous shut in pressures isips were clipped at the level of sv due to fracture rotation below the sands the regime is strike slip with both horizontal stresses showing lateral uniformity despite substantial variations in topography the magnitude of sh in the sand beds themselves and a lower limestone remains at least as great as sv despite the decline in shale stress hence stress contrasts between these beds and neighboring shales become pronounced with depth the contrast in sh and sh between the lowermost sand and the immediately underlying shale is at least 6 and 14 5 mpa respectively sh levels in the lower strike slip regime are about 1 75 times greater than sh and are less than the value required to initiate slippage on favourably oriented frictional interfaces for the upper thrust regime and the sand and limestone beds however the inferred lower bound on sh is close to the slippage threshold for a coulomb friction coefficient of 0 6 the orientation of sh is ene with a standard deviation of 20 fracture traces were usually splayed occasionally spanning 30 of well bore no systematic correlation between mean orientation and lithology is evident significantly different orientations were obtained for adjacent tests in which almost identical islps were observed suggesting that the fractures quickly reorient themselves to propagate normal to the least principal stress direction similarly vertical traces were observed in those tests where isips apparently reflect sv suggesting that rotation to horizontal was rapid

determination of stress state in deep subsea formation by combination of hydraulic fracturing in situ test and core analysis a case study in the iodp expedition 319 1 in situ test of hydraulic fracturing hf provides the only way to observe in situ stress magnitudes directly the maximum and minimum horizontal stresses shmax and shmin are determined from critical borehole pressures i e the reopening pressure pr and the shut in pressure ps etc observed during the test however there is inevitably a discrepancy between actual and measured values of the critical pressures and this discrepancy is very significant for pr for effective measurement of pr it is necessary for the fracturing system to have a sufficiently small compliance a diagnostic procedure to evaluate whether the compliance of the employed fracturing system is appropriate for shmax determination from pr was developed furthermore a new method for stress measurement not restricted by the system compliance and pr is herein proposed in this method the magnitudes and orientations of shmax and shmin are determined from i the cross sectional shape of a core sample and ii ps obtained by the hf test performed near the core depth these ideas were applied for stress measurement in a central region of the kumano fore arc basin at a water depth of 2054 m using a 1 6 km riser hole drilled in the integrated ocean drilling program iodp expedition 319 as a result the stress decoupling through a boundary at 1285 m below seafloor was detected the boundary separates new upper layers and old lower ones with an age gap of 1 8 ma which is possibly the accretionary prism the stress state in the lower layers is consistent with that observed in the outer edge of accretionary prism

development of borehole jack fracturing technique for in situ stress measurement abstract in order to measure crustal stresses at great depths of more than 1000 m from the ground surface we have been promoting research and development of borehole jack fracturing technique when a borehole wall is loaded by a borehole jack a pair of new fractures will be induced oppositely in parallel to the borehole axis after unloading if the same place on the borehole wall is loaded again by the jack the pair of fractures will be opened again two principal stresses and the orientation of crustal stress in the plane perpendicular to the borehole axis are determined by the re opening pressures and the orientation of the fractures respectively this technique is similar to a hydraulic fracturing from the viewpoint of analysis the features of this technique are that it is possible to produce a pair of axial fractures in an arbitrary direction possible to measure a displacement of the fracture opening and as a result it is possible to determine the re opening pressure accurately this paper describes our results of numerical analyses laboratory experiments and field tests

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

satellite measurements through the center of a substorm surge measurements have been made of electric and magnetic fields plasma drifts and electron precipitation within a surge at the westward leading edge of the auroral bulge at the peak of the substorm expansion phase the trajectory of the de 2 satellite over the auroral emissions is determined from nearly simultaneous observations with the imager on the de 1 satellite at a higher altitude the electric field and plasma drift measurements have enabled us to deduce the basic configuration of the ionospheric electric potential or plasma convection around the surge the electric potential shows that the bulge is associated with a protrusion of the dawn convection cell into the dusk cell poleward of the harang discontinuity this protrusion contains a westward electric field that strongly enhances the westward electrojet current by the creation of a cowling channel this westward electric field and the associated cowling current appear to terminate within the surge which contains an intense upward field aligned current the magnetic field measurements show that the region containing this field aligned current is shaped more like a cylinder rather than a long sheet the total current is found to exceed one half million amperes

continuation of a deep borehole stress measurement profile near the san andreas fault 2 hydraulic fracturing stress measurements at black butte mojave desert california hydraulic fracturing stress measurements were obtained in the black butte drill hole 18 km northeast of the san andreas fault in the mojave desert at depths from 251 to 635 m in all tests the least and greatest horizontal principal stresses s h and s h respectively exceeded the vertical stress s indicating a thrust faulting stress regime a single good quality hydraulic fracture impression from 309 m depth indicates an s h direction of n41 e 10 this s h direction should be interpreted with caution because it is based on only one observation this orientation is fairly compatible with nearby surface stress measurements but is incompatible with most of the hydraulic fracturing stress orientations reported from comparable depths in the mojave desert and is not favorable for right lateral slip on either the san andreas fault or nw striking faults present farther to the east the stress regime measured in the black butte hole is comparable to that measured at nearby shallow depths but differs from the strike slip or transitional strike slip to thrust faulting stress regime present at similar depths in two nearby holes crystallaire 4 km northeast of fhe san andreas fault and hi vista 32 km northeast of the san andreas fault the s h direction measured in these holes is approximately 60 counterclockwise of that observed in the black butte hole the differences in stress magnitudes and orientation among these holes substantiate previous indications of local variations in stress in the upper kilometer of the crust in this area and cast doubt on the validity of linear elastic models in which the effects of the san andreas fault dominate the stress field in the western mojave desert

electrodynamic parameters in the nighttime sector during auroral substorms the characteristics of the large scale electrodynamic parameters field aligned currents facs electric fields and electron precipitation which are associated with auroral substorm events in the nighttime sector have been obtained through a unique analysis which places the ionospheric measurements of these parameters into the context of a generic substorm determined from global auroral images a generic bulge type auroral emission region has been deduced from auroral images taken by the dynamics explorer 1 de 1 satellite during a number of isolated substorms and the form has been divided into six sectors based on the peculiar emission characteristics in each sector west of bulge surge horn surge middle surge eastern bulge and east of bulge by comparing the location of passes of the dynamics explorer 2 de 2 satellite to the simultaneously obtained auroral images each pass is placed onto the generic aurora the organization of de 2 data in this way has systematically clarified peculiar characteristics in the electrodynamic parameters an upward net current mainly appears in the surge with little net current in the surge horn and the west of bulge the downward net current is distributed over wide longitudinal regions from the eastern bulge to the east of bulge near the poleward boundary of the expanding auroral bulge a pair of oppositely directed fac sheets is observed with the downward fac on the poleward side this downward fac and most of the upward fac in the surge and the middle surge are associated with narrow intense antisunward convection corresponding to an equatorward directed spikelike electric field this pair of currents decreases in amplitude and latitudinal width toward dusk in the surge and the west of bulge and the region 1 and 2 facs become embedded in the sunward convection region the upward fac region associated with the spikelike field on the poleward edge of the bulge coincides well with intense electron precipitation and aurora appearing in this western and poleward portion of the bulge the convection reversal is sharp in the west of bulge and surge horn sectors and near the high latitude boundary of the upward region 1 fac in the surge the convection reversal is near the low latitude boundary of the upward region 1 with a near stagnation region often extending over a large interval of latitude in the eastern bulge and east of bulge sectors the region 1 and 2 facs are located in the sunward convection region while a spikelike electric field occasionally appears poleward of the aurora but usually not associated with a pair of fac sheets in the eastern bulge magnetic field data show complicated fac distributions which correspond to current segments and filamentary currents

stress regimes in the gulf coast offshore louisiana data from well bore breakout analysis maximum horizontal stress shmax orientations were mapped for the northern gulf of mexico offshore louisiana using borehole breakouts which are commonly used by petroleum and drilling engineers to determine the stress regime for well bore stability analyses and reservoir stimulation the breakout signal is subtle but indicates shmax orientations largely parallel to the strike of the progradational clastic sedimentary wedge in a general northeast southwest direction however significant anomalies exist in the breakout orientations the main anomaly being orthogonal to the principal shmax direction these anomalies were attributed to local perturbations around faults and salt structures and to the effect of the underlying salt the correspondence among the orientation of faults salt intrusions and shmax from breakouts with the strike of the clastic wedge confirms the existence of stress anisotropy in the northern gulf of mexico shelf sediments stress anisotropy is caused by the geometry and orientation of the prograding wedge and its associated gravity driven extension

isrm suggested methods for rock stress estimation part 3 hydraulic fracturing hf and or hydraulic testing of pre existing fractures htpf contents1 introduction 10112 summaryoftestmethods 10123 assumptionsandlimitations 10123 1 hydraulicfracturing hf 10123 2 hydraulictestsonpre existingfractures htpf 10124 apparatus 10134 1 fractureorientationdetectiondevices 10145 personnelquali cation 10146 equipmentperformance 10147 hydraulicfracturingtestprocedure 10148 hydraulic fracturedelineation 10168 1 impressionpackertechnique 10168 2 geophysicalimagingtechnique 10179 obtainingcriticalpressureparametersfromthepressure timerecords 101710 stresscalculations 101810 1 hydraulicfracturing hf 101810 2 hydraulictestsonpre existingfractures 101911 reportingofresults 1019references 1020

sonic properties as a signature of overpressure in the marcellus gas shale of the appalachian basin abstractthe marcellus formation a devonian gas shale in the appalachian basin is a heterogeneous rock as the result of a complex depositional diagenetic and deformational history although it is overpressured over a large portion of its economic area the origin and distribution of pore pressure within the gas shale are not well understood we have used the sonic properties of the marcellus and statistical analyses to tackle this problem the sonic data come from a suite of 53 wells including a calibration well in the appalachian basin we first analyze the influence of various extrinsic and intrinsic parameters on sonic velocities with univariate regression analyses the sonic velocities of the marcellus in the calibration well generally decrease with an increase in gamma ray american petroleum institute api and increase with density and effective stress basin wide median sonic velocities generally decrease with an increase in median gamma ray api and pore pressure and increase with burial depth

comprehensive atlas of stress trajectory patterns and stress magnitudes around cylindrical holes in rock bodies for geoscientific and geotechnical applications abstract this study presents a comprehensive atlas comprising maps and graphs which quantify the wide range of possible stress states around circular cylindrical cavities when subjected to the full range of far field stress conditions and internal net pressures a greater understanding of the principal stress trajectories and related stress magnitudes around stressed cylindrical holes in rock formations significantly aids the design of safer and more stable wellbores advances our capacities in hydraulic fracturing and provides additional insights into the nature and stability of volcanic dikes salt domes and caverns used for hydrocarbon and other fluid storage the algorithms used in our study systematically non dimensionalize the analytical kirsch equations using two critical parameters to scale the variable boundary conditions 1 the bi axial stress scalar describes the far field stresses at all depths and for each of the formations penetrated by a cylindrical borehole or natural liquid filled pipe by describing the anisotropy of the two far field principal stresses and 2 the frac number f specifies the net pressure on the cylindrical hole as it relates to the natural background stress in the host rock and the pressure in the penetrated formation at each depth manipulation of these two governing parameters f enables us to view and monitor the stress trajectories in the host rock around a cylindrical hole at any given depth additionally the neutral point locations and their effects on the principal stress orientations near the cylindrical hole have been more thoroughly investigated than in earlier studies the neutral points form the boundaries for the stress cage in overbalanced cylindrical cavities and for the complimentary fracture cage in underbalanced cavities the understanding of stresses induced around cylindrical holes in a rock formation is paramount in geotechnical analyses and stability appraisals of manmade cylindrical holes as well as for the geoscientific interpretation of piercing by cylindrical fluid filled pipes our stress atlas aims to provide fundamental insight and practical support for such analyses

appalachian stress study 2 analysis of devonian shale core some implications for the nature of contemporary stress variations and alleghanian deformation in devonian rocks detailed stress measurements in three boreholes penetrating horizontally bedded devonian silt stones sandstones and limestones above a prominent salt decollement in the appalachian plateau of western new york have revealed variations in horizontal stress magnitudes which correlate with lithologic and stratigraphic units in all wells high differential stress levels up to 20 mpa were found in shales of very high clay content contrary to the proposition that such materials have negligible long term strength elastic modulus data show that stiffer beds generally host higher stress levels and suggest that sand shale stress contrasts result in large part from elastic shortening of the section in response to regional ene compression no correlation between stress and poisson s ratio was found however a major systematic drop in stress level within the generally massive shales which occurs across a group of sand beds near the base of the rhinestreet formation appears to be of different origin the stress offset corresponds to the top of a section which we conclude on the basis of local and regional total strain data derived from chlorite fabric measurements once hosted abnormally high pore pressures the total strain data also suggest the entire section above the salt has been uniformly shortened during alleghanian compression to explain the stress discontinuity two kinematic patterns for alleghanian deformation of the devonian section are proposed both involving abnormal pore pressure development in the sub rhinestreet section in response to limited drainage of fluid drainage of this paleo overpressure is the best available explanation of the stress offset although an additional remnant stress component must also be present to satisfy the stress data precisely

joint initiation in bedded clastic rocks abstract two general types of sedimentary structures serve as initiation points ip for cross fold joints in siltstone beds of the ithaca formation appalachian plateau new york planar bed boundaries and geometrically complex features such as bedforms trace fossils and soft sediment deformation structures the relative abundance of each ip type varies with the stratigraphic position of the jointed body when they are mechanically isolated by thicker shales siltstone bodies have a higher percentage of ip associated with complex features this association supports the hypothesis that sedimentary structures serve to concentrate stress thus becoming preferred sites for primary joint initiation secondary joint initiation occurs at planar bed boundaries through interaction with pre existing joints in adjacent siltstone beds that are mechanically coupled to form composite joints the propagation path of joints passing through planar bed boundaries of composite joints can be traced backward to primary ip at complex sedimentary structures modeling suggests that when beds are mechanically coupled the modest joint tip stress concentration across a shale layer e g

microseismicity and permeability enhancement of hydrogeologic structures during massive fluid injections into granite at 3 km depth at the soultz hdr site summary a high rate injection of 20 000 m3 of water into granite between 2 8 and 3 4 km depth at the soultz hot dry rock hdr test site in france in 1993 september led to a 200 fold increase in borehole transmissivity and produced a subvertical cloud of microseismicity of dimensions 0 5 km wide 1 2 km long 1 5 km high and oriented 25 nw the resulting data set is unusually complete and well suited to studying permeability creation enhancement processes in crystalline rock and the utility of microseismic data for revealing them although the microseismic cloud defined using joint hypocentre determination jhd locations was diffuse and showed little structure application of the collapsing method showed it to be composed largely of discrete tubes and planes that propagated coherently one prominent structure that extended 350 m downwards from the vicinity of a flow inlet early in the injection and that appears to contain a major flow path was subjected to detailed investigation to establish its hydrogeologic nature and the mechanisms underpinning its inferred permeability enhancement high resolution microseismic mapping techniques i e multiplets and clustering showed it to be a subvertical nnw sse striking fracture zone of width 10 20 m the strike and scale of the structure identifies it as a member of a family of hydrothermally altered cataclastic shear structures that constitute the primary permeable paths for fluid migration within the rock mass both under ambient and forced fluid flow conditions the microseismicity occurred on subvertical small scale fractures within the cataclastic shear zone whose azimuths scatter within 22 of parallel to the parent structure although the structure is likely to have been naturally permeable to some degree its permeability appears to have been significantly enhanced as a consequence of the injection the most likely mechanism of permeability enhancement which is in accord with the strong preference for the microseismicity to grow downwards involves strike slip shearing which produced the opening of vertical tubes at along strike jogs in the fault the so called hill mesh seismic moment release averaged over the structure suggests shear displacements of at least 0 3 mm occurred which are sufficient to generate aperture changes that are hydraulically significant the preponderance of discrete structures within the microseismic cloud after collapsing suggests that significant flow and permeability enhancement i e stimulation within the rock mass is largely confined to the interiors of shear zones that appear to have a spacing of approximately 100 m

permeability creation and damage due to massive fluid injections into granite at 3 5 km at soultz 2 critical stress and fracture strength 1 the stress acting on fractures in a 3 5 km deep borehole in granite is examined to place constraints on fracture strength and to evaluate whether permeable fractures tended to be critically stressed two data sets consisting of some 500 fractures are analyzed the first considers the permeable impermeable fracture populations in their natural state the second considers the populations after the hole had been subject to a prolonged 9 mpa overpressure in two major injections which increased the number of permeable fractures from 18 to more than 95 for both data sets it is found that permeable fractures are critically stressed inasmuch as they support levels of shear stress that would be verging on failure if their strength were governed by a coulomb friction law with a coefficient of 0 6 1 0 however the vast majority of impermeable fractures are also critically stressed implying that critical stressing is a necessary but not a sufficient condition for permeability to develop most permeable fractures showed evidence of shear failure after the injections and tended to occur in hydrothermally altered cataclastic shear zones suggesting that they may have been relatively weak because of the presence of illite to prevent failure of the impermeable fractures during the injections requires a cohesion of at least 6 5 mpa to augment a maximum allowable friction coefficient of 1 0 these are probably early mode 1 fractures that are sealed primarily with calcite the strength of the shear zones appears to be significantly greater than the illite rich fractures from which they are composed this imposes constraints on their internal architecture

in situ stress measurements and regional stress field assessment in the xinjiang candidate area for china s hlw disposal abstract altogether 180 hydraulic fracturing hf in situ stress measurements were conducted in a pre selected area in the xinjiang uygur autonomous region china which is being considered as one of the candidate areas for china s high level radioactive waste hlw repository in situ stress magnitudes and orientations in three sub areas i e yamansu tianhu and aqishan sub areas were obtained from 163 valid measurement points in eight sub vertical boreholes at depths ranging from 17 to 647 m and the variation of the in situ stress components with depth was analyzed the measurement results show that the maximum horizontal stress is larger than the vertical stress within most of the measurement depth ranges indicating that the regional stress field is dominated by tectonic horizontal stress rather than by the overburden stress v within the range of the tested depths the stress field is characterized by h v h where h and h are the maximum and minimum horizontal stresses respectively fracture impression results reveal that the maximum horizontal stress is dominantly oriented in the nee sww direction which is in general in agreement with the orientation of regional tectonic stress field generated from movement of the earth s tectonic plates and is consistent with those interpreted by gps measurements and focal mechanism solutions in addition an abnormal phenomenon in performing stress measurements in one borehole was noticed and the result was discussed using borehole injection test data

in situ stress estimates from hydraulic fracturing and direct observation of crack orientation estimates of in situ stress in g tunnel rainier mesa nevada test site have been obtained with hydraulic fracturing techniques this work represents a nontraditional use of hydraulic fracturing in that it was performed primarily in horizontal boreholes drilled into the formation from access drifts tunneled into the mesa rather than the usual operation performed at depth in vertical boreholes drilled from the surface several operations were performed in essentially orthogonal triads of boreholes located at a point a significant feature of this work is the mineback operation in which the borehole is mined out to reveal the actual fracture during the hydraulic fracturing operation colored dye was added to the fracturing fluid which left an easily detectable stain on the fracture surface direct observation of the fracture orientation away from the borehole establishes the direction of the minimum compressive in situ stress and the plane of the other two principal stresses in one borehole which was not aligned along a principal stress direction a modified mineback operation revealed a fracture plane that twisted as it grew away from the borehole aligning itself finally in a plane determined by the in situ stresses this twisting is consistent with theoretical predictions of borehole stresses under these conditions this same mineback also showed that fracture of the formation initiated at or under one of the packers indicating that the packers may have an effet on in situ stress estimates general observations of over 100 tests performed in g tunnel show that under the sloping portion of the mesa fracture planes are not vertical but dip in a direction tending to parallel the mesa slope deep into the tunnel and well under the flat part of the mesa fractures are essentially vertical with strikes approximately n45 e which substantiates the usual vertical fracture assumption in traditional fracture operations performed at depth in vertical boreholes advantages limitations and problem areas associated with extracting in situ stress fields from hydraulic fracture pressure records are discussed in detail

the relationship between closure pressures from fluid injection tests and the minimum principal stress in strong rocks abstract closure pressures measured during injection tests such as mini fracs are normally considered an accurate measure of the minimum in situ principal stress magnitude this paper presents stress strength and image log data from the australian cooper basin which suggests that in reservoirs with high in situ stress high tensile strength and weak geological fabrics interpreted closure pressures may be significantly greater than the minimum principal stress closure pressures interpreted from mini frac injection tests in the cooper basin suggest the minimum principal stress varies from 12 4 27 2 mpa km 0 55 1 2 psi ft to better understand the reasons for this variation in closure pressure image logs and mini frac data from 13 treatment zones and core from seven of these treatment zones were analysed the analysis revealed that treatment zones with high measured closure pressures 18 1 mpa km 0 8 psi ft high treating pressures 31 6 mpa km 1 4 psi ft and high measured hydraulic fracture complexity existed in reservoirs with high tensile rock strength 7 mpa 1015 psi and geological fabrics planes of weakness including natural fractures conversely treatment zones with lower measured closure stress 19 mpa km 0 84 psi ft and low hydraulic fracture complexity occurred in reservoirs with lower tensile strength and or no geological fabrics we suggest that closure pressures in rocks with high tensile strength and weak geological fabrics may not be representative of the minimum principal stress magnitude in the cooper basin where they are associated with hydraulic fracture complexity rather they reflect the normal stress incident on pre existing weaknesses that are exploited by hydraulic fluid during the mini frac injection

the role of salt in fold and thrust belts davis d m and engelder t 1985 the role of salt in fold and thrust belts in n l carter and s uycda editors collision tectonics deformation of continental lithosphere tectonophysics 119 67 88 the style of deformation in thin skinned fold and thrust belts is critically dependent upon the resistance to sliding along the detachment between the mass of deforming sediments and the underlying rocks evaporites can provide an extremely weak horizon within which a basal detachment can form and along which only a relatively small shear traction can be supported fold and thrust belts that form atop a salt layer such as the appalachian plateau the franklin mountains in northwestern canada and the jura of the alps among others share several readily observable characteristics as predicted by a simple mechanical model for fold and thrust belts a detachment in salt permits a thrust belt to have an extremely narrow cross sectional taper in addition predicted orientations of the principal stress axes over a salt dtcollement are consistent with the commonly observed lack of a consistently dominant vergence direction of structures within the thrust belt other common attributes of salt basal thin skinned deformation include the presence of several widely but regularly spaced folds and abrupt changes in deformational style at the edge of the salt basin

in situ spatiotemporal measurements of the detailed azimuthal substructure of the substorm current wedge the substorm current wedge scw is a fundamental component of geomagnetic substorms models tend to describe the scw as a simple line current flowing into the ionosphere toward dawn and out of the ionosphere toward dusk linked by a westward electrojet we use multispacecraft observations from perigee passes of the cluster 1 and 4 spacecraft during a substorm on 15 january 2010 in conjunction with ground based observations to examine the spatial structuring and temporal variability of the scw at this time the spacecraft traveled east west azimuthally above the auroral region we show that the scw has significant azimuthal substructure on scales of 100 km at altitudes of 4000 7000 km we identify 26 individual current sheets in the cluster 4 data and 34 individual current sheets in the cluster 1 data with cluster 1 passing through the scw 120 240 s after cluster 4 at 1300 2000 km higher altitude both spacecraft observed large scale regions of net upward and downward field aligned current consistent with the large scale characteristics of the scw although sheets of oppositely directed currents were observed within both regions we show that the majority of these current sheets were closely aligned to a north south direction in contrast to the expected east west orientation of the preonset aurora comparing our results with observations of the field aligned current associated with bursty bulk flows bbfs we conclude that significant questions remain for the explanation of scw structuring by bbf driven wedgelets our results therefore represent constraints on future modeling and theoretical frameworks on the generation of the scw

measurement related uncertainties in overcoring data at the aspo hrl sweden part 2 biaxial tests of csiro hi overcore samples this paper is the second dealing with measurement related uncertainties of overcoring data undertaken at the aspo hard rock laboratory and focuses on the biaxial test data from csiro hi overcore sa

a case for neotectonic joints along the niagara escarpment in a search for proof that some joints on the appalachian plateau are neotectonic orientation data were collected on more than 1800 fractures in exposures of the lockport dolomite in western new york and southern ontario of greatest interest was a late forming ene joint set present in the central and western portions of the field area these joints increase in abundance with proximity to the niagara escarpment the shape of the east west trending niagara escarpment is defined by a series of asymmetric reentrants with a strong ene linear trend reflecting the bedrock joint pattern the joints appear to have formed near the earth s surface in response to low tensile stresses developed in bedrock adjacent to the retreating escarpment since the current shape and position of the escarpment were controlled by glacial activity during the last 3 m y it is possible that the ene joints and reentrants are neotectonic features

effects of birkeland current limitation on high latitude convection patterns it is shown how the high latitude convection pattern may be modified by substorm enhanced polarization electric fields these are generated whenever the flow of those birkeland currents which are associated with ionospheric conductivity gradients is limited such birkeland currents are fed mainly by the enhanced pedersen current in the evening and morning sectors of the auroral oval and by the enhanced hall current around local midnight as the current limitation increases the ionospheric potential represented here by a symmetric two cell pattern will rotate clockwise and deform as will the associated birkeland current distribution which will attain roughly a symmetric shape relative to the noonmidnight magnetic meridian the resulting patterns are shown to agree well with observations a pronounced westward intrusion of the equipotential contours occurs in the auroral oval and may be associated with the westward traveling surge this feature does not however require any assumed longitudinal conductivity gradients rather it falls out naturally when the closure of the enhanced pedersen current is also taken into account

27 august 2001 substorm preonset phenomena two main onsets field aligned current systems and plasma flow channels in the ionosphere and in the magnetosphere we supplement the results of the 27 august 2001 substorm studied earlier in the series of papers described is the plasma flow in the nightside ionosphere from the near polar region from the polar cap to the auroral oval during the substorm preonset phase and two expansion onsets eo1 and eo2 produced by reconnection in the closed tail magnetic reconnection mr1 and in the open tail lobes mr2 respectively we discuss the location of the mr2 region is it near middle and or distant tail and the eo2 trigger mechanism the upward substorm current wedge field aligned current fac and the downward fac in the polar cap dusk sector that were both produced by different types of magnetosphere ionosphere feedback instability are found to provide the main contribution to the system of facs during eo1 and eo2 also we obtain the estimates for the eo1 and eo2 power and energy addressed are the variations in the tail lobe magnetic flux and their variations association with eo2 in addition we describe a 3 d system of mesoscale cells each of which involves a plasma vortex and a local fac maximum the cells of this system in the inner magnetosphere and in the tail lobes intensify one after other within 2 min interval at last we substantiate the assumption that the fast plasma flow recorded by the cluster satellites 7 min prior to eo1 was a bursty bulk flow from the most distant tail

in situ stress from breakouts in the danish sector of the north sea abstract borehole breakouts from 26 wells situated in the danish central graben and one well situated in the northwestern section of the ringkobing fyn high have been studied the data provide information on the orientation of the contemporary principal horizontal stresses down to depths of 4 7 km statistics were used to obtain a mean azimuth of breakouts for each well and each individual breakout was weighted by its length data quality of breakouts is assigned according to the world stress map quality ranking scheme the breakout data reveal that s h orientations in the danish central graben are extremely scattered no regional trend in mean s h orientation can be identified as a result of the large standard deviation 77 similar scatterings of s h orientations have been reported from adjacent regions of the north sea the results indicate that the state of stress in the sedimentary cover of the danish central graben is influenced by a local stress generating mechanism rather than platewide stress transmissions mechanisms three units with different states of stress have been identified and found to correspond somewhat with the lithostratigraphic divisions of the post chalk group chalk group and pre chalk group

joint interaction with embedded concretions joint loading configurations inferred from propagation paths abstract the interaction between propagating joints and embedded concretions in a devonian black shale near seneca lake ny permits identification of the loading configurations responsible for two joint sets of different ages striking at nearly the same orientation the earlier set consists of systematic joints cut by later alleghanian joints of the appalachian plateau the later set consists of non systematic curving cross joints that abut these same alleghanian joints field evidence shows that concretions functioned as stiff inclusions in a compliant black shale as a consequence of this elastic contrast local perturbations in the remote stress field persisted around the concretions during burial tectonic deformation and exhumation these stress perturbations influenced joint propagation paths of both joint sets our conclusions about loading configurations are based on finite element modeling of the effect of the local stress perturbation on concretion modified joint propagation modeling shows that the local stress perturbation from a thermoelastic loading was responsible for deflecting cross joints away from concretions in a curved propagation path near the concretion this load configuration also led to arrest of cross joints before they penetrated the shale concretion interface at greater distances from the concretion the propagation path of cross joints was controlled by the contemporary tectonic stress field the interface between concretions and the surrounding shale was strongly bonded as indicated by the crossing of the interface by some of the systematic ene joints higher compressive stress levels within the concretions relative to the shale suppressed joint development in the concretion causing the arrest of those joints once they had driven across the interface and a short distance into the concretion numerical modeling shows that interface penetration by the systematic ene joints is consistent with a fluid load the same loading configuration postulated for the subsequent alleghanian joints the traces of the systematic ene joints align on opposite sides of concretions rather than curving toward the concretion as predicted by two dimensional models of the fluid load co planar traces are indicative of large planar joints propagating in plane around the concretion making it energetically inefficient for the crack front to curve as it enters the local stress perturbation near the concretion a fluid load for the systematic ene joints came from high pore pressure during the pre alleghanian stages of burial of the devonian catskill delta complex

evaluation of measurement related uncertainties in the analysis of overcoring rock stress data from aspo hrl sweden a case study a re evaluation of overcoring results from the borre probe and the csiro hi cell was carried out at the aspo hrl the re analysis revealed a number of measurement related uncertainties for the two cells e g bonding temperature effects and identification of elastic parameters these uncertainties were corrected and the re analyzed strains were then used to determine the in situ stress field using standard least squares technique the results indicate lower stress magnitudes compared to the original interpretations and the results are in good agreement with existing hydraulic fracturing data as well as with theoretical vertical stress overall the re analysis indicates that the stress field at aspo hrl is well constrained and relatively consistent with depth

discordant paleomagnetic poles from the canadian coast plutonic complex regional tilt rather than large scale displacement stratigraphic petrologic and isotopic data indicate that parts of the coast plutonic complex and the north cascade range have been tilted northeast side up by angles of approximately 30 degree about north northwest trending axes these tilts can account for discordant paleomagnetic directions observed in mid cretaceous plutons from these regions without large scale displacement relative to north america

integrated 3 d stress determination by hydraulic fracturing in multiple inclined boreholes beneath an underground cavern abstract this paper presents a complete three dimensional stress determination using hydraulic fracturing data from three inclined boreholes drilled from the floor of an underground cavern at a depth of about 100 m both conventional hydraulic fracturing hf and hydraulic testing of pre existing fractures htpf were carried out at all test points to acquire reliable data and conduct the integrated stress analysis we determined 3 d stress states using a numerical inversion code that integrates the entire data set from hf and htpf methods and employs a nonlinear least squares optimization routine based on a modified levenberg marquardt method and a finite difference jacobian algorithm we present the trend of complete three dimensional stress states with depth expressed by correlation equations we compare the 3 d stress inversion result for the integration of all data from the three boreholes with the results determined independently for each individual borehole the results showed that the maximum principal stress was subhorizontal and oriented approximately nne ssw and the ratio of maximum to minimum principal stress was 2 0 on average the inverted scatter of misfit remained within 10 for both the integrated analysis of the entire data set and the measurement data for each individual borehole from these findings we conclude that the 3 d stress determination made by integration of hf and htpf data from multiple inclined boreholes resulted in a reliable inversion of the 3 d stress field

geophysical log responses and their correlation with bed to bed stress contrasts in paleozoic rocks appalachian plateau new york a 1 km profile of in situ stress and geophysical log data was acquired in the wilkins well to study the relationship between rock properties and in situ stress contrasts the wilkins well penetrates devonian clastic rocks on the appalachian plateau near the town of south canisteo new york open hole hydraulic fracture stress measurements were made in stratigraphic sequences where geophysical logs indicated significant bed to bed variations in elastic and lithologic properties analysis of stress magnitudes and interval averaged geophysical data shows that principal horizontal stress magnitudes correlate directly with elastic stiffness and inversely with clay content a similar relation is found for older paleozoic strata penetrated by a well at auburn new york correlations between stress magnitude and geophysical properties observed in the wilkins and auburn wells provide strong evidence that bed to bed stress variations arise from a uniform ene wsw directed strain acting on beds of different young s modulus rather than from variations in rock shear strength because of their high young s modulus sandstones siltstones and limestones in the northern appalachian basin are likely to be stronger barriers to hydraulic fracture propagation than shales porosity logs in the wilkins well show that the large decrease in horizontal stress found at the base of the rhine street formation occurs where shales are less compacted the correlation with undercompaction is consistent with a paleo overpressure drainage mechanism as the cause for the stress decrease

recent stress field and neotectonics in the eastern jura mountains switzerland the regional stress field and its local variation were determined for the northern part of central switzerland fig 1 by using overcoring techniques doorstopper triaxial strain cell and observations of breakouts in deep boreholes the results are compared with fault plane solutions of earthquakes and with the orientation of horizontal stylolites in the northern part of central switzerland the nw se orientation of the maximum horizontal stress sh which is characteristic for central europe was observed only in the crystalline basement in the folded jura and south of it in one well the greatest principal horizontal stress above the triassic decollement horizon is oriented approximately in a n s to nne ssw direction this direction persists into the western tabular jura and the southernmost rhine graben only in the eastern part of the tabular jura the greatest principal horizontal stress shows a nnw sse to nw se orientation comparison of the near surface stress field as determined by in situ stress measurements and borehole breakouts with the directions of horizontal stylolites generated during the evolution of the folded jura indicates that the orientation of the recent stress field near the earth s surface is the same as that which prevailed during the upper miocene to lower pliocene the central part of northern switzerland is therefore the first area in central europe where it is possible to demonstrate that the near surface stress field is decoupled from that in the crystalline basement the difference in the orientation of the greatest principal horizontal stress is about 50

revisiting the hubbert rubey pore pressure model for overthrust faulting inferences from bedding parallel detachment surfaces within middle devonian gas shale the appalachian basin usa abstract both bedding parallel slickensides and cleavage duplexes are forms of mesoscopic scale detachment faulting populating black marcellus and geneseo burket and intervening gray mahantango shales of the middle devonian a section known for abnormal pore pressure below the appalachian plateau the abundance and the orientation of slickensides and cleavage duplexes in the more organic rich black shale relative to gray shale suggests that maturation related abnormal pore pressure facilitates detachment a mesoscopic manifestation of the hubbert rubey pore pressure model for overthrust faulting the former are discrete slip surfaces whereas the latter consists of nested anastomosing slip surfaces either cutting through bedding or on disrupted bedding surfaces stacked as mesoscopic versions of thrust duplexes cleavage duplexes are between a few cm and over 1 m thick with their hanging walls commonly transported toward the appalachian foreland regardless of local limb dip cleavage duplexes are most common near the stratigraphic maximum flooding surface the organic rich section most prone to develop maturation related pore pressure in the middle devonian gas shales bedding parallel slickensides are somewhat more evenly distributed in the black shale but also found in overlying gray shale in both black and gray shales slickensides are more abundant on the limbs of folds an indication of pore pressure related flexural slip folding on the macroscopic scale the pine mountain block of the southern appalachian mountains was enabled by a basal detachment cutting along the upper devonian chattanooga black shale which has a thermal maturity sufficient for the generation of abnormal pore pressure the pine mountain block is a large scale overthrust showing little evidence of collapse of the hinterland side a credible example of a pore pressure aided overthrust fault block of the type envisioned by the hubbert rubey model

photoelastic and numerical investigation of stress distributions around fault models under biaxial compressive loading conditions abstract this paper presents the results of a photoelastic and numerical study of stress distributions contours and trajectories around fault models of various geometries submitted to a biaxial compressive load it aims to describe typical biaxial stress behaviours and emphasize the existing differences with the well known uniaxial compressive load case stress trajectories are sometimes shown by joint sets acting as markers of a paleostress field and they can be interpreted by particular shallow tension compression situations at depth fractures can be reactivated or can dilate under conditions of triaxial compression and behaviour is essentially controlled by a high stress ratio high 3 1 ratio in spite of the potential importance of such stress states on fracture permeability and fluid flow and although they are frequently found at depth in a reservoir context such stress conditions are poorly investigated particularly in terms of stress perturbations the presented analogue experiments consisted of compression tests done on polymethylmethacrylate pmma plates this material has mechanical properties comparable to those of brittle rocks in the upper crust and presents birefringence the samples contained open defects acting as faults and the stress trajectories around these faults were investigated using a photoelastic device comparable numerical experiments were realised with a finite element code franc 2d using frictionless fault models first the effect of an increasing biaxial compressive load ratio 3 1 on stress trajectories around an isolated open defect was explored it was shown that the stress trajectories were drastically modified when 3 1 increased from 0 2 to 0 4 this result being consistent with previous studies in particular when 3 1 was superior or equal to 0 4 external isotropic points around where trajectories diverged called repulsive isotropic points appeared near the tips but away from the defects they tended to move away from the defects towards the main load direction when 3 1 increased the described isotropic points were points of stress trajectory divergence i e points where stress decreased implying that zones around them were strongly unfavourable to shear reactivation second stress trajectories around fault models of various geometries oblique isolated defect dilational jog compressive jog and complex patterns were studied the applied biaxial compressive load ratio being 0 7 these biaxial stress trajectories were compared with similar uniaxial stress trajectories in order to explore the existing differences between the two regimes it enabled new stress trajectory geometries to be described and interpreted in particular numerous external repulsive isotropic points were observed and defect tips were shown to be zones of high convergence of stress trajectories furthermore in contrast to the uniaxial compressive load case stress trajectories and stress contours were geometrically similar within dilational and compressive jogs under biaxial compressive load for both jog types the centre of the overlapping zone and the areas along the wall of a jog defining fault and facing the overlapping tip of the other fault proved to be zones of low mean stress implying that fluids may migrate towards them from the tips of the faults in response to mean stress gradients furthermore the centre of the jogs exhibiting a high differential stress was a favourable area in terms of fracture reactivation which may facilitate fluids transfer and storage it was also observed that for both the uniaxial and biaxial compressive loads isotropic zones were localised at the acute angle between branching defects

separation of cowling channel and local closure currents in the vicinity of a substorm breakup spiral 1 the substorm breakup spiral demarcates the westernmost terminator of the substorm current wedge a long standing debate has been devoted to the question of to what extent the strong upward field aligned currents fac in that spiral are either closed by downward fac through pedersen currents flowing radially to the center of the spiral local closure or by currents that flow westward though a cowling channel which extends in the region eastward of the spiral cowling closure in particular studies using ground based and space based instrumentation came to systematically different results in order to address this question we reanalyze a pseudobreakup spiral event on 3 february 1999 2204 ut that has been observed with the miracle network of ground based instruments in fennoscandia and for which spatial results of electric fields conductances and currents have been calculated by amm et al 2001 we apply a new technique that allows under certain geometrical assumptions the current systems to separate in the vicinity of the breakup spiral into their local closure and cowling closure parts the result shows that for the event analyzed 68 of the upward fac in the spiral is closed via the local closure current system and the remaining 32 via the cowling closure current system therefore the local closure mechanism is the dominating provider of the upward fac in the breakup spiral overall however a localized area at the western flank of the spiral where conductance gradients are largest exists in which the cowling closure dominates and feeds more than 80 of the upward fac in that area further studies are required in order to examine the generality of the results obtained for our specific event

integrating borehole breakout dimensions strength criteria and leak off test results to constrain the state of stress across the chelungpu fault taiwan article the paper describes the computation of the maximum horizontal stress h magnitude in the vicinity of the chelungpu fault taiwan host of the slip zone during the chi chi earthquake mw 7 6 1999 the scientific hole b intercepts the chelungpu fault at 1136 m at the depths of logged breakouts 940 1310 m the vertical stress v as estimated from density logs increases linearly with depth from 22 to 31 mpa a series of leak off tests yielded two reliable shut in pressures 23 7 mpa at 1085 m and 29 8 mpa at 1279 m which are lower than the estimated v albeit by only 2 1 and 0 6 mpa respectively in our analysis the shut in pressures were considered to represent estimates of the least horizontal principal stresses h at the respective depths and consequently the test induced fractures were assumed to have been vertical principal stress directions had been previously determined by others 105 155 for the maximum horizontal stress h

new developments in the integrated stress determination method and their application to rock stress data at the aspo hrl sweden the integrated stress determination method isdm is a powerful tool for estimating the regional stress tensor from in situ measurements of local stress tensors using a wide variety of stress measu

estimation of rock mass deformation modulus using variations in transmissivity and rqd with depth discontinuity normal stiffness and deformation modulus of large scale rock masses are very difficult to determine a method for estimation of discontinuity normal stiffness based on the decrease in transmissivity with depth has been proposed by the authors in a former paper in the current study the method is further developed by accounting for the changes in both discontinuity aperture and frequency with depth which are key factors that cause the transmissivity to decrease with depth the discontinuity frequency can be estimated from rqd measurements which are readily available in most geotechnical investigations the transmissivity data from packer tests are usually available in geotechnical investigations for hydropower plants for a rock mass in a dam site mainly controlled by lithostatic stress based on transmissivity and rqd data at different depths the change in discontinuity aperture with depth can be linked to the change in aperture with stress which defines the normal stiffness of discontinuities in the case study the discontinuity normal stiffness is successfully estimated by using transmissivity and rqd data and the result shows that the normal stiffness increases with stress depth and the rate of normal stiffness versus stress depth decreases with stress depth which is consistent with experimental studies the estimated normal stiffness has been utilized to calculate the rock mass deformation modulus using an equivalent model the result of deformation modulus by the proposed method is close to that obtained by using in situ measurements as well as by using empirical models relating rqd to deformation modulus

hydrofracturing stress measurements in the iceland research drilling project drill hole at reydarfjordur iceland two independent suites of hydrofracturing stress measurements were conducted in the top 600 m of the iceland research drilling project deep hole at reydarfjordur east iceland as indicated by the continuously extracted drill core the tested section consists of tertiary subaerial tholeiitic lava flows cut by many basaltic dikes the density of the basalt was used to estimate the vertical stress v in megapascals at depth d in meters v 0 027d in the 33 to 270 m depth range the horizontal principal stresses hmax and hmin as calculated from hydrofracturing results increase gradually with depth but at a lower rate than the vertical stress hmin 2 1 0 016d hmax 3 3 0 016d in the 300 to 550 m depth range a major perturbation occurs with both horizontal stress magnitudes rapidly rising between 300 and 400 m and then uniformly decreasing between 400 and 550 m to a level apparently predictable by the relationships obtained in the top 270 m conditions favoring normal faulting v hmax hmin were established in the 540 to 580 m range and are conjectured to persist at greater depths since the vertical stress gradient is higher than that of hmax and hmin this would be in accord with the predominant fault type in east iceland the relative low horizontal stress values and the small difference between them are supportive of ocean floor spreading theories and contradict near surface high compressive stresses previously determined by hast 1973 in east iceland the large increase in horizontal stress in the range of 300 to 500 m depth is not understood a correlation appears to exist between the zone of almost unintruded thick lava flows 200 500 m and that of high stresses an intuitive model suggesting locked in stresses in this zone is proposed the hydrofracturing determined overall mean direction of hmax is n50 e while a more selective sampling of results yields n40 e the measured stress directions appear to be reasonably compatible with the local structure such as dike direction nne and lava dip wsw significantly hmax direction is subparallel to the n20 e trend of the nearest fissure swarms in the axial rift zone this almost coincidence of directions like the measured stress magnitudes is also in agreement with the model of spreading lithospheric plates being pulled apart

paleomagnetism of the upper cretaceous strata of mount tatlow evidence for 3000 km of northward displacement of the eastern coast belt british columbia this study contributes the first paleomagnetic data from dated cretaceous units of the coast belt 51 3 n 123 8 w in which paleohorizontal bedding surfaces can be identified with certainty we sampled 38 sites through a 1 5 km stratigraphic interval of cenomanian silverquick sedimentary strata and cenomanian to campanian powell creek volcanics in the tatlow syncline in the tyaughton methow basin magnetizations at 21 sites have high unblocking temperatures tub and pass the tilt test precision k 5 before untilting maximum k 48 after 100 untilting magnetic minerals of volcanics show original textures mean declinations in volcanic and sedimentary rocks are statistically identical and polarity is consistently normal the mean inclination 55 4 3 4 probability p 0 05 of this pretilting magnetization is 20 1 4 0 shallower than that expected if the rocks were deposited in their present position relative to cratonic north america field and laboratory observations rule out other causes for these anomalous shallow inclinations these rocks therefore have been displaced northward by 3000 500 km relative to cratonic north america since mid cretaceous time this displacement is almost 3 times greater than that estimated from mid cretaceous volcanics in the now adjacent intermontane belt evidently the two belts were not amalgamated until latest cretaceous or early tertiary time these results are inconsistent with all previous geologically based tectonic reconstructions of the cordillera magnetizations at eight sites in silverquick and powell creek beds have low tub and directional characteristics similar to those in three younger diorite intrusions reversed and normal polarities occur the rocks are commonly highly altered and maximum precision occurs after 40 untilting the magnetizations therefore are interpreted as overprints of latest cretaceous age acquired during folding

observations of hydraulic fracturing in soils through field testing and numerical simulations hydraulic fracturing is a potential cause of leakage of earth dams or loss of fluid in drilling and field permeability testing the effect of hydraulic fracturing on soil grouting is also a major c

stress orientation profile to 3 5 km depth near the san andreas fault at cajon pass california a detailed profile of the orientation of the apparent maximum horizontal compressive stress was constructed over the depth interval 1 75 3 46 km in the cajon pass drill hole located 4 2 km from the san andreas fault in southern california the profile is based on the analysis of stress induced well bore breakouts and consists of 32 616 orientation determinations at a basic sampling interval of 4 cm the azimuth of the apparent maximum horizontal compressive stress around the borehole is 057 s d 19 depth dependent variations in breakout azimuths from a few degrees to as much as 100 occur over depth intervals of several centimeters to hundreds of meters in the borehole with wavelengths showing a self similar distribution over a range of depth intervals the variations in breakout orientation at depth seem to reflect stress fluctuations associated with active faults penetrated by the drill hole assuming linear elastic behavior perturbations in stress magnitude and orientation were computed for predrilling slip on these faults fault stress drops limited to the ambient shear stress i e up to total stress drop along with occasional local extreme stress drops and geometrical complications can explain the variations in breakout orientations in the drill hole the penetrative stress inhomogeneity may suggest that the average orientation of borehole breakouts in cajon pass which indicates left lateral shear on planes parallel to the san andreas fault may not be representative of the state of stress near the fault at depth in this region rather the stress orientation profile indicates the superposition of numerous local stress perturbations on an average stress field which is characterized by an absence of appreciable right lateral shear on planes parallel to the san andreas combining these observations with the highly variable stress state in shallow boreholes in the western mojave desert it is suggested that the heterogeneous stress field in this region representing a spectrum of seismic events may extend over substantial distances across and along the san andreas fault zone if the average measured breakout orientation in cajon pass is representative of the state of stress near the san andreas in this region then substantial left lateral shear stress is resolved on planes parallel to the fault in contradiction to the sense of its long term motion we use an elastic dislocation model to compute the net stress change near the cajon pass drill site since just prior to the large 1812 earthquake taking into account coseismic slip in major earthquakes and aseismic accumulation of slip on the san andreas and san jacinto faults the model suggests that the net change in fault parallel shear stress during the current earthquake cycle in the cajon pass area is nearly negligible the difference between the measured and computed results requires that additional left lateral shear stress be superimposed on a generally fault normal maximum horizontal compressive stress the results suggest that during the great fort tejon earthquake of 1857 left lateral shear stress parallel to the san andreas fault in the region of cajon pass may have played a role in terminating the southeastward extent of the rupture propagation

relation between seismic source parameters and mechanical properties of rocks a case study this study attempts to determine the relation between source parameters and mechanical properties of the rock matrix in which the microseismic events occur for this purpose accurate geological mechanical and seismological data were acquired on a gas field experiencing induced seismicity due to its reservoir pressure drop more than 30 deep boreholes depth greater than 4 km are concentrated in a 10 10 5 km volume providing core samples for both geological and mechanical assessment in this study we focus on induced seismic events recorded by the local seismic network over a three year long period characteristics of the seismic sources were obtained using spectral analysis and a dynamic model of failure results point out correlation between physical parameters of the seismic sources and the geomechanical properties of the rocks involved maximal static stress drops are found to be associated with the mechanical strength of the geological strata where the rupture occurs the fracture size using a circular model of failure is also found to be dependent on the geomechanical setting it is found that the size of the seismic fractures is dependent on the layer thickness and the prefracturation of the medium both factors influencing the extension of preexisting discontinuities the parameters of the seismic sources also show important changes when the gas reservoir is reached the reservoir unit experienced a 45 mpa pore fluid pressure drop over a period of 20 years

the convection electric field in auroral substorms dynamics explorer 2 de 2 electric field and ion drift data are used in a statistical study of the ionospheric convection electric field in bulge type auroral substorms thirty one individual de 2 substorm crossings were carefully selected and organized by the use of global auroral images obtained by de 1 the selected passes which occurred during substorm expansion phase maximum or early recovery phase cover the entire nighttime substorm the organization of the data used the method developed by fujii et al 1994 which divided the data into six local time sectors covering the nighttime substorm region following the procedures employed in the paper by gjerloev and hoffman 2000b the latitudinal width and location of each auroral oval crossing was then adjusted to fit the sector average in addition to the detailed study of the characteristics of the field within each sector this database enabled us to compile a model of the ionospheric convection electric field the characteristics of the premidnight convection reversal show a pronounced local time dependency far west of the surge it is a fairly well defined point reversal or convection shear approaching the surge and within the surge it is a region of weak electric fields increasing in width toward midnight that separates regions of equatorward and poleward electric fields therefore we adopt the term harang region rather than the harang discontinuity for the premidnight convection reversal a relatively narrow convection channel is coincident with the highest conductances located just poleward of the harang region this channel drives the substorm current wedge component of the westward electrojet in the surge and middle surge sectors it is present in all premidnight passes and consequently is an integral part of the three dimensional substorm current wedge system

horizontal slip along alleghanian joints of the appalachian plateau evidence showing that mild penetrative strain does little to change the pristine appearance of early joints abstract some alleghanian joints in black shales of the geneseo and middlesex formations of the catskill delta complex finger lakes district new york slipped horizontally up to 8 cm horizontal slip is measured by the offset of ene striking joints alleghanian joints striking 330 350 display a right lateral slip with an average value of 1 9 cm while joints striking 004 010 slip in the left lateral sense with an average value of 1 3 cm the maximum horizontal stress sh driving this slip falls between 350 and 004 the orientation of local alleghanian layer parallel shortening as indicated by both disjunctive and pencil cleavage by commonality of orientation we infer that slip on alleghanian joints is driven contemporaneously with layer parallel shortening if so the offset ene striking joints predate the alleghanian stress field these observations mean that both pre alleghanian and early alleghanian joints persist through a period of penetrative strain

joint two dimensional observations of ground magnetic and ionospheric electric fields associated with auroral zone currents 32 auroral zone currents during the passage of a westward travelling surge

late cretaceous paleogeography of wrangellia paleomagnetism of the mccoll ridge formation southern alaska revisited comment comment introduction stamatakos et al 2001 1 revisited the paleomagnetism of the maccoll ridge formation in alaska finding smaller northerly transport than a prior study concluding that their result is incompatible with studies supporting large scale transport of coast cascade belt terranes along

uncertainty in the maximum principal stress estimated from hydraulic fracturing measurements due to the presence of the induced fracture the classical theory for hydraulic fracturing stress measurements assumes an ideal case with a linear elastic homogenous and isotropic medium and a fracture that reopens distinctly when the minimum tangential borehole stress is exceeded the induced fracture disturbs this ideal picture in several aspects which are important for the evaluation of the maximum horizontal principal stress using the fracture reopening pressure this disturbance can be attributed to the fracture normal stiffness and the initial hydraulic fracture permeability in this paper the hydraulic fracturing reopening test is studied by coupled hydromechanical modeling that takes into account an induced fracture that is incompletely closed the result shows that with realistic equipment compliance the apparent fracture reopening evaluated from the well pressure is close to the magnitude of the minimum horizontal principal stress with little or no correlation to the maximum horizontal principal stress this observation suggests that the determination of maximum principal stress by hydraulic fracturing using the reopening pressure is very uncertain

a new parameter to assess hydromechanical effects in single hole hydraulic testing and grouting grouting or filling of the open voids in fractured rock is done by introducing a fluid grout through boreholes under pressure the grout may be either a newtonian fluid or a bingham fluid the penetration of the grout and the resulting pressure profile may give rise to hydromechanical effects which depend on factors such as the fracture aperture the pressure at the borehole and the rheological properties of the grout in this paper we postulate that a new parameter a which is the areal integral of the fluid pressure change over the fracture plane is an appropriate measure to describe the change in fracture aperture volume due to a change in effective stress in many cases analytic expressions are available to calculate pressure profiles and the a parameter the approach is verified using a fully coupled hydromechanical simulator for the case of a newtonian fluid results of the verification exercise show that the new approach is reasonable and that the a parameter is a good measure for the fracture volume change i e the larger the a parameter the larger the fracture volume change in an almost linear fashion to demonstrate the application of the approach short duration hydraulic tests and constant pressure grouting are studied it is concluded that using analytic expressions for penetration lengths and pressure profiles to calculate the a parameter provides a possibility to describe a complex situation and compare discuss and weigh the impact of hydromechanical couplings for different alternative grouting strategies further the analyses identify an effect of high pressure grouting that of uncontrolled grouting of larger fractures and insufficient or less than expected sealing of finer fractures under certain grouting conditions

formation of regional cross fold joints in the northern appalachian plateau abstract fold axis parallel elongation associated with the development of arcuate fold and thrust belts is proposed as a causal mechanism for syn orogenic cross fold joints such a mechanism can be coupled with other joint propagation models providing a widely applicable resolution to the enigmatic origin of regional systematic joints the fold axis parallel elongation model is compatible with kinematic indicators as recorded by a sequence of cross fold joints and related deformational fabrics in the central and northern appalachian plateau modeling of an arcuate tectonic boundary subjected to normal compressive loading demonstrates that tangential tensile stress can be large enough to initiate cross fold joints on the convex side of the tectonic boundary simulated stress trajectories from boundary element modeling bear a strong resemblance to the stress trajectories inferred from the regional cross fold joint patterns in the central and northern appalachian plateau modeling also displays a cratonward decrease in both the tangential stress and the tangential strain such a decrease is consistent with the deformation styles observed in the central and northern appalachian plateau

quantitative comparison of microfabric and magnetic fabric in black shales from the appalachian plateau western pennsylvania u s a abstract anisotropy of magnetic susceptibility ams has been shown to be a good proxy for crystallographic preferred orientation texture however it is not clear in detail how different factors e g modal composition and preferred orientation define the total ams in a rock black shale samples from a drill core on the appalachian plateau in western pennsylvania were analyzed with respect to their microfabric as determined by texture and microstructure and ams low and high field ams which was measured at room temperature reveals that the ams of the samples is dominated by the para and diamagnetic phases synchrotron diffraction was applied to determine the texture of all relevant mineral phases incorporated in the samples muscovite and chlorite pole figures show single maxima perpendicular to the foliation reflecting dominant flattening strain from these textures and the modal compositions ams models were calculated using the intrinsic magnetic anisotropy of the single crystals the modeled and measured ams are comparable in terms of their principal directions and shapes of the anisotropy ellipsoid for the dominantly paramagnetic samples with the maximum susceptibility axes oriented subhorizontal to the ne sw both textures and ams indicate that the samples have undergone largely bedding compaction with a weak tectonic overprint linked to the alleghenian orogeny our results suggest that the microfabric and the magnetic fabrics on the plateau 200 km away from the allegheny front detect the tectonic shortening the ams modeling demonstrates how the contribution of the intrinsic ams of each mineral controls the ams of the whole rock

drilling induced core fractures and in situ stress the relationship between the shapes of drilling induced core fractures and the in situ state of stress is developed the stress concentrations at the well bore bottom are first determined using a complete three dimensional finite element analysis existing in situ compressional stresses generate large tensions in the immediate vicinity of the bottom hole which are sufficient to rupture the rock tensile fracture trajectories within these concentrated stress fields are predicted using a simple model of fracture propagation these modeled fracture trajectories resemble well the observed shapes of drilling induced core disking petal and petal centerline fractures further this agreement suggests that both the shape of the drilling induced fracture and the location at which it initiates depends on the in situ stress state existing in the rock mass prior to drilling the core fractures contain substantial information on in situ stress conditions in all faulting regimes the coring induced fractures initiate near the bit cut except for most cases under thrust faulting regime where the fracture initiates on the well bore axis further under thrust faulting conditions only disk fractures appear possible both petal and disking fractures can be produced in strike slip and normal faulting regimes depending upon the relative magnitudes between the least compressive horizontal principal stress and the vertical overburden stress the predicted fracture shapes are in good qualitative agreement with observations of drilling induced fractures described in the literature from laboratory experiments and field programs in which in situ stresses are measured by other means the relationship of the morphology of coring induced fractures and in situ stresses suggests that the fractures can be used as independent complementary indicators in identifying stress regimes

complex rotation of maximum horizontal stress in the wufeng longmaxi shale on the eastern margin of the sichuan basin china implications for predicting natural fractures abstract the in situ stress state of the eastern margin of the sichuan basin where there is a complicated fault zone is poorly understood this study offers vital insights into the in situ stress state based on information from 17 wellbores in the wufeng longmaxi shale near the eastern boundary fault i e the qiyueshan fault of the sichuan basin china drilling induced tensile fractures and borehole breakouts revealed a decrease in the angle between the shmax maximum horizontal stress orientation and fault strike when the wellbores are close to the damage zone of the qiyueshan fault this stress rotation may be caused by the lateral contrast in the poisson s ratio between the country rock and damage zone of the qiyueshan fault a high angle between the shmax orientation and fault strike also appeared in the individual fault blocks of the qiyueshan fault as a result of local stress concentration the higher magnitudes of horizontal stresses indicate stronger lateral compression in the qiyueshan fault zone owing to the lateral compression of a sandwich consisting of a high e young s modulus roof and low e shale intervals natural fractures mainly appeared in the low e shale intervals when the wellbore was close to the fault damage zone the low fracture intensity in the low e shale intervals of the wellbores located at the boundaries of the stress concentration blocks may be associated with an extensional stepover process

an overview of rock stress measurement methods this paper presents an overview of methods that have been used to estimate the state of stress in rock masses with the emphasis on methods applicable to hard rocks and scandinavia rock stress is a difficult quantity to estimate because the rock stress measuring techniques consist of perturbing the rock measuring displacements or hydraulic parameters and converting the measured quantities into rock stresses there are two main types of method those that disturb the in situ rock conditions i e by inducing strains deformations or crack opening pressures and those that are based on observation of rock behaviour without any major influence from the measuring method the most common methods are briefly described including their application areas and limiting factors

adverse effects of pore pressure drainage on stress measurements performed in deep tunnels an example from the lower kihansi hydroelectric power project tanzania abstract we describe a campaign to measure stress within a gneissic ridge near the edge of the tanzanian craton the measurements were conducted to determine whether the level of minimum principal stress along the trajectory of a hydropower pressure tunnel attained the level of 10 mpa as required to leave the tunnel unlined extrapolation of early hydrofracture and overcoring measurements conducted mostly in boreholes drilled behind face suggested that the requisite level was not quite met however the minimum principal stress estimates from the instantaneous shut in pressures isips of the hydrofracture tests generally showed large scatter and were sometimes less than the estimated pre disturbance pore pressure it was suspected that the stress tests were affected by stress alteration around the tunnel due to pore pressure drainage to test this hydrofracture and hydro jacking tests were conducted in two long horizontal holes drilled ahead of the excavation face into relatively undrained rock and a third vertical hole drilled behind the face all were near the critical location where the pressure tunnel enters the powerhouse the minimum principal stress estimates in the ahead of face holes were found to be much higher than those from the behind face hole consistent with the hypothesis estimates of the drawdown in pore pressure about the tunnel and test holes suggested a coupling factor between drawdown and minimum principal stress magnitude of approximately unity this is slightly higher than predicted from an hydromechanical model that explains the coupling between the pore pressure and stress fields in terms of fracture compliance and poro elasticity of intact blocks nonetheless both results demonstrate the importance of accounting for drawdown when conducting stress measurements in boreholes drilled from tunnels we also note some instances where hydrofracture and hydro jacking tests yielded different estimates for the minimum principal stress magnitude

geohazards of tunnel excavation in interbedded layers under high in situ stress abstract in situ stress is one of the most important parameters that affect rock mass stability during tunnel excavation high in situ stress may induce a series of geohazards such as rockburst in strong rock and squeezing in weak rock this paper presents a case study of a tunnel excavated using a tunnel boring machine across interbedded strong and weak rock layers under high in situ stress in pakistan to estimate the induced geohazards during tunnel excavation in situ stress was measured and the stability of the tunnel was analyzed using the finite element method in situ stress measured in the strong rock layers shows that the maximum compressive stress is very high and its direction is almost perpendicular to the tunnel axis using a simple analytical method stress distribution in weak layers was determined and the results show that the magnitude of the maximum compressive stress in the weak layers is much less than that in strong layers and the direction is almost parallel with the tunnel axis the fem simulation reveals that the redistributed stress due to the tunnel excavation is more concentrated in strong layers and less concentrated in weak layers both the initial in situ stress distribution and the stress redistribution after tunnel excavation predict geohazard in strong layers rather than in weak layers this point is consistent with the field records which indicated that rockbursts were frequent in the strong rock layers and no squeezing occurred in the weak rock layers

regional fractures i a mechanism for the formation of regional fractures at depth in flat lying reservoirs 1 regional fractures may form slowly at depth in extension and parallel to regional horizontal tectonic compression such fracture sets can be common in reservoirs despite the absence of flexure of the strata and are commonly important contributors to reservoir permeability such fractures make good targets for deviated wells high formation pore pressure is crucial to creating low effective confining stresses and brittle rock properties in the reservoirs conditions conducive to fracturing despite deep burial but pore pressure does not exceed the least principal stress and does not cause tensile fracturing the mechanism of natural hydraulic fracturing is incompatible with the characteristics of most regional fracture sets rather many regional fractures form duri g far field compression initiate at locally induced tensile stresses caused by flaws in the rock and propagate in the plane of the maximum and intermediate compressive stresses the differential stress required for initiation and propagation of regional fractures is well below that necessary for shear failure in the presence of high pore pressure fractures can be held open at depth by a tectonic basinwide dilatancy of the strata and the open void space commonly becomes mineralized

a comparative simulation study of coupled thm processes and their effect on fractured rock permeability around nuclear waste repositories this paper presents an international multiple code simulation study of coupled thermal hydrological and mechanical thm processes and their effect on permeability and fluid flow in fractured rock around heated underground nuclear waste emplacement drifts simulations were conducted considering two types of repository settings 1 open emplacement drifts in relatively shallow unsaturated volcanic rock and 2 backfilled emplacement drifts in deeper saturated crystalline rock the results showed that for the two assumed repository settings the dominant mechanism of changes in rock permeability was thermal mechanically induced closure reduced aperture of vertical fractures caused by thermal stress resulting from repository wide heating of the rock mass the magnitude of thermal mechanically induced changes in permeability was more substantial in the case of an emplacement drift located in a relatively shallow low stress environment where the rock is more compliant allowing more substantial fracture closure during thermal stressing however in both of the assumed repository settings in this study the thermal mechanically induced changes in permeability caused relatively small changes in the flow field with most changes occurring in the vicinity of the emplacement drifts

seismic fracture anisotropy in the earth s crust an overview fluid filled fractures and microcracks are the most stress sensitive component of crustal rock and often determine the pathways and volume of crustal fluid movement if we are to comprehend the role of fractures and fluids in tectonic processes to formulate an accurate hypothesis for phenomena precursory to catastrophic seismic failure or to monitor hydrocarbon and geothermal reservoirs for the presence or absence of major fluid pathways we must understand how seismic waves interact with the fracture crack and microcrack structures within the rock mass a salient feature of fracture related rock structure is the effective seismic anisotropy of aligned fractures and microcracks with the advent of large volume digital seismic data sets it is feasible to use polarized shear waves to explore the anisotropic fracture properties of the crust crustal fracture structures which may be monitored by polarized shear waves range in dimensions from the 10 100 km scale of crustal stress orientation and preseismic stress buildup through the 100 1000 m scale of fluid reservoir structure and characterization and the 1 10 scale of mining and geoengineering rock strain monitoring to millimeter and micrometer or possibly smaller fluid filled inclusions determining the elastic response of the intact rock mass to focus developments in seismic anisotropic studies of crustal fracture phenomena the american geophysical union and the society of exploration geophysicists sponsored a chapman conference on seismic anisotropy in the earth s crust held may 31 to june 4 1988 at the lawrence berkeley laboratory of the university of california this was the third international workshop on seismic anisotropy 3iwsa nineteen conference papers are presented in this special section of the journal of geophysical research

observations of the electric field fine structure associated with the westward traveling surge and large scale auroral spirals the characteristics of the fine scale electric field associated with the westward traveling surge and large scale auroral spirals and surges are investigated using high resolution electric field magnetic field particle and uv imager observations from four eveningside auroral oval crossings by the freja satellite three of the crossings were associated with signatures of auroral substorms and one crossing went directly through the head of a surge close in time and space to substorm onset three passes were adjacent to auroral spiral formations one poleward of and one equatorward of such forms and one through the multiple arc region near the front of an extended region of auroral activity the ambient electric field was found to intensify in the direction toward the spiral head or the center of the auroral activity region over a region comparable to the size of the visible auroral forms these results confirm previous findings that the spiral or surge head is associated with negative space charge and an intense upward field aligned current the fourth pass directly through the surge head reveals a very complicated structure of the surge region narrowly structured intense up to 700 mv m and mostly converging electric fields associated with intense electron precipitation of both high and medium energies and balanced field aligned currents up to 30 a m2 are seen near the edge of the surge head and adjacent to auroral structures in the wake these narrow regions are embedded within more extended regions of intense high energy electron precipitation but very weak electric fields and field aligned currents according to some existing models of the surge a pronounced westward electric field component and a southward polarisation electric field is expected within the entire high conductivity region but evidence in support of this was not found in the data rather these suggest that a significant part of the upward surge current is closed by distributed downward field aligned currents from the near surroundings the freja electric field is typically seen to intensify at the edges of or in between bright auroral structures and to decrease within the arcs similar to what is observed in the ionosphere the surge electric field is however much more intense than previously observed or anticipated at these altitudes with characteristics rather similar to those observed in the auroral acceleration region since the particle data indicate that most of the acceleration takes place above freja altitudes it seems as if freja traversed the lower part of the auroral acceleration region associated with the surge

correlation between magnetic anisotropy and fabric for devonian shales on the appalachian plateau the magnetic anisotropy of devonian black shale samples was measured from two cores drilled in the appalachian plateau the mineralogy of the shales is predominantly clay with small quantities of quartz and minor amounts of opaques and chlorite magnetite is the predominant ferromagnetic mineral present in the samples the magnetic fabric was measured at both room temperature and liquid nitrogen temperature and is dominated by a well defined bedding vertical compaction and a lesser defined magnetic lineation measurements of the anisotropy of magnetic susceptibility ams at liquid nitrogen temperature which enhances the paramagnetic contribution in the rock showed a strong increase in both the bulk susceptibility and susceptibility differences this increase suggests that the ams is controlled by the paramagnetic minerals particularly the clays and chlorite strain was measured from the orientation of basal planes of the chlorite crystals by texture goniometry good correlations have been found 1 between the orientation of the magnetic lineation and the long axes of the chlorite crystals and 2 between the degree of magnetic foliation and the amount of vertical compaction the magnetic lineation also agrees well with the direction of seismic anisotropy over the plateau the anisotropy of the anhysteretic remanence which expresses the anisotropy due to the ferromagnetic component in the rocks shows a weaker correlation with the amount of vertical compaction a weak magnetic lineation suggests that the magnetite grains were aligned during a deformation phase which post dates the main alleghanian orogeny the magnetic anisotropy of the devonian shales mirrors the compaction and tectonic fabric on the appalachian plateau

nightside ionospheric electrodynamics associated with substorms pfisr and themis asi observations 1 observations from the newly available poker flat incoherent scatter radar have been used to study the ionospheric electrodynamics during the substorm expansion phase substorm events selected have been divided into three categories based on onset location relative to the radar and repeatable features have been identified west of onset westward flows increased in response to the passage of the westward traveling surge wts poleward of them the enhanced flows occurred within the sub auroral polarization streams region of downward region 2 currents which extend equatorward from near the equatorward boundary of electron precipitation strong e region ionization increase associated with the wts occurred poleward of the enhanced westward flows suggesting that poleward pedersen currents fed by the downward region 2 field aligned current fac further equatorward contribute at least partially to closure of the upward fac of the wts an enhanced eastward flow channel developed east of onset e region ionization decreased poleward of this flow peak implying formation of downward fac and further increased equatorward of the flow peak suggesting strengthening of preexisting region 2 upward fac these observations indicate that the downward fac formed at onset at least partially closes via the upward region 2 currents the enhanced eastward flows decreased after passage of the auroral bulge and the reduced e region ionization increased again when onset occurred within the radar fov the thin breakup arc of onset occurred at the center of the harang reversal while if the optical onset was poleward of the radar fov the harang reversal was observed after the onset a 2 d picture of the evolution of ionospheric electrodynamics was generated by synthesizing observations from the three categories which provides a schematic description of the relationship between region 2 and substorm expansion electrodynamics

fringe cracks key structures for the interpretation of the progressive alleghanian deformation of the appalachian plateau vertical joints in devonian clastic sedimentary rocks of the finger lakes area of new york state are ornamented with arrays of fringe cracks that reveal the complex deformational history of the appalachian plateau detachment sheet during the alleghanian orogeny three types of fringe cracks were mapped gradual twist hackles abrupt twist hackles and kinks gradual twist hackles are curviplanar en echelon fringe cracks that propagate with an overall vertical direction within the bed hosting the parent crack and are found in all clastic lithologies of the detachment sheet abrupt twist hackles propagate as planar features in thick shale beds above or below the siltstone beds hosting parent joints kinks propagate horizontally as planar surfaces from the tips of parent joints in siltstone beds the breakdown of the parent joint into either gradual or abrupt twist hackles depends on the orientation and magnitude of the remote stress field internal fluid pressure and the elastic properties of the bed the twist angle of gradual twist hackles is larger in coarser clastic beds indicating that stress and internal pressure are more important parameters than elastic properties in controlling breakdown assuming that the vertical stress axis s v equals 78 mpa at 3 km burial depth the difference in twist angle between sandstone and shale beds is used to estimate the maximum horizontal stress difference in the shale beds as s h s h 2 5 mpa when s h s h 12 mpa in sandstone beds the twist angle of the fringe cracks and the abutting relationships of parent joints give an indication of the overall change in stress field orientation within the detachment sheet during alleghanian tectonics these parent joints indicate a regional clockwise stress rotation of alleghanian age concordant with the twist angle of fringe cracks throughout the western part of the study area a counterclockwise twist angle in the eastern portion indicates a local stress attributed to drag where no salt was available to detach the eastern edge of the plateau sheet the clockwise change in stress orientation is consistent with the rotation in stress orientation found in the anthracite belt of the pennsylvania valley and ridge but is opposite to the sense of rotation in the southwestern portion of the detachment sheet western pennsylvania and west virginia the two regional rotation domains are separated by the juniata culmination

appalachian stress study 3 regional scale stress variations and their relation to structure and contemporary tectonics using commercial hydrofracture data to supplement published stress data we map regional variations in least horizontal stress magnitude sh and orientation within the paleozoic sediments of the appalachian plateau systematic trends revealed by the data are examined for correlations with structure the study area includes a prominent salt detachment and tectonic history the dominant stress component present at all depths including basement throughout the region is ene oriented compression and is most likely of tectonic origin no deviations from this rule are found that are statistically significant near surface 600 m stresses are consistently greater than the overburden with strike slip regime conditions prevailing below however where the salina salt beds are present thrust regime conditions extend deeper reaching the salt in most places where data are available limited to where the salt is shallower than 1 5 km at the salt a discontinuity in the trend of sh with depth marks the abrupt transition to the underlying strike slip regime characterized by least stress ratios i e rh sh sv in the range 0 5 0 7 and extending to basement the comparatively high stress found above the salt may reflect a difference between the strain histories of the overlying and underlying sections facilitated by mechanical decoupling by the salt and perhaps includes a component of stress remnant from nw oriented alleghanian shortening of the upper section the lower subsalt stress regime extends beyond the eastern salt margin and displays remarkable uniformity over distances of hundreds of kilometers this is particularly evident across ohio where an rh value of 0 6 0 05 appears valid down to basement north of ohio values increase markedly whereas to the south of the salt pinch out in west virginia and eastern kentucky values as low as 0 35 are found in devonian shales at 1 1 5 km depth although values deeper in the section may be higher the northward increasing trend can be reasonably well modeled as the effect of persisting lithospheric flexure due to the pleistocene ice sheet however flexure was much greater in the past and as devonian shales in south ohio west virginia are currently only marginally stable a modest increase over the present flexure would be expected to cause shear failure this suggests that the very low stress in the shales of south ohio west virginia is of different origin and more recent than the pleistocene

mesoscale ionospheric electrodynamics observed with the miracle network 1 analysis of a pseudobreakup spiral a short lived auroral spiral is observed on february 3 1999 around 2204 ut using the multi instrument array for ionosphere magnetosphere coupling studies miracle network of ground based instruments in fennoscandia which consists of magnetometers coherent scatter radars and all sky cameras four minutes later after this spiral has subsided the auroral bulge of a fully developed substorm that has set on earlier over russia intrudes into the miracle field of view from the east hence the auroral spiral precedes the substorm spatially i e it is located westward of the substorm bulge rather than temporarily since this spiral is associated with a localized and short lived substorm type auroral development we interpret it as a pseudobreakup spiral which is subsequently covered by the intruding auroral bulge during the decaying phase of the pseudobreakup activity in contrast to growth phase pseudobreakups that occur temporally before a full breakup this pseudobreakup is observed spatially ahead westward of an already evolving substorm bulge our instantaneous spatial analysis of the distributions of ionospheric electrodynamic parameters associated with this spiral shows that all key features are similar to the ionospheric electrodynamics of the westward traveling surge sharp decrease of conductance toward the west from hall conductance values up to 30 s in the spiral to less than 2 s west of it a cluster of upward field aligned currents facs in the spiral area with magnitudes up to 5 a km 2 and anticlockwise spiraling horizontal currents around it the upward facs diverging at least 100 ka of current in total are fed to a large extent by a pseudosubstorm electrojet carried by westward flowing hall currents no signs of local current closure by downward facs from the immediate vicinity of the spiral are found our results are inconsistent with the earlier conjecture that insufficient ionospheric conductance would prevent the pseudobreakup to evolve to a full breakup since the solar wind data do not provide clear evidence for being a driver of the pseudobreakup breakup sequence the driver has to be an internal magnetospheric one