anisotropy and transverse isotropy a number of authors in the exploration literature have written about anisotropy but have restricted their discussions to wave propagation through rock having transverse isotropy with a vertical symmetry axis this note shows that there are fundamental differences between transverse isotropy when the symmetry axis is vertical normal to the free surface and more general anisotropy with an azimuthal variation of properties these differences are important now that effective azimuthal shear wave anisotropy resulting from aligned cracks and pores is becoming recognized as a significant property of crustal rocks

s local wave seismic anisotropy in the forearc above the subducted nazca plate between 33 s and 34 5 s s wave splitting from local earthquakes within the nazca plate that are deeper than the interplate seismogenic zone enabled the determination of the fast velocity direction and the lag time t in the forearc of the overriding plate data were collected from 20 seismic stations most of which were temporary deployed between 33 5 s and 34 5 s and included part of the normal subduction section to the south and part of the transitional section to flat subduction to the north the fast velocity direction has a complex pattern with three predominant directions northwest southeast north south and northeast southwest and relatively high t a quality evaluation of the highest measurements enabled us to identify possible cycle skipping in some of the measurements which could be responsible for the large observed lag time we consider that most of the anisotropy that was observed in the forearc is probably located in the mantle wedge and a minor part is located in the crust the complex pattern of splitting parameters when the anisotropy is associated at the mantle wedge could be the result of three dimensional variations in the subducting nazca plate at these latitudes also similarities between the splitting parameters and the principal compressional stress direction from pliocene and quaternary rocks suggest that the anisotropy in the crust could originate by tectonic local stress

fluid inclusion studies of the abawso gold prospect near the ashanti belt ghana the major ghanaian lode gold deposits are preferentially aligned along the western and eastern contacts of the kumasi basin with the ashanti and sefwi belts respectively the investigated area of the abawso small scale concession covering the workings of the old ettadom mine is situated 3 km west of the lithological contact of the birimian metavolcanic rocks of the akropong belt in the east with the birimian metasedimentary rocks of the kumasi basin in the west the rocks of the abawso concession represent a steeply nw dipping limb of a se verging anticline with an axis plunging to the sw quartz veining occurs predominantly in the form of en echelon dilatational veins along nne ssw striking shear zones of a few metres width and shows evidence of brittle and ductile deformation also stockwork style quartz veining occurs in the vicinity of the main shaft of the old ettadom mine hydrothermal alteration includes sericitisation sulphidation and locally carbonatisation the auriferous quartz veins mainly follow the trend of brittle to ductile deformed quartz veins however some occur in stockwork fluid inclusion studies reveal a large number of h2o inclusions along intragranular trails in auriferous quartz vein samples as well as an overall dominance of h2o and h2o co2 inclusions over co2 inclusions textural observations and physico chemical fluid inclusion properties indicate post entrapment modifications for all quartz vein samples due to grain boundary migration recrystallisation this process is interpreted to be responsible for the generation of the co2 inclusions from a h2o co2 parent fluid in comparison with mineralisation at the ashanti and prestea deposits which are characterised by co2 n2 inclusions the observed inclusion assemblage may be due to a shallower crustal level of mineralisation or different degrees and styles of recrystallisation or a less pronounced development of laminated quartz veins due to comparably restricted pressure fluctuations furthermore the microthermometric observations allow the reconstruction of a possible retrograde p t path depicting near isothermal decompression in the p t range of the brittle ductile transition

anisotropy detected in shallow clays using shear wave splitting in a vsp survey1 shear waves can provide valuable information about seismic anisotropy on entering an anisotropic medium a shear wave generally splits shear wave splitting into a fast and a slow quasi shear wave with polarizations fixed by the elastic properties of the medium and direction of travel if the medium contains planar discontinuities with common normals the fast shear wave will be suitably propagated if its polarization lies in the plane of the discontinuities measuring this polarization using a vsp geometry with oriented three component geophones in the borehole offers the possibility of monitoring the orientation and density of the discontinuities as a function of depth such a shear wave vsp was carried out in an uncased 0 3 m diameter borehole drilled to a depth of 120 m in the north of the netherlands the upper 80 m of the sequence consisting of a glacial till and sands and clays of pleistocene age was studied the clays in this sequence have been subjected to glacial deformation and as a result are overconsolidated and locally fissured in our shallow vsp experiment shear wave splitting and therefore anisotropy was identified at various geophone depths for one source offset hodograms showed a consistent polarization of the fast shear wave component over a large depth interval under the assumption that the anisotropy was caused by planar discontinuities with common normals this polarization direction gives the strike of the fissures in this interval the polarization direction of the fast s wave did not correspond exactly with the strike which was obtained from geological information on the fissures the geological information was from undisturbed oriented 70 mm core samples taken at 3 m intervals in the borehole the discrepancy however could be explained in terms of dipping fissures and such a dip was confirmed by the geological and geotechnical information the orientation of fissures is an important factor in the directional deformation and strength characteristics of clays as far as geotechnical behaviour is concerned this study thus illustrates a practical application of shear wave splitting observed in shallow shear wave vsp for geology and geotechnical engineering

reflection seismology over azimuthally anisotropic media recent surveys have shown that azimuthal anisotropy due most plausibly to aligned fractures has an important effect on seismic shear waves previous work had discussed these effects on vsp data the same effects are seen in surface recording of reflections at small to moderate angles of incidence the anisotropic effects one different polarization components of vertically traveling shear waves permit the recognition and estimation of very small degrees of azimuthal anisotropy of order 2 1 percent as in an interferometer anisotropic effects on traveltime yield estimates of anisotropy which are averages over large depth intervals often raw field data must be corrected for these effects before the reflectors may be imaged two variations of a rotational algorithm to determine the principal time series are derived anisotropic effects on moveout lead to abnormal moveout unless the survey line is parallel to the fractures anisotropic effects on reflection amplitude permit the recognition and estimation of anisotropy hence fracture intensity differences at the reflecting horizon i e with high vertical resolution

core log integration studies in hole a of taiwan chelungpu fault drilling project summary taiwan chelungpu fault drilling project tcdp was initiated to understand the physical mechanisms involved in the large displacements of the 1999 taiwan chi chi earthquake continuous measurements of cores including laboratory work and a suite of geophysical downhole logs including p and s wave sonic velocity gamma ray electrical resistivity density temperature electrical borehole images and dipole shear sonic imager were acquired in hole a over the depth of 500 2003 m integrated studies of cores and logs facilitate qualitative and quantitative comparison of subsurface structures and physical properties of rocks a total of 10 subunits were divided on the basis of geophysical characteristics generally formation velocity and temperature increase with depth as a result of the overburden and thermal gradient respectively gamma ray resistivity formation density shear velocity anisotropy and density derived porosity are primarily dependent on the lithology zones with changes of percentage of shear wave anisotropy and the fast shear polarization azimuth deduced from dipole shear imager dsi are associated with the appearance of fractures steep bedding and shear zones the fast shear wave azimuth is in good agreement with overall dip of the bedding approximately 30 towards se and maximum horizontal compressional direction particularly in the kueichulin formation showing strong shear wave velocity anisotropy bedding parallel fractures are prevalent within cores whereas minor sets of high angle nnw sse trending with n and s dipping fractures are sporadically distributed the fault zone at depth 1111 m fza1111 is the chi chi earthquake slip zone and could be a fluid conduit after the earthquake the drastic change in fast shear wave polarization direction across the underlying non active sanyi thrust at depth 1710 m reflects changes in stratigraphy physical properties and structural geometry

interpretation of synthetic common depth point gathers for a single anisotropic layer analysis of synthetic traveltime gathers shows that anisotropy may have a large enough effect on p sh and sv propagation to alter significantly the interpretation of the subsurface below the anisotropic layers consequently if anisotropy exists below a seismic line it is important to estimate the anisotropic parameters correctly we discuss the effects of anisotropy on seismic waves and present a method for estimating the elastic constants of a transversely isotropic layer from p and sh arrival time gathers the technique may be extended to more general anisotropic symmetries by analyzing gathers from several azimuths to illustrate the possible effect of anisotropy on exploration surveys p sh and sv velocity variations are calculated for several types of anisotropic sedimentary fabrics alignments due to bedding shale lithology and dry parallel cracks may have similar velocity variations fabrics with other configurations of cracks may still possess overall transversely isotropic symmetry but t

high co2 content of fluid inclusions in gold mineralisations in the ashanti belt ghana a new category of ore forming fluids a reply fluid inclusions were studied in samples from the ashanti konongo southern cross prestea abosso damang and ayanfuri gold deposits in the ashanti belt ghana primary fluid inclusions in quartz from mineralised veins of the ashanti prestea konongo southern cross and abosso damang deposits contain almost exclusively volatile species the primary setting of the gaseous i e the fluid components co2 ch4 and n2 fluid inclusions in clusters and intragranular trails suggests that they represent the mineralising fluids microthermometric and raman spectroscopic analyses of the inclusions revealed a co2 dominated fluid with variable contents of n2 and traces of ch4 water content of most inclusions is below the detection limits of the respective methods used aqueous inclusions are rare in all samples with the exception of those from the granite hosted ayanfuri mineralisation here inclusions associated with the gold mineralisation contain a low salinity co2 and low salinity 6 eq wt nacl however fluid inclusions in quartz from the gold mineralisations in the ashanti belt point to distinctly different fluid compositions specifically the predominance of co2 and co2 h2o have to be emphasized fluid systems with this unique bulk composition were apparently active over more than 200 km along strike of the ashanti belt fluids rich in co2 may present a hitherto unrecognised new category of ore forming fluids

evaluation of anisotropy by shear wave splitting the polarizations of three component shear wavetrains carry unique information about the internal structure of the rock through which they pass specifically commonly observed shear wave splitting may contain information about the orientation of crack distributions this information cannot usually be recovered from shear waves recorded at the free surface because of interference with the interaction of the shear wave with the surface even for nearly vertical incidence however shear wave splitting in synthetic three component vertical seismic profiles in some cases may be interpreted directly in terms of the direction of strike of vertical cracks because the human eye is not skilled at identifying the phase relationships between three component seismograms played out conventionally as parallel time series the polarizations are displayed in orthogonal sections of the particle displacements to facilitate recognition and evaluation of the shear wave splitting estimating the orientations of cracks an

measurements of seismic anisotropy and fracture compliances in synthetic fractured media ultrasonic velocities were measured on a stack of synthetic material plexiglass at low 90 120 khz long wavelength range and high 480 khz short wavelength range frequencies plates were pressed together with uniaxial normal stress these measurements were repeated under different uniaxial pressures for two cases 1without circular rubber inclusions between synthetic fractures and 2with circular rubber inclusions between fractures calculation of anisotropy parameters and also normal and shear interface fracture compliances using linear calculus show reasonable variations anisotropy and compliances decreases as overburden pressure increases suggesting that anisotropy is caused by compliances at higher frequencies synthetic fractures seem to be stiffer than at lower frequencies for the long and short wavelength ranges similar variations were observed for anisotropy parameters and fracture compliances vs measurements in different directions show that for the case without rubber inclusions there is a good approximation of vti medium whereas for the case with inclusions there is a significant difference between vs travelling in directions parallel and perpendicular to the plates

sulfidation of organic matter associated with gold mineralization pueblo viejo dominican republic the pueblo viejo district is one of the largest producers of precious metals in the world yielding more than 11 000 kg of au annually gold mineralization at pueblo viejo is hosted in spilite and coarse elastic and finely laminated fine grained carbonaceous sedimentary rocks of the lower cretaceous los ranchos formation mineralization was accompanied by sulfidation as evidenced by i the occurrence of siderite distal to mineralization and pyrite proximal to mineralization 2 increased s fe ratios associated with au mineralization 3 the occurrence of native s in and adjacent to mineralization and 4 the presence of sulfidized organic matter organo s compounds in mineralized rocks organic matter in the carbonaceous sedimentary rocks comprises vitrinite and pyrobitumen rock eval pyrolysis data indicate that this organic matter is overmature hi 55 the organic matter is s rich up to 14 wt s in rocks where all fe has been sulfidized to pyrite x ray absorption near edge structure xanes analysis indicates that the c bonded s is thiophenic early disseminated au mineralization is postulated to have occurred in response to sulfidation of ferrous fe to form pyrite the consequent decline in the activity of h2s in the mineralizing fluid would have destabilized au bisulfide complexes and caused deposition of gold the restriction of s rich organic matter to rocks in which all fe occurs as pyrite indicates that sulfidation of organic matter postdates sulfidation of ferrous fe and therefore deposition of much of the au

origin and evolution of the um egat and dungash orogenic gold deposits egyptian eastern desert evidence from fluid inclusions in quartz shear zone related mesothermal gold deposits of um egat and dungash in the egyptian eastern desert are hosted by greenschist facies metavolcanic and or metasedimentary rocks of pan african age both deposits comprise boudinaged quartz veins that show evidence of incipient recrystallization and are similar in alteration style structural control and mineralogy the ore mineralogy includes pyrite arsenopyrite pyrrhotite chalcopyrite galena gold occurs both in the veins usually included in arsenopyrite pyrite or pyrrhotite next to fragments of altered country rocks or disseminated in the alteration haloes arsenic in arsenopyrite and aliv in chlorite geothermometers indicate that wall rock alteration and ore mineral precipitation occurred at temperatures between 400 and 250 c fluid inclusions in vein quartz occur in clusters or along trails three types of fluid inclusions were identified based on petrography and laser micro raman spectroscopy 1 two phase carbonic inclusions with co2 ch4 n2 h2o 2 two phase aqueous inclusions and 3 three phase aqueous carbonic inclusions with ch4 final melting of ice tmf for most inclusions occurs at temperatures between 4 and 0 c indicating a low salinity 300 c but cluster into two distinct groups for each type of inclusion inclusions from the same trail or cluster are commonly characterized by different degrees of fill or different th values field petrographic and microthermometric data suggest that low salinity aqueous carbonic fluids interacted with graphite bearing metasedimentary rocks to form ch4 at t 400 c and p 3 kbars these reduced fluids leached gold as they circulated through the metavolcanic rocks carrying it in the form of bisulfide complexes interaction of these aqueous carbonic fluids with the country rocks caused hydrothermal alteration and precipitated gold bearing sulfides in the alteration zones a drop of pressure during the migration of these fluids to shallower depths along the shear zones led to phase separation at t 300 c and p 2 3 kbars quartz crystallizing over a range of lower temperatures and pressures trapped carbonic and aqueous fluids as separate inclusions in clusters along pseudosecondary and secondary trails postdepositional deformation caused decrepitation of some inclusions and the stretching and leakage of others increasing th to 250 c deformation also remobilized the gold depositing it as globules of higher fineness in secondary sites

flash vaporization during earthquakes evidenced by gold deposits fluids flowing through cavities in earth s crust can deposit gold thermo mechanical modelling of a fluid filled cavity that expands suddenly during an earthquake shows that the drop in pressure would cause the fluid to vaporize and deposit the gold almost instantaneously

seismic velocity anisotropy and fluid pressure in the barbados accretionary wedge from an offset vertical seismic profile with seabed sources 1 the state of compaction and fluid pressure in the barbados accretionary wedge near its toe at ocean drilling program site 949 were investigated by modeling travel times of seismic waves from ocean bottom shots to a borehole geophone array the model constrained by a three dimensional seismic survey and well logs shows 1 a velocity gradient of about 1 1 25 s 1 in the uppermost 180 230 m of the wedge 2 a zone of variable but no net change in velocity between 230 and 350 m depth 3 a low velocity zone 40 50 m thick just above the decollement at 391 m and 4 a displacement of the low velocity zone by thrust faults pore fluid pressure sections derived from p wave velocity show that the upper half of the wedge is normally pressured while the lower half is overpressured the 160 m thick underconsolidated basal zone shows anisotropy which increases downward the lowest 40 50 m has velocity varying 1 azimuthally 3 being fastest in the direction of plate convergence and 2 in the vertical plane 2 5 horizontal faster than vertical after correction for the effect of anisotropy in the derivation of effective stress from seismic velocity the calculated pore fluid pressure ratio does not exceed 0 9 and in the lowest 40 50 m of the basal zone is between 0 71 and 0 82 with fluid pressure hydrostatic lithostatic pressure hydrostatic between 0 5 and 0 65 in accordance with in situ measurements of fluid pressure in the decollement zone beneath these indicate that the accretionary wedge is stronger and less overpressured than was previously supposed

orogenic gold formation in an evolving decompressing hydrothermal system genesis of the samut gold deposit eastern desert egypt abstract samut is a vein type gold mineralization in the eastern desert of egypt which shares many geologic features with other orogenic deposits in this region the deposit has a long history of mining activity dating back to ancient times but so far has received little attention from the standpoint of geologic study here we present new field petrographic microthermometric and microanalytical data from the samut gold deposit to evaluate its genesis and fluid evolution vein quartz at samut shows abundant evidence of precipitation coinciding with early phases of ductile deformation that show transitions to increasing brittle fracturing and fragmentation through time in some samples brecciation of the vein quartz is intense and shows evidence of repeated episodes of fragmentation this brittle style of deformation culminates with latest stage brittle opening and loss of confining pressure resulting in open space filling textures backscattered electron images and electron microprobe analysis reveal three varieties of pyrite in the mineralized quartz veins py i py ii and py iii laser ablation icpms analysis of pyrites from mineralized quartz veins reveals that the earlier py i is low in arsenic and gold whereas py ii and py iii are enriched in both elements anhedral py ii is particularly gold rich and gives way to blocky euhedral py iii that is rich in arsenic but not as rich in gold compared to py ii fluid inclusions in the least brittle deformed vein quartz indicate that the veins formed at minimum temperatures of 300 c early in the hydrothermal system the calculated fluid pressures at homogenization of the aqueous carbonic inclusions furnish a wide range throughout the paragenesis perhaps on the order of 1 to 3 kbar the 34s values of pyrites imply sulfur isotopic composition of the mineralizing fluid on the order of 0 6 to 4 9 overlapping with the broad range of 34s values calculated for ore fluids in orogenic gold deposits worldwide together these results support a general model in which gold and sulfide deposition was coinciding with an ongoing syn deformational vein formation and tending towards the late stages of vein growth prior to loss of confinement free milling gold appears to be mostly related to the later stages of brittle fracturing and re sealing of the veins during repeated fluid release as the background fluid pressure evolved from earlier dominantly lithostatic to later fully hydrostatic conditions

fluid immiscibility in natural processes use and misuse of fluid inclusion data ii interpretation of fluid inclusion data in terms of immiscibility abstract phase equilibrium analysis of the relevant systems together with the application of the principles of chemical equilibrium put severe constraints on the interpretation of fluid inclusion data in terms of immiscibility in part i following from that point the major limits on the accuracy and even the validity of fluid inclusion quantitative data and their interpretation in terms of fluid composition and density are briefly discussed the practical implications of the general constraints temperature pressure topology of the fluid systems are envisaged emphasis is laid on some important consequences such as the use of isochore intercepts and the possible resulting interpretation of fluid mixing rather than unmixing the case of highly saline inclusions the identification and interpretation of heterogeneous trapping the composition and density constraints on coexisting fluids are presented and illustrated by natural examples taking into account all the measurable parameters in fluid inclusions volume temperature and nature of phase transitions more or less complete individual spectroscopic analyses all the available experimental data and all the theoretical constraints may be long and difficult however it is most generally very informative and productive although part of these data is often sufficient to deny unmixing nevertheless a final example on metamorphic fluids demonstrates how such an approach can prove and also characterize a fluid unmixing during a geologic process

minor and trace elements as indicators of solute provenance and flow routing in a subglacial hydrological system mitchell a c brown g h fuge r 2006 minor and trace elements as indicators of solute provenance and flow routing in a subglacial hydrological system hydrological processes 20 4 877 897 sponsorship university of wales aberystwyth studentship am university of wales aberystwyth research grants 56 95 and 36 96 ghb and rf and nerc grant no gr3 11216

stress induced upper crustal anisotropy in southern california 1 we use an automated method to analyze shear wave splitting from local earthquakes recorded by the southern california seismic network between 2000 and 2005 the observed fast directions of upper crustal anisotropy generally are consistent with the direction of maximum horizontal compression hmax suggesting that one major mechanism of anisotropy in the top 20 km of crust under southern california is regional stress however at other stations fast directions are aligned with the trends of regional faulting and local alignment of anisotropic bedrock splitting delay times range widely within 0 2 s these upper crustal anisotropy observations together with previous studies of sks shear wave splitting surface waves and receiver functions suggest different mechanisms of anisotropy at different depths under southern california anisotropy in the upper crust appears to be in response to the current horizontal maximum compression hmax which differs from the cause of anisotropy in the lower crust and mantle we also explore possible temporal variations in upper crustal anisotropy associated with preearthquake stress changes or stress changes excited by surface waves of great earthquakes but do not observe any clear temporal variations in fast directions or time delays

geochemical weathering at the bed of haut glacier d arolla switzerland a new model waters were sampled from 17 boreholes at haut glacier d arolla during the 1993 and 1994 ablation seasons three types of concentrated subglacial water were identified based on the relative proportions of ca 2c h co 3 and so4 2 to si type a waters are the most solute rich and have the lowest relative proportion of si they are believed to form in hydrologically inefficient areas of a distributed drainage system most solute is obtained from coupled sulphide oxidation and carbonate dissolution so cd it is possible that there is a subglacial source of o2 perhaps from gas bubbles released during regelation because the high so4 2 levels found up to 1200 eq l are greater than could be achieved if sulphides are oxidized by oxygen in saturated water at 0 c c 414 eq l a more likely alternative is that sulphide is oxidized by fe 3c in anoxic environments if this is the case exchange reactions involving fe iii and fe ii from silicates are possible these have the potential to generate relatively high concentrations of hco3 with respect to so4 2 formation of secondary weathering products such as clays may explain the low si concentrations of type a waters type b waters were the most frequently sampled subglacial water they are believed to be representative of waters flowing in more efficient parts of a distributed drainage system residence time and reaction kinetics help determine the solute composition of these waters the initial water rock reactions are carbonate and silicate hydrolysis and there is exchange of divalent cations from solution for monovalent cations held on surface exchange sites hydrolysis is followed by so cd the so4 2 concentrations usually are 414 eq l although some range up to 580 eq l which suggests that elements of the distributed drainage system may become anoxic type c waters were the most dilute yet they were very turbid their chemical composition is characterized by low so4 2 h co3 ratios and high ph type c waters were usually artefacts of the borehole chemical weathering environment true type c waters are believed to flow through sulphide poor basal debris particularly in the channel marginal zone the composition of bulk runoff was most similar to diluted type b waters at high discharge and was similar to a mixture of type b and c waters at lower discharge these observations suggest that some supraglacial meltwaters input to the bed are stored temporarily in the channel marginal zone during rising discharge and are released during declining flow little of the subglacial chemical weathering we infer is associated with the sequestration of atmospheric co2 t he progression of reactions is from carbonate and silicate hydrolysis through sulphide oxidation by first oxygen and then fe iii which drives further carbonate and silicate weathering a crude estimate of the ratio of carbonate to silicate weathering following hydrolysis is 4 1 we speculate that microbial oxidation of organic carbon also may occur both sulphide oxidation and microbial oxidation of organic carbon are likely to drive the bed towards suboxic conditions hence we believe that subglacial chemical weathering does not sequester significant quantities of atmospheric co2 and that one of the key controls on the rate and magnitude of solute acquisition is microbial activity which catalyses the reduction of fe iii and the oxidation of fes2 copyright 2002 john wiley sons ltd

radiation patterns of a shear wave vibrator in near surface shale two shallow surface to borehole field tests were conducted to investigate the radiation patterns of a shear wave vibrator in near surface shale in the in line direction for which baseplate motion is transverse to the source geophone raypath the vibrator produces a broadband high amplitude signal of the sh type and virtually no signal of the sv or p types the amplitude of the sh radiation is approximately constant as a function of polar angle but it falls off as the cosine of the azimuthal angle measured with respect to the in line direction in the broadside direction the vibrator produces a strong uncontaminated signal of the sv type the radiation pattern of the sv signature is lobate in the polar plane and peaks at an incident angle of 30 to 35 degrees in the horizontal plane the sv radiation varies as the sine of the azimuthal angle measured with respect to the in line direction the field tests were conducted on a permian shale in oklahoma and an eocene shale in east texas comparison between sh and sv wave surfaces indicates that both shales behave approximately like transversely isotropic solids with a vertical axis of symmetry theoretical wave surfaces are fit to the observed data by using elastic moduli determined from the field measurements and by making the assumption that the near surface shale at each field site is a single homogeneous inherently transversely isotropic material the analyses indicate that horizontal sh group velocity is about 30 percent greater than horizontal sv group velocity in the permian shale and about 25 percent greater in the eocene shale

modelling orthorhombic anisotropic effects for reservoir fracture characterization of a naturally fractured tight carbonate reservoir onshore texas usa in this study we present a step by step theoretical modelling approach using established seismic wave propagation theories in anisotropic media to generate unique anisotropic reflection patterns observed from three dimensional pure mode pressure 3d pp full azimuth and full offset seismic reflection data acquired over a naturally fractured tight carbonate field onshore texas usa our aim is to gain an insight into the internal structures of the carbonate reservoir responsible for the observed anisotropic reflection patterns from the generated model we were able to establish that the observed field seismic reflection patterns indicate azimuthal anisotropy in the form of crack induced shear wave splitting and variation in p wave velocity with offset and azimuth amplitude variation with azimuth avaz analysis also confirmed multi crack sets induced anisotropy which is characteristic of orthorhombic symmetry evident as multiple bright and dim amplitude azimuth directions as well as complete reversal of bright amplitude to dim amplitude azimuth direction as the angle of incidence increases from near 15 to mid 30 offsets finally we fitted the generated p wave velocity into an ellipse to determine the intensity and orientation n26e of the open crack set as well as the direction of the minimum in situ stress axis n116e within the reservoir the derived information served as an aid for the design of horizontal well paths that would intercept open fractures and ensure production optimization of the carbonate reservoir which was on production decline despite reservoir studies that indicate un depleted reserves

subglacial drilling at black rapids glacier alaska u s a drilling method and sample descriptions we employed a commercial wireline drill rig to investigate the subglacial conditions of black rapids glacier a well studied surge type glacier in the central alaska range the four main goals were to assess the capabilities of the commercial drilling industry for sampling subglacial material to investigate the basal morphology to determine the subglacial geology and to emplace borehole instruments the drilling was done in an area where seasonal and secular variations in speed are large and where seismic studies suggested the presence of a till layer four holes were drilled at three locations to a maximum depth of 620 m three holes yielded samples of basal ice and till although recovery of the latter was generally poor bedrock was sampled in one or possibly two of the holes in the area sampled the glacier is underlain by a till layer some 4 7 m thick confirming the seismic interpretation it consists ofa sandy matrix at least 20 30 of which comprises larger clasts limited samples ofthe matrix indicate that near the top ofthe till the porosity is 40 and that some of the pore water is frozen geologic studies suggest that the drilling area lies to the north of the denali fault a major tectonic boundary followed by the glacier and that most of the till is locally derived with transport distances of 2 km

shear waves in lithosphere studies on the territory of the u s s r abstract thousands of kilometers of dss profiles were compiled with three component stations in the u s s r the results showed that the usual shots in holes or water basins normally create not only p waves but shear waves as well the most favourable condition for their generation is the presence of a sharp seismic boundary near the source for example the basement surface the s wave field as a rule contains all the types of reflected and refracted waves that are found in the p field the difference is in lower frequencies they are lower by one third somewhat higher intensity and greater variability of amplitudes of shear waves when recording the s waves the major information is obtained from the velocity relation v p v s with depth and laterally it reveals that three major factors are affecting this relation the degree of rock fissuring the composition of rocks and the temperatures at depth reduction of fissuring with depth i e with greater distances from the source results in an overall drop of as the composition of the uppermost crust changes the urals increases in blocks composed of basic rocks this value shows little changes on the baltic and ukrainian shields on the siberian platform first increases with depth from 1 71 up to 1 76 probably due to the dilatancy effect and then it decreases to values less than 1 7 in the lower crust and upper mantle in western siberia grows with depth reaching 1 79 in the lower crust it is somewhat reduced in the mantle but still above 1 7 this can be ascribed apparently to a higher temperature regime of the lower crust in western siberia in many regions a splitting of the shear waves of different polarization is observed due to velocity anisotropy this was found for the first refracted waves in the urals and for reflected waves from the m boundary on the siberian platform in both cases anisotropy is associated with the crust on the ukrainian and baltic shields the difference in the velocities of sh and sv waves was not recorded

black box modeling of the subglacial water system measurements of water pressure beneath trapridge glacier yukon territory canada yield the following generalizations about subglacial conditions in the studied region 1 even over short distances the subglacial water system is highly heterogeneous 2 the subglacial water system consists of at least two distinct components which we refer to as the connected and unconnected water systems 3 regions of the glacier bed can switch back and forth from being part of the connected or part of the unconnected water system 4 large spatial pressure gradients can exist within the unconnected water system and between the connected and unconnected systems 5 rapid pressure variations can occur in the unconnected water system 6 pressure variations in the unconnected water system do not match those in the connected system and can in fact be strongly anticorrelated with pressure variations in the connected system if the water pressure variations in the connected system are viewed as a forcing and those in the unconnected system as a response to this forcing the input output relation between forcing and response can be efficiently represented as a low order nonlinear ordinary differential equation the response of the unconnected system to forcing from the connected system is governed by time constants having approximate magnitudes of 1 7 hours and 7 4 hours that we believe are associated with process rates for substrate compression and pore water diffusion respectively

fluid immiscibility in natural processes use and misuse of fluid inclusion data abstract many occurrences of fluid immiscibility in natural geologic systems have been reported recently most often from fluid inclusion studies however the interpretation of fluid inclusion data in terms of immiscibility sometimes suffers from ambiguity of the vocabulary insufficient knowledge of the immiscibility constraints and insufficient knowledge of the topology in the tpx space of natural fluid systems for such reasons some authors have been misled to erroneous interpretations the term chemical immiscibility is unambiguously redefined as a multiphase multicomponent equilibrium the consequences of this definition are directly derived from the phase rule and concern the possible equations that relate the various parameters temperature pressure volumes compositions to each other these equations already put constraints on the topology of the phase equilibria in fluid systems a particular expression of the phase rule is proposed which takes into account the multiphase constant bulk volume constant bulk composition constraints in fluid inclusions the consequences of such an expression are of major importance in fluid inclusion studies the phase relations of some simple systems that approximate quite efficiently the natural complex fluids are then detailed h2o nacl co2 ch4 h2o co2 h2o co2 nacl the effects of these topologies and of the supplementary constraints constant bulk composition and constant bulk volume assumed for fluid inclusions isopleth isochoric systems are discussed

three term taylor series for t2 x2 curves of p and s waves over layered transversely isotropic ground the arrival time curve of a reflection from a horizontal interface beneath a homogeneous isotropic layer is a hyperbola in the x t domain if the subsurface is one dimensionally inhomogeneous horizontally layered or if some or all of the layers are transversely isotropic with vertical axis of symmetry the statement is no longer strictly true though the arrival time curves are still hyperbola like in the case of transverse isotropy however classical interpretation of these curves fails interval velocities calculated from t 2 x 2 curves do not always approximate vertical velocities and therefore cannot be used to calculate depths of reflectors to study the relationship between velocities calculated from t 2 x 2 curves and the true velocities of a transversely isotropic layer we approximate t 2 x 2 curves over a vertically inhomogeneous transversely isotropic medium by a three term taylor series and calculate expressions for these terms as a function of the elastic parameters it is shown that both inhomogeneity and transverse isotropy affect slope and curvature of t 2 x 2 curves for p waves the effect of transverse isotropy is that the t 2 x 2 curves are convex upwards for sv waves the curves are convex downwards for sh waves transverse isotropy has no effect on curvature

archean lode gold deposits fluid flow and chemical evolution in vertically extensive hydrothermal systems abstract the range of conditions of formation of lode gold deposits from the sub greenschist to the lower granulite facies in archean greenstone belts and the generally steeply plunging vertically continuous pipe like or tabular geometries of individual deposits indicate long distance hydrothermal fluid advection along well defined channelways in the upper and middle crust from presently available gold solubility data destabilisation of gold bisulphide complexes through h 2 s loss from the fluid to the wallrock was the dominant gold precipitation mechanism within these hydrothermal systems as a whole this inference is supported by the s au ratios of ores sulphur and au precipitation in the hydrothermal system is estimated to be relatively inefficient with only 10 50 of s or au contained in the fluid precipitated over any kilometre length of fluid channelway the relative inefficiency of gold precipitation allowed mineralisation over a significant depth range in a crustal profile

orogenic gold common or evolving fluid and metal sources through time abstract orogenic gold deposits of all ages from paleoarchean to tertiary show consistency in chemical composition they are the products of aqueous carbonic fluids with typically 5 20 mol co 2 although unmixing during extreme pressure fluctuation can lead to entrapment of much more co 2 rich fluid inclusions in some cases ore fluids are typically characterized by significant concentrations of ch 4 and or n 2 common estimates of 0 01 0 36 mol h 2 s a near neutral ph of 5 5 and salinities of 3 7 wt nacl equiv with na k ca mg this fluid composition consistency favors an ore fluid produced from a single source area and rules out mixing of fluids from multiple sources as significant in orogenic gold formation nevertheless there are broad ranges in more robust fluid inclusion trapping temperatures and pressures between deposits that support a model where this specific fluid may deposit ore over a broad window of upper to middle crustal depths much of the reported isotopic and noble gas data is inconsistent between deposits leading to the common equivocal interpretations from studies that have attempted to define fluid and metal source areas for various orogenic gold provinces fluid stable isotope values are commonly characterized by the following ranges 1 18 o for precambrian ores of 6 to 11 and for phanerozoic ores of 7 to 13 2 d and 34 s values that are extremely variable 3 13 c values that range from 11 to 2 and 4 15 n of 10 to 24 for the neoarchean 6 5 to 12 for the paleoproterozoic and 1 5 to 10 for the phanerozoic secular variations in large scale earth processes appear to best explain some of the broad ranges in the o s and n data fluid rock interaction particularly in ore trap areas may cause important local shifts in the o s and c ratios the extreme variations in d mainly reflect measurements of hydrogen isotopes by bulk extraction of waters from numerous fluid inclusion generations many which are not related to ore formation radiogenic isotopes such as those of pb sr nd sm and os measured on hydrothermal minerals are even more difficult to interpret for defining metal source particularly as the low salinity ore fluids transport limited amounts of these elements and significant amounts of these may be locally added to the minerals during alteration reactions at the sites of gold deposition noble gas and halogen data are equally equivocal fluid exsolution from granitoids emplaced into the upper and middle crust metamorphism of the crust or fluids entering trans crustal fault zones from below the crust all remain as permissive scenarios associated with orogenic gold formation as the abundant geochemical data are equivocal however geological and geochronological data weigh heavily against a magmatic hydrothermal model in the upper to middle crust there is no universal temporal association between orogenic gold and magmatism and where there is an overlap in age there is no specific type of magmatism consistently associated with gold formation nor element zonation around any specific pluton a crustal metamorphic model for fluid and metal sources is very consistent with geological geochronological and geochemical data although metamorphism on a regional scale that releases these components into major fault zones can be associated with many processes along active continental margins these can include crustal thickening and radiogenic heating slab rollback and heating during crustal extension or subduction of a spreading ridge heating the base of an accretionary prism in rare examples where phanerozoic orogenic gold deposits are hosted in precambrian high grade metamorphic terranes fluids and metals must however enter a transcrustal fault system from a sub crustal source this could either be a devolatilized subducted relatively flat perhaps stalled slab and its overlying sediment or the corner of the fertilized mantle wedge that releases a fluid during a thermal event

a review of shear wave splitting in the crack critical crust summary over the last 15 years it has become established that crack induced stress aligned shear wave splitting with azimuthal anisotropy is an inherent characteristic of almost all rocks in the crust this means that most in situ rocks are pervaded by fluid saturated microcracks and consequently are highly compliant the evolution of such stress aligned fluid saturated grain boundary cracks and pore throats in response to changing conditions can be calculated in some cases with great accuracy using anisotropic poro elasticity ape ape is tightly constrained with no free parameters yet dynamic modelling with ape currently matches a wide range of phenomena concerning anisotropy stress shear waves and cracks in particular ape has allowed the anisotropic response of a reservoir to injection to be calculated predicted with hindsight and the time and magnitude of an earthquake to be correctly stress forecast the reason for this calculability and predictability is that the microcracks in the crust are so closely spaced that they form critical systems this crack critical crust leads to a new style of geophysics that has profound implications for almost all aspects of pre fracturing deformation of the crust and for solid earth geophysics and geology we review past present and speculate about the future of shear wave splitting in the crack critical crust shear wave splitting is seen to be a dynamic measure of the deformation of the rock mass there is some good news and some bad news for conventional geophysics many accepted phenomena are no longer valid at high spatial and temporal resolution a major effect is that the detailed crack geometry changes with time and varies from place to place in response to very small previously negligible changes however at least in some circumstances the behaviour of the rock in the highly complex inhomogeneous earth may be calculated and the response predicted opening the way to possible control by feedback the need is to devise ways to exploit these new opportunities in the crack critical crust recent observations from the smsites project at husavik in northern iceland gathered while this review was being written display the extraordinarily sensitivity of in situ rock to small changes at great distances the effects are far too large to occur in a conventional elastic brittle crust and their presence confirms the highly compliant nature of the crack critical crust

the effect of the aspect ratio on crack induced anisotropy1 media containing aligned cracks show anisotropy with respect to elastic wave propagation there are several models describing the wave propagation in cracked media most of them only valid for cracks with small aspect ratios one of these models crampin s model is compared with a model valid for all aspect ratios nishizawa s model the elastic constants and the group velocities are compared for both dry and liquid filled inclusions with aspect ratios ranging from 0 0001 flat cracks up to 1 spheres the difference between both models is small for small aspect ratios but becomes larger for increasing aspect ratios at a crack density of 0 05 both models give within an error of 5 the same results for aspect ratios up to 0 3 therefore crampin s model can be applied to a large range of cracked media even if the aspect ratio of the inclusions is not small the variation of the anisotropy as a function of the aspect ratio can be studied using thomsen s dimensionless parameters e and y they show how inclusions with large aspect ratios result in elliptical anisotropy