drilling deep into young oceanic crust hole 504b costa rica rift hole 504b is by far the deepest hole yet drilled into the oceanic crust in situ and it therefore provides the most complete ground truth now available to test our models of the structure and evolution of the upper oceanic crust cored in the eastern equatorial pacific ocean in 5 9 m y old crust that formed at the costa rica rift hole 504b now extends to a total depth of 1562 3 m below seafloor penetrating 274 5 m of sediments and 1287 8 m of basalts the site was located where the rapidly accumulating sediments impede active hydrothermal circulation in the crust as a result the conductive heat flow approaches the value of about 200 mw m predicted by plate tectonic theory and the in situ temperature at the total depth of the hole is about 165 c the igneous section was continuously cored but recovery was poor averaging about 20 the recovered core indicates that this section includes about 575 m of extrusive lavas underlain by about 200 m of transition into over 500 m of intrusive sheeted dikes the latter have been sampled in situ only in hole 504b the igneous section is composed predominantly of magnesium rich olivine tholeiites with marked depletions in incompatible trace elements nearly all of the basalts have been altered to some degree but the geochemistry of the freshest basalts is remarkably uniform throughout the hole successive stages of on axis and off axis alteration have produced three depth zones characterized by different assemblages of secondary minerals 1 the upper 310 m of extrusives characterized by oxidative seafloor weathering 2 the lower extrusive section characterized by smectite and pyrite and 3 the combined transition zone and sheeted dikes characterized by greenschist facies minerals a comprehensive suite of logs and downhole measurements generally indicate that the basalt section can be divided on the basis of lithology alteration and porosity into three zones that are analogous to layers 2a 2b and 2c described by marine seismologists on the basis of characteristic seismic velocities many of the logs and experiments suggest the presence of a 100 to 200 m thick layer 2a comprising the uppermost rubbly pillow lavas which is the only significantly permeable interval in the entire cored section layer 2b apparently corresponds to the lower section of extrusive lavas in which original porosity is partially sealed as a result of alteration nearly all of the logs and experiments showed significant changes in in situ physical properties at about 900 1000 m below seafloor within the transition between extrusives and sheeted dikes indicating that this lithostratigraphic transition corresponds closely to that between seismic layers 2b and 2c and confirming that layer 2c consists of intrusive sheeted dikes a vertical seismic profile conducted during leg 111 indicates that the next major transition deeper than the hole now extends that between the sheeted dikes of seismic layer 2c and the gabbros of seismic layer 3 which has never been sampled in situ may be within reach of the next drilling expedition to hole 504b therefore despite recent drilling problems deep in the hole current plans now include revisiting hole 504b for further drilling and experiments when the ocean drilling program returns to the eastern pacific in 1991

deformation pattern in a massive ponded lava flow at odp iodp site 1256 a core and log approach 1 a complete in situ section of upper oceanic crust from extrusive lavas through dikes into gabbros has been recently drilled for the first time in 15 ma old crust created at the superfast spreading east pacific rise epr the upper igneous basement cored at ocean drilling program odp integrated ocean drilling program iodp holes 1256c and 1256d consists of sheet flows and massive flows including a ponded lava flow unit lava pond interpreted as a thick lava flow delivered off axis and accumulated in a topographic depression fine scale structural analysis shows that the lava pond is much less fractured than the deeper lavas probably as the effect of structure spacing that is larger than in the underlying lava flows in hole 1256d structures are mainly subhorizontal in the lava pond and become steeper downhole toward the sheeted dike complex this reflects the distance of the lava pond from the stress field produced by the intrusion of dikes at depth inducing mostly subvertical eruptive fractures and fracture network and the distance of the site from active rifting at the epr contrasting structural patterns in the lava pond and in the deeper parts of the drilled section are reflected in contrasting physical properties patterns as obtained by shipboard and downhole logging data electrical resistivity compressional velocity and bulk density are generally higher whereas natural radioactivity and neutron porosity are lower in the lava pond than in the deeper hole the occurrence of a thick massive lava flow at the top part of the volcanic section affects the deformation pattern of the entire drilled crust and consequently its permeability regime which in turn controls the geometry and distribution of hydrothermal circulation and basalt alteration

velocity structure in upper ocean crust at hole 504b from vertical seismic profiles hole 504b provides the only opportunity to directly correlate seismic velocity structure to the lithology and physical properties of upper ocean crust providing a baseline for comparison with seismic measurements elsewhere we determine p and s velocities from vertical seismic profiles vsps obtained on ocean drilling program odp legs 111 and 148 four issues are considered the location of the seismic layer 2 3 boundary p to s wave conversion by scattering transverse isotropy and poisson s ratio as an indicator of lithology porosity and structure 1 in the p velocity profile the change in slope marking the layer 2 3 boundary coincides with the top of the sheeted dike unit seismic layer 2 is composed of the extrusives and the lithologic transition zone the layer in which flows and dikes interfinger 2 even in these normal incident vsps several second arrivals with velocities indicative of vertically polarized shear energy are observed p to s wave conversion within the upper 110 m of basement occurs by scattering from surface roughness and volume heterogeneities and does not depend on angle of incidence as predicted by plane boundary transmission coefficient analysis 3 vertical velocities determined from the vsp differ by 10 from horizontal velocities obtained from the oblique seismic experiment ose on deep sea drilling project dsdp leg 92 the p wave velocity structure is determined by small and intermediate 1 cm pore structure with no measurable anisotropy the large scale well oriented vertical fractures which are formed tectonically do not have a detectable effect on compressional wave velocities 4 high poisson s ratio in the upper 300 m of basement coincides with an extrusive layer composed of pillows and thin flows low poisson s ratio at 850 1150 m below seafloor mbsf coincides with the downhole decrease in bulk porosity caused by the transition from extrusives to dikes relatively large aspect ratio cracks are required to produce such low values of poisson s ratio the cracks were likely created by hydraulic fracturing when hot dikes encountered low temperature seawater

crustal construction and magma chamber properties along the eastern lau spreading center abstract the l scan active source seismic tomography experiment maps the crustal structure and magmatic system along a 120 km long section of the back arc eastern lau spreading center elsc where the ridge undergoes abrupt changes in morphology and composition associated with increasing proximity to the tofua volcanic arc using this dataset we picked 197 000 p wave travel times from 57 seismic airgun lines recorded at 83 ocean bottom seismograph stations and inverted for a 3 d p wave velocity image the seismic images reveal a prominent but narrow seismic low velocity volume lvv located beneath all surveyed ridge segments consistent with the high temperatures and melt of a crustal magmatic system crustal magmatic systems thus underlie spreading axes even where previous seismic reflection surveys did not detect magma lens reflectors accounting for the heat source of high temperature hydrothermal vents in these areas the top of the lvv closely follows the ridge axis and steps across three overlapping spreading centers as the offset of the overlap increases the lvv becomes increasingly discontinuous across the ridge limbs surprisingly the lvv is as much as twice as wide but deeper in the northern part of the ridge system where the crust is thinner as compared to the lvv beneath the southern segments where the crust is relatively thicker the width of the lvv may be modulated by the degree of deep hydrothermal activity or temporal variations in melt supply and thus may not correlate directly with the average melt flux as indicated by crustal thickness over the past 4 myr the location of the ridge has swept across different mantle compositional domains and the crust produced at the ridge formed a zoned pattern the interpretation is that thick highly porous volcanic layers with felsic compositions cap regions of thick crust thinner volcanic layers of basaltic composition cap regions of thinner crust the zonal pattern indicates that the influence of slab derived water on crustal construction has substantially decreased over time

instantaneous outbursts in underground coal mines an overview and association with coal type instantaneous outbursts in underground coal mines have occurred in at least 16 countries involving both methane ch4 and carbon dioxide co2 the precise mechanisms of an instantaneous outburst are still unresolved but must consider the effects of stress gas content and physico mechanical properties of the coal other factors such as mining methods e g development heading into the coal seam and geological features e g coal seam disruptions from faulting can combine to exacerbate the problem prediction techniques continue to be unreliable and unexpected outburst incidents resulting in fatalities are a major concern for underground coal operations gas content thresholds of 9 m 3 t for ch4 and 6 m 3 t for co2 are used in the sydney basin to indicate outburst prone conditions but are reviewed on an individual mine basis and in mixed as situations data on the sorption behaviour of bowen basin coals from australia have provided an explanation for the conflicting results obtained by coal face desorption indices used for outburst proneness assessment a key factor appears to be different desorption rates displayed by banded coals which is supported by both laboratory and mine site investigations dull coal bands with high fusinite and semifusinite contents tend to display rapid desorption from solid coal for a given pressure drop the opposite is true for bright coal bands with high vitrinite contents and dull coal bands with high inertodetrinite contents consequently when face samples of dull fusinite or semifusinite rich coal of small particle size are taken for desorption testing much gas has already escaped and low readings result the converse applies for samples taken from coal bands with high vitrinite and or inertodetrinite contents in terms of outburst potential it is the bright vitrinite rich and the dull inertodetrinite rich sections of a coal seam that appear to be more outburst prone this is due to the ability of the solid coal to retain gas even after pressure reduction creating a gas content gradient across the coal face sufficient to initiate an outburst once the particle size of the coal is reduced rapid gas desorption can then take place c 1998 elsevier science

compressional and shear wave anisotropy in the oceanic lithosphere the ngendei seismic refraction experiment summary synthetic seismogram modelling of seismic refraction data from the 1983 ngendei expedition to the south pacific indicate seismic anisotropy both within the top kilometre of oceanic crust and in the uppermost mantle we calculate average p and s wave velocity versus depth functions for two orthogonal azimuths n3ooe approximately aligned with the fast mantle direction and n120 e close to the fast crustal direction probable lateral heterogeneities at the ngendei site prevent perfect matching of data and synthetic waveforms crustal anisotropy is indicated by a 28 pattern of both p and s wave travel times as a function of azimuth and is consistent with an approximate 0 2 km s difference in p wave velocities and 0 1 km s difference in s wave velocities upper mantle anisotropy is characterized by pn velocity differences of 7 95 8 4 km s l and a nearly uniform s velocity of 4 65 km s we use these velocity profiles and traveltime data to calculate bounds on the properties of the elastic constants of the crust and upper mantle the crustal anisotropy can be explained by a model involving aligned cracks parallel to the original spreading ridge resulting in a fast direction perpendicular to the fossil spreading direction the upper mantle anisotropy is consistent with aligned olivine crystal models in which the fast direction is parallel to the fossil spreading direction for both of these models we use bounds from the ngendei data to place constraints on the physical properties of the lithosphere

off axis hydrothermal circulation parametric tests of a refined model of processes at deep sea drilling project ocean drilling program site 504 we have developed a refined model of coupled heat and fluid flow to numerically simulate off axis hydrothermal circulation through the upper oceanic crust the new representation includes mesh elements with curved sides and noncentered nodes these curvilinear elements allow improved simulation of topography at the seafloor and within underlying sediment and basaltic layers curvilinear simulations of circulation at deep sea drilling project ocean drilling program site 504 confirm the broad conclusions derived from the rectilinear simulations of fisher et al 1990 but also suggest that permeability within the upper few hundred meters of crust around hole 504b must be concentrated within several narrow zones this interpretation is consistent with core observations and wireline logs from hole 504b within these highly permeable zones absolute permeability may be several orders of magnitude higher than the bulk permeability measured for the upper crust as a whole a series of detailed parametric simulations was conducted to determine the quantitative importance of basal heat flow seafloor bathymetry basement relief and differential sediment thickness in modifying the geometry and intensity of off axis hydrothermal circulation these studies reveal that basement relief and differential sediment thickness are more important than seafloor bathymetry in enhancing off axis convection within the seafloor observed natural correlations between seafloor bathymetry and heat flow may exist because bathymetry is a proxy for the other two important parameters basement relief and differential sediment thickness off axis convection at geochemically significant velocities is possible even with heat input appropriate for some of the oldest oceanic crust a thick sediment layer over older crust may cause enough conductive refraction at the seafloor to mask the variations in seafloor heat flow often associated with off axis hydrothermal convection

photogeologic study of a segment of the east pacific rise axis near 21 n latitude geologic and structural interpretations based primarily on photographic data collected by the marine physical laboratory deep tow instrument the woods hole oceanographic institution angus and the french manned submersible cyana show that extensional tectonic activity modifies and locally replaces extrusive volcanic processes along a 6 km segment of the east pacific rise axial zone near lat 21 n relatively young pillow flows dominate the spreading axis however fissures are present throughout the axial zone and there is a light dusting of pelagic sediment on even the youngest flows in contrast the axial zone immediately southwest in the rise project study area is free of fissures and sediment photographic evidence for the existence of undiscovered active hydrothermal vents or narrow zones of diffuse hydrothermal emissions includes anomalously high concentrations of common deep sea organisms not the exotic hydrothermal vent communities some of these are coincident with high 3 he values and positive temperature anomalies discovered in deep tow water samples lupton and others 1980 estimation of the absolute age of pillow flows based on the sedimentation rate and the volume of interpillow sediment ponds suggests that the average pillow flow with a sediment cover of 70 is approximately 1 000 yr old and that a pillow flow will be completely covered with sediment in about 40 000 yr this relation indicates that the extrusive terrane of the inner axial zone in the study area although undergoing extensional modification is considerably less than 1 000 yr old visual geologic data collected by the several deep sea photographic systems are complementary in nature and the best geologic interpretations are made through use of more than one system to provide various photographic perspectives and scales of observation

balancing the amount and composition of gas contained in the pore space of cupriferous rocks from the perspective of economy and safety balancing gas contained in the pore space of rocks is extremely essential predominantly to establishments dealing with extraction of mineral resources one of the main methods of evaluating the amount and composition of gas contained in the pore space of a rock is releasing the gas as a result of comminuting the investigated rock material in the case of cupriferous rocks effective comminution is a very difficult task due to the strength properties of these rocks the present paper discusses the results of studies into the gas content of cupriferous rocks obtained by means of an original device named the gpr analyzer the research was done on 41 samples of dolomites and anhydrites from various areas of two copper mines located in poland rudna and polkowice sieroszowice for all samples on the basis of microscope analyses performed on cuts and polished sections the open porosity closed porosity and total porosity were determined in the case of the dolomite samples the total porosity variability fell in the range of 4 75 23 05 and in the case of the anhydrite samples in the range of 3 87 16 60 the maximum gas content of the dolomite samples was 166 67 cm3 kg and of the anhydrite samples 84 66 cm3 kg in some of the studied samples the presence of methane was confirmed toxic gases such as h2s were not found the main gas in the pore space of the investigated rocks was nitrogen knowing the amount of the released gas and the value of the closed porosity in the investigated samples the authors were able to estimate the pore pressure of the gas whose maximum value was 0 583 mpa

detailed structure of an oceanic normal fault a multiscalar approach at dsdp odp site 504 seismic data downhole measurements images and core obtained at dsdp odp site 504 in the eastern equatorial pacific reveal the presence of a normal fault about 500 m into basement this north dipping structure is sub vertical in the sediment and dips about 45 toward the ridge in the hole similar faults are expected from seismic data to cut this deep hole within the sheeted dikes and close to the base of the hole where the drilling was abruptly terminated the combined analysis of geophysical and geological data reveals a 300 m thick fault zone with modified physical and mineralogical properties the fault was initially identified from a 4 step in magnetic field inclination at 800 mbsf and the lowest values of electrical resistivity in basement the fault also exhibits modified mechanical properties and a high open porosity fraction while fracturing mapped from electrical images decreases slightly versus depth in basement the fracture density increases with increasing depth within the fault zone the basalt magnetic properties are strongly modified near the base of the fault zone from 900 to 1050 mbsf possibly in relation with early faulting and the circulation of high temperature fluids to the contrary present ruptures appear to be located near the top of the interval and suggest a propagation of the deformation toward the hanging wall with time

in situ electrical resistivity and bulk porosity of the oceanic crust costa rica rift in situ electrical resistivity was measured in dsdp hole 504b to a depth of 1 013 m into oceanic basement apparent resistivities are about 10 m in about 600 m of pillow lavas sharply increasing to nearly 1 000 m in the underlying dykes bulk porosities calculated from resistivities show a distinct layering apparently corresponding to geophysical layers 2a 2b and 2c

oceanic basement structure sediment thickness and heat flow near hole 504b a new seismic reflection survey around hole 504b the deepest borehole in ocean crust reveals active faulting possible volcanic centers and a lateral change in the relationship of heat flow and basement structure near the borehole migration of single channel and multichannel seismic profiles collected in a 25 by 25 km grid with a 1 km line spacing significantly improved the resolution of basement structure and sediment thickness west of hole 504b heat flow is high above east west lineated basement ridges whereas heat flow to the east is normal above ridges and high above two buried basement knolls the difference is probably due to lateral variations in sediment thickness small buried basement knolls are common and may have been point sources for lava flows hole 504b lies in a flat floored basin that slopes gently upward to the west a recently active fault 1 1 1 2 km south of hole 504b is indicated by sediment reflector discontinuities that extend up to the seafloor the fault strikes east west and crosses a buried volcanic knoll where holes 678b and 896a were drilled regionally basement relief north of hole 504b is 100 to 150 m lower than to the south which we attribute to an increased spreading rate obtained from dating published local magnetic anomaly patterns with a recent timescale we find at least five graben structures resembling failed rifts which may have formed in response to asymmetric spreading or to the change in tectonic stress accompanying the spreading rate change south facing scarps on basement ridges are as common as north facing scarps sediment thickness is highly correlated to basement depth due to preferential deposition in topographic lows when the crust was 1 2 ma old and to later winnowing

lateral heterogeneity in the seismic structure of upper oceanic crust western north atlantic we carried out a borehole seismic experiment at deep sea drilling project hole 418a in order to study the vertical and lateral velocity variations in 100 m y old oceanic crust we clamped a seismometer successively at five depths between 41 m and 430 m within the extrusive basalt layer while air gun and explosive sources were fired on eight radial lines and four concentric circles to 8 km range our travel time data do not give consistent evidence of anisotropy however our experimental error may mask anisotropy of up to 0 2 0 3 km s velocities from inflection point analysis sonic log and reflectivity synthetic seismograms support results of inversion of radial line p arrival times velocity increases with an almost linear gradient of 1 3 s 1 from 4 6 km s at the top of basement to 1 8 km depth the velocity profile at 418a does not differ significantly at the 95 confidence level from that at 417d located 7 5 km away seismic velocities at shorter length scales do vary laterally travel time anomalies indicate that seismic velocity in the upper 0 5 km increases laterally northwest to 3 km range other data indicate that this velocity anomaly in the extrusive layer may be due to alteration controlled by topography or to primary porosity variations

magma productivity and early seafloor spreading rate correlation on the northern voring margin norway constraints on mantle melting abstract continental rifting at the voring margin off mid norway was initiated during the earliest eocene 54 ma and large volumes of magmatic rocks were emplaced during and after continental breakup in 2003 a marine survey collecting ocean bottom seismometer single channel reflection and magnetic data was conducted on the norwegian margin to constrain continental breakup and early seafloor spreading processes the profile described here crosses the northern part of the voring plateau and the crustal velocity model was constructed through a combination of ray tracing and forward gravity modeling the latter corrected for the thermal effects remaining from the seafloor spreading we found a maximum igneous crustal thickness of 18 km decreasing to 6 5 km over the first 6 m y after continental breakup both the volume and the duration of excess magmatism are about twice as large as that of the more margin south of the east jan mayen fracture zone which offsets the two margin segments by 170 km a similar reduction in magmatism occurs to the north over an along margin distance of 150 km to the lofoten margin but without a margin offset both the geochemical data and the mean p wave velocity indicate that there is active mantle upwelling combined with a moderate temperature increase during the earliest mantle melting at the voring margin the mean p wave velocity versus crustal thickness also indicates that there is a transition from convection dominated to temperature dominated magma production 2 m y after breakup the magnetic data were used to derive plate half spreading rates for the northern voring margin which are very similar to that obtained at the more margin there is a strong correlation between magma productivity and early plate spreading rate suggesting a common cause a model for the breakup related magmatism should be able to explain this correlation but also the magma production peak at breakup the along margin magmatic segmentation and the active mantle upwelling proposed end member hypotheses comprise elevated upper mantle temperatures caused by a hot mantle plume or edge driven small scale convection fluxing mantle rocks through the melt zone edge driven convection does not easily explain these observations but a mantle plume model in which buoyant plume material flows laterally to pond in the rift topography at the base of the lithosphere close to breakup time is promising when the continents break apart the hot and buoyant plume material can flow up into the rift zone from surrounding areas as the rift transits to drift and the excess temperature of this material will then cause excess magmatism which dies off as the rift restricted material is spent the buoyancy of the plume material may in addition cause active upwelling which can increase the melting furthermore and also increase the force on the plate boundaries to enhance plate spreading rate this conceptual model explains how both excess magmatism and spreading rate will be reduced similarly with time as the plume material is consumed by plate spreading and thus correlate

a finite element computation of seismic diffraction about the rise axis magma chamber in a previous study models of the crust and magma chamber at 12 50 n on the east pacific rise were derived with a two dimensional ray tracing analysis of rise normal refraction lines the best model featured a 4 km wide axial magma chamber but the data contained diffraction effects that were not modeled by geometric ray theory this raised questions as to whether this model would reproduce the waveform data and whether diffraction might be obscuring a much larger magma chamber we have tested the model with a finite element calculation of synthetic seismograms this method incorporates full wave effects including diffraction the synthetics accurately reproduce the data and exhibit diffraction phenomena similar to those in the data the previous travel time analysis was largely successful and was only slightly compromised by diffraction diffraction in rise normal refraction experiments is therefore not obscuring a large low velocity magma chamber

three dimensional upper crustal heterogeneity and anisotropy around hole 504b from seismic tomography we have used over 16 000 compressional wave travel times recorded on 11 ocean bottom seismometers to determine the magnitude of p wave heterogeneity and anisotropy in the upper 2 km of the oceanic crust in an 150 km2 area around hole 504b the deepest hole drilled into the oceanic crust our best fitting one dimensional isotropic upper crustal velocity model for the 150 km2 area around the drill site is similar to velocity models at hole 504b determined using downhole sonic logs vertical seismic profiles oblique seismic experiments and conventional refraction profiling there is no evidence seismic layer 2 is anomalously thin at hole 504b suggesting that the occurrence of the seismic layer 2 3 boundary within the upper sheeted dike section which has been documented in hole 504b is a general feature of this 5 9 ma crust on the scale of a few kilometers and averaged over seismic wavelengths of a few hundred meters the heterogeneity in upper crustal p wave velocity around hole 504b is comparatively small within the 571 m thick extrusive section maximum p wave velocity differences are 300 400 m s 1 a 6 8 velocity anomaly these velocity differences decrease to 1 km into the crust the highest upper crustal velocities are found beneath basement ridges west and south of hole 504b while the lowest upper crustal velocities occur beneath the flanking basement troughs there is an azimuthal dependence to the travel time residuals slow direction n s which is consistent with 2 4 crack induced anisotropy in the upper crust the magnitude of upper crustal heterogeneity observed near hole 504b is significantly less than that found in zero age crust at the mid atlantic ridge but comparable to that observed off axis in crust formed at faster spreading ridges

gas emission prediction and recovery in underground coal mines abstract strata gas can be released and captured from non active and active gas resources either from virgin or relaxed strata both prior to and when mining activities take place the high and irregular gas emissions associated with high production longwall mining have provided a need to optimise the methods used to predict these gas levels and the ventilation requirements for gas dilution a forecast of gas emissions during development drivage and longwall mining indicated possible gas and ventilation problems requiring the introduction of various gas drainage techniques and in maintaining the necessary air quantities in ventilation systems to satisfy the statutory gas limitations for various coal production rates although there are sound principles used in world recognised methods of gas emission prediction a new approach developed from long term practical experience in underground gassy coal mine practices and gas rock mechanics studies appear most suitable for local conditions and mining systems in use the lunagas floorgas and roofgas geomechanical and gas emission models offer an effective solution to these problems both programs are the most advanced engineering numerical tools available to calculate gas source contributions to total gassiness and improve the accuracy and quality of gas control gas capture technologies and ventilation system design

seismic velocity structure at deep sea drilling project site 504b panama basin evidence for thin oceanic crust we present an analysis of wide angle reflection refraction data collected in the immediate vicinity of deep sea drilling project hole 504b in the panama basin currently the deepest drill hole 1 288 km into oceanic crust the data were acquired with a 1785 inch3 air gun array and fixed gain sonobuoy receivers and consist of four intersecting profiles shot along three different azimuths near normal incidence multichannel seismic mcs reflection data were acquired simultaneously observed p and s wave arrivals out to maximum ranges of 30 km provide constraints on the velocity structure of the middle and lower crust and on total crustal thickness comparison of the travel times and amplitudes of the p and s wave arrivals on all four profiles revealed important similarities which were modeled using the reflectivity synthetic seismogram method forward modeling shows that in contrast to standard oceanic velocity models a velocity depth profile that better explains the observed data is characterized by high velocity gradients up to 0 6 km s km in the middle crust a 1 8 km thick low velocity zone vp 7 1 6 7 km s immediately above moho and a total crustal thickness of only 5 km interpretation of the high velocity gradients in the middle crust is constrained by the observation of p wave amplitude focusing at ranges of 16 19 km although not as well developed in comparison to the p wave arrivals s wave arrivals show similar focusing total crustal thickness is constrained by the combined interpretation of a p wave wide angle reflection event observed at a range of 16 28 km and an mcs reflection event with a crustal travel time of 1 4 1 5 s although these events cannot be directly correlated their travel times are consistent with the assumption that both have a common origin amplitude modeling of the wide angle event demonstrates that these events are generated at the moho

cracked media poisson s ratio and the structure of the upper oceanic crust summary theoretical models of water filled cracks within a solid are in general agreement and indicate that the poisson s ratio of the composite material will typically increase for thin cracks with aspect ratios less than a but will decrease for thick cracks with aspect ratios between and i near spherical pores cause little change in poisson s ratio this is true both for randomly oriented isotropic crack models and at certain directions for aligned crack models which predict seismic anisotropy observed anomalous values of poisson s ratio in oceanic layer 2 can often be explained using these models as an effect of cracks and do not necessarily require the presence of rocks of unusual composition in particular the low poisson s ratios reported by spudich orcutt 1980 and au clowes 1984 can approximately but not always exactly be fitted with a model consisting of thick cracks within higher poisson s ratio rock more typical of the upper oceanic crust however this interpretation is near the limit of existing theories of cracked material so other explanations for the anomalous observations must also be considered

seismic attenuation structure of the east pacific rise near 9 30 n we describe a spectral technique to measure the apparent attenuation of compressional waveforms recorded during an active seismic tomography experiment centered at 9 32 n on the east pacific rise over 3500 estimates of t are obtained from 0 4 to 0 7 s long windows aligned with crustal p phases including diffractions above and below a midcrustal magma chamber and moho reflections which cross the rise axis at a range of lower crustal depths we apply a smoothest model inversion algorithm to the t measurements to derive images of apparent crustal q 1 both 20 km off axis and within a 16 16 km area centered on the rise crest the models resolve regions of high attenuation in the uppermost crust and in a low q zone which extends from midcrustal to lower crustal depths beneath the rise axis off axis q values in the upper 1 km average 35 50 while at depths greater than 2 3 km q is at least 500 1000 the high levels of attenuation in the uppermost crust probably result from the combined effect of factional fluid flow and scattering mechanisms within 1 3 km of the rise axis q increases markedly in the uppermost 1 km to about 65 if the increase in attenuation off axis is entirely due to the 300 m increase in the thickness of layer 2a extrusives required by seismic velocity measurements then q in layer 2a must be about 10 20 no measurements of q are obtained in the immediate vicinity of the 1 6 km deep axial magma lens because no wave paths cross the rise axis through this region the diffractions beneath the magma chamber and the moho reflections require a low q region with minimum q values of 20 50 which extends from no more than 2 5 km depth to the base of the crust these values are similar to laboratory measurements of q obtained at solidus temperatures and constrain the low q region to contain no more than a few percent melt the axial magma chamber which comprises a melt lens and an underlying crystal mush zone must be confined to a narrow 1 km thick region through which no rise crossing paths pass inversions for along axis structure in the low q anomaly show a 20 25 increase in attenuation at 2 3 km depth north of 9 34 n but resolve no such trend at 4 6 km depth the along axis variations may reflect the recent history of volcanic eruptions and hydrothermal cooling and do not require systematic along axis variations in magma supply

lateral heterogeneity in the upper oceanic crust at deep sea drilling project site 504 the lateral variability of the upper oceanic crust can be studied by mapping the amplitude or power content of arrivals from a dense array of identical sources to a single receiver in this paper we present the results from a borehole seismic experiment at deep sea drilling project site 504 in the eastern equatorial pacific seismic amplitude anomalies of 20 db and more are present with a horizontal correlation length of 1 3 km finite difference synthetic seismogram techniques are used to predict seismic wave propagation through realistic bathymetry basement relief and lateral variations within basement a three dimensional model of the velocity structure around the borehole can be constructed the resultant model has horizontal gradients greater than 2 0 s 1 in the upper crust these gradients are comparable to the vertical gradients normally associated with oceanic crust assuming that velocity anomalies in the upper crust reflect changes in porosity the resultant structure provides constraints on the hydrothermal circulation at the site in addition the finite difference modeling indicates that polarity reversals can be generated by lateral velocity variations near the source and receiver

vegetation dynamics during the pleistocene holocene transition in the central great plains usa the holocene pleistocene transition in the upland loess mantled regions of the central great plains is punctuated by the brady soil which separates the late pleistocene peoria loess and the holocene bignell loess previous research on the brady soil at the old wauneta roadcut site in southwestern nebraska has produced paleoenvironmental information based on well constrained luminescence and radiocarbon ages stable carbon isotope data and chemical and physical data while the research indicated high effective moisture during formation of the brady soil and a shift to warm season c4 vegetation from the cool season c3 dominated vegetation of the peoria loess those data do not provide any detail as to plant community composition and significant underlying climatic inferences assemblages of phytoliths and other biosilicates extracted from the brady soil provide specific information on vegetation communities and indicate shifts of plant taxa comprising these assemblages short cell phytolith count data rev

dsdp hole 504b the first reference section over 1 km through layer 2 of the oceanic crust hole 504b provides a reference section for correlation of in situ petrological and geophysical studies with regional crustal models based on dredged samples and remote surveys and at least for the medium spreading rate costa rica rift hole 504b confirms that the main features of the upper part of an idealized ophiolite sequence occurs in the oceanic crust

a mechanism for decoupling within the oceanic lithosphere revealed in the troodos ophiolite contrasting kinematic histories recorded in the sheeted dykes and underlying plutonic rocks of the troodos ophiolite provide a new perspective on the mechanical evolution of oceanic spreading centres the kinematic framework of the decoupling zone that partitions deformation between the sheeted dykes and plutonics contrasts with low angle detachment models for slow spreading ridges based on continental rift analogues a model for the generation of multiple horizontal decoupling horizons linked by planar normal faults demonstrates new possibilities for the kinematic and rheological significance of seismic reflectors in oceanic lithosphere

smooth inversion of vsp traveltime data vertical seismic profile vsp direct arrivals provide an insitu measurement of traveltime with depth into the earth in this note we describe a weighted damped least squares inversion of vsp traveltimes for a smooth velocity depth function that inherently reveals the resolution of the data smooth velocity depth profiles of this type are suitable for migration or as a starting models for waveform inversion or tomography the application of this inversion is particularly simple requiring only the value of the damping parameter to be determined and this value is determined from residual statistics

an investigation of seismic anisotropy due to cracks in the upper oceanic crust at 45 n mid atlantic ridge summary a detailed seismic refraction survey using three 3 component ocean bottom seismographs over young 1 1 3 4 ma old mid atlantic ridge at 45 n recorded compressional wave velocity anisotropy in the upper crust of about 5 per cent and associated particle motion deviations of up to 8 the magnitudes and azimuths of these anisotropic variations suggest that they are generated by sets of subparallel near vertical water filled cracks in layer 2 the directions of the cracks cannot be determined uniquely but they lie between the seafloor spreading and the ridge axis directions in the range 30 60 with respect to the ridge axis we explain this unexpected crack orientation by the occurrence of cracking and fracturing induced in the upper crust as it moves away from the spreading axis the principal horizontal stresses responsible for this fracturing are parallel and perpendicular to the ridge axis the apparently negligible seismic influence exerted by ridge parallel sets of shallow cracks observed at the ridge axis is explained by their infilling by sediment and hydrothermal precipitates and by their closure under ridge normal compression we find a normal oceanic crustal velocity structure that can be explained by decreasing numbers of open cracks with increasing depth through the upper 2 km of the crust lateral variability across the 25 km square survey is limited in magnitude typically 0 05 s variations in delay times and the observed travel times are consistent with a model of the crust in which the isovelocity surfaces at depth are a subdued copy of the basement topography

structure porosity and stress regime of the upper oceanic crust sonic and ultrasonic logging of dsdp hole 504b abstract the layered structure of the oceanic crust is characterized by changes in geophysical gradients rather than by abrupt layer boundaries correlation of geophysical logs and cores recovered from dsdp hole 504b provides some insight into the physical properties which control these gradient changes borehole televiewer logging in hole 504b provides a continuous image of wellbore reflectivity into the oceanic crust revealing detailed structures not apparent otherwise due to the low percentage of core recovery physical characteristics of the crustal layers 2a 2b and 2c such as the detailed sonic velocity and lithostratigraphic structure are obtained through analysis of the sonic borehole televiewer and electrical resistivity logs a prediction of bulk hydrated mineral content consistent with comparison to the recovered material suggests a change in the nature of the alteration with depth data from the sonic borehole televiewer electrical resistivity and other porosity sensitive logs are used to calculate the variation of porosity in the crustal layers 2a 2b and 2c several of the well logs which are sensitive to the presence of fractures and open porosity in the formation indicate many zones of intense fracturing interpretation of these observations suggests that there may be a fundamental pattern of cooling induced structure in the oceanic crust

in situ evidence for the nature of the seismic layer 2 3 boundary in oceanic crust the igneous oceanic crust is typically thought of as comprising two layers an upper crust seismic layer 2 characterized by a rapid increase in seismic velocity with depth and a thicker lower crust seismic layer 3 which is distinguished from layer 2 by both a higher p wave velocity 6 69 0 26 km s l and a much smaller vertical velocity gradient 1 8 km continuous hole drilled into the oceanic crust11 13 and find that at this location the seismic layer 2 3 boundary lies within the sheeted dyke complex where it is associated with gradual downhole changes in crustal porosity and alteration not a litho logical transition from sheeted dykes to gabbro

velocity porosity relationships in the upper oceanic crust theoretical considerations we consider here the application of rock physics theories to investigate relationships between seismic velocities and porosities in the shallow oceanic crust classical hashin shtrikman limits ignore void shapes and are too broad to provide useful constraints on velocities and porosities making some assumptions about the distribution of void shapes improves the constraints theories which ignore crack crack interactions underestimate the effects of porosities on velocities thus providing upper bounds on velocities and porosities self consistent theories overestimate crackcrack interactions and so provide lower bounds at the high porosities required to reduce basalt from a p velocity of 7km s in massive form to the 2 2km s observed in zero age oceanic crust however the bounds are too far apart to be useful the theories are strictly valid only for very small porosities using an algorithm proposed by cheng for iteratively building up porosity to create a highly porous medium analogous to differential computation methods traditionally used to improve upon the self consistent approach we have devised two hybrid theories which we term extended walsh and extended kuster toksoz these two theories remain approximately valid at the high porosities of oceanic crustal layer 2a to provide useful upper and lower bounds on velocity for a given porosity and pore aspect ratio distribution we attempt the inverse problem determining porosity from a given velocity using on bottom refraction data collected on the flank of the east pacific rise for 120ka material with a p velocity of 2 5km s if our assumptions regarding the aspect ratio distribution are correct porosity lies somewhere between 24 and 34 resolution on slower zero age crust 2 2km s is poorer there we predict a porosity between 26 and 43 use of shear wave information would tighten these bounds

multi scalar structure at dsdp odp site 504 costa rica rift iii faulting and fluid circulation constraints from integration of fms images geophysical logs and core data

evidence for active normal faulting on 5 9 ma crust near hole 504b on the southern flank of the costa rica rift single channel and multichannel seismic reflection data show evidence for recent movement of faults on 5 9 ma crust near ocean drilling program odp hole 504b on the southern flank of the costa rica rift 200 km from the ridge axis these faults which are associated with n85 w trending ridge parallel basement escarpments can be traced upward into the thick overlying sedimentary section that blankets volcanic crust in this area the offset of sedimentary horizons indicates the style and history of faulting it consistently shows the downdropped side to the north signifying inward facing normal faults or grabenlike structures indicative of crustal extension perpendicular to the ridge axis although most of the movement on these faults appears to have occurred in young crust near the ridge axis many of these faults have a long history of activity with cumulative displacements of several tens of metres occurring over the past several million years these results are inconsistent with inferences from borehole stress measurements made in hole 504b that the crust in this area is in horizontal compression or with previous assumptions that crustal extension at mid ocean ridges is limited to within 10 20 km of the spreading axis although the broad zone of crustal extension on the south flank of the costa rica rift could reflect anomalous stresses within the nazca plate several independent lines of evidence suggest that the active tectonic zone of crustal extension and normal faulting at mid ocean ridges may be significantly wider than previously suspected

hydrothermal alteration of a 1 km section through the upper oceanic crust deep sea drilling project hole 504b mineralogy chemistry and evolution of seawater basalt interactions deep sea drilling project hole 504b penetrates 1076 m into oceanic layer 2 and is the first hole to pass through the transition from pillow basalts altered at low temperatures into hydrothermally metamorphosed sheeted dikes alteration of the crust at site 504 occurred in four stages related to the movement of the crust away from the spreading axis 1 dikes reacted with seawater 200 300 c in the upwelling zone of an axial convection cell at the spreading axis resulting in the formation of greenschist facies parageneses in veins and host rocks mixing of the upwelling hydrothermal fluids with seawater circulating in the overlying more permeable pillow section occurred in the upper part of the lithologic transition zone causing a steep temperature gradient at the base of the pillow section secondary minerals formed in the lower pillow section from the resultant reducing mixed fluids at temperatures of around 100 c at the same time the initial effects of seafloor weathering began in the upper 320 m of the pillow section at low temperatures 50 c and under conditions of open circulation of oxidizing seawater 2 following refracturing of the dikes a second stage of axial upwelling occurred hydrothermal fluids 200 380 c were probably similar to those presently sampled from spreading ridges on the seafloor mixing of these fluids with seawater in the lithologic transition zone caused deposition of quartz epidote and sulfides in veins and formation of a sulfide rich stockworklike zone within this transition zone alteration of the pillow section proceeded under the prior conditions with the effects of seafloor weathering extending progressively downward into the crust 3 seawater recharge penetrated to depths of at least 1075 5 m subbasement and deposited anhydrite locally in veins 4 off axis alteration of the dikes was characterized by formation of zeolites calcite and prehnite in veins and host rocks from more highly evolved and lower temperature 100 250 c fluids alteration in the upper pillow section evolved from seafloor weathering conditions to more reducing and rock dominated as cracks in the basalt were sealed with secondary minerals and the basement was covered with a layer of sediment zeolites and calcite were the last phases to form throughout the pillow section sealing of the crust to convective cooling also allowed conductive reheating of the crust from below calculations of seawater crustal chemical fluxes from whole rock data are complicated by the low recovery and heterogeneity of alteration effects but indicate that basalt seawater interactions are a sink for seawater mg and k and a source for si and ca

anisotropic seismic scattering in old atlantic crust an oblique seismic experiment ose was carried out at ocean drilling program odp hole 418a in the western north atlantic to investigate vertical lateral and azimuthal structural variations in the upper portions of relatively old ocean crust original analysis of the data for travel time anisotropy did not show systematic azimuthal trends in seismic structure except at shallow depths immediately adjacent to the borehole subsequent analysis of the ose data presented here reveals a correlation between scattering strengths versus azimuth and spreading direction of the crust analyses are performed on body and coda waves and include coda energy and normalized coda energy calculations spectral analysis coda dropoff rates and cross correlation analysis all of the analyses show trends indicating an increase in seismic scattering along directions parallel to the paleospreading direction these results suggest that evidence of crustal formation processes is still present and seismically detectable after 108 m y of crustal evolution scattering is likely caused by crustal cracks and or aligned heterogeneities indicating that either large crustal cracks are still present in the crust or that homogenization of the crust through hydrothermal deposition of secondary minerals is not complete at this site

geothermal regimes of the costa rica rift east pacific investigated by drilling dsdp ipod legs 68 69 and 70 drilling durin deep sea drilling project legs 68 69 and 70 on the southern limb of the costa rica rift was used to study geothermal processes in the ocean crust two areas were drilled one was a geothermally hot site on 6 2 m y old crust where topography is smooth heat flow is close to that predicted by conductive cooling of the lithosphere 200 mwm 2 and hydrothermal circulation may be sealed within the crust the other was on 3 9 m y old crust where rough topography is associated with low heat flow 15 to 50 mwm 2 and possible open convection of sea water at both sites about 250 m of siliceous calcareous sediments overlies igneous basement in the hot area it blankets the topography whereas in the cold area basement outcrops still occur operations included numerous down hole experiments in both areas and hydraulic piston coring of a 230 m sediment section in the hot area diagenesis of the sediments appears closely related to temperature at the hot site chert was found near basement and the chemistry of pore fluids sampled from both sediments and basement is strongly influenced by reactions within the basement strong lateral gradients in the composition of pore fluids occur in the sediments at the cold site no chert was found and bacterial processes within the sediment dominated the chemistry of the pore fluids basaltic basement in both areas consists mainly of pillow lavas and thin flows with occasional more massive units the basalt is relatively magnesian the degree of alteration is very small in the cold area but much more extensive in the hot area ease of drilling also shows a strong contrast basement penetration reached 562 m in the hot area and was halted because of lack of time at the cold site 43 m of basement was cored only with difficulty the most intensive in hole experiments were conducted in the hot area successful runs with the borehole televiewer allowed basement lithology to be determined and showed the presence of more and less fractured zones pulse tests using a single borehole packer gave values of basement permeability of about 2 to 40 millidarcies numerous temperature logs established a broadly conductive in situ temperature gradient with temperatures reaching 120 c at 562 m into the basement however anomalously low temperatures in the upper part of the hole which persisted after drilling disturbance had decayed away showed that cold ocean water was flowing down the hole and into the basement at about 90 m below the base of the sediments at rates of about 80 to 100 m hr the packer records indicate a pressure at this depth of 10 bars below hydrostatic costa rica rift united scientific team made up of shipboard participants on legs 68 501 69 and 70 who were in reverse alphabetical order

three dimensional seismic structure of the mid atlantic ridge 35 n evidence for focused melt supply and lower crustal dike injection 1 we gathered seismic refraction and wide angle reflection data from several active source experiments that occurred along the mid atlantic ridge near 35 n and constructed three dimensional anisotropic tomographic images of the crust and upper mantle velocity structure and crustal thickness the tomographic images reveal anomalously thick crust 8 9 km and a low velocity bull s eye from 4 to 10 km depth beneath the center of the ridge segment the velocity anomaly is indicative of high temperatures and a small amount of melt up to 5 and likely represents the current magma plumbing system for melts ascending from the mantle in addition at the segment center seismic anisotropy in the lower crust indicates that the crust is composed of partially molten dikes that are surrounded by regions of hot rock with little or no melt fraction our results indicate that mantle melts are focused at mantle depths to the segment center and that melt is delivered to the crust via dikes in the lower crust our results also indicate that the segment ends are colder receive a reduced magma supply and undergo significantly greater tectonic stretching than the segment center

coal and gas outbursts in footwalls of reverse faults abstract instantaneous outbursts in coal mines involve catastrophic and forceful expulsion of coal and large quantities of gas creating a major mining hazard other researchers have studied relationships between coal outbursts and many geological factors such as coal seam thickness coal rank coal seam age current burial depth and distance to plutons with different conclusions for different coal fields and regions because of the complexity of the causative mechanisms coal outbursts nearly always have a close relationship to geologic structures the present research was designed to determine the location and characteristics of specific danger zones associated with reverse faults four coal mines in the pingdingshan coal field henan province china were investigated in detail coal and gas outbursts associated with reverse faults nearly always occurred in the footwalls additional data were also used from two mines in sichuan province and one in henan province the first important observation of the geological structural phenomena was that coal seams in the footwalls of the reverse faults underwent greater tectonic deformation than those in the hanging walls coal lost its banded character and was physically transformed into microstructurally altered forms categorized as cataclastic coal granular coal and mylonitic coal outbursts always occurred within a zone of this tectonically altered coal surrounding the fault second the zone of tectonic alteration in the footwalls extended farther from the fault than that in the hanging walls third where present in association with reverse faults zones of higher coalbed methane content nearly always occurred in the footwalls these three factors are the principal controls on coal and gas outbursts associated with reverse faults through their application coal safety experts can narrow the predicted outburst danger zone to a smaller area

inversion for elliptically anisotropic velocity using vsp reflection traveltimes this paper presents a traveltime inversion approach using the reflection traveltimes from offset vsp data to reconstruct the horizontal and vertical velocities for stratified anisotropic media the inverse problem is reduced to a set of linear equations and solved by the singular value decomposition svd technique the validity of this inversion scheme is verified using two sets of synthetic data simulated using a finite difference method one for an isotropic model and the other for an elliptically anisotropic model the inversion result demonstrates that our anisotropic velocity inversion scheme may be applied to both isotropic and anisotropic media the method is finally applied to a real offset vsp data set acquired in an oilfield in northwestern china

deformation metamorphism of bituminous and anthracite coals from china abstract tectonic displacement of coal seams in china has resulted in faulting parallel to coal bedding displacement along these faults caused significant comminution of the coal on the footwall contributing to various mining problems the worst of which is catastrophic failure or outbursting of the working face during mining the granular texture and mostly unconsolidated nature of the coal suggests that faulting occurred relatively late in the coalification sequence at a time of maximum tectonic stress coal samples taken on either side of the fault plane normal and deformed coal layers were obtained in an effort to establish what influence these tectonic stresses might have had on coal properties as well as what they might reveal about the influence of tectonic pressure on organic maturity sample sets were collected within coal beds from undisturbed and adjacent deformed layers including 21 bituminous samples from the pingdingshan coal field and nine anthracite samples from the jiaozuo coal field the tieshenggou coal mine of the yuxi coal field in henan province the beijing xishan coal field the baijiao mine of the furong coal field in sichuang province and the baisha coal field in hunan province china results from vitrinite reflectance proximate and ultimate analyses show some differences in reflectance hydrogen content and nitrogen content of anthracite coal no significant difference was found between volatile matter yields of normal and deformed coal specimens gc measurements of the saturated hydrocarbon fraction of chloroform extracts from bituminous coals showed that lower molecular weight carbon fragments were concentrated in the deformed samples therefore although changes in the gross chemical properties of the deformed coal were insignificant some modification of the chemical structure is seen to have occurred as a result of exposure to tectonic pressure

anisotropy in the oceanic lithosphere theory and observations from the ngendei seismic refraction experiment in the south west pacific summary p wave travel time data from a seismic refraction experiment on the 1983 ngendei expedition to the south pacific indicate anisotropy at two levels in the oceanic lithosphere in the upper mantle p wave velocities vary between 8 0 and 8 5 km s 1 with the fast direction at n30 e crustal anisotropy within layer 2 is characterized by azimuthal p wave velocity differences of about 0 2 0 4 km s 1 with the fast direction at n120 e orthogonal to the upper mantle anisotropy the observed anisotropy is consistent with a model in which aligned olivine crystals cause anisotropy in the upper mantle and aligned cracks within layer 2 cause anisotropy in the crust we examine a model of p wave propagation from a point source in an isotropic layer above an anisotropic half space and show that under these conditions anisotropy can cause seismic ray paths to deviate from the vertical plane connecting source and receiver this deviation has a negligible effect on seismic travel times but must be taken into account in modelling p wave polarization anomalies p wave polarization anomalies within the anisotropic half space are the sum of the particle motion deviation from the ray path and ray path deviation from the source receiver azimuth and are typically range dependent p wave polarization anomalies will also occur in the overlying isotropic layer purely as a result of the ray path deviation with greater deviations at long ranges a simple model of upper mantle anisotropy 6kms 1 crust above 8 0 8 6 km s 1 anisotropic mantle produces surface polarization anomalies of 1 5 3 5 in practice p wave polarization anomalies are difficult to observe because they are small compared to observed scatter in polarization data and are sensitive to differences in horizontal seismometer gain levels we examine the implications of anisotropic ray path deviations on travel times and p wave polarizations for the ngendei data as well as some previous marine seismic refraction experiments

the effects of temperature pressure and alteration on seismic properties of diabase dike rocks from dsdp odp hole 504b a 0 4 to 0 5 s 1 in situ vertical velocity gradient in the diabase dike section of dsdp odp hole 504b has been attributed to the effects of increasing pressure and downhole changes of mineralogy caused by hydrothermal alteration an analysis of velocities measured in diabase samples as a function of pressure shows that increasing pressure accounts for a gradient of at most 0 1 s 1 neither laboratory data nor a voigt reuss hill average velocity model in which vp pyx vp amph shows a significant variation of velocity with depth composition and pressure combined account for a gradient not greater than 0 2 s 1 these results indicate that either the elastic moduli of the amphibole minerals in these rocks are higher than those of the pyroxene phase vp amph vp pyx or the seismic properties of the dikes are affected by a network of thin large scale cracks

estimating range to a vocalizing fin whale using the timing and amplitude of multipath arrivals a semi automated method is described to range to vocalizing fin whales using the timing and amplitude of multipath arrivals measured on seafloor receivers calls are detected and multipath arrivals identified with a matched filter multipath times and relative amplitudes are predicted as a function of range by ray tracing because the direct and first water column multiple arrivals are not always observed different hypotheses for the observed arrival paths must be considered for two arrivals an amplitude threshold is used to determine if the first arrival is the direct path and if not the call is disregarded as distant when three or more arrivals are detected three hypotheses for the paths of arrivals are considered the solution is the hypothesis and range that minimizes the timing and optionally amplitude ratio or absolute amplitude residual the method is tested with data from two ocean bottom seismometers one on the juan de fuca ridge and the other in the cascadia basin solutions obtained by

physical properties and seismic structure of izu bonin mariana fore arc crust results from iodp expedition 352 and comparison with oceanic crust most of the well preserved ophiolite complexes are believed to form in supra subduction zone ssz settings we compare physical properties and seismic structure of ssz crust at the izu bonin mariana ibm fore arc with oceanic crust drilled at holes 504b and 1256d to evaluate the similarities of ssz and oceanic crust expedition 352 basement consists of fore arc basalt fab and boninite lavas and dikes p wave sonic log velocities are substantially lower for the ibm fore arc mean values 3 1 3 4 km s compared to holes 504b and 1256d mean values 5 0 5 2 km s at depths of 0 300 m below the sediment basement interface for similar porosities lower p wave sonic log velocities are observed at the ibm fore arc than at holes 504b and 1256d we use a theoretical asperity compression model to calculate the fractional area of asperity contact af across cracks af values are 0 021 0 025 at the ibm fore arc and 0 074 0 080 at holes 504b and 1256d for similar depth intervals 0 300 m within basement the af values indicate more open but not necessarily wider cracks in the ibm fore arc than for the oceanic crust at holes 504b and 1256d which is consistent with observations of fracturing and alteration at the expedition 352 sites seismic refraction data constrains a crustal thickness of 10 15 km along the ibm fore arc implications and inferences are that crust composing ophiolites formed at ssz settings could be thick and modified after accretion and these processes should be considered when using ophiolites as an analog for oceanic crust this article is protected by copyright all rights reserved

seismic anisotropy in the upper oceanic crust seismic anisotropy in the upper oceanic crust is observed in borehole data obtained at deep sea drilling project dsdp site 504b on dsdp leg 92 particle motion analysis of converted shear wave arrivals from explosive sources at various azimuths reveals a set of patterns which is indicative of hexagonally isotropic structure with a horizontal symmetry axis there are four diagnostic patterns 1 along symmetry axes where vertically polarized shear waves sv are generated but horizontally polarized shear waves sh are not generated the particle motions are purely vertical 2 for azimuths at which both sv and sh are generated and the sh velocity is significantly faster than sv a cruciform pattern with horizontal first motion is observed 3 for azimuths at which both are generated and the sv velocity is significantly faster than sh a cruciform pattern with vertical first motion is observed and 4 for azimuths at which both are generated and sv and sh velocities are similar elliptical particle motions are observed the shear wave particle motions and compressional wave travel times from a 2 km radius circle are consistent with an anisotropic model with hexagonal symmetry the compressional wave velocity has a two theta azimuthal variation between 4 0 and 5 0 km s the symmetry axis is horizontal with an azimuth of n20 w 10 the spreading direction at the site 6 m y age is north south the observed seismic anisotropy is most probably caused by the preferred orientation of large scale fractures and fissures in upper layer 2 which were created in the early stages of crustal development by near axis extensional processes and normal block faulting

passive off axis convection through the southern flank of the costa rica rift pore fluids are passively convecting through young oceanic sediments and crust around deep sea drilling project dsdp site 504 on the southern flank of the costa rica rift as inferred from a variety of geological geochemical and geothermal observations the presence of a fluid circulation system is supported by new data collected on ocean drilling program odp leg 111 and a predrilling survey cruise over the heavily sedimented 5 9 ma site during the latter elongated heat flow anomalies were mapped subparallel to structural strike with individual measurements of twice the regional mean value and strong lateral and vertical geochemical gradients were detected in pore waters squeezed from sediment cores also there is a strong correlation between heat flow bathymetry sediment thickness and inferred fluid velocities up through the sediments on an earlier dsdp leg an 8 bar underpressure was measured in the upper 200 m of basement beneath thick sediment cover although the forces which drive passive circulation are not well understood it has generally been thought that the length scale of heat flow variations provides a good indication of the depth of hydrothermal circulation within the oceanic crust this assumption was based on analytical and numerical analyses of relatively simple porous media models deep crustal convection had been inferred near site 504 based on the geometry of surface heat flow anomalies with a wavelength of 4 7 km but appears to be precluded by low crustal permeability as measured in dsdp hole 504b the widely varied geothermal and hydrogeological observations near site 504 are readily explained by a model which combines 1 basement relief 2 irregular sediment drape 3 largely conductive heat transfer through the sediments overlying the crust and 4 thermal and geochemical homogenization of pore fluids at the sediment basement interface which results from 5 topographically induced passive hydrothermal circulation with large aspect ratio convection cells this convection involves mainly the permeable upper 200 300 m of crust the deeper crust is not involved this convection is induced through a combination of buoyancy fluxes due to heating from below and topographic variations on the seafloor and at the basement sediment interface this model was designed to incorporate data from both near the sediment surface and deep within boreholes it is successful in duplicating numerous field observations

architecture of the volcanic layer from the costa rica rift constraints from core log integration 1 an understanding of the architecture of the ocean crust s volcanic layer is critical since this records the variations in volcanic activity during formation and subsequently controls the movement of hydrothermal fluids through this layer unfortunately core recovery in the majority of ocean drilling program odp deep sea drilling program basement holes is generally low and biased toward more massive lithologies consequently the identification and estimation of lithological diversity within the volcanic layer is only possible through the integration of core and wire line logging results wire line logging provides nearly continuous records of both physical and chemical properties of the borehole wall which can be used to identify the various volcanic sedimentary lithologies which then can be calibrated in regions of enhanced core recovery using this approach the architecture of the volcanics from odp hole 896a has been reconstructed this new model illustrates a hiatus in volcanism marked by a thick breccia horizon which probably marks the transition from axis to off axis volcanism

finite difference seismic modeling of axial magma chambers we tested the feasibility of using finite difference methods to model seismic propagation at 10 hz through a two dimensional representation of an axial magma chamber with a thin liquid lid our technique produces time series of displacement or pressure at seafloor receivers to mimic a seismic refraction experiment and snapshots of p and s energy propagation the results indicate that our implementation is stable for models with sharp velocity contrasts and complex geometries we observe a high energy downward traveling shear phase observable only with borehole receivers that would be useful in studying the nature and shape of magma chambers the ability of finite difference methods to model high order wave phenomena makes this method ideal for testing velocity models of spreading axes and for planning near axis drilling of the east pacific rise in order to optimize the benefits from shear wave imaging of sub axis structure

shear wave velocity structure of the oceanic lithosphere from ocean bottom seismometer studies summary a comprehensive interpretation of two shear wave refraction profiles recorded on ocean bottom seismometers provides velocity depth models for the oceanic crust and upper mantle the data were obtained as part of an investigation of lithospheric structure carried out in the nootka fault zone area off the west coast of canada one dimensional s wave velocity depth models are derived from the data by trial and error comparisons with wkbj synthetic seismograms the results of this study confirm crustal characteristics interpreted previously from the compressional wave data layers 3a and 2b in the upper crust are characterized by rapid increases in seismic velocities shear wave velocity increases from 2 2 to 3 6 km s l over an interval of 3 5 km although the exact nature of the shearwave velocity gradient is uncertain in contrast the shear velocity in layer 3a remains fairly constant with depth at values near 3 6km s no velocity discontinuity is observed at the moho instead the transition from crust to upper mantle is marked by the presence of a velocity gradient zone layer 3b the shear wave velocities in the upper mantle are 4 5 and 4 6 km s l in directions approximately parallel to the spreading ridge and parallel to the direction of spreading respectively this low degree of velocity anisotropy contrasts sharply with the 10 per cent anisotropic effect observed for the pn velocities combining the p wave interpretation with this s wave study enables evaluation of poisson s ratio as a function of depth except for low values of poisson s ratio found in the upper crust the seismic properties of the oceanic crust and upper mantle interpreted in this study agree remarkably well with those of laboratory measurements on ophiolite samples

the use of selected research methods to describe the pore space of dolomite from copper ore mine poland basic knowledge of the characteristics of copper bearing rock such as dolomite is essential to locate those areas of the deposit which have different structural and textural properties those regions can be important in terms of the assessment of the possibilities of gas accumulating in them as well as in terms of gasogeodynamic hazard to better understand those threats it is necessary to locate monitor and analyse those areas in detail this article characterises the structural and textural parameters of dolomite from the polkowice sieroszowice copper mine in poland the study involved five samples from various areas of the mine a number of research methods were selected reflected and transmitted light microscopy ms computer microtomography micro ct gas adsorption porosimetry lpna mercury porosimetry mip helium and quasi fluid pycnometry pycno he dryflo each of the methods examined a different scope of the pore size which enabled to achieve a full view of the porous nature of those rocks we determined their porosity open closed total surface area as well as mean size and volume of the pores also we studied the character and the pore size distribution from a few nm to a few mm comprehensive dolomite properties analyses showed that these rocks are characterised by high structural variability they have mesopores and macropores but few micropores the analyses presented in this paper are determined by a large petrographic diversity of the deposits containing dolomite this article is an example of a comprehensive approach to the rock analysis in copper ore mines

age variations of oceanic crust poisson s ratio inversion and a porosity evolution model porosity in the oceanic crust is one of the most important factors influencing measured seismic velocities porosity is particularly important in the uppermost young crust where rapid variations in velocities with depth and crustal age are observed knowledge of the concentration and aspect ratios of inferred crack populations can be improved considerably if estimates of poisson s ratio are available from observations of compressional and shear seismic velocities p and s in this paper i present a joint seismic waveform inversion for p and s velocities are found while maximizing or minimizing poisson s ratio using a hypothesis testing mechanism i apply this method to ocean bottom hydrophone data in 140 ma atlantic crust the resulting solution corridor agrees with laboratory measurements without the low poisson s ratio anomalies at depths of 0 8 1 5 km found by spudich and orcutt 1980 and au and clowes 1984 on younger 15 ma pacific crust compiling other published p and s solutions an age dependent pattern emerges none of the solutions for crust older than 60 ma display the poisson s ratio anomaly i propose a simple crustal evolution model using thin and thick cracks to explain these observations thin cracks preferentially close at shallow depths in the crust producing the localized poisson s ratio anomaly sealing of all cracks by hydrothermal deposits as the crust ages restores the seismic velocities to consistency with laboratory measurements this model is consistent with similar models of crack populations and their evolution from shallow measurements