experimental model for a seismic landmine detection system a laboratory scale experimental model has been developed and tested for a system that uses artificially generated high frequency seismic waves in conjunction with a radar based noncontact displacement sensor to detect buried landmines the principle of operation of the system is to measure the transient displacement field very close to a mine location in this way the absorption and the geometrical spreading of the seismic waves have not reduced the effects of the mine by using a seismic excitation the system exploits the large difference between the elastic properties of a mine and the surrounding soil this difference causes seismic wave interactions in the vicinity of a mine to be quite distinctive and provides a method for imaging mines and distinguishing them from typical buried clutter images of a variety of simulated and inert anti tank and anti personnel mines have been formed using this system burial scenarios involving natural clutter rocks and sticks light surface vegetation localized burial effects and multiple mines in close proximity have been studied none of these scenarios appears to pose serious problems for detection performance

effects of contact line movement on the dissipation of waves in partially saturated rocks a model is presented to describe the attenuation of seismic waves in a partially saturated rock in particular we include the dissipation due to contact line movement in some cases this can dominate the dissipation mechanisms the model includes both dissipation due to viscosity and contact line movement this additional dissipation can increase the specific dissipation function by an order of magnitude this could account for some of the observed difference between theory and experiment an exact calculation of the dissipation due to contact line movement in an in viscid fluid is also presented

seismic velocities in fractured rocks an experimental verification of hudson s theory flow of fluids in many hydrocarbon reservoirs aquifers is enhanced by the presence of cracks and fractures these cracks could be detected by their effects on propagation of compressional and shear waves through the reservoir several theories including hudson s claim to predict the seismic effects of cracks although hudson s theory has already been used to calculate crack densities from seismic survey s the predictions of the theory have not yet been tested experimentally on rocks containing a known crack distribution this paper describes an experimental verification of the theory the rock used carrara marble was chosen for its uniformity and low porosity so that the effect of cracks would not be obscured by other influences cracks were induced by loading of laboratory specimens velocities of compressional and shear waves were measured by ultrasound at 0 85 mhz in dry and water saturated specimens at high and low effective pressures the cracks were then counted in polished sections of the specimens in dry specimens with both dry and saturated cracks hudson s theory overpredicted observed crack densities by a constant amount that is attributed to the observed value being systematically underestimated the theory made poor predictions for fully saturated specimens shear wave splitting caused by anisotropy duemore to both crystal and crack alignment was observed cracks were seen to follow grain boundaries rather than the direction of maximum compression due to loading the results demonstrate that hudson s theory may be used in some cases to determine crack and fracture densities from compressional and shear wave velocity data less

sediments and history of the rottnest shelf southwest australia a swell dominated non tropical carbonate margin abstract the rottnest shelf is a narrow wave dominated open shelf on the passive continental margin of southwest australia adjacent to a hinterland of low relief and sluggish drainage high physical energy low nutrients in cool subtropical waters and rapid postglacial transgression have limited carbonate productivity restricted grain types and reworked the transgressed surface to form only a thin given sufficient time the rottnest shelf could recover from drowning and form blanket like skeletal carbonates thin

scaling of sedimentation rates and drowning of reefs and carbonate platforms the rule that sedimentation rates decrease as the time interval increases has been examined by a procedure that avoids the mathematical problem of plotting a variable time against its inverse the test confirms that the scaling trend has real physical meaning in the time range of 10 1 108 yr sedimentation rates change roughly proportionally to the inverse of the square root of time this report proposes and evaluates the hypothesis that the growth potential of reefs and carbonate platforms follows the same scaling trend the trend implies that a growth of 104 m yr sustained over 103 yr an upper limit of reef growth in the holocene translates into 102 m yr sustained over 107 yr decrease of the growth potential with increasing time solves the paradox of drowned platforms if the decrease results largely from environmental factors and not simply from increase of the time fraction of subaerial exposure

acoustic measures of partial gas saturation in tight sandstones the acoustic properties of tight sandstones are very sensitive to partial gas water saturation measurements at 5 khz show that compressional velocity drops sharply with undersaturation extensional and shear losses at 5 khz are large in both fully and partially saturated sands the shear loss qs 1 declines linearly with increasing gas content the extensional loss qe 1 peaks at 0 80 water saturation q 1 is frequency dependent the attenuation is well explained by hydrodynamic relaxation in the grain contacts the measurements indicate velocity dispersions consistent with this mechanism ultrasonic velocities at 500 khz are roughly 10 25 higher than acoustic velocities at 5 khz the ultrasonic velocities exhibit a weak dependence on saturation frame moduli are unrelaxed at these frequencies

experimental measurements of seismic attenuation in microfractured sedimentary rock ultrasonic compressional and shear wave attenuation in water saturated carrara marble increase with increasing crack density and decreasing effective pressure between 0 4 and 1 0 mhz empirical linear relationships between 1 q and crack density cd were found to be cd 1 96 or 0 63 x 1 q for compressional waves andcd 6 7 or 1 5 x 1 q for shear waves

spectrum analysis of seismic surface waves and its applications in seismic landmine detection in geophysics spectrum analysis of surface waves sasw refers to a noninvasive method for soil characterization however the term spectrum analysis can be used in a wider sense to mean a method for determining and identifying various modes of seismic surface waves and their properties such as velocity polarization etc surface waves travel along the free boundary of a medium and can be easily detected with a transducer placed on the free surface of the boundary a new method based on vector processing of space time data obtained from an array of triaxial sensors is proposed to produce high resolution multimodal spectra from surface waves then individual modes can be identified in the spectrum and reconstructed in the space time domain also reflected waves can be separated easily from forward waves in the spectrum domain this new sasw method can be used for detecting and locating landmines by analyzing the reflected waves for resonance processing examples are presented for numerically generated data experimental data collected in a laboratory setting and field data

stratigraphic sedimentology of tertiary cool water limestones se australia abstract mid tertiary limestones exposed as sea cliffs along the coast of victoria southeastern australia accumulated on the inner part of a cool water distally steepened ramp or open shelf all sediments are bioclastic dominated by particles of bryozoans echinoderms benthic foraminifers brachiopods and molluscs shallow grainy facies also contain coralline algae quartz and glauconite while deep muddy facies include ahermatypic corals ostracodes sponge spicules planktic foraminifers and terrigenous clays the fundamental depositional unit is a meter scale subtidal cycle or rhythm shallow shelf cycles are shallowing upward cross bedded or burrowed grainstone capped by a marine cemented hardground or quartz granule lag while mid shelf cycles are upward coarsening and thickening proximal to distal tempestites such cycles are interpreted to form by eustatically driven climatically controlled oscillations in abrasion wave base and swell wave base deep shelf rhythms are interbedded bryozoan marl and calcareous clay thought to be generated by climatically influenced fluctuations in carbonate productivity and terrigenous dilution lowstand systems tracts are depositional or erosional sequence boundaries are complex multigeneration mostly submarine surfaces condensed cycle boundaries that can be traced offshore into multiple omission surfaces and conformable contacts basinward facies shifts result in deposition of condensed lowstand wedges of stacked shallowing upward grainstone cycles transgressive systems tracts are thick bryozoan marl calcareous clay rhythms associated with facies backstepping and abrupt deepening such rhythmites typically grade stratigraphically upward into tempestite cycles highstand systems tracts are highly progradational wedge shaped rock bodies of shallowing upward grainy cycles these limestones provide the critical link of stratigraphy between modern cool water carbonate shelf deposits and the older mesozoic and paleozoic record of similar sediments

the environments of production and deposition of calcareous sediments on the shelf west of scotland the dominant calcareous organisms and sediment characteristics are described for eight different physical settings on the shelf west of scotland each having a different depth substrate and degree of hydrodynamic exposure the principal sites of carbonate production are on shallow rocky substrates where barnacles molluscs echinoderms and serpulids are the dominant calcareous organisms in sheltered shallow sandy zones molluscs echinoderms and benthic foraminiferans are the active producers though the sediments are commonly barnacle rich where tidal currents are enhanced between islands and the waves are suppressed calcareous red algae phymatolithon calcareum and mussel shells build localised banks in deep open shelf water molluscs are the major skeletal contributor to the sediment though on rocky sea beds bryozoans serpulids and echinoderms are important the major sites of deposition are where persistent hydro and aero dynamic conditions sweep together grains from active production sites e g sand ribbons or beaches and dunes adjacent to shallow rocky platforms or in sinks where the physiographic configuration favours the deposition and retention of locally produced sediment or sediment derived from suspension the well sorted cross bedded beach and dune sands commonly contain 75 caco3 in sheltered depressions bioturbated muds accumulate with up to 30 calcite silt which is probably the breakdown product of barnacles and benthic foraminiferans

barnacle dominated limestone with giant cross beds in a non tropical tide swept pliocene forearc seaway hawke s bay new zealand abstract pliocene non tropical widespread and locally thick up to 100 m limestones occur in hawke s bay eastern north island where they are intimately associated with very thick 5 km terrigenous dominated neogene sequences that formed in a tectonically active convergent margin setting the non tropical character of the limestones is shown unequivocally by 1 the complete dominance of skeletal calcarenites and calcirudites 2 the occurrence of oyster banks as the only in situ organic structures 3 the dominance of barnacles epifaunal molluscs bryozoans echinoderms foraminifers brachiopods and calcareous red algae as skeletal components and 4 the preponderance of calcite over aragonite in the mineralogy of the skeletal grains and cements the abundance of barnacle fragments in the limestones and the related exclusive occurrence of only one major organic association a barnacle epifaunal bivalve bryozoan assemblage is striking and unusual given the extent of the limestones pecten and oyster valves acted as substrates for barnacle attachment and their growth was promoted by strong tidal paleocurrents that swept the depositional setting a long 450 km narrow 30 50 km forearc basin seaway which formed between an actively deforming subduction complex to the east and an uplifting structural ridge to the west synsedimentary deformation promoted limestone formation on the margins of the seaway by creating current swept clastic free submarine ridges that acted as the sites of carbonate production tidal flows dispersed the carbonate constituents and organised them into a wide spectrum of tide influenced cross bedded and horizontal structures most spectacular are occurrences of giant tabular cross beds with sets 10 40 m thick and foreset dips of 7 36 some interpreted as the deposits of major sand bars on carbonate deltas marginal to the mouths of saddles traversing the rising antiforms and others analogous to modern linear sand ridges the small to large scale planar and trough cross beds and the horizontal and lenticular beds that are invariably associated with the giant cross beds and dominate most sections represent mainly the deposits of sand waves and sand sheets at inner to mid shelf depths in the seaway

sedimentation rates and growth potential of tropical cool water and mud mound carbonate systems carbonate fixation in the ocean proceeds in three basic modes abiotically biotically induced with organic trigger and biotically controlled where organisms determine timing location and composition of the product the three modes combine in a variety of ways to produce carbonate sediment when viewed on the scale of formations and global facies belts three benthic carbonate production systems or factories emerge 1 the tropical shallow water system dominated by biotically controlled mainly photoautotrophic and abiotic precipitates 2 the cool water system dominated by biotically controlled mainly heterotrophic precipitates and 3 the mud mound system dominated by abiotic and biotically induced mainly microbial precipitates the sedimentation rates of all three factories decrease as the time span of observation increases this scaling trend is not just an artifact of ratio correlation it is almost certainly related to the episodic pulsating nature of sedimentation the growth potential of the three systems can be estimated by drawing the envelope of the maximum observed rates of aggradation the tropical system shows the highest rates 10

seismic properties of rocks with distributions of small cracks summary hudson s theory of multiple scattering for the dynamic properties of cracked rock is applied to media containing cracks of varying orientation radius and aspect ratio angular variation of velocity and attenuation due to parallel and nearly parallel cracks and cracks with normals at a fixed angle to a given axis are presented anisotropy gradually decreases as the distribution of orientations is widened from exactly parallel to random increasing the aspect ratio of thin saturated cracks generally causes the elastic properties to approach those due to dry gas filled cracks for fluid filled cracks the line singularity where the birefringent shear waves have equal velocity moves closer to the symmetry axis as either the distribution of orientations is widened or the aspect ratio is increased calculated attenuations due to rayleigh scattering and viscous dissipation in crack filling fluids are many orders of magnitude less than reported attenuations in the earth viscous dissipation varies linearly with frequency and aspect ratio of thin fluid filled cracks a power law distribution of crack radii intended to represent that found in the earth s crust has no effect on the angular variations of seismic velocities or viscous attenuation but scattering attenuation is sensitive to the power law exponent lognormal distributions of aspect ratio also corresponding to observations in real rock affect velocity as well as attenuation velocities derived from the theory are compared with those measured in laboratory specimens with known geometries of cracks

mid dinantian waulsortian buildups in the dublin basin ireland abstract the sedimentary history and biostratigraphic setting of waulsortian carbonate buildups of the feltrim limestone formation late courceyan to early chadian within the dublin basin are described there is no unique precursor or successor facies to this formation and the massive waulsortian banks are composed predominantly of peloidal skeletal wackestones and lime mudstones with packstones near the tops of banks these banks form tabular bodies of moderate relief and are interbedded with thin shales and argillaceous crinoidal limestones of inter bank facies in the southwest of the basin inter bank facies are rare and the bank facies have abundant stromatactis cavities and uniquely at roselawn a fauna of rugose corals all buildups in the dublin basin have phase c and or d component assemblages of lees and miller 1985 and are interpreted as accumulating in moderately shallow water depths near or within the photic zone isopachs for the feltrim limestone formation show a ne sw trending axial depocentre where the waulsortian facies is in excess of 400 m thickness deposition appears to have taken place on this double sided ramp in a manner similar to the model of lees 1982 for belgium and southern britain soft sediment deformation such as large scale slumping shale injections and water escape structures not previously recorded from these rocks is widespread the upper surface of the feltrim limestone formation is fissured and displays a prominent erosion surface termination of waulsortian facies deposition and influx of terrigenous sediment was caused by rapid uplift attributed to chadian tectonism however eustatic sea level fall cannot be ruled out as a partial cause of the demise of the waulsortian

paleoreef maps evaluation of a comprehensive database on phanerozoic reefs to get a better understanding of controls on reef development through time we created a comprehensive database on phanerozoic reefs the database currently comprises 2470 reefs and contains information about geographic position paleoposition age reef type dimensions environmental setting paleontological and petrographical features and reservoir quality of each buildup reef data were analyzed in two qualitatively different ways the first type of analysis was by visualization of paleogeographic reef distribution maps five examples late devonian early permian late triassic late jurassic middle miocene are presented to show the potential of paleoreef maps for paleogeographic and paleoclimatological reconstructions the second type of analysis was a numerical processing of coded reef characteristics to realize major trends in reef evolution and properties of reef carbonates the analysis of paleolatitudinal reef distributions through time shows pronounced asymmetries in some time slices probably related to climatic asymmetries rather than controlled by plate tectonic evolution alone the dominance of particular reef builders through time suggests that there are seven cycles of phanerozoic reef development first curves for the phanerozoic distribution of bioerosion in reefs bathymetric setting and debris potential of reefs are presented the observed pattern in the temporal and spatial distribution of reefs with reservoir quality may assist in hydrocarbon exploration statistical tests on the dependencies of reefal reservoir quality suggest that large size high debris potential low paleolatitude high amount of marine aragonite cement and a platform shelf edge setting favor reservoir quality reefal reservoirs are significantly enhanced in times of high evaporite sedimentation elevated burial of organic carbon low oceanic crust production low atmospheric co2 content and cool paleoclimate as well as when they are present in aragonite oceans

non tropical carbonate deposits on the modern new zealand shelf abstract extensive ca 50 000 km 2 shallow marine platforms 70 caco 3 despite proximity to a tectonically active plate margin in these regions the terrigenous sediment supply is presently low and growth of epibenthos is fostered by firm substrates rock gravels shells seaweeds and the generally energetic nature and high nutrient levels of open shelf waters rapid transition into adjacent terrigenous dominated facies is characteristic irrespective of water depth the carbonates are coarse grained and fragmental carbonate mud is rare calcite dominates over aragonite high mg calcite widespread off northern new zealand is rare in the south skeletal material is dominated by bryozoans and bivalve molluscs with significant local contributions from foraminifers barnacles calcareous red algae and echinoderms the name bryomol is suggested for this distinctive temperate region skeletal carbonate facies which can be usefully subdivided based on dominant zoarial growth forms of the bryozoan component known to be habitat related bioerosion is an important mechanism of skeletal fragmentation and degradation many grains especially aragontic bivalves are infested by endolithic borers and have low preservation potential ages of skeletal material in the surficial deposits range from more than 20 000 years b p to modern which is consistent with both low rates of carbonate production and sediment accumulation and the wide range in preservation state of grains some data suggest that the skeletal carbonates are dispersed and mixed mainly during infrequent movement of sand ribbons sand waves and sand sheets driven by storm assisted tidal flows tracts of modern palimpsest and relict carbonates can occur in juxtaposition the facies characteristics of the new zealand shelf carbonate deposits contrast significantly with those of the classical bahaman type carbonate model however they are similar to those reported from many other mid to high latitude carbonate shelves and afford good analogues for most onland occurrences of new zealand cenozoic limestones

estimating slowness dispersion from arrays of sonic logging waveforms acoustic wave propagation in a fluid filled borehole is affected by the type of rock which surrounds the hole more specifically the slowness dispersion of the various body wave and borehole modes depends to some extent on the properties of the rock we have developed a technique for estimating the dispersion relations from data acquired by full waveform digital sonic array well logging tools the technique is an extension of earlier work and is based on a variation of the well known prony method of exponential modeling to estimate the spatial wavenumbers at each temporal frequency this variation known as the forward backward method of linear prediction models the spatial propagation by purely real valued wavenumbers the prony exponential model is derived from the physics of borehole acoustics under the assumption that the formation does not vary in the axial or azimuthal dimensions across the aperture of the receiver array but can vary arbitrarily in the radial dimension the exponential model fits

petrogenesis of cenozoic temperate water calcarenites south australia a model for meteoric shallow burial diagenesis of shallow water calcite sediments abstract the gambler limestone is one of several extensive shallow water shelf carbonates of eocene to miocene age exposed along the southern margin of australia it is a muddy to grainy bryozoan calcarenite with accessory benthonic foraminifers and echinoids the sediments originally composed almost entirely of calcite or mg calcite have been in vadose and phreatic environments for over 10 ma yet are virtually unlithified the only cements of any consequence are epitaxial on echinoids numerous karst features dolines caves speleothems and surface karren attest to prolonged residence in the meteoric zone the gambler limestone is presently one of the best fresh water aquuifers in australia most flow occurs through intergranular pores in sediments with over 30 porosity cementation is by minor intergranular pressure solution which has developed under overburden of less than 100 m the overlying naracoorte limestone miocene a calcarenite of warmer water aspect contains numerous aragonite molds and is well cemented we propose that such cool water limestones are a better model for the meteoric diagenesis of calcite sediments of all ages than are aragonite rich tropical sediments it is probable that many similar early and middle paleozoic calcite limestones may have been in the meteoric zone for prolonged periods yet contain little or no petrographic or geochemical record of such exposure

origin of a cool water oligo miocene deep shelf limestone eucla platform southern australia the abrakurrie limestone is an areally extensive bryozoan rich unit within the eucla platform a tertiary carbonate shelf which caps the central part of the southern australian continental margin the onshore portion the topic of this study has been exposed since middle miocene time and lies beneath the nullarbor plain the rocks are fine sand to granule sized calcarenites composed of bryozoans bivalves benthic foraminifera and echinoids with lesser numbers of brachiopods solitary corals and serpulids they conspicuously lack significant numbers of planktonic foraminifera and coralline algae most bryozoan remains are from delicate branching cyclostomes although delicate branching robust branching foliose fenestrate multilaminar encrusting and free living cheilostomes are variably abundant in specific units the poorly lithified sequence is punctuated by well cemented layers with erosional tops which are interpreted as hardgrounds the limestone is interpreted as a cool water deep shelf deposit which accumulated in water depths generally greater than 50 m on the inner part of the eucla platform a model which involves deposition and cementation on a carbonate shelf swept by open ocean swells is proposed to explain the style of sedimentation the shelf is envisaged as partitioned by the depth of the zone of wave abrasion sediments were produced throughout but accumulated only below this depth when the seafloor was above this depth it was an environment of cementation and erosion the vertical sequence correlates in a general way with the global sea level model for the mid cenozoic while accumulation rates for southern australian carbonates are similar to rates of cool water carbonate deposition elsewhere c 2 5 cm kyr 1 the rate of abrakurrie accumulation is much less suggesting that significant time periods are represented by the hardgrounds

the impact of saturation on seismic dispersion in shales laboratory measurements previous studies found a significant increase of acoustic velocities between seismic and ultrasonic frequencies seismic dispersion for shales which would have to be taken into account when comparing seismic or sonic field data with ultrasonic measurements in the laboratory we report on a series of experiments performed with partially saturated mancos shale and pierre shale i in which the influence of water saturation on acoustic velocities and seismic dispersion was investigated the experiments were carried out in a triaxial set up allowing for combined measurements of quasi static rock deformation ultrasonic velocities and dynamic elastic stiffness at seismic frequencies under deviatoric stresses prior to testing the rock samples were preconditioned in desiccators at different relative humidities for both shale types we present and discuss experimental results that demonstrate strong saturation and frequency dependence of dynamic young s moduli poisson s ratios and thomsen s anisotropy parame

shear wave velocity and attenuation from pulse echo studies of berea sandstone the pulse echo spectral ratio technique has been adapted to the determination of ultrasonic shear wave attenuation in sandstone at variable states of saturation and pressure the method can measure shear attenuation coefficients in the range 0 5 db cm to 8 db cm to within 0 5 db cm for the berea sandstone this range corresponds to values of the shear quality factor qs between 10 and 100 spectra of qs show that between 600 and 1100 khz qs decreases with frequency particularly at high pressures up to 70 mpa ultrasonic shear wave attenuation in a 90 water saturated sample was intermediate between that for dry samples and the relatively high attenuation in fully saturated rock strong pressure dependence is seen in the shear attenuation for all saturation states indicating a dominant role of dissipation mechanisms operating within open and compliant cracks substantial shear attenuation remains at the highest effective pressure applied to the saturated sample which may be due to a more global fluid flow loss mechanism scattering losses as described by weak scattering theories for compressional waves do not appear to be dominant at these frequencies

bioclastic carbonate sedimentation on a high latitude tide dominated shelf northeast orkney islands scotland abstract shell sands and gravels cover much of the shallow orkneys shelf at 59 n accumulating locally into 30 m high banks at rates up to 540 g m2 yr 67 cm l 000 yr overall the orkney shelf sedimentation rate is approximately 10 cm l 000 yr compared with 3 cm 1 000 yr for the entire scottish continental shelf major sandbanks are located off headlands that produce circulation loops in the tidal flow regional sandwave orientations reveal a clockwise transport of sediment around the islands probably resulting from storm wave reinforcement of the tidal asymmetry combined with the net inflow of atlantic water into the north sea carbonate production is high from the modiolus epifauna modiolus shell gravels commonly containing glycymeris pass laterally into comminuted shell sands within the euphotic zone down to 40 m dead shells are weakened by echinoid biting algal boring and limpet grazing boring by fungi and clionid sponges and grazing by chitons are common but not depth restricted sediments contain 89 95 carbonate on the level bottom offshore but 94 99 in sandwaves mean values for the main skeletal components are bivalves 46 barnacles 18 bryozoans 11 and serpulids 7 calcareous algal gravels occur in sheltered areas less than 20 m deep bryozoa typify lower energy offshore environments while more durable barnacle and serpulid debris is concentrated in sandwave fields the sediments are dominantly calcitic and have a high preservation potential

anatomy of a modern open ocean carbonate slope northern little bahama bank the open ocean carbonate slope north of little bahama bank consists of a relatively steep 4 upper slope between water depths of 200 and 900 m and a more gentle 1 2 lower slope between depths of 900 and 1300 m the upper slope is dissected by numerous small submarine canyons 50 150 m in relief that act as a line source for the downslope transport of coarse grained carbonate debris the lower slope is devoid of any well defined canyons but does contain numerous small 1 5 m hummocks of uncertain origin and numerous larger 5 40 m patchily distributed ahermatypic coral mounds sediments along the upper slope have prograded seaward during the cenozoic as a slope front fill seismic facies of fine grained peri platform ooze surface sediments show lateral gradation of both grain size and carbonate mineralogy with the fine fraction derived largely from the adjacent shallow water platform near surface sedimentary facies along the upper slope display a gradual downslope decrease in the degree of submarine cementation from well lithified hardgrounds to patchily cemented nodular ooze to unlithified peri platform ooze controlled by lateral variations in diagenetic potential and or winnowing by bottom currents submarine cementation stabilizes the upper part of the slope allowing upbuilding of the platform margin and controls the distribution of submarine slides as well as the headward extent of submarine canyons where unlithified sediments are heavily bioturbated and are locally undergoing dolomitization upper slope sediments are also conditioned eustatically resulting in vertical cyclic sequences of diagenetically unstable aragonite and magnesian calcite rich and stable calcite rich carbonates that may explain the well bedded nature of ancient peri platform ooze sequences lower slope sediments have prograded seaward during the cenozoic as a chaotic fill seismic facies of coarse grained carbonate turbidites and debris flow deposits with subordinate amounts of peri platform ooze coarse clasts are internally derived from fine grained upper slope sediments via incipient cementation submarine sliding and the generation of sediment gravity flows gravity flows bypass the upper slope via a multitude of canyons and are deposited along the lower slope as a wedge shaped apron of debris parallel to the adjacent shelf edge consisting of a complex spatial arrangement of localized turbidites and debris flow deposits a proximal apron facies of thick mud supported debris flow deposits plus thick coarse grained ta turbidites grades seaward into a distal apron facies of thinner grain supported debris flow deposits and thinner finer grained ta b turbidites with increasing proportions of peri platform ooze both the geomorphology and sedimentary facies relationships of the carbonate apron north of little bahama bank differ significantly from the classic submarine fan model as such a carbonate apron model offers an alternative to the fan model for palaeoenvironmental analysis of ancient open ocean carbonate slope sequences

seismic wave propagation in cracked porous media summary the movement of interstitial fluids within a cracked solid can have a significant effect on the properties of seismic waves of long wavelength propagating through the solid we consider three distinct mechanisms of wave induced fluid flow flow through connections between cracks in an otherwise non porous material fluid movement within partially saturated cracks and diffusion from the cracks into a porous matrix material in each case the cracks may be aligned or randomly oriented leading respectively to anisotropic or isotropic wave speeds and attenuation factors in general seismic velocities exhibit behaviour that is intermediate between that of empty cracks and that of isolated liquid filled cracks if fluid flow is significant in the range of frequencies for which considerable fluid flow occurs there is high attenuation and dispersion of seismic waves fluid flow may be on either a wavelength scale or a local scale depending on the model and whether the cracks are aligned or randomly oriented resulting in completely different effects on seismic wave propagation a numerical analysis shows that all models can have an effect over the exploration seismic frequency range

the world s most spectacular carbonate mud mounds middle devonian algerian sahara abstract in the southern ahnet basin of algeria a unique middle devonian underwater scenery is revealed which consists of 191 mud mounds 15 mud ridges and 2 mud atolls distributed over nine discrete areas of a shallow pelagic carbonate shelf all the buildups are totally exhumed thus perfectly exhibiting their original morphologies slope angles elevations and the pattern of the onlapping strata as well as their relationship to the surrounding off buildup sequences types of buildups include 1 small mud mounds 1 5 m high 10 20 m diameter 2 large mud mounds up to 40 m high up to 200 m diameter 3 lens shaped mounds 1 30 m thick 10 15 slopes 4 mud ridges up to 100 m high 150 8500 m long and 5 semicircular mud atolls 2500 2800 m long 20 30 m high the buil ups were constructed during a short time span in the early givetian part of the lower varcus zone probably not exceeding a few hundred thousand years sedimentation rates in coeval off buildup areas were at least 3 10 times lower the lithology of the buildups is a massive boundstone with a high proportion of irregular spar filled cavities stromatactis but only very scattered potential frame builders the carbonate mud is considered to be an in situ cyanobacterial precipitate although evidence of such is poor organic components of the buildups are not significantly different from those of the off buildup lithologies and include numerous crinoid ossicles some tabulate and solitary rugose corals brachiopods mollusc debris trilobites and very rare sponge spicule and bryozoans the total absence of stromatoporoids large tabulate and colonial rugose corals green algae and micritic envelopes around grains suggests growth of the buildups in the lower part of the photic zone 100 200 m they are onlapped by a discontinuous sequence a few meters thick of skeletal packstones and grainstones and locally orthoceratid floatstones that were deposited later in the givetian a widespread rise of sea level in the early frasnian buried both buildups and adjacent areas under shales and siltstones postdepositional alteration of the mud buildups includes 1 formation of stromatactis cavities probably by dewatering and internal erosion 2 cementation of stromatactis cavities and interparticle pore spaces by early high mg calcite and late blocky low mg calcite 3 neomorphic transformation of mg calcitic mud and aragonitic skeletal debris 4 occasional formation of neptunian dikes 5 slight erosion of buildup surfaces 6 compaction and slumping of onlapping capping beds and 7 local dolomitization the formation and growth of the mud build ups is envisaged as a sequence of stages starting with patchy coral thickets evolving into small mud mounds that coalesced into larger mounds by continuous vertical and lateral growth closely spaced mounds amalgamated into mud ridges and mud atolls regular nw se and n s patterns of some mound rows and ridges show a relationship of buildup formation to rejuvenated late precambrian tectonic lineaments which reappear in the hercynian orogeny the isotopic signature of matrix and early ements of mound rocks however does not support the idea of fluid migration and hydrothermal co2 supply during buildup growth

squirt flow in fully saturated rocks we estimate velocity frequency dispersion and attenuation in fully saturated rocks by employing the squirt flow mechanism of solid fluid interaction in this model pore fluid is squeezed from thin soft cracks into the surrounding large pores information about the compliance of these soft cracks at low confining pressures is extracted from high pressure velocity data the frequency dependence of squirt induced pressure in the soft cracks is linked with the porosity and permeability of the soft pore space and the characteristic squirt flow length these unknown parameters are combined into one expression that is assumed to be a fundamental rock property that does not depend on frequency the appropriate value of this expression for a given rock can be found by matching our theoretical predictions with the experimental measurements of attenuation or velocity the low frequency velocity limits as given by our model are identical to those predicted by gassmann s formula the high frequency limits may sign

dynamic poroelasticity a unified model with the squirt and the biot mechanisms the velocities and attenuation of seismic and acoustic waves in rocks with fluids are affected by the two most important modes of fluid solid interaction 1 the biot mechanism where the fluid is forced to participate in the solid s motion by viscous friction and inertial coupling and 2 the squirt flow mechanism where the fluid is squeezed out of thin pores deformed by a passing wave traditionally both modes have been modeled separately with the biot mechanism treated in a macroscopic framework and the squirt flow examined at the individual pore level we offer a model which treats both mechanisms as coupled processes and relates p velocity and attenuation to macroscopic parameters the biot poroelastic constants porosity permeability fluid com pressibility and viscosity and a newly introduced microscale parameter a characteristic squirt flow length the latter is referred to as a fundamental rock property that can be determined experimentally we show that the squirt flow mechanism dominates the biot mechanism and is responsible for measured large velocity dispersion and attenuation values the model directly relates p velocity and attenuation to measurable rock and fluid properties therefore it allows one to realistically interpret velocity dispersion and or attenuation in terms of fluid properties changes e g viscosity during thermal enhanced oil recovery eor or to link seismic measurements to reservoir properties as an example of the latter transformation we relate permeability to attenuation and achieve good qualitative correlation with experimental data

preliminary steps toward formation of a generalized budget for cold water carbonates high latitude carbonate sediments were once considered isolated exceptions to the tropical rule they are now being studied throughout the world and preliminary descriptive models have been proposed constituents composition and distribution of cold water carbonate sediments have been described and they are diagnostically different from their tropical counterparts the rates of depositional and post depositional processes may differ as well the present study of the rates of gross production net production and diagenesis was undertaken at shell beach in the gulf of maine 43 45 n 69 45 w a shelly pocket beach surface and subsurface sediments were collected and analyzed for caco3 content assemblage and alteration population density of dominant organisms and the weight of skeletal material in typical adults were determined transport of sediment was observed gross production by dominant organisms was calculated to be 1600 g caco3 m2 y over the area of production net production while less exact was on the order of 13 130 g caco3 m2 y the difference between these is explained by strong destructive diagenesis dominantly biological and at a rate of 80 1300 g caco3 m3 y the large gap between gross and net production seems to be characteristic of cold water carbonate sediments as does the strongly destructive diagenesis the gap must be remembered when assessing fossil cold water populations

stromatolites associated with coralgal communities in holocene high energy reefs thick laminar to columnar stromatolitic growths encrust typical high energy coralgal reef communities in a french polynesian outer reef edge the reef communities accumulated during vertical growth from 5870 100 yr b p to the present at depths never exceeding 5 m below mean sea level oxygen and carbon isotope records and high calcification rates of stromatolite encrusted corals strongly suggest that for about the past 6000 yr the local environmental conditions were optimal for reef development accordingly stromatolites can grow in healthy holocene reefs because similar stromatolites have been recorded in other cenozoic reefs we suggest that microbial structures may have played a more prominent role in cenozoic reef growth than is currently recognized

growth history of stromatolites in a holocene fringing reef stocking island bahamas abstract a stromatolite and algal ridge reef complex 2 1 m thick fringes the east coast of stocking island exuma cays bahamas this reef was established on a pleistocene calcarenite terrace about 4500 yr b p stromatolites which occur in back reef and reef flat zones are up to 1 m thick and were constructed by cyanobacterial dominated communities study of the growth history of these stromatolites ranging in scope from facies analyses to details of microfabric construction presents new perspectives on stromatolite formation lithologies identified in eight cores from across the stocking island reef complex together with plots of 13 radiocarbon dates in relation to a bahamian sea level curve indicate that this reef began as an intertidal vermetid gastropod buildup subsequent flooding of the pleistocene terrace allowed the branching coralline alga neogoniolithon strictum to overgrow the vermetids and eventually form an emergent algal ridge about 1500 years ago shifting sands accumulated in the lee of this ridge and excluded most benthic communities and herbivores thereby promoting growth of cyanobacterial mats that formed stromatolite buildups with a decrease in wave energy over the last 500 years possibly due to the growth of offshore patch reefs the urchin echinometra lucunter coloni ed the algal ridge resultant bioerosion by this urchin destroyed the emergent part of the ridge and is now undercutting the seaward edge of the stromatolite buildups lamination in the stocking island stromatolites results from early lithificafion processes in cyanobacterial mats possibly in response to biogeochemical changes in the mats during hiatuses in sediment accretion these processes which create partially indurated laminae with a distinct microstructure involve precipitation of thin micrite crusts intense microboring along a surface below this crust micritization of sediment grains and precipitation of point contact cement between micritized grains introduction of turf algae to the cyanobacterial mat community disrupts formation of the lithified laminae thereby inhibiting stromatolite development

geomorphic and tectonic process rates effects of measured time interval analysis of the original numerical components of diverse geomorphic and tectonic process rates indicates that process rate is not independent of measured time interval this implies that direct comparison of geologic process rates measured over variable time intervals may not be valid and that conclusions based on those comparisons regarding changes in rates through geologic time may be misleading in some cases calculation of statistically derived scaling functions is one approach that can be used to account for this measurement interval bias and to improve comparisons of geologic process rate data when this approach is used a 10 4 or 10 5 yr variation in the measured time interval accounts for approximately one order of magnitude change in the measured process rate the magnitude of the correction can substantially affect conclusions of studies that compare uncorrected process rates calculated from variable time intervals