spatial sensitivity functions for rapid numerical simulation of borehole sonic measurements in vertical wells abstractborehole sonic measurements are routinely used to measure dynamic elastic properties of rock formations surrounding a wellbore however shoulder bed effects mud filtrate invasion and near wellbore damage often influence the measurements and bias the interpretations inversion based methods can reduce the influence of complex geometric conditions in the estimation of formation properties from sonic logs but they require fast forward modeling algorithms we have developed new spatial sensitivity functions for rapid modeling of borehole sonic measurements which were equivalent to the green s functions of borehole modal wave measurements an adaptive finite element fe method was used to calculate 1d axial vertical and 2d axial radial sensitivity functions which combined the dependence of sonic logs on the wave mode frequency formation properties and logging instrument geometry semianalytical formulations were also used to calculate the sensitivity functions which involved modeling the

frequency domain finite element simulations of 2d sonic wireline borehole measurements acquired in fractured and thinly bedded formations we simulated wireline borehole sonic waveforms to appraise modal frequency dispersions across fractured and thinly bedded formations simulations included monopole and dipole sources of excitation and explicitly took into account the borehole the mandrel tool and casing whenever present calculations were performed in the frequency domain with a highly accurate finite element method that automatically generates optimal grids for each problem frequency combination the method guarantees solutions at significantly reduced computational time with relative energy errors below 0 5 even in the presence of singularities in the solution that can originate from complex geometries high material contrasts and simultaneous presence of large and fine structures such a high accuracy in the simulation of sonic waveforms is necessary to accurately quantify the effects of fractures and thin beds on acoustic logs simulations indicate that fractures mainly influence propagation modes related to the formation shear velocity in slow thinly bedded formations effective properties are similar to those of average layer properties as predicted by the reuss lower bound however presence of intralayer interfaces gives rise to multiple reflections that can deleteriously affect the estimation of elastic properties with dispersion processing casing effectively functions as a low pass frequency filter of sonic waveforms significantly distorting the original borehole modes hence the estimation of elastic properties

the interaction of tube waves with borehole fractures part ii analytical models we solve numerically the equations of elastodynamics that govern the propagation of waves in a fluid filled borehole intersected by one or more fluid filled fractures thus extending earlier work on formations presumed to be rigid the model is axis symmetric and it allows for arbitrary radial and vertical variations in the elastic properties of the formation we have developed a novel gridding scheme that takes advantage of the assumed thinness of the fracture compared to any relevant wavelength this technique obviates the necessity for a mesh within the fracture itself and thereby saves computational time and memory we illustrate our technique by means of examples with single and double fractures with or without washouts as well as examples with variable width fractures we compute the frequency dependent reflection and transmission coefficients of tube waves by fractures from the time waveforms generated by the present method major conclusions of this work are 1 reflections of tube waves by fr

frequency domain simulation of logging while drilling borehole sonic waveforms numerical simulation of sonic logging while drilling lwd borehole measurements is challenging because of significant wave propagation effects due to the massive drilling collar occupying a large portion of the borehole in addition the internal structure of the lwd tool can have a significant impact on the measured dispersions of stoneley and quadrupole modes the collar is typically constructed with a set of inner periodic grooves which act as a mechanical filter to attenuate undesirable collar modes reliable numerical simulation and interpretation of lwd sonic waveforms requires that all features and dimensions of the drilling collar be included in the simulation model furthermore the presence of the drilling collar can prompt numerical instabilities due to backward propagating modes in the perfectly matched layer pml commonly used to truncate the computational domain this problem can be circumvented with the implementation of artificial viscoelastic attenuation in the collar whenever the simulations are intended to reproduce only wave propagation within the surrounding rock formations in addition reliable modeling of borehole wave propagation in the presence of high contrasts in material properties and the internal structure of the lwd collar requires a numerical method capable of accurately and stably resolving all spectral scales present in the model we implemented an automatic hp adaptive finite element method in the frequency domain combined with a pml technique to simulate lwd sonic logging measurements examples of the application verified the accuracy and reliability of the simulated borehole and formation propagation modes in the presence of casing and internal structures in the lwd collar the presence of steel casing and quality of casing formation bond significantly influence the propagation modes excited in a borehole however it is still possible to estimate the formation shear slowness using monopole and quadrupole sources regardless of the quality of cement bond in fast formations assessment of the formation compressional slowness was significantly impeded by the strong pipe mode estimation of formation shear slowness in slow formations is practically impossible due to the presence of casing and a strong annulus mode when the quality of casing bond is poor

high ground water levels and dune development in central australia tl dates from gypsum and quartz dunes around lake lewis napperby northern territory abstract an episode of high lake levels prior to the last maximum glaciation has been identified at many localities in wastern australia similar events have been recognized at playa lakes in central australia where gypsum dunes along playa margins formed during one or more episodes of high groundwater discharge with a large influx of calcium sulphate at lake lewis exposures at two islands show similar sediment sequences three pedogenic gypcrete layers interbedded with aeolian quartz and gypsum sand horizons form three units within gypsum dunes up to 7 m high the lowest unit has cliffed edges buried by the upper units indicating a significant time break four tl dates coarse grained quartz show that this lowest unit was deposited at or before 70 80 ka the middle unit of mixed gypsum and quartz sand capped by gypcrete represents the major phase of gypsum dune formation and 6 tl dates range from 33 to 46 ka with overlapping error bars these are slightly younger but statistically similar to tl dates from 39 to 59 ka of the shoreline gypsum dune at lake amadeus in the same region the top unit of the two islands up to 1 m thick has not yet been well dated one date is inconsistent with the well dated middle layer below possibly because of incomplete bleaching and has been rejected the other date 17 5 ka is much younger which possibly indicates a minor and local reactivation of old gypsum sediments at the lake margin there are quartz dunes overlying the gypsum dunes and a buried aeolian quartz sand layer occurs in a lake margin terrace these represent reactivation of the regional quartz dune field after the major gypsum dune formation two consistent tl dates 21 4 ka and 23 6 ka indicate that regional dunes were active at about the time of the last glacial maximum

initiation of the australian summer monsoon 14 000 years ago abstract definitive evidence for the onset of monsoon moisture in northern australia following the arid phase of the last glacial maximum has been elusive in large part due to the lack of long continuous sediment records here we report new evidence from two separate drainage basins in the kimberley region of monsoonal northwestern australia at both sites radiocarbon ages on basal biogenic materials in flood deposits indicate stream flow as high or higher than maximum historic levels began about 14 cal ka bp these results suggest that the australian summer monsoon was fully active several millennia earlier than previously expected the early onset of the monsoon and the widespread evidence for diminished monsoon activity in the late holocene cannot be reconciled with primary forcing of the australian monsoon by changes in low latitude summer insolation over australia

the timing of linear dune activity in the strzelecki and tirari deserts australia abstract linear dunes occupy more than one third of the australian continent but the timing of their formation is poorly understood in this study we collected 82 samples from 26 sites across the strzelecki and tirari deserts in the driest part of central australia to provide an optically stimulated luminescence chronology for these dunefields the dunes preserve up to four stratigraphic horizons bounded by palaeosols which represent evidence for multiple periods of reactivation punctuated by episodes of increased environmental stability dune activity took place in episodes around 73 66 35 32 22 18 and 14 10 ka intermittent partial mobilisation persisted at other times throughout the last 75 ka and dune activity appears to have intensified during the late holocene dune construction occurred when sediment was available for aeolian transport in the strzelecki and tirari deserts this coincided with cold arid conditions during marine isotope stage mis 4 late mis 3 and mis 2 and the warm dry climates of the late pleistocene holocene transition period and late holocene localised influxes of sediment on active floodplains and lake floors during the relatively more humid periods of mis 5 also resulted in dune formation the timing of widespread dune reactivation coincided with glaciation in southeastern australia along with cooler temperatures in the adjacent oceans and antarctica

aeolian landscapes in central australia gypsiferous and quartz dune environments from lake amadeus there are two different dune systems in central australia regional quartz dunefields and transverse gypsiferous dunes associated with playa lakes these two systems especially gypsiferous dunes at lake amadeus the largest playa in central australia provide a sedimentary geomorphological and environmental history of the region during the late quaternary the gypsifierous dunes consist of a surficial gypcrete overlying an aeolian sediment sequence below a mixture of gypsum sand and quartz sand no clay pellets have been found in the dune sequence in significant contrast to the gypsiferous clay dunes in other parts of australia three possible models of the environmental controls of gypsiferous dune formation are discussed the most plausible one suggests simultaneous gypsum precipitation and deflation sandsized gypsum was precipitated in a groundwater seepage zone around the playa margin during seasonally high water tables and these crystals were deflated onto land during dry intervals forming the marginal gypsiferous dunes these processes require conditions of high regional water table strong climatic seasonality and probably a windier and overall wetter climate at least two separate gypsiferous duneforming episodes can be recognized the age of formation of the younger one has been dated by thermoluminescence at 44 54 ka the gypcrete crust capping the dunes is characterized by intergrown microcrystalline gypsum crystals showing evidence of leaching dissolution and recrystallization it is interpreted as a pedogenic product formed during a stable period after accumulation of the gypsiferous dune after the construction of the younger gypsiferous dune there was a major episode of activation of regional quartz dunefields which formed thick quartz sand mantles overlying gypsiferous dunes on both playa margins and the dune islands within the playa an equivalent aeolian sand layer was deposited within the playa soil structures in this unit indicate that the sand sheet over the playa was later colonized by vegetation activation of the regional dunefields suggests a major period of dry climate which although not dated may correlate with the last glacial maximum identified as a period of maximum aridity from 25 to 18 ka at other sites in australia

inversion based interpretation of borehole sonic measurements using semianalytical spatial sensitivity functions abstractone of the major challenges in the interpretation of sonic logs is the presence of bed boundary shoulder bed effects which can decrease the accuracy of the estimated elastic properties due to spatial averaging caused by the sonic receiver array across thin beds a reliable approach to account for shoulder bed effects in the interpretation of sonic logs is the implementation of forward modeling and inversion techniques we have developed an efficient inversion based interpretation method for the estimation of in situ rock formation elastic properties from sonic logs acquired in vertical wells the method uses semianalytical spatial sensitivity functions for fast forward modeling of sonic slowness logs and estimates formation elastic properties by iteratively matching the available logs with numerical simulations due to the intrinsic vertical resolution limits of sonic logs the inversion method requires predefined bed boundaries using other high resolution well logs we first developed a workfl

formation of analcime and moganite at lake lewis central australia significance of groundwater evolution in diagenesis abstract lake lewis basin in the northern territory is a hydrologically closed intermontane basin in which clay rich palaeolacustrine sediments are infused with highly evolved brines acquisition of dissolved silica and sodium by groundwaters from the proterozoic arunta granite gneiss terrain and from arkosic alluvium and weathering products sourced from these bedrocks is important to diagenesis at the salt lake supersaturation of groundwaters with respect to sio 2 and the availability of catalytic ions coupled with high rates of evaporation favour neoformation of opal and chalcedonic silica from solution near the groundwater discharge zone moganite a distinctive silica polymorph that is commonly associated with evaporites has been identified amongst precipitated silica species down gradient from the zone of silica precipitation beneath the playa brines are sio 2 deficient long periods of weathering and diagenesis of lacustrine clays immersed in semi stagnant brines has transformed detrital minerals into substantial proportions of amorphous or gelatinous aluminosilicate material aluminosilicate phases have become metastable in the presence of evolved sio 2 deficient interstitial brine analcime a sodium bearing zeolite group mineral is stable in the presence of the na rich brine and is crystallizing authigenically below the watertable from the amorphous aluminosilicate material in the lacustrine sediment no volcanic rocks or pyroclastic sediments are involved in this diagenetic occurrence of analcime the appearance of analcime in the evolution of the salt lake system probably commenced within the last 100 000 years and is occurring in the present day hydrologic and climatic regime of this arid zone area

sticky dunes in a wet desert formation stabilisation and modification of the australian desert dunefields independent dating evidence and dune morphology indicate great stability of the australian dunefields most dunefields have seen only minor superficial modification since they were formed up to 1 million years ago despite quite large changes in climate conditions this stability may be partly due to the relatively dense vegetation cover on australian dunes under the marginally arid climate but new studies supported by many older observations suggest that sticky dunes where sand grains are bonded or cemented may form under a broader range of wind climates than widely thought and have greater resistance to reworking new mapping of the australian continental dunefields from satellite imagery shows a previously unrecognised diversity of dune morphologies dune orientation continuity connectedness crest planform crest sharpness spacing and setting all show patterns of variation over the continent these are consistent with the overall low sand supply and variable wind climate that contribute to the dominance of longitudinal dunes but also with only superficial modification of the dunes after their initial formation the longevity of the dunes is likely also partly due to the stabilisation of dune sand by pedogenesis the bonding of sand by pedogenic calcium carbonate gypsum silica and translocated clays the extremely low mobility of the sand dunes has led to preservation of dunes of great age with stacked accretionary units and multiple palaeosols

fracture evaluation using reflected stoneley wave arrivals we use the low frequency reflected stoneley wave mode to locate permeable fractures intersecting a borehole and to estimate their effective apertures assuming a model in which the average aperture of the fracture is roughly constant theoretical work relates the magnitude of the stoneley wave reflectivity to an effective fracture width we treat both the case of a horizontal fracture and the case of a fracture crossing the borehole at an angle laboratory experiments verify the analytic solution for the case of a horizontal fracture full waveform array sonic data were also acquired in a wellbore with a long recording time 25 5 ms in order to capture the late stoneley wave arrivals the data processing involves computation of the stoneley wave reflectivity response using the measured direct and reflected stoneley wave arrivals a least squares fit to the arrival time of the reflected wave arrivals is used to estimate the locations of permeable fractures and the effective width of the fractures is esti

parabolic halite dunes on the salar de uyuni bolivia abstract palaeoenvironmental interpretation in desert and evaporitic sediments is often ambiguous consequently the recognition of primary sedimentary structures that indicate subaerial exposure or shallow marine conditions are therefore of crucial importance when interpreting marginal sedimentary evaporite facies parabolic halite dunes described here for the first time are observed on the salt crust of the largest salt flat in the world the salar de uyuni in bolivia the halite dunes are relatively small 0 7 1 3 m in width and length well defined lobate to hemicyclic parabolic dunes and are formed directly on the salt crust a model describing the formation and migration of the dunes in relation to brine level and climatic conditions is presented

spatial variability and hydrologic evolution of australian lake basins analogue for pleistocene hydrologic change and evaporite formation abstract processes evident in basins across hydrologic gradients from humid to arid regions today provide analogues for systems that may have operated in some individual basins during periods of past hydrologic change thus lakes in humid regions today with low salinity sedimentary facies and with morphology controlled largely by wave action may closely resemble past lacustral pluvial phases experienced in zones which today are relatively arid and saline conversely playas controlled today by groundwater and in some cases subject to defletion may resemble those arid and dune building stages experienced by basins in sub humid regions today as they underwent glacial age desiccation in any such sequence of spatial or temporal gradients from humid to more arid conditions the frequency of basin filling and drying modulates the rate of evaporitive loss of interstitial waters from the exposed basin surface this in turn controls the salinity evolution of the interstitial and local groundwaters the notion of closed systems for terminal lakes while satisfactory for analysis of surface water budgets is quite inapplicable to the evolution of brines where surfacewater groundwater exchange during the high salinity drying stages becomes a controlling factor affecting all salt budgets in these systems evaporites may only be preserved when the sub surface waters have evolved to salinites near saturation for the salts concerned if sub surface waters are frequently diluted by surfacewater groundwater exchange evaporites will not be preserved in the record even though the surface system appears hydrologically closed the process of interstitial water evolution which controls efflorescence deflation and salt weathering exerts a dominating influence on basin geomorphic expression as well as on hydrochemical fractionation

simulation and interpretation of sonic waveforms in high angle and horizontal wells abstractwe have developed a new method to process and interpret sonic waveforms acquired in high angle ha and horizontal hz wells in the vicinity of layer boundaries numerical simulations are examined for ha hz fluid filled borehole models in which sonic tools with monopole and dipole sources operate across a horizontal bed boundary in hard and soft formations simulations are performed with a 3d time domain finite difference method assuming a sonic logging tool in which each receiver station consists of eight azimuthal receivers instead of conventional processing for monopole and dipole waveforms which takes the average and difference respectively of waveforms acquired with azimuthal receivers we analyze the waveforms acquired individually by each azimuthal receiver for dipole sources we refer to the dispersive signals recorded by individual receivers as pseudo flexural waves and we first process them with a weighted spectral semblance method to obtain the frequency dispersion curves we t

multipole sources in boreholes penetrating anisotropic formations numerical and experimental results a 2 5 d velocity stress finite difference code is described that models acoustic propagation in a borehole penetrating a generally anisotropic formation the excitation may be a dilatation monopole or a point force dipole in an arbitrary direction the anisotropic formation is homogeneous along the axis of the borehole but may be inhomogeneous in the transverse plane the borehole cross section and location of the source in the borehole are arbitrary synthetic time domain waveforms are displayed for arrays of monopole and dipole receivers deployed along the borehole axis in both fast and slow anisotropic formations the specific anisotropy model employed for the numerical results is a transversely isotropic ti formation with its axis of symmetry inclined with respect to the borehole axis by an arbitrary angle flexural shear and stoneley wave slowness and attenuation estimates are extracted from the synthetic waveforms using a variant of prony s method for a range of borehole inclinations relative

finite difference synthetic acoustic logs the finite difference method is a powerful technique for studying the propagation of elastic waves in boreholes even for the simple case of an open borehole with vertical homogeneity the snapshot format of the method displays clearly the interaction between the borehole and the rock and the origin and evolution of phases we present an outline of the finite difference method applied to the acoustic logging problem including a boundary condition formulation for liquid solid cylindrical interfaces which is correct to second order in the space increments absorbing boundaries based on the formulations of reynolds 1978 and clayton and engquist 1977 were used to reduce reflections from the grid boundaries results for a vertically homogeneous sharp interface model are compared with the discrete wavenumber method and excellent agreement is obtained the technique is also demonstrated by considering sharp and continuous transitions damaged zones at the borehole wall and by considering the effects of wa

inversion based method to mitigate noise in borehole sonic logs abstracta major challenge in the interpretation of seismic measurements and sonic logs is the presence of deleterious noise that impacts the quality and reliability of the estimated seismic wavelets and seismic inversion products spatial averaging effects and borehole drilling damage can also bias the estimation of in situ stress and elastic properties from sonic logs we have developed an inversion based method to mitigate processing errors spatial averaging effects and borehole environmental noise on sonic logs which does not require arbitrary numerical filters effective medium theory models or time consuming waveform reprocessing the inversion based method estimates layer by layer slownesses via joint inversion of shear and compressional logs measured in a vertical well and it uses the estimated slownesses of the assumed horizontal layers to model noise mitigated sonic logs by making use of geometric and physical constraints for noise reduction implicit in the inversion based method we obtain

borehole wave propagation in three dimensions in this paper the three dimensional finite difference method is used to simulate borehole wave propagations in an isotropic as well as an anisotropic formation the finite difference results agree excellently with the analytic solutions of a point force source in the transversely isotropic medium the finite difference synthetics are also in very good agreement with the discrete wave number solutions for fluid filled borehole wave propagation the finite difference synthetics are compared with ultrasonic lab measurements in a scaled borehole model the borehole is drilled along the x axis in an orthorhombic phenolite solid both monopole and dipole logs agree well the observations of the shear wave splitting in the dipole logs are confirmed by the finite difference simulations the 3 d finite difference method is applied to the fluid filled borehole wave propagation in the tilted isotropic formation and in the orthorhombic phenolite formation in a borehole drilled along the z axis in a phenolite formati

revision of the late middle pleistocene stratigraphy and palaeoclimate in poland abstract the aim of this paper is a critical verification of the late middle pleistocene stratigraphy in poland in connection with stratigraphical schemes in the neighbouring countries although a terrestrial record of the middle pleistocene is almost complete in western and southern europe it is full of gaps in central and eastern europe eight key sites in poland and one in belarus were selected to compile environmental and palaeoclimate data for a revised stratigraphy from the termination of mis 12 to mis 6 and the chronology was based on correlation with oxygen isotope stratigraphy and osl ages the late middle pleistocene record starts with the holsteinian lake sequences some of which have been initiated already at the end of mis 12 persisted through mis 11 and terminated in mid mis 10 in the examined sites mis 11 is represented also by deposits of meandering rivers in a temperate climate in contrast there is no evidence for deposition in most of mis 10 and in mis 9 a warmer and presumably wet episode at the beginning of mis 8 lead to local erosion followed by development of small lakes modest ice sheet advance into central europe occurred in the second part of mis 8 as reflected by a till glaciofluvial and glaciolacustrine deposits and presumably also by permafrost and ice wedges in western belarus mis 7 seems to have been generally warm and wet when small lakes and meandering rivers could develop such mild conditions were interrupted with cool and probably drier episodes when braided rivers and limited pedogenesis occurred saalian ice sheet cover of central europe in mis 6 was widespread but even more importantly in this stage periglacial environments were dominant as shown by continuous permafrost intensive aeolian activity and development of ice wedges and involutions braided rivers developed during short wetter intervals

thermoluminescence dating study of quartz in aeolian sediments from southeastern australia abstract using 90 125 m grains of quartz extracted from sediments the equivalent dose absorbed since burial was derived by exposing aliquots of quartz to a sunlamp regenerating the tl growth curve by artificial irradiation and comparing the natural tl of the sample being dated as well as that of an adjacent surface sample to this curve the age can then be calculated once the dose rate is known the method is tested successfully on known age aeolian sediments retaining their original bedding but reveals some difficulties in trying to use tl to date aeolian sediments for which the original bedding has been destroyed anomalous fading of tl in quartz is reported this latter difficulty was circumvented by using a delay of several weeks between the irradiation and heating of the quartz

comparative uranium thorium and thermoluminescence dating of weathered quaternary alluvium in the tropics of northern australia abstract thermoluminescence tl age determinations of alluvial sediments in the tropics are evaluated by comparison with u th age determinations of pedogenic accumulations in the alluvium of the lower gilbert river a large fan delta in the wet dry tropics of northern queensland australia this study extends u th dating by applying it not only to calcretes but also to fe mn oxyhydroxide oxide accumulations while a direct correlation cannot be made between u th dates from secondary minerals and tl dates from the host sediments both sets of data show broad consistency in addition to providing a minima for acceptable tl ages u th dates are useful for determining the chronology of pedogenesis diagenesis they show that calcretes and ferricretes have formed under similar climatic conditions in the wet dry tropics of northern australia during the late pleistocene beneath about 5 12 m the gilbert fan delta consists of an extensive sand body older than 85 000 yr and probably about 120 000 yr in age representative of a period of major fluvial activity not repeated since this time above are muds and fine sandy muds that extend uninterrupted to the present surface except in the downstream fan where they are bisected by a thin unit of medium sand that tl dates at 40 000 50 000 yr b p a system of sandy distributary channels over the fan surface represents an early holocene fluvial phase probably more active than at present

late quaternary environmental changes in eastern and central australia and their climatic interpretation abstract the march of environmental changes in eastern australia from the last interglacial to the holocene is reviewed palynologic evidence suggests that forest composition fluctuated substantially during oxygen isotope oi substages 5a to 5e but physical evidence for this period is as yet poorly defined during oi stage 3 and possibly part of 4 inland lakes were larger and the hydrologic balance was wetter than at any time since conditions were coldest and dune activity was maximal throughout the continent during oi stage 2 but driest conditions as judged from lake levels occurred at the termination of the pleistocene when temperatures were rising rapidly lags between changes of boundary conditions and responses of natural systems are clearly seen in holocene behaviour of coasts and reefs and the effects of extreme events rather than climatic changes are seen in some sedimentary sequences establishment of the chronology of deposits older than about 35 ka and forging links between the terrestrial and deep sea records are major challenges for future research

aeolian fluvial interaction and climate change source bordering dune development over the past 100 ka on cooper creek central australia abstract this study provides an interpretation of interrelated quaternary fluvial and aeolian activity related to climate change on cooper creek in the lake eyre basin in southwestern queensland central australia the extensive muddy floodplain is characterised by buried sandy palaeochannels now almost entirely invisible but stratigraphically connected to source bordering dunes that emerge as distinctive sandy islands through the floodplain surface luminescence dating has identified pronounced periods of fluvial activity represented by abundant sandy alluvium from marine isotope stages mis 8 3 while all these sandy fluvial episodes on cooper creek were much more powerful than anything subsequent they appear to be ranked in order of declining activity mis 8 6 saw reworking of almost the entire floodplain whereas subsequent phases of reworking were far less extensive source bordering dunes were derived from active sandy channels in late mis 5 85 80 ka and mid mis 3 50 40 ka after 40 ka sand channel activity largely ceased and the floodplains and channels were inundated with mud isolating the dunes as emergent features although aeolian reworking of the upper parts of some dunes has continued to the present they show remarkable resilience having survived without appreciable migration for at least 40 ka whilst the channels once determined the location of source bordering dunes in an interesting role reversal the remnant dunes now determine the position of many contemporary flood channels and waterholes by deflection and confinement of overbank flows

multipole borehole acoustic waveforms synthetic logs with beds and borehole washouts we give a two dimensional 2 d velocity stress finite difference vs fd formulation for the multipole borehole acoustic logging problem irregular boreholes horizontal bedding and axially varying alteration are encompassed by the model excellent agreement is obtained with transform techniques for constant radius boreholes and homogeneous formations the method is used to generate waveforms for many positions of transmitter and receiver array relative to beds and washouts as in logging first motion and semblance techniques are applied to the waveforms to extract formation slownesses resulting in synthetic multipole logs which demonstrate the response of ideal multipole logging tools and signal processing to several environmental effects for monopole compressional first motion logs residual slowness errors remain at borehole washouts after borehole compensation these errors increase as the effective measure point in the waveforms is moved back in time from true first motion admitting greater in

gypsum ground a new occurrence of gypsum sediment in playas of central australia abstract there are many playas dry salt lakes in arid central australia regional rainfall about 250 mm y and pan evaporation around 3000 mm y highly soluble salts such as halite only appear as a thin several centimetres thick white ephemeral efflorescent crust on the dry surface gypsum is the major evaporite precipitating both at present and preserved in sediment sequences one type of gypsum deposit forms a distinctive surface feature which is here termed gypsum ground it consists of a thick up to 80 cm gypsum zone which rises from the surrounding smooth white playa surface and is overlain by a heaved brown crust the gypsum zone with an average gypsum content above 60 consists of pure gypsum sublayers and interlayered clastic bands of sandy clay the gypsum crystals are highly corroded especially in the direction parallel to the c axis and on the upper sides where illuviated clay has accumulated in corrosion hollows overgrowth parallel to the a and b axes is very common forming highly discoidal habits these secondary changes corrosion and overgrowth are well developed in the vadose zone and absent from crystals below the long term watertable depth around 40 cm these crystal characteristics indicate a rainwater leaching process at lake amadeus one of the largest playas 800 km2 of central australia such gypsum ground occupies 16 of the total area the gypsum ground is interpreted as an alteration of a pre existing gypsum deposit which probably extended across the whole playa before breaking down leaving a playa marginal terrace and several terrace islands within the gypsum ground this pre existing gypsum deposit preserved in the residual islands consists of pure pale sand sized lenticular crystals it is believed to have been deposited during an episode of high regional watertable causing active groundwater seepage and more frequent surface brine in the playa a later fall in watertable probably resulting from climatic change caused the degradation of the gypsum deposit by dissolution and leaching processes the common distribution of the gypsum ground and marginal terraces in the playas of central australia demonstrates the extent of this hydrologic and climatic history

thermoluminescence and excess 226ra decay dating of late quaternary fluvial sands east alligator river australia thermoluminescence tl dating was applied to seven samples of siliceous fluvial sands from the east alligator river of northern australia giving ages ranging from modern to 6000 yr b p two methods of estimating the equivalent dose ed total bleach and regenerative were applied to the 90 to 125 m quartz fraction of the samples in order to determine the reliability and internal consistency of the technique high resolution and spectroscopy were used to measure radionuclide contents these measurements revealed an excess 226ra activity compared with 230th this excess decreased with depth and was used directly to derive mean sedimentation rates and thus sediment ages both this method and one 14c date confirmed the validity of the tl values which increased systematically with depth and were consistent with site stratigraphy tl was of limited use in the dating of these late holocene deposits because of age uncertainties of 500 to 1600 yr resulting from a significant residual ed this residual probably resulted from incomplete bleaching during reworking upstream of the sampling site for pleistocene deposits the residual ed will be less significant because of higher total eds and tl dates will be correspondingly more accurate