paleomagnetism of the upper ordovician juniata formation of the central appalachians revisited again two components of magnetization were isolated in the upper ordovician juniata formation sampled in the area of the pennsylvania salient the thermally distributed reversed polarity b component was most likely acquired during alleghenian deformation and although it is poorly grouped it is similar to other appalachian synfolding magnetizations the pre alleghenian age c magnetization is entirely of normal polarity and shows a difference in declinations between the mean magnetizations isolated on the northern and southern limbs of the salient of 24 23 this anomaly is consistent with the sense and magnitude of declination anomalies observed in pre alleghenian magnetizations isolated in other throughgoing appalachian red beds of silurian devonian and early carboniferous age the mean inclination of 44 7 suggests a paleolatitude of about 26 s for the central appalachians in the late ordovician this paleolatitude fits a trend of southward motion of north america from the ordovician to the early devonian followed by northward drift through the remainder of the paleozoic

impact of water saturation on seismoelectric transfer functions a laboratory study of coseismic phenomenon s u m m a r y seismic waves propagating in a porous medium under favourable conditions generate measurable electromagnetic fields due to electrokinetic effects it has been proposed following experimental and numerical studies that these so called seismoelectromagnetic couplings depend on pore fluid properties the theoretical frame describing these phenomena are based on the original biot s theory assuming that pores are fluid filled we study here the impact of a partially saturated medium on amplitudes of those seismoelectric couplings by comparing experimental data to an effective fluid model we have built a 1 m length scale experiment designed for imbibition and drainage of an homogeneous silica sand the experimental setup includes a seismic source accelerometers electric dipoles and capacitance probes in order to monitor seismic and seismoelectric fields during water saturation apparent velocities and frequency spectra in the kilohertz range are derived from seismic and electrical measurements during experiments in varying saturation conditions amplitudes of seismic and seismoelectric waves and their ratios i e transfer functions are discussed using a spectral analysis performed by continuous wavelet transform the experiments reveal that amplitude ratios of seismic to coseismic electric signals remain rather constant as a function of the water saturation in the s w 0 2 0 9 range consistently with theoretically predicted transfer functions

comment on palaeomagnetic results from volcanic rocks of the shelve inlier wales evidence for a wide late ordovician iapetus ocean in britain by c mccabe and j e t channell in a recent paper mccabe and channell 1 report a late ordovician palaeomagnetic pole from the shelve inlier wales and argue for a iapetus ocean of 30 degrees latitudinal width between laurentia and southern britain in late ordovician times they further suggest that southern britain lay in high latitudes adjacent to the northern margin of gondwana at this time unlike mccabe and channell we contend a primary llanvirn mid ordovician remanence age for the shelve volcanics additionally several important points regarding the palaeomagnetism of southern britain may have been overlooked by mccabe and channell in particular re examination of a previous palaeomagnetic study of the shelve inlier 2 reveals a dual polarity remanence which passes a fold test at 95 confidence and indicates temperature southerly palaeolatitudes of 32s 8

further paleomagnetic results from the builth wells ordovician inlier wales a characteristic magnetization has been isolated from 18 sites in middle ordovician igneous and sedimentary rocks from the builth inlier wales two conglomerate tests demonstrate that the characteristic magnetization in two of the volcanic units was acquired early perhaps during initial emplacement six sites from the northern and central parts of the inlier give a mean direction of d 153 8 i 69 8 k 45 8 95 10 that passes a regional fold test however 12 sites from the southern end of the inlier in and near the llanelwedd quarry yield a significantly different direction after correction for local tilt d 176 4 i 44 5 k 42 4 95 6 7 the llanelwedd quarry data are anomalous with respect to both results from the northern parts of the inlier and those obtained from approximately coeval rocks from the shelve inlier we have searched for an explanation for the discrepancy between the magnetization directions of the two areas and have now ruled out some possibilities including superimposed magnetization components at one of the two areas the llanelwedd quarry lies within a wedge of dominantly andesitic volcanic rocks that appear only locally and which are overlain by two unconformities that are also not present elsewhere in the inlier one possible explanation for the anomalous nature of the llanelwedd quarry magnetization is that the strata were not horizontal at the time of magnetization either due to early volcano tectonic tilting or primary dips on the flanks of a volcano another possibility is that the llanelwedd quarry rocks were extruded rapidly enough that the geocentric coaxial dipole field hypothesis cannot be applied results from cenozoic volcanic rocks from the western united states suggest that even highly anomalous fields can be preserved virtually unchanged over a number of separate flow events we currently regard this to be the most viable explanation for the anomalous nature of the llanelwedd quarry magnetization we conclude that the data from the 6 northern sites represent the best estimate of the middle ordovician axial dipole field for the inlier

modification of streaming potential by precipitation of calcite in a sand water system laboratory measurements in the ph range from 4 to 12 spontaneous potentials associated with volcanic activity are often interpreted by means of the electrokinetic potential which is usually positive in the flow direction i e zeta potential of the rock is negative the water rock interactions in hydrothermal zones alter the primary minerals leading to the formation of secondary minerals this work addresses the study of calcite precipitation in a sand composed of 98 quartz and 2 calcite using streaming potential measurements the precipitation of calcite as a secondary mineral phase inferred by high calcite saturation indices and by a fall in permeability has a significant effect on the electrokinetic behaviour leading to a significant reduction in the zeta potential in absolute value and even a change in sign the measured decrease in zeta potential from 16 mv to 27 4 mv takes place as the ph rises from 4 to 7 while it remains constant at 25 1 mv as the ph increases from 8 to 10 5 for ph higher than 10 5 calcite precipitates and is expected to coat the quartz surface the measured zeta potential vary from 17 to 8 mv for ph ranging from 10 6 to 11 7 depending on the amount of precipitated calcite indicated by the decrease in permeability the observed change in sign of the electrical surface potential rules out the usual qualitative interpretation of sp anomalies in order to determine fluid circulations even at ph lower than 9 if calcite is widely present as a secondary mineral phase since the electrical surface potential of calcite depends also on co2 partial pressure and ca2 therefore sp anomalies as measured in hydrothermal field without mineralogical analyses of hydrothermal deposits and without geochemical fluid survey should be interpreted with caution

seismic attenuation and velocity dispersion in heterogeneous partially saturated porous rocks summary using a numerical approach we explore wave induced fluid flow effects in partially saturated porous rocks in which the gas water saturation patterns are governed by mesoscopic heterogeneities associated with the dry frame properties the link between the dry frame properties and the gas saturation is defined by the assumption of capillary pressure equilibrium which in the presence of heterogeneity implies that neighbouring regions can exhibit different levels of saturation to determine the equivalent attenuation and phase velocity of the synthetic rock samples considered in this study we apply a numerical upscaling procedure which permits to take into account mesoscopic heterogeneities associated with the dry frame properties as well as spatially continuous variations of the pore fluid properties the multiscale nature of the fluid saturation is taken into account by locally computing the physical properties of an effective fluid which are then used for the larger scale simulations we consider two sets of numerical experiments to analyse such effects in heterogeneous partially saturated porous media where the saturation field is determined by variations in porosity and clay content respectively in both cases we also evaluate the seismic responses of corresponding binary patchy type saturation patterns our results indicate that significant attenuation and modest velocity dispersion effects take place in this kind of media for both binary patchy type and spatially continuous gas saturation patterns and in particular in the presence of relatively small amounts of gas the numerical experiments also show that the nature of the gas distribution patterns is a critical parameter controlling the seismic responses of these environments since attenuation and velocity dispersion effects are much more significant and occur over a broader saturation range for binary patchy type gas water distributions this analysis therefore suggests that the physical mechanisms governing partial saturation should be accounted for when analysing seismic data in a poroelastic framework in this context heterogeneities associated with the dry frame properties which do not play important roles in wave induced fluid flow processes per se should be taken into account since they may determine the kind of gas distribution pattern taking place in the porous rock

subtraction of powerline harmonics from geophysical records harmonic noise generated by power lines and electric railways has plagued geophysicists for decades the noise occurs as electric and magnetic fields at the fundamental frequency of power transmission typically 60 hz in north america and its harmonics it may be recorded directly during time domain measurements of electric and magnetic felds or indirectly by geophone cables during the acquisition of seismic data

vertical seismoelectric profiling in a borehole penetrating glaciofluvial sediments 1 seismoelectric signals have been measured as a function of depth in a borehole penetrating glaciofluvial sands silts and glacial till using a broadband surface seismic source and a downhole electrode array transient electric field pulses with amplitudes of 1 to 4 v m accompanied the arrival of seismic p waves at the electrodes but no simultaneous interfacial signals were observed above the noise floor of approximately 0 2 v m the co seismic effect was strongest in a sand and gravel layer where its amplitude is consistent with the predictions of a simplified theoretical model normalization of the amplitude logs by measurements of seismic particle velocity and electrical conductivity enhanced their sensitivity to changes in lithology and porosity the results of this experiment suggest that co seismic seismoelectric effects show potential as a porosity permeability logging tool in the borehole environment

active seismoelectric exploration of glaciers 1 repeatable and strong seismoelectric signals were recorded on glacier de tsanfleuron switzerland using a vertical sounding geometry electromagnetic waves are inferred to be generated by electrokinetic conversion of seismic energy within the snow pack and near the dry wet ice and ice bed interfaces a simple gradient based scheme allows such electrokinetic interface responses eirs to be isolated from noise eirs depend sensitively on the azimuthal orientation of the receiving array of electrical dipoles seismoelectric techniques promise to allow mapping even of thin water bearing strata within or beneath glaciers or frozen ground estimation of hydraulic or fluid properties of such strata as well as monitoring of ice fracturing or basal properties and processes at improved spatial resolution

nature of surface electrical conductivity in natural sands sandstones and clays the electrical conductivity of rocks results from conduction through the bulk solution occupying the pores and from surface conduction occurring at the fluid grain interface the nature of the surface electrical conductivity of shaly and clean sands and sandstones is examined surface conduction is characterized by the specific surface conductance which is the sum of three contributions i conduction within the electrical diffuse layer which makes a negligible contribution to the total specific surface conductance ii conduction in the stern layer which is shown to vary significantly with the salinity of the pore fluid at low salinities 10 6 to 10 3 mol 1 1 but becomes independent of salinity at higher salinities iii a mechanism operating directly on the mineral surface independent of salinity and perhaps associated with proton transfer at salinities higher than 10 3 mol 1 1 and at 25 c the specific surface conductance of quartz and clays is equal to 8 9 10 9 s and 2 5 10 9 s respectively equations describing the influence of surface conductivity and microstructure upon the macroscopic electrical conductivity of sands sandstones and shales are also developed

episodic late quaternary palaeogully erosion in northern kwazulu natal south africa abstract hillslope deposits in northern kwazulu natal south africa are incised by gullies dongas which expose sequences of buried palaeosols developed within a succession of discontinuity bounded colluvial sedimentary units infilled and buried palaeogully topography attests to at least four geomorphic cycles comprising gully cut and fill and palaeosol formation on hillslopes in the region during the past 135 ka past attempts at dating colluvial stratigraphic units were limited to radiocarbon dating of associated palaeosol organic matter and authigenic soil carbonates this study presents a preliminary set of infra red stimulated luminescence irsl age determinations pertaining to deposition of the succession of colluvial units infilling and burying palaeogullies at several sites of stratigraphic geochronological and palaeoenvironmental importance in the region interpretation of the irsl and 14 c dates in the context of cyclical hillslope processes suggests that past episodes of palaeogully erosion and infill were not synchronous and individual palaeosol landsurfaces were diachronous during the late pleistocene

direct laboratory observation of patchy saturation and its effects on ultrasonic velocities maximizing the recovery of known hydrocarbon reserves is one of the biggest challenges facing the petroleum industry today optimal production strategies require accurate monitoring of production induced changes of reservoir saturation and pressure over the life of the field time lapse seismic technology is increasingly used to map these changes in space and time however until now interpretation of time lapse seismic data has been mostly qualitative in order to allow accurate estimation of the saturation it is necessary to know the quantitative relationship between fluid saturation and seismic characteristics elastic moduli velocity dispersion and attenuation the problem of calculating acoustic properties of rocks saturated with a mixture of two fluids has attracted considerable interest gist 1994 mavko and nolen hoeksema 1994 knight et al 1998 for a comprehensive review of theoretical and experimental studies of the patchy saturation problem see toms et al 2006

seismoelectric imaging of the vadose zone of a sand aquifer we have acquired a 300 m seismoelectric section over an unconfined aquifer to demonstrate the effectiveness of interfacial signals at imaging interfaces in shallow sedimentary environments the seismoelectric data were acquired by using a 40 kg accelerated weight drop source and a 24 channel seismoelectric recording system composed of grounded dipoles preamplifiers and seismographs in the shot records interfacial signals were remarkably clear they arrived simultaneously at offsets as far as 40 m from the seismic source the most prominent signal was generated at the water table at a depth of approximately 14 m and had peak amplitudes on the order of 1 v m a weaker response was generated at a shallower interface that is interpreted to be a water retentive layer the validity of these two laterally continuous events and of other discontinuous events indicative of vadose zone heterogeneity is corroborated by the presence of reflections exhibiting similar characteristics in a ground penetrating rada

high resolution shallow seismic experiments in sand part ii velocities in shallow unconsolidated sand we conducted a shallow high resolution seismic reflection and refraction experiment on a sandy beach the depth of investigation was about 2 m we interpret the data using the hertz mindlin contact theory combined with gassmann s equation these were used to obtain the vertical velocity profile then the profile was computed from seismic data using the turning rays approximation the normal moveout nmo velocity at the depth of 2 m matches the velocity profile as a result we developed a method to invert measured velocity from first arrivals i e velocity versus distance into velocity versus depth using only one adjustable parameter this parameter contains all the information about the internal structure and elasticity of the sand the lowest velocity observed was about 40 m s it is noteworthy that the theoretical lower bound for velocity in dry sand with air is as low as 13 m s we find that modeling sand as a quartz sphere pack does not quantitatively agree with the measured data however the the

late quaternary hillslope evolution recorded in eastern south african colluvial badlands abstract infrared stimulated luminescence irsl ages have been obtained from a sequence of sandy colluvial deposits in northern kwazulu natal comparison of the irsl ages on sand sized potassium rich feldspars with radiocarbon dates on a horizon organic matter from buried palaeosols within the colluvial succession shows good agreement between the dating techniques when compared with palaeoenvironmental records the data suggest that the colluvium accumulated during arid stages of the late quaternary whilst pedogenesis repeatedly occurred at intervals of hillslope stability which may reflect periods of greater humidity temperature does not seem to have been a forcing factor in the landscape development extensive colluviation took place during the arid last glacial maximum but the occurrence of several palaeosols indicates that it was a time of fluctuating climatic conditions

experimental studies of seismoelectric conversions in fluid saturated porous media when seismic waves generate a relative fluid solid motion in a fluid saturated porous medium the moving charges streaming current in the electric double layer induce an electromagnetic em field this paper first experimentally confirms that the coupling between the seismic wave and the electromagnetic field in the kilohertz range is electrokinetic in nature seismoelectric signals are measured in homogeneous cylindrical porous rock samples and multilayered models the seismoelectric signals in homogeneous rock are electric fields that move along with the acoustic wave the mechanism of the seismoelectric conversion is completely different from the piezoelectric effect of quartz grains the seismoelectric sensitivity with respect to salinity of the saturant has been experimentally determined the amplitude of seismoelectric signals increases as the saturant conductivity decreases the seismoelectric effects are generated by two different mechanisms both the em radiation and the electric potential generated at an interface and within a porous medium respectively were measured as the p wave at ultrasonic frequencies passes through the layered models our experimental results demonstrate that seismoelectric effects exist and are measurable in the kilohertz range the paper concludes with a comparison of experimental data and modeled data in a three layer porous model seismoelectric measurements could be an effective means of obtaining transport coefficients such as hydraulic permeability and other porous rock properties

paleomagnetic results from volcanic rocks of the shelve inlier wales evidence for a wide late ordovician iapetus ocean in britain abstract in order to shed light on the early paleozoic paleogeography of iapetus bordering continents samples for paleomagnetic study were collected from middle ordovician llanvirn volcanic rocks of the shelve inlier welsh borderland the folding within the inlier is constrained to late ordovician ashgill time thus offering a fold test with tight age brackets after stepwise thermal demagnetization a pre folding characteristic magnetization component was isolated from 95 specimens the mean of 11 site mean directions for the characteristic magnetization gives d i 116 4 67 9 k 89 4 95 4 9 and the paleomagnetic pole lies at 26 6 n 36 1 e p 6 9 m 8 2 the fold test is positive at the 99 confidence level indicating that the magnetization was acquired during the middle or late ordovician we infer from geological considerations that this magnetization is not a primary thermoremanence but was acquired during late ordovician time as a result of a thermochemical disturbance of that age our results indicate that southern britain was near 51 south latitude during the later ordovician since middle to late ordovician paleolatitudes for gondwana and laurentia are well determined our result can be used to test various scenarios for the closure of iapetus we conclude that 1 the iapetus ocean in britain still had a latitudinal width of some 30 at this time thus rendering a postulated ordovician closure unlikely 2 southern britain avalon and hercynian europe all occupied high latitudes during the later ordovician consistent with a paleoposition near the northern margin of gondwana and 3 the acadian orogeny in britain and north america could have been the result of the collision of gondwana and laurentia during early devonian time

a note on the age and pyroxene chemistry of the igneous rocks of the shelve inlier welsh borderland geological survey electron microprobe analyses of clinopyroxenes and isotopic age determinations have revealed that the intrusive dolerites and basic intermediate lavas of the shelve ordovician inlier are part of a co magmatic suite which show transitional affinities between tholeiites and alkali basalts the ages of most of the intrusives are shown to be mid ordovician

evaluation of luminescence dating procedures applied to late holocene colluvium near st paul s mission natal south africa colluvial deposits provide an interesting challenge for luminescence dating techniques because of the short and varied light exposure histories of the grains prior to deposition results of thermoluminescence tl and infrared stimulated luminescence irsl measurements are presented for two stacked colluvial deposits from natal a radiocarbon date for bulk organic matter in the a horizon of the soil formed within the surface of the lower unit provides a means of assessing the suitability of various luminescence procedures for such material the overlying colluvium contains grains bleached to varying extents as shown by the range of eds for the coarse grain feldspar separate obtained using the single aliquot irsl method for this sample all age estimates either by irsl or tl are too large compared with the radiocarbon age indicating that even the irsl signal was not zeroed in all grains before they were buried the underlying colluvium contains grains bleached more uniformly resulting in a smaller ra

palaeomagnetic study of the derbyshire lavas and intrusions central england definition of carboniferous apparent polar wander abstract the remanence in extrusive volcanic rocks of lower carboniferous visean age in the pennines of central england has been reinvestigated employing detailed thermal demagnetisation and incorporating new geological knowledge of field relationships indications of primary remanence identified by early studies are supported and four successive reversals in the asbian brigantian succession about 335 ma recognised a mean reversed direction of d i 203 36 10 sites r 9 75 yields a pole position at 336 e 14 s dp dm 6 10 grouping improves slightly when correction is made for mild intra carboniferous folding and widespread uniformity of polarity is recognised within individual units sporadic variscan overprinting increases in importance to the southeast intrusive dolerite bodies yield two contrasting directions of magnetisation in the central part of the derbyshire pericline mixed polarity directions mean d i 211 37 4 sites r 3 92 comparable to the lavas imply a lower carboniferous age of emplacement in the se upper permian directions of reversed polarity mean d i 221 23 define a later intrusive event or comprehensive overprinting the primary lower carboniferous pole positions accord with contemporaneous poles from the midland valley of scotland and demonstrate that carboniferous and younger tectonic movements are palaeomagnetically undetectable at least between these two areas apparent polar wander apw movement is confirmed between lower and upper carboniferous times at about 335 320 ma the lower carboniferous limestones have a remanence entirely younger than depositional ages and are therefore unreliable indicators of carboniferous magnetostratigraphy and tectonics silicified limestones prove to have remanences closer to the primary lower carboniferous field and appear to have been shielded from the effects of later overprinting the apw swathe identified from north american carboniferous rocks coincides in polarity and position with the latter part of the british carboniferous path on the pangaean reconstruction

wavelet analysis for geophysical applications wavelet transforms originated in geophysics in the early 1980s for the analysis of seismic signals since then significant mathematical advances in wavelet theory have enabled a suite of applications in diverse fields in geophysics the power of wavelets for analysis of nonstationary processes that contain multiscale features detection of singularities analysis of transient phenom ena fractal and multifractal processes and signal com pression is now being exploited for the study of several processes including space time precipitation remotely sensed hydrologic fluxes atmospheric turbulence can opy cover land surface topography seafloor bathymetry and ocean wind waves it is anticipated that in the near future significant further advances in understanding and modeling geophysical processes will result from the use of wavelet analysis in this paper we review the basic properties of wavelets that make them such an attractive and powerful tool for geophysical applications we dis cuss continuous discrete orthogonal wavelets and wavelet packets and present applications to geophysical processes

bayesian inversion of joint sh seismic and seismoelectric data to infer glacier system properties abstract in glacial studies properties such as glacier thickness and the basement permeability and porosity are key to understand the hydrological and mechanical behaviour of the system the seismoelectric method could potentially be used to determine key properties of glacial environments here we analytically model the generation of seismic and seismoelectric signals by means of a shear horizontal seismic wave source on top of a glacier overlying a porous basement considering a one dimensional setting we compute the seismic waves and the electrokinetically induced electric field we then analyse the sensitivity of the seismic and electromagnetic data to relevant model parameters namely depth of the glacier bottom porosity permeability shear modulus and saturating water salinity of the glacier basement moreover we study the possibility of inferring these key parameters from a set of very low noise synthetic data adopting a bayesian framework to pay particular attention to the uncertainty of the model parameters mentioned above we tackle the resolution of the probabilistic inverse problem with two strategies 1 we compute the marginal posterior distributions of each model parameter solving multidimensional integrals numerically and 2 we use a markov chain monte carlo algorithm to retrieve a collection of model parameters that follows the posterior probability density function of the model parameters given the synthetic data set both methodologies are able to obtain the marginal distributions of the parameters and estimate their mean and standard deviation the markov chain monte carlo algorithm performs better in terms of numerical stability and number of iterations needed to characterize the distributions the inversion of seismic data alone is not able to constrain the values of porosity and permeability further than the prior distribution in turn the inversion of the electric data alone and the joint inversion of seismic and electric data are useful to constrain these parameters as well as other glacial system properties furthermore the joint inversion reduces the uncertainty of the model parameters estimates and provides more accurate results

kinematics of strike slip faulting builth inlier mid wales abstract strike slip faulting in the builth ordovician inlier is demonstrated by large scale maps of the llanelwedd quarries near builth wells and by fault plane and slickenline data in the main quarry steep nnw striking strike slip faults dominate the structure together with significant strike slip displacement on the w dipping bedding surfaces and bedding parallel faults a zone of steep n striking extensional dip slip faults links two of the strike slip strands and there is a weaker e striking set of strike slip faults when the four fault sets are rotated so that the regional bedding is horizontal three become vertical and one horizontal probably their attitude during active life in late ordovician to early silurian time they formed a linked fault system capable of accommodating three dimensional bulk strain the fault flats have the same kinematic role in a strike slip system as lateral ramps or transfer faults in dip slip systems in the nearby gelli cadwgan quarry strike slip faults are again dominant but strike e or ese this heterogeneity of fault pattern in the southern builth inlier resolves into more homogeneous domains with areas of 0 1 to 0 5 km 2 separated by e to ne striking dextral strike slip faults domainal structure an important general feature of strike slip tectonics may be present on a variety of scales

late quaternary earthquake chronology from luminescence dating of colluvial and alluvial deposits of the arava valley israel abstract luminescence ages help provide a time scale for recurrent surface ruptures on a normal fault in the southern arava israel the dated samples come from alluvium and colluvium along a fault scarp north of elat the ages obtained mainly by infrared stimulation of sand size potassium feldspar grains accord with geomorphic age estimates from soils and degraded scarps four large earthquakes occurred between 37 000 and 14 000 yr ago five smaller earthquakes occurred thereafter continuing until recent times the mean recurrence interval for earthquakes of magnitudes greater than 6 is about 4000 yr

early devonian pre acadian magnetization directions in lower old red sandstone of south wales uk summary two components of magnetization have been resolved from the lower old red sandstone of south wales all 39 sampled sites lie north of the variscan front and all but three are largely unaffected by variscan folding thirteen sites yield a high unblocking temperature magnetization component which pre dates acadian mid devonian folding the site mean direction for this component dec 232 2 inc 31 9 95 8 5 yields a pole position lat 7 3 s long 306 7 e which can be assigned an early devonian age a lower unblocking temperature component is present at 38 of the sampled sites this component post dates both the acadian and variscan folding has reversed polarity and a mean direction dec 193 8 inc 7 2 95 2 6 which corresponds to a pole position lat 40 4 s long 338 0 e consistent with a latest carboniferous or early permian age the early devonian pole position confirms the existence of a siluro devonian inflexion in the apparent polar wander path apwp for southern britain which coincides with an inflexion in the apwp for scotland britain north of the iapetus suture indicating the closure of the iapetus ocean by siluro devonian time the early devonian palaeolatitude 17 s 5 is consistent with an early devonian configuration in which gondwana laurentia baltica and avalonia were part of a single supercontinent

an analytical solution to assess the sh seismoelectric response of the vadose zone fil monachesi leonardo bruno universidad nacional de rio negro sede alto valle instituto de investigaciones en paleobiologia y geologia argentina consejo nacional de investigaciones cientificas y tecnicas argentina

luminescence dating and its application to key pre late devensian sites in scotland abstract relatively few sites have been identified in scotland that contain records that predate the peak of the late devensian glaciation ca 18 ka bp where such records have been identified they have assumed a key role in developing our understanding of climate change in scotland radiocarbon dating has been the primary chronological tool applied to these sites but the reliability of age determinations in excess of 30 ka is questionable in this paper we describe the results of a programme which applies luminescence dating methods to eight key pre late devensian sites in scotland methods were developed and tested to enable the identification of samples for which luminescence dating was only able to provide an upper limit on the age because of the limited light exposure that the sample experienced at deposition these new approaches were applied to four sites where independent age control was available this meant that the choice of sediments from test sites was restricted to those sediments which were formed in the last 40 000 years and hence were amenable to radiocarbon dating the luminescence age estimates for the control sites and the pre late devensian sites are discussed within the context of any existing chronological control on a site by site basis

curvelet based seismoelectric data processing seismoelectromagnetic conversions are induced through electrokinetic phenomena occurring when passing seismic waves induce relative fluid to solid displacements information on porous material properties provided by the type i coseismic electric field accompanying surface and body waves is limited to the vicinity of the electrical receivers as opposed to the type ii electromagnetic interface response ir which can help detect porous contrasts and or fluid changes within the subsurface at depth for general field geometries the problem is that type ii disturbances exhibit lower amplitudes than type i fields and are difficult to access directly hence separating both types of waves is a critical step when processing seismoelectric data synthetic seismograms and electrograms generated with a full waveform seismoelectric forward modelling code written by garambois dietrich enabled us to study the behaviour of the ir s characteristic dipolar amplitude pattern recovered through filtering the definitive version is available at www blackwell synergy com to better preserve the ir amplitudes we have developed a new filtering strategy based on the fast discrete curvelet transform seismic or seismoelectric wave fronts can be optimall described using this multiscale decomposition over multidirectional anisotropic needle shape structures we have built a mask in the curvelet domain zeroing out seismoelectric samples corresponding to non zero samples from the accelerogram thus taking advantage of the relationship between seismic and seismoelectric waves for type i fields this mask consists of a threshold function combined with a gaussian distribution promoting horizontal i e zero slowness directions when applied to synthetic data this filter enabled to successfully extract the ir although less altering its dipolar radiation pattern than the conventional dip based techniques this strategy was successfully applied to a seismoelectric data set acquired in sedimentary deposits and permitted to isolate an ir generated at the water table

estimating seismic velocities at ultrasonic frequencies in partially saturated rocks seismic velocities in rocks at ultrasonic frequencies depend not only on the degree of saturation but also on the distribution of the fluid phase at various scales within the pore space two scales of saturation heterogeneity are important 1 saturation differences between thin compliant pores and larger stiffer pores and 2 differences between saturated patches and undersaturated patches at a scale much larger than any pore we propose a formalism for predicting the range of velocities in partially saturated rocks that avoids assuming idealized pore shapes by using measured dry rock velocity versus pressure and dry rock porosity versus pressure the pressure dependence contains all of the necessary information about the distribution of pore compliances for estimating effects of saturation at the finest scales where small amounts of fluid in the thinnest most compliant parts of the pore space stiffen the rock in both compression and shear increasing both p and s wave velocities in approximately th

statistical significance of the fold test in palaeomagnetism summary the significance of the change in direction dispersion on applying the fold test in palaeomagnetism is used as the criterion for deciding whether or not the fold test has statistical significance a set of tables is compiled which can be used for quick application of the test

ordovician magnetostratigraphy a correlation of global data palaeomagnetic studies where primary magnetizations are established from well dated rocks have been compiled to construct a magnetostratigraphic time scale for the ordovician period an excellent correlation of magnetozones is observed between independent studies of the baltic and south siberian platform sequences supplemental datasets from other continents concur with the baltic and siberian polarity data early ordovician times were mainly characterized by a reverse field rapid reversals then occurred during llanvirn and llandeilo times late ordovician times were dominated by a normal polarity field some gaps remain in the polarity record notably for late caradoc early ashgill and mid tremadoc times

synfolding magnetization in the jurassic preuss sandstone wyoming idaho utah thrust belt the jurassic preuss sandstone exposed in five thrust plates of the wyoming idaho utah thrust belt carries directions of remanent magnetization that group most tightly after only partial unfolding field petrographic and rock magnetic evidence indicates that the carrier of this magnetization is detrital low ti titanomagnetite the detrital titanomagnetite was remagnetized at low temperatures 75 150 c probably completely during folding anisotropy of magnetic susceptibility and petrographic observations indicate that the detrital titanomagnetite has been affected by tectonic strain we suggest that low temperature remagnetization of the detrital titanomagnetite was either a viscous partial thermoremanent magnetization the acquisition of which was enhanced by stress or a piezoremanent magnetization that involved stress induced movement of domain walls during intracrystalline strain or was a combination of the two mechanisms stress may promote remagnetization at temperatures much lower than predicted by current theoretical models other mechanisms such as acquisition of chemical remanent magnetization during folding deflection of a prefolding magnetization by internal strain or combination of components of magnetization with different direction cannot account for the geometry of magnetization in the preuss the locus of acquisition of synfolding magnetization in the preuss migrated in conjunction with deformation in the thrust belt a model is presented in which synfolding magnetization was acquired during cooling and folding as strata moved up thrust ramps a lack of reverse polarity directions remains a puzzling feature of the remanence the remanent direction is tentatively interpreted to reflect the predominant polarity state during its acquisition over an extended rather than a discrete time period during folding in late cretaceous and early tertiary periods of predominantly normal polarity

a palaeomagnetic study of the builth wells llandrindod wells ordovician inlier wales palaeogeographic and structural implications summary discrepancies exist in the southern british ordovician palaeomagnetic data set with individual studies indicating either high southerly or temperate southerly palaeolatitudes although previous palaeomagnetic investigations from the builth inlier have delineated a three component remanence structure doubts have been raised concerning i the magnetization age of the supposed primary component p ii the structural setting of the sampling area llanelwedd quarry within a zone of strike slip deformation and iii whether detailed step wise demagnetization has fully separated the various magnetization components in addressing these problems we present new palaeomagnetic results as follows i a positive palaeomagnetic conglomerate test establishing a pre late llanvirn age for component p ii an enlarged geographic sampling embracing several tectonic domains indicating that relative rotations linked to strike slip tectonism are minor within the builth sampling area and iii evidence that component p is uncontaminated by overprints at some sites suggesting the resulting palaeolatitude of 35 s to be representative for the builth inlier we also present evidence for local syn volcanic deformation within llanvirn volcanics and for intrusion related deformation of llanvirn shales when the revised llanvirn pole from the inlier 3 s 4 e is combined with other data from southern britain in a statistical spline analysis a mean palaeolatitude of 44 5 s is calculated for builth in llanvirn times a palaeoreconstruction for 468 474 ma c mid llanvirn is presented utilizing this analysis

optical dating of tsunami laid sands abstract the ages of some tsunami deposits can be determined by optical dating a key requirement being that the deposits are derived from sediment that was reworked and exposed to daylight by tidal currents waves wind or bioturbation during the last years before the tsunami measurements have been made using 1 4 ev infrared excitation of k feldspar grains separated from samples of prehistoric tsunami sand sheets and modern analogs of tsunami source sediments at four sites in washington state and british columbia source sands gave equivalent doses indicative of recent exposure to daylight tsunami sand at cultus bay washington yielded an optical age of 1285 95 yr calendric years before a d 1995 1 at 2 this age overlaps the range of from 1030 to 1100 yr determined through a combination of high precision radiocarbon dating and stratigraphic correlation tsunami sands at three sites near tofino and port alberni on vancouver island british columbia have optical ages of 260 20 325 25 and 335 45 yr historical records and radiocarbon dating show that the sand at each of the three sites is between 150 and 400 yr old these optical ages support the hypothesis that the vancouver island sands were deposited by a tsunami generated by a large earthquake on the cascadia subduction zone about 300 yr ago

palaeozoic palaeomagnetic studies in the welsh basin recent advances in the last two years new palaeomagnetic data fro m wale s have resulted i n radical revision of the ordovician palaeogeography of eastern avalonia part of the southern margin of the iapetus ocean combined with palaeozoic palaeomagnetic data from laurentia and gondwana these data suggest that eastern avalonia was a peri gondwanide high latitude continental fragment during at least part of ordovician time with a palaeolatitude of about 62 s and 51 s in arenig and llanvirn time respectively this implies a latitudinal width of the early ordovician iapetus ocean between eastern avalonia and laurentia of at least 30 geological evidence for the proximity of eastern avalonia and laurentia suggests that the intervening iapetus ocean closed during silurian time from late llandovery to early ludlow recent palaeolatitude data from the iapetus bordering continents are consistent wit h closure b y middle to late silurian time new pre acadian early devonian palaeomagnetic data from the old red sandstone places the welsh basin at about 17 s consistent with a palaeogeography in which laurentia baltica avalonia armorica and possibly gondwana wer e part o f a singl e supercontinent pervasiv e lat e carboniferous early permia n remagnetization affect s the welsh basin the remagnetization i s probably associated with fluids emanating from th e variscan thrust front w e do not observe remagnetization associate d with acadian orogen y and the remagnetizations which have been studied in more detail in north america appear to be a unique feature of the variscan hercynian alleghenian orogeny

crosshole seismoelectric measurements in borehole models with fractures laboratory experiments are performed in crossborehole models with fractures to investigate seismoelectric conversions in the fractures using an electrode in a borehole we measure the electric field induced by an acoustic wave and we investigate the relationship between the electric signal and the fracture aperture using ultrasonic fracture borehole models the experimental results confirm that a radiating electromagnetic em wave is induced by a guide wave at a fracture between an acoustic source and electric receiver boreholes the position of a vertical or inclined fracture between two boreholes can be determined using the arrival times of the em wave and the formation velocity by placing the acoustic source at a different depth and recording electric signals with real or synthetic arrays of acoustic and electric receivers in the second borehole crosshole seismoelectric measurements might be a technique for investigating fractures between boreholes more directly than traditional acoustic measurements

dependence of seismoelectric amplitudes on water content summary the expectation behind seismoelectric field measurements is to achieve a combination of the sensitivity of electrical properties to water content and permeability and the high spatial resolution of seismic surveys a better understanding of the physical processes and a reliable quantification of the conversion between seismic energy and electric energy are necessary and need to take into account the effect of water content especially for shallow subsurface investigations we performed a field survey to quantify the seismoelectric signals as the water content changed we measured seismoelectric signals induced by seismic wave propagation by repeating the observations on the same two profiles during several months the electrical resistivity was monitored to take into account the water content variations weshowthatthehorizontalcomponentoftheseismoelectricfield normalizedwithrespectto the horizontal component of the seismic acceleration is inversely proportional to the electrical resistivity 0 42 0 25 assuming that the observed resistivity changes depend only on the water content this result implies that the electrokinetic coefficient should increase with increasing water saturation taking into account the water saturation and combining our results with the archie law for the resistivity in non saturated conditions the normalized seismoelectric field is a power law of the effective saturation with the exponent 0 42 0 25 n where n is archie s saturation exponent

experimental quantification of the seismoelectric transfer function and its dependence on conductivity and saturation in loose sand under certain circumstances seismic propagation within porous media may be associated to a conversion of mechanical into electromagnetical energy known as a seismoelectromagnetic phenomemon the propagation of fast compressional p waves is more specifically associated to manifestations of a seismoelectric field linked to fluid flow within the pores the analysis of seismoelectric phenomena which requires combining the theory of electrokinetics to biot s theory of poroelasticity provides us with a transfer function noted e u that links the coseismic seismoelectric field e to the seismic acceleration u in order to measure the transfer function we have developed an experimental set up enabling seismoelectric laboratory observation in unconsolidated quartz sand within the kilohertz range the investigation focused on the impact of fluid conductivity and water saturation over the coseismic seismoelectric field during the experiment special attention was given to the accuracy of electric field measurements we concluded that in order to obtain a reliable estimate of the electric field amplitude the dipole from which the potential differences are measured should be of much smaller length than the wavelength of the propagating seismic field time lapse monitoring of the seismic velocities and seismoelectric transfer functions were performed during imbibition and drainage experiments in all cases the quantitative analysis of the seismoelectric transfer function e u was in good agreement with theoretical predictions while investigating saturation variations from the residual water saturation to full saturation we showed that the e u ratio undergoes a switch in polarity at a particular saturation s also implying a sign change of the filtration traducing a reversal of the relative fluid displacement with respect to the frame this sign change at critical saturation s stresses a particular behaviour of the poroelastic medium the dropping of the coseismic electric field to zero traduces the absence of relative pore fluid displacements representative of a biot dynamically compatible medium we concluded from our experimental study in loose sand that measurements of the coseismic seismoelectric coupling may provide information on fluid distribution within the pores and that the reversal of the seismoelectric field may be used as an indicator of the dynamically compatible state of the medium

palaeomagnetic data from the borrowdale volcanic group volcano tectonics and late ordovician palaeolatitudes nineteen palaeomagnetic sampling sites from the borrowdale volcanic group between borrowdale and langdale english lake district yield high unblocking temperature magnetization components which postdate the structural tilt and therefore postdate the formation of the broad block faulted syncline which characterizes this part of the central lake district the mean magnetization direction before tilt correction dec 347 2 inc 61 9 95 6 9 corresponds to a palaeolatitude of 43 s 5 7 the magnetization is associated with pervasive late ordovician hydrothermal alteration and is certainly pre devonian based on comparison with available palaeomagnetic data from southern britain this implies that the block faulted synclinal structure of the borrowdale langdale region is pre acadian in age consistent with recent interpretations of late ordovician volcano tectonics based on mapping and volcanological studies

streaming potential during drainage and imbibition total are thanked for partially funding jackson under the total chairs programme at imperial college london and for funding vinogradov shell are thanked for funding leinov the department of earth science and engineering at imperial college london are thanked for supporting zhang with a janet watson scholarship damien jougnot frederic perrier and an anonymous reviewer are thanked for their careful and thorough reviews the data for this paper are available in supporting information or by contacting the corresponding author m d jackson imperial ac uk

anatomy of a seismoelectric conversion measurements and conceptual modeling in boreholes penetrating a sandy aquifer 1 conversions of compressional seismic waves to electric fields have been measured in two boreholes drilled in an unconfined sandy aquifer on the gnangara mound near perth australia the seismoelectric conversions at both field sites occurred in the vicinity of the water table at 13 m depth and yielded maximum amplitudes of 1 v m using a sledgehammer source on surface partially cemented layers inferred from geological and geophysical logs straddle the water table and may play a role in generating the conversion and influencing its amplitude distribution the dense vertical sampling used in these borehole experiments reveals spatial and temporal polarity reversals of the interfacial signal which provide new evidence in support of the conceptual model for seismoelectric conversions at interfaces we demonstrate that the growth rate of the source zone and its maximum vertical extent below the water table are encoded in the polarity of the interfacial signal these experiments confirm that vertical seismoelectric profiling can be used to gain further insight into seismoelectric conversions and characteristics of interfaces that makes them amenable to detection

the pontesford lineament welsh borderland the ne trending pontesford lineament centres on the pontesford linley and adjacent faults in the shelve longmynd inlier to the ne it is usually drawn along the se edge of the cheshire basin and to the sw along a variety of lines parallel to or converging with the church stretton fault new mapping and reassessment of published data suggest that sw of the pontesford linley fault the lineament is logically drawn along the clun forest disturbance a post pridoli late silurian fold fault belt then across the nw side of the llandrindod inlier to flank the tywi lineament the lineament was intermittently active at least from mid ordovician through to triassic time with both strike slip and dip slip displacements that involved the upper part of the precambrian basement late ordovician and early silurian strike slip was particularly important the lineament controlled the sites of several silurian sedimentary basins ordovician contrasts across the lineament can be explained either by contemporary fault control or by subsequent large displacements juxtaposing unlike terranes the long history of reactivation and basement control on the pontesford lineament is typical of lineaments throughout wales and the borderland it suggests that both caledonian and variscan structural trends n of the variscan front may be inherited from older pre silurian or even precambrian basement structures

the occurrence and origin of late paleozoic remagnetization in the sedimentary rocks of north america although it has been known for 25 years that some paleozoic sedimentary rock units in europe and north america were remagnetized during pennsylvanian or permian time it is only very recently that the complex and widespread nature of the late paleozoic remagnetization phenomenon has been generally acknowledged it is now recognized that many paleozoic paleomagnetic poles for north america that were once considered reliable are in fact the result of remagnetization and as a consequence the paleomagnetic data base for the paleozoic is undergoing rapid and drastic revision the causes of late paleozoic remagnetization in north america are currently the focus of much interest and active research the remagnetization can reside in either hematite or magnetite and different remagnetization mechanisms have been important in different settings chemical remagnetization processes some related to specific diagenetic events are dominant in hematite bearing sandstones and carbonates in magnetite bearing carbonates both chemical and thermoviscous remagnetization processes appear to have been important but it is difficult to determine which process is the dominant one in some settings some of the observed remagnetizations can be linked to the migration of chemically active and perhaps hot fluids during the mountain building events that affected much of the continent during the late paleozoic paleomagnetic studies promise to be important in assessing the role of orogeny in driving fluid migrations within sedimentary basins and in constraining the age of the migrations and the nature of the fluids

tectonic rotation within the british paratectonic caledonides and early palaeozoic location of the orogen magnetizations of late ordovician caradoc early ashgill age in the paratectonic caledonides of wales and the lake district are directed westerly and northerly respectively they identify c 55 of relative rotation of probable acadian age southward subduction of iapetus ocean crust occurred in ordovician times beneath a latitudinally oriented orogen sited in mid southerly latitudes with the borrowdale and snowdonia volcanic provinces forming parallel fore arc and back arc lineaments respectively mid ordovician to mid devonian palaeomagnetic poles from the orogen correlate with contemporaneous gondwana data to identify a former location sited west of the south american perimeter and not near to north africa as assumed in previous reconstructions

the palaeogeographic evolution of southern britain during early palaeozoic times a reconciliation of palaeomagnetic and biogeographic evidence abstract palaeomagnetic biogeographic and palaeoclimatic evidence together suggest that in the early ordovician southern britain lay off the west african margin of gondwana at around 60 s the combination of these disciplines reduces latitude uncertainties to around 5 to 10 degrees rifting of avalonia from gondwana was followed during most of the ordovician by northward drift as avalonia and baltica converged with subduction of tornquist sea crust both continents had similar moderate northward drift rates 3 6 cm yr so their convergence probably had a considerable longitudinal component silurian and early devonian data indicate that northward drift of avalonia and baltica continued after their collision and that southern britain reached a latitude of around 20 s by c 400 ma

evidence of the theoretically predicted seismo magnetic conversion summary seismo electromagnetic phenomena in porous media arise from seismic wave induced fluid motion in the pore space which perturbs the equilibrium of the electric double layer this paper describes with details the original experimental apparatus built within the ultra shielded chamber of the low noise underground laboratory of rustrel france we measured seismo magnetic conversions in moist sand using two induction magnetometers and a pneumatic seismic source to generate the seismic wave propagation we ensured to avoid the magnetometer vibrations which could induce strong disturbances from induction origin interpretation of the data is improved by an analytical description of phase velocities for fast pf and slow ps longitudinal modes transverse mode s as well as the extensional mode due to the cylindrical geometry of the sample the purpose of this paper is to provide elements to measure correctly coseismic seismomagnetic fields and to specify their amplitude the seismic arrivals recorded in the sample showing a 1200 1300 m s 1 velocity have been associated to p and extensional waves the measured seismo magnetic arrivals show a velocity of about 800 m s 1 close to the calculated phase velocity of s waves therefore we show that the seismo magnetic field is associated to the transverse part of the propagation as theoretically predicted by pride 1994 but never measured up to now moreover the combined experimental and analytical approaches lead us to the conclusion that the measured seismo magnetic field is probably about 0 035 nt for a 1 m s 2 seismic source acceleration 0 1 g

seismoelectric wave propagation numerical modelling in partially saturated materials to better understand and interpret seismoelectric measurements acquired over vadose environments both the existing theory and the wave propagation modelling programmes available for saturated materials should be extended to partial saturation conditions we propose here an extension of pride s equations aiming to take into account partially saturated materials in the case of a water air mixture this new set of equations was incorporated into an existing seismoelectric wave propagation modelling code originally designed for stratified saturated media this extension concerns both the mechanical part using a generalization of the biot gassmann theory and the electromagnetic part for which dielectric permittivity and electrical conductivity were expressed against water saturation the dynamic seismoelectric coupling was written as a function of the streaming potential coefficient which depends on saturation using four different relations derived from recent laboratory or theoretical studies in a second part this extended programme was used to synthesize the seismoelectric response for a layered medium consisting of a partially saturated sand overburden on top of a saturated sandstone half space subsequent analysis of the modelled amplitudes suggests that the typically very weak interface response ir may be best recovered when the shallow layer exhibits low saturation we also use our programme to compute the seismoelectric response of a capillary fringe between a vadose sand overburden and a saturated sand half space our first modelling results suggest that the study of the seismoelectric ir may help to detect a sharp saturation contrast better than a smooth saturation transition in our example a saturation contrast of 50 per cent between a fully saturated sand half space and a partially saturated shallow sand layer yields a stronger ir than a stepwise decrease in saturation

chronology and controls of donga gully formation in the upper blood river catchment kwazulu natal south africa evidence for a climatic driver of erosion dongas gullies and badland type terrain are found extensively across interior south africa but the controls on their formation remain contested establishing the timing of donga formation is essential for developing a clearer understanding of the relative importance of natural e g climatic and human e g overgrazing controls here we present optically stimulated luminescence osl ages for colluvial sediments exposed in the sidewalls of dongas and for tributary fan sediments deposited downslope of dongas in the upper blood river catchment eastern south africa the tributary fan ages enable the timing of donga formation to be established since fan sedimentation is a direct result of upslope donga erosion the osl ages indicate that net colluviation and intervening palaeosol formation occurred from prior to 22 ka until 1 6 ka from 0 89 ka to present there has been a shift to net erosion characterised by widespread donga formation and tributary fan sedimentation contemporary donga formation i

palaeomagnetism of late ordovician igneous intrusions from the northern welsh borderlands implications to motion of eastern avalonia and regional rotations the palaeomagnetism of a late ordovician dolerite suite widely distributed in the northern welsh borderlands has been investigated to evaluate i regional rotations associated with later deformations and ii lower palaeozoic latitudes of eastern avalonia only local effects of acadian and variscan overprinting are observed in this region and most bodies appear to preserve a primary cooling related magnetization magnetic properties in the breidden dolerite are related to primary igneous differentiation and a dual polarity characteristic remanence d i 333 1 71 6 6 sites 95 10 4 predates late ordovician folding dolerites elsewhere in the breidden hills inlier record the same reversal of remanence and the combined mean d i 314 3 72 8 is rotated clockwise from the mean remanence d i 291 5 59 5 21 sites 95 6 0 in a similar dolerite suite within the adjoining shelve inlier which is also of dual polarity and predates late ordovician folding the difference in magnetic declinations is similar to the rotation of late ordovician fold axes and fracture systems between the two inliers and is probably the resultant effect of local rotation possibly about a constraining bend in the severn valley fault system the moel y golfa andesite of caradocian soudleyan age has a remanence d i 294 3 50 4 13 samples 95 2 8 which study of an adjoining volcanic conglomerate shows to be of primary cooling related origin the equivalent palaeolatitude is 31 s in early caradocian times a similar estimate is derived from a probable diagenetic overprint in the contemporaneous castle hill conglomerate at montgomery the mynydd bryn andesite in the berwyn inlier of probable similar origin and age to moel y golfa has a magnetization of opposite polarity but similar inclination d i 239 1 47 4 14 samples 86 5 5 declination differences between these volcanic outcrops are a possible signature of later block rotations between the lower palaeozoic inliers and identify the importance of terrane definition in analysis of lower palaeozoic palaeomagnetic results from the caledonides a dolerite sheet complex at hendre in the berwyn hills has a younger remanence d i 182 3 3 0 20 samples 95 7 8 probably of mid carboniferous age the collective upper ordovician results define a rapid movement of the avalonian plate probably spanning no more than 10 ma from 30 s in early caradocian times to 57 s prior to folding of the dolerites in late caradocian to early ashgill times this motion into higher southerly palaeolatitudes marks a reversal of the motion of avalonia into more shallow southerly palaeolatitudes between arenig and llanvirn times and shows that ordovician closure of the iapetus ocean was more complex than recognized hitherto

seismoelectric effects due to mesoscopic heterogeneities 1 while the seismic effects of wave induced fluid flow due to mesoscopic heterogeneities have been studied for several decades the role played by these types of heterogeneities on seismoelectric phenomena is largely unexplored to address this issue we have developed a novel methodological framework which allows for the coupling of wave induced fluid flow as inferred through numerical oscillatory compressibility tests with the pertinent seismoelectric conversion mechanisms simulating the corresponding response of a water saturated sandstone sample containing mesoscopic fractures we demonstrate for the first time that these kinds of heterogeneities can produce measurable seismoelectric signals under typical laboratory conditions given that this phenomenon is sensitive to key hydraulic and mechanical properties we expect that the results of this pilot study will stimulate further exploration on this topic in several domains of the earth environmental and engineering sciences

superimposed recent permo carboniferous and ordovician palaeomagnetic remanence in the builth volcanic series wales the builth volcanic series of llanvirnian age in llanelwedd quarries mid wales carries three components of natural remanent magnetisation component p regarded as primary is a thermochemical remanence directed at d 181 7 i 54 5 95 4 4 relative to bedding component s is a secondary component with in situ d 178 7 i 6 7 95 5 4 and is believed to be a low temperature chemical remanence crm of permo carboniferous age component r is directed close to the present geomagnetic field and is believed to be a recent viscous remanence vrm the results are of interest for three reasons first they are an unusually good example of multi component nrm analysis the three components being so clearly discriminated by thermal demagnetisation because they have almost completely separate blocking temperature ranges second they provide evidence of a permo carboniferous event possibly a mild thermal or hydrothermal pulse promoting crm acquisition some 40 km north of the hercynian orogenic front third they illustrate very clearly the importance of detailed demagnetisation this work revises the pole position for these rocks by 10 and removes an obstacle to the palaeomagnetic recognition of the 1000 km wide iapetus ocean cutting britain in ordovician time

laboratory monitoring of p waves in partially saturated sand energy dissipation is observed on seismic data when a wave propagates through a porous medium involving different frequency regimes depending on the nature of rock and fluid types we focus here on the role of partial fluid saturation in unconsolidated porous media looking in particular at p wave phase velocity and attenuation the study consists in running an experiment in a sand filled tank partially saturated with water seismic propagation in the tank is generated in the khz range by hitting a steel ball on a granite plate seismic data are recorded by buried accelerometers and injecting or extracting water controls the partial saturation several imbibition drainage cycles were performed between the water and gas residual saturations a continuous wavelet transform applied on seismic records allowed us to extract the direct p wave at each receiver we observe an hysteresis in phase velocities and inverse quality factors between imbibition and drainage phase velocities and inverse quality factors are then jointly inverted to get a final poro visco elastic model of the partially saturated sand that satisfactorily reproduces the data the model formulation consists in generalizing the biot theory to effective properties of the fluid and medium permeability and bulk modulus in order to properly explain the phase velocity variation as a function of the saturation the strong level of attenuation measured experimentally is further explained by an anelastic effect due to grain to grain sliding adding to biot s losses this study shows that fluid distribution at microscopic scale has strong influence on the attenuation of direct p waves at macroscopic scale and confirms that seismic prospection may be a powerful tool for the characterization of transport phenomena in porous media

a revised palaeozoic apparent polar wander path for southern britain eastern avalonia summary a revised apparent polar wander path apwp is presented for palaeozoic southern britain the new path based on a structural reinterpretation of existing data combined with new data from wales differs significantly from previous estimates in the following ways 1 the 1 ocus of the ordovician path segment is extended by approximately 25 degrees of arc 2 revised time calibration of the apwp suggests more ordovician apw than previously calculated and 3 the new path implies middle ordovician separation across the british sector of iapetus to be greater than previous palaeomagnetic estimates based on apwp analyses c 3300 km northern britain c 133 southern britain c 45s

reply to comment of a trench and t h torsvik on paleomagnetic results from volcanic rocks of the shelve inlier wales evidence for a wide late ordovician iapetus ocean in britain

luminescence dating of hillslope deposits a review abstract luminescence dating determines the last exposure to light of quartz and feldspar mineral grains and thus the time a sediment was laid down prerequisite is that the sediment grains were sufficiently exposed to light prior to deposition for eolian deposits this seems a fair assumption although exceptions may well exist for other depositional environments light exposure prior to burial is often restricted in duration intensity and spectral composition and may result in an age overestimation recent developments now allow dating of sediments that were exposed to only little light prior to deposition the main developments are single aliquot techniques and statistical tests that allow extraction of valid age estimates even from deposits in which bleached and unbleached grains are mixed these developments in combination with the fact that the great majority of sediments near the earth s surface are not of eolian origin have resulted in the large interest in the luminescence dating of non eolian deposits here we review possibilities and challenges of luminescence dating of colluvial deposits we summarize techniques for the detection and handling of insufficiently bleached sediments a common condition in colluvial deposits we give an overview how luminescence dating has been applied on hillslope sediments for reconstructing landscape response to climate change to analyze past fault activity and unravel human induced soil erosion we discuss sampling strategies and give guidelines as to which techniques are best suited to establish luminescence chronologies for colluvium

geophysical applications of electrokinetic conversion in volume 1 of geophysics 1936 r r thompson of humble oil and refining company proposed that the coupling of seismic and electrical energy can be used as an exploration tool subsequent efforts were directed toward using electric field antennas as geophones small scale studies concluded that antennas are not as sensitive as conventional geophones and their response is highly variable depending on the local soil conditions by 1959 martner and sparks of pan american petroleum reported in geophysics a systematic study of seismoelectric coupling using explosives at various depths their results were the first to show that the conversion of seismic to electromagnetic energy at the water table could be detected using surface antennas no studies have systematically explored the full potential of electroseismic methods using the same level of experimental sophistication used in seismic exploration

quality assurance in luminescence dating recent advances in luminescence dating have led to increasing application of the technique to sediments from a wide range of depositional environments many of which are characterised by rapid transport and deposition under turbid conditions the complete zeroing of the luminescence signal by exposure to light is critical to obtaining an accurate age for deposition of the sediment before any geomorphological or palaeoenvironmental reconstructions can be undertaken based on this chronology comparison of luminescence dates with independent age control is rare due to a lack of suitable material in natural contexts and thus the accuracy of the luminescence dating technique has yet to be proven in many depositional environments there is therefore a clear need for both quality assurance in luminescence dating and a method for assessing the accuracy of each date this paper describes and demonstrates a quality assurance method that can be used as part of a routine dating programme and which can discriminate against samples which would give inaccurate dates

dating the lower paleolithic open air site of holon israel by luminescence and esr techniques abstract the open air acheulian site in holon israel was dated by the luminescence methods and by electron spin resonance esr situated in the coastal plain quaternary kurkar group the holon site was first excavated in the late 1960s when typical lower paleolithic lithics and middle pleistocene fauna were found in order to date the site new test pits were dug adjacent to the earlier excavations and the archaeological bed was exposed in a section comprising a series of paleosols and aeolianites alkali feldspars separated from the sediments were dated using the infrared stimulated luminescence and thermoluminescence signals and quartz was dated using the optically stimulated luminescence signal the age of the archaeological bed is constrained by two samples to 198 000 22 000 201 000 17 000 yr the age of the base of the section is 240 000 29 000 yr and the age of the top is 81 000 8000 yr two teeth from the archaeological bed recovered from the original excavation collection yielded an average esr age of 204 000 16 000 yr calculated using the linear uptake model which is in a very good agreement with the luminescence ages these dates place holon within the range of other late acheulian and acheulo yabrudian sites in this region such as tabun e younger chronology yabrud i archaeological level 18 and berekhat ram

palaeomagnetism of the early devonian lower old red sandstones of south wales implications to variscan overprinting and differential regional rotations component structures of magnetic remanence in the old red sandstones of south wales are related to tectonic history by detailed study of three contrasting areas 1 remote from 2 bordering and 3 within the variscan orogenic belt a uniform southerly shallow directed component dominates remanence in each sector in area 1 where it is considered most representative of the regional field this magnetisation has a mean di 191 1 8 0 151 samples in situ 95 2 equivalent to a pole position at 341 1 e 41 2 s fold tests show that it was acquired at an intermediate stage of orogenesis and during the carboniferous permian reversed superchron it is interpreted as a pervasive magnetic overprint imparted by northward fluid migration consequent upon loading of the orogen to the south remote from the orogenic front a higher blocking temperature dual polarity remanence of probable lower devonian age has sporadically survived di 246 38 27 samples tilt adjusted 95 7 2 corresponding to a pole position at 297 e 3 n this remanence is entirely replaced close to the frontal zone of the orogen within the orogen low blocking temperature components are more prominent and chemical alteration obscures the highest blocking temperature portion of the remanence spectrum this region was deformed into upright folds before the magnetising event following remanence acquisition continuing deformation produced asymmetry in fold structure producing a plunge and steepening of the southern limbs structures in the south were already more deformed than those in the north at the time of magnetisation implying that the causative fluid front moved northwards with time the palaeomagnetic vectors are rotated clockwise to the west in south wales and in part record the effects of regional dextral transpression the largest rotation however is observed within the same thrust block and apparent differential rotation here is probably a consequence of strain

reply to comment on dependence of shear wave seismoelectrics on soil textures a numerical study in the vadose zone by f i zyserman l b monachesi and l jouniaux by revil a

paleomagnetic results from the lower devonian llandstadwell formation dyfed wales abstract a paleomagnetic investigation of the lower devonian llandstadwell formation in southwest wales has yielded a characteristic direction declination 196 inclination 5 that passes a fold test comparison of the corresponding virtual geomagnetic pole 334 e 39 s to previously published poles for great britain indicates that the magnetization is secondary and likely to be of carboniferous age no evidence for post hercynian rotation of pembrokeshire is indicated by our data

seismoelectric and seismomagnetic measurements in fractured borehole models seismoelectric and seismomagnetic fields generated by seismic waves in fluid saturated fractured borehole models are experimentally investigated with an electrode and a hall effect sensor in a borehole with a horizontal fracture the stoneley and flexural waves induce seismoelectric and seismomagnetic fields on the borehole wall and an electromagnetic wave propagating at light speed at the horizontal fracture in a borehole with a vertical fracture the acoustic field generated by a monopole or dipole source is similar to that in a borehole without a vertical fracture however the acoustic wave propagating along the vertical fracture induces seismoelectric and seismomagnetic fields whose apparent velocities are equal to that of a stoneley wave experimental results show that two different kinds of electric and magnetic fields are generated by acoustic waves in borehole models with horizontal and or vertical fractures one is an electromagnetic wave propagating with light speed the second is a stationary or localized seismoelectric and seismomagnetic field seismoelectric and seismomagnetic measurements might be a new logging technique for exploring fractures in a borehole

two new ordovician volcanic centres in the shelve inlier powys wales summary as a direct result of new work in the shelve ordovician inlier two new important extrusive andesitic members of a revised stapeley volcanic formation are described these members were originally regarded as extrusive however they have for over half a century been considered intrusive and a brief literature review attempts to account for this anomaly a palaeogeographic reconstruction suggests that the members represent the remains of two llanvirn volcanic islands which following subsidence were overlapped and buried by later llanvirn sedimentation this reinterpretation considerably increases the volume of andesitic material known to have been erupted in llanvirn times in this part of the welsh borderland

streaming potential and permeability of saturated sandstones under triaxial stress consequences for electrotelluric anomalies prior to earthquakes the streaming potential due to fluid circulation in rock was measured on saturated sediments fontainebleau sandstones the electrokinetic coupling coefficient which is the ratio of the streaming potential and the excess pore pressure is proportional to the fluid resistivity additionally for a fluid conductivity of 10 3 s m the electrokinetic coupling coefficient varies from 10 to 6642 mv 0 1 mpa for sample permeability in the range of permeabilities from 0 15 10 15 to 1220 10 15 m2 the different values of the electrokinetic coupling coefficient have been explained by the effect of increasing surface conductivity which becomes nonnegligible compared to fluid conductivity for low permeability when the sample is deformed under triaxial stress up to failure the vertical permeability along the principal stress drops by about 0 20 0 1 mpa when failure occurs the typical variation of the electrokinetic coupling coefficient is a large increase beginning with the onset of the localization of the shear band at about 75 of the yield stress and stopping at the failure this increase of the electrokinetic coupling coefficient is due to an increase of potential in the shear zone when new surfaces are created and connected possible consequences of our results are given concerning the electrical fields which could appear during the preparation of an earthquake it is shown that in some cases self potential anomalies reported in the deformed zone preceding an earthquake occurrence could be due to an increase of the electrokinetic coupling coefficient from 75 of the yield stress to rupture in the vicinity of one of the electrodes any variation of fluid resistivity or permeability in the vicinity of one electrode could change the electrokinetic coupling coefficient inducing a surface electrokinetic potential anomaly in regard to the interpretation of the electrokinetic effect which occurs at large distance from the epicenter a larger electrokinetic potential anomaly could be measured between electrodes situated along a vertical fluid flow for instance in a shallow borehole an electrokinetic potential anomaly up to 30 mv for a fluid conductivity of 0 01 s m and a rock permeability of 10 12 m2 could be observed with a change of the underground water table level as slight as 50 cm 50 mbar moreover if the permeability between the electrodes is increased by a factor of 8 103 the electrokinetic coupling coefficient could be enhanced by a factor up to 650

high resolution shallow seismic experiments in sand part i water table fluid flow and saturation a high resolution very shallow seismic reflection and refraction experiment was conducted to investigate the seismic response of groundwater level changes in beach sand in situ a fixed 10 m long receiver array was used for repeated seismic profiling direct measurements of water level in a monitoring well and moisture content in the sand were taken as well the water table in the well changed by about 1 m in slightly delayed response to the nearby ocean tides in contrast inversion of the seismic data yielded a totally different picture the reflection from the water table at high tide appeared at a later time than the reflection at low tide this unexpected discrepancy can be reconciled using gassmann s equation a low velocity layer must exist between the near surface dry sand and the deeper and much faster fully saturated sand this low velocity layer coincides with the newly saturated zone and is caused by a combination of the sand s high density close to that of fully saturated sand and its hig

experimental measurements of seismoelectric signals in borehole models s u m m a r y an experimental system is built for the electrokinetic measurements with a small scaled seismoelectric detector and a high resolution digitizer 1 ms s 1 22 bits the acoustic and seismoelectric experiments are carried out in different boreholemodels at the high frequency of 90 khz in the laboratory all the localized seismoelectric signals that accompany compressional wave shear wave and stoneley wave are first clearly observed with a monopole source in sandstone boreholes that are saturated by tap water the amplitudes of these signals are measured in the range of 1 120 v which is useful for designing the seismoelectric logging instruments then the amplitude ratio of electric signal to acoustic pressure rep for each of the three waves is calculated and compared with the theoretical simulations based on the experimental data we find that seismoelectric logging signals as well as rep become stronger at the more permeable borehole model we also find that seismoelectric logging signals are more sensitive to permeability and porosity comparedwith acoustic logging signals therefore this study verifies the feasibility of seismoelectric well logging and further indicates that the seismoelectric logging technique might be a preferable method to estimate formation parameters in the field measurements

palaeomagnetic study of the lower cambrian longmyndian sediments and tuffs welsh borderlands summary this study covers detailed thermal chemical and alternating field demagnetization from 50 sites distributed through all units of the longmyndian sedimentary succession with the exception of the stretton shales and eight sites in the tuff bands comprising the batch volcanics the resultant effect of treatment is to remove a low blocking temperature steep component and isolate a coherent remanence direction d 116 i 76 the total nrm is composite and not an adequate indicator of the palaeofield chemical demagnetization indicates that both hematite and magnetite grains contribute to this component and these two phases are magnetized in the same direction a negative fold test shows that this remanence was acquired during or after the longmyndian folding formation of the major magnetic constituent authigenic magnetite is linked to dewatering during rapid uplift following the folding which is dated by both the rb sr and fission track methods at c 525 ma the study defines a palaeopole of this age remote from the later apw path for britain and links the late precambrian lower cambrian path defmed from basement rocks of england and wales with the ordovician and younger results palaeomagnetic results from tuff bands within the sediments and lower silurian age intrusions cutting the outcrop are also reported

luminescence dating in less than ideal conditions case studies from klasies river main site and duinefontein south africa abstract luminescence dating results are reported for two south african archaeological sites where changes in the dose rate through time are apparent a subtraction procedure involving analysis of both quartz and potassium feldspar extracts is applied to detect and correct for these changes at the middle stone age site of klasies river the dose rate changes inferred from geologic evidence for shell dissolution turn out not to be significant having occurred rapidly after deposition the dates here range from 50 115 ka in broad agreement with other evidence the important howiesons poort component here appears to date somewhat later about 55 60 ka than at several other sites in south africa at the acheulian site of duinefontein the dose rate changes also thought to be caused by shell dissolution are on the other hand very significant the subtraction ages here 125 300 ka agree with independent chronological assessments