review of magnetic and electric field effects near active faults and volcanoes in the u s a abstract synchronized measurements of geomagnetic field have been recorded along 800 km of the san andreas fault and in the long valley caldera since 1974 and during eruptions on mount st helens since 1980 for shorter periods of time continuous measurements of geoelectric field measurements have been made on mount st helens and near the san andreas fault where moderate seismicity and fault slip frequently occurs significant tectonic and volcanic events for which nearby magnetic and electric field data have been obtained include 1 two moderate earthquakes m l 5 8 for which magnetometers were close enough to expect observable signals about three source lengths 2 one moderate earthquake m s 7 3 for which magnetometers were installed as massive fluid outflow occurred during the post seismic phase 3 numerous fault creep events and moderate seismicity 4 a major explosive volcanic eruption and numerous minor extrusive eruptions and 5 an episode of aseismic uplift for one of the two earthquakes with m l 5 8 seismomagnetic effects of 1 3 and 0 3 nt were observed for this event magnetometers were optimally located near the epicenter and the observations obtained are consistent with simple seismomagnetic models of the event similar models for the other event indicate that the expected seismomagnetic effects are below the signal resolution of the nearest magnetometer precursive tectonomagnetic effects were recorded on two independent instruments at distances of 30 and 50 km from a m l 5 2 earthquake longer term changes were recorded in one region in southern california where a moderate m l 5 9 earthquake has since occurred surface observations of fault creep events have no associated magnetic or electrical signature above the present measurement precision 0 25 nt and 0 01 respectively and are consistent with near surface fault failure models of these events longer term creep is sometimes associated with corresponding longer term magnetic field perturbations correlated changes in gravity magnetic field areal strain and uplift occurred during episodes of aseismic deformation in southern california primarily between 1979 and 1983 because the relationships between these parameters agrees with those calculated from simple deformation and tectonomagnetic models the preferred explanation appeals to short term strain episodes independently detected in each data set an unknown source of meteorologically generated noise in the strain gravity and uplift data and an unknown but correlated disturbance in the absolute magnetic data might also explain the data no clear observations of seismoelectric or tectonoelectric effects have yet been reported the eruption of mount st helens generated large oscillatory fields and 9 2 nt offset on the only surviving magnetometer a large scale traveling magnetic disturbance passed through the san andreas array from 1 to 2 h after the eruption subsequent extrusive eruptions generated small precursory magnetic changes in some cases these data are consistent with a simple volcanomagnetic model magneto gas dynamic effects and a blast excited traveling ionospheric disturbance traveling ionospheric disturbances tids also generated by earthquake related atmospheric pressure waves may explain many electromagnetic disturbances apparently associated with earthquakes local near fault magnetic field transients rarely exceed a few nt at periods of a few minutes and longer

quantitative analysis of reservoir rocks by microfocus x ray computerised tomography abstract microfocus computer tomography ct is a non destructive imaging technique that allows visualisation of internal features within non transparent objects such as sedimentary rocks with resolution as good as 10 m in three dimensions this technique is highly superior to medical ct widely used in geology and material sciences superior resolution is achieved by decreasing the x ray spot size which allows a decrease in the penumbra effect and an increase in primary magnification by placing the sample close to the x ray source since polychromatic x ray sources are used the technique is not free of artefacts application of filtering techniques is recommended to circumvent these artefacts and to achieve good quality images for quantitative analysis application of a dual energy approach allows quantification of density as well as effective atomic number distribution of internal features as the technique acquires data in 3d ct provides an ideal tool to link 2d thin section petrography to 3d petrophysical measurements or to be used within an upscaling approach a number of applications and the use of dual energy density and effective atomic number characterisations are presented attention is also paid to some of the limitations of this technique

effects of microstructure on water imbibition in sandstones using x ray computed tomography and neutron radiography capillary imbibition in variably saturated porous media is important in defining displacement processes and transport in the vadose zone and in low permeability barriers and reservoirs nonintrusive imaging in real time offers the potential to examine critical impacts of heterogeneity and surface properties on imbibition dynamics neutron radiography is applied as a powerful imaging tool to observe temporal changes in the spatial distribution of water in porous materials we analyze water imbibition in both homogeneous and heterogeneous low permeability sandstones dynamic observations of the advance of the imbibition front with time are compared with characterizations of microstructure via high resolution x ray computed tomography ct pore size distribution mercury intrusion porosimetry and permeability of the contrasting samples we use an automated method to detect the progress of wetting front with time and link this to square root of time progress these data are used to estimate the effect of microstructure on water sorptivity from a modified lucas washburn equation moreover a model is established to calculate the maximum capillary diameter by modifying the hagen poiseuille and young laplace equations based on fractal theory comparing the calculated maximum capillary diameter with the maximum pore diameter from high resolution ct shows congruence between the two independent methods for the homogeneous silty sandstone but less effectively for the heterogeneous sandstone finally we use these data to link observed response with the physical characteristics of the contrasting media homogeneous versus heterogeneous and to demonstrate the sensitivity of sorptivity expressly to tortuosity rather than porosity in low permeability sandstones

impact of sedimentology and diagenesis on the petrophysical properties of a tight oolitic carbonate reservoir the case of the oolithe blanche formation bathonian paris basin france the oolithe blanche formation was studied in three quarries located at the south eastern edge of the paris basin france heterogeneities in reservoir properties were assessed through a sedimentological diagenetic and petrophysical study the relationships between depositional settings diagenesis and petrophysical properties were analysed using detailed petrographic studies image analysis nano ctscans and petrophysical measurements the carbonate reservoir pore network is mainly controlled by intraparticle microporosity which ensures the connectivity with interparticle meso and macroporosity early cementation vs early compaction processes mainly grain interpenetration may have considerable influence on fluid flow properties and parameters such as permeability acoustic velocities and tortuosity better reservoir properties are found when compaction processes begin before cementation from statistical analyses e g principal component analysis and linear discriminant analysis a sedimentological diagenetic and petrophysical model is proposed that is in a good agreement with the geological model developed from field work

magnetic and electric fields associated with changes in high pore pressure in fault zones application to the loma prieta ulf emissions we determined the electric and magnetic fields generated during failure of faults containing sealed compartments with pore pressures ranging from hydrostatic to lithostatic levels exhumed fault studies and strain measurement data limit the possible size of these compartments to less than 1 km in extent rupture of seals between compartments produces rapid pore pressure changes and fluid flow and may create fractures that propagate away from the high pressure compartment along the fault face nonuniform fluid flow results from pressure decrease in the fracture from crack generated dilatancy partial blockage by silica deposition and clearing as pressure increases a direct consequence of this unsteady fluid flow may be associated transient magnetic signals caused by electrokinetic piezomagnetic and magnetohydrodynamic effects models of these processes for fault geometries with 1 km high pressure compartments show that electrokinetic effects are several orders of magnitude larger than the other mechanisms the electrokinetic signals produced by this unsteady flow are comparable in magnitude and frequency to the magnetic signals observed prior to the ml 7 1 loma prieta earthquake of october 18 1989 provided fracture lengths are less than 200 m

permeability anisotropy and its relations with porous medium structure 1 the complete permeability tensor of 18 porous rock cores was determined by means of x ray tomography monitoring during the displacement of a salty tracer to study the effect of the pore space geometry on the anisotropy of permeability we compared the three dimensional shape of the invasion front with the x ray porosity maps obtained before injection the samples clean and shale bearing sandstones limestones and volcanic rocks belong to a broad variety of granulometry and pore space geometry their porosity ranges from 12 to 57 and their permeability ranges from 1 5 10 14 to 4 10 12 m2 for sandstones the permeability anisotropy is well correlated with the presence of bedding for volcanic rocks it is clearly related to the orientation of vesicles or cracks however for limestones no evident link between the geometry of the porous network and the permeability anisotropy appears probably because of the influence of the nonconnected porosity that does not contribute to the hydraulic transport this systematic work evidences the ability and the limitations of the tracer method to characterize the anisotropy of permeability in the laboratory in a simple and rapid way

morphology of red creek wyoming an arid region anastomosing channel system red creek in the red desert area of the great divide basin wyoming is an arid region anastomosing stream the narrow deep and sinuous main channel is flanked by anastomosing flood channels or anabranches most anabranches are initiated at meander bends the primary mechanism of anabranch initiation is avulsion during overbank floods anabranch enlargement occurs by headward erosion anabranches act as distributary channels during floods when water and sediment from overbank flows are transported to and deposited on the floodplain via the anabranches during periods of low discharges the anabranches act as tributaries to the main channel transporting runoff from the floodplain and surrounding hillslopes to the main channel of red creek aggradation is occurring in the main channel and on the floodplain throughout the study reach infilling of the main channel occurs primarily by lateral accretion while the floodplain accretes vertically through deposition of overbank sediment from the main channel and anabranches infilling of the main channel may cause avulsion of the main channel into an anabranch the abandoned main channel segment may then fill completely or act as an anabranch because lateral migration of channels is inhibited by the high cohesion of the silt and clay channel sediment periodic avulsion is the primary form of lateral mobility in the system

pre seismic electromagnetic phenomena in the framework of percolation and fractal theories abstract understanding the electromagnetic response to geodynamic processes occurring in the earth s upper crust in particular pre seismic and seismic processes is a challenging task in modern geophysics there is increasing evidence that seismo electromagnetic sem phenomena are difficult to describe quantitatively by linear models using averaged parameters of the medium such as electrical and hydraulic conductivities because the upper crust is highly inhomogeneous at all scales porous and can be fully or partially water saturated the most natural way to describe its parameters is via fractal theoretic and percolation theoretic models recent studies indicate that the electrokinetic effect is the most likely driving mechanism for the various types of sem signals here we considered the hydraulic electric and electrokinetic conductances of a porous water saturated medium as a function of porosity and moisture content utilizing a percolation fractal approach we show that the electric conductivity and electrokinetic current in such a medium are both proportional to c 2 and c 2 where c and c are the critical values of porosity and moisture content respectively this behavior admits the possibility of a relatively large change in the respective electric and electrokinetic parameters due to a small change in the mechanical strain field this is significant because it may account for the appearance of some types of sem signals at large distances from the earthquake origin which is the main deficiency of most models indeed the anticipated strain changes related to pre earthquake processes is usually very small except near the focal area the expected averaged electromagnetic response also would be very small unless a local underground water system exists not necessarily near the focal area but which is close to critical point s we discuss the conditions under which electrotelluric and geomagnetic variations can accompany mechanical disturbances in the earth s crust

theory of electrokinetic effects occurring at the final stage in the preparation of a tectonic earthquake crustal deformation processes occur with relative rapidity when short term precursors appear at the final preparatory stage of a tectonic earthquake this paper contains calculations of crustal bulk strain before an earthquake based on the model of a rigid inclusion the data give initial conditions for the problem of fluid percolation in a two phase medium accompanied by electrokinetic effects the solution provides a description of systems of currents arising in the vicinity of the source region an analysis of this theoretical evidence yields information on the behaviour of short term precursors in telluric currents and the geomagnetic field

seismic electric signals ses and earthquakes a review of an updated van method and competing hypotheses for ses generation and earthquake triggering abstract electromagnetic phenomena are sometimes associated with seismic events but earthquake prediction using seismic electric signals ses has not been seriously considered since the early 1990s there are several causes 1 false alarms that have created panics in greece and 2 a strong critique of the varotsos alexopoulos nomicos van method used there an updated van method that includes time series analysis has made successful medium range predictions and short range predictions when coupled with seismic time series analysis four types of potential mechanisms to create precursory ses relying on deformation effects temperature effects ore mineral movement or groundwater variation are reviewed data relevant to each are insufficient to favor a single mechanism records of the number of false positive and missed events for an updated van method have not been well maintained false positives appear to be abundant and render the method unusable co seismic groundwater fluctuations are addressed and two novel hypotheses of earthquake trigger mechanism are also presented ionospherically induced electric and resultant magnetic fields to weaken rock thereby causing failure and volatiles liberated from minerals additionally promoting rock failure recommendations for further study are included

physical properties of the variations of the electric field of the earth preceding earthquakes ii determination of epicenter and magnitude abstract as reported in the preceding paper a transient change of the electric field of the earth seismic electric signal hereafter called ses appears many hours before an earthquake eq by measuring this change in a given direction and dividing it with a suitable relative effective resistivity one obtains a quantity that reflects the current density in this direction measurements in two directions e w and n s give the relative signal intensity jrel at the station under consideration by measuring jrel at a number of stations and considering that it attenuates according to a 1 r law the epicenter can be determined with an accuracy usually around 100 km once the epicenter has been determined the product jrel r can be evaluated so that the magnitude m can be estimated by resorting to an empirical log jrel r versus m plot the uncertainty of m is around 0 5 units following sobolev 1975 and for the statistics to be beyond any doubt predictions were officially documented before the eq occurrence for 23 earthquakes with a magnitude equal or greater than ms 5 0 two events were missed the present method is compared to other electrical methods used in china japan and soviet union a number of problems concerning the origin of the effect its directivity and the attenuation with distance remain open for further studies

morphology and recent sediments of the lower anastomosing reaches of the attawapiskat river the attawapiskat river is one of the major low gradient rivers which cross the flat wetlands of the hudson bay lowland it has a nival regime leading to strong short duration spring floods during which considerable amount of sediment is carried for the remainder of the year only organic matter is transported in solution and suspension the land is isostatically rebounding after the pleistocene glaciations the river downcuts through the softer holocene estuarine basal clays and the pleistocene tills to the paleozoic carbonate bedrock the river develops an anastomosing pattern in the lower reaches and it acquires an irregular meandering character inland it does not construct a significantly thick delta as the river borne sediments are dispersed along the coasts by tides longshore currents and ice rafting the river is subjected to frequent ice jams in its lower reaches as warmer southern waters flow toward the still frozen mouth and sea during the jams secondary channels become active floodings develop and sedimentation occurs on the levees ice scours and ice rafted materials affect the river banks greatly the principal sedimentary environments within the river consist of 1 erosional shoals covered by boulder and pebble lags over till 2 sorted coarse to medium sized sands in junction bars downstream from islands these sands frequently develop gas bubbles and they contain characteristic fining upward sequences from boulder pavements at the base grading up into rippled sand with abundant organic matter grading up into laminated silt and sand and organic matter 3 secondary channels which develop thin silty and clay drapes on eroded hard substratum and are filled eventually by thick peats and 4 river banks which show well developed regressive sequences from pleistocene tills at the base overlain by estuarine sparsely fossiliferous clays capped by thin conglomeratic units with abundant reworked macoma balthica shells grading upward into upper tidal flat sand irregularly laminated sand and silt of marshes and regular interbedding of sand silt and organic layers of the levees at the top

effective darcy scale contact angles in porous media imbibing solutions of various surface tensions 1 surface tensions of high salinity solutions are significantly different from those of pure water our objective was to develop and test a methodology to determine whether these surface tension effects predictably alter imbibition into dry and moist porous media static and dynamic experiments were performed using four grades of quartz sand to determine the effects of solution salinity on imbibition results were quantified as apparent contact angles between the sand and three solutions pure water 5 molal nano3 and n hexane contact angles determined using a static method in initially air dried sand ranged from 23 to 31 with the same values found for both water and the nano3 solution effective contact angles determined for the air dried sand using a dynamic method based on a modified version of the green and ampt model were about twice those found using the static method averaging 45 and 62 for water and the nano3 solution respectively in prewetted sands the dynamic imbibition data yielded apparent contact angles of 2 and 21 for water and the nano3 solution respectively with the latter value comparing well to a predicted value of 25 for the nano3 solution solely on the basis of surface tension contrast the results of this study indicate that on the darcy scale saline solutions appear to follow the relationship of nonzero contact angles with other miscible fluids of different surface tensions used to prewet the sand grains in agreement with the macroscale infiltration results of weisbrod et al 2004

the meandering braided river threshold a reappraisal abstract the concept of a threshold discharge slope q s combination that separates braided from meandering streams is critically re examined partly by review of previous data and partly by analysis of new data from south island new zealand it is argue d that discriminant functions that use mean annual discharge provide a poor physical basis for interpretations based on natural processes while bankfull discharge is also deficient because it is not a fully independent variable the common use of channel rather than valley slope adds a further misleading component because channel slope is partly dependent on channel pattern sinuosity most previous studies have inadequately treated the role of bed particle size in pattern discrimination failing to recognize that active gravel streams must plot higher on a q s chart than sand bed channels irrespective of pattern because of the greater power requirements for bed material movement within any one size class of bed material there is no evidence to indicate a clear discrimination between braiding and meandering only a weak statistical association between pattern and slope discharge values while this association may be explicable in terms of the high shear stress and stream power that accompany high qs values promoting braiding the real prerequisite for braiding appears to be high loads of bed calibre material producing wide shallow channels a factor that is only partly controlled by stream power accordingly the search for a pattern threshold based on discharge and slope seems to be a futile exercise an interpretation of existing pattern discriminant functions from the perspective of the threshold shear stress for particle movement is made it is concluded that in effect these functions merely state that gravel bed streams are more likely to be braided than channels in finer sediment statistically this appears to be valid and reasons for that difference are offered based on the balance between imposed bed material load and channel capacity

x ray imaging of water motion during capillary imbibition geometry and kinetics of water front in intact and damaged porous rocks 1 imaging fluid flow in porous rocks is a constant challenge in earth sciences we present the results of an experimental study to image capillary imbibition processes using an industrial x ray scanner to investigate fluid motion processes in relation with microstructural properties for various porous rocks two dimensional density maps obtained at a constant time step of about 3 s in standard capillary rise experiments on intact and damaged rock samples are analyzed both sandstones and carbonate rocks have been studied the geometry of the water front depends on the sample geometry but also on the rock microstructure and the dynamics of capillary processes as its curvature changes during water rising we discuss the effect of anisotropy heterogeneity microstructures and driving forces on the kinetics and geometry of the water front once a good knowledge of the intact rock behavior was achieved mechanical damage was induced in creep experiments at increasing stress levels with a continuous recording of acoustic emissions the density maps obtained during imbibition under the scanner were analyzed to show how the mechanical deformation alters the fluid displacement patterns we found that the velocity and the geometry of the waterfront are strongly disturbed in comparison to the intact rock however our study was not really conclusive on the effect of fractures on fluid flow patterns conducting capillary imbibition experiments coupled with imaging techniques using an x ray scanner is a powerful technique to image fluid flow patterns in reservoir rocks and how they are affected by deformation

feasibility of monitoring hydraulic fracturing using time lapse audio magnetotellurics abstracthydraulic fracturing is widely used for initiating and subsequently propagating fractures in reservoir strata by means of a pressurized fluid to release oil and gas or to store industry waste downhole or surface microseismic monitoring is commonly used to characterize the hydraulically induced fractures however in some cases downhole microseismic monitoring can be difficult due to the limitation imposed by boreholes surface microseismic monitoring often faces difficulties acquiring high signal to noise ratio data because of the on site noise from hydraulic fracturing process research and field observations indicate that injecting conductive slurry or water into a strata may generate distinct time lapse electromagnetic anomalies between pre and posthydraulic fracturing these anomalies provide a means for characterizing the hydraulic fracturing using time lapse electromagnetic methods we examined the time lapse variation over an hour one day one month and two years of observed audio magn

laboratory measurements anomalous 0 1 0 5 hz streaming potential under geochemical changes implications for electrotelluric precursors to earthquakes streaming potentials resulting from flow of various salt solutions in rock were measured on saturated sediments fontainebleau sandstones the streaming potential v was found to be proportional to the driving pore pressure p pulses of amplitude 15 40 mv in the frequency range of 0 1 to 0 5 hz were observed when the conductivity of the injected water was decreased and the fluid flow rate was relatively low corresponding to a darcian velocity of 17 to 30 cm h the amplitudes of these pulses are 47 to 133 of the corresponding steady components of the v values such geochemically induced effects may possibly be responsible for the frequency signals from 0 1 to 0 5 hz that were sometimes observed before an earthquake

anastomosing river deposits sedimentation rates and basin subsidence magdalena river northwestern colombia south america situated in a tectonically active foreland basin the magdalena river consists of vertically accreting levee confined channels and adjacent extensive wetlands which are interpreted as an anastomosing river sedimentary system equivalent rates of basin filling and subsidence average 3 8 mm yr 1 based on 18 14c dates from five bore holes drilled to depths of 55 m and sediment transport budgets from 35 years of measurement located in a savanna tropical climate anastomosing river deposits of the magdalena are remarkably similar to the anastomosing deposits of the upper columbia river in a temperate cold climate in western canada suggesting that climate is not a controlling factor of anastomosis the geometry of anastomosing channel fills in the magdalena consists of stratigraphically non uniform low sinuous narrow stringers of sand up to 30 m thick by 600 m wide a width depth ratio of 20 thin 1 2 m off channel crevasse splay sand sheets extend laterally up to 10 km distance when buried both sand deposits become encased by lacustrine or marsh mud to form stratigraphic traps while there are few modern anastomosing river systems as compared to braiding and meandering there may be a disproportionately large number of ancient anastomosed fluvial rock sequences due to the rapid rate of vertical accretion such a different depositional style and geometry of sand bodies have considerable significance in the interpretation of some ancient fluvial rock sequences because it provides an alternative to the meandered and braided river deposition models

remote monitoring of the mechanical instability induced by fluid substitution and water weakening in the laboratory abstract we studied the effect of fluid injection on the mechanical behaviour of the poorly consolidated and layered sherwood sandstone under varying stresses with micro seismic ms monitoring in order to highlight possible weakening effects water and inert oil have been injected into critically loaded samples to assess their effect on strength and elastic properties derive the ultrasonic signature of the saturation front for each fluid and the potential development of damage to this end the specimens were instrumented with 16 ultrasonic p wave transducers used for both passive and active monitoring during loading and fluid injection a first set of injection tests in hydrostatic conditions using either water or inert oil has been performed on samples subjected to low confining pressure water invasion in the pore space induces a significant decrease of the p wave velocity whereas oil invasion shows a velocity increase the velocity decrease associated with water injection is analysed in terms of attenuation mechanisms and corresponding critical frequencies a second series of injection tests with the same fluids has been performed during creep tests on critically loaded samples the development of mechanical instability inducing micro seismic activity is observed only when water is injected into the sample the recorded micro seismic events are spatially and temporally located thanks to the dedicated velocity models accounting for the initially homogeneous sample anisotropy and for the heterogeneous velocity field associated with fluid migration within the sample the consistency between the relocated clusters of events and the final damage pattern is verified thanks to x ray computed tomography images of the samples taken post mortem

electromagnetic precursors to earthquakes in the ulf band a review of observations and mechanisms despite over 2 decades of international and national monitoring of electrical signals with the hope of detecting precursors to earthquakes the scientific community is no closer to understanding why precursors are observed only in some cases laboratory measurements have demonstrated conclusively that self potentials develop owing to fluid flow and that both resistivity and magnetization change when rocks are stressed however field experiments have had much less success many purported observations of low frequency electrical precursors are much larger than expectations based on laboratory results in some cases no precursors occurred prior to earthquakes or precursory signals were reported with no corresponding coseismic signals nonetheless the field experiments are in approximate agreement with laboratory measurements maximum resistivity changes of a few percent have been observed prior to some earthquakes in china but the mechanism causing those changes is still unknown anomalous electric and magnetic fields associated with fluid flow prior to earthquakes may have been observed finally piezomagnetic signals associated with stress release in earthquakes have been documented in measurements of magnetic fields

results of ultra low frequency magnetic field measurements during the guam earthquake of 8 august 1993 we report the results of measurements of ultra low frequency magnetic noise during a large earthquake ms 7 1 at guam of 8 august 1993 depth 60 km the ulf observing system is located in the guam island about 65 km from the epicenter several distinct features of this analysis are summarized 1 we have proposed rather sophisticated statistical analyses monthly mean standard deviation in order to estimate the wave intensity and polarization i e ratio z h 2 a comparison between the ulf wave activity and kp is useful in distinguishing between the space geomagnetic pulsations and non space emissions 3 then the use of the ratio z h is found to be of essential importance in discrimating the emissions presumably of seismic origin from space plasma waves 4 the statistical analysis of the temporal evolution of this ratio has yielded that it shows a broad maximum only about one month before the earthquake and this suggests that the emissions during this period are very likely to be magnetic precursors 5 the temporal variation of z component is similar to that for the loma prieta earthquake such that it shows a broad maximum ten days two weeks before the earthquake and another increase a few days before the earthquake 6 the emissions presumably associated with the earthquake are of noise like nature and their main frequency is 0 02 0 05 hz with maximum intensity 0 1 nt

low frequency magnetic field measurements near the epicenter of the ms 7 1 loma prieta earthquake the authors report the results of measurements of low frequency magnetic noise by two independent monitoring systems prior to the occurrence of the m sub s 7 1 loma prieta earthquake of 17 october 1989 their measurements cover 25 narrow frequency bands in the more than six decade frequency range 0 01 hz 32 khz with a time resolution varying from a half hour in the ulf range 0 01 10 hz to one second in the elf vlf range 10 hz 32 khz the ulf system is located near corralitos about 7 km from the epicenter the elf vlf system is located on the stanford campus about 52 km from the epicenter analysis of the elf vlf system is located on the stanford campus about 52 km from the epicenter analysis of the elf vlf data has revealed no precursor activity that they can identify at this time however the ulf data have some distinctive and anomalous features first a narrow band signal appeared in the range 0 05 0 2 hz around september 12 and persisted until the appearance of the second anomalous feature which consisted of a substantial increase in the noise background starting on 5 october and covering almost the entire frequency range of the ulf system third there was an anomalousmore dip in the noise background in the range 0 2 5 hz starting one day ahead of the earthquake finally and perhaps most compelling there was an increase to an exceptionally high level of activity in the range 0 01 0 5 hz starting approximately three hours before the earthquake there do not appear to have been any magnetic field fluctuations originating in the upper atmosphere that can account for this increase further while the systems are sensitive to motion seismic measurements indicate that there were no significant shocks preceding the quake thus the various anomalous features in the data and in particular the large amplitude increase in activity starting three hours before the quake may have been magnetic precursors less

influence of mechanical damage on fluid flow patterns investigated using ct scanning imaging and acoustic emissions techniques 1 in order to get some insight into the influence of mechanical deformation on fluid flow patterns we performed series of x ray ct scanning runs during capillary imbibition experiments to image the water front displacement in bentheim sandstone samples a three steps procedure was followed 1 intact rock samples were tested 2 the samples were deformed in creep experiments under triaxial conditions with recording of acoustic emissions 3 the deformed samples experienced a second capillary imbibition test with ct scanning imaging using image analysis tools relevant flow parameters like the height velocity and curvature of the water front were measured stress induced damage had a significant impact on the flow patterns however contrasted results were obtained localized damage evidenced by acoustic emissions clustering resulted in one experiment in local disturbance of the flow patterns whereas in the second one a fracture detected by ct scan did not modify the local flow geometry

predicting water permeability in sedimentary rocks from capillary imbibition and pore structure this study was financed by the spanish ministry of science and innovation cgl2011 25162 a pre doctoral research fellowship was awarded to c pla for this project

alluvial river response to neotectonic deformation in louisiana and mississippi repeat geodetic surveys show uplift of the monroe and wiggins anticlines in louisiana and mississippi there are deformed quaternary terraces which indicate long term deformation in the valleys of the alluvial rivers that cross these structures and there are floodplain and channel convexities that provide evidence of modern deformation in addition the channels show significant variations of morphology sinuosity gradient and depth and behavior appropriate to reaches of increased and decreased valley slope these alluvial rivers are adjusting to modern deformation and their adjustment confirms two geodetic leveling anomalies

streaming potential properties of westerly granite with applications this paper reports on laboratory measurements of the streaming potential properties of common minerals and in particular of westerly granite data are presented for these solids in contact with simple electrolytic solutions and in one case for temperature variation in the range 5 70 c for 1 1 electrolytes the ratio of the streaming potential coupling coefficient to fluid resistivity was approximately 4 mv atm m and for 2 1 electrolytes 2 mv atm m the effect of temperature was small 0 05 mv c temperature increase and indicates that the zeta potential was independent of temperature preliminary experimental results using mixed electrolytic solutions nacl cacl2 and two phase fluid flow are also described two phase flow can enhance streaming potentials by as much as a factor of 3 or 4 the laboratory data are used to interpret streaming potentials in geological problems such as earthquake prediction geothermal prospecting using the self potential method and surface conductivity contribution to the electrical resistivity of rocks

interrelations of the petrophysical sedimentological and microstructural properties of the oolithe blanche formation bathonian saline aquifer of the paris basin abstract the paris basin seems a suitable location for co2 capture and storage given both the amount of co2 produced and the availability of depleted fields and deep saline aquifers this study investigates the petrophysical properties in relation to the sedimentary and diagenetic environment of the oolithe blanche formation middle jurassic a deep saline aquifer considered a potential candidate for co2 storage because of the scarcity of core data in the paris basin our investigation was based on the study of field analogues in the south east part of the paris basin the oolithe blanche formation bathonian is 70 80 m thick eighteen oriented blocks were collected from three outcrops selected for the different sedimentological facies identified in the formation their petrophysical properties were measured porosity pore size distribution derived from mercury injection tests permeability and capillary imbibition parameters the sedimentary and diagenetic facies were defined by using optical microscopy coupled with image analysis on thin sections and sem the oolithe blanche formation is composed of oolitic and bioclastic limestones deposited in a shallow marine setting three main sedimentological facies were defined the oolitic shoal facies the tide dominated facies and the prograding oolitic facies they exhibit subtle variations in composition and in the distribution of carbonate textures porosity measured using the water saturation triple weight method ranges from 6 to 34 permeability values are low between 0 1 md and 9 md the data from mercury intrusion porosimetry show that the distribution of the pore throat diameter is either unimodal microporosity only or bimodal macro and microporosity microporosity is intraparticle intraooid and macroporosity is interparticle and is related to dolo dedolomitization variations of petrophysical parameters can be explained by the microstructure now these latter results calibrated by studying field analogues must be extended to borehole data and core data to optimize our knowledge of the deep saline aquifer of the oolithe blanche formation without such investigations it will be impossible to estimate the suitability of this deep saline aquifer for co2 storage

geoelectric potential changes possible precursors to earthquakes in japan whether electromagnetic precursors to earthquakes eqs exist is an important question not only for eq prediction but also for understanding the physical processes of eq generation slow transient geoelectric potential changes have been observed before several recent eqs in japan in most cases they appeared 1 19 days before the eqs and their duration and intensity were several minutes to 1 h and 1 2 mv 100 m the changes appeared before five of all six eqs with magnitude 5 that occurred within 20 km of our stations during the observation period changes were also detected at greater epicentral distances up to 75 km before two other eqs including one eq of magnitude 4 7 which was preceded by a signal simultaneously recorded at three widely separated stations these geoelectric potential changes have been distinguished through the following criteria from a multitude of other changes which were noise of various origins i the selected changes were proportional in amplitude to the length of the recording station s short 100 m dipoles and were simultaneously detected also on long 1 10 km dipoles when the latter were in operation ii no such changes occurred during the observation period that were not followed by eqs although the eq precursory nature of these geoelectric potential changes is admittedly unproven it seems that the present results warrant continued serious research into the occurrence generation and transmission of these signals and their possible causal relationship to eqs

ulf electromagnetic precursors for an earthquake at biak indonesia on february 17 1996 ulf electromagnetic emissions associated with a large earthquake occurred at biak island indonesia at 5h 59m ut on february 17 1996 magnitude mw 8 2 and depth 20 km from usgs catalog have been investigated on the basis of ulf magnetic observations at two stations biak and darwin in australia about 1 200 km apart though a simple analysis of magnetic field intensity horizontal and vertical magnetic field components at both stations was found to be closely correlated with geomagnetic kp activity a detailed analysis of the difference of h and z components at the two stations the polarization analysis z h and fractal analysis frequency spectrum slope at these two stations has yielded that the ulf emissions in the frequency range from 5 mhz to 30 mhz about 1 5 1 0 months before the quake are likely to be a precursory signature of the quake with its intensity on the order of 0 2 0 3 nt

ultrasonic monitoring of spontaneous imbibition experiments acoustic signature of fluid migration capillary rise experiments spontaneous imbibition tests were conducted in the laboratory with ultrasonic and x ray monitoring on the sherwood sandstone and the majella grainstone the aim was to provide a direct comparison between the variation in seismic attributes amplitude velocity spectral content and energy and the actual fluid distribution in the rock two pairs of ultrasonic p wave sensors located at different heights on a cylindrical rock specimen recorded every 5 s the waveforms when capillary forces make water rise up into the rock from the bottom in contact with a water tank simultaneously computerized tomography scan images of a vertical cross section were also recorded two important results were found i the amplitude of the first p wave arrival is impacted by the upward moving fluid front before the p wave velocity is while the fluid front has not yet reached the sensors level in contrast the p wave velocity decreases when the fluid front reaches the fresnel clearance zone the spectral analysis of the waveforms shows that the peak frequency amplitude is continuously decreasing without noticeable frequency shift ii a methodology based on the calculation of the analytical signal and instantaneous phase was designed to decompose each waveform into discrete wavelets associated with direct or reflected waves the energy carried by the wavelets is very sensitive to the fluid substitution process the coda wavelets related to reflections on the bottom end face of the specimen are impacted as soon as imbibition starts and can be used as a precursor for the arriving fluid

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

anastomosis and the continuum of channel pattern anastomosing rivers are characterized by multiple channels separated by islands excised from the floodplain their status relative to the continuum concept of channel pattern is assessed with channel pattern defined in terms of three variables flow strength bank erodibility and relative sediment supply using an ordinal scaling l ow m oderate h igh the traditional forms of straight meandering and braided have respective representations of l l l m l m l m and h h m h in terms of those variables the anastomosing pattern is on average represented by l l m h but not so definitively as other forms specification of the third element sediment supply is particularly hampered by the paucity of data but aggradation a characteristic of many anastomosing rivers can be thought of as symptomatic of a moderately high rate of supply relative to the ability for onward transport a sufficiently high rate of supply to a channel with low flow strength and resistant banks would induce shoaling and or lateral constriction that locally forces flow out of the main channel and ultimately leads to the cutting of anabranches a flow regime characterized by concentrated floods of relatively large magnitude is also regarded as highly conducive to the formation of new channels where low bank erodibility constrains channel capacity anastomosis may in certain cases represent a transitional form of channel pattern but there is no denying the longevity of some anastomosing systems

physical properties of the variations of the electric field of the earth preceding earthquakes i abstract in an earlier paper we described the physical properties of the preseismic transient variations in the electrotelluric field the present letter clarifies a few points that were not properly emphasized in our previous publications furthermore recent results like that of the destructive kalamata earthquake on september 13 1986 indicate that the duration of a preseismic electric signal may reach considerably high values lasting up to several hours

earthquake rupture as a critical point consequences for telluric precursors abstract earthquakes are related to rupture phenomena in complex statistical systems of cracks we first review some available information suggesting that rupture in earthquakes is a critical point the consequences of this hypothesis on several phenomena are then discussed in particular we focus our attention on a simple theory of electroseismic activity based on mechano electrical transduction effects such as the piezoelectric or the pressure induced polarization effect the neighbourhood of the rupture threshold is characterized by a specific response which could correspond to telluric wave activity observed as precursors of earthquakes

plausibility of long distance electrotelluric precursors to earthquakes a systematic analysis of the plausibility of different classes of models which could account for low frequency precursory electrotelluric signals observable at large distances from a large earthquake is carried out first we consider linear models for which either a stationary electrical field is generated by a current dipole in the source volume or a static deformation field results from a precursory strain in the source volume linearly inducing a streaming potential effect near the electrode in the former case only the conjunction of very favorable circumstances may lead to an electrical field larger than 10 mv km at more than 100 km with the constraint that no anomaly exceeds a few volts per kilometer in the source area it indeed requires a powerful electrical dipole current 104 a length 1 km which might be produced by a streaming potential effect associated with the connection of nonhydrostatic fluid reservoirs pore pressure difference up to 100 mpa a two dimensional channeling of the electrical currents from the source to the electrode area which would result from high conductivity horizontal layers and finally a local amplification factor of typically 100 near the electrodes which might be produced by a narrowing of the conductive channel for an elastic long distance interaction an electric field of 10 mv km at a distance 10 times the source length 100 km away from a m 6 earthquake requires a precursory deformation larger than the coseismic deformation if no strong local amplification is considered alternatively nonlinear models are proposed and discussed assuming that the crust is stuffed with small scale local instabilities fractures and nonhydrostatic fluid reservoirs among which a number are close to their instability threshold these instabilities may follow their own evolutionary process inducing sporadic fluid flow and hence streaming potential effect in the vicinity of the electrodes without any relation to a remote tectonic activity however these instabilities may be sensitive to very small changes in the stress field and hence be triggered by the elastic precursory deformation of a remote seismic source this requires that the precursory deformation near the instability is at least larger than the tidal strain 10 7 this necessary condition is met at a distance of up to 10 times the source dimension when the precursory strain source is only 10 of the coseismic deformation provided that the strain source is double the size of the seismic fault itself this model could account for the absence of self potential anomalies related to the earthquake itself as a large proportion of the available instabilities could have been swept away by the precursory strain furthermore with such a model a precursory strain in the source area could induce a detectable change in the background seismic activity over a vast area which may give a new interpretation of the times of increased probability observations for short distances the triggering of fluid instabilities by strain perturbation and the associated streaming potential effect may well be a promising model for explaining low frequency electric and magnetic precursors

electric and magnetic phenomena observed before the volcano seismic activity in 2000 in the izu island region japan significant anomalous changes in the ultra low frequency range 0 01 hz were observed in both geoelectric and geomagnetic fields before the major volcano seismic activity in the izu island region japan the spectral intensity of the geoelectric potential difference between some electrodes on niijima island and the third principal component of geomagnetic field variations at an array network in izu peninsula started to increase from a few months before the onset of the volcano seismic activity culminating immediately before nearby magnitude 6 class earthquakes appearance of similar changes in two different measurements conducted at two far apart sites seems to provide information supporting the reality of preseismic electromagnetic signals

comparison of the anisotropic behaviour of undeformed sandstones under dry and saturated conditions abstract this article presents a systematic analysis of the anisotropic behaviours of the bentheim and rothbach sandstones using ultrasonic p wave velocity electrical conductivity and magnetic susceptibility measurements for each sandstone the data were obtained from three core samples drilled perpendicularly to each other and tested in dry and water saturated conditions for acoustic and magnetic investigations the same statistical analysis was applied in order to present the data on comparable stereoplots surprisingly the bentheim sandstone which appeared homogeneous at macroscopic scale showed a stronger elastic and electrical anisotropy than the rothbach sandstone in which cross laminations were clearly identified as confirmed by a sedimentary magnetic fabric a discussion on the velocity contrasts between dry and saturated samples led us to consider two different origins for the observed anisotropies first by comparing electrical and acoustic properties in the bentheim sandstone we conclude that the nature of the anisotropic behaviour is linked to the anisotropy of pore shape the inclusion model developed by kachanov kachanov m 1993 elastic solids with many cracks and related problems advances in applied mechanics vol 30 academic press boston ma pp 259 445 accounts for our observations if one considers that the pore space is made of parallel flat pores with moderate pore aspect ratio second acoustic electrical and magnetic properties indicate that the observed anisotropy in the rothbach sandstone can be attributed to the matrix and more specifically to cementation we modified the dvorkin and nur geophysics 61 5 1996 1363 model of cemented granular media by introducing a spatially variable contact length and the model suggests that a very small variability of cemented contact length is enough to account for the observed p wave velocity anisotropy we emphasise the fact that combining several kinds of measurements is of great help in capturing the nature of the anisotropic behaviour of porous rocks

microfabric and anisotropy of elastic waves in sandstone an observation using high resolution x ray microtomography abstract petrophysical experiments using acoustic velocities to characterise anisotropies of mechanical behaviour of rocks are of essential relevance to understand the geomechanical behaviour of sandstone reservoirs under changing stress fields here we present high resolution x ray microtomography ct as a supplementary research tool to interpret anisotropic ultrasound velocities in sandstones with variation of isotopic stress specimens of two lower cretaceous sandstones localities bentheim and obernkirchen both germany have been used in petrophysical laboratory experiments under dry conditions to study ultrasonic sound velocities frequency of signal input 1 mhz subsequently oriented micro plugs drilled from the sandstone samples were investigated using high resolution x ray microtomography by means of image processing of the reconstructed scan images geometric attributes such as mean structural thickness orientation and tortuosity were evaluated from the ct data for both pore space and grain skeleton our observations clearly indicate the different roles of pore space and grain skeleton in regard to the propagation of ultrasonic waves because the pores do not transmit the waves it was sufficient to investigate the average thickness of this fabric element in contrast as the ultrasonic waves traverse the rock via the adjacent grains it was necessary to survey the actual travel lengths of seismic waves in the sandstone grain skeleton

permeability dependence of streaming potential in rocks for various fluid conductivities streaming potentials have been measured on sandstone and limestone samples in a large range of permeabilities the electrokinetic coupling coefficient increases with permeability and we explain this effect by the related variation of surface conductivity a model is proposed to study this effect for various fluid conductivities and it is shown that the dependence of the electrokinetic coupling coefficient on permeability is stronger for high fluid resistivity and is weaker for lower fluid resistivity when fluid resistivity is below 1 m permeability and streaming potential are no more related