electromagnetic induction studies in the eyre peninsula south australia magnetic field fluctuations have been recorded by an array of portable three component magnetometers at 60 sites across the eyre peninsula in south australia between december 1993 and march 1995 an additional 54 magnetometer data records collected prior to 1989 and described by milligan 1989 and milligan white chamalaun 1989 were included in the analysis a major conductive feature in the crust first noted by white milligan 1984 as the eyre peninsula anomaly epa is re examined to assess its continuity to the north of the original arrays and to investigate its relationship with major tectonic features magnetic field time series were converted to induction arrows in the frequency domain these induction arrows were initially inverted using the minimum structure 2 d occam approach to estimate the electrical conductance of the crust following this thin sheet forward modelling was used to examine the relationship between the conductance and the dominant tectonic features the principal results of the modelling are that a narrow conductive feature extends inland from the coast about 160 km before terminating and the conductance is in the range 3000 to 10 000 s which decreases inland a strong correlation exists between the electrical conductance of the eyre peninsula and bouguer gravity anomalies and in particular the epa is coincident with a significant bouguer gravity gradient there is also good agreement between the locations of the foci of earthquakes of magnitude greater than 4 0 and the epa we believe that the anomaly is associated with a geological fracture in the precambrian upper crust as a result of crustal extension prior to the rifting of australia from antarctica in the jurassic 160 ma

coast effect distortion of marine magnetotelluric data insights from a pilot study offshore northeastern japan abstract we report on strong coast effect distortions observed for broadband marine magnetotelluric mt data collected on the forearc offshore northeastern japan eight days of horizontal electric and magnetic fields recorded at eight seafloor stations and the horizontal magnetic fields from a land remote station were processed with a robust multiple station algorithm yielding good mt responses and inter station transfer functions at periods of 7 10 000 s transverse electric te mode responses have cusps in apparent resistivity and negative phases at periods around 1000 s while the transverse magnetic tm mode responses are galvanically depressed below the te responses an analysis of inter station transfer functions confirms that the apparent resistivity cusps are a magnetic field rather than electric field phenomenon consisting of an amplitude minimum and rapid phase change around a characteristic frequency poynting vectors for a te coast effect model study illustrate that the anomalous phases are associated with energy diffusing back up to the seafloor from below after being turned around from its usual downward propagating trajectory by inductive coupling between the conductive ocean and the resistive seafloor along the continental margin we show that the characteristic frequency and position of the te mode apparent resistivity cusps are determined by a relatively simple combination of the electrical resistivity of the seafloor the depth of the ocean and the distance from the coastline by including coastlines and bathymetry in 2d inversion we recover the seafloor conductivity structure along the forearc demonstrating that broadband data can constrain the thickness of conductive forearc sediments and the underlying high resistivity associated with the mantle wedge and subducting oceanic lithosphere

sea effect correction in magnetotelluric mt data and its application to mt soundings carried out in jeju island korea summary when magnetotelluric mt data are obtained in the vicinity of the coast the surrounding sea makes it difficult to interpret subsurface structures especially at the deep parts of the subsurface we apply an iterative method to remove the sea effect the iterative method was originally developed to remove the distortion due to topographic changes from mt data recorded on the seafloor the iterative sea effect correction method is carried out in two steps the first corrects the sea effect whereas the second inverts the sea effect corrected responses the two steps are alternatively carried out until the criterion for either the inversion or the seaeffect correction is satisfied because the surrounding 3 d sea bathymetry is only incorporated into forward modelling for the sea effect correction it can be more robust than the method that directly incorporates 3 d sea bathymetry into a model space for inversion the synthetic examples show that the sea effect correction method yields an inverted model comparable to the true model by applying the sea effect correction method to real field data acquired in jeju island korea we also demonstrate that the sea effect correction method effectively removes the sea effects from the 1 d and 2 d real field data which helps enhance the inversion results on the basis of these results it may be concluded that the iterative sea effect correction method can be used as a promising technique for recovering the true response of the subsurface in mt data suffering from sea effects

electromagnetic induction in new zealand analogue model and field results abstract the behaviour of electric and magnetic variations over north island new zealand is studied with the aid of a laboratory analogue model the source field frequencies used in the analogue modelling simulate naturally occurring geomagnetic variations of 5 120 min periods in phase and quadrature magnetic and electric fields for a selection of traverses for the modelled region of north island are presented since north island is of a relatively narrow cross section the field responses even for inland locations are expected to show strongly the effects of the surrounding ocean the irregular coastlines as well as the strait between north and south islands lead to coastal and inland field anomalies due to induced currents being deflected and channelled to produce localized current densities the comparison of model results with field station measurements obtained earlier individually by ingham and by midha for sites in the northeastern central and southern near cook strait regions of north island demonstrates the large role the ocean has in the observed field responses differences in the model and field results at some sites are expected and should reflect the effects of the local geology and the conductive substructure related to the complex tectonics of the region not simulated in the model

conductivity and fluids in the oceanic upper mantle abstract three deep electromagnetic soundings in the north pacific ocean were re examined our inversion of existing data showed the presence of conductors below the three sites in very good agreement with previous results we showed that h 2 o rich fluids as well as partial melting account for the conductivities observed this led us to determine temperatures ranging firstly from 1050 to 1150 c when only h 2 o rich fluids were present and secondly from 1300 to 1350 c for partial melting only we suggest that the upper mantle below the two oldest sites 30 and 72 ma is characterized by about 1 of partial melting in the presence of about 1 of h 2 o rich fluids below the third site that nearest to the ridge zone 1 ma the degree of partial melting might reach 10 the presence of 1 of h 2 o rich fluids would reduce the effect of the asthenospheric outflows on the lithospheric magmatism this agrees with the principle of thermodynamic decoupling of the asthenosphere and lithosphere the weakening effect of fluids particularly water on upper mantle rheology is also discussed we suggest that the asthenosphere and lithosphere might be decoupled mechanically which if true would affect small scale convection below the oceanic lithosphere

upper crustal resistivity structure of the east pacific rise near 13 n an active source electromagnetic em sounding has been conducted on the axis of the east pacific rise epr at 13 10 n 1d inversion and modelling techniques seeking resistivity as a function of depth have been applied to 8 hz amplitude data collected along the ridge crest resistivity is seen to increase monotonically between 50 m and 1 km below the seafloor increasing from 1 m to around 90 m we observe no intrinsic difference in upper crustal resistivity structure between the rise axis and 100 000 year old crust inferred surface porosities of 20 are larger than those recorded in 5 9 my old crust in dsdp hole 504b our data do not require and lack sufficient information for the reliable inclusion of a conductive termination to the model below 1 2 km

a theory for strong long lived squall lines abstract we study herein the mechanics of long lived line oriented precipitating cumulus convection squall lines using two and three dimensional numerical models of moist convection these models used in juxtaposition enable us to address the important theoretical issue of whether a squall line is a system of special long lived cells or whether it is a long lived system of ordinary short lived cells our review of the observational literature indicates that the latter is the most consistent paradigm for the vast majority of cases but on occasion a squall line may be composed of essentially steady supercell thunderstorms the numerical experiments presented herein show that either type of squall line may develop from an initial line like disturbance depending on the magnitude and orientation of the environmental shear with respect to the line with shallow shear oriented perpendicular to the line a long lived line evolves containing individually short lived cells our analysis of this type o

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

on the vertical gradient and associated heterogeneity in mantle electrical conductivity abstract equivalent scalar magnetotelluric response functions obtained from the robust analysis of geomagnetic field daily mean values at 16 magnetic observatories have been systematically inverted data from these sites were found to contain sufficient one dimensional 1 d data consistent with required source field assumptions for meaningful interpretation and were also found to have acceptably well bounded response function confidence limits previous work is reviewed which indicates that significant lateral heterogeneity in mantle conductivity structure is required by the data and that most of these data are adequately modeled by a series of simple two layered earth models with a conductivity jump at depths of 500 1000 km regional variations in upper mantle conductivity have previously been suggested by systematic inversion using the h algorithm these features are examined in the restricted model space of conductance delta functions the statistical significance of these variations is also investigated new evidence is found here for the coast effect at very long periods as well as related evidence for the presence of electrically conductive zones in the upper mantle beneath the pacific ocean semi norm minimization inversion techniques are applied to directly place strong constraints on the sharpness of the vertical conductivity gradient in the mid mantle the mid mantle conductivity jump is required to be abrupt with values beneath this mid mantle transition zone of o 1 s m 1 at all sites

approximations for the 2 d coast effect on marine magnetotelluric data marine natural source electromagnetic data acquired on continental margins are often of considerable scientific and commercial interest however the large conductivity contrast between the ocean and coast causes this type of data to be severely distorted for a 2 d coastal model this distortion is most pronounced for the marine magnetotelluric and geomagnetic response function derived from induced currents flowing parallel to the coast a maximal distortion occurs for a given period at a specific distance from the coast and causes severe anomalies in the magnitude and phase of the response functions based on a modelling study we empirically relate the characteristic period and characteristic distance to physical parameters such as the ocean depth and the host resistivity based on a simple analytical approach we test these approximations and show that maximum distortion occurs when destructive interference between the ocean and host response is at its highest while the coast effect causes a large distortion in the marine responses we show through a resolution analysis that it does not mask subsurface conductivity anomalies but in fact increases the sensitivity to the seafloor

regional induction in scotland an example of three dimensional numerical modelling using the thin sheet approximation abstract a new algorithm recently devised for the solution of three dimensional problems of electromagnetic induction by a uniform source in a non uniform thin sheet at the surface of a conducting half space is used in a numerical study of regional induction in scotland limitations imposed by grid size and the conductivity of the lower crust restrict the investigation to short periods and for comparison with the published results from a recent magneto telluric survey of scotland a period of 25 s is chosen here the variations of the field components along parallel traverses across scotland are plotted for two perpendicular polarizations of the inducing field the coastlines and other conductivity structures included in the model notably the great glen fault are clearly revealed by these plots and the importance of vertical as well as horizontal deflections of the induced currents at a conductivity discontinuity is demonstrated real induction vectors are drawn at a number of grid points whose positions correspond approximately to the sites of actual field stations agreement with the corresponding induction vectors given by the field data is quite good at the coastal stations and near the great glen fault but otherwise the calculated vectors at inland stations are much shorter than those obtained from field data apparent resistivities phases and the azimuth of the major principal axis of impedence are also calculated for each site but there is poor agreement with measured values which show a greater degree of anisotropy it seems that for many sites the numerical grid is too coarse to model the surface conductivity distribution in sufficient detail to provide realistic estimates of these parameters

controlled source electromagnetic sounding of the oceanic lithosphere the attenuation of ionospheric signals in the frequency range 0 06 24 hz by sea water effectively precludes using the magnetotel luric method to study the electrical structure of the upper oceanic lithosphere we have carried out a dipole dipole electromagnetic sounding in the north pacific by injecting electromagnetic signals into the ocean and sea bed the crust at the site is 25 myr old and has a thin sediment cover the technique similar to that used in earlier work1 2 involves dragging a horizontal dipole antenna along the sea floor the electric fields that propagated through the resistive basement were detected by seafloor receivers at ranges of 10 65 km as the ambient electric field is very small varying from 10 18 v2 m 2 hz 1 at 0 1 hz to 10 24 v2 m2 hz 1 above 1 hz ref 3 the controlled source signals could be easily monitored our data are consistent with a simple one dimensional earth model consisting of a 3 7 km thick crustal layer of moderate conductivity 0 001 s m 1 underlain by a thicker region of very low conductivity 2 10 5 s m 1 the results suggest an upper mantle water content of at most 0 1 by volume

a comparison of electromagnetic distortion and resolution of upper mantle conductivities beneath continental europe and the mediterranean using islands as windows abstract to investigate the potential of utilising islands as windows through which oceanic mantle conductivities might be imaged simultaneous magnetotelluric mt measurements were carried out continuously over a 3 month duration at two continental stations one in the swabian alps southern germany the other in the bourgogne central france and on the mediterranean islands of montecristo mchr and mallorca mall the mt transfer functions to 1 5 cycles per day cpd and single site z h transfer functions for the first four harmonics of the solar quiet sq daily field variations are presented the transfer functions derived at the island sites are corrected for the distortive and inductive effects arising from the mediterranean bathymetry relative to the transfer functions calculated at the continental stations using three dimensional thin sheet modelling the corrections are formulated in terms of the magnetic perturbation tensor w static shifts in the mt transfer functions are corrected using the z h transfer functions and by comparison with hypothesised resistivity jumps at the transition zones models are derived jointly from the residual mt and z h transfer functions constraints on the resistivities of the upper mantle are investigated despite the widely different tectonic settings no significant differences in the conductivities of the upper mantle below 100 km are detected the disadvantages of utilising widely spaced islands as windows to the mantle are highlighted by departures from one dimensionality in the complex tectonic setting of the mediterranean region

frontogenesis in a fluid with horizontal density gradients the adjustment under gravity of a fluid containing a horizontal density gradient is described the fluid is initially at rest and the resulting motion is calculated as the flow accelerates driven by the baroclinic density field two forms of the initial density distribution are considered in the first the initial horizontal gradient is constant a purely horizontal motion develops as the isopycnals rotate towards the horizontal the vertical density gradient increases continually with time but the horizontal density gradient remains unchanged the horizontal velocity has a uniform vertical shear and the gradient richardson number is constant in space and decreases monotonically with time to the second density distribution consists of a piecewise constant gradient with a jump in the gradient along a vertical isopycnal the density is continuous in this case frontogenesis is predicted to occur on the isopycnal between the two constant density gradient regions and the timescale for the formation of a front is determined laboratory experiments are reported which confirm the results of these calculations in addition lock exchange experiments have been carried out in which the horizontal mean gradient is represented by a series of step density differences separated by vertical gates

electrical conductivity imaging of the philippine sea upper mantle using seafloor magnetotelluric data abstract we performed a three year seafloor electromagnetic survey in the philippine sea including the western edge of the pacific ocean to image electrical features of a deep mantle slab stagnating in the transition zone and the surrounding mantle in three dimensions 3 d the project iterated one year deployment of ocean bottom electromagnetometers obems using a total of 37 instruments installed at 18 sites the data obtained have been analyzed in the order of their recovery based on a magnetotelluric mt method in this study we attempt to obtain a one dimensional 1 d electrical conductivity model beneath the philippine sea and the pacific region separately that can be used as a reference model in the first step toward the 3 d analysis the resultant 1 d models show three main features 1 the conductivity in the shallower 200 km of the upper mantle depths of the two regions contrasts sharply which is qualitatively consistent with the large difference in lithospheric age 2 the conductivity at 200 300 km depth in both regions is more or less the same at approximately 0 3 s m 1 3 the conductivity just below 400 km depth is higher for the philippine sea mantle than for the pacific mantle the conductivity structure can be interpreted in terms of the thermal structure mantle hydration and existence of partial melt using experimental results for the conductivity of mantle minerals if the conductivity is interpreted simply as the effect of temperature the mantle beneath the philippine sea could be hotter than the dry solidus of mantle peridotite and thus partially molten however beneath the pacific region the present analysis suggests that the partial melting is not required under the assumed peridotitic composition even if we consider mantle hydration

an application of thin sheet electromagnetic modelling to the tasman sea abstract the method of thin sheet approximation is invoked for the ocean layer to model and interpret magnetotelluric mt data observed on the floor of the tasman sea between australia and new zealand a technique to remove 3d distortion from the observed seafloor mt data is developed from the thin sheet model mt responses the de distorted data then appear isotropic and are reinverted using 1d procedures this approach thus assists interpretation through a combination of 3d forward modelling procedures and 1d inversion a seafloor sediment layer is shown to have a strong effect on observed seafloor mt data also an exercise is carried out to compare the results given by the vertical gradient sounding vgs method with seafloor mt data the electrical conductivity structure beneath the tasman sea is analysed in terms of an upper mantle lithosphere and asthenosphere and a highly conducting lower mantle the observed anisotropy in seafloor mt data is diagnostic of a lithospheric conductivity of less than 10 4 s m 1 conductivity rises by two orders of magnitude below 80 km to greater than 10 2 s m 1 and is probably related to an asthenospheric layer in the upper mantle seismic interpretations similarly place a low velocity zone below a depth of 70 km at a depth of approximately 400 km the conductivity is approximately 1 s m 1 consistent with global estimates for the lower mantle there is little evidence for major change with age in the structure across the tasman sea

microburst modelling and scaling a microburst can be modeled by releasing a volume of fluid that is slightly heavier than the ambient fluid allowing it to fall onto a horizontal surface vorticity develops on the sides of this parcel as it descends and causes it to roll up into a turbulent vortex ring which impinges on the ground such a model exhibits many of the features of naturally occurring microbursts which are a hazard to aviation in this paper this model is achieved experimentally by releasing a volume of salt water into fresh water from a cylindrical dispenser when care is taken with the release the spreading rate of the surface outflow is measurable and quite repeatable despite the fact that the flow is turbulent an elementary numerical approximation to this model based on inviscid vortex dynamics has also been developed a scaling law is proposed which allows experiments with different fluid densities to be compared with each other and with the numerical results more importantly the scaling law makes it possible to compare the model results with real microbursts

the continental slope experiment along the tasman project profile southeast australia abstract three seafloor magnetometers made recordings for up to 95 days between july and october 1986 at site across the continental slope of southeast australia along the profile of the 1984 tasman project of seafloor magnetotelluric exploration tpsme land magnetometers at tpsme sites at the coast and canberra were reactivated to give simultaneous recordings the seafloor magnetometers were at depths of 140 m 2240 m and 3380 m and spanned the continental shelf between the coastline and the closest inshore tpsme ocean floor site depth 4850 m this experiment complements the tpsme by giving a much sharper definition of the geomagnetic coast effect in this critical region data are presented in the form of parkinson arrows for comparison with previously derived tpsme results along the profile they show a strong coast effect with the maximum mid way down the continental slope the situation is closely two dimensional and using this approximation some simple models have been computed one which gives a relatively good fit at a period of 1 h comprises simply a conductive ocean overlying a uniform conductor at depth further work will be needed to determine whether lateral conductivity structure at depth is required to fit the data more closely

the late diagenetic conversion of pyrite to magnetite by organically complexed ferric iron abstract the geochemistry of late stage diagenetic replacement of pyrite by magnetite was investigated by laboratory experiments of pyrite dissolution and magnetite precipitation involving organically complexed ferric iron the design of the experiments is based on the hypothesized geochemical conditions for in situ diagenesis in the pennsylvanian age belden formation an organic rich carbonate in northwestern colorado with pyrite grains rimmed with magnetite that contains a cretaceous paleopole position the geochemistry is applicable to both deep and near surface reactions involving pyrite magnetite and organic matter and bears upon the geologic interpretation of secondary chemical remanent magnetizations crm aqueous solutions of ferric ligand complexes and pyrite were heated 60 c for 60 days in the absence of light and at low partial pressures of oxygen additional experiments with each ligand were performed both in the presence of bentonite and without ferric iron ligand only raising the ph to 8 5 produced a ferrous hydroxide precipitate and subsequent heating 90 c resulted in the formation of magnetite ferric complexes of oxalate salicylate and acetylacetonate resulted in pyrite dissolution and magnetite formation the activity of strongly reducing ligands e g catechol or unsaturated fe iii exchange capacity particularly in experiments with bentonite or with solutions of uncomplexed ligands can result in an effective reduction of ferric iron concentration and prevent pyrite dissolution and or interfere with the crystallization of magnetite the bentonite acted to adsorb dissolved species and also underwent direct dissolution especially by the oxalate ligands although these reactions could occur at any depth a continuing supply of molecular oxygen more available in near surface settings would result in the formation of hematite or the production of acidic sulfate conditions whereas at greater depths or restricted porosity the redox gradation is halted at magnetite the results of this study thus provide a likely geochemical mechanism for the replacement of pyrite by magnetite during the latter stages of diagenesis

a two dimensional interpretation of the geomagnetic coast effect of southeast australia observed on land and seafloor abstract data from detailed investigations of the geomagnetic coast effect across the southeast australian continental margin are interpreted in terms of a two dimensional numerical model there is both land and seafloor control on this model and both e pol and b pol mode responses are incorporated the good fit to the data of a model comprising ocean water and marine sediments is improved when the sub ocean electrical conductivity profile is allowed to differ from the continental conductivity profile a final model is determined by an inversion procedure based on systematic search the main characteristic of this model is that an increase in conductivity occurs at a shallower depth beneath the ocean than beneath the land some 100 km beneath ocean 200 km beneath land the southeast continental margin of australia is considered to have formed by passive rifting at the time of the opening of the tasman sea a depth of 100 km for the base of the oceanic lithosphere corresponds well to the age of rifting some 80 ma ago the contrast with the continental profile suggests an electrical asthenosphere relatively deeper beneath southeast australia

the coast effect in terms of deviated electric currents a numerical study abstract a numerical thin sheet model is used to study the coast effect and to determine the magnitude of the near surface part of the anomalous field theoretical induction arrows are computed for australia california japan and the british isles and compared with the observed arrows this comparison shows that the main part of the coast effect and in some cases the whole coast effect is due to the deviation of currents flowing in the ocean by the upper part of the resistive continents it is suggested that the term coast effect should refer to the anomalous field whose source is near surface the coast effect so defined behaves like a regional anomaly superimposed on other anomalous features of the transient geomagnetic field a numerical determination of the geometrical features of the coast effect allows consideration of its elimination this elimination is performed for california and japan a correlation between the resulting residual and heat flow data is apparent in both cases

mapping the upper mantle three dimensional modeling of earth structure by inversion of seismic waveforms a method is presented for the inversion of waveform data for the three dimensional distribution of seismic wave velocities the method is applied to data from the global digital networks international deployment of accelerometers global digital seismograph network the selected data set consists of some 2000 seismograms corresponding to 53 events and 870 paths the moment tensors of the earthquakes are determined through an iterative procedure which minimizes the corrupting influence of lateral heterogeneity a global model is constructed for shear wave velocity expanded up to degree and order 8 in spherical harmonics and described by a cubic polynomial in depth for the upper 670 km of the earth s mantle although no a priori information is incorporated the model predictions reproduce much of what is known about the dispersion of mantle waves for example high phase velocities for shields low velocities at ridges and a strong degree 2 pattern for rayleigh waves since the method makes use of complete waveforms overtone data are also included it is shown that the model is reproducible in that substantially the same model can be constructed from each half of the total data set considered independently the model shows that shields and ridges are major features in the depth interval 25 250 km the ridges of the southern pacific and the larger shields persist to 350 km but the southeast indian rise is underlain by a high velocity anomaly at this depth as is much of the mid atlantic ridge at 450 650 km the major features are a broad region of high velocities incorporating south america much of the south atlantic and parts of west africa a broad region of low velocities in the central and eastern pacific high velocities in the western pacific and a low velocity anomaly beneath the red sea and the gulf of aden in the absence of a crustal correction degrees 2 and 3 show a high positive correlation with the geoid paradoxically this is largely destroyed when the distribution in crustal thickness is taken into account spherical harmonic degrees 4 7 show a significant negative correlation

the mechanical and electrical properties of earth s asthenosphere abstract an adequate theory of continental drift can be based on heat transfer theory but it does demand the acceptance of a large downward revision of traditional estimates of average upper mantle temperatures and a consistent understanding of lithosphere and asthenosphere in terms of a difference in rheological behaviour under prolonged non hydrostatic stress the recognition that an extremely viscous average state of the upper mantle is self regulating both requires and permits an explanation of magma generation at a strictly limited rate when averaged for the whole planet over a few years in terms of unsteady and local deformational heating the activity of water as a reducer of silicate creep resistance is used to develop the hypothesis that water produced by an amphibole dehydration has been effectively trapped in the earth and is the underlying cause of a low seismic q 50 and an electrical conductivity 10 2 10 1 ohm 1 m 1 at depths of 100 km at the predicted low horizontally avera temperatures the conductivity contrast of rock and aqueous solutions is very large and mantle electrical conductivity studies now look best suited to explore this trapping process and the distinctly recognisable possibility that the uptake of ocean water in the subduction process exceeds the rate of loss that can be explained purely through magmatic activity

electrical conductivity of olivine a dunite and the mantle laboratory studies of the electrical conductivity of rocks and minerals are vital to the interpretation of electromagnetic soundings of the earth s mantle to date the most reliable data have been collected from single crystals we have extended these studies with electrical conductivity measurements on a dunite from north carolina in the temperature range of 600 1200 c and under controlled oxygen fugacity observations of conductivity as a function of oxygen fugacity and temperature demonstrate that conduction in the dunite is indistinguishable from conduction in single olivine crystals thus the common practice of exaggerating the single crystal conductivities to account for conduction by grain boundary phases in the mantle is unnecessary because the dunite conductivity is consistent with that published for single crystals under similar conditions we have made a combined analysis of these data conductivity as a function of temperature between 600 and 1450 c displays three conduction mechanisms whose activation energies may be recovered by nonlinear least squares fitting yielding activation energies of 0 21 2 56 10 19 j 0 13 1 60 ev below 720 c 2 56 0 02 10 19 j 1 60 0 01 ev between 720 c and 1500 c and 11 46 0 90 10 19 j 7 16 0 56 ev above 1500 c the behavior of conductivity as a function of oxygen fugacity is well explained by a model in which an o2 independent population of charge carriers is supplemented at high oxygen fugacities with a population that is proportional to o20 3 this parametrization produces a clear correlation of the o2 dependent term with iron content which is otherwise obscured by variations in conductivity among olivines

an analogue model study of electromagnetic induction in the tasmania region abstract laboratory analogue model magnetic measurements are carried out for a model of the region including tasmania bass strait with its highly conductive deep sedimentary basins and the south coast of mainland australia the model source frequencies used simulate naturally occurring geomagnetic variations of periods 5 120 min in phase and quadrature magnetic hx hy and hz field measurements for the modelled region are presented for an approximately uniform overhead horizontal source field for e polarization electric field of the source in the n s direction and for h polarization electric field of the source in the e w direction large anomalous in phase and quadrature model magnetic fields are observed over bass strait and the coastal regions at short periods for both e and h polarization but with increasing period the field anomalies decrease more rapidly for e polarization than for h polarization the difference in response with polarization for the bass strait region is attributed to current induced in the deep ocean for all periods being channelled through bass strait for h polarization but not for e polarization the persistent large coastal field anomalies elsewhere for h polarization can be accounted for by the coastal current concentrations due to currents induced in the deep ocean for all periods deflected to the south and to the north by the shelving sea floor and channelled through bass strait and around the southern coast of tasmania the phenomena of current deflection and channelling for h polarization for the geometry of the southern australia coastline and associated ocean bathymetry is particularly effective in producing field anomalies for a large period range the coastal horizontal hx and hy field anomalies present for e polarization at short periods and for h polarization at all periods do not extend far inland and thus for inland station sites somewhat removed from the coast should not present serious problems for magnetic soundings in field work the sharp vertical field hz gradient over tasmania at short periods which is predominantly in the e w direction for e polarization and the n s direction for h polarization is strongly frequency dependent becoming almost undetectable at 60 min the behaviour of the hz field gradients however are very similar from traverse to traverse over inland tasmania and thus the effects of the ocean should not present too serious a problem in the interpretation of field station studies the discrepancies between model and field station results should be useful in mapping geological boundaries in the region

correction of seafloor magnetotelluric data for topographic effects during inversion 1 the large contrast in electrical conductivity between seawater and the underlying seafloor accumulates boundary electric charges which can severely distort observed electric and magnetic fields for marine magnetotelluric mt studies correcting this topographic effect is critical to obtaining accurate conductivity models for the mantle previously correction for topography was based on the thin sheet approximation which breaks down at periods under 1000 s in the deep ocean this paper introduces an analysis method for seafloor mt data which combines removal of three dimensional 3 d topographic effects with inversion of the data for 2 d structure the observed mt impedance is first corrected to a flat lying seafloor datum using the observed bathymetry without invoking the thin sheet approximation the corrected mt response is then inverted in a flat seafloor model space because of coupling between topographic effects and deeper structure the correction and inversion steps are iterated until changes in each become small the procedure is verified using synthetic and real data tests for synthetic 3 d topography over a half space show that the method closely recovers the true half space model after a few iterations the procedure is also applied to real data collected in the mantle electromagnetic and tomography melt experiment on the east pacific rise at 17 s

a new technique for the incorporation of seafloor topography in electromagnetic modelling summary we describe a new technique for incorporating seafloor topography in electromagnetic modelling it is based on a transformation of the topographic relief into a change in electrical conductivity and magnetic permeability within a flat seafloor since the technique allows us to model arbitrary topographic changes without extra grid cells and any restriction by vertical discretization we can model very precise topographic changes easily without an extra burden in terms of computer memory or calculation time the reliability and stability of the technique are tested by comparing the magnetotelluric responses from two synthetic seafloor topography models with three different approaches these incorporate the topographic change in terms of 1 the change in conductance using the thin sheet approximation 2 a series of rectangular block like steps and 3 triangular finite elements the technique is easily applied to any modelling method including 3d modelling allowing us to model complex structure in the earth while taking full account of the 3d seafloor topography

barotropic flow of a warm core ring from seafloor electric measurements a seafloor array of recorders spanning the tasman sea has measured the electric field signature of a major warm core ring embedded in the east australian current theory for motional electromagnetic induction allows the electric measurements to be interpreted in terms of barotropic water velocity water transport and kinetic energy estimates thus obtained show a concentration of mesoscale activity near the australian coast the total circulation of the particular ring is estimated to be of the order of 100 sv

mantle dynamics beneath the east pacific rise at 17 s insights from the mantle electromagnetic and tomography melt experiment 1 the electromagnetic data from the mantle electromagnetic and tomography melt experiment are inverted for a two dimensional transversely anisotropic conductivity structure that incorporates a correction for three dimensional topographic effects on the magnetotelluric responses the model space allows for different conductivity values in the along strike cross strike and vertical directions along with imposed constraints of model smoothness and closeness among the three directions anisotropic models provide a slightly better fit to the data for a given level of model smoothness and are more consistent with other geophysical and laboratory data the preferred anisotropic model displays a resistive uppermost 60 km thick mantle independent of plate age except in the vicinity of the ridge crest in most inversions a vertically aligned sheet like conductor at the ridge crest is especially prominent in the vertical conductivity its presence suggests that the melt is more highly concentrated and connected in the vertical direction immediately beneath the rise axis the melt zone is at least 100 km wide and is asymmetric having a greater extent to the west off axis and to the east of the ridge the mantle is more conductive in the direction of plate spreading at depths greater than 60 km the flat resistive conductive boundary at 60 km agrees well with the inferred depth of the dry solidus of peridotite and the deeper conductive region is consistent with the preferred orientation of olivine inferred from seismic observations this suggests that the uppermost 60 km represents the region of mantle that has undergone melting at the ridge and has been depleted of water dissolved hydrogen by contrast the underlying mantle has retained a significant amount of water

free carbon electrical conductivity in the earth s mantle laboratory measurements indicate that solid carbon either amorphous or as graphite enhances the electrical conductivity of rocks carbon in quantities of only a few ppm providing it forms an interconnected phase could give rise to electrical conductivities of the order seen in the high conductivity layers in the earth s mantle this possibility is consistent with carbon petrology in mantle rocks and puts the formation and migration of abiogenic carbon compounds into a broader context

magnetic and electric fields produced in the sea during geomagnetic disturbances to understand geomagnetic effects on systems with long conductors it is necessary to know the electric field those systems experience for surface conductors such as power systems and pipelines this can easily be calculated from the magnetic field variations at the surface using the surface impedance of the earth however for calculating the electric fields in pipelines and submarine cables at the seafloor it is necessary to take account of the attenuating effect of the conducting seawater assuming that the fields are vertically propagating plane waves we derive the transfer functions between the electric and magnetic fields at the seafloor and the magnetic field variations at the sea surface these transfer functions are then used with surface magnetic field data to determine the power spectra of the seafloor magnetic and electric fields in a shallow sea depth 100 m and in the deep ocean depth 5 km for different values of the kp magnetic activity index for the period range considered 2 min to 3 hrs the spectral characteristics of the seafloor magnetic and electric fields for a 100 m deep sea are very similar to those of the surface fields for the deep ocean the seafloor spectra show a faster decrease in spectral density with increasing frequency compared to the surface fields the results obtained are shown to be consistent with seafloor observations assessment of the seafloor electric fields produced by different levels of geomagnetic activity can be useful in the design of the power feed equipment for submarine cables and cathodic protection for undersea pipelines

the tasman project of seafloor magnetotelluric exploration experiment and observations abstract a major experiment in natural geomagnetic induction has been carried out by making seafloor magnetotelluric observations on a line of sites between australia and new zealand simultaneous observations were made by land magnetometers on the australian continent the large volume of data recorded shows a great variety of phenomena of electromagnetic induction in the earth and sea parkinson arrows and magnetotelluric impedances have been computed for the seafloor sites the parkinson arrows show a strong coast effect near the australian continent and a weaker coast effect over the submerged lord howe rise the occurrence of smoothly varying arrows across the tasman sea suggests a simple large scale pattern for electromagnetic induction in the tasman sea the interpretation of parkinson arrows at seafloor sites is shown to require special care due to the attenuation and phase shift effects caused by the ocean on seafloor horizontal magnetic field fluctuations the magnetotelluric impedances exhibit a strong anisotropy at all sites in the tasman sea the orientation of the impedance axes is constant across the sea and the anisotropy may be attributed to the two dimensional shape of the tasman sea calculations based on the distance scale of the magnetotelluric anisotropy interpreted as an ocean continent boundary effect give a lower bound for the integrated resistivity of the tasman sea oceanic lithosphere of order 107 m2 corresponding to an estimate of 2 102 m for its average resistivity

geomagnetic induction in the tasman sea and electrical conductivity structure beneath the tasman seafloor summary during the tasman project of seafloor magnetotelluric exploration recordings were made of the natural magnetic and electric field variations along a line of seafloor sites crossing the tasman sea using these data and additional magnetic field recordings made on the australian continent the pattern of geomagnetic induction in the tasman sea region is examined an investigation of the seafloor magnetotelluric impedance tensor indicates the geomagnetic induction is influenced by the large scale 3 d conductivity structure of the region this result is confirmed by a study of impedance estimates obtained using the vertical gradient sounding method in which the seafloor impedance is determined from the attenuation of the horizontal magnetic field variations through the ocean inversion of the e polarization impedance component from sites in the central tasman sea indicates the conductivity structure at depths greater than about 100 km is laterally homogeneous in this region it is probable that the conductivity structure at these sites includes a high conducting layer similar to one observed in the pacific ocean one seafloor site lying on the tasmantid seamount chain exhibits an increased conductivity at depths of less than 100 km the increased conductivity may be due to heating of the lithosphere by a hotspot one proposed source of the seamount chain

conductivity structure of oceanic upper mantle beneath the pacific plate summary ocean bottom magnetotelluric responses from three ages on the pacific plate have been inverted to reveal the presence of a conductive zone whose depth is a function of lithospheric age the conductivity achieves its maximum value 0 1 sm 1 at depths near 70 120 and 180 km for lithospheric ages of 1 30 and 72 myr respectively the existence of this conductive structure has firmly been established by constructing a number of different acceptable models and also by obtaining unique averages over depth of the conductivity an explanation in terms of partial melting in the upper mantle has produced lower bounds for the volume fraction of melt which must exist in order to produce the high conductivity beneath jdf t 1 myr the lower bound lies in the range 0 04 0 12 beneath cal t 30 myr it lies in the range 0 02 0 10 and beneath ncp t 72 myr it lies in the range 0 008 0 02 the distributions of partial melt show that the major portion of melt beneath jdf lies between 40 and 90 km and that melting has ceased by depths of about 170 km at all three sites geotherms have been computed and they are qualitatively similar to those predicted from a convective system where the temperature increases rapidly through a boundary layer to achieve a maximum value and then possibly decreases with depth the depth at which the maximum temperature is reached is about 125 km for cal and about 175 km for ncp and the maximum temperatures are estimated to be 1400 1500 c lastly as an independent check upon the partial melt explanation the location and magnitude of the seismic low velocity zone has been correlated positively with the computed distribution of partial melt at all three ocean bottom sites

the determination of resistivity and porosity of the sediment and fractured basalt layers near the juan de fuca ridge summary in the summer of 1984 an electrical survey using magnetometric off shore electrical sounding moses was conducted at two sites in middle valley part of the northern juan de fuca ridge complex moses has been designed to minimize the difficulties inherent in electrical surveys of the crust below the electrically conductive sea layer site 1 at 48 32n 128 42w is in the central part of the turbidite filled basin using a two layer model of conductive sediments overlying a fractured basalt basement the sediment resistivity and thickness were found to be 0 82 0 06 m and 1800 300 m respectively the basement resistivity although not well constrained by the data is consistent with the results obtained at site 2 site 2 located at 48 10n 128 50w has a thinner sediment layer which appears to vary with position the sediment conductivity thickness product is the parameter determined by the data if the sediment resistivity were the same as at site 1 the sediment thickness would be 140 30 m to the se of site 2 and 240 55 m to the nw the fractured basalt basement has a resistivity of 8 5 3 4 m and is at least 1000 m thick using temperature corrected pore fluid resistivity the calculated porosity is found to vary from 62 per cent at the top to 21 per cent at the base of the sediments and is 8 per cent in the basement these values are in good agreement with estimates from seismic velocities for a thick turbidite sequence in a nearby sediment filled basin and determined for layer 2a b basalts in dsdp hole 504b respectively

melt segregation from partially molten source regions the importance of melt density and source region size the compressibility of basic melt at 1 atmosphere is about an order of magnitude higher than that of mantle minerals consequently the density contrast between melt and the principal residual crystals in mantle source regions is expected to decrease with increasing source region depth the increasingly olivine normative character of primary melts produced at greater depths is also expected to result in a decrease in this density contrast with increasing source region depth once vertical permeability is established by melt generated during partial melting buoyancy driven melt percolation can under some circumstances segregate melt from the residual crystals in its source region on a geologically rapid time scale limits to this process are provided by cooling of the source region freezing melt in and rigidity of the crystalline matrix mechanically trapping melt source region size influences these limits strongly consequently small partially molten diapirs km in diameter may be able to trap large melt fractions 30 but larger source regions would be unable to do so the reduction in density contrast with pressure reduces the buoyant force driving melt percolation and provides another limit to melt segregation diapirs at depth may thus stably contain large fractions of melt but may decompress and unload their melt during ascent this effect would be enhanced in small diapirs and may be relevant to the genesis of komatiitic magma melt compression may also be a factor in explaining why the very different maximum depths inferred for typical basic melt segregation from source regions on different planets 500 km on the moon 250 km on mars 100 km on earth correspond to similar pressures 25 35 kbar at greater pressures melt may no longer be capable under ordinary conditions of segregating upwards by buoyancy this may also help to explain why depleted peridotites overlie more fertile peridotites and how deep regions of the mantle are able to remain fertile over geologic time

the electrical conductivity of the oceanic upper mantle summary previous inversions of sea floor magnetotelluric mt sounding data have predicted upper mantle electrical conductivities which are more than an order of magnitude higher than laboratory measurements of the conductivity of olivine would suggest and controlled source field electromagnetic csem soundings require a lithospheric mantle conductivity of 3 x lop5 sm which is so low that the electromagnetic em coast effect would produce more anisotropy in mt soundings than is observed we address these issues by constructing an olivine mantle model for conductivity and examining the inversion of mt data from three sea floor sites and show that the incompatibilities can be largely resolved if the effects of oceans and coastlines are considered our mantle model is based on recent measurements of olivine conductivity the conductivity of tholeiite melt a thermal model for the upper mantle based on lithospheric cooling and the temperature of a a 3 olivine transition the pyrolite model of mantle petrology and conductivities derived from csem sounding we propose archie s law with exponent 2 and interconnected tubes as realistic lower and upper bounds for the effect of partial melt on rock conductivity and archie s law with exponent 1 5 as the preferred estimate the 1 d forward response of this model is not compatible with observed sea floor mt data three data sets presented by oldenburg 1981 pass a test for one dimensionality based on the size of the residuals when fit with parker s d algorithm but two of the three soundings fail a test for independence of residuals we also find that the presence of the upper mantle high conductivity zone previously inferred from these data is highly dependent on data misfit and not required when the misfit criterion is relaxed a reasonable amount re inversions of the mt data produce models which are incompatible with our petrological model of mantle conductivity however by adding an ocean with various coastlines of simple geometry to our petrological model and solving the forward 3 d problem using thin sheet analysis we predict mt responses which are distorted in a manner that is remarkably similar to the observed data

long period magnetotelluric measurements near the central california coast a land locked view of the conductivity structure under the pacific ocean summary telluric data from hollister california were combined with magnetic data from the fresno california magnetic observatory to determine the mt response at long periods the shifted eigenstate analysis of latorraca et al 1986 was used to determine the maximum apparent resistivities and corresponding eih phases for periods from 1 to 30 h the apparent resistivity and phase at hollister are greatly influenced by the ocean with the mantle branch beginning at a period of about 12 h in typical continental areas the mantle branch usually begins at periods of 10s or less a maximum likelihood inversion algorithm was used to invert the observed data and to determine the sensitivity of the model parameters the data required a resistivity thickness product of approximately 1 x lo9 q m2 1 x lo6 q m km for the oceanic lower crust and upper mantle the oceanic mantle shows a rather small gradient of conductivity from approximately 200 to 640 km in depth and a rapid conductivity increase below 640km the sensitivity analysis shows the data are most sensitive to the oceanic parameters except for the electric field amplitudes which are also sensitive to the resistivities under the observation site

inversion of ocean bottom magnetotelluric data revisited three ocean bottom magnetotelluric data sets from sites on the pacific plate are reinterpreted the initial analysis found a correlation between the lithospheric age and the depth to a conductive zone beneath each site that work also suggested that the resistivity increased below the conductor this analysis which includes new methods for constructing one dimensional conductivity models shows that the postulated increase in resistivity is not demanded by the data it also reveals an unexpectedly large nonuniqueness inherent in the interpretation of these data the previously reported trends with lithospheric age still exist but they are not as strong as initially believed finally it is shown rigorously that the different age sites are distinct in that no one dimensional model can account for all three data sets

a density current parameterization coupled with emanuel s convection scheme part i the models abstract the aim of the present series of papers is to develop a density current parameterization for global circulation models this first paper is devoted to the presentation of this new wake parameterization coupled with emanuel s convective scheme the model represents a population of identical circular cold pools the wakes with vertical frontiers the wakes are cooled by the precipitating downdrafts while the outside area is warmed by the subsidence induced by the saturated drafts the budget equations for mass energy and water yield evolution equations for the prognostic variables the vertical profiles of the temperature and humidity differences between the wakes and their exterior they also provide additional terms for the equations of the mean variables the driving terms of the wake equations are the differential heating and drying due to convective drafts the action of the convection on the wakes is implemented by splitting the convective tendency and attributing the effect of the precip

regional conductance map of andaman and nicobar region abstract to image the electrical conductivity distribution fluxgate magnetometers are operated at five sites in andaman and nicobar region transfer functions are estimated for the period range 8 128 min from nighttime transient geomagnetic variations using robust regression analysis the observed induction arrows in andaman islands are found to point towards east despite deep sea located towards its west this indicates that fore arc basin andaman nicobar deep is more conducting than the region of outer non volcanic island arc thin sheet model requires the conductance of 10 000 35 000 s with increase conductivity towards the south for explaining the observed induction pattern the observed induction pattern at andaman nicobar stations can be explained in terms of high conducting cretaceous tertiary sediments filling the andaman nicobar deep high conductivity over invisible bank has been attributed to the partial melts volatile fluids derived from the subducting indian plate that are intruding into the eastern margin of fore arc basin through the west andaman fault waf the induction pattern at great nicobar station campbell bay may be related to the highly conducting sediments filling the mergui basin along with mafic intrusions also crustal transition occurs below the mergui terrace at the malayan coast contributing to the enhanced conductivity anomaly

a seafloor magnetotelluric sounding in the tasman sea this paper reports an analysis of electromagnetic data from a site occupied on the floor of the tasman sea during an australia u s research program the tasman project of seafloor magnetotelluric exploration this site 37 33 s 155 58 e depth 4450 m is in the central portion of the tasman sea and near both a fossil spreading ridge and the tasmantid seamount chain the extensive electric and magnetic data sets covering 60 days exhibit the signals of a most energetic oceanic environment conspicuous features of the electric field include an unusually large component induced by mesoscale fluid motions as well as sporadically occurring high frequency signal probably induced by ocean turbulence these motional fields contribute a significant background noise to the ionospheric storm and substorm magnetic activity from which the seafloor magnetotelluric impedance tensor is estimated but permit a usable frequency band of 0 06 to 5 5 cph over this band the seafloor impedance is found to exhibit frequency independent skew and anisotropy the skew suggests the presence of some nearby lateral conductivity heterogeneity the anisotropy is characterised by a principal axes ratio of 3 5 the major axis being oriented 24 clockwise from true north tentative inversion of the data implies the presence of an unusually high conductance in the uppermost seafloor altogether information on upper mantle conductivity may reach a 500 km depth the principal magnetotelluric axes are approximately aligned with both the trend of the seamount chain and with the australian coastline the ensemble of data from eight other sites occupied in the project should permit an assessment of the relative contribution of these two features

on the electrical conductivity of the oceanic lithosphere abstract below the bottom of the sediments the oceanic lithosphere is difficult to sound electromagnetically because the resistivity is high compared to over and under lying structures there are three methods which can contribute information 1 1 magnetotelluric methods provide the impedance as a function of frequency for signals of sufficiently low frequency to penetrate through the ocean in the deep sea the upper frequency limit is a few cph 2 2 a quasi static electric field can be measured at the ocean floor the field is generated by charges built up by the motion of ocean water currents through the geomagnetic field by providing a current path below the ocean the lithospheric conductivity tends to reduce this field in order to interpret the measurements one needs to know the spatial character of the water motions 3 3 an artificial em source which transmits at frequencies near 1 hz can be used at the ocean floor energy propagation through the lithosphere has been detected on the ocean floor to horizontal distances of 18 km detection is aided by the very low noise level at the high frequencies concerned estimates of the typical conductivity distribution throughout the ocean crust are given