is the asthenosphere electrically anisotropic abstract two techniques for resolving the conductance of an electrically conductive asthenosphere are presented the first technique combines observatory electromagnetic data in the period range of the daily variation and longer which provides penetration depths of 400 km and deeper with laboratory data of the conductivity of upper mantle materials the conductance can be estimated by this technique but the geometry e g the exact depth is not resolved the second technique extends the classical magnetotelluric mt tensor estimation and decomposition to the period range 1000 30 000 s if an electrical asthenosphere exists then in this period range the electromagnetic field penetrates into it 1000 s but also through it 30 000 s the concept of regional strike directions which has been successfully used to obtain crustal anisotropy directions is again employed at these long periods the surprising result is that in the depth range of the asthenosphere a strong directional dependence of the conductance again occurs although large arrays will be necessary in order to prove that this is really conductivity anisotropy in the asthenosphere first field results from arrays 100 200 km in extent are encouraging the concept of a superposition of crustal and upper mantle anisotropy explains to some extent why earlier magnetotelluric determinations of the asthenosphere had little success only in the fortunate case where a mt experiment was performed not parallel to the direction of the crustal conductor but parallel to the asthenospheric one is the second structure resolved electrical anisotropy in the uppermost mantle supports the concept of intracrystalline water which allows for a contribution of hydrogen diffusity to the conductivity of olivine this diffusity is highly anisotropic with respect to the crystal axis and if the crystals are partly aligned the conductivity measured with mt is also direction dependent

the fallacy of the shallow water problem in marine csem exploration the recent explosion of activity in offshore controlled source electromagnetic csem exploration has shown both theoretically and in practice for the time harmonic case that shallow water environments depths less than say 300 m can pose a significant challenge for detection and characterization of thin resistive targets in the subsurface because the weakly attenuated atmospheric response overprints the weaker target response as an alternative the transient csem experiment is considered to explore the nature of the airwave signature when viewed from the perspective of csem time series the signature of a thin isolated resistive horizon is computed by using the quasi analytic solution for a 1d earth in response to excitation by a horizontal electric dipole antenna that signature clearly seen in the transient data generally lies in the time interval between the arrivalof the airwave and the late time seawater response the fact that either decreasing the water depth or increasing the source r

marine controlled source electromagnetics with geothermal purposes central gulf of california mexico abstract we acquired and interpreted data from two controlled source electromagnetic profiles in the gulf of california off the coast of santa rosalia baja california sur mexico this area is interesting from the geothermal point of view due to its proximity to the guaymas basin the la reforma caldera and the las tres virgenes volcano the first with intense submarine hydrothermal manifestations and the final one with a geothermal field in production the survey consisted of the deployment of 10 ocean bottom electromagnetic receivers distributed along two 14 km long profiles these sensors recorded the two horizontal components of the electric and magnetic fields the source consisted of a horizontal 200 m electric dipole towed behind the vessel the processed data were inverted to estimate the subsurface distribution of electrical resistivity using a finite element two dimensional multi frequency program resulting in adequate fits between observed and calculated data to estimate how well the models are constrained by the data sensitivity tests were applied to the models the tests were based on the jacobian matrix and a perturbation analysis of selected features of the model they show that the model in its central part is well constrained by the data down to a depth of about 2 5 km with the perturbation approach 16 resistivity features out of the 19 tested turned out to be well constrained by the data the estimated resistivity distribution was compared with the information obtained from other geophysical methods bathymetry gravity and 2d multi channel seismic reflection resulting in reasonably good correlations between them for the first profile we find a remarkable similarity with the current conceptual models proposed in the literature for a volcanic geothermal system where an altered conductive clay cap overlays another with higher electrical resistivity the second profile shows agreements with anomalies detected with echo sounder bathymetry suggesting the existence of hydrothermal manifestations

structure of the crust in the vicinity of the banggong nujiang suture in central tibet from indepth magnetotelluric data 1 magnetotelluric data from a 150 km long profile crossing the banggong nujiang suture bns central tibet acquired as part of the international deep profiling of tibet and the himalaya indepth project have been examined for crustal and upper mantle structure strike and dimensionality analyses demonstrate that regional scale electrical structures are two dimensional and oriented approximately parallel to surface geological strike as seen elsewhere in tibet the double thickness crust is generally characterized by resistive upper crust hundreds to thousands of ohm meters overlying conductive middle and lower crust tens to hundreds of ohm meters but in detail there are lateral variations at all levels regionally a northward transition from thick 45 km to thin 15 km resistive upper crust coincides with 1 the surface trace of the bns 2 a prominent strand of the karakorum jiali fault system 3 northward decrease in upper mantle seismic velocities and increase in attenuation and 4 pronounced northward onset of seismic polarization anisotropy the latter two seismological features have been taken to mark the northern limit of indian mantle lithosphere thrust beneath southern tibet on the basis of our electrical model we speculate that 1 the resistive upper crustal root beneath the neogene lunpola and duba basins was produced by crustal shortening localized along the northern edge of the lhasa terrane 2 the low midcrustal resistivity beneath the bns reflects enhanced neogene melting and or metamorphic dewatering of relatively fertile subduction zone complex rocks 3 observed steep upper crustal low resistivity anomalies are produced by hydrothermal fluids within active faults localized within and adjacent to the bns and 4 these strike slip and extensional fault arrays are surface manifestations of lithosphere penetrating shear localized along the northern edge of the underthrust indian plate

direct reservoir parameter estimation using joint inversion of marine seismic ava and csem data a new joint inversion algorithm to directly estimate reservoir parameters is described this algorithm combines seismic amplitude versus angle ava and marine controlled source electromagnetic csem data the rock properties model needed to link the geophysical parameters to the reservoir parameters is described errors in the rock properties model parameters measured in percent introduce errors of comparable size in the joint inversion reservoir parameter estimates tests of the concept on synthetic one dimensional models demonstrate improved fluid saturation and porosity estimates for joint ava csem data inversion compared to ava or csem inversion alone comparing inversions of ava csem and joint ava csem data over the north sea troll field at a location with well control shows that the joint inversion produces estimated gas saturation oil saturation and porosity that is closest as measured by the rms difference l1 norm of the difference and net over the interval to the logged values whereas csem inversion provides the closest estimates of water saturation

on the ionospheric effects of asthenospheric earthquakes abstract characteristic fof2 variations during preparation of the asthenospheric hindukush earthquake of 12 september 1983 are investigated stable effects in f2 layer critical frequency variations are revealed which can be considered as earthquake precursors the existence of stable enhancement of fof2 night values 1 5 2 days before an earthquake shock is demonstrated as well as a steep decrease of fof2 evening values 1 day before the earthquake these phenomena are regionally restricted

electromagnetic induction by ocean currents bempex abstract the theory of electromagnetic induction by ocean water currents is reviewed using a modal formulation with an emphasis on interactions of the induced fields with the conducting earth and on their association with distinct dynamically significant parts of the water velocity field it is shown that the dominant electromagnetic mode at low frequencies a recent major experiment to study the low frequency variability of the ocean using electromagnetic induction principles is described denoted by the acronym bempex for barotropic electromagnetic and pressure experiment this effort involves 41 seafloor magnetometers horizontal and vertical electrometers and pressure recorders covering an area 1100 km east west by 1000 km north south in the north pacific for 11 months the major oceanographic objectives of bempex are multifold and include a description of the wind forced barotropic variability of the deep ocean studies of the frequency wavenumber characteristics of certain oscillations of basin wide scale and tests of predictions from existing models of wind forcing

electrical conductivity and crustal deformation from magnetotelluric results in the region of the araguainha impact brazil abstract in order to obtain information about the deep structure of large meteorite craters a series of mt soundings were carried out in the region of the araguainha impact crater located in the northern section of the parana basin mato grosso brazil the mt responses in the short period range from 0 001 to 1 s show one dimensional behaviour in contrast to the longer periods where data are multi dimensional at periods between 1 and 10 s the splitting of the apparent resistivity and phase curves of different polarisations is accompanied by a strong increase of the phase sensitive regional skew parameter to values up to 0 5 the results of two and three dimensional modelling reveal a disk shaped body embedded within a layer of 5000 m the resistivity of the body ranging between 20 m and 500 m lies significantly below the bulk value of the upper crust the anomaly is believed to be caused by impact induced faulting and brecciation of the crust reaching to depths of 3 to 7 km the horizontal extent of this zone is 16 km corresponding to about 40 of the total crater diameter at the surface the long period data t 10s reveal a heterogeneous lower crust of enhanced conductivity at depths between 15 and 30 km which is not related to the impact event it is likely to be a feature typical of the basement in the area of the araguainha structure

electromagnetic active source sounding near the east pacific rise an active source electrical conductivity sounding was carried out near the east pacific rise at 21 n the average electrical conductivity from 2 km beneath the seafloor into the upper mantle was found to be about 0 004 s m there is no evidence of fully molten magma at a distance of 7 km from the spreading axis the inferred conductivity with depth of the crustal rock is consistent with brace s model of connected seawater filled pores in rocks heated from beneath

ferropericlase crystallization under upper mantle conditions ferropericlase is a common lower mantle phase that can occur as inclusions in superdeep diamonds however it has been proposed that some of these inclusions could be formed in the upper mantle brey et al 2004 to explore this hypothesis two types of experiments on ferropericlase crystallization were conducted at 5 12 gpa and 1500 1700 c 1 equilibrium experiments with ca mg fe carbonate mgo olivine mixtures and 2 sandwich experiments containing a layer of metallic iron overlain by olivine with a carbonate layer on top the equilibrium experiments produced a series of carbonate silicate melts saturated in olivine and ferropericlase these melts contain 2 12 wt sio2 which correlates negatively with cao and co2 contents and positively with temperature a comparison of the composition of the carbonate silicate melts with literature data for melts saturated in olivine and low ca pyroxene indicate that melts saturated in ferropericlase cannot be produced by crystallization of melts derived by melting carbonated harzburgite or lherzolite at upper mantle conditions on the other hand the sandwich experiments reveal that ferropericlase and diamond or metastable graphite can crystallize simultaneously during the reduction of carbonate silicate melt the resulting ferropericlase crystallizing in equilibrium with olivine will be richer in fe compared with lower mantle ferropericlase in equilibrium with bridgmanite thus the considerable variation in mg values observed for some suites of ferropericlase inclusions in diamond could in part be attributable to ferropericlase formation in the upper mantle

delineating ore forming rock using a frequency domain controlled source electromagnetic method abstract several magmatic ni cu deposits associated with permian triassic mafic ultramafic intrusions have been discovered during the last three decades in the southern part of the central asian orogenic belt the northeastern margin of tarim and the margin of the north china craton the accurate delineations of the large resistive rock masses are crucial for locating potential ore bodies when they are located beneath rock masse the controlled source electromagnetic method csem is an important tool for investigating resistivity contrast in earth subsurface in this study we present a comprehensive example of using frequency domain csem survey to delineate the resistive rock masses that are related to the forming of orebody the horizontal electric field which is more sensitive to resistive target than the other components is used in our study we first built a geophysical model of the survey area based on prior confirmed geophysical and geological information then both forward and inverse modeling were performed in order to assess the detectability of the resistive ore forming rock masses in the model finally this study acquired field data in the kalatongke ni cu deposit which is located in the kalatongke district of the southern section of the central asian orogenic belt and is specifically part of china s southern altai orogenic belt the resistive ore forming rock masses were successfully identified during the survey process and further verified by the drilling results it was observed that both the synthetic and field examples indicated that the adopted frequency domain csem method successfully measured the electrical fields and could be utilized to explore deeply buried resistive targets in future prospecting activities

electrical anisotropy of south african lithosphere compared with seismic anisotropy from shear wave splitting analyses abstract electrical anisotropy in southern africa inferred from the analysis of magnetotelluric mt data recorded as part of the southern african mt experiment samtex is compared with seismic anisotropy inferred from an sks shear wave splitting study in the same region given the vastly varying penetration depths in the survey area electrical anisotropy is derived in terms of approximate depth rather than frequency electrical anisotropy directions for crustal depths 45 km are not consistent with the fast axis directions inferred from the sks analyses upper mantle electrical results appear to be mostly a consequence of the geometry of large scale geological structures and provide evidence that some crustal structures are distinct at depth while others seem to be confined to the crust our results indicate that the causative region for the seismic anisotropy in the lithospheric mantle has either a correspondingly weak electrical anisotropic signature or is more prominent at greater lithospheric depths than those we investigate here

hydrothermal circulation at the cleft vance overlapping spreading center results of a magnetometric resistivity survey we report on a magnetometric resistivity sounding carried out in the overlapping spreading center between the cleft and vance segments of the juan de fuca ridge the data collected reveal a strong three dimensionality in the crustal electrical resistivity structure on wavelengths of a few kilometers areas of reduced crustal electrical resistivities with values approaching that of seawater are seen beneath the neovolcanic zones of both active spreading centers we interpret these reduced resistivities as evidence of active hydrothermal circulation within the uppermost 1 km of hot young oceanic crust

1d inversion and resolution analysis of marine csem data we present results from an investigation into 1d inversion of controlled source electromagnetic csem data based on inspection of a data set we formulate a simple empirical noise model described by a few pragmatically determined parameters we also investigate the effects of transmitter height above the seafloor and include the data uncertainty resulting from varying transmitter height in our noise model based on the noise model and assumptions about the available data we analyze model parameter uncertainty estimates derived from the a posteriori model covariance matrix for a resistive layer buried at depth we find that the layer parameters uncertainty primarily depends on the depth of burial and the thickness of the layer we then formulate quantitative bounds for these parameters within which we have a small uncertainty of the parameters of the resistive layer the depth of burial and the transverse resistance of the layer become better determined the higher its resistance we invert a field data

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

imaging the continental upper mantle using electromagnetic methods abstract the internal structure of the continental lithosphere holds the key to its creation and development and this internal structure can be determined using appropriate seismic and electromagnetic methods these two are complementary in that the seismic parameters usually represent bulk properties of the rock whereas electrical conductivity is primarily a function of the connectivity of a minor constituent of the rock matrix such as the presence of a conducting mineral phase e g carbon in graphite form or of a fluid phase e g partial melt or volatiles in particular conductivity is especially sensitive to the top of the asthenosphere generally considered to be a region of interconnected partial melt knowledge of the geometry of the lithosphere asthenosphere boundary is important as this boundary partially controls the geodynamic processes that create modify and destroy the lithosphere accordingly collocated seismic and electromagnetic experiments result in superior knowledge than would be obtained from using each on its own this paper describes the state of knowledge of the continental upper mantle obtained primarily from the natural source magnetotelluric technique and outlines how hypotheses and models regarding the development of cratonic lithosphere can be tested using deep probing electromagnetic surveying the resolution properties of the method show the difficulties that can be encountered if there is conducting material in the crust examples of data and interpretations from various regions around the globe are discussed to demonstrate the correlation of electromagnetic and seismic observations of the lithosphere asthenosphere boundary also the observations from laboratory measurements on candidate mineralogies representative of the mantle such as olivine are presented

controlled source audiomagnetotellurics in geothermal exploration theoretical and field tests indicate that the controlled source audiomagnetotelluric csamt method provides an efficient means of delineating the shallow resistivity pattern above a hydrothermal system utilizing a transmitter overcomes the main limitation of conventional audiomagnetotellurics variable and unreliable natural source fields reliable csamt measurements can be made with a simple scalar receiver our calculations for a half space show that the plane wave assumption is valid when the transmitter is more than 3 skin depths away in the broadside configuration and more than 5 skin depths away in the collinear configuration three dimensional 3 d numerical modeling results for a bipole source 5 skin depths away compare well with those for a plane wave source showing that the method is valid a csamt survey at the roosevelt hot springs geothermal area in utah produced apparent resistivity contour maps at four frequencies 32 98 977 and 5208 hz these maps show the same features as those of a

an alternative interpretation of lower mantle mineral associations in diamonds diamonds containing ferropericlase mg fe o and other silicate enstatite mg fe sio3 in particular assemblages are generally believed to be derived from the earth s lower mantle on the basis of the observed ratio between ferropericlase and enstatite inclusions and the feo content of these ferropericlases it is concluded that most of these minerals entrapped in diamonds may not represent the lithology of the lower mantle itself as has been suggested by many investigators instead ferropericlases in these diamonds represent most likely the disproportionate product of ferromagnesite mg fe co3 which underwent a decarbonation reaction to form both diamond and ferropericlase simultaneously in the lower mantle the wide variation in the mg of ferropericlase inclusions in diamonds is attributed to the decarbonation loop of the mgco3 feco3 solid solutions some of the enstatite inclusions coexisting with these ferropericlases in the same diamond may represent the most abundant mineral species of mg fe sio3 perovskite in the lower mantle the latter mineral phase experienced a retrogressive transition into enstatite during the transport of diamonds to the earth s surface

ten years of marine csem for hydrocarbon exploration marine controlled source electromagnetic csem surveying has been in commercial use for predrill reservoir appraisalandhydrocarbonexplorationfor10 years althougha recent decrease has occurred in the number of surveys and publications associated with this technique the method has become firmly established as an important geophysical tool intheoffshoreenvironment thisisaconsequenceoftwoimportant aspects associated with the physics of the method first it is sensitive to high electrical resistivity which although not an unambiguous indicator of hydrocarbons is an important property of economically viable reservoirs second although the method lacks the resolution of seismic wave propagation it has a much better intrinsic resolution than potential field methods such as gravity and magnetic surveying which until now have been the primary nonseismicdatasetsusedinoffshoreexploration althoughbymany measures marine csem is still in its infancy the reliability and noise floors of the instrument systems have improved significantly over the last decade and interpretation methodology has progressed from simple anomaly detection to 3d anisotropic inversion of multicomponent data using some of theworld sfastestsupercomputers researchdirectionspresently include tackling the airwave problem in shallow water by applying time domain methodology continuous profiling tools and the use of csem for reservoir monitoring during production

electrical structure of the crust below the deccan flood basalts india inferred from magnetotelluric soundings summary the indian shield has experienced major stable continental region scr earthquakes in the recent past which occurred in the deccan volcanic province until now the deeper crust below the deccan traps where thick basaltic layers cover large areas of the indian shield remains poorly understood a magnetotelluric mt study covering a broad period range was conducted along a n s running profile of 330 km length to achieve more insights into the nature of the crustal electrical structure below the flood basalts after dimensionality analysis and decomposition 2 d inversions for te and tm modes were carried out the basalt cover characterized by at first glance surprisingly low electrical resistivities is found to have an average thickness of 400 m with the exception of central parts of the profile where it reaches up to 700 m the crust is in general highly resistive but several subvertical zones of enhanced conductivity were delineated in the middle to lower crust which are tentatively explained as images of hidden partly reactivated faults fractures of the precambrian basement sensitivity analysis indicates a high resolution of these features an alternative approach incorporating macro anisotropy is additionally considered and corresponding 2 d anisotropic forward modelling confirms the existence of successions of conductive dykes in the deep crust

a numerical evaluation of electromagnetic methods in geothermal exploration the size and low resistivity of the clay cap associated with a geothermal system create a target well suited for electromagnetic em methods and also make electrical detection of the underlying geothermal reservoir a challenge using 3 d numerical models we evaluate four em techniques for use in geothermal exploration magnetotellurics mt controlled source audio magnetotellurics csamt long offset time domain em lotem and short offset time domain em tem our results show that all of these techniques can delineate the clay cap but none can be said to unequivocally detect the reservoir we do find however that the em anomaly from a deep conductive reservoir overlain by a larger more conductive clay cap is caused by the presence of the electric charge at conductivity boundaries rather than electromagnetic induction this means that for detection of the reservoir methods such as mt which rely on electric field measurements are superior to those where only the magnetic field is measured t

magnetotelluric soundings over a precambrian contact in australia summary the willyama complex near broken hill represents a major exposure of the precambrian terrain in australia however the eastern limits are obscured by recent cover in the murray darling basin the nature and extent of the contact with the younger basement to the east is unknown magnetotelluric data have now been obtained as an aid to regional mapping profiles are based on observations at 14 sites in a traverse east of broken hill apparent resistivities presented as pseudosections reflect the major structural units identified from surface geology continuity between sites is indicated from similarity in apparent resistivities and lateral changes east of the block are generally gradational characteristic divergences in orthogonal resistivity components are consistent with 2 d structure the foliated precambrian complex appears to form a semi vertical contact at the margin of the murray darling basin thus generating the required anisotropy at depths from 15 35 km seismic evidence indicates an equivalent structural contact possibly extending into the mantle

asymptotic airwave modeling for marine controlled source electromagnetic surveying marine controlled source electromagnetic csem surveying also called seabed logging is a technique for finding resistive layers in the subsurface which can be hydrocarbon saturated reservoirs the em signal from the horizontal electric dipole source induces an airwave along the air sea surface that interferes with the signal from the subsurface and dominates at larger offsets the airwave commonly denotes the energy that propagates from the source via the atmosphere to the receiver on the seabed the airwave component is especially problematic in shallow waters where it is less attenuated during its up and down propagation in the water column than for deeper waters an asymptotically derived space domain equation that describes the airwavecomponent in a water half space bounded by air is well known from the em literature we demonstrate that by taking into account the reflections and reverberations of the airwave component in the water column between the seabed and the sea surface at both the source

the detection of lateral anisotropy of the ocean floor by electromagnetic methods summary in a typical marine controlled source electromagnetic csem experiment one or more self contained receivers capable of measuring small variations in the electromagnetic field are located in an array on the sea floor a remote horizontal electric dipole is towed close to the sea floor by a surface vessel variations in the current through the dipole cause correlated variations in the electric and magnetic fields at the receivers the variations contain information about the distribution of electrical conductivity of the subsurface rocks the electrical conductivity is related to such critical physical parameters as porosity temperature and fluid content published studies of the response of the sea floor to electromagnetic excitation have not included any systematic variation in the sea floor conductivity with horizontal direction a horizontally isotropic oceanic lithosphere is not an appropriate assumption particularly in the vicinity of the ridge here for example in the near surface layers basalts are extruded onto the sea floor within the very narrow fissured neo volcanic zone the fissures may be filled with sea water and sediment which are better conductors than typical basalt alternatively as time progresses they may become sealed with hydrothermal mineral deposits which can be less conductive than the host rocks in both instances the electrical conductivity of the composite material in the direction oriented along the fisures parallel to the ridge will be greater than that in the transverse direction measures of anisotropy on sea floor of varying age are an indication of the history of the tectonic activity at the ridge the electromagnetic effects of lateral anisotropy are examined through the study of the response of a simple double half space model an upper half space representing sea water is connected to a lower half space representing anisotropic crustal material a general theory describing the excitation of this model by any transient electromagnetic source is developed and the step on responses for systems composed of a horizontal electric dipole transmitter and horizontal magnetic and electric field receivers are computed the results of previous work indicate that following a change in current in the transmitter two distinct changes in horizontal electric field strength separated in time are observed at the receiver the first is caused by the diffusion of the electromagnetic field through the relatively resistive sea floor the second is caused by diffusion through the relatively conductive sea water the times at which the two events occur are robust measures of the isotropic conductivity of the sea floor and the sea water respectively the results of this study show that shape and time of the initial change in the electric field varies systematically with both the average conductivity and the anisotropy of the sea floor for some orientations of the transmitter and the receiver the effect of increasing the anisotropy is similar to the effect of increasing the average conductivity the initial event is delayed however there are orientations for which increasing the anisotropy actually results in an earlier arrival natural science and engineering research council in canada

the electrical conductivity distribution beneath vancouver island a region of active plate subduction magnetotelluric mt and geomagnetic depth sounding gds data were recorded at 25 locations across vancouver island canada over the subducting juan de fuca plate the gds data respond to a number of local conductive features at frequencies above 10 hz while at longer periods the in phase and quadrature induction arrows are colinear and are aligned approximately normal to the continental shelf with a maximum amplitude response near the southwest coast interpretation of the mt data show the presence of a conducting zone at depths greater than 20 km beneath vancouver island and correlating with a strong seismic reflector extensive testing with two dimensional models shows the mt and gds data are consistent with a proposed geoelectrical structure containing a conducting layer extending from the pacific ocean dipping beneath vancouver island and in electrical contact with the mainland conductive region the layer has a horizontal conductance of 200 s that may decrease to 100 s beneath the northeast side of vancouver island related geophysical studies suggest the conductive zone is above the actual thrust surface of the juan de fuca plate the conducting layer is most likely the result of cracks and pores filled with saline fluids which are supplied bymore water subducted with the oceanic crust and by dehydration reactions the presence of fluids has significant implications for thrust earthquakes and for metamorphic reactions that occur in subduction zones the conductance of the upper mantle between 90 and 390 km is best modeled at approximately 1 500 s less

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

magnetotelluric image of the crust and upper mantle in the backarc of the northwestern argentinean andes magnetotelluric data from the backarc of the central andes in nw argentinawere re examined by employing impedance tensor decomposition and 2 d inversion and modelling techniques the data in the period range of 50 15 000 s were collected on a profile of 220 km length reaching from the eastern cordillera across the santa barbara system to the andean foreland of the argentinean chaco after a dimensionality analysis data from most sites were treated as regional 2 d the exception was the eastern section of the profile where the magnetotelluric transfer functions for periods 1000 s reflect a 3 d earth application of two tensor decomposition schemes yielded a regional strike direction of n s which is the azimuth of the central andean mountain chains several 2 d models were obtained by pseudo and full 2 d occam inversion schemes special emphasis was placed on the inversion of phase data to reduce the influence of static shifts in the apparent resistivity data the smooth inversion models all show a good conductor at depth a final model was then calculated using a finite element forward algorithm the most prominent feature of the resulting model is a conductor which rises from depths of 180 km below the chaco region to 80 km beneath the santa barbara system and the eastern cordillera its interpretation as a rise of the electrical asthenosphere is supported by seismic attenuation studies magnetotelluric results surface heat flow distribution in the area and the electrical properties of crustal and mantle rocks suggest that the upper mantle is predominantly ductile beneath the eastern cordillera and the western santa barbara system this generally agrees with anelastic seismic attenuation models of the area and is useful in discriminating between models of q quality factor distribution

3d modelling of electrical anisotropy from electromagnetic array data hypothesis testing for different upper mantle conduction mechanisms abstract upper mantle electrical conductivities provide besides seismic velocities or thermal diffusivities another source of information on upper mantle structure and composition we present the results of a long period magnetotelluric dataset consisting of 64 sites covering an area of 300 km 400 km in central germany mapping electrical conductivities at the base of the lithosphere to seperate the effect of crustal and upper mantle structures we interprete the obtained dataset by 3d conductivity modelling we observe high conductivities and anisotropies at asthenospheric upper mantle depth with main bulk conductivities of 1 2 s m in west east and 4 1 0 3 s m in north south direction differing by about two orders of magnitude in comparing with results from laboratory measurements on electrical conductivites for various conduction mechanisms we are able to confine the conduction mechanisms possibly dominating at these depths with melt volume fractions of 1 only partial melt can be excluded as origin of the high upper mantle conductivities instead conductivity enhancement due to hydrogen diffusion along the olivine 1 0 0 axis is favoured as the dominant conduction mechanism at the base of the lithosphere

northern east pacific rise magnetic anomaly and bathymetric framework the oceanic crust in the eastern pacific between 7on and 30on and east of 127ow contains a fairly complete history of the spreading centers associated with the east pacific rise since 25 m y b p late oligocene in this paper we have summarized the seafloor spreading magnetic anomaly data and the bathymetric data that reflect the record of this tectonic history the well defined magnetic lineations north of the clarion fracture zone in the mouth of the gulf of california and on the east flank of the east pacific rise epr are carefully examined and used to provide a guide for interpreting the spreading pattern between the clarion and clipperton fracture zones southward of the rivera fracture zone over the mathematician ridge and over the entire epr east of the mathematician ridge between the rivera and siqueiros fracture zones the bathymetric data provide a trace of the fracture zone pattern in each of the above mentioned areas the fracture zone bathymetry and the seafloor spreading magnetic lineations on the epr south of the rivera fracture zone have a distinctive fanning pattern caused by close poles of rotation and plate boundary reorganizations all these data provide a good record of the plate reorganizations in the middle miocene at magnetic anomaly 5a time 12 5 to 11 m y b p in the late miocene at magnetic anomaly 3 4 time 6 5 m y b p and in the pliocene at magnetic anomaly 2 3 time 3 5 m y b p several abandoned spreading centers including the mathematician ridge were left behind as a result of these reorganizations the mathematician ridge is shown to be a set of ridges and trough whose origin is related to the tectonic activity associated with each of the above mentioned reorganizations since anomaly 5a

first results from a marine controlled source electromagnetic survey to detect gas hydrates offshore oregon 1 submarine gas hydrate is a hazard to drilling a potential hydrocarbon resource and has been implicated as a factor in both submarine slope stability and climate change bulk in situ electrical resistivities evaluated from electromagnetic surveys have the potential to provide an estimate of the total hydrate volume fraction more directly than by using seismic and well log data we conducted a marine controlled source electromagnetic sounding at hydrate ridge oregon usa in august 2004 electromagnetic fields transmitted by a deep towed horizontal electric dipole source were measured by a linear array of 25 seafloor electromagnetic receivers positioned 600 m apart to produce a dense coverage in the recorded electric field data results are presented in simple form by apparent resistivity pseudosections which produce an approximate image of lateral resistivity variations across the study region resistivity values are consistent with those from well logs collected in the area and pseudosection features are correlated with seismic reflectors archie s law based on pseudosection apparent resistivities predicts volumetric hydrate concentrations vary from 0 30 across the ridge

marine electromagnetic methods a new tool for offshore exploration electromagnetic survey methods used for decades mainly for mining exploration on land have burst upon the offshore hydrocarbon exploration scene over the past five years this move driven by the costs and challenges associated with the deepwater exploration environment has left many companies scrambling to understand new and unfamiliar technologies and has resulted in a sudden expansion of the portfolios of the few nonseismic geophysicists left in the oil industry of the two main methods magnetotelluric mt surveys are at least somewhat familiar from their use on land as a reconnaissance tool and for problem solving in areas of poor seismic performance and to a large extent can be applied to the marine environment in a similar way to their terrestrial counterparts the other dominant technology controlled source electromagnetic csem sounding has featured much less in land exploration but more importantly behaves so very differently in the deepwater marine environment that it might as well

the conductivity of the crust and mantle beneath the kapuskasing uplift electrical anisotropy in the upper mantle summary kapuskasing uplift has been interpreted as an oblique cross section of up to 25km of crust and thus provides the opportunity to examine the properties of exposed mid to lower crustal material a magnetotelluric mt survey mapped a remarkably uniform upper crust and provided no evidence for upper crustal conductive zones that could be related to the often observed increase in mid and lower crustal conductivity the only shallow conductive anomaly is related to the ivanhoe lake cataclastic zone but its electrical signature does not appear to extend more than one kilometer in depth a regional apparent resistivity curve was determined from mt data and confirmed by subsequent controlled source electromagnetic surveys regional curves are essential for proper structural interpretation but are often difficult to determine because of electric field distortions the data show a decrease in resistivity at depths below 15 km which is typical of continental crust in many areas of the world there is a clear difference in orthogonal apparent resistivity and phase curves at periods greater than 10s which is most pronounced in a n65 e and n25 w coordinate frame two different models are proposed to explain the data the first consists of a conducting 2 d slab of approximately 50 km width in the lower crust and striking n65 e the strike direction is consistent with a number of regional structural trends but there is no other direct supporting evidence for it the second model invokes either micro or macro electrical anisotropy in the upper mantle there is considerable evidence for seismic anisotropy in the region the electrical anisotropy could be the result of preferential conduction along the c axis of olivine crystals hydrogen dissolved in the olivine lattice with preferential conduction along the a axis or possible alignment of dykes joints and faults in the upper mantle if the anisotropy is related to strain induced orientations of crystals or fabric it will provide evidence for the motions of the mantle associated with plate tectonics

magnetotelluric regional strike a new method for determining regional strikes from the magnetotelluric impedance tensor z and tipper t is presented it involves the minimization of weighted sums of the squared magnitudes of elements of z or t over all frequencies and all stations of interest when applied to data from the mexicali valley baja california around the cerro prieto geothermal field for a particular weighting function the method yielded orientations that agree to within 2 9 degrees for three lines with a total of 16 stations the consistency of orientations can be attributed in part to the pronounced two dimensional 2 d geologic characteristics of the area but the techniques also proved stable at stations near three dimensional 3 d inhomogeneities

controlled electromagnetic sources for measuring electrical conductivity beneath the oceans 1 forward problem and model study exact closed form expressions for the electromagnetic induction fields produced by vertical and horizontal current sources in the conducting ocean overlying a one dimensional earth are derived from the maxwell equations numerical methods for the evaluation of the solutions are given including correction for the finite size of real sources simple models of the electrical conductivity structure of the ocean crust and lithosphere are deduced from geologic petrologic and laboratory data and their electromagnetic response is modeled horizontal electric dipole sources produce much larger field amplitudes than their vertical counterparts for a given frequency and range and the horizontal electric field offers superior received signal performance reflections of electromagnetic waves from the sea surface and thermocline must be considered for low enough frequencies or long ranges estimates of the ambient noise level from natural electromagnetic sources in the frequency range 0 01 10 hz are presented the ability of controlled sources to determine features of the conductivity of the ocean crust and upper mantle especially low conductivity zones is demonstrated if the mantle conductivity is low enough horizontal ranges of 50 km and conductivity estimates to over 20 km depth can be achieved

electrical resistivity of the northern australian lithosphere crustal anisotropy or mantle heterogeneity abstract interpretations of continental magnetotelluric mt data are increasingly invoking electrical resistivity anisotropy in the crustal and or mantle lithosphere or deeper in the asthenosphere to explain non uniform responses in this paper we show that seven shallow marine 10 000 m in the proterozoic western end of the survey grading to 1000 m beneath the eastern australian phanerozoic shear wave velocity tomographic models show 10 higher velocities beneath the proterozoic craton compared with phanerozoic continental lithosphere suggesting that the variations in resistivity may be due to higher temperatures beneath the eastern side and or an increased volatile content

enhanced subsurface response for marine csem surveying one of the main challenges of using marine controlled source electromagnetic csem sounding for hydrocarbon detection has been a relatively low resolution and restricted depth penetration in addition the use of csem in shallow waters has been perceived as particularly difficult a new robust method for enhancing marine csem subsurface response is particularly useful in shallow waters the method is designed to reduce acquisition imprints and attenuate the dominant primary airwave contribution synthetic examples show that it is possible to detect thin deeply buried 3d resistive bodies in a shallow water environment with a complex resistivity structure the results highlight the importance of a sufficient signal to noise ratio

two types of marine controlled source electromagnetic transmitters marine controlled source electromagnetic methods are used to derive the electrical properties of a wide range of sub seafloor targets including gas hydrate reservoirs in most marine controlled source electromagnetic surveys the deep tow transmitter is used with a long horizontal electric dipole being towed above the seafloor which is capable of transmitting dipole moments in the order of up to several thousand amperemetres the newly developed deployed transmitter uses two horizontal orthogonal electrical dipoles and can land on the seafloor it can transmit higher frequency electromagnetic signals can provide accurate transmission orientation and can obtain higher signal stacking which compensates for the shorter source dipole length in this paper we present the study key technologies and implementation details of two new marine controlled source electromagnetic transmitters the deep tow transmitter and the deployed transmitter we also present the results of a marine controlled source electromagnetic experiment conducted from april to may 2014 in the south china sea using both the deep tow transmitter and the deployed transmitter which show that the two types of marine transmitters can be used as effective source for gas hydrate exploration

analysis of tec measurements above active seismic regions abstract ionospheric perturbations by natural geophysical activity such as volcanic eruptions and earthquakes have been studied since the great alaskan earthquake in 1964 measurements made from the ground show a variation of the critical frequency of the ionosphere layers before and after the shock in this paper we present an experimental investigation of the electron density variations before earthquakes when the low altitude satellite topex poseidon is above active seismic regions fluctuations in the integrated electron density or tec total electron content between the satellite and the ground are measured to correct the altimetry measurement the satellite was launched on august 10 1992 and an analysis of the tec fluctuations above the main earthquakes ms 5 is performed up to the end of 1995 due to different constraints the number of earthquakes considered in this study is only 706 increases as well as decreases of tec are observed above some epicentres three events are detailed and a statistical study is performed with the doris data the corresponding number of tec perturbations correlated in space and in time up to 48 h before these earthquakes is 238 i e 34 of the cases a comparison is made with tec data randomly selected during the lifetime of the satellite and ionospheric perturbations are observed in 16 of the cases this study indicates that it is possible to use tec measurements to survey these phenomena

audiomagnetotelluric characterization of range front faults snake range nevada two controlled source audio magnetotelluric csamt profiles were collected on the eastern flank of the snake range in eastern nevada across geologically complex terrain to investigate the suspected presence of faults along the range front the location of the range bounding faults is not easily determined on geologic grounds because of the presence of extensive young sedimentary cover and overall geologic complexity characterization of the presence location and structural style of the range front faults is critical to assessment of connectivity of groundwater aquifers near the mountain front and in adjacent alluvial basins a total of 48 csamt soundings were recorded along two lines that were chosen to maximize subsurface geologic information two generations of faults were interpreted based on the csamt data an older low angle fault that is cut by a younger more steeply dipping fault lack of deep boreholes in the region required that the subsurface interpretation rely on analogy from surface outcrops within and adjacent to the study area the success of the csamt method as applied in this study hinged on near ideal collection conditions the relatively high contrast in electrical resistivity provided by the rock types involved and well developed geologic conceptual models of the region

controlled source electromagnetic sounding in shallow water principles and applications the marine controlled source electromagnetic csem method is being applied to the problem of detecting and characterizing hydrocarbons in a variety of settings until recently its use was confined to deepwater water depths greater than approximately 300 m because of the interaction of signals with the atmosphere in shallower water depths the purpose of this study was to investigate using a simple 1d analytical analysis the physics of csem in shallow water this approach demonstrates that it is difficult to simply decouple signals that have interacted with the earth from those that have interacted with the air using either frequency domain or time domain methods stepping away from wavelike approaches which if applied without care can be misleading for the diffusive fields of csem we demonstrate an effective way to mitigate the effect of the air in shallow water surveys by decomposing the em signal into modes and using only the mode least affected by interaction with the atmosphere such decompos

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

resistivity imaging of controlled source audiofrequency magnetotelluric data the controlled source audiofrequency magnetotelluric csamt method is an electromagnetic em method where a transmitter a grounded electric bipole or a current loop is placed far away from the receiver sites if the transmitter is located at distances greater than 3 5 skin depths the plane wave approximation is valid and the techniques used for natural source mt interpretation can be applied goldstein and strangway 1975 sandberg and hohmann 1982 the csamt method can be employed in a detailed survey by closely spacing a number of receiver sites along a traverse the borehole csamt technique is proposed in west and ward 1988 to enhance the ability of the surface csamt method to detect subsurface inhomogeneities in these cases two dimensional 2 d smooth model inversion rodi et al 1984 sasaki 1989 degroot hedlin and constable 1990 would be particularly useful for deriving a resistivity model from the far field data and for presenting the survey results in the form of an image

navigating marine electromagnetic transmitters using dipole field geometry the marine controlled source electromagnetic csem technique has been adopted by the hydrocarbon industry to characterize the resistivity of targets identified from seismic data prior to drilling over the years marine controlled source electromagnetic has matured to the point that four dimensional or time lapse surveys and monitoring could be applied to hydrocarbon reservoirs in production or to monitor the sequestration of carbon dioxide marine controlled source electromagnetic surveys have also been used to target shallow resistors such as gas hydrates these novel uses of the technique require very well constrained transmitter and receiver geometry in order to make meaningful and accurate geologic interpretations of the data current navigation in marine controlled source electromagnetic surveys utilize a long base line or a short base line acoustic navigation system to locate the transmitter and seafloor receivers if these systems fail then rudimentary navigation is possible by assuming the transmitter follows in the ship s track however these navigational assumptions are insufficient to capture the detailed orientation and position of the transmitter required for both shallow targets and repeat surveys in circumstances when acoustic navigation systems fail we propose the use of an inversion algorithm that solves for transmitter geometry this algorithm utilizes the transmitter s electromagnetic dipole radiation pattern as recorded by stationary close range 1000 m receivers in order to model the geometry of the transmitter we test the code with a synthetic model and validate it with data from a well navigated controlled source electromagnetic survey over the scarborough gas field in australia

a model of lower crustal electrical anisotropy for the pontiac subprovince of the canadian shield summary magnetotelluric data from the pontiac subprovince of the canadian shield indicate that the crustal electrical conductivity structure of the region is very uniform because the data are distorted by near surface electrical heterogeneities the exact parameters of the resistivity structure may vary for different static shift corrections but the same sequence of layers is retained an approximate one dimensional model of the pontiac based on inversion of the effective impedance recorded at each site consists of a thin conductive weathered layer underlain by at least 12 km of resistive upper crust at 5000 rm or more a mid crustal layer of 200 qm extends to a depth of 25 km or more below which there is evidence of an increase in the bulk resistivity in the lower crust and or upper mantle however this last layer displays marked non one dimensional magnetotelluric responses which appear to be quite uniform along the profile similar responses have been seen in other parts of the canadian shield and the baltic shield this feature in the mt data can be explained by azimuthal electrical anisotropy or by a two dimensional conductive structure the absence of any large vertical magnetic field responses suggests that anisotropy is the more likely cause modelling the anisotropy shows that a ratio of horizontal resistivities between 1 6 and 1 13 is representative of the lower crust and or the upper mantle in the area the higher conductivity is found or electric fields measured in the east west direction subparallel to the tectonic fabric of the shield in this region

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

the diamonds of south australia abstract diamonds in south australia occur in kimberlites at eurelia orroroo and in placer deposits which include the springfield basin and the historic echunga goldfield to identify the kimberlitic and mantle sources of the placer diamonds and to determine any possible connections between the placer diamonds and the diamonds from the eurelia kimberlites we examined the physical and compositional characteristics and the mineral inclusion content of 122 diamonds from the springfield basin and 43 diamonds from kimberlites at eurelia additional morphological data for three echunga diamonds are also given most of the diamonds from the springfield basin are similar to the diamonds from eurelia with respect to their crystal shapes surface textures and colors the diamond populations from both areas are characterized by a high abundance of low nitrogen

sensitivity study of marine csem data for reservoir production monitoring using numerical and analytical modeling we assess the feasibility of marine controlled source electromagnetic csem data for monitoring the flooding front during water flooding of an oil reservoir we discuss the ability of time lapse csem data to resolve changes in the electric conductivity small enough to be of interest for monitoring purposes measurement and modeling errors are discussed briefly and analytical calculations comparing time lapse signals with a certain type of time lapse modeling errors are performed numerical calculations performed with a volume integral equation method are used to study the effect on the electromagnetic em fields of relevant conductivity changes the numerical investigation includes robustness with respect to survey parameter values discrimination between different front shapes and ability to overcome time lapse error simulated signals are found to be strong enough to overtake typical measurement errors and are fairly robust toward perturbations of survey parameters it is found analytically and numerically that certain modeling errors experience a high degree of time lapse cancellation

an introduction to marine controlled source electromagnetic methods for hydrocarbon exploration early development of marine electromagnetic methods dating back about 80 years was driven largely by defense military applications and use for these purposes continues to this day deepwater frequency domain electric dipole dipole controlled source electromagnetic csem methods arose from academic studies of the oceanic lithosphere in the 1980s and although the hydrocarbon exploration industry was aware of this work the shallow water environments being explored at that time were not ideally suited for its use low oil prices and increasingly successful results from 3d seismic methods further discouraged investment in costly alternative geophysical data streams these circumstances changed in the late 1990s when both statoil and exxonmobil began modeling studies and fieldtrials of csem surveying in deep water around 1000 m or deeper specifically for characterizing the resistivity of previously identified drilling targets trials offshore angola in 2000 2002 by both these companies showed that

decomposition of electromagnetic fields into upgoing and downgoing components this paper gives a unified treatment of electromagnetic em field decomposition into upgoing and downgoing components forconductiveandnonconductivemedia wheretheelectromagneticdataaremeasuredonaplaneinwhichtheelectricpermittivity magneticpermeability andelectricalconductivityareknown constants with respect to space and time above and below the plane of measurement the medium can be arbitrarily inhomogeneousandanisotropic in particular the proposed decomposition theory applies to marine em low frequency data acquired for hydrocarbon mapping where the upgoing components of the recordedfield guided and refracted from the reservoir that are of interest for the interpretation the direct source field the refracted airwave induced by the source the reflected field from the sea surface and most magnetotelluric noise traveling downward just below the seabed are field components that are considered to be noise in electromagneticmeasurements the viability and validity of the decomposition method is demonstrated using modeled and real marine em data also termed seabed logging sbl data the synthetic data are simulated in a model that is fairly representative of the geologic area wheretherealsblwerecollected theresultsfromthesynthetic data study therefore are used to assist in the interpretation of the realdatafromanareawith320 mwaterdepthaboveaknowngas province offshore norway the effect of the airwave is seen clearly in measured data after field decomposition just below the seabed the upgoing component of the recorded electric field has almost linear phase indicating that most of the effect of the airwavecomponenthasbeenremoved

high frequency seismo electromagnetic effects abstract electromagnetic perturbations caused by natural geophysical activity such as earthquakes and volcanic eruptions have been studied since the great alaskan earthquake in 1964 in recent years interest has been increasing because when a precursor event occurs it normally does so less than a few hours before the shock this paper presents a review of the high frequency seismo electromagnetic phenomena already observed and occurring before as well as after the shock the term high frequency is to be taken in the seismic sense i e larger than a few hertz mechanisms that might produce these seismo electromagnetic emissions will be described it is shown that at the moment a causal relationship between electromagnetic signals and seismic activity still remains to be demonstrated theoretical as well as intensive experimental studies are necessary to understand the physical processes responsible for seismo or volcano electric variations electrical resistivity variations electromagnetic effects associated with dilatance as well as electromagnetic fields generated during laboratory experiments or ionospheric disturbances

detecting hydrocarbon reservoirs from csem data in complex settings application to deepwater sabah malaysia controlled source electromagnetic csem field surveys offer a geophysical method to discriminate between high and low hydrocarbon saturations in a potential reservoir however the same geological processes that create the possible hydrocarbon reservoir may also create topography and near surface variations of resistivity e g shallow gas or hydrates that can complicate the interpretation of csem data in this paper we discuss the interpretation of such data over a thrust belt prospect in deepwater sabah malaysia we show that detailed modeling of the key scenarios can help us understand the contributions of topography near surface hydrates and possible hydrocarbons at reservoir depth complexity at the surface and at depth requires a 3d electromagnetic modeling code that can handle realistic ten million cell models this has been achieved by using an iterative solver based on a multigrid preconditioner finite difference approach with frequency dependent grid adaptation

northeastern pacific mantle conductivity profile from long period magnetotelluric sounding using hawaii to california submarine cable data we present results of a long period magnetotelluric mt investigation of the electrical structure beneath the eastern north pacific the electric field data consist of 2 years of continuously recorded voltages across an unpowered 4000 km long submarine telephone cable haw 1 extending from point arena california to oahu hawaii the electric field measurements are coherent to some degree with magnetic field measurements from honolulu observatory at periods of 0 1 to 45 days this coherence is enhanced at long periods over that observed with point electric field sensors due to horizontal averaging of the motional electric fields of spatial scale smaller than the cable length significantly diminishing their effect robust controlled leverage mt response estimates and their jacknife confidence limits are computed for the haw 1 to honolulu data an equivalent scalar mt response obtained from honolulu magnetic variations data is used to correct the haw 1 mt response for static shift and to extend the mt response estimate to periods of 100 days the composite response function satisfies necessary and sufficient conditions for consistency with a one dimensional conductivity structure and is most sensitive to structure between 150 and 1000 km inversion of the mt response reveals a conductive zone 0 05 0 1 s m between 150 and 400 km depth and a positive gradient below 500 km these observations are consistent with previous mt studies in the north pacific this upper mantle conductivity is too high to be explained by solid state conduction in dry olivine using reasonable mantle geotherms calculations based on measurements of hydrogen solubility and diffusivity in olivine indicate that h dissolved in olivine possibly combined with a lattice preferred orientation consistent with measured seismic anisotropy provide sufficient conductivity enhancement to explain the inversion results the high conductivity may also be explained by the presence of gravitationally stable partial melt comparison of the haw 1 results with long period mt studies conducted on land reveals differences in upper mantle conductivity between different tectonic regimes in particular the upper mantle beneath the pacific ocean is considerably more conductive than that beneath the canadian shield and similar in conductivity to that beneath the basin and range

a marine electromagnetic survey to detect gas hydrate at hydrate ridge oregon gas hydrates are a potential energy resource and hazard for drilling and infrastructure yet estimates of global volume vary by over three orders of magnitude hydrates are electrically resistive compared to water saturated sediment and so electromagnetic methods provide an additional tool to seismic surveys and drilling for determining hydrate saturations a marine electromagnetic survey was carried out at hydrate ridge oregon usa with the aim of testing the use of controlled source electromagnetic csem and magnetotelluric mt methods to map gas hydrate and free gas below the gas hydrate stability zone a 2 d csem inversion supports the scenario deduced from previous seismic and drilling results which indicate two mechanisms of hydrate emplacement a transport dominated and reaction dominated regime a prominent resistive region of 2 5 4 m at a depth of about 130 mbsf near the seismic bottom simulating reflector bsr suggests that 27 to 46 per cent of the bulk volume is filled with hydrate depending on whether archie s law or the hashin strikman bounds are used this is representative of a reaction dominated regime for hydrate emplacement and where a significant low velocity zone exists based on a seismic tomography inversion suggests large quantities of free gas below the bsr electrical resistivity logging while drilling lwd data show general agreement with the csem inversion model except for a csem derived resistive region at seismic horizon a known to transport free gas into the gas hydrate stability zone inversion of mt data collected simultaneously during the csem survey provides a complimentary low resolution image of the shallow sediments and shows folding in the accretionary complex sediments similar to that imaged by a tomographic seismic velocity model

mapping thin resistors and hydrocarbons with marine em methods insights from 1d modeling the use of marine controlled source electromagnetic em csem sounding to detect thin resistive layers at depths below the seafloor has been exploited recently to assess the resistivity of potential hydrocarbon reservoirs before drilling we examine the sensitivity of the csem method to such layers with forward and inverse modeling in one and three dimensions the 3d modeling demonstrates that if both source and receivers are over a tabular 3d target 1d modeling predicts the observed response to very high accuracy experimental design can thus be based on 1d analysis in which hundreds of range and frequency combinations can be computed to find the optimal survey parameters for a given target structure modeling in three dimensions shows that the vertical electric field response is largest over the edges of a 3d target the 3d modeling also suggests that a target body needs to have a diameter twice the burial depth to be reliably seen by csem sounding a simple air wave model energy propagating from source to receiver via the atmosphere allows the effects of the target layer and atmosphere to be separated and shows where sensitivity to the target is diminished or lost because of finite water depth as a function of range frequency and seafloor resistivity unlike dc resistivity sounding the marine csem method is not completely t equivalent and in principle can resolve resistivity and thickness separately smooth inversion provides an estimate of the method s resolving power and highlights the fact that although the radial csem fields contain most of the sensitivity to the thin resistive target inverted alone they produce only increasing resistivity with depth inclusion of the radial mode csem data forces the recovery of the thin resistor but magnetotelluric data can be used more effectively to achieve the same result

electrical conductivity variations around the palmdale section of the san andreas fault zone our analysis of a decade of observations from the palmdale california telluric array is presented the array is a system of large scale telluric field measurements used for the passive monitoring of electrical conductivity variations over a large region centered over the san andreas fault zone northeast of los angeles since 1985 systematic changes in the relationships between the telluric field measurements around the array have been observed these changes amounted to 1 to 2 by the time the array was closed down in 1989 with the fields measured along the fault and northeast of the fault showing signal decreases relative to the fields southwest of the fault because of the possible implications of these changes for conductivity variations related to tectonic activity on the san andreas we have attempted to sort out the geography of the zones that could cause the changes the simplest though nonunique interpretation of the observed variations in the telluric relationships is a steady increase in the conductivity of the lower crustal region of the fault zone from two dimensional models of the telluric response around the array we infer that an increase of about 60 milli siemens in the conductance of the lower crustal fault zone could account for the changes seen over the period 1985 to 1989 assuming changes were in a kilometer wide zone and that the lower crustal fluids were very conductive this could represent a strain of about 0 4 10 6 the surface strain would be much smaller and would be lost in surface strain noise because of the possibility that fluid pressure changes are involved these small strains could still be significant for earthquake cycle phenomena alternatively the observed changes could have been produced by increases in the conductivity of the upper crust of the mojave desert northeast of the fault or by decreases in the conductivity of the upper crust in the san gabriel mountains southwest of the fault to produce the observed telluric changes however the additional conductivity and the associated strain changes would have to be much larger the strain needed to accommodate the increased conductivity on the mojave side would have to be of the order of 6 10 4 which seems unrealistically high smaller strains could account for conductivity changes in the san gabriel mountains but these strains would have to be compressive water level changes are not considered to be a possible cause of the conductivity changes because the measurement scale is much larger than the typical scale of water level changes and no seasonally correlated changes in the telluric fields were observed

deep electrical conductivity structures of the appalachian orogen in the southeastern u s long period magnetotelluric data across the southeastern appalachians image deep crustal and upper mantle relics of ancient orogenic events inversions of the responses show 1 beneath the appalachian mountains there is a sub horizontal conductor at 15 20 km depth which dips to the southeast at the surface trace of the brevard fault 2 at the location of the central piedmont suture there is a crustal conductor which dips towards the southeast interpreted as a structure related to the acadian suture 3 upper mantle conductors were found at 80 km depth northwest of the blue ridge and at 140 km depth southeast of the eastern piedmont between these there is a northwest dipping resistive gap possibly representing the remnant structure of the alleghanian collision

three methods for mitigating airwaves in shallow water marine controlled source electromagnetic data in the past several years marine controlled source electromagnetic mcsem techniques have been applied successfully in deep water depth 1 km for oil and gas exploration the application of this technology in shallow water is challenged however because of airwaves that mask the signal from the target reservoir at depth based upon the understanding that an airwave is a lateral wave which can be analytically expressed in a dual half space resistivity model we propose three airwave mitigation approaches to reduce the effects of these airwaves on mcsem data in the em x bucking approach the effect of the airwaves can be bucked out from two measurements by using the analytic expression of the airwave the frequency derivative de dfreq approach takes advantages of the unique characteristics of the airwaves in frequency domain enhancing the reservoir signals while suppressing the airwave the magnetotelluric mt stripping method uses the plane wave feature of the airwaves and subtraction of

electrical conductivity of san carlos olivine along 100 under oxygen and pyroxene buffered conditions and implications for defect equilibria the electrical conductivity along 100 of single crystal san carlos olivine was measured as a function of temperature between 1100 and 1200 c and oxygen fugacity between 10 6 and 10 0 5 pa at 1200 c and either with pyroxene buffered or without self buffered an added natural pyroxene buffer from a san carlos lherzolite under these temperature and o2 conditions electrical conduction in the self buffered sample is attributed to polarons fe and electrons e and in the pyroxene buffered sample is attributed to polarons fe and magnesium vacancies v mg over the range of temperature and o2 investigated the electrical conductivity of the self buffered sample is given by sb 100 2 27 s m e 0 55 ev kt o20 18 306 3 s m e 2 25 ev kt o2 0 18 and for the pyroxene buffered sample by pb 100 0 18 s m e 0 34 ev kt o20 17 15 2 s m e 1 3 ev kt where k is boltzmann s constant t is in kelvin and o2 is in atmospheres the conductivity of the pyroxene buffered sample is lower than that of the self buffered sample primarily as a result of a decrease in the polaron concentration the electrical conductivity of both samples was found to decrease irreversibly once the samples experienced an oxygen fugacity more reducing than approximately the wustite magnetite buffer electron microprobe analyses indicate that this effect results from loss of iron from the olivine samples to the iridium electrodes a series conduction model based on the observed compositional gradient adequately accounts for the magnitude of the irreversible conductivity decrease and limits the thickness of any surficial pyroxene phase to 0 1 m mantle temperature profiles based on laboratory measurements of self buffered samples predict temperatures of the order of between 25 and 150 c colder depending on the ambient oxygen fugacity than those based on measurements of pyroxene buffered samples