electromagnetic induction studies recent developments in electromagnetic induction studies of the lithosphere and the asthenosphere are reviewed attention is given to geoelectrical studies of active tectonic areas in terms of the major zones of crustal extension the basin and range province along western regions of north america and the rio grande rift studies have also been performed of tectonic activity around iceland the salton trough and cerro prieto and the subduction zones of the cascade mountains volcanic belt where magnetotelluric and geomagnetic variation studies have been done geomagnetic variations experiments have been reported in the central appalachians and submarine electromagnetic studies along the juan de fuca ridge controlled source electromagnetic and dc resistivity investigations have been carried out in nevada hawaii and in the adirondacks mountains laboratory examinations on the conductivity of representative materials over a broad range of temperature pressure and chemistry are described

the effect of hydrocarbon saturation on resistivity distribution and csem response abstract the controlled source electromagnetic csem method can be used to map resistivity differences in the sub surface with the purpose of identifying hydrocarbon filled reservoirs prior to drilling exploration wells in this study 15 exploration wells in the norwegian sea were analysed to investigate the relationship between reservoir properties hydrocarbon saturation resistivity and subsequently csem response two discovery wells linerle and falk with similar reservoir properties but different reservoir resistivity were further compared and evaluated through forward csem modelling to identify possible causes of the observed contrasting resistivities the results demonstrate that hydrocarbon saturation which can significantly affect the resistivity of hydrocarbon filled reservoirs may change rapidly over short distances controlling factors include 1 oil density and viscosity 2 reservoir quality and 3 the location of the reservoir relative to the transition zone associated with the oil water contact owc high oil density and poor reservoir quality commonly result in a thicker saturation transition zone than low oil densities and high permeability if the reservoir section is located within the transition zone the average resistivity can be significantly reduced low average resistivity in the linerle reservoir is found to be caused by low hydrocarbon saturation as a combined result of high density oil reservoir heterogeneity and that the reservoir section is situated within the oil water transition zone above the owc higher resistivity in the falk reservoir which has similar thickness and burial depth is attributed to better reservoir properties i e higher permeabilities lighter oil and the location of the reservoir section well above the saturation transition zone associated with the owc the ability to understand and predict key fluid and reservoir parameters is critical for appropriate application of the csem technology in hydrocarbon exploration

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

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

exploration of mid ocean ridges with a frequency domain electromagnetic system summary electromagnetic survey methods using controlled sources have been applied to ocean floor and mid ocean ridge surveys and have shed new light on the distribution of fluids within these structures the present state of knowledge of the electrical nature of oceanic crust is reviewed and some alternative mid ocean ridge electromagnetic surveys are simulated using a numerical modelling algorithm to see what further information they could provide this simulation considers the response of both large scale crustal targets such as magma bodies and zones of hydrothermal circulation and also the effect of near surface heterogeneity that may spatially alias the data the magnitude of this heterogeneity is estimated and it is shown that it could be large enough to prevent useful structural constraints being obtained if the effect spatial aliasing was negligible or could be removed a trans ridge experiment could delinate zones of partial melting and hydrothermal circulation within the oceanic crust

increasing the sensitivity of controlled source electromagnetics with synthetic aperture controlled source electromagnetics csem has been used as a derisking tool in the hydrocarbon exploration industry we apply the concept of synthetic aperture to the lowfrequency electromagnetic field in csem synthetic aperture sources have been used in radar imaging for many years using the synthetic aperture concept big synthetic sources can be constructed by adding the response to small sources building blocks in different ways and consequently big sources with different radiation patterns can be created we show that the detectability of hydrocarbons is significantly enhanced by applying synthetic aperture to csem data more challenging targets such as deep reservoirs 4km belowsea floor can be detected the synthetic aperture technique also increases the sensitivity of the field to subsurface targets in the towing streamer acquisition we also show that a pseudovertical source orthogonally distributed dipole pairs can be constructed synthetically and that the detection capability of this pseudovertical source is increased by applying field steering the synthetic aperture concept opens a new line of research in csem with the freedom to design suitable synthetic aperture sources for a given purpose

cenozoic subsidence and uplift of continents from time varying dynamic topography time varying global dynamic topography has been computed for the entire cenozoic the spherical viscous flow model is driven by a distribution of density heterogeneity based on mesozoic and cenozoic subduction by using the calculated topography the uplift and subsidence histories of five regions fixed to the north american indian australian indonesian and south american continents are computed predicted vertical motions are in qualitative agreement with observed values except for india and south america for australia and north america the predicted periods of uplift and subsidence are out of phase with respect to observations but display the correct overall trend the latest phase of uplift for north america is directly related to the cessation of kula plate subduction after 48 ma dynamic topography depends on the viscosity structure of the mantle a lower mantle that is 50 times more viscous than the upper mantle yields the best agreement between modeled and predicted vertical motions the presence of a low viscosity channel beneath the lithosphere has only a small influence on our results compared to dynamic topography the time varying geoid is insignificant in controlling relative sea level

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

unified structure in quaternary climate the quaternary climate record exhibits a structure of superimposed aperiodic oscillations starting at the 11 yr sunspot cycle and spaced by powers of 2 in period through the major 90 000 yr glacial cycle climate cycles that do not fall in this structure typically correspond to harmonics of the structure oscillations the inclusion of the known solar cycles and the presence of increased abundances of cosmogenic radionuclides at many structure periods suggest that the structure is related to long period solar variability

power spectrum of sea level change over fifteen decades of frequency 1 the power spectrum of relative sea level change has been estimated over more than 15 orders of magnitude in frequency from a frequency of 1 591 ma to a frequency of 1 5 s although there are still regions of the spectrum where data sampling and duration do not allow the power to be calculated most notably between periods of 100 1000 years the general shape of the spectrum is that in which the power depends on the square of the reciprocal frequency apart from periods between 1 and 100 years where the power spectrum falls off less steeply with increasing frequency a spectrum in which power depends on reciprocal frequency squared is the same spectrum as that calculated from a random walk signal of finite length there are some causes that have defined frequencies such as those associated with tides and the milankovitch cycles of the ice ages but there is also a continuum of relative sea level change that requires other causes the implications of this are discussed in the light of global change and heating of the lithosphere from the bottom

crustal processes at spreading centers recent discoveries and research on mid ocean ridges mor s in the last four years have greatly increased our knowledge of this fundamental type of plate boundary the last iugg review of mor research was in 1975 aumento et al 1975 and its omission as a topic in 1978 may have reflected a feeling that spreading centers were reasonably well understood since that time however there has been a surfeit of important new observations and exciting research and there is now a turmoil of innovative ideas about the processes which create and leave their imprint on two thirds of the earth s crust new seismic measurements have been made and interpreted as evidence for the existence of shallow axial magma chambers amc beneath intermediate total opening rate 5 9 cm yr and fast 9 18 cm yr spreading centers fig 1 yet there is controversy as to whether they are steady state whether they persist beneath slow ridges as well in the steady state what the dimensions and shape of the chamber are its relation to oceanic crustal stratigraphy and its relation to observations in ophiolites black smoker hydrothermal vents have been discovered in the pacific spewing forth 350 c fluids blackened by polymetallic sulfide precipitates this discovery is embroiled in debate as to how long the vents last whether they occur at slow spreading rates as well whether the enormous heat flux of the vents freeze the magma chamber even at fast spreading rates their influence on geochemical balances in the ocean and the value of the sulfide deposits commercially it has been found that our initial view of mor s as two dimensional linear structures has served us well as an initial working model but must be significantly modified to account for variations in tectonic volcanic and hydrothermal activity along strike as well as for episodicity of these processes deep sea drilling project dsdp samples have raised fundamental questions about formation of the magnetic source layer hydrothermal circulation and fault mechanisms along the mor spreading centers which penetrate existing lithosphere along strike as propagating rifts have modified our views of how plate boundaries reorient themselves and how v shaped magnetic and topographic trends are formed we have also found that transform faults may modify spreading center processes for some distance away from their intersections with mor s and that rigid plate transform faults may fail to develop at fast spreading centers under certain conditions resulting in en echelon overlapping of spreading centers this review will focus on these developments and several others only example references to these topics will be given in the text and many important topics will not be discussed directly e g petrology and geochemistry mantle convection research on lithosphere older than 1 m y however i have tried to compile and have included an extensive bibliography for u s research on a broader range of mor topics for mid 1978 to mid 1982 which is the main purpose of this report pre 1978 references are occasionally included where appropriate abstracts are not included

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

the ramesses experiment iii controlled source electromagnetic sounding of the reykjanes ridge at 57 45 n summary a controlled source electromagnetic sounding survey centred on an axial volcanic ridge avr segment of the reykjanes ridge at 57 45aen was performed as part of the ramesses experiment low frequency 0 35 11 hz electromagnetic signals were transmitted through the crust to an array of horizontal electric field recorders at the seafloor to ranges of 15 km from the source which was a 100 m long horizontal electric dipole towed at heights of 50 80 m from the seafloor coincident seismic and magnetotelluric studies were conducted during the rest of the ramesses experiment data were interpreted using a combination of 1 d forward modelling and inversion and iterative forward modelling in two dimensions on the axis of the avr the resistivity at the seafloor is 1 v m there is a steep resistivity gradient in the upper few hundred metres of the crust with the resistivity reaching approximately 10 v ma t a depth of 500 m in order to explain the low resistivities the upper layer of the crust must be heavily fractured and saturated with sea water the resistivity increases with distance from the axis as the porosity decreases with increasing crustal age the most intriguing feature in the data is the large diverence in amplitude between fields transmitted along and across the avr axis a significant zone of low resistivity material is required at approximately 2 km depth beneath the ridge crest in order to explain this diverence it is coincident with the low velocity zone required by the seismic data and has a total electrical conductance in excellent agreement with the results of the magnetotelluric study the low resistivity zone can be explained by the presence of a body of partially molten basalt in the crust taken together these results provide the first clear evidence for a crustal magma chamber at a slow spreading mid ocean ridge the data constrain the melt fraction within the body to be at least 20 per cent with a melt volume suycient to feed crustal accretion at this segment of the ridge for of the order of 20 000 years since this body would freeze in the order of 1500 years this finding lends support to the hypothesis that at slow spreading rates crustal accretion is a cyclic process accompanying periodic influxes of melt from the mantle to a crustal melt reservoir

electrical resistivity structure of the valu fa ridge lau basin from marine controlled source electromagnetic sounding summary in december 1995 we carried out a comprehensive controlled source electromagnetic survey of the valu fa ridge at 22u25k si n the lau basin the valu fa ridge is ab ack arc spreading centre of intermediate spreading rate and is a site of extensive hydrothermal activity seismic studies have imaged a melt lens at an average depth of 3 2 km below the seafloor surrounded by a zone of lowered seismic velocity interpreted as a region of partial melt in the crust the electromagnetic experiment was part of a multidisciplinary study which included wide angle and reflection seismics bathymetry and potential field measurements electromagnetic signals at frequencies between 0 25 and 40 hz were transmitted from a horizontal electric dipole towed close to the seafloor and were recorded by an array of 11 sea bottom receivers at ranges of up to 20 km from the source over 80 hr of data consisting of the magnitude of the horizontal electric field at the seafloor were collected these data have extremely low scatter compared to similar data from previous surveys the data were interpreted using a combination of 1 and 2 d forward modelling and inversion the vertical resistivity gradient in the upper crust at the valu fa ridge is abnormally low with resistivities of less than 10 v m observed throughout layer 2 of the crust to a depth of 3 km this is significantly more conductive at depth than the axis of the slow spreading reykjanes ridge at 57u45kn and the fastspreading east pacific rise at 13un where similar data sets have been collected in the past although the structure of layer 2 is well constrained by the electromagnetic data its extremely low resistivity causes rapid attenuation of electromagnetic signals diffusing through it and hence the data are not sensitive to the structure in layer 3 in particular the structure of the melt lens or surrounding low velocity zone the seismic velocity structure of the valu fa ridge determined from the coincident wide angle seismic study is similar to that observed at other mid ocean ridges with a steep seismic velocity gradient through layer 2 although overall velocities are slightly lower the seismic velocity anomaly calculated relative to an average off axis structure is also small this suggests that the very low resistivities observed at the axis are not caused by an upper crust of abnormally high porosity however hot and or saline fluids permeating the crust can explain the low resistivities without affecting the seismic velocity since the conductive region extends unbroken from 3 km depth to the seafloor it is probable that these fluids circulate to or close to the magma chamber itself

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

splitting marine controlled source electromagnetic responses into sea and subseafloor contributions grounding the air wave abstractwe have found how the effects of the air wave in marine controlled source electromagnetic csem methods gradually vanish in the sea for shallow waters and how at the same time they gradually grow below the seafloor in an effort to comprehend existing detectability definitions the transition from sea to land is smooth because the sea becomes a thin conductive layer when the water depth is smaller than the skin depth in the sea we consider the problem of detecting resistive layers at depth associated with hydrocarbon reservoirs particularly in shallow water explorations and specifically on how the air wave affects detection our analysis is based on an integral representation of the electric field in terms of its sensitivity to changes in the electrical conductivity of a 1d profile two dimensional images of the integrands are obtained by plotting the integrand as a function of depth for different offsets results include the expected growth of the inhibiting effect of the sea as the water d

transient electromagnetic responses in seafloor with triaxial anisotropy summary electrical anisotropy of young oceanic crust at mid ocean ridges is detectable by observation of the rate and geometry of the diffusion of electromagnetic fields the anisotropy in electrical properties arises from the presence of conductive seawater in an interconnected network of mostly ridge parallel cracks in this paper we first justify the choice of a triaxial model to represent young oceanic crust with three distinct electrical conductivities in the vertical strike and spreading directions we then present an algorithm to calculate the transient electromagnetic responses generated by an electric dipole source over such a triaxially anisotropic seafloor we show that if the transient passages are measured with three distinct electric dipole dipole configurations it is possible to discern all three unknown conductivities independently of each other

use of marine controlled source electromagnetic sounding for sub basalt exploration the problem of imaging sedimentary structure obscured by high velocity layers such as carbonate basalt or salt using conventional seismic techniques is well known when this problem is encountered in offshore areas marine electromagnetic data can provide valuable alternative and complementary constraints on the structure we concentrate on the use of marine controlled source electromagnetic csem sounding in the frequency domain the feasibility of mapping sub basalt sedimentary structure using this technique is explored by means of modelling studies which mimic a type of survey which could readily be employed in practice as a starting point the problem is addressed in terms of 1d resistivity structure we show that sub basalt sediments can be detected and their depth of burial quantified to within 200 m in the examples shown the presence of small scale inhomogeneities in the basalt which cause much of the scattering in seismic data is shown to have no appreciable effect on the ability of the csem data to detect the sediments the modelling is then extended to 2 5 dimensions again the presence of sub basalt sediments can be readily detected and their properties and geometry quantified especially if the electromagnetic data are combined with constraints from complementary geophysical techniques

heat flow on the western flank of the east pacific rise at 21 n we report 48 new heat flow measurements on the western flank of the east pacific rise at 21 n the stations were taken in 1 to 30 m of sediment on 0 4 to 1 4 m y old crust the low average measured value of 173 mw 2 including 12 tilted minimum values is about 1 3 the mean heat flow predicted for crust of this age range by cooling plate theory this may be partly due to a bias of measurement locations in sediment ponds nevertheless 4 km running averages of measured heat flows projected normally to the axis display an oscillatory trend suggestive of cellular hydrothermal circulation in the basement we sampled no more than one cycle of the apparent heat flow modulation which is of 15 km wavelength and 200 mw m2 amplitude this variation appears to be uncorrelated with the topographic variation of 10 km wavelength assuming that the porosity sensitive electromagnetic results of young and cox 1981 reflect a permeability distribution that restricts the depth of circulation to 1 4 km average temperatures of circulation may be 50 60 c deeper higher temperature circulation is allowable if a more uniform permeability depth distribution holds

ordered hierarchy versus scale invariance in sequence stratigraphy sequence stratigraphy has been applied in a wide range of scales of time and space from decimeter thick layers formed within hours to kilometer thick basin fills formed during hundreds of millions of years the traditional approach to practice sequence stratigraphy in this wide range of scales is to subdivide the sediment piles into an ordered hierarchy of sequence cycles of different duration and different architecture an alternative are scale invariant models with fractal characteristics published data confirm two predictions of the ordered hierarchy model sequences of very short duration 200 106 years are symmetrical transgressive regressive cycles however the sequence record in the range of 1 104 200 106 years the principal domain of sequence stratigraphy shows a rather irregular succession of sequences with variable symmetry and bounded by flooding surfaces or exposure surfaces for these time scales scale invariant models are a good first approximation particularly because the evidence for scale invariance and randomness in the stratigraphic record is strong frequency spectra of sea level change as well as rates of sedimentation and rates of accommodation change plotted against length of observation span show basic trends indistinguishable from random walk these trends combined with scale invariant sequence models may be the most efficient tools for across the board predictions on sequences and for locating islands of order in the sequence record

sea level variations global sedimentation rates and the hypsographic curve sea level changes during the neozoic have been estimated by two different methods the first involves measuring the amount of present day land area which was flooded during the past and using the present day hypsographic curve to estimate the amount of sea level rise necessary to produce this flooding the second involves the estimation of the changing volume of mid oceanic ridges through time and estimating sea level changes after having allowed for isostatic adjustment a difference in sea level of 170 m is obtained from the two methods for the cretaceous 80 100 m y b p this is equivalent to a difference in continental flooding of 24 mm 2 using the present day hypsographic curve we attempt to explain this difference firstly by allowing for the fact that the present day ocean basins have more sediment in them than did the cretaceous ocean basins this produces a sea level change in the opposite direction to that produced by the reduction in mid ocean ridge volume since the cretaceous secondly we suggest another large factor in producing the difference is that the present day hypsographic curve is not the correct one to use when studying sea level stands in the cretaceous present day average continental heights are closely related to continental areas accepting this principle if continents are joined together in the past their average height must be greater and so their hypsographic curve must be steeper a given sea level rise would produce less continental flooding during times of continental aggregation than it would today

marine controlled source electromagnetic sounding 2 the pegasus experiment the marine controlled source electromagnetic sounding method developed over the past 15 years at scripps institution of oceanography employs a towed seafloor electric dipole transmitter of moment 4 104 am and multiple free vehicle seafloor electric field recorders a survey of 40 ma normal oceanic lithosphere in the northeast pacific using frequencies of 0 25 to 24 hz and synchronous stacking of 0 25 to 12 hour duration detected signals at transmitter receiver ranges between 5 and 95 km one dimensional electrical conductivity structure is recovered from the data using the occam process of nonlinear regularized inversion repeated inversion of a model terminated with an essentially infinite conductor or resistor demonstrates that the maximum depth of inference for this experiment is about 30 km well into the upper mantle with bounds placed on conductivity to depths of 60 km structure shallower than about 1 km is comparable to that obtained by a similar experiment on the east pacific rise and by borehole logging with a sharp increase in resistivity at depths of 600 800 m although strictly our experiment is sensitive only to integrated square root of conductivity or total attenuation in the surface layers the lower crust and upper mantle has a resistivity between 2 and 7 104 m and a transverse resistance of at least 109 m2 suggesting at most 0 3 volume fraction of free water in the lower crust and some form of conductivity enhancement over mineral conductivity in the uppermost mantle although resolution is weak below 30 km our data are compatible with a dry olivine model of mantle conductivity temperature

constraints on ocean volume change since the archean water may be added to earth s surface by the bombardment of large numbers of small comets increasing free water on earth s surface deepening the oceans continental freeboard is tied to the ocean surface because of the balance between erosion mountain building addition of water will also affect continental thickness area i calculate the effect of increasing ocean volume on the balance between continental oceanic area one model also includes crustal volume growth significant oceanic depths are known to exist during the archean because sulphide deposits requiring deep oceans occur in greenstone belts these observations prohibit the constant addition of large quantities of water comparable to the present day ocean volume throughout earth history but smaller quantities could be added

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

the tectonic factor in sea level change a countervailing view it is now demonstrable that many unconformity bounded assemblages of strata the sequences and groupings of sequences of seismic stratigraphy can be correlated subglobally the facies tracts that are identifiable within sequences clearly represent depositional environments that bear a systematic relationship to the strandline and to water depths and thus to sea levels lacking a more suitable explanation for the intercontinental correlation of sequences it has become widely accepted that eustatic changes of sea level are responsible for the interruptions in sedimentation that separate sequences and for the shifts of coastal onlap and of facies tracts implicit in sequence analysis students of cratonic basins are convinced that continental interiors are subject to a tectonic evolution of positive and negative vertical movements and that these control relative sea level and the distribution of sedimentary environments proof of this control is buried hi the sedimentary fills of cratonic basins but it has been difficult to produce such proof in a form digestible by neo neptunist doubters digitization of isopach maps of successive stratal units produces thickness data amenable to numerical study for example the logarithms of thicknesses of a stratal unit commonly exhibit a uniform areal distribution the hallmark of a bivariate normal distribution this revelation makes available a number of gaussian statistical measures as descriptors of time variable basin geometry extraction and analysis of geometric parameters lead to the realization that basin evolution necessarily including relative sea levels and the transgression and progradation of strandlines and sedimentary environments has a strong tectonic component that gives evidence of synchrony of widely separated basins on separate cratons secular variation in radial heat flow leading to episodic inflation and deflation of continents is suggested as a possible globally effective mechanism

phanerozoic marine inundation of continents driven by dynamic topography above subducting slabs a spherical model of mantle flow constrained by the locations of trenches can be used to predict the dynamic topography of the earth s surface and hence the marine inundation of continents for past periods of high sea level the predicted geographical pattern of flooding correlates well with the geological record the high spatial correlation may result from increased plate velocities at these times leading to increased rates of subduction subsidence and inundation at convergent margins

induction of currents in long submarine cables by natural phenomena the influence of induced currents on long submarine cables has been observed for nearly a century the currents originate from time variations in the magnetic field of external origin magnetospheric atmospheric tides and earth and ocean tides steady earth currents of internal origin have also been proposed as a possible source the induced currents will depend upon the length depth and geomagnetic location of the cables present understanding of the phenomena is reviewed also discussed are potential scientific problems which might be addressed with data from such long cables and possible limitations on the information obtained such problems include studies of 1 the deep conductivity structure of the earth 2 telluric currents flowing in mid oceanic ridges 3 the spatial extent of external source fields and 4 a possible role of electromagnetic eddy currents in fluctuations of the earth s spin rate

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

eustasy and epeirogeny of continents on time scales between about 1 and 100 m y eustatic changes are believed to be the cause of the extensive flooding of the continents by shallow marine seas during the cretaceous and much of the paleozoic the possible causes of eustatic change which can be easily quantified over the past 100 m y have been estimated in order to arrive at a value for the freeboard decrease of the continents these causes in order of decreasing importance are ridge crest volume thermal uplift associated with excessive volcanism in the pacific during the cretaceous formation of continental ice sheets the collision of india with asia and the cooling of the oceans several effects act in the opposite direction and they can be grouped together because they are all due to there being larger quantities of sediments in the ocean basins today than there were 100 m y ago another effect causing a freeboard increase is the thinning of continental masses during continental breakup and the formation of new ocean basins the total change in sea level after having allowed for isostatic response is 202 m during the last 80 m y flooding of the continents can also be used to determine sea level changes with help from modern hypsographic curves when this is done it is found that the freeboard decrease is considerably less than that calculated from volume estimates although the inaccuracy of the various data would allow these estimates to be not significantly different it is likely that the estimate of freeboard decrease using flooded areas and modern hypsographic data is an underestimate due to the probability that the hypsographic curve has become less steep with time individual estimates of flooding from continents require considerably different amounts of freeboard change the common signal is usually assumed to be the eustatic signal whereas when this common signal is removed from the freeboard estimate for an individual continent the remaining signal is the epeirogenic signal epeirogenic movements can be as large as 150 m and can occur on time scales of a few tens of millions of years other examples of epeirogenic variations are also discussed we have developed a rough power spectrum for epeirogenic movements of the continents this appears to have 1 f variation we suggest that the spectrum could be produced by temperature changes at the bottom of the lithosphere

electric dipole fields over an anisotropic seafloor theory and application to the structure of 40 ma pacific ocean lithosphere summary seismic anisotropy has been detected in the oceanic crust and upper mantle and likewise it is geologically reasonable to expect that a certain amount of lateral anisotropy exists in sea oor electrical properties anisotropy in earth properties can often lead to surprising eiects on geophysical responses that are not anticipated from simple isotropic theories here we investigate the eiects of lateral anisotropy on the frequency domain controlled source electromagnetic csem response of a uniaxially conducting non magnetic sea oor excited by a horizontal electric dipole whose moment is oriented obliquely with respect to the electrical strike direction a paradox of anisotropy is observed in which the sea oor electric eld strength is enhanced in the most conductive direction of the sea oor this enhancement is opposite to what one would expect based on naive isotropic theory we also show that it is possible in certain circumstances to extract the along strike electrical conductivity from marine controlled source electromagnetic data using only isotropic modelling the extraction of across strike conductivity however requires full anisotropic modelling the physical insight into electromagnetic induction in uniaxial media that is presented here should greatly assist the geological interpretation of marine csem experimental data applying our algorithm to the pegasus data set csem data collected over 40 ma paci c ocean lithosphere produces a model with conductivity in the fossil spreading direction that is seven times greater than the conductivity perpendicular to spreading strain aligned mineralogical fabric as predicted by tectonic modelling would explain our result with enhanced conductivities caused by hydrogen conduction along the olivine a axis or connected accumulations of trace conductors such as graphite or magnetite

using csem to monitor production from a complex 3d gas reservoir a synthetic case study the marine controlled source electromagnetic csem method is most commonly used in targeted exploration or appraisal studies as a tool for detecting and delineating hydrocarbon reservoirs see for example constable and srnka 2007 whereas seismic surveys can detect the structures that may contain hydrocarbons with great accuracy distinguishing hydrocarbon fluids from water within these structures is more problematic originally developed in the late 1970s young and cox 1981 the csem method uses a high powered horizontal electric dipole hed to transmit a low frequency electromagnetic signal through the sea floor to an array of multicomponent electromagnetic receivers bulk electrical resistivity of the sea floor can be determined at scales of a few tens of meters to depths of several kilometers in many cases this resistivity is driven by the fluid content and saturation of subsurface formations

a numerical comparison of time and frequency domain marine electromagnetic methods for hydrocarbon exploration in shallow water in shallow water the frequency domain controlled source electromagnetic method is subject to airwave saturation that strongly limits the sensitivity to resistive hydrocarbon targets at depth it has been suggested that time domain csem may offer an improved sensitivity and resolution of these deep targets in the presence of the airwave in order to examine and test these claims this work presents a side by side investigation of both methods with a main focus on practical considerations and how these effect the resolution of a hydrocarbon reservoir synthetic noisy data for both time domain and frequency domain methods are simulated using a realistic frequency dependent noise model and frequency dependent scaling for representative source waveforms the synthetic data studied here include the frequency domain response from a compact broadband waveform the time domain step response from a low frequency square wave and the time domain impulse response obtained from pseudo random binary sequences these data are used in a systematic resolution study of each method as a function of water depth relative noise and stacking length the results indicate that the broadband frequency domain data have the best resolution for a given stacking time whereas the time domain data require prohibitively longer stacking times to achieve similar resolution

calibration of the c 14 timescale over the past 30 000 years using mass spectrometric u th ages from barbados corals uranium thorium ages obtained by mass spectrometry from corals raised off the island of barbados confirm the high precision of this technique over at least the past 30 000 years comparison of the u th ages with 14c ages obtained on the holocene samples shows that the u th ages are accurate because they accord with the dendrochronological calibration before 9 000 yr bp the 14c ages are systematically younger than the u th ages with a maximum difference of 3 500 yr at 20 000 yrbp the u th technique thus provides a way of calibrating the radiocarbon timescale beyond the range of dendrochronological calibration

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

on the electrical nature of the axial melt zone at 13 n on the east pacific rise the first controlled source electromagnetic experiment directly on a ridge with the potential to identify the presence of an axial melt body beneath a fast spreading center was conducted at 13on on the east pacific rise epr in 1989 transmission for 36 hours was achieved by a deep towed horizontal electric dipole source of moment 6000 am operating at frequencies between 1 4 and 8 hz signals from the source were recorded by seven seafloor electric field receivers positioned both along the ridge crest and 5 km to the east on 100 000 year old crust data above ambient noise levels were obtained at ranges of up to 10 kin the results of modeling observed electric field amplitudes reveal that resistivities in the uppermost crust are very low 1 tim indicating a heavily fractured high porosity surficial layer below this topmost layer the upper 2 km of crust is found to be moderately resistive 100 tim we find no evidence for a large conductive axial melt body with dimensions on the order of kilometers in the middle or upper crust if a partial melt body is present which is continuous along strike and which comprises a connected and therefore conductive melt texture it must be of very limited volumetric extent this picture is consistent with recently proposed models of a thin sill like melt lens with across strike dimensions of no more than 1 km and probably with smaller vertical extent the larger region below the sill characterized by low seismic velocities must contain at best a very small melt fraction distributed in isolated pockets providing further evidence that the epr at 13on is currently in a state of relative magmatic quiescence

the marine controlled source electromagnetic method in shallow water we analyzed marine controlled source electromagnetic csem acquisition in shallow water by analytic and numerical calculations the problem at hand was the fact that the amplitude of the airwave due to a horizontal electric dipole becomes larger as the water depth is reduced the amplitude of the scattered field from a buried thin resistor would be difficult to extract from the increased total field if the scattered field amplitude were independent of water depth however we identified and explained a compensating effect where the amplitude of the scattered field increases with reduced water depth this amplification effect makes marine csem surveys in shallow water feasible we derived an approximate expression for the total field for a simple but realistic conductivity model this expression explains why the amplitudes of the term that describes the background field contribution and the term that gives the scattered field from the thin resistor increase when the water depth is reduced our results and sensitivity analysis indicated that the sensitivity of marine csem data acquired in a water depth of 40 m is comparable to the sensitivity of marine csem data acquired in a water depth of 300 m

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

a magnetotelluric sounding across vancouver island detects the subducting juan de fuca plate magnetotelluric1 mt and geomagnetic2 depth sounding measurements have been made at 18 sites 7 with remote reference3 across vancouver island fig 1 beneath which the juan de fuca plate is underthrusting4 5 vancouver island is part of the old accretionary wrangellia terrane6 and is located in the central portion of a 350 km forearc region that extends from the sediment filled trench7 to the garibaldi volcanic belt8 gravity9 and seismic refraction studies10 suggest that the descending plate has a shallow northeasterly dip of 8 16 with perhaps higher dips beneath northeastern vancouver island a high resolution seismic reflection survey11 along lines 1 and 3 fig 1 located a zone of strong acoustic reflections the e horizon11 near the top of the subducting juan de fuca plate at depths of 23 34 km our model of the broad band mt data defines a sloping highly conducting zone also near the top of the subducting plate at the same depths as the seismic e horizon the high electrical conductivity in this zone the e conductor is the result of saline fluid within the pore spaces of sedimentary and mafic materials of the upper oceanic crust that have been subducted to those depths beneath vancouver island these are the first data which clearly define a dipping conductive layer associated with the boundary region between converging plates they have significant implications for thrust earthquakes and metamorphic reactions that occur in subduction zones

episodic euxinia in the changhsingian late permian of south china evidence from framboidal pyrite and geochemical data abstract a multiproxy study of a new upper permian lower triassic section xiaojiaba in sichuan province china documents large changes in marine productivity redox conditions and detrital input prior to the latest permian mass extinction marine productivity as proxied by total organic carbon content toc biogenic sio 2 and excess barium displays a long term decline through most of the changhsingian stage late late permian culminating in very low values around the permian triassic boundary concurrently redox proxies including pyrite framboid 34 s py mo auth and u auth and c org p document a shift from suboxic to dysoxic oxic conditions that was interrupted by several episodes of benthic euxinia and detrital siliciclastic proxies al hf nb and rees suggest an increased flux of weathered material from land areas the long term changes in productivity redox conditions and terrigenous detrital fluxes were probably caused by a regional sea level fall across the south china craton on the other hand the brief euxinic episodes occurring during the late permian had oceanographic causes probably related to the transient upward expansion of the chemocline at the top of the oceanic oxygen minimum zone these euxinic episodes may have been harbingers of the more widespread anoxia that developed concurrently with the latest permian mass extinction and that may have played a major role in triggering the largest biotic crisis of the phanerozoic

vertical vertical controlled source electromagnetic instrumentation and acquisition abstract vertical vertical controlled source electromagnetic is an alternative to other techniques for providing three dimensional resistivity images of the subsurface it utilizes a large and powerful vertical dipole transmitter and arrays of e field receivers with vertical and horizontal dipole sensors the necessary instrumentation and acquisition procedures which differ strongly from other controlled source electromagnetic methods are described in the paper

stochastic and deterministic controls on stratigraphic completeness and fidelity compilations of sedimentation rates determined across a large range of time spans and different sedimentary environments attest to the discontinuous nature of sedimentation and support the reasoning that hiatuses pervade sedimentary successions at multiple spatial and temporal scales the relationship between sedimentation rate and time span of observation also emphasises how stratigraphic completeness is a time span dependent concept the scaling of stratigraphic completeness with time span alludes to the likely stochastic and fractal nature of the stratigraphic record it also implies that the relationship between stratal thickness and time is inherently non linear and this has obvious implications for the way we should expect temporal processes to be recorded stratigraphically order within the stratigraphic record may arise from deterministic processes in stratigraphy most notably through astronomically influenced sedimentation intuitively a strongly ordered stratigraphic record due to the presence of cyclicity in sedimentation increases stratigraphic completeness at time spans equal to and longer than the time span of the cyclicity and minimises the non linearity between stratal thickness and time here geological data and simple modelling are used to support this supposition but these results also highlight how stochastic variability inherent in depositional processes may sensitise certain successions to missed cycles and incompleteness even in the presence of strong cyclicity this stochastic variability has the potential to mask evidence of astronomical forcing of sedimentation it is also shown how hiatuses affect the ability of a succession to faithfully record temporal processes and this is true regardless of whether hiatuses are deterministically controlled or random or indeed whether a succession is considered complete or not consequently stratigraphic completeness and stratigraphic fidelity are not synonymous terms

time variations of the earth s magnetic field from daily to secular variations in time of the earth s natural magnetic field have a frightfully wide spectrum ranging over more than 20 orders of magnitude they extend from well over 103 hz where they merge into the radioelectric noise spectrum to probably more than 100 m y in rough summary the higher frequencies stem from external sources in the ionosphere and above into the magnetosphere whereas the longer periods arise from internal sources primarily fluid motions in the core at depths in excess of 2900 km methods for separating external and internal signals have been available since the time of gauss 1 838 but the accuracy the space distribution and the time duration of available data are often not sufficient to unambiguously solve the problem for all types of variations although periods shorter than 1 yr should clearly be assigned to external sources the extent to which the spectra of the external and internal parts overlap is the subject of recent debate and progress currie 1966 1 968 proposed that there was little overlap and a cutoff of 3 7 yr a long debated l l yr contribution due to the solar cycle was clearly demonstrated by currie 1973a courtillot le mouei 1976a b and alldredge 1976 and the cutoff period was thus rejected to at least 20 yr the situation seemed to be confused again when courtillot et al 1978a provided evidence for sharp impulses of 1 yr duration i n the internal so called secular variation the debate that followed over the reality of brief internal events is one of the topics addressed in this review we first provide a short historical account of successive discoveries

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

glacial cycles and astronomical forcing narrow spectral features in ocean sediment records offer strong evidence that the cycles of glaciation were driven by astronomical forces two million years ago the cycles match the 41 000 year period of earth s obliquity this supports the croll milankovitch theory which attributes the cycles to variations in insolation but for the past million years the spectrum is dominated by a single 100 000 year feature and is a poor match to the predictions of insolation models the spectrum can be accounted for by a theory that derives the cycles of glaciation from variations in the inclination of earth s orbital plane

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

sea level higher than present 3500 years ago on the northern main hawaiian islands new data from an emerged coastal bench and associated fossil beach on kapapa island oahu hawaii preserve a detailed history of middle to late holocene sea level these include 29 new calibrated radiocarbon ages and elevations indicating mean sea level reached a maximum position of 2 00 0 35 m ca 3500 yr b p these results correlate with additional evidence from hawaii and other pacific islands and provide constraints on oahu s long term uplift rate 0 03 0 07 mm yr previously based solely on pleistocene age shorelines our sea level reconstruction is consistent with geophysical model predictions of earth s geoid response to the last deglaciation and with observations of increased antarctic ice volume during the late holocene

synthetic aperture controlled source electromagnetics controlled source electromagnetics csem has been used as a de risking tool in the hydrocarbon exploration industry although there have been successful applications of csem this technique is still not widely used in the industry because the limited types of hydrocarbon reservoirs csem can detect in this paper we apply the concept of synthetic aperture to csem data synthetic aperture allows us to design sources with specific radiation patterns for different purposes the ability to detect reservoirs is dramatically increased after forming an appropriate synthetic aperture antenna consequently the types of hydrocarbon reservoirs that csem can detect are significantly extended because synthetic apertures are constructed as a data processing step there is no additional cost for the csem acquisition synthetic aperture has potential for simplifying and reducing the cost of csem acquisition we show a data example that illustrates the increased sensitivity obtained by applying synthetic aperture csem source

a fractal analysis of quaternary cenozoic mesozoic and late pennsylvanian sea level changes sea level changes are related to both climatic variations and tectonic movements the fractal dimensions of several sea level curves were compared to a modern climatic fractal dimension of 1 26 established for annual precipitation records a similar fractal dimension 1 22 based on 18o 16o in deep sea sediments has been suggested to characterize climatic change during the past 2 m y our analysis indicates that sea level changes over the past 150 000 to 250 000 years also exhibit comparable fractal dimensions sea level changes for periods longer than about 30 m y are found to produce fractal dimensions closer to unity and missourian late pennsylvanian sea level changes yield a fractal dimension of 1 41 the fact that these sea level curves all possess fractal dimensions less than 1 5 indicates that sea level changes exhibit nonperiodic long run persistence the different fractal dimensions calculated for the various time periods could be the result of a characteristic overprinting of the sediment record by prevailing processes during deposition for example during the quaternary glacio eustatic sea level changes correlate well with the present climatic signature during the missourian however mechanisms such as plate reorganization may have dominated resulting in a significantly different fractal dimension

intra annual variation in the stable oxygen and carbon and trace element composition of sclerosponges 1 this paper presents data to support the presence of 1 intra annual signals in the chemical composition 18o and sr ca of the skeletons of sclerosponges from the bahamas and 2 variable rates of skeletal accretion these conclusions are based on data obtained by using a microsampling method for the stable oxygen and carbon isotopes in which material was extracted at a resolution of one sample every 34 m and a laser microprobe which obtained trace element data every 20 m sr mg and pb an age model was established using a combination of changes in the concentration of pb the change in the 13c of the skeleton of the sclerosponges and u th isotopic measurements these methods yield a mean growth rate of 220 m yr but suggest that the growth rate in this particular sclerosponge was not constant the calculated growth rate is within error identical to that determined by u th methods the variable growth rate was confirmed through spectral analysis of the 18o and sr ca data that showed peaks corresponding to the annual cycle in these parameters as well as peaks corresponding to growth rates of approximately 128 212 270 and 400 m yr the presence of these additional frequencies suggests a growth rate between approximately 100 and 300 m yr these conclusions were supported by modeling of oxygen isotopic data measured on a scleractinian coral as well as model isotope data generated on synthetic time series these findings have important implications for the use of sclerosponges as proxies of paleoclimate because they emphasize the need for a precise yearly chronology in order that proxy data can be compared with climatic variables

three dimensional spherical models of layered and whole mantle convection we present numerical calculations of three dimensional spherical shell thermal convection for constant viscosity and stratified viscosity models of whole layer and two layer mantle convection these four examples are intended to provide theoretical guidance for determining the style of convection that is occurring in earth s mantle an impermeable interface between the upper and lower converting shells in the two layer solutions is placed at a depth of 670 km to coincide with the mantle seismic discontinuity that divides the upper and lower mantle the interface results in an internal thermal boundary layer that raises the mean temperature in the lower shell by about 1400 k compared to the whole layer solutions the patterns of convection in the upper part of the whole layer solutions are dominated by narrow arcuate sheetlike downflows in a background of weak upflow in contrast the upper shells of the two layer solutions have complicated networks of convective rolls with the upflows and downflows having very similar structure the structure of convection in the lower shells is similar to that in the lower part of the whole layer solutions an increase decrease in the viscosity in the lower upper shell decreases increases the convective velocity in the lower upper shell and increases decreases the horizontal scale in the lower upper shell the upper and lower shells are viscously coupled at the 670 km interface when viscosity is the same in the two shells however when viscosity in the lower shell is 30 times greater than that in the upper shell the coupling is partially viscous and partially thermal based on the horizontal structure of subduction zones on earth s surface and on tomographic images of temperature variations in earth s mantle we conclude that the style of convection in earth s mantle is more like that of the whole mantle models

controlled source electromagnetic three dimensional grid modelling based on a complex resistivity structure of the seafloor effects of acquisition parameters and geometry of multi layered resistors a comprehensive controlled source electromagnetic csem modelling study based on complex resistivity structures in a deep marine geological setting is conducted the study demonstrates the effects of acquisition parameters and multi layered resistors on csem responses three dimensional 3d finite difference time domain fdtd grid modelling is used for csem sensitivity analysis interpolation of normalized csem responses provides attributes representing relative sensitivity of the modelled structures modelling results show that fine grid 1 1 km receiver spacing provides good correlations between csem responses and the modelled structures irrespective of source orientation the resolution of csem attributes decreases for receiver spacing 2 2 km when using only in line data broadside data in the grid geometry increase data density by 100 approximately 200 by filling in in line responses and improve the resolution of csem attributes optimized source orientation i e oblique to the strike of an elongated resistor improves the structural definition of the csem anomalies for coarse grid geometries receiver spacing 3 3 km the study also shows that a multi resistor anomaly is not simply the summation but a cumulative response with mutual interference between constituent resistors the combined response of constituent resistors is approximately 50 higher than the cumulative response of the multi resistor for 0 5 hz at 4000 m offset a gradual inverse variation of offset and frequency allows differentiation of csem anomalies for multi layered resistors similar frequency offset variations for laterally persistent high resistivity facies show visual continuity with varying geometric expressions 3d grid modelling is an effective and adequate tool for csem survey design and sensitivity analysis

csem technology as a value driver for hydrocarbon exploration abstract the use of the controlled source electromagnetic csem method as a risk reduction tool in marine hydrocarbon exploration is gaining increased acceptance in the oil industry this is related to the ability to map resistivity contrasts in the sub surface and thus aid the detection of hydrocarbons which are typically more resistive than surrounding rocks whereas acoustic seismic energy allows for mapping sub surface structures that may contain hydrocarbons electromagnetic em energy can often say something about the fluids contained within the structures numerous successful csem case stories have been published over the past several years however there are also quite a few stories about failure cases although not well documented in the literature such failure cases may reflect the lack of understanding of the csem technology and how csem data can act as a value driver in order to understand this it is necessary to handle uncertainties and decisions associated with the technology these include geological uncertainties noise models survey designs forward modelling parameters inversion migration parameters and pre processing of real data a proper handling of these uncertainties and decisions will aid in defining and constraining the chance of geologic success and geologic success case net present value for prospects prior to drilling wells as such the csem technology has a significant potential to increase exploration efficiency if applied correctly

late pleistocene holocene sediments on the spanish continental shelves model for very high resolution sequence stratigraphy abstract sequence stratigraphy analysis of high resolution seismic profiles geopulse uniboom and 3 5 khz of late pleistocene holocene sediments has been carried out on five sectors of the spanish continental margin four types of depositional settings are distinguished in these sectors 1 low subsident ramps alboran margin cadiz gulf 2 high subsident 2m kyr ramps alicante valencia 3 ria type morphology on the atlantic passive margin ria de muros and 4 fault scarp morphology systems with subsidence balearic margin a type 1 sequence is interpreted in all these sectors being composed of lowstand systems tract transgressive systems tract and highstand systems tract this conforms to the basic concepts of sequence stratigraphy and each systems tract correlates with a particular part of the last eustatic hemicycle characteristic shelf features such as terraces terraces with beach deposits and progradational sediment wedges evidence a complex stacking of lesser sub sequences in all the systems tracts which must be related to very short period sea level stillstands and fall we propose a very high resolution sequence stratigraphy model in which the last sea level hemicycle is punctuated by p cycles 4500 years which give rise to the neo glacial events h cycles 2200 950 years and c cycles 500 50 years these cycles interact with each other thus establishing the placing of the high and low sea levels this attention to detail 1 explains sedimentary evolution on both the shelf and upper slope during late pleistocene holocene time 2 illustrates some departures from the classical sequence stratigraphy model and 3 also demonstrates that the late pleistocene holocene eustatic curve is not one simple transgression but is modulated by three differing period cycle groups below the milankovitch band our model is delimited by fluctuating sea level during pleistocene holocene times such features should be identifiable on any continental margin however localized features occur due to subsidence and continental shelf morphology which determ ine the location and depth of sedimentary bodies generated in each eustatic cycle

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