titanium geophysics the application of induced polarization to sea floor mineral exploration titanium is abundant in the earth s crust but it can be economically extracted from only a limited group of minerals principally rutile tio2 and ilmenite fetio3 both found mainly in fossil beach complex placer deposits both minerals have only a weak magnetic susceptibility insufficient to permit correlation between magnetic surveys and known titanium rich deposits however ilmenite shows an unusually strong induced polarization ip response whereas the ip response of rutile is relatively weak ip spectral signatures for ilmenite acquired in laboratory and field settings are also distinctly different from those of other polarizing materials for instance pyrite a nonfloating towed streamer ip system was designed and deployed in surveys off the coasts of virginia and georgia when the cable lies on the sea floor calculations indicate that only about 8 percent of the injected current actually finds its way into the underlying sediment partly because of this high transmitted current to injecte

mapping 3d salt using the 2d marine magnetotelluric method case study from gemini prospect gulf of mexico the dominant salt body at gemini prospect gulf of mexico has been analyzed by seismic methods revealing a complex 3d salt volume at depths 1 to 5 km beneath the mud line because of the high contrast in electrical conductivity between the salt and surrounding sediments gemini is an attractive target for electromagnetic interrogation using a broadband magnetotelluric mt sensor package developed at the scripps institution of oceanography data in the period band of 1 to 3000 s were collected at 42 sites in a series of profiles over gemini one of which was directly over a linear ridgelike salt feature striking roughly northwest southeast and another orthogonal to it these two profiles reveal that the strongest mt response arises when the electric field is oriented northeast southwest we test the suitability of 2d inversion of these data for recovering the true salt structure by examining inversions of both actual data and synthetic 3d mt responses derived from the seismically inferred salt volume occam inversions of the northeast southwest component result in resistivity images that generally agree with the seismic data whereas inversions of the complementary component yield significantly poorer fidelity disagreement is greatest 1 2 km along the salt sides and base depth errors for top of salt are less than 500 m although thin deep salt 1k m thick at 5 km depth is not well resolved the inversions reveal a resistive basement and a shallow subseabed environment rich in electrical heterogeneity that is weakly if at all suggested by the seismic data a notable exception is a correlation between a previously uninterpreted seismic reflector and the base of a shallow resistivity anomaly whose presence is consistent with gas accumulation near the hydrate stability zone

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

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

first results of the moses experiment sea sediment conductivity and thickness determination bute inlet british columbia by magnetometric offshore electrical sounding a static electrical method has been developed to determine the electrical resistivity of crustal rock beneath the sea the transmitter is a vertical long wire bipole extending from the sea surface to the sea floor a commutated current generated on the ship is fed to two large electrodes one near the sea surface the other at the end of a long insulated wire the return path for the current is through the sea and the subjacent crust the receiver is a self contained remote microprocessor controlled magnetometer which is deployed from the ship to the sea floor and subsequently recovered the data are measurements of the azimuthal component of the magnetic field as a function of transmitter receiver horizontal separation the acronym moses has been coined for the method the choice of the name moses is appropriate because the system geometry is carefully arranged to remove many of the adverse effects of the relatively conductive sea water in particular accurate estimates of sea floor resistivity ar

integrating long offset transient electromagnetics lotem with seismics in an exploration environment1 the applications of electromagnetics have increased in the past two decades because of an improved understanding of the methods improved service availability and the increased focus of exploration in the more complex reservoir characterization issues for electromagnetic methods surface applications for hydrocarbon exploration and production are still a special case while applications in borehole and airborne research and for engineering and environmental objectives are routine in the past electromagnetic techniques in particular deep transient electromagnetics made up a completely different discipline in geophysics although many of the principles are similar to the seismic one with an understanding of the specific problems related to data processing initially and then acquisition the inclusion of principles learned from seismics happened almost naturally initially the data processing was very similar to seismic full waveform processing the hardware was also changed to include multichannel acquisition systems and the field procedures became very similar to seismic surveying as a consequence the integration and synergism of the interpretation process is becoming almost automatic the long offset transient electromagnetic lotem technique will be summarized from the viewpoint of its similarity to seismics the complete concept of the method will also be reviewed an interpretation case history that integrates seismic and lotem from a hydrocarbon area in china clearly demonstrates the limitations and benefits of the method

marine magnetotellurics for petroleum exploration part ii numerical analysis of subsalt resolution in areas where seismic imaging of the base of salt structures is difficult seaborne electromagnetic techniques offer complementary as well as independent structural information numerical models of 2 d and 3 d salt structures demonstrate the capability of the marine magnetotelluric mt technique to map the base of the salt structures with an average depth accuracy of better than 10 the mapping of the base of the salt with marine mt is virtually unaffected by internal variation within the salt three dimensional anticlinal structures with a horizontal aspect ratio greater than two can be interpreted adequately via two dimensional inversions marine mt can distinguish between salt structures which possess deep vertical roots and those which do not one measure of the relative accuracy of mt and seismic methods can be made by considering the vertical and lateral position errors in the locations of interfaces caused by neglecting velocity anisotropy in migration for the shallow part of the section where two way travel times are on the order of 1 s the vertical and lateral position errors in the locations of salt sediment interfaces from 2 d mt inversion is more than twice the expected migration error in reflectors in transversely isotropic sediments such as those in the gulf of mexico deeper in the section where two way times are on the order of 4 s lateral position errors in migration become comparable to those of the mt inverse whereas seismic vertical position errors remain more than a factor of two smaller than mt errors this analysis shows that structural mapping accuracy would be improved using mt and seismic together

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

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

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

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

remote reservoir resistivity mapping a method for surface estimation of reservoir properties wherein location of and average earth resistivities above below and horizontally adjacent to the subsurface geologic formation are first determined using geological and geophysical data in the vicinity of the subsurface geologic formation then dimensions and probing frequency for an electromagnetic source are determined to substantially maximize transmitted vertical and horizontal electric currents at the subsurface geologic formation using the location and the average earth resistivities next the electromagnetic source is activated at or near surface approximately centered above the subsurface geologic formation and a plurality of components of electromagnetic response is measured with a receiver array geometrical and electrical parameter constraints are determined using the geological and geophysical data finally the electromagnetic response is processed using the geometrical and electrical parameter constraints to produce inverted vertical and horizontal resistivity depth images optionally the inverted resistivity depth images may be combined with the geological and geophysical data to estimate the reservoir fluid and shaliness properties

marine magnetotellurics for base of salt mapping gulf of mexico field test at the gemini structure a sea floor magnetotelluric mt survey was conducted over the gemini subsalt petroleum prospect in the gulf of mexico gom to demonstrate that the base of salt can be mapped using marine magnetotelluric mmt methods the high contrast in electrical resistivity between the salt and the surrounding sediments provides an excellent target for mmt the gemini salt body located at 28 46 n 88 36 w is a relatively complex shape buried 2 5 km below the sea floor in 1 km deep water its geometry has been previously determined using 3 d seismic prestack depth migration with well log control in order not to confuse limitations in interpretation technique with limitations in data acquisition numerical forward and inverse modeling guided the survey design to locate a profile that would be amenable to 2 d inversion even though the body was clearly 3 d the seismic imaging of the base of salt along the chosen profile is considered good thus providing a good control for testing the mt method in many other ar

the assessment of marine gas hydrates through electrical remote sounding hydrate without a bsr marine gas hydrates prevalent in offshore sediments in canada and japan are a possible hydrocarbon resource a hazard to drilling and a source of a major greenhouse gas quantitative estimates of hydrate concentrations in deep sea sediment are difficult to obtain by conventional methods we have sought novel techniques specifically designed for assessment such as transient electric dipole dipole electromagnetics the latter method is based on the assumption of a reduced electrical conductivity in hydrate rich zones field trails of new apparatus on the cascadia margin have proven successful excellent data were collected and analysed using a differential phase method to reduce systematic error apparent resistivities collected on three lines demonstrate that the resistivity of the seafloor is remarkably uniform over the whole survey area the average hydrate concentration deduced with the aid of a reference model based on the electrical logs of odp holes 888 and 889 is about 17 26 of pore space 9 13 of sediment volume in the 100 m interval above the bsr the values are consistent with those obtained by other analyses further the presence of hydrate is predicted in a region to the east of 889b where there is no visible bsr

simultaneous core sample measurements of elastic properties and resistivity at reservoir conditions employing a modified triaxial cell a feasibility study adaptations of existing triaxial cells for ultrasonic p and s wave measurements are well documented this paper proposes further modification of such a cell so that also resistivity measurements can be carried out simultaneously at reservoir conditions by employing the top cap and the pedestal of the cell as electrodes axial resistivity measurements are now feasible in order to minimize the polarization effect of this two electrode arrangement careful analyses have been carried out to optimize the choice of electrode coating and measurement frequency band radial resistivity measurements are also included in the system by introducing a strap electrode system in a reservoir under production changes in both saturations temperature if steam injection and stresses can take place therefore the modified triaxial system should be able to measure the integrated effects on the acoustic parameters and electric responses caused by variations in each of these parameters the feasibility of the system to obtain such reliable information is demonstrated employing a small selection of core samples in the future such combined measurements on reservoir core samples can be used to link both seismic and electromagnetic observations to the actual earth model and constrain both modelling and inversion

remote sensing of hydrocarbon layers by seabed logging sbl results from a cruise offshore angola detecting and assessing hydrocarbon reservoirs without the need to drill test wells is of major importance to the petroleum industry seismic methods have traditionally been used in this context but the results can be ambiguous another approach is to use electromagnetic sounding methods that exploit the resistivity differences between a reservoir containing highly resistive hydrocarbons and one saturated with conductive saline fluids modeling presented by eidesmo et al 2002 demonstrates that by using seabed logging sbl a special application of frequency domain controlled source electromagnetic csem sounding the existence or otherwise of hydrocarbon bearing layers can be determined and their lateral extent and boundaries can be quantified such information provides valuable complementary constraints on reservoir geometry and characteristics obtained by seismic surveying

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

buried paleo channels on the new jersey continental margin channel porosity structures from electromagnetic surveying abstract we report on a marine electromagnetic em survey across two portions of the new jersey continental margin that have been previously shown to contain buried paleo channels the em method used provides bulk porosity estimates to depths of around 20 m below the seafloor and is thus able to place porosity constraints on the nature of the channel infill and the contrast in physical properties across the channel boundaries our data show that a key condition for the channels to have an electrical signature is that they incise an underlying regional unconformity r thought to represent a subaerially eroded surface exposed during the late wisconsinan glaciation channels that cut r are seen through increases in apparent porosity another seismically imaged channel sequence which lies within the outer shelf sediment wedge sequence above r does not have an electrical signature indicating that the sediments above and below the channel boundaries have similar physical properties

marine magnetotellurics for petroleum exploration part i a sea floor equipment system induction in electrically conductive seawater attenuates the magnetotelluric mt fields and coupled with a minimum around 1 hz in the natural magnetic field spectrum leads to a dramatic loss of electric and magnetic field power on the sea floor at periods shorter than 1000 s for this reason the marine mt method traditionally has been used only at periods of 103 to 105 s to probe deep mantle structure rarely does a sea floor mt response extend to a 100 s period to be useful for mapping continental shelf structure at depths relevant to petroleum exploration however mt measurements need to be made at periods between 1 and 1000 s this can be accomplished using ac coupled sensors induction coils for the magnetic field and an electric field amplifier developed for marine controlled source applications the electrically quiet sea floor allows the attenuated electric field to be amplified greatly before recording in deep 1 km water motional noise in magnetic field sensors appears not to be a proble

subspace inversion of electromagnetic data application to mid ocean ridge exploration controlled source electromagnetic csem surveys have the ability to provide tomo graphic images of electrical conductivity within the earth the interpretation of such data sets has long been hampered by inadequate modelling and inversion techniques in this paper a subspace inversion technique is described that allows electric dipole dipole data to be inverted for a 2 d electrical conductivity model more efficiently than with existing techniques the subspace technique is validated by comparison with conventional inversion methods and by inverting data collected over the east pacific rise in 1989 a model study indicates that with adequate data a variety of possible mid ocean ridge conductivity models could be distinguished on the basis of a csem survey

waveform inversion for lateral heterogeneities using multimode surface waves summary we propose a waveform inversion scheme that can be used to invert strong structural heterogeneities the method is based on a ray approximation for surface waves strong structural lateral variations are modelled by vertical discontinuities therefore a geologically heterogeneous region is partitioned into a number of lateral homogeneous subregions synthetic seismograms are calculated by modal summation over incident and transmitted modes with this method the complete waveforms of surface waves including mode cross branch coupling multipathing and scattering are considered in the inversion we test the method with several geophysically realistic structural models an example of waveform inversion for real data a wave path across the iberian peninsula and the oceanic structure off the french coast owing to the gibraltar earthquake ms 5 7 is presented the method allows us to obtain the information represented by different set of structures along the same source receiver minor arc through which the waves have propagated the method is applicable to different models of lateral heterogeneity in its present development it is most appropriate for inverting structures around tectonic scenarios such as continental margins grabens and discontinuities between major plates

crustal resistivity structure at 9 50 n on the east pacific rise results of an electromagnetic survey 1 we report the first results of an extensive electromagnetic survey of the east pacific rise epr at 9 50 n which used the magnetometric resistivity mmr technique to measure the electrical resistivity structure of the seafloor in the vicinity of the spreading center ten seafloor magnetometers were deployed in areas of known hydrothermal activity in axial sites devoid of venting and further off axis to a distance of approximately 4 km data collected at off axis sites show higher seafloor resistivities than at axial sites this response is opposite to that expected from porosity controlled resistivity structure with a thicker high porosity extrusive layer 2a off axis as required by seismic data an explanation for the reduced axial resistivities is that the uppermost few hundred meters of crust are much hotter beneath the ridge crest than a few kilometers off axis lowering the pore fluid resistivity

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

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

estimation of seafloor electromagnetic receiver orientation abstractif the orientation of a seafloor electromagnetic em instrument is unknown it has to be estimated afterwards relatively early in the processing here we present a method to estimate the horizontal and vertical orientation of seafloor electromagnetic receivers three component magnetic recordings of the naturally occurring magnetotelluric field are utilized to find the tilt pitch and azimuth angle relative to a given reference estimation of angles is controlled by minimum difference criteria precautionary measures are necessary to eliminate the subsurface response in the recordings of the em field components the proposed method is applied to synthetic and real data demonstrating that even in difficult geologic settings the simple characteristics of the incident natural em field allow us to quickly estimate the orientation of the instruments

on the theory of sea floor conductivity mapping using transient electromagnetic systems the electrical conductivity of the sea floor is usually much less than that of the seawater above it a theoretical study of the transient step on responses of some common controlled source electromagnetic systems to adjoining conductive half spaces shows that two systems the horizontal in line electric dipole dipole and horizontal coaxial magnetic dipole dipole are capable of accurately measuring the relatively low conductivity of the sea floor in the presence of seawater for these systems the position in time of the initial transient is indicative of the conductivity of the sea floor while at distinctly later times a second characteristic of the transient is a measure of the seawater conductivity the diagnostic separation in time between the two parts of the transient response does not occur for many other systems including several systems commonly used for exploration on land a change in the conductivity of the sea floor produces a minor perturbation in what is essentially a seawater response some transient responses which could be observed with a practical deep towed coaxial magnetic dipole dipole system located near the sea floor are those for the half space the layer over a conductive or resistive basement and the half space with an intermediate resistive zone the system response to two adjoining half spaces representing seawater and sea floor respectively is derived analytically the solution is valid for all time provided the conductivity ratio is greater than about ten or less than about one tenth the analytic theory confirms the validity of numerical evaluations of closed form solutions to these layered earth models a lateral conductor such as a vertical infinite conductive dike outcropping at the sea floor delays the arrival of the initial crustal transient response the delay varies linearly with the conductance of the dike this suggests that time delay could be inverted directly to give a measure of the anomalous integrated conductance of the sea floor both between and in the vicinity of the transmitter and the receiver dipoles

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

geometry and kinematics of the arlar strike slip fault sw qaidam basin china new insights from 3 d seismic data abstract the right stepping en echelon kunbei fault system consisting of the hongliuquan arlar xiii and kunbei faults features importantly in the tectonic evolution of the southwest qaidam basin the 3d seismic cross section interpretation and key horizon similarity attribute analysis show that the nww striking arlar fault is composed of two steeply dipping faults f1 and f2 in cross sectional view and branches eastward into two west dipping faults f5 and f6 in map view the fact that the strata above t 2 the base of the early miocene xiayoushashan formation become dramatically thinner toward the high points of the arlar fault suggests an early miocene initiation ca 22 0 ma for this fault as indicated by the windowed amplitude related attribute maps the arlar fault and f5 one branch of the arlar fault have cumulative sinistral offsets of 8 6 8 7 km and 5 2 5 4 km respectively these considerably greater strike slip components in conjunction with the respective maximum true dip slip displacements of