the rise and fall of early oil field technology the torsion balance gradiometer today elementary physics students take for granted such quantities as big g the universal gravitational constant in fact in the late 1700s the value of this quantity was unknown and the quest to determine it led to some of the earliest geophysical instrumentation just after the revolutionary war in the united states cavendish developed the first system to measure the universal gravitational constant the familiar big g unfortunately for geologists at this time still mostly gentlemen scientists this apparatus produced data which were difficult to interpret geologically and it was far too large and cumbersome for field use the geologic limitation was that the system only measured the horizontal derivative of a horizontal component of the gravity field a quantity which by itself is difficult to interpret thus no applications of this elegant yet laboratory bound instrument emerged

isopycnal and diapycnal circulation of the upper equatorial atlantic ocean in 1983 1984 isopycnal circulation and diapycnal processes in the equatorial atlantic ocean are investigated using eight cruises from november 1982 to july 1984 an eastward decrease of the transport of the equatorial undercurrent is observed averaging 18 2 106 m3 s 1 at 35 w to 10 2 106 m3 s 1 at 4 w there is also a meridional convergence at the undercurrent level and a larger meridional divergence in the surface layer of 15 106 m3 s 1 between 35 w and 4 w in the eastern equatorial atlantic the undercurrent core density as well as the salinity above the undercurrent experience a large seasonal cycle during the season where the eastern atlantic thermocline is closest to the surface and surface waters are coldest the current peaks in denser waters and subsurface salinity maximum are weaker an analysis of the salinity on isopycnal surfaces indicates that there is significant diapycnal mixing in the upper thermocline in the eastern atlantic the upper thermocline vertical heat diffusivity coefficient scaled over 1 of latitude varies between 3 cm2 s 1 during the upwelling season and nearly 0 early in the year this seasonal mixing is an important element of undercurrent dynamics the turbulent heat flux at the base of the surface layer averages 50 w m 2 between 1 5 n and 1 5 s the surface layer heat budget implies an average oceanic gain of 60 w m 2 between 1 5 n and 1 5 s which could result from exchanges with the atmosphere the heat flux and fresh water seasonal cycles needed to close the budgets are not realistic however the eastern equatorial atlantic thermocline seasonal upwelling is associated with a large inflow into the surface layer over the two years this flow averaging 11 12 106 m3 s 1 originates from the upper thermocline and diapycnal transports near the core of the undercurrent or below are found to be small below the core of the undercurrent we also find that mixing is not intense for instance at 26 5 in the upper part of the thermostad vertical heat diffusivity is only 0 6 cm2 s 1 assuming that mixing takes place over 1 of latitude the uncertainties on these budgets are however large and assumptions on the dynamics that were used could not be checked

a new moment tensor decomposition for seismic events in anisotropic media summary investigating the mechanisms of small seismic sources usually consists of three steps determining the moment tensor of the source decomposing the moment tensor into parameters that can be interpreted in terms of physical mechanisms and displaying those parameters this paper concerns the second and third steps two existing methods the riedesel jordan and hudson pearce rogers parameters and displays are reviewed compared and contrasted and advantages and disadvantages of the two methods are discussed one disadvantage is that neither method takes into consideration the effect of anisotropy on the interpretation in microseisms anisotropy can be important a new procedure based on the biaxial decomposition of the potency tensor is introduced which explicitly allows for anisotropy and interprets the moment tensor in terms of an isotropic pressure change and a displacement discontinuity on a fault it is shown that this interpretation is always possible for any moment tensor whatever the anisotropy to compare the pressure change with the displacement discontinuity it is useful to be able to determine the volume change from the pressure source in any medium this depends on the embedded bulk modulus which differs from the normal bulk modulus the embedded modulus in isotropic media is well known and the equivalent anisotropic result is derived in this paper interpreting a seismic source in terms of the volume change due to a pressure change and a displacement discontinuity on a fault allows a simple 3 d graphical glyph to be used to display the interpretation

local vertical seismic profiling vsp elastic reverse time migration and migration resolution salt flank imaging with transmitted p to s waves to avoid the defocusing effects of propagating waves through salt and overburden with an inaccurate overburden velocity model we introduce a vertical seismic profiling vsp local elastic reverse time migration rtm method for salt flank imaging by transmitted p to s waves this method back projects the transmitted ps waves using a local velocity model around the well until they are in phase with the back projected pp waves at the salt boundaries the merits of this method are that it does not require the complex overburden and salt body velocities and it automatically accounts for source side statics in addition the method accounts for kinematic and dynamic effects including anisotropy absorption and all other unknown rock effects outside of this lo cal subsalt velocity model numerical tests on an elastic salt model and offset 2d vsp data in the gulf of mexico using a finite difference time domain staggered grid rtm scheme partly demonstrate the effectiveness of this method over interferometry

the south atlantic anticyclone as a key player for the representation of the tropical atlantic climate in coupled climate models the key role of the south atlantic anticyclone saa on the seasonal cycle of the tropical atlantic is investigated with a regionally coupled atmosphere ocean model for two different coupled domains both domains include the equatorial atlantic and a large portion of the northern tropical atlantic but one extends southward and the other northwestward the saa is simulated as internal model variability in the former and is prescribed as external forcing in the latter in the first case the model shows significant warm biases in sea surface temperature sst in the angola benguela front zone if the saa is externally prescribed these biases are substantially reduced the biases are both of oceanic and atmospheric origin and are influenced by ocean atmosphere interactions in coupled runs the strong sst austral summer biases are associated with a weaker saa which weakens the winds over the southeastern tropical atlantic deepens the thermocline and prevents the local coastal upwelling of colder water the biases in the basins interior in this season could be related to the advection and eddy transport of the coastal warm anomalies in winter the deeper thermocline and atmospheric fluxes are probably the main biases sources biases in incoming solar radiation and thus cloudiness seem to be a secondary effect only observed in austral winter we conclude that the external prescription of the saa south of 20 s improves the simulation of the seasonal cycle over the tropical atlantic revealing the fundamental role of this anticyclone in shaping the climate over this region

window constrained inversion of gravity gradient tensor data using dike and contact models we have developed a constrained inversion technique for interpretation of gravity gradient tensor data for dike and contact models striking in the y direction the measured g xz and g zz com

response of the surface tropical atlantic ocean to wind forcing abstract we use 10 years of satellite data sea level pressure surface winds and absolute dynamic topography adt together with argo inferred monthly mean values of near surface velocity and water transport to examine how the tropical system of near surface zonal currents responds to wind forcing the data is analyzed using complex hilbert empirical orthogonal functions confirming that most of the variance has annual periodicity with maximum amplitudes in the region spanned by the seasonal displacement of the inter tropical convergence zone itcz the adt mirrors the shape of the upper isopycnals hence becoming a good indicator of the amount of water stored in the upper ocean within about 3 from the equator where the coriolis force is small there is year long meridional ekman transport divergence that would lead to the eastward transport of the equatorial undercurrent and its northern and southern branches beyond 3 of latitude and at least as far as 20 the convergence of the ekman transport generally causes a poleward positive adt gradient which sustains the westward south equatorial current sec the sole exception occurs in summer between 8 n and 12 n when an ekman transport divergence develops and depletes de amount of surface water resulting in an adt ridge valley system which reverses the adt gradient and drives the eastward north equatorial countercurrent necc at latitudes 4 9 n in late fall divergence ceases and the necc drains the adt ridge so the adt gradient again becomes positive and the sec reappears the seasonal evolution of a tilted itcz controls the surface water fluxes the wind induced transports set the surface divergence convergence which then drive the adt and through the adt gradients create the geostrophic jets that close the water balance

polar anisotropy from walkway vsps polar anisotropy also called vertical transverse isotropy vti causes a dependence of velocity on propagation direction resulting from layering or microstructure shales commonly exhibit this type of anisotropy and the magnitude of the effect can be large a stack of isotropic layers will also exhibit the effects of polar anisotropy such a medium is then said to be effectively anisotropic it is well known that polar anisotropy can have a large effect on seismic imaging and neglecting its effect causes mispositioning and degraded focusing of reflections it can also impact avo demands on polar anisotropy measurements are different depending on the problem for correct seismic imaging migration average or effective properties are needed for avo interpretation local properties are also important both types of polar anisotropy measurements are possible with walkaway multioffset vertical vsp data

bhp develops airborne gravity gradiometer for mineral exploration bhp s project falcon a research effort specifically aimed at building the world s first airborne gravity gradiometer agg system has developed two fully operational systems that are capable of sensitive high resolution detection of small variations in the earth s gravity at the level required in exploration for mineral and hydrocarbon resources

anisotropic elastic full waveform inversion of walkaway vertical seismic profiling data from the arabian gulf abstract borehole seismic addresses the need for high resolution images and elastic parameters of the subsurface full waveform inversion of vertical seismic profile data is a promising technology with the potential to recover quantitative information about elastic properties of the medium full waveform inversion has the capability to process the entire wavefield and to address the wave propagation effects contained in the borehole data multi component measurements anisotropic effects compressional and shear waves and transmitted converted and reflected waves and multiples full waveform inversion therefore has the potential to provide a more accurate result compared with conventional processing methods we present a feasibility study with results of the application of high frequency up to 60 hz anisotropic elastic full waveform inversion to a walkaway vertical seismic profile data from the arabian gulf full waveform inversion has reproduced the majority of the wave events and recovered a geologically plausible layered model with physically meaningful values of the medium

a whirling ecosystem in the equatorial atlantic 1 the equatorial pacific and atlantic oceans exhibit remarkable meridional undulations in temperature and chlorophyll fronts visible from space over thousands of kilometers and often referred to as tropical instability waves here we present new observations of an ecosystem ranging through three trophic levels phytoplankton zooplankton and small pelagic fish whirling within a tropical vortex of the atlantic ocean and associated with such undulations cold nutrient and biologically rich equatorial waters are advected northward and downward to form sharp fronts visible in all tracers and trophic levels the equatorward recirculation experiences upwelling at depth with the pycnocline and ecosystem progressively moving toward the surface to reconnect with the equatorial water mass the observations thus indicate that it is a fully three dimensional circulation that dominates the distribution of physical and biological tracers in the presence of tropical instabilities and maintains the cusp like shapes of temperature and chlorophyll observed from space

main processes of the atlantic cold tongue interannual variability the interannual variability of the atlantic cold tongue act is studied by means of a mixed layer heat budget analysis a method to classify extreme cold and warm act events is proposed and applied to ten various analysis and reanalysis products this classification allows 5 cold and 5 warm act events to be selected over the period 1982 2007 cold warm act events are defined by the presence of negative positive sea surface temperature sst anomalies at the center of the equatorial atlantic in late boreal spring preceded by negative positive zonal wind stress anomalies in the western equatorial atlantic an ocean general circulation model capable of reconstructing the interannual variability of the act correctly is used to demonstrate that cold act events develop rapidly from may to june mainly due to intense cooling by vertical mixing and horizontal advection the simulated cooling at the center of the basin is the result of the combined effects of non local and local processes the non local process is an upwelling associated with an eastward propagating kelvin wave which makes the mixed layer more shallow and preconditions the upper layers to be cooled by an intense heat loss at the base of the mixed layer which is amplified by a stronger local injection of energy from the atmosphere the early cooling by vertical mixing in march is also shown to be a good predictor of june cooling in july horizontal advection starts to warm the mixed layer abnormally and damps sst anomalies the advection anomalies which result from changes in the horizontal temperature gradient are associated in some cases with the propagation of rossby waves along the equator during warm act events processes are reversed generating positive sst anomalies a downwelling kelvin wave triggers stratification anomalies and mixed layer depth anomalies amplified by a weaker injection of energy from the atmosphere in may june in july warm act events are abnormally cooled due to negative horizontal advection anomalies resulting from processes similar to those that occur during cold act events this additional cooling process extends the period of cooling of the act reducing sst anomalies

localized s inversion of time domain electromagnetic data interpretation of time domain electromagnetic tdem data over inhomogeneous geological structures is a challenging problem of geophysical exploration the most widely used approach of interpreting tdem data is based on the smooth 1 d layered resistivity inversion we have developed an effective technique of fast tdem inversion based on thin sheet conductance approximation that we call s inversion in this paper we extend the s inversion technique approximating the conductivity cross section by adding a local inhomogeneous disk with an excess conductance s to the horizontal conductive thin sheet used in s inversion localized s inversion determines the distribution of this excess conductance as a function of a depth and a horizontal coordinate this new method takes into account the limited horizontal extent of the inhomogeneities localizing inversion the numerical modeling results and inversion of practical tdem data demonstrate that the method resolves local geological targets better than traditiona

why were sea surface temperatures so different in the eastern equatorial atlantic in june 2005 and 2006 abstract a comparison of june 2005 and june 2006 sea surface temperatures in the eastern equatorial atlantic exhibits large variability in the properties of the equatorial cold tongue with far colder temperatures in 2005 than in 2006 this difference is found to result mainly from a time shift in the development of the cold tongue between the two years easterlies were observed to be stronger in the western tropical atlantic in april may 2005 than in april may 2006 and these winds favorably preconditioned oceanic subsurface conditions in the eastern atlantic however it is also shown that a stronger than usual intraseasonal intensification of the southeastern trades was responsible for the rapid and early intense cooling of the sea surface temperatures in mid may 2005 over a broad region extending from 20 w to the african coast and from 6 s to the equator this particular event underscores the ability of local intraseasonal wind stress variability in the gulf of guinea to initiate the cold tongue seaso

atmospheric response to sea surface temperature in the eastern equatorial atlantic at quasi biweekly time scales the surface wind response to sea surface temperature sst and sst meridional gradient is investigated in the gulf of guinea by using daily observations and re analyses in the 2000 2009 decade with a focus on boreal spring and summer months may to august where quasi biweekly fluctuations in the position of the northern front of the equatorial cold tongue induce quasi biweekly equatorial sst anomalies following a large scale wind acceleration deceleration an equatorial sst cold warm anomaly is created within a few days in order to explain the local atmospheric response to this sst anomaly the two following mechanisms are invoked first a colder warmer ocean decreases increases the vertical stability in the marine atmospheric boundary layer which favours a weaker stronger surface wind and second a negative positive anomaly of sst meridional gradient induces a positive negative anomaly of the sea level pressure meridional gradient which decelerates accelerates the surface wind the first mechanism has an immediate effect in the equatorial belt between 1 s and 1 n and to a lesser extent between 3 s and 1 s whereas the second takes 1 or 2 days to adjust and damps anomalous southeasterlies up to 800 hpa in the low troposphere between 7 s and 1 n through reversed anomalies of meridional sst and pressure gradient this negative feedback leads to weaker stronger winds in the southeastern tropical atlantic which forces the opposite phase of the oscillation within about 1 week around the equator where the amplitude of the oscillation is found to be maximal both mechanisms combine to maximize the wind response to the front fluctuations between the equator and the coast a low level secondary atmospheric circulation takes control of the surface wind acceleration or deceleration around 3 n which reduces the influence of the sst front fluctuations

on the annual cycle of equatorial upwelling in the central atlantic ocean abstract the annual cycle of the upper ocean s vertical velocity component w on the equator at 28 w is examined by integrating the continuity equation using current meter data from the seasonal response of the equatorial atlantic experiment the annual cycle consists in part of an intense but brief 1 month upwelling season beginning with the onset of strong easterly wind stress in boreal spring this upwelling is followed by weaker downwelling during the summer despite the persistence of strong easterly wind stress the record length averaged w profile shows that maximum upwelling 0 6 10 3 cm s 1 is located slightly above the core of the equatorial undercurrent and downwelling is located below the base of the thermocline the standard deviations are about tenfold the magnitude of the means independent evidence supporting these results are that 1 sea surface temperature sst is related to w during the springtime changes in easterly wind stress with the observed and computed isotherm displacem

three dimensional regularized focusing inversion of gravity gradient tensor component data we develop a new method for interpretation of tensor gravity field component data based on regularized focusing inversion the focusing inversion makes its possible to reconstruct a sharper image of the geological target than conventional maximum smoothness inversion this new technique can be efficiently applied for the interpretation of gravity gradiometer data which are sensitive to local density anomalies the numerical modeling and inversion results show that the resolution of the gravity method can be improved significantly if we use tensor gravity data for interpretation we also apply our method for inversion of the gradient gravity data collected by bhp billiton over the cannington ag pb zn orebody in queensland australia the comparison with the drilling results demonstrates a remarkable correlation between the density anomaly reconstructed by the gravity gradient data and the true structure of the orebody this result indicates that the emerging new geophysical technology of the airborne gravity gradient observations can improve significantly the practical effectiveness of the gravity method in mineral exploration

variability of the mixed layer heat budget in the eastern equatorial atlantic during 2005 2007 as inferred using argo floats we examine the variability of the sea surface temperatures in the eastern equatorial atlantic during 2005 2007 by using argo profiling floats prediction and research moored array in the atlantic buoys and satellite in situ and atmospheric data sets the eastern equatorial atlantic characterized by shallow mixed layers all yearlong is divided into nine boxes of nearly equal surface area with respect to the dynamics and thermodynamics in this region monthly mixed layer heat budgets are computed in each box from 10 day argo profiles in all the boxes the net surface heat flux is one of the main causes of the seasonal evolution of sea surface temperatures for the 3 studied years the amount of short wave radiation penetrating through the base of the mixed layer and horizontal heat advection may locally contribute to the temperature variability while entrainment has a weaker contribution to balance the heat budget a residual term exists which includes all processes that cannot be calculated with observations as well as the possible errors in the other terms this residual is more intense in the cold tongue and the northern region and exhibits a clear seasonal cycle with minimum negative values in boreal summer and maximum values in winter this residual compares well with available observations of vertical turbulent mixing collected during etude de la circulation oceanique et des echanges ocean atmosphere dans le golfe de guinee campaigns 2005 2007 in the eastern equatorial atlantic when assuming that the residual is mostly associated with vertical turbulent mixing it can be conjectured that turbulent mixing is a significant cooling source in the cold tongue and north of the equator

dynamics of upwelling annual cycle in the equatorial atlantic ocean the annual upwelling is an important component of the equatorial atlantic annual cycle a simple theory is proposed using the framework of zebiak cane zc ocean model for insights into the dynamics of the upwelling annual cycle it is demonstrated that in the atlantic equatorial region this upwelling is dominated by ekman processing in the west whereas in the east it is primarily owing to shoaling and deepening of the thermocline resulting from equatorial mass meridional recharge discharge and zonal redistribution processes associated with wind driven equatorial ocean waves this wind driven wave upwelling plays an important role in the development of the annual cycle in the sea surface temperature sst of the cold tongue in the eastern equatorial atlantic

dynamics of simulated atlantic upwelling annual cycle in cmip5 models as one of the prominent climatic component of tropical climate system the annual cycle of the equatorial atlantic cold tongue region has been a subject of significant research in this work we examine the fidelity of the key part of the annual cycles namely the upwelling annual cycles from the simulations of the coupled model intercomparison project phase 5 cmip5 utilizing the simple framework we have recently developed based on the zebiak cane zc ocean model we first tested the validity of the simple framework of upwelling as a combination of the local wind driven ekman upwelling and nonlocal wind driven wave upwelling it is demonstrated that the theoretically reconstructed upwelling annual cycles to large extent are in good agreement with the simulations in these coupled climatic models comparing with observations we show that the semi annual component of the upwelling is systematically too strong this significant bias is attributable to the dynamic amplification to the wind driven wave upwelling originated from a less significant bias in the semi annual harmonic component in the central equatorial wind field the latter is dynamically amplified in the wind driven wave upwelling owing to its relatively fast timescale thus the realistic simulations of equatorial annual cycles may sensitively depend on good simulations of the semi annual harmonic components in equatorial wind fields

focusing iterative migration of gravity gradiometry data acquired in the nordkapp basin barents sea abstract geological interpretation based on gravity gradiometry data constitutes a very challenging problem rigorous 3d inversion is the main technique used in quantitative interpretation of the gravity gradiometry data an alternative approach to the quantitative analysis of the gravity gradiometry data is based on 3d smooth potential field migration this rapid imaging approach however has the shortcomings of providing smooth images since it is based on direct integral transformation of the observed gravity tensor data another limitation of migration transformation is related to the fact that in a general case the gravity data generated by the migration image do not fit the observed data well in this paper we describe a new approach to rapid imaging that allows us to produce the density distribution which adequately describes the observed data and at the same time images the structures with anomalous densities having sharp boundaries this approach is based on the basic theory of potential field migration with a focusing stabilizer in the framework of regularized scheme which iteratively transfers the observed gravity tensor field into an image of the density distribution in the subsurface formations the results of gravity migration can also be considered as an a priori model for conventional inversion subsequently we demonstrate the practical application of migration imaging using both synthetic and real gravity gradiometry data sets acquired for the nordkapp basin in the barents sea

a statistical review of mudrock elastic anisotropy abstract mudrocks defined to be fine grained siliclastic sedimentary rocks such as siltstones claystones mudstones and shales are often anisotropic due to lamination and microscopic alignments of clay platelets the resulting elastic anisotropy is often non negligible for many applications in the earth sciences such as wellbore stability well stimulation and seismic imaging anisotropic elastic properties reported in the open literature have been compiled and statistically analysed correlations between elastic parameters are observed which will be useful in the typical case that limited information on a rock s elastic properties is known for example it is observed that the highest degree of correlation is between the horizontal elastic stiffnesses c11 and c66 the results of statistical analysis are generally consistent with prior observations in particular it is observed that thomsen s and parameters are almost always positive thomsen s and parameters are well correlated thomsen s is most frequently small and thomsen s is generally larger than thomsen s these observations suggest that the typical range for the elastic properties of mudrocks span a sub space less than the five elastic constants required to fully define a vertical transversel isotropic medium principal component analysis confirms this and that four principal components can be used to span the space of observed elastic parameters

coupling between the atlantic cold tongue and the west african monsoon in boreal spring and summer 1 the formation of the atlantic cold tongue act is the dominant seasonal sea surface temperature signal in the eastern equatorial atlantic eea a comprehensive analysis of variability in its spatial extent temperature and onset is presented then the physical mechanisms which initiate act onset as well as the feedbacks from the act to the maritime boundary layer and how the act influences the onset of the west african monsoon wam are discussed we argue that in the eea the air sea coupling between the act and wam occurs in two phases from march to mid june the act results from the intensification of the southeastern trades associated with the st helena anticyclone steering of surface winds by the basin shape of the eea imparts optimal wind stress for generating the maximum upwelling south of the equator during the second phase mid june august wind speeds north of the equator increase as a result of the northward progression of the intensifying trades and as a result of significant surface heat flux gradients produced by the differential cooling between the act and the tropical waters circulating in the gulf of guinea gg it is anticipated that the atmospheric divergence induced at low levels north of the equator reduces convection over the gg and that increased northward winds shift convection over land correlations between the act and the wam onset dates over the last 26 years 1982 2007 measure as much as 0 8 this suggests that the act plays a key role in the wam onset

satellite derived surface current divergence in relation to tropical atlantic sst and wind the relationships between tropical atlantic ocean surface currents and horizontal mass divergence sea surface temperature sst and winds on monthly to annual time scales are described for the time period from 1993 through 2003 surface horizontal mass divergence upwelling is calculated using surface currents estimated from satellite sea surface height surface vector wind and sst data with a quasi linear steadystate model geostrophic and ekman dynamical contributions are considered the satellite derived surface currents match climatological drifter and ship drift currents well and divergence patterns are consistent with the annual north south movement of the intertropical convergence zone itcz and equatorial cold tongue evolution while the zonal velocity component is strongest the meridional velocity component controls divergence along the equator and to the north beneath the itcz zonal velocity divergence is weaker but nonnegligible along the equator a strong divergence upwelling season in the central eastern equatorial atlantic peaks in may while equatorial sst is cooling within the cold tongue in addition a secondary weaker and shorter equatorial divergence occurs in november also coincident with a slight sst cooling the vertical transport at 30 m depth averaged across the equatorial atlantic ocean between 2 s and 2 n for the record length is 15 6 10 6 m 3 s 1 results are consistent with what is known about equatorial upwelling and cold tongue evolution and establish a new method for observing the tropical upper ocean relative to geostrophic and ekman dynamics at spatial and temporal coverage characteristic of satellite based observations