historical development of the gravity method in exploration the gravity method was the first geophysical technique to be used in oil and gas exploration despite being eclipsed by seismology it has continued to be an important and sometimes crucial constraint in a number of exploration areas in oil exploration the gravity method is particularly applicable in salt provinces overthrust and foothills belts underexplored basins and targets of interest that underlie high velocity zones the gravity method is used frequently in mining applications to map subsurface geology and to directly calculate ore reserves for some massive sulfide orebodies there is also a modest increase in the use of gravity techniques in specialized investigations for shallow targets gravimeters have undergone continuous improvement during the past 25 years particularly in their ability to function in a dynamic environment this and the advent of

the quantum event of oceanic crustal accretion impacts of diking at mid ocean ridges seafloor diking eruptive events represent the irreducible quantum events of upper oceanic crustal accretion they record events by which a large portion of the oceanic crust has formed through geological history since 1993 the u s navy s real time sound surveillance system has allowed location of ongoing acoustic signatures of dike emplacement and basalt eruptions at ridge crests in the northeast pacific these diking eruptive events trigger a sequence of related rapidly evolving physical chemical and biological processes magmatic volatiles released during these events may provide nutrients for communities of subseafloor microorganisms some of which thrive in high temperature anaerobic environments many of the organisms identified from these systems are archaea if microorganisms can thrive in the water saturated pores and cracks within deep volcanically active portions of our planet other hydrothermally active planets may harbor similar life forms

seamount loading and stress in the ocean lithosphere 2 viscoelastic and elastic viscoelastic models analytical solutions are presented for the deformation and stress state of a horizontally stratified earth subject to normal loads of size and wavelength that are characteristic of seamounts the models investigated include layered elastic plates homogeneous viscoelastic plates and elastic over viscoelastic models in all cases the composite models overlie an inviscid half space the principal advantage of the laminated elastic models over homogeneous elastic plates is that they can result in a substantial reduction in the maximum stress differences in those parts of the plate least capable of supporting large stress differences authors

uplift by thermal expansion of the lithosphere summary green s function solutions are derived for the surface heat flow and the uplift caused by conductive heating when axially symmetric variations in heat flow occur at the base of the lithosphere the solutions determined for different space time dependence of the heat flux into the lithosphere illustrate that 1 when the lateral variations in heat flux occur over short distances in comparison to the lithospheric thickness the amplitude of the uplift and of the anomalous surface heat flow are attenuated in comparison to the changes that would follow uniform variations this effect is slightly more important for the surface heat flow which spreads out over larger distances 2 there is a time lag between the uplift which starts instantly and the change in the surface heat flow it is suggested that the model could be applied to the study of plateaux uplift of moderate amplitude such as the black hills or the adirondacks and possibly to the heating event and uplift that may have preceded the subsidence of intracontinental basins

crustal magnetization reveals subsurface structure of juan de fuca ridge hydrothermal vent fields a near bottom geophysical survey on the endeavour segment of the northern juan de fuca ridge shows that regions of well defined low crustal magnetization are strongly correlated with both active and extinct submarine hydrothermal vent sites in particular at the main endeavour field we find discrete magnetization lows associated with each cluster of vents magnetization lows are directly centered beneath the vent clusters and have diameters of 100 m which implies a near vertical narrow pipe like source region located directly beneath the surface expression of the vent edifices lows are also separated from each other by only 200 m which further implies highly focused zones magnetization lows are also associated with inactive and extinct vent areas which indicates that alteration of the magnetic minerals in the crust rather than necessarily temporary thermal demagnetization is the primary process responsible for the low magnetization these narrow pipe like bodies are highly characteristic of alteration pipes found in ophiolites and are indicative of hydrothermal fluid up flow zones thus each magnetization low may define an individual upwelling zone with distinct subsurface plumbing and thermal structure the crustal magnetization patterns provide important constraints on the geometry of the subsurface plumbing beneath these hydrothermal vent systems at the main endeavour field magnetization lows are distributed along the trend of the rift valley in a semiregular pattern with a spacing of 200 m arguing that upward flow may be partitioned into regularly spaced intervals along the axis of the rift valley

instability waves observed on the equator in the atlantic ocean during 1983 a packet of surface confined westward propagating waves apparently generated by barotropic instability of the surface currents was observed in the equatorial atlantic during 1983 initial wavenumber analyses and energetics calculations are presented local working by the horizontal reynolds stress was large enough and at the correct time to account for the wave packet s existence the amplitude of the waves was comparable to the mean currents implying that the waves are of primary importance in the momentum balance of the seasonally varying equatorial surface currents

resolution of the scripps noaa marine gravity field from satellite altimetry the july 1995 declassification of the entire geosat gm satellite altimeter data set enabled a joint scripps noaa effort to compute a new version 7 2 marine gravity field on a 2 minute grid this gravity field covers the world oceans between 72 n and 72 s and is derived from a combination of ers 1 and geosat gm and erm data an earlier noaa geosat only gravity field solution was confined to the southern latitudes because the 1992 declassification was limited to gm data south of 30 s a simple coherence analysis between accurately navigated ship gravity profiles and comparable gravity profiles obtained from the gravity grids reveals that the scripps noaa gravity field is coherent with ship gravity down to 23 30 km this slight increase in resolution over the previous noaa geosat only gravity field short wavelength resolution of 26 30 km implies that the increased spatial coverage provided by the ers i altimeter when combined with geosat improves the solution coherence analyses between satellite gravity and ship topography and ship gravity and ship topography show that even shorter wavelength gravity anomalies 13 km are present in sea surface measurements made by ship even so the scripps noaa marine gravity field does an excellent job of resolving most of the short wavelength gravity anomalies covering the world oceans

a seafloor and sea surface gravity survey of axial volcano seafloor and sea surface gravity measurements are used to model the internal density structure of axial volcano seafloor measurements made at 53 sites within and adjacent to the axial volcano summit caldera provide constraints on the fine scale density structure shipboard gravity measurements made along 540 km of track line above axial volcano and adjacent portions of the juan de fuca ridge provide constraints on the density over a broader region and on the isostatic compensation the seafloor gravity anomalies give an average density of 2 7 g cm 3 for the uppermost portion of axial volcano the sea surface gravity anomalies yield a local compensation parameter of 23 significantly less than expected for a volcanic edifice built on zero age lithosphere three dimensional ideal body models of the seafloor gravity measurements suggest that low density material with a density contrast of at least 0 15 g cm 3 may be located underneath the summit caldera the data are consistent with low density material at shallow depths near the southern portion of the caldera dipping downward to the north the correlation of shallow low density material and surface expressions of recent volcanic activity fresh lavas and high temperature hydrothermal venting suggests a zone of highly porous crust seminorm minimization modeling of the surface gravity measurements also suggest a low density region under the central portion of axial volcano the presence of low density material beneath axial caldera suggests a partially molten magma chamber at depth

a towed marine gravity meter for near bottom surveys gravity is measured presently on the sea surface and on the sea floor surface gravity suffers from loss of resolution over the deep ocean because the perturbing source masses are far from the observer bottom measurements recover this resolution but suffer from poor coverage because of the time needed for each measurement we have constructed a gravimetry system that combines the rapid data collection capability of a moving platform with the high resolution gained by locating the observations near the bottom this gravity sensor is tethered to a ship and towed just above the sea floor the instrument consists of a lacoste and romberg shipboard gravity meter modified to fit inside a pressure case that is mounted on a platform designed for towing stability we have tested it in a survey in the san diego trough a 1000 m deep sedimented valley in the pacific ocean in the california continental borderlands multiple gravity tracklines collected there at a depth of 935 m show a resolution of a few tenths of a mgal the new instrument will be useful for surveys of features whose lateral extent is equal to or less than the ocean depth

a seafloor gravity profile across the tag hydrothermal mound a series of on bottom gravity stations were completed across the tag hydrothermal mound during a dive program with the japanese shinkai 6500 submersible the mound appears as a gravity high 2 mgal with respect to the surrounding seafloor reflecting the high density sulfides of which it is comprised three dimensional modeling of the data shows no evidence of a dense sulfide root extending deep into the seafloor below the mound which would be associated with a broad highly altered stockwork zone sulfide thickness appears to be greatest 50m to the north of the mound the thickness decreases sharply to the south reaching a steady value of about 10m just to the south of the kremlin area the mass of sulfides within the mound is estimated at 3 million tonnes the surrounding basaltic seafloor appears to have a density of 2400 100 kgm 3 indicating a surficial porosity of between 20 30

measurements beneath an antarctic ice shelf using an autonomous underwater vehicle the cavities beneath antarctic ice shelves are among the least studied regions of the world ocean yet they are sites of globally important water mass transformations here we report results from a mission beneath fimbul ice shelf of an autonomous underwater vehicle the data reveal a spatially complex oceanographic environment an ice base with widely varying roughness and a cavity periodically exposed to water with a temperature significantly above the surface freezing point the results of this the briefest of glimpses of conditions in this extraordinary environment are already reforming our view of the topographic and oceanographic conditions beneath ice shelves holding out great promises for future missions from similar platforms

a new sea floor gravimeter a new reservoir management application uses precise time lapse gravity measurements on the sea floor to detect seawater infiltration in offshore natural gas fields during production reservoir models for the north sea troll field predict gravity changes as large as 0 060 mgal within a 3 5 year period we have constructed and deployed a new instrument the rovdog remotely operated vehicle deployed deep ocean gravimeter system for this application because the measurements must be relocated accurately within 3 cm we required a gravimeter which could be handled by an rov and placed atop sea floor benchmarks we have built an instrument based upon the scintrex cg 3m land gravimeter motorized gimbals level the gravimeter sensor within a watertight pressure case precision quartz pressure gauges provide depth information a shipboard operator remotely controls the instrument and monitors the data the system error budget considers both instrumental and field measurement uncertainties the instrument prototy

thermoelastic bending of the lithosphere implications for basin subsidence summary thermoelastic stresses are capable of producing significant lithospheric deflection a systematic approach to modelling this effect is presented in which the lithosphere is presumed to behave as a thin elastic or viscoelastic plate on a fluid substrate and lateral variations in basal heat flow induce both vertical buoyant loads and thermoelastic bending moments the amount of uplift or subsidence produced by a given heat source depends on a number of factors including the strength duration and lateral extent of the thermal anomaly and the thickness density rigidity and viscosity of the plate the mechanical response of the plate is characterized by two distinct length scales one for shearing the other for bending and the deformation produced depends critically on the scaled width of the heat source the plate acts as a low pass spatial filter in response to thermal loads but has a narrow band pass filter response to applied moments the temporal response to an abrupt change in basal temperature or heat flow also depends on the ratio of the thermal diffusion time for the plate versus the viscoelastic relaxation time of the material a rapid localized increase in basal heat flow will initially produce a central depression and a peripheral uplift thus while it is often assumed that the main depositional phase in platform basins is associated with the cooling and contraction of the lithosphere following an earlier thermal doming event it is shown that if conditions are right a basin may also form in direct response to the initial heating event either way subsequent thermal equilibration and viscous stress relaxation will tend to modify the initial deflection profile

porosity of the upper edifice of axial seamount seamounts are not solid basalt structures but have relatively high porosities in their upper crustal sections at axial seamount off the coast of oregon united states we used on bottom gravity measurements with a bell gravity meter within deep sea submersible alvin to determine a porosity of 31 for the uppermost 100 m of the edifice the southwestern caldera wall has a porosity of 22 and the caldera floor has a slightly higher porosity of 33 seafloor observations and models indicate that these high porosities result from large scale structural features such as lava tubes and cracks large lava drain backs or regions of open pillow basalts in the near subsurface these high porosity zones can affect the subsurface permeability and models of hydrothermal upflow zones explain observed localized gravity anomalies the variety of hydrothermal alteration hydrothermally active areas and open porous features appears to be related to the high porosity that is inferred from geophysical measurements on this active seafloor volcano

comments on on the vulnerability of lithospheric plates to mid plate volcanism reply to comments by p r vogt by h n pollack i g gass r s thorpe and d s chapman

on bottom gravity profile across the east pacific rise crest at 21 north twenty on bottom gravity stations were taken with a lacoste romberg model g gravity meter inside the submersible alvin over a 6 km long cross strike profile which straddles the east pacific rise spreading center at 21 n the residual bouguer anomaly shows a 1 5 mgal low with a total half width of 1 7 km centered about the spreading axis and hydrothermal vents uncertainties in the ocean depth are believed to contribute a standard error of about 0 3 to 0 4 mgal for the gravity data the observed elevation factor of 0 1552 mgal m indicates that the upper ocean crust bulk density is 2 62 0 09g cm3 on the other hand 90 rock samples give a density of 2 90 0 01g cm3 this suggests that the largescale porosity of the upper crust is 14 percent the gravity anomaly was inverted to find the ideal body with the smallest maximum density contrast which satisfies the data and their associated errors this body extends from the sea floor to 2 500 m depth and has a density contrast of 0 043g cm3 if the density contr

comparison of marine gravity from shipboard and high density satellite altimetry along the mid atlantic ridge 30 5 35 5 s we compare new marine gravity fields derived from satellite altimetry with shipboard measurements over a region of more than 120 000 square kilometers in the central south atlantic newly declassified satellite data were employed to construct free air anomaly maps on 0 05 degree grids sandwell and smith 1992 marks et al 1993 an extensive gravity and bathymetry dataset from four cruises along the mid atlantic ridge from 30 5 35 5os provides a benchmark for testing the two dimensional resolution and accuracy of the satellite measurements where their crosstrack spacing is near their widest the satellite gravity signal is coherent with bathymetry in this region down to wavelengths of 26 km t2 0 5 compared to 12 5 km for shipboard gravity residuals between the shipboard and satellite datasets have a roughly normal distribution the standard deviation of satellite gravity with respect to shipboard measurements is nearly 7 mgal in a region of 140 mgal total variation whereas the internal standard deviation at crossovers for gps navigated shipboard data is 1 8 mgal the differences between shipboard and satellite data are too large to use satellite gravity to determine crustal thickness variations within a typical ridge segment

age dependent porosity of young upper oceanic crust insights from seafloor gravity studies of recent volcanic eruptions 1 an october 1997 alvin program collected a series of seafloor gravity stations over the february 1996 volcanic eruption site on the northern gorda ridge 46 41 n 126 47 w these measurements yielded estimates of the density and porosity of mid ocean ridge crust in the first few decades following formation the estimated crustal density for this pillow eruption of 2270 260 kg m3 is very low but in good agreement with the estimated densities of other recent seafloor volcanic flows rock matrix densities measured from hand samples retrieved from the study area give a bulk porosity for this flow of 34 16 this data when included with similar estimates of upper crustal porosity argue for a rapid evolution of upper crustal porosities from very high values 30 for recent eruptions to much lower values 10 15 where the seafloor is older than 0 5 million years these variations of density and porosity are of a magnitude consistent with previously observed changes in seismic compressional wave velocity and permeability of ocean crust

continental collision tectonics evidence from the himalaya and the neighbouring regions abstract the evolution of the himalaya and the tectonics of neighbouring regions are generally believed to be due to continent to continent collision of the eurasian and the indian plates during the last decade many new data have been obtained and examined and a suite of models has been developed to explain the evolution of the himalaya and the neighbouring regions these models could broadly be classified into three categories the subdivision of the himalaya into five main structural units is now well accepted a tectogenetic diagram of the himalaya is briefly discussed ophiolites are reported to occur all along the indus tsangpo suture zone results of geochemical analysis of an ophiolite sequence from dras in kashmir himalaya are presented surface wave dispersion and attenuation investigations have revealed the crustal and upper mantle structure below the himalaya and tibet plateau the finding of low velocity layers is consistent with the concept that horizontal compression has been responsible for creating these weak zones and the inferred elevated temperatures in the vicinity of the moho discontinuity below tibet earthquake focal mechanism solutions and interpretation of landsat imagery indicate that convergence of the indian plate with eurasia continues and is accommodated partly by crustal shortening and partly by lateral mass movement at the northwest and the northeast margins of the indian plate palaeomagnetic data from the himalaya belt and central asia can be interpreted in support of molnar and tapponnier s model of continental collision

continuous near bottom gravity measurements made with a bgm 3 gravimeter in dsv alvin on the east pacific rise crest near 9 31 n and 9 50 n a bell bgm 3 gravimeter has been used to collect continuous underway near bottom 3 to 10 m altitude gravity measurements from the deep diving submersible dsv alvin during surveys on the east pacific rise epr crest near 9 31 n and 9 50 n closely spaced 20 to 30 m gravity measurements were made along transects up to 8 km long in both regions repeatability of measurements made at the same location on different dives is 0 3 mgal along track spatial resolution of anomalies is 130 160 m with the limiting factors being precision and sampling rate of the pressure gauge depth data used to calculate vertical accelerations of the submersible the average upper crustal density of the ridge crest determined from the relationship between depth and free water gravity anomalies varies greatly between 9 31 n and 9 50 n average upper crustal densities of 2410 kg m 3 for the 9 50 n area and 2690 kg m 3 for the 9 31 n area were calculated the different densities are not due to differing geometry of the layer 2a 2b boundary or a regional cross axis gravity gradient differences in porosity of the shallow crustal rocks or a difference in the proportion of low density extrusives to higher density dikes and sills within layer 2a in these two areas are the likely causes of the different upper crustal densities bouguer gravity anomalies near the epr axis are primarily small amplitude 0 5 2 mgal are a few hundred meters across and appear to be lineated parallel to the axis larger amplitude bouguer anomalies of up to 4 mgal were found at a few locations across the crestal plateau and are associated with pillow ridges composed of lavas which are clearly younger than the surrounding seafloor these ridges have distinct chemical compositions compared to lavas from the axial summit collapse trough asct at the same latitude probable sources of the 0 5 to 2 mgal anomalies observed on the summit plateau include areas of collapsed and fissured terrain and dike swarms feeding melt through layer 2a to the surface a grid survey of the ridge axis near 9 50 n shows bouguer anomalies lineated along the axis suggesting that dike swarms do contribute to the observed bouguer anomalies the along axis continuity of the gravity anomalies is disrupted at a 75 m offset of the asct suggesting that shallow feeders of lava to the surface may be segmented on a finer scale than the deeper crustal magmatic system this initial study confirms the ability to conduct high resolution near bottom continuous gravity measurements from alvin it also provides important information on how the shallow crustal structure of a fast spreading mid ocean ridge develops and how it varies with the surface morphology

thickness of a submarine lava flow determined from near bottom magnetic field mapping by autonomous underwater vehicle magnetic field surveys obtained near the seafloor can map the boundaries of recent volcanic eruptions and can provide thickness estimates of these lava flow units independent of bathymetry differencing methods magnetic thickness estimation requires knowledge of the intensity of magnetization of the new lava and surrounding terrain but this can be satisfactorily obtained by representative sampling of the various volcanic units while bathymetry differencing requires pre existing data to assess the thickness of new lava eruptions magnetic surveys can be obtained after an eruption has occurred in this study near bottom magnetic surveys were obtained using an autonomous underwater vehicle auv which operates without a tether or human intervention auv technology offers rapid deployment and an efficient surveying approach for remotely mapping recent lava eruption sites on the seafloor

thermal isostasy response of a moving lithosphere to a distributed heat source spreading ridges and hot spot swells are identified by their high surface heat flow shallow seafloor and high geopotential to understand these and other thermotectonic features the oceanic lithosphere is modeled as a thermomechanical boundary layer moving through a three dimensional time independent heat source the heat source mimics the heat advection associated with a spreading ridge or hot spot without introducing the nonlinearities of these flow processes the fourier transforms of three green s functions response functions which relate the three observable fields to their common heat source are determined analytically each of these reponse functions is highly anisotropic because the lithosphere is moving with respect to the source however the ratio of the gravity response function to the topography response function i e gravity topography transfer function is nearly isotropic and has a maximum lying between the flexural wavelength and 2 times the thickness of the thermal boundary layer the response functions are most useful for determining the surface heat flow seafloor topography and geopotential for complex lithospheric thermal structures in practice these three observables are calculated by multiplying the fourier transform of the heat source by the appropriate response function and inverse transforming the products almost any time independent thermotectonic feature can be modeled using this technique included in this report are examples of spreading ridges and thermal swells although more complex geometries such as ridges offset by transform faults and rrr type triple junctions can also be modeled because forward modeling is both linear and computationally simple the inverse of this technique could be used to infer some basic characteristics of the heat source directly from the observed fields

direct measurements of oceanic crustal density at the northern juan de fuca ridge density and porosity of oceanic crust are critically important to understanding the circulation of hydrothermal fluid at mid ocean ridges to determine these parameters directly we completed a fixed on bottom gravity survey on the endeavour segment of the juan de fuca ridge which is a well studied spreading center that is both hydrothermally and tectonically active a bell aerospace gravity meter mounted within the submersible alvin was used to make gravity measurements from the floor of the axial valley to the summit of the west axial ridge analysis of these data gave a density of 2240 50 kg m 3 for the upper 130 m of crust comparison of the outcrop density with that of recovered rock samples resulted in a calculated porosity of 38 4 forward modeling of the seafloor gravity data indicated that both the valley floor and bounding walls have unusually low density crust that is flanked by higher density crust

density and porosity of the upper oceanic crust from seafloor gravity measurements the exposure of 1300 meters of upper oceanic crust at the blanco fracture zone allows near bottom gravity measurements to determine the in situ density of the seafloor as a function of depth gravity measurements along the north wall of the blanco depression indicate an outcrop density of 2530 kg m for the upper 800 meters of crust and a calculated porosity of 23 the lower 500 meters of crust 800 to 1300 meters below the sea floor has a measured density of 2710 130 kg m and a porosity of 14 these data indicate that most of the extrusive volcanic oceanic crust is highly porous and can act as an aquifer and large scale reservoir for hydrothermal fluids these direct crustal density measurements also support previous interpretations that low seismic velocities observed in layer 2 are due to the high porosity of the upper extrusive section

role of membrane stresses in the support of planetary topography the principal purpose of this paper is to examine whether membrane stresses can support topographic loads on planetary elastic lithospheres it is found that the ability of a spherical shell to support loads through membrane stresses is determined by the nondimensional parameter ed gr2 where d is the thickness of the elastic lithosphere is the density difference between the mantle and crust and r is the radius of the planetary body when this parameter is large membrane stresses can fully support topographic loads without flexure and when it is small the influence of membrane stresses can be neglected solutions of the equation governing the behavior of a spherical shell are obtained for a topographic load expressed in terms of spherical harmonics spherical harmonic expansions of the measured gravity and topography for mars and the moon are compared with the theory it is found that for mars the support of topography is primarily due to membrane stresses for n 8 and for the moon for n 17 for mars the data for 4 n 7 give 0 5 for the moon the data have considerable scatter that is attributed to the mascons but generally correlate with 0 5 if bending stresses are neglected the governing equation for the deflection of the spherical shell is legendre s equation a general solution is obtained for an axisymmetric load this solution is applied to the tharsis region on mars the 60 65 compensation of this region requires that 0 6 the well defined fracture pattern surrounding the tharsis region is attributed to tensional membrane stresses

seafloor gravity evidence for hydrothermal alteration of the sediments in middle valley juan de fuca ridge abstract gravity data collected at two different levels sea surface and seafloor are compared and interpreted to characterize the effect of hydrothermal circulation on the sediment density in middle valley a deeply sedimented spreading center on the juan de fuca ridge the sea surface gravity data constrain density variations at depth beneath the seafloor whereas sea bottom measurements are more sensitive to shallow sources at least two different types of hydrothermal signatures in the sediments can be distinguished from the gravity data short wavelength anomalies associated with sulfide deposits and broader anomalies associated with areas of lithified sediments in middle valley three distinct gravity anomalies were identified a an anomaly over a sulfide mound south of bent hill shows that the sulfide body extends to depths of 120 to 180 m and has been fed by several near surface conduits b an anomaly at the base of the fault bordering the valley to the east is interpreted as a regional anomaly combined with the local effect of lithified sediments and possibly sulfide deposits c an anomaly paralleling an intra valley fault that limits the deepest part of the graben is interpreted as indicating lithification of the upper sediment layer a high heat flow anomaly is located 1 to 2 km east of this fault suggesting that sediment lithification occurred in a wide band above the fault and eastward to the current high heat flow area due to the progressive migration of the hot fluid circulation

free air gravity over the hoggar massif northwest africa evidence for alteration of the lithosphere abstract analysis of the free air gravity field over the broadly elevated hoggar region in northwest africa suggests that the uplift is similar in origin to the midplate hotspot swells observed on the seafloor the hoggar massif is a dome of precambrian basement approximately 1 km high 1000 km wide and capped by late tertiary alkali basalt volcanos gravity measurements reveal a broad free air high with maximum amplitude of approximately 40 mgal which is coincident with the areal extent of the hoggar using the reasonable assumption that the hoggar is in local isostatic balance calculations indicate that the amplitude of the gravity anomaly is best explained if the isostatic root of the elevated area is 60 km below ground surface this root depth is similar to that observed for both the hawaiian swell in the pacific and the bermuda rise in the atlantic and is shallower than the expected base of the lithosphere the gravity data suggest that the lithosphere beneath the hoggar has been reduced in density perhaps by reheating as inferred for oceanic swells

evaluation of the bgm 3 sea gravity meter system onboard r v conrad the first bell aerospace bgm 3 marine gravity meter system available for academic use was installed on r v robert d conrad in february 1984 the bgm 3 system consists of a forced feedback accelerometer mounted on a gyrostabilized platform its sensor requiring no cross coupling correction is a significant improvement over existing beam and spring type sea gravimeters such as the gss 2 a gravity survey over the wallops island test range together with the results of subsequent cruises allow evaluation of the precision accuracy and capabilities of the new system over the test range the bgm 3 data were compared directly to data obtained by a gss 2 meter onboard r v conrad the rms discrepancy between free air gravity anomaly values at intersecting ship tracks of r v conrad was 0 38 mgal for bgm 3 compared to 1 60 mgal for the gss 2 moreover bgm 3 s platform recovered from abrupt changes in ship s heading more rapidly than did the platform of gss 2 the principal factor limiting the accuracy of se

the density structure associated with oceanic crustal rifting at the hess deep a seafloor and sea surface gravity study abstract seafloor and sea surface gravity measurements provide new constraints on the lithospheric structure of the hess deep at the triple junction between the pacific cocos and nazca plates as the cocos nazca ridge propagates westward breaking into young oceanic crust created at the east pacific rise rocks attributed to the base of the crust or to the upper mantle are exposed on the seafloor at hess deep the tip of the propagator seafloor gravity data were collected along two north south profiles the western profile runs through the deepest part of the hess deep valley and the eastern profile runs over the tip of the cocos nazca neovolcanic ridge shipboard gravity track lines provide coverage of the rift valley from the northern to the southern rim bouguer gravity anomalies are negative over the deepest part of the basin and positive north of it at the intra rift ridge where gabbros and hartzburgites have been drilled seafloor and sea surface gravity data are modeled together in a 2d stochastic inversion and a 3d ideal body inversion when interpreted in the light of geologic data gravity results favor an asymmetric emplacement mode for the deep origin rocks the alteration of the mantle material playing a key role in the gravity signal in addition new magma is emplaced from the east forming a neovolcanic ridge over strongly serpentinized peridotite the density distribution shows that positive buoyancy cannot be the driving force for magma ascent through the upper crust

crustal structure of the mid atlantic ridge south of the kane fracture zone from seafloor and sea surface gravity data seafloor and sea surface gravity data are inverted together to construct a model for the near axis crustal structure of a slow spreading ridge the seafloor data set offers two main advantages it allows us to recover shorter wavelengths signal and to constrain the value of a potential field at two different levels the model we propose here would not have been derived from sea surface data alone it is based on a dense sea surface gravity coverage and on 121 sea bottom gravity measurements collected in the mid atlantic ridge at kane mark area during the hydrosnake 1988 and gravinaute 1993 cruises the primary goal of the seafloor surveys was to test for the presence of a magma reservoir beneath the axial neovolcanic ridge first a forward two dimensional 2 d model of the crustal structure across the axis is fit to observed gravity anomalies using constraints from geological and structural observations bouguer anomalies computed from sea bottom measurements and downward continuation of sea surface measurements both constrain the forward modeling this forward model is the starting point of a 2 d monte carlo inversion of seafloor and sea surface data in addition to the crustal thickness variations along axis our data document the amplitude variations of the crustal thickness and or its density in the across axis direction the model resulting from our inversion exhibits several features of the crustal structure in the mark area 1 the presence of a low density ap 300 50 kg m 3 body beneath the neovolcanic ridge is suggested and could correspond to a magma chamber or more probably to a highly hydrothermally fissured zone 2 both long and short wavelength gravity signals exhibit a difference between the western and eastern sides of the axial domain the mean value and the amplitude of bouguer anomalies are higher on the western part this difference suggests that axial processes in this area are very asymmetric 3 abyssal hills are not associated with a single gravity signature for instance on the west side of the axis one of the explored hills has no bouguer anomaly and is interpreted as a neovolcanic ridge whereas the others are associated with a shifted bouguer anomaly high and are interpreted as having thinner magmatic crust 4 the last feature of the crustal fabric we document here is the asymmetric emplacement of some deep rocks outcrops in the mark area we find that pink hill a topographic high where serpentinized peridotites are outcropping is much more serpentinized on its east flank toward the axial valley than on its west flank alteration occurring mainly by fluid circulation through faulted zones the asymmetric serpentinization suggests that deep origin rocks have outcropped by means of a main fault zone and are not emplaced by diapirism

structure of oceanic core complexes constraints from seafloor gravity measurements made at the atlantis massif 1 using the dsv alvin the relative seafloor gravimeter rovdog was deployed at 18 sites on the atlantis massif located at the ridge transform intersection of the mid atlantic ridge and the atlantis transform fault near 30 n 42 w these data along with previously collected shipboard gravity and bathymetry provide constraints on the density structure of this oceanic core complex a series of quasi 3 d forward models suggests that symmetric east and west dipping density interfaces bound the core of the massif with dip angles of 16 24 in the east and 16 28 in the west creating a wedge with a density of 3150 3250 kg m3 the dip angle in the east is steeper than that of the surface slope suggesting that the detachment fault surface does not coincide with the density boundary the resulting low density layer is interpreted as a zone of serpentinization

porosity of very young oceanic crust from sea floor gravity measurements porosity and density are two of the most important but under determined parameters in studies of igneous oceanic crust lacking quantitative measurements of crustal densities but with plausible expectations of substantial variation with age previous investigators have used estimates that have varied widely to address this problem more rigorously a recent alvin program obtained 133 on bottom gravity stations to calculate crustal densities for three recent volcanic eruptions on the juan de fuca ridge in the summer of 1995 a bell aerospace bgm 3 gravity meter was mounted in alvin and made fixed station gravity measurements on 11 transect lines from three distinct volcanic environments including 1 the recent coaxial segment volcanic eruption that occurred in 1993 2 two nearby flows with less well determined ages but dated post 1981 and 3 several older adjacent areas on the juan de fuca axial ridge densities for the recent eruptive units ranged between 2280 and 2450 kg m while the upper 50 meters of the older unfractured surrounding flows were approximately 2800 kg m laboratory grain densities measured from rocks collected in the study area allow crustal porosities to be calculated which are anomalously high for the recent flows in comparison to the surrounding axial ridge porosities for the three youngest flows range between 29 36 while the adjacent unfractured regions of the surrounding axial region had porosities near 10 two localized regions of intense surficial crustal fracturing in older crust located just south of the 1993 eruption site also had high porosities of 26 28 this high variability indicates that the evolution of crustal porosities from young to older oceanic crust is not related only to the collapse and infilling of macroscopic voids but that tectonic activity can significantly modify the porosity of older crustal structures

upper crustal densities derived from sea floor gravity measurements northern juan de fuca ridge a transect of sea floor gravity stations has been analyzed to determine upper crustal densities on the endeavour segment of the northern juan de fuca ridge data were obtained using alvin along a corridor perpendicular to the axis of spreading over crustal ages from 0 to 800 000 years calculated elevation factors from the gravity data show an abrupt increase in density with age distance for the upper 200 m of crust this density change is interpreted as a systematic reduction in bulk porosity of the upper crustal section from 23 for the axial ridge to 10 for the off axis flanking ridges the porosity decrease is attributed to the collapse and filling of large scale voids as the abyssal hills move out of the crustal formation zone forward modeling of a plausible density structure for the near axis region agrees with the observed anomaly data only if the model includes narrow along strike low density regions adjacent to both inner and outer flanks of the abyssal hills the required low density zones could be regions of systematic upper crustal fracturing and faulting that were mapped by submersible observers and side scan sonar images and whose presence was suggested by the distribution of heat flow data in the same area

dynamics and navigation of autonomous underwater vehicles for submarine gravity surveying we investigated the effect of autonomous underwater vehicle auv dynamics and navigation on underway submarine gravimetry our research was motivated by the need to obtain spatially dense marine gravity measurements close to the source of subkilometer scale geologic features in the shallow oceanic crust such measurements have been previously obtained for instance with piloted submarines and towed sleds however the high cost and in the case of on bottom measurements poor spatial sampling preclude routine acquisition of these measurements continuous underway gravity surveys with auvs is a compelling cost effective option but this method requires separating the auv accelerations from the measured gravity we show that auvs with a large distance between the center of buoyancy and the center of gravity have lower vertical accelerations than torpedo shaped auvs and consequentially are better suited for underway gravity surveys furthermore state estimators which combine sensor measurements and models of the vehicle s motion provide superior estimates of the vehicle s vertical accelerations than methods used in previous underway submarine gravity surveys we simulated the use of these navigation methods in detecting dike swarms at the east pacific rise analysis showed that we can shorten filters used in reducing gravity data and consequentially provide improved measurements of the free water anomaly with a minimal detectable spatial wavelength approximately 65 lower than previously reported results