geological modelling from field data and geological knowledge part ii modelling validation using gravity and magnetic data inversion the analysis of multiple data sets is required to select a realistic 3d geological model among an infinite number of possibilities an inverse method that aims at describing the 3d geometry of geological objects is presented the method takes into account the geology and the physical properties of rocks while respecting the topological properties of an a priori model the a priori model is built from the geological data set and its geometry is largely dependent upon assumptions about inaccessible geology at depth this method referred to as total litho inversion is a generalized 3d inversion that results in quantifying the lithology and the distribution of rock property in a probabilistic way its application is demonstrated through i a simple synthetic case and ii the relative distribution characterization of granites and diorites in an orogenic domain

onshore offshore structure and hydrocarbon potential of the south yellow sea abstract recent high resolution airborne gravity data taken over the south yellow sea and its western onshore offshore transition zone combined with ground gravity data taken over the onshore area subei basin china show that the south basin of the south yellow sea and the subei basin correspond to the same gravity low anomaly magnetic data also support our interpretations both areas have similar strata structures and hydrocarbon potential and form a large cenozoic terrestrial sedimentary basin controlled by the tanlu fault cenozoic terrestrial strata are well developed in the south basin of the south yellow sea and thick meso paleozoic marine strata are preserved in the central uplift area future hydrocarbon exploration in the south yellow sea should focus on the cenozoic continental sequence especially the paleogene in the south basin as well as the meso paleozoic marine rocks in the central uplift area the western part of the middle depression and middle and western parts of the north depression in the south basin of the south yellow sea have the greatest potential for hydrocarbon accumulation

incremental emplacement of the late jurassic midcrustal lopolith like qitianling pluton south china revealed by ams and bouguer gravity data modes of magma emplacement may provide key information on geodynamic tectonic settings of magmatism when the threshold of magma volume and duration of emplacement process is crossed brittle ductile crustal deformation behavior and volumes and rates of magma transfer in the crust the majority of upper crustal granitic intrusions are incrementally emplaced as laccoliths or sills while midcrustal or deeper level granitic intrusions appear to be predominantly emplaced as sills or lopoliths but relatively few examples have been studied we present a case study on the jurassic qitianling pluton from south china which was constructed in the upper part of the middle crust at a depth of 10 15 km to characterize its emplacement mechanism and evolution we have combined field and microscopic observations anisotropy of magnetic susceptibility measurements and bouguer gravity anomaly modeling the main findings are 1 the generally undeformed granite country rock contacts with a narrow thermal aureole 2 irregularly wavy and gradational contacts between the main granite facies 3 dominant subhorizontal magnetic fabrics in the granites with a scattered magnetic foliation 4 rare occurrence of dykes and 5 a subcircular surface and lopolith like pluton geometry accordingly we suggest that the qitianling pluton forms an 25 km wide and 5 km thick lopolith that was incrementally emplaced during a period of tectonic quiescence most likely by under accretion of multiple magma sheets deflected from magma pulses that ascended through pipe like channels the mineralogical and compositional variation of the pluton may be the result of in situ fractionation fractionation at deeper crustal level or variation in source derived melt compositions

on the use of different methods for estimating magnetic depth magnetic depth estimation plays an important role in magnetic interpretation a complete quantitative interpretation of potential fields data aims to estimate three types of information about sources of geologic interest the depth the dimension and the contrast in the relevant physical property such an interpretation suffers from inherent ambiguity it is impossible to obtain all three types of information simultaneously without other a priori information in many applications we are often interested in depth more than either dimension or physical property contrast thus different quick methods have been developed over half a century to estimate the magnetic depth these methods work for simplified source geometries dimensions and are independent of the susceptibility contrast the depths estimated by some method can be used as the final quantitative solution in some ideal situations i e the anomaly is well isolated and the noise is insignificant or well removed it is more often that estimated depths provide a good starting point for a genuine structural interpretation e g an interactive modeling or a constrained inversion in petroleum exploration for example the structural surface interpreted from magnetic depth estimates is often the best available approximation to the true crystalline i e metamorphic igneous basement configuration basement depth or equivalently sedimentary thickness is a primary exploration risk parameter estimates of basement depth are directly applicable to basin modeling e g source rock volume estimation and thermal maturity applications e g source rock burial depth basement structure inferred from magnetic depth estimates provides insight into the evolution of more recent sedimentary features e g sub minibasin compartmentalization salt structure distribution kinesis localization of reservoir bearing structures in areas where the inherited basement fabric architecture has affected either continuously or episodically basin evolution and development examples of the latter are the prolific passive rifted margins of the southern atlantic ocean a basin s plumbing often exploits faults and fractures within the sedimentary

integrated 3d geophysical and geological modelling of the hercynian suture zone in the champtoceaux area south brittany france this paper combines geological knowledge and geophysical imagery at the crustal scale to model the 3d geometry of a segment of the hercynian suture zone of western europe in the champtoceaux area brittany france the champtoceaux complex consists of a stack of metamorphic nappes of gneisses and micaschists with eclogite bearing units the exhumation of the complex during early carboniferous times was accompanied by deformation during regional dextral strike slip associated with a major hercynian shear zone the south armorican shear zone sasz dextral shearing produced a km scale antiformal structure with a steeply dipping axial plane and a steeply eastward plunging axis armor 2 deep seismic profile shows that the regional structure was cut by a set of faults with northward thrusting components based on the seismic constraint direct 2d crustal scale modelling was performed throughout the champtoceaux fold on seven radial gravity profiles also using geological data and density measurements from field and drill hole samples the 3d integration of the cross sections the digitised geological map and the structural information foliation dips insure the geometrical and topological consistency of all sources of data the 2d information is interpolated to the whole 3d space using a geostatistical analysis finally the 3d gravity contribution of the resulting model is computed taking into account densities for each modelled geological body and compared to the bouguer anomaly the final 3d model is thus compatible with the seismic and gravity data as well as with geological data main geological results derived from the modelling are i the overall 3d geometry of the south dipping thrust system interpreted on the seismic profile emphasises northward thrusting and folding of the champtoceaux complex which was coeval with strike slip along the south armorican shear zone ii the gravity modelling suggests the presence of a relatively dense body below the champtoceaux complex that could be interpreted as a result of relative uplift of midcrustal material during thrusting along the e w trending wrench thrust system iii the northern limb of the champtoceaux anticline is a relatively shallow feature and iv vigneux synkinematic granitic body is a laccolith sheared and rooted along the southern branch of the sasz and spreads away from the strike slip zone within weak country rocks d 2004 elsevier b v all rights reserved

continuous wavelet transform theoretical aspects and application to aeromagnetic data at the huanghua depression dagang oilfield china we use the continuous wavelet transform based on complex morlet wavelets which has been developed to estimate the source distribution of potential fields for magnetic anomalies of adjacent sources they always superimpose upon each other in space and wavenumber making the identification of magnetic sources problematic therefore a scale normalization factor a n is introduced on the wavelet coefficients to improve resolution in the scalogram by theoretical modelling we set up an approximate linear relationship between the pseudo wavenumber and source depth the influences of background field random noise and magnetization inclination on the continuous wavelet transform of magnetic anomalies are also discussed and compared with the short time fourier transform results synthetic examples indicate that the regional trend has little effect on our method while the influence of random noise is mainly imposed on shallower sources with higher wavenumbers the source horizontal position will be affected by the change of magnetization direction whereas the source depth remains unchanged after discussing the performance of our method by showing the results of various synthetic tests we use this method on the aeromagnetic data of the huanghua depression in central china to define the distribution of volcanic rocks the spectrum slices in different scales are used to determine horizontal positions of volcanic rocks and their source depths are estimated from the modulus maxima of complex coefficients which is in good accordance with drilling results

subaqueous calderas in the archean abitibi greenstone belt an overview and new ideas abstract the 300 700 km abitibi greenstone belt in canada contains numerous world class archean volcanogenic massive sulfide vms deposits yet documentation of archean subaqueous calderas hosting such deposits is lacking the modern sunrise deposit in the myojin knoll caldera of the izu bonin arc shows that submarine calderas are first order sites for vms the hunter mine and normetal calderas as well as the blake river megacaldera complex are reviewed with respect to geometry regional geology physical volcanology dyke emplacement and hydrothermal carbonate alteration these subaqueous calderas are placed into the geodynamic context of the abitibi greenstone belt the abitibi belt displays a complex history of oceanic arc formation evolution collision and fragmentation with numerous types of volcanic edifices and sedimentary basins controlled by oblique subduction and mantle plumes analogue models yielding new insights into the sequence of caldera forming events and their synvolcanic fault patterns are applied to explain archean examples underpressure experiments develop a consistent sequence of fault patterns that compare favourably with natural caldera events during subsidence two sets of faults propagate from the magma chamber margin 1 early reverse outward dipping faults and 2 normal inward dipping faults the largest massive sulfide deposits form preferentially at the caldera margin based on experimental studies and detailed facies mapping the hunter mine caldera is of the segmented piecemeal variety and the normetal caldera is akin to a piston structure the blake river megacaldera contains 1 the e w trending misema 2 the nw trending new senator and 3 the ne trending noranda calderas with the latter two being nested graben type calderas consistent with oblique archean subduction the poorly documented hydrothermal carbonate alteration is discussed and a new exploration model for calderas is presented the alteration study was evaluated at three levels microprobe analysis geochemical staining of drill core and whole rock analyses there were two principal stages of alteration related to vms formation an early silicification phase developed either at depth and or at the seawater interface whereby silicification was bedding parallel in fine grained volcaniclastic deposits near the water surface a pervasive and extensive semi conformable carbonate alteration zone with a discordant focused root zone along faults evolved subsequently three distinct carbonate pairings were observed 1 proximal siderite sideroplesite fe ankerite next to the vms deposit 2 an intermediate ankerite fe dolomite zone and 3 a distal calcite dolomite zone the carbonate phase is associated with the mineralizing event transitions between carbonate species are subtle and changes are indicated by mineral assemblage overlap the silica carbonate hydrothermal alteration pattern is a replacement product and is far more widespread than has been documented this alteration can be easily confused with primary carbonates and chemical precipitation deposits e g banded iron formation if detailed volcano sedimentary facies mapping with alteration has not been conducted and if outcrop is insufficient

ice cover landscape setting and geological framework of lake vostok east antarctica abstract lake vostok located beneath more than 4 km of ice in the middle of east antarctica is a unique subglacial habitat and may contain microorganisms with distinct adaptations to such an extreme environment melting and freezing at the base of the ice sheet which slowly flows across the lake controls the flux of water biota and sediment particles through the lake the influx of thermal energy however is limited to contributions from below thus the geological origin of lake vostok is a critical boundary condition for the subglacial ecosystem we present the first comprehensive maps of ice surface ice thickness and subglacial topography around lake vostok the ice flow across the lake and the landscape setting are closely linked to the geological origin of lake vostok our data show that lake vostok is located along a major geological boundary magnetic and gravity data are distinct east and west of the lake as is the roughness of the subglacial topography the physiographic setting of the lake has important consequences for the ice flow and thus the melting and freezing pattern and the lake s circulation lake vostok is a tectonically controlled subglacial lake the tectonic processes provided the space for a unique habitat and recent minor tectonic activity could have the potential to introduce small but significant amounts of thermal energy into the lake

emplacement in an extensional setting of the mont lozere borne granitic complex se france inferred from comprehensive ams structural and gravity studies the emplacement mode and setting of the late hercynian mont lozere borne granitic complex french massif central which consists of several plutons is investigated structural and anisotropy of magnetic susceptibility ams studies have been carried out to characterize the internal fabrics of the granitic plutons throughout the pont de montvert borne pluton an e w trending magnetic lineation is well developed in the host rock and the thermal aureole a conspicuous e w trending lineation is interpreted as an evidence of a late orogenic extensional event to the east of the pluton the ams fabric is characterized by values of anisotropy degree p around 4 5 with a prolate ellipsoid and subsolidus structures whereas to the west the p parameter is weaker with an oblate ellipsoid and purely magmatic microstructures a gravity investigation allows determination of the 3d shape of the pluton the western part of the granitic complex is thicker than the eastern one and is interpreted as the feeder zone this suggests an eastward spreading of the magma the consistency between regional stretching and directions of ams lineations in the pluton and the shape of the complex deduced from gravity strongly argues that the emplacement mode of the complex was influenced by the regional extensional tectonic setting during the collapse of the hercynian belt

lithologic tomography an application to geophysical data from the cadomian belt of northern brittany france abstract a probabilistic description of subsurface lithologic structures can be established by inverting multidisciplinary geophysical data constrained by geological and geostatistical priors the methodology is based on the joint modelling of several media properties and on a statistical description of the relationships between them the information provided by the geophysical data and the geological and geostatistical priors is represented by probability density functions pdf that are combined into a posterior pdf composed by 1 a prior pdf in the space of the primary lithologic model parameters 2 a pdf of the secondary physical model parameters conditional to the primary model parameters and 3 a joint likelihood function that is the product of the independent likelihood functions for each observed geophysical field applying a markov chain sampling method enables a large sample of joint models to be generated from the posterior pdf the true configuration of the media is then determined from the representation of models pulled from the chain and the elaboration of statistics from the large sample of posterior joint models the method was used to invert gravity and magnetic data jointly characterising the mass density field the magnetic susceptibility field and the lithotype field along two 2 d sections of the geological units in the cadomian belt of northern brittany besides generating 10 6 joint models consistent with the observations and priors some features of the joint models and the statistical tomographic images provided additional insights to the geologic configuration of the area for example the main cadomian thrust shows an irregular geometry that could have resulted from the belt emplacement and or from variscan tectonism and the hercynian granitic intrusion shows a deep subsurface continuation the cosimulation of magnetic susceptibility and mass density inside each lithologic region was performed according to a multivariate gaussian model or a mixed multivariate gaussian functions model that was developed specially to describe multimodal distributed properties

a multidisciplinary study of a syntectonic pluton close to a major lithospheric scale fault relationships between the montmarault granitic massif and the sillon houiller fault in the variscan french massif central 2 gravity aeromagnetic investigations and 3 d geologic modeling 1 new gravity and aeromagnetic investigations have been carried out to understand the emplacement mechanisms of a granitic pluton and the relationships with a nearby lithospheric scale fault this paper concerns the second part of a methodological multidisciplinary study and complements previous geochronologic and anisotropy of magnetic susceptibility ams studies on the same pluton in the northern part of the variscan french massif central fmc the montmarault massif crops out along the sillon houiller fault shf bouguer and aeromagnetic anomaly maps imply thickening of the pluton along the shf and suggest laccolitic spreading northwestward on the basis of petrophysical measurements direct two dimensional 2 d joint gravity and magnetic modeling has been performed along 10 cross sections in order to quantitatively constrain the 3 d pluton geometry and its relationships with surrounding geologic units these geophysical cross sections new structural information field and ams measurements and petrophysical data have been integrated into a regional 3 d geological and geophysical model altogether the results obtained from geochronology petromagnetic fabrics part 1 gravity and aeromagnetic investigations as well as 3 d modeling part 2 demonstrate that the montmarault pluton was emplaced during the namurian period along the shf our results further show that at that time in response to a nw se regional extension if the shf existed it behaved as a normal fault mylonites attesting for synmagmatic normal motion on the northeastern part of the montmarault pluton strengthen this tectonic scenario during the late carboniferous the fmc experienced ne sw extension along the shf by 80 km of brittle left lateral wrench offset this second tectonic event is well recorded in the stephanian coal basins which were formed along nw se listric brittle faults and constrain the present day shape of the montmarault pluton

aeromagnetic constraints on the geostructural interpretation of the southern part of the sardinian rift italy abstract aeromagnetic data are interpreted to better understand the relationship between the southern part of the oligo miocene rift and the plio quaternary campidano graben on sardinia particular reference is made to the geostructural location of the campidano graben the aeromagnetic data have been interpreted by applying three different methods to the original survey data spectral analysis analytical signal and werner deconvolution the results of this study give first order estimates of the depth of the magnetic basement and position of major dykes in the study area and on the efficiency of the inversion methods used

magnetic basement gravity guided magnetic source depth analysis and interpretation we present a new integrated approach to the interpretation of magnetic basement that is based on recognition of characteristic patterns in distributions and alignments of magnetic source depth solutions above and below the surface of magnetic basement this approach integrates a quantitative analysis of depth solutions obtained by 2d werner deconvolution of the magnetic data with a qualitative evaluation of the bouguer gravity anomalies the crystalline metamorphic basement and sedimentary cover have different origins tectonic histories lithologies and magnetic properties these differences result in different geometries of magnetic sources associated with faults fracture zones igneous intrusions erosional truncations subcrop edges and other structural discontinuities properly tuned 2d werner deconvolution is able to resolve the intra sedimentary and intra basement magnetic source geometries into distinctly different distributions and alignments of calculated depth solutions an empirical set of criteria basement indicators was developed for identification and correlation of the basement surface the ambiguity of basement correlation with limited or non existent well control which is common for onshore frontier and offshore explorations can be reduced by incorporating the bouguer gravity data into the process of correlation

thermal evolution of the intracratonic paris basin insights from 3d basin modelling abstract the thermal evolution of the paris basin pb has been widely studied using 1d 2d and more rarely 3d thermal models it is well documented that the pb experienced higher temperatures in the past compared to what is currently observed however a quantitative analysis of the main processes and parameters that affect the temperature distribution at the basin scale and over time is still not available in this study through basin modeling which accounts for the main processes of the thermal evolution of sedimentary basins we analyze and quantify the role of the different geological mechanisms by discriminating the causes of abnormal temperatures during the late mesozoic this is done with a 3d basin model built from base moho to present day topography using the temisflow basin modelling software the model includes thermal processes within an evolving upper crust defined by three main structural domains each crustal sector presents radiogenic heat production conductivity and thickness values which are used as input parameters to reproduce the paleo and present day basal heat flow variations observed in the basin the model calculates heat flow through time in both crust and sedimentary column where the crust is coupled with the geological evolution of the basin this approach allows estimating the eroded thickness during the main tertiary uplift event and therefore the maximum temperature in the late cretaceous the model is constrained by different types of paleo thermo chronometers and by 52 wells that are regionally distributed over the entire basin resulting in a new regional thermal history of the pb the amount of missing section in the cretaceous chalk which mainly affected the eastern part of the basin is increased by up to 500m compared with previous studies and constitutes the key controlling factor of the temperature evolution this new regional thermal history of the paris basin may be important for further analysis of the hc generation from the lower jurassic toarcian source rock and bring new insights into the geothermal potential of the basin

weighted euler deconvolution of gravity data euler deconvolution is used for rapid interpretation of magnetic and gravity data it is particularly good at delineating contacts and rapid depth estimation the quality of the depth estimation depends mostly on the choice of the proper structural index and adequate sampling of the data the structural index is a function of the geometry of the causative bodies for gravity surveys station distribution is in general irregular and the gravity field is aliased this results in erroneous depth estimates by weighting the euler equations by an error function proportional to station accuracies and the interstation distance it is possible to reject solutions resulting from aliasing of the field and less accurate measurements the technique is demonstrated on bouguer anomaly data from the charlevoix region in eastern canada

flood disturbance recovery and inter flood incision on a large sand bed river abstract the lower 800 km of the missouri river has been incising for decades however the 2011 flood scoured more sediment than the previous 12 yr combined the river rebounded in the following two years re depositing over 70 of the sediment scoured during the 2011 flood historic evidence suggests that past missouri river floods may have followed similar scour rebound patterns this paper analyzes 16 channel surveys from 1987 to 2014 including 50 723 total cross sections to document the morphological history of the lower 800 km of the missouri river over the last thirty years including flood disturbance and long term trends the last thirty years on the missouri river included two basic morphologic regimes steady incision during low to moderate flow periods and rapid scour rebound responses to floods surveys collected during the 2011 flood demonstrate that the river scoured throughout the flood including the falling limb and did not begin a rebound phase until after the flood the analysis also identifies sediment sinks associated with each of these scour regimes flood scour and incision between floods that exceed the total volume eroded from the bed during those periods results suggest that floodplain deposition may induce the supply limitation that drives flood scour on this large sand bed river additionally aggregate mining removes substantially more sediment than the total river incision between floods and the reaches with maximum scour during the 1993 and 2011 floods correspond to the zones of aggregate mining

emplacement of dikes sills and crustal magma chambers at divergent plate boundaries abstract a model of dike emplacement at divergent plate boundaries is developed which predicts that below the uppermost 1 3 km of the oceanic crust the number and lengths of dikes in any particular swarm should increase but that the thickness should decrease with depth in the crust these predictions are supported by field observations it is proposed that rapid injections of dikes at divergent plate boundaries may temporarily alter the normal stress field in such a way that at a certain level in the crust subsequent dikes change into sills such sills may develop into crustal magma chambers either when many nearby sills combine into a large one or when an initial sill absorbs the magma of many dikes that enter it in a rapid succession while the sill is liquid for the fastest spreading ridges the initial thickness of a sill that is to develop into a crustal chamber need only attain 20 m whereas at the slowest spreading ridges the initial thickness must exceed 170 m because thin sills are much more likely to form than thick sills the probability of sills developing into crustal chambers increases with dike intrusion frequency hence with spreading rate furthermore the model predicts first that with increasing spreading rate the length of the chamber along the ridge axis should increase and second that the most likely site for magma chambers is at depths of 2 4 km both these predictions are supported by available data

mapping of a buried basement combining aeromagnetic gravity and petrophysical data the substratum of southwest paris basin france aeromagnetic and gravity data have proven to be among the most effective methods for mapping deeply buried basin basement interfaces however the data interpretation generally suffers from ambiguities due to the non uniqueness of the gravity and magnetic signatures here we tie the gravity and magnetic signatures with a petrophysical characterization of the lithologies outcropping around the french paris basin our methodology investigates the lithology and structure of its hidden variscan substratum at the junction between the armorican massif and massif central our approach is based on the combination of potential field data magnetic susceptibilities measured in the field density values of sample rocks and information documented in boreholes in order to propose a new interpretative geological map of the buried substratum of the paris basin the petrophysical description is combined with geophysical patterns of the substratum mapped through statistical unsupervised classification of suitably selected magnetic and gravity maps the first step of interpretation consists in extending the outcropping major structures below the meso cenozoic sedimentary cover of the paris basin the litho structural units in between these major structures are then interpreted separately the second step consists in assigning lithologies within each unit with respect to its magnetization and density as derived from the petrophysical compilation and mapping its extension under cover integrating punctual borehole information overall with a special emphasis on relating geophysical signatures and petrophysical characteristics of litho structural units this methodology permits a precise structural and lithological cartography of a segment of the buried variscan substratum in the southwestern part of the paris basin this approach reveals i the limited eastward extension of central brittany ii the eastward extension of the major cholet fault iii the emphasis on n150e n160e striking fault and their n30e conjugates controlling the opening of permo carboniferous basins and iv the eastward extension of the eo variscan suture

2d joint inversion of geophysical data using petrophysical clustering and facies deformation geologic expertise and petrophysical relationships can be brought together to provide prior information while inverting multiple geophysical data sets the merging of such information can result in more realistic solution in the distribution of the model parameters we have evaluated the geophysical inverse problem in terms of gaussian random fields with mean functions controlled by petrophysical relationships and covariance functions controlled by a prior geologic cross section including the definition of spatial boundaries for the geologic facies the petrophysical relationship problem is formulated as a regression problem upon each facies the inversion of the geophysical data is performed in a bayesian framework we have developed the usefulness of this strategy using a first synthetic case for which we have performed a joint inversion of gravity and galvanometric resistivity data with the stations located at the ground surface the joint inversion was used to recover the density and resistivity distributions of the subsurface in a second step we considered the possibility that the facies boundaries were deformable and their shapes were inverted as well we used the level set approach to perform such deformation preserving a priori topological properties of the facies throughout the inversion with the help of a priori facies petrophysical relationships and the topological characteristics of each facies we made a posteriori inference about multiple geophysical tomograms based on their corresponding geophysical data misfits we have applied this method to a second synthetic case showing that we can recover the heterogeneities inside the facies the mean values for the petrophysical properties and to some extent the facies boundaries using the 2d joint inversion of gravity and galvanometric resistivity data

fast mapping of magnetic basement depth structure and nature using aeromagnetic and gravity data combined methods and their application in the paris basin assessment of deep buried basin basement relationships using geophysical data is a challenge for the energy and mining industries as well as for geothermal or co2 storage purposes in deep environments few methods can provide geological information magnetic and gravity data remain among the most informative and cost effective methods here in order to derive fast first order information on the basement basin interface we propose a combination of existing and original approaches devoted to potential field data analysis namely we investigate the geometry i e depth and structure and the nature of a deep buried basement through a case study sw of the paris basin joint processing of new high resolution magnetic data and up to date gravity data provides an updated overview of the deep basin first the main structures of the magnetic basement are highlighted using euler deconvolution and are interpreted in a structural sketch map the new high resolution aeromagnetic map actually offers a continuous view of regional basement structures and reveals poorly known and complex deformation at the junction between major domains of the variscan collision belt second werner deconvolution and an ad hoc post processing analysis allow the extraction of a set of magnetic sources at or close to the basin basement interface interpolation of these sources together with the magnetic structural sketch provides a werner magnetic basement map displaying realistic 3d patterns and basement depths consistent with data available in deep petroleum boreholes the last step of processing was designed as a way to quickly combine gravity and magnetic information and to simply visualize first order petrophysical patterns of the basement lithology this is achieved through unsupervised classification of suitably selected gravity and magnetic maps and as compared to previous work provides a realistic and updated overview of the cartographic distribution of density magnetization of basement rocks altogether the three steps of processing proposed in this paper quickly provide relevant information on a deep buried basement in terms of structure geometry and nature through petrophysics notwithstanding limitations of the proposed procedure are raised in the case of the paris basin for instance this study does not provide proper information on pre mesozoic basins some of which have been sampled in deep boreholes

identification of sources of potential fields with the continuous wavelet transform complex wavelets and application to aeromagnetic profiles in french guiana a continuous wavelet technique has been recently introduced to analyze potential fields data first we summarize the theory which primarily consists of interpreting potential fields via the properties of the upward continued derivative field using complex wavelets to analyze magnetic data gives an inverse scheme to find the depth and homogeneity degree of local homogeneous sources and the inclination of their magnetization vector this is analytically applied on several local and extended synthetic magnetic sources the application to other potential fields is also discussed then profiles crossing dikes and faults are extracted from the recent high resolution aeromagnetic survey of french guiana and analyzed using complex one dimensional wavelets maps of estimated depth to sources and their magnetization inclination and homogeneity degree are proposed for a region between cayenne and kourou

characterization of geological boundaries using 1 d wavelet transform on gravity data theory and application to the himalayas we investigate the use of the continuous wavelet transform for gravity inversion the wavelet transform operator has recently been introduced in the domain of potential fields both as a filtering and a source analysis tool here we develop an inverse scheme in the wavelet domain designed to recover the geometric characteristics of density heterogeneities described by simple shaped sources the 1 d analyzing wavelet we use associates the upward continuation operator and linear combinations of derivatives of any order in the gravity case we first demonstrate how to localize causative sources using simple geometric constructions both the upper part of the source and the whole source can be studied when considering low or high altitudes respectively the homogeneity degree of the source is deduced without prior information and allows us to infer its shape introducing complex wavelets we derive analytically the scaling behavior of the wavelet coefficients for the dyke and the step sources the modulus term is used in an inversion procedure to recover the thickness of the source the phase term provides its dip this analysis is performed on gravity data we measured along a profile across the himalayas in nepal good agreement of our results with well documented thrusting structures demonstrates the applicability of the method to real data also deeper less constrained structures are characterized

euler deconvolution using differential similarity transformations of gravity or magnetic anomalies euler deconvolution of gravity and magnetic anomalies can be used to estimate the coordinates of a simple point source and the level of a constant background in the deconvolved field by testing a series of structural indices by using differential similarity transformations dsts joint estimations of the source coordinates the structural index and the coefficients of a linear background trend become possible dsts calculated from the derivatives of the interpreted field depend on the position of a chosen central point of similarity with respect to the source position on this basis techniques for euler deconvolution of anomalous fields are presented the theoretical formulations of dst and the deduced sets of equations for euler deconvolution are tested and verified on model and field examples of magnetic anomalies caused by one point and two point sources such as thin and thick dikes semi infinite and finite sills contacts steps faulted beds etc the possibility of an optimum estimation of the structural index can be used to define a second acceptance criterion along with the criterion of relative standard deviations the joint application of these two criteria ensures the selection of reliable results the suggested dst techniques are suitable for initial rapid estimations of the source type depth and plane location they may be applied to either profile or gridded data acquired from onecomponent measurements as well as from gradient or three component measurements additional data on the density or magnetization of the sources are not necessary

the historical development of the magnetic method in exploration the magnetic method perhaps the oldest of geophysical exploration techniques blossomed after the advent of airborne surveys in world war ii with improvements in instrumentation navigation and platform compensation it is now possible to map the entire crustal section at a variety of scales from strongly magnetic basement at regional scale to weakly magnetic sedimentary contacts at local scale methods of data filtering display and interpretation have also advanced especially with the availability of low cost high performance personal computers and color raster graphics the magnetic method is the primary exploration tool in the search for minerals in other arenas the magnetic method has evolved from its sole use for mapping basement structure to include a wide range of new applications such as locating intrasedimentary faults defining subtle lithologic contacts mapping salt domes in weakly magnetic sediments and better defining targets through 3d inversion these new applications have increased the method s utility in all realms of exploration in the search for minerals oil and gas geothermal resources and groundwater and for a variety of other purposes such as natural hazards assessment mapping impact structures and engineering and environmental studies

evolution of an active sea floor massive sulphide deposit hydrothermal circulation at oceanic spreading ridges causes sea water to penetrate to depths of 2 to 3 km in the oceanic crust where it is heated to 400 c before venting at spectacular black smokers these hydrothermal systems exert a strong influence on ocean chemistry1 yet their structure longevity and magnitude remain largely unresolved2 the active transatlantic geotraverse tag deposit at 26 n on the mid atlantic ridge is one of the largest oldest and most intensively studied of the massive sulphide mounds that accumulate beneath black smoker fields here we report ages of sulphides and anhydrites from the recently drilled3 tag substrate structures determined from 234u 230th systematics analysed by thermal ionization mass spectrometry the new precise ages combined with existing data4 5 show that the oldest material 11 000 to 37 000 years old forms a layer across the centre of the deposit with younger material 2 300 7 800 years old both above and below this stratigraphy confirms that much of the sulphide and anhydrite are precipitated within the mound by mixing of entrained sea water with hydrothermal fluid6 the age distribution is consistent with episodic activity of the hydrothermal system recurring at intervals of up to 2 000 years

stratigraphy and u pb zircon geochronology of kidd creek implications for the formation of giant volcanogenic massive sulphide deposits and the tectonic history of the abitibi greenstone belt within the context of a structural stratigraphic analysis of the giant kidd creek cu zn ag deposit precise u pb zircon ages have been obtained for five critical rock units associated with the deposit massive flow banded footwall rhyolite is dated at 2714 3 1 2 ma slightly younger than a previously published age of 2717 2 ma on nearby rhyolite breccia extrusive rhyolite in the immediate hanging wall to the main ore lenses is dated at 2711 5 1 2 ma leaving an interval of one to several million years for the formation of the deposit we consider the demonstrated longevity of the kidd creek hydrothermal system as a major control on its giant size age constraints together with the size of the deposit suggest a time averaged rate of base metal sulphide deposition of about 10 100 t year peak rates were likely one to possibly two orders of magnitude above time averaged rates hence 100 1000 t year or higher detrital zircon ages on greywackes in the structural footwall to the deposit support structu