amplitude variation with offset and prestack waveform inversion a direct comparison using a real data example from the rock springs uplift wyoming usa abstractrecent advances in seismic data acquisition and processing allow routine extraction of offset angle dependent reflection amplitudes from prestack seismic data for quantifying subsurface lithologic and fluid properties amplitude variation with offset avo inversion is the most commonly used practice for such quantification although quite successful avo has a few shortcomings primarily due to the simplicity in its inherent assumptions and for any quantitative estimation of reservoir properties they are generally interpreted in combination with other information in recent years waveform based inversions have gained popularity in reservoir characterization and depth imaging going beyond the simple assumptions of avo and using wave equation solutions these methods have been effective in accurately predicting the subsurface properties developments of these waveform inversions have so far been along two lines 1 the methods that use a locally 1d model of the subsurface for each common midpo

geochemical behavior under tropical weathering of the barama mazaruni greenstone belt at omai gold mine guiana shield abstract mineralogical petrographical and geochemical studies of the weathering profile have been carried out at omai au mine guyana the area is underlain by felsic to mafic volcanic and sedimentary rocks of the barama mazaruni supergroup part of the paleoproterozoic greenstone belts of the guiana shield tropical rainy climate has favoured extensive lateritization processes and formation of a deeply weathered regolith the top of the weathering profile consists of lateritic gravel or is masked by the pleistocene continental deltaic berbice formation mineralogical composition of regolith consists mainly of kaolinite goethite and quartz and subordinately sericite feldspar hematite pyrite smectite heavy minerals and uncommon mineral phases nacrite ephesite corrensite guyanaite a specific feature of the weathering profile at omai is the preservation of fresh hydrothermal pyrite in the saprolith horizon chemical changes during the weathering processes depend on various physicochemical and structural parameters consequently the depth should not be the principal criterion for comparison purposes of the geochemical behavior within the weathering profile but rather an index that measures the degree of supergene alteration that has affected each analyzed sample independently of the depth of sampling thus the mineralogical index of alteration mia can provide more accurate information about the behavior of major and trace elements in regolith as opposed to unweathered bedrock it can also aid in establishing a quantitative relationship between intensity of weathering and mobility leaching or accumulation of each element in each analyzed sample at omai some major and trace elements that are commonly considered as immobile ex tio2 zr etc during weathering could become mobile in several rock types and cannot be used to calculate the mass and volume balance in addition due to higher immobile element ratios the weathered felsic volcanic rocks plotted in identification diagrams are shifted towards more mafic rock types and a negative adjustment of 20 units is necessary for correct classification in contrast these elements could aid in defining the material source in sedimentary rocks affected by weathering generally the rare earth element ree patterns of the bedrock are preserved in the saprolith horizon this can represent a potentially useful tool for geochemical exploration in tropical terrains strong negative ce and tb anomalies are displayed by weathered pillowed andesites which are explained by the influence of the water rock ratio

p wave anisotropy from azimuthal avo and velocity estimates using 3d seismic data from saudi arabia a horizontal well is most productive in tight reservoirs when it intersects a large number of vertical fractures yet strata near the borehole remain mechanically stable azimuthal velocity analysis and p wave amplitude versus offset avo using 3d wide azimuth prestack surface seismic data provide a remote yet detailed way to map a fracture pattern away from well control we estimate fracture direction and relative fracture intensity from such data at two gas fields in saudi arabia our results show a small azimuthal variation in p wave velocity maximum 5 and a larger variation in azimuthal avo at the reservoir larger than 100 computed fracture attributes for field 1 show a consistent east west fracture direction however in field 2 fracture azimuth is variable but generally east west and north south with the strongly anisotropic north south orientation correlating with faults and areas of large structural curvature in the reservoir in both fields azimuthal avo analysis shows a more consisten

reflection seismology over azimuthally anisotropic media recent surveys have shown that azimuthal anisotropy due most plausibly to aligned fractures has an important effect on seismic shear waves previous work had discussed these effects on vsp data the same effects are seen in surface recording of reflections at small to moderate angles of incidence the anisotropic effects one different polarization components of vertically traveling shear waves permit the recognition and estimation of very small degrees of azimuthal anisotropy of order 2 1 percent as in an interferometer anisotropic effects on traveltime yield estimates of anisotropy which are averages over large depth intervals often raw field data must be corrected for these effects before the reflectors may be imaged two variations of a rotational algorithm to determine the principal time series are derived anisotropic effects on moveout lead to abnormal moveout unless the survey line is parallel to the fractures anisotropic effects on reflection amplitude permit the recognition and estimation of anisotropy hence fracture intensity differences at the reflecting horizon i e with high vertical resolution

seismic characterization of multiple fracture sets does orthotropy suffice as geophysicists rely increasingly on effective media theories to characterize naturally fractured reservoirs they become more and more interested in evaluating the accuracy of different theories estimating their limits of applicability and assessing their usefulness for practice with this in mind we compare two popular seismological theories of hudson and schoenberg with the theory of kachanov developed in the context of mechanics of materials by performing finite element simulations of effective media for models that contain several sets of nonintersecting circular vertical fractures embedded in otherwise isotropic host rock we examine the accuracy of these theories our numeric study reveals that predictions of both the first and second order hudson s theories are typically inferior to those of others especially when the fractures are dry while on average the theories of schoenberg and kachanov fit finite element computations with comparable accuracy the latter appears to be more useful

reflection transmission coefficients and azimuthal anisotropy in marine seismic studies summary introduce two methods for inferring the direction of vertical fractures from marine seismic data first we derive the necessary reflection and transmission coefficients for an interface between a liquid and an azimuthally anisotropic solid next we show that multicomponent ocean bottom seismometer data from surface airgun sources along two perpendicular shot lines can be rotated into the principal directions of azimuthal anisotropy to determine the orientation of vertical fractures finally we show that p wave amplitude versus offset avo depends on the orientation of the shot line with respect to the vertical fractures thus p wave avo can also be used to determine fracture orientation

avo inversion part 1 isolating rock property contrasts seismic amplitudes and the degree to which they change with offset have proven more effective at high grading prospects and providing substantial risk reduction than using structural information alone the discriminating power of amplitudes comes from changes observed in rock properties when hydrocarbons displace the omnipresent subsurface fluid brine these rock property changes in turn control the strength of reflections and the avo response

p wave and s wave azimuthal anisotropy at a naturally fractured gas reservoir bluebell altamont field utah reflection p and s wave data were used in an investigation to determine the relative merits and strengths of these two data sets to characterize a naturally fractured gas reservoir in the tertiary upper green river formation the objective is to evaluate the viability of p wave seismic to detect the presence of gas filled fractures estimate fracture density and orientation and compare the results with estimates obtained from the s wave data the p wave response to vertical fractures must be evaluated at different source receiver azimuths travelpaths relative to fracture strike two perpendicular lines of multicomponent reflection data were acquired approximately parallel and normal to the dominant strike of upper green river fractures as obtained from outcrop core analysis and borehole image logs the p wave amplitude response is extracted from prestack amplitude variation with offset avo analysis which is compared to isotropic model avo responses of gas sand versus brine sand in the upper green

anisotropic geometrical spreading correction for wide azimuth p wave reflections compensation for geometrical spreading along a raypath is one of the key steps in avo amplitude variation with offset analysis in particular for wide azimuth surveys here we propose an efficient methodology to correct long spread wide azimuth reflection data for geometrical spreading in stratified azimuthally anisotropic media the p wave geometrical spreading factor is expressed through the reflection traveltime described by a nonhyperbolic moveout equation that has the same form as in vti transversely isotropic with a vertical symmetry axis media the adapted vti equation is parameterized by the normal moveout nmo ellipse and the azimuthally varying anellipticity parameter to estimate the moveout parameters we apply a 3d nonhyperbolic semblance algorithm of vasconcelos and tsvankin that operates simultaneously with traces at all offsets and azimuths the estimated moveout parameters are used as the input in our geometrical spreading computation numerical tests for models composed of orthorhombic layers with strong depthvarying velocity anisotropy confirm the high accuracy of our travetime fitting procedure and therefore of the geometricalspreading correction because our algorithm is based entirely on the kinematics of reflection arrivals it can be incorporated readily into the processing flow of azimuthal avo analysis in combination with the nonhyperbolic moveout inversion we apply our method to wide azimuth p wave data collected at the weyburn field in canada the geometrical spreading factor for the reflection from the top of the fractured reservoir is clearly influenced by azimuthal anisotropy in the overburden which should cause distortions in the azimuthal avo attributes this case study confirms that the azimuthal variation of the geometrical spreading factor often is comparable to or exceeds that of the reflection coefficient

prediction of some weathering trends of plutonic and volcanic rocks based on thermodynamic and kinetic considerations abstract the exposed crust consists mainly of plagioclase 35 quartz 20 k feldspar 11 volcanic glass 12 biotite 8 and muscovite 5 quartz is a resistate thus feldspars and glass represent approximately 75 percent of the labile minerals the weathering characteristics of these constituents are summarized in the context of thermodynamic mass balance and kinetic considerations experimentally determined release rate constants were used to predict the proportions of ca na and k released by feldspars of plutonic rocks granites to gabbros to weathering solutions the chemical weathering trends of the weathered residues calculated from the kinetic data conform closely to the initial trends observed in some recent weathering profiles demonstrating the accuracy of the predictions since the weathering of feldspars is controlled by processes that should not change through geological time the relative release rates of ca na and k from the feldspars of granitic rocks can be calculated for future and past episodes of continental weathering experimentally determined release rate constants are not available for a wide range of volcanic glass compositions but the limited data indicate that compositional trends are predictable in weathering profiles developed on volcanic rocks the kinetic data available for rhyolitic glasses accurately predict the initial weathering trends observed in a recent rhyolite weathering profile

interpreting seismic data in the presence of azimuthal anisotropy or azimuthal anisotropy in the presence of the seismic interpretation measuring p wave azimuthal anisotropy has been in the recent past an elusive task therefore the interpreter ignored this attribute of the seismic data and left the subject to the research and technology group geophysicists conversely the interpreting geophysicist knew that when measured the anisotropy could yield important reservoir properties related to fractures and stress fields however little did we suspect that p wave azimuthal anisotropy both in velocity and avo would change our perception of data interpretation as is happening now

seismic reservoir description using hybrid seismic inversion a 3d case study from the maria ines oeste field argentina maria ines oeste an oil and gas field in argentina s magallanes foreland basin also called austral basin produces from maria inees sandstones that are known to represent the base of a second order transgressive system tract tst deposited during late maastrichtian times in a coastal to open shelf environment figure 1 outlines the basic tectonic framework of the austral basin

characteristics of residual soils in singapore as formed by weathering residual soils are weathering products of rocks that are commonly found under unsaturated conditions the properties of residual soils are a function of the degree of weathering a series of index properties engineering properties mercury porosimetry tests and scanning electron microscope sem examinations were performed on residual soils from two major geological formations in singapore the results indicate that the variation in the index and engineering properties as well as microstructural characteristics of the residual soils with depth can be related to the degree of weathering an increase in the degree of weathering results in an increase in pore volume and produces a larger range of pore size distribution the variation in the pore volume and the pore size distribution through a profile of weathered rock can be used as an indicative measure of the variation in the degree of weathering with depth

the new geophysics new geophysics of critically microcracked rock is a new understanding of fluid rock deformation that matches much of the behaviour of in situ rock observed throughout the earth s crust and uppermost mantle the behaviour of rock in new geophysics is self similar can be monitored calculated predicted even in principle controlled prevails in almost all rocks and has butterfly effect sensitivity all of these remarkable features except controllability have been observed in some cases many times however new geophysics is innovative controversial and currently ridiculed and violently opposed schopenhauer stages 1 and 2 and is difficult to get published via consensus driven peer review consequently in the last 10 years various aspects of new geophysics have been published in some 20 different journals and conference proceedings making it difficult to access by the general reader this review presents a synopsis of what is believed to be a fundamental advance in solid earth geoscience

hybrid seismic inversion a reconnaissance tool for deepwater exploration when exploring new areas it is desirable to have a robust inversion methodology that can obtain reliable results in a reasonable time in addition it is also desirable that such inversion be run with or without any a priori well information this is particularly important in deep water where well information is sparse and seismic data is the only information on which the initial exploration strategy can be based

fractured reservoir modeling from seismic to simulator a reality characterizing carbonate reservoirs has traditionally been a difficult task when the reservoir performance is strongly influenced by an often poorly constrained fracture system studies have shown that the geometry of the fracture network is a dominant control of fractured reservoir performance direct fracture data only exist at the well and so a long running quest has been the accurate description of the geometry and impact of fractures in the interwell reservoir zone a range of different approaches has been used to constrain the expected distribution of subseismic fractures in the reservoir including such techniques as geomechanical modeling palinplastic reconstruction and curvature analysis however these techniques are all attempting to predict the distribution of fracture based upon indirect observations or assumptions what is ideally required is an approach that can directly sample the whole reservoir in order to determine the properties of the fracture network

determination of the principal directions of azimuthal anisotropy from p wave seismic data in an azimuthally anisotropic medium the principal directions of azimuthal anisotropy are the directions along which the quasi p and the quasi s waves propagate as pure p and s modes when azimuthal anisotropy is induced by oriented vertical fractures imposed on an azimuthally isotropic background two of these principal directions correspond to the directions parallel and perpendicular to the fractures s waves propagating through an azimuthally anisotropic medium are sensitive to the direction of their propagation with respect to the principal directions as a result primary or mode converted multicomponent s wave data are used to obtain the principal directions apart from high acquisition cost processing and interpretation of multicomponent data require a technology that the seismic industry has not fully developed anisotropy detection from conventional p wave data on the other hand has been limited to a few qualitative studies of the amplitude variation with offset avo for different azimutha

rapid computation of multioffset vertical seismic profile synthetic seismograms for layered media by rearranging the formulas for the responses of buried sources and receivers in the kennett reflectivity algorithm we have obtained a new algorithm that is very efficient for computing multioffset vsp synthetic seismograms the rearrangement of the response formulas is quite general inasmuch as it applies to both isotropic and anisotropic kennett codes our new algorithm and the original single receiver algorithm can both be made to run much faster on vector computers by taking the layer loop out of the p loop but leaving it inside the frequency loop the resulting vector codes can still compute the response of media with frequency dependent velocities

fracture detection in a carbonate reservoir using a variety of seismic methods we combine various methods to estimate fracture orientation in a carbonate reservoir located in southwest venezuela the methods we apply include the 2 d rotation analysis of 2 d p s data along three different azimuths amplitude variation with offset avo of 2 d p wave data along the same three azimuths normal moveout nmo analysis of the same 2 d data and both 3 d azimuthal avo and nmo analysis of 3 d p wave data recorded in the same field the results of all methods are compared against measures of fracture orientation obtained from formation microscanner logs recorded at four different locations in the field regional and local measures of maximum horizontal stress and the alignment of the major faults that cross the field p s data yield fracture orientations that follow the regional trend of the maximum horizontal stress and are consistent with fracture orientations measured in the wells around the carbonate reservoir azimuthal avo analysis yields a similar regional trend as that obtained from the p s data but the resolution is lower local variations in fracture orientation derived from 3 d avo show good correlation with local structural changes in contrast due to the influence of a variety of factors including azimuthal anisotropy and lateral heterogeneity in the overburden azimuthal nmo analysis over the 3 d p wave data yields different orientations compared to those obtained by other methods it is too early to say which particular method is more appropriate and reliable for fracture characterization the answer will depend on factors that range from local geological conditions to additional costs for acquiring new information

geomechanical and geochemical changes during early stages of weathering of karamu basalt new zealand abstract variations in geochemical and geomechanical characteristics were measured through a basalt weathering sequence exposed at an abandoned quarry at karamu near hamilton new zealand geochemical measurements show that at the early stages of weathering fresh to slightly weathered reductions in the concentrations cao and mgo are rapid although they are not accompanied by major mineralogical change during later stages of weathering secondary clay minerals form following well described mineralogical changes fresh basalt is strong uniaxial compressive strength 262 mn m 2 point load index 5 59 with well developed closely spaced columnar jointing slightly weathered material is distinguished in the field only by slight discolouration along joint planes but laboratory tests show a dramatic loss of strength in slightly weathered rock point load index 0 41 cbr 43 a causal relationship between the loss of cations and loss of strength in the early stages of weathering is suggested this stage of weathering is diffusion controlled whereby cations are lost from the constituent primary minerals and replaced by h this process disrupts the lattice structure and causes a marked loss of strength implications are that slightly weathered rock which appears competent in the field is likely to have much lower strength and durability than assumed from field evidence

avo inversion part 2 isolating rock property contrasts part 1 of this article tle march 2001 described a method for extracting rock property contrasts from seismic data this has opened many doors previously thought to have been closed to the explorationist this paper will address issues such as determining hydrocarbon saturation production effects nonbright pay and s wave seismic from p wave acquisition and processing these are pertinent problems in today s industry and many require great expense to solve

fracture characterization at valhall application of p wave amplitude variation with offset and azimuth avoa analysis to a 3d ocean bottom data set the delineation and characterization of fracturing is important in the successful exploitation of many hydrocarbon reservoirs such fracturing often occurs in preferentially aligned sets if the fractures are of subseismic scale this may result in seismic anisotropy thus measurements of anisotropy from seismic data may be used to delineate fracture patterns and investigate their properties here fracture induced anisotropy is investigated in the valhall field which lies in the norwegian sector of the north sea this field is a chalk reservoir with good porosity but variable permeability where fractures may significantly impact production e g during waterflooding to investigate the nature of fracturing in this reservoir p wave amplitude variation with offset and azimuth avoa is analyzed in a 3d ocean bottom cable obc data set in general 3d ocean bottom seismic obs acquisition leads to patchy coverage in offset and azimuth and this must be addressed when considering such data to overcome this challenge and others associated with 3d obs acquisition a new method for processing and analysis is presented for example a surface fitting approach which involves analyzing azimuthal variations in avo gradients is used to estimate the orientation and magnitude of the fracture induced anisotropy this approach is also more widely applicable to offset azimuth analysis of other attributes e g traveltimes and any data set where there has been true 3d data acquisition land or marine using this new methodology we derive high resolution maps of p wave anisotropy from the avoa analysis for the top chalk reflection at valhall these anisotropy maps show coherent but laterally varying trends synthetic avoa modeling using effective medium models indicates that if this anisotropy is from aligned fracturing the fractures are likely liquid filled with small aspect ratios and the fracture density must be high furthermore we show that the fracture normal direction is parallel to the direction of most positive avo gradient in other situations the reverse can be true i e the fracture normal direction can be parallel to the direction of the most negative avo gradient effective medium modeling or comparisons with anisotropy estimates from other approaches e g azimuthal variations in velocity must therefore be used to resolve this ambiguity the inferred fracture orientations and anisotropy magnitudes show a degree of correlation with the positions and alignments of larger scale faults which are estimated from 3d coherency analysis overall this work demonstrates that significant insight may be gained into the alignment and character of fracturing and the stress field variations throughout a field using this high resolution avoa method

prediction of compressional wave velocity using regression and neural network modeling and estimation of stress orientation in bokaro coalfield india velocity of compressional wave v p of coal and non coal lithology is predicted from five wells from the bokaro coalfield cf india shear sonic travel time logs are not recorded for all wells under the study area shear wave velocity vs is available only for two wells one from east and other from west bokaro cf the major lithologies of this cf are dominated by coal shaly coal of barakar formation this paper focuses on the a relationship between vp and vs b prediction of vp using regression and neural network modeling and c estimation of maximum horizontal stress from image log coal characterizes with low acoustic impedance ai as compared to the overlying and underlying strata the cross plot between ai and vp vs is able to identify coal shaly coal shale and sandstone from wells in bokaro cf the relationship between vp and vs is obtained with excellent goodness of fit r 2 ranging from 0 90 to 0 93 linear multiple regression and multi layered feed forward neural network mlfn models are developed for prediction vp from two wells using four input log parameters gamma ray resistivity bulk density and neutron porosity regression model predicted vp shows poor fit from r 2 0 28 to good fit r 2 0 79 with the observed velocity mlfn model predicted vp indicates satisfactory to good r2 values varying from 0 62 to 0 92 with the observed velocity maximum horizontal stress orientation from a well at west bokaro cf is studied from formation micro imager fmi log breakouts and drilling induced fractures difs are identified from the fmi log breakout length of 4 5 m is oriented towards n60 w whereas the orientation of difs for a cumulative length of 26 5 m is varying from n15 e to n35 e the mean maximum horizontal stress in this cf is towards n28 e

an accurate ray based offset to angle transform from normal moveout uncorrected multicomponent data in a transversely isotropic medium with vertical symmetry axis a new ray based approach for converting multicomponent normal moveout uncorrected offset domain prestack seismic data into angles is presented from any two way zero offset time sample and a given angle of incidence the exact ray and a few neighboring rays that contribute to the given angle of incidence are traced through the medium to their corresponding source and receiver locations to compute their raypath lengths traveltimes and offsets a polynomial then is fitted locally between these computed offsets and their corresponding traveltimes next the traveltimes for the actual offsets on data that lie within these traced rays are computed from this local polynomial fit and their corresponding time samples are partially stacked and moved to their two way zero offset times repetition of this procedure for all zero offset time samples and for all desired angles produces an angle gather the procedure allows computation of both primary p p and mode converted p sv angle gathers also by calculating the exact raypath length an accurate geometric spreading compensation can be computed and applied to these gathers by using the methodology for a horizontally stratified transversely isotopic medium with a vertical symmetry axis a vti medium it is possible to extract a much more accurate angle domain response from the ray based method than is possible from a traditional nmo based method accuracy out to large angles for both primary and mode converted reflection suggests that this ray based transform potentially could be used in a multicomponent prestack waveform inversion scheme

use of avoa data to estimate fluid indicator in a vertically fractured medium microstructural attributes of cracks and fractures such as crack density aspect ratio and fluid infill determine the elastic properties of a medium containing a set of parallel vertical fractures although the tangential weakness t of the fractures does not vary with the fluid content the normal weakness n exhibits significant dependence on fluid infill based on linear slip theory we used the ratio g n t termed the fluid indicator as a quantitative measure of the fluid content in the fractures with g representing the square of the ratio of s and p wave velocity in the unfractured medium we used a born formalism to derive the sensitivity to fracture weakness of pp and ps reflection coefficients for an interface separating an unfractured medium from a vertically fractured medium our formulae reveal that the pp reflection coefficient does not depend on the 2d microcorrugation surface roughness with ridges and valleys parallel to the fracture strike whereas the ps reflection coefficien

a critique of the schellmann definition and classification of laterite abstract schellman s definition and classification of laterite are based on the sio2 al2o3 and fe2o3 contents of weathered formations in comparison to the chemical composition of the underlying rocks from which the weathered materials are assumed to be derived this approach is open to misinterpretation because many regolith materials have formed by lateral transport both physical and chemical it ignores the role of absolute accumulation it pays little attention to the morphological characteristics of laterites that give clues to their origins it ignores the detailed mineralogical compositions of weathered materials and laterite and it does not permit field identification understanding of geology stratigraphy geomorphic evolution mineralogy and micromorphology are essential ingredients of regolith investigations chemical analysis alone is insufficient schellmann s chemical classification seems appropriate only to a small subset of potential laterites in which the whole profile is of bedrock and saprolite and where there has been no lateral movement of solids or solutions schellmann s definition demands formation by tropical weathering which eliminates ferruginous duricrusts formed outside the tropics his approach produces confusion by grouping disparate ferruginous aluminous materials together as laterite the range of applications of the term laterite is so broad that it has become meaningless and the schellmann approach has not resolved this issue

tropical chemical weathering of hillslope deposits and bedrock source in the aburra valley northern colombian andes abstract tropical chemical weathering produces extensive lateritization and formation of deep weathering profiles both processes are fundamental to landscape evolution and slope instability the aburra valley of the northern colombian andes is characterized by tropical conditions the valley slopes are mostly covered by hillslope deposits originating from four basement rock suites which comprise contrasting granitoid volcanic sedimentary ophiolitic and metamorphic sources respectively tropical chemical weathering of the aburra hillslope deposits and their respective bedrock were examined using x ray fluorescence and x ray diffraction analysis to document and quantify their chemical weathering profiles compositions and mineralogical properties the chemical index of alteration cia loss on ignition loi and the mobiles index imob were used to quantify the degree of weathering of hillslope deposits and bedrock source weathering trends were analyzed using a cn k and a cnk fm diagrams the material mantling the slopes in the aburra valley records an intense weathering history chemical weathering is characterized by increased development of clay minerals kaolinite halloysite and iron and aluminum sesquioxides lateritization characterizes the final stage of the weathering profiles concentrations of cao na2o k2o decrease markedly in the weathering products compared to the fresh bedrock source whereas concentrations of al2o3 fe2o3 and mgo increase significantly cia ratios of matrix slope deposits derived from all four sources near 100 whereas those of boulder slope deposits and saprolites are lower but exceed source rock values different a cn k weathering paths are evident for each lithotype validating the correlation established between the hillslope deposits and their various parents chemical weathering indices in some samples are strongly influenced by the presence of sesquioxides as reflected by high loi anomalously low cia and varying enrichment trends on the a cnk fm diagrams consequently different chemical indices based on different criteria need to be combined to obtain best results as illustrated here by the combination of loi cia and imob the overall results suggest that tropical conditions have dominated for a long time in the northern colombian andes leading to uniformly high weathering indices in matrix slope deposits irrespective of parent lithotype prolonged warm and humid conditions could thus be responsible for the weathering and remobilization of extensive old hillslope deposits during the quaternary however in addition to the influence of climatic factors tectonism has also undoubtedly influenced slope evolution in the aburra valley

magmatic underplating in the canary archipelago abstract the location of the canary archipelago at the east edge of the north atlantic a few hundred kilometres from the west african coast has for years posed a serious difficulty in understanding the complex interaction between long lasting volcanic activity and one of the oldest seafloor basements 150 ma on earth many different hypotheses have been proposed in the last two decades to understand the genesis of the canary archipelago there is an increasing acceptance to explain the archipelago as the product of the slow passage of the african plate over a mantle hotspot in rival tectonic models stretching and thinning of the lithosphere would determine the islands as independent volcanic blocks we review recent geophysical and geological evidence supporting a mantle plume origin for the canary islands based on the recognition of crustal thickening by mafic intrusions produced by the interaction between the mantle plume and the old oceanic lithosphere beneath the canary archipelago

azimuthal anisotropy analysis of p wave seismic data and estimation of the orientation of the in situ stress fields an example from the rock springs uplift wyoming usa abstractestimating the orientation and magnitude of maximum and minimum horizontal in situ stress is important for characterizing naturally fractured unconventional and carbon sequestered reservoirs for naturally fractured reservoirs they are needed to guide directional drilling for unconventional reservoirs they are used for optimal placements of hydraulic fractures and for carbon sequestered reservoirs they are used to avoid fracturing of overlying seal rocks in addition a knowledge of stress fields can be used to induce fractures within the target reservoirs and enhance additional storage for carbon sequestration experiments the orientation and magnitude of in situ stress can be calculated at the well locations for locations away from the wells analysis of the azimuthal dependence of the amplitude variation with angle gradient or azimuthal angle stacks are used to quantify anisotropy which are then related with well data and other geologic information for stress estimation such azimutha

detection of fracture orientation using azimuthal variation of p wave avo responses azimuthally dependent p wave amplitude variation with offset avo responses can be related theoretically to open fracture orientation and have been suggested as a geophysical tool to identify fracture orientation in fractured oil and gas reservoirs a field experiment conducted recently over a fractured reservoir in the barinas basin venezuela provides data for an excellent test of this approach three lines of data were collected in three different azimuths and three component receivers were used the distribution of fractures in this reservoir was obtained previously using measurements of shear wave splitting from p s converted waves from the same data set in this work we use p wave data to see if the data can yield the same information using azimuthal variation of p wave avo responses results obtained from the azimuthal p wave avo analysis corroborate the fracture orientation obtained previously using p s converted waves this analysis with field data is an example of the high potential of p waves to detect fracture effects on seismic wave propagation

converted wave seismic exploration methods multicomponent seismic recording measurement with vertical and horizontal component geophones and possibly a hydrophone or microphone captures the seismic wavefield more completely than conventional single element techniques in the last several years multicomponent surveying has developed rapidly allowing creation of converted wave or p s images these make use of downgoing p waves that convert on reflection at their deepest point of penetration to upcoming s waves survey design for acquiring p s data is similar to that for p waves but must take into account subsurface vp vs values and the asymmetric p s ray path p s surveys use conventional sources but require several times more recording channels per receiving location some special processes for p s analysis include anisotropic rotations s wave receiver statics asymmetric and anisotropic binning nonhyperbolic velocity analysis and nmo correction p s to p p time transformation p s dip moveout prestack migration with two velocities and wa