resolution of reservoir scale electrical anisotropy from marine csem data a combination of 1d and 3d forward and inverse solutions is used to quantify the sensitivity and resolution of conventional controlled source electromagnetic csem data collected using a horizontal electric dipole source to transverse electric anisotropy located in a deep water exploration reservoir target because strongly anisotropic shale layers have a vertical resistivity that can be comparable to many reservoirs we examined how csem can discriminate confounding shale layers through their characteristically lower horizontal resistivity forward modeling indicated that the sensitivity to reservoir level anisotropy is very low compared with the sensitivity to isotropic reservoirs especially when the reservoir is deeper than about 2 km below the seabed however for 1d models where the number of inversion parameters can be fixed to be only a few layers both vertical and horizontal resistivity of the reservoir can be well resolved using a stochastic inversion we found that the resolution of horizonta

direct reservoir parameter estimation using joint inversion of marine seismic ava and csem data a new joint inversion algorithm to directly estimate reservoir parameters is described this algorithm combines seismic amplitude versus angle ava and marine controlled source electromagnetic csem data the rock properties model needed to link the geophysical parameters to the reservoir parameters is described errors in the rock properties model parameters measured in percent introduce errors of comparable size in the joint inversion reservoir parameter estimates tests of the concept on synthetic one dimensional models demonstrate improved fluid saturation and porosity estimates for joint ava csem data inversion compared to ava or csem inversion alone comparing inversions of ava csem and joint ava csem data over the north sea troll field at a location with well control shows that the joint inversion produces estimated gas saturation oil saturation and porosity that is closest as measured by the rms difference l1 norm of the difference and net over the interval to the logged values whereas csem inversion provides the closest estimates of water saturation

reservoir property mapping and monitoring from joint inversion of time lapse seismic electromagnetic and production data abstractwe have developed a deterministic multiphysics joint inversion approach integrating seismic electromagnetic em and production data to map relevant reservoir properties such as permeability and porosity and the time evolution of the flooding front movement i e saturation changes with time these measurements are complementary in terms of their sensitivity to individual reservoir properties and their coverage of reservoir volumes as a consequence integration reduces ambiguities in the interpretation in the workflow a reservoir model is first built based on prior information the production data are simulated by evolving the model in time based on the known well control strategy simultaneously the temporal and spatial distribution of fluid properties such as saturation salt concentration density and pressure are also obtained from the forward modeling these properties together with in situ rock properties are transformed to formation resistivity and elastic properties using pres

the sierra de mil cumbres michoacan mexico transitional volcanism between the sierra madre occidental and the trans mexican volcanic belt abstract the sierra de mil cumbres is a miocene volcanic range located in central mexico in the north eastern part of the state of michoacan near the city of morelia structurally it is a ene trending horst that covers an area of 1022 km2 approximately 20 km wide 60 km long and contains exposures of chemically bimodal volcanism in the form of ignimbrites lava domes lava flows cinder cones and related deposits the main volcanic manifestations of this range are the la escalera caldera 16 3 23 ma the garnica volcanic complex 18 3 17 9 ma the atecuaro caldera 16 3 19 4 ma and the indaparapeo volcanic complex 14 1 17 5 ma the sierra de mil cumbres stands in space and time at the intersection between the miocene recent trans mexican volcanic belt and the late cretaceous early miocene sierra madre occidental and so provides new insights into the geological evolution of central mexico arc volcanism in the sierra de mil cumbres was initiated by a massive nnw sse extension probably during the counterclockwise rotation of the sierra madre occidental new geological mapping stratigraphic analysis detailed geochemistry and 40ar 39ar geochronology demonstrates that this intra plate volcanism was emplaced between 14 and 23 ma

eruption rates and compositional trends at los humeros volcanic center puebla mexico the los humeros volcanic center located 180 km east of mexico city is one of several pleistocene silicic centers in the back arc portion of the mexican neovolcanic belt the eruptive history at los humeros suggests that average eruptive rates increased with time and exceeded rates of regeneration of differentiated magma silicic volcanism began 0 47 ma ago with extrusion of high silica rhyolite domes shortly thereafter at 0 46 ma ago 115 km3 of magma zoned from high silica rhyolite to rhyodacite were erupted resulting in formation of the xaltipan ignimbrite and collapse of the 21 15 km los humeros caldera high silica rhyolite domes then erupted along the northwestern ring fracture zone of the caldera they are covered by the faby tuff a dominantly rhyodacitic sequence of plinian fall deposits erupted from the southeast quadrant of the los humeros caldera dated at 0 24 ma a second major caldera forming event occurred 0 1 ma ago with the eruption of the zaragoza tuff a nonwelded ignimbrite zoned from rhyodacite to andesite eruption of these 12 km3 of magma led to collapse of the 10 km diameter los potreros caldera which is nested within the older los humeros caldera between 0 04 and 0 03 ma ago an arc of andesitic scoria cones concentrated along the southern ring fracture zone of the los humeros caldera fed lavas that flowed southward from the volcanic center and andesite lavas built two small shields between the eastern rims of the two calderas approximately 6 km of andesitic magma were extruded during this stage activity continued up to 0 02 ma ago with eruption of 10 km3 of rhyodacitic to andesitic lava flows from centers near the northern margin of the los potreros caldera in the area between the eastern rims of the two calderas and within a broad band where the southern segments of the inferred los humeros and los potreros ring fracture zones nearly coincide simultaneous venting of rhyodacitic and andesitic tephra within this band led to formation of the 1 7 km diameter el xalapazco caldera minor fault bounded uplift of the southeastern quadrant of the los potreros caldera followed the latest stage of volcanic activity is represented by the eruption of 0 25 km3 of olivine basalt lavas along the southern ring fracture zone of the los humeros caldera and on the floors of the los potreros and el xalapazco calderas erupted magmas show an overall trend with time toward more mafic compositions volumetric eruptive rates increased from 0 06 km3 per thousand years 0 25 ma ago to 0 2 km3 per thousand years in the last 0 1 ma the increase in eruptive rate may have been the result of a progressive decrease in the structural integrity of the roof zone of the system as successive caldera forming eruptions reactivated old zones of weakness and created new ones an increasingly disrupted roof allowed mafic and intermediate magmas to reach the surface relatively rapidly decreasing their residence time in a high level chamber and thus the time available for their differentiation

boiling over dense pyroclastic density currents during the formation of the 100 km 3 huichapan ignimbrite in central mexico stratigraphic and lithofacies analysis abstract a lithofacies analysis of the huichapan ignimbrite has been undertaken to evaluate its depositional history from large pyroclastic density currents the huichapan ignimbrite is a massive ignimbrite sheet with a maximum runout of at least 55 km and thickness variations between 6 and 80 m the lower portion of the huichapan ignimbrite consists of a large plateau 100 km3 69 km3 as dense rock equivalent dre of massive ignimbrites with welding variations from densely welded to partly welded devitrification and high temperature vapor phase alteration the lower part grades laterally to moderately welded and non devitrified ignimbrites these variations are interpreted as the sedimentation of density stratified pyroclastic density currents erupted as boiling over pulses from the huichapan donguinyo caldera complex at a continuous rate supporting deposition by quasi steady progressive aggradation of sustained and hot currents to the north of the caldera the lower portion of the ignimbrite consists of a small plateau

geology of los azufres caldera mexico and its relationships with regional tectonics ferrari l gardufio v h pasquar6 g and tibaldi a 1991 geology of los azufres caldera mexico and its relationships with regional tectonics in s p verma editor calderas genesis structure and unrest j volcanol geotherm res 47 129 148 the los azufres geothermal field is one of several silicic centres of the mexican volcanic belt mvb for which a caldera structure was suggested geological and structural surveys in a wide area surrounding los azufres reveal that this complex is situated in an area of unusual concentration of acid volcanism consisting of four pyroclastic units and several dome complexes although a complete caldera structure is not morphologically evident several facts suggest a long collapse history at los a zufres the geothermal field lies at the centre of a subcircular depressed area 27 26 km in size filled by a distinctive fluviolaeustrine sequence middle to late miocene rocks bound to the south and to the north of this depression and are encountered only at depth inside it four large ignimbritic suites of latest miocene and pliocenc age outcrop outside the depression volume estimations of these pyroclastic products are comparable with the missing volume of the depressed area no alternative caldera structures exist in a radius of 200 km from los azufres dacitic to rhyolitic lavas principally extruded as dome complexes were emplaced inside this depression during the pleistocene los azufres is therefore interpreted as a nested caldera of latest miocene and pliocene age the recent regional tectonic evolution of the central sector of mvb comprises a late miocene early pliocene left lateral transcurrent phase followed by a late pliocene quatern ary transtensional one the silicic volcanism occurring between 6 1 and 2 8 ma can be linked to the first phase while in the adjacent areas of mvb a volcanic hiatus can be recognized the following transtensional phase reached the los azufres area only during the pleistocene and disrupted the caldera structure normal faults developed during this period controlled the uprising of basic magma which partly interacted with the remaining differentiated one and produced part of the recent intracaldera cycle

experimental investigation of spudcan penetration in multi layer clays with interbedded sand layers highly layered soils are prevalent in several areas of current offshore activity and in particular in emerging provinces and fields however no investigation has been carried out for spudcan penetration in stratified sediments with more than two layers this paper reports centrifuge modelling of spudcan foundations penetrating through 3 6 clay layers with interbedded stronger clay silica sand and carbonate sand layers the aim was to provide insight into the behaviour of spudcan foundations in stratified sediments comprising layers of different drainage conditions and mineralogy the motivation for this paper emanated directly from the future needs identified by the recent revised version of iso 19905 1 it was found that the downward soil deformation was extended into 3 4 underlying layers of sand and clay with thickness ratios t d less than 0 6 the limiting squeezing depth was 0 17 0 18d when approaching stiff clay moderate clay layers and 0 28d when approaching dense sand stiff clay layers these

uncertainty reduction through geologically conditioned petrophysical constraints in joint inversion abstractwe have developed a joint geophysical inversion workflow that aims to improve subsurface imaging and decrease uncertainty by integrating petrophysical constraints and geologic data in this framework probabilistic geologic modeling is used as a source of information to condition the petrophysical constraints spatially and to derive starting models the workflow then uses petrophysical measurements to constrain the values retrieved by geophysical joint inversion the different sources of constraints are integrated into a least squares framework to capture and integrate information related to geophysical petrophysical and geologic data this allows us to quantify the posterior state of knowledge and to calculate posterior statistical indicators to test this workflow using geologic field data we have generated a set of geologic models which we used to derive a probabilistic geologic model in this synthetic case study we found that the integration of geologic information and petrophysical con

the dynamic history of the trans mexican volcanic belt and the mexico subduction zone abstract the trans mexican volcanic belt tmvb is a 1000 km long neogene continental arc showing a large variation in composition and volcanic style and an intra arc extensional tectonics it overlies the rivera and cocos slabs which display marked changes in geometry geophysical studies indicate that lithospheric mantle is very thin or absent beneath the forearc and arc the fluids from the slab are released in a 40 to 100 km wide belt beneath the frontal part of the arc and the lower crust beneath the arc is partially molten east of 101 w the tmvb is built on a precambrian to paleozoic crust with thickness of 50 55 km west of 101 w the tmvb is underlain by jurassic to cenozoic marine and continental arcs with a 35 40 km thick crust the evolution of the tmvb occurred in four stages 1 from 20 to 10 ma the initial andesitic arc moved inland showing progressively drier melting and eventually slab melting suggesting flattening of the subducted slab 2 since 11 ma a pulse of mafic volcanism migrated from west to east reaching the gulf of mexico by 7 ma this mafic lavas marks the lateral propagation of a slab tear triggered by cessation of subduction beneath baja california 3 thereafter the volcanic front started moving trenchward with a marked phase of silicic volcanism between 7 5 and 3 ma local emplacement of small volume intraplate like basalts since 5 ma and development of extensional faulting these features are related to slab rollback enhancing asthenophere flux into the mantle wedge and promoting partial melting of the crust 4 the modern arc consists of a frontal belt dominated by flux and slab melting and a rear belt characterized by more differentiated rocks or by mafic lavas with little or no evidence of subduction fluids but higher asthenosphere fingerprint

bayesian inversion of marine csem data with a trans dimensional self parametrizing algorithm summary the posterior distribution of earth models that fit observed geophysical data convey information on the uncertainty with which they are resolved from another perspective the non uniqueness inherent in most geophysical inverse problems of interest can be quantified by examining the posterior model distribution converged upon by a bayesian inversion in this work we apply a reversible jump markov chain monte carlo method to sample the posterior model distribution for the anisotropic 1 d seafloor conductivity constrained by marine controlled source electromagnetic data unlike conventional gradient based inversion approaches our algorithm does not require any subjective choice of regularization parameter and it is self parametrizing and trans dimensional in that the number of interfaces with a resistivity contrast at depth is variable as are their positions a synthetic example demonstrates how the algorithm can be used to appraise the resolution capabilities of various electromagnetic field components for mapping a thin resistive reservoir buried beneath anisotropic conductive sediments a second example applies the method to survey data collected over the pluto gas field on the northwest australian shelf a benefit of our bayesian approach is that subsets of the posterior model probabilities can be selected to test various hypotheses about the model structure without requiring further inversions as examples the subset of model probabilities can be viewed for models only containing a certain number of layers or for models where resistive layers are present between a certain interval as suggested by other geological constraints such as seismic stratigraphy or nearby well logs

overexploitation of groundwater resources in the faulted basin of queretaro mexico a 3d deformation and stress analysis abstract the city of queretaro is located on a continental basin filled since the oligocene with lacustrine and alluvial sediments pyroclastic deposits and interbedded fractured basalts the graben structure of the basin was formed by two major north south trending normal faults among which the thicknesses of the filling materials vary many tens of meters in close distances hence important differences of hydraulic and mechanical properties characterize the various geologic units groundwater has been strongly withdrawn over the last three decades in the study area with a decline of the piezometric level exceeding 100 m because of the high variability of the geologic deposits the piezometric decrease and consequently the effective stress increase are characterized by a large spatial variability piezometric variations are also due to faults that strongly impact on groundwater flow dynamics the deformation and effective stress variability has caused large differential subsidence causing ground fracturing that has damaged the urban infrastructure of the city of queretaro this complex geological setting has been properly accounted into a three dimensional 3d flow and geomechanical modeling approach to quantify the displacement deformation and stress fields caused by water withdrawal the static geologic model was accurately defined using geological logs from extraction wells field mapping of faults fractures and the integration of major structures reported in previous geophysical works the model has been calibrated using observed groundwater levels and land settlement records the simulations have spanned the period from 1970 to 2011 the modeling results highlight that the areas where large differential subsidence and horizontal displacements developed correspond to the portions of the city where ground fractures are observed normal and shear components of the stress field changes accumulate along the discontinuity surfaces at depth providing evidence that large piezometric declines can be a key factor triggering the fault reactivation the spatial relationship between major withdrawals discontinuities of the geologic structure and accumulation of large stress and strain fields clearly emerged from the outcomes of the 3d geomechanical model

volcano stratigraphy and physical characterization of the san mateo pomez unit cerritos member temascalcingo volcano central mexico abstract the temascalcingo volcano is a dacitic stratovolcano located at the eastern part of the acambay graben a tectonic basin in the central sector of the trans mexican volcanic belt tmvb which is an active mostly calc alkaline volcanic arc that crosses mexico from the pacific ocean to the gulf of mexico this study characterises the explosive activity of the san mateo pumice eruption that is the most recent and largest paroxysmal episode of this volcano and the last unit of the cerritos member indeed this eruption is associated with an undated debris avalanche event consisting of a major sector collapse developed on the western flank of the volcano stratigraphic data suggest a highly explosive eruption that generated a relatively thick up to 10 m and widespread pumice fallout deposit trending to the ne and an aerial distribution of at least 100 km2 this subplinian type eruption of vei 4 lasted about 9 h and produced a plume height up to 12 km above the vent i e 14 km above sea level this type of explosive eruption is relatively common in the tmvb so a comprehensive geological study with the corresponding numerical simulations like the one that was carried out contributes to the understanding of the explosive volcanism of modest size stratovolcanoes that are common in the tmvb

long lived magmatic phases at los azufres volcanic center mexico abstract the los azufres volcanic complex has had a long eruptive history of 1 5 myr or more less than 3 myr subsequent to the building of a large miocene and pliocene andesitic complex since about 1 5 ma its development consists of two main periods of volcanic activity the first period from 1 5 to 0 8 ma has two magmatic cycles about 200 000 years in duration characterized by acidic followed basaltic volcanism such a magmatic evolution suggests successive emptyings of a shallow reservoir in which differentiation previously occurred about 0 6 ma major deep magma supplies were probably responsible for the resurgent doming which led to uplift of the southern part of the caldera from 0 6 ma to present the second period exposes volcanic products grading from basalts to rhyolites considering the recent age of the last ignimbrites 26 000 to 29 000 years one can assume that the volcanic activity related to a voluminous differentiated magma body at shallow depth has not yet ended especially in the southern area in and around the resurgent zone occupied by the geothermal field

bayesian inversion of joint sh seismic and seismoelectric data to infer glacier system properties abstract in glacial studies properties such as glacier thickness and the basement permeability and porosity are key to understand the hydrological and mechanical behaviour of the system the seismoelectric method could potentially be used to determine key properties of glacial environments here we analytically model the generation of seismic and seismoelectric signals by means of a shear horizontal seismic wave source on top of a glacier overlying a porous basement considering a one dimensional setting we compute the seismic waves and the electrokinetically induced electric field we then analyse the sensitivity of the seismic and electromagnetic data to relevant model parameters namely depth of the glacier bottom porosity permeability shear modulus and saturating water salinity of the glacier basement moreover we study the possibility of inferring these key parameters from a set of very low noise synthetic data adopting a bayesian framework to pay particular attention to the uncertainty of the model parameters mentioned above we tackle the resolution of the probabilistic inverse problem with two strategies 1 we compute the marginal posterior distributions of each model parameter solving multidimensional integrals numerically and 2 we use a markov chain monte carlo algorithm to retrieve a collection of model parameters that follows the posterior probability density function of the model parameters given the synthetic data set both methodologies are able to obtain the marginal distributions of the parameters and estimate their mean and standard deviation the markov chain monte carlo algorithm performs better in terms of numerical stability and number of iterations needed to characterize the distributions the inversion of seismic data alone is not able to constrain the values of porosity and permeability further than the prior distribution in turn the inversion of the electric data alone and the joint inversion of seismic and electric data are useful to constrain these parameters as well as other glacial system properties furthermore the joint inversion reduces the uncertainty of the model parameters estimates and provides more accurate results

probabilistic 3 d time lapse inversion of magnetotelluric data application to an enhanced geothermal system summary surface based monitoring of mass transfer caused by injections and extractions in deep boreholes is crucial to maximize oil gas and geothermal production inductive electromagnetic methods such as magnetotellurics are appealing for these applications due to their large penetration depths and sensitivity to changes in fluid conductivity and fracture connectivity in this work we propose a 3 d markov chain monte carlo inversion of time lapse magnetotelluric data to image mass transfer following a saline fluid injection the inversion estimates the posterior probability density function of the resulting plume and thereby quantifies model uncertainty to decrease computation times we base the parametrization on a reduced legendre moment decomposition of the plume a synthetic test shows that our methodology is effective when the electrical resistivity structure prior to the injection is well known the centre of mass and spread of the plume are well retrieved we then apply our inversion strategy to an injection experiment in an enhanced geothermal system at paralana south australia and compare it to a 3 d deterministic time lapse inversion the latter retrieves resistivity changes that are more shallow than the actual injection interval whereas the probabilistic inversion retrieves plumes that are located at the correct depths and oriented in a preferential north south direction to explain the time lapse data the inversion requires unrealistically large resistivity changes with respect to the base model we suggest that this is partly explained by unaccounted subsurface heterogeneities in the base model from which time lapse changes are inferred

information on elastic parameters obtained from the amplitudes of reflected waves seismic amplitude variation with offset data contain information on the elastic parameters of geological layers as the general solution of the inverse problem consists of a probability over the space of all possible earth models we look at the probabilities obtained using amplitude variation with offset avo data for different choices of elastic parameters a proper analysis of the information in the data requires a nontrivial definition of the probability defining the state of total ignorance on different elastic parameters seismic velocities lame s parameters etc we conclude that mass density seismic impedance and poisson s ratio constitute the best resolved parameter set when inverting seismic amplitude variation with offset data

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

pressure and fluid saturation prediction in a multicomponent reservoir using combined seismic and electromagnetic imaging this paper presents a method for combining seismic and electromagnetic em measurements to predict changes in water saturation pressure and co2 gas oil ratio in a reservoir undergoing co2 flood crosswell seismic and em data sets taken before and during co2 flooding of an oil reservoir are inverted to produce crosswell images of the change in compressional velocity shear velocity and electrical conductivity during a co2 injection pilot study a rock properties model is developed using measured log porosity fluid saturations pressure temperature bulk density sonic velocity and electrical conductivity the parameters of the rock properties model are found by an l1 norm simplex minimization of predicted and observed differences in compressional velocity and density a separate minimization using archie s law provides parameters for modeling the relations between water saturation porosity and electrical conductivity the rock properties model is used to generate relationships between changes in

joint inversion of rock properties from sonic resistivity and density well log measurements well log data are processed in order to derive subsurface physical parameters namely rock porosity fluid saturations and permeability this step involves the selection and inversion of experimental constitutive equations which are the link between the rock parameters and geophysical measurements in this paper we investigate the rock parameter observability and the reliability of well log data processing we present a visual analysis of the constitutive equations and of the inverse problem conditioning when using independently or jointly log data from different domains the existence of a common set of rock properties cross properties that influence different measurements makes it possible to reduce the ambiguities of the interpretation we select a test case in a reservoir scenario and we explore how to determine rock porosity and fluid saturation from sonic conductivity and density logs we propose a bayesian joint inversion procedure which is able to control the conditioning problems to efficiently take into account input data and model uncertainty and to provide a confidence interval for the solution the inversion procedure is validated on a real well log data set

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

coarse scale resistivity for saturation estimation in heterogeneous reservoirs based on archie s formula relations between porosity saturation and resistivity are essential when estimating in place hydrocarbon volumes the most common relation is the archie formula during the last decade several new coarse scale electromagnetic techniques for resistivity measurements have been developed applications of coarse scale resistivity must consider the underlying geologic variability of porosity and saturation to be used in such relations i have investigated the effect of small scale logging scale variability in porosity and water saturation on coarse scale resistivity for archie relations an analytical expression for coarse scale resistivity is derived based on the assumption of lognormal distributed porosity and saturation similarly coarse scale water saturation is derived assuming lognormal distributed porosity and resistivity the results suggest that the coarse scale equivalent archie resistivity is normally less than the geometric average and larger than the harmonic average of the heterogeneous res

joint inversion of marine seismic ava and csem data using statistical rock physics models and markov random fields joint inversion of seismic ava and csem data requires rock physics relationships to link seismic attributes to electric properties ideally we can connect them through reservoir parameters e g porosity and water saturation by developing physical based models such as gassmann s equations and archie s law using nearby borehole logs this could be difficult in the exploration stage because information available is typically insufficient for choosing suitable rock physics models and for subsequently obtaining reliable estimates of the associated parameters the use of improper rock physics models and the inaccuracy of the estimates of model parameters may cause misleading inversion results conversely it is easy to derive statistical relationships among seismic and electric attributes and reservoir parameters from distant borehole logs in this study we developed a bayesian model to jointly invert seismic ava and csem data for reservoir parameters using statistical rock physics models the spatial dependence of geophysical and reservoir parameters were carried out by lithotypes through markov random fields we applied the developed model to a synthetic case that simulates a co2 monitoring application we derived statistical rock physics relations from borehole logs at one location and estimated seismic p and s wave velocity ratio acoustic impedance density electric resistivity lithotypes porosity and water saturation at three different locations by conditioning to seismic ava and csem data comparison of the inversion results with their corresponding true values showed that the correlation based statistical rock physics models provide significant information for improving the joint inversion results

seismoelectric and electroseismic responses to a point source in a marine stratified model abstract we investigate the seismoelectric electroseismic wavefields excited by a point source in an air seawater three layered porous medium configuration containing a hydrocarbon layer the results show that if an explosive source for excitation is used receivers at seafloor can record the coseismic electromagnetic fields accompanying the p s fluid acoustic waves and the interface responses converted from the acoustic waves at seafloor interface and from the seismic waves at the interfaces beneath the seafloor employing a vertical electric dipole source shows that with the exception of the interface responses converted from electromagnetic waves at seafloor the interface responses converted from transmitted electromagnetic waves at the interfaces beneath the seafloor can also be identified given that the strength of the explosive source is within excitation capability of industry air guns the generated interface responses from the hydrocarbon layer can be detected by current electromagnetic sensors considering the low ambient noise at the seafloor our results demonstrate the feasibility of the seismoelectric method applied to marine hydrocarbon exploration electroseismic modelling results suggest that it is not practical to employ this method to prospect marine hydrocarbon layer due to the weak interface response signal unless a much larger current is injected into seafloor

volcanic stratigraphy of the los azufres geothermal area mexico abstract the los azufres volcanic center located 200 km northwest of mexico city is one of a number of pleistocene silicic volcanic centers with active geothermal systems that lie north of the axis of the mexican neovolcanic belt silicic volcanism began approximately 1 m y ago with eruption of the agua fria rhyodacite to high silica rhyolite lava domes and flows they are covered by rhyodacite and dacite lava domes and flows of the san andres volcano dated at 0 3 m y a north to northeast trending set of high angle normal faults is offset by a younger and more prominent east trending set that parallels the principal regional structural trend these faults cut all but the youngest volcanic rocks of the center the yerbabuena rhyodacites to high silica rhyolites these morphologically well preserved lava domes are approximately 0 15 m y old and are probably associated with air fall tuffs that blanket the western portion of the volcanic center two episodes of high heat flow suggested by studies of drill core samples and fluids from the los azufres geothermal field may correspond to two different episodes of magma injection to high levels the most recent one resulting in eruption of the yerbabuena lavas

value of information of seismic amplitude and csem resistivity we propose a method for computing the value of information in petroleum exploration a field in which decisions regarding seismic or electromagnetic data acquisition and processing are critical we estimate the monetary value in a certain context of a seismic amplitude or electromagnetic resistivity data set before purchasing the data the method is novel in the way we incorporate spatial dependence to solve large scale real world problems by integrating the decision theoretical concept of value of information with rock physics and statistics the method is based on a statistical model for saturation and porosity on a lattice along the top reservoir our model treats these variables as spatially correlated the porosity and saturation are tied to the seismic and electromagnetic data via nonlinear rock physics relations we efficiently approximate the posterior distribution for the reservoir variables in a bayesian model by fitting a gaussian at the posterior mode for transformed versions of saturation and porosity the value of information is estimated based on the prior and posterior distributions the possible revenues from the reservoir and the cost of drilling wells we illustrate the method with three examples

uncertainty analysis for the integration of seismic and controlled source electro magnetic data abstract we study the appraisal problem for the joint inversion of seismic and controlled source electro magnetic csem data and utilize rock physics models to integrate these two disparate data sets the appraisal problem is solved by adopting a bayesian model and we incorporate four representative sources of uncertainty these are uncertainties in 1 seismic wave velocity 2 electric conductivity 3 seismic data and 4 csem data the uncertainties in porosity and water saturation are quantified by a posterior random sampling in the model space of porosity and water saturation in a marine one dimensional structure we study the relative contributions from the four individual sources of uncertainty by performing several statistical experiments the uncertainties in the seismic wave velocity and electric conductivity play a more significant role on the variation of posterior uncertainty than do the seismic and csem data noise the numerical simulations also show that the uncertainty in porosity is most affected by the uncertainty in the seismic wave velocity and that the uncertainty in water saturation is most influenced by the uncertainty in electric conductivity the framework of the uncertainty analysis presented in this study can be utilized to effectively reduce the uncertainty of the porosity and water saturation derived from the integration of seismic and csem data

bayesian inversion of marine controlled source electromagnetic data offshore vancouver island canada this paper applies nonlinear bayesian inversion to marine controlled source electromagnetic csem data collected near two sites of the integrated ocean drilling program iodp expedition 311 on the northern cascadia margin to investigate subseafloor resistivity structure related to gas hydrate deposits and cold vents the cascadia margin off the west coast of vancouver island canada has a large accretionary prism where sediments are under pressure due to convergent plate boundary tectonics gas hydrate deposits and cold vent structures have previously been investigated by various geophysical methods and seabed drilling here we invert time domain csem data collected at sites u1328 and u1329 of iodp expedition 311 using bayesian methods to derive subsurface resistivity model parameters and uncertainties the bayesian information criterion is applied to determine the amount of structure number of layers in a depth dependent model that can be resolved by the data the parameter space is sampled with the metropolis hastings algorithm in principal component space utilizing parallel tempering to ensure wider and efficient sampling and convergence nonlinear inversion allows analysis of uncertain acquisition parameters such as time delays between receiver and transmitter clocks as well as input electrical current amplitude marginalizing over these instrument parameters in the inversion accounts for their contribution to the geophysical model uncertainties one dimensional inversion of time domain csem data collected at measurement sites along a survey line allows interpretation of the subsurface resistivity structure the data sets can be generally explained by models with 1 to 3 layers inversion results at u1329 at the landward edge of the gas hydrate stability zone indicate a sediment unconformity as well as potential cold vents which were previously unknown the resistivities generally increase upslope due to sediment erosion along the slope inversion results at u1328 on the middle slope suggest several vent systems close to bullseye vent in agreement with ongoing interdisciplinary observations

joint petrophysical inversion of electromagnetic and full waveform seismic data abstractaccurate determination of reservoir petrophysical parameters is of great importance for reservoir monitoring and characterization we developed a joint inversion approach for the direct estimation of in situ reservoir petrophysical parameters such as porosity and fluid saturations by jointly inverting electromagnetic and full waveform seismic measurements full waveform seismic inversions allow the exploitation of the full content of the data so that a more accurate geophysical model can be inferred electromagnetic data are linked to porosity and fluid saturations through archie s equations whereas seismic data are linked to them through rock physics fluid substitution equations for seismic modeling we used an acoustic approximation sensitivity studies combined with inversion tests show that seismic data are mainly sensitive to porosity distribution whereas electromagnetic data are more sensitive to fluid saturation distribution the separate inversion of electromagnetic or seismic data is h

deep penetrating spudcan foundations on layered clays centrifuge tests installation of independent legged jack up rigs in sea bed sediments where a strong layer overlays weaker soil can lead to catastrophic punch through with potential leg buckling or toppling of the unit although approximate methods of analysis exist for estimating the penetration resistance these do not account for the distortion of the upper layer as it punches through into the lower layer and there has been only limited visual depiction of the failure mechanism that occurs for the case of strong clay overlying weaker soil this paper reports results of centrifuge model tests undertaken to provide insight into spudcan foundation behaviour during undrained vertical penetration through a stronger clay layer into weaker material varying the strength ratio between lower and upper soil layers the thickness of the upper layer relative to the spudcan diameter and the strength gradient of the lower layer the model tests included half spudcan tests against a transparent window allowing the soil flow to be

trans dimensional bayesian inversion of controlled source electromagnetic data in the german north sea this paper presents the first controlled source electromagnetic survey carried out in the german north sea with a recently developed seafloor towed electrical dipole dipole system i e hydra ii controlled source electromagnetic data are measured processed and inverted in the time domain to estimate an electrical resistivity model of the sub seafloor the controlled source electromagnetic survey targeted a shallow phase reversed seismic reflector which potentially indicates free gas to compare the resistivity model to reflection seismic data and draw a combined interpretation we apply a trans dimensional bayesian inversion that estimates model parameters and uncertainties and samples probabilistically over the number of layers of the resistivity model the controlled source electromagnetic data errors show time varying correlations and we therefore apply a non toeplitz data covariance matrix in the inversion that is estimated from residual analysis the geological interpretation drawn from controlled source electromagnetic inversion results and borehole and reflection seismic data yield resistivities of 1 m at the seafloor which are typical for fine grained marine deposits whereas resistivities below 20 mbsf increase to 2 4 m and can be related to a transition from fine grained holocene age to unsorted coarse grained and compacted glacial sediments pleistocene age interface depths from controlled source electromagnetic inversion generally match the seismic reflector related to the contrast between the different depositional environments resistivities decrease again at greater depths to 1 m with a minimum resistivity at 300 mbsf where a seismic reflector that marks a major flooding surface of late miocene age correlates with an increased gamma ray count indicating an increased amount of fine grained sediments we suggest that the grain size may have a major impact on the electrical resistivity of the sediment with lower resistivities for fine grained sediments concerning the phase reversed seismic reflector that was targeted by the survey controlled source electromagnetic inversion results yield no indication for free gas below it as resistivities are generally elevated above the reflector we suggest that the elevated resistivities are caused by an overall decrease in porosity in the glacial sediments and that the seismic reflector could be caused by an impedance contrast at a thin low velocity layer controlled source electromagnetic interface depths near the reflector are quite uncertain and variable we conclude that the seismic interface cannot be resolved with the controlled source electromagnetic data but the thickness of the corresponding resistive layer follows the trend of the reflector that is inclined towards the west

a method of moments approach to modeling the electromagnetic response of multiple steel casings in a layered earth in oil and gas production environments controlled source electromagnetics can be used to aid brownfield exploration development and reservoir monitoring efforts however such environments typically have many highly conductive steel cased wells in the area of interest we present a modeling algorithm using a method of moments approach to calculate the electromagnetic response of multiple 3d steel cased wells of arbitrary geometry in a layered earth conductivity model this approach involves dividing each casing into a collection of segments each carrying a uniform current density a matrix is computed that describes how the casing segments interact with each other electromagnetically then we solve a linear system for the current within each casing segment given a transmitter of arbitrary frequency and location from these currents we are able to solve for the secondary electromagnetic fields produced solely by the casings at any point in our layered model and add these to the primary fields produce

resistivity image beneath an area of active methane seeps in the west svalbard continental slope the arctic continental margin contains large amounts of methane in the form of methane hydrates the west svalbard continental slope is an area where active methane seeps have been reported near the landward limit of the hydrate stability zone the presence of bottom simulating reflectors bsrs on seismic reflection data in water depths greater than 600 m suggests the presence of free gas beneath gas hydrates in the area resistivity obtained from marine controlled source electromagnetic csem data provides a useful complement to seismic methods for detecting shallow hydrate and gas as they are more resistive than surrounding water saturated sediments we acquired two csem lines in the west svalbard continental slope extending from the edge of the continental shelf 250 m water depth to water depths of around 800 m high resistivities 5 12 m observed above the bsr support the presence of gas hydrate in water depths greater than 600 m high resistivities 3 4 m at 390 600 m water depth also suggest possible hydrate occurrence within the gas hydrate stability zone ghsz of the continental slope in addition high resistivities 4 8 m landward of the ghsz are coincident with high amplitude reflectors and low velocities reported in seismic data that indicate the likely presence of free gas pore space saturation estimates using a connectivity equation suggest 20 50 per cent hydrate within the lower slope sediments and less than 12 per cent within the upper slope sediments a free gas zone beneath the ghsz 10 20 per cent gas saturation is connected to the high free gas saturated 10 45 per cent area at the edge of the continental shelf where most of the seeps are observed this evidence supports the presence of lateral free gas migration beneath the ghsz towards the continental shelf

the los azufres caldera mexico comment on the paper by l ferrari v h garduno g pasquare and a tibaldi or an attempt to understand the volcanic structure

estimating gas saturation in a thin layer by using frequency dependent amplitude versus offset modelling various models have been proposed to link partial gas saturation to seismic attenuation and dispersion suggesting that the reflection coefficient should be frequencydependent in many cases of practical importance previous approaches to studying this phenomenon typically have been limited to single interface models here we propose a modelling technique that allows us to incorporate frequency dependent reflectivity into convolutional modelling with this modelling framework seismic data can be synthesised from well logs of velocity density porosity and water saturation this forward modelling could act as a basis for inversion schemes aimed at recovering gas saturation variations with depth we present a bayesian inversion scheme for a simple thin layer case and a particular rock physics model and show that although the method is very sensitive to prior information and constraints both gas saturation and layer thickness theoretically can be estimated in the case of interfering reflections

time series analysis of diffusion interferometry data and its application to bayesian inversion of synthetic borehole pressure data interferometry methods in exploration geophysics are premised on a powerful theoretical foundation in which the ambient noise observed at discrete locations can be manipulated through either crosscorrelation or convolution to yield the response of a virtual active source experiment i e the empirical green s function abbreviated as egf without the actual deployment of physical sources sources of the ambient background may be either naturally occurring or engineered regardless the theory for diffusive systems requires them to be volumetrically distributed over an infinite domain the central question is then what region for the ambient sources matters most for good egf estimation here we build on previous work in frequency domain egf estimation by extending the analysis to the time domain where the broadband response is driven by the continuum of diffusive length scales therein analysis of the double half space diffusion model simulating a lithologic contact demonstrates that sources between the two receiver locations have the most impact on egf accuracy and that when either of the receivers is close to the lithologic contact the sources must also extend more deeply into the high diffusivity side to maintain accuracy we further examine the suitability for inversion of egf signals built with the limited and finite ambient source distributions expected in actual exploration scenarios one dimensional bayesian inversion of singlewell and crosswell configurations of a three layered system simulating a reservoir layer between two impermeable layers revealed that transient egf signals were reliably invertible when sources were constrained to the middle reservoir layer in production monitoring settings natural sources originating from pumping and subsequent flow related physics pressure diffusion electrochemical and seismoelectric effects etc this result suggests that egf signals may be a useful measure of reservoir properties

numerical investigation of bearing capacity due to spudcan penetration in sand overlying clay numerical studies were carried out to investigate the bearing capacity of spudcan foundations of offshore jack up rigs in sand overlying normally consolidated clay by using the coupled eulerian lag

a bayesian model for gas saturation estimation using marine seismic ava and csem data we develop a bayesian model to jointly invert marine seismic amplitude versus angle ava and controlled source electromagnetic csem data for a layered reservoir model we consider the porosity and fluid saturation of each layer in the reservoir the bulk and shear moduli and density of each layer not in the reservoir and the electrical conductivity of the overburden and bedrock as random variables we also consider prestack seismic ava data in a selected time window as well as real and quadrature components of the recorded electrical field as data using markov chain monte carlo mcmc sampling methods wedraw a large number of samples from the joint posterior distribution function with these samples we obtain not only the estimates of each unknown variable but also various types of uncertainty information associated with the estimation this method is applied to both synthetic and field data to investigate the combined use of seismic ava and csem data for gas saturation estimation results show th

estimating spudcan penetration resistance in stiff soft stiff clay abstractthe paper presents a new mechanism based two step approach for assessing spudcan penetration resistance in three layer stiff soft stiff clay sediments an alternative approach is also propo

spudcan penetration analysis for case histories in clay this paper reports advances in the prediction of spudcan resistance profiles during installation and preloading based on the soil flow mechanisms observed in model tests and numerical analysis and taking into account strain softening and rate dependency on the soil strength deep penetration of spudcan foundations was simulated for 14 case histories reported from different locations in the gulf of mexico where the soils were single layer predominantly normally to lightly overconsolidated clay axisymmetric and three dimensional analyses were carried out using two large deformation finite element ldfe approaches modifying the simple elastic perfectly plastic tresca soil model to allow strain softening and strain rate dependency of the shear strength the field data were compared with the results of the finite element analyses and with predictions using various design approaches excellent agreement was found particularly when a mechanism based design approach was used that differentiated between conditions before and after backflow of soil above the penetrating spudcan

application of the gauss theorem to the study of silicic calderas the calderas of la primavera los azufres and los humeros mexico abstract we explored applications including limitations of gauss s theorem to the study of silicic calderas first it enables us to determine the mass deficiency from calderas mass deficiency itself has also other potential applications it enables to make qualitative comparisons between calderas we can use the mass deficiency to test in a quick way and as a preliminary step to a formal gravity inversion for the feasibility of caldera types of simple geometry i e piston subsidence and funnel models this application can be done in a straightforward way once the mass deficiency has been determined for this purpose the mass deficiency is converted to the volume of material missing at the caldera subsequently for example this volume and the respective caldera diameter enable us to estimate the height of the cylinder fitting the piston subsidence model if the obtained parameters are congruent with the known geology and geophysical information then the model may be considered further in the inversion of the gravity data for the detailed structure other simple models i e the funnel model can also be analyzed in this way in particular when working with a piston subsidence caldera type the gauss theorem enables us to estimate the caldera collapse very difficult to obtain based on geologic information alone these possible uses of gauss s theorem are illustrated with the calderas of la primavera los azufres and los humeros caldera mexico the obtained mass deficiency from these calderas follow the linear mass deficiency diameter trend observed for other calderas in particular because of their diameters and mass deficiencies la primavera and krakatau calderas can be considered equiparable this comparison is of the most importance considering that la primavera is located in the neighbourhood of a metropolis guadalajara city since geophysical studies have already established a piston subsidence model for these calderas we assessed gauss s theorem used to estimate their collapses the respective estimated subsidences for la primavera 600 m los humeros 500 m and los azufres calderas 800 and 1200 m respectively are supported by borehole data these relatively small subsidences in relation to many calderas in western us can be explained by the presence of cylindrical to sub spherical magma chambers of smaller size than those underlying the western us calderas of yellowstone valles and long valley for example in an epicontinental setting of young quaternary rocks against precambrian cratonic basement in western us a reassessment of all the available geological and geophysical information was also done in particular the caldera limits were checked and corrected when necessary based on available 2 d gravity modeling gravity gradients and the location of the volcanic vents regional tectonic lineaments might explain many of the particular details of their structures la primavera caldera might have formed through a piecemeal subsidence controlled by the pre caldera tectonics los humeros caldera is an example of a caldera that evolved by incremental growth as the vulsiniani calderas in italy the los azufres caldera comprises two contiguous calderas emplaced in a regional tectonic depression

bayesian wavelet estimation from seismic and well data a bayesian method for wavelet estimation from seismic and well data is developed the method works both on stacked data and on prestack data in form of angle gathers the seismic forward model is based on the convolutional model where the reflectivity is calculated from the well logs possible misties between the seismic traveltimes and the time axis of the well logs errors in the log measurements and seismic noise are included in the model the estimated wavelets are given as probability density functions such that uncertainties of the wavelets are an integral part of the solution the solution is not analytically obtainable and is therefore computed by markov chain monte carlo simulation an example from sleipner field shows that the estimated wavelet has higher amplitude compared to wavelet estimation where well log errors are neglected and the uncertainty of the estimated wavelet is lower

stochastic inversion of magnetotelluric data using a sharp boundary parameterization and application to a geothermal site we developed a bayesian model to invert magnetotelluric mt data using a 2d sharp boundary parameterization we divided the 2d cross section into layers and considered the locations of interfaces and resistivity of the regions formed by the interfaces as random variables we assumed that those variables are independent in the vertical direction and dependent along the lateral direction whose spatial dependence is described by either pairwise difference or multivariate gaussian priors we used a parallel adaptive finite element algorithm to rapidly forward simulate frequency domain mt responses of the 2d resistivity structure and used markov chain monte carlo methods to draw many samples from the joint posterior probability distribution we applied the bayesian model to a synthetic case that mimics a geothermal exploration scenario our results demonstrated that the developed method is effective in estimating the resistivity and depths to interfaces and in quantifying uncertainty on the estimates we also applied the developed method to the field mt data collected from the darajat geothermal site in indonesia we compared our inversion results with those obtained from a deterministic inversion of 3d mt data they are consistent even if the two inversion methods are very different and the amount of information used for inversion is different

the amazcala caldera queretaro mexico geology and geochronology abstract the amazcala caldera is located 30 km ne of queretaro city near amazcala state of queretaro this caldera is the northernmost caldera within the central sector of the mexican volcanic belt mvb it has a 11 14 km2 elliptical shape and was formed 7 3 6 6 ma ago all caldera products are rhyolites the oldest caldera unit the ezequiel montes pumice emp is a widespread pumice fallout emplaced around 7 3 ma ago an isopach map of the emp shows two dispersal axes oriented to the sw and se with respect to the caldera the emp is 5 m thick 40 km to the sw and 35 km to the se of the caldera an isopleth map of the emp shows that pumice fragments increase in size toward the caldera from 1 cm at 40 km to 25 cm near the caldera rim the emp is a regional stratigraphic marker the colon ignimbrite dated at 7 3 0 5 ma is stratigraphically above the emp it consists of several ash flow units interbedded with minor pumice fall lapilli and ash with a minimum thickness of about 80 m at colon the caldera rim is occupied by several rhyolite lava domes and flows some of which extend 10 km from the rim these domes contain parts of fresh aphyric obsidian the last caldera event is an intracaldera rhyolitic dome near the ne rim at about 6 6 ma the dome is 4 2 km2 and is elongated in the ne direction the amazcala caldera is 480 km from the middle america trench and represents the farthest inland caldera in the central sector of the mvb its age of 7 3 6 6 ma indicates that it is the oldest caldera of the mvb so far reported this confirms the general view that the volcanic activity of the mvb initiated at its northern margin and then migrated southward in time

fault controlled geothermal fluids of the northern trans mexican volcanic belt a geochemical and isotopic study of the los geysers field valley of queretaro mexico abstract this study focuses on the geochemical and isotopic composition of natural spring waters and gases discharging at los geysers thermal area northern trans mexican volcanic belt tmvb and its relations to the regional tectonic framework water is of na hco3 type and samples were collected at temperatures ranging from 48 c to 98 c and ph from 6 8 to 9 3 stable isotope compositions 10 0 to 8 0 for 18o and 75 6 to 69 7 for d deviate from the global and local meteoric lines suggesting fractionation by steam loss the 87sr 86sr values of the thermal waters range from 0 70664 to 0 70698 indicating that most of the strontium comes from the dissolution of carbonate belonging to the late berriasian to early valanginian in the guanajuato conglomerate group and a minor component of less radiogenic sr from paleogene neogene volcano sedimentary and volcanic rocks solute geothermometers reveal a reservoir temperature around 164 c a mixture between air and hydrothermal co2 composes the bubbling gases helium isotopes data r ra 0 96 1 71 and 4he 20ne ratios 0 53 9 41 reveal that helium is dominated by a radiogenic component with a considerable mantle he contribution up to 20 8 since there is no active volcanism at the northern tmvb mantle he is best explained by transport of volatiles along active faults reaching the crust mantle boundary the conceptual hydrogeochemical model involves a thermal aquifer at 1000 m depth which is recharged by regional meteoric water flow a high thermal gradient 153 c km and fast fluid upflow to the surface along discontinuities related to regional horst and graben tectonics

joint avo inversion wavelet estimation and noise level estimation using a spatially coupled hierarchical bayesian model the main objective of the avo inversion is to obtain posterior distributions for p wave velocity s wave velocity and density from specified prior distributions seismic data and well log data the inversion problem also involves estimation of a seismic wavelet and the seismic noise level the noise model is represented by a zero mean gaussian distribution specified by a covariance matrix a method for joint avo inversion wavelet estimation and estimation of the noise level is developed in a bayesian framework the stochastic model includes uncertainty of both the elastic parameters the wavelet and the seismic and well log data the posterior distribution is explored by markov chain monte carlo simulation using the gibbs sampler algorithm the inversion algorithm has been tested on a seismic line from the heidrun field with two wells located on the line the use of a coloured seismic noise model resulted in about 10 lower uncertainties for the p wave velocity s wave velocity and density compared with a white noise model the uncertainty of the estimated wavelet is low in the heidrun example the effect of including uncertainty of the wavelet and the noise level was marginal with respect to the avo inversion results

reply to blaauw et al boslough daulton gill et al and hardiman et al younger dryas impact proxies in lake cuitzeo mexico blaauw et al 1 take issue with our age depth model for the cuitzeo core they state that no offset for our accelerator mass spectrometry ams 14c dates was quantified that our identification of the cieneguillas tephra is doubtful that we used an outdated calibration model and they object to our rejection of six ams dates in the anomalous zone

undrained bearing capacity factors for conical footings on clay introduction the bearing capacity of circular foundations on undrained clay is of fundamental importance in many geotechnical problems in particular there are a number of designs of offshore foundations where the foundation can be treated approximately as a large circular footing for instance some gravity bases the spudcan foundations of jack up units and the more recently developed caisson foundations in most cases the footing is not placed at the ground surface and it is important to take into account the depth of embedment furthermore the base of a spudcan is generally not flat but approximates a shallow cone for foundations on soft clays the effect of the increase of strength of the soil with depth needs to be taken into account and this is particularly important for large foundations the purpose of this note is to present calculations of bearing capacity factors for shallow circular foundations accounting for embedment cone angle rate of increase of strength with depth and surface roughness of the foundation the results have widespread application particularly in the offshore industry the soil is assumed to be rigid plastic with yield determined by the tresca condition with an undrained strength su the method of characteristics is used for the bearing capacity calculation as described by shield 1955 eason shield 1960 houlsby 1982 and houlsby wroth 1982a for application to undrained axisymmetric problems some previous results have been published for this problem using similar numerical techniques e g houlsby wroth 1982b salencon matar 1982 houlsby wroth 1983 tani craig 1995 martin 2001 but the study presented here involves a much more comprehensive coverage of the parameters where comparisons can be made with the previous solutions the factors differ by up to about 0 5 which gives some indication of the level of accuracy attainable with this numerical technique exceptionally the rough footing results given by tani craig 1995 are higher by up to about 5 but this may be due to a problem with their numerical integration procedures see martin randolph 2001

anatomy of the xaltipan ignimbrite at los humeros volcanic complex the largest eruption of the trans mexican volcanic belt abstract the xaltipan ignimbrite ca 290 km3 dre represents the major explosive event from los humeros volcanic complex lhvc that can be ranked as the largest caldera forming eruption of the trans mexican volcanic belt tmvb this eruption marked the onset of an explosive period of lhvc producing the collapse of the main los humeros caldera at 164 4 2 ka this paper provides a detailed characterization of the xaltipan ignimbrite anatomy and eruptive chronology that records a multi phase single eruption this eruption resulted in more complex architecture and lithofacies variations of the ignimbrite than previously reported to support these findings we present two representative composite xaltipan stratigraphic columns based on two regional reference sections together with a welding lithofacies map digital mapping energy line projections geologic sections and high resolution petrophysical characterization micro ct he pycnometry additionally we have reevaluated the magnitude dre volume and the total erupted mass of the deposit by taking into account critical primary and post emplacement processes such as lithic concentration welding grade and bulk loss due to erosion the internal architecture of the xaltipan ignimbrite is composed of two flow units and two pumice fallout deposits a basal one and an intercalated layer which formed as a result of a voluminous and sequential eruption that can be resumed in four eruptive phases the initial stage of the eruption produced a basal pumice fallout bpf phase 1 derived from a buoyant plume that eventually collapsed generating pyroclastic density current pdc deposits with intra ignimbrite pumice fallout beds this event was followed by the climactic caldera forming boiling over type eruptive phase phase 2 that generated voluminous pdcs of the lower flow unit lfu as the eruption waned a brief eruptive hiatus occurred immediately followed by a second explosive event in the western rim of the caldera with a boiling over type eruptive column depositing a discrete pumice fallout layer ipf phase 3 which later collapsed depositing less voluminous pdc fu2 in the western flank of the los humeros caldera phase 4 the anatomy of the deposits shows that the xaltipan ignimbrite was emplaced in different behaviors radially in the proximal parts and channelized and confined in the distal parts showing significant welding variations that range from non to highly welded conditions welding ranks i vi controlled mainly by the paleotopography the size calculations indicate that the deposits have an aerial extent of ca 4600 km2 an ejected bulk volume of ca 330 km3 with a dre volume of ca 290 km3 and an ejected mass of 1 1 1012 kg which points out to a paroxysmal eruption vei 6 7