Bayesian lithology/fluid inversion—comparison of two algorithms

Algorithms for inversion of seismic prestack AVO data into lithology-fluid classes in a vertical profile are evaluated. The inversion is defined in a Bayesian setting where the prior model for the lithology-fluid classes is a Markov chain, and the likelihood model relates seismic data and elastic material properties to these classes. The likelihood model is approximated such that the posterior model can be calculated recursively using the extremely efficient forward–backward algorithm. The impact of the approximation in the likelihood model is evaluated empirically by comparing results from the approximate approach with results generated from the exact posterior model. The exact posterior is assessed by sampling using a sophisticated Markov chain Monte Carlo simulation algorithm. The simulation algorithm is iterative, and it requires considerable computer resources. Seven realistic evaluation models are defined, from which synthetic seismic data are generated. Using identical seismic data, the approximate marginal posterior is calculated and the exact marginal posterior is assessed. It is concluded that the approximate likelihood model preserves 50% to 90% of the information content in the exact likelihood model.     

Late Cretaceous – Early Tertiary sedimentation and tectonic inversion in the southern Netherlands

Analysis of the Upper Cretaceous stratigraphy of the Peel Block reveals the basin development of the block to have been influenced by both the inversion of the Roer Valley Graben and Central Netherlands Basin, and the overall Late Cretaceous transgression. Sediments of Santonian to Danian age were deposited on the block. These sediments are compared with the detailed lithostratigraphy of southern Limburg, where Late Cretaceous strata are exposed. Four successions can be recognised in southern Limburg. The two oldest successions, the Santonian Oploo Formation (new name, proposed in the present contribution) and the mainly Early Campanian Vaals Formation, are restricted to the central and northern parts of the block. These siliciclastic formations were deposited under the influence of inversion of the Roer Valley Graben and the Central Netherlands Basin, as well as under the influence of a rising sea level. Towards the north, sands of the Oploo Formation grade into marls and chalks of the Ommelanden Formation. The two youngest successions comprise the largely Late Campanian to Maastrichtian Gulpen and Maastricht Formations and the Danian Houthem Formation. These chalk formations were deposited under the influence of regional subsidence during a sea-level highstand. Subsequent to deposition of the Houthem Formation, a regional regression triggered a change from shallow-marine carbonate to paralic siliciclastic deposition.     

EM2INV — A finite difference based algorithm for two-dimensional inversion of geoelectromagnetic data

The paper presents an efficient finite difference based 2D-inversion algorithm, EM2INV, for geoelectromagnetic data. The special features of the algorithm are

Moho depth determination beneath the Zagros Mountains from 3D inversion of gravity data

Due to its geological and economic importance, the Zagros Mountains have been investigated by many researchers during the last decades. Nevertheless, in spite of all the studies conducted on the region, there are still some controversial problems concerning the structure of the Zagros Mountains, including crustal depths, demanding more insights into understanding the crustal constraints of the region. Accordingly, we have conducted a gravity study to determine Moho depth map of the Zagros Mountains region, including its major structural domains from the coastal plain of the Persian Gulf to central Iran. The employed data are the densest and most accurate terrestrial gravity data set observed until now with the precision of 5 μGal and resolution of 5 arc-minute by 5 arc-minute. To image Moho depth variations, gravity inversion software GROWTH2.0 is used, proposing the possibility to model stratified structures by means of a semi-objective exploratory 3D inversion approach. The obtained results reveal the crustal thickness of ~?30–35 km underneath the southwestern most Zagros Fold-Thrust Belt increasing northeastward to 48 km. The maximum Moho depth is estimated ~?62 km below the Zagros Mountains belt along the Main Zagros Thrust. Northeast of the study area, an average crustal thickness of 46 km is computed beneath Urumieh–Dokhtar magmatic arc and central Iran.     

Research on the inversion of elemental abundances from Chang’E-2 X-ray spectrometry data

The elemental abundances of lunar surface are the important clues to study the formation and evolution history of the Moon. In 2010, China＇s Chang＇E-2 （CE-2） lunar orbiter carried a set of X-ray spectrometer （XRS） to investigate the elemental abundances of the lunar surface. During CE-2＇s life span around the Moon, the XRS ex- perienced several events of solar flare. The X-ray solar monitor onboard recorded the spectra of solar X-rays at the same time. In this paper, we introduced the XRS instrument and data product. We analyzed the characteristics of the XRS data. Using the data obtained during an M solar flare event which had occurred on Feb. 16, 201 l, we derived the elemental abundances ofMg, A1, Si, Ca and Fe of the lunar surface in the Oceanus Procellarum. Finally, we dis- cussed the factors that influence the accuracy of the inversion.     

Detecting a magnesiowüstite phase transition in the lower mantle by inversion of geomagnetic data

Global geomagnetic data are inverted for detecting a high-conductivity layer at depths of 1500–2000 km to test the hypothesis of a magnesiowüstite phase transition in the lower mantle. We present the results of processing of both synthetic and global data—average monthly values of the geomagnetic field from 1920 to 2009. The inverted global data are consistent with the possible existence of a high-conductivity layer at great depths in the lower mantle.     

Importance of coloured inversion technique for thin hydrocarbon sand reservoir detection – A case in mid Cambay basin

Cambay basin is an intra-cratonic rift graben formed as a result of rifting which was occurred in late Cretaceous with Deccan lava eruption through linear trending NNW to SSE directional basin. The Deccan basalt forms the basement over which more than 7–11 km thick piles of Tertiary sediments have been deposited during syn-rift and post-rift phases of basin development. Cambay basin has been considered as one of the significant hydrocarbon prolific basin in India. The biggest challenge in current days for this basin is further exploration or exploration under development stage in small marginal field or unexplored left out areas in the basin part as most of the areas are already explored/discovered by various small to big E&;P (exploration &; production) industrial players. In this present study one such small marginal field has been chosen for “Exploration under Development” portfolio in mid Cambay basin. The amount of oil-in-place volume, investment and techno-economics analysis of small marginal field has made this study area. In view of further hydrocarbon exploration in this area this kind of study will provide a robust support in limited dataset. The reservoir sand quality of the study area is discrete, thin and less permeable. This kind of sand body detection through classical seismic interpretation approach is difficult and there will be always a big amount of uncertainties for findings the pay reservoir sand. In view of the limitation of available data and challenging geological setup of the reservoir, a quantitative approach has been taken to detect the thin reservoir sand in this study area. Primarily coloured inversion technique has been applied on post-stack seismic data based upon well to seismic correlation and reservoir sand detection in seismic interpretation and well log property analysis. This technique has produced higher detectability impedance/property volume with respect to normal post-stack seismic data signature. Based on high contrast impedance/elastic property further seismic based attribute analysis on reservoir section has been performed. The attribute analysis has been made along surface and 3D seismic data level, provided clear image about the thin hydrocarbon sand reservoir. Based upon quantitative interpretation approach coloured inverted volume the prospect was chosen for further drilling in the study area and drilling of that sand was turned to be a hydrocarbon discovery prospect. The unconventional approaches e.g. coloured inversion with limited dataset for this kind of small marginal field has potential to find the hydrocarbon.     

Regional sandstone-type uranium mineralization rooted in Oligo–Miocene tectonic inversion in the Songliao Basin,NE China

The tectonic inversion of the Songliao Basin during the Oligo–Miocene may have played an important role in controlling the development of sandstone-type uranium deposits (SUDs). Here we investigate drill holes along a southeast to northwest section in this basin based on apatite fission-track (AFT) and zircon fission-track (ZFT) techniques. We present 50 data from 15 deep boreholes at different depths between 665 and 3956 m and different structural units including grabens and horsts formed in the Early Cretaceous beneath the basin. The results of the effective AFT ages are 100 ± 11 to 2.3 ± 0.4 Ma (P(x²) > 5%) and ZFT ages are 97.5–20.4 Ma (including binomial peak ages). These results reveal that the basin underwent two distinct stages of rapid cooling after Late Cretaceous. In the first stage, during the Late Cretaceous–Early Paleogene (~80–50 Ma), tectonic uplift occurred in all of the structural units including grabens and horsts, which was marked by an unconformity between the latest Cretaceous Mingshui and the Eocene Yi'an formations. In the second stage, during the Oligo–Miocene (~40–10 Ma), tectonic uplift occurred mainly in the grabens but not in the horsts, corresponding with a few sediments of the Neogene Da'an and Taikang formations. We propose that the folds and the thrust faults mostly characterize in the second stage indicating a major tectonic inversion in the basin. The shifting of the two stages was probably in response to differences in the subduction angles and directions of motion of the Paleo-Pacific Plate from the southeast. Combined with previous information, it was demonstrated that most of the U mineralization ages are younger than 40 Ma, with a peak in the Miocene or later (<20 Ma). We thus propose that the SUDs have been redistributed and redeposited locally in successive stages during and after the Oligo–Miocene tectonic inversion.     

Intrusion versus inversion—a 3D density model of the southern rim of the Northwest German Basin

An unsolved problem of regional importance for both the evolution and structure of the Northwest German Basin is the existence or non-existence of the so-called Bramsche Massif. Explaining the nature of this massif and the cause of a related strong, positive Bouguer anomaly (Bramsche Anomaly) is critical. In the study described here, we tested an existing “intrusion model” against a newer “inversion model” in the southern Northwest German Basin. In the intrusion model, the strongly-positive Bouguer anomaly represents the gravity effect of an intrusion at depths between 6 and 10 km. More recent interpretations invoke tectonic inversion rather than intrusion to explain increased burial and the low level of hydrocarbon maturity found in boreholes. We tested these different interpretations by constructing 3D forward density models to 15 km depth. The intrusion model was updated and adjusted to incorporate recent data and we also modelled pre-Zechstein structures using different scenarios. The final model has a very good fit between measured and modelled gravity fields. Based on currently available seismic and structural models, as well as borehole density measurements, we show that the positive Bouguer anomaly cannot be modeled without a high-density, intrusive-like body at depth. However, further in-sight into the crustal structures of the Bramsche region requires more detailed investigations. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Precise estimation of pressure–temperature paths from zoned minerals using Markov random field modeling: theory and synthetic inversion

The chemical zoning profile in metamorphic minerals is often used to deduce the pressure–temperature (P–T) history of rock. However, it remains difficult to restore detailed paths from zoned minerals because thermobarometric evaluation of metamorphic conditions involves several uncertainties, including measurement errors and geological noise. We propose a new stochastic framework for estimating precise P–T paths from a chemical zoning structure using the Markov random field (MRF) model, which is a type of Bayesian stochastic method that is often applied to image analysis. The continuity of pressure and temperature during mineral growth is incorporated by Gaussian Markov chains as prior probabilities in order to apply the MRF model to the P–T path inversion. The most probable P–T path can be obtained by maximizing the posterior probability of the sequential set of P and T given the observed compositions of zoned minerals. Synthetic P–T inversion tests were conducted in order to investigate the effectiveness and validity of the proposed model from zoned Mg–Fe–Ca garnet in the divariant KNCFMASH system. In the present study, the steepest descent method was implemented in order to maximize the posterior probability using the Markov chain Monte Carlo algorithm. The proposed method successfully reproduced the detailed shape of the synthetic P–T path by eliminating appropriately the statistical compositional noises without operator’s subjectivity and prior knowledge. It was also used to simultaneously evaluate the uncertainty of pressure, temperature, and mineral compositions for all measurement points. The MRF method may have potential to deal with several geological uncertainties, which cause cumbersome systematic errors, by its Bayesian approach and flexible formalism, so that it comprises potentially powerful tools for various inverse problems in petrology.     

Tectonic inversion in a segmented foreland basin from extensional to piggy back settings: The Tucumán basin in NW Argentina

The Tucumán foreland basin is bounded by: 1) basement cored ranges with elevations over 6000 m in the west; 2) inverted extensional grabens to the north; 3) basement thrust blocks in the south and 4) basement cored small ranges in the east. This foreland basin is located between two geological provinces: the Sierras Pampeanas and the Santa Bárbara system.Cretaceous Salta rifting extended southwards covering the entire eastern part of the province of Tucumán in NW Argentina. Syn-rift and post-rift deposits can be recognized in accordance with their architectural geometries. Foreland basin sediments progressively covered the rift deposits as the Andean orogen propagated towards the east.Despite some early studies, the Tucumán basin is poorly documented. For the present study, 44 sections of 2D seismic surveys amounting to more than 730 km were used to describe the structure and the depositional evolution of the basin. The present structure is the result of a long sequence of events that includes a compressional deformation during the Paleozoic, a rifting stage during the Cretaceous and a foreland stage during the late Cenozoic. Although tectonic inversion, which has played a role during the foreland stage since the Miocene, can be observed in many sectors of the basin, it is more prominent along the margins. Reactivation of old basement discontinuities and inversion of Cretaceous normal faults produced the compartmentalization of the foreland, giving rise to the present shape of the Tucumán basin. This evolution is recorded in the Neogene deposits.     

Transient inversion during the opening stageof the Wilson cycle “Sardic phase” in the Iberian Variscides –Stratigraphic and tectonic record

The Variscides of Iberia have a bilateral symmetry with east vergence in the eastern branch and west vergence in the western, on both sides of a Centro-Iberian Zone (CIZ), with predominant steep axial planes. All the structures curve around the Ibero-Armorican Arc (IAA). Unconformities in the sedimentary sequences of Cambrian to Early Ordovician age were ascribed to “Sardic phase” by correlation with similar tectonosedimentary events in Sardinia. Recent studies showed diachronism between these events in Sardinia and Iberia but migration of major geodynamic regime in time may be due to regional variation of major events at plate tectonic scale. We studied in detail two critical areas in the CIZ, the Marão anticline in the NE and the Amêndoa-Carvoeiro synform in the SW. Two unconformities can be put in evidence, as elsewhere in CIZ. A stronger lower unconformity of a Volcano-Sedimentary Complex of Lower Arenig (and Tremadocian?) age on top of a Cambrian clastic sequence with flysch characteristics; and a milder upper unconformity of Armorican Quartzite of Arenig age on both the Volcano-Sedimentary Complex and the Cambrian sequences. The lithostratigraphy of the studied areas is described and correlated with other areas in Iberia. The Volcano-Sedimentary Complex and coeval magmatic bodies with bimodal composition are briefly described. The Sardic event corresponds to folds with steep axial planes at high angles to Variscan structures that produce the penetrative cleavage that cut across the unconformity surfaces. Sardic thrusts are also present and can be explained by thin-skinned compressive tectonics. Sardic folds and thrusts suggest a brief period of transient inversion between a major extensional regime from Cambrian to Devonian. The obliquity of Sardic structures to Variscan compression suggests a component of transpression during the Sardic tectonic event, corresponding to a tectonically enhanced unconformity near the Cambro-Ordovician boundary. The transient Sardic inversion is interpreted in terms of a break-up unconformity related to the migration of an intracratonic rift; in the Ordovician this rift moves into the SW of Ossa Morena Zone (OMZ) and since then become the SW Iberia suture during the Variscan Wilson cycle. This migration induced transient compression and dextral strike-slip in the major boundary between CIZ and OMZ due to presence of incipient primary curvature in this segment of IAA.     

Waveform inversion and focal mechanisms of two weak earthquakes in Cordillera Principal (Argentina) between 35° and 35.5° S

Only few (six) focal mechanism, in CMT Catalog, have been so far known for intraplate shallow events in the Andean chain close to Chile–Argentina state border at latitudes ∼35° S. We add two more mechanisms, depths and moment magnitudes by carefully analyzing full waveforms of weak events recorded by broad-band stations of the Chile Argentina Geophysical Experiment (southern profile). The moment magnitudes of both events (Mw = 3.6 and 3.7) are lower than the duration magnitudes (Md = 4.0 and 4.29) reported by NEIC. The source depth, constrained by waveforms for one of the studied events (5.5–8.5 km) seems to be considerably shallower than the hypocenter depth located by means of arrival times (∼20 km). The waveform analysis was complemented by first-motion polarities which resulted in an uncertainty assessment of the focal mechanism. Event 1 (2001-11-03) has a strike-slip mechanism with a small normal component and almost vertical nodal planes in the north-south and east-west directions. The north-south nodal plane could be related to the Calabozos faults system. Event 2 (2002-02-16) has a strike-slip mechanism with a small thrust component. The latter event (its subhorizontal nodal plane) could be associated with the El Diablo-El Fierro fault system. Dextral strike-slip solutions are consistent with recent studies in the area.     

New basin modelling results from the Polish part of the Central European Basin system: implications for the Late Cretaceous–Early Paleogene structural inversion

Combined subsidence and thermal 1D modelling was performed on six well-sections located in the north-western Mid-Polish Trough/Swell in the eastern part of the Central European Basin system. The modelling allowed constraining quantitatively both the Mesozoic subsidence and the magnitude of the Late Cretaceous–Paleocene inversion and erosion. The latter most probably reached 2,400 m in the Mid-Polish Swell area. The modelled Upper Cretaceous thickness did not exceed 500 m, and probably corresponded to 200–300 m in the swell area as compared with more than 2,000 m in the adjacent non-inverted part of the basin. Such Upper Cretaceous thickness pattern implies early onset of inversion processes, probably in the Late Turonian or Coniacian. Our modelling, coupled with previous results of stratigraphic and seismic studies, demonstrates that the relatively low sedimentation rates in the inverted part of the basin during the Late Cretaceous were the net result of several discrete pulses of non-deposition and/or erosion that were progressively more pronounced towards the trough axis. The last phase of inversion started in the Late Maastrichtian and was responsible for the total amount of erosion, which removed also the reduced Upper Cretaceous deposits. According to our modelling results, a Late Cretaceous heat-flow regime which is similar to the present-day conditions (about 50 mW/m²) was responsible for the observed organic maturity of the Permian-Mesozoic rocks. This conclusion does not affect the possibility of Late Carboniferous–Permian and Late Permian–Early Triassic thermal events.     

Was the lethal eruption of Lake Nyos related to a double CO2/H2O density inversion?

The 1986 lethal eruption of Lake Nyos (Cameroon) was caused by a sudden inversion between deep, CO₂-loaded bottom lake waters and denser, gas-free surface waters. A deep CO₂ source has been found in fluid inclusions which occur predominantly in clinopyroxenes from lherzolitic mantle xenoliths, brought to the surface by the last erupted alkali basalts. P–T conditions of CO₂ trapping correspond to a gas density equal (or higher) than that of liquid water. It is suggested that this dense CO₂, found in many ultrabasic mantle xenoliths worldwide, has accumulated at km depth, below a column of descending lake water. It may remain in a stable state for a long period, as long as the temperature is above the density inversion temperature for pure H₂O/CO₂ systems. At an estimated depth of about 3 km, cooling by descending waters (to about 30 °C) induces a density inversion for the upper part of the CO₂ reservoir. This causes a constant, regular upstream of low-density CO₂ which, in its turn, feeds the shallower lake density inversion.