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The best petrophysical models are based on direct measurements from the core. Unfortunately, core is not available in many cases, either for economic, logistical, or historical reasons. In this study, we needed to construct a detailed Field Development Plan (FDP) for the small, marginal B-9 field in the Western Offshore Basin, India, which did not merit core acquisition. The objective is to propose a workflow for building a petrophysical model with limited data sets instead of a typical FDP workflow. After analyzing the assumptions, limitations, and uncertainties involved in the petrophysical model, we used advanced petrophysical logs to reduce uncertainty and create a robust petrophysical model. We carried out a log-based petrophysical study to determine the volume of shale, porosity, saturation, and permeability. The advanced petrophysical logs including spectroscopy, nuclear magnetic resonance (NMR), formation pressures, and well testing data are used to calibrate the petrophysical model. Spectroscopy data are used to calibrate the mineralogical volumes and grain density, whereas the porosity is calibrated from NMR data. We calibrated log-derived permeability results with NMR permeability and mobility from well test data. We used heterogeneous rock analysis on petrophysical outputs to carry out petrophysical rock typing (PRT). This has helped in establishing the porosity-permeability relationship and saturation-height model for each PRT. In the absence of irreducible water saturation ([Formula: see text]) information from the core, NMR-derived [Formula: see text] is calculated and then used to calibrate the saturation model. Log-derived permeability and saturation are estimated, which agrees well with the available testing data. This provided a robust petrophysical model that served as a basis for geologic static and reservoir dynamic models. The gas-down-to and water-up-to methods are used to establish the contacts. The resulting saturation height model agreed well with the saturations derived from the log, which gave us confidence in our dynamic model.

Analysis of cores from the Lower Gondwana basin in eastern India confirms the presence of abundant organic matter, indicating a high shale gas prospective zone. Shale is intrinsically (transversely) anisotropic due to the preferential orientation of anisotropic clay platelets depending on the geological processes of deposition. Such anisotropy needs to be incorporated in modeling for an accurate explanation of observed seismic anisotropy in shale. Combined anisotropic formulations of self-consistent approximation (SCA) and differential effective medium (DEM) theory is the most suitable among existing theories to deal with the complex microstructure of shale. We start by predicting the bulk and rigidity moduli of clay mineral aggregates as 21 GPa and 10 GPa from our model, which are difficult to measure in the laboratory due to the small crystal size of clay. A concept of host medium (HM) is presented here, which is constituted of clay mineral aggregates, kerogen, and unconnected pores. Other minerals are added in the biconnected composite constituted of HM and connected pores filled with water. An orientation distribution function (ODF) of clay particles is determined using the combined SCA-DEM theory from the observed ultrasonic velocity measurements. Our model shows strong intrinsic anisotropy at the shallow depth that decreases with depth because of the changes in the microstructure of the shale. The P-velocity predicted from our model, widely used Biot–Gassmann theory (BGT) and Biot–Gassmann theory modified by Lee (BGTL) match well with the measured data where P-wave anisotropy is insignificant. We also predict from our model the volume of kerogen and total organic carbon as 26–43% and 6–8%, respectively.

In present study, three recently developed geostatistical methods, single attribute analysis, multi-attribute analysis and probabilistic neural network algorithm have been used to predict porosity in inter well region for Blackfoot field, Alberta, Canada, an offshore oil field. These techniques make use of seismic attributes, generated by model based inversion and colored inversion techniques. The principle objective of the study is to find the suitable combination of seismic inversion and geostatistical techniques to predict porosity and identification of prospective zones in 3D seismic volume. The porosity estimated from these geostatistical approaches is corroborated with the well log porosity. The results suggest that all the three implemented geostatistical methods are efficient and reliable to predict the porosity but the multi-attribute and probabilistic neural network analysis provide more accurate and high resolution porosity sections. A low impedance (6000–8000 m/s g/cc) and high porosity (> 15%) zone is interpreted from inverted impedance and porosity sections respectively between 1060 and 1075 ms time interval and is characterized as reservoir. The qualitative and quantitative results demonstrate that of all the employed geostatistical methods, the probabilistic neural network along with model based inversion is the most efficient method for predicting porosity in inter well region.

Porosity is the most crucial parameter to assess the potential of fine grain rocks like shale. Shale has very low connected porosity and permeability, which controls the fluid flow and migration. Therefore, detection of connected and closed pore network in digital images can help to evaluate porosity and permeability of rock. The objective of this paper is to characterize Gondwana shale of eastern India in micro-scale, that is, in terms of its porosity, pore-structure and pore size distribution for its shale gas potential. Watershed-transform has been performed to segment pores, throat and mineral grains by using 2-D Scan Electron Microscopy (SEM) images of Gondwana shale of Barren-Measures Formation. The watershed transform, when coupled with morphological operators and a customized disk shape structuring element, segments the pores and throats efficiently and estimates the porosity of Gondwana shale for the very first time. Three Gondwana shale samples from different borehole depth is used for this study. Finally, porosity measured in a laboratory using Mercury Injection Capillary Pressure (MICP) test, is compared with the numerically modeled watershed porosity values, which shows good agreement between the two sets of results with precisely segmented pore bodies and throat bodies at appropriate locations. Most of the pores in the sample are found to be in a mesopore category (2–50 nm). The porosity values measured using MICP ranges from 5.01% to 6.53% whereas the porosity estimated from watershed image segmentation ranges from 5.21% to 6.91%.

A prominent magnetic low is seen over the Himalayan–Tibetan plateau and the northern Indian shield region in satellite anomaly data. Here we model the sources for the observed magnetic low in the region. The induced magnetic field is modelled based on a crustal magnetisation model computed in a Geographical Information System, with horizontal layers containing susceptibility derived from the geology and a tectonic map of the region and global seismic crustal thickness model. The modelling results indicate that the upper crust of the Himalayan–Tibetan plateau is magnetised while much of the middle crust and the lower crust appears non-magnetic. Thermal modelling corroborates the predictions, placing the Curie isotherm in the southern Tibetan Plateau no deeper than 10 km. The results suggest that the sources of the magnetic low observed in the region are likely to be a thin magnetised crust, because of the elevated Curie isotherm in this region. The Indian shield region is believed to extend up to the foothills of the Himalayas in the north, but most of the shield is obscured by a thick sediment cover of the Gangetic Plains to the north. The magnetic field modelling suggests a weakly magnetic crust in the region indicating either the Indian shield may not extend up to the base of the Himalayas or the lower crust in the region has been modified.

In many parts of the world sedimentary horizons with potential for hydrocarbon are located below flood basalt provinces. However, the presence of high velocity basaltic overburden makes delineation of sediments difficult due to the low velocity layer problem. Electrical and electromagnetic methods have been used in such scenarios because of the good electrical conductivity contrast between basalts and underlying sediments. However, mapping of the target sediments becomes difficult when the layer is thin as the data errors due to inherent noise lead to equivalent solutions. To tackle such difficult situations, a joint inversion scheme incorporating seismic reflection and refraction, magnetotelluric and deep electrical resistivity datasets is presented. Efficacy of the scheme is tested for a model comprising a thin sedimentary layer sandwiched between a thick basalt cover and a granitic basement. The results indicate that the parameters of the target sedimentary layer are either poorly resolved or equivalent solutions are obtained by the inversion of individual datasets. Joint inversions of seismic reflection (RFLS) and refraction (RFRS), or DC and MT dataset pairs provide improved results and the range of equivalent solutions is narrowed down. Combination of any three of the above datasets leads to further narrowing of this range and improvements in mean model estimates. Joint inversion incorporating all the datasets is found to yield good estimates of the structure. Resolution analysis is carried out to appraise estimates of various model parameters obtained by jointly inverting different combinations of datasets.

Estimating depth to the bottom of the magnetic crust (synonymously called the Curie isotherm) on a regional scale from long wavelength magnetic anomalies requires that large areas of survey data be used for the calculations. There is still no consensus on the minimum survey area required to arrive at a reliable estimate of the Curie isotherm depth. From the available aeromagnetic data over India, depth to the Curie isotherm is estimated using spectral techniques. We found that the significant spectral maxima required to calculate the depth to the Curie isotherm existed only for 4° × 4° blocks in southern peninsular India up to 18 °N latitude and for 5° × 5° blocks for the rest of the region in Central India. As aeromagnetic data coverage over India is incomplete, we also calculate the magnetic crustal thickness from the lithospheric model of the CHAMP satellite data for the whole country, using an iterative forward modeling approach. The calculated Curie isotherm is shallow in the mobile belts and deeper in the cratons in both the derivations. As the Curie isotherms calculated from the two data sets collected at very different altitudes using totally different techniques, match reasonably well, it lends credence to the methodology adopted. The derived Curie isotherm depth map is in accordance with the basic structural trend of the major tectonic units within the Indian subcontinent. A comparison is made of the calculated Curie isotherm depth with Moho depths along available DSS profiles over India and it is found that the Curie depth is generally shallower than the Moho depth implying that it possibly represents a thermal boundary rather than a compositional change. Further, we find that high magnitude earthquakes are associated with high gradients in Curie depth.

It is generally believed that an anomaly should be expected over the continent–ocean (C–O) boundary in the global magnetic anomaly maps. However, such maps prepared at satellite altitude do not show an anomaly over most of the C–O boundary. We address this issue by a forward modelling technique. We use a Geographic Information System (GIS) technique to integrate information of the various geological units of continents and oceans, the seismic crustal structure, and standard susceptibility values of the rock types and compute a global vertically integrated susceptibility (VIS) model. In addition, a remanent magnetisation model is used for the oceanic crust. Combining the induced and remanent magnetisation models, the crustal field anomaly is predicted at a satellite altitude of 400 km for spherical harmonic degrees 16–80. The results show that an anomaly is indeed not predicted over the C–O boundaries where the oceanic crust is flanked by continental regions Phanerozoic in age. We demonstrate that, largely, a C–O anomaly over these regions is not even expected in the global magnetic anomaly map for degrees 1–80, confirming that it is not a long-wavelength feature masked by the core field. An anomaly, however, is predicted over some parts of the C–O boundary where the flanking geological provinces over the continents are Precambrian in age. We argue that the VIS values for the continents and oceans are largely comparable and therefore no anomaly is expected over most of the C–O boundary.

The central Red Sea rift is considered to be an embryonic ocean. It is characterised by high heat flow, with more than 90% of the heat flow measurements exceeding the world mean and high values extending to the coasts — providing good prospects for geothermal energy resources. In this study, we aim to map the depth to the Curie isotherm (580 °C) in the central Red Sea based on magnetic data. A modified spectral analysis technique, the “de-fractal spectral depth method” is developed and used to estimate the top and bottom boundaries of the magnetised layer. We use a mathematical relationship between the observed power spectrum due to fractal magnetisation and an equivalent random magnetisation power spectrum. The de-fractal approach removes the effect of fractal magnetisation from the observed power spectrum and estimates the parameters of depth to top and depth to bottom of the magnetised layer using iterative forward modelling of the power spectrum. We applied the de-fractal approach to 12 windows of magnetic data along a profile across the central Red Sea from onshore Sudan to onshore Saudi Arabia. The results indicate variable magnetic bottom depths ranging from 8.4 km in the rift axis to about 18.9 km in the marginal areas. Comparison of these depths with published Moho depths, based on seismic refraction constrained 3D inversion of gravity data, showed that the magnetic bottom in the rift area corresponds closely to the Moho, whereas in the margins it is considerably shallower than the Moho. Forward modelling of heat flow data suggests that depth to the Curie isotherm in the centre of the rift is also close to the Moho depth. Thus Curie isotherm depths estimated from magnetic data may well be imaging the depth to the Curie temperature along the whole profile. Geotherms constrained by the interpreted Curie isotherm depths have subsequently been calculated at three points across the rift — indicating the variation in the likely temperature profile with depth.

We construct a model of remanence for the oceans, combine it with a model of induced magnetization for the whole Earth from a previous study, compute the predicted lithospheric geomagnetic field and compare the result with a model, MF7, that is based on satellite data. Remanence is computed by assigning magnetizations to the oceanic lithosphere acquired at the location and time of formation. The magnetizing field is assumed to be an axial dipole that switches polarity with the reversal time scale. The magnetization evolves with time by decay of thermal remanence and acquisition of chemical remanence. The direction of remanence is calculated by Euler rotation of the original geomagnetic field direction with respect to an absolute reference frame, significantly improving previous results which did not include realistic oceanic magnetization computed this way. Remanence only accounts for 24 per cent of the energy of the oceanic magnetization, the induced magnetization being dominant, increasing slightly to 30 per cent of the part of the magnetization responsible for generating geomagnetic anomalies and 39 per cent of the Lowes energy of the geomagnetic anomalies. This is because our model of oceanic crust and lithosphere is fairly uniform, and a uniform layer magnetized by a magnetic field of internal origin produces no external field. The largest anomalies are produced by oceanic lithosphere magnetized during the Cretaceous Normal Superchron. Away from ridges and magnetic quiet zones the prediction fails to match the MF7 values; these are also generally, but not always, somewhat smaller than the observations. This may indicate that the magnetization estimates are too small, in which case the most likely error is in the poorly-known magnetization deep in the crust or upper mantle, or it may indicate some other source such as locally underplated continental lithosphere or anomalous oceanic crust, or even small-scale core fields.

SUMMARY The Earth’s magnetic field is mainly produced within the Earth’s liquid and electrically conducting core, as a result of a process known as the geodynamo. Many other sources also contribute to the magnetic signal accessible to observation at the Earth’s surface, partly obscuring the main core magnetic field signal. Thanks to a series of very successful satellites and to advances in magnetic field modelling techniques, considerable progress has, however, been made in the recent years toward better identifying the signal of each of these sources. In particular, temporal changes in the field of internal origin happen to be detectable now in spherical harmonic degrees up to, perhaps, 16. All of these changes are usually attributed to changes in the core field itself, the secular variation, on the ground that the lithospheric magnetization cannot produce such signals. It has, however, been pointed out, on empirical grounds, that temporal changes in the field of internal origin produced by the induced part of the lithospheric magnetization could dominate the core field signal beyond degree 22. This short note revisits this issue by taking advantage of our improved knowledge of the small-scale field changes and of the likely sources of the lithospheric field. We rely on a simple extrapolation of the observed spatial spectrum of the field changes beyond degree 16 and use a forward approach based on a recent geological model of lithospheric magnetization. This leads us to confirm that the main cause of the observed changes in the field of internal origin up to some critical degree, N C, is indeed likely to be the secular variation of the core field, but that the signal produced by the time-varying lithospheric field is bound to dominate and conceal the time-varying core signal beyond that critical degree, in very much the same way the permanent component of the lithospheric field dominates and conceals the permanent component of the core field beyond degree 14. All uncertainties taken into account, we estimate N C to lie between 22 and 24. We, however, also note that in practice, the main limitation to the observation of the core field small-scale secular variation is not so much its concealing by the field of lithospheric origin but its fast changing nature and small magnitude. This leads us to conclude that whereas cumulative small-scale lithospheric field changes might be detected some day, detection of core-field secular variation beyond degree 18 is likely to remain a severe challenge for some time.

International audience; A long standing question in geomagnetism is whether the time variation of the induced crustal field is a detectable quantity and, if so, at which spatial wavelengths. We tackle this problem with the help of a forward modeling approach using a vertically integrated susceptibility (VIS) grid of theEarth’s crust. For spherical harmonic degrees 15–90, we estimate the root mean square of the crustal magnetic field secular variation to amount 0.06–0.12 nT/yr at the terrestrial surface between epochs 1960–2002.5. The geographical distribution of the signal shows absolute values reaching0.65–1.30 nT/yr over South America. Unfortunately, most of the world magnetic observatories currently lie on quasistationary features where the crustal field signal variations are expected to be very low. However, this long sought signal could be detected over well chosen regions, provided that satellite, observatory, and repeat station measurements are available over several decades.

Marine and airborne magnetic anomaly data have been collected for more than half a century, providing global coverage of the Earth. Furthermore, the German CHAMP satellite is providing increasingly accurate information on large-scale magnetic anomalies. The World Digital Magnetic Anomaly Map project is an international effort to integrate all available near-surface and satellite magnetic anomaly data into a global map database. Teams of researchers were invited to produce candidate maps using a common pool of data sets. Here we present the National Geophysical Data Center (NGDC) candidate. To produce a homogeneous map, the near-surface data were first line-leveled and then merged by Least Squares Collocation. Long wavelengths were found to agree surprisingly well with independent satellite information. This validates our final processing step of merging the short-wavelength part of the near-surface data with long-wavelength satellite magnetic anomalies.

[1] Six years of low-orbit CHAMP satellite magnetic measurements have provided an exceptionally high-quality data resource for lithospheric magnetic field modeling and interpretation. Here we describe the fifth-generation satellite-only magnetic field model MF5. The model extends to spherical harmonic degree 100. As a result of careful data selection, extensive corrections, filtering, and line leveling, the model has low noise levels, even if evaluated at the Earth's surface. The model is particularly suited for inferring large-scale structure and composition of the lithosphere. It is also meant to serve as the long-wavelength part of continental- and global-scale marine and aeromagnetic anomaly maps.

International audience; We report here a simple but practical method to combine systematically the magnetic surveys of disparate specifications from marine, ground, aeromagnetic and satellite platforms that aims to be helpful in the framework of the world digital magnetic anomaly map project. Because the satellite coverage and data quality is more uniform than near-surface surveys, it is adopted as the base map for wavelengths larger than 400 km and up to spherical harmonic degree inclusive of 16 (not, vert, similar 2500 km). After checking that satellite models and large aeromagnetic wavelengths are not consistent, we filter out wavelengths longer than 400 km in aeromagnetic compilations. Existing regional grids and, when available, individual surveys, are merged to give a more uniform spatial variation of the magnetic anomalies. The effects of merging the grids are examined by plotting profiles and difference maps of the overlapping regions. A preliminary global map is obtained after combining the magnetic anomaly map with the downward continued CHAMP satellite derived magnetic anomaly map. Forthcoming availability of new datasets, as well as improvements in the procedure, will help to produce increasingly reliable maps.

Current models of continental crust favor an increase in magnetization with depth. Here we report a counter example from the Canadian Cordillera where almost a full thickness of non-magnetic continental crust is suggested by joint interpretation of magnetic and seismic data. The magnetic field over the Cordillera is characterized by complex, short-wavelength ( 100 km) portion of the Cordilleran field is subdued and mainly featureless, and suggests a lack of magnetic sources at greater depths. Seismic reflection and refraction data from three major transects in the Yukon and British Columbia, Canada support this interpretation and indicate that sedimentary-like formations make up the majority of the crust. The dominance of shallow, upper crustal magnetization in the Canadian Cordillera contrasts with the generally-held view that the lower continental crust is the primary source for long-wavelength magnetic anomalies. Sources for these anomalies are often assumed to be located in the lower crust when surface magnetizations are insufficient to produce such anomalies or no correlation exists between the magnetic field and the mapped surface geology. The Canadian Cordillera appears to be an example of a non-magnetic lower crust overlain by a more magnetic upper crust that is, however, not magnetized strongly enough to produce significant long-wavelength magnetic anomalies.

[1] Reliable global crustal field anomaly maps produced from magnetic measurements of the CHAMP satellite mission now allow for quantitative geological studies of crustal structure and composition. We have developed a GIS based forward modeling technique to model these anomaly maps. On the basis of the geologic and tectonic maps of the world, laboratory susceptibility values of the occurring rock types, and the seismic thickness of the crust, a vertically integrated susceptibility grid is generated in the GIS system. In addition, a remanent magnetization grid is computed for the oceanic crust using a digital isochron map of the ocean floor and rotation models of the paleoplates. Combining the global VIS and remanent magnetization grids, the vertical magnetic field anomaly is computed at satellite altitude and compared with the corresponding CHAMP magnetic anomaly map. Over the oceans, induced and remanent magnetization explains well the prominent observed anomalies over the Cretaceous quiet zones. We also find a good agreement between predicted and observed anomalies over the continents. Remaining discrepancies between the predicted and observed anomalies can be used to adjust poorly known boundaries and the composition of the buried Precambrian provinces, until the recomputed anomalies fit the observed anomalies. The feasibility of this approach is demonstrated on Greenland, the West African Craton, Bangui in central Africa, and the Kolyma-Omolon Block in Siberia. We conclude that quantitative information on the lateral extent, the composition and the thickness of the lower crust within a Precambrian province can thus be inferred from the new satellite magnetic anomaly maps.

Satellite missions over more than three decades, have added immensely to our understanding of the geomagnetic field. Here, we give a comparison of the lithospheric field models, prepared by various workers from POGO, Magsat, Orsted and CHAMP satellite data. Mapping the global lithospheric anomalies requires careful reductions which include the main field, its secular variation (SV) and corrections for the external magnetospheric and ionospheric contributions to the data recorded by satellite. The discrepancies between the various maps are mainly due to differences in the rigor with which the data is processed, in the selection of the magnetically quiet periods and in the algorithms used to estimate the spherical harmonic coefficients of scalar potential or the total intensity of the field. The models also strongly differ in how the magnetospheric and ionospheric contributions have been detected and eliminated in order to correct the equatorial, mid-latitude and polar latitude data. We plot the anomaly of the total intensity of the field at 450 km altitude (except ALP94 which is at 400 km).