Review of Ray Theory Applications in Modelling and Imaging of Seismic Data

Throughout the last twenty years, 3D seismic ray modelling has developed from a research tool to a more operational tool that has gained growing interest in the petroleum industry. Various areas of application have been established and new ones are under development. Many of these applications require a modelling system with flexible, robust and efficient modelling algorithms in the core. The present paper reviews the basic elements of such a system, based on the open model concept and the wavefront construction technique. In the latter, ervený's dynamic ray tracing is an intrinsic part. The modelling system can be used for generating ray attributes and synthetic seismograms for realistic 3D surveys with tens of thousands of shots and receivers. Moreover, some other types of application areas are illustrated: Production of Green's functions for prestack depth migration and hybrid modelling (combined ray and finite-difference modelling), attribute mapping and illumination analysis, both for survey planning and interpretation. Finally, the concepts of isochron rays and velocity rays related to seismic isochrons have been introduced recently, with very interesting future applications.     

Numerical modelling of direct current electrical resistivity for the characterisation of cracks in soils

Fractures or cracks in rocks and soils are thin planar features of high resistivity, embedded in a more conductive bulk volume. In soils, several processes, particularly swelling and shrinkage of clays, result in cracks and can regenerate macroporosity. Their detection and the assessment of their evolution necessitate the use of non-invasive techniques. Although cracks have a major influence on the electrical resistivity of bulk material their complete geometrical characterisation achieved through the use of electrical resistivity measurements remains difficult due to the lack of appropriate inversion models. An inversion model based on the method of moments (MoM) has been developed, and is presented here. Direct simulations demonstrate the model's ability to reconstruct real experimental data. The influence of various object parameters on the model's accuracy was tested, showing that the model is sensitive to all the geometric characteristics of a crack. The inversion of both simulated and experimental data has demonstrated the model's ability to calculate the initial position, width and depth of several different cracks.     

Bathymay : la structure sous-marine de Mayotte révélée par l'imagerie multifaisceaux

The BATHYMAY marine survey was carried out in January 2004 around the French volcanic island of Mayotte, onboard the research vessel Marion Dufresne 2. Multibeam bathymetry revealed for the first time the morphology of the whole outer slope. Preliminary interpretations show broad canyons that deeply incise the slope, numerous volcanoes and huge landslides. Large submarine plateaus are also highlighted, bounded by steep cliffs and active normal faults. A model for Mayotte is suggested, in which eruptive activity, sometimes influenced by normal faulting, was concentrated along a N140° regional line (the Comoros archipelago axis), with landslide activity concentrated on the northeastern and southwestern flanks of this line. To cite this article: J.-C. Audru et al., C. R. Geoscience 338 (2006).     

Transformation of Monoaromatic hydrocarbons to organic acids in anoxic groundwater environment

The transformation of benzene and a series of alkylbenzenes was studied in anoxic groundwater of a shallow glacial-outwash aquifer near Bemidji, Minnesota, U.S.A. Monoaromatic hydrocarbons, the most water-soluble components of crude oil, were transported downgradient of an oil spill, forming a plume of contaminated groundwater. Organic acids that were not original components of the oil were identified in the anoxic groundwater. The highest concentrations of these oxidized organic compounds were found in the anoxic plume where a decrease in concentrations of structurally related alkylbenzenes was observed. These results suggest that biological transformation of benzene and alkylbenzenes to organic acid intermediates may be an important attenuation process in anoxic environments. The transformation of a complex mixture of hydrocarbons to a series of corresponding oxidation products in an anoxic subsurface environment provides new insight into in situ anaerobic degradation processes.     

Evaluating the sulfur isotopic composition of biodegraded petroleum: The case of the Western Canada Sedimentary Basin

The sulfur isotopic composition (δ³⁴S) of petroleum is believed to be affected mainly by sulfur incorporation reactions into the sedimentary organic matter during the early diagenesis. However, secondary processes could affect the original δ³⁴S of oil under the effect of thermal maturity or of the microbial activity of biodegraded reservoirs. In this study, the different processes that may affect the δ³⁴S of in-reservoir oils were assessed based on the sulfur content and isotopes of a series of oil and core samples coming from various reservoirs of the Lower Cretaceous Mannville Group, Western Canada Sedimentary Basin (WCSB). Based on the molecular study, these samples appear to have reached various levels of maturity and biodegradation, ranging from 0 to 6.5 on the biodegradation scale of Peters and Moldowan. In addition, mixing of organic matter coming from different source rocks was identified based on the comparison with extensive correlation studies performed in the WCSB.Investigation of the δ³⁴S shows a trend that seems a priori correlated to the level of biodegradation. However, a careful interpretation of molecular and sulfur isotope data leads to the conclusion that the observed δ³⁴S variations have rather to be ascribed to contributions of oils generated by various source rocks. Alternatively, variations of δ³⁴S could neither be related to maturity differences nor to kinetic effects during organic sulfur compounds biodegradation. In the case of some specific core samples showing a common origin based on biomarker study, δ³⁴S variations might not be related to different sources but to secondary sulfur incorporation/exchange processes occurring within the reservoir. These processes would involve reduced sulfur species from bacterial sulfate reduction formed in situ or migrated into Mannville reservoirs. This hypothesis is supported by laboratory experiments showing sulfur exchange/incorporation under plausible conditions for shallow reservoirs.