The use of OBC for CSEM is proven to work, and in fact to work better than nodes. Work is now in progress on using a towed streamer or equivalent platform.

ABSTRACTThis paper discusses the asymptotic behaviour of the electromagnetic fields received on the sea‐bed (target response), as well as the fields distributed inside a thin resistive target, generated by a horizontal electric dipole above the sea‐bed in marine controlled‐source electromagnetics for hydrocarbon exploration. It is found that the guided wave supported by a thin resistive target can be expressed as a single‐mode exponential function. A simple closed‐form expression is derived to relate the single‐mode wavenumber of the guided wave to the model parameters: the resistivity and thickness of the target layer, the sea‐bed resistivity and the frequency. When the air‐wave is removed, the guided wave is dominant among the fields received on the sea‐bed at far offset. Hence the wavenumber of the guided wave can be calculated from the fields measured on the sea‐bed. The closed‐form expression can then be used to invert the target property from the calculated wavenumber and hence, can be considered as a hydrocarbon indicator.

We have developed a 2.5D finite-element modeling (FEM) method for marine controlled-source electromagnetic (CSEM) applications in stratified anisotropic media. The main feature of the method is that delta sources are used to solve the governing partial differential equations for cases with and without a resistive target and to obtain the difference of these two solutions as the scattered field from the target. The total field is then the sum of the analytical background field calculated with a 1D modeling method and the difference or scattered field mentioned above. Compared with a conventional direct solution (using delta sources directly in a 2.5D formulation), the new method has smaller near-field error as a result of the source singularity and smaller boundary reflections. The new method does not require a dense mesh in the source region, which thereby reduces the total number of variables to be solved. In this way, the modeling time can be kept within a few minutes for some cases. We show that the maximum relative error of the calculation can be kept within 2% for targets at depths of approximately [Formula: see text]. The method is valid for stratified anisotropic media. The anisotropic modeling examples show that (1) marine CSEM is predominantly sensitive to target vertical resistivity and not to target horizontal resistivity, provided that the targets are thin, horizontal, high-resistivity layers and (2) marine CSEM is sensitive to the horizontal resistivity of the conductive sediments surrounding the target (e.g., the overburden).

PreviousNext No AccessSEG Technical Program Expanded Abstracts 2007Azimuth decomposition of SBL dataAuthors: Frank A. MaaøSvein Erling JohnstadPål T. GabrielsenMartin PanznerFrank A. MaaøEMGSSearch for more papers by this author, Svein Erling JohnstadHydro O&ESearch for more papers by this author, Pål T. GabrielsenEMGSSearch for more papers by this author, and Martin PanznerEMGSSearch for more papers by this authorhttps://doi.org/10.1190/1.2792484 SectionsAboutPDF/ePub ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InRedditEmail Abstract Seabed logging data are traditionally acquired along a line and can often be compared and interpreted directly. When data is acquired with different geometry and the source is no longer pointing towards the receiver this interpretation can be difficult. We consider an azimuth decomposition which takes into account the 3D acquisition geometry and that recovers the inline response under a plane layer assumption. We show that this approach can be used when this assumption is violated. However, the azimuth angle must be restricted under such conditions.Permalink: https://doi.org/10.1190/1.2792484FiguresReferencesRelatedDetailsCited By3D CSEM modeling and time-lapse sensitivity analysis for subsurface CO2 storageAnwar H. Bhuyian, Martin Landrø, and Ståle E. Johansen8 August 2012 | GEOPHYSICS, Vol. 77, No. 5Hydrocarbon reservoir thickness resolution in 3D CSEM anisotropic inversionJan Petter Morten, Astrid Kornberg Bjørke, and Anh Kiet Nguyen21 October 2010Frontier exploration by electromagnetic scanning — A deep water exampleJoanne Suffert, Pranaya Sangvai, Friedrich Roth, Anil Tyagi, and Rabi Bastia15 December 2008 SEG Technical Program Expanded Abstracts 2007ISSN (print):1052-3812 ISSN (online):1949-4645Copyright: 2007 Pages: 3124 publication data© 2007 Copyright © 2007 Society of Exploration GeophysicistsPublisher:Society of Exploration Geophysicists HistoryPublished: 14 Sep 2007 CITATION INFORMATION Frank A. Maaø, Svein Erling Johnstad, Pål T. Gabrielsen, and Martin Panzner, (2007), "Azimuth decomposition of SBL data," SEG Technical Program Expanded Abstracts : 564-568. https://doi.org/10.1190/1.2792484 Plain-Language Summary PDF DownloadLoading ...

In seabed logging applications, only three types of sources: VED, HED-R (in-line) and HMD-P (cross line) are sensitive in detecting the target - a thin horizontal hydrocarbon layer. The other three types of the sources: VMD, HED-P (cross line), HMD-R are not sensitive in detecting the target.

Analytical results show that the scattered field from a hydrocarbon layer takes the form of an exponentially decay function. Both the real part and the imaginary part of the complex propagation constant are functions of target properties (resistivity and thickness etc.), and can be used as the hydrocarbon indicators.We have shown the correlation between the position of the pole existing in the TM reflection coefficient and the decay rate of a scattered field. By investigating the variance of the pole position versus the model parameters, we have derived an approximate expression, which relates the measurable complex propagation constant of a scattered field to the hydrocarbon and seabed properties.

ABSTRACTA recently developed laboratory method allows for simultaneous imaging of fluid distribution and measurements of acoustic‐wave velocities during flooding experiments. Using a specially developed acoustic sample holder that combines high pressure capacity with good transparency for X‐rays, it becomes possible to investigate relationships between velocity and fluid saturation at reservoir stress levels. High‐resolution 3D images can be constructed from thin slices of cross‐sectional computer‐tomography scans (CT scans) covering the entire rock‐core volume, and from imaging the distribution of fluid at different saturation levels. The X‐ray imaging clearly adds a new dimension to rock‐physics measurements; it can be used in the explanation of variations in measured velocities from core‐scale heterogeneities. Computer tomography gives a detailed visualization of density regimes in reservoir rocks within a core. This allows an examination of the interior of core samples, revealing inhomogeneities, porosity and fluid distribution. This mapping will not only lead to an explanation of acoustic‐velocity measurements; it may also contribute to an increased understanding of the fluid‐flow process and gas/liquid mixing mechanisms in rock. Immiscible and miscible flow in core plugs can be mapped simultaneously with acoustic measurements. The effects of core heterogeneity and experimentally introduced effects can be separated, to clarify the validity of measured velocity relationships.

A seabed logging R&D study was conducted on the Troll Field in December 2003 – January 2004. Different EM waveforms and frequencies were tested to optimize the hydrocarbon response from this shallow water gas and oil field. The survey consists of 41 receivers, deployed along a line crossing the Oil Province, the Western Gas Province and the Eastern Gas Province of the Troll Field. All receivers recorded two orthogonal components of the horizontal electric field, and 12 receivers measured in addition two orthogonal components of the horizontal magnetic field. The R&D study was carried out as an attempt to qualify seabed logging for shallow waters (less than 500 m) and optimize acquisition parameters for such cases. Up until now the marine controlled source EM method has been qualified for use in deep waters (more than 1000 m water depth) mainly because of the disturbing influence from the air wave in shallower water areas. The Troll Field seabed logging data is of excellent quality and helps in solving problems related to shallow water applications of this method.

H014 Controlled Source Electromagnetic Imaging on the Grane Field North Sea. Abstract 1 In Autumn 2004 Hydro Oil and Energy in collaboration with OHM Ltd carried out a controlled source electromagnetic imaging (CSEMI) survey on the Grane Field North Sea. Electric field signals were recorded by a total of 13 receivers deployed on and around the target structure for two source tow lines. Good quality data were recovered to sourcereceiver separations of 10-13 km. Initial interpretation indicates that in spite of the shallow water in the area (c. 120 m) the responses are sensitive to sub-seafloor structure over the depth

1 A05 REMOTE CHARACTERIZATION OF HYDROCARBON FILLED RESERVOIRS AT THE TROLL FIELD BY SEA BED LOGGING Introduction SeaBed Logging (SBL) is a remote sensing technique which gives information about subsurface resistivity variations by the use of electromagnetic energy. The method has been demonstrated both theoretically (Kong et al. 2002) and in practice by several calibration and commercial surveys (Ellingsrud et al. 2002 Røsten et al. 2002 Amundsen et al. 2004 and Wicklund and Fanavoll 2004). This abstract presents preliminary results from a scientific SBL survey across the Troll Field. The overall objective of the study is to obtain an improved

C-12 VELOCITY SATURATION AND FLUID DISTRIBUTION MEASURED BY A 4D-CT LABORATORY METHOD Summary 1 A recently developed laboratory method allows for simultaneous imaging of fluid distribution and measurement of acoustic wave velocities during flooding experiments. Using a specially designed acoustic sample holder that combines high pressure capacity with good transparency for X-rays it is possible to investigate velocity-fluid saturation relations at reservoir stress levels. High-resolution 3-D images are constructed from thin slices of cross-sectional CT-scans covering the entire rock core volume showing the distribution of fluid at different saturation levels. The system is briefly described and examples are presented from

P058 A SEA BED LOGGING (SBL) CALIBRATION SURVEY OVER THE ORMEN LANGE GAS FIELD Introduction 1 Remote resistivity sensing of buried resistive layers in conductive sediments a concept called Sea Bed Logging (SBL) has been demonstrated both theoretically by Kong et al. (2002) and Eidesmo et al. (2002) and in practice by a survey over a known oil field offshore Angola in November 2000 (Ellingsrud et al. 2002). The Angola survey was run over an area which is ideal for the SBL technique mainly due to large water depth and shallow reservoir. In November 2002 ElectroMagnetic GeoServices (emgs) established by

M-25 A METHOD OF INVESTIGATING THE EFFECT OF MUD INVASION ON THE ACOUSTIC AND DENSITY LOGS BRIAN A. FARRELLY SVEIN E. JOHNSTAD and REIDAR KANESTRØM Summary 1 In this paper we have demonstrated how wireline logs can be combined with the results of core analysis to investigate the effect of mud filtrate on the density and acoustic logs. The density log is most seriously affected by the mud filtrate but the P - waves are also significantly influenced. The invasion of mud filtrate is related to the permeability. Since the synthetic seismic response is computed from the wireline logs it

B-7 IMPROVEMENTS IN PORE FLUID SUBSTITUTION Abstract 1 The conventional way of performing pore fluid substitution is to use Gassmann's equation which requires knowledge of the dry rock properties as well as matrix parameters. We have introduced an alternative approach based on lithological models where all the necessary parameters are implicit in the model. This is a very efficient procedure which automatically takes care of the effect of lithological variations. To overcome some of the weaknesses in connection with the application of Gassmann's equation we have combined the equation with a dry rock model. This combination can be done in