Svein Ellingsrud

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.

Decomposition of electromagnetic fields into upgoing and downgoing components

This paper gives a unified treatment of electromagnetic EM field decomposition into upgoing and downgoing components forconductiveandnonconductivemedia,wheretheelectromagneticdataaremeasuredonaplaneinwhichtheelectricpermittivity,magneticpermeability,andelectricalconductivityareknown constants with respect to space and time. Above and below the plane of measurement, the medium can be arbitrarily inhomogeneousandanisotropic. In particular, the proposed decomposition theory applies to marine EM, low-frequency data acquired for hydrocarbon mapping where the upgoing components of the recordedfield guided and refracted from the reservoir, that are of interest for the interpretation. The direct-source field, the refracted airwave induced by the source, the reflected field from the sea surface, and most magnetotelluric noise traveling downward just below the seabed are field components that are considered to be noise in electromagneticmeasurements. The viability and validity of the decomposition method is demonstrated using modeled and real marine EM data, also termed seabed logging SBL data. The synthetic data are simulated in a model that is fairly representative of the geologic area wheretherealSBLwerecollected.Theresultsfromthesynthetic data study therefore are used to assist in the interpretation of the realdatafromanareawith320-mwaterdepthaboveaknowngas province offshore Norway. The effect of the airwave is seen clearly in measured data. After field decomposition just below the seabed, the upgoing component of the recorded electric field has almost linear phase, indicating that most of the effect of the airwavecomponenthasbeenremoved.

Low-frequency electromagnetic fields in applied geophysics: Waves or diffusion?

Low-frequency electromagnetic (EM) signal propagation in geophysical applications is sometimes referred to as diffusion and sometimes as waves. In the following we discuss the mathematical and physical approaches behind the use of the different terms. The basic theory of EM wave propagation is reviewed. From a frequency-domain description we show that all of the well-known mathematical tools of wave theory, including an asymptotic ray-series description, can be applied for both nondispersive waves in nonconductive materials and low-frequency waves in conductive materials. We consider the EM field from an electric dipole source and show that a common frequency-domain description yields both the undistorted pulses in nonconductive materials and the strongly distorted pulses in conductive materials. We also show that the diffusion-equation approximation of low-frequency EM fields in conductive materials gives the correct mathematical description, and this equation has wave solutions. Having considered both a wave-picture approach and a diffusion approach to the problem, we discuss the possible confusion that the use of these terms might lead to.

On the orientation and absolute phase of marine CSEM receivers

Receiver orientation can be recovered from electric and/or magnetic data if it is not directly measured. A receiver dropped on the seabed will end up with an arbitrary orientation, which means that the recorded electric and magnetic x- and y-components will point in arbitrary directions. We demonstrate how both electric and magnetic data can be used to rotate the field data to a coordinate system where the x-direction points in the inline or towline direction or 180° with respect to this direction. The amplitudes of electric and magnetic marine CSEM data are highly offset dependent so we introduce a median filtering approach to handle this problem. An inspection of the electric and/or magnetic phase after normalization with the source-current phase can resolve the remaining problem of the 180 degree spatial rotation. The result is electric and magnetic data where the x-component points in the positive towline direction. We analyze the case of lost temporal synchronization between receivers and the transmi...

Combined Depth Migration and Constrained Inversion of Low Frequency Electromagnetic Data

In order to migrate electromagnetic data from a low frequency controlled source, 3D electromagnetic Green functions should be used since the near-fleld efiects may be large. Imaging principles of the correlation type do not have su-cient depth sensitivity to be used in a one-pass migration step. To increase the depth sensitivity, a non-local operator is introduced in the imaging condition. This operator accounts for the lateral propagation of the EM fleld in the high resistivity reservoir. The non-local operator depends on two parameters related to the resistivity and thickness of an assumed resistivity anomaly. We propose to estimate these parameters from a limited set of forward modeling operations. DOI: 10.2529/PIERS060907093858