Processing of marine controlled-source electromagnetic data
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The idea to use controlled-source electromagnetics in a marine environment for identification of possible hydrocarbons in the subsurface has been developed during the last decade. This imposes renewed insight and interest into the propagation theory of electromagnetic fields in layered media, acquisition setup for optimal data recordings and solid processing of the data for interpretation. In a shallow water environment the part of the electromagnetic field that propagates unattenuated through the air has a profound impact on the recorded data, rendering the interpretation difficult. This document contains articles on processing of marine controlled-source electromagnetic data by removing unwanted signals due to the water layer, which holds no or little information about the subsurface. The main focus in this thesis is removal of the so called airwave and the effect of the water layer from acquired CSEM data, such that the processed data are ready for interpretation or for further processing. First, we used an asymptotically derived space-domain equation that describes the source-induced airwave component in a water half-space bounded by air that is well known from the electromagnetic literature. We demonstrate that by taking into account the reflections and reverberations of the airwave component in the water column between the seabed and the sea surface at the source side and the receiver side, the airwave effect can be generalized to a water layer. Then we show how the source-induced airwave may affect the marine controlled-source electromagnetic recordings and how the airwave component in principle can be suppressed by a modeling-and-subtraction approach. Second, we present a theory to eliminate from recorded multi-component source, multi-component receiver marine electromagnetic measurements the effect of the physical source radiation pattern and the scattering response of the waterlayer. The multi-component sources are assumed to be orthogonally aligned above the receivers at the seabottom. Other than the position of the sources, no source characteristics are required. The integral equation method, which for short is denoted by Lorentz water-layer elimination, follows from Lorentz’ reciprocity theorem. It requires information only of the electromagnetic parameters at the receiver level to decompose the electromagnetic measurements into upgoing and downgoing constituents. Lorentz water-layer elimination replaces the water layer with a homogeneous half-space with properties equal to those of the sea-bed. The source is redatumed to the receiver depth. Third, a method for removing the water layer effect from a recorded controlledsource electromagnetic line survey is presented. Since Lorentz water-layer elimination requires data recorded over a 2D grid on the seafloor, the method is not directly applicable on data from a controlled-source electromagnetic line profile survey. We show that line profile data can be extrapolated or mapped onto a 2D grid when the dipole source is towed with an angle to the sail line, so that Lorentz water-layer elimination can be applied. Our method which is based on a plane layer assumption, is verified through numerical examples. And finally, we present cross-property relations between electrical conductivity and seismic velocity, stiffness moduli, and density. This can be obtained by expressing the porosity in terms of those properties. There are many possible ways to combine the constitutive equations to obtain a relation, each one representing a given type of rock. The relations depend on the assumptions to obtain the constitutive equations. In the electromagnetic case, the equations involve Archie’s law and its modifications for a conducting frame, the Hashin-Shtrikman (HS) bounds, and the self-similar and complex refractionindex method (CRIM) models. In the elastic case, the stress-strain relations are mainly based on the time-average equation, the HS bounds, and the Gassmann equation. Also, expressions for dry rocks and for anisotropic media, using Backus averaging, are analyzed. The relations are applied to shale saturated with brine (overburden) and to sandstone saturated with oil (reservoir). Tests with sections of a North Sea well log show that the best fit is given by the relation between the Gassmann velocity and the CRIM, selfsimilar, and Archie models for the conductivity.