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.