On the orientation and absolute phase of marine CSEM receivers

Abstract

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 [Formula: see text] 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 transmitter and show that the proper absolute phase can be recovered approximately by producing precalculated tables for zero or minimum offset of the electric and/or magnetic phase. These tables depend on the frequency, the transmitter length, the water conductivity, and the distance between the receiver and the midpoint of the transmitter. These four quantities are measured during a marine CSEM survey. The method requires electric and/or magnetic data that are not saturated at short offsets. The magnetic zero or minimum offset phase shows less variation with these four parameters than the electric zero or minimum offset phase. Hence, if magnetic data are available, they are preferable to electric data for this type of processing.