Removal of Sea Surface Related Wavefields from CSEM Data

Abstract

For a controlled-source electromagnetic (CSEM) survey in a shallow water envi- ronment, the presence of the sea surface signiflcantly hinders the interpretation of the measured data. The electromagnetic (EM) waveflelds are disturbed by the sea surface. The removal of the sea-surface related wave phenomena from the data is an important step in order to robustly interpret the collected data. We propose a processing method by which the sea-surface related multiples would be removed, while a priori knowledge of the EM source wavelet becomes super∞u- ous. The governing equations are obtained from an appropriate application of the EM reciprocity theorem that relates on one side the EM flelds in the actual measurement conflguration including the sea surface and on the other side the EM flelds in a desired source conflguration and in the absence of the sea surface, where the water layer is extended to inflnity. In a controlled-source EM (CSEM) survey (1), it is necessary to interpret the measurements in such a way that a prediction of the presence of hydrocarbons in the sedimentary layers can be made. However, in a shallow water environment, the presence of the sea surface hinders this interpretation. Electromagnetic waveflelds are partly re∞ected and partly transmitted by the sea surface. This means that the source signal is contaminated by its so-called source ghost signal and that the received signals are contaminated by the so-called receiver ghost. Further the receiver ghost can be considered as secondary source signals that are transmitted in the earth. Hence, removal of all these sea surface related electromagnetic waveflelds from the data is a prerequisite step before actual interpretation of the data can take place. In this paper we show that an appropriate use of the electromagnetic reciprocity theorem leads to the mathematical equations for a consistent removal of sea-surface related waveflelds. The actual EM wavefleld is denoted as f ^ E(x); ^ H(x)g in the frequency domain (with frequency parameter s = j!). The position in space is denoted as x = fx1;x2;x3g. The coordinates x1 and x2 denote the horizontal directions, while x3 denotes the vertical direction pointing into the earth. The wavefleld is generated by an electromagnetic source in the sea at x S = fx1;x2;x S g. On the sea bottom (not necessarily a horizontal plane), we measure the EM wavefleld re∞ected by the earth geology at the sea bottom (see Figure 1, left). We assume that there exists a horizontal plane at x3 = x obs between the source level and the sea bottom. It is assumed that the sea water is homogeneous with complex permittivity ^ and real constant permeability . In this domain, outside the sources, the electromagnetic flelds satisfy Maxwell's equations r £ ^