Quantifying the effect of the air/water interface in marine active source EM

Abstract

The marine controlled source EM surveying method has become an accepted tool for deep water exploration for oil and gas reserves. In shallow water (<500m) data are complicated by the signal which interacts with the water–air interface which can dominate the response at the receiver. By decomposing the 1-D response to an impulsive current dipole source in the time domain and frequency domain I separate the response into: (1) an earth response, (2) a direct arrival, (3) a coupled airwave which travels through the air and (4) a surface coupling term which travels through the earth. The last two terms are coupled to the sea surface as well as to the earth resistivity structure but one travels through the air between source and receiver and the other only through the earth. Using a range of simple models I quantify the effect of these four terms in the time domain and the frequency domain. The results show that in shallow water the total response is significantly larger than in very deep water and that a large part of this extra energy comes from surface coupling, which is reflected at the sea surface and does not propagate through the air but through the earth. As a result, this term is highly sensitive to the resistivity of the earth. This means that the sea surface in shallow water not only significantly increases the signal strength of CSEM data but also enhances the sensitivity to subsurface resistivity structure. Compared with the surface coupling term, the coupled part of the airwave contains very little information about the earth, and is limited to the near surface.Time domain separation of the airwave from the surface coupling response results in greater sensitivity to a deep resistive target than frequency domain separation although there is also reasonable sensitivity in the frequency domain.