Asymptotic airwave modeling for marine controlled-source electromagnetic surveying

Abstract

Marine controlled-source electromagnetic (CSEM) surveying, also called seabed logging, is a technique for finding resistive layers in the subsurface, which can be hydrocarbon-saturated reservoirs. The EM signal from the horizontal electric dipole source induces an airwave along the air/sea surface that interferes with the signal from the subsurface and dominates at larger offsets. The airwave commonly denotes the energy that propagates from the source via the atmosphere to the receiver on the seabed. The airwave component is especially problematic in shallow waters, where it is less attenuated during its up-and-down propagation in the water column than for deeper waters. An asymptotically derived space-domain equation that describes the airwavecomponent in a water half-space bounded by air is well known from the EM 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 both the source side and the receiver side, this equation can be generalized to a water layer. The new model we present can serve as an integrity check for other modeling schemes. Our airwave model shows how the airwave component depends on water depth, source-receiver range, frequency, and seabed resistivity. The model is verified by extensive numerical testing. Finally, we show how the source-induced airwave may affect the marine CSEM recordings and how the airwave component in principle can be suppressed by a modeling-and-subtraction approach.