In the past several years, marine controlled-source electromagnetic (MCSEM) techniques have been applied successfully in deep water (depth > 1 km) for oil and gas exploration. The application of this technology in shallow water is challenged, however, because of “airwaves” that mask the signal from the target reservoir at depth. Based upon the understanding that an airwave is a lateral wave, which can be analytically expressed in a dual-half-space resistivity model, we propose three airwave-mitigation approaches to reduce the effects of these airwaves on MCSEM data. In the EM “x-bucking” approach, the effect of the airwaves can be “bucked” out from two measurements by using the analytic expression of the airwave. The frequency derivative (dE/dFreq) approach takes advantages of the unique characteristics of the airwaves in frequency domain, enhancing the reservoir signals while suppressing the airwave. The magnetotelluric (MT) stripping method uses the plane-wave feature of the airwaves and subtraction of the lateral wave electric component, which is obtained from measured marine MT impedance and controlled-source electromagnetics (CSEM) data, to generate a new data set in which the effects of the airwaves are removed substantially. By comparing the detectability, which is defined as the ratio of inline Ex fields between a reservoir model and a corresponding baseline model, for a reservoir target in deep water versus shallow water with a moderate 2D bathymetry, we show that the effects of the airwaves in shallow water can be reduced in the data, leading to greater reservoir detectability. In addition, these approaches have been applied successfully to a real shallow water MCSEM data set in which the detectability to the deeper resistive basement is enhanced.
Source modification for efficiency enhancement of marine controlled-source electromagnetic method
