Over the past several years, Marine Controlled-Source EM (MCSEM) techniques have been successfully applied in deep water (water depth >1 km) for oil/gas exploration. In contrast, application of this technology in shallow water, although available, is challenged due to ‘airwaves’ that mask the signal from the target reservoir at depth. Based upon ‘lateral wave’ theory, we propose three airwave-mitigation approaches to reduce the effects of these arrivals on MCSEM data. By comparing the detectability of a target in deep water versus shallow water for models including bathymetry, we show that the airwave effects in a shallow water environment can be reduced leading to a greater reservoir detectability.
Marine Controlled-Source ElectroMagnetics (MCSEM) has been applied as a useful tool to derisk oil/gas exploration in the ocean environment. This method uses a high-powered horizontal electric dipole (HED) to transmit a low-frequency (0.01 – 10 Hz) EM signal through seawater column and seafloor. The source is typically towed just above an array of multicomponent EM receivers that are deployed on seafloor to record the generated EM responses. By analyzing these EM responses, the bulk electrical resistivity of seafloor sediments can be estimated both as a function of lateral position and more importantly, depth. Because oil is relatively resistive compared to conductive sediments, the resulting resistivity analysis can be used to determine the probability that a given region of the subsurface contains hydrocarbon [Young and Cox, 1981; Chave and Cox, 1982; Eidesmo et al., 2002; Srnka et al., 2006].
The depth of the seawater column has strong influence on measured EM signals. Because of this, early applications of MCSEM for hydrocarbon exploration concentrated on targets in deep water scenarios, where the water depth is generally greater than 1 km [Andreis and MacGregor, 2008]. Conventional transmitters are towed deep to maximize EM coupling between sources, receivers and reservoir targets, as well as to mitigate the so-called “airwave” effects [Constable, 2003; Lu et al., 2005; LØseth et al., 2008]. The airwave effects are notorious in shallow water, where the useful signals from the reservoir targets might be totally masked by the airwaves, which generally are thought to contain little information about the subsurface.
Because the airwave masks the deeper reservoir signal, much effort has been devoted to this problem. Several techniques have been published for dealing with frequency-domain (FD) EM data; these include subtraction of a background model response from a total field [Lu et al., 2005], up- and down-ward separation of the measured fields [Amundsen, 2003], measurement of vertical electric fields [Constable, 2003], combining spatial derivatives of the measured electric and magnetic fields [MacGregor and Sinha, 2004] , direct computation of the airwaves [Weidelt, 2007; Nordskag and Amundsen, 2007], using crossed-dipole sources [LØseth and Amundsen, 2007], and using reciprocity/decomposition of EM fields [van den Burg et al., 2008]. In the time-domain (TD), the airwaves are considered to be separable in the early time data [Ziolkowski and Wright, 2007], although their effects still might prevail and interfere with other components in the mid time range [Weiss, 2007].
