The marine controlled-source electromagnetic (CSEM) method is being applied to the problem of detecting and characterizing hydrocarbons in a variety of settings. Until recently, its use was confined to deepwater (water depths greater than approximately [Formula: see text]) because of the interaction of signals with the atmosphere in shallower water depths. The purpose of this study was to investigate, using a simple 1D analytical analysis, the physics of CSEM in shallow water. This approach demonstrates that it is difficult to simply decouple signals that have interacted with the earth from those that have interacted with the air using either frequency-domain or time-domain methods. Stepping away from wavelike approaches, which if applied without care can be misleading for the diffusive fields of CSEM, we demonstrate an effective way to mitigate the effect of the air in shallow water surveys by decomposing the EM signal into modes and using only the mode least affected by interaction with the atmosphere. Such decomposition is straightforward in a 1D earth, and we demonstrate that the approach remains valid in higher dimensional structures. We also show that the coupling between signals diffusing through the earth and those that have interacted with air can be used to our advantage in the interpretation of marine CSEM data.

A B S T R A C TThe controlled-source electromagnetic (CSEM) and magnetotelluric method (MT) are two techniques that can be jointly used to explore the resistivity structure of the earth. Such methods have, in recent years, been applied in marine environments to the exploration and appraisal of hydrocarbons. In many situations the electric properties of the earth are anisotropic, with differences between resistivity in the vertical direction typically much higher than those in the horizontal direction. In cases such as this, the two modes of the time-harmonic electromagnetic field are altered in different ways, implying that the sensitivity to the earth resistivity may vary significantly from one particular resistivity component (scalar, horizontal or vertical) to another, depending on the measurement configuration (range, azimuth, frequency or water depth). In this paper, we examine the sensitivity of the electromagnetic field to a vertically anisotropic earth for a typical set of configurations, compare inversion results of synthetic data characterizing a vertically anisotropic earth obtained using the isotropic and anisotropic assumptions and show that correctly accounting for anisotropy can prevent artefacts in inversion results.

The marine controlled-source electromagnetic (CSEM) sounding method is rapidly gaining acceptance as an exploration tool for detecting and delineating hydrocarbon reservoirs. Whereas seismic surveys can detect the structures that may contain hydrocarbons with great accuracy, distinguishing hydrocarbon fluids from water within these structures is more problematic. As a result, less than a third of exploration wells result in a commercial discovery.Originally developed in the late 1970s (Young and Cox, 1981), the CSEM method uses a high-powered horizontal electric dipole to transmit a low-frequency electromagnetic signal through the seafloor to an array of multicomponent electromagnetic receivers ( Figure 1). The direct signal through the water column is rapidly attenuated, with the result that at source-receiver separations of more than a few hundred meters, the signals received are dominated by fields that have interacted with the earth. By studying the received signal as the source is towed through the array, the bulk electrical resistivity of the seafloor can be determined at scales of a few tens of meters to depths of several kilometers. Transmission frequencies are typically between 0.01 and 10 Hz. At such low frequencies, the behavior of electromagnetic fields in the earth is governed by the diffusion equation. Resolution criteria based on wavelength arguments can therefore be misleading.The resistivity of silicate minerals is extremely high, and so the bulk resistivity measured in a CSEM survey is in general controlled by the volume, properties, and distribution of more conductive fluid phases (e.g., Schmeling, 1986). This makes the CSEM method ideal for studying fluid-dominated geologic systems (e.g., Greer, 2001; MacGregor et al., 2001). Marine sediments saturated with seawater typically have resistivities of 1-5 Ω-m. Replacing the seawater with resistive hydrocarbons can result in an increase in the bulk resistivity of the formation by 1 to 2 orders of magnitude. CSEM sounding exploits this dramatic change in physical properties to distinguish water-bearing formations from those containing hydrocarbons.CSEM surveys have been used successfully in a variety of settings including West Africa, Southeast Asia, the Gulf of Mexico and the North Atlantic (e.g., Ellingsrud et al., 2002; Srnka et al., 2005). Early surveys concentrated on Tertiary reservoir systems in deepwater areas (e.g., Ellingsrud et al., 2002). These demonstrated that in areas of relatively simple geologic structure, including deepwater turbidites and channel systems, positive results could be obtained from CSEM surveys. However, these settings represent only a small proportion of potential exploration targets. In particular to date the method has been limited to relatively deep water (300 m or more, e.g., Johansen et al., 2005). In shallow water, signals that have interacted with the (extremely resistive) air can have a severe impact on the recorded signals. This noise (known as the "airwave") dominates the CSEM response at source/rece...

The marine controlled-source electromagnetic (CSEM) method is most commonly used in targeted exploration or appraisal studies as a tool for detecting and delineating hydrocarbon reservoirs (see, for example, Constable and Srnka 2007). Whereas seismic surveys can detect the structures that may contain hydrocarbons with great accuracy, distinguishing hydrocarbon fluids from water within these structures is more problematic. Originally developed in the late 1970s (Young and Cox, 1981), the CSEM method uses a high-powered horizontal electric dipole (HED) to transmit a low-frequency electromagnetic signal through the sea floor to an array of multicomponent electromagnetic receivers. Bulk electrical resistivity of the sea floor can be determined at scales of a few tens of meters to depths of several kilometers. In many cases this resistivity is driven by the fluid content and saturation of subsurface formations.

The marine CSEM method is being applied to the problem of detecting and characterizing hydrocarbons in a variety of settings. Until recently its use was confined to deep water (water depths greater than approximately 300m), because of the interaction of signals with the atmosphere in shallower water depths. This interaction is often described as an airwave: a signal free of information about the earth, which contaminates the signal. Based on this interpretation and inspired by the land electromagnetic surveys case, a possible solution is going from the classic frequency domain CSEM to the time domain one in order to decouple the earth from the air. The purpose of this presentation is to compare frequency and time domain methods in the marine case.