High salt concentrations and small targets in complicated geology often found in the Gulf of Mexico have been an impediment to the application of marine CSEM techniques, until recent advances in operations, acquisition hardware and advanced processing techniques permit 3D-mapping of complex resistivity distributions. Using two examples from a recent prelease sale campaign, we demonstrate the successful application of the entire value chain from customized acquisition grids, multi-frequency acquisition to processing of data including wide-azimuth lines with reliable phase and amplitude, and subsequent inversion-based 3D interpretation. Advanced interpretation is based on an iterative Hessian-based inversion with quasi-Newton update and fast finite-difference time-domain modeling. Inversion results are used to construct resistivity distribution consistent with 3D-seismic images. During the campaign, we successfully addressed the client’s need to resolve small targets (2 km x 2 km) with low-resistivity pay (??<5 Om), many found in the vicinity (<1 km) of large salt bodies.
Marine controlled-source electromagnetic (CSEM) methods for hydrocarbon detection relying on a horizontal electric bipole (HED) emitting a predefined low frequency spectrum (0.05-10 Hz), and the recorded electromagnetic fields by ocean bottom receivers, have been used in hydrocarbon exploration on a commercial scale since 2002 (Eidesmo et al., 2002). The sensitivity to hydrocarbons is due to the relative enhancement of the transverse magnetic component of the received electromagnetic signal through a partial waveguide effect by buried resistors, which can be either hydrocarbon deposits or other resistive bodies.
Marine CSEM has become a method for 3D imaging of areas with complex geologies, which is applied by many major oil companies, either as a stand-alone frontier exploration tool (Monk et al., 2008; Suffert et al., 2008) or in conjunction with, or addition to other geophysical probes. Recent published case studies for the latter include Carrazone et al. (2008), Price et al. (2008) Plessix and van der Sman (2008), Zach and Frenkel (2009), and Zach et al.(2009).
Advances in hardware and operations have resulted in a vast improvement in data quality, permitting the acquisition of well-defined and repeatable grids of seabed receivers with complex towing patterns including the acquisition of wide-azimuth data with consistent magnitude and phase (Zach et al., 2008b). Such high-fidelity physical measurements enable the effective use of 3D inversion techniques. They allow for imaging of multiple resistive bodies in the subsurface. The majority of 3D CSEM inversion techniques rely on iterative optimization where the gradient of a misfit functional with respect to a discrete conductivity grid is computed during each iteration. The present approach employs the quasi-Newton method described in Zach et al. (2008a). It is based on the gradient calculation developed by Støren et al. (2008) and the fast finite-difference time-domain modeling code by Maaø (2007). Other notable recent contributions to CSEM inversion methodology include Commer and Newman (2008) on joint CSEM and MT inversion, Jing et al. (2008), which shows the importance of anisotropy in many surveys, as well as Norman et al. (2008) for joint interpretation with seismic data (see also additional references in Zach et al. (2008a)).