Increasing the Accuracy of Marine CSEM Survey Planning and Data Interpretation Using Novel 3D TTI Anisotropic Resistivity Modeling

Abstract

Abstract The marine Controlled-Source Electromagnetic (CSEM) method has been evolving into a subsurface resistivity imaging tool for increasingly complex geological settings. It allows recovery of subsurface resistivity, a key hydrocarbon (HC) indicator, using modeling- and inversion-based interpretation of EM data acquired on the seafloor. Application of CSEM can reduce risk and optimize drilling operations, as well as improve estimates of HC reserves. The CSEM workflow is comprised of feasibility study, survey design, data acquisition, processing, and interpretation. Survey design and data interpretation rely heavily on 3DEM modeling to properly select and plan surveys and recover the 3D subsurface resistivity image. Currently, simplified 3D Vertical Transverse Isotropic (VTI) resistivity models are applied to simulate EM fields over complex anisotropic subsurface geological structures. This may lead to incorrect assessment of the feasibility of a potential CSEM survey, as well as mispositioning of targets and erroneous assessments of their parameters during interpretation of acquired marine CSEM data. Therefore, there is a need to start using 3D Tilted Transverse Isotropic (TTI) models, which provide a much more accurate description of complex anisotropic resistivity structures than the widely used VTI models. To perform simulations of TTI models, we use a 3D finite-difference (FD) modeling method that allows simulating arbitrary 3D anisotropic models. We can apply our workflow to build a 3D TTI model or alternatively utilize a 3D resistivity/conductivity volume determined from a geological/resistivity model with each cell or a set of cells described by the 9-component conductivity tensor. Our method provides simultaneous accurate computation of all the components of the EM fields at any number of frequencies and hence offers fast 3D TTI simulation for complex marine CSEM applications. In this paper, to demonstrate the difference between the TTI and VTI, we used 3D benchmark models with different orientations of the vertical-horizontal background resistivity (Rv, Rh) pair. The TTI effects were assessed using comparisons of simulations for TTI and the corresponding VTI models. The presented 3D modeling results show that ignoring the TTI effect, if it exists, can lead to incorrect conclusions at the feasibility study and data inversion steps of the CSEM workflow. This study makes the case that application of 3D CSEM TTI modeling can improve the reliability of CSEM by increasing the accuracy of survey planning and data interpretation, which, in turn, will accelerate the adoption of marine CSEM technology.