Detection of Offshore Resources using Marine Controlled-source Electromagnetic Methods

Abstract

A marine controlled-source electromagnetic (CSEM) survey using an electric dipole in frequency domain has become popular for hydrocarbon (HC) exploration, where EM responses are directly related to the electrically resistive property of HC bearing strata in otherwise conductive marine sediments. Possible targets of the marine CSEM survey, other than HC or gas hydrate (GH) embedded in sediments, may be shallow sea-bottom sedimentation and hydrothermal mineral deposits under the deep sea. Computer programs have been developed to evaluate EM responses for a one-dimensional (1D) model with multiple source and receiver dipoles that are finite in length in both frequency- and time-domain. The time-domain solution can be obtained by applying an inverse fast Fourier transform (FFT) to frequency-domain fields for efficiency. Frequency-domain responses are first obtained for 10 logarithmically equidistant frequencies per decade, and then cubic spline interpolated to get the FFT input. The phase curve must be made to be continuous prior to the spline interpolation. The spline interpolated data are convolved with a source current waveform prior to FFT. Using the frequency-domain code, I conducted sensitivity analysis of marine CSEM methods to a GH layer in the shallow section. From these numerical experiments, I found that there are plenty of useful offset ranges and frequencies where amplitude difference is large enough to detect the target layer. Furthermore, an effect of airwaves is almost absent in amplitude difference. With the use of time-domain code, I calculated step-off responses for 1D HC reservoir models. Although the vertical electric field has much smaller amplitude of signal than the horizontal field, vertical currents resulting from a vertical transmitter are sensitive to resistive layers. The modeling showed a significant difference in step-off responses between HC- and water-filled reservoirs, and the contrast can be recognized at late times at relatively short offsets. A maximum contrast occurs at more than 4 s, being delayed with the depth of the HC layer. I examined step-off responses for a layered model and compare the characteristics of horizontal and vertical loop systems for detecting hydrothermal deposits. The feasibility study showed that transient EM (TEM) responses are very sensitive to a highly conductive layer. Time-domain target responses are larger and appear earlier in horizontal magnetic fields than in vertical ones, although the vertical field has 2 – 3 times larger magnitude than the horizontal one. An inverse problem is formulated with the Gauss-Newton method and solved with the damped and smoothness-constrained least-squares approach. The test example for a marine hydrothermal TEM survey demonstrated that the depth extent, conductivity and thickness of the highly conductive layer are well resolved. Finally, I investigated the 3D frequency-domain electromagnetic responses of a 100 m thick, 5 km in diameter disk-shaped hydrocarbon reservoir buried at a depth of 1 km below the seafloor. From the numerical results, I recognized that a 3D effect of the reservoir typically produces a transition zone in comparison with 1D model responses. The transition zone decreases with the airwave effect as the depth of water becomes shallow. As the source frequency increases, the sensitivity to the reservoir increases, whereas the amplitude decreases and falls at higher than 1 Hz below the current system noise floor. Broadside electric fields for a 10-km diameter disk model are only about 5 % of in-line electric fields for the 5-km disk model. The T-equivalence is observed at such a low frequency of 1 Hz for the thin resistive tabular target, whose response varies almost linearly with the target thickness and resistivity even in the transition zone.