Modeling the Response of a Seafloor Antenna in the Limits of Low Frequency and Shallow Water

Abstract

In this paper we describe a novel FDFD (flnite-difierence frequency-domain) method for computing the 2-D earth response of a 3-D subsea horizontal electric dipole (HED) source. It represents scattered fleld solutions to Maxwell's equations and uses a 1-D three-layer analytical solution for the primary fleld calculations. Comparison with the 1-D industry type of program CSEM1D gave accurate flts. DOI: 10.2529/PIERS060821043723 The basis of the Sea Bed Logging (SBL) approach is the use of a mobile horizontal electric dipole (HED) source and an array of sea∞oor electric fleld receivers (3). The transmitting dipole emits a low frequency electromagnetic signal (fundamental frequency typically ranging from 0.25 to 2Hz) that difiuses outwards, and the method relies on the large resistivity contrast between hydrocarbon- saturated reservoirs and the surrounding sedimentary layers. The generated electromagnetic sound- ings can in general be divided into three main contributions: direct EM fleld, guided modes (associ- ated with high-resistivity zones like hydrocarbons) and air waves (cf. Fig. 2(a)). In case of shallow water depths (e.g., 200meters or less) the presence of strong airwaves will mask the subsurface signals. In order to develop improved data processing schemes for such cases, modeling tools are needed that can provide controlled test data. In addition, modeling is important when planning new SBL surveys and also when carrying out feasibility analysis of possible new prospects. Since electromagnetic modeling based on time-domain methods give prohibitive large computation times in case of an air layer we concentrate on frequency-domain (methods). Several EM-modeling pro- grams have been developed (1,5,7)). However, none of these codes have neither been tailored to the SBL-case nor been documented to work accurately for the case of a low frequency operating sea∞oor antenna deployed in shallow water.