Navigating marine electromagnetic transmitters using dipole field geometry

Abstract

The marine controlled source electromagnetic (CSEM) technique has been adopted by the hydrocarbon industry to characterize the resistivity of targets identified from seismic data prior to drilling. Over the years, marine controlled source electromagnetic has matured to the point that four-dimensional or time lapse surveys and monitoring could be applied to hydrocarbon reservoirs in production, or to monitor the sequestration of carbon dioxide. Marine controlled source electromagnetic surveys have also been used to target shallow resistors such as gas hydrates. These novel uses of the technique require very well constrained transmitter and receiver geometry in order to make meaningful and accurate geologic interpretations of the data. Current navigation in marine controlled source electromagnetic surveys utilize a long base line, or a short base line, acoustic navigation system to locate the transmitter and seafloor receivers. If these systems fail, then rudimentary navigation is possible by assuming the transmitter follows in the ship's track. However, these navigational assumptions are insufficient to capture the detailed orientation and position of the transmitter required for both shallow targets and repeat surveys. In circumstances when acoustic navigation systems fail we propose the use of an inversion algorithm that solves for transmitter geometry. This algorithm utilizes the transmitter's electromagnetic dipole radiation pattern as recorded by stationary, close range (<1000 m), receivers in order to model the geometry of the transmitter. We test the code with a synthetic model and validate it with data from a well navigated controlled source electromagnetic survey over the Scarborough gas field in Australia.