During December 2003 - January 2004 a sea bed logging acquisition was conducted across the North Sea Troll Field. Different EM wave forms and frequencies were tested to optimize the hydrocarbon response from this shallow water gas and oil field. The survey consists of 41 receivers, deployed along a line crossing the Oil Province, the Western Gas Province and the Eastern Gas Province of the Troll Field. All receivers recorded two orthogonal components of the horizontal electric field, and 12 receivers measured also two orthogonal components of the horizontal magnetic field.
The R&D study was carried out as an attempt to qualify sea bed logging for shallow waters (less than 500 m) and optimize acquisition parameters for such cases. Up until now the marine controlled source EM method has been qualified for use in deep waters (more than 1000 m water depth) mainly because of the disturbing influence from the air wave in shallower water areas. The Troll Field seabed logging data is of excellent quality and helps solving problems related to shallow water applications of this method.
SeaBed Logging (SBL) is a remote sensing technique, which gives information about subsurface resistivity variations by the use of electromagnetic energy. The method has been demonstrated both theoretically (Kong et al., 2002) and in practice, by several calibration and commercial surveys (Ellingsrud et al., 2002 and RÃ¸sten et al., 2002). This abstract presents selected results from a scientific SBL survey across the Troll Field. The overall objective of the study was to obtain an improved understanding of the SBL measurements in shallow waters and thereby improve the interpretation methods.
The surveys are performed by towing an electromagnetic source above an array of receivers deployed on the seabed. The source is a horizontal electric dipole which emits a low frequency, continuous electromagnetic signal both into the seawater and downwards into the subsurface. The array of seabed receivers records the transmitted signals, which depend on the resistivity structure of the subsurface. A detailed description of the method is given in Eidesmo et al. (2002) and Farrelly et al. (2004).
The receivers are dropped from the survey vessel and sinks freely down to the seabed. Acoustic ultra short baseline communication (USBL) is used to establish the exact receiver positions. The receivers are held in position at the seabed by a concrete anchor. After the recording period, an acoustic signal from the vessel triggers a realise mechanism, causing the receivers to release from their anchors and float back to the surface. Figure 1 shows photographs of a receiver when dropped from the survey vessel and the EM source at the surface. The receivers consist roughly of a buoyancy system (five yellow spheres at the top of the receiver), a data acquisition unit (white cubic box), an anchor (grey plate) and removable horizontal sensors. The system may include up to two pairs of orthogonal electric sensors (long yellow arms) and two magnetic sensors (short grey cylinders).