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Through accurate large-scale test experiments, direct comparisons of node and cable-based sensor responses have been performed. Though short conceptual cables were used, significant differences in the vector responses of the two types of sensors were shown. The vector fidelity of the nodes is very good, whereas for the cables, it is not. The vector fidelity of the nodes is also confirmed by data acquired offshore. Regarding studies of anisotropy effects and the low frequency content of PS converted data, the cable effects observed on the first break and PS reflected data have also been observed on commercial systems. Introduction The potential of using converted (PS) waves to image through a gas chimney was demonstrated for the first time in 1994 by Berg et al. [1]. Since then, the importance of converted waves and the need for proper equipment to record such shear waves, have been proven time and again. Nowadays, several largescale offshore 4C-3D surveys are performed every year. Both the oil and the service companies agree that in many cases, high quality 4C-3D data are necessary to meet the mapping objectives predicted by using the 4C methods. As converted waves cannot be transmitted through water, the sensors have to be put on the seafloor. This poses several new challenges. Obviously, it is not as simple as for surface surveys, where the survey vessels simply tow long streamers. To record shear wave data correctly, the sensors will have to be well coupled to the seabed. Furthermore, the irregularities of the seafloor may make it impossible to put sensors on a straight line. Some locations may have a fragile seabed (for instance coral reefs) where special care must be taken when placing the sensors. Finally, as converted waves convey a direction of movement, the sensors must record this information correctly. Accurate measurement of this direction of movement is what we call vector fidelity. A set of test results will show weaknesses and benefits of a couple of common sensor strategies regarding their vector fidelity. Basically, one might divide the sensors into those that are cable-based and those that are node-based. The vector fidelity will be tested, and its significance will be evaluated. Theory and Definitions A couple of definitions and ways to test for vector fidelity are given. Vector fidelity. Mjaaland et al. [2]: Vector fidelity is defined as that property of multi component seismic receivers wherein a given particle motion impulse applied parallel to one of the components registers only on that component, and wherein the same impulse applied parallel to the other components give the same response, so that the various components can be combined according to the rules of vector algebra. Node. A node is the underwater equivalent of a land geophone. In addition, a node usually consists of a hydrophone as well. The node has a skirt to achieve good coupling, and the connecting wires should be so light and flexible that they don't have any significant influence on the response of the sensors.

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Seabed data acquisition methods offer numerous advantages over towed streamer data. These advantages can lead to improved static and dynamic reservoir characterization. By recording complete vector field data at the sea floor with full azimuth acquisition improved shallow resolution, signal-to-noise ratio, spectral content, deep imaging and 3D illumination can be achieved. Also in the presence of obstacles such as production facilities a regular coverage can be assured. Autonomous node technology has been developed to a fully commercial system. It has demonstrated improved imaging of complex reservoir with both pressure (PP) and converted shear (PS) with stable and consistent measurements achieved by very well planted nodes into the sea floor and full azimuth acquisition with densely sampled shots. It has been experienced that the background response from well planted nodes can be repeated in a 4C-4D scenario when the coupling conditions are the same. The vector fidelity in the node system will secure this behavior. In addition, the accurate positioning and re-positioning of the nodes under realistic water depth ranges gives positioning accuracy close to permanently buried cable systems. An experiment performed on the Volve field in the North Sea with pairs of nodes planted side by side clearly confirmed the high degree of stability in the coupling and the repeatability of the measurements from all components. At 100 m water depth all the planted nodes were within a short radius around the pre-plot position. A cost sensitivity study of different 4C-4D node scenarios depending of field size, water depths and node spacing indicates that, for larger field sizes (300-600km2 receiver coverage), the alternative use of nodes could be significantly more cost effective than permanently buried cable systems. Moreover, there are advantages linked to the acquisition geometry, operation, zero equipment life time risk and low initial investment. Introduction Marine seismic exploration and reservoir imaging have been through numerous stages of adjustments and improvements. Towed streamer surveys from 2D to 3D and now to 4D dominate the offshore seismic survey with a well established technology which remains the most common acquisition with narrow azimuth coverage. New techniques such as " single sensor recording?? (Egan et al, 2005), " over-under?? (Singh et al, 1996) and " wide azimuth?? (Campbell et al, 2002) have recently delivered impressive results. These techniques have raised the cost and complexity to more traditionally " simple?? towed streamer operations.