Vector Fidelity in Ocean Bottom Seismic Systems
Authors: Berg E.W. , Rykkelid J.I. , Woje G. , Svendsen Ø. 
Published: October 2008

Abstract
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.