Figure 2.21: An artist’s impression of nodes on the seabed.
Te third system is a combination of the first two: ‘nodes on a rope’, or steel wire, to increase the speed of deployment. Te nodes can be accurately placed on the seafloor anywhere from the shoreline to deeper waters, with any desired interval.
One of the major requirements in 4C-OBS experiments is
that the geophones are well coupled to the sea floor in order to record both high-quality P-waves and S-waves. Because shear waves do not travel through water, the geophones must be in direct contact with the seabed to capture the motion of the seabed and not the change of pressure in seawater. Te process of ensuring direct contact between the geophones and the seabed or any other solid material is called coupling. Another important requirement of OBS surveys is the
so-called vector fidelity. Tis can be defined as that property of a three-component geophone sensor system where a given particle-motion impulse applied parallel to one of the sensor components registers only on that component, and wherein the same impulse applied parallel to the other components gives the same response, so that the various components can be combined according to the rules of vector algebra.
Figure 2.22: Magseis offers small and light nodes inserted into a steel cable to enhance the efficiency of OBS operations.
