Page 215 - Acquisition and Processing of Marine Seismic Data

206 3. NOISE IN MARINE SEISMICS

FIG. 3.36 An example stack section with side-sweep reflection above the seabed. Inset schematically shows diffractions
from out-of-plane scattering points.

3.10.4 Noise From Other Marine
Vehicles changes with the distance to the noise source.
Fig. 3.38 shows a number of successive shots
Hydrophones are so sensitive that they can with propeller noise interference from an oil
perceive the signal propagating from nearby tanker approaching from the aft of the seismic
vessels’ propellers. Experience has shown that vessel and passing through one side towards
this noise can be received from long distances the bow. At first, the propeller noise of the
from the vessel, e.g., at approximately 5 miles tanker is linear from far to near traces, indicating
away from the propeller. Their amplitude is that the tanker is far behind the seismic vessel.
rather low with respect to the amplitudes of Then it approaches to the sideward of the
the genuine reflections, so the noise from other streamer and the propeller noise appears as
marine vessels is generally visible at early times symmetrical hyperbolas on the shot records at
of the shots corresponding to the water column the center in Fig. 3.38. As the tanker moves away
before the seabed arrival. to the bow of the seismic vessel, the propeller
These appear as semisymmetrical continuous noise becomes linear again from near to far
hyperbolas on the shot gathers if the propeller is traces, indicating that the tanker is far ahead of
close enough (Fig. 3.37). However, its character the streamer.