Page 183 - Acquisition and Processing of Marine Seismic Data

174 3. NOISE IN MARINE SEISMICS

from vessel’s propeller shows up, and, if we machinery occurs due to the transformation of
remove this operational noise, then powerline vibrations from the main engine(s), power gen-
interference becomes visible. Sometimes it is not erators, or air compressors into the water col-
possible to completely remove all of these noise umn. These vibrations appear on the data
amplitudes of different dip, amplitude or fre- spectrum as narrow bands proportional to the
quency characteristics from the data using the speed and revolution of the main engines, and
conventional processing techniques. their amplitudes depend on several agents such
Marine seismic data contain individual noise as specifications, mounting and stability of the
types that are quite different from those obser- engines. Hydrodynamic noise arises from the
ved on land records. Understanding the source movement of the vessel and may be effective
and physical specifications of noise has key on the near offset traces, since it is not transmit-
importance in the determination of processing ted for long distances.
steps and the parameters in removing the partic- The noise from propeller is important for seis-
ular noise types. Main purpose of seismic data mic data because the propellers directly operate
processing is simple: to suppress the noise and in the water and they produce a continuous low
make genuine reflections more prominent: in amplitude noise in the water column, which cre-
other words, to increase the signal/noise ratio ates linear events at near-offset traces. This noise
(S/N). Definition of the noise embedded in the can be distinguished asperiodic high amplitudes
data and investigation of its characteristics are in the amplitude spectrum, which are propor-
important for the processors to determine the tional to the propeller’s rotation speed. The peak
suitable processing steps to remove a particular frequencyofthepropellernoisecanbecalculated
noise type. The main characteristic features of using f ¼ nR/60, where n is the number of pro-
the noise components are their trace-by-trace peller blades and R is the rotation speed of the
consistency and dip on the shot/CDP gathers propeller in rpm. For instance, a propeller with
or on the stack sections, and their frequency three blades rotating at 300 rpm produces an
and amplitude specifications. In this section, operational noise at approximately 15 Hz center
most of the noise types encountered in marine frequency.Fig.3.3showslinearoperationalnoise
seismic data are defined and discussed in detail and its mean amplitude spectrum, prominent at
by means of their appearance and dip on the the water column, and late arrivals of near traces.
shot records as well as on stack sections, and Thenoiserepeatsitselfperiodicallyrelativetothe
their amplitude and frequency characteristics. rotation of the propeller and has a dominant fre-
Several noise examples from real seismic data quency of approximately 20 Hz. Although it is
are introduced and analyzed. In most cases, not possible to completely avoid recording this
before/after representations are also included. noise, hydrophone arrays along the streamer
and long tow distances from the vessel may
reduce the noise level. In practice, reflection
3.1 OPERATIONAL NOISE amplitudes in near traces are strong enough to
suppress operational noise and relatively weak
Operational noise originates directly from a amplitudes of the noise are masked under strong
seismic vessel’s hydrodynamic movement, its amplitudes of genuine reflections. The opera-
engine(s), machinery, and propeller(s). The tional noise, however, becomes evident for late
shape, amplitude and frequency characteristics arrivals where reflection amplitudes cease, and
of operational noise primarily depend on the amplitude restoration processes such as auto-
dimensions, tow speed and construction form mated gain control (AGC) increases the noise
of the vessel. Operational noise from onboard amplitude.