Page 216 - Acquisition and Processing of Marine Seismic Data

3.10 OTHER NOISE TYPES 207

FIG. 3.37 (A) Propeller noise (P) from a nearby container vessel, and (B) noise record showing the propeller noise of the
seismic ship’s support vessel.

Transport ships do not transmit signals into single or narrow frequency pulse, but has a fre-
the water column, and their propellers are the quency band (e.g., from 40 to 180 Hz) which
only source of noise for seismic surveys. Navy almost coincides with the seismic bandwidth
vessels, especially minesweepers, and subma- (Fig. 3.39B). The linear noise is periodic and
rines, however, can use different sonar systems, there are approximately 80 ms between the con-
which transmit acoustic pulses into the ocean secutive pulses.
with various frequency and amplitude ranges. Excessive gain applications or AGC may
These pulses may constitute serious noise on increase their amplitudes, and the propeller
the seismic data. Fig. 3.39A shows extremely noise from other vessels can be mostly attenu-
high-amplitude linear noise interference from ated during the stacking process. In this case,
the sonar signal of a possible submarine located because the noise is linear and its dip is quite
at the bow side of the vessel. The f-k spectrum of different from those of primary reflection
the interference indicates that the noise is not a amplitudes, one can easily remove this type of