Vessel Noise from Western Geophysical's
1998 Programme in Alaska
Lawson[1] describes some detailed measurements that were made of the
noise generated by three vessels during a 1998 seismic programme
conducted for Western Geophysical in Alaska. A boat-based hydrophone
was used to measure vessel sounds at several distances from the
vessels Arctic Star and Saber Tooth. An air-dropped
sonobuoy was used to measure sounds at a single, longer distance of
3 km from the Western Frontier. All three vessels were in
shallow water and underway at the times of the recordings. No airgun
was in use when the measurements were made.
The figures below show both the 1/3 octave band levels and the
spectral densities for the three vessels. The background noise level
is also shown. For the Saber Tooth and the Arctic Star,
which were recorded at several ranges, the levels decreased consistently
with range, as expected. At frequencies below 100-125 Hz, the received
levels decreased rapidly with decreasing frequency. The authors attribute
this to the low-frequency cutoff effect of shallow water. At frequencies
above 100 Hz the noise from the Arctic Star was consistently higher
than the Saber Tooth, despite the fact that the Saber Tooth
was moving more rapidly. Arctic Star is a conventional vessel with
propellers whereas Saber Tooth is a shallow draft vessel with water
jet propulsion.
Source Levels were estimated by 1/3 octave band, for frequencies of 100 Hz
to 10 kHz. At lower frequencies the rate of attenuation was too great for
the Source Level to be estimated. The narrowband spectra did not exhibit
tonal structure, which was put down to the high attenuation rate at low
frequencies, and the low tonal noise level of water jet propulsion systems.
The figure below illustrates broadband SPL versus range for the three vessels
recorded in 1998, with the 1996 data for Peregrine.
- Lawson J W (1999)
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