July 27, 2004 WASHINGTON - Scientists investigating the possible effect of underwater seismic pulses on marine mammals have conducted a series of tests, designed to better understand the force of sound waves generated by shipboard airguns. These instruments are used by some 100 vessels worldwide to penetrate into the seabed for oil exploration and geophysical research, with an estimated 15 to 20 active on any given day.
Researchers from the Lamont-Doherty Earth Observatory of Columbia University conducted tests in the northern Gulf of Mexico in 2003, using the 20-gun array aboard their research vessel, Maurice Ewing. Their results will be published July 27 in Geophysical Research Letters (GRL), a journal of the American Geophysical Union. While most other scientific research ships can only deploy much smaller systems, Maurice Ewing's 20-gun capacity is comparable to those aboard many industry ships. It provides the flexibility to design source arrays of many different sizes and power, allowing scientists to look deep below the ocean's surface to study problems as diverse as earthquake prediction and the ocean's role in the carbon cycle.
Maya Tolstoy, lead author of the study, writes that the researchers covered sound frequencies of concern to many species of marine mammals. The GRL paper focuses in part on beaked whales, a relatively little known family of 18 species found, often at great depths, in all of the world's oceans.
Only one event of stranding of beaked whales that might be related to airgun activity has been recorded, Tolstoy writes, but there is solid evidence that some other species of whales avoid the acoustic output of seismic systems at distances up to 20 kilometers [10 miles]. In 2002, two beaked whales were stranded in the Gulf of California at around the time R/V Maurice Ewing was conducting acoustic research in the area, but a causal relationship has not been established. Some beaked whale strandings have occurred in connection with Navy sonar operations at frequencies of around three kilohertz, Tolstoy says.
Recognizing that different species of marine mammals may be sensitive to different sound frequencies and decibel levels, the researchers are seeking to better understand how strongly and how far sound pulses travel, both in shallow and deep water. The Maurice Ewing cruise was a step in that direction. Using hydrophones (underwater sound detectors), the scientists found that previous models had overestimated the deep water impact of low frequency sound waves, while underestimating their impact in shallow water. This, they say, is because reverberations play a significant role in received sound levels in shallow water, which had not previously been incorporated into the models.
During the experiments, the ship's airguns were fired in various combinations, in order to determine the radius of the emitted sound in the water at various frequencies and decibel levels. Tolstoy notes that since passage of the Marine Mammal Protection Act in 1972, ships operating in U.S. waters must adhere to increasingly strict guidelines, in order to minimize impacts on whales and other animals. This includes the gradual ramping up of a seismic array over 30 to 60 minutes, to warn whales and give them time to leave the area.
The National Marine Fisheries Service, part of the National Oceanic and Atmospheric Administration, issues permits to vessels whose operations may "harass" marine mammals, an industry term describing any activity that alters a mammal's normal behavior. The Service issues guidelines as to the radius in which sound pulses might be received at various high decibel levels. In the past, these radii were calculated with computer models. The experiments conducted aboard the Maurice Ewing were the first in which they were actually measured, using well calibrated broadband instruments. Beaked whales are most sensitive to sound in the one-to-20 kilohertz frequency range. Most airgun energy is much lower frequency than that, around five to 100 hertz, and the energy, or decibel, level of sound pulses drops sharply above that frequency.
Tolstoy says that ocean scientists take the threat to whales seriously. "Lamont-Doherty and the National Science Foundation, which funded our study, have already responded to the new data by adjusting the safety radius when they do research involving seismic sound generation," she says. "If a whale enters this radius, they shut down operations until it leaves."
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