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."

A copy of this study, published in Geophysical Research Letters, can be purchased at service@agu.org. Journalists and public information officers should contact Harvey Leifert at HLeifert@agu.org for a free copy. Please refer to the following reference: 10.1029/2004GL020234.

2003

Contact:
Mariellen Gallagher

Lamont-Doherty to Conduct Comprehensive Underwater Measurements of Human-Made Acoustic Sources at Different Frequencies
Data to Be Applied to the Protection of Marine Mammals

Lamont-Doherty Earth Observatory (LDEO) scientists have joined a multi-year research program of the U.S. Department of Interior Minerals Management Service and the Office of Naval Research to provide a better understanding of human-made acoustic sound and its impact on whales.

There is a paucity of data on how different sound levels radiate in different geographical surroundings, and how marine mammals may respond to these different levels. A report from The National Academies' National Research Council states that the "Impact of noise on marine mammals remains unclear." Funds should be designated for "research into how human-generated sounds may affect marine mammals," and "needed are better models to predict the noise levels that will be generated in the ocean by particular human activities." The Academies' study was requested by the National Ocean Partnership Program and sponsored by the Office of Naval Research, the National Oceanic and Atmospheric Administration, the National Science Foundation, and the U.S. Geological Survey. To view this report, click HERE.

Toward furthering scientific knowledge, Lamont researchers deployed listening devices in the Gulf of Mexico to measure sound levels received from controlled sound sources. The data will allow researchers the ability to model how sound spreads in varying topography and water depth. This is essential data needed to establish accurate baseline information for marine mammal tolerance. It will provide the science community with a critical tool to minimize risk to marine mammals when conducting research.

All seismic research aboard Lamont's Research Vessel Ewing must meet a strict set of procedures, above and beyond what is required by permit, designed to minimize interactions between the sound sources and marine mammals. The procedures include the employment of trained marine mammal observers on bridge watch prior to and during all sound operations; the observance of an established safety zone for marine mammals appropriate to the type of acoustic activity to be conducted; and the activation of sound sources at their lowest levels with prescribed increases to allow animals to detect, track, and avoid operations. In addition, the Gulf of Mexico cruise will employ passive monitoring systems to ensure that no whales are in the area of research. To view LDEO Marine Mammal Protection Procedures, click HERE.

Concurrent with this research, the Sperm Whale Seismic Study (SWSS) program is studying the impact of sound sources on sperm whales. SWSS is a three-year program of the Minerals Management Service and Texas A&M Research Foundation with financial support from the Office of Naval Research and the International Association of Geophysical Contractors. An international community of researchers is attaching both satellite and digital tags (D-Tags) to sperm whales in order to study long-term broad-scale and short-term detailed responses to controlled sound sources in the Gulf of Mexico. SWSS researchers attach temporary tracking devices to sperm whales to measure their behavioral response to controlled exposure to specific sounds.

The need for definitive data on the impact of human-made acoustic sources on marine mammals is of critical concern to the oceanographic community. In addition to participation in the SWSS program, Lamont is engaged in other programs to further scientific understanding of marine mammal response to acoustic research, and is working with specialists in the field of marine mammals to help inform future researcher programs. To view more information on LDEO marine mammal initiatives, click HERE.

Marine acoustic surveys are fundamental tools guiding explorations of this planet. Acoustic and seismic research has contributed more to understanding Earth's physical history, natural hazard potential, and climate systems than perhaps all other scientific technology combined. The ability to map the ocean floor and sub-floor revolutionized earth sciences 50 years ago with the discovery that continents break apart and tectonic plates shift. Surveys have revealed the globe-encircling volcanic mid-ocean ridge system, earthquake-producing boundaries of crustal plates, drowned shorelines, and submarine landslide deposits. The data that is being collected today is critical to informed decision making regarding our future.

Lamont-Doherty Earth Observatory

The Lamont-Doherty Earth Observatory is one of the world's leading research centers examining the planet from its core to its atmosphere, across every continent and every ocean. From global climate change to earthquakes, volcanoes, environmental hazards and beyond, Observatory scientists provide the knowledge of Earth systems that must inform the future health and habitability of our planet.