Page 15 - Fall 2005

k l
ATOC
p n
r
o
Pioneer
e
f
Kauai
Fig. 2. The ATOC array. (From Ref. 7 © 2005 Acoustical Society of America.)
tions! Statistical analysis of the aerial survey data showed some small shifts in the distribution of humpback (and possibly sperm) whales away from the sources during transmission periods. Statistical analyses of the behavioral data revealed some small changes in the behavior of humpback whales in response to the playback of ATOC-like sounds and to the transmissions of the ATOC Kauai source. The conclusion was that the minor effects that were observed would not adversely impact either the survival of an individual whale or the status of the North Pacific humpback whale population.
North Pacific Acoustic Laboratory (NPAL)
Encouraged by the ATOC results, we decided to under- take the effort needed to obtain the approvals and permits required to resume transmissions from the Kauai source. The ATOC permits allowed for two years of transmissions from each source. The Kauai transmissions had begun on 30 October 1997 and been terminated on 3 October 1999. We began the arduous process of obtaining the required author- izations in 1999 (Fig. 3). Transmissions from the Kauai source finally resumed on 24 January 2002 as one component of the North Pacific Acoustic Laboratory program.7 The Kauai time series are therefore now over seven years long, albeit with a substantial gap while we were obtaining the authorizations needed to resume transmissions.
Even at long time and large spatial scales the ocean is highly variable. A path from Kauai to California (receiver f) shows a modest cooling trend (longer travel times) until the present time (Fig. 4). A path to the northwest (receiver k)
ATOC Marine Mammal Research Program (MMRP). Thus, permission for climate-oriented research was to be subse- quent to studies relating to the possible effects of sound on marine life.
Nonetheless, analyses of data from the ATOC project demonstrated that acoustic thermometry is a powerful tool for making routine measurements of large-scale ocean temperature variability and heat content, as originally hypothesized.6 We found that acoustic travel times could be measured with a preci- sion of about 20–30 msec at 3000–5000 km ranges. For com- parison, the total travel time for an underwater acoustic signal over 5000 km is nearly an hour. ATOC measurements proved to be more precise than originally thought possible. The initial concern that acoustic scattering from small-scale ocean struc- ture, such as internal waves, might make accurate measure- ments of acoustic travel times impossible at 3000–5000 km ranges proved to be unfounded. The travel times can then be used to estimate the range- and depth-averaged temperature with a precision of about 0.010°C.
The ATOC Marine Mammal Research Program was sig- nificantly expanded during the process of obtaining the nec- essary authorizations. In the end the California and Hawaii ATOC MMRPs consisted of multiple components, including (i) aerial surveys to determine any changes in the abundance and distribution of marine mammals, (ii) elephant seal tag- ging studies to determine any changes in elephant seal migra- tory or diving behavior, (iii) playback studies to humpback whales off the Kona-Kohala coast of Hawaii designed to look for behavioral changes in response to ATOC-like sounds, (iv) visual observations of humpback whale abundance, distribu- tion, and behavior north of Kauai, (v) undersea acoustic recordings made with seafloor data recorders north of Kauai to determine any changes in humpback vocalizations, (vi) auditory measurements on small toothed whales (odonto- cetes) to determine their sensitivity to the frequencies trans- mitted by the ATOC sources, and (vii) playback studies to fish at the Bodega Bay Marine Laboratory designed to look for behavioral changes in response to ATOC-like sounds. None of the studies found any overt or obvious short-term changes in abundance, distribution, behavior, or vocaliza-
Fig. 3. Peter Worcester, North Pacific Acoustic Laboratory (NPAL) principal inves- tigator, with the environmental documentation prepared in the course of obtaining the authorizations needed to operate a low-frequency sound source off the north shore of Kauai to do a second phase of research on the feasibility and value of large- scale acoustic thermometry. Obtaining the required authorizations took nearly three years and cost in excess of half a million dollars.
Acoustic Remote Sensing of Ocean Gyres 13