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Geophysical Survey of Hawaiian Coral Reefs
In early October, CMGP researchers from Menlo Park completed a two-week geophysical survey offshore from the Hawaiian Islands, where coral reefs are in decline. Walter Barnhardt, Bruce Richmond, Pat Hart, Larry Kooker, and Mike Boyle sailed on the R/V Wailoa with nearly every piece of high-resolution sub-bottom gear in the USGS arsenal (plus several systems that the USGS doesn't own). Prior to the survey, and for the first time in the field, CMGP tested acoustic systems using a calibrated hydrophone as required under a permit from the National Marine Fisheries Service. The terms of the permit are meant to ensure that marine mammals are not harmed by research sound sources. Two days of testing determined the 160-dB safety zone for marine mammals, that is, the distance from the sound source at which the sound-pressure level had decreased to 160 decibels. The safe distances were 4 m for a Chirp system, 30 m for a boomer, and 100 m for a mini-sparker. If marine mammals were observed closer to the sound source than these distances, the system would have to be shut down and data collection temporarily halted. A team of three independent observers was on board to watch and warn of the approach of marine mammals. No whales were sighted and no shutdowns occurred.
The investigations focused on three study areas along the leeward coast of Molokai, and the windward and leeward coasts of Oahu (Mamala and Kailua Bays). The main objective was a better understanding of the geologic evolution of fringing reefs that have formed since the end of the last Ice Age. During that period, sea-level rise has flooded formerly exposed parts of older pre-Holocene reefs and generated a complex, three-dimensional structure of biogenic materials. Coral reefs present special challenges for geologic studies. Reef growth is highly variable over small spatial scales, and widely spaced cores may not accurately resolve patterns of coral accumulation. With assistance from University of Hawaii researchers Eric Grossman (now with CMGP in Santa Cruz) and Chip Fletcher, we used seismic-reflection techniques to target and successfully image sections of a Holocene(?) reef up to 30 m thick. The most notable finding was the presence of a continuous, low-relief reflection that underlies extensive areas of reef off Molokai. Seismic profiles traced the buried surface parallel to shore for nearly the entire length of the island (approximately 40 km) and seaward to a depth of more than 130 m. As with any good science project, we returned home with as many new questions as answers. What is the nature of this marker horizon? Is it a wave-cut platform etched into older limestone? Are we imaging the upper surface of volcanic rocks? Planning is already underway on how to determine the acoustic velocities, compositions, and ages of the units that comprise Hawaiian reefs.
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in this issue:
Sediment Study on the Columbia River
Woods Hole's First Annual Open House Shark Festival and Sanctuary Celebration 2001
USGS Mendenhall Post-doc Fellowship
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U. S. Department of the Interior | U.S. Geological Survey
cover story:
