Florence L. Wong, Michael R. Hamer, Monty A. Hampton, Michael E. Torresan

Bottom Characteristics of an Ocean Disposal Site off Honolulu, Hawaii: Time-based Navigational Trackline Data Managed by Routes and Events

 
 
Abstract
Introduction
Data
GIS Analysis
Results and Conclusions
Acknowledgments
References
External Links

TABLES
1 2 3 4 5

FIGURES
1 2 3 4 5 6 7

Contact:
F.L. Wong
M.R. Hamer
M.A. Hampton
M.E. Torresan

ABSTRACT

Mamala Bay, offshore of Honolulu, Hawaii, has been a disposal area for dredged materials from nearby Pearl and Honolulu Harbors for more than a century. The U.S. Geological Survey, the U.S. Army Corps of Engineers, and the U.S. Environmental Protection Agency are studying the dredged materials and their impact on the marine environment. Data collection includes geophysical profiling and imaging, bottom video and still photography, sediment sampling, analyses of chemical and physical sediment properties, and evaluation of the nature of and impact on the benthic fauna. Synthesis of this information begins with the establishment of a geographic framework for the data.

Ship-based oceanographic surveys typically operate several data-collection systems: navigation, bathymetry, camera, seismic-reflection, sidescan sonar, and physical sampling. All data gathered during the survey are geographically located by referencing their time of collection with the time of a navigational "fix" from satellites (GPS) and/or transponders. After the field activity is completed, time is also used to annotate interpreted data, for example, geologic structures from seismic-reflection profiles or sidescan-sonar images, or seafloor features from photographs. With time as the measure, routes and related events are used to extract and analyze data along segments of tracklines (line events) or at discrete points (point events). In addition to enabling time as the primary measure, route treatment of tracklines preserves the start-to-end continuity that is ordinarily fragmented by building arc topology. Depending on the data set, the event tables may be converted to standard line and point coverages.

Oriented bottom photographs that are time-referenced are one product of this study. Bottom current direction and bottom roughness were determined from the appearance of the seafloor in each photograph. A plot of the inferred current directions as a function of time along the camera tracklines reveals a complex pattern with a dominant northwest-trending swath through the area. Sediment textures were determined from the photographs and also from sample grain-size analyses. These textures provide confirmation, along survey tracks, of the textures interpreted from the sidescan-sonar image of the area. Correlation of observed sediment textures with image data paves the way for construction of a sediment map of the whole area, both along- and between-track.

U.S. Department of the Interior, U.S. Geological Survey, Western Region Coastal and Marine Geology