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Table of Contents:

OF 2004-1014

Disc Contents

Geologic
    Discussion

• Introduction

• Field Program

• Field Equipment

• Interpretation

• Discussion

• Summary

• Acknowledgments

Data/GIS

Metadata/Data
    Catalog

References Cited

Contacts

Field Equipment

The geophysical data were collected using an SIS-1000 sidescan-sonar and subbottom profiling system. The system was developed by Datasonics (now part of Benthos Inc .). It consists of a vehicle containing sidescan-sonar and subbottom transducers, a tow cable, and a digital data acquisition system. The sidescan-sonar imagery and subbottom data were logged digitally by an ISIS system developed by Triton/Elics. The sidescan-sonar data were processed and mapped in the field using techniques summarized by Danforth and others (1991) and Paskevich (1996) to create a sidescan-sonar mosaic. This digital mosaic served as the base map for the ground-truth component of the field program. The chirp seismic-reflection profiles that were collected simultaneously with the sidescan-sonar imagery were converted to SEG-Y format and then JPEG images were made of each profile. These profiles enable the measurement of the thickness of sediment that has accumulated in the reservoir since impoundment.

Navigation of the ESTERO during the 2000 field program was by GPS receivers using the Precise Positioning Service (PLGR), and real-time differential GPS was used during the 2002 field program. Navigational accuracy for these receivers was better than 10 m. Although the location of the ship was known to within 10 m, the positional accuracy of the sidescan-sonar imagery was not as good. The vehicle was deployed from a boom off the bow of the ESTERO at a depth of approximately 2 m. This mode of deployment resulted in the vehicle and ship locations being nearly identical. However, smoothing of the navigation data during data processing and rubber-sheeting individual sidescan-sonar lines during the mosaicking procedure introduced further error to the completed mosaic. Aerial photographs collected prior to completion of the dam show roads along both sides of the river, which have been submerged since completion of the dam. Comparison of the mosaic of aerial photographs with the USGS topographic maps that were published after completion of the dam show that, beyond the extent of the reservoir, features such as roads and buildings line up mostly within 20 m of each other. The roads that now are submerged show on the sidescan-sonar imagery, and although there are some localized areas where the difference in road location between the aerial photograph and the sidescan-sonar image is as much as 80-95 m, for most of the reservoir the difference is less than 20 m. Most of the areas with large offsets were bounded by long stretches where the roads on the two mosaics were within 20 m of each other. Offsets that exceeded 40 m mostly coincided with sinuous sections of the ship track. Much of the discrepancy between the mosaics appears to be due to smoothing of the navigation data during processing of the sidescan-sonar data, which straightened sinuous sections of the track lines. For much of the sidescan-sonar mosaic, features appear to be within 20-25 m of their proper location.

The ground-truth part of the program employed the SeaBOSS imaging and sampling system (Blackwood and Parolski, 2001) in 2000, and a sampler with a video camera attached to it during 2002. The system used during 2002 was designed and constructed by staff at the Columbia River Research Laboratory, but was similar in function to the SeaBOSS system.

The acquisition systems used during the 2000 survey are further described on the USGS Woods Hole Science center Seafloor Mapping Web page (http://woodshole.er.usgs.gov/operations/sfmapping/).