The Gulf of the Farallones: Sidescan-Sonar Imagery

In 1989, the U.S. Geological Survey (USGS) began a major geologic and
oceanographic investigation of the Gulf of the Farallones National Marine
Sanctuary continental shelf system, designed to evaluate and monitor human
impacts on the marine environment (Karl and others, 2002). The study region is
located off the central California coast, adjacent to San Francisco Bay and
encompasses the Gulf of Farallones Marine Sanctuary. Geologic mapping of this
area included the use of various remote sensing and sampling techniques such as
shallow sub-bottom profiling, sidescan-sonar and bathymetric mapping, gravity
core and grab sampling, and photography. These data were used to define the
surficial sediment distribution, underlying structure and sea floor morphology
of the study area.

The primary focus of this report is to present a georeferenced, digital
sidescan-sonar mosaic of the study region. The sidescan-sonar data were
acquired with the AMS-120 (120 kHz) sidescan-sonar system during USGS cruise
F9-89-NC. The dataset covers approximately 1000 km² of the continental shelf
between Point Reyes, California and Half Moon Bay, California, extending west
to the continental shelf break near the Farallon Islands. The sidescan-sonar
mosaic displays a heterogenous sea-floor environment, containing outcropping
rock, ripples, dunes, lineations and depressions, as well as flat, featureless
sea floor (Karl and others, 2002). Location of Sidescan-Sonar Mosaic Figure 3.
Outline of the Gulf of Farallones AMS-120 sidescan-sonar study region. These
data, along with sub-bottom interpretation (Chin and others, 1997) and ground
truth data (Maher and others, 1991) define the geologic framework of the
region. The sidescan-sonar mosaic can be used with supplemental remote sensing
and sampling data as a base for future research, helping to define the local
current regime and predominant sediment transport directions and forcing
conditions within the Gulf of Farallones.

[Summary provided by the USGS.]