Scope of Work
Sonar, 3.5kHz 1,
Apx 1: Statistics 1
Apx 1: Statistics 2
Apx 2: Equipment 1
Apx 2: Equipment 2
THE K1-93-HW ACOUSTIC SURVEY
Throughout the remainder of this report shipboard operations from the 1993 USGS
survey are reported in Julian Day and Greenwich Mean Time (JD/GMT). In some cases local
time and date are also shown. The geophysical and navigational instrumentation and methods
are explained in greater detail in Appendix 2.
SCOPE OF WORK
The original scope of work presented by the USGS, Branch of Pacific Marine Geology
(PMG) to the COE and EPA stated that the principal objective and primary products of the
acoustic survey are maps of the seafloor, including a sidescan sonar mosaic and bathymetric
map. These goals were achieved; primary products include a detailed bathymetric map (Chase
and others, 1994), and a sidescan sonar mosaic of the Mamala Bay seafloor that delimits the
general extent of the acoustically-resolvable dredged material deposits (figures 5, 6, and plate
1). There are no major acoustic data gaps, and there now exists a firm foundation and basis for
further studies. Also included as a product is a characterization of the seafloor substrate in and
adjacent to the disposal sites, interpreted from 3.5 kHz high-resolution subbottom profiles.
Navigation is a critical element of any acoustic marine survey because the location of
specific seafloor features imaged or profiled must be accurately known so that identical features
on adjacent sidescan swaths coincide, and other data sets can be registered to the sonographic
mosaic. Also, it is often necessary to reoccupy specific sites for sampling purposes.
The Global Positioning System (GPS) was used to navigate the ship during the survey,
and provided nearly 24 hours per day coverage. When GPS was not in service, LORAN-C or
transit satellites were used for positioning. Steering of the ship was aided by a trackline-
following display on monitors located on the bridge and at the navigation station in the
geophysics lab (Gann, 1992; Appendix 2). Nominal accuracy of the GPS system used is 100
m, but may be as good as 20 m (Appendix 2). Most lines were run east-west/west-east
(figures 3 and 4), as we were unable to run any north-south lines owing to the northeasterly
trade winds and the resulting sea state.
An EG&G Model SMS 960 Seafloor Mapping System and 59-kHz SMS 990 sidescan
sonar towfish were used to obtain the plan-view image of the seafloor shown in figure 5.
Specific details of the sonar system, data collection, processing, and interpretation are
discussed in Appendix 2. The system was set at a 1-km swath, and sidescan tracklines were
spaced 800 m apart, providing 20% overlap between adjacent swaths (figures 3 and 4). The
processed mosaic is shown in figures 5, 6, and plate 1. Plate 1 is a Mercator projection at
1:40000 scale, and has a resolution of 1.3 m per pixel, and the bathymetry is taken from Chase
and others (1994).