Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the 

U.S. Government

This map was printed on an electronic plotter directly from digital files. Dimensional calibration may vary between electronic plotters 

and between X and Y directions on the same plotter, and paper may change size due to atmospheric conditions; therefore, scale and 

proportions may not be true on plots of this map.

For sale by U.S. Geological Survey, Information Services, Box 25286, Federal Center, Denver, CO 80225, 1–888–ASK–USGS

Digital files available at https://doi.org/10.3133/ofr20181023

Suggested Dartnell, P., and Kvitek, R.G., 2018, Acoustic backscatter, Offshore of Gaviota map area, California, sheet 3 in Johnson, S.Y., 

Dartnell, P., Cochrane, G.R., Hartwell, S.R., Golden, N.E., Kvitek, R.G., and Davenport, C.W. (S.Y. Johnson and S.A. Cochran, eds.), 

California State Waters Map Series—Offshore of Gaviota, California: U.S. Geological Survey Open-File Report 2018–1023, pamphlet 

41 p., 9 sheets, scale 1:24,000, https://doi.org/10.3133/ofr20181023.

ISSN 2331-1258 (online)

https://doi.org/10.3133/ofr20181023

Figure 1. Map showing areas of multibeam-echosounder and bathymetric-sidescan surveys (pink 

shading) and publicly available onshore topographic-lidar data (yellow shading). Also shown are 

data-collecting agencies (FP, Fugro Pelagos; USGS, U.S. Geological Survey) and dates of surveys if 

known.

USGS, 2007, 2008

Gaviota

State Park

FP, 2008

120°20' 120°15'

34°30'

34°25'

20

DISCUSSION

This acoustic-backscatter map of the Offshore of Gaviota map area in southern California was generated 

from acoustic-backscatter data collected by the U.S. Geological Survey (USGS) and by Fugro Pelagos (fig. 1) 

in 2007 and 2008, using a combination of 400-kHz Reson 7125, 240-kHz Reson 8101, and 100-kHz Reson 

8111 multibeam echosounders, as well as a 234-kHz SEA SWATHplus bathymetric sidescan-sonar system. 

These mapping missions combined to collect acoustic-backscatter data from about the 10-m isobath to beyond 

the 3-nautical-mile limit of California’s State Waters.

During the USGS mapping missions, GPS data with real-time-kinematic corrections were combined with 

measurements of vessel motion (heave, pitch, and roll) in a CodaOctopus F190 attitude-and-position system to 

produce a high-precision vessel-attitude packet. This packet was transmitted to the acquisition software in real 

time and combined with instantaneous sound-velocity measurements at the transducer head before each ping. 

The returned samples were projected to the seafloor using a ray-tracing algorithm that works with previously 

measured sound-velocity profiles. Statistical filters were applied to discriminate seafloor returns (backscatter 

intensity) from unintended targets in the water column. The backscatter data were postprocessed in 2016 using 

SonarWiz software that normalizes for time-varying signal loss and beam-directivity differences. Thus, the 

raw 16-bit backscatter data were gain-normalized to enhance the backscatter of the SWATHplus system. The 

data were exported in Imagine format, imported into a geographic information system (GIS), and converted to 

a GRID at 2-m resolution.

During the Fugro Pelagos mapping missions, an Applanix POS-MV (Position and Orientation System for 

Marine Vessels) was used to accurately position the vessels during data collection, and it also accounted for 

vessel motion such as heave, pitch, and roll, with navigational input from GPS receivers. Smoothed Best 

Estimated Trajectory (SBET) files were postprocessed from logged POS-MV files. Sound-velocity profiles 

were collected with an Applied Microsystems (AM) SVPlus sound velocimeter. Soundings were corrected for 

vessel motion using the Applanix POS-MV data, for variations in water-column sound velocity using the 

AM SVPlus data, and for variations in water height (tides) and heave using the postprocessed SBET data 

(California State University, Monterey Bay, Seafloor Mapping Lab, 2016). The Reson backscatter data were 

postprocessed using Geocoder software. The backscatter intensities were radiometrically corrected (including 

despeckling and angle-varying gain adjustments), and the position of each acoustic sample was geometrically 

corrected for slant range on a line-by-line basis. After the lines were corrected, they were mosaicked into 

0.5-m resolution images (California State University, Monterey Bay, Seafloor Mapping Lab, 2016). The 

mosaics were then exported as georeferenced TIFF images, imported into a GIS, and converted to GRIDs at 

2-m resolution. 

The acoustic-backscatter imagery from each different mapping system and processing method were 

merged into their own individual grids. These individual grids, which cover different areas, were displayed in 

a GIS to create this composite acoustic-backscatter map. On the map, brighter tones indicate higher backscatter intensity, and darker tones indicate lower backscatter intensity. The intensity represents a complex interaction between the acoustic pulse and the seafloor, as well as characteristics within the shallow subsurface, 

providing a general indication of seafloor texture and composition. Backscatter intensity depends on the 

acoustic source level; the frequency used to image the seafloor; the grazing angle; the composition and 

character of the seafloor, including grain size, water content, bulk density, and seafloor roughness; and some 

biological cover. Harder and rougher bottom types such as rocky outcrops or coarse sediment typically return 

stronger intensities (high backscatter, lighter tones), whereas softer bottom types such as fine sediment return 

weaker intensities (low backscatter, darker tones). Ripple patterns and straight lines in some parts of the map 

area are data-collection and -processing artifacts.

The onshore-area image was generated by applying an illumination having an azimuth of 300° and from 

45° above the horizon to 2-m-resolution topographic-lidar data from National Oceanic and Atmospheric 

Administration Office for Coastal Management’s Digital Coast (available at http://www.csc.noaa.gov/

digitalcoast/data/coastallidar/) and to 10-m-resolution topographic-lidar data from the U.S. Geological 

Survey’s National Elevation Dataset (available at http://ned.usgs.gov/).

REFERENCE CITED

California State University, Monterey Bay, Seafloor Mapping Lab, 2016, Southern California 2008 CSMP 

surveys: California State University, Monterey Bay, Seafloor Mapping Lab Data Library, accessed 

October 2016 at http://seafloor.otterlabs.org/SFMLwebDATA_SURVEYMAP.htm.

EXPLANATION

Backscatter intensity

High

Low

Area of “no data”—Areas near shoreline not mapped owing to insufficient high-resolution 

seafloor mapping data; areas beyond 3-nautical-mile limit of California’s State Waters were 

not mapped as part of California Seafloor Mapping Program

3-nautical-mile limit of California’s State Waters

Bathymetric contour (in meters)—Derived from modified 2-m-resolution bathymetry grid. 

Contour interval: 10 m

120°30' 120° 119°30'

34°

34°30'

Limit of California’s

State Waters

Kilometers 0 5 10

Santa Barbara Channel

Area of

Map

Santa Barbara CALIF.

MAP LOCATION

Anacapa Is.

Santa Cruz Is.

Santa Rosa Is.

San Miguel Is.

1

U.S. Geological Survey;

2

California State University, Monterey Bay, Seafloor Mapping Lab

Acoustic Backscatter, Offshore of Gaviota Map Area, California

By

Peter Dartnell1 and Rikk G. Kvitek2

2018

ONE MILE = 0.869 NAUTICAL MILES

BATHYMETRIC CONTOUR INTERVAL 10 METERS

1 .5 0 1 KILOMETER

1000 0 1000 2000 3000 4000 5000 6000 7000 FEET

1 1/2 0 1 MILE

SCALE 1:24 000

APPROXIMATE MEAN

DECLINATION, 2018

MAGNETIC NORTH

TRUE NORTH

12 / ° 1 2

Acoustic-backscatter imagery by Peter Dartnell, 2016 (data collected 

by U.S. Geological Survey in 2007 and 2008 and by Fugro Pelagos in 

2008). Bathymetric contours by Peter Dartnell, 2016

GIS database and digital cartography by Nadine E. Golden and 

Stephen R. Hartwell

Manuscript approved for publication February 12, 2018

Onshore elevation data from National Oceanic and 

Atmospheric Administration (NOAA) Office for Coastal 

Management's Digital Coast (available at 

http://www.csc.noaa.gov/digitalcoast/data/coastallidar/) and 

from U.S. Geological Survey’s National Elevation Dataset 

(available at http://ned.usgs.gov/). California's State Waters 

limit from NOAA Office of Coast Survey

Universal Transverse Mercator projection, Zone 10N

NOT INTENDED FOR NAVIGATIONAL USE

20

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90

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150

190

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120

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140

160

170

10

 Santa Ynez Mountains

 Gaviota

State Park

120°20' 120°15'

34°30'

34°25'

120°20' 120°15'

34°25'

34°30'

Open-File Report 2018–1023

Sheet 3 of 9

Pamphlet accompanies map

U.S. Department of the Interior

U.S. Geological Survey