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 http://pubs.usgs.gov/sim/3281/

Suggested Citation: Dartnell, P., Kvitek, R.G., Phillips, E.L., and Cochrane, G.R., 2013, Acoustic backscatter, Offshore of Santa Barbara 

map area, California, sheet 3 in Johnson, S.Y., Dartnell, P., Cochrane, G.R., Golden, N.E., Phillips, E.L., Ritchie, A.C., Greene, H.G., 

Krigsman, L.M., Kvitek, R.G., Dieter, B.E., Endris, C.A., Seitz, G.G., Sliter, R.W., Erdey, M.D., Gutierrez, C.I., Wong, F.L., Yoklavich, M.M., 

Draut, A.E., Hart, P.E., and Conrad, J.E. (S.Y. Johnson and S.A. Cochran, eds.), California State Waters Map Series—Offshore of Santa 

Barbara, California: U.S. Geological Survey Scientific Investigations Map 3281, pamphlet 45 p., 11 sheets, scale 1:24,000, 

http://dx.doi.org/10.3133/sim3281.

ISSN 2329-132X (online)

http://dx.doi.org/10.3133/sim3281

US Army Corps

of Engineers

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

topographic-lidar surveys (orange diagonal lines), and publicly available interferometric synthetic aperture 

radar (ifSAR) topography (yellow shading). Also shown are data-collecting agencies (CSUMB, California 

State University, Monterey Bay, Seafloor Mapping Lab; USACE, U.S. Army Corps of Engineers; USGS, U.S. 

Geological Survey) and dates of surveys if known.

Santa Barbara

USGS, 2006

USGS, 

 2005 USGS, 2006

CSUMB, 2007

119°45' 119°40'

34°25'

34°20'

USACE, 2009

20

DISCUSSION 

This acoustic-backscatter map of the Offshore of Santa Barbara map area in southern California was 

generated from backscatter data collected by California State University, Monterey Bay (CSUMB), and by the 

U.S. Geological Survey (USGS) (fig. 1). Most of the offshore area was mapped by CSUMB in the summer of 

2007, using a 244-kHz Reson 8101 multibeam echosounder. Smaller areas in the far-east nearshore, as well as 

further offshore to the west and in the southeast outer shelf area, were mapped by the USGS in 2005 and 2006, 

using a combination of 468-kHz (2005) and 117-kHz (2006) SEA (AP) Ltd. SWATHplus-M phasedifferencing sidescan sonars. 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 CSUMB mapping mission, an Applanix position and motion compensation system (POS/MV) 

was used to accurately position the vessel during data collection, and it also accounted for vessel motion such 

as heave, pitch, and roll (position accuracy, ±2 m; pitch, roll, and heading accuracy, ±0.02°; heave accuracy, 

±5%, or 5 cm). NavCom 2050 GPS receiver (CNAV) data were used to account for tidal-cycle fluctuations, 

and 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) using vertical-position 

data from the CNAV receiver. Backscatter data were postprocessed using CARIS7.0/Geocoder software. 

Geobars were created for each survey line using the beam-averaging engine. 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. The contrast and brightness of some 

geobars were adjusted to better match the surrounding geobars. Individual geobars were mosaicked together at 

2-m resolution using the auto-seam method. The mosaics were then exported from CARIS as georeferenced 

TIFF images, imported into a GIS, and converted to GRIDs.

During the USGS mapping missions, differential GPS (DGPS) data were combined with measurements 

of vessel motion (heave, pitch, and roll) in a CodaOctopus F180 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 the raw samples that discriminate the 

seafloor returns (soundings and backscatter intensity) from unintended targets in the water column. The 

backscatter data were postprocessed using USGS software (D.P. Finlayson, written commun., 2011) 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 resulting normalizedamplitude values were rescaled to 16-bit and gridded into GeoJPEGs using GRID Processor Software, then 

imported into a GIS and converted to GRIDs. 

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 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 sediment type. 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). The differences in backscatter intensity that are apparent in some areas of 

the map are due to the different frequencies of mapping systems, as well as different processing techniques.

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

45° above the horizon to coastal airborne topographic-lidar data, as well as to publicly available, 3-m-resolution, 

interferometric synthetic aperture radar (ifSAR) data, available from National Oceanic and Atmospheric 

Administration (NOAA) Coastal Service Center’s Digital Coast, at http://csc-s-maps-q.csc.noaa.gov/dataviewer/

viewer.html (last accessed April 5, 2011).

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 10-m-resolution bathymetry grid. 

Contour interval: 10 m

CALIF.

0 4 8 Nautical Miles

Kilometers 0 5 10

120° 119°40' 119°20' 119°

34°

34°20'

MAP LOCATION

Ventura

Santa Barbara

Anacapa

Island

Santa Cruz Island Santa Rosa Island

Santa Barbara Channel

Limit of California’s

State Waters

Area of

Map

1

U.S. Geological Survey;

2

California State University, Monterey Bay, Seafloor Mapping Lab

Acoustic Backscatter, Offshore of Santa Barbara Map Area, California

By

Peter Dartnell,1 Rikk G. Kvitek,2 Eleyne L. Phillips,1 and Guy R. Cochrane1

2013

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, 2013

MAGNETIC NORTH

12 / ° 1 2

TRUE NORTH

Acoustic-backscatter imagery collected by California State University, 

Monterey Bay, Seafloor Mapping Lab in 2007 (reprocessed by Peter 

Dartnell, 2010) and by USGS in 2005 and 2006. Bathymetric contours by 

Andrew C. Ritchie, 2011

GIS database and digital cartography by Nadine E. Golden and Eleyne 

L. Phillips

Edited by Taryn A. Lindquist

Manuscript approved for publication December 12, 2013

Onshore elevation data from NOAA Coastal Services Center (data collected by 

EarthData International in 2002-2003) and from U.S. Army Corps of Engineers 

(data collected by Fugro Pelagos in 2009). California's State Waters limit from 

NOAA Office of Coast Survey

Universal Transverse Mercator projection, Zone 11N

NOT INTENDED FOR NAVIGATIONAL USE

20

10

40

30

60

50

80

70

50

60

50

Santa Barbara

119°45' 119°40'

34°25'

34°20'

119°45' 119°40'

34°25'

34°20'

Scientific Investigations Map 3281

Sheet 3 of 11

Pamphlet accompanies map

U.S. Department of the Interior

U.S. Geological Survey