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://dx.doi.org/10.3133/ofr20151088 or http://pubs.usgs.gov/of/2015/1088/

Suggested Citation: Dartnell, P., Erdey, M.D., and Kvitek, R.G., 2015, Acoustic backscatter, Offshore of Tomales Point map area, 

California, sheet 3 in Johnson, S.Y., Dartnell, P., Golden, N.E., Hartwell, S.R., Greene, H.G., Erdey, M.D., Cochrane, G.R., Watt, J.T., 

Kvitek, R.G., Manson, M.W., Endris, C.A., Dieter, B.E., Krigsman, L.M., Sliter, R.W., Lowe, E.N., and Chin, J.L. (S.Y. Johnson and S.A. 

Cochran, eds.), California State Waters Map Series—Offshore of Tomales Point, California: U.S. Geological Survey Open-File Report 

2015–1088, pamphlet 38 p., 10 sheets, scale 1:24,000, http://dx.doi.org/10.3133/ofr20151088.

ISSN 2331-1258 (online)

http://dx.doi.org/10.3133/ofr20151088

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

shading) and topographic-lidar surveys (yellow shading). Also shown are data-collecting agencies 

(CSUMB, California State University, Monterey Bay, Seafloor Mapping Lab; FP, Fugro Pelagos; USGS, 

U.S. Geological Survey) and dates of surveys. Yellow line shows limit of California’s State Waters.

FP, 2007

 

 CSUMB, 

2005, 2007

38°15'

123°5' 123° 122°55'

38°10'

 USGS,

 2004–2005

Limit of California’s 

 State Waters

10

DISCUSSION

This acoustic-backscatter map of the Offshore of Tomales Point map area in northern California was 

generated from backscatter data collected by Fugro Pelagos, by California State University, Monterey Bay 

(CSUMB), and by the U.S. Geological Survey (USGS) (fig. 1). Mapping was completed between 2004 and 

2010, using a combination of 200-kHz and 400-kHz Reson 7125 and 244-kHz Reson 8101 multibeam 

echosounders, as well as 234-kHz and 468-kHz SEA SWATHplus bathymetric sidescan-sonar systems. 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 Fugro Pelagos and CSUMB mapping missions, an Applanix POS MV (Position and Orientation System for Marine Vessels) 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). KGPS altitude data (Fugro Pelagos, StarFix HP & XP units; 

CSUMB, NavCom 2050) 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 KGPS receivers. Backscatter data were postprocessed using Geocoder within Cairs HIPS and SIPS software. Georeferenced Backscatter 

Rasters (GeoBaRs) were created for each survey line using the beam average or time series data types. 

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. Individual 

GeoBaRs were mosaicked into 1- or 2-m resolution images. Overlap between parallel lines was resolved using 

a priority table based on the distance of each sample from the ship track, with samples closest to- and furthest 

from- the ship track given the lowest priority. An anti-aliasing algorithm was also applied. The mosaics were 

then exported from CARIS as georeferenced TIFF images, imported into a geographic information system 

(GIS), and converted to GRIDs at 2-m resolution.

For the USGS mapping missions within Tomales Bay (Anima and others, 2008), differential GPS (DGPS) 

data were combined with measurements of vessels 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 

discriminate seafloor returns (soundings) 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 gainnormalized to enhance the backscatter of the SWATHplus system. The resulting normalized-amplitude 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. 

Note that parallel lines of higher backscatter intensity throughout the map area are data-collection and processing artifacts.

Bathymetric contours were generated at 10-m intervals from a modified 2-m-resolution bathymetric 

surface (see sheet 1 of this report). The original surface was smoothed using the Focal Mean tool in ArcGIS 

and a circular neighborhood with a radius of 20 to 30 m (depending on the area). The contours were generated 

from this smoothed surface using the ArcGIS Spatial Analyst Contour tool. The most continuous contour 

segments were preserved; smaller segments and isolated island polygons were excluded from the final output. 

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

45° above the horizon to 2- and 3-m-resolution topographic-lidar data available from National Oceanic and 

Atmospheric Administration (NOAA) Coastal Service Center’s Digital Coast and the U.S. Geological Survey, 

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

REFERENCE CITED

Anima, R.J., Chin, J.L., Finlayson, D.P., McGann, M.L., and Wong, F.L., 2008, Interferometric sidescan 

bathymetry, sediment and foraminiferal analyses; a new look at Tomales Bay, California: U.S. Geological 

Survey Open-File Report 2008–1237, 33 p., available at http://pubs.usgs.gov/of/2008/1237/.

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 2-m-resolution bathymetry grid. Contour 

interval: 10 m 

123°20 123°

38°20'

38°

0 5 10 Kilometers

0 5 10 Nautical Miles

Area of

Map

Limit of California’s

State Waters

Point Reyes

Tomales Bay

Bodega

Bay

CALIF.

MAP LOCATION

Bodega

Harbor

1

U.S. Geological Survey; 

2

California State University Monterey Bay, Seafloor Mapping Lab

Acoustic Backscatter, Offshore of Tomales Point Map Area, California

By

Peter Dartnell,1 Mercedes D. Erdey,1 and Rikk G. Kvitek2

2015

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

14°

SCALE 1:24 000

APPROXIMATE MEAN

DECLINATION, 2015

MAGNETIC 

 NORTH

TRUE NORTH

Acoustic-backscatter data collected by Fugro Pelagos in 2007, by California State University, 

Monterey Bay, Seafloor Mapping Lab in 2007–2010, and by U.S. Geological Survey in 2004–2005. 

Bathymetric contours by Mercedes D. Erdey, 2013

GIS database and digital cartography by Nadine E. Golden

Manuscript approved for publication May 4, 2015

Onshore elevation data from California Coastal Conservancy, 

available at http://www.csc.noaa.gov/digitalcoast/data/

coastallidar/, and from U.S. Geological Survey, 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

10

20

40

30

50

60

70

10

10

10

10

10

10

Tomales Point

 Point Reyes

National Seashore

Tomales Bay

Bodega 

 Bay

Toms

Point Keys Creek

Abbots

Lagoon

123°5' 123° 122°55'

38°15'

38°10' 38°10'

38°15'

123°5' 123° 122°55'

Open-File Report 2015–1088

Sheet 3 of 10

Pamphlet accompanies map

U.S. Department of the Interior

U.S. Geological Survey