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/ofr20151232

Suggested Citation: Dartnell, P., Kvitek, R.G., Ritchie, A.C., and Finlayson, D.P., 2015, Acoustic backscatter, Offshore of Pigeon Point 

map area, California, sheet 3 in Cochrane, G.R., Watt, J.T., Dartnell, P., Greene, H.G., Erdey, M.D., Dieter, B.E., Golden, N.E., Johnson, 

S.Y., Endris, C.A., Hartwell, S.R., Kvitek, R.G., Davenport, C.W., Krigsman, L.M., Ritchie, A.C., Sliter, R.W., Finlayson, D.P., and Maier, K.L. 

(G.R. Cochrane and S.A. Cochran, eds.), California State Waters Map Series—Offshore of Pigeon Point, California: U.S. Geological 

Survey Open-File Report 2015–1232, pamphlet 40 p., 10 sheets, scale 1:24,000, http://dx.doi.org/10.3133/ofr20151232.

ISSN 2331-1258 (online) 

http://dx.doi.org/10.3133/ofr20151232

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

shading) and onshore 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 if known. Yellow line shows limit of California’s State 

Waters.

CSUMB,

 2007

 FP, 

 2006, 2007

USGS, 2009

Año Nuevo 

State Park

Pigeon Point

Limit of California’s 

 State Waters

122o25'

37o10'

122o0'

37o05'

20

DISCUSSION

This acoustic-backscatter map of the Offshore of Pigeon Point map area in central California was 

generated from acoustic-backscatter data collected by California State University, Monterey Bay (CSUMB), 

by Fugro Pelagos, and by the U.S. Geological Survey (USGS) (fig. 1). Mapping was completed between 2006 

and 2009, using a combination of 400-kHz Reson 7125 and 244-kHz Reson 8101 multibeam echosounders, as 

well as a 234-kHz 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 CSUMB and Fugro Pelagos multibeam 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 (position accuracy, ±2 m; pitch, roll, and 

heading accuracy, ±0.02°; heave accuracy, ±5%, or 5 cm). To account for tidal-cycle fluctuations, Fugro 

Pelagos used StarFix HP and XP real-time kinematic (RTK) GPS receivers, and CSUMB used a NavCom 

2050 receiver; in addition, 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 RTK receivers. The multibeam backscatter data were postprocessed using 

Geocoder, within which 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 1- or 2-mresolution images. Overlap between parallel lines was resolved using a priority table whose values were based 

on the distance of each sample from the ship track, with the samples that were closest to and furthest from the 

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

exported as georeferenced TIFF images, imported into a geographic information system (GIS), and converted 

to GRIDs at 2-m resolution.

During the USGS mapping mission, GPS data with real-time-kinematic corrections 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 discriminate seafloor returns (soundings 

and backscatter intensity) from unintended targets in the water column (Ritchie and others, 2010). 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 map, on which 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 an 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). 

Note that the differences in backscatter intensity that are apparent in some areas of the map are due to the 

different frequencies of the mapping systems, as well as to different processing techniques. 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 sheets 1 and 2 of this report). The merged surface was smoothed using the Focal Mean tool in 

ArcGIS and a circular neighborhood that has a radius of between 20 and 30 m (depending on the location). 

The contours were generated from this smoothed surface using the Spatial Analyst Contour tool in ArcGIS. 

The most continuous contour segments were preserved; smaller segments and isolated island polygons were 

excluded from the final output. The contours were then clipped to the boundary of the map area.

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 California Coastal Conservancy 

(available from National Oceanic and Atmospheric Administration [NOAA] Coastal Service Center’s Digital 

Coast 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

Ritchie, A.C., Finlayson, D.P., and Logan, J.B., 2010, Swath bathymetry surveys of the Monterey Bay area 

from Point Año Nuevo to Moss Landing, San Mateo, Santa Cruz, and Monterey Counties, California: 

U.S. Geological Survey Data Series 514, available at http://pubs.usgs.gov/ds/514/.

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

37°

36°40'

122°20' 122°

Santa Cruz

Monterey

Moss 

Landing

Pigeon

Point

Limit of California’s

State Waters

Area of

Map

CALIF.

MAP LOCATION

0 5 10 Kilometers

0 5 10Nautical Miles

121°40'

Monterey

Bay

1

U.S. Geological Survey; 

2

California State University, Monterey Bay, Seafloor Mapping Lab

Acoustic Backscatter, Offshore of Pigeon Point Map Area, California

By

Peter Dartnell,1 Rikk G. Kvitek,2 Andrew C. Ritchie,1 and David P. Finlayson1 

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

SCALE 1:24 000

APPROXIMATE MEAN

DECLINATION, 2015

MAGNETIC NORTH

13 / ° 1 2

TRUE NORTH

Acoustic-backscatter data collected by Fugro Pelagos in 2006–2007, by California State 

University, Monterey Bay, Seafloor Mapping Lab in 2007 (data available at 

http://seafloor.otterlabs.org/index.html), and by U.S. Geological Survey in 2009 (data 

available at http://pubs.usgs.gov/ds/514/). Bathymetric contours by Mercedes D. Erdey, 

2014

GIS database and digital cartography by Nadine E. Golden, 2013

Manuscript approved for publication December 4, 2015

Onshore elevation data from California Coastal Conservancy (available 

from National Oceanic and Atmospheric Administration [NOAA] Coastal 

Service Center’s Digital Coast 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

10

30

40

50

60

70

70

10

10

10

10

10

10

20

Año Nuevo 

 State Park

Pigeon Point

122°25' 122°20'

122°25' 122° 20'

37°10'

37°05'

37°05'

37°10'

Open-File Report 2015–1232

Sheet 3 of 10

Pamphlet accompanies map

U.S. Department of the Interior

U.S. Geological Survey