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/3306/

Suggested Citation: Bretz, C.K., Kvitek, R.G., Dartnell, P., and Phillips, E.L., 2014, Acoustic backscatter, Offshore of San Gregorio map 

area, California, sheet 3 in Cochrane, G.R., Dartnell, P., Greene, H.G., Watt, J.T., Golden, N.E., Endris, C.A., Phillips, E.L., Hartwell, S.R., 

Johnson, S.Y., Kvitek, R.G., Erdey, M.D., Bretz, C.K., Manson, M.W., Sliter, R.W., Ross, S.L., Dieter, B.E., and Chin, J.L. (G.R. Cochrane 

and S.A. Cochran, eds.), California State Waters Map Series—Offshore of San Gregorio, California: U.S. Geological Survey Scientific 

Investigations Map 3306, pamphlet 38 p., 10 sheets, scale 1:24,000, http://dx.doi.org/10.3133/sim3306.

ISSN 2329-132X (online)

http://dx.doi.org/10.3133/sim3306

CSUMB, 2007

San Gregorio

State Beach

 Pescadero

State Beach

122°30'

37°20'

122°25'

37°15'

FP, 

2006, 2007

Figure 1. Map showing areas of multibeam-echosounder surveys (pink shading) and onshore topographiclidar surveys (yellow shading). Also shown are data-collecting agencies (CSUMB, California State University, 

Monterey Bay, Seafloor Mapping Lab; FP, Fugro Pelagos) and dates of surveys if known.

Photoscience,

2005

10

DISCUSSION

This acoustic-backscatter map of the Offshore of San Gregorio map area in northern California was 

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

(CSUMB) (fig. 1). Mapping was completed in 2006 and 2007, using a combination of 400-kHz Reson 7125 

and 244-kHz Reson 8101 multibeam echosounders. These mapping missions combined to collect acousticbackscatter data from about the 10-m isobath to beyond the 3-nautical-mile limit of California’s State Waters.

During all the 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, CSUMB used NavCom 2050 GPS receiver 

(CNAV) data, and Fugro Pelagos used KGPS data (GPS data with real-time kinematic corrections); 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 KGPS receivers. Backscatter data were postprocessed using Geocoder version 3.2 (Fugro 

Pelagos modified test release 16). Within Geocoder, 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 a 1-m-resolution image. 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.

The acoustic-backscatter grids were combined 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 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). 

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

surface (see sheet 1 of this report). The most continuous contour segments were preserved; smaller segments 

and isolated island polygons were excluded from the final output. Contours were smoothed using a polynomial 

approximation with exponential kernel algorithm and a tolerance value of 60 m. 

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

45° above the horizon to 1-m-resolution topographic-lidar data collected by Photoscience in 2005 for the U.S. 

Geological Survey and the County of San Mateo.

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

Kilometers 0 5 10

0 5 10Nautical Miles

122°40' 122°20'

38°

37°40'

37°20'

San

Francisco

Bay

San

Francisco

Area of

Map

Limit of California’s

State Waters

CALIF.

MAP LOCATION

1

California State University, Monterey Bay, Seafloor Mapping Lab;

2

U.S. Geological Survey 

Acoustic Backscatter, Offshore of San Gregorio Map Area, California

By

Carrie K. Bretz,1 Rikk G. Kvitek,1 Peter Dartnell,2 and Eleyne L. Phillips2

2014

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, 2014

MAGNETIC NORTH

13 / ° 1 2

TRUE NORTH

Onshore elevation data collected by Photoscience in 2005 for U.S. 

Geological Survey and County of San Mateo. California's State 

Waters limit from NOAA Office of Coast Survey

Universal Transverse Mercator projection, Zone 10N

NOT INTENDED FOR NAVIGATIONAL USE

Acoustic-backscatter imagery by Peter Dartnell, 2012 (data collected by Fugro Pelagos in 2006 and 2007 and 

by California State University, Monterey Bay, Seafloor Mapping Lab in 2007). Bathymetric contours by Carrie 

K. Bretz, 2008

GIS database and digital cartography by Nadine E. Golden

Edited by Taryn A. Lindquist

Manuscript approved for publication June 30, 2014

10

20

30

30

30

30

40

40

50

60

San Gregorio

 State Beach

 Pescadero

 State Beach

122°30' 122°25'

122°30' 122°25'

37°20'

37°15'

37°20'

37°15'

Scientific Investigations Map 3306

Sheet 3 of 10

Pamphlet accompanies map

U.S. Department of the Interior

U.S. Geological Survey