Introduction

Researchers of the sea-floor mapping

facility at the U.S. Geological Survey

(USGS) Woods Hole Field Center in

Woods Hole, Mass., use state-of-the-art

technology to produce accurate geologic

maps of the sea floor (figs. 1, 2). In addition to basic bathymetry and morphology,

sea-floor maps may contain information

about the distribution of sand resources,

patterns of coastal erosion, pathways of

pollutant transport, and geologic controls

on marine biological habitats. The maps

may also show areas of human impacts,

such as disturbance by bottom fishing

and pollution caused by offshore waste

disposal. The maps provide a framework

for scientific research and provide critical

information to decisionmakers who oversee resources in the coastal ocean.

The strength of the USGS mapping

capability is in using advanced technology to streamline at-sea operations and

integrate all newly acquired data in the

field. Data processing, archiving, and

initial interpretations are now completed

at sea, thus saving time and money during the research effort and eliminating

costly and time-consuming postcruise

editing. As a result, usable map products

are provided to scientists and decisionmakers at the end of a fieldwork effort

rather than weeks or months later.

Sea-Floor Mapping Facility

Researchers of the sea-floor mapping facility develop and apply modern acoustic and optical remote-sensing

techniques to acquire detailed geologic

information about the sea floor. The

sea-floor mapping group is headed by a

research geophysicist and is staffed by

eight geologists and five field engineers who carry out data acquisition

and processing and generate interpretive products. The facility maintains a

range of equipment appropriate for

mapping in either shallow coastal water

or the deep ocean. At present, most

mapping is conducted in water depths

less than 100 meters.

Data-Acquisition Capability

Sea-floor mapping involves using

acoustic and optical systems to remotely

acquire information about the topography,

composition, and structure of the sea floor.

Differential Global Positioning Systems

(DGPS) are used to navigate the research

vessel and acquisition equipment. 

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet FS–039–02

June 2002

The Sea-Floor Mapping Facility at the U.S. Geological Survey

Woods Hole Field Center, Woods Hole, Massachusetts

SEISMIC REFLECTION 

(subsurface coverage)

SIDESCAN-SONAR 

(areal coverage)

GPS U.S. Coast Guard differential corrections

Hydrophone

Seismic

source

Sidescan

tow vehicle

Research vessel

Echo sounder/

3.5-kHz profiler

Swath 

bathymetry

255 

0 

 

Bathymetry

 (meters)

-3

-15

Vertical exaggeration = 300x

Sun illumination = Southwest

13 kilometers

9 kilometers

Backscatter

(DN)

Shore

N

Sidescan-sonar imagery

draped over bathymetry

Bathymetry

South

Carolina

Study

area

Figure 1. The U.S. Geological Survey collects data from a research vessel

for mapping the sea floor and its underlying geology. Differential Global

Positioning Systems (DGPS) guide the vessel and equipment positions. A

single-beam echo sounder measures water depth and provides a continuous profile of the sea floor below the vessel. A 3.5-kHz profiler sends and

receives sound pulses that penetrate about 5–10 meters into the sea floor.

An interferometric bathymetric swath sonar system measures water depth

and the intensity of sound reflected from the sea floor; the hull-mounted

transducer sends out a fan of sound, which is reflected from the sea floor

and received at the transducer. In high-resolution seismic-reflection profiling, a towed sound source transmits acoustic pulses that are reflected off

the sea floor and the layers beneath. Towed hydrophones (shown above) or

hydrophones built into the sound source receive the returned signal. Sidescan-sonar systems map the intensity of sound reflected from the sea floor

on either side of a towed vehicle that emits a fan of sound. The reflections

provide an image of the sea floor and information on sediment types.

Figure 2. Three-dimensional perspective images of the sea floor off

Myrtle Beach, S.C. These images were produced onboard a research

vessel as part of an ongoing investigation of coastal erosion hazards.

The color image shows bathymetry derived from an interferometric

bathymetric sonar system. The black-and-white image combines sidescan-sonar data with the bathymetric data to help researchers visualize

the distribution of sediment types and sea-floor features. Backscatter,

represented as a digital number (DN), displays the intensity of sound

reflected from the sea floor. High backscatter (strong reflections,

generally from rock or coarse sand) is displayed as light-gray tones;

low backscatter (weak reflections, generally from fine sand or mud)

is displayed as dark-gray tones.
Data-acquisition systems include

the following:

• Sidescan-sonar systems (fig. 1),

which map the intensity of sound

(backscatter) reflected from the sea

floor, providing images of sea-floor

features and information about seafloor sediments. Typical survey

swath widths range from a few hundred meters in shallow water to

several kilometers in deep water.

The USGS sea-floor mapping facility currently operates two sidescansonar systems: a swept-frequency

(CHIRP) sidescan sonar (100–120

kilohertz, kHz) and a dual-frequency sidescan sonar (100/500 kHz).

• Multibeam echo sounder and interferometric bathymetric swath sonar

systems, which measure water

depth and the intensity of sound

reflected from the sea floor. Combining bathymetric data with

backscatter intensity allows scientists to visualize the sea floor in

great detail (fig. 2). The USGS seafloor mapping facility currently

operates single-beam echo sounders

(200 kHz) and an interferometric

bathymetric swath sonar (234 kHz).

Multibeam echo sounders of various frequencies are utilized through

cooperative studies.

• Seismic-reflection systems, which

help to determine sediment thickness and composition by sending

acoustic energy into the sea floor.

Depth of penetration and resolution

depend on both the strength and

frequency of the sound source.

Equipment used for these tasks

includes a 3.5-kHz profiler, a

swept-frequency (CHIRP) seismicreflection system (2–7 kHz), a

boomer seismic-reflection system

(300 Hz–3 kHz), and a 15-cubicinch watergun seismic-reflection

system (20 Hz–1.5 kHz).

After data are acquired remotely,

scientists sample the sea floor to “groundtruth” the geophysical data. The USGS

sea-floor mapping facility in Woods Hole

uses the SEABed Observation and Sampling System (SEABOSS) and other

equipment to obtain sediment samples,

still photographs, and video images of the

sea floor. More information is online at

http://pubs.usgs.gov/factsheet/fs142-00/.

Data-Processing Capability

Software systems are used to

decode, process, edit, display, archive,

and analyze data both at sea and on

shore. Sidescan-sonar, seismic-reflection, and bathymetric data are processed

as they are collected onboard ship. Data

are displayed in real time to check data

quality and to ensure complete coverage

of the sea floor.

The data-acquisition and data-processing equipment can be mobilized

and deployed (typically in a day or

less) on a variety of research vessels.

On larger vessels, two self-contained

vans (8 by 20 feet) provide space for

data-acquisition and data-processing

computers (fig. 3). Space on small

boats commonly limits the at-sea work

to data acquisition (fig. 4); processing

systems are set up on land, and data are

exchanged at the end of each workday.

Mapping operations are usually carried

out during day trips on smaller boats

(less than 50 feet long); larger vessels

allow 24-hour operations.

Archive Capability

Sidescan-sonar, seismic-reflection,

bathymetric, and navigation raw data

from each research cruise are archived

at sea on compact discs (CD-ROM and

DVD-ROM). At-sea interpretations of

these data are generally stored within a

geographic information system (GIS)

and distributed to principal investigators

and collaborators. Paper copies of preliminary results are printed onboard by

using large-format plotters.

—Rebecca E. Deusser, William C.

Schwab, and Jane F. Denny

For more information, please contact:

William C. Schwab 

(E-mail: bschwab@usgs.gov) or 

William W. Danforth

(E-mail: bdanforth@usgs.gov)

U.S. Geological Survey

384 Woods Hole Road

Woods Hole, MA 02543–1598

Telephone: 508–548–8700

Fax: 508–457–2309

Web site: http://woodshole.er.usgs.gov

Figure 3. USGS scientists work at sea inside

a self-contained data-processing van, which

can fit on larger vessels. Photograph by Chris

Polloni, USGS, 1999.

Figure 4. Small vessels are used to map the

geology of the sea floor in shallow water

near the coast. Sea-floor mapping systems

are either rigidly mounted to the vessel, as

shown above, or towed behind the vessel on

a conducting cable. Photograph by Dann

Blackwood, USGS, 2000.