On July 29, 1865, the steamship

Brother Jonathan was wrecked on an

uncharted rock pinnacle near Crescent City,

California. All but 19 of the 244 people

aboard were killed, a tragedy that might

have been averted had the captain known

the rock was there. For as long as humans

have ventured onto the sea, knowledge of

the ocean bottom has been essential to their

safety and success. 

Today, nautical charts made by the

National Oceanic and Atmospheric Administration (NOAA) help mariners recognize

and avoid navigational hazards. A variety of

sea-floor base maps, constructed by the U.S.

Geological Survey (USGS) and other

research groups, help scientists identify

more far-reaching hazards—such as offshore earthquake faults, unstable undersea

slopes, and pathways for the movement of

sediment and pollutants.

Base maps are fundamental tools for

studying not just offshore hazards but also

natural resources and the offshore environment. They provide a context for recording

and interpreting scientific data. For example, when scientists pull a sample of rock or

sediment from the sea floor, they want to

know: where is it from? is it from the bottom of a channel, from a small hill, or from

a flat plain? The answers to such simple

questions, which are necessary to help scientists understand the processes that shape

the sea floor, are provided by accurate base

maps.

Sea-floor base maps made over the past

century range widely in accuracy. Older

navigation systems resulted in features

being mapped many meters or even kilometers from their actual geographic locations.

Systems used to measure the distance to the

sea floor resulted in water depths that might

be off a few meters or tens of meters.

Depending on the spatial resolution of the

mapping system, objects below a certain

size—such as shipwrecks or even undersea

mountains—might not appear at all.

Methods of sea-floor mapping have

improved over time (see back page). By the

mid-1990’s a new generation of sea-floor

mapping systems, called high-resolution

multibeam systems, had been developed.

These systems send a fan of sound energy

toward the sea floor and receive reflected

sound through multiple narrow beams, each

pointed at a different angle. They produce

sea-floor maps of unprecedented accuracy.

On the continental shelf (the extension of

the continent below sea level), these systems can detect features as small as 1 meter

across, and locate them to within 1 meter of

their true geographic location and less than

50 centimeters of their true depth below sea

level. The new systems not only provide

bathymetric (depth) data, but also record the

amount of sound energy returned to the

mapping system from the sea floor, termed

“backscatter.” Backscatter data can help scientists identify the materials that make up

the sea floor, such as rock, sand, or mud. 

Exposing the Sea Floor: High-Resolution Multibeam Mapping

Along the U.S. Pacific Coast

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 013-00

2000

What would you see if youcould drain the oceans and

take aerial photographs of the

sea floor? You would see views

like those produced by the U.S.

Geological Survey (USGS) using

high-resolution multibeam mapping systems. The USGS is mapping the sea floor off populated

areas, such as Southern

California and the Hawaiian

Islands, and under coastal and

inland water bodies, such as San

Francisco Bay and Lake Tahoe.

The resulting new images support studies of biological habitats, offshore earthquake and

landslide hazards, waste disposal, movement of sediment and

pollutants, and other coastal and

marine issues.

Los Angeles Santa 

Monica

Newport

 Beach

Long

 Beach

Sea floor off the Los Angeles area. Sea floor is in shades of blue and green. The Palos Verdes Peninsula is

in the center of the coastline, Santa Monica Bay to the left, and San Pedro Bay to the right. From Santa

Monica to Newport Beach is about 65 kilometers (40 miles) as the crow flies. Slopes look steeper than they

really are; vertical exaggeration is 20 to 1. (Image constructed from data collected in 1996, 1998, and 1999.)

A hull-mounted high-resolution multibeam mapping

system sends sound energy toward the sea floor

and receives reflected sound through multiple

narrow beams. As the ship moves forward (toward

lower left), it maps a swath of sea floor.
Angel 

Island

Tiburon

Peninsula

Golden Gate

San Francisco

Harding Rk

Shag Rk

Arch Rk Alcatraz Island

 San 

 Francisco 

 Bay

1 kilometer

1 mile

 Marin

Peninsula

Harding

Rock Shag Rock

Arch Rock

Outline of rocks shallower 

than 55 feet (17 meters)

Reference mesh of

50-yd (46-meter) squares

Vertical exaggeration: 10x

Some rock pinnacles on the floor 

of San Francisco Bay that pose 

hazards to navigation

N

Some Multibeam Mapping Images Produced by the USGS

To see more images, visit http://walrus.wr.usgs.gov/pacmaps/

Floor of Central San Francisco Bay

This image shows the floor of central San

Francisco Bay—the deep part of the bay,

where ocean-going vessels traverse the shipping lanes and strong tidal currents sculpt the

bay floor. Collected in late 1996 and early

1997, the multibeam data used to produce

this image provide a first look at such a large

area of the bay floor in such detail.

Because multibeam data are digital, they

can be manipulated in many ways. In this

image, shadows have been added as if the

sun were shining from the west. The resulting “shaded relief” makes details of the bayfloor topography easier to see and interpret.

The new bay-floor data can help answer

some important questions:

• How do currents move sediment on the

bay floor?

• Where can sediment dredged from harbors and shipping channels be most

safely disposed of?

• What types of bay floor provide habitats for fish and shellfish?

• How much rock must be trimmed from

pinnacles on the bay floor to ensure that

big ships do not run aground on them?

(See “Rock Pinnacles in San Francisco

Bay,” below.)

On-land portions of this image (tinted green) were

created from topographic data, so the image shows no

roads, buildings, or vegetation. Light blue areas are parts

of San Francisco Bay and adjacent waters that were not

mapped during this study.

Rock Pinnacles in San Francisco Bay

On the shaded-relief image above, you can see

three rock pinnacles rising from the floor of San

Francisco Bay northwest of Alcatraz Island. They

are shown in an oblique view at left, made from

the same data used to make the shaded-relief

image. Now under study is a plan to lower these

and other rock pinnacles in the bay to promote safe

ship travel and to prevent oil spills that might

occur if a ship struck one of these rocks. 

The detailed bathymetric data collected during

multibeam mapping allowed the USGS to estimate

the amount of rock that would have to be removed

to lower these pinnacles to a safe depth for deepdraft vessels. From Harding Rock, for example,

about 38,000 cubic meters of rock would have to

be removed. This information is important for

agencies planning to remove and dispose of the

material. 

(For more information, see USGS Open-File Report 98-139,

http://sfbay.wr.usgs.gov/access/mapping/multibeam/)
Incline

Village

Tahoe

City

South

Lake Tahoe

0 1 mi

0 1 km

Emerald

Bay

South Lake

Tahoe

 To 

Incline 

Village

To Emerald Bay

Pearl

Harbor

Honolulu Int'l

Airport Honolulu

Mamala Bay

2 kilometers

2 miles

(on left) or in oblique views (on right). This

oblique view looks south-southwest from

near Incline Village toward the south shore

of the lake (vertical exaggeration is 3 to 1).

(For more information, images, and a

computer-generated fly-through movie, see

http://blt.wr.usgs.gov/.)

cooperation between Federal agencies and

other groups conducting research in the

Lake Tahoe basin. The new base map will

help all researchers in the basin conduct and

coordinate their studies.

The pair of images above shows how the

digital data can be presented in map view

Bottom of Lake Tahoe

The USGS mapped the bottom of Lake

Tahoe on the California-Nevada border in

summer of 1998—the first time since it had

been roughly mapped in 1923. The mapping

was undertaken in response to a 1997

Presidential Executive Order calling for

Coronado

 Bank

San Diego

 Loma

Sea Valley

Mission Bay 

N

In 1998, the USGS conducted multibeam mapping off several of

the Hawaiian Islands. This image shows backscatter data collected in

Mamala Bay, south of Oahu. Backscatter is a measure of how much

sound energy is reflected from the sea floor. Hard surfaces, such as

rock and sand, reflect more sound energy than soft surfaces, such as

mud. In this image, harder materials are bright, softer materials dark.

Tiny, circular light patches in the middle of the bay floor are piles of

material that was dredged from Pearl and Honolulu Harbors over the

past century and dumped in Mamala Bay. The dumped material is

resting on natural sediment.

This and other maps produced by the USGS will help the U.S.

Army Corps of Engineers and the U.S. Environmental Protection

Agency determine the volume and ultimate fate of materials disposed at sea-floor sites off the Hawaiian Islands.

Sea Floor off San Diego

The image below shows the sea floor off San Diego, California,

which was mapped by the USGS in 1998. The undersea ridge labeled

“Coronado Bank” is a sliver of continental crust shifted into its

present position by movement along faults. Several of those faults

run through Loma Sea Valley, between Coronado Bank and the

narrow continental shelf off the San Diego area. 

Eventually, USGS scientists plan to map the entire stretch of coast

between the Mexican border (right edge of this image) and Santa

Monica (just north of Los Angeles; see figure at top of first page).

The slopes in this image look steeper than they really are; vertical

exaggeration is 20 to 1.

Mamala Bay, off Honolulu, Hawaii
Until the 20th century, seafarers used

weighted lines to measure water depth. They

assumed the distance to the sea floor was the

same as the length of line that had to be let

out before the weight hit bottom. They often

recovered samples of sea-floor sediment as

well, clinging to wax coatings on the

weights or picked up by mechanical sampling devices. 

These depth measurements, or soundings, were taken routinely only in rivers and

coastal waters. Deep-ocean soundings were

rare, took hours to perform, and could be

wildly inaccurate. The weight of the line

needed to measure great depths exceeded

that of the end weight, making it difficult to

tell when the weight hit bottom. Also, undersea currents and drift of the vessel caused

the line to slant as it dropped toward the bottom, making the length of line let out greater

than the vertical distance to the sea floor.

Today sound energy is used to measure

distances in water. The modern term for this

approach is “sonar” (SOund NAvigation and

Ranging). Devices that use sound to measure

distances in water were first invented in the

early 20th century. They made it possible to

measure water depths by a technique called

“echo sounding.”

In the simplest form of echo sounding,

a pulse of sound sent from a ship travels to

the sea floor and bounces back to a receiver

on the ship. The time lapse between sending

the sound pulse and receiving its echo is

used to calculate water depth at that point.

In World War II, the U.S. Navy introduced continuous echo sounding, in which

pulses of sound were sent at short intervals

and the echoes converted into marks on a

recording chart as a vessel was under way.

The result was a continuous profile of the

sea floor along the line that marked the

ship’s track. 

In the 1950’s, the Navy developed

sidescan sonar to look for mines and other

explosive devices. An instrument towed

behind a ship sent out a fan of sound extending to either side of the ship’s track.

Returning echoes produced an image of features along a swath of sea floor, the ship’s

track being in the center of the swath. By

combining overlapping swaths of sidescansonar data, scientists produced the first photograph-like views of the sea floor. 

Sidescan sonar also provides clues

about what materials make up the sea floor.

Hard materials such as rock and sand reflect

more sound back to the sidescan receiver

than soft materials such as mud. Scientists

can study bright and dark areas in sidescansonar images and make inferences about

what kinds of sea-floor materials have been

imaged. Sea-floor samples are taken to confirm the scientists’ interpretations.

Although sidescan sonar provides valuable views of the sea floor, it does not provide information on water depth. Also,

researchers must make assumptions about

the location of the towed sidescan device relative to the position of the ship, in order to

determine the geographic location of areas

depicted in sidescan-sonar images. 

These problems are not present in the

high-resolution multibeam swath-mapping

systems developed in the 1990’s. In these

systems, a hull-mounted transducer sends

out a fan of sound, then records sound

reflected from the sea floor through a set of

narrow receivers aimed at different angles.

These systems collect bathymetric (waterdepth) data of such high resolution that they

can distinguish depths that differ by just a

few tens of centimeters (just a few centimeters in shallow water). At the same time, the

systems collect “backscatter” data, a measure of the amount of sound energy returned

from the sea floor. Because the transducer is

mounted on the ship’s hull, both the bathymetric and the backscatter data are firmly

tied to the ship’s position, and compensations can be made for the ship’s roll and

other movements. Using differential GPS

(Global Positioning System), high-resolution

multibeam mapping can pinpoint sea-floor

locations to within one meter. It can detect

features as small as a meter across.

Multibeam mapping is part of the

USGS Coastal and Marine Geology

Program’s ongoing efforts to provide information that will assist the careful management of our coasts and seas.

Authors: James V. Gardner, Peter Dartnell, 

Helen Gibbons, Duncan MacMillan

Contributors: Paul R. Carlson, Pat S. Chavez, Jr., 

John L. Chin, Stephen L. Eittreim, Deborah Lee

Soltesz, Michael E. Torresan, Michael E. Field,

Miguel Velasco, Florence L. Wong

COOPERATING ORGANIZATIONS

University of New Brunswick

U.S. Environmental Protection Agency

U.S. Army Corps of Engineers

U.S. Navy

Minerals Management Service

National Oceanic and Atmospheric Administration

Orange County Sanitation District

Orange County Public Facilities and Resources Department

Los Angeles County Sanitation District

City of Long Beach

Port of Long Beach

San Diego Metropolitan Waste Water Department

Massachusetts Institute of Technology

University of Hawaii Institute of Marine Biology

Two views of the floor of Santa Monica Bay

created from high-resolution multibeam data

(compare oblique view on front page). Left-hand

image shows bathymetry, right-hand image shows

backscatter. Brighter areas on backscatter image

represent harder materials, probably rock and

sand. Darker areas are softer materials, such as

mud. Scientists are working to better understand

the relationship between sea-floor materials and

the backscatter images they produce, with the

ultimate goal of using backscatter data to make

maps of the materials on the sea floor. 

2 kilometers

2 miles

Marina 

 Del Rey

Santa Monica

2 kilometers

2 miles

Marina 

 Del Rey

Santa Monica

0 1 kilometer

0 1 mile

Santa Cruz

Monterey Bay

N

Image of sea floor off Santa Cruz, California, created from sidescan-sonar data collected in 1995 and

1996. Parallel bands in broad V-shape are swaths of

sidescan-sonar data. Dark areas are muddy sediment. Bright areas are outcrops of bedrock and

patches of sand. Light line angling southwestward

from Santa Cruz is an outfall pipe.

A Brief History of Sea-Floor Mapping

For more information contact:

U.S. Geological Survey, MS-999

345 Middlefield Road

Menlo Park, CA 94025

(650) 329-5042

http://walrus.wr.usgs.gov/