U.S. Geological Survey
Fact Sheet 039-02
Online Version 1.0

The Sea-Floor Mapping Facility at the U.S. Geological Survey
Woods Hole Field Center, Woods Hole, Massachusetts

By Rebecca E. Deusser, William C. Schwab, and Jane F. Denny

Introduction

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 (figs. 1, 2). 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 at left) 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.

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

Data-Acquisition Capability

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 sea-floor 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 sidescan-sonar 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 sea-floor 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) seismic-reflection system (2-7 kHz), a boomer seismic-reflection system (300 Hz-3 kHz), and a 15-cubic-inch watergun seismic-reflection system (20 Hz-1.5 kHz).

Data-Processing Capability

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.

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.

Archive Capability

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/