The bottom of the ocean is mapped for a number of environmental, research, and commercial applications, such as navigational charting, oil and gas exploration, and marine and coastal resource management. Monitoring the health of the seafloor has recently attracted more attention due to increased demand for this information from fishing, the oil and gas industry, and other commercial interests. A suite of acoustic and visual tools is used to study the geology of the seabed and sub-surface layers. These tools can define the geologic framework (seabed morphology [the shape and general landform of the seafloor], surficial sediment distribution, and underlying geologic structure) of a particular region, which then forms the base for further studies of the seabed, such as on sediment transport, coastal erosion, and benthic habitats.

Geologic Seafloor Mapping Methods
The primary tools used for seafloor mapping are acoustic (or sonar) systems that transmit and receive an acoustic pulse from a device called a transducer, which is mounted on a survey vessel or towed in a separate tow-vehicle (called, appropriately, the "fish"). Sonar systems map a narrow strip or swath of the seafloor perpendicular to the ship's track. Surveys are designed so that multiple adjacent lines can yield 100 percent sonar coverage of the seafloor. The travel time of the acoustic pulse (from the transducer to the seafloor, and back) and the strength of the return signals are used to measure the depth to the seafloor (bathymetry), depth to sub-surface sediment layers (sub-bottom), and the "reflectance" of the seafloor (intensity of backscattered energy) (Figure 1). Sonar systems operate at various frequencies. The purpose of the survey and depth of the study area determine the type of sonar system used for a seafloor mapping investigation. Generally, most sonar systems used in mapping areas in the continental shelf operate in the 30-500 kHz range. Lower frequency sonars are typically used in deep water applications because there is less absorption of the sonar pulse, so the lower frequency acoustic pulse can travel greater distances. Sub-bottom information is acquired using very low frequency (Hz-12kHz) systems that are able to penetrate the seafloor and locate underlying geologic structures.

Optical techniques, such as photography and video, are used to collect data about the surface of the seafloor and to "ground truth" the sonar data by correlating the photographic images with features in the sonar record. For example, one kind of seafloor mapping system, a sidescan-sonar, measures the strength of the reflected acoustic signal. This reflected signal, known as backscatter intensity, allows geologists to infer the composition (or grain size) of the surface sediments (such as cobble, sand, gravel, or bedrock). Photography and seafloor sediment samples are used to relate backscatter intensity to a physical sample of the seafloor. In general, low backscatter intensity is measured in fine-grained areas, and stronger backscatter intensity in regions of rock or coarse-grained sediments. Within a map of backscatter intensity, dark tones usually represent areas of low backscatter intensity (generally fine-grained sediments), and lighter tones represent areas of high backscatter intensity (generally coarse-grained sediments) (Figure 2). The combination of remote sensing, or sonar mapping, and direct sampling (photography, sediment samples) of the seafloor allows geologists to define the sediments and underlying structure of the seafloor.

Figure 1.
Marine data collection systems used in seafloor mapping. Differential global positioning systems (DGPS) provide navigation, and locate both the survey vessel and the collected data. A single-beam echo sounder measures water depth and provides a continuous profile of the seafloor directly below the vessel. A 3.5-kHz sub-bottom profiler sends and receives sound pulses that penetrate 5-10 meters into the seafloor. An interferometric bathymetric swath sonar system measures water depth and the intensity of sound reflected from the seafloor; the hull-mounted transducer sends out a fan of sound, which is reflected from the seafloor and received at the transducer. In high-resolution seismic-reflection profiling, a towed sound source transmits acoustic pulses that are reflected off the seafloor and layers beneath. Towed hydrophones (shown left) or hydrophones built into the sound source receive the returned signal. Sidescan-sonar systems map the intensity of the sound reflected from the seafloor on either side of the towed vehicle that emits a fan of sound. The reflections provide an image of the seafloor and information on sediment types. (Source: USGS)

Analysis and Visualization
While the tools and techniques for acquiring bathymetry and backscatter intensity are well established, those for extracting quantitative information from these data to define the geology and seafloor habitat are continually developing. Interpretation of marine remotely sensed data is challenging due to factors that affect the data while it is being acquired (e.g., wind, ship noise, sea state) and the historically subjective nature of visual interpretation. Visual analysis of sonar data can reveal areas of similar characteristics. However, a rigorous quantitative analysis using photographs or surface sediment grabs to examine biological characteristics of the seafloor can yield more repeatable results that can then be used to identify and define areas of unique geologic and benthic characteristics. All of this information can then be processed using a geographic information system (GIS) to generate habitat maps of the seafloor. Data collected on the seafloor geology (seabed morphology, sediment distribution, and underlying structure) and biology (e.g., species density and community structure) are entered into a GIS that facilitates further analysis and interpretation (Figure 3). Mapping seafloor habitats is an evolving process, but identifying distinct geological areas that provide the substrate for animal populations is a fundamental component. As methods are developed to identify biological habitats, the maps of surficial geology will prove invaluable.

Seafloor Mapping Projects in Massachusetts
The U.S. Geological Survey (USGS), in cooperation with the Massachusetts Office of Coastal Zone Management and the National Oceanic and Atmospheric Administration (NOAA), began a three-year project designed to collect geologic seafloor data within the coastal waters along the North Shore of Massachusetts and Boston Harbor. This project complements existing seafloor data collected by USGS and NOAA within the Stellwagen Bank National Marine Sanctuary, and western Massachusetts Bay from 1994-2000. These data and resulting interpretations will be applied to further marine research and coastal and marine resource management projects, by federal, state, private sector, and academic organizations. These investigations will facilitate comprehensive study and monitoring of the seafloor environment.

Figure 2.
Sidescan-sonar mosaic from Stellwagen Bank National Marine Sanctuary. Areas of low backscatter intensity (soft sediments) are shown as dark tones and areas of high backscatter intensity (rough surfaces such as cobble and rock) are shown as lighter tones.

Figure 3.
Far left: Color sun-illuminated bathymetry. Depths are vertically exaggerated five times. Illumination source is from the northwest at 45 degrees altitude. Middle: Sidescan-sonar mosaic of the same area illustrating areas of low backscatter (dark) and areas of high backscatter (white).
Right: Same area showing interpreted bottom types from analysis of depth, backscatter, bottom photography, and sediment sampling data.

To learn more about this project, visit the project website at http://woodshole.er.usgs.gov/project-pages/coastal_mass/index.htm.

Additional Resources:
USGS National Geologic Studies of Benthic Habitats, Northeastern United States
http://woodshole.er.usgs.gov/project-pages/stellwagen/

High-Resolution Geologic Mapping of the Sea Floor Offshore of Massachusetts (USGS)
http://woodshole.er.usgs.gov/project-pages/coastal_mass/index.htm

Sea-Floor Mapping Technology (USGS)
http://woodshole.er.usgs.gov/operations/sfmapping/index.html

Gulf of Maine Mapping Initiative
http://gulfofmaine.org/council/publications/gommifactsheet2002.pdf