U.S. Exclusive Economic Zone (EEZ)
GLORIA Mapping Program

Introduction

In March 1983, President Reagan signed a proclamation establishing an Exclusive Economic Zone (EEZ) extending the 200 nautical mile from the coasts of the United States, Puerto Rico, the Northern Mariana Islands, and the U.S. territories and possessions. In 1984, the U.S. Geological Survey (USGS) began a program to map these areas of the EEZ To accomplish the mapping of the deepwater portion (depths greater than 400 m) of this area, the USGS selected the long-range sidescan sonar system GLORIA (Geological LOng-Range Inclined Asdic). At that time the GLORIA system was owned and operated by British colleagues at the United Kingdom's Institute of Oceanographic Sciences (IOS), now the Southampton Oceanography Centre, Challenger Division. The GLORIA system was developed specifically to map the morphology and texture of seafloor features in the deep ocean.

GLORIA-II Sidescan Sonar System

The GLORIA system is a digital sidescan sonar system capable of producing digital image maps of the seafloor from reflected sound waves. The sidescan sonar system was used aboard the research vessel R/V FARNELLA and is shown with the GLORIA fish mounted on the stern of the ship.

GLORIA creates images (sonographs) that are a record of the acoustic backscatter from the ocean floor. These images are created by transmitting a series of sound pulses and recording their echoes from the sea floor as the collecting ship moves along a set course. The sound source and receivers are built into a "fish" that is towed about 200 meters behind a ship. This electronic mapping system emits a signal (pulse) every 30 seconds. The pulse travels through the ocean to the bottom where it is partly absorbed and partly reflected by the seafloor. The returned signal is then recorded by shipboard computers. In general, the returned signal is recorded in a range of 0-255 with strong returns recorded as higher values and weak returns recorded as lower values. The darkness or brightness of a feature or an area on the sonographs and completed mosaics is therefore a function of how much sound is reflected from the seafloor.

The USGS selected sidescan sonar as the mapping tool because it could be used to obtain information on geologic processes. The intensity of the back-scattered sound from the seafloor is a function of the gradient or slope of the seafloor, of the surface roughness, and of the sediment characteristics such as texture. Another advantage to the GLORIA system was because the system provides information from a swath of seafloor, large areas could be mapped quickly. More detailed information and specifications of the GLORIA-II system is available from Somers and others (1978). The GLORIA imagery provides the first broad-scale view of sea-floor features and the effects of sedimentary processes that have been unknown or poorly known until recently. These data are a unique set of basic information that will support many future studies by government, academic, and industry workers.

The recorded digital data are processed and used to construct digital maps of the seafloor such as the one shown (Gulf of Mexico mosaic #13). Seafloor variation is depicted by the light and dark areas representing the high and low returned signal values, and can be interpreted by a geologist.

Digital Processing

The imagery contained in these mosaics is from computer processed, digitally collected sidescan sonographs. The sidescan sonographs or swaths were first processed to correct for geometric and radiometric distortions that exists in the raw data. These major source of geometric distortions were: 1- water-depth offset; 2- slant-range geometry; 3- aspect distortion; and changes in the ship's speed. Radiometric corrections applied were: 1- a shading correction to correct for the attenuation of the sonar energy in water as a function of range; 2- a power correction for very-near-nadir data because of slow buildup in the energy signal transmitted by the system; 3- a speckle-noise correction; and 4- removal of striping noise. A more detailed description of the digital processing is available from Chavez (1986).

Digital Mosaicking

After the sidescan sonar data swaths were processed to correct them geometrically and radiometrically, they were ready for digital mosaicking. The final results of the digital mosaicking steps were to create mosaics of a 2degree X 2degree (or smaller) for the survey area. This resulted in 16 mosaics for the U.S. Gulf of Mexico, 21 mosaics covering the U.S. Atlantic East Coast and 36 for the U.S. Pacific West Coast.

To begin, the swaths, approximately 6 hours in length, were spliced end-to-end to make a continuous line segment were the portions of the ship's heading remained generally constant. The swaths were tone matched by applying a contrast stretch to minimize the seam where they were joined. Control points were then selected from the newly created strips. The strips were then georeferenced within it's map area. This step was repeated for each strip of sonar data for the specific area.

After all the strips for a given area were georeferenced, the adjacent lines segments were stenciled together to create a continuous mosaic. In cases where further registration of the strips were required to align seafloor features, additional ground control points and rubber sheeting was done prior to the stenciling. When the swaths were aligned, interactive stenciling of the swaths were done by drawing a polygon around the area of the swath to be retained. The stenciled line segments were then combined, sequentially building the composite map.

Information contained in these pages have been summarized from a variety of sources. More detailed descriptions of the EEZ, GLORIA program and sidescan sonar system, data processing and mosaicking techniques, and geologic interpretations may be obtained from previously published sources. A few of these sources have been listed here.

 GLORIA Publications
 Additional References