Figure 43. The U.S. EEZ Johnston Atoll area GLORIA sidescan-sonar mosaic.

Background

Johnston Island, approximately 717 miles west-southwest of Honolulu, marks the center of the circular U.S. EEZ Johnston Atoll area and consists of the area shown in the figure to the left (fig. 43). The area shown in the upper right area of the image and connected to the Johnston Atoll mosaic via the transit lines are portions of the Hawaii II Central Hawaiian Ridge dataset and were included in the final mosaic area.

GLORIA data for the U.S. EEZ Johnston Atoll area were acquired during three consecutive surveys beginning December 6, 1990 and ending February 21, 1991. When surveying was concluded a total of 16 digital mosaics of approximately a 2-degree by 2-degree size area were completed for the area. The GLORIA image of the Johnston Atoll area covers approximately 477,000 sq km of seafloor.

The first cruise (F-13-90-CP) (Ryan and others, 1992) departed Honolulu and operated from December 6 to 22, 1990. Before reaching the Johnston Atoll area a resurvey of the Necker Ridge in the Hawaii II Central Hawaiian Ridge was completed. Though surveyed on previous cruises, the line orientation over Necker Ridge resulted in a series of doughnut holes and it was determined to resurvey Necker Ridge prior to continuing to the the Johnston Atoll area.

The second cruise (F-1-91-CP) began January 5, 1991 and was completed January 28, 1991. The third and final cruise (F-2-91-CP) began January 31, 1991 and finished three week later on February 21, 1991.

Figure 44. Map showing field activities for GLORIA sidescan-sonar data collection in the U.S. Exclusive Economic Zone (EEZ) Johnston Atoll area.