The U.S. Naval Oceanographic Office conducted a bathymetric survey in January 1972 over the western Puerto Rico Trench by using a highly directional, multiple-beam sonar system. The system is composed of 15 independent, 2⅔° beams, of which two corresponding side beams may be displayed on one graphic recorder while the vertical beam is displayed on a second recorder.
Seven north-south tracklines across the trench provided accurate topographic profiles that delineate the character of the walls and contained abyssal plain. Prominent fault scarps and flat benches, the results of normal faulting, dominate the topography of the north wall. Two or three major ridges on the south wall, which are more or less continuous in an east-west direction, constitute a barrier to downslope sediment transportation and cause sediment ponding. The abyssal plain has a maximum relief of only 15 fm (uncorrected for variations in sound velocity). Because the abyssal plain slopes both east and south, its resultant strike is east-northeast. This strike also approximates the axis of maximum depth of the trench.
The axis of maximum depth of the western Puerto Rico Trench was determined for 70 n mi. The method required only one east-west trackline along the abyssal plain; the vertical beam and two corresponding side beams were utilized.