Cruise Report: R/V Surf Surveyor Cruise S1-00-CL, Mapping the Bathymetry of Crater Lake, Oregon, 2000
EM1002 Operational Modes
There are several operational modes available for the EM1002. The differences in the modes are a function of pulse length, beam spacing, and angular sector. The pulse length controls the amount of energy transmitted into the water column. The system can be operated in an “equiangular” (EA) mode in which the beams are spaced at equal angles apart, resulting in a non-linear (increasing spacing away from nadir) spacing of sonar footprints on the seafloor. The system can also be operated in an “equidistant” (EDBS) mode in which the beams are spaced such that the sonar footprints are equally spaced in the across-track profile. The EDBS geometry is achieved by generating variable beamangular spacings. Although EDBS has advantages in data handling (i.e., provides even sounding density), there are two limitations. The beams in the 140° and 150° modes are spaced wider than their beam widths and results in incomplete coverage that produces a striping close to nadir. This problem disappears as the swath width closes to ~120°. However, the second limitation occurs because of attitude uncertainties and imperfect refraction models that can result in sounding errors that grow with angle from the vertical. Because these limitations render the outermost beams less reliable than for the EA mode, we preferred to use the EA mode.
The Crater Lake survey was carried out in the EA mode. In the EA mode, the EM1002 was operated with a 0.2 ms pulse length in waters less than 150 m deep, and the swath width was constrained to 120° swath. In waters deeper than 150 m, the EM1002 switched to a 0.7 ms pulse length and restricted to an 800-m swath width.