The LINZ specifications require three hits along track and three across track for a given target size. Whilst the operator has little controlover vessel motion, vessel speed is an important factor. The water depth and consequent ping rate combined with vessel motion and speed are the determining factors in the eventual along-track sounding density. The SeaBat systems in general are designed to ping as fast as possible for any given range scale. The need to wait for the acoustic round-trip time prior to the next acoustic transmission is directly related to water depth and swath angle, the two of which determine the maximum slant range. The ensonified footprint on the sea floor from which a single sounding is derived increases with water depth. The receiver sample rate (assuming a short enough pulse length is used) defines the range resolution of a system, which is considered to be its ability to discriminate depth differences.
The along-track (projector) beamwidth contributes to the spatial resolution but also directly affects the along-track sounding density, which, in turn, may restrict vessel speed under certain conditions. Narrow receiver beams provide spatial resolution and, as one sounding per beam is generated, a combination of a large number of narrow receiver beams provides the best opportunity to detect a target. One sounding per receiver beam is generated, the number of beams varying according to system and operating mode. Both the SeaBat 71 have a nadir beam width of 0.5°, spreading to 1° at ☖0° steering angle as the effective aperture decreases. These parameters are examined in more detail below.Ī multi-beam system uses a combination of a narrow (1°) along-track projector beam and multiple receiver beams in the cross-track plane. The ability to detect and delineate an object is determined by a combination of: Having developed a new multi-beam echosounder (MBES) system, RESON wanted to compare the performance of this system in a field test with their SeaBat 8125 MBES.
The increased density in the outer beams maintains a high number of soundings across the swath and can significantly reduce the amount of overlap required, thus reducing survey time and costs. The availability of equi-distant beam-forming significantly increases productivity, especially in survey operations where a number of soundings per grid cell is specified. The LINZ specifications, which are built on the IHO S-44 specifications, specify a minimum target size to be detected and they also define a minimum number of hits on that target in order to assure detection in the reduced data set. Land Information New Zealand (LINZ) is the National Hydrographic Authority of New Zealand. With beams formed in equi-angle mode, sounding spacing is most dense at nadir and decreases towards the edge of the swath, whereas in equi-distant mode nadir density is maintained across the entire swath. This article describes a comparison survey with two SeaBat MBESs: one with equi-angle mode and one operated in equi-distant mode. A multi-beam echosounder (MBES) with an equi-distance possibility increases the number of hits at the sides of the swath. In practice, the surveyor has two options to increase the number of hits: reduce the range to increase number of hits or slow down the vessel to increase sounding density. Survey specifications such as the Land Information New Zealand (LINZ) require a specified number of hits on a given target size and a specified overlap in Special Order areas.