Multibeam Sonar System Development Effort
Multibeam Patch Testing Using Angle Independent Imagery

Measuring the time for a packet (pulse) of sound waves to travel from a shipborne transmitter, be reflected from the seabed, and travel back to a shipborne receiver is a technique that has been used for many years to measure water depth. The conventional single-beam echo sounder, developed in the 1930's and still in wide use today, uses sound waves propagating vertically downward and returning vertically upwards to give the depth directly beneath the survey vessel. Running a series of lines at a specified spacing while obtaining such sounding constitutes a conventional survey of depths in an area. An obvious limitation of this conventional surveying with a single-beam echo sounder is that no quantitative depth information is obtained between survey lines.

Recent advances in sonar technology have provided echo sounders with multiple beams. These systems employ sound waves propagating at angles which vary from vertical to nearly horizontal. The locations on the bottom where echoes are generated cover a swath whose port to starboard width may be equal to many times the water depth. By adjusting the survey tracklines such that adjacent swaths are overlapping, it is possible to obtain quantitative depth information for 100% of the bottom in the survey area. Unfortunately the interpretation of the time for a packet (pulse) of sound waves to travel from a shipborne transmitter, produce echoes at the seabed, and travel back to a shipborne receiver is not so straightforward in the case of multibeam echo sounders as it is in the case of single beam echo sounders.

HSTP has an ongoing project which addresses problems associated with making operational survey systems based on multibeam echo sounders. The relationship between water depth, acoustic travel times and the propagation angle of the packet of sound waves relative to the vertical is highly nonlinear. This means that careful attention must be given to the physics of propagating sound waves in a media with nonuniform sound speeds. It also means that information about the heave, roll, pitch and heading of the multibeam echo sounder must be taken into account when drawing conclusions regarding depths that stem from interpretations of the multibeam acoustic travel times.

One common source of depth bias errors in multibeam echo sounders stems from less than perfect knowledge of the location and as-installed mounting angles of the transducer in the ship's reference plane and in turn relative to local gravity (direction of vertical). HTP has developed field procedures and software programs specifically designed to calibrate the mechanical mounting angles and thereby reduce bias errors in the survey depth.

HSTP has developed processing algorithms for extracting accurate locations and depths of survey soundings from the multibeam echo sounder data in conjunction with data from other sensors that provide heave, roll, pitch, ship heading, ship's position, and sound speed profiles. Corrections for refraction effects in the stratified ocean medium are particularly important because acoustic ray-paths tend to become less stable as the propagation angles become more nearly horizontal. HSTP's development of data processing algorithms has matured to a point permitting out-sourcing of some software development to the private sector.

Research continues on many aspects of integrated multibeam sonar survey systems, including the development of operational techniques to best utilize this technology in the field.

Revised Thursday October 25 2001by OCS Webmaster