Synthetic aperture sonar to help in hunting sea mines

SAS has better resolution than real aperture sonar (RAS), which is currently the most widespread form of side scan sonar in use. RAS transmits pings, receives echoes and then paints a strip of pixels on a computer screen. RAS repeats this pattern until it has an image of the seafloor. This technology is readily available, and relatively cheap, but its resolution over long ranges is not good enough to suit the Navy’s mine hunting needs.

RAS sensors emit acoustic frequencies that are relatively high and are therefore quickly absorbed by the seawater. SAS uses lower frequency acoustics, which can travel farther underwater. Upgrading to SAS improves the range at which fine resolution pictures can be produced.

“RAS can give you a great looking picture but it can only see out 30 to 50 meters,” Cook said. “For the same resolution, SAS can see out to 300 meters.”

SAS does not paint a line-by-line picture of the sea floor like RAS. Instead, SAS pings several times and then records the echoes on a hard drive for post-processing. Once the AUV surfaces, the hard drive is removed and the data is analyzed by computers in a complex signal processing effort. The signal processing converts the pings into a large, fine-resolution image of the seafloor. The commonly accepted measure for fine resolution is a pixel size of 1 inch by 1 inch, which is what SAS can achieve.

Tests of SAS in AUVs have produced fine-resolution images of sunken ships, aircraft, and pipelines. But when looking at an image of the seafloor from above, operators might have difficulty discerning the identity of simple objects. For example, certain mines have a circular cross section. When looking at a top-down image, an operator might not be able to tell the difference between a mine and a discarded tire. To discern if that circular-shaped object is a threat, operators consider the shadow that an object casts in the sonar image. A mine will cast a shadow that is easy to distinguish from those cast by clutter objects such as tires. The shadow contrast research will be used to help ensure that this distinction is as clear as possible.

“There are other more complicated models that the Navy uses that will do this sort of calculation, but it takes too long,” Cook said. “We have developed a compact model that will allow you to predict contrast very quickly.”

The release notes that improving contrast prediction can have a ripple effect in mine hunting capability. Naval officers will be better able to plan missions by predicting how good the shadows will be in a certain environment. This can lead to improved imagery, power conservation, and better performance for automatic target recognition software.

Mines are plentiful and easy to make. Some mines explode on contact. Others are more sophisticated, exploding or deploying torpedoes when their sensors detect certain acoustic, magnetic or pressure triggers. Some can destroy a ship in 200 feet of water.

“Mines are a terrible problem. They lie in wait on the seafloor, so you want to go find them with as few people in the process as possible, which is why we’re driven towards these autonomous vehicles with synthetic aperture sonar,” Cook said.

This research is supported by the Office of Naval Research, but the release notes that any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the Office of Naval Research.

— Read more in D. Cook et al. “Synthetic aperture sonar contrast,” in John S. Papodakis and Leif Bjorno, eds, Proceedings of the First International Conference and Exhibition on Underwater Acoustics (June 2013): 143-50; Z. G. Lowe et al., “Multipath ray tracing model for shallow water acoustics,” in Proceedings of the 11th European Conference Underwater Acoustics, ECUA2012 (July 2012)