Kongsberg subsidiary Mesotech has developed a unique design to tackle the problem of sonar beam defocusing, producing more accurate and detailed results
While sonar has been used on ROVs for over 30 years, improvements in technology have led to major advances in accuracy, depth rating and resolution. Yet despite those advancements, the basic transducer design has not been altered much since the mid 1990s.
Early designs used fixed frequencies and exposed the transducer to the sea water. However, this also left other crucial components exposed, and any failure of the protective o-ring seals could result in damage to the equipment. To combat this, in the 1990s designers housed the transducer in an oil-filled dome, providing protection from water but preventing more accurate results.
Doing so, however, introduced a new problem. The difference in temperature between the oil and the surrounding water led to defocusing of the sonar beam in warmer shallower water, and in deeper, colder water. This defocusing becomes more extreme as water depth increases – an acute problem as firms increasingly take ROVs and other equipment to these environments.
Despite improvements in the technology – variable-frequency wide-band units now provide greatly improved resolution, range and can produce wider beam widths – the ultimate accuracy is limited by the oil-filled dome.
Speed of sound
Konsgberg Mesotech is the Canadian subsidiary of the Norwegian-headquartered maritime firm, and specialises in sonar other and acoustic instrumentation and equipment. Noting the problem of beam defocusing, the firm’s engineers set about investigating ways to correct it.
The issue arises from the fact that the acoustic properties of oil are very different from those of seawater. In seawater, rises in pressure, depth or the salinity of seawater will increase the velocity of sound – yet these velocities do not change inside the dome. As a result, the two velocities only converge at a water depth of around 500m, before deviating significantly.
A sound wave will also refract as it passes from one medium to another – as measured according to Snell’s Law. This means that as the transducer signal passes out of the oil and into water (and then vice versa when it is received), the beam will refract, causing the signal to be defocused. This produces blurred results. Refraction also intensifies with depth, meaning defocusing will be worse the deeper the ROV dives.
Mesotech’s solution to this is refreshingly analogue. Designers fixed a water-filled acoustic lens on top of the transducer and inside the dome which corrects the refraction of the beam – working in much the same manner as an optical contact lens.
The result, it claims, is a reduction in beam width and significant improvements in resolution and usable operating range – illustrated here in the beam pattern graph.
That increased image quality is of major benefit to operators pushing further into deepwater, especially in application environments with poor visibility.
The technology will be rolled out later this year, beginning with the company’s 171 4,000m lightweight domed sonar model.
Contact: Daryl Morse, International Sales Manager – Underwater Vehicle Instruments