How do the Origin 65 ADCP acoustics work?
Conventional low frequency ADCPs utilise large monolithic transducers to achieve beam widths of typically 4-5.5°. In contrast, Origin 65 ADCP acoustics use a ‘satellite dish’ design that involves a small transducer with a wide (38°) beam that’s bounced off a parabolic reflector. This process focusses the beamwidth to 3.8° and it is this narrower beam that is used to measure current profiles. The narrower beam width of Origin 65 provides an extra 1.2 dB of gain compared to equivalent conventional designs at the same frequency.
What are the risks associated with sediment deposits on the parabolic reflectors?
When Origin 65 is deployed to the seabed, over the course of a mission it is possible that a layer of sediment may gather on the upper reflector surface. This would affect conventional designs less because the front face of the transducer is more angled with respect to the vertical so it’s harder for sediment to accumulate. However, there are several reasons why sediment collection in the reflector is not likely to impact acoustic performance significantly.
First, the sediment loading would have to be substantial to affect the acoustic performance. A thin layer of sediment with substantial water loading would result in a slight increase in noise at worst, and the systematic contribution from sidelobe pickup (which is present in conventional transducer designs as well) is a far larger effect.
Second, the shape of the Origin 65 beams is mainly determined by the geometry of the reflector. To disrupt the acoustic performance, the collected sediment would have to form a layer dense enough, and with a large enough reflectivity, to reflect sound in a similar manner to steel. In addition, the layer would have to form a surface of different geometry to the reflector parabola, whereas uniform depositions of sediment would likely accumulate equally across the reflector surface. These disruptive requirements are unlikely to be met in most deep water environments.
Thirdly, the curvature of the reflectors is relatively shallow – a depth of around 3 cm over a diameter of 40 cm does not provide a large volume, and it would be easy for currents to push all but the largest, stickiest sediment out of the reflector, or at least rearrange the deposition uniformly across the surface.
It is of course still possible for biofouling to accumulate over the acoustic projectors and the reflector surfaces in the same way as for conventional transducers. Anti biofouling treatments can be applied to the projectors and reflectors to mitigate this effect. However, for many deep water sites even this may not be necessary as the colder temperatures and lack of nutrients and sunlight will hinder growth of biofouling. Therefore, it is unlikely that this effect will degrade the performance of Origin 65 even over multi year deployments.
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