Choosing the right equipment when designing a mooring system is critical to ensuring stability, durability, and reliable data collection. Below, we outline some of the most common challenges our customers encounter, along with best practices for addressing them.

Metals

This issue can be split into two parts: one is the mixing of metals, and the other is the awareness of the magnetic properties in your rig.

When using metals of any kind in your rig, it's crucial to ensure that you don't mix different types of metals as they will begin to corrode. For instance, combining acid-resistant metal and galvanized metal can lead to pitting, where the metals start corroding each other, and this could result in lost instruments. If you're uncertain about your metals, it's recommended to isolate between them by using a length of rope, for example. 

Magnetic materials can interfere with the compass in your instrument. Being mindful of the materials near your device and running a quality compass calibration are two crucial steps to ensure that your compass is accurate. You can follow this link for a guide on compass calibration.

Drag down

"Drag down" is an occurrence in which ocean currents seize hold of the entire rig, dragging it down. This results in the instrument measuring its own velocity instead of the actual current, which in return gives false measurements. The telltale signs of drag down can be identified in the data by a noticeable increase in pitch, velocity, and pressure, as demonstrated in the example below:

Figure 1: Example of ADCP data affected by drag down. Note the increase in pressure (top), pitch (center, blue) and speed (bottom)

To reduce the risk of drag down, there are several measures that can be taken. One effective strategy is to carefully consider the amount of mass the current can take hold of. Are there ways to reduce the mass, such as using a thinner rope or reorganizing the rig to make it shorter?

Buoys that lie on the surface of the water can often be problematic, as they are exposed to strong currents and waves. If the mooring is not appropriately weighted or if the line is under too much tension, the rig may be at risk of being lifted and carried away by the current, or snapping. 

Staggering

To minimize the risk of interference between multiple instruments deployed in the same area, it is recommended to stagger your instruments. This can be achieved by setting a time interval between the starting times of the instruments, ensuring that they do not ping at the same time. Staggering the instruments in this way helps to reduce the likelihood of one instrument picking up the pings from another, leading to more accurate data collection. For further details on this topic, please see this FAQ.

Physical beam interference

One common reason for users having to redeploy their instruments is due to physical interference with the beams. The size of a beam increases as it travels further away from the instrument and is affected by blanking, cell size, and beam geometry. 

Figure 2: Example of a deployment setup affected by obstructions. The cells marked in red will experience significant data loss.

Any physical obstructions within the beams can result in visible data errors, such as interference from other instruments, growth on the seabed, or buoys. Such interference can be recognized in the data by a constant line at a different speed than the rest of the data.

Figure 3: Example signal strength of a single beam that is obstructed at a range of 160m - 180m from the instrument. 

To prevent such errors, it is crucial to plan well for deployment by becoming familiar with the deployment area and double-checking the beam's direction on the instrument. It is essential to ensure that nothing on the rig interferes with the pings.

Related FAQ's:

Sidelobe interference