The AquaPro HR works with two different pulses that should not be in the water at the same time. If this happens and the echo from first pulse overpowers the echo from the second pulse, a “pulse-to-pulse interference” occurs, and your data collection might be compromised.
To avoid pulse-to-pulse interference, the user should balance the multiple following adjustable parameters: depth cell size, profile length, pulse distance, sample rate and distance to the boundary. The boundary will change depending on whether the instrument is mounted looking downward or upward as follows:
- Downward-looking AquaPro HR at a known distance from bed: this is the easiest way to guarantee good data from your AquaPro HR as the “Distance to the bottom” is not subjected to tidal variations and the frame height can be measured prior to the deployment. For this reason, the downward-looking configuration is particularly recommended for sites subjected to large tidal variations. Additionally, the pulse might dissipate if the bottom consists of sand or silt particles, avoiding echo and bad measurements consequently. Hard bottom (rock, coral reef) might cause some echo interference.
- Upward looking in deep water: defined as profile range being less than half of total depth. In this situation, pulse-to-pulse interference is not a concern.
- Upward looking in shallow water: acoustic interference from the surface might be a concern. Differently from the water/sediment interface, the water/air interface eco might not die off, as it can be a strong acoustic reflector, which could contribute to additional noise in the dataset. Additionally, tidal variations could be an issue as distance to boundary changes over time. To be conservative in avoiding interference, the deployment software sets pulse distance to half the depth. Upward-looking AquaPro HR in shallow water has a minimum water depth operation of the order 2-3 times the profiling range.
Whatever setup is chosen, the instrument tilt should be looked at carefully. It is recommended that one would take a level and a compass to fieldwork. The level would guarantee that at the initial setup the instrument has a low tilt. Tilt over 10 degrees may result in bad data. The compass is to guarantee that whatever coordinate configuration was selected during the planning can still be changed in post-processing. The compass should be aligned with the “x” datum on the instrument.
Let’s look at a few examples and how to avoid mistakes during the AquaPro HR deployment:
- User 1 configured their AquaPro HR to an upward-looking deployment in shallow water with a profile range of 3 meters and 4 meters of pulse distance. During the field campaign, they changed their mind and deployed the instrument facing downwards at 6 m deep keeping the original configuration plan. Near-bed data had low correlation and a high number of bad data, which corresponds to the upper cells. What happened?
- Firstly, sediment and water/air interface both behave as strong and clear boundaries, which is not a problem for most Nortek instruments. For the AquaPro HR, however, the water/air interface can generate a strong echo, and interfere with the data, just like a rocky bed.
- As the pulse distance (4m) is smaller than the full depth (6m), that means that both signals are in the water column at the same time, and the instrument is listening to both signals. A higher pulse distance than the full profile range guarantees that the first signal dies off before the second is transmitted.
What should User 1 have done? Flipping the instrument upside down isn’t generally a problem. However, the best solution to get good results is always making a thorough deployment plan and sticking to the original plan during the field campaign to guarantee good results.
- User 2 wants to get as much data near the surface as possible, so they deployed an upward-looking AquaPro HR and set the Distance to surface twice as large as it actually is. As a consequence, correlation is low in the upper layers with bad data as both signals are in the water at the same time.
What should User 2 have done? Using the Standard tab is enough for most deployments. Stick to the configuration and the data collection will most likely guarantee good data acquisition.
- User 3 wanted to study the sediment/water boundary layer and deployed the AquaPro HR downward looking in a shallow environment (2 m deep). They configured the instrument with a distance to the bottom shorter than it actually was measured in the field. Near-bed data had low correlation and unreliable values. What happened? The second pulse was transmitted too early and both pulses were in the water at the same time for the upper cells, which is near bed in this case.
What should User 3 have done? If the distance to bottom is not precisely known, it is better to slightly overestimate it. This would increase pulse distance, and as consequence, the second pulse will be sent later than it should, losing data in the first cells (near instrument) instead of the last cells (near-bed).