Cells

A velocity profile is a set of velocity measurements in a sequence of depth cells. The cell size specifies the vertical length of each depth cell in the profile, thus the cell size defines the depth resolution. A greater number of smaller cells give more details about the vertical variation of currents throughout the water column. Each cell represents the average of the return signal for a given period of time corresponding to that cell size. The current meters measure the water current in one defined sampling area.

Selecting a proper cell size depends on what the objective with the deployment is. If the instrument is deployed in shallow water it is usually of interest to get as much detail in the profile as possible, and therefore selecting a small cell size is key. In deeper waters, where optimal range may be the goal, increasing the cell size to the maximum may be a good approach. The length of the transmitted sound pulse is equal to the configured cell size and a larger cell size will therefore lead to more energy sent out by the instrument which again will result in a better signal strength and longer range. A larger cell size will also have a larger volume of scattering particles to reflect more of the transmitted signal, thus more information to calculate average velocities from. This leads to the important fact that the standard deviation (precision) of the velocity measurement is inversely proportional to the depth cell size. See the precision and measurement uncertainty chapter for more info.

Cell position

The depth cell does not give equal weight to all points within the cell, but is weighted towards the middle. As shown in Figure 1 the instrument applies approximate triangular weighting to each measurement cell. To find the exact location of the depth cell, relative to the instrument, you have to take both blanking distance and cell size into account and the the \( n^{th} \) cell is centered at a vertical distance from the transducer given by the following formula:

Example:

If the blanking distance is set to 1 m and the cell size is 0.5 m, the center of the first cell (n=1) is located at \(1 m + 1 \times 0.5 m = 1.5 m \) from the instrument. The full extent of the measurement area for the cell is between 1 m and 2 m. Correspondingly, the center of the second cell will be at 2 m. 

Note that these numbers are projections along the vertical axis, the numbers along the beam axis are larger by a factor of \(cos (\alpha)^{-1} \) due to the transducer geometry, where \( \alpha \) is the angle of the beam relative to the vertical. The principle of triangular weighting is the same for a single point current meter as for a profiling instrument.

Tilt correction

During current measurements the water column is, divided into several segments referred to as cells. This division allows velocities to be measured at various depths. For the best data quality, instruments should be kept stable looking straight upward or downward without any tilt. When the instrument is tilted, the pointing angle of the instrument beams will be shifted so that the same cell from different beams are not located at the same depth level. Keep in mind that one of the key assumptions when measuring current velocities with an ADCP is that the flow is horizontally homogeneous so that several beams at different angles can be used to resolve different velocity components at individual depth levels. When the instrument is tilted the same cell from different beams will be located at different depths, and when data from these cells are combined residual error in the profile may occur.

To avoid these errors in your data, it is possible to remap the cells for each beam so that cells at equal depth are matched and velocities will be computed at that level. This process is called bin mapping and uses the pitch and roll information from the instrument tilt sensor.

For details on the quality control during post-processing, please see this guide.