Ý Notes

Slide Show

Outline

1	Blueprint of the Velocity Profile
2	A Velocity Profile describes how water current velocity varies through a vertical range of the water column. The profile is composed of a series of 3-d vectors uniformly spaced through the depths reached by acoustic signals emitted by the ADCP. The following slides describe the geometric relationship between the Velocity Profile and the depths where backscattering sources produce the acoustic echoes.
3	Velocity Profile Pieces (1) Transmit Time (2) Blank Time (3) Time Record of Echoes (4) Range Gates (5) Measured Depth Layers (6) Depth Cells
4	Definitions (Figure)
5	Transmit duration is set (default) to match the user choice for Depth Cell size. An intermediate step is that Transmit duration is set by the Range Gate duration. As we will see, the Range Gate time slots in the echo record are set by the Depth Cell size.
6	Blank period is a factory- recommended value (default). After the ADCP transmits, a brief pause or blank in the record is required to allow acoustic transients or ringing (like a bell) to dissipate. Any echoes recorded during this ring-down period are contaminated by these transients that are unrelated to the water in motion (i.e., Doppler shift = 0). Any Doppler shifts computed with such contaminated acoustic data will be biased low.
7	A Range Gate is a time slot in the echo record. These time slots (1) are associated with specific layers in the water column (2) record echoes originating from their assigned layers (3) sample the center of their layer most and top & bottom least (4) overlap with adjacent depth layers, resulting in some mutual information in successive Range Gates.
8	Velocity Profile Pieces (Figure)
9	Velocity Profile Layers (Figure)
10	A Depth Cell generally has size = 50% of the layer heard by a Range Gate and is centered in that layer. Most of the echoes heard by a Range Gate, however, originate from the region within the Depth Cell (due to the spatial-weighting described in the last slide). As a result, data reported for a Depth Cell are predominantly affected by those depths mapped by the cell.
11	Depth Cell Size (Figure)
12	From the Doppler-shifted echo data in each Range Gate, an average velocity is calculated. The result is assigned to the associated Depth Cell. Although adjoining Depth Cells do not overlap, their associated Range Gates share some mutual information. Successive velocities in the Velocity Profile are therefore slightly correlated.