ADCP data examples

Contents

Bubbles
Interference Examples
Triggering (K-Sync)

Bubbles

Bubbles affect ADCP data in several ways:

they can block sound (reducing the remaining ping to a shallow profile)
they can cause profiles to be shorter than usual
affected profiles can biased towards zero (acting as a reflective layer which is moving at approximately the same speed as the ship

Combining these effects, one can end up with shallow profiles that are strongly biased towards zero. After adding the ship speed to the profile (to get ocean velocities), the short profiles are strongly biased in the direction of motion, resulting in a bias in the averaged profiles.

Bubbles cause biases and reduced range in the ADCP data.

(above): This example shows the data range for both instruments (OS150 and OS75) and both ping types (Broadband and Narrowband) when the ship was travelling at about 10kts during a period of deteriorating weather. The ship was heading NNW and the wind was astern. Wind speed was 10-15kts during the part shown, but increased later in the day.

(above): This example shows a speed test, with a clear decrease in range as ship speed increased. The next two plots also look at this time range.

(above): This is a single ping look at the data from the previous example, as the ship speed increases and the bubbles kick in. The velocities have had a mean removed, to center them around zero.

(above) Speed test: background noise increases with speed, range decreases for both instruments. The OS75 is badly affected starting around 6.5kts for this test. The actual ship speed at which the OS75 range plummets, will depend in scattering and sea state.

(above) Heading test: range varied as a function of heading, with a decrase in range in both instruments, using both ping types.

Interference Examples

The following figures are examples of acoustic interference on ADCP data. They are all laid out with the same scheme:

upper left: beam 1 backscatter (“RSSI”) (color panel plot)

lower left: “interference mask” created from amplitude spikes (eg. in beam1 backscatter)

upper center: beam 1 velocity (color panel plot)

lower center: final computed ocean velocity (oriented in ship's forward direction)

upper right: beam velocity plotted versus depth; “bad” values (as determined from the “interference mask” are plotted in red

lower right: final computed ocean velocity plotted versus depth

This treatment is applied to all beams, before the transformation to horizontal and vertical coordinates. None of the data flagged with red dots will be used in CODAS processing. Additional single-ping editing is applied to the velocities after they are converted into horizontal velocities and rotated into earth coordinates.

The next 3 plots show annotated versions of specific interference.