4.1. Typical ranges of RDI instruments

4.1.1. Operations

Minimum ocean depth for trustworthy Bridge Plot currents:

wh1200	6m
wh600	6m
wh300	11m
sv300	11m
nb150	30m
os150bb	20m
os150nb	30m
os75bb	30m
os75nb	55m
os38bb	42m
os38nb	81m

4.1.2. Science

In good conditions, these are the shallowest bin and typical maximum range of ocean currents from different RDI ADCPs. The parameters used to calculate the shallowest depth are on the right. Don’t forget to add the transducer depth (on the hull) to the numbers below.

	shallowest bin	typical max range	bin size	beam angle	transducer depth
wh1200	4m	12-15m	0.5m	20deg	3m
wh600	4m	30-35m	1m	20deg	3m
wh300	7m	50-80m	2m	20deg	5m
sv300	7m	50-80m	2m	20deg	5m
nb150	11m	300-400m	8m	30deg	5m
os150bb	8m	150-300m	4m	30deg	5m
os150nb	11m	250-350m	8m	30deg	5m
os75bb	11m	400-550m	8m	30deg	5m
os75nb	18m	600-750m	16m	30deg	5m
os38bb	15m	600-1000m	12m	30deg	5m
os38nb	25m	800-1400m	24m	30deg	5m

Considerations:

These numbers are based on

• typical UHDAS settings

• nominal hull depth

UHDAS settings are optimized for these instruments. The vertical extent of the data collected is usually set to be 10%-15% deeper than the deepest typical range if the ADCP velocity data.

Data near the bottom are not useful, due to sidelobe interference. For a 30deg beam angle, the bottom 15% is bad and must be discarded.

Shallow

• The shallowest bin is most likely to be distorted by ringing or bubbles, so it is not used in the Bridge Plot.

• In water shallower than the shallowest possible bin, data below the bottom will register as “strong velocities” and should be ignored. This is a particular problem if the Bridge is using the Bridge Plot to judge ocean currents on their way out of a harbor. We usually need 2 bins of water data in shallow water, to get 1 bin of good data (the bottom impacts the deepest bin).

• The Bridge Plot is usually bins 2-3, to avoid the shallowest bin, so in shallow water, we would need 3-4 bins of data to have reliable bridge plot velocities.

Deep

• Sonar penetration can be blocked or distorted by bubbles. Bubbles can result in low Percent Good and reduced range (and sometimes biases).

• Sonar reflection relies on scatterering off bugs or density gradients. If the water is too clear (eg. Arctic) range will be reduced.

• Range can vary with the time of day: Bugs migrate down during sunlight, so range can vary by 10%-20% between night (deeper) and day (when the bugs are shallower)