What the instrument measures
An echo sounder sends a short acoustic pulse downward from a transducer mounted on or through the hull, and listens for the echo from the bottom. What it measures is time: how long the pulse took to go down and come back. Depth is that time multiplied by the speed of sound in the water, divided by two.
A single-beam sounder does this in one cone pointing straight down. Logged continuously as the vessel moves, and tied to a satellite position, the result is a profile: a line of depths along the track.
From a reading to a depth
The number on the sounder’s display is not yet a depth that can be compared with a chart or with another survey. Three corrections stand between them.
- Transducer depth. The sounder measures from the transducer, not from the surface. The depth of the transducer below the waterline has to be added, and it changes with the vessel’s load.
- Speed of sound. Sound travels through water at roughly 1,400 to 1,550 metres per second, depending on temperature and salinity. A sounder set for the wrong value is wrong in proportion: one per cent off in speed, one per cent off in depth.
- Water level. A river’s surface rises and falls. To compare two profiles, or a profile with a chart, depths are reduced to a common reference level using the water level recorded at the time of the survey.
Motion adds a fourth. Pitch, roll and heave move the transducer, and in rough water the depths need a motion sensor, or at least careful reading.
What a single line cannot see
The cone of a single-beam sounder is narrow, and the patch it covers on the bottom grows with depth. With an 8-degree beam, that patch is about 1.4 metres across in 10 metres of water. Whatever lies to the side of it — a shoal between two survey lines, a sunken hull, a bar forming at the edge of the channel — does not appear in the profile.
Schematic · no scale, no values. Left: one beam in section. Right: survey lines seen from above.
- Survey line: depths recorded
- Beam footprint on the bottom
- Feature between lines: not recorded
Surveys handle this by running lines close together and across the channel, and by interpolating between them. Interpolation is an estimate. The closer the lines, the smaller the space in which something can hide; it never becomes zero.
The bottom itself can be uncertain. Over soft sediment, a high-frequency pulse tends to return from the top of the soft layer and a low-frequency pulse from deeper down. Dual-frequency sounders record both, and the difference between them is information, not noise.
A profile has a date
On a river, the bottom moves. Sand and silt are carried downstream, bars form and migrate, and the deepest line of the channel — the thalweg — shifts with the season. A profile describes the bottom on the day it was surveyed, along the line it was surveyed. Months later, with the river in flood or at low water, it may describe something that is no longer there.
That does not make profiles less useful. It means each one should travel with its date, its line, its instrument and its corrections, and be read as evidence for a decision at that date.
What it can support
A well-run profile can support a decision about a crossing, show where the channel was deepest on the day, confirm that a reach has changed since the last survey, or set a baseline for the next one. Reading it that way, with its limits stated, is most of the work.
