Measuring depths with echosounders and sonars
Before the Second World War
Until the Second World War, the leadline was the main method of measuringdepths and determining the type of seabed. The leadline is a cord with a lead weight at one end and depth marks along its length. It was lowered vertically to the seabed, and the depth mark on the cord at the water’s surface was recorded.
To determine the type of seabed, tallow was pressed into the hollow end of the lead weight. When the leadline was dropped into the water, material that adhered to the tallow would provide a physical sample of the seabed.
An echosounder sends sound pulses through water to measure water depth. The water’s depth is calculated by recording the time between the emission of the sound and the reception of an echo as well as the speed at which the sound travels through water. The echosounder first was developed for military purposes, but by the mid-1930s it was used widely for hydrography in Canada.
Early echosounders were quite crude and had low resolution (large sonar footprint). They required constant attention to get moderate performance. Advances in digital circuitry and digital signal processing led to modern survey echosounders with high resolution, high precision and good long-time frequency stability. Improvements in transducer materials and design led to narrow-shaped beams capable of resolving small targets on the seabed.
When the echosounders replaced the leadline, there was no longer regular sampling of the seabed type. Instead, CHS hydrographers have developed ways to use acoustic backscatter to determine the type of seabed.
Because the single-beam echosounder is aimed vertically, it cannot capture information on depth or hazards in the spaces between sounding lines. Sidescan sonar grew out of the idea of tilting a broad-beam echosounder to one side to produce a time series of acoustic returns. This technology has narrow along-track beam width and high across-track resolution.
Sidescan sonars work well for identifying seabed obstacles between sounding lines, but cannot locate those targets precisely or measure the least depth accurately. This inexactness arises from the technical assumption used in sidescan sonar measurements that the seabed is flat. Fortunately, the single-beam echosounder and sidescan sonar are complementary.
Another approach used by CHS is the sweep system, which deploys many single-beam echosounders (SBES) equally spaced along a boom, or some booms, of a vessel. The sweep system provides total coverage of the seabed, at least at some depths, while still acquiring precise depth and position of each measured sounding.
This approach, however, has some constraints. The operation of large booms is awkward and restricts the manoeuvrability of a ship. While the sweep system has a far larger coverage range than the single-beam echosounder, it is still limited by the width of the ship’s booms.
The swath system represents that class of sonars that can obtain multiple across-track soundings from the same array of transducers—usually a transmitter array and at least one receiver array. A swath system does not have the physical constraints of a sweep system. And its angular coverage of the seabed makes it far more efficient than a sweep system as the depth increases.
Multibeam echosounders (MBES) are one class of swath system. Like single-beam echosounders (SBES), they transmit a shaped acoustic pulse into the water column. Unlike SBES, the transmit beam (or ping) is very wide in the across-track direction and very narrow in the along-track direction. The receive beams—and there are many receive beams for each ping—are very narrow in the across-track direction and somewhat wider in the along-track direction. The combination of these two beam patterns results in wide coverage across-track with very high spatial resolution. This system overcomes all the shortcomings of combined single-beam echosounders and sidescan sonars, as well as the limitations of sweep systems.
Phase-measuring bathymetric sidescan sonar
This type of sidescan sonar, also called a phase-differencing sidescan sonar, collects a time series of amplitudes from a side-looking array in the same way that a standard sidescan sonar does. But unlike a standard sidescan, multiple receiver arrays and sophisticated signal processing allow the angle of the arriving signal to be determined. Phase-measuring bathymetric sidescan sonars have the advantage of a very wide look angle (about 10 times water depth, versus about four times water depth for most multibeam echosounders). They also have very high-resolution acoustic backscatter imagery. Their primary disadvantage at present is the increased noise in the data compared with MBES. And like a regular sidescan sonar, this type of measurement still has a gap directly below the sonar where no data are collected. CHS uses the phase-measuring bathymetric sidescan sonar mainly for very shallow, nearshore water, for applications such as habitat mapping.
Single-beam echosounders, multibeam echosounders and sidescan sonars can measure two parameters: round-trip time for a pulse of sound to reflect from the seabed and the strength of the returned (backscattered) signal. These parameters are depicted by the darkness of an echosounder trace. Lighter traces indicate that more sound is being absorbed by the seabed. Hydrographers have taken this idea one step further, and associate higher (darker) acoustic backscatter returns with a harder seabed and lower (lighter) acoustic backscatter returns with a softer seabed.
This idea has led to much CHS research in acoustic seabed classification, in which the type of seabed material can be determined from the strength of the backscattered acoustic signal. Typically, a classification based on acoustic backscatter needs to be tested by physically sampling—called grab sampling or ground-truthing—the seabed.