Depth sounders and sonar: interpreting echoes and choosing a system

Mar 4, 2019

3 min read

Updated: Oct 11

Fishing is an ancient art practiced by our ancestors thousands of years before Christ. Several techniques — including patience, hahaha — have been used since ancient Egypt to hook, catch and hunt schools of fish. But the question is: How do you identify fish underwater?

Sonar uses sound to obtain information about the underwater environment. Its history includes military, scientific, navigation and fishing applications; this review does not attribute its invention to one date or application.

Furuno is among the manufacturers cited in the original edition on fish-finding equipment. Consult each model’s documentation for its features and operating limits.

Lowrance also appeared in the history presented by the original edition. Older models and their commercial importance are historical context, not evidence of current system performance or availability.

Portable Lowrance Fish Lo-K-Tor depth sounder with a circular display in an open case.

FISHFINDER SYSTEM

A fishfinder system basically consists of a display — in some models with an integrated sounder module, otherwise with a separately sold module — and a transducer. The transducer converts electrical signals into transmitted radio-frequency pulses that travel through the water. When these pulses strike a target, they echo and return to the originating transducer, which converts them back into electrical signals to plot the image below the waterline on a display. This makes it possible to detect underwater objects and identify the bottom type.

A depth sounder uses a transducer to convert electrical signals into acoustic pulses that travel through water. Some of the sound energy reflected by targets returns to the transducer and is converted into an electrical signal for processing. The system can estimate depth and display echoes. These are not radiofrequency pulses travelling through water; target identification and interpretation depend on the equipment, settings and conditions.

Here are some technologies available in the marine market to equip your vessel’s Electronic Navigation System for your next fishing trip, voyage, bathymetric survey or underwater bottom survey.

The technology examples and videos below belong to the original edition. For fishing, navigation or a specific survey, assess the objective and requirements before selecting a transducer, module and display; these examples are not an up-to-date catalog.

CHIRP

CHIRP (Compressed High-Intensity Radiated Pulse), meaning Compressed High-Intensity Radiated Pulse, is one of the most advanced sounder technologies available in the marine market. Unlike standard sounder systems that transmit one frequency pulse at a time, limiting target clarity and resolution, CHIRP transmits a continuous sweep of frequencies from low to high and then interprets them individually. When this continuous frequency sweep is transmitted by a CHIRP-compatible sounder system, the returning echoes provide a wider range of information. Consequently, the images are much clearer, with high resolution and target detection, making it possible to distinguish large fish, tightly packed schools of smaller fish and fish near the seabed, rocks, holes, vegetation and at great depths.

On compatible CHIRP systems, each transmission can sweep a frequency range defined for that equipment and transducer. Echo processing can improve target separation compared with appropriate single-frequency configurations. CHIRP does not mean continuously transmitting every frequency from the lowest to the highest. Frequency range, power, installation, depth, noise and water conditions affect the reading; detection or identification of every fish or object is not guaranteed.

Animation of a vessel emitting sonar pulses and receiving returning echoes.

Animation of a vessel emitting groups of sonar waves and receiving their echoes.

As noted above, choosing the ideal transducer is directly related to its intended purpose: underwater bottom surveys, bathymetric mapping, depth, temperature and water-current speed readings, or fishing in rivers, lakes, nearshore or at depths of more than 1,000 meters.

Transducer selection depends on the application and compatibility with the module and display. Depth range, coverage, frequency, installation and water conditions limit the result. Operation beyond 1,000 metres is not a universal capability and should only be specified when documented for the relevant configuration and conditions.

The historical selection below includes examples from Lowrance, Simrad, Garmin, Furuno and Raymarine. Confirm each model's features and suitability for the intended purpose in the manufacturer's sources.

LOWRANCE ACTIVE IMAGING

SIMRAD FORWARDSCAN

LOWRANCE LIVESIGHT

RAYMARINE ELEMENT

RAYMARINE REALVISION 3D

GARMIN ULTRA HIGH-DEFINITION SCANNING SONAR

GARMIN PANOPTIX PS51-TH

GARMIN PANOPTIX LIVESCOPE

FURUNO MULTIBEAM SONAR

REFERENCES

NAVEGAR E PESCAR

GARMIN