Side-Scan Sonar
Side-scan sonar is a remote sensing technique that uses sound waves to create detailed images of the seafloor. It involves towing a sonar device behind a survey vessel, which emits sound pulses and records the returning echoes. The resulting images provide valuable information about the seafloor's texture, composition, and features, such as shipwrecks, pipelines, and geological formations. This data is essential for habitat mapping, environmental monitoring, and archaeological investigations.
1- Introduction
The SIDE SCAN SONAR is a particular active sonar device designed mainly for research and mapping of the seabed for underwater archaeology and other purposes. We will not go into the details of the data collected and visually presented by the apparatus, data that are abundantly available on the net, but we will investigate the machine and how it allows accurate surveys of the profiles of the finds lying on the seabed.
In this page we will try to illustrate the "side scan sonar" by comparing it with a generic active sonar installed on surface ships to better understand the operating philosophy and the physical structure that characterizes this particular apparatus. The comparison between the two will also allow to highlight the acoustic problems that differentiate the two detection systems.
Figure 1: Side Scan Sonar Investigation.
2- Mode of action of the side scan sonar
The side scan sonar must allow very high angular discrimination in order to be able to identify echoes coming from reflecting elements of the seabed, angularly very close to each other, at the purpose of:
- creating nautical charts and the detection and identification of underwater objects
- Researching the bathymetric characteristics of the sites.
- Investigate the problems of marine archeology
- help locate and identify underwater artifacts of anthropic origin
- Classify the backdrops based on the type of deposit material.
- Detect items of debris and other seabed obstructions dangerous to navigation
- Monitor seabed installations of the oil and gas industry.
The operating mode of this particular sonar is illustrated in the following figure which we are about to comment on:
The pilot vessel, with the equipment which constitutes the control and data processing part of the side scan sonar, is trolling, at a speed between 2 and the 6 knots, a small light vehicle which houses the wet parts of the sonar.
The set of emission/reception transducers of the and the associated electronics interface with the dry part through a special data and signal interchange cable in order to create the lateral scanning acoustic beams.
The navigation depth of is kept constant around 100-200 m from the bottom for site depths not exceeding 1000 m.
During the path it emits, at programmed cadence, impulsive acoustic signals which suitably illuminate the background (lateral scanning beams) in order to obtain a series of return echoes which, received by the acoustic bases of, generate impulsive voltages to be processed for seabed mapping.
The acoustic scanning operation of the seabed takes place, in principle, according to the sequence:
1- (VL) emits an acoustic pulse capable of illuminating, with calibrated thickness "lzi", the two sections of the seabed, the right section and the left section, the part of the seabed below the vertical of (VL) is excluded.
2- The echoes of the small areolas that form the illuminated sections are received by the hydroponic bases of (VL) which transform them into electrical voltages; These, suitably amplified, filtered and transformed with suitable A/D converters, are sent to the sonar calculation and processing system on (NP).
3- The calculation system, on the basis of the differences in the arrival times of the single echoes, identifies the positions of the small areolas along the illuminated band "zfi" for the translation into video format of the first scanning strip.
4- (VL), pulled by (NP), continues the advance and the sequence repeats itself.
Each four actions 1), 2), 3), 4) corresponds to a new scanning strip suitably positioned next to the previous one to achieve the cascade video presentation effect; see in figure 7 a map from the life of a seabed in which some scanning strips are highlighted.
Figure 2: Side-Scan Sonar results.
The algorithms studied for the processing of the signals received by the (VL) are the prerogative of the various specialized branches of underwater archaeology, we just have to examine the operation of the side scan sonar from the point of view of the acoustic beam formation technique.
The working frequencies of the side scan sonars are generally variable from 80 Khz to 600 Khz; lower frequency values are used for object discovery (low resolution), higher frequencies for their classification (high resolution).