New High Frequency Side Scan Sonar

Tags: sss, sonar

Side scan sonar is becoming a common sight in ports and harbors.
Inset photo: 1200 kHz side scan image of bicycle on river bottom.

Side scan is the ideal tool for search and survey operations because it produces detailed images of the underwater environment regardless of water clarity. In addition, the sonar is able to search large areas quickly, scanning several hundred feet of ocean, lake, or river bottom with each pass of the boat. The sonar does this by transmitting an acoustic beam from a towfish which sweeps the bottom and reflects off any objects lying there. The reflected beam returns to the fish and is sent topside where vivid color images are displayed and stored on a notebook computer. Connecting a GPS allows position coordinates to be captured with the sonar data. Side scan can easily locate a variety of targets including sunken boats, submerged vehicles, and drowning victims.

JW Fishers, a company specializing in underwater search equipment, has added a new high frequency side scan system to their line. The new 1200 kHz sonar produces extremely detailed images of even small and soft targets such as old wood wrecks, areas of scattered debris, or a drowning victim. When the 1200 kHz was towed over a bicycle that had been disposed of in a waterway, not only were the bike's frame and wheels clearly visible, but also the seat and other features. The new sonar is available as a single frequency system or a dual frequency side scan with two sets of transducers in one towfish. Putting two sets in one fish provides tremendous versatility allowing the operator to switch between frequencies at any time during operation. The lower frequency is capable of scanning long ranges, but with less resolution. High frequency has a shorter scan range, but produces highly detailed images. In a typical search operation an initial scan is done with the low frequency. When a potential target is located, the operator switches to the high frequency and makes a closer pass yielding an exact identification of the object. The two frequencies available for coupling with the 1200 kHz in a dual frequency system are the 600 kHz and the 100 kHz. The 600 kHz provides an excellent combination of range and resolution with a maximum scan of 200 feet per side (400 foot swath), yet with the capability of detecting small targets. The 100 kHz has the longest range, up to 2,000 feet per side (4,000 foot swath), making it ideal for scanning large areas when searching for big targets such as downed aircraft or sunken ships.

Fire and police departments consider side scan an essential piece of equipment for their dive teams. Sonar can save hours of dive time and dramatically increases the safety factor. Towing the sonar over an area produces a detailed map of the underwater terrain so divers know exactly what's down there before entering the water. Even in zero visibility environments the clarity of the image is the same. A limitation of conventional side scan systems is they look only at the bottom; but a rotating transducer option on Fishers sonars let them scan vertical structures as well. This means a team can survey the face of a dam, seawall, pier, bridge support, or ship's hull to look for damage or any "unnatural" device like an explosive that may have been attached to the structure. A significant number of state emergency management agencies and sheriffs departments have received homeland security grants to acquire this equipment to aid in the protection of ports and crucial infrastructure.

Another option for Fishers side scan systems are Sonar Map Coverage software which shows the track of the boat as it moves over the search area and the size of the area being scanning, ensuring no part of the area is missed. The map can be displayed in a small window laid directly over the side scan track, or displayed alone, and GPS position coordinates are automatically captured with the sonar data. During playback the sonar track can be overlaid onto a nautical chart showing the precise location of the search operation.

A few of the many teams using Fishers side scan sonars are Columbia County Sheriffs Department in New York, the Union Fire Company in Pennsylvania, Webster Rescue Squad in Massachusetts, Livingston Parish Office of Homeland Security in Louisiana, Midland County Sheriffs Department in Michigan, Wheeling Police Department in West Virginia, Chautauqua County Emergency Services in New York, Miami-Dade Police Recovery Unit in Florida and the US Navy's EOD Technology Division.