Underwater survey technologies for inspection of underwater conditions

AquaCoustic uses a wide variety of technologies in order to create accurate assessments of a range of underwater conditions. All surveys use a combination of technologies as necessary for the requirements of the project.

• Digital sonar
• Scanning or side-scan sonar
• Sub-bottom profiling systems
• Differential global positioning system (DGPS)
• Robotic total station
• Laser
• Digital CCTV
• Special camera lenses
• Gas sensor
• Solid state bearing and inclinometer
• Electronic tide gauge
• Long tether

Digital sonar

Sonar is a basic tool of underwater surveys.

A programmable, narrow-beam sonar head generates an acoustic pulse. The sound frequency and pulse can be modified and calibrated to suit conditions. When the sound pulse hits a target, it bounces back and the echo appears as a data point. Data points are calculated using the angle of the sonar head, the speed of sound in water and the travel time.

Consecutive profiles or cross-sections of the pipe, surface or other targeted feature are built as the rotating sonar head mechanically sweeps about its axis and travels along a pre-determined path. When operating within a ±45° angle of incidence to the targeted feature, the overall accuracy of our sonar system is considered to be better than 0.5% of range.

The sonar system can be deployed from a robotic tractor, skid, float or small boat.

Data from the sonar, attitude sensor and other technologies is fed into our software and used to calculate X, Y and Z information, and can then be produced in a variety of formats. When the sonar is combined with a CCTV camera, the sonar image is displayed on screen as a picture in picture.

Using sonar, data can be gathered from a vertical dam face, across the bottom, under overhangs or other vertical structures, or inside pipes and sewers.

Scanning or side-scan sonar

AquaCoustic uses side-scan sonar to investigate seafloor conditions such as scour patterns, obstructions, the type and size of debris, and the condition of pipelines and cables. Side-scan is useful in detecting debris items and other obstructions that may be hazardous to shipping or to seafloor installations by the oil and gas industry. Side-scan sonar is also used for dredging operations and environmental studies.

Side-scan uses a sonar device that emits fan-shaped pulses toward the seafloor across a wide angle perpendicular to the path of the sensor through the water, which may either be towed from a surface vessel, or the sensor may be rotated about a stationary point. The intensity of the acoustic reflections from the seafloor is recorded in a series of cross-track slices. When stitched together along the direction of motion, these slices form an image of the sea bottom within the swath (coverage width) of the beam, and the resulting images can be combined and overlaid on a 3D wire mesh surface.

Side-scan data are frequently acquired along with sub-bottom profiler data.

Sub-bottom profiling systems

Especially useful in surveys of river crossings, sub-bottom profiling systems measure the positions of objects that exist below the bottom. A sound source emits a signal downwards into the water and a receiver records the reflected return. Some of the acoustic signal will penetrate the bottom and be reflected when it encounters a change in density such as a pipeline or sediment layers. The system uses this reflected energy to provide information on features beneath the bottom.

Sub-bottom profiling systems can be useful for calculating pipe cover or exposure, or areas of concern. The penetration depth depends on the hardness of the overlying layers and the presence of gas deposits, such as methane.

Differential global positioning system (DGPS)

Accurate positioning allows precise geo referencing and accurate collation of data from multiple technology sources.

The Differential Global Positioning System (DGPS) is an enhancement to the Global Positioning System. It uses a network of fixed, ground-based reference stations to broadcast the difference between the positions indicated by the satellite systems and the known fixed positions.

Depending on the accuracy required, AquaCoustic uses a Trimble Pro XRS (sub metre) or RTK GPS (centimetre) receiver. The Pro XRS offers GPS, WAAS, EGNOS, satellite differential, and beacon capabilities. It also uses patented multipath rejection technology to remove multipath signals in reflective environments.

Robotic total station

For tracking and positioning to centimetre accuracy we use a Trimble 5600 or Trimble S6 robotic total station in combination with the sonar head prism. The S6 has a spin rate of 115 degrees per second and a distance accuracy of +/- 0.8 mm +1 ppm.

Laser

AquaCoustic uses proprietary laser measurement devices specifically to obtain data on pipe profiles above the water line. Combining sonar and laser data provides a complete pipe profile, while adding digital CCTV images enables post-survey review of actual pipe condition.

Digital CCTV

AquaCoustic uses digital CCTV cameras to provide high resolution images that do not suffer degradation in transmission or when copied. The images can be digitally enhanced for greater clarity, and can be easily catalogued and reviewed.

In the case of dam and reservoir surveys, cameras can be pole mounted and lowered 50 metres into the water. The advantages of this technique over remotely operated vehicles (ROVs) is that the position and heading of the camera is known and there are no thrusters to stir up the bottom.

Images acquired in this way can also be displayed in combination with sonar information. The sonar image can be displayed as a picture in picture (PIP) on the CCTV screen. The main screen and PIP window can be swapped to show details of the sonar image, and the sonar overlay image can be turned on and off as needed. The images are recorded in digital format to DVD’s.

Special camera lenses

For review of the entire interior surface of a pipe or sewer, survey cameras can be fitted with special lenses. The recorded images can be enhanced and reviewed off-site after completion of the survey. This not only provides a permanent visual record, but also shortens the time on site, reducing cost, and allows more detailed review at a later date.

Gas sensor

If allowed to build up in confined or poorly ventilated areas, toxic gases such as hydrogen sulfide (H₂S) are not only lethal but corrode pipes and fittings. AquaCoustic deploys gas sensors to measure H₂S and other gas levels, like methane (CH₄), carbon monoxide (CO) and oxygen (O), in sewers from manhole to manhole.

Solid state bearing and inclinometer

Some projects may require the use of a three-axis, tilt-compensated, heading, pitch and roll sensor. Together with a two-axis accelerometer, the sensor can transmit attitude information ten times a second (10 Hz) which is time-stamped to the other acquired data. The unit compensates for hard iron distortions, ferrous objects, and stray fields.

Electronic tide gauge

To track water elevation we us a submersible pressure transmitter. This device stores pressure, temperature and time data, and the data is also monitored in real time via a radio link. The tide gauge is calibrated on site by following our written procedures for field calibration.

Long tether

AquaCoustic specializes in the use of remotely operated vehicles and long tethers up to one mile in length. Integrity of sensor data and quality of digital CCTV images is assured by using a fibre-optic core within the armored tether.

Long tethers not only allow efficient remote data collection in hard-to-reach environments, but also have practical uses such as avoiding traffic congestion around survey target areas.