What is underwater surveying?

Whether it is a piece of land or a body of water, surveying is the practice of accurately determining the area of the Earth’s surface. In surveying, a three-dimensional position is measured and fixed. Almost every type of construction project, including maritime and dredging projects, must include surveying as an essential element of planning and execution. Dredging and maritime infrastructure projects require underwater or hydrographic surveys as part of the data collection process to assess the seafloor and subsea conditions and validate the dredging accuracy.  

What tools are used for surveying?

In dredging and maritime infrastructure projects, underwater or hydrographic surveys are used to determine the seafloor and subsea conditions, as well as to determine dredging accuracy. In addition to echo sounding, density profilers and satellite positioning systems, hydrographic or underwater surveying technologies include a wide range of techniques. A surveying method most commonly used is echo-sounding. Bathymetric charts are used for both maritime safety and scientific or engineering purposes. For specialized applications, you can then extract additional information. 

What is Echo-sounding? 

Echo-sounding uses sound pulses to transmit information into the water. During pulse recurrence, the length of time between emissions and returns is recorded. A measurement of water depth is made with this. The echosounder is also used to study fish and aquatic habitats. The hydro-acoustic assessment of fish biomass and spatial distributions can either be conducted using mobile surveys from boats or using stationary transducers to monitor  passing fish. Echosounders can either be single beam or multi beam echosounders.

What is a single beam survey? 

One way of examining one particular part of the sea bed directly below the recording vessel is with a single-beam system, which consists of a transmitter and a receiver. A bathymetric map can be made by sailing parallel to survey lines and measuring the area along each line. The system can be used both in shallow and deeper waters. Acoustic sounding on the sea bed can be highly accurate, but when muddy layers are encountered, the accuracy is reduced. A lot depends on the sonar frequency (kHz) used. Multibeam echo sounding systems are typically used when a high-density seabed coverage is required. 

What is a multi-beam survey? 

Nautical Charting with Multibeam Data

Echo-sounding with multiple beams operates similarly to single beam echo-sounding, however it utilizes a swath of many sound beams instead of just one. Multi-beam systems can have up to 1024 beams, whose outputs can be correlated together so that the sound transmitted in a particular direction or arriving from that direction is amplified. The seafloor’s surface can be accurately determined by measuring the sound travel time between the sender and the seafloor, among other parameters. When using a multi-beam echo sounder on a vessel it requires precise positioning for each beam and this is captured by a high-end Inertial Navigation System (INS) 

What is a Side Scan Sonar?

A side scan sonar provides a picture of the seafloor over a large area. In addition to mapping the seabed, this is an efficient tool for detecting and identifying underwater objects, like rocks, pipelines, and debris. Unlike echo-sounding, it is not a depth measurement method.

What is LIDAR?

LiDAR is a sensing technology that uses light pulses in the form of a laser to measure variable distances from the Earth,  providing three-dimensional information about the surface characteristics and shape of Earth. This helps mapping professionals to study artificial as well as natural environments for numerous purposes. LiDAR (light detection and ranging, a play on the word radar) is used to survey dredging placement areas, assist with hydrographic surveys and collect data that will help analyse beach erosion. It has been used to characterise various benthic habitats in coral reef ecosystems. LiDAR survey data can help determine the current topography and dredge material capacity of placement areas. 

What other tools are used for surveying?

The silt layers in some harbours and access channels are loosely packed. Many ships with deep drafts can sometimes navigate this so-called fluid mud. A density level must then be determined for the fluid mud, but using electroacoustic survey tools is difficult to determine this density level. Surveys can be conducted with a density profiler to establish the densities and in these cases, Navigable depth is defined as the physical level in the fluid mud below which deep-draft vessels can still operate. For example, a towfish with pressure sensors and transmission gauges could serve as an in-situ density profiler. With the ‘navigable depth’ concept, maintenance dredging can be reduced by an average of 30%, resulting in savings for port authorities. 

How accurate are surveys?

To successfully complete a project, a survey must be accurate. In order to obtain accurate results, it is necessary to take into account the quality of the sounding system installation, the skill level of the operators, the type of bed surface being surveyed, the depth and nature of the water and the general structure of the survey system. Surveys need to be performed with strict guidelines with respect to vertical and horizontal accuracy. These issues should be addressed early in a project to avoid unnecessary risks and costs for the client and contractor. 

Total Hydrographic’s knowledgeable professionals have completed many of these types of projects for our clients over the course of a number of years. Make sure you get in touch with us if you are planning a project that requires surveys of water bodies.