Techniques and instruments used in Hydrographic Survey

Hydrographic Survey

Hydrography is the science which determines the physical features and the navigable portions of the Earth's surface adjoining coastal areas. Surveyors study bodies of water to see what the floor looks like.

Techniques and Instruments used in Hydrographic Survey :

Many instruments are used in hydrographic survey like multibeam sonars, LIDAR, multibeam echo sounders(MBES), global positioning system (GPS), laser scanners etc. Among these instruments some are discussed ahead which are mostly used.

1. Light Detection and Ranging:

LIDAR is used to measure the elevation or the depths of water bodies by analyzing the reflection of laser light off an object or seafloor. 

Bathymetric LIDAR is used to determine is used to determine the depths of water bodies by measuring the time delay between the pulse of transmission and its return signals.

2.Multibeam Sonars:

Multibeam echo sounders(MBES) or sonar systems transmit sound energy which strike seafloor and reflected back and in this way depths of water bodies is calculated.

Mostly MBES systems have the ability of recording acoustic backscattter data.

3. Laser Scanners:

 Laser scanners use similar technology as bathymetric lidar. They are used to measure and locate features such as rocks, islets and small islands. The head of the laser creates sixteen laser beams, which will fall on the target object, reflect off and then are received back by the laser head. There is a computer attached that uses the collected data with precise positioning and altitude data to evaluate the elivation and position of the object. These laser unlike Lidar cannot pnetrate sea water.

    Nowadays Mobile Laser Scanners (MLS) are also being used which enables to gather accurate data in circumstances may have been impossible or dangerous in past such as under bridges, break waters, dam faces etc.

4. Sound Velocity Profilers(CTD Unit):

 Salinity, temperature and depth of water can affect the density of water. Different types of sound velocity profilers are used to measure density of water in a particulae region.

     The Conductivity, Temperature and Depth (CTD) unit consists of different integrated sensors that enables the user to sample many density profiles and transfer them to computer. Pressure sensor is installed to measure depth and a small impeler to laboriously draw water acros temperature reading thermistor and conductivity-measuring circuits. By measuring conductivity, pressure, and temperature, simultaneously, the CTD can calculate the sound speed in water using the Chen-Millero equation. Hydrographers should only use CTD unit when survey platform is stationary.

5. Sonar-mites Echo Sounder: 

 SonarMites’s Echo Sounder is one of the most advanced intrument being used in hydrographic survey. It is the world’s first suver grade echo sounder to be truly portable. It provides an excellent solution when surveying, measuring and identifying the physical features and depths of seas, lakes and rivers by using bluetooth technology and communicating directly with Viva controllers. 

      It fits on to a detail pole which acts as an extension, providing an easy to use method to survey on water. The surveying can be done from a boat or with the HyDrone-RCV portable remote control boat platform. The Sonarmite Echo Sounder is currently being used on projects in the european countries for river surveys with the use of robotic total stations.

References:

• Combined project on Hydrographic Survey by Taha Nasir, Rameez, Anas & Talha Shafique.