In different areas we use different survey specifications. Measurements will be performed in accordance with IHO S-44 standards, Dutch standards for hydrographic surveys, and special clients requirements.
Survey Plan (development)
While survey planning, close cooperation with our clients is needed. Important issues must be discussed in order to achieve the specific task.
First we want to know the following information.
· Area of the survey
· Type of survey
· Scale of chart to be produced
· Scope of time of the survey
· Usefully background ( e.g. satellite photography, topographic maps, tides)
· Limitations (e.g. budget, position systems limitations..)
· Survey limits
· Requirements, resolution and accuracy
· How the survey report is to be rendered
· Tidal datum, Horizontal datum, projection and grid
· Wrecks in the area
· Survey priorities and other relevant guidance or instruction
Hydrographic measurements can be made in various ways.
Data is collected by our eight well-equipped vessels. The geometry of each vessel is well known. First we ensure our system is calibrated, and in case of MBES, the patch test gives good corrections, measurement before we get started. We can provide data using Single Beam Echo Sounder, Multibeam Echo Sounder, Side Scan Sonar, Silas, Laser Scanning, Magnetometer, Echoscoop, Acoustic Doppler Current Profiler (ADCP). For data accusation software like QINSy, PDS2000 are used.
Data processing is done under strict quality control criteria. Partially data cleaning is already done onboard. Final data processing and plotting are realized at the office.
Singlebeam echosounder (SBES)
To determinate the depth of the seafloor, a single or multi beam echo sounder is used. Single beam Echo sounder emits an acoustic signal (sound wave) into the water. The two-way travel time of the signal is measured. The depth is calculated from the equation D=1/2*v*t where `v´ is the speed of sound in the water column and it is measured by the Sound Velocity Profiler (SVP). A SBES measures only one point per pulse and creates an image of soil directly beneath the vessel (see figure on the right). To get a full seafloor coverage a multi beam echo sounder is used. The result of measurements can be represented in charts or digital data format.
Multibeam Echosounder (MBES)
The principles of operation of a multibeam sounder are similar to SBES but instant of transmitting and receiving a single vertical acoustic signal, the signal will be received by an array of receivers. In this way a MBES produces a swath of soundings across the longitudinal axis of the survey vessel. For the accurate computation of depth measurement all vessel movements must be detected and took into account. The motion sensor, also named attitude sensor, is constantly measuring roll, pitch and heave movements. These measured movements are corrected in the acquisition software to get an actual sea floor image.
Side Scan sonar
A Side Scan sonar is a visualization tool providing acoustic reflecting images of the seabed. Objects that rise above the seafloor intercept the pulse and produce shadows in the sonar image. Those shadows can be used to determine the height and the shape of the object. A side scan sonar can be hull-mounted, mounted on an pole to the side of the vessel or towed behind the vessel on a towfish. It is used for object detection like mines, pipeline or shipwreck.
The magnetometer is a measurement instrument used to measure the strength and/or direction of the magnetic field. The magnetic field varies from place to place and is influenced by local mass spots. With a magnetometer additional evidence of the existence of ferrous material on or below the seafloor can be provided. It is used to detect shipwrecks and other submerged objects.
Acoustic Doppler Current Profiler (ADCP)
The Acoustic Doppler Current Profiler (ADCP) measures the speed and direction of currents. The ADCP exploits the Doppler Effect by emitting a sequence of high frequency pulses of sound with known direction that scatter off of moving particles in the water. It has 4 acoustic transducers that emit and receive acoustical pulses from 4 different directions. It is used to determine the amount of sediments in the water column and for the computation of the speed of the current in the water column.
A laser scanning is installed on a survey vessel to have the possibility to create a 3D-Modell not only containing the surface under the water but also to model the objects above the water. It is used to model buildings, locks, quay walls or bridges.
Silas measurements are performed by two SingleBeam transducers which operate at high and low frequencies at the same time. Mostly it is used in 33 kHz and 210 kHz. Combination of this two frequencies allows to penetrate the ground. Silas is used to obtain more information about water depth, silt layers, sub bottom characteristics and to determinate objects like pipeline, cable, wreck etc.
Echoscope is a combination of Side Scan Sonar and Multibeam Echosounder . This sonar device delivers real time, high resolution 3D underwater image. It can be used in operations like dredging, object removal and pipeline monitoring. Because of good visualization of work environment, risk of accident during underwater construction can be reduced.