BSH

Survey methods

The BSH makes use of various methods for surveying. By the single beam echo sounder, the water depth is measured with an electro-acoustic sound signal using the time that elapses between the emission of a sound pulse (waterborne sound) and the arrival of the sound waves reflected from the sea floor (echo).

The multibeam echo sounder, on the other hand, emits several hundred measuring beams. The fan detects a strip of the seabed along the measuring profile. From this, a digital terrain model can be created.

A new method is the aircraft laser scanning method (Airborne Lidar Bathymetry) for determining the topography of water soils in waters with low to medium water depth and low turbidity. Laser scanners simultaneously scan the water surface and the water bottom in the green wavelength range.

Survey methods in detail

Single beam echo sounder

The single beam echo sounder is used as a measuring instrument for the detection of larger areas in a short time.

An acoustic signal is emitted by a sensor to the bottom of the lake and the transit time to the arrival of the echo is measured. Depending on the frequency used, different soil horizons are recorded.

As a rule, the sensors are built into the boat hull for this purpose. By measuring the sensors, the position and height of the sensors are linked to the antenna position of the satellite positioning system.

By using correction parameters for positioning via satellite measurement procedures and calibrating the echo sounder system with measured waterborne sound velocities, highly accurate positions in position and height for the measured sea floor points are calculated in real time. This produces about 10 measured ground points per second.



The single beam echo sounder is used to record the sea floor in profiles. These profiles are arranged in the presumed gradient direction of the sea floor in order to be able to detect changes in gradient well. The distance between the mostly parallel profiles depends on the unevenness of the terrain and the desired accuracy with which the seabed is to be surveyed. The advantage of single beam echo sounder measurements is that the survey results are immediately available after calculation of the corrections mentioned above. A further evaluation in short time is possible, so that this procedure is suitable for the fast recording of the sea floor topography in larger areas and in areas of high variability. The disadvantage here is that there is no measured information between the measured profiles and therefore the accuracy and resolution of the survey are very limited compared to a full-area survey with mutlibeam echo sounders, for example.

Multibeam echo sounder

The multibeam echo sounder is a derivative of the single beam echo sounder. Depending on the depth, the multibeam echo sounder can be used to measure a wide strip across the ship on the seabed in a short time. The basic principle of the acoustic measurement is similar to that of the single beam echo sounder. With this method, a sensor integrated in the ship's hull emits several acoustic signals in a fan and then captures the signals reflected from the sea floor. The depth of the seabed is calculated from the different transit times of the signals. Several hundred data points per second are recorded simultaneously over the width of the fan. With the help of this method, a terrain model of the measured seabed is created.

The acoustic signal is refracted at the water layers due to their different density, temperature and salinity. Therefore, waterborne sound profiles are required to correct the position of the measured points on the sea floor. These are recorded with special waterborne sound velocity probes before and after the survey and attached to the data. During the survey, the high-precision positioning of the ship is carried out by means of a satellite positioning system. The ship inertial navigation system, in turn, precisely records the ship's position angles (roll, pitch, yaw). This highly accurate information is attached to the measured data in real time. Together, these systems guarantee high-precision position coordinates of the measured data.

The measurement of the seabed is carried out with multibeam echo sounders, if possible covering the entire area. The profiles are selected in such a way that the measured strips overlap at the edges in order to mutually control the measurements.

On the screen on board, the terrain model is built up in real time from the measurement results during the survey. The data is then cleaned and evaluated. The evaluation is carried out in a short time for the production of nautical charts, but can be very time-consuming for other applications, such as scientific analyses and highly accurate representations of the seabed topography.

As the depth of the seafloor decreases, the width of the fan decreases. Therefore, the multibeam echo sounder is preferably used in deeper areas. However, it is hardly used in very shallow wadden areas. There, a surface model of the tidal flats is recorded using laser scanners.

Position determination

Today, the position of surveying vessels is determined almost exclusively by satellite positioning systems. These satellite positioning systems, also called GNSS, are known by the freely available US-American GPS service. However, the accuracy of a commercial GNSS receiver is ±2-10 meters. In this form, these GNSS devices are therefore only of limited use for surveying and are only used in deep areas far from the coast in accordance with IHO specifications.

For high-precision measurements in coastal and safety-relevant areas, professional GNSS devices with so-called real-time kinematics are used. The real-time corrections are usually sent by mobile radio from a base reference station to the GNSS device on the moving ship. For the surveying, own temporary reference stations are set up as required or the permanently installed reference stations of the satellite positioning service of the German National Survey (SAPOS) are used. The received corrections are converted internally and thus deliver an accuracy in the small centimetre range.
The depth data recorded by echo sounder are corrected in real time on board with the GNSS positioning information as well as the position information of the ship in order to obtain high-precision depth information.

New survey techniques

New survey techniques and alternative measuring platforms make surveying significantly more efficient. The modern methods already in use today in hydrographic surveying and wreck search include, for example, airborne laser bathymetry and satellite image data evaluation, whereby these are to be regarded as complementary measuring methods due to limitations caused by the measuring principle (such as maximum achievable penetration depth in laser bathymetry or expected 3D coordinate accuracy in satellite image data evaluation). They cannot completely replace hydroacoustic methods.

Laser bathymetry

The aircraft laser scanning method effectively determines the topography of water bodies in waters with low to medium water depths and low turbidity. The method is based on the simultaneous scanning of the water surface and the seabed by laser scanners in the green wavelength range. The turbidity of the North Sea and the Baltic Sea currently limits the applicability of the measurement method to shallow water up to a maximum depth of about 10 m. The laser bathymetry is used in the North Sea and the Baltic Sea. Laser bathymetry is thus a supplement to the usual ship-supported hydroacoustic survey methods. The BSH is currently investigating the extent to which laser bathymetry methods and systems can contribute to sea surveying in coastal areas.

Satellite-based sensors and unmanned platforms

Satellites provide valuable information about the composition of the earth's and sea's surface as well as the atmosphere. Satellite-derived bathymetry (SDB) refers to the extraction of sea floor topography from satellite image data. For shallow waters, this can be measured directly by combining optical satellite image data from different spectral bands. This results in an interesting option for hydrographic surveying at the BSH: Optical SDS methods can continuously measure shallow water areas with a water depth of up to about 10 m in high temporal resolution. The effort required for surveying is very low; however, the extractable depth values are less accurate and reliable compared to data acquired with hydroacoustic methods. Satellite-based remote sensing data are therefore particularly suitable for planning surveying operations. Better knowledge of the changes in the seabed allows the ship's operating time to be prioritised much more efficiently and repeat measurements to be planned more specifically.

In addition, the use of unmanned survey platforms is being tested. In addition to autonomous surveying boats (unmanned surface vehicle, USV) and underwater vehicles (unmanned underwater vehicle, UUV), flying robots ("drones", unmanned aerial vehicle, UAV) will also be used in the future. UAV-supported aerial surveys provide up-to-date aerial maps (orthophotos) through the evaluation of overlapping colour aerial photographs. With these, operations can be better planned, especially for highly variable areas such as the dry falling Wadden Sea in the North Sea.

New and further development

In addition to the investigation of alternative equipment carriers for surveying under special conditions and the testing of new methods in sea surveying, the new and further development of fundamental methodological approaches plays an important role in order to meet the increased demands on the quality of topographical information. In interdisciplinary cooperations, for example, the sound velocities necessary for an exact calculation of water depths are modelled and optimised on the basis of sound velocity profiles measured in situ. In a further project (www.famosproject.eu), tidal feeding is further developed with the aid of satellite-supported positioning methods. For this purpose, a current (geoid) model will be provided which accurately and reliably describes the Earth's gravitational force in the Baltic Sea region and is indispensable for today's satellite-based navigation.