Hydrographic Surveying
Also known as a Bathymetric survey. With the use of computers this survey discipline allows huge amounts of data to be collected very quickly. Post processing to allow for tidal influence when working at sea allows us to compensate for the height of the tide at any given time during the survey so that the depths produced are referenced to the same datum.
Our equipment include:
- High frequency echo sounders that record depths at a rate of 6 soundings per second.
- DGPS is used for positioning the echo sounder horizontally on the water and is accurate to 1m. The DGPS is also used for navigating the boat to the particular area of water that is to be surveyed
- The equipment is flexible enough to fit on boats from about 3m to 15m in length.
Who would use our service?
- Dredge operators for pre and post dredge surveys
- Port authorities or Councils who need to monitor the depths of channels or rivers or water adjacent to wharf, or boat ramps etc
- Hydrologists who need to monitor sand movements or who are doing a hydrology study or need to know how much water is in given area.
- Marine Engineers for the designing of new or improved groynes jetties or dredging programs.
How is it done?
The Hydrographic survey equipment that we use is the Ceeman survey system provided by Bruttour International.
This system combines a DGPS receiver, an echo sounder, and data recorder into one unit. The position and depth data is recorded on a solid state data logger in real time, and is also linked to a laptop computer which displayed the current position of the boat in relation to the river banks, marina or beach or the required survey lines.
The survey can be carried out from any sized boat, which allows flexibility to ensure we are providing the best most cost effective solutions for each project.
The survey system recorded the position of the boat every two seconds and recorded depths at a rate of six soundings per second using a high frequency sounder.
The survey speed is approximately 4 knots, or 2 metres per second, resulting in 3 soundings per metre of line run. The position of the boat in relation to the required run line is available to the coxswain in real time and is used to navigate the boat along predetermined lines.
Survey lines can be run at a nominal spacing of 2.5 to 100m in two directions perpendicular to each other or where directed or as required depending on the client brief. The line data can be thinned so that there is a sounding every metre along the line, this data is transferred into Terramodel software, where a digital terrain model is created to digitally represent the sea bed depths and provide a contour map of the sea floor. The software was then used to create a plot of contours at 0.25m to 1m intervals.
Plan Data
The shore details or river banks can be incorporated onto the maps as well making them easier to read. The data can be supplied by the State Data Centre or can be prepared by us carrying out a feature survey as well.
Consultancy
Hydrographic or underwater surveying involves solving a number of unique problems, necessitating the use of specialised equipment to perform the work, and specialised sensors and software to monitor the progress of work.
Depths are measured by sound pulses from an echo sounder, and a thorough understanding of the physical properties of the transmission of sound in water and the equipment used to perform these measurements is essential to achieving accurate data.
The sounder is mounted on a vessel which is positioned in reference to a mapping grid by a positioning system capable of providing a continuous record of the track of the vessel. By accurately recording the time of both the position and depth record, a plan showing geo referenced depths reduced to a specified vertical datum can be derived.
From the brief description above it can be seen that the operation is a complex procedure involving a large number of sensors with varying characteristics, which have to be calibrated and synchronised to achieve a meaningful result.
Accurate hydrographic survey is necessary for safe navigation, dredging and underwater construction projects, and the duties of a consultant in this field includes supplying the principal with a suitable specification for producing survey data for any proposed project, supervising the collection of this data and the positioning of any works, and the presentation of the data in a suitable format.
Additional responsibilities arise in the monitoring of time schedules, assessing the suitability of plant and equipment, including data collection software, the maintenance of specified safety and environmental protection standards, provision of advice and reports during the course of a project, and liaison with all parties to assist with the timely completion of a project.
