HYDROGRAPHIC SURVEY
survey are in two main areas. First, the difficulty of mobilisation of survey assets caused by the remoteness of the survey site, limited access to the banks and the lack of piers. Te dam may be some distance from the nearest road, requiring all the equipment to be carried over rough land. Second, running the survey lines is difficult because of the limited depth of water (sometimes less than a metre over much of the dam periphery) along with the presence of hazards to navigation such as trees or submerged structures. Often during the survey the boat will need to navigate close to a shore covered by dense vegetation and around the hydroelectric plant structures. Tese difficulties exclude the possibility of using a standard survey launch; something much lighter and man–launchable is required, but it still has to carry a fully capable hydrographic survey rig.
Choice of a suitable integrated hydrographic package was key to successful completion of the contracts. Te requirements were for wide swath bathymetric mapping to better than IHO special order specifications along with co-registered side-scan images for structure inspection, with reasonable productivity in water depths often less than 1metre and data collection up to the waterline. Te GeoSwath Plus (GS+) interferometric sonar from GeoAcoustics Ltd (a Kongsberg Maritime Company) was chosen. Te GeoSwath Plus sonar allows wide swath bathymetry and side scan data to be collected from small vessels of opportunity in water depths up to 200m beneath the transducers. Te system was designed to meet the need for a wide swath system capable of high productivity surveys in very shallow waters, and can achieve swath widths over 12 times water depth. Te GeoSwath provides high accuracy, high resolution bathymetry (depth measurement accuracy exceeding the latest IHO specifications), and co-registered side-scan data. GeoSwath versions are now also available for mounting on small ROVs and AUVs. One of the great advantages of the GS+ for this type of small boat work is its compactness. Most of the ancillary sensors are integrated into the transducer V-Plate which saves space, saves time during installation and allows pre-calibration of the offsets between the sensors. Full calibration on-site would often have been difficult because of the small extent of the survey areas. Another advantage of the GeoSwath is the robustness of the V-Plate and transducers; more than once an accidental grounding left the system undamaged.
Te equipment sometimes had to be hand carried
Fig. 2. Surveying a small dam near Volci, Italy.
Image: Pangea srl
For this contract Pangea chose two vessels: a 3.7metres long aluminium boat and a 4.30m inflatable boat. Te advantages of the aluminium boat is its lightness and strength; it is easily transportable, is straightforward to launch and robust around obstacles. However it was found to be fairly unstable especially when loaded with crew and numerous instruments that raised the centre of gravity. Te advantage of the inflatable boat was the stability, allowing good mobility when the boat was fully loaded and operating. However it was heavier and less resilient, leading to transport difficulties where waterside access was limited and the greater likelihood of damage during launching or around obstacles. In order to limit the weight and the quantity of equipment to be carried to the site a small 2HP outboard engine was often used (or even oars in the smallest sites). In the larger surveys and in the rivers a 15HP outboard engine was used.
8
www.engineerlive.com
over rough terrain to the waterside, which is only feasible for a compact system like the GeoSwath Plus. Te boat was separately craned in or manhandled to the water. At each survey site the transit to the waterside, installation of equipment on the vessel and system tests took about 2 hours in total. Data acquisition usually lasted 1–2 hours (sometimes much less on the smaller dams), and dismantling took about
Fig. 3. Example of a side scan pass by a dam structure. Image: Pangea srl
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36