English to French Tech/Engineering Surveying at sea survey

Hydrologic survey
(3) Using five points survey method, the vertical line of current survey depth respectively are Surface (0.5m below water surface), 0.2H, 0.6H, 0.8H and Seabed (0.5～1.0m upon seabed ), record the depth at the same time.
(4) Use the //////Fish lead///// (about 15~25kg) or material without magnetism as heavy load.

From a document detailing the studies to be carried out for the construction of a port.

Il s'agit de poids mais existe-t-il un terme specifique pour ce type de poids ?

Thanks for your help.