Abstract: Method for determining the thickness of an upper layer of a fluid mass comprising at least this upper layer and a lower layer lying thereunder, such as a waterway with a navigable upper layer and a viscous mud layer located thereunder, wherein the viscosity of the fluid of the lower layer is greater than the viscosity of the fluid of the upper layer, wherein a body freely movable in the fluid mass is moved through the fluid mass at a velocity such that the body enters a state of equilibrium substantially at the interface between the lower and upper layer; the depth position of the body is determined at successive points in order to know the thickness of the upper layer at these points; and the resistivity of the fluid in the vicinity of the body is measured at successive points.

Abstract: Method for measuring a rheological transition level between a first layer with first rheological properties, typically a fluid mud layer, and a second layer lying thereunder and having different, second rheological properties, typically a solid mud layer, wherein using a cable an object is lowered through the first layer into the second layer lying thereunder. A dynamic quantity of the moving cable with object, influenced by the rheological properties, or a change of this quantity, is measured. It is determined on the basis of this measurement when the object is located at the rheological transition level.