Abstract: A method of and apparatus for determining the topography and/or volume of a layer of sludge on the bottom of an oil tank, for example, in which a sonar head having a velocimeter is immersed in the oil, and a fan-shaped beam of acoustic energy is swept across the layer of sludge. Also the velocity of sound in the oil is measured, preferably at a plurality of heights over a relatively short vertical range to determine the velocity gradient, which is then extrapolated for the entire depth of the oil. Processing means determine, from the extrapolated velocity gradient, the topography and/or volume of the sludge. The method provides an accurate indication of the distribution and volume of sludge across the whole of the bottom of an oil tank, taking into account changes in the speed and attenuation of acoustic energy due to, for example, the temperature gradient in the oil.