Quantitative Sludge Level Measurement in Storage Tanks

We are seeking solutions to determine sludge levels in external floating roof tanks and 30-40m diameter fixed roof tanks to be able quantify and understand profile of sludge in tanks for better planning of tank cleaning works. 

Key objective is to have quantitative understanding and not just qualitative understanding on the sludge profile in the tanks.

Preference to have proposed technology that can be used in non intrusive manner while tank is still in operation. Feel free to propose solutions depending on your company's capabilities and products.