Apparatus is provided featuring a signal processor configured to receive from an array of ultrasonic transponders around a tank, container or flow pipe signaling containing information about acoustic waves of energy penetrating a fluid in the tank, container or flow pipe; and determine using tomography a three-dimensional image of layers of the fluid in the tank, container or flow pipe based at least partly on the signalling received and. The signal processor provides corresponding signal containing information about the three-dimensional image of the layers of the fluid in the tank, container or flow pipe. The array is an obliquely oriented array of ultrasonic transponders arranged on an oblique plane to the axis of the tank or column; or the array is placed normal to the flow around the exterior of the flow pipe.