A method and apparatus for measuring a parameter of a flow passing through a pipe is provided, wherein the apparatus includes at least two spatial array of sensors disposed at different axial locations along the pipe, wherein each of the sensors provide a signal indicative of unsteady pressure created by coherent structures convecting with the flow within the pipe at a corresponding axial location of the pipe. The apparatus also includes a signal processor configured to determine the flow rate at the circumference location of each sensor array in response to the respective measured unsteady pressures. The signal processor compares the velocity of the flow at each respective location and provides a signal indicative the presence of solids settled at the bottom of the pipe and/or the level of the settled solids in the pipe, in response to an uncharacteristic increase in the velocity of a lower portion of the flow in comparison to the velocity measured above the lower portion of the flow.