Classification of mined material and detection of end of mining based on sound

Problem: Marine diamonds are found in a gravel layer that lies under other sediments and on top of a "footwall" layer that is typically clay.  Mining techniques use various forms of suction, such as a suction drill, to remove the sediment and gravel and extract diamonds on board a ship.   There is currently no precise way of knowing when the end of the gravel layer has been reached during mining.  Manual observations are made of the mined material on board a ship to roughly determine the depth of the gravel, but these observations are subjectiv