Marine seismic sources that send sound pulses into the sub-sea floor sediments. The sound pulses bounce off the sea floor and subsequent buried sediment layers according to differences in their acoustic impedance (hardness) and in turn, their density. Parametric sub-bottom profilers are non-linear systems that transmit two different higher frequencies that interact during sound propagation to generate a resultant lower operating frequency with narrow bandwidth. They typically operate at frequencies greater than 2 kHz and penerate to several hundreds of metres.