Abstract: The invention provides an apparatus (10) and method for measuring the velocity and amplitude of a pressure wave (310) from an acoustic pulse (300) through layers of sediment (140) located below the mudline (130) of the seafloor (120). The apparatus (10) includes an implosive seismic source (30) for injecting acoustic pulses (300) into a region of the sediments (140), a plurality of vertically spaced sensors (50) located vertically below the seismic source (30) for sensing when the pressure waves (130) from the acoustic pulses (300) impact the sensor (50) or the amplitude of the pressure wave (310), and a digital recording system (100) for recording the signals from the sensors (50).