Abstract

Detection of the time of arrival of a harbour porpoise click at several synchronized broadband hydrophones, attached to a drifter, provides a method for locating porpoise and assessing any interactions with tidal turbine installations. Presently we show how two synchronized hydrophones, arranged as a vertical array, can be used to obtain both the depth of a porpoise and its range from the hydrophones. Our method applies whenever time of arrival for a porpoise click can be measured along both direct paths and paths reflected from either the sea surface or seafloor. Comparison of signal level with porpoise range from the array indicated that source levels were consistent with previous findings for wild porpoises. Signals reflected from the seafloor appeared to suffer from a high degree of scattering but their leading edge was sometimes clear and useful. Reflections from the sea surface were relatively clean and more commonly useful. The calculated porpoise depth was below/above the level of the hydrophone array whenever reflections were from the sea surface/floor, corresponding to the expected orientation of a porpoise that is rising/diving, respectively. Time of arrival is likely to be more uncertain for reflected signals than for those taking a direct path from porpoise to hydrophone. Nevertheless, using reflected signals can greatly increase the effective aperture of a hydrophone array so it may be advantageous to consider the arrival times of reflected signals even when an array has many more than two hydrophones.