A doppler speed log for marine vessels transmits fore and aft pulses of high frequency sonic energy. The log normally operates in a mode such that a gating pulse permits signal processing circuits to respond only to signals derived from reflections from minute scatterers in the water occurring at a specified distance from the hull of the vessel. If the vessel is in shallow water, a variable threshold comparator permits the signal processing circuits to respond to reflections from the sea bottom when the magnitude of these reflected signals exceeds the threshold level. The processing circuit forms fore and aft enabling pulses each having a duration equal to the time required for the reception of a specified number of cycles of sonic energy reflected from the fore and aft directions, respectively. Pulses from a calibrated clock source are applied to an up-down counter during the time that only one of the enabling pulses exists. The readings corresponding to a specified number of transmitted pulses are accumulated and displayed on a suitable readout means.