Evaluation of multipath effects on depth measurements provided by acoustic transmitters in shallow water

Abstract

Measurement errors caused by multipath effects are one of the problems of acoustic biotelemetry, especially in shallow waters because both the sea surface and bottom can be major boundaries reflecting ultrasonic pulses. We conducted an experiment using transmitters with a pressure sensor to examine occurrence probabilities of multipath effects in a shallow sea area. We deployed seven transmitters with a 1.28-s signal transmitting interval in the same location, and two receivers located 18.5 m (R1) and 38.0 m (R2) horizontally from the transmitters. In order to solve the problems derived from multipath effects, we applied a state-space model to the depth data to estimate the correct values. Then we compared data availabilities, which were percentages of the number of correct values to the number of actual transmissions, between the acquired data and the model estimation. In 15 min recording, the mean proportions of the correct values for all transmitters were 88.4% for R1 and 86.3% for R2. The data availabilities became significantly larger from 65.7% to 96.7% with R1, and from 64.6% to 93.3% with R2 by applying the model. This suggests that we can reduce the measurement errors caused by multipath effects by post processing.