Comparing Modeled and Measured Bathymetric Uncertainties: Effect of Doppler and Baseline Decorrelation

Abstract

Nowadays Multi-Beam Echo-Sounder (MBES) derived bathymetry is used for a large range of applications. However, these measurements are affected by the uncertainties inherent to the MBES. Since the development of the depth uncertainty prediction model, MBES systems have been improved noticeably. The present contribution addresses the importance of modifying the vertical uncertainty prediction model based on the most recent insights of the error contributors.The received signal is affected by a Doppler frequency shift due to the constant movement of the MBES. This induces an error on the beamsteering which is not corrected for by the manufacturer, and hence its contribution is a first-order effect. The phenomenon of baseline decorrelation is encountered in the MBES interferometry step in which the full MBES receiving array is divided into two sub-arrays and the phase difference of the signals arriving at these sub-arrays is determined. The slightly different angular directions of the two received signals together with the finite signal footprint reduces the coherence between them, leading to a deterioration in the precision of the MBES derived depth.In this contribution, we compare the predictions for the bathymetric uncertainty with and without accounting for the Doppler effect and baseline decorrelation with the uncertainties estimated from real observations. To this end, data was acquired with EM2040c dual head MBES (manufactured by Kongsberg) with the center frequency of 300 kHz in water depths of around 2 m, 10 m and 30 m with pulse lengths of 27 µs, 54 µs and 134 µs in Oosterschelde estuary, the Netherlands. In general, the predicted and measured uncertainties are in the same order of magnitude. It is seen that the contribution of the Doppler effect increases with depth and beam angle and, if not accounted for, can potentially lead to an underestimation of the total vertical uncertainty budget. Including the contribution of the baseline decorrelation improves the agreement between the modeled and measured uncertainties for the beams close to nadir.