On the investigation of vertical uncertainty of depth sounding in a shallow environment with muddy seabed: Preliminary results from a launch operation of a dual-frequency echosounder

Abstract

Hydrographic echosounder has been the standard instrument that provides a measure of water depths. In a muddy environment, this detection is not as straightforward as it seems; low gradient of acoustic impedance presence within the water-sediment interface resulting in vertical separation of liquid-solid boundary detected by different frequencies of depth sounding system. In this study, we investigate the depths measured by a dual-frequency hydrographic echosounder in a muddy environment, coupled with a simultaneous probing of the water-seabed interface by means of a free-falling cone penetrometer. We intend to understand the extent of the uncertainty of a depth-sounding system to precisely locate the liquid-solid boundary within the water-seabed interface, specifically at Patimban coasts, situated in the north coast part of Java Island, where muddy sediments dominate the seabed. From our investigation, we found that standard high-frequency sounding (200 kHz) underestimates the physical depth by 0.26 ± 0.17 m, while standard low-frequency sounding (24 kHz) overestimates the physical depth by 0.23 ± 0.19 m and tends to give inconsistent measures. Our study suggests the importance of considering these measures of discrepancy when depth sounding is being carried out in a muddy environment.