The importance of sound velocity determination for bathymetric survey

Bathymetric surveys are carried out whenever there is a need to know the exact morphological trend of the seabed. For a correct operation of the echo sounder, which uses the principle of acoustic waves to scan the bottom and determine the depth, it is important to accurately determine the sound velocity in water, as it varies according to specific parameters (Density, Temperature, and Pressure). In this work, we want to analyse the role of sound velocity determination in bathymetric survey and its impact on the accuracy of depth measurement. The experiments are conducted on data set provided by “Istituto Idrografico della Marina Militare Italiana” (IIM), the official Hydrographic Office for Italy, and acquired in the Ligurian sea. In our case, the formulas of Chen & Millero (UNESCO), Medwin, and Mackenzie were applied. The introduction of errors on chemical-physical parameters of the water column (Temperature, Pressure, Salinity, Depth) simulating inaccurate measurements, produces considerable impacts on sound velocity determination and subsequently a decrease of the depth value accuracy. The results remark the need to use precise probes and accurate procedures to obtain reliable depth data.

The Development of Closure Dike as Countermeasure of Sedimentation in the Wonogiri Reservoir, Indonesia

The problem of reservoir sedimentation occurs worldwide, including in the Wonogiri reservoir, Indonesia. The reservoir was built from 1977 to 1980, but the dam faces a severe problem of sedimentation. Sediment inflow in the Wonogiri Reservoir comes from several sources, especially from Keduang River, a tributary from Mount Lawu. The sediment management problems are generally complicated and different in every place. In some conditions, it will be possible to prevent sediment from entering the reservoir by adding an internal barrier as applied in the Wonogiri Reservoir. The reservoir is divided into two reservoirs by a closure dike equipped with an overflow dike and a new spillway. This paper will describe the development of closure dike based on field observation and data obtained from the Ministry of Public Works and Housing. Moreover, this paper will also describe the sedimentation status of the Wonogiri reservoir based on the recent bathymetric data. The development of closure dike already completed, which consists of Closure Dike A (700 m), B (700 m), C (302 m), and Overflow Dike (298 m). The recent bathymetric survey revealed that the capacity of effective storage of the main reservoir is 322.84 MCM.

ICESat-2 Marine Bathymetry: Extraction, Refraction Adjustment and Vertical Accuracy as a Function of Depth in Mid-Latitude Temperate Contexts

Nearshore bathymetric data are used in many coastal monitoring applications, but acquisition conditions can be challenging. Shipborne surveys are prone to the risk of grounding in shallow waters, and scheduled airborne surveys often fail to coincide with optimal atmospheric and water conditions. As an alternative, since its launch in 2018, ICESat-2 satellite laser profile altimetry data provide free and readily available data on a 91-day repeat cycle, which may contain incidental bathymetric returns when suitable environmental conditions prevail. In this paper, the vertical accuracy of extracted, refraction-adjusted ICESat-2 nearshore marine bathymetric data is evaluated at four test sites in a Northern hemisphere, temperate latitude location. Multiple ICEsat-2 bathymetric values that occurred in close horizontal proximity to one another were averaged at a spatial scale of 1 m and compared with Multibeam Echosounder bathymetric survey data and Global Navigation Satellite System reference data. Mean absolute errors of less than 0.15 m were observed up to depths of 5 m, with errors of less than 0.24 m (to 6 m), 0.39 m (to 7 m) and 0.52 m (to 10 m). The occurrence of larger bathymetric errors with depth, which increase to 0.54 m at maximum photon depths of 11 m, appears to be primarily related to reduced numbers of geolocated photons with depth. The accuracies achieved up to 6 m suggest that the manual extraction, refraction adjustment and bathymetric filtering steps were effective. Overall, the results suggest that ICESat-2 bathymetric data accuracy may be sufficient to be considered for use in nearshore coastal monitoring applications where shipborne and airborne bathymetric data might otherwise be applied.

The Importance of Under-Keel Sound Velocity Sensor in Measuring Water Depth with Multibeam Echosounder

The basic and most commonly used application of modern multibeam echosounders (MBES) is the bathymetric survey. Surface sound velocity errors introduce errors on beam steering angles and consequently errors in depth and position values. Due to systematic malfunction and troubleshooting of the sound velocity sensor (SVS) on board Polish Navy hydrographic ship Arctowski, attempts to solve the problem were made. All the inspections and cleaning of the sensor were performed with the use of divers or while staying in the shipyard. Diving work did not always bring the expected results and periodic ship docking was quite expensive. The article shows the importance of the SVS sensor in bathymetric measurements using multibeam echosounder. Selected problems of the sensor operation and temporary solutions were presented. The paper provides a description of practical solutions implemented aboard the navy ship Arctowski. The idea and implementation were the result of the author’s experience gained during 18 years of service on board that ship.

Diagnóstico Batimétrico do Reservatório da Usina Hidrelétrica de Caçu-GO

O presente trabalho teve como objetivo avaliar as mudanças e os níveis de sedimentação ocorridos no reservatório da Usina Hidrelétrica (UHE) Caçu, entre janeiro de 2014 e março de 2015. Os procedimentos empregados para avaliar o processo de sedimentação do reservatório envolveram o diagnóstico dos locais assoreados por meio de levantamento batimétrico e análise da velocidade de escoamento da água com o uso do acoustic doppler current profiler (ADCP). Os resultados obtidos permitiram identificar alterações no perfil topográfico do reservatório, sendo a perda de volume inferior a 0,5% ao ano. O local de maior acúmulo de sedimentos encontra-se no trecho inicial do reservatório (primeiros 6km), ocorrendo a formação de bancos de areia. Os valores máximos de velocidade de escoamento da água nos períodos avaliados variaram entre 0,858m/s a 1,979 m/s. Os valores mínimos da velocidade de escoamento foram detectados nas áreas entre o córrego do Caju e a barragem da usina hidrelétrica de Caçu. The Bathymetric Diagnosis of the Hydroelectric Power Station from Caçu – GO. A B S T R A C TThis research job has as its goal to evaluate the changes and the sedimentation levels occurred in the reservoir of Hydroelectric Power Station (UHE) Caçu, between January 2014 and March 2015. The procedures employed to evaluate the sedimentation process of the reservoir involved the diagnosis of the locals silted through the bathymetric survey and analysis of the water drainage speed with the use of the acoustic doppler current profiler (ADCP). The results obtained permitted to identify changings on the topographic profile of the reservoir, being the loss of the volume under 0,5% per year. The local of the largest sediment accumulation is located in the initial excerpt (first 6 kilometers) occurring the formation of sandbanks. The maximum speed values of water drainage in the evaluated periods varied between 0,858m/s to 1,979 m/s. The minimum values of drainage speed were detected in the areas between the Caju water stream and the dam of the Hydroelectric Power Station of Caçu.Keywords: Sedimentation. Water Speed. ADCP.

Structural Analysis and Optimization of Hybrid Unmanned Amphibian Aerial Vehicle

Hybrid Unmanned Amphibian Aerial Vehicle (HUAAV) is a hybrid unmanned system consisting of a multi-rotor and a hovercraft. Water quality management, hydrographic survey, bathymetric survey and related application areas still employs time consuming and erroneous traditional methods of using boats to access the test sites. The application of unmanned systems will significantly increase the efficiency of this process by getting access to remote locations of both air and water. This paper deals with the design optimization and finite element structural analysis to validate the structural capability of the hybrid amphibian aerial vehicle for operational conditions. The Reserve Factors (RF) for all the components in the amphibian aerial vehicle are calculated and then optimized.

Seafloor Geodesy From Repeated Multibeam Bathymetric Surveys: Application to Seafloor Displacement Caused by the 2011 Tohoku-Oki Earthquake

Repeated multibeam bathymetric surveys played an important role for understanding the distribution of coseismic seafloor displacement caused by the March 11, 2011 Mw 9.0 Tohoku-oki earthquake. After the earthquake, we collected bathymetric data along the same tracks obtained before the earthquake. The selected tracks were crossing the trench and extending from the landward to seaward trench slopes. We examined the seafloor displacement on the landward relative to the seaward by means of the difference in bathymetry before and after the earthquake. The multibeam bathymetric survey has the advantage of areal coverage. The repeated surveys clarified the areal distribution of the coseismic seafloor displacement. In the main rupture area, very large seafloor displacement was observed. Sharp bathymetric change at the trench axis provided solid evidence that the fault slip on the shallowest part of the megathrust reached the trench axis and peaked at the trench axis. The very large displacement is limited to the particular area. Smaller seafloor displacements were observed in the area tens of kilometers away from the main rupture area. We present methods and results of the repeated multibeam bathymetric surveys and an application to the seafloor displacement caused by the 2011 Tohoku-oki earthquake in the northern Japan Trench. Less than several meters in seafloor vertical displacements and less than 20 m in seafloor horizontal displacement were estimated in the northern Japan Trench. The estimated smaller displacements are comparable in magnitude to error of the seafloor displacement observation from our bathymetric survey. Nevertheless, three adjacent survey tracks showed coherent relative differences in seafloor elevation, which suggests the relative difference enables us to discuss the along-track variation in seafloor displacement in the area. However, our survey was affected by uncertainties of roll and pitch biases and sound velocity errors. Well-prepared repeated multibeam bathymetric survey for the purpose of seafloor geodesy could lead to a higher resolution and more accurate result. Repeated acquisition of high resolution and accuracy bathymetric data using state-of-the-art technology will be important to quantitative discussion of the seafloor displacements caused by even smaller magnitude earthquakes and tsunamis.