Abstract
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.
Highlights
As reported in Huet (2009), “Hydrography is the branch of applied sciences, which deals with the measurement, and description of the physical features of oceans, seas coastal areas, lakes and rivers, as well as with the prediction of their evolution, for the primary purpose of safety of navigation and all other marine activities, including economic development, security and defence, scientific research, and environmental protection” [1]
The bathymetric surveys belong to the family of hydrographic surveys and are carried out whenever there is a need to precisely know the morphological trend of the seabed [4], [5]
We proceeded with the calculation of the sound velocity in water using three of the formulas previously described (Del Grosso formula is not applicable because the analyzed depths are less than 1000 m)
Summary
As reported in Huet (2009), “Hydrography is the branch of applied sciences, which deals with the measurement, and description of the physical features of oceans, seas coastal areas, lakes and rivers, as well as with the prediction of their evolution, for the primary purpose of safety of navigation and all other marine activities, including economic development, security and defence, scientific research, and environmental protection” [1]. A correct functioning of the echo sounder is very important for accurate depth measurement, but an accurate determination of the sound velocity in water is necessary [9]. The average value of the sound velocity in water is accepted as 1500 m/s, calculated taking as reference the nominal conditions of the water, characterized by a temperature of 0 ° C, a salinity of 35 ppt (parts per thousand) and a pressure of 760 mmHg [20], [21] This average value, can oscillate according to the characteristics of the water, varying between 1387 m/s and 1529 m/s [17]. Empirical formulas require physical and chemical parameters, such as depth (D), temperature (T), salinity (S), pressure (P) These parameters can be measured with different types of instruments. There are many different formulas available to calculate the sound velocity in water, and the most popular and accurate are Chen & Millero (1977) [22]-[25], Del Grosso (1974) [26]-[28], Mackenzie (1981) [29]-[31] and Medwin (1975) [32], [33]
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