**1. Introduction**

Submarine sand waves are approximately regular undulating landforms [1] formed by the movement of sandy sediments under various marine hydrodynamic forces, such as ocean currents [2], tidal currents [3] and internal waves [4]. Submarine sand waves are widely distributed across continental slopes [5,6], continental shelves [7,8], straits [9,10], gulfs [11], and other geomorphic units around the world, in shallow to deep seas. Under the action of ocean dynamics, sand waves undergo periodic migration movements, and rates can reach nearly 70 meters per year [12,13]. From this process, vertical deformation may spur the suspension or burial of submarine cables [3,14] and submarine pipelines [8,15], which can seriously damage them. Therefore, the observation and study of the geomorphic morphology of seabed sand waves has attracted the attention of numerous researchers.

At present, the positioning and repeated measurement of water depths is used to observe the migration of seabed sand waves. By analyzing high-precision digital terrain model (DTM) data measured at several time points in a study area, the average rate and direction of sand wave migration for a given period can be obtained [16,17]. DTM data obtained from multi-beam water depth measurements can also be used to calculate the rate of seabed sand wave migration by profile analysis [13,18]. Franzetti [7] used the spatial cross-correlation three-dimensional analysis approach to analyze and calculate the horizontal migration rate and the vertical variation of sand waves. Zhou [19] also used three sets of repeated multi-beam sounding data for 2011–2013 to study the first migration and changes of giant sand wave fields in the Taiwan Shoal in the northern South China Sea. However, because these methods depend on ship operations, they cannot be used to observe continuous changes in height, due to weather, sea conditions and cost constraints.

In-situ integrated observation techniques have been developed to better understand processes and mechanisms of sand wave migration at finer scales since the late 20th Century. Fixed flow meters and small bottom observation platforms, equipped with scan sonar and acoustic backscattering, have been used to observe sand wave surfaces, the dynamic sand wave processes on bottom surfaces [20], variations in sediment velocity on sand wave surfaces [21] and sand ripple variations with rising tides [22]. However, because these acoustic or optical instruments are not only susceptible to the concentration of suspended sediments in near-surface water, but also generate a lot of power consumption during the actual operation, they might not always meet the need for continuous on-site observations in sand wave areas.

Due to their stability and environmental adaptability, pressure sensors have increasingly been used for sea floor height measurements [23–26] and vertical seabed deformation observations [27–29] since the 1990s. Japan's MH21 plan for the exploitation of natural gas hydrates in Japan's seabed involves formation subsidence monitoring at a precision level of 10 mm [30]. In the North Sea, high-precision water pressure measurement technology is used to monitor seabed subsidence [31]. These successful cases highlight possibilities to apply pressure sensing technology in the study of seabed sand wave migration.

In view of this summary of past research, this paper focuses on the application of pressure sensing techniques in the study of sand wave migration. Based on the principle of vertical pressure change caused by sand wave migration, two observation methods were designed and verified by indoor water flume tests. The work presented in this paper can guide the monitoring and early detection of submarine sand wave migration, and be used to better understand mechanisms of submarine sand wave migration.
