Using particle motion to estimate sediment properties

Gopu R Potty – gpotty@uri.edu

Department of Ocean Engineering, University of Rhode Island, Narragansett, RI, 02879, United States

James H Miller – miller@uri.edu

Popular version of 3pPA4 – Estimation of seabed properties at the New England Mud Patch using vector acoustic measurements
Presented at the 190th ASA Meeting
Read the abstract at https://eppro01.ativ.me/web/planner.php?id=ASASPRING2026

–The research described in this Acoustics Lay Language Paper may not have yet been peer reviewed–

Shear is one of the fundamental mechanical parameter that bridges geological, engineering, and environmental aspects of the seafloor influencing loss of acoustic energy in addition to other factors such as seafloor stability, load bearing capacity, sediment transport and deposition. Shear wave velocity is one of the parameters which characterizes shear strength of the sediments. In this study we use waves propagating along the seabed (interface waves) to estimate the shear speed of the sediments.

Interface waves:
Interface waves are waves which travel along an interface between two media. Examples include Rayleigh waves (waves which travel along land) and Scholte waves (waves along seabed). Figure 1 shows a typical scenario in which a sensor on the seabed will measure Scholte waves in addition to acoustic waves along different paths (direct, surface reflected etc.).

Fig. 1: Schematic of a typical scenario in which a sensor on the seabed measures interface waves in addition to acoustic waves along different paths. Right panel shows the OBX sensor package.

The Scholte waves have the following characteristics:

1. They have maximum amplitude at the water-sediment interface (seabed). The data used in this study is from a receiver deployed on the seabed.
2. Particles in the medium traces an elliptical path in water and sediment.
3. The magnitude of the particle motion decreases exponentially as a function of distance from the interface in both media.
4. The ratio of the horizontal to vertical component of the particle motion is strongly correlated to the shear velocity and thickness of the sediment. In this study we have used this characteristics of the Scholte wave to estimate the shear velocity in the sediment.

We measured the particle velocities along three mutually orthogonal directions associated with Scholte waves using a senor package (Ocean Bottom Recorder or OBX, shown in the right panel of Figure 1) deployed on the seabed during an experiment in 2022 in the New England Mud patch (NEMP), 200 km south of Martha’s Vineyard in 70 m of water depth. As the name implies, NEMP has a layer of mud/clay sediments on top of sand. Many types of sources generated sound at different frequency bands in addition to sources of opportunity such as ships passing close to the experimental area. Figure 2 shows an example of the motion (velocity in mm/s) of the particle measured by the OBX during the experiment. This represents the motion of the particle for a short period of time (~ 1 seconds) in a narrow frequency band.

Fig.2: The trace of the particle motion (hodogram) in the source-to-receiver direction (radial, shown in pink), in the vertical direction (normal to the seabed, shown in yellow). The red curve shows the path of the particle in the vertical plane containing the source and receiver.

The strong correlation of horizontal to vertical ratio (HVSR) of the particle motion to shear speed in the sediment and sediment layer thickness is demonstrated using simulated data in Figure 3. Particle motion data were simulated for a ocean environment as shown in the left panel of Figure 3. Sound speeds in the water column, sediment and basement were assumed as 1500 m/s, 1495 m/s and 1750 m/s respectively. The shear speeds in the sediment and basement were assumed as 50 m/s and 300 m/s respectively. Densities in the water column, sediment and basement were assumed as 1025 kg/m3, 1650 kg/m3 and 2000 kg/m3 respectively.

Fig.3: Ratio of the horizontal to vertical (HVSR) particle motion amplitude as a function of frequency (right panel). Particle motion was simulated for an ocean environment as shown in the left panel.

The particle velocities of the Scholte waves for this environment were generated using a numerical model and ratio of the horizontal to vertical component of the particle motion amplitudes were calculated as a function of frequency (Figure 3; right panel). The HVSR curve shows a dominant peak at 2 Hz which correspond to the shear resonant frequency. The data measured in the NEMP experiment is used to calculate the HVSR and then identify the peak in the frequency versus HVSR curve. HVSR is then modelled for various shear speeds and layer thicknesses. The shear speed which produces the best data-model match (particularly the peak frequency) is then estimated.