**3. Results**

#### The test was applied to TPRFD and BPRSS groups, and observation points were set at the crest (A1, A2) and trough (B1, B2) (Figure 2) of each group to compare the applicability of the two methods at *3.1. Measurements Made by the Fixed-depth Total Pressure Recorder (TPRFD)*

different positions. With the exception of the pressure sensor, the layout and test conditions of the instruments used in the two groups of tests remained the same. In this test, a hydrodynamic force was applied over two stages. First, waves with a frequency of 34 Hz and a height of 7.9 cm were applied for one hour, and then waves with a frequency of 50 Hz and a height of 12.0 cm were applied for 25 minutes (in the experiment without any measuring instrument in advance, through direct Shown by the echo ranging group data and experimental records, the shape of the sandy bed underwent significant migration and deformation after continuous wave loading (Figure 5). The height of the bottom bed surface at observation point A<sup>1</sup> decreased by roughly 7.0 cm, while that at B<sup>1</sup> increased by approximately 7.9 cm. Wave crest A and trough B moved roughly 3.2 cm along the wave propagation direction.

**Figure 4.** Dish-shaped float. (**a**) Appearance; (**b**) internal structure.

wave propagation direction.

**3. Results**

**3. Results**

had reached a stable state). In addition, to evaluate the accuracy of the two methods, we applied echo ranging, which has been widely used in seabed deformation measurement [33,34], as a control group. A freestyle sonar altimeter (AA400, EofE Ultrasonics Co., Ltd., Korea) and an ultrasonic terrain scanner were used for the echo ranging group (Table 1). The ultrasonic terrain scanner was used to collect bottom bed morphological data before and after each test, while the AA400 made real-time

had reached a stable state). In addition, to evaluate the accuracy of the two methods, we applied echo ranging, which has been widely used in seabed deformation measurement [33,34], as a control group. A freestyle sonar altimeter (AA400, EofE Ultrasonics Co., Ltd., Korea) and an ultrasonic terrain scanner were used for the echo ranging group (Table 1). The ultrasonic terrain scanner was used to collect bottom bed morphological data before and after each test, while the AA400 made real-time

*J. Mar. Sci. Eng.* **2020**, *8*, x FOR PEER REVIEW 6 of 12

Shown by the echo ranging group data and experimental records, the shape of the sandy bed underwent significant migration and deformation after continuous wave loading (Figure 5). The height of the bottom bed surface at observation point A<sup>1</sup> decreased by roughly 7.0 cm, while that at B<sup>1</sup> increased by approximately 7.9 cm. Wave crest A and trough B moved roughly 3.2 cm along the

Shown by the echo ranging group data and experimental records, the shape of the sandy bed underwent significant migration and deformation after continuous wave loading (Figure 5). The height of the bottom bed surface at observation point A<sup>1</sup> decreased by roughly 7.0 cm, while that at B<sup>1</sup> increased by approximately 7.9 cm. Wave crest A and trough B moved roughly 3.2 cm along the

observations of the bottom bed height at the observation point.

observations of the bottom bed height at the observation point.

*3.1. Measurements Made by the Fixed-depth Total Pressure Recorder (TPRFD)*

*3.1. Measurements Made by the Fixed-depth Total Pressure Recorder (TPRFD)*

**Figure 5.** Bed morphology measured by echo ranging for the TPRFD group. **Figure 5.** Bed morphology measured by echo ranging for the TPRFD group. **Figure 5.** Bed morphology measured by echo ranging for the TPRFD group.

From the measurement results of echo ranging and the TPRFD (Figure 6), overall terrain changes can be divided into two processes: (a) a sharp period of change of 0–20 min, and (b) a slow adjustment period of 20–90 min. The experimental records show that wave crest flattening and wave trough filling processes mainly occurred during the period of sharp change. During this period, the positions of wave crests and troughs moved roughly 2 cm along the wave propagation direction, and the terrain was gentle overall. The height remained basically stable, and the terrain slowly moved in the From the measurement results of echo ranging and the TPRFD (Figure 6), overall terrain changes can be divided into two processes: (a) a sharp period of change of 0–20 min, and (b) a slow adjustment period of 20–90 min. The experimental records show that wave crest flattening and wave trough filling processes mainly occurred during the period of sharp change. During this period, the positions of wave crests and troughs moved roughly 2 cm along the wave propagation direction, and the terrain was gentle overall. The height remained basically stable, and the terrain slowly moved in the wave propagation direction and then moved roughly 3.2 cm relative to the original terrain. From the measurement results of echo ranging and the TPRFD (Figure 6), overall terrain changes can be divided into two processes: (a) a sharp period of change of 0–20 min, and (b) a slow adjustment period of 20–90 min. The experimental records show that wave crest flattening and wave trough filling processes mainly occurred during the period of sharp change. During this period, the positions of wave crests and troughs moved roughly 2 cm along the wave propagation direction, and the terrain was gentle overall. The height remained basically stable, and the terrain slowly moved in the wave propagation direction and then moved roughly 3.2 cm relative to the original terrain.

**Figure 6.** Comparison of results measured by echo ranging and the TPRFD. *3.2. Measurements Made by the Surface Synchronous Bottom Pressure Recorder (BPRSS)*

*3.2. Measurements Made by the Surface Synchronous Bottom Pressure Recorder (BPRSS)* The bed height data measured by echo ranging (Figure 7) show that the height of crest A before

The bed height data measured by echo ranging (Figure 7) show that the height of crest A before migration was 20.19 cm, while the trough B reached 11.50 cm. After migration, the height at initial crest A dropped by 8.2 cm, the height of trough B increased by 6.4 cm, and the sand wave migrated roughly 3.4 cm in the wave direction. migration was 20.19 cm, while the trough B reached 11.50 cm. After migration, the height at initial crest A dropped by 8.2 cm, the height of trough B increased by 6.4 cm, and the sand wave migrated roughly 3.4 cm in the wave direction.

**Figure 7.** Bed morphology measured by echo ranging for the BPRSS group. the first 20 minutes of the TPRFD measurement at crest A. During this period, the results measured by **Figure 7.** Bed morphology measured by echo ranging for the BPRSS group.

obvious stability period. During the test period of 85 minutes, heights at observation points A<sup>2</sup> and B2 underwent four step-like changes. According to the pressure data, the height of the bed at B<sup>2</sup> was increased by roughly 5 cm, falling 1.4 cm below test records. The height of the bed at A<sup>1</sup> dropped by

**Figure 8.** Comparison of results measured by the BPRSS and echo ranging.

It is worth noting that there was a significant response delay to the change in elevation within

*4.1. The comparison of the Results Measured by Pressure Sensor Techniques and Echo Ranging*

8.3 cm, which is consistent with test records.

**4. Discussion**

roughly 3.4 cm in the wave direction.

The data measured by the BPRSS and echo ranging (Figure 8) show that the height of the float at the position used in this test continuously rose or descended in a stepwise manner, without an obvious stability period. During the test period of 85 min, heights at observation points A<sup>2</sup> and B<sup>2</sup> underwent four step-like changes. According to the pressure data, the height of the bed at B<sup>2</sup> was increased by roughly 5 cm, falling 1.4 cm below test records. The height of the bed at A<sup>1</sup> dropped by 8.3 cm, which is consistent with test records. The data measured by the BPRSS and echo ranging (Figure 8) show that the height of the float at the position used in this test continuously rose or descended in a stepwise manner, without an obvious stability period. During the test period of 85 minutes, heights at observation points A<sup>2</sup> and B2 underwent four step-like changes. According to the pressure data, the height of the bed at B<sup>2</sup> was increased by roughly 5 cm, falling 1.4 cm below test records. The height of the bed at A<sup>1</sup> dropped by 8.3 cm, which is consistent with test records.

*J. Mar. Sci. Eng.* **2020**, *8*, x FOR PEER REVIEW 7 of 12

**Figure 6.** Comparison of results measured by echo ranging and the TPRFD.

The bed height data measured by echo ranging (Figure 7) show that the height of crest A before migration was 20.19 cm, while the trough B reached 11.50 cm. After migration, the height at initial crest A dropped by 8.2 cm, the height of trough B increased by 6.4 cm, and the sand wave migrated

*3.2. Measurements Made by the Surface Synchronous Bottom Pressure Recorder (BPRSS)*

**Figure 8.** Comparison of results measured by the BPRSS and echo ranging. **Figure 8.** Comparison of results measured by the BPRSS and echo ranging.

#### **4. Discussion 4. Discussion**

experiments.

#### *4.1. The comparison of the Results Measured by Pressure Sensor Techniques and Echo Ranging 4.1. The comparison of the Results Measured by Pressure Sensor Techniques and Echo Ranging*

It is worth noting that there was a significant response delay to the change in elevation within the first 20 minutes of the TPRFD measurement at crest A. During this period, the results measured by It is worth noting that there was a significant response delay to the change in elevation within the first 20 min of the TPRFD measurement at crest A. During this period, the results measured by the TPRFD and echo ranging have similar changes in elevation, but there is a delay of about 5 min. From the experimental records, this phenomenon may have occurred because the weight measured by the TPRFD is the total weight of overlying sand in an area of 100 mm × 100 mm, while the height measured by echo ranging is a point height (Figure 9). For this reason, for the point height of sharp terrain changes, rendering the response sensitivity of the fiber optic pressure sensor is insufficient. Therefore, quantitative research into the process of this phenomenon, by increasing the number of acoustic ranging points on the plane where the optical fiber pressure sensor is located or reducing the surface area of the sensor, is needed in the following research. *J. Mar. Sci. Eng.* **2020**, *8*, x FOR PEER REVIEW 8 of 12the TPRFD and echo ranging have similar changes in elevation, but there is a delay of about 5 minutes. From the experimental records, this phenomenon may have occurred because the weight measured by the TPRFD is the total weight of overlying sand in an area of 100 mm × 100 mm, while the height measured by echo ranging is a point height (Figure 9). For this reason, for the point height of sharp terrain changes, rendering the response sensitivity of the fiber optic pressure sensor is insufficient. Therefore, quantitative research into the process of this phenomenon, by increasing the number of acoustic ranging points on the plane where the optical fiber pressure sensor is located or reducing the surface area of the sensor, is needed in the following research.

**Figure 9.** Measurement state of echo ranging and optical fiber pressure sensor. that, while the two methods reflect height changes that are consistent with actual height changes of **Figure 9.** Measurement state of echo ranging and optical fiber pressure sensor.

both sides of the floating ball accelerated, increasing the erosion of the sediment on both sides of the floating ball (Figure 10); (b) because the density of the float is slightly greater than that of seawater, relative to the downward movement at the wave crest, the upward movement of the float at the trough is subjected to more resistance. It should be noted that only the impact of the existing floating ball on the movement of the bottom sediment, and whether and to what extent the movement of the sediment in other areas is affected, still needs to be explored by designing more controlled

**Figure 10.** Bottom depression formed by the bottom of the floating ball at the trough.

A comparison of the two methods with the observation results of the echo ranging group shows

*4.2. The Accuracy of Pressure Sensor Techniques in Reflecting Elevation*

A comparison of measurement results from the BPRSS and the TPRFD shows that there are two reasons for this stepwise manner: (a) due to the presence of the floating ball, the water flow speed on both sides of the floating ball accelerated, increasing the erosion of the sediment on both sides of the floating ball (Figure 10); (b) because the density of the float is slightly greater than that of seawater, relative to the downward movement at the wave crest, the upward movement of the float at the trough is subjected to more resistance. It should be noted that only the impact of the existing floating ball on the movement of the bottom sediment, and whether and to what extent the movement of the sediment in other areas is affected, still needs to be explored by designing more controlled experiments. A comparison of measurement results from the BPRSS and the TPRFD shows that there are two reasons for this stepwise manner: (a) due to the presence of the floating ball, the water flow speed on both sides of the floating ball accelerated, increasing the erosion of the sediment on both sides of the floating ball (Figure 10); (b) because the density of the float is slightly greater than that of seawater, relative to the downward movement at the wave crest, the upward movement of the float at the trough is subjected to more resistance. It should be noted that only the impact of the existing floating ball on the movement of the bottom sediment, and whether and to what extent the movement of the sediment in other areas is affected, still needs to be explored by designing more controlled experiments.

*J. Mar. Sci. Eng.* **2020**, *8*, x FOR PEER REVIEW 8 of 12

the TPRFD and echo ranging have similar changes in elevation, but there is a delay of about 5 minutes. From the experimental records, this phenomenon may have occurred because the weight measured by the TPRFD is the total weight of overlying sand in an area of 100 mm × 100 mm, while the height measured by echo ranging is a point height (Figure 9). For this reason, for the point height of sharp terrain changes, rendering the response sensitivity of the fiber optic pressure sensor is insufficient. Therefore, quantitative research into the process of this phenomenon, by increasing the number of acoustic ranging points on the plane where the optical fiber pressure sensor is located or reducing

the surface area of the sensor, is needed in the following research.

**Figure 10.** Bottom depression formed by the bottom of the floating ball at the trough. **Figure 10.** Bottom depression formed by the bottom of the floating ball at the trough.

#### *4.2. The Accuracy of Pressure Sensor Techniques in Reflecting Elevation 4.2. The Accuracy of Pressure Sensor Techniques in Reflecting Elevation*

A comparison of the two methods with the observation results of the echo ranging group shows that, while the two methods reflect height changes that are consistent with actual height changes of A comparison of the two methods with the observation results of the echo ranging group shows that, while the two methods reflect height changes that are consistent with actual height changes of the observation points as a whole, there are significant differences in the accuracy of the observations. Therefore, to quantify the accuracy of the height observations of the BPRSS and TPRFD, we define the observation error and accuracy α as:

$$\text{error} = H\_{\text{PR}} - H\_{\text{echo ranging}} \tag{4}$$

$$\alpha = \frac{\overline{\mathcal{H}}(\text{error})}{\overline{\mathcal{H}}\_{\text{echo ranging}}} \times 100\% \,\tag{5}$$

where "*HPR*" is the height calculated using the pressure observation method, and "*H*echo ranging" is the height measured by echo ranging.

The two methods produced crest and trough observations with accuracy levels of more than 90%, with the most accurate (98.1%) observation obtained using the pressure floating ball method at the trough (Figure 11). The largest error (2.7 cm) was made in the 17th min of observing the crest using the fiber-optic pressure sensor. With the corresponding delay occurring after 15 min, the overall observation accuracy level decreased to 91.4%, representing the least accurate observation. In addition, the two methods exhibit the following two characteristics in terms of accuracy levels: (a) observations of the trough are more accurate than those of the crest, and (b) the BPRSS method is more accurate than the TPRFD method. These findings show that while seabed elevation can be measured from the weight of overlying sand, this approach is less accurate than using water pressure to reflect elevation.

observation error and accuracy α as:

height measured by echo ranging.

the observation points as a whole, there are significant differences in the accuracy of the observations. Therefore, to quantify the accuracy of the height observations of the BPRSS and TPRFD, we define the

> H(error) Hecho ranging

where "" is the height calculated using the pressure observation method, and "echo ranging" is the

The two methods produced crest and trough observations with accuracy levels of more than 90%, with the most accurate (98.1%) observation obtained using the pressure floating ball method at the trough (Figure 11). The largest error (2.7 cm) was made in the 17th minute of observing the crest using the fiber-optic pressure sensor. With the corresponding delay occurring after 15 min, the overall observation accuracy level decreased to 91.4%, representing the least accurate observation. In addition, the two methods exhibit the following two characteristics in terms of accuracy levels: (a) observations of the trough are more accurate than those of the crest, and (b) the BPRSS method is more accurate than the TPRFD method. These findings show that while seabed elevation can be measured

α =

error = − echo ranging, (4)

× 100%, (5)

**Figure 11.** Comparison of the accuracy of the BPRSS and TPRFD methods.

### **Figure 11.** Comparison of the accuracy of the BPRSS and TPRFD methods. *4.3. Applicability of In Situ Observations*

*4.3. Applicability of In Situ Observations* A method's reliability is essential to practical observations. Our experiments show that while the BPRSS method, using the floating ball as a carrier, can more accurately reflect bottom bed elevation, it affects the migration of sand waves. When the underflow velocity of active sand wave areas reaches 58 cm/s [16], a floating ball with a density slightly higher than the density of the seawater can generate up and down movement under the action of the current. Increasing the density to reduce this effect may exacerbate the disturbance of the sand wave migration process. Therefore, its optimal density is difficult to determine, making the authenticity of the migration process reflected A method's reliability is essential to practical observations. Our experiments show that while the BPRSS method, using the floating ball as a carrier, can more accurately reflect bottom bed elevation, it affects the migration of sand waves. When the underflow velocity of active sand wave areas reaches 58 cm/s [16], a floating ball with a density slightly higher than the density of the seawater can generate up and down movement under the action of the current. Increasing the density to reduce this effect may exacerbate the disturbance of the sand wave migration process. Therefore, its optimal density is difficult to determine, making the authenticity of the migration process reflected in observation results difficult to confirm.

in observation results difficult to confirm. In this respect, as a fixed-depth total pressure recorder with an optical fiber pressure sensor is buried in the sea floor, sediment migration on the sea floor surface is less likely to be interrupted. Furthermore, because the sand wave has a common height of approximately 0.4–5 m and a wavelength of approximately 5–100 m [35], it is much larger than that of the bed model used in this In this respect, as a fixed-depth total pressure recorder with an optical fiber pressure sensor is buried in the sea floor, sediment migration on the sea floor surface is less likely to be interrupted. Furthermore, because the sand wave has a common height of approximately 0.4–5 m and a wavelength of approximately 5–100 m [35], it is much larger than that of the bed model used in this experiment. Therefore, the effect of the response delay phenomenon exhibited by the TPRFD method used in this experiment should be greatly reduced. Therefore, the TPRFD method is more applicable for field applications. However, if this technique is to be applied to the field observation, there are still many unresolved problems. How large is this noise caused by bottom current and potential turbulence relative to the measured noise? Further, how to ensure that the TPRFD is always inside the sand wave, and not exposed to current during the sand wave migration process. A designed field observation device, equipped with a TPFRD observation probe and some instruments for measuring hydrodynamics, may help to explore these issues. A multistage penetrating method can also be used to ensure that the TPRFD is always located inside the sand waves [36]. Nevertheless, more relevant subjects in pressure sensing techniques for observing seabed deformation caused by submarine sand wave migration are still expected to be studied and explored.
