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Coastal Engineering Journal, Vol. 57, No. 2 (2015) 1550004 (17 pages)c World Scientiﬁc Publishing Company and Japan Society of Civil Engineers
DOI: 10.1142/S0578563415500047

Development of a Fiber-Optic Cable Monitoring
System for Storm-Generated Bathymetric
Change in the Surf Zone

Shuro Yoshikawa∗,§ , Hide Sakaguchi∗,¶ , Shinichi Akutagawa†, ,Yuichi Machijima‡,∗∗ and Zhao Yue‡,††∗Department of Mathematical Science and Advanced Technology,
Japan Agency for Marine Earth Science and Technology,
3173-25 Showa-Machi, Kanazawa-ku, Yokohama 236-0001, Japan†

Department of Civil Engineering,
Graduate School of Engineering, Kobe University,
Rokkodai 1-1, Nada, Kobe 657-8501, Japan‡LAZOC Inc., 1-5-3, Nishikameari,Katsushika-ku, Tokyo, Japan§s-yoshikawa@jamstec.go.jp¶
sakaguchih@jamstec.go.jp
cadax@kobe-u.ac.jp∗∗
machijima@lazoc.jp††
zhao@lazoc.jp
Received 7 October 2014
Accepted 25 March 2015
Published 8 June 2015

A monitoring system, called the optic sediment sensor (OSS), for study of the detailed processes of bathymetric changes in the surf zone has been developed. The OSS system consistsof an array of ﬁber gap sensors (FGSs), a support structure (steel pipes), an electronics unitthat transmits and receives light through ﬁber-optic cables, and a data acquisition system.
The FGSs are mounted in a 2-m long vertical steel pipe; the sensor spacing is 10 cm. Steelpipes containing FGSs were embedded in the seaﬂoor at two places at water depths of
4–5 m beneath a pier at the Hazaki Oceanographical Research Station (HORS) (Port and§ Corresponding author.

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Airport Research Institute) on the Sudahama Coast, facing the Paciﬁc Ocean, Japan. The
electronics unit and the data acquisition system were installed in an observation room on
the pier. The OSS systems successfully recorded well-deﬁned and consecutive bathymetricchanges caused by storm waves (maximum signiﬁcant wave-height = 3 m); e.g. reversalof seabed erosion and deposition within a day could be recognized. If the OSS system
could be installed in many more locations along the pier, the consecutive process of theformation, migration, and deformation of the whole of the longshore bar may be clariﬁed.
Keywords: Optic sediment sensor; bathymetric change; monitoring; high-energy surf zone.

1. Introduction
Development of a monitoring system that can record the detailed processes ofbathymetric change and sediment transport is necessary for progress in studies ofnearshore morphodynamics, modern sedimentary processes, and coastal engineering
works. In general, bathymetric changes and related sediment transport are studied
on the basis of survey results obtained before and after an event such as a storm or
tsunami [e.g. Yoshikawa and Nemoto, 2010, 2014; Goto et al., 2011]. This method,
however, cannot be used to clarify the timing, volume, and manner of sedimentmigration during a single event. Several devices have been created for continuous
monitoring of nearshore features and sediment transport in the nearshore zone.
For example, Traykovski et al. [1999] observed continuous morphological changes inwave orbital ripples for six weeks at a water depth of 11 m using an autonomous
rotary side-scan sonar system mounted on a tripod. To estimate the suspended sediment concentration in the swash zone, optical backscatter sensors (OBS) have beendeveloped [Downing et al., 1981]. Masselink et al. [2007] investigated the dynamics
of wave ripples using multiple sensors including a sand ripple proﬁler and OBS.
Those systems provide very high-resolution data on morphological change and sediment transport, but are unsuitable for investigation of large-scale sediment erosionand deposition that produces a longshore bar generated by high-energy waves in thesurf zone. If the change in water depth is large (e.g. more than 1 m) over a shorttime interval, the monitoring system will be destroyed, lost, or submerged. Oncethe monitoring system is buried below the seaﬂoor, it cannot be used to observebathymetric changes and sediment transport.
In this study, a new instrument containing plastic ﬁber-optic cables, which canbe mainly used to monitor bathymetric changes in the high-energy shallow marine
environment has been developed. Field testing was conducted at the Hazaki Oceanographical Research Station (HORS) (Fig. 1) owned by the Port and Airport ResearchInstitute (PARI) on the Sudahama Coast, facing the Paciﬁc Ocean, Japan. This area
is suitable for our research objectives because active sediment transport that createsand deforms the longshore bar has been observed [e.g. Kuriyama, 2002; Kuriyama
et al., 2006].
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Fig. 1. Location of the study area. The newly developed monitoring system for research on stormgenerated bathymetric change and sediment transport was placed at the Hazaki OceanographicalResearch Station (HORS; http://www.pari.go.jp/en/about/facilities/hors.html) owned by the PARIon the Sudahama Coast, central Japan. The aerial photograph is a Google Earth image taken on
29 March 2012.

2. Instruments, Methods, and Study Site
2.1. Concept of the sensing technique using light
The backscatter intensity is important for observations of sediment transport andconcentration in water using light [e.g. Downing et al., 1981; Beach et al., 1992; Foster et al., 2000]. Herein, this method of detecting the backscatter intensity is called
the twin ﬁber method (Fig. 2). Use of the method for detection of local migration of
sandy particles has been established in laboratory experiments [Akutagawa et al.,2014]. The equipment consists of two plastic ﬁber-optic cables with a diameter of
1 mm (Fig. 2). The ﬁrst cable (Fiber-1) is used to supply light to the point of observation; the other cable (Fiber-2) collects light reﬂected back from surfaces of nearby
particles and carries it back to a photosensor. Therefore, the area of observation at
the tips of the ﬁbers is on the scale of millimeters [Akutagawa et al., 2014].
In this study, a gap between two ﬁber cables aligned in a straight line is used tomeasure the attenuation of light intensity. In this way, a ﬁber gap sensor (FGS) is
formed (shown in Fig. 2 as the “gap method.”) The concept of the sensing technique
is illustrated in Fig. 3. The light intensity at the emitting and receiving sides aredenoted as LE and LR , respectively. As the light from the emitting side is scattered
at a wide angle, LR is generally smaller than LE . When the gap is fully in seawater,
LR remains stable at a certain level. If suspended sediment particles enter the gap,
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Fig. 2. Diagram showing the sensor created using ﬁber-optic cables and an LED.

Fig. 3. Diagram illustrating the sensing technique using the gap method.

LR is aﬀected by the movement of those particles and its value is altered randomly.
When the gap is completely ﬁlled by sediments below the seaﬂoor, LR becomesalmost zero. Therefore, by monitoring ﬂuctuations in LR , it is possible to determine
the location of the seaﬂoor at any given time. In a strict sense, LR is aﬀected not
only by LE and the presence of sediment in the gap, but also by sunlight. However,
it is demonstrated later that this eﬀect is negligible in this study.
2.2. OSS system configuration
The newly developed device, called the optic sediment sensor (OSS) system, consistsof four main components: An FGS array formed as a unit assembly with a 10-mm
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Fig. 4. Illustrations and photographs of the undersea system of the monitoring device. The diﬀusion
and reception of light by the ﬁber-optic cables within the gap is also illustrated at the top of theﬁgure.

gap between the emitting and receiving sides of ﬁber-optic cables supported in a ring
made of polyoxymethylene (Fig. 4); a support structure (steel pipes); an electronics
unit that transmits light from the LED (OSM5XNE1E1S, http://www.optosupply.
com/EN/search.asp; Opto Supply Ltd.) and receives returning light through theﬁber-optic cables (PGS-CD1002-22-E, http://www.toray.co.jp/english/electronic/
index.html; TORAY Industries, Inc.); and a data acquisition system. The diameterof the steel pipe that composes the FGS array is 48.6 mm. A schematic representation of the undersea OSS system is illustrated in Fig. 4.
The gap spacing of the FGS was decided upon based on the following reasoning:(1) shell fragments and some gravels would probably be easily caught in the gap if
the spacing was less than 10 mm; (2) the desire to avoid the diﬀusion of the light
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(described below), and (3) the light intensity (LR ) becomes high as the distance
between the emitting and receiving sides decreases. In the present system, the eﬀect
of inter-channel (FGS) interference on the light intensity (LR ) is thought to be small
because of the range of the acceptance angle (maximum 60◦ ) and numerical aperture(0.5) of the present ﬁber cable, the vertical length of the ring that supports the FGS(15 mm), and the gap spacing (Fig. 4). Even if the angle of light diﬀusion from theemitting side reaches 60◦ , the diﬀusion area is still within the ring, as shown in theuppermost part of Fig. 4.
The newly developed FGSs were mounted in a 2-m long vertical steel pipe; thespace between the sensors was 10 cm (Fig. 4). The FGS-containing steel pipes wereembedded in the seaﬂoor at two locations at water depths of 4–5 m beneath a pierat HORS. The pier is aligned perpendicular to the coastline. The systems on theseaﬂoor were called OSS systems No. 1 and No. 2, with system No. 1 being 15 m
nearer to the shore. A bundle of ﬁber-optic cables with stainless steel wire ran
from the steel pipe on the seaﬂoor to the pier, and the cables were routed into an
observation room on the pier (Fig. 4). The digital processing unit was installed inthis room.
The digital processing unit is composed of the electronics unit and the dataacquisition system. The electronics unit consists of two sensor boxes (Box 1 and 2).
Box 1 contains light emitting diodes (LEDs) that emit light through the emitting
parts of the FGSs through the ﬁber-optic cables. The photosensors in Box 2 receive
light from the receiving parts of the FGSs, yielding ampliﬁed electrical currents thatare ﬁnally converted into digital signals. These signals are processed by a microcomputer in Box 2. The digital signals are transferred from Box 2 to the data acquisition
system (PC). The PC controls the processing in Box 2; i.e. the sampling rate canbe changed by the PC.
During the survey period by the OSSs, remote-control operation of the digital
data acquisition system was conducted from an oﬃce in Yokosuka city, about 115 km
west-southwest of HORS.
2.3. Sediments around the survey site
Near the HORS, the bottom sediment from shoreline to a water depth of about 6 m ismainly composed of ﬁne to medium sand (0.18–0.5 mm in median diameter) basedon repeated sediment sampling along the pier with a cross-shore interval of 10 m
[Katoh et al., 1990]. The beach sediments that are the likely source of the shallow
seaﬂoor sediments near the HORS are mainly chert, shale, sandstone, quartz, shellfragments, and iron minerals [Sudo, 2006].
3. Laboratory Experiments
Laboratory experiments were conducted to determine the detection ability of sensors
using the gap method, the diﬀerence in the detection ability of the gap method
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Fig. 5. Photographs of the experiment using a cylinder and sandy sediments obtained on the seaﬂoor
at HORS. An illustration of the sensors using the gap method for the experiment is also shown.

and the twin ﬁber method, and the eﬀect of sunlight on the light intensity. The
experiments used a cylinder about 1-m long and sediments obtained from the seaﬂoor
at HORS. In the cylinder, the gaps were placed at heights of 30 and 60 cm, and arecalled Gap-1 and Gap-2 (Fig. 5). The gap between the emitting and receiving sidesof the ﬁber-optic cables was 10 mm. This was the same setting for the device asused in the ﬁeld experiments. Experiments were conducted outside the laboratory
in sunlight, and inside the laboratory with the lights turned oﬀ. The latter condition
simulates ﬁeld measurements obtained at night.
First, the sediments were slowly fed into the top of the cylinder at a time.
Changes in light intensity (LR ) during sediment settling were detected at Gap-1 andGap-2 (Fig. 6). At Gap-1, a high amplitude of LR with attenuation of the intensity
resulting from passage of sediment through the gap was observed. The higher LR
values during the ﬂuctuations indicate lower concentration of sediments at the Gap1 depth at that time, resulting in minor disturbances in the amount of light between
the emitting and receiving parts of the gap. In the second round of sediment supply
to Gap-2, the sediments were also fed slowly at a time. At Gap-2 (a height of 60 cm),a similar change to that at Gap-1 occurred. A drop in LR immediately before thesecond round was caused because small quantity of the sediments had been dropped
from the top of the cylinder. The LR values in the gaps become zero when the gaps
are completely buried in the sediment.
In addition, the LR measured using the gap method is about 10 times higher thanthat measured using the twin ﬁber method, indicating that the former method ismore useful for studies of sediment concentrations. A higher initial value is desirable
for measurement of the LR because attenuation of the light intensity (transmission
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Fig. 6. Result of the experiment based on the sensors using the gap method. The light intensity isa dimensionless value.

loss) becomes large on increasing the length of the plastic ﬁber-optic cables. Furthermore, Akutagawa et al. [2014] reported that ﬂuctuation in the light intensity
was detected by the twin ﬁber method even if the tip of the cables are completely
within ﬁne-grained sediments (90% of the particles are smaller than 0.25 mm on
average); a sensor (tip of the cable) reacted to slight motions of the sediments infront of the tip when the ﬁber cable was inserted into, and removed from the sediments. The reaction seen using the twin ﬁber method is unfavorable for monitoring
the seabed in the present study area. The seaﬂoor and sub-seaﬂoor sediments are
unstable because of high energy waves and currents.
Regardless of the presence or absence of sunlight, the same results were observed.
The initial values of LR at the gaps were similar: the value of LR in sunlight isapproximately 1120 at Gap-1 and 1190 at Gap-2; LR is approximately 1135 at Gap1 and 1230 at Gap-2 inside the laboratory with the lights turned oﬀ.
4. Field Experiments
In this research, the ﬁeld data consist of data obtained by the OSS systems, wave
observation data, and weekly bathymetric cross sections investigated by PARI.
The measurements from the OSS systems were obtained between 20 January and3 March, 2014, although OSS system No. 1 was destroyed on 16 February because of
breakage of the ﬁber cables, probably due to storm waves. Based on the bathymetriccross section data, there was no signiﬁcant change in the bathymetry between 20 January and 3 February, and a longshore bar developed between 3 February and 3 March(Fig. 7). The wave data observed at the pier (Fig. 8), about 30 m oﬀshore from OSS
system No. 2, indicate that high incident waves (signiﬁcant wave-height > 2 m) thatare likely to have been related to the formation of the longshore bar were frequently
observed in the latter period (from 3 February to 3 March), particularly between 4
and 18 February because of the passage of several low-pressure systems.
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Fig. 7. Temporal changes in the bathymetric cross section, surveyed by PARI. The locations of
OSS systems No. 1 and No. 2 are also shown.

Fig. 8. Wave conditions (measured by PARI) during the survey period of the OSS systems.

This section presents the bathymetric changes identiﬁed by the OSS systems
between 3 and 17 February, when the most remarkable development of the longshore
bar was observed based on the data shown in the cross section (Fig. 7). The dataobtained during this period are suitable for conﬁrming the availability of the systems,
because the OSS systems were located in the depositional zone of formation of thelongshore bar and its oﬀshore-directed migration (Fig. 7). The FGS channel numbers
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increase upward: ch 1–21 are in OSS system No. 1, and ch 23–43 are in OSS system
No. 2. It should be noted that the initial values in these channels are not the same,
because the optical axis alignments of the ﬁbers on the emitting and receiving sides
were not necessarily perfect. The diﬀerence in the length of the ﬁber cables, thedegree of cable bending between the sensor box in the observation room and sensor
array in the seaﬂoor, and the processing accuracy of planarization of the end faceof the cables are also possible causes of the diﬀerence in the initial value of the lightintensity (LR ). However, the amount of change relative to the initial value should
be obtained in the present system, in contrast to the general measurement that
evaluates the absolute value.
The OSS data described below can be interpreted in the same way as the resultsof the laboratory experiments: LR of the given channels is zero when the channels areburied completely below the seaﬂoor, although no attenuation of LR will be observed
for channels located above the seaﬂoor where suspended materials are rarely present.
In addition, the attenuation rate depends on the increase or decrease in the amountof suspended sediment at the depth of the given FGS channels. Figures 9–11 show
time-series of the position of the FGS channel immediately above the seaﬂoor basedon the LR value and the threshold LR value (tentatively deﬁned in this study).
When the LR value is greater than 1, the FGS channel is located just on or above theseaﬂoor. LR values ﬂuctuate from zero to > 1 when a given FGS channel is locatednear the seaﬂoor, because of repeated exposure and burial of the FGS channel at thissurface. Thus, the particular channel immediately above the seaﬂoor will frequently
vary (Fig. 9).
On 4 February, when the signiﬁcant wave-height brieﬂy reached approximately
2.7 m (Fig. 8), ch 7, 8, 30–32 were exposed above the seaﬂoor (Fig. 9); i.e. about
20 cm of erosion occurred at OSS system No. 1, and 30 cm of erosion at OSS system
No. 2. The exposure was instantaneous. When an FGS is located near the seaﬂoor,
a high amplitude of LR is observed, as shown in the intensity values for ch 7, 8, 33,
and 34. Rapid attenuation was occasionally observed in ch 8 (10 am and 4 pm on 5
February; Fig. 9), despite ﬂuctuations in the LR value in ch 7. This may have resulted
from spatial and temporal variations in the suspended sediment concentration near
the seaﬂoor. Gradual attenuation of LR , as observed in ch 33 and 34, during theperiod between 11 pm on 4 February and midnight on 6 February may have beencaused by bending of the ﬁber-optic cables by waves and wind. Similar attenuations
or ﬂuctuations were observed after this period.
Between 6 and 10 February, when the signiﬁcant wave-height increased fromabout 0.7 to 2.9 m (Fig. 8), burial of ch 7–13 and ch 30–33, and subsequent exposure of ch 10–13 and ch 30–33, were observed (Fig. 10). The transition time from
deposition to erosion was 16 h in OSS system No. 1, and about 10 h in OSS system
No. 2.
Between 10 and 17 February, the signiﬁcant wave-height was mostly 2–3 m(Fig. 8). Changes in water depth were small until the night of 12 February, when
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Fig. 9. Changes in light intensity detected by the OSS systems between midnight on 3 February
and midnight on 6 February. The light intensity is a dimensionless value. The sampling interval is
20 s during this period. The number of the FGS channel immediately above the seaﬂoor (measured
at 5 min intervals) is shown in the lower portions of the two ﬁgure parts.

ch 9 and 10 were buried. In OSS system No. 2, no channels were buried or exposed
during this period (Fig. 11). In contrast, dramatic changes in water depth wereobserved between 13 and 17 February. Burial of ch 12–21 occurred over about 21 h,and burial of ch 29–34 over 26 h, although the upper limit of deposition in OSS system No. 1 is uncertain because no FGSs were placed above ch 21. Rapid depositionin OSS system No. 1 began approximately 3 h before the start of the depositionin OSS system No. 2. In addition, seaﬂoor erosion after the substantial deposition
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Fig. 10. Changes in light intensity detected by the OSS systems between midnight on 6 February
and midnight on 10 February. The light intensity is a dimensionless value. The sampling interval is
30 s during this period. The number of the FGS channels immediately above the seaﬂoor (measured
at 5 min intervals) is shown in the lower portions of the two parts of the ﬁgure.

was recognized during this period as well as between 6 and 10 February. The timerequired for burial and exposure of one FGS channel was practically instantaneous(a few seconds).
The change in the number of the FGS channel immediately above the seaﬂoor isgenerally consistent with the change based on the survey of the bathymetric cross
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Fig. 11. Changes in light intensity detected by the OSS systems between midnight on 10 February
and noon on 17 February. The light intensity is a dimensionless value. The sampling interval is 60 s
during this period. The number of the FGS channel immediately above the seaﬂoor (measured at
5 min intervals) is shown in the lower portions of the two parts of the ﬁgure.

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section near the OSS system, although a minor diﬀerence (ca. < 20 cm) in themeasured depth value between both datasets remains. For example, on 20 and 27
January, and 3 and 12 February, the water depth was 4.27, 4.49, 4.29 and 3.84 m,
respectively, near OSS system No. 1, whereas the channel number immediately above
the seaﬂoor was ch 10 on 20 January, ch 8 on 27 January, ch 9 on 3 February, andch 12 on 12 February in OSS system No. 1. These channel numbers were the locations
at about 11 am on the given days, when the surveys were conducted by PARI. The
reason for the minor diﬀerence in each water depth value probably relates to the
distance (about 3.5 m) between the location of the bathymetric survey by PARI andthat of the OSS system; the former survey was conducted on the south side of thepier, but the OSS system was on the north side.
5. Discussion and Conclusions
A new monitoring system, the OSS, was designed using ﬁber-optic cables and
LEDS. The system is designed to mainly investigate the detailed processes of stormgenerated bathymetric change. Fiber-optic sensors have often been used for multidisciplinary research projects. In research on sediment transport and bed level change,ﬁber-optic sensors have been used for measurements of the backscatter intensityof light as described above, ﬂuctuations in temperature between ﬂowing water andsaturated soil [Manzoni et al., 2011], and strain [Zarafshan et al., 2012]. Two of thelatter type of sensor are used for the monitoring of bridge pier scouring, and werebased on a ﬁber Bragg grating (FBG) sensor. The main diﬀerence between the FGB
and the OSS used in the present study is whether the cut in the sensor cable thattransmits and receives the data signal is present or not. In contrast to the FGB
system, which has sensors in various places in the cable, the tips of the cables are
the sensing parts in the OSS system. The proposed OSS system, if installed at anexisting pier, is likely to be of value in monitoring within the high-energy surf zone,
because dramatic changes in the water depth caused by the passage of low-pressure
systems can be clearly monitored without breakdown of the OSS or collapse of thesteel pipes, apart from breakage of the ﬁber-optic cables. As the distance between
the FGSs can probably be decreased to about 1.5 cm based on the consideration of
their inter-channel interference, a higher-resolution of seabed level change will bepossible by increasing the number of FGS used.
The data from the OSS system indicate that the longshore bar at HORS wasformed by repeated sediment erosion and deposition over very short time intervals
during storms, whereas the bathymetric cross sections indicate that the formationand oﬀshore-directed migration of the bar results simply from sediment deposition.
In addition, reversal of seabed erosion and deposition within a day could be recognized using the data from the OSS systems. Instantaneous exposure and burialof one channel was also observed. This information could not have been obtained
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locations (e.g. 20–30 m intervals) along a pier, and so perpendicular to the shoreline,
where a longshore bar is present, such as the HORS, the consecutive process of theformation, migration, and deformation of the whole of the bar will be clariﬁed. In
addition, the OSS system should be suitable for monitoring local erosion around thebase of coastal structures; e.g. jetties, oﬀshore breakwaters, and piers. However, an
online system for sending data wirelessly from the seaﬂoor, or an oﬄine approach
using automatic data acquisition and storage within the underwater system, will
have to be developed if sensor cables cannot be run from the seaﬂoor above the surface (or on land) because of the lack of an observation room near the measurement
system (unlike HORS).
In future, it will be necessary to develop measures to protect the ﬁber-optic cables
against damage, and this may avoid attenuation of the initial value of light intensity.
One possible countermeasure against cable damage is as follows. As the breakageof the ﬁber cables occurred near the sea surface, all of the ﬁber cables could betaken out from the lower part of the FGS array below the seaﬂoor, and then covered
with a protecting tube. They could then be routed from the array to the bottom of
the support structure of the pier. Then, the cables with the tube attached to the
support structure can run from the seaﬂoor to the pier. Their burial depth should be
below the lower limit of seaﬂoor erosion, as determined from previous bathymetricdata obtained at the HORS. Furthermore, monitoring any change in the positionof the pipes during the survey period may also be required, although a change inthe position of the pipes probably did not occur in the present survey period as: (1)the stainless steel wires were keeping the position that were ran from the steel pipeson the seaﬂoor to right above the pier; and (2) observations by divers revealed thatthe pipes had not moved. Moreover, the OSS systems must have a high withdrawal
resistance for the pipe because the penetration depth below the seaﬂoor of the pipes,
including the support pipes, is 2 m initially. The convex structures of the FGS rings
are an additional factor for increase of the withdrawal resistance for the sensor pipe.
In addition, examination of the relationship between light intensity and sedimentconcentration will be required using OSS ﬁeld data and data measured simultaneously by a turbidity meter, or sampling of the suspended sediment within seawater[Sternberg et al., 1986; Katayama et al., 1999], or a combination of these. For example, Katayama et al. [1999] reported a time-series of suspended sediment concentration and grain size distribution in sediment from continuous seawater sampling bypump at HORS. As the ﬂuctuations in light intensity can be measured immediately
above the seaﬂoor during storms, the OSS system may be a promising device for the
study of both bathymetric change and sediment concentration in water, although
the eﬀect of ﬂow modiﬁcation probably caused by the steel pipe and the ring of
the FGS on measurement of suspended sediment concentration must be examinedfurther.
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Acknowledgments
We thank Dr. Satoshi Nakamura and Mr. Masayuki Banno of PARI for support
while using HORS, and Mikuniya Construction Co., Ltd., for installation work on thepipes. The data for bathymetric cross sections and wave observations at HORS were
provided by PARI. We also thank Dr. Kuang-An Chang and anonymous reviewers
for their helpful comments, which led to improvements in the manuscript. We express
our deep gratitude to these individuals and organizations.

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