PROCEEDINGS of the23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

A Study of Acoustic Characteristics at Sea Bottom Sediment
Including Organic Matter
Hanako OGASAWARA1; Kazuyoshi MORI; Hiroshi Yagi
1

National Defense Academy of Japan, JapanABSTRACTAt shallow area, organic rich sediment is piled up and run away according to the tidal stream or upper sea
water movement. Authors measure the acoustic characteristics of organic rich sediment to monitor themovement of those sediment at the sea bottom. The bottom statement environmental changes affect benthicactivities. Sample sediment was obtained from the Ariake Sea in Japan, mostly consist with cray and manyorganic matters, was measured its acoustic speed and attenuation with the frequency of 2 MHz in laboratory.
At the same time, the grain size distribution was also measured. This result will help to understand thereflected waves from the sea bottom and sound propagation simulation in models.
Keywords: Sediment Properties, Underwater sound

1. INTRODUCTION
Assessment of the impact on coastal environments caused by global warming has mainly been done
so far based on disappearance of sandy beach areas by sea surface elevation, and changes of biota byelevated water temperature. However, it is required to carry out composite environmental assessment
to understand unequivocal ongoing global warming. At the boundary of water and seabed, there are
mixed layer with water and particle of sediments. The particle includes not only sand, mad, silt but alsoorganic matter such as plankton and algae. The thickness and density of the boundary area effectbenthic ecosystem. Therefore, it is important to monitor temporal and spatial change of the boundary
configuration. Acoustic is one of the best methods for underwater monitoring.
Multipath transmission is one of the major reasons to deteriorate of underwater soundcommunication 1, 2). Usually, reflected waves arrives a receiver after the direct signal from a source.
But in case of short travel distance or very shallow area, there is not so much time lag between thedirect signal and the reflected signals. Therefore, as reflected signals overlap with the direct signal,
signal-to-noise ratio decrease because of reflected signals. Sometimes, as coherent with reflectedsignals and a direct signal makes destructive interference, sound signal from the source cannot bereceived at the receiver 3) . It is very important to understand how much the reflected signals affect tothe main direct signal. Calm sea surface could be assumed as a mirror acoustically. According to thesea state or wind speed, many approximate formula or sea surface model were studied to explain theeffect of sea surface reflection and scattering 4, 5). The same methods are also applied to understand thesea bottom reflection 6) . But as the sound propagation of the sediment at the bottom sometimes very
similar behavior to sea water especially the sediment consists with mad and contained much water.
Moreover, some sound wave incident into the sediment and refracts upper side then they re-enter to thesea water. To understand such behavior of the sound at the sea bottom, it is necessary to know acoustic
characteristics such as sound speed, sound attenuation of the sediment and their dependency of thedepth from the sea bottom.
In this paper, we suggest acoustic monitoring method for the bottom boundary layer and get asample data as the first step. Although the measurement used only one frequency and particle size wasalmost uniform, this experiment will useful to assess boundary condition in future work.

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ogasawar@nda.ac.jp

2. EXPERIMENTAL METHODS
2.1 Sampling Area
The sediment sample was obtained from the Ariake Sea in Japan. As the sea is surrounded by land
and there are several rivers, many dirt is carried from rivers. The depth of the Ariake Sea is less than 50m, and tidal elevation exceed more than 4 m. The rich mad sediment is good place for marine creatures
especially for benthos. For many years, the sediment conditions such as pH, density, and thickness ofthe boundary layer has been monitored with core samples by Research center for fisheries andenvironment in the Ariake and Yatsushiro bays, Seikai National Fisheries Research Institute toinvestigate environmental changes and understanding their impacts on biological production. The
sediment sample was obtained by a diver at the monthly sampling by the research center. In addition,
we use the sample obtained other area of the Ariake Sea provided by Dr. Nakagawa.
Before sound speed measurement, the samples were screened by wet sieve analysis. At first,
sample on the sound speed measurement system was put on a scale to measure weight of the system
including the sample. As the weight of the system without any sample was measured in advance, thesample weight M can be calculated by the subtraction of the weight of the empty measurement system.
Then, the sample was screened by wet sieve analysis. Three sieves (0.6 mm, 0.15 mm, and 0.075 mm)
were used for this screening. They were set on a motor-driven shaking apparatus (Tsutsui: VSS-50).
All sieves were dried by an electric dryer for 24 hours and measured each of the weight to get particle
size distribution. The sum of all particle size weight becomes total particle weight and the difference
between the wet sample weights indicates water weight M V in the sediment sample. Suppose all spaceamong the particle in the sediment were filled with water, the porosity of the sediment ߚ can bewritten asߚൌܸ௏ൈ ͳͲͲܸ(1)where, V v and V are volume of the space and that of the total including the space, respectivel 8). The
total volume V is the obtained from the inner volume of a pipe and the volume of the space is calculated
from the weight of water in the sediment and water density. Although the particle size has distributed,
average density of the particles ρ r is obtained as‫ ܯ‬െ ‫ܯ‬௏ߩ௥ ൌ(2)ܸ െ ܸ௏
Table 1 shows all weight according to the particle weight distribution. We could screen samplesof two different places in the Ariake Sea. As the sieves mesh size is not enough fine, we could notseparate silt and clay. But the percentage of sand was less than 5%, sediment samples werecategorized as clay or silt According to the soil texture triangle provided by United States
Department of Agricultur 9).
Table 1 – Particle size distribution of sample sediments
Particle size [μm]Weight ratio [%]Sample A

Sample B

600 Ӎ

2.85

1.25

150-600

3.68

0.74

75-15038.85

1.1675<54.62

96.84

2.2 Measurement System
In this study, acoustical characteristics, especially the sound speed changes according to the densityof sediment matters in water are the most interesting topic. As the obtained sample includes different
grain size sediment, authors decided to compare the difference between the upper side and the lower
side of the sample. Therefore, a new measurement system to sound speed of a sample in a 4 cm
diameter cylinder with 3 cm height is required. Figure 1 shows a diagram of the sound speed4455

measurement system. A transmitter and a receiver (KGK: 2 MHz) were faced each other across thesample with 3 cm distance fixed with acrylic pipe which has the same diameter and thickness to that ofused at the core sampling. As sound speed varies according to temperature, it is important to control
temperature of the sample. Therefore, a part of the measurement system including the sample which isindicated by dashed line in Fig. 1 was put into an incubator to control the temperature but it was broken
in this experiment. To monitor temperature changes of the sample, a K-type thermocouple was inserted
into the sample from the upper side of the pipe. The burst signal was sending from the transmitter andreceived at the receiver located opposite side of the pipe. The career frequency was 2 MHz which isresonance frequency of the transmitter and the receiver. Both sending and receiving signals weremonitored by an oscilloscope (Agilent: 33220A) to measure travel time. After recording the receiving
signal through the oscilloscope, peak time was measured as the difference of phase from oscilloscope
monitor.

Figure 1 – A diagram of sediment sound speed measuring system.

3. RESULTS
3.1 Distance Calibration
To get the accurate distance between the transceiver and the receiver, water was stored in theacrylic pipe and measured travel time and the temperature at that time. The sound speed was calculated
from measured temperature with UNESCO equation. From measured travel time and calculated soundspeed, the distance between the transceiver and the receiver was 35 mm.
3.2 Sound Speed Measurement
At the boundary layer, small size particles mast float with some density distribution. To recreate
the boundary layer, add the sediment of the smallest particle size into water of 70 ml and stir it for 1min. Then the liquid put into the acrylic pipe and measured travel time. The travel time measured 5times and took their average to calculate sound speed. Figure 2 shows measured sound speed
according to the amount of sediment in water. Unfortunately, the thermostatic chamber could not
keep temperature with some trouble, the liquid temperature was not same throughout the allexperiment. As the temperature also affect the sound speed, Figure 3 shows ratio to pure water at themeasured temperature. The sound speed of the pure water was calculated from UNESCO equation 7).
From Fig. 3, sound speed clearly decreases when the sediment weight percentage increases.
In the same way, sound attenuation of received signal to the sending signal was monitored asshown in Figure 4. When the added sediments amount over 15͙, the received signal could notconfirm from the oscilloscope. As the density of the particles increase, sound scattered and could notreach to the receiver side.

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4. CONCLUSIONSound Speed [m/s]

In this paper, the grain size distribution of the Ariake Bay was investigated and measure theacoustic properties with frequency of 2 MHz. In this method, it is possible to measure sound speed
and attenuation according to the contained sediment amount. As it was measured only single
frequency, we will carry out the same experiment with different frequency. A lso, as the contained
sediment material was limited the particle size, more close-to-reality contamination is better for
future experiments.

Weight Percentage [‰]Sound Speed ratio to pure water

Figure 2 – Estimated sound speed according to the weight percentage.

Weight Percentage [‰]Figure 3 – Ratio of liquid sound speed to pure water according to the weight percentage.

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Signal attenuation ratio to pure water

Weight Percentage [‰]ACKNOWLEDGEMENTS
This work was partly supported by JSPS KAKEN Grant Number 17H03317. The authors aregrateful to members in Research Center for Fisheries and Environment in the Ariake and Yatsushiro
bays in Seikai National Fisheries Research Insttitute, FRA for providing precious sediment samples inthe Ariake bay. The autors also thank Dr. Yasuyuki Nakagawa to providing his correction of sedimentsin the Ariake bay. The authors are grateful to Dr. Masahiro Kimura for giving us many of hisknowledge of sediment acoustic analysis methods.

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