**5. Thermal properties**

### **5.1 Thermal conductivity variation with depth**

Figure 8 shows the variation of thermal conductivity at different depths in the Ariake sea. In the samples of tidal flats (sample 1 and sample 2), the variation is more prominent than the other samples collected from deep sea. This is probably due to much turbulent in the tidal flat mud in the region and introduces various kinds of matter during the tidal water movement as well as the direct exposure to the sun light during the ebb tide. All the samples show great variations in the sub surface (0-0.20 m) region but less variation in deeper region. Thermal conductivity of mud varies with soil texture, water content and organic matter content (Hamdeh and Reeder, 2000). The water content of the Ariake mud is always over 130% in different depths, which indicates that the conductivity of the Ariake mud is not affected by the water content at different depths.

sulfide materials (Richard and Morse, 2005). Actually there are many factors which are liable to produce AVS in some specific regions. However, the laboratory test showed that due to the acid treatment practice the AVS value increased in the tidal flat mud (Moqsud et al. 2007). So the conceptual image of acid treatment practice and the seasonal variation of

Figure 7 illustrates the conceptual image of the pore water movement in the tidal mud due to the seasonal variation of temperature. During spring and summer, the temperature at the shallow depth of the Iida tidal mud of the Ariake sea was higher than that of deeper depth, whereas opposite phenomenon was found during autumn and winter. The temperature gradient in the mud causes pore water to move in the vapor phase from a higher temperature site to a lower temperature site. The vapor condenses at the lower temperature area and becomes water, which increases the total head and drives the water liquid phase from lower temperature site to the higher temperature site (Nassar et al. 2000). Aforementioned process is

titled coupled heat-pore water vapor-pore water liquid flow, as shown in Fig. 7.

Pore water liquid

High total pressure head

Fig. 7. Proposed concept of coupled heat-pore water vapor-pore water liquid flow in tidal flat

Figure 8 shows the variation of thermal conductivity at different depths in the Ariake sea. In the samples of tidal flats (sample 1 and sample 2), the variation is more prominent than the other samples collected from deep sea. This is probably due to much turbulent in the tidal flat mud in the region and introduces various kinds of matter during the tidal water movement as well as the direct exposure to the sun light during the ebb tide. All the samples show great variations in the sub surface (0-0.20 m) region but less variation in deeper region. Thermal conductivity of mud varies with soil texture, water content and organic matter content (Hamdeh and Reeder, 2000). The water content of the Ariake mud is always over 130% in different depths, which indicates that the conductivity of the Ariake mud is not

Heat

Low

Autumn ~ Winter

High temperature Pore water liquid

pressure head

High total

pressure head Condensation

Evaporation Low total

Pore water vapor

Low total pressure head

temperatureSpring ~ Summer

Hot water Cold water16°C - 7°C

temperature are thought to be rational.

Heat

High temperature

Mud depth

**5. Thermal properties** 

temperature

Condensation Low

**5.1 Thermal conductivity variation with depth** 

affected by the water content at different depths.

Pore water vapor

16°C - 29°C

Evaporation

**4.4 Proposed mechanisms of pore water movement** 

Fig. 8. Variation of thermal conductivity with depth in the Ariake sea

#### **5.2 Thermal diffusivity variation with depth**

Figure 9 shows the variation of thermal diffusivity with depth for all the Ariake mud. It is seen that in the tidal flats (sample 1 and sample 2); the thermal diffusivity varied much at the different depths. On the other hand in the case of deep sea mud sample (sample 3, sample 4 and sample 5) the thermal diffusivity was constant at different depths. This is due to a small chance in turbulence in the deep sea bed floor. However, in the tidal flat area, during the low tide, the tidal mud is exposed directly to the sunlight, and during the ebb tide, a lot of foreign matters come and disturb the homogeneity in the mud of the tidal mud layers. It is seen that in the deep sea mud, the value of thermal diffusivity is always in 0.12 x 10-6 m2/s. In the tidal flat, the peak was reached at 0.13 x 10-6 m2/s at different depths.

Fig. 9. Variation of thermal diffusivity with depth

#### **5.3 Volumetric heat capacity variation with depth**

The volumetric heat capacity of the tidal mud refers to the value which indicates the ability to store heat. If the volumetric heat capacity of a soil is high then the soil is more stable in terms of temperature change or the thermal environment. Figure 10 illustrates the variation of volumetric heat capacity with depth of the various samples. Sample 2 shows a great variation in volumetric heat capacity. The peak shows at 0.35 m depth and value is about 6.3 MJ/m3 °C. Clay soil generally has higher volumetric heat capacity than sandy soil for the same water content and soil density (Hamed, 2003). Volumetric heat capacity is very important for the acid infected tidal mud. Sulphate reducing bacteria (SRB) plays an important role in the geo-environmental condition of the Ariake Sea. These Bacteria like the layer where the volumetric heat capacity is higher (Moqsud et al.2006). Because in that layer it shows the more stable condition which is liked by the bacteria.

Fig. 10. Variation of volumetric heat capacity with depth

The temperature of underground soil is affected mainly by the soil thermal properties (Nassar et al., 2006) and these properties play a significant role in the geo-environmental condition in the global environment. The thermal properties of the mud are also induced by the mineralogical matter presence in the mud. The effects of this mineral matter on the thermal properties of the Ariake sea mud needs further study.
