11:00 AM - 1:00 PM
[ACG42-P04] Adaptations and limitations of combined geophysical method for the investigation of submarine groundwater discharge in shallow coastal waters.
Keywords:submarine groundwater discharge, Thermal Image, Electrical exploration, Drone
Various methods have been used to study submarine groundwater discharge (SGD) in coastal areas. However, due to the difficulty in visually capturing SGD and the uneven extent of its impact, the assessment of its impact on coastal ecosystems has been slow in progress. It is important to visualize such heterogeneous fields in order to accurately understand the impact of SGD. Since 2015, we have conducted high-resolution observations of sea surface temperature using drones and thermal cameras, survey of resistivity distribution under the seabed by using a towing electric exploration system, and direct measurements using electrical conductometers and thermometers in shallow coastal waters in several regions, to develop a method for visualizing the distribution and characteristics of SGD in shallow coastal waters. These results have been presented in JpGU sessions from 2018 to 2021. In this presentation, we summarize these results, add new observation data to them, and report the results of our study on the effectiveness and limitations of these survey methods for SGD surveys.
Observations using thermal cameras mounted on drones enable high-resolution temperature mapping and are ideal for capturing detailed sea surface temperatures.
However, the phenomenon of SGD does not necessarily change the temperature of the water surface, and temperature changes occurring only near the seabed cannot be detected at all. Therefore, this method is effective only when a strong SGD plume phenomenon is observed. On the other hand, it is easy to check the difference in temperature between the target area and its surroundings, and is suitable for a simple understanding of the difference between the surrounding and thermal environment. The horizontal resolution of the towed resistivity survey is very rough because of the observation by lines. However, it is suitable for confirming the possibility of the existence of SGD on the seabed, because the difference in the resistivity between freshwater and seawater is remarkable. In particular, it is very effective in the early stages of an SGD survey because it can find the distribution area of "potential SGD" that exists under the seabed where there is no discharge. However, this observation alone is not enough to determine whether or not there is an discharging. The direct observation method using a conductivity meter and a thermometer can reliably detect SGD inflow, but the observation resolution is very rough and does not cover the existence of SGD in the target area.
Thus, each observation method has its own adaptive limitations, and in the case of these individual observations, even if SGD is present, it may not be detected. However, by combining these observations, it was found that it is possible to observe SGDs without missing them. The order of survey is particularly important. First, overhead drone observations are conducted to check for strong signals and to determine whether there is a difference in water surface temperature between the target area and surrounding areas. Next, a towed resistivity survey will be conducted to identify possible locations of SGD, and direct measurements will be made using conductivity meters and thermometers at possible SGD locations. By observing in this order, it was possible to determine the presence or absence of SGD and the spatial extent of influence of temperature and salinity with considerable certainty. In addition, by integrating these results and mapping them in three dimensions on a computer, it is possible to visualize SGD, which is a heterogeneous phenomenon. However, this observation method is limited in the area that can be surveyed at a time. The area that can be surveyed by this method is at most a few kilometers square in one day.
Observations using thermal cameras mounted on drones enable high-resolution temperature mapping and are ideal for capturing detailed sea surface temperatures.
However, the phenomenon of SGD does not necessarily change the temperature of the water surface, and temperature changes occurring only near the seabed cannot be detected at all. Therefore, this method is effective only when a strong SGD plume phenomenon is observed. On the other hand, it is easy to check the difference in temperature between the target area and its surroundings, and is suitable for a simple understanding of the difference between the surrounding and thermal environment. The horizontal resolution of the towed resistivity survey is very rough because of the observation by lines. However, it is suitable for confirming the possibility of the existence of SGD on the seabed, because the difference in the resistivity between freshwater and seawater is remarkable. In particular, it is very effective in the early stages of an SGD survey because it can find the distribution area of "potential SGD" that exists under the seabed where there is no discharge. However, this observation alone is not enough to determine whether or not there is an discharging. The direct observation method using a conductivity meter and a thermometer can reliably detect SGD inflow, but the observation resolution is very rough and does not cover the existence of SGD in the target area.
Thus, each observation method has its own adaptive limitations, and in the case of these individual observations, even if SGD is present, it may not be detected. However, by combining these observations, it was found that it is possible to observe SGDs without missing them. The order of survey is particularly important. First, overhead drone observations are conducted to check for strong signals and to determine whether there is a difference in water surface temperature between the target area and surrounding areas. Next, a towed resistivity survey will be conducted to identify possible locations of SGD, and direct measurements will be made using conductivity meters and thermometers at possible SGD locations. By observing in this order, it was possible to determine the presence or absence of SGD and the spatial extent of influence of temperature and salinity with considerable certainty. In addition, by integrating these results and mapping them in three dimensions on a computer, it is possible to visualize SGD, which is a heterogeneous phenomenon. However, this observation method is limited in the area that can be surveyed at a time. The area that can be surveyed by this method is at most a few kilometers square in one day.