5:15 PM - 6:30 PM
[AHW24-P04] Examination of groundwater level measurement method by microtremor array exploration
Keywords:Microtremor exploration, groundwater level, repeated measurements
1. Introduction
One of the geophysical exploration methods is "microtremor exploration", which estimates the S wave velocity by constantly observing microtremors on the surface of the earth, and requires an S wave velocity structure in the vertical one-dimensional direction at the exploration point. Since the S wave velocity is determined by the physical properties of the ground, the geological structure such as the distribution of the aquifer and the basement can be grasped from the vertical distribution of the S wave velocity.
On the other hand, the ground is composed of three phases: a solid phase part such as soil and rock, a liquid phase part such as groundwater and soil water, and a gas phase part existing in the unsaturated zone near the ground surface. Therefore, if the ratio of the liquid phase portion in the ground (≈ groundwater level) changes, it is expected that the S wave velocity will change.
Takahashi (2019) reviewed and model-analyzed the relationship between S-wave velocity and saturation of rocks. From past experiments in the literature, in the case of sandstone and shale, the S wave velocity decreases with increasing water saturation, in limestone there is no change in S wave velocity due to water saturation, and in granite, S wave with increasing water saturation. It showed a tendency to increase in speed. On the other hand, although the experimental results for the unconsolidated sedimentary layer were not obtained, it is possible that the fluctuation of the water content has an effect on the S wave velocity from the large pore space in comparison with the consolidated layer.
Therefore, in this study, the relationship between the groundwater level change and the S wave velocity distribution was investigated by repeating the measurement of the groundwater level and the micro-array tremor exploration at the same point at the same time.
2. Method
Repeated tremor exploration at fixed points was conducted nine times from June to November 2014 and once each in August 2019 and January 2020 at two points (H54, R50) located in the alluvial fan area in the upper reaches of the Ashigara Plain. A total of 11 micro-array microtremor surveys were conducted. In addition, the groundwater level was continuously measured at the shallow groundwater observation wells attached to these points.
Microtremor observation was performed using JU210 and JU410 manufactured by Hakusan Kogyo with a sampling interval of 200 Hz. The results of microtremor exploration were analyzed according to the method of Senna et. al, (2014).
3. Results and discussion
As a result of 11 microtremor surveys, the maximum exploration depth was 68.9 m at H54 and 86.9 m at R50, and the S wave velocity at maximum depth was 965 m/s at H54 and 940 m/s at R50. It showed almost the same S wave velocity. The groundwater level during tremor exploration changed from -12.73 to -15.30 m at H54 and from -7.24 to -8.41 m at R50.
The S wave velocity near the groundwater table is 370 to 880 m/s at the H54 point and 230 to 370 m/s at the R50 point. The results show that the S wave velocity differs greatly depending on the exploration time. In addition, explorations in which the S wave velocity changed up and down across the vicinity of the water table were observed in 8 out of 11 explorations at H54 and 10 out of 11 at R50.
Factors that cause the S-wave velocity at the same depth to change significantly for each microtremor exploration include errors in exploration and analysis such as installation errors of microtremors for each microtremor exploration and waveform reading errors in analysis, as well as changes in soil temperature and inadequacy. Factors such as differences in soil moisture content in the saturated zone are possible. In the future, in addition to examining these error factors, we will also examine other geological and climatic conditions.
One of the geophysical exploration methods is "microtremor exploration", which estimates the S wave velocity by constantly observing microtremors on the surface of the earth, and requires an S wave velocity structure in the vertical one-dimensional direction at the exploration point. Since the S wave velocity is determined by the physical properties of the ground, the geological structure such as the distribution of the aquifer and the basement can be grasped from the vertical distribution of the S wave velocity.
On the other hand, the ground is composed of three phases: a solid phase part such as soil and rock, a liquid phase part such as groundwater and soil water, and a gas phase part existing in the unsaturated zone near the ground surface. Therefore, if the ratio of the liquid phase portion in the ground (≈ groundwater level) changes, it is expected that the S wave velocity will change.
Takahashi (2019) reviewed and model-analyzed the relationship between S-wave velocity and saturation of rocks. From past experiments in the literature, in the case of sandstone and shale, the S wave velocity decreases with increasing water saturation, in limestone there is no change in S wave velocity due to water saturation, and in granite, S wave with increasing water saturation. It showed a tendency to increase in speed. On the other hand, although the experimental results for the unconsolidated sedimentary layer were not obtained, it is possible that the fluctuation of the water content has an effect on the S wave velocity from the large pore space in comparison with the consolidated layer.
Therefore, in this study, the relationship between the groundwater level change and the S wave velocity distribution was investigated by repeating the measurement of the groundwater level and the micro-array tremor exploration at the same point at the same time.
2. Method
Repeated tremor exploration at fixed points was conducted nine times from June to November 2014 and once each in August 2019 and January 2020 at two points (H54, R50) located in the alluvial fan area in the upper reaches of the Ashigara Plain. A total of 11 micro-array microtremor surveys were conducted. In addition, the groundwater level was continuously measured at the shallow groundwater observation wells attached to these points.
Microtremor observation was performed using JU210 and JU410 manufactured by Hakusan Kogyo with a sampling interval of 200 Hz. The results of microtremor exploration were analyzed according to the method of Senna et. al, (2014).
3. Results and discussion
As a result of 11 microtremor surveys, the maximum exploration depth was 68.9 m at H54 and 86.9 m at R50, and the S wave velocity at maximum depth was 965 m/s at H54 and 940 m/s at R50. It showed almost the same S wave velocity. The groundwater level during tremor exploration changed from -12.73 to -15.30 m at H54 and from -7.24 to -8.41 m at R50.
The S wave velocity near the groundwater table is 370 to 880 m/s at the H54 point and 230 to 370 m/s at the R50 point. The results show that the S wave velocity differs greatly depending on the exploration time. In addition, explorations in which the S wave velocity changed up and down across the vicinity of the water table were observed in 8 out of 11 explorations at H54 and 10 out of 11 at R50.
Factors that cause the S-wave velocity at the same depth to change significantly for each microtremor exploration include errors in exploration and analysis such as installation errors of microtremors for each microtremor exploration and waveform reading errors in analysis, as well as changes in soil temperature and inadequacy. Factors such as differences in soil moisture content in the saturated zone are possible. In the future, in addition to examining these error factors, we will also examine other geological and climatic conditions.