5:15 PM - 7:15 PM
[AHW31-P07] Case study of salinity anomalies investigation with multi geophysical exploration methods at Kujukuri coastal area
Keywords:Kujukuri Plain, Salinity-fresh water interface, TDEM, Microtremor array survey, Microgravity survey
[Purpose]
Further understandings of fresh/saline groundwater are necessary for agriculture, industries and so on. Especially seawater intrusion significant impact on building and living at coastal area. So, the identification and monitoring of fresh/saline groundwater an issue for the safeguard of these area. This study was focused on fresh/saline groundwater interface position using geophysical exploration methods. We performed several methods such as gravity, time domain electromagnetic survey (TDEM), microtremor survey in the middle-northern part of Kujukuri plain, Chiba Prefecture. Observed field data and several quick interpretations are shown below.
[Material and Methods]
Gravity, TDEM and Microtremor surveys were conducted in two parts. Gravity survey was conducted from August 23 to August 29, 2024. TDEM and Microtremor surveys were conducted from September 18 to 24, 2024. The observation points shown in Fig.1A.
Scintrex CG-6 was used in Gravity survey. Both dense microgravity survey(50 m interval) and regional gravity survey(500 m interval) were conducted. The survey area is 6 km * 0.5 km rectangle extending in a northwest-southeast direction. 75 points were measured in this area. For the acquired data, applied tidal, temperature, tilt, drift corrections, atmospheric pressure and latitude corrections using SEEQUENT software Oasis montaj.
ABEM GroundTEM10i was used in TDEM soundings. Total of 10 points were measured along the gravity survey line. All measurements were conducted in The transmitter loop size is fixed in 20 m x 20 m. The receiver loop size is also fixed in 3 m x 3 m with four turns. All measurements were conducted in central loop arrangement.
Both LE-3D Lite seismometer (Lenarts seismometer) and SmartSolo IGU-BD3C-5 seismometer (SmartSolo seismometer) were used in Microtremor survey so as to compare receiver response in same observation points. Observation points were the same location as TDEM soundings. More than 60 minutes observation were conducted with triangular and L-shaped array.
[Results and Conclusion]
In the microtremor survey, the Lennart and SmartSolo seismometers obtained roughly similar waveforms (Fig. 1(B)). In addition, amplitude spectrum on the low frequency side is prominent. This suggests that influences of sea waves were successfully observed.
In the gravity survey, the Bouguer anomaly (assumed density: 2.20 g/cm3) was estimated. Bouguer anomaly shows a low gravity anomaly at south-west part of survey area, It is almost consistent with the sedimentary basin structures that ware down towards South-West direction. (Figure 1(A)).
In the TDEM soundings, the apparent resistivity was calculated using late time decay curves by Fitterman (1989). Fig. 1(C) shows equivalent investigation depth estimated by A.I. Ammar et al. (2021). The apparent resistivity changes depending on the distance from the coast and investigation depth. Resistivity inversion is expected for further interpretation.
Finally, Three geophysical explorations were conducted for further understandings of fresh/saline groundwater interface. Integrated interpretation of density (gravity) ,resistivity(TDEM), and Share wave speed (microtremor) is expected in future study. Contribution for complex fresh/saline groundwater flow mechanics is also expected.
[References]
Mitsuhata Y., Uchida T., Matsuo K., Marui A., Kusunose K. (2006): Various-scale electromagnetic investigations of high-salinity zones in a coastal plain, Society of Exploration Geophysicist, Geophysics VOL71, NO6
Ammar, A. I., Gomaa, M., Kamal, K., A. (2021): Applying of SP, DC-Resistivity, DC-TDIP and TDEM soundings in high saline coastal aquifer, Heliyon, Volume7, Issue7.
Further understandings of fresh/saline groundwater are necessary for agriculture, industries and so on. Especially seawater intrusion significant impact on building and living at coastal area. So, the identification and monitoring of fresh/saline groundwater an issue for the safeguard of these area. This study was focused on fresh/saline groundwater interface position using geophysical exploration methods. We performed several methods such as gravity, time domain electromagnetic survey (TDEM), microtremor survey in the middle-northern part of Kujukuri plain, Chiba Prefecture. Observed field data and several quick interpretations are shown below.
[Material and Methods]
Gravity, TDEM and Microtremor surveys were conducted in two parts. Gravity survey was conducted from August 23 to August 29, 2024. TDEM and Microtremor surveys were conducted from September 18 to 24, 2024. The observation points shown in Fig.1A.
Scintrex CG-6 was used in Gravity survey. Both dense microgravity survey(50 m interval) and regional gravity survey(500 m interval) were conducted. The survey area is 6 km * 0.5 km rectangle extending in a northwest-southeast direction. 75 points were measured in this area. For the acquired data, applied tidal, temperature, tilt, drift corrections, atmospheric pressure and latitude corrections using SEEQUENT software Oasis montaj.
ABEM GroundTEM10i was used in TDEM soundings. Total of 10 points were measured along the gravity survey line. All measurements were conducted in The transmitter loop size is fixed in 20 m x 20 m. The receiver loop size is also fixed in 3 m x 3 m with four turns. All measurements were conducted in central loop arrangement.
Both LE-3D Lite seismometer (Lenarts seismometer) and SmartSolo IGU-BD3C-5 seismometer (SmartSolo seismometer) were used in Microtremor survey so as to compare receiver response in same observation points. Observation points were the same location as TDEM soundings. More than 60 minutes observation were conducted with triangular and L-shaped array.
[Results and Conclusion]
In the microtremor survey, the Lennart and SmartSolo seismometers obtained roughly similar waveforms (Fig. 1(B)). In addition, amplitude spectrum on the low frequency side is prominent. This suggests that influences of sea waves were successfully observed.
In the gravity survey, the Bouguer anomaly (assumed density: 2.20 g/cm3) was estimated. Bouguer anomaly shows a low gravity anomaly at south-west part of survey area, It is almost consistent with the sedimentary basin structures that ware down towards South-West direction. (Figure 1(A)).
In the TDEM soundings, the apparent resistivity was calculated using late time decay curves by Fitterman (1989). Fig. 1(C) shows equivalent investigation depth estimated by A.I. Ammar et al. (2021). The apparent resistivity changes depending on the distance from the coast and investigation depth. Resistivity inversion is expected for further interpretation.
Finally, Three geophysical explorations were conducted for further understandings of fresh/saline groundwater interface. Integrated interpretation of density (gravity) ,resistivity(TDEM), and Share wave speed (microtremor) is expected in future study. Contribution for complex fresh/saline groundwater flow mechanics is also expected.
[References]
Mitsuhata Y., Uchida T., Matsuo K., Marui A., Kusunose K. (2006): Various-scale electromagnetic investigations of high-salinity zones in a coastal plain, Society of Exploration Geophysicist, Geophysics VOL71, NO6
Ammar, A. I., Gomaa, M., Kamal, K., A. (2021): Applying of SP, DC-Resistivity, DC-TDIP and TDEM soundings in high saline coastal aquifer, Heliyon, Volume7, Issue7.