Electromagnetic exploration in high-salinity groundwater zones: case studies from volcanic and soft sedimentary sites in coastal Japan
Koichi Suzuki 1 4 Yukiko Kusano 2 Ryota Ochi 3 Nariaki Nishiyama 3 Tomochika Tokunaga 2 Kazuhiro Tanaka 31 Central Research Institute of Electric Power Industry, 1646, Abiko, Abiko-shi, Chiba 270–1194, Japan.
2 Department of Environment Systems, The University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa-shi, Chiba 277–8563, Japan.
3 Department of Geosphere Sciences, Facility of Science, Yamaguchi University, 1677-1 Yoshida Yamaguchi-shi, Yamaguchi 753–8512, Japan.
4 Corresponding author. Email: k-suzuki@criepi.denken.or.jp
Exploration Geophysics 48(2) 95-109 https://doi.org/10.1071/EG15121
Submitted: 27 November 2015 Accepted: 30 November 2015 Published: 20 January 2016
Abstract
Estimating the spatial distribution of groundwater salinity in coastal plain regions is becoming increasingly important for site characterisation and the prediction of hydrogeological environmental conditions resulting from radioactive waste disposal and underground CO2 storage. In previous studies of the freshwater–saltwater interface, electromagnetic methods were used for sites characterised by unconsolidated deposits or Neocene soft sedimentary rocks. However, investigating the freshwater–saltwater interface in hard rock sites (e.g. igneous areas) is more complex, with the permeability of the rocks greatly influenced by fractures. In this study, we investigated the distribution of high-salinity groundwater at two volcanic rock sites and one sedimentary rock site, each characterised by different hydrogeological features. Our investigations included (1) applying the controlled source audio-frequency magnetotelluric (CSAMT) method and (2) conducting laboratory tests to measure the electrical properties of rock core samples. We interpreted the 2D resistivity sections by referring to previous data on geology and geochemistry of groundwater. At the Tokusa site, an area of inland volcanic rocks, low resistivity zones were detected along a fault running through volcanic rocks and shallow sediments. The results suggest that fluids rise through the Tokusa-Jifuku Fault to penetrate shallow sediments in a direction parallel to the river, and some fluids are diluted by rainwater. At the Oki site, a volcanic island on a continental shelf, four resistivity zones (in upward succession: low, high, low and high) were detected. The results suggest that these four zones were formed during a transgression–regression cycle caused by the last glacial period. At the Saijo site, located on a coastal plain composed of thick sediments, we observed a deep low resistivity zone, indicative of fossil seawater remnant from a transgression after the last glacial period. The current coastal plain formed in historical times, following which fresh water penetrated the upper parts of the fossil seawater zone to form a freshwater aquifer ~200 m in thickness.
Key words: coastal area, deep fluid, electromagnetic exploration, freshwater–saltwater interface, island on a continental shelf, resistivity.
References
Chiba, T., Kaneko, N., and Shikano, K., 2000, Geology of the Urago district: a map on a scale of 1 : 50,000: Geological Survey of Japan Open-File Report, 74 pp. [in Japanese].Goto, H., and Nakata, T., 1998, Discovery of active fault traces based on dip-slip distribution pattern of strike-slip faults: a case study on Kawakami and Okamura faults of the Median Tectonic Line active fault system in Shikoku: Active Fault Research, 17, 132–140
Ishida, S., Tsuchihara, T., Yoshimoto, S., Minakawa, H., Masumoto, T., and Imaizumi, M., 2011, Estimate of the amount of freshwater lens in Tarama Island, Japan: Irrigation, Drainage and Rural Engineering Journal, 79, 7–18
Ishida, S., Yoshimoto, S., Kohayashi, S., Koda, K., and Nakazato, H., 2013, Application of geophysical exploration for investigation of freshwater lens in small island: Jiban Kogakkai Ronbun Hokokushu, 61, 32–35
Katsube, T. J., and Hume, J. P., 1989, Electrical resistivities of rocks from Chalk River, in M. D. Thomas, and D. F. Dixon, eds., Proceedings of a workshop on geophysical and related geoscientific research at Chalk River, Ontario: Atomic Energy of Canada Limited, Report Vol. 9085, no. 9, 105–114.
Kusano, Y., Tokunaga, T., Asai, K., Takahashi, A. H., Morikawa, N., and Yasuhara, M., 2014, Occurrence of old groundwater in a volcanic island on a continental shelf: an example from Nakano-shima Island, Oki-Dozen, Japan: Journal of Hydrology, 511, 295–309
| Occurrence of old groundwater in a volcanic island on a continental shelf: an example from Nakano-shima Island, Oki-Dozen, Japan:Crossref | GoogleScholarGoogle Scholar | 1:CAS:528:DC%2BC2cXls1Wqsro%3D&md5=1905ba9229a4c595757964e617d9faa2CAS |
Mitsuhata, Y., 2006, Electromagnetic investigation of high-salinity groundwater zones in coastal plains: a case study of the Kujukuri coastal plain: The Journal of Geography, 115, 416–424
| Electromagnetic investigation of high-salinity groundwater zones in coastal plains: a case study of the Kujukuri coastal plain:Crossref | GoogleScholarGoogle Scholar | 1:CAS:528:DC%2BD28XhtlelsrvN&md5=294e2f4ac7f03f4419c03aec16128a61CAS |
Morioka, T., Sagawa, A., Kanaori, Y., and Tanaka, T., 2007, Characteristics and activity of a southwestern part of the Tokusa-Jifuku Fault and the northeastmost part of the western Mt. Kido Fault in Central Yamaguchi Prefecture, southwest Japan: Journal of the Japan Society of Engineering Geology, 48, 35–47
| Characteristics and activity of a southwestern part of the Tokusa-Jifuku Fault and the northeastmost part of the western Mt. Kido Fault in Central Yamaguchi Prefecture, southwest Japan:Crossref | GoogleScholarGoogle Scholar |
Murakami, H., 2010, Geochemical studies on the groundwater in southwest Shimane Prefecture and northeast Yamaguchi Prefecture, southwest Japan: Master Thesis of Graduate School of Science and Engineering, Yamaguchi University [in Japanese].
Nishiyama, N., Tanaka, K., and Suzuki, K., 2014, Visualization of deep-seated fluid flow in Tokusa Basin, Yamaguchi Prefecture: Japan Geoscience Union Meeting 2014, SCG62-07.
Nowroozi, A. A., Stephen, A. B., and Henderson, P., 1999, Salt water intrusion into the freshwater aquifer in the eastern shore of Virginia: a reconnaissance electrical resistivity survey: Journal of Applied Geophysics, 42, 1–22
| Salt water intrusion into the freshwater aquifer in the eastern shore of Virginia: a reconnaissance electrical resistivity survey:Crossref | GoogleScholarGoogle Scholar |
Ochi, R., Tanaka, K., Suzuki, K., and Tokumasu, M., 2013, Study of geological structure and groundwater flow of the Saijo-plain: Proceedings of JSEG Conference, pp. 151–152 [in Japanese].
Sagawa, A., Aiyama, K., Kanaori, Y., and Tanaka, T., 2008, Characteristics and activity of the Sakota-Ikumo and the Tokusa-Jifuku Faults in Central-Eastern Yamaguchi Prefecture, Westernmost Honshu Island of Japan: Journal of the Japan Society of Engineering Geology, 49, 78–93
| Characteristics and activity of the Sakota-Ikumo and the Tokusa-Jifuku Faults in Central-Eastern Yamaguchi Prefecture, Westernmost Honshu Island of Japan:Crossref | GoogleScholarGoogle Scholar |
Sakai, H., and Ohki, Y., 1978, Hot spring in Japan: Science, 48, 41–52
| 1:CAS:528:DyaE1cXksVKgsrs%3D&md5=bf97d7ab4d223aff25daddf75b76a0e1CAS |
Stewart, M. T., 1999, Geophysical investigations, in J. Bear, A. H.-D. Cheng, S. Sorek, D. Ouazar, and I. Herrera, eds., Seawater intrusion in coastal aquifers: Kluwer Academic Publishers, 9–50.
Suzuki, K., 2003, Study on resistivity of crystalline and sedimentary samples: the influence of pore fluid resistivity and bulk surface conduction on rock resistivity: Butsuri Tansa, 56, 107–116
Suzuki, K., Tokuyasu, S., and Tanaka, K., 2009, Underground structure of mud volcanoes in Tokamachi City, Niigata Prefecture, determined by electromagnetic exploration, and geographical and geological surveys: The Journal of Geography, 118, 373–389
| Underground structure of mud volcanoes in Tokamachi City, Niigata Prefecture, determined by electromagnetic exploration, and geographical and geological surveys:Crossref | GoogleScholarGoogle Scholar | 1:CAS:528:DC%2BC3cXhtVOls77J&md5=a038a4a0a6e121745af0c797390720ceCAS |
Suzuki, K., Nishiyama, N., and Tanaka, K., 2014a, Investigation of geological structure and salinity of groundwater along the Tokusa-Jifuku Fault using the CSAMT survey: Proceedings of the 130th SEGJ Conference, 145–148 [in Japanese].
Suzuki, K., Kusano, Y., and Tokunaga, T., 2014b, Investigation of geological structure and salinity of groundwater at the Basaltic Island using the CSAMT survey: Proceedings of the 130th SEGJ Conference, 155–158 [in Japanese].
Suzuki, K., Ochi, R., Tanaka, K., Tokumasu, M., Kubota, K., and Jyomori, A., 2014c, Investigation of salinity of groundwater at the alluvial plane near coastal area in Saijo-shi using the CSAMT survey (2): Proceedings of the 130th SEGJ Conference, 149–152 [in Japanese].
Tsukimori, K., 1984, Hydrogeology of Nakano-shima, Dozen, Oki Islands: Geological report of Shimane University, 3, 161–165 [in Japanese].
Uchida, T., and Ogawa, Y., 1993, Development of Fortran code for two dimensional magneto-telluric inversion with smoothness constraint: Geological Survey of Japan Open-File Report, 205 pp.
Ueda, T., Mitsuhata, Y., Uchida, T., Mrui, A., and Ohsawa, K., 2014, A new marine magnetotelluric measurement system in a shallow-water environment for hydrogeological study: Journal of Applied Geophysics, 100, 23–31
| A new marine magnetotelluric measurement system in a shallow-water environment for hydrogeological study:Crossref | GoogleScholarGoogle Scholar |
Van Dam, J. C., and Meulenkamp, J. J., 1967, Some results of the geo-electrical resistivity method in ground water investigation in the Netherlands: Geophysical Prospecting, 15, 92–115
| Some results of the geo-electrical resistivity method in ground water investigation in the Netherlands:Crossref | GoogleScholarGoogle Scholar |
Yamashita, M., and Hallof, P. G., 1985, CSAMT case histories with a multi-channel CSAMT system and discussion of near-field data correction: 55th Annual International Meeting, SEG, Expanded Abstracts, 276–278.