Electromagnetic exploration in high-salinity groundwater zones: case studies from volcanic and soft sedimentary sites in coastal Japan

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.

In order to evaluate the freshwater–saltwater interface of three sample areas in the Chugoku-Shikoku region of western Japan, we conducted CSAMT surveys at locations with varying hydrogeological features, and performed laboratory tests to measure the electrical properties of rock core samples from each site.

Key words::

• coastal area, deep fluid, electromagnetic exploration, freshwater–saltwater interface, island on a continental shelf, resistivity.

Acknowledgements

For assistance with the Tokusa fieldwork, the authors extend their thanks to the students of Yamaguchi University. For assistance with the Oki fieldwork, the authors extend their thanks to Mr. Matsuda of the Ama-cho office and Mr. Mogi of Tokyo University. For assistance with the Saijo fieldwork, the authors extend their thanks to Mr. Tokumasu, Ms. Aono, Mr. Sugino and Ms. Manabe of the Saijo-shi office, Mr. Kubota of the Central Research Institute of Electric Power Industry, Dr Jyomori and Mr. Takahashi of Neo-science Co., Ltd. and Mr. Komatsubara of Yamaguchi University. The authors also thank Mr. Takahashi of the Denrhoku Calculation Center Co., Ltd. for the 2D inversion using the CSAMT data, and Mr. Taniguchi of the Ceres Co., Ltd. for the laboratory tests of rock core samples.