This paper reviews methods of measuring soil volumetric water contents and soil hydraulic properties, such as unsaturated hydraulic conductivity and water retention curve, in the vadose zone with non-invasive surface ground penetrating radar (GPR). While saturated hydraulic conductivities were estimated in a number of studies from GPR measurements, there are only a limited number of studies performed to determine soil hydraulic properties such as water retention curves using GPR measurements. In this paper, we mainly reviewed stepped-frequency-continuous-wave radar which can estimate soil hydraulic properties. Though its importance has been recognized for long time, no study has attempted to fully characterize the spatial distribution of soil hydraulic properties from GPR measurements. Since obtaining spatial distribution of soil hydraulic properties is an inter-disciplinary topic, we need to build a multi-disciplinary cooperative system for effective synergies among geophysicists, soil physicists, hydrologists, and geostatisticians to address current issues.

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  Time-lapse cross-borehole measurements using ground penetrating radar (GPR) were conducted to investigate changes in soil moisture distributions beneath an infiltration pit for artificial groundwater recharge through a vadose zone of sand gravel soil in Nagaoka city, Japan. We measured electromagnetic wave velocity with the geometry that transmitter and receiver antennas set at the same depth, and estimated changes in the water content profile using Toppfs equation. The velocity of wetting front and the increment of soil water were then estimated. We then estimated soil water content and found it agrees well with the values obtained by a series of direct measurements using core samples. The estimated seepage rate almost coincide with observed one. The estimated soil water content profile shows that the soil seems to be not uniform. It was expected that the fine and less permeable upper layer controls the infiltration rate as a hydraulic resistive layer. As a result, the increase in the water content in the lower layer was limited to the relatively low level. The cross-borehole radar monitoring was an efficient method for quantitative estimation of the recharge process in the vadose zone.

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Hotta Stockade Site is a wooden palisade site. The excavation clarified that the river flowed in this site. This river trace has the depth of about 30cm near the surface, and this river trace is the channel of the last stage. In addition, the sandy silt is in the depth of about 2m from the surface, and the gravel layer exists below it. This gravel layer recorded a large flood event. The purpose of this study is to clarify the old river trace and riverbed expected by the sandy silt and gravel layer by the technique of electrical exploration method.
The horizontal and vertical electrical prospecting is applied to the place where the underground structures are defined by the excavation, and features of the resistivity change are grasped. By this preliminary research, 30 Ωm layer obtained by vertical electrical prospecting corresponds to the clay layer or the blue gray silt layer. The part in which the upper surface in clay layer rapidly deepened from the 0.5-0.7 meter was regarded as a boundary in the riverbed. The position of the beginning of the sandy silt and the clay layer was illustrated by the horizontal electrical prospecting with the electrode spacing 3 to 5 meters. The similar examination was carried out in north-south and east-west direction in a survey area, and estimated the distributional configurations of the riverbed.
The river of the last stage can be expected over the upper sandy silt in natural ground clay layer. Since the material of the clay is included, its resistivity shows lower value. In the horizontal electrical prospecting of electrical exploration, the river trace of the last stage could be probed, if electrode interval was set to about 1m. The candidate of the river of the last stage is the part which shows higher resistivity compared to the clay layer in the base rock, and shows lower resistivity than the circumference. The part in which the resistivity was lower than 60 Ohm-m in the range of 1m depth from the surface was determined to be a channel in the last stage. Southern and northern boundaries in estimated riverbed and channels in the last stage agreed well with the excavation. The result of the hand boring was also concordant with these results.

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Electrical resistivity measurements crossing a shoreline have been tested in the Kujukuri coastal area in order to estimate geological and hydrogeological structure related to a freshwater discharge and a seawater intrusion. We laid down a multi-core cable equipped with spring-type electrodes for the seafloor measurements and set up a land-seafloor continuous measurement line. The high density pole-pole array was adopted for the data acquisition. The 2-D inversion of the obtained data revealed a resistivity structure transecting the coast at depths up to 40m. The resistivities under the seafloor increase gradually with the depth, and some heterogeneous resistive zones are recognized. They imply a gradual decrease of porosity in the vertical direction and the existence of discharge zones of freshwater, respectively. The resitivities under the ground decrease gradually with the depth, which implies the existence of shallow freshwater aquifer and deep saline water aquifer. It is deemed that the saline water consists of intrusive seawater and trapped fossil seawater. The high density resistivity measurements crossing a shoreline is effective for the estimation of geological and hydrogeological structure in a coastal area. The spatial resolution must be improved by arranging a denser electrode array.

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Resistivity monitoring by two-dimensional resistivity survey was conducted to observe the behavior of the high electric conductivity groundwater in the Ryukyu Limestone distribution region. The data was measured by pole-pole electrode array, 36 scans with about two hours each. The apparent resistivity cross-section diagrams do not show much change between different scans. However, after noisy data were eliminated, the average change rate in apparent resistivity with time shows a well-regulated change in a cycle of about 24 and 12 hours varying within a range of 7%. These changes corresponded to the opposite phase of the pattern of diurnal inequality in the prediction record of tide at the near site. Further time-lapse tomography analysis reveals that the resistivity changing parts are mainly around the groundwater level, which could be explained as the effects of the tidal fluctuation. This work demonstrates that the resistivity monitoring is effective for understanding the behavior of groundwater salinization in the likewise islands and coast regions.

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The Omaehama tideland is formed around the mouth of the Shukugawa River draining into the Osaka bay, in Nishinomiya City. It appears at low tide and forms about 150m long fan-shaped tideland. VLF-MT measurements have been carried out along a line in the tideland to reveal behavior of shallow groundwater, which have been repeated eight times: Nov. 2005, Jan. 2006, Apr. 2006, May 2006, Aug. 2006, Nov. 2007, Feb. 2007, and Apr. 2007. By comparing the observed apparent resistivity and phase profiles, different features are recognized. It is deemed that they can be accounted for fresh and salt groundwater distributions which may be related to the tidal sea level change and the precipitation in the hinterland.

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Recently, resistivity method is frequently applied to monitoring of groundwater flow at various fields, for example, change of groundwater level by heavy rainfall at landslide-slope areas, behavior of rainfall penetrating into layers buried by industry rejection, behavior of water penetrating to layers at cultivation tests of groundwater, movement of boundary between fresh water and seawater at the seacoast. However, in order to monitoring fast flow of groundwater in soft-sediments, it is difficult to measure a large number of data sets by 3D resitivtiy configurations within a few minutes using conventional survey systems An effective 3D data acquisition system for electrical resistivity survey has been developed, in which a new sampling method has been adopted, enabling 240 stations set to be measured simultaneously using analogue circuit of Fourier transformation and high frequency sine wave (max. 5kHz). For example, it takes only 5 minutes to complete to measure all potential data sets (120×119=14,280) in case of 120 stations by pole-pole array. In order to confirm performance of the system, the monitoring test was carried out by injecting saline into cavity, which was positioned at the center of survey lines in our office ground. The 60 electrodes were placed along two lines (each line has 30 electrodes) and electrode interval is 1m. The cavity is estimated to occur based on old tunnel excavated in loam layer (late Pleistocene). The continuous survey was started a few minutes before injection of saline and finished 20 minutes after injection. The interval of the measurements was 2.5 minutes. The CaCl₂ solution (1000ℓ, 1.4Ωm) was injected within 110 minutes. The total number of 51 data sets was measured and one data set contains 3,540 potential data. The 51 resistivity sections were analyzed by 2D inversion and the 50 resistivity change rate sections versus the resistivity section before injection were calculated. As a result, we can monitor behavior of saline penetrating into the cavity as continuous resitivity change sections. It is expected that the new data acquisition system contribute to monitor fast flow of groundwater as 3D resistivity imaging.

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