Hiroomi Nakazato

Cross-hole georadar monitoring for moisture distribution and migration in soil beneath an infiltration pit: a case study of an artificial groundwater recharge test in Niigata, Japan

Continuous monitoring by time-lapse and repetitive measurements using cross-hole geo-radar was conducted to investigate soil moisture distribution and migration beneath infiltration pit for artificial groundwater recharge. This monitoring enabled us to clarify the infiltration process from the infiltration pit into the vadose zone in a quantitative, nondestructive, and noninvasive way. The infiltration pit was 2.0 x 2.0 m square and 2.3 m deep between 2 boreholes in gravel soil. The groundwater table was at about —10 m. We monitored the veitical distribution of electromagnetic wave traveltime beneath the infiltration pit by repetitive measurements using cross-hole geo-radar profiling with zero-offset gathering. Traveltime was distinctly retarded from the upper layer to the deeper one after ponding of the pit. The downward retardation velocity of the infiltration rate into the soil was estimated at8 x 10-2cm/s. The estimated values for water content and water seepage rate in the soil were almost coincident with the directly measured values. In our case of an infiltration pit test, cross-hole geo-radar monitoring was an efficient, noninvasive method for visualizing the infiltration process and estimating water migration properties of the soil on a macro scale.

Three-dimensional inversion of in-line resistivity data for monitoring a groundwater recharge experiment in a pyroclastic plateau

An artificial groundwater recharge experiment was conducted in a pyroclastic plateau in Kagoshima Prefecture in Japan, and time-lapse electrical resistivity data were collected to monitor the recharge process. In the experiment, time-efficient in-line resistivity surveys were performed along four intersecting lines, because a large amount of water was released from two recharge areas and a relatively fast migration of water into the vadose zone was expected. The migration of recharged water may be estimated from changes in electrical resistivity because resistivity in the vadose zone is largely controlled by water saturation variations there. The geological setting at the experiment site was interpreted from the resistivity distribution inverted from the in-line survey data, which were obtained before the recharge experiment. The resistivity distribution showed an approximately layered structure, which could be correlated with four borehole logs in the area. Three-dimensional (3D) distributions of the resistivity change ratio were derived through constrained nonlinear ratio inversion. Three-dimensional inversion of the in-line resistivity data was more suitable than two-dimensional inversion to describe the 3D phenomena associated with groundwater recharge. During the recharge experiment, the zones of decreased resistivity shifted with time, indicating non-uniform penetration of water from the recharge areas into the ground and a horizontal flow of the recharged water, especially in the secondary Shirasu layer, which comprises lacustrine or marine sediments of pyroclastic origin. These interpretations agree with the variation in water content observed in a borehole. An artificial groundwater recharge experiment was conducted in a pyroclastic plateau in Kagoshima Prefecture in Japan, and time-efficient in-line resistivity surveys were performed along four intersecting lines. The zones of decreased resistivity shifted with time, indicating non-uniform penetration of water from the recharge areas and a horizontal flow of the recharged water.

Investigation of the line arrangement of 2D resistivity surveys for 3D inversion

We have conducted numerical and field experiments to investigate the applicability of electrode configurations and line layouts commonly used for two-dimensional (2D) resistivity surveys to 3D inversion. We examined three kinds of electrode configurations and two types of line arrangements, for 16 resistivity models of a conductive body in a homogeneous half-space. The results of the numerical experiment revealed that the parallel-line arrangement was effective in identifying the approximate location of the conductive body. The orthogonal-line arrangement was optimal for identifying a target body near the line intersection. As a result, we propose that parallel lines are useful to highlight areas of particular interest where further detailed work with an intersecting line could be carried out. In the field experiment, 2D resistivity data were measured on a loam layer with a backfilled pit. The reconstructed resistivity image derived from parallel-line data showed a low-resistivity portion near the backfilled pit. When an orthogonal line was added to the parallel lines, the newly estimated location of the backfilled pit coincided well with the actual location. In a further field application, we collected several 2D resistivity datasets in the Nojima Fault area in Awaji Island. The 3D inversion of these datasets provided a resistivity distribution corresponding to the geological structure. In particular, the Nojima Fault was imaged as the western boundary of a low-resistivity belt, from only two orthogonal lines. We have conducted numerical and field experiments to investigate the applicability of electrode configurations and line layouts commonly used for 2D resistivity surveys to 3D inversion. We propose that parallel lines are useful to highlight areas of particular interest where further detailed work with an intersecting line could be carried out.

A Case Study of Behavior Observation of Landslide Induced by Snowmelt After an Earthquake

The landslide movement was recognized on April 19th, 2011 in Shimizu, Tokamachi city after the earthquake of M6.7 with the epicenter in Sakae village, Nagano Prefecture, occurred on March 12th, 2011. This landslide was probably induced by the snowmelt after the earthquake. This landslide is a reactive landslide, and the bedrock of this landslide is the Pliocene mudstone. The size of this landslide block is 100 m in width, 500 m in length, and 20 m in depth. Only this landslide greatly acted though a lot of landslide blocks were distributed around this landslide. To estimate the risk for the construction work of disaster recovery, the displacement monitoring using GPS and the groundwater level observation were executed in advance. The displacements were only detected when it rained heavily. The advanced landslide monitoring system performed in this case was effective to investigate for the disaster recovery safely because participants can share remotely landslide displacement data on semi-real time.

Inversion of airborne EM data accounting for terrain and inaccurate flight height

Summary When airborne electromagnetic (AEM) surveys are carried out over mountainous areas, not only are magnetic data affected by terrain, but altimeter data are inaccurate. In this case, the resistivity models obtained from inversion are misleading unless both effects are properly accounted for. We present an inversion algorithm that simultaneously solves for resistivity and flight height. The method can incorporate topography into the model. As starting estimates of the height, we employ the apparent height determined from magnetic data at the highest frequency. Constraints are imposed both on the smoothness of the flight path and the closeness to the starting estimates of the height, in addition to those acting on the model. The synthetic example shows that when topography is incorporated, the result of simultaneous inversion is very close to that obtained from the inversion using the correct height. It also indicates that when topography is not incorporated, simultaneous inversion is less affected by topographic effects than the inversion that uses the correct height. This is bec ause some portions of topographic effects are compensated for by the flight height. Application of our inversion to a field data set over a landslide area produces a 2-D model that is consistent with the result of inversion of ground dc resistivity data.

Estimating high hydraulic conductivity locations through a 3D simulation of water flow in soil and a resistivity survey

In this study, we propose a method to estimate high hydraulic conductivity locations that uses 3D simulation of soil water flow and in-line resistivity survey data acquired during a groundwater recharge experiment, and we apply this method to numerical and field experiments. The high hydraulic conductivity locations are estimated from a combination of field-observed and simulated apparent resistivities using the following simple steps. (1) Assuming that high hydraulic conductivity zones exist in the first layer, simulations of saturated-unsaturated seepage are conducted for several possible water-flow models that have high hydraulic conductivity zones in different locations. (2) The simulated volumetric water contents are converted into bulk resistivities, which are used to produce apparent resistivity data through simulation of a resistivity survey. (3) The differences between the simulated apparent resistivities and the field-observed data are examined, and the best-fit hydraulic conductivity model is identified by minimising the above differences. In the numerical experiment, 3D inversion of the simulated resistivity survey provides an image of the preferential flow, although the infiltration locations are unclear. Comparing the field model with the possible models, the high hydraulic conductivity location in the field model corresponds to the high hydraulic conductivity location in the possible model with the minimum errors. In the field, an in-line resistivity survey was conducted during a groundwater recharge experiment on a pyroclastic plateau. The 3D inversion of the in-line resistivity survey data provides an image of the preferential flow. Comparing the field apparent resistivity data with the simulated apparent resistivity data, the high hydraulic conductivity location of the possible model that provides the minimum error corresponds to the recharge water range, whereas the hydraulic conductivity location of the possible model that gives the maximum errors corresponds to ranges with no recharge water. These results indicate that it is possible to estimate high hydraulic conductivity locations using 3D simulations of the soil water flow and a resistivity survey. We propose a simple method for estimating high hydraulic conductivity locations. The proposed method uses the 3D simulations of soil water flow and resistivity survey during a groundwater recharge experiment. Results of numerical and field experiments indicate that the proposed method estimates the high hydraulic conductivity locations more precisely compared with 3D inversion of in-line data.