In relation to the Great East Japan Earthquake (the 2011 off the Pacific coast of Tohoku Earthquake), a number of abnormal phenomena, including rumbling of the ground, groundwater eruption and ground subsidence, occurred in areas of abandoned coal mines in Iwaki City, Fukushima Prefecture, in March 2011. The Society of Exploration Geophysicists of Japan (SEGJ) was consulted by the Iwaki City Office about professional knowledge and advices on these geological hazards. After the discussion in the Board of Directors, SEGJ decided to carry out emergency geophysical surveys in Iwaki City. The target area of the surveys was first limited in the disaster areas in abandoned coal mines. However, at the beginning of the field work, a large earthquake (after shock), with a magnitude of 7.0, hit Iwaki City on April 11, 2011. The survey targets were expanded to include the active faults and the stricken areas by the after shock.
 In this paper, assuming that our society will conduct emergency geophysical surveys at an occurrence of large-scale natural disasters in the future, we reviewed the activities and results of the emergency surveys conducted in Iwaki City, including how SEGJ decided the execution of the emergency surveys and how the survey contents were modified as the situation of the survey areas changed. In addition, in order to examine the effectiveness of the emergency surveys with respect to the needs of the City Office and the local people who live in the disaster areas, we conducted a self evaluation on successful results and unsolved problems of the surveys, by applying the five items of business evaluation defined by the Development Assistance Committee of the Organization for Economic Co-operation and Development (OECD). We also pointed out that it is essential to conduct such evaluation at each of the emergency surveys that our society will conduct in the future.

View full abstract

 In landfill management projects, such as stabilization and management of groundwater pollution caused by leachate, it is very important to visualize permeating water paths non-destructively. In addition, incinerator ash containing cesium at levels below 8,000 Bq/kg has been placed in controlled landfill sites since the 2011 Fukushima nuclear accident. These sites have been built so that rainwater can easily permeate through the fill to accelerate stabilization. On the other hand, cesium is very soluble in water. Therefore, it is also necessary to clarify the water paths in the fills to prevent cesium from escaping into the environment. However, landfills are composed of various types of waste that are heterogeneously distributed, so a suitable visualization technique has not yet been proposed. We propose that changes in leachate content and quality might be reflected in the resistivity of fills. Therefore, we measured resistivity accompanying leachate level changes both in a laboratory setting and in field resistivity prospecting. In the laboratory experiment, a simulated landfill layer consisting mainly of incinerator ash was prepared in a rectangular tank. The resistivity prospecting methods were applied at high and low leachate levels. After measurement, a number of undisturbed core samples were taken from the fills based on resistivity profiles. We measured the hydraulic conductivity and main chemical components for each sample to elucidate parameters that affected resistivity. In the field experiment, resistivity was measured before and after forced drainage in an industrial landfill site to check the results of the laboratory test. The leachate table was not deeper than 1 m. At first, the resistivity at this level was measured along 4 prospecting lines, and drainage work by pumping up leachate from a collecting pit was performed for about 30 days. After that, resistivity along the same lines was measured, three-dimensional inverse analysis was applied to the apparent resistivity data, and 3-D profiles were generated. Differential profiles between areas with high and low leachate levels were also calculated. The results showed that it is difficult to identify a permeable zone using a simple resistivity profile. However the water path in the fills can be visualized. Because the resistivity's rate of change in the leachate table levels is closely related to the permeability of the fills in the laboratory experiment and to the conductivity of the leachate in the field experiment. Furthermore, it became clear that water permeated almost vertically in fills toward the filtration layer with the drainage pipes.

View full abstract

 There are many activities of post-disaster rehabilitation that can become more efficient by subsurface surveys using ground-penetrating radar (GPR). In the present paper, case studies of GPR survey for various activities, such as archeological surveys, near-surface exploration and search for lost articles by Tsunami, are shown. For archaeological surveys and bear-surface exploration, GPR survey can quickly and easily detect and locate object to be investigated, and it makes the following investigations with excavation more efficient. This is illustrated with the examples of archaeological surveys in Nobiru-Chikkou site (Higashimatsushima-town, Miyagi) and Kassenhara tomb cluster (Yamamoto-town, Miyagi) and a near-surface exploration in Iwaki-city, Fukushima. For the search operation of missing people and lost articles, it is demonstrated that GPR can contribute by detecting and locating buried object. Further, GPR systems used for these activities are presented.

View full abstract

  In the first article, we described flow properties and flow mechanisms of CO₂ in porous sandstones in most of the potential reservoirs. The flow and trapping of CO₂ in porous sandstones are primarily controlled by capillary pressure and the different scales of inhomogeneity in geomaterials: from pore networks to geological structures. Following the view points in the first article, we present our experimental investigation on the relationship between P-wave velocity and CO₂ saturation in a porous sandstone. We conducted a simultaneous measurement of P-wave velocity and mapping of CO₂/brine saturation in a rock sample by employing a medical X-ray CT scanner and a pressure vessel that was designed for velocity measurements during fluid injection into a rock sample. The P-wave velocity and CO₂/brine saturation relationship differs between CO₂ drainage and brine imbibition, suggesting that CO₂ saturation in rock cannot be uniquely determined by P-wave velocity. We tried to interpret the non-unique relationship by applying a continuous random patchy saturation model, which is essentially a combined model of poroelasticity and the theory of seismic wave scattering in random inhomogeneous media. To understand the model, we first present the basic relationships for describing elasticity in fluid-saturated porous media. Then we present an example which is interpreted by employing the random patchy saturation model. In the last, we discuss applications of the relationships for CO₂ storage reservoirs.
 Followings are the points of this article: 1. Clusters of CO₂ exist in pores and form P-wave velocity inhomogeneity in brine saturated rock. 2. The relationship between P-wave velocity and CO₂ saturation depends on the sizes of CO₂ clusters in pores. 3. The scattered Biot slow wave (the second-kind wave) appears due to the velocity inhomogeneity, which is responsible for velocity changes in CO₂/brine saturated porous rock. 4. The relationship between P-wave velocity and CO₂ saturation in pore depends on the relationship between characteristic cluster size and the wave number of P wave. 5. When considering the relationship between CO₂ saturation and P-wave velocity in a reservoir, all scales of inhomogeneity, from pore size to geologic scales, should be considered because of the regional bias of the CO₂ saturation.

View full abstract

[in Japanese]

View full abstract