In analysis of airborne gravity gradiometry, both the simulation that can calculate precise gravity gradient using subsurface density distribution and the inversion that can estimate subsurface density using observed data are necessary. Both the new simulation method that can treat complex density anomalies ant the new inversion method that is not based on the least-squares method were proposed in this paper.
 Most of three dimensional (3-D) gravity calculations depend on the calculation method using cuboids or rectangular prisms in previous researches. However it is slightly difficult to approximate 3-D terrains and anomalous bodies with complex figure using superposition of cuboids or prisms. For example, it is difficult for the traditional method to construct smooth inclination surface in case of an inclined dyke because an inclined dyke is often approximated by the step-like anomalous body using a lot of cuboids. The new technique to calculate gravity anomaly that is suitable for 3-D terrain and anomalous body with complex figure is proposed in this paper. This technique uses numerical integration using hexahedral element in order to calculate gravity gradiometry anomaly. As the result of numerical simulation using the new 3-D program, it is found that the boundary of density anomaly can be estimated by the distribution of gravity gradient of z-direction (g_zz). And horizontally changed density anomaly can be selectively extracted by using gravity gradient of x-direction and y-direction (g_xx and g_yy). In the simulation of airborne gravity gradiometry, it is found that the peak values of gravity gradient are influenced by the flight height. But the distribution shapes of gravity gradient are not influenced by the flight height. As the result of simulation using the inclined bodies, it is found that inclination angle might be estimated quantitatively from the profile of gravity gradient.
 In addition to the new 3-D simulation method, gravity gradiometry correlation tomography that can estimate 3-D density contrast is proposed in this paper. The correlation tomography can rapidly calculate 3-D correlation factor that reflects relative density distribution although it cannot calculate density distribution directly. As the result of correlation tomography using two density anomalies that have high and low densities, it is confirmed that positive and negative anomalies appeared around the center of high and low density anomalies respectively. The correlation tomography of gravity gradiometry will become a useful tool to analyze gravity gradiometry data as a simplified or auxiliary method for 3-D inversion.

View full abstract

 Recently, landslide due to natural disasters such as large-scale typhoons, or local torrential rain as a consequence of climate change, has become a serious problem for effective and detailed management of infrastructure safety. Particularly, slopes at risk of large-scale landslide in the future should be evaluated across the entire slopes for the behavior of groundwater infiltration from heavy rainfall. It is very important to establish monitoring techniques to visualize rainwater infiltration across entire slopes for the long term management of landslide risks. The authors developed a new electrical exploration system for remote-controlled measurement. We conducted a continuous electrical survey over a period of two years at two slope sites in order to monitor rainwater infiltration of landslide slopes. Based on resistivity changes during rainfall events of 50–100 mm, the findings indicate that rainwater rapidly infiltrated shallower sediments to depths of a few meters during rainfalls, and evaporated slowly after rainfall ceased. We also estimate that rainwater penetrated slowly into deeper layers. The sampling period was limited to 50–60 days, due to frequent methodological challenges such as the severing of cables by wild animals and rock falls, and instrument failure caused by the thunderbolts at two sites. Furthermore, we were unable to continue the measurement during heavy rainfall events of more than a few hundred millimeters. The biggest factor is that the instruments were not protected from thunderbolts during storms. It is recommended that future studies should incorporate protective circuits for thunderbolts to double or triple insulation.

View full abstract

 Electrical prospecting that investigates subsurface resistivity distribution is used in many fields. It is helpful in discriminating a stratum and a rock mass, because the range of the resistivity of rocks and soils is considerably wide and because difference in the character of rocks and soils is reflected in resistivity. In order to collect electrical data efficiently, when the survey area has a low or high resistivity, it is important to use an electrical survey system that is suitable for the region. For example, when the resistivity of the survey area is high, an electrical survey system with a high output voltage is required, because the contact resistance is generally high. On the contrary, using an electrical survey system that can send a large electric current is required for a low resistivity area, because the measured electric potential is low.
 I have designed and developed some multichannel electrical survey systems to cope with the various characteristics of survey areas and for various investigation purposes. For example, the high-voltage and small-current systems were used in resistive areas, whereas the low-voltage and large-current systems were used in conductive areas. The high-output multichannel system was applied for monitoring a deep resistivity structure. The results show that the electrical exploration data were acquired efficiently and high-precision resistivity structures were obtained using suitable systems

View full abstract

 Deep saline aquifer formations are expected to have a large potential for carbon dioxide (CO₂) storage. Host rocks of the aquifers are mostly porous sandstones. Laboratory experiments on CO₂/brine displacement flow in porous sandstones under aquifer's temperature and pressure conditions are important for better understanding the CO₂ trapping mechanism in aquifers. A high resolution X-ray CT scanner gives in situ images of CO₂ and brine distributions in porous sandstone. The images give important clues for clarifying mechanisms of CO₂ trapping in porous sandstones. In this article, we discuss mechanisms of CO₂/brine displacements revealed by employing a high-resolution medical CT scanner, and then in the succeeding article, we discuss changes of seismic velocity during the displacements. Considering the role of capillary pressure in porous sandstone and the inhomogeneity revealed in reservoir rocks by petrological and sedimentological analyses, the following points are important for the CO₂ migration and trapping in deep saline aquifers. 1. Locally-biased CO₂ flow paths appear during CO₂ injection in brine-saturated sandstone because the fine-scaled local fluctuation (about mm sizes) in pore-size distribution. 2. In simultaneous flow of CO₂ and brine, tapping and flow behaviors of CO₂ depend on the directional relationship between anisotropy in porosity distribution and flow direction. 3. The above phenomena arise from slight differences in capillary pressure accompanied by differences in pore size distributions. 4. Flow and trapping of CO₂ are governed by sizes of CO₂ clusters in pore spaces. 5. Sizes of CO₂ clusters in pores are different between the processes of CO₂ injection and brine reinjection. 6. Heterogeneity scales from pore size to geologic structure affect CO₂ flow behavior in the reservoir at CO₂ storage sites. 7. When fractures or large pore sized channels exist in a reservoir and its surrounding formations, capillary pressure has no effect on CO₂ flow.

View full abstract

[in Japanese]

View full abstract