Kyosuke Onishi

Earthquake fault of the 26 May 2006 Yogyakarta earthquake observed by SAR interferometry

We analyzed synthetic aperture radar interferometry (InSAR) to reveal surface deformation associated with the 26 May 2006 Yogyakarta earthquake, for which the fault location and geometry have not been clearly determined. Our results demonstrate that surface deformation occurred ∼10 km east of the Opak River fault thought to be the source of the May 2006 event and that the probable causative fault delineated in this study is consistent with aftershock epicenters determined by a temporary seismic network. The trace of the causative fault bends at its southern termination toward the Opak River fault as if it were a splay. Our data demonstrate that another probable slip plane extends across Yogyakarta and that the heavily damaged areas covered by young volcanic deposits may have undergone subsidence during the earthquake.

Application Seismic Interferometry to Natural Earthquakes Measured By Small-scale Array

Seismic interferometry synthesizes the Green’s function between two receivers by calculating cross-correlation of transmitted responses measured at their locations. This Green’s function means the wavefield propagating from one receiver to the other receiver. By changing the crosscorrelation pairs, pseudo shot-gather records can be synthesized at arbitrary receiver locations. General relations between the reflection and transmission response were studied by Claerbout (1968) for the one-dimensional case and by Wapenaar (2003) for the three-dimensional case.

Applying differential analysis to cross-well seismic survey for monitoring CO2 sequestration

Summary Capturing CO2 directly from large stationary sources such as thermal power plants and subsequently storing it in a nearby aquifer can be the most efficient way to reduce CO2 emissions into the atmosphere at a lower cost, and monitoring injected CO2 is important to verify practical effectiveness of the CO2 storage. The pilot-scale CO2 sequestration experiment has been undertaken at the Nagaoka gas and oil field, in Niigata Prefecture, Japan and we conducted time-lapse cross-well seismic tomography to detect the spread of injected CO2. When super-critical CO2 spreads into porous media saturated with brine water, the seismic velocity always decreases, which is confirmed theoretically and experimentally. Therefore, CO2 flood area can be estimated from seismic tomography records before and after CO2 injection. However, the tomography records before and after CO2 injection include not only the variation due to CO2 flood, but also differences occurred from the location and property of sources and receivers and they result analysis errors. Thus, in this study, we applied the differential analysis to cross-well seismic tomography for reducing the difference due to measurement conditions and getting more accurate monitoring results. In the normal procedure, velocity differences are calculated after applying inversion processing to each record separately, but in this differential analysis, first forward modeling is applied using the velocity distribution inverted from one referential record, second the set of the first arrival time is obtained from the modeling result and finally the inversion processing is analyzed from the data set of the first arrival time added with the difference of travel-times from the referential record. In the result, we can have an analysis result in which the distribution of CO2 flooding is clearly identified along with a caprock under the inversion analysis with the constrained condition of no velocity enhancement.

Seafloor receiver function analysis for hybrid dataset composed of both refraction survey and earthquake records

In this study, we present two types of simulation models that consist of a sediment layer, a crust layer, and a sloped interface that are marked as seismic velocity discontinuities. Seismometers are aligned on the seafloor. One model has only surface seismic sources and the other has buried seismic sources that are equivalent to natural earthquakes. For every shot or earthquake, we obtain either refracted or converted waves from the velocity interfaces. We estimate a receiver function for each shot or earthquake for our sub-seafloor structural analysis. After these processing, the receiver function analysis is justified to estimate velocity interface and the applicable method of refraction wave analysis. For the first models, we confirmed that our receiver function could estimate the depth and shape of the velocity interface precisely. The refracted waves propagate beneath the velocity interfaces and generate both refracted compressional and converted shear waves that are used to estimate receiver functions at each location of seismometers. Our result indicates that the receiver function analysis can be applied to refraction seismic survey data using artificial seismic sources located near the surface above the target structure. Finally, we include seismic records from the buried sources in our receiver function analysis, and combine results from refraction survey and conventional earthquake data. Although data processing such as seismic migration becomes necessary, our results demonstrate that receiver function analysis can be applied not only for conventional earthquake records but also for artificial seismic surveys. Our analysis to synthetic data demonstrates the effectiveness of the method and the necessity of fair wave decomposition techniques to separate both compressional and shear waves from observed wavefield.

Imaging Underground Structure Using Receiver Function For P-S Converted Waves

Receiver function analysis is known as a method frequently practiced to utilize both vertical and horizontal components of seismic records to reveal subsurface structure using earthquake signals in natural seismology. In the receiver function analysis, a receiver function is estimated by the deconvolution of P-SV horizontal record with the vertical component. This receiver function is used to measure the phase delay of P-SV converted against P-P waves. Finally, the phase delay tells the depth of an interface that caused PSV conversion. It is obvious that the receiver function analysis is a method based on ray-theory for a horizontally stratified earth model. We tried to expand the methodology to an imaging tool to utilize the advantage of the receiver function analysis to use both P and SV waves recorded by tri-component geophones. Also, our modification makes it possible to use not only earthquake records but also artificial signals generated by active sources. We found that the method of receiver function can be extended as a general exploration method for imaging underground geological or geophysical interfaces that causing P-SV mode conversion in traveling seismic waves.

Identifying damaged areas inside a masonry monument using a combined interpretation of resistivity and ground-penetrating radar data

The Bayon Complex in the Angkor heritage site, Cambodia, has been damaged by weathering. To plan its long-term preservation, it is essential to investigate its internal structure and the degree of damage within the masonry monument. This study shows results of ground-penetrating radar (GPR) and electrical exploration surveys, and an interpreted section of the internal structure and moisture distribution in the masonry monument. The GPR can detect boundaries between stone blocks and between stone blocks and compacted soil. Electrical resistivity can indicate moisture distribution with high reliability in combination with GPR sections. The top surface zone of the terrace structure of this monument is composed of three layers of stone blocks, and the zone below a depth of 55–60 cm is composed of compacted soil. Rainwater penetrates into the terrace through gaps between the stone blocks and drains from vertical walls through cavities in the top part of the compacted soil. Damaged areas are limited to a part of the terrace, and a large area has remained in good condition. This study shows that a combination of electrical resistivity and GPR data is useful for investigating the internal structures and classifying the degree of damage to old stone structures. This study shows results of ground-penetrating radar and electrical exploration surveys, and an interpreted section of the internal structure and moisture distribution in the Bayon Complex in the Angkor heritage site, Cambodia. Rainwater penetrates into the monument through gaps between stone blocks and drains from vertical walls through cavities in the top part.

Classifying destruction areas in a stone structure from joint interpretation of resistivity and ground-penetrating radar data

The Bayon Complex in the Angkor Heritage Site, Cambodia has been destructed by weathering effect. Investigating the structure and the progressive degree of destruction inside the masonry monument is needed to preserve it for a long time. This study shows the survey results of electric exploration and groundpenetrating radar (GPR) and the interpreted sections of internal structure and moisture distribution. GPR can detect boundaries between stone blocks and a boundary between a stone block and a compacted soil layer. Electric resistivity can estimate moisture distribution using composed structures estimated from the GPR. The top subsurface area of the terrace structure of the monument is composed of three layers of stone blocks and the area below the depth of 55 cm or 60 cm is composed of compacted soil. Rainwater invades into the terrace through between stone blocks and drains from a vertical wall through cavities in the top zone of the compacted soil. Destruction areas are limited in a part of the terrace and wide areas keep good conditions. This study shows that the joint interpretation of resistivity and GPR data is useful to investigate internal structures and to classify destruction degrees of old stone structures.

Availability of Fresnel volume migration to three‐component seismic reflection data using tau‐P transforms

In seismic reflection surveys, the superposition of Pand Swaves often influences the resolution of obtained images. We, therefore, needs to first decompose multi-component data before imaging procedure even in Fresnel volume migration. For decomposition processing of refracted Pand Swaves, we need incident angles of received elastic waves. In this study, we calculated the incident angle for each receiver using tangential angles, one of which using amplitudes of acquired three component data and the other using tau-P transform. After the comparison, we used the latter considering that both incident angle and medium wave slowness could be estimated in the acquisition. Also, we use incident angles to the Fresnel volume migration. We found that the slant stack method showed better results for the data in which the secondary P-P reflected wave and first P-S reflected waves are overlapped.

Measuring Electrical Resistivity Variations in a Sandstone Specimen Injected with Gas, Liquid, and Supercritical CO2

Carbon dioxide sequestration into a deep aquifer is considered one of the most effective methods to solve the global warming problem. To realize the geological sequestration of CO2, we need to reduce injection cost and increase the accuracy of characterizing reservoirs for CO2 sequestration using inexpensive geophysical exploration methods. The interpretation of geophysical exploration records needs corresponding relationships between physical properties observed from geophysical exploration and CO2 saturation and phase state. We monitored the behavior of gas, liquid, and supercritical CO2 injected into a sandstone specimen saturated with brine by measuring resistivity variations. We made an experimental apparatus that reproduces the pressure of CO2 sequestration reservoirs. A cylindrical sample of Berea Sandstone (5 cm [1.9 in.] in diameter and 12 cm [4.7 in.] in length) was used in this experiment. Two current electrodes were set on both ends of the specimen. Five electrodes for measuring electric potential were installed on the side of the rock specimen. The side of the specimen was coated with silicone rubber. Carbon dioxide (gas, liquid, and supercritical phases) was flooded through the sandstone specimen at three different flow rates. We monitored the behavior of the resistivity as it changed over time in the sandstone as affected by the CO2. The saturations estimated from resistivity are nearly equal to the values calculated from actual outflow volumes. This result shows the high reliability of electric exploration to monitor CO2 saturation. The replacement ratio and storage volume of liquid CO2 are higher than those of supercritical CO2.

The estimation of subsurface carbon stocks for the Kyoto Protocol by ground-penetrating radar

Every advanced country will have to make efforts to reduce greenhouse gases in the atmosphere in compliance with the Kyoto Protocol. It has been decided in Bonn that the countries can estimate the amount of forest absorption generously, because it is difficult to achieve the target for the reduction of carbon dioxide emissions only by means of innovative environmental technology and enhanced energy efficiency in the society. However, uncertainty about the amount of carbon stocks in forests is large. As it is desirable to leave out uncertainty and estimate the amount conservatively in the spirit of the Kyoto Protocol, an accurate estimation would lead to large amounts of the estimated carbon stocks. In the forest, the underground carbon stock is larger than the amount above the ground and its uncertainty is very large. Until now, there has not been an effective method to estimate underground carbon stocks, and the only relatively good one is the method using models from the vegetation above ground. Thus, we suggest an estimating method using ground-penetrating radar (GPR) for improving the estimation method. Using the nondestructive inspection technology like GPR, we can reduce the estimation cost and obtain the quantified data. GPR surveys can classify the soil layers and identify the roots of trees (Rokugawa et al. (2000)).