The 22nd International Symposium on Recent Advances in Exploration Geophysics (RAEG 2018)

Ground Penetrating Rader (GPR) is a nondestructive testing method to visualize shallow subsurface using reflection of downgoing electromagnetic (EM) waves generated on the surface. The processing of GPR data has been developed using that of reflection seismology and the method is applied to survey wide area in an expeditious way, GPR has been widely used in many engineering fields. Quantitative analyses based on physical properties of buried targets have, however, not been well attempted in spite of increasing interest to the identification of subsurface materials in recent years. Since the combination of conductivity, permittivity, and magnetic permeability contrasts of material discontinuities causes the attenuation and the dispersion of electromagnetic waves that propagate through, reflected electromagnetic signals could be utilized to the material identification. In this study, we employed the Cole-Cole equation to analyze the attenuation and the dispersion of electromagnetic reflection signals for possible material identification using what are called the Cole parameters, i.e., physical properties. Since there are many materials for which the Cole parameters are not estimated, we employed a method using reflection signals for a two-layered earth model, that has a top layer of a material with the known Cole parameters and the second layer for which the Cole parameters are estimated. We use the dry soil as the second layer to estimate the physical properties using the Cole-Cole equation. Although we use very wide range of frequency in this study as well to discuss the optimum frequency band, we get good estimation result from the frequency band from 10⁷Hz to 10⁸Hz, which is usually used in GPR.

For improving the production of conventional oil and shale gas, the practice of hydraulic fracturing has been increasing in recent years. In addition, hydraulic fracturing is used for the development of geothermal energy known as hot dry rock (HDR) geothermal power, and enhanced geothermal system (EGS), and for measuring the rock failure strength and the orientation of principal stress direction, etc. On the other hand, hydraulic fracturing has some environmental impact, such as pollution caused by chemical substances in injected proppant or fluid, induced seismicity, etc. Since it is necessary to minimize the environmental impact, techniques to predict propagating directions and distances of fractures to be generated hydraulically, which are known still very difficult, have been waited for. In this paper, we demonstrate the influence of differential stress and the anisotropy using numerical experiments based on distinct element method (DEM) combined with smooth joint model (SJM). Hydraulic fractures in general propagate in the direction of maximum principal stress on large differential stress conditions. As the differential stress decreased, the propagating directions hydraulic fractures curves to the direction of bedding plane, i.e., anisotropic direction of weak rock strength, and sometimes fractures branch to plural directions. These results suggest that the behavior and propagating direction of hydraulic fractures are strongly influenced by both the differential stress and the rock strength anisotropy in the underground shallow layer.

In natural earthquake tomography targeting a single volcano, temporary seismic observation is carried out in order to improve resolution in general. However, since the number of observation points and observation period are limited, it is important to grasp the presumable resolution in advance. Therefore, we conducted to examine how much resolution we can estimate the underground velocity structure by performing temporary seismic observation with how many observation points and observation period (the number of earthquakes), when carrying out natural seismic tomography on Hachijojima, Kozushima and Niijima of volcanic islands in Tokyo by using numerical experiments, a checkerboard resolution test (CRT). CRT is one of the reasonable tests to show the resolution of the natural earthquake tomography and to evaluate the influence of parameters with good resolution (a pair of grid point spacing, observation points and number of earthquakes) on the resolution. As the results of parametric study using CRT, we have found that in every island I have researched, when setting a pair of some grid point spacing, some number of observation points and some number of earthquakes, they were possible to image with a resolution of some degree. Considering that temporary seismic observation will be carried out in volcanic islands, Tokyo, in the future, the data obtained in this study will be very important for establishing the observation plan.

Recently, foam-assisted enhanced oil recovery (EOR) has drawn attention due to its effectiveness. However, the method to explore how the subsurface sweep foam front moves has not been fully studied yet. In the previous study, our numerical results indicated that subsurface foam distribution could be detected by seismic method with an amplitude versus offset (AVO) analysis. Since the effectiveness of the seismic method for capturing the location of foam front has been validated, our next step is to extend its applicability to monitor the movement of foam-saturated zone in a quantitative way. In this study, we conducted numerical experiments to examine the effectiveness of wave-theoretical seismic methods to time-lapse monitoring. We set 2D subsurface models supposing foam-assisted EOR with CO2 and water injection. We make synthetic data sets for these two models with different position of sweep foam front. We take the difference of waveforms between before and after the advancement of foam sweep front, and then, back propagate these residuals as sources. From the correlation of the forward and the backward wave fileds, we got the images of the vertical wave field which exaggerate the diffraction caused by the difference of position of foam sweep front. This result indicates that seismic exploration could detect the location of physical property change due to the advancement of sweep foam front in the practice of foam-assisted EOR.

The sanding is recognized as one of the main problems in producing hydrocarbon resources. Although it is important to predict the occurrence of the sanding during production, the prediction or countermeasure still remains to be established. In this study, we conduct numerical simulation of fluid-solid multi-phase flow in order to investigate the factors which affect the occurrence of the sanding. Our numerical model is a fluid path consists of a set of planar plates of a finite length filled by fluid and a mound of small solid grains. A fluid flow is produced by the pressure gradient with different magnitudes between the both end of the path. We found that a few small grains leave the mound with a high pressure gradient whereas no floating grains were observed with a low pressure gradient. We also observed a reasonable change in the flow velocity and relative permeability when the floating of small grains occurs. This result indicates that the sanding can be detected by observing the time variation of flow rate during production.

A novel scheme we proposed for analyzing S-wave azimuthal anisotropic angle in the subsurface below the seafloor has been applied to a tilted transversally isotropic medium of horizontal axis of symmetry (HTI). The proposed method utilizes the virtual source method to acquired data set with single ocean bottom seismometer and an air-gun array. To evaluate the effectiveness of our scheme, we conducted the numerical experiments for 3D model with a tilted anisotropic target. We applied this method to the synthetic data to make the virtual cross-dipole data at each shot location. Finally we applied the Alford rotation to estimate an azimuthal angle of the anisotropy of the target layer. Our numerical results show the limitations of the Alford rotation. Especially, the Alford rotation assumes that a dipole signal in one direction has an amplitude equivalent to the other dipole to the other normal direction in the cross dipole measurements. Since our approach could not assume the equality of the amplitudes for each of the cross dipole signals, we conclude that a strategy using full waveforms needs to be taken into consideration for estimating azimuthal angle of tilted targets.