The 21st International Symposium on Recent Advances in Exploration Geophysics (RAEG 2017)

In recent years, development of a monitoring technique (Passive Seismic Emission Tomography method: PSET) using vibration generated in the reservoir has been widely progressed. However, monitoring with PSET only estimates the extent of the reservoir. Moreover, its theoretical knowledge is lacking at present. On the other hand, we proposed a method to monitor underground from vibration caused by fluid flow inside the reservoir. We are conducting research based on the hypothesis that the difference in the fluid in the reservoir is reflected to the seismic wave generated in the reservoir. In the previous research, we showed that it is possible to visualize the reservoir area by measuring the seismic wave caused by the two-phased oil and water flow for a long time. In this study, we calculate seismic waves using the Green’s function occurring with water-oil two-phase flow and gas-oil two-phase flow. As a result, the vibration caused by the flow of the fluid not only indicates the flowing area but also information on the properties of the fluid. It is suggested that this result may be effective as a method to monitor the change of the fluid properties in the reservoir associated with oil production and enhanced oil recovery method.

For exploring subsurface resources such as oil or natural gas reservoirs, seismic reflection survey has been widely implemented in order to image subsurface structures. In recent years, utilization of S-wave or converted wave is required for estimating lithology or petrophysical properties of reservoir rock. However, such an analysis of S-wave seismograms had been relatively difficult. On the other hand, equivalent offset migration (EOM) is one of the prestack time migrations and has been found to be effective method for imaging S-wave information on the common scatter point (CSP) gather with recorded horizontal component in our previous study. Furthermore, S-wave AVO effect has also been confirmed by the amplitude reversal of S-wave event on the CSP gathers. Therefore, we propose the procedure of accurate estimation of densities and shear modulus with S-wave source. First, we conduct numerical experiment with a 2D layer model using horizontal point force to obtain horizontal component seismic data, in which we can get higher S/N data about S-wave. Second, we implement EOM with those data to get CSP gather, and calculate each cross-correlated value versus incident angle as observed waveform information. Third, in contrast, we generate calculated waveform information as a function of incident angle and physical properties with geometrical spreading, radiation pattern and S-wave reflection coefficient. Finally, we can estimate the optimal solutions by minimizing the misfit from the both information.

We have confirmed the interaction among phased array antennas in our previous study. In this paper, we would like to discuss one of the powerful and practical schemes of using phased array antennas applied to near surface exploration using ground penetrating radar (GPR). GPR emits electromagnetic (EM) waves to the subsurface and to measure signals reflected back from buried anomalies for the estimation of the positions and shapes of the anomalies. Although phased array antennas could generate EM waves whose signal-to-noise ratio is superior to that emitted by the conventional GPR antenna, there has been a risk of the interaction among plural antennas could lower the signal-to-noise ratio due to electrical currents induced by magnetic field generated by the other antennas and it was necessary to confirm the level of the interaction or the unfavorable currents caused by the other antennas. We conduct a numerical simulation to evaluate the mutual coupling of each pair of phased array antennas to determine the best alignment of antennas in the design to maximize the level of emitted signals. We then conduct another simulation with heterogeneous ground model using our designed antenna in order to confirm the effectiveness of the phased array antenna in practice. Our results show that the phased array antenna enhances the signal-to-noise ratio compared to the conventional antenna and has a higher sensitivity to the targets located lateral to the survey lines with low interaction level between the neighboring two antennas. We conclude that the phased array antenna has a potential to be used as a new radar source for GPR.

It is necessary to obtain an accurate underground velocity structure to grasp the image of subsurface in seismic survey. Among various estimation methodologies, borehole logging is one of the best ways accurately to estimate the rock elastic properties of the ground around the wellbore. In the conventional study, the combination of Alford rotation with slowness time coherence (STC) has been applied to estimate both the formation velocity and the azimuth angles under the existence of azimuthal anisotropy in the formation. However, it has been revealed the approach with Alford rotation could fail or gives improper estimates when the axis of symmetry of the anisotropic later does not lie in the plane orthogonal to the well axis.

In this study, we conduct numerical simulation for transversely isotropic medium (TI) which has 5 independent stiffness elements in 3-dimensional logging model. In recent years, full waveform inversion (FWI) has been focused which could estimate physical properties by using all information of waveforms. We investigate the feasibility of FWI to detect the orientation and dip of TI. We introduce the Euler angles into TI to estimate the stiffness parameters by FWI instead of estimating the stiffness parameters as orthorhombic medium under a hypothesis that the stable solution can be obtained by introducing the Euler angles. This approach can reduce unknowns in FWI, i.e. computational efficiency and stability of inversion could be improved. The result clearly indicates that the FWI for anisotropic medium is effective in order to detect the shear wave anisotropy and stable solution could be obtained according to misfit function even when the anisotropic layer has the dip and orientation.

We newly developed a finite-difference (FD) operator for the frequency-domain acoustic wave propagation. This operator uses a stretched stencil to avoid the numerical anisotropy. In general, direct solvers for sparse matrices are used in exploration geophysical community because they have advantage over iterative ones, i.e. multi-source configuration can be simply implemented. In the frequency-domain modling, the computational costs (calculation time and computational memory) depend on not only the number of neighbors but also the bandwidth of the impedance matrix. So usage of higher-order scheme is not always conducive to the improvement of the computational costs. In the present study, we use a stretched stencil of FD operator not to increase the bandwidth in the impedance matrix. Coefficients of the stencil are determined by a minimization process. We investigate the accuracy of our new scheme using dispersion analysis and numerical experiment. They show that the proposed scheme can improve not only accuracy but also efficiency compared to the conventional 9-point scheme.