The 18th International Symposium on Recent Advances in Exploration Geophysics (RAEG 2014)

Electrical prospecting is a method usually attempted for groundwater exploration. However, the method requires the stability of electrical contact resistance of electrode to the surrounding earth materials. Therefore, it sometimes becomes difficult to conduct a survey in areas where the contact resistance stability could not be assured. In this study, we propose to employ the electromagnetic propagation, which could be detected without any use of electrode, and to apply the phase-shift method as in seismic data processing. We apply the method to the horizontal magnetic components with multiple frequencies in order to image subsurface resistivity structures to locate groundwater. The survey is conducted more easily and shortly, if only the magnetic sensors above the surface are enough for estimating the structures. As in the seismic migration, both upward/downward imaging and the exploding reflector concepts can be applied to the horizontal magnetic components. The synthetic data examples show that the migration method is effective for imaging the conductive anomaly. However, it is necessary to select appropriate frequency bands and to understand correct resistivity structures to some extent in order to estimate the subsurface ones. We conclude that this technique gives an approximate resistivity structures quickly and that the migration of magnetic components is expected to provide information on the subsurface. This method is also useful for geological interpretations and for an initial model of the more complicated inversion method.

In this study, we develop a 3D inversion scheme for the transient self potential (SP) data to estimate hydraulic conductivity, streaming current coefficient and specific storage. We test our inversion scheme with the synthetic data according to the pumping test. Our inversion could reconstruct the structure of hydraulic conductivity properly. However, the resolution of the estimation image of specific storage is not as clear as the image of hydraulic conductivity. To detect the factor that decides the resolution in the estimation image of the specific storage, we check the variation of the sensitivity of specific storage during the pumping. The specific storage is sensitive just after the beginning and the termination of the pumping. The stage when the specific storage is sensitive has been kept in the shorter duration than the stage of the hydraulic conductivity. The resolution of estimated image would be affected by the pattern of the transient SP data and pumping data. To improve the resolution in the estimation image of specific storage, the inversion of the synthetic transient SP data according to the repetitive pumping and injection is performed. The inversion image of the specific storage was improved under this condition.

Scaling is a key phenomenon that obstructs fluid circulation in plumbing system. Our goal in this study is to evaluate the importance of both chemical kinetic and hydrodynamic effects on silica deposition. For qualitative and quantitative discussion, we estimate the amount and the distribution of silica deposition with these two processes based on the numerical simulation and compare with the observation from a field experiment. We solve the fluid, temperature and dissolved silica concentration field by using the lattice Boltzmann method. From our simulation results, it is found that the effects of the hydrodynamic process are very important to reproduce the growth of scale qualitatively, whereas the simple chemical kinetic deposition could not sufficiently contribute to the real silica deposition. It is, therefore, necessary to emphasize the hydrodynamic effect should be take into account for reproducing silica scaling.

Unsteady fluid dynamics in Newtonian and non-Newtonian fluid is the main concern of aeronautical, mechanical, chemical, resource, and civil engineering fields, etc. Seismic stimulation is known as one of the Enhanced Oil Recovery (EOR) methods and is regarded as one of unsteady flow problems. Numerous observations show that seismic stimulation to oil reservoir may improve oil production. However, for the application of seismic EOR efficiently, we need to understand the characteristics of changing apparent viscosity of fluid in geological formation. In this study, we attempt to demonstrate the apparent viscousity of fluid in laminar flow under oscillating boundary condition with the models of a single pore throat and porous media. In this study, we set up two hypotheses: one is pressure disturbance causes apparent viscosity change and the other is the apparent viscosity of porous media can be estimated in the use of apparent viscosity of single pore throats. Then, We find pressure disturbance causes apparent viscosity change as a function of the incident angle of shear waves in either vertical or horizontal and the tortuosity of pore throats. And we finally find that the change in the apparent viscosity for a network system of pores and pore throats could be different from that for a single pore throat model due to local pressure disturbance.

Full waveform inversion (FWI) produces subsurface images by minimizing the misfit between observed data and calculated model using iterative local optimization algorithms like conjugate gradient method. This approach requires a starting model which should appear in the neighborhood of the global solution of the FWI problem to ensure that the modeled waveforms are less than half a period away from the recorded data. Usually, reflection traveltime tomography is used to create a long-wavelength background velocity model for starting FWI iterations. In this paper we suggest an alternative method to develop the starting model for FWI by using a reflectivity inversion technique. A depth section of migrated data is used to extract the reflection coefficients and impedance section, then the impedance section is converted to velocity model by considering a known density model. The reflectivity inversion can detect subsurface geological structures very well and on the other hand, an approximate known density model is a fair assumption for FWI and does not dramatically affect the long-wavelength model. We applied our method on a part of Marmousi2 model in order to develop P and S wave velocity models via elastic full waveform inversion in the frequency domain.

Seismic Full-Waveform Tomography (FWT) method has been used to estimate velocity structure in the subsurface using acoustic wave equation. In elastic FWT, density is usually estimated using an empirical formula or is fixed to a constant value. Almost all elastic FWT studies have ignored the influence of density that could be an important parameter. The objective of this study is to investigate the difficulty of estimating density structure and propose a new approach. In this study, we perform series of numerical simulations in order to investigate the important factor in the inversion of density structure. Our results show that it is difficult to estimate an accurate density model in the density sorely inversion because the density structure is less effective to the waveform, and the density parameter is more strongly related to the Vp parameter than Vs. We propose s new strategy based on the separations of P-wave and S-wave, and of two inversion stages: the first stage to invert Vp and density structures simultaneously, and the second one to invert Vp, Vs and density together. We conclude that this strategy indicates effective way to estimate an accurate density structure.

The fluid distribution in the hydrocarbon reservoir affects waveforms acquired in seismic reflection method. A reflected wave changes its waveform at the transition zone of the interface of two different fluids as a function of volume fraction of the two. AVO is in general used to estimate the difference in the P and S wave velocities for the interfacing two media at the interface without any assumptions on the existence of the transition zone. The consideration of the effect of the volume fraction of a fluid to the other in the waveform could be a key for evaluating the fluid mixture around the fluid contact in the reservoir. Therefore, we try to use the waveform directly to estimate fluid distribution in the transition zone that has not been done in the practice of AVO. In our research, we consider the effects of the transition zone at a gas-water contact (GWC) in a horizontally stratified medium on seismic waveforms. The numerical simulation reveals that the fluid distribution of transition zone distorts the seismic waveform both in amplitude and in phase. We developed a full-waveform AVO inversion method to apply to the fluid substitution problems to see if the method is applicable to estimate the fluid contact with the transition zone, while the conventional AVO only utilizes the amplitude derived from observed data. The simulation results imply the advantages in the estimation of the parameters including the thickness of the transition zone under that assumption of linear trend in the volume fraction in a contrast porosity condition. We suggest that the phase information should be used simultaneously for the inversion process to get the closer contact image.

In the developing of the hydrothermal or shale oil and gas reservoir, hydraulic fracturing method is widely used today. It creates fracture nets around the well and improves the permeability, which enhances the production of those resources. However, it is challenging task to predict the direction and area of the hydraulic fracturing due to the complicated in-situ stress field. Fracturing to the unexpected area might cause induced seismicity or environmental pollution. It is, therefore, quite important to estimate how fractures are creates in the reservoir to get the idea of the production of resources and prevent the environmental pollution. We focused on how the pre-existing fractures propagate with the steady and non-steady injection pressures. In this study, especially we could adopt the multiple pre-existing fractures, which exist symmetrically around the borehole. To deal with the multiple fractures in the hydraulic fracturing simulation is much important to understand the realistic behavior of the fracture propagation. We simulated how the fractures could propagate with the steady and non-steady hydraulic pressure during the water injection. We could obtain two results from our numerical simulations: First, the curvature of the fracture trace depends on hydraulic pressure with the both cases of the steady and non-steady pressure, but no matter how the fluid pressure is, the orientation of fracture propagation finally converges to that of maximum principal stress. Second, the transition of the stress field follows the change of the injection pressure after some seconds, which means the delayed response of the fracture propagation in the non-steady hydraulic pressure condition. The simulation scheme of this study can deal with multiple fractures so that it could realize the quantitative estimation of fracture propagation in the complex stress region including various scales of fractures in the future.