[S-SS09] 地震波伝播:理論と応用

[SSS09-08] Joint inversion of seismic wave exploration and muography

★Invited Papers

*児玉 匡史¹、横田 俊之¹、松島 潤²、Hamid Basiri²、Ali Mohammed³、Bouchaala Fateh³、Eleslambouly Ahmed³、田中 宏幸⁴ (1.産業技術総合研究所、2.東京大学大学院、3.ハリファ大学、4.東京大学地震研究所)

キーワード：ミュオン、地震波、ジョイントインバージョン、密度、弾性定数

Seismic wave exploration is one of the main geophysical methods used in fluid resource exploration and carbon dioxide capture and storage monitoring. While seismic wave exploration is highly sensitive to the presence of fluids, it is difficult to distinguish between commercial, highly saturated gas layers and less saturated gas layers, suggesting that its sensitivity to changes in gas saturation is limited. The elastic wave velocities of the two types of elastic waves, namely P and S waves, are determined by trade-off relationship among three parameters, elastic constants (i.e. bulk and shear modulus) and density, and thus have uncertainty. Separating the elastic wave velocity into density and elastic constants provides information that is useful for estimating gas saturation and rock mechanical properties. However, additional information on density and elastic constants is required for the separation. Muography is a density estimation method that uses cosmic ray muons, which can penetrate objects on the order of kilometers. The higher the density of objects, the more difficult it is for muons to penetrate them, and by counting the number of transmitted muons, the average density of the transmission path can be estimated. In a previous study on integrated analysis of muography and seismic wave exploration, the applicability was preliminarily evaluated using one-dimensional observation model of seismic and muography. We extended this method to tomographic estimation of elastic constants and performed numerical experiments to evaluate its applicability and the effect of the bending of the ray path of the elastic wave by performing joint inversion with rock physics relationships between properties as constraints. Multiple observation geometries with muon detectors and seismic receivers in vertical and horizontal boreholes and seismic sources on the ground were considered for the numerical model of a subsurface with anomalies, the first arrival travel time and number of muon observations were calculated numerically and used for estimation of the elastic wave velocity, density, and elastic constants. In the inversion method, the posterior distribution of each property is estimated by updating the a priori distribution with the likelihood function based on the observed data. The estimated density, elastic wave velocity, and elastic constants were improved by the joint inversion in terms of accuracy and uncertainty. The results from different velocity models also suggest the need to consider the effects of elastic wave propagation in joint inversions.