Three dimensional magnetotelluric inversion using heterogenious smoothness-constraint least-squares method

不均質平滑化拘束条件付き三次元MTインバージョン法の開発

根木 健之*; 水永 秀樹*; 浅森 浩一; 梅田 浩司

Negi, Tateyuki*; Mizunaga, Hideki*; Asamori, Koichi; Umeda, Koji

将来の地層処分システムに重大な影響を及ぼす可能性がある現象の潜在的なリスクを排除するためには、地表からの調査の段階において、地下深部における震源断層などの存否や構造をあらかじめ確認しておくための調査技術が必要となる。本研究では、震源断層などの地殻内の物性境界を適切に推定可能な三次元MTインバージョン法を構築するため、インバージョンにおいて平滑化拘束条件として用いるラプラシアン・フィルタ行列の各要素を反復計算の過程で修正するアルゴリズムを考案し、これを組み込んだ三次元インバージョン法を開発した。さらに、地下深部の物性境界の検出に対し、本手法が従来のインバージョン法に比して有効であることを数値テストにより示した。

Electromagnetic surveys have been widely used for several decades in geothermal, mining and oil exploration or disaster prevention. More recently, three-dimensional surveys have been increasingly carried out. However, they require the large-scale inversion consuming significant computation time. Traditional inversion algorithms based on the linear sensitivity matrix consume time to update the sensitivity matrix in the several iterations. In the presentation, we show a rapid calculation algorithm for the electromagnetic inversion procedure that allows to bypass updating of the sensitivity matrix. In the procedure, a fixed sensitivity matrix for a homogeneous half-space is used in all iterations. Instead of the fixing sensitivity matrix, the Laplace filter coefficients in the each element are updated based on the resistivity smoothness calculated by reconstructed structure. This substitution is not only expected to reduce the computing time required for the large-scale inversion such as for the three dimensional surveys, but also expected to allow the reconstruction for sharp boundary structure. We applied the algorithm for the three dimensional magnetotelluric inversion algorithms to confirm the effectiveness. In the presentation, we show the calculation results using several synthetic data examples with different structures and conditions.