| 講演要旨(和文) | | VLF-EM法は、地表で観測された磁場成分から水平方向の比抵抗分布図の作成には長けているが、比抵抗断面の推定は困難である。本研究では、磁気探査などで用いられる正規化勾配法を単一の周波数で求められた地表の磁場成分に適用し、比抵抗異常体の位置を特定、さらにはNFG値を磁場成分に重み付けることで擬似的比抵抗断面図の作成に成功した。低比抵抗異常体として与えた真の比抵抗値より高く推定されてはいるが、周囲よりも比抵抗値の低い領域がモデルの異常体の位置とほぼ一致することが確認できる。次に、地震探査で用いられるフェイズ・シフト法を地表の磁場成分に適用し、地下の比抵抗構造の可視化を試みた。こちらは先述の方法とは違い、複数の周波数を用いる必要があるが、計算コストをかける事なく簡便に地下構造を推定できる。これらの手法を組み合わせることで、これから取り組む予定の全波形インバージョンの初期モデルに役立つと思われる。 |
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| | 講演要旨(英文) | | VLF-EM method has been the powerful tool for mapping subsurface geological structures because of its low cost and short survey terms. However, it has not been tested to estimate a pseudo-resistivity section, both the apparent resistivity and the depth of conductive anomaly by using the measured magnetic components with a single frequency. In this study, the Normalized Full Gradient (NFG) method, generally used for the downward continuation of the potential filed data, was applied to the magnetic components at the surface. We confirmed that the weighted apparent resistivity values are lower in the vicinity of low resistivity anomaly than the surrounding area, although the estimated value is a little higher than the original value. Also, the phase-shift method, which is used in seismic migration, is applied to the horizontal magnetic components with multiple frequencies. The synthetic example shows that the migration method is effective for imaging the anomaly. We conclude that our simple technique give an approximate subsurface resistivity structures quickly, which is useful for geological interpretations and also for an initial model of the fullwave form inversion. |
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